Publications

Research on sphingolipids has proliferated exponentially over the past couple of decades, as exemplified in the findings reported at the International Leopoldina Symposium on Lipid Signaling held in Frankfurt in late 2023. Most researchers in the field study how sphingolipids function in regulating a variety of cellular processes and, in particular, how they are dysregulated in numerous human diseases; however, I now propose that we implement a more holistic research program in our study of sphingolipids, which embraces a sense of awe and wonder at the complexities and beauty of sphingolipids and of sphingolipid metabolism. I will outline the chemical complexity of sphingolipids, their modes of interaction within the lipid bilayer, and their biosynthetic pathways. I will then briefly touch upon the ability of current neo-Darwinian mechanisms to explain the emergence of both sphingolipids and of the complex pathways that generate them. Although such discussion is normally considered taboo in biological circles, I nevertheless submit that in-depth analysis of the minutiae of metabolic pathways, such as those of the sphingolipid biosynthetic pathway, raises challenges to current neo-Darwinian mechanisms that should not be shunned or ignored.

Almost all attempts to date at gene therapy approaches for monogenetic disease have used the amino acid sequences of the natural protein. In the current study, we use a designed, thermostable form of glucocerebrosidase (GCase), the enzyme defective in Gaucher disease (GD), to attempt to alleviate neurological symptoms in a GD mouse that models type 3 disease, i.e. the chronic neuronopathic juvenile subtype. Upon injection of an AAVrh10 (adeno-associated virus, serotype rh10) vector containing the designed GCase (dGCase) into the left lateral ventricle of Gba^−/−;Gba^tg mice, a significant improvement in body weight and life-span was observed, compared to injection of the same mouse with the wild type enzyme (wtGCase). Moreover, a reduction in levels of glucosylceramide (GlcCer), and an increase in levels of GCase activity were seen in the right hemisphere of Gba^−/−;Gba^tg mice, concomitantly with a significant improvement in motor function, reduction of neuroinflammation and a reduction in mRNA levels of various genes shown previously to be elevated in the brain of mouse models of neurological forms of GD. Together, these data pave the way for the possible use of modified proteins in gene therapy for lysosomal storage diseases and other monogenetic disorders.

In eukaryotes, the de novo synthesis of sphingolipids (SLs) consists of multiple sequential steps which are compartmentalized between the endoplasmic reticulum and the Golgi apparatus. Studies over many decades have identified the enzymes in the pathway, their localization, topology and an array of regulatory mechanisms. However, little is known about the evolutionary forces that underly the generation of this complex pathway or of its anteome, i.e., the metabolic pathways that converge on the SL biosynthetic pathway and are essential for its activity. After briefly describing the pathway, we discuss the mechanisms by which the enzymes of the SL biosynthetic pathway are targeted to their different subcellular locations, how the pathway per se may have evolved, including its compartmentalization, and the relationship of the pathway to eukaryogenesis. We discuss the circular interdependence of the evolution of the SL pathway, and comment on whether current Darwinian evolutionary models are able to provide genuine mechanistic insight into how the pathway came into being.

Gaucher disease (GD) is a lysosomal storage disorder (LSD) caused by the defective activity of acid β-glucosidase (GCase) which results from mutations in GBA1. Neurological forms of GD (nGD) can be generated in mice by intra-peritoneal injection of conduritol B-epoxide (CBE) which irreversibly inhibits GCase. Using this approach, a number of pathological pathways have been identified in mouse brain by RNAseq. However, unlike transcriptomics, proteomics gives direct information about protein expression which is more likely to provide insight into which cellular pathways are impacted in disease. We now perform non-targeted, mass spectrometry-based quantitative proteomics on brains from mice injected with 50 mg/kg body weight CBE for 13 days. Of the 5038 detected proteins, 472 were differentially expressed between control and CBE-injected mice of which 104 were selected for further analysis based on higher stringency criteria. We also compared these proteins with differentially expressed genes (DEGs) identified by RNAseq. Some lysosomal proteins were up-regulated as was interferon signaling, whereas levels of ion channel related proteins and some proteins associated with neurotransmitter signaling were reduced, as was cholesterol metabolism. One protein, transglutaminase 1 (TGM1), which is elevated in a number of neurodegenerative diseases, was absent from the control group but was found at high levels in CBE-injected mice, and located in the extracellular matrix (ECM) in layer V of the cortex and intracellularly in Purkinje cells in the cerebellum. Together, the proteomics data confirm previous RNAseq data and add additional mechanistic understanding about cellular pathways that may play a role in nGD pathology.

The unique biophysical and biochemical properties of lipids render them crucial in most models of the origin of life (OoL). Many studies have attempted to delineate the prebiotic pathways by which lipids were formed, how micelles and vesicles were generated, and how these micelles and vesicles became selectively permeable towards the chemical precursors required to initiate and support biochemistry and inheritance. Our analysis of a number of such studies highlights the extremely narrow and limited range of conditions by which an experiment is considered to have successfully modeled a role for lipids in an OoL experiment. This is in line with a recent proposal that bias is introduced into OoL studies by the extent and the kind of human intervention. It is self-evident that OoL studies can only be performed by human intervention, and we now discuss the possibility that some assumptions and simplifications inherent in such experimental approaches do not permit determination of mechanistic insight into the roles of lipids in the OoL. With these limitations in mind, we suggest that more nuanced experimental approaches than those currently pursued may be required to elucidate the generation and function of lipids, micelles and vesicles in the OoL.

A number of genetic risk factors have been identified over the past decade for Parkinsons Disease (PD), with variants in GBA prominent among them. GBA encodes the lysosomal enzyme that degrades the glycosphingolipid, glucosylceramide (GlcCer), with the activity of this enzyme defective in Gaucher disease. Based on the ill-defined relationship between glycosphingolipid metabolism and PD, we now analyze levels of various lipids by liquid chromatography/electrospray ionization-tandem mass spectrometry in four brain regions from age- and sex-matched patient samples, including idiopathic PD, PD patients with a GBA mutation and compare both to control brains (n=21 for each group) obtained from individuals who died from a cause unrelated to PD. Of all the glycerolipids, sterols, and (glyco)sphingolipids (251 lipids in total), the only lipid class which showed significant differences were the gangliosides (sialic acid-containing complex glycosphingolipids), which were elevated in 3 of the 4 PD-GBA brain regions. There was no clear correlation between levels of individual gangliosides and the genetic variant in Gaucher disease [9 samples of severe (neuronopathic), 4 samples of mild (non-neuronopathic) GBA variants, and 8 samples with low pathogenicity variants which have a higher risk for development of PD]. Most brain regions, i.e. occipital cortex, cingulate gyrus, and striatum, did not show a statistically significant elevation of GlcCer in PD-GBA. Only one region, the middle temporal gyrus, showed a small, but significant elevation in GlcCer concentration in PD-GBA. We conclude that changes in ganglioside, but not in GlcCer levels, may contribute to the association between PD and GBA mutations.

Dihydroceramide is a lipid molecule generated via the action of (dihydro)ceramide synthases (CerSs), which use two substrates, namely sphinganine and fatty acyl-CoAs. Sphinganine is generated via the sequential activity of two integral membrane proteins located in the endoplasmic reticulum. Less is known about the source of the fatty acyl-CoAs, although a number of cytosolic proteins in the pathways of acyl-CoA generation modulate ceramide synthesis via direct or indirect interaction with the CerSs. In this study, we demonstrate, by proteomic analysis of immunoprecipitated proteins, that fatty acid transporter protein 2 (FATP2) (also known as very long-chain acyl-CoA synthetase) directly interacts with CerS2 in mouse liver. Studies in cultured cells demonstrated that other members of the FATP family can also interact with CerS2, with the interaction dependent on both proteins being catalytically active. In addition, transfection of cells with FATP1, FATP2, or FATP4 increased ceramide levels although only FATP2 and 4 increased dihydroceramide levels, consistent with their known intracellular locations. Finally, we show that lipofermata, an FATP2 inhibitor which is believed to directly impact tumor cell growth via modulation of FATP2, decreased de novo dihydroceramide synthesis, suggesting that some of the proposed therapeutic effects of lipofermata may be mediated via (dihydro)ceramide rather than directly via acyl-CoA generation. In summary, our study reinforces the idea that manipulating the pathway of fatty acyl-CoA generation will impact a wide variety of down-stream lipids, not least the sphingolipids, which utilize two acyl-CoA moieties in the initial steps of their synthesis.

Cell function is highly dependent on membrane structure, organization, and fluidity. Therefore, methods to probe the biophysical properties of biological membranes are required. Determination of generalized polarization (GP) values using Laurdan in fluorescence microscopy studies is one of the most widely-used methods to investigate changes in membrane fluidity in vitro and in vivo. In the last couple of decades, there has been a major increase in the number of studies using Laurdan GP, where several different methodological approaches are used. Such differences interfere with data interpretation inasmuch as it is difficult to validate if Laurdan GP variations actually reflect changes in membrane organization or arise from biased experimental approaches. To address this, we evaluated the influence of different methodological details of experimental data acquisition and analysis on Laurdan GP. Our results showed that absolute GP values are highly dependent on several of the parameters analyzed, showing that incorrect data can result from technical and methodological inconsistencies. Considering these differences, we further analyzed the impact of cell variability on GP determination, focusing on basic cell culture conditions, such as cell confluency, number of passages and media composition. Our results show that GP values can report alterations in the biophysical properties of cell membranes caused by cellular adaptation to the culture conditions. In summary, this study provides thorough analysis of the factors that can lead to Laurdan GP variability and suggests approaches to improve data quality, which would generate more precise interpretation and comparison within individual studies and among the literature on Laurdan GP.

Mutations in GBA, which encodes the lysosomal enzyme glucocerebrosidase, are the highest genetic risk factor for Parkinson's disease (PD), although the mechanistic link between GBA mutations and PD is unknown. An attractive hypothesis is that the lipid substrate of glucocerebrosidase, glucosylceramide, accumulates in patients with PD with a GBA mutation (PD-GBA). Despite the availability of new and accurate methods to quantitatively measure brain glucosylceramide levels, there is little evidence that glucosylceramide, or its deacetylated derivative, glucosylsphingosine, accumulates in human PD or PD-GBA brain or cerebrospinal fluid. Thus, a straightforward association between glucosylceramide levels and the development of PD does not appear valid, necessitating the involvement of other cellular pathways to explain this association, which could involve defects in lysosomal function.

Sphingolipids can be differentiated from other membrane lipids by the distinctive chemistry of the sphingoid long chain base (LCB), which is generated by the condensation of an amino acid (normally but not always serine) and a fatty acyl CoA (normally palmitoyl CoA) by the pyridoxal phosphate-dependent enzyme, serine palmitoyl transferase (SPT). The first five carbon atoms of the sphingoid LCB, herein defined as the sphingoid motif, are largely responsible for the unique chemical and biophysical properties of sphingolipids since they can undergo a relatively large number (compared to other lipid species) of molecular interactions with other membrane lipids, via hydrogen-bonding, charge-pairing, hydrophobic and van der Waals interactions. These interactions are responsible, for instance, for the association of sphingolipids with cholesterol in the membrane lipid bilayer. Here, we discuss some of the unique properties of this sphingoid motif, and in addition to outlining how this structural motif drives intra-bilayer interactions, discuss the atomic details of the interactions with two critical players in the biosynthetic pathway, namely SPT, and the ceramide transport protein, CERT. In the former, the selectivity of sphingolipid synthesis relies on a hydrogen bond interaction between Lys379 of SPTLC2 and the l-serine sidechain hydroxyl moiety. In the latter, the entire sphingoid motif is stereoselectively recognized by a hydrogen-bonding network involving all three sphingoid motif heteroatoms. The remarkable selectivity of these interactions, and the subtle means by which these interactions are modified and regulated in eukaryotic cells raises a number of challenging questions about the generation of these proteins, and of their interactions with the sphingoid motif in evolutionary history.

Niemann-Pick disease type C (NPC) is a complex and rare pathology, which is mainly associated to mutations in the NPC1 gene. This disease is phenotypically characterized by the abnormal accumulation of multiple lipid species in the acidic compartments of the cell. Due to the complexity of stored material, a clear molecular mechanism explaining NPC pathophysiology is still not established. Abnormal sphingosine accumulation was suggested as the primary factor involved in the development of NPC, followed by the accumulation of other lipid species. To provide additional mechanistic insight into the role of sphingosine in NPC development, fluorescence spectroscopy and microscopy were used to study the biophysical properties of biological membranes using different cellular models of NPC. Addition of sphingosine to healthy CHO-K1 cells, in conditions where other lipid species are not yet accumulated, caused a rapid decrease in plasma membrane and lysosome membrane fluidity, suggesting a direct effect of sphingosine rather than a downstream event. Changes in membrane fluidity caused by addition of sphingosine were partially sustained upon impaired trafficking and metabolization of cholesterol in these cells, and could recapitulate the decrease in membrane fluidity observed in NPC1 null Chinese Hamster Ovary (CHO) cells (CHO-M12) and in cells with pharmacologically induced NPC phenotype (treated with U18666A). In summary, these results show for the first time that the fluidity of the membranes is altered in models of NPC and that these changes are in part caused by sphingosine, supporting the role of this lipid in the pathophysiology of NPC.

Gaucher disease (GD) is caused by homozygous mutations in the GBA1 gene, which encodes the lysosomal β-glucosidase (GBA) enzyme. GD affects several organs and tissues, including the brain in certain variants of the disease. Heterozygous GBA1 variants are a major genetic risk factor for developing Parkinson's disease. The RIPK3 kinase is relevant in GD and its deficiency improves the neurological and visceral symptoms in a murine GD model. RIPK3 mediates necroptotic-like cell death: it is unknown whether the role of RIPK3 in GD is the direct induction of necroptosis or if it has a more indirect function by mediating necrosis-independent. Also, the mechanisms that activate RIPK3 in GD are currently unknown. In this study, we show that c-Abl tyrosine kinase participates upstream of RIPK3 in GD. We found that the active, phosphorylated form of c-Abl is increased in several GD models, including patient's fibroblasts and GBA null mice. Furthermore, its pharmacological inhibition with the FDA-approved drug Imatinib decreased RIPK3 signaling. We found that c-Abl interacts with RIPK3, that RIPK3 is phosphorylated at a tyrosine site, and that this phosphorylation is reduced when c-Abl is inhibited. Genetic ablation of c-Abl in neuronal GD and GD mice models significantly reduced RIPK3 activation and MLKL downstream signaling. These results showed that c-Abl signaling is a new upstream pathway that activates RIPK3 and that its inhibition is an attractive therapeutic approach for the treatment of GD.

