Large-scale annotated dataset for cochlear hair cell detection and classification.

Our sense of hearing is mediated by cochlear hair cells, of which there are two types organized in one row of inner hair cells and three rows of outer hair cells. Each cochlea contains 5-15 thousand terminally differentiated hair cells, and their survival is essential for hearing as they do not regenerate after insult. It is often desirable in hearing research to quantify the number of hair cells within cochlear samples, in both pathological conditions, and in response to treatment. Machine learning can be used to automate the quantification process but requires a vast and diverse dataset for effective training. In this study, we present a large collection of annotated cochlear hair-cell datasets, labeled with commonly used hair-cell markers and imaged using various fluorescence microscopy techniques. The collection includes samples from mouse, rat, guinea pig, pig, primate, and human cochlear tissue, from normal conditions and following in-vivo and in-vitro ototoxic drug application. The dataset includes over 107,000 hair cells which have been identified and annotated as either inner or outer hair cells. This dataset is the result of a collaborative effort from multiple laboratories and has been carefully curated to represent a variety of imaging techniques. With suggested usage parameters and a well-described annotation procedure, this collection can facilitate the development of generalizable cochlear hair-cell detection models or serve as a starting point for fine-tuning models for other analysis tasks. By providing this dataset, we aim to give other hearing research groups the opportunity to develop their own tools with which to analyze cochlear imaging data more fully, accurately, and with greater ease.

Large-scale annotated dataset for cochlear hair cell detection and classification.

Our sense of hearing is mediated by cochlear hair cells, localized within the sensory epithelium called the organ of Corti. There are two types of hair cells in the cochlea, which are organized in one row of inner hair cells and three rows of outer hair cells. Each cochlea contains a few thousands of hair cells, and their survival is essential for our perception of sound because they are terminally differentiated and do not regenerate after insult. It is often desirable in hearing research to quantify the number of hair cells within cochlear samples, in both pathological conditions, and in response to treatment. However, the sheer number of cells along the cochlea makes manual quantification impractical. Machine learning can be used to overcome this challenge by automating the quantification process but requires a vast and diverse dataset for effective training. In this study, we present a large collection of annotated cochlear hair-cell datasets, labeled with commonly used hair-cell markers and imaged using various fluorescence microscopy techniques. The collection includes samples from mouse, human, pig and guinea pig cochlear tissue, from normal conditions and following in-vivo and in-vitro ototoxic drug application. The dataset includes over 90'000 hair cells, all of which have been manually identified and annotated as one of two cell types: inner hair cells and outer hair cells. This dataset is the result of a collaborative effort from multiple laboratories and has been carefully curated to represent a variety of imaging techniques. With suggested usage parameters and a well-described annotation procedure, this collection can facilitate the development of generalizable cochlear hair cell detection models or serve as a starting point for fine-tuning models for other analysis tasks. By providing this dataset, we aim to supply other groups within the hearing research community with the opportunity to develop their own tools with which to analyze cochlear imaging data more fully, accurately, and with greater ease.

TRPA1 activation in non-sensory supporting cells contributes to regulation of cochlear sensitivity after acoustic trauma.

TRPA1 channels are expressed in nociceptive neurons, where they detect noxious stimuli, and in the mammalian cochlea, where their function is unknown. Here we show that TRPA1 activation in the supporting non-sensory Hensen's cells of the mouse cochlea causes prolonged Ca2+ responses, which propagate across the organ of Corti and cause long-lasting contractions of pillar and Deiters' cells. Caged Ca2+ experiments demonstrated that, similar to Deiters' cells, pillar cells also possess Ca2+-dependent contractile machinery. TRPA1 channels are activated by endogenous products of oxidative stress and extracellular ATP. Since both these stimuli are present in vivo after acoustic trauma, TRPA1 activation after noise may affect cochlear sensitivity through supporting cell contractions. Consistently, TRPA1 deficiency results in larger but less prolonged noise-induced temporary shift of hearing thresholds, accompanied by permanent changes of latency of the auditory brainstem responses. We conclude that TRPA1 contributes to the regulation of cochlear sensitivity after acoustic trauma.

