Induction of cell proliferation by fibroblast and insulin-like growth factors in pure rat inner ear epithelial cell cultures

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

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

Regeneration of Hair Cells in the Human Vestibular System

The vestibular system is a critical part of the human balance system, malfunction of this system will lead to balance disorders, such as vertigo. Mammalian vestibular hair cells, the mechanical receptors for vestibular function, are sensitive to ototoxic drugs and virus infection, and have a limited restorative capacity after damage. Considering that no artificial device can be used to replace vestibular hair cells, promoting vestibular hair cell regeneration is an ideal way for vestibular function recovery. In this manuscript, the development of human vestibular hair cells during the whole embryonic stage and the latest research on human vestibular hair cell regeneration is summarized. The limitations of current studies are emphasized and future directions are discussed.

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.

Progress in protecting vestibular hair cells

Vestibular hair cells are mechanosensory receptors that are capable of detecting changes in head position and thereby allow animals to maintain their posture and coordinate their movement. Vestibular hair cells are susceptible to ototoxic drugs, aging, and genetic factors that can lead to permanent vestibular dysfunction. Vestibular dysfunction mainly results from the injury of hair cells, which are located in the vestibular sensory epithelium. This review summarizes the mechanisms of different factors causing vestibular hair cell damage and therapeutic strategies to protect vestibular hair cells.

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

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

Development and regeneration of vestibular hair cells in mammals

Vestibular sensation is essential for gaze stabilization, balance, and perception of gravity. The vestibular receptors in mammals, Type I and Type II hair cells, are located in five small organs in the inner ear. Damage to hair cells and their innervating neurons can cause crippling symptoms such as vertigo, visual field oscillation, and imbalance. In adult rodents, some Type II hair cells are regenerated and become re-innervated after damage, presenting opportunities for restoring vestibular function after hair cell damage. This article reviews features of vestibular sensory cells in mammals, including their basic properties, how they develop, and how they are replaced after damage. We discuss molecules that control vestibular hair cell regeneration and highlight areas in which our understanding of development and regeneration needs to be deepened.

Directed Differentiation of Human Embryonic Stem Cells Toward Placode-Derived Spiral Ganglion-Like Sensory Neurons

The ability to generate spiral ganglion neurons (SGNs) from stem cells is a necessary prerequisite for development of cell-replacement therapies for sensorineural hearing loss. We present a protocol that directs human embryonic stem cells (hESCs) toward a purified population of otic neuronal progenitors (ONPs) and SGN-like cells. Between 82% and 95% of these cells express SGN molecular markers, they preferentially extend neurites to the cochlear nucleus rather than nonauditory nuclei, and they generate action potentials. The protocol follows an in vitro stepwise recapitulation of developmental events inherent to normal differentiation of hESCs into SGNs, resulting in efficient sequential generation of nonneuronal ectoderm, preplacodal ectoderm, early prosensory ONPs, late ONPs, and cells with cellular and molecular characteristics of human SGNs. We thus describe the sequential signaling pathways that generate the early and later lineage species in the human SGN lineage, thereby better describing key developmental processes. The results indicate that our protocol generates cells that closely replicate the phenotypic characteristics of human SGNs, advancing the process of guiding hESCs to states serving inner-ear cell-replacement therapies and possible next-generation hybrid auditory prostheses. © Stem Cells Translational Medicine 2017;6:923-936.

Induction of Functional Hair-Cell-Like Cells from Mouse Cochlear Multipotent Cells

In this paper, we developed a two-step-induction method of generating functional hair cells from inner ear multipotent cells. Multipotent cells from the inner ear were established and induced initially into progenitor cells committed to the inner ear cell lineage on the poly-L-lysine substratum. Subsequently, the committed progenitor cells were cultured on the mitotically inactivated chicken utricle stromal cells and induced into hair-cell-like cells containing characteristic stereocilia bundles. The hair-cell-like cells exhibited rapid permeation of FM1-43FX. The whole-cell patch-clamp technique was used to measure the membrane currents of cells differentiated for 7 days on chicken utricle stromal cells and analyze the biophysical properties of the hair-cell-like cells by recording membrane properties of cells. The results suggested that the hair-cell-like cells derived from inner ear multipotent cells were functional following differentiation in an enabling environment.

Transcription factor STOX1 regulates proliferation of inner ear epithelial cells via the AKT pathway

OBJECTIVES

Storkhead box 1 (STOX1) belongs to the forkhead family of transcription factors, and is reported to be involved in apoptosis of Caenorhabditis elegans. However, up to now the precise role of STOX1 in mammalian epithelial development has not been established. Here, we report that it plays an important role in regulation of proliferation of inner ear epithelial cells.

MATERIALS AND METHODS

Immunohistochemistry and reverse transcription-PCR assays were used to determine expression pattern of STOX1 in the mouse inner ear. Furthermore, its overexpression and knockdown effects on mouse inner ear epithelial cells were studied using RT-PCR, immunofluorescence, MTT assay, BrdU labelling and western blotting.

RESULTS

Storkhead box 1 was selectively expressed in epithelial cells, but not in stromal cells of the inner ear. Its over-expression enhanced cell proliferation and sphere formation, however, STOX1 knockdown inhibited cell proliferation and sphere formation in purified utricular epithelial cells in culture. Consistently, several cell cycle regulatory genes such as for PCNA, cyclin A and cyclin E, were up-regulated by STOX1 over-expression. Furthermore, biochemical analyses indicated that proliferation-promoting effects induced by STOX1 were mediated via phosphorylation of AKT in these cells.

CONCLUSIONS

Taken together, we demonstrate that STOX1 is a novel stimulatory factor for inner ear epithelial cell proliferation and might be an important target to be considered in regeneration or repair of inner ear epithelium.

