Deficit of mitogen-activated protein kinase phosphatase 1 (DUSP1) accelerates progressive hearing loss

Canonical MAPK signaling in auditory neuropathy

Auditory neuropathy (AN) is an under-recognized form of hearing loss characterized by lesions in inner hair cells (IHCs), ribbon synapses and spiral ganglion neurons (SGNs). The lack of a targeted therapy for AN has increased the need for a better understanding of the pathogenic mechanism of AN. As mitogen-activated protein kinase (MAPK) signaling is ubiquitous in many biological processes, its alteration may facilitate the pathogenesis of multiple sites in AN. Here, we summaries the characteristics of AN under different molecular bases and first explore the mechanism of MAPK at different lesion sites. Alterations of extracellular signal-regulated kinase (ERK)/MAPK occur in IHCs and SGNs, whereas modulations of p38 and c-Jun NH2-terminal kinase (JNK) were found in ribbon synapses and SGNs. In conclusion, inductive MAPK alterations in the pathogenesis and development of AN are likely to represent a potential therapeutic target to guide the development of treatments.

A rare haplotype of the GJD3 gene segregating in familial Meniere's disease interferes with connexin assembly

BACKGROUND

Familial Meniere's disease (FMD) is a rare polygenic disorder of the inner ear. Mutations in the connexin gene family, which encodes gap junction proteins, can also cause hearing loss, but their role in FMD is largely unknown.

METHODS

We retrieved exome sequencing data from 94 individuals in 70 Meniere's disease (MD) families. Through gene burden analysis, we calculated the enrichment of rare variants (allele frequency < 0.05) in connexins genes in FMD individuals compared with the reference population. The connexin monomer and the homomeric connexon structural models were predicted using AlphaFold2 and HDOCK. RT-qPCR and immunofluorescence were done in mice cochleae to identify expression of the mouse ortholog candidate gene Gjd3.

RESULTS

We found an enrichment of rare missense variants in the GJD3 gene when comparing allelic frequencies in FMD (N = 94) with the Spanish reference population (OR = 3.9[1.92-7.91], FDR = 2.36E-03). In the GJD3 sequence, we identified a rare haplotype (TGAGT) composed of two missense, two synonymous, and one downstream variant. This haplotype was found in five individuals with FMD, segregating in three unrelated families with a total of ten individuals; and in another eight MD individuals. GJD3 encodes the gap junction protein delta 3, also known as human connexin 31.9 (Cx31.9). The protein model predicted that the NP_689343.3:p.(His175Tyr) missense variant could modify the interaction between connexins and the connexon assembly, affecting the homotypic GJD3 gap junction between cells. Our studies in mice revealed that Gjd3-encoding Gjd3 or mouse connexin 30.2 (Cx30.2)-was expressed in the organ of Corti and vestibular organs, particularly in the tectorial membrane, the base of inner and outer hair cells and the nerve fibers.

CONCLUSIONS

The present results describe a novel association between GJD3 and FMD, providing evidence that FMD is related to changes in the inner ear channels, and supporting a new role of tectorial membrane proteins in MD.

Calcitriol alleviates noise-induced hearing loss by regulating the ATF3/DUSP1 signalling pathway

BACKGROUND

Calcitriol (Cal) is the most active metabolite of vitamin D and has antioxidant and anti-inflammatory properties. The aim of this study was to investigate the role of Cal in noise-induced hearing loss (NIHL) to further elucidate the mechanism of noise-induced oxidative stress in the mouse cochlea.

METHODS

C57BL/6 J mice were given six intraperitoneal injections of Cal (500 ng/kg/d). After 14 days of noise exposure, auditory brainstem response (ABR) thresholds, and the cochlear outer hair cell loss rate were analysed to evaluate auditory function. Real-time fluorescence quantitative PCR, immunofluorescence and western blotting were performed in vitro after the treatment of cochlear explants with 100 µM tert-butyl hydroperoxide (TBHP) for 2.5 h and HEI-OC1 cells with 250 µM TBHP for 1.5 h.