(1) Background: six mammalian ceramide synthases (CerS1-6) determine the acyl chain length of sphingolipids (SLs). Although ceramide levels are increased in murine allergic asthma models and in asthmatic patients, the precise role of SLs with specific chain lengths is still unclear. The role of CerS2, which mainly synthesizes C22-C24 ceramides, was investigated in immune responses elicited by airway inflammation using CerS2 null mice. (2) Methods: asthma was induced in wild type (WT) and CerS2 null mice with ovalbumin (OVA), and inflammatory cytokines and CD4 (cluster of differentiation 4)+ T helper (Th) cell profiles were analyzed. We also compared the functional capacity of CD4+ T cells isolated from WT and CerS2 null mice. (3) Results: CerS2 null mice exhibited milder symptoms and lower Th2 responses than WT mice after OVA exposure. CerS2 null CD4+ T cells showed impaired Th2 and increased Th17 responses with concomitant higher T cell receptor (TCR) signal strength after TCR stimulation. Notably, increased Th17 responses of CerS2 null CD4+ T cells appeared only in TCR-mediated, but not in TCR-independent, treatment. (4) Conclusions: altered Th2/Th17 immune response with higher TCR signal strength was observed in CerS2 null CD4+ T cells upon TCR stimulation. CerS2 and very-long chain SLs may be therapeutic targets for Th2-related diseases such as asthma.

Background: Mucolipidosis type IV (MLIV), an ultra-rare neurodevelopmental and neurodegenerative disorder, is caused by mutations in the MCOLN1 gene, which encodes the late endosomal/lysosomal transient receptor potential channel TRPML1 (mucolipin 1). The precise pathophysiogical pathways that cause neurological disease in MLIV are poorly understood. Recently, the first post-mortem brain sample became available from a single MLIV patient, and in the current study we performed mass spectrometry (MS)-based proteomics on this tissue with a view to delineating pathological pathways, and to compare with previously-published data on MLIV, including studies using the Mcoln1^−/− mouse. Results: A number of pathways were altered in two brain regions from the MLIV patient, including those related to the lysosome, lipid metabolism, myelination, cellular trafficking and autophagy, mTOR and calmodulin, the complement system and interferon signaling. Of these, levels of some proteins not known previously to be associated with MLIV were altered, including APOD, PLIN4, ATG and proteins related to interferon signaling. Moreover, when proteins detected by proteomics in the human brain were compared with their orthologs detected in the Mcoln1^−/− mouse by RNAseq, the results were remarkably similar. Finally, analysis of proteins in human and mouse CSF suggest that calbindin 1 and calbindin 2 might be useful as biomarkers to help chart the course of disease development. Conclusions: Despite the sample size limitations, our findings are consistent with the relatively general changes in lysosomal function previously reported in MLIV, and shed light on new pathways of disease pathophysiology, which is required in order to understand the course of disease development and to determine the efficacy of therapies when they become available for this devastating disease.

BACKGROUND: Niemann-Pick type C (NPC, MIM #257220) is a neuro-visceral disease, caused predominantly by pathogenic variants in the NPC1 gene. Here we studied patients with clinical diagnosis of NPC but inconclusive results regarding the molecular analysis.METHODS: We used a Next-Generation Sequencing (NGS)-panel followed by cDNA analysis. Latter, we used massively parallel single-cell RNA-seq (MARS-Seq) to address gene profiling changes and finally the effect of different variants on the protein and cellular levels.RESULTS: We identified novel variants and cDNA analysis allowed us to establish the functional effect of a silent variant, previously reported as a polymorphism. We demonstrated that this variant induces the skipping of exon 11 leading to a premature stop codon and identified it in NPC patients from two unrelated families. MARS-Seq analysis showed that a number of upregulated genes were related to the unfolded protein response (UPR) and endoplasmic reticulum (ER) stress in one specific patient. Also, for all analyzed variants, the NPC1 protein was partially retained in the ER.CONCLUSION: We showed that the NPC1 silent polymorphism (p.V562V) is a disease-causing variant in NPC and that the UPR is upregulated in an NPC patient.

The backbone of all sphingolipids (SLs) is a sphingoid long-chain base (LCB) to which a fatty acid is
N-acylated. Considerable variability exists in the chain length and degree of saturation of both of these hydrophobic chains, and recent work has implicated ceramides with different LCBs and
N-acyl chains in distinct biological processes; moreover, they may play different roles in disease states and possibly even act as prognostic markers. We now demonstrate that the half-life, or turnover rate, of ceramides containing diverse
N-acyl chains is different. By means of a pulse-labeling protocol using stable-isotope, deuterated free fatty acids, and following their incorporation into ceramide and downstream SLs, we show that very-long-chain (VLC) ceramides containing C24:0 or C24:1 fatty acids turn over much more rapidly than long-chain (LC) ceramides containing C16:0 or C18:0 fatty acids due to the more rapid metabolism of the former into VLC sphingomyelin and VLC hexosylceramide. In contrast, d16:1 and d18:1 ceramides show similar rates of turnover, indicating that the length of the sphingoid LCB does not influence the flux of ceramides through the biosynthetic pathway. Together, these data demonstrate that the
N-acyl chain length of SLs may not only affect membrane biophysical properties but also influence the rate of metabolism of SLs so as to regulate their levels and perhaps their biological functions.

Both monogenic diseases and viral infections can manifest in a broad spectrum of clinical phenotypes that range from asymptomatic to lethal, suggesting that other factors modulate disease severity. Here, we examine the interplay between the genetic neuronopathic Gaucher's disease (nGD), and neuroinvasive Sindbis virus (SVNI) infection. Infection of nGD mice with SVNI had no influence on nGD severity. However, nGD mice were more resistant to SVNI infection. Significantly different inflammatory responses were seen in nGD brains when compared with SVNI brains: the inflammatory response in the nGD brains consisted of reactive astrocytes and microglia with no infiltrating macrophages, but the inflammatory response in the brains of SVNI-infected mice was characterized by infiltration of macrophages and altered activation of microglia and astrocytes. We suggest that the innate immune response activated in nGD confers resistance against viral infection of the CNS.

The lysosome is a central player in the cell, acting as a clearing house for macromolecular degradation, but also plays a critical role in a variety of additional metabolic and regulatory processes. The lysosome has recently attracted the attention of neurobiologists and neurologists since a number of neurological diseases involve a lysosomal component. Among these is Parkinson's disease (PD). While heterozygous and homozygous mutations in GBA1 are the highest genetic risk factor for PD, studies performed over the past decade have suggested that lysosomal loss of function is likely involved in PD pathology, since a significant percent of PD patients have a mutation in one or more genes that cause a lysosomal storage disease (LSD). Although the mechanistic connection between the lysosome and PD remains somewhat enigmatic, significant evidence is accumulating that lysosomal dysfunction plays a central role in PD pathophysiology. Thus, lysosomal dysfunction, resulting from mutations in lysosomal genes, may enhance the accumulation of α-synuclein in the brain, which may result in the earlier development of PD.

Bi-allelic mutations in the glucocerebrosidase gene (GBA1) cause Gaucher's disease, the most common human lysosomal storage disease. We previously reported a marked increase in miR-155 transcript levels and early microglial activation in a zebrafish model of Gaucher's disease (gba1(-/-)). miR-155 is a master regulator of inflammation and has been implicated in a wide range of different neurodegenerative disorders. The observed miR-155 upregulation preceded the subsequent development of widespread pathology with marked neuroinflammation, closely resembling human Gaucher's disease pathology. We now report similar increases of miR-155 expression in mammalian models of GD, confirming that miR-155 upregulation is a shared feature in glucocerebrosidase (GCase) deficiency across different species. Using CRISPR/Cas9 mutagenesis we then generated a miR-155 mutant zebrafish line (miR-155(-/-)) with completely abolished miR-155 expression. Unexpectedly, loss of miR-155 did not mitigate either the reduced lifespan or the robust inflammatory phenotypes of gba1(-/-) mutant zebrafish. Our data demonstrate that neither neuroinflammation nor disease progression in GCase deficiency are dependent on miR-155 and suggest that miR-155 inhibition would not be a promising therapeutic target in Gaucher's disease.

Sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is a lipid raft enzyme that regulates plasma membrane (PM) fluidity. Here we report that SMPDL3b excess, as observed in podocytes in diabetic kidney disease (DKD), impairs insulin receptor isoform B-dependent pro-survival insulin signaling by interfering with insulin receptor isoforms binding to caveolin-1 in the PM. SMPDL3b excess affects the production of active sphingolipids resulting in decreased ceramide-1-phosphate (C1P) content as observed in human podocytes in vitro and in kidney cortexes of diabetic db/db mice in vivo. Podocyte-specific Smpdl3b deficiency in db/ db mice is sufficient to restore kidney cortex C1P content and to protect from DKD. Exogenous administration of C1P restores IR signaling in vitro and prevents established DKD progression in vivo. Taken together, we identify SMPDL3b as a modulator of insulin signaling and demonstrate that supplementation with exogenous C1P may represent a lipid therapeutic strategy to treat diabetic complications such as DKD.

CD1d-restricted invariant natural killer T (iNKT) cells represent a heterogeneous population of lipid-reactive T cells that are involved in many immune responses, mediated through T-cell receptor (TCR)-dependent and/or independent activation. Although numerous microbial lipid antigens (Ags) have been identified, several lines of evidence have suggested the existence of relevant Ags of endogenous origin. However, the identification of their precise nature as well as the molecular mechanisms involved in their generation are still highly controversial and ill defined. Here, we identified two mammalian gangliosidesnamely monosialoganglioside GM3 and disialoganglioside GD3as endogenous activators for mouse iNKT cells. These glycosphingolipids are found in Toll-like receptor-stimulated dendritic cells (DC) as several species varying in their N-acyl fatty chain composition. Interestingly, their ability to activate iNKT cells is highly dependent on the ceramide backbone structure. Thus, both synthetic GM3 and GD3 comprising a d18:1-C24:1 ceramide backbone were able to activate iNKT cells in a CD1d-dependent manner. GM3 and GD3 are not directly recognized by the iNKT TCR and required the Ag presenting cell intracellular machinery to reveal their antigenicity. We propose a new concept in which iNKT cells can rapidly respond to pre-existing self-molecules after stress-induced structural changes in CD1d-expressing cells. Moreover, these gangliosides conferred partial protection in the context of bacterial infection. Thus, this report identified new biologically relevant lipid self-Ags for iNKT cells.Author summary Invariant natural killer T (iNKT) cells are a population of unconventional T lymphocytes that activate rapidly during inflammation due to their innate-like features. They are unconventional since they do not react to peptidic antigens (Ags) presented by classical major histocompatibility complex (MHC) molecules; instead, they recognize lipid-based Ags in the context of the MHC class I-like molecule CD1d. While numerous Ags of microbial origins have been described, their endogenous Ags are far less understood and remain a matter of strong debate. Here, we report that engagement of an innate receptor on the Ag-presenting cells leads to modulation of their lipid metabolism. This results in an enrichment of particular glycosphingolipid species that differ in both the nonpolar tail and polar head structures. Among those, two species have the potential to activate iNKT cells in a CD1d-dependent manner after further intracellular modifications. Based on these data, we propose a concept that iNKT cells can rapidly respond to pre-existing self-molecules after stress-induced changes in CD1d-expressing cells. Given the presence of closely related molecules in some pathological conditions such as cancer, it will be interesting to evaluate the biological relevance of these Ags in disease states.

Lipids display large structural complexity, with ∼40,000 different lipids identified to date, ∼4000 of which are sphingolipids. A critical factor determining the biological activities of the sphingolipid, ceramide, and of more complex sphingolipids is their
N-acyl chain length, which in mammals is determined by a family of six ceramide synthases (CerS). Little information is available about the CerS regions that determine specificity toward different acyl-CoA substrates. We previously demonstrated that substrate specificity resides in a region of ∼150 residues in the Tram-Lag-CLN8 domain. Using site-directed mutagenesis and biochemical analyses, we now narrow specificity down to an 11-residue sequence in a loop located between the last two putative transmembrane domains (TMDs) of the CerS. The specificity of a chimeric protein, CerS5
(299-309→CerS2), based on the backbone of CerS5 (which generates C16-ceramide), but containing 11 residues from CerS2 (which generates C22-C24-ceramides), was altered such that it generated C22-C24 and other ceramides. Moreover, a chimeric protein, CerS4
(291-301→CerS2), based on CerS4 (which normally generates C18-C22 ceramides) displayed significant activity toward C24:1-CoA. Additional data supported the notion that substitutions of these 11 residues alter the specificities of the CerS toward their cognate acyl-CoAs. Our findings may suggest that this short loop may restrict adjacent TMDs, leading to a more open conformation in the membrane, and that the CerS acting on shorter acyl-CoAs may have a longer, more flexible loop, permitting TMD flexibility. In summary, we have identified an 11-residue region that determines the acyl-CoA specificity of CerS.

The mechanisms by which lung structural cells survive toxic exposures to cigarette smoke (CS) are not well defined but may involve proper disposal of damaged mitochondria by macroautophagy (mitophagy), processes that may be influenced by proapoptotic ceramide (Cer) or its precursor dihydroceramide (DHC). Human lung epithelial and endothelial cells exposed to CS exhibited mitochondrial damage, signaled by phosphatase and tensin homologinduced putative kinase 1 (PINK1) phosphorylation, autophagy, and necroptosis. Although cells responded to CS by rapid inhibition of DHC desaturase, which elevated DHC levels, palmitoyl (C16)Cer also increased in CSexposed cells. Whereas DHC augmentation triggered autophagy without cell death, the exogenous administration of C16Cer was sufficient to trigger necroptosis. Inhibition of Cergenerating acid sphingomyelinase reduced both CSinduced PINK1 phosphorylation and necroptosis. When exposed to CS, Pinkldeficient (Pink1−/−) mice, which are protected from airspace enlargement compared with wildtype littermates, had blunted C16Cer elevations and less lung necroptosis. CSexposed Pink1−/− mice also exhibited significantly increased levels of lignoceroyl (C24)DHC, along with increased expression of Cer synthase 2 (CerS2), the enzyme responsible for its production. This suggested that a combination of high C24DHC and low C16Cer levels might protect against CSinduced necroptosis. Indeed, CerS2−/− mice, which lack C24DHC at the expense of increased C16Cer, were more susceptible to CS, developing airspace enlargement following only 1 month of exposure. These results implicate DHCs, in particular, C24DHC, as protective against CS toxicity by enhancing autophagy, whereas C16Cer accumulation contributes to mitochondrial damage and PINKlmediated necroptosis, which may be amplified by the inhibition of C24DHCproducing CerS2. Mizumura, K., Justice, M.J., Schweitzer, K.S., Krishnan, S., Bronova, I., Berdyshev, E. V., Hubbard, W.C., PewznerJung, Y., Futerman, A. H., Choi, A. M. K., Petrache, I. Sphingolipid regulation of lung epithelial cell mitophagy and necroptosis during cigarette smoke exposure.

