Identification of factors that maintain mammalian outer hair cells in adult organ of Corti explants

IGF-1 Mediated Neuroprotective Effects of Olfactory-Derived Mesenchymal Stem Cells on Auditory Hair Cells

IMPORTANCE

Mesenchymal stem cells (MSCs) have the capability of providing ongoing paracrine support to degenerating tissues. Since MSCs can be extracted from a broad range of tissues, their specific surface marker profiles and growth factor secretions can be different. We hypothesized that MSCs derived from different sources might also have different neuroprotective potential.

OBJECTIVE

In this study, we extracted MSCs from rodent olfactory mucosa and compared their neuroprotective effects on auditory hair cell survival with MSCs extracted from rodent adipose tissue.

METHODS

Organ of Corti explants were dissected from 41 cochlea and incubated with olfactory mesenchymal stem cells (OMSCs) and adipose mesenchymal stem cells (AMSCs). After 72 hours, Corti explants were fixed, stained, and hair cells counted. Growth factor concentrations were determined in the supernatant and cell lysate using Enzyme-Linked Immunosorbent Assay (ELISA).

RESULTS

Co-culturing of organ of Corti explants with OMSCs resulted in a significant increase in inner and outer hair cell stereocilia survival, compared to control. Comparisons between both stem cell lines, showed that co-culturing with OMSCs resulted in superior inner and outer hair cell stereocilia survival rates over co-culturing with AMSCs. Assessment of growth factor secretions revealed that the OMSCs secrete significant amounts of insulin-like growth factor 1 (IGF-1). Co-culturing OMSCs with organ of Corti explants resulted in a 10-fold increase in IGF-1 level compared to control, and their secretion was 2 to 3 times higher compared to the AMSCs.

CONCLUSIONS

This study has shown that OMSCs may mitigate auditory hair cell stereocilia degeneration. Their neuroprotective effects may, at least partially, be ascribed to their enhanced IGF-1 secretory abilities compared to AMSCs.

Updates to the guinea pig animal model for in-vivo auditory neuroscience in the low-frequency hearing range

For gaining insight into general principles of auditory processing, it is critical to choose model organisms whose set of natural behaviors encompasses the processes being investigated. This reasoning has led to the development of a variety of animal models for auditory neuroscience research, such as guinea pigs, gerbils, chinchillas, rabbits, and ferrets; but in recent years, the availability of cutting-edge molecular tools and other methodologies in the mouse model have led to waning interest in these unique model species. As laboratories increasingly look to include in-vivo components in their research programs, a comprehensive description of procedures and techniques for applying some of these modern neuroscience tools to a non-mouse small animal model would enable researchers to leverage unique model species that may be best suited for testing their specific hypotheses. In this manuscript, we describe in detail the methods we have developed to apply these tools to the guinea pig animal model to answer questions regarding the neural processing of complex sounds, such as vocalizations. We describe techniques for vocalization acquisition, behavioral testing, recording of auditory brainstem responses and frequency-following responses, intracranial neural signals including local field potential and single unit activity, and the expression of transgenes allowing for optogenetic manipulation of neural activity, all in awake and head-fixed guinea pigs. We demonstrate the rich datasets at the behavioral and electrophysiological levels that can be obtained using these techniques, underscoring the guinea pig as a versatile animal model for studying complex auditory processing. More generally, the methods described here are applicable to a broad range of small mammals, enabling investigators to address specific auditory processing questions in model organisms that are best suited for answering them.

Pegylated Insulin-Like Growth Factor 1 attenuates Hair Cell Loss and promotes Presynaptic Maintenance of Medial Olivocochlear Cholinergic Fibers in the Cochlea of the Progressive Motor Neuropathy Mouse

