A synaptogenic adhesion code for myelin specificity

University Of Colorado Denver, Aurora CO

Investigators

Abstract

PROJECT SUMMARY Human brain function requires that neuronal axons are ensheathed by myelin, a specialized, lipid-rich membrane produced by oligodendrocytes, one of the principal glial cell types of the central nervous system. Myelin, the white matter of the brain, insulates axons, improving electrical impulse conduction, and supports neuronal health. Importantly, white matter abnormalities are commonly associated with many neuropsychiatric disorders, including autism spectrum disorder (ASD). The cellular and molecular mechanisms that cause abnormal formation or maintenance of myelin in these disorders is entirely unknown. Genome association and sequencing studies have identified numerous genetic variants associated with ASD and other disorders. Many of these variants are predicted to impair proteins that promote the formation and function of neuronal synapses. Intriguingly, oligodendrocytes express genes that encode canonical postsynaptic proteins, often at higher levels than neurons. Our work has provided some of the initial evidence that these proteins function in axonal ensheathment by myelin membrane. These observations raise the novel possibility that genetic variants that impair the function of postsynaptic molecules disrupt myelination by altering the interaction of myelin sheaths with their target axons. In this project we will investigate three synaptic adhesion molecules: Ncam1, Cadm1, and Cadm4. Our main objective is to determine whether these proteins guide myelin formation on specific axons. We will use zebrafish as our model system because it enables us to manipulate gene functions and directly observe axon-myelin interactions in living animals using confocal microscopy. We will use this exploratory R21 mechanism to create powerful new tools for visualizing gene expression and protein localization in vivo and creating loss-of-function mutations in oligodendrocytes. Specifically, in Aim 1 we will use an innovative CRISPR- Cas9 mediated chromosomal insertion strategy to create transgenic reporters that reveal axon-myelin interactions marked by adhesion protein expression and we will determine subcellular localization of adhesion proteins in oligodendrocytes. These studies will identify the specific axons and oligodendrocytes that express adhesive partners that might promote axon-myelin interaction. In Aim 2 we will build a dual transgene system to create loss-of-function mutations of genes encoding Ncam1, Cadm1, and Cadm4 in oligodendrocyte lineage cells. These experiments will reveal whether variants of the genes encoding these proteins might contribute to the myelin abnormalities associated with ASD and other neuropsychiatric disorders. Altogether, this project will improve our understanding of mechanisms that regulate myelination, test the possibility that synaptic protein dysfunction contributes directly to myelin abnormalities in disease, and create powerful new tools for highly detailed future investigations of axoglial interactions.