Apoptosis-related genes change their expression with age and hearing loss in the mouse cochlea

Probucol-bile acid nanoparticles: a novel approach and promising solution to prevent cellular oxidative stress in sensorineural hearing loss

The use of antioxidants could thus prove an effective medication to prevent or facilitate recovery from oxidative stress-induced sensorineural hearing loss (SNHL). One promising strategy to prevent SNHL is developing probucol (PB)-based nanoparticles using encapsulation technology and administering them to the inner ear via the established intratympanic route. The preclinical, clinical and epidemiological studies support that PB is a proven antioxidant that could effectively prevent oxidative stress in different study models. Such findings suggest its applicability in preventing oxidative stress within the inner ear and its associated neural cells. However, several hurdles, such as overcoming the blood-labyrinth barrier, ensuring sustained release, minimising systemic side effects and optimising targeted delivery in the intricate inner ear structures, must be overcome to efficiently deliver PB to the inner ear. This review explores the background and pathogenesis of hearing loss, the potential of PB in treating oxidative stress and its cellular mechanisms, and the obstacles linked to inner ear drug delivery for effectively introducing PB to the inner ear.

Environmental noise-induced cardiovascular, metabolic and mental health disorders: a brief review

BACKGROUND

Environmental noise is a pervasive pollutant that is one of the greatest environmental threats to mental, physiological and psychological well-being and has a significant global health burden associated with it. Many epidemiological studies indicate long-term relationship of noise pollution with wide range of metabolic, cardio-vascular and respiratory disorders and diseases.

OBJECTIVE

The goal of this study was to thoroughly analyse available literature on public health implications and various underlying biological mechanisms associated with ambient noise exposure, taking into account both objective and subjective measures of noise exposure.

METHODS

A search of literature for review on environmental noise and associated cardiovascular, mental health and metabolic implications on human health was done using Web of Science, Google Scholar and PubMed databases.

DISCUSSION

Experimental studies indicate that noise exposure leads to endocrine effects, increased incidence of diabetes, impairment of cognitive performance, sleep disturbance and annoyance. Epidemiological evidence indicates that high levels of noise, particularly at night, may cause arterial hypertension and endothelial dysfunction due to higher level of stress hormones and oxidative stress. An increased incidence of cardio-vascular diseases like myocardial infarction, heart rate, ischemic heart disease, stroke and heart failure is associated with noise-induced mental stress. Furthermore, psychological and mental health issues like anxiety and depression are also related with exposure to noise pollution.

CONCLUSION

This article summarises a comprehensive and systematic knowledge established in recent noise research with the spotlight on cardiovascular, metabolic and mental health disorders of environmental noise, providing unique understanding into underlying mechanisms.

Low-Dose Resveratrol Inhibits RIPK3-Mediated Necroptosis and Delays the Onset of Age-Related Hearing Loss

Background: To investigate the pathophysiology of age-related hearing loss (ARHL) and the mechanism of resveratrol (RSV) in prevention and treatment of ARHL.

Methods: C57BL/6 mice of different ages were used in this study. Auditory brainstem response (ABR) was performed to assess hearing and identify abnormalities. Surface preparation and hair cell-specific marker Myo VIIa were employed to evaluated cochlear hair cell losses. Scanning electron microscopy (SEM) was to observe the microstructure of the organ of Corti (OC). The expression of related proteins in the RIPK1/RIPK3/MLKL pathway in cochlear tissue was detected by immunofluorescence.

Results: In old mice (15 months), the ABR threshold increased significantly compared with the young mice. After 50 mg/kg RSV intervention, the hearing threshold of the old mice was significantly reduced at 8 kHz and 12 kHz as well as click. 100 mg/kg RSV led to a statistically significant reduction in hearing threshold only at clicks, whereas 300 mg/kg RSV showed no difference at all frequencies tested. In terms of cochlear hair cell loss, the damage of OHC and IHC was severe in old mice, but the damage was evidently reduced in RSV 50 mg/kg group. Notably, in the RSV 300 mg/kg group, the loss and disorientation of both the OHCs and IHCs were aggravated. Under SEM, a large number of OHCs were lost in the old group, but increased significantly in the RSV 50 mg/kg group, and even the OHCs were more seriously damaged in the RSV 300 mg/kg group. Furthermore, immunofluorescence showed that 50 mg/kg RSV significantly reduced the expression of RIPK3, RIPK1, and MLKL in the cochlea during aging, especially in necroptosis-sensitive regions OCs and SGN.

Conclusion: Low-dose RSV inhibited RIPK3-mediated necroptosis in aging cochlea and delayed the onset of ARHL, which was a promising therapeutic strategy for ARHL.

Prestin and electromotility may serve multiple roles in cochlear outer hair cells

Outer hair cells (OHCs) are innervated by both medial olivocochlear (MOC) efferents and type II afferents, which also innervate supporting cells to form a local neural network. It has also been demonstrated that prestin provides the molecular basis for OHC somatic electromotility, amplifying movements within the organ of Corti. Although not anticipated, early-onset OHC loss was found in two prestin transgenic mouse models that either lack prestin protein or lack electromotility. To uncover the molecular pathways that evoke OHC death, we profiled the coding transcriptome of OHCs from wildtype (WT), prestin-knockout (KO), and 499-knockin (KI) mice using single-cell RNA sequencing (scRNA-seq). scRNA-Seq transcriptomics and pathway analyses did not reveal common pathways associated with OHC loss observed in prestin-KO and 499-KI mice. Clustering enrichment analysis showed that increased gene expression in OHCs from prestin-KO mice was associated with lipid metabolic processes and cell death pathways. These mRNA profiles likely contribute to the OHC loss observed in prestin-KO mice and support the notion that prestin is also a structural protein, important for the normal plasma membrane compartmentalization that is essential to establish MOC efferent synapses. In contrast, the mRNA profile of OHCs from 499-KI mice did not provide a rational explanation of the early-onset OHC loss in this mutant. OHCs from 499-KI mice have normal plasma membrane compartmentalization and normal OHC-MOC contacts. However, 499 prestin lacks electromotility and appears to change the local neural network around OHCs, as more synaptic markers were found near neighboring supporting cells when compared to WT and prestin-KO mice. Thus, OHCs in prestin-KOs (no prestin protein, no electromotility) and 499-KIs (prestin protein present, no electromotility) may influence local neuronal networks in different ways. Collectively, our data suggest that prestin and its motile properties are important for OHC survival and the maintenance of local afferent/efferent circuits, as well as for its role in cochlear amplification.

Thymoquinone ameliorates age-related hearing loss in C57BL/6J mice by modulating Sirt1 activity and Bak1 expression

Age-related hearing loss (AHL) is the most common sensory disorder of aged population. Currently, one of the most important sources of experimental medicine for AHL is medicinal plants. This study performed the first investigation of the effect of thymoquinone (TQ), a potent antioxidant, on AHL. Here, we used inbred C57BL/6J mice (B6 mice) as a successful experimental model of the early onset of AHL. The behavioral assessment of hearing revealed that the injection of a high dose of TQ (40 mg/kg; TQ40) significantly improved the auditory sensitivity of B6 mice at all tested frequencies (8, 16 and 22 kHz). Histological sections of cochlea from B6 mice injected with a low dose (20 mg/kg; TQ20) and high dose showed relatively less degenerative signs in the modiolus, hair cells and spiral ligaments, the main constituents of the cochlea. In addition, TQ40 completely restored the normal pattern of hair cells in B6 mice, as shown in scanning electron micrographs. Our data indicated that TQ20 and TQ40 reduced levels of Bak1-mediated apoptosis in the cochlea of B6 mice. Interestingly, the level of Sirt1, a positive regulator of autophagy, was significantly increased in B6 mice administered TQ40. In conclusion, TQ relieves the symptoms of AHL by downregulating Bak1 and activating Sirt1 in the cochlea of B6 mice.

