siRNA targeting Hes5 augments hair cell regeneration in aminoglycoside-damaged mouse utricle

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

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

Gene therapy: an emerging therapy for hair cells regeneration in the cochlea

Sensorineural hearing loss is typically caused by damage to the cochlear hair cells (HCs) due to external stimuli or because of one's genetic factors and the inability to convert sound mechanical energy into nerve impulses. Adult mammalian cochlear HCs cannot regenerate spontaneously; therefore, this type of deafness is usually considered irreversible. Studies on the developmental mechanisms of HC differentiation have revealed that nonsensory cells in the cochlea acquire the ability to differentiate into HCs after the overexpression of specific genes, such as Atoh1, which makes HC regeneration possible. Gene therapy, through in vitro selection and editing of target genes, transforms exogenous gene fragments into target cells and alters the expression of genes in target cells to activate the corresponding differentiation developmental program in target cells. This review summarizes the genes that have been associated with the growth and development of cochlear HCs in recent years and provides an overview of gene therapy approaches in the field of HC regeneration. It concludes with a discussion of the limitations of the current therapeutic approaches to facilitate the early implementation of this therapy in a clinical setting.

Vestibular Deficits in Deafness: Clinical Presentation, Animal Modeling, and Treatment Solutions

The inner ear is responsible for both hearing and balance. These functions are dependent on the correct functioning of mechanosensitive hair cells, which convert sound- and motion-induced stimuli into electrical signals conveyed to the brain. During evolution of the inner ear, the major changes occurred in the hearing organ, whereas the structure of the vestibular organs remained constant in all vertebrates over the same period. Vestibular deficits are highly prevalent in humans, due to multiple intersecting causes: genetics, environmental factors, ototoxic drugs, infections and aging. Studies of deafness genes associated with balance deficits and their corresponding animal models have shed light on the development and function of these two sensory systems. Bilateral vestibular deficits often impair individual postural control, gaze stabilization, locomotion and spatial orientation. The resulting dizziness, vertigo, and/or falls (frequent in elderly populations) greatly affect patient quality of life. In the absence of treatment, prosthetic devices, such as vestibular implants, providing information about the direction, amplitude and velocity of body movements, are being developed and have given promising results in animal models and humans. Novel methods and techniques have led to major progress in gene therapies targeting the inner ear (gene supplementation and gene editing), 3D inner ear organoids and reprograming protocols for generating hair cell-like cells. These rapid advances in multiscale approaches covering basic research, clinical diagnostics and therapies are fostering interdisciplinary research to develop personalized treatments for vestibular disorders.

Regeneration of Hair Cells in the Human Vestibular System

The vestibular system is a critical part of the human balance system, malfunction of this system will lead to balance disorders, such as vertigo. Mammalian vestibular hair cells, the mechanical receptors for vestibular function, are sensitive to ototoxic drugs and virus infection, and have a limited restorative capacity after damage. Considering that no artificial device can be used to replace vestibular hair cells, promoting vestibular hair cell regeneration is an ideal way for vestibular function recovery. In this manuscript, the development of human vestibular hair cells during the whole embryonic stage and the latest research on human vestibular hair cell regeneration is summarized. The limitations of current studies are emphasized and future directions are discussed.

The crosstalk between the Notch, Wnt, and SHH signaling pathways in regulating the proliferation and regeneration of sensory progenitor cells in the mouse cochlea

Sensory hair cells (HCs) are highly susceptible to damage by noise, ototoxic drugs, and aging. Although HCs cannot be spontaneously regenerated in adult mammals, previous studies have shown that signaling pathways are involved in HC regeneration in the damaged mouse cochlea. Here, we used a Notch antagonist (DAPT), a Wnt agonist (QS11), and recombinant Sonic hedgehog (SHH) protein to investigate their concerted actions underlying HC regeneration in the mouse cochlea after neomycin-induced damage both in vivo and in vitro. With DAPT, the numbers of HCs increased, and supporting cell (SC) proliferation was seen in both the intact and damaged cochlear sensory epithelia, while these numbers were unchanged in the presence of QS11. When simultaneously treated with DAPT and QS11, the number of HCs increased dramatically, and much greater SC proliferation was seen in the cochlear epithelium. In transgenic mice with both Notch1 conditional knockout and β-catenin over-expression, cochlear SC proliferation and HC regeneration were more obvious than in either Notch1 knockout or β-catenin over-expressing mice separately. When cochleae were treated with DAPT, QS11, and SHH together, SC proliferation was even greater, and this proliferation was seen in both the HC region and the greater epithelial ridge. High-throughput RNA sequencing was used to identify the differentially expressed genes between all groups, and the results showed that the SHH and Wnt signaling pathways are involved in SC proliferation. Our study suggests that co-regulation of the Notch, Wnt, and SHH signaling pathways promotes extensive cell proliferation and regeneration in the mouse cochlea.

