Regulation of Pax6 expression is conserved between mice and flies

Pax6 plays a key role in visual system development throughout the metazoa and the function of Pax6 is evolutionarily conserved. However, the regulation of Pax6 expression during eye development is largely unknown. We have identified two physically distinct promoters in mouse Pax6, P0 and P1, that direct differential Pax6 expression in the developing eye. P0-initiated transcripts predominate in lens placode and corneal and conjunctival epithelia, whereas P1-initiated transcripts are expressed in lens placode, optic vesicle and CNS, and only weakly in corneal and conjunctival epithelia. To further investigate their tissue-specific expression, a series of constructs for each promoter were examined in transgenic mice. We identified three different regulatory regions which direct distinct domains of Pax6 expression in the eye. A regulatory element upstream of the Pax6 P0 promoter is required for expression in a subpopulation of retinal progenitors and in the developing pancreas, while a second regulatory element upstream of the Pax6 P1 promoter is sufficient to direct expression in a subset of post-mitotic, non-terminally differentiated photoreceptors. A third element in Pax6 intron 4, when combined with either the P0 or P1 promoter, accurately directs expression in amacrine cells, ciliary body and iris. These results indicate that the complex expression pattern of Pax6 is differentially regulated by two promoters acting in combination with multiple cis-acting elements. We have also tested whether the regulatory mechanisms that direct Pax6 ocular expression are conserved between mice and flies. Remarkably, when inserted upstream of either the mouse Pax6 P1 or P0 promoter, an eye-enhancer region of the Drosophila eyeless gene, a Pax6 homolog, directs eye- and CNS-specific expression in transgenic mice that accurately reproduces features of endogenous Pax6 expression. These results suggest that in addition to conservation of Pax6 function, the upstream regulation of Pax6 has also been conserved during evolution.