Mutually regulated expression of Pax6 and Six3 and its implications for the Pax6 haploinsufficient lens phenotype

Abstract

Pax6 is a key regulator of eye development in vertebrates and invertebrates, and heterozygous loss-of-function mutations of the mouse Pax6 gene result in the Small eye phenotype, in which a small lens is a constant feature. To provide an understanding of the mechanisms underlying this haploinsufficient phenotype, we evaluated in Pax6 heterozygous mice the effects of reduced Pax6 gene dosage on the activity of other transcription factors regulating eye formation. We found that Six3 expression was specifically reduced in lenses of Pax6 heterozygous mouse embryos. Interactions between orthologous genes from the Pax and Six families have been identified in Drosophila and vertebrate species, and we examined the control of Pax6 and Six3 gene expression in the developing mouse lens. Using in vitro and transgenic approaches, we found that either transcription factor binds regulatory sequences from the counterpart gene and that both genes mutually activate their expression. These studies define a functional relationship in the lens in which Six3 expression is dosage-dependent on Pax6 and where, conversely, Six3 activates Pax6. Accordingly, we show a rescue of the Pax6 haploinsufficient lens phenotype after lensspecific expression of Six3 in transgenic mice. This phenotypic rescue was accompanied by cell proliferation and activation of the platelet-derived growth factor ␣-R͞cyclin D1 signaling pathway. Our findings thus provide a mechanism implicating gene regulatory interactions between Pax6 and Six3 in the tissue-specific defects found in Pax6 heterozygous mice.homeodomain proteins ͉ transcription factors ͉ eye development

The Six1 homeoprotein stimulates tumorigenesis by reactivation of cyclin A1

“…Members of the Six family have been implicated in the proliferation of pluripotent precursor cells during development (3,(5)(6)(7)(8)(9)(10). Loss-of-function of Six1 in mice results in a reduction in size or the absence of various organs, because of a decrease in proliferation and an increase in apoptosis (6-8, 39, 40).…”

“…The Six family of homeobox genes has been implicated in the proliferation of progenitor populations before cell type specification (3)(4)(5)(6)(7)(8)(9)(10). Six6 represses the cyclin-dependent kinase inhibitor p27, thereby promoting progenitor cell proliferation in the pituitary gland and the retina (3).…”

Anterior eye development and ocular mesenchyme: new insights from mouse models and human diseases

“…There is evidence that the expression of c-Maf(62) and Six3 (75) is regulated by Pax6 and that Pax6 is able to form specific complexes with both factors. (76,77) Moreover, Pax6 itself may be such a target gene, as there are autoregulative mechanisms of Pax6 gene transcription, which are mediated by Pax6-binding sites in the Pax6 promoter (78) and/or the distal tissue-preferred 3′-enhancer of the Pax6 gene (75,79) The fact that Pax6 is able to form complexes with other homeodomain-containing proteins such as Six3(77) raises the distinct possibility that Pax6 might form similar complexes with itself. Such Pax6-Pax6 complexes might be critically required for activating the transcription of some selected target genes, but might be less available or difficult to assemble under conditions of haploinsufficiency.…”

Signaling during lens regeneration

“…These data suggest that Six-3, along with Pax-6, is also at the top of the hierarchy of genes that is responsible for lens induction. Indeed, it has been shown that Six-3 and Pax-6 can mutually activate one another through binding sites in their respective enhancer elements [50].…”