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

Automated threshold detection for auditory brainstem responses: comparison with visual estimation in a stem cell transplantation study

BACKGROUND

Auditory brainstem responses (ABRs) are used to study auditory acuity in animal-based medical research. ABRs are evoked by acoustic stimuli, and consist of an electrical signal resulting from summated activity in the auditory nerve and brainstem nuclei. ABR analysis determines the sound intensity at which a neural response first appears (hearing threshold). Traditionally, threshold has been assessed by visual estimation of a series of ABRs evoked by different sound intensities. Here we develop an automated threshold detection method that eliminates the variability and subjectivity associated with visual estimation.

RESULTS

The automated method is a robust computational procedure that detects the sound level at which the peak amplitude of the evoked ABR signal first exceeds four times the standard deviation of the baseline noise. Implementation of the procedure was achieved by evoking ABRs in response to click and tone stimuli, under normal and experimental conditions (adult stem cell transplantation into cochlea). Automated detection revealed that the threshold shift from pre- to post-surgery hearing levels was similar in mice receiving stem cell transplantation or sham injection for click and tone stimuli. Visual estimation by independent observers corroborated these results but revealed variability in ABR threshold shifts and significance levels for stem cell-transplanted and sham-injected animals.

CONCLUSION

In summary, the automated detection method avoids the subjectivity of visual analysis and offers a rapid, easily accessible http://axograph.com/source/abr.html approach to measure hearing threshold levels in auditory brainstem response.

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.

Quo vadis, hair cell regeneration?

Hearing loss is a global health problem with profound socioeconomic impact. We contend that acquired hearing loss is mainly a modern disorder caused by man-made noise and modern drugs, among other causes. These factors, combined with increasing lifespan, have exposed a deficit in cochlear self-regeneration that was irrelevant for most of mammalian evolution. Nevertheless, the mammalian cochlea has evolved from phylogenetically older structures, which do have the capacity for self-repair. Moreover, nonmammalian vertebrates can regenerate auditory hair cells that restore sensory function. We will offer a critical perspective on recent advances in stem cell biology, gene therapy, cell cycle regulation and pharmacotherapeutics to define and validate regenerative medical interventions for mammalian hair cell loss. Although these advances are promising, we are only beginning to fully appreciate the complexity of the many challenges that lie ahead.

New ectopic vestibular hair cell-like cells induced by Math1 gene transfer in postnatal rats

Overexpression of Math1 has been demonstrated to induce robust ectopic new hair cells in greater epithelial ridge (GER) and lesser epithelial ridge (LER) of the postnatal rats' cochlear in vitro. In spite of the similarities between cochlear and vestibular epithelia in origin and structure, no similar results have been reported in the non-sensory region of vestibular epithelia in vitro. In the study, the adenoviral vectors inserted with Math1 gene were constructed to examine their effect on vestibular epithelia in postnatal rats. In vivo, the adenovirus vectors administered in vestibular perilymphatic or endolymphatic space, their transduction efficiency and other indexes are different. We set up a culture construction to simulate and show the differences. In the study, we also developed a new dissection protocol to be quick to harvest the whole maculae and cristae. The new ectopic vestibular hair cell-like cells induced by overexpression of Math1, were found to appear in the non-sensory region of the postnatal rats' vestibular epithelia as observed in the cochlear, and the number of the new cells was different when a different virus administration was simulated in vestibular perilymphatic or endolymphatic space, suggesting that the cells found could have the capability to differentiate into new hair cells. Our study might pave the way for further in vivo studies on vestibular Math1 adenoviral vector gene transfer.

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.

Feathers and fins: non-mammalian models for hair cell regeneration

Death of mechanosensory cells in the inner ear results in two profound disabilities: hearing loss and balance disorders. Although mammals lack the capacity to regenerate hair cells, recent studies in mice and other rodents have offered valuable insight into strategies for stimulating hair cell regeneration in mammals. Investigations of model organisms that retain the ability to form new hair cells after embryogenesis, such as fish and birds, are equally important and have provided clues as to the cellular and molecular mechanisms that may block hair cell regeneration in mammals. Here, we summarize studies on hair cell regeneration in the chicken and the zebrafish, discuss specific advantages of each model, and propose future directions for the use of non-mammalian models in understanding hair cell regeneration.

