Egg-jelly signal molecules for triggering the acrosome reaction in starfish spermatozoa

It was in the early 1950s that J.C. Dan discovered the acrosome reaction in sea urchins, starfishes and several other marine invertebrates at Misaki Marine Biological Station on the Pacific coast of Japan. We now know that in many animals including mammals the acrosome reaction is an essential, and probably the most central, change in spermatozoa for fertilization. Starfish spermatozoa undergo the acrosome reaction upon encountering the jelly coat consisting of glycoproteins, steroid saponins oligopeptides and inorganic components. To induce the acrosome reaction, three egg jelly components act in concert on the spermatozoa: a highly sulfated glycoprotein named acrosome reaction-inducing substance (ARIS), a group of sulfated steroidal saponins named Co-ARIS, and a group of glutamine-rich tetratriacontapeptides named sperm activating peptide (SAP). The action of ARIS is quite species-specific due to the specificity of ARIS-receptors in a restricted domain of the sperm surface and depends very much on sulfated saccharide chains. Co-ARIS is not much species-specific and its action depends on the sulfate group and steroid side chain. SAPs have a ring of 25 residues and increase the intracellular pH of spermatozoa. None of them can induce the acrosome reaction by itself in normal sea water, but ARIS does induce it in high Ca²⁺ or high pH sea water. Although a combination of ARIS and either Co-ARIS or SAP induces the acrosome reaction in normal sea water, all three are required to mimic the full activity of dissolved jelly coat.