Localized disorganization of the cochlear inner hair cell synaptic region after noise exposure

Detecting Noise-Induced Cochlear Synaptopathy by Auditory Brainstem Response in Tinnitus Patients With Normal Hearing Thresholds: A Meta-Analysis

Noise-induced cochlear synaptopathy (CS) is defined as a permanent loss of synapses in the auditory nerve pathway following noise exposure. Several studies using auditory brainstem response (ABR) have indicated the presence of CS and increased central gain in tinnitus patients with normal hearing thresholds (TNHT), but the results were inconsistent. This meta-analysis aimed to review the evidence of CS and its pathological changes in the central auditory system in TNHT. Published studies using ABR to study TNHT were reviewed. PubMed, EMBASE, and Scopus databases were selected to search for relevant literature. Studies (489) were retrieved, and 11 were included for meta-analysis. The results supported significantly reduced wave I amplitude in TNHT, whereas the alternations in wave V amplitude were inconsistent among the studies. Consistently increased V/I ratio indicated noise-induced central gain enhancement. The results indicated the evidence of noise-induced cochlear synaptopathy in tinnitus patients with normal hearing. However, inconsistent changes in wave V amplitude may be explained by that the failure of central gain that triggers the pathological neural changes in the central auditory system and/or that increased central gain may be necessary to generate tinnitus but not to maintain tinnitus.

Aligned Organization of Synapses and Mitochondria in Auditory Hair Cells

Recent studies have revealed great functional and structural heterogeneity in the ribbon-type synapses at the basolateral pole of the isopotential inner hair cell (IHC). This feature is believed to be critical for audition over a wide dynamic range, but whether the spatial gradient of ribbon morphology is fine-tuned in each IHC and how the mitochondrial network is organized to meet local energy demands of synaptic transmission remain unclear. By means of three-dimensional electron microscopy and artificial intelligence-based algorithms, we demonstrated the cell-wide structural quantification of ribbons and mitochondria in mature mid-cochlear IHCs of mice. We found that adjacent IHCs in staggered pairs differ substantially in cell body shape and ribbon morphology gradient as well as mitochondrial organization. Moreover, our analysis argues for a location-specific arrangement of correlated ribbon and mitochondrial function at the basolateral IHC pole.

MANF supports the inner hair cell synapse and the outer hair cell stereocilia bundle in the cochlea

The authors show in the mouse how the auditory hair cell structural maintenance is perturbed by the inactivation of Manf and the concomitant ER stress, causing early-onset, progressive hearing loss.

Failure in the structural maintenance of the hair cell stereocilia bundle and ribbon synapse causes hearing loss. Here, we have studied how ER stress elicits hair cell pathology, using mouse models with inactivation of Manf (mesencephalic astrocyte-derived neurotrophic factor), encoding an ER-homeostasis-promoting protein. From hearing onset, Manf deficiency caused disarray of the outer hair cell stereocilia bundle and reduced cochlear sound amplification capability throughout the tonotopic axis. In high-frequency outer hair cells, the pathology ended in molecular changes in the stereocilia taper region and in strong stereocilia fusion. In high-frequency inner hair cells, Manf deficiency degraded ribbon synapses. The altered phenotype strongly depended on the mouse genetic background. Altogether, the failure in the ER homeostasis maintenance induced early-onset stereociliopathy and synaptopathy and accelerated the effect of genetic causes driving age-related hearing loss. Correspondingly, MANF mutation in a human patient induced severe sensorineural hearing loss from a young age onward. Thus, we present MANF as a novel protein and ER stress as a mechanism that regulate auditory hair cell maintenance in both mice and humans.

Mechanical overstimulation causes acute injury and synapse loss followed by fast recovery in lateral-line neuromasts of larval zebrafish

Excess noise damages sensory hair cells, resulting in loss of synaptic connections with auditory nerves and, in some cases, hair-cell death. The cellular mechanisms underlying mechanically induced hair-cell damage and subsequent repair are not completely understood. Hair cells in neuromasts of larval zebrafish are structurally and functionally comparable to mammalian hair cells but undergo robust regeneration following ototoxic damage. We therefore developed a model for mechanically induced hair-cell damage in this highly tractable system. Free swimming larvae exposed to strong water wave stimulus for 2 hr displayed mechanical injury to neuromasts, including afferent neurite retraction, damaged hair bundles, and reduced mechanotransduction. Synapse loss was observed in apparently intact exposed neuromasts, and this loss was exacerbated by inhibiting glutamate uptake. Mechanical damage also elicited an inflammatory response and macrophage recruitment. Remarkably, neuromast hair-cell morphology and mechanotransduction recovered within hours following exposure, suggesting severely damaged neuromasts undergo repair. Our results indicate functional changes and synapse loss in mechanically damaged lateral-line neuromasts that share key features of damage observed in noise-exposed mammalian ear. Yet, unlike the mammalian ear, mechanical damage to neuromasts is rapidly reversible.

