Apoptosis in inner ear sensory hair cells

Hair cell apoptosis and deafness in Tmc1 mutations

Transmembrane channel-like protein 1 (TMC1), a pore-forming component of the mechano-electrical transducer (MET) channel in cochlear outer hair cells, is subject to numerous mutations causing deafness and hair cell death. We studied mice harboring semidominant mutations Tmc1 p.T416K, p.M412K, and p.D569N, which all display functional MET channels at postnatal day (P)6 but become deaf by P21. Early signs of concomitant hair cell apoptosis were assayed in neonatal Tmc1 mutants by labeling with Calcein-acetomethyl ester (AM), MitoTracker, and Annexin V, the latter labeling scramblase externalization of phosphatidyl serine. Reduced labeling with Calcein-AM was correlated with reduced MitoTracker, the targeting of mitochondria being confirmed with the uncoupling agent carbonylcyanidep-trifluoromethoxyphenylhydrazone, and use of MitoLight to monitor mitochondrial membrane potential. These markers demonstrated mitochondrial dysfunction in Tmc1 mutants, even at P6 when MET currents were still present. Acoustic brainstem responses established that Tmc1 p.D569N and Tmc1 p.M412K mice were deaf by P15 and Tmc1 p.T416K by P21. Two methods of blocking the stereociliary PMCA2 Ca2+ pump both elicited scramblase activity, suggesting that apoptosis is promoted by elevation of hair bundle [Ca2+]. Reduced PMCA2 density was found in the stereocilia of Tmc1 mutants and was correlated with a decrease in MET channel Ca2+ permeability. Cre-Lox excision of the mutant M412K exon at P1 fully preserved hearing across all frequencies by P19 and promoted recovery to wild type of PMCA2 density. These results demonstrate that hair cells in Tmc1 mutants have embarked on apoptosis at P6 and argue for connections between stereociliary PMCA2 density, hair cell apoptosis, and deafness.

A OHCs-Targeted Strategy for PEDF Delivery in Noise-Induced Hearing Loss

Noise-induced hearing loss (NIHL) results from prolonged exposure to intense noise, causing damage to sensory outer hair cells (OHCs) and spiral ganglion neurons (SGNs). The blood labyrinth barrier (BLB) hinders systemic drug delivery to the inner ear. This study applied a retro-auricular round window membrane (RWM) method to bypass the BLB, enabling the transport of macromolecular proteins into the inner ear. Pigment epithelium-derived factor (PEDF), which has anti-inflammatory and neuroprotective properties, is conjugated to a prestin-targeting peptide 2 (PrTP2) using N-succinimidyl-3-maleimidopropionate (SMP) to form PrTP2-SMP/PEDF. This compound specifically targeted Prestin and accumulated around OHCs for sustained release, effectively reducing OHC and SGN loss. Functional and structural tests, including auditory brainstem response (ABR), confocal microscopy, and scanning electron microscopy (SEM), revealed significant hearing restoration and cellular protection. Additionally, the results of enzyme-linked immunosorbent assay (ELISA), Annexin V and propidium iodide (PI) staining and immunoblotting show that noise exposure may induce pyroptosis in the cochlea by activating the NOD-like receptor protein 3 (NLRP3)-apoptosis-associated speck-like protein containing a CARD (ASC) - cysteinyl aspartate specific proteinase (Caspase-1) pathway and PrTP2-SMP/PEDF alleviates the inflammatory response by inhibiting pyroptosis. Toxicity analysis indicates no adverse effects, suggesting that PrTP2-SMP/PEDF has a promising therapeutic prospective for NIHL.

ARC protects cochlear hair cells from neomycin-induced ototoxicity via the Ras/JNK signaling pathway

The present study was designed to investigate the role and mechanism of the Apoptosis repressor with caspase recruitment domain (ARC) in protecting the neomycin-induced hair cell damage. HEI-OC1 cells and basilar membrane culture were applied to determine the effect of ARC. Plasmid transfection was used to regulate the ARC or Ras expression. We have found the ARC overexpression in HEI-OC1 cells can increase the cell viability and decrease cell apoptosis after neomycin injury. The cleaved caspase 3 was reduced in ARC overexpression group after neomycin treatment. The p-CREB expression was increased in ARC overexpression group, while the p-c-Jun expression was decreased after neomycin incubation. In HEI-OC1 cells and basilar membranes, JNK and Ras inhibitions both can reduce ARC expression, and Ras overexpression can increase the ARC expression. This study indicates that ARC can protect the hair cells from neomycin-induced apoptosis through Ras/JNK signaling pathway. Our findings provide new insights in preventing cochlear HC death after drug-induced ototoxicity.

Nanodelivery of antioxidant Agents: A promising strategy for preventing sensorineural hearing loss

Sensorineural hearing loss (SNHL), often stemming from reactive oxygen species (ROS) generation due to various factors such as ototoxic drugs, acoustic trauma, and aging, remains a significant health concern. Oxidative stress-induced damage to the sensory cells of the inner ear, particularly the non-regenerating hair cells, is a critical pathologic mechanism leading to SNHL. Despite the proven efficacy of antioxidants in mitigating oxidative stress, their clinical application for otoprotection is hindered by the limitations of conventional drug delivery methods. This review highlights the challenges associated with systemic and intratympanic administration of antioxidants, including the blood-labyrinthine barrier, restricted permeability of the round window membrane, and inadequate blood flow to the inner ear. To overcome these hurdles, the application of nanoparticles as a delivery platform for antioxidants emerges as a promising solution. Nanocarriers facilitate indirect drug delivery to the cochlea through the round and oval window membrane, optimising drug absorption while reducing dosage, Eustachian tube clearance, and associated side effects. Furthermore, the development of nanoparticles carrying antioxidants tailored to the intracochlear environment holds immense potential. This literature research aimed to critically examine the root causes of SNHL and ROS overproduction in the inner ear, offering insights into the application of nanoparticle-based drug delivery systems for safeguarding sensorineural hair cells. By focusing on the intricate interplay between oxidative stress and hearing loss, this research aims to contribute to the advancement of innovative therapeutic strategies for the prevention of SNHL.

