The aging cochlea: Towards unraveling the functional contributions of strial dysfunction and synaptopathy

Effects of Age on Responses of Principal Cells of the Mouse Anteroventral Cochlear Nucleus in Quiet and Noise

Older listeners often report difficulties understanding speech in noisy environments. It is important to identify where in the auditory pathway hearing-in-noise deficits arise to develop appropriate therapies. We tested how encoding of sounds is affected by masking noise at early stages of the auditory pathway by recording responses of principal cells in the anteroventral cochlear nucleus (AVCN) of aging CBA/CaJ and C57BL/6J mice in vivo. Previous work indicated that masking noise shifts the dynamic range of single auditory nerve fibers (ANFs), leading to elevated tone thresholds. We hypothesized that such threshold shifts could contribute to increased hearing-in-noise deficits with age if susceptibility to masking increased in AVCN units. We tested this by recording the responses of AVCN principal neurons to tones in the presence and absence of masking noise. Surprisingly, we found that masker-induced threshold shifts decreased with age in primary-like units and did not change in choppers. In addition, spontaneous activity decreased in primary-like and chopper units of old mice, with no change in dynamic range or tuning precision. In C57 mice, which undergo early-onset hearing loss, units showed similar changes in threshold and spontaneous rate at younger ages, suggesting they were related to hearing loss and not simply aging. These findings suggest that sound information carried by AVCN principal cells remains largely unchanged with age. Therefore, hearing-in-noise deficits may result from other changes during aging, such as distorted across-channel input from the cochlea and changes in sound coding at later stages of the auditory pathway.

Age-related changes in olivocochlear efferent innervation in gerbils

Introduction

Age-related hearing difficulties have a complex etiology that includes degenerative processes in the sensory cochlea. The cochlea comprises the start of the afferent, ascending auditory pathway, but also receives efferent feedback innervation by two separate populations of brainstem neurons: the medial olivocochlear and lateral olivocochlear pathways, innervating the outer hair cells and auditory-nerve fibers synapsing on inner hair cells, respectively. Efferents are believed to improve hearing under difficult conditions, such as high background noise. Here, we compare olivocochlear efferent innervation density along the tonotopic axis in young-adult and aged gerbils (at ~50% of their maximum lifespan potential), a classic animal model for age-related hearing loss.

Methods

Efferent synaptic terminals and sensory hair cells were labeled immunohistochemically with anti-synaptotagmin and anti-myosin VIIa, respectively. Numbers of hair cells, numbers of efferent terminals, and the efferent innervation area were quantified at seven tonotopic locations along the organ of Corti.

Results

The tonotopic distribution of olivocochlear innervation in the gerbil was similar to that previously shown for other species, with a slight apical cochlear bias in presumed lateral olivocochlear innervation (inner-hair-cell region), and a broad mid-cochlear peak for presumed medial olivocochlear innervation (outer-hair-cell region). We found significant, age-related declines in overall efferent innervation to both the inner-hair-cell and the outer-hair-cell region. However, when accounting for the age-related losses in efferent target structures, the innervation density of surviving elements proved unchanged in the inner-hair-cell region. For outer hair cells, a pronounced increase of orphaned outer hair cells, i.e., lacking efferent innervation, was observed. Surviving outer hair cells that were still efferently innervated retained a nearly normal innervation.

Discussion

A comparison across species suggests a basic aging scenario where outer hair cells, type-I afferents, and the efferents associated with them, steadily die away with advancing age, but leave the surviving cochlear circuitry largely intact until an advanced age, beyond 50% of a species' maximum lifespan potential. In the outer-hair-cell region, MOC degeneration may precede outer-hair-cell death, leaving a putatively transient population of orphaned outer hair cells that are no longer under efferent control.

Effects of age and noise exposure history on auditory nerve response amplitudes: A systematic review, study, and meta-analysis

Auditory nerve (AN) function has been hypothesized to deteriorate with age and noise exposure. Here, we perform a systematic review of published studies and find that the evidence for age-related deficits in AN function is largely consistent across the literature, but there are inconsistent findings among studies of noise exposure history. Further, evidence from animal studies suggests that the greatest deficits in AN response amplitudes are found in noise-exposed aged mice, but a test of the interaction between effects of age and noise exposure on AN function has not been conducted in humans. We report a study of our own examining differences in the response amplitude of the compound action potential N1 (CAP N1) between younger and older adults with and without a self-reported history of noise exposure in a large sample of human participants (63 younger adults 18-30 years of age, 103 older adults 50-86 years of age). CAP N1 response amplitudes were smaller in older than younger adults. Noise exposure history did not appear to predict CAP N1 response amplitudes, nor did the effect of noise exposure history interact with age. We then incorporated our results into two meta-analyses of published studies of age and noise exposure history effects on AN response amplitudes in neurotypical human samples. The meta-analyses found that age effects across studies are robust (r = -0.407), but noise exposure effects are weak (r = -0.152). We conclude that noise exposure effects may be highly variable depending on sample characteristics, study design, and statistical approach, and researchers should be cautious when interpreting results. The underlying pathology of age-related and noise-induced changes in AN function are difficult to determine in living humans, creating a need for longitudinal studies of changes in AN function across the lifespan and histological examination of the AN from temporal bones collected post-mortem.

The Stria Vascularis: Renewed Attention on a Key Player in Age-Related Hearing Loss

Age-related hearing loss (HL), or presbycusis, is a complex and heterogeneous condition, affecting a significant portion of older adults and involving various interacting mechanisms. Metabolic presbycusis, a type of age-related HL, is characterized by the dysfunction of the stria vascularis, which is crucial for maintaining the endocochlear potential necessary for hearing. Although attention on metabolic presbycusis has waned in recent years, research continues to identify strial pathology as a key factor in age-related HL. This narrative review integrates past and recent research, bridging findings from animal models and human studies, to examine the contributions of the stria vascularis to age-related HL. It provides a brief overview of the structure and function of the stria vascularis and then examines mechanisms contributing to age-related strial dysfunction, including altered ion transport, changes in pigmentation, inflammatory responses, and vascular atrophy. Importantly, this review outlines the contribution of metabolic mechanisms to age-related HL, highlighting areas for future research. It emphasizes the complex interdependence of metabolic and sensorineural mechanisms in the pathology of age-related HL and highlights the importance of animal models in understanding the underlying mechanisms. The comprehensive and mechanistic investigation of all factors contributing to age-related HL, including cochlear metabolic dysfunction, remains crucial to identifying the underlying mechanisms and developing personalized, protective, and restorative treatments.

