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

[Organoids-the key to novel therapies for the inner ear? German version]

The sensitivity and the complexity of the human inner ear in conjunction with the lack of regenerative capacity are the main reasons for hearing loss and tinnitus. Progress in the development of protective and regenerative therapies for the inner ear often failed in the past not least due to the fact that no suitable model systems for cell biological and pharmacological in vitro studies were available. A novel technology for creating "mini-organs", so-called organoids, could solve this problem and has now also reached inner ear research. It makes it possible to produce inner ear organoids from cochlear stem/progenitor cells, embryonic and induced pluripotent stem cells that mimic the structural characteristics and functional properties of the natural inner ear. This review focuses on the biological basis of these inner ear organoids, the current state of research and the promising prospects that are now opening up for basic and translational inner ear research.

Adult porcine (Sus scrofa) derived inner ear cells: Characteristics in in-vitro cultures

There is a need for an animal model that closely parallels human cochlea gestational development. This study aims to document porcine inner ear anatomy, and in vitro porcine derived inner ear cell culture characteristics. Twenty-four temporal bone were harvested from 12 adult pigs (Sus scrofa). Six were formalin fixed and their maximal diameters were measured. The cochlea duct length was determined by the insertion length of a Nucleus 22 cochlear implant in two bones. Four formalin fixed bones were sectioned for histology. Cochlear and vestibular tissues were harvested from non-fixed bones, cultured and characterized at different passages (P). Gene and protein expression of multipotent stem/progenitor (Nestin and Sox2), inner ear hair (Myosin VIIa, Prestin) and supporting (Cytokeratin 18 and Vimentin) cell markers were determined. The porcine cochlea was a 3.5 turn spiral. There was a separate vestibular compartment. The cochlear mean maximal diameter and height was 7.99 and 3.77 mm, respectively. Sphere forming cells were identified on phase-contrast microscopy. The relative mRNA expression levels of KRT18, MYO7A and SLC26A5 were significantly positively correlated in cochlear cultures; and MYO7A and SLC26A5; SOX2 and KRT18; NES and SLC26A5 genes were positively correlated in vestibular cultures (p = .037, Spearman correlation [τ] = .900). Inner ear sensory and stem cell characteristics persist in passaged porcine inner ear cells. Further work is required to establish the usefulness of porcine inner ear cell cultures to the study of human inner ear disorders.

A Novel in vitro Model Delineating Hair Cell Regeneration and Neural Reinnervation in Adult Mouse Cochlea

The study of an adult mammalian auditory system, such as regeneration, has been hampered by the lack of an in vitro system in which hypotheses can be tested efficiently. This is primarily due to the fact that the adult inner ear is encased in the toughest bone of the body, whereas its removal leads to the death of the sensory epithelium in culture. We hypothesized that we could take advantage of the integral cochlear structure to maintain the overall inner ear architecture and improve sensory epithelium survival in culture. We showed that by culturing adult mouse cochlea with the (surrounding) bone intact, the supporting cells (SCs) survived and almost all hair cells (HCs) degenerated. To evaluate the utility of the explant culture system, we demonstrated that the overexpression of Atoh1, an HC fate-determining factor, is sufficient to induce transdifferentiation of adult SCs to HC-like cells (HCLCs). Transdifferentiation-derived HCLCs resemble developmentally young HCs and are able to attract adult ganglion neurites. Furthermore, using a damage model, we showed that degenerated adult ganglions respond to regenerated HCLCs by directional neurite outgrowth that leads to HCLC-neuron contacts, strongly supporting the intrinsic properties of the HCLCs in establishing HCLC-neuron connections. The adult whole cochlear explant culture is suitable for diverse studies of the adult inner ear including regeneration, HC-neuron pathways, and inner ear drug screening.

Murine cochlear cell sorting and cell-type-specific organoid culture

Neonatal mouse cochlear duct cells can proliferate and grow in vitro into inner ear organoids. Distinctive cochlear duct cell types have different organoid formation capacities. Here, we provide a flow cytometric cell-sorting method that allows the subsequent culture of individual cochlear cell populations. For the efficient culture of the sorted cells, we provide protocols for growing free-floating inner ear organoids, the adherence of organoids to a substrate, and the expansion of organoid-derived inner ear colonies.

For complete details on the use and execution of this protocol, please refer to Kubota et al. (2021).

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Flow cytometric sorting of mouse cochlear cells

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Culture of sorted cochlear cell populations and growth of inner ear organoids

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Adherent growth of inner ear organoid-derived colonies

Greater epithelial ridge cells are the principal organoid-forming progenitors of the mouse cochlea

In mammals, hearing loss is irreversible due to the lack of regenerative potential of non-sensory cochlear cells. Neonatal cochlear cells, however, can grow into organoids that harbor sensory epithelial cells, including hair cells and supporting cells. Here, we purify different cochlear cell types from neonatal mice, validate the composition of the different groups with single-cell RNA sequencing (RNA-seq), and assess the various groups' potential to grow into inner ear organoids. We find that the greater epithelial ridge (GER), a transient cell population that disappears during post-natal cochlear maturation, harbors the most potent organoid-forming cells. We identified three distinct GER cell groups that correlate with a specific spatial distribution of marker genes. Organoid formation was synergistically enhanced when the cells were cultured at increasing density. This effect is not due to diffusible signals but requires direct cell-to-cell contact. Our findings improve the development of cell-based assays to study culture-generated inner ear cell types.

