Intracellular anions as the voltage sensor of prestin, the outer hair cell motor protein

Infrared light stimulates the cochlea through a mechanical displacement detected and amplified by hair cells

Although cochlear implants (CI) are the standard of care for profound sensorineural hearing loss they are technically constrained by the tendency of electrical current to spread within the fluid-filled chambers of the cochlea. This limits the resolution of individual electrodes and patients' perceptions of complex sounds. Infrared irradiation has been proposed as an alternative to electrical stimulation because it can elicit auditory responses while being spatially constrained, theoretically promising higher-fidelity hearing for the deaf. However, conflicting reports locate the site of infrared excitation at spiral ganglia neurons or hair cells. We use a combination of genetic, pharmacological, optical, and electrophysiological tools to determine the site of action of infrared irradiation. Infrared-evoked cochlear potentials are composed of two peaks: one driven by hair cells (the microphonic) and a second driven by spiral ganglion neurons (the neural response). Manipulations that prevented hair cell synaptic activity abolished the neural component, while manipulations blocking hair cell mechanotransduction abolished all responses, suggesting a mechanical component to the infrared response. Optical coherence tomography (OCT) confirmed that infrared irradiation creates a mechanical stimulus that is both amplified and detected by hair cells. Because infrared irradiation does not stimulate spiral ganglion neurons directly, it is unlikely to replace the electrical CI.

Deciphering enhancers of hearing loss genes for efficient and targeted gene therapy of hereditary deafness

Hereditary hearing loss accounts for about 60% of congenital deafness. Although adeno-associated virus (AAV)-mediated gene therapy shows substantial potential for treating genetic hearing impairments, there remain significant concerns regarding the specificity and safety of AAV vectors. The sophisticated nature of the cochlea further complicates the challenge of precisely targeting gene delivery. Here, we introduced an AAV-reporter-based in vivo transcriptional enhancer reconstruction (ARBITER) workflow, enabling efficient and reliable dissection of enhancers. With ARBITER, we successfully demonstrated that the conserved non-coding elements (CNEs) within the gene locus collaboratively regulate the expression of Slc26a5, which was further validated using knockout mouse models. We also assessed the potential of identified enhancers to treat hereditary hearing loss by conducting gene therapy in Slc26a5 mutant mice. Based on the original Slc26a5 enhancer with limited efficiency, we engineered a highly efficient and outer hair cell (OHC)-specific enhancer, B8, which successfully restored hearing of Slc26a5 knockout mice.

Affinity purification of the outer hair cell motor protein prestin using His-tag

OBJECTIVE

The high sensitivity and broad frequency selectivity of mammalian hearing are associated with the somatic motility of outer hair cells (OHCs) in the cochlea. This motility is considered to be induced by conformational changes of the motor protein prestin expressing in the lateral plasma membrane of OHCs. Since its identification in 2000, prestin has been actively investigated and its structure and function have gradually been elucidated. These successes are partly due to the development of efficient expression and purification system of the membrane proteins including prestin. To obtain further understandings of prestin, the development of various types of such systems will be essential. However, recent study protocols on membrane proteins have often employed HEK293 cells and have become complexed with expression genes carrying several proteins and peptides for stabilization and purification of the expressed proteins. In the present study, a simple expression and purification system using Chinese hamster ovary (CHO) cells and Hi-tag was developed.

METHODS

Full length gerbil prestin was transfected into modified mammalian expression vectors with C-terminal 6 × His-tag. After drug selection with G418 for 4 weeks, single colonies were isolated by limiting dilution method. Cell lines highly expressing prestin were selected (named 3D5, 4D7 and 3C8). These cells were gently disrupted using a Dounce tissue grinder. Membrane fractions were extracted by ultracentrifugation and affinity chromatography was performed. The efficiency of the purification process was evaluated by quantitative Western blotting using a standard protein.

RESULTS

Among the cell lines constructed, Western blotting analysis showed bands at around 100 kDa and the highest intensity was confirmed from the 3C8 cell line, indicating that this cell line has the highest expression of prestin molecules. The membrane fraction was therefore extracted from this cell line and subjected to the following purification procedure. It was found that 78.7 μg of prestin was purified from 2.0 × 109 CHO cells.

CONCLUSION

In the present study, 78.7 μg of prestin was purified from 2.0 × 109 CHO cells, which stably expressing 6 × His-tagged prestin, by extracting cell membrane fractions and standard affinity chromatography for His-tag.

Chloride binding to prestin does not influence very high-frequency complex nonlinear capacitance (cNLC) in the mouse outer hair cell

Prestin (SLC26a5) function evolved to enhance auditory sensitivity and frequency selectivity by providing mechanical feedback via outer hair cells (OHC) into the organ of Corti. Its effectiveness is governed by the voltage-dependent kinetics of the protein's charge movements, namely, nonlinear capacitance (NLC). We study the frequency response of NLC in the mouse OHC, a species with ultrasonic hearing. We find that the characteristic frequency cut-off (F is ) for the mouse in near 27 kHz. Single point mutations within the chloride binding pocket of prestin (e.g., S396E, S398E) lack the protein's usual anion susceptibility. In agreement, we now show absence of anion binding in these mutants through molecular dynamics (MD) simulations. NLC F is in the S396E knock-in mouse is unaltered, indicating that high frequency activity is not governed by chloride, but more likely by viscoelastic loads within the membrane. We also show that the allosteric action of chloride does not underlie piezoelectric-like behavior in prestin, since tension sensitivity of S396E NLC is comparable to that of WT. Because prestin structures of all species studied to-date are essentially indistinguishable, with analogous chloride binding pockets, auditory requirements of individual species for cochlear amplification likely evolved to enhance prestin performance by modifying, not its protein-anion interaction, but instead external mechanical loads on the protein.

Significance

Prestin is believed to provide cochlear amplification in mammals that possess a wide range of frequency sensitivities. Previously, chloride anions have been shown to control prestin kinetics at frequencies below 10 kHz. However, now we find that chloride binding is not influential for prestin kinetics in the very high range of the mouse. We suggest that such high frequency prestin performance is governed by impinging mechanical loads within the membrane, and not interactions with anions.

The frequency dependence of prestin-mediated fast electromotility for mammalian cochlear amplification

Prestin's voltage-driven motor activity confers sound-elicited somatic electromotility in auditory outer hair cells (OHCs) and is essential for the exquisite sensitivity and frequency selectivity of mammalian hearing. Lack of prestin results in hearing threshold shifts across frequency, supporting the causal association of variants in the prestin-coding gene, SLC26A5 , with human hearing loss, DFNB61. However, cochlear function can tolerate reductions in prestin-mediated OHC electromotility. We found that two deafness-associated prestin variants, p.A100T and p.P119S, do not deprive prestin of its fast motor function but significantly reduce membrane expression, leading to large reductions in OHC electromotility that were only ∼30% of wildtype (WT). Mice harboring these missense variants suffered congenital hearing loss that was worse at high frequencies; however, they retained WT-like auditory brainstem response thresholds at 8 kHz, which is processed at the apex of the mouse cochlea. This observation suggests the increasing importance of prestin-driven cochlear amplification at higher frequencies relevant to mammalian hearing. The observation also suggests the promising clinical possibility that small enhancements of OHC electromotility could significantly ameliorate DFNB61 hearing loss in human patients.