Single-unit data for sensory neuroscience: Responses from the auditory nerve of young-adult and aging gerbils

This dataset was collected to study the functional consequences of age-related hearing loss for the auditory nerve, which carries acoustic information from the periphery to the central auditory system. Using high-impedance glass electrodes, raw voltage traces and spike times were recorded from more than one thousand single fibres of the auditory nerve of young-adult, middle-aged, and old Mongolian gerbils raised in a quiet environment. The dataset contains not only responses to simple acoustic stimuli to characterize the fibres, but also to more complex stimuli, such as speech logatomes in background noise and Schroeder-phase stimuli. A software toolbox is provided to search through the dataset, to plot various analysed outcomes, and to give insight into the analyses. This dataset may serve as a valuable resource to test further hypotheses about age-related hearing loss. Additionally, it can aid in optimizing available computational models of the auditory system, which can contribute to, or eventually even fully replace, animal experiments.

Effects of Age and Noise Exposure History on Auditory Nerve Response Amplitudes: A Systematic Review, Study, and Meta-Analysis

Auditory nerve (AN) function has been hypothesized to deteriorate with age and noise exposure. Here, we perform a systematic review of published studies and find that the evidence for age-related deficits in AN function is largely consistent across the literature, but there are inconsistent findings among studies of noise exposure history. Further, evidence from animal studies suggests that the greatest deficits in AN response amplitudes are found in noise-exposed aged mice, but a test of the interaction between effects of age and noise exposure on AN function has not been conducted in humans. We report a study of our own examining differences in the response amplitude of the compound action potential N1 (CAP N1) between younger and older adults with and without a self-reported history of noise exposure in a large sample of human participants (63 younger adults 18-30 years of age, 103 older adults 50-86 years of age). CAP N1 response amplitudes were smaller in older than younger adults. Noise exposure history did not appear to predict CAP N1 response amplitudes, nor did the effect of noise exposure history interact with age. We then incorporated our results into two meta-analyses of published studies of age and noise exposure history effects on AN response amplitudes in neurotypical human samples. The meta-analyses found that age effects across studies are robust (r=-0.407), but noise-exposure effects are weak (r=-0.152). We conclude that noise-exposure effects may be highly variable depending on sample characteristics, study design, and statistical approach, and researchers should be cautious when interpreting results. The underlying pathology of age-related and noise-induced changes in AN function are difficult to determine in living humans, creating a need for longitudinal studies of changes in AN function across the lifespan and histological examination of the AN from temporal bones collected post-mortem.

The crucial role of diverse animal models to investigate cochlear aging and hearing loss

Age-related hearing loss affects a large and growing segment of the population, with profound impacts on quality of life. Age-related pathology of the cochlea-the mammalian hearing organ-underlies age-related hearing loss. Because investigating age-related changes in the cochlea in humans is challenging and often impossible, animal models are indispensable to investigate these mechanisms as well as the complex consequences of age-related hearing loss on the brain and behavior. In this review, we advocate for a comparative and interdisciplinary approach while also addressing the challenges of comparing age-related hearing loss across species with varying lifespans. We describe the experimental advantages and limitations as well as areas for future research in well-established models of age-related hearing loss, including mice, rats, gerbils, chinchillas, and birds. We also indicate the need to expand characterization of age-related hearing loss in other established animal models, especially guinea pigs, cats, and non-human primates, in which auditory function is well characterized but age-related cochlear pathology is understudied. Finally, we highlight the potential of emerging animal models for advancing our understanding of age-related hearing loss, including deer mice, with their notably extended lifespans and preserved hearing, naked mole rats, with their exceptional longevity and extensive vocal communications, as well as zebrafish, which offer genetic tractability and suitability for drug screening. Ultimately, a comparative and interdisciplinary approach in auditory research, combining insights from various animal models with human studies, is key to robust and reliable research outcomes that better advance our understanding and treatment of age-related hearing loss.

Cochlear Ribbon Synapses in Aged Gerbils

In mammalian hearing, type-I afferent auditory nerve fibers comprise the basis of the afferent auditory pathway. They are connected to inner hair cells of the cochlea via specialized ribbon synapses. Auditory nerve fibers of different physiological types differ subtly in their synaptic location and morphology. Low-spontaneous-rate auditory nerve fibers typically connect on the modiolar side of the inner hair cell, while high-spontaneous-rate fibers are typically found on the pillar side. In aging and noise-damaged ears, this fine-tuned balance between auditory nerve fiber populations can be disrupted and the functional consequences are currently unclear. Here, using immunofluorescent labeling of presynaptic ribbons and postsynaptic glutamate receptor patches, we investigated changes in synaptic morphology at three different tonotopic locations along the cochlea of aging gerbils compared to those of young adults. Quiet-aged gerbils showed about 20% loss of afferent ribbon synapses. While the loss was random at apical, low-frequency cochlear locations, at the basal, high-frequency location it almost exclusively affected the modiolar-located synapses. The subtle differences in volumes of pre- and postsynaptic elements located on the inner hair cell's modiolar versus pillar side were unaffected by age. This is consistent with known physiology and suggests a predominant, age-related loss in the low-spontaneous-rate auditory nerve population in the cochlear base, but not the apex.

Cochlear metabolomics, highlighting novel insights of purine metabolic alterations in age-related hearing loss

Aging is an inevitable phase in mammals that leads to health impairments, including hearing loss. Age-related hearing loss (AHL) leads to psychosocial problems and cognitive decline in the elderly. In this study, mean thresholds of auditory brainstem responses (ABR) and distortion-product otoacoustic emissions (DPOAE) increased at multiple frequencies in aged rats (14 months old) compared to young rats (2 months old). Using untargeted ultra-high performance liquid chromatography-mass spectroscopy (LC-MS), we quantified molecular metabolic markers in the cochlea of aged rats with hearing loss. A total of 137 different metabolites were identified in two groups, highlighting several prominent metabolic pathways related to purine metabolism; glycine, serine, and threonine metabolism; arginine and proline metabolism; and pyrimidine metabolism. In addition, the beneficial effects of purine supplementation were demonstrated in a mimetic model of senescent marginal cells (MCs). Overall, altered metabolic profiling is both the cause and manifestation of pathology, and our results suggest that cellular senescence and dysfunctional cochlear metabolism may contribute to the progression of AHL. These findings are seminal in elucidating the pathophysiological mechanisms underlying AHL and serve as a basis for future clinical predictions and interventions in AHL.

