Exploration of the Mechanism of Action of Human Oviduct-Specific Glycoprotein (OVGP1) in Regulating Sperm Capacitation
Oviductal Secretions , OVGP1 , Sperm Capacitation , Tyrosine Phosphorylation , Intracellular Calcium
The present research is of significance to the field of fertility treatment as better understanding the mechanism that regulates the function of oviduct-specific glycoprotein (OVGP1) could lead to its supplementation to culture medium used in assisted reproductive technologies, potentially improving the fertilizing competence of sperm. The mammalian oviductal non-ciliated secretory cells synthesize and secrete a major glycoprotein known as oviductin or OVGP1. Various mammalian in vitro studies have implicated native OVGP1 in enhancing sperm capacitation, sperm motility, sperm penetration and sperm-egg binding. Our laboratory has successfully produced recombinant human OVGP1 (rHuOVGP1) which has been shown to enhance tyrosine phosphorylation of sperm proteins, a biochemical hallmark of capacitation that takes place in the sperm tail, enhance the potential of sperm undergoing acrosome reaction, and increase sperm-egg binding. In the present study, we set out to explore the mechanism of human OVGP1 in enhancing key events of sperm capacitation. A major mechanism of inducing sperm capacitation is through the increase of intracellular calcium concentration ([Ca2+]i). The cation channels of sperm (CatSper) have been found to control the entry of Ca2+ ions into the sperm tail during capacitation and to be required for male fertility. Progesterone (P4) is known to bind to the plasma membrane P4 receptor of sperm and activate the CatSper channels, thus inducing the influx of Ca2+. We hypothesized that human OVGP1 enhances tyrosine phosphorylation of sperm proteins through increasing [Ca2+]i, similar to P4, via the CatSper channels during human sperm capacitation. Results showed that the presence of rHuOVGP1 in the capacitating medium was able to further increase the level of [Ca2+]i in human sperm. Additionally, treatment of sperm with rHuOVGP1 further increases and sustains the level of [Ca2+]i in sperm following treatment with P4. The inhibition of CatSper channels with the selective inhibitor HC-056456 impedes the effects of rHuOVGP1 on [Ca2+]i. Furthermore, the present study showed that P4 alone and in combination with rHuOVGP1 can further enhance the level of sperm protein tyrosine-phosphorylation. In summary, OVGP1 and P4 function most effectively in combination to enhance sperm capacitation through upregulating [Ca2+]i and protein tyrosine-phosphorylation.