Asymmetric mechanotransduction by hair cells of the zebrafish lateral line.

In the lateral line system, water motion is detected by neuromast organs, fundamental units that are arrayed on a fish's surface. Each neuromast contains hair cells, specialized mechanoreceptors that convert mechanical stimuli, in the form of water movement, into electrical signals. The orientation of hair cells' mechanosensitive structures ensures that the opening of mechanically gated channels is maximal when deflected in a single direction. In each neuromast organ, hair cells have two opposing orientations, enabling bi-directional detection of water movement. Interestingly, Tmc2b and Tmc2a proteins, which constitute the mechanotransduction channels in neuromasts, distribute asymmetrically so that Tmc2a is expressed in hair cells of only one orientation. Here, using both in vivo recording of extracellular potentials and calcium imaging of neuromasts, we demonstrate that hair cells of one orientation have larger mechanosensitive responses. The associated afferent neuron processes that innervate neuromast hair cells faithfully preserve this functional difference. Moreover, Emx2, a transcription factor required for the formation of hair cells with opposing orientations, is necessary to establish this functional asymmetry within neuromasts. Remarkably, loss of Tmc2a does not impact hair cell orientation but abolishes the functional asymmetry as measured by recording extracellular potentials and calcium imaging. Overall, our work indicates that oppositely oriented hair cells within a neuromast employ different proteins to alter mechanotransduction to sense the direction of water motion.

Multiple plasma membrane reporters discern LHFPL5 region that blocks trafficking to the plasma membrane.

The mechano-electrical transduction (MET) channel of the inner ear receptor cells, termed hair cells, is a protein complex that enables our senses of hearing and balance. Hair cell MET requires an elaborate interplay of multiple proteins that form the MET channel. One of the MET complex components is the transmembrane protein LHFPL5, which is required for hair cell MET and hearing. LHFPL5 is thought to form a multi-protein complex with other MET channel proteins, such as PCDH15, TMIE, and TMC1. Despite localizing to the plasma membrane of stereocilia, the mechanosensing organelles of hair cells, LHFPL5 requires its binding partner within the MET complex, PCDH15, to localize to the stereocilia tips in hair cells and to the plasma membrane in heterologous cells. Using the Aquaporin 3-tGFP reporter (AGR) for plasma membrane localization, we found that a region within extracellular loop 1, which interacts with PCDH15, precludes the trafficking of AGR reporter to the plasma membrane in heterologous cell lines. Our results suggest that the presence of protein partners may mask endoplasmic reticulum retention regions or enable the proper folding and trafficking of the MET complex components, to facilitate expression of the MET complex at the stereocilia membrane.

Mitochondrial calcium uniporter is essential for hearing and hair cell preservation in congenic FVB/NJ mice.

Mitochondrial Ca2+ regulates a wide range of cell processes, including morphogenesis, metabolism, excitotoxicity, and survival. In cochlear hair cells, the activation of mechano-electrical transduction and voltage-gated Ca2+ channels result in a large influx of Ca2+. The intracellular rise in Ca2+ is partly balanced by the mitochondria which rapidly uptakes Ca2+ via a highly selective channel comprised of the main pore-forming subunit, the mitochondrial Ca2+ uniporter (MCU), and associated regulatory proteins. MCU thus contributes to Ca2+ buffering, ensuring cytosolic homeostasis, and is posited to have a critical role in hair cell function and hearing. To test this hypothesis, Ca2+ homeostasis in hair cells and cochlear function were investigated in FVB/NJ mice carrying the knockout allele of Mcu (Mcu+/- or Mcu-/-). The Mcu knockout allele, which originated in C57BL/6 strain cosegregated along with Cdh23ahl allele to the FVB/NJ strain, due to the close proximity of these genes. Neither Mcu+/- nor Mcu-/- genotypes affected cochlear development, morphology, or Ca2+ homeostasis of auditory hair cells in the first two postnatal weeks. However, Mcu-/- mice displayed high-frequency hearing impairment as early as 3 weeks postnatal, which then progressed to profound hearing loss at all frequencies in about 6 months. In Mcu+/- mice, significantly elevated ABR thresholds were observed at 6 months and 9 months of age only at 32 kHz frequency. In three-month-old Mcu-/- mice, up to 18% of the outer hair cells and occasionally some inner hair cells were missing in the mid-cochlear region. In conclusion, mitochondrial Ca2+ uniporter is not required for the development of cochlea in mice, but is essential for hearing and hair cell preservation in congenic FVB/NJ mice.

