Atoh1 expression defines activated progenitors and differentiating hair cells during avian hair cell regeneration

Hair cell regeneration, reinnervation, and restoration of hearing thresholds in the avian hearing organ

Hearing starts, at the cellular level, with mechanoelectrical transduction by sensory hair cells. Sound information is then transmitted via afferent synaptic connections with auditory neurons. Frequency information is encoded by the location of hair cells along the cochlear duct. Loss of hair cells, synapses, or auditory neurons leads to permanent hearing loss in mammals. Birds, in contrast, regenerate auditory hair cells and functionally recover from hearing loss. Here, we characterized regeneration and reinnervation in sisomicin-deafened chickens and found that afferent neurons contact regenerated hair cells at the tips of basal projections. In contrast to development, synaptic specializations are established at these locations distant from the hair cells' bodies. The protrusions then contracted as regenerated hair cells matured and became functional 2 weeks post-deafening. We found that auditory thresholds recovered after 4-5 weeks. We interpret the regeneration-specific synaptic reestablishment as a location-preserving process that might be needed to maintain tonotopic fidelity.

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.

An essential signaling cascade for avian auditory hair cell regeneration

Hearing loss is a chronic disease affecting millions of people worldwide, yet no restorative treatment options are available. Although non-mammalian species can regenerate their auditory sensory hair cells, mammals cannot. Birds retain facultative stem cells known as supporting cells that engage in proliferative regeneration when surrounding hair cells die. Here, we investigated gene expression changes in chicken supporting cells during auditory hair cell death. This identified a pathway involving the receptor F2RL1, HBEGF, EGFR, and ERK signaling. We propose a cascade starting with the proteolytic activation of F2RL1, followed by matrix-metalloprotease-mediated HBEGF shedding, and culminating in EGFR-mediated ERK signaling. Each component of this cascade is essential for supporting cell S-phase entry in vivo and is integral for hair cell regeneration. Furthermore, STAT3-phosphorylation converges with this signaling toward upregulation of transcription factors ATF3, FOSL2, and CREM. Our findings could provide a basis for designing treatments for hearing and balance disorders.

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.

Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration in adult mice

Sox2 is a transcription factor that is necessary in the mammalian inner ear for development of sensory hair cells and supporting cells. Sox2 is expressed in supporting cells of adult mammals, but its function in this context is poorly understood. Given its role in the developing inner ear, we hypothesized that Sox2 is required in vestibular supporting cells for regeneration of type II hair cells after damage. Using adult mice, we deleted Sox2 from Sox9-CreER-expressing supporting cells prior to diphtheria toxin-mediated hair cell destruction and used fate-mapping to assess regeneration. In utricles of control mice with normal Sox2 expression, supporting cells regenerated nearly 200 hair cells by 3 weeks post-damage, which doubled by 12 weeks. In contrast, mice with Sox2 deletion from supporting cells had approximately 20 fate-mapped hair cells at 3 weeks post-damage, and this number did not change significantly by 12 weeks, indicating regeneration was dramatically curtailed. We made similar observations for saccules and ampullae. We found no evidence that supporting cells lacking Sox2 had altered cellular density, morphology, or ultrastructure. However, some Sox2-negative supporting cell nuclei appeared to migrate apically but did not turn on hair cell markers, and type I hair cell survival was higher. Sox2 heterozygotes also had reduced regeneration in utricles, but more hair cells were replaced than mice with Sox2 deletion. Our study determined that Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration but found no other major requirements for Sox2 in adult supporting cells.

The ubiquitin-proteasome system in normal hearing and deafness

Post-translational modifications of proteins are essential for the proper development and function of many tissues and organs, including the inner ear. Ubiquitination is a highly selective post-translational modification that involves the covalent conjugation of ubiquitin to a substrate protein. The most common outcome of protein ubiquitination is degradation by the ubiquitin-proteasome system (UPS), preventing the accumulation of misfolded, damaged, and excess proteins. In addition to proteasomal degradation, ubiquitination regulates other cellular processes, such as transcription, translation, endocytosis, receptor activity, and subcellular localization. All of these processes are essential for cochlear development and maintenance, as several studies link impairment of UPS with altered cochlear development and hearing loss. In this review, we provide insight into the well-oiled machinery of UPS with a focus on its confirmed role in normal hearing and deafness and potential therapeutic strategies to prevent and treat UPS-associated hearing loss.

Loss of Mst1/2 activity promotes non-mitotic hair cell generation in the neonatal organ of Corti

Mammalian sensory hair cells (HCs) have limited capacity for regeneration, which leads to permanent hearing loss after HC death. Here, we used in vitro RNA-sequencing to show that the Hippo signaling pathway is involved in HC damage and self-repair processes. Turning off Hippo signaling through Mst1/2 inhibition or Yap overexpression induces YAP nuclear accumulation, especially in supporting cells, which induces supernumerary HC production and HC regeneration after injury. Mechanistically, these effects of Hippo signaling work synergistically with the Notch pathway. Importantly, the supernumerary HCs not only express HC markers, but also have cilia structures that are able to form neural connections to auditory regions in vivo. Taken together, regulating Hippo suggests new strategies for promoting cochlear supporting cell proliferation, HC regeneration, and reconnection with neurons in mammals.