Fibroblast growth factors stimulate hair growth through β-catenin and Shh expression in C57BL/6 mice

Growth factors are involved in the regulation of hair morphogenesis and cycle hair growth. The present study sought to investigate the hair growth promoting activities of three approved growth factor drugs, fibroblast growth factor 10 (FGF-10), acidic fibroblast growth factor (FGF-1), and basic fibroblast growth factor (FGF-2), and the mechanism of action. We observed that FGFs promoted hair growth by inducing the anagen phase in telogenic C57BL/6 mice. Specifically, the histomorphometric analysis data indicates that topical application of FGFs induced an earlier anagen phase and prolonged the mature anagen phase, in contrast to the control group. Moreover, the immunohistochemical analysis reveals earlier induction of β-catenin and Sonic hedgehog (Shh) in hair follicles of the FGFs-treated group. These results suggest that FGFs promote hair growth by inducing the anagen phase in resting hair follicles and might be a potential hair growth-promoting agent.

C-MYC transcriptionally amplifies SOX2 target genes to regulate self-renewal in multipotent otic progenitor cells

Sensorineural hearing loss is caused by the loss of sensory hair cells and neurons of the inner ear. Once lost, these cell types are not replaced. Two genes expressed in the developing inner ear are c-Myc and Sox2. We created immortalized multipotent otic progenitor (iMOP) cells, a fate-restricted cell type, by transient expression of C-MYC in SOX2-expressing otic progenitor cells. This activated the endogenous C-MYC and amplified existing SOX2-dependent transcripts to promote self-renewal. RNA-seq and ChIP-seq analyses revealed that C-MYC and SOX2 occupy over 85% of the same promoters. C-MYC and SOX2 target genes include cyclin-dependent kinases that regulate cell-cycle progression. iMOP cells continually divide but retain the ability to differentiate into functional hair cells and neurons. We propose that SOX2 and C-MYC regulate cell-cycle progression of these cells and that downregulation of C-MYC expression after growth factor withdrawal serves as a molecular switch for differentiation.

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A single factor, C-MYC, induces self-renewal in SOX2-expressing otic progenitors

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C-MYC transcriptionally amplifies SOX2 target genes

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SOX2 modulates transcription of cell-cycle genes

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Immortalized multipotent otic progenitors can differentiate into otic cell types

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.

Nestin expression and reactive phenomena in the mouse cochlea after kanamycin ototoxicity

Following injury to the adult mammalian cochlea, hair cells cannot be spontaneously replaced. Nonetheless, the postnatal cochlea contains progenitor cells, distinguished by the expression of nestin, which are able to proliferate and form neurospheres in vitro. Such resident progenitors might be endowed with reparative potential. However, to date little is known about their behaviour in situ following hair cell injury. Using adult mice and ex vivo cochlear cultures, we sought to determine whether: (i) resident cochlear progenitors respond to kanamycin ototoxicity and compensate for it; and (ii) the reparative potential of cochlear progenitors can be stimulated by the addition of growth factors. Morphological changes of cochlear tissue, expression of nestin mRNA and protein and cell proliferation were investigated in these models. Our observations show that ototoxic injury has modest effects on nestin expression and cell proliferation. On the other hand, the addition of growth factors to the injured cochlear explants induced the appearance of nestin-positive cells in the supporting cell area of the organ of Corti. The vast majority of nestin-expressing cells, however, were not proliferating. Growth factors also had a robust stimulatory effect on axonal sprouting and the proliferative response, which was more pronounced in injured cochleae. On the whole, our findings indicate that nestin expression after kanamycin ototoxicity is related to tissue reactivity rather than activation of resident progenitors attempting to replace the lost receptors. In addition, administration of growth factors significantly enhances tissue remodelling, suggesting that cochlear repair may be promoted by the exogenous application of regeneration-promoting substances.

Correct timing of proliferation and differentiation is necessary for normal inner ear development and auditory hair cell viability

BACKGROUND

Hearing restoration through hair cell regeneration will require revealing the dynamic interactions between proliferation and differentiation during development to avoid the limited viability of regenerated hair cells. Pax2-Cre N-Myc conditional knockout (CKO) mice highlighted the need of N-Myc for proper neurosensory development and possible redundancy with L-Myc. The late-onset hair cell death in the absence of early N-Myc expression could be due to mis-regulation of genes necessary for neurosensory formation and maintenance, such as Neurod1, Atoh1, Pou4f3, and Barhl1.

RESULTS

Pax2-Cre N-Myc L-Myc double CKO mice show that proliferation and differentiation are linked together through Myc and in the absence of both Mycs, altered proliferation and differentiation result in morphologically abnormal ears. In particular, the organ of Corti apex is re-patterned into a vestibular-like organization and the base is truncated and fused with the saccule.

CONCLUSIONS

These data indicate that therapeutic approaches to restore hair cells must take into account a dynamic interaction of proliferation and differentiation regulation of basic Helix-Loop-Helix transcription factors in attempts to stably replace lost cochlear hair cells. In addition, our data indicate that Myc is an integral component of the evolutionary transformation process that resulted in the organ of Corti development.

Coupling the cell cycle to development and regeneration of the inner ear

Cell cycle exit and acquirement of a postmitotic state is essential for the proper development of organs. In the present review, we examine the role of the cell cycle control in the sensory epithelia of the mammalian inner ear. We describe the roles of the core cell cycle regulators in the proliferation of prosensory cells and in the initiation and maintenance of terminal mitosis of the sensory epithelia. We also discuss how other intracellular signalling may influence the cell cycle. Finally, we address the question of whether manipulations of the cell cycle may have the potential to create replacement cells for the damaged inner sensory epithelia.