Altered neural encoding of vowels in noise does not affect behavioral vowel discrimination in gerbils with age-related hearing loss

Introduction

Understanding speech in a noisy environment, as opposed to speech in quiet, becomes increasingly more difficult with increasing age. Using the quiet-aged gerbil, we studied the effects of aging on speech-in-noise processing. Specifically, behavioral vowel discrimination and the encoding of these vowels by single auditory-nerve fibers were compared, to elucidate some of the underlying mechanisms of age-related speech-in-noise perception deficits.

Methods

Young-adult and quiet-aged Mongolian gerbils, of either sex, were trained to discriminate a deviant naturally-spoken vowel in a sequence of vowel standards against a speech-like background noise. In addition, we recorded responses from single auditory-nerve fibers of young-adult and quiet-aged gerbils while presenting the same speech stimuli.

Results

Behavioral vowel discrimination was not significantly affected by aging. For both young-adult and quiet-aged gerbils, the behavioral discrimination between /eː/ and /iː/ was more difficult to make than /eː/ vs. /aː/ or /iː/ vs. /aː/, as evidenced by longer response times and lower d' values. In young-adults, spike timing-based vowel discrimination agreed with the behavioral vowel discrimination, while in quiet-aged gerbils it did not. Paradoxically, discrimination between vowels based on temporal responses was enhanced in aged gerbils for all vowel comparisons. Representation schemes, based on the spectrum of the inter-spike interval histogram, revealed stronger encoding of both the fundamental and the lower formant frequencies in fibers of quiet-aged gerbils, but no qualitative changes in vowel encoding. Elevated thresholds in combination with a fixed stimulus level, i.e., lower sensation levels of the stimuli for old individuals, can explain the enhanced temporal coding of the vowels in noise.

Discussion

These results suggest that the altered auditory-nerve discrimination metrics in old gerbils may mask age-related deterioration in the central (auditory) system to the extent that behavioral vowel discrimination matches that of the young adults.

Effects of hearing loss, age, noise exposure, and listening skills on envelope regularity discrimination

The envelope regularity discrimination (ERD) test assesses the ability to discriminate irregular from regular amplitude modulation (AM). The measured threshold is called the irregularity index (II). It was hypothesized that the II at threshold should be almost unaffected by the loudness recruitment that is associated with cochlear hearing loss because the effect of recruitment is similar to multiplying the AM depth by a certain factor, and II values depend on the amount of envelope irregularity relative to the baseline modulation depth. To test this hypothesis, the ERD test was administered to 60 older adults with varying degrees of hearing loss, using carrier frequencies of 1 and 4 kHz. The II values for the two carrier frequencies were highly correlated, indicating that the ERD test was measuring a consistent characteristic of each subject. The II values at 1 and 4 kHz were not significantly correlated with the audiometric thresholds at the corresponding frequencies, consistent with the hypothesis. The II values at 4 kHz were significantly positively correlated with age. There was an unexpected negative correlation between II values and a measure of noise exposure. This is argued to reflect the confounding effects of listening skills.

Human blood-labyrinth barrier model to study the effects of cytokines and inflammation

Hearing loss is one of the 10 leading causes of disability worldwide. No drug therapies are currently available to protect or restore hearing. Inner ear auditory hair cells and the blood-labyrinth barrier (BLB) are critical for normal hearing, and the BLB between the systemic circulation and stria vascularis is crucial for maintaining cochlear and vestibular homeostasis. BLB defects are associated with inner ear diseases that lead to hearing loss, including vascular malformations, inflammation, and Meniere's disease (MD). Antibodies against proteins in the inner ear and cytokines in the cochlea, including IL-1α, TNF-α, and NF-kβ, are detected in the blood of more than half of MD patients. There is also emerging evidence of inner ear inflammation in some diseases, including MD, progressive sensorineural hearing loss, otosclerosis, and sudden deafness. Here, we examined the effects of TNF-α, IL6, and LPS on human stria vascularis-derived primary endothelial cells cultured together with pericytes in a Transwell system. By measuring trans-endothelial electrical resistance, we found that TNF-α causes the most significant disruption of the endothelial barrier. IL6 had a moderate influence on the barrier, whereas LPS had a minimal impact on barrier integrity. The prominent effect of TNF-α on the barrier was confirmed in the expression of the major junctional genes responsible for forming the tight endothelial monolayer, the decreased expression of ZO1 and OCL. We further tested permeability using 2 μg of daptomycin (1,619 Da), which does not pass the BLB under normal conditions, by measuring its passage through the barrier by HPLC. Treatment with TNF-α resulted in higher permeability in treated samples compared to controls. LPS-treated cells behaved similarly to the untreated cells and did not show differences in permeability compared to control. The endothelial damage caused by TNF-α was confirmed by decreased expression of an essential endothelial proteoglycan, syndecan1. These results allowed us to create an inflammatory environment model that increased BLB permeability in culture and mimicked an inflammatory state within the stria vascularis.

Human blood-labyrinth barrier on a chip: a unique in vitro tool for investigation of BLB properties

Hearing loss is one of the leading causes of disability worldwide, usually as a result of hair cell damage in the inner ear due to aging, acoustic trauma, or exposure to antibiotics or chemotherapy. No drug therapies can protect or restore hearing and current in vitro and animal models used in drug discovery have a very low success rate, mostly due to major differences in anatomy and accessibility of the inner ear environment between species. The blood-labyrinth barrier (BLB) in the stria vascularis is a highly specialized capillary network that controls exchanges between the blood and interstitial space in the cochlea. The BLB is critical for normal hearing, functioning as a physical, transport, and metabolic barrier. To address its complexity and accessibility, we created the first micro-engineered human model of BLB on a chip using autogenous progenitor cells from adult temporal bones. We successfully isolated the BLB from post-mortem human tissue and established an endothelial cell and pericyte culture system on a BLB chip. Using biocompatible materials, we fabricated sustainable two chamber chips. We validated the size-dependent permeability limits of our BLB model by measuring the permeability to daptomycin (molecular weight 1.6 kDa) and midazolam (molecular weight 325.78 Da). Daptomycin did not pass through the BLB layer, whereas midazolam readily passed through the BLB in our system. Thus, our BLB-chip mimicked the integrity and permeability of human stria vascularis capillaries. This represents a major step towards establishing a reliable model for the development of hearing loss treatments.

