Analysis of the gene regulatory mechanism controlling Pax6 expression in teleost and mammals
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The homeodomain transcription factor Pax6 is crucial to the proper development and maintenance of the brain, spinal cord, olfactory system, endocrine pancreas and the eye. A large body of evidence has demonstrated that Pax6 is of particular importance to the visual system and is part of an ancient genetic network of transcriptional regulators that controls various steps during oculogenesis in creatures as diverse as insects and mammals. Within the developing retina, the center piece of the visual system in vertebrates, the Pax6 gene plays several important roles, including control of cell proliferation, maintenance of the retinogenic potential of progenitor cells, and cell fate specification. All of those functions have been attributed to a Pax6 gene product containing a paired-domain, a homeodomain and a C-terminal transactivation domain. However, it was recently postulated that the different aspects of Pax6 function are mediated by different isoforms of the Pax6 protein; however, little is known about the spatiotemporal expression of Pax6 isoforms in the retina. Interestingly, a Pax6 isoform lacking the paired-DNA binding domain was recently identified but its exact expression domains and function remain elusive. In this study, I have used modified bacterial artificial chromosomes (BAC) from zebrafish and mouse to distinguish the expression pattern of paired containing and paired-less Pax6 transcripts in retina. In both zebrafish and mice, the spatial and temporal onset of expression of these transcripts suggests that the paired-less isoform is involved in the cell fate decision leading to the generation of amacrine cells. Furthermore, I present evidence that the two Pax6 isoforms are differentially expressed within amacrine subpopulations, possibly controlling sub-cell type specification events therein. Furthermore, in an effort to determine the extent of conservation of mechanism controlling Pax6 expression, I have established a complement of transgenic mouse lines, carrying either the zebrafish Pax6a or Pax6b locus. Using this cross species comparison, I show that the gene regulatory control mechanism directing Pax6 activity has been highly conserved within the vertebrate lineage. In addition, my data suggests that the Pax6 loci in zebrafish have been retained due to degenerative and complementary mutations in essential tissue specific regulatory elements.