Changes in plasma membrane structure and electromotile properties in prestin deficient outer hair cells

Two new mouse alleles of Ocm and Slc26a5

The genes Ocm (encoding oncomodulin) and Slc26a5 (encoding prestin) are expressed strongly in outer hair cells and both are involved in deafness in mice. However, it is not clear if they influence the expression of each other. In this study, we characterise the auditory phenotype resulting from two new mouse alleles, Ocmtm1e and Slc26a5tm1Cre. Each mutation leads to absence of detectable mRNA transcribed from the mutant allele, but there was no evidence that oncomodulin regulates expression of prestin or vice versa. The two mutants show distinctive patterns of auditory dysfunction. Ocmtm1e homozygotes have normal auditory brainstem response thresholds at 4 weeks old followed by progressive hearing loss starting at high frequencies, while heterozygotes show largely normal thresholds until 6 months of age, when signs of worse thresholds are detected. In contrast, Slc26a5tm1Cre homozygotes have stable but raised thresholds across all frequencies tested, 3 to 42 kHz, at least from 4 to 8 weeks old, while heterozygotes have raised thresholds at high frequencies. Distortion product otoacoustic emissions and cochlear microphonics show deficits similar to auditory brainstem responses in both mutants, suggesting that the origin of hearing impairment is in the outer hair cells. Endocochlear potentials are normal in the two mutants. Scanning electron microscopy revealed normal development of hair cells in Ocmtm1e homozygotes but scattered outer hair cell loss even at 4 weeks old when thresholds appeared normal, indicating that there is not a direct relationship between numbers of outer hair cells present and auditory thresholds.

The pathogenic roles of the p.R130S prestin variant in DFNB61 hearing loss

DFNB61 is a recessively inherited nonsyndromic hearing loss caused by mutations in SLC26A5, the gene that encodes the voltage-driven motor protein, prestin. Prestin is abundantly expressed in the auditory outer hair cells that mediate cochlear amplification. Two DFNB61-associated SLC26A5 variants, p.W70X and p.R130S, were identified in patients who are compound heterozygous for these nonsense and missense changes (SLC26A5W70X/R130S ). Our recent study showed that mice homozygous for p.R130S (Slc26a5R130S/R130S ) suffer from hearing loss that is ascribed to significantly reduced motor kinetics of prestin. Given that W70X-prestin is nonfunctional, compound heterozygous Slc26a5R130S/- mice were used as a model for human SLC26A5W70X/R130S . By examining the pathophysiological consequences of p.R130S prestin when it is the sole allele for prestin protein production, we determined that this missense change results in progressive outer hair cell loss in addition to its effects on prestin's motor action. Thus, this study defines the pathogenic roles of p.R130S prestin and identifies a limited time window for potential clinical intervention. KEY POINTS: The voltage-driven motor protein, prestin, is encoded by SLC26A5 and expressed abundantly in cochlear outer hair cells (OHCs). The importance of prestin for normal hearing was demonstrated in mice lacking prestin; however, none of the specific SLC26A5 variants identified to date in human patients has been experimentally demonstrated to be pathogenic. In this study we used both cell lines and a mouse model to define the pathogenic role of compound heterozygous p.W70X (c.209G>A) and p.R130S (c.390A>C) SLC26A5 variants identified in patients with moderate to profound hearing loss. As in patients, mice carrying one copy of p.R130S Slc26a5 showed OHC dysfunction and progressive degeneration, which results in congenital progressive hearing loss. This is the first functional study reporting pathogenic SLC26A5 variants and pointing to the presence of a therapeutic time window for potential clinical interventions targeting the affected OHCs before they are lost.

