Hair Cells Research Articles

DMXL2 Is Required for Endocytosis and Recycling of Synaptic Vesicles in Auditory Hair Cells.

Ribbon synapses of inner hair cells (IHCs) are uniquely designed for ultrafast and indefatigable neurotransmission of the sound. The molecular machinery ensuring the efficient, compensatory recycling of the synaptic vesicles (SVs), however, remains elusive. This study showed that hair cell knock-out of murine Dmxl2, whose human homolog is responsible for nonsyndromic sensorineural hearing loss DFNA71, resulted in auditory synaptopathy by impairing synaptic endocytosis and recycling. The mutant mice in the C57BL/6J background of either sex had mild hearing loss with severely diminished wave I amplitude of the auditory brainstem response. Membrane capacitance measurements of the IHCs revealed deficiency in sustained synaptic exocytosis and endocytic membrane retrieval. Consistent with the electrophysiological findings, 3D electron microscopy reconstruction showed reduced reserve pool of SVs and endocytic compartments, while the membrane-proximal and ribbon-associated vesicles remain intact. Our results propose an important role of DMXL2 in hair cell endocytosis and recycling of the SVs.

Developmental cochlear defects are involved in early-onset hearing loss in A/J mice.

A/J mice exhibited a severe hearing loss (HL) at juvenile stage. Up-to-date, studies on HL in A/J mice have mostly focused on the damage or dysfunction of hair cells (HCs), spiral ganglion neurons (SGNs), and stereocilia. We examined A/J mice at the early postnatal stage and found that the damage and the loss of outer hair cells (OHCs) are not severe enough to explain the profound HL observed at this age, which suggests that other cochlear defects may be responsible for HL. To better understand the mechanisms of early-onset HLin A/J mice, we characterized the pathology of the cochlea from postnatal day 3 to day 21. Our results showed defects in cochlear HC stereocilia and MET channel function as early as 3 days old. We also found abnormal localization and a significant reduction in the number of ribbon synapses in 2-week-old A/J mice. There are also abnormalities in the cochlear nerve innervation and terminal swellings in 3-week-old A/J mice. All of the abnormalities of cochlear existed in the A/J mice were identified in the juvenile stage and occurred before HCs or auditory nerve loss and was the initial pathological change. Our results suggest that developmental defects and subsequent cochlear degeneration are responsible for early-onset hearing loss in A/J mice.

The role of GDF15 in attenuating noise-induced hidden hearing loss by alleviating oxidative stress

Noise-induced hidden hearing loss (HHL) is a newly uncovered form of hearing impairment that causes hidden damage to the cochlea. Patients with HHL do not have significant abnormalities in their hearing thresholds, but they experience impaired speech recognition in noisy environments. However, the mechanisms underlying HHL remain unclear. In this study, we developed single-cell transcriptome profiles of the cochlea of mice with HHL, detailing changes in individual cell types. Our study revealed a transient threshold shift, reduced auditory brainstem response wave I amplitude, and decreased number of ribbon synapses in HHL mice. Our findings suggest elevated oxidative stress and GDF15 expression in cochlear hair cells of HHL mice. Notably, the upregulation of GDF15 attenuated oxidative stress and auditory impairment in the cochlea of HHL mice. This suggests that a therapeutic strategy targeting GDF15 may be efficacious against HHL.Graphical HHL mice had a transient threshold shift, reduced ABR wave I amplitude, and decreased number of ribbon synapses.HHL mice's cochlear hair cells exhibited increased oxidative stress and elevated GDF15 expression.Upregulation of GDF15 attenuated oxidative stress and auditory damage in the cochlea of HHL mice, implying that GDF15-targeted treatment techniques may be useful for HHL.

