Regulation of Pax6 expression is conserved between mice and flies

Pax6 plays a key role in visual system development throughout the metazoa a nd the function of Pax6 is evolutionarily conserved. However, the regulatio n of Pax6 expression during eye development is largely unknown. We have ide ntified two physically distinct promoters in mouse Pax6, P0 and P1, that di rect differential Pax6 expression in the developing eye. PO-initiated trans cripts predominate in lens placode and corneal and conjunctival epithelia, whereas Pi-initiated transcripts are expressed in lens placode, optic vesic le and CNS, and only weakly in corneal and conjunctival epithelia, To furth er investigate their tissue-specific expression, a series of constructs for each promoter were examined in transgenic mice. We identified three differ ent regulatory regions which direct distinct domains of Pax6 expression in the eye. A regulatory element upstream of the Pax6 PO promoter is required for expression in a subpopulation of retinal progenitors and in the develop ing pancreas, while a second regulatory element upstream of the Pax6 P1 pro moter is sufficient to direct expression in a subset of post-mitotic, non-t erminally differentiated photoreceptors, A third element in Pax6 intron 4, when combined with either the PO or P1 promoter, accurately directs express ion in amacrine cells, ciliary body and iris. These results indicate that t he complex expression pattern of Pax6 is differentially regulated by two pr omoters acting in combination with multiple cis-acting elements. We have also tested whether the regulatory mechanisms that direct Pax6 ocul ar expression are conserved between mice and flies. Remarkably, when insert ed upstream of either the mouse Pax6 PI or PO promoter, an eye-enhancer reg ion of the Drosophila eyeless gene, a Pax6 homolog, directs eye- and CNS-sp ecific expression in transgenic mice that accurately reproduces features of endogenous Pax6 expression. These results suggest that in addition to cons ervation of Pax6 function, the upstream regulation of Pax6 has also been co nserved during evolution.