Recent advances in hair cell regeneration research

PURPOSE OF REVIEW

This review discusses recent progress in research that seeks to understand the regeneration of hair cells and highlights findings that may hold importance for the eventual development of regenerative therapies for hearing and balance impairments.

RECENT FINDINGS

Signaling via the Notch receptor and the basic helix-loop-helix transcription factors has important roles in the development and regeneration of hair cells. The cytoskeletal properties and cell-matrix interactions of supporting cells in mice of different ages may hold part of the explanation for the age-related differences in their proliferative responses to damage and the differences between mammals and nonmammals in hair cell regeneration. Progress also has been made in deriving stem cells from inner ear tissues and other sources and in the evaluation of their potential uses as sources of new hair cells and as tools for biomedical research.

SUMMARY

Much has been accomplished since the discovery of postembryonic hair cell production and hair cell regeneration in nonmammals decades ago. No therapies for hair cell regeneration are under clinical trials, but research is yielding potentially important discoveries that are likely to lead to the development of therapeutic methods for inducing hair cell regeneration in the mammalian inner ear.

Isolation of sphere-forming stem cells from the mouse inner ear

The mammalian inner ear has very limited ability to regenerate lost sensory hair cells. This deficiency becomes apparent when hair cell loss leads to hearing loss as a result of either ototoxic insult or the aging process. Coincidently, with this inability to regenerate lost hair cells, the adult cochlea does not appear to harbor cells with a proliferative capacity that could serve as progenitor cells for lost cells. In contrast, adult mammalian vestibular sensory epithelia display a limited ability for hair cell regeneration, and sphere-forming cells with stem cell features can be isolated from the adult murine vestibular system. The neonatal inner ear, however, does harbor sphere-forming stem cells residing in cochlear and vestibular tissues. Here, we provide protocols to isolate sphere-forming stem cells from neonatal vestibular and cochlear sensory epithelia as well as from the spiral ganglion. We further describe procedures for sphere propagation, cell differentiation, and characterization of inner ear cell types derived from spheres. Sphere-forming stem cells from the mouse inner ear are an important tool for the development of cellular replacement strategies of damaged inner ears and are a bona fide progenitor cell source for transplantation studies.

Directed differentiation of mouse cochlear neural progenitors in vitro

Multipotent cochlear neural progenitors (CNPs) in the organ of Corti hold the promise for cell replacement in degenerative hearing disorders. However, not much is known about the CNPs and the specific conditions for their differentiation. Here we isolate the CNPs from the postnatal day 1 organ of Corti in mice and demonstrate their capability to self-renew and to differentiate into hair cell-like and neuronal cell-like phenotypes under the guidance of sonic hedgehog (SHH), epidermal growth factor (EGF), retinoic acid (RA), and brain-derived neurotrophic factor (BDNF), herein termed SERB (abbreviation of SHH, EGF, RA, and BDNF) in an asymmetric or symmetric manner from clonal isolates. Differentiation of CNPs into hair cells by SERB was dependent on the ERK signaling pathway, whereas the differentiation of CNPs into neurons by SERB was not. This work develops a new in vitro methodology for the maintenance and self-regeneration of CNPs for future design of regenerative strategies for hearing disorders.

Inner ear drug delivery for auditory applications

Many inner ear disorders cannot be adequately treated by systemic drug delivery. A blood-cochlear barrier exists, similar physiologically to the blood-brain barrier, which limits the concentration and size of molecules able to leave the circulation and gain access to the cells of the inner ear. However, research in novel therapeutics and delivery systems has led to significant progress in the development of local methods of drug delivery to the inner ear. Intratympanic approaches, which deliver therapeutics to the middle ear, rely on permeation through tissue for access to the structures of the inner ear, whereas intracochlear methods are able to directly insert drugs into the inner ear. Innovative drug delivery systems to treat various inner ear ailments such as ototoxicity, sudden sensorineural hearing loss, autoimmune inner ear disease, and for preserving neurons and regenerating sensory cells are being explored.