Non-linguistic auditory speech processing

OBJECTIVES

A method for testing auditory processing of non-linguistic speech-like stimuli was developed and evaluated.

DESIGN

Monosyllabic words were temporally reversed and distorted. Stimuli were matched for spectrum and level. Listeners discriminated between distorted and undistorted stimuli.

STUDY SAMPLE

Three groups were tested. The Normal group was comprised of 12 normal-hearing participants. The Senior group was comprised of 12 seniors. The Hearing Loss group was comprised of 12 participants with thresholds of at least 35 dB HL at one or more frequencies.

RESULTS

The Senior group scored lower than the Normal group, and the Hearing Loss group scored lower than the Senior group. Scores for forward compressed speech were slightly higher than backward compressed speech but the difference was not statistically significant. Retest scores were slightly higher than scores on the first test, but the difference was not statistically significant.

CONCLUSIONS

Large differences in discrimination of distorted speech were observed among the three groups. Age and hearing loss separately affected performance. The depressed performance of the Senior group may be a result of "hidden hearing loss" that is attributed to cochlear synaptopathy. The backward-distorted speech task may be a useful non-linguistic test of speech processing that is language independent.

Conditional Ablation of Glucocorticoid and Mineralocorticoid Receptors from Cochlear Supporting Cells Reveals Their Differential Roles for Hearing Sensitivity and Dynamics of Recovery from Noise-Induced Hearing Loss

Endogenous glucocorticoids (GC) are known to modulate basic elements of cochlear physiology. These include both noise-induced injury and circadian rhythms. While GC signaling in the cochlea can directly influence auditory transduction via actions on hair cells and spiral ganglion neurons, evidence also indicates that GC signaling exerts effects via tissue homeostatic processes that can include effects on cochlear immunomodulation. GCs act at both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Most cell types in the cochlea express both receptors sensitive to GCs. The GR is associated with acquired sensorineural hearing loss (SNHL) through its effects on both gene expression and immunomodulatory programs. The MR has been associated with age-related hearing loss through dysfunction of ionic homeostatic balance. Cochlear supporting cells maintain local homeostatic requirements, are sensitive to perturbation, and participate in inflammatory signaling. Here, we have used conditional gene manipulation techniques to target Nr3c1 (GR) or Nr3c2 (MR) for tamoxifen-induced gene ablation in Sox9-expressing cochlear supporting cells of adult mice to investigate whether either of the receptors sensitive to GCs plays a role in protecting against (or exacerbating) noise-induced cochlear damage. We have selected mild intensity noise exposure to examine the role of these receptors related to more commonly experienced noise levels. Our results reveal distinct roles of these GC receptors for both basal auditory thresholds prior to noise exposure and during recovery from mild noise exposure. Prior to noise exposure, auditory brainstem responses (ABRs) were measured in mice carrying the floxed allele of interest and the Cre recombinase transgene, but not receiving tamoxifen injections (defined as control (no tamoxifen treatment), versus conditional knockout (cKO) mice, defined as mice having received tamoxifen injections. Results revealed hypersensitive thresholds to mid- to low-frequencies after tamoxifen-induced GR ablation from Sox9-expressing cochlear supporting cells compared to control (no tamoxifen) mice. GR ablation from Sox9-expressing cochlear supporting cells resulted in a permanent threshold shift in mid-basal cochlear frequency regions after mild noise exposure that produced only a temporary threshold shift in both control (no tamoxifen) f/fGR:Sox9iCre+ and heterozygous f/+GR:Sox9iCre+ tamoxifen-treated mice. A similar comparison of basal ABRs measured in control (no tamoxifen) and tamoxifen-treated, floxed MR mice prior to noise exposure indicated no difference in baseline thresholds. After mild noise exposure, MR ablation was initially associated with a complete threshold recovery at 22.6 kHz by 3 days post-noise. Threshold continued to shift to higher sensitivity over time such that by 30 days post-noise exposure the 22.6 kHz ABR threshold was 10 dB more sensitive than baseline. Further, MR ablation produced a temporary reduction in peak 1 neural amplitude one day post-noise. While supporting cell GR ablation trended towards reducing numbers of ribbon synapses, MR ablation reduced ribbon synapse counts but did not exacerbate noise-induced damage including synapse loss at the experimental endpoint. GR ablation from the targeted supporting cells increased the basal resting number of Iba1-positive (innate) immune cells (no noise exposure) and decreased the number of Iba1-positive cells seven days following noise exposure. MR ablation did not alter innate immune cell numbers at seven days post-noise exposure. Taken together, these findings support differential roles of cochlear supporting cell MR and GR expression at basal, resting conditions and especially during recovery from noise exposure.

Linking Cerebrovascular Dysfunction to Age-Related Hearing Loss and Alzheimer’s Disease—Are Systemic Approaches for Diagnosis and Therapy Required?

Alzheimer’s disease (AD), the most common cause of dementia in the elderly, is a neurodegenerative disorder associated with neurovascular dysfunction, cognitive decline, and the accumulation of amyloid β peptide (Aβ) in the brain and tau-related lesions in neurons termed neurofibrillary tangles (NFTs). Aβ deposits and NFT formation are the central pathological hallmarks in AD brains, and the majority of AD cases have been shown to exhibit a complex combination of systemic comorbidities. While AD is the foremost common cause of dementia in the elderly, age-related hearing loss (ARHL) is the most predominant sensory deficit in the elderly. During aging, chronic inflammation and resulting endothelial dysfunction have been described and might be key contributors to AD; we discuss an intriguing possible link between inner ear strial microvascular pathology and blood–brain barrier pathology and present ARHL as a potentially modifiable and treatable risk factor for AD development. We present compelling evidence that ARHL might well be seen as an important risk factor in AD development: progressive hearing impairment, leading to social isolation, and its comorbidities, such as frailty, falls, and late-onset depression, link ARHL with cognitive decline and increased risk of dementia, rendering it tempting to speculate that ARHL might be a potential common molecular and pathological trigger for AD. Additionally, one could speculate that amyloid-beta might damage the blood–labyrinth barrier as it does to the blood–brain barrier, leading to ARHL pathology. Finally, there are options for the treatment of ARHL by targeted neurotrophic factor supplementation to the cochlea to improve cognitive outcomes; they can also prevent AD development and AD-related comorbidity in the future.