Angiotensin-(1-7) plays an important role in regulating spermatogenesis in Trachemys scripta elegans under salinity stress

Elevation in water salinity can threaten the spermatogenesis and fertility of freshwater animals. The role of the renin-angiotensin system (RAS) in regulating spermatogenesis has attracted considerable attention. Our previous study found that red-eared sliders (Trachemys scripta elegans), could survive in 10 PSU water for over 1 year. To understand the chronic impact of salinity on testicular spermatogenesis and underlying mechanisms, male T. s. elegans were subjected to treatment with water of 5 PSU and 10 PSU for a year, and spermatogenesis and regulation of the RAS signal pathway was assessed. Results showed induced inflammation in the testes of T. s. elegans in the 10 PSU group, as evidenced by a decrease in the number of testicular germ cells from 1586 to 943. Compared with the control group, the levels of proinflammatory genes, including TNF-α, IL-12A and IL-6 were elevated 3.1, 0.3, and 1.4 times, respectively, in animals exposed to 10 PSU water. Testicular antiapoptotic processes of T. s. elegans might involve the vasoactive peptide angiotensin-(1-7) in the RAS, as its level was significantly increased from 220.2 ng ml-1 in controls to 419.2 ng ml-1 in the 10 PSU group. As expected, specific inhibitor (A-779) for the Ang-(1-7) acceptor effectively prevented the salinity-induced upregulation of genes encoding anti-inflammatory and antiapoptotic factors (TGF-β1, Bcl-6) in the testis of the 10 PSU animals, whereas it promoted the upregulation of proinflammatory and proapoptotic factors (TNF-α, IL-12A, IL-6, Bax and caspase-3). Our data indicated that Ang-(1-7) attenuates the effect of salinity on inflammation and apoptosis of the testis in T. s. elegans. A new perspective to prevent salinity-induced testis dysfunction is provided.

Ginsenoside Rc from Panax Ginseng Ameliorates Palmitate-Induced UB/OC-2 Cochlear Cell Injury

By 2050, at least 700 million people will require hearing therapy while 2.5 billion are projected to suffer from hearing loss. Sensorineural hearing loss (SNHL) arises from the inability of the inner ear to convert fluid waves into neural electric signals because of injury to cochlear hair cells that has resulted in their death. In addition, systemic chronic inflammation implicated in other pathologies may exacerbate cell death leading to SNHL. Phytochemicals have emerged as a possible solution because of the growing evidence of their anti-inflammatory, antioxidant, and anti-apoptotic properties. Ginseng and its bioactive molecules, ginsenosides, exhibit effects that suppress pro-inflammatory signaling and protect against apoptosis. In the current study, we investigated the effects of ginsenoside Rc (G-Rc) on UB/OC-2 primary murine sensory hair cell survival in response to palmitate-induced injury. G-Rc promoted UB/OC-2 cell survival and cell cycle progression. Additionally, G-Rc enhanced the differentiation of UB/OC-2 cells into functional sensory hair cells and alleviated palmitate-induced inflammation, endoplasmic reticulum stress, and apoptosis. The current study offers novel insights into the effects of G-Rc as a potential adjuvant for SNHL and warrants further studies elucidating the molecular mechanisms.

Potentials and limitations of pharmaceutical and pharmacological applications of bile acids in hearing loss treatment

Hearing loss is a worldwide epidemic, with approximately 1.5 billion people currently struggling with hearing-related conditions. Currently, the most wildly used and effective treatments for hearing loss are primarily focus on the use of hearing aids and cochlear implants. However, these have many limitations, highlighting the importance of developing a pharmacological solution that may be used to overcome barriers associated with such devices. Due to the challenges of delivering therapeutic agents to the inner ear, bile acids are being explored as potential drug excipients and permeation enhancers. This review, therefore, aims to explore the pathophysiology of hearing loss, the challenges in treatment and the manners in which bile acids could potentially aid in overcoming these challenges.

Alantolactone Inhibits Melanoma Cell Culture Viability and Migration and Promotes Apoptosis by Inhibiting Wnt/β-Catenin Signaling

BACKGROUND

Melanoma is a highly invasive and metastatic malignant tumor originating from melanocytes and is associated with a poor prognosis. Surgical resection and chemotherapy are currently the main therapeutic options for malignant melanoma; however, their efficacy is poor, highlighting the need for the development of new, safe, and effective drugs for the treatment of this cancer.

OBJECTIVE

To investigate the effects of alantolactone (ALT) on the proliferative, migratory, invasive, and apoptotic ability of malignant melanoma cells and explore its potential anticancer mechanism.

METHODS

Melanoma cells (A375 and B16) were treated with different concentrations (4, 6, 8, and 10 μmol/L) of ALT, with DMSO and no treatment serving as controls. The effects of the different concentrations of the drug on cell proliferation were assessed by crystal violet staining and CCK-8 assay. The effects on cell migration and invasion were detected by wound healing and Transwell assays, respectively. Flow cytometry was used to evaluate the effects of the drug on apoptosis and the cell cycle. ALT target genes in melanoma were screened using network pharmacology. Western blotting was used to measure the expression levels of the proliferation-related protein PCNA; the apoptosisrelated proteins Bax, Bcl-2, and caspase-3; the invasion and metastasis-related proteins MMP-2, MMP-7, MMP-9, vimentin, E-cadherin, and N-cadherin; and the canonical Wnt signaling pathway-related proteins β-catenin, c-Myc, and p-GSK3β. In addition, an l model of melanoma was established by the subcutaneous injection of A375 melanoma cells into nude mice, following which the effects of ALT treatment on malignant melanoma were determined in vivo.

RESULTS

Compared with the controls, the proliferative, migratory, and invasive capacity of ALT-treated melanoma cells was significantly inhibited, whereas apoptosis was enhanced (P<0.01), showing effects that were exerted in a dose-dependent manner. The expression levels of the pro-apoptotic proteins Bax and caspase-3, as well as those of the interstitial marker E-cadherin, were upregulated in melanoma cells irrespective of the ALT concentration (P<0.05). In contrast, the expression levels of the anti-apoptotic protein Bcl-2, the proliferation-related protein PCNA, and the invasion and metastasis-related proteins MMP-2, MMP-7, MMP-9, N-cadherin, and vimentin were downregulated (P<0.05). The network pharmacology results indicated that GSK3β may be a key ALT target in melanoma. Meanwhile, western blotting assays showed that ALT treatment markedly suppressed the expression of β-catenin as well as that of its downstream effector c-Myc, and could also inhibit GSK3β phosphorylation.

