Atoh1 expression and function during auditory hair cell regeneration in post-hatch chickens

A Low Dose of Rapamycin Promotes Hair Cell Differentiation by Enriching SOX2+ Progenitors in the Neonatal Mouse Inner Ear Organoids

PURPOSE

To investigate the impact of rapamycin on the differentiation of hair cells.

METHODS

Murine cochlear organoids were derived from cochlear progenitor cells. Different concentrations of rapamycin were added into the culture medium at different proliferation and differentiation stages.

RESULTS

Rapamycin exhibited a concentration-dependent reduction in the proliferation of these inner ear organoids. Nevertheless, organoids subjected to a 10-nM dose of rapamycin demonstrated a markedly increased proportion of hair cells. Furthermore, rapamycin significantly upregulated the expression of markers associated with both hair cells and supporting cells, including ATOH1, MYO7A, and SOX2. Mechanistic studies revealed that rapamycin preferentially suppressed cells without Sox2 expression during the initial proliferation stage, thereby augmenting and refining the population of SOX2+ progenitors. These enriched progenitors were predisposed to differentiate into hair cells during the later stages of organoid development. Conversely, the use of the mTOR activator MHY 1485 demonstrated opposing effects.

CONCLUSION

Our findings underscore a practical strategy for enhancing the generation of inner ear organoids with a low dose of rapamycin, achieved by enriching SOX2+ progenitors in an in vitro setting.

Hair Cell Regeneration: From Animals to Humans

Cochlear hair cells convert sound into electrical signals that are relayed via the spiral ganglion neurons to the central auditory pathway. Hair cells are vulnerable to damage caused by excessive noise, aging, and ototoxic agents. Non-mammals can regenerate lost hair cells by mitotic regeneration and direct transdifferentiation of surrounding supporting cells. However, in mature mammals, damaged hair cells are not replaced, resulting in permanent hearing loss. Recent studies have uncovered mechanisms by which sensory organs in non-mammals and the neonatal mammalian cochlea regenerate hair cells, and outlined possible mechanisms why this ability declines rapidly with age in mammals. Here, we review similarities and differences between avian, zebrafish, and mammalian hair cell regeneration. Moreover, we discuss advances and limitations of hair cell regeneration in the mature cochlea and their potential applications to human hearing loss.

Stem Cell-Based Hair Cell Regeneration and Therapy in the Inner Ear

Hearing loss has become increasingly prevalent and causes considerable disability, thus gravely burdening the global economy. Irreversible loss of hair cells is a main cause of sensorineural hearing loss, and currently, the only relatively effective clinical treatments are limited to digital hearing equipment like cochlear implants and hearing aids, but these are of limited benefit in patients. It is therefore urgent to understand the mechanisms of damage repair in order to develop new neuroprotective strategies. At present, how to promote the regeneration of functional hair cells is a key scientific question in the field of hearing research. Multiple signaling pathways and transcriptional factors trigger the activation of hair cell progenitors and ensure the maturation of newborn hair cells, and in this article, we first review the principal mechanisms underlying hair cell reproduction. We then further discuss therapeutic strategies involving the co-regulation of multiple signaling pathways in order to induce effective functional hair cell regeneration after degeneration, and we summarize current achievements in hair cell regeneration. Lastly, we discuss potential future approaches, such as small molecule drugs and gene therapy, which might be applied for regenerating functional hair cells in the clinic.

Experimental drugs for the prevention or treatment of sensorineural hearing loss

INTRODUCTION

Sensorineural hearing loss results in irreversible loss of inner ear hair cells and spiral ganglion neurons. Reduced sound detection and speech discrimination can span all ages, and sensorineural hearing rehabilitation is limited to amplification with hearing aids or cochlear implants. Recent insights into experimental drug treatments for inner ear regeneration and otoprotection have paved the way for clinical trials in order to restore a more physiological hearing experience. Paired with the development of innovative minimally invasive approaches for drug delivery to the inner ear, new, emerging treatments for hearing protection and restoration are within reach.

AREAS COVERED

This expert opinion provides an overview of the latest experimental drug therapies to protect from and to restore sensorineural hearing loss.

EXPERT OPINION

The degree and type of cellular damage to the cochlea, the responsiveness of remaining, endogenous cells to regenerative treatments, and the duration of drug availability within cochlear fluids will determine the success of hearing protection or restoration.

