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Can you still see the for the molecules? Jonathan F Ashmore* and Fabio Mammano†

It is now established that the mammalian cochlea uses active of excitation via the auditory nerve to amplification of incoming sound to achieve sensitivity. Cellular the central nervous system nuclei. It is thought that OHCs details are emerging slowly. Recent studies of sensory hair generate forces that counteract the viscous drag of the fluid cells have highlighted the possible molecular bases for and the component cells that surround the basilar mem- amplification and the components of sensitivity regulation brane. To do this, OHCs must sense the sub-nanometre within the cochlea: a synthesis is likely to depend on effective deflections of the basilar membrane and then feed back and physiologically informed modelling. forces into its vibration pattern with the right phase and amplitude to enhance local motion. This is not a trivial Addresses problem in engineering terms, and the processes also have *Department of Physiology, University College London, Gower Street, to be fast enough to work at acoustic frequencies. London WC1E 6BT, UK; e-mail: [email protected] †Biophysics Laboratory and INFM, SISSA International School for How good are cochlear models? Advanced Studies, via Beirut 2-4, 34014 Trieste, Italy; e-mail: [email protected] One of the most effective ways of knowing the function of any cluster of cells or proteins in a complex structured Current Opinion in Neurobiology 2001, 11:449–454 system like the cochlea is to model it mathematically. 0959-4388/01/$ — see front matter Modelling the cochlea is therefore an exercise in modelling © 2001 Elsevier Science Ltd. All rights reserved. biological complexity. Many of the motions of the cochlear • Abbreviations partition involve many components [2 ]. In particular, it is AChR acetylcholine receptor important to identify precisely the mechanics and the IHC inner composition of the tectorial membrane — a gelatinous OHC outer hair cell structure that overlies the that is crucial for the transfer of mechanical stimuli to the sensory hair Introduction bundles. Mutations in genes that encode tectorial membrane In a post-genomic era, assigning functions to the products proteins cause hearing losses [3]. When one of these genes, of genes is a pressing issue. This concern is paramount in α-tectorin, is knocked out, it causes changes of cochlear the cochlea, where major advances in understanding the physiology in the mouse compatible with simple ideas of genetics of hearing loss have been made [1]. With many tectorial membrane function [4••]. It may also be critical to complete and near-complete genome sequences now take full account of the three-dimensional structure of the available, it should, in principle, be easier to identify genes organ of Corti [5•,6]. Finally, as suspected over 10 years ago, associated with cochlear function and development. In there are differences between the mechanics of the apical hearing, the intimate relationship between cochlear and basal cochlear regions [7]. To integrate such data, we performance and single-molecule function is very direct need to have good models of the cochlea that are informed because the stimulus that activates sensory receptors is a by realistic biological constraints. An understanding of the mechanical deformation that is measured in molecular cochlea also means knowing how it responds to acoustic scales. Yet the function of the proteins associated with the transients and not just to steady tones. It has proved difficult 28 genes that have to date been shown to affect hearing is to solve the equations that describe the cochlea efficiently really only understood in rudimentary terms. This applies for time varying inputs. Such solutions are particularly as much with structural proteins associated with hearing important for understanding otoacoustic emissions, the loss as with transcription factors, where elucidating the phenomenon of sound being re-emitted from the ear canal. cascade of signalling pathways is a prerequisite for under- These are developments for the future. standing cochlear development. Here, we concentrate on a few selected topics that have a bearing on how mammalian Molecules for mechano-electrical transduction hearing works. For more than a decade, the holy grail of hair cell physiology has been to identify the mechano-electric transducing channel Hair-cell function: a brief overview at