The retinoblastoma gene pathway regulates the postmitotic state of hair cells of the mouse inner ear

Recent Therapeutic Progress and Future Perspectives for the Treatment of Hearing Loss

Up to 1.5 billion people worldwide suffer from various forms of hearing loss, with an additional 1.1 billion people at risk from various insults such as increased consumption of recreational noise-emitting devices and ageing. The most common type of hearing impairment is sensorineural hearing loss caused by the degeneration or malfunction of cochlear hair cells or spiral ganglion nerves in the inner ear. There is currently no cure for hearing loss. However, emerging frontier technologies such as gene, drug or cell-based therapies offer hope for an effective cure. In this review, we discuss the current therapeutic progress for the treatment of hearing loss. We describe and evaluate the major therapeutic approaches being applied to hearing loss and summarize the key trials and studies.

Age-Related Hearing Loss: Sensory and Neural Etiology and Their Interdependence

Age-related hearing loss (ARHL) is a common, increasing problem for older adults, affecting about 1 billion people by 2050. We aim to correlate the different reductions of hearing from cochlear hair cells (HCs), spiral ganglion neurons (SGNs), cochlear nuclei (CN), and superior olivary complex (SOC) with the analysis of various reasons for each one on the sensory deficit profiles. Outer HCs show a progressive loss in a basal-to-apical gradient, and inner HCs show a loss in a apex-to-base progression that results in ARHL at high frequencies after 70 years of age. In early neonates, SGNs innervation of cochlear HCs is maintained. Loss of SGNs results in a considerable decrease (~50% or more) of cochlear nuclei in neonates, though the loss is milder in older mice and humans. The dorsal cochlear nuclei (fusiform neurons) project directly to the inferior colliculi while most anterior cochlear nuclei reach the SOC. Reducing the number of neurons in the medial nucleus of the trapezoid body (MNTB) affects the interactions with the lateral superior olive to fine-tune ipsi- and contralateral projections that may remain normal in mice, possibly humans. The inferior colliculi receive direct cochlear fibers and second-order fibers from the superior olivary complex. Loss of the second-order fibers leads to hearing loss in mice and humans. Although ARHL may arise from many complex causes, HC degeneration remains the more significant problem of hearing restoration that would replace the cochlear implant. The review presents recent findings of older humans and mice with hearing loss.

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.

An Integrated Perspective of Evolution and Development: From Genes to Function to Ear, Lateral Line and Electroreception

Four sensory systems (vestibular, lateral line, electroreception, auditory) are unique and project exclusively to the brainstem of vertebrates. All sensory neurons depend on a common set of genes (Eya1, Sox2, Neurog1, Neurod1) that project to a dorsal nucleus and an intermediate nucleus, which differentiate into the vestibular ear, lateral line and electroreception in vertebrates. In tetrapods, a loss of two sensory systems (lateral line, electroreception) leads to the development of a unique ear and auditory system in amniotes. Lmx1a/b, Gdf7, Wnt1/3a, BMP4/7 and Atoh1 define the lateral line, electroreception and auditory nuclei. In contrast, vestibular nuclei depend on Neurog1/2, Ascl1, Ptf1a and Olig3, among others, to develop an independent origin of the vestibular nuclei. A common origin of hair cells depends on Eya1, Sox2 and Atoh1, which generate the mechanosensory cells. Several proteins define the polarity of hair cells in the ear and lateral line. A unique connection of stereocilia requires CDH23 and PCDH15 for connections and TMC1/2 proteins to perceive mechanosensory input. Electroreception has no polarity, and a different system is used to drive electroreceptors. All hair cells function by excitation via ribbons to activate neurons that innervate the distinct target areas. An integrated perspective is presented to understand the gain and loss of different sensory systems.

Human induced pluripotent stem cells and CRISPR/Cas-mediated targeted genome editing: Platforms to tackle sensorineural hearing loss

Hearing loss (HL) is a major global health problem of pandemic proportions. The most common type of HL is sensorineural hearing loss (SNHL) which typically occurs when cells within the inner ear are damaged. Human induced pluripotent stem cells (hiPSCs) can be generated from any individual including those who suffer from different types of HL. The development of new differentiation protocols to obtain cells of the inner ear including hair cells (HCs) and spiral ganglion neurons (SGNs) promises to expedite cell-based therapy and screening of potential pharmacologic and genetic therapies using human models. Considering age-related, acoustic, ototoxic, and genetic insults which are the most frequent causes of irreversible damage of HCs and SGNs, new methods of genome editing (GE), especially the CRISPR/Cas9 technology, could bring additional opportunities to understand the pathogenesis of human SNHL and identify novel therapies. However, important challenges associated with both hiPSCs and GE need to be overcome before scientific discoveries are correctly translated to effective and patient-safe applications. The purpose of the present review is (a) to summarize the findings from published reports utilizing hiPSCs for studies of SNHL, hence complementing recent reviews focused on animal studies, and (b) to outline promising future directions for deciphering SNHL using disruptive molecular and genomic technologies.

Usher Syndrome: Genetics and Molecular Links of Hearing Loss and Directions for Therapy

Usher syndrome (USH) is an autosomal recessive (AR) disorder that permanently and severely affects the senses of hearing, vision, and balance. Three clinically distinct types of USH have been identified, decreasing in severity from Type 1 to 3, with symptoms of sensorineural hearing loss (SNHL), retinitis pigmentosa (RP), and vestibular dysfunction. There are currently nine confirmed and two suspected USH-causative genes, and a further three candidate loci have been mapped. The proteins encoded by these genes form complexes that play critical roles in the development and maintenance of cellular structures within the inner ear and retina, which have minimal capacity for repair or regeneration. In the cochlea, stereocilia are located on the apical surface of inner ear hair cells (HC) and are responsible for transducing mechanical stimuli from sound pressure waves into chemical signals. These signals are then detected by the auditory nerve fibers, transmitted to the brain and interpreted as sound. Disease-causing mutations in USH genes can destabilize the tip links that bind the stereocilia to each other, and cause defects in protein trafficking and stereocilia bundle morphology, thereby inhibiting mechanosensory transduction. This review summarizes the current knowledge on Usher syndrome with a particular emphasis on mutations in USH genes, USH protein structures, and functional analyses in animal models. Currently, there is no cure for USH. However, the genetic therapies that are rapidly developing will benefit from this compilation of detailed genetic information to identify the most effective strategies for restoring functional USH proteins.

