Delayed mitochondrial dysfunction in apoptotic hair cells in chinchilla cochleae following exposure to impulse noise

Pharmaceutical otoprotection strategies to prevent impulse noise-induced hearing loss

One of the ongoing challenges for hearing researchers is successful protection of the ear from noise injury. For decades, the most effective methods have been based on modifying the acoustic properties of the noise, either by reducing noise output from various sources, interfering in the acoustic exposure path with environmental controls, or altering the noise dose for the individual with personal hearing protection devices. Because of the inefficiencies of some of the acoustic modification procedures, pharmaceutical otoprotection is targeted at making the cochlea less susceptible to injury. Short-duration, high-level impulse noises, typically caused by small-scale explosions, cause different sets of injuries in the ear than long-duration, low-variance noise exposures. Therefore, the expectation is that the ears exposed to impulse noise may need different pharmaceutical interventions, both in type of compounds used and the time course of administration of the compounds. The current review discusses four different classes of compounds that have been tested as impulse noise otoprotectants. In the process of describing those experiments, particular emphasis is placed on the acoustic properties of the impulses used, with the goal of providing context for evaluating the relevance of these different models to human impulse noise-induced hearing loss.

Role of Calcium-Sensing Receptor in Mechanotransducer-Channel-Mediated Ca2+ Influx in Hair Cells of Zebrafish Larvae

The calcium-sensing receptor (CaSR) is an extracellular Ca²⁺ sensor that plays a critical role in maintaining Ca²⁺ homeostasis in several organs, including the parathyroid gland and kidneys. In this study, through in situ hybridization, the expression of CaSR mRNA was found in the neuromasts of zebrafish larvae. Immunohistochemistry further demonstrated that the CaSR protein was present in neuromast hair cell stereocilia and basolateral membranes. Based on the expression and subcellular localization of the CaSR in hair cells, we hypothesized that the CaSR is expressed in zebrafish lateral-line hair cells to regulate mechanotransducer (MET)-channel-mediated Ca²⁺ entry. Using the scanning ion-selective electrode technique, MET-channel-mediated Ca²⁺ influx at the stereocilia of hair cells was measured in intact larvae. Ca²⁺ influx was suppressed after larvae were pretreated with a CaSR activator (R-568) or high-Ca²⁺ (HCa) medium. Gene knockdown by using morpholino oligonucleotides decreased CaSR expression in hair cells and eliminated the effects of R-568 and HCa on Ca²⁺ influx. In addition, we found that treatment with R-568 attenuated neomycin-induced hair cell death. This study is the first to demonstrate that the CaSR is involved in mechanotransduction in zebrafish hair cells.

Variation analysis of transcriptome changes reveals cochlear genes and their associated functions in cochlear susceptibility to acoustic overstimulation

Individual variation in the susceptibility of the auditory system to acoustic overstimulation has been well-documented at both the functional and structural levels. However, the molecular mechanism responsible for this variation is unclear. The current investigation was designed to examine the variation patterns of cochlear gene expression using RNA-seq data and to identify the genes with expression variation that increased following acoustic trauma. This study revealed that the constitutive expressions of cochlear genes displayed diverse levels of gene-specific variation. These variation patterns were altered by acoustic trauma; approximately one-third of the examined genes displayed marked increases in their expression variation. Bioinformatics analyses revealed that the genes that exhibited increased variation were functionally related to cell death, biomolecule metabolism, and membrane function. In contrast, the stable genes were primarily related to basic cellular processes, including protein and macromolecular syntheses and transport. There was no functional overlap between the stable and variable genes. Importantly, we demonstrated that glutamate metabolism is related to the variation in the functional response of the cochlea to acoustic overstimulation. Taken together, the results indicate that our analyses of the individual variations in transcriptome changes of cochlear genes provide important information for the identification of genes that potentially contribute to the generation of individual variation in cochlear responses to acoustic overstimulation.

Cell-cell junctions: a target of acoustic overstimulation in the sensory epithelium of the cochlea

Background

Exposure to intense noise causes the excessive movement of the organ of Corti, stretching the organ and compromising sensory cell functions. We recently revealed changes in the transcriptional expression of multiple adhesion-related genes during the acute phases of cochlear damage, suggesting that the disruption of cell-cell junctions is an early event in the process of cochlear pathogenesis. However, the functional state of cell junctions in the sensory epithelium is not clear. Here, we employed graded dextran-FITC, a macromolecule tracer that is impermeable to the organ of Corti under physiological conditions, to evaluate the barrier function of cell junctions in normal and noise-traumatized cochlear sensory epithelia.

