Evidence for a medial K+ recycling pathway from inner hair cells

Age-related changes in P2Y receptor signalling in mouse cochlear supporting cells

Our sense of hearing depends on the function of a specialised class of sensory cells, the hair cells, which are found in the organ of Corti of the mammalian cochlea. The unique physiological environment in which these cells operate is maintained by a syncitium of non-sensory supporting cells, which are crucial for regulating cochlear physiology and metabolic homeostasis. Despite their importance for cochlear function, the role of these supporting cells in age-related hearing loss, the most common sensory deficit in the elderly, is poorly understood. Here, we investigated the age-related changes in the expression and function of metabotropic purinergic receptors (P2Y1 , P2Y2 and P2Y4 ) in the supporting cells of the cochlear apical coil. Purinergic signalling in supporting cells is crucial during the development of the organ of Corti and purinergic receptors are known to undergo changes in expression during ageing in several tissues. Immunolabelling and Ca2+ imaging experiments revealed a downregulation of P2Y receptor expression and a decrease of purinergic-mediated calcium responses after early postnatal stages in the supporting cells. An upregulation of P2Y receptor expression was observed in the aged cochlea when compared to 1 month-old adults. The aged mice also had significantly larger calcium responses and displayed calcium oscillations during prolonged agonist applications. We conclude that supporting cells in the aged cochlea upregulate P2Y2 and P2Y4 receptors and display purinergic-induced Ca2+ responses that mimic those observed during pre-hearing stages of development, possibly aimed at limiting or preventing further damage to the sensory epithelium. KEY POINTS: Age-related hearing loss is associated with lower hearing sensitivity and decreased ability to understand speech. We investigated age-related changes in the expression and function of metabotropic purinergic (P2Y) receptors in cochlear non-sensory supporting cells of mice displaying early-onset (C57BL/6N) and late-onset (C3H/HeJ) hearing loss. The expression of P2Y1 , P2Y2 and P2Y4 receptors in the supporting cells decreased during cochlear maturation, but that of P2Y2 and P2Y4 was upregulated in the aged cochlea. P2Y2 and P2Y4 receptors were primarily responsible for the ATP-induced Ca2+ responses in the supporting cells. The degree of purinergic expression upregulation in aged supporting cells mirrored hearing loss progression in the different mouse strains. We propose that the upregulation of purinergic-mediated signalling in the aged cochlea is subsequent to age-related changes in the hair cells and may act as a protective mechanism to limit or to avoid further damage to the sensory epithelium.

Three-dimensional cultured ampullae from rats as a screening tool for vestibulotoxicity: Proof of concept using styrene

Numerous ototoxic drugs, such as some antibiotics and chemotherapeutics, are both cochleotoxic and vestibulotoxic (causing hearing loss and vestibular disorders). However, the impact of some industrial cochleotoxic compounds on the vestibular receptor, if any, remains unknown. As in vivo studies are long and expensive, there is considerable need for predictive and cost-effective in vitro models to test ototoxicity. Here, we present an organotypic model of cultured ampullae harvested from rat neonates. When cultured in a gelatinous matrix, ampulla explants form an enclosed compartment that progressively fills with a high-potassium (K+) endolymph-like fluid. Morphological analyses confirmed the presence of a number of cell types, sensory epithelium, secretory cells, and canalar cells. Treatments with inhibitors of potassium transporters demonstrated that the potassium homeostasis mechanisms were functional. To assess the potential of this model to reveal the toxic effects of chemicals, explants were exposed for either 2 or 72 h to styrene at a range of concentrations (0.5-1 mM). In the 2-h exposure condition, K+ concentration was significantly reduced, but ATP levels remained stable, and no histological damage was visible. After 72 h exposure, variations in K+ concentration were associated with histological damage and decreased ATP levels. This in vitro 3D neonatal rat ampulla model therefore represents a reliable and rapid means to assess the toxic properties of industrial compounds on this vestibular tissue, and can be used to investigate the specific underlying mechanisms.

Cochlin Deficiency Protects Aged Mice from Noise-Induced Hearing Loss

Several studies have shown that type IV fibrocytes, located in the spiral ligament, degenerate first after noise exposure. Interestingly, this is the region where Coch expression is most abundant. As it is suggested that cochlin plays a role in our innate immune system, our goal is to investigate hearing thresholds and inner ear inflammation after noise exposure in Coch knockout (Coch^−/−) mice compared to Coch wildtype (Coch^+/+) mice. Animals were randomly allocated to a noise exposure group and a control group. Vestibular and auditory testing was performed at 48 h and one week after noise exposure. Whole mount staining and cryosectioning of the cochlea was performed in order to investigate hair cells, spiral ganglion neurons, inner ear inflammation, Coch expression and fibrocyte degeneration. Hearing assessment revealed that Coch^+/+ mice had significantly larger threshold shifts than Coch^−/− mice after noise exposure. We were unable to identify any differences in hair cells, neurons, fibrocytes and influx of macrophages in the inner ear between both groups. Interestingly, Coch expression was significantly lower in the group exposed to noise. Our results indicate that the absence of Coch has a protective influence on hearing thresholds after noise exposure, but this is not related to reduced inner ear inflammation in the knockout.

Charging Up the Periphery: Glial Ionic Regulation in Sensory Perception

The peripheral nervous system (PNS) receives diverse sensory stimuli from the environment and transmits this information to the central nervous system (CNS) for subsequent processing. Thus, proper functions of cells in peripheral sense organs are a critical gate-keeper to generating appropriate animal sensory behaviors, and indeed their dysfunction tracks sensory deficits, sensorineural disorders, and aging. Like the CNS, the PNS comprises two major cell types, neurons (or sensory cells) and glia (or glia-like supporting neuroepithelial cells). One classic function of PNS glia is to modulate the ionic concentration around associated sensory cells. Here, we review current knowledge of how non-myelinating support cell glia of the PNS regulate the ionic milieu around sensory cell endings across species and systems. Molecular studies reviewed here suggest that, rather than being a passive homeostatic response, glial ionic regulation may in fact actively modulate sensory perception, implying that PNS glia may be active contributors to sensorineural information processing. This is reminiscent of emerging studies suggesting analogous roles for CNS glia in modulating neural circuit processing. We therefore suggest that deeper molecular mechanistic investigations into critical PNS glial functions like ionic regulation are essential to comprehensively understand sensorineural health, disease, and aging.

Electrochemical properties of the non-excitable tissue stria vascularis of the mammalian cochlea are sensitive to sounds

