The role of progesterone-induced hyperactivation in the detachment of bull sperm from the oviduct reservoir

For optimum fertility, sperm should reside in the bovine female reproductive tract for a number of hours before the onset of ovulation in order to ensure sperm have successfully acquired the ability to fertilise the oocyte. The formation of the sperm reservoir is aided by the complexity of the convoluted isthmus and the molecular recognition between the fucose receptor on the sperm plasma membrane and the carbohydrate residues on the oviduct epithelium. Sperm are then sequentially released from the reservoirs at a time that coincides with ovulation and these release mechanisms are not well understood in mammals, particularly in bovines. Therefore, the aim of this study was to elucidate the mechanisms that trigger the detachment of bull sperm from oviduct epithelial cells. Capacitation of frozen-thawed sperm in vitro resulted in a reduced binding affinity to bovine oviductal epithelial cell (BOEC) explants when treated with heparin, caffeine or heparin and caffeine in combination (P<0.001). When non-capacitated sperm were co-incubated with BOEC explants, high binding affinity was apparent and the release of sperm was stimulated through capacitating factors (heparin) and hyperactivation of sperm through treatment with caffeine (P<0.001). As the cumulus cells of the bovine oocyte are known to secrete progesterone (P4) it was hypothesised that P4 could induce hyperactivation and thus the release of sperm bound to the oviduct epithelium. Progesterone-treated sperm displayed a heightened level of hyperactivated motility (P<0.001) which was suppressed when calcium (Ca²⁺) influx was inhibited through Mibefradil (non-specific Ca²⁺ channel antagonist), NNC 550396 (CatSper specific Ca²⁺ channel antagonist), or when extracellular Ca²⁺ was chelated using Ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; P<0.001). Hyperactivation induced by P4 stimulated the release of bound sperm from BOEC explants (P<0.01), and this detachment required an influx of extracellular Ca²⁺. Furthermore, we investigated the mode of action of P4 on bull sperm and the detachment of sperm from BOEC explants by P4, which was inhibited by Mifepristone (progesterone membrane receptor antagonist) and AG205 (progesterone membrane component 1 specific antagonist; PGRMC1, P<0.01). These findings suggest the presence of a P4 membrane receptor on bull sperm and that P4 is capable of inducing the release of sperm from the bovine oviductal epithelium. This P4 induced release is mediated by extracellular Ca²⁺. This study has increased our understanding of sperm interaction with the female reproductive tract and may aid in the development of fertility biomarkers in bovine reproduction.