Hair cell regeneration from inner ear progenitors in the mammalian cochlea

Cochlear hair cells (HCs) are the mechanoreceptors of the auditory system, and because these cells cannot be spontaneously regenerated in adult mammals, hearing loss due to HC damage is permanent. However, cochleae of neonatal mice harbor some progenitor cells that retain limited ability to give rise to new HCs in vivo. Here we review the regulatory factors, signaling pathways, and epigenetic factors that have been reported to play roles in HC regeneration in the neonatal mammalian cochlea.

Hearing loss risk and DNA methylation signatures in preschool children following lead and cadmium exposure from an electronic waste recycling area

Experimental studies have uncovered chemical exposure-induced ototoxicity, but population-based hearing risk assessment especially for early-life exposure to heavy metals and relevant biological mechanism remains unclear. We aimed to measure lead (Pb) and cadmium (Cd) levels, blood DNA methylations of Rb1, CASP8 and MeCP2 and hearing in 116 preschool children 3- to 7-years of age from an e-waste and a reference area, and to evaluate the association of exposures with hearing loss potentially affected by epigenetic modifications. A higher median Pb level but not Cd was found in the exposed group than the reference group. Average hearing thresholds in either ear of the exposed children were higher. Higher promoter methylation levels at cg02978827 and position +14, and lower at position +4 of Rb1 were found in the exposed group. Pb was positively correlated with chewing pencil habit while negatively correlated with washing hands before dinner. Slightly negative trends of promoter methylations in Rb1 and CASP8, while a strong positive trend of MeCP2 promoter methylation, were found along with increasing Pb and Cd levels. Logistic analyses showed the adjusted OR of Pb for hearing loss in the left ear and both ears was 1.46 (95% CI: 1.12, 1.91) and 1.40 (95% CI: 1.06, 1.84), respectively. Our results show an elevated Pb level, altered promoter DNA methylations and hearing ability in children of e-waste areas, suggesting that epigenetic changes of specific genes involves in the development of the auditory system during early exposure to environmental chemicals.

Using Sox2 to alleviate the hallmarks of age-related hearing loss

Age-related hearing loss (ARHL) is the most prevalent sensory deficit. ARHL reduces the quality of life of the growing population, setting seniors up for the enhanced mental decline. The size of the needy population, the structural deficit, and a likely research strategy for effective treatment of chronic neurosensory hearing in the elderly are needed. Although there has been profound advancement in auditory regenerative research, there remain multiple challenges to restore hearing loss. Thus, additional investigations are required, using novel tools. We propose how the (1) flat epithelium, remaining after the organ of Corti has deteriorated, can be converted to the repaired-sensory epithelium, using Sox2. This will include (2) developing an artificial gene regulatory network transmitted by (3) large viral vectors to the flat epithelium to stimulate remnants of the organ of Corti to restore hair cells. We hope to unite with our proposal toward the common goal, eventually restoring a functional human hearing organ by transforming the flat epithelial cells left after the organ of Corti loss.

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.

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.

Approaches for the study of epigenetic modifications in the inner ear and related tissues

DNA methylation and histone modifications such as methylation, acetylation, and phosphorylation, are two types of epigenetic modifications that alter gene expression. These additions to DNA regulatory elements or to the tails of histones can be inherited or can also occur de novo. Since epigenetic modifications can have significant effects on various processes at both the cellular and organismal level, there has been a rapid increase in research on this topic throughout all fields of biology in recent years. However, epigenetic research is relativity new for the inner ear field, likely due to the limited number of cells present and their quiescent nature. Here, we provide an overview of methods used to detect DNA methylation and histone modifications with a focus on those that have been validated for use with limited cell numbers and a discussion of the strengths and limitations for each. We also provide examples for how these methods have been used to investigate the epigenetic landscape in the inner ear and related tissues.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

Understanding Molecular Evolution and Development of the Organ of Corti Can Provide Clues for Hearing Restoration

The mammalian hearing organ is a stereotyped cellular assembly with orderly innervation: two types of spiral ganglion neurons (SGNs) innervate two types of differentially distributed hair cells (HCs). HCs and SGNs evolved from single neurosensory cells through gene multiplication and diversification. Independent regulation of HCs and neuronal differentiation through expression of basic helix-loop-helix transcription factors (bHLH TFs: Atoh1, Neurog1, Neurod1) led to the evolution of vestibular HC assembly and their unique type of innervation. In ancestral mammals, a vestibular organ was transformed into the organ of Corti (OC) containing a single row of inner HC (IHC), three rows of outer HCs (OHCs), several unique supporting cell types, and a peculiar innervation distribution. Restoring the OC following long-term hearing loss is complicated by the fact that the entire organ is replaced by a flat epithelium and requires reconstructing the organ from uniform undifferentiated cell types, recapitulating both evolution and development. Finding the right sequence of gene activation during development that is useful for regeneration could benefit from an understanding of the OC evolution. Toward this end, we report on Foxg1 and Lmx1a mutants that radically alter the OC cell assembly and its innervation when mutated and may have driven the evolutionary reorganization of the basilar papilla into an OC in ancestral Therapsids. Furthermore, genetically manipulating the level of bHLH TFs changes HC type and distribution and allows inference how transformation of HCs might have happened evolutionarily. We report on how bHLH TFs regulate OHC/IHC and how misexpression (Atoh1-Cre; Atoh1f/kiNeurog1) alters HC fate and supporting cell development. Using mice with altered HC types and distribution, we demonstrate innervation changes driven by HC patterning. Using these insights, we speculate on necessary steps needed to convert a random mixture of post-mitotic precursors into the orderly OC through spatially and temporally regulated critical bHLH genes in the context of other TFs to restore normal innervation patterns.