RESULTS

In vivo experiments confirmed that Cal pretreatment mitigated NIHL and outer hair cell death. The in vitro results demonstrated that Cal significantly reduced TBHP-induced cochlear auditory nerve fibre degradation and spiral ganglion neuron damage. Moreover, treatment with Cal inhibited the expression of oxidative stress-related factors (3-NT and 4-HNE) and DNA damage-related factors (γ-H2A.X) and attenuated TBHP-induced apoptosis in cochlear explants and HEI-OC1 cells. A total of 1479 upregulated genes and 1443 downregulated genes were screened in cochlear tissue 1 h after noise exposure. The level of transcription factor 3 (ATF3) was significantly elevated in HEI-OC1 cells after TBHP stimulation. Gene Transcription Regulation Database （GTRD）and Cistrome database analyses revealed that the downstream target gene of ATF3 is dual specificity phosphatase 1 (DUSP1). Cistrome DB Toolkit database results showed that the transcription factor of DUSP1 was ATF3. In addition, the ChIP-PCR results indicated that ATF3 might be a direct transcription factor of DUSP1.

CONCLUSION

The results of our study suggest that Cal attenuates NIHL and inhibits noise-induced apoptosis by regulating the ATF3/DUSP1 signalling pathway.

A murine model for the del(GJB6-D13S1830) deletion recapitulating the phenotype of human DFNB1 hearing impairment: generation and functional and histopathological study

Inherited hearing impairment is a remarkably heterogeneous monogenic condition, involving hundreds of genes, most of them with very small (< 1%) epidemiological contributions. The exception is GJB2, the gene encoding connexin-26 and underlying DFNB1, which is the most frequent type of autosomal recessive non-syndromic hearing impairment (ARNSHI) in most populations (up to 40% of ARNSHI cases). DFNB1 is caused by different types of pathogenic variants in GJB2, but also by large deletions that keep the gene intact but remove an upstream regulatory element that is essential for its expression. Such large deletions, found in most populations, behave as complete loss-of-function variants, usually associated with a profound hearing impairment. By using CRISPR-Cas9 genetic edition, we have generated a murine model (Dfnb1em274) that reproduces the most frequent of those deletions, del(GJB6-D13S1830). Dfnb1em274 homozygous mice are viable, bypassing the embryonic lethality of the Gjb2 knockout, and present a phenotype of profound hearing loss (> 90 dB SPL) that correlates with specific structural abnormalities in the cochlea. We show that Gjb2 expression is nearly abolished and its protein product, Cx26, is nearly absent all throughout the cochlea, unlike previous conditional knockouts in which Gjb2 ablation was not obtained in all cell types. The Dfnb1em274 model recapitulates the clinical presentation of patients harbouring the del(GJB6-D13S1830) variant and thus it is a valuable tool to study the pathological mechanisms of DFNB1 and to assay therapies for this most frequent type of human ARNSHI.

DUSP1/MKP-1 represents another piece in the P2X7R intracellular signaling puzzle in cerebellar cells: our last journey with Mª Teresa along the purinergic pathways of Eden

The P2X7 receptor (P2X7R) stands out within the purinergic family as it has exclusive pharmacological and regulatory features, and it fulfills distinct roles depending on the type of stimulation and cellular environment. Tonic activation of P2X7R promotes cell proliferation, whereas sustained activation is associated with cell death. Yet strikingly, prolonged P2X7R activation in rat cerebellar granule neurons and astrocytes does not affect cell survival. The intracellular pathways activated by P2X7Rs involve proteins like MAPKs, ERK1/2 and p38, and interactions with growth factor receptors could explain their behavior in populations of rat cerebellar cells. In this study, we set out to characterize the intracellular mechanisms through which P2X7Rs and Trk receptors, EGFR (epidermal growth factor receptor) and BDNFR (brain-derived neurotrophic factor receptor), regulate the dual-specificity phosphatase DUSP1. In cerebellar astrocytes, the regulation of DUSP1 expression by P2X7R depends on ERK and p38 activation. EGFR stimulation can also induce DUSP1 expression, albeit less strongly than P2X7R. Conversely, EGF was virtually ineffective in regulating DUSP1 in granule neurons, a cell type in which BDNF is the main regulator of DUSP1 expression and P2X7R only induces a mild response. Indeed, the regulation of DUSP1 elicited by BDNF reflects the balance between both transcriptional and post-transcriptional mechanisms. Importantly, when the regulation of DUSP1 expression is compromised, the viability of both astrocytes and neurons is impaired, suggesting this phosphatase is essential to maintain proper cell cytoarchitecture and functioning.