Progenitor Cells from the Adult Human Inner Ear

Loss of inner ear hair cells leads to incurable balance and hearing disorders because these sensory cells do not effectively regenerate in humans. A potential starting point for therapy would be the stimulation of quiescent progenitor cells within the damaged inner ear. Inner ear progenitor/stem cells, which have been described in rodent inner ears, would be principal candidates for such an approach. Despite the identification of progenitor cell populations in the human fetal cochlea and in the adult human spiral ganglion, no proliferative cell populations with the capacity to generate hair cells have been reported in vestibular and cochlear tissues of adult humans. The present study aimed at filling this gap by isolating colony-forming progenitor cells from surgery- and autopsy-derived adult human temporal bones in order to generate inner ear cell types in vitro. Sphere-forming and mitogen-responding progenitor cells were isolated from vestibular and cochlear tissues. Clonal spheres grown from adult human utricle and cochlear duct were propagated for a limited number of generations. When differentiated in absence of mitogens, the utricle-derived spheres robustly gave rise to hair cell-like cells, as well as to cells expressing supporting cell-, neuron-, and glial markers, indicating that the adult human utricle harbors multipotent progenitor cells. Spheres derived from the adult human cochlear duct did not give rise to hair cell-like or neuronal cell types, which is an indication that human cochlear cells have limited proliferative potential but lack the ability to differentiate into major inner ear cell types. Anat Rec, 303:461-470, 2020. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

The biological strategies for hearing re-establishment based on the stem/progenitor cells

The cochlea is the essential organ for hearing and includes both auditory sensory hair cells and spiral ganglion neurons. The discovery of inner ear stem cell brings hope to the regeneration of hair cell and spiral ganglion neuron as well as the followed hearing re-establishment. Thus the investigation on characteristics of inner ear stem/progenitor cells and related regulating clue is important to make such regeneration a reality. In addition, attempts have also been made to transplant exogenous stem cells into the inner ear to restore hearing function. In this review, we describe recent advances in the characterization of mammalian inner ear progenitor/stem cells and the mechanisms of regulating their proliferation and differentiation, and summarize studies that have used exogenous stem cells to repair damaged hair cells and neurons in the inner ear.

[Strategies for a regenerative therapy of hearing loss. German version]

Despite impressive technical progress in the field of conventional hearing aids and implantable hearing systems, the hopes for the treatment of inner ear diseases such as hearing loss and tinnitus have become increasingly directed toward regenerative therapeutic approaches. This review discusses the currently most promising strategies for hair cell regeneration in the inner ear to treat hearing loss, including stem cell-based, gene transfer-based, and pharmacological interventions. Furthermore, previous milestones and ground-breaking work in this scientific field are identified. After many years of basic research, the first clinical trials with a regenerative therapeutic approach for hearing-impaired patients were recently initiated. Although there is still a long and bumpy road ahead until a true breakthrough is achieved, it seems more realistic than ever that regenerative therapies for the inner ear will find their way into clinical practice.

Decreased expression of TERT correlated with postnatal cochlear development and proliferation reduction of cochlear progenitor cells

Cochlear progenitor cells are considered as one of the best candidates for hair cell regeneration, thus, the regulation of cochlear progenitor cell proliferation has become a focus in this field. Several genes expressed in the inner ear during postnatal development have been demonstrated to be involved in maintaining the proliferative potential of progenitor cells, but the mechanism for regulating the proliferation and differentiation of cochlear progenitor cells remains poorly understood. Telomerase reverse transcriptase (TERT) has rate limiting telomerase activity and the overexpression of TERT has been shown to promote cell proliferation in series of cell lines. The aim of the present study was to evaluate the expression of TERT in the postnatal development of the cochlea and progenitor cells. The results demonstrated that TERT was expressed in the basilar membranes during the first postnatal week. In vitro, TERT expression in progenitor cells reached a maximum at day 4 after culture and decreased as the culture time prolonged or the cell passage number increased. These results led us to hypothesize that TERT may be involved in the development of the cochlea and in maintaining the proliferation ability of progenitor cells.

Microarray analyses of otospheres derived from the cochlea in the inner ear identify putative transcription factors that regulate the characteristics of otospheres

Various tissues possess tissue-specific stem/progenitor cells, including the inner ears. Stem/progenitor cells of the inner ear can be isolated as so-called otospheres from differentiated cells using a sphere forming assay. Although recent studies have demonstrated the characteristics of otospheres to some extent, most of the features of these cells are unknown. In this report, we describe the findings of transcriptome analyses with a cDNA microarray of otospheres derived from the cochleae of the inner ears of neonatal mice in order to clarify the gene expression profile of otic stem/progenitor cells. There were common transcription factors between otospheres and embryonic stem cells, which were supposed to be due to the stemness of otospheres. In comparison with the cochlear sensory epithelium, the otospheres shared characteristics with the cochlea, although several transcription factors specific for otospheres were identified. These transcription factors are expected to be essential for maintaining the characteristics of otospheres, and appear to be candidate genes that promote the direct conversion of cells into otic stem/progenitor cells.

Advances in translational inner ear stem cell research

Stem cell research is expanding our understanding of developmental biology as well as promising the development of new therapies for a range of different diseases. Within hearing research, the use of stem cells has focused mainly on cell replacement. Stem cells however have a broad range of other potential applications that are just beginning to be explored in the ear. Mesenchymal stem cells are an adult derived stem cell population that have been shown to produce growth factors, modulate the immune system and can differentiate into a wide variety of tissue types. Potential advantages of mesenchymal/adult stem cells are that they have no ethical constraints on their use. However, appropriate regulatory oversight seems necessary in order to protect patients from side effects. Disadvantages may be the lack of efficacy in many preclinical studies. But if proven safe and efficacious, they are easily translatable to clinical trials. The current review will focus on the potential application on mesenchymal stem cells for the treatment of inner ear disorders.

