Intracellular signals that control cell proliferation in mammalian balance epithelia: key roles for phosphatidylinositol-3 kinase, mammalian target of rapamycin, and S6 kinases in preference to calcium, protein kinase C, and mitogen-activated protein kinase

Hear the sounds: The role of G Protein-Coupled Receptors in the cochlea

Sound is converted by hair cells in the cochlea into electrical signals, which are transmitted by spiral ganglion neurons (SGNs) and heard by the auditory cortex. G protein-coupled receptors (GPCRs) are crucial receptors that regulate a wide range of physiological functions in different organ and tissues. The research of GPCRs in the cochlea is essential for the understanding of the cochlea development, hearing disorders, and the treatment for hearing loss. Recently, several GPCRs have been found to play important roles in the cochlea. Frizzleds and Lgrs are dominant GPCRs that regulate stem cell self-renew abilities. Moreover, Frizzleds and Celsrs have been demonstrated to play core roles in the modulation of cochlear planar cell polarity (PCP). In addition, hearing loss can be caused by mutations of certain GPCRs, such as Vlgr1, Gpr156, S1P2 and Gpr126. And A1, A2A and CB2 activation by agonists have protective functions on noise- or drug-induced hearing loss. Here, we review the key findings of GPCR in the cochlea, and discuss the role of GPCR in the cochlea, such as stem cell fate, PCP, hearing loss, and hearing protection.

Regeneration of Hair Cells in the Human Vestibular System

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

mTOR Signaling in the Inner Ear as Potential Target to Treat Hearing Loss

Hearing loss affects many people worldwide and occurs often as a result of age, ototoxic drugs and/or excessive noise exposure. With a growing number of elderly people, the number of people suffering from hearing loss will also increase in the future. Despite the high number of affected people, for most patients there is no curative therapy for hearing loss and hearing aids or cochlea implants remain the only option. Important treatment approaches for hearing loss include the development of regenerative therapies or the inhibition of cell death/promotion of cell survival pathways. The mammalian target of rapamycin (mTOR) pathway is a central regulator of cell growth, is involved in cell survival, and has been shown to be implicated in many age-related diseases. In the inner ear, mTOR signaling has also started to gain attention recently. In this review, we will emphasize recent discoveries of mTOR signaling in the inner ear and discuss implications for possible treatments for hearing restoration.

Characterizing Adult Cochlear Supporting Cell Transcriptional Diversity Using Single-Cell RNA-Seq: Validation in the Adult Mouse and Translational Implications for the Adult Human Cochlea

Hearing loss is a problem that impacts a significant proportion of the adult population. Cochlear hair cell (HC) loss due to loud noise, chemotherapy and aging is the major underlying cause. A significant proportion of these individuals are dissatisfied with available treatment options which include hearing aids and cochlear implants. An alternative approach to restore hearing would be to regenerate HCs. Such therapy would require a recapitulation of the complex architecture of the organ of Corti, necessitating regeneration of both mature HCs and supporting cells (SCs). Transcriptional profiles of the mature cell types in the cochlea are necessary to can provide a metric for eventual regeneration therapies. To assist in this effort, we sought to provide the first single-cell characterization of the adult cochlear SC transcriptome. We performed single-cell RNA-Seq on FACS-purified adult cochlear SCs from the Lfng^EGFP adult mouse, in which SCs express GFP. We demonstrate that adult cochlear SCs are transcriptionally distinct from their perinatal counterparts. We establish cell-type-specific adult cochlear SC transcriptome profiles, and we validate these expression profiles through a combination of both fluorescent immunohistochemistry and in situ hybridization co-localization and quantitative polymerase chain reaction (qPCR) of adult cochlear SCs. Furthermore, we demonstrate the relevance of these profiles to the adult human cochlea through immunofluorescent human temporal bone histopathology. Finally, we demonstrate cell cycle regulator expression in adult SCs and perform pathway analyses to identify potential mechanisms for facilitating mitotic regeneration (cell proliferation, differentiation, and eventually regeneration) in the adult mammalian cochlea. Our findings demonstrate the importance of characterizing mature as opposed to perinatal SCs.