Auditory changes in awake guinea pigs exposed to overcompressed music

Exposure to loud sound during leisure time is identified as a significant risk factor for hearing by health authorities worldwide. The current standard that defines unsafe exposure rests on the equal-energy hypothesis, according to which the maximum recommended exposure is a tradeoff between level and daily exposure duration, a satisfactory recipe except for strongly non-Gaussian intense sounds such as gunshots. Nowadays, sound broadcast by music and videoconference streaming services makes extensive use of numerical dynamic range compression. By filling in millisecond-long valleys in the signal to prevent competing noise from masking, it pulls sound-level statistics away from a Gaussian distribution, the framework where the equal-energy hypothesis emerged.Auditory effects of a single 4 hour exposure to the same music were compared in two samples of guinea pigs exposed either to its original or overcompressed version played at the same average level of 102 dBA allowed by French regulations. Apart from a temporary shift of otoacoustic emissions at the lowest two frequencies 2 and 3 kHz, music exposure had no detectable cochlear effect, as monitored at 1, 2 and 7 days post-exposure. Conversely, middle-ear muscle strength behaved differentially as the group exposed to original music had fully recovered one day after exposure whereas the group exposed to overcompressed music remained stuck to about 50% of baseline even after 7 days. Subsamples were then re-exposed to the same music as the first time and sacrificed for density measurements of inner-hair-cell synapses. No difference in synaptic density was found compared to unexposed controls with either type of music.The present results show that the same music piece, harmless when played in its original version, induces a protracted deficit of one auditory neural pathway when overcompressed at the same level. The induced disorder does not seem to involve inner-hair cell synapses.

Combinatorial protection of cochlear hair cells: not too little but not too much.

A number of drugs are toxic to the cochlear sensory cells known as hair cells (HCs), resulting in hearing loss. Treatment with survival-promoting growth factors, antioxidants, and inhibitors of cell death pathways or proteinases have been shown to reduce HC damage in in vivo and/or in vitro animal models. Conversely, translation to humans has often been disappointing. This may be due to the complexity of intracellular damage processes. We hypothesized that combining treatments targeting different cellular processes would be more effective. Using an in vitro model of gentamicin ototoxicity for murine cochlear hair cells, we screened all 56 possible combinations of inhibitors targeting five different cell damage mechanisms, plus the activator of one cell survival pathway, each of which have been shown to be singly effective in preventing HC loss in experimental studies. A high dose of gentamicin (200 μM) was used over three days in culture. All compounds were added at a dosage below that required for significant protection in the assay, and only this single dose was then employed. This was done so that we could more easily detect interactive, as opposed to additive, effects. Increasing protection of hair cells was observed as combinations of compounds were increased from two to four factors, although not all combinations were equally protective. The optimal combination of four compounds consisted of an anti-oxidant, an apoptosis inhibitor, an autophagy inhibitor and a protective growth factor. Increasing the number of factors to five or six resulted in decreased protection. The results support the hypothesis that targeting multiple cellular damage or survival pathways provides more an effective hair cell protection approach. The results help to identify critical interactions among the cellular processes that operate in gentamicin ototoxicity. They also suggest that inhibiting too many biological processes impairs functions critical to HC survival, resulting in decreased protection.

Foxg1a is required for hair cell development and regeneration in the zebrafish lateral line.

Mechanosensory hair cells located in the inner ear mediate the sensations of hearing and balance. If damaged, mammalian inner ear hair cells are unable to regenerate, resulting in permanent sensory deficits. Aquatic vertebrates like zebrafish (Danio rerio) have a specialized class of mechanosensory hair cells found in the lateral line system, allowing them to sense changes in water current. Unlike mammalian inner ear hair cells, lateral line hair cells can robustly regenerate following damage. In mammals, the transcription factor Foxg1 functions to promote normal development of the inner ear. Foxg1a is expressed in lateral line sensory organs in zebrafish larvae, but its function during lateral line development and regeneration has not been investigated. Our study demonstrates that mutation of foxg1a results in slower posterior lateral line primordium migration and delayed neuromast formation. In developing and regenerating neuromasts, we find that loss of Foxg1a function results in reduced hair cell numbers, as well as decreased proliferation of neuromast cells. Foxg1a specifically regulates the development and regeneration of Islet1-labeled hair cells. These data suggest that Foxg1 may be a valuable target for investigation of clinical hair cell regeneration.

The role of cilia in the development, survival, and regeneration of hair cells.

Mutations impacting cilia genes lead to a class of human diseases known as ciliopathies. This is due to the role of cilia in the development, survival, and regeneration of many cell types. We investigated the extent to which disrupting cilia impacted these processes in lateral line hair cells of zebrafish. We found that mutations in two intraflagellar transport (IFT) genes, ift88 and dync2h1, which lead to the loss of kinocilia, caused increased hair cell apoptosis. IFT gene mutants also have a decreased mitochondrial membrane potential, and blocking the mitochondrial uniporter causes a loss of hair cells in wild-type zebrafish but not mutants, suggesting mitochondria dysfunction may contribute to the apoptosis seen in these mutants. These mutants also showed decreased proliferation during hair cell regeneration but did not show consistent changes in support cell number or proliferation during hair cell development. These results show that the loss of hair cells seen following disruption of cilia through either mutations in anterograde or retrograde IFT genes appears to be due to impacts on hair cell survival but not necessarily development in the zebrafish lateral line.

