Quantitative Relations between Outer Hair Cell Electromotility and Nonlinear Capacitance

The electrically evoked somatic motility of outer hair cells （OHC）, briefly termed OHC electromotility, plays a crucial role in cochlear amplification that underlies the remarkably high sensitivity and frequency selectivity of the mammalian hearing. Accompanying OHC electromotility is a voltage-dependent gating charge movement within the cell lateral membrane, manifested as a measurable nonlinear capacitance （NLC） in OHCs. The electromotility and NLC of OHCs are highly correlated by sharing a common molecular substrate, the motor protein prestin. In this study, we systematically characterized the quantitative relationship between OHC electromotility and NLC in their voltage dependences for the purpose of further understanding the electromechanical transduction in OHCs. The results demonstrated that the two possess differing voltage dependences with the V^2 of electromotility consistently being -20 mV depolarized in comparison with that of NLC although their slope factors a are statistically identical. Further investigations showed that the initial state of OHCs influences the voltage dependence of electromotility but not that of NLC, indicating that some biophysical factors other than the motor protein per se are involved in the process of OHC length changes. We proposed that the cytoskeletal spectrin-actin framework underneath the OHC plasma membrane and the cell＇ s turgor are the two most probable factors that cause the voltage-dependence discrepancy between OHC electromotility and NLC.

The role of Rho GTPase family in cochlear hair cells and hearing

Rho GTPases are essential regulators of the actin cytoskeleton.They are involved in various physiological and biochemical processes such as the regulation of cytoskeleton dynamics,development,proliferation,survival,and regeneration.During the development of cochlear hair cells,Rho GTPases are activated by various extracellular signals through membrane receptors to further stimulate multiple downstream effectors.Specifically,RhoA,Cdc42,and Rac1,members of the classical subfamily of the Rho GTPase family,regulate the development and maintenance of cilia by inducing the polymerization of actin monomers and stabilizing actin filaments.In addition,they also regulate the normal morphology orientation of ciliary bundles in auditory hair cells,which is an important element of cell polarity regulation.Moreover,the actin-related pathways mediated by RhoA,Cdc42,and Rac1 also play a role in the motility of outer hair cells,indicating that the function of Rho GTPases is crucial in the highly polar auditory sensory system.In this review,we focus on the expression of RhoA,Cdc42,and Rac1 in cochlear hair cells and how these small molecules participate in ciliary bundle morphogenesis and cochlear hair cell movement.We also discuss the progress of current research investigating the use of these small molecules as drug targets for deafness treatment.

Distribution of Prestin on Outer Hair Cell Basolateral Surface

Prestin has been identified as a motor protein responsible for outer hair cell (OHC) electromotility and is expressed on the OHC surface. Previous studies revealed that OHC electromotility and its associated nonlinear capacitance were mainly located at the OHC lateral wall and absent at the apical cuticular plate and the basal nucleus region. Immunofluorescent staining for prestin also failed to demonstrate prestin expression at the OHC basal ends in whole-mount preparation of the organ of Corti. However, there lacks a definitive demonstration of the pattern of prestin distribution. The OHC lateral wall has a trilaminate organization and is composed of the plasma membrane, cortical lattice, and subsurface cisternae. In this study, the location of prestin proteins in dissociated OHCs was examined using immunofluorescent staining and confocal microscopy. We found that prestin was uniformly expressed on the basolateral surface, including the basal pole. No staining was seen on the cuticular plate and stereocilia. When co-stained with a membrane marker di-8-ANEPPS, prestin-labeling was found to be in the outer layer of the OHC lateral wall. After separating the plasma membrane from the underlying subsurface cisternae using a hypotonic extracellular solution, prestin-labeling was found to be in the plasma membrane, not the subsurface cisternae. The data show that prestin is expressed in the plasma membrane on the entire OHC basolateral surface.