Impaired Glycolysis Promotes AlcoholExposure-Induced Apoptosis in HEI-OC1 Cells via Inhibition of EGFR Signaling

Glucose metabolism is an important metabolic pathway in the auditory system. Chronic alcohol exposure can cause metabolic dysfunction in auditory cells during hearing loss. While alcohol exposure has been linked to hearing loss, the mechanism by which impaired glycolysis promotes cytotoxicity and cell death in auditory cells remains unclear. Here, we show that the inhibition of epidermal growth factor receptor (EGFR)-induced glycolysis is a critical mechanism for alcohol exposure-induced apoptosis in HEI-OC1 cells. The cytotoxicity via apoptosis was significantly increased by alcohol exposure in HEI-OC1 cells. The glycolytic activity and the levels of hexokinase 1 (HK1) were significantly suppressed by alcohol exposure in HEI-OC1 cells. Mechanistic studies showed that the levels of EGFR and AKT phosphorylation were reduced by alcohol exposure in HEI-OC1 cells. Notably, HK1 expression and glycolytic activity was suppressed by EGFR inhibition in HEI-OC1 cells. These results suggest that impaired glycolysis promotes alcohol exposure-induced apoptosis in HEI-OC1 cells via the inhibition of EGFR signaling.

Renewed proliferation in adult mouse cochlea and regeneration of hair cells

The adult mammalian inner ear lacks the capacity to divide or regenerate. Damage to inner ear generally leads to permanent hearing loss in humans. Here, we present that reprogramming of the adult inner ear induces renewed proliferation and regeneration of inner ear cell types. Co-activation of cell cycle activator Myc and inner ear progenitor gene Notch1 induces robust proliferation of diverse adult cochlear sensory epithelial cell types. Transient MYC and NOTCH activities enable adult supporting cells to respond to transcription factor Atoh1 and efficiently transdifferentiate into hair cell-like cells. Furthermore, we uncover that mTOR pathway participates in MYC/NOTCH-mediated proliferation and regeneration. These regenerated hair cell-like cells take up the styryl dye FM1-43 and are likely to form connections with adult spiral ganglion neurons, supporting that Myc and Notch1 co-activation is sufficient to reprogram fully mature supporting cells to proliferate and regenerate hair cell-like cells in adult mammalian auditory organs.

ERBB2 signaling drives supporting cell proliferation in vitro and apparent supernumerary hair cell formation in vivo in the neonatal mouse cochlea

In mammals, cochlear hair cells are not regenerated once they are lost, leading to permanent hearing deficits. In other vertebrates, the adjacent supporting cells act as a stem cell compartment, in that they both proliferate and differentiate into de novo auditory hair cells. Although there is evidence that mammalian cochlear supporting cells can differentiate into new hair cells, the signals that regulate this process are poorly characterized. We hypothesize that signaling from the epidermal growth factor receptor (EGFR) family may play a role in cochlear regeneration. We focus on one such member, ERBB2, and report the effects of expressing a constitutively active ERBB2 receptor in neonatal mouse cochlear supporting cells, using viruses and transgenic expression. Lineage tracing with fluorescent reporter proteins was used to determine the relationships between cells with active ERBB2 signaling and cells that divided or differentiated into hair cells. In vitro, individual supporting cells harbouring a constitutively active ERBB2 receptor appeared to signal to their neighbouring supporting cells, inducing them to down-regulate a supporting cell marker and to proliferate. In vivo, we found supernumerary hair cell-like cells near supporting cells that expressed ERBB2 receptors. Both supporting cell proliferation and hair cell differentiation were largely reproduced in vitro using small molecules that we show also activate ERBB2. Our data suggest that signaling from the receptor tyrosine kinase ERBB2 can drive the activation of secondary signaling pathways to regulate regeneration, suggesting a new model where an interplay of cell signaling regulates regeneration by endogenous stem-like cells.

Inhibition of mTOR by Rapamycin Results in Auditory Hair Cell Damage and Decreased Spiral Ganglion Neuron Outgrowth and Neurite Formation In Vitro

Rapamycin is an antifungal agent with immunosuppressive properties. Rapamycin inhibits the mammalian target of rapamycin (mTOR) by blocking the mTOR complex 1 (mTORC1). mTOR is an atypical serine/threonine protein kinase, which controls cell growth, cell proliferation, and cell metabolism. However, less is known about the mTOR pathway in the inner ear. First, we evaluated whether or not the two mTOR complexes (mTORC1 and mTORC2, resp.) are present in the mammalian cochlea. Next, tissue explants of 5-day-old rats were treated with increasing concentrations of rapamycin to explore the effects of rapamycin on auditory hair cells and spiral ganglion neurons. Auditory hair cell survival, spiral ganglion neuron number, length of neurites, and neuronal survival were analyzed in vitro. Our data indicates that both mTOR complexes are expressed in the mammalian cochlea. We observed that inhibition of mTOR by rapamycin results in a dose dependent damage of auditory hair cells. Moreover, spiral ganglion neurite number and length of neurites were significantly decreased in all concentrations used compared to control in a dose dependent manner. Our data indicate that the mTOR may play a role in the survival of hair cells and modulates spiral ganglion neuronal outgrowth and neurite formation.