Virally-Mediated Enhancement of Efferent Inhibition Reduces Acoustic Trauma in Wild Type Murine Cochleas.

Noise-induced hearing loss (NIHL) poses an emerging global health problem with only ear protection or sound avoidance as preventive strategies. In addition, however, the cochlea receives some protection from medial olivocochlear (MOC) efferent neurons, providing a potential target for therapeutic enhancement. Cholinergic efferents release ACh (Acetylycholine) to hyperpolarize and shunt the outer hair cells (OHCs), reducing sound-evoked activation. The (α9)2(α10)3 nicotinic ACh receptor (nAChR) on the OHCs mediates this effect. Transgenic knock-in mice with a gain-of-function nAChR (α9L9'T) suffer less NIHL. α9 knockout mice are more vulnerable to NIHL but can be rescued by viral transduction of the α9L9'T subunit. In this study, an HA-tagged gain-of-function α9 isoform was expressed in wildtype mice in an attempt to reduce NIHL. Synaptic integration of the virally-expressed nAChR subunit was confirmed by HA-immunopuncta in the postsynaptic membrane of OHCs. After noise exposure, α9L9'T-HA injected mice had less hearing loss (auditory brainstem response (ABR) thresholds and threshold shifts) than did control mice. ABRs of α9L9'T-HA injected mice also had larger wave1 amplitudes and better recovery of wave one amplitudes post noise exposure. Thus, virally-expressed α9L9'T combines effectively with native α9 and α10 subunits to mitigate NIHL in wildtype cochleas.

Cingulin-nonmuscle myosin interaction plays a role in epithelial morphogenesis and cingulin nanoscale organization.

Cingulin (CGN) tethers nonmuscle myosin 2B (NM2B; heavy chain encoded by MYH10) to tight junctions (TJs) to modulate junctional and apical cortex mechanics. Here, we studied the role of the CGN-nonmuscle myosin 2 (NM2) interaction in epithelial morphogenesis and nanoscale organization of CGN by expressing wild-type and mutant CGN constructs in CGN-knockout Madin-Darby canine kidney (MDCK) epithelial cells. We show that the NM2-binding region of CGN is required to promote normal cyst morphogenesis of MDCK cells grown in three dimensions and to maintain the C-terminus of CGN in a distal position with respect to the ZO-2 (or TJP2)-containing TJ submembrane region, whereas the N-terminus of CGN is localized more proximal to the TJ membrane. We also show that the CGN mutant protein that causes deafness in human and mouse models is localized at TJs but does not bind to NM2B, resulting in decreased TJ membrane tortuosity. These results indicate that the interaction between CGN and NM2B regulates epithelial tissue morphogenesis and nanoscale organization of CGN and suggest that CGN regulates the auditory function of hair cells by organizing the actomyosin cytoskeleton to modulate the mechanics of the apical and junctional cortex.

Targeting the NLRP3 inflammasome in cochlear macrophages protects against hearing loss in chronic suppurative otitis media

The activation of the NLRP3 inflammasome has been linked to several inflammatory and autoinflammatory diseases. Despite cases of potential hearing improvement in immune-mediated diseases, direct evidence of the efficacy of targeting this mechanism in the inner ear is still lacking. Previously, we discovered that macrophages are associated with Sensorineural Hearing loss (SNHL) in Chronic Suppurative Otitis Media (CSOM), the leading cause of this permanent hearing loss in the developing world and incurring costs of $4 to $11 billion dollars in the United States. However, the underlying mechanism remained unknown. Here, we investigate how macrophages drive permanent hearing loss in CSOM. We first confirmed the occurrence of NLRP3 inflammasome activation in cochlear macrophages in CSOM. We then revealed that Outer Hair Cells (OHCs) were protected in CSOM by macrophage depletion and subsequently confirmed the same protection in the NLRP3 knockout condition. Furthermore, we showed that therapeutic inhibition of NLRP3 inflammasome activation and downstream inhibition of IL-1β protects OHCs in CSOM. Collectively, our data demonstrates that the main driver for hearing loss in CSOM is NLRP3 inflammasome activation in cochlear macrophages and this is therapeutically targetable, leading the way for the development of interventions to prevent the leading cause of permanent hearing loss and a costly disease in the developed world.

TMC7 deficiency causes acrosome biogenesis defects and male infertility in mice.