Regeneration in the Auditory Organ in Cuban and African Dwarf Crocodiles (Crocodylus rhombifer and Osteolaemus tetraspis) Can We Learn From the Crocodile How to Restore Our Hearing?

Background: In several non-mammalian species, auditory receptors undergo cell renewal after damage. This has raised hope of finding new options to treat human sensorineural deafness. Uncertainty remains as to the triggering mechanisms and whether hair cells are regenerated even under normal conditions. In the present investigation, we explored the auditory organ in the crocodile to validate possible ongoing natural hair cell regeneration.

Materials and Methods: Two male Cuban crocodiles (Crocodylus rhombifer) and an adult male African Dwarf crocodile (Osteolaemus tetraspis) were analyzed using transmission electron microscopy and immunohistochemistry using confocal microscopy. The crocodile ears were fixed in formaldehyde and glutaraldehyde and underwent micro-computed tomography (micro-CT) and 3D reconstruction. The temporal bones were drilled out and decalcified.

Results: The crocodile papilla basilaris contained tall (inner) and short (outer) hair cells surrounded by a mosaic of tightly connected supporting cells coupled with gap junctions. Afferent neurons with and without ribbon synapses innervated both hair cell types. Supporting cells occasionally showed signs of trans-differentiation into hair cells. They expressed the MAFA and SOX2 transcription factors. Supporting cells contained organelles that may transfer genetic information between cells, including the efferent nerve fibers during the regeneration process. The tectorial membrane showed signs of being replenished and its architecture being sculpted by extracellular exosome-like proteolysis.

Discussion: Crocodilians seem to produce new hair cells during their life span from a range of supporting cells. Imposing efferent nerve fibers may play a role in regeneration and re-innervation of the auditory receptors, possibly triggered by apoptotic signals from wasted hair cells. Intercellular signaling may be accomplished by elaborate gap junction and organelle systems, including neural emperipolesis. Crocodilians seem to restore and sculpt their tectorial membranes throughout their lives.

From Bench to Booth: Examining Hair-Cell Regeneration Through an Audiologist's Scope

Damage to auditory hair cells is a key feature of sensorineural hearing loss due to aging, noise exposure, or ototoxic drugs. Though hair-cell loss is permanent in humans, research in bird species led to the discovery that analogous hair cells of the avian basilar papilla are able to regenerate after being damaged by ototoxic agents. Regeneration appears to occur through a combination of the mitotic expansion of a precursor population of supporting cells and direct transdifferentiation of supporting cells into functioning hair cells. This review will synthesize the relevant anatomy and pathophysiology of sensorineural hearing loss, the historical observations that led to the genesis of the hair-cell regeneration field, and perspectives on initial human hair-cell regeneration trials.

Avian auditory hair cell regeneration is accompanied by JAK/STAT-dependent expression of immune-related genes in supporting cells

The avian hearing organ is the basilar papilla that, in sharp contrast to the mammalian cochlea, can regenerate sensory hair cells and thereby recover from deafness within weeks. The mechanisms that trigger, sustain and terminate the regenerative response in vivo are largely unknown. Here, we profile the changes in gene expression in the chicken basilar papilla after aminoglycoside antibiotic-induced hair cell loss using RNA-sequencing. We identified changes in gene expression of a group of immune-related genes and confirmed with single-cell RNA-sequencing that these changes occur in supporting cells. In situ hybridization was used to further validate these findings. We determined that the JAK/STAT signaling pathway is essential for upregulation of the damage-response genes in supporting cells during the second day after induction of hair cell loss. Four days after ototoxic damage, we identified newly regenerated, nascent auditory hair cells that express genes linked to termination of the JAK/STAT signaling response. The robust, transient expression of immune-related genes in supporting cells suggests a potential functional involvement of JAK/STAT signaling in sensory hair cell regeneration.

Molecular Pathways Modulating Sensory Hair Cell Regeneration in Adult Mammalian Cochleae: Progress and Perspectives

Noise-induced, drug-related, and age-related disabling hearing loss is a major public health problem and affect approximately 466 million people worldwide. In non-mammalian vertebrates, the death of sensory hair cells (HCs) induces the proliferation and transdifferentiation of adjacent supporting cells into new HCs; however, this capacity is lost in juvenile and adult mammalian cochleae leading to permanent hearing loss. At present, cochlear implants and hearing devices are the only available treatments and can help patients to a certain extent; however, no biological approach or FDA-approved drug is effective to treat disabling hearing loss and restore hearing. Recently, regeneration of mammalian cochlear HCs by modulating molecular pathways or transcription factors has offered some promising results, although the immaturity of the regenerated HCs remains the biggest concern. Furthermore, most of the research done is in neonates and not in adults. This review focuses on critically summarizing the studies done in adult mammalian cochleae and discusses various strategies to elucidate novel transcription factors for better therapeutics.

