Effects of growth factors on the hair cells after ototoxic treatment of the neonatal mammalian cochlea in vitro

FOXO3/TGF-β signal-dependent ciliogenesis and cell functions during differentiation of temperature-sensitive mouse cochlear precursor hair cells

The transcription factor FOXO3 is necessary to preserve cochlear hair cells. Growth factors, including TGF-β, closely contribute to cochlear hair cell regeneration. In the present study, to investigate the roles of FOXO3 in the ciliogenesis and cell functions of cochlear hair cells, UB/OC-2 temperature-sensitive mouse cochlear precursor hair cells were treated with TGF-β receptor type 1 inhibitor EW-7197 or EGF receptor inhibitor AG-1478 after transfection with or without siRNA-FOXO3a. GeneChip analysis revealed that treatment with EW-7197 increased Foxo3 genes and decreased genes of Smads. During cell differentiation, treatment with EW-7197 or AG-1478 induced an increase in length of cilia-like structures that were positive for acetylated tubulin and inhibited cell migration. Treatment with EW-7197 also increased cell metabolism measured as mitochondrial basal respiration (oxygen consumption rate). The effects of EW-7197 were stronger than those of AG-1478. Knockdown of FOXO3 prevented the growth of cilia-like structures induced by EW-7197 or AG-1478 and induced cell migration under treatment with EW-7197. No change of the epithelial cell polarity molecule PAR3 was observed with any treatment. Treatment with the antimicrobial agent amikacin prevented the growth of cilia-like structures induced by EW-7197 and induced apoptosis. Pretreatment with the glucocorticoid dexamethasone inhibited the apoptosis induced by amikacin. This in vitro model of mouse cochlear hair cells suggests that FOXO3/TGF-β signaling plays a crucial role in ciliogenesis and cell functions during differentiation of cochlear hair cells. This model is useful for analysis of the mechanisms of hearing loss and to find therapeutic agents to prevent it.

In vitro culture of mammalian inner ear hair cells

Auditory function in vertebrates depends on the transduction of sound vibrations into electrical signals by inner ear hair cells. In general, hearing loss resulting from hair cell damage is irreversible because the human ear has been considered to be incapable of regenerating or repairing these sensory elements following severe injury. Therefore, regeneration and protection of inner ear hair cells have become an exciting, rapidly evolving field of research during the last decade. However, mammalian auditory hair cells are few in number, experimentally inaccessible, and barely proliferate postnatally in vitro. Various in vitro primary culture systems of inner ear hair cells have been established by different groups, although many challenges remain unresolved. Here, we briefly explain the structure of the inner ear, summarize the published methods of in vitro hair cell cultures, and propose a feasible protocol for culturing these cells, which gave satisfactory results in our study. A better understanding of in vitro hair cell cultures will substantially facilitate research involving auditory functions, drug development, and the isolation of critical molecules involved in hair cell biology.

Protein Synthesis Inhibition and Activation of the c-Jun N-Terminal Kinase Are Potential Contributors to Cisplatin Ototoxicity

Cisplatin has been regarded as an effective and versatile chemotherapeutic agent for nearly 40 years. Though the associated dose-dependent ototoxicity is known, the cellular mechanisms by which cochleovestibular hair cell death occur are not well understood. We have previously shown that aminoglycoside ototoxicity is mediated in part by cytosolic protein synthesis inhibition. Despite a lack of molecular similarity, aminoglycosides were shown to elicit similar stress pathways to cisplatin. We therefore reasoned that there may be some role of protein synthesis inhibition in cisplatin ototoxicity. Employing a modification of the bioorthogonal noncanonical amino acid tagging (BONCAT) method, we evaluated the effects of cisplatin on cellular protein synthesis. We show that cisplatin inhibits cellular protein synthesis in organ of Corti explant cultures. Similar to what was found after gentamicin exposure, cisplatin activates both the c-Jun N-terminal kinase (JNK) and mammalian target of rapamycin (mTOR) pathways. In contrast to aminoglycosides, cisplatin also inhibits protein synthesis in all cochlear cell types. We further demonstrate that the multikinase inhibitor sorafenib completely prevents JNK activation, while providing only moderate hair cell protection. Simultaneous stimulation of cellular protein synthesis by insulin, however, significantly improved hair cell survival in culture. The presented data provides evidence for a potential role of protein synthesis inhibition in cisplatin-mediated ototoxicity.

Development of an electrode for the artificial cochlear sensory epithelium

An artificial cochlear sensory epithelium has been developed on the basis of a new concept that the piezoelectric membrane, which converts mechanical distortion into electricity, can mimic the function of the inner hair cell and basilar membrane of the mammalian cochlea. Our previous research demonstrated that the piezoelectric membrane generated electrical outputs in response to the sound stimulation after implantation into the guinea pig cochlea, whereas electrodes for the stimulation of spiral ganglion neurons have not been fabricated, and a method to fix the device in the cochlea is also required to show proof-of-concept. In the present study, to achieve proof-of-concept of hearing recovery by implantation of the artificial cochlear sensory epithelium, we fabricated new electrodes that stick into the cochlear modiolus, which also play a role in the fixation of the device in the cochlea. The efficacy of new electrodes for fixation of the device in the cochlea and for the stimulation of spiral ganglion neurons was estimated in guinea pigs. Four weeks after implantation, we confirmed that the devices were in place. Histological analysis of the implanted cochleae revealed inconspicuous fibrosis and scar formation compared with the sham-operated specimens (n = 5 for each). The terminal deoxynucleotidyl transferase dUTP nick end labeling method was used to assess cell death due to surgical procedures in the cochleae that were harvested after 1 day (n = 6) and 7 days (n = 6) of implantation; there was no significant increase in apoptotic cell death in the implanted cochleae compared with sham-operated cochleae. In seven animals, serial measurements of electrically evoked auditory brainstem responses were obtained, with the electrode positioned in the scala tympani and with the electrode inserted into the cochlear modiolus. With the insertion of electrodes into the cochlear modiolus, significant reduction was achieved in the thresholds of electrically evoked auditory brainstem responses compared with those placed in the scala tympani (p = 0.028). These findings indicated that the new electrodes efficiently fixed the device in the cochlea and were able to stimulate spiral ganglion neurons.

Sox2-CreER mice are useful for fate mapping of mature, but not neonatal, cochlear supporting cells in hair cell regeneration studies

Studies of hair cell regeneration in the postnatal cochlea rely on fate mapping of supporting cells. Here we characterized a Sox2-CreER knock-in mouse line with two independent reporter mouse strains at neonatal and mature ages. Regardless of induction age, reporter expression was robust, with CreER activity being readily detectable in >85% of supporting cells within the organ of Corti. When induced at postnatal day (P) 28, Sox2-CreER activity was exclusive to supporting cells demonstrating its utility for fate mapping studies beyond this age. However, when induced at P1, Sox2-CreER activity was also detected in >50% of cochlear hair cells, suggesting that Sox2-CreER may not be useful to fate map a supporting cell origin of regenerated hair cells if induced at neonatal ages. Given that this model is currently in use by several investigators for fate mapping purposes, and may be adopted by others in the future, our finding that current protocols are effective for restricting CreER activity to supporting cells at mature but not neonatal ages is both significant and timely.

A randomized controlled clinical trial of topical insulin-like growth factor-1 therapy for sudden deafness refractory to systemic corticosteroid treatment

BACKGROUND

To date, no therapeutic option has been established for sudden deafness refractory to systemic corticosteroids. This study aimed to examine the efficacy and safety of topical insulin-like growth factor-1 (IGF-1) therapy in comparison to intratympanic corticosteroid therapy.

METHODS

We randomly assigned patients with sudden deafness refractory to systemic corticosteroids to receive either gelatin hydrogels impregnated with IGF-1 in the middle ear (62 patients) or four intratympanic injections with dexamethasone (Dex; 58 patients). The primary outcome was the proportion of patients showing hearing improvement (10 decibels or greater in pure-tone average hearing thresholds) 8 weeks after treatment. The secondary outcomes included the change in pure-tone average hearing thresholds over time and the incidence of adverse events.

