Isolation and characterization of a downstream target of Pax6 in the mammalian retinal primordium

The transcription factor Pax6 is required for eye morphogenesis in humans, mice and insects, and can induce ectopic eye formation in vertebrate and in vertebrate organisms. Although the role of Pax6 has intensively been studie d, only a limited number of genes have been identified that depend on Pax6 activity for their expression in the mammalian visual system. Using a large -scale in situ hybridization screen approach, we have identified a novel ge ne expressed in the mouse optic vesicle. This gene, Necab, encodes a putati ve cytoplasmic Ca2+-binding protein and coincides with Pax6 expression patt ern in the neural ectoderm of the optic vesicle and in the forebrain pretec tum. Remarkably, Necab expression is absent in both structures in Pax6 muta nt embryos. By contrast, the optic vesicle-expressed homeobox genes Rx, Six 3, Otx2 and Lhx2 do not exhibit an altered expression pattern. Using gain-o f-function experiments, we show that, Pax6 can induce ectopic expression of Necab, suggesting that Necab is a direct or indirect transcriptional targe t of Pax6. In addition, we have found that Necab misexpression can induce e ctopic expression of the homeobox gene Chx10, a transcription factor implic ated in retina development. Taken together, our results provide evidence th at Necab is genetically downstream of Pax6 and that it is a part of a signa l transduction pathway in retina development.