The role of sphingolipids (SLs) in the immune system has come under increasing scrutiny recently due to the emerging contributions that these important membrane components play in regulating a variety of immunological processes. The acyl chain length of SLs appears particularly critical in determining SL function. Here, we show a role for very-long acyl chain SLs (VLC-SLs) in invariant natural killer T (iNKT) cell maturation in the thymus and homeostasis in the liver. Ceramide synthase 2-null mice, which lack VLC-SLs, were susceptible to a hepatotropic strain of lymphocytic choriomeningitis virus, which is due to a reduction in the number of iNKT cells. Bone marrow chimera experiments indicated that hematopoietic-derived VLC-SLs are essential for maturation of iNKT cells in the thymus, whereas parenchymal-derived VLC-SLs are crucial for iNKT cell survival and maintenance in the liver. Our findings suggest a critical role for VLC-SL in iNKT cell physiology.

Sphingolipids (SLs) have been extensively investigated in biomedical research due to their role as bioactive molecules in cells. Here, we describe the effect of a SL analog, jaspine B (JB), a cyclic anhydrophytosphingosine found in marine sponges, on the gastric cancer cell line, HGC-27. JB induced alterations in the sphingolipidome, mainly the accumulation of dihydrosphingosine, sphingosine, and their phosphorylated forms due to inhibition of ceramide synthases. Moreover, JB provoked atypical cell death in HGC-27 cells, characterized by the formation of cytoplasmic vacuoles in a time and dose-dependent manner. Vacuoles appeared to originate from macropinocytosis and triggered cytoplasmic disruption. The pan-caspase inhibitor, z-VAD, did not alter either cytotoxicity or vacuole formation, suggesting that JB activates a caspase-independent cell death mechanism. The autophagy inhibitor, wortmannin, did not decrease JB-stimulated LC3-II accumulation. In addition, cell vacuolation induced by JB was characterized by single-membrane vacuoles, which are different from double-membrane autophagosomes. These findings suggest that JB-induced cell vacuolation is not related to autophagy and it is also independent of its action on SL metabolism.

Ceramide and more complex sphingolipids constitute a diverse group of lipids that serve important roles as structural entities of biological membranes and as regulators of cellular growth, differentiation, and development. Thus, ceramides are vital players in numerous diseases including metabolic and cardiovascular diseases, as well as neurological disorders. Here we show that acyl-coenzyme A-binding protein (ACBP) potently facilitates very-long acyl chain ceramide synthesis. ACBP increases the activity of ceramide synthase 2 (CerS2) by more than 2-fold and CerS3 activity by 7-fold. ACBP binds very-long-chain acyl-CoA esters, which is required for its ability to stimulate CerS activity. We also show that high-speed liver cytosol from wild-type mice activates CerS3 activity, whereas cytosol from ACBP knock-out mice does not. Consistently, CerS2 and CerS3 activities are significantly reduced in the testes of ACBP−/− mice, concomitant with a significant reduction in long- and very-long-chain ceramide levels. Importantly, we show that ACBP interacts with CerS2 and CerS3. Our data uncover a novel mode of regulation of very-long acyl chain ceramide synthesis by ACBP, which we anticipate is of crucial importance in understanding the regulation of ceramide metabolism in pathogenesis.

The lysosomal acid β-glucosidase GBA1 and the non-lysosomal β-glucosidase GBA2 degrade glucosylceramide (GlcCer) to glucose and ceramide in different cellular compartments. Loss of GBA2 activity and the resulting accumulation of GlcCer results in male infertility, whereas mutations in the GBA1 gene and loss of GBA1 activity cause the lipid-storage disorder Gaucher disease. However, the role of GBA2 in Gaucher disease pathology and its relationship to GBA1 is not well understood. Here, we report a GBA1-dependent down-regulation of GBA2 activity in patients with Gaucher disease. Using an experimental approach combining cell biology, biochemistry, and mass spectrometry, we show that sphingosine, the cytotoxic metabolite accumulating in Gaucher cells through the action of GBA2, directly binds to GBA2 and inhibits its activity. We propose a negative feedback loop, in which sphingosine inhibits GBA2 activity in Gaucher cells, preventing further sphingosine accumulation and, thereby, cytotoxicity. Our findings add a new chapter to the understanding of the complex molecular mechanism underlying Gaucher disease and the regulation of β-glucosidase activity in general.

In the lysosomal storage disorder Gaucher disease (GD), glucosylceramide (GlcCer) accumulates due to the defective activity of glucocerebrosidase. A subset of GD patients develops neuropathology. We now show mislocalization of Limp2-positive puncta and a large reduction in the number of Lamp1-positive puncta, which are associated with impaired tubulin. These changes occur at an early stage in animal models of GD, prior to development of overt symptoms and considerably earlier than neuronal loss. Altered lysosomal localization and cytoskeleton disruption precede the neuroinflammatory pathways, axonal dystrophy and neuronal loss previously characterized in neuronal forms of GD.

Glucosylceramide (GlcCer) plays an active role in the regulation of various cellular events. Moreover, GlcCer is also a key modulator of membrane biophysical properties, which might be linked to the mechanism of its biological action. In order to understand the biophysical implications of GlcCer on membranes of living cells, we first studied the effect of GlcCer on artificial membranes containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), sphingomyelin (SM) and cholesterol (Chol). Using an array of biophysical methods, we demonstrate that at lower GlcCer/Chol ratios, GlcCer stabilizes SM/Chol-enriched liquid-ordered domains. However, upon decreasing the Chol content, GlcCer significantly increased membrane order through the formation of gel domains. Changes in pH disturbed the packing properties of GlcCer-containing membranes, leading to an increase in membrane fluidity and reduced membrane electronegativity. To address the biophysical impact of GlcCer in biological membranes, studies were performed in wild type and in fibroblasts treated with conduritol-B-epoxide (CBE), which causes intracellular GlcCer accumulation, and in fibroblasts from patients with type I Gaucher disease (GD). Decreased membrane fluidity was observed in cells containing higher levels of GlcCer, such as in CBE-treated and GD cells. Together, we demonstrate that elevated GlcCer levels change the biophysical properties of cellular membranes, which might compromise membrane-associated cellular events and be of relevance for understanding the pathology of diseases, such as GD, in which GlcCer accumulates at high levels.

Endothelial oxidative stress develops with aging and reactive oxygen species impair endotheliumdependent relaxation (EDR) by decreasing nitric oxide (NO) availability. Endothelial KCa3.1, which contributes to EDR, is upregulated by H2O2. We investigated whether KCa3.1 upregulation compensates for diminished EDR to NO during agingrelated oxidative stress. Previous studies identified that the levels of ceramide synthase 5 (CerS5), sphingosine, and sphingosine 1phosphate were increased in aged wildtype and CerS2 mice. In primary mouse aortic endothelial cells (MAECs) from aged wildtype and CerS2 null mice, superoxide dismutase (SOD) was upregulated, and catalase and glutathione peroxidase 1 (GPX1) were downregulated, when compared to MAECs from young and agematched wildtype mice. Increased H2O2 levels induced Fyn and extracellular signalregulated kinases (ERKs) phosphorylation and KCa3.1 upregulation. Catalase/GPX1 double knockout (catalase−/−/GPX1−/−) upregulated KCa3.1 in MAECs. NO production was decreased in aged wildtype, CerS2 null, and catalase−/−/GPX1−/− MAECs. However, KCa3.1 activationinduced, NGnitrolarginine, and indomethacinresistant EDR was increased without a change in acetylcholineinduced EDR in aortic rings from aged wildtype, CerS2 null, and catalase−/−/GPX1−/− mice. CerS5 transfection or exogenous application of sphingosine or sphingosine 1phosphate induced similar changes in levels of the antioxidant enzymes and upregulated KCa3.1. Our findings suggest that, during agingrelated oxidative stress, SOD upregulation and downregulation of catalase and GPX1, which occur upon altering the sphingolipid composition or acyl chain length, generate H2O2 and thereby upregulate KCa3.1 expression and function via a H2O2/Fynmediated pathway. Altogether, enhanced KCa3.1 activity may compensate for decreased NO signaling during vascular aging.

Diseases caused by single-gene mutations can display substantial phenotypic variability, which may be due to genetic, environmental, or epigenetic modifiers. Here, we induce Gaucher disease (GD), a rare inherited metabolic disorder, by injecting 15 inbred mouse strains with a low dose of a chemical inhibitor of acid beta-glucosidase, the enzyme defective in GD. Different mouse strains exhibit widely different lifespans, which is unrelated to levels of acid beta-glucosidase's substrate accumulation. Genome-wide association reveals a number of candidate risk loci, including a marker within Grin2b, which in combination with another marker allows us to predict the lifespan of additional mouse strains. An antagonist of the NMDA receptor (encoded by Grin2b) significantly increases the lifespan of GD mice that would otherwise have lived for a short time. Our data identify putative modifier genes that may be involved in determining GD severity, which might help elucidate phenotypic variability between patients with similar GD mutations.

Great interest has been shown in understanding the pathology of Gaucher disease (GD) due to the recently discovered genetic relationship with Parkinson's disease. For such studies, suitable animal models of GD are required. Chemical induction of GD by inhibition of acid βglucosidase (GCase) using the irreversible inhibitor conduritol Bepoxide (CBE) is particularly attractive, although few systematic studies examining the effect of CBE on the development of symptoms associated with neurological forms of GD have been performed. We now demonstrate a correlation between the amount of CBE injected into mice and levels of accumulation of the GD substrates, glucosylceramide and glucosylsphingosine, and show that disease pathology, indicated by altered levels of pathological markers, depends on both the levels of accumulated lipids and the time at which their accumulation begins. Gene array analysis shows a remarkable similarity in the gene expression profiles of CBEtreated mice and a genetic GD mouse model, the Gbaflox/flox;nestinCre mouse, with 120 of the 144 genes upregulated in CBEtreated mice also upregulated in Gbaflox/flox;nestinCre mice. We also demonstrate that various aspects of neuropathology and some behavioural abnormalities can be arrested upon cessation of CBE treatment during a specific time window. Together, our data demonstrate that injection of mice with CBE provides a rapid and relatively easy way to induce symptoms typical of neuronal forms of GD. This is particularly useful when examining the role of specific biochemical pathways in GD pathology, since CBE can be injected into mice defective in components of putative pathological pathways, alleviating the need for timeconsuming crossing of mice. Copyright © 2016 Pathological Society of Great Britain and Ireland.

Glucosylceramide (GlcCer), one of the simplest glycosphingolipids, plays key roles in physiology and pathophysiology. It has been suggested that GlcCer modulates cellular events by forming specialized domains. In this study, we investigated the interplay between GlcCer and cholesterol (Chol), an important lipid involved in the formation of liquid-ordered (l_o) phases. Using fluorescence microscopy and spectroscopy, and dynamic and electrophoretic light scattering, we characterized the interaction between these lipids in different pH environments. A quantitative description of the phase behavior of the ternary unsaturated phospholipid/Chol/GlcCer mixture is presented. The results demonstrate coexistence between l_o and liquid-disordered (l_d) phases. However, the extent of l_o/l_d phase separation is sparse, mainly due to the ability of GlcCer to segregate into tightly packed gel domains. As a result, the phase diagram of these mixtures is characterized by an extensive three-phase coexistence region of fluid (l_d-phospholipid enriched)/l_o (Chol enriched)/gel (GlcCer enriched). Moreover, the results show that upon acidification, GlcCer solubility in the l_o phase is increased, leading to a larger l_o/l_d coexistence region. Quantitative analyses allowed us to determine the differences in the composition of the phases at neutral and acidic pH. These results predict the impact of GlcCer on domain formation and membrane organization in complex biological membranes, and provide a background for unraveling the relationship between the biophysical properties of GlcCer and its biological action.

Tumor necrosis factor alpha (TNF alpha) is an inflammatory cytokine that plays an intimate role in septic shock. Injection of high levels of lipopolysaccharide induces septic shock and death in mice within 30 h, whereas ceramide synthase 2 (CerS2) null mice, defective in the synthesis of very-long acyl chain ceramides, die within similar to 10 h. The augmented rate of death of CerS2 null mice is due to elevated levels of TNF alpha secretion as a result of enhanced activity of TNF alpha-converting enzyme (TACE). We discuss the relationship between the sphingolipid acyl chain length and TACE activity and the relevance of this data to septic shock.

Ceramide synthases (CerS) synthesise ceramides of defined acyl chain lengths, which are thought to mediate cellular processes in a chain length-dependent manner. In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), we observed a significant elevation of CerS2 and its products, C24-ceramides, in CD11b(+) cells (monocytes and neutrophils) isolated from blood. This result correlates with the clinical finding that CerS2 mRNA expression and C24-ceramide levels were significantly increased by 2.2- and 1.5-fold, respectively, in white blood cells of MS patients. The increased CerS2 mRNA/C24-ceramide expression in neutrophils/monocytes seems to mediate pro-inflammatory effects, since a specific genetic deletion of CerS2 in blood cells or a total genetic deletion of CerS2 significantly delayed the onset of clinical symptoms, due to a reduced infiltration of immune cells, in particular neutrophils, into the central nervous system. CXCR2 chemokine receptors, expressed on neutrophils, promote the migration of neutrophils into the central nervous system, which is a prerequisite for the recruitment of further immune cells and the inflammatory process that leads to the development of MS. Interestingly, neutrophils isolated from CerS2 null EAE mice, as opposed to WT EAE mice, were characterised by significantly lower CXCR2 receptor mRNA expression resulting in their reduced migratory capacity towards CXCL2. Most importantly, G-CSF-induced CXCR2 expression was significantly reduced in CerS2 null neutrophils and their migratory capacity was significantly impaired. In conclusion, our data strongly indicate that G-CSF-induced CXCR2 expression is regulated in a CerS2-dependent manner and that CerS2 thereby promotes the migration of neutrophils, thus, contributing to inflammation and the development of EAE and MS.

