Mutations in Myo7a cause hereditary deafness in mice and humans. We describe the effects of two mutations, Myo7a(6J) and Myo7a(4626SB), on mechano-electrical transduction in cochlear hair cells. Both mutations result in two major functional abnormalities that would interfere with sound transduction. The hair bundles need to be displaced beyond their physiological operating range for mechanotransducer channels to open. Transducer currents also adapt more strongly than normal to excitatory stimuli. We conclude that myosin VIIA participates in anchoring and holding membrane-bound elements to the actin core of the stereocilium. Myosin VIIA is therefore required for the normal gating of transducer channels.

We quanti¢ed the molecular energies and forces involved in opening and closing of mechanoelectrical transducer channels in hair cells using a novel generally applicable method. It relies on a thermodynamic description of the free energy of an ion channel in terms of its open probability. The molecular gating force per channel as re£ected in hair bundle mechanics is shown to equal kT/I(X ) Â dI(X )/dX, where I is the transducer current and X the de£ection of the hair bundle. We applied the method to previously measured I(X ) curves in mouse outer hair cells (OHCs) and vestibular hair cells (VHCs). Contrary to current models of transduction, gating of the transducer channel was found to involve only a ¢nite range of free energy (510 kT), a consequence of our observation that the channel has a ¢nite minimum open probability of ca. 1% for inhibitory bundle de£ections. The maximum gating forces per channel of both cell types were found to be comparable (ca. 300^500 f N). Because of di¡erences in passive restoring forces, gating forces result in very limited mechanical nonlinearity in OHC bundles compared to that in VHC bundles. A kinetic model of channel activation is proposed that accounts for the observed transducer currents and gating forces. It also predicts adaptation-like e¡ects and spontaneous bundle movements ensuing from changes in state energy gaps possibly related to interactions of the channel with calcium ions.

The gating-spring theory of hair cell mechanotransduction channel activation was first postulated over twenty years ago. The basic tenets of this hypothesis have been reaffirmed in hair cells from both auditory and vestibular systems and across species. In fact, the basic findings have been reproduced in every hair cell type tested. A great deal of information regarding the structural, mechanical, molecular and biophysical properties of the sensory hair bundle and the mechanotransducer channel has accumulated over the past twenty years. The goal of this review is to investigate new data, using the gating spring hypothesis as the framework for discussion. Mechanisms of channel gating are presented in reference to the need for a molecular gating spring or for tethering to the intra-or extracellular compartments. Dynamics of the sensory hair bundle and the presence of motor proteins are discussed in reference to passive contributions of the hair bundle to gating compliance. And finally, the molecular identity of the channel is discussed in reference to known intrinsic properties of the native transducer channel.

Cupular vibration in the lateral-line canal of fish was measured in response to motion of the fluid in the canal by laser-heterodyne interferometry. The results show that the mechanical output/input ratio of the cupula depends on the stimulus amplitude; the cupula thus behaves nonlinearly. The nonlinearity is due to the hair bundles, since it disappears when the cupula is uncoupled from the underlying hair cells. A model ofcupular dynamics in which the behavior of the gating springs of the hair cells is incorporated predicts nonlinear responses that are similar to the measurements, suggesting that the nonlinear behavior of the cupula may be attributed to the opening and closing of the transduction channels of the hair cells.Vertebrates utilize sensory organs with hair cells to detect a variety of mechanical stimuli from their environment. Examples of these are the auditory, vestibular, and lateral-line systems. The characteristic organelles of hair cells are the stereocilia, pivoting around their insertion points in the cuticular plate that forms the apical end of the cell. Deflection of the stereocilia opens and closes the mechanoelectrical transduction channels (for reviews see refs. 1-3).The channels responsible for gating the transduction current are located somewhere near the tips of the hair bundles (4, 5) and are possibly associated with the mechanical links connecting the tips of the individual stereocilia in a hair bundle (6). Deflection of the hair bundle would produce mechanical stress in these tip links and could therefore open a transduction channel attached to it.Corey and Hudspeth (7) proposed a two-state model of the kinetics of transduction channels in frog saccular hair cells, in which it is assumed that the free energy difference between the two states varies linearly with the deflection of the hair bundle, so that deflection in the hair bundle's depolarizing direction increasingly favors the population of open states. An elastic element governed by Hooke's law exhibits the required free energy relationship with displacement. Howard and Hudspeth (8) considered the mechanics of such elastic elements (gating springs) positioned between the stereocilia, like the tip links, and predicted a hair bundle stiffness that varies with hair bundle deflection, having a minimum at a bundle position at which half of the transduction channels are open. This predicted nonlinearity in stiffness was confirmed by measurements on isolated frog saccular hair cells (8).Comparable nonlinearities in stiffness of hair cell bundles were observed in cochlear cultures (9).In the lateral-line canals of fish, the bundles of the sensory hair cells are attached to the base of a cupula, a dome-shaped structure. Water motion around the animal is coupled to the canal fluid, which displaces the cupula, thus producing hair bundle deflection. The dynamics of the cupula has been investigated in vivo by laser-heterodyne interferometry (10, 11), and it was found that the displacement of the cupula in response to canal fluid mo...

