Publications

Myelination of neuronal axons is essential for nervous system development. Myelination requires dramatic cytoskeletal dynamics in oligodendrocytes, but how actin is regulated during myelination is poorly understood. We recently identified serum response factor (SRF)-a transcription factor known to regulate expression of actin and actin regulators in other cell types-as a critical driver of myelination in the aged brain. Yet, a major gap remains in understanding the mechanistic role of SRF in oligodendrocyte lineage cells. Here, we show that SRF is required cell autonomously in oligodendrocytes for myelination during development. Combining ChIP-seq with RNA-seq identifies SRF-target genes in oligodendrocyte precursor cells and oligodendrocytes that include actin and other key cytoskeletal genes. Accordingly, SRF knockout oligodendrocytes exhibit dramatically reduced actin filament levels early in differentiation, consistent with its role in actin-dependent myelin sheath initiation. Surprisingly, oligodendrocyte-restricted loss of SRF results in upregulation of gene signatures associated with aging and neurodegenerative diseases. Together, our findings identify SRF as a transcriptional regulator that controls the expression of cytoskeletal genes required in oligodendrocytes for myelination. This study identifies an essential pathway regulating oligodendrocyte biology with high relevance to brain development, aging, and disease.

Here, we provide a step-by-step protocol for the collection and intracerebroventricular infusion of cerebrospinal fluid (CSF) in mice. We describe steps to withdraw CSF quickly and abundantly while avoiding blood contamination. Using the Lynch coil technique, we gain functional insights into the collected CSF by slowly infusing minimal amounts of CSF directly to the lateral ventricles of aged mice. This protocol is versatile and can be used to infuse drugs, antibodies, or scarce biological compounds. 

Myelination of neuronal axons is essential for nervous system development. Myelination requires dramatic cytoskeletal dynamics in oligodendrocytes, but how actin is regulated during myelination is poorly understood. We recently identified serum response factor (SRF)a transcription factor known to regulate expression of actin and actin regulators in other cell typesas a critical driver of myelination in the aged brain. Yet, a major gap remains in understanding the fundamental role of SRF in oligodendrocyte lineage cells. Here we show that SRF is required cell autonomously in oligodendrocytes for myelination during development. Combining ChIP-seq with RNA-seq identifies SRF-target genes in OPCs and oligodendrocytes that include actin and other key cytoskeletal genes. Accordingly, SRF knockout oligodendrocytes exhibit dramatically reduced actin filament levels early in differentiation, consistent with its role in actin-dependent myelin sheath initiation. Together, our findings identify SRF as a transcriptional regulator that controls the expression of cytoskeletal genes required in oligodendrocytes for myelination. This study identifies a novel pathway regulating oligodendrocyte biology with high relevance to brain development, aging, and disease.

Recent understanding of how the systemic environment shapes the brain throughout life has led to numerous intervention strategies to slow brain ageing¹³. Cerebrospinal fluid (CSF) makes up the immediate environment of brain cells, providing them with nourishing compounds^4,5. We discovered that infusing young CSF directly into aged brains improves memory function. Unbiased transcriptome analysis of the hippocampus identified oligodendrocytes to be most responsive to this rejuvenated CSF environment. We further showed that young CSF boosts oligodendrocyte progenitor cell (OPC) proliferation and differentiation in the aged hippocampus and in primary OPC cultures. Using SLAMseq to metabolically label nascent mRNA, we identified serum response factor (SRF), a transcription factor that drives actin cytoskeleton rearrangement, as a mediator of OPC proliferation following exposure to young CSF. With age, SRF expression decreases in hippocampal OPCs, and the pathway is induced by acute injection with young CSF. We screened for potential SRF activators in CSF and found that fibroblast growth factor 17 (Fgf17) infusion is sufficient to induce OPC proliferation and long-term memory consolidation in aged mice while Fgf17 blockade impairs cognition in young mice. These findings demonstrate the rejuvenating power of young CSF and identify Fgf17 as a key target to restore oligodendrocyte function in the ageing brain.