Gaucher disease, a recessive inherited metabolic disorder caused by defects in the gene encoding glucosylceramidase (GlcCerase), can be divided into three subtypes according to the appearance of symptoms associated with central nervous system involvement. We now identify a protein, glycoprotein non-metastatic B (GPNMB), that acts as an authentic marker of brain pathology in neurological forms of Gaucher disease. Using three independent techniques, including quantitative global proteomic analysis of cerebrospinal fluid (CSF) in samples from Gaucher disease patients that display neurological symptoms, we demonstrate a correlation between the severity of symptoms and GPNMB levels. Moreover, GPNMB levels in the CSF correlate with disease severity in a mouse model of Gaucher disease. GPNMB was also elevated in brain samples from patients with type 2 and 3 Gaucher disease. Our data suggest that GPNMB can be used as a marker to quantify neuropathology in Gaucher disease patients and as a marker of treatment efficacy once suitable treatments towards the neurological symptoms of Gaucher disease become available.

Background & Aims: Sortilin traffics newly synthesized molecules from the trans- Golgi apparatus along secretory pathways to endosomes, lysosomes or to the cell surface. Sortilin trafficking of acid sphingomyelinase (aSMase) may regulate ceramide levels, a major modulator of insulin signalling. We therefore tested whether sortilin deficiency reduces hepatic and adipose tissue aSMase activity, improving insulin sensitivity in diet-induced obesity (DIO). Methods: DIO in C57BL/6 (WT) and sortilin(-/-) mice was induced by high-fat diet feeding for 10 weeks. Results: Sortilin(-/-) mice gained less body weight and less visceral fat, despite similar food intake compared to WT type mice and had enhanced glucose uptake in insulin tolerance tests, which was further corroborated by enhanced hepatic pAkt expression. Sortilin deficiency led to attenuated hepatic steatosis, reduced expression of genes involved in lipogenesis, ceramide synthesis and inflammatory cytokine production and reduced activity of ceramide synthase 5/6 (CerS5/6). Sortilin(-/-) mice had reduced hepatic aSMase activity under both steady-state and DIO. Likewise, sortilin(-/-) hepatocytes displayed hypersensitivity to insulin, due to enhanced insulin receptor downstream signalling. In adipose tissue, sortilin(-/-) mice exhibited lower expression of inflammatory cytokines and lower expression and activity of CerS5/6. As in liver, adipose tissue displayed increased insulin signalling, accompanied by attenuated aSMase activity. Conclusions: Sortilin deficiency induces a beneficial metabolic phenotype in liver and adipose tissue upon DIO, mediated in part by reduced aSMase activity.

Inhibition of ceramide synthesis prevents diabetes, steatosis, and cardiovascular disease in rodents. Six different ceramide synthases (CerS) that differ in tissue distribution and substrate specificity account for the diversity in acyl-chain composition of distinct ceramide species. Haploinsufficiency for ceramide synthase 2 (CerS2), the dominant isoform in the liver that preferentially makes very-long-chain (C22/C24/C24:1) ceramides, led to compensatory increases in long-chain C16-ceramides and conferred susceptibility to diet-induced steatohepatitis and insulin resistance. Mechanistic studies revealed that these metabolic effects were likely due to impaired β-oxidation resulting from inactivation of electron transport chain components. Inhibiting global ceramide synthesis negated the effects of CerS2 haploinsufficiency in vivo, and increasing C16-ceramides by overexpressing CerS6 recapitulated the phenotype in isolated, primary hepatocytes. Collectively, these studies reveal that altering sphingolipid acylation patterns impacts hepatic steatosis and insulin sensitivity and identify CerS6 as a possible therapeutic target for treating metabolic diseases associated with obesity.

The sphingolipidoses are a group of inherited lysosomal storage diseases in which sphingolipids accumulate due to the defective activity of one or other enzymes involved in their degradation. For most of the sphingolipidoses, little is known about the molecular mechanisms that lead to disease, which has negatively impacted attempts to develop therapies for these devastating human diseases. Use of both genetically-modified animals, ranging from mice to larger mammals, and of novel cell culture systems, is of utmost importance in delineating the molecular mechanisms that cause pathophysiology, and in providing tools that enable testing the efficacy of new therapies. In this review, we discuss eight sphingolipidoses, namely Gaucher disease, Fabry disease, metachromatic leukodystrophy, Krabbe disease, Niemann-Pick diseases A and B, Farber disease, GM1 gangliosidoses, and GM2 gangliosidoses, and describe the tools that are currently available for their study. This article is part of a Special Issue entitled Tools to study lipid functions.

Ceramide is located at a key hub in the sphingolipid metabolic pathway and also acts as an important cellular signaling molecule. Ceramide contains one acyl chain which is attached to a sphingoid long chain base via an amide bond, with the acyl chain varying in length and degree of saturation. The identification of a family of six mammalian ceramide synthases (CerS) that synthesize ceramide with distinct acyl chains, has led to significant advances in our understanding of ceramide biology, including further delineation of the role of ceramide in various pathophysiologies in both mice and humans. Since ceramides, and the complex sphingolipids generated from ceramide, are implicated in disease, the CerS might potentially be novel targets for therapeutic intervention in the diseases in which the ceramide acyl chain length is altered. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Therapy resistance is a major limitation to the successful treatment of cancer. Here, we identify Bcl2-like 13 (Bcl2L13), an atypical member of the Bcl-2 family, as a therapy susceptibility gene with elevated expression in solid and blood cancers, including glioblastoma (GBM). We demonstrate that mitochondria-Associated Bcl2L13 inhibits apoptosis induced by a wide spectrum of chemo- and targeted therapies upstream of Bcl2-Associated X protein activation and mitochondrial outer membrane permeabilization in vitro and promotes GBM tumor growth in vivo. Mechanistically, Bcl2L13 binds to proapoptotic ceramide synthases 2 (CerS2) and 6 (CerS6) via a unique C-terminal 250-Aa sequence located between its Bcl-2 homology and membrane anchor domains and blocks homo- and heteromeric CerS2/6 complex formation and activity. Correspondingly, CerS2/6 activity and Bcl2L13 abundance are inversely correlated in GBM tumors. Thus, our genetic and functional studies identify Bcl2L13 as a regulator of therapy susceptibility and point to the Bcl2L13-CerS axis as a promising target to enhance responses of therapy-refractory cancers toward conventional and targeted regimens currently in clinical use.

Gaucher's disease (GD), an inherited metabolic disorder caused by mutations in the glucocerebrosidase gene (GBA), is the most common lysosomal storage disease. Heterozygous mutations in GBA are a major risk factor for Parkinson's disease. GD is divided into three clinical subtypes based on the absence (type 1) or presence (types 2 and 3) of neurological signs. Type 1 GD was the first lysosomal storage disease (LSD) for which enzyme therapy became available, and although infusions of recombinant glucocerebrosidase (GCase) ameliorate the systemic effects of GD, the lack of efficacy for the neurological manifestations, along with the considerable expense and inconvenience of enzyme therapy for patients, renders the search for alternative or complementary therapies paramount. Glucosylceramide and glucosylsphingosine accumulation in the brain leads to massive neuronal loss in patients with neuronopathic GD (nGD) and in nGD mouse models. However, the mode of neuronal death is not known. Here, we show that modulating the receptor-interacting protein kinase-3 (Ripk3) pathway markedly improves neurological and systemic disease in a mouse model of GD. Notably, Ripk3 deficiency substantially improved the clinical course of GD mice, with increased survival and motor coordination and salutary effects on cerebral as well as hepatic injury.

Objective: Ceramides are precursors of complex sphingolipids (SLs), which are important for normal functioning of both the developing and mature brain. Altered SL levels have been associated with many neurodegenerative disorders, including epilepsy, although few direct links have been identified between genes involved in SL metabolism and epilepsy. Methods: We used quantitative realtime PCR, Western blotting, and enzymatic assays to determine the mRNA, protein, and activity levels of ceramide synthase 2 (CERS2) in fiibroblasts isolated from parental control subjects and from a patient diagnosed with progressive myoclonic epilepsy (PME). Mass spectrometry and fluorescence microscopy were used to examine the effects of reduced CERS2 activity on cellular lipid composition and plasma membrane functions. Results: We identify a novel 27 kb heterozygous deletion including the CERS2 gene in a proband diagnosed with PME. Compared to parental controls, levels of CERS2 mRNA, protein, and activity were reduced by ~50% in fibroblasts isolated from this proband, resulting in significantly reduced levels of ceramides and sphingomyelins containing the very long-chain fatty acids C24:0 and C26:0. The change in SL composition was also reflected in a reduction in cholera toxin B immunofluorescence, indicating that membrane composition and function are altered. Interpretation: We propose that reduced levels of CERS2, and consequently diminished levels of ceramides and SLs containing very long-chain fatty acids, lead to development of PME.

The involvement of ceramide in death receptor-mediated apoptosis has been widely examined with most studies focusing on the role of ceramide generated from sphingomyelin hydrolysis. We now analyze the effect of the ceramide acyl chain length by studying tumor necrosis factor a receptor-1 (TNFR1)-mediated apoptosis in a ceramide synthase 2 (CerS2) null mouse, which cannot synthesize very-long acyl chain ceramides. CerS2 null mice were resistant to lipopolysaccharide/galactosaminemediated fulminant hepatic failure even though TNFα secretion from macrophages was unaffected. Cultured hepatocytes were also insensitive to TNFα-mediated apoptosis. In addition, in both liver and in hepatocytes, caspase activities were not elevated, consistent with inhibition of TNFR1 pro-apoptotic signaling. In contrast, Fas receptor activation resulted in the death of CerS2 null mice. Caspase activation was blocked because of the inability of CerS2 null mice to internalize the TNFR1; whereas Fc-TNFα was internalized to a perinuclear region in hepatocytes from wild-type mice, no internalization was detected in CerS2 null mice. Our results indicate that altering the acyl chain composition of sphingolipids inhibits TNFR1 internalization and inhibits selective pro-apoptotic downstream signaling for apoptosis.

Very long chain (C22-C24) ceramides are synthesized by ceramide synthase 2 (CerS2). A CerS2 null mouse displays hepatopathy because of depletion of C22-C24 ceramides, elevation of C16-ceramide, and/or elevation of sphinganine. Unexpectedly, CerS2 null mice were resistant to acetaminophen-induced hepatotoxicity. Although there were a number of biochemical changes in the liver, such as increased levels of glutathione and multiple drug-resistant protein 4, these effects are unlikely to account for the lack of acetaminophen toxicity. A number of other hepatotoxic agents, such as D-galactosamine, CCl4, and thioacetamide, were also ineffective in inducing liver damage. All of these drugs and chemicals require connexin (Cx) 32, a key gap junction protein, to induce hepatotoxicity. Cx32 was mislocalized to an intracellular location in hepatocytes from CerS2 null mice, which resulted in accelerated rates of its lysosomal degradation. This mislocalization resulted from the altered membrane properties of the CerS2 null mice, which was exemplified by the disruption of detergent-resistant membranes. The lack of acetaminophen toxicity and Cx32 mislocalization were reversed upon infection with recombinant adeno-associated virus expressing CerS2. We establish that Gap junction function is compromised upon altering the sphingolipid acyl chain length composition, which is of relevance for understanding the regulation of drug-induced liver injury.

Background: Myristate is a novel potential substrate for sphingoid base synthesis. Results: Myocardial sphingoid base synthesis utilizes myristate; these sphingolipids are functionally non-redundant with canonical sphingoid bases. Conclusion: d16:0 and d16:1 sphingolipids constitute an appreciable proportion of cardiac dihydrosphingosine and dihydroceramide, with distinct biological roles. Significance: This pool of sphingolipids may play a heretofore unsuspected role in myocardial pathology or protection.

Scope: Fumonisins are mycotoxins produced by Fusarium species. The predominant derivative, fumonisin B1 (FB1), occurs in food and feed and is of health concern due to its hepatotoxic and carcinogenic effects. However, the role of FB1 metabolites on the mechanism of the toxicity, the inhibition of the ceramide synthesis, is unknown. The aim of this study was to identify new fumonisin metabolites and to evaluate their cytotoxic potential. Methods and results: MS, molecular biology, and in vitro enzyme assays were used to investigate fumonisin metabolism in mammalian cells overexpressing human ceramide synthase (CerS) genes. N-acyl-FB1 derivatives were detected as new metabolites in cultured cells at levels of up to 10 pmol/mg of protein. The N-acylation of FB1 and hydrolyzed FB1 was analyzed in several cell lines, including cells overexpressing CerS. The acyl-chain length of the N-acyl fumonisins depends on the CerS isoform acylating them. The N-acyl fumonisins are more cytotoxic than the parent fumonisin B1. Conclusion: The identification of N-acyl fumonisins with various acyl chain lengths together with the observed cytotoxicity of these compounds is a new aspect of fumonisin-related toxicity. Therefore, these new metabolites might play an important role in the mode of action of fumonisins.

Glucosylceramide (GlcCer), a relevant intermediate in the pathways of glycosphingolipid metabolism, plays key roles in the regulation of cell physiology. The molecular mechanisms by which GlcCer regulates cellular processes are unknown, but might involve changes in membrane biophysical properties and formation of lipid domains. In the present study, fluorescence spectroscopy, confocal microscopy and surface pressure-area (π-A) measurements were used to characterize the effect of GlcCer on the biophysical properties of model membranes. We show that C16:0-GlcCer has a high tendency to segregate into highly ordered gel domains and to increase the order of the fluid phase. Monolayer studies support the aggregation propensity of C16:0-GlcCer. π-A isotherms of single C16:0-GlcCer indicate that bilayer domains, or crystal-like structures, coexist within monolayer domains at the air-water interface. Mixtures with POPC exhibit partial miscibility with expansion of the mean molecular areas relative to the additive behavior of the components. Moreover, C16:0-GlcCer promotes morphological alterations in lipid vesicles leading to formation of flexible tubule-like structures that protrude from the fluid region of the bilayer. These results support the hypothesis that alterations in membrane biophysical properties induced by GlcCer might be involved in its mechanism of action.