Specializations of intercellular junctions are associated with the presence and absence of hair cell regeneration in ears from six vertebrate classes

Sensory hair cell losses lead to hearing and balance deficits that are permanent for mammals, but temporary for nonmammals because supporting cells in their ears give rise to replacement hair cells. In mice and humans, vestibular supporting cells grow exceptionally large circumferential F-actin belts and their junctions express E-cadherin in patterns that strongly correlate with postnatal declines in regeneration capacity. In contrast, chicken supporting cells retain thin F-actin belts throughout life and express little E-cadherin. To determine whether the junctions in chicken ears might be representative of other ears that also regenerate hair cells, we investigated inner ears from dogfish sharks, zebrafish, bullfrogs, Xenopus, turtles, and the lizard, Anolis. As in chickens, the supporting cells in adult zebrafish, Xenopus, and turtle ears retained thin circumferential F-actin belts and expressed little E-cadherin. Supporting cells in adult sharks and bullfrogs also retained thin belts, but were not tested for E-cadherin. Supporting cells in adult Anolis exhibited wide, but porous webs of F-actin and strong E-cadherin expression. Anolis supporting cells also showed some cell cycle reentry when cultured. The results reveal that the association between thin F-actin belts and low E-cadherin is shared by supporting cells in anamniotes, turtles, and birds, which all can regenerate hair cells. Divergent junctional specializations in supporting cells appear to have arisen independently in Anolis and mammals. The presence of webs of F-actin at the junctions in Anolis appears compatible with supporting cell proliferation, but the solid reinforcement of the F-actin belts in mammals is associated with its absence.

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

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

A historical to present-day account of efforts to answer the question: "what puts the brakes on mammalian hair cell regeneration?"

Hearing and balance deficits often affect humans and other mammals permanently, because their ears stop producing hair cells within a few days after birth. But production occurs throughout life in the ears of sharks, bony fish, amphibians, reptiles, and birds allowing them to replace lost hair cells and quickly recover after temporarily experiencing the kinds of sensory deficits that are irreversible for mammals. Since the mid 1970s, researchers have been asking what puts the brakes on hair cell regeneration in mammals. Here we evaluate the headway that has been made and assess current evidence for alternative mechanistic hypotheses that have been proposed to account for the limits to hair cell regeneration in mammals.

Over half the hair cells in the mouse utricle first appear after birth, with significant numbers originating from early postnatal mitotic production in peripheral and striolar growth zones

Many non-mammalian vertebrates produce hair cells throughout life and recover from hearing and balance deficits through regeneration. In contrast, embryonic production of hair cells declines sharply in mammals where deficits from hair cell losses are typically permanent. Hair cell density estimates recently suggested that the vestibular organs of mice continue to add hair cells after birth, so we undertook comprehensive counting in murine utricles at different ages. The counts show that 51% of the hair cells in adults arise during the 2 weeks after birth. Immature hair cells are most common near the neonatal macula's peripheral edge and striola, where anti-Ki-67 labels cycling nuclei in zones that appear to contain niches for supporting-cell-like stem cells. In vivo lineage tracing in a novel reporter mouse where tamoxifen-inducible supporting cell-specific Cre expression switched tdTomato fluorescence to eGFP fluorescence showed that proteolipid-protein-1-expressing supporting cells are an important source of the new hair cells. To assess the contributions of postnatal cell divisions, we gave mice an injection of BrdU or EdU on the day of birth. The labels were restricted to supporting cells 1 day later, but by 12 days, 31% of the labeled nuclei were in myosin-VIIA-positive hair cells. Thus, hair cell populations in neonatal mouse utricles grow appreciably through two processes: the progressive differentiation of cells generated before birth and the differentiation of new cells arising from divisions of progenitors that progress through S phase soon after birth. Subsequent declines in these processes coincide with maturational changes that appear unique to mammalian supporting cells.

Concise review: Inner ear stem cells--an oxymoron, but why?

Hearing loss, caused by irreversible loss of cochlear sensory hair cells, affects millions of patients worldwide. In this concise review, we examine the conundrum of inner ear stem cells, which obviously are present in the inner ear sensory epithelia of nonmammalian vertebrates, giving these ears the ability to functionally recover even from repetitive ototoxic insults. Despite the inability of the mammalian inner ear to regenerate lost hair cells, there is evidence for cells with regenerative capacity because stem cells can be isolated from vestibular sensory epithelia and from the neonatal cochlea. Challenges and recent progress toward identification of the intrinsic and extrinsic signaling pathways that could be used to re-establish stemness in the mammalian organ of Corti are discussed.

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.