the tips of hair cell . This channel is unique The mammalian cochlea is a fluid-filled duct, where the because of its distribution and gating by an extracellular tip separation of sound into component frequencies arises link [8••]. However, this distinct feature makes the assay of from the mechanical properties of the basilar membrane, a candidate genes particularly problematic, although new tech- collagenous membrane that bisects the duct. The basilar nologies offer promise for activating the channel [9•]. Taking membrane is a macroscopic structure but its mechanics are hints from other species, recent progress has been the identi- determined by the cellular properties of hair cells, the fication of a Drosophila mutant in which mechano-transduction outer hair cells (OHCs) in particular. A second class of hair in the sensory bristles is defective. The mutation occurs in a cells, the inner hair cells (IHCs), act as the primary sensory gene (nompC) coding a protein — named from ‘no mechano- cells of the cochlea and send their signals about the pattern receptor potential’ [10••] — which contains a large number of 450 Sensory systems

Figure 1

(a) A cross section of the cochlear duct (a) shows the organ of Corti, with the hair cells — IHCs and OHCs — riding upon the basilar IHC membrane (BM). A shear displacement from the tectorial membrane (TM) provides the TM stimulus for the stereocilia. (b) Hair cell forces. A complete cycle of upward and downward deflection of the basilar membrane occurs during one cycle of the sound wave OHC entering the ear. Upward deflection corresponds to a rarifaction of the sound DC pressure wave in the ear canal. Upward movement of the BM deflects OHC stereocilia to open mechano-electric- BM transduction (met) channels at the tips. Two possible feedback schemes, stereocilial and somatic, are possible. (c) Stereocilial forces: channel gating causes entry of ions that act on the met channel or associated proteins to generate force in the tip links and hence a bundle force (Fs) that augments the movement of the OHC [20•,21•]. (d) Somatic forces: voltage-gated conformational changes (b) (c) in the lateral membrane yield a cell-shortening Displacement force (Fb) in OHCs that is mediated either (i) by an area motor scheme with the motor molecules containing an intrinsic voltage •• F Met sensor [25,27 ] or (ii) by a flexing of the s channels Stereocilia plasma membrane [31], with the membrane surface charge altering the curvature. These feedback forces, being driven by membrane voltage, are delayed relative to the stereocilial displacement. As a result, both of these feedback schemes provide essentially positive feedback. In the case of somatic forces, the OHC Fb OHC feeds back longitudinal cell body forces on the basilar membrane through Deiters’ (d) (i) (ii) cells (DC). The ionic currents flowing though ACh the transducer channel exit through + IKn K -channels at the cell base (IKn [36] and • IKAch IKACh [44 ]).

DC Area Flexolectric model motor

Motion of BM

Current Opinion in Neurobiology

ankyrin repeats and which is a member of the TRP super- which the presence of molecular motors such as myosin family that contains many transmembrane signalling suggests that the stereocilial bundle behaves like a motile molecules. The ankyrin repeats could signify either binding structure. Of the 38 myosin isoforms now known from sites to putative intracellular structures or be part of the gating mammalian genomes, at least five are present in hair cells. structure (as found in other proteins such as latrotoxin). Some are clearly associated with the processes of Although there are several promising candidates for the adaptation to a mechanical stimulus [12], whereas others vertebrate transducer, some of which may even have the high determine the unique cellular organization around the calcium permeability of the hair cell transducer [11], the transducer. Myosin VIIa, the first to be associated with a search is still on. known genetic defect [13], appears to be involved in membrane trafficking at the apical membrane [14]. At least The mechano-electrical transduction step may also be a one myosin-linking protein has been described [15,16], source of mechanical amplification within the cochlea. suggesting the existance of a complex of myosins acting This suggestion comes from non-mammalian species in together in a hair cell. Can you still see the cochlea for the molecules? Ashmore and Mammano 451

