Hair cell regeneration in sensory epithelia from the inner ear of a urodele amphibian

Transdifferentiation is uncoupled from progenitor pool expansion during hair cell regeneration in the zebrafish inner ear

Death of mechanosensory hair cells in the inner ear is a common cause of auditory and vestibular impairment in mammals, which have a limited ability to regrow these cells after damage. In contrast, non-mammalian vertebrates including zebrafish can robustly regenerate hair cells following severe organ damage. The zebrafish inner ear provides an understudied model system for understanding hair cell regeneration in organs that are highly conserved with their mammalian counterparts. Here we quantitatively examine hair cell addition during growth and regeneration of the larval zebrafish inner ear. We used a genetically encoded ablation method to induce hair cell death and observed gradual regeneration with correct spatial patterning over two weeks following ablation. Supporting cells, which surround and are a source of new hair cells, divide in response to hair cell ablation, expanding the possible progenitor pool. In parallel, nascent hair cells arise from direct transdifferentiation of progenitor pool cells uncoupled from progenitor division. These findings reveal a previously unrecognized mechanism of hair cell regeneration with implications for how hair cells may be encouraged to regenerate in the mammalian ear.

On the value of diverse organisms in auditory research: From fish to flies to humans

Historically, diverse organisms have contributed to our understanding of auditory function. In recent years, the laboratory mouse has become the prevailing non-human model in auditory research, particularly for biomedical studies. There are many questions in auditory research for which the mouse is the most appropriate (or the only) model system available. But mice cannot provide answers for all auditory problems of basic and applied importance, nor can any single model system provide a synthetic understanding of the diverse solutions that have evolved to facilitate effective detection and use of acoustic information. In this review, spurred by trends in funding and publishing and inspired by parallel observations in other domains of neuroscience, we highlight a few examples of the profound impact and lasting benefits of comparative and basic organismal research in the auditory system. We begin with the serendipitous discovery of hair cell regeneration in non-mammalian vertebrates, a finding that has fueled an ongoing search for pathways to hearing restoration in humans. We then turn to the problem of sound source localization - a fundamental task that most auditory systems have been compelled to solve despite large variation in the magnitudes and kinds of spatial acoustic cues available, begetting varied direction-detecting mechanisms. Finally, we consider the power of work in highly specialized organisms to reveal exceptional solutions to sensory problems - and the diverse returns of deep neuroethological inquiry - via the example of echolocating bats. Throughout, we consider how discoveries made possible by comparative and curiosity-driven organismal research have driven fundamental scientific, biomedical, and technological advances in the auditory field.

Deletion of Notch1 during Cochlear Maturation Leads to Rapid Supporting Cell Death and Profound Deafness

The sensory region of the mammalian hearing organ contains two main cell types-hair cells and supporting cells. During development, Notch signaling plays an important role in whether a cell becomes either a hair cell or supporting cell by mediating lateral inhibition. However, once the cell fate decisions have been determined, little is understood about the role Notch plays in cochlear maturation. Here, we report that deletion of Notch1 from the early postnatal mouse cochlea in both male and female animals resulted in profound deafness at 6 weeks of age. Histologic analyses at 6 weeks revealed significant hair cell and supporting cell loss throughout the Notch1-deficient cochlea. Early analyses revealed a reduction in supporting cells in the outer hair cell region between postnatal day (P) 2 and P6, without a comparable increase in outer hair cell number, suggesting a mechanism other than lateral inhibition. Consistent with this, we found apoptotic cells in the outer supporting cell region of the cochlea at P1 and P2, indicating that Notch1 is required for outer supporting cell survival during early cochlear maturation. Interestingly, inner supporting cell types were not lost after Notch1 deletion. Surprisingly, we do not detect outer hair cell loss in Notch1 mutants until after the onset of hearing, around P14, suggesting that hair cell loss is caused by loss of the supporting cells. Together, these results demonstrate that Notch1 is required for supporting cell survival during early maturation and that loss of these cells causes later loss of the hair cells and cochlear dysfunction.SIGNIFICANCE STATEMENT During development, Notch signaling has been shown to be critical in regulating the cell fate choices between hair cells and supporting cells. However, little is known about how Notch functions after those cell fate choices are made. Here, we examine the role of Notch1 in the maturing cochlea. We demonstrate that deletion of Notch1 results in profound deafness by 6 weeks of age. Histologic analyses revealed rapid supporting cell death shortly after Notch1 deletion, followed by eventual loss of the hair cells. These results reveal an unexpected role for Notch in supporting cell survival during cochlear maturation.

Differential regulation of mammalian and avian ATOH1 by E2F1 and its implication for hair cell regeneration in the inner ear

The mammalian inner ear has a limited capacity to regenerate its mechanosensory hair cells. This lack of regenerative capacity underlies the high incidence of age-related hearing loss in humans. In contrast, non-mammalian vertebrates can form new hair cells when damage occurs, a mechanism that depends on re-activation of expression of the pro-hair cell transcription factor Atoh1. Here, we show that members of the E2F transcription factor family, known to play a key role in cell cycle progression, regulate the expression of Atoh1. E2F1 activates chicken Atoh1 by directly interacting with a cis-regulatory region distal to the avian Atoh1 gene. E2F does not activate mouse Atoh1 gene expression, since this regulatory element is absent in mammals. We also show that E2F1 expression changes dynamically in the chicken auditory epithelium during ototoxic damage and hair cell regeneration. Therefore, we propose a model in which the mitotic regeneration of non-mammalian hair cells is due to E2F1-mediated activation of Atoh1 expression, a mechanism which has been lost in mammals.

Transcription Factor Reprogramming in the Inner Ear: Turning on Cell Fate Switches to Regenerate Sensory Hair Cells

Non-mammalian vertebrates can restore their auditory and vestibular hair cells naturally by triggering the regeneration of adjacent supporting cells. The transcription factor ATOH1 is a key regulator of hair cell development and regeneration in the inner ear. Following the death of hair cells, supporting cells upregulate ATOH1 and give rise to new hair cells. However, in the mature mammalian cochlea, such natural regeneration of hair cells is largely absent. Transcription factor reprogramming has been used in many tissues to convert one cell type into another, with the long-term hope of achieving tissue regeneration. Reprogramming transcription factors work by altering the transcriptomic and epigenetic landscapes in a target cell, resulting in a fate change to the desired cell type. Several studies have shown that ATOH1 is capable of reprogramming cochlear non-sensory tissue into cells resembling hair cells in young animals. However, the reprogramming ability of ATOH1 is lost with age, implying that the potency of individual hair cell-specific transcription factors may be reduced or lost over time by mechanisms that are still not clear. To circumvent this, combinations of key hair cell transcription factors have been used to promote hair cell regeneration in older animals. In this review, we summarize recent findings that have identified and studied these reprogramming factor combinations for hair cell regeneration. Finally, we discuss the important questions that emerge from these findings, particularly the feasibility of therapeutic strategies using reprogramming factors to restore human hearing in the future.