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

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

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

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

The challenge of hair cell regeneration

Sensory hair cells of the inner ear are responsible for translating auditory or vestibular stimuli into electrical energy that can be perceived by the nervous system. Although hair cells are exquisitely mechanically sensitive, they can be easily damaged by excessive stimulation by ototoxic drugs and by the effects of aging. In mammals, auditory hair cells are never replaced, such that cumulative damage to the ear causes progressive and permanent deafness. In contrast, non-mammalian vertebrates are capable of replacing lost hair cells, which has led to efforts to understand the molecular and cellular basis of regenerative responses in different vertebrate species. In this review, we describe recent progress in understanding the limits to hair cell regeneration in mammals and discuss the obstacles that currently exist for therapeutic approaches to hair cell replacement.

CD44 is a marker for the outer pillar cells in the early postnatal mouse inner ear

Cluster of differentiation antigens (CD proteins) are classically used as immune cell markers. However, their expression within the inner ear is still largely undefined. In this study, we explored the possibility that specific CD proteins might be useful for defining inner ear cell populations. mRNA expression profiling of microdissected auditory and vestibular sensory epithelia revealed 107 CD genes as expressed in the early postnatal mouse inner ear. The expression of 68 CD genes was validated with real-time RT-PCR using RNA extracted from microdissected sensory epithelia of cochleae, utricles, saccules, and cristae of newborn mice. Specifically, CD44 was identified as preferentially expressed in the auditory sensory epithelium. Immunohistochemistry revealed that within the early postnatal organ of Corti, the expression of CD44 is restricted to outer pillar cells. In order to confirm and expand this finding, we characterized the expression of CD44 in two different strains of mice with loss- and gain-of-function mutations in Fgfr3 which encodes a receptor for FGF8 that is essential for pillar cell development. We found that the expression of CD44 is abolished from the immature pillar cells in homozygous Fgfr3 knockout mice. In contrast, both the outer pillar cells and the aberrant Deiters' cells in the Fgfr3 ( P244R/ ) (+) mice express CD44. The deafness phenotype segregating in DFNB51 families maps to a linkage interval that includes CD44. To study the potential role of CD44 in hearing, we characterized the auditory system of CD44 knockout mice and sequenced the entire open reading frame of CD44 of affected members of DFNB51 families. Our results suggest that CD44 does not underlie the deafness phenotype of the DFNB51 families. Finally, our study reveals multiple potential new cell type-specific markers in the mouse inner ear and identifies a new marker for outer pillar cells.

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

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

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.

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.

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

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

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.

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

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

Developmental changes in cell-extracellular matrix interactions limit proliferation in the mammalian inner ear

Hair cell losses can produce severe hearing and balance deficits in mammals and nonmammals alike, but nonmammals recover after epithelial supporting cells divide and give rise to replacement hair cells. Here, we describe cellular changes that appear to underlie the permanence of hair cell deficits in mammalian vestibular organs. In sensory epithelia isolated from the utricles of embryonic day 18 (E18) mice, supporting cells readily spread and proliferated, but spreading and proliferation were infrequent in supporting cells from postnatal day 6 (P6) mice. Cellular spreading and proliferation were dependent on alpha6 integrin, which disappeared from lateral cell membranes by P6 and colocalized with beta4 integrin near the basement membrane at both ages. In the many well-spread, proliferating E18 supporting cells, beta4 was localized at cell borders, but it was localized to hemidesmosome-like structures in the columnar, nondividing supporting cells that were prevalent in P6 cultures. We treated cultures with phorbol myristate acetate (PMA) to activate protein kinase C (PKC) in an initial test of the possibility that maturational changes in supporting cell cytoskeletons or their anchorage might restrict the proliferation of these progenitor cells in the developing mammalian inner ear. That treatment triggered the disassembly of the hemidesmosome-like beta4 structures and resulted in significantly increased cellular spreading and S-phase entry in the P6 epithelia. The results suggest that maturational changes in cytoskeletal organization and anchorage restrict proliferation of mammalian supporting cells whose counterparts are the progenitors of replacement hair cells in nonmammals, thereby leaving mammals vulnerable to persistent sensory deficits caused by hair cell loss.

A PI3K pathway mediates hair cell survival and opposes gentamicin toxicity in neonatal rat organ of Corti

