Spatial and temporal expression pattern during sea urchin embryogenesis of a gene coding for a protease homologous to the human protein BMP-1 and to the product of the Drosophila dorsal-ventral patterning gene tolloid.

Abstract

A cDNA clone coding for a sea urchin embryonic protein was isolated from a prehatching blastula lambda gt11 library. The predicted translation product is a secreted 64 x 10(3) Mr enzyme designated as BP10. The protein contains several domains: a signal peptide, a putative propeptide, a catalytic domain with an active center typical of a Zn(2+)-metalloprotease, an EGF-like domain and two internal repeats similar to repeated domains found in the C1s and C1r serine proteases of the complement cascade. The BP10 protease is constructed with the same domains as the human bone morphogenetic protein BMP-1, a protease described as a factor involved in bone formation, and as the recently characterized product of the tolloid gene which is required for correct dorsal-ventral patterning of the Drosophila embryo. The transcription of the BP10 gene is transiently activated around the 16- to 32-cell stage and the accumulation of BP10 transcripts is limited to a short period at the blastula stage. By in situ hybridization with digoxygenin-labelled RNA probes, the BP10 transcripts were only detected in a limited area of the blastula, showing that the transcription of the BP10 gene is also spatially controlled. Antibodies directed against a fusion protein were used to detect the BP10 protein in embryonic extracts. The protein is first detected in early blastula stages, its level peaks in late cleavage, declines abruptly before ingression of primary mesenchyme cells and remains constant in late development. The distribution of the BP10 protein during its synthesis and secretion was analysed by immunostaining blastula-stage embryos. The intracellular localization of the BP10 staining varies with time. The protein is first detected in a perinuclear region, then in an apical and submembranous position just before its secretion into the perivitelline space. The protein is synthesized in a sharply delimited continuous territory spanning about 70% of the blastula. Comparison of the size and orientation of the labelled territory in the late blastula with the fate map of the blastula stage embryo shows that the domain in which the BP10 gene is expressed corresponds to the presumptive ectoderm. Developing embryos treated with purified antibodies against the BP10 protein and with synthetic peptides derived from the EGF-like domain displayed perturbations in morphogenesis and were radialized to various degrees. These results are consistent with a role for BP10 in the differentiation of ectodermal lineages and subsequent patterning of the embryo. On the basis of these results, we speculate that the role of BP10 in the sea urchin embryo might be similar to that of tolloid in Drosophila. We discuss the idea that the processes of spatial regulation of gene expression along the animal-vegetal in sea urchin and dorsal-ventral axes in Drosophila might have some similarities and might use common elements.