Cochlear epithelial of dog fetuses: a new source of multipotent stem cells

Hearing loss caused by the damage of cochlea sensory cells or neurons is a common human disease, but also affects dogs and other animals. To test their progenitor nature as potential value for future therapies, we characterized cells derived from the cochlear epithelium in dog fetuses. In total, 8 fetuses of 35-40 days of gestation, derived from castration campaigns, were investigated. Cells were analysed by the MTT colorimetric assay and in regard to cell cycle, differentiation capacities, immunophenotypes and qPCR analysis. In culture, cells had a fibroblast-like morphology. Phenotypic immunocharacterization showed positive staining for mesenchymal stem cell and pluripotency markers and were negative for hematopoietic cell markers. Cells possessed differentiation capacity for the three main cell lineages: osteogenic, adipogenic and chondrogenic, altogether indicating their nature as mesenchymal stem cells. Thus, cells derived from fetal cochlear tissues indeed may provide valuable sources of progenitor cells for cell therapy of canine deafness and other diseases.

Culture conditions have an impact on the maturation of traceable, transplantable mouse embryonic stem cell-derived otic progenitor cells

The generation of replacement inner ear hair cells (HCs) remains a challenge and stem cell therapy holds the potential for developing therapeutic solutions to hearing and balance disorders. Recent developments have made significant strides in producing mouse otic progenitors using cell culture techniques to initiate HC differentiation. However, no consensus has been reached as to efficiency and therefore current methods remain unsatisfactory. In order to address these issues, we compare the generation of otic and HC progenitors from embryonic stem (ES) cells in two cell culture systems: suspension vs. adherent conditions. In the present study, an ES cell line derived from an Atoh1-green fluorescent protein (GFP) transgenic mouse was used to track the generation of otic progenitors, initial HCs and to compare these two differentiation systems. We used a two-step short-term differentiation method involving an induction period of 5 days during which ES cells were cultured in the presence of Wnt/transforming growth factor TGF-β inhibitors and insulin-like growth factor IGF-1 to suppress mesoderm and reinforce presumptive ectoderm and otic lineages. The generated embryoid bodies were then differentiated in medium containing basic fibroblast growth factor (bFGF) for an additional 5 days using either suspension or adherent culture methods. Upon completion of differentiation, quantitative polymerase chain reaction analysis and immunostaining monitored the expression of otic/HC progenitor lineage markers. The results indicate that cells differentiated in suspension cultures produced cells expressing otic progenitor/HC markers at a higher efficiency compared with the production of these cell types within adherent cultures. Furthermore, we demonstrated that a fraction of these cells can incorporate into ototoxin-injured mouse postnatal cochlea explants and express MYO7A after transplantation. Copyright © 2016 John Wiley & Sons, Ltd.

Combinatorial enzymatic digestion with thermolysin and collagenase type I improved the isolation and culture effects of hair cell progenitors from rat cochleae

The high incidence of hearing loss in human combined with the lack of hair cell regeneration in mammalian cochleae had got the attention to manipulate stem/progenitor cells to participate in hair cell regeneration for years. Cochlear progenitor cells are considered as the best candidate for hair cell regeneration. However, there is not any effective and feasible way to separate hair cell progenitors from rat cochleae, yet. In this study, we tried to isolate single epithelial cells from rat basilar membrane by combinatorial enzymatic digestion with thermolysin and collagenase type I. The results showed that the harvested single cells gave rise to otospheres with features of stem cells and could be induced to differentiate into hair cells. Significantly, more otospheres of epithelial origin were obtained by digesting with thermolysin and collagenase type I. The combinatorial enzymatic digestion would be a potential method for hair cell progenitor isolation and culture with broad applications.

Transplantation and survival of mouse inner ear progenitor/stem cells in the organ of Corti after cochleostomy of hearing-impaired guinea pigs: preliminary results

In mammals, damage to sensory receptor cells (hair cells) of the inner ear results in permanent sensorineural hearing loss. Here, we investigated whether postnatal mouse inner ear progenitor/stem cells (mIESCs) are viable after transplantation into the basal turns of neomycin-injured guinea pig cochleas. We also examined the effects of mIESC transplantation on auditory functions. Eight adult female Cavia porcellus guinea pigs (250-350g) were deafened by intratympanic neomycin delivery. After 7 days, the animals were randomly divided in two groups. The study group (n=4) received transplantation of LacZ-positive mIESCs in culture medium into the scala tympani. The control group (n=4) received culture medium only. At 2 weeks after transplantation, functional analyses were performed by auditory brainstem response measurement, and the animals were sacrificed. The presence of mIESCs was evaluated by immunohistochemistry of sections of the cochlea from the study group. Non-parametric tests were used for statistical analysis of the data. Intratympanic neomycin delivery damaged hair cells and increased auditory thresholds prior to cell transplantation. There were no significant differences between auditory brainstem thresholds before and after transplantation in individual guinea pigs. Some mIESCs were observed in all scalae of the basal turns of the injured cochleas, and a proportion of these cells expressed the hair cell marker myosin VIIa. Some transplanted mIESCs engrafted in the cochlear basilar membrane. Our study demonstrates that transplanted cells survived and engrafted in the organ of Corti after cochleostomy.