Gentamicin is well known to promote hair cell death in inner ear, but it also appears to activate opposing pathways that promote hair cell survival. In combination with others, our previous work has indicated that a K-Ras/Rac/JNK pathway is important for hair cell death and an H-Ras/Raf/MEK/Erk pathway is involved in promoting hair cell survival (Battaglia et al., Neuroscience 122(4):1025-1035, 2003). However, these data also suggested that a Ras-independent survival pathway for activation of MEK might be stimulated by gentamicin. To investigate alternatives to the Ras/Raf/MEK/Erk pathway in promoting hair cell survival, cochlear explants were exposed to gentamicin combined with several inhibitors of alternative pathways (LY294002, calphostin C, SH-6, U73122). When exposed to gentamicin with the PI3K inhibitor LY294002 (10, 50 microM), the protein kinase C (PKC) inhibitor calphostin C (50, 100 nM) or the PKB/Akt inhibitor SH-6 (5, 10 microM), hair cell damage was significantly increased compared to gentamicin alone. By Western blotting, strong PKB/Akt activation was observed in the organ of Corti following exposure to 50 microM gentamicin for 6 h. In addition, PKC activation by 12-O-tetradecanoylphorbol-13-acetate protected outer hair cells from gentamicin induced cell death. In contrast, the phospholipase C-gamma (PLCgamma) inhibitor U73122 (2, 5 microM) did not affect hair cell damage when combined with gentamicin. Also, phosphorylation of PLCgamma was not increased in the organ of Corti following gentamicin treatment, as evaluated by Western blot. The results indicate that PI3K promotes hair cell survival via its downstream targets, PKC and PKB/Akt. This suggests that both Ras-dependent and Ras-independent survival pathways are involved during gentamicin exposure. In contrast, PLCgamma activation of PKC does not appear to play a role.

Kanamycin alters cytoplasmic and nuclear phosphoinositide signaling in the organ of Corti in vivo

Aminoglycoside antibiotics strongly bind to phosphoinositides and affect their membrane distribution and metabolism. Kanamycin treatment also disrupts Rac/Rho signaling pathways to the actin cytoskeleton in the mouse inner ear in vivo. Here, we investigate the influence of kanamycin on phosphoinositide signaling in sensory cells (hair cells) of the mouse cochlea. Immunoreactivity to phosphatidylinositol-3,4,5-trisphosphate (PIP3) decreased in the organ of Corti, especially in outer hair cells, after 3-7 days of drug treatment, whereas imunoreactivity to phosphatidylinositol-4,5-bisphosphate (PIP2) increased. Immunoreactivity to PIP2 was present at the apical poles of outer hair cells, but appeared in their nuclei only after drug treatment. Furthermore, nuclear PIP2 formed a complex with histone H3 and attenuated its acetylation in outer hair cells. In agreement with reduced PIP3 signaling, phosphorylated Akt decreased in both the cytoplasm and nuclei of outer hair cells after kanamycin treatment. This study suggests that kanamycin disturbs the balance between PIP2 and PIP3, modifies gene transcription via histone acetylation and diminishes the PI3K/Akt survival pathway. These actions may contribute to the death of outer hair cells, which is a consequence of chronic kanamycin treatment.

A Meniere's disease gene linked to chromosome 12p12.3

Meniere's disease (MD) is characterized by spontaneous attacks of vertigo, fluctuating sensorineural hearing loss, tinnitus, and aural fullness. The majority of patients with MD appear sporadic but 5%-13% of the cases have a family history for the disease. The cause of both the sporadic and inherited forms of MD remains unclear despite a number of candidate genes defined from their association with hearing loss. We have performed a genome wide linkage scan on a large Swedish family segregating MD in five generations. Five candidate regions with a lod score of >1 were identified. Two additional families with autosomal dominant MD were analyzed for linkage to these regions and a cumulative Z(max) of 3.46 was obtained for a single region on chromosome 12p. In two of the three families, a shared haplotype was found to extend over 1.7 Mb which suggests a common ancestral origin. Within this region, a single recombination event restricts the candidate region to 463 kb.

A disorganized innervation of the inner ear persists in the absence of ErbB2

ErbB2 protein is essential for the development of Schwann cells and for the normal fiber growth and myelin formation of peripheral nerves. We have investigated the fate of the otocyst-derived inner ear sensory neurons in the absence of ErbB2 using ErbB2 null mutants. Afferent innervation of the ear sensory epithelia shows numerous fibers overshooting the organ of Corti, followed by a reduction of those fibers in near term embryos. This suggests that mature Schwann cells do not play a role in targeting or maintaining the inner ear innervation. Comparable to the overshooting of nerve fibers, sensory neurons migrate beyond their normal locations into unusual positions in the modiolus. They may miss a stop signal provided by the Schwann cells that are absent as revealed with detailed histology. Reduction of overshooting afferents may be enhanced by a reduction of the neurotrophin Ntf3 transcript to about 25% of wild type. Ntf3 transcript reductions are comparable to an adult model that uses a dominant negative form of ErbB4 expressed in the supporting cells and Schwann cells of the organ of Corti. ErbB2 null mice retain afferents to inner hair cells possibly because of the prominent expression of the neurotrophin Bdnf in developing hair cells. Despite the normal presence of Bdnf transcript, afferent fibers are disoriented near the organ of Corti. Efferent fibers do not form an intraganglionic spiral bundle in the absence of spiral ganglia and appear reduced and disorganized. This suggests that either ErbB2 mediated alterations in sensory neurons or the absence of Schwann cells affects efferent fiber growth to the organ of Corti.