Cell proliferation during hair cell regeneration induced by Math1 in vestibular epithelia in vitro

Hair cell regeneration is the fundamental method of correcting hearing loss and balance disorders caused by hair cell damage or loss. How to promote hair cell regeneration is a hot focus in current research. In mammals, cochlear hair cells cannot be regenerated and few vestibular hair cells can be renewed through spontaneous regeneration. However, Math₁ gene transfer allows a few inner ear cells to be transformed into hair cells in vitro or in vivo. Hair cells can be renewed through two possible means in birds: supporting cell differentiation and transdifferentiation with or without cell division. Hair cell regeneration is strongly associated with cell proliferation. Therefore, this study explored the relationship between Math₁-induced vestibular hair cell regeneration and cell division in mammals. The mouse vestibule was isolated to harvest vestibular epithelial cells. Ad-Math₁-enhanced green fluorescent protein (EGFP) was used to track cell division during hair cell transformation. 5-Bromo-2′-deoxyuridine (BrdU) was added to track cell proliferation at various time points. Immunocytochemistry was utilized to determine cell differentiation and proliferation. Results demonstrated that when epithelial cells were in a higher proliferative stage, more of these cells differentiated into hair cells by Math₁ gene transfer. However, in the low proliferation stage, no BrdU-positive cells were seen after Math₁ gene transfer. Cell division always occurred before Math₁ transfection but not during or after Math₁ transfection, when cells were labeled with BrdU before and after Ad-Math₁-EGFP transfection. These results confirm that vestibular epithelial cells with high proliferative potential can differentiate into new hair cells by Math₁ gene transfer, but this process is independent of cell proliferation.

Hedgehog Signaling Promotes the Proliferation and Subsequent Hair Cell Formation of Progenitor Cells in the Neonatal Mouse Cochlea

Hair cell (HC) loss is the major cause of permanent sensorineural hearing loss in mammals. Unlike lower vertebrates, mammalian cochlear HCs cannot regenerate spontaneously after damage, although the vestibular system does maintain limited HC regeneration capacity. Thus HC regeneration from the damaged sensory epithelium has been one of the main areas of research in the field of hearing restoration. Hedgehog signaling plays important roles during the embryonic development of the inner ear, and it is involved in progenitor cell proliferation and differentiation as well as the cell fate decision. In this study, we show that recombinant Sonic Hedgehog (Shh) protein effectively promotes sphere formation, proliferation, and differentiation of Lgr5+ progenitor cells isolated from the neonatal mouse cochlea. To further explore this, we determined the effect of Hedgehog signaling on cell proliferation and HC regeneration in cultured cochlear explant from transgenic R26-SmoM2 mice that constitutively activate Hedgehog signaling in the supporting cells of the cochlea. Without neomycin treatment, up-regulation of Hedgehog signaling did not significantly promote cell proliferation or new HC formation. However, after injury to the sensory epithelium by neomycin treatment, the over-activation of Hedgehog signaling led to significant supporting cell proliferation and HC regeneration in the cochlear epithelium explants. RNA sequencing and real-time PCR were used to compare the transcripts of the cochleae from control mice and R26-SmoM2 mice, and multiple genes involved in the proliferation and differentiation processes were identified. This study has important implications for the treatment of sensorineural hearing loss by manipulating the Hedgehog signaling pathway.

Axon guidance pathways served as common targets for human speech/language evolution and related disorders

Human and several nonhuman species share the rare ability of modifying acoustic and/or syntactic features of sounds produced, i.e. vocal learning, which is the important neurobiological and behavioral substrate of human speech/language. This convergent trait was suggested to be associated with significant genomic convergence and best manifested at the ROBO-SLIT axon guidance pathway. Here we verified the significance of such genomic convergence and assessed its functional relevance to human speech/language using human genetic variation data. In normal human populations, we found the affected amino acid sites were well fixed and accompanied with significantly more associated protein-coding SNPs in the same genes than the rest genes. Diseased individuals with speech/language disorders have significant more low frequency protein coding SNPs but they preferentially occurred outside the affected genes. Such patients' SNPs were enriched in several functional categories including two axon guidance pathways (mediated by netrin and semaphorin) that interact with ROBO-SLITs. Four of the six patients have homozygous missense SNPs on PRAME gene family, one youngest gene family in human lineage, which possibly acts upon retinoic acid receptor signaling, similarly as FOXP2, to modulate axon guidance. Taken together, we suggest the axon guidance pathways (e.g. ROBO-SLIT, PRAME gene family) served as common targets for human speech/language evolution and related disorders.

Carbonic Anhydrase Inhibitors Induce Developmental Toxicity During Zebrafish Embryogenesis, Especially in the Inner Ear

In vertebrates, carbonic anhydrases (CAs) play important roles in ion transport and pH regulation in many organs, including the eyes, kidneys, central nervous system, and inner ear. In aquatic organisms, the enzyme is inhibited by various chemicals present in the environment, such as heavy metals, pesticides, and pharmaceuticals. In this study, the effects of CA inhibitors, i.e., sulfonamides [ethoxyzolamide (EZA), acetazolamide (AZA), and dorzolamide (DZA)], on zebrafish embryogenesis were investigated. In embryos treated with the sulfonamides, abnormal development, such as smaller otoliths, an enlarged heart, an irregular pectoral fin, and aberrant swimming behavior, was observed. Especially, the development of otoliths and locomotor activity was severely affected by all the sulfonamides, and EZA was a consistently stronger inhibitor than AZA or DZA. In the embryos treated with EZA, inner ear hair cells containing several CA isoforms, which provide HCO₃^- to the endolymph for otolith calcification and maintain an appropriate pH there, were affected. Acridine orange/ethidium bromide staining indicated that the hair cell damage in the inner ear and pectral fin is due to apoptosis. Moreover, RNA measurement demonstrated that altered gene expression of cell cycle arrest- and apoptosis-related proteins p53, p21, p27, and Bcl-2 occurred even at 0.08 ppm with which normal development was observed. This finding suggests that a low concentration of EZA may affect embryogenesis via the apoptosis pathway. Thus, our findings demonstrated the importance of potential risk assessment of CA inhibition, especially regarding the formation of otoliths as a one of the most sensitive organs in embryogenesis.