Noise Exposure Potentiates Exocytosis From Cochlear Inner Hair Cells

Noise-induced hearing loss has gained relevance as one of the most common forms of hearing impairment. The anatomical correlates of hearing loss, principally cell damage and/or death, are relatively well-understood histologically. However, much less is known about the physiological aspects of damaged, surviving cells. Here we addressed the functional consequences of noise exposure on the capacity of inner hair cells (IHCs) to release synaptic vesicles at synapses with spiral ganglion neurons (SGNs). Mice of either sex at postnatal day (P) 15–16 were exposed to 1–12 kHz noise at 120 dB sound pressure level (SPL), for 1 h. Exocytosis was measured by tracking changes in membrane capacitance (ΔCm) from IHCs of the apical cochlea. Upon IHC depolarization to different membrane potentials, ΔC_m showed the typical bell-shaped curve that mirrors the voltage dependence of Ca²⁺ influx, in both exposed and unexposed cells. Surprisingly, from IHCs at 1-day after exposure (d.a.e.), we found potentiation of exocytosis at the peak of the bell-shaped curve. The increase in exocytosis was not accompanied by changes in whole-cell Ca²⁺ influx, suggesting a modification in coupling between Ca²⁺ channels and synaptic vesicles. Consistent with this notion, noise exposure also changed the Ca²⁺-dependence of exocytosis from linear to supralinear. Noise exposure did not cause loss of IHCs, but did result in a small reduction in the number of IHC-SGN synapses at 1-d.a.e. which recovered by 14-d.a.e. In contrast, a strong reduction in auditory brainstem response wave-I amplitude (representing synchronous firing of SGNs) and distortion product otoacoustic emissions (reflecting outer hair cell function) indicated a profound hearing loss at 1- and 14-d.a.e. To determine the role of glutamate release in the noise-induced potentiation of exocytosis, we evaluated vesicular glutamate transporter-3 (Vglut3) knock-out (KO) mice. Unlike WT, IHCs from Vglut3^KO mice showed a noise-induced reduction in ΔC_m and Ca²⁺ influx with no change in the Ca²⁺-dependence of exocytosis. Together, these results indicate that traumatic noise exposure triggers changes of IHC synaptic function including a Vglut3-dependent potentiation of exocytosis.

Maturation of Heterogeneity in Afferent Synapse Ultrastructure in the Mouse Cochlea

Auditory nerve fibers (ANFs) innervating the same inner hair cell (IHC) may have identical frequency tuning but different sound response properties. In cat and guinea pig, ANF response properties correlate with afferent synapse morphology and position on the IHC, suggesting a causal structure-function relationship. In mice, this relationship has not been fully characterized. Here we measured the emergence of synaptic morphological heterogeneities during maturation of the C57BL/6J mouse cochlea by comparing postnatal day 17 (p17, ∼3 days after hearing onset) with p34, when the mouse cochlea is mature. Using serial block face scanning electron microscopy and three-dimensional reconstruction we measured the size, shape, vesicle content, and position of 70 ribbon synapses from the mid-cochlea. Several features matured over late postnatal development. From p17 to p34, presynaptic densities (PDs) and post-synaptic densities (PSDs) became smaller on average (PDs: 0.75 to 0.33; PSDs: 0.58 to 0.31 μm²) and less round as their short axes shortened predominantly on the modiolar side, from 770 to 360 nm. Membrane-associated synaptic vesicles decreased in number from 53 to 30 per synapse from p17 to p34. Anatomical coupling, measured as PSD to ribbon distance, tightened predominantly on the pillar side. Ribbons became less spherical as long-axes lengthened only on the modiolar side of the IHC, from 372 to 541 nm. A decreasing gradient of synaptic ribbon size along the modiolar-pillar axis was detected only at p34 after aligning synapses of adjacent IHCs to a common reference frame (median volumes in nm³ × 10⁶: modiolar 4.87; pillar 2.38). The number of ribbon-associated synaptic vesicles scaled with ribbon size (range 67 to 346 per synapse at p34), thus acquiring a modiolar-pillar gradient at p34, but overall medians were similar at p17 (120) and p34 (127), like ribbon surface area (0.36 vs. 0.34 μm²). PD and PSD morphologies were tightly correlated to each other at individual synapses, more so at p34 than p17, but not to ribbon morphology. These observations suggest that PDs and PSDs mature according to different cues than ribbons, and that ribbon size may be more influenced by cues from the IHC than the surrounding tissue.