Ginkgo Biloba Extract Attenuates Oxidative Stress and Apoptosis in Mouse Cochlear Neural Stem Cells

In the organ or Corti, oxidative stress could result in damage to the hearing, and neural stem cells (NSCs) hold great therapeutic potential in treating hearing loss. Ginkgo biloba extract (GBE) has been widely shown to exhibit anti-oxidative and anti-apoptotic effects in treatments of neural damage and disorder. Using hydrogen peroxide to induced oxidative stress as a model, we investigated the anti-oxidative role of GBE in isolated mouse cochlear NSCs. GBE treatment was found to significantly promote viability of NSCs, by markedly attenuating hydrogen peroxide induced oxidative stress. In addition, this anti-oxidative function of GBE was also able to prevent mitochondrial depolarization and subsequent apoptosis. Moreover, the anti-apoptotic role of GBE was mediated by antagonizing the intrinsic mitochondrial apoptotic pathway, where GBE could reverse the changes in key intrinsic apoptosis pathway factors including Bcl-2, Bax, and Caspase-3. Our data provided the first report on the beneficial role of GBE in protecting cochlear NSCs, by attenuating oxidative stress triggered intrinsic apoptosis, therefore supporting the potential therapeutic value of GBE in preventing oxidative stress-related hearing loss. Copyright © 2016 John Wiley & Sons, Ltd.

Initiating Differentiation in Immortalized Multipotent Otic Progenitor Cells

Use of human induced pluripotent stem cells (iPSC) or embryonic stem cells (ESC) for cell replacement therapies holds great promise. Several limitations including low yields and heterogeneous populations of differentiated cells hinder the progress of stem cell therapies. A fate restricted immortalized multipotent otic progenitor (iMOP) cell line was generated to facilitate efficient differentiation of large numbers of functional hair cells and spiral ganglion neurons (SGN) for inner ear cell replacement therapies. Starting from dissociated cultures of single iMOP cells, protocols that promote cell cycle exit and differentiation by basic fibroblast growth factor (bFGF) withdrawal were described. A significant decrease in proliferating cells after bFGF withdrawal was confirmed using an EdU cell proliferation assay. Concomitant with a decrease in proliferation, successful differentiation resulted in expression of molecular markers and morphological changes. Immunostaining of Cdkn1b (p27(KIP)) and Cdh1 (E-cadherin) in iMOP-derived otospheres was used as an indicator for differentiation into inner ear sensory epithelia while immunostaining of Cdkn1b and Tubb3 (neuronal β-tubulin) was used to identify iMOP-derived neurons. Use of iMOP cells provides an important tool for understanding cell fate decisions made by inner ear neurosensory progenitors and will help develop protocols for generating large numbers of iPSC or ESC-derived hair cells and SGNs. These methods will accelerate efforts for generating otic cells for replacement therapies.

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.

Cell cycle reactivation of cochlear progenitor cells in neonatal FUCCI mice by a GSK3 small molecule inhibitor

Due to the lack of regenerative capacity of the mammalian auditory epithelium, sensory hair cell loss results in permanent hearing deficit. Nevertheless, a population of tissue resident stem/progenitor cells has been recently described. Identification of methods to trigger their activity could lead to exploitation of their potential therapeutically. Here we validate the use of transgenic mice reporting cell cycle progression (FUCCI), and stemness (Lgr5-GFP), as a valuable tool to identify regulators of cell cycle re-entry of supporting cells within the auditory epithelium. The small molecule compound CHIR99021 was used to inhibit GSK3 activity. This led to a significant increase in the fraction of proliferating sphere-forming cells, labeled by the FUCCI markers and in the percentage of Lgr5-GFP + cells, as well as a selective increase in the fraction of S-G2-M cells in the Lgr5 + population. Using whole mount cultures of the organ of Corti we detected a statistically significant increment in the fraction of proliferating Sox2 supporting cells after CHIR99021 treatment, but only rarely appearance of novel MyoVIIa+/Edu + hair cells. In conclusion, these tools provide a robust mean to identify novel regulators of auditory organ regeneration and to clarify the contribution of stem cell activity.

Pseudo-immortalization of postnatal cochlear progenitor cells yields a scalable cell line capable of transcriptionally regulating mature hair cell genes

The mammalian cochlea is a highly specialized organ within the inner ear. Sensory hair cells (HC) in the cochlea detect and transduce sound waves into electrical impulses that are sent to the brain. Studies of the molecular pathways regulating HC formation are hindered by the very sparse nature of HCs, where only ~3300 are found within an entire mouse cochlea. Current cell lines mimic certain aspects of HCs but lack terminal HC marker expression. Here we successfully “pseudo-immortalized” cochlear progenitor cells using the “conditional reprogramming” technique. These cells, termed “Conditionally Reprogrammed Otic Stem Cells” (CR-OSC), are able to bypass the senescence inherent to cochlear progenitor cells without genetic alterations, allowing for the generation of over 15 million cells from a single cochlea. These cells can be differentiated and up-regulate both early and terminal differentiation genes associated with HCs, including the terminal HC differentiation marker prestin. CR-OSCs also respond to known HC cues, including upregulation of HC genes in response to Atoh1 overexpression, and upregulation of prestin expression after thyroid hormone application. Overall, we describe the creation of a HC line capable of regulated expression of HC genes that can easily be recreated in any laboratory from any mouse of interest.

Neural stem/progenitor cell properties of glial cells in the adult mouse auditory nerve

The auditory nerve is the primary conveyor of hearing information from sensory hair cells to the brain. It has been believed that loss of the auditory nerve is irreversible in the adult mammalian ear, resulting in sensorineural hearing loss. We examined the regenerative potential of the auditory nerve in a mouse model of auditory neuropathy. Following neuronal degeneration, quiescent glial cells converted to an activated state showing a decrease in nuclear chromatin condensation, altered histone deacetylase expression and up-regulation of numerous genes associated with neurogenesis or development. Neurosphere formation assays showed that adult auditory nerves contain neural stem/progenitor cells (NSPs) that were within a Sox2-positive glial population. Production of neurospheres from auditory nerve cells was stimulated by acute neuronal injury and hypoxic conditioning. These results demonstrate that a subset of glial cells in the adult auditory nerve exhibit several characteristics of NSPs and are therefore potential targets for promoting auditory nerve regeneration.

