Cochlear implants: the view from the brain

The cochlear implant arguably is the most successful neural prosthesis. Studies of the responses of the central auditory system to prosthetic electrical stimulation of the cochlea are revealing the success with which electrical stimulation of a deaf ear can mimic acoustic stimulation of a normal-hearing ear. Understanding of the physiology of central auditory structures can lead to improved restoration of hearing with cochlear implants. In turn, the cochlear implant can be exploited as an experimental tool for examining central hearing mechanisms isolated from the effects of cochlear mechanics and transduction.     

顺铂耳毒性大鼠耳聋模型的研究

摘要 目的：探讨顺铂对大鼠造成的听力损伤及耳蜗细胞形态学变化。方法：体内实验,运用顺铂腹腔注射的方法,连续七天注射,通过听性脑干反应检测,观察顺铂对不同日龄的大鼠听力损伤情况;测听后取耳蜗,通过基底膜铺片和冰冻切片的免疫荧光染色,观察听力损伤后对耳蜗毛细胞和螺旋神经元的影响。体外实验,耳蜗器官培养免疫荧光染色,观察顺铂对耳蜗毛细胞和螺旋神经元的影响。结果：顺铂具有耳毒性,会对大鼠听力造成损伤,高频听力损伤更加严重,而且对不同日龄的大鼠造成的听力损失不同,小日龄的大鼠对顺铂耳毒性更加敏感。体内实验,顺铂耳毒性造成听力损失,会引起大鼠耳蜗毛细胞的缺失,但未观察到明显的螺旋神经元缺失,也没有观察到明显的Cleaved caspase-3阳性螺旋神经元细胞。体外实验,可以观察到顺铂同时引起毛细胞和螺旋神经元产生明显的损伤。结论：体、内外实验,都可以建立稳定的顺铂耳毒性大鼠耳聋模型,对研究顺铂损伤耳蜗毛细胞的发生机制和保护奠定了实验基础。     

Expression and function of pannexins in the inner ear and hearing

Pannexin (Panx) is a gene family encoding gap junction proteins in vertebrates. So far, three isoforms (Panx1, 2 and 3) have been identified. All of three Panx isoforms express in the cochlea with distinct expression patterns. Panx1 expresses in the cochlea extensively, including the spiral limbus, the organ of Corti, and the cochlear lateral wall, whereas Panx2 and Panx3 restrict to the basal cells of the stria vascularis in the lateral wall and the cochlear bony structure, respectively. However, there is no pannexin expression in auditory sensory hair cells. Recent studies demonstrated that like connexin gap junction gene, Panx1 deficiency causes hearing loss. Panx1 channels dominate ATP release in the cochlea. Deletion of Panx1 abolishes ATP release in the cochlea and reduces endocochlear potential (EP), auditory receptor current/potential, and active cochlear amplification. Panx1 deficiency in the cochlea also activates caspase-3 cell apoptotic pathway leading to cell degeneration. These new findings suggest that pannexins have a critical role in the cochlea in regard to hearing. However, detailed information about pannexin function in the cochlea and Panx mutation induced hearing loss still remain largely undetermined. Further studies are required.     

Loss And Survival of Spiral Ganglion Neurons in the Guinea Pig After Intracochlear Perfusion with Aminoglycosides