An Age-Related Hearing Protection Locus on Chromosome 16 of BXD Strain Mice.

Inbred mouse models are widely used to study age-related hearing loss (AHL). Many genes associated with AHL have been mapped in a variety of strains. However, little is known about gene variants that have the converse function-protective genes that confer strong resistance to hearing loss. Previously, we reported that C57BL/6J (B6) and DBA/2J (D2) strains share a common hearing loss allele in Cdh23. The cadherin 23 (Cdh23) gene is a key contributor to early-onset hearing loss in humans. In this study, we tested hearing across a large family of 54 BXD strains generated from B6 to D2 crosses. Five of 54 strains maintain the normal threshold (20 dB SPL) even at 2 years old-an age at which both parental strains are essentially deaf. Further analyses revealed an age-related hearing protection (ahp) locus on chromosome 16 (Chr 16) at 57~76 Mb with a maximum LOD of 5.7. A small number of BXD strains at 2 years with good hearing correspond roughly to the percentage of humans who have good hearing at 90 years old. Further studies to define candidate genes in the ahp locus and related molecular mechanisms involved in age-related resilience or resistance to AHL are warranted.

Tmc proteins are essential for zebrafish hearing where Tmc1 is not obligatory.

Perception of sound is initiated by mechanically gated ion channels at the tips of stereocilia. Mature mammalian auditory hair cells require transmembrane channel-like 1 (TMC1) for mechanotransduction, and mutations of the cognate genetic sequences result in dominant or recessive heritable deafness forms in humans and mice. In contrast, zebrafish lateral line hair cells, which detect water motion, require Tmc2a and Tmc2b. Here, we use standard and multiplex genome editing in conjunction with functional and behavioral assays to determine the reliance of zebrafish hearing and vestibular organs on Tmc proteins. Surprisingly, our approach using multiple mutant alleles demonstrates that hearing in zebrafish is not dependent on Tmc1, nor is it fully dependent on Tmc2a and Tmc2b. Hearing however is absent in triple-mutant zebrafish that lack Tmc1, Tmc2a and Tmc2b. These outcomes reveal a striking resemblance of Tmc protein reliance in the vestibular sensory epithelia of mammals to the maculae of zebrafish. Moreover, our findings disclose a logic of Tmc use where hearing depends on a complement of Tmc proteins beyond those employed to sense water motion.

Genomic non-redundancy of the mir-183/96/182 cluster and its requirement for hair cell maintenance.

microRNAs are important regulators of gene expression. In the retina, the mir-183/96/182 cluster is of particular interest due to its robust expression and studies in which loss of the cluster caused photoreceptor degeneration. However, it is unclear which of the three miRNAs in the cluster are ultimately required in photoreceptors, whether each may have independent, contributory roles, or whether a single miRNA from the cluster compensates for the loss of another. These are important questions that will not only help us to understand the role of these particular miRNAs in the retina, but will deepen our understanding of how clustered microRNAs evolve and operate. To that end, we have developed a complete panel of single, double, and triple mir-183/96/182 mutant zebrafish. While the retinas of all mutant animals were normal, the triple mutants exhibited acute hair cell degeneration which corresponded with impaired swimming and death at a young age. By measuring the penetrance of this phenotype in each mutant line, we determine which of the three miRNAs in the cluster are necessary and/or sufficient to ensure normal hair cell development and function.

Unconventional secretory pathway activation restores hair cell mechanotransduction in an USH3A model.