The myosin associated with the transducer is myosin 1β: arguments have been used to show that another compo- this isoform is localised at the stereocilial tips. It is thought nent of the OHC membrane, a sugar transporter GLUT5, to mediate adaptation by adjusting tip-link tension is unlikely to be the primary component of the motor. [17,18•]. As part of a force generator, it could equally well move the stereocilia to feed back energy [19,20•]. The The question of how works to produce forces remains issue is whether the binding and unbinding of myosin to unanswered. The protein contains a so-called ‘sulphate- its actin substrate at the mechano-electric transducer site is transport motif’, although there is no evidence to indicate fast enough to work at the highest acoustic frequencies. An this function in hair cells. Genomic database searches have alternative possibility is that calcium entry through the identified prestin as being a member of a much larger super- transducer, a ‘mechano-enzyme’ in its own right, may gate family, SLC26A, of anion transporters, which includes a close the transducer and hence generate forces. In turtles, a fast relative ‘pendrin’ (SLC26A4), mutations in which are known component of bundle movement occurs on a sub-milli- to be responsible for a recessive hearing loss (DFNB4) [30]. second time scale when the stereocilia are deflected On the basis of superfamily membership, prestin (SLC26A5) [21•,22]: the newly discovered feature is the speed of the is likely to contain 12 transmembrane α-helices. This con- process. Similar results are also found in non-acoustic hair clusion now has support from experiments where amino- and cells [23]. Rapid force feedback is required as part of any carboxy-terminal regions have been tagged and found to be cochlear amplifier: it is a complex design problem to on the same side of the membrane. Such α-helices would ensure that the forces are correct in magnitude and sign all form a compact structure within the membrane, perhaps along the cochlear partition. For this reason, a force-gener- undergoing changes of area between two stable configura- ating step associated with electromotility of the cell body tions and thus generating forces. The intramembrane region seems most likely in the mammalian cochlea. is likely to be conserved between OHC species, with small differences at the carboxy-terminal end determining species- Molecules for electro-mechanical transduction specific targetting. As with cation transporters, the dipole of In the mammalian cochlea, OHCs act as part of a mechan- the motor molecule needs to be no more than an external ical feedback loop that interacts with cochlear mechanics. anion moving into a deep pore within the electric field of the A significant body of evidence now shows that OHCs membrane but no further. In this model, SLC26A5 is there- counteract the dissipative forces within the cochlea, and fore a membrane motor by virtue of being a transporter with result in enhanced sensitivity to sound, ‘the cochlear an incomplete transport cycle. amplifier’. An added bonus is that there is improved frequency selectivity, as each section of the basilar mem- There are other models for OHC electromotility that do brane acts locally like a high quality resonator with a much not depend explicitly on structural features of a protein. more restricted vibration pattern. This depends on the One of these models, based on data of lateral protein OHCs being able to generate forces along their lengths. diffusion in the OHC membrane, suggests that membrane As electromotile elements, OHCs can be driven voltage produces membrane curvature as a result of at frequencies >80 kHz [24•]. The underlying cellular unequal charge distribution, and is consistent with data processes depend on a molecular motor associated with the showing that membrane labels differentially diffuse [31]. OHC basolateral membrane. The motor mechanism has The choice between such OHC models will depend on many of the characteristics of biological piezoelectricity numerical predictions of each scheme. The OHC mem- [25,26]: a voltage generates a force; conversely, pulling on branes still seems to hold a number of surprises. the motor generates a voltage. Molecules to modulate hair cells in the cochlea There have been important steps taken recently towards The cochlea works like a mechanical spectrum analyser identifying this motor molecule. By constructing a differen- because the basilar membrane exhibits a stiffness gradient tial OHC