Roles of HDACs and MEF2 in Adult Hippocampal Neurogenesis
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The maintenance of the resident adult neural stem/progenitor cell (NSPC) pool depends on the precise balance of proliferation, differentiation, and maintenance of the undifferentiated state. Identifying the mechanisms that regulate this balance in adult hippocampal NSPCs can provide insight into basic neurogenesis principles important for tissue homeostasis and preventing tumor formation. Pharmacological inhibition of histone deacetylases (HDACs), a class of histone-modifying enzymes, have promising effects in cancer cells, yet the specific roles of individual HDACs in adult NSPCs are unclear. In this dissertation, I focus on dissecting the roles of two different HDACs in adult hippocampal neurogenesis: the Class I HDAC, HDAC3 and the Class IIa HDAC, HDAC5 as well as the Class IIa HDAC binding partner, myocyte enhancer factor 2 (MEF2). Using conditional knockout (cKO) mice and in vitro cell culture, I show that histone deacetylase 3 (HDAC3) is required for the proliferation of adult NSPCs. Detailed cell cycle analysis of NSPCs from Hdac3 cKO mice reveals a defect in cell cycle progression through G2/M phase, but not S phase. Moreover, HDAC3 controls G2/M phase progression mainly through post-translational stabilization of the G2/M cyclin- dependent kinase-1 (CDK1). These results demonstrate that HDAC3 plays a critical role in NSPC proliferation. HDAC5 is the most abundant Class IIa HDAC in adult dentate gyrus. HDAC5 is only expressed in immature and mature neurons. Using Hdac5 knockout mice and in vitro cell culture, I show that HDAC5 is necessary and sufficient to restrict the neuronal differentiation of NSPCs. However, the detailed mechanisms are yet to be determined. Class IIa HDACs bind to myocyte enhancer factor 2 (MEF2) in the nucleus to repress transcription of pro-neuronal genes. Thus, we also examined the function of Mef2 genes in adult hippocampal neurogenesis. In adult hippocampus, the three most highly expressed MEF2 proteins are MEF2A, 2C, and 2D, which are expressed in immature and mature neurons similar to HDAC5. We have shown that one synthetic small molecule, Isoxazole-9 (Isx-9) could trigger neuronal differentiation robustly in vitro and in vivo. Inducible knockout of all three Mef2 genes specifically in NSPCs and their progeny revealed their critical roles in mediating Isx-9 induced neurogenesis and baseline neurogenesis. In summary, these results demonstrate that HDACs and MEF2 control different stages of adult hippocampal neurogenesis and suggest that strategies aimed at pharmacological modulation of these proteins may be beneficial for tissue regeneration and controlling tumor cell growth in mammalian brain.