Calcium entry into stereocilia drives adaptation of the mechanoelectrical transducer current of mammalian cochlear hair cells

Abstract

Mechanotransduction in the auditory and vestibular systems depends on mechanosensitive ion channels in the stereociliary bundles that project from the apical surface of the sensory hair cells. In lower vertebrates, when the mechanoelectrical transducer (MET) channels are opened by movement of the bundle in the excitatory direction, Ca 2+ entry through the open MET channels causes adaptation, rapidly reducing their open probability and resetting their operating range. It remains uncertain whether such Ca 2+ -dependent adaptation is also present in mammalian hair cells. Hair bundles of both outer and inner hair cells from mice were deflected by using sinewave or step mechanical stimuli applied using a piezo-driven fluid jet. We found that when cochlear hair cells were depolarized near the Ca 2+ reversal potential or their hair bundles were exposed to the in vivo endolymphatic Ca 2+ concentration (40 μM), all manifestations of adaptation, including the rapid decline of the MET current and the reduction of the available resting MET current, were abolished. MET channel adaptation was also reduced or removed when the intracellular Ca 2+ buffer 1,2-Bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) was increased from a concentration of 0.1 to 10 mM. The findings show that MET current adaptation in mouse auditory hair cells is modulated similarly by extracellular Ca 2+ , intracellular Ca 2+ buffering, and membrane potential, by their common effect on intracellular free Ca 2+ .H earing and balance depend on the transduction of mechanical stimuli into electrical signals. This process depends on the opening of mechanoelectrical transducer (MET) channels located at the tips of the shorter of pairs of adjacent stereocilia (1), which are specialized microvilli-like structures that form the hair bundles that project from the upper surface of hair cells (2,3). Deflection of hair bundles in the excitatory direction (i.e., toward the taller stereocilia) stretches specialized linkages, the tip-links, present between adjacent stereocilia (3-5), opening the MET channels. In hair cells from lower vertebrates, open MET channels reclose during constant stimuli via an initial fast adaptation mechanism followed by a much slower, myosin-based motor process, both of which are driven by Ca 2+ entry through the channel itself (6-13). In mammalian auditory hair cells, MET current adaptation seems to be mainly driven by the fast mechanism (14-16), although the exact process by which it occurs is still largely unknown. The submillisecond speed associated with the adaptation kinetics of the MET channels in rat and mouse cochlear hair cells (17,18) indicates that Ca 2+ , to cause adaptation, has to interact directly with a binding site on the channel or via an accessory protein (16). However, a recent investigation on rat auditory hair cells has challenged the view that Ca 2+ entry is required for fast adaptation, and instead proposed an as-yetundefined mechanism involving a Ca 2+ -independent reduction in the viscoelastic force of ele...

Neuroplastin Isoform Np55 Is Expressed in the Stereocilia of Outer Hair Cells and Required for Normal Outer Hair Cell Function

“…Nptn encodes a transmembrane protein that is a member of the Ig superfamily and is expressed in two isoforms generated by alternative splicing of the primary transcript (Beesley et al, 2014). The longer Np65 isoform contains three extracellular Ig domains, while the shorter Np55 isoform lacks the N-terminal Ig1 domain (Fig.…”

“…In contrast, OHCs act as amplifiers that increase the motion of the organ of Corti (Fettiplace and Hackney, 2006). Somatic electromotility, length changes in the soma of OHCs driven by hair cell depolarization (Fettiplace and Hackney, 2006;Ashmore, 2008), and hair bundle motility, driven by the gating of the transducer channels (Fettiplace and Hackney, 2006;, are both thought to contribute to amplification.…”

“…In a N-ethyl-N-nitrosaurea (ENU) mutagenesis screen, we have now generated a deaf mouse line, audio-1, which carries a recessive mutation in the neuroplastin gene (Nptn), a member of the Ig superfamily that is expressed in two isoforms (Beesley et al, 2014). The longer Np65 isoform contains three extracellular Ig domains and is expressed in the CNS (Beesley et al, 2014).…”

“…The longer Np65 isoform contains three extracellular Ig domains and is expressed in the CNS (Beesley et al, 2014). Recent studies suggest that Np65 may regulate the properties of synapses connecting IHCs with spiral ganglion neurons (SGNs; Carrott et al, 2016).…”

Homeostatic enhancement of sensory transduction

“…To minimize its effects on the dynamical response of the bundle to sinusoidal stimulation, the homeostatic mechanism operates slowly. Because experimental manipulations have been shown to affect adaptation, homeostatic feedback is applied to the models' adaptation mechanisms, although other choices are possible (31)(32)(33). Additional motivation for the homeostatic mechanisms is provided in Discussion.…”