Evidence of the presence of calcium/calmodulin-dependent protein kinase IV in human sperm and its involvement in motility regulation

The mechanisms involved in the regulation of mammalian sperm motility are not well understood. Calcium ions (Ca²⁺) have been suggested to play a key role in the maintenance of motility; nevertheless, how Ca²⁺ modulates this process has not yet been completely characterized. Ca²⁺ can bind to calmodulin and this complex regulates the activity of multiple enzymes, including Ca²⁺/calmodulin-dependent protein kinases (CaM kinases). Results from this study confirmed that the presence of Ca²⁺ in the incubation medium is essential for maintaining human sperm motility. The involvement of CaM kinases in Ca²⁺ regulation of human sperm motility was evaluated using specific inhibitors (KN62 and KN93) or their inactive analogues (KN04 and KN92 respectively). Sperm incubation in the presence of KN62 or KN93 led to a progressive decrease in the percentage of motile cells; in particular, incubation with KN62 also reduced sperm motility parameters. These inhibitors did not alter sperm viability, protein tyrosine phosphorylation or the follicular fluid-induced acrosome reaction; however, KN62 decreased the total amount of ATP in human sperm. Immunological studies showed that Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) is present and localizes to the human sperm flagellum. Moreover, CaMKIV activity increases during capacitation and is inhibited in the presence of KN62. This report is the first to demonstrate the presence of CaMKIV in mammalian sperm and suggests the involvement of this kinase in the regulation of human sperm motility.