CONCLUSION

ALT can effectively inhibit the culture viability, migration, and invasion of A375 and B16 melanoma cells while also promoting their apoptosis. ALT may exert its anti-melanoma effects by inhibiting the Wnt/β-catenin signaling pathway. Combined, our data indicate that ALT has the potential as an effective and safe therapeutic drug for the treatment of melanoma.

Characterization of EPO H131S as a key mutation site in the hypoxia-adaptive evolution of Gymnocypris dobula

Erythropoietin (EPO) is a glycoprotein hormone involved in proerythropoiesis, antioxidation, and antiapoptosis. It also contributes to cellular immune function in high-altitude species, such as the schizothoracine fish Gymnocypris dobula. Six mutation sites previously identified in EPO from G. dobula (GD-EPO) were injected into zebrafish embryos, and their effects were compared with EPO from the low-altitude schizothoracine Schizothorax prenanti. The key mutation site in GD-EPO was identified as H131S. Under hypoxic conditions, the levels of superoxide dismutase and malondialdehyde were decreased, whereas that of nitric oxide was increased in zebrafish injected with GD-EPO compared with those injected with S. prenanti-EPO (SP-EPO). The results suggest that EPO in high-altitude schizothoracine species is both antioxidative and antiapoptotic, driven by the H131S mutation site. Thus, this enhanced the ability of this species to adapt to the high-altitude hypoxic environment. These results provide a basis for investigating further the hypoxia adaptation mechanisms of teleosts.

Protective Effects of Vitamin C against Neomycin-Induced Apoptosis in HEI-OC1 Auditory Cell

Ototoxic hearing loss results from hair cell death via reactive oxygen species (ROS) overproduction and consequent apoptosis. We investigated the effects of vitamin C (VC) on neomycin-induced HEI-OC1 cell damage, as well as the mechanism of inhibition. HEI-OC1 cells were treated with neomycin or with vitamin C (VC). The results indicated that VC had a protective effect on neomycin-induced HEI-OC1 cell death. Mechanistically, VC decreased neomycin-induced ROS generation, suppressed cell death, and increased cell viability. VC inhibited neomycin-induced apoptosis, ameliorated neomycin reduced antiapoptotic Bcl-2 expression, and suppressed neomycin increased expression of proapoptotic Bax, caspase-3 cleavage and caspase-8. TUNEL labeling demonstrated that VC blocked neomycin-induced apoptosis. Further study revealed that the effect of VC on neomycin-induced hair cell death was through interference with JNK activation and p38 phosphorylation. These results indicate that VC via suppressed ROS generation, which inhibited cell death by counteracting apoptotic signaling induced by neomycin in cells. Hence, VC is a potential candidate for protection agent against neomycin-induced HEI-OC1 cell ototoxicity.

Ameliorative effect of taxifolin on gentamicin-induced ototoxicity via down-regulation of apoptotic pathways in mouse cochlear UB/OC-2 cells

BACKGROUND

Taxifolin is a flavanonol with efficacious cytoprotective properties, such as anti-inflammatory, antioxidant, anticancer, hepatoprotective, and nephroprotective effects. However, the potential protective effects of taxifolin against gentamicin-induced ototoxicity have not been confirmed. In this study, the possible mechanisms underlying the effects of taxifolin on gentamicin-induced death of UB/OC-2 cochlear cells were investigated.

METHODS

Mouse cochlear UB/OC-2 cells with or without taxifolin pretreatment were exposed to gentamicin, and the effects on cytotoxicity, reactive oxygen species (ROS) production, mitochondrial permeability transition, and apoptotic marker expression were examined using biochemical techniques, flow cytometry, western blotting, and fluorescent staining.

RESULTS

Little or no apparent effect of taxifolin on cell viability was observed at concentrations less than 40 μM. Further investigations showed that gentamicin significantly inhibited cell viability in a concentration-dependent manner. Pretreatment with taxifolin attenuated gentamicin-induced lactate dehydrogenase release, as well as cellular cytotoxicity. In addition, taxifolin significantly prevented gentamicin-induced cell damage by decreasing ROS production, stabilizing mitochondrial membrane potential, and downregulating the mitochondrial pathway of apoptosis.

CONCLUSION

In summary, pretreatment with taxifolin is effective for mitigating gentamicin-induced apoptotic cell death mediated by the mitochondrial pathway. Our data suggest that taxifolin provides a new approach to combat gentamicin-induced ototoxicity.

Carbonic anhydrase inhibitor induces otic hair cell apoptosis via an intrinsic pathway and ER stress in zebrafish larvae

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CA inhibitor EZA causes lateral line organ death in zebrafish larvae.

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Neuromast hair cells are especially sensitive to EZA during embryo development.

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EZA induces apoptosis in otic hair cells via an intrinsic pathway and ER stress.

Carbonic anhydrase (CA) catalyzes reversible hydration of CO₂ to HCO₃⁻ to mediate pH and ion homeostasis. Some chemical pollutants have been reported to have inhibitory effects on fish CA. In this study, we investigated effects of a CA inhibitor ethoxyzolamide (EZA) on neuromasts development during zebrafish embryogenesis, since embryogenesis in aquatic organisms can be particularly sensitive to water pollution. EZA caused alteration of pH and calcium concentration and production of reactive oxygen species (ROS) in larvae, and induced apoptosis in hair cells especially in the otic neuromast, in which CA2 was distributed on the body surface. mRNA levels of apoptotic genes and caspase activities were increased by EZA, whereas anti-oxidants and apoptotic inhibitors, Bax, NF-κB, and p53 inhibitors significantly relieved the induction of hair cell death. Also, mRNA levels of Bip and CHOP, which are induced in response to ER stress, were upregulated by EZA, suggesting that EZA induces otic hair cell apoptosis via the intrinsic mitochondrial pathway and ER stress. Our results demonstrated an essential role of CA in neuromast development via maintenance of ion transport and pH, and that the CA, which is directly exposed to the ambient water, shows marked sensitivity to EZA.