Ginkgo Biloba Extract Enhances Differentiation and Performance of Neural Stem Cells in Mouse Cochlea

Ginkgo biloba extract (GBE) has been widely used for treatment of neural damage and disorders. Neural stem cells (NSCs) hold promise as a treatment of hearing loss caused by neural damage. However, the biological functions of GBE in modulating NSC behaviors in the cochlea are still largely elusive. In this study, we sought to explore the effects of GBE on the differentiation and performance of NSCs from mouse cochlea. Our data showed that GBE treatment promotes cell survival and NSC proliferation. In addition, GBE treatment also increases NSC differentiation to neurons and enhances the performance of mature neural networks evident by the increased frequency of calcium oscillation. Moreover, neurite outgrowth is also dramatically increased upon GBE treatment. Overall, our study demonstrates the positive regulatory role of GBE in NSC proliferation and differentiation into functional neurons in vitro, supporting the potential therapeutic use of GBE in hearing loss recovery.

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.

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

Clonal colony formation from spiral ganglion stem cells

Neural stem cells from the central nervous system have the distinct capacity to give rise to clonal neurospheres. These clonal spheres are derived from a single clone-forming cell and represent homogenous, pure cell colonies. Recently, stem/progenitor cells have been isolated from the spiral ganglion of the inner ear using sphere-forming assays. However, the clonality of spiral ganglion-derived spheres has not yet been addressed in detail. Here, we report the isolation of clonal colonies from the spiral ganglion of early postnatal mice. We analyze sphere clonality using coculture experiments with transgenic cells, a semisolid assay, and culture of single cells in isolation. Our data show that sphere clonality differs in primary and secondary cultures and indicate that clonal sphere formation is dependent on specific culture parameters. We also show that the initiation of clonal colony formation does not require cell-to-cell interactions or paracrine signaling from surrounding cells. Generation of clonal colonies from spiral ganglion stem/progenitor cells might be crucial for future clinical applications because pure cell populations are considered to be more efficient and safe for therapeutic use than chimeric, heterogeneous spheres.

Spiral ganglion stem cells can be propagated and differentiated into neurons and glia

The spiral ganglion is an essential functional component of the peripheral auditory system. Most types of hearing loss are associated with spiral ganglion cell degeneration which is irreversible due to the inner ear's lack of regenerative capacity. Recent studies revealed the existence of stem cells in the postnatal spiral ganglion, which gives rise to the hope that these cells might be useful for regenerative inner ear therapies. Here, we provide an in-depth analysis of sphere-forming stem cells isolated from the spiral ganglion of postnatal mice. We show that spiral ganglion spheres have characteristics similar to neurospheres isolated from the brain. Importantly, spiral ganglion sphere cells maintain their major stem cell characteristics after repeated propagation, which enables the culture of spheres for an extended period of time. In this work, we also demonstrate that differentiated sphere-derived cell populations not only adopt the immunophenotype of mature spiral ganglion cells but also develop distinct ultrastructural features of neurons and glial cells. Thus, our work provides further evidence that self-renewing spiral ganglion stem cells might serve as a promising source for the regeneration of lost auditory neurons.

Comparing the cultivated cochlear cells derived from neonatal and adult mouse

Background

Previous reports showed the presence of limited numbers of stem cells in neonatal murine cochlear sensory epithelia and these cells are progressively lost during the postnatal development. The goal of this study was to investigate whether stem cells can be derived from mature mouse cochleae under suspension culture conditions, and to analyze the expression of the stem cell and inner ear progenitor cell markers in cells dissociated from neonatal and adult mouse organs of Corti.

Methods

Organs of Corti were dissected from postnatal day 1 (P1) or postnatal day 60 (P60) mouse. The dissociated cells were cultivated under suspension cultures conditions. Reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry were conducted for phenotype characterization.

Results

The number of cochlear stem cells (otospheres) yielded from P1 organ of Corti was significantly higher than that of the P60 organ of Corti. RT-PCR analyses showed that the stem markers, such as nanog, sox2, klf4, and nestin can be found to be distributed similarly in the cells derived from both of organisms, but the inner ear developmental/progenitor cell markers showed lower expression in P60 organ of Corti compared to P1. Immunocytochemistry results also revealed the evidence that P60 otospheres lacking of differentiation potential in vitro, which opposed to the strong differentiation potential of otospheres at P1 stage.

Conclusions

Our findings suggest that the loss of numbers and features of stem cells in the adult organ of Corti is associated with the substantial down-regulation of inner ear progenitor key-markers during maturation of the cells in organ of Corti.

Molecular mechanisms and potentials for differentiating inner ear stem cells into sensory hair cells

In mammals, hair cells may be damaged or lost due to genetic mutation, infectious disease, chemical ototoxicity, noise and other factors, causing permanent sensorineural deafness. Regeneration of hair cells is a basic pre-requisite for recovery of hearing in deaf animals. The inner ear stem cells in the organ of Corti and vestibular utricle are the most ideal precursors for regeneration of inner ear hair cells. This review highlights some recent findings concerning the proliferation and differentiation of inner ear stem cells. The differentiation of inner ear stem cells into hair cells involves a series of signaling pathways and regulatory factors. This paper offers a comprehensive analysis of the related studies.