Stepwise fate conversion of supporting cells to sensory hair cells in the chick auditory epithelium

In contrast to mammals, the avian cochlea, specifically the basilar papilla, can regenerate sensory hair cells, which involves fate conversion of supporting cells to hair cells. To determine the mechanisms for converting supporting cells to hair cells, we used single-cell RNA sequencing during hair cell regeneration in explant cultures of chick basilar papillae. We identified dynamic changes in the gene expression of supporting cells, and the pseudotime trajectory analysis demonstrated the stepwise fate conversion from supporting cells to hair cells. Initially, supporting cell identity was erased and transition to the precursor state occurred. A subsequent gain in hair cell identity progressed together with downregulation of precursor-state genes. Transforming growth factor β receptor 1-mediated signaling was involved in induction of the initial step, and its inhibition resulted in suppression of hair cell regeneration. Our data provide new insights for understanding fate conversion from supporting cells to hair cells in avian basilar papillae.

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.

Differentiation of embryonic stem cells into a putative hair cell-progenitor cells via co-culture with HEI-OC1 cells

Several studies have shown how different cell lines can influence the differentiation of stem cells through co-culture systems. The House Ear Institute-Organ of Corti 1 (HEI-OC1) is considered an important cell line for in vitro auditory research. However, it is unknown if HEI-OC1 cells can promote the differentiation of embryonic stem cells (ESCs). In this study, we investigated whether co-culture of ESCs with HEI-OC1 cells promotes differentiation. To this end, we developed a co-culture system of mouse ESCs with HEI-OC1 cells. Dissociated or embryonic bodies (EBs) of ESCs were introduced to a conditioned and inactivated confluent layer of HEI-OC1 cells for 14 days. The dissociated ESCs coalesced into an EB-like form that was smaller than the co-cultured EBs. Contact co-culture generated cells expressing several otic progenitor markers as well as hair cell specific markers. ESCs and EBs were also cultured in non-contact setup but using conditioned medium from HEI-OC1 cells, indicating that soluble factors alone could have a similar effect. The ESCs did not form into aggregates but were still Myo7a-positive, while the EBs degenerated. However, in the fully differentiated EBs, evidence to prove mature differentiation of inner ear hair cell was still rudimentary. Nevertheless, these results suggest that cellular interactions between ESCs and HEI-OC1 cells may both stimulate ESC differentiation.

Single-Cell Sequencing Applications in the Inner Ear

Genomics studies face specific challenges in the inner ear due to the multiple types and limited amounts of inner ear cells that are arranged in a very delicate structure. However, advances in single-cell sequencing (SCS) technology have made it possible to analyze gene expression variations across different cell types as well as within specific cell groups that were previously considered to be homogeneous. In this review, we summarize recent advances in inner ear research brought about by the use of SCS that have delineated tissue heterogeneity, identified unknown cell subtypes, discovered novel cell markers, and revealed dynamic signaling pathways during development. SCS opens up new avenues for inner ear research, and the potential of the technology is only beginning to be explored.

Initiation of Supporting Cell Activation for Hair Cell Regeneration in the Avian Auditory Epithelium: An Explant Culture Model

Sensorineural hearing loss is a common disability often caused by the loss of sensory hair cells in the cochlea. Hair cell (HCs) regeneration has long been the main target for the development of novel therapeutics for sensorineural hearing loss. In the mammalian cochlea, hair cell regeneration is limited, but the auditory epithelia of non-mammalian organisms retain the capacity for hair cell regeneration. In the avian basilar papilla (BP), supporting cells (SCs), which give rise to regenerated hair cells, are usually quiescent. Hair cell loss induces both direct transdifferentiation and mitotic division of supporting cells. Here, we established an explant culture model for hair cell regeneration in chick basilar papillae and validated it for investigating the initial phase of hair cell regeneration. The histological assessment demonstrated hair cell regeneration via direct transdifferentiation of supporting cells. Labeling with 5-ethynyl-2′-deoxyuridine (EdU) revealed the occurrence of mitotic division in the supporting cells at specific locations in the basilar papillae, while no EdU labeling was observed in newly generated hair cells. RNA sequencing indicated alterations in known signaling pathways associated with hair cell regeneration, consistent with previous findings. Also, unbiased analyses of RNA sequencing data revealed novel genes and signaling pathways that may be related to the induction of supporting cell activation in the chick basilar papillae. These results indicate the advantages of our explant culture model of the chick basilar papillae for exploring the molecular mechanisms of hair cell regeneration.