Cerebrospinal fluid (CSF) proteins and their structures have been implicated in aging and neurodegenerative diseases. In the present study, we used limited proteolysismass spectrometry (LiPMS) to screen for new aging-associated changes in the CSF proteome using a modified analysis. We found 38 protein groups that change in abundance with aging, predominantly immunoglobulins of the IgM subclass. We discovered six high-confidence candidates that underwent structural changes with aging, of which Kng1, Itih2, Lp-PLA₂ and 14-3-3 proteins have binding partners or chemical forms known previously to change in the brains of patients with Alzheimers disease. Orthogonal validation by western blotting identified that the LiPMS hit Cd5l forms a covalent complex with IgM in mouse and human CSF, the abundance of which increases with aging. In human CSF, SOMAmer probe signals for all six LiPMS hits were associated with cognitive function and/or biomarkers of neurodegeneration, especially 14-3-3 proteins YWHAB and YWHAZ. Together, our findings show that LiPMS can uncover age-related structural changes in CSF with relevance to neurodegeneration.

The ability to slow or reverse biological ageing would have major implications for mitigating disease risk and maintaining vitality¹. Although an increasing number of interventions show promise for rejuvenation², their effectiveness on disparate cell types across the body and the molecular pathways susceptible to rejuvenation remain largely unexplored. Here we performed single-cell RNA sequencing on 20 organs to reveal cell-type-specific responses to young and aged blood in heterochronic parabiosis. Adipose mesenchymal stromal cells, haematopoietic stem cells and hepatocytes are among those cell types that are especially responsive. On the pathway level, young blood invokes new gene sets in addition to reversing established ageing patterns, with the global rescue of genes encoding electron transport chain subunits pinpointing a prominent role of mitochondrial function in parabiosis-mediated rejuvenation. We observed an almost universal loss of gene expression with age that is largely mimicked by parabiosis: aged blood reduces global gene expression, and young blood restores it in select cell types. Together, these data lay the groundwork for a systemic understanding of the interplay between blood-borne factors and cellular integrity.

Lysosome dysfunction is a shared feature of rare lysosomal storage diseases and common age-related neurodegenerative diseases. Microglia, the brain-resident macrophages, are particularly vulnerable to lysosome dysfunction because of the phagocytic stress of clearing dying neurons, myelin, and debris. CD22 is a negative regulator of microglial homeostasis in the aging mouse brain, and soluble CD22 (sCD22) is increased in the cerebrospinal fluid of patients with Niemann-Pick type C disease (NPC). However, the role of CD22 in the human brain remains unknown. In contrast to previous findings in mice, here, we show that CD22 is expressed by oligodendrocytes in the human brain and binds to sialic acid-dependent ligands on microglia. Using unbiased genetic and proteomic screens, we identify insulin-like growth factor 2 receptor (IGF2R) as the binding partner of sCD22 on human myeloid cells. Targeted truncation of IGF2R revealed that sCD22 docks near critical mannose 6-phosphate-binding domains, where it disrupts lysosomal protein trafficking. Interfering with the sCD22-IGF2R interaction using CD22 blocking antibodies ameliorated lysosome dysfunction in human NPC1 mutant induced pluripotent stem cell-derived microglia-like cells without harming oligodendrocytes in vitro. These findings reinforce the differences between mouse and human microglia and provide a candidate microglia-directed immunotherapeutic to treat NPC.

Ageing is characterized by a progressive loss of physiological integrity, leading to impaired function and increased vulnerability to death¹. Despite rapid advances over recent years, many of the molecular and cellular processes that underlie the progressive loss of healthy physiology are poorly understood². To gain a better insight into these processes, here we generate a single-cell transcriptomic atlas across the lifespan of Mus musculus that includes data from 23 tissues and organs. We found cell-specific changes occurring across multiple cell types and organs, as well as age-related changes in the cellular composition of different organs. Using single-cell transcriptomic data, we assessed cell-type-specific manifestations of different hallmarks of ageingsuch as senescence³, genomic instability⁴ and changes in the immune system². This transcriptomic atlaswhich we denote Tabula Muris Senis, or Mouse Ageing Cell Atlasprovides molecular information about how the most important hallmarks of ageing are reflected in a broad range of tissues and cell types.