AAV8-mediated Atoh1 overexpression induces dose-dependent regeneration of vestibular hair cells in adult mice

Vestibular hair cells (HCs) are mechanoreceptors for the detection of head movement. Vestibular HCs of adult mammals never completely regenerate after damage, resulting in vestibular dysfunction. Overexpression of Atoh1 is effective for inducing HC regeneration. However, method of clinical feasibility and improvement of regenerative extent are both in need. Here we used an adeno-associated virus (AAV) serotype 8 vector of two different titers to overexpress Atoh1 in the injured utricles of adult mice. One month after virus inoculation, abundant myosin VIIa-positive cells and immature stereocilia were observed. Quantitative analyses revealed that Atoh1 overexpression replenished vestibular HCs in a dose-dependent manner. Vectors of a higher titer increased the number of myosin VIIa-positive cells compared to those of lower titer. Moreover, only Atoh1 overexpression in the higher titer group enhanced stereocilium regeneration, which is an important step in the maturation of regenerated HCs. Although the current treatment failed to initiate functional recovery of the animals, our results prompt further improvements in the recovery of vestibular dysfunction by AAV.

Enhanced Inner-Ear Organoid Formation from Mouse Embryonic Stem Cells by Photobiomodulation

Photobiomodulation (PBM) stimulates different types of stem cells to migrate, proliferate, and differentiate in vitro and in vivo. However, little is known about the effects of PBM on the differentiation of embryonic stem cells (ESCs) toward the otic lineage. Only a few reports have documented the in vitro differentiation of ESCs into inner-ear hair cells (HCs) due to the complexity of HCs compared with other target cell types. In this study, we determined the optimal condition to differentiate the ESCs into the otic organoid using different culture techniques and PBM parameters. The efficiency of organoid formation within the embryoid body (EB) was dependent on the cell density of the hanging drop. PBM, using 630 nm wavelength light-emitting diodes (LEDs), further improved the differentiation of inner-ear hair cell-like cells coupled with reactive oxygen species (ROS) overexpression. Transcriptome analysis showed the factors that are responsible for the effect of PBM in the formation of otic organoids, notably, the downregulation of neural development-associated genes and the hairy and enhancer of split 5 (Hes5) gene, which inhibits the differentiation of prosensory cells to hair cells. These data enrich the current differentiation protocols for generating inner-ear hair cells.

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.

Effects of 3,3'-Iminodipropionitrile on Hair Cell Numbers in Cristae of CBA/CaJ and C57BL/6J Mice

This study examines absolute hair cell numbers in the cristae of C57BL/6J mice and CBA/CaJ mice from weaning to adulthood as well as the dose required for 3,3'-iminodiproprionitrile (IDPN)-injury of the cristae in C57BL/6J mice and CBA/CaJ mice, the two mouse strains most commonly used by inner ear researchers. In cristae of CBA/CaJ and C57BL/6J mice, no loss of hair cells was observed up to 24 weeks. In both strains, dose-dependent loss of hair cells was observed 7 days after IDPN treatment of 2-month-old mice (IC₅₀ = 16.1 mmol/kg in C57BL/6J mice vs. 25.21 mmol/kg in CBA/CaJ mice). Four-month-old C57BL/6J mice exposed to IDPN developed dose-dependent vestibular dysfunction as indicated by increased activity and circling behavior in open field tests and by failure to swim 7 days after treatment. IDPN-hair cell injury in C57BL/6J mice and CBA/CaJ mice represents a fast and predictable experimental model for the study of vestibular degeneration and a platform for the testing of vestibular therapies.