Fursultiamine Prevents Drug-Induced Ototoxicity by Reducing Accumulation of Reactive Oxygen Species in Mouse Cochlea

Drug-induced hearing loss is a major type of acquired sensorineural hearing loss. Cisplatin and aminoglycoside antibiotics have been known to cause ototoxicity, and excessive accumulation of intracellular reactive oxygen species (ROS) are suggested as the common major pathology of cisplatin- and aminoglycoside antibiotics-induced ototoxicity. Fursultiamine, also called thiamine tetrahydrofurfuryl disulfide, is a thiamine disulfide derivative that may have antioxidant effects. To evaluate whether fursultiamine can prevent cisplatin- and kanamycin-induced ototoxicity, we investigated their preventive potential using mouse cochlear explant culture system. Immunofluorescence staining of mouse cochlear hair cells showed that fursultiamine pretreatment reduced cisplatin- and kanamycin-induced damage to both inner and outer hair cells. Fursultiamine attenuated mitochondrial ROS accumulation as evidenced by MitoSOX Red staining and restored mitochondrial membrane potential in a JC-1 assay. In addition, fursultiamine pretreatment reduced active caspase-3 and TUNEL signals after cisplatin or kanamycin treatment, indicating that fursultiamine decreased apoptotic hair cell death. This study is the first to show a protective effect of fursultiamine against cisplatin- and aminoglycoside antibiotics-induced ototoxicity. Our results suggest that fursultiamine could act as an antioxidant and anti-apoptotic agent against mitochondrial oxidative stress.in cochlear hair cells.

Melatonin protects cochlear hair cells from nicotine-induced injury through inhibiting apoptosis, inflammation, oxidative stress and endoplasmic reticulum stress

Hearing loss positively links with cigarette smoking. However, the involved mechanism and treatment strategies are largely unrevealed. This study aimed to investigate the damaging effect of nicotine on cochlear hair cells, reveal the underlying mechanism and evaluate the therapeutic effect of melatonin on nicotine-induced injury. The results showed that nicotine induced cytotoxicity of House Ear Institute-Organ of Corti 1 (HEI-OC1) cochlear hair cells in a dose-dependent manner (0, 2.5, 5, 10, 20, 40 and 80 μM). Functional investigations showed that nicotine (10 μM) stimulation dramatically promoted apoptosis, inflammatory response, oxidative stress and endoplasmic reticulum stress in HEI-OC1 cells. Moreover, melatonin treatment dose-dependently alleviated the nicotine-induced cytotoxicity in HEI-OC1 cells (0, 10 25, 50 and 100 μM). Further investigation showed that melatonin (100 μM) effectively attenuated the nicotine-induced apoptosis, inflammation, oxidative stress and endoplasmic reticulum stress in HEI-OC1 cells. Collectively, we demonstrated that nicotine induced apoptosis, inflammation, oxidative stress and endoplasmic reticulum stress of cochlear hair cells in an in vitro cell model. Melatonin showed protective effect on these aspects, suggesting that melatonin may be a potential agent for treating smoking-induced hearing loss.

A Review on Recent Advancement on Age-Related Hearing Loss: The Applications of Nanotechnology, Drug Pharmacology, and Biotechnology

Aging is considered a contributing factor to many diseases such as cardiovascular disease, Alzheimer’s disease, and hearing loss. Age-related hearing loss, also termed presbycusis, is one of the most common sensory impairments worldwide, affecting one in five people over 50 years of age, and this prevalence is growing annually. Associations have emerged between presbycusis and detrimental health outcomes, including social isolation and mental health. It remains largely untreatable apart from hearing aids, and with no globally established prevention strategies in the clinical setting. Hence, this review aims to explore the pathophysiology of presbycusis and potential therapies, based on a recent advancement in bile acid-based bio-nanotechnologies. A comprehensive online search was carried out using the following keywords: presbycusis, drugs, hearing loss, bile acids, nanotechnology, and more than 150 publications were considered directly relevant. Evidence of the multifaceted oxidative stress and chronic inflammation involvement in cellular damage and apoptosis that is associated with a loss of hair cells, damaged and inflamed stria vascularis, and neuronal signalling loss and apoptosis continues to emerge. New robust and effective therapies require drug delivery deeper into the various layers of the cochlea. Bile acid-based nanotechnology has gained wide interest in its permeation-enhancing ability and potential for numerous applications in treating presbycusis.

Primed to die: an investigation of the genetic mechanisms underlying noise-induced hearing loss and cochlear damage in homozygous Foxo3-knockout mice

The prevalence of noise-induced hearing loss (NIHL) continues to increase, with limited therapies available for individuals with cochlear damage. We have previously established that the transcription factor FOXO3 is necessary to preserve outer hair cells (OHCs) and hearing thresholds up to two weeks following mild noise exposure in mice. The mechanisms by which FOXO3 preserves cochlear cells and function are unknown. In this study, we analyzed the immediate effects of mild noise exposure on wild-type, Foxo3 heterozygous (Foxo3+/-), and Foxo3 knock-out (Foxo3-/-) mice to better understand FOXO3's role(s) in the mammalian cochlea. We used confocal and multiphoton microscopy to examine well-characterized components of noise-induced damage including calcium regulators, oxidative stress, necrosis, and caspase-dependent and caspase-independent apoptosis. Lower immunoreactivity of the calcium buffer Oncomodulin in Foxo3-/- OHCs correlated with cell loss beginning 4 h post-noise exposure. Using immunohistochemistry, we identified parthanatos as the cell death pathway for OHCs. Oxidative stress response pathways were not significantly altered in FOXO3's absence. We used RNA sequencing to identify and RT-qPCR to confirm differentially expressed genes. We further investigated a gene downregulated in the unexposed Foxo3-/- mice that may contribute to OHC noise susceptibility. Glycerophosphodiester phosphodiesterase domain containing 3 (GDPD3), a possible endogenous source of lysophosphatidic acid (LPA), has not previously been described in the cochlea. As LPA reduces OHC loss after severe noise exposure, we treated noise-exposed Foxo3-/- mice with exogenous LPA. LPA treatment delayed immediate damage to OHCs but was insufficient to ultimately prevent their death or prevent hearing loss. These results suggest that FOXO3 acts prior to acoustic insult to maintain cochlear resilience, possibly through sustaining endogenous LPA levels.

