Spatial specification of mammalian eye territories by reciprocal transcriptional repression of Pax2 and Pax6

We have studied the molecular basis of the Pax2 and Pax6 function in the es tablishment of visual system territories. Loss-of-function mutants have rev ealed crucial roles for Pax2 in the generation of the optic stalk and for P ax6 in the development of the optic cup. Ectopic expression of Pax6 in the optic stalk under control of Pax2 promoter elements resulted in a shift of the optic cup/optic stalk boundary indicated by the presence of retinal pig mented cells on the optic stalk. By studying mouse embryos at early develop mental stages we detected an expansion of Pax2, expression domain in the Pa x6(-/-) mutant and of Pax6 expression domain in the Pax2(-/-) embryo. These results suggest that the position of the optic cup/optic stalk boundary de pends on Pax2 and Pax6 expression, hinting at a possible molecular interact ion. Using gel shift experiments, we confirmed the presence of Pax2- and Pa x6-binding sites on the retina enhancer of the Pax6 gene and on the Pax2 up stream control region, respectively. Cotransfection experiments revealed a reciprocal inhibition of Pax2 promoter/enhancer activity by Pax6 protein an d vice versa, Based on our findings, we propose a model for Pax gene regula tion that establishes the proper spatial regionalization of the mammalian v isual system.