Excitation of fluorescent dyes inactivates the outer hair cell integral membrane motor protein prestin and betrays its lateral mobility

Maturation of Voltage-induced Shifts in SLC26a5 (Prestin) Operating Point during Trafficking and Membrane Insertion

Prestin (SLC26a5) is an integral membrane motor protein in outer hair cells (OHC) that underlies cochlear amplification. As a voltage-dependent protein, it relies on intrinsic sensor charge to respond to transmembrane voltage (receptor potentials), thereby effecting conformational changes. The protein's electromechanical actively is experimentally monitored as a bell-shaped nonlinear capacitance (NLC), whose magnitude peaks at a characteristic voltage, V_h. This voltage denotes the midpoint of prestin's charge-voltage (Q-V) Boltzmann distribution and region of maximum gain of OHC electromotility. It is an important factor in hearing capabilities for mammals. A variety of biophysical forces can influence the distribution of charge, gauged by shifts in V_h, including prior holding voltage or membrane potential. Here we report that the effectiveness of prior voltage augments during the delivery of prestin to the membranes in an inducible HEK cell line. The augmentation coincides with an increase in prestin density, maturing at a characteristic membrane areal density of 870 functional prestin units per square micrometer, and is likely indicative of prestin-prestin cooperative interactions.

Outer Hair Cell Lateral Wall Structure Constrains the Mobility of Plasma Membrane Proteins

Nature’s fastest motors are the cochlear outer hair cells (OHCs). These sensory cells use a membrane protein, Slc26a5 (prestin), to generate mechanical force at high frequencies, which is essential for explaining the exquisite hearing sensitivity of mammalian ears. Previous studies suggest that Slc26a5 continuously diffuses within the membrane, but how can a freely moving motor protein effectively convey forces critical for hearing? To provide direct evidence in OHCs for freely moving Slc26a5 molecules, we created a knockin mouse where Slc26a5 is fused with YFP. These mice and four other strains expressing fluorescently labeled membrane proteins were used to examine their lateral diffusion in the OHC lateral wall. All five proteins showed minimal diffusion, but did move after pharmacological disruption of membrane-associated structures with a cholesterol-depleting agent and salicylate. Thus, our results demonstrate that OHC lateral wall structure constrains the mobility of plasma membrane proteins and that the integrity of such membrane-associated structures are critical for Slc26a5’s active and structural roles. The structural constraint of membrane proteins may exemplify convergent evolution of cellular motors across species. Our findings also suggest a possible mechanism for disorders of cholesterol metabolism with hearing loss such as Niemann-Pick Type C diseases.

Nature’s fastest motor is the cochlear outer hair cell (OHC) in the mammalian inner ear. These cells can contract and elongate thousands of times per second. Slc26a5 (prestin) is the essential protein in the fast motor and resides in the plasma membrane of OHC lateral wall. Slc26a5 undergoes voltage-dependent conformational changes associated with the rapid changes in cell length to increase mammalian hearing sensitivity. However, it remains unclear how Slc26a5 transfers the force created to the entire cell. In this study, we show the importance of association between Slc26a5 and specialized membrane structures of the OHC lateral wall. Mobility of Slc26a5 was normally constrained in membrane-associated structures and disruption of these structures by a cholesterol depleting reagent and salicylate liberated Slc26a5 and four other heterologously expressed membrane proteins. These observations provide evidence that OHC lateral wall structure constrains the mobility of plasma membrane proteins and such membrane-associated structures are critical for Slc26a5’s functional roles. Our findings also shed light on other cellular motors across species and suggest a mechanism for cholesterol metabolic disorders in humans.

Membrane cholesterol strongly influences confined diffusion of prestin

Prestin is the membrane motor protein that drives outer hair cell (OHC) electromotility, a process that is essential for mammalian hearing. Prestin function is sensitive to membrane cholesterol levels, and numerous studies have suggested that prestin localizes in cholesterol-rich membrane microdomains. Previously, fluorescence recovery after photobleaching experiments were performed in HEK cells expressing prestin-GFP after cholesterol manipulations, and revealed evidence of transient confinement. To further characterize this apparent confined diffusion of prestin, we conjugated prestin to a photostable fluorophore (tetramethylrhodamine) and performed single-molecule fluorescence microscopy. Using single-particle tracking, we determined the microscopic diffusion coefficient from the full time course of the mean-squared deviation. Our results indicate that prestin undergoes diffusion in confinement regions, and that depletion of membrane cholesterol increases confinement size and decreases confinement strength. By interpreting the data in terms of a mathematical model of hop-diffusion, we quantified these cholesterol-induced changes in membrane organization. A complementary analysis of the distribution of squared displacements confirmed that cholesterol depletion reduces prestin confinement. These findings support the hypothesis that prestin function is intimately linked to membrane organization, and further promote a regulatory role for cholesterol in OHC and auditory function.