Key Signaling Pathways Regulate the Development and Survival of Auditory Hair Cells

The loss of auditory sensory hair cells (HCs) is the most common cause of sensorineural hearing loss (SNHL). As the main sound transmission structure in the cochlea, it is necessary to maintain the normal shape and survival of HCs. In this review, we described and summarized the signaling pathways that regulate the development and survival of auditory HCs in SNHL. The role of the mitogen-activated protein kinase (MAPK), phosphoinositide-3 kinase/protein kinase B (PI3K/Akt), Notch/Wnt/Atoh1, calcium channels, and oxidative stress/reactive oxygen species (ROS) signaling pathways are the most relevant. The molecular interactions of these signaling pathways play an important role in the survival of HCs, which may provide a theoretical basis and possible therapeutic interventions for the treatment of hearing loss.

Progenitor cell therapy for acquired pediatric nervous system injury: Traumatic brain injury and acquired sensorineural hearing loss

While cell therapies hold remarkable promise for replacing injured cells and repairing damaged tissues, cell replacement is not the only means by which these therapies can achieve therapeutic effect. For example, recent publications show that treatment with varieties of adult, multipotent stem cells can improve outcomes in patients with neurological conditions such as traumatic brain injury and hearing loss without directly replacing damaged or lost cells. As the immune system plays a central role in injury response and tissue repair, we here suggest that multipotent stem cell therapies achieve therapeutic effect by altering the immune response to injury, thereby limiting damage due to inflammation and possibly promoting repair. These findings argue for a broader understanding of the mechanisms by which cell therapies can benefit patients.

SYPL1 Inhibits Apoptosis in Pancreatic Ductal Adenocarcinoma via Suppression of ROS-Induced ERK Activation

Synaptophysin-like 1 (SYPL1) is a neuroendocrine-related protein. The role of SYPL1 in pancreatic ductal adenocarcinoma (PDAC) and the underlying molecular mechanism remain unclarified. Here, after analyzing five datasets (GSE15471, GSE16515, GSE28735, TCGA, and PACA-AU) and 78 PDAC patients from Sun Yat-sen University Cancer Center, we demonstrated that SYPL1 was upregulated in PDAC and that a high level of SYPL1 indicated poor prognosis. Bioinformatics analysis implied that SYPL1 was related to cell proliferation and cell death. To validate these findings, gain-of-function and loss-of-function experiments were carried out, and we found that SYPL1 promoted cell proliferation in vitro and in vivo and that it protected cells from apoptosis. Mechanistic studies revealed that sustained extracellular-regulated protein kinase (ERK) activation was responsible for the cell death resulting from knockdown of SYPL1. In addition, bioinformatics analysis showed that the expression of SYPL1 positively correlated with antioxidant activity. Reactive oxygen species (ROS) were upregulated in cells with SYPL1 knockdown and vice versa. Upregulated ROS led to ERK activation and cell death. These results suggest that SYPL1 plays a vital role in PDAC and promotes cancer cell survival by suppressing ROS-induced ERK activation.

Type 1 Diabetes Induces Hearing Loss: Functional and Histological Findings in An Akita Mouse Model

The relationship between type 1 diabetes and hearing loss is not well known, although based on many pathological studies, type 2 diabetes induced hearing loss is associated with microcirculation problems in the inner ear. The purpose of this study was to investigate the correlation between type 1 diabetes and hearing loss through hearing function and immunohistochemical analyses using type 1 diabetic Akita or wild-type (WT) mice. The Akita mice had a significant increase in hearing thresholds, blood glucose, and insulin tolerance compared to WT mice. Histological analysis showed that the loss of cells and damage to mitochondria in the spiral ganglion neurons of Akita mice were significantly increased compared to WT. Also, the stria vascularis showed decreased thickness, loss of intermediate cells, and disturbance in blood capillary shape in the Akita mice. Moreover, a reduction in type I, II, and IV fibrocytes and Na⁺/K⁺-ATPase α1 expression in spiral ligament was also observed. Cleaved caspase-3 expression was highly expressed in spiral ganglion neurons. In conclusion, hearing loss in type 1 diabetes is caused not only by ion imbalance and blood flow disorders of cochlear endolymph, but through the degenerative nervous system via apoptosis-mediated cell death.

Ectopic expression of ROR1 prevents cochlear hair cell loss in guinea pigs with noise-induced hearing loss

Noise-induced hearing loss (NIHL) is one of the most frequent disabilities in industrialized countries. Evidence shows that hair cell loss in the auditory end organ is responsible for the majority of various ear pathological conditions. The functional roles of the receptor tyrosine kinase ROR1 have been underscored in various tumours. In this study, we evaluated the ability of ROR1 to influence cochlear hair cell loss of guinea pigs with NIHL. The NIHL model was developed in guinea pigs, with subsequent measurement of the auditory brainstem response (ABR). Gain-of-function experiments were employed to explore the role of ROR1 in NIHL. The interaction between ROR1 and Wnt5a and their functions in the cochlear hair cell loss were further analysed in response to alteration of ROR1 and Wnt5a. Guinea pigs with NIHL demonstrated elevated ABR threshold and down-regulated ROR1, Wnt5a and NF-κB p65. The up-regulation of ROR1 was shown to decrease the cochlear hair cell loss and the expression of pro-apoptotic gene (Bax, p53) in guinea pig cochlea, but promoted the expression of anti-apoptotic gene (Bcl-2) and the fluorescence intensity of cleaved-caspase-3. ROR1 interacted with Wnt5a to activate the NF-κB signalling pathway through inducing phosphorylation and translocation of p65. Furthermore, Wnt5a overexpression decreased the cochlear hair cell loss. Collectively, this study suggested the protection of overexpression of ROR1 against cochlear hair cell loss in guinea pigs with NIHL via the Wnt5a-dependent NF-κB signalling pathway.

MicroRNAs in Noise-Induced Hearing Loss and their Regulation by Oxidative Stress and Inflammation

Noise exposure (NE) has been recognized as one of the causes of sensorineural hearing loss (SNHL), which can bring about irreversible damage to sensory hair cells in the cochlea, through the launch of oxidative stress pathways and inflammation. Accordingly, determining the molecular mechanism involved in regulating hair cell apoptosis via NE is essential to prevent hair cell damage. However, the role of microRNAs (miRNAs) in the degeneration of sensory cells of the cochlea during NE has not been so far uncovered. Thus, the main purpose of this study was to demonstrate the regulatory role of miRNAs in the oxidative stress pathway and inflammation induced by NE. In this respect, articles related to noise-induced hearing loss (NIHL), oxidative stress, inflammation, and miRNA from various databases of Directory of Open Access Journals (DOAJ), Google Scholar, PubMed; Library, Information Science & Technology Abstracts (LISTA), and Web of Science were searched and retrieved. The findings revealed that several studies had suggested that up-regulation of miR-1229-5p, miR-451a, 185-5p, 186 and down-regulation of miRNA-96/182/183 and miR-30b were involved in oxidative stress and inflammation which could be used as biomarkers for NIHL. There was also a close relationship between NIHL and miRNAs, but further research is required to prove a causal association between miRNA alterations and NE, and also to determine miRNAs as biomarkers indicating responses to NE.

