Proliferation of Functional Hair Cells in Vivo in the Absence of the Retinoblastoma Protein

Cell cycle perturbation uncouples mitotic progression and invasive behavior in a post-mitotic cell

The acquisition of the post-mitotic state is crucial for the execution of many terminally differentiated cell behaviors during organismal development. However, the mechanisms that maintain the post-mitotic state in this context remain poorly understood. To gain insight into these mechanisms, we used the genetically and visually accessible model of C. elegans anchor cell (AC) invasion into the vulval epithelium. The AC is a terminally differentiated uterine cell that normally exits the cell cycle and enters a post-mitotic state before initiating contact between the uterus and vulva through a cell invasion event. Here, we set out to identify the set of negative cell cycle regulators that maintain the AC in this post-mitotic, invasive state. Our findings revealed a critical role for CKI-1 (p21CIP1/p27KIP1) in redundantly maintaining the post-mitotic state of the AC, as loss of CKI-1 in combination with other negative cell cycle regulators-including CKI-2 (p21CIP1/p27KIP1), LIN-35 (pRb/p107/p130), FZR-1 (Cdh1/Hct1), and LIN-23 (β-TrCP)-resulted in proliferating ACs. Remarkably, time-lapse imaging revealed that these ACs retain their ability to invade. Upon examination of a node in the gene regulatory network controlling AC invasion, we determined that proliferating, invasive ACs do so by maintaining aspects of pro-invasive gene expression. We therefore report that the requirement for a post-mitotic state for invasive cell behavior can be bypassed following direct cell cycle perturbation.

EBF1 Limits the Numbers of Cochlear Hair and Supporting Cells and Forms the Scala Tympani and Spiral Limbus during Inner Ear Development

Early B-cell factor 1 (EBF1) is a basic helix-loop-helix transcription factor essential for the differentiation of various tissues. Our single-cell RNA sequencing data suggest that Ebf1 is expressed in the sensory epithelium of the mouse inner ear. Here, we found that the murine Ebf1 gene and its protein are expressed in the prosensory domain of the inner ear, medial region of the cochlear duct floor, otic mesenchyme, and cochleovestibular ganglion. Ebf1 deletion in mice results in incomplete formation of the spiral limbus and scala tympani, increased number of cells in the organ of Corti and Kölliker's organ, and aberrant course of the spiral ganglion axons. Ebf1 deletion in the mouse cochlear epithelia caused the proliferation of SOX2-positive cochlear cells at E13.5, indicating that EBF1 suppresses the proliferation of the prosensory domain and cells of Kölliker's organ to facilitate the development of appropriate numbers of hair and supporting cells. Furthermore, mice with deletion of cochlear epithelium-specific Ebf1 showed poor postnatal hearing function. Our results suggest that Ebf1 is essential for normal auditory function in mammals.

Cell cycle perturbation uncouples mitotic progression and invasive behavior in a post-mitotic cell

The acquisition of the post-mitotic state is crucial for the execution of many terminally differentiated cell behaviors during organismal development. However, the mechanisms that maintain the post-mitotic state in this context remain poorly understood. To gain insight into these mechanisms, we used the genetically and visually accessible model of C. elegans anchor cell (AC) invasion into the vulval epithelium. The AC is a terminally differentiated uterine cell that normally exits the cell cycle and enters a post-mitotic state, initiating contact between the uterus and vulva through a cell invasion event. Here, we set out to identify the set of negative cell cycle regulators that maintain the AC in this post-mitotic, invasive state. Our findings revealed a critical role for CKI-1 (p21CIP1/p27KIP1) in redundantly maintaining the post-mitotic state of the AC, as loss of CKI-1 in combination with other negative cell cycle regulators-including CKI-2 (p21CIP1/p27KIP1), LIN-35 (pRb/p107/p130), FZR-1 (Cdh1/Hct1), and LIN-23 (β-TrCP)-resulted in proliferating ACs. Remarkably, time-lapse imaging revealed that these ACs retain their ability to invade. Upon examination of a node in the gene regulatory network controlling AC invasion, we determined that proliferating, invasive ACs do so by maintaining aspects of pro-invasive gene expression. We therefore report that the requirement for a post-mitotic state for invasive cell behavior can be bypassed following direct cell cycle perturbation.

Reprogramming by drug-like molecules leads to regeneration of cochlear hair cell-like cells in adult mice

Strategies to overcome irreversible cochlear hair cell (HC) damage and loss in mammals are of vital importance to hearing recovery in patients with permanent hearing loss. In mature mammalian cochlea, co-activation of Myc and Notch1 reprograms supporting cells (SC) and promotes HC regeneration. Understanding of the underlying mechanisms may aid the development of a clinically relevant approach to achieve HC regeneration in the nontransgenic mature cochlea. By single-cell RNAseq, we show that MYC/NICD "rejuvenates" the adult mouse cochlea by activating multiple pathways including Wnt and cyclase activator of cyclic AMP (cAMP), whose blockade suppresses HC-like cell regeneration despite Myc/Notch activation. We screened and identified a combination (the cocktail) of drug-like molecules composing of small molecules and small interfering RNAs to activate the pathways of Myc, Notch1, Wnt and cAMP. We show that the cocktail effectively replaces Myc and Notch1 transgenes and reprograms fully mature wild-type (WT) SCs for HC-like cells regeneration in vitro. Finally, we demonstrate the cocktail is capable of reprogramming adult cochlea for HC-like cells regeneration in WT mice with HC loss in vivo. Our study identifies a strategy by a clinically relevant approach to reprogram mature inner ear for HC-like cells regeneration, laying the foundation for hearing restoration by HC regeneration.

