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

[Advances in stem cell inner ear transplantation]

Sensorineural hearing loss has long been one of the common diseases in the field of otology. With the increasing research on stem cell therapy, the experiments and applications of stem cell inner ear transplantation are developing rapidly, with some remarkable results and some questions to be considered. The source of stem cells and the transplantation route are crucial, and the immune rejection in the post-transplantation period should not be ignored. This paper will review the issues related to stem cell source, transplantation route and immune rejection in inner ear transplantation, hoping to provide new ideas for research in the field of stem cell inner ear transplantation.

A review of the pathology and treatment of TBI and PTSD

This literature review focuses on the underlying pathophysiology of TBI and PTSD symptoms, while also examining the plethora of stem cell treatment options to ameliorate these neuronal and functional changes. As more veterans return suffering from TBI and/or PTSD, it is vital that researchers discover novel therapies to mitigate the detrimental symptoms of both diagnoses. A variety of stem cell treatments have been studied and offer hopeful options for TBI and PTSD recovery.

Growth factors with valproic acid restore injury-impaired hearing by promoting neuronal regeneration

Spiral ganglion neurons (SGNs) are primary auditory neurons in the spiral ganglion that transmit sound information from the inner ear to the brain and play an important role in hearing. Impairment of SGNs causes sensorineural hearing loss (SNHL), and it has been thought until now that SGNs cannot be regenerated once lost. Furthermore, no fundamental therapeutic strategy for SNHL has been established other than inserting devices such as hearing aids and cochlear implants. Here we show that the mouse spiral ganglion contains cells that are able to proliferate and indeed differentiate into neurons in response to injury. We suggest that SRY-box transcription factor 2/SRY-box transcription factor 10-double-positive (Sox2/Sox10-double-positive) Schwann cells sequentially started to proliferate, lost Sox10 expression, and became neurons, although the number of new neurons generated spontaneously was very small. To increase the abundance of new neurons, we treated mice with 2 growth factors in combination with valproic acid, which is known to promote neuronal differentiation and survival. This treatment resulted in a dramatic increase in the number of SGNs, accompanied by a partial recovery of the hearing loss induced by injury. Taken together, our findings offer a step toward developing strategies for treatment of SNHL.

Mesenchymal Stem Cell-Induced Anti-Neuroinflammation Against Traumatic Brain Injury

Traumatic brain injury (TBI) is a pervasive and damaging form of acquired brain injury (ABI). Acute, subacute, and chronic cell death processes, as a result of TBI, contribute to the disease progression and exacerbate outcomes. Extended neuroinflammation can worsen secondary degradation of brain function and structure. Mesenchymal stem cell transplantation has surfaced as a viable approach as a TBI therapeutic due to its immunomodulatory and regenerative features. This article examines the role of inflammation and cell death in ABI as well as the effectiveness of bone marrow-derived mesenchymal stem/stromal cell (BM-MSC) transplants as a treatment for TBI. Furthermore, we analyze new studies featuring transplanted BM-MSCs as a neurorestorative and anti-inflammatory therapy for TBI patients. Although clinical trials support BM-MSC transplants as a viable TBI treatment due to their promising regenerative characteristics, further investigation is imperative to uncover innovative brain repair pathways associated with cell-based therapy as stand-alone or as combination treatments.

Cochlear Synaptic Degeneration and Regeneration After Noise: Effects of Age and Neuronal Subgroup

In CBA/CaJ mice, confocal analysis has shown that acoustic overexposure can immediately destroy synapses between auditory-nerve fibers (ANFs) and their peripheral targets, the inner hair cells (IHCs), and that years later, a corresponding number of ANF cell bodies degenerate. In guinea pig, post-exposure disappearance of pre-synaptic ribbons can be equally dramatic, however, post-exposure recovery to near-baseline counts has been reported. Since confocal counts are confounded by thresholding issues, the fall and rise of synaptic ribbon counts could represent “regeneration,” i.e., terminal retraction, re-extension and synaptogenesis, or “recovery,” i.e., down- and subsequent up-regulation of synaptic markers. To clarify, we counted pre-synaptic ribbons, assessed their juxtaposition with post-synaptic receptors, measured the extension of ANF terminals, and quantified the spatial organization and size gradients of these synaptic elements around the hair cell. Present results in guinea pigs exposed as adults (14 months), along with prior results in juveniles (1 month), suggest there is post-exposure neural regeneration in the guinea pig, but not the CBA/CaJ mouse, and that this regenerative capacity extends into adulthood. The results also show, for the first time, that the acute synaptic loss is concentrated on the modiolar side of IHCs, consistent with a selective loss of the high-threshold ANFs with low spontaneous rates. The morphological similarities between the post-exposure neurite extension and synaptogenesis, seen spontaneously in the guinea pig, and in CBA/CaJ only with forced overexpression of neurotrophins, suggest that the key difference may be in the degree of sustained or injury-induced expression of these signaling molecules in the cochlea.

