Challenges in Cell-Based Therapies for the Treatment of Hearing Loss

AAV-mediated Gpm6b expression supports hair cell reprogramming

Irreversible damage to hair cells (HCs) in the cochlea leads to hearing loss. Cochlear supporting cells (SCs) in the murine cochlea have the potential to differentiate into HCs. Neuron membrane glycoprotein M6B (Gpm6b) as a four-transmembrane protein is a potential regulator of HC regeneration according to our previous research. In this study, we found that AAV-ie-mediated Gpm6b overexpression promoted SC-derived organoid expansion. Enhanced Gpm6b prevented the normal decrease in SC plasticity as the cochlea develops by supporting cells re-entry cell cycle and facilitating the SC-to-HC transformation. Also, overexpression of Gpm6b in the organ of Corti through the round window membrane injection facilitated the trans-differentiation of Lgr5+ SCs into HCs. In conclusion, our results suggest that Gpm6b overexpression promotes HC regeneration and highlights a promising target for hearing repair using the inner ear stem cells combined with AAV.

CFTR potentiator ivacaftor protects against noise-induced hair cell loss by increasing Nrf2 and reducing oxidative stress

Over-production of reactive oxygen species (ROS) in the inner ear can be triggered by a variety of pathological events identified in animal models after traumatic noise exposure. Our previous research found that inhibition of the AMP-activated protein kinase alpha subunit (AMPKα) protects against noise-induced cochlear hair cell loss and hearing loss by reducing ROS accumulation. However, the molecular pathway through which AMPKα exerts its antioxidative effect is still unclear. In this study, we have investigated a potential target of AMPKα and ROS, cystic fibrosis transmembrane conductance regulator (CFTR), and the protective effect against noise-induced hair cell loss of an FDA-approved CFTR potentiator, ivacaftor, in FVB/NJ mice, mouse explant cultures, and HEI-OC1 cells. We found that noise exposure increases phosphorylation of CFTR at serine 737 (p-CFTR, S737), which reduces wildtype CFTR function, resulting in oxidative stress in cochlear sensory hair cells. Pretreatment with a single dose of ivacaftor maintains CFTR function by preventing noise-increased p-CFTR (S737). Furthermore, ivacaftor treatment increases nuclear factor E2-related factor 2 (Nrf2) expression, diminishes ROS formation, and attenuates noise-induced hair cell loss and hearing loss. Additionally, inhibition of noise-induced AMPKα activation by compound C also diminishes p-CFTR (S737) expression. In line with these in-vivo results, administration of hydrogen peroxide to cochlear explants or HEI-OC1 cells increases p-CFTR (S737) expression and induces sensory hair cell or HEI-OC1 cell damage, while application of ivacaftor halts these effects. Although ivacaftor increases Nrf2 expression and reduces ROS accumulation, cotreatment with ML385, an Nrf2 inhibitor, abolishes the protective effects of ivacaftor against hydrogen-peroxide-induced HEI-OC1 cell death. Our results indicate that noise-induced sensory hair cell damage is associated with p-CFTR. Ivacaftor has potential for treatment of noise-induced hearing loss by maintaining CFTR function and increasing Nrf2 expression for support of redox homeostasis in sensory hair cells.

Stem cells as potential therapeutics for hearing loss

Hearing impairment is a global health problem. Stem cell therapy has become a cutting-edge approach to tissue regeneration. In this review, the recent advances in stem cell therapy for hearing loss have been discussed. Nanomaterials can modulate the stem cell microenvironment to augment the therapeutic effects further. The potential of combining nanomaterials with stem cells for repairing and regenerating damaged inner ear hair cells (HCs) and spiral ganglion neurons (SGNs) has also been discussed. Stem cell-derived exosomes can contribute to the repair and regeneration of damaged tissue, and the research progress on exosome-based hearing loss treatment has been summarized as well. Despite stem cell therapy's technical and practical limitations, the findings reported so far are promising and warrant further investigation for eventual clinical translation.

Enhanced survival of hypoimmunogenic otic progenitors following intracochlear xenotransplantation: repercussions for stem cell therapy in hearing loss models

Stem cell replacement holds the potential for sensorineural hearing loss (SNHL) treatment. However, its translation into clinical practice requires strategies for improving stem cell survival following intracochlear transplantation. Considering recent findings showing that the inner ear contains a resident population of immune cells, we hypothesized that immune evasion would improve the survival and residence time of transplanted stem cells in the cochlea, potentially leading to better outcomes. To test this, we leveraged genetic engineering techniques to develop hypoimmunogenic human-induced pluripotent stem cells (hi-iPSC), which lack human leukocyte antigen expression. We found that gene editing does not affect the biological properties of hi-iPSCs, including their capacity to differentiate into otic neural progenitors (ONPs). Compared to wild-type ONPs, more hypoimmunogenic ONPs (derived from hi-iPSCs) were found in the inner ear of immunocompetent mice ten days following cochlear xenotransplantation. This approach may open a new avenue for experimental and clinical SNHL treatments.