Tissue repair brakes: A common paradigm in the biology of regeneration

To date, most attention on tissue regeneration has focused on the exploration of positive cues promoting or allowing the engagement of natural cellular restoration upon injury. In contrast, the signals fostering cell identity maintenance in the vertebrate body have been poorly investigated; yet they are crucial, for their counteraction could become a powerful method to induce and modulate regeneration. Here we review the mechanisms inhibiting pro-regenerative spontaneous adaptive cell responses in different model organisms and organs. The pharmacological or genetic/epigenetic modulation of such regenerative brakes could release a dormant but innate adaptive competence of certain cell types and therefore boost tissue regeneration in different situations.

Regenerating hair cells in vestibular sensory epithelia from humans

Human vestibular sensory epithelia in explant culture were incubated in gentamicin to ablate hair cells. Subsequent transduction of supporting cells with ATOH1 using an Ad-2 viral vector resulted in generation of highly significant numbers of cells expressing the hair cell marker protein myosin VIIa. Cells expressing myosin VIIa were also generated after blocking the Notch signalling pathway with TAPI-1 but less efficiently. Transcriptomic analysis following ATOH1 transduction confirmed up-regulation of 335 putative hair cell marker genes, including several downstream targets of ATOH1. Morphological analysis revealed numerous cells bearing dense clusters of microvilli at the apical surfaces which showed some hair cell-like characteristics confirming a degree of conversion of supporting cells. However, no cells bore organised hair bundles and several expected hair cell markers genes were not expressed suggesting incomplete differentiation. Nevertheless, the results show a potential to induce conversion of supporting cells in the vestibular sensory tissues of humans.

The inner ear contains our balance system (the vestibular system) and our hearing organ (the cochlea). Their sensing units, the hair cells, detect movement or sound waves. A loss of hair cells is a major cause of inner ear disorders, such as dizziness, imbalance and deafness.

When hair cells die, supporting cells that surround them close the ‘wound’ to repair the tissue. In fish, amphibians, reptiles and birds, the supporting cells can replace lost hair cells, but in mammals – including humans – hair cells are unable to regenerate in the cochlea, so hearing loss is permanent. However, previous research has shown that in certain mammals, spontaneous replacement of lost hair cells in the vestibular system can occur, but not enough to lead to a full recovery.

Scientists have been able to convert supporting cells in the vestibular system of mice into hair cells by using either certain chemicals, or by introducing a specific gene into the supporting cells. In the mouse embryo, this gene, called Atoh1, switches on a signalling pathway in the inner ear, through which a non-specialised precursor cell becomes a hair cell. Inducing hair cell regeneration could be a therapy for inner ear disorders. Therefore, Taylor et al. wanted to find out if such procedures would work in inner ear tissue from humans.

The researchers collected intact tissue samples from the vestibular system of patients who had undergone surgery to have a tumour removed, which would normally destroy the inner ear. All existing hair cells were removed so that mainly supporting cells remained. Then, the tissue was either treated with chemicals that increased the production of hair cells or received the gene ATOH1. The results showed that the cells containing the gene were able to develop many features characteristic of hair cells. And a smaller number of hair cells treated with the chemicals also started to develop hair cell-like features. A gene analysis after the ATOH1 transfer revealed a number of active genes known to be markers of hair cells, but also several inactive ones. This suggests that additional factors are necessary for generating fully functional hair cells.

Dizziness and balance disorders present a major health care burden, particularly in the elderly population. Yet, they are often disregarded and overlooked. This study suggests that hair cell regeneration could be a feasible therapy for some forms of balance disorders linked to loss of vestibular hair cells. More research is needed to identify the other factors at play to test if hair cell regeneration in the cochlea could be used to treat hearing impairment.

MicroRNAs and Presbycusis

Presbycusis (age-related hearing loss) is the most universal sensory degenerative disease in elderly people caused by the degeneration of cochlear cells. Non-coding microRNAs (miRNAs) play a fundamental role in gene regulation in almost every multicellular organism, and control the aging processes. It has been identified that various miRNAs are up- or down-regulated during mammalian aging processes in tissue-specific manners. Most miRNAs bind to specific sites on their target messenger-RNAs (mRNAs) and decrease their expression. Germline mutation may lead to dysregulation of potential miRNAs expression, causing progressive hair cell degeneration and age-related hearing loss. Therapeutic innovations could emerge from a better understanding of diverse function of miRNAs in presbycusis. This review summarizes the relationship between miRNAs and presbycusis, and presents novel miRNAs-targeted strategies against presbycusis.

Elastic force restricts growth of the murine utricle

Dysfunctions of hearing and balance are often irreversible in mammals owing to the inability of cells in the inner ear to proliferate and replace lost sensory receptors. To determine the molecular basis of this deficiency we have investigated the dynamics of growth and cellular proliferation in a murine vestibular organ, the utricle. Based on this analysis, we have created a theoretical model that captures the key features of the organ’s morphogenesis. Our experimental data and model demonstrate that an elastic force opposes growth of the utricular sensory epithelium during development, confines cellular proliferation to the organ’s periphery, and eventually arrests its growth. We find that an increase in cellular density and the subsequent degradation of the transcriptional cofactor Yap underlie this process. A reduction in mechanical constraints results in accumulation and nuclear translocation of Yap, which triggers proliferation and restores the utricle’s growth; interfering with Yap’s activity reverses this effect.