Excitable cochlear hair cells convert the mechanical energy of sounds into the electrical signals necessary for neurotransmission. The key process is cellular depolarization via K⁺ entry from K⁺ -enriched endolymph through hair cells' mechanosensitive channels. Positive 80 mV potential in endolymph accelerates the K⁺ entry, thereby sensitizing hearing. This potential represents positive extracellular potential within the epithelial-like stria vascularis; the latter potential stems from K⁺ equilibrium potential (E_K ) across the strial membrane. Extra- and intracellular [K⁺ ] determining E_K are likely maintained by continuous unidirectional circulation of K⁺ through a putative K⁺ transport pathway containing hair cells and stria. Whether and how the non-excitable tissue stria vascularis responds to acoustic stimuli remains unclear. Therefore, we analysed a cochlear portion for the best frequency, 1 kHz, by theoretical and experimental approaches. We have previously developed a computational model that integrates ion channels and transporters in the stria and hair cells into a circuit and described a circulation current composed of K⁺ . Here, in this model, mimicking of hair cells' K⁺ flow induced by a 1 kHz sound modulated the circulation current and affected the strial ion transport mechanisms; the latter effect resulted in monotonically decreasing potential and increasing [K⁺ ] in the extracellular strial compartment. Similar results were obtained when the stria in acoustically stimulated animals was examined using microelectrodes detecting the potential and [K⁺ ]. Measured potential dynamics mirrored the E_K change. Collectively, because stria vascularis is electrically coupled to hair cells by the circulation current in vivo too, the strial electrochemical properties respond to sounds. KEY POINTS: A highly positive potential of +80 mV in K⁺ -enriched endolymph in the mammalian cochlea accelerates sound-induced K⁺ entry into excitable sensory hair cells, a process that triggers hearing. This unique endolymphatic potential represents an E_K -based battery for a non-excitable epithelial-like tissue, the stria vascularis. To examine whether and how the stria vascularis responds to sounds, we used our computational model, in which strial channels and transporters are serially connected to those hair cells in a closed-loop circuit, and found that mimicking hair cell excitation by acoustic stimuli resulted in increased extracellular [K⁺ ] and decreased the battery's potential within the stria. This observation was overall verified by electrophysiological experiments using live guinea pigs. The sensitivity of electrochemical properties of the stria to sounds indicates that this tissue is electrically coupled to hair cells by a radial ionic flow called a circulation current.

Cochlear apical morphology in toothed whales: Using the pairing hair cell-Deiters' cell as a marker to detect lesions

The apex or apical region of the cochlear spiral within the inner ear encodes for low-frequency sounds. The disposition of sensory hair cells on the organ of Corti is largely variable in the apical region of mammals, and it does not necessarily follow the typical three-row pattern of outer hair cells (OHCs). As most underwater noise sources contain low-frequency components, we expect to find most lesions in the apical region of the cochlea of toothed whales, in cases of permanent noise-induced hearing loss. To further understand how man-made noise might affect cetacean hearing, there is a need to describe normal morphological features of the apex and document interspecific anatomic variations in cetaceans. However, distinguishing between apical normal variability and hair cell death is challenging. We describe anatomical features of the organ of Corti of the apex in 23 ears from five species of toothed whales (harbor porpoise Phocoena phocoena, spinner dolphin Stenella longirostris, pantropical spotted dolphin Stenella attenuata, pygmy sperm whale Kogia breviceps, and beluga whale Delphinapterus leucas) by scanning electron microscopy and immunofluorescence. Our results showed an initial region where the lowest frequencies are encoded with two or three rows of OHCs, followed by the typical configuration of three OHC rows and three rows of supporting Deiters' cells. Whenever two rows of OHCs were detected, there were usually only two corresponding rows of supporting Deiters' cells, suggesting that the number of rows of Deiters' cells is a good indicator to distinguish between normal and pathological features.

An in vitro model to assess the peripheral vestibulotoxicity of aromatic solvents

Epidemiological and experimental studies indicate that a number of aromatic solvents widely used in the industry can affect hearing and balance following chronic exposure. Animal studies demonstrated that long-term exposure to aromatic solvents directly damages the auditory receptor within the inner ear: the cochlea. However, no information is available on their effect on the vestibular receptor, which shares many structural features with the cochlea and is also localized in inner ear. The aim of this study was to use an in vitro approach to assess and compare the vestibular toxicity of different aromatic solvents (toluene, ethylbenzene, styrene and ortho-, meta-, para-xylene), all of which have well known cochleotoxic properties. We used a three-dimensional culture model of rat utricles ("cysts") with preserved functional sensory and secretory epithelia, and containing a potassium-rich (K⁺) endolymph-like fluid for this study. Variations in K⁺ concentrations in this model were considered as biomarkers of toxicity of the substances tested. After 72 h exposure, o-xylene, ethylbenzene and styrene decreased the K⁺ concentration by 78 %, 37 % and 28 %, respectively. O- xylene and styrene both caused histopathological alterations in secretory and sensory epithelial areas after 72 h exposure, whereas no anomalies were observed in ethylbenzene-exposed samples. These in vitro results suggest that some widely used aromatic solvents might have vestibulotoxic properties (o-xylene, styrene and ethylbenzene), whereas others may not (p-xylene, m-xylene, toluene). Our results also indicate that variations in endolymphatic K⁺ concentration may be a more sensitive marker of vestibular toxicity than histopathological events. Finally, this study suggests that cochleotoxic solvents might not be necessarily vestibulotoxic, and vice versa.

Translational and interdisciplinary insights into presbyacusis: A multidimensional disease

There are multiple etiologies and phenotypes of age-related hearing loss or presbyacusis. In this review we summarize findings from animal and human studies of presbyacusis, including those that provide the theoretical framework for distinct metabolic, sensory, and neural presbyacusis phenotypes. A key finding in quiet-aged animals is a decline in the endocochlear potential (EP) that results in elevated pure-tone thresholds across frequencies with greater losses at higher frequencies. In contrast, sensory presbyacusis appears to derive, in part, from acute and cumulative effects on hair cells of a lifetime of environmental exposures (e.g., noise), which often result in pronounced high frequency hearing loss. These patterns of hearing loss in animals are recognizable in the human audiogram and can be classified into metabolic and sensory presbyacusis phenotypes, as well as a mixed metabolic+sensory phenotype. However, the audiogram does not fully characterize age-related changes in auditory function. Along with the effects of peripheral auditory system declines on the auditory nerve, primary degeneration in the spiral ganglion also appears to contribute to central auditory system aging. These inner ear alterations often correlate with structural and functional changes throughout the central nervous system and may explain suprathreshold speech communication difficulties in older adults with hearing loss. Throughout this review we highlight potential methods and research directions, with the goal of advancing our understanding, prevention, diagnosis, and treatment of presbyacusis.

Cochlear spiral ganglion neuron degeneration following cyclodextrin-induced hearing loss

Because cyclodextrins are capable of removing cholesterol from cell membranes, there is growing interest in using these compounds to treat diseases linked to aberrant cholesterol metabolism. One compound, 2-hydroxypropyl-beta-cyclodextrin (HPβCD), is currently being evaluated as a treatment for Niemann-Pick Type C1 disease, a rare, fatal neurodegenerative disease caused by the buildup of lipids in endosomes and lysosomes. HPβCD can reduce some debilitating symptoms and extend life span, but the therapeutic doses used to treat the disease cause hearing loss. Initial studies in rodents suggested that HPβCD selectively damaged only cochlear outer hair cells during the first week post-treatment. However, our recent in vivo and in vitro studies suggested that the damage could become progressively worse and more extensive over time. To test this hypothesis, we treated rats subcutaneously with 1, 2, 3 or 4 g/kg of HPβCD and waited for 8-weeks to assess the long-term histological consequences. Our new results indicate that the two highest doses of HPβCD caused extensive damage not only to OHC, but also to inner hair cells, pillar cells and other support cells resulting in the collapse and flattening of the sensory epithelium. The 4 g/kg dose destroyed all the outer hair cells and three-fourths of the inner hair cells over the basal two-thirds of the cochlea and more than 85% of the nerve fibers in the habenula perforata and more than 80% of spiral ganglion neurons in the middle of basal turn of the cochlea. The mechanisms that lead to the delayed degeneration of inner hair cells, pillar cells, nerve fibers and spiral ganglion neurons remain poorly understood, but may be related to the loss of trophic support caused by the degeneration of sensory and/or support cells in the organ of Corti. Despite the massive damage to the cochlear sensory epithelium, the blood vessels in the stria vascularis and the vestibular hair cells in the utricle and saccule remained normal.