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.

Adult porcine (Sus scrofa) derived inner ear cells: Characteristics in in-vitro cultures

There is a need for an animal model that closely parallels human cochlea gestational development. This study aims to document porcine inner ear anatomy, and in vitro porcine derived inner ear cell culture characteristics. Twenty-four temporal bone were harvested from 12 adult pigs (Sus scrofa). Six were formalin fixed and their maximal diameters were measured. The cochlea duct length was determined by the insertion length of a Nucleus 22 cochlear implant in two bones. Four formalin fixed bones were sectioned for histology. Cochlear and vestibular tissues were harvested from non-fixed bones, cultured and characterized at different passages (P). Gene and protein expression of multipotent stem/progenitor (Nestin and Sox2), inner ear hair (Myosin VIIa, Prestin) and supporting (Cytokeratin 18 and Vimentin) cell markers were determined. The porcine cochlea was a 3.5 turn spiral. There was a separate vestibular compartment. The cochlear mean maximal diameter and height was 7.99 and 3.77 mm, respectively. Sphere forming cells were identified on phase-contrast microscopy. The relative mRNA expression levels of KRT18, MYO7A and SLC26A5 were significantly positively correlated in cochlear cultures; and MYO7A and SLC26A5; SOX2 and KRT18; NES and SLC26A5 genes were positively correlated in vestibular cultures (p = .037, Spearman correlation [τ] = .900). Inner ear sensory and stem cell characteristics persist in passaged porcine inner ear cells. Further work is required to establish the usefulness of porcine inner ear cell cultures to the study of human inner ear disorders.

Cellular autophagy, the compelling roles in hearing function and dysfunction

Sensorineural hearing loss (SNHL) is currently a major health issue. As one of the most common neurodegenerative diseases, SNHL is associated with the degradation of hair cells (HCs), spiral ganglion neurons (SGNs), the stria vascularis, supporting cells and central auditory system cells. Autophagy is a highly integrated cellular system that eliminates impaired components and replenishes energy to benefit cellular homeostasis. Etiological links between autophagy alterations and neurodegenerative diseases, such as SNHL, have been established. The hearing pathway is complex and depends on the comprehensive functions of many types of tissues and cells in auditory system. In this review, we discuss the roles of autophagy in promoting and inhibiting hearing, paying particular attention to specific cells in the auditory system, as discerned through research. Hence, our review provides enlightening ideas for the role of autophagy in hearing development and impairment.

Is there an unmet medical need for improved hearing restoration?

Hearing impairment, the most prevalent sensory deficit, affects more than 466 million people worldwide (WHO). We presently lack causative treatment for the most common form, sensorineural hearing impairment; hearing aids and cochlear implants (CI) remain the only means of hearing restoration. We engaged with CI users to learn about their expectations and their willingness to collaborate with health care professionals on establishing novel therapies. We summarize upcoming CI innovations, gene therapies, and regenerative approaches and evaluate the chances for clinical translation of these novel strategies. We conclude that there remains an unmet medical need for improving hearing restoration and that we are likely to witness the clinical translation of gene therapy and major CI innovations within this decade.

Dual-Specificity Phosphatase 14 Regulates Zebrafish Hair Cell Formation Through Activation of p38 Signaling Pathway

Most cases of acquired hearing loss are due to degeneration and subsequent loss of cochlear hair cells. Whereas mammalian hair cells are not replaced when lost, in zebrafish, they constantly renew and regenerate after injury. However, the molecular mechanism among this difference remains unknown. Dual-specificity phosphatase 14 (DUSP14) is an important negative modulator of mitogen-activated protein kinase (MAPK) signaling pathways. Our study was to investigate the effects of DUSP14 on supporting cell development and hair cell regeneration and explore the potential mechanism. Our results showed that dusp14 gene is highly expressed in zebrafish developing neuromasts and otic vesicles. Behavior analysis showed that dusp14 deficiency resulted in hearing defects in zebrafish larvae, which were reversed by dusp14 mRNA treatment. Moreover, knockdown of dusp14 gene caused a significant decrease in the number of neuromasts and hair cells in both neuromast and otic vesicle, mainly due to the inhibition of the proliferation of supporting cells, which results in a decrease in the number of supporting cells and ultimately in the regeneration of hair cells. We further found significant changes in a series of MAPK pathway genes through transcriptome sequencing analysis of dusp14-deficient zebrafish, especially mapk12b gene in p38 signaling. Additionally, inhibiting p38 signaling effectively rescued all phenotypes caused by dusp14 deficiency, including hair cell and supporting cell reduction. These results suggest that DUSP14 might be a key gene to regulate supporting cell development and hair cell regeneration and is a potential target for the treatment of hearing loss.

