Implication of the human Binder of SPerm Homolog 1 (BSPH1) protein in capacitation

Binder of SPerm (BSP) proteins are a family of proteins expressed exclusively in the male reproductive tract (seminal vesicles or epididymis) of several mammalian species. They are known to promote capacitation, a sperm maturation step essential for fertilization. Our recent studies have shown that in human, the Binder of SPerm Homolog 1 (BSPH1) is expressed solely in epididymal tissues. The goal of the current study was to characterize BSPH1 and evaluate its effect on different sperm functions. A human recombinant BSPH1 (rec-BSPH1) was produced, purified and refolded. Rec-BSPH1 was found to share many characteristics with other members of the BSP superfamily, as it was able to bind gelatin and heparin as well as capacitate sperm. Rec-BSPH1 had no effect on sperm acrosome reaction or any sperm motility parameters. Native BSPH1 was localized on the equatorial segment, post-acrosomal segment and neck of ejaculated human sperm. Rec-BSPH1, following incubation with washed ejaculated human sperm, exhibited binding patterns similar to the native protein. These results show that the human epididymal BSPH1 shares many biochemical and functional characteristics with BSP proteins secreted by seminal vesicles of ungulates, and behaves similarly to its murine epididymal orthologue BSPH1. This study of human BSPH1 brings us one step closer to understanding the importance of this protein in male fertility.