Evolution of molecular determinants for calcium permeability of the mammalian hair cell nicotinic acetylcholine receptor

LIPOVSEK M; MILLAR NS; KATZ E; ELGOYHEN AB

New Orleans

Congreso; Meeting of the Society for Neuroscience; 2012

Society for Neuroscience Organization

The α9α10 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 α9α10 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 α9α10 nAChRs (PCa/PNa<2) is much lower than that of its mammalian counterpart and similar to that of α4β2 receptors (Lipovsek et al., 2012. PNAS. 109:4308). This may follow from the differential evolutionary history recently described for vertebrate α9α10 nAChRs (Franchini and Elgoyhen, 2006. Mol Phylog and Evol. 41:622) and provides theopportunity 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 α9α10 nAChRs. These results do not support the notion that transmembrane region 2 is solely responsible for calcium permeability of nAChRs, showing that ions are selected along the entire conduction pathway.