[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.

Stem Cells: A New Hope for Hearing Loss Therapy

Permanent hearing loss was considered which cannot be cured since cochlear hair cells and primary afferent neurons cannot be regenerated. In recent years, due to the in-depth study of stem cell and its therapeutic potential, regenerating auditory sensory cells is made possible. By using two strategies of endogenous stem cell activation and exogenous stem cell transplantation, researchers hope to find methods to restore hearing function. However, there are complex factors that need to be considered in the in vivo application of stem cell therapy, such as stem cell-type choice, signaling pathway regulations, transplantation approaches, internal environment of the cochlea, and external stimulation. After years of investigations, some theoretic progress has been made in the treatment of hearing loss using stem cells, but there are also many problems which limited its application that need to be solved. Understanding the future perspective of stem cell therapy in hearing loss, solving the encountered problems, and promoting its development are the common goals of audiological researchers. In this review, we present critical experimental findings of stem cell therapy on treatment of hearing loss and intend to bring hope to researchers and patients.

Valproic acid promotes the neuronal differentiation of spiral ganglion neural stem cells with robust axonal growth

Hearing loss occurs with the loss of hair cells of the cochlea and subsequent degeneration of spiral ganglion neurons (SGNs). Regeneration of SGNs is a potentially promising therapeutic approach to hearing loss in addition to the use of a cochlear implant (CI), because this device stimulates SGNs directly to restore hearing bypassing the missing hair cells. The presence of SGN-neural stem cells (NSCs) has been reported in adult human and mice. These cells have the potential to become SGNs and thus represent a cellular foundation for regeneration therapies for hearing loss. Valproic acid (VPA) has been shown to influence the neural differentiation of NSCs through multiple signaling pathways involving glycogen synthase kinase3β (GSK3β). Our present study therefore aimed to modulate the neural differentiation potential of SGN-NSCs by treatment with VPA. We here report that a clinically relevant concentration of 1 mM VPA induced the differentiation of basic fibroblast growth factor (bFGF)-treated P1- and P14-SGN-NSCs into neuronal and glial cells, confirmed by neuronal marker (Tuj1 and MAP2) and glial cell marker (GFAP and S100β) detection. VPA-treated cells also promoted much longer neurite outgrowth compared to differentiated cells cultured without bFGF. The effects of VPA on the regulation of differentiation may be related to the activation of the Wnt/β-catenin signaling pathway, but not the inhibition of histone deacetylases (HDACs). We propose that VPA has the potential to convert SGN-NSCs into SGNs and thereby restore hearing when combined with a CI.

Generation of Otic Sensory Neurons from Mouse Embryonic Stem Cells in 3D Culture

The peripheral hearing process taking place in the cochlea mainly depends on two distinct sensory cell types: the mechanosensitive hair cells and the spiral ganglion neurons (SGNs). The first respond to the mechanical stimulation exerted by sound pressure waves on their hair bundles by releasing neurotransmitters and thereby activating the latter. Loss of these sensorineural cells is associated with permanent hearing loss. Stem cell-based approaches aiming at cell replacement or in vitro drug testing to identify potential ototoxic, otoprotective, or regenerative compounds have lately gained attention as putative therapeutic strategies for hearing loss. Nevertheless, they rely on efficient and reliable protocols for the in vitro generation of cochlear sensory cells for their implementation. To this end, we have developed a differentiation protocol based on organoid culture systems, which mimics the most important steps of in vivo otic development, robustly guiding mouse embryonic stem cells (mESCs) toward otic sensory neurons (OSNs). The stepwise differentiation of mESCs toward ectoderm was initiated using a quick aggregation method in presence of Matrigel in serum-free conditions. Non-neural ectoderm was induced via activation of bone morphogenetic protein (BMP) signaling and concomitant inhibition of transforming growth factor beta (TGFβ) signaling to prevent mesendoderm induction. Preplacodal and otic placode ectoderm was further induced by inhibition of BMP signaling and addition of fibroblast growth factor 2 (FGF2). Delamination and differentiation of SGNs was initiated by plating of the organoids on a 2D Matrigel-coated substrate. Supplementation with brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) was used for further maturation until 15 days of in vitro differentiation. A large population of neurons with a clear bipolar morphology and functional excitability was derived from these cultures. Immunostaining and gene expression analysis performed at different time points confirmed the transition trough the otic lineage and final expression of the key OSN markers. Moreover, the stem cell-derived OSNs exhibited functional electrophysiological properties of native SGNs. Our established in vitro model of OSNs development can be used for basic developmental studies, for drug screening or for the exploration of their regenerative potential.