Recent Advancements in the Regeneration of Auditory Hair Cells and Hearing Restoration

Neurosensory responses of hearing and balance are mediated by receptors in specialized neuroepithelial sensory cells. Any disruption of the biochemical and molecular pathways that facilitate these responses can result in severe deficits, including hearing loss and vestibular dysfunction. Hearing is affected by both environmental and genetic factors, with impairment of auditory function being the most common neurosensory disorder affecting 1 in 500 newborns, as well as having an impact on the majority of elderly population. Damage to auditory sensory cells is not reversible, and if sufficient damage and cell death have taken place, the resultant deficit may lead to permanent deafness. Cochlear implants are considered to be one of the most successful and consistent treatments for deaf patients, but only offer limited recovery at the expense of loss of residual hearing. Recently there has been an increased interest in the auditory research community to explore the regeneration of mammalian auditory hair cells and restoration of their function. In this review article, we examine a variety of recent therapies, including genetic, stem cell and molecular therapies as well as discussing progress being made in genome editing strategies as applied to the restoration of hearing function.

Development and regeneration of vestibular hair cells in mammals

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

Supernumerary human hair cells-signs of regeneration or impaired development? A field emission scanning electron microscopy study

BACKGROUND

Current attempts to regenerate cochlear sensorineural structures motivate further inspection of the human organ of hearing. Here, we analyzed the supernumerary inner hair cell (sIHC), a possible sign of regeneration and cell replacement.

METHODS

Human cochleae were studied using field emission scanning electron microscopy (FESEM; maximum resolution 2 nm) obtained from individuals aged 44, 48, and 58 years with normal sensorineural pure-tone average (PTA) thresholds (PTA <20 dB). The wasted tissue was harvested during trans-cochlear approaches and immediately fixed for ultrastructural analysis.

RESULTS

All specimens exhibited sIHCs at all turns except at the extreme lower basal turn. In one specimen, it was possible to image and count the inner hair cells (IHCs) along the cochlea representing the 0.2 kHz-8 kHz region according to the Greenwood place/frequency scale. In a region with 2,321 IHCs, there were 120 scattered one-cell losses or 'gaps' (5%). Forty-two sIHCs were present facing the modiolus. Thirty-eight percent of the sIHCs were located near a 'gap' in the IHC row (±6 IHCs).

CONCLUSIONS

The prevalence of ectopic inner hair cells was higher than expected. The morphology and placement could reflect a certain ongoing regeneration. Further molecular studies are needed to verify if the regenerative capacity of the human auditory periphery might have been underestimated.

Mammalian Cochlear Hair Cell Regeneration and Ribbon Synapse Reformation

Hair cells (HCs) are the sensory preceptor cells in the inner ear, which play an important role in hearing and balance. The HCs of organ of Corti are susceptible to noise, ototoxic drugs, and infections, thus resulting in permanent hearing loss. Recent approaches of HCs regeneration provide new directions for finding the treatment of sensor neural deafness. To have normal hearing function, the regenerated HCs must be reinnervated by nerve fibers and reform ribbon synapse with the dendrite of spiral ganglion neuron through nerve regeneration. In this review, we discuss the research progress in HC regeneration, the synaptic plasticity, and the reinnervation of new regenerated HCs in mammalian inner ear.

Proteomics studies in inner ear disorders: pathophysiology and biomarkers

Although proteomics has been exploited in a wide range of diseases for identification of biomarkers and pathophysiological mechanisms, there are still biomedical disciplines such as otology where proteomics platforms are underused due to technical challenges and/or complex features of the disease. Thus, in the past few years, healthcare and scientific agencies have advocated the development and adoption of proteomic technologies in otological research. However, few studies have been conducted and limited literature is available in this area. Here, we present the state of the art of proteomics in otology, discussing the substantial evidence from recent experimental models and clinical studies in inner-ear conditions. We also delineate a series of critical issues including minute size of the inner ear, delicacy and poor accessibility of tissue that researchers face while undertaking otology proteomics research. Furthermore, we provide perspective to enhance the impact and lead to the clinical implementation of these proteomics-based strategies.

The quest for restoring hearing: Understanding ear development more completely

Neurosensory hearing loss is a growing problem of super-aged societies. Cochlear implants can restore some hearing, but rebuilding a lost hearing organ would be superior. Research has discovered many cellular and molecular steps to develop a hearing organ but translating those insights into hearing organ restoration remains unclear. For example, we cannot make various hair cell types and arrange them into their specific patterns surrounded by the right type of supporting cells in the right numbers. Our overview of the topologically highly organized and functionally diversified cellular mosaic of the mammalian hearing organ highlights what is known and unknown about its development. Following this analysis, we suggest critical steps to guide future attempts toward restoration of a functional organ of Corti. We argue that generating mutant mouse lines that mimic human pathology to fine-tune attempts toward long-term functional restoration are needed to go beyond the hope generated by restoring single hair cells in postnatal sensory epithelia.