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

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

Neuregulin induces proliferation of neural progenitor cells via PLC/PKC pathway

Nestin-expressing neural progenitor cells (NPCs) have been isolated from hippocampus of brains and propagated with epidermal growth factor and basic fibroblast growth factor (bFGF). However, the underlying signaling mechanisms regulating NPC proliferation remain elusive. Here we showed that neuregulinbeta1 (NRG), like bFGF, effectively promoted the proliferation of hippocampus-derived NPCs and maintained the progenitor states of NPCs. Activation of protein kinase C (PKC), a downstream effector of phospholipase C (PLC), with 12-O-tetradecanoylphorbol-13-acetate (TPA) mimicked the NRG-induced proliferation of NPCs. The synergic effect of TPA plus NRG on neurosphere growth further prompted us to find that NRG induced NPC propagation through PLC/PKC signaling pathway. ErbB4, an important functional receptor of NRG, had an interaction with PLCgamma1 protein. In addition, inactivation of PLC pathway led to severe proliferative suppression of NPCs. Our study suggests that activation of PLC/PKC pathway plays an essential role in the NRG-induced proliferation of hippocampus-derived NPCs.

Transcription profiling of inner ears from Pou4f3(ddl/ddl) identifies Gfi1 as a target of the Pou4f3 deafness gene

Pou4f3 (Brn3.1, Brn3c) is a class IV POU domain transcription factor that has a central function in the development of all hair cells in the human and mouse inner ear sensory epithelia. A mutation of POU4F3 underlies human autosomal dominant non-syndromic progressive hearing loss DFNA15. Through a comparison of inner ear gene expression profiles of E16.5 wild-type and Pou4f3 mutant deaf mice using a high density oligonucleotide microarray, we identified the gene encoding growth factor independence 1 (Gfi1) as a likely in vivo target gene regulated by Pou4f3. To validate this result, we performed semi-quantitative RT-PCR and in situ hybridizations for Gfi1 on wild-type and Pou4f3 mutant mice. Our results demonstrate that a deficiency of Pou4f3 leads to a statistically significant reduction in Gfi1 expression levels and that the dynamics of Gfi1 mRNA abundance closely follow the pattern of expression for Pou4f3. To examine the role of Gfi1 in the pathogenesis of Pou4f3-related deafness, we performed comparative analyses of the embryonic inner ears of Pou4f3 and Gfi1 mouse mutants using immunohistochemistry and scanning electron microscopy. The loss of Gfi1 results in outer hair cell degeneration, which appears comparable to that observed in Pou4f3 mutants. These results identify Gfi1 as the first downstream target of a hair cell specific transcription factor and suggest that outer hair cell degeneration in Pou4f3 mutants is largely or entirely a result of the loss of expression of Gfi1.

Hair cell re-growth

Hair cell loss is usually a function of age, noise, ototoxic drugs and genetics. Therapeutic strategies fall into two categories: protection and regeneration. Protective methods include targeted application of growth factors and other agents to promote cell survival, and systemic application of drugs to prevent activation of programmed cell death. These strategies are related to treatments that cause predictable damage, such as the use of aminoglycoside antibiotics. The challenge of hair cell regeneration is more difficult. Instead of preventing cell loss, we must consider methods of stimulating cell division and hair cell differentiation from existing cells. We need to know much more about the molecular mechanisms that govern these processes so that we can identify potential targets for specific drugs or gene therapies. One method of approaching the issue is to combine in vitro models of developing hair cells with genomic and proteomic technologies. The benefits of hair cell re-growth may not be limited to full replacement of pre-existing cells. Surrogate hair cells may help to maintain cochlear innervation, even if they do not detect sound, and this property could be harnessed to improve the performance of cochlear implants.

Planar and vertical signals control cellular differentiation and patterning in the mammalian cochlea

The sensory epithelium of the mammalian cochlea is composed of a regular mosaic of sensory hair cells and nonsensory supporting cells. During development, differentiation occurs in a gradient that progresses along the axis of the cochlea from base to apex. To begin to identify some of the factors that regulate this developmental process, the potential roles of planar and vertical signals were examined during early stages of cochlear development. We demonstrate roles for both underlying mesenchymal cells and adjacent epithelial cells in the differentiation and patterning of the sensory epithelium, and in particular in the development of mechanosensory hair cells. As development proceeds, the requirements for both planar and vertical signals decrease, and development of the sensory epithelium becomes essentially independent from these cues. Finally, we demonstrate that the temporal gradient of cellular differentiation is not dependent on planar signals within the developing sensory epithelium.