Styrene alters potassium endolymphatic concentration in a model of cultured utricle explants

Despite well-documented neurotoxic and ototoxic properties, styrene remains commonly used in industry. Its effects on the cochlea have been extensively studied in animals, and epidemiological and animal evidence indicates an impact on balance. However, its influence on the peripheral vestibular receptor has yet to be investigated. Here, we assessed the vestibulotoxicity of styrene using an in vitro model, consisting of three-dimensional cultured newborn rat utricles filled with a high‑potassium (K⁺) endolymph-like fluid, called "cysts". K⁺ entry in the cyst ("influx") and its exit ("efflux") are controlled by secretory cells and hair cells, respectively. The vestibular epithelium's functionality is thus linked to K⁺ concentration, measured using a microelectrode. Known inhibitors of K⁺ efflux and influx validated the model. Cysts were subsequently exposed to styrene (0.25; 0.5; 0.75 and 1 mM) for 2 h or 72 h. The decrease in K⁺ concentration measured after both exposure durations was dose-dependent, and significant from 0.75 mM styrene. Vacuoles were visible in the cytoplasm of epithelial cells from 0.5 mM after 2 h and from 0.25 mM after 72 h. The results presented here are the first evidence that styrene may deregulate K⁺ homeostasis in the endolymphatic space, thereby altering the functionality of the vestibular receptor.

Calcium signaling in interdental cells during the critical developmental period of the mouse cochlea

The tectorial membrane (TM), a complex acellular structure that covers part of the organ of Corti and excites outer hair cells, is required for normal hearing. It consists of collagen fibrils and various glycoproteins, which are synthesized in embryonic and postnatal development by different cochlear cell types including the interdental cells (IDCs). At its modiolar side, the TM is fixed to the apical surfaces of IDCs, which form the covering epithelium of the spiral limbus. We performed confocal membrane imaging and Ca²⁺ imaging in IDCs of the developing mouse cochlea from birth to postnatal day 18 (P18). Using the fluorescent membrane markers FM 4-64 and CellMask™ Deep Red on explanted whole-mount cochlear epithelium, we identified the morphology of IDCs at different z-levels of the spiral limbus. Ca²⁺ imaging of Fluo-8 AM-loaded cochlear epithelia revealed spontaneous intracellular Ca²⁺ transients in IDCs at P0/1, P4/5, and P18. Their relative frequency was lowest on P0/1, increased by a factor of 12.5 on P4/5 and decreased to twice the initial value on P18. At all three ages, stimulation of IDCs with the trinucleotides ATP and UTP at 1 and 10 μM elicited Ca²⁺ transients of varying amplitude and shape. Before the onset of hearing, IDCs responded with robust Ca²⁺ oscillations. At P18, after the onset of hearing, ATP stimulation either caused Ca²⁺ oscillations or an initial Ca²⁺ peak followed by a plateau while the UTP response was unchanged from that at pre-hearing stage. Parameters of spontaneous and nucleotide-evoked Ca²⁺ transients such as amplitude, decay time and duration were markedly reduced during cochlear development, whereas the kinetics of the Ca²⁺ rise did not show relevant changes. Whether low-frequency spontaneous Ca²⁺ transients are necessary for the formation and maintenance of the tectorial membrane e.g. by regulating gene transcription needs to be elucidated in further studies.

Hearing consequences in Gjb2 knock-in mice: implications for human p.V37I mutation

Human p.V37I mutation of GJB2 gene was strongly correlated with late-onset progressive hearing loss, especially among East Asia populations. We generated a knock-in mouse model based on human p.V37I variant (c.109G>A) that recapitulated the human phenotype. Cochlear pathology revealed no significant hair cell loss, stria vascularis atrophy or spiral ganglion neuron loss, but a significant change in the length of gap junction plaques, which may have contributed to the observed mild endocochlear potential (EP) drop in homozygous mice lasting lifetime. The cochlear amplification in homozygous mice was compromised, but outer hair cells' function remained unchanged, indicating that the reduced amplification was EP- rather than prestin-generated. In addition to ABR threshold elevation, ABR wave I latencies were also prolonged in aged homozygous animals. We found in homozygous IHCs a significant increase in I_Ca but no change in Ca²⁺ efficiency in triggering exocytosis. Environmental insults such as noise exposure, middle ear injection of KCl solution and systemic application of furosemide all exacerbated the pathological phenotype in homozygous mice. We conclude that this Gjb2 mutation-induced hearing loss results from 1) reduced cochlear amplifier caused by lowered EP, 2) IHCs excitotoxicity associated with potassium accumulation around hair cells, and 3) progression induced by environmental insults.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

Prevalence of Mutations in Deafness-Causing Genes in Cochlear Implanted Patients with Profound Nonsyndromic Sensorineural Hearing Loss in Shandong Province, China

The mutations of GJB2, SLC26A4, and mtDNA12SrRNA are the most common inherited causes of nonsyndromic sensorineural hearing loss (NSHL) in China, yet previous genetic screenings were mainly carried on patients with moderate-to-profound impairment. We aimed to detect the mutation frequencies in NSHL population within a more specified range of severity. Patients with profound NSHL who had undergone cochlear implantation in the Shandong Provincial Hospital (Shandong, China) were recruited. The majority (n  =  472) were between 0.7 and 6 years old, and the remaining (n  =  63) were between 6 and 70 years old. In total, 115 mutation alleles of the three genes were screened with SNP scan assay. Of the patients, 19.44% (104/535) were found to have GJB2 mutations, and the most common allele was c.235delC, followed by c.299_300delAT and c.109G>A. SLC26A4 mutations were detected in 13.46% patients (72/535), and the most common allele was c.919-2A>G (IVS7-2A>G), followed by c.1174A>T and c.2168A>G. Seven patients (1.31%) carried mutations in mtDNA12SrRNA, with the alleles of m.1555A>G and m.1494C>T. We found the allele frequency of c.109G>A (GJB2) was relatively lower in the profound NSHL population in comparison to the moderate-to-profound ones, and the c.1174A>T (SLC26A4) relatively higher. It suggests those mutations may be connected with the degree of deafness, which needs more observations and analyses to support.

Organ of Corti and Stria Vascularis: Is there an Interdependence for Survival?

Cochlear hair cells and the stria vascularis are critical for normal hearing. Hair cells transduce mechanical stimuli into electrical signals, whereas the stria is responsible for generating the endocochlear potential (EP), which is the driving force for hair cell mechanotransduction. We questioned whether hair cells and the stria interdepend for survival by using two mouse models. Atoh1 conditional knockout mice, which lose all hair cells within four weeks after birth, were used to determine whether the absence of hair cells would affect function and survival of stria. We showed that stria morphology and EP remained normal for long time despite a complete loss of all hair cells. We then used a mouse model that has an abnormal stria morphology and function due to mutation of the Mitf gene to determine whether hair cells are able to survive and transduce sound signals without a normal electrochemical environment in the endolymph. A strial defect, reflected by missing intermediate cells in the stria and by reduction of EP, led to systematic outer hair cell death from the base to the apex after postnatal day 18. However, an 18-mV EP was sufficient for outer hair cell survival. Surprisingly, inner hair cell survival was less vulnerable to reduction of the EP. Our studies show that normal function of the stria is essential for adult outer hair cell survival, while the survival and normal function of the stria vascularis do not depend on functional hair cells.