The pathogenic variant c.144T>G (p.N48K) in the clarin1 gene (CLRN1) results in progressive loss of vision and hearing in Usher syndrome IIIA (USH3A) patients. CLRN1 is predicted to be an essential protein in hair bundles, the mechanosensory structure of hair cells critical for hearing and balance. When expressed in animal models, CLRN1 localizes to the hair bundle, whereas glycosylation-deficient CLRN1N48K aggregates in the endoplasmic reticulum, with only a fraction reaching the bundle. We hypothesized that the small amount of CLRN1N48K that reaches the hair bundle does so via an unconventional secretory pathway and that activation of this pathway could be therapeutic. Using genetic and pharmacological approaches, we find that clarin1 knockout (clrn1KO/KO ) zebrafish that express the CLRN1c.144T>G pathogenic variant display progressive hair cell dysfunction, and that CLRN1N48K is trafficked to the hair bundle via the GRASP55 cargo-dependent unconventional secretory pathway (GCUSP). On expression of GRASP55 mRNA, or on exposure to the drug artemisinin (which activates GCUSP), the localization of CLRN1N48K to the hair bundles was enhanced. Artemisinin treatment also effectively restored hair cell mechanotransduction and attenuated progressive hair cell dysfunction in clrn1KO/KO larvae that express CLRN1c.144T>G , highlighting the potential of artemisinin to prevent sensory loss in CLRN1c.144T>G patients.

A molecular basis for water motion detection by the mechanosensory lateral line of zebrafish.

Detection of water motion by the lateral line relies on mechanotransduction complexes at stereocilia tips. This sensory system is comprised of neuromasts, patches of hair cells with stereociliary bundles arranged with morphological mirror symmetry that are mechanically responsive to two opposing directions. Here, we find that transmembrane channel-like 2b (Tmc2b) is differentially required for mechanotransduction in the zebrafish lateral line. Despite similarities in neuromast hair cell morphology, three classes of these cells can be distinguished by their Tmc2b reliance. We map mechanosensitivity along the lateral line using imaging and electrophysiology to determine that a hair cell's Tmc2b dependence is governed by neuromast topological position and hair bundle orientation. Overall, water flow is detected by molecular machinery that can vary between hair cells of different neuromasts. Moreover, hair cells within the same neuromast can break morphologic symmetry of the sensory organ at the stereocilia tips.

Modeling and Preventing Progressive Hearing Loss in Usher Syndrome III.

Usher syndrome type III (USH3) characterized by progressive loss of vision and hearing is caused by mutations in the clarin-1 gene (CLRN1). Clrn1 knockout (KO) mice develop hair cell defects by postnatal day 2 (P2) and are deaf by P21-P25. Early onset profound hearing loss in KO mice and lack of information about the cochlear cell type that requires Clrn1 expression pose challenges to therapeutic investigation. We generated KO mice harboring a transgene, TgAC1, consisting of Clrn1-UTR (Clrn1 cDNA including its 5' and 3' UTR) under the control of regulatory elements (Atoh1 3' enhancer/ß-globin basal promoter) to direct expression of Clrn1 in hair cells during development and down regulate it postnatally. The KO-TgAC1 mice displayed delayed onset progressive hearing loss associated with deterioration of the hair bundle structure, leading to the hypothesis that hair cell expression of Clrn1 is essential for postnatal preservation of hair cell structure and hearing. Consistent with that hypothesis, perinatal transfection of hair cells in KO-TgAC1 mice with a single injection of AAV-Clrn1-UTR vector showed correlative preservation of the hair bundle structure and hearing through adult life. Further, the efficacy of AAV-Clrn1 vector was significantly attenuated, revealing the potential importance of UTR in gene therapy.

Inhibition of Mitochondrial Division Attenuates Cisplatin-Induced Toxicity in the Neuromast Hair Cells.