The potential of stem cells for the restoration of auditory function in humans

Hearing loss is one of the most common disabilities, affecting approximately 10% of the population. Hair cells and spiral ganglion neurons are usually damaged in most cases of hearing loss. Currently, there is virtually no biological approach to replace damaged hearing cells. Recent developments in stem cell technology provide new opportunities for the treatment of deafness. Two major strategies have been investigated: differentiation of endogenous stem cells into new hair cells; and introduction of exogenous cells into the inner ear to substitute injured hearing neurons. Although there is still a learning curve in stem cell-based replacement, the probability exists to utilize personalized stem cells to eventually provide a novel intervention for patients with deafness in future clinical research trials.

Reprogramming of mouse cochlear cells by transcription factors to generate induced pluripotent stem cells

As an initial step for using technology derived from induced pluripotent stem cells (iPSCs) in the field of inner ear therapeutics, we examined the potential of four transcription factors, Oct3/4, Sox2, Klf4, and c-Myc, which are employed in the generation of iPSCs, for dedifferentiating cochlear epithelial cells. Otospheres, which are sphere-forming cells derived from dissociated cochlear epithelial cells of neonatal mice, were used as a cell source. The four transcription factors were introduced into otospheres using retroviral vectors. Virally transduced otospheres formed embryonic stem cell-like colonies that expressed markers for pluripotent stem cells and were capable of differentiating into the three germ layers in vivo and in vitro. These findings illustrate that viral transduction of four transcription factors can lead to reprogramming of cochlear epithelial cells, which may contribute to future studies of dedifferentiation of cochlear epithelial cells in tissue and identification of key molecules for otic induction.

The genetics of hair cell development and regeneration

Sensory hair cells are exquisitely sensitive vertebrate mechanoreceptors that mediate the senses of hearing and balance. Understanding the factors that regulate the development of these cells is important, not only to increase our understanding of ear development and its functional physiology but also to shed light on how these cells may be replaced therapeutically. In this review, we describe the signals and molecular mechanisms that initiate hair cell development in vertebrates, with particular emphasis on the transcription factor Atoh1, which is both necessary and sufficient for hair cell development. We then discuss recent findings on how microRNAs may modulate the formation and maturation of hair cells. Last, we review recent work on how hair cells are regenerated in many vertebrate groups and the factors that conspire to prevent this regeneration in mammals.

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.

Otospheres derived from neonatal mouse cochleae retain the progenitor cell phenotype after ex vivo expansions

Because of their limited regenerative potential, cochlear hair cell loss is one of the major causes of permanent hearing loss in mammals. However, recent studies have shown that postnatal cochlear epithelia retain the progenitor cells that form otospheres. Otospheres are capable of self-renewing and differentiating into inner ear cell lineages, thereby suggesting a promising source for hair cell regeneration. We investigated retention of the progenitor cell phenotype in otospheres after ex vivo expansion, which is crucial for transplantation approaches. Reverse transcriptase-polymerase chain reaction and immunocytochemical analyses showed that otospheres derived from neonatal mice retained expression of stem and cochlear cell markers. After in vitro differentiation, otosphere-consisting cells differentiated into hair cell phenotypes after ex vivo expansion. However, the capacity of otospheres for self-renewal weakened with subsequent generations of ex vivo expansion. Our results indicate that ex vivo expanded-otospheres are useful experimental tools for studying hair cell regeneration in transplantation approaches and that the mechanisms for retention of the progenitor cell phenotype in otospheres should be investigated.

Transcriptomic analysis of the developing and adult mouse cochlear sensory epithelia

The adult mammalian cochlea lacks regenerative ability and the irreversible degeneration of cochlear sensory hair cells leads to permanent hearing loss. Previous data show that early postnatal cochlea harbors stem/progenitor-like cells and shows a limited regenerative/repair capacity. These properties are progressively lost later during the postnatal development. Little is known about the genes and pathways that are potentially involved in this difference of the regenerative/repair potentialities between early postnatal and adult mammalian cochlear sensory epithelia (CSE). The goal of our study is to investigate the transcriptomic profiles of these two stages. We used Mouse Genome 430 2.0 microarray to perform an extensive analysis of the genes expressed in mouse postnatal day-3 (P3) and adult CSE. Statistical analysis of microarray data was performed using SAM (Significance Analysis of Microarrays) software. We identified 5644 statistically significant differentially expressed transcripts with a fold change (FC) >2 and a False Discovery Rate (FDR) ≤0.05. The P3 CSE signature included 3,102 transcripts, among which were known genes in the cochlea, but also new transcripts such as, Hmga2 (high mobility group AT-hook 2) and Nrarp (Notch-regulated ankyrin repeat protein). The adult CSE overexpressed 2,542 transcripts including new transcripts, such as Prl (Prolactin) and Ar (Androgen receptor), that previously were not known to be expressed in the adult cochlea. Our comparative study revealed important genes and pathways differentially expressed between the developing and adult CSE. The identification of new candidate genes would be useful as potential markers of the maintenance or the loss of stem cells and regenerative/repair ability during mammalian cochlear development.

Stemness of the organ of Corti relates to the epigenetic status of Sox2 enhancers

In the adult mammalian auditory epithelium, the organ of Corti, loss of sensory hair cells results in permanent hearing loss. The underlying cause for the lack of regenerative response is the depletion of otic progenitors in the cell pool of the sensory epithelium. Here, we show that an increase in the sequence-specific methylation of the otic Sox2 enhancers NOP1 and NOP2 is correlated with a reduced self-renewal potential in vivo and in vitro; additionally, the degree of methylation of NOP1 and NOP2 is correlated with the dedifferentiation potential of postmitotic supporting cells into otic stem cells. Thus, the stemness the organ of Corti is related to the epigenetic status of the otic Sox2 enhancers. These observations validate the continued exploration of treatment strategies for dedifferentiating or reprogramming of differentiated supporting cells into progenitors to regenerate the damaged organ of Corti.