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.

Advances and challenges in adeno-associated viral inner-ear gene therapy for sensorineural hearing loss

There is growing attention and effort focused on treating the root cause of sensorineural hearing loss rather than managing associated secondary characteristic features. With recent substantial advances in understanding sensorineural hearing-loss mechanisms, gene delivery has emerged as a promising strategy for the biological treatment of hearing loss associated with genetic dysfunction. There are several successful and promising proof-of-principle examples of transgene deliveries in animal models; however, there remains substantial further progress to be made in these avenues before realizing their clinical application in humans. Herein, we review different aspects of development, ongoing preclinical studies, and challenges to the clinical transition of transgene delivery of the inner ear toward the restoration of lost auditory and vestibular function.

Cell-type identity of the avian cochlea

In contrast to mammals, birds recover naturally from acquired hearing loss, which makes them an ideal model for inner ear regeneration research. Here, we present a validated single-cell RNA sequencing resource of the avian cochlea. We describe specific markers for three distinct types of sensory hair cells, including a previously unknown subgroup, which we call superior tall hair cells. We identify markers for the supporting cells associated with tall hair cells, which represent the facultative stem cells of the avian inner ear. Likewise, we present markers for supporting cells that are located below the short cochlear hair cells. We further infer spatial expression gradients of hair cell genes along the tonotopic axis of the cochlea. This resource advances neurobiology, comparative biology, and regenerative medicine by providing a basis for comparative studies with non-regenerating mammalian cochleae and for longitudinal studies of the regenerating avian cochlea.

Transcriptomic characterization of dying hair cells in the avian cochlea

Sensory hair cells are prone to apoptosis caused by various drugs including aminoglycoside antibiotics. In mammals, this vulnerability results in permanent hearing loss because lost hair cells are not regenerated. Conversely, hair cells regenerate in birds, making the avian inner ear an exquisite model for studying ototoxicity and regeneration. Here, we use single-cell RNA sequencing and trajectory analysis on control and dying hair cells after aminoglycoside treatment. Interestingly, the two major subtypes of avian cochlear hair cells, tall and short hair cells, respond differently. Dying short hair cells show a noticeable transient upregulation of many more genes than tall hair cells. The most prominent gene group identified is associated with potassium ion conductances, suggesting distinct physiological differences. Moreover, the dynamic characterization of >15,000 genes expressed in tall and short avian hair cells during their apoptotic demise comprises a resource for further investigations toward mammalian hair cell protection and hair cell regeneration.

Signaling pathways (Notch, Wnt, Bmp and Fgf) have additive effects on hair cell regeneration in the chick basilar papilla after streptomycin injury in vitro: Additive effects of signaling pathways on hair cell regeneration

Hair cells can be regenerated after damage by transdifferentiation in which a supporting cell directly differentiates into a hair cell without mitosis. However, such regeneration is at the cost of exhausting the support cells in the mammalian mature cochlea. Thus, more effective methods should be found to promote mitotic regeneration but partially preserve support cells after damage. To address the issue, we first injured hair cells in the chick basilar papillae (BP) by treatment with streptomycin in vitro. We then compared the mitotic regeneration on the neural side in the middle part of BP after treatment with a pharmacological inhibitor or agonist of the Notch (DAPT), Wnt (LiCl), Bmp (Noggin) or Fgf (SU5402) signaling pathway, with that after treatment with combinations of two or three inhibitors or agonist of these pathways. Our results indicate that treatments with a single inhibitor or agonist of the Notch, Wnt, Bmp or Fgf signaling pathway could significantly increase mitotic regeneration as well as direct transdifferentiation. The results also show that hair cells (Myosin 7a+), support cells (Sox2+) and mitotically regenerated hair cells (Myosin 7a+/Sox2+/BrdU+) increased significantly on the neural side in the middle part of BP after two or three combinations of the inhibition of Notch, Bmp or Fgf signaling pathway or the activation of Wnt signaling pathway, besides the reported coregulatory effects of Notch and Wnt signaling. The study of the effects of systematic combinations of pathway modulators provided more insight into hair cell regeneration from mitosis.

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.

Developmental and Functional Hair Cell-Like Cells Induced by Atoh1 Overexpression in the Adult Mammalian Cochlea In Vitro

Hair cells (HCs) in the mammalian cochleae cannot spontaneously regenerate once damaged, resulting in permanent hearing loss. It has been shown that Atoh1 overexpression induces hair cell-like cells (HCLCs) in the cochlea of newborn rodents, but this is hard to achieve in adult mammals. In this study, we used a three-dimensional cochlear culture system and an adenoviral-mediated delivery vector to overexpress Atoh1 in adult mouse cochleae. HCLCs were successfully induced from 3 days after virus infection (3 DVI) in vitro, and the number increased with time. HCLCs were myosin7a positive and distinguishable from remnant HCs in a culture environment. Meanwhile, patch-clamp results showed that noninactive outward potassium currents (sustained outward potassium currents) could be recorded in HCLCs and that their magnitude increased with time, similar to normal HCs. Furthermore, transient HCN currents were recorded in some HCLCs, indicating that the HCLCs experienced a developmental stage similar to normal HCs. We also compared the electrophysiological features of HCLCs from adult mice with native HCs and found the HCLCs gradually matured, similar to the normal HCs. Meanwhile, HCLCs from adult mice possessed the same bundles as developmental HCs. However, these HCLCs did not express prestin, which is a special marker for outer hair cells (OHCs), even at 13 DVI. These results demonstrate that Atoh1 overexpression induces HCLC formation in the adult mammalian cochlea and that these HCLCs were functional and experienced a developmental process similar to that of normal HCs.