Cisplatin and other related platinum antineoplastic drugs are commonly used in the treatment of a variety of cancers in both adults and children but are often associated with severe side effects, including hearing loss. Cisplatin's ototoxic effects are multifaceted, culminating in irreversible damage to the mechanosensory hair cells in the inner ear. Platinum drugs act on cancerous cells by forming nuclear DNA adducts, which may initiate signaling leading to cell cycle arrest or apoptosis. Moreover, it was reported that cisplatin may induce mitochondrial DNA damage in non-cancerous cells. Therefore, protecting mitochondria may alleviate cisplatin-induced insult to non-proliferating cells. Thus, it is important to identify agents that shield the mitochondria from cisplatin-induced insult without compromising the anti-tumor actions of the platinum-based drugs. In this study we tested the protective properties of mitochondrial division inhibitor, mdivi-1, a derivative of quinazolinone and a regulator of mitochondrial fission. Interestingly, it has been reported that mdivi-1 increases the apoptosis of cells that are resistant to cisplatin. The ability of mdivi-1 to protect hair cells against cisplatin-induced toxicity was evaluated in a fish model. Wild-type (Tübingen strain), cdh23 mutant, and transgenic pvalb3b::GFP zebrafish stably expressing GFP in the hair cells were used in this study. Larvae at 5-6 days post fertilization were placed in varying concentrations of cisplatin (50-200 µM) and/or mdivi-1 (1-10 µM) for 16 h. To evaluate hair cell's viability the number of hair bundles per neuromast were counted. To assess hair cell function, we used the FM1-43 uptake assay and recordings of neuromast microphonic potentials. The results showed that mdivi-1 protected hair cells of lateral line neuromasts when they were challenged by 50 µM of cisplatin: viability of hair cells increased almost twice from 19% ± 1.8% to 36% ± 2.0% (p < 0.001). No protection was observed when higher concentrations of cisplatin were used. In addition, our data were in accord with previously reported results that functional mechanotransduction strongly potentiates cisplatin-induced hair cell toxicity. Together, our results suggest that mitochondrial protection may prevent cisplatin-induced damage to hair cells.

ER stress inhibitor attenuates hearing loss and hair cell death in Cdh23erl/erl mutant mice.

Hearing loss is one of the most common sensory impairments in humans. Mouse mutant models helped us to better understand the mechanisms of hearing loss. Recently, we have discovered that the erlong (erl) mutation of the cadherin23 (Cdh23) gene leads to hearing loss due to hair cell apoptosis. In this study, we aimed to reveal the molecular pathways upstream to apoptosis in hair cells to exploit more effective therapeutics than an anti-apoptosis strategy. Our results suggest that endoplasmic reticulum (ER) stress is the earliest molecular event leading to the apoptosis of hair cells and hearing loss in erl mice. We also report that the ER stress inhibitor, Salubrinal (Sal), could delay the progression of hearing loss and preserve hair cells. Our results provide evidence that therapies targeting signaling pathways in ER stress development prevent hair cell apoptosis at an early stage and lead to better outcomes than those targeting downstream factors, such as tip-link degeneration and apoptosis.

Zebrafish Models for the Mechanosensory Hair Cell Dysfunction in Usher Syndrome 3 Reveal That Clarin-1 Is an Essential Hair Bundle Protein.

Usher syndrome type III (USH3) is characterized by progressive loss of hearing and vision, and varying degrees of vestibular dysfunction. It is caused by mutations that affect the human clarin-1 protein (hCLRN1), a member of the tetraspanin protein family. The missense mutation CLRN1(N48K), which affects a conserved N-glycosylation site in hCLRN1, is a common causative USH3 mutation among Ashkenazi Jews. The affected individuals hear at birth but lose that function over time. Here, we developed an animal model system using zebrafish transgenesis and gene targeting to provide an explanation for this phenotype. Immunolabeling demonstrated that Clrn1 localized to the hair cell bundles (hair bundles). The clrn1 mutants generated by zinc finger nucleases displayed aberrant hair bundle morphology with diminished function. Two transgenic zebrafish that express either hCLRN1 or hCLRN1(N48K) in hair cells were produced to examine the subcellular localization patterns of wild-type and mutant human proteins. hCLRN1 localized to the hair bundles similarly to zebrafish Clrn1; in contrast, hCLRN1(N48K) largely mislocalized to the cell body with a small amount reaching the hair bundle. We propose that this small amount of hCLRN1(N48K) in the hair bundle provides clarin-1-mediated function during the early stages of life; however, the presence of hCLRN1(N48K) in the hair bundle diminishes over time because of intracellular degradation of the mutant protein, leading to progressive loss of hair bundle integrity and hair cell function. These findings and genetic tools provide an understanding and path forward to identify therapies to mitigate hearing loss linked to the CLRN1 mutation. SIGNIFICANCE STATEMENT: Mutations in the clarin-1 gene affect eye and ear function in humans. Individuals with the CLRN1(N48K) mutation are born able to hear but lose that function over time. Here, we develop an animal model system using zebrafish transgenesis and gene targeting to provide an explanation for this phenotype. This approach illuminates the role of clarin-1 and the molecular mechanism linked to the CLRN1(N48K) mutation in sensory hair cells of the inner ear. Additionally, the investigation provided an in vivo model to guide future drug discovery to rescue the hCLRN1(N48K) in hair cells.

