Role of CD9 in Sperm-Egg Fusion and Its General Role in Fusion Phenomena

Abstract

In fertilization, two types of sex cells or gametes – a sperm and an egg – unite in a stepwise manner to form a mother cell, which is capable of developing naturally into a new individual. Notably, the “membrane fusion” that occurs intercellularly between a sperm and an egg is essential for fertilization. A sperm factor that is delivered into the egg cytoplasm through fusion serves to activate a signaling pathway; this leads to the resumption of meiosis in the egg. In mammals, sperm-egg fusion is partly mediated by two integral membrane proteins, sperm Izumo (Inoue et al. 2005) and egg cluster of differentiation 9 (CD9) (Kaji et al. 2000, Le Naour et al. 2000, Miyado et al. 2000), and the roles played by both are critical but yet unknown. A recent study (Miyado et al. 2000) showed that CD9-containing vesicles are released from wild-type eggs, and that these exosome-like vesicles induce fusion between sperm and CD9-null eggs in vitro, even though CD9-null eggs are highly refractory to sperm-egg fusion. This result provides compelling evidence for the crucial involvement of CD9-containing, fusion-facilitating vesicles in sperm-egg fusion and offers new insight into both gamete fusion and other membrane fusion events.