噪声所致听力损失现象的物种差异及可能生理机制研究进展

噪声广泛存在于人和动物的生活环境中,从无脊椎动物到哺乳动物乃至人类,都会受到噪声的负面影响.强烈的噪声会损伤听觉系统的结构和功能,引起噪声性听力损失(noise-induced hearing loss,NIHL).本文对噪声性听力损失的类型、影响因素、噪声所致不同程度听力损失形成的可能机制进行了总结,发现NIHL主要与突触结构肿胀、谷氨酸引起的可逆兴奋性中毒以及活性氧引起的氧化应激、细胞凋亡、带状体损伤、α激动型鸟嘌呤核苷酸结合蛋白(guanine nucleotide binding protein alpha stimulating,GNAS)基因的mRNA及其上游lncRNA Sept7的表达量上调等因素有关.比较噪声暴露后不同物种听力损失情况的差异,发现鱼类和鸟类由于具有毛细胞再生能力而能够较快从听力损伤中恢复,啮齿类较容易受到噪声影响,而回声定位鲸类噪声暴露后的暂时性听觉阈移较小,非常有趣的是回声定位蝙蝠在噪声高强度暴露后未表现出暂时性听觉阈移的现象.上述结论提示,对不同物种的比较生理研究可深入揭示NIHL机制,并为听力保护以及噪声所致的听力损伤后修复等提供理论参考.     

The voltage-gated potassium channel subfamily KQT member 4 (KCNQ4) displays parallel evolution in echolocating bats

Bats are the only mammals that use highly developed laryngeal echolocation, a sensory mechanism based on the ability to emit laryngeal sounds and interpret the returning echoes to identify objects. Although this capability allows bats to orientate and hunt in complete darkness, endowing them with great survival advantages, the genetic bases underlying the evolution of bat echolocation are still largely unknown. Echolocation requires high-frequency hearing that in mammals is largely dependent on somatic electromotility of outer hair cells. Then, understanding the molecular evolution of outer hair cell genes might help to unravel the evolutionary history of echolocation. In this work, we analyzed the molecular evolution of two key outer hair cell genes: the voltage-gated potassium channel gene KCNQ4 and CHRNA10, the gene encoding the α10 nicotinic acetylcholine receptor subunit. We reconstructed the phylogeny of bats based on KCNQ4 and CHRNA10 protein and nucleotide sequences. A phylogenetic tree built using KCNQ4 amino acid sequences showed that two paraphyletic clades of laryngeal echolocating bats grouped together, with eight shared substitutions among particular lineages. In addition, our analyses indicated that two of these parallel substitutions, M388I and P406S, were probably fixed under positive selection and could have had a strong functional impact on KCNQ4. Moreover, our results indicated that KCNQ4 evolved under positive selection in the ancestral lineage leading to mammals, suggesting that this gene might have been important for the evolution of mammalian hearing. On the other hand, we found that CHRNA10, a gene that evolved adaptively in the mammalian lineage, was under strong purifying selection in bats. Thus, the CHRNA10 amino acid tree did not show echolocating bat monophyly and reproduced the bat species tree. These results suggest that only a subset of hearing genes could underlie the evolution of echolocation. The present work continues to delineate the genetic bases of echolocation and ultrasonic hearing in bats.     

Parallel signatures of sequence evolution among hearing genes in echolocating mammals: an emerging model of genetic convergence

Recent findings of sequence convergence in the Prestin gene among some bats and cetaceans suggest that parallel adaptations for high-frequency hearing have taken place during the evolution of echolocation. To determine if this gene is an exception, or instead similar processes have occurred in other hearing genes, we have examined Tmc1 and Pjvk, both of which are associated with non-syndromic hearing loss in mammals. These genes were amplified and sequenced from a number of mammalian species, including echolocating and non-echolocating bats and whales, and were analysed together with published sequences. Sections of both genes showed phylogenetic signals that conflicted with accepted species relationships, with coding regions uniting laryngeal echolocating bats in a monophyletic clade. Bayesian estimates of posterior probabilities of convergent and divergent substitutions provided more direct evidence of sequence convergence between the two groups of laryngeal echolocating bats as well as between echolocating bats and dolphins. We found strong evidence of positive selection acting on some echolocating bat species and echolocating cetaceans, contrasting with purifying selection on non-echolocating bats. Signatures of sequence convergence and molecular adaptation in two additional hearing genes suggest that the acquisition of high-frequency hearing has involved multiple loci.     