Perspectives on Human Hearing Loss, Cochlear Regeneration, and the Potential for Hearing Restoration Therapies

Most adults who acquire hearing loss find it to be a disability that is poorly corrected by current prosthetics. This gap drives current research in cochlear mechanosensory hair cell regeneration and in hearing restoration. Birds and fish can spontaneously regenerate lost hair cells through a process that has become better defined in the last few years. Findings from these studies have informed new research on hair cell regeneration in the mammalian cochlea. Hair cell regeneration is one part of the greater problem of hearing restoration, as hearing loss can stem from a myriad of causes. This review discusses these issues and recent findings, and places them in the greater social context of need and community.

Vascular endothelial growth factor is required for regeneration of auditory hair cells in the avian inner ear

Hair cells in the auditory organ of the vertebrate inner ear are the sensory receptors that convert acoustic stimuli into electrical signals that are conveyed along the auditory nerve to the brainstem. Hair cells are highly susceptible to ototoxic drugs, infection, and acoustic trauma, which can cause cellular degeneration. In mammals, hair cells that are lost after damage are not replaced, leading to permanent hearing impairments. By contrast, supporting cells in birds and other non-mammalian vertebrates regenerate hair cells after damage, which restores hearing function. The cellular mechanisms that regulate hair cell regeneration are not well understood. We investigated the role of vascular endothelial growth factor (VEGF) during regeneration of auditory hair cells in chickens after ototoxic injury. Using RNA-Seq, immunolabeling, and in situ hybridization, we found that VEGFA, VEGFC, VEGFR1, VEGFR2, and VEGFR3 were expressed in the auditory epithelium, with VEGFA expressed in hair cells and VEGFR1 and VEGFR2 expressed in supporting cells. Using organotypic cultures of the chicken cochlear duct, we found that blocking VEGF receptor activity during hair cell injury reduced supporting cell proliferation as well as the numbers of regenerated hair cells. By contrast, addition of recombinant human VEGFA to organ cultures caused an increase in both supporting cell division and hair cell regeneration. VEGF's effects on supporting cells were preserved in isolated supporting cell cultures, indicating that VEGF can act directly upon supporting cells. These observations demonstrate a heretofore uncharacterized function for VEGF signaling as a critical positive regulator of hair cell regeneration in the avian inner ear.

Novel insights into inner ear development and regeneration for targeted hearing loss therapies

Sensorineural hearing loss is the most common sensory deficit in humans. Despite the global scale of the problem, only limited treatment options are available today. The mammalian inner ear is a highly specialized postmitotic organ, which lacks proliferative or regenerative capacity. Since the discovery of hair cell regeneration in non-mammalian species however, much attention has been placed on identifying possible strategies to reactivate similar responses in humans. The development of successful regenerative approaches for hearing loss strongly depends on a detailed understanding of the mechanisms that control human inner ear cellular specification, differentiation and function, as well as on the development of robust in vitro cellular assays, based on human inner ear cells, to study these processes and optimize therapeutic interventions. We summarize here some aspects of inner ear development and strategies to induce regeneration that have been investigated in rodents. Moreover, we discuss recent findings in human inner ear development and compare the results with findings from animal models. Finally, we provide an overview of strategies for in vitro generation of human sensory cells from pluripotent and somatic progenitors that may provide a platform for drug development and validation of therapeutic strategies in vitro.

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.

Stem Cells and the Bird Cochlea-Where Is Everybody?

In sharp contrast to the adult mammalian cochlea, which lacks regenerative ability, the mature avian cochlea, or basilar papilla (BP) is capable of complete recovery from hearing loss after damage. Avian sensory hair cell regeneration relies on rousing quiescent supporting cells to proliferate or transdifferentiate after hair cell death. Unlike mammalian cochlear supporting cells, which have clearly defined subtypes, avian BP supporting cells are deceptively indistinguishable and molecular markers have yet to be identified. Despite the importance of supporting cells as the putative stem cells in avian regeneration, it is unknown whether all supporting cells possess equal capability to give rise to a hair cell or if a specialized subpopulation exists. In this perspective, we reinvigorate the concept of a stem cell in the BP, and form comparisons to other regenerating tissues that show cell-cycle reentry after damage. Special emphasis is given to the structure of the BP and how anatomy informs both the potential, intrinsic heterogeneity of the supporting cell layer as well as the choice between mitotic and nonmitotic regenerative strategies.

