The exocytotic acrosome reaction (AR) is a mandatory step in sperm-egg interactions in most metazoans. The molecules present in egg coats play crucial roles in AR induction. These signaling molecules are mainly complex carbohydrates, which are either protein-bound or -free. The structural elucidation of these glycans is in progress. The new knowledge gained should lead to new insights into the molecular mechanisms of AR induction. The modifications of the AR-inducing glycans involve O-glycosylation, fucosylation, sulfation and sialylation. These modifications are conserved from the lowest deuterostomes to the highest vertebrates. The evidence suggests that the AR is mediated by a carbohydrate-protein interaction. Here the primary structures and important features of these AR inducing carbohydrates present in both invertebrate and vertebrate egg coats are reviewed.
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.
Naturally occurring polysialic acids (polySias), a unique homopolymer of sialic acid (Sia) residues, have a large structural diversity, primarily arising from the diversities in Sia components as well as in the intersialyl linkages. The α2,8-linked polySia on the neural cell adhesion molecules (NCAM) is the most extensively studied and recognized as a regulator of cell-cell interactions during development and differentiation in vertebrate brain. However, little is known about the occurrence and functions of polySia other than the α2,8-linked polySia on NCAM. Recently, it has been shown that polySia occurs in nature more frequently than previously recognized, as analytical methods to detect polySia structures have been greatly improved. Using these methods, two different types of polySia, α2,8- and α2,9-linked polySia, have been discovered on the sea urchin sperm, although the α2,5Oglycolyl-linked polySia was already known in sea urchin egg. This is the first example of the co-localization of polySia with different intersialyl linkages in the same cell. The α2,9-linked polySia-containing glycoprotein of sea urchin sperm flagella was identified and named “flagellasialin”. The α2,9-linked polySia on flagellasialin is suggested as a regulator of intracellular Ca2+ and sperm motility. In addition, the α2,5Oglycolyl-linked polySia has recently been shown to increase intracellular pH and potentiate acrosome reaction of sea urchin sperm. These results indicate that these polySia structures in sea urchin gametes have new functions other than the negative regulator for the NCAM-mediated cell adhesion. In this review, we summarize the recent advances in structural and functional studies of polySia in sea urchin gametes.
Neural plasticity is necessary for the expression and maintenance of higher brain functions. While most of the experimental approach to the plasticity still remains phenomenalism, analyses at the molecular level have gradually progressed, especially concerning synaptic plasticity. The analyses revealed that several carbohydrate structures play important roles in the synaptic plasticity. This review describes the roles of 5 of these species of carbohydrates.