Roles of CD9 and CD9-Containing Exosomes in Sperm-Egg Membrane Fusion

Abstract: In fertilization, two types of gametes - sperm and egg-unite via a stepwise approach to create a fertilized cell, which is capable of naturally developing into a new individual. Notably, “membrane fusion” occurring intercellularly between a sperm and an egg is essential for fertilization. In mammals, sperm-egg fusion is, at least in part, mediated by two integral membrane proteins, sperm Izumo and egg CD9, and their roles are critical but unelucidated. A recent study showed that CD9-containing vesicles are released from wild-type eggs, and then exosome-like vesicles induce fusion between sperm and CD9-deficient eggs in vitro, even though CD9-deficient eggs are highly refractory to spermegg fusion. This result provides compelling evidence for the crucial involvement of CD9-containing, fusion-facilitating vesicles in sperm-egg fusion, and offers a new insight into gamete fusion and other membrane fusion events.