Specific sugars on oviduct cells bind porcine sperm and regulate sperm calcium and lifespan

Abstract Text:

Upon semen deposition in the female reproductive tract, a fraction of the sperm is transported to the isthmus and utero-tubal junction to form a functional sperm reservoir. This reservoir can modulate sperm Ca²⁺, lengthen sperm lifespan and release sperm so that fertilization occurs, even if insemination and ovulation are not well synchronized. The function of this reservoir is often inadequate in swine, which necessitates the common practice of multiple inseminations during the course of each estrus period. Previous work testing a few small oligosaccharides suggested that oviduct sugars (glycans) bound sperm.  Using an array containing hundreds of glycans, we found that each glycan that bound sperm contained one of two motifs, either biantennary 6-sialylated lactosamine or Lewis X trisaccharide.  Both motifs were present in the oviduct isthmus; in some cases they were both in single large glycans.  Fluoresceinated versions of these glycans bound to the sperm head very specifically.  If we blocked either glycan on oviduct cells, sperm binding to these cells was reduced.  Thus, oviduct glycan binding was necessary and sufficient to bind sperm.  Because intracellular free Ca²⁺ is an important regulator of sperm function, we examined whether oviduct glycans affected Ca²⁺, using a fluorescent probe.  Both glycans suppressed the increase in intracellular Ca²⁺  that normally accompanies capacitation.  Consistent with this, glycans also reduced the normal increase in sperm protein tyrosine phosphorylation that occurs during capacitation.  As these results might be expected to prolong sperm lifespan, we examined whether glycans would improve sperm viability, in vitro.  When coupled to beads, both glycan motifs bound sperm and lengthened sperm viability above control glycans or fibronectin, a protein that binds sperm.  Initial investigations into receptor candidates have identified several sperm proteins with glycan affinity including lactadherin/SED1, a protein also implicated in binding to the zona pellucida.  After binding to the oviduct reservoir, the fertilizing sperm must be released to fertilize eggs.  What triggers sperm release is controversial.  We have found that progesterone activates release of half of the sperm bound to oviduct cells and that this release depends on Ca²⁺ influx through the major Ca²⁺channels in sperm, the CatSper channels.  Together these results elucidate how sperm are stored and released from the oviduct.  This fundamental information provides insight into methods to improve sperm lifespan in the oviduct, develop novel ways to lengthen sperm storage outside the oviduct, and improve reproductive efficiency.

Keywords: Sialic acid, fertility, capacitation, Lewis X, glycans