Transcriptional Regulation of Neuronal Fate Specification by Proneural Genes in the Developing Neocortex
Creators: name: Dennis, Daniel
Publisher: name: Graduate Studies
Year: 2017
Subjects: subject: Neuroscience
subject: proneural
subject: Neocortex
subject: Development

description: The neocortex is a relatively recently evolved region of the mammalian central nervous system (CNS) that is involved in the processing of sensory information, motor control, coordination, language, and spatial reasoning. Performing these complex functions requires the coordinated activities of a variety of different neurons and glia, which are generated in a strict temporal order during development. One class of intrinsic determinants that control the timing of cortical neurogenesis are the proneural basic helix-loop-helix (bHLH) transcription factors encoded by Neurog1, Neurog2, and Ascl1. The activities of these proneural genes are tightly regulated, ensuring that neocortical neurons are born at their correct times and with their proper identities. I began my PhD project by investigating the connection between Neurog2 and Ascl1 and the postmitotic derepression circuit, a molecular circuit for determining neuronal subtype identities. I found a redundant function for Neurog2 and Ascl1 in specifying a subcerebral layer V fate (Ctip2+, Fezf2+), while repressing layer VI (Tbr1+) and upper layer II-IV (Satb2+) identities (Chapter 2). This work expanded our knowledge of Neurog2’s role in neuronal subtype specification and identified a new role for Ascl1 in specifying cortical neuronal identities. In the next part of my thesis, I examined the function of Neurog1 in neocortical development, using loss- and gain-of-function assays (Chapter 3). Very little work had previously been done to elucidate Neurog1 function in the neocortex. I found that while Neurog1 and Neurog2 share a redundant capacity for specifying deep-layer (Tbr1+, Ctip2+) neuronal identities, co-expression of Neurog1 and Neurog2 creates a neurogenesis-limiting regulatory interaction. Further, I showed that Neurog1 and Neurog2 proteins physically interact. Finally, using Neurog1-/- mice, I identified several target genes that are influenced by Neurog1 expression, including several Notch pathway genes. In my final data chapter (Chapter 4), I characterized the changes in progenitor and neuronal subtypes in Neurog1-/-;Neurog2-/- double mutants (Chapter 4). I found a reduction of both germinal zone and cortical plate size in Neurog1-/-;Neurog2-/- cortices, and I identified severe defects in the radial fibers of Neurog1-/-;Neurog2-/- progenitors. Moreover, I found that deep layer neurons were reduced in number in Neurog1-/-;Neurog2-/- cortices, whereas Cux1/2+ upper-layer neurons were expanded. Taken together these data suggest that there is an acceleration of differentiation in double mutants, which will be further examined in the future. In summary, my PhD work has allowed a broader understanding of proneural function in the developing neocortex. This brings us closer to knowing the molecular network that allows cells to go from cortical progenitors to distinct subtypes of cortical projection neurons and the regulatory interactions that influence these choices.