The progressive motor neuropathy (PMN) mouse is a model of an inherited motor neuropathy disease with progressive neurodegeneration. Axon degeneration associates with homozygous mutations of the TBCE gene encoding the tubulin chaperone E protein. TBCE is responsible for the correct dimerization of alpha and beta-tubulin. Strikingly, the PMN mouse also develops a progressive hearing loss after normal hearing onset, characterized by degeneration of the auditory nerve and outer hair cell (OHC) loss. However, the development of this neuronal and cochlear pathology is not fully understood yet. Previous studies with pegylated insulin-like growth factor 1 (peg-IGF-1) treatment in this mouse model have been shown to expand lifespan, weight, muscle strength, and motor coordination. Accordingly, peg-IGF-1 was evaluated for an otoprotective effect. We investigated the effect of peg-IGF-1 on the auditory system by treatment starting at postnatal day 15 (p15). Histological analysis revealed positive effects on OHC synapses of medial olivocochlear (MOC) neuronal fibers and a short-term attenuation of OHC loss. Peg-IGF-1 was able to conditionally restore the disorganization of OHC synapses and maintain the provision of cholinergic acetyltransferase in presynapses. To assess auditory function, frequency-specific auditory brainstem responses and distortion product otoacoustic emissions were recorded in animals on p21 and p28. However, despite the positive effect on MOC fibers and OHC, no restoration of hearing could be achieved. The present work demonstrates that the synaptic pathology of efferent MOC fibers in PMN mice represents a particular form of “efferent auditory neuropathy.” Peg-IGF-1 showed an otoprotective effect by preventing the degeneration of OHCs and efferent synapses. However, enhanced efforts are needed to optimize the treatment to obtain detectable improvements in hearing performances.

Molecular and Clinical Significance of Fibroblast Growth Factor 2 in Development and Regeneration of the Auditory System

The fibroblast growth factor 2 (FGF2) is a member of the FGF family which is involved in key biological processes including development, cellular proliferation, wound healing, and angiogenesis. Although the utility of the FGF family as therapeutic agents has attracted attention, and FGF2 has been studied in several clinical contexts, there remains an incomplete understanding of the molecular and clinical function of FGF2 in the auditory system. In this review, we highlight the role of FGF2 in inner ear development and hearing protection and present relevant clinical studies for tympanic membrane (TM) repair. We conclude by discussing the future implications of FGF2 as a potential therapeutic agent.

Caspase inhibitor z-VAD-FMK increases the survival of hair cells after Actinomycin-D-induced damage in vitro

Actinomycin-D (Act-D) is a highly effective chemotherapeutic agent that induces apoptosis in systemic tissues. Act-D combined with other chemotherapeutic agents exhibits ototoxic effects and causes hearing impairment. To investigate the potential toxic effects of Act-D in the inner ear, we treated cochlear organotypic cultures with varying concentrations of Act-D for different durations. For the first time, we found that Act-D specifically induced HC loss and apoptosis in a dose- and time-dependent manner but not neuronal degeneration. Co-treatment with benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (z-VAD-FMK), a pan cysteine protease inhibitor, significantly reduced HC loss and apoptosis induced by Act-D, indicating increased cell survival. Taken together, Act-D exposure has ototoxic effects on the auditory system, while z-VAD-FMK prevents Act-D-induced hair cell damage.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

A kinase inhibitor library screen identifies novel enzymes involved in ototoxic damage to the murine organ of Corti

Ototoxicity is a significant side effect of a number of drugs, including the aminoglycoside antibiotics and platinum-based chemotherapeutic agents that are used to treat life-threatening illnesses. Although much progress has been made, the mechanisms that lead to ototoxic loss of inner ear sensory hair cells (HCs) remains incompletely understood. Given the critical role of protein phosphorylation in intracellular processes, including both damage and survival signaling, we screened a library of kinase inhibitors targeting members of all the major families in the kinome. Micro-explants from the organ of Corti of mice in which only the sensory cells express GFP were exposed to 200 μM of the ototoxic aminoglycoside gentamicin with or without three dosages of each kinase inhibitor. The loss of sensory cells was compared to that seen with gentamicin alone, or without treatment. Of the 160 inhibitors, 15 exhibited a statistically significant protective effect, while 3 significantly enhanced HC loss. The results confirm some previous studies of kinase involvement in HC damage and survival, and also highlight several novel potential kinase pathway contributions to ototoxicity.

