Pax6 is regulated by Meis and Pbx homeoproteins during pancreatic development.

Pancreatic development depends on the transcription factor Pax6, which controls islet cell differentiation and hormone production. To understand the regulation of Pax6 pancreatic expression, we have identified a minimal Pax6 pancreatic enhancer and show that it contains a composite binding site for Meis and Pbx homeoproteins. We further show that Meis proteins are expressed during pancreatic development, and together with Pbx, are able to form a synergistic binding complex on the Pax6 pancreatic enhancer. When tested in transgenic mice, both the Meis and Pbx sites are essential for Pax6 pancreatic enhancer activity, and the composite site can be functionally replaced by a consensus Meis-Pbx sequence. In addition, analysis of Pbx1 and Pbx2 knockout mice demonstrates that, during pancreatic islet formation, Pax6 expression becomes dependent upon Pbx1 and Pbx2 function. As Meis homeoproteins have been previously demonstrated to regulate Pax6 expression during lens development, these results suggest a conserved mechanism of Pax6 regulation by Meis homeoproteins in two different organs.

Cre-loxp fate-mapping of Pax6 enhancer active retinal and pancreatic progenitors.

Pax6 plays important roles in the control of ocular and pancreatic development. We identified a 450 bp Pax6 enhancer that contains two interacting sequences: a 274 bp fragment sufficient for expression in retinal progenitors and an adjacent 156 bp fragment required for expression in pancreatic progenitors. Since this enhancer is only transiently expressed during embryogenesis, a Cre-loxP fate-mapping strategy was used to investigate the developmental potential of these progenitors. Surprisingly, the labeled retinal precursors predominantly gave rise to horizontal cells, indicating a cell lineage role in horizontal cell differentiation. In the pancreas, all enhancer-specific cells were restricted to endocrine and ductal cell lineages. This result lends support to a model whereby Pax6-expressing progenitors contribute to the adult pancreatic islets and ducts. The progenitor cell-specificity of this enhancer will be useful in studies that require either cell-specific expression or conditional gene inactivation in these cell populations.

Meis homeoproteins directly regulate Pax6 during vertebrate lens morphogenesis.

Pax6 is a pivotal regulator of eye development throughout Metazoa, but the direct upstream regulators of vertebrate Pax6 expression are unknown. In vertebrates, Pax6 is required for formation of the lens placode, an ectodermal thickening that precedes lens development. Here we show that the Meis1 and Meis2 homeoproteins are direct regulators of Pax6 expression in prospective lens ectoderm. In mice, Meis1 and Meis2 are developmentally expressed in a pattern remarkably similar to Pax6 and their expression is Pax6-independent. Biochemical and transgenic experiments reveal that Meis1 and Meis2 bind a specific sequence in the Pax6 lens placode enhancer that is required for its activity. Furthermore, Pax6 and Meis2 exhibit a strong genetic interaction in lens development, and Pax6 expression is elevated in lenses of Meis2-overexpressing transgenic mice. When expressed in embryonic lens ectoderm, dominant-negative forms of Meis down-regulate endogenous Pax6. These results contrast with those in Drosophila, where the single Meis homolog, Homothorax, has been shown to negatively regulate eye formation. Therefore, despite the striking evolutionary conservation of Pax6 function, Pax6 expression in the vertebrate lens is uniquely regulated.