Abstract
We demonstrate for the first time that a stable, micron-scale segregation of focal enrichments of sterols exists at physiological temperature in the plasma membrane of live murine and human sperm. These enrichments of sterols represent microheterogeneities within this membrane domain overlying the acrosome. Previously, we showed that cholera toxin subunit B (CTB), which binds the glycosphingolipid, G(M1), localizes to this same domain in live sperm. Interestingly, the G(M1) undergoes an unexplained redistribution upon cell death. We now demonstrate that G(M1) is also enriched in the acrosome, an exocytotic vesicle. Transfer of lipids between this and the plasma membrane occurs at cell death, increasing G(M1) in the plasma membrane without apparent release of acrosomal contents. This finding provides corroborative support for an emerging model of regulated exocytosis in which membrane communications might occur without triggering the "acrosome reaction." Comparison of the dynamics of CTB-bound endogenous G(M1) and exogenous BODIPY-G(M1) in live murine sperm demonstrate that the sub-acrosomal ring (SAR) functions as a specialized diffusion barrier segregating specific lipids within the sperm head plasma membrane. Our data show significant differences between endogenous lipids and exogenous lipid probes in terms of lateral diffusion. Based on these studies, we propose a hierarchical model to explain the segregation of this sterol- and G(M1)-enriched domain in live sperm, which is positioned to regulate sperm fertilization competence and mediate interactions with the oocyte. Moreover, our data suggest potential origins of subtypes of membrane raft microdomains enriched in sterols and/or G(M1) that can be separated biochemically.