Shedding light on the control of CatSper Ca2+ channels by cAMP and chemicals used to probe cAMP signaling

The sperm-specific CatSper channel controls the influx of Ca2+ into the flagellum and, thereby, the swimming behavior of sperm. A hallmark of human CatSper is its polymodal activation by membrane voltage, intracellular pH, and oviductal hormones. Whether CatSper is also activated by signaling pathways involving an increase of cAMP and ensuing activation of protein kinase A (PKA) is, however, a matter of controversy. Here, using kinetic ion-sensitive fluorimetry and patch-clamp recordings, we study transmembrane Ca2+ flux and membrane currents in human sperm from healthy donors and from patients that lack functional CatSper channels. We show that human CatSper is neither activated by intracellular cAMP directly nor indirectly by the cAMP/PKA-signaling pathway. Moreover, we demonstrate that non-physiological concentrations of cAMP and membrane-permeable cAMP analogs used to mimic the action of intracellular cAMP activate human CatSper from the outside via a previously unknown extracellular cyclic nucleotide-binding site. Finally, we demonstrate that the effects of common PKA inhibitors on human CatSper rest on off-target drug actions on CatSper itself rather than on inhibition of PKA. We conclude that the concept of an intracellular cAMP/PKA-activation of CatSper is primarily based on unspecific effects of chemical probes used to interfere with cAMP signaling. Altogether, our findings solve several controversial issues, which has important bearings on future studies of cAMP and Ca2+ signaling and the ligand-control of CatSper in sperm.