MicroRNA-182 regulates otocyst-derived cell differentiation and targets T-box1 gene

BACKGROUND

Recently, in vitro and in vivo models have identified that microRNAs (miRNAs), which are extensively expressed in the inner ear, play important roles in inner ear development and function. However, the function of miRNA in vertebrate tissue is not well understood.

RESULTS

The current study used an in vitro model of embryonic mouse inner ear in a stem/progenitor cell culture to demonstrate that: 1) miR-182 is expressed during differentiation of inner ear stem/progenitor cell into a hair cell-like fate, 2) ectopic miR-182 promotes inner ear stem/progenitor cell differentiation into a hair cell-like fate, and 3) the function of miR-182 may be associated with its putative target Tbx1, a transcription factors that have been implicated in inner ear development and hair cell fate.

CONCLUSIONS

Our findings suggest that miR-182 could regulate inner ear progenitor cell differentiation and that miRNAs are important regulators of hair cell differentiation, providing new targets for hair cell repair.

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.

Isolating LacZ-expressing cells from mouse inner ear tissues using flow cytometry

Isolation of specific cell types allows one to analyze rare cell populations such as stem/progenitor cells. Such an approach to studying inner ear tissues presents a unique challenge because of the paucity of cells of interest and few transgenic reporter mouse models. Here, we describe a protocol using fluorescence-conjugated probes to selectively label LacZ-positive cells from the neonatal cochleae. The most common underlying pathology of sensorineural hearing loss is the irreversible damage and loss of cochlear sensory hair cells, which are required to transduce sound waves to neural impulses. Recent evidence suggests that the murine auditory and vestibular organs harbor stem/progenitor cells that may have regenerative potential. These findings warrant further investigation, including identifying specific cell types with stem/progenitor cell characteristics. The Wnt signaling pathway has been demonstrated to play a critical role in maintaining stem/progenitor cell populations in several organ systems. We have recently identified Wnt-responsive Axin2-expressing cells in the neonatal cochlea, but their function is largely unknown. To better understand the behavior of these Wnt-responsive cells in vitro, we have developed a method of isolating Axin2-expressing cells from cochleae of Axin2-LacZ reporter mice. Using flow cytometry to isolate Axin2-LacZ positive cells from the neonatal cochleae, we could in turn execute a variety of experiments on live cells to interrogate their behavior as stem/progenitor cells. Here, we describe in detail the steps for the microdissection of neonatal cochlea, dissociation of these tissues, labeling of the LacZ-positive cells using a fluorogenic substrate, and cell sorting. Techniques for dissociating cochleae into single cells and isolating cochlear cells via flow cytometry have been described. We have made modifications to these techniques to establish a novel protocol to isolate LacZ-expressing cells from the neonatal cochlea.

Sensory epithelial cells acquire features of prosensory cells via epithelial to mesenchymal transition

Epithelial to mesenchymal transition (EMT) plays a critical role during normal development and in adult tissue repair. It is known that immortalized epithelial cells can undergo an EMT and become cancer stem cells, and that epithelial cells from mouse pancreatic islet and avian inner ear can acquire mesenchymal traits in vitro via EMT. However, it is unclear whether epithelial cells from mammalian sensory system can undergo an EMT and obtain features of stem/progenitor cells. In this study, we used mouse utricle sensory epithelial cells (MUCs) as a mammalian cell model to address this issue. When cultured on 2-dimensional substrates, dissociated MUCs gradually lost their columnar shape and started to expand on the substrate with downregulation of expression of epithelial junction markers and upregulation of genes and proteins that are widely shown in mesenchymal cells. Moreover, MUCs expressed genes and proteins that are usually presented in prosensory epithelial cells and stem cells. These MUCs showed potential to differentiate into epithelial cells via a reverse EMT when they were forced to suspend in culture medium. Our findings reveal that sensory epithelial cells from mammalian tissue can undergo an EMT to become cells expressing features of stem cells that can be induced to become epithelial cells via a reverse EMT. The outcomes of this study may provide a novel approach to generate epithelial progenitors for use in cell replacement therapy to treat a number of human diseases, such as hearing loss and vision loss.

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.

Serial analysis of gene expression in the chicken otocyst

The inner ear arises from multipotent placodal precursors that are gradually committed to the otic fate and further differentiate into all inner ear cell types, with the exception of a few immigrating neural crest-derived cells. The otocyst plays a pivotal role during inner ear development: otic progenitor cells sub-compartmentalize into non-sensory and prosensory domains, giving rise to individual vestibular and auditory organs and their associated ganglia. The genes and pathways underlying this progressive subdivision and differentiation process are not entirely known. The goal of this study was to identify a comprehensive set of genes expressed in the chicken otocyst using the serial analysis of gene expression (SAGE) method. Our analysis revealed several hundred transcriptional regulators, potential signaling proteins, and receptors. We identified a substantial collection of genes that were previously known in the context of inner ear development, but we also found many new candidate genes, such as SOX4, SOX5, SOX7, SOX8, SOX11, and SOX18, which previously were not known to be expressed in the developing inner ear. Despite its limitation of not being all-inclusive, the generated otocyst SAGE library is a practical bioinformatics tool to study otocyst gene expression and to identify candidate genes for developmental studies.

Dynamic changes in microRNA expression during differentiation of rat cochlear progenitor cells in vitro

OBJECTIVE

Cochlear progenitor cells could be used to explore the cochlea developmental mechanism and for cell replacement therapy in deafness. MicroRNAs are small, noncoding RNAs that could regulate the cell fate of stem cells, as well as cellular proliferation, differentiation and maturation. An expression profile analysis of microRNAs is necessary to understand their complex roles in differentiating cochlear progenitor cells.

