Evidence for Both Tyrosine Kinase and G-Protein-Coupled Pathways Leading to Starfish Egg Activation

To investigate possible pathways leading to egg activation at fertilization, the ability of exogenously introduced tyrosine kinase and G-protein-coupled recaptors to mimic events of fertilization was examined. Oocytes of the starfish Asterina miniatawere injected with RNA for a chimeric receptor consisting of the extracellular domain of the β form of the mouse platelet-derived growth factor (PDGF) receptor and the transmembrane/intracellular domain of the human fibroblast growth factor (FGF) receptor, or with RNA for the rat serotonin 1c receptor. These oocytes were cultured for 1 to 3 days and then matured with 1-methyladenine. In response to PDGF or serotonin, the injected eggs underwent responses like those at fertilization: cortical granule exocytosis, a rise in intracellular free calcium, and DNA synthesis. Some of these artificially activated eggs cleaved, and some of the PDGF-activated eggs were observed to form larvae, A PDGF/FGF receptor with a point mutation which eliminated its ability to interact with phospholipase C-γ did not cause fertilization-like responses. Thus components of a signaling pathway involving phospholipase C-γ, characteristic of tyrosine kinase receptors, as well as components of a pathway involving a G-protein and phospholipase C-β, characteristic of G-protein-coupled receptors, appear to be present in starfish eggs. Either or both could function in egg activation at fertilization.