Dabrafenib protects from cisplatin-induced hearing loss in a clinically relevant mouse model

The widely used chemotherapy cisplatin causes permanent hearing loss in 40%-60% of patients with cancer. One drug, sodium thiosulfate, is approved by the FDA for use in pediatric patients with localized solid tumors for preventing cisplatin-induced hearing loss, but more drugs are desperately needed. Here, we tested dabrafenib, an FDA-approved BRAF kinase inhibitor and anticancer drug, in a clinically relevant multidose cisplatin mouse model. The protective effects of dabrafenib, given orally twice daily with cisplatin, were determined by functional hearing tests and cochlear outer hair cell counts. Toxicity of the drug cotreatment was evaluated, and levels of phosphorylated ERK were measured. A dabrafenib dose of 3 mg/kg BW, twice daily, in mice, was determined to be the minimum effective dose, and it is equivalent to one-tenth of the daily FDA-approved dose for human cancer treatment. The levels of hearing protection acquired, 20-25 dB at the 3 frequencies tested, in both female and male mice, persisted for 4 months after completion of treatments. Moreover, dabrafenib exhibited a good in vivo therapeutic index (> 25), protected hearing in 2 mouse strains, and diminished cisplatin-induced weight loss. This study demonstrates that dabrafenib is a promising candidate drug for protection from cisplatin-induced hearing loss.

The Stria Vascularis in Mice and Humans Is an Early Site of Age-Related Cochlear Degeneration, Macrophage Dysfunction, and Inflammation

Age-related hearing loss, or presbyacusis, is a common degenerative disorder affecting communication and quality of life for millions of older adults. Multiple pathophysiologic manifestations, along with many cellular and molecular alterations, have been linked to presbyacusis; however, the initial events and causal factors have not been clearly established. Comparisons of the transcriptome in the lateral wall (LW) with other cochlear regions in a mouse model (of both sexes) of "normal" age-related hearing loss revealed that early pathophysiological alterations in the stria vascularis (SV) are associated with increased macrophage activation and a molecular signature indicative of inflammaging, a common form of immune dysfunction. Structure-function correlation analyses in mice across the lifespan showed that the age-dependent increase in macrophage activation in the stria vascularis is associated with a decline in auditory sensitivity. High-resolution imaging analysis of macrophage activation in middle-aged and aged mouse and human cochleas, along with transcriptomic analysis of age-dependent changes in mouse cochlear macrophage gene expression, support the hypothesis that aberrant macrophage activity is an important contributor to age-dependent strial dysfunction, cochlear pathology, and hearing loss. Thus, this study highlights the SV as a primary site of age-related cochlear degeneration and aberrant macrophage activity and dysregulation of the immune system as early indicators of age-related cochlear pathology and hearing loss. Importantly, novel new imaging methods described here now provide a means to analyze human temporal bones in a way that had not previously been feasible and thereby represent a significant new tool for otopathological evaluation.SIGNIFICANCE STATEMENT Age-related hearing loss is a common neurodegenerative disorder affecting communication and quality of life. Current interventions (primarily hearing aids and cochlear implants) offer imperfect and often unsuccessful therapeutic outcomes. Identification of early pathology and causal factors is crucial for the development of new treatments and early diagnostic tests. Here, we find that the SV, a nonsensory component of the cochlea, is an early site of structural and functional pathology in mice and humans that is characterized by aberrant immune cell activity. We also establish a new technique for evaluating cochleas from human temporal bones, an important but understudied area of research because of a lack of well-preserved human specimens and difficult tissue preparation and processing approaches.

