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K. B. Moore, T. J. van Raay, M. L. Vetter; Regulation of Sox2 Expression in the Developing Xenopus Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4426.
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The transcription factor Sox2 is the most common causative gene of micro-and anophthalmia identified to date. Understanding the pathology of Sox2-related eye disorders requires a complete understanding of how Sox2 is expressed and functions in normal eye development. In the Xenopus retina, transient Sox2 expression is required for the transition to neural competence, for initiating proneural gene expression and for neuronal differentiation to occur (van Raay et al., 2005). Sox2 expression in the optic vesicle is dependent upon Wnt/β-catenin signaling, but it is not known whether this regulation is direct or how it is mediated. To address this we focused on understanding the regional control of Sox2 expression within the optic vesicle.
In chick, the Sox2 N3 enhancer mediates expression in the optic vesicle and dienencephalon. To gain further insight into the mechanisms regulating Sox2 expression in retinal progenitors, we PCR-cloned an enhancer from optic vesicle stage Xenopus head genomic DNA using primers designed to the putative X. tropicalis N3 enhancer (Uchikawa et al., 2003). We cloned this enhancer into a promoterless GFP reporter construct (pG1) containing a c-fos minimal promoter (Sox2N3::GFP). To test whether the enhancer drives expression of GFP within the optic vesicles, we generated Xenopus embryos transgenic for Sox2N3::GFP using the nuclear transfer method (Kroll & Amaya, 1996) and assayed for transgene expression at optic vesicle and cup stages.
We isolated a 583 bp enhancer from Xenopus laevis that is 89% identical to the chick N3 enhancer and 98% identical to a region 5’ to the Xenopus tropicalis Sox2 coding sequence. The Xenopus N3 enhancer drives expression of GFP within the optic vesicles of transgenic embryos (N= 42/85, optic vesicle st 23) as well as within later optic cup stages (N=20/42, optic cup st 34). There are 7 conserved TCF/LEF binding sites with the N3 enhancer consistent with our results showing Sox2 optic vesicle expression is Wnt/β-catenin dependent.
Regional control of Sox2 is expression is conserved among vertebrates and is regulated by the N3 enhancer. The Sox2N3 enhancer may be Wnt/β-catenin- dependent in the early optic vesicle, thus providing a pathway by which Wnt and Sox2 control retinal progenitor progression.
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