The Piezo channel is a mechano-sensitive complex component in the mammalian inner ear hair cell

The inner ear is the hub where hair cells (HCs) transduce sound, gravity, and head acceleration stimuli to the brain. Hearing and balance rely on mechanosensation, the fastest sensory signals transmitted to the brain. The mechanoelectrical transducer (MET) channel is the entryway for the sound-balance-brain interface, but the channel-complex composition is not entirely known. Here, we report that the mouse utilizes Piezo1 (Pz1) and Piezo2 (Pz2) isoforms as MET-complex components. The Pz channels, expressed in HC stereocilia, and cell lines are co-localized and co-assembled with MET complex partners. Mice expressing non-functional Pz1 and Pz2 at the ROSA26 locus have impaired auditory and vestibular traits that can only be explained if the Pzs are integral to the MET complex. We suggest that Pz subunits constitute part of the MET complex and that interactions with other MET complex components yield functional MET units to generate HC MET currents.

The conformational cycle of prestin underlies outer-hair cell electromotility

The voltage-dependent motor protein prestin (also known as SLC26A5) is responsible for the electromotive behaviour of outer-hair cells and underlies the cochlear amplifier1. Knockout or impairment of prestin causes severe hearing loss2-5. Despite the key role of prestin in hearing, the mechanism by which mammalian prestin senses voltage and transduces it into cellular-scale movements (electromotility) is poorly understood. Here we determined the structure of dolphin prestin in six distinct states using single-particle cryo-electron microscopy. Our structural and functional data suggest that prestin adopts a unique and complex set of states, tunable by the identity of bound anions (Cl- or SO42-). Salicylate, a drug that can cause reversible hearing loss, competes for the anion-binding site of prestin, and inhibits its function by immobilizing prestin in a new conformation. Our data suggest that the bound anion together with its coordinating charged residues and helical dipole act as a dynamic voltage sensor. An analysis of all of the anion-dependent conformations reveals how structural rearrangements in the voltage sensor are coupled to conformational transitions at the protein-membrane interface, suggesting a previously undescribed mechanism of area expansion. Visualization of the electromotility cycle of prestin distinguishes the protein from the closely related SLC26 anion transporters, highlighting the basis for evolutionary specialization of the mammalian cochlear amplifier at a high resolution.

Expression of a membrane-targeted fluorescent reporter disrupts auditory hair cell mechanoelectrical transduction and causes profound deafness

The reporter mT/mG mice expressing a membrane-targeted fluorescent protein are becoming widely used to study the auditory and vestibular system due to its versatility. Here we show that high expression levels of the fluorescent mtdTomato reporter affect the function of the sensory hair cells and the auditory performance of mT/mG transgenic mice. Auditory brainstem responses and distortion product otoacoustic emissions revealed that adult mT/mG homozygous mice are profoundly deaf, whereas heterozygous mice present high frequency loss. We explore whether this line would be useful for studying and visualizing the membrane of auditory hair cells by airyscan super-resolution confocal microscopy. Membrane localization of the reporter was observed in hair cells of the cochlea, facilitating imaging of both cell bodies and stereocilia bundles without altering cellular architecture or the expression of the integral membrane motor protein prestin. Remarkably, hair cells from mT/mG homozygous mice failed to uptake the FM1-43 dye and to locate TMC1 at the stereocilia, indicating defective mechanotransduction machinery. Our work emphasizes that precautions must be considered when working with reporter mice and highlights the potential role of the cellular membrane in maintaining functional hair cells and ensuring proper hearing.