Identification of a key residue in Kv7.1 potassium channel essential for sensing external potassium ions

A glutamate at position 290 in the human K_v7.1 S5-pore linker is required for its inhibition by high concentrations of extracellular potassium.

K_v7.1 voltage-gated K⁺ (K_v) channels are present in the apical membranes of marginal cells of the stria vascularis of the inner ear, where they mediate K⁺ efflux into the scala media (cochlear duct) of the cochlea. As such, they are exposed to the K⁺-rich (∼150 mM of external K⁺ (K⁺_e)) environment of the endolymph. Previous studies have shown that K_v7.1 currents are substantially suppressed by high K⁺_e (independent of the effects of altering the electrochemical gradient). However, the molecular basis for this inhibition, which is believed to involve stabilization of an inactivated state, remains unclear. Using sequence alignment of S5-pore linkers of several K_v channels, we identified a key residue, E290, found in only a few K_v channels including K_v7.1. We used substituted cysteine accessibility methods and patch-clamp analysis to provide evidence that the ability of K_v7.1 to sense K⁺_e depends on E290, and that the charge at this position is essential for K_v7.1’s K⁺_e sensitivity. We propose that K_v7.1 may use this feedback mechanism to maintain the magnitude of the endocochlear potential, which boosts the driving force to generate the receptor potential of hair cells. The implications of our findings transcend the auditory system; mutations at this position also result in long QT syndrome in the heart.

KCNK5 channels mostly expressed in cochlear outer sulcus cells are indispensable for hearing

In the cochlea, K(+) is essential for mechano-electrical transduction. Here, we explore cochlear structure and function in mice lacking K(+) channels of the two-pore domain family. A profound deafness associated with a decrease in endocochlear potential is found in adult Kcnk5(-/-) mice. Hearing occurs around postnatal day 19 (P19), and completely disappears 2 days later. At P19, Kcnk5(-/-) mice have a normal endolymphatic [K(+)] but a partly lowered endocochlear potential. Using Lac-Z as a gene reporter, KCNK5 is mainly found in outer sulcus Claudius', Boettcher's and root cells. Low levels of expression are also seen in the spiral ganglion, Reissner's membrane and stria vascularis. Essential channels (KCNJ10 and KCNQ1) contributing to K(+) secretion in stria vascularis have normal expression in Kcnk5(-/-) mice. Thus, KCNK5 channels are indispensable for the maintenance of hearing. Among several plausible mechanisms, we emphasize their role in K(+) recycling along the outer sulcus lateral route.

Cellular and Deafness Mechanisms Underlying Connexin Mutation-Induced Hearing Loss - A Common Hereditary Deafness

Hearing loss due to mutations in the connexin gene family, which encodes gap junctional proteins, is a common form of hereditary deafness. In particular, connexin 26 (Cx26, GJB2) mutations are responsible for ~50% of non-syndromic hearing loss, which is the highest incidence of genetic disease. In the clinic, Cx26 mutations cause various auditory phenotypes ranging from profound congenital deafness at birth to mild, progressive hearing loss in late childhood. Recent experiments demonstrate that congenital deafness mainly results from cochlear developmental disorders rather than hair cell degeneration and endocochlear potential reduction, while late-onset hearing loss results from reduction of active cochlear amplification, even though cochlear hair cells have no connexin expression. However, there is no apparent, demonstrable relationship between specific changes in connexin (channel) functions and the phenotypes of mutation-induced hearing loss. Moreover, new experiments further demonstrate that the hypothesized K⁺-recycling disruption is not a principal deafness mechanism for connexin deficiency induced hearing loss. Cx30 (GJB6), Cx29 (GJC3), Cx31 (GJB3), and Cx43 (GJA1) mutations can also cause hearing loss with distinct pathological changes in the cochlea. These new studies provide invaluable information about deafness mechanisms underlying connexin mutation-induced hearing loss and also provide important information for developing new protective and therapeutic strategies for this common deafness. However, the detailed cellular mechanisms underlying these pathological changes remain unclear. Also, little is known about specific mutation-induced pathological changes in vivo and little information is available for humans. Such further studies are urgently required.

Gap junctional coupling is essential for epithelial repair in the avian cochlea

The loss of auditory hair cells triggers repair responses within the population of nonsensory supporting cells. When hair cells are irreversibly lost from the mammalian cochlea, supporting cells expand to fill the resulting lesions in the sensory epithelium, an initial repair process that is dependent on gap junctional intercellular communication (GJIC). In the chicken cochlea (the basilar papilla or BP), dying hair cells are extruded from the epithelium and supporting cells expand to fill the lesions and then replace hair cells via mitotic and/or conversion mechanisms. Here, we investigated the involvement of GJIC in the initial epithelial repair process in the aminoglycoside-damaged BP. Gentamicin-induced hair cell loss was associated with a decrease of chicken connexin43 (cCx43) immunofluorescence, yet cCx30-labeled gap junction plaques remained. Fluorescence recovery after photobleaching experiments confirmed that the GJIC remained robust in gentamicin-damaged explants, but regionally asymmetric coupling was no longer evident. Dye injections in slice preparations from undamaged BP explants identified cell types with characteristic morphologies along the neural-abneural axis, but these were electrophysiologically indistinct. In gentamicin-damaged BP, supporting cells expanded to fill space formerly occupied by hair cells and displayed more variable electrophysiological phenotypes. When GJIC was inhibited during the aminoglycoside damage paradigm, the epithelial repair response halted. Dying hair cells were retained within the sensory epithelium and supporting cells remained unexpanded. These observations suggest that repair of the auditory epithelium shares common mechanisms across vertebrate species and emphasize the importance of functional gap junctions in maintaining a homeostatic environment permissive for subsequent hair cell regeneration.

Neuroprotective effects of cutamesine, a ligand of the sigma-1 receptor chaperone, against noise-induced hearing loss

The sigma-1 receptor, which is expressed throughout the brain, provides physiological benefits that include higher brain function. The sigma-1 receptor functions as a chaperone in the endoplasmic reticulum and may control cell death and regeneration within the central nervous system. Cutamesine (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine dihydrochloride) is a ligand selective for this receptor and may mediate neuroprotective effects in the context of neurodegenerative disease. We therefore assessed whether cutamesine protects the inner ear from noise-induced or aging-associated hearing loss. Immunohistochemistry and Western blotting revealed that the sigma-1 receptor is present in adult cochlea. We treated mice with 0, 3, or 30 mg/kg cutamesine from 10 days before noise exposure until the end of the study. All subjects were exposed to a 120-dB, 4-kHz octave-band noise for 2 hr. We assessed auditory thresholds by measuring the auditory-evoked brainstem responses at 4, 8, and 16 kHz, prior to and 1 week, 1 month, or 3 months following noise exposure. For the aging study, measurements were made before treatment was initiated and after 3 or 9 months of cutamesine treatment. Damage to fibrocytes within the cochlear spiral limbus was assessed by quantitative histology. Cutamesine significantly reduced threshold shifts and cell death within the spiral limbus in response to intense noise. These effects were not dose or time dependent. Conversely, cutamesine did not prevent aging-associated hearing loss. These results suggest that cutamesine reduces noise-induced hearing loss and cochlear damage during the acute phase that follows exposure to an intense noise.

Recognition and control of the progression of age-related hearing loss

Recent breakthroughs have provided notable insights into both the pathogenesis and therapeutic strategies for age-related hearing loss (ARHL). Simultaneously, these breakthroughs enhance our knowledge about this neurodegenerative disease and raise the question of whether the disorder is preventable or even treatable. Discoveries relating to ARHL have revealed a unique link between ARHL and the underlying pathologies. Therefore, we need to better understand the pathogenesis or the mechanism of ARHL and learn how to take full advantage of various therapeutic strategies to prevent the progression of ARHL.