Atoh1 Directs Regeneration and Functional Recovery of the Mature Mouse Vestibular System

Utricular hair cells (HCs) are mechanoreceptors required for vestibular function. After damage, regeneration of mammalian utricular HCs is limited and regenerated HCs appear immature. Thus, loss of vestibular function is presumed irreversible. Here, we found partial HC replacement and functional recovery in the mature mouse utricle, both enhanced by overexpressing the transcription factor Atoh1. Following damage, long-term fate mapping revealed that support cells non-mitotically and modestly regenerated HCs displaying no or immature bundles. By contrast, Atoh1 overexpression stimulated proliferation and widespread regeneration of HCs exhibiting elongated bundles, patent mechanotransduction channels, and synaptic connections. Finally, although damage without Atoh1 overexpression failed to initiate or sustain a spontaneous functional recovery, Atoh1 overexpression significantly enhanced both the degree and percentage of animals exhibiting sustained functional recovery. Therefore, the mature, damaged utricle has an Atoh1-responsive regenerative program leading to functional recovery, underscoring the potential of a reprogramming approach to sensory regeneration.

The mature mouse utricle, which detects linear acceleration, displays limited regeneration, but whether function returns is unknown. Sayyid et al. show that regenerated hair cells appear and mature over months, resulting in a limited, unsustained functional recovery. Atoh1 overexpression enhances regeneration and leads to a sustained recovery of vestibular function.

LIN28B/let-7 control the ability of neonatal murine auditory supporting cells to generate hair cells through mTOR signaling

Mechano-sensory hair cells within the inner ear cochlea are essential for the detection of sound. In mammals, cochlear hair cells are only produced during development and their loss, due to disease or trauma, is a leading cause of deafness. In the immature cochlea, prior to the onset of hearing, hair cell loss stimulates neighboring supporting cells to act as hair cell progenitors and produce new hair cells. However, for reasons unknown, such regenerative capacity (plasticity) is lost once supporting cells undergo maturation. Here, we demonstrate that the RNA binding protein LIN28B plays an important role in the production of hair cells by supporting cells and provide evidence that the developmental drop in supporting cell plasticity in the mammalian cochlea is, at least in part, a product of declining LIN28B-mammalian target of rapamycin (mTOR) activity. Employing murine cochlear organoid and explant cultures to model mitotic and nonmitotic mechanisms of hair cell generation, we show that loss of LIN28B function, due to its conditional deletion, or due to overexpression of the antagonistic miRNA let-7g, suppressed Akt-mTOR complex 1 (mTORC1) activity and renders young, immature supporting cells incapable of generating hair cells. Conversely, we found that LIN28B overexpression increased Akt-mTORC1 activity and allowed supporting cells that were undergoing maturation to de-differentiate into progenitor-like cells and to produce hair cells via mitotic and nonmitotic mechanisms. Finally, using the mTORC1 inhibitor rapamycin, we demonstrate that LIN28B promotes supporting cell plasticity in an mTORC1-dependent manner.

Spatiotemporally controlled overexpression of cyclin D1 triggers generation of supernumerary cells in the postnatal mouse inner ear

The retinoblastoma family of pocket proteins (pRBs), composed of Rb1, p107, and p130 are negative regulators of cell-cycle progression. The deletion of any individual pRB in the auditory system triggers hair cells' (HCs) and supporting cells' (SCs) proliferation to different extents. Nevertheless, accessing their combined role in the inner ear through conditional or complete knockout methods is limited by the early mortality of the triple knockout. In quiescent cells, hyperphosphorylation and inactivation of the pRBs are maintained through the activity of the Cyclin-D1-cdk4/6 complex. Cyclin D1 (CycD1) is expressed in the embryonic and neonatal inner ear. In the mature organ of Corti (OC), CycD1 expression is significantly downregulated, paralleling the OC mitotic quiescence. Earlier studies showed that CycD1 overexpression leads to cell-cycle reactivation in cultures of inner ear explants. Here, we characterize a Cre-activated, Doxycycline (Dox)-controlled, conditional CycD1 overexpression model, which when bred to a tetracycline-controlled transcriptional activator and the Atoh1-cre mouse lines, allow for transient CycD1 overexpression and pRBs' downregulation in the inner ear in a reversible fashion. Analyses of postnatal mice's inner ears at various time points revealed the presence of supernumerary cells throughout the length of the cochlea and in the vestibular end-organs. Notably, most supernumerary cells were observed in the inner hair cells' (IHCs) region, expressed myosin VIIa (M7a), and showed no signs of apoptosis at any of the time points analyzed. Auditory and vestibular phenotypes were similar between the different genotypes and treatment groups. The fact that no significant differences were observed in auditory and vestibular function supports the notion that the supernumerary cells detected in the adult mice cochlea and macular end-organs may not impair auditory functions.