Transcription co-factor LBH is necessary for the survival of cochlear hair cells

Hearing loss affects ∼10% of adults worldwide. Most sensorineural hearing loss is caused by the progressive loss of mechanosensitive hair cells (HCs) in the cochlea. The molecular mechanisms underlying HC maintenance and loss remain poorly understood. LBH, a transcription co-factor implicated in development, is abundantly expressed in outer hair cells (OHCs). We used Lbh-null mice to identify its role in HCs. Surprisingly, Lbh deletion did not affect differentiation and the early development of HCs, as nascent HCs in Lbh knockout mice had normal looking stereocilia. The stereocilia bundle was mechanosensitive and OHCs exhibited the characteristic electromotility. However, Lbh-null mice displayed progressive hearing loss, with stereocilia bundle degeneration and OHC loss as early as postnatal day 12. RNA-seq analysis showed significant gene enrichment of biological processes related to transcriptional regulation, cell cycle, DNA damage/repair and autophagy in Lbh-null OHCs. In addition, Wnt and Notch pathway-related genes were found to be dysregulated in Lbh-deficient OHCs. Our study implicates, for the first time, loss of LBH function in progressive hearing loss, and demonstrates a critical requirement of LBH in promoting HC survival in adult mice.

The effect of AP-2δ on transcription of the Prestin gene in HEI-OC1 cells upon oxidative stress

Background

The study aimed to investigate the effect of oxidative stress on Prestin expression, and explore the transcription factors (TFs) that are involved in regulating the expression of Prestin in House Ear Institute-Organ of Corti 1 (HEI-OC1) cells upon oxidative stress.

Methods

Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were used to detect the expression level of Prestin. Reverse chromatin immunoprecipitation (reverse ChIP) assay was performed to identify proteins that could bind to the Prestin gene. Small interfering RNA (siRNA) and chromatin immunoprecipitation (ChIP) experiments were used to further verify the results. HEI-OC1 cells were incubated with four different concentrations of tert-butyl hydroperoxide (t-BHP) for 24 h or 48 h to construct the oxidative stress model.

Results

Oxidative stress induced Prestin increase at the mRNA level but with a concomitant decrease at the protein level. TF activating enhancer binding protein-2δ (AP-2δ) screened by reverse ChIP assay was demonstrated to bind to transcriptional start site 1441 of the Prestin promoter region and negatively regulate the expression of Prestin by siRNA and ChIP experiments. Furthermore, AP-2δ was down-regulated under oxidative stress.

Conclusions

In conclusion, oxidative stress inhibits the expression of Prestin protein, and the transcription mechanism is triggered to compensate for the loss of Prestin protein. AP-2δ is one of the important TFs that suppresses transcription of the Prestin gene, and AP-2δ suppression further boosted Prestin mRNA activation under oxidative stress.

Electronic supplementary material

The online version of this article (10.1186/s11658-019-0170-0) contains supplementary material, which is available to authorized users.

Deletion of Limk1 and Limk2 in mice does not alter cochlear development or auditory function

Inherited hearing loss is associated with gene mutations that result in sensory hair cell (HC) malfunction. HC structure is defined by the cytoskeleton, which is mainly composed of actin filaments and actin-binding partners. LIM motif-containing protein kinases (LIMKs) are the primary regulators of actin dynamics and consist of two members: LIMK1 and LIMK2. Actin arrangement is directly involved in the regulation of cytoskeletal structure and the maturation of synapses in the central nervous system, and LIMKs are involved in structural plasticity by controlling the activation of the actin depolymerization protein cofilin in the olfactory system and in the hippocampus. However, the expression pattern and the role of LIMKs in mouse cochlear development and synapse function also need to be further studied. We show here that the Limk genes are expressed in the mouse cochlea. We examined the morphology and the afferent synapse densities of HCs and measured the auditory function in Limk1 and Limk2 double knockout (DKO) mice. We found that the loss of Limk1 and Limk2 did not appear to affect the overall development of the cochlea, including the number of HCs and the structure of hair bundles. There were no significant differences in auditory thresholds between DKO mice and wild-type littermates. However, the expression of p-cofilin in the DKO mice was significantly decreased. Additionally, no significant differences were found in the number or distribution of ribbon synapses between the DKO and wild-type mice. In summary, our data suggest that the Limk genes play a different role in the development of the cochlea compared to their role in the central nervous system.

