How does neuronal contact mediate astrocyte transcriptional maturation?

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ABSTRACT Developing a healthy functioning brain depends on the coordination of neuronal circuit formation by glial cells called astrocytes. Previous work has shown that astrocytes, born from the same neural stem cells after the end of neurogenesis, require contact with neurons to mature morphologically, functionally, and transcriptionally. Astrocytic expression of the cell adhesion molecule neuroligin 2 (Nlgn2) is necessary for neuronal contact- dependent astrocyte morphogenesis and synaptogenesis. These findings led us to investigate how neuronal contact mediates astrocyte morphogenesis and synaptogenic functions. To address this question, we performed multiplexed indexed T7 chromatin immunoprecipitation (MintChIP) sequencing to measure histone modifications across the astrocyte genome. These experiments found that the chromatin modification landscape changes significantly over postnatal astrocyte development. Interestingly, when we measured histone modifications in astrocytes from Nlgn2 KO mice, we found a substantial decrease in the H3K4me1 and H3K4me3 modifications compared to wildtype astrocytes. Based on these preliminary findings, this proposal will test the hypothesis that epigenetic histone modifications are the mechanistic link between neuronal contact and astrocyte transcriptional maturation. Specifically, aim 1 will test how histone modifications and transcriptional maturation change when astrocytes are cultured with or without neurons and whether inhibiting histone-modifying enzymes is sufficient to prevent astrocyte transcriptional maturation. Aim 2 will investigate the molecular mechanisms of Nlgn2-dependent astrocyte transcriptional maturation. In particular, we will test the hypothesis that the chromatin remodeler Chd8 is required for Nlgn2-dependent H3K4 methylation and that preventing this H3K4 methylation through astrocyte-specific Chd8 knockout will prevent astrocyte transcriptional maturation. The successful completion of these aims will determine the extent to which histone modifications regulate astrocyte maturation and further our understanding of the molecular mechanism responsible for Nlgn2-induced gene expression changes in astrocytes. These findings will also be highly relevant to future work in neurodevelopmental disorders as astrocyte-regulated synaptogenesis, and epigenetic regulation of gene expression are emerging as prominent aspects of conditions like autism spectrum disorders. Finally, this project will provide an excellent training experience for Mr. Justin Savage through its intersection of epigenetics and glial cell biology.