The activation and maintenance of Pax2 expression at the mid-hindbrain boundary is controlled by separate enhancers.

Pax2 is the earliest known gene to be expressed throughout the mid-hindbrain region in late gastrula embryos of the mouse and is essential for the formation of an organizing center at the midbrain-hindbrain boundary (MHB), which controls midbrain and cerebellum development. We have used transgenic analysis to identify three MHB-specific enhancers in the upstream region of the mouse Pax2 gene. A 120 bp enhancer (at -3.7 kb) in cooperation with the endogenous promoter was sufficient to induce transgene expression in the anterior neural plate of late gastrula embryos, while it was already inactivated again at the MHB during somitogenesis. The activity of this early enhancer was severely reduced by mutation of three homeodomain-binding sites, two of which are part of a recognition sequence for POU homeodomain proteins. Oct3/4 (Pou5f1), the mouse ortholog of zebrafish Pou2, efficiently bound to this sequence, suggesting its involvement in the regulation of the early Pax2 enhancer. Starting at the four-somite stage, Pax2 is expressed at the MHB under the control of two enhancers located at -4.1 kb and -2.8 kb. The distal late enhancer contains a 102 bp sequence that is not only highly conserved between the mouse and pufferfish Pax2 genes, but also contributes to the enhancer activity of both genes in transgenic mice. The proximal 410 bp enhancer, which overlaps with a kidney-specific regulatory element, contains a functional Pax2/5/8-binding site and thus maintains Pax2 expression at the MHB under auto- and cross-regulatory control by Pax2/5/8 proteins. Importantly, the early and proximal late enhancers are not only sufficient but also necessary for expression at the MHB in the genomic context of the Pax2 locus, as their specific deletion interfered with correct temporal expression of a large Pax2 BAC transgene. Hence, separate enhancers under the control of distinct transcription factors activate and maintain Pax2 expression at the MHB.