SIGNIFICANCE

Prestin is abundantly expressed in the auditory outer hair cells and is essential for normal cochlear operation. Hence, reduction of prestin expression is often taken as indicative of reduced cochlear function in diseased or aged ears. However, this assumption overlooks the fact that cochlear function can tolerate large reductions in prestin motor activity. DFNB61 mouse models generated and characterized in this study provide an opportunity to gauge the amount of prestin motor activity needed to sustain normal hearing sensitivity. This knowledge is crucial not only for understanding the pathogenic roles of deafness-associated variants that impair OHC electromotility but also for unraveling how prestin contributes to cochlear amplification.

The molecular principles underlying diverse functions of the SLC26 family of proteins

Mammalian SLC26 proteins are membrane-based anion transporters that belong to the large SLC26/SulP family, and many of their variants are associated with hereditary diseases. Recent structural studies revealed a strikingly similar homodimeric molecular architecture for several SLC26 members, implying a shared molecular principle. Now a new question emerges as to how these structurally similar proteins execute diverse physiological functions. In this study, we sought to identify the common versus distinct molecular mechanism among the SLC26 proteins using both naturally occurring and artificial missense changes introduced to SLC26A4, SLC26A5, and SLC26A9. We found: (i) the basic residue at the anion binding site is essential for both anion antiport of SLC26A4 and motor functions of SLC26A5, and its conversion to a nonpolar residue is crucial but not sufficient for the fast uncoupled anion transport in SLC26A9; (ii) the conserved polar residues in the N- and C-terminal cytosolic domains are likely involved in dynamic hydrogen-bonding networks and are essential for anion antiport of SLC26A4 but not for motor (SLC26A5) and uncoupled anion transport (SLC26A9) functions; (iii) the hydrophobic interaction between each protomer's last transmembrane helices, TM14, is not of functional significance in SLC26A9 but crucial for the functions of SLC26A4 and SLC26A5, likely contributing to optimally orient the axis of the relative movements of the core domain with respect to the gate domains within the cell membrane. These findings advance our understanding of the molecular mechanisms underlying the diverse physiological roles of the SLC26 family of proteins.

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.

SLC26 Anion Transporters

Solute carrier family 26 (SLC26) is a family of functionally diverse anion transporters found in all kingdoms of life. Anions transported by SLC26 proteins include chloride, bicarbonate, and sulfate, but also small organic dicarboxylates such as fumarate and oxalate. The human genome encodes ten functional homologs, several of which are causally associated with severe human diseases, highlighting their physiological importance. Here, we review novel insights into the structure and function of SLC26 proteins and summarize the physiological relevance of human members.

The molecular principles underlying diverse functions of the SLC26 family of proteins

Mammalian SLC26 proteins are membrane-based anion transporters that belong to the large SLC26/SulP family, and many of their variants are associated with hereditary diseases. Recent structural studies revealed a strikingly similar homodimeric molecular architecture for several SLC26 members, implying a shared molecular principle. Now a new question emerges as to how these structurally similar proteins execute diverse physiological functions. In this study we sought to identify the common vs. distinct molecular mechanism among the SLC26 proteins using both naturally occurring and artificial missense changes introduced to SLC26A4, SLC26A5, and SLC26A9. We found: (i) the basic residue at the anion binding site is essential for both anion antiport of SLC26A4 and motor functions of SLC26A5, and its conversion to a nonpolar residue is crucial but not sufficient for the fast uncoupled anion transport in SLC26A9; (ii) the conserved polar residues in the N- and C-terminal cytosolic domains are likely involved in dynamic hydrogen-bonding networks and are essential for anion antiport of SLC26A4 but not for motor (SLC26A5) and uncoupled anion transport (SLC26A9) functions; (iii) the hydrophobic interaction between each protomer's last transmembrane helices, TM14, is not of functional significance in SLC26A9 but crucial for the functions of SLC26A4 and SLC26A5, likely contributing to optimally orient the axis of the relative movements of the core domain with respect to the gate domains within the cell membrane. These findings advance our understanding of the molecular mechanisms underlying the diverse physiological roles of the SLC26 family of proteins.

Folding of prestin's anion-binding site and the mechanism of outer hair cell electromotility

Prestin responds to transmembrane voltage fluctuations by changing its cross-sectional area, a process underlying the electromotility of outer hair cells and cochlear amplification. Prestin belongs to the SLC26 family of anion transporters yet is the only member capable of displaying electromotility. Prestin's voltage-dependent conformational changes are driven by the putative displacement of residue R399 and a set of sparse charged residues within the transmembrane domain, following the binding of a Cl- anion at a conserved binding site formed by the amino termini of the TM3 and TM10 helices. However, a major conundrum arises as to how an anion that binds in proximity to a positive charge (R399), can promote the voltage sensitivity of prestin. Using hydrogen-deuterium exchange mass spectrometry, we find that prestin displays an unstable anion-binding site, where folding of the amino termini of TM3 and TM10 is coupled to Cl- binding. This event shortens the TM3-TM10 electrostatic gap, thereby connecting the two helices, resulting in reduced cross-sectional area. These folding events upon anion binding are absent in SLC26A9, a non-electromotile transporter closely related to prestin. Dynamics of prestin embedded in a lipid bilayer closely match that in detergent micelle, except for a destabilized lipid-facing helix TM6 that is critical to prestin's mechanical expansion. We observe helix fraying at prestin's anion-binding site but cooperative unfolding of multiple lipid-facing helices, features that may promote prestin's fast electromechanical rearrangements. These results highlight a novel role of the folding equilibrium of the anion-binding site, and help define prestin's unique voltage-sensing mechanism and electromotility.

Elevator-like movements of prestin mediate outer hair cell electromotility

The outstanding acuity of the mammalian ear relies on cochlear amplification, an active mechanism based on the electromotility (eM) of outer hair cells. eM is a piezoelectric mechanism generated by little-understood, voltage-induced conformational changes of the anion transporter homolog prestin (SLC26A5). We used a combination of molecular dynamics (MD) simulations and biophysical approaches to identify the structural dynamics of prestin that mediate eM. MD simulations showed that prestin samples a vast conformational landscape with expanded (ES) and compact (CS) states beyond previously reported prestin structures. Transition from CS to ES is dominated by the translational-rotational movement of prestin's transport domain, akin to elevator-type substrate translocation by related solute carriers. Reversible transition between CS and ES states was supported experimentally by cysteine accessibility scanning, cysteine cross-linking between transport and scaffold domains, and voltage-clamp fluorometry (VCF). Our data demonstrate that prestin's piezoelectric dynamics recapitulate essential steps of a structurally conserved ion transport cycle.

