Regionalized lipid diffusion in the plasma membrane of mammalian spermatozoa

The plasma membrane of mammalian spermatozoa shows pronounced lateral asymm etry with many glycoproteins restricted to specific domains. Some of these antigens are freely diffusing throughout the membrane whereas others appear static in position. It is not clear whether these concepts also apply to m embrane lipids. In this investigation we have used fluorescence recovery af ter photobleaching (FRAP) techniques to spatially resolve lipid dynamics in various surface domains of 5 species of mammalian spermatozoa (bull, boar, ram, mouse, and guinea pig). Sperm plasma membranes were loaded with 5-(N- octadecanoyl)aminofluorescein (ODAF) reporter probe, and its diffusion was measured in various domains by FRAP analysis. Results showed that in live b ull, boar, ram, and mouse spermatozoa, diffusion coefficients (D) were sign ificantly higher over the acrosome and postacrosome than on the midpiece an d principal piece of the tail. In dead or permeabilized cells, on the other hand, large immobile phases developed, particularly on the sperm tail, tha t severely reduced D values, ODAF diffusion was also sensitive to temperatu re and cross-linking of protein components within the membrane with parafor maldehyde. Guinea pig spermatozoa were different in almost all respects fro m those of the other species tested. It is concluded that lipid diffusion i n the plasma membrane of live spermatozoa varies significantly between surf ace domains, because of either compositional heterogeneity, or differences in bilayer disposition, or the presence of intramembranous barriers that im pede free exchange between domains. This study emphasizes the important rol e of membrane lipids in regulating polarized migration of sperm surface ant igens during developmental processes such as maturation and capacitation.