Wnt signaling mediates reorientation of outer hair cell stereociliary bundles in the mammalian cochlea

In the mammalian cochlea, stereociliary bundles located on mechanosensory hair cells within the sensory epithelium are unidirectionally oriented. Development of this planar polarity is necessary for normal hearing as stereociliary bundles are only sensitive to vibrations in a single plane; however, the mechanisms governing their orientation are unknown. We report that Wnt signaling regulates the development of unidirectional stereociliary bundle orientation. In vitro application of Wnt7a protein or inhibitors of Wnt signaling, secreted Frizzled-related protein 1 or Wnt inhibitory factor 1, disrupts bundle orientation. Moreover, Wnt7a is expressed in a pattern consistent with a role in the polarization of the developing stereociliary bundles. We propose that Wnt signaling across the region of developing outer hair cells gives rise to planar polarity in the mammalian cochlea.

Hair cell regeneration: winging our way towards a sound future

The discovery of hair cell regeneration in the inner ear of birds provides new optimism that there may be a treatment for hearing and balance disorders. In this review we describe the process of hair cell regeneration in birds; including restoration of function, recovery of perception and what is currently known about molecular events, such as growth factors and signalling systems. We examine some of the key recent findings in both birds and mammals.

Fibroblast growth factor signaling regulates pillar cell development in the organ of corti

One of the most striking aspects of the cellular pattern within the sensory epithelium of the mammalian cochlea is the presence of two rows of pillar cells in the region between the single row of inner hair cells and the first row of outer hair cells. The factors that regulate pillar cell development have not been determined; however, previous results suggested a key role for fibroblast growth factor receptor 3 (FGFR3). To examine the specific effects of FGFR3 on pillar cell development, we inhibited receptor activation in embryonic cochlear explant cultures. Results indicated that differentiation of pillar cells is dependent on continuous activation of FGFR3. Moreover, transient inhibition of FGFR3 did not inhibit the pillar cell fate permanently, because reactivation of FGFR3 resulted in the resumption of pillar cell differentiation. The effects of increased FGFR3 activation were determined by exposing cochlear explants to FGF2, a strong ligand for several FGF receptors. Treatment with FGF2 led to a significant increase in the number of pillar cells and to a small increase in the number of inner hair cells. These effects were not dependent on cellular proliferation, suggesting that additional pillar cells and inner hair cells were a result of increased recruitment into the prosensory domain. These results indicate that FGF signaling plays a critical role in the commitment and differentiation of pillar cells. Moreover, the position of the pillar cells appears to be determined by the activation of FGFR3 in a subset of the progenitor cells that initially express this receptor.

Expression of heregulin and ErbB/Her receptors in adult chinchilla cochlear and vestibular sensory epithelium

Immunolabeling of heregulin, a growth factor that enhances cell proliferation in damaged utricles, and one of its binding receptors, ErbB-2, has been briefly described in the P3 rat cochlea and utricle [Zheng et al. (1999) J. Neurocytol. 28, 901-912]. However, little is known about the distribution of heregulin and its three binding receptors in adult animals. Here we describe the immunolabeling patterns for heregulin, ErbB-2, ErbB-3 and ErB-4 in the cochlea, spiral ganglion, utricle and saccule of the adult chinchilla using confocal microscopy. Heregulin immunolabeling was intense along the apical pole of Deiters cells and Hensen cells and along the membrane of supporting cells of the utricle and saccule; light immunolabeling was present in the outer layer of the spiral prominence and cytoplasm of spiral ganglion neurons. In the cochlea, intense to moderate ErbB-2 immunolabeling was evident in the cytoplasm of pillar cells, outer hair cells (OHCs), border cells, stria vascularis and spiral ligament; moderate ErbB-2 immunolabeling was present in the cytoplasm of the hair cell and supporting cell layers of the utricle and saccule. In the cochlea, light ErbB-3 immunolabeling was present in the inner hair cells, OHCs, marginal and intermediate cell layers of the stria vascularis and spiral ganglion neurons; moderate ErbB-3 immunolabeling was present in the cytoplasm of hair cells and supporting cells of the utricle and saccule. In the cochlea, utricle and saccule, ErbB-4 immunolabeling was intense in the nuclei and light to moderate in the cytoplasm and membrane of sensory cells and supporting cells. These results suggest that heregulin acting through ErbB receptors and various receptor complexes may play an important role in cell proliferation and survival in the cochlea and vestibular system.