Simultaneous zygotic inactivation of multiple genes in mouse through CRISPR/Cas9-mediated base editing

In vivo genetic mutation has become a powerful tool for dissecting gene function; however, multi-gene interaction and the compensatory mechanisms involved can make findings from single mutations, at best difficult to interpret, and, at worst, misleading. Hence, it is necessary to establish an efficient way to disrupt multiple genes simultaneously. CRISPR/Cas9-mediated base editing disrupts gene function by converting a protein-coding sequence into a stop codon; this is referred to as CRISPR-stop. Its application in generating zygotic mutations has not been well explored yet. Here, we first performed a proof-of-principle test by disrupting Atoh1, a gene crucial for auditory hair cell generation. Next, we individually mutated vGlut3 (Slc17a8), otoferlin (Otof) and prestin (Slc26a5), three genes needed for normal hearing function. Finally, we successfully disrupted vGlut3, Otof and prestin simultaneously. Our results show that CRISPR-stop can efficiently generate single or triple homozygous F0 mouse mutants, bypassing laborious mouse breeding. We believe that CRISPR-stop is a powerful method that will pave the way for high-throughput screening of mouse developmental and functional genes, matching the efficiency of methods available for model organisms such as Drosophila.

Prestin Contributes to Membrane Compartmentalization and Is Required for Normal Innervation of Outer Hair Cells

Outer hair cells (OHC) act as amplifiers and their function is modified by medial olivocochlear (MOC) efferents. The unique OHC motor protein, prestin, provides the molecular basis for somatic electromotility, which is required for sensitivity and frequency selectivity, the hallmarks of mammalian hearing. Prestin proteins are the major component of the lateral membrane of mature OHCs, which separates apical and basal domains. To investigate the contribution of prestin to this unique arrangement, we compared the distribution of membrane proteins in OHCs of wildtype (WT) and prestin-knockout (KO) mice. In WT, the apical protein PMCA2 was exclusively localized to the hair bundles, while it was also found at the lateral membrane in KOs. Similarly, a basal protein KCNQ4 did not coalesce at the base of OHCs but was widely dispersed in mice lacking prestin. Since the expression levels of PMCA2 and KCNQ4 remained unchanged in KOs, the data indicate that prestin is required for the normal distribution of apical and basal membrane proteins in OHCs. Since OHC synapses predominate in the basal subnuclear region, we also examined the synaptic architecture in prestin-KO mice. Although neurite densities were not affected, MOC efferent terminals in prestin-KO mice were no longer constrained to the basal pole as in WT. This trend was evident as early as at postnatal day 12. Furthermore, terminals were often enlarged and frequently appeared as singlets when compared to the multiple clusters of individual terminals in WT. This abnormality in MOC synaptic morphology in prestin-KO mice is similar to defects in mice lacking MOC pathway proteins such as α9/α10 nicotinic acetylcholine receptors and BK channels, indicating a role for prestin in the proper establishment of MOC synapses. To investigate the contribution of prestin’s electromotility, we also examined OHCs from a mouse model that expresses non-functional prestin (499-prestin). We found no changes in PMCA2 localization and MOC synaptic morphology in OHCs from 499-prestin mice. Taken together, these results indicate that prestin, independent of its motile function, plays an important structural role in membrane compartmentalization, which is required for the formation of normal efferent-OHC synapses in mature OHCs.

Characterization of Hair Cell-Like Cells Converted From Supporting Cells After Notch Inhibition in Cultures of the Organ of Corti From Neonatal Gerbils

