In the starfish, Asterias amurensis, the cooperation of three components of the egg jelly, namely ARIS (acrosome reaction-inducing substance), Co-ARIS and asterosap, is responsible for the induction of acrosome reaction. For the induction, ARIS alone is enough in high-Ca2+ or high-pH seawater, but, besides ARIS, the addition of either Co-ARIS or asterosap is required in normal seawater. Asterosap transiently increased both the intracellular pH (pHi) and Ca2+ ([Ca2+]i), while ARIS slightly elevated the basal level of [Ca2+]i. However, a sustained elevation of [Ca2+]i and acrosome reaction occurred if sperm were simultaneously treated with ARIS and asterosap. EGTA inhibited the sustained [Ca2+]i elevation and acrosome reaction. The sustained [Ca2+]i elevation and acrosome reaction were highly susceptible to SKF96365 and Ni2+, specific blockers of the store-operated Ca2+ channel (SOC). These results suggest that sustained [Ca2+]i elevation, mediated by the SOC-like channel, is a prerequisite for the acrosome reaction. In high-pH seawater, ARIS alone induced a prominent [Ca2+]i increase and acrosome reaction, which were similarly sensitive to SKF96365. The acrosome reaction was effectively induced by ARIS alone when pHi was artificially increased to more than 7.7. Asterosap increased pHi from 7.6±0.1 to 7.7±0.1. Furthermore, the sustained [Ca2+]i elevation and acrosome reaction, induced by a combination of ARIS and asterosap, were drastically inhibited by a slight reduction in pHi. Taking these results into account, we suggest that an asterosap-induced pHi elevation is required for triggering the ARIS-induced sustained [Ca2+]i elevation and consequent acrosome reaction.
Structure of acrosome reaction-inducing steroidal saponins from the egg jelly of the starfish, Asterias amurensis.