A brief history of hair cell regeneration research and speculations on the future

Millions of people worldwide suffer from hearing and balance disorders caused by loss of the sensory hair cells that convert sound vibrations and head movements into electrical signals that are conveyed to the brain. In mammals, the great majority of hair cells are produced during embryogenesis. Hair cells that are lost after birth are virtually irreplaceable, leading to permanent disability. Other vertebrates, such as fish and amphibians, produce hair cells throughout life. However, hair cell replacement after damage to the mature inner ear was either not investigated or assumed to be impossible until studies in the late 1980s proved this to be false. Adult birds were shown to regenerate lost hair cells in the auditory sensory epithelium after noise- and ototoxic drug-induced damage. Since then, the field of hair cell regeneration has continued to investigate the capacity of the auditory and vestibular epithelia in vertebrates (fishes, birds, reptiles, and mammals) to regenerate hair cells and to recover function, the molecular mechanisms governing these regenerative capabilities, and the prospect of designing biologically-based treatments for hearing loss and balance disorders. Here, we review the major findings of the field during the past 25 years and speculate how future inner ear repair may one day be achieved.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.

MYC gene delivery to adult mouse utricles stimulates proliferation of postmitotic supporting cells in vitro

The inner ears of adult humans and other mammals possess a limited capacity for regenerating sensory hair cells, which can lead to permanent auditory and vestibular deficits. During development and regeneration, undifferentiated supporting cells within inner ear sensory epithelia can self-renew and give rise to new hair cells; however, these otic progenitors become depleted postnatally. Therefore, reprogramming differentiated supporting cells into otic progenitors is a potential strategy for restoring regenerative potential to the ear. Transient expression of the induced pluripotency transcription factors, Oct3/4, Klf4, Sox2, and c-Myc reprograms fibroblasts into neural progenitors under neural-promoting culture conditions, so as a first step, we explored whether ectopic expression of these factors can reverse supporting cell quiescence in whole organ cultures of adult mouse utricles. Co-infection of utricles with adenoviral vectors separately encoding Oct3/4, Klf4, Sox2, and the degradation-resistant T58A mutant of c-Myc (c-MycT58A) triggered significant levels of supporting cell S-phase entry as assessed by continuous BrdU labeling. Of the four factors, c-MycT58A alone was both necessary and sufficient for the proliferative response. The number of BrdU-labeled cells plateaued between 5–7 days after infection, and then decreased ∼60% by 3 weeks, as many cycling cells appeared to enter apoptosis. Switching to differentiation-promoting culture medium at 5 days after ectopic expression of c-MycT58A temporarily attenuated the loss of BrdU-labeled cells and accompanied a very modest but significant expansion of the sensory epithelium. A small number of the proliferating cells in these cultures labeled for the hair cell marker, myosin VIIA, suggesting they had begun differentiating towards a hair cell fate. The results indicate that ectopic expression of c-MycT58A in combination with methods for promoting cell survival and differentiation may restore regenerative potential to supporting cells within the adult mammalian inner ear.

Hearing restoration in a deaf animal model with intravenous transplantation of mesenchymal stem cells derived from human umbilical cord blood

OBJECTIVE

This study was performed to confirm the effect of transplantation of human umbilical cord blood mesenchymal stem cells (UCB-MSCs) on hearing restoration in a sensorineural hearing loss (SNHL) animal model.

MATERIAL AND METHODS

UCB was collected from pregnant women after obtaining consent, and mesenchymal stem cells (MSCs) were extracted. We established an SNHL model and transplanted UCB-MSCs through the brachial vein of the guinea pigs. The animals were divided into 4 groups: animals with normal hearing, animals with SNHL, animals with SNHL and injected with saline, and animals with SNHL and transplanted with UCB-MSCs. Hearing tests were conducted at 1, 3, and 5 weeks, and the results were compared by grading auditory brainstem response (ABR) recordings and distortion product otoacoustic emissions (DPOAEs) for each treatment. Lastly, cochlear pathological features were examined, and surface preparations and morphological changes in each animal model were compared using hematoxylin and eosin staining and light microscopy studies.

RESULTS

In SNHL group, decreased DPOAEs and increased ABR threshold were noted. Furthermore, in the SNHL group, ABR hearing thresholds were unconverted and were similar to those observed in deafness. The transplanted UCB-MSC group showed a significant improvement in hearing threshold (40 dB) compared to that in all the SNHL group (80-90 dB). Examination of the SNHL animals' cochlear morphological features demonstrated a noticeable lack of spiral ganglion cells and also showed degenerated outer hair cells. However, the transplanted UCB-MSCs showed an increase in spiral ganglion and hair cells.

CONCLUSION

Intravenous transplantation of UCB-MSCs can enhance hearing thresholds, outer-hair cells and increase the number of spiral ganglion neurons (SGNs).

The myc road to hearing restoration

Current treatments for hearing loss, the most common neurosensory disorder, do not restore perfect hearing. Regeneration of lost organ of Corti hair cells through forced cell cycle re-entry of supporting cells or through manipulation of stem cells, both avenues towards a permanent cure, require a more complete understanding of normal inner ear development, specifically the balance of proliferation and differentiation required to form and to maintain hair cells. Direct successful alterations to the cell cycle result in cell death whereas regulation of upstream genes is insufficient to permanently alter cell cycle dynamics. The Myc gene family is uniquely situated to synergize upstream pathways into downstream cell cycle control. There are three Mycs that are embedded within the Myc/Max/Mad network to regulate proliferation. The function of the two ear expressed Mycs, N-Myc and L-Myc were unknown less than two years ago and their therapeutic potentials remain speculative. In this review, we discuss the roles the Mycs play in the body and what led us to choose them to be our candidate gene for inner ear therapies. We will summarize the recently published work describing the early and late effects of N-Myc and L-Myc on hair cell formation and maintenance. Lastly, we detail the translational significance of our findings and what future work must be performed to make the ultimate hearing aid: the regeneration of the organ of Corti.

