Alternative promoter use in eye development: the complex role and regulation of the transcription factor MITF

Abstract

During vertebrate eye development, the transcription factor MITF plays central roles in neuroepithelial domain specification and differentiation of the retinal pigment epithelium. MITF is not a single protein but represents a family of isoforms generated from a common gene by alternative promoter/exon use. To address the question of the role and regulation of these isoforms, we first determined their expression patterns in developing mouse eyes and analyzed the role of some of them in genetic models. We found that two isoforms, A-and J-Mitf, are present throughout development in both retina and pigment epithelium, whereas H-Mitf is detected preferentially and D-Mitf exclusively in the pigment epithelium. We further found that a genomic deletion encompassing the promoter/exon regions of H-, D-and B-Mitf leads to novel mRNA isoforms and proteins translated from internal start sites. These novel proteins lack the normal, isoform-specific N-terminal sequences and are unable to support the development of the pigment epithelium, but are capable of inducing pigmentation in the ciliary margin and the iris. Moreover, in mutants of the retinal Mitf regulator Chx10 (Vsx2), reduced cell proliferation and abnormal pigmentation of the retina are associated with a preferential upregulation of H-and D-Mitf. This retinal phenotype is corrected when H-and D-Mitf are missing in double Mitf/Chx10 mutants. The results suggest that Mitf regulation in the developing eye is isoform-selective, both temporally and spatially, and that some isoforms, including H-and D-Mitf, are more crucial than others in effecting normal retina and pigment epithelium development.

Modeling early retinal development with human embryonic and induced pluripotent stem cells

“…The next phase in retinal specification in vivo occurs with the formation of the optic vesicles from the paired eye fields. At this stage, all cells that will give rise to either the neural retina or the RPE express the transcription factor Mitf (17). The subset of Mitfϩ cells destined to become neural retina subsequently down-regulate Mitf in response to the onset of Chx10 (also called Vsx2) expression (18,19).…”

Characterization of Three-Dimensional Retinal Tissue Derived from Human Embryonic Stem Cells in Adherent Monolayer Cultures

“…The distribution of OTX2 [+] cell nuclei, which are typically found in PR precursors [54,85], resembled that in 6-10-week-old human fetal retina [81]. RPE pigmentation together with strong upregulation of several key RPE signature genes [106] (MITF [RPE isoform] [107], DCT, RPE-65, TYR, TYRP, and even PMEL) suggests the formation of an RPE layer around the hESCderived retinal tissue, providing additional evidence for self-organization and polarization of this tissue. We noted that PMEL (human homolog of the Silver gene in mice), which is responsible for melanocyte granule formation, was upregulated about sevenfold by 6 weeks in hESCderived retinal tissue.…”

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

“…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.…”