Eleonora Katz

High calcium permeability and calcium block of the α9 nicotinic acetylcholine receptor

At the synapse between olivocochlear efferent fibers and outer hair cells (OHCs) of the cochlea, a non-classical ionotropic cholinergic receptor allows Ca(2+) entry into the hair cell, thus activating a Ca(2+)-sensitive K(+) current which hyperpolarizes the cells membrane. In the mammalian ear, this leads to a reduction in basilar membrane motion, altering auditory nerve fiber activity and reducing the dynamic range of hearing. The alpha9 nicotinic acetylcholine receptor (nAChR) subunit mediates synaptic transmission between cholinergic olivocochlear fibers and OHCs. Given that Ca(2+) is a key player at this inhibitory synapse, we evaluated the permeability to Ca(2+) of the recombinant alpha9 receptor expressed in Xenopus laevis oocytes and the modulation of its activity by extracellular Ca(2+). Our results show that the alpha9 receptor is highly permeable to Ca(2+) and that this cation potently blocks monovalent currents through this channel (IC(50)=100 microM, at -70 mV) in a voltage-dependent manner. At a Ca(2+) concentration similar to that found in the perilymph bathing the base of the OHCs, approximately 90% of the Na(+) current through the alpha9 receptor is blocked, suggesting that one of the main functions of this channel could be to provide a pathway for Ca(2+) influx.

Effects of Ca2+ channel blocker neurotoxins on transmitter release and presynaptic currents at the mouse neuromuscular junction

1 The effects of the voltage‐dependent calcium channel (VDCC) blockers ω‐agatoxin IVA (ω‐AgaIVA), ω‐conotoxin GVIA (ω‐CgTx), ω‐conotoxin MVIIC (ω‐MVIIC) and ω‐conotoxin MVIID (ω‐MVIID) were evaluated on transmitter release in the mouse diaphragm preparation. The effects of ω‐AgaIVA and ω‐MVIIC were also evaluated on the perineurial calcium and calcium‐dependent potassium currents, ICa and IK(Ca), respectively, in the mouse levator auris preparation. 2 The P‐ and Q‐type VDCC blocker ω‐AgaIVA (100u2003nM) and P‐ Q‐ and N‐type channel blockers ω‐MVIIC (1u2003μM) and ω‐MVIID (3u2003μM) strongly reduced transmitter release (>80–90% blockade) whereas the selective N‐type channel blocker ω‐CgTx (5u2003μM) was ineffective. 3 The process of release was much more sensitive to ω‐MVIIC (IC50=39u2003nM) than to ω‐MVIID (IC50=1.4u2003μM). After almost completely blocking transmitter release (quantal content ∼0.3% of its control value) with 3u2003μMω‐MVIIC, elevating the external [Ca2+] from 2 to 10u2003mM induced an increase of ∼20 fold on the quantal content of the endplate potential (e.p.p.) (from 0.2±0.04 to 4.8±1.4). 4 Nerve‐evoked transmitter release in a low Ca2+‐high Mg2+ medium (low release probability, quantal content = 2±0.1) had the same sensitivity to ω‐AgaIVA (IC50=16.8u2003nM) as that in normal saline solutions. In addition, K+‐evoked transmitter release was also highly sensitive to the action of this toxin (IC50=11.5u2003nM; 100u2003nM >95% blockade). The action of ω‐AgaIVA on transmitter release could be reversed by toxin washout if the experiments were carried out at 31–33°C. Conversely, the effect of ω‐AgaIVA persisted even after two hours of toxin washout at room temperature. 5 Both the calcium and calcium‐dependent potassium presynaptic currents, ICa and IK(Ca), respectively, were highly sensitive to low concentrations (10–30u2003nM) of ω‐AgaIVA. The ICa and the IK(Ca) were also strongly reduced by 1u2003μMω‐MVIIC. The most marked difference between the action of these two toxins was the long incubation times required to achieve maximal effects with ω‐MVIIC. 6 In summary these results provide more evidence that synaptic transmission at the mammalian neuromuscular junction is mediated by Ca2+ entry through P‐ and/or Q‐type calcium channels.

The nicotinic receptor of cochlear hair cells: a possible pharmacotherapeutic target?