Neural differentiation of bone marrow-derived mesenchymal stem cells: applicability for inner ear therapy

Background and Objectives

Regeneration or replacement of damaged hair cells and neurons in the cochlea might be an ideal treatment for sensorineural hearing loss (SNHL). The purpose of the present study was to investigate whether mesenchymal stem cells (MSCs), derived from the bone marrow of rats, could differentiate into auditory hair cells and neurons.

Materials and Methods

The centrifuge gradient method was used to isolate MSCs from the bone marrow of rats. To confirm whether bone marrow-derived MSCs can differentiate into neuronal cells, culture medium with glial cell-derived neurotrophic factor, brain-derived neurotrophic factor and neurotrophin-3 for 14 days. In addition, immunofluorescence staining and RT-PCR were performed for characterization of the neurospheres and differentiated cells from 7 and 14 day cultures.

Results

The results showed that MSCs could differentiate into neuron-positive and hair cell-positive cells, using different compositions of growth factors. And RT-PCR result was identified high or low of gene expression all these differentiated cells.

Conclusions

Rat bone marrow-derived MSCs differentiated into neuronal progenitor cells. These cells might be useful for the treatment of SNHL.

Regulated reprogramming in the regeneration of sensory receptor cells

Vision, olfaction, hearing, and balance are mediated by receptors that reside in specialized sensory epithelial organs. Age-related degeneration of the photoreceptors in the retina and the hair cells in the cochlea, caused by macular degeneration and sensorineural hearing loss, respectively, affect a growing number of individuals. Although sensory receptor cells in the mammalian retina and inner ear show only limited or no regeneration, in many nonmammalian vertebrates, these sensory epithelia show remarkable regenerative potential. We summarize the current state of knowledge of regeneration in the specialized sense organs in both nonmammalian vertebrates and mammals and discuss possible areas where new advances in regenerative medicine might provide approaches to successfully stimulate sensory receptor cell regeneration. The field of regenerative medicine is still in its infancy, but new approaches using stem cells and reprogramming suggest ways in which the potential for regeneration may be restored in individuals suffering from sensory loss.

Variations in shape-sensitive restriction points mirror differences in the regeneration capacities of avian and mammalian ears

When inner ear hair cells die, humans and other mammals experience permanent hearing and balance deficits, but non-mammalian vertebrates quickly recover these senses after epithelial supporting cells give rise to replacement hair cells. A postnatal decline in cellular plasticity appears to limit regeneration in mammalian balance organs, where declining proliferation responses are correlated with decreased spreading of supporting cells on artificial and native substrates. By culturing balance epithelia on substrates that differed in flexibility, we assessed spreading effects independent of age, showing a strong correlation between shape change and supporting cell proliferation. Then we made excision wounds in utricles cultured from young and old chickens and mice and compared quantified levels of spreading and proliferation. In utricles from young mice, and both young and old chickens, wounds re-epithelialized in <24 hours, while those in utricles from mature mice took three times longer. More cells changed shape in the fastest healing wounds, which accounted for some differences in the levels of proliferation, but inter-species and age-related differences in shape-sensitive restriction points, i.e., the cellular thresholds for shape changes that promote S-phase, were evident and may be particularly influential in the responses to hair cell losses in vivo.

In vitro cultured primary cells from a human utricle explant possesses hair cell like characteristics

The utricle is the enlarged portion of the membranous labyrinth of the inner ear and is essential for balance. It comprises of fine hair cells (mechanoreceptors), supporting cells and calcareous otoliths. Utricle cells are considered to be post-mitotic and possess a limited capacity for regeneration. Unlike birds and reptiles, mammalian mechanosensory hair cells do not regenerate. The in vitro culture of primary cells from the utricle and other inner ear structures of mammals have proven difficult. Presented here for the first time is the culture of primary cells derived from an explant of an adult human utricle, without any intervention or manipulation. Cells were proliferative until cellular quiescence occurred during passage six. Cell morphology was atypical of epithelial cells, appearing as a homogenous, slightly elongated population. Analysis of cultured utricle cells by immunofluorescent staining (IF) and reverse transcriptase polymerase chain reaction (RT-PCR) have shown these cells to possess epithelial (Epithelium-specific ets-1 (ESE-1)), supporting hair cell (p27(Kip1)), and hair cell specific (Atoh1 and Myosin VI) markers. Additionally, RT-PCR revealed positive gene expression for the proliferation control marker fibroblast growth factor receptor 1 (FGFR1) and negative gene expression for E-cadherin (CDH1), a vestibular cell differentiation marker.

Retention of progenitor cell phenotype in otospheres from guinea pig and mouse cochlea

BACKGROUND

Culturing otospheres from dissociated organ of Corti is an appropriate starting point aiming at the development of cell therapy for hair cell loss. Although guinea pigs have been widely used as an excellent experimental model for studying the biology of the inner ear, the mouse cochlea has been more suitable for yielding otospheres in vitro. The aim of this study was to compare conditions and outcomes of otosphere suspension cultures from dissociated organ of Corti of either mouse or guinea pig at postnatal day three (P3), and to evaluate the guinea pig as a potential cochlea donor for preclinical cell therapy.

METHODS

Organs of Corti were surgically isolated from P3 guinea pig or mouse cochlea, dissociated and cultivated under non-adherent conditions. Cultures were maintained in serum-free DMEM:F12 medium, supplemented with epidermal growth factor (EGF) plus either basic fibroblast growth factor (bFGF) or transforming growth factor alpha (TGFα). Immunofluorescence assays were conducted for phenotype characterization.