METHODS

The microRNAs microarray was used to analyze microRNA expression changes while differentiating cochlear progenitor cells. Quantitative real-time polymerase chain reaction was used to confirm and compare the results of the microarray and to detect the expression pattern of several microRNAs during the differentiation of neural stem cells.

RESULTS

Nearly 100 microRNAs were identified from the microarray. Most showed changes in expression levels as cochlear progenitor cell differentiation progressed. The quantitative real-time polymerase chain reaction result demonstrated that the miR-183 family exhibits cell-specific expression in cochlear progenitor cells compared with neural stem cells.

CONCLUSIONS

The temporal regulation of these microRNAs indicated that they might play different roles in differentiating cochlear progenitor cells, and that specific microRNAs might influence the cell fate determination of cochlear progenitor cells.

Intrinsic regenerative potential of murine cochlear supporting cells

The lack of cochlear regenerative potential is the main cause for the permanence of hearing loss. Albeit quiescent in vivo, dissociated non-sensory cells from the neonatal cochlea proliferate and show ability to generate hair cell-like cells in vitro. Only a few non-sensory cell-derived colonies, however, give rise to hair cell-like cells, suggesting that sensory progenitor cells are a subpopulation of proliferating non-sensory cells. Here we purify from the neonatal mouse cochlea four different non-sensory cell populations by fluorescence-activated cell sorting (FACS). All four populations displayed proliferative potential, but only lesser epithelial ridge and supporting cells robustly gave rise to hair cell marker-positive cells. These results suggest that cochlear supporting cells and cells of the lesser epithelial ridge show robust potential to de-differentiate into prosensory cells that proliferate and undergo differentiation in similar fashion to native prosensory cells of the developing inner ear.

Hypoxia enhances the stemness markers of cochlear stem/progenitor cells and expands sphere formation through activation of hypoxia-inducible factor-1 alpha

Unlike neural stem cells that maintain populations in the adult brains of both rodents and humans, cochlear stem cells appear to diminish in number after birth and may become quiescent in adult mammalian cochleae. Hypoxia has been observed to promote an undifferentiated cell state in various stem cell populations; however, little is known about such an effect on cochlear stem/progenitor cells (SPCs). The aims of this study were to assess the effect of hypoxia on cochlear SPCs and to examine the impact of hypoxia-inducible factor-1 alpha (Hif-1a) on regulating such an effect. Our data demonstrate that hypoxic culturing for 24 h significantly increased sphere formation and viability of cochlear SPCs compared with those cultured under normoxic conditions. Concurrent with these proliferation promotion effects are changes in the expression of multiple stemness and cell-cycle quiescent associated gene targets, including Abcg2, nestin, p27(Kip1)and Vegf. Knockdown of Hif-1a expression by small-interfering RNA inhibited hypoxia-induced cochlear SPC expansion and resulted in downregulation of Vegf, Abcg2, and nestin and upregulation of p27(Kip1) gene expression. These results suggest that Hif-1a plays an important role in the stimulation of the proliferation of cochlear SPCs, which confers a great benefit of expanding cochlear SPCs via hypoxic conditions.

[Characterization of stem cells derived from the neonatal auditory sensory epithelium]

BACKGROUND

In contrast to regenerating hair cell-bearing organs of nonmammalian vertebrates the adult mammalian organ of Corti appears to have lost its ability to maintain stem cells. The result is a lack of regenerative ability and irreversible hearing loss following auditory hair cell death. Unexpectedly, the neonatal auditory sensory epithelium has recently been shown to harbor cells with stem cell features. The origin of these cells within the cochlea's sensory epithelium is unknown.

MATERIAL AND METHODS

We applied a modified neurosphere assay to identify stem cells within distinct subregions of the neonatal mouse auditory sensory epithelium. Sphere cells were characterized by multiple markers and morphologic techniques.

RESULTS

Our data reveal that both the greater and the lesser epithelial ridge contribute to the sphere-forming stem cell population derived from the auditory sensory epithelium. These self-renewing sphere cells express a variety of markers for neural and otic progenitor cells and mature inner ear cell types.

CONCLUSION

Stem cells can be isolated from specific regions of the auditory sensory epithelium. The distinct features of these cells imply a potential application in the development of a cell replacement therapy to regenerate the damaged sensory epithelium.

Curing hearing loss: Patient expectations, health care practitioners, and basic science

Millions of patients are debilitated by hearing loss, mainly caused by degeneration of sensory hair cells in the cochlea. The underlying reasons for hair cell loss are highly diverse, ranging from genetic disposition, drug side effects, traumatic noise exposure, to the effects of aging. Whereas modern hearing aids offer some relief of the symptoms of mild hearing loss, the only viable option for patients suffering from profound hearing loss is the cochlear implant. Despite their successes, hearing aids and cochlear implants are not perfect. Particularly frequency discrimination and performance in noisy environments and general efficacy of the devises vary among individual patients. The advent of regenerative medicine, the publicity of stem cells and gene therapy, and recent scientific achievements in inner ear cell regeneration have generated an emerging spirit of optimism among scientists, health care practitioners, and patients. In this review, we place the different points of view of these three groups in perspective with the goal of providing an assessment of patient expectations, health care reality, and potential future treatment options for hearing disorders.

LEARNING OUTCOMES

(1) Readers will be encouraged to put themselves in the position of a hearing impaired patient or family member of a hearing impaired person. (2) Readers will be able to explain why diagnosis of the underlying pathology of hearing loss is difficult. (3) Readers will be able to list the main directions of current research aimed to cure hearing loss. (4) Readers will be able to understand the different viewpoints of patients and their relatives, health care providers, and scientists with respect to finding novel treatments for hearing loss.