Molecular Mechanism of sperm-egg membrane fusion in fertilization

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Fertilization is an essential process that naturally produces a cell capable for developing into a new individual. However, a molecular mechanism that underlying membrane interaction of sperm and egg still remains a mystery in mammals. CD9, a tetraspan-membrane protein (tetraspanin), expressed in a wide variety of cells, is reported to function in many cellular events. CD9 associates with other tetraspanins and some kinds of membrane proteins to form complexes at the plasma membrane. In fertilization, CD9 is required for sperm-egg fusion, and also implicated in other fusion events. When inseminated, eggs from CD9-/-females bound sperm normally, but were greatly impaired in fusion with sperm, but not developmental processes. Since CD9 can associate with integrins, models of fertilization have posited that CD9 acts in association with alpha6betal in fusion. However, analyses of deficient eggs have sublated that integrins are essential for sperm-egg fusion. A new model in which CD9 acts by itself, or interacts with egg protein(s) other than integrins, to function in sperm-egg fusion, was proposed. CD9 was exclusively localized to microvilli, not concentrated of actin-filaments. Electron microscopy also revealed that CD9-/-eggs had a severe defect in microvilli formation. When a vector, expressing EGFP- tagged CD9 under oocyte-specific promoter, was transferred into CD9-/-females, both gametes fusion and microvilli formation were restored. Microvilli, visualized by EGFP-CD9, was vitally monitored and defined to function in gametes fusion. These results indicate that CD9 is a key element for the formation of microvilli required for sperm-egg fusion.