Genetic and pharmacological intervention for treatment/prevention of hearing loss

Twenty years ago it was first demonstrated that birds could regenerate their cochlear hair cells following noise damage or aminoglycoside treatment. An understanding of how this structural and functional regeneration occurred might lead to the development of therapies for treatment of sensorineural hearing loss in humans. Recent experiments have demonstrated that noise exposure and aminoglycoside treatment lead to apoptosis of the hair cells. In birds, this programmed cell death induces the adjacent supporting cells to undergo regeneration to replace the lost hair cells. Although hair cells in the mammalian cochlea undergo apoptosis in response to noise damage and ototoxic drug treatment, the supporting cells do not possess the ability to undergo regeneration. However, current experiments on genetic manipulation, gene therapy, and stem cell transplantation suggest that regeneration in the mammalian cochlea may eventually be possible and may 1 day provide a therapeutic tool for hearing loss in humans.

LEARNING OUTCOMES

The reader should be able to: (1) Describe the anatomy of the avian and mammalian cochlea, identify the individual cell types in the organ of Corti, and distinguish major features that participate in hearing function, (2) Demonstrate a knowledge of how sound damage and aminoglycoside poisoning induce apoptosis of hair cells in the cochlea, (3) Define how hair cell loss in the avian cochlea leads to regeneration of new hair cells and distinguish this from the mammalian cochlea where there is no regeneration following damage, and (4) Interpret the potential for new approaches, such as genetic manipulation, gene therapy and stem cell transplantation, could provide a therapeutic approach to hair cell loss in the mammalian cochlea.

Hair cell regeneration

The mammalian inner ear largely lacks the capacity to regenerate hair cells, the sensory cells required for hearing and balance. Recent studies in both lower vertebrates and mammals have uncovered genes and pathways important in hair cell development and have suggested ways that the sensory epithelia could be manipulated to achieve hair cell regeneration. These approaches include the use of inner ear stem cells, transdifferentiation of nonsensory cells, and induction of a proliferative response in the cells that can become hair cells.

Regenerating cochlear hair cells: quo vadis stem cell

Many elderly people worldwide lose the neurosensory part of their ear and turn deaf. Cochlear implants to restore some hearing after neurosensory hearing loss are, at present, the only therapy for these people. In contrast to this therapy, replacement of hair cells via stem cell therapies holds the promise for a cure. We review here current insights into embryonic, adult, and inducible stem cells that might provide cells for seeding the cochlea with the hope of new hair cell formation. We propose a two-step approach using a first set of transcription factors to enhance the generation of inducible pluripotent stem (iPS) cells and a second set of factors to initiate the differentiation of hair cells. Recent evidence regarding ear development and stem cell research strongly suggest that microRNAs will be an important new regulatory factor in both iPS cell formation and differentiation to reprogram cells into hair cells. In addition, we highlight currently insurmountable obstacles to the successful transformation of stem cells into hair cell precursors and their injection into the cochlear canal to replace lost hair cells.

Stem cell transplantation for auditory nerve replacement

The successful function of cochlear prostheses depends on activation of auditory nerve. The survival of auditory nerve neurons, however, can vary widely in candidates for cochlear implants and influence implant efficacy. Stem cells offer the potential for improving the function of cochlear prostheses and increasing the candidate pool by replacing lost auditory nerve. The first phase of studies for stem cell replacement of auditory nerve has examined the in vitro survival and differentiation as well as in vivo differentiation and survival of exogenous embryonic and tissue stem cells placed into scala tympani and/or modiolus. These studies are reviewed and new results on in vivo placement of B-5 mouse embryonic stem cells into scala tympani of the guinea pig cochleae with differentiation into a glutamatergic neuronal phenotype are presented. Research on the integration and connections of stem cell derived neurons in the cochlea is described. Finally, an alternative approach is considered, based on the use of endogenous progenitors rather than exogenous stem cells, with a review of promising findings that have identified stem cell-like progenitors in cochlear and vestibular tissues to provide the potential for auditory nerve replacement.

