The Progress Of Studies On Oocyte Membrane Molicules Involved In Sperm-Oocyte Fusion

Sperm-oocyte fusion includes a series of cellular interactions culminating with the fusion of gamete membranes, creating a zygote. It is one of the most impressive events in sexual reproduction, and the elucidation of its molecular mechanism has fascinated researchers for a long time. Because of the limitation of materials and difficulties in analyzing membrane protein-protein interactions, many attempts have failed to reach this goal. Recent studies involving gene targeting have clearly demonstrated the various molecules that are involved in sperm-oocyte binding and fusion. oocyte integrins, are necessary for the sperm-oocyte interaction. Recently, several studies have focused the spotlight on CD9 and glycosylphosphatidylinositol (GPI)-anchored proteins on oocytes, as candidate molecules involved in sperm-oocyte fusion. Lack of, or interference with the function of, these proteins can disrupt the sperm-oocyte fusion without changing the binding. In this review we highlight the candidate molecules involved in the sperm-oocyte interaction suggested from the recent progress in this research field.The cell-surface molecule CD9, a member of the transmembrane-4 superfamily, interacts with the integrin family and other membrane proteins. and is postulated to participate in cell migration and adhesion. Expression of CD9 enhances membrane fusion between muscle cells and promotes viral infection in some cells. Fertilization also involves membrane fusion, between gametes. In mammals, the sperm binds to microvilli on the oocyte surface, and sperm-oocyte membrane fusion first occurs around the equatorial region of the sperm head. The fused membrane is then disrupted, and the sperm nucleus as well as the cytoplasm is incorporated into the oocyte. The tetraspanins CD9, KAI-1/CD82, and CD63 are involved in metastasis suppression, an effect that may be related to their association with beta1 integrins. Knockout mice lacking CD9 were created to evaluate the physiological importance of CD9. CD9^-/- females displayed a severe reduction of fertility. Oocytes were ovulated but were not successfully fertilized because sperm did not fuse with the oocytes from CD9^-/- females. Thus, CD9 appears to be essential for sperm-oocyte fusion, a process involving the CD9-associated integrin alpha6betal. CD9 is expressed on the plasma membrane of the mouse oocyte, and an anti-CD9 monoclonal antibody inhibits sperm-oocyte surface interactions. Kaiji generated CD9 mice and found that homozygous mutant females were infertile. Sperm-oocyte binding was normal, but sperm-oocyte fusion was almost entirely inhibited in oocytes from CD9 females. Intracellular Ca² oscillations, which signal fertilization, were absent in almost all mutant oocytes; in rare cases, a response occurred after a long time period. In normal animals, CD9 molecules were expressed on the oocyte microvilli and became densely concentrated at the sperm attachment site. Thus, our results show that CD9 is important in the gamete fusion process at fertilization.Glycosylphosphatidylinositol-anchored proteins on the oocyte surface have been proposed to play a role in gamete fusion on the basis of in vitro experiments. Coonrod treated the oocyte with PI-PLC , the results demonstrate that GPI-anchored proteins are required for gamete fusion. Alfieri JA tested this hypothesis by asking if oocyte GPI-anchored proteins are required for fertilization in vivo. Oocyte-specific knockout mice were created using the Cre/loxP system to delete a portion of the Pig-a gene, which encodes an enzyme involved in GPI anchor biosynthesis. Conditional Pig-a-knockout females are infertile, and oocytes recovered from the females after mating are unfertilized. In in vitro assays, the knockout oocytes are severely deficient in their ability to fuse with sperm. These results demonstrate that GPI-anchored proteins are required for gamete fusion, too.Integrin alpha6betal is a strong candidate for a sperm receptor on the oocyte plasma membrane. However, the ability of the oocyte integrin alpha6betal to interact with molecules on intact sperm has not yet been proven. Takahashi' s report, possible involvement of integrin alpha6betal in sperm-oocyte interactions was examined by biochemical and immunocytochemical analyses. To identify oocyte molecules that specifically interact with sperm, they first incubated sperm with biotin-labeled oocyte surface proteins. Under this condition, solubilized proteins from oocytes inhibited sperm-oocyte fusion. they next examined the localization of integrin alpha6 and beta1 subunits before and after fertilization by Confocal laser microscopy. these results strongly suggest that the integrin alpha6betal is involved in sperm-oocyte binding leading to fusion via direct association of the integrin alpha6 with sperm. Ziyyat' s study show that CD9 controls the redistribution of some membrane proteins including the alpha 6 beta1 integrin into clusters that may be necessary for gamete fusion. Integrin alpha6betal may be prduce a marked effect as a member of the muti-molecule complex.The studies of sperm-oocyte fusion will largely assists the development of ART.