Whole transcriptome sequencing analysis of blood plasma-derived exosomes from immune-related hearing loss

BACKGROUND

Early diagnosis of immune-related hearing loss and timely treatment can prevent structural damage to the inner ear and contribute to hearing retention. Exosomal miRNAs, lncRNAs and proteins have great prospects as novel biomarkers for clinical diagnosis. Our study aimed to investigate the molecular mechanisms of exosomes or exosomal ceRNA regulatory networks in immune-related hearing loss.

METHODS

An immune-related hearing loss mice model was constructed by injection with inner ear antigen, and then the blood plasma samples of the mice were collected for exosomes isolation by ultra-centrifugation. Subsequently, the different exosomes were sent for whole transcriptome sequencing using Illumina platform. Finally, a ceRNA pair was chosen for validation by RT-qPCR and dual luciferase reporter gene assay.

RESULTS

The exosomes were successfully extracted from the blood samples of the control and the immune-related hearing loss mice. After sequencing, 94 differentially expressed (DE) lncRNAs, 612 DEmRNAs, and 100 DEmiRNAs were found in the immune-related hearing loss-associated exosomes. Afterwards, ceRNA regulatory networks consisting of 74 lncRNAs, 28 miRNAs and 256 mRNAs were proposed, and the genes in the ceRNA regulatory networks were significantly enriched in 34 GO terms of biological processes and 9 KEGG pathways. Finally, Gm9866 and Dusp7 were significantly up-regulated, while miR-185-5p level was declined in the exosomes from immune-related hearing loss, and Gm9866, miR-185-5p and Dusp7 interacted with each other.

CONCLUSIONS

Gm9866-miR-185-5p-Dusp7 was confirmed to be closely correlated with the occurrence and progression of immune-related hearing loss.

Effect of miR-101 on the Proliferation and Apoptosis of Goat Hair Follicle Stem Cells

The Yangtze River Delta white goat is a rare goat species capable of producing high-quality brush hair. Dual specificity protein phosphatase 1 (DUSP1) may play a role in the formation of high-quality brush hair, as evidenced by our previous research. We investigated the potential mechanisms that regulate the proliferation and apoptosis of goat hair follicle stem cells. We particularly focused on the relationship between DUSP1 and miR-101, which directly targets DUSP1, predicted and screened through bioinformatics websites. Then, fluorescence assays, flow cytometry, RT-qPCR, and Western blotting were used to investigate the effects of miR-101 on the proliferation and apoptosis of hair follicle stem cells. We found that miR-101 overexpression significantly decreased (p < 0.01) apoptosis and promoted the proliferation of hair follicle stem cells. Furthermore, the overexpression of miR-101 increased (p < 0.05) the mRNA and protein expression levels of the proliferation-related gene (PCNA) and anti-apoptotic gene (Bcl-2), and it decreased (p < 0.05) the mRNA and protein expression levels of the apoptotic gene (Bax). In conclusion, miR-101 can promote the proliferation of and inhibit the apoptosis of hair follicle stem cells by targeting DUSP1, which provides a theoretical basis for further elucidating the molecular mechanism that regulates the production of high-quality brush hair of Yangtze River Delta white goats.

Insulin-like Growth Factor 1 Signaling in Mammalian Hearing

Insulin-like growth factor 1 (IGF-1) is a peptide hormone belonging to the insulin family of proteins. Almost all of the biological effects of IGF-1 are mediated through binding to its high-affinity tyrosine kinase receptor (IGF1R), a transmembrane receptor belonging to the insulin receptor family. Factors, receptors and IGF-binding proteins form the IGF system, which has multiple roles in mammalian development, adult tissue homeostasis, and aging. Consequently, mutations in genes of the IGF system, including downstream intracellular targets, underlie multiple common pathologies and are associated with multiple rare human diseases. Here we review the contribution of the IGF system to our understanding of the molecular and genetic basis of human hearing loss by describing, (i) the expression patterns of the IGF system in the mammalian inner ear; (ii) downstream signaling of IGF-1 in the hearing organ; (iii) mouse mutations in the IGF system, including upstream regulators and downstream targets of IGF-1 that inform cochlear pathophysiology; and (iv) human mutations in these genes causing hearing loss.

