Low temperature-induced dimerization of the bovine sperm serine protease, BSp66

Serine protease activity, bovine sperm protease, 66 kDa (BSp66), is present in hamster sperm and is involved in sperm–zona interaction

Bovine sperm protease, 66 kDa (BSp66) is a serine protease previously characterized in bovine spermatozoa. Like other proteases, it may be present in sperm from other mammalian species different from bovine, playing a role in the fertilization process. In this study, we looked for BSp66 in hamster spermatozoa using heterologous antibodies against bovine BSp66. An immunoreactive protein was detected by Western blotting in mature and immature sperm. The detected protein had two isoforms similar to the ones reported in bovine sperm. Furthermore, indirect immune detection by fluorescence and electron microscopy assays, showed BSp66 signal at the acrosomal region similar to bovine sperm. As it was determined in bovine sperm, the acrosomal reaction displays the antigen within the acrosomal content. When live hamster sperm was incubated with polyclonal antibody against bovine BSp66 a decrease in the number of sperm bound to zona pellucida in homologous IVF and an impairment of head–head agglutination, were observed. These results suggest that a protease homologous to bovine BSp66 is present in golden hamster spermatozoa, with a conserved molecular mass and cellular location. Moreover, hamster BSp66 is probably involved in zona pellucida recognition.

BSp66 protease is widespread in the acrosomal region of sperm from several mammalian species

Fertilization in mammals comprises a sequence of events leading to the fusion of sperm and oocyte membranes. Although proteases are known to be involved in this process, their role in fertilization is controversial. There is extensive work on the characterization of proteolytic systems, including serine proteases, which demonstrates that acrosomal proteases can be distinguished among the sperm of different mammalian species on the basis of the gelatin-hydrolyzing activity on SDS-PAGE by the quantity and variety of the enzymes. In this report, we investigated the occurrence and activity of the serine protease BSp66, previously characterized in bovine spermatozoa, in various mammalian sperm. A protein with a molecular mass of 66 kDa cross-reacted with heterologous antibodies against bovine BSp66 when sperm extracts of several mammalian species were analyzed by Western blot. In agreement, proteolytic activity corresponding to the molecular mass of BSp66 was detected by gelatin zymography in all the species analyzed. This protein was located on the acrosomal region of sperm cells by immunofluorescence methods. We concluded that BSp66 is widespread in mammalian sperm, with a conserved location in the acrosomal region.

Partial purification and characterization of a trypsin-like serine protease from bovine sperm

Sperm proteolytic activities are relevant in the enzymatic mechanism of fertilization. Several authors have suggested the presence of serine proteases other than acrosin in mice and human spermatozoa. In this work we describe the characterization of a partially purified bovine sperm serine protease BSp66 and its dimmer, BSp120. Partial purification of the monomer was performed from fresh spermatozoa, while the dimer form of the protease was obtained from cryopreserved spermatozoa. The Mr of BSp120 and BSp66 estimated by zymography and gel filtration chromatography were 120 and 66 kDa, respectively. They were positively stained by Schiff-PAS reagent for glycoproteins and they both digested synthetic peptides with basic amino acids in the P1 site. Polyclonal antibodies against acrosin or proacrosin did not cross-react neither with BSp120, nor BSp66. In addition, antibodies raised in our laboratory against BSp120 and BSp66 did not recognize acrosin or proacrosin suggesting that they are not antigenically related proteins. Also, no cross- reactivity was detected with proteins in the range of 120-66 kDa when antibodies against the proteasome were used. The cellular localization of this protease by optical immunocytochemistry using specific antibodies revealed a positive signal in the apical portion of the sperm head suggesting acrosomal or membrane localization. The evidences presented here characterize BSp66 as a trypsin-like serine protease, a putative new member of this highly redundant proteolytic system of the sperm acrosome.

Immunolocalization of bovine sperm protease BSp120 by light and electron microscopy during capacitation and the acrosome reaction: Its role in in vitro fertilization

Mammalian fertilization involves various steps in which the participation of specific enzymes has been demonstrated by numerous studies. Acrosin is one of the most widely acrosomal protease in mammalian spermatozoa studied, including bovine; however, other proteases have also been described. A new trypsin-like serine protease named bovine serine protease of 120 kDa (BSp120) and its pre-cursor BSp66 (66 kDa) were identified in bovine spermatozoa. Cytological and ultrastructural immunolocalization studies on BSp120 were performed in live and fixed cells. Immunoflorescence assays with specific polyclonal antibodies revealed localization of BSp120 on the sperm head, with a signal homogeneously distributed over the acrosome resembling a horseshoe. After the acrosome reaction, sperm showed a patchy pattern in the acrosomal cap. Immune electron microscopy analysis indicated that BSp120 is located over the head plasma membrane of capacitated spermatozoa and acrosome reacting spermatozoa. To assess BSp120 function in sperm-oocyte interaction, in vitro fertilization studies were conducted. Oocytes were incubated with spermatozoa pre-treated with anti-BSp120, anti-guinea pig acrosin, and anti-BSp120 plus anti-guinea pig acrosin. Pre-treatment of bovine spermatozoa with antibodies towards each protein did not significantly modify fertilization rates. However, when both anti-acrosin and anti-BSp120 antibodies were simultaneously added, there was a significant decrease in the fertilization rate, suggesting that both enzymes may be required for fertilization. Altogether, the results from the present study described the localization of BSp120 over the acrosome of bovine sperm, and suggest its involvement in fertilization.