Direct cellular reprogramming and inner ear regeneration

INTRODUCTION

Sound is integral to communication and connects us to the world through speech and music. Cochlear hair cells are essential for converting sounds into neural impulses. However, these cells are highly susceptible to damage from an array of factors, resulting in degeneration and ultimately irreversible hearing loss in humans. Since the discovery of hair cell regeneration in birds, there have been tremendous efforts to identify therapies that could promote hair cell regeneration in mammals.

AREAS COVERED

Here, we will review recent studies describing spontaneous hair cell regeneration and direct cellular reprograming as well as other factors that mediate mammalian hair cell regeneration.

EXPERT OPINION

Numerous combinatorial approaches have successfully reprogrammed non-sensory supporting cells to form hair cells, albeit with limited efficacy and maturation. Studies on epigenetic regulation and transcriptional network of hair cell progenitors may accelerate discovery of more promising reprogramming regimens.

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.

Characterization of Adult Vestibular Organs in 11 CreER Mouse Lines

Utricles are vestibular sense organs that encode linear head movements. They are composed of a sensory epithelium with type I and type II hair cells and supporting cells, sitting atop connective tissue, through which vestibular nerves project. We characterized utricular Cre expression in 11 murine CreER lines using the ROSA26^tdTomato reporter line and tamoxifen induction at 6 weeks of age. This characterization included Calbindin2^CreERT2, Fgfr3-iCreER^T2, GFAP-A-CreER™, GFAP-B-CreER™, GLAST-CreER^T2, Id2^CreERT2, Otoferlin^CreERT2, Parvalbumin^CreERT2, Prox1^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2. Otoferlin^CreERT2 mice had inducible Cre activity specific to hair cells. GLAST-CreER^T2, Id2^CreERT2, and Sox9-CreER^T2 had inducible Cre activity specific to supporting cells. Sox2^CreERT2 had inducible Cre activity in supporting cells and most type II hair cells. Parvalbumin^CreERT2 mice had small numbers of labeled vestibular nerve afferents. Calbindin2^CreERT2 mice had labeling of most type II hair cells and some type I hair cells and supporting cells. Only rare (or no) tdTomato-positive cells were detected in utricles of Fgfr3-iCreER^T2, GFAP-A-CreER™, GFAP-B-CreER™, and Prox1^CreERT2 mice. No Cre leakiness (tdTomato expression in the absence of tamoxifen) was observed in Otoferlin^CreERT2 mice. A small degree of leakiness was seen in GLAST-CreER^T2, Id2^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2 lines. Calbindin2^CreERT2 mice had similar tdTomato expression with or without tamoxifen, indicating lack of inducible control under the conditions tested. In conclusion, 5 lines-GLAST-CreER^T2, Id2^CreERT2, Otoferlin^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2-showed cell-selective, inducible Cre activity with little leakiness, providing new genetic tools for researchers studying the vestibular periphery.

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.

Sox2 haploinsufficiency primes regeneration and Wnt responsiveness in the mouse cochlea

During development, Sox2 is indispensable for cell division and differentiation, yet its roles in regenerating tissues are less clear. Here, we used combinations of transgenic mouse models to reveal that Sox2 haploinsufficiency (Sox2haplo) increases rather than impairs cochlear regeneration in vivo. Sox2haplo cochleae had delayed terminal mitosis and ectopic sensory cells, yet normal auditory function. Sox2haplo amplified and expanded domains of damage-induced Atoh1+ transitional cell formation in neonatal cochlea. Wnt activation via β-catenin stabilization (β-cateninGOF) alone failed to induce proliferation or transitional cell formation. By contrast, β-cateninGOF caused proliferation when either Sox2haplo or damage was present, and transitional cell formation when both were present in neonatal, but not mature, cochlea. Mechanistically, Sox2haplo or damaged neonatal cochleae showed lower levels of Sox2 and Hes5, but not of Wnt target genes. Together, our study unveils an interplay between Sox2 and damage in directing tissue regeneration and Wnt responsiveness and thus provides a foundation for potential combinatorial therapies aimed at stimulating mammalian cochlear regeneration to reverse hearing loss in humans.

Aminoglycoside Damage and Hair Cell Regeneration in the Chicken Utricle

In this study, we present a systematic characterization of hair cell loss and regeneration in the chicken utricle in vivo. A single unilateral surgical delivery of streptomycin caused robust decline of hair cell numbers in striolar as well as extrastriolar regions, which in the striola was detected very early, 6 h post-insult. During the initial 12 h of damage response, we observed global repression of DNA replication, in contrast to the natural, mitotic hair cell production in undamaged control utricles. Regeneration of hair cells in striolar and extrastriolar regions occurred via high rates of asymmetric supporting cell divisions, accompanied by delayed replenishment by symmetric division. While asymmetric division of supporting cells is the main regenerative response to aminoglycoside damage, the detection of symmetric divisions supports the concept of direct transdifferentiation where supporting cells need to be replenished after their phenotypic conversion into new hair cells. Supporting cell divisions appear to be well coordinated because total supporting cell numbers throughout the regenerative process were invariant, despite the initial large-scale loss of hair cells. We conclude that a single ototoxic drug application provides an experimental framework to study the precise onset and timing of utricle hair cell regeneration in vivo. Our findings indicate that initial triggers and signaling events occur already within a few hours after aminoglycoside exposure. Direct transdifferentiation and asymmetric division of supporting cells to generate new hair cells subsequently happen largely in parallel and persist for several days.

