Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, C04
Transducer-transporter coupling in the cochlear sensory cells
P. Mistrik1,2, N. Daudet1, J. Ashmore1,2
1. Ear Institute, UCL, London, United Kingdom. 2. Department of Neuroscience, Physiology and Pharmacology, UCL, London, United Kingdom.
Prestin (SLC26A5) is a molecular actuator found in mammalian cochlear outer hair cells (OHCs) and implicated in the amplification of sound in the inner ear. One of the issues has been that OHCs need to serve as a universal power source over a broad frequency range, and it has often been claimed that the receptor potentials will be so attenuated by the membrane electrical filtering that other mechanisms need to be invoked at high acoustic frequencies. To address this point we have used a large-scale computational model of cochlear current flow to analyse the transmembrane potentials in OHCs when the cells are embedded in the intact cochlea (Mistrik P et al., 2009). This in silico model has allowed us to conclude that the known gradient in OHC conductance between cochlear base and apex effectively counteracts the single-cell membrane capacitance at high frequencies. The calculated attenuation of the OHC receptor potential was 6 dB/decade (instead of 21 dB/decade) in the presence (or absence, respectively) of the gradient. This low attenuation suggests that forces generated by OHCs can indeed provide power input over the whole auditory frequency range. A further consideration of the contribution of prestin to cochlear function was guided by the analysis of its amino acid sequence. Prestin belongs to a family SLC26 of solute carriers, members of which exchange halides for SO42- or HCO3-. To determine if prestin is also a bicarbonate transporter, we linked prestin to super-ecliptic pHluorin to provide a sensitive fluorescence probe of intracellular pH (pHin) near the plasma membrane. Prestin was expressed heterologously in CHO cells. Measuring the initial rate of the pHin recovery from the CO2-induced acidification showed that pHin recovered 4 times faster in cells transfected with prestin, but only in the presence of extracellular HCO3-. Such acceleration required extracellular Cl- to be lowered to 2 mM to establish a gradient to drive HCO3- into the cell. Such a mechanism was simulated by a computational model of CO2- HCO3- exchange. The transport process was significantly reduced by extracellular application of 10 mM salicylate, an agent which inhibits OHC forces. These data therefore suggest that prestin does act as a weak HCO3-/ Cl- antiporter, effects anticipated to be much greater in OHCs than in CHO cells as a result of the 30x higher copy number for prestin. The proposal suggests that intracellular chloride in OHCs may be under metabolic control. These results indicate that, in addition to participating in wide band cochlear sound amplification, prestin may also be involved in many slower time scale (>10s) cochlear phenomena where changes in OHC stiffness and turgor pressure have been implicated.
Where applicable, experiments conform with Society ethical requirements