Background: Ceramide synthase 2 null mice, which cannot synthesize very-long chain ceramides, display severe hepatopathy. Results: These mice have elevated sphinganine and altered N-acyl chain ceramides that disrupt mitochondrial function by modifying respiratory chain activity. Conclusion: Alteration of mitochondrial sphingolipids results in formation of reaction oxygen species in liver. Significance: Ceramides with defined acyl chains influence oxidative stress signaling pathways.

To identify novel inhibitors of sphingomyelin (SM) metabolism, a new and selective high throughput microscopy-based screening based on the toxicity of the SM-specific toxin, lysenin, was developed. Out of a library of 2011 natural compounds, the limonoid, 3-chloro-8β-hydroxycarapin-3,8-hemiacetal (CHC), rendered cells resistant to lysenin by decreasing cell surface SM. CHC treatment selectively inhibited the de novo biosynthesis of SM without affecting glycolipid and glycerophospholipid biosynthesis. Pretreatment with brefeldin A abolished the limonoid-induced inhibition of SM synthesis suggesting that the transport of ceramide (Cer) from the endoplasmic reticulum to the Golgi apparatus is affected. Unlike the Cer transporter (CERT) inhibitor HPA-12, CHC did not change the transport of a fluorescent short chain Cer analog to the Golgi apparatus or the formation of fluorescent and short chain SM from the corresponding Cer. Nevertheless, CHC inhibited the conversion of de novo synthesized Cer to SM.We show that CHC specifically inhibited the CERT-mediated extraction of Cer from the endoplasmic reticulum membranes in vitro. Subsequent biochemical screening of 21 limonoids revealed that some of them, such as 8β-hydroxycarapin-3,8-hemiacetal and gedunin, which exhibits anti-cancer activity, inhibited SM biosynthesis and CERT-mediated extraction of Cer from membranes. Model membrane studies suggest that 8β-hydroxycarapin- 3,8-hemiacetal reduced the miscibility of Cer with membrane lipids and thus induced the formation of Cerrich membrane domains. Our study shows that certain limonoids are novel inhibitors of SM biosynthesis and suggests that some biological activities of these limonoids are related to their effect on the ceramide metabolism.

Gaucher's disease, the most common lysosomal storage disorder, is caused by the defective activity of glucocerebrosidase, the lysosomal hydrolase that degrades glucosylceramide. The neuronopathic forms of Gaucher's disease are characterized by severe neuronal loss, astrocytosis and microglial proliferation, but the cellular and molecular pathways causing these changes are not known. In the current study, we delineate the role of neuroinflammation in the pathogenesis of neuronopathic Gaucher's disease and show significant changes in levels of inflammatory mediators in the brain of a neuronopathic Gaucher's disease mouse model. Levels of messenger RNA expression of interleukin-1β, tumour necrosis factor-α, tumour necrosis factor-α receptor, macrophage colony-stimulating factor and transforming growth factor-β were elevated by up to ∼30-fold, with the time-course of the increase correlating with the progression of disease severity. The most significant elevation was detected for the chemokines CCL2, CCL3 and CCL5. Blood-brain barrier disruption was also evident in mice with neuronopathic Gaucher's disease. Finally, extensive elevation of nitrotyrosine, a hallmark of peroxynitrite (ONOO ^-) formation, was observed, consistent with oxidative damage caused by macrophage/microglia activation. Together, our results suggest a cytotoxic role for activated microglia in neuronopathic Gaucher's disease. We suggest that once a critical threshold of glucosylceramide storage is reached in neurons, a signalling cascade is triggered that activates microglia, which in turn releases inflammatory cytokines that amplify the inflammatory response, contributing to neuronal death.

Little is known about the effects of altering sphingolipid (SL) acyl chain structure and composition on the biophysical properties of biological membranes. We explored the biophysical consequences of depleting very long acyl chain (VLC) SLs in membranes prepared from lipid fractions isolated from a ceramide synthase 2 (CerS2)-null mouse, which is unable to synthesize C22-C24 ceramides. We demonstrate that ablation of CerS2 has different effects on liver and brain, causing a significant alteration in the fluidity of the membrane and affecting the type and/or extent of the phases present in the membrane. These changes are a consequence of the depletion of VLC and unsaturated SLs, which occurs to a different extent in liver and brain. In addition, ablation of CerS2 causes changes in intrinsic membrane curvature, leading to strong morphological alterations that promote vesicle adhesion, membrane fusion, and tubule formation. Together, these results show that depletion of VLC-SLs strongly affects membrane biophysical properties, which may compromise cellular processes that critically depend on membrane structure, such as trafficking and sorting.

Ceramide is an important bioactive sphingolipid involved in a variety of biological processes. The mechanisms by which ceramide regulates biological events are not fully understood, but may involve alterations in the biophysical properties of membranes. We now examine the properties of ceramide with different acyl chains including long chain (C16- and C18-), very long chain (C24-) and unsaturated (C18:1- and C24:1-) ceramides, in phosphatidylcholine model membranes. Our results show that i) saturated ceramides have a stronger impact on the fluid membrane, increasing its order and promoting gel/fluid phase separation, while their unsaturated counterparts have a lower (C24:1-) or no (C18:1-) ability to form gel domains at 37 °C; ii) differences between saturated species are smaller and are mainly related to the morphology and size of the gel domains, and iii) very long chain ceramides form tubular structures likely due to their ability to form interdigitated phases. These results suggest that generation of different ceramide species in cell membranes has a distinct biophysical impact with acyl chain saturation dictating membrane lateral organization, and chain asymmetry governing interdigitation and membrane morphology.

Recently, two studies were published that examined the structure of the acid-β-glucosidase N370S mutant, the most common mutant that causes Gaucher disease. One study used the experimental tool of X-ray crystallography, and the other utilized molecular dynamics (MD). The two studies reinforced each other through the similarities in their findings, but each approach also added some unique information. Both studies report that the conformation of active site loop 3 changes, due to an altered hydrogen bonding network; however, the MD study produced additional data concerning the flexibility of loop 1 and the catalytic residues that are not observed in the other study.

Sphingolipids (SLs) act as signaling molecules and as structural components in both neuronal cells and myelin. We now characterize the biochemical, histological, and behavioral abnormalities in the brain of a mouse lacking very long acyl (C22-C24) chain SLs. This mouse, which is defective in the ability to synthesize C22-C24-SLs due to ablation of ceramide synthase 2, has reduced levels of galactosylceramide (GalCer), a major component of myelin, and in particular reduced levels of non-hydroxy-C22-C24-GalCer and 2-hydroxy-C22-C24- GalCer. Noteworthy brain lesions develop with a time course consistent with a vital role for C22-C24-GalCer in myelin stability. Myelin degeneration and detachment was observed as was abnormal motor behavior originating from a subcortical region. Additional abnormalities included bilateral and symmetrical vacuolization and gliosis in specific brain areas, which corresponded to some extent to the pattern of ceramide synthase 2 expression, with astrogliosis considerably more pronounced than microglial activation. Unexpectedly, unidentified storage materials were detected in lysosomes of astrocytes, reminiscent of the accumulation that occurs in lysosomal storage disorders. Together, our data demonstrate a key role in the brain for SLs containing very long acyl chains and in particular GalCer with a reduction in their levels leading to distinctive morphological abnormalities in defined brain regions.

Cyclodextrin-based host-guest chemistry has been exploited to facilitate co-crystallization of recombinant human acid β-glucosidase (β-glucocerebrosidase, GlcCerase) with amphiphilic bicyclic nojirimycin analogues of the sp²-iminosugar type. Attempts to co-crystallize GlcCerase with 5-N,6-O-[N-(n-octyl)iminomethylidene]nojirimycin (NOI-NJ) or with 5-N,6-S-[N-(n-octyl)iminomethylidene]-6-thionojirimycin (6S-NOI-NJ), two potent inhibitors of the enzyme with promising pharmacological chaperone activity for several Gaucher disease-associated mutations, were unsuccessful probably due to the formation of aggregates that increase the heterogeneity of the sample and affect nucleation and growth of crystals. Cyclomaltoheptaose (β-cyclodextrin, βCD) efficiently captures NOI-NJ and 6S-NOI-NJ in aqueous media to form inclusion complexes in which the lipophilic tail is accommodated in the hydrophobic cavity of the cyclooligosaccharide. The dissociation constant of the complex of the amphiphilic sp²-iminosugars with βCD is two orders of magnitude higher than that of the corresponding complex with GlcCerase, allowing the efficient transfer of the inhibitor from the βCD cavity to the GlcCerase active site. Enzyme-inhibitor complexes suitable for X-ray analysis were thus grown in the presence of βCD. In contrast to what was previously observed for the complex of GlcCerase with the more basic derivative, 6-amino-6-deoxy-5-N,6-N-[N-(n-octyl)iminomethylidene]nojirimycin (6N-NOI-NJ), the β-anomers of both NOI-NJ and 6S-NOI-NJ were seen in the active site, even though the α-anomer was exclusively detected both in aqueous solution and in the corresponding βCD:sp²-iminosugar complexes. Our results further suggest that cyclodextrin derivatives might serve as suitable delivery systems of amphiphilic glycosidase inhibitors in a biomedical context.

Gaucher disease is caused by the defective activity of the lysosomal hydrolase, glucosylceramidase. Although the x-ray structure of wild type glucosylceramidase has been resolved, little is known about the structural features of any of the >200 mutations. Various treatments for Gaucher disease are available, including enzyme replacement and chaperone therapies. The latter involves binding of competitive inhibitors at the active site to enable correct folding and transport of the mutant enzyme to the lysosome. We now use molecular dynamics, a set of structural analysis tools, and several statistical methods to determine the flexible behavior of the N370S Gaucher mutant at various pH values, with and without binding the chaperone, N-butyl- deoxynojirimycin. We focus on the effect of the chaperone on the whole protein, on the active site, and on three important structural loops, and we demonstrate how the chaperone modifies the behavior of N370S in such a way that it becomes more active at lysosomal pH. Our results suggest a mechanism whereby the binding of N-butyl-deoxynojirimycin helps target correctly folded glucosylceramidase to the lysosome, contributes to binding with saposin C, and explains the initiation of the substrate-enzyme complex. Such analysis provides a new framework for determination of the structure of other Gaucher disease mutants and suggests new approaches for rational drug design.

In mammals, ceramide, a key intermediate in sphingolipid metabolism and an important signaling molecule, is synthesized by a family of six ceramide synthases (CerS), each of which synthesizes ceramides with distinct acyl chain lengths. There are a number of common biochemical features between the CerS, such as their catalytic mechanism, and their structure and intracellular localization. Different CerS also display remarkable differences in their biological properties, with each of them playing distinct roles in processes as diverse as cancer and tumor suppression, in the response to chemotherapeutic drugs, in apoptosis, and in neurodegenerative diseases.

Lysosomal storage disorders are caused by the defective activity of lysosomal proteins, which results in the lysosomal accumulation of undegraded metabolites. Sphingolipid storage disorders are a subgroup of lysosomal storage disorders, in which unmetabolized sphingolipids and glycosphingolipids accumulate. Sphingolipid storage disorders are normally associated with devastating neurodegeneration and death at an early age. Despite years of study of the genetic and molecular bases of sphingolipid storage disorders, little is known about the events that lead from lipid accumulation to pathology. Research over the past few years has demonstrated that sphingolipid storage can result in multiple direct or indirect effects on various cellular compartments and on biochemical pathways. This article offers a guided tour through the cellular scenes that may be of relevance to sphingolipid storage disorder pathogenesis and to the development of new therapeutic approaches.

We have generated a mouse that cannot synthesize very long acyl chain (C22-C24) ceramides (Pewzner-Jung, Y., Park, H., Laviad, E. L., Silva, L. C., Lahiri, S., Stiban, J., Erez-Roman, R., Brugger, B., Sachsenheimer, T., Wieland, F. T., Prieto, M., Merrill, A. H., and Futerman, A. H. (2010) J. Biol. Chem. 285, 10902-10910) due to ablation of ceramide synthase 2 (CerS2). As a result, significant changes were observed in the sphingolipid profile of livers from these mice, including elevated C16-ceramide and sphinganine levels. We now examine the functional consequences of these changes. CerS2 null mice develop severe nonzonal hepatopathy from about 30 days of age, the age at which CerS2 expression peaks in wild type mice, and display increased rates of hepatocyte apoptosis and proliferation. In older mice there is extensive and pronounced hepatocellular anisocytosis with widespread formation of nodules of regenerative hepatocellular hyperplasia. Progressive hepatomegaly and noninvasive hepatocellular carcinoma are also seen from -10 months of age. Even though CerS2 is found at equally high mRNA levels in kidney and liver, there are no changes in renal function and no pathological changes in the kidney. High throughput analysis of RNA expression in liver revealed up-regulation of genes associated with cell cycle regulation, protein transport, cell-cell interactions and apoptosis, and down-regulation of genes associated with intermediary metabolism, such as lipid and steroid metabolism, adipocyte signaling, and amino acid metabolism. In addition, levels of the cell cycle regulator, the cyclin dependent-kinase inhibitor p21^WAF1/CIP1, were highly elevated, which occurs by at least two mechanisms, one of which may involve p53. We propose a functional rationale for the synthesis of sphingolipids with very long acyl chains in liver homeostasis and in cell physiology.