Folding of Prestin's Anion-Binding Site and the Mechanism of Outer Hair Cell Electromotility

Prestin responds to transmembrane voltage fluctuations by changing its cross-sectional area, a process underlying the electromotility of outer hair cells and cochlear amplification. Prestin belongs to the SLC26 family of anion transporters yet is the only member capable of displaying electromotility. Prestin's voltage-dependent conformational changes are driven by the putative displacement of residue R399 and a set of sparse charged residues within the transmembrane domain, following the binding of a Cl - anion at a conserved binding site formed by amino termini of the TM3 and TM10 helices. However, a major conundrum arises as to how an anion that binds in proximity to a positive charge (R399), can promote the voltage sensitivity of prestin. Using hydrogen-deuterium exchange mass spectrometry, we find that prestin displays an unstable anion-binding site, where folding of the amino termini of TM3 and TM10 is coupled to Cl - binding. This event shortens the TM3-TM10 electrostatic gap, thereby connecting the two helices, resulting in reduced cross-sectional area. These folding events upon anion-binding are absent in SLC26A9, a non-electromotile transporter closely related to prestin. Dynamics of prestin embedded in a lipid bilayer closely match that in detergent micelle, except for a destabilized lipid-facing helix TM6 that is critical to prestin's mechanical expansion. We observe helix fraying at prestin's anion-binding site but cooperative unfolding of multiple lipid-facing helices, features that may promote prestin's fast electromechanical rearrangements. These results highlight a novel role of the folding equilibrium of the anion-binding site, and helps define prestin's unique voltage-sensing mechanism and electromotility.

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 ( SLC26A5 W70X/R130S ). Our recent study showed that mice homozygous for p.R130S ( Slc26a5 R130S/R130S ) suffer from hearing loss that is ascribed to significantly reduced motor kinetics of prestin. Given that W70X-prestin is nonfunctional, compound heterozygous Slc26a5 R130S/- mice were used as a model for human SLC26A5 W70X/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 fully defines the pathogenic roles for the p.R130S prestin, which points to the presence of a limited time window for potential clinical intervention.

Megahertz Sampling of Prestin (SLC26a5) Voltage-Sensor Charge Movements in Outer Hair Cell Membranes Reveals Ultrasonic Activity that May Support Electromotility and Cochlear Amplification

Charged moieties in the outer hair cell (OHC) membrane motor protein, prestin, are driven by transmembrane voltage to power OHC electromotility (eM) and cochlear amplification (CA), an enhancement of mammalian hearing. Consequently, the speed of prestin's conformational switching constrains its dynamic influence on micromechanics of the cell and the organ of Corti. Corresponding voltage-sensor charge movements in prestin, classically assessed as a voltage-dependent, nonlinear membrane capacitance (NLC), have been used to gauge its frequency response, but have been validly measured only out to 30 kHz. Thus, controversy exists concerning the effectiveness of eM in supporting CA at ultrasonic frequencies where some mammals can hear. Using megahertz sampling of guinea pig (either sex) prestin charge movements, we extend interrogations of NLC into the ultrasonic range (up to 120 kHz) and find an order of magnitude larger response at 80 kHz than previously predicted, indicating that an influence of eM at ultrasonic frequencies is likely, in line with recent in vivo results (Levic et al., 2022). Given wider bandwidth interrogations, we also validate kinetic model predictions of prestin by directly observing its characteristic cut-off frequency under voltage-clamp as the intersection frequency (Fis), near 19 kHz, of the real and imaginary components of complex NLC (cNLC). The frequency response of prestin displacement current noise determined from either the Nyquist relation or stationary measures aligns with this cut-off. We conclude that voltage stimulation accurately assesses the spectral limits of prestin activity, and that voltage-dependent conformational switching is physiologically significant in the ultrasonic range.SIGNIFICANCE STATEMENT The motor protein prestin powers outer hair cell (OHC) electromotility (eM) and cochlear amplification (CA), an enhancement of high-frequency mammalian hearing. The ability of prestin to work at very high frequencies depends on its membrane voltage-driven conformation switching. Using megahertz sampling, we extend measures of prestin charge movement into the ultrasonic range and find response magnitude at 80 kHz an order of magnitude larger than previously estimated, despite confirmation of previous low pass characteristic frequency cut-offs. The frequency response of prestin noise garnered by the admittance-based Nyquist relation or stationary noise measures confirms this characteristic cut-off frequency. Our data indicate that voltage perturbation provides accurate assessment of prestin performance indicating that it can support cochlear amplification into a higher frequency range than previously thought.

The auditory efferent system in mosquitoes

Whilst acoustic communication forms an integral component of the mating behavior of many insect species, it is particularly crucial for disease-transmitting mosquitoes; swarming males rely on hearing the faint sounds of flying females for courtship initiation. That males can hear females within the din of a swarm is testament to their fabulous auditory systems. Mosquito hearing is highly frequency-selective, remarkably sensitive and, most strikingly, supported by an elaborate system of auditory efferent neurons that modulate the auditory function - the only documented example amongst insects. Peripheral release of octopamine, serotonin and GABA appears to differentially modulate hearing across major disease-carrying mosquito species, with receptors from other neurotransmitter families also identified in their ears. Because mosquito mating relies on hearing the flight tones of mating partners, the auditory efferent system offers new potential targets for mosquito control. It also represents a unique insect model for studying auditory efferent networks. Here we review current knowledge of the mosquito auditory efferent system, briefly compare it with its counterparts in other species and highlight future research directions to unravel its contribution to mosquito auditory perception.

Lipid-mediated prestin organization in outer hair cell membranes and its implications in sound amplification

Prestin is a high-density motor protein in the outer hair cells (OHCs), whose conformational response to acoustic signals alters the shape of the cell, thereby playing a major role in sound amplification by the cochlea. Despite recent structures, prestin's intimate interactions with the membrane, which are central to its function remained unresolved. Here, employing a large set (collectively, more than 0.5 ms) of coarse-grained molecular dynamics simulations, we demonstrate the impact of prestin's lipid-protein interactions on its organization at densities relevant to the OHCs and its effectiveness in reshaping OHCs. Prestin causes anisotropic membrane deformation, which mediates a preferential membrane organization of prestin where deformation patterns by neighboring copies are aligned constructively. The resulting reduced membrane rigidity is hypothesized to maximize the impact of prestin on OHC reshaping. These results demonstrate a clear case of protein-protein cooperative communication in membrane, purely mediated by interactions with lipids.

