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

Gene therapy for hearing loss: Current status and future prospects of non-viral vector delivery systems

Current therapeutic options for hearing loss rely on hearing aids, ossiculoplasty or cochlear implants. These devices have limitations, particularly in noisy acoustic environments. Therefore, interest in exploring aetiological treatments to improve not only auditory perception but also the quality of life of those affected is increasing. Gene therapy is a promising aetiological treatment that can fully restore auditory function. The success of gene therapy relies on the efficient delivery of therapeutic genes or genetic modifications to the cells of the inner ear that are designed to repair or replace defective genes and restore normal hearing function. Two main strategies for gene therapy involve the use of recombinant viral vectors and nonviral delivery vehicles. Owing to their excellent diffusion properties and compatibility with sensory cells, recombinant viral vectors, particularly adeno-associated viruses (AAVs), have dominated gene therapy in the cochlea. However, recombinant viral vectors have several drawbacks, such as limited transgene size, immunogenicity (particularly in neonates), and potential need for repeat administration. Nonviral vectors, such as cationic lipids and polymeric nanoparticles, are potential attractive alternatives. Nonviral vectors have several advantages, including low immunogenicity and unlimited transgene size. Recent studies have demonstrated significant auditory recovery in vivo using nonviral vectors in murine models. However, nonviral vectors are not as efficient as viral vectors in transferring genetic material. An alternative to nanoparticles is the use of other methods, such as electroporation. The main advantage of electroporation is that it can be used in combination with cochlear implantation and can target surface cells, but this method has a risk of cell damage. The goal of this review is to provide valuable insights into the current state of research on nonviral vectors for inner ear gene therapy and propose the exploration of innovative and effective gene therapy strategies for the treatment of hearing loss.

Protective Effects of Gastrodin Against Gentamicin-Induced Vestibular Damage by the Notch Signaling Pathway

PURPOSE

Gentamicin is a broad-spectrum antibiotic commonly used in clinical practice. However, the drug causes side effects of ototoxicity, leading to disruption in balance functionality. This study investigated the effect of gastrodin, a prominent compound present in Gastrodia, and the underlying mechanism on the development of gentamicin-induced vestibular dysfunction.

METHODS

Wild-type C57BL/6 mice were randomly assigned to three groups: control, gentamicin, and gentamicin + gastrodin groups. The extent of gentamicin-induced vestibular impairment was assessed through a series of tests including the swimming test, contact righting reflex test, and air-righting reflex. Alterations in vestibular hair cells were monitored through immunofluorescence assay, and cellular apoptosis was observed using TUNEL staining. The mRNA and protein expression of Notch1, Jagged1, and Hes1 was quantified through qRT-PCR, immunofluorescence, and western blot analyses.

RESULTS

Gentamicin treatment led to pronounced deficits in vestibular function and otolith organ hair cells in mice. Nevertheless, pretreatment with gastrodin significantly alleviated these impairments. Additionally, the Notch signaling pathway was activated by gentamicin in the utricle, contributing to a notable increase in the expression levels of apoptosis-associated proteins. By contrast, gastrodin treatment effectively suppressed the Notch signaling pathway, thereby mitigating the occurrence of apoptosis.

CONCLUSION

Collectively, these findings underscore the crucial role of gastrodin in safeguarding against gentamicin-induced vestibular dysfunction through the modulation of the Notch signaling pathway. This study suggests the potential of gastrodin as a promising therapeutic agent for preventing vestibular injuries.

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

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

Mapping of Notch signaling in the developing organ of Corti in common marmosets

Introduction

The well-regulated development of the sensory epithelium is essential for hearing. This process involves the specification of a pro-sensory epithelium containing common progenitors that differentiate into hair and supporting cells. Notch signaling is one of the most critical pathways during these processes, and its modification is thought to be a feasible approach for treating hearing loss. Despite interspecies differences between rodents and primates or humans, most of our current knowledge regarding cochlear development has been obtained from rodent models.

Methods

We therefore examined and mapped the expression patterns of Notch signal components in the developing cochlea of the common marmoset (Callithrix jacchus), a small monkey species native to the New World, a primate model animal.

Results

In contrast to the preserved expression patterns of the Notch signaling components in the hair cell differentiation between primates and rodents, we unveiled relatively large interspecies differences during the maturation of supporting cells.

Discussion

This improved knowledge of Notch signaling during primate cochlear development will facilitate the development of future regenerative therapies.

