Mechanism Regulating Ca2+-dependent Mechanosensory Behaviour in Sea Urchin Spermatozoa

Flagellar movement of the sea urchin sperm is regulated by intracellular Ca2+. Flagellasialin, a polysialic acid-containing glycoprotein, as well as other membrane proteins seems responsible for the Ca2+ control. To elucidate the mechanism of Ca2+ dynamics underlying flagellar movement, we analysed the sperm's mechanosensory behavioural responses by using microtechniques. In sea water containing 10 mM Ca2+, the sperm swim in circular paths. When a mechanical stimulus was applied to the sperm head with a glass microstylus, the sperm showed a series of flagellar responses, consisting of a stoppage of beating (quiescence) and a recovery of swimming in a straight path, followed by swimming in a circular path again; as the result the sperm avoided the obstacle. Ca2+-imaging with Fluo-4 showed that the intracellular Ca2+ was high in the quiescence and gradually decreased after that. The effects of blockers and antibodies against candidate components revealed that the Ca2+ influx was induced by Ca2+ channels and the Ca2+ efflux was induced by a flagellasialin-related Ca2+-efflux system, plasma membrane Ca2+-ATPases and the K+-dependent Na+/Ca2+ exchanger. The results show that the Ca2+-dependent mechanosensory behaviour of the sea urchin sperm is regulated by organized functioning of the membrane environment including the plasma membrane proteins and flagellasialin.