GLYCOLIPID MIGRATION FROM THE APICAL TO THE EQUATORIAL SUBDOMAINS OF THE SPERM HEAD PLASMA-MEMBRANE PRECEDES THE ACROSOME REACTION - EVIDENCE FOR A PRIMARY CAPACITATION EVENT IN BOAR SPERMATOZOA

In order to extend the static information of immunolabelling sulphogal actolipids in fixed boar spermatozoa, a fluorescent sulphogalactolipid analogue, galactose(3-sulphate)-beta 1-1'[(N-lissamine rhodaminyl)-12 -aminododecanoyl]-sphingosine, was incorporated into plasma membranes of living spermatozoa and its lateral distribution over the sperm head was studied. The fluorescent lipid was enriched in the apical ridge s ubdomain of freshly ejaculated sperm cells, After sperm binding to the zona pellucida the lipid redistributed to the equatorial segment of t he sperm surface, A similar shift occurred during capacitation in vitr o with 2 mM CaCl2 or with 4% (w/v) bovine serum albumin. The desulphat ed derivative galactose-beta 1-1'[(N-lissamine rhodaminyl)-12-aminodod ecanoyl]-sphingosine was also incorporated into the plasma membrane of freshly ejaculated sperm cells and clearly stained the apical ridge s ubdomain and the (pre)-equatorial subdomains of the sperm heads, The d esulphogalactolipid analogue showed a slightly faster migration to the equatorial segment of the sperm plasma membrane than did its sulphate d counterpart, The measured fluorescence intensity distributions corre lated linearly with the spatial probe distribution, which was checked by fluorescence lifetime imaging microscopy, The observed migration of the incorporated glycolipids precedes the acrosome reaction and is on e of the underlying molecular events likely to be important in the pro cess of sperm capacitation, The results of this study suggest that lip id phase segregation is an important driving force for the organizatio n of the sperm head plasma membrane into subdomains.