Depicting pseudotime-lagged causality across single-cell trajectories for accurate gene-regulatory inference

Identifying the causal interactions in gene-regulatory networks requires an accurate understanding of the time-lagged relationships between transcription factors and their target genes. Here we describe DELAY (short for Depicting Lagged Causality), a convolutional neural network for the inference of gene-regulatory relationships across pseudotime-ordered single-cell trajectories. We show that combining supervised deep learning with joint probability matrices of pseudotime-lagged trajectories allows the network to overcome important limitations of ordinary Granger causality-based methods, for example, the inability to infer cyclic relationships such as feedback loops. Our network outperforms several common methods for inferring gene regulation and, when given partial ground-truth labels, predicts novel regulatory networks from single-cell RNA sequencing (scRNA-seq) and single-cell ATAC sequencing (scATAC-seq) data sets. To validate this approach, we used DELAY to identify important genes and modules in the regulatory network of auditory hair cells, as well as likely DNA-binding partners for two hair cell cofactors (Hist1h1c and Ccnd1) and a novel binding sequence for the hair cell-specific transcription factor Fiz1. We provide an easy-to-use implementation of DELAY under an open-source license at https://github.com/calebclayreagor/DELAY.

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.

Altering gene expression using antisense oligonucleotide therapy for hearing loss

Hearing loss affects more than 430 million people, worldwide, and is the third most common chronic physical condition in the United States and Europe (GBD Hearing Loss Collaborators, 2021; NIOSH, 2021; WHO, 2021). The loss of hearing significantly impacts motor and cognitive development, communication, education, employment, and overall quality of life. The inner ear houses the sensory organs for both hearing and balance and provides an accessible target for therapeutic delivery. Antisense oligonucleotides (ASOs) use various mechanisms to manipulate gene expression and can be tailor-made to treat disorders with defined genetic targets. In this review, we discuss the preclinical advancements within the field of the highly promising ASO-based therapies for hereditary hearing loss disorders. Particular focus is on ASO mechanisms of action, preclinical studies on ASO treatments of hearing loss, timing of therapeutic intervention, and delivery routes to the inner ear.

Single-cell transcriptome analysis reveals three sequential phases of gene expression during zebrafish sensory hair cell regeneration

Loss of sensory hair cells (HCs) in the mammalian inner ear leads to permanent hearing and vestibular defects, whereas loss of HCs in zebrafish results in their regeneration. We used single-cell RNA sequencing (scRNA-seq) to characterize the transcriptional dynamics of HC regeneration in zebrafish at unprecedented spatiotemporal resolution. We uncovered three sequentially activated modules: first, an injury/inflammatory response and downregulation of progenitor cell maintenance genes within minutes after HC loss; second, the transient activation of regeneration-specific genes; and third, a robust re-activation of developmental gene programs, including HC specification, cell-cycle activation, ribosome biogenesis, and a metabolic switch to oxidative phosphorylation. The results are relevant not only for our understanding of HC regeneration and how we might be able to trigger it in mammals but also for regenerative processes in general. The data are searchable and publicly accessible via a web-based interface.

Cochlea cell-specific marker expression upon in vitro Hes1 knockdown

NOTCH pathway proteins, including the transcriptional factor HES1, play crucial roles in the development of the inner ear by means of the lateral inhibition mechanism, in which supporting cells have their phenotype preserved while they are prevented from becoming hair cells. Genetic manipulation of this pathway has been demonstrated to increase hair cell number. The present study aimed to investigate gene expression effects in hair cells and supporting cells after Hes1-shRNA lentivirus transduction in organotypic cultures of the organ of Corti from postnatal-day-3 mice. Forty-eight hours after in vitro knockdown, Hes1 gene expression was reduced at both mRNA and protein levels. Myo7a (hair cell marker) and Sox2 (progenitor cell marker) mRNA levels also significantly increased. The modulation of gene expression in the organ of Corti upon Hes1 knockdown is consistent with cell phenotypes related to lateral inhibition mechanism interference in the inner ear. The lentivirus-based expression of Hes1-shRNA is a valuable strategy for genetic interference in the organ of Corti and for future evaluation of its efficacy in protocols aiming at the regeneration of hair cells in vivo.