Cryo-EM structures of thermostabilized prestin provide mechanistic insights underlying outer hair cell electromotility

Outer hair cell elecromotility, driven by prestin, is essential for mammalian cochlear amplification. Here, we report the cryo-EM structures of thermostabilized prestin (PresTS), complexed with chloride, sulfate, or salicylate at 3.52-3.63 Å resolutions. The central positively-charged cavity allows flexible binding of various anion species, which likely accounts for the known distinct modulations of nonlinear capacitance (NLC) by different anions. Comparisons of these PresTS structures with recent prestin structures suggest rigid-body movement between the core and gate domains, and provide mechanistic insights into prestin inhibition by salicylate. Mutations at the dimeric interface severely diminished NLC, suggesting that stabilization of the gate domain facilitates core domain movement, thereby contributing to the expression of NLC. These findings advance our understanding of the molecular mechanism underlying mammalian cochlear amplification.

How much prestin motor activity is required for normal hearing?

Prestin (SLC26A5) is a membrane-based voltage-dependent motor protein responsible for outer hair cell (OHC) somatic electromotility. Its importance for mammalian cochlear amplification has been demonstrated using mouse models lacking prestin (prestin-KO) and expressing dysfunctional prestin, prestinV499G/Y501H (499-prestin-KI). However, it is still not elucidated how prestin contributes to the mechanical amplification process in the cochlea. In this study, we characterized several prestin mouse models in which prestin activity in OHCs was variously manipulated. We found that near-normal cochlear function can be maintained even when prestin activity is significantly reduced, suggesting that the relationship between OHC electromotility and the peripheral sensitivity to sound may not be linear. This result is counterintuitive given the large threshold shifts in prestin-KO and 499-prestin-KI mice, as reported in previous studies. To reconcile these apparently opposing observations, we entertain a voltage- and turgor pressure-based cochlear amplification mechanism that requires prestin but is insensitive to significant reductions in prestin protein expression. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

Coupling between outer hair cell electromotility and prestin sensor charge depends on voltage operating point

The OHC drives cochlear amplification, and prestin activity is the basis. The frequency response of nonlinear capacitance (NLC), which is a ratiometric measure of prestin's voltage-sensor charge movement (dQp/dVm), depends on the location of AC voltage excitation along prestin's operating voltage range, being slowest at the voltage (Vh) where NLC peaks. Here we directly investigate the coupling between prestin charge movement (Qp) and electromotility (eM) at frequencies up to 6.25 kHz, and find tight correspondence between the two at operating voltages displaced from Vh. Near Vh, however, eM shows a slower frequency response than Qp. We reason that coupling is more susceptible to molecular/cellular loads at Vh, where prestin compliance is expected to be maximal. Recent cryo-EM studies have begun to shed light on structural features of prestin that impact its performance against loads. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

On the frequency response of prestin charge movement in membrane patches

Outer hair cell (OHC) nonlinear membrane capacitance derives from voltage-dependent sensor charge movements within the membrane protein prestin (SLC26a5) that drive OHC electromotility. The ability of the protein to influence hearing depends on its reaction to membrane receptor potentials across auditory frequency. Estimates of prestin's frequency response have been evaluated by several groups out to tens of kHz in voltage-clamped macro-patches of OHC membrane. The response is a power function of frequency that is down 40 dB at 77 kHz. Despite these observations, concerns remain that the macro-patch approach is flawed due to mechanical constraints of pipette solution column load or patch size itself. In the absence of these influences, prestin's frequency response is posited by some to be ultrasonic in nature. Here we evaluate the influence of these putative confounding factors on prestin's frequency response. We show that neither pipette column height nor negative or positive pipette pressure substantially influence total sensor charge frequency response. Additionally, patch surface area has negligible influence. We conclude that the speed of voltage-driven conformational changes in prestin within the plasma membrane is accurately assessed with the macro-patch technique, permitting investigations of membrane characteristics that can substantially alter prestin's performance bandwidth. We illustrate significant alterations in bandwidth by perturbation of membrane fluidity and chloride anion concentration. Finally, we speculate that OHC membrane characteristics may differ along the tonotopic axis of the cochlea to tune nonlinear membrane capacitance frequency cutoffs.

In vivo CRISPR-Cas9-mediated DNA chop identifies a cochlear outer hair cell-specific enhancer

Cochlear outer hair cells (OHCs) are essential for hearing. A short, OHC-specific enhancer is necessary but not yet available for gene therapeutic applications in OHC damage. Such damage is a major cause of deafness. Prestin is a motor protein exclusively expressed in OHCs. We hypothesized that the cis-regulatory DNA fragment deletion of Slc26a5 would affect its expression. We tested this hypothesis by conducting CRISPR/Cas9-mediated large DNA fragment deletion of mouse Slc26a5 intron regions. First, starting from a ~13 kbp fragment, step-by-step, we narrowed down the sequence to a 1.4 kbp segment. By deleting either a 13 kbp or 1.4 kbp fragment, we observed delayed Prestin expression. Second, we showed that 1.4 kbp was an OHC-specific enhancer because enhanced green fluorescent protein (EGFP) was highly and specifically expressed in OHCs in a transgenic mouse where EGFP was driven by the 1.4 kbp segment. More importantly, specific EGFP was also driven by its homologous 398 bp fragment in human Slc26a5. This suggests that the enhancer is likely to be evolutionarily conserved across different species.

Single particle cryo-EM structure of the outer hair cell motor protein prestin

The mammalian outer hair cell (OHC) protein prestin (Slc26a5) differs from other Slc26 family members due to its unique piezoelectric-like property that drives OHC electromotility, the putative mechanism for cochlear amplification. Here, we use cryo-electron microscopy to determine prestin’s structure at 3.6 Å resolution. Prestin is structurally similar to the anion transporter Slc26a9. It is captured in an inward-open state which may reflect prestin’s contracted state. Two well-separated transmembrane (TM) domains and two cytoplasmic sulfate transporter and anti-sigma factor antagonist (STAS) domains form a swapped dimer. The transmembrane domains consist of 14 transmembrane segments organized in two 7+7 inverted repeats, an architecture first observed in the bacterial symporter UraA. Mutation of prestin’s chloride binding site removes salicylate competition with anions while retaining the prestin characteristic displacement currents (Nonlinear Capacitance), undermining the extrinsic voltage sensor hypothesis for prestin function.

Prestin, expressed in outer hair cell (OHC), belongs to the Slc26 transporter family and functions as a voltage-driven motor that drives OHC electromotility. Here, the authors report cryo-EM structure and characterization of gerbil prestin, with insights into its mechanism of action.