Cochlear Repair by Transplantation of Human Cord Blood CD133+ Cells to Nod-Scid Mice Made Deaf with Kanamycin and Noise

We investigated the fate of human cord blood CD133+ hematopoietic stem cells (HSC) transplanted intravenously (IV) into irradiated nodscid mice previously made deaf by ototoxic treatment with kanamycin and/or intense noise, to verify whether HSC engraft the cochlea and contribute to inner ear restoration, in vivo. We tested the presence of HLA.DQα1 by PCR, used for traceability of engrafted cells, finding evidence that HSC migrated to various host tissues, including the organ of Corti (OC). By histology, antibody and lectin-staining analysis, we confirmed that HSC IV transplantation in mice previously damaged by ototoxic agents correlated with the repair process and stimulation ex novo of morphological recovery in the inner ear, while the cochlea of control oto-injured, nontransplanted mice remained seriously damaged. Dual color FISH analysis also provided evidence of positive engraftment in the inner ear and in various mouse tissues, also revealing small numbers of heterokaryons, probably derived from fusion of donor with endogenous cells, for up to 2 months following transplantation. These observations offer the first evidence that transplanted human HSC migrating to the inner ear of oto-injured mice may provide conditions for the resumption of deafened cochlea, emerging as a potential strategy for inner ear rehabilitation.

Cochlear stem/progenitor cells from a postnatal cochlea respond to Jagged1 and demonstrate that notch signaling promotes sphere formation and sensory potential

Hair cells and supporting cells of the mammalian cochlea terminally differentiate during development. Recent in vitro evidence suggests the presence of hair cell progenitors in the postnatal cochlea. Phenotypic properties of these cells and factors that promote their ability to generate spheres in aggregate cultures have not been reported. We define an in vitro system that allows stem/progenitor cells harvested from the early postnatal cochlea to develop into spheres. These spheres contain Abcg2, Jagged1 and Notch1 positive progenitor cells that can divide and generate new hair cell-like cells, i.e. immunopositive for specific hair cell markers, including Myosin VI, Myosin VIIa, Math1 and ability to uptake FM1-43. We demonstrate that reducing Notch signaling with a gamma secretase inhibitor decreases the number of spheres generated following treatment of the stem/progenitor cell cultures. Additionally, activation of Notch by an exogenous soluble form of a Notch ligand, i.e. Jagged1 protein, promotes sphere formation and the sensory potential of cochlear stem/progenitor cells. Our findings suggest that Notch1/Jagged1 signaling plays a role in maintaining a population of Abcg2 sensory stem/progenitor cells in the postnatal cochlea.

[Regenerative medicine in the treatment of sensorineural hearing loss]

Regenerative medicine offers the prospect of causal treatment of sensorineural hearing loss. In humans, the loss of sensory hair cells is irreversible and results in chronic hearing loss. Other vertebrates, particularly birds, have the capability to spontaneously regenerate lost sensory hair cells and restore hearing. In the bird model, regeneration of hair cells is based on the proliferation of supporting cells. In mammals, supporting cells have lost their proliferative capacity and are terminally differentiated. To gain an understanding about regeneration of hair cells in mammals, cell division of supporting cells has to be controlled. Gene disruption of the cell cycle inhibitor p27(Kip1) allows supporting cell proliferation in the organ of Corti in vivo. Furthermore, in vitro studies indicate that newly generated cells may differentiate into hair cells after p27(Kip1) disruption. Other current methods to induce hair cell regeneration include the gene transfer of Math1 and transplantation of stem cells to the inner ear.

