Fechner, S., Alvarez, L., Bönigk, W., Müller, A., Berger, T., Pascal, R., Trötschel, C., Poetsch, A., Stölting, G., Siegfried, K.R., Kremmer, E., Seifert, R., Kaupp, U.B. (2015) A K(+)-selective CNG channel orchestrates Ca2(+) signalling in zebrafish sperm. eLIFE. 4.
Calcium in the flagellum controls sperm navigation. In sperm of marine invertebrates and mammals, Ca2(+) signalling has been intensely studied, whereas for fish little is known. In sea urchin sperm, a cyclic nucleotide-gated K(+) channel (CNGK) mediates a cGMP-induced hyperpolarization that evokes Ca2(+) influx. Here, we identify in sperm of the freshwater fish Danio rerio a novel CNGK family member featuring non-canonical properties. It is located in the sperm head rather than the flagellum and is controlled by intracellular pH, but not cyclic nucleotides. Alkalization hyperpolarizes sperm and produces Ca2(+) entry. Ca2(+) induces spinning-like swimming, different from swimming of sperm from other species. The 'spinning' mode probably guides sperm into the micropyle, a narrow entrance on the surface of fish eggs. A picture is emerging of sperm channel orthologues that employ different activation mechanisms and serve different functions. The channel inventories probably reflect adaptations to species-specific challenges during fertilization.