Phylogenetic differences in calcium permeability of the auditory hair cell cholinergic nicotinic receptor

Abstract

The α9 and α10 cholinergic nicotinic receptor subunits assemble to form the receptor that mediates efferent inhibition of hair cell function within the auditory sensory organ, a mechanism thought to modulate the dynamic range of hearing. In contrast to all nicotinic receptors, which serve excitatory neurotransmission, the activation of α9α10 produces hyperpolarization of hair cells. An evolutionary analysis has shown that the α10 subunit exhibits signatures of positive selection only along the mammalian lineage, strongly suggesting the acquisition of a unique function. To establish whether mammalian α9α10 receptors have acquired distinct functional properties as a consequence of this evolutionary pressure, we compared the properties of rat and chicken recombinant and native α9α10 receptors. Our main finding in the present work is that, in contrast to the high (pCa 2+ /pMonovalents ∼10) Ca 2+ permeability reported for rat α9α10 receptors, recombinant and native chicken α9α10 receptors have a much lower permeability (∼2) to this cation, comparable to that of neuronal α4β2 receptors. Moreover, we show that, in contrast to α10, α7 as well as α4 and β2 nicotinic subunits are under purifying selection in vertebrates, consistent with the conserved Ca 2+ permeability reported across species. These results have important consequences for the activation of signaling cascades that lead to hyperpolarization of hair cells after α9α10 gating at the cholinergic-hair cell synapse. In addition, they suggest that high Ca 2+ permeability of the α9α10 cholinergic nicotinic receptor might have evolved together with other features that have given the mammalian ear an expanded high-frequency sensitivity.evolution | ionotropic receptor | ligand-gated channel | outer hair cell N icotinic cholinergic receptors (nAChRs) are transmembrane allosteric proteins involved in the physiological responses to the neurotransmitter acetylcholine (ACh) (1). They are assembled from five identical (homopentamer) or different (heteropentamer) polypeptide chains arranged symmetrically around a central axis that lines a channel pore (1). The selective pressures for maintaining a wide diversity of nAChR subunits remain unknown (2, 3). The pentameric structure and function of muscle nAChR is well established; however, although much progress has been achieved, the possible combinatorial assemblies of neuronal subunits and their stoichiometry, distribution and function in the central nervous system remain subjects of ongoing investigation (4). Uniquely among these nonmuscle nAChRs, a clear function has been identified for the α9 and α10 nAChR subunits. These subunits assemble to form a pentameric receptor with a likely α9 2 α10 3 stoichiometry, which serves cholinergic neurotransmission at the synapse between efferent fibers and hair cells of the auditory and vestibular end organs (5-8). Efferent function has been studied in cochlear hair cells, where it is thought to inhibit outer hair cell (OHC) electromotility and so reduce cochlear amplification (9). In co...

Cloning, synthesis, and characterization of αO-conotoxin GeXIVA, a potent α9α10 nicotinic acetylcholine receptor antagonist

“…In contrast, the ribbon and bead isomers have sharper spectra indicative of either single conformations or fast exchange between conformations. The ribbon isomer contains a well-defined loop of 12 residues restrained by a disulfide bond and featuring a 3 10 -helix between Tyr12 and Arg16.…”

“…Surprisingly, the most active disulfide isomer of GeXIVA is the bead isomer, comprising, according to NMR analysis, two wellresolved but uncoupled disulfide-restrained loops. The ribbon isomer is almost as potent but has a more rigid structure built around a short 3 10 -helix. In contrast to most α-conotoxins, the globular isomer is the least potent and has a flexible, multiconformational nature.…”

“…Reduced function of α9α10 nAChR is associated with a beneficial alteration of immune function in animal models of chronic pain and experimental autoimmune encephalomyelitis (8,9). In vertebrates, the α9 and/or α10 subunit might be a primordial ancestor of the nAChR family (10,11).…”

α9α10 Acetylcholine receptors: structure and functions

“…Mammalian recombinant and native 910 nAChRs have relative Ca 2+ to monovalent cation permeability ratios close to or higher than 10 [14,[50][51][52] . In contrast to this, the relative Ca 2+ to monovalent cation permeability ratio for the chicken recombinant 910 nAChR is much lower, a ratio of 2, which is similar to other nAChRs that do not contain the 910 subunits (50).…”

“…Mammalian recombinant and native 910 nAChRs have relative Ca 2+ to monovalent cation permeability ratios close to or higher than 10 [14,[50][51][52] . In contrast to this, the relative Ca 2+ to monovalent cation permeability ratio for the chicken recombinant 910 nAChR is much lower, a ratio of 2, which is similar to other nAChRs that do not contain the 910 subunits (50). The authors conclude that the evolutionary processes that were responsible for the increased Ca 2+ permeability of mammalian 910 nAChRs were specific to the mammalian lineage and happened concomitant with other adaptations that conferred high frequency hearing to mammals [27,[48][49][50] .…”