Evolution of Molecular Determinants for Calcium Permeability of the Mammalian Hair Cell Nicotinic Acetylcholine Receptor

LIPOVSEK M; MILLAR NS; KATZ E; ELGOYHEN AB

Congreso; 42nd Annual Meeting, Society for Neuroscience; 2012

Society for Neuroscience

The á9á10 cholinergic nicotinic receptor (nAChR) mediates efferent inhibition of hair cell function within the auditory sensory organ, a mechanism thought to modulate the dynamic range of hearing. The Ca2+ permeability of the mammalian á9á10 nAChR is among the highest reported for a ligand-gated ion channel (PCa/PNa~10), in accordance with its known function as the source of Ca2+ entry to cochlear hair cells (Elgoyhen et al., 2001. PNAS. 98: 3501). Surprisingly, we have reported that the calcium permeability of chicken á9á10 nAChRs (PCa/PNa<2) is much lower than that of its mammalian counterpart and similar to that of á4â2 receptors (Lipovsek et al., 2012. PNAS. 109: 4308). This may follow from the differential evolutionary history recently described for vertebrate á9á10 nAChRs (Franchini and Elgoyhen, 2006) and provides the opportunity to analyze the determinants of Ca2+ permeability by searching for the differences in the amino acid sequence that natural selection (and not arbitrary experimental mutagenesis) has fixed. Through the generation of chimeric receptors and site directed mutagenesis we show that residues located in the extracellular vestibule and the TM1-TM2 loop are determinants of Ca2+ permeability of á9á10 nAChRs. These results do not support the notion that transmembrane region 2 is solely responsible for calcium permeability of nAChRs, but rather support recent findings showing that ions are selected along the entire conduction pathway (Peters et al., 2010. J Physiology. 588: 587).