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基本过程。然而，在哺乳动物中，精子和卵子膜相互作用的分子机制仍然是一个谜。CD9是一种四胺基膜蛋白，在多种细胞中表达，据报道在许多细胞事件中发挥功能。CD9与其他Tetraspanins和某些膜蛋白在质膜上形成复合体。在受精过程中，CD9是精子-卵子融合所必需的，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，但在与精子的融合中严重受损，但不影响发育过程。由于CD9可以与整合素相关，受精模型已经假设CD9与α-β1在融合中起关联作用。然而，对缺陷卵子的分析已经否定了整合素对精子-卵子融合至关重要的观点。提出了一种新的精卵融合模型，即CD9单独作用或与卵蛋白(S)相互作用，而不是整合素。CD9仅定位于微绒毛，不集中于肌动蛋白细丝。电子显微镜还显示CD9-/-卵在微绒毛形成方面存在严重缺陷。当在卵母细胞特异性启动子下表达EGFP标记的CD9的载体被转移到CD9-/-雌体中时，配子融合和微绒毛形成都得到了恢复。EGFP-CD9显示的微绒毛在配子融合过程中发挥重要作用。这些结果表明，CD9是精卵融合所需微绒毛形成的关键元件。

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.