Mechanosensory hair cells of the organ of Corti transmit information regarding sound to the central nervous system by way of peripheral afferent neurons. In return, the central nervous system provides feedback and modulates the afferent stream of information through efferent neurons. The medial olivocochlear efferent system makes direct synaptic contacts with outer hair cells and inhibits amplification brought about by the active mechanical process inherent to these cells. This feedback system offers the potential to improve the detection of signals in background noise, to selectively attend to particular signals, and to protect the periphery from damage caused by overly loud sounds. Acetylcholine released at the synapse between efferent terminals and outer hair cells activates a peculiar nicotinic cholinergic receptor subtype, the alpha9alpha10 receptor. At present no pharmacotherapeutic approaches have been designed that target this cholinergic receptor to treat pathologies of the auditory system. The potential use of alpha9alpha10 selective drugs in conditions such as noise-induced hearing loss, tinnitus and auditory processing disorders is discussed.

Stoichiometry of the alpha9alpha10 nicotinic cholinergic receptor.

The α9 and α10 nicotinic cholinergic subunits assemble to form the receptor that mediates synaptic transmission between efferent olivocochlear fibers and hair cells of the cochlea. They are the latest vertebrate nicotinic cholinergic receptor (nAChR) subunits that have been cloned, and their identification has established a distant early divergent branch within the nAChR gene family. The α10 subunit serves as a “structural” component leading to heteromeric α9α10 nAChRs with distinct properties. We now have probed the stoichiometry of recombinant α9α10 nAChRs expressed in Xenopus oocytes. We have made use of the analysis of the population of receptors assembled from a wild-type subunit and its partner α9 or α10 subunit bearing a reporter mutation of a valine to threonine at position 13′ of the second transmembrane domain (TM2). Because the mutation increased the sensitivity of the receptor for acetylcholine (ACh) but mutations at different subunits were not equivalent, the number of α9 and α10 subunits could be inferred from the number of components in compound concentration-response curves to ACh. The results were confirmed via the analysis of the effects of a mutation to threonine at position 17′ of TM2. Because at this position the mutations at different subunits were equivalent, the stoichiometry was inferred directly from the shifts in the ACh EC50 values. We conclude that the recombinant α9α10 receptor is a pentamer with a (α9)2(α10)3 stoichiometry.

The efferent medial olivocochlear-hair cell synapse.

Amplification of incoming sounds in the inner ear is modulated by an efferent pathway which travels back from the brain all the way to the cochlea. The medial olivocochlear system makes synaptic contacts with hair cells, where the neurotransmitter acetylcholine is released. Synaptic transmission is mediated by a unique nicotinic cholinergic receptor composed of α9 and α10 subunits, which is highly Ca2+ permeable and is coupled to a Ca2+-activated SK potassium channel. Thus, hyperpolarization of hair cells follows efferent fiber activation. In this work we review the literature that has enlightened our knowledge concerning the intimacies of this synapse.

Block of the α9 nicotinic receptor by ototoxic aminoglycosides

Abstract In the present study, we report that the α9 nicotinic acetylcholine receptor (nAChR) expressed in Xenopus laevis oocytes is reversibly blocked by aminoglycoside antibiotics. The aminoglycosides tested blocked the α9 nAChR in a concentration-dependent manner with the following rank order of potency: neomycin>gentamicin>streptomycin>amikacin>kanamycin. The antagonistic effect of gentamicin was not overcome by increasing the concentration of acetylcholine (ACh), indicative of a non-competitive type of block. Blockage of ACh-evoked currents by gentamicin was found to be voltage-dependent, being more potent at hyperpolarized than at depolarized holding potentials. Furthermore, gentamicin blockage was dependent upon the extracellular Ca2+ concentration, shown by the fact that increments in extracellular Ca2+ significantly reduced the potency of this aminoglycoside to block the α9 nAChR. Possible mechanisms of blockage by the aminoglycosides are discussed. The present results suggest that the initial reversible actions of aminoglycosides at the organ of Corti, such as the elimination of the olivocochlear efferent function, are due in part to the interaction with the native α9-containing cholinergic receptor of the outer hair cells.

