THE MOUSE HOMEODOMAIN PROTEIN PHOX2 REGULATES NCAM PROMOTER ACTIVITY IN CONCERT WITH CUX CDP AND IS A PUTATIVE DETERMINANT OF NEUROTRANSMITTER PHENOTYPE/

Transcriptional regulation of the gene encoding the cell adhesion rece ptor NCAM (neural cell adhesion molecule), a putative effector molecul e of a variety of morphogenetic events, is likely to involve important regulators of morphogenesis. Here we identify two mouse homeodomain p roteins that bind to an upstream regulatory element in the Ncam promot er: Cux, related to Drosophila cut and human CDP, and Phox2, a novel p rotein with a homeodomain related to that of the Drosophila paired gen e. In transient transfection experiments, Cux was found to be a strong inhibitor of Ncam promoter activity, and this inhibition could be rel ieved by simultaneously overexpressing Phox2 These results suggest tha t the Ncam gene might be a direct target of homeodomain proteins and p rovide a striking example of regulatory cross-talk between homeodomain proteins of different classes. Whereas the expression pattern of Cux/ CDP includes many NCAM-negative sites, Phox2 expression was restricted to cells also expressing Ncam or their progenitors. The localisation data thus strongly reinforce the notion that Phox2 plays a role in tra nscriptional activation of Ncam in Phox2-positive cell types. In the p eripheral nervous system, Phox2 was strongly expressed in all ganglia of the autonomic nervous system and more weakly in some cranial sensor y ganglia, but not in the sensory ganglia of the trunk. Phox2 transcri pts were detected in the primordia of sympathetic ganglia as soon as t hey form. Phox2 expression in the brain was confined to spatially rest ricted domains in the hindbrain, which correspond to the noradrenergic and adrenergic nuclei once they are identifiable. All Phox2-expressin g components of the peripheral nervous system are at least transiently adrenergic or noradrenergic. In the developing brain, Phox2 was expre ssed at all known locations of (nor)adrenergic neurones and of their p recursors. These results suggest that Phox2, in addition to regulating the NCAM gene, may be part of the regulatory cascade that controls th e differentiation of neurons towards this neurotransmitter phenotype.