Electrically driven motor in the outer hair cell: Effect of a mechanical constraint

Abstract

The outer hair cell has a unique voltagedependent motility associated with charge transfer across the plasma membrane. To examine mechanical changes in the membrane that are coupled with such charge movements, we digested the undercoating of the membrane with trypsin. We inf lated the cell into a sphere and constrained the surface area by not allowing volume changes. We found that this constraint on the membrane area sharply reduced motor-associated charge movement across the membrane, demonstrating that charge transfer is directly coupled with membrane area change. This electromechanical coupling in the plasma membrane must be the key element for the motile mechanism of the outer hair cell.The outer hair cell changes the length of its cylindrical cell body in response to changes in the membrane potential (1-5), reaching a frequency of at least 20 kHz (6, 7). Because of its strategic location in the inner ear connecting the basilar membrane and the reticular lamina, this mechano-receptor cell presumably serves as an actuator in a feedback loop affecting vibrations of these tissues and thereby modulating the sensitivity of the mammalian ear (see refs. 8 and 9 for review). For this reason, the mechanism of this motility and its biological function have been a focus of intense research. For operating at high frequencies, a direct transduction mechanism would be advantageous. Indeed, the cell appears to be driven by a membrane-based motor that directly uses electrical energy available at the plasma membrane. The lateral membrane of the cell has charges that are transferable across the membrane (10-12), analogous to gating charges of voltage-gated ion transporters. These charges enable the cell to obtain electrical energy. If the electrical energy obtained by charge transfer across the membrane is directly converted into mechanical energy in a manner similar to piezoelectricity, the charge transfer must be reciprocally affected by an externally applied tension. That has indeed been experimentally verified (12-14), leading to estimates of the area change of a single motor unit (7,12,13).However, the simple characterization of the motor as described above is somewhat uncertain because of the structural complexity of the lateral membrane of the outer hair cell. The structural complexity obscures the molecular identity of the motor. In addition, the lateral membrane, where the motor resides, has an intricate structure of many elements (see ref. 15 for review) whose role in motility is uncertain. These factors leave uncertainties in the magnitude of tension that affects the motor.To characterize the elements of the motile mechanism in the plasma membrane, we examine a simplified system, in which the cortical cytoskeleton is digested with trypsin (13,16,17). The cells treated internally with trypsin can no longer maintain their ordinary cylindrical shape and become spherical when the internal pressure is elevated. The conspicuous length changes (up to 5%), which are elicited by changes in the membrane potential of ...

Evidence That Prestin Has at Least Two Voltage-dependent Steps

“…5G. If the diameter of monomeric prestin in the membrane were estimated to be 5ϳ7 nm as expected from the diameter of a potentially tetrameric prestin complex observed in the lateral membrane of OHCs (10ϳ14 nm (28)), the total unitary displacement of a single prestin molecule (0.20 ϩ 0.34) nm is expected to change the membrane area 3ϳ4 nm 2 , which agrees with the estimates by others (2ϳ4 nm 2 (29,30)). …”

Non-uniform mapping of stress-induced, motility-related charge movement in the outer hair cell plasma membrane

“…This phenomenon may play a role in reducing mechanically generated gating currents when the cell is collapsed from its cylindrical shape; however, from our data it appears that mechanically evoked gating currents are reduced in a graded fashion as intracellular pressure is decreased. Even more confounding is the observation that Q max derived from voltage stimulation appears to decrease as intracellular pressure is increased [14,22] until, under extreme conditions, charge movement can be blocked substantially [1]. Of course, these findings might suggest an opposite result from what we found.…”

Functional motor microdomains of the outer hair cell lateral membrane

“…We noted that motor sensitivity to membrane stress appeared greater after cortical cytoskeleton destruction with trypsin [14]. Adachi and Iwasa [1] quantified this sensitivity change by degree of V pkcm shift, and found that sensitivity was about six times greater than normal, i.e., 155 mV/kPa compared to normal whole cell values of 25 mV/kPa [2,14], which may account for their observed step-like C m decrease [1]. Our present data may provide an alternative explanation for the effects of stress on the whole cell as opposed to patch nonlinear capacitance.…”

“…It has been argued that changes in peak C m arise from membrane tension working through an anisotropic cytoskeleton or working on an anisotropic motor [9,13]; indeed, isolated patch recordings, where tension can be applied isotropically, show only a shift in V pkcm [9,16]. Nevertheless, even in OHCs that have become spherical due to intracellular trypsin treatment, and where applied tension should be isotropic, peak C m is altered by turgor pressure [1,14]. We noted that motor sensitivity to membrane stress appeared greater after cortical cytoskeleton destruction with trypsin [14].…”