Calcium entry does not drive fast mechanotransduction adaptation in cochlear hair cells

Abstract

37Sound detection in auditory sensory hair cells depends on the deflection of the 38 stereocilia hair bundle, which opens mechano-electric transduction (MET) channels. 39 Adaptation is hypothesized to be a critical property of MET that contributes to the wide 40 dynamic range and sharp frequency selectivity of the auditory system. Historically, 41 adaptation was hypothesized to have multiple mechanisms, all of which require calcium 42 entry through MET channels. Our recent work using a stiff probe to displace hair bundles 43 showed that the fastest adaptation mechanism (fast adaptation) does not require 44 calcium entry. Using a fluid-jet stimulus, others obtained data showing only a calcium-45 dependent fast adaptation response. Here, we identified the source of this discrepancy. 46Because the hair cell response to a hair bundle stimulus depends critically on the 47 magnitude and time course of the hair bundle deflection, we developed a high-speed 48 imaging technique to quantify this deflection. The fluid jet delivers a force stimulus, and 49 step-like force stimuli lead to a complex time course of hair bundle displacement 50 (mechanical creep), which affects the hair cell's macroscopic MET current response by 51 masking the time course of the fast adaptation response. Modifying the fluid-jet stimulus 52to generate a step-like hair bundle displacement produced rapidly adapting currents that 53 did not depend on membrane potential. This indicated that fast adaptation does not 54 depend on calcium entry. We also confirmed the presence of a calcium-dependent slow 55 adaptation process. These results confirm the existence of multiple adaptation 56 processes: a fast adaptation that is not driven by calcium entry and a slower calcium-57 dependent process. 58 Significance Statement: 59 3Mechanotransduction by sensory hair cells represents a key first step for the sound 60 sensing ability in vertebrates. The sharp frequency tuning and wide dynamic range of 61 sound sensation are hypothesized to require a mechanotransduction adaptation 62 mechanism. For decades, it had been accepted that all adaptation mechanisms require 63 calcium entry into hair cells. However, more recent work indicated that the apparent 64 calcium dependence of the fastest adaptation differs with the method of cochlear hair 65 cell stimulation. Here, we reconcile existing data and show that calcium entry does not 66 drive the fastest adaptation process, independent of the stimulation method. 67 Introduction: 68 Inner ear hair bundles comprise an array of graded length stereocilia that are 69 organized in a staircase manner. Positive hair bundle deflection increases tension in a 70 filamentous tip link that connects adjacent rows of stereocilia (1-3). Changes in tip-link 71 tension result in the gating of mechano-electric transduction (MET) ion channels. 72 MET adaptation is hypothesized to extend the dynamic range of the cell, control 73 the channel's operating point, and filter incoming stimuli (4-6). Adaptation is identified 74 by two phenom...