Whole organ culture of the postnatal sensory inner ear in simulated microgravity

Among the three major biological in vitro models, cell culture, tissue culture, and organ culture, the latter provides the closest approximation to the in vivo situation, but also requires the most demanding culture conditions. Due to its small size and complex tissue architecture, the mammalian inner ear provides a particular challenge to the development of whole organ culture. Using a rotating bioreactor system with simulated microgravity conditions, the entire mouse inner ear organ can be maintained in culture for up to seven days with preservation of sensory organ morphology and robust marker protein expression in sensory hair cells. Controlled sensory cell lesions can be induced by the ototoxic agent, neomycin sulphate, as a toxicologic model of hair cell degeneration and hair cell loss. The results demonstrate that simulated microgravity organ culture of the inner ear affords an in vitro model for the investigation of developmental, regulatory, and differentiation processes, as well as toxicological, biotechnological, and pharmaceutical screening applications within the normal and pathologic sensory hearing organ.

Pharmacological inhibition of Notch signaling in the mature guinea pig cochlea

Recent studies using explant cultures have demonstrated that pharmacological inhibition of Notch signaling by gamma-secretase inhibitors generates supernumerary hair cells in embryonic or neonatal cochleae. The aim of this study was to examine the effects of such pharmacological inhibition on mature auditory epithelia in vivo. Normal adult guinea pig auditory epithelia exhibited weak or no immunoreactivity for Notch1 and Jagged1, whereas ototoxic treatment caused the upregulation of these molecules in damaged auditory epithelia. Local application of a gamma-secretase inhibitor in damaged cochleae generated ectopic hair cells in mature auditory epithelia. These findings indicate that pharmacological inhibition of Notch signaling is a possible strategy for hair cell regeneration in adult auditory epithelia.

Sox2 and JAGGED1 expression in normal and drug-damaged adult mouse inner ear

Inner ear hair cells detect environmental signals associated with hearing, balance, and body orientation. In humans and other mammals, significant hair cell loss leads to irreversible hearing and balance deficits, whereas hair cell loss in nonmammalian vertebrates is repaired by the spontaneous generation of replacement hair cells. Research in mammalian hair cell regeneration is hampered by the lack of in vivo damage models for the adult mouse inner ear and the paucity of cell-type-specific markers for non-sensory cells within the sensory receptor epithelia. The present study delineates a protocol to drug damage the adult mouse auditory epithelium (organ of Corti) in situ and uses this protocol to investigate Sox2 and Jagged1 expression in damaged inner ear sensory epithelia. In other tissues, the transcription factor Sox2 and a ligand member of the Notch signaling pathway, Jagged1, are involved in regenerative processes. Both are involved in early inner ear development and are expressed in developing support cells, but little is known about their expressions in the adult. We describe a nonsurgical technique for inducing hair cell damage in adult mouse organ of Corti by a single high-dose injection of the aminoglycoside kanamycin followed by a single injection of the loop diuretic furosemide. This drug combination causes the rapid death of outer hair cells throughout the cochlea. Using immunocytochemical techniques, Sox2 is shown to be expressed specifically in support cells in normal adult mouse inner ear and is not affected by drug damage. Sox2 is absent from auditory hair cells, but is expressed in a subset of vestibular hair cells. Double-labeling experiments with Sox2 and calbindin suggest Sox2-positive hair cells are Type II. Jagged1 is also expressed in support cells in the adult ear and is not affected by drug damage. Sox2 and Jagged1 may be involved in the maintenance of support cells in adult mouse inner ear.

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.

The potential of amniotic membrane/amnion-derived cells for regeneration of various tissues