Novel Nanoencapsulation Technology and its Potential Role in Bile Acid-Based Targeted Gene Delivery to the Inner Ear

Hearing loss impacts a large proportion of the global population. Damage to the inner ear, in particular the sensitive hair cells, can impact individuals for the rest of their lives. There are very limited options for interventions after damage to these cells has occurred. Targeted gene delivery may provide an effective means to trigger appropriate differentiation of progenitor cells for effective replacement of these sensitive hair cells. There are several hurdles that need to be overcome to effectively deliver these genes. Nanoencapsulation technology has previously been used for the delivery of pharmaceuticals, proteins and nucleic acids, and may provide an effective means of delivering genes to trigger appropriate differentiation. This review investigates the background of hearing loss, current advancements and pitfalls of gene delivery, and how nanoencapsulation may be useful.

Hearing loss drug discovery and medicinal chemistry: Current status, challenges, and opportunities

Hearing loss is a severe high unmet need condition affecting more than 1.5 billion people globally. There are no licensed medicines for the prevention, treatment or restoration of hearing. Prosthetic devices, such as hearing aids and cochlear implants, do not restore natural hearing and users struggle with speech in the presence of background noise. Hearing loss drug discovery is immature, and small molecule approaches include repurposing existing drugs, combination therapeutics, late-stage discovery optimisation of known chemotypes for identified molecular targets of interest, phenotypic tissue screening and high-throughput cell-based screening. Hearing loss drug discovery requires the integration of specialist therapeutic area biology and otology clinical expertise. Small molecule drug discovery projects in the global clinical portfolio for hearing loss are here collated and reviewed. An overview is provided of human hearing, inner ear anatomy, inner ear delivery, types of hearing loss and hearing measurement. Small molecule experimental drugs in clinical development for hearing loss are reviewed, including their underpinning biology, discovery strategy and activities, medicinal chemistry, calculated physicochemical properties, pharmacokinetics and clinical trial status. SwissADME BOILED-Egg permeability modelling is applied to the molecules reviewed, and these results are considered. Non-small molecule hearing loss assets in clinical development are briefly noted in this review. Future opportunities in hearing loss drug discovery for human genomics and targeted protein degradation are highlighted.

Supporting Cells and Their Potential Roles in Cisplatin-Induced Ototoxicity

Cisplatin is a known ototoxic chemotherapy drug, causing irreversible hearing loss. Evidence has shown that cisplatin causes inner ear damage as a result of adduct formation, a proinflammatory environment and the generation of reactive oxygen species within the inner ear. The main cochlear targets for cisplatin are commonly known to be the outer hair cells, the stria vascularis and the spiral ganglion neurons. Further evidence has shown that certain transporters can mediate cisplatin influx into the inner ear cells including organic cation transporter 2 (OCT2) and the copper transporter Ctr1. However, the expression profiles for these transporters within inner ear cells are not consistent in the literature, and expression of OCT2 and Ctr1 has also been observed in supporting cells. Organ of Corti supporting cells are essential for hair cell activity and survival. Special interest has been devoted to gap junction expression by these cells as certain mutations have been linked to hearing loss. Interestingly, cisplatin appears to affect connexin expression in the inner ear. While investigations regarding cisplatin-induced hearing loss have been focused mainly on the known targets previously mentioned, the role of supporting cells for cisplatin-induced ototoxicity has been overlooked. In this mini review, we discuss the implications of supporting cells expressing OCT2 and Ctr1 as well as the potential role of gap junctions in cisplatin-induced cytotoxicity.

Application of New Materials in Auditory Disease Treatment

Auditory diseases are disabling public health problems that afflict a significant number of people worldwide, and they remain largely incurable until now. Driven by continuous innovation in the fields of chemistry, physics, and materials science, novel materials that can be applied to hearing diseases are constantly emerging. In contrast to conventional materials, new materials are easily accessible, inexpensive, non-invasive, with better acoustic therapy effects and weaker immune rejection after implantation. When new materials are used to treat auditory diseases, the wound healing, infection prevention, disease recurrence, hair cell regeneration, functional recovery, and other aspects have been significantly improved. Despite these advances, clinical success has been limited, largely due to issues regarding a lack of effectiveness and safety. With ever-developing scientific research, more novel materials will be facilitated into clinical use in the future.

Insights into the molecular mechanisms regulating mammalian hair cell regeneration

PURPOSE OF REVIEW

To give an overview of recent advances in mammalian auditory hair cell regeneration.

RECENT FINDINGS

Supporting cells act as progenitors to regenerate hair cells in the prehearing mammalian cochlea but not in the mature cochlea. To overcome this developmental obstacle, manipulation of multiple genes and intracellular pathways has been investigated, which has obtained promising data. This review focuses on recent advances in auditory hair cell regeneration, including synergic gene regulation associated with Atoh1 and Notch signaling, epigenetics, and functional recovery of regenerated hair cells. Co-manipulation of genes critical for hair cell development and cell cycle re-entry, including Atoh1, Isl1, Pou4f3, Gata3, Gfi1, P27kip1, RB, Myc, and Notch-signaling genes, has generated hair cell-like cells in the adult cochlea both in vitro and in vivo. The epigenetic mechanism has been studied in hair cell development and regeneration. Regeneration of hair cell function has a very limited progress, which lacks in-vitro and in-vivo electrophysiology data.

SUMMARY

Regeneration of adult auditory hair cells remains a major challenge. Manipulation of multiple genes and pathways together with epigenetic regulation might potentially regenerate functional hair cells in the adult mammalian cochlea.