Achievements and Challenges in Transplantation of Mesenchymal Stem Cells in Otorhinolaryngology

Otorhinolaryngology enrolls head and neck surgery in various tissues such as ear, nose, and throat (ENT) that govern different activities such as hearing, breathing, smelling, production of vocal sounds, the balance, deglutition, facial animation, air filtration and humidification, and articulation during speech, while absence of these functions can lead to high morbidity and even mortality. Conventional therapies for head and neck damaged tissues include grafts, transplants, and artificial materials, but grafts have limited availability and cause morbidity in the donor site. To improve these limitations, regenerative medicine, as a novel and rapidly growing field, has opened a new therapeutic window in otorhinolaryngology by using cell transplantation to target the healing and replacement of injured tissues. There is a high risk of rejection and tumor formation for transplantation of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs); mesenchymal stem cells (MSCs) lack these drawbacks. They have easy expansion and antiapoptotic properties with a wide range of healing and aesthetic functions that make them a novel candidate in otorhinolaryngology for craniofacial defects and diseases and hold immense promise for bone tissue healing; even the tissue sources and types of MSCs, the method of cell introduction and their preparation quality can influence the final outcome in the injured tissue. In this review, we demonstrated the anti-inflammatory and immunomodulatory properties of MSCs, from different sources, to be safely used for cell-based therapies in otorhinolaryngology, while their achievements and challenges have been described too.

Selective ablation of cochlear hair cells promotes engraftment of human embryonic stem cell-derived progenitors in the mouse organ of Corti

BACKGROUND

Hearing loss affects 25% of the population at ages 60-69 years. Loss of the hair cells of the inner ear commonly underlies deafness and once lost this cell type cannot spontaneously regenerate in higher vertebrates. As a result, there is a need for the development of regenerative strategies to replace hair cells once lost. Stem cell-based therapies are one such strategy and offer promise for cell replacement in a variety of tissues. A number of investigators have previously demonstrated successful implantation, and certain level of regeneration of hair and supporting cells in both avian and mammalian models using rodent pluripotent stem cells. However, the ability of human stem cells to engraft and generate differentiated cell types in the inner ear is not well understood.

METHODS

We differentiate human pluripotent stem cells to the pre-placodal stage in vitro then transplant them into the mouse cochlea after selective and complete lesioning of the endogenous population of hair cells.

RESULTS

We demonstrate that hair cell ablation prior to transplantation leads to increased engraftment in the auditory sensory epithelium, the organ of Corti, as well as differentiation of transplanted cells into hair and supporting cell immunophenotypes.

CONCLUSION

We have demonstrated the feasibility of human stem cell engraftment into an ablated mouse organ of Corti.

A human induced pluripotent stem cell-based modular platform to challenge sensorineural hearing loss

The sense of hearing depends on a specialized sensory organ in the inner ear, called the cochlea, which contains the auditory hair cells (HCs). Noise trauma, infections, genetic factors, side effects of ototoxic drugs (ie, some antibiotics and chemotherapeutics), or simply aging lead to the loss of HCs and their associated primary neurons. This results in irreversible sensorineural hearing loss (SNHL) as in mammals, including humans; the inner ear lacks the capacity to regenerate HCs and spiral ganglion neurons. SNHL is a major global health problem affecting millions of people worldwide and provides a growing concern in the aging population. To date, treatment options are limited to hearing aids and cochlear implants. A major bottleneck for development of new therapies for SNHL is associated to the lack of human otic cell bioassays. Human induced pluripotent stem cells (hiPSCs) can be induced in two-dimensional and three-dimensional otic cells in vitro models that can generate inner ear progenitors and sensory HCs and could be a promising preclinical platform from which to work toward restoring SNHL. We review the potential applications of hiPSCs in the various biological approaches, including disease modeling, bioengineering, drug testing, and autologous stem cell based-cell therapy, that offer opportunities to understand the pathogenic mechanisms of SNHL and identify novel therapeutic strategies.