Activation of PI3K signaling prevents aminoglycoside-induced hair cell death in the murine cochlea

Loss of sensory hair cells of the inner ear due to aminoglycoside exposure is a major cause of hearing loss. Using an immortalized multipotent otic progenitor (iMOP) cell line, specific signaling pathways that promote otic cell survival were identified. Of the signaling pathways identified, the PI3K pathway emerged as a strong candidate for promoting hair cell survival. In aging animals, components for active PI3K signaling are present but decrease in hair cells. In this study, we determined whether activated PI3K signaling in hair cells promotes survival. To activate PI3K signaling in hair cells, we used a small molecule inhibitor of PTEN or genetically ablated PTEN using a conditional knockout animal. Hair cell survival was challenged by addition of gentamicin to cochlear cultures. Hair cells with activated PI3K signaling were more resistant to aminoglycoside-induced hair cell death. These results indicate that increased PI3K signaling in hair cells promote survival and the PI3K signaling pathway is a target for preventing aminoglycoside-induced hearing loss.

IGF-1 deficiency causes atrophic changes associated with upregulation of VGluT1 and downregulation of MEF2 transcription factors in the mouse cochlear nuclei

Insulin-like growth factor 1 (IGF-1) is a neurotrophic protein that plays a crucial role in modulating neuronal function and synaptic plasticity in the adult brain. Mice lacking the Igf1 gene exhibit profound deafness and multiple anomalies in the inner ear and spiral ganglion. An issue that remains unknown is whether, in addition to these peripheral abnormalities, IGF-1 deficiency also results in structural changes along the central auditory pathway that may contribute to an imbalance between excitation and inhibition, which might be reflected in abnormal auditory brainstem responses (ABR). To assess such a possibility, we evaluated the morphological and physiological alterations in the cochlear nucleus complex of the adult mouse. The expression and distribution of the vesicular glutamate transporter 1 (VGluT1) and the vesicular inhibitory transporter (VGAT), which were used as specific markers for labeling excitatory and inhibitory terminals, and the involvement of the activity-dependent myocyte enhancer factor 2 (MEF2) transcription factors in regulating excitatory synapses were assessed in a 4-month-old mouse model of IGF-1 deficiency and neurosensorial deafness (Igf1 (-/-) homozygous null mice). The results demonstrate decreases in the cochlear nucleus area and cell size along with cell loss in the cochlear nuclei of the deficient mouse. Additionally, our results demonstrate that there is upregulation of VGluT1, but not VGAT, immunostaining and downregulation of MEF2 transcription factors together with increased wave II amplitude in the ABR recording. Our observations provide evidence of an abnormal neuronal cytoarchitecture in the cochlear nuclei of Igf1 (-/-) null mice and suggest that the increased efficacy of glutamatergic synapses might be mediated by MEF2 transcription factors.

Application of insulin-like growth factor-1 in the treatment of inner ear disorders

Sensorineural hearing loss (SNHL) is considered an intractable disease, given that hair and supporting cells (HCs and SCs) of the postnatal mammalian cochlea are unable to regenerate. However, with progress in regenerative medicine in the 21st century, several innovative approaches for achieving regeneration of inner ear HCs and SCs have become available. These methods include stem cell transplantation, overexpression of specific genes, and treatment with growth factors. Insulin-like growth factor-1 (IGF-1) is one of the growth factors that are involved in the development of the inner ear. Treatment with IGF-1 maintains HC numbers in the postnatal mammalian cochlea after various types of HC injuries, with activation of two major pathways downstream of IGF-1 signaling. In the aminoglycoside-treated neonatal mouse cochlear explant culture, promotion of the cell-cycle in SCs as well as inhibition of HC apoptosis was observed in the IGF-1-treated group. Activation of downstream molecules was observed in SCs and, in turn, SCs contribute to the maintenance of HC numbers. Using comprehensive analysis of the gene expression, the candidate effector molecules of the IGF-1 signaling pathway in the protection of HCs were identified as Netrin1 and Gap43. Based on these studies, a clinical trial has sought to investigate the effects of IGF-1 on SNHL. Sudden SNHL (SSHL) that was refractory to systemic steroids was treated with IGF-1 in a gelatin hydrogel and the outcome was compared with a historical control of hyperbaric oxygen therapy. The proportion of patients showing hearing improvement was significantly higher in the IGF-1-treatment group at 24 weeks after treatment than in the control group. A randomized clinical trial is ongoing to compare the effect of IGF-1 treatment with that of intra-tympanic steroids for SSHL that is refractory to systemic steroids.