Selective ablation of inner hair cells and subsequent in-situ hair cell regeneration in the neonatal mouse cochlea

Loss of hair cells (HCs) accounts for most sensorineural hearing loss, and regeneration of cochlear HCs is considered as the ultimate strategy for restoring hearing. Several lines of evidence have shown that Lgr5+ progenitor cells can spontaneously regenerate new HCs after HC loss at the neonatal stage, and most of which are immature. IHCs are resistant to ototoxic drugs and noise and cannot be ablated efficiently in order to precisely investigate IHC regeneration in existing hearing injury models, and thus we generated a new transgenic mouse model by inserting diphtheria toxin receptor (DTR) under the control of the Vglut3 promoter. In this model, IHCs were selectively ablated in a dose-dependent manner after the injection of diphtheria toxin (DT) at the neonatal stage, while OHCs remained intact with normal hair bundle structures until adulthood. With this IHC-specific injury model, we observed HC regeneration from Lgr5+ progenitors after IHC ablation at the neonatal stage. Some of the newly generated HCs replaced the lost IHCs in-situ and re-build the structure of the organ of Corti through the asymmetrical mitosis of progenitor cells. While, the majority of the regenerated HCs did not survive until adulthood, and the loss of spiral ganglion neurons was observed after the IHC ablation, which led to profound hearing loss after DT injection in Vglut3^DTR+ mice at the neonatal stage. The model presented here shows promise for investigating the mechanisms behind IHC loss and subsequent regeneration.

Defined Mathematical Relationships Among Cancer Cells Suggest Modular Growth in Tumor Progression and Highlight Developmental Features Consistent With a Para-Embryonic Nature of Cancer

Several similarities between the embryo development and the cancer process suggest the para-embryonic nature of tumors. Starting from an initial cancer stem cell (i-CSC) as a para-embryonic stem cell (p-ESC), a hierarchic sequence of CSCs (CSC1s, CSC2s, CSC3s) and non-CSCs [cancer progenitor cells (CPCs), cancer differentiated cells (CDCs)] would be generated, mimicking an ectopic rudimentary ontogenesis. Such a proposed heterogeneous cell hierarchy within the tumor structure would suggest a tumor growth model consistent with experimental data reported for mammary tumors. By tabulating the theoretical data according to this model, it is possible to identify defined mathematical relationships between cancer cells (CSCs and non-CSCs) that are surprisingly similar to experimental data. Moreover, starting from this model, it is possible to speculate that, during progression, tumor growth would occur in a modular way that recalls the propagation of tumor spheres in vitro. All these considerations favor a comparison among normal blastocysts (as in vitro embryos), initial avascular tumors (as in vivo abnormal blastocysts) and tumor spheres (as in vitro abnormal blastocysts). In conclusion, this work provides further support for the para-embryonic nature of the cancer process, as recently theorized.

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.

Renewed proliferation in adult mouse cochlea and regeneration of hair cells

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

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.

Comparing Sensory Organs to Define the Path for Hair Cell Regeneration

Deafness or hearing deficits are debilitating conditions. They are often caused by loss of sensory hair cells or defects in their function. In contrast to mammals, nonmammalian vertebrates robustly regenerate hair cells after injury. Studying the molecular and cellular basis of nonmammalian vertebrate hair cell regeneration provides valuable insights into developing cures for human deafness. In this review, we discuss the current literature on hair cell regeneration in the context of other models for sensory cell regeneration, such as the retina and the olfactory epithelium. This comparison reveals commonalities with, as well as differences between, the different regenerating systems, which begin to define a cellular and molecular blueprint of regeneration. In addition, we propose how new technical advances can address outstanding questions in the field.

Changes in chromatin accessibility ensure robust cell cycle exit in terminally differentiated cells

During terminal differentiation, most cells exit the cell cycle and enter into a prolonged or permanent G0 in which they are refractory to mitogenic signals. Entry into G0 is usually initiated through the repression of cell cycle gene expression by formation of a transcriptional repressor complex called dimerization partner (DP), retinoblastoma (RB)-like, E2F and MuvB (DREAM). However, when DREAM repressive function is compromised during terminal differentiation, additional unknown mechanisms act to stably repress cycling and ensure robust cell cycle exit. Here, we provide evidence that developmentally programmed, temporal changes in chromatin accessibility at a small subset of critical cell cycle genes act to enforce cell cycle exit during terminal differentiation in the Drosophila melanogaster wing. We show that during terminal differentiation, chromatin closes at a set of pupal wing enhancers for the key rate-limiting cell cycle regulators Cyclin E (cycE), E2F transcription factor 1 (e2f1), and string (stg). This closing coincides with wing cells entering a robust postmitotic state that is strongly refractory to cell cycle reactivation, and the regions that close contain known binding sites for effectors of mitogenic signaling pathways such as Yorkie and Notch. When cell cycle exit is genetically disrupted, chromatin accessibility at cell cycle genes remains unaffected, and the closing of distal enhancers at cycE, e2f1, and stg proceeds independent of the cell cycling status. Instead, disruption of cell cycle exit leads to changes in accessibility and expression of a subset of hormone-induced transcription factors involved in the progression of terminal differentiation. Our results uncover a mechanism that acts as a cell cycle–independent timer to limit the response to mitogenic signaling and aberrant cycling in terminally differentiating tissues. In addition, we provide a new molecular description of the cross talk between cell cycle exit and terminal differentiation during metamorphosis.

The longer a cell remains in G0, the more refractory it becomes to re-entering the cell cycle. This study shows that in terminally differentiated cells in vivo, regulatory elements at genes encoding just three key cell cycle regulators (cycE, e2f1 and stg) become inaccessible, limiting their aberrant activation and maintaining a prolonged, robust G0.

[Strategies for a regenerative therapy of hearing loss. German version]

Despite impressive technical progress in the field of conventional hearing aids and implantable hearing systems, the hopes for the treatment of inner ear diseases such as hearing loss and tinnitus have become increasingly directed toward regenerative therapeutic approaches. This review discusses the currently most promising strategies for hair cell regeneration in the inner ear to treat hearing loss, including stem cell-based, gene transfer-based, and pharmacological interventions. Furthermore, previous milestones and ground-breaking work in this scientific field are identified. After many years of basic research, the first clinical trials with a regenerative therapeutic approach for hearing-impaired patients were recently initiated. Although there is still a long and bumpy road ahead until a true breakthrough is achieved, it seems more realistic than ever that regenerative therapies for the inner ear will find their way into clinical practice.