Use of Radical Oxygen Species Scavenger Nitrones to Treat Oxidative Stress-Mediated Hearing Loss: State of the Art and Challenges

Nitrones are potent antioxidant molecules able to reduce oxidative stress by trapping reactive oxygen and nitrogen species. The antioxidant potential of nitrones has been extensively tested in multiple models of human diseases. Sensorineural hearing loss has a heterogeneous etiology, genetic alterations, aging, toxins or exposure to noise can cause damage to hair cells at the organ of Corti, the hearing receptor. Noxious stimuli share a battery of common mechanisms by which they cause hair cell injury, including oxidative stress, the generation of free radicals and redox imbalance. Therefore, targeting oxidative stress-mediated hearing loss has been the subject of much attention. Here we review the chemistry of nitrones, the existing literature on their use as antioxidants and the general state of the art of antioxidant treatments for hearing loss.

Depletion of microglia mitigates cerebrovascular dysfunction in diet-induced obesity mice

Obesity is frequently associated with cerebrovascular dysfunction; however, the underlying mechanism remains less well understood. In this study, by using pharmacological approaches, we show that neuroinflammation involving microglia plays an important role in obesity-related cerebrovascular dysfunction. PLX3397 treatment, which leads to depletion of microglia, reduced the wall thickness and collagen deposition in the basilar artery of diet-induced obesity (DIO) mice. Besides, the phosphorylation of endothelial nitric oxide synthase (eNOS) at Ser1177 was enhanced, suggesting improved endothelial function of the basilar artery. The wire myography data show that acetylcholine-elicited relaxation of basilar artery isolated from DIO mice was improved after the treatment with PLX3397. Moreover, our data demonstrate that brain administration of IL-18 impaired cerebrovascular function in mice with normal body weight. Together, these data suggest that neuroinflammation involving microglia is important in obesity-related vascular dysfunction in the brain.NEW & NOTEWORTHY We reported that microglia, the resident immune cells in the brain, contribute to obesity-related cerebrovascular dysfunction in mice. Moreover, we showed that excessive IL-18 can lead to vascular dysfunction in mouse brain.

Dual-Specificity Phosphatase 1 (DUSP1) Has a Central Role in Redox Homeostasis and Inflammation in the Mouse Cochlea

Stress-activated protein kinases (SAPK) are associated with sensorineural hearing loss (SNHL) of multiple etiologies. Their activity is tightly regulated by dual-specificity phosphatase 1 (DUSP1), whose loss of function leads to sustained SAPK activation. Dusp1 gene knockout in mice accelerates SNHL progression and triggers inflammation, redox imbalance and hair cell (HC) death. To better understand the link between inflammation and redox imbalance, we analyzed the cochlear transcriptome in Dusp1-/- mice. RNA sequencing analysis (GSE176114) indicated that Dusp1-/- cochleae can be defined by a distinct profile of key cellular expression programs, including genes of the inflammatory response and glutathione (GSH) metabolism. To dissociate the two components, we treated Dusp1-/- mice with N-acetylcysteine, and hearing was followed-up longitudinally by auditory brainstem response recordings. A combination of immunofluorescence, Western blotting, enzymatic activity, GSH levels measurements and RT-qPCR techniques were used. N-acetylcysteine treatment delayed the onset of SNHL and mitigated cochlear damage, with fewer TUNEL+ HC and lower numbers of spiral ganglion neurons with p-H2AX foci. N-acetylcysteine not only improved the redox balance in Dusp1-/- mice but also inhibited cytokine production and reduced macrophage recruitment. Our data point to a critical role for DUSP1 in controlling the cross-talk between oxidative stress and inflammation.