The senses of hearing and balance depend upon hair cells, the sensory receptors of the inner ear. Hair cells transduce mechanical stimuli into electrical activity. Loss of hair cells as a result of aging or exposure to noise and ototoxic drugs is the major cause of noncongenital hearing and balance deficits. In the ear of non-mammals, lost hair cells can spontaneously be replaced by production of new hair cells from conversion of supporting cells. Although supporting cells in adult mammals have lost that capability, neonatal supporting cells are able to convert to hair cells after inhibition of Notch signaling. We questioned whether Notch inhibition is sufficient to convert supporting cells to functional hair cells using electrophysiology and electron microscopy. We showed that pharmacological inhibition of the canonical Notch pathway in the cultured organ of Corti prepared from neonatal gerbils induced stereocilia formation in supporting cells (defined as hair cell-like cells or HCLCs) and supernumerary stereocilia in hair cells. The newly emerged stereocilia bundles of HCLCs were functional, i.e., able to respond to mechanical stimulation with mechanotransduction (MET) current. Transmission electron microscopy (TEM) showed that HCLCs converted from pillar cells maintained the pillar cell shape and that subsurface cisternae, normally observed underneath the cytoskeleton in outer hair cells (OHCs), was not present in Deiters’ cells-derived HCLCs. Voltage-clamp recordings showed that whole-cell currents from Deiters’ cells-derived HCLCs retained the same kinetics and magnitude seen in normal Deiters’ cells and that nonlinear capacitance (NLC), an electrical hallmark of OHC electromotility, was not detected from any HCLCs measured. Taken together, these results suggest that while Notch inhibition is sufficient for promoting stereocilia bundle formation, it is insufficient to convert neonatal supporting cells to mature hair cells. The fact that Notch inhibition led to stereocilia formation in supporting cells and supernumerary stereocilia in existing hair cells appears to suggest that Notch signaling may regulate stereocilia formation and stability during development.

Hair Cell Transduction, Tuning, and Synaptic Transmission in the Mammalian Cochlea

Sound pressure fluctuations striking the ear are conveyed to the cochlea, where they vibrate the basilar membrane on which sit hair cells, the mechanoreceptors of the inner ear. Recordings of hair cell electrical responses have shown that they transduce sound via submicrometer deflections of their hair bundles, which are arrays of interconnected stereocilia containing the mechanoelectrical transducer (MET) channels. MET channels are activated by tension in extracellular tip links bridging adjacent stereocilia, and they can respond within microseconds to nanometer displacements of the bundle, facilitated by multiple processes of Ca2+-dependent adaptation. Studies of mouse mutants have produced much detail about the molecular organization of the stereocilia, the tip links and their attachment sites, and the MET channels localized to the lower end of each tip link. The mammalian cochlea contains two categories of hair cells. Inner hair cells relay acoustic information via multiple ribbon synapses that transmit rapidly without rundown. Outer hair cells are important for amplifying sound-evoked vibrations. The amplification mechanism primarily involves contractions of the outer hair cells, which are driven by changes in membrane potential and mediated by prestin, a motor protein in the outer hair cell lateral membrane. Different sound frequencies are separated along the cochlea, with each hair cell being tuned to a narrow frequency range; amplification sharpens the frequency resolution and augments sensitivity 100-fold around the cell's characteristic frequency. Genetic mutations and environmental factors such as acoustic overstimulation cause hearing loss through irreversible damage to the hair cells or degeneration of inner hair cell synapses. © 2017 American Physiological Society. Compr Physiol 7:1197-1227, 2017.

Membrane prestin expression correlates with the magnitude of prestin-associated charge movement

Full expression of electromotility, generation of non-linear capacitance (NLC), and high-acuity mammalian hearing require prestin function in the lateral wall of cochlear outer hair cells (OHCs). Estimates of the number of prestin molecules in the OHC membrane vary, and a consensus has not emerged about the correlation between prestin expression and prestin-associated charge movement in the OHC. Using an inducible prestin-expressing cell line, we demonstrate that the charge density, but not the voltage at peak capacitance, directly correlates with the amount of prestin in the plasma membrane. This correlation is evident in studies involving a controlled increase of prestin expression with time after induction and inducer dose-response. Conversely, membrane prestin levels and charge density gradually decline together following the reduction of prestin levels from a steady state by removal of the inducer. Thus, charge density directly correlates with the level of membrane prestin expression, whereas changing membrane levels of prestin have no effect on the voltage at peak capacitance in this inducible prestin-expressing cell line.