Atoh1 is required in supporting cells for regeneration of vestibular hair cells in adult mice

In amniotes, head movements are encoded by two types of vestibular hair cells (type I and type II) with unique morphology, physiology, and innervation. After hair cell destruction in mature rodents, supporting cells regenerate some type II hair cells, but no type I hair cells are replaced. The transcription factor Atoh1 is required for hair cell development, and Atoh1 is upregulated in supporting cells, the hair cell progenitors, in mature chickens and mice following hair cell damage. We investigated whether Atoh1 is required for type II hair cell regeneration in adult mice after genetic ablation of hair cells. First, we used a knock-in Atoh1 reporter to demonstrate that supporting cells in the utricle, a vestibular organ that detects linear acceleration of the head, upregulate Atoh1 expression by 7 days after hair cell destruction was initiated. Next, we labeled supporting cells prior to damage and fate-mapped them over time to test whether conditional deletion of Atoh1 from supporting cells prevented them from converting into hair cells after damage. In mice with normal Atoh1 expression, fate-mapped supporting cells in the adult utricle gave rise to hundreds of type II hair cells after hair cell destruction, but they did not form new type I hair cells. By contrast, mice with Atoh1 deletion prior to hair cell damage had only 10-20 fate-mapped type II hair cells per utricle at 3 weeks post-damage, and numbers did not change at 12 weeks after hair cell destruction. Supporting cells had normal cell shape and nuclear density up to 12 weeks after Atoh1 deletion. Similar observations were made in two other vestibular organs, the saccule and the lateral ampulla. Our findings demonstrate that Atoh1 is necessary in adult mouse supporting cells for regeneration of type II vestibular hair cells and that deletion of Atoh1 from supporting cells prior to damage does not appear to induce supporting cells to die or to proliferate.

High-resolution transcriptional dissection of in vivo Atoh1-mediated hair cell conversion in mature cochleae identifies Isl1 as a co-reprogramming factor

In vivo direct conversion of differentiated cells holds promise for regenerative medicine; however, improving the conversion efficiency and producing functional target cells remain challenging. Ectopic Atoh1 expression in non-sensory supporting cells (SCs) in mouse cochleae induces their partial conversion to hair cells (HCs) at low efficiency. Here, we performed single-cell RNA sequencing of whole mouse sensory epithelia harvested at multiple time points after conditional overexpression of Atoh1. Pseudotemporal ordering revealed that converted HCs (cHCs) are present along a conversion continuum that correlates with both endogenous and exogenous Atoh1 expression. Bulk sequencing of isolated cell populations and single-cell qPCR confirmed 51 transcription factors, including Isl1, are differentially expressed among cHCs, SCs and HCs. In transgenic mice, co-overexpression of Atoh1 and Isl1 enhanced the HC conversion efficiency. Together, our study shows how high-resolution transcriptional profiling of direct cell conversion can identify co-reprogramming factors required for efficient conversion.

The ongoing ATOH1 gene therapy clinical trial offers promise for hearing restoration in humans. However, in animal models, Atoh1-mediated sensory regeneration is inefficient and incomplete. Here we performed high-resolution gene expression profiling of single cochlear cells at multiple time points in a mouse model whereby we discovered a continuous regeneration process that leads to the formation of immature sensory cells. We identified 51 key reprogramming transcription factors that may increase the efficiency and completion of the regeneration process and confirmed that Isl1 in transgenic mice promotes Atoh1-mediated sensory regeneration as a co-reprogramming factor. Our studies identify molecular mechanisms and novel co-reprogramming factors for sensory restoration in humans with irreversible hearing loss.