Organization of membrane motor in outer hair cells: an atomic force microscopic study

Using atomic force microscopy, we imaged the cytosolic surface of the lateral plasma membrane of outer hair cells from guinea pigs’ inner ear. We used a “cell-free” preparation, in which a patch of plasma membrane was firmly attached to a substrate and the cytoplasmic face was exposed. The membrane patches contained densely packed particles whose diameter, after correcting for the geometry of the probing tip, was ∼10 nm. The particles were predominantly aligned unidirectionally with spacing of ∼36 nm. The density of the particle was ∼850 μm⁻², which could be an underestimate presumably due to the method of sample preparation. Antibody-labeled specimens showed particles more elevated than unlabeled preparation indicative of primary and secondary antibody complexes. The corrected diameters of these particles labeled with anti-actin were ∼12 nm while that with antiprestin were ∼8 nm. The alignment pattern in antiprestin-labeled specimens resembled that of the unlabeled preparation. Specimens labeled with actin antibodies did not show such alignment. We interpret that the particles observed in the unlabeled membranes correspond to the 10-nm particles reported by electron microscopy and that these particles contain prestin, a member of the SLC26 family, which is essential for electromotility.

Prestin up-regulation in chronic salicylate (aspirin) administration: an implication of functional dependence of prestin expression

Salicylate (aspirin) can reversibly eliminate outer hair cell (OHC) electromotility to induce hearing loss. Prestin is the OHC electromotility motor protein. Here we report that, consistency with increase in distortion product otoacoustic emission, long-term administration of salicylate can increase prestin expression and OHC electromotility. The prestin expression at the mRNA and protein levels was increased by three- to four-fold. In contrast to the acute inhibition, the OHC electromotility associated charge density was also increased by 18%. This incremental increase was reversible. After cessation of salicylate administration, the prestin expression returned to normal. We also found that long-term administration of salicylate did not alter cyclooxygenase (Cox) II expression but down-regulated NF-kappaB and increased nuclear transcription factors c-fos and egr-1. The data suggest that prestin expression in vivo is dynamically up-regulated to increase OHC electromotility in long-term administration of salicylate via the Cox-II-independent pathways.

Probing conformational changes of prestin with thiol-reactive optical switches

Thiol-reactive optical switch probes were used to examine conformational changes of prestin-based membrane motor. Because this motor is based on mechanoelectric coupling similar to piezoelectricity, the motile activity can be monitored by charge movements across the plasma membrane, which appears as nonlinear capacitance. When the plasma membrane is conjugated with the probes, optically induced spiro-merocyanine transition positively shifted nonlinear capacitance of outer hair cells and prestin-transfected cells by approximately 10 mV. These shifts were reversible and were eliminated by pretreatment with iodoacetamide. However, they were little affected by pretreatment with biotin maleimide, which cannot reach the cytoplasmic surface. Our results showed that merocyanine states, with a larger dipole moment, interact with the motor's extended conformation stronger than with the compact conformation by 1.6 x 10(-21) J/molecule. The interaction sites are near the cytoplasmic side of the motor protein.

ATP activates P2x receptors and requires extracellular Ca(++) participation to modify outer hair cell nonlinear capacitance

Intracochlear ATP is an important mediator in regulating hearing function. ATP can activate ionotropic purinergic (P2x) and metabotropic purinergic (P2y) receptors to influence cell functions. In this paper, we report that ATP can activate P2x receptors directly to modify outer hair cell (OHC) electromotility, which is an active cochlear amplifier determining hearing sensitivity and frequency selectivity in mammals. We found that ATP, but not UTP, a P2y receptor agonist, reduced the OHC electromotility-associated nonlinear capacitance (NLC) and shifted its voltage dependence to the right (depolarizing) direction. Blockage of the activation of P2x receptors by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suramin, and 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) could block the ATP effect. This modification also required extracellular Ca(++) participation. Removal of extracellular Ca(++) abolished the ATP effect. However, chelation of intracellular Ca(++) concentration by a fast calcium-chelating reagent 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM) did not affect the effect of ATP on NLC. The effect is also independent of K(+) ions. Substitution of Cs(+) for intracellular or extracellular K(+) did not affect the ATP effect. Our findings indicate that ATP activates P2x receptors instead of P2y receptors to modify OHC electromotility. Extracellular Ca(++) is required for this modification.