Loss of cochlear hair cells results in a loss of ganglion cells and further neurodegenerative changes throughout the auditory pathway. Understanding more about the early stages of ganglion cell loss in vivo may lead to ways of ameliorating or preventing the loss of these neurons. To examine these stages, the effects of intracochlear perfusion with aminoglycoside antibiotics on the organ of Corti and spiral ganglion cells were evaluated in young adult guinea pigs at survival periods ranging from 1 hour to 12 weeks, using immunocytochemical and ultrastructural techniques. At 1 hour survival a base-to-apex gradient of damage was indicated in the cochlea by the appearance of severely damaged hair cells and injured ganglion cells in the basal coil while in the apical coil, hair cells were damaged but intact and ganglion cells appeared normal. By 4 hours the appearance of severely disrupted hair cells and damaged ganglion cells had extended throughout the cochlea. The ultrastructural appearance of many injured ganglion cells demonstrated features characteristic of cell death including condensed cytoplasm, non-marginal clumping of nuclear chromatin, and wrinkled nuclear membrane. Despite the loss of many ganglion cells, a population of these cells remained at 12 weeks survival. These contained large amounts of rough endoplasmic reticulum, were unmyelinated apart from the central process and were surrounded by satellite cells. These features are typical of ganglion cells during development, before the onset of hearing. Immunolabelling of cochlear whole mounts after hair cell destruction with protein gene product 9.5 (PGP 9.5) revealed the presence of neural elements in the organ of Corti at up to 12 weeks survival. These may associated with the remaining ganglion cells. In these surviving ganglion cells, the intense labelling with PGP 9.5 together with the increase in rough endoplasmic reticulum, indicates the presence of active protein synthesis which may be connected with their survival.     

The auditory sensory epithelium: the instrument of sound perception

The auditory sensory epithelium is the specialized region of the cochlear epithelium that transduces sound. It is composed of a highly ordered, repeated array of mechanosensory hair cells and nonsensory supporting cells that run along the length of the cochlea. On the apical surface of the hair cells is a specialized structure called the hair bundle that deflects in response to sound vibration, resulting in depolarization of the hair cell and neurotransmitter release. Formation of the auditory sensory epithelium during embryogenesis involves strict control of both cell proliferation and cell patterning. Misregulation of these events can lead to congenital hearing loss, and damage to the auditory sensory epithelium during adult life can lead to adult-onset deafness. This paper reviews recent data on the formation of the auditory sensory epithelium during embryogenesis, the identification of components of the sound transduction apparatus, and advances in the treatment of hearing impairment.     

Immediate and Delayed Cochlear Neuropathy after Noise Exposure in Pubescent Mice

Moderate acoustic overexposure in adult rodents is known to cause acute loss of synapses on sensory inner hair cells (IHCs) and delayed degeneration of the auditory nerve, despite the completely reversible temporary threshold shift (TTS) and morphologically intact hair cells. Our objective was to determine whether a cochlear synaptopathy followed by neuropathy occurs after noise exposure in pubescence, and to define neuropathic versus non-neuropathic noise levels for pubescent mice. While exposing 6 week old CBA/CaJ mice to 8-16 kHz bandpass noise for 2 hrs, we defined 97 dB sound pressure level (SPL) as the threshold for this particular type of neuropathic exposure associated with TTS, and 94 dB SPL as the highest non-neuropathic noise level associated with TTS. Exposure to 100 dB SPL caused permanent threshold shift although exposure of 16 week old mice to the same noise is reported to cause only TTS. Amplitude of wave I of the auditory brainstem response, which reflects the summed activity of the cochlear nerve, was complemented by synaptic ribbon counts in IHCs using confocal microscopy, and by stereological counts of peripheral axons and cell bodies of the cochlear nerve from 24 hours to 16 months post exposure. Mice exposed to neuropathic noise demonstrated immediate cochlear synaptopathy by 24 hours post exposure, and delayed neurodegeneration characterized by axonal retraction at 8 months, and spiral ganglion cell loss at 8-16 months post exposure. Although the damage was initially limited to the cochlear base, it progressed to also involve the cochlear apex by 8 months post exposure. Our data demonstrate a fine line between neuropathic and non-neuropathic noise levels associated with TTS in the pubescent cochlea.     