Presbycusis: An Update on Cochlear Mechanisms and Therapies

Age-related hearing impairment (ARHI), also referred to as presbycusis, is the most common sensory impairment seen in the elderly. As our cochlea, the peripheral organ of hearing, ages, we tend to experience a decline in hearing and are at greater risk of cochlear sensory-neural cell degeneration and exacerbated age-related hearing impairments, e.g., gradual hearing loss, deterioration in speech comprehension (especially in noisy environments), difficulty in the localization sound sources, and ringing sensations in the ears. However, the aging process does not affect people uniformly; nor, in fact, does the aging process appear to be uniform even within an individual. Here, we outline recent research into chronological cochlear age in healthy people, and exacerbated hearing impairments during aging due to both extrinsic factors including noise and ototoxic medication, and intrinsic factors such as genetic predisposition, epigenetic factors, and aging. We review our current understanding of molecular pathways mediating ARHL and discuss recent discoveries in experimental hearing restoration and future prospects.

MicroRNAs in acquired sensorineural hearing loss

OBJECTIVE

This review summarises the current literature on the role of microRNAs in presbyacusis (age-related hearing loss) and sudden sensorineural hearing loss.

METHODS

Medline, PubMed, Web of Science and Embase databases were searched for primary English-language studies, published between 2000 and 2017, which investigated the role of microRNAs in the pathogenesis of presbyacusis or sudden sensorineural hearing loss. Quality of evidence was assessed using the National Institutes of Health quality assessment tool.

RESULTS

Nine of 207 identified articles, 6 of good quality, satisfied the review's inclusion criteria. In presbyacusis, microRNAs in pro-apoptotic and autophagy pathways are upregulated, while microRNAs in proliferative and differentiation pathways are downregulated. Evidence for microRNAs having an aetiological role in sudden hearing loss is limited.

CONCLUSION

A shift in microRNA expression, leading to reduced cellular activity and impaired inner-ear homeostasis, may contribute to the pathogenesis of presbyacusis.

Bcl-2 is a prognostic marker and its silencing inhibits recurrence in ameloblastomas

OBJECTIVES

Ameloblastomas are the most common odontogenic epithelial tumors with high recurrence rate. The aim of this study was to identify apoptosis-related genes with recurrence of ameloblastomas and to evaluate its feasibility as a prognostic marker and as a target molecule preventing from recurrence.

MATERIALS AND METHODS

Public microarray data were analyzed. To evaluate their expression in ameloblastoma patients, immunohistochemical staining was performed in 89 human ameloblastoma tissues. Quantitative PCR was performed by use of ameloblastoma cell line (AM-1). Fluorescence activated cell sorting analysis and western blotting were conducted following transfection with siRNA. Further, AM-1 cells were implanted in the renal subcapsular layer of immunodeficient mice.

RESULTS

Microarray data analysis revealed that osteoprotegerin (OPG) and B-cell lymphoma 2 (Bcl-2) were the two most upregulated genes in ameloblastoma. Only Bcl-2 expression was significantly (p = 0.020) associated with recurrence in conservative treatment group (n = 17) among 89 patients. Silencing of Bcl-2 increased apoptosis in AM-1 cells in vitro and inhibited tumor nodule formation of AM-1 cells in vivo.

CONCLUSION

These results suggest that Bcl-2 expression is a useful biomarker to predict recurrence of ameloblastomas, and as a therapeutic target molecule to prevent recurrence of ameloblastoma.

Decreased Expression of TRPV4 Channels in HEI-OC1 Cells Induced by High Glucose Is Associated with Hearing Impairment

PURPOSE

Previous reports have shown that hyperglycemia-induced inhibition of transient receptor potential vanilloid sub type 4 (TRPV4), a transient receptor potential ion channel, affects the severity of hearing impairment (HI). In this study, we explored the role of TRPV4 in HI using HEI-OC1 cells exposed to high glucose (HG).

MATERIALS AND METHODS

HEI-OC1 cells were cultured in a HG environment (25 mM D-glucose) for 48 hours, and qRT-PCR and Western blotting were used to analyze the expression of TRPV4 at the mRNA and protein level. TRPV4 agonist (GSK1016790A) or antagonist (HC-067047) in cultured HEI-OC1 cells was used to obtain abnormal TRPV4 expression. Functional TRPV4 activity was assessed in cultured HEI-OC1 cells using the MTT assay and a cell death detection ELISA.

RESULTS

TRPV4 agonists exerted protective effects against HG-induced HI, as evidenced by increased MTT levels and inhibition of apoptosis in HEI-OC1 cells. TRPV4 overexpression significantly increased protein levels of phosphorylated p38 mitogen-activated protein kinase (p-p38 MAPK), while TRPV4 antagonists had the opposite effect. Our results indicated that TRPV4 is a hyperglycemia-related factor that can inhibit cell proliferation and promote cell apoptosis by activating the MAPK signaling pathway in HEI-OC1 cells.

CONCLUSION

Our results show that the overexpression of TRPV4 can attenuate cell death in HEI-OC1 cells exposed to HG.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

Genome-wide identification of directed gene networks using large-scale population genomics data

Identification of causal drivers behind regulatory gene networks is crucial in understanding gene function. Here, we develop a method for the large-scale inference of gene-gene interactions in observational population genomics data that are both directed (using local genetic instruments as causal anchors, akin to Mendelian Randomization) and specific (by controlling for linkage disequilibrium and pleiotropy). Analysis of genotype and whole-blood RNA-sequencing data from 3072 individuals identified 49 genes as drivers of downstream transcriptional changes (Wald P < 7 × 10-10), among which transcription factors were overrepresented (Fisher's P = 3.3 × 10-7). Our analysis suggests new gene functions and targets, including for SENP7 (zinc-finger genes involved in retroviral repression) and BCL2A1 (target genes possibly involved in auditory dysfunction). Our work highlights the utility of population genomics data in deriving directed gene expression networks. A resource of trans-effects for all 6600 genes with a genetic instrument can be explored individually using a web-based browser.

A Novel Dorsal Slit Approached Non-Ischemic Partial Nephrectomy Method for a Renal Tissue Regeneration in a Mouse Model

BACKGROUND

Kidney ischemia-reperfusion (IR) via laparotomy is a conventional method for kidney surgery in a mouse model. However, IR, an invasive procedure, can cause serious acute and chronic complications through apoptotic and inflammatory pathways. To avoid these adverse responses, a Non-IR and dorsal slit approach was designed for kidney surgery.

METHODS

Animals were divided into three groups, 1) sham-operated control; 2) IR, Kidney IR via laparotomy; and 3) Non-IR, Non-IR and dorsal slit. The effects of Non-IR method on renal surgery outcomes were verified with respect to animal viability, renal function, apoptosis, inflammation, fibrosis, renal regeneration, and systemic response using histology, immunohistochemistry, real-time polymerase chain reaction, serum chemistry, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining, and Masson's trichrome staining.