DOI:http://dx.doi.org/10.7554/eLife.25681.001

Development and regeneration of vestibular hair cells in mammals

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

The Transcriptomics to Proteomics of Hair Cell Regeneration: Looking for a Hair Cell in a Haystack

Mature mammals exhibit very limited capacity for regeneration of auditory hair cells, while all non-mammalian vertebrates examined can regenerate them. In an effort to find therapeutic targets for deafness and balance disorders, scientists have examined gene expression patterns in auditory tissues under different developmental and experimental conditions. Microarray technology has allowed the large-scale study of gene expression profiles (transcriptomics) at whole-genome levels, but since mRNA expression does not necessarily correlate with protein expression, other methods, such as microRNA analysis and proteomics, are needed to better understand the process of hair cell regeneration. These technologies and some of the results of them are discussed in this review. Although there is a considerable amount of variability found between studies owing to different species, tissues and treatments, there is some concordance between cellular pathways important for hair cell regeneration. Since gene expression and proteomics data is now commonly submitted to centralized online databases, meta-analyses of these data may provide a better picture of pathways that are common to the process of hair cell regeneration and lead to potential therapeutics. Indeed, some of the proteins found to be regulated in the inner ear of animal models (e.g., IGF-1) have now gone through human clinical trials.

Changes in Regenerative Capacity through Lifespan

Most organisms experience changes in regenerative abilities through their lifespan. During aging, numerous tissues exhibit a progressive decline in homeostasis and regeneration that results in tissue degeneration, malfunction and pathology. The mechanisms responsible for this decay are both cell intrinsic, such as cellular senescence, as well as cell-extrinsic, such as changes in the regenerative environment. Understanding how these mechanisms impact on regenerative processes is essential to devise therapeutic approaches to improve tissue regeneration and extend healthspan. This review offers an overview of how regenerative abilities change through lifespan in various organisms, the factors that underlie such changes and the avenues for therapeutic intervention. It focuses on established models of mammalian regeneration as well as on models in which regenerative abilities do not decline with age, as these can deliver valuable insights for our understanding of the interplay between regeneration and aging.

The Wnt and Notch signalling pathways in the developing cochlea: Formation of hair cells and induction of regenerative potential

The Wnt and Notch signalling pathways control proliferation, specification, and cell fate choices during embryonic development and in adult life. Hence, there is much interest in both signalling pathways in the context of stem cell biology and tissue regeneration. In the developing ear, the Wnt and Notch signalling pathways specify otic cells and refine the ventral boundary of the otic placode. Since both signalling pathways control events essential for the formation of sensory cells, such as proliferation and hair cell differentiation, these pathways could hold promise for the regeneration of hair cells in adult mammalian cochlea. Indeed, modulating either the Wnt or Notch pathways can trigger the regenerative potential of supporting cells. In the neonatal mouse cochlea, Notch-mediated regeneration of hair cells partially depends on Wnt signalling, which implies an interaction between the pathways. This review presents how the Wnt and Notch signalling pathways regulate the formation of sensory hair cells and how modulating their activity induces regenerative potential in the mammalian cochlea.

Characterizing human vestibular sensory epithelia for experimental studies: new hair bundles on old tissue and implications for therapeutic interventions in ageing

Balance disequilibrium is a significant contributor to falls in the elderly. The most common cause of balance dysfunction is loss of sensory cells from the vestibular sensory epithelia of the inner ear. However, inaccessibility of inner ear tissue in humans severely restricts possibilities for experimental manipulation to develop therapies to ameliorate this loss. We provide a structural and functional analysis of human vestibular sensory epithelia harvested at trans-labyrinthine surgery. We demonstrate the viability of the tissue and labeling with specific markers of hair cell function and of ion homeostasis in the epithelium. Samples obtained from the oldest patients revealed a significant loss of hair cells across the tissue surface, but we found immature hair bundles present in epithelia harvested from patients >60 years of age. These results suggest that the environment of the human vestibular sensory epithelium could be responsive to stimulation of developmental pathways to enhance hair cell regeneration, as has been demonstrated successfully in the vestibular organs of adult mice.

Robust regeneration of adult zebrafish lateral line hair cells reflects continued precursor pool maintenance

We have examined lateral line hair cell and support cell maintenance in adult zebrafish when growth is largely complete. We demonstrate that adult zebrafish not only replenish hair cells after a single instance of hair cell damage, but also maintain hair cells and support cells after multiple rounds of damage and regeneration. We find that hair cells undergo continuous turnover in adult zebrafish in the absence of damage. We identify mitotically-distinct support cell populations and show that hair cells regenerate from underlying support cells in a region-specific manner. Our results demonstrate that there are two distinct support cell populations in the lateral line, which may help explain why zebrafish hair cell regeneration is extremely robust, retained throughout life, and potentially unlimited in regenerative capacity.

In vitro preparation of newt inner ear sensory epithelia as a model for repair and regeneration

The sensory "hair" cells of the inner ear transform sound energy into electrical signals, but are readily lost through aging, excessive noise, and ototoxic agents. The newt provides an excellent model in which to explore regeneration and whilst loss of hair cells from inner ear epithelia does not require whole organ regeneration, new hair cells are generated from differentiated supporting cells that transdifferentiate without an intervening mitotic event. Here we describe the methods for maintaining the sensory epithelia in long term culture; for the use of the aminoglycoside, gentamicin, to kill the hair cells; and for the examination of the tissue by electron microscopy or fluorescence microscopy. Demembranation of the epithelium reveals the underlying ultrastructure of the tissue for examination by scanning electron microscopy (SEM) and is a technique that can be utilized with immunogold labelling.

Gap junctional coupling is essential for epithelial repair in the avian cochlea

The loss of auditory hair cells triggers repair responses within the population of nonsensory supporting cells. When hair cells are irreversibly lost from the mammalian cochlea, supporting cells expand to fill the resulting lesions in the sensory epithelium, an initial repair process that is dependent on gap junctional intercellular communication (GJIC). In the chicken cochlea (the basilar papilla or BP), dying hair cells are extruded from the epithelium and supporting cells expand to fill the lesions and then replace hair cells via mitotic and/or conversion mechanisms. Here, we investigated the involvement of GJIC in the initial epithelial repair process in the aminoglycoside-damaged BP. Gentamicin-induced hair cell loss was associated with a decrease of chicken connexin43 (cCx43) immunofluorescence, yet cCx30-labeled gap junction plaques remained. Fluorescence recovery after photobleaching experiments confirmed that the GJIC remained robust in gentamicin-damaged explants, but regionally asymmetric coupling was no longer evident. Dye injections in slice preparations from undamaged BP explants identified cell types with characteristic morphologies along the neural-abneural axis, but these were electrophysiologically indistinct. In gentamicin-damaged BP, supporting cells expanded to fill space formerly occupied by hair cells and displayed more variable electrophysiological phenotypes. When GJIC was inhibited during the aminoglycoside damage paradigm, the epithelial repair response halted. Dying hair cells were retained within the sensory epithelium and supporting cells remained unexpanded. These observations suggest that repair of the auditory epithelium shares common mechanisms across vertebrate species and emphasize the importance of functional gap junctions in maintaining a homeostatic environment permissive for subsequent hair cell regeneration.

