Origin of the integrin-mediated signal transduction. Functional studies with cell cultures from the sponge Suberites domuncula.

Sponges (phylum Porifera) represent the phylogenetically oldest metazoan animals. Recently, from the marine sponge Geodia cydonium a first cDNA encoding a putative integrin receptor molecule was isolated. In the present study basic functional experiments have been conducted to test the hypothesis that in sponges integrin polypeptides also function as adhesion molecules and as outside-in signaling molecules. The sponge Suberites domuncula has been used for the experiments because from this sponge only has a cell culture been established. Here we report that aggregation factor (AF)-mediated cell-cell adhesion is blocked by the RGDS peptide which is known to interact with beta integrin. Both RGDS and AF were found to stimulate DNA synthesis within 24 h. The beta subunit of the integrin receptor was cloned from S. domuncula; the estimated 91-kDa molecule comprises the characteristic signatures. Evolutionary conservation of the beta integrin was assessed by comparison with corresponding beta integrin subunits from evolutionary higher metazoan taxa. Addition of RGDS or of AF to isolated cells of S. domuncula causes a rapid (within 1-2 min) increase in the intracellular Ca2+ concentration which is further augmented in the presence of Ca2+. Furthermore, incubation of the cells with RGDS or AF causes an activation of the GTP-binding protein Ras. In addition it is shown that after a prolonged incubation of the cells with RGDS and AF the expression of the genes coding for Ras and for calmodulin is upregulated. These results suggest that the integrin receptor functions in the sponge system not only as adhesion molecule but also as a molecule involved in outside-in signaling.