library, separating out inner hair cells from OHCs along its length. This gradient ensures that different sound in the gerbil cochlea, Zheng et al. [27••] have identified a frequencies map onto different places. As well as this classic protein, termed ‘prestin’, that confers electromotile proper- ‘mechanical’ gradient, the geometry of the organ of Corti, the ties on transfected cells. Kidney cells expressing prestin size of the hair cell bodies and the length of the stereocilia all show the gating charge movements, or equivalently, a voltage vary systematically along the length of the cochlea. A dependent membrane capacitance, characteristic of OHCs. researcher who has tried to measure any membrane properties along the cochlea knows that cells from the basal end in par- A number of gaps in the evidence for ‘prestin’ have since ticular do not survive well in short term tissue culture. There been plugged. That expressed prestin exhibits a force hys- are, in addition, gradients of K+ channel expression in the teresis is shown in OHCs [28]. It has also been shown that mammalian cochlea, a situation that mirrors the case in turtle antibodies raised against gerbil prestin label the lateral [32•]. At least three types of K+ channel exist in mammalian membrane of OHCs [29•]. These latter data also demon- OHCs, the details of which may depend upon the species. strate that prestin, during development of the rat cochlea, is expressed with a time course that matches that of the One of the K+ currents, responsible for maintaining a large emergence of OHC electromotility. Such developmental resting potential in the hair cells, is the voltage-gated channel 452 Sensory systems

KCNQ4 [33•,34•]. This channel is expressed prominently in how the structure of individual molecules contribute to the OHCs and mutations result in a non-syndromic, autosomal fast processes that allow us to hear. We might also be on the dominant, progressive hearing loss (DFNA2 in humans). verge of understanding how, during development, cellular There is molecular and electrophysiological evidence that signalling cascades result in a device of the necessary and KCNQ4 expresses itself in a gradient along the cochlea [35]; organised complexity of the mammalian cochlea. one of the more intriguing issues is how this gradient, or indeed any cochlear gradient, might be set up. There is Update evidence that channel properties change during cochleo- Several important results on the molecular basis of OHC genesis [36] therefore a complex orchestration of channel motility have appeared since our Opinion article was type must take place during development. written. The OHC motor protein prestin has been identi- fied in rat [49] and shares considerable homology with the OHCs also contain a distinct K+ channel that is the target of prestin identified in gerbil where it was first identified by modulation by the efferent system. A long-standing Zheng et al. [27••]. The expression of the prestin clone conundrum in cochlear physiology has been the role of the allowed a clear demonstration, by tagging the amino-ter- cholinergic efferent system, the fibre pathway that termi- minal with an HA tag and the carboxy-terminal with a GFP nates on OHCs (and from separate nuclei, on IHC afferent tag, that prestin has an even number of transmembrane terminals). There has been a long-running debate as to helices with the ends both in the cytoplasm [50]. It thus whether efferents protect against noise damage [37], or shares much greater structural homology with the other even enhance signals in a noisy background. Postnatal members of the pendrin transporter family than originally de-efferentation does not significantly alter cochlear appeared to be the case. The mouse prestin has also been development [38] and molecular biological techniques sequenced in other laboratories and it seems it will not be have been unable to clarify the efferent function role. The long before we know what the consequences of knocking ACh receptor (AChR) on OHCs, α9 AChR, can be out the gene will be in an animal model. expressed in oocytes [39]. It is highly permeable to calcium [40,41] so that ACh-induced calcium rises can be detected In a second recent paper from same group, the mechanism of in OHCs [42]. Nevertheless, knockouts of the α9 AChR, action of the motor protein has been considerably clarified yield only a weak phenotype [43•]. The possibility that the [51]. As predicted from membership of the SLC26A super- native AChR is not