Morphological and morphometric characterization of direct transdifferentiation of support cells into hair cells in ototoxin-exposed neonatal utricular explants

We have studied aminoglycoside-induced vestibular hair-cell renewal using long-term culture of utricular macula explants from 4-day-old rats. Explanted utricles were exposed to 1 mM of gentamicin for 48 h, during 2nd and 3rd days in vitro (DIV), and then recovering in unsupplemented medium. Utricles were harvested at specified time points from the 2nd through the 28th DIV. The cellular events that occurred within hair cell epithelia during the culture period were documented from serial sectioned specimens. Vestibular hair cells (HCs) and supporting cells (SCs) were systematically counted using light microscopy (LM) with the assistance of morphometric software. Ultrastructural observations were made from selected specimens with transmission electron microscopy (TEM). After 7 DIV, i.e. four days after gentamicin exposure, the density of HCs was 11% of the number of HCs observed in non-gentamicin-exposed control explants. At 28 DIV the HC density was 61% of the number of HCs observed in the control group explant specimens. Simultaneously with this increase in HCs there was a corresponding decline in the number of SCs within the epithelium. The proportion of HCs in relation to SCs increased significantly in the gentamicin-exposed explant group during the 5th to the 28th DIV period of culture. There were no significant differences in the volume estimations of the gentamicin-exposed and the control group explants during the observed period of culture. Morphological observations showed that gentamicin exposure induced extensive loss of HCs within the epithelial layer, which retained their intact apical and basal linings. At 7 to 14 DIV (i.e. 3-11 days after gentamicin exposure) a pseudostratified epithelium with multiple layers of disorganized cells was observed. At 21 DIV new HCs were observed that also possessed features resembling SCs. After 28 DIV a new luminal layer of HCs with several layers of SCs located more basally characterized the gentamicin-exposed epithelium. No mitoses were observed within the epithelial layer of any explants. Our conclusion is that direct transdifferentiation of SCs into HCs was the only process contributing to the renewal of HCs after gentamicin exposure in these explants of vestibular inner ear epithelia obtained from the labyrinths of 4-day-old rats.

Aging and regeneration in vertebrates

Aging is marked by changes that affect organs and resident stem cell function. Shorting of telomeres, DNA damage, oxidative stress, deregulation of genes and proteins, impaired cell-cell communication, and an altered systemic environment cause the eventual demise of cells. At the same time, reparative activities also decline. It is intriguing to correlate aging with the decline of regenerative abilities. Animal models with strong regenerative capabilities imply that aging processes might not be affecting regeneration. In this review, we selectively present age-dependent changes in stem/progenitor cells that are vital for tissue homeostasis and repair. In addition, the aging effect on regeneration following injury in organs such as lung, skeletal muscle, heart, nervous system, cochlear hair, lens, and liver are discussed. These tissues are also known for diseases such as heart attack, stroke, cognitive impairment, cataract, and hearing loss that occur mostly during aging in humans. Conclusively, vertebrate regeneration declines with age with the loss of stem/progenitor cell function. Future studies on improving the function of stem cells, along with studies in fish and amphibians where regeneration does not decline with age, will undoubtedly provide insights into both processes.

Growth factors have a protective effect on neomycin-induced hair cell loss

We have demonstrated that selected growth factors are involved in regulating survival and proliferation of progenitor cells derived from the neonatal rat organ of Corti (OC). The protective and regenerative effects of these defined growth factors on the injured organ of Corti were therefore investigated. The organ of Corti dissected from the Wistar rat pups (P3-P5) was split into apical, middle, and basal parts, explanted and cultured with or without neomycin and growth factors. Insulin-like growth factor-1 (IGF-1), fibroblast growth factor-2 (FGF-2), and epidermal growth factor (EGF) protected the inner hair cells (IHCs) and outer hair cells (OHCs) from neomycin ototoxicity. Using EGF, IGF-1, and FGF-2 alone induced no protective effect on the survival of auditory hair cells. Combining 2 growth factors (EGF + IGF-1, EGF + FGF-2, or IGF-1 + FGF-2) gave statistically protective effects. Similarly, combining all three growth factors effectively protected auditory hair cells from the ototoxic insult. None of the growth factors induced regeneration of hair cells in the explants injured with neomycin. Thus various combinations of the three defined factors (IGF-1, FGF-2, and EGF) can protect the auditory hair cells from the neomycin-induced ototoxic damage, but no regeneration was seen. This offers a possible novel approach to the treatment of hearing loss.

Inner ear tissue preservation by rapid freezing: improving fixation by high-pressure freezing and hybrid methods

In the preservation of tissues in as ‘close to life’ state as possible, rapid freeze fixation has many benefits over conventional chemical fixation. One technique by which rapid freeze-fixation can be achieved, high pressure freezing (HPF), has been shown to enable ice crystal artefact-free freezing and tissue preservation to greater depths (more than 40 μm) than other quick-freezing methods. Despite increasingly becoming routine in electron microscopy, the use of HPF for the fixation of inner ear tissue has been limited. Assessment of the quality of preservation showed routine HPF techniques were suitable for preparation of inner ear tissues in a variety of species. Good preservation throughout the depth of sensory epithelia was achievable. Comparison to chemically fixed tissue indicated that fresh frozen preparations exhibited overall superior structural preservation of cells. However, HPF fixation caused characteristic artefacts in stereocilia that suggested poor quality freezing of the actin bundles. The hybrid technique of pre-fixation and high pressure freezing was shown to produce cellular preservation throughout the tissue, similar to that seen in HPF alone. Pre-fixation HPF produced consistent high quality preservation of stereociliary actin bundles. Optimising the preparation of samples with minimal artefact formation allows analysis of the links between ultrastructure and function in inner ear tissues.