SOD2 Alleviates Hearing Loss Induced by Noise and Kanamycin in Mitochondrial DNA4834-deficient Rats by Regulating PI3K/MAPK Signaling

Superoxide dismutase 2 (SOD2)-mediated gene therapy has significant protective effects against kanamycin-induced hearing loss and hair cell loss in the inner ear, but the underlying mechanisms are still unclear. Herein, an in vivo aging model of mitochondrial DNA (mtDNA)4834 deletion mutation was established using D-galactose, and the effects of noise or kanamycin on inner ear injury was investigated. Rats subjected to mtDNA4834 mutation via D-galactose administration showed hearing loss characterized by the disruption of inner ear structure (abnormal cell morphology, hair cell lysis, and the absence of the organ of Corti), increased SOD2 promoter methylation, and an increase in the degree of apoptosis. Exposure to noise or kanamycin further contributed to the effects of D-galactose. SOD2 overexpression induced by viral injection accordingly counteracted the effects of noise and kanamycin and ameliorated the symptoms of hearing loss, suggesting the critical involvement of SOD2 in preventing deafness and hearing-related conditions. The PI3K and MAPK signaling pathways were also regulated by noise/kanamycin exposure and/or SOD2 overexpression, indicating that they may be involved in the therapeutic effect of SOD2 against age-related hearing loss.

Claudin h Is Essential for Hair Cell Morphogenesis and Auditory Function in Zebrafish

Hereditary hearing loss caused by defective hair cells is one of the most common congenital diseases, whose nosogenesis is still unclear because many of the causative genes remain unidentified. Claudins are one kind of transmembrane proteins that constitute the most important components of the tight junctions and paracellular barrier and play important roles in neurodevelopment. In this study, we investigated the function of claudin h in morphogenesis and auditory function of the hair cell in zebrafish. The results of in situ hybridization showed that claudin h was specifically localized in the otic vesicle and neuromasts in zebrafish embryos. The deficiency of claudin h caused significant reduction of otic vesicle size and loss of utricle otolith. Moreover, the startle response and vestibulo-ocular reflex experiments revealed that loss of claudin h led to serious hearing loss and vestibular dysfunction. Importantly, the confocal microscopy observation found that compared to the control zebrafish, the claudin h morphants and mutants displayed significantly reduced the number of cristae hair cells and shortened kinocilia. Besides, the deficiency of claudin h also caused the loss of hair cells in neuromasts which could be rescued by injecting claudin h mRNA into the mutant embryos at one cell stage. Furthermore, the immunohistochemistry experiments demonstrated remarkable apoptosis of hair cells in the neuromasts, which might contribute to the loss of hair cells number. Overall, these data indicated that claudin h is indispensable for the development of hair cells, vestibular function, and hearing ability of zebrafish.

An Implanted Vestibular Prosthesis Improves Spatial Orientation in Animals with Severe Vestibular Damage

Gravity is a pervasive environmental stimulus, and accurate graviception is required for optimal spatial orientation and postural stability. The primary graviceptors are the vestibular organs, which include angular velocity (semicircular canals) and linear acceleration (otolith organs) sensors. Graviception is degraded in patients with vestibular damage, resulting in spatial misperception and imbalance. Since minimal therapy is available for these patients, substantial effort has focused on developing a vestibular prosthesis or vestibular implant (VI) that reproduces information normally provided by the canals (since reproducing otolith function is very challenging technically). Prior studies demonstrated that angular eye velocity responses could be driven by canal VI-mediated angular head velocity information, but it remains unknown whether a canal VI could improve spatial perception and posture since these behaviors require accurate estimates of angular head position in space relative to gravity. Here, we tested the hypothesis that a canal VI that transduces angular head velocity and provides this information to the brain via motion-modulated electrical stimulation of canal afferent nerves could improve the perception of angular head position relative to gravity in monkeys with severe vestibular damage. Using a subjective visual vertical task, we found that normal female monkeys accurately sensed the orientation of the head relative to gravity during dynamic tilts, that this ability was degraded following bilateral vestibular damage, and improved when the canal VI was used. These results demonstrate that a canal VI can improve graviception in vestibulopathic animals, suggesting that it could reduce the disabling perceptual and postural deficits experienced by patients with severe vestibular damage.SIGNIFICANCE STATEMENT Patients with vestibular damage experience impaired vision, spatial perception, and balance, symptoms that could potentially respond to a vestibular implant (VI). Anatomic features facilitate semicircular canal (angular velocity) prosthetics but inhibit approaches with the otolith (linear acceleration) organs, and canal VIs that sense angular head velocity can generate compensatory eye velocity responses in vestibulopathic subjects. Can the brain use canal VI head velocity information to improve estimates of head orientation (e.g., head position relative to gravity), which is a prerequisite for accurate spatial perception and posture? Here we show that a canal VI can improve the perception of head orientation in vestibulopathic monkeys, results that are highly significant because they suggest that VIs mimicking canal function can improve spatial orientation and balance in vestibulopathic patients.

Human induced pluripotent stem cells and CRISPR/Cas-mediated targeted genome editing: Platforms to tackle sensorineural hearing loss

Hearing loss (HL) is a major global health problem of pandemic proportions. The most common type of HL is sensorineural hearing loss (SNHL) which typically occurs when cells within the inner ear are damaged. Human induced pluripotent stem cells (hiPSCs) can be generated from any individual including those who suffer from different types of HL. The development of new differentiation protocols to obtain cells of the inner ear including hair cells (HCs) and spiral ganglion neurons (SGNs) promises to expedite cell-based therapy and screening of potential pharmacologic and genetic therapies using human models. Considering age-related, acoustic, ototoxic, and genetic insults which are the most frequent causes of irreversible damage of HCs and SGNs, new methods of genome editing (GE), especially the CRISPR/Cas9 technology, could bring additional opportunities to understand the pathogenesis of human SNHL and identify novel therapies. However, important challenges associated with both hiPSCs and GE need to be overcome before scientific discoveries are correctly translated to effective and patient-safe applications. The purpose of the present review is (a) to summarize the findings from published reports utilizing hiPSCs for studies of SNHL, hence complementing recent reviews focused on animal studies, and (b) to outline promising future directions for deciphering SNHL using disruptive molecular and genomic technologies.