RESULTS

The Non-IR group showed 100% viability with mild elevation of serum blood urea nitrogen and creatinine values at day 1 after surgery, whereas the IR group showed 20% viability and lethal functional abnormality. Histologically, renal tubule epithelial cell injury was evident on day 1 in the IR group, and cellular apoptosis enhanced TUNEL-positive cell number and Fas/caspase-3 and KIM-1/NGAL expression. Inflammation and fibrosis were high in the IR group, with enhanced CD4/CD8-positive T cell infiltration, inflammatory cytokine secretion, and Masson's trichrome stain-positive cell numbers. The Non-IR group showed a suitable microenvironment for renal regeneration with enhanced host cell migration, reduced immune cell influx, and increased expression of renal differentiation-related genes and anti-inflammatory cytokines. The local renal IR influenced distal organ apoptosis and inflammation by releasing circulating pro-inflammatory cytokines.

CONCLUSION

The Non-IR and dorsal slit method for kidney surgery in a mouse model can be an alternative surgical approach for researchers without adverse reactions such as apoptosis, inflammation, fibrosis, functional impairment, and systemic reactions.

Acute Noise Exposure Is Associated With Intrinsic Apoptosis in Murine Central Auditory Pathway

Noise that is capable of inducing the hearing loss (NIHL) has a strong impact on the inner ear structures and causes early and most obvious pathophysiological changes in the auditory periphery. Several studies indicated that intrinsic apoptotic cell death mechanisms are the key factors inducing cellular degeneration immediately after noise exposure and are maintained for days or even weeks. In addition, studies demonstrated several changes in the central auditory system following noise exposure, consistent with early apoptosis-related pathologies. To clarify the underlying mechanisms, the present study focused on the noise-induced gene and protein expression of the pro-apoptotic protease activating factor-1 (APAF1) and the anti-apoptotic B-cell lymphoma 2 related protein a1a (BCL2A1A) in the cochlear nucleus (CN), inferior colliculus (IC) and auditory cortex (AC) of the murine central auditory pathway. The expression of Bcl2a1a mRNA was upregulated immediately after trauma in all tissues investigated, whereas the protein levels were significantly reduced at least in the auditory brainstem. Conversely, acute noise has decreased the expression of Apaf1 gene along the auditory pathway. The changes in APAF1 protein level were not statistically significant. It is tempting to speculate that the acoustic overstimulation leads to mitochondrial dysfunction and induction of apoptosis by regulation of proapoptotic and antiapoptotic proteins. The inverse expression pattern on the mRNA level of both genes might reflect a protective response to decrease cellular damage. Our results indicate the immediate presence of intrinsic apoptosis following noise trauma. This, in turn, may significantly contribute to the development of central structural deficits. Auditory pathway-specific inhibition of intrinsic apoptosis could be a therapeutic approach for the treatment of acute (noise-induced) hearing loss to prevent irreversible neuronal injury in auditory brain structures and to avoid profound deficits in complex auditory processing.

Loss of IDH2 Accelerates Age-related Hearing Loss in Male Mice

Isocitrate dehydrogenase (IDH) 2 participates in the TCA cycle and catalyzes the conversion of isocitrate to α-ketoglutarate and NADP⁺ to NADPH. In the mitochondria, IDH2 also plays a key role in protecting mitochondrial components from oxidative stress by supplying NADPH to both glutathione reductase (GSR) and thioredoxin reductase 2 (TXNRD2). Here, we report that loss of Idh2 accelerates age-related hearing loss, the most common form of hearing impairment, in male mice. This was accompanied by increased oxidative DNA damage, increased apoptotic cell death, and profound loss of spiral ganglion neurons and hair cells in the cochlea of 24-month-old Idh2^−/− mice. In young male mice, loss of Idh2 resulted in decreased NADPH redox state and decreased activity of TXNRD2 in the mitochondria of the inner ear. In HEI-OC1 mouse inner ear cell lines, knockdown of Idh2 resulted in a decline in cell viability and mitochondrial oxygen consumption. This was accompanied by decreased NADPH redox state and decreased activity of TXNRD2 in the mitochondria of the HEI-OC1 cells. Therefore, IDH2 functions as the principal source of NADPH for the mitochondrial thioredoxin antioxidant defense and plays an essential role in protecting hair cells and neurons against oxidative stress in the cochlea of male mice.

The Role of MicroRNAs in Environmental Risk Factors, Noise-Induced Hearing Loss, and Mental Stress

SIGNIFICANCE

MicroRNAs (miRNAs) are important regulators of gene expression and define part of the epigenetic signature. Their influence on every realm of biomedicine is established and progressively increasing. The impact of environment on human health is enormous. Among environmental risk factors impinging on quality of life are those of chemical nature (toxic chemicals, heavy metals, pollutants, and pesticides) as well as those related to everyday life such as exposure to noise or mental and psychosocial stress. Recent Advances: This review elaborates on the relationship between miRNAs and these environmental risk factors.

CRITICAL ISSUES

The most relevant facts underlying the role of miRNAs in the response to these environmental stressors, including redox regulatory changes and oxidative stress, are highlighted and discussed. In the cases wherein miRNA mutations are relevant for this response, the pertinent literature is also reviewed.

FUTURE DIRECTIONS

We conclude that, even though in some cases important advances have been made regarding close correlations between specific miRNAs and biological responses to environmental risk factors, a need for prospective large-cohort studies is likely necessary to establish causative roles. Antioxid. Redox Signal. 28, 773-796.

Protective Effects of Silymarin Against Age-Related Hearing Loss in an Aging Rat Model

Age-related hearing loss (ARHL) is one of the most common chronic degenerative disorders. Several studies have indicated that supplementation with some antioxidants can slow down the progression of ARHL. Despite several lines of evidence about the potent antioxidant and anti-aging effects of silymarin, its protective effect against ARHL has not evaluated yet. The aim of the current study was to investigate the effects of silymarin in prevention of ARHL in a d-Galactose-induced aging rat model for the first time. 45 male wistar rats aged 3-month old were divided into 5 groups: group 1, 2 and 3 received 500 mg/kg/day d-Gal plus 100, 200 and 300 mg/kg/day silymarin respectively for 8 weeks, placebo group received 500 mg/kg/day d-Gal plus propylene glycol as placebo, and control group received normal saline during this period of time. Auditory brainstem responses were measured at several frequencies (4, 6, 8, 12 and 16 kHz) before and after the intervention. Placebo group and group 3 showed significant ABR threshold increase across frequencies of 4, 6, 16 kHz compared with the other groups (P < 0.05). However, rats treated with silymarin 100 and 200 mg/kg/day plus d-Gal did not show any significant ABR threshold shifts. Similarly, ABR amplitude of P2 at 4, 8 kHz and P1, P4 at 4 kHz in the placebo group and group 3 were decreased significantly compared with other groups (P < 0.05). However, no significant differences are found in ABR absolute and inter-peak latencies between groups (P > 0.05). The findings indicates that silymarin with doses of 100 and 200 mg/kg/day has protective effect against ARHL and it can be supplemented into the diet of older people to slow down the progression of age-related hearing loss.

Age-related hearing loss: prevention of threshold declines, cell loss and apoptosis in spiral ganglion neurons

Age-related hearing loss (ARHL) -presbycusis - is the most prevalent neurodegenerative disease and number one communication disorder of our aged population; and affects hundreds of millions of people worldwide. Its prevalence is close to that of cardiovascular disease and arthritis, and can be a precursor to dementia. The auditory perceptual dysfunction is well understood, but knowledge of the biological bases of ARHL is still somewhat lacking. Surprisingly, there are no FDA-approved drugs for treatment. Based on our previous studies of human subjects, where we discovered relations between serum aldosterone levels and the severity of ARHL, we treated middle age mice with aldosterone, which normally declines with age in all mammals. We found that hearing thresholds and suprathreshold responses significantly improved in the aldosterone-treated mice compared to the non-treatment group. In terms of cellular and molecular mechanisms underlying this therapeutic effect, additional experiments revealed that spiral ganglion cell survival was significantly improved, mineralocorticoid receptors were upregulated via post-translational protein modifications, and age-related intrinsic and extrinsic apoptotic pathways were blocked by the aldosterone therapy. Taken together, these novel findings pave the way for translational drug development towards the first medication to prevent the progression of ARHL.