Ear morphology of the frog-eating bat (Trachops cirrhosus,family: Phyllostomidae): Apparent specializations for low-freqency hearing

The frog-eating bat (Trachops cirrhosus) is unusual among bats studied because of its reliance on low-frequency (<5 kHz) sounds emitted by frogs for prey localization. We investigated the ear of this bat in order to identify anatomical features that might serve as adaptations for low-frequency hearing. Trachops cirrhosus has a variety of anatomical features that might enhance low-frequency hearing, either by increasing sensitivity to low-frequency sounds or expanding the total frequency range to include lower frequencies. These bats have long pinnae, and a long and wide basilar membrane. The basal portion of the basilar membrane is much stiffer than the apical portion, and the basal portion of the tectorial membrane is more massive than the apical portion. There is also a concentration of mass in the apical portion of the cochlea. T. cirrhosus possesses the largest number of cochlear neurons reported for any mammal, the second highest density of cochlear neurons innervation known among mammals, and three peaks of cochlear neuron density. Other bats have two peaks of cochlear neuron density, lacking the apical concentration, while other mammals usually have only one. T. cirrhosus differs from most other small mammals and bats in characteristics of the apical portion of the cochlea, i.e., that area where the place theory of hearing predicts that low frequencies are detected.     

Characteristics of echolocating bats’auditory stereocilia length, compared with other mammals

The stereocilia of the Organ of Corti in 4 different echolocating bats, Myotis adversus, Murina leuco-gaster, Nyctalus plancyi (Nyctalus velutinus), and Rhinolophus ferrumequinum were observed by using scanning electron microscopy (SEM). Stereocilia lengths were estimated for comparison with those of non-echolocating mammals. The specialized lengths of outer hair cells (OHC) stereocilia in echolocating bats were shorter than those of non-echolocating mammals. The specialized lengths of inner hair cells (IHC) stereocilia were longer than those of outer hair cells stereocilia in the Organ of Corti of echolocating bats. These characteristics of the auditory stereocilia length of echolocating bats represent the fine architecture of the electromotility process, helping to adapt to high frequency sound and echolocation.     

Characteristics of echolocating bats’ auditory stereocilia length,compared with other mammals

The stereocilia of the Organ of Corti in 4 different echolocating bats, Myotis adversus, Murina leucogaster, Nyctalus plancyi (Nyctalus velutinus), and Rhinolophus ferrumequinum were observed by using scanning electron microscopy (SEM). Stereocilia lengths were estimated for comparison with those of non-echolocating mammals. The specialized lengths of outer hair cells (OHC) stereocilia in echolocating bats were shorter than those of non-echolocating mammals. The specialized lengths of inner hair cells (IHC) stereocilia were longer than those of outer hair cells stereocilia in the Organ of Corti of echolocating bats. These characteristics of the auditory stereocilia length of echolocating bats represent the fine architecture of the electromotility process, helping to adapt to high frequency sound and echolocation.     

Characteristics of echolocating bats’ auditory stereocilia length, compared with other mammals

The stereocilia of the Organ of Corti in 4 different echolocating bats, Myotis adversus, Murina leucogaster, Nyctalus plancyi (Nyctalus velutinus), and Rhinolophus ferrumequinum were observed by using scanning electron microscopy (SEM). Stereocilia lengths were estimated for comparison with those of non-echolocating mammals. The specialized lengths of outer hair cells (OHC) stereocilia in echolocating bats were shorter than those of non-echolocating mammals. The specialized lengths of inner hair cells (IHC) stereocilia were longer than those of outer hair cells stereocilia in the Organ of Corti of echolocating bats. These characteristics of the auditory stereocilia length of echolocating bats represent the fine architecture of the electromotility process, helping to adapt to high frequency sound and echolocation. These authors contributed equally to this work Supported by the National Natural Science Foundation of China (Grant No. 30430120) and Foundation of President of the Chinese Academy of Sciences     

Protective role of hydrogen sulfide against noise-induced cochlear damage: a chronic intracochlear infusion model

Background

A reduction in cochlear blood flow plays an essential role in noise-induced hearing loss (NIHL). The timely regulation of cochlear perfusion determines the progression and prognosis of NIHL. Hydrogen sulfide (H₂S) has attracted increasing interest as a vasodilator in cardiovascular systems. This study identified the role of H₂S in cochlear blood flow regulation and noise protection.

