Regulation of boar sperm function by the oviduct – formation of a sperm reservoir, modulation of Ca 2+ influx, and release from storage
In mammals, an elaborate series of physiological events take place before fertilization of the female gamete. Upon insemination, a small fraction of the sperm deposited in the female reproductive tract is transported to the lower oviduct (isthmus) to form a functional sperm reservoir. The sperm reservoir extends sperm lifespan, prevents polyspermic fertilization, and is implicated in the regulation of capacitation. Once capacitated, a small number of sperm is released from the storage site to form a pool that can bind to the egg. The precise identity of the molecules that mediate the interactions between spermatozoa and epithelial cells of the isthmus are not currently known. Similarly, the molecules and mechanisms involved in the sperm release from the reservoir are poorly understood. It has been proposed, however, that oviduct glycans play a major role in the molecular interaction that leads to the formation of the sperm reservoir. A glycan array previously demonstrated that porcine sperm bound predominantly to motifs containing either biantennary α2,6 sialylated lactosamine (SiLN) or LewisX (LeX) trisaccharide present in the luminal epithelium of the isthmus. Functional data generated by blocking the interaction between uncapacitated sperm and isthmic oviduct cells in vitro strongly suggests that the glycoconjugates containing SiLN and LeX are at least partially involved in the formation of sperm reservoir in swine. These glycans, involved in binding, are also implicated in the regulation of the capacitation process by extending sperm lifespan. Proper calcium ion uptake is a key element for the sperm to acquire hypermotility, which is essential in the final steps of capacitation. Uncapacitated sperm binding to SiLN and LeX reduce intracellular Ca2+ levels under capacitating conditions, which is compatible with the notion that sperm binding to the isthmic epithelium suppresses the normal Ca2+ increase in sperm that occurs during capacitation, thus extending sperm lifespan. The stimuli associated with separation of sperm and epithelial ligands, including glycans, on the oviductal isthmus are not entirely recognized. The endocrine milieu, changes in local secretory activity, and cumulus-oocyte complexes might participate in the events accompanying sperm release. This work demonstrated that progesterone stimulates sperm Ca2+ influx through CatSper, a sperm-specific channel. Moreover, these data suggest that progesterone and CatSper channels are involved in the process of sperm detachment from oviductal cells in vitro. Progesterone treatment of sperm bound to isthmic epithelial cells stimulated sperm detachment and inactivation of progesterone-responsive CatSper channels blocked the effect of progesterone on sperm release. Collectively, these data suggest that: 1) sperm binding to porcine epithelial cells of the oviductal isthmus to form a functional sperm reservoir is at least partially mediated by SiLN and LeX glycan residues; 2) the glycans involved in the formation of the sperm reservoir suppress Ca2+ entry, thereby regulating sperm capacitation; and 3) progesterone-activated CatSper channels stimulate Ca2+ influx in boar sperm and detachment from oviductal explants. In general, the understanding of the molecular interactions that regulate sperm capacitation and lead sperm to fertilize the egg properly is paramount in the advancement of basic reproductive science in several species. This work improved the understanding of fertilization in a species in which ovulation is not always well synchronized with semen deposition. Moreover, especially in species like swine, this knowledge might soon contribute to improvements in conditions in which porcine semen is stored.
swine, sperm, oviduct, sperm reservoir, glycans, capacitation
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign