Simultaneous gentamicin-mediated damage and Atoh1 overexpression promotes hair cell regeneration in the neonatal mouse utricle

Loss of hair cells from vestibular epithelium results in balance dysfunction. The current therapeutic regimen for vestibular diseases is limited. Upon injury or Atoh1 overexpression, hair cell replacement occurs rapidly in the mammalian utricle, suggesting a promising approach to induce vestibular hair cell regeneration. In this study, we applied simultaneous gentamicin-mediated hair cell ablation and Atoh1 overexpression to induce neonatal utricular hair cell formation in vitro. We confirmed that type I hair cells were the primary targets of gentamicin. Furthermore, injury and Atoh1 overexpression promoted hair cell regeneration in a timely and efficient manner through robust viral transfection. Hair cells regenerated with type II characteristics in the striola and type I/II characteristics in non-sensory regions. Rare EdU⁺/myosin7a⁺ cells in sensory regions and robust EdU⁺/myosin7a⁺ signals in ectopic regions indicate that transdifferentiation of supporting cells in situ, and mitosis and differentiation of non-sensory epithelial cells in ectopic regions, are sources of regenerative hair cells. Distinct regeneration patterns in in situ and ectopic regions suggested robust plasticity of vestibular non-sensory epithelium, generating more developed hair cell subtypes and thus providing a promising stem cell-like source of hair cells. These findings suggest that simultaneously causing injury and overexpressing Atoh1 promotes hair cell regeneration efficacy and maturity, thus expanding the understanding of ectopic plasticity in neonatal vestibular organs.

Knockdown of Foxg1 in Sox9+ supporting cells increases the trans-differentiation of supporting cells into hair cells in the neonatal mouse utricle

Foxg1 plays important roles in regeneration of hair cell (HC) in the cochlea of neonatal mouse. Here, we used Sox9-CreER to knock down Foxg1 in supporting cells (SCs) in the utricle in order to investigate the role of Foxg1 in HC regeneration in the utricle. We found Sox9 an ideal marker of utricle SCs and bred Sox9^CreER/+Foxg1^loxp/loxp mice to conditionally knock down Foxg1 in utricular SCs. Conditional knockdown (cKD) of Foxg1 in SCs at postnatal day one (P01) led to increased number of HCs at P08. These regenerated HCs had normal characteristics, and could survive to at least P30. Lineage tracing showed that a significant portion of newly regenerated HCs originated from SCs in Foxg1 cKD mice compared to the mice subjected to the same treatment, which suggested SCs trans-differentiate into HCs in the Foxg1 cKD mouse utricle. After neomycin treatment in vitro, more HCs were observed in Foxg1 cKD mice utricle compared to the control group. Together, these results suggest that Foxg1 cKD in utricular SCs may promote HC regeneration by inducing trans-differentiation of SCs. This research therefore provides theoretical basis for the effects of Foxg1 in trans-differentiation of SCs and regeneration of HCs in the mouse utricle.

YAP Mediates Hair Cell Regeneration in Balance Organs of Chickens, But LATS Kinases Suppress Its Activity in Mice

Loss of sensory hair cells causes permanent hearing and balance deficits in humans and other mammals, but for nonmammals such deficits are temporary. Nonmammals recover hearing and balance sensitivity after supporting cells proliferate and differentiate into replacement hair cells. Evidence of mechanical differences between those sensory epithelia and their supporting cells prompted us to investigate whether the capacity to activate YAP, an effector in the mechanosensitive Hippo pathway, correlates with regenerative capacity in acceleration-sensing utricles of chickens and mice of both sexes. After hair cell ablation, YAP accumulated in supporting cell nuclei in chicken utricles and promoted regenerative proliferation, but YAP remained cytoplasmic and little proliferation occurred in mouse utricles. YAP localization in supporting cells was also more sensitive to shape change and inhibition of MST1/2 in chicken utricles than in mouse utricles. Genetic manipulations showed that in vivo expression of the YAP-S127A variant caused robust proliferation of neonatal mouse supporting cells, which produced progeny that expressed hair cell markers, but proliferative responses declined postnatally. Expression of YAP-5SA, which more effectively evades inhibitory phosphorylation, resulted in TEAD-dependent proliferation of striolar supporting cells, even in adult utricles. Conditional deletion of LATS1/2 kinases abolished the inhibitory phosphorylation of endogenous YAP and led to striolar proliferation in adult mouse utricles. The findings suggest that damage overcomes inhibitory Hippo signaling and facilitates regenerative proliferation in nonmammalian utricles, whereas constitutive LATS1/2 kinase activity suppresses YAP-TEAD signaling in mammalian utricles and contributes to maintaining the proliferative quiescence that appears to underlie the permanence of sensory deficits.SIGNIFICANCE STATEMENT Loud sounds, ototoxic drugs, infections, and aging kill sensory hair cells in the ear, causing irreversible hearing loss and balance deficits for millions. In nonmammals, damage evokes shape changes in supporting cells, which can divide and regenerate hair cells. Such shape changes are limited in mammalian ears, where supporting cells develop E-cadherin-rich apical junctions reinforced by robust F-actin bands, and the cells fail to divide. Here, we find that damage readily activates YAP in supporting cells within balance epithelia of chickens, but not mice. Deleting LATS kinases or expressing YAP variants that evade LATS-mediated inhibitory phosphorylation induces proliferation in supporting cells of adult mice. YAP signaling eventually may be harnessed to overcome proliferative quiescence that limits regeneration in mammalian ears.

Aminoglycoside Damage and Hair Cell Regeneration in the Chicken Utricle

In this study, we present a systematic characterization of hair cell loss and regeneration in the chicken utricle in vivo. A single unilateral surgical delivery of streptomycin caused robust decline of hair cell numbers in striolar as well as extrastriolar regions, which in the striola was detected very early, 6 h post-insult. During the initial 12 h of damage response, we observed global repression of DNA replication, in contrast to the natural, mitotic hair cell production in undamaged control utricles. Regeneration of hair cells in striolar and extrastriolar regions occurred via high rates of asymmetric supporting cell divisions, accompanied by delayed replenishment by symmetric division. While asymmetric division of supporting cells is the main regenerative response to aminoglycoside damage, the detection of symmetric divisions supports the concept of direct transdifferentiation where supporting cells need to be replenished after their phenotypic conversion into new hair cells. Supporting cell divisions appear to be well coordinated because total supporting cell numbers throughout the regenerative process were invariant, despite the initial large-scale loss of hair cells. We conclude that a single ototoxic drug application provides an experimental framework to study the precise onset and timing of utricle hair cell regeneration in vivo. Our findings indicate that initial triggers and signaling events occur already within a few hours after aminoglycoside exposure. Direct transdifferentiation and asymmetric division of supporting cells to generate new hair cells subsequently happen largely in parallel and persist for several days.

Dose-dependent cochlear and vestibular toxicity of trans-tympanic cisplatin in the rat

In vivo studies are needed to study cisplatin ototoxicity and to evaluate candidate protective treatments. Rats and mice are the preferred species for toxicological and pharmacological pre-clinical research, but systemic administration of cisplatin causes high morbidity in these species. We hypothesized that trans-tympanic administration of cisplatin would provide a good model for studying its auditory and vestibular toxicity in the rat. Cisplatin was administered by the trans-tympanic route in one ear (50μl, 0.5-2mg/ml) of rats of both sexes and two different strains. Cochlear toxicity was corroborated by histological means. Vestibular toxicity was demonstrated by behavioral and histological analysis. Cisplatin concentrations were assessed in inner ear after trans-tympanic and i.v. administration. In all experiments, no lethality and only scant body weight loss were recorded. Cisplatin caused dose-dependent cochlear toxicity, as demonstrated by hair cell counts in the apical and middle turns of the cochlea, and vestibular toxicity, as demonstrated by behavioral analysis and hair cell counts in utricles. High concentrations of cisplatin were found in the inner ear after trans-tympanic administration. In comparison, i.v. administration resulted in lower inner ear concentrations. We conclude that trans-tympanic administration provides an easy, reproducible and safe model to study the cochlear and vestibular toxicity of cisplatin in the rat. This route of exposure may be useful to address particular questions on cisplatin induced ototoxicity and to test candidate protective treatments.

Live imaging the phagocytic activity of inner ear supporting cells in response to hair cell death

Hearing loss and balance disorders affect millions of people worldwide. Sensory transduction in the inner ear requires both mechanosensory hair cells (HCs) and surrounding glia-like supporting cells (SCs). HCs are susceptible to death from aging, noise overexposure, and treatment with therapeutic drugs that have ototoxic side effects; these ototoxic drugs include the aminoglycoside antibiotics and the antineoplastic drug cisplatin. Although both classes of drugs are known to kill HCs, their effects on SCs are less well understood. Recent data indicate that SCs sense and respond to HC stress, and that their responses can influence HC death, survival, and phagocytosis. These responses to HC stress and death are critical to the health of the inner ear. Here we have used live confocal imaging of the adult mouse utricle, to examine the SC responses to HC death caused by aminoglycosides or cisplatin. Our data indicate that when HCs are killed by aminoglycosides, SCs efficiently remove HC corpses from the sensory epithelium in a process that includes constricting the apical portion of the HC after loss of membrane integrity. SCs then form a phagosome, which can completely engulf the remaining HC body, a phenomenon not previously reported in mammals. In contrast, cisplatin treatment results in accumulation of dead HCs in the sensory epithelium, accompanied by an increase in SC death. The surviving SCs constrict fewer HCs and display impaired phagocytosis. These data are supported by in vivo experiments, in which cochlear SCs show reduced capacity for scar formation in cisplatin-treated mice compared with those treated with aminoglycosides. Together, these data point to a broader defect in the ability of the cisplatin-treated SCs, to preserve tissue health in the mature mammalian inner ear.

Growth hormone promotes hair cell regeneration in the zebrafish (Danio rerio) inner ear following acoustic trauma

Background

Previous microarray analysis showed that growth hormone (GH) was significantly upregulated following acoustic trauma in the zebrafish (Danio rerio) ear suggesting that GH may play an important role in the process of auditory hair cell regeneration. Our objective was to examine the effects of exogenous and endogenous GH on zebrafish inner ear epithelia following acoustic trauma.

Methodology/Principal Findings

We induced auditory hair cell damage by exposing zebrafish to acoustic overstimulation. Fish were then injected intraperitoneally with either carp GH or buffer, and placed in a recovery tank for either one or two days. Phalloidin-, bromodeoxyuridine (BrdU)-, and TUNEL-labeling were used to examine hair cell densities, cell proliferation, and apoptosis, respectively. Two days post-trauma, saccular hair cell densities in GH-treated fish were similar to that of baseline controls, whereas buffer-injected fish showed significantly reduced densities of hair cell bundles. Cell proliferation was greater and apoptosis reduced in the saccules, lagenae, and utricles of GH-treated fish one day following trauma compared to controls. Fluorescent in situ hybridization (FISH) was used to examine the localization of GH mRNA in the zebrafish ear. At one day post-trauma, GH mRNA expression appeared to be localized perinuclearly around erythrocytes in the blood vessels of the inner ear epithelia. In order to examine the effects of endogenous GH on the process of cell proliferation in the ear, a GH antagonist was injected into zebrafish immediately following acoustic trauma, resulting in significantly decreased cell proliferation one day post-trauma in all three zebrafish inner ear end organs.

Conclusions/Significance

Our results show that exogenous GH promotes post-trauma auditory hair cell regeneration in the zebrafish ear through stimulating proliferation and suppressing apoptosis, and that endogenous GH signals are present in the zebrafish ear during the process of auditory hair cell regeneration.

The Pharmacology of a Novel Topical Retinoid, BMY 30123: Comparison with Tretinoin

Preclinical studies pertaining to the pharmacology and toxicology of BMY 30123 (4-acetamidophenyl retinoate) are reported. BMY 30123 is a novel compound which has topical retinoid activity. This compound exhibits lower toxicity, both local and systemic, than other clinically used topical retinoids such as tretinoin (all-trans retinoic acid) in animal models. BMY 30123 is effective in a number of retinoid sensitive skin models including the rhino mouse utriculi reduction assay, the mouse epidermal hyperplasia model and in the suppression of DNA synthesis in mouse skin stimulated with phorbol ester. BMY 30123 was equipotent with tretinoin in these topical models. In the rhino mouse model the ED30 values for BMY 30123 and tretinoin were 0·037 and 0·015 Mm, respectively. In addition, BMY 30123 was active in the UVB-induced photodamaged mouse model, another retinoid sensitive model. One of the problems associated with topically applied tretinoin is local irritation. Therefore, for topical therapy to be optimal, it is important to reduce or minimize local irritation. Repeated applications of BMY 30123 to rabbit skin resulted in low skin irritation. The first perceptible signs of skin irritation produced by BMY 30123 occurred at a dose 10 times higher than that observed for tretinoin. BMY 30123 also exhibits low retinoid activity after oral or i.p. administration in mice and produced no signs of hypervitaminosis A-related toxicity at twenty times the no effect dose of tretinoin. Because retinoids are effective modulators of epidermal growth and differentiation, this compound should be useful for the treatment of cutaneous disorders that exhibit altered epidermal differentiation such as acne, psoriasis, ichthyosis and epithelial tumours. While BMY 30123 and tretinoin are shown here to be equipotent in animal efficacy models, the low skin irritation activity gives BMY 30123, at a minimum, a 10-fold enhancement in the therapeutic index relative to tretinoin and suggests that retinoid efficacy and skin irritation are separable phenomena.

Thyroid hormone mediates otolith growth and development during flatfish metamorphosis

Flatfish begin life as bilaterally symmetrical larvae that swim up-right, then abruptly metamorphose into asymmetrically shaped juveniles with lateralized swimming postures. Flatfish metamorphosis is mediated entirely by thyroid hormone (TH). Changes in flatfish swim posture are thought to be regulated via vestibular remodeling, although the influence of TH on teleost inner ear development remains unclear. This study addresses the role of TH on the development of the three otolith end-organs (sacculus, utricle, and lagena) during southern flounder (Paralichthys lethostigma) metamorphosis. Compared with pre-metamorphosis, growth rates of the sacculus and utricle otoliths increase dramatically during metamorphosis in a manner that is uncoupled from general somatic growth. Treatment of P. lethostigma larvae with methimazol (a pharmacological inhibitor of endogenous TH production) inhibits growth of the sacculus and utricle, whereas treatment with TH dramatically accelerates their growth. In contrast with the sacculus and utricle otoliths that begin to form and mineralize during embryogenesis, a non-mineralized lagena otolith is first visible 10-12 days after hatching. The lagena grows during pre- and pro-metamorphosis, then abruptly mineralizes during metamorphic climax. Mineralization of the lagena, but not growth, can be induced with TH treatment, whereas treatment with methimazol completely inhibits lagena mineralization without inhibiting its growth. These findings suggest that during southern flounder metamorphosis TH exerts differential effects on growth and development among the three types of otolith.

Hsp70 inhibits aminoglycoside-induced hair cell death and is necessary for the protective effect of heat shock

Sensory hair cells of the inner ear are sensitive to death from aging, noise trauma, and ototoxic drugs. Ototoxic drugs include the aminoglycoside antibiotics and the antineoplastic agent cisplatin. Exposure to aminoglycosides results in hair cell death that is mediated by specific apoptotic proteins, including c-Jun N-terminal kinase (JNK) and caspases. Induction of heat shock proteins (Hsps) is a highly conserved stress response that can inhibit JNK- and caspase-dependent apoptosis in a variety of systems. We have previously shown that heat shock results in a robust upregulation of Hsps in the hair cells of the adult mouse utricle in vitro. In addition, heat shock results in significant inhibition of both cisplatin- and aminoglycoside-induced hair cell death. In our system, Hsp70 is the most strongly induced Hsp, which is upregulated over 250-fold at the level of mRNA 2 h after heat shock. Therefore, we have begun to examine the role of Hsp70 in mediating the protective effect of heat shock. To determine whether Hsp70 is necessary for the protective effect of heat shock against aminoglycoside-induced hair cell death, we utilized utricles from Hsp70.1/3 (-/-) mice. While heat shock inhibited gentamicin-induced hair cell death in wild-type utricles, utricles from Hsp70.1/3 (-/-) mice were not protected. In addition, we have examined the role of the major heat shock transcription factor, Hsf1, in mediating the protective effect of heat shock. Utricles from Hsf1 (-/-) mice and wild-type littermates were exposed to heat shock followed by gentamicin. The protective effect of heat shock on aminoglycoside-induced hair cell death was only observed in wild-type mice and not in Hsf1 (-/-) mice. To determine whether Hsp70 is sufficient to protect hair cells, we have utilized transgenic mice that constitutively overexpress Hsp70. Utricles from Hsp70-overexpressing mice and wild-type littermates were cultured in the presence of varying neomycin concentrations for 24 h. The Hsp70-overexpressing utricles were significantly protected against neomycin-induced hair cell death at moderate to high doses of neomycin. This protective effect was achieved without a heat shock. Taken together, these data indicate that Hsp70 and Hsf1 are each necessary for the protective effect of heat shock against aminoglycoside-induced death. Furthermore, overexpression of Hsp70 alone significantly inhibits aminoglycoside-induced hair cell death.

Identification of genetic and chemical modulators of zebrafish mechanosensory hair cell death

Inner ear sensory hair cell death is observed in the majority of hearing and balance disorders, affecting the health of more than 600 million people worldwide. While normal aging is the single greatest contributor, exposure to environmental toxins and therapeutic drugs such as aminoglycoside antibiotics and antineoplastic agents are significant contributors. Genetic variation contributes markedly to differences in normal disease progression during aging and in susceptibility to ototoxic agents. Using the lateral line system of larval zebrafish, we developed an in vivo drug toxicity interaction screen to uncover genetic modulators of antibiotic-induced hair cell death and to identify compounds that confer protection. We have identified 5 mutations that modulate aminoglycoside susceptibility. Further characterization and identification of one protective mutant, sentinel (snl), revealed a novel conserved vertebrate gene. A similar screen identified a new class of drug-like small molecules, benzothiophene carboxamides, that prevent aminoglycoside-induced hair cell death in zebrafish and in mammals. Testing for interaction with the sentinel mutation suggests that the gene and compounds may operate in different pathways. The combination of chemical screening with traditional genetic approaches is a new strategy for identifying drugs and drug targets to attenuate hearing and balance disorders.

Loss of sensory hair cells in the inner ear is observed in the majority of hearing and balance disorders, affecting the health of more than 600 million people worldwide. Exposure to environmental toxins and certain pharmaceutical drugs such as aminoglycoside antibiotics and some cancer chemotherapy agents account for many of these hearing and balance problems. Variation in the genetic makeup between individuals plays a major role in establishing differences in susceptibility to environmental agents that damage the inner ear. Using zebrafish larvae, we developed a screen to uncover genes leading to differences in antibiotic-induced death of hair cells and to identify compounds that protect hair cells from damage. The combination of chemical screening with traditional genetic approaches offers a new strategy for identifying drugs and drug targets to attenuate hearing and balance disorders.

Closure of supporting cell scar formations requires dynamic actin mechanisms

In many vertebrate inner ear sensory epithelia, dying sensory hair cells are extruded, and the apices of surrounding supporting cells converge to re-seal the epithelial barrier between the electrochemically-distinct endolymph and perilymph. These cellular mechanisms remain poorly understood. Dynamic microtubular mechanisms have been proposed for hair cell extrusion; while contractile actomyosin-based mechanisms are required for cellular extrusion and closure in epithelial monolayers. The hypothesis that cytoskeletal mechanisms are required for hair cell extrusion and supporting cell scar formation was tested using bullfrog saccules incubated with gentamicin (6h), and allowed to recover (18h). Explants were then fixed, labeled for actin and cytokeratins, and viewed with confocal microscopy. To block dynamic cytoskeletal processes, disruption agents for microtubules (colchicine, paclitaxel) myosin (Y-27632, ML-9) or actin (cytochalasin D, latrunculin A) were added during treatment and recovery. Microtubule disruption agents had no effect on hair cell extrusion or supporting cell scar formation. Myosin disruption agents appeared to slow down scar formation but not hair cell extrusion. Actin disruption agents blocked scar formation, and largely prevented hair cell extrusion. These data suggest that actin-based cytoskeletal processes are required for hair cell extrusion and supporting cell scar formation in bullfrog saccules.

Effect of Intratympanic Application of Aminoglycosides on Click-Evoked Myogenic Potentials in Guinea Pigs

OBJECTIVES

Although numerous studies have identified damage to the cochlear and vestibular end organs as the primary site of aminoglycoside-induced ototoxicity, the effect on the saccule remains poorly understood, possibly due to lack of monitoring saccular function in experimental animals. Therefore, this study applied three kinds of aminoglycosides into the tympanic space of guinea pigs to examine their toxic impact on the saccule by way of click-evoked myogenic potential test coupled with morphologic assessment.

DESIGN

Albino guinea pigs were treated with saline, gentamicin, tobramycin, or amikacin, with 10 animals assigned to each group. Each compound was injected directly overlying but not through the round window membrane on the left ear, with the right ear serving as a control. One week after injection, each animal underwent auditory brain stem response, caloric test, and click-evoked myogenic potential test. Animals were then killed for morphologic assessment through the use of light and electron microscopic examinations.

RESULTS

The animals treated with saline, gentamicin, tobramycin, or amikacin exhibited abnormal auditory brain stem response in 0%, 30%, 100%, and 30% of cases; abnormal caloric responses were found in 0%, 100%, 40%, and 40% of cases; absent click-evoked myogenic potentials were found in 0%, 100%, 30%, and 40% of cases, respectively. Gentamicin and other groups differed significantly in abnormal rates of caloric responses and click-evoked myogenic potentials. Morphologic study of the gentamicin-treated animals confirmed that the absence of click-evoked myogenic potential originated from the lesion in the saccular macula.

CONCLUSIONS

Gentamicin represents the dominant susceptibility of aminoglycoside-induced vestibulotoxicity for eliminating both semicircular canal and saccular functions. This study further confirms the findings of human studies in which the caloric and vestibular evoked myogenic potentials responses were monitored to assess the abolition of vestibular function in patients treated with intratympanic gentamicin injection.

The coupling of acetylcholine-induced BK channel and calcium channel in guinea pig saccular type II vestibular hair cells

Molecular biological studies and electrophysiological data have demonstrated that acetylcholine (ACh) is the principal cochlear and vestibular efferent neurotransmitter among mammalians. However, the functional roles of ACh in type II vestibular hair cells (VHCs II) among mammalians are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9-containing nAChR)-activated small conductance, calcium-dependent potassium current (SK) in cochlear hair cells and frog saccular hair cells. The activation of SK current was necessary for the calcium influx through the alpha9-containing nAChR. Recently, we have demonstrated that ACh-induced big conductance, calcium-dependent potassium current (BK) was present in VHCs II of the vestibular end-organ of guinea pig. In this study, the nature of calcium influx for the activation of ACh-induced BK current in saccular VHCs II of guinea pig was investigated. Following extracellular perfusion of ACh, saccular VHCs II displayed a sustained outward current, which was sensitive to iberiotoxin (IBTX). High concentration of apamin failed to inhibit the current amplitude of ACh-induced outward current. Intracellular application of Cs(+) completely abolished the current evoked by ACh. ACh-induced current was potently inhibited by nifedipine, nimodipine, Cd(2+) and Ni(2+), respectively. The inhibition potency of these four calcium channel antagonists was nimodipine>nifedipine>cadmium>nickel. The L-type Ca(2+) channels agonist, (-)-Bay-K 8644 mimicked the effect of ACh and activated an IBTX-sensitive current. In addition, partial VHCs II displayed a biphasic waveform. In conclusion, the present data showed that in the guinea pig saccular VHCs II, ACh-induced BK channel was coupled with the calcium channel, but not the receptor. The perfusion of ACh will drive the opening of calcium channels; the influx of calcium ions will then activate the BK current.

Developmental changes in two voltage-dependent sodium currents in utricular hair cells

Two kinds of sodium current (I(Na)) have been separately reported in hair cells of the immature rodent utricle, a vestibular organ. We show that rat utricular hair cells express one or the other current depending on age (between postnatal days 0 and 22, P0-P22), hair cell type (I, II, or immature), and epithelial zone (striola vs. extrastriola). The properties of these two currents, or a mix, can account for descriptions of I(Na) in hair cells from other reports. The patterns of Na channel expression during development suggest a role in establishing the distinct synapses of vestibular hair cells of different type and epithelial zone. All type I hair cells expressed I(Na,1), a TTX-insensitive current with a very negative voltage range of inactivation (midpoint: -94 mV). I(Na,2) was TTX sensitive and had less negative voltage ranges of activation and inactivation (inactivation midpoint: -72 mV). I(Na,1) dominated in the striola at all ages, but current density fell by two-thirds after the first postnatal week. I(Na,2) was expressed by 60% of hair cells in the extrastriola in the first week, then disappeared. In the third week, all type I cells and about half of type II cells had I(Na,1); the remaining cells lacked sodium current. I(Na,1) is probably carried by Na(V)1.5 subunits based on biophysical and pharmacological properties, mRNA expression, and immunoreactivity. Na(V)1.5 was also localized to calyx endings on type I hair cells. Several TTX-sensitive subunits are candidates for I(Na,2).

M-like K+ currents in type I hair cells and calyx afferent endings of the developing rat utricle

Type I vestibular hair cells have large K+ currents that, like neuronal M currents, activate negative to resting potential and are modulatable. In rodents, these currents are acquired postnatally. In perforated-patch recordings from rat utricular hair cells, immature hair cells [younger than postnatal day 7 (P7)] had a steady-state K+ conductance (g(-30)) with a half-activation voltage (V1/2) of -30 mV. The size and activation range did not change in maturing type II cells, but, by P16, type I cells had added a K conductance that was on average fourfold larger and activated much more negatively. This conductance may comprise two components: g(-60) (V1/2 of -60 mV) and g(-80) (V1/2 of -80 mV). g(-80) washed out during ruptured patch recordings and was blocked by a protein kinase inhibitor. M currents can include contributions from KCNQ and ether-a-go-go-related (erg) channels. KCNQ and erg channel blockers both affected the K+ currents of type I cells, with KCNQ blockers being more potent at younger than P7 and erg blockers more potent at older than P16. Single-cell reverse transcription-PCR and immunocytochemistry showed expression of KCNQ and erg subunits. We propose that KCNQ channels contribute to g(-30) and g(-60) and erg subunits contribute to g(-80). Type I hair cells are contacted by calyceal afferent endings. Recordings from dissociated calyces and afferent endings revealed large K+ conductances, including a KCNQ conductance. Calyx endings were strongly labeled by KCNQ4 and erg1 antisera. Thus, both hair cells and calyx endings have large M-like K+ conductances with the potential to control the gain of transmission.

Disruption and restoration of cell-cell junctions in mouse vestibular epithelia following aminoglycoside treatment

The intracellular junction complexes, which consist of tight junctions (TJ), adherens junctions (AJ), and desmosomes, mediate cell-cell adhesion in epithelial cells. E-cadherin, which is a major component of AJ, plays a role not only in the maintenance of cell-cell junctions, but also in repressing cell proliferation. In this study, we examined changes of E-cadherin expression in mouse vestibular epithelia following local application of neomycin using immunohistochemistry and western blotting, and morphology of cell-cell junctions by transmission electron microscopy (TEM). Immunohistochemistry and western blotting revealed down-expression of E-cadherin and its consecutive recovery. TEM demonstrated temporal disruption of cell-cell junctions. Morphology of cell-cell junctions was more rapidly restored than recovery of E-cadherin expression. Transient disruption of cell-cell junctions and down-expression of E-cadherin is a rational response for the deletion of dying hair cells, and may be associated with a limited capacity for cell proliferations in mammalian vestibular epithelia following their rapid restoration.

2,3-Dihydroxybenzoic acid attenuates kanamycin-induced volume reduction in mouse utricular type I hair cells

The aminoglycoside kanamycin is a commonly used antibiotic, but unfortunately it is oto- and nephrotoxic in large doses. The negative effects are thought to be due to the formation of free radicals which is why strong antioxidants and iron chelators like 2,3-dihydroxybenzoic acid (DHB) are of great interest. This study estimates cellular quantitative changes in the utricular macula of mice following systemic treatment with kanamycin alone or in combination with DHB. The animals were injected with either saline, kanamycin or kanamycin + DHB for 15 days and perfusion fixed three weeks after last injection. Total volume of the utricle, as well as total number of hair and supporting cells, were estimated on light microscopic sections. Total volume and mean volume of hair cell types I and II and supporting cells were estimated on digital transmission electron micrographs. Total volume of the utricular macula, hair cell type I and supporting cells decreased significantly in animals injected with kanamycin but not in animals co-treated with DHB. Hair and supporting cell numbers remained unchanged in all three groups. In conclusion, the kanamycin-induced volume reduction of type I hair cells was attenuated by DHB.

Recurrent paroxysmal positional vertigo related to oral contraceptive treatment

Benign paroxysmal positional vertigo (BPPV) is a high-prevalence vestibular end-organ disorder caused by the detachment of utricular otoconia which float in the posterior or lateral semicircular canal. In the majority of cases the etiology of BPPV is unknown and it may follow viral infection, vascular disorders or head trauma. BPPV may be recurrent, with some authors demonstrating a correlation between recurrence and female gender. We report herein on ten cases (out of 289 diagnoses of BPPV) of recurrent idiopathic BPPV, occurring in healthy women receiving oral contraceptive treatment, which ceased after treatment suspension. It has been hypothesized that the impaired water and electrolyte balance, the variations of endolymphatic pH and the impairment of glucose or lipid metabolism induced by oral contraceptive treatment may cause otoconial degeneration and subsequent otoconia detachment and BPPV. The rarity of the finding (10/289) could account for the poor attention paid to the hormonal pathogenesis of BPPV.

Gentamicin induced nitric oxide-related oxidative damages on vestibular afferents in the guinea pig

Gentamicin is a well-known ototoxic aminoglycoside. However, the mechanism underlying this ototoxicity remains unclear. One of the mechanisms which may be responsible for this ototoxicity is excitotoxic damage to hair cells. The overstimulation of the N-methyl-d-aspartate (NMDA) receptors increases the production of nitric oxide (NO), which induces oxidative stress on hair cells. In order to determine the mechanism underlying this excitotoxicity, we treated guinea pigs with gentamicin by placing gentamicin (0.5 mg) pellets into a round window niche. After the sacrifice of the animals, which occurred at 3, 7 and 14 days after the treatment, the numbers of hair cells in the animals were counted with a scanning electron microscope. We then performed immunostaining using neuronal nitric oxide synthase (nNOS), inducible NOS (iNOS) and nitrotyrosine antibodies. The number of hair cells in the animals was found to decrease significantly after 7 days. nNOS and iNOS expression levels were observed to have increased 3 days after treatment. Nitrotyrosine was expressed primarily at the calyceal afferents of the type I hair cells 3 days after treatment. Terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) staining revealed positive hair cells 3 days after treatment. Our results suggest that inner ear treatment with gentamicin may upregulate nNOS and iNOS to induce oxidative stress in the calyceal afferents of type I hair cells, via nitric oxide overproduction.

Streptidine, a metabolic derivative produced after administration of streptomycin in vivo, is vestibulotoxic in rats

Streptomycin is the treatment of choice in developing countries for patients suffering from tuberculosis or other infectious diseases. However, it produces incapacitating vestibular symptoms whose onset is delayed and gradual. This observation led to the notion that a streptomycin metabolic derivative and not the antibiotic itself is the damaging agent for the inner ear. To study further the existence of this ototoxic metabolite, chronic treatment with streptomycin or its putative derivative streptidine was carried out in young male Long Evans rats. The presence of streptomycin or streptidine in the blood of animals of either experimental group was assessed by high performance liquid chromatography and analysis of swimming behavior was used to evaluate vestibular damage. Features of the sensory epithelium and quantification of hair cells were attained in sections of the utricular organ of all groups by light microscopy. After 25, 35 and 45 days of treatment with streptomycin, a metabolite with the same chromatographic properties as the streptidine standard run in parallel was identified in the blood of rats. Concentrations of the metabolite were 2.26 microg/ml on the 25th day and around 8.0 microg/ml in both the 35th and the 45th day of treatment, while streptomycin was below its detection level at either period. In streptidine-treated rats, the concentration of this compound was 1.0, 1.84 and 4.94 microg/ml on the 25th, 35th and 45th treatment days, respectively. Treatment with either streptomycin or streptidine resulted in similar abnormal swimming patterns and histological alterations of the utricular epithelium. Loss of hair cells was roughly equivalent even though streptidine was administered in a dose 90% lower than streptomycin. The gradual appearance of streptidine as a metabolic derivative of the antibiotic in the blood of rats or the administration of this compound alone, causing similar functional and structural vestibular deterioration seen in streptomycin-treated animals, supports the notion that streptidine is a potential contributor to ototoxicity after prolonged antibiotic administration.

Molecular diversity of voltage-gated sodium channel alpha subunits expressed in neuronal and non-neuronal excitable cells

In order to investigate the role of molecular diversity of voltage-activated sodium channel alpha-subunits in excitability of neuronal and non-neuronal cells, we carried out patch-clamp recordings and single-cell RT-PCR on two different types of mammalian excitable cells i.e. hippocampal neurons and non-neuronal utricular epithelial hair cells. In each cell type, multiple different combinations of sodium channel alpha-subunits exist from cell to cell despite similar sodium current properties. The mRNA isoforms, Nav1.2 and Nav1.6, are the most frequently detected by single cell analysis in the two cell types while Nav1.3 and Nav1.7 are also moderately expressed in embryonic hippocampal neurons and in neonatal utricular hair cells respectively. By investigating the particular alternate splice isoforms of Nav1.6 occurring at the exon 18 of the mouse orthologue SCN8A, we revealed that this subunit co-exist in the two cell types under different alternative spliced isoforms. The expression of non-functional isoforms of Nav1.6 in utricular epithelial hair cells excludes the involvement of this subunit in supporting their excitability. Thus, from a functional point of view, the present results suggest that, at the single cell level, both neuronal and non-neuronal excitable cells expressed different and complex patterns of sodium channel gene transcripts but this diversity alone cannot explain the sodium current properties of these cell types.

