Sperm membrane protein profiles of fertile and infertile men: Identification and characterization of fertility-associated sperm antigen

Background: Male infertility is the major cause of conception failure in about 25% of all infertile couples. Understanding the causes of male infertility depends to a certain extent on the proteins present on the spermatozoa. The study aim was to investigate first, whether there is any difference in the expression of sperm membrane proteins between fertile and infertile males; and second, whether there is any functional significance of these proteins in the spermatozoa. Methods: Six different protocols were employed to extract sperm membrane proteins. A 57 kDa protein was identified and purified using different chromatographic techniques. The homogeneity and isoelectric point of the protein was confirmed by 2D-electrophoresis. The protein was characterized by immunofluorescence, ELISA, flow cytometry, SDS-PAGE and Western blot analysis. The role of 57 kDa protein in sperm-oocyte binding was studied in vitro. Results: All six sperm extracts of normozoospermic and infertile subjects showed 16-18 major and 12-15 minor protein bands. However, in one of the methods, the lysis buffer containing N-octyl-β-D-glycopyranoside (NOG) resulted in an additional protein band at the 57 kDa region in 95% of normozoospermic samples. The protein was either absent (∼80%) or negligible (∼20%) in infertile subjects. The protein was localized to the head of non-acrosome-reacted spermatozoa (NAR), and shifted to the equatorial segment in acrosome-reacted (AR) spermatozoa. The antibody directed against purified 57 kDa protein inhibited binding of human sperm to zona-free hamster oocytes in a dose-dependent manner. Conclusions: The results suggest that lack and/or low expression of 57 kDa protein may be one of the reasons for infertility in men. Therefore, the protein could be used as a marker for sperm quality in men.