A large-conductance calcium-selective mechanotransducer channel in mammalian cochlear hair cells.
ABSTRACT Sound stimuli are detected in the cochlea by opening of hair cell mechanotransducer (MT) channels, one of the few ion channels not yet conclusively identified at a molecular level. To define their performance in situ, we measured MT channel properties in inner hair cells (IHCs) and outer hair cells (OHCs) at two locations in the rat cochlea tuned to different characteristic frequencies (CFs). The conductance (in 0.02 mM calcium) of MT channels from IHCs was estimated as 260 pS at both low-frequency and mid-frequency positions, whereas that from OHCs increased with CFs from 145 to 210 pS. The combination of MT channel conductance and tip link number, assayed from scanning electron micrographs, accounts for variation in whole-cell current amplitude for OHCs and its invariance for IHCs. Channels from apical IHCs and OHCs having a twofold difference in unitary conductance were both highly calcium selective but were distinguishable by a small but significant difference in calcium permeability and in their response to lowering ionic strength. The results imply that the MT channel has properties possessed by few known candidates, and its diversity suggests expression of multiple isoforms.
- SourceAvailable from: Giun-Yi Hung[Show abstract] [Hide abstract]
ABSTRACT: Zebrafish lateral-line hair cells are an in vivo model for studying hair cell development, function, and ototoxicity. However, molecular identification and properties of the mechanotransducer (MET) channel in hair cells are still controversial. In this study, a noninvasive electrophysiological technique, the scanning ion-electrode technique (SIET), was applied for the first time to investigate properties of MET channels in intact zebrafish embryos. Using a Ca(2+)-selective microelectrode to deflect hair bundles and simultaneously record the Ca(2+) flux, the inward Ca(2+) flux was detected at stereocilia of hair cells in 2~4-days post-fertilization embryos. The Ca(2+) influx was blocked by MET channel blockers (BAPTA, La(3+), Gd(3+) and curare). In addition, 10 µM aminoglycoside antibiotics (neomycin and gentamicin) were found to effectively block the Ca(2+) influx within 10 min. Elevating the external Ca(2+) level (0.2 to 2 mM) neutralized the effects of neomycin and gentamicin. However, elevating the Mg(2+) level up to 5 mM neutralized the blockade of gentamicin but not neomycin. This study demonstrated MET channel-mediated Ca(2+) entry at hair cells and showed the SIET to be a sensitive approach for functionally assaying the MET channel in zebrafish.AJP Cell Physiology 09/2013; · 3.71 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Sound stimuli elicit movement of the stereocilia that make up the hair bundle of cochlear hair cells, putting tension on the tip links connecting the stereocilia and thereby opening mechanotransducer (MT) channels. Tmc1 and Tmc2, two members of the transmembrane channel-like family, are necessary for mechanotransduction. To assess their precise role, we recorded MT currents elicited by hair bundle deflections in mice with null mutations of Tmc1, Tmc2, or both. During the first postnatal week, we observed a normal MT current in hair cells lacking Tmc1 or Tmc2; however, in the absence of both isoforms, we recorded a large MT current that was phase-shifted 180°, being evoked by displacements of the hair bundle away from its tallest edge rather than toward it as in wild-type hair cells. The anomalous MT current in hair cells lacking Tmc1 and Tmc2 was blocked by FM1-43, dihydrostreptomycin, and extracellular Ca(2+) at concentrations similar to those that blocked wild type. MT channels in the double knockouts carried Ca(2+) with a lower permeability than wild-type or single mutants. The MT current in double knockouts persisted during exposure to submicromolar Ca(2+), even though this treatment destroyed the tip links. We conclude that the Tmc isoforms do not themselves constitute the MT channel but are essential for targeting and interaction with the tip link. Changes in the MT conductance and Ca(2+) permeability observed in the absence of Tmc1 mutants may stem from loss of interaction with protein partners in the transduction complex.The Journal of General Physiology 10/2013; · 4.73 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Tip links between adjacent stereocilia are believed to gate mechano-electrical transducer (MET) channels and mediate the electrical responses of sensory hair cells. We found that mouse auditory hair cells that lack tip links due to genetic mutations or exposure to the Ca(2+) chelator BAPTA can, however, still respond to mechanical stimuli. These MET currents have unusual properties and are predominantly of the opposite polarity relative to those measured when tip links are present. There are other striking differences, for example, the channels are usually all closed when the hair cell is not stimulated and the currents in response to strong stimuli can be substantially larger than normal. These anomalous MET currents can also be elicited early in development, before the onset of mechano-electrical transduction with normal response polarity. Current-voltage curves of the anomalous MET currents are linear and do not show the rectification characteristic of normal MET currents. The permeant MET channel blocker dihydrostreptomycin is two orders of magnitude less effective in blocking the anomalous MET currents. The findings suggest the presence of a large population of MET channels with pore properties that are distinct from those of normal MET channels. These channels are not gated by hair-bundle links and can be activated under a variety of conditions in which normal tip-link-mediated transduction is not operational.Journal of Neuroscience 04/2014; 34(16):5505-14. · 6.91 Impact Factor