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

Abstract

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.