Integral bHLH factor regulation of cell cycle exit and RGC differentiation.

Abstract

In the developing mouse embryo, the bHLH transcription factor Neurog2 is transiently expressed by retinal progenitor cells and required for the initial wave of neurogenesis. Remarkably, another bHLH factor, Ascl1, normally not present in the embryonic Neurog2 retinal lineage, can rescue the temporal phenotypes of Neurog2 mutants. Here we show that Neurog2 simultaneously promotes terminal cell cycle exit and retinal ganglion cell differentiation, using mitotic window labeling and integrating these results with retinal marker quantifications. We also analyzed the transcriptomes of E12.5 GFP-expressing cells from Neurog2GFP/+ , Neurog2GFP/GFP , and Neurog2Ascl1KI/GFP eyes, and validated the most significantly affected genes using qPCR assays. Our data support the hypothesis that Neurog2 acts at the top of a retinal bHLH transcription factor hierarchy. The combined expression levels of these downstream factors are sufficiently induced by ectopic Ascl1 to restore RGC genesis, highlighting the robustness of this gene network during retinal ganglion cell neurogenesis. Developmental Dynamics 247:965-975, 2018. © 2018 Wiley Periodicals, Inc.