Prestin forms tetramer with each subunit being mechanically independent

Prestin is the motor protein of cochlear outer hair cells （OHCs）. It is able to perform rapid and reciprocal electromechanical conversion that underlies OHC electromotility. Due to the inadequate size of a single prestin molecule to form the2 nm intramembraneous protein particles （IMPs） in the OHC lateral membrane （LM）, the possibility of prestin oligomerization has been proposed. It has been suggested that prestin molecules form highorder oligomers, most likely as the tetramer, in heterologous systems. In OHCs, however, the oligomeric structure of prestin remains unclear. Here we calculated the prestin-related charge density in both gerbil and guinea pig OHCs through measuring their nonlinear capacitance （NLC） and LM surface area, showing that the average charge density （22, 608 μm-2 in gerbils; 19, 460 μm-2 in guinea pigs） is statistically 4 times the average density of IMPs （5,686 μm-2 in gerbils; 5, 000 μm-2 in guinea pigs）. This suggests that each IMP contains four prestin molecules based upon the notion that each prestin transfers a single elementary charge, implying that prestin forms tetramers in OHCs. To determine whether the prestin tetramer functions as a mechanical unit, we subsequently compared the slope factors （α） of electromotility and NLC simultaneously measured from the same OHC, showing that the α values of the two are statistically the same. This suggests that each prestin molecule in the tetramer is mechanically independent and equally contributes to OHC electromotility.

An Overview of Electrically Evoked Otoacoustic Emissions in the Mammalian Cochlea

Alternating currents injected into the cochlea are able to evoke outer hair cell-mediated basilar membrane motion, thus give rise to production of otoacoustic emissions. This electrically evoked otoacoustic emission(EEOAE) provides a useful tool for the research of out hair cell electromotility in vivo. This article reviews the research work on EEOAEs in mammals. Features of the EEOAEs and theories of their generation are introduced. Methods of EEOAE measurement are also described.

Direct current pulse elicits basilar membrane vibration in guinea pigs

Objective:To study the electromotility of the basilar membrane (BM) of guinea pigs in vivo. Methods :A pair of platinum-iridium wire electrodes were deposited into the holes drilled into the scala vestibuli and scala tympani on the basal turn of cochlea. The organ of Corti was stimulated with rectangular, constant current pulses . The displacement and velocity of BM were measured with laser doppler velocimeter. Results: The electrically elicited displacement of BM moved toward the scala where the electrode was positively charged. The waveform of BM displacement generally corresponded to the shape of the rectangular pulse of electric current. Ringing responses could be seen at the onset and offset of current pulse reflecting transient responses of the organ of Corti. In the cochlea of hearing-impaired or dead animal, direct current (DC) could still elicit a BM displacement but the ringing response was attenuated or disappeared. This phenomenon was probably due to metabolic disturbance in the damaged outer hair cells. In the sensitive cochlea, the BM vibration induced with direct current was similar to that induced by acoustic stimulation, and the BM moves in a traveling wave pattern. Conclusion: The findings of this experiment implicated that the DC stimulation of the cochlea conduces the contraction or elongation of OHCs. The electromotility of OHCs provides sufficient force to displace the BM. In the electrically stimulated normal cochlea,transient response of OHCs can induce resonant vibration at the same frequency as that Of the characteristic frequency (CF) of a partition in the BM. The vibration should be an active process of energy depletion associated with the cochlear amplifier.The vibration of BM can propagate to other partition of BM according to the traveling wave theory. This characteristic has laid the foundation for the electromotile hearing and electrically evoked otoacoustic emission.

Prestin-Mediated Frequency Selectivity Does not Cover Ultrahigh Frequencies in Mice

In mammals,the piezoelectric protein,Prestin,endows the outer hair cells(OHCs)with electromotility(eM),which confers the capacity to change cellular length in response to alterations in membrane potential.Together with basilarmembrane resonance and possible stereociliary motility,Prestin-based OHC eM lays the foundation for enhancing cochlear sensitivity and frequency selectivity.However,it remains debatable whether Prestin contributes to ultrahigh-frequency hearing due to the intrinsic nature of the cel's low-pass features.The low-pass_property of mouse OHC eM is based on the finding that eM magnitude dissipates within the frequency bandwidth of human speech.In this study,we examined the role of Prestin in sensing broad-range frequencies(4-80 kHz)in mice that use ultrasonic hearing and vocalization(to>100 kHz)for social communication.The audiometric measurements in mice showed that ablation of Prestin did not abolish hearing at frequencies>40 kHz.Acoustic associative behavior tests confirmed that Prestin-knockout mice can learn ultrahigh-frequency sound-coupled tasks,similar to control mice.Ex vivo cochlear Ca2+imaging experiments demonstrated that without Prestin,the OHCs still exhibit ultrahigh-frequency transduction,which in contrast,can be abolished by a universal cation channel blocker,Gadolinium.In vivo salicylate treatment disrupts hearing at frequencies<40 kHz but not ultrahigh-frequency hearing.By pharmacogenetic manipulation,we showed that specific ablation of the OHCs largely abolished hearing at frequencies>40 kHz.These findings demonstrate that cochlear OHCs are the target cells that support ultrahigh-frequency transduction,which does not require Prestin.