Regional abundances of binder of sperm (BSP) proteins are negatively associated with the quality of frozen-thawed bovine spermatozoa.

In brief: The localization and abundance of the sperm BSP proteins correlate with in vitro fertility in domestic bulls used in artificial insemination service. Abstract: Binder of sperm (BSP) proteins, secreted mainly by the accessory sex glands, are the major protein family present in bovine seminal plasma and on the sperm surface after ejaculation. In vivo, BSP proteins facilitate sperm capacitation and sperm reservoir formation; however, their impact on sperm function within the in vitro systems is less clear. Therefore, this biomarker-based study aimed to characterize the localization and abundance of BSP proteins from in vitro processed frozen-thawed bovine spermatozoa. Using image-based flow cytometry and Western blotting, BSP protein localization, abundance, membrane and acrosomal integrity were investigated in the supernatant (nonmotile) and pellet (motile) fractions of gradient-separated bull spermatozoa. Spermatozoa from the supernatant fraction had high enrichment of all BSP proteins investigated (BSP1, BSP3, BSP5; P < 0.05) when compared to the pellet fraction. In the pellet fraction, BSP1 and BSP3 bound predominately to the acrosomal region, whereas BSP5 had a high affinity for the midpiece. However, in the supernatant fraction, BSP proteins predominately coated the entire sperm surface resulting in the loss of regional specificity. High BSP protein abundance in the spermatozoa also correlated with acrosome and membrane damage. Whereas a high abundance of BSP5 correlated with low embryo cleavage rates, high abundance of BSP1 on the sperm head coincided with a high blastocyst rate. Therefore, changes in the quantity and localization of specific BSP proteins could act as potential biomarkers of sperm quality and fertility.