Stem cells and combination therapy for the treatment of traumatic brain injury

TBI is a nondegenerative, noncongenital insult to the brain from an external mechanical force; for instance a violent blow in a car accident. It is a complex injury with a broad spectrum of symptoms and has become a major cause of death and disability in addition to being a burden on public health and societies worldwide. As such, finding a therapy for TBI has become a major health concern for many countries, which has led to the emergence of many monotherapies that have shown promising effects in animal models of TBI, but have not yet proven any significant efficacy in clinical trials. In this paper, we will review existing and novel TBI treatment options. We will first shed light on the complex pathophysiology and molecular mechanisms of this disorder, understanding of which is a necessity for launching any treatment option. We will then review most of the currently available treatments for TBI including the recent approaches in the field of stem cell therapy as an optimal solution to treat TBI. Therapy using endogenous stem cells will be reviewed, followed by therapies utilizing exogenous stem cells from embryonic, induced pluripotent, mesenchymal, and neural origin. Combination therapy is also discussed as an emergent novel approach to treat TBI. Two approaches are highlighted, an approach concerning growth factors and another using ROCK inhibitors. These approaches are highlighted with regard to their benefits in minimizing the outcomes of TBI. Finally, we focus on the consequent improvements in motor and cognitive functions after stem cell therapy. Overall, this review will cover existing treatment options and recent advancements in TBI therapy, with a focus on the potential application of these strategies as a solution to improve the functional outcomes of TBI.

Single-Cell Transcriptome Analysis of Developing and Regenerating Spiral Ganglion Neurons

The spiral ganglion neurons (SGNs) of the cochlea are essential for our ability to hear. SGN loss after exposure to ototoxic drugs or loud noise results in hearing loss. Pluripotent stem cell-derived and endogenous progenitor cell types have the potential to become SGNs and are cellular foundations for replacement therapies. Repurposing transcriptional regulatory networks to promote SGN differentiation from progenitor cells is a strategy for regeneration. Advances in the Fludigm C1 workflow or Drop-seq allow sequencing of single cell transcriptomes to reveal variability between cells. During differentiation, the individual transcriptomes obtained from single-cell RNA-seq can be exploited to identify different cellular states. Pseudotemporal ordering of transcriptomes describes the differentiation trajectory, allows monitoring of transcriptional changes and determines molecular barriers that prevent the progression of progenitors into SGNs. Analysis of single cell transcriptomes will help develop novel strategies for guiding efficient SGN regeneration.

Role of Endogenous Bone Marrow Stem Cells Mobilization in Repair of Damaged Inner Ear in Rats

Background and Objectives

The utilization of the stem cells is widely used in the last few years in different fields of medicine, either by external transplantation or endogenous mobilization, most of these studies still experimental on animals; few were tried on human as in the spinal cord injury or myocardial infarction. As regard its use in the inner ear, stem cell transplantation was examined in many previous studies, while the mobilization idea is a new method to be experimented in inner ear hair cell regeneration. The present work assessed the possibility of mobilizing endogenous bone marrow derived stem cells (SCs) in rats using granulocyte colony stimulating factor (G-CSF) to induce regeneration and repair to experimentally damaged inner ear hair cells by Amikacin injection.

Methods

The study included thirty adult Sprague Dawley male rats. Experimental induction of inner ear damage was done by repeated intratympanic injection of amikacin sulfate. Mobilization of bone marrow SCs was provoked by subcutaneous injection of GCSF. Cochlear integrity, induction of hearing loss and functional recovery of sensory hearing loss were assessed using Distortion Product Otoacoustic Emission (DPOAEs). The morphological alteration and recovery of the organ of Corti was assessed histologically using the light and scanning electron microscopes.

Results

After six month duration, there was improvement in 50% of the sensorineural DPOAE results. Functional recovery coincided with the repair of structural components of organ of Corti.