Growth factors have a protective effect on neomycin-induced hair cell loss

We have demonstrated that selected growth factors are involved in regulating survival and proliferation of progenitor cells derived from the neonatal rat organ of Corti (OC). The protective and regenerative effects of these defined growth factors on the injured organ of Corti were therefore investigated. The organ of Corti dissected from the Wistar rat pups (P3-P5) was split into apical, middle, and basal parts, explanted and cultured with or without neomycin and growth factors. Insulin-like growth factor-1 (IGF-1), fibroblast growth factor-2 (FGF-2), and epidermal growth factor (EGF) protected the inner hair cells (IHCs) and outer hair cells (OHCs) from neomycin ototoxicity. Using EGF, IGF-1, and FGF-2 alone induced no protective effect on the survival of auditory hair cells. Combining 2 growth factors (EGF + IGF-1, EGF + FGF-2, or IGF-1 + FGF-2) gave statistically protective effects. Similarly, combining all three growth factors effectively protected auditory hair cells from the ototoxic insult. None of the growth factors induced regeneration of hair cells in the explants injured with neomycin. Thus various combinations of the three defined factors (IGF-1, FGF-2, and EGF) can protect the auditory hair cells from the neomycin-induced ototoxic damage, but no regeneration was seen. This offers a possible novel approach to the treatment of hearing loss.

Protective effect of systemic administration of erythropoietin on auditory brain stem response and compound action potential thresholds in an animal model of cochlear implantation

OBJECTIVES

An animal model of cochlear implantation has been developed, and the hearing threshold was evaluated after different surgical procedures. The effect of perioperative systemic administration of erythropoietin on the hearing loss induced by cochlear implantation was tested.

METHODS

Twenty-nine guinea pigs with normal hearing underwent implantation of a 254-microm-diameter array through a cochleostomy. The effects on hearing of cochleostomy and transient and long-term array implantation (21 days) were assessed by testing of the auditory brain stem responses and compound action potentials. Eleven implanted animals received intraperitoneal administration of erythropoietin. Selected computed tomographic scans and cochlear histologic studies were performed 1 month after implantation to confirm proper placement of the array. The erythropoietin concentration at the time of surgery was assessed in samples of perilymph, cerebrospinal fluid, and blood.

RESULTS

The cochleostomy and transient array insertion had no effect on hearing thresholds. Long-term array implantation induced a stable decrease of hearing threshold (30 dB), a decrease that was reduced by 12 dB in erythropoietin-treated animals. The erythropoietin-treated animals had better hearing preservation at higher frequencies. Fibrosis surrounding the array was seen in both groups.

CONCLUSIONS

The hearing loss observed was probably due to the presence of the array in the cochlea. The intraperitoneal injection of erythropoietin improved the hearing threshold shift induced by implantation.

Ischemia-reperfusion injury of the cochlea: pharmacological strategies for cochlear protection and implications of glutamate and reactive oxygen species

A large amount of energy produced by active aerobic metabolism is necessary for the cochlea to maintain its function. This makes the cochlea vulnerable to blockade of cochlear blood flow and interruption of the oxygen supply. Although certain forms of human idiopathic sudden sensorineural hearing loss reportedly arise from ischemic injury, the pathological mechanism of cochlear ischemia-reperfusion injury has not been fully elucidated. Recent animal studies have shed light on the mechanisms of cochlear ischemia-reperfusion injury. It will help in the understanding of the pathology of cochlear ischemia-reperfusion injury to classify this injury into ischemic injury and reperfusion injury. Excitotoxicity, mainly observed during the ischemic period, aggravates the injury of primary auditory neurons. On the other hand, oxidative damage induced by hydroxyl radicals and nitric oxide enhances cochlear reperfusion injury. This article briefly summarizes the generation mechanisms of cochlear ischemia-reperfusion injury and potential therapeutic targets that could be developed for the effective management of this injury type.

Effects of systemic administration of methylprednisolone on residual hearing in an animal model of cochlear implantation

CONCLUSION

Delivery of intramuscular injection of methylprednisolone around the implantation surgery improved the hearing threshold shift induced by cochlear implantation.

OBJECTIVES

During electroacoustic cochlear implantation surgery, the residual hearing is not preserved in about 15% of cases. In this study, we tested the effects of intramuscular administration of methylprednisolone on the hearing loss induced by cochlear implantation in a model of guinea pig cochlear implantation.