Age-related transcriptome changes in Sox2+ supporting cells in the mouse cochlea

Background

Inner ear supporting cells (SCs) in the neonatal mouse cochlea are a potential source for hair cell (HC) regeneration, but several studies have shown that the regeneration ability of SCs decreases dramatically as mice age and that lost HCs cannot be regenerated in adult mice. To better understand how SCs might be better used to regenerate HCs, it is important to understand how the gene expression profile changes in SCs at different ages.

Methods

Here, we used Sox2^GFP/+ mice to isolate the Sox2+ SCs at postnatal day (P)3, P7, P14, and P30 via flow cytometry. Next, we used RNA-seq to determine the transcriptome expression profiles of P3, P7, P14, and P30 SCs. To further analyze the relationships between these age-related and differentially expressed genes in Sox2+ SCs, we performed gene ontology (GO) analysis.

Results

Consistent with previous reports, we also found that the proliferation and HC regeneration ability of isolated Sox2+ SCs significantly decreased as mice aged. We identified numerous genes that are enriched and differentially expressed in Sox2+ SCs at four different postnatal ages, including cell cycle genes, signaling pathway genes, and transcription factors that might be involved in regulating the proliferation and HC differentiation ability of SCs. We thus present a set of genes that might regulate the proliferation and HC regeneration ability of SCs, and these might serve as potential new therapeutic targets for HC regeneration.

Conclusions

In our research, we found several genes that might play an important role in regulating the proliferation and HC regeneration ability of SCs. These datasets are expected to serve as a resource to provide potential new therapeutic targets for regulating the ability of SCs to regenerate HCs in postnatal mammals.

New molecular therapies for the treatment of hearing loss

An estimated 466 million people suffer from hearing loss worldwide. Sensorineural hearing loss is characterized by degeneration of key structures of the sensory pathway in the cochlea such as the sensory hair cells, the primary auditory neurons and their synaptic connection to the hair cells - the ribbon synapse. Various strategies to protect or regenerate these sensory cells and structures are the subject of intensive research. Yet despite recent advances in our understandings of the capacity of the cochlea for repair and regeneration there are currently no pharmacological or biological interventions for hearing loss. Current research focusses on localized cochlear drug, gene and cell-based therapies. One of the more promising drug-based therapies is based on neurotrophic factors for the repair of the ribbon synapse after noise exposure, as well as preventing loss of primary auditory neurons and regrowth of the auditory neuron fibers after severe hearing loss. Drug therapy delivery technologies are being employed to address the specific needs of neurotrophin and other therapies for hearing loss that include the need for high doses, long-term delivery, localised or cell-specific targeting and techniques for their safe and efficacious delivery to the cochlea. Novel biomaterials are enabling high payloads of drugs to be administered to the cochlea with subsequent slow-release properties that are proving to be beneficial for treating hearing loss. In parallel, new gene therapy technologies are addressing the need for cell specificity and high efficacy for the treatment of both genetic and acquired hearing loss with promising reports of hearing recovery. Some biomaterials and cell therapies are being used in conjunction with the cochlear implant ensuring therapeutic benefit to the primary neurons during electrical stimulation. This review will introduce the auditory system, hearing loss and the potential for repair and regeneration in the cochlea. Drug delivery to the cochlea will then be reviewed, with a focus on new biomaterials, gene therapy technologies, cell therapy and the use of the cochlear implant as a vehicle for drug delivery. With the current pre-clinical research effort into therapies for hearing loss, including clinical trials for gene therapy, the future for the treatment for hearing loss is looking bright.

Antisense Oligonucleotides for the Treatment of Inner Ear Dysfunction

Antisense oligonucleotides (ASOs) have shown potential as therapeutic molecules for the treatment of inner ear dysfunction. The peripheral sensory organs responsible for both hearing and equilibrium are housed within the inner ear. Hearing loss and vestibular balance problems affect a large portion of the population and limited treatment options exist. Targeting ASOs to the inner ear as a therapeutic strategy has unique pharmacokinetic and drug delivery opportunities and challenges. Here, we review ASO technology, delivery, disease targets, and other key considerations for development of this therapeutic approach.