Susceptibility of outer hair cells to cholesterol chelator 2-hydroxypropyl-β-cyclodextrine is prestin-dependent

Niemann-Pick type C1 disease (NPC1) is a fatal genetic disorder caused by impaired intracellular cholesterol trafficking. Recent studies reported ototoxicity of 2-hydroxypropyl- β-cyclodextrin (HPβCD), a cholesterol chelator and the only promising treatment for NPC1. Because outer hair cells (OHCs) are the only cochlear cells affected by HPβCD, we investigated whether prestin, an OHC-specific motor protein, might be involved. Single, high-dose administration of HPβCD resulted in OHC death in prestin wildtype (WT) mice whereas OHCs were largely spared in prestin knockout (KO) mice in the basal region, implicating prestin's involvement in ototoxicity of HPβCD. We found that prestin can interact with cholesterol in vitro, suggesting that HPβCD-induced ototoxicity may involve disruption of this interaction. Time-lapse analysis revealed that OHCs isolated from WT animals rapidly deteriorated upon HPβCD treatment while those from prestin-KOs tolerated the same regimen. These results suggest that a prestin-dependent mechanism contributes to HPβCD ototoxicity.

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.

Glutamate transporter homolog-based model predicts that anion-π interaction is the mechanism for the voltage-dependent response of prestin

Prestin is the motor protein of cochlear outer hair cells. Its unique capability to perform direct, rapid, and reciprocal electromechanical conversion depends on membrane potential and interaction with intracellular anions. How prestin senses the voltage change and interacts with anions are still unknown. Our three-dimensional model of prestin using molecular dynamics simulations predicts that prestin contains eight transmembrane-spanning segments and two helical re-entry loops and that tyrosyl residues are the structural specialization of the molecule for the unique function of prestin. Using site-directed mutagenesis and electrophysiological techniques, we confirmed that residues Tyr(367), Tyr(486), Tyr(501), and Tyr(508) contribute to anion binding, interacting with intracellular anions through novel anion-π interactions. Such weak interactions, sensitive to voltage and mechanical stimulation, confer prestin with a unique capability to perform electromechanical and mechanoelectric conversions with exquisite sensitivity. This novel mechanism is completely different from all known mechanisms seen in ion channels, transporters, and motor proteins.

Reduction in noise-induced functional loss of the cochleae in mice with pre-existing cochlear dysfunction due to genetic interference of prestin

Various cochlear pathologies, such as acoustic trauma, ototoxicity and age-related degeneration, cause hearing loss. These pre-existing hearing losses can alter cochlear responses to subsequent acoustic overstimulation. So far, the knowledge on the impacts of pre-existing hearing loss caused by genetic alteration of cochlear genes is limited. Prestin is the motor protein expressed exclusively in outer hair cells in the mammalian cochlea. This motor protein contributes to outer hair cell motility. At present, it is not clear how the interference of prestin function affects cochlear responses to acoustic overstimulation. To address this question, a genetic model of prestin dysfunction in mice was created by inserting an internal ribosome entry site (IRES)-CreERT2-FRT-Neo-FRT cassette into the prestin locus after the stop codon. Homozygous mice exhibit a threshold elevation of auditory brainstem responses with large individual variation. These mice also display a threshold elevation and a shift of the input/output function of the distortion product otoacoustic emission, suggesting a reduction in outer hair cell function. The disruption of prestin function reduces the threshold shifts caused by exposure to a loud noise at 120 dB (sound pressure level) for 1 h. This reduction is positively correlated with the level of pre-noise cochlear dysfunction and is accompanied by a reduced change in Cdh1 expression, suggesting a reduction in molecular responses to the acoustic overstimulation. Together, these results suggest that prestin interference reduces cochlear stress responses to acoustic overstimulation.