SK Current, Expressed During the Development and Regeneration of Chick Hair Cells, Contributes to the Patterning of Spontaneous Action Potentials

Chick hair cells display calcium (Ca2+)-sensitive spontaneous action potentials during development and regeneration. The role of this activity is unclear but thought to be involved in establishing proper synaptic connections and tonotopic maps, both of which are instrumental to normal hearing. Using an electrophysiological approach, this work investigated the functional expression of Ca2+-sensitive potassium [IK(Ca)] currents and their role in spontaneous electrical activity in the developing and regenerating hair cells (HCs) in the chick basilar papilla. The main IK(Ca) in developing and regenerating chick HCs is an SK current, based on its sensitivity to apamin. Analysis of the functional expression of SK current showed that most dramatic changes occurred between E8 and E16. Specifically, there is a developmental downregulation of the SK current after E16. The SK current gating was very sensitive to the availability of intracellular Ca2+ but showed very little sensitivity to T-type voltage-gated Ca2+ channels, which are one of the hallmarks of developing and regenerating hair cells. Additionally, apamin reduced the frequency of spontaneous electrical activity in HCs, suggesting that SK current participates in patterning the spontaneous electrical activity of HCs.

Progress in understanding the structural mechanism underlying prestin's electromotile activity

Prestin (SLC26A5), a member of the SLC26 transporter family, is the molecular actuator that drives OHC electromotility (eM). A wealth of biophysical data indicates that eM is mediated by an area motor mechanism, in which prestin molecules act as elementary actuators by changing their area in the membrane in response to changes in membrane potential. The area changes of a large and densely packed population of prestin molecules sum up, resulting in macroscopic cellular movement. At the single protein level, this model implies major voltage-driven conformational rearrangements. However, the nature of these structural dynamics remained unknown. A main obstacle in elucidating the eM mechanism has been the lack of structural information about SLC26 transporters. The recent emergence of several high-resolution cryo-EM structures of prestin as well as other SLC26 transporter family members now provides a reliable picture of prestin's molecular architecture. Thus, SLC26 transporters including prestin generally are dimers, and each protomer is folded according to a 7+7 transmembrane domain inverted repeat (7TMIR) architecture. Here, we review these structural findings and discuss insights into a potential molecular mechanism. Most important, distinct conformations were observed when purifying and imaging prestin bound to either its physiological ligand, chloride, or to competitively inhibitory anions, sulfate or salicylate. Despite differences in detail, these structural snapshots indicate that the conformational landscape of prestin includes rearrangements between the two major domains of prestin's transmembrane region (TMD), core and scaffold ('gate') domains. Notably, distinct conformations differ in the area the TMD occupies in the membrane and in their impact on the immediate lipid environment. Both effects can contribute to generate membrane deformation and thus may underly electromotility. Further functional studies will be necessary to determine whether these or similar structural rearrangements are driven by membrane potential to mediate piezoelectric activity. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

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.

Molecular mechanism of prestin electromotive signal amplification

Hearing involves two fundamental processes: mechano-electrical transduction and signal amplification. Despite decades of studies, the molecular bases for both remain elusive. Here, we show how prestin, the electromotive molecule of outer hair cells (OHCs) that senses both voltage and membrane tension, mediates signal amplification by coupling conformational changes to alterations in membrane surface area. Cryoelectron microscopy (cryo-EM) structures of human prestin bound with chloride or salicylate at a common "anion site" adopt contracted or expanded states, respectively. Prestin is ensconced within a perimeter of well-ordered lipids, through which it induces dramatic deformation in the membrane and couples protein conformational changes to the bulk membrane. Together with computational studies, we illustrate how the anion site is allosterically coupled to changes in the transmembrane domain cross-sectional area and the surrounding membrane. These studies provide insight into OHC electromotility by providing a structure-based mechanism of the membrane motor prestin.

A novel theoretical framework reveals more than one voltage-sensing pathway in the lateral membrane of outer hair cells

Outer hair cell (OHC) electromotility amplifies acoustic vibrations throughout the frequency range of hearing. Electromotility requires that the lateral membrane protein prestin undergo a conformational change upon changes in the membrane potential to produce an associated displacement charge. The magnitude of the charge displaced and the mid-reaction potential (when one half of the charge is displaced) reflects whether the cells will produce sufficient gain at the resting membrane potential to boost sound in vivo. Voltage clamp measurements performed under near-identical conditions ex vivo show the charge density and mid-reaction potential are not always the same, confounding interpretation of the results. We compare the displacement charge measurements in OHCs from rodents with a theory shown to exhibit good agreement with in silico simulations of voltage-sensing reactions in membranes. This model equates the charge density to the potential difference between two pseudo-equilibrium states of the sensors when they are in a stable conformation and not contributing to the displacement current. The model predicts this potential difference to be one half of its value midway into the reaction, when one equilibrium conformation transforms to the other pseudo-state. In agreement with the model, we find the measured mid-reaction potential to increase as the charge density decreases to exhibit a negative slope of ∼1/2. This relationship suggests that the prestin sensors exhibit more than one stable hyperpolarized state and that voltage sensing occurs by more than one pathway. We determine the electric parameters for prestin sensors and use the analytical expressions of the theory to estimate the energy barriers for the two voltage-dependent pathways. This analysis explains the experimental results, supports the theoretical approach, and suggests that voltage sensing occurs by more than one pathway to enable amplification throughout the frequency range of hearing.

State dependent effects on the frequency response of prestin's real and imaginary components of nonlinear capacitance

The outer hair cell (OHC) membrane harbors a voltage-dependent protein, prestin (SLC26a5), in high density, whose charge movement is evidenced as a nonlinear capacitance (NLC). NLC is bell-shaped, with its peak occurring at a voltage, V_h, where sensor charge is equally distributed across the plasma membrane. Thus, V_h provides information on the conformational state of prestin. V_h is sensitive to membrane tension, shifting to positive voltage as tension increases and is the basis for considering prestin piezoelectric (PZE). NLC can be deconstructed into real and imaginary components that report on charge movements in phase or 90 degrees out of phase with AC voltage. Here we show in membrane macro-patches of the OHC that there is a partial trade-off in the magnitude of real and imaginary components as interrogation frequency increases, as predicted by a recent PZE model (Rabbitt in Proc Natl Acad Sci USA 17:21880–21888, 2020). However, we find similar behavior in a simple 2-state voltage-dependent kinetic model of prestin that lacks piezoelectric coupling. At a particular frequency, F_is, the complex component magnitudes intersect. Using this metric, F_is, which depends on the frequency response of each complex component, we find that initial V_h influences F_is; thus, by categorizing patches into groups of different V_h, (above and below − 30 mV) we find that F_is is lower for the negative V_h group. We also find that the effect of membrane tension on complex NLC is dependent, but differentially so, on initial V_h. Whereas the negative group exhibits shifts to higher frequencies for increasing tension, the opposite occurs for the positive group. Despite complex component trade-offs, the low-pass roll-off in absolute magnitude of NLC, which varies little with our perturbations and is indicative of diminishing total charge movement, poses a challenge for a role of voltage-driven prestin in cochlear amplification at very high frequencies.

