Sperm interactions with heparin-like GAG may facilitate capacitation in vivo by inducing loss or modification of sperm surface molecules involved in sperm-oviduct binding. We in fact showed that GAG from fluids derived from dominant follicles were very efficient in competing for carbohydrate binding to the sperm surface (valid for all biotinylated carbohydrates tested). A chemically prepared GAG fraction from follicular fluid was less efficient in competing for carbohydrate binding to the sperm surface than unprocessed follicular fluid. The tertiary structure of glycoproteins (containing GAG), which is lost after purification, is probably important for optimal bioactivity. This finding may indicate that the complete tertiary structure of glycosylated proteins is required for full bioactivity of glycosidic structures involved in sperm adhesion to the oviduct epithelial cells. It is also worth noting that various proteoglycans from follicular fluid promote capacitation and acrosome reaction in bovine spermatozoa.
In fact, in fertility clinics, follicular fluid from dominant follicles is used for optimizing IVF. In vivo, however, these processes take place in the oviduct. Two obvious changes in the oviduct just before ovulation are i) GAG secretion especially at the ip-silateral side to the ovulating follicle in the isthmus and ii) infiltration of follicular fluid in the ampulla of the ipsilateral oviduct. Therefore, GAG are likely involved in modulating sperm-oviduct interactions (sperm release) and probably facilitates sperm activation and surface remodeling, which is important for sperm-zona interaction.
In conclusion, the newly developed biochemical assay for the quantitative detection of carbohydrate binding properties of the SPM did elucidate how carbohydrate moieties are involved in the sperm-oviduct adhesion as well as in the in vivo capacitation-induced release of activated sperm cells. The assay can additionally be used to determine exposed carbohydrate moieties that are involved in sperm-zona binding.