RESULTS

The TGFα group presented a number of spheres significantly higher than the bFGF group. Although mouse cultures yielded more cells per sphere than guinea pig cultures, sox2 and nestin distributed similarly in otosphere cells from both organisms. We present evidence that otospheres retain properties of inner ear progenitor cells such as self-renewal, proliferation, and differentiation into hair cells or supporting cells.

CONCLUSIONS

Dissociated guinea pig cochlea produced otospheres in vitro, expressing sox2 and nestin similarly to mouse otospheres. Our data is supporting evidence for the presence of inner ear progenitor cells in the postnatal guinea pig. However, there is limited viability for these cells in neonatal guinea pig cochlea when compared to the differentiation potential observed for the mouse organ of Corti at the same developmental stage.

Acceleration of articular cartilage repair by combined gene transfer of human insulin-like growth factor I and fibroblast growth factor-2 in vivo

INTRODUCTION

Improving the biochemical and structural qualities of the new tissue that fills deep osteochondral defects is critical to enhance articular cartilage repair. We developed a novel molecular therapy to increase articular cartilage repair based on a combined strategy to stimulate chondrogenesis by co-transfection of the human insulin-like growth factor I (IGF-I) and fibroblast growth factor 2 (FGF-2) in a xenogenic transplantation model.

MATERIALS AND METHODS

NIH 3T3 cells were transfected with expression plasmid vectors containing a cDNA for the E. coli lacZ gene (lacZ implants), the human IGF-I gene (IGF-I implants) or both the human IGF-I and FGF-2 genes (IGF-I/FGF-2 implants). The expression patterns of the transgenes were monitored in vitro for 21 days. LacZ, IGF-I and IGF-I/FGF-2 implants were transplanted into osteochondral defects in the trochlear groove of rabbits. At 3 weeks, the quality of articular cartilage repair was evaluated qualitatively and quantitatively.

RESULTS

Both IGF-I and IGF-I/FGF-2 implants secreted increased levels of the corresponding recombinant proteins in vitro. In vivo, transplantation of the co-transfected IGF-I/FGF-2 implants increased the DNA content of the repair tissue, accelerated the formation of the subchondral bone and improved articular cartilage repair in a magnitude that was larger than with IGF-I alone or when compared to lacZ implants.

CONCLUSION

These results suggest that gene delivery of a combination of IGF-I and FGF-2 to cartilage defects may be more beneficial than application of IGF-I alone.

cAMP-induced auditory supporting cell proliferation is mediated by ERK MAPK signaling pathway

Sensorineural hearing deficiencies result from the loss of auditory hair cells. This hearing loss is permanent in humans and mammals because hair cells are not spontaneously replaced. In other animals such as birds, this is not the case. Damage to the avian cochlea evokes proliferation of supporting cells and the generation of functionally competent replacement hair cells. Signal transduction pathways are clinically useful as potential therapeutic targets, so there is significant interest in identifying the key signal transduction pathways that regulate the formation of replacement hair cells. In a previous study from our lab, we showed that forskolin (FSK) treatment induces auditory supporting cell proliferation and formation of replacement hair cells in the absence of sound or aminoglycoside treatment. Here, we show that FSK-induced supporting cell proliferation is mediated by cell-specific accumulation of cyclic adenosine monophosphate (cAMP) in avian supporting cells and the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway. By a combination of immunostaining and pharmacological analyses, we show that FSK treatment increases cAMP levels in avian auditory supporting cells and that several ERK MAP inhibitors effectively block FSK-induced supporting cell proliferation. Next, we demonstrate by Western blotting and immunostaining analyses the expression of several ERK MAPK signaling molecules in the avian auditory epithelium and the cell-specific expression of B-Raf in avian auditory supporting cells. Collectively, these data suggest that FSK-induced supporting cell proliferation in the avian auditory epithelium is mediated by increases of cAMP levels in supporting cells and the cell-specific expression of the ERK MAPK family member B-Raf in supporting cells.

Human fetal auditory stem cells can be expanded in vitro and differentiate into functional auditory neurons and hair cell-like cells

In the quest to develop the tools necessary for a cell-based therapy for deafness, a critical step is to identify a suitable stem cell population. Moreover, the lack of a self-renovating model system for the study of cell fate determination in the human cochlea has impaired our understanding of the molecular events involved in normal human auditory development. We describe here the identification and isolation of a population of SOX2+OCT4+ human auditory stem cells from 9-week-old to 11-week-old fetal cochleae (hFASCs). These cells underwent long-term expansion in vitro and retained their capacity to differentiate into sensory hair cells and neurons, whose functional and electrophysiological properties closely resembled their in vivo counterparts during development. hFASCs, and the differentiating protocols defined here, could be used to study developing human cochlear neurons and hair cells, as models for drug screening and toxicity and may facilitate the development of cell-based therapies for deafness.

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.

Stem/progenitor cells derived from the cochlear sensory epithelium give rise to spheres with distinct morphologies and features

Nonmammalian vertebrates regenerate lost sensory hair cells by means of asymmetric division of supporting cells. Inner ear or lateral line supporting cells in birds, amphibians, and fish consequently serve as bona fide stem cells resulting in high regenerative capacity of hair cell-bearing organs. Hair cell regeneration does not happen in the mammalian cochlea, but cells with proliferative capacity can be isolated from the neonatal cochlea. These cells have the ability to form clonal floating colonies, so-called spheres, when cultured in nonadherent conditions. We noticed that the sphere population derived from mouse cochlear sensory epithelium cells was heterogeneous, consisting of morphologically distinct sphere types, hereby classified as solid, transitional, and hollow. Cochlear sensory epithelium-derived stem/progenitor cells initially give rise to small solid spheres, which subsequently transition into hollow spheres, a change that is accompanied by epithelial differentiation of the majority of sphere cells. Only solid spheres, and to a lesser extent, transitional spheres, appeared to harbor self-renewing stem cells, whereas hollow spheres could not be consistently propagated. Solid spheres contained significantly more rapidly cycling Pax-2-expressing presumptive otic progenitor cells than hollow spheres. Islet-1, which becomes upregulated in nascent sensory patches, was also more abundant in solid than in hollow spheres. Likewise, hair cell-like cells, characterized by the expression of multiple hair cell markers, differentiated in significantly higher numbers in cell populations derived from solid spheres. We conclude that cochlear sensory epithelium cell populations initially give rise to small solid spheres that have self-renewing capacity before they subsequently convert into hollow spheres, a process that is accompanied by loss of stemness and reduced ability to spontaneously give rise to hair cell-like cells. Solid spheres might, therefore, represent the most suitable sphere type for cell-based assays or animal model transplantation studies aimed at development of cell replacement therapies.