Methodology/Principal Findings

The gene and protein expression of the H₂S synthetase cystathionine-γ-lyase (CSE) in the rat cochlea was examined using immunofluorescence and real-time PCR. Cochlear CSE mRNA levels varied according to the duration of noise exposure. A chronic intracochlear infusion model was built and artificial perilymph (AP), NaHS or DL-propargylglycine (PPG) were locally administered. Local sodium hydrosulfide (NaHS) significantly increased cochlear perfusion post-noise exposure. Cochlear morphological damage and hearing loss were alleviated in the NaHS group as measured by conventional auditory brainstem response (ABR), cochlear scanning electron microscope (SEM) and outer hair cell (OHC) count. The highest percentage of OHC loss occurred in the PPG group.

Conclusions/Significance

Our results suggest that H₂S plays an important role in the regulation of cochlear blood flow and the protection against noise. Further studies may identify a new preventive and therapeutic perspective on NIHL and other blood supply-related inner ear diseases.     

An autocorrelation model of bat sonar

Their sonar system allows echolocating bats to navigate with high skill through a complex, three- dimensional environment at high speed and low light. The auditory analysis of the echoes of their ultrasonic sounds requires a detailed comparison of the emission and echoes. Here an auditory model of bat sonar is introduced and evaluated against a set of psychophysical phantom-target, echo-acoustic experiments. The model consists of a relatively detailed simulation of auditory peripheral processing in the bat, Phyllostomus discolor, followed by a functional module consisting of a strobed, normalised, autocorrelation in each frequency channel. The model output is accumulated in a sonar image buffer. The model evaluation is based on the comparison of the image-buffer contents generated in individually simulated psychophysical trials. The model provides reasonably good predictions for both temporal and spectral behavioural sonar processing in terms of sonar delay-, roughness, and phase sensitivity and in terms of sensitivity to the temporal separations in two-front targets and the classification of spectrally divergent phantom targets.     

Hearing vulnerability after noise exposure in a mouse model of reactive oxygen species overproduction

Previous studies have convincingly argued that reactive oxygen species (ROS ) contribute to the development of several major types of sensorineural hearing loss, such as noise‐induced hearing loss (NIHL ), drug‐induced hearing loss, and age‐related hearing loss. However, the underlying molecular mechanisms induced by ROS in these pathologies remain unclear. To resolve this issue, we established an in vivo model of ROS overproduction by generating a transgenic (TG ) mouse line expressing the human NADPH oxidase 4 (NOX 4, NOX 4‐ TG mice), which is a constitutively active ROS ‐producing enzyme that does not require stimulation or an activator. Overproduction of ROS was detected at the cochlea of the inner ear in NOX 4 ‐TG mice, but they showed normal hearing function under baseline conditions. However, they demonstrated hearing function vulnerability, especially at high‐frequency sounds, upon exposure to intense noise, which was accompanied by loss of cochlear outer hair cells (OHC s). The vulnerability to loss of hearing function and OHC s was rescued by treatment with the antioxidant Tempol. Additionally, we found increased protein levels of the heat‐shock protein 47 (HSP 47) in models using HEK 293 cells, including H₂O₂ treatment and cells with stable and transient expression of NOX 4. Furthermore, the up‐regulated levels of Hsp47 were observed in both the cochlea and heart of NOX 4 ‐TG mice. Thus, antioxidant therapy is a promising approach for the treatment of NIHL . Hsp47 may be an endogenous antioxidant factor, compensating for the chronic ROS overexposure in vivo , and counteracting ROS ‐related hearing loss.