Disruption of ion-trafficking system in the cochlear spiral ligament prior to permanent hearing loss induced by exposure to intense noise: possible involvement of 4-hydroxy-2-nonenal as a mediator of oxidative stress

Noise-induced hearing loss is at least in part due to disruption of endocochlear potential, which is maintained by various K⁺ transport apparatuses including Na⁺, K⁺-ATPase and gap junction-mediated intercellular communication in the lateral wall structures. In this study, we examined the changes in the ion-trafficking-related proteins in the spiral ligament fibrocytes (SLFs) following in vivo acoustic overstimulation or in vitro exposure of cultured SLFs to 4-hydroxy-2-nonenal, which is a mediator of oxidative stress. Connexin (Cx)26 and Cx30 were ubiquitously expressed throughout the spiral ligament, whereas Na⁺, K⁺-ATPase α1 was predominantly detected in the stria vascularis and spiral prominence (type 2 SLFs). One-hour exposure of mice to 8 kHz octave band noise at a 110 dB sound pressure level produced an immediate and prolonged decrease in the Cx26 expression level and in Na⁺, K⁺-ATPase activity, as well as a delayed decrease in Cx30 expression in the SLFs. The noise-induced hearing loss and decrease in the Cx26 protein level and Na⁺, K⁺-ATPase activity were abolished by a systemic treatment with a free radical-scavenging agent, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, or with a nitric oxide synthase inhibitor, N^ω-nitro-L-arginine methyl ester hydrochloride. In vitro exposure of SLFs in primary culture to 4-hydroxy-2-nonenal produced a decrease in the protein levels of Cx26 and Na⁺, K⁺-ATPase α1, as well as Na⁺, K⁺-ATPase activity, and also resulted in dysfunction of the intercellular communication between the SLFs. Taken together, our data suggest that disruption of the ion-trafficking system in the cochlear SLFs is caused by the decrease in Cxs level and Na⁺, K⁺-ATPase activity, and at least in part involved in permanent hearing loss induced by intense noise. Oxidative stress-mediated products might contribute to the decrease in Cxs content and Na⁺, K⁺-ATPase activity in the cochlear lateral wall structures.

[Water regulation in the cochlea : Do molecular water channels facilitate potassium-dependent sound transduction?]

BACKGROUND

Sound transduction in the cochlea critically depends on the circulation of potassium ions (K(+)) along so-called "K(+) recycling routes" between the endolymph and perilymph. These K(+) currents generate high ionic and osmotic gradients, which potentially impair the excitability of sensory hair cells and threaten cell survival in the entire cochlear duct. Molecular water channels-aquaporins (AQP)-are expressed in all cochlear supporting cells along the K(+) recycling routes; however, their significance for osmotic equilibration in cochlear duct cells is unknown.

METHODS

The diffusive and osmotic water permeabilies of Reissner's membrane, the organ of Corti and the entire cochlear duct epithelium were determined. Expression of the potassium channel Kir4.1 and the water channel AQP4 in the cochlear duct was investigated by immunohistochemistry.

RESULTS

The calculated water permeability values indicate the extent of AQP-facilitated water flux across the cochlear duct epithelium. Immunohistochemically, Kir4.1 and AQP4 were found to colocalize in distinct membrane domains of supporting cells along the K(+)-recycling routes.

CONCLUSION

These observations suggest the presence of a rapid AQP-mediated water exchange between the endolymph, the cells of the cochlear duct and the perilymph. The subcellular colocalization of Kir4.1 and AQP4 in epithelial supporting cells indicates functional coupling of potassium and water flow in the cochlea. Finally, this offers an explanation for the hearing impairment observed in individuals with mutations in the AQP4 gene.

Sox10 expressing cells in the lateral wall of the aged mouse and human cochlea

Age-related hearing loss (presbycusis) is a common human disorder, affecting one in three Americans aged 60 and over. Previous studies have shown that presbyacusis is associated with a loss of non-sensory cells in the cochlear lateral wall. Sox10 is a transcription factor crucial to the development and maintenance of neural crest-derived cells including some non-sensory cell types in the cochlea. Mutations of the Sox10 gene are known to cause various combinations of hearing loss and pigmentation defects in humans. This study investigated the potential relationship between Sox10 gene expression and pathological changes in the cochlear lateral wall of aged CBA/CaJ mice and human temporal bones from older donors. Cochlear tissues prepared from young adult (1–3 month-old) and aged (2–2.5 year-old) mice, and human temporal bone donors were examined using quantitative immunohistochemical analysis and transmission electron microscopy. Cells expressing Sox10 were present in the stria vascularis, outer sulcus and spiral prominence in mouse and human cochleas. The Sox10⁺ cell types included marginal and intermediate cells and outer sulcus cells, including those that border the scala media and those extending into root processes (root cells) in the spiral ligament. Quantitative analysis of immunostaining revealed a significant decrease in the number of Sox10⁺ marginal cells and outer sulcus cells in aged mice. Electron microscopic evaluation revealed degenerative alterations in the surviving Sox10⁺ cells in aged mice. Strial marginal cells in human cochleas from donors aged 87 and older showed only weak immunostaining for Sox10. Decreases in Sox10 expression levels and a loss of Sox10⁺ cells in both mouse and human aged ears suggests an important role of Sox10 in the maintenance of structural and functional integrity of the lateral wall. A loss of Sox10⁺ cells may also be associated with a decline in the repair capabilities of non-sensory cells in the aged ear.

Distribution and phenotype of GJB2 mutations in 102 Sicilian patients with congenital non syndromic sensorineural hearing loss

OBJECTIVE

To evaluate the frequency of GJB2 mutations and their correlation with phenotype in Sicilian non-syndromic sensorineural hearing loss (NSHL) patients.

DESIGN

Sequencing of the coding region, basal promoter, exon 1, and donor splice site of the GJB2 gene; screening for the presence of the two common GJB6 deletions.

STUDY SAMPLE

A cohort of 102 Sicilian NSHL patients.

RESULTS

Fifteen different mutations in GJB2 and seventeen different genotypes were detected. No GJB6 mutations were found. The hearing impairment was profound in the 64.72% of probands (mean PTA0.25-4 kHz of 88.82 ± 26.52 dB HL). A total of 81.37% of patients harboured at least one c.35delG allele; c.167delT and c.-23 + 1G> A were identified in 10.78% and the 9.8% of patients respectively; c.35delG homozygotes presented more severe hearing impairment (75.59% of profound hearing loss) and a higher mean PTA0.25-4 kHz (96.79 ± 21.11 dB HL) with respect to c.35delG/non-c.35delG and c.35delG/Wt patients (P < 0.05).

CONCLUSIONS

This work underlines the role of c.35delG, c.167delT and c.-23 + 1G> A as the most frequent causes of NSHL in Sicily. The c.35delG frequency found is similar to those reported in other populations of the Mediterranean area. The analysis of genetic and audiologic data confirmed a variability in the phenotype associated to a single genotype.

Prevalence of GJB2 mutations in the Silk Road region of China and a report of three novel variants

CONCLUSION

The c.235delC of GJB2 gene is the hotspot mutation of the hearing loss population in the Silk Road region of China. It is high time that some active interventions (such as hearing aids or cochlear implant) are provided to improve their language ability and quality of life.