Results

Exposure to an impulse noise of 155 dB (peak sound pressure level) caused a site-specific disruption in the intercellular junctions within the sensory epithelium of the chinchilla cochlea. The most vulnerable sites were the junctions among the Hensen cells and between the Hensen and Deiters cells within the outer zone of the sensory epithelium. The junction clefts that formed in the reticular lamina were permeable to 40 and 500 but not 2,000 kDa dextran-FITC macromolecules. Moreover, this study showed that the interruption of junction integrity occurred in the reticular lamina and also in the basilar membrane, a site that had been considered to be resistant to acoustic injury. Finally, our study revealed a general spatial correlation between the site of sensory cell damage and the site of junction disruption. However, the two events lacked a strict one-to-one correlation, suggesting that the disruption of cell-cell junctions is a contributing, but not the sole, factor for initiating acute sensory cell death.

Conclusions

Impulse noise causes the functional disruption of intercellular junctions in the sensory epithelium of the chinchilla cochlea. This disruption occurs at an early phase of cochlear damage. Understanding the role of this disruption in cochlear pathogenesis will require future study.

Increased activity of mitochondrial respiratory chain complex in noise-damaged rat cochlea

CONCLUSION

Functional changes of mitochondrial respiratory chain (MRC) and hearing threshold shifts were observed after noise exposure and the pattern was different depending on the extent of the noise damage.

OBJECTIVES

The aim of this study was to reveal any functional changes of the MRC enzyme in outer hair cells after noise damage.

METHODS

Sprague-Dawley rats were used and divided into three groups including normal control group and noise exposure for 1 h and 5 h. White band noise was used with 120 dB SPL. We measured hearing threshold and activity of succinate dehydrogenase (SDH) before noise exposure, and immediately and 1, 3, 7, and 14 days after noise exposure in each group. Immunostaining using monoclonal anti-MRC complex V subunit α was performed. Transmission electron microscopy (TEM) was carried out.

RESULTS

The hearing threshold level changed depending on the exposure time in both the noise-exposed groups. In the 1 h group, hearing was improved after 1 day, and in the 5 h group, hearing was measured after 7 days. Changes in SDH activity were observed at the third day in the 1 h group and at the seventh day in the 5 h group. The expression level of MRC complex V was increased after noise exposure. TEM showed mitochondrial damage.

Membrane disruption: an early event of hair cell apoptosis induced by exposure to intense noise

Membrane leakage has been found in hair cells undergoing apoptosis following exposure to intense noise. However, it is not known whether this membrane damage is the consequence of apoptotic degeneration or direct mechanical stress. The current study was designed to investigate whether membrane damage occurred before the onset of apoptosis and to determine the level of the membrane damage. Chinchillas were exposed to an impulse noise at 155 dB peak SPL. The noise-induced membrane damage was assessed functionally, using membrane tracers with graded molecular sizes (propidium iodide and FITC-dextrans with molecular sizes of 3, 40, 500, and 2000 kDa), and morphologically, using DiO staining and semithin sections. The study revealed two major findings. First, exposure to intense noise caused a rapid increase in membrane permeability, and the onset of membrane leakage preceded the manifestation of nuclear condensation. This indicates that the early membrane damage observed in apoptosis is the direct consequence of mechanical stress. Second, the level of membrane damage was severe, allowing the entry of 3 kDa and 40 kDa FITC-dextrans, but the membrane was not completely broken down, as evidenced by the preservation of the ability to exclude 500 kDa and 2000 kDa FITC-dextran molecules and the maintenance of the cell boundary. Notably, despite the membrane damage, hair cells continue to undergo the apoptotic process, leading to the generation of a type of apoptosis with early membrane damage.

The impact of mitochondrial energetic dysfunction on apoptosis in outer hair cells of the cochlea following exposure to intense noise

Previous studies have shown that exposure to intense noise causes outer hair cells (OHCs) to die, primarily through the process of apoptotic degeneration. The current study was designed to examine the regulatory role of mitochondrial bioenergetic function in controlling the initiation and execution of the apoptotic process of OHCs. Chinchilla cochleae were treated with 3-nitropropionic acid (3-NP, 20 or 50mM), an irreversible inhibitor of succinate dehydrogenase (SDH), to inhibit the mitochondrial energy production before and after exposure to 75 pairs of impulses at 155dB pSPL. Comparison of the noise-exposed cochleae treated with and without 3-NP revealed that the inhibition of SDH activity delayed nuclear degradation in apoptotic OHCs. However, the initiation of apoptosis appeared to be undeterred. There was no major shift of cell death pathways from apoptosis to necrosis, although a small portion of OHCs showed signs of secondary necrosis. Collectively, the results of the study suggest that, while the mitochondrial energetic function plays an important role in regulating the apoptotic process, its dysfunction has a limited influence on the suppression of apoptotic induction in OHCs following exposure to intense noise.