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Routine high pressure freezing can preserve large depths of inner ear tissue.

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Stereocilial actin preserved by rapid freezing exhibits characteristic artefacts.

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Hybrid methods of fixation improved structural preservation of stereocilial actin.

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Improved fixation will reduce artefacts in ultrastructural studies of the inner ear.

Sensational placodes: neurogenesis in the otic and olfactory systems

For both the intricate morphogenetic layout of the sensory cells in the ear and the elegantly radial arrangement of the sensory neurons in the nose, numerous signaling molecules and genetic determinants are required in concert to generate these specialized neuronal populations that help connect us to our environment. In this review, we outline many of the proteins and pathways that play essential roles in the differentiation of otic and olfactory neurons and their integration into their non-neuronal support structures. In both cases, well-known signaling pathways together with region-specific factors transform thickened ectodermal placodes into complex sense organs containing numerous, diverse neuronal subtypes. Olfactory and otic placodes, in combination with migratory neural crest stem cells, generate highly specialized subtypes of neuronal cells that sense sound, position and movement in space, odors and pheromones throughout our lives.

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.

Lead roles for supporting actors: critical functions of inner ear supporting cells

Many studies that aim to investigate the underlying mechanisms of hearing loss or balance disorders focus on the hair cells and spiral ganglion neurons of the inner ear. Fewer studies have examined the supporting cells that contact both of these cell types in the cochlea and vestibular end organs. While the roles of supporting cells are still being elucidated, emerging evidence indicates that they serve many functions vital to maintaining healthy populations of hair cells and spiral ganglion neurons. Here we review recent studies that highlight the critical roles supporting cells play in the development, function, survival, death, phagocytosis, and regeneration of other cell types within the inner ear. Many of these roles have also been described for glial cells in other parts of the nervous system, and lessons from these other systems continue to inform our understanding of supporting cell functions. This article is part of a Special Issue entitled "Annual Reviews 2013".

Filling the silent void: genetic therapies for hearing impairment

The inner ear cytoarchitecture forms one of the most intricate and delicate organs in the human body and is vulnerable to the effects of genetic disorders, aging, and environmental damage. Owing to the inability of the mammalian cochlea to regenerate sensory hair cells, the loss of hair cells is a leading cause of deafness in humans. Millions of individuals worldwide are affected by the emotionally and financially devastating effects of hearing impairment (HI). This paper provides a brief introduction into the key role of genes regulating inner ear development and function. Potential future therapies that leverage on an improved understanding of these molecular pathways are also described in detail.

Proliferative regeneration of zebrafish lateral line hair cells after different ototoxic insults

Sensory hair cells in the zebrafish lateral line regenerate rapidly and completely after damage. Previous studies have used a variety of ototoxins to kill lateral line hair cells to study different phenomena including mechanisms of hair cell death and regeneration. We sought to directly compare these ototoxins to determine if they differentially affected the rate and amount of hair cell replacement. In addition, previous studies have found evidence of proliferative hair cell regeneration in zebrafish, but both proliferation and non-mitotic direct transdifferentiation have been observed during hair cell regeneration in the sensory epithelia of birds and amphibians. We sought to test whether a similar combination of regenerative mechanisms exist in the fish. We analyzed the time course of regeneration after treatment with different ototoxic compounds and also labeled dividing hair cell progenitors. Certain treatments, including cisplatin and higher concentrations of dissolved copper, significantly delayed regeneration by one or more days. However, cisplatin did not block all regeneration as observed previously in the chick basilar papilla. The particular ototoxin did not appear to affect the mechanism of regeneration, as we observed evidence of recent proliferation in the majority of new hair cells in all cases. Inhibiting proliferation with flubendazole blocked the production of new hair cells and prevented the accumulation of additional precursors, indicating that proliferation has a dominant role during regeneration of lateral line hair cells.

Microarray analysis of microRNA expression during axolotl limb regeneration

Among vertebrates, salamanders stand out for their remarkable capacity to quickly regrow a myriad of tissues and organs after injury or amputation. The limb regeneration process in axolotls (Ambystoma mexicanum) has been well studied for decades at the cell-tissue level. While several developmental genes are known to be reactivated during this epimorphic process, less is known about the role of microRNAs in urodele amphibian limb regeneration. Given the compelling evidence that many microRNAs tightly regulate cell fate and morphogenetic processes through development and adulthood by modulating the expression (or re-expression) of developmental genes, we investigated the possibility that microRNA levels change during limb regeneration. Using two different microarray platforms to compare the axolotl microRNA expression between mid-bud limb regenerating blastemas and non-regenerating stump tissues, we found that miR-21 was overexpressed in mid-bud blastemas compared to stump tissue. Mature A. mexicanum (“Amex”) miR-21 was detected in axolotl RNA by Northern blot and differential expression of Amex-miR-21 in blastema versus stump was confirmed by quantitative RT-PCR. We identified the Amex Jagged1 as a putative target gene for miR-21 during salamander limb regeneration. We cloned the full length 3′UTR of Amex-Jag1, and our in vitro assays demonstrated that its single miR-21 target recognition site is functional and essential for the response of the Jagged1 gene to miR-21 levels. Our findings pave the road for advanced in vivo functional assays aimed to clarify how microRNAs such as miR-21, often linked to pathogenic cell growth, might be modulating the redeployment of developmental genes such as Jagged1 during regenerative processes.

Identification of modulators of hair cell regeneration in the zebrafish lateral line

The external location of the zebrafish lateral line makes it a powerful model for studying mechanosensory hair cell regeneration. We have developed a chemical screen to identify FDA-approved drugs and biologically active compounds that modulate hair cell regeneration in zebrafish. Of the 1680 compounds evaluated, we identified two enhancers and six inhibitors of regeneration. The two enhancers, dexamethasone and prednisolone, are synthetic glucocorticoids that potentiated hair cell numbers during regeneration and also induced hair cell addition in the absence of damage. BrdU analysis confirmed that the extra hair cells arose from mitotic activity. We found that dexamethasone and prednisolone, like other glucocorticoids, suppress zebrafish caudal fin regeneration, indicating that hair cell regeneration occurs by a distinctly different process. Further analyses of the regeneration inhibitors revealed that two of the six, flubendazole and topotecan, significantly suppress hair cell regeneration by preventing proliferation of hair cell precursors. Flubendazole halted support cell division in M-phase, possibly by interfering with normal microtubule activity. Topotecan, a topoisomerase inhibitor, killed both hair cells and proliferating hair cell precursors. A third inhibitor, fulvestrant, moderately delayed hair cell regeneration by reducing support cell proliferation. Our observation that hair cells do not regenerate when support cell proliferation is impeded confirms previous observations that cell division is the primary route for hair cell regeneration after neomycin treatment in zebrafish.