The Repression of Atoh1 by Neurogenin1 during Inner Ear Development

Atonal homolog 1 (Atoh1) and Neurogenin1 (Neurog1) are basic Helix-Loop-Helix (bHLH) transcription factors crucial for the generation of hair cells (HCs) and neurons in the inner ear. Both genes are induced early in development, but the expression of Atoh1 is counteracted by Neurog1. As a result, HC development is prevented during neurogenesis. This work aimed at understanding the molecular basis of this interaction. Atoh1 regulation depends on a 3'Atoh1-enhancer that is the site for Atoh1 autoregulation. Reporter assays on chick embryos and P19 cells show that Neurog1 hampers the autoactivation of Atoh1, the effect being cell autonomous and independent on Notch activity. Assay for Transposase-Accessible Chromatin with high throughput sequencing (ATAC-Seq) analysis shows that the region B of the 3'Atoh1-enhancer is accessible during development and sufficient for both activation and repression. Neurog1 requires the regions flanking the class A E-box to show its repressor effect, however, it does not require binding to DNA for Atoh1 repression. This depends on the dimerization domains Helix-1 and Helix-2 and the reduction of Atoh1 protein levels. The results point towards the acceleration of Atoh1 mRNA degradation as the potential mechanism for the reduction of Atoh1 levels. Such a mechanism dissociates the prevention of Atoh1 expression in neurosensory progenitors from the unfolding of the neurogenic program.

Diphtheria Toxin-Induced Cell Death Triggers Wnt-Dependent Hair Cell Regeneration in Neonatal Mice

Cochlear hair cells (HCs), the sensory cells that respond to sound, do not regenerate after damage in adult mammals, and their loss is a major cause of deafness. Here we show that HC regeneration in newborn mouse ears occurred spontaneously when the original cells were ablated by treatment with diphtheria toxin (DT) in ears that had been engineered to overexpress the DT receptor, but was not detectable when HCs were ablated in vivo by the aminoglycoside antibiotic neomycin. A variety of Wnts (Wnt1, Wnt2, Wnt2b, Wnt4, Wnt5a, Wnt7b, Wnt9a, Wnt9b, and Wnt11) and Wnt pathway component Krm2 were upregulated after DT damage. Nuclear β-catenin was upregulated in HCs and supporting cells of the DT-damaged cochlea. Pharmacological inhibition of Wnt decreased spontaneous regeneration, confirming a role of Wnt signaling in HC regeneration. Inhibition of Notch signaling further potentiated supporting cell proliferation and HC differentiation that occurred spontaneously. The absence of new HCs in the neomycin ears was correlated to less robust Wnt pathway activation, but the ears subjected to neomycin treatment nonetheless showed increased cell division and HC differentiation after subsequent forced upregulation of β-catenin. These studies suggest, first, that Wnt signaling plays a key role in regeneration, and, second, that the outcome of a regenerative response to damage in the newborn cochlea is determined by reaching a threshold level of Wnt signaling rather than its complete absence or presence.

SIGNIFICANCE STATEMENT

Sensory HCs of the inner ear do not regenerate in the adult, and their loss is a major cause of deafness. We found that HCs regenerated spontaneously in the newborn mouse after diphtheria toxin (DT)-induced, but not neomycin-induced, HC death. Regeneration depended on activation of Wnt signaling, and regeneration in DT-treated ears correlated to a higher level of Wnt activation than occurred in nonregenerating neomycin-treated ears. This is significant because insufficient regeneration caused by a failure to reach a threshold level of signaling, if true in the adult, has the potential to be exploited for development of clinical approaches for the treatment of deafness caused by HC loss.

Microarray analyses of otospheres derived from the cochlea in the inner ear identify putative transcription factors that regulate the characteristics of otospheres

Various tissues possess tissue-specific stem/progenitor cells, including the inner ears. Stem/progenitor cells of the inner ear can be isolated as so-called otospheres from differentiated cells using a sphere forming assay. Although recent studies have demonstrated the characteristics of otospheres to some extent, most of the features of these cells are unknown. In this report, we describe the findings of transcriptome analyses with a cDNA microarray of otospheres derived from the cochleae of the inner ears of neonatal mice in order to clarify the gene expression profile of otic stem/progenitor cells. There were common transcription factors between otospheres and embryonic stem cells, which were supposed to be due to the stemness of otospheres. In comparison with the cochlear sensory epithelium, the otospheres shared characteristics with the cochlea, although several transcription factors specific for otospheres were identified. These transcription factors are expected to be essential for maintaining the characteristics of otospheres, and appear to be candidate genes that promote the direct conversion of cells into otic stem/progenitor cells.