Overexpression of Bcl-2 prevents neomycin-induced hair cell death and caspase-9 activation in the adult mouse utricle in vitro

Mechanosensory hair cells of the inner ear are especially sensitive to death induced by exposure to aminoglycoside antibiotics. This aminoglycoside-induced hair cell death involves activation of an intrinsic program of cellular suicide. Aminoglycoside-induced hair cell death can be prevented by broad-spectrum inhibition of caspases, a family of proteases that mediate apoptotic and programmed cell death in a wide variety of systems. More specifically, aminoglycoside-induced hair cell death requires activation of caspase-9. Caspase-9 activation requires release of mitochondrial cytochrome c into the cytoplasm, indicating that aminoglycoside-induced hair cell death is mediated by the mitochondrial (or "intrinsic") cell death pathway. The Bcl-2 family of pro-apoptotic and anti-apoptotic proteins are important upstream regulators of the mitochondrial apoptotic pathway. Bcl-2 is an anti-apoptotic protein that localizes to the mitochondria and promotes cell survival by preventing cytochrome c release. Here we have utilized transgenic mice that overexpress Bcl-2 to examine the role of Bcl-2 in neomycin-induced hair cell death. Overexpression of Bcl-2 significantly increased hair cell survival following neomycin exposure in organotypic cultures of the adult mouse utricle. Furthermore, Bcl-2 overexpression prevented neomycin-induced activation of caspase-9 in hair cells. These results suggest that the expression level of Bcl-2 has important effects on the pathway(s) important for the regulation of aminoglycoside-induced hair cell death.

Steroid-Dependent Auditory Plasticity Leads to Adaptive Coupling of Sender and Receiver

For seasonally breeding vertebrates, reproductive cycling is often coupled with changes in vocalizations that function in courtship and territoriality. Less is known about changes in auditory sensitivity to those vocalizations. Here, we show that nonreproductive female midshipman fish treated with either testosterone or 17beta-estradiol exhibit an increase in the degree of temporal encoding of the frequency content of male vocalizations by the inner ear that mimics the reproductive female's auditory phenotype. This sensory plasticity provides an adaptable mechanism that enhances coupling between sender and receiver in vocal communication.

Induction of cell proliferation and beta-catenin expression in rat utricles in vitro

Limited regenerative response occurs in mammalian vestibular epithelia, although vestibular hair cell regeneration in mammals has been demonstrated under a variety of experimental conditions. Beta-catenin is known to play an important role in both cell-cell adhesion and signal transduction associated with cell proliferation. This study evaluated cell proliferation activity in mammalian vestibular epithelia in organotypic culture and examined the involvement of beta-catenin in cell proliferation in vestibular epithelia. After 72 h of culture, utricles with or without induction of cell proliferation were examined. Cell proliferation was induced by brief exposure to forskolin and supplementation with fetal bovine serum. Cell proliferation activity was assessed by bromodeoxyuridine (BrdU) labeling. Immunohistochemistry was employed for analysis of cellular distribution of beta-catenin. In utricles cultured without induction of cell proliferation. BrdU labeling was not found in vestibular epithelia. Expression of beta-catenin was found in the area corresponding to the distribution of adherens junctions in vestibular epithelia. However, BrdU labeling was identified in sensory epithelia of utricles following induction of cell proliferation, although the number of BrdU-positive cells in sensory epithelia was very limited. Accumulation of beta-catenin was occasionally found in proliferating cells in sensory epithelia; however, it was not always found in BrdU-positive cells. The present findings suggest that beta-catenin may play a role in the induction of cell proliferation in mammal vestibular epithelia.

Characterization of leukocyte subtypes in chicken inner ear sensory epithelia

Human hearing and balance require intact inner ear sensory hair cells, which transduce mechanical stimuli into electrical signals that are transmitted to the brain. Loss of hair cells after birth in mammals is irreversible, whereas birds are able to regenerate hair cells after insult and demonstrate ongoing hair cell production in the vestibular epithelia. Leukocytes reside in undamaged sensory epithelia of the avian inner ear and increase in number after trauma, prior to the proliferation of hair cell progenitors. It has been hypothesized that leukocyte-produced growth factors or cytokines may be involved in triggering hair cell regeneration. Little is known about the specific leukocyte subtypes present in avian ear. Immunohistochemistry with a panel of monoclonal antibodies to chicken leukocytes was used to identify leukocyte subtypes in normal posthatch chicken ear sensory epithelia. The responsiveness of the leukocytes to aminoglycoside-induced damage was also observed. Based on immunocytochemical and morphological criteria, we quantified leukocyte subtypes in normal and drug-damaged auditory and vestibular sensory epithelia. Data indicate that lymphocytes (B and T cells) do not reside in normal or drug-damaged ear sensory epithelia at 1-3 days post insult but are present in adjacent nonsensory tissues. The most common leukocytes in inner ear sensory epithelia are ramified cells of the myeloid lineage. Many of these are MHC class II positive, and a small percentage are mature tissue macrophages. An absence of leukocytes in lesioned areas of the auditory sensory epithelium suggests they may not play a critical role in triggering hair cell regeneration.

Voltage-gated outward K currents in frog saccular hair cells

A biophysical analysis of the voltage-gated K (Kv) currents of frog saccular hair cells enzymatically isolated with bacterial protease VIII was carried out, and their contribution to the cell electrical response was addressed by a modeling approach. Based on steady-state and kinetic properties of inactivation, two distinct Kv currents were found: a fast inactivating IA and a delayed rectifier IDRK. IA exhibited a strongly hyperpolarized inactivation V(1/2) (-83 mV), a relatively rapid single exponential recovery from inactivation (taurec of approximately 100 ms at -100 mV), and fast activation and deactivation kinetics. IDRK showed instead a less-hyperpolarized inactivation V(1/2) (-48 mV), a slower, double-exponential recovery from inactivation (taurec1 approximately 490 ms and taurec2 approximately 4,960 ms at -100 mV), and slower activation and deactivation kinetics. Steady-state activation gave a V(1/2) and a k of -46.2 and 8.2 mV for IA and -48.3 and 4.2 mV for IDRK. Both currents were not appreciably blocked by bath application of 10 mM TEA, but were inhibited by 4-AP, with IDRK displaying a higher sensitivity. IDRK also showed a relatively low affinity to linopirdine, being half blocked at approximately 50 microM. Steady-state and kinetic properties of IDRK and IA were described by 2nd- and 3rd-order Hodgkin-Huxley models, respectively. The goodness of our quantitative description of the Kv currents was validated by including IA and IDRK in a theoretical model of saccular hair cell electrical activity and by comparing the simulated responses with those obtained experimentally. This thorough description of the IDRK and IA will contribute toward understanding the role of these currents in the electrical response on this preparation.

Voltage-gated Na+ channel activation induces both action potentials in utricular hair cells and brain-derived neurotrophic factor release in the rat utricle during a restricted period of development

The mammalian utricular sensory receptors are commonly believed to be non-spiking cells with electrical activity limited to graded membrane potential changes. Here we provide evidence that during the first post-natal week, the sensory hair cells of the rat utricle express a tetrodotoxin (TTX)-sensitive voltage-gated Na+ current that displays most of the biophysical and pharmacological characteristics of neuronal Na+ current. Single-cell RT-PCR reveals that several alpha-subunit isoforms of the Na+ channels are co-expressed within a single hair cell, with a major expression of Nav1.2 and Nav1.6 subunits. In neonatal hair cells, 30 % of the Na+ channels are available for activation at the resting potential. Depolarizing current injections in the range of the transduction currents are able to trigger TTX-sensitive action potentials. We also provide evidence of a TTX-sensitive activity-dependent brain-derived neurotrophic factor (BDNF) release by early post-natal utricle explants. Developmental analysis shows that Na+ currents decrease dramatically from post-natal day 0 (P0) to P8 and become almost undetectable at P21. Concomitantly, depolarizing stimuli fail to induce both action potential and BDNF release at P20. The present findings reveal that vestibular hair cells express neuronal-like TTX-sensitive Na+ channels able to generate Na+-driven action potentials only during the early post-natal period of development. During the same period an activity-dependent BDNF secretion by utricular explants has been demonstrated. This could be an important mechanism involved in vestibular sensory system differentiation and synaptogenesis.

Caspase inhibitors promote vestibular hair cell survival and function after aminoglycoside treatment in vivo

The sensory hair cells of the inner ear undergo apoptosis after acoustic trauma or aminoglycoside antibiotic treatment, causing permanent auditory and vestibular deficits in humans. Previous studies have demonstrated a role for caspase activation in hair cell death and ototoxic injury that can be reduced by concurrent treatment with caspase inhibitors in vitro. In this study, we examined the protective effects of caspase inhibition on hair cell death in vivo after systemic injections of aminoglycosides. In one series of experiments, chickens were implanted with osmotic pumps that administrated the pan-caspase inhibitor z-Val-Ala-Asp(Ome)-fluoromethylketone (zVAD) into inner ear fluids. One day after the surgery, the animals received a 5 d course of treatment with streptomycin, a vestibulotoxic aminoglycoside. Direct infusion of zVAD into the vestibule significantly increased hair cell survival after streptomycin treatment. A second series of experiments determined whether rescued hair cells could function as sensory receptors. Animals treated with streptomycin displayed vestibular system impairment as measured by a greatly reduced vestibulo-ocular response (VOR). In contrast, animals that received concurrent systemic administration of zVAD with streptomycin had both significantly greater hair cell survival and significantly increased VOR responses, as compared with animals treated with streptomycin alone. These findings suggest that inhibiting the activation of caspases promotes the survival of hair cells and protects against vestibular function deficits after aminoglycoside treatment.