IGF-1 Haploinsufficiency Causes Age-Related Chronic Cochlear Inflammation and Increases Noise-Induced Hearing Loss

Insulin-like growth factor 1 (IGF-1) deficiency is an ultrarare syndromic human sensorineural deafness. Accordingly, IGF-1 is essential for the postnatal maturation of the cochlea and the correct wiring of hearing in mice. Less severe decreases in human IGF-1 levels have been associated with other hearing loss rare genetic syndromes, as well as with age-related hearing loss (ARHL). However, the underlying mechanisms linking IGF-1 haploinsufficiency with auditory pathology and ARHL have not been studied. Igf1-heterozygous mice express less Igf1 transcription and have 40% lower IGF-1 serum levels than wild-type mice. Along with ageing, IGF-1 levels decreased concomitantly with the increased expression of inflammatory cytokines, Tgfb1 and Il1b, but there was no associated hearing loss. However, noise exposure of these mice caused increased injury to sensory hair cells and irreversible hearing loss. Concomitantly, there was a significant alteration in the expression ratio of pro- and anti-inflammatory cytokines in Igf1^+/- mice. Unbalanced inflammation led to the activation of the stress kinase JNK and the failure to activate AKT. Our data show that IGF-1 haploinsufficiency causes a chronic subclinical proinflammatory age-associated state and, consequently, greater susceptibility to stressors. This work provides the molecular bases to further understand hearing disorders linked to IGF-1 deficiency.

All-Trans Retinoic Acid Attenuates Blue Light-Induced Apoptosis of Retinal Photoreceptors by Upregulating MKP-1 Expression

The study investigated the antiapoptotic effects of all-trans retinoic acid (RA) on retinal degeneration caused by exposure to blue light. Sprague-Dawley rats received intraperitoneal injections of RA and, if necessary, the mitogen-activated protein kinase phosphotase-1(MKP-1) inhibitor, (E)-2-benzylidene-3-(cyclohexylamino)-2, 3-dihydro-1H-inden-1-one (BCI), or the retinoic acid receptor (RAR) antagonist, AGN 193109. Retinal damage was induced by 24 h of continuous exposure to blue light. Haematoxylin and eosin staining and electroretinography were performed to measure retinal thickness and retinal function before and at 3 days and 7 days after light exposure. The retinal protein expression levels of phosphorylated c-Jun N-terminal kinase (JNK), phosphorylated nuclear factor-κB, MKP-1, Bim, Bax, and cleaved caspase-3 were also measured. Terminal-deoxynucleotidyl-transferase-mediated deoxyuridine triphosphate-biotin nick end labelling (TUNEL) staining and immunofluorescent staining of cleaved caspase-3 were also performed to evaluate photoreceptor apoptosis. The administration of RA significantly mitigated retinal dysfunction and the decrease in the outer nuclear layer (ONL) thickness at 3 days and 7 days after light exposure. RA also reduced the percentage of TUNEL-positive nuclei in the ONL and cleaved caspase-3 immunofluorescence intensity at 3 days after light exposure. Light exposure increased the retinal expression of proapoptotic proteins (Bim, Bax, and cleaved caspase-3), which was attenuated by RA. Moreover, RA enhanced the expression of MKP-1 and inhibited the phosphorylation of JNK, which were attenuated by the inhibition of RAR. The inhibitory effects of RA on blue light-induced photoreceptor apoptosis were abrogated by the MKP-1inhibitor. Our results indicate that RA alleviates photoreceptor loss following blue light exposure, at least partly, by the MKP-1/JNK pathway, which may serve as a therapeutic target for relieving retinal degeneration.

SCN11A gene deletion causes sensorineural hearing loss by impairing the ribbon synapses and auditory nerves

Background

The SCN11A gene, encoded Nav1.9 TTX resistant sodium channels, is a main effector in peripheral inflammation related pain in nociceptive neurons. The role of SCN11A gene in the auditory system has not been well characterized. We therefore examined the expression of SCN11A in the murine cochlea, the morphological and physiological features of Nav1.9 knockout (KO) ICR mice.

Results

Nav1.9 expression was found in the primary afferent endings beneath the inner hair cells (IHCs). The relative quantitative expression of Nav1.9 mRNA in modiolus of wild-type (WT) mice remains unchanged from P0 to P60. The number of presynaptic CtBP2 puncta in Nav1.9 KO mice was significantly lower than WT. In addition, the number of SGNs in Nav1.9 KO mice was also less than WT in the basal turn, but not in the apical and middle turns. There was no lesion in the somas and stereocilia of hair cells in Nav1.9 KO mice. Furthermore, Nav1.9 KO mice showed higher and progressive elevated ABR threshold at 16 kHz, and a significant increase in CAP thresholds.