Reinforcement of cell junctions correlates with the absence of hair cell regeneration in mammals and its occurrence in birds

Debilitating hearing and balance deficits often arise through damage to the inner ear's hair cells. For humans and other mammals, such deficits are permanent, but nonmammalian vertebrates can quickly recover hearing and balance through their innate capacity to regenerate hair cells. The biological basis for this difference has remained unknown, but recent investigations in wounded balance epithelia have shown that proliferation follows cellular spreading at sites of injury. As mammalian ears mature during the first weeks after birth, the capacity for spreading and proliferation declines sharply. In seeking the basis for those declines, we investigated the circumferential bands of F-actin that bracket the apical junctions between supporting cells in the gravity-sensitive utricle. We found that those bands grow much thicker as mice and humans mature postnatally, whereas their counterparts in chickens remain thin from hatching through adulthood. When we cultured utricular epithelia from chickens, we found that cellular spreading and proliferation both continued at high levels, even in the epithelia from adults. In contrast, the substantial reinforcement of the circumferential F-actin bands in mammals coincides with the steep declines in cell spreading and production established in earlier experiments. We propose that the presence of thin F-actin bands at the junctions between avian supporting cells may contribute to the lifelong persistence of their capacity for shape change, cell proliferation, and hair cell replacement and that the postnatal reinforcement of the F-actin bands in maturing humans and other mammals may have an important role in limiting hair cell regeneration.

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.

Development and regeneration of hair cells

The vertebrate inner ear is derived from the otic placode and undergoes a complicated series of morphogenetic processes to differentiate into an elaborate structure harboring mechanosensory epithelia featuring hair cells, the mechanoreceptors of hearing and balance. Recently, the principal mechanisms producing hair cells and the key molecules involved in their fate determination and differentiation have been gradually unveiled. The in-depth understanding of hair cell development is consequently providing clues to strategies for mammalian hair cell regeneration. Among them, the identification and characterization of progenitor cells for the hair cell lineage, which is just emerging, is of particular interest. Herein, we review the molecular mechanisms of inner ear development with particular focus on perspectives for hair cell regeneration.

Basic helix-loop-helix gene Hes6 delineates the sensory hair cell lineage in the inner ear

The basic helix-loop-helix (bHLH) gene Hes6 is known to promote neural differentiation in vitro. Here, we report the expression and functional studies of Hes6 in the inner ear. The expression of Hes6 appears to be parallel to that of Math1 (also known as Atoh1), a bHLH gene necessary and sufficient for hair cell differentiation. Hes6 is expressed initially in the presumptive hair cell precursors in the cochlea. Subsequently, the expression of Hes6 is restricted to morphologically differentiated hair cells. Similarly, the expression of Hes6 in the vestibule is in the hair cell lineage. Hes6 is dispensable for hair cell differentiation, and its expression in inner ear hair cells is abolished in the Math1-null animals. Furthermore, the introduction of Hes6 into the cochlea in vitro is not sufficient to promote sensory or neuronal differentiation. Therefore, Hes6 is downstream of Math1 and its expression in the inner ear delineates the sensory lineage. However, the role of Hes6 in the inner ear remains elusive.

Insulin-like growth factor binding protein (IGFBP)-mediated hair cell survival on the mouse utricle exposed to neomycin: the roles of IGFBP-4 and IGFBP-5

CONCLUSION

This study suggests for the first time that 1) IGF-I, IGFBP-4, and -5 alone and IGF-I+IGFBP-5 mixture stimulated hair cell survival and prevented neomycin-induced hair cell loss in the sensory epithelial culture of mouse utricles, 2) When administered together, IGFBP-4 diminished the effect of IGF-I, 3) In P3-5 mice utricle, IGF-I, IGFBP-4, and IGFBP-5 are expressed in the cytoplasm of hair cells. And Insulin/IGF-I Receptor is expressed in the nucleus of hair cells.

OBJECTIVES

Several growth factors have been demonstrated to protect auditory sensory cells in vitro and in vivo from aminoglycoside toxicity. IGF-I is one of the most well-known mitogenic and protective substance working in the inner ear. However, there are no reports available regarding the function of IGFBPs in the inner ear. In the present study, the effects of IGFBP-4 and -5 on hair cell survival were investigated in mouse utriclular organ cultures.

MATERIALS AND METHODS

The amount of cellular damage and cell viability in vestibular organs were assessed by counting hair cells stained with a rhodamine-phalloidin probe. The expressions of IGFBP-4, IGFBP-5, IGF-IR, and IGF-I were localized by immunohistochemistry.