Spontaneous hair-cell renewal following gentamicin exposure in postnatal rat utricular explants

We have established an in vitro model of long-time culture of 4-day-old rat utricular maculae to study aminoglycoside-induced vestibular hair-cell renewal in the mammalian inner ear. The explanted maculae were cultured for up to 28 days on the surface of a membrane insert system. In an initial series of experiments utricles were exposed to 1 mM of gentamicin for 48 h and then allowed to recover in unsupplemented medium or in medium supplemented with the anti-mitotic drug aphidicolin. In a parallel control series, explants were not exposed to gentamicin. Utricles were harvested at specified time points from the second through the 28th day in vitro. Whole-mount utricles were stained with phalloidin-fluorescein isothiocyanate and their stereociliary bundles visualized and counted. In a second experimental series 2'-bromo-5'deoxyuridine labeling was used to confirm the antimitotic efficacy of aphidicolin. Loss of hair-cell stereociliary bundles was nearly complete 3 days after exposure to gentamicin, with the density of stereociliary bundles only 3-4% of their original density. Renewal of hair-cell bundles was abundant (i.e. 15x increase) in cultures in unsupplemented medium, with a peak of stereociliary bundle renewal reached after 21 days in vitro. A limited amount of hair-cell renewal also occurred in the presence of the anti-mitotic drug, aphidicolin. These results suggest that spontaneous renewal of hair-cell stereociliary bundles following gentamicin damage in utricular explants predominantly follows a pathway that includes mitotic events, but that a small portion of the hair-cell stereociliary bundle renewal does not require mitotic activity.

Histological preparation of developing vestibular otoconia for scanning electron microscopy

The unique nature of vestibular otoconia as calcium carbonate biominerals makes them particularly susceptible to chemical deformation during histological processing. We fixed and stored otoconia from all three otolith endorgans of embryonic, hatchling and adult Japanese quail in glutaraldehyde containing either phosphate or non-phosphate buffers for varying lengths of time and processed them for scanning electron microscopy. Otoconia from all age groups and otolith endorgans processed in 0.1 M phosphate buffer (pH 7.4) showed abnormal surface morphology when compared to acetone fixed controls. Otoconia processed in 0.1 M sodium cacodylate or HEPES buffered artificial endolymph (pH 7.4) showed normal morphology that was similar to controls. The degree of otoconial deformation was directly related to the time exposed to phosphate buffer. Short duration exposure produced particulate deformations while longer exposures resulted in fused otoconia that formed solid sheets. Otoconial surface deformation and fusing was independent of the glutaraldehyde component of the histological processing. These findings should help vestibular researchers to develop appropriate histological processing protocols in future studies of otoconia.

A novel technique for inducing local inner ear damage

With significant development of mouse genomics and the availability of transgenic and knockout mice, the mouse will be the preferred animal model for inner ear research. However, few studies have used mice as experimental animals for examination of hair cell degeneration, because of their relative resistance to ototoxic agents and difficulties in surgical treatment. This study presents a model for induction of apoptotic cell death in sensory epithelia of the mouse inner ear using injection of neomycin into the posterior semicircular canal. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay revealed that local application of neomycin produced sufficient induction of apoptotic cell death in both auditory and vestibular epithelia over a definite time course. Supplementation of the general caspase-inhibitor significantly reduced induction of TUNEL-positive cells, indicating caspase-dependency of apoptotic cell death observed in the present model. In addition, the approach to the posterior semicircular canal was an easy technique, and sham-operation induced no significant induction of TUNEL-positive cells. This model, hence, enables the use of various genetic tools in studies for mechanisms of hair cell apoptosis.

Gentamicin tympanoclysis: effects on the labyrinthine sensory cells

OBJECTIVES

The objective of the study was to determine whether a selective vestibular hair cell toxicity with sparing of the cochlear hair cells could be achieved by infusing different concentrations of gentamicin into the middle ears of adult cats.

STUDY DESIGN

Prospective experimental animal study treating only the left ear of each cat, the right ear serving as individual control.

METHODS

Gentamicin solution at concentrations of either 30 or 3 mg/mL was infused daily into the left middle ear of adult cats until overt ataxia occurred. After 1 month or 6 months, each cat was killed and its temporal bones prepared for optical microscopy.

RESULTS

Animals treated with 30 mg/mL gentamicin until ataxic required a median of five daily doses. These animals had clear-cut cochlear basal turn hair cell losses accompanying toxic lesions in the utricle and cristae. In contrast, animals treated with 3 mg/mL gentamicin until ataxic required an average of 19 daily doses. These animals had lesions restricted to the utricle and cristae with sparing of the cochlea hair cells. Animals that failed to develop ataxia manifested neither lesions of the cochlear nor vestibular hair cells.

CONCLUSION

Gentamicin tympanoclysis in the cat animal model, using a dilute solution and continued once daily until clinical ataxia occurs, is capable of producing selective vestibular hair cell toxicity while sparing cochlea hair cells.

Characterization of adaptation motors in saccular hair cells by fluctuation analysis

The mechanical sensitivity of hair cells, the sensory receptors of the vestibular and auditory systems, is maintained by adaptation, which resets the transducer to cancel the effects of static stimuli. Adaptation motors in hair cells can be experimentally activated by externally applying a transduction channel blocker to the hair bundle, causing the hair bundle to move in the negative direction. We studied the variance in the position of the hair bundle during these displacements and found that it increases as the bundle moves to its new position. Often the variance peaks, and then declines to a steady-state value. We describe both displacement and variance with a model in which a motor acting on the bundle takes approximately 3.6-nm steps whose frequency (approximately 22 s(-1)) declines with the motor's load.

Caspase activation in hair cells of the mouse utricle exposed to neomycin

Aminoglycoside exposure results in the apoptotic destruction of auditory and vestibular hair cells. This ototoxic hair cell death is prevented by broad-spectrum caspase inhibition. We have used in situ substrate detection, immunohistochemistry, and specific caspase inhibitors to determine which caspases are activated in the hair cells of the adult mouse utricle in response to neomycin exposure in vitro. In addition, we have examined the hierarchy of caspase activation. Our data indicate that both upstream caspase-8 and upstream caspase-9, as well as downstream caspase-3 are activated in hair cells exposed to neomycin. The inhibition of caspase-9-like activity provided significant protection of hair cells exposed to neomycin, whereas the inhibition of caspase-8-like activity was not effective in preventing neomycin-induced hair cell death. In addition, caspase-9 inhibition prevented the activation of downstream caspase-3, whereas the inhibition of caspase-8 did not. These data indicate that caspase-9 is the primary upstream caspase mediating neomycin-induced hair cell death in this preparation.

Alteration of E-cadherin and beta-catenin in mouse vestibular epithelia during induction of apoptosis

The aim of this study was to examine if adhesion molecules had relation with degeneration and regeneration processes of mammalian vestibular epithelia. The distribution of E-cadherin and beta-catenin was immunohistochemically examined in normal and aminoglycoside-treated utricles of mice. E-cadherin and beta-catenin linearly expressed between epithelial cells in normal specimens. Aminoglycoside injury resulted in temporal alteration in distribution of these molecules with induction of apoptosis in hair cells. Degradation of both molecules was widely observed in vestibular epithelia, while some supporting cells exhibited accumulation of beta-catenin. After completion of induction of apoptosis, expression of these adhesion molecules was normal in distribution. These findings suggest that the E-cadherin-beta-catenin complex plays roles in degeneration and subsequent repair processes in vestibular epithelia affected by aminoglycosides.

The alpha9/alpha10-containing nicotinic ACh receptor is directly modulated by opioid peptides, endomorphin-1, and dynorphin B, proposed efferent cotransmitters in the inner ear

Opioid peptides have been detected in the auditory and vestibular efferent neurons where they colocalize with the major neurotransmitter, acetylcholine. We investigated the function of opioids to modulate neurotransmission mediated by hair cell's alpha9/alpha10-containing nicotinic acetylcholine receptors (alpha9/alpha10nAChRs). The endogenous opioid peptides, endomorphin-1 (mu agonist) and dynorphin B (kappa agonist), but not a delta agonist [D-Pen2,D-Pen-5]enkephalin, inhibited the acetylcholine-evoked currents in frog saccular hair cells and rat inner hair cells. This inhibition was noncompetitive, voltage-independent, and was accompanied by an acceleration of the rate of current decay. Selective mu- and kappa-opioid receptor antagonists did not block the inhibition, although partial reduction by naloxone was observed. All opioid antagonists tested also reduced the acetylcholine response. Endomorphin-1 and dynorphin B inhibited the acetylcholine-evoked currents in alpha9/alpha10-expressing Xenopus oocytes. Because oocytes lack opioid receptors, it provides strong evidence for the direct interaction of opioid peptides with alpha9/alpha10nAChR.