Regenerative medicine is a new field based on the use of stem cells to generate biological substitutes and improve tissue functions, restoring damaged tissue with high proliferability and differentiability. It is of interest as a potential alternative to complicated tissue/organ transplantation. Recently, amnion-derived cells have been reported to have multipotent differentiation ability, and these cells have attracted attention as a cell source for cell-transplantation therapy. The amnion possesses considerable advantageous characteristics: the isolated cells can differentiate into all three germ layers; they have low immunogenicity and anti-inflammatory functions; and they do not require the sacrifice of human embryos for their isolation, thus avoiding the current controversies associated with the use of human embryonic stem cells. Moreover, we developed human amniotic cell-sheets using a novel culture surface coated with a noncytotoxic, temperature-responsive elastic protein-based polymer. We also generated a "hyper-dry-amnion", which has already been applied clinically in the ophthalmological field. Compared to cryopreserved fresh amnion, "hyper-dry-amnion" is easy to handle and has started to bring good results to patients. These materials from the amnion are also expected to open a new field in tissue engineering. Thus, amnion, which had been discarded after parturition, has started to be appreciated as an attractive material in the field of regenerative medicine. In this review, the most recent and relevant clinical and experimental data about the use of amniotic membrane and cells derived from it are described.

LIF promotes neurogenesis and maintains neural precursors in cell populations derived from spiral ganglion stem cells

Background

Stem cells with the ability to form clonal floating colonies (spheres) were recently isolated from the neonatal murine spiral ganglion. To further examine the features of inner ear-derived neural stem cells and their derivatives, we investigated the effects of leukemia inhibitory factor (LIF), a neurokine that has been shown to promote self-renewal of other neural stem cells and to affect neural and glial cell differentiation.

Results

LIF-treatment led to a dose-dependent increase of the number of neurons and glial cells in cultures of sphere-derived cells. Based on the detection of developmental and progenitor cell markers that are maintained in LIF-treated cultures and the increase of cycling nestin-positive progenitors, we propose that LIF maintains a pool of neural progenitor cells. We further provide evidence that LIF increases the number of nestin-positive progenitor cells directly in a cell cycle-independent fashion, which we interpret as an acceleration of neurogenesis in sphere-derived progenitors. This effect is further enhanced by an anti-apoptotic action of LIF. Finally, LIF and the neurotrophins BDNF and NT3 additively promote survival of stem cell-derived neurons.

Conclusion

Our results implicate LIF as a powerful tool to control neural differentiation and maintenance of stem cell-derived murine spiral ganglion neuron precursors. This finding could be relevant in cell replacement studies with animal models featuring spiral ganglion neuron degeneration. The additive effect of the combination of LIF and BDNF/NT3 on stem cell-derived neuronal survival is similar to their effect on primary spiral ganglion neurons, which puts forward spiral ganglion-derived neurospheres as an in vitro model system to study aspects of auditory neuron development.

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.

Survival and stimulation of neurite outgrowth in a serum-free culture of spiral ganglion neurons from adult mice

We have developed a reliable protocol for the serum-free dissociation and culture of spiral ganglion neurons from adult mice, an important animal model for patients with post-lingual hearing loss. Pilot experiments indicated that the viability of spiral ganglion cells in vitro depended critically on the use of Hibernate medium with B27 supplement. With an optimized protocol, we obtained 2 x 10(3) neurons immediately after dissociation, or about one-fifth of those present in the intact spiral ganglion. After four days in culture, 4% of the seeded neurons survived without any exogenous growth factors other than insulin. This yield was highly reproducible in five independent experiments and enabled us to measure systematically the numbers and lengths of the regenerating neurites. Furthermore, the survival rate compared well to the few published protocols for culturing adult spiral ganglion neurons from other species. Enhanced survival and neurite outgrowth upon the addition of brain-derived neurotrophic factor and leukemia inhibitory factor demonstrated that both are potent stimulants for damaged spiral ganglion neurons in adults. This responsiveness to exogenous growth factors suggested that our culture protocol will facilitate the screening of molecular compounds as potential treatments for sensorineural hearing loss.

The human fetal cochlea can be a source for auditory progenitors/stem cells isolation

The development of new stem cell-based technologies is creating new hopes in regenerative medicine. Hearing-impaired individuals should benefit greatly from the development of a cell-based regenerative strategy to treat deafness. An important achievement would be to develop a human-based system that could bring the advances made in animal models closer to clinical application. In this work, we have explored the suitability of the developing fetal cochlea to be used as a source for the extraction of auditory progenitor/stem cells. We have established cultures that express critical markers such as NESTIN, SOX2, GATA3 and PAX2. These cultures can be expanded in vitro for several months and differentiating markers such as ATOH1/HATH1 and POU4F3/BRN3C can be induced by manipulating the culture conditions using specific growth factors such as bFGF, EGF and retinoic acid.