Application of fluorescence recovery after photobleaching to study prestin lateral mobility in the human embryonic kidney cell

The transmembrane protein prestin is crucial to outer hair cell (OHC) electromotility and contributes to the sensitivity and frequency selectivity of mammalian hearing. The molecular mechanisms of electromotility remain unclear, but prestin is purported to function as both a voltage sensor and a molecular motor. Understanding the role of prestin requires characterizing its organization and behavior in the plasma membrane. Fluorescence recovery after photobleaching (FRAP) provides a powerful means to quantitatively study molecular diffusion. However, OHCs are inherently fragile ex vivo, and dynamic studies of prestin require model systems, such as human embryonic kidney (HEK) cells, expressing fluorescently labeled prestin. Utilizing this system, we provide the first direct, quantitative measurement of prestin lateral mobility. The results show remarkably different diffusion behavior for prestin-green fluorescent protein (GFP) as compared to a control protein, human somatostatin receptor 5 (SSTR5). Prestin-GFP FRAP experiments reveal immobile fractions approaching 50%, low effective diffusion coefficients, and recovery times slower than those of SSTR5. Secondary bleaching of a region reveals distinctly different diffusion parameters, which we propose reflect the transient confinement of prestin in the HEK cell. Although uncharacterized, intermolecular interactions between prestin and the membrane and/or cytoskeleton may be important for the unique properties of prestin in electromotile OHCs.

Prestin is expressed on the whole outer hair cell basolateral surface

Prestin has been identified as a motor protein responsible for outer hair cell (OHC) electromotility. Previous experiments revealed that OHC electromotility and its associated nonlinear capacitance resided in the OHC lateral wall and was not detected at the apical cuticular plate and basal region. In this experiment, the distribution of prestin in adult mouse, rat, and guinea pig OHCs was re-examined by use of immunofluorescent staining and confocal microscopy. We found that prestin labeling was located at the whole OHC basolateral wall, including the basal plasma membrane. However, staining at the basal membrane was weak. As compared with the intensity at the lateral wall, the intensities of prestin labeling at the membrane at the nuclear level and basal pole were 80.5% and 61.1%, respectively. Prestin labeling was not found at the cuticular plate and stereocilia. The prestin labeling was also absent in the cytoplasm and nuclei. The OHC lateral wall above the nuclear level is composed of the plasma membrane, cortical lattice, and subsurface cisternae. By co-staining with di-8-ANEPPS, prestin labeling was found at the outer layer of the OHC lateral wall, which was further evidenced by use of a hypotonic challenge to separate the plasma membrane from the underlying subsurface cisternae. The data revealed that prestin is expressed at the whole OHC basolateral membrane. Prestin in the basal plasma membrane may provide a reservoir on the OHC surface for prestin-recycling and may also facilitate performing its hypothesized transporter function.

Photometric recording of transmembrane potential in outer hair cells

Cochlear outer hair cells (OHCs) are polarized epithelial cells that have mechanoelectrical transduction channels within their apical stereocilia and produce electromotile force along their lateral wall. Phase shifts, or time delays, in the transmembrane voltage occurring at different axial locations along the cell may contribute to our understanding of how these cells operate at auditory frequencies. We developed a method to optically measure the phase of the OHC transmembrane potential using the voltage-sensitive dye (VSD) di-8-ANEPPS. The exit aperture of a fibre-optic light source was driven in two dimensions so that a 24 microm spot of excitation light could be positioned along the length of the OHC. We used the whole-cell patch-clamp technique in the current-clamp mode to stimulate the OHC at the base. The photometric response and the voltage response were monitored with a photodetector and patch-clamp amplifier, respectively. The photometric response was used to measure the regional changes in the membrane potential in response to maintained (dc) and sinusoidal (ac) current stimuli applied at the base of the cell. We used a neutral density filter to lower the excitation light intensity and reduce phototoxicity. A sensitive detector and lock-in amplifier were used to measure the small ac VSD signal. This permitted measurements of the ac photometric response below the noise floor of the static fluorescence. The amplitude and phase components of the photometric response were recorded for stimuli up to 800 Hz. VSD data at 400-800 Hz show the presence of a small phase delay between the stimulus voltage at the base of the cell and the local membrane potential measured along the lateral wall. Results are consistent with the hypothesis that OHCs exhibit inhomogeneous membrane potentials that vary with position in analogy with the voltage in nerve axons.