A novel thyroid hormone receptor isoform, TRβ2-46, promotes SKP2 expression and retinoblastoma cell proliferation

Retinoblastoma is a childhood retinal tumor that develops from cone photoreceptor precursors in response to inactivating RB1 mutations and loss of functional RB protein. The cone precursor's response to RB loss involves cell type-specific signaling circuitry that helps to drive tumorigenesis. One component of the cone precursor circuitry, the thyroid hormone receptor β2 (TRβ2), enables the aberrant proliferation of diverse RB-deficient cells in part by opposing the down-regulation of S-phase kinase-associated protein 2 (SKP2) by the more widely expressed and tumor-suppressive TRβ1. However, it is unclear how TRβ2 opposes TRβ1 to enable SKP2 expression and cell proliferation. Here, we show that in human retinoblastoma cells TRβ2 mRNA encodes two TRβ2 protein isoforms: a predominantly cytoplasmic 54-kDa protein (TRβ2-54) corresponding to the well-characterized full-length murine Trβ2 and an N-terminally truncated and exclusively cytoplasmic 46-kDa protein (TRβ2-46) that starts at Met-79. Whereas TRβ2 knockdown decreased SKP2 expression and impaired retinoblastoma cell cycle progression, re-expression of TRβ2-46 but not TRβ2-54 stabilized SKP2 and restored proliferation to an extent similar to that of ectopic SKP2 restoration. We conclude that TRβ2-46 is an oncogenic thyroid hormone receptor isoform that promotes SKP2 expression and SKP2-dependent retinoblastoma cell proliferation.

p27Kip1 down-regulation as achieved by two clinically feasible means did not induce proliferation of supporting cells in the rat neonatal cochlea in vivo

In mammals, the cochlear sensory epithelium becomes quiescent early during development. After the first postnatal week, there is no cell replacement or proliferation, and severe damage leads to permanent deafness. Supporting cells' trans-differentiation has been suggested as a way to regenerate cochlear hair cells after damage. However, they are also needed for proper functionality. Cdkn1b (p27^Kip1) participates in the cochlear terminal mitosis state achieved during development. Its expression is maintained in adult supporting cells and its postnatal deletion has induced cochlear proliferation in vitro and in vivo. Therefore, its manipulation has been proposed as a feasible way to induce proliferation of supporting cells after birth. Nevertheless, the literature is scarce regarding feasible methods to directly decrease p27^Kip1 in the clinical domain. The effects of p27^Kip1 knockdown using viral vectors are not completely elucidated and no pharmacological approaches to decrease p27^Kip1 in the cochlea have been tested in vivo before. This study explores the ability of p27^Kip1 messenger knockdown and pharmacological transcriptional inhibition to induce proliferation of supporting cells in the P0 neonatal rat cochlea in vivo. Respectively, lentiviral vectors transducing shRNA against p27^Kip1 were administered into the scala media or Alsterpaullone 2-Cyanoethyl into the round window niche. Cell markers and gene expression were assessed through immunostaining and qRT-PCR. Despite both methods significantly decreasing p27^Kip1 expression in vivo, signs of toxicity in the organ of Corti were not found; however, relevant proliferation was not found either. Finally, cochlear damage was added to increase the response in vitro, achieving only a mild to moderate proliferation induction. We conclude that our approaches were not able to stimulate the recall of supporting cell proliferation despite significantly decreased p27^Kip1 levels in vivo. Considering the evaluation of the cochlea at a very responsive stage, we propose that the level of isolated modification of p27^Kip1 expression in living mammals achievable through these approaches is insufficient to induce proliferation of supporting cells. Future proliferation induction experiments in the cochlea should study other methods and genes.

Gene Delivery into the Inner Ear and Its Clinical Implications for Hearing and Balance

The inner ear contains many types of cell, including sensory hair cells and neurons. If these cells are damaged, they do not regenerate. Inner ear disorders have various etiologies. Some are related to aging or are idiopathic, as in sudden deafness. Others occur due to acoustic trauma, exposure to ototoxic drugs, viral infections, immune responses, or endolymphatic hydrops (Meniere's disease). For these disorders, inner ear regeneration therapy is expected to be a feasible alternative to cochlear implants for hearing recovery. Recently, the mechanisms underlying inner ear regeneration have been gradually clarified. Inner ear cell progenitors or stem cells have been identified. Factors necessary for regeneration have also been elucidated from the mechanism of hair cell generation. Inducing differentiation of endogenous stem cells or inner ear stem cell transplantation is expected. In this paper, we discuss recent approaches to hair cell proliferation and differentiation for inner ear regeneration. We discuss the future road map for clinical application. The therapies mentioned above require topical administration of transgenes or drug onto progenitors of sensory cells. Developing efficient and safe modes of administration is clinically important. In this regard, we also discuss our development of an inner ear endoscope to facilitate topical administration.

Age-dependent dormant resident progenitors are stimulated by injury to regenerate Purkinje neurons

Outside of the neurogenic niches of the brain, postmitotic neurons have not been found to undergo efficient regeneration. We demonstrate that mouse Purkinje cells (PCs), which are born at midgestation and are crucial for development and function of cerebellar circuits, are rapidly and fully regenerated following their ablation at birth. New PCs are produced from immature FOXP2+ Purkinje cell precursors (iPCs) that are able to enter the cell cycle and support normal cerebellum development. The number of iPCs and their regenerative capacity, however, diminish soon after birth and consequently PCs are poorly replenished when ablated at postnatal day five. Nevertheless, the PC-depleted cerebella reach a normal size by increasing cell size, but scaling of neuron types is disrupted and cerebellar function is impaired. Our findings provide a new paradigm in the field of neuron regeneration by identifying a population of immature neurons that buffers against perinatal brain injury in a stage-dependent process.