Ageing is the single greatest cause of disease and death worldwide, and understanding the associated processes could vastly improve quality of life. Although major categories of ageing damage have been identifiedsuch as altered intercellular communication, loss of proteostasis and eroded mitochondrial function¹these deleterious processes interact with extraordinary complexity within and between organs, and a comprehensive, whole-organism analysis of ageing dynamics has been lacking. Here we performed bulk RNA sequencing of 17 organs and plasma proteomics at 10 ages across the lifespan of Mus musculus, and integrated these findings with data from the accompanying Tabula Muris Senis²or Mouse Ageing Cell Atlaswhich follows on from the original Tabula Muris³. We reveal linear and nonlinear shifts in gene expression during ageing, with the associated genes clustered in consistent trajectory groups with coherent biological functionsincluding extracellular matrix regulation, unfolded protein binding, mitochondrial function, and inflammatory and immune response. Notably, these gene sets show similar expression across tissues, differing only in the amplitude and the age of onset of expression. Widespread activation of immune cells is especially pronounced, and is first detectable in white adipose depots during middle age. Single-cell RNA sequencing confirms the accumulation of T cells and B cells in adipose tissueincluding plasma cells that express immunoglobulin Jwhich also accrue concurrently across diverse organs. Finally, we show how gene expression shifts in distinct tissues are highly correlated with corresponding protein levels in plasma, thus potentially contributing to the ageing of the systemic circulation. Together, these data demonstrate a similar yet asynchronous inter- and intra-organ progression of ageing, providing a foundation from which to track systemic sources of declining health at old age.

The vascular interface of the brain, known as the bloodbrain barrier (BBB), is understood to maintain brain function in part via its low transcellular permeability¹³. Yet, recent studies have demonstrated that brain ageing is sensitive to circulatory proteins^4,5. Thus, it is unclear whether permeability to individually injected exogenous tracersas is standard in BBB studiesfully represents blood-to-brain transport. Here we label hundreds of proteins constituting the mouse blood plasma proteome, and upon their systemic administration, study the BBB with its physiological ligand. We find that plasma proteins readily permeate the healthy brain parenchyma, with transport maintained by BBB-specific transcriptional programmes. Unlike IgG antibody, plasma protein uptake diminishes in the aged brain, driven by an age-related shift in transport from ligand-specific receptor-mediated to non-specific caveolar transcytosis. This age-related shift occurs alongside a specific loss of pericyte coverage. Pharmacological inhibition of the age-upregulated phosphatase ALPL, a predicted negative regulator of transport, enhances brain uptake of therapeutically relevant transferrin, transferrin receptor antibody and plasma. These findings reveal the extent of physiological protein transcytosis to the healthy brain, a mechanism of widespread BBB dysfunction with age and a strategy for enhanced drug delivery.

Microglia become progressively activated and seemingly dysfunctional with age, and genetic studies have linked these cells to the pathogenesis of a growing number of neurodegenerative diseases. Here we report a striking buildup of lipid droplets in microglia with aging in mouse and human brains. These cells, which we call lipid-droplet-accumulating microglia (LDAM), are defective in phagocytosis, produce high levels of reactive oxygen species and secrete proinflammatory cytokines. RNA-sequencing analysis of LDAM revealed a transcriptional profile driven by innate inflammation that is distinct from previously reported microglial states. An unbiased CRISPRCas9 screen identified genetic modifiers of lipid droplet formation; surprisingly, variants of several of these genes, including progranulin (GRN), are causes of autosomal-dominant forms of human neurodegenerative diseases. We therefore propose that LDAM contribute to age-related and genetic forms of neurodegeneration.

Focusing on microtubule heterogeneity and brain specificity allowed for initial discoveries of multiple tubulin isotypes four decades ago. Methods evolved from using radioactive labelling and single cell cultures to monoclonal antibodies recognizing discrete forms of tubulin in single neurons. With the advantage of molecular cloning and fluorescent protein tagging, essential components for microtubule dynamics/stability and function were identified, including activity-dependent neuroprotective protein, ADNP and its peptide snippet, NAP (drug candidate, davunetide/CP201). ADNP/NAP through the SxIP motif interact with microtubule end binding proteins EB1 and EB3 to increase microtubule dynamics, axonal transport and dendritic spine formation. Recent transcriptomic analysis of the young mouse brain at the single cell level enabled characterization of cell-type specific cytoskeleton related gene signatures (e.g., tubulin transcripts, microtubule-associated protein Tau, Mapt and microtubule end binding protein, EB3, Mapre3) at unprecedented detail. Here, we review these findings with a methodological perspective to highlight how cutting-edge techniques have allowed us to disentangle cytoskeleton dynamics in health and disease.