Comparative Molecular Dynamics Investigation of the Electromotile Hearing Protein Prestin

The mammalian protein prestin is expressed in the lateral membrane wall of the cochlear hair outer cells and is responsible for the electromotile response of the basolateral membrane, following hyperpolarisation or depolarisation of the cells. Its impairment marks the onset of severe diseases, like non-syndromic deafness. Several studies have pointed out possible key roles of residues located in the Transmembrane Domain (TMD) that differentiate mammalian prestins as incomplete transporters from the other proteins belonging to the same solute-carrier (SLC) superfamily, which are classified as complete transporters. Here, we exploit the homology of a prototypical incomplete transporter (rat prestin, rPres) and a complete transporter (zebrafish prestin, zPres) with target structures in the outward open and inward open conformations. The resulting models are then embedded in a model membrane and investigated via a rigorous molecular dynamics simulation protocol. The resulting trajectories are analyzed to obtain quantitative descriptors of the equilibration phase and to assess a structural comparison between proteins in different states, and between different proteins in the same state. Our study clearly identifies a network of key residues at the interface between the gate and the core domains of prestin that might be responsible for the conformational change observed in complete transporters and hindered in incomplete transporters. In addition, we study the pathway of Cl− ions in the presence of an applied electric field towards their putative binding site in the gate domain. Based on our simulations, we propose a tilt and shift mechanism of the helices surrounding the ion binding cavity as the working principle of the reported conformational changes in complete transporters.

Salicylate-Induced Changes in Hearing Thresholds in Mongolian Gerbils Are Correlated With Tinnitus Frequency but Not With Tinnitus Strength

Tinnitus is an auditory phantom percept without external sound sources. Despite the high prevalence and tinnitus-associated distress of affected patients, the pathophysiology of tinnitus remains largely unknown, making prevention and treatments difficult to develop. In order to elucidate the pathophysiology of tinnitus, animal models are used where tinnitus is induced either permanently by noise trauma or transiently by the application of salicylate. In a model of trauma-induced tinnitus, we have suggested a central origin of tinnitus-related development of neuronal hyperactivity based on stochastic resonance (SR). SR refers to the physiological phenomenon that weak subthreshold signals for given sensors (or synapses) can still be detected and transmitted if appropriate noise is added to the input of the sensor. The main objective of this study was to characterize the neurophysiological and behavioral effects during salicylate-induced tinnitus and compare these to the conditions within the trauma model. Our data show, in line with the pharmacokinetics, that hearing thresholds generally increase 2 h after salicylate injections. This increase was significantly stronger within the region of best hearing compared to other frequencies. Furthermore, animals showed behavioral signs of tinnitus during that time window and frequency range as assessed by gap prepulse inhibition of the acoustic startle reflex (GPIAS). In contrast to animals with noise trauma-induced tinnitus, salicylate-induced tinnitus animals showed no correlation between hearing thresholds and behavioral signs of tinnitus, indicating that the development of tinnitus after salicylate injection is not based on SR as proposed for the trauma model. In other words, salicylate-induced tinnitus and noise trauma-induced tinnitus are not based on the same neurophysiological mechanism.

Identification and characterization of amphibian SLC26A5 using RNA-Seq

Background

Prestin (SLC26A5) is responsible for acute sensitivity and frequency selectivity in the vertebrate auditory system. Limited knowledge of prestin is from experiments using site-directed mutagenesis or domain-swapping techniques after the amino acid residues were identified by comparing the sequence of prestin to those of its paralogs and orthologs. Frog prestin is the only representative in amphibian lineage and the studies of it were quite rare with only one species identified.

Results

Here we report a new coding sequence of SLC26A5 for a frog species, Rana catesbeiana (the American bullfrog). In our study, the SLC26A5 gene of Rana has been mapped, sequenced and cloned successively using RNA-Seq. We measured the nonlinear capacitance (NLC) of prestin both in the hair cells of Rana’s inner ear and HEK293T cells transfected with this new coding gene. HEK293T cells expressing Rana prestin showed electrophysiological features similar to that of hair cells from its inner ear. Comparative studies of zebrafish, chick, Rana and an ancient frog species showed that chick and zebrafish prestin lacked NLC. Ancient frog’s prestin was functionally different from Rana.

Conclusions

We mapped and sequenced the SLC26A5 of the Rana catesbeiana from its inner ear cDNA using RNA-Seq. The Rana SLC26A5 cDNA was 2292 bp long, encoding a polypeptide of 763 amino acid residues, with 40% identity to mammals. This new coding gene could encode a functionally active protein conferring NLC to both frog HCs and the mammalian cell line. While comparing to its orthologs, the amphibian prestin has been evolutionarily changing its function and becomes more advanced than avian and teleost prestin.

Drug distribution along the cochlea is strongly enhanced by low-frequency round window micro vibrations

The cochlea’s inaccessibility and complex nature provide significant challenges to delivering drugs and other agents uniformly, safely and efficiently, along the entire cochlear spiral. Large drug concentration gradients are formed along the cochlea when drugs are administered to the middle ear. This undermines the major goal of attaining therapeutic drug concentration windows along the whole cochlea. Here, utilizing a well-known physiological effect of salicylate, we demonstrate a proof of concept in which drug distribution along the entire cochlea is enhanced by applying round window membrane low-frequency micro vibrations with a probe that only partially covers the round window. We provide evidence of enhanced drug influx into the cochlea and cochlear apical drug distribution without breaching cochlear boundaries. It is further suggested that ossicular functionality is not required for the effective drug distribution we report. The novel method presented here of local drug delivery to the cochlea could be implemented when ossicular functionality is absent or impeded and can be incorporated in clinically approved auditory protheses for patients who suffer with conductive, sensorineural or mixed hearing loss.

A Wirelessly Controlled Scalable 3D-Printed Microsystem for Drug Delivery

Here we present a 3D-printed, wirelessly controlled microsystem for drug delivery, comprising a refillable microreservoir and a phase-change peristaltic micropump. The micropump structure was inkjet-printed on the back of a printed circuit board around a catheter microtubing. The enclosure of the microsystem was fabricated using stereolithography 3D printing, with an embedded microreservoir structure and integrated micropump. In one configuration, the microsystem was optimized for murine inner ear drug delivery with an overall size of 19 × 13 × 3 mm3. Benchtop results confirmed the performance of the device for reliable drug delivery. The suitability of the device for long-term subcutaneous implantation was confirmed with favorable results of implantation of a microsystem in a mouse for six months. The drug delivery was evaluated in vivo by implanting four different microsystems in four mice, while the outlet microtubing was implanted into the round window membrane niche for infusion of a known ototoxic compound (sodium salicylate) at 50 nL/min for 20 min. Real-time shifts in distortion product otoacoustic emission thresholds and amplitudes were measured during the infusion, demonstrating similar results with syringe pump infusion. Although demonstrated for one application, this low-cost design and fabrication methodology is scalable for use in larger animals and humans for different clinical applications/delivery sites.

