HUMAN ZONA-PELLUCIDA RECOGNITION ASSOCIATED WITH REMOVAL OF SIALIC-ACID FROM HUMAN SPERM SURFACE

The ability of human spermatozoa recovered from highly motile sperm fr actions to bind wheat germ agglutinin (WGA) after discontinuous Percol l gradient centrifugation was studied. WGA could bind to almost all mo tile spermatozoa, whereas fewer than 25% of spermatozoa could bind pea nut (PNA) and concanavalin A (Con A) agglutinin, two lectins that spec ifically bind acrosomal membranes. After removal of the plasma membran e with 0.04% Triton X100, WGA, PNA and Con A bound more than 80% of sp ermatozoa, but binding sites for WGA on the anterior acrosomal region were markedly reduced. The expression of sialic acid on human sperm pl asma membrane was demonstrated, since WGA, which specifically recogniz es both sialic acid (NeuNAc) and N-acetylglucosamine (GlcNAc), bound a lmost all intact motile spermatozoa, whereas succinylated WGA, which r ecognizes only GlcNAc, bound less than 10% of intact motile spermatozo a. Moreover, binding of WGA was compared with that of three other lect ins (Sambucus nigra, SNA; Maackia amurensis, MAL and Limulus polyphemu s, LPA) with specificity for different NeuNAc linkages. Only SNA, whic h requires the presence of the disaccharide structure NeuNAc alpha(2,6 ) Gal/GalNAc, showed a positive correlation with sperm motility as obs erved with WGA. Moreover, there was a strong inhibition of WGA binding on spermatozoa preincubated with bovine submaxillary mucin containing (2,6)-linked NeuNAc. These results demonstrate the presence of NeuNAc alpha(2,6) Gal/GalNAc glycoconjugate sequences on the plasma membrane of the motile human spermatozoon. Treatment of spermatozoa with Arthr obacter ureafaciens neuraminidase to cleave NeuNAc residues led to a d ose-dependent decrease of WGA binding at the sperm surface and to the enhancement of sperm attachment to the zona pellucida. We hypothesize that the release of sialic acid from the sperm plasma membrane could b e one of the capacitation events necessary for unmasking certain sperm surface antigens implicated in zona pellucida recognition.