Lateral diffusion anisotropy and membrane lipid/skeleton interaction in outer hair cells

The organization of the plasma membrane of cells in lipid domains affects the way the membrane interacts with the underlying protein skeleton, which in turn affects the lateral mobility of lipid and protein molecules in the membrane. Membrane fluidity properties can be monitored by various approaches, the most versatile of which is fluorescence recovery after photobleaching (FRAP). We extended previous FRAP experiments on isolated cochlear outer hair cells (OHCs) by analyzing the two-dimensional pattern of lipid diffusion in the lateral membrane of these cells. We found that membrane lipid mobility in freshly isolated OHCs is orthotropic, diffusion being faster in the axial direction of the cell and slower in the circumferential direction. Increasing the cell's turgor pressure by osmotic challenge reduced the axial diffusion constant, but had only a slight effect on circumferential diffusion. Our results suggest that lipid mobility in the OHC plasma membrane is affected by the presence of the cell's orthotropic membrane skeleton. This effect could reflect interaction with spectrin filaments or with other membrane skeletal proteins. We also performed a number of FRAP measurements in temporal bone preparations preserving the structural integrity of the hearing organ. The diffusion rates measured for OHCs in this preparation were in good agreement with those obtained in isolated OHCs, and comparable to the mobility rates measured on the sensory inner hair cells. These observations support the idea that the plasma membranes of both types of hair cells share similar highly fluid phases in the intact organ. Lipid mobility was significantly slower in the membranes of supporting cells of the organ of Corti, which could reflect differences in lipid phase or stronger hindrance by the cytoskeleton in these membranes.

Gap junctional hemichannel-mediated ATP release and hearing controls in the inner ear

Connexin gap junctions play an important role in hearing function, but the mechanism by which this contribution occurs is unknown. Connexins in the cochlea are expressed only in supporting cells; no connexin expression occurs in auditory sensory hair cells. A gap junctional channel is formed by two hemichannels. Here, we show that connexin hemichannels in the cochlea can release ATP at levels that account for the submicromolar concentrations measured in the cochlear fluids in vivo. The release could be increased 3- to 5-fold by a reduction of extracellular Ca2+ or an increase in membrane stress, and blocked by gap junctional blockers. We also demonstrated that extracellular ATP at submicromolar levels apparently affected outer hair cell (OHC) electromotility, which is an active cochlear amplifier determining cochlear sensitivity to sound stimulation in mammals. ATP reduced OHC electromotility and the slope factor of the voltage dependence and shifted the operating point to reduce the active amplifier gain. ATP also reduced the generation of distortion products. Immunofluorescent staining showed that purinergic receptors P2x2 and P2x7 were distributed on the OHC surface. Blockage of P2 receptors eliminated the effect of ATP on the OHC electromotility. The data revealed that there is a hemichannel-mediated, purinergic intercellular signaling pathway between supporting cells and hair cells in the cochlea to control hearing sensitivity. The data also demonstrated a potential source of ATP in the cochlea.

New tunes from Corti's organ: the outer hair cell boogie rules

The amplification of acoustic stimuli is a feature of hair cells that evolved early on in vertebrates. Though standard stereocilia mechanisms to promote such amplification may persist in the mammal, an additional mechanism evolved to enhance high frequency sensation. Only in mammals, a special cell type, the outer hair cell, arose that possesses a remarkably fast somatic mechanical response, which probably endows the passive cochlea with a boost in sensitivity by a factor of 100 (40dB), at least. Experiments conducted over the past few years have shed light on many aspects of outer hair cell electromotility, including the molecular identification of the motor, the effects of a knockout, and underlying mechanisms of action. A review of this remarkable progress is attempted.