Generation of a Retinoblastoma (Rb)1-inducible dominant-negative (DN) mouse model

Retinoblastoma 1 (Rb1) is an essential gene regulating cellular proliferation, differentiation, and homeostasis. To exert these functions, Rb1 is recruited and physically interacts with a growing variety of signaling pathways. While Rb1 does not appear to be ubiquitously expressed, its expression has been confirmed in a variety of hematopoietic and neuronal-derived cells, including the inner ear hair cells (HCs). Studies in transgenic mice demonstrate that complete germline or conditional Rb1 deletion leads to abnormal cell proliferation, followed by massive apoptosis; making it difficult to fully address Rb1's biochemical activities. To overcome these limitations, we developed a tetracycline-inducible TetO-CB-myc6-Rb1 (CBRb) mouse model to achieve transient and inducible dominant-negative (DN) inhibition of the endogenous RB1 protein. Our strategy involved fusing the Rb1 gene to the lysosomal protease pre-procathepsin B (CB), thus allowing for further routing of the DN-CBRb fusion protein and its interacting complexes for proteolytic degradation. Moreover, reversibility of the system is achieved upon suppression of doxycycline (Dox) administration. Preliminary characterization of DN-CBRb mice bred to a ubiquitous rtTA mouse line demonstrated a significant inhibition of the endogenous RB1 protein in the inner ear and in a number of other organs where RB1 is expressed. Examination of the postnatal (P) DN-CBRb mice inner ear at P10 and P28 showed the presence of supernumerary inner HCs (IHCs) in the lower turns of the cochleae, which corresponds to the described expression domain of the endogenous Rb1 gene. Selective and reversible suppression of gene expression is both an experimental tool for defining function and a potential means to medical therapy. Given the limitations associated with Rb1-null mice lethality, this model provides a valuable resource for understanding RB1 activity, relative contribution to HC regeneration and its potential therapeutic application.

DNA damage signaling regulates age-dependent proliferative capacity of quiescent inner ear supporting cells

Supporting cells (SCs) of the cochlear (auditory) and vestibular (balance) organs hold promise as a platform for therapeutic regeneration of the sensory hair cells. Prior data have shown proliferative restrictions of adult SCs forced to re-enter the cell cycle. By comparing juvenile and adult SCs in explant cultures, we have here studied how proliferative restrictions are linked with DNA damage signaling. Cyclin D1 overexpression, used to stimulate cell cycle re-entry, triggered higher proliferative activity of juvenile SCs. Phosphorylated form of histone H2AX (γH2AX) and p53 binding protein 1 (53BP1) were induced in a foci-like pattern in SCs of both ages as an indication of DNA double-strand break formation and activated DNA damage response. Compared to juvenile SCs, γH2AX and the repair protein Rad51 were resolved with slower kinetics in adult SCs, accompanied by increased apoptosis. Consistent with the in vitro data, in a Rb mutant mouse model in vivo, cell cycle re-entry of SCs was associated with γH2AX foci induction. In contrast to cell cycle reactivation, pharmacological stimulation of SC-to-hair-cell transdifferentiation in vitro did not trigger γH2AX. Thus, DNA damage and its prolonged resolution are critical barriers in the efforts to stimulate proliferation of the adult inner ear SCs.

Opportunities and limits of the one gene approach: the ability of Atoh1 to differentiate and maintain hair cells depends on the molecular context

Atoh1 (Math1) was the first gene discovered in ear development that showed no hair cell (HC) differentiation when absent and could induce HC differentiation when misexpressed. These data implied that Atoh1 was both necessary and sufficient for hair cell development. However, other gene mutations also result in loss of initially forming HCs, notably null mutants for Pou4f3, Barhl1, and Gfi1. HC development and maintenance also depend on the expression of other genes (Sox2, Eya1, Gata3, Pax2) and several genes have been identified that can induce HCs when misexpressed (Jag1) or knocked out (Lmo4). In the ear Atoh1 is not only expressed in HCs but also in some supporting cells and neurons that do not differentiate into HCs. Simple removal of one gene, Neurod1, can de-repress Atoh1 and turns those neurons into HCs suggesting that Neurod1 blocks Atoh1 function in neurons. Atoh1 expression in inner pillar cells may also be blocked by too many Hes/Hey factors but conversion into HCs has only partially been achieved through Hes/Hey removal. Detailed analysis of cell cycle exit confirmed an apex to base cell cycle exit progression of HCs of the organ of Corti. In contrast, Atoh1 expression progresses from the base toward the apex with a variable delay relative to the cell cycle exit. Most HCs exit the cell cycle and are thus defined as precursors before Atoh1 is expressed. Atoh1 is a potent differentiation factor but can differentiate and maintain HCs only in the ear and when other factors are co-expressed. Upstream factors are essential to regulate Atoh1 level of expression duration while downstream, co-activated by other factors, will define the context of Atoh1 action. We suggest that these insights need to be taken into consideration and approaches beyond the simple Atoh1 expression need to be designed able to generate the radial and longitudinal variations in hair cell types for normal function of the organ of Corti.

Auditory hair cell-specific deletion of p27Kip1 in postnatal mice promotes cell-autonomous generation of new hair cells and normal hearing

Hearing in mammals relies upon the transduction of sound by hair cells (HCs) in the organ of Corti within the cochlea of the inner ear. Sensorineural hearing loss is a widespread and permanent disability due largely to a lack of HC regeneration in mammals. Recent studies suggest that targeting the retinoblastoma (Rb)/E2F pathway can elicit proliferation of auditory HCs. However, previous attempts to induce HC proliferation in this manner have resulted in abnormal cochlear morphology, HC death, and hearing loss. Here we show that cochlear HCs readily proliferate and survive following neonatal, HC-specific, conditional knock-out of p27(Kip1) (p27CKO), a tumor suppressor upstream of Rb. Indeed, HC-specific p27CKO results in proliferation of these cells without the upregulation of the supporting cell or progenitor cell proteins, Prox1 or Sox2, suggesting that they remain HCs. Furthermore, p27CKO leads to a significant addition of postnatally derived HCs that express characteristic synaptic and stereociliary markers and survive to adulthood, although a portion of the newly derived inner HCs exhibit cytocauds and lack VGlut3 expression. Despite this, p27CKO mice exhibit normal hearing as measured by evoked auditory brainstem responses, which suggests that the newly generated HCs may contribute to, or at least do not greatly detract from, function. These results show that p27(Kip1) actively maintains HC quiescence in postnatal mice, and suggest that inhibition of p27(Kip1) in residual HCs represents a potential strategy for cell-autonomous auditory HC regeneration.