RESULTS

When treated with IGF-I, IGFBP-4, or IGFBP-5 for 24 h, explant culture showed hair cell survival rates of 136+/-18%, 140+/-15%, and 133+/-6%, respectively, compared to controls. Neomycin (1 mM) induced hair cell loss resulted in 45+/-17% of hair cell survival. However, pre-treatment of IGF-I, IGFBP-4, or -5 before neomycin insult showed survival rates of 113+/-14%, 98+/-8%, and 73+/-24%, respectively. Similar to IGF-I, IGFBP-4 and IGFBP-5 were significantly protective. IGFBP-4 and -5 immunoreactivities were observed in the cytoplasm of normal explanted vestibular hair cells as well as in the P3 mouse utricular hair cells in vivo.

Inner ear hair cells produced in vitro by a mesenchymal-to-epithelial transition

Sensory hair cell loss is a major contributor to disabling hearing and balance deficits that affect >250 million people worldwide. Sound exposures, infections, drug toxicity, genetic disorders, and aging all can cause hair cell loss and lead to permanent sensory deficits. Progress toward treatments for these deficits has been limited, in part because hair cells have only been obtainable via microdissection of the anatomically complex internal ear. Attempts to produce hair cells in vitro have resulted in reports of some success but have required transplantation into embryonic ears or coculturing with other tissues. Here, we show that avian inner ear cells can be cultured and passaged for months, frozen, and expanded to large numbers without other tissues. At any point from passage 6 up to at least passage 23, these cultures can be fully dissociated and then aggregated in suspension to induce a mesenchymal-to-epithelial transition that reliably yields new polarized sensory epithelia. Those epithelia develop numerous hair cells that are crowned by hair bundles, composed of a single kinocilium and an asymmetric array of stereocilia. These hair cells exhibit rapid permeance to FM1-43, a dye that passes through open mechanotransducing channels. Because a vial of frozen cells can now provide the capacity to produce bona fide hair cells completely in vitro, these discoveries should open new avenues of research that may ultimately contribute to better treatments for hearing loss and other inner ear disorders.

Stem cell therapy for hearing loss: Math1 overexpression in VOT-E36 cells

HYPOTHESIS

VOT-E36 cells acquire mechanosensitivity after mammalian atonal homolog 1 (Math1) overexpression.

BACKGROUND

VOT-E36 cells are derived from a population of epithelial cells in the ventral region of the otocyst at embryonic Day 10.5, before hair cell differentiation. These cells express a number of specific molecular markers for hair cells under both proliferation and differentiation states. Overexpression of Math1 can convert nonsensory epithelial cells into hair cells in the cochlea. Based on this information, we tested whether VOT-E36 cells can be converted into hair cells by Math1 overexpression.

METHODS

Using reverse transcriptase-polymerase chain reaction-based analysis, we first compared the expression patterns of various molecular markers for hair cell development in VOT-E36 cells between proliferation and differentiation states, and also before and after overexpression of Math1. Subsequently, with a standard calcium imaging method, we examined whether VOT-E36 cells overexpressing Math1 could detect mechanical vibrations and activate spiral ganglion neurons in a coculture model. In addition, using confocal and scanning electron microscopes, we examined morphologic changes of VOT-E36 cells after Math1 overexpression.

RESULTS

Consistent with previous reports, this study has shown that VOT-E36 cells express a number of specific molecular markers for hair cells in both proliferation and differentiation states. Under appropriate culture conditions, Math1 is transiently expressed in this cell line during conditional differentiation. In VOT-E36 cells overexpressing Math1, the normal expression pattern of certain molecular markers for mature hair cells is partially restored. Interestingly, after coculture with spiral ganglion neurons, VOT-E36 cells overexpressing Math1 are able to respond to mechanical vibrations and activate spiral ganglion neurons. Possible molecular mechanisms underlying this novel finding have been explored.

CONCLUSION

Math1 overexpression can partially restore presumably downstream signaling cascades for normal hair cell differentiation in VOT-E36 cells, which are able to detect mechanical vibrations after being cocultured with spiral ganglion neurons.

Proliferative responses to growth factors decline rapidly during postnatal maturation of mammalian hair cell epithelia

Millions of lives are affected by hearing and balance deficits that arise as a consequence of sensory hair cell loss. Those deficits affect mammals permanently, but hearing and balance recover in nonmammals after epithelial supporting cells divide and produce replacement hair cells. Hair cells are not effectively replaced in mammals, but balance epithelia cultured from the ears of rodents and adult humans can respond to hair cell loss with low levels of supporting cell proliferation. We have sought to stimulate vestibular proliferation; and we report here that treatment with glial growth factor 2 (rhGGF2) yields a 20-fold increase in cell proliferation within sheets of pure utricular hair cell epithelium explanted from adult rats into long-term culture. In epithelia from neonates, substantially greater proliferation responses are evoked by rhGGF2 alone, insulin alone and to a lesser degree by serum even during short-term cultures, but all these responses progressively decline during the first 2 weeks of postnatal maturation. Thus, sheets of utricular epithelium from newborn rats average > 40% labelling when cultured for 72 h with bromo-deoxyuridine (BrdU) and either rhGGF2 or insulin. Those from 5- and 6-day-olds average 8-15%, 12-day-olds average < 1% and after 72 h there is little or no labelling in epithelia from 27- and 35-day-olds. These cells are the mammalian counterparts of the progenitors that produce replacement hair cells in nonmammals, so the postnatal quiescence described here is likely to be responsible for at least part of the mammalian ear's unique vulnerability to permanent sensory deficits.