Molecular Behavior of HMGB1 in the Cochlea Following Noise Exposure and in vitro

Noise-induced hearing loss (NIHL) is characterized by cellular damage to the inner ear, which is exacerbated by inflammation. High-mobility group box 1 (HMGB1), a representative damage-associated molecular pattern (DAMP), acts as a mediator of inflammation or an intercellular messenger according to its cellular localization. Blocking or regulating HMGB1 offers an attractive approach in ameliorating NIHL. However, the precise therapeutic intervention must be based on a deeper understanding of its dynamic molecular distribution and function in cochlear pathogenesis after acoustic trauma. Here, we have presented the spatiotemporal dynamics of the expression of HMGB1, exhibiting distribution variability in specific cochlear regions and cells following noise exposure. After gene manipulation, we further investigated the characteristics of cellular HMGB1 in HEI-OC1 cells. The higher cell viability observed in the HMGB1 knocked-down group after stimulation with H₂O₂ indicated the possible negative effect of HMGB1 on cellular lifespan. In conclusion, this study demonstrated that HMGB1 is involved in NIHL pathogenesis and its molecular biology has essential and subtle influences, preserving a translational potential for pharmacological intervention.

A Newly-Synthesized Chalcone Derivative of Ligustrazine Induces Caspase-Dependent and Apoptosis-Inducing Factor-Dependent Apoptosis in Cochlear Hair Cells

Objective: A newly synthesized derivative of ligustrazine chalcone, named as Z11, has shown a variety of promising biological activities. Here we aim to explore the effects of Z11 on the cochlear hair cells (HCs). Methods: Immunostaining and transmission electron microscopy (TEM) were used to examine the survival of HCs and their morphological changes. Furthermore, apoptosis was detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and the mRNA expression of apoptosis related genes including Caspase-9, Caspase-3, Bcl-2, Bax and Apaf1 were measured by RT-PCR. In addition, the protein expression of cleaved-Caspas-3 and cleaved-Caspase-9 were analyzed by Western blot respectively, and the protein expressionof AIF and cleaved-Caspase-3 were assessed by immunofluorescence as well. Results: Immunostaining showed that Z11 was ototoxic to mouse cochlear hair cells and significantly triggered cell death in a concentration-, time- and location-dependent manner. TUNEL assays evidenced that Z11 exerts its cytotoxicity through induction of apoptosis of cochlear hair cells in vitro. Immunofluorescence and western blot assay showed that Z11 activated the translation of apoptosis-inducing factor (AIF) and Caspase-9/Caspase-3 dependent apoptotic pathway in cochlear hair cells (HCs). Conclusion:These findings suggest that Z11 exhibits its ototoxicity through inducing apoptosis of HCs via both Caspase-dependent and AIF translocation pathways in mouse cochlear cultures.

Congenital Diseases of DNA Replication: Clinical Phenotypes and Molecular Mechanisms

Deoxyribonucleic acid (DNA) replication can be divided into three major steps: initiation, elongation and termination. Each time a human cell divides, these steps must be reiteratively carried out. Disruption of DNA replication can lead to genomic instability, with the accumulation of point mutations or larger chromosomal anomalies such as rearrangements. While cancer is the most common class of disease associated with genomic instability, several congenital diseases with dysfunctional DNA replication give rise to similar DNA alterations. In this review, we discuss all congenital diseases that arise from pathogenic variants in essential replication genes across the spectrum of aberrant replisome assembly, origin activation and DNA synthesis. For each of these conditions, we describe their clinical phenotypes as well as molecular studies aimed at determining the functional mechanisms of disease, including the assessment of genomic stability. By comparing and contrasting these diseases, we hope to illuminate how the disruption of DNA replication at distinct steps affects human health in a surprisingly cell-type-specific manner.

Oral Antioxidant Vitamins and Magnesium Limit Noise-Induced Hearing Loss by Promoting Sensory Hair Cell Survival: Role of Antioxidant Enzymes and Apoptosis Genes

Noise induces oxidative stress in the cochlea followed by sensory cell death and hearing loss. The proof of principle that injections of antioxidant vitamins and Mg2+ prevent noise-induced hearing loss (NIHL) has been established. However, effectiveness of oral administration remains controversial and otoprotection mechanisms are unclear. Using auditory evoked potentials, quantitative PCR, and immunocytochemistry, we explored effects of oral administration of vitamins A, C, E, and Mg2+ (ACEMg) on auditory function and sensory cell survival following NIHL in rats. Oral ACEMg reduced auditory thresholds shifts after NIHL. Improved auditory function correlated with increased survival of sensory outer hair cells. In parallel, oral ACEMg modulated the expression timeline of antioxidant enzymes in the cochlea after NIHL. There was increased expression of glutathione peroxidase-1 and catalase at 1 and 10 days, respectively. Also, pro-apoptotic caspase-3 and Bax levels were diminished in ACEMg-treated rats, at 10 and 30 days, respectively, following noise overstimulation, whereas, at day 10 after noise exposure, the levels of anti-apoptotic Bcl-2, were significantly increased. Therefore, oral ACEMg improves auditory function by limiting sensory hair cell death in the auditory receptor following NIHL. Regulation of the expression of antioxidant enzymes and apoptosis-related proteins in cochlear structures is involved in such an otoprotective mechanism.

Molecular Mechanisms and Biological Functions of Autophagy for Genetics of Hearing Impairment

The etiology of hearing impairment following cochlear damage can be caused by many factors, including congenital or acquired onset, ototoxic drugs, noise exposure, and aging. Regardless of the many different etiologies, a common pathologic change is auditory cell death. It may be difficult to explain hearing impairment only from the aspect of cell death including apoptosis, necrosis, or necroptosis because the level of hearing loss varies widely. Therefore, we focused on autophagy as an intracellular phenomenon functionally competing with cell death. Autophagy is a dynamic lysosomal degradation and recycling system in the eukaryotic cell, mandatory for controlling the balance between cell survival and cell death induced by cellular stress, and maintaining homeostasis of postmitotic cells, including hair cells (HCs) and spiral ganglion neurons (SGNs) in the inner ear. Autophagy is considered a candidate for the auditory cell fate decision factor, whereas autophagy deficiency could be one of major causes of hearing impairment. In this paper, we review the molecular mechanisms and biologic functions of autophagy in the auditory system and discuss the latest research concerning autophagy-related genes and sensorineural hearing loss to gain insight into the role of autophagic mechanisms in inner-ear disorders.

