LvNotch signaling mediates secondary mesenchyme specification in the sea urchin embryo

Cell-cell interactions are thought to regulate the differential specification of secondary mesenchyme cells (SMCs) and endoderm in the sea urchin embryo. The molecular bases of these interactions, however, are unknown. We have previously shown that the sea urchin homologue of the LIN-12/Notch receptor, LvNotch, displays dynamic patterns of expression within both the presumptive SMCs and endoderm during the blastula stage, the time at which these two cell types are thought to be differentially specified (Sherwood, D. R. and McClay, D. R. (1997) Development 124, 3363-3374). The LIN-12/Notch signaling pathway has been shown to mediate the segregation of numerous cell types in both invertebrate and vertebrate embryos. To directly examine whether LvNotch signaling has a role in the differential specification of SMCs and endoderm, we have overexpressed activated and dominant negative forms of LvNotch during early sea urchin development. We show that activation of LvNotch signaling increases SMC specification, while loss or reduction of LvNotch signaling eliminates or significantly decreases SMC specification. Furthermore, results from a mosaic analysis of LvNotch function as well as endogenous LvNotch expression strongly suggest that LvNotch signaling acts autonomously within the presumptive SMCs to mediate SMC specification. Finally, we demonstrate that the expansion of SMCs seen with activation of LvNotch signaling comes at the expense of presumptive endoderm cells, while loss of SMC specification results in the endoderm expanding into territory where SMCs usually arise. Taken together, these results offer compelling evidence that LvNotch signaling directly specifies the SMC fate, and that this signaling is critical for the differential specification of SMCs and endoderm in the sea urchin embryo.