Effects of Diet and Lifestyle on Audio-Vestibular Dysfunction in the Elderly: A Literature Review

BACKGROUND

The world's age-related health concerns continue to rise. Audio-vestibular disorders, such as hearing loss, tinnitus, and vertigo, are common complaints in the elderly and are associated with social and public health burdens. Various preventative measures can ease their impact, including healthy food consumption, nutritional supplementation, and lifestyle modification. We aim to provide a comprehensive summary of current possible strategies for preventing the age-related audio-vestibular dysfunction.

METHODS

A PubMed, Embase, and Cochrane review databases search was conducted to identify the relationship between diet, lifestyle, and audio-vestibular dysfunction. "Diet", "nutritional supplement", "lifestyle", "exercise", "physical activity", "tinnitus", "vertigo" and "age-related hearing loss" were used as keywords.

RESULTS

Audio-vestibular dysfunction develops and progresses as a result of age-related inflammation and oxidative stress. Diets with anti-inflammatory and antioxidant effects have been proposed to alleviate this illness. A high-fat diet may induce oxidative stress and low protein intake is associated with hearing discomfort in the elderly. Increased carbohydrate and sugar intake positively correlate with the incidence of audio-vestibular dysfunction, whereas a Mediterranean-style diet can protect against the disease. Antioxidants in the form of vitamins A, C, and E; physical activity; good sleep quality; smoking cessation; moderate alcohol consumption; and avoiding noise exposure are also beneficial.

CONCLUSIONS

Adequate diet or nutritional interventions with lifestyle modification may protect against developing audio-vestibular dysfunction in elderly individuals.

Immediate Effects of (Simulated) Age-Related Hearing Loss on Cognitive Processing and Performance for the Backward-Digit-Span Task

The recall of auditorily presented sequences of digits in reverse order (also known as the Backward Digit Span, BDS) is considered to reflect a person's information storage and processing abilities which have been linked to speech-in-noise intelligibility. However, especially in aging research and audiology, persons who are administered the BDS task are often affected by hearing loss (HL). If uncorrected, HL can have immediate assessment-format-related effects on cognitive-test performance and can result, in the long term, in neuroplastic changes impacting cognitive functioning. In the present study, an impairment-simulation approach, mimicking mild-to-moderate age-related HLs typical for persons aged 65, 75, and 85 years, was used in 19 young normal-hearing participants to evaluate the impact of HL on cognitive performance and the cognitive processes probed by the BDS task. Participants completed the BDS task in several listening conditions, as well as several commonly used visual tests of short-term and working memory. The results indicated that BDS performance was impaired by a simulated HL representing that of persons aged 75 years and above. In the normal-hearing condition, BDS performance correlated positively with both performance on tests of short-term memory and performance on tests of working memory. In the listening condition simulating moderate HL (as experienced by the average 85-year-old person), BDS performance only correlated with performance on working-memory tests. In conclusion, simulated (and, by extrapolation, actual) age-related HL negatively affects cognitive-test performance and may change the composition of the cognitive processes associated with the completion of a cognitive task.

Physiological Evidence for Delayed Age-related Hearing Loss in Two Long-lived Rodent Species (Peromyscus leucopus and P. californicus)

Deer mice (genus Peromyscus) are an emerging model for aging studies due to their longevity relative to rodents of similar size. Although Peromyscus species are well-represented in genetic, developmental, and behavioral studies, relatively few studies have investigated auditory sensitivity in this genus. Given the potential utility of Peromyscus for investigations of age-related changes to auditory function, we recorded auditory brainstem responses (ABRs) in two Peromyscus species, P. californicus, and P. leucopus, across the lifespan. We compared hearing sensitivity and ABR wave metrics measured in these species with measurements from Mus musculus (CBA/CaJ strain) to assess age-related effects on hearing across species. Recordings in young animals showed that all species had similar hearing ranges and thresholds with peak sensitivity ranging from 8 to 16 kHz; however, P. californicus and P. leucopus were more sensitive to frequencies below 8 kHz. Although M. musculus showed significant threshold shifts across a broad range of frequencies beginning at middle age and worsening among old individuals, older Peromyscus mice retained good sensitivity to sound across their lifespan. Middle-aged P. leucopus had comparable thresholds to young for frequencies below 24 kHz. P. leucopus also had notably large ABRs that were robust to age-related amplitude reductions, although response latencies increased with age. Old P. californicus were less sensitive to mid-range tones (8-16 kHz) than young individuals; however, there were no significant age-effects on ABR amplitudes or latencies in this species. These results indicate that longevity in Peromyscus mice may be correlated with delayed aging of the auditory system and highlight these species as promising candidates for longitudinal hearing research.

The impact of age-related hearing loss on structural neuroanatomy: A meta-analysis

This meta-analysis investigated the association between age-related hearing loss and structural neuroanatomy, specifically changes to gray matter volume. Hearing loss is associated with increased risk of cognitive decline. Hence, understanding the effects of hearing loss in older age on brain health is essential. We reviewed studies which compared older participants with hearing loss (age-related hearing loss: ARHL) to older adults without clinical hearing loss (no-ARHL), on neuroanatomical outcomes, specifically gray matter (GM) volume as measured by magnetic resonance imaging. A total of five studies met the inclusion criteria, three of which were included in an analysis of whole-brain gray matter volume (ARHL group n = 113; no-ARHL group n = 138), and three were included in analyses of lobe-wise gray matter volume (ARHL group n = 139; no-ARHL group n = 162). Effect-size seed-based d mapping software was employed for whole-brain and lobe-wise analysis of gray matter volume. The analysis indicated there was no significant difference between adults with ARHL compared to those with no-ARHL in whole-brain gray matter volume. Due to lacking stereotactic coordinates, the level of gray matter in specific neuroanatomical locations could only be observed at lobe-level. These data indicate that adults with ARHL show increased gray matter atrophy in the temporal lobe only (not in occipital, parietal, or frontal), compared to adults with no-ARHL. The implications for theoretical frameworks of the hearing loss and cognitive decline relationship are discussed in relation to the results. This meta-analysis was pre-registered on PROSPERO (CRD42021265375). Systematic Review Registration: https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=265375, PROSPERO CRD42021265375.