Noise exposure immediately activates cochlear mitogen-activated protein kinase signaling.

Noise-induced hearing loss (NIHL) is a major public health issue worldwide. Uncovering the early molecular events associated with NIHL would reveal mechanisms leading to the hearing loss. Our aim is to investigate the immediate molecular responses after different levels of noise exposure and identify the common and distinct pathways that mediate NIHL. Previous work showed mice exposed to 116 decibels sound pressure level (dB SPL) broadband noise for 1 h had greater threshold shifts than the mice exposed to 110 dB SPL broadband noise, hence we used these two noise levels in this study. Groups of 4-8-week-old CBA/CaJ mice were exposed to no noise (control) or to broadband noise for 1 h, followed by transcriptome analysis of total cochlear RNA isolated immediately after noise exposure. Previously identified and novel genes were found in all data sets. Following exposure to noise at 116 dB SPL, the earliest responses included up-regulation of 243 genes and down-regulation of 61 genes, while a similar exposure at 110 dB SPL up-regulated 155 genes and down-regulated 221 genes. Bioinformatics analysis indicated that mitogen-activated protein kinase (MAPK) signaling was the major pathway in both levels of noise exposure. Nevertheless, both qualitative and quantitative differences were noticed in some MAPK signaling genes, after exposure to different noise levels. Cacna1b , Cacna1g , and Pla2g6 , related to calcium signaling were down-regulated after 110 dB SPL exposure, while the fold increase in the expression of Fos was relatively lower than what was observed after 116 dB SPL exposure. These subtle variations provide insight on the factors that may contribute to the differences in NIHL despite the activation of a common pathway.

Molecular remodeling of tip links underlies mechanosensory regeneration in auditory hair cells.

Sound detection by inner ear hair cells requires tip links that interconnect mechanosensory stereocilia and convey force to yet unidentified transduction channels. Current models postulate a static composition of the tip link, with protocadherin 15 (PCDH15) at the lower and cadherin 23 (CDH23) at the upper end of the link. In terminally differentiated mammalian auditory hair cells, tip links are subjected to sound-induced forces throughout an organism's life. Although hair cells can regenerate disrupted tip links and restore hearing, the molecular details of this process are unknown. We developed a novel implementation of backscatter electron scanning microscopy to visualize simultaneously immuno-gold particles and stereocilia links, both of only a few nanometers in diameter. We show that functional, mechanotransduction-mediating tip links have at least two molecular compositions, containing either PCDH15/CDH23 or PCDH15/PCDH15. During regeneration, shorter tip links containing nearly equal amounts of PCDH15 at both ends appear first. Whole-cell patch-clamp recordings demonstrate that these transient PCDH15/PCDH15 links mediate mechanotransduction currents of normal amplitude but abnormal Ca(2+)-dependent decay (adaptation). The mature PCDH15/CDH23 tip link composition is re-established later, concomitant with complete recovery of adaptation. Thus, our findings provide a molecular mechanism for regeneration and maintenance of mechanosensory function in postmitotic auditory hair cells and could help identify elusive components of the mechanotransduction machinery.

Fibroblast growth factor receptor 3 regulates microtubule formation and cell surface mechanical properties in the developing organ of Corti.

