The role of cholesterol efflux in regulating the fertilization potential of mammalian spermatozoa.

spermatogenesis and spermiogenesis, mam-malian spermatozoa leaving the testis appear to be morphologically mature but clearly are immature from a functional standpoint; that is, they have acquired neither progressive motility nor the ability to fertilize a metaphase II–arrested egg. Although progressive motil-ity is acquired and signaling pathways mature during sperm transit through the epididymis, complete fertilization capacity in vivo is conferred only during residence in the female reproductive tract. Similar observations have been made using a variety of in vitro assays, suggesting that a series of events, some initiated by environmental cues, confer on sperm the ability to fertilize the egg. This acquired capacity to fertilize was first observed by Austin (1) and Chang (2), who demonstrated that freshly ejaculated sperm cannot fertilize eggs until they reside in the female reproductive tract for a finite period of time. All of the cellular events that allow the ejaculated sperm to fertilize an egg were subsumed into a single phenomenon that was termed " capacitation. " The ability to capacitate sperm in vitro has been of great importance to both scientists and clinicians, who have used it to study the basic biology of fertilization and to develop various assisted reproductive technologies for humans and other species. Work by many investigators has established that the process of fertilization, not surprisingly, represents a series of elegant intercellular communication and cellular activation events (3–5). Sperm functions such as motility and capacitation in the female reproductive tract are likely modulated by environmental cues in the luminal fluid, as well as by interactions with oviductal epithelium or other female tissues (6). When sperm arrive in the oviduct and encounter the ovulated, metaphase II–arrested egg enclosed in its cumulus cell matrix, a complex series of cell-cell and cell-ECM interactions ensues, initiating cellular signaling events that permit the fusion of the sperm and egg plasma membranes. Several of these cell-matrix and cell-cell interactions involve novel gamete surface proteins and matrices. Signal transduction events leading to gamete activation, in particular sperm acrosomal exocytosis and egg cortical granule secretion, share some features with signaling events described in somatic cells. Sperm membrane cholesterol efflux contributes to one such novel signaling mechanism that controls sperm capacitation, and the details of this effect are now beginning to be understood at the molecular level. Knowledge of how cholesterol efflux occurs in these cells, as well as how this efflux is integrated with transmembrane sig-naling to regulate …