Acid Extrusion from Human Spermatozoa is Mediated by Flagellar Hv1 Proton Channel

Human spermatozoa are quiescent in the male reproductive system and must undergo activation once introduced into the female reproductive tract. After ejaculation, and during their transit through the female reproductive tract, sperm motility is initiated and then hyperactivated to allow the spermatozoa to penetrate through the viscous oviductal mucus and the egg's protective vestments. These processes are known to require alkalinization of sperm cytoplasm, but the mechanism responsible for transmembrane proton extrusion has remained unknown due to the inability to measure membrane conductances in human sperm. Here, by successfully patch clamping human spermatozoa, we show that proton channel Hv1 is their dominant proton conductance. Hv1 is confined to the principal piece of the sperm flagellum, where it is expressed at an unusually high density. Robust flagellar Hv1-dependent proton conductance is activated by membrane depolarization, an alkaline extracellular environment, and removal of extracellular zinc, a potent Hv1 blocker.Surprisingly, endocannabinoid anandamide was also found to modulate Hv1 activity in the sperm cells as well as in the heterologously expression system. Hv1 allows only outward transport of protons and is therefore dedicated to inducing intracellular alkalinization and activating spermatozoa. In contrast to the large Hv1 current in human sperm cells, the amplitude of the outward current recorded under similar conditions from mouse spermatozoa was 30 times smaller; therefore, mouse sperms seem to have a different mechanism for acid extrusion.In human sperm, Hv1-induced intracellular alkalinisation should activate pH-dependent Ca2+ channel CatSper and control intracellular Ca2+ concentration. Finally, since Hv1 lies upstream in the signalling cascades leading to sperm activation, hyperactivation, and capacitation, Hv1 is an attractive target for the control of male fertility.