Canalostomy As a Surgical Approach to Local Drug Delivery into the Inner Ears of Adult and Neonatal Mice

Local delivery of therapeutic drugs into the inner ear is a promising therapy for inner ear diseases. Injection through semicircular canals (canalostomy) has been shown to be a useful approach to local drug delivery into the inner ear. The goal of this article is to describe, in detail, the surgical techniques involved in canalostomy in both adult and neonatal mice. As indicated by fast-green dye and adeno-associated virus serotype 8 with the green fluorescent protein gene, the canalostomy facilitated broad distribution of injected reagents in the cochlea and vestibular end-organs with minimal damage to hearing and vestibular function. The surgery was successfully implemented in both adult and neonatal mice; indeed, multiple surgeries could be performed if required. In conclusion, canalostomy is an effective and safe approach to drug delivery into the inner ears of adult and neonatal mice and may be used to treat human inner ear diseases in the future.

Outlook and future of inner ear therapy

Drug delivery to the inner ear is an ideal method to treat a wide variety of otologic conditions. A broad range of potential applications is just beginning to be explored. New approaches combine principles of inner ear pharmacokinetics with emerging technologies of drug delivery including novel delivery systems, drug-device combinations, and new categories of drugs. Strategies include cell-specific targeting, manipulation of gene expression, local activation following systemic delivery, and use of stem cells, viral vectors, and gene editing systems. Translation of these therapies to the clinic remains challenging given the potential risks of intracochlear and intralabyrinthine trauma, our limited understanding of the etiologies of particular inner ear disorders, and paucity of accurate diagnostic tools at the cellular level. This review provides an overview of future methods, delivery systems, disease targets, and clinical considerations required for translation to clinical medicine.

Rhesus Cochlear and Vestibular Functions Are Preserved After Inner Ear Injection of Saline Volume Sufficient for Gene Therapy Delivery

Sensorineural losses of hearing and vestibular sensation due to hair cell dysfunction are among the most common disabilities. Recent preclinical research demonstrates that treatment of the inner ear with a variety of compounds, including gene therapy agents, may elicit regeneration and/or repair of hair cells in animals exposed to ototoxic medications or other insults to the inner ear. Delivery of gene therapy may also offer a means for treatment of hereditary hearing loss. However, injection of a fluid volume sufficient to deliver an adequate dose of a pharmacologic agent could, in theory, cause inner ear trauma that compromises functional outcome. The primary goal of the present study was to assess that risk in rhesus monkeys, which closely approximates humans with regard to middle and inner ear anatomy. Secondary goals were to identify the best delivery route into the primate ear from among two common surgical approaches (i.e., via an oval window stapedotomy and via the round window) and to determine the relative volumes of rhesus, rodent, and human labyrinths for extrapolation of results to other species. We measured hearing and vestibular functions before and 2, 4, and 8 weeks after unilateral injection of phosphate-buffered saline vehicle (PBSV) into the perilymphatic space of normal rhesus monkeys at volumes sufficient to deliver an atoh1 gene therapy vector. To isolate effects of injection, PBSV without vector was used. Assays included behavioral observation, auditory brainstem responses, distortion product otoacoustic emissions, and scleral coil measurement of vestibulo-ocular reflexes during whole-body rotation in darkness. Three groups (N = 3 each) were studied. Group A received a 10 μL transmastoid/trans-stapes injection via a laser stapedotomy. Group B received a 10 μL transmastoid/trans-round window injection. Group C received a 30 μL transmastoid/trans-round window injection. We also measured inner ear fluid space volume via 3D reconstruction of computed tomography (CT) images of adult C57BL6 mouse, rat, rhesus macaque, and human temporal bones (N = 3 each). Injection was well tolerated by all animals, with eight of nine exhibiting no signs of disequilibrium and one animal exhibiting transient disequilibrium that resolved spontaneously by 24 h after surgery. Physiologic results at the final, 8-week post-injection measurement showed that injection was well tolerated. Compared to its pretreatment values, no treated ear's ABR threshold had worsened by more than 5 dB at any stimulus frequency; distortion product otoacoustic emissions remained detectable above the noise floor for every treated ear (mean, SD and maximum deviation from baseline: -1.3, 9.0, and -18 dB, respectively); and no animal exhibited a reduction of more than 3 % in vestibulo-ocular reflex gain during high-acceleration, whole-body, passive yaw rotations in darkness toward the treated side. All control ears and all operated ears with definite histologic evidence of injection through the intended site showed similar findings, with intact hair cells in all five inner ear sensory epithelia and intact auditory/vestibular neurons. The relative volumes of mouse, rat, rhesus, and human inner ears as measured by CT were (mean ± SD) 2.5 ± 0.1, 5.5 ± 0.4, 59.4 ± 4.7 and 191.1 ± 4.7 μL. These results indicate that injection of PBSV at volumes sufficient for gene therapy delivery can be accomplished without destruction of inner ear structures required for hearing and vestibular sensation.