Styrene targets sensory and neural cochlear function through the crossroad between oxidative stress and inflammation

BACKGROUND

Although styrene is an established ototoxic agent at occupational exposure levels, the mechanisms of styrene toxicity in the auditory system are still unclear.

OBJECTIVES

The aim of this study was to identify the consequences of styrene chronic exposure in cochlear structures, looking for the mechanisms of ototoxicity of this organic compound and focusing on cell targets and oxidative stress/inflammatory processes.

METHODS

Male adult Wistar rats were exposed to styrene (400 mg/kg by gavage for 5 days/week, 3 consecutive weeks). Hearing loss was evaluated by measuring auditory brainstem responses (ABR), morphological analysis were performed to evaluate hair cell and spiral ganglion neuron survival, as well as synaptic damage. Analysis of apoptotic (p53) and inflammatory (NF-κB, TNF-α, IL-1β and IL-10) mediators were performed by immunofluorescence analysis and western blot.

RESULTS

Styrene ototoxic effects induced a hearing loss of about 35-40 dB. Immunofluorescence and western blotting analyses demonstrated that styrene administration induced redox imbalance and activated inflammatory processes, targeting sensory hair cell and neural dysfunction by a cross-talk between oxidative and inflammatory mediators.

DISCUSSION

Major findings connect styrene ototoxicity to an interplay between redox imbalance and inflammation, leading to the intriguing assumption of a mixed sensory and neural styrene-induced ototoxicity. Thus, in a clinical perspective, data reported here have important implications for styrene risk assessment in humans.

Molecular Assembly and Structural Plasticity of Sensory Ribbon Synapses-A Presynaptic Perspective

In the mammalian cochlea, specialized ribbon-type synapses between sensory inner hair cells (IHCs) and postsynaptic spiral ganglion neurons ensure the temporal precision and indefatigability of synaptic sound encoding. These high-through-put synapses are presynaptically characterized by an electron-dense projection-the synaptic ribbon-which provides structural scaffolding and tethers a large pool of synaptic vesicles. While advances have been made in recent years in deciphering the molecular anatomy and function of these specialized active zones, the developmental assembly of this presynaptic interaction hub remains largely elusive. In this review, we discuss the dynamic nature of IHC (pre-) synaptogenesis and highlight molecular key players as well as the transport pathways underlying this process. Since developmental assembly appears to be a highly dynamic process, we further ask if this structural plasticity might be maintained into adulthood, how this may influence the functional properties of a given IHC synapse and how such plasticity could be regulated on the molecular level. To do so, we take a closer look at other ribbon-bearing systems, such as retinal photoreceptors and pinealocytes and aim to infer conserved mechanisms that may mediate these phenomena.

Near-infrared-light pre-treatment attenuates noise-induced hearing loss in mice

Noise induced hearing loss (NIHL) is accompanied by a reduction of cochlear hair cells and spiral ganglion neurons. Different approaches have been applied to prevent noise induced apoptosis / necrosis. Physical intervention is one technique currently under investigation. Specific wavelengths within the near-infrared light (NIR)-spectrum are known to influence cytochrome-c-oxidase activity, which leads in turn to a decrease in apoptotic mechanisms. It has been shown recently that NIR can significantly decrease the cochlear hair cell loss if applied daily for 12 days after a noise exposure. However, it is still unclear if a single NIR-treatment, just before a noise exposure, could induce similar protective effects. Therefore, the present study was conducted to investigate the effect of a single NIR-pre-treatment aimed at preventing or limiting NIHL. The cochleae of adult NMRI-mice were pre-treated with NIR-light (808 nm, 120 mW) for 5, 10, 20, 30 or 40 minutes via the external ear canal. All animals were noised exposed immediately after the pre-treatment by broad band noise (5–20 kHz) for 30 minutes at 115 dB SPL. Frequency specific ABR-recordings to determine auditory threshold shift were carried out before the pre-treatment and two weeks after the noise exposure. The amplitude increase for wave IV and cochlear hair cell loss were determined. A further group of similar mice was noise exposed only and served as a control for the NIR pre-exposed groups. Two weeks after noise exposure, the ABR threshold shifts of NIR-treated animals were significantly lower (p < 0.05) than those of the control animals. The significance was at three frequencies for the 5-minute pre-treatment group and across the entire frequency range for all other treatment groups. Due to NIR light, the amplitude of wave four deteriorates significantly less after noise exposure than in controls. The NIR pre-treatment had no effect on the loss of outer hair cells, which was just as high with or without NIR-light pre-exposure. Relative to the entire number of outer hair cells across the whole cochlea, outer hair cell loss was rather negligible. No inner hair cell loss whatever was detected. Our results suggest that a single NIR pre-treatment induces a very effective protection of cochlear structures from noise exposure. Pre-exposure of 10 min seems to emerge as the optimal dosage for our experimental setup. A saturated effect occurred with higher dosage-treatments. These results are relevant for protection of residual hearing in otoneurosurgery such as cochlear implantation.