OBJECTIVES

The first gene to be identified for humans with nonsyndromic hearing loss was GJB2 gene. We investigated the prevalence of GJB2 mutations in the Silk Road region of China to study the mutation spectrum in this area.

METHODS

Bidirectional sequencing was carried out for all PCR products of samples. The statistical analysis was carried out using SAS 9.0.1 software.

RESULTS

Pathogenic mutations were identified in 243 of 2398 patients, including 168 homozygous mutations and 75 compound heterozygous mutations. Three variants (c.225G>T, c.521G>A, and c.557C>T) are novel mutations.

Functional analysis and regulation of purified connexin hemichannels

Gap-junction channels (GJCs) are aqueous channels that communicate adjacent cells. They are formed by head-to-head association of two hemichannels (HCs), one from each of the adjacent cells. Functional HCs are connexin hexamers composed of one or more connexin isoforms. Deafness is the most frequent sensineural disorder, and mutations of Cx26 are the most common cause of genetic deafness. Cx43 is the most ubiquitous connexin, expressed in many organs, tissues, and cell types, including heart, brain, and kidney. Alterations in its expression and function play important roles in the pathophysiology of very frequent medical problems such as those related to cardiac and brain ischemia. There is extensive information on the relationship between phosphorylation and Cx43 targeting, location, and function from experiments in cells and organs in normal and pathological conditions. However, the molecular mechanisms of Cx43 regulation by phosphorylation are hard to tackle in complex systems. Here, we present the use of purified HCs as a model for functional and structural studies. Cx26 and Cx43 are the only isoforms that have been purified, reconstituted, and subjected to functional and structural analysis. Purified Cx26 and Cx43 HCs have properties compatible with those demonstrated in cells, and present methodologies for the functional analysis of purified HCs reconstituted in liposomes. We show that phosphorylation of serine 368 by PKC produces a partial closure of the Cx43 HCs, changing solute selectivity. We also present evidence that the effect of phosphorylation is highly cooperative, requiring modification of several connexin subunits, and that phosphorylation of serine 368 elicits conformational changes in the purified HCs. The use of purified HCs is starting to provide critical data to understand the regulation of HCs at the molecular level.

Heptanol application to the mouse round window: a model for studying cochlear lateral wall regeneration

OBJECTIVE

Identify cells supporting cochlear lateral wall regeneration.

STUDY DESIGN

Prospective controlled trial.

SETTING

Laboratory. Human presbyacusis occurs, in part, secondary to age-related degeneration of cochlear lateral wall structures such as the stria vascularis and spiral ligament fibrocytes. This degeneration is likely linked to the diminished regenerative capacity of lateral wall cells with age. While lateral wall regeneration is known to occur after an acute insult, this process remains poorly understood and the cells capable of self-replication unidentified. We hypothesized that spiral ligament fibrocytes constitute these proliferative cells.

SUBJECTS AND METHODS

To test the hypothesis, an acute ototoxic insult was created in 65 normal-hearing, young adult mice via cochlear exposure to heptanol. Sacrifice occurred at 1 to 60 days posttreatment. Auditory brainstem responses, 5-ethynyl-2'-deoxyuridine assay, and immunostaining were used to assess regeneration.

RESULTS

Posttreatment hearing thresholds were elevated in nearly all treated mice. Selective fibrocyte apoptosis and strial injury were observed at the time of peak hearing loss around 1 to 7 days posttreatment. Cellular proliferation was detected in the region of type II fibrocytes during this time. Hearing thresholds plateaued at 7 days posttreatment followed by a significant recovery of both hearing and morphologic appearance. Permanent outer hair cell degeneration was observed.

CONCLUSIONS

Heptanol application to the round window of young adult mice is a rapid, selective, and reliable technique for investigating proliferation in the cochlear lateral wall. The data indirectly showed that spiral ligament fibrocytes may be the proliferative cells of the cochlear lateral wall. Further studies of this process are needed.

Kanamycin-furosemide ototoxicity in the mouse cochlea: a 3-dimensional analysis

OBJECTIVE

Administration of an aminoglycoside antibiotic and loop diuretic causes damage to hair cells in the organ of Corti, resulting in their death and the death of their corresponding spiral ganglion neurons. While this phenomenon has been studied previously, analysis of its effects in the whole cochlea has not been reported. The authors sought to evaluate the effects of a combination dose of kanamycin and furosemide in mice cochlea using an imaging system and computer analysis that allowed for nondestructive, whole-cochlea visualization.

STUDY DESIGN

Study using an animal model.

SETTING

Cochlear analysis laboratory.

SUBJECTS AND METHODS

Five mice received kanamycin and furosemide and 3 mice received saline. Cochleas were harvested and imaged with scanning thin-sheet laser imaging microscopy (sTSLIM) to analyze sensory cells and cochlea structures.

RESULTS

The drug-treated animals showed substantial loss of inner hair cells and complete outer hair cell loss. All treated mice showed spiral ganglion neuron loss with fewer neurons than control animals and decreased cell density in the middle turn of the cochlea. The spiral ligament and spiral limbus in the treated animals also showed a decrease in fibrocyte cell density in the middle to apical portion of the cochlea. The stria vascularis appeared normal in all animals.

CONCLUSION

Imaging methods that allow for whole-cochlea analysis provide insight into changes that occur in the cochlea after ototoxic insult. Trends that may not be apparent in cross-section samples of the cochlea can be observed. Computer analysis of these trends allows them to be assessed accurately.

Mesenchymal stem cell transplantation to the mouse cochlea as a treatment for childhood sensorineural hearing loss

OBJECTIVE

There is no treatment established for congenital sensorineural hearing loss because the majority of the cases are hereditary. Although congenital sensorineural hearing loss is thought to be hereditary, this hearing loss occur postnatally. We hypothesized that the transplantation of MSCs (mesenchymal stem cells) to the cochlea would be an effective therapy for stopping or delaying the progression of sensorineural hearing loss in childhood.

METHODS

Cultured mouse MSCs were labeled with EGFP (enhanced green fluorescence protein) using retroviruses. EGFP-MSCs were transplanted into the posterior semicircular canal of mice at 2-3 weeks (young group) and 24-26 weeks (adult group) of age by a novel perilymphatic perfusion technique. Engraftment of MSCs was evaluated immunohistologically at 1 week and 2 weeks after transplantation.

RESULTS

In young mice, migrated MSCs were detected in the cochlea tissue by immunofluorescence for EGFP and by immunohistochemistry for fibronectin. The differentiation of migrated MSCs into fibrocyte-like cells was demonstrated by immunofluorescence for connexin 26. There were no adverse effects on auditory function by MSC transplantation, and the auditory brain stem responses threshold did not significantly shift after surgery. In contrast, neither MSC migration nor differentiation was detected in the adult mice canal after MSC transplantation.

CONCLUSION

The bone marrow derived MSCs were successfully transplanted into the cochlea of young mice by the perilymphatic perfusion technique and were further differentiated into fibrocyte-like cells without any adverse effects on auditory function.