Hearing loss drug discovery and medicinal chemistry: Current status, challenges, and opportunities

Hearing loss is a severe high unmet need condition affecting more than 1.5 billion people globally. There are no licensed medicines for the prevention, treatment or restoration of hearing. Prosthetic devices, such as hearing aids and cochlear implants, do not restore natural hearing and users struggle with speech in the presence of background noise. Hearing loss drug discovery is immature, and small molecule approaches include repurposing existing drugs, combination therapeutics, late-stage discovery optimisation of known chemotypes for identified molecular targets of interest, phenotypic tissue screening and high-throughput cell-based screening. Hearing loss drug discovery requires the integration of specialist therapeutic area biology and otology clinical expertise. Small molecule drug discovery projects in the global clinical portfolio for hearing loss are here collated and reviewed. An overview is provided of human hearing, inner ear anatomy, inner ear delivery, types of hearing loss and hearing measurement. Small molecule experimental drugs in clinical development for hearing loss are reviewed, including their underpinning biology, discovery strategy and activities, medicinal chemistry, calculated physicochemical properties, pharmacokinetics and clinical trial status. SwissADME BOILED-Egg permeability modelling is applied to the molecules reviewed, and these results are considered. Non-small molecule hearing loss assets in clinical development are briefly noted in this review. Future opportunities in hearing loss drug discovery for human genomics and targeted protein degradation are highlighted.

Temporal acuity is preserved in the auditory midbrain of aged mice

Impaired temporal resolution of the central auditory system has long been suggested to contribute to speech understanding deficits in the elderly. However, it has been difficult to differentiate between direct age-related central deficits and indirect effects of confounding peripheral age-related hearing loss on temporal resolution. To differentiate this, we measured temporal acuity in the inferior colliculus (IC) of aged CBA/J and C57BL/6 mice, as a model of aging with and without concomitant hearing loss. We used two common measures of auditory temporal processing: gap detection as a measure of temporal fine structure and amplitude-modulated noise as a measure of envelope sensitivity. Importantly, auditory temporal acuity remained precise in the IC of old CBA/J mice when no or only minimal age-related hearing loss was present. In contrast, temporal acuity was only indirectly reduced by the presence of age-related hearing loss in aged C57BL/6 mice, not by affecting the brainstem precision, but by affecting the signal-to-noise ratio of the neuronal activity in the IC. This demonstrates that indirect effects of age-related peripheral hearing loss likely remain an important factor for temporal processing in aging in comparison to 'pure' central auditory decline itself. It also draws attention to the issue that the threshold difference between 'nearly normal' or 'clinically normal' hearing aging subjects in comparison to normal hearing young subjects still can have indirect effects on central auditory neural representations of temporal processing.

The global hearing therapeutic pipeline: 2021

Hearing loss is a serious condition affecting more than 1.5 billion people globally. Many affected people benefit from the use of devices, such as hearing aids, but these do not restore natural hearing, and many users still struggle to follow speech in the presence of background noise. Consequently, there is rapid growth in work to discover therapeutics to address this need. Our analysis of the therapeutic pipeline for inner ear and central processing disorders identified 23 assets in clinical trials and 56 in preclinical development, of which 25% have entered the pipeline in the past three years. The innovative potential of this pipeline is encouraging, but there are translational hurdles to be overcome. We highlight challenges for the pipeline and comment on opportunities to support and strengthen it.

Age-related decline in cochlear ribbon synapses and its relation to different metrics of auditory-nerve activity

Loss of inner hair cell-auditory nerve fiber synapses is considered to be an important early stage of neural presbyacusis. Mass potentials, recorded at the cochlear round window, can be used to derive the neural index (NI), a sensitive measure for pharmacologically-induced synapse loss. Here, we investigate the applicability of the NI for measuring age-related auditory synapse loss in young-adult, middle-aged, and old Mongolian gerbils. Synapse loss, which was progressively evident in the 2 aged groups, correlated weakly with NI when measured at a fixed sound level of 60 dB SPL. However, the NI was confounded by decreases in single-unit firing rates at 60 dB SPL. NI at 30 dB above threshold, when firing rates were similar between age groups, did not correlate with synapse loss. Our results show that synapse loss is poorly reflected in the NI of aged gerbils, particularly if further peripheral pathologies are present. The NI may therefore not be a reliable clinical tool to assess synapse loss in aged humans with peripheral hearing loss.

Speech audiometry in noise: SNR Loss per age-group in normal hearing subjects

OBJECTIVES

The present study aimed to determine normal SNR values per age group for the 50% speech reception threshold in noise (SNR Loss) on the VRB (Vocale Rapide dans le Bruit: rapid speech in noise) test.

MATERIAL AND METHODS

Two hundred patients underwent pure-tone threshold and VRB speech-in-noise audiometry. Six ages groups were distinguished: 20-30, 30-40, 40-50, 50-60, 60-70 and>70 years. All subjects had normal hearing for age according to ISO 7029. SNR Loss was measured according to age group.

RESULTS

Mean SNR Loss ranged from -0.37dB in the youngest age group (20-30 years) to +6.84dB in the oldest (>70 years). Range and interquartile range increased with age: 3.66 and 1.49dB respectively for 20-30 year-olds; 6 and 3.5dB for>70 year-olds. Linear regression between SNR Loss and age showed a coefficient R² of 0.83.

CONCLUSION

The present study reports SNR Loss values per age group in normal-hearing subjects (ISO 7029), confirming that SNR Loss increases with age. Scatter also increased with age, suggesting that other age-related factors combine with inner-ear aging to impair hearing in noise.

Psychometric function for speech-in-noise tests accounts for word-recognition deficits in older listeners

Speech-in-noise (SIN) understanding in older age is affected by hearing loss, impaired central auditory processing, and cognitive deficits. SIN-tests measure these factors' compound effects by a speech reception threshold, defined as the signal-to-noise ratio required for 50% word understanding (SNR50). This study compared two standard SIN tests, QuickSIN (n = 354) in young and older adults and BKB-SIN (n = 139) in older adults (>60 years). The effects of hearing loss and age on SIN understanding were analyzed to identify auditory and nonauditory contributions to SIN loss. Word recognition in noise was modelled with individual psychometric functions using a logistic fit with three parameters: the midpoint (SNRα), slope (β), and asymptotic word-recognition deficit at high SNR (λ). The parameters SNRα and λ formally separate SIN loss into two components. SNRα characterizes the steep slope of the psychometric function at which a slight SNR increase provides a considerable improvement in SIN understanding. SNRα was discussed as being predominantly affected by audibility and low-level central auditory processing. The parameter λ describes a shallow segment of the psychometric function at which a further increase in the SNR provides modest improvement in SIN understanding. Cognitive factors in aging may contribute to the SIN loss indicated by λ.