METHODS

Eleven guinea pigs with normal hearing were implanted with a 254 μm diameter silicone array through a cochleostomy, and the effects on hearing of longstanding array insertion (21 days) were assessed. Six of the implanted animals received intramuscular administration of methylprednisolone. Auditory brainstem response recordings were performed before and up to 21 days after the cochlear implantation. CT scans were performed in some animals 1 month after implantation.

RESULTS

CT scans confirmed that the array was well positioned in tested animals. From days 3 to 21, a hearing loss of about 30 dB on all frequencies was observed in the implanted nontreated group. This hearing loss remained stable during the whole follow-up period. Compared with implanted nontreated animals, the hearing threshold shift decreased by 12 dB in animals treated with methylprednisolone.

Dexamethasone treatment of tumor necrosis factor-alpha challenged organ of Corti explants activates nuclear factor kappa B signaling that induces changes in gene expression that favor hair cell survival

The objective was to determine the role of nuclear factor kappa B (NFκB) in dexamethasone base (DXMb) protection of auditory hair cells from tumor necrosis factor-alpha (TNFα)-induced loss on gene expression and cell signaling levels. Organ of Corti (OC) explants from 3-day-old rats were cultured under one of the following conditions: (1) media only--no treatment; (2) media+TNFα; (3) media+TNFα+DXMb; (4) media+TNFα+DXMb+NFκB-Inhibitor (NFκB-I); or (5) media+TNFα+DXMb+NFκBI-Scrambled control (NFκBI-C). A total of 60 organ of Corti explants (OC) were stained with FITC-Phalloidin after 96 h in culture (conditions 1-5) for hair cell counts and imaging of surface characteristics. A total of 108 OC were used for gene expression studies (i.e. B-actin, Bax, Bcl-2, Bcl-xl, and TNFR1) after 0, 24, or 48 h in vitro (conditions 1-4). A total of 86 OC were cultured (conditions 1-3) for 48 h, 36 of which were used for phosphorylated NFκB (p-NFκB) ELISA studies and 50 for whole mount anti-p-NFκB immunostain experiments. TNFα+DXMb exposed cultures demonstrated significant upregulation in anti-apoptotic Bcl-2 and Bcl-xl genes and downregulation in pro-apoptotic Bax gene expression; DXMb treatment of TNFα explants also lowered the Bax/Bcl-2 ratio and inhibited TNFR1 upregulation. After inhibiting NFκB activity with NFκB-I, the gene expression profile following TNFα+DXMb treatment now mimics that of TNFα-challenged OC explants. The levels of p-NFκB and the degree of nuclear translocation are significantly greater in TNFα+DXMb exposed OC explants than observed in the TNFα and control groups in the middle+basal turns of OC explants. These findings were supported by the results of the hair cell counts and the imaging results obtained from the whole mount OC specimens. DXMb protects against TNFα-induced apoptosis of auditory hair cells in vitro via activation of NFκB signaling in hair cell nuclei, and regulation of the expression levels of anti- and pro-apoptotic genes and a pro-inflammatory gene.

Expression of Insulin-like Growth Factors in a Mouse Model of Salicylate Ototoxicity

OBJECTIVES

Insulin-like growth factors (IGFs) are known to be neurotrophic factors, and they efficiently signal to cells to grow, differentiate and survive. The purpose of study was to identify the expressions of IGFs in mice with salicylate ototoxicity, which is a typical reversible hearing loss model.

METHODS

The mice were given intraperitoneal injections of salicylate (400 mg/kg) and about a 30 dB threshold shift was achieved at 3 hours. The expressions of IGF-1 and 2 were confirmed by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and Western blotting. Localization of IGFs was confirmed using confocal immunofluorescence imaging. For in-vitro study on the HEI-OC1 auditory cells, the cell viability was calculated and the apoptotic features of the nuclei were observed with Hoechst staining.

RESULTS

The expressions of the IGFs mRNA and protein were significantly increased in the salicylate ototoxicity groups compared with that of the normal control group. Salicylate induced apoptosis and decreased viability of the HEI-OC1 auditory cells in a time- and dose-dependent manner. The expressions of IGFs were localized in the stria vascularis, and these IGFs play a protective role in the in-vivo condition of salicylate ototoxicity.