RESULTS

In the IGF-1 group, 66.7% (95% confidence interval [CI], 52.9-78.6%) of the patients showed hearing improvement compared to 53.6% (95% CI, 39.7-67.0%) of the patients in the Dex group (P = 0.109). The difference in changes in pure-tone average hearing thresholds over time between the two treatments was statistically significant (P = 0.003). No serious adverse events were observed in either treatment group. Tympanic membrane perforation did not persist in any patient in the IGF-1 group, but did persist in 15.5% (95% CI, 7.3-27.4%) of the patients in the Dex group (P = 0.001).

CONCLUSIONS

The positive effect of topical IGF-1 application on hearing levels and its favorable safety profile suggest utility for topical IGF-1 therapy in patients with sudden deafness.

TRIAL REGISTRATION

UMIN Clinical Trials Registry Number UMIN000004366, October 30th, 2010.

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.

EGFR signaling is required for regenerative proliferation in the cochlea: conservation in birds and mammals

Proliferation and transdifferentiaton of supporting cells in the damaged auditory organ of birds lead to robust regeneration of sensory hair cells. In contrast, regeneration of lost auditory hair cells does not occur in deafened mammals, resulting in permanent hearing loss. In spite of this failure of regeneration in mammals, we have previously shown that the perinatal mouse supporting cells harbor a latent potential for cell division. Here we show that in a subset of supporting cells marked by p75, EGFR signaling is required for proliferation, and this requirement is conserved between birds and mammals. Purified p75+ mouse supporting cells express receptors and ligands for the EGF signaling pathway, and their proliferation in culture can be blocked with the EGFR inhibitor AG1478. Similarly, in cultured chicken basilar papillae, supporting cell proliferation in response to hair cell ablation requires EGFR signaling. In addition, we show that EGFR signaling in p75+ mouse supporting cells is required for the down-regulation of the cell cycle inhibitor p27(Kip1) (CDKN1b) to enable cell cycle re-entry. Taken together, our data suggest that a conserved mechanism involving EGFR signaling governs proliferation of auditory supporting cells in birds and mammals and may represent a target for future hair cell regeneration strategies.

Topical insulin-like growth factor 1 treatment using gelatin hydrogels for glucocorticoid-resistant sudden sensorineural hearing loss: a prospective clinical trial

BACKGROUND

Sudden sensorineural hearing loss (SSHL) is a common condition in which patients lose the hearing in one ear within 3 days. Systemic glucocorticoid treatments have been used as standard therapy for SSHL; however, about 20% of patients do not respond. We tested the safety and efficacy of topical insulin-like growth factor 1 (IGF1) application using gelatin hydrogels as a treatment for SSHL.

METHODS

Patients with SSHL that showed no recovery to systemic glucocorticoid administration were recruited. We applied gelatin hydrogels, impregnated with recombinant human IGF1, into the middle ear. The primary outcome measure was the proportion of patients showing hearing improvement 12 weeks after the test treatment. The secondary outcome measures were the proportion of patients showing improvement at 24 weeks and the incidence of adverse events. The null hypothesis was that 33% of patients would show hearing improvement, as was reported for a historical control after hyperbaric oxygen therapy.

RESULTS

In total, 25 patients received the test treatment at a median of 23 days (range 15-32) after the onset of SSHL, between 2007 and 2009. At 12 weeks after the test treatment, 48% (95% CI 28% to 69%; P = 0.086) of patients showed hearing improvement, and the proportion increased to 56% (95% CI 35% to 76%; P = 0.015) at 24 weeks. No serious adverse events were observed.

CONCLUSIONS

Topical IGF1 application using gelatin hydrogels is well tolerated and may be efficacious for hearing recovery in patients with SSHL that is resistant to systemic glucocorticoids.

Brain-derived nerve growth factor in the treatment of sensorineural hearing loss

OBJECTIVES/HYPOTHESIS

A possible medical treatment for sensorineural hearing loss using brain-derived nerve growth factor (BDNF) was explored. The hypothesis is that direct intracochlear application of BDNF will result in improved hearing.

STUDY DESIGN

Animal research study.

METHODS

Significant hearing loss was created using cisplatin in 11 guinea pigs. One month later, bilateral cochleostomies were performed placing 0.05 microg of BDNF in one cochlea of each animal prior to plugging with connective tissue. The other cochlea served as a control. Auditory brain-stem response (ABR) testing was then carried out for three months at 6,000, 8,000, 12,000, and 24,000 Hz.