The ceramide synthase (CerS) enzymes are key regulators of ceramide homeostasis. CerS1 is central to regulating C18 ceramide which has been shown to be important in cancer and the response to chemotherapeutic drugs. Previous work indicated that some drugs induced a novel and specific translocation of CerS1 from the endoplasmic reticulum to the Golgi apparatus. We now show that diverse stresses such as UV light, DTT, as well as drugs with different mechanisms of action induce CerS1 translocation. The stresses cause a specific cleavage of the CerS1 enzyme, and the cleavage is dependent on the action of the proteasome. Inhibition of proteasome function inhibits stress-induced CerS1 translocation, indicating that this proteolytic cleavage precedes the translocation. Modulation of protein kinase C activity shows that it plays a central role in regulating CerS1 translocation. Analysis of the C-terminus of the CerS1 protein shows that several KxKxx motifs are not involved in regulating stress induced translocation. The study suggests that diverse stresses initiate responses through different signaling pathways, which ultimately converge to regulate CerS1 localization. The data provide an increasingly detailed understanding of the regulation of this important enzyme in normal and stressed cells and support the idea that it is uniquely regulated with respect to the other CerS enzymes.

Ceramide is an important bioactive lipid, intimately involved in many cellular functions, including the regulation of cell death, and in cancer and chemotherapy. Ceramide is synthesized de novo from sphinganine and acyl CoA via a family of 6 ceramide synthase enzymes, each having a unique preference for different fatty acyl CoA substrates and a unique tissue distribution. However, little is known regarding the regulation of these important enzymes. In this study we focus on ceramide synthase 1 (CerS1) which is the most structurally and functionally distinct of the enzymes, and describe a regulatory mechanism that specifically controls the level of CerS1 via ubiquitination and proteasome dependent protein turnover. We show that both endogenous and ectopically expressed CerS1 have rapid basal turnover and that diverse stresses including chemotherapeutic drugs, UV light and DTT can induce CerS1 turnover. The turnover requires CerS1 activity and is regulated by the opposing actions of p38 MAP kinase and protein kinase C (PKC). p38 MAP kinase is a positive regulator of turnover, while PKC is a negative regulator of turnover. CerS1 is phosphorylated in vivo and activation of PKC increases the phosphorylation of the protein. This study reveals a novel and highly specific mechanism by which CerS1 protein levels are regulated and which directly impacts ceramide homeostasis.

FTY720, a sphingosine analog, is in clinical trials as an immunomodulator. The biological effects of FTY720 are believed to occur after its metabolism to FTY720 phosphate. However, very little is known about whether FTY720 can interact with and modulate the activity of other enzymes of sphingolipid metabolism. We examined the ability of FTY720 to modulate de novo ceramide synthesis. In mammals, ceramide is synthesized by a family of six ceramide synthases, each of which utilizes a restricted subset of acyl-CoAs. We show that FTY720 inhibits ceramide synthase activity in vitro by noncompetitive inhibition toward acyl-CoA and uncompetitive inhibition toward sphinganine; surprisingly, the efficacy of inhibition depends on the acyl-CoA chain length. In cultured cells, FTY720 has a more complex effect, with ceramide synthesis inhibited at high (500 nM to 5 μM) but not low (

In mammalian cells, glucosylceramide (GlcCer), the simplest glycosphingolipid, is hydrolyzed by the lysosomal enzyme acid β-glucosidase (GlcCerase). In the human metabolic disorder Gaucher disease, GlcCerase activity is significantly decreased owing to one of approximately 200 mutations in the GlcCerase gene. The most common therapy for Gaucher disease is enzyme replacement therapy (ERT), in which patients are given intravenous injections of recombinant human GlcCerase; the Genzyme product Cerezyme® has been used clinically for more than 15 years and is administered to approximately 4000 patients worldwide. Here we review the crystal structure of Cerezyme® and other recombinant forms of GlcCerase, as well as of their complexes with covalent and non-covalent inhibitors. We also discuss the stability of Cerezyme®, which can be altered by modification of its N-glycan chains with possible implications for improved ERT in Gaucher disease.

We previously observed that gangliosides GM2, GM1, and GM3 inhibit Ca ²⁺-uptake via the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) in neurons and in brain microsomes. We now systematically examine the effect of various gangliosides and their analogs on Ca²⁺-uptake via SERCA and demonstrate that an exposed carboxyl group on the ganglioside sialic acid residue is required for inhibition. Thus, asialo-GM2 and asialo-GM1 have no inhibitory effect, and modifications of the carboxyl group of GM1 and GM2 into a hydroxymethyl residue (CH₂OH), a methyl ester (COOCH₃) or a taurine-conjugated amide (CONHCH₂CH₂SO₃H) drastically diminish their inhibitory activities. We also demonstrate that the saccharides must be attached to a ceramide backbone in order to inhibit SERCA as the ceramide-free ganglioside saccharides only inhibit SERCA to a minimal extent. Finally, we attempted to use the ceramide-free ganglioside saccharides to antagonize the effects of the gangliosides on SERCA; although some reversal was observed, the inhibitory effects of the gangliosides were not completely abolished.

A monoclonal antibody raised against mixed monolayers of 60:40 mol% cholesterol/C16-ceramide of known structure was used to label cholesterol/ceramide-rich domains in cell membranes. The antibody, Cer-Chol 405F specifically recognizes the mixed crystalline and homogeneous phase in monolayers, but it does not interact with either of the components separately. It interacts differently with mixed monolayers that contain ceramides of different acyl chain length. When used on cells, the antibody labeling is sensitive to changes in cholesterol and ceramide levels, as well as to over-expression of specific ceramides; this is in agreement with the results that were obtained on lipid monolayers. This represents a proof of concept of the applicability of a new approach to the structural characterization of lipid microdomains in cell membranes. The approach consists of raising antibodies that recognize specific structural organizations of lipids in artificial mixtures, characterizing the antibody/ordered domain complexes in vitro, and subsequently using them to detect the presence of the same (or similar) domains in cell membranes.

With its role in intracellular protein transport already known, the FAPP2 protein has now also been implicated in lipid transfer and synthesis. What is more, these two FAPP2-mediated events seem to be linked.

Sphingolipids and glycosphingolipids are emerging as major players in many facets of cell physiology and pathophysiology. We now present an overview of sphingolipid biochemistry and physiology, followed by a brief presentation of recent advances in translational research related to sphingolipids. In discussing sphingolipid biochemistry, we focus on the structure of sphingolipids, and their biosynthetic pathways - the recent identification of most of the enzymes in this pathway has led to significant advances and better characterization of a number of the biosynthetic steps, and the relationship between them. We then discuss some roles of sphingolipids in cell physiology, particularly those of ceramide and sphingosine-1-phosphate, and mention current views about how these lipids act in signal transduction pathways. We end with a discussion of sphingolipids and glycosphingolipids in the etiology and pathology of a number of diseases, such as cancer, immunity, cystic fibrosis, emphysema, diabetes, and sepsis, areas in which sphingolipids are beginning to take a central position, even though many of the details remain to be elucidated.

We have recently shown that phosphatidylcholine (PC) metabolism is altered in a macrophage model of Gaucher disease. We now demonstrate that treatment of macrophages with conduritol-B-epoxide (CBE), a glucocerebrosidase inhibitor, results in elevated activity of CTP:phosphocholine cytidylyltransferase (CCT), the rate-limiting enzyme in the pathway of PC biosynthesis. Furthermore, we provide evidence for a role for CCT in Gaucher macrophage growth by using macrophages derived from a genetically modified mouse which lacks a specific CCT isoform, CCTα, in macrophages. Upon CBE-treatment, macrophage size, analyzed by microscopy and by FACS, was significantly increased in macrophages from control mice, but did not increase, or increased to a much lower extent, in CCTα-/- macrophages. Together, these results suggest that the increase in PC biosynthesis is mediated via CCTα, and suggests a possible role for macrophage CCTα in Gaucher disease pathology.

Gaucher disease is caused by mutations in the gene encoding acid-beta-glucosidase. A recombinant form of this enzyme, Cerezyme((R)), is used to treat Gaucher disease patients by 'enzyme- replacement therapy'. Crystals of Cerezyme((R)) after its partial deglycosylation were obtained earlier and the structure was solved to 2.0 angstrom resolution [Dvir et al. (2003), EMBO Rep. 4, 704 - 709]. The crystal structure of unmodified Cerezyme1 is now reported, in which a substantial number of sugar residues bound to three asparagines via N-glycosylation could be visualized. The structure of intact fully glycosylated Cerezyme((R)) is virtually identical to that of the partially deglycosylated enzyme. However, the three loops at the entrance to the active site, which were previously observed in alternative conformations, display additional variability in their structures. Comparison of the structure of acid-beta-glucosidase with that of xylanase, a bacterial enzyme from a closely related protein family, demonstrates a close correspondence between the active-site residues of the two enzymes.

Acid β -glucosidase (EC 3.2.1.45, D-glucosyl- N -acylsphingosine glucohydrolase, glucocerebrosidase, GlcCerase), the enzyme defective in Gaucher disease, is a peripheral lysosomal membrane protein that hydrolyzes the β -glucosyl linkage of glucosylceramide (GlcCer). GlcCerase requires the coordinate action of saposin C and negatively charged lipids for maximal activity. ^1,2 Enzyme replacement therapy (ERT) with Cerezyme ®, a recombinant human GlcCerase, ³ is the main treatment for Type 1 Gaucher disease. Although attempts at structural prediction had been made earlier, ^4,5 the lack of an experimental 3-D structure of GlcCerase hampered attempts to establish its catalytic mechanism and to analyze the structural relationships between the mutations, levels of residual enzyme activity, and disease severity. The recent determination in our laboratories of the x-ray structures of GlcCerase at 2.0 Å resolution ⁶ and, subsequently, of a conjugate with an irreversible inhibitor, ⁷ conduritol B epoxide (CBE), has paved the way for detailed structural analysis of GlcCerase. In this chapter we will review the two structures, and discuss the insights that they provide that may help in designing second-generation GlcCerases for ERT.

Sandhoff disease, a neurodegenerative disorder characterized by the intracellular accumulation of GM2 ganglioside, is caused by mutations in the hexosaminidase β-chain gene resulting in a hexosaminidase A (αβ) and B (ββ) deficiency. A bicistronic lentiviral vector encoding both the hexosaminidase α and β chains (SIV.ASB) has previously been shown to correct the β-hexosaminidase deficiency and to reduce GM2 levels both in transduced and cross-corrected human Sandhoff fibroblasts. Recent advances in determining the neuropathophysiological mechanisms in Sandhoff disease have shown a mechanistic link between GM2 accumulation, neuronal cell death, reduction of sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA) activity, and axonal outgrowth. To examine the ability of the SIV.ASB vector to reverse these pathophysiological events, hippocampal neurons from embryonic Sandhoff mice were transduced with the lentivector. Normal axonal growth rates were restored, as was the rate of Ca²⁺ uptake via the SERCA and the sensitivity of the neurons to thapsigargin-induced cell death, concomitant with a decrease in GM2 and GA2 levels. Thus, we have demonstrated that the bicistronic vector can reverse the biochemical defects and down-stream consequences in Sandhoff neurons, reinforcing its potential for Sandhoff disease in vivo gene therapy.

More than 40 lysosomal disorders are known and classified into broad categories based on the nature of the undegraded metabolites. In this review, the defective enzymatic activities are analyzed, and the pathology of the disorders are described. Various treatments are discussed, including enzyme replacement therapy, which is the most successful treatment available today.

Gaucher disease is an inherited metabolic disorder caused by mutations in the lysosomal enzyme acid-β-glucosidase (GlcCerase). We recently determined the x-ray structure of GlcCerase to 2.0 Å resolution (Dvir, H., Harel, M., McCarthy, A. A., Toker, L., Silman, I., Futerman, A. H., and Sussman, J. L. (2003) EMBO Rep. 4, 704-709) and have now solved the structure of GlcCerase conjugated with an irreversible inhibitor, conduritol-B-epoxide (CBE). The crystal structure reveals that binding of CBE to the active site does not induce a global conformational change in GlcCerase and confirms that Glu ³⁴⁰ is the catalytic nucleophile. However, only one of two alternative conformations of a pair of flexible loops (residues 345-349 and 394-399) located at the entrance to the active site in native GlcCerase is observed in the GlcCerase-CBE structure, a conformation in which the active site is accessible to CBE. Analysis of the dynamics of these two alternative conformations suggests that the two loops act as a lid at the entrance to the active site. This possibility is supported by a cluster of mutations in loop 394-399 that cause Gaucher disease by reducing catalytic activity. Moreover, in silico mutational analysis demonstrates that all these mutations stabilize the conformation that limits access to the active site, thus providing a mechanistic explanation of how mutations in this loop result in Gaucher disease.

Gaucher disease, the most common lysosomal storage disorder, is caused by the defective activity of the lysosomal enzyme, acid-β-glucosidase (GlcCerase), leading to accumulation of glucosylceramide (GlcCer), particularly in cells of the macrophage lineage. Nearly 200 mutations in GlcCerase have been described, but for the most part, genotype-phenotype correlations are weak, and little is known about the down-stream biochemical changes that occur upon GlcCer accumulation that result in cell and tissue dysfunction. In contrast, the clinical course of Gaucher disease has been well described, and at least one treatment is available, namely enzyme replacement therapy. One other treatment, substrate reduction therapy, has recently been marketed, and others are in early stages of development. This review, after discussing pathological mechanisms, evaluates the advantages and disadvantages of existing therapies.

Glycosphingolipid storage disorders are inborn errors of metabolism caused by the defective activity of degradative enzymes in lysosomes. In this review, we summarize studies performed over the past few years attempting to define the secondary and down-stream biochemical and cellular pathways affected in GSL storage disorders that are responsible for neuronal dysfunction, a characteristic of most of these disorders. We focus mainly on the regulation of intracellular calcium homeostasis and phospholipid biosynthesis. These studies may help unravel new roles for glycosphingolipids in the regulation of normal cell physiology, as well as suggest potential new therapeutic options in the glycosphingolipid and other lysosomal storage disorders.

Lysosomal storage disorders, of which more than 40 are known, are caused by the defective activity of lysosomal proteins, which results in the intra-lysosomal accumulation of undegraded metabolites. Despite years of study of the genetic and molecular bases of lysosomal storage disorders, little is known about the events that lead from this intra-lysosomal accumulation to pathology. Here, we summarize the biochemistry of lysosomal storage disorders. We then discuss downstream cellular pathways that are potentially affected in these disorders and that might help us to delineate their pathological mechanisms.