Potential of Gene and Cell Therapy for Inner Ear Hair Cells

Sensorineural hearing loss is caused by the loss of sensory hair cells (HCs) or a damaged afferent nerve pathway to the auditory cortex. The most common option for the treatment of sensorineural hearing loss is hearing rehabilitation using hearing devices. Various kinds of hearing devices are available but, despite recent advancements, their perceived sound quality does not mimic that of the "naïve" cochlea. Damage to crucial cochlear structures is mostly irreversible and results in permanent hearing loss. Cochlear HC regeneration has long been an important goal in the field of hearing research. However, it remains challenging because, thus far, no medical treatment has successfully regenerated cochlear HCs. Recent advances in genetic modulation and developmental techniques have led to novel approaches to generating HCs or protecting against HC loss, to preserve hearing. In this review, we present and review the current status of two different approaches to restoring or protecting hearing, gene therapy, including the newly introduced CRISPR/Cas9 genome editing, and stem cell therapy, and suggest the future direction.

microRNA‑183 is involved in the differentiation and regeneration of Notch signaling‑prohibited hair cells from mouse cochlea

Auditory hair cell regeneration following injury is critical to hearing restoration. The Notch signaling pathway participates in the regulation of inner ear development and cell differentiation. Recent evidence suggests that microRNA (miR)-183 has a similar role in the inner ear. However, it is unclear how Notch signaling functions in hair cell regeneration in mammals and if there is cross-talk between Notch signaling and miR-183. The present study used a gentamicin-induced cochlear injury mouse model. Gentamicin-induced damage of the hair cells activated the Notch signaling pathway and downregulated miR-183 expression. Notch signaling inhibition by the γ-secretase inhibitor, 24-diamino-5-phenylthiazole (DAPT), attenuated gentamicin-induced hair cell loss and reversed the downregulation of miR-183 expression. Further investigation revealed that the novel hair cells produced, induced by DAPT, were derived from transdifferentiated supporting cells. Additionally, myosin VI-positive hair cell numbers were increased by Notch signaling inhibition in in vitro experiments with cultured neonatal mouse inner ear precursor cells. This effect was reversed by miR-183 inhibition. These findings indicate that the Notch signaling pathway served a repressing role during the regeneration of hair cells. Inhibiting this signal improved hair cell regeneration in the gentamicin-damaged cochlear model. miR-183 was demonstrated to be involved in hair cell differentiation and regeneration, and was required for the differentiation of the Notch-inhibited hair cells.

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.

DNER modulates the length, polarity and synaptogenesis of spiral ganglion neurons via the Notch signaling pathway

The Delta/Notch-like epidermal growth factor-related receptor (DNER) serves an important role in the developing central nervous system. However, the actions of DNER in the development of the spiral ganglion in the inner ear have yet to be elucidated. Wild-type C57BL/6 mice were housed and time-mated for use in the present study. Primary neuronal cultures were prepared using spiral ganglion progenitors isolated from the modiolus of postnatal day 1 (P1) mice. DNER recombinant lentiviral vectors were constructed and transfected into the cultured primary neurons. The relative proportion of differentiated neurons and the length of their neurites were evaluated using microscopy. The results of the present study demonstrated that DNER was expressed in spiral ganglion neurons (SGNs) that exhibited significant polarity in the early differentiation stages; DNER expression gradually decreased until the polarity was lost on week 35. The in vitro expression of DNER was revealed to be similar to that in vivo. When DNER expression was silenced using RNA interference, the polarity of the differentiated neurons was altered and they exhibited significantly reduced dendritic length. In addition, the proportion of bipolar neurons was decreased compared with the control group. Furthermore, the expression of α-synuclein and the GluR2/3 subunits of the α-amin-o-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptor were also reduced in cultured neurons in which DNER was silenced. Notch1 was co-expressed with DNER in SGNs isolated from P1 mice. The indirect Notch inhibitor N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester also affected the polarity and the formation of protrusions, and reduced the expression of DNER and glial fibrillary acidic protein in SGNs. In conclusion, the present study demonstrated that DNER was expressed in SGNs and appeared to be involved in the mechanisms underlying neuronal polarity and neuritogenesis, via a Notch-dependent signaling pathway.

SKP2 Activation by Thyroid Hormone Receptor β2 Bypasses Rb-Dependent Proliferation in Rb-Deficient Cells

Germline RB1 mutations strongly predispose humans to cone precursor-derived retinoblastomas and strongly predispose mice to pituitary tumors, yet shared cell type-specific circuitry that sensitizes these different cell types to the loss of RB1 has not been defined. Here we show that the cell type-restricted thyroid hormone receptor isoform TRβ2 sensitizes to RB1 loss in both settings by antagonizing the widely expressed and tumor-suppressive TRβ1. TRβ2 promoted expression of the E3 ubiquitin ligase SKP2, a critical factor for RB1-mutant tumors, by enabling EMI1/FBXO5-dependent inhibition of SKP2 degradation. In RB1 wild-type neuroblastoma cells, endogenous Rb or ectopic TRβ2 was required to sustain SKP2 expression as well as cell viability and proliferation. These results suggest that in certain contexts, Rb loss enables TRβ1-dependent suppression of SKP2 as a safeguard against RB1-deficient tumorigenesis. TRβ2 counteracts TRβ1, thus disrupting this safeguard and promoting development of RB1-deficient malignancies. Cancer Res; 77(24); 6838-50. ©2017 AACR.

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.

Recent advances in cochlear hair cell regeneration-A promising opportunity for the treatment of age-related hearing loss

The objective of this paper is to review current information regarding the treatment of age-related hearing loss by using cochlear hair cell regeneration. Recent advances in the regeneration of the inner ear, including the usefulness of stem cells, are also presented. Based on the current literature, cochlear cell regeneration may well be possible in the short term and cochlear gene therapy may also be useful for the treatment of hearing loss associated with ageing. The present review provide further insight into the pathogenesis of Inner Ear senescence and aged-related hearing loss and facilitate the development of therapeutic strategies to repair hair cells damaged by ageing. More research will be needed in order to translate them into an effective treatment for deafness linked to cochlear senescence in humans.