Differential distribution of stem cells in the auditory and vestibular organs of the inner ear

The adult mammalian cochlea lacks regenerative capacity, which is the main reason for the permanence of hearing loss. Vestibular organs, in contrast, replace a small number of lost hair cells. The reason for this difference is unknown. In this work we show isolation of sphere-forming stem cells from the early postnatal organ of Corti, vestibular sensory epithelia, the spiral ganglion, and the stria vascularis. Organ of Corti and vestibular sensory epithelial stem cells give rise to cells that express multiple hair cell markers and express functional ion channels reminiscent of nascent hair cells. Spiral ganglion stem cells display features of neural stem cells and can give rise to neurons and glial cell types. We found that the ability for sphere formation in the mouse cochlea decreases about 100-fold during the second and third postnatal weeks; this decrease is substantially faster than the reduction of stem cells in vestibular organs, which maintain their stem cell population also at older ages. Coincidentally, the relative expression of developmental and progenitor cell markers in the cochlea decreases during the first 3 postnatal weeks, which is in sharp contrast to the vestibular system, where expression of progenitor cell markers remains constant or even increases during this period. Our findings indicate that the lack of regenerative capacity in the adult mammalian cochlea is either a result of an early postnatal loss of stem cells or diminishment of stem cell features of maturing cochlear cells.

Differential enhancement of neural and photoreceptor cell differentiation of cultured pineal cells by FGF-1, IGF-1, and EGF

There are several common features between the pineal organ and the lateral eye in their developmental and evolutionary aspects. The avian pineal is a photoendocrine organ that originates from the diencephalon roof and represents a transitional type between the photosensory organ of lower vertebrates and the endocrine gland of mammals. Previous cell culture studies have shown that embryonic avian pineal cells retain a wide spectrum of differentiative capacities, although little is known about the mechanisms involved in their fate determination. In the present study, we investigated the effects of various cell growth factors on the differentiation of photoreceptor and neural cell types using pineal cell cultures from quail embryos. The results show that IGF-1 promotes differentiation of rhodopsin-immunoreactive cells, but had no effect on neural cell differentiation. Simultaneous administration of EGF and IGF-1 further enhanced differentiation of rhodopsin-immunoreactive cells, although the mechanism of the synergistic effect is unknown. FGF-1 did not stimulate proliferation of neural progenitor cells, but intensively promoted and maintained expression of a neural cell phenotype. FGF-1 appeared to lead to the conversion from an epithelial (endocrinal) to a neuronal type. It also enhanced phenotypic expression of retinal ganglion cell markers but rather suppressed expression of an amacrine cell marker. These results indicate that growth factors are important regulatory cues for pineal cell differentiation and suggest that they play roles in determining the fate of the pineal organ and the eye. It can be speculated that the differences in environmental cues between the retina and pineal may result in the transition of the pineal primordium from a potentially ocular (retinal) organ to a photoendocrine organ.

Bone marrow mesenchymal stem cells are progenitors in vitro for inner ear hair cells

Stem cells have been demonstrated in the inner ear but they do not spontaneously divide to replace damaged sensory cells. Mesenchymal stem cells (MSC) from bone marrow have been reported to differentiate into multiple lineages including neurons, and we therefore asked whether MSCs could generate sensory cells. Overexpression of the prosensory transcription factor, Math1, in sensory epithelial precursor cells induced expression of myosin VIIa, espin, Brn3c, p27Kip, and jagged2, indicating differentiation to inner ear sensory cells. Some of the cells displayed F-actin positive protrusions in the morphology characteristic of hair cell stereociliary bundles. Hair cell markers were also induced by culture of mouse MSC-derived cells in contact with embryonic chick inner ear cells, and this induction was not due to a cell fusion event, because the chick hair cells could be identified with a chick-specific antibody and chick and mouse antigens were never found in the same cell.

Einfluss von Ischämie/Hypoxie auf die HIF-1-Aktivität und Expression von hypoxieabhängigen Genen in der Kochlea der neugeborenen Ratte

BACKGROUND

Transcription factor HIF-1 (hypoxia-inducible factor-1) regulates the expression of genes which are involved in glucose supply, growth, metabolism, redox reactions and blood supply. Hypoxia and ischemia play an important role in the pathogenesis of tinnitus and hearing loss. Therefore, HIF-1 activity and the expression of HIF-1 dependent genes in the cochlea were examined under ischemic and hypoxic conditions.

MATERIAL AND METHODS

For the HIF-1 analysis, single-cell cultures of the organ of Corti (OC), stria vascularis (SV) and modiolus (MOD) were used. mRNA expression was analyzed in the organotypic culture using a microarray technique (RN U34-chip, Affymetrix).

RESULTS

Ischemia (hypoxia without glucose) and pure hypoxia increase the HIF-1 activity identically, with the highest increase found in MOD and OC. The HIF-1 alpha mRNA levels were found to be higher in SV than in the OC and MOD. During culturing, there is a clear increase in HIF-1 alpha mRNA and the expression of a number of HIF-1 dependent genes, such as Gapdh/glyceraldehyde-3-phosphate dehydrogenase, Slc2a1/solute carrier family 2 (facilitated glucose transporter), member 1, Tf/transferrin and Tfrc/transferrin receptor, in all three regions. In SV, MOD and OC, increase in the expression of Hmox1/hemoxygenase 1, Nos2/nitric oxide synthase, inducible and Tfrc is particularly high. Hypoxia (5 h) results in an increased expression of Igf2/Insulin-like growth factor 2.

CONCLUSION

The present data underline the contribution of radical forming processes to the pathogenesis of inner ear diseases. For experimental research, it is important to note that organotypic culture may be coupled with hypoxia.

Keynote review: The auditory system, hearing loss and potential targets for drug development

There is a huge potential market for the treatment of hearing loss. Drugs are already available to ameliorate predictable, damaging effects of excessive noise and ototoxic drugs. The biggest challenge now is to develop drug-based treatments for regeneration of sensory cells following noise-induced and age-related hearing loss. This requires careful consideration of the physiological mechanisms of hearing loss and identification of key cellular and molecular targets. There are many molecular cues for the discovery of suitable drug targets and a full range of experimental resources are available for initial screening through to functional analysis in vivo. There is now an unparalleled opportunity for translational research.