Alterations in astrocyte function such as a pro-inflammatory phenotype are associated with Alzheimers disease (AD). We had shown impairments in the ability of aged astrocytes isolated from 5xFAD mice to clear and uptake amyloid-β (Aβ) as well as to support neuronal growth. Senescent cells accumulate with age and exhibit a senescence-associated secretory phenotype, which includes secretion of pro-inflammatory cytokines. In this study, we predicted that with age, astrocytes in 5xFAD mice would exhibit a cellular senescence phenotype that could promote neurodegeneration. We found an age-dependent increase in senescent astrocytes adjacent to Aβ plaques in 5xFAD mice. Inhibition of nuclear factor kappa-light-chain-enhancer of activated B cells reduced interelukin-6 secretion by senescent astrocytes and resulted in improved neuronal support. Moreover, senescent astrocytes exhibited an increase in the induction of the TGF-β1-SMAD2/3 pathway, and inhibition of this pathway resulted in a reduction of cellular senescence. We also discovered that soluble Aβ42 induced astrocyte senescence in young naïve mice in a SMAD2/3-dependent manner. Our results suggest an important role of astrocyte senescence in AD and its role in mediating the neurotoxicity properties of astrocytes in AD and related neurodegenerative diseases.

To gain unfettered insight into one of the scourges of our aging societies, Mathys and colleagues in Nature (Mathys et al., 2019) illuminate the brain transcriptome of Alzheimer's disease at single-cell resolution. Their findings implicate oligodendrocytes, a cell type largely neglected in Alzheimer's disease research, and sex in the disease in intriguing ways.

Microglia maintain homeostasis in the central nervous system through phagocytic clearance of protein aggregates and cellular debris. This function deteriorates during ageing and neurodegenerative disease, concomitant with cognitive decline. However, the mechanisms of impaired microglial homeostatic function and the cognitive effects of restoring this function remain unknown. We combined CRISPRCas9 knockout screens with RNA sequencing analysis to discover age-related genetic modifiers of microglial phagocytosis. These screens identified CD22, a canonical B cell receptor, as a negative regulator of phagocytosis that is upregulated on aged microglia. CD22 mediates the anti-phagocytic effect of α2,6-linked sialic acid, and inhibition of CD22 promotes the clearance of myelin debris, amyloid-β oligomers and α-synuclein fibrils in vivo. Long-term central nervous system delivery of an antibody that blocks CD22 function reprograms microglia towards a homeostatic transcriptional state and improves cognitive function in aged mice. These findings elucidate a mechanism of age-related microglial impairment and a strategy to restore homeostasis in the ageing brain.

Here we present a compendium of single-cell transcriptomic data from the model organism Mus musculus that comprises more than 100,000 cells from 20 organs and tissues. These data represent a new resource for cell biology, reveal gene expression in poorly characterized cell populations and enable the direct and controlled comparison of gene expression in cell types that are shared between tissues, such as T lymphocytes and endothelial cells from different anatomical locations. Two distinct technical approaches were used for most organs: one approach, microfluidic droplet-based 3-end counting, enabled the survey of thousands of cells at relatively low coverage, whereas the other, full-length transcript analysis based on fluorescence-activated cell sorting, enabled the characterization of cell types with high sensitivity and coverage. The cumulative data provide the foundation for an atlas of transcriptomic cell biology.

In Alzheimer's disease (AD), astrocytes undergo morphological changes ranging from atrophy to hypertrophy, but the effect of such changes at the functional level is still largely unknown. Here, we aimed to investigate whether alterations in astrocyte activity in AD are transient and depend on their microenvironment, or whether they are irreversible. We established and characterized a new protocol for the isolation of adult astrocytes and discovered that astrocytes isolated from old 5xFAD mice have higher GFAP expression than astrocytes derived from WT mice, as observed in vivo. We found high C1q levels in brain sections from old 5xFAD mice in close vicinity to amyloid plaques and astrocyte processes. Interestingly, while old 5xFAD astrocytes are impaired in uptake of soluble Aβ42, this effect was reversed upon an addition of exogenous C1q, suggesting a potential role for C1q in astrocyte-mediated Aβ clearance. Our results suggest that scavenger receptor B1 plays a role in C1q-facilitated Aβ uptake by astrocytes and that expression of scavenger receptor B1 is reduced in adult old 5xFAD astrocytes. Furthermore, old 5xFAD astrocytes show impairment in support of neuronal growth in co-culture and neurotoxicity concomitant with an elevation in IL-6 expression. Further understanding of the impact of astrocyte impairment on AD pathology may provide insights into the etiology of AD.