Therapeutic Potential of Wnt and Notch Signaling and Epigenetic Regulation in Mammalian Sensory Hair Cell Regeneration

Hearing loss is one of the most prevalent sensory deficits worldwide and can result from the death of mechanosensory hair cells that transduce auditory signals in the cochlea. The mammalian cochlea lacks the capacity to regenerate these hair cells once damaged, and currently there are no biological therapies for hearing loss. Understanding the signaling pathways responsible for hair cell development can inform regenerative strategies and identify targets for treating hearing loss. The canonical Wnt and Notch pathways are critical for cochlear development; they converge on several key molecules, such as Atoh1, to regulate prosensory specification, proliferation, hair cell differentiation, and cellular organization. Much work has focused on Wnt and Notch modulation in the neonatal mouse cochlea, where they can promote hair cell regeneration. However, this regenerative response is limited in the adult cochlea and this might be attributed to age-dependent epigenetic modifications. Indeed, the epigenetic status at key gene loci undergoes dynamic changes during cochlear development, maturation, and aging. Therefore, strategies to improve regenerative success in the adult cochlea might require the modulation of Wnt, Notch, or other pathways, as well as targeted epigenetic modifications to alter the activity of key genes critical for supporting cell proliferation or transdifferentiation.

Neuroprotective effect of Notch pathway inhibitor DAPT against focal cerebral ischemia/reperfusion 3 hours before model establishment

As an inhibitor of the Notch signaling pathway, N-[N-(3,5-difluorohenacetyl)-l-alanyl]-S-phenylglycine tert-butyl ester (DAPT) may protect brain tissue from serious ischemic injury. This study aimed to explore neuroprotection by DAPT after cerebral ischemia/reperfusion (I/R) injury. DAPT was intraperitoneally injected 3 hours before the establishment of a focal cerebral I/R model in the right middle cerebral artery of obstructed mice. Longa scores were used to assess neurological changes of mice. Nissl staining and TdT-mediated dUTP-biotin nick-end labeling staining were used to examine neuronal damage and cell apoptosis in the right prefrontal cortex, while immunofluorescence staining was used to detect glial fibrillary acidic protein- and Notch1-positive cells. Protein expression levels of Hes1 and Hes5 were detected by western blot assay in the right prefrontal cortex. Our results demonstrated that DAPT significantly improved neurobehavioral scores and relieved neuronal morphological damage. DAPT decreased the number of glial fibrillary acidic protein- and Notch1-positive cells in the right prefrontal cortex, while also reducing the number of apoptotic cells and decreasing interleukin-6 and tumor necrosis factor-α contents, and simultaneously downregulating Hes1 and Hes5 protein expression. These findings verify that DAPT alleviates pathological lesions and strengthens the anti-inflammatory response after cerebral I/R injury. Thus, DAPT might be developed as an effective drug for the prevention of cerebral I/R injury.

Innovative pharmaceutical approaches for the management of inner ear disorders

The sense of hearing is essential for permitting human beings to interact with the environment, and its dysfunctions can strongly impact on the quality of life. In this context, the cochlea plays a fundamental role in the transformation of the airborne sound waves into electrical signals, which can be processed by the brain. However, several diseases and external stimuli (e.g., noise, drugs) can damage the sensorineural structures of cochlea, inducing progressive hearing dysfunctions until deafness. In clinical practice, the current pharmacological approaches to treat cochlear diseases are based on the almost exclusive use of systemic steroids. In the last decades, the efficacy of novel therapeutic molecules has been proven, taking advantage from a better comprehension of the pathological mechanisms underlying many cochlear diseases. In addition, the feasibility of intratympanic administration of drugs also permitted to overcome the pharmacokinetic limitations of the systemic drug administration, opening new frontiers in drug delivery to cochlea. Several innovative drug delivery systems, such as in situ gelling systems or nanocarriers, were designed, and their efficacy has been proven in vitro and in vivo in cochlear models. The current review aims to describe the art of state in the cochlear drug delivery, highlighting lights and shadows and discussing the most critical aspects still pending in the field.

Regeneration of Cochlear Hair Cells and Hearing Recovery through Hes1 Modulation with siRNA Nanoparticles in Adult Guinea Pigs

Deafness is commonly caused by the irreversible loss of mammalian cochlear hair cells (HCs) due to noise trauma, toxins, or infections. We previously demonstrated that small interfering RNAs (siRNAs) directed against the Notch pathway gene, hairy and enhancer of split 1 (Hes1), encapsulated within biocompatible poly(lactic-co-glycolic acid) nanoparticles (PLGA NPs) could regenerate HCs within ototoxin-ablated murine organotypic cultures. In the present study, we delivered this sustained-release formulation of Hes1 siRNA (siHes1) into the cochleae of noise-injured adult guinea pigs. Auditory functional recovery was measured by serial auditory brainstem responses over a nine-week follow-up period, and HC regeneration was evaluated by immunohistological evaluations and scanning electron microscopy. Significant HC restoration and hearing recovery were observed across a broad tonotopic range in ears treated with siHes1 NPs, beginning at three weeks and extending out to nine weeks post-treatment. Moreover, both ectopic and immature HCs were uniquely observed in noise-injured cochleae treated with siHes1 NPs, consistent with de novo HC production. Our results indicate that durable cochlear HCs were regenerated and promoted significant hearing recovery in adult guinea pigs through reversible modulation of Hes1 expression. Therefore, PLGA-NP-mediated delivery of siHes1 to the cochlea represents a promising pharmacologic approach to regenerate functional and sustainable mammalian HCs in vivo.

