Chx10repression ofMitfis required for the maintenance of mammalian neuroretinal identity

Abstract

During vertebrate eye development, the cells of the optic vesicle (OV)become either neuroretinal progenitors expressing the transcription factor Chx10, or retinal pigment epithelium (RPE) progenitors expressing the transcription factor Mitf. Chx10 mutations lead to microphthalmia and impaired neuroretinal proliferation. Mitf mutants have a dorsal RPE-to-neuroretinal phenotypic transformation, indicating that Mitfis a determinant of RPE identity. We report here that Mitf is expressed ectopically in the Chx10or-J/or-J neuroretina(NR), demonstrating that Chx10 normally represses the neuroretinal expression of Mitf. The ectopic expression of Mitf in the Chx10or-J/or-J NR deflects it towards an RPE-like identity; this phenotype results not from a failure of neuroretinal specification, but from a partial loss of neuroretinal maintenance. Using Chx10 and Mitf transgenic and mutant mice, we have identified an antagonistic interaction between Chx10 and Mitf in regulating retinal cell identity. FGF (fibroblast growth factor) exposure in a developing OV has also been shown to repress Mitf expression. We demonstrate that the repression of Mitfby FGF is Chx10 dependent, indicating that FGF, Chx10 and Mitf are components of a pathway that determines and maintains the identity of the NR.

Analysis of opo cis-regulatory landscape uncovers Vsx2 requirement in early eye morphogenesis

“…This TF was initially identified in mutant mice and human patients as an important regulator of retinal precursors' proliferation and lineage specification 13,14 . Later studies in mice found that Vsx2 played an essential role in the specification of the neural retina domain, mainly by repressing Mitf family members Mitf and Tfec, and hence preventing the activation of the RPE genetic programme 17,18,20 . A number of knockdown studies in zebrafish have shown that the reported role for Vsx2 in mammals is, at least, partially conserved in teleosts.…”

“…Similar phenotypes were also observed in human Vsx2 mutations 14 and in vsx2-depleted embryos in zebrafish 15,16 . More recent studies on early stages of eye development have shown that Vsx2 represses Mitf and hence RPE identity, thus acting as a central component of the gene regulatory network involved in the specification of the neural retina domain [17][18][19][20] . Interestingly, the FGF-induced reprogramming of the RPE lineage does not occur in null mutant mice for Vsx2 (ref.…”

“…Interestingly, the FGF-induced reprogramming of the RPE lineage does not occur in null mutant mice for Vsx2 (ref. 18), thus indicating that Vsx2 is an essential downstream node in the MAPK-FGF pathway, responsible for neural retina determination.…”

Chx10 is required to block photoreceptor differentiation but is dispensable for progenitor proliferation in the postnatal retina

“…The central retina is less affected, but data here show that, by P3, division is severely reduced. The early defect in RPC expansion probably ref lects a requirement for Chx10 to regulate one or more genes that directly or indirectly affect proliferation (9,13,15,17). Deregulation of these genes early in retinogenesis probably has enormous indirect consequences on late-stage development; thus, the or J model cannot reveal whether Chx10 plays a role either in proliferation of late-stage RPCs or in driving bipolar cell differentiation.…”

“…However, this finding is only indirect evidence that Chx10 promotes their differentiation because it is unclear whether their absence reflects a direct role for Chx10 in bipolar differentiation or whether the severe defects in proliferation, which alter expression of many genes (15)(16)(17), has an indirect effect on late-stage neurogenesis. These cells are the last retinal neurons born (12); thus, the severe proliferation defect in the or J retina may deplete RPCs before bipolar cell specification.…”