Transmembrane channel-like (TMC) proteins are a highly conserved ion channel family consisting of eight members (TMC1-TMC8) in mammals. TMC1/2 are components of the mechanotransduction channel in hair cells, and mutations of TMC1/2 cause deafness in humans and mice. However, the physiological roles of other TMC proteins remain largely unknown. Here, we show that Tmc7 is specifically expressed in the testis and that it is required for acrosome biogenesis during spermatogenesis. Tmc7-/- mice exhibited abnormal sperm head, disorganized mitochondrial sheaths, and reduced number of elongating spermatids, similar to human oligo-astheno-teratozoospermia. We further demonstrate that TMC7 is colocalized with GM130 at the cis-Golgi region in round spermatids. TMC7 deficiency leads to aberrant Golgi morphology and impaired fusion of Golgi-derived vesicles to the developing acrosome. Moreover, upon loss of TMC7 intracellular ion homeostasis is impaired and ROS levels are increased, which in turn causes Golgi and endoplasmic reticulum stress. Taken together, these results suggest that TMC7 is required to maintain pH and ion homeostasis, which is needed for acrosome biogenesis. Our findings unveil a novel role for TMC7 in acrosome biogenesis during spermiogenesis.

Artificial basilar membrane/hair cell integrated acoustic system for keyword spotting in noisy environments inspired by human cochlea

We report a novel speech recognition method using a noise-robust acoustic sensor system that integrates a spatially frequency-separating sensor with a nonlinear amplification algorithm, mimicking the cochlea’s basilar membrane and hair cells. The multichannel piezoelectric artificial basilar membrane (ABM) sensor detects specific sound frequencies with high sensitivity over 0.2–6 kHz. The signal processing model of the Artificial Hair Cell inspired by the signal transduction mechanism of inner hair cells, simultaneously enhances the frequency selectivity of ABM sensors and improves noise robustness. In a 0 dB SNR noisy environment, it effectively detected the voice signal with a maximum SNR of 57 dB. Furthermore, we converted the frequency-separated signals for speech sounds in various noisy environments into heatmap images and utilized them as input for a CNN-based speech recognition algorithm. Consequently, our system demonstrated noise-robust recognition performance with 94 % accuracy, even in noisy environments.

Comparative Histopathologic Analysis of Inner Ear Damage in Meningitis: Otogenic Versus Meningogenic Routes.

To distinguish the patterns of inner ear changes between meningogenic and otogenic routes in meningitis cases. Our hypothesis is that pinpointing distinct patterns linked to each route could aid in the development of diagnostic strategies and targeted therapies. Temporal bones (TBs) from patients with a history of meningitis and histopathological evidence of labyrinthitis were divided into two groups (otogenic and meningogenic). Inner ear histopathological examination was performed to identify qualitative and semi-quantitative changes. This assessment encompassed inflammation patterns, indications of early ossification, hair cell loss, and alterations in the lateral wall, round window membrane, cochlear aqueduct and vestibular aqueduct. Thirty-six TBs were included in the study (otogenic, 21; meningogenic, 15). Generalized labyrinthitis was more common in otogenic cases (100% vs. 53%, p < 0.001). Early signs of cochlear ossification were exclusively observed in otogenic cases (9 TBs). The spiral ligament of otogenic cases has shown a uniform loss of fibrocytes across all cochlear turns, while meningogenic cases showed more severe loss in the apical turn. Otogenic cases exhibited a higher prevalence of severe inflammation of the cochlear aqueduct and endolymphatic sac. Meningogenic cases showed more severe loss of vestibular hair cells in the otolithic organs. Otogenic cases displayed a higher prevalence of changes in the spiral ligament and signs of early ossification, whereas meningogenic cases were associated with a higher degree of vestibular damage. Our findings emphasize the importance of considering the infection route and its implications for timely diagnosis and development of pathology-oriented treatment strategies. NA Laryngoscope, 2024.

Pathogenesis and New Pharmacological Approaches to Noise-Induced Hearing Loss: A Systematic Review.