The Repression of Atoh1 by Neurogenin1 during Inner Ear Development

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

Characterization of Lgr5+ Progenitor Cell Transcriptomes after Neomycin Injury in the Neonatal Mouse Cochlea

Lgr5+ supporting cells (SCs) are enriched hair cell (HC) progenitors in the cochlea. Both in vitro and in vivo studies have shown that HC injury can spontaneously activate Lgr5+ progenitors to regenerate HCs in the neonatal mouse cochlea. Promoting HC regeneration requires the understanding of the mechanism of HC regeneration, and this requires knowledge of the key genes involved in HC injury-induced self-repair responses that promote the proliferation and differentiation of Lgr5+ progenitors. Here, as expected, we found that neomycin-treated Lgr5+ progenitors (NLPs) had significantly greater HC regeneration ability, and greater but not significant proliferation ability compared to untreated Lgr5+ progenitors (ULPs) in response to neomycin exposure. Next, we used RNA-seq analysis to determine the differences in the gene-expression profiles between the transcriptomes of NLPs and ULPs from the neonatal mouse cochlea. We first analyzed the genes that were enriched and differentially expressed in NLPs and ULPs and then analyzed the cell cycle genes, the transcription factors, and the signaling pathway genes that might regulate the proliferation and differentiation of Lgr5+ progenitors. We found 9 cell cycle genes, 88 transcription factors, 8 microRNAs, and 16 cell-signaling pathway genes that were significantly upregulated or downregulated after neomycin injury in NLPs. Lastly, we constructed a protein-protein interaction network to show the interaction and connections of genes that are differentially expressed in NLPs and ULPs. This study has identified the genes that might regulate the proliferation and HC regeneration of Lgr5+ progenitors after neomycin injury, and investigations into the roles and mechanisms of these genes in the cochlea should be performed in the future to identify potential therapeutic targets for HC regeneration.

siRNA-loaded biodegradable nanocarriers for therapeutic MAPK1 silencing against cisplatin-induced ototoxicity

Ototoxicity represents a major adverse side-effect of cis-diamminedichloroplatinum-II (cisplatin, CDDP). The mitogen-activated protein kinase (MAPK) pathway is thought to play a central role in potentiating the apoptotic effect of CDDP within the cochlea. We hypothesized that prophylactic inhibition of MAPK signaling, using small interfering RNA (siRNA), might confer a protective effect against CDDP-induced apoptosis within the auditory sensory epithelia. To enhance the therapeutic utility of this approach, we synthesized biocompatible siMAPK1-loaded nanoparticles (NPs) and performed physicochemical characterizations for size, morphology, drug loading and release kinetics, using dynamic light scattering, electron microscopy and spectrophotometric analyses, respectively. Our findings show 183.88±6.26 nm-sized spherical siMAPK1-loaded NPs with -27.12±6.65mV zeta potential and 112.78±0.24pmol/mg of siMAPK1 loading that exhibit a sustained release profile for prolonged therapeutic efficacy. Synthesized NPs were validated for biocompatibility and prophylactically protected against CDDP-induced cytotoxicity in HEI-OC1 cells and hair cell loss in murine organotypic cochlear explants. Our study confirms a pivotal role for MAPK1 signaling as a potentiating factor for CDDP-induced apoptosis and cochlear hair cell loss, and highlights siMAPK1 NP treatment as a therapeutic strategy for limiting the ototoxic side-effects associated with systemic CDDP administration.