CONCLUSION

IGFs were highly expressed in the mice with salicylate ototoxicity, and this expression was mainly focused in the stria vascularis in the salicylate intoxicated mice. The systemic action of IGFs, which were expressed in the vascular-rich stria vascularis, can act as a major protective mechanism in a mouse model of salicylate ototoxicity.

Cellular targets of estrogen signaling in regeneration of inner ear sensory epithelia

Estrogen signaling in auditory and vestibular sensory epithelia is a newly emerging focus propelled by the role of estrogen signaling in many other proliferative systems. Understanding the pathways with which estrogen interacts can provide a means to identify how estrogen may modulate proliferative signaling in inner ear sensory epithelia. Reviewed herein are two signaling families, EGF and TGFbeta. Both pathways are involved in regulating proliferation of supporting cells in mature vestibular sensory epithelia and have well characterized interactions with estrogen signaling in other systems. It is becoming increasingly clear that elucidating the complexity of signaling in regeneration will be necessary for development of therapeutics that can initiate regeneration and prevent progression to a pathogenic state.

Involvement of platelet-derived growth factor receptor-beta in maintenance of mesenchyme and sensory epithelium of the neonatal mouse inner ear

Platelet-derived growth factor receptor (PDGFR) signaling has been demonstrated to play a pivotal role in early embryonic development. Although the expression of PDGF in the inner ear has been studied by RT-PCR, how PDGFR is involved there remains largely unclear. In the current study, we used the antagonistic anti-PDGFR-beta antibody, APB5, to investigate the role of PDGFR-beta in the neonatal mouse inner ear. PDGFR-beta was detected immunohistochemically in the mesenchymal tissue adjacent to the sensory epithelium of the inner ear, and a ligand for PDGFR-beta was detected around the sensory epithelium. To determine whether this expression plays a functional role, we injected APB5 into neonates to block the function of PDGFR-beta. Mesenchymal tissue defects and abnormal capillaries with irregular shapes, especially in the cochlear lateral wall, were detected in APB5-treated mice. The results of a TUNEL assay revealed that not only the adjacent mesenchymal cells but also the sensory epithelial cells underwent cell death. These results indicate that PDGFR-beta signals are required for the survival of the capillary and mesenchymal cells in the neonatal mouse inner ear and also indirectly implicate these signals in the survival of the sensory epithelium.

Childhood hearing is associated with growth rates in infancy and adolescence

It is known that shorter stature is associated with sensorineural hearing loss; and that cochlear development is associated with activity of IGF 1, as are many important aspects of neurodevelopment. We hypothesized that this relation might be extrapolated to a normally hearing group, and that the strongest relation between hearing level and growth rate would be in late puberty, when serum IGF-1 levels are highest. We examined the statistical relation between childhood hearing threshold and rate of growth in height at different times during the life course up to age 32. We found mixed support for the hypothesis. The strongest relations were observed in late puberty, at the ages which previous research shows are associated with the highest serum concentrations of IGF-1 in males and females, but also in infancy and early childhood. The association between hearing and height is present in a normally hearing, general population sample, and is associated with growth in late adolescence. Our findings support the idea that childhood hearing threshold may be predictive of IGF-1 mediated developmental characteristics.

The complexity of age-related hearing impairment: contributing environmental and genetic factors

Age-related hearing impairment (ARHI) is the most common sensory impairment seen in the elderly. It is a complex disorder, with both environmental as well as genetic factors contributing to the impairment. The involvement of several environmental factors has been partially elucidated. A first step towards the identification of the genetic factors has been made, which will result in the identification of susceptibility genes, and will provide possible targets for the future treatment and/or prevention of ARHI. This paper aims to give a broad overview of the scientific findings related to ARHI, focusing mainly on environmental and genetic data in humans and in animal models. In addition, methods for the identification of contributing genetic factors as well as possible future therapeutic strategies are discussed.

