Regulation of BMP signaling by the BMP1/TLD-related metalloprotease, SpAN

We have used the Xenopus embryo as a test system for analyzing the activity of SpAN, a sea urchin metalloprotease in the astacin family containing BMP 1 and tolloid. Embryos expressing SpAN initiated gastrulation on a time sca le indistinguishable from controls, but invagination of the vegetal pole wa s subsequently delayed by several hours. At tailbud stages the most severel y affected embryos were completely ventralized, lacking all dorsal structur es. Molecular analysis of injected embryos, using probes for both dorsal (x gsc and xnot) and ventral (xhox3 and xwnt8) mesoderm, indicates that SpAN v entralizes dorsal mesoderm during gastrula stages. These results mirror tho se previously obtained with BMP4, suggesting that SpAN may enhance the acti vity of this ventralizing factor. Consistent with this suggestion, we have shown that SpAN blocks the dorsalizing activity of noggin and chordin, two inhibitory binding proteins for BMP4, but not that of a dominant-negative r eceptor for BMP4. In contrast, a dominant-negative SpAN, in which the metal loprotease domain has been deleted, dorsalizes ventral mesoderm, a phenotyp e that can be rescued by coexpressing either SpAN or XBMP1. This suggests t hat SpAN is mimicking a Xenopus metalloprotease responsible for regulating the activity of Xenopus BMPs during gastrulation. Moreover, our results rai se the possibility that SpAN may function to facilitate BMP signaling in ea rly sea urchin embryos. (C) 1999 Academic Press.