Abstract
Lipid rafts are ordered membrane microdomains enriched in cholesterol, glycolipids and saturated phospholipids. Lipid rafts also assemble molecules involved in cell adhesion and signaling, and they are thus considered platforms for these phenomena. This has made the lipid raft topic a very active research field. However, a number of issues remain controversial in this research line. Advanced imaging studies have recently indicated that individual lipid/membrane rafts are of nanometer sizes. This suggests that rafts isolated from cells as low-density membrane fractions may rather represent “macrorafts” due to coalescence of individual rafts. Further controversy lies in the use of Triton X-100 in raft isolation, as the detergent may induce lipid microdomain formation. Investigators in the field are moving towards using milder non-ionic detergents or physical forces in raft isolation. Sperm-egg interaction is an ideal system to attest the implication of lipid rafts in cell adhesion/signaling. Recently, we have shown that Triton X-100 resistant membrane rafts of capacitated pig sperm have direct binding to pig zona pellucida (ZP) with a Kd value much lower than that of non-capacitated sperm rafts-ZP binding. These results may partially explain the enhanced ZP binding ability of capacitated sperm, relative to non-capacitated sperm. Another possible contribution to the greater ZP binding ability of capacitated sperm is their possession of higher lipid raft levels despite a cholesterol efflux occurring during capacitation. Significantly, GM1 ganglioside, normally used as a raft marker of somatic cells, does not exist in capacitated sperm rafts. Rather, 70% of male germ cell specific sulfogalacotosylglycerolipid (SGG) is present in isolated capacitated sperm rafts, making it an attractive candidate as a sperm raft marker. Nonetheless, since sperm lipid rafts used in these studies were isolated by the Triton X-100 treatment method, work should be repeated using lipid rafts prepared from sperm by a physical force (e.g., nitrogen cavitation). Finally, advanced imaging studies should be performed with an appropriate sperm raft marker to determine whether individual sperm lipid raft microdomains are of nanometer sizes, like somatic cell rafts.