Functional Parameters of Prestin Are Not Correlated With the Best Hearing Frequency

Among the vertebrate lineages with different hearing frequency ranges, humans lie between the low-frequency hearing (1 kHz) of fish and amphibians and the high-frequency hearing (100 kHz) of bats and dolphins. Little is known about the mechanism underlying such a striking difference in the limits of hearing frequency. Prestin, responsible for cochlear amplification and frequency selectivity in mammals, seems to be the only candidate to date. Mammalian prestin is densely expressed in the lateral plasma membrane of the outer hair cells (OHCs) and functions as a voltage-dependent motor protein. To explore the molecular basis for the contribution of prestin in hearing frequency detection, we collected audiogram data from humans, dogs, gerbils, bats, and dolphins because their average hearing frequency rises in steps. We generated stable cell lines transfected with human, dog, gerbil, bat, and dolphin prestins (hPres, dPres, gPres, bPres, and nPres, respectively). The non-linear capacitance (NLC) of different prestins was measured using a whole-cell patch clamp. We found that the Q_max/C_lin of bPres and nPres was significantly higher than that of humans. The V_1/2 of hPres was more hyperpolarized than that of nPres. The z values of hPres and bPres were higher than that of nPres. We further analyzed the relationship between the high-frequency hearing limit (F_max) and the functional parameters (V_1/2, z, and Q_max/C_lin) of NLC among five prestins. Interestingly, no significant correlation was found between the functional parameters and F_max. Additionally, a comparative study showed that the amino acid sequences and tertiary structures of five prestins were quite similar. There might be a common fundamental mechanism driving the function of prestins. These findings call for a reconsideration of the leading role of prestin in hearing frequency perception. Other intriguing kinetics underlying the hearing frequency response of auditory organs might exist.

Prestin amplifies cardiac motor functions

Cardiac cells generate and amplify force in the context of cardiac load, yet the membranous sheath enclosing the muscle fibers-the sarcolemma-does not experience displacement. That the sarcolemma sustains beat-to-beat pressure changes without experiencing significant distortion is a muscle-contraction paradox. Here, we report that an elastic element-the motor protein prestin (Slc26a5)-serves to amplify actin-myosin force generation in mouse and human cardiac myocytes, accounting partly for the nonlinear capacitance of cardiomyocytes. The functional significance of prestin is underpinned by significant alterations of cardiac contractility in Prestin-knockout mice. Prestin was previously considered exclusive to the inner ear's outer hair cells; however, our results show that prestin serves a broader cellular motor function.

Calmodulin binds to the STAS domain of SLC26A5 prestin with a calcium-dependent, one-lobe, binding mode

SLC26A5 transporter prestin is fundamental for the higher hearing sensitivity and frequency selectivity of mammals. Prestin is a voltage-dependent transporter found in the cochlear outer hair cells responsible for their electromotility. Intracellular chloride binding is considered essential for voltage sensitivity and electromotility. Prestin is composed by a transmembrane domain and by a cytosolic domain called STAS. There is evidence of a calcium/calmodulin regulation of prestin mediated by the STAS domain. Using different biophysical techniques, namely SEC, CD, ITC, MST, NMR and SAXS, here we demonstrate and characterize the direct interaction between calmodulin and prestin STAS. We show that the interaction is calcium-dependent and that involves residues at the N-terminal end of the "variable loop". This is an intrinsically disordered insertion typical of the STAS domains of the SLC26 family of transporters whose function is still unclear. We derive a low-resolution model of the STAS/CaM complex, where only one lobe of calmodulin is engaged in the interaction, and build a model for the entire dimeric prestin in complex with CaM, which can use the unoccupied lobe to interact with other regions of prestin or with other regulatory proteins. We show that also a non-mammalian STAS can interact with calmodulin via the variable loop. These data start to shed light on the regulatory role of the STAS variable loop of prestin.

Motor Proteins and Spermatogenesis

Unlike the intermediate filament- and septin-based cytoskeletons which are apolar structures, the microtubule (MT) and actin cytoskeletons are polarized structures in mammalian cells and tissues including the testis, most notable in Sertoli cells. In the testis, these cytoskeletons that stretch across the epithelium of seminiferous tubules and lay perpendicular to the basement membrane of tunica propria serve as tracks for corresponding motor proteins to support cellular cargo transport. These cargoes include residual bodies, phagosomes, endocytic vesicles and most notably developing spermatocytes and haploid spermatids which lack the ultrastructures of motile cells (e.g., lamellipodia, filopodia). As such, these developing germ cells require the corresponding motor proteins to facilitate their transport across the seminiferous epithelium during the epithelial cycle of spermatogenesis. Due to the polarized natures of these cytoskeletons with distinctive plus (+) and minus (-) end, directional cargo transport can take place based on the use of corresponding actin- or MT-based motor proteins. These include the MT-based minus (-) end directed motor proteins: dyneins, and the plus (+) end directed motor proteins: kinesins, as well as the actin-based motor proteins: myosins, many of which are plus (+) end directed but a few are also minus (-) end directed motor proteins. Recent studies have shown that these motor proteins are essential to support spermatogenesis. In this review, we briefly summarize and evaluate these recent findings so that this information will serve as a helpful guide for future studies and for planning functional experiments to better understand their role mechanistically in supporting spermatogenesis.

Kinetic Membrane Model of Outer Hair Cells

The effectiveness of outer hair cells (OHCs) in amplifying the motion of the organ of Corti, and thereby contributing to the sensitivity of mammalian hearing, depends on the mechanical power output of these cells. Electromechanical coupling in OHCs, which enables these cells to convert electrical energy into mechanical energy, has been analyzed in detail using isolated cells using primarily static membrane models. The mechanical output of OHCs was previously evaluated by developing a kinetic theory based on a simplified one-dimensional model for OHCs. Here, a kinetic description of OHCs is extended by using the membrane model, which was used for analyzing in vitro experiments. This theory predicts, for systems without inertial load, that elastic load enhances positive shift of voltage dependence of the membrane capacitance because of turgor pressure. The effect of turgor pressure increases with increasing elastic load. For systems with inertia, the magnitude of mechanical power output could be ∼5% higher than the value predicted by the one-dimensional model at the optimal turgor pressure.