Cochlear pathology, sensory cell death and regeneration

Loss of cochlear hair cells leads to permanent hearing loss. Hair cells may degenerate due to hereditary or environmental causes, or a combination of the two. Cochlear supporting cells actively participate in the process of hair cell elimination and scar formation by rapidly expanding and sealing the reticular lamina, the barrier between endolymph and perilymph. This scarring process helps preserve the remaining hair cells and hearing. Anti-apoptotic agents, anti-oxidants and several growth factors have been shown to protect hair cells and hearing against environmental insults. Characterization of the genes that regulate the development of the inner ear and its response to trauma has been helpful in designing strategies for enhancing protection of the inner ear and for inducing hair cell regeneration. This chapter discusses the potential for some of these approaches.

Application of new biological approaches to stimulate sensory repair and protection

The inner ear governs hearing and balance via six sense organs, each composed of a few thousand mechanosensory hair cells. Most inner ear disorders involve irreversible loss of hair cells and their associated nerves. They are a function of age, genetic abnormalities and environmental factors such as noise and the use of ototoxic drugs. The genetics and cell biology of the inner ear have revealed some key molecular mechanisms of development and sensory degeneration that raise hopes for new therapeutic approaches to the regeneration of sensory function. This review highlights these advances and the approaches that might be taken to effect protection and repair. It concludes with the suggestion that we can expect tangible, practical progress towards the clinic over the next 5-10 years and that, to provide the training and skills required to take full advantage of emerging technologies, we should forge much closer links between specialist clinicians and basic scientists.

Return of potassium ion channels in regenerated hair cells: possible pathways and the role of intracellular calcium signaling

Recent electrophysiological studies in pigeon have demonstrated that potassium channels are completely functional in regenerated type II hair cells at 21 days post-treatment (PT) with ototoxic doses of streptomycin. The currents return in the order they appear during development. The mixture of ionic currents in a regenerated type II hair cell in a particular region of the neuroepithelium is the same as in its ancestor in that region. The return of currents in regenerated type I hair cells is more complicated. The dominant conductance gKI is not present until after 70 days PT. Before 70 days, the ionic currents in type I hair cells resemble those of regenerated type II hair cells, suggesting that the ionic currents in type II hair cells might be precursors of the ionic currents in regenerated type I hair cells. New data show that at one year PT, the kinetics and drug sensitivity of the dominant K+ conductance in type I hair cells are identical to gKI. Supporting cells, believed to be the precursors of regenerated type II hair cells, have effectively no voltage-gated outward potassium channels, suggesting that regenerated type II hair cells must develop these channels de novo. The next step is to understand the mechanisms by which the potassium channel protein is synthesized, migrates through the cytosol, and is inserted into the plasmalemma of regenerating hair cells. These mechanisms are unknown. We propose that intracellular calcium is involved in this process, as well as in the differentiation, proliferation, and gene regulation of precursor cells fated to become hair cells.

Regeneration in avian hair cell epithelia: identification of intracellular signals required for S-phase entry

Balance epithelia in birds closely resemble their mammalian counterparts, but their cells turnover rapidly and they quickly regenerate hair cells, leading to functional recovery from damage that would be permanent for a mammal. We isolated and cultured sheets of the chicken's utricular epithelium in bromo-deoxyuridine and specific inhibitors of different intracellular signalling pathways to identify signals that influence turnover and regeneration. Synthesis (S-phase) entry was effectively blocked by inhibition of PI3-K, TOR or MAPK, and significantly decreased by inhibitors of PKC. Comparisons indicate that activated PI3-K and TOR are required for S-phase entry in both avian and mammalian balance epithelia, but activation of the MAPK pathway appears to have a more significant role in avian utricles than in mammals. The dissimilarities in the requirements for these signalling pathways do not appear sufficient to explain the marked difference in regenerative capacity between the ears of birds and mammals.

Brief treatments with forskolin enhance s-phase entry in balance epithelia from the ears of rats

In the ears of mammals, hair cell loss results in permanent hearing and balance deficits, whereas in fish, amphibians, and birds, the production of replacement hair cells can restore those modalities. In avian ears, continuous exposures to forskolin trigger cell proliferation and the regeneration of hair cells, so we investigated the effect of forskolin on sensory epithelia cultured from the ears of mammals. Continuous 72 hr exposures to forskolin failed to induce proliferation in neonatal rat utricles, but brief (</=1 hr) exposures to forskolin or Br-cAMP did. Proliferation occurred only in media that contained serum. Forskolin also augmented the mitogenic effects of glial growth factor 2. The S-phase entry induced by forskolin was blocked by monensin and bafilomycin, two compounds that can inhibit the recycling of membrane receptors. The results are consistent with the hypothesis that in mammalian vestibular epithelia elevated cAMP induces S-phase entry by increasing the number of growth factor receptors at the plasma membrane.