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Chai-An Mao, Jang-Hyeon Cho, Jing Wang, Zhiguang Gao, Ping Pan, Diana Lozano, Laura J. Frishman, William H. Klein; Reprogramming Retinal Progenitor Cells by Exchanging Neurod1 for Atoh7/Math5. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3937.
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© ARVO (1962-2015); The Authors (2016-present)
In response to changes in the local environment, vertebrate retina progenitor cells (RPCs) continuously adjust their intrinsic programs to acquire distinct competency states. Atoh7/Math5, a basic helix-loop-helix (bHLH) proneural transcription factor, is first expressed in a subset of RPCs and sets their competency for a retinal ganglion cell (RGC) fate. Soon afterward, RGC differentiation begins in a subpopulation of Atoh7-expressing RPCs. At the same time, several other bHLH factors required for establishing non-RGC fate are expressed in distinct subpopulations of RPCs. The mosaic spatiotemporal pattern of the bHLH factors mirrors the competency state in the RPCs. The process by which an individual RPC destine for one fate yet surrounded by RPCs destine for other fates is exceedingly complex and is not well understood. In this study, we determine whether RPC fate can be reprogrammed by replacing one bHLH factor with another.
Previously, we replaced Atoh7 with Neurod1, a bHLH factor required for amacrine cell fate, and showed that Neurod1 could replace Atoh7’s function in specifying RGC fate, suggesting that Atoh7+ RPCs are largely programmed by intrinsic mechanisms that are not solely dependent on a particular bHLH gene. Here, we performed the converse experiment by replacing Neurod1 with Atoh7 by gene targeting.
Retinas from Neurod1Atoh7 mice produced approximately twenty percent more RGCs than retinas from wild type mice. In the absence of endogenous Atoh7, Neurod1Atoh7 activated RGC genes, and partially restored the optic nerve. The Neurod1Atoh7-induced RGC axons projected to the lateral geniculate nucleus (LGN) and superior colliculus (SC). Full field scotopic (dark-adapted) electroretinograms (ERG) analysis showed that RGCs in Neurod1Atoh7 mice partially rescue the ERG defects found in Atoh7-/- retinas. Moreover, expression of Atoh7 from the NeuroD1Atoh7/Atoh7 alleledid not rescue any of the phenotypic defects associated with Neurod1-/- retinas.
We conclude that Atoh7 alone is sufficient to redirect an RPC destine for an anacrine cell fate to an RGC fate, and that early arising RPCs are inherently programmed to produce RGCs, although they require Atoh7 to do so. Together with our previous results, we propose that in early retinogenesis, most RPCs have the developmental potential to differentiate into RGCs. Although the extrinsic environment plays a role in this process, subtle intrinsic differences must exist among different RPC subpopulations which can be overcome by the expression of Atoh7.
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