Past events

Mammalian brains store and update quantitative internal variables. Primates and rodents, for example, have an internal sense of whether they are 1 or 10 meters away from a landmark and whether a ripe fruit is twice or four times as appetizing as a less ripe counterpart. Such quantitative internal signals are the basis of cognitive function, however, our understanding of how the brain stores and updates these variables remains fragmentary. In this talk, I will discuss imaging and perturbation experiments in tethered, walking Drosophila. The goal of these experiments is to determine how internal variables are calculated by the tiny Drosophila brain and how these variables influence behavior. Specifically, in the Drosophila central complex a set of heading neurons have been described, whose activity tracks the fly’s orientation, similar to head direction cells in rodents. However, the circuit architecture that gives rise to these orientation tracking properties remains largely unknown in any species. I will describe a set of clockwise- and counterclockwise-shifting neurons whose wiring and calcium dynamics provide a means to rotate the heading system’s angular estimate over time. Shifting neurons are required for the heading system to properly track the fly's movements in the dark, and, their stimulation induces a rotation of the heading signal in the expected direction and by the expected amount. The central features of this circuit are analogous to models proposed for head-direction cells in rodents and may thus inform how neural systems, in general, perform addition.

The hundreds of sex differences found in the brain beg the question as to how they develop and what is their function. Factors that cause sex differences in the brain are sex chromosomal gene expression, gonadal hormones, and environmental interactions. Parsimony dictates that these factors act directly on the brain. In fact, available literature on sexual differentiation of the mammalian brain by and large considers just two organs: the gonads and the brain. This perspective, which leaves out all other body parts, misleads us in several ways. First, there is accumulating evidence that all organs are sexually differentiated, and that sex differences in peripheral organs affect the brain. For example, there are sex differences in muscles, adipose tissue, the liver, immune system, gut, kidneys, bladder, and placenta that directly affect the nervous system and behavior. Sex differences may therefore develop in part because brains reside in fundamentally different bodies. This has consequences for brain function as well. Brains may generate different output autonomously, but if they are wired up to different bodies, similar output will have different consequences. To generate similar behaviors, the nervous system may have to compensate by giving different commands. This interaction between body and brain has to be taken into account for a full understanding of the development as well as function of sex differences in the brain. Considering the consequences of this interaction also provides possible explanations for the often remarkable sex differences in neurological and behavioral disorders. These principles will be demonstrated by discussing the development and function of sex differences in vasopressin signaling in brain and body.

Neurodevelopmental disorders encompass a wide range of childhood-onset medical conditions caused by different genetic mutations and interaction with environmental factors, affect ~2% of the population, and are a leading cause of intellectual disability and autism spectrum disorder. Evidence is accumulating that either loss or gain in dosage of proteins involved in cognitive and behavioural processes can be deleterious to the nervous system by causing a failure in the ability to maintain neuronal homeostasis. My studies are focused on the MECP2 duplication syndrome, one of the most common genomic rearrangements in males, characterized by autism, intellectual disability, motor dysfunction, anxiety, epilepsy, recurrent respiratory tract infections and early death. To determine whether the phenotypes of MECP2 duplication are reversible upon normalization of MeCP2 levels, I first generated and characterized a new mouse model that over-expresses a conditional allele of Mecp2 that could be deleted in the adult animal (Nature 2015). Upon normalization of MeCP2 in adult symptomatic mice, several phenotypes were rescued at the behavioral, physiological, and molecular levels. Next, I reduced MeCP2 using an antisense oligonucleotide (ASO) strategy, which has greater translational potential. I found that ASO treatment induced a broad phenotypic rescue in adult symptomatic MECP2 duplication mice, abolished abnormal EEG discharges and behavioral seizures, and corrected abnormal gene expression in the hippocampus. I am currently characterizing a novel “humanized” mouse model of MECP2 duplication syndrome that will precisely mimic the human condition by having two copies of human MECP2 and no copies of the mouse gene. These mice will serve as the ideal model for preclinical tests as they represent the closest construct validity model for the human condition. In addition, I am generating and characterizing neurons and cortical spheroids induced from patients’ derived pluripotent stem cells (iPSCs).