     

Noise‐induced cochlear F‐actin depolymerization is mediated via ROCK2/p‐ERM signaling

Our previous work has suggested that traumatic noise activates Rho‐GTPase pathways in cochlear outer hair cells (OHCs), resulting in cell death and noise‐induced hearing loss (NIHL). In this study, we investigated Rho effectors, Rho‐associated kinases (ROCKs), and the targets of ROCKs, the ezrin‐radixin‐moesin (ERM) proteins, in the regulation of the cochlear actin cytoskeleton using adult CBA/J mice under conditions of noise‐induced temporary threshold shift (TTS) and permanent threshold shift (PTS) hearing loss, which result in changes to the F/G‐actin ratio. The levels of cochlear ROCK2 and p‐ERM decreased 1 h after either TTS‐ or PTS‐noise exposure. In contrast, ROCK2 and p‐ERM in OHCs decreased only after PTS‐, not after TTS‐noise exposure. Treatment with lysophosphatidic acid, an activator of the Rho pathway, resulted in significant reversal of the F/G‐actin ratio changes caused by noise exposure and attenuated OHC death and NIHL. Conversely, the down‐regulation of ROCK2 by pretreatment with ROCK2 siRNA reduced the expression of ROCK2 and p‐ERM in OHCs, exacerbated TTS to PTS, and worsened OHC loss. Additionally, pretreatment with siRNA against radixin, an ERM protein, aggravated TTS to PTS. Our results indicate that a ROCK2‐mediated ERM‐phosphorylation signaling cascade modulates noise‐induced hair cell loss and NIHL by targeting the cytoskeleton.

     

Peripheral control of acoustic signals in the auditory system of echolocating bats.

Many species of echolocating bats emit intense orientation sounds. If such intense sounds directly stimulated their ears, detection of faint echoes would be impaired. Therefore, possible mechanisms for the attenuation of self-stimulation were studied with Myotis lucifugus. The acoustic middle-ear-muscle reflex could perfectly and transiently regulate the amplitude of an incoming signal only at its beginning. However, its shortest latency in terms of electromyograms and of the attenuation of the cochlear microphonic was 3-4 and 4-8 msec, respectively, so that these muscles failed to attenuate orientation signals by the reflex. The muscles, however, received a message from the vocalization system when the bat vocalized, and contracted synchronously with vocalization. The duration of the contraction-relaxation was so short that the self-stimulation was attenuated, but the echoes were not. The tetanus-fusion frequency of tha stapedium muscle ranged between 260 and 320/sec. Unlike the efferent fibres in the lateral-line and vestibular systems, the olivo-cochlear bundle showed no sign of attenuation of self-stimulation.     

Reg3b在大鼠耳蜗的分布及噪声刺激后的表达变化

目的：探讨Reg3b在大鼠耳蜗中的分布情况及在噪声刺激前后的表达变化,为治疗噪声性聋提供新思路。方法：30只健康成年SD大鼠,分为噪声暴露组和正常对照组,利用110dBSPL宽频稳态白噪声对噪声组进行噪声暴露,通过免疫组织荧光技术,观察Reg3b在正常及噪声刺激后成年sD大鼠耳蜗内的分布情况。采用实时定量PCR技术（Realtime-PCR）方法检测大鼠接受噪声刺激前后Reg3b在耳蜗内的表达变化。结果：免疫组织荧光技术提示,Reg3b在噪声暴露后主要表达于大鼠耳蜗的内毛细胞、外毛细胞,以及螺旋神经节处,而正常大鼠耳蜗中Reg3b表达不明显或呈阴性表达。与噪声刺激前相比,噪声刺激后,Reg3b在mRNA水平表达较噪声前明显提高。结论：Reg3b在耳蜗内的分布及在噪声刺激后的表达显著升高提示其在噪声诱导的细胞死亡及对抗噪声损伤方面具有一定作用,可能成为治疗感音神经性聋的新靶点。     