Signaling and Transcription Factors during Inner Ear Development: The Generation of Hair Cells and Otic Neurons

Integration between cell signals and bHLH transcription factors plays a prominent role during the development of hair cells of the inner ear. Hair cells are the sensory receptors of the inner ear, responsible for the mechano-transduction of sound waves into electrical signals. They derive from multipotent progenitors that reside in the otic placode. Progenitor commitment is the result of cell signaling from the surrounding tissues that result in the restricted expression of SoxB1 transcription factors, Sox2 and Sox3. In turn, they induce the expression of Neurog1 and Atoh1, two bHLH factors that specify neuronal and hair cell fates, respectively. Neuronal and hair cell development, however, do not occur simultaneously. Hair cell development is prevented during neurogenesis and prosensory stages, resulting in the delay of hair cell development with respect to neuron production. Negative interactions between Neurog1 and Atoh1, and of Atoh1 with other bHLH factors driven by Notch signaling, like Hey1 and Hes5, account for this delay. In summary, the regulation of Atoh1 and hair cell development relies on interactions between cell signaling and bHLH transcription factors that dictate cell fate and timing decisions during development. Interestingly, these mechanisms operate as well during hair cell regeneration after damage and during stem cell directed differentiation, making developmental studies instrumental for improving therapies for hearing impairment.

Selection of cell fate in the organ of Corti involves the integration of Hes/Hey signaling at the Atoh1 promoter

Determination of cell fate within the prosensory domain of the developing cochlear duct relies on the temporal and spatial regulation of the bHLH transcription factor Atoh1. Auditory hair cells and supporting cells arise in a wave of differentiation that patterns them into discrete rows mediated by Notch-dependent lateral inhibition. However, the mechanism responsible for selecting sensory cells from within the prosensory competence domain remains poorly understood. We show in mice that rather than being upregulated in rows of cells, Atoh1 is subject to transcriptional activation in groups of prosensory cells, and that highly conserved sites for Hes/Hey repressor binding in the Atoh1 promoter are needed to select the hair cell and supporting cell fate. During perinatal supporting cell transdifferentiation, which is a model of hair cell regeneration, we show that derepression is sufficient to induce Atoh1 expression, suggesting a mechanism for priming the 3' Atoh1 autoregulatory enhancer needed for hair cell expression.

Co-regulation of the Notch and Wnt signaling pathways promotes supporting cell proliferation and hair cell regeneration in mouse utricles

This work sought to determine the crosstalk between the Notch and Wnt signaling pathways in regulating supporting cell (SC) proliferation and hair cell (HC) regeneration in mouse utricles. We cultured postnatal day (P)3 and P60 mouse utricles, damaged the HCs with gentamicin, and treated the utricles with the γ-secretase inhibitor DAPT to inhibit the Notch pathway and with the Wnt agonist QS11 to active the Wnt pathway. We also used Sox2-CreER, Notch1-flox (exon 1), and Catnb-flox (exon 3) transgenic mice to knock out the Notch pathway and activate the Wnt pathway in Sox2+ SCs. Notch inhibition alone increased SC proliferation and HC number in both undamaged and damaged utricles. Wnt activation alone promoted SC proliferation, but the HC number was not significantly increased. Here we demonstrated the cumulative effects of Notch inhibition and Wnt activation in regulating SC proliferation and HC regeneration. Simultaneously inhibiting Notch and overexpressing Wnt led to significantly greater SC proliferation and greater numbers of HCs than manipulating either pathway alone. Similar results were observed in the transgenic mice. This study suggests that the combination of Notch inhibition and Wnt activation can significantly promote SC proliferation and increase the number of regenerated HCs in mouse utricle.

Genetic Correction of Induced Pluripotent Stem Cells From a Deaf Patient With MYO7A Mutation Results in Morphologic and Functional Recovery of the Derived Hair Cell-Like Cells

The genetic correction of induced pluripotent stem cells (iPSCs) induced from somatic cells of patients with sensorineural hearing loss (caused by hereditary factors) is a promising method for its treatment. The correction of gene mutations in iPSCs could restore the normal function of cells and provide a rich source of cells for transplantation. In the present study, iPSCs were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T; P-iPSCs), the asymptomatic father of the patient (MYO7A c.1184G>A mutation; CF-iPSCs), and a normal donor (MYO7A(WT/WT); C-iPSCs). One of MYO7A mutation sites (c.4118C>T) in the P-iPSCs was corrected using CRISPR/Cas9. The corrected iPSCs (CP-iPSCs) retained cell pluripotency and normal karyotypes. Hair cell-like cells induced from CP-iPSCs showed restored organization of stereocilia-like protrusions; moreover, the electrophysiological function of these cells was similar to that of cells induced from C-iPSCs and CF-iPSCs. These results might facilitate the development of iPSC-based gene therapy for genetic disorders.