Defining the cellular environment in the organ of Corti following extensive hair cell loss: a basis for future sensory cell replacement in the Cochlea

Background

Following the loss of hair cells from the mammalian cochlea, the sensory epithelium repairs to close the lesions but no new hair cells arise and hearing impairment ensues. For any cell replacement strategy to be successful, the cellular environment of the injured tissue has to be able to nurture new hair cells. This study defines characteristics of the auditory sensory epithelium after hair cell loss.

Methodology/Principal Findings

Studies were conducted in C57BL/6 and CBA/Ca mice. Treatment with an aminoglycoside-diuretic combination produced loss of all outer hair cells within 48 hours in both strains. The subsequent progressive tissue re-organisation was examined using immunohistochemistry and electron microscopy. There was no evidence of significant de-differentiation of the specialised columnar supporting cells. Kir4.1 was down regulated but KCC4, GLAST, microtubule bundles, connexin expression patterns and pathways of intercellular communication were retained. The columnar supporting cells became covered with non-specialised cells migrating from the outermost region of the organ of Corti. Eventually non-specialised, flat cells replaced the columnar epithelium. Flat epithelium developed in distributed patches interrupting regions of columnar epithelium formed of differentiated supporting cells. Formation of the flat epithelium was initiated within a few weeks post-treatment in C57BL/6 mice but not for several months in CBA/Ca's, suggesting genetic background influences the rate of re-organisation.

Conclusions/Significance

The lack of dedifferentiation amongst supporting cells and their replacement by cells from the outer side of the organ of Corti are factors that may need to be considered in any attempt to promote endogenous hair cell regeneration. The variability of the cellular environment along an individual cochlea arising from patch-like generation of flat epithelium, and the possible variability between individuals resulting from genetic influences on the rate at which remodelling occurs may pose challenges to devising the appropriate regenerative therapy for a deaf patient.

Growth hormone promotes hair cell regeneration in the zebrafish (Danio rerio) inner ear following acoustic trauma

Background

Previous microarray analysis showed that growth hormone (GH) was significantly upregulated following acoustic trauma in the zebrafish (Danio rerio) ear suggesting that GH may play an important role in the process of auditory hair cell regeneration. Our objective was to examine the effects of exogenous and endogenous GH on zebrafish inner ear epithelia following acoustic trauma.

Methodology/Principal Findings

We induced auditory hair cell damage by exposing zebrafish to acoustic overstimulation. Fish were then injected intraperitoneally with either carp GH or buffer, and placed in a recovery tank for either one or two days. Phalloidin-, bromodeoxyuridine (BrdU)-, and TUNEL-labeling were used to examine hair cell densities, cell proliferation, and apoptosis, respectively. Two days post-trauma, saccular hair cell densities in GH-treated fish were similar to that of baseline controls, whereas buffer-injected fish showed significantly reduced densities of hair cell bundles. Cell proliferation was greater and apoptosis reduced in the saccules, lagenae, and utricles of GH-treated fish one day following trauma compared to controls. Fluorescent in situ hybridization (FISH) was used to examine the localization of GH mRNA in the zebrafish ear. At one day post-trauma, GH mRNA expression appeared to be localized perinuclearly around erythrocytes in the blood vessels of the inner ear epithelia. In order to examine the effects of endogenous GH on the process of cell proliferation in the ear, a GH antagonist was injected into zebrafish immediately following acoustic trauma, resulting in significantly decreased cell proliferation one day post-trauma in all three zebrafish inner ear end organs.

Conclusions/Significance

Our results show that exogenous GH promotes post-trauma auditory hair cell regeneration in the zebrafish ear through stimulating proliferation and suppressing apoptosis, and that endogenous GH signals are present in the zebrafish ear during the process of auditory hair cell regeneration.

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.

Transcriptomic analysis of the zebrafish inner ear points to growth hormone mediated regeneration following acoustic trauma

Background

Unlike mammals, teleost fishes are capable of regenerating sensory inner ear hair cells that have been lost following acoustic or ototoxic trauma. Previous work indicated that immediately following sound exposure, zebrafish saccules exhibit significant hair cell loss that recovers to pre-treatment levels within 14 days. Following acoustic trauma in the zebrafish inner ear, we used microarray analysis to identify genes involved in inner ear repair following acoustic exposure. Additionally, we investigated the effect of growth hormone (GH) on cell proliferation in control zebrafish utricles and saccules, since GH was significantly up-regulated following acoustic trauma.

Results

Microarray analysis, validated with the aid of quantitative real-time PCR, revealed several genes that were highly regulated during the process of regeneration in the zebrafish inner ear. Genes that had fold changes of ≥ 1.4 and P -values ≤ 0.05 were considered significantly regulated and were used for subsequent analysis. Categories of biological function that were significantly regulated included cancer, cellular growth and proliferation, and inflammation. Of particular significance, a greater than 64-fold increase in growth hormone (gh1) transcripts occurred, peaking at 2 days post-sound exposure (dpse) and decreasing to approximately 5.5-fold by 4 dpse. Pathway Analysis software was used to reveal networks of regulated genes and showed how GH affected these networks. Subsequent experiments showed that intraperitoneal injection of salmon growth hormone significantly increased cell proliferation in the zebrafish inner ear. Many other gene transcripts were also differentially regulated, including heavy and light chain myosin transcripts, both of which were down-regulated following sound exposure, and major histocompatability class I and II genes, several of which were significantly regulated on 2 dpse.

Conclusions

Transcripts for GH, MHC Class I and II genes, and heavy- and light-chain myosins, as well as many others genes, were differentially regulated in the zebrafish inner ear following overexposure to sound. GH injection increased cell proliferation in the inner ear of non-sound-exposed zebrafish, suggesting that GH could play an important role in sensory hair cell regeneration in the teleost ear.