The mechanical frequency selectivity of the cupula located in the supraorbital lateral line canal and the frequency selectivity of the hair cells driven by the cupula were measured simultaneously in vivo. Laser interferometry was used to measure cupular mechanics and extracellular receptor potentials were recorded to determine hair cell frequency selectivity. Results were obtained from two teleost fish species, the ruffe (Acerina cernua L.), a European temperate zone freshwater fish, and the tropical African knife fish (Xenomiystus nigri). In both species cupular displacement grows with increasing frequency of canal fluid displacement, reaching a maximum at 115 Hz in the ruffe and at 460 Hz in the African knife fish. Cupular best frequencies were independent of temperature. Cut-off frequencies of hair cell frequency selectivity were found to depend on temperature with a Q10 of 1.75, ranging from 116 Hz (4 degrees C) to 290 Hz (20 degrees C), as established in the ruffe. At normal habitat temperatures of the two fish species (ruffe, 4 degrees C; African knife fish, 28 degrees C), this results in hair cell cut-off frequencies that match the two different cupular best frequencies remarkably well. This match suggests adjusted signal transfer in these two peripheral stages of canal lateral line transduction.

Amiloride is a known blocker of the mechano-electrical transduction current in sensory hair cells. Measurements of cupular motion in the lateral line organ of ¢sh now show that amiloride concurrently changes the micromechanical properties of the hair cell bundles. The e¡ects of amiloride on the mechanics and receptor potentials of the hair cells resemble those previously observed for the aminoglycoside drug dihydrostreptomycin (DHSM) and are similarly antagonized by Ca 2+ . We hypothesize that amiloride and DHSM act on hair cells in two correlated ways, which manifest themselves in both the electrical and mechanical properties of the transduction process. One action is the reduction of the transduction current with a concurrent increase in hair bundle sti¡ness. The other action is a shift of the hair cell's operating point on a current^displacement curve, with a concomitant shift along the associated hair bundle sti¡ness^displacement curve. The latter action has the opposite e¡ect to that of the ¢rst and thus may lead, at relatively low blocker concentrations, to both an increase of transduction current and a decrease in hair bundle sti¡ness.

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In vivo measurements of the motion of the skin covering the supraorbital lateral line canal of the ruffe are presented. Stimulation of the lateral line with local oscillatory water motion evokes displacement of the skin which exhibits a characteristic node-antinode pattern along the length of the canal. Frequency responses of the skin show tuning with resonance frequencies comparable with the 'best frequencies' of the cupula, suggesting that cupular dynamics is involved in the generation of the node-antinode pattern. Indeed, the experiments show that the node-antinode pattern is caused by the presence of the cupulae in the canal. A hydrodynamic model describing the dynamics of the skin is presented. The model accounts for many of the observed phenomena, and leads to the conclusion that the motion of the skin reflects the micromechanical properties of the sensory bundles of the hair cells underlying the cupula.