Tracking the Molecular Evolution of Calcium Permeability in a Nicotinic Acetylcholine Receptor

Nicotinic acetylcholine receptors are a family of ligand-gated nonselective cationic channels that participate in fundamental physiological processes at both the central and the peripheral nervous system. The extent of calcium entry through ligand-gated ion channels defines their distinct functions. The α9α10 nicotinic cholinergic receptor, expressed in cochlear hair cells, is a peculiar member of the family as it shows differences in the extent of calcium permeability across species. In particular, mammalian α9α10 receptors are among the ligand-gated ion channels which exhibit the highest calcium selectivity. This acquired differential property provides the unique opportunity of studying how protein function was shaped along evolutionary history, by tracking its evolutionary record and experimentally defining the amino acid changes involved. We have applied a molecular evolution approach of ancestral sequence reconstruction, together with molecular dynamics simulations and an evolutionary-based mutagenesis strategy, in order to trace the molecular events that yielded a high calcium permeable nicotinic α9α10 mammalian receptor. Only three specific amino acid substitutions in the α9 subunit were directly involved. These are located at the extracellular vestibule and at the exit of the channel pore and not at the transmembrane region 2 of the protein as previously thought. Moreover, we show that these three critical substitutions only increase calcium permeability in the context of the mammalian but not the avian receptor, stressing the relevance of overall protein structure on defining functional properties. These results highlight the importance of tracking evolutionarily acquired changes in protein sequence underlying fundamental functional properties of ligand-gated ion channels.

Activation of presynaptic GABA(B(1a,2)) receptors inhibits synaptic transmission at mammalian inhibitory cholinergic olivocochlear-hair cell synapses.

The synapse between olivocochlear (OC) neurons and cochlear mechanosensory hair cells is cholinergic, fast, and inhibitory. The inhibitory sign of this cholinergic synapse is accounted for by the activation of Ca2+-permeable postsynaptic α9α10 nicotinic receptors coupled to the opening of hyperpolarizing Ca2+-activated small-conductance type 2 (SK2)K+ channels. Acetylcholine (ACh) release at this synapse is supported by both P/Q- and N-type voltage-gated calcium channels (VGCCs). Although the OC synapse is cholinergic, an abundant OC GABA innervation is present along the mammalian cochlea. The role of this neurotransmitter at the OC efferent innervation, however, is for the most part unknown. We show that GABA fails to evoke fast postsynaptic inhibitory currents in apical developing inner and outer hair cells. However, electrical stimulation of OC efferent fibers activates presynaptic GABAB(1a,2) receptors [GABAB(1a,2)Rs] that downregulate the amount of ACh released at the OC–hair cell synapse, by inhibiting P/Q-type VGCCs. We confirmed the expression of GABABRs at OC terminals contacting the hair cells by coimmunostaining for GFP and synaptophysin in transgenic mice expressing GABAB1–GFP fusion proteins. Moreover, coimmunostaining with antibodies against the GABA synthetic enzyme glutamic acid decarboxylase and synaptophysin support the idea that GABA is directly synthesized at OC terminals contacting the hair cells during development. Thus, we demonstrate for the first time a physiological role for GABA in cochlear synaptic function. In addition, our data suggest that the GABAB1a isoform selectively inhibits release at efferent cholinergic synapses.

Properties of mutated murine α4β2 nicotinic receptors linked to partial epilepsy

We characterized, by electrophysiological methods, two biophysical properties of murine recombinant alpha4beta2 nicotinic acetylcholine receptors (nAChR) bearing a mutation (alpha4:+L264alpha4:beta2 or alpha4:S252Falpha4:beta2) linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). Sensitivity to acetylcholine (ACh) was increased by the S252F substitution expressed in heterozygosis (alpha4:S252Falpha4:beta2) but was markedly reduced when this mutation was expressed in homozygosis (S252Falpha4:beta2). ACh sensitivity was not altered by the +L264 insertion. Moreover, receptor desensitization was significantly increased by both mutations expressed in heterozygosis. These results are in general agreement to those of rat and human recombinant receptors bearing the same mutations, thus contributing to validate the use of knock-in mice harboring ADNFLE mutations as models to study this pathology.