Peroxynitrite induces apoptosis of mouse cochlear hair cells via a Caspase-independent pathway in vitro

Peroxynitrite (ONOO-) is a potent and versatile oxidant implicated in a number of pathophysiological processes. The present study was designed to investigate the effect of ONOO- on the cultured cochlear hair cells (HCs) of C57BL/6 mice in vitro as well as the possible mechanism underlying the action of such an oxidative stress. The in vitro primary cultured cochlear HCs were subjected to different concentrations of ONOO-, then, the cell survival and morphological changes were examined by immunofluorescence and transmission electron microscopy (TEM), the apoptosis was determined by Terminal deoxynucleotidyl transferase dUNT nick end labeling (TUNEL) assay, the mRNA expressions of Caspase-3, Caspase-8, Caspase-9, Apaf1, Bcl-2, and Bax were analyzed by RT-PCR, and the protein expressions of Caspase-3 and AIF were assessed by immunofluorescence. This work demonstrated that direct exposure of primary cultured cochlear HCs to ONOO- could result in a base-to-apex gradient injury of HCs in a concentration-dependent manner. Furthermore, ONOO- led to much more losses of outer hair cells than inner hair cells mainly through the induction of apoptosis of HCs as evidenced by TEM and TUNEL assays. The mRNA expressions of Caspase-8, Caspase-9, Apaf1, and Bax were increased and, meanwhile, the mRNA expression of Bcl-2 was decreased in response to ONOO- treatment. Of interesting, the expression of Caspase-3 had no significant change, whereas, the expression alteration of AIF was observed. These results suggested that ONOO- can effectively damage the survival of cochlear HCs via triggering the apoptotic pathway. The findings from this work suggest that ONOO--induced apoptosis is mediated, at least in part, via a Caspase-independent pathway in cochlear HCs.

Involvement of p53 and Bcl-2 in sensory cell degeneration in aging rat cochleae

CONCLUSION

p53 and Bcl-2 (B-cell lymphoma 2) are involved in the process of sensory cell degeneration in aging cochleae.

OBJECTIVE

To determine molecular players in age-related hair cell degeneration, this study examined the changes in p53 and Bcl-2 expression at different stages of apoptotic and necrotic death of hair cells in aging rat cochleae.

METHODS

Young (3-4 months) and aging (23-24 months) Fisher 344/NHsd rats were used. The thresholds of the auditory brainstem response (ABR) were measured to determine the auditory function. Immunolabeling was performed to determine the expression of p53 and Bcl-2 proteins in the sensory epithelium. Propidium iodide staining was performed to determine the morphologic changes in hair cell nuclei.

RESULTS

Aging rats exhibited a significant elevation in ABR thresholds at all tested frequencies (p < 0.001). The p53 and Bcl-2 immunoreactivity was increased in aging hair cells showing the early signs of apoptotic changes in their nuclei. The Bcl-2 expression increase was also observed in hair cells displaying early signs of necrosis. As the hair cell degenerative process advanced, p53 and Bcl-2 immunoreactivity became reduced or absent. In the areas where no detectable nuclear staining was present, p53 and Bcl-2 immunoreactivity was absent.

Cellular mechanisms of noise-induced hearing loss

Exposure to intense sound or noise can result in purely temporary threshold shift (TTS), or leave a residual permanent threshold shift (PTS) along with alterations in growth functions of auditory nerve output. Recent research has revealed a number of mechanisms that contribute to noise-induced hearing loss (NIHL). The principle cause of NIHL is damage to cochlear hair cells and associated synaptopathy. Contributions to TTS include reversible damage to hair cell (HC) stereocilia or synapses, while moderate TTS reflects protective purinergic hearing adaptation. PTS represents permanent damage to or loss of HCs and synapses. While the substrates of HC damage are complex, they include the accumulation of reactive oxygen species and the active stimulation of intracellular stress pathways, leading to programmed and/or necrotic cell death. Permanent damage to cochlear neurons can also contribute to the effects of NIHL, in addition to HC damage. These mechanisms have translational potential for pharmacological intervention and provide multiple opportunities to prevent HC damage or to rescue HCs and spiral ganglion neurons that have suffered injury. This paper reviews advances in our understanding of cellular mechanisms that contribute to NIHL and their potential for therapeutic manipulation.

Expression levels of the BAK1 and BCL2 genes highlight the role of apoptosis in age-related hearing impairment

Age-related hearing impairment (ARHI) is a progressive and a common sensory disorder in the elderly and will become an increasingly important clinical problem given the growing elderly population. Apoptosis of cochlear cells is an important factor in animal models of ARHI. As these cells cannot regenerate, their loss leads to irreversible hearing impairment. Identification of molecular mechanisms can facilitate disease prevention and effective treatment. In this study, we compared the expression of the genes BAK1 and BCL2 as two arms of the intrinsic apoptosis pathway between patients with ARHI and healthy subjects. ARHI and healthy subjects were selected after an ear nose throat examination, otoscopic investigation, and pure tone audiometry. RNA was extracted from peripheral blood samples, and relative gene expression levels were measured using quantitative real-time polymerase chain reaction. BAK1 and the BAK1/BCL2 ratio were statistically significantly upregulated in the ARHI subjects. The BAK1/BCL2 ratio was positively correlated with the results of the audiometric tests. Our results indicate that BAK-mediated apoptosis may be a core mechanism in the progression of ARHI in humans, similar to finding in animal models. Moreover, the gene expression changes in peripheral blood samples could be used as a rapid and simple biomarker for early detection of ARHI.

Anti-apoptotic treatment in mouse models of age-related hearing loss

Age-related hearing loss (AHL), or presbycusis, is the most common neurodegenerative disorder and top communication deficit of the aged population. Genetic predisposition is one of the major factors in the development of AHL. Generally, AHL is associated with an age-dependent loss of sensory hair cells, spiral ganglion neurons and stria vascularis cells in the inner ear. Although the mechanisms leading to genetic hearing loss are not completely understood, caspase-family proteases function as important signals in the inner ear pathology. It is now accepted that mouse models are the best tools to study the mechanism of genetic hearing loss or AHL. Here, we provide a brief review of recent studies on hearing improvement in mouse models of AHL by anti-apoptotic treatment.

Effects of Erlong Zuoci decoction on the age-related hearing loss in C57BL/6J mice

ETHNOPHARMACOLOGICAL RELEVANCE

Erlong Zuoci decoction (ELZCD), a typical traditional Chinese medicine (TCM) prescription, has long been clinically used in treatment of deafness and tinnitus with the syndrome of "kidney yin deficiency". However, there are few studies to investigate its pharmacological mechanisms. Until now, there is not report about its effects on the age-related hearing loss (ARHL).

AIM OF STUDY

The present study was conducted to observe the effects of ELZCD on the ARHL in C57BL/6J mice and explore the mechanisms.