Bone morphogenetic protein 4 antagonizes hair cell regeneration in the avian auditory epithelium

Permanent hearing loss is often a result of damage to cochlear hair cells, which mammals are unable to regenerate. Non-mammalian vertebrates such as birds replace damaged hair cells and restore hearing function, but mechanisms controlling regeneration are not understood. The secreted protein bone morphogenetic protein 4 (BMP4) regulates inner ear morphogenesis and hair cell development. To investigate mechanisms controlling hair cell regeneration in birds, we examined expression and function of BMP4 in the auditory epithelia (basilar papillae) of chickens of either sex after hair cell destruction by ototoxic antibiotics. In mature basilar papillae, BMP4 mRNA is highly expressed in hair cells, but not in hair cell progenitors (supporting cells). Supporting cells transcribe genes encoding receptors for BMP4 (BMPR1A, BMPR1B, and BMPR2) and effectors of BMP4 signaling (ID transcription factors). Following hair cell destruction, BMP4 transcripts are lost from the sensory epithelium. Using organotypic cultures, we demonstrate that treatments with BMP4 during hair cell destruction prevent supporting cells from upregulating expression of the pro-hair cell transcription factor ATOH1, entering the cell cycle, and fully transdifferentiating into hair cells, but they do not induce cell death. By contrast, noggin, a BMP4 inhibitor, increases numbers of regenerated hair cells. These findings demonstrate that BMP4 antagonizes hair cell regeneration in the chicken basilar papilla, at least in part by preventing accumulation of ATOH1 in hair cell precursors.

ADAM10 and γ-secretase regulate sensory regeneration in the avian vestibular organs

The loss of sensory hair cells from the inner ear is a leading cause of hearing and balance disorders. The mammalian ear has a very limited ability to replace lost hair cells, but the inner ears of non-mammalian vertebrates can spontaneously regenerate hair cells after injury. Prior studies have shown that replacement hair cells are derived from epithelial supporting cells and that the differentiation of new hair cells is regulated by the Notch signaling pathway. The present study examined molecular influences on regeneration in the avian utricle, which has a particularly robust regenerative ability. Chicken utricles were placed in organotypic culture and hair cells were lesioned by application of the ototoxic antibiotic streptomycin. Cultures were then allowed to regenerate in vitro for seven days. Some specimens were treated with small molecule inhibitors of γ-secretase or ADAM10, proteases which are essential for transmission of Notch signaling. As expected, treatment with both inhibitors led to increased numbers of replacement hair cells. However, we also found that inhibition of both proteases resulted in increased regenerative proliferation. Subsequent experiments showed that inhibition of γ-secretase or ADAM10 could also trigger proliferation in undamaged utricles. To better understand these phenomena, we used RNA-Seq profiling to characterize changes in gene expression following γ-secretase inhibition. We observed expression patterns that were consistent with Notch pathway inhibition, but we also found that the utricular sensory epithelium contains numerous γ-secretase substrates that might regulate cell cycle entry and possibly supporting cell-to-hair cell conversion. Together, our data suggest multiple roles for γ-secretase and ADAM10 in vestibular hair cell regeneration.

Hearing sensitivity differs between zebrafish lines used in auditory research

Zebrafish are increasingly used in auditory studies, in part due to the development of several transgenic lines that express hair cell-specific fluorescent proteins. However, it is largely unknown how transgene expression influences auditory phenotype. We previously observed reduced auditory sensitivity in adult Brn3c:mGFP transgenic zebrafish, which express membrane-bound green fluorescent protein (GFP) in sensory hair cells. Here, we examine the auditory sensitivity of zebrafish from multiple transgenic and background strains. We recorded auditory evoked potentials in adult animals and observed significantly higher auditory thresholds in three lines that express hair cell-specific GFP. There was no obvious correlation between hair cell density and auditory thresholds, suggesting that reduced sensitivity was not due to a reduction in hair cell density. FM1-43 uptake was reduced in Brn3c:mGFP fish but not in other lines, suggesting that a mechanotransduction defect may be responsible for the auditory phenotype in Brn3c animals, but that alternate mechanisms underlie the increased AEP thresholds in other lines. We found reduced prepulse inhibition (a measure of auditory-evoked behavior) in larval Brn3c animals, suggesting that auditory defects develop early in this line. We also found significant differences in auditory sensitivity between adults of different background strains, akin to strain differences observed in mouse models of auditory function. Our results suggest that researchers should exercise caution when selecting an appropriate zebrafish transgenic or background strain for auditory studies.