NaHS Protects Cochlear Hair Cells from Gentamicin-Induced Ototoxicity by Inhibiting the Mitochondrial Apoptosis Pathway

Aminoglycoside antibiotics such as gentamicin could cause ototoxicity in mammalians, by inducing oxidative stress and apoptosis in sensory hair cells of the cochlea. Sodium hydrosulfide (NaHS) is reported to alleviate oxidative stress and apoptosis, but its role in protecting aminoglycoside-induced hearing loss is unclear. In this study, we investigated the anti-oxidant and anti-apoptosis effect of NaHS in in vitro cultured House Ear Institute-Organ of Corti 1 (HEI-OC1) cells and isolated mouse cochlea. Results from cultured HEI-OC1 cells and cochlea consistently indicated that NaHS exhibited protective effects from gentamicin-induced ototoxicity, evident by maintained cell viability, hair cell number and cochlear morphology, reduced reactive oxygen species production and mitochondrial depolarization, as well as apoptosis activation of the intrinsic pathway. Moreover, in the isolated cochlear culture, NaHS was also demonstrated to protect the explant from gentamicin-induced mechanotransduction loss. Our study using multiple in vitro models revealed for the first time, the potential of NaHS as a therapeutic agent in protecting against aminoglycoside-induced hearing loss.     

Building the mammalian cochlea — an overview

The mammalian cochlea is a highly intricate organ responsible for hearing. Numerous specialized cell types residing in the cochlear participate in processing and relaying sound information to the brain. In general, cells in the cochlea are divided into three major types: sensory, neural, and non-sensory. Sensory cells are a group of cells in the organ of Corti consisting of hair cells and supporting cells. Sensory hair cells play a primary role in detecting and processing sound in the form of vibrations. Neural cells are the neurons and glia in the spiral (cochlear) ganglion that relay the processed sound signals in the form of a neurotransmitter to the brain. Other non-sensory cells include all other cell types providing architectural and functional support. Building a functional cochlea requires tightly orchestrated, spatial and temporal regulation of gene expressions. Disruption of the normal gene expression patterns can cause developmental failure of the organ, which can lead to permanent hearing loss. Thus, comprehensive understanding of genes contributing to cochlear development is crucial for elucidating the pathological mechanisms of hearing loss. This article is intended to provide an overview of mammalian cochlear development, focusing on genes involved in its early patterning.     

Arctigenin protects against neuronal hearing loss by promoting neural stem cell survival and differentiation

Neuronal hearing loss has become a prevalent health problem. This study focused on the function of arctigenin (ARC) in promoting survival and neuronal differentiation of mouse cochlear neural stem cells (NSCs), and its protection against gentamicin (GMC) induced neuronal hearing loss. Mouse cochlea was used to isolate NSCs, which were subsequently cultured in vitro. The effects of ARC on NSC survival, neurosphere formation, differentiation of NSCs, neurite outgrowth, and neural excitability in neuronal network in vitro were examined. Mechanotransduction ability demonstrated by intact cochlea, auditory brainstem response (ABR), and distortion product optoacoustic emissions (DPOAE) amplitude in mice were measured to evaluate effects of ARC on GMC‐induced neuronal hearing loss. ARC increased survival, neurosphere formation, neuron differentiation of NSCs in mouse cochlear in vitro. ARC also promoted the outgrowth of neurites, as well as neural excitability of the NSC‐differentiated neuron culture. Additionally, ARC rescued mechanotransduction capacity, restored the threshold shifts of ABR and DPOAE in our GMC ototoxicity murine model. This study supports the potential therapeutic role of ARC in promoting both NSCs proliferation and differentiation in vitro to functional neurons, thus supporting its protective function in the therapeutic treatment of neuropathic hearing loss in vivo.     