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

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

Molecular mechanisms and potentials for differentiating inner ear stem cells into sensory hair cells

In mammals, hair cells may be damaged or lost due to genetic mutation, infectious disease, chemical ototoxicity, noise and other factors, causing permanent sensorineural deafness. Regeneration of hair cells is a basic pre-requisite for recovery of hearing in deaf animals. The inner ear stem cells in the organ of Corti and vestibular utricle are the most ideal precursors for regeneration of inner ear hair cells. This review highlights some recent findings concerning the proliferation and differentiation of inner ear stem cells. The differentiation of inner ear stem cells into hair cells involves a series of signaling pathways and regulatory factors. This paper offers a comprehensive analysis of the related studies.

High-throughput screening reveals alsterpaullone, 2-cyanoethyl as a potent p27Kip1 transcriptional inhibitor

p27^Kip1 is a cell cycle inhibitor that prevents cyclin dependent kinase (CDK)/cyclin complexes from phosphorylating their targets. p27^Kip1 is a known tumor suppressor, as the germline loss of p27^Kip1 results in sporadic pituitary formation in aged rodents, and its presence in human cancers is indicative of a poor prognosis. In addition to its role in cancer, loss of p27^Kip1 results in regenerative phenotypes in some tissues and maintenance of stem cell pluripotency, suggesting that p27^Kip1 inhibitors could be beneficial for tissue regeneration. Because p27^Kip1 is an intrinsically disordered protein, identifying direct inhibitors of the p27^Kip1 protein is difficult. Therefore, we pursued a high-throughput screening strategy to identify novel p27^Kip1 transcriptional inhibitors. We utilized a luciferase reporter plasmid driven by the p27^Kip1 promoter to transiently transfect HeLa cells and used cyclohexamide as a positive control for non-specific inhibition. We screened a “bioactive” library consisting of 8,904 (4,359 unique) compounds, of which 830 are Food and Drug Administration (FDA) approved. From this screen, we successfully identified 111 primary hits with inhibitory effect against the promoter of p27^Kip1. These hits were further refined using a battery of secondary screens. Here we report four novel p27^Kip1 transcriptional inhibitors, and further demonstrate that our most potent hit compound (IC₅₀ = 200 nM) Alsterpaullone 2-cyanoethyl, inhibits p27^Kip1 transcription by preventing FoxO3a from binding to the p27^Kip1 promoter. This screen represents one of the first attempts to identify inhibitors of p27^Kip1 and may prove useful for future tissue regeneration studies.

Disrupting Rb-Raf-1 interaction inhibits hair cell regeneration in zebrafish lateral line neuromasts

Zebrafish neuromast is an ideal model for investigating hair cell (HC) death and regeneration following ototoxic insults. HC undergoes rapid and robust replacement in larval zebrafish after neomycin damage. However, the origin of new HCs remains unclear. Our data showed that asymmetric cell division was involved in the process of HC regeneration in zebrafish lateral line neuromasts. Furthermore, a small molecule RRD251, which disrupted the physical interaction between RB and Raf-1 and then blocked the phosphorylation of Rb, could have inhibited the HC regeneration from supporting cell proliferation. Our results indicate that Rb-Raf-1 interaction plays an important role in spontaneous HC regeneration in zebrafish.

Correct timing of proliferation and differentiation is necessary for normal inner ear development and auditory hair cell viability

BACKGROUND

Hearing restoration through hair cell regeneration will require revealing the dynamic interactions between proliferation and differentiation during development to avoid the limited viability of regenerated hair cells. Pax2-Cre N-Myc conditional knockout (CKO) mice highlighted the need of N-Myc for proper neurosensory development and possible redundancy with L-Myc. The late-onset hair cell death in the absence of early N-Myc expression could be due to mis-regulation of genes necessary for neurosensory formation and maintenance, such as Neurod1, Atoh1, Pou4f3, and Barhl1.

RESULTS

Pax2-Cre N-Myc L-Myc double CKO mice show that proliferation and differentiation are linked together through Myc and in the absence of both Mycs, altered proliferation and differentiation result in morphologically abnormal ears. In particular, the organ of Corti apex is re-patterned into a vestibular-like organization and the base is truncated and fused with the saccule.

CONCLUSIONS

These data indicate that therapeutic approaches to restore hair cells must take into account a dynamic interaction of proliferation and differentiation regulation of basic Helix-Loop-Helix transcription factors in attempts to stably replace lost cochlear hair cells. In addition, our data indicate that Myc is an integral component of the evolutionary transformation process that resulted in the organ of Corti development.

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

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

Notch inhibition induces cochlear hair cell regeneration and recovery of hearing after acoustic trauma

Hearing loss due to damage to auditory hair cells is normally irreversible because mammalian hair cells do not regenerate. Here, we show that new hair cells can be induced and can cause partial recovery of hearing in ears damaged by noise trauma, when Notch signaling is inhibited by a γ-secretase inhibitor selected for potency in stimulating hair cell differentiation from inner ear stem cells in vitro. Hair cell generation resulted from an increase in the level of bHLH transcription factor Atoh1 in response to inhibition of Notch signaling. In vivo prospective labeling of Sox2-expressing cells with a Cre-lox system unambiguously demonstrated that hair cell generation resulted from transdifferentiation of supporting cells. Manipulating cell fate of cochlear sensory cells in vivo by pharmacological inhibition of Notch signaling is thus a potential therapeutic approach to the treatment of deafness.

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.

