Combined albumin and bicarbonate induces head-to-head sperm agglutination which physically prevents equine sperm-oviduct binding

In many species, sperm binding to oviduct epithelium is believed to be an essential step in generating a highly fertile capacitated sperm population primed for fertilization. In several mammalian species, this interaction is based on carbohydrate-lectin recognition. D-galactose has previously been characterized as a key molecule that facilitates sperm-oviduct binding in the horse. We used oviduct explant and oviduct apical plasma membrane (APM) assays to investigate the effects of various carbohydrates; glycosaminoglycans; lectins; S-S reductants; and the capacitating factors albumin, Ca2+ and HCO3- on sperm-oviduct binding in the horse. Carbohydrate-specific lectin staining indicated that N-acetylgalactosamine, N-acetylneuraminic acid (sialic acid) and D-mannose or D-glucose were the most abundant carbohydrates on equine oviduct epithelia, whereas D-galactose moieties were not detected. However, in a competitive binding assay, sperm-oviduct binding density was not influenced by any tested carbohydrates, glycosaminoglycans, lectins or D-penicillamine, nor did the glycosaminoglycans induce sperm tail-associated protein tyrosine phosphorylation. Furthermore, N-glycosidase F (PNGase) pretreatment of oviduct explants and APM did not alter sperm-oviduct binding density. By contrast, a combination of the sperm-capacitating factors albumin and HCO3- severely reduced (>10-fold) equine sperm-oviduct binding density by inducing rapid head-to-head agglutination, both of which events were independent of Ca2+ and an elevated pH (7.9). Conversely, neither albumin and HCO3- nor any other capacitating factor could induce release of oviduct-bound sperm. In conclusion, a combination of albumin and HCO3- markedly induced sperm head-to-head agglutination which physically prevented stallion sperm to bind to oviduct epithelium.