Prevention and Rehabilitation of Old Age Deafness

Hearing impairment is one of the most common sensory deficit affecting 466 million people globally and in majority of old age people it can not corrected. Since presbycusis is always associated with diminished cognition power resulting in two fold loss in understanding of speech. There is no treatment available till date to regenerate the hair cells but certainly we can augment hearing by preventing and regenerating (apoptosis) atrophy of stria vascularis, spiral neural cells degeneration, atrophy of auditory nerve and cerebral cortex by modified greeva, skandh chalan, dynamic neurobics, tratak (focused concentration), Bhramari, Kumbhak along with mindful relaxation technique.

Characterization of rare spindle and root cell transcriptional profiles in the stria vascularis of the adult mouse cochlea

The stria vascularis (SV) in the cochlea generates and maintains the endocochlear potential, thereby playing a pivotal role in normal hearing. Knowing transcriptional profiles and gene regulatory networks of SV cell types establishes a basis for studying the mechanism underlying SV-related hearing loss. While we have previously characterized the expression profiles of major SV cell types in the adult mouse, transcriptional profiles of rare SV cell types remained elusive due to the limitation of cell capture in single-cell RNA-Seq. The role of these rare cell types in the homeostatic function of the adult SV remain largely undefined. In this study, we performed single-nucleus RNA-Seq on the adult mouse SV in conjunction with sample preservation treatments during the isolation steps. We distinguish rare SV cell types, including spindle cells and root cells, from other cell types, and characterize their transcriptional profiles. Furthermore, we also identify and validate novel specific markers for these rare SV cell types. Finally, we identify homeostatic gene regulatory networks within spindle and root cells, establishing a basis for understanding the functional roles of these cells in hearing. These novel findings will provide new insights for future work in SV-related hearing loss and hearing fluctuation.

The slc4a2b gene is required for hair cell development in zebrafish

Hair cells (HCs) function as important sensory receptors that can detect movement in their immediate environment. HCs in the inner ear can sense acoustic signals, while in aquatic vertebrates HCs can also detect movements, vibrations, and pressure gradients in the surrounding water. Many genes are responsible for the development of HCs, and developmental defects in HCs can lead to hearing loss and other sensory dysfunctions. Here, we found that the solute carrier family 4, member 2b (slc4a2b) gene, which is a member of the anion-exchange family, is expressed in the otic vesicles and lateral line neuromasts in developing zebrafish embryos. An in silico analysis showed that the slc4a2b is evolutionarily conserved, and we found that loss of function of slc4a2b resulted in a decreased number of HCs in zebrafish neuromasts due to increased HC apoptosis. Taken together, we conclude that slc4a2b plays a critical role in the development of HCs in zebrafish.

Serum Per- and Polyfluoroalkyl Substances Are Associated with Increased Hearing Impairment: A Re-Analysis of the National Health and Nutrition Examination Survey Data

Although studies have shown that per- and polyfluoroalkyl substances (PFAS) are potential environmental ototoxicants, epidemiologic study has been limited. I conducted a cross-sectional study to re-examine the associations between PFAS and hearing impairment. Data were obtained from the National Health and Nutrition Examination Survey (NHANES) 1999–2000, 2003-06, 2009-12, and 2015-16. Perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorohexane sulfonic acid (PFHxS), and perfluorononanoic acid (PFNA) were measured in serum samples. Participants were divided into quartiles for each PFAS. Air conduction pure-tone audiometry was administered. Hearing impairment (1: yes, 0: no) was defined as a hearing threshold of more than 25 dB at 500, 1000, 2000, 4000, and 8000 Hz in the worse ear. I assessed the relation of serum PFAS with hearing impairment by the generalized linear mixed model with a logit link and binary distribution. Tests for linear trend across quartiles of serum PFAS were conducted using the median serum PFAS in each quartile as a continuous variable. After adjusting for age, sex, body mass index, education, ethnicity group, and family income, I found positive correlations between PFOA and hearing impairment at 2000 Hz (p-trend < 0.01) and 3000 Hz (p-trend = 0.02); between PFOS and hearing impairment at 500 Hz (p-trend < 0.01), 2000 Hz (p-trend < 0.0001) and 3000 Hz (p-trend = 0.02); between PFNA and hearing impairment at 2000 Hz (p-trend = 0.05), 3000 Hz (p-trend < 0.01), 4000 Hz (p-trend = 0.02), and 8000 Hz (p-trend < 0.01); between PFHxS and hearing impairment at 500 Hz (p-trend = 0.04), 1000 Hz (p-trend = 0.03), and 2000 Hz (p-trend < 0.01). However, some of the findings were not significant when only comparing the highest with the lowest quartile of PFASs. In conclusion, several background serum PFASs are positively correlated with hearing impairment in the United States adult population.

Role of melatonin in prevention of age-related hearing loss

INTRODUCTION

Age-related hearing loss (ARHL) is a consequence of aging of the auditory system. The best known mechanism of cell death in ARHL is apoptosis due to increased production of reactive oxygen species. In this context, it is hypothesized that melatonin, owing to its high antioxidant potential and its action in the mitochondria, helps prevent or delay outer hair cell dysfunction (HCD).

AIMS

To evaluate the effect of melatonin on the prevention of HCD dysfunction in the ARHL process in a susceptible murine C57BL/6J model.

METHOD

C57BL/6J animals were divided into two groups: control (CG) and melatonin (MG). The CG received a saline and ethanol solution and the MG, melatonin (10 mg/kg/day). The solutions were offered daily (50 μl) orally over a 10-month period. Distortion Product Otoacoustic Emissions (DPOAE) measurements were conducted once a month.

RESULTS

There was a decrease in DPOAE values in both groups over time and a differentiation between them from the 10th month of life onwards. At 10 months, the MG maintained higher DPOAE values than the CG at all frequencies tested.

CONCLUSION

The use of melatonin has otoprotective effects on HCD in the ARHL process in the C57BL/6J model.