Migration and differentiation of mouse embryonic stem cells transplanted into mature cochlea of rats with aminoglycoside-induced hearing loss

CONCLUSION

Mouse embryonic stem cells (ESCs) transplanted into the scala tympani are able to migrate in the cochlea of rats deafened with aminoglycoside and partly restore the structure of sensory epithelia of the inner ear.

OBJECTIVES

To explore the migration and differentiation of enhanced green fluorescence protein (EGFP)-expressing ESCs by transplanting them into the scala tympani of rats with amikacin sulfate-induced hearing loss.

METHODS

Adult Sprague-Dawley (SD) rats were deafened with amikacin sulfate. Mouse ESCs expressing EGFP (EGFP-ESCs) were transplanted into the scala tympani. The migration and differentiation were observed at different time points.

RESULTS

EGFP-ESCs transplanted into normal cochlea did not migrate, but those in the amikacin-damaged cochlea could survive and migrate into the scala media and the vestibular cisterna. For the first time, we observed that the EGFP-ESCs migrated into the scala media, took the place of the organ of Corti, and formed a structure just like the cochlear tunnel. Some grafted stem cells even expressed myosin VIIa, the molecular marker of hair cells. Some nerve fibers reached to the bottom of the hair cell-like cells. The ESCs migrated into the vestibule and restored the sensory epithelia of the ampullary crest. The number of the transplanted ESCs reduced over the 6 week period of the study.

Filling the silent void: genetic therapies for hearing impairment

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

New strategies for the restoration of hearing loss: challenges and opportunities

INTRODUCTION

For most types of hearing impairments, a definitive therapy would rest on the ability to restore hair cells and the spiral ganglion neurons. The only established technique to treat deafness is based on the functional replacement of hair cells with a cochlear implant, but this still has important limitations.

SOURCES OF DATA

A systematic revision of the relevant literature is presented.

AREAS OF AGREEMENT

New curative strategies, ranging from stem cells to gene and molecular therapy, are under development.

AREAS OF CONTROVERSY

Although still experimental, they have delivered some initial promissory results that allow us to look at them with cautious optimism.

GROWING POINTS

The isolation of human auditory cells, the generation of protocols to control their differentiation into sensory lineages, their promising application in vivo and the identification of key genes to target molecularly offer an exciting landscape.

AREAS TIMELY FOR DEVELOPING RESEARCH

In this chapter, I discuss the latest advances in the field and how they are being translated into a clinical application.

MYC gene delivery to adult mouse utricles stimulates proliferation of postmitotic supporting cells in vitro

The inner ears of adult humans and other mammals possess a limited capacity for regenerating sensory hair cells, which can lead to permanent auditory and vestibular deficits. During development and regeneration, undifferentiated supporting cells within inner ear sensory epithelia can self-renew and give rise to new hair cells; however, these otic progenitors become depleted postnatally. Therefore, reprogramming differentiated supporting cells into otic progenitors is a potential strategy for restoring regenerative potential to the ear. Transient expression of the induced pluripotency transcription factors, Oct3/4, Klf4, Sox2, and c-Myc reprograms fibroblasts into neural progenitors under neural-promoting culture conditions, so as a first step, we explored whether ectopic expression of these factors can reverse supporting cell quiescence in whole organ cultures of adult mouse utricles. Co-infection of utricles with adenoviral vectors separately encoding Oct3/4, Klf4, Sox2, and the degradation-resistant T58A mutant of c-Myc (c-MycT58A) triggered significant levels of supporting cell S-phase entry as assessed by continuous BrdU labeling. Of the four factors, c-MycT58A alone was both necessary and sufficient for the proliferative response. The number of BrdU-labeled cells plateaued between 5–7 days after infection, and then decreased ∼60% by 3 weeks, as many cycling cells appeared to enter apoptosis. Switching to differentiation-promoting culture medium at 5 days after ectopic expression of c-MycT58A temporarily attenuated the loss of BrdU-labeled cells and accompanied a very modest but significant expansion of the sensory epithelium. A small number of the proliferating cells in these cultures labeled for the hair cell marker, myosin VIIA, suggesting they had begun differentiating towards a hair cell fate. The results indicate that ectopic expression of c-MycT58A in combination with methods for promoting cell survival and differentiation may restore regenerative potential to supporting cells within the adult mammalian inner ear.

The myc road to hearing restoration

Current treatments for hearing loss, the most common neurosensory disorder, do not restore perfect hearing. Regeneration of lost organ of Corti hair cells through forced cell cycle re-entry of supporting cells or through manipulation of stem cells, both avenues towards a permanent cure, require a more complete understanding of normal inner ear development, specifically the balance of proliferation and differentiation required to form and to maintain hair cells. Direct successful alterations to the cell cycle result in cell death whereas regulation of upstream genes is insufficient to permanently alter cell cycle dynamics. The Myc gene family is uniquely situated to synergize upstream pathways into downstream cell cycle control. There are three Mycs that are embedded within the Myc/Max/Mad network to regulate proliferation. The function of the two ear expressed Mycs, N-Myc and L-Myc were unknown less than two years ago and their therapeutic potentials remain speculative. In this review, we discuss the roles the Mycs play in the body and what led us to choose them to be our candidate gene for inner ear therapies. We will summarize the recently published work describing the early and late effects of N-Myc and L-Myc on hair cell formation and maintenance. Lastly, we detail the translational significance of our findings and what future work must be performed to make the ultimate hearing aid: the regeneration of the organ of Corti.