Characterization of the transcriptome of nascent hair cells and identification of direct targets of the Atoh1 transcription factor

Hair cells are sensory receptors for the auditory and vestibular system in vertebrates. The transcription factor Atoh1 is both necessary and sufficient for the differentiation of hair cells, and is strongly upregulated during hair-cell regeneration in nonmammalian vertebrates. To identify genes involved in hair cell development and function, we performed RNA-seq profiling of purified Atoh1-expressing hair cells from the neonatal mouse cochlea. We identified >600 enriched transcripts in cochlear hair cells, of which 90% have not been previously shown to be expressed in hair cells. We identified 233 of these hair cell genes as candidates to be directly regulated by Atoh1 based on the presence of Atoh1 binding sites in their regulatory regions and by analyzing Atoh1 ChIP-seq datasets from the cerebellum and small intestine. We confirmed 10 of these genes as being direct Atoh1 targets in the cochlea by ChIP-PCR. The identification of candidate Atoh1 target genes is a first step in identifying gene regulatory networks for hair-cell development and may inform future studies on the potential role of Atoh1 in mammalian hair cell regeneration.

Fgf-signaling-dependent Sox9a and Atoh1a regulate otic neural development in zebrafish

Fibroblast growth factors (Fgfs) play important roles in developmental processes of the inner ear, including the ontogeny of the statoacoustic ganglia (SAG) and hair cells. However, the detailed genetic mechanism(s) underlying Fgf/Fgfr-dependent otic neural development remains elusive. Using conditional genetic approaches and inhibitory small molecules, we have revealed that Fgfr-PI3K/Akt signaling is mainly responsible for zebrafish SAG development and have determined that Sox9a and Atoh1a act downstream of Fgfr-Akt signaling to specify and/or maintain the otic neuron fate during the early segmentation stage. Sox9a and Atoh1a coregulate numerous downstream factors identified through our ChIP-seq analyses, including Tlx2 and Eya2. Fgfr-Erk1/2 signaling contributes to ultricular hair cell development during a critical period between 9 and 15 hours postfertilization. Our work reveals that a genetic network of the previously known sensory determinant Atoh1 and the neural crest determinant Sox9 plays critical roles in SAG development. These newly uncovered roles for Atoh1and Sox9 in zebrafish otic development may be relevant to study in other species.

Molecular cloning and functional characterisation of chicken Atonal homologue 1: a comparison with human Atoh1

BACKGROUND INFORMATION

The vertebrate basic helix-loop-helix transcription factor Atoh1 is essential for maturation and survival of mechanosensory hair cells of the inner ear, neurogenesis, differentiation of the intestine, homeostasis of the colon and is implicated in cancer progression. Given that mutations in Atoh1 are detected in malignant tumours, study of functionally different Atoh1 alleles and homologues might yield useful avenues for investigation. The predicted sequence of chicken Atoh1 (cAtoh1) has large regions of dissimilarity to that of mammalian Atoh1 homologues. We hypothesise that cAtoh1 might have intrinsic functional differences to mammalian Atoh1.

RESULTS

In this study, we cloned and sequenced the full open reading frame of cAtoh1. In overexpression experiments, we show that this sequence is sufficient to generate a cAtoh1 protein capable of inducing hair cell markers when expressed in nonsensory regions of the developing inner ear, and that morpholino-mediated knock-down using a section of the sequence 5' to the start codon inhibits differentiation of hair cells in the chicken basilar papilla. Furthermore, we compare the behaviour of cAtoh1 and human Atoh1 (hAtoh1) in embryonic mouse cochlear explants, showing that cAtoh1 is a potent inducer of hair cell differentiation and that it can overcome Sox2-mediated repression of hair cell differentiation more effectively than hAtoh1.

CONCLUSIONS

cAtoh1 is both necessary and sufficient for avian mechanosensory hair cell differentiation. The non-conserved regions of the cAtoh1 coding region have functional consequences on its behaviour.