Age-related hearing loss pertaining to potassium ion channels in the cochlea and auditory pathway

Age-related hearing loss (ARHL) is the most prevalent sensory deficit in the elderly and constitutes the third highest risk factor for dementia. Lifetime noise exposure, genetic predispositions for degeneration, and metabolic stress are assumed to be the major causes of ARHL. Both noise-induced and hereditary progressive hearing have been linked to decreased cell surface expression and impaired conductance of the potassium ion channel K_V7.4 (KCNQ4) in outer hair cells, inspiring future therapies to maintain or prevent the decline of potassium ion channel surface expression to reduce ARHL. In concert with K_V7.4 in outer hair cells, K_V7.1 (KCNQ1) in the stria vascularis, calcium-activated potassium channels BK (KCNMA1) and SK2 (KCNN2) in hair cells and efferent fiber synapses, and K_V3.1 (KCNC1) in the spiral ganglia and ascending auditory circuits share an upregulated expression or subcellular targeting during final differentiation at hearing onset. They also share a distinctive fragility for noise exposure and age-dependent shortfalls in energy supply required for sustained surface expression. Here, we review and discuss the possible contribution of select potassium ion channels in the cochlea and auditory pathway to ARHL. We postulate genes, proteins, or modulators that contribute to sustained ion currents or proper surface expressions of potassium channels under challenging conditions as key for future therapies of ARHL.

Linking anatomical and physiological markers of auditory system degeneration with behavioral hearing assessments in a mouse (Mus musculus) model of age-related hearing loss

Age-related hearing loss is a very common sensory disability, affecting one in three older adults. Establishing a link between anatomical, physiological, and behavioral markers of presbycusis in a mouse model can improve the understanding of this disorder in humans. We measured age-related hearing loss for a variety of acoustic signals in quiet and noisy environments using an operant conditioning procedure and investigated the status of peripheral structures in CBA/CaJ mice. Mice showed the greatest degree of hearing loss in the last third of their lifespan, with higher thresholds in noisy than in quiet conditions. Changes in auditory brainstem response thresholds and waveform morphology preceded behavioral hearing loss onset. Loss of hair cells, auditory nerve fibers, and signs of stria vascularis degeneration were observed in old mice. The present work underscores the difficulty in ascribing the primary cause of age-related hearing loss to any particular type of cellular degeneration. Revealing these complex structure-function relationships is critical for establishing successful intervention strategies to restore hearing or prevent presbycusis.

Age-related changes in the temporal processing of acoustical signals in the auditory cortex of rats

Age-related hearing loss is manifested primarily by a decreased sensitivity to faint sounds, that is, by elevation of the hearing thresholds. Nevertheless, aging also affects the ability of the auditory system to process temporal parameters of the sound stimulus. To explore the precision and reliability of auditory temporal processing during aging, responses to several types of sound stimuli were recorded from neurons of the auditory cortex (AC) of young and aged anaesthetized Fischer 344 rats. In response to broad-band noise bursts, the aged rats exhibited larger response magnitudes, a higher proportion of monotonic units, and also a larger variability of response magnitudes, suggesting a lower stability of the rate code. Of primary interest were the responses to temporally structured stimuli (amplitude-modulated (AM) noise, frequency-modulated (FM) tones, and click trains) recorded separately in the right and left AC. Significant differences of temporal processing were already found between the neuronal responses in the left and right AC in the young animals: for the click trains, the left hemisphere exhibited a greater responsiveness to higher repetition rates, lower vector strength values, and a lower similarity of responses. The two hemispheres were also affected differently by aging. In the right hemisphere, neurons in the aged animals displayed worse synchronization with the AM noise and clicks, but better synchronization with the FM tone. In the left hemisphere, neuronal synchronization with the stimulus modulation improved at a higher age for all three stimuli. The results show that the ability of the aging auditory system to process temporal parameters of the stimulus strongly depends on the stimulus type and on laterality. Furthermore, the commonly reported age-related decline in the temporal processing ability cannot be regarded as general as, at least at the neuronal level in the AC, objective measures of the temporal representation often exhibit age-related improvement instead of deterioration.

Uncertainty-based informational masking in a vowel discrimination task for young and old Mongolian gerbils

Informational masking emerges with processing of complex sounds in the central auditory system and can be affected by uncertainty emerging from trial-to-trial variation of stimulus features. Uncertainty can be non-informative but confusing and thus mask otherwise salient stimulus changes resulting in increased discrimination thresholds. With increasing age, the ability for processing of such complex sound scenes degrades. Here, 6 young and 4 old gerbils were tested behaviorally in a vowel discrimination task. Animals were trained to discriminate between sequentially presented target and reference vowels of the vowel pair/I/-/i/. Reference and target vowels were generated shifting the three formants of the reference vowel in steps towards the formants of the target vowels. Non-informative but distracting uncertainty was introduced by random changes in location, level, fundamental frequency or all three features combined. Young gerbils tested with uncertainty for the target or target and reference vowels showed similar informational masking effects for both conditions. Young and old gerbils were tested with uncertainty for the target vowels only. Old gerbils showed no threshold increase discriminating vowels without uncertainty in comparison with young gerbils. Introducing uncertainty, vowel discrimination thresholds increased for young and old gerbils and vowel discrimination thresholds increased most when presenting all three uncertainty features combined. Old gerbils were more susceptible to non-informative uncertainty and their thresholds increased more than thresholds of young gerbils. Gerbils' vowel discrimination thresholds are compared to human performance in the same task (Eipert et al., 2019).

