Chromatin regulation of cell fate transitions during cortical development

University Of Michigan At Ann Arbor, Ann Arbor MI

Investigators

Abstract

PROJECT SUMMARY/ABSTRACT The cerebral cortex – the brain’s center for conscious perceptions, thoughts, and actions – is supported by a myriad of cells distinct identities. These identities are established during cortical development in a temporally organized manner: neural progenitor cells (NPCs) first give rise to deep layer neurons, then to upper layer neurons, and finally transition into gliogenesis to produce astrocytes. To generate the correct type of cell at the correct time, NPCs must keep track of where they are in the sequence. However, the mechanisms that directs when NPCs transition from one production mode to another during sequential neurogenesis remain incompletely understood. Some groups have proposed the Polycomb Repressive Complex 2 (PRC2) as a potential mechanism. PRC2 is thought to be involved in the mitotic inheritance of the histone post-translational modification (PTM) H3K27me3, which is associated with transcriptional repression. In fact, when essential PRC2 subunit Eed was deleted at the onset of neurogenesis, NPCs bypassed generation of late-born upper-layer neurons and prematurely transitioned to astrocyte generation. To directly test if PRC2 is the mechanism that transitions NPCs through the neurogenesis sequence, we first conditionally deleted Eed at the onset of neurogenesis, then re-expressed it by in utero electroporation (IUE) at E15.5 – 5 days after conditional deletion of Eed – when upper layer neurons are typically generated. Remarkably, re-expression of Eed in NPCs enabled them to correctly generate upper-layer neurons appropriate for E15.5, effectively rescuing cKO phenotypes. This surprising finding supported the possibility that temporal information successfully progressed in NPCs over the 5 days during which H3K27me3 deposition was absent. These exciting preliminary data form the basis of my hypothesis that PRC2 is required for interpreting the correct stage of neurogenesis, but NPC temporal progression is mediated by non-PRC2 mechanisms. Here, I aim to investigate this hypothesis by: 1) mechanistically separating the roles of PRC2 in transcriptional repression from its functions in histone PTM inheritance across NPC divisions; and 2) identifying non-PRC2 transcriptional mechanisms and histone PTMs that can progress the neurogenic sequence. Together, the proposed work will delineate the roles of PRC2 in neurogenesis, deliver developmentally-staged NPC-specific transcriptomic and functional genomic data, and identify candidate mechanisms of sequential neurogenesis for future follow-up studies. Importantly, these studies will provide excellent conceptual and technical training towards my long-term goal of pursuing academic research in molecular and developmental neuroscience.