Noise-induced Cochlear Synaptopathy with and Without Sensory Cell Loss

Prior work has provided extensive documentation of threshold sensitivity and sensory hair cell losses after noise exposure. It is now clear, however, that cochlear synaptic loss precedes such losses, at least at low-moderate noise doses, silencing affected neurons. To address questions of whether, and how, cochlear synaptopathy and underlying mechanisms change as noise dose is varied, we assessed cochlear physiologic and histologic consequences of a range of exposures varied in duration from 15 min to 8 h and in level from 85 to 112 dB SPL. Exposures delivered to adult CBA/CaJ mice produced acute elevations in hair cell- and neural-based response thresholds ranging from trivial (∼5 dB) to large (∼50 dB), followed by varying degrees of recovery. Males appeared more noise vulnerable for some conditions of exposure. There was little to no inner hair cell (IHC) loss, but outer hair cell (OHC) loss could be substantial at highest frequencies for highest noise doses. Synapse loss was an early manifestation of noise injury and did not scale directly with either temporary or permanent threshold shift. With increasing noise dose, synapse loss grew to ∼50%, then declined for exposures yielding permanent hair cell injury/loss. All synaptopathic, but no non-synaptopathic exposures produced persistent neural response amplitude declines; those additionally yielding permanent OHC injury/loss also produced persistent reductions in OHC-based responses and exaggerated neural amplitude declines. Findings show that widespread cochlear synaptopathy can be present with and without noise-induced sensory cell loss and that differing patterns of cellular injury influence synaptopathic outcomes.

Audiometric notch for the prediction of early occupational hearing loss and its association with the interleukin-1beta genotype

Objectives

Noise-induced hearing loss (NIHL) is a frequent and irreversible industrial-health problem, the early diagnosis of which can prevent hearing deterioration, especially of speech frequencies. This study aimed to assess hearing impairment in workers occupationally exposed to noise. Audiometric notches and IL-1β gene polymorphisms were evaluated.

Methods

The study included 98 workers employed in a textile factory. Pure-tone audiometric testing was conducted for all workers, using a manual pure-tone diagnostic audiometer, and the noise level was measured at four different work sites in a spinning section. IL-1β gene polymorphism was determined using PCR-RFLP methods.

Results

Workers were exposed to a mean noise level of 105.5 dB. As many as 27.6% of them complained of tinnitus and the majority suffered from some degree of hearing loss in either ear, but none of them exceeded 60 dB. Audiometric notches were detected in either one or both ears. Only 28.6% of workers showed the absence of notches. The TT genotype of IL-1β polymorphisms was dominant in 49% of the workers, whereas TC was predominant in 39.8% and CC in 11.2%. The CC genotype was associated with smoking (54.5%) and audiometric notches (100%). The IL-1β genotype distribution showed no significant difference with or without tinnitus.

Conclusion

This study showed an association between the CC genotype and smoking as well as audiometric notches in workers occupationally exposed to noise. Workers showing audiometric notches should be followed up regularly. Further studies are required to confirm the interrelationship amongst tinnitus, audiometric notches, and IL-1β genotypes.

Targeting dysregulation of redox homeostasis in noise-induced hearing loss: Oxidative stress and ROS signaling

Hearing loss caused by exposure to recreational and occupational noise remains a worldwide disabling condition and dysregulation of redox homeostasis is the hallmark of cochlear damage induced by noise exposure. In this review we discuss the dual function of ROS to both promote cell damage (oxidative stress) and cell adaptive responses (ROS signaling) in the cochlea undergoing a stressful condition such as noise exposure. We focus on animal models of noise-induced hearing loss (NIHL) and on the function of exogenous antioxidants to maintaining a physiological role of ROS signaling by distinguishing the effect of exogenous "direct" antioxidants (i.e. CoQ₁₀, NAC), that react with ROS to decrease oxidative stress, from the exogenous "indirect" antioxidants (i.e. nutraceutics and phenolic compounds) that can activate cellular redox enzymes through the Keap1-Nrf2-ARE pathway. The anti-inflammatory properties of Nrf2 signaling are discussed in relation to the ROS/inflammation interplay in noise exposure. Unveiling the mechanisms of ROS regulating redox-associated signaling pathways is essential in providing relevant targets for innovative and effective therapeutic strategies against NIHL.