Genomic instability and proliferation/survival pathways in RB1-deficient malignancies

Genomic instability (GIN) is a hallmark of most cancer cells. However, compared to most human cancer cell types, the retinoblastoma tumor cells show a relatively stable genome. The fundamental basis of this genomic stability has yet to be elucidated, and the role of certain proteins involved in cell cycle regulation may be the key to the development of these specific genotypes. We examined whether thyroid hormone receptor beta 1 and 2 (TRβ1 and TRβ2), known to regulate tumorigenesis, and PTTG1, a mitotic checkpoint protein, play a role in maintaining genomic stability in retinoblastoma. In order to elucidate the role of these proteins in development of aneuploidy/polyploidy, an indicator of GIN, we first studied comparative GIN in retinoblastomas and multiple RB mutant cancer cell lines using single nucleotide polymorphism (SNP) analysis. We then utilized pLKO lentiviral vectors to selectively modify expression of the targeted cell cycle proteins and interpret their effect on downstream cell cycle proteins and their relative effects on the development of polyploidy in multiple tumor cell lines. The SNP analysis showed that retinoblastomas displayed relatively fewer genomic copy number changes as compared to other RB1-deficient cancer cell lines. Both TRβ1 and TRβ2 knockdown led to accumulation of E2F1 and PTTG1 and increased GIN as demonstrated by an increase in polyploidy. Downregulation of PTTG1 led to a relative decrease in GIN while upregulation of PTTG1 led to a relative increase in GIN. Knockdown of E2F1 led to a downstream decrease in PTTG1 expression. Rb-knockdown also upregulated E2F1 and PTTG1 leading to increased GIN. We showed that Rb is necessary for PTTG1 inhibition and genomic stability. A relatively stable genome in retinoblastoma tumor cells is maintained by TRβ1 and TRβ2-mediated PTTG1 inhibition, counteracting Rb-deficiency-related GIN. TRβ1, TRβ2 and Rb-KD all led to the downstream PTTG1 accumulation, apparently through an activation of E2F1 resulting in extensive genomic instability as seen in other Rb-deficient tumors.

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.

Adenovirus Vectors Target Several Cell Subtypes of Mammalian Inner Ear In Vivo

Mammalian inner ear harbors diverse cell types that are essential for hearing and balance. Adenovirus is one of the major vectors to deliver genes into the inner ear for functional studies and hair cell regeneration. To identify adenovirus vectors that target specific cell subtypes in the inner ear, we studied three adenovirus vectors, carrying a reporter gene encoding green fluorescent protein (GFP) from two vendors or with a genome editing gene Cre recombinase (Cre), by injection into postnatal days 0 (P0) and 4 (P4) mouse cochlea through scala media by cochleostomy in vivo. We found three adenovirus vectors transduced mouse inner ear cells with different specificities and expression levels, depending on the type of adenoviral vectors and the age of mice. The most frequently targeted region was the cochlear sensory epithelium, including auditory hair cells and supporting cells. Adenovirus with GFP transduced utricular supporting cells as well. This study shows that adenovirus vectors are capable of efficiently and specifically transducing different cell types in the mammalian inner ear and provides useful tools to study inner ear gene function and to evaluate gene therapy to treat hearing loss and vestibular dysfunction.

Cochlear hair cell regeneration after noise-induced hearing loss: Does regeneration follow development?

Noise-induced hearing loss (NIHL) affects a large number of military personnel and civilians. Regenerating inner-ear cochlear hair cells (HCs) is a promising strategy to restore hearing after NIHL. In this review, we first summarize recent transcriptome profile analysis of zebrafish lateral lines and chick utricles where spontaneous HC regeneration occurs after HC damage. We then discuss recent studies in other mammalian regenerative systems such as pancreas, heart and central nervous system. Both spontaneous and forced HC regeneration occurs in mammalian cochleae in vivo involving proliferation and direct lineage conversion. However, both processes are inefficient and incomplete, and decline with age. For direct lineage conversion in vivo in cochleae and in other systems, further improvement requires multiple factors, including transcription, epigenetic and trophic factors, with appropriate stoichiometry in appropriate architectural niche. Increasing evidence from other systems indicates that the molecular paths of direct lineage conversion may be different from those of normal developmental lineages. We therefore hypothesize that HC regeneration does not have to follow HC development and that epigenetic memory of supporting cells influences the HC regeneration, which may be a key to successful cochlear HC regeneration. Finally, we discuss recent efforts in viral gene therapy and drug discovery for HC regeneration. We hope that combination therapy targeting multiple factors and epigenetic signaling pathways will provide promising avenues for HC regeneration in humans with NIHL and other types of hearing loss.

Role of Wnt and Notch signaling in regulating hair cell regeneration in the cochlea

Sensory hair cells in the inner ear are responsible for sound recognition. Damage to hair cells in adult mammals causes permanent hearing impairment because these cells cannot regenerate. By contrast, newborn mammals possess limited regenerative capacity because of the active participation of various signaling pathways, including Wnt and Notch signaling. The Wnt and Notch pathways are highly sophisticated and conserved signaling pathways that control multiple cellular events necessary for the formation of sensory hair cells. Both signaling pathways allow resident supporting cells to regenerate hair cells in the neonatal cochlea. In this regard, Wnt and Notch signaling has gained increased research attention in hair cell regeneration. This review presents the current understanding of the Wnt and Notch signaling pathways in the auditory portion of the inner ear and discusses the possibilities of controlling these pathways with the hair cell fate determiner Atoh1 to regulate hair cell regeneration in the mammalian cochlea.