a homomer but a heteromer of two family, prestin is a choride-bicarbonate transporter with an specialised cochlear receptor units, α9 and α10, leaves the incomplete transport cycle. Using both expressed rat prestin case unsettled until the double knockout is performed. (rPrestin) and patches from rat OHC lateral membrane, it has now been shown that the dipole moment of the OHC motor The final target of the intracellular calcium rise is a K+ chan- arises from intracellular anions entering a pore in the protein, nel presumed to be the small conductance channel SK2 and thereby crossing the membrane electric field presum- because of its sensitivity to apamin [44•]. This channel is ably distorting the protein. This is exemplified in Figure 1 functional early in development [45]. Activation of efferents scheme d(i). It is similar to an earlier proposal, based on by this route produces a hyperpolarization and could serve to charge relaxation times, that the motor was a cation trans- clamp OHCs at a hyperpolarized potential. However, more porter with a deep pore [52]. The rPrestin data show that subtle and slower effects of efferent activity can be detected chloride binds to prestin with a Km of 6 mM and bicarbonate as consequences of cytoskeletal alterations induced within binds with an affinity of 44 mM. The data also show that OHCs [46]. This ACh-dependant signalling pathway salicylate binds competitively to the intracellular site with an includes Rho and other small GTPases much admired by the affinity 200 times higher than chloride. This result provides cell biology laboratories [47]. As these signalling molecules a convincing explanation of why salicylate inhibits OHC are known to play a role in bundle morphogenesis [48], it motility; by implication it also provides a molecular explana- looks as though hair cells are about to receive the serious tion for the clinical observation that aspirin at sufficient doses attention of a larger cell biological community. reversibly reduces hearing sensitivity. The elegance of this simple scheme is a compelling reason to believe that we are Conclusions close to a molecular explanation of how the motor works. The emerging interfaces between the genetics, biophysical description of single sensory and non-sensory cells, and Acknowledgement cochlear models all indicate a growing complexity of auditory The work was supported in part by a Wellcome Trust Collaborative Grant. processing in the periphery. It seems likely that some of the molecular events that determine auditory transduction will References and recommended reading be resolved relatively soon but a detailed description of Papers of particular interest, published within the annual period of review, events occurring in cells at the high-frequency end of the have been highlighted as: cochlea is a much harder proposition: there, cellular events • of special interest •• of outstanding interest are occurring either at or near the temporal limits of most current measurement technologies. What we can hope for 1. Friedman T, Battey J, Kachar B, Riazuddin S, Noben-Trauth K, Griffith A, Wilcox E: Modifier genes of hereditary hearing loss. Curr over the next few years is a much deeper understanding of Opin Neurobiol 2000, 10:487-493. Can you still see the cochlea for the molecules? Ashmore and Mammano 453

2. Hemmert W, Zenner HP, Gummer AW: Three-dimensional motion 17. Garcia JA, Yee AG, Gillespie PG, Corey DP: Localization of myosin- • of the organ of Corti. Biophys J 2000, 78:2285-2297. Iβ near both ends of tip links in frog saccular hair cells. J Neurosci An elegant measurement of the motion components in an in situ preparation 1998, 18:8637-8647. of the organ of Corti. The paper describes both radial shear and perpendicu- lar motions of the tectorial membrane and organ of Corti are measured for the 18. Gillespie PG, Gillespie SK, Mercer JA, Shah K, Shokat KM: • β first time by combining position sensing measurements in three dimensions. Engineering of the myosin-l nucleotide-binding pocket to create selective sensitivity to N(6)-modified ADP analogs. J Biol Chem 3. Steel KP: A take on the tectorial membrane. Nat Genet 2000, 1999, 274:31373-31381. 24:104. Gillespie et al. use protein engineering to alter the ATP-binding pocket of myosin 1β with the intention of studying the involvement of this protein at 4. Legan PK, Lukashkina VA, Goodyear RJ, Kossi M, Russell IJ, stereocilial tips. This paper provides the best evidence so far that this myosin •• Richardson GP: A targeted deletion in alpha-tectorin reveals that is employed in transducer adaptation. the tectorial membrane is required for the gain and timing of cochlear feedback. Neuron 2000, 28:273-285. 