CONCLUSION

alpha9/alpha10nAChR is a target for modulation by endomorphin-1 and dynorphin B, efferent cotransmitters in the inner ear.

Inhibition of caspases prevents ototoxic and ongoing hair cell death

Sensory hair cells die after acoustic trauma or ototoxic insults, but the signal transduction pathways that mediate hair cell death are not known. Here we identify several important signaling events that regulate the death of vestibular hair cells. Chick utricles were cultured in media supplemented with the ototoxic antibiotic neomycin and selected pharmacological agents that influence signaling molecules in cell death pathways. Hair cells that were treated with neomycin exhibited classically defined apoptotic morphologies such as condensed nuclei and fragmented DNA. Inhibition of protein synthesis (via treatment with cycloheximide) increased hair cell survival after treatment with neomycin, suggesting that hair cell death requires de novo protein synthesis. Finally, the inhibition of caspases promoted hair cell survival after neomycin treatment. Sensory hair cells in avian vestibular organs also undergo continual cell death and replacement throughout mature life. It is unclear whether the loss of hair cells stimulates the proliferation of supporting cells or whether the production of new cells triggers the death of hair cells. We examined the effects of caspase inhibition on spontaneous hair cell death in the chick utricle. Caspase inhibitors reduced the amount of ongoing hair cell death and ongoing supporting cell proliferation in a dose-dependent manner. In isolated sensory epithelia, however, caspase inhibitors did not affect supporting cell proliferation directly. Our data indicate that ongoing hair cell death stimulates supporting cell proliferation in the mature utricle.

Leupeptin protects cochlear and vestibular hair cells from gentamicin ototoxicity

Calpains, a family of calcium-activated proteases that breakdown proteins, kinases, phosphatases and transcription factors, can promote cell death. Since leupeptin, a calpain inhibitor, protected against hair cell loss from acoustic overstimulation, we hypothesized that it might protect cochlear and vestibular hair cells against gentamicin (GM) ototoxicity. To test this hypothesis, mouse organotypic cultures from the cochlea, maculae of the utricle and the crista of the semicircular canal (P1-P3) were treated with different doses of GM (0.1-3 mM) alone or in the presence of leupeptin (0.1-3 mM). The percentage of outer hair cells (OHCs) and inner hair cells (IHCs) decreased with increasing doses of GM between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced GM-induced damage to IHCs and OHCs; this protective effect was dose-dependent. GM also significantly reduced hair cell density in the crista and utricle in a dose-dependent manner between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced hair cell loss in the crista and utricle for GM concentrations between 0.1 and 3 mM. These results suggest that one of the early steps in GM ototoxicity may involve calcium-activated proteases that lead to the demise of cochlear and vestibular hair cells.

Ongoing cell death and immune influences on regeneration in the vestibular sensory organs

Hair cells in the vestibular organs of birds have a relatively short life span. Mature hair cells appear to die spontaneously and are then quickly replaced by new hair cells that arise from the division of epithelial supporting cells. A similar regenerative mechanism also results in hair cell replacement after ototoxic damage. The cellular basis of hair cell turnover in the avian ear is not understood. We are investigating the signaling pathways that lead to hair cell death and the relationship between ongoing cell death and cell production. In addition, work from our lab and others has demonstrated that the avian inner ear contains a resident population of macrophages and that enhanced numbers of macrophages are recruited to sites of hair cells lesions. Those observations suggest that macrophages and their secretory products (cytokines) may be involved in hair cell regeneration. Consistent with that suggestion, we have found that treatment with the anti-inflammatory drug dexamethasone reduces regenerative cell proliferation in the avian ear, and that certain macrophage-secreted cytokines can influence the proliferation of vestibular supporting cells and the survival of statoacoustic neurons. Those results suggest a role for the immune system in the process of sensory regeneration in the inner ear.

Glycerol affects vestibular evoked myogenic potentials in Meniere's disease

OBJECTIVES

to show that abnormal vestibular evoked myogenic potentials on the sternocleidomastoid muscle (SCM) in patients with unilateral Meniere's disease are caused by endolymphatic hydrops.

SUBJECTS

six normal volunteers and 17 patients with unilateral Meniere's disease were examined.

METHODS

click-evoked myogenic potentials were recorded with surface electrodes over each SCM. Responses evoked by clicks recorded after oral administration of glycerol (1.3 g/kg body weight) were compared with those recorded before administration.

RESULTS

the change rate of the p13-n23 amplitude was calculated. The mean+standard deviation (S.D.) of the change rate was 3.52+14.6% in normal subjects. On the unaffected side of patients the change rates were within the normal range (within the mean+/-2S.D.) in 13 patients, and three ears showed significant decrease. Only one ear showed significant increase. On the affected side, five ears showed significant increase of the amplitude while two ears showed significant decrease after oral administration of glycerol. Effects on evoked myogenic potentials were independent of those on pure tone hearing.

CONCLUSION

vestibular evoked myogenic potentials in some patients with unilateral Meniere's disease were improved by oral administration of glycerol. This result suggests that abnormal vestibular evoked myogenic potentials in patients with unilateral Meniere's disease could result from endolymphatic hydrops.

Recycling of the insulin-sensitive glucose transporter GLUT4. Access of surface internalized GLUT4 molecules to the perinuclear storage compartment is mediated by the Phe5-Gln6-Gln7-Ile8 motif

The insulin-sensitive glucose transporter GLUT4 is translocated to the plasma membrane in response to insulin and recycled back to the intracellular store(s) after removal of the hormone. We have used clonal 3T3-L1 fibroblasts and adipocyte-like cells stably expressing wild-type GLUT4 to characterize (a) the intracellular compartment where the bulk of GLUT4 is intracellularly stored and (b) the mechanisms involved in the recycling of endocytosed GLUT4 to the store compartment. Surface internalized GLUT4 is targeted to a large, flat, fenestrated saccular structure resistant to brefeldin A that localized to the vicinity of the Golgi complex is sealed to endocytosed transferrin (GLUT4 storage compartment). Recycling of endocytosed GLUT4 was studied by comparing the cellular distributions of antibody/biotin tagged GLUT4 and GLUT4(Ser(5)), a mutant with the Phe(5)-Gln(6)-Gln(7)-Ile(8) inactivated by the substitution of Ser for Phe(5). Ablation of the Phe(5)-Gln(6)-Gln(7)-Ile(8) inhibits the recycling of endocytosed GLUT4 to the GLUT4 store compartment and results in its transport to late endosomes/lysosomes where it is rapidly degraded.

Brief treatments with forskolin enhance s-phase entry in balance epithelia from the ears of rats

In the ears of mammals, hair cell loss results in permanent hearing and balance deficits, whereas in fish, amphibians, and birds, the production of replacement hair cells can restore those modalities. In avian ears, continuous exposures to forskolin trigger cell proliferation and the regeneration of hair cells, so we investigated the effect of forskolin on sensory epithelia cultured from the ears of mammals. Continuous 72 hr exposures to forskolin failed to induce proliferation in neonatal rat utricles, but brief (</=1 hr) exposures to forskolin or Br-cAMP did. Proliferation occurred only in media that contained serum. Forskolin also augmented the mitogenic effects of glial growth factor 2. The S-phase entry induced by forskolin was blocked by monensin and bafilomycin, two compounds that can inhibit the recycling of membrane receptors. The results are consistent with the hypothesis that in mammalian vestibular epithelia elevated cAMP induces S-phase entry by increasing the number of growth factor receptors at the plasma membrane.

Basic fibroblast growth factor inhibits cell proliferation in cultured avian inner ear sensory epithelia

Postembryonic production of inner ear hair cells occurs after insult in nonmammalian vertebrates. Recent studies suggest that the fibroblast family of growth factors may play a role in stimulating cell proliferation in mature inner ear sensory epithelium. Effects of acidic fibroblast growth factor (FGF-1) and basic fibroblast growth factor (FGF-2) were tested on progenitor cell division in cultured auditory and vestibular sensory epithelia taken from posthatch chickens. The effects of heparin, a glycosaminoglycan that often potentiates the effects of the FGFs, were also assessed. Tritiated-thymidine autoradiographic techniques and 5-bromo-2;-deoxyuridine (BrdU) immunocytochemistry were used to identify cells synthesizing DNA. The terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-biotin nick-end-label (TUNEL) method was used to identify apoptotic cells. TUNEL and overall counts of sensory epithelial cell density were used to assess possible cytotoxic effects of the growth factors. FGF-2 inhibited DNA synthesis in vestibular and auditory sensory epithelia and was not cytotoxic at the concentrations employed. FGF-1 did not significantly alter sensory epithelial cell proliferation. Heparin by itself inhibited DNA synthesis in the vestibular sensory epithelia and failed to potentiate the effects of FGF-1 or FGF-2. Heparin was not cytotoxic at the concentrations employed. Results presented here suggest that FGF-2 may be involved in inhibiting cell proliferation or stimulating precursor cell differentiation in avian inner ear sensory epithelia.