Glial scarring, formed by reactive astrocytes, is one of the major impediments for regeneration after spinal cord injury (SCI). Reactive astrocytes become hypertrophic, proliferate and secrete chondroitin sulphate proteoglycans into the extracellular matrix (ECM). Many studies have demonstrated that epidermal growth factor receptors (EGFR) can mediate astrocyte reactivity after neurotrauma. Previously we showed that there is crosstalk between nucleolin and EGFR that leads to increased EGFR activation followed by increased cell proliferation. Treatment with the nucleolin inhibitor GroA (AS1411) prevented these effects in vitro and in vivo. In this study, we hypothesized that similar interactions may mediate astrogliosis after SCI. Our results demonstrate that nucleolin and EGFR interaction may play a pivotal role in mediating astrocyte proliferation and reactivity after SCI. Moreover, we demonstrate that treatment with GroA reduces EGFR activation, astrocyte proliferation and chondroitin sulphate proteoglycans secretion, therefore promoting axonal regeneration and sprouting into the lesion site. Our results identify, for the first time, a role for the interaction between nucleolin and EGFR in astrocytes after SCI, indicating that nucleolin inhibitor GroA may be used as a novel treatment after neurotrauma. (Figure presented.) A major barrier for axonal regeneration after spinal cord injury is glial scar created by reactive and proliferating astrocytes. EGFR mediate astrocyte reactivity. We showed that inhibition of nucleolin by GroA, reduces EGFR activation, which results in attenuation of astrocyte reactivity and proliferation in vivo and in vitro. EGFR, epidermal growth factor receptor.

Multiple EGF-like domains 10 (Megf10) is a class F scavenger receptor (SR-F3) expressed on astrocytes and myosatellite cells, and recessive mutations in humans result in early-onset myopathy, areflexia, respiratory distress, and dysphagia (EMARDD). Here we report that Megf10-deficient mice have increased apoptotic cells in the developing cerebellum and have impaired phagocytosis of apoptotic cells by astrocytes ex vivo.We also report that cells transfected with Megf10 gain the ability to phagocytose apoptotic neurons and that Megf10 binds with high affinity to C1q, an eat-me signal for apoptotic cells. In contrast, cells expressing Megf10 with EMARDD mutations have impaired apoptotic cell clearance and impaired binding to C1q. Our studies reveal that Megf10 is a receptor for C1q and identify a novel role for Megf10 in clearance of apoptotic cells in the mammalian developing brain with potential relevance to EMARDD patients and other CNS disorders.

Alzheimer's disease (AD) is the most prevalent form of dementia in elderly. Genetic studies revealed allelic segregation of the apolipoprotein E (ApoE) gene in sporadic AD and in families with higher risk of AD. The mechanisms underlying the pathological effects of ApoE4 are not yet entirely clear. Several studies indicate that autophagy, which plays an important role in degradation pathways of proteins, organelles and protein aggregates, may be impaired in AD. In the present study, we investigated the effects of ApoE4 versus the ApoE3 isoform on the process of autophagy in mouse-derived astrocytes. The results obtained reveal that under several autophagy-inducing conditions, astrocytes expressing ApoE4 exhibit lower autophagic flux compared to astrocytes expressing ApoE3. Using an in situ model, we examined the role of autophagy and the effects thereon of ApoE4 in the elimination ofAβ plaques from isolated brain sections of transgenic 5xFADmice. This revealed that ApoE4 astrocytes eliminate Aβ plaques less effectively than the corresponding ApoE3 astrocytes. Additional experiments showed that the autophagy inducer, rapamycin, enhances Aβ plaque degradation by ApoE4 astrocytes whereas the autophagy inhibitor, chloroquine, blocks Aβ plaque degradation by ApoE3 astrocytes. Taken together, these findings show that ApoE4 impairs autophagy in astrocyte cultures and that this effect is associated with reduced capacity to clear Aβ plaques. This suggests that impaired autophagy may play a role in mediating the pathological effects of ApoE4 in AD.