Mature mice lacking Rbl2/p130 gene have supernumerary inner ear hair cells and supporting cells

Adult mammalian auditory hair cells (HCs) and their associated supporting cells (SCs) do not proliferate, and HC death leads to irreversible neurosensory hearing loss and balance impairment. In nonmammalian vertebrates, loss of HCs induces mitotic proliferation of adjacent nonsensory SCs and/or direct SC transdifferentiation to generate replacement cells. This results in the structural and functional recovery of the nonmammalian sensory systems. Potential replacement of mammalian auditory HCs, either by transplanting cells or by transforming existing cells through molecular therapy, has long been proposed. However, HC replacement strategies with clear therapeutic potential remain elusive. The retinoblastoma (pRB) family of cell cycle regulators, Rb1, Rbl1 (p107), and Rbl2 (p130), regulate the G(1)- to S-phase transition in proliferating cells. In the inner ear, the biochemical and molecular pathways involving pRBs, particularly p107 and p130, are relatively unexplored and their therapeutic suitability is yet to be determined. In this study, we analyzed the cochleae of adult p130 knock-out (p130(-/-)) mice and showed that lack of the p130 gene results in extra rows of HCs and SCs in the more apical regions of the cochlea. No evidence of transdifferentiation of these supernumerary SCs into HCs was observed in the p130(-/-) mouse. Nevertheless, unscheduled proliferation of SCs in the adult p130(-/-) cochlea coupled to downregulation of bona fide cell cycle inhibitors provides a mechanistic basis for the role of p130 as a regulator of SC and HC mitotic quiescence in the more apical regions of the cochlea. Interestingly, p130(-/-) mice exhibited nearly normal peripheral auditory sensitivity.

MicroRNA181a plays a key role in hair cell regeneration in the avian auditory epithelium

Specialized sensory-transducing hair cells regenerate in response to injury in non-mammalian vertebrates such as birds and fish but not in mammals. Previous work has shown that overexpression of microRNA181a (miR181a) in cultured chicken basilar papillae, the avian counterpart of the cochlea, is sufficient to stimulate proliferation with production of new hair cells. The present study investigates the role of miR181a in hair cell regeneration after injury in explants of chicken auditory epithelia. Basilar papillae were explanted from 0-day-old chickens and transfected with either anti-miR181a, which knocks down endogenous miR181a, or a non-targeting miRNA and cultured with streptomycin to eliminate all hair cells from the epithelium. Labeling with BrdU was used to quantify proliferation. Explants exposed to streptomycin and transfected with anti-miR181a had significantly fewer BrdU positive cells than basilar papillae treated with streptomycin and transfected with a non-targeting miRNA. Activated caspase-3 and myosin VI labeling were used to show that the pattern of hair cell death and loss, respectively, were not affected by anti-miR181a transfection. MiR181a downregulation therefore seems to dimish the proliferative component of hair cell regeneration rather than prevent hair cell death following ototoxic injury.

Identification of the hair cell soma-1 antigen, HCS-1, as otoferlin

Hair cells, the mechanosensitive receptor cells of the inner ear, are critical for our senses of hearing and balance. The small number of these receptor cells in the inner ear has impeded the identification and characterization of proteins important for hair cell function. The binding specificity of monoclonal antibodies provides a means for identifying hair cell-specific proteins and isolating them for further study. We have generated a monoclonal antibody, termed hair cell soma-1 (HCS-1), which specifically immunolabels hair cells in at least five vertebrate classes, including sharks and rays, bony fish, amphibians, birds, and mammals. We used HCS-1 to immunoprecipitate the cognate antigen and identified it as otoferlin, a member of the ferlin protein family. Mutations in otoferlin underlie DFNB9, a recessive, nonsyndromic form of prelingual deafness characterized as an auditory neuropathy. Using immunocytochemistry, we find that otoferlin is associated with the entire basolateral membrane of the hair cells and with vesicular structures distributed throughout most of the hair cell cytoplasm. Biochemical assays indicate that otoferlin is tightly associated with membranes, as it is not solubilized by alterations in calcium or salt concentrations. HCS-1 immunolabeling does not co-localize with ribeye, a constituent of synaptic ribbons, suggesting that otoferlin may, in addition to its proposed function in synaptic vesicle release, play additional roles in hair cells.

Notch signaling and Atoh1 expression during hair cell regeneration in the mouse utricle

The mammalian vestibular epithelium has a limited capacity for spontaneous hair cell regeneration. The mechanism underlying the regeneration is not well understood. Because the Notch signaling pathway mediates the formation of the sensory epithelial mosaic patterning during ear development, it may also play a role in hair cell regeneration in the mature mammalian vestibular epithelium after a lesion. To investigate the process of spontaneous regeneration in the vestibular epithelium vis-à-vis changes in Notch signaling, we induced a unilateral lesion by infusing streptomycin into the mouse posterior semicircular canal, and examined Notch signaling molecules and their mRNA expression levels by immunohistochemistry and quantitative real-time polymerase chain reaction (qRTPCR), respectively. We detected Jagged1 in supporting cells in both normal and lesioned utricles. Atoh1, a marker for early developing hair cells, was absent in the intact mature tissue, but re-appeared after the lesion. Many cells were either positive for both Atoh1 and myosin VIIa, or for one of them. qRTPCR data showed a post trauma decrease of Hes5 and an increase in Atoh1. Atoh1 up-regulation may either be a result of Hes5 down-regulation or mediated by another signaling pathway.

Gene expression analysis of forskolin treated basilar papillae identifies microRNA181a as a mediator of proliferation

Background

Auditory hair cells spontaneously regenerate following injury in birds but not mammals. A better understanding of the molecular events underlying hair cell regeneration in birds may allow for identification and eventually manipulation of relevant pathways in mammals to stimulate regeneration and restore hearing in deaf patients.