Myc and Fgf Are Required for Zebrafish Neuromast Hair Cell Regeneration

Unlike mammals, the non-mammalian vertebrate inner ear can regenerate the sensory cells, hair cells, either spontaneously or through induction after hair cell loss, leading to hearing recovery. The mechanisms underlying the regeneration are poorly understood. By microarray analysis on a chick model, we show that chick hair cell regeneration involves the activation of proliferation genes and downregulation of differentiation genes. Both MYC and FGF are activated in chick hair cell regeneration. Using a zebrafish lateral line neuromast hair cell regeneration model, we show that the specific inhibition of Myc or Fgf suppresses hair cell regeneration, demonstrating that both pathways are essential to the process. Rapid upregulation of Myc and delayed Fgf activation during regeneration suggest a role of Myc in proliferation and Fgf in differentiation. The dorsal-ventral pattern of fgfr1a in the neuromasts overlaps with the distribution of hair cell precursors. By laser ablation, we show that the fgfr1a-positive supporting cells are likely the hair cell precursors that directly give rise to new hair cells; whereas the anterior-posterior fgfr1a-negative supporting cells have heightened proliferation capacity, likely to serve as more primitive progenitor cells to replenish lost precursors after hair cell loss. Thus fgfr1a is likely to mark compartmentalized supporting cell subtypes with different capacities in renewal proliferation and hair cell regeneration. Manipulation of c-MYC and FGF pathways could be explored for mammalian hair cell regeneration.

Cortical neurons gradually attain a post-mitotic state

Once generated, neurons are thought to permanently exit the cell cycle and become irreversibly differentiated. However, neither the precise point at which this post-mitotic state is attained nor the extent of its irreversibility is clearly defined. Here we report that newly born neurons from the upper layers of the mouse cortex, despite initiating axon and dendrite elongation, continue to drive gene expression from the neural progenitor tubulin α1 promoter (Tα1p). These observations suggest an ambiguous post-mitotic neuronal state. Whole transcriptome analysis of sorted upper cortical neurons further revealed that neurons continue to express genes related to cell cycle progression long after mitotic exit until at least post-natal day 3 (P3). These genes are however down-regulated thereafter, associated with a concomitant up-regulation of tumor suppressors at P5. Interestingly, newly born neurons located in the cortical plate (CP) at embryonic day 18-19 (E18-E19) and P3 challenged with calcium influx are found in S/G2/M phases of the cell cycle, and still able to undergo division at E18-E19 but not at P3. At P5 however, calcium influx becomes neurotoxic and leads instead to neuronal loss. Our data delineate an unexpected flexibility of cell cycle control in early born neurons, and describe how neurons transit to a post-mitotic state.

Differential effects of Cdh23(753A) on auditory and vestibular functional aging in C57BL/6J mice

The C57BL/6J (B6) mouse strain carries a cadherin 23 mutation (Cdh23(753A), also known as Ahl), which affects inner ear structures and results in age-related hearing loss. The B6.CAST strain harbors the wild type Cdh23 gene, and hence, the influence of Ahl is absent. The purpose of the present study was to characterize the effect of age and gender on gravity receptor function in B6 and B6.CAST strains and to compare functional aging between auditory and vestibular modalities. Auditory sensitivity declined at significantly faster rates than gravity receptor sensitivity for both strains. Indeed, vestibular functional aging was minimal for both strains. The comparatively smaller loss of macular versus cochlear sensitivity in both the B6 and B6.CAST strains suggests that the contribution of Ahl to the aging of the vestibular system is minimal, and thus very different than its influence on aging of the auditory system. Alternatively, there exist unidentified genes or gene modifiers that serve to slow the degeneration of gravity receptor structures and maintain gravity receptor sensitivity into advanced age.

Regulation of cell polarity determinants by the Retinoblastoma tumor suppressor protein

In addition to their canonical roles in the cell cycle, RB family proteins regulate numerous developmental pathways, although the mechanisms remain obscure. We found that Drosophila Rbf1 associates with genes encoding components of the highly conserved apical-basal and planar cell polarity pathways, suggesting a possible regulatory role. Here, we show that depletion of Rbf1 in Drosophila tissues is indeed associated with polarity defects in the wing and eye. Key polarity genes aPKC, par6, vang, pk, and fmi are upregulated, and an aPKC mutation suppresses the Rbf1-induced phenotypes. RB control of cell polarity may be an evolutionarily conserved function, with important implications in cancer metastasis.

Where hearing starts: the development of the mammalian cochlea

The mammalian cochlea is a remarkable sensory organ, capable of perceiving sound over a range of 10(12) in pressure, and discriminating both infrasonic and ultrasonic frequencies in different species. The sensory hair cells of the mammalian cochlea are exquisitely sensitive, responding to atomic-level deflections at speeds on the order of tens of microseconds. The number and placement of hair cells are precisely determined during inner ear development, and a large number of developmental processes sculpt the shape, size and morphology of these cells along the length of the cochlear duct to make them optimally responsive to different sound frequencies. In this review, we briefly discuss the evolutionary origins of the mammalian cochlea, and then describe the successive developmental processes that lead to its induction, cell cycle exit, cellular patterning and the establishment of topologically distinct frequency responses along its length.

Coordinating cell proliferation and differentiation: Antagonism between cell cycle regulators and cell type-specific gene expression

Cell proliferation and differentiation show a remarkable inverse relationship. Precursor cells continue division before acquiring a fully differentiated state, while terminal differentiation usually coincides with proliferation arrest and permanent exit from the division cycle. Mechanistic insight in the temporal coordination between cell cycle exit and differentiation has come from studies of cells in culture and genetic animal models. As initially described for skeletal muscle differentiation, temporal coordination involves mutual antagonism between cyclin-dependent kinases that promote cell cycle entry and transcription factors that induce tissue-specific gene expression. Recent insights highlight the contribution of chromatin-regulating complexes that act in conjunction with the transcription factors and determine their activity. In particular SWI/SNF chromatin remodelers contribute to dual regulation of cell cycle and tissue-specific gene expression during terminal differentiation. We review the concerted regulation of the cell cycle and cell type-specific transcription, and discuss common mutations in human cancer that emphasize the clinical importance of proliferation versus differentiation control.