19. Martin P, Hudspeth AJ: Active hair-bundle movements can amplify The authors combined a targetted deletion of α-tectorin, a major structural a hair cell’s response to oscillatory mechanical stimuli. Proc Natl component of the tectorial membrane, with measurement of cochlear Acad Sci USA 1999, 96:14306-14311. mechanics. They conclude that the structural mass of the tectorial mem- brane is plays a critical role in the macroscopic mechanics. This one of the 20. Martin P, Mehta AD, Hudspeth AJ: Negative hair-bundle stiffness • first papers where a gene only expressed in the cochlea is knocked out. betrays a mechanism for mechanical amplification by the hair cell. Proc Natl Acad Sci USA 2000, 97:12026-12031. 5. Nilsen KE, Russell IJ: The spatial and temporal representation of a The authors use a displacement-clamp system to measure the mechanical • tone on the guinea pig basilar membrane. Proc Natl Acad Sci USA properties of individual frog hair bundles. These demonstrate negative slope 2000, 97:11751-11758. stiffness, suggesting a system that can amplify mechanical stimuli, a mecha- The use of a specialized interferometer to probe in vivo the differences in nism that can be extrapolated to the mammalian cochlea. radial motion of the basilar membrane is described here. The nonlinear con- tribution of OHCs to basilar membrane movements is determined by the 21. Ricci AJ, Crawford AC, Fettiplace R: Active hair bundle motion • comparison of the pre- and post-mortem responses. linked to fast transducer adaptation in auditory hair cells. J Neurosci 2000, 20:7131-7142. 6. Nilsen KE, Russell IJ: Timing of cochlear feedback: spatial and Hair bundle motion in voltage-clamped turtle hair cells is described here in temporal representation of a tone across the basilar membrane. the search for a mechanical correlate of fast adaptation. The data show a Nat Neurosci 1999, 2:642-648. very fast component that was sensitive to external calcium concentration and abolished by blocking the transducer channels with dihydrostreptomycin. 7. Khanna SM, Hao LF: Nonlinear vibrations in the apex of guinea-pig cochlea. Int J Solids Struct 2001, 38:1919-1933. 22. Fettiplace R, Ricci AJ, Hackney CM: Clues to the cochlear amplifier from the turtle ear. Trends Neurosci 2001, 24:169-175. 8. Kachar B, Parakkal M, Kurc M, Zhao YD, Gillespie PG: High- •• resolution structure of hair-cell tip links. Proc Natl Acad Sci USA 23. Marquis RE, Hudspeth AJ: Effects of extracellular Ca2+ 2000, 97:13336-13341. concentration on hair-bundle stiffness and gating-spring integrity This paper provides spectacular high-resolution images of the tip link in hair in hair cells. Proc Natl Acad Sci USA 1997, 94:11923-11928. cell stereocilia, providing convincing evidence that the link is either a helical polymer or a braided pair of filamentous macromolecules, and is thus likely 24. Frank G, Hemmert W, Gummer AW: Limiting dynamics of high- • to be relatively stiff and inextensible. The properties of this protein may deter- frequency electromechanical transduction of outer hair cells. Proc mine some of the cochlear non-linearities. Natl Acad Sci USA 1999, 96:4420-4425. This study shows that isolated OHCs in the microchamber configuration 9. Langer MG, Koitschev A, Haase H, Rexhausen U, Horber JK, (i.e. sucked into the opening of a pipette and stimulated electrically) are able • Ruppersberg JP: Mechanical stimulation of individual stereocilia of to counteract fluid forces with almost constant displacement amplitude and living cochlear hair cells by atomic force microscopy. phase up to frequencies well above their place-frequency on the basilar Ultramicroscopy 2000, 82:269-278. membrane. This is a prerequisite for considering OHCs as part of a fast A novel, and the first, use of a custom-made atomic force microscope to image cochlear feedback loop. and measure some of the elastic properties of non-fixed hair cell stereocilia. 25. Iwasa KH: Effect of membrane motor on the axial stiffness of 10. Walker RG, Willingham AT, Zuker CS: A Drosophila the cochlear outer hair cell. J Acoust Soc Am 2000, •• mechanosensory transduction channel. Science 2000, 107:2764-2766. 287:2229-2234. 