Robust postmortem survival of murine vestibular and cochlear stem cells

Potential treatment strategies of neurodegenerative and other diseases with stem cells derived from nonembryonic tissues are much less subjected to ethical criticism than embryonic stem cell-based approaches. Here we report the isolation of inner ear stem cells, which may be useful in cell replacement therapies for hearing loss, after protracted postmortem intervals. We found that neonatal murine inner ear tissues, including vestibular and cochlear sensory epithelia, display remarkably robust cellular survival, even 10 days postmortem. Similarly, isolation of sphere-forming stem cells was possible up to 10 days postmortem. We detected no difference in the proliferation and differentiation potential between stem cells isolated directly after death and up to 5 days postmortem. At longer postmortem intervals, we observed that the potency of sphere-derived cells to spontaneously differentiate into mature cell types diminishes prior to the cells losing their potential for self-renewal. Three-week-old mice also displayed sphere-forming stem cells in all inner ear tissues investigated up to 5 days postmortem. In summary, our results demonstrate that postmortem murine inner ear tissue is suited for isolation of stem cells.

The potential role of endogenous stem cells in regeneration of the inner ear

Stem cells in various mammalian organs retain the capacity to renew themselves and may be able to restore damaged tissue. Their existence has been proven by genetic tracer studies that demonstrate their differentiation into multiple tissue types and by their ability to self-renew through proliferation. Stem cells from the adult nervous system proliferate to form clonal floating colonies called spheres in vitro, and recent studies have demonstrated sphere formation by cells in the cochlea in addition to the vestibular system and the auditory ganglia, indicating that these tissues contain cells with stem cell properties. The presence of stem cells in the inner ear raises the hope of regeneration of mammalian inner ear cells but is difficult to correlate with the lack of spontaneous regeneration seen in the inner ear after tissue damage. Loss of stem cells postnatally in the cochlea may correlate with the loss of regenerative capacity and may limit our ability to stimulate regeneration. Retention of sphere forming ability in adult vestibular tissues suggests that the limited capacity for repair may be attributed to the continued presence of progenitor cells. Future strategies for regeneration must consider the distribution of endogenous stem cells in the inner ear and whether the tissue retains cells with the capacity for regeneration.

Engraftment and differentiation of embryonic stem cell-derived neural progenitor cells in the cochlear nerve trunk: growth of processes into the organ of Corti

Hearing loss in mammals is irreversible because cochlear neurons and hair cells do not regenerate. To determine whether we could replace neurons lost to primary neuronal degeneration, we injected EYFP-expressing embryonic stem cell-derived mouse neural progenitor cells into the cochlear nerve trunk in immunosuppressed animals 1 week after destroying the cochlear nerve (spiral ganglion) cells while leaving hair cells intact by ouabain application to the round window at the base of the cochlea in gerbils. At 3 days post transplantation, small grafts were seen that expressed endogenous EYFP and could be immunolabeled for neuron-specific markers. Twelve days after transplantation, the grafts had neurons that extended processes from the nerve core toward the denervated organ of Corti. By 64-98 days, the grafts had sent out abundant processes that occupied a significant portion of the space formerly occupied by the cochlear nerve. The neurites grew in fasciculating bundles projecting through Rosenthal's canal, the former site of spiral ganglion cells, into the osseous spiral lamina and ultimately into the organ of Corti, where they contacted hair cells. Neuronal counts showed a significant increase in neuronal processes near the sensory epithelium, compared to animals that were denervated without subsequent stem cell transplantation. The regeneration of these neurons shows that neurons differentiated from stem cells have the capacity to grow to a specific target in an animal model of neuronal degeneration.

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.