Notch Signaling Limits Supporting Cell Plasticity in the Hair Cell-Damaged Early Postnatal Murine Cochlea

In mammals, auditory hair cells are generated only during embryonic development and loss or damage to hair cells is permanent. However, in non-mammalian vertebrate species, such as birds, neighboring glia-like supporting cells regenerate auditory hair cells by both mitotic and non-mitotic mechanisms. Based on work in intact cochlear tissue, it is thought that Notch signaling might restrict supporting cell plasticity in the mammalian cochlea. However, it is unresolved how Notch signaling functions in the hair cell-damaged cochlea and the molecular and cellular changes induced in supporting cells in response to hair cell trauma are poorly understood. Here we show that gentamicin-induced hair cell loss in early postnatal mouse cochlear tissue induces rapid morphological changes in supporting cells, which facilitate the sealing of gaps left by dying hair cells. Moreover, we provide evidence that Notch signaling is active in the hair cell damaged cochlea and identify Hes1, Hey1, Hey2, HeyL, and Sox2 as targets and potential Notch effectors of this hair cell-independent mechanism of Notch signaling. Using Cre/loxP based labeling system we demonstrate that inhibition of Notch signaling with a γ- secretase inhibitor (GSI) results in the trans-differentiation of supporting cells into hair cell-like cells. Moreover, we show that these hair cell-like cells, generated by supporting cells have molecular, cellular, and basic electrophysiological properties similar to immature hair cells rather than supporting cells. Lastly, we show that the vast majority of these newly generated hair cell-like cells express the outer hair cell specific motor protein prestin.     

Reduction in Noise-Induced Functional Loss of the Cochleae in Mice with Pre-Existing Cochlear Dysfunction Due to Genetic Interference of Prestin

Various cochlear pathologies, such as acoustic trauma, ototoxicity and age-related degeneration, cause hearing loss. These pre-existing hearing losses can alter cochlear responses to subsequent acoustic overstimulation. So far, the knowledge on the impacts of pre-existing hearing loss caused by genetic alteration of cochlear genes is limited. Prestin is the motor protein expressed exclusively in outer hair cells in the mammalian cochlea. This motor protein contributes to outer hair cell motility. At present, it is not clear how the interference of prestin function affects cochlear responses to acoustic overstimulation. To address this question, a genetic model of prestin dysfunction in mice was created by inserting an internal ribosome entry site (IRES)-CreER^T2-FRT-Neo-FRT cassette into the prestin locus after the stop codon. Homozygous mice exhibit a threshold elevation of auditory brainstem responses with large individual variation. These mice also display a threshold elevation and a shift of the input/output function of the distortion product otoacoustic emission, suggesting a reduction in outer hair cell function. The disruption of prestin function reduces the threshold shifts caused by exposure to a loud noise at 120 dB (sound pressure level) for 1 h. This reduction is positively correlated with the level of pre-noise cochlear dysfunction and is accompanied by a reduced change in Cdh1 expression, suggesting a reduction in molecular responses to the acoustic overstimulation. Together, these results suggest that prestin interference reduces cochlear stress responses to acoustic overstimulation.     

Adenosine amine congener mitigates noise-induced cochlear injury

Hearing loss from noise exposure is a leading occupational disease, with up to 5% of the population at risk world-wide. Here, we present a novel purine-based pharmacological intervention that can ameliorate noise-induced cochlear injury. Wistar rats were exposed to narrow-band noise (8–12 kHz, 110 dB SPL, 2–24 h) to induce cochlear damage and permanent hearing loss. The selective adenosine A₁ receptor agonist, adenosine amine congener (ADAC), was administered intraperitoneally (100 μg/kg/day) at time intervals after noise exposure. Hearing thresholds were assessed using auditory brainstem responses and the hair cell loss was evaluated by quantitative histology. Free radical damage in the organ of Corti was assessed using nitrotyrosine immunohistochemistry. The treatment with ADAC after noise exposure led to a significantly greater recovery of hearing thresholds compared with controls. These results were upheld by increased survival of sensory hair cells and reduced nitrotyrosine immunoreactivity in ADAC-treated cochlea. We propose that ADAC could be a valuable treatment for noise-induced cochlear injury in instances of both acute and extended noise exposures.     