Atoh1 as a Coordinator of Sensory Hair Cell Development and Regeneration in the Cochlea

Cochlear sensory hair cells (HCs) are crucial for hearing as mechanoreceptors of the auditory systems. Clarification of transcriptional regulation for the cochlear sensory HC development is crucial for the improvement of cell replacement therapies for hearing loss. Transcription factor Atoh1 is the key player during HC development and regeneration. In this review, we will focus on Atoh1 and its related signaling pathways (Notch, fibroblast growth factor, and Wnt/β-catenin signaling) involved in the development of cochlear sensory HCs. We will also discuss the potential applicability of these signals for the induction of HC regeneration.

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.

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

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

Insights into sensory hair cell regeneration from the zebrafish lateral line

Loss of mechanosensory hair cells in the inner ear leads to loss of hearing. In humans this results in permanent deafness, as mammals are largely unable to regenerate hair cells. In contrast, zebrafish robustly regenerate hair cells in the sensory lateral line and ear and recent gene expression and time-lapse analyses of cell behaviors at the single cell level have greatly advanced our understanding of the mechanisms responsible for hair cell regeneration. In the lateral line, hair cell regeneration is controlled via dynamic interactions between Notch and Wnt/β-catenin signaling, and likely also between Fgf and the retinoic acid signaling pathways. Less is known about what initiates regeneration and we discuss potential pathways that may trigger proliferation after hair cell damage.

A central to peripheral progression of cell cycle exit and hair cell differentiation in the developing mouse cristae

The inner ear contains six distinct sensory organs that each maintains some ability to regenerate hair cells into adulthood. In the postnatal cochlea, there appears to be a relationship between the developmental maturity of a region and its ability to regenerate as postnatal regeneration largely occurs in the apical turn, which is the last region to differentiate and mature during development. In the mature cristae there are also regional differences in regenerative ability, which led us to hypothesize that there may be a general relationship between the relative maturity of a region and the regenerative competence of that region in all of the inner ear sensory organs. By analyzing adult mouse cristae labeled embryonically with BrdU, we found that hair cell birth starts in the central region and progresses to the periphery with age. Since the peripheral region of the adult cristae also maintains active Notch signaling and some regenerative competence, these results are consistent with the hypothesis that the last regions to develop retain some of their regenerative ability into adulthood. Further, by analyzing embryonic day 14.5 inner ears we provide evidence for a wave of hair cell birth along the longitudinal axis of the cristae from the central regions to the outer edges. Together with the data from the adult inner ears labeled with BrdU as embryos, these results suggest that hair cell differentiation closely follows cell cycle exit in the cristae, unlike in the cochlea where they are uncoupled.

Developmental regulation of planar cell polarity and hair-bundle morphogenesis in auditory hair cells: lessons from human and mouse genetics