Temporal Coding of Single Auditory Nerve Fibers Is Not Degraded in Aging Gerbils

People suffering from age-related hearing loss typically present with deficits in temporal processing tasks. Temporal processing deficits have also been shown in single-unit studies at the level of the auditory brainstem, midbrain, and cortex of aged animals. In this study, we explored whether temporal coding is already affected at the level of the input to the central auditory system. Single-unit auditory nerve fiber recordings were obtained from 41 Mongolian gerbils of either sex, divided between young, middle-aged, and old gerbils. Temporal coding quality was evaluated as vector strength in response to tones at best frequency, and by constructing shuffled and cross-stimulus autocorrelograms, and reverse correlations, from responses to 1 s noise bursts at 10-30 dB sensation level (dB above threshold). At comparable sensation levels, all measures showed that temporal coding was not altered in auditory nerve fibers of aging gerbils. Furthermore, both temporal fine structure and envelope coding remained unaffected. However, spontaneous rates were decreased in aging gerbils. Importantly, despite elevated pure tone thresholds, the frequency tuning of auditory nerve fibers was not affected. These results suggest that age-related temporal coding deficits arise more centrally, possibly due to a loss of auditory nerve fibers (or their peripheral synapses) but not due to qualitative changes in the responses of remaining auditory nerve fibers. The reduced spontaneous rate and elevated thresholds, but normal frequency tuning, of aged auditory nerve fibers can be explained by the well known reduction of endocochlear potential due to strial dysfunction in aged gerbils.SIGNIFICANCE STATEMENT As our society ages, age-related hearing deficits become ever more prevalent. Apart from decreased hearing sensitivity, elderly people often suffer from a reduced ability to communicate in daily settings, which is thought to be caused by known age-related deficits in auditory temporal processing. The current study demonstrated, using several different stimuli and analysis techniques, that these putative temporal processing deficits are not apparent in responses of single-unit auditory nerve fibers of quiet-aged gerbils. This suggests that age-related temporal processing deficits may develop more central to the auditory nerve, possibly due to a reduced population of active auditory nerve fibers, which will be of importance for the development of treatments for age-related hearing disorders.

Temporal Coding of Single Auditory Nerve Fibers Is Not Degraded in Aging Gerbils

People suffering from age-related hearing loss typically present with deficits in temporal processing tasks. Temporal processing deficits have also been shown in single-unit studies at the level of the auditory brainstem, midbrain, and cortex of aged animals. In this study, we explored whether temporal coding is already affected at the level of the input to the central auditory system. Single-unit auditory nerve fiber recordings were obtained from 41 Mongolian gerbils of either sex, divided between young, middle-aged, and old gerbils. Temporal coding quality was evaluated as vector strength in response to tones at best frequency, and by constructing shuffled and cross-stimulus autocorrelograms, and reverse correlations, from responses to 1 s noise bursts at 10-30 dB sensation level (dB above threshold). At comparable sensation levels, all measures showed that temporal coding was not altered in auditory nerve fibers of aging gerbils. Furthermore, both temporal fine structure and envelope coding remained unaffected. However, spontaneous rates were decreased in aging gerbils. Importantly, despite elevated pure tone thresholds, the frequency tuning of auditory nerve fibers was not affected. These results suggest that age-related temporal coding deficits arise more centrally, possibly due to a loss of auditory nerve fibers (or their peripheral synapses) but not due to qualitative changes in the responses of remaining auditory nerve fibers. The reduced spontaneous rate and elevated thresholds, but normal frequency tuning, of aged auditory nerve fibers can be explained by the well known reduction of endocochlear potential due to strial dysfunction in aged gerbils.SIGNIFICANCE STATEMENT As our society ages, age-related hearing deficits become ever more prevalent. Apart from decreased hearing sensitivity, elderly people often suffer from a reduced ability to communicate in daily settings, which is thought to be caused by known age-related deficits in auditory temporal processing. The current study demonstrated, using several different stimuli and analysis techniques, that these putative temporal processing deficits are not apparent in responses of single-unit auditory nerve fibers of quiet-aged gerbils. This suggests that age-related temporal processing deficits may develop more central to the auditory nerve, possibly due to a reduced population of active auditory nerve fibers, which will be of importance for the development of treatments for age-related hearing disorders.

New molecular therapies for the treatment of hearing loss

An estimated 466 million people suffer from hearing loss worldwide. Sensorineural hearing loss is characterized by degeneration of key structures of the sensory pathway in the cochlea such as the sensory hair cells, the primary auditory neurons and their synaptic connection to the hair cells - the ribbon synapse. Various strategies to protect or regenerate these sensory cells and structures are the subject of intensive research. Yet despite recent advances in our understandings of the capacity of the cochlea for repair and regeneration there are currently no pharmacological or biological interventions for hearing loss. Current research focusses on localized cochlear drug, gene and cell-based therapies. One of the more promising drug-based therapies is based on neurotrophic factors for the repair of the ribbon synapse after noise exposure, as well as preventing loss of primary auditory neurons and regrowth of the auditory neuron fibers after severe hearing loss. Drug therapy delivery technologies are being employed to address the specific needs of neurotrophin and other therapies for hearing loss that include the need for high doses, long-term delivery, localised or cell-specific targeting and techniques for their safe and efficacious delivery to the cochlea. Novel biomaterials are enabling high payloads of drugs to be administered to the cochlea with subsequent slow-release properties that are proving to be beneficial for treating hearing loss. In parallel, new gene therapy technologies are addressing the need for cell specificity and high efficacy for the treatment of both genetic and acquired hearing loss with promising reports of hearing recovery. Some biomaterials and cell therapies are being used in conjunction with the cochlear implant ensuring therapeutic benefit to the primary neurons during electrical stimulation. This review will introduce the auditory system, hearing loss and the potential for repair and regeneration in the cochlea. Drug delivery to the cochlea will then be reviewed, with a focus on new biomaterials, gene therapy technologies, cell therapy and the use of the cochlear implant as a vehicle for drug delivery. With the current pre-clinical research effort into therapies for hearing loss, including clinical trials for gene therapy, the future for the treatment for hearing loss is looking bright.

Therapeutic Potential of Wnt and Notch Signaling and Epigenetic Regulation in Mammalian Sensory Hair Cell Regeneration

Hearing loss is one of the most prevalent sensory deficits worldwide and can result from the death of mechanosensory hair cells that transduce auditory signals in the cochlea. The mammalian cochlea lacks the capacity to regenerate these hair cells once damaged, and currently there are no biological therapies for hearing loss. Understanding the signaling pathways responsible for hair cell development can inform regenerative strategies and identify targets for treating hearing loss. The canonical Wnt and Notch pathways are critical for cochlear development; they converge on several key molecules, such as Atoh1, to regulate prosensory specification, proliferation, hair cell differentiation, and cellular organization. Much work has focused on Wnt and Notch modulation in the neonatal mouse cochlea, where they can promote hair cell regeneration. However, this regenerative response is limited in the adult cochlea and this might be attributed to age-dependent epigenetic modifications. Indeed, the epigenetic status at key gene loci undergoes dynamic changes during cochlear development, maturation, and aging. Therefore, strategies to improve regenerative success in the adult cochlea might require the modulation of Wnt, Notch, or other pathways, as well as targeted epigenetic modifications to alter the activity of key genes critical for supporting cell proliferation or transdifferentiation.