LOCALIZATION AND CHARACTERIZATION OF BLASTOCOELIC EXTRACELLULAR-MATRIX ANTIGENS IN EARLY SEA-URCHIN EMBRYOS AND EVIDENCE FOR THEIR PROTEOLYTIC MODIFICATION DURING GASTRULATION

Previously, results were presented showing a spatiotemporal expression of matrix metalloproteases consistent with a role in remodeling the b lastocoelic extracellular matrix (bECM) of the gastrulating sea urchin embryo [35]. In the present work, we provide evidence suggesting that the bECM is in fact the substrate for developmentally regulated prote olysis. Monoclonal antibody (mAb) LG11C7 was generated against testicu lar tissue of the sea urchin: Strongylocentrotus purpuratus, and recog nizes extracellular matrix antigens overlying the perivisceral epithel ium. Indirect immunofluorescence microscopy shows that mAb LG11C7 cros s-reacts with components of the basal lamina-lining the blastocoeles o f early embryos and Western immunoblots of detergent extracts indicate that it recognizes gastrula-stage antigens with M(r)s of 158, 68, and 37 kDa. Glycosidase treatments reveal that the embryonal antigens con tain multiple N-linked oligosaccharides. Developmental studies employi ng immunoprecipitations and Western blot analyses of staged embryonal detergent extracts show that the 68-kDa antigen appears between 18 and 24 h after fertilization and is accompanied by a substantial increase in the 37-kDa antigen. Thus, the appearances of the 68 and 37-kDa ant igens occur during the blastula-gastrula transition, and their spatiot emporal expression is similar to that of the matrix metalloproteases r eported previously. The appearance of the 68-kDa antigen and the incre ase in the 37-kDa antigen may be blocked by exposing the embryos to th e metalloprotease inhibitor 1,10-phenanthroline, which also blocks gas trulation reversibly. These results suggest (1) that the 68- and 37-kD a antigens are products of developmentally regulated proteolysis of a basal laminar glycoprotein, and (2) that this proteolysis is required for the cell-cell/cell-matrix interactions and morphogenetic movements associated with normal gastrulation in the sea urchin embryo.