Noise-induced hearing loss (NIHL) is responsible for significant adverse effects on cognition, quality of life and work, social relationships, motor skills, and other psychological aspects. The severity of NIHL depends on individual patient characteristics, sound intensity, and mainly the duration of sound exposure. NIHL leads to the production of a reactive oxygen (ROS) inflammatory response and the activation of apoptotic pathways, DNA fragmentation, and cell death. In this situation, antioxidants can interact with free radicals as well as anti-apoptotics or anti-inflammatory substances and stop the reaction before vital molecules are damaged. Therefore, the aim of this study was to analyze the effects of different pharmacological treatments, focusing on exogenous antioxidants, anti-inflammatories, and anti-apoptotics to reduce the cellular damage caused by acoustic trauma in the inner ear. Experimental animal studies using these molecules have shown that they protect hair cells and reduce hearing loss due to acoustic trauma. However, there is a need for more conclusive evidence demonstrating the protective effects of antioxidant/anti-inflammatory or anti-apoptotic drugs' administration, the timeline in which they exert their pharmacological action, and the dose in which they should be used in order to consider them as therapeutic drugs. Further studies are needed to fully understand the potential of these drugs as they may be a promising option to prevent and treat noise-induced hearing loss.

Ototoxicity-Alleviating and Cytoprotective Allomelanin Nanomedicine for Efficient Sensorineural Hearing Loss Treatment.

Sensorineural hearing loss (SNHL) represents a significant clinical challenge, predominantly attributed to oxidative stress-related mechanisms. In this work, we report an innovative antioxidant strategy for mitigating SNHL, utilizing synthetically engineered allomelanin nanoparticles (AMNPs). Empirical evidence elucidates AMNPs' profound capability in free radical neutralization, substantiated by a significant decrement in reactive oxygen species (ROS) levels within HEI-OC1 auditory cells exposure to cisplatin or hydrogen peroxide (H2O2). Comparative analyses reveal that AMNPs afford protection against cisplatin-induced and noise-induced auditory impairments, mirroring the effect of dexamethasone (DEX), a standard pharmacological treatment for acute SNHL. AMNPs exhibit notable cytoprotective properties for auditory hair cells (HCs), effectively preventing ototoxicity from cisplatin or H2O2 exposure, as confirmed by both in vitro assays and cultured organ of Corti studies. Further in vivo research corroborates AMNPs' ability to reverse auditory brainstem response (ABR) threshold shifts resulting from acoustic injury, concurrently reducing HCs loss, ribbon synapse depletion, and spiral ganglion neuron degeneration. The therapeutic benefits of AMNPs are attributed to mitigating oxidative stress and inflammation within the cochlea, with transcriptome analysis indicating downregulated gene expression related to these processes post-AMNPs treatment. The pronounced antioxidative and anti-inflammatory effects of AMNPs position them as a promising alternative to DEX for SNHL treatment.

Presynaptic Nrxn3 is essential for ribbon-synapse maturation in hair cells.

Hair cells of the inner ear and lateral-line system rely on specialized ribbon synapses to transmit sensory information to the central nervous system. The molecules required to assemble these synapses are not fully understood. We show that Nrxn3, a presynaptic adhesion molecule, is crucial for ribbon-synapse maturation in hair cells. In both mouse and zebrafish models, the loss of Nrxn3 results in significantly fewer intact ribbon synapses. We show in zebrafish that, initially, Nrxn3 loss does not alter pre- and postsynapse numbers but, later, synapses fail to pair, leading to postsynapse loss. We also demonstrate that Nrxn3 subtly influences synapse selectivity in zebrafish lateral-line hair cells that detect anterior flow. Loss of Nrxn3 leads to a 60% loss of synapses in zebrafish, which dramatically reduces pre- and postsynaptic responses. Despite fewer synapses, auditory responses in zebrafish and mice are unaffected. This work demonstrates that Nrxn3 is a crucial and conserved molecule required for the maturation of ribbon synapses. Understanding how ribbon synapses mature is essential to generating new therapies to treat synaptopathies linked to auditory or vestibular dysfunction.

Hearing Evaluations in Children With Retinoblastoma Treated With Intra-arterial Carboplatin Chemotherapy: A Single Institution Review.