Cell cycle reactivation of cochlear progenitor cells in neonatal FUCCI mice by a GSK3 small molecule inhibitor

Due to the lack of regenerative capacity of the mammalian auditory epithelium, sensory hair cell loss results in permanent hearing deficit. Nevertheless, a population of tissue resident stem/progenitor cells has been recently described. Identification of methods to trigger their activity could lead to exploitation of their potential therapeutically. Here we validate the use of transgenic mice reporting cell cycle progression (FUCCI), and stemness (Lgr5-GFP), as a valuable tool to identify regulators of cell cycle re-entry of supporting cells within the auditory epithelium. The small molecule compound CHIR99021 was used to inhibit GSK3 activity. This led to a significant increase in the fraction of proliferating sphere-forming cells, labeled by the FUCCI markers and in the percentage of Lgr5-GFP + cells, as well as a selective increase in the fraction of S-G2-M cells in the Lgr5 + population. Using whole mount cultures of the organ of Corti we detected a statistically significant increment in the fraction of proliferating Sox2 supporting cells after CHIR99021 treatment, but only rarely appearance of novel MyoVIIa+/Edu + hair cells. In conclusion, these tools provide a robust mean to identify novel regulators of auditory organ regeneration and to clarify the contribution of stem cell activity.

The goya mouse mutant reveals distinct newly identified roles for MAP3K1 in the development and survival of cochlear sensory hair cells

Mitogen-activated protein kinase, MAP3K1, plays an important role in a number of cellular processes, including epithelial migration during eye organogenesis. In addition, studies in keratinocytes indicate that MAP3K1 signalling through JNK is important for actin stress fibre formation and cell migration. However, MAP3K1 can also act independently of JNK in the regulation of cell proliferation and apoptosis. We have identified a mouse mutant, goya, which exhibits the eyes-open-at-birth and microphthalmia phenotypes. In addition, these mice also have hearing loss. The goya mice carry a splice site mutation in the Map3k1 gene. We show that goya and kinase-deficient Map3k1 homozygotes initially develop supernumerary cochlear outer hair cells (OHCs) that subsequently degenerate, and a progressive profound hearing loss is observed by 9 weeks of age. Heterozygote mice also develop supernumerary OHCs, but no cellular degeneration or hearing loss is observed. MAP3K1 is expressed in a number of inner-ear cell types, including outer and inner hair cells, stria vascularis and spiral ganglion. Investigation of targets downstream of MAP3K1 identified an increase in p38 phosphorylation (Thr180/Tyr182) in multiple cochlear tissues. We also show that the extra OHCs do not arise from aberrant control of proliferation via p27KIP1. The identification of the goya mutant reveals a signalling molecule involved with hair-cell development and survival. Mammalian hair cells do not have the ability to regenerate after damage, which can lead to irreversible sensorineural hearing loss. Given the observed goya phenotype, and the many diverse cellular processes that MAP3K1 is known to act upon, further investigation of this model might help to elaborate upon the mechanisms underlying sensory hair cell specification, and pathways important for their survival. In addition, MAP3K1 is revealed as a new candidate gene for human sensorineural hearing loss.

Summary: The ENU-derived mouse mutant goya, reveals, for the first time, multiple roles of MAP3K1 in cochlear development and correct auditory function.

Sensory hair cell development and regeneration: similarities and differences

Sensory hair cells are mechanoreceptors of the auditory and vestibular systems and are crucial for hearing and balance. In adult mammals, auditory hair cells are unable to regenerate, and damage to these cells results in permanent hearing loss. By contrast, hair cells in the chick cochlea and the zebrafish lateral line are able to regenerate, prompting studies into the signaling pathways, morphogen gradients and transcription factors that regulate hair cell development and regeneration in various species. Here, we review these findings and discuss how various signaling pathways and factors function to modulate sensory hair cell development and regeneration. By comparing and contrasting development and regeneration, we also highlight the utility and limitations of using defined developmental cues to drive mammalian hair cell regeneration.

Neural-induced human mesenchymal stem cells promote cochlear cell regeneration in deaf Guinea pigs

Objectives

In mammals, cochlear hair cell loss is irreversible and may result in a permanent sensorineural hearing loss. Secondary to this hair cell loss, a progressive loss of spiral ganglion neurons (SGNs) is presented. In this study, we have investigated the effects of neural-induced human mesenchymal stem cells (NI-hMSCs) from human bone marrow on sensory neuronal regeneration from neomycin treated deafened guinea pig cochleae.

Methods

HMSCs were isolated from the bone marrow which was obtained from the mastoid process during mastoidectomy for ear surgery. Following neural induction with basic fibroblast growth factor and forskolin, we studied the several neural marker and performed electrophysiological analysis. NI-hMSCs were transplanted into the neomycin treated deafened guinea pig cochlea. Engraftment of NI-hMSCs was evaluated immunohistologically at 8 weeks after transplantation.

Results

Following neural differentiation, hMSCs expressed high levels of neural markers, ionic channel markers, which are important in neural function, and tetrodotoxin-sensitive voltage-dependent sodium currents. After transplantation into the scala tympani of damaged cochlea, NI-hMSCs-injected animals exhibited a significant increase in the number of SGNs compared to Hanks balanced salt solution-injected animals. Transplanted NI-hMSCs were found within the perilymphatic space, the organ of Corti, along the cochlear nerve fibers, and in the spiral ganglion. Furthermore, the grafted NI-hMSCs migrated into the spiral ganglion where they expressed the neuron-specific marker, NeuN.

Conclusion

The results show the potential of NI-hMSCs to give rise to replace the lost cochlear cells in hearing loss mammals.