Cytoskeletal protein mRNA expression in the chick utricle after treatment in vitro with aminoglycoside antibiotics: effects of insulin, iron chelators and cyclic nucleotides

In birds, spontaneous recovery of the hair cells of the inner ear can occur after damage induced by aminoglycoside antibiotics. The factors that influence this recovery and the process of hair cell regeneration itself have until recently been investigated largely by morphological and histological methods. The aim of this work was to use a molecular biological approach to the analysis of hair cell regeneration by measuring the changes that occur in expression of mRNA for hair cell-specific cytoskeletal proteins fimbrin and class III beta-tubulin, along with that for beta-actin, in the utricle of chicks after hair cell damage both in vitro and in vivo. Utricles were removed from 1-day-old chicks and incubated with the aminoglycoside antibiotics gentamicin or neomycin (both 1 mM), or chicks were injected intraperitoneally with 100 mg/kg gentamicin or neomycin for 4 consecutive days. At the end of the treatment periods, total RNA was extracted from single utricles, reverse transcribed to cDNA and the cDNA amplified by PCR with primers for beta-actin, fimbrin and class III beta-tubulin. Co-amplification allowed quantitative comparison of mRNA between fimbrin, or class III beta-tubulin and beta-actin from the same utricle. Both aminoglycosides, either after 48 h in vitro or immediately after treatment in vivo, caused a significant decrease in the expression of fimbrin mRNA and class III beta-tubulin mRNA, relative to beta-actin mRNA, which itself increased. Light and electron microscopy confirmed that this corresponded to loss of, and damage to, hair cells. The relative expression of fimbrin and class III beta-tubulin mRNAs was partly restored almost to control levels 4 days after cessation of treatment in vivo and fully normalised by 4 weeks, by which time hair cells appeared normal. However, their relative expression remained depressed 4 days after removal of antibiotic in vitro. The iron chelator desferrioxamine (100 microM) in vitro prevented the aminoglycoside-induced reduction in relative expression of mRNA for both fimbrin and class III beta-tubulin. Neither insulin (5 microM) nor a combination of dibutyryl cyclic AMP (5 mM) and the phosphodiesterase inhibitor IBMX (0.5 mM) prevented the decrease in relative expression of the mRNAs for the hair cell-specific proteins, but both treatments allowed their partial recovery in vitro during the 4-day-period after removal of aminoglycoside. It is concluded that the cells of the sensory epithelium of the chick utricle subjected to aminoglycoside-induced damage undergo a process in which mRNA expression is switched away from the production of functional proteins and towards proteins necessary for structural re-organisation. The restoration of mRNA expression to a normal pattern is promoted by the growth factor insulin and by cyclic AMP.

Overexpression of Math1 induces robust production of extra hair cells in postnatal rat inner ears

For mammalian cochlear hair cells, fate determination is normally completed by birth. We report here that overexpression of Math1, a mouse homolog of the Drosophila gene atonal, in postnatal rat cochlear explant cultures resulted in extra hair cells. Surprisingly, we found that the source of the ectopic hair cells was columnar epithelial cells located outside the sensory epithelium in the greater epithelial ridge, which normally give rise to inner sulcus cells. Moreover, Math1 expression also facilitated conversion of postnatal utricular supporting cells into hair cells. Thus Math1 was sufficient for the production of hair cells in the ear, and immature postnatal mammalian inner ears retained the competence to generate new hair cells.

Heregulin enhances regenerative proliferation in postnatal rat utricular sensory epithelium after ototoxic damage

Hair cell loss due to acoustic and ototoxic damage often leads to hearing and balance impairments. Although a spontaneous event in chicks and lower vertebrates, hair cell replacement occurs at a much lower frequency in mammals presumably due to a very low rate of supporting cell proliferation following injury. We report here that heregulin, a member of the neuregulin family, dramatically enhances proliferation of supporting cells in postnatal rat utricular epithelial sheet cultures after gentamicin treatment, as revealed by bromo-deoxyuridine (BrdU) immunocytochemistry. A dose-dependent study shows that the maximal effects of heregulin are achieved at 3 nM. The mitogenic effects of heregulin are confirmed in utricular whole mount cultures. Autoradiography of the utricular whole mount cultures shows that heregulin also enhances the number of tritiated thymidine-labeled cells within the hair cell layer. TaqMan quantitative RT-PCR analysis and immunocytochemistry reveal that heregulin and its binding receptors (ErbB-2, ErbB-3 and ErbB-4) are expressed in the inner ear sensory epithelium. Of several ligands activating various ErbB receptors, including heregulin, neuregulin-3, beta-cellulin, heparin binding-epidermal growth factor (HB-EGF), transforming growth factor-alpha (TGF-alpha) and EGF, heregulin shows the most potent mitogenic effects on supporting cells. Because neuregulin-3 that signals only through ErbB-4 does not show an effect, these data suggest that activation of the ErbB-2-ErbB-3 heterodimeric complexes, rather than ErbB-4, is critical for the proliferative response in the utricular sensory epithelium. In addition, gentamicin treatment induces an upregulation of heregulin mRNA. Considered together, heregulin may play an important role in hair cell regeneration following ototoxic damage.

Immune cytokines and dexamethasone influence sensory regeneration in the avian vestibular periphery

Prior studies have shown that macrophages are recruited to sites of hair cell lesions in the avian inner ear in vitro (Warchol, 1997) and in vivo (Bhave et al., 1998). Although the avian ear has a high capacity for sensory regeneration (Oberholtzer & Corwin, 1997; Stone et al., 1998), the role of macrophages in the regenerative process is uncertain. The present study examined the possible influence of macrophages and immune cytokines on regenerative proliferation in the avian utricle, one of the sensory endorgans of the vestibular system. Utricles from post-hatch chicks were placed in organ culture and hair cell lesions were created by incubation in neomycin. The cultures were then maintained for an additional 24-48 hours in vitro, and some cultures were treated with dexamethasone, which inhibits macrophage activation and cytokine production. Following fixation, resident macrophages were identified by immunoreactivity to CD68. Labeled macrophages were present in all specimens and increased numbers of macrophages were observed following neomycin treatment. Regenerative proliferation in dexamethasone-treated specimens was reduced by about 50%, relative to untreated controls. Additional experiments showed that two macrophage secretory products-TGF-alpha and TNF-alpha-enhanced the proliferation of utricular supporting cells. The results are consistent with a role for macrophages in hair cell regeneration.

Distribution and time course of hair cell regeneration in the pigeon utricle

Vestibular and cochlear regeneration following ototoxic insult from aminoglycoside antibiotics has been well documented, particularly in birds. In the present study, intraotic application of a 2 mg streptomycin paste was used to achieve complete vestibular hair cell destruction in pigeons (Columba livia) while preserving regenerative ability. Scanning electron microscopy was used to quantify hair cell density longitudinally during regeneration in three different utricular macula locations, including the striola, central and peripheral regions. The utricular epithelium was void of stereocilia (indicating hair cell loss) at 4 days after intraotic treatment with streptomycin. At 2 weeks the stereocilia began to appear randomly and mostly in an immature form. However, when present most kinocilia were polarized toward the developing striola. Initially, regeneration occurred more rapidly in the central and peripheral regions of the utricle as compared to the striola. As regeneration proceeded from 2 to 12 weeks, hair cell density in the striola region equaled the density noted in the central and peripheral regions. At 24 weeks, hair cell density of the central and peripheral regions was equal to normal values, however the striola region had a slightly greater hair cell density than that observed for normal animals.

Ionic currents and current-clamp depolarisations of type I and type II hair cells from the developing rat utricle

Ionic currents and the voltage response to injected currents were studied in an acutely dissected preparation of the rat utricle between birth and postnatal day 12 (PN12). Based upon morphological criteria, the sensory cells examined were divided into two classes, "type I" and "type 2 category," the latter of which may include some immature type I cells. The former group comprises a clearly defined electrophysiological population, with one large outwardly rectifying potassium conductance that is sensitive to 4-aminopyridine (4-AP), insensitive to tetraethylammonium (TEA) and displays voltage-dependent activation kinetics. In the absence of enzymatic dissociation procedures, and with the epithelium left largely intact, the mean half activation of this conductance was -30.3 mV at PN3, and -37.5 mV at PN12. At both stages it was almost entirely turned off at -74 mV. Omission of ATP from the intracellular solution appeared to prevent rundown of this conductance. Type II category hair cells formed a more heterogeneous population, exhibiting a distinct TEA-sensitive delayed rectifier potassium conductance; the rapidly activating and inactivating IA; an inward rectifier; and inward sodium currents at around PN3. Both cell types depolarised strongly in response to injected currents, with time courses reflecting the activation kinetics of their major outward conductances.