MATERIALS AND METHODS

ELZCD was fed to C57BL/6J mice from 3 months to 6 months in ELZCD group as a dose of 6g/kg/d. And the same volume of saline was fed to mice in ARHL group. 3-months-old C57BL/6J mice were used as control group. High performance liquid chromatography (HPLC) was used for the quality control of ELZCD. Auditory brainstem response (ABR) was used to assess the hearing function of mice. The morphologic changes were observed by hematoxylin eosin (HE) staining. Apoptosis was tested by terminal dexynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) method. Mitochondrial damage was detected by transmission electron microscopy (TEM). Quantitative RT-PCR (qRT-PCR) was used to observe the mRNA expression of p53 and Bak. Fluorescence immunohistochemical technique was used to test the protein expression of p53 and Bak.

RESULTS

The hearing threshold of ARHL group was higher than that of control group (P<0.001) and ELZCD decreased the rise of hearing threshold levels of ARHL mice (P<0.001), which suggested ELZCD inhibited the hearing loss of ARHL mice. HE staining showed that ELZCD decreased the spiral ganglion (SG) cell damage and loss in ARHL. TUNEL test showed that the apoptotic SG cells increased in ARHL group compared to control group and decreased in ELZCD group compared to ARHL group. TEM observation showed that mitochondrial damage was obvious in SG cells of ARHL group and ELZCD inhibited the mitochondrial damage. The qRT-PCR results showed that the mRNA expression of p53 and Bak in ARHL group increased compared to that of control group (P<0.05), and ELZCD reduced the elevated mRNA expression levels of p53 and Bak (P<0.01, P<0.05). In addition, ELZCD inhibited the increased proteins expression (green fluorescence) of p53 and Bak.

CONCLUSION

The results demonstrated that ELZCD prevented ARHL in C57BL/6J mice and p53/Bak-mediated mitochondrial apoptosis of SG cells might be involved in the mechanisms.

Differential effects of Cdh23(753A) on auditory and vestibular functional aging in C57BL/6J mice

The C57BL/6J (B6) mouse strain carries a cadherin 23 mutation (Cdh23(753A), also known as Ahl), which affects inner ear structures and results in age-related hearing loss. The B6.CAST strain harbors the wild type Cdh23 gene, and hence, the influence of Ahl is absent. The purpose of the present study was to characterize the effect of age and gender on gravity receptor function in B6 and B6.CAST strains and to compare functional aging between auditory and vestibular modalities. Auditory sensitivity declined at significantly faster rates than gravity receptor sensitivity for both strains. Indeed, vestibular functional aging was minimal for both strains. The comparatively smaller loss of macular versus cochlear sensitivity in both the B6 and B6.CAST strains suggests that the contribution of Ahl to the aging of the vestibular system is minimal, and thus very different than its influence on aging of the auditory system. Alternatively, there exist unidentified genes or gene modifiers that serve to slow the degeneration of gravity receptor structures and maintain gravity receptor sensitivity into advanced age.

Gene expression profiling of the inner ear

The identification of transcriptional differences has served as an important starting point in understanding the molecular mechanisms behind biological processes and systems. The developmental biology of the inner ear, the biology of hearing and of course the pathology of deafness are all processes that warrant a molecular description if we are to improve human health. To this end, technological innovation has meant that larger scale analysis of gene transcription has been possible for a number of years now, extending our molecular analysis of genes to beyond those that are currently in vogue for a given system. In this review, some of the contributions gene profiling has made to understanding developmental, pathological and physiological processes in the inner ear are highlighted.

Circulating miR-34a levels correlate with age-related hearing loss in mice and humans

Age-related hearing loss (AHL) is a progressive neurodegenerative disease that is largely silent in its initial stages. There is no sensitive blood biomarker for diagnosis or early detection of AHL. MicroRNAs (miRNAs or miRs) are abundant and highly stable in blood, and have been recently described as powerful circulating biomarkers in a wide range of diseases. In the present study, we identified concordant increases in miR-34a levels in the cochlea, auditory cortex, and plasma of C57BL/6 mice during aging. These increases were accompanied by elevated hearing thresholds and greater loss of hair cells. Levels of miR-34a targets, silent information regulator 1 (SIRT1), B-cell lymphoma-2 (Bcl-2), and E2F transcription factor 3 (E2F3), in the cochlea, auditory cortex, and plasma decreased with aging inversely to miR-34a. Moreover, plasma miR-34a levels were significantly higher in patients with AHL compared with controls who had normal hearing and had a receiver-operating characteristic curve that distinguished AHL patients from controls. However, SIRT1, Bcl-2, and E2F3 showed no correlation with AHL in humans. In summary, circulating miR-34a level may potentially serve as a useful biomarker for early detection of AHL.

The Mouse Round-window Approach for Ototoxic Agent Delivery: A Rapid and Reliable Technique for Inducing Cochlear Cell Degeneration

Investigators have utilized a wide array of animal models and investigative techniques to study the mammalian auditory system. Much of the basic research involving the cochlea and its associated neural pathways entails exposure of model cochleae to a variety of ototoxic agents. This allows investigators to study the effects of targeted damage to cochlear structures, and in some cases, the self-repair or regeneration of those structures. Various techniques exist for delivery of ototoxic agents to the cochlea. When selecting a particular technique, investigators must consider a number of factors, including the induction of inadvertent systemic toxicity, the amount of cochlear damage produced by the surgical procedure itself, the type of lesion desired, animal survivability, and reproducibility/reliability of results. Currently established techniques include parenteral injection, intra-peritoneal injection, trans-tympanic injection, endolymphatic sac injection, and cochleostomy with perilymphatic perfusion. Each of these methods has been successfully utilized and is well described in the literature; yet, each has various shortcomings. Here, we present a technique for topical application of ototoxic agents directly to the round window niche. This technique is non-invasive to inner ear structures, produces rapid onset of reliably targeted lesions, avoids systemic toxicity, and allows for an intra-animal control (the contra-lateral ear). Results stemming from this approach have helped deeper understanding of auditory pathophysiology, cochlear cell degeneration, and regenerative capacity in response to an acute injury. Future investigations may use this method to conduct interventional studies involving gene therapy and stem cell transplantation to combat hearing loss.

Neuronal erythropoietin overexpression protects mice against age-related hearing loss (presbycusis)

So far, typical causes of presbycusis such as degeneration of hair cells and/or primary auditory (spiral ganglion) neurons cannot be treated. Because erythropoietin's (Epo) neuroprotective potential has been shown previously, we determined hearing thresholds of juvenile and aged mice overexpressing Epo in neuronal tissues. Behavioral audiometry revealed in contrast to 5 months of age, that 11-month-old Epo-transgenic mice had up to 35 dB lower hearing thresholds between 1.4 and 32 kHz, and at the highest frequencies (50-80 kHz), thresholds could be obtained in aged Epo-transgenic only but not anymore in old C57BL6 control mice. Click-evoked auditory brainstem response showed similar results. Numbers of spiral ganglion neurons in aged C57BL6 but not Epo-transgenic mice were dramatically reduced mainly in the basal turn, the location of high frequencies. In addition, there was a tendency to better preservation of inner and outer hair cells in Epo-transgenic mice. Hence, Epo's known neuroprotective action effectively suppresses the loss of spiral ganglion cells and probably also hair cells and, thus, development of presbycusis in mice.