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

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

Inner ear barriers to nanomedicine-augmented drug delivery and imaging

There are several challenges to inner ear drug delivery and imaging due to the existence of tight biological barriers to the target structure and the dense bone surrounding it. Advances in imaging and nanomedicine may provide knowledge for overcoming the existing limitations to both the diagnosis and treatment of inner ear diseases. Novel techniques have improved the efficacy of drug delivery and targeting to the inner ear, as well as the quality and accuracy of imaging this structure. In this review, we will describe the pathways and biological barriers of the inner ear regarding drug delivery, the beneficial applications and limitations of the imaging techniques available for inner ear research, the behavior of engineered nanomaterials in inner ear applications, and future perspectives for nanomedicine-based inner ear imaging.

Sox2-CreER mice are useful for fate mapping of mature, but not neonatal, cochlear supporting cells in hair cell regeneration studies

Studies of hair cell regeneration in the postnatal cochlea rely on fate mapping of supporting cells. Here we characterized a Sox2-CreER knock-in mouse line with two independent reporter mouse strains at neonatal and mature ages. Regardless of induction age, reporter expression was robust, with CreER activity being readily detectable in >85% of supporting cells within the organ of Corti. When induced at postnatal day (P) 28, Sox2-CreER activity was exclusive to supporting cells demonstrating its utility for fate mapping studies beyond this age. However, when induced at P1, Sox2-CreER activity was also detected in >50% of cochlear hair cells, suggesting that Sox2-CreER may not be useful to fate map a supporting cell origin of regenerated hair cells if induced at neonatal ages. Given that this model is currently in use by several investigators for fate mapping purposes, and may be adopted by others in the future, our finding that current protocols are effective for restricting CreER activity to supporting cells at mature but not neonatal ages is both significant and timely.

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.

Differential regulation of Hes/Hey genes during inner ear development

Notch signaling plays a crucial role during inner ear development and regeneration. Hes/Hey genes encode for bHLH transcription factors identified as Notch targets. We have studied the expression and regulation of Hes/Hey genes during inner ear development in the chicken embryo. Among several Hes/Hey genes examined, only Hey1 and Hes5 map to the sensory regions, although with salient differences. Hey1 expression follows Jag1 expression except at early prosensory stages while Hes5 expression corresponds well to Dl1 expression throughout otic development. Although Hey1 and Hes5 are direct Notch downstream targets, they differ in the level of Notch required for activation. Moreover, they also differ in mRNA stability, showing different temporal decays after Notch blockade. In addition, Bmp, Wnt and Fgf pathways also modify Hey1 and Hes5 expression in the inner ear. Particularly, the Wnt pathway modulates Hey1 and Jag1 expression. Finally, gain of function experiments show that Hey1 and Hes5 cross-regulate each other in a complex manner. Both Hey1 and Hes5 repress Dl1 and Hes5 expression, suggesting that they prevent the transition to differentiation stages, probably by preventing Atoh1 expression. In spite of its association with Jag1, Hey1 does not seem to be instrumental for lateral induction as it does not promote Jag1 expression. We suggest that, besides being both targets of Notch, Hey1 and Hes5 are subject to a rather complex regulation that includes the stability of their transcripts, cross regulation and other signaling pathways.

Ligand-dependent Notch signaling strength orchestrates lateral induction and lateral inhibition in the developing inner ear

During inner ear development, Notch exhibits two modes of operation: lateral induction, which is associated with prosensory specification, and lateral inhibition, which is involved in hair cell determination. These mechanisms depend respectively on two different ligands, jagged 1 (Jag1) and delta 1 (Dl1), that rely on a common signaling cascade initiated after Notch activation. In the chicken otocyst, expression of Jag1 and the Notch target Hey1 correlates well with lateral induction, whereas both Jag1 and Dl1 are expressed during lateral inhibition, as are Notch targets Hey1 and Hes5. Here, we show that Jag1 drives lower levels of Notch activity than Dl1, which results in the differential expression of Hey1 and Hes5. In addition, Jag1 interferes with the ability of Dl1 to elicit high levels of Notch activity. Modeling the sensory epithelium when the two ligands are expressed together shows that ligand regulation, differential signaling strength and ligand competition are crucial to allow the two modes of operation and for establishing the alternate pattern of hair cells and supporting cells. Jag1, while driving lateral induction on its own, facilitates patterning by lateral inhibition in the presence of Dl1. This novel behavior emerges from Jag1 acting as a competitive inhibitor of Dl1 for Notch signaling. Both modeling and experiments show that hair cell patterning is very robust. The model suggests that autoactivation of proneural factor Atoh1, upstream of Dl1, is a fundamental component for robustness. The results stress the importance of the levels of Notch signaling and ligand competition for Notch function.

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

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

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.