RESULTS

ABR thresholds were better in the treated ear for all frequencies. Threshold differences were statistically significantly better two months after treatment (general linear model, repeated measures P = .045).

CONCLUSIONS

Intracochlear application of BDNF may prevent hearing loss.

A model for mammalian cochlear hair cell differentiation in vitro: effects of retinoic acid on cytoskeletal proteins and potassium conductances

We have established a model for the in-vitro differentiation of mouse cochlear hair cells and have used it to explore the influence of retinoic acid on proliferation, cytoskeletal proteins and voltage-gated potassium conductances. The model is based on the conditionally immortal cell line University of Sheffield/ventral otocyst-epithelial cell line clone 36 (US/VOT-E36), derived from ventral otic epithelial cells of the mouse at embryonic day 10.5 and transfected with a reporter for myosin VIIa. Retinoic acid did not increase cell proliferation but led to up-regulation of myosin VIIa and formation of prominent actin rings that gave rise to numerous large, linear actin bundles. Cells expressing myosin VIIa had larger potassium conductances and did not express the cyclin-dependent kinase inhibitor p27(kip1). US/VOT-E36 endogenously expressed the voltage-gated potassium channel alpha-subunits Kv1.3 and Kv2.1, which we subsequently identified in embryonic and neonatal hair cells in both auditory and vestibular sensory epithelia in vivo. These subunits could underlie the embryonic and neonatal delayed-rectifiers recorded in nascent hair cells in vivo. Kv2.1 was particularly prominent on the basolateral membrane of cochlear inner hair cells. Kv1.3 was distributed throughout all hair cells but tended to be localized to the cuticular plates. US/VOT-E36 recapitulates a coherent pattern of cell differentiation under the influence of retinoic acid and will provide a convenient model for screening the effects of other extrinsic factors on the differentiation of cochlear epithelial cell types in vitro.

Leupeptin, a Calpain Inhibitor, Protects Inner Ear Hair Cells from Aminoglycoside Ototoxicity

Inner ear hair cells play a major role in the auditory pathway that converts sound stimulation into electrical signals, and then into a neural code. However this function is often lost by aminoglycoside ototoxicity. The injury of inner ear hair cells from aminoglycoside treatment is considered apoptosis, and caspase is an important participant in the apoptosis pathway in many organs. It has been reported that calpain, a calcium-dependent protease, is essential for mediation and promotion of cell death. The purpose of the present study was to investigate effects of caspase and calpain inhibitors on the inner ear hair cells after aminoglycoside treatment, and to explore the cell death pathway. Cochlea explant cultures were prepared from mice of postnatal 6 days, cultured with neomycin and/or protease inhibitors, and then stained with phalloidin-fluorescein isothiocyanate (phalloidin-FITC), which was used as a marker to identify surviving hair cells. We demonstrated that neomycin (0.1-1 mM) reduced the number of outer hair cells in a dose-dependent manner. Furthermore, we showed that leupeptin, a calpain inhibitor, significantly protects against the neomycin-induced loss of outer hair cells, whereas a caspase inhibitor was effective only against a lower concentration of neomycin (0.2 mM). Using the TdT-mediated dUTP-biotin nick and labeling method, we also found that a calpain inhibitor, but not a caspase inhibitor, prevents apoptotic DNA fragmentation after treatment with 1 mM neomycin. These results suggest that calpain, rather than caspase, may be responsible for apoptosis induced by aminoglycoside. Thus, leupeptin may prevent hearing loss from aminoglycoside ototoxity.

In vitro growth and differentiation of mammalian sensory hair cell progenitors: a requirement for EGF and periotic mesenchyme

The sensory hair cells and supporting cells of the organ of Corti are generated by a precise program of coordinated cell division and differentiation. Since no regeneration occurs in the mature organ of Corti, loss of hair cells leads to deafness. To investigate the molecular basis of hair cell differentiation and their lack of regeneration, we have established a dissociated cell culture system in which sensory hair cells and supporting cells can be generated from mitotic precursors. By incorporating a Math1-GFP transgene expressed exclusively in hair cells, we have used this system to characterize the conditions required for the growth and differentiation of hair cells in culture. These conditions include a requirement for epidermal growth factor, as well as the presence of periotic mesenchymal cells. Lastly, we show that early postnatal cochlear tissue also contains cells that can divide and generate new sensory hair cells in vitro.