SIGNIFICANCE

Induced pluripotent stem cells (iPSCs) were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T). One of the MYO7A mutation sites (c.4118C>T) in the iPSCs was corrected using CRISPR/Cas9. The genetic correction of MYO7A mutation resulted in morphologic and functional recovery of hair cell-like cells derived from iPSCs. These findings confirm the hypothesis that MYO7A plays an important role in the assembly of stereocilia into stereociliary bundles. Thus, the present study might provide further insight into the pathogenesis of sensorineural hearing loss and facilitate the development of therapeutic strategies against monogenic disease through the genetic repair of patient-specific iPSCs.

Epigenetic regulation of Atoh1 guides hair cell development in the mammalian cochlea

In the developing cochlea, sensory hair cell differentiation depends on the regulated expression of the bHLH transcription factor Atoh1. In mammals, if hair cells die they do not regenerate, leading to permanent deafness. By contrast, in non-mammalian vertebrates robust regeneration occurs through upregulation of Atoh1 in the surviving supporting cells that surround hair cells, leading to functional recovery. Investigation of crucial transcriptional events in the developing organ of Corti, including those involving Atoh1, has been hampered by limited accessibility to purified populations of the small number of cells present in the inner ear. We used µChIP and qPCR assays of FACS-purified cells to track changes in the epigenetic status of the Atoh1 locus during sensory epithelia development in the mouse. Dynamic changes in the histone modifications H3K4me3/H3K27me3, H3K9ac and H3K9me3 reveal a progression from poised, to active, to repressive marks, correlating with the onset of Atoh1 expression and its subsequent silencing during the perinatal (P1 to P6) period. Inhibition of acetylation blocked the increase in Atoh1 mRNA in nascent hair cells, as well as ongoing hair cell differentiation during embryonic organ of Corti development ex vivo. These results reveal an epigenetic mechanism of Atoh1 regulation underlying hair cell differentiation and subsequent maturation. Interestingly, the H3K4me3/H3K27me3 bivalent chromatin structure observed in progenitors persists at the Atoh1 locus in perinatal supporting cells, suggesting an explanation for the latent capacity of these cells to transdifferentiate into hair cells, and highlighting their potential as therapeutic targets in hair cell regeneration.

Establishment of a model of cochlear lesions in rats to study potential gene therapy for sensorineural hearing loss

INTRODUCTION

Sensorineural hearing loss seriously influences a patient's daily life, and no effective treatments exist to date. Gene therapy is a potential treatment for regenerating hair cells to restore hearing.

METHODS

In this study, we established a cochlear lesions model to study hair cell regeneration by co-administration of kanamycin and furosemide. After the injections, we assessed the survival of outer hair cells (OHC), inner hair cells (IHC), supporting cells (SC), spiral ganglion neurons (SGN) and peripheral axons. Moreover, we used two viral vectors to detect the transgene distribution.

RESULTS

Our results showed at 12h post-treatment, numerous OHC were missing in the basal turn. At 24h post-treatment, all OHCs in basal half of the cochlea were lost, and by 48h, OHC loss had spread to the apical coil. Four days after the injections, all OHCs were absent. At 1mo post-treatment, the organ of Corti had collapsed. In contrast, most of the SC remained 4d after the injections. The loss of SGN and peripheral axons was consistent with this time course post-treatment. The results of transgene distribution suggested the correlative gene can be transferred into the organ of Corti using adenoviruses (AdV) vectors and lentiviruses (LV) vectors in our cochlear lesion model.

COMPARISON WITH EXISTING METHOD(S)

We assessed the details of HC death at more time point and chosen the time point for gene transfer in this model.

CONCLUSIONS

We conclude that this cochlear lesion model would be suitable for the study of hair cell regeneration.

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.

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.

LSD1 is Required for Hair Cell Regeneration in Zebrafish

Lysine-specific demethylase 1 (LSD1/KDM1A) plays an important role in complex cellular processes such as differentiation, proliferation, apoptosis, and cell cycle progression. It has recently been demonstrated that during development, downregulation of LSD1 inhibits cell proliferation, modulates the expression of cell cycle regulators, and reduces hair cell formation in the zebrafish lateral line, which suggests that LSD1-mediated epigenetic regulation plays a key role in the development of hair cells. However, the role of LSD1 in hair cell regeneration after hair cell loss remains poorly understood. Here, we demonstrate the effect of LSD1 on hair cell regeneration following neomycin-induced hair cell loss. We show that the LSD1 inhibitor trans-2-phenylcyclopropylamine (2-PCPA) significantly decreases the regeneration of hair cells in zebrafish after neomycin damage. In addition, immunofluorescent staining demonstrates that 2-PCPA administration suppresses supporting cell proliferation and alters cell cycle progression. Finally, in situ hybridization shows that 2-PCPA significantly downregulates the expression of genes related to Wnt/β-catenin and Fgf activation. Altogether, our data suggest that downregulation of LSD1 significantly decreases hair cell regeneration after neomycin-induced hair cell loss through inactivation of the Wnt/β-catenin and Fgf signaling pathways. Thus, LSD1 plays a critical role in hair cell regeneration and might represent a novel biomarker and potential therapeutic approach for the treatment of hearing loss.

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.