Age-related hearing impairment and the triad of acquired hearing loss

Understanding underlying pathological mechanisms is prerequisite for a sensible design of protective therapies against hearing loss. The triad of age-related, noise-generated, and drug-induced hearing loss displays intriguing similarities in some cellular responses of cochlear sensory cells such as a potential involvement of reactive oxygen species (ROS) and apoptotic and necrotic cell death. On the other hand, detailed studies have revealed that molecular pathways are considerably complex and, importantly, it has become clear that pharmacological protection successful against one form of hearing loss will not necessarily protect against another. This review will summarize pathological and pathophysiological features of age-related hearing impairment (ARHI) in human and animal models and address selected aspects of the commonality (or lack thereof) of cellular responses in ARHI to drugs and noise.

Impaired unfolded protein response in the degeneration of cochlea cells in a mouse model of age-related hearing loss

Endoplasmic reticulum (ER) stress triggers the unfolded protein response (UPR) to prevent the accumulation of proteins in an aberrant conformation. The UPR can restore homeostasis by upregulating ER chaperones, such as glucose-regulated protein 78kD (GRP78), to refold the incorrectly handled protein, and by degrading the misfolded proteins via the ubiquitin-proteasome and autophagy-lysosome system. ER stress was recently demonstrated to be involved in the pathogenesis of age-related diseases. In this study, we measured the expression levels of GRP78 and ubiquitinated proteins in the cochleae of young C57BL/6 mice and aged mice to assess the capacity of the UPR. The lower expression of GRP78 and the increased number of ubiquitinated proteins observed in the cochleae of aged mice suggested that the capacity of the UPR was impaired and that the cell death pathway was activated. We found a markedly increased expression of the ER-related pro-apoptotic factor C/EBP homologous protein (CHOP) in the cochleae of aged mice, whereas the level of cleaved caspase-12 did not differ between the two groups. In addition, the cleavage of caspase-9, caspase-3 and poly [ADP-ribose] polymerase 1 was significantly increased in the aged cochleae, suggesting the activation of apoptosis in the cochleae resulting from the cross-talk between the ER and mitochondria through CHOP. These results indicated that impaired UPR in the cochleae of aged C57BL/6 mice resulting in ER stress may lead to apoptosis that is dependent on the mitochondrial pathway and that ER stress induced apoptosis may not be mediated by caspase-12.

Attenuation of hearing loss in DBA/2J mice by anti-apoptotic treatment

DBA/2J mice are characterized by early onset hearing loss at about 3-4 weeks of age. Mutations in cadherin 23 (Cdh23) and fascin-2 (Fscn2) are responsible for the phenotypes, but the underlying mechanism is unknown. In the present study, DBA/2J mice displayed progressive hair cell loss and degeneration of spiral ganglion neurons (SGNs) after 2 weeks of age; however, the mRNA level of Caspase-3 in the inner ears was much higher at 2 weeks of age than that at 4 or 8 weeks of age. Moreover, transcriptional levels of Caspase-3 and Caspase-9 in the inner ears of DBA/2J mice were significantly higher than those of C57BL/6J mice at 2 or 8 weeks of age. Immunohistochemistry localized Caspase-3 and Caspase-9 mainly to the hair cells, SGNs and stria vascularis of the cochleae. To determine the significance of caspase-dependent apoptosis in the hearing loss, the pan-caspase inhibitor Z-VAD-FMK was given intraperitoneally to DBA/J2 mice over an 8-week period starting at one week of age. Blockage of caspases preserved hearing in the mice by more than 10 dB (dB) sound pressure level (SPL) of the ABR thresholds and significantly reduced outer hair cell loss at the basal turns of the cochleae. These results demonstrate that apoptosis in the cochleae of DBA/J2 mice contributes to the early onset of hearing loss, which can be attenuated by anti-apoptotic treatment.

Mechanisms of sensorineural cell damage, death and survival in the cochlea

The majority of acquired hearing loss, including presbycusis, is caused by irreversible damage to the sensorineural tissues of the cochlea. This article reviews the intracellular mechanisms that contribute to sensorineural damage in the cochlea, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. These data have primarily been generated in hearing loss not directly related to age. However, there is evidence that similar mechanisms operate in presbycusis. Moreover, accumulation of damage from other causes can contribute to age-related hearing loss (ARHL). Potential therapeutic interventions to balance opposing but interconnected cell damage and survival pathways, such as antioxidants, anti-apoptotics, and pro-inflammatory cytokine inhibitors, are also discussed.

Molecular basis of hair cell loss

Mechanisms that lead to the death of hair cells are reviewed. Exposure to noise, the use of ototoxic drugs that damage the cochlea and old age are accompanied by hair cell death. Outer hair cells are often more susceptible than inner hair cells, partly because of an intrinsically greater susceptibility; high frequency cells are also more vulnerable. A common factor in hair cell loss following age-related changes and exposure to ototoxic drugs or high noise levels is the generation of reactive oxygen species, which can trigger intrinsic apoptosis (the mitochondrial pathway). However, hair cell death is sometimes produced via an extracellular signal pathway triggering extrinsic apoptosis. Necrosis and necroptosis also play a role and, in various situations in which cochlear damage occurs, a balance exists between these possible routes of cell death, with no one mechanism being exclusively activated. Finally, the numerous studies on these mechanisms of hair cell death have led to the identification of many potential therapeutic agents, some of which have been used to attempt to treat people exposed to damaging events, although clinical trials are not yet conclusive. Continued work in this area is likely to lead to clinical treatments that could be used to prevent or ameliorate hearing loss.

Overexpression of X-Linked Inhibitor of Apoptotic Protein (XIAP) reduces age-related neuronal degeneration in the mouse cochlea

Previously, we showed that age-related hearing loss (AHL) was delayed in C57BL6 mice overexpressing X-Linked Inhibitor of Apoptotic Protein (XIAP), and the delayed AHL was associated with attenuated hair cell (HC) loss in XIAP-overexpressing mice. Similar to other reports, the HC loss in aged mice was restricted to the basal turn in this previous study, and occurred slightly at the apical end of the cochlea, showing considerably less spread than the frequency region of hearing loss. In the present study, we examined whether and how AHL is related to the degeneration of neuronal innervation of the cochlea and if the overexpression of XIAP exerts a protective effect against age-related degeneration in both afferent and efferent cochlear neurites. In contrast to HC loss, degeneration of both afferent and efferent neurites spread to the middle turns of the cochlea. Moreover, XIAP-overexpressing mice lost fewer HC afferent dendrites and efferent axons, as well as fewer spiral ganglion neurons (SGNs) between 3– 14 months of age in comparison to wild-type littermates. The results suggest that age-related degeneration of cochlear neurites may be independent of HC loss. Further, the inhibition of apoptosis by XIAP appears to reduce degeneration of both afferent and efferent cochlear neurites.

Genes involved in immunity and apoptosis are associated with human presbycusis based on microarray analysis

CONCLUSIONS

Genes involved in immunity and apoptosis were associated with human presbycusis. CCR3 and GILZ played an important role in the pathogenesis of presbycusis, probably through regulating chemokine receptor, T-cell apoptosis, or T-cell activation pathways.

OBJECTIVES

To identify genes associated with human presbycusis and explore the molecular mechanism of presbycusis.

METHODS

Hearing function was tested by pure-tone audiometry. Microarray analysis was performed to identify presbycusis-correlated genes by Illumina Human-6 BeadChip using the peripheral blood samples of subjects. To identify biological process categories and pathways associated with presbycusis-correlated genes, bioinformatics analysis was carried out by Gene Ontology Tree Machine (GOTM) and database for annotation, visualization, and integrated discovery (DAVID). Quantitative RT-PCR (qRT-PCR) was used to validate the microarray data.

