Calmodulin and CaMKII in the sperm principal piece: Evidence for a motility-related Calcium/Calmodulin pathway

Both cyclic AMP (cAMP)/protein kinase A (PKA) and calcium (Ca2+) signaling pathways are known to be involved in the regulation of motility in mammalian sperm. Calmodulin (CaM) is a ubiquitous Ca2+ sensor that has been implicated in the acrosome reaction. In this report, we identify an insoluble pool of CaM in sperm and show that the protein, in addition to its presence in the acrosome, is found in the principal piece of the flagellum. These findings are consistent with, though not proof of, the presence of a pool of CaM in the fibrous sheath. The Ca2+/CaM-dependent protein kinase lip (CaMKII beta), which is a downstream target of Ca2+/CaM, similarly localizes to the principal piece. In addition, we confirm earlier reports that a CaM inhibitor decreases sperm motility. However, we find that this inhibition can be largely reversed by stimulation of PKA if substrates for oxidative respiration are present in the medium. Our results suggest that the Ca2+/CaM/CaMKII signaling pathway in the sperm principal piece is involved in regulating sperm motility, and that this pathway functions either in parallel with or upstream of the cAMP/PKA pathway.