Hearing loss is the most common and costly sensory defect in humans and genetic causes underlie a significant proportion of affected individuals. In mammals, sound is detected by hair cells (HCs) housed in the cochlea of the inner ear, whose function depends on a highly specialized mechanotransduction organelle, the hair bundle. Understanding the factors that regulate the development and functional maturation of the hair bundle is crucial for understanding the pathophysiology of human deafness. Genetic analysis of deafness genes in animal models, together with complementary forward genetic screens and conditional knock-out mutations in essential genes, have provided great insights into the molecular machinery underpinning hair-bundle development and function. In this review, we highlight recent advances in our understanding of hair-bundle morphogenesis, with an emphasis on the molecular pathways governing hair-bundle polarity and orientation. We next discuss the proteins and structural elements important for hair-cell mechanotransduction as well as hair-bundle cohesion and maintenance. In addition, developmental signals thought to regulate tonotopic features of HCs are introduced. Finally, novel approaches that complement classic genetics for studying the molecular etiology of human deafness are presented. WIREs Dev Biol 2016, 5:85-101. doi: 10.1002/wdev.202 For further resources related to this article, please visit the WIREs website.

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.

Culture and properties of adipose-derived mesenchymal stem cells: characteristics in vitro and immunosuppression in vivo

OBJECTIVE

To compare the two sources of adipose and bone marrow derived mesenchymal stem cells (BMSCs and AMSCs) in immune regulation and to evaluate the therapeutic effects of AMSCs on Con A induced hepatitis and the possible mechanism involved in it.

METHODS

We isolated bone marrow and adipose derived mesenchymal stem cells respectively and compared their differences on T lymphocyte activation, proliferation and suppression. We also test the anti-apoptosis ability of AMSCs on LO2 cell line. The effects of intravenous infusion of AMSCs on liver damage were also tested and we detected donor AMSCs in liver of recipient and their effects on the activity of intrahepatic NKT cells.

RESULTS

BMSCs and AMSCs were similar in cell phenotype and the difference existed only in the expression of CD106. The results showed that the capacity of suppressing T cells proliferation and activation was weakened in AMSCs. AMSCs ameliorated liver damage and this effect was time and dose dependent. We detected donor AMSCs in liver of recipient which suggested tissue damage could be a clue for AMSCs migration. We also found AMSCs suppress the activity of intrahepatic NKT cells, but this suppress effects was not restricted in liver only, but the whole body.

CONCLUSION

Cell origin and abundance are decisive factors in stem cells applications and with the same premise of AMSCs and BMSCs, adipose tissue is a more promising origin source of stem cells. The immunoregulatory features of MSCs might play an important role in various MSCs cellular therapies.

Green Tea Polyphenols Protects Cochlear Hair Cells from Ototoxicity by Inhibiting Notch Signalling

Notch signalling pathway plays an essential role in the development of cochlea, which inhibits the proliferation of hair cells. Epigallocatechin-3-gallate (EGCG) is the most abundant polyphenol in green tea, which presents strong antioxidant activation and has been applied for anti-cancer and anti-inflammatory. In this study, we treated the cochlear explant cultures with EGCG and found that EGCG can protect cochlear hair cells from ototoxic drug gentamicin. We demonstrated that EGCG could down-regulate the expression of Notch signalling pathway target genes, such as Hes1, Hes5, Hey1 and Hey5. However, the Notch pathway ligands such as Delta1, Jag1 and Jag2 were not affected by EGCG. To further illustrate the mechanism of recover cochlear hair cells, we demonstrated that EGCG inhibited the activity of γ-secrectase to suppress Notch signalling pathway and promoted the proliferation and regeneration of hair cells in the damaged cochlea. Our results suggest for the first time the role of EGCG as an inhibitor of the Notch signalling pathway, and support its potential value in hearing-impaired recovery in clinical therapy.

Nonviral Reprogramming of Human Wharton's Jelly Cells Reveals Differences Between ATOH1 Homologues

The transcription factor atonal homolog 1 (ATOH1) has multiple homologues that are functionally conserved across species and is responsible for the generation of sensory hair cells. To evaluate potential functional differences between homologues, human and mouse ATOH1 (HATH1 and MATH-1, respectively) were nonvirally delivered to human Wharton's jelly cells (hWJCs) for the first time. Delivery of HATH1 to hWJCs demonstrated superior expression of inner ear hair cell markers and characteristics than delivery of MATH-1. Inhibition of HES1 and HES5 signaling further increased the atonal effect. Transfection of hWJCs with HATH1 DNA, HES1 siRNA, and HES5 siRNA displayed positive identification of key hair cell and support cell markers found in the cochlea, as well as a variety of cell shapes, sizes, and features not native to hair cells, suggesting the need for further examination of other cell types induced by HATH1 expression. In the first side-by-side evaluation of HATH1 and MATH-1 in human cells, substantial differences were observed, suggesting that the two atonal homologues may not be interchangeable in human cells, and artificial expression of HATH1 in hWJCs requires further study. In the future, this line of research may lead to engineered systems that would allow for evaluation of drug ototoxicity or potentially even direct therapeutic use.