RESULTS

Microarray analysis identified 469 up-regulated genes and 323 down-regulated genes. Both the dominant biological processes by Gene Ontology (GO) analysis and the enriched pathways by Kyoto encyclopedia of genes and genomes (KEGG) and BIOCARTA showed that genes involved in immunity and apoptosis were associated with presbycusis. In addition, CCR3, GILZ, CXCL10, and CX3CR1 genes showed consistent difference between groups for both the gene chip and qRT-PCR data. The differences of CCR3 and GILZ between presbycusis patients and controls were statistically significant (p < 0.05).

Inner ear symptoms and disease: pathophysiological understanding and therapeutic options

In recent years, huge advances have taken place in understanding of inner ear pathophysiology causing sensorineural hearing loss, tinnitus, and vertigo. Advances in understanding comprise biochemical and physiological research of stimulus perception and conduction, inner ear homeostasis, and hereditary diseases with underlying genetics. This review describes and tabulates the various causes of inner ear disease and defines inner ear and non-inner ear causes of hearing loss, tinnitus, and vertigo. The aim of this review was to comprehensively breakdown this field of otorhinolaryngology for specialists and non-specialists and to discuss current therapeutic options in distinct diseases and promising research for future therapies, especially pharmaceutic, genetic, or stem cell therapy.

Hair cell overexpression of Islet1 reduces age-related and noise-induced hearing loss

Isl1 is a LIM-homeodomain transcription factor that is critical in the development and differentiation of multiple tissues. In the mouse inner ear, Isl1 is expressed in the prosensory region of otocyst, in young hair cells and supporting cells, and is no longer expressed in postnatal auditory hair cells. To evaluate how continuous Isl1 expression in postnatal hair cells affects hair cell development and cochlear function, we created a transgenic mouse model in which the Pou4f3 promoter drives Isl1 overexpression specifically in hair cells. Isl1 overexpressing hair cells develop normally, as seen by morphology and cochlear functions (auditory brainstem response and otoacoustic emissions). As the mice aged to 17 months, wild-type (WT) controls showed the progressive threshold elevation and outer hair cell loss characteristic of the age-related hearing loss (ARHL) in the background strain (C57BL/6J). In contrast, the Isl1 transgenic mice showed significantly less threshold elevation with survival of hair cells. Further, the Isl1 overexpression protected the ear from noise-induced hearing loss (NIHL): both ABR threshold shifts and hair cell death were significantly reduced when compared with WT littermates. Our model suggests a common mechanism underlying ARHL and NIHL, and provides evidence that hair cell-specific Isl1 expression can promote hair cell survival and therefore minimize the hearing impairment that normally occurs with aging and/or acoustic overexposure.

The role of mitochondria in age-related hearing loss

Age-related hearing loss (ARHL), the hearing loss associated with aging, is a vital problem in present society. The severity of hearing loss is possibly associated with the degeneration of cochlear cells. Mitochondria play a key role in the energy supply, cellular redox homeostasis, signaling, and regulation of programmed cell death. In this review, we focus on the central role of mitochondria in ARHL. The mitochondrial redox imbalance and mitochondrial DNA mutation might collaboratively involve in the process of cochlear senescence in response to the aging stress. Subsequent responses, including alteration of mitochondrial biogenesis, mitophagy, apoptosis and paraptosis, participate in the aging process from different respects.

Tumor necrosis factor-alpha-mutant mice exhibit high frequency hearing loss

Exogenous tumor necrosis factor-alpha (TNF-α) plays a role in auditory hair cell death by altering the expression of apoptosis-related genes in response to noxious stimuli. Little is known, however, about the function of TNF-α in normal hair cell physiology. We, therefore, investigated the cochlear morphology and auditory function of TNF-α-deficient mice. Auditory evoked brainstem response showed significantly higher thresholds, especially at higher frequencies, in 1-month-old TNF-α(-/-) mice as compared to TNF-α(+/-) and wild type (WT); hearing loss did not progress further from 1 to 4 months of age. There was no difference in the gross morphology of the organ of Corti, lateral wall, and spiral ganglion cells in TNF-α(-/-) mice compared to WT mice at 4 months of age, nor were there differences in the anatomy of the auditory ossicles. Outer hair cells were completely intact in surface preparations of the organ of Corti of TNF-α(-/-) mice, and synaptic ribbon counts of TNF-α(-/-) and WT mice at 4 months of age were similar. Reduced amplitudes of distortion product otoacoustic emissions, however, indicated dysfunction of outer hair cells in TNF-α(-/-) mice. Scanning electron microscopy revealed that stereocilia were sporadically absent in the basal turn and distorted in the middle turn. In summary, our results demonstrate that TNF-α-mutant mice exhibit early hearing loss, especially at higher frequencies, and that loss or malformation of the stereocilia of outer hair cells appears to be a contributing factor.

Modulation of Mcl-1 expression reduces age-related cochlear degeneration

Mcl-1 is an anti-apoptotic member of the Bcl-2 family that modulates apoptosis-related signaling pathways and promotes cell survival. We have previously demonstrated a reduction of Mcl-1 expression in aging cochleae. To investigate whether restoring Mcl-1 expression would reduce aging-related cochlear degeneration, we developed a rat model of Mcl-1 overexpression. A plasmid encoding human Mcl-1/enhanced green fluorescent protein was applied to the round window of the cochlea. This in vivo treatment transfected both the sensory and supporting cells of the cochlear sensory epithelium and enhanced Mcl-1 expression at both the mRNA and the protein level. The upregulation of Mcl-1 expression reduced the progression of age-related cochlear dysfunction and sensory cell death. Furthermore, the transfection of Mcl-1 exerted its protective effect by suppressing cochlear apoptosis at the mitochondrial level. This study demonstrates that the genetic modulation of Mcl-1 expression reduces the progression of age-related cochlear degeneration.

Identifying microRNAs involved in degeneration of the organ of corti during age-related hearing loss

MicroRNAs (miRNAs), a class of short non-coding RNAs that regulate the expression of mRNA targets, are important regulators of cellular senescence and aging. We questioned which miRNAs are involved in age-related degeneration of the organ of Corti (OC), the auditory sensory epithelium that transduces mechanical stimuli to electrical activity in the inner ear. Degeneration of the OC is generally accepted as the main cause of age-related hearing loss (ARHL), a progressive loss of hearing in individuals as they grow older. To determine which miRNAs are involved in the onset and progression of ARHL, miRNA gene expression in the OC of two mouse strains, C57BL/6J and CBA/J, was compared at three different ages using GeneChip miRNA microarray and was validated by real-time PCR. We showed that 111 and 71 miRNAs exhibited differential expression in the C57 and CBA mice, respectively, and that downregulated miRNAs substantially outnumbered upregulated miRNAs during aging. miRNAs that had approximately 2-fold upregulation included members of miR-29 family and miR-34 family, which are known regulators of pro-apoptotic pathways. In contrast, miRNAs that were downregulated by about 2-fold were members of the miR-181 family and miR-183 family, which are known to be important for proliferation and differentiation, respectively. The shift of miRNA expression favoring apoptosis occurred earlier than detectable hearing threshold elevation and hair cell loss. Our study suggests that changes in miRNA expression precede morphological and functional changes, and that upregulation of pro-apoptotic miRNAs and downregulation of miRNAs promoting proliferation and differentiation are both involved in age-related degeneration of the OC.