Conclusions

These data suggest a role of Nav1.9 in regulating the function of ribbon synapses and the auditory nerves. The impairment induced by Nav1.9 gene deletion mimics the characters of cochlear synaptopathy.

G6PD overexpression protects from oxidative stress and age-related hearing loss

Aging of the auditory system is associated with the incremental production of reactive oxygen species (ROS) and the accumulation of oxidative damage in macromolecules, which contributes to cellular malfunction, compromises cell viability, and, ultimately, leads to functional decline. Cellular detoxification relies in part on the production of NADPH, which is an important cofactor for major cellular antioxidant systems. NADPH is produced principally by the housekeeping enzyme glucose‐6‐phosphate dehydrogenase (G6PD), which catalyzes the rate‐limiting step in the pentose phosphate pathway. We show here that G6PD transgenic mice (G6PD‐Tg), which show enhanced constitutive G6PD activity and NADPH production along life, have lower auditory thresholds than wild‐type mice during aging, together with preserved inner hair cell (IHC) and outer hair cell (OHC), OHC innervation, and a conserved number of synapses per IHC. Gene expression of antioxidant enzymes was higher in 3‐month‐old G6PD‐Tg mice than in wild‐type counterparts, whereas the levels of pro‐apoptotic proteins were lower. Consequently, nitration of proteins, mitochondrial damage, and TUNEL⁺ apoptotic cells were all lower in 9‐month‐old G6PD‐Tg than in wild‐type counterparts. Unexpectedly, G6PD overexpression triggered low‐grade inflammation that was effectively resolved in young mice, as shown by the absence of cochlear cellular damage and macrophage infiltration. Our results lead us to propose that NADPH overproduction from an early stage is an efficient mechanism to maintain the balance between the production of ROS and cellular detoxification power along aging and thus prevents hearing loss progression.

Drug development for noise-induced hearing loss

INTRODUCTION

Excessive exposure to noise is a common occurrence that contributes to approximately 50% of the non-genetic hearing loss cases. Researchers need to develop standardized preclinical models and identify molecular targets to effectively develop prevention and curative therapies.

AREAS COVERED

In this review, the authors discuss the many facets of human noise-induced pathology, and the primary experimental models for studying the basic mechanisms of noise-induced damage, making connections and inferences among basic science studies, preclinical proofs of concept and clinical trials.

EXPERT OPINION

Whilst experimental research in animal models has helped to unravel the mechanisms of noise-induced hearing loss, there are often methodological variations and conflicting results between animal and human studies which make it difficult to integrate data and translate basic outcomes to clinical practice. Standardization of exposure paradigms and application of -omic technologies will contribute to improving the effectiveness of transferring newly gained knowledge to clinical practice.

Deficit of mitogen-activated protein kinase phosphatase 1 (DUSP1) accelerates progressive hearing loss

Mitogen-activated protein kinases (MAPK) such as p38 and the c-Jun N-terminal kinases (JNKs) are activated during the cellular response to stress signals. Their activity is regulated by the MAPK-phosphatase 1 (DUSP1), a key component of the anti-inflammatory response. Stress kinases are well-described elements of the response to otic injury and the otoprotective potential of JNK inhibitors is being tested in clinical trials. By contrast, there are no studies exploring the role of DUSP1 in hearing and hearing loss. Here we show that Dusp1 expression is age-regulated in the mouse cochlea. Dusp1 gene knock-out caused premature progressive hearing loss, as confirmed by auditory evoked responses in Dusp1^-/- mice. Hearing loss correlated with cell death in hair cells, degeneration of spiral neurons and increased macrophage infiltration. Dusp1^-/- mouse cochleae showed imbalanced redox status and dysregulated expression of cytokines. These data suggest that DUSP1 is essential for cochlear homeostasis in the response to stress during ageing.