Lipid Rafts Function in Ca2+ Signaling Responsible for Activation of Sperm Motility and Chemotaxis in the Ascidian Ciona intestinalis

Lipid rafts are specialized membrane microdomains that function as signaling platforms across plasma membranes of many animal and plant cells. Although there are several studies implicating the role of lipid rafts in capacitation of mammalian sperm, the function of these structures in sperm motility activation and chemotaxis remains unknown. In the ascidian Ciona intestinalis, egg-derived sperm activating-and attracting-factor (SAAF) induces both activation of sperm motility and sperm chemotaxis to the egg. Here we found that a lipid raft disrupter, methyl-beta-cyclodextrin (MCD), inhibited both SAAF-induced sperm motility activation and chemotaxis. MCD-inhibited both SAAF-promoted synthesis of intracellular cyclic AMP and sperm motility induced by ionophore-mediated Ca2+ entry, but not that induced by valinomycin-mediated hyperpolarization. Ca2+-imaging revealed that lipid raft disruption inhibited Ca2+ influx upon activation of sperm motility. The Ca2+-activated adenylyl cyclase was clearly inhibited by MCD in isolated lipid rafts. The results suggest that sperm lipid rafts function in signaling upstream of cAMP synthesis, most likely in SAAF-induced Ca2+ influx, and are required for Ca2+-dependent pathways underlying activation and chemotaxis in Ciona sperm.