A gradient of Bmp7 specifies the tonotopic axis in the developing inner ear

The auditory systems of animals that perceive sounds in air are organized to separate sound stimuli into their component frequencies. Individual tones then stimulate mechanosensory hair cells located at different positions on an elongated frequency (tonotopic) axis. During development, immature hair cells located along the axis must determine their tonotopic position in order to generate frequency-specific characteristics. Expression profiling along the developing tonotopic axis of the chick basilar papilla (BP) identified a gradient of Bmp7. Disruption of that gradient in vitro or in ovo induces changes in hair cell morphologies consistent with a loss of tonotopic organization and the formation of an organ with uniform frequency characteristics. Further, the effects of Bmp7 in determination of positional identity are shown to be mediated through activation of the Mapk, Tak1. These results indicate that graded, Bmp7-dependent, activation of Tak1 signalling controls the determination of frequency-specific hair cell characteristics along the tonotopic axis.

In vivo generation of immature inner hair cells in neonatal mouse cochleae by ectopic Atoh1 expression

Regeneration of auditory hair cells (HCs) is a promising approach to restore hearing. Recent studies have demonstrated that induced pluripotent stem cells/embryonic stem cells or supporting cells (SCs) adjacent to HCs can be converted to adopt the HC fate. However, little is known about whether new HCs are characteristic of outer or inner HCs. Here, we showed in vivo conversion of 2 subtypes of SCs, inner border cells (IBs) and inner phalangeal cells (IPhs), to the inner HC (IHC) fate. This was achieved by ectopically activating Atoh1, a transcription factor necessary for HC fate, in IBs/IPhs at birth. Atoh1+ IBs/IPhs first turned on Pou4f3, another HC transcription factor, before expressing 8 HC markers. The conversion rate gradually increased from ∼2.4% at 1 week of age to ∼17.8% in adult. Interestingly, new HCs exhibited IHC characteristics such as straight line–shaped stereociliary bundles, expression of Fgf8 and otoferlin, and presence of larger outward currents than those of outer HCs. However, new HCs lacked the terminal differentiation IHC marker vGlut3, exhibited reduced density of presynaptic Cbtp2 puncta that had little postsynaptic GluR2 specialization, and displayed immature IHC outward currents. Our results demonstrate that the conversion rate of IBs/IPhs in vivo by Atoh1 ectopic expression into the IHC fate was higher and faster and the conversion was more complete than that of the 2 other SC subtypes underneath the outer HCs; however, these new IHCs are arrested before terminal differentiation. Thus, IBs/IPhs are good candidates to regenerate IHCs in vivo.

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.

Hair cell generation by notch inhibition in the adult mammalian cristae

Balance disorders caused by hair cell loss in the sensory organs of the vestibular system pose a significant health problem worldwide, particularly in the elderly. Currently, this hair cell loss is permanent as there is no effective treatment. This is in stark contrast to nonmammalian vertebrates who robustly regenerate hair cells after damage. This disparity in regenerative potential highlights the need for further manipulation in order to stimulate more robust hair cell regeneration in mammals. In the utricle, Notch signaling is required for maintaining the striolar support cell phenotype into the second postnatal week. Notch signaling has further been implicated in hair cell regeneration after damage in the mature utricle. Here, we investigate the role of Notch signaling in the mature mammalian cristae in order to characterize the Notch-mediated regenerative potential of these sensory organs. For these studies, we used the γ-secretase inhibitor, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT), in conjunction with a method we developed to culture cristae in vitro. In postnatal and adult cristae, we found that 5 days of DAPT treatment resulted in a downregulation of the Notch effectors Hes1 and Hes5 and also an increase in the total number of Gfi1(+) hair cells. Hes5, as reported by Hes5-GFP, was downregulated specifically in peripheral support cells. Using lineage tracing with proteolipid protein (PLP)/CreER;mTmG mice, we found that these hair cells arose through transdifferentiation of support cells in cristae explanted from mice up to 10 weeks of age. These transdifferentiated cells arose without proliferation and were capable of taking on a hair cell morphology, migrating to the correct cell layer, and assembling what appears to be a stereocilia bundle with a long kinocilium. Overall, these data show that Notch signaling is active in the mature cristae and suggest that it may be important in maintaining the support cell fate in a subset of peripheral support cells.

