Spatial and temporal regulation of neural stem cells in the developing brain by PRDM16

Abstract 

The mammalian brain comprises a large number and diversity of neurons and non-neural cells. Most of these cell types derive from a single sheet of neuroepithelium (NE) that contains a limited pool of neural stem cells (NSCs).  NSCs in the developing brain are under complex regulation to achieve maximum daughter cell type diversity. In this thesis, we investigate how NSCs are regulated spatially and temporally by a critical transcription factor (TF), PRDM16, in the developing mouse brain.  

In the first paper, we study the mechanisms that guide specification of an essential brain structure, the Choroid plexus (ChP).  The ChP localizes at the brain dorsal midline, and the cell identity of the ChP epithelium depends on the induction by Bone morphogenetic protein (BMP) morphogens. NSCs at the presumptive ChP exit cell cycle, lose neural potential and subsequently obtain the ChP epithelial cell identity. We found that PRDM16 is required for ChP epithelium specification, as proliferative NSCs failed to acquire quiescence in the Prdm16 mutant brain. To investigate the molecular mechanisms controlling NSC proliferation and quiescence, we determined transcriptional states of proliferative versus quiescent NSCs and profiled genomic binding patterns for the BMP signaling effectors SMAD4 and pSMAD1/5/8. We found that BMP signaling suppresses cell proliferation genes in NSCs, and this activity requires PRDM16 co-binding to the enhancers of these target genes. One of the PRDM16-SMAD co-regulated genes is the Wnt pathway ligand Wnt7b. Using a single-cell-resolution fluorescent in situ technique, we confirm that Wnt7band Wnt activity are upregulated in Prdm16 mutant ChP epithelial cells and that the increased Wnt activity correlates with NSC over-proliferation. Together, our work reveals the mechanism by which BMP signaling induces NSC quiescence and defines a regulatory circuit composed of BMP, Wnt signaling and PRDM16 in the specification of the ChP epithelium.  

NSCs sequentially generate cortical projection neurons in the neocortex. How the temporal progression of these stem cells is regulated remains unclear. We identify an essential role of PRDM16 in determining the transition of early to late neurogenesis. Prdm16 deficiency leads to delayed NSC progression, neuronal subtype mis-specification and severe defects in laminar organization. Our genomic profiling of PRDM16 binding sites reveals its enrichment at distal regulatory elements of genes that are differentially expressed between early and late neurogenesis. We further show that PRDM16 represses target gene expression by restricting the chromatin accessibility of the enhancer regions. Cell cycle regulators and genes involved in neuronal migration are direct targets of PRDM16 in regulating NSC behavior and neuronal specification.  

Together, the findings have broadened our understanding of spatial and temporal regulation on NSC identity by the interplay of morphogens and TFs. In particular, we reveal the molecular mechanisms by which BMP signaling together with PRDM16 induces stem cell quiescence to obtain tissue-specific identity and show the temporal dynamics of cortical NSC gene expression programs controlled by PRDM16.