IDENTIFICATION AND LOCALIZATION OF A SEA-URCHIN NOTCH HOMOLOG - INSIGHTS INTO VEGETAL PLATE REGIONALIZATION AND NOTCH RECEPTOR REGULATION

The specifications of cell types and germ-layers that arise from the v egetal plate of the sea urchin embryo are thought to be regulated by c ell-cell interactions, the molecular basis of which are unknown. The N otch intercellular signaling pathway mediates the specification of num erous cell fates in both invertebrate and vertebrate development. To g ain insights into mechanisms underlying the diversification of vegetal plate cell types, we have identified and made antibodies to a sea urc hin homolog of Notch (LvNotch). We show that in the early blastula emb ryo, LvNotch is absent from the vegetal pole and concentrated in basol ateral membranes of cells in the animal half of the embryo. However, i n the mesenchyme blastula embryo LvNotch shifts strikingly in subcellu lar localization into a ring of cells which surround the central veget al plate, This ring of LvNotch delineates a boundary between the presu mptive secondary mesoderm and presumptive endoderm, and has an asymmet ric bias towards the dorsal side of the vegetal plate, Experimental pe rturbations and quantitative analysis of LvNotch expression demonstrat e that the mesenchyme blastula vegetal plate contains both animal/vege tal and dorsoventral molecular organization even before this territory invaginates to form the archenteron, Furthermore, these experiments s uggest roles for the Notch pathway in secondary mesoderm and endoderm lineage segregation, and in the establishment of dorsoventral polarity in the endoderm, Finally, the specific and differential subcellular e xpression of LvNotch in apical and basolateral membrane domains provid es compelling evidence that changes in membrane domain localization of LvNotch are an important aspect of Notch receptor function.