A mutation in CCDC50, a gene encoding an effector of epidermal growth factor-mediated cell signaling, causes progressive hearing loss

We previously mapped a novel autosomal dominant deafness locus, DFNA44, by studying a family with postlingual, progressive, nonsyndromic hearing loss. We report here on the identification of a mutation in CCDC50 as the cause of hearing loss in the family. CCDC50 encodes Ymer, an effector of epidermal growth factor (EGF)-mediated cell signaling that is ubiquitously expressed in different organs and has been suggested to inhibit down-regulation of the EGF receptor. We have examined its expression pattern in mouse inner ear. Western blotting and cell transfection results indicate that Ymer is a soluble, cytoplasmic protein, and immunostaining shows that Ymer is expressed in a complex spatiotemporal pattern during inner ear development. In adult inner ear, the expression of Ymer is restricted to the pillar cells of the cochlea, the stria vascularis, and the vestibular sensory epithelia, where it shows spatial overlap with the microtubule-based cytoskeleton. In dividing cells, Ymer colocalizes with microtubules of the mitotic apparatus. We suggest that DFNA44 hearing loss may result from a time-dependent disorganization of the microtubule-based cytoskeleton in the pillar cells and stria vascularis of the adult auditory system.

Vector-mediated delivery of bcl-2 prevents degeneration of auditory hair cells and neurons after injury

OBJECTIVE

To test the hypothesis that bcl-2 prevents oxidative stress-induced apoptosis of auditory sensory cells in explants of the organ of Corti and dissociated cell cultures of the spiral ganglion.

METHODS

Organ of Corti explants and dissociated spiral ganglion cell cultures obtained from 3-day-old (P3) rats or adult spiral ganglion cell cultures from 28-day-old (P28) rats were transduced with vectors containing a human bcl-2 gene. Cultures were then exposed to neomycin, cisplatin or subjected to withdrawal of neurotrophin supplementation. Outcome measures included hair cell and neuron counts, mitochondrial membrane potential and a histological measure of apoptosis.

RESULTS

Expression of bcl-2 in the organ of Corti explants and neuronal cell cultures provided a significant level of protection against cell death. Bcl-2 expression in the organ of Corti explants also protected mitochondria from loss of membrane potential and blocked an early step in the commitment of hair cells to apoptosis.

CONCLUSION

Expression of bcl-2 in cochlear tissues protects sensory cells from a variety of insults that have been demonstrated to damage the inner ear.

Insulin-induced vascular endothelial growth factor expression is mediated by the NADPH oxidase NOX3

Vascular endothelial growth factor (VEGF) is the most potent stimulatory factor of angiogenesis. Its expression is induced by reactive oxygen species (ROS) in hypoxic conditions and by insulin in normoxic cells. Both ROS and insulin can activate mitogen-activated protein kinases (MAPKs) and induce the transcriptional factor Sp1, components that are essential for VEGF gene expression. The aim of this study was to investigate the role of ROS producing NADPH oxidase enzymes (NOX-es) in insulin-regulated VEGF gene activation. To achieve this goal we chose HepG2 cells as our model system as these cells express the NADPH oxidase isoform NOX3 and respond to insulin stimulation with enhanced ROS production and mRNA transcription and production of VEGF. We demonstrate that in control cells insulin stimulation leads to H2O2 generation, a biphasic activation of p42/44 MAPK and the induction of both Sp1 and HIF-1alpha. Transfection of NOX3-specific siRNA abrogates H2O2 production and inhibits exclusively the second phase of p42/44 MAPK phosphorylation and Sp1 DNA binding and thus prevents upregulation of VEGF-A mRNA expression. In conclusion, our results demonstrate that NOX3, a ROS generating NADPH oxidase, plays an integral role in insulin-induced p42/44 MAPK signal transmission and VEGF-A production.

Cochlear Protection by Local Insulin-Like Growth Factor-1 Application Using Biodegradable Hydrogel

OBJECTIVE

The aim of this experimental study was to examine the potential of local recombinant human insulin-like growth factor-1 (rhIGF-1) application through a biodegradable hydrogel for the treatment of cochleae.

METHODS

A hydrogel immersed with rhIGF-1 was placed on the round window membrane of Sprague-Dawley rats while a hydrogel immersed with physiological saline was applied to control animals. On day 3 after drug application, the animals were exposed to white noise at 120 dB sound pressure level (SPL) for 2 hours. Cochlear function was monitored using measurements of auditory brain stem responses (ABRs) at frequencies of 8, 16, and 32 kHz. The temporal bones were collected 7 or 30 days after noise exposure and the loss of hair cells was quantitatively analyzed.

RESULTS

Local rhIGF-1 treatment significantly reduced the elevation of ABR thresholds on days 7 and 30 after noise exposure. Histologic analysis revealed that local rhIGF-1 treatment significantly prohibited the loss of outer hair cells.