Gaucher disease, an inherited metabolic disorder caused by mutations in the gene encoding acid-β-glucosidase (GlcCerase), is a multi-system disease whose manifestations include anemia, thrombocytopenia, hepatosplenomegaly, bone pathology and, in some cases, neurological signs. Enzyme replacement therapy (ERT) using recombinant GlcCerase (Cerezyme®) alleviates many disease symptoms and is used by ~3000 patients worldwide, and substrate-reduction therapy (SRT) using the glycolipid synthesis inhibitor N-butyldeoxynojirimycin [NB-DNJ (Zavesca®)] has been approved recently for patients for whom ERT is unsuitable. It is our opinion that a multiplicity of treatment strategies is required for the management of Gaucher disease. In this article, we discuss the pros and cons of currently available treatments, and suggest complementary therapies arising from the determination of the X-ray structure of Cerezyme® and from delineation of secondary biochemical pathways affected in Gaucher disease.

The glycosphingolipid lysosomal storage diseases are a group of monogenic human disorders caused by the impaired catalytic activity of enzymes responsible for glycosphingolipid catabolism. Clinical presentation of the diseases is heterogeneous, with little obvious correlation between the kind of accumulating glycosphingolipid and disease progression or pathogenesis. In this review, we discuss clinical symptoms of this group of diseases, and attempt to link disease progression and pathology with the biochemical and cellular pathways that may be potentially altered in the diseases.

Overexpression of upstream of growth and differentiation factor 1 (uog1), a mammalian homolog of the yeast longevity assurance gene (LAG1), selectively induces the synthesis of stearoyl-containing sphingolipids in mammalian cells (Venkataraman, K., Riebeling, C., Bodennec, J., Riezman, H., Allegood, J. C., Sullards, M. C., Merrill, A. H. Jr., and Futerman, A. H. (2002) J. Biol. Chem. 277, 35642-35649). Gene data base analysis subsequently revealed a new subfamily of proteins containing the Lag1p motif, previously characterized as translocating chain-associating membrane (TRAM) protein homologs (TRH). We now report that two additional members of this family regulate the synthesis of (dihydro)ceramides with specific fatty acid(s) when overexpressed in human embryonic kidney 293T cells. TRH1 or TRH4-overexpression elevated [ ³H](dihydro)ceramide synthesis from L-[3-³H]serine and the increase was not blocked by the (dihydro)ceramide synthase inhibitor, fumonisin B₁ (FB₁). Analysis of sphingolipids by liquid chromatography-electrospray tandem mass spectrometry revealed that TRH4 overexpression elevated mainly palmitic acid-containing sphingolipids whereas TRH1 overexpression increased mainly stearic acid and arachidic acid, which in both cases were further elevated upon incubation with FB₁. A similar fatty acid specificity was obtained upon analysis of (dihydro)ceramide synthase activity in vitro using various fatty acyl-CoA substrates, although in a FB ₁-sensitive manner. Moreover, in homogenates from TRH4-overexpressing cells, sphinganine, rather than sphingosine was the preferred substrate, whereas no preference was seen in homogenates from TRH1-overexpressing cells. These findings lend support to our hypothesis (Venkataraman, K., and Futerman, A. H. (2002) FEBS Lett. 528, 3-4) that Lag1p family members regulate (dihydro)ceramide synthases responsible for production of sphingolipids containing different fatty acids.

Sandhoff disease is a lysosomal storage disease in which ganglioside GM2 accumulates because of a defective β-subunit of β-hexosaminidase. This disease is characterized by neurological manifestations, although the pathogenic mechanisms leading from GM2 accumulation to neuropathology are largely unknown. We now examine the viability, development and rates of neurite growth of embryonic hippocampal neurones cultured from a mouse model of Sandhoff disease, the Hexb-/- mouse. GM2 was detected by metabolic labelling at low levels in wild type (Hexb+/+) neurones, and increased by approximately three-fold in Hexb-/- neurones. Hexb-/- hippocampal neurones were as viable as their wild type counterparts and, moreover, their developmental programme was unaltered because the formation of axons and of the minor processes which eventually become dendrites was similar in Hexb-/- and Hexb+/+ neurones. In contrast, once formed, a striking difference in the rate of axonal and minor process growth was observed, with changes becoming apparent after 3 days in culture and highly significant after 5 days in culture. Analysis of various parameters of axonal growth suggested that a key reason for the decreased rate of axonal growth was because of a decrease in the formation of collateral axonal branches, the major mechanism by which hippocampal axons elongate in culture. Thus, although the developmental programme with respect to axon and minor process formation and the viability of hippocampal neurones are unaltered, a significant decrease occurs in the rate of axonal and minor process growth in Hexb-/- neurones. These results appear to be in contrast to dorsal root ganglion neurones cultured from 1-month-old Sandhoff mice, in which cell survival is impaired but normal outgrowth of neurones occurs. The possible reasons for these differences are discussed.

We recently demonstrated that elevation of intracellular glucosylceramide (GlcCer) levels results in increased functional Ca²⁺ stores in cultured neurons, and suggested that this may be due to modulation of ryanodine receptors (RyaRs) by GlcCer (Korkotian, E., Schwarz, A., Pelled, D., Schwarzmann, G., Segal, M. and Futerman, A. H. (1999) J. Biol. Chem. 274, 21673-21678). We now systematically examine the effects of exogenously added GlcCer, other glycosphingolipids (GSLs) and their lyso-derivatives on Ca²⁺ release from rat brain microsomes. GlcCer had no direct effect on Ca²⁺ release, but rather augmented agonist-stimulated Ca²⁺ release via RyaRs, through a mechanism that may involve the redox sensor of the RyaR, but had no effect on Ca²⁺ release via inositol 1,4,5-trisphosphate receptors. Other GSLs and sphingolipids, including galactosylceramide, lactosylceramide, ceramide, sphingomyelin, sphingosine 1-phosphate, sphinganine 1-phosphate, and sphingosylphosphorylcholine had no effect on Ca²⁺ mobilization from rat brain microsomes, but both galactosylsphingosine (psychosine) and glucosylsphingosine stimulated Ca²⁺ release, although only galactosylsphingosine mediated Ca²⁺ release via the RyaR. Finally, we demonstrated that GlcCer levels were ∼10-fold higher in microsomes prepared from the temporal lobe of a type 2 Gaucher disease patient compared with a control, and Ca²⁺ release via the RyaR was significantly elevated, which may be of relevance for explaining the pathophysiology of neuronopathic forms of Gaucher disease.

The roles of sphingolipids, and particularly of the complex glycosphingolipids (GSLs), the gangliosides, have been studied for many years in neurons, glia, and cell lines derived from these tissues, due to their abundance in tissues of neuronal origin. More recently, significant attention has been paid to the simple sphingolipids, particularly ceramide, glucosylceramide (GlcCer), and sphingosine-1-phosphate (S1P), each of which appears to be involved in the regulation of specific aspects of neuronal proliferation, differentiation, survival and apoptosis. In this review, we will summarize studies performed in our laboratory over the past few years using cultured hippocampal neurons in an attempt to define the precise roles of these lipids, and to define their mechanisms of action by identifying down-stream targets with which they interact. We will also discuss work suggesting that complex GSLs, such as gangliosides GM2 and GD3, can also regulate neuronal development, although the down-stream targets with which they interact are less well defined. Our work will be reviewed in light of studies from other laboratories, with particular emphasis on the use of models of sphingolipid storage diseases to determine how these lipids affect neuronal function.

Glucosylceramide (GlcCer) accumulates in the inherited metabolic disorder, Gaucher disease, because of the defective activity of lysosomal glucocerebrosidase. We previously demonstrated that upon GlcCer accumulation, cultured hippocampal neurons exhibit modified growth patterns, altered endoplasmic reticulum density, and altered calcium release from intracellular stores. We here examined the relationship between GlcCer accumulation and phospholipid synthesis. After treatment of neurons with an active site-directed inhibitor of glucocerebrosidase, or in neurons obtained from a mouse model of Gaucher disease, [14C]methyl choline incorporation into [14C]phosphatidylcholine ([14C]PC) and [14C]sphingomyelin was elevated, as were [14C]CDP-choline levels, suggesting that CTP:phosphocholine cytidylyltransferase (CCT) is activated. Indeed, CCT activity was elevated in neurons that had accumulated GlcCer. GlcCer, but not galactosylceramide (GalCer), stimulated CCT activity in rat brain homogenates, and significantly higher levels of CCT were membrane associated in cortical homogenates from a mouse model of Gaucher disease compared with wild-type mice. Because CCT mRNA and protein levels were unaltered in either neurons or brain tissue that had accumulated GlcCer, it appeared likely that GlcCer activates CCT by a post-translational mechanism. This was verified by examination of the effect of GlcCer on CCT purified about 1200-fold from rat brain. GlcCer stimulated CCT activity, with stimulation observed at levels as low as 2.5 mol% and with maximal activation reached at 10 mol%. In contrast, GalCer had no effect. Together, these data demonstrate that GlcCer directly activates CCT, which results in elevated PC synthesis, which may account for some of the changes in growth rates observed upon neuronal GlcCer accumulation.

The longevity assurance gene (LAG1) and its homolog (LAC1) are required for acyl-CoA-dependent synthesis of ceramides containinn-g very long acyl chain (e.g. C26) fatty acids in yeast, and a homolog of LAG1, ASC1, confers resistance in plants to fumonisin B₁, an inhibitor of ceramide synthesis. To understand further the mechanism of regulation of ceramide synthesis, we now characterize a mammalian homolog of LAG1, upstream of growth and differentiation factor-1 (uog1). cDNA clones of uog1 were obtained from expression sequence-tagged clones and sub-cloned into a mammalian expression vector. Transient transfection of human embryonic kidney 293T cells with uog1 followed by metabolic labeling with [4,5-³H]sphinganine or L-3-[³H]serine demonstrated that uog1 conferred fumonisin B₁ resistance with respect to the ability of the cells to continue to produce ceramide. Surprisingly, this ceramide was channeled into neutral glycosphingolipids but not into gangliosides. Electrospray tandem mass spectrometry confirmed the elevation in sphingolipids and revealed that the ceramides and neutral glycosphingolipids of uog1-transfected cells contain primarily stearic acid (C18), that this enrichment was further increased by FB₁, and that the amount of stearic acid in sphingomyelin was also increased. UOG1 was localized to the endoplasmic reticulum, demonstrating that the fatty acid selectivity and the fumonisin B₁ resistance are not due to a subcellular localization different from that found previously for ceramide synthase activity. Furthermore, in vitro assays of uog1-transfected cells demonstrated elevated ceramide synthase activity when stearoyl-CoA but not palmitoyl-CoA was used as substrate. We propose a role for UOG1 in regulating C18-ceramide (N-stearoyl-sphinganine) synthesis, and we note that not only is this the first case of ceramide formation in mammalian cells with such a high degree of fatty acid specificity, but also that the N-stearoyl-sphinganine produced by UOG1 most significantly impacts neutral glycosphingolipid synthesis.

Binding of nerve growth factor (NGF) to the p75 neurotrophin receptor (p75) in cultured hippocampal neurons has been reported to cause seemingly contrasting effects, namely ceramide-dependent axonal outgrowth of freshly plated neurons, versus Jun kinase (Jnk)-dependent cell death in older neurons. We now show that the apoptotic effects of NGF in hippocampal neurons are observed only from the 2nd day of culture onward. This switch in the effect of NGF is correlated with an increase in p75 expression levels and increasing levels of ceramide generation as the cultures mature. NGF application to neuronal cultures from p75^exonIII-/- mice had no effect on ceramide levels and did not affect neuronal viability. The neutral sphingomyelinase inhibitor, scyphostatin, inhibited NGF-induced ceramide generation and neuronal death, whereas hippocampal neurons cultured from acid sphingomyelinase^-/- mice were as susceptible to NGF-induced death as wild type neurons. The acid ceramidase inhibitor, (1S,2R)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol, enhanced cell death, supporting a role for ceramide itself and not a downstream lipid metabolite. Finally, scyphostatin inhibited NGF-induced Jnk phosphorylation in hippocampal neurons. These data indicate an initiating role of ceramide generated by neutral sphingomyelinase in the diverse neuronal responses induced by binding of neurotrophins to p75.

Ceramide (Cer) is a key intermediate in the synthetic and degradative pathways of sphingolipid metabolism, and is also an important second messenger. Natural Cer exists in the D-erythro configuration. Three additional, non-natural stereoisomers exist, but conflicting reports have appeared concerning their metabolism. We now compare the stereospecificity of three enzymes in the sphingolipid biosynthetic pathway, namely dihydroceramide (dihydroCer), sphingomyelin (SM) and glucosylceramide synthases, in subcellular fractions and in cultured cells. The L-erythro enantiomers of sphinganine, dihydroCer and Cer do not act as substrates for any of the three enzymes. In contrast, the diastereoisomer, L-threo-sphinganine, is acylated by dihydroCer synthase, and L-threo-dihydroCer and L-threo-Cer are both metabolized to dihydroSM and SM, respectively, but not to dihydroglucosylceramide and glucosylceramide. No significant difference was detected in the ability of SM synthase to metabolize Cer containing a short (hexanoyl) versus long acyl chain (palmitoyl), demonstrating that short-acyl chain Cers mimic their natural counterparts, at least in the sphingolipid biosynthetic pathway.

Much discussion has recently centred around the biochemical mechanisms by which ceramide is produced in signalling pathways. Since ceramide is virtually insoluble in aqueous solutions, the biological effects of ceramide should be considered in the context of its generation within the membrane lipid bilayer. To this end, we now summarize recent data describing some biophysical properties of ceramide that are of relevance for understanding the mode of ceramide action as a second messenger, and, as a consequence, how the site(s) of ceramide generation might impact upon its role in signalling. Copyright (C) 2000 Elsevier Science Ltd.

Recently a putative mammalian neutral-sphingomyelinase was cloned [Tomiuk et al. (1998) Proc. Natl. Acad. Sci. USA 95, 3638-3643; GenBank accession number AJ222801]. We have overexpressed this enzyme in cultured cells and demonstrate, using four different tagged constructs, that it is localized at the endoplasmic reticulum and not at the plasma membrane. This localization precludes a role for enzyme AJ222801 in the sphingomyelin cycle. Furthermore, a recent publication demonstrated that this enzyme has lyso-platelet activating factor (PAF) phospholipase C activity [Sawai et al. (1999) J. Biol. Chem. 274, 38131-38139]. Together, these data suggest a role for enzyme AJ222801 in the regulation of PAF metabolism. Copyright (C) 2000 Federation of European Biochemical Societies.