Fibroblast Growth Factor (Fgf) signaling is involved in the exquisite cellular patterning of the developing cochlea, and is necessary for proper hearing function. Our previous data indicate that Fgf signaling disrupts actin, which impacts the surface stiffness of sensory outer hair cells (OHCs) and non-sensory supporting pillar cells (PCs) in the organ of Corti. Here, we used Atomic Force Microscopy (AFM) to measure the impact of loss of function of Fgf-receptor 3, on cytoskeletal formation and cell surface mechanical properties. We find a 50% decrease in both OHC and PC surface stiffness, and a substantial disruption in microtubule formation in PCs. Moreover, we find no change in OHC electromotility of Fgfr3-deficient mice. To further understand the regulation by Fgf-signaling on microtubule formation, we treated wild-type cochlear explants with Fgf-receptor agonist Fgf2, or antagonist SU5402, and find that both treatments lead to a significant reduction in ß-Tubulin isotypes I&II. To identify downstream transcriptional targets of Fgf-signaling, we used QPCR arrays to probe 84 cytoskeletal regulators. Of the 5 genes significantly upregulated following treatment, Clasp2, Mapre2 and Mark2 impact microtubule formation. We conclude that microtubule formation is a major downstream effector of Fgf-receptor 3, and suggest this pathway impacts the formation of fluid spaces in the organ of Corti.

TRPA1-mediated accumulation of aminoglycosides in mouse cochlear outer hair cells.

Aminoglycoside ototoxicity involves the accumulation of antibiotic molecules in the inner ear hair cells and the subsequent degeneration of these cells. The exact route of entry of aminoglycosides into the hair cells in vivo is still unknown. Similar to other small organic cations, aminoglycosides could be brought into the cell by endocytosis or permeate through large non-selective cation channels, such as mechanotransduction channels or ATP-gated P2X channels. Here, we show that the aminoglycoside antibiotic gentamicin can enter mouse outer hair cells (OHCs) via TRPA1, non-selective cation channels activated by certain pungent compounds and by endogenous products of lipid peroxidation. Using conventional and perforated whole-cell patch clamp recordings, we found that application of TRPA1 agonists initiates inward current responses in wild-type OHCs, but not in OHCs of homozygous Trpa1 knockout mice. Similar responses consistent with the activation of non-selective cation channels were observed in heterologous cells transfected with mouse Trpa1. Upon brief activation with TRPA1 agonists, Trpa1-transfected cells become loaded with fluorescent gentamicin-Texas Red conjugate (GTTR). This uptake was not observed in mock-transfected or non-transfected cells. In mouse organ of Corti explants, TRPA1 activation resulted in the rapid entry of GTTR and another small cationic dye, FM1-43, in OHCs and some supporting cells, even when hair cell mechanotransduction was disrupted by pre-incubation in calcium-free solution. This TRPA1-mediated entry of GTTR and FM1-43 into OHCs was observed in wild-type but not in Trpa1 knockout mice and was not blocked by PPADS, a non-selective blocker of P2X channels. Notably, TRPA1 channels in mouse OHCs were activated by 4-hydroxynonenal, an endogenous molecule that is known to be generated during episodes of oxidative stress and accumulate in the cochlea after noise exposure. We concluded that TRPA1 channels may provide a novel pathway for the entry of aminoglycosides into OHCs.

Actin-bundling protein TRIOBP forms resilient rootlets of hair cell stereocilia essential for hearing.

Inner ear hair cells detect sound through deflection of mechanosensory stereocilia. Each stereocilium is supported by a paracrystalline array of parallel actin filaments that are packed more densely at the base, forming a rootlet extending into the cell body. The function of rootlets and the molecules responsible for their formation are unknown. We found that TRIOBP, a cytoskeleton-associated protein mutated in human hereditary deafness DFNB28, is localized to rootlets. In vitro, purified TRIOBP isoform 4 protein organizes actin filaments into uniquely dense bundles reminiscent of rootlets but distinct from bundles formed by espin, an actin crosslinker in stereocilia. We generated mutant Triobp mice (Triobp(Deltaex8/Deltaex8)) that are profoundly deaf. Stereocilia of Triobp(Deltaex8/Deltaex8) mice develop normally but fail to form rootlets and are easier to deflect and damage. Thus, F-actin bundling by TRIOBP provides durability and rigidity for normal mechanosensitivity of stereocilia and may contribute to resilient cytoskeletal structures elsewhere.