NAP (NAPVSIPQ, davunetide) is a microtubule stabilizing peptide drug candidate. Here, we set out to identify NAP-like peptides that provide neuroprotection and reduce tau pathology. NAP-like peptides were derived using publically available search engines, which identified sequence homologies in the microtubule subunit tubulin and in the microtubule associated protein, tau. NATLSIHQ (NAT) and STPTAIPQ were derived from tubulin, and TAPVPMPD (TAP) was derived from tau. All peptides provided neuroprotection against the Alzheimer's disease (AD) toxin, the amyloid-β 1-42 peptide, although NAT and TAP were much more potent than STPTAIPQ. NAT also protected astrocytes, while STPTAIPQ was active only at micromolar concentrations. Because NAT and TAP were much more potent than STPTAIPQ in neuroprotection, those peptides were also tested for inhibition of tau-like aggregation (the second protein hallmark pathology of AD). Both NAT and TAP inhibited tau-like aggregation, with NAT being active over a very broad concentration range. NAT also protected in vivo in a frontotemporal dementia transgenic mouse model (Tau-Tg), when tested at the age of ~10 months. Results showed significantly decreased levels of the NAP parent protein, activity-dependent neuroprotective protein in the cerebral cortex of the Tau-Tg which was increased back to normal levels by NAT treatment. This was coupled to protection of Brain-Body weight ratio in the compromised Tau-Tg. With AD being the major tauopathy and with tau taking part in frontotemporal dementia, novel NAP derivatives that reduce tauopathy and provide neuroprotection are of basic and clinical interest.

The clearance of apoptotic cells is critical for the control of tissue homeostasis; however, the full range of receptors on phagocytes responsible for the recognition of apoptotic cells remains to be identified. Here we found that dendritic cells (DCs), macrophages and endothelial cells used the scavenger receptor SCARF1 to recognize and engulf apoptotic cells via the complement component C1q. Loss of SCARF1 impaired the uptake of apoptotic cells. Consequently, in SCARF1-deficient mice, dying cells accumulated in tissues, which led to a lupus-like disease, with the spontaneous generation of autoantibodies to DNA-containing antigens, activation of cells of the immune system, dermatitis and nephritis. The discovery of such interactions of SCARF1 with C1q and apoptotic cells provides insight into the molecular mechanisms involved in the maintenance of tolerance and prevention of autoimmune disease.

Axon pruning during development is essential for proper wiring of the mature nervous system, but itsregulation remains poorly understood. We have identified an immunoglobulin superfamily (IgSF) transmembrane protein, Plum, that is cell autonomously required for axon pruning of mushroom body (MB) γ neurons and for ectopic synapse refinement at the developing neuromuscular junction in. Drosophila. Plum promotes MB γ neuron axon pruning by regulating the expression of Ecdysone Receptor-B1, a key initiator of axon pruning. Genetic analyses indicate that Plum acts to facilitate signaling of Myoglianin, a glial-derived TGF-β, on MB γ neurons upstream of the type-I TGF-β receptor Baboon. Myoglianin, Baboon, and Ecdysone Receptor-B1 are also required for neuromuscular junction ectopic synapse refinement. Our study highlights both IgSF proteins and TGF-β facilitation as key promoters of developmental axon elimination and demonstrates a mechanistic conservation between MB axon pruning during metamorphosis and the refinement of ectopic larval neuromuscular connections

Astrocytes are the most abundant cells in the brain and play an important role in the homeostasis and maintenance of the brain. Furthermore, astrocytes play a key role in brain protection and in functional recovery from injuries. Impairment in astrocytes activity may promote neurodegeneration and, eventually, retraction of neuronal synapses, which leads to cognitive deficits found in neurodegenrative diseases, such as Alzheimer's disease. Alzheimer's disease (AD) is the most common type of dementia affecting more than 18 million people worldwide. The main cause of AD is generally attributed to the increased production and accumulation of amyloid-β (Aβ), in association with neurofibrillary tangle (NFT) formation. In AD patient's brain, reactive astrocytes are integral components of neuritic plaques. Astrocytic activation seems to be particularly prominent around Aβ deposits both in the brain parenchyma and in the cerebrovasculature. Furthermore, recent evidence from AD patients suggests that pathological changes in the morphology of astrocyte occur prior to the appearance of Aβ plaques. The focus of this review is on astrocytic cells and their role in the progression of AD.