<Emphasis Type="Italic">OPA1</Emphasis>, the disease gene for optic atrophy type Kjer,is expressed in the inner ear

Autosomal dominant optic atrophy (adOA) is the most common form of hereditary optic neuropathy. The majority of cases are associated with mutations in the OPA1 gene. A few cases of adOA are known to be associated with moderate progressive hearing loss. To gain insight into the pathogenesis of this hearing loss, we performed expression analyses of OPA1 in the rat auditory and vestibular organ. In cochlear tissue, several splice variants of OPA1 were detected, which are also expressed in retinal tissue. OPA1 mRNA and protein was found in the hair cells and ganglion cells of the cochlea and vestibular organ. In ganglion cells, OPA1 mRNA and protein was already detectable at birth, whereas in the organ of Corti OPA1 mRNA and protein was up-regulated after birth and reached mature-like expression level during the onset of hearing. Comparison of an antibody directed to the mitochondrial marker protein HSP60 with antibodies directed to different amino acid stretches of OPA1 revealed a sub-cellular distribution of OPA1 in areas of significant density of mitochondria. The data suggest that defects in OPA1 cause hearing disorders due to a progressing metabolic disturbance of hair and ganglion cells in the inner ear. Stefanie Bette and Ulrike Zimmermann contributed equally to this work.     

Neurotrophin Gene Therapy for Sustained Neural Preservation after Deafness

The cochlear implant provides auditory cues to profoundly deaf patients by electrically stimulating the residual spiral ganglion neurons. These neurons, however, undergo progressive degeneration after hearing loss, marked initially by peripheral fibre retraction and ultimately culminating in cell death. This research aims to use gene therapy techniques to both hold and reverse this degeneration by providing a sustained and localised source of neurotrophins to the deafened cochlea. Adenoviral vectors containing green fluorescent protein, with or without neurotrophin-3 and brain derived neurotrophic factor, were injected into the lower basal turn of scala media of guinea pigs ototoxically deafened one week prior to intervention. This single injection resulted in localised and sustained gene expression, principally in the supporting cells within the organ of Corti. Guinea pigs treated with adenoviral neurotrophin-gene therapy had greater neuronal survival compared to contralateral non-treated cochleae when examined at 7 and 11 weeks post injection. Moreover; there was evidence of directed peripheral fibre regrowth towards cells expressing neurotrophin genes after both treatment periods. These data suggest that neurotrophin-gene therapy can provide sustained protection of spiral ganglion neurons and peripheral fibres after hearing loss.     

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

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

Cochlear Inner Hair Cell Ribbon Synapse is the Primary Target of Ototoxic Aminoglycoside Stimuli

The ribbon synapses of inner hair cells (IHCs) play an important role in sound encoding and neurotransmitter release. However, it remains unclear whether IHC ribbon synapse plasticity can be interrupted by ototoxic aminoglycoside stimuli. Here, we report that quantitative changes in the number of IHC ribbon synapses and hearing loss occur in response to gentamicin treatment in mice. Using 3D reconstruction, we were able to calculate the number of IHC ribbon synapses after ototoxic gentamicin exposure. Mice were injected intraperitoneally with a low dose of gentamicin (100 mg/kg) once a day for 14 days. Double immunostaining was used to identify IHC ribbon synapses; histopathology and scanning electron microscopy were used to observe the morphology of cochlear hair cells and spiral ganglion neurons (SGNs), the hearing threshold shifts were recorded by auditory brainstem response examinations. Our study shows that the maximal number of IHC ribbon synapses appeared at the 7th day after treatment, followed by a significant reduction after the 7th day regardless of ongoing treatment. Correspondingly, the maximal elevation of hearing threshold was observed at the 7th day after treatment. Meanwhile, additional cochlear components included OHCs, IHCs, and SGNs were unaffected, suggesting that IHC ribbon synapses are more susceptible to ototoxic aminoglycoside stimulation. Our study indicated that quantitative changes in the number of IHC ribbon synapses is critical response to lower dose of ototoxic stimulation, and may contribute to moderate hearing loss. Additionally, our data indcated that ribbon synaptic plasticity may require the quantitative changes to play self-protective role adapted to ototoxic aminoglycoside stimuli.     

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。