Generation of hair cells in neonatal mice by β-catenin overexpression in Lgr5-positive cochlear progenitors

Mammalian hair cells do not regenerate, and their loss is a major cause of deafness. We recently identified leucine-rich repeat containing, G-protein-coupled receptor 5 (Lgr5)-expressing cochlear supporting cells with the capacity for self-renewal and hair cell differentiation in vitro. We found that these cells, a subset of cochlear supporting cells, were responsive to Wnt signaling. Here we asked whether these Lgr5-positive cells, despite their lack of contribution to hair cell replacement after degenerative loss, could be driven by forced expression of β-catenin to act as hair cell progenitors in vivo. We showed that forced stabilization of β-catenin in supporting cells in neonatal animals resulted in proliferation of supporting cells and generation of hair cells. Although β-catenin expression was increased by genetic means in all supporting cells, entry to the cell cycle and differentiation to hair cells of the normally postmitotic cells was restricted to the Lgr5-positive population. Our finding suggests that Wnt/β-catenin can drive Lgr5-positive cells to act as hair cell progenitors, even after their exit from the cell cycle and apparent establishment of cell fate.

Lentivirus carrying the Atoh1 gene infects normal rat cochlea

Lentivirus carrying the Atoh1 gene can infect Corti's organ and express a hair-like cell surface marker in the supporting cell area. However, expression of the gene carried by adenovirus is instantaneous, which undoubtedly limits its clinical application. Lentivirus acts as a carrier that can stably and continuously express genes. In this study, the cochlear structure and hearing level were not affected, and Atoh1 gene carried by lentivirus promoted the production of hair-like cells in the cochlear supporting cell area. This led to expression of the hair-like cell surface marker myosin 7a 30 days after lentivirus carrying Atoh1 was microinjected into the cochlear round window of rats.

The genetics of hair cell development and regeneration

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

The role of Atonal transcription factors in the development of mechanosensitive cells

Mechanosensation is an evolutionarily ancient sensory modality seen in all main animal groups. Mechanosensation can be mediated by sensory neurons or by dedicated receptor cells that form synapses with sensory neurons. Evidence over the last 15-20 years suggests that both classes of mechanosensory cells can be specified by the atonal class of basic helix-loop-helix transcription factors. In this review we discuss recent work addressing how atonal factors specify mechanosensitive cells in vertebrates and invertebrates, and how the redeployment of these factors underlies the regeneration of mechanosensitive cells in some vertebrate groups.

Inner ear supporting cells: rethinking the silent majority

Sensory epithelia of the inner ear contain two major cell types: hair cells and supporting cells. It has been clear for a long time that hair cells play critical roles in mechanoreception and synaptic transmission. In contrast, until recently the more abundant supporting cells were viewed as serving primarily structural and homeostatic functions. In this review, we discuss the growing information about the roles that supporting cells play in the development, function and maintenance of the inner ear, their activities in pathological states, their potential for hair cell regeneration, and the mechanisms underlying these processes.

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.

Notch signaling in mammalian hair cell regeneration

In the inner ear, Notch signaling has been shown to have two key developmental roles. The first occurs early in otic development and defines the prosensory domains that will develop into the six sensory organs of the inner ear. The second role occurs later in development and establishes the mosaic-like pattern of the mechanosensory hair cells and their surrounding support cells through the more well-characterized process of lateral inhibition. These dual developmental roles have inspired several different strategies to regenerate hair cells in the mature inner ear organs. These strategies include (1) modulation of Notch signaling in inner ear stem cells in order to increase hair cell yield, (2) activation of Notch signaling in order to promote the formation of ectopic sensory regions in normally non-sensory regions within the inner ear, and (3) inhibition of Notch signaling to disrupt lateral inhibition and allow support cells to transdifferentiate into hair cells. In this review, we summarize some of the promising studies that have used these various strategies for hair cell regeneration through modulation of Notch signaling and some of the challenges that remain in developing therapies based on hair cell regeneration.

Patterning and cell fate in the inner ear: a case for Notch in the chicken embryo

The development of the inner ear provides a beautiful example of one basic problem in development, that is, to understand how different cell types are generated at specific times and domains throughout embryonic life. The functional unit of the inner ear consists of hair cells, supporting cells and neurons, all deriving from progenitor cells located in the neurosensory competent domain of the otic placode. Throughout development, the otic placode resolves into the complex inner ear labyrinth, which holds the auditory and vestibular sensory organs that are innervated in a highly specific manner. How does the early competent domain of the otic placode give rise to the diverse specialized cell types of the different sensory organs of the inner ear? We review here our current understanding on the role of Notch signaling in coupling patterning and cell fate determination during inner ear development, with a particular emphasis on contributions from the chicken embryo as a model organism. We discuss further the question of how these two processes rely on two modes of operation of the Notch signaling pathway named lateral induction and lateral inhibition.