Inhibition of Ser/Thr phosphatases induces capacitation-associated signaling in the presence of Src kinase inhibitors.

Signaling events leading to mammalian sperm capacitation rely on activation/deactivation of proteins by phosphorylation. This cascade includes soluble adenylyl cyclase, an atypical bicarbonate-stimulated adenylyl cyclase, and is mediated by protein kinase A and the subsequent stimulation of protein tyrosine phosphorylation. Recently, it has been proposed that the capacitation-associated increase in tyrosine phosphorylation is governed by Src tyrosine kinase activity. This conclusion was based mostly on the observation that Src is present in sperm and that the Src kinase family inhibitor SU6656 blocked the capacitation-associated increase in tyrosine phosphorylation. Results in the present manuscript confirmed these observations and provided evidence that these inhibitors were also able to inhibit protein kinase A phosphorylation, sperm motility, and in vitro fertilization. However, the block of capacitation-associated parameters was overcome when sperm were incubated in the presence of Ser/Thr phosphatase inhibitors such as okadaic acid and calyculin-A at concentrations reported to affect only PP2A. Altogether, these data indicate that Src is not directly involved in the observed increase in tyrosine phosphorylation. More importantly, this work presents strong evidence that capacitation is regulated by two parallel pathways. One of them requiring activation of protein kinase A and the second one involving inactivation of Ser/Thr phosphatases.