Rewiring the retinal ganglion cell gene regulatory network: Neurod1 promotes retinal ganglion cell fate in the absence of Math5

Abstract

*Retinal progenitor cells (RPCs) express basic helix-loop-helix (bHLH) factors in a strikingly mosaic spatiotemporal pattern, which is thought to contribute to the establishment of individual retinal cell identity. Here, we ask whether this tightly regulated pattern is essential for the orderly differentiation of the early retinal cell types and whether different bHLH genes have distinct functions that are adapted for each RPC. To address these issues, we replaced one bHLH gene with another. Math5 is a bHLH gene that is essential for establishing retinal ganglion cell (RGC) fate. We analyzed the retinas of mice in which Math5 was replaced with Neurod1 or Math3, bHLH genes that are expressed in another RPC and are required to establish amacrine cell fate. In the absence of Math5, Math5Neurod1 Neurod1-KI/Math3-KI allele did not result in enhanced amacrine cell production. These results were unexpected because they indicated that bHLH genes, which are currently thought to have evolved highly specialized functions, are nonetheless able to adjust their functions by interpreting the local positional information that is programmed into the RPC lineages. We conclude that, although Neurod1 and Math3 have evolved specialized functions for establishing amacrine cell fate, they are nevertheless capable of alternative functions when expressed in foreign environments.KEY WORDS: Retinal ganglion cells, Retinal progenitor cells, bHLH genes, Math5 (Atoh7), Neurod1, Math3 (Neurod4)

Neurog1 can partially replace Atoh1 to differentiate and maintain hair cells in a disorganized organ of Corti

“…Our data suggest that HC precursors behave like RGCs (Mao et al, 2008) and show limited flexibility to respond to the distantly as previously suggested (Jahan et al, 2010). miR-96, an essential miRNA for stereocilia differentiation, shows no expression changes in HCs (E-F′).…”

“…This inability of Neurog1 to maintain and differentiate HCs beyond that achieved with even transient Atoh1 expression (Pan et al, 2012a) contrasts with work on retinal ganglion cells (RGCs), in which the Atoh1 paralog, Atoh7, was replaced by Neurod1 (Mao et al, 2008). Neurod1 can replace Atoh7, possibly because RGC precursors are pre-programmed to differentiate as RGCs independently of the type of bHLH TF (Mao et al, 2013).…”

“…Replacing Atoh7 with Neurod1 leads to normal differentiation of RGCs, whereas replacing Neurod1 by Atoh7 alters the cell fate of amacrine and photoreceptor cells into RGCs (Mao et al, 2013(Mao et al, , 2008, indicating context dependency of gene actions. We knocked Neurog1 into the Atoh1 locus to test whether a bHLH TF that is exclusively associated with proliferative precursors in the ear and brain (Imayoshi and Kageyama, 2014;Ma et al, 2000) can function in differentiation to maintain or alter HC precursor differentiation.…”

“…In molecular terms, the pairing of Atoh1/Neurog1 is as different as that of Atoh7/Neurod1 (52% versus 60% sequence similarity), suggesting that overall binding differences might not fully explain these very different results in eyes and ears. We reasoned that failure of functional replacement of Atoh1 by Neurog1 in the ear (Jahan et al, 2012), compared with Atoh7 by Neurod1 in the retina (Mao et al, 2008), or Atoh1 by fly atonal (Wang et al, 2002), could relate either to 'self-regulation' of Atoh1 via its enhancer (Helms et al, 2000) or to a unique functional requirement of Atoh1/Atonal protein to initiate HC differentiation.…”

Reprogramming amacrine and photoreceptor progenitors into retinal ganglion cells by replacing Neurod1 with Atoh7

“…Previously, we tested whether replacing one bHLH gene with another could redirect RPCs from one cell fate into another (Mao et al, 2008b). We inserted Neurod1 into the Atoh7 locus and showed that Neurod1 replaced the function of Atoh7 in specifying RGC fate.…”