Cloning and Expression of A Small Conductance Ca-Activated K Channel from The Mouse Cochlea: Coexpression with α9/α10 Acetylcholine Receptors

Functional interactions between ligand-gated, voltageand Ca-activated ion channels are essential to the properties of excitable cells and thus to the working of the nervous system. The outer hair cells in the mammalian cochlea receive efferent inputs from the brainstem through cholinergic nerve fibers that form synapses at their base. The acetylcholine released from these efferent fibers activates fast inhibitory postsynaptic currents mediated, to some extent, by small conductance Ca-activated K channels (SK) that had not been cloned. We here report the cloning, characterization and expression of a complete SK2 cDNA from the mouse cochlea. The cDNAs of the mouse cochlea α9 and α10 acetylcholine receptors were also obtained, sequenced and co-expressed with the SK2 channels. Human cultured cell lines transfected with SK2 yielded Ca-sensitive K current that was blocked by dequalinium chloride and apamin, known blockers of SK channels. Xenopus oocytes injected with SK2 in-vitro transcribed RNA, under conditions where only outward K currents could be recorded, expressed an outward current that was sensitive to EGTA, dequalinium chloride and apamin. In HEK-293 cells co-transfected with cochlear SK2 plus α9/α10 receptors, acetylcholine induced an inward current followed by a robust outward current. The results indicate that SK2 and the α9/α10 acetylcholine receptors are sufficient to partly recapitulate the native hair cell efferent synaptic response.