Analysis of the binding of cholera toxin to ganglioside GM1 in both living and fixed neurons, and comparison with the distribution of defined axonal and dendritic proteins, demonstrates that ganglioside GM1 is distributed in a non-polarized manner over the axonal and dendritic plasma membranes of mature, cultured hippocampal neurons. Likewise, ganglioside GD1b is also distributed in a non-polarized manner. These results suggest that a recent report [Ledesma, M.D. et al. EMBO J. 18 (1999) 1761-1771] proposing that ganglioside GM1 is highly enriched on the axonal versus dendritic membrane of hippocampal neurons may need to be re-evaluated. Copyright (C) 1999 Federation of European Biochemical Societies.

Sphingolipid-rich rafts play an essential role in the posttranslational (Borchelt, D. R., Scott, M., Taraboulos, A., Stahl, N., and Prusiner, S. B. (1990) J. Cell Biol. 110, 743-752)) formation of the scrapie prion protein PrP(Sc) from its normal conformer PrP(c) (Taraboulos, A., Scott, M., Semenov, A., Avrahami, D., Laszlo, L., Prusiner, S. B., and Avraham, D. (1995) J. Cell Biol. 129, 121-132). We investigated the importance of sphingolipids in the metabolism of the PrP isoforms in scrapie-infected ScN2a cells. The ceramide synthase inhibitor fumonisin B₁ (FB₁) reduced both sphingomyelin (SM) and ganglioside GM1 in cells by up to 50%, whereas PrP(Sc) increased by 3-4- fold. Whereas FB₁ profoundly altered the cell lipid composition, the raft residents PrP(c), PrP(Sc), caveolin 1, and GM1 remained insoluble in Triton X-100. Metabolic radiolabeling demonstrated that PrP(c) production was either unchanged or slightly reduced in FB₁-treated cells, whereas PrP(Sc) formation was augmented by 3-4-fold. To identify the sphingolipid species the decrease of which correlates with increased PrP(Sc), we used two other reagents. When cells were incubated with sphingomyelinase for 3 days, SM levels decreased, GM1 was unaltered, and PrP(Sc) increased by 3-4-fold. In contrast, the glycosphingolipid inhibitor PDMP reduced PrP(Sc) while increasing SM. Thus, PrP(Sc) seems to correlate inversely with SM levels. The effects of SM depletion contrasted with those previously obtained with the cholesterol inhibitor lovastatin, which reduced PrP(Sc) and removed it from detergent-insoluble complexes.

A series of D-erythro- and L-threo-ceramide analogues was synthesized and investigated for their ability to reverse the inhibitory effects of fumonisin B₁ (FB₁) on axonal growth in hippocampal neurons. The analogues contained either a C₇ side chain or a phenyl group substituted for the C₁₃ residue present in naturally occurring ceramides, while the N-acyl chain length was reduced from C₁₆-C₂₄ to C₂-C₈. D-erythro-Ceramide 18a with a C₇ side chain and an N-acetyl residue and D-erythro-ceramide 20c with an aromatic side chain and an N-hexanoyl residue were most active in reversing the inhibitory effects of FB₁ on axonal growth, although the mechanism remains unclear.

Axonal growth can be disrupted by various treatments that inhibit the synthesis of membrane components or their delivery by microtubule-based transport. In cultured hippocampal neurons, a direct correlation exists between the synthesis of sphingolipids, and particularly the simplest glycosphingolipid, glucosylceramide, and the ability of growth factors to stimulate axonal growth [S. Boldin, A.H. Futerman, J. Neurochem. 68 (1997) 882-885]. We now demonstrate that dendritic growth in hippocampal neurons also requires ongoing sphingolipid synthesis. Upon incubation with fumonisin B₁ (FB₁), an inhibitor of acylation of sphingoid long-chain bases, dendritic growth rates are ~ 25% slower than those of control cells, resulting in neurons with shorter dendritic arbors and less dendritic branch points per cell, and readily apparent differences in morphology compared to control cells after 10-14 days in culture. In contrast, FB₁ had no effect on the initial growth of the minor processes, which are destined to become dendrites, even in cells in which FB₁ affected the rate of axon growth. Inhibition of sphingolipid degradation, by incubation with conduritol-B- epoxide (an inhibitor of glucosylceramide degradation) had no effect on dendrite or minor process growth at any stage of development, and no aberrant neurite or ectopic dendrite formation was observed. Together, these data demonstrate that normal dendrite growth in hippocampal neurons requires sphingolipid synthesis, although the molecular requirements for sphingolipid synthesis may differ from those in axons.

Many studies have examined the localization of gangliosides using anti- ganglioside antibodies, although widely differing conclusions have been reached. We now demonstrate that the apparent localization of gangliosides can be greatly influenced by the fixation method. Using monoclonal antibody (MAb) A2B5 (which reacts with a variety of gangliosides), hippocampal neurons were labeled at the cell surface when incubated with the antibody before fixation, but when incubated after fixation the cells displayed a variety of labeling patterns, depending on the fixation method. Biochemical analysis demonstrated that some of the fixatives (particularly acetone and methanol) significantly reduced or completely depleted cellular gangliosides, implying that the immunoreactivity observed with A2B5, and with other antibodies, was not due to gangliosides. When neurons were incubated with an anti-GD1b antibody prefixation, uniform labeling of the plasma membrane was observed, but after ganglioside depletion using biochemical inhibitors of ganglioside synthesis no cell surface labeling was detected. However, even in cells depleted of gangliosides, labeling of both the cell surface and intracellular compartments was observed when the anti-GD1b antibody was applied after fixation. Moreover, after fixation, antibodies to GM4 and GD2 reacted with hippocampal neurons, although these gangliosides are absent from these neurons. In contrast, the JONES antibody (which reacts with 9-O-acetylated GD3) labeled neurons with a similar pattern, essentially irrespective of the fixation method. These observations demonstrate that great care must be taken in assigning gangliosides to specific cell populations or to intracellular locations solely on the basis of use of anti-ganglioside antibodies, and suggest that optimal fixation conditions must be established for each anti- ganglioside antibody.

Previous studies have shown that hippocampal neurons cultured at high density express g-bungarotoxin binding sites and have α7 nicotinic acetylcholine receptor subunit immunoreactivity [Barrantes, G.E., Rogers, A.T., Lindstrom, J. and Wonnacott, S., Brain Res., 672 (1995) 228-236]. We now examine both of these parameters in well-characterized hippocampal neurons cultured at sufficiently low densities to resolve individual neurons and their processes. The specific binding of [α-¹²⁵I]bungarotoxin is first detectable after 3 days in culture and increases during the next 12 days in culture, reaching a maximum of approximately 30,000 binding sites per cell. This is accompanied, over the same timecourse, by an increase in immunoreactivity for two antibodies that specifically bind to the α7 subunit. Both cell bodies and processes were labelled by 9 days in culture. The timecourse of α7-type nicotinic receptor expression resembles that previously described for synapse formation in hippocampal cultures.

Changes in the levels and types of gangliosides occur during neuronal differentiation and development, but no studies have correlated these changes with defined events in neuronal morphogenesis. Here, we have analyzed the relationship between ganglioside synthesis and the development of axons and dendrites in polarized neurons, using hippocampal neurons cultured in such a way that axons and dendrites are generated by a defined sequence of events and in which there is virtually no contamination by glial cells. Neurons were labeled with [4,5-³H]dihydrosphingosine, which was rapidly incorporated into cells and metabolized to ³H-labeled glycosphingolipids. The rate of ³H- labeled glycosphingolipid synthesis was directly proportional to the initial rate of [4,5-³H]dihydrosphingosine uptake and was linear versus time for up to 9 h of incubation. The major changes in ³H-labeled ganglioside synthesis occurred during the period of axonogenesis and rapid axon growth. During axonogenesis, there was a significant increase in the synthesis of complex gangliosides (i.e. G(M1), G(D1a), G(D1b), and G(T1b)) with a corresponding reduction in the synthesis of glucosylceramide and ganglioside G(D3). During the stage of rapid axon growth, the ratio of a- to b-series gangliosides increased significantly. However, during dendritogenesis, dendrite growth, and synaptogenesis, there was little change in ganglioside synthesis, with a small and gradual increase in the ratio of a- to b-series gangliosides and an increase in the synthesis of gangliosides G(D1a) and G(T1b). These results indicate that despite major changes in neuronal morphology and functionality as neurons mature, changes in ganglioside synthesis are restricted to early stages of neuronal development, namely axonogenesis and rapid axon elongation.

Sphingolipids, particularly gangliosides, are enriched in neuronal membranes where they have been implicated as mediators of various regulatory events. We recently provided evidence that sphingolipid synthesis is necessary to maintain neuronal growth by demonstrating that in hippocampal neurons, inhibition of ceramide synthesis by Fumonisin B₁ (FB₁) disrupted axonal outgrowth (Harel, R. and Futerman, A. H. (1993) J. Biol. Chem. 268, 14476-14481). We now analyze further the relationship between neuronal growth and sphingolipid metabolism by examining the effect of an inhibitor of glucosylceramide synthesis, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) and by examining the effects of both FB₁ and PDMP at various stages of neuronal development. No effects of FB₁ or PDMP were observed during the first 2 days in culture, but by day 3 axonal morphology was significantly altered, irrespective of the time of addition of the inhibitors to the cultures. Cells incubated with FB₁ or PDMP had a shorter axon plexus and less axonal branches. FB₁ appeared to cause a retraction of axonal branches between days 2 and 3, although long term incubation had no apparent effect on neuronal morphology or on the segregation of axonal or dendritic proteins. In contrast, incubation of neurons with conduritol B-epoxide, an inhibitor of glucosylceramide degradation, caused an increase in the number of axonal branches and a corresponding increase in the length of the axon plexus. A direct correlation was observed between the number of axonal branch points per cell and the extent of inhibition of either sphingolipid synthesis or degradation. These results suggest that sphingolipids play an important role in the formation or stabilization of axonal branches.

Glucosylceramide, a degradation product of complex glycosphingolipids, is hydrolysed in lysosomes by glucocerebrosidase (GlcCerase). Mutations in the human GlcCerase gene cause a reduction in GlcCerase activity and accumulation of glucosylceramide, which results in the onset of Gaucher disease, the most common lysosomal storage disease. Significant clinical heterogeneity is observed in Gaucher disease, with three main types known, but no clear correlation has been reported between the different types and levels of residual GlcCerase activity. We now demonstrate that a correlation exists by using a radioactive, short-acyl chain substrate, N-(1-[¹⁴C]hexanoyl)-D-erythro-glucosylsphingosine ([¹⁴C]hexanoyl-GlcCer). This substrate rapidly transferred into biological membranes in the absence of detergent and was hydrolyzed to N-(1-[¹⁴C]hexanoyl)-D-erythro-sphingosine ([¹⁴C]hexanoyl-Cer) both in vitro and in situ, with an acid pH optimum. A strict correlation was observed between levels of [¹⁴C]hexanoyl-GlcCer hydrolysis and Gaucher type in human skin fibroblasts. The mean residual activity measured in vitro for 3 h incubation in type 1 Gaucher fibroblasts (the mild form of the disease) was 46.3 ± 4.6 nmol of [¹⁴C]hexanoyl-Cer formed per mg protein (n = 9), and in type 2 and 3 fibroblasts (the neuronopathic forms of the disease) was 19.6 ± 6.5 (n = 9). A similar correlation was observed when activity was measured in situ, suggesting that the clinical severity of a lysosomal storage disease is related to levels of residual enzyme activity.

Neuronal growth is regulated by both extracellular and cellular determinants and is believed to proceed by the addition of new membrane material at the growth cone. To determine whether lipid synthesis is necessary to maintain neuronal growth, we have examined the effect of Fumonisin B₁, an inhibitor of ceramide synthesis, on the development of cultured hippocampal neurons. Fumonisin B₁ inhibits ceramide synthesis in hippocampal neurons both in vivo and in vitro. Ganglioside synthesis and content was reduced after Fumonisin B₁ treatment, and ganglioside GD_1b was not detectable at the cell surface by immunofluorescence. Inhibition of sphingolipid synthesis by Fumonisin B₁ had a significant effect on axonal growth. Between days 2-3 in culture, mean axon length increased from 170 to 240 μm, but in Fumonisin-treated cells, no increase in axon length was observed. Addition of a fluorescent derivative of ceramide together with Fumonisin B₁ reversed this effect, confirming that Fumonisin B₁ acts via inhibition of ceramide synthase. Further, ceramide by itself caused a significant increase in axon length. We discuss three possible mechanisms by which inhibition of sphingolipid synthesis could disrupt axonal growth, among them the possibility that ongoing sphingolipid synthesis is necessary to provide new membrane material to the growing axon.

The ability of phosphatidylinositol-specific phospholipase C (PIPLC) to solubilize acetylcholinesterase (AChE) in the electromotor system of adult Torpedo ocellata and in the developing electric organ was examined. PIPLC solubilizes significant amounts of the membrane-bound G₂ form of AChE throughout embryonic development of the electric organ, as it does in the adult electric organ, the AChE of which we have shown to contain covalently bound inositol in its membrane-anchoring domain. In the electromotor system of the mature fish, PIPLC solubilizes almost quantitatively the AChE dimer in the electromotor axon as in the electric organ itself, but the corresponding fraction in the electric lobe is almost totally resistant to the phospholipase. This finding implies that the covalently bound phosphatidylinositol is added concomitantly with axonal transport. A substantial part of the G₂ form in back muscle is sensitive to PIPLC, whereas the G₄ tetrameter of Torpedo brain is completely resistant.

The temperature-dependence of the catalytic activity of acetylcholinesterase (AChE) from rat erythrocyte-ghost membranes and from Torpedo electric-organ membranes was examined. In the case of rat erythrocyte AChE, a non-linear Arrhenius plot was observed both before and after solubilization by a phosphatidylinositol-specific phospholipase C or by proteinase treatment. Similarly, no significant differences were observed in Arrhenius plots of Torpedo electric-organ AChE before or after solubilization. These results support our suggestion that the catalytic subunit of AChE does not penetrate deeply into the lipid bilayer of the plasma membrane and also suggest that care must be taken in ascribing break points in Arrhenius plots of membrane-bound enzymes to changes in their lipid environment.