Prestin at year 14: progress and prospect

Prestin, the motor protein of cochlear outer hair cells, was identified 14 years ago. Prestin-based outer hair cell motility is responsible for the exquisite sensitivity and frequency selectivity seen in the mammalian cochlea. Prestin is the 5th member of an eleven-member membrane transporter superfamily of SLC26A proteins. Unlike its paralogs, which are capable of transporting anions across the cell membrane, prestin primarily functions as a motor protein with unique capability of performing direct and reciprocal electromechanical conversion on microsecond time scale. Significant progress in the understanding of its structure and the molecular mechanism has been made in recent years using electrophysiological, biochemical, comparative genomics, structural bioinformatics, molecular dynamics simulation, site-directed mutagenesis and domain-swapping techniques. This article reviews recent advances of the structural and functional properties of prestin with focus on the areas that are critical but still controversial in understanding the molecular mechanism of how prestin works: The structural domains for voltage sensing and interaction with anions and for conformational change. Future research directions and potential application of prestin are also discussed. This article is part of a Special Issue entitled <Annual Reviews 2014>.

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.

Normal hearing sensitivity at low-to-middle frequencies with 34% prestin-charge density

The mammalian outer hair cells (OHCs) provide a positive mechanical feedback to enhance the cochlea's hearing sensitivity and frequency selectivity. Although the OHC-specific, somatic motor protein prestin is required for cochlear amplification, it remains unclear whether prestin can provide sufficient cycle-by-cycle feedback. In cochlear mechanical modeling, varying amounts of OHC motor activity should provide varying degrees of feedback efficiency to adjust the gain of cochlear amplifier at resonant frequencies. Here we created and characterized two new prestin-hypomorphic mouse models with reduced levels of wild-type prestin. OHCs from these mice exhibited length, total elementary charge movement (Q_max), charge density, and electromotility intermediate between those of wild-type and prestin-null mice. Remarkably, measurements of auditory brainstem responses and distortion product otoacoustic emissions from these mice displayed wild-type like hearing sensitivities at 4–22 kHz. These results indicate that as low as 26.7% Q_max, 34.0% charge density and 44.0% electromotility in OHCs were sufficient for wild-type-like hearing sensitivity in mice at 4–22 kHz, and that these in vitro parameters of OHCs did not correlate linearly with the feedback efficiency for in vivo gain of the cochlear amplifier. Our results thus provide valuable data for modeling cochlear mechanics and will stimulate further mechanistic analysis of the cochlear amplifier.

Prestin in HEK cells is an obligate tetramer

The unusual membrane motor protein prestin is essential for mammalian hearing and for the survival of cochlear outer hair cells. While prestin has been demonstrated to be a homooligomer, by Western blot and FRET analyses, the stoichiometry of self association is unclear. Prestin, coupled to the enhanced green fluorescent protein, was synthesized and membrane targeted in human embryonic kidney cells by plasmid transfection. Fragments of membrane containing immobilized fluorescent molecules were isolated by osmotic lysis. Diffraction-limited fluorescent spots consistent in size with single molecules were observed. Under continuous excitation, the spots bleached to background in sequential and approximately equal-amplitude steps. The average step count to background levels was 2.7. A binomial model of prestin oligomerization indicated that prestin was most likely a tetramer, and that a fraction of the green fluorescent protein molecules was dark. As a positive control, the same procedure was applied to cells transfected with plasmids coding for the human cyclic nucleotide-gated ion channel A3 subunit (again coupled to the enhanced green fluorescent protein), which is an obligate tetramer. The average step count for this molecule was also 2.7. This result implies that in cell membranes prestin oligomerizes to a tetramer.

