Molecular Mechanism of sperm-egg membrane fusion in fertilization

受精是自然产生能够发育成新个体的细胞的重要过程。然而，哺乳动物中精子和卵子膜相互作用的分子机制仍然是一个谜。CD9是一种四跨膜蛋白（tetraspan-membrane protein, tetraspanin），在多种细胞中表达，据报道在许多细胞事件中起作用。CD9与其他四跨蛋白和一些膜蛋白结合在质膜上形成复合物。在受精过程中，CD9是精卵融合所必需的，也与其他融合事件有关。当受精时，来自CD9-/-雌性的卵子正常结合精子，但在与精子融合时受到严重损害，而不是发育过程。由于CD9可以与整合素结合，受精模型假设CD9在融合中与α - 6 - β蛋白结合。然而，对缺陷卵子的分析表明，整合素对精卵融合至关重要。提出了一种新的模型，其中CD9本身起作用，或与除整合素以外的卵子蛋白相互作用，在精子-卵子融合中起作用。CD9仅局限于微绒毛，而不集中于肌动蛋白丝。电镜还显示CD9-/-卵在微绒毛形成方面存在严重缺陷。当载体在卵母细胞特异性启动子下表达EGFP标记的CD9时，将其转移到CD9-/-雌性中，配子融合和微绒毛形成都得以恢复。EGFP-CD9显示的微绒毛在配子融合中起重要作用。这些结果表明，CD9是形成精卵融合所需的微绒毛的关键因素。

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.