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The functional expression of the protein osmotically activated channel (VR-OAC), a candidate vertebrate confers electromotile properties on transfected kidney cells that are essen- osmoreceptor. Cell 2000, 103:525-535. tially indistinguishable from OHC motor properties. This paper was the first to positively identify the OHC motor. 12. Eatock RA: Adaptation in hair cells. Annu Rev Neurosci 2000, 23:285-314. 28. Santos Sacchi J, Shen WX, Zheng J, Dallos P: The outer hair cell lateral membrane motor, prestin, shows hysteresis. Biophys J 13. Hasson T: Molecular motors: sensing a function for myosin-VIIa. 2001, 80:346A. Curr Biol 1999, 9:R838-R841. 29. Belyantseva IA, Adler HJ, Curi R, Frolenkov GI, Kachar B: Expression 14. Richardson GP, Forge A, Kros CJ, Marcotti W, Becker D, Williams DS, • and localization of Prestin and the sugar transporter GLUT-5 Thorpe J, Fleming J, Brown SD, Steel KP: A missense mutation in during development of electromotility in cochlear outer hair cells. myosin VIIA prevents aminoglycoside accumulation in early J Neurosci 2000, 20:RC116 (1-5). postnatal cochlear hair cells. Ann New York Acad Sci 1999, This paper positively identifies, by immunohistochemistry, prestin in the 884:110-124. lateral membrane of OHCs. The results also show that several other proposed motor protein candidates are expressed progressively in OHCs 15. Bitner-Glindzicz M, Lindley KJ, Rutland P, Blaydon D, Smith VV, during cochlear development, but not with the appropriate time course. Milla PJ, Hussain K, Furth-Lavi J, Cosgrove KE, Shepherd RM et al.: A recessive contiguous gene deletion causing infantile 30. 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32. Jones EM, Gray-Keller M, Fettiplace R: The role of Ca2+-activated K+ cochlear cell line from the immortomouse. J Physiol 2000, • channel spliced variants in the tonotopic organization of the turtle 527:49-54. cochlea. J Physiol 1999, 518:653-665. A description of splice variants of the K(Ca) channels found in hair cells 42. Evans MG, Lagostena L, Darbon P, Mammano F: Cholinergic control 2+ taken from along the length of the turtle auditory papilla. The differential of membrane conductance and intracellular free Ca in outer expression of alternatively spliced α-subunit variants and a non-uniform hair cells of the guinea pig cochlea. Cell Calcium 2000, distribution of a β-subunit produces a range of K(Ca) channel properties 28:195-203. needed to explain the tonotopic organization of the turtle cochlea. 43. Vetter DE, Liberman MC, Mann J, Barhanin J, Boulter J, Brown MC, • α 33. Kharkovets T, Hardelin JP, Safieddine S, Schweizer M, El Amraoui A, Saffiote-Kolman J, Heinemann SF, Elgoyhen AB: Role of 9 nicotinic • Petit C, Jentsch TJ: KCNQ4, a K+ channel mutated in a form of ACh receptor subunits in the development and function of dominant deafness, is expressed in the inner ear and the central cochlear efferent innervation. Neuron 1999, 23:93-103. auditory pathway. Proc Natl Acad Sci USA 2000, 97:4333-4338. A knockout mouse study of the α9 AChR, the presumed target receptor of This study demonstrates that KCNQ4 is expressed widely in neurons of the the medial cochlear efferent system. The weak phenotype — with only small auditory pathway, as well as in OHCs at a site on the basolateral membrane changes in the axon terminal arborizations — may point to a second receptor where the dominant OHC K+ channel is found. It further identifies a gene yet to be identified. + that underlies two K currents, IKn and IKL, described in the cochlear and vestibular systems. 44. Oliver D, Klocker N, Schuck J, Baukrowitz T, Ruppersberg JP, • Fakler B: Gating of Ca2+-activated K+ channels controls fast 34. Kubisch C, Schroeder BC, Friedrich T, Lutjohann B, El Amraoui A, inhibitory synaptic transmission at auditory outer hair cells. • Marlin S, Petit C, Jentsch TJ: KCNQ4, a novel potassium channel Neuron 2000, 26:595-601. expressed in sensory outer hair cells, is mutated in dominant Here, a novel experimental system is used to study the efferent synapse onto deafness. Cell 1999, 96:437-446. OHCs. The study demonstrates that ACh acts by activating apamin-sensitive First identification of KCNQ4, and its association with a dominant form of small conductance channels (SK2). The experimental system uses a rat cochlear deafness, DFNA2. The K+ channel protein was identified fortuitously in the brain slice system where the hair cells and their innervation remain intact. cochlea as part of a larger search for K+ M-current expression. 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