Auditory Brainstem Responses (ABR) of Rats during Experimentally Induced Tinnitus: Literature Review

Tinnitus is a subjective phantom sound perceived only by the affected person and a symptom of various auditory and non-auditory conditions. The majority of methods used in clinical and basic research for tinnitus diagnosis are subjective. To better understand tinnitus-associated changes in the auditory system, an objective technique measuring auditory sensitivity-the auditory brainstem responses (ABR)-has been suggested. Therefore, the present review aimed to summarize ABR's features in a rat model during experimentally induced tinnitus. PubMed, Web of Science, Science Direct, and Scopus databanks were searched using Medical Subject Heading (MeSH) terms: auditory brainstem response, tinnitus, rat. The search identified 344 articles, and 36 of them were selected for the full-text analyses. The experimental protocols and results were evaluated, and the gained knowledge was synthesized. A high level of heterogeneity between the studies was found regarding all assessed areas. The most consistent finding of all studies was a reduction in the ABR wave I amplitude following exposure to noise and salicylate. Simultaneously, animals with salicylate-induced but not noise-induced tinnitus had an increased amplitude of wave IV. Furthermore, the present study identified a need to develop a consensus experimental ABR protocol applied in future tinnitus studies using the rat model.

Dynamic changes of serum protein in rats with acute intoxication of Chinese cobra snake venom by proteomic analysis

To elucidate the toxic mechanism of snake venom at the protein level, proteomics technology was applied to investigate the effect of venom on circulation in the mammalian body. Temporal proteomic analysis was performed to profile the dynamic changes in the sera of Sprague–Dawley rats administered with Chinese cobra venom or saline. Using 8-plex iTRAQ analysis, 392 and 636 serum proteins were identified to be linearly upregulated or downregulated over time in the low-dose group and high-dose group, respectively. These proteins were mainly associated with the acute phase response pathway, complement system, and liver X receptor (LXR)/retinoid X receptor (RXR) and farnesoid X receptor (FXR)/RXR activation pathways. Compared with the low-dose group, the immune response and integrin pathways were inhibited in the high-dose group, although no obvious effect was observed. With consistently higher or lower expression in the high-dose group compared to the low-dose group throughout the whole process of venom poisoning, two proteins, Kininogen-1 (KNG1) and orosomucoid 1 (ORM1), which are involved in metabolism and immune response, occupied a core position in the pathway network and are considered venom dose-dependent biomarker candidates.

The cochlear outer hair cell speed paradox

Cochlear outer hair cells (OHCs) are among the fastest known biological motors and are essential for high-frequency hearing in mammals. It is commonly hypothesized that OHCs amplify vibrations in the cochlea through cycle-by-cycle changes in length, but recent data suggest OHCs are low-pass filtered and unable to follow high-frequency signals. The fact that OHCs are required for high-frequency hearing but appear to be throttled by slow electromotility is the "OHC speed paradox." The present report resolves this paradox and reveals origins of ultrafast OHC function and power output in the context of the cochlear load. Results demonstrate that the speed of electromotility reflects how fast the cell can extend against the load, and does not reflect the intrinsic speed of the motor element itself or the nearly instantaneous speed at which the coulomb force is transmitted. OHC power output at auditory frequencies is revealed by emergence of an imaginary nonlinear capacitance reflecting the phase of electrical charge displacement required for the motor to overcome the viscous cochlear load.

Age-related changes in the biophysical and morphological characteristics of mouse cochlear outer hair cells

KEY POINTS

Age-related hearing loss (ARHL) is a very heterogeneous disease, resulting from cellular senescence, genetic predisposition and environmental factors (e.g. noise exposure). Currently, we know very little about age-related changes occurring in the auditory sensory cells, including those associated with the outer hair cells (OHCs). Using different mouse strains, we show that OHCs undergo several morphological and biophysical changes in the ageing cochlea. Ageing OHCs also exhibited the progressive loss of afferent and efferent synapses. We also provide evidence that the size of the mechanoelectrical transducer current is reduced in ageing OHCs, highlighting its possible contribution in cochlear ageing.

ABSTRACT

Outer hair cells (OHCs) are electromotile sensory receptors that provide sound amplification within the mammalian cochlea. Although OHCs appear susceptible to ageing, the progression of the pathophysiological changes in these cells is still poorly understood. By using mouse strains with a different progression of hearing loss (C57BL/6J, C57BL/6NTac, C57BL/6NTacCdh23+ , C3H/HeJ), we have identified morphological, physiological and molecular changes in ageing OHCs (9-12 kHz cochlear region). We show that by 6 months of age, OHCs from all strains underwent a reduction in surface area, which was not a sign of degeneration. Although the ageing OHCs retained a normal basolateral membrane protein profile, they showed a reduction in the size of the K+ current and non-linear capacitance, a readout of prestin-dependent electromotility. Despite these changes, OHCs have a normal Vm and retain the ability to amplify sound, as distortion product otoacoustic emission thresholds were not affected in aged, good-hearing mice (C3H/HeJ, C57BL/6NTacCdh23+ ). The loss of afferent synapses was present in all strains at 15 months. The number of efferent synapses per OHCs, defined as postsynaptic SK2 puncta, was reduced in aged OHCs of all strains apart from C3H mice. Several of the identified changes occurred in aged OHCs from all mouse strains, thus representing a general trait in the pathophysiological progression of age-related hearing loss, possibly aimed at preserving functionality. We have also shown that the mechanoelectrical transduction (MET) current from OHCs of mice harbouring the Cdh23ahl allele is reduced with age, highlighting the possibility that changes in the MET apparatus could play a role in cochlear ageing.

Echolocating Whales and Bats Express the Motor Protein Prestin in the Inner Ear: A Potential Marker for Hearing Loss

Prestin is an integral membrane motor protein located in outer hair cells of the mammalian cochlea. It is responsible for electromotility and required for cochlear amplification. Although prestin works in a cycle-by-cycle mode up to frequencies of at least 79 kHz, it is not known whether or not prestin is required for the extreme high frequencies used by echolocating species. Cetaceans are known to possess a prestin coding gene. However, the expression and distribution pattern of the protein in the cetacean cochlea has not been determined, and the contribution of prestin to echolocation has not yet been resolved. Here we report the expression of the protein prestin in five species of echolocating whales and two species of echolocating bats. Positive labeling in the basolateral membrane of outer hair cells, using three anti-prestin antibodies, was found all along the cochlear spiral in echolocating species. These findings provide morphological evidence that prestin can have a role in cochlear amplification in the basolateral membrane up to 120–180 kHz. In addition, labeling of the cochlea with a combination of anti-prestin, anti-neurofilament, anti-myosin VI and/or phalloidin and DAPI will be useful for detecting potential recent cases of noise-induced hearing loss in stranded cetaceans. This study improves our understanding of the mechanisms involved in sound transduction in echolocating mammals, as well as describing an optimized methodology for detecting cases of hearing loss in stranded marine mammals.

Characterisation of the static offset in the travelling wave in the cochlear basal turn

In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset’s magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.

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.