LvNotch signaling plays a dual role in regulating the position of the ectoderm-endoderm boundary in the sea urchin embryo

The molecular mechanisms guiding the positioning of the ectoderm-endoderm b oundary along the animal-vegetal axis of the sea urchin embryo remain large ly unknown, We report here a role for the sea urchin homolog of the Notch r eceptor, LvNotch, in mediating the position of this boundary. Overexpressio n of an activated form of LvNotch throughout the embryo shifts the ectoderm -endoderm boundary more animally along the animal-vegetal axis, whereas exp ression of a dominant negative form shifts the border vegetally. Mosaic exp eriments-that-target activated and dominant negative forms of LvNotch into individual blastomeres of the early embryo, combined with lineage analyses, further reveal that LvNotch signaling mediates the position of this bounda ry by distinct mechanisms within the animal versus vegetal portions of the embryo. In the animal region of the embryo, LvNotch signaling acts cell aut onomously to promote endoderm formation more animally, while in the vegetal portion, LvNotch signaling also promotes the ectoderm-endoderm boundary mo re animally, but through a cell non-autonomous mechanism, We further demons trate that vegetal LvNotch signaling controls the localization of nuclear b eta -catenin at the ectoderm-endoderm boundary. Based on these results, we propose that LvNotch signaling promotes the position of the ectoderm-endode rm boundary more animally via two mechanisms: (1) a cell-autonomous functio n within the animal region of the embryo, and (2) a cell non-autonomous rol e in the vegetal region that regulates a signal(s) mediating ectoderm-endod erm position, possibly through the control of nuclear beta -catenin at the boundary.