Otoprotection to Implanted Cochlea Exposed to Noise Trauma With Dexamethasone Eluting Electrode

Cochlear implantation (CI) is now widely used to provide auditory rehabilitation to individuals having severe to profound sensorineural hearing loss (SNHL). However, CI can lead to electrode insertion trauma (EIT) that can cause damage to sensory cells in the inner ear resulting in loss of residual hearing. Even with soft surgical techniques where there is minimal macroscopic damage, we can still observe the generation of molecular events that may initiate programmed cell death via various mechanisms such as oxidative stress, the release of pro-inflammatory cytokines, and activation of the caspase pathway. In addition, individuals with CI may be exposed to noise trauma (NT) due to occupation and leisure activities that may affect their hearing ability. Recently, there has been an increased interest in the auditory community to determine the efficacy of drug-eluting electrodes for the protection of residual hearing. The objective of this study is to determine the effect of NT on implanted cochlea as well as the otoprotective efficacy of dexamethasone eluting electrode to implanted cochlea exposed to NT in a guinea pig model of CI. Animals were divided into five groups: EIT with dexamethasone eluting electrode exposed to NT; EIT exposed to NT; NT only; EIT only and naïve animals (control group). The hearing thresholds were determined by auditory brainstem recordings (ABRs). The cochlea was harvested and analyzed for transcript levels of inflammation, apoptosis and fibrosis genes. We observed that threshold shifts were significantly higher in EIT, NT or EIT + NT groups compared to naive animals at all the tested frequencies. The dexamethasone eluting electrode led to a significant decrease in hearing threshold shifts in implanted animals exposed to NT. Proapoptotic tumor necrosis factor-α [TNF-α, TNF-α receptor 1a (TNFαR1a)] and pro-fibrotic transforming growth factor β1 (TGFβ) genes were more than two-fold up-regulated following EIT and EIT + NT compared to the control group. The use of dexamethasone releasing electrode significantly decreased the transcript levels of pro-apoptotic and pro-fibrotic genes. The dexamethasone releasing electrode has shown promising results for hearing protection in implanted animals exposed to NT. The results of this study suggest that dexamethasone releasing electrode holds great potential in developing effective treatment modalities for NT in the implanted cochlea.

Evidence Supporting the Hypothesis That Inflammation-Induced Vasospasm Is Involved in the Pathogenesis of Acquired Sensorineural Hearing Loss

Sensorineural hearing loss is mainly acquired and affects an estimated 1.3 billion humans worldwide. It is related to aging, noise, infection, ototoxic drugs, and genetic defects. It is essential to identify reversible and preventable causes to be able to reduce the burden of this disease. Inflammation is involved in most causes and leads to tissue injury through vasospasm-associated ischemia. Vasospasm is reversible. This review summarized evidence linking inflammation-induced vasospasm to several forms of acquired sensorineural hearing loss. The link between vasospasm and sensorineural hearing loss is directly evident in subarachnoid haemorrhage, which involves the release of vasoconstriction-inducing cytokines like interleukin-1, endothelin-1, and tumour necrosis factor. These proinflammatory cytokines can also be released in response to infection, autoimmune disease, and acute or chronically increased inflammation in the ageing organism as in presbyacusis or in noise-induced cochlear injury. Evidence of vasospasm and hearing loss has also been discovered in bacterial meningitis and brain injury. Resolution of inflammation-induced vasospasm has been associated with improvement of hearing in autoimmune diseases involving overproduction of interleukin-1 from inflammasomes. There is mainly indirect evidence for vasospasm-associated sensorineural hearing loss in most forms of systemic or injury- or infection-induced local vascular inflammation. This opens up avenues in prevention and treatment of vascular and systemic inflammation as well as vasospasm itself as a way to prevent and treat most forms of acquired sensorineural hearing loss. Future research needs to investigate interventions antagonising vasospasm and vasospasm-inducing proinflammatory cytokines and their production in randomised controlled trials of prevention and treatment of acquired sensorineural hearing loss. Prime candidates for interventions are hereby inflammasome inhibitors and vasospasm-reducing drugs like nitric oxide donors, rho-kinase inhibitors, and magnesium which have the potential to reduce sensorineural hearing loss in meningitis, exposure to noise, brain injury, arteriosclerosis, and advanced age-related and autoimmune disease-related inflammation.

DNA methylation dynamics during embryonic development and postnatal maturation of the mouse auditory sensory epithelium

The inner ear is a complex structure responsible for hearing and balance, and organ pathology is associated with deafness and balance disorders. To evaluate the role of epigenomic dynamics, we performed whole genome bisulfite sequencing at key time points during the development and maturation of the mouse inner ear sensory epithelium (SE). Our single-nucleotide resolution maps revealed variations in both general characteristics and dynamics of DNA methylation over time. This allowed us to predict the location of non-coding regulatory regions and to identify several novel candidate regulatory factors, such as Bach2, that connect stage-specific regulatory elements to molecular features that drive the development and maturation of the SE. Constructing in silico regulatory networks around sites of differential methylation enabled us to link key inner ear regulators, such as Atoh1 and Stat3, to pathways responsible for cell lineage determination and maturation, such as the Notch pathway. We also discovered that a putative enhancer, defined as a low methylated region (LMR), can upregulate the GJB6 gene and a neighboring non-coding RNA. The study of inner ear SE methylomes revealed novel regulatory regions in the hearing organ, which may improve diagnostic capabilities, and has the potential to guide the development of therapeutics for hearing loss by providing multiple intervention points for manipulation of the auditory system.

Tauroursodeoxycholic acid attenuates gentamicin-induced cochlear hair cell death in vitro

Gentamycin is one of the most clinically used aminoglycoside antibiotics which induce intrinsic apoptosis of hair cells. Tauroursodeoxycholic acid (TUDCA) is known as safe cell-protective agent in disorders associated with apoptosis. We aimed to investigate the protective effects of TUDCA against gentamicin-induced ototoxicity. House Ear Institute-Organ of Corti 1(HEI-OC1) cells and explanted cochlear tissue were treated with gentamicin and TUDCA, followed by serial analyses including cell viability assay, hair cell staining, qPCR, ELISA and western blotting to determine the cell damage by the parameters relevant to cell apoptosis and endoplasmic reticulum stress. TUDCA significantly attenuated gentamicin-induced cell damage in cultured HEI-OC1 cells and explanted cochlear hair cells. TUDCA alleviated gentamicin-induced cell apoptosis, supported by the decreased Bax/Bcl2 ratio compared with that of gentamicin treated alone. TUDCA decreased gentamicin-induced nitric oxide production and protein nitration in both models. In addition, TUDCA suppressed gentamicin-induced endoplasmic reticulum stress as reflected by inversing the expression levels of Binding immunoglobulin protein (Bip), CCAAT/-enhancer-binding protein homologous protein (CHOP) and Caspase 3. TUDCA attenuated gentamicin-induced hair cell death by inhibiting protein nitration activation and ER stress, providing new insights into the new potential therapies for sensorineural deafness.