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.

Transforming growth factor-alpha-induced cellular changes in organotypic cultures of juvenile, amikacin-treated rat organ of corti

Hair cell losses in the mammalian cochlea following an ototoxic insult are irreversible. However, past studies have shown that amikacin treatment in rat cochleae resulted in the transient presence of atypical Deiters' cells (ACs) in the damaged organ of Corti. These ACs arise through a transformation of Deiters' cells, which produce, at their apical pole, densely packed microvilli reminiscent of early-differentiating stereociliary bundles. The ACs do not, however, express typical hair cell markers such as parvalbumin or calbindin. The present study was designed to determine whether specific growth factors could influence the survival and differentiation of these ACs and stimulate hair cell regeneration processes in vitro. Apical-medial segments of organ of Corti of juvenile amikacin-treated rats were established as organotypic cultures, and the effects of epidermal growth factor (EGF), insulin-like growth factor 1 (IGF-1), transforming growth factor-alpha (TGFalpha), and retinoic acid were studied using morphological and molecular approaches. Our results indicate that TGFalpha supports the survival of the damaged organ of Corti and influences ACs differentiation in vitro, possibly acting through reorganization of the actin cytoskeleton. These effects could be directly mediated through activation of the EGF receptor, which is expressed by supporting cells in the mature organ of Corti. TGFalpha does not, however, allow the ACs to progress towards a hair cell phenotype.

FGFR3 expression during development and regeneration of the chick inner ear sensory epithelia

Several studies suggest fibroblast growth factor receptor 3 (FGFR3) plays a role in the development of the auditory epithelium in mammals. We undertook a study of FGFR3 in the developing and mature chicken inner ear and during regeneration of this epithelium to determine whether FGFR3 shows a similar pattern of expression in birds. FGFR3 mRNA is highly expressed in most support cells in the mature chick basilar papilla but not in vestibular organs of the chick. The gene is expressed early in the development of the basilar papilla. Gentamicin treatment sufficient to destroy hair cells in the basilar papilla causes a rapid, transient downregulation of FGFR3 mRNA in the region of damage. In the initial stages of hair cell regeneration, the support cells that reenter the mitotic cycle in the basilar papilla do not express detectable levels of FGFR3 mRNA. However, once the hair cells have regenerated in this region, the levels of FGFR3 mRNA and protein expression rapidly return to approximate those in the undamaged epithelium. These results indicate that FGFR3 expression changes after drug-induced hair cell damage to the basilar papilla in an opposite way to that found in the mammalian cochlea and may be involved in regulating the proliferation of support cells.

Spatio-temporal distribution of cellular retinoid binding protein gene transcripts in the developing and the adult cochlea. Morphological and functional consequences in CRABP- and CRBPI-null mutant mice

The expression patterns of the mouse cellular retinoid binding protein genes were investigated by in situ hybridization analysis in the inner ear from 10.5 days post coïtum (dpc) up to the adult stage. The cellular retinoic acid binding protein II (CRABPII) and cellular retinol binding protein I (CRBPI) were present in a widespread and abundant pattern in cochlear structures during embryogenesis. Expression of the cellular retinoic acid binding protein I (CRABPI) is restricted during development in Kölliker's organ whilst cellular retinol binding protein II (CRBPII) is only visible after birth with a ubiquitous distribution in most regions of the cochlea including nervous components. No CRABP or CRBP transcripts were observed in the auditory receptors. Morphological observations of CRBPI- and CRABPI/CRABPII-null mutant fetus at 18.5 dpc do not show any structural modification at the level of the organ of Corti. Furthermore, electrophysiological tests performed by measuring distorsion-product otoacoustic emissions and auditory brainstem evoked responses did not present significant alteration of the auditory function for the different types of mutants. The expression of retinoid binding proteins in cochlear structures during embryogenesis could suggest important roles for these proteins during ontogenesis and morphogenesis of the inner ear. Despite these observations, morphological and functional data from mutant mice did not present obvious modifications of the cochlear structures and auditory thresholds. It is therefore unlikely that CRABPs and CRBPI are directly involved in development of the cochlea and hair cell differentiation.