The role of tetraspanin complexes in egg-sperm fusion

Abstract

Cell fusion occurs when cells unit their membranes and share their –cytoplasm. Cell fusion may occur between genetically identical or different cells. Gamete fusion is a short event of the organism life cycle but it is essential for all organisms that depend on sexual reproduction for the maintenance of the species. The fusion of gametes having inherited one half of the genetic material of their respective parents ensures the diversity of individuals within a population. This diversity is increased by the exchange of genetic material between homologous chromosomes during meiosis. Therefore gamete fusion is a critical biological step in the life cycle that has recently raised more interest due to the possibility of treating patients with a fertility defect by in vitro methods. Molecular mechanisms underlying the fusion process are far from evident since (1) gene-knock out technologies developed to test in vivo the data from in vitro fertilization (IVF) studies failed to confirm the essential role of most of previously candidate molecules and (2) the surface proteins involved are deprived of fusogen properties. Indeed, among the large number of in vivo tested proteins, only CD9 and CD81 tetraspanins on egg and Izumo on sperm have been shown to be essential in mammalian sperm-egg membrane fusion and none of these molecules contain a fusion peptide.