Hyperactivation of stallion sperm is required for successful in vitro fertilization of equine oocytes

Capacitation is a complex and not well-understood process that encompasses all the molecular changes sperm must undergo to successfully fertilize an oocyte. In vitro fertilization has remained elusive in the horse, as evidenced by low in vitro fertilization (IVF) rates (0%-33%); moreover, only two foals have ever been produced using IVF. Incubation of stallion sperm in modified Whittens supplemented with bovine serum albumin and sodium bicarbonate yielded significant rates of time-dependent protein tyrosine phosphorylation and induced acrosomal exocytosis, consistent with capacitation. The objective of this study was to characterize stallion sperm hyperactivation and to test whether hyperactivation of capacitated sperm supported equine IVF. Treatment of sperm with procaine, an anesthetic shown to induce hyperactivation in other mammalian species, resulted in the decrease of three motility variables indicative of hyperactivation: straight line velocity (P = 0.029), straightness (P = 0.001), and linearity (P = 0.002). We demonstrated that procaine-induced hyperactivation was not regulated by changes in protein tyrosine phosphorylation and that it did not induce acrosomal exocytosis in capacitated sperm compared with calcium ionophore (P > 0.05), similar to findings in the bovine. Most notably, by coupling our capacitating conditions with the induction of hyperactivation using procaine, we have achieved the novel result of substantial and reproducible percentages of fertilized mare oocytes (60.7%) in our IVF experiments. Conversely, sperm incubated in capacitating conditions but not treated with procaine did not fertilize (0%). These results support the hypothesis that capacitation and hyperactivation are required for successful IVF in the equine.