Free radical scavengers vitamins A, C, and E plus magnesium reduce noise trauma

Free radical formation in the cochlea plays a key role in the development of noise-induced hearing loss (NIHL). The amount, distribution, and time course of free radical formation have been defined, including a clinically significant formation of both reactive oxygen species and reactive nitrogen species 7-10 days after noise exposure. Reduction in cochlear blood flow as a result of free radical formation has also been described. Here we report that the antioxidant agents vitamins A, C, and E act in synergy with magnesium to effectively prevent noise-induced trauma. Neither the antioxidant agents nor the magnesium reliably reduced NIHL or sensory cell death with the doses we used when these agents were delivered alone. In combination, however, they were highly effective in reducing both hearing loss and cell death even with treatment initiated just 1 h before noise exposure. This study supports roles for both free radical formation and noise-induced vasoconstriction in the onset and progression of NIHL. Identification of this safe and effective antioxidant intervention that attenuates NIHL provides a compelling rationale for human trials in which free radical scavengers are used to eliminate this single major cause of acquired hearing loss.     

Sonar-induced temporary hearing loss in dolphins

There is increasing concern that human-produced ocean noise is adversely affecting marine mammals, as several recent cetacean mass strandings may have been caused by animals'' interactions with naval ‘mid-frequency’ sonar. However, it has yet to be empirically demonstrated how sonar could induce these strandings or cause physiological effects. In controlled experimental studies, we show that mid-frequency sonar can induce temporary hearing loss in a bottlenose dolphin (Tursiops truncatus). Mild-behavioural alterations were also associated with the exposures. The auditory effects were induced only by repeated exposures to intense sonar pings with total sound exposure levels of 214 dB re: 1 μPa² s. Data support an increasing energy model to predict temporary noise-induced hearing loss and indicate that odontocete noise exposure effects bear trends similar to terrestrial mammals. Thus, sonar can induce physiological and behavioural effects in at least one species of odontocete; however, exposures must be of prolonged, high sound exposures levels to generate these effects.     

Parallel evolution of KCNQ4 in echolocating bats

High-frequency hearing is required for echolocating bats to locate, range and identify objects, yet little is known about its molecular basis. The discovery of a high-frequency hearing-related gene, KCNQ4, provides an opportunity to address this question. Here, we obtain the coding regions of KCNQ4 from 15 species of bats, including echolocating bats that have higher frequency hearing and non-echolocating bats that have the same ability as most other species of mammals. The strongly supported protein-tree resolves a monophyletic group containing all bats with higher frequency hearing and this arrangement conflicts with the phylogeny of bats in which these species are paraphyletic. We identify five parallel evolved sites in echolocating bats belonging to both suborders. The evolutionary trajectories of the parallel sites suggest the independent gain of higher frequency hearing ability in echolocating bats. This study highlights the usefulness of convergent or parallel evolutionary studies for finding phenotype-related genes and contributing to the resolution of evolutionary problems.     

Acoustic trauma increases cochlear and hair cell uptake of gentamicin

Background

Exposure to intense sound or high doses of aminoglycoside antibiotics can increase hearing thresholds, induce cochlear dysfunction, disrupt hair cell morphology and promote hair cell death, leading to permanent hearing loss. When the two insults are combined, synergistic ototoxicity occurs, exacerbating cochlear vulnerability to sound exposure. The underlying mechanism of this synergism remains unknown. In this study, we tested the hypothesis that sound exposure enhances the intra-cochlear trafficking of aminoglycosides, such as gentamicin, leading to increased hair cell uptake of aminoglycosides and subsequent ototoxicity.

Methods

Juvenile C57Bl/6 mice were exposed to moderate or intense sound levels, while fluorescently-conjugated or native gentamicin was administered concurrently or following sound exposure. Drug uptake was then examined in cochlear tissues by confocal microscopy.

Results

Prolonged sound exposure that induced temporary threshold shifts increased gentamicin uptake by cochlear hair cells, and increased gentamicin permeation across the strial blood-labyrinth barrier. Enhanced intra-cochlear trafficking and hair cell uptake of gentamicin also occurred when prolonged sound, and subsequent aminoglycoside exposure were temporally separated, confirming previous observations. Acute, concurrent sound exposure did not increase cochlear uptake of aminoglycosides.

Conclusions

Prolonged, moderate sound exposures enhanced intra-cochlear aminoglycoside trafficking into the stria vascularis and hair cells. Changes in strial and/or hair cell physiology and integrity due to acoustic overstimulation could increase hair cell uptake of gentamicin, and may represent one mechanism of synergistic ototoxicity.