Transcriptomic analysis of the developing and adult mouse cochlear sensory epithelia

The adult mammalian cochlea lacks regenerative ability and the irreversible degeneration of cochlear sensory hair cells leads to permanent hearing loss. Previous data show that early postnatal cochlea harbors stem/progenitor-like cells and shows a limited regenerative/repair capacity. These properties are progressively lost later during the postnatal development. Little is known about the genes and pathways that are potentially involved in this difference of the regenerative/repair potentialities between early postnatal and adult mammalian cochlear sensory epithelia (CSE). The goal of our study is to investigate the transcriptomic profiles of these two stages. We used Mouse Genome 430 2.0 microarray to perform an extensive analysis of the genes expressed in mouse postnatal day-3 (P3) and adult CSE. Statistical analysis of microarray data was performed using SAM (Significance Analysis of Microarrays) software. We identified 5644 statistically significant differentially expressed transcripts with a fold change (FC) >2 and a False Discovery Rate (FDR) ≤0.05. The P3 CSE signature included 3,102 transcripts, among which were known genes in the cochlea, but also new transcripts such as, Hmga2 (high mobility group AT-hook 2) and Nrarp (Notch-regulated ankyrin repeat protein). The adult CSE overexpressed 2,542 transcripts including new transcripts, such as Prl (Prolactin) and Ar (Androgen receptor), that previously were not known to be expressed in the adult cochlea. Our comparative study revealed important genes and pathways differentially expressed between the developing and adult CSE. The identification of new candidate genes would be useful as potential markers of the maintenance or the loss of stem cells and regenerative/repair ability during mammalian cochlear development.

In vivo proliferative regeneration of balance hair cells in newborn mice

The regeneration of mechanoreceptive hair cells occurs throughout life in non-mammalian vertebrates and allows them to recover from hearing and balance deficits that affect humans and other mammals permanently. The irreversibility of comparable deficits in mammals remains unexplained, but often has been attributed to steep embryonic declines in cellular production. However, recent results suggest that gravity-sensing hair cells in murine utricles may increase in number during neonatal development, raising the possibility that young mice might retain sufficient cellular plasticity for mitotic hair cell regeneration. To test for this we used neomycin to kill hair cells in utricles cultured from mice of different ages and found that proliferation increased tenfold in damaged utricles from the youngest neonates. To kill hair cells in vivo, we generated a novel mouse model that uses an inducible, hair cell-specific CreER allele to drive expression of diphtheria toxin fragment A (DTA). In newborns, induction of DTA expression killed hair cells and resulted in significant, mitotic hair cell replacement in vivo, which occurred days after the normal cessation of developmental mitoses that produce hair cells. DTA expression induced in 5-d-old mice also caused hair cell loss, but no longer evoked mitotic hair cell replacement. These findings show that regeneration limits arise in vivo during the postnatal period when the mammalian balance epithelium's supporting cells differentiate unique cytological characteristics and lose plasticity, and they support the notion that the differentiation of those cells may directly inhibit regeneration or eliminate an essential, but as yet unidentified pool of stem cells.

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

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

Understanding the evolution and development of neurosensory transcription factors of the ear to enhance therapeutic translation

Reconstructing a functional organ of Corti is the ultimate target towards curing hearing loss. Despite the impressive technical gains made over the last few years, many complications remain ahead for the two main restoration avenues: in vitro transformation of pluripotent cells into hair cell-like cells and adenovirus-mediated gene therapy. Most notably, both approaches require a more complete understanding of the molecular networks that ensure specific cell types form in the correct places to allow proper function of the restored organ of Corti. Important to this understanding are the basic helix-loop-helix (bHLH) transcription factors (TFs) that are highly diverse and serve to increase functional complexity but their evolutionary implementation in the inner ear neurosensory development is less conspicuous. To this end, we review the evolutionary and developmentally dynamic interactions of the three bHLH TFs that have been identified as the main players in neurosensory evolution and development, Neurog1, Neurod1 and Atoh1. These three TFs belong to the neurogenin/atonal family and evolved from a molecular precursor that likely regulated single sensory cell development in the ectoderm of metazoan ancestors but are now also expressed in other parts of the body, including the brain. They interact extensively via intracellular and intercellular cross-regulation to establish the two main neurosensory cell types of the ear, the hair cells and sensory neurons. Furthermore, the level and duration of their expression affect the specification of hair cell subtypes (inner hair cells vs. outer hair cells). We propose that appropriate manipulation of these TFs through their characterized binding sites may offer a solution by itself, or in conjunction with the two other approaches currently pursued by others, to restore the organ of Corti.

Twists in views on RB functions in cellular signaling, metabolism and stem cells

One-quarter of a century ago, identification of the human retinoblastoma gene (RB) loci proved Knudson's 'two-hit theory' that tumor suppressor genes exist. Since then, numerous works delineated crucial roles for the RB protein (pRB)-E2F transcription factor complex in G1-S phase transition. In addition, discovering the relationship between pRB and tissue-specific transcription factors enabled a better understanding of how cell cycle exit and terminal differentiation are coupled. Recent works provoked many exciting twists in views on pRB functions during cancer initiation and progression beyond its previously well-appreciated roles. Various mitogenic and cytostatic cellular signals appeared to modulate pRB functions and thus affect a wide variety of effector molecules. In addition, genetic studies in mice as well as other creatures incessantly force us to revise our views on pRB functions. This review will focus particularly on the roles of pRB in regulating intracellular signaling, cell metabolism, chromatin function, stem cells and cancer stem cells.

Stem cell therapy for the inner ear: recent advances and future directions

In vertebrates, perception of sound, motion, and balance is mediated through mechanosensory hair cells located within the inner ear. In mammals, hair cells are only generated during a short period of embryonic development. As a result, loss of hair cells as a consequence of injury, disease, or genetic mutation, leads to permanent sensory deficits. At present, cochlear implantation is the only option for profound hearing loss. However, outcomes are still variable and even the best implant cannot provide the acuity of a biological ear. The recent emergence of stem cell technology has the potential to open new approaches for hair cell regeneration. The goal of this review is to summarize the current state of inner ear stem cell research from a viewpoint of its clinical application for inner ear disorders to illustrate how complementary studies have the potential to promote and refine stem cell therapies for inner ear diseases. The review initially discusses our current understanding of the genetic pathways that regulate hair cell formation from inner ear progenitors during normal development. Subsequent sections discuss the possible use of endogenous inner ear stem cells to induce repair as well as the initial studies aimed at transplanting stem cells into the ear.