THE XENOPUS HOMOLOG OF DROSOPHILA SUPPRESSOR OF HAIRLESS MEDIATES NOTCH SIGNALING DURING PRIMARY NEUROGENESIS

The X-Notch-1 receptor, and its putative ligand, X-Delta-1, are though t to mediate an inhibitory cell-cell interaction, called lateral inhib ition, that limits the number of primary neurons that form in Xenopus embryos. The expression of Xenopus ESR-1, a gene related to Drosophila Enhancer of split, appears to be induced by Notch signaling during th is process. To determine how the activation of X-Notch-1 induces ESR-1 expression and regulates primary neurogenesis, we isolated the Xenopu s homolog of Suppressor of Hairless (X-Su(H)), a component of the Notc h signaling pathway in Drosophila. Using animal cap assays, we show th at X-Su(H) induces ESR-1 expression, perhaps directly, when modified b y the addition of ankyrin repeats. Using a DNA binding mutant of X-Su( H), we show that X-Su(H) activity is required for induction of ESR-1. Finally, expression of the DNA binding mutant in embryos leads to a ne urogenic phenotype as well as increased expression of both X-Delta-1 a nd XNGNR1, a proneural gene expressed during primary neurogenesis. The se results suggest that activation of X-Su(H) is a key step in the Not ch signaling pathway during primary neurogenesis in Xenopus embryos.