Conclusions

SCs mobilization by G-CSF is a promising alternative method for replacement therapy in sensorineural hearing loss.

Cisplatin exposure damages resident stem cells of the mammalian inner ear

BACKGROUND

Cisplatin is a widely used chemotherapeutic agent that can also cause ototoxic injury. One potential treatment for cisplatin-induced hearing loss involves the activation of endogenous inner ear stem cells, which may then produce replacement hair cells. In this series of experiments, we examined the effects of cisplatin exposure on both hair cells and resident stem cells of the mouse inner ear.

RESULTS

Treatment for 24 hr with 10 µM cisplatin caused significant loss of hair cells in the mouse utricle, but such damage was not evident until 4 days after the cisplatin exposure. In addition to killing hair cells, cisplatin treatment also disrupted the actin cytoskeleton in remaining supporting cells, and led to increased histone H2AX phosphorylation within the sensory epithelia. Finally, treatment with 10 µM cisplatin appeared to have direct toxic effects on resident stem cells in the mouse utricle. Exposure to cisplatin blocked the proliferation of isolated stem cells and prevented sphere formation when those cells were maintained in suspension culture.

CONCLUSION

The results suggest that inner ear stem cells may be injured during cisplatin ototoxicity, thus limiting their ability to mediate sensory repair.

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

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

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

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

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.

Biomaterials in cochlear implants

The cochlear implant (CI) represents, for almost 25 years now, the gold standard in the treatment of children born deaf and for postlingually deafened adults. These devices thus constitute the greatest success story in the field of ‘neurobionic’ prostheses. Their (now routine) fitting in adults, and especially in young children and even babies, places exacting demands on these implants, particularly with regard to the biocompatibility of a CI’s surface components. Furthermore, certain parts of the implant face considerable mechanical challenges, such as the need for the electrode array to be flexible and resistant to breakage, and for the implant casing to be able to withstand external forces.

As these implants are in the immediate vicinity of the middle-ear mucosa and of the junction to the perilymph of the cochlea, the risk exists – at least in principle – that bacteria may spread along the electrode array into the cochlea. The wide-ranging requirements made of the CI in terms of biocompatibility and the electrode mechanism mean that there is still further scope – despite the fact that CIs are already technically highly sophisticated – for ongoing improvements to the properties of these implants and their constituent materials, thus enhancing the effectiveness of these devices.

This paper will therefore discuss fundamental material aspects of CIs as well as the potential for their future development.

Connecting the ear to the brain: Molecular mechanisms of auditory circuit assembly

Our sense of hearing depends on precisely organized circuits that allow us to sense, perceive, and respond to complex sounds in our environment, from music and language to simple warning signals. Auditory processing begins in the cochlea of the inner ear, where sounds are detected by sensory hair cells and then transmitted to the central nervous system by spiral ganglion neurons, which faithfully preserve the frequency, intensity, and timing of each stimulus. During the assembly of auditory circuits, spiral ganglion neurons establish precise connections that link hair cells in the cochlea to target neurons in the auditory brainstem, develop specific firing properties, and elaborate unusual synapses both in the periphery and in the CNS. Understanding how spiral ganglion neurons acquire these unique properties is a key goal in auditory neuroscience, as these neurons represent the sole input of auditory information to the brain. In addition, the best currently available treatment for many forms of deafness is the cochlear implant, which compensates for lost hair cell function by directly stimulating the auditory nerve. Historically, studies of the auditory system have lagged behind other sensory systems due to the small size and inaccessibility of the inner ear. With the advent of new molecular genetic tools, this gap is narrowing. Here, we summarize recent insights into the cellular and molecular cues that guide the development of spiral ganglion neurons, from their origin in the proneurosensory domain of the otic vesicle to the formation of specialized synapses that ensure rapid and reliable transmission of sound information from the ear to the brain.

Drug delivery for treatment of inner ear disease: current state of knowledge

Delivery of medications to the inner ear has been an area of considerable growth in both the research and clinical realms during the past several decades. Systemic delivery of medication destined for treatment of the inner ear is the foundation on which newer delivery techniques have been developed. Because of systemic side effects, investigators and clinicians have begun developing and using techniques to deliver therapeutic agents locally. Alongside the now commonplace use of intratympanic gentamicin for Meniere's disease and the emerging use of intratympanic steroids for sudden sensorineural hearing loss, novel technologies, such as hydrogels and nanoparticles, are being explored. At the horizon of inner ear drug-delivery techniques, intracochlear devices that leverage recent advances in microsystems technology are being developed to apply medications directly into the inner ear. Potential uses for such devices include neurotrophic factor and steroid delivery with cochlear implantation, RNA interference technologies, and stem-cell therapy. The historical, current, and future delivery techniques and uses of drug delivery for treatment of inner ear disease serve as the basis for this review.