Headbobber: a combined morphogenetic and cochleosaccular mouse model to study 10qter deletions in human deafness

The recessive mouse mutant headbobber (hb) displays the characteristic behavioural traits associated with vestibular defects including headbobbing, circling and deafness. This mutation was caused by the insertion of a transgene into distal chromosome 7 affecting expression of native genes. We show that the inner ear of hb/hb mutants lacks semicircular canals and cristae, and the saccule and utricle are fused together in a single utriculosaccular sac. Moreover, we detect severe abnormalities of the cochlear sensory hair cells, the stria vascularis looks severely disorganised, Reissner's membrane is collapsed and no endocochlear potential is detected. Myo7a and Kcnj10 expression analysis show a lack of the melanocyte-like intermediate cells in hb/hb stria vascularis, which can explain the absence of endocochlear potential. We use Trp2 as a marker of melanoblasts migrating from the neural crest at E12.5 and show that they do not interdigitate into the developing strial epithelium, associated with abnormal persistence of the basal lamina in the hb/hb cochlea. We perform array CGH, deep sequencing as well as an extensive expression analysis of candidate genes in the headbobber region of hb/hb and littermate controls, and conclude that the headbobber phenotype is caused by: 1) effect of a 648 kb deletion on distal Chr7, resulting in the loss of three protein coding genes (Gpr26, Cpmx2 and Chst15) with expression in the inner ear but unknown function; and 2) indirect, long range effect of the deletion on the expression of neighboring genes on Chr7, associated with downregulation of Hmx3, Hmx2 and Nkx1.2 homeobox transcription factors. Interestingly, deletions of the orthologous region in humans, affecting the same genes, have been reported in nineteen patients with common features including sensorineural hearing loss and vestibular problems. Therefore, we propose that headbobber is a useful model to gain insight into the mechanisms underlying deafness in human 10qter deletion syndrome.

Korean red ginseng ameliorates acute 3-nitropropionic acid-induced cochlear damage in mice

3-Nitropropionic acid (3-NP), a mitochondrial toxin, has been reported to induce an acute cochlear damage. Korean red ginseng (KRG) is known to have protective effects from some types of hearing loss. This study aimed to observe the protective effect of KRG in an ototoxic animal model using 3-NP intratympanic injection. BALB/c mice were classified into 5 groups (n=15) and dose-dependent toxic effects after intratympanic injection with 3-NP (300-5000 mM) on the left ear were investigated to determine the appropriate toxicity level of 3-NP. For observation of the protective effects of KRG, 23 mice were grouped into 3-NP (500 mM, n=12) and KRG+3-NP groups (300 mg/kg KRG for 7 days before 500 mM 3-NP administration, n=11). Auditory brain response (ABR) and cochlear morphological evaluations were performed before and after drug administration. The ABR thresholds in the 800-5000 mM groups exceeded the maximum recording limit at 16 and 32 kHz 1 day after 3-NP administration. The ABR threshold in the 500 mM 3-NP+KRG group was significantly lower than that in the 500 mM 3-NP group from post 1 week to 1 month. The mean type II fibrocyte counts significantly differed between the control and 3-NP groups and between the 3-NP and 3-NP+KRG groups. Spiral ganglion cell degeneration in the 3-NP group was more severe than that in the 3-NP+KRG group. This animal model exhibited a dose-dependent hearing loss with histological changes. KRG administration ameliorated the deterioration of hearing by 3-NP.

ATP-dependent intercellular Ca2+ signaling in the developing cochlea: facts, fantasies and perspectives

Hearing relies on a sensitive mechanoelectrical transduction process in the cochlea of the inner ear. The cochlea contains sensory, secretory, neural, supporting and epithelial cells which are all essential to the sound transduction process. It is well known that a complex extracellular purinergic signaling system contributes to cochlear homeostasis, altering cochlear sensitivity and neural output via ATP-gated ion channels (P2X receptors) and G protein-coupled P2Y receptors. This review focuses on the emerging roles of ATP that are currently under investigation in the developing sensory epithelium, with particular emphasis on the link between ATP release, Ca(2+) signaling, the expression and function of gap junction proteins connexin26 and connexin30, and the acquisition of hearing.

Amniotic membrane: from structure and functions to clinical applications

Amniotic membrane (AM) or amnion is a thin membrane on the inner side of the fetal placenta; it completely surrounds the embryo and delimits the amniotic cavity, which is filled by amniotic liquid. In recent years, the structure and function of the amnion have been investigated, particularly the pluripotent properties of AM cells, which are an attractive source for tissue transplantation. AM has anti-inflammatory, anti-bacterial, anti-viral and immunological characteristics, as well as anti-angiogenic and pro-apoptotic features. AM is a promoter of epithelialization and is a non-tumorigenic tissue and its use has no ethical problems. Because of its attractive properties, AM has been applied in several surgical procedures related to ocular surface reconstruction and the genito-urinary tract, skin, head and neck, among others. So far, the best known and most auspicious applications of AM are ocular surface reconstruction, skin applications and tissue engineering. However, AM can also be applied in oncology. In this area, AM can prevent the delivery of nutrients and oxygen to cancer cells and consequently interfere with tumour angiogenesis, growth and metastasis.

Human connexin30.2/31.3 (GJC3) does not form functional gap junction channels but causes enhanced ATP release in HeLa cells

Gap junctional intercellular communication has numerous functions, each of which meets the particular needs of organs, tissues, or groups of cells. Connexins (CXs) are homologous four-transmembrane-domain proteins that are the major components of gap junctions. CX30.2/CX31.3 (GJC3) is a relatively new member of the CX protein family. Until now, however, the functional characteristics of CX30.2/CX31.3 have been unclear. To elucidate the properties of CX30.2/CX31.3 channels, their subcellular localization in HeLa cells, their effectiveness in dye transfer, and function on channels were investigated. In the immunofluorescent assay, cells that express CX30.2/CX31.3-GFP exhibited continuous fluorescence along the apposed cell membranes, rather than punctated fluorescence in contacting membranes between two cells. Surprisingly, dyes that can be capable of being permeated by CX26 GJ, according to a scrape loading dye transfer assay in previous studies, are impermeated by CX30.2/CX31.3 GJ, suggesting a difference between the characteristics of CX30.2/CX31.3 GJ and CX26 GJ. Furthermore, a significant amount of ATP was released from the HeLa cells that stably expressed CX30.2/CX31.3, in a medium with low calcium ion concentration, suggesting a hemichannel-based function for CX30.2/CX31.3. Based on these findings, we suggest that CX30.2/CX31.3 shares functional properties with pannexin (hemi) channels rather than gap junction channels of other CXs.

A novel missense mutation in the connexin30 causes nonsyndromic hearing loss

Dysfunctional gap junctions caused by GJB2 (CX26) and GJB6 (CX30) mutations are implicated in nearly half of nonsyndromic hearing loss cases. A recent study identified a heterozygous mutation, c.119C>T (p.A40V), in the GJB6 gene of patients with nonsyndromic hearing loss. However, the functional role of the mutation in hearing loss remains unclear. In this study, analyses of cell biology indicated that a p.A40V missense mutation of CX30 causes CX30 protein accumulation in the Golgi body rather than in the cytoplasmic membrane. The tet-on protein expression system was used for further study of mutant proteins in CX30 and CX30A40V co-expressions and in CX26 and CX30A40V co-expressions. The p.A40V missense mutation exerted a dominant negative effect on both normal CX30 and CX26, which impaired gap junction formation. Moreover, computer-assisted modeling suggested that this p.A40V mutation affects the intra molecular interaction in the hydrophobic core of Trp44, which significantly alters the efficiency of gap junction formation. These findings suggest that the p.A40V mutation in CX30 causes autosomal-dominant nonsyndromic hearing loss. These data provide a novel molecular explanation for the role of GJB6 in hearing loss.

