Bull sperm binding to oviductal epithelium is mediated by a Ca2+-dependent lectin on sperm that recognizes Lewis-a trisaccharide.

Abstract

Sperm binding to oviductal epithelium produces a reservoir in vivo that may serve to maintain sperm fertility and provide sperm for fertilization when ovulation occurs. Previously, it was determined that bull sperm binding could be blocked by fucoidan and its component fucose; furthermore, treatment of epithelium with fucosidase prevented binding. The present study was conducted to further characterize binding. Because fucose would probably be present on the epithelium as part of oligosaccharide moieties of glycoproteins and/or glycolipids, competitive binding inhibition activity was tested for fucose in five linkages commonly found in oligosaccharides. Binding inhibition was assayed by determining the concentration of motile, frozen/thawed sperm bound to fresh epithelial explants in the presence of test inhibitors. Initially, 5 monosaccharides were tested at 30 mM (fucose, mannose, sialic acid, glucose, N-acetyl glucosamine, and galactose), and only fucose significantly reduced sperm binding compared to vehicle control (p = 0.03). Of the oligosaccharides tested (lacto-N-fucopentaose I, 3-fucosyllactose, Lewis-X, Lewis-a, and GlcNAcbeta1-4[Fucalpha1-6]GlcNAc-O-Me), only Lewis-a significantly reduced binding, and it did so in a dose-dependent fashion (p = 0.009 at 12.5 mM). Ca2+ dependency of binding was examined. Sperm were incubated with explants in Tyrode's albumin lactate pyruvate (TALP) containing 2 mM CaCl2 or lacking CaCl2 and containing 2 mM EGTA. Sperm-binding density was reduced significantly in EGTA (p < 0.03) but could be restored by readdition of CaCl2. Also, live sperm were labeled with the oligosaccharide ligand Lewis-a conjugated to fluorescein isothiocyanate-tagged polyacrylamide. Sperm exhibited labeling on the head only in the presence of Ca2+. Labeling could be blocked by fucose or Lewis-a-polyacrylamide. It was concluded that bull sperm bind to an oligosaccharide ligand on the oviductal epithelium that resembles Lewis-a and that binding is Ca2+-dependent.