Acrosomal Enzymes

Abstract

Sperm-egg interaction is a species-specific event, which is initiated by the recognition and binding of complementary molecule(s) present on sperm surface (receptor) and the surface of the zona-pellucida (ZP ligand). This is a carbohydrate-mediated signal transduction event, which is believed to be a pre-requisite that enables the acrosome reacted spermatozoa to penetrate the ZP before the egg is fertilized. The hydrolytic enzymes released at the site of sperm-egg binding, alongwith the enhanced thrust generated by the hyperactivation of the bound spermatozoa are important factors that regulate the penetration of wna pellucida and fusion of the gametes. Evidences strongly suggest that sperm-egg binding leading to the acrosomal exocytosis is a complex event that likely reflects interaction between multiple sperm surface receptors and multivalent ZP. The ZP in a species is composed of three glycoproteins namely, ZP1, ZP2, and ZP3. These glycoproteins at least in the mouse are synthesized and secreted during oogenesis. It is generally accepted that hydrolytic enzymes, such as arylsulfatase, &glucuronidase, N-acetylglucosaminidase and hyaluronidase, and perhaps proteases aid in the process of dispersion of the cumulus mass by hydrolyzing hyaluronic acid andlor polypeptide substrates. The protease group of enzymes includes serine proteases, proacrosin, acrosin, metalloproteases, ADAM proteins, dipeptidylpeptidase 11, aspartic protease, cathepsin D, cysteine proteases such as calpain, CRES, cathepsins B, L, and S-like. Many of these enzymes are released during acrosome reaction when the outer acrosomal membrane and the overlapping sperm plasma membrane fuse, allowing the release of the acrosomal contents. In addition to the glycohydrolases and , proteinases the sperm acrosome contains esterases, sulfatases, phosphatases, and phospholipases. Some of these enzymes have been described in sufficient details elsewhere (Zaneveld and de Jonge, 1991), whereas phospholipases, metalloproteases, and ADAM proteins have been discussed in chapters 22,25 and 26 respectively. ACROSOME BIOGENESISThe acrosome of spermatowan is formed during spermiogenesis phase of spermatogenesis that lasts for about 20 days in rat. The Golgi apparatus is actively involved in the formation of sperm acrosome during Golgi phase and cap phase. During acrosome phase and maturation phase, the acrosomic system undergoes structural transformation and condensation, but without participation of Golgi apparatus. During Golgi phase, the proacrosomic granules appear as electron dense material and coalesce into larger proacrosomic granules (step 1-2).. . oligosaccharides on ZP3 that aggregate sperm-bound receptors, thus eliciting intracellular signals. On the sperm surface, it binds to terminal GlcNAc residues of 0-linked oligosaccharides of ZP3 perturbing GalTase or its ZP3 ligand affects sperm-zona binding (Gong et al., 1995). It seems that GalTase serves as a generalized gamete receptor in mammals. The PNA-positive glycoprotein of 135-150 kDa present on plasma me...