Engineered pendrin protein, an anion transporter and molecular motor

Pendrin and prestin both belong to a distinct anion transporter family called solute carrier protein 26A, or SLC26A. Pendrin (SLC26A4) is a chloride-iodide transporter that is found at the luminal membrane of follicular cells in the thyroid gland as well as in the endolymphatic duct and sac of the inner ear, whereas prestin (SLC26A5) is expressed in the plasma membrane of cochlear outer hair cells and functions as a unique voltage-dependent motor. We recently identified a motif that is critical for the motor function of prestin. We questioned whether it was possible to create a chimeric pendrin protein with motor capability by integrating this motility motif from prestin. The chimeric pendrin was constructed by substituting residues 160-179 in human pendrin with residues 156-169 from gerbil prestin. Non-linear capacitance and somatic motility, two hallmarks representing prestin function, were measured from chimeric pendrin-transfected human embryonic kidney 293 cells using the voltage clamp technique and photodiode-based displacement measurement system. We showed that this 14-amino acid substitution from prestin was able to confer pendrin with voltage-dependent motor capability despite the amino acid sequence disparity between pendrin and prestin. The molecular mechanism that facilitates motor function appeared to be the same as prestin because the motor activity depended on the concentration of intracellular chloride and was blocked by salicylate treatment. Radioisotope-labeled formate uptake measurements showed that the chimeric pendrin protein retained the capability to transport formate, suggesting that the gain of motor function was not at the expense of its inherent transport capability. Thus, the engineered pendrin was capable of both transporting anions and generating force.

Evidence that prestin has at least two voltage-dependent steps

Prestin is a voltage-dependent membrane-spanning motor protein that confers electromotility on mammalian cochlear outer hair cells, which is essential for normal hearing of mammals. Voltage-induced charge movement in the prestin molecule is converted into mechanical work; however, little is known about the molecular mechanism of this process. For understanding the electromechanical coupling mechanism of prestin, we simultaneously measured voltage-dependent charge movement and electromotility under conditions in which the magnitudes of both charge movement and electromotility are gradually manipulated by the prestin inhibitor, salicylate. We show that the observed relationships of the charge movement and the physical displacement (q-d relations) are well represented by a three-state Boltzmann model but not by a two-state model or its previously proposed variant. Here, we suggest a molecular mechanism of prestin with at least two voltage-dependent conformational transition steps having distinct electromechanical coupling efficiencies.

Review series: The cell biology of hearing

Mammals have an astonishing ability to sense and discriminate sounds of different frequencies and intensities. Fundamental for this process are mechanosensory hair cells in the inner ear that convert sound-induced vibrations into electrical signals. The study of genes that are linked to deafness has provided insights into the cell biological mechanisms that control hair cell development and their function as mechanosensors.

Prestin forms oligomer with four mechanically independent subunits

Prestin is the motor protein of cochlear outer hair cells (OHCs) with the unique capability of performing direct, rapid, and reciprocal electromechanical conversion. Prestin consists of 744 amino acids with a molecular mass of approximately 81.4 kDa. The predicted membrane topology and molecular mass of a single prestin molecule appear inadequate to account for the size of intramembrane particles (IMPs) expressed in the OHC membrane. Although recent biochemical evidence suggests that prestin forms homo-oligomers, most likely as a tetramer, the oligomeric structure of prestin in OHCs remains unclear. We obtained the charge density of prestin in the gerbil OHCs by measuring their nonlinear capacitance (NLC). The average charge density (22,608 microm(-2) measured was four times the average IMP density (5686 microm(-2) reported in the freeze-fracture study. This suggests that each IMP contains four prestin molecules, based on the general notion that each prestin transfers a single elementary charge. We subsequently compared the voltage dependency and the values of slope factor of NLC and somatic motility simultaneously measured from the same OHCs to determine whether NLC and motility are fully coupled and how prestin subunits function within the tetramer. We showed that the voltage dependency and slope factors of NLC and motility were not statistically different, suggesting that NLC and motility are fully coupled. The fact that the slope factor is the same between NLC and motility suggests that each prestin monomer in the tetramer is in parallel, each interacting independently with cytoplasmic or other partners to facilitate the mechanical response.