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.

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.