Ca2+ homeostasis defects and hereditary hearing loss

Ca(2+) acts as a fundamental signal transduction element in inner ear, delivering information about sound, acceleration and gravity through a small number of mechanotransduction channels in the hair cell stereocilia and voltage activated Ca(2+) channels at the ribbon synapse, where it drives neurotransmission. The mechanotransduction process relies on the endocochlear potential, an electrical potential difference between endolymph and perilymph, the two fluids bathing respectively the apical and basolateral membrane of the cells in the organ of Corti. In mouse models, deafness and lack or reduction of the endocochlear potential correlate with ablation of connexin (Cx) 26 or 30. These Cxs form heteromeric channels assembled in a network of gap junction plaques connecting the supporting and epithelial cells of the organ of Corti presumably for K(+) recycle and transfer of key metabolites, for example, the Ca(2+) -mobilizing second messenger IP(3) . Ca(2+) signaling in these cells could play a crucial role in regulating Cx expression and function. Another district where Ca(2+) signaling alterations link to hearing loss is hair cell apex, where ablation or missense mutations of the PMCA2 Ca(2+) -pump of the stereocilia cause deafness and loss of balance. If less Ca(2+) is exported from the stereocilia, as in the PMCA2 mouse mutants, Ca(2+) concentration in endolymph is expected to fall causing an alteration of the mechanotransduction process. This may provide a clue as to why, in some cases, PMCA2 mutations potentiated the deafness phenotype induced by coexisting mutations of cadherin-23 (Usher syndrome type 1D), a single pass membrane Ca(2+) binding protein that is abundantly expressed in the stereocilia.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

The human deafness-associated connexin 30 T5M mutation causes mild hearing loss and reduces biochemical coupling among cochlear non-sensory cells in knock-in mice

Mutations in the GJB2 and GJB6 genes, respectively, coding for connexin26 (Cx26) and connexin30 (Cx30) proteins, are the most common cause for prelingual non-syndromic deafness in humans. In the inner ear, Cx26 and Cx30 are expressed in different non-sensory cell types, where they largely co-localize and may form heteromeric gap junction channels. Here, we describe the generation and characterization of a mouse model for human bilateral middle/high-frequency hearing loss based on the substitution of an evolutionarily conserved threonine by a methionine residue at position 5 near the N-terminus of Cx30 (Cx30T5M). The mutation was inserted in the mouse genome by homologous recombination in mouse embryonic stem cells. Expression of the mutated Cx30T5M protein in these transgenic mice is under the control of the endogenous Cx30 promoter and was analysed via activation of the lacZ reporter gene. When probed by auditory brainstem recordings, Cx30(T5M/T5M) mice exhibited a mild, but significant increase in their hearing thresholds of about 15 dB at all frequencies. Immunolabelling with antibodies to Cx26 or Cx30 suggested normal location of these proteins in the adult inner ear, but western blot analysis showed significantly down-regulated the expression levels of Cx26 and Cx30. In the developing cochlea, electrical coupling, probed by dual patch-clamp recordings, was normal. However, transfer of the fluorescent tracer calcein between cochlear non-sensory cells was reduced, as was intercellular Ca(2+) signalling due to spontaneous ATP release from connexin hemichannels. Our findings link hearing loss to decreased biochemical coupling due to the point-mutated Cx30 in mice.

Modifiers of hearing impairment in humans and mice

Lack of penetrance and variability of expression are common findings in nonsyndromic hearing loss with autosomal dominant mode of inheritance, but are also seen with recessive inheritance. Now we know that genotype cannot necessarily predict phenotype due to the complexity of the genome, the proteome interacting with the transcriptome, and the dynamically coupled systems that are involved. The contribution of genetic background to phenotypic diversity reflects the additive and interactive (epistasis) effects of multiple genes. Because, individual genes do not act alone but rather in concert with many other genes, it is not surprising that, modifier genes are common source of phenotypic variation in human populations. They can affect the phenotypic outcome of a given genotype by interacting in the same or in a parallel biological pathway as the disease gene. These modifier genes modulate penetrance, dominance, pleiotropy or expressivity in individuals with Mendelian traits and can also be exerted by influencing the severity, the penetrance, the age of onset and the progression of a disease. In this review, we focus on modifier genes that specifically affect hearing loss phenotypes in humans as well as those described in mice. We also include examples of digenic inheritance of deafness, because additive or interactive effects can also result from interaction between two mutant genes.

ATP-mediated potassium recycling in the cochlear supporting cells

Gap junction-mediated K(+) recycling in the cochlear supporting cell has been proposed to play a critical role in hearing. However, how potassium ions enter into the supporting cells to recycle K(+) remains undetermined. In this paper, we report that ATP can mediate K(+) sinking to recycle K(+) in the cochlear supporting cells. We found that micromolar or submicromolar levels of ATP could evoke a K(+)-dependent inward current in the cochlear supporting cells. At negative membrane potentials and the resting membrane potential of -80 mV, the amplitude of the ATP-evoked inward current demonstrated a linear relationship to the extracellular concentration of K(+), increasing as the extracellular concentration of K(+) increased. The inward current also increased as the concentration of ATP was increased. In the absence of ATP, there was no evoked inward current for extracellular K(+) challenge in the cochlear supporting cells. The ATP-evoked inward current could be inhibited by ionotropic purinergic (P2X) receptor antagonists. Application of pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 50 microM) or pre-incubation with an irreversible P2X7 antagonist oxidized ATP (oATP, 0.1 mM) completely abolished the ATP-evoked inward current at the negative membrane potential. ATP also evoked an inward current at cell depolarization, which could be inhibited by intracellular Cs(+) and eliminated by positive holding potentials. Our data indicate that ATP can activate P2X receptors to recycle K(+) in the cochlear supporting cells at the resting membrane potential under normal physiological and pathological conditions. This ATP-mediated K(+) recycling may play an important role in the maintenance of cochlear ionic homeostasis.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11302-010-9184-9) contains supplementary material, which is available to authorized users.

Enhanced expression of C/EBP homologous protein (CHOP) precedes degeneration of fibrocytes in the lateral wall after acute cochlear mitochondrial dysfunction induced by 3-nitropropionic acid

We previously reported that treatment of the rat cochlea with a mitochondrial toxin, 3-nitropropionic acid (3-NP), causes temporary to permanent hearing loss depending on the amount of the drug. Furthermore, apoptosis of cochlear lateral wall fibrocytes, which are important for maintaining the endolymph, is a predominant pathological feature in this animal model. 3-NP is known to induce oxidative stress as well as neuronal apoptosis. C/EBP homologous protein gene (chop) is one of the marker genes induced during endoplasmic reticulum (ER) stress, and is also considered to be involved in apoptosis. To elucidate the molecular mechanism of cochlear fibrocyte apoptosis induced by 3-NP, we studied spatiotemporal expression of C/EBP homologous protein (CHOP) and other signaling molecules related to ER stress as well as the appearance of apoptotic cells in the cochlear lateral wall after 3-NP treatment. Quantitative real-time PCR revealed that chop and activating transcription factor 4 gene (atf-4) showed marked increase within 6h, whereas expression of other ER stress-responsive genes such as grp78 and grp94 did not change. Immunohistochemistry showed that 3-NP treatment caused up-regulation of CHOP, especially in type II and type IV fibrocytes, followed by the appearance of terminal deoxynucleotidyl transferase mediated dUTP nick end-labeling (TUNEL)-positive apoptotic cells in the same confined area. Thus, apoptosis of lateral wall fibrocytes induced by 3-NP is likely to be mediated by induction of CHOP. These results contribute clarification of pathological mechanism of cochlear fibrocytes and may lead to development of novel therapeutic strategy for hearing loss.