CONCLUSIONS

These findings demonstrate that local IGF-1 application through the biodegradable hydrogel has the potential for protection of cochleae from noise trauma.

Recombinant human erythropoietin prevents ischemia-induced apoptosis and necrosis in explant cultures of the rat organ of Corti

This study was designed to evaluate the effect of recombinant human erythropoietin (rhEPO), insulin-like growth factor-1 (rhIGF-1) and epidermal growth factor (rhEGF) on ischemia-induced hair cell loss in an organotypic cochlea culture. The apical, middle and basal parts of the organs of Corti (newborn rat, postnatal days 3-5) were exposed to ischemia (3.5 h) in glucose-free artificial perilymph (pO2 10-20 mmHg) with or without growth factors. Controls were exposed to normoxia. Twenty-four hours after the onset of ischemia, the cultures were stained using tetramethyl rhodamine isothiocyanate (TRITC) phalloidin (hair cells), propidium iodide (membrane integrity) and apoptosis detection kit (DNA-fragmentation). Ischemia (3.5 h) induced a hair cell loss of 20 and 40% in the middle and basal cochlear parts, respectively, and an increase of the numbers of PI-stained and DNA-fragmented nuclei (controls 0-1, ischemia 4-7 nuclei/100 microm). The basal part was more affected than the apical one. rhEPO and rhIGF-1 significantly attenuated the ischemia-induced hair cell loss by reducing processes involved in apoptosis and necrosis. rhEPO has been in clinical use for more than a decade and found to be well tolerated. Therefore, rhEPO could be an effective drug for the prevention of hearing loss via a hair cell protective mechanism.

Trophic effects of insulin-like growth factor-I (IGF-I) in the inner ear

Insulin-like growth factors (IGFs) have a pivotal role during nervous system development and in its functional maintenance. IGF-I and its high affinity receptor (IGF1R) are expressed in the developing inner ear and in the postnatal cochlear and vestibular ganglia. We recently showed that trophic support by IGF-I is essential for the early neurogenesis of the chick cochleovestibular ganglion (CVG). In the chicken embryo otic vesicle, IGF-I regulates developmental death dynamics by regulating the activity and/or levels of key intracellular molecules, including lipid and protein kinases such as ceramide kinase, Akt and Jun N-terminal kinase (JNK). Mice lacking IGF-I lose many auditory neurons and present increased auditory thresholds at early postnatal ages. Neuronal loss associated to IGF-I deficiency is caused by apoptosis of the auditory neurons, which presented abnormally increased levels of activated caspase-3. It is worth noting that in man, homozygous deletion of the IGF-1 gene causes sensory-neural deafness. IGF-I is thus necessary for normal development and maintenance of the inner ear. The trophic actions of IGF-I in the inner ear suggest that this factor may have therapeutic potential for the treatment of hearing loss.

The inhibition of cyclin-dependent kinases induces differentiation of supernumerary hair cells and Deiters' cells in the developing organ of Corti

In the embryonic day 19 organs of Corti, we showed that roscovitine, a chemical inhibitor of cyclin-dependent kinases (CDKs), significantly increased the number of hair cells (HCs) and corresponding supporting cells (SCs) by triggering differentiation of precursor cells without interacting with cell proliferation. The effect of roscovitine was mimicked by other CDK1, 2, 5, and 7 inhibitors but not by CDK4/6 and mitogen-activated protein kinase pathway antagonists. Immunohistochemical analysis indicated that roscovitine-specific intracellular targets, CDK1, 2, 5, and 7, were expressed in the organ of Corti and especially in Hensen's cells. Affinity chromatography studies showed a tight correlation between the protein levels of CDK1/2 and 5 and the rate of roscovitine-induced supernumerary cells in the organ of Corti. In addition, we demonstrated that basal CDK activity was higher and more roscovitine-sensitive at developmental stages that are selectively permissive for the emergence of supernumerary cells. These results suggest that CDKs are involved in the normal development of the organ of Corti and that, at least in E19 embryos, inhibition of CDKs is sufficient to trigger the differentiation of HCs and corresponding SCs, presumably from the Hensen's cell progenitors and/or from progenitors located in the greater epithelial ridge area.