Noise stresses the junctions to deaf

Sitting in the rainforest in Costa Rica, you can hear rain, rushing water, howling monkeys, birds and crickets. All are in abundance and although they are tantalizing to all your senses, your sense of hearing picks up what you cannot see with your eyes or feel with your hands.

A comparison of the proliferative capacity and ultrastructure of proliferative cells from the cochleae of newborn rats of different ages

OBJECTIVE

Recent reports have shown that multipotent stem cells/progenitor cells that are capable of proliferation and regeneration are present in mammalian cochleae. However, progenitor cells have not been isolated from the adult cochlea. We examined the proliferative potential of cells derived from neonatal rats of various ages. The determination of the differences between the proliferative cells from rats of different ages may provide clues to the mechanisms controlling the destiny of these cells.

METHODS

Proliferative cells were isolated from the cochleae of 1-, 7-, and 14-day-old rats, and the proliferative capacity and ultrastructure of the cells from each age group were assessed using flow cytometry and transmission electron microscopy, respectively.

RESULTS

During the first two postnatal weeks, the number of proliferative cells gradually fell to zero. This decrease occurred in parallel with the impairment of the proliferative capacity of the cells and the accumulation of proliferative cells in G0/G1. In addition, some of the cells exited the cell cycle by means of gradual maturity and apoptosis.

CONCLUSIONS

Our study suggests that cochlear proliferative cells are remnants of the progenitor cells that originally gave rise to the sensory epithelium. The disappearance of the cochlear proliferative cells in adult mammalian cochleae may result from their differentiation and/or apoptosis.

Deafness genes in Israel: implications for diagnostics in the clinic

The identification of the molecular basis of deafness in the last decade has made a remarkable impact on genetic counseling and diagnostics for the hearing impaired population. Since the discovery of the most prevalent form of deafness associated with mutations in the GJB2 (connexin 26) gene, many other genes have been found worldwide, with a subset of these, including unique mutations, in Israel. Here, we review the current status of deafness genes in Israel and report one known mutation in a syndromic form of deafness, Usher syndrome, described in the Jewish Israeli population for the first time. In the future, the identification of specific mutations may be relevant for specific types of treatment.

Gene therapy for the inner ear: challenges and promises

Since the recognition of genes as the discrete units of heritability, and of DNA as their molecular substrate, the utilization of genes for therapeutic purposes has been recognized as a potential means of correcting genetic disorders. The tools of molecular biology, which allow the manipulation of DNA sequence, provided the means to put this concept into practice. However, progress in the implementation of these ideas has been slow. Here we review the history of the idea of gene therapy and the complexity of genetic disorders. We also discuss the requirements for sequence-based therapy to be accomplished for different types of inherited diseases, as well as the methods available for gene manipulation. The challenges that have limited the applications of gene therapy are reviewed, as are ethical concerns. Finally, we discuss the promise of gene therapy to address inherited and acquired disorders of the inner ear.

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.

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.

Protein expression of pigment-epithelium-derived factor in rat cochlea

Pigment-epithelium-derived factor (PEDF) is a 50-kDa glycoprotein with well-recognised expression in various mammalian organs showing diverse (e.g. anti-angiogenic and neuroprotective) activities. However, at present, no information is available regarding the potential function of this cytokine in the inner ear. As a first approach to investigating whether PEDF is involved in cochlear function, we have explored its protein expression in the rat cochlea by immunocytochemistry. Our results show that PEDF expression in the cochlea is most prominent in the basilar membrane below the organ of Corti, in the lateral wall (especially in the stria vascularis), in ganglion neurons, and in the endothelia of blood vessels. Our findings on its distribution in the cochlea suggest that PEDF in the basilar membrane prevents blood vessel formation that would disturb cochlear micromechanics and would interfere with the mechano-electrical transduction in the organ of Corti. In cochlear ganglion neurons, PEDF might serve a neuroprotective function possibly protecting these neurons from excessive glutamate released by the inner hair cells. Our data constitute the first report on the morphological protein distribution of this multifunctional molecule in the rat cochlea and suggest its role in important functions of the internal ear.

[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.

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.