To determine whether the administration of intra-arterial carboplatin affected the hearing of children with retinoblastoma. Children with retinoblastoma who were treated with intra-arterial carboplatin chemotherapy were included. Hearing tests before chemotherapy including tympanometry, distortion product otoacoustic emissions, and audiogram (if achievable) were performed and repeated 3 to 9 months after concluding intra-arterial therapy. The study was approved by the institutional review board. Patients were identified from the retinoblastoma clinic when the treatment plan included intra-arterial carboplatin chemotherapy. Children were excluded if they had previous intra-arterial carboplatin or preexisting hearing loss but were included if they had systemic carboplatin and dosing was available. Tympanometry was performed to rule out inner ear fluid. All examinations were performed by a certified audiologist with the same equipment, calibrated regularly by a certified technician. Twenty-two children (32 eyes) were evaluable. Because most children are diagnosed at a young age and are unable to participate in an audiogram, distortion product otoacoustic emission measurement was the primary measurement. No child displayed hearing loss. Intra-arterial chemotherapy with carboplatin did not cause ototoxicity in any child by distortion product otoacoustic emission measurement in contrast to systemic chemotherapy where ototoxicity is common. Distortion product otoacoustic emission levels were essentially unchanged from before to after intra-arterial chemotherapy in children with retinoblastoma. These findings suggest that intra-arterial carboplatin does not affect outer hair cell function, and distortion product otoacoustic emission tests can provide useful information when monitoring children at risk of developing carboplatin ototoxicity. [J Pediatr Ophthalmol Strabismus. 20XX;X(X):XXX-XXX.].

SUB-immunogold-SEM reveals nanoscale distribution of submembranous epitopes

Electron microscopy paired with immunogold labeling is the most precise tool for protein localization. However, these methods are either cumbersome, resulting in small sample numbers and restricted quantification, or limited to identifying protein epitopes external to the membrane. Here, we introduce SUB-immunogold-SEM, a scanning electron microscopy technique that detects intracellular protein epitopes proximal to the membrane. We identify four critical sample preparation factors contributing to the method’s sensitivity. We validate its efficacy through precise localization and high-powered quantification of cytoskeletal and transmembrane protein distribution. We evaluate the capabilities of SUB-immunogold-SEM on cells with highly differentiated apical surfaces: (i) auditory hair cells, revealing the presence of nanoscale MYO15A-L rings at the tip of stereocilia; and (ii) respiratory multiciliate cells, mapping the distribution of the SARS-CoV-2 receptor ACE2 along the motile cilia. SUB-immunogold-SEM extends the application of SEM-based nanoscale protein localization to the detection of intracellular epitopes on the exposed surfaces of any cell.

LOXHD1 is indispensable for maintaining TMC1 auditory mechanosensitive channels at the site of force transmission

Hair cell bundles consist of stereocilia arranged in rows of increasing heights, connected by tip links that transmit sound-induced forces to shorter stereocilia tips. Auditory mechanotransduction channel complexes, composed of proteins TMC1/2, TMIE, CIB2, and LHFPL5, are located at the tips of shorter stereocilia. While most components can interact with the tip link in vitro, their ability to maintain the channel complexes at the tip link in vivo is uncertain. Return, using mouse models, we show that an additional component, LOXHD1, is essential for keeping TMC1-pore forming subunits at the tip link but is dispensable for TMC2. Using SUB-immunogold-SEM, we showed that TMC1 localizes near the tip link but mislocalizes without LOXHD1. LOXHD1 selectively interacts with TMC1, CIB2, LHFPL5, and tip-link protein PCDH15. Our results demonstrate that TMC1-driven mature auditory channels require LOXHD1 to stay connected to the tip link and remain functional, while TMC2-driven developmental channels do not.

Myosin XVA isoforms participate in the mechanotransduction-dependent remodeling of the actin cytoskeleton in auditory stereocilia.

Auditory hair cells form precise and sensitive staircase-like actin protrusions known as stereocilia. These specialized microvilli detect deflections induced by sound through the activation of mechano-electrical transduction (MET) channels located at their tips. At rest, a small MET channel current results in a constant calcium influx, which regulates the morphology of the actin cytoskeleton in the shorter 'transducing' stereocilia. However, the molecular mechanisms involved in this novel type of activity-driven plasticity in the stereocilium cytoskeleton are currently unknown. Here, we tested the contribution of myosin XVA (MYO15A) isoforms. We used electron microscopy to evaluate morphological changes in the cytoskeleton of auditory hair cell stereocilia after the pharmacological blockage of resting MET currents in cochlear explants from mice that lacked one or all isoforms of MYO15A. Hair cells lacking functional MYO15A isoforms did not exhibit MET-dependent remodeling in their stereocilia cytoskeleton. In contrast, hair cells that only lack the long isoform of MYO15A exhibited increased MET-dependent stereocilia remodeling, including remodeling in stereocilia from the tallest 'non-transducing' row of the bundle. We conclude that MYO15A isoforms not only enable but also fine-tune the MET-dependent remodeling of the actin cytoskeleton in transducing stereocilia and contribute to the stability of the tallest row.