Gene-expression analysis of hair cell regeneration in the zebrafish lateral line

Deafness caused by the terminal loss of inner ear hair cells is one of the most common sensory diseases. However, nonmammalian animals (e.g., birds, amphibians, and fish) regenerate damaged hair cells. To understand better the reasons underpinning such disparities in regeneration among vertebrates, we set out to define at high resolution the changes in gene expression associated with the regeneration of hair cells in the zebrafish lateral line. We performed RNA-Seq analyses on regenerating support cells purified by FACS. The resulting expression data were subjected to pathway enrichment analyses, and the differentially expressed genes were validated in vivo via whole-mount in situ hybridizations. We discovered that cell cycle regulators are expressed hours before the activation of Wnt/β-catenin signaling following hair cell death. We propose that Wnt/β-catenin signaling is not involved in regulating the onset of proliferation but governs proliferation at later stages of regeneration. In addition, and in marked contrast to mammals, our data clearly indicate that the Notch pathway is significantly down-regulated shortly after injury, thus uncovering a key difference between the zebrafish and mammalian responses to hair cell injury. Taken together, our findings lay the foundation for identifying differences in signaling pathway regulation that could be exploited as potential therapeutic targets to promote either sensory epithelium or hair cell regeneration in mammals.

Gene therapy for deafness

Hearing loss is the most common sensory deficit in humans and can result from genetic, environmental or combined etiologies that prevent normal function of the cochlea, the peripheral sensory organ. Recent advances in understanding the genetic pathways that are critical for the development and maintenance of cochlear function, as well as the molecular mechanisms that underlie cell trauma and death, have provided exciting opportunities for modulating these pathways to correct genetic mutations, to enhance the endogenous protective pathways for hearing preservation and to regenerate lost sensory cells with the possibility of ameliorating hearing loss. A number of recent animal studies have used gene-based therapies in innovative ways toward realizing these goals. With further refinement, some of the protective and regenerative approaches reviewed here may become clinically applicable.

Hair cell generation by notch inhibition in the adult mammalian cristae

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

Regeneration of mammalian cochlear and vestibular hair cells through Hes1/Hes5 modulation with siRNA

The Notch pathway is a cell signaling pathway determining initial specification and subsequent cell fate in the inner ear. Previous studies have suggested that new hair cells (HCs) can be regenerated in the inner ear by manipulating the Notch pathway. In the present study, delivery of siRNA to Hes1 and Hes5 using a transfection reagent or siRNA to Hes1 encapsulated within poly(lactide-co-glycolide acid) (PLGA) nanoparticles increased HC numbers in non-toxin treated organotypic cultures of cochleae and maculae of postnatal day 3 mouse pups. An increase in HCs was also observed in cultured cochleae and maculae of mouse pups pre-conditioned with a HC toxin (4-hydroxy-2-nonenal or neomycin) and then treated with the various siRNA formulations. Treating cochleae with siRNA to Hes1 associated with a transfection reagent or siRNA to Hes1 delivered by PLGA nanoparticles decreased Hes1 mRNA and up-regulated Atoh1 mRNA expression allowing supporting cells (SCs) to acquire a HC fate. Experiments using cochleae and maculae of p27(kip1)/-GFP transgenic mouse pups demonstrated that newly generated HCs trans-differentiated from SCs. Furthermore, PLGA nanoparticles are non-toxic to inner ear tissue, readily taken up by cells within the tissue of interest, and present a synthetic delivery system that is a safe alternative to viral vectors. These results indicate that when delivered using a suitable vehicle, Hes siRNAs are potential therapeutic molecules that may have the capacity to regenerate new HCs in the inner ear and possibly restore human hearing and balance function.

Notch signaling in mammalian hair cell regeneration

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