Methodology/Principal Findings

Gene expression was profiled in forskolin treated (i.e., proliferating) and quiescent control auditory epithelia of post-hatch chicks using an Affymetrix whole-genome chicken array after 24 (n = 6), 48 (n = 6), and 72 (n = 12) hours in culture. In the forskolin-treated epithelia there was significant (p<0.05; >two-fold change) upregulation of many genes thought to be relevant to cell cycle control and inner ear development. Gene set enrichment analysis was performed on the data and identified myriad microRNAs that are likely to be upregulated in the regenerating tissue, including microRNA181a (miR181a), which is known to mediate proliferation in other systems. Functional experiments showed that miR181a overexpression is sufficient to stimulate proliferation within the basilar papilla, as assayed by BrdU incorporation. Further, some of the newly produced cells express the early hair cell marker myosin VI, suggesting that miR181a transfection can result in the production of new hair cells.

Conclusions/Significance

These studies have identified a single microRNA, miR181a, that can cause proliferation in the chicken auditory epithelium with production of new hair cells.

Supporting cell division is not required for regeneration of auditory hair cells after ototoxic injury in vitro

In chickens, nonsensory supporting cells divide and regenerate auditory hair cells after injury. Anatomical evidence suggests that supporting cells can also transdifferentiate into hair cells without dividing. In this study, we characterized an organ culture model to study auditory hair cell regeneration, and we used these cultures to test if direct transdifferentiation alone can lead to significant hair cell regeneration. Control cultures (organs from posthatch chickens maintained without streptomycin) showed complete hair cell loss in the proximal (high-frequency) region by 5 days. In contrast, a 2-day treatment with streptomycin induced loss of hair cells from all regions by 3 days. Hair cell regeneration proceeded in culture, with the time course of supporting cell division and hair cell differentiation generally resembling in vivo patterns. The degree of supporting cell division depended upon the presence of streptomycin, the epithelial region, the type of culture media, and serum concentration. On average, 87% of the regenerated hair cells lacked the cell division marker BrdU despite its continuous presence, suggesting that most hair cells were regenerated via direct transdifferentiation. Addition of the DNA polymerase inhibitor aphidicolin to culture media prevented supporting cell division, but numerous hair cells were regenerated nonetheless. These hair cells showed signs of functional maturation, including stereociliary bundles and rapid uptake of FM1-43. These observations demonstrate that direct transdifferentiation is a significant mechanism of hair cell regeneration in the chicken auditory after streptomycin damage in vitro.

Inner ear protection and regeneration: a 'historical' perspective

PURPOSE OF REVIEW

In evaluating strategies to preserve or regenerate the cochlea, understanding the process of labyrinthine injury on a cellular and molecular level is crucial. Examination of inner ear injury reveals mechanism-specific types of damage, often at specific areas within the cochlea. Site-specific interventions can then be considered.

RECENT FINDINGS

The review will briefly summarize the historical perspective of advancements in hearing science through 2006. Areas of research covered include hair cell protection, hair cell regeneration, spiral ganglion cell regeneration, and stria vascularis metabolic regulation.

SUMMARY

The review will briefly summarize the early development of a few such site-specific interventions for inner ear functional rehabilitation, for work done prior to 2006. The outstanding reviews of cutting edge research from this year's and last year's Hearing Science section of Current Opinion in Otolaryngology - Head and Neck Surgery can then be understood and appreciated in a more informed manner.

Epigenetic influences on sensory regeneration: histone deacetylases regulate supporting cell proliferation in the avian utricle

The sensory hair cells of the cochlea and vestibular organs are essential for normal hearing and balance function. The mammalian ear possesses a very limited ability to regenerate hair cells and their loss can lead to permanent sensory impairment. In contrast, hair cells in the avian ear are quickly regenerated after acoustic trauma or ototoxic injury. The very different regenerative abilities of the avian vs. mammalian ear can be attributed to differences in injury-evoked expression of genes that either promote or inhibit the production of new hair cells. Gene expression is regulated both by the binding of cis-regulatory molecules to promoter regions as well as through structural modifications of chromatin (e.g., methylation and acetylation). This study examined effects of histone deacetylases (HDACs), whose main function is to modify histone acetylation, on the regulation of regenerative proliferation in the chick utricle. Cultures of regenerating utricles and dissociated cells from the utricular sensory epithelia were treated with the HDAC inhibitors valproic acid, trichostatin A, sodium butyrate, and MS-275. All of these molecules prevent the enzymatic removal of acetyl groups from histones, thus maintaining nuclear chromatin in a "relaxed" (open) configuration. Treatment with all inhibitors resulted in comparable decreases in supporting cell proliferation. We also observed that treatment with the HDAC1-, 2-, and 3-specific inhibitor MS-275 was sufficient to reduce proliferation and that two class I HDACs--HDAC1 and HDAC2--were expressed in the sensory epithelium of the utricle. These results suggest that inhibition of specific type I HDACs is sufficient to prevent cell cycle entry in supporting cells. Notably, treatment with HDAC inhibitors did not affect the differentiation of replacement hair cells. We conclude that histone deacetylation is a positive regulator of regenerative proliferation but is not critical for avian hair cell differentiation.

Cell proliferation follows acoustically-induced hair cell bundle loss in the zebrafish saccule

Fishes are capable of regenerating sensory hair cells in the inner ear after acoustic trauma. However, a time course of auditory hair cell regeneration has not been established for zebrafish. Adult zebrafish (Danio rerio) were exposed to a 100 Hz pure tone at 179 dB re 1 microPa RMS for 36 h and then allowed to recover for 0-14 days before morphological analysis. Hair cell bundle loss and recovery were determined using phalloidin to visualize hair bundles. Cell proliferation was quantified through bromodeoxyuridine (BrdU) labeling. Immediately following sound exposure, zebrafish saccules exhibited significant hair bundle damage (e.g., splayed, broken, and missing stereocilia) and loss (i.e., missing bundles and lesions in the epithelia) in the caudal region. Hair bundle counts increased over the course of the experiment, reaching pre-treatment levels at 14 days post-sound exposure (dpse). Low levels of proliferation were observed in untreated controls, indicating that some cells of the zebrafish saccule are mitotically active in the absence of a damaging event. In sound-exposed fish, cell proliferation peaked two dpse in the caudal region, and to a lesser extent in the rostral region. This proliferation was followed by an increase in numbers of cuticular plates with rudimentary stereocilia and immature-like hair bundles at 7 and 14 dpse, suggesting that at least some of the saccular cell proliferation resulted in newly formed hair cells. This study establishes a time course of hair cell bundle regeneration in the zebrafish inner ear and demonstrates that cell proliferation is associated with the regenerative process.

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

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