Cyclic Nucleotide-gated Channels on the Flagellum Control Ca²⁺ Entry into Sperm

Cyclic nucleotide-gated (CNG) channels are key elements of cGMP- and cAMP-signaling pathways in vertebrate photoreceptor cells and in olfactory sensory neurons, respectively. These channels form heterooligomeric complexes composed of at least two distinct subunits (α and β). The α subunit of cone photoreceptors is also present in mammalian sperm. Here we identify one short and several long less abundant transcripts of β subunits in testis. The α and β subunits are expressed in a characteristic temporal and spatial pattern in sperm and precursor cells. In mature sperm, the α subunit is observed along the entire flagellum, whereas the short β subunit is restricted to the principal piece of the flagellum. These findings suggest that different forms of CNG channels coexist in the flagellum. Confocal microscopy in conjunction with the Ca²⁺ indicator Fluo-3 shows that the CNG channels serve as a Ca²⁺ entry pathway that responds more sensitively to cGMP than to cAMP. Assuming that CNG channel subtypes differ in their Ca²⁺ permeability, dissimilar localization of α and β subunits may give rise to a pattern of Ca²⁺ microdomains along the flagellum, thereby providing the structural basis for control of flagellar bending waves.