Capacitation-dependent reorganization of microdomains in the apical sperm head plasma membrane: Functional relationship with zona binding and the zona-induced acrosome reaction

For sperm to successfully fertilize on oocyte, it needs to pass through certain steps prior to, during and after initial recognition of the zona pellucida (ZP). During capacitation, the surface of the sperm head becomes remodelled, priming it to bind to the ZP and subsequently to undergo the ZP-induced acrosome reaction. During capacitation, sperm ZP-binding proteins are ordered in functional protein complexes that only emerge at the apical tip of the sperm head plasma membrane; this is also functionally the exclusive sperm surface area involved in primary ZP binding. After primary ZP binding, the same area is probably involved in the induciton of the acrosome reaction. A combination of biochemicla and proteomic membrane protein techniques have enabled us to dissect and highly purify the apical sperm plasma membrane area from control and capacitated sperm cells. The actual ZP-binding proteins identified predominantly belonged to the sperm membrane-associated family members of spermadhesins (AQN-3) and were present in the aggregating lipid ordered membrane microdomains (lipid rafts) that emerged during invitro capacitation in the apical ridge area of the sperm head plasma membrane. This clustering of these rafts was dependent on the presence of bicarbonate removal was restricted to the non-craft membrane fraction of the sperm plasma membrane, but did not cause any depletion of cholesterol in the raft membrane fraction. Interestingly, sperm SNARE proteins (both VAMP from the outer acrosomal membrane, as well syntaxin from the apical sperm head plasma membrane) shared lateral redistribution properties, along with the ZP-binding protein complex and raft marker proteins. All of these were recovered after capacitation in the detergent-resistant membrane preparations from sperm thought to represent membrane lipid rafts. We inferred that the capacitation-dependent formation of an aggregated lipid ordered aical ridge surface area in the sperm head plasma membrane was not only relevant for ZP-binding, but also for the ZP-induced acrosome reaction. (C) 2008 Elsevier Inc. All rights reserved.