Understanding and treating paediatric hearing impairment

Sensorineural hearing impairment is the most frequent form of hearing impairment affecting 1-2 in 1000 newborns and another 1 in 1000 adolescents. More than 50% of congenital hearing impairment is of genetic origin and some forms of monogenic deafness are likely targets for future gene therapy. Good progress has been made in clinical phenotyping, genetic diagnostics, and counselling. Disease modelling, e.g. in transgenic mice, has helped elucidate disease mechanisms underlying genetic hearing impairment and informed clinical phenotyping in recent years. Clinical management of paediatric hearing impairment involves hearing aids, cochlear or brainstem implants, signal-to-noise improvement in educational settings, speech therapy, and sign language. Cochlear implants, for example, have much improved the situation of profoundly hearing impaired and deaf children. Nonetheless there remains a major unmet clinical need for improving hearing restoration. Preclinical studies promise that we will witness clinical trials on gene therapy and a next generation of cochlear implants during the coming decade. Moreover, progress in generating sensory hair cells and neurons from stem cells spurs disease modelling, drug screening, and regenerative approaches. This review briefly summarizes the pathophysiology of paediatric hearing impairment and provides an update on the current preclinical development of innovative approaches toward improved hearing restoration.

miR‑125/CDK2 axis in cochlear progenitor cell proliferation

Hearing loss ranks fourth among the principal causes of disability worldwide, and manipulation of progenitor cells may be a key strategy for hair cell regeneration. The present study investigated the role and mechanism of miR‑125 on the proliferation of cochlear progenitor cells (CPCs). CPCs were isolated from the cochleae of neonatal rats, and their morphology was observed. Furthermore, the differentiation ability of CPCs was determined by assessing the expression of 5‑bromodeoxyuridine (BrdU), nestin and myosin VII by immunofluorescence. The expression levels of miR‑125 and cyclin‑dependent kinase 2 (CDK2) as well as the cell proliferation of CPCs were assessed. In addition, following gain‑ and loss‑of‑function assays, the cell cycle was examined by flow cytometry, and the expression levels of miR‑125, CDK2, proliferating cell nuclear antigen (PCNA) and nestin were determined by reverse transcription‑quantitative PCR and western blotting. The binding sites between miR‑125 and CDK2 were predicted by TargetScan and identified by the dual luciferase reporter assay. The results demonstrated that different types of progenitor spheres were observed from CPCs with positive expression of BrdU, nestin and myosin VII. Following in vitro incubation for 2, 4 and 7 days, the spheres were enlarged, and CPC proliferation gradually increased and reached a plateau after further incubation for 3 days. Furthermore, the expression levels of nestin and PCNA in CPCs increased and then decreased during in vitro incubation for 2, 4 and 7 days. Following this incubation, the expression levels of miR‑125 in CPCs decreased; thereafter, its expression increased, and the expression pattern was different from that of CDK2. In addition, miR‑125 overexpression in CPCs decreased the expression of CDK2 and the number of cells in the S phase. Different expression patterns were found in CPCs in response to the miR‑125 knockdown. In addition, miR‑125 directly targeted CDK2. Simultaneous knockdown of miR‑125 and CDK2 enhanced CPC proliferation compared with CDK2 knockdown alone. Taken together, the findings from the present study suggested that miR‑125 may inhibit CPC proliferation by downregulating CDK2. The present study may provide a novel therapeutic direction for treatment of hearing loss.

Stem Cell-Based Therapeutic Approaches to Restore Sensorineural Hearing Loss in Mammals

The hair cells that reside in the cochlear sensory epithelium are the fundamental sensory structures responsible for understanding the mechanical sound waves evoked in the environment. The intense damage to these sensory structures may result in permanent hearing loss. The present strategies to rehabilitate the hearing function include either hearing aids or cochlear implants that may recover the hearing capability of deaf patients to a limited extent. Therefore, much attention has been paid on developing regenerative therapies to regenerate/replace the lost hair cells to treat the damaged cochlear sensory epithelium. The stem cell therapy is a promising approach to develop the functional hair cells and neuronal cells from endogenous and exogenous stem cell pool to recover hearing loss. In this review, we specifically discuss the potential of different kinds of stem cells that hold the potential to restore sensorineural hearing loss in mammals and comprehensively explain the current therapeutic applications of stem cells in both the human and mouse inner ear to regenerate/replace the lost hair cells and spiral ganglion neurons.

Emerging approaches for restoration of hearing and vision

Impairments of vision and hearing are highly prevalent conditions limiting the quality of life and presenting a major socioeconomic burden. For long, retinal and cochlear disorders have remained intractable for causal therapies, with sensory rehabilitation limited to glasses, hearing aids, and electrical cochlear or retinal implants. Recently, the application of gene therapy and optogenetics to eye and ear has generated hope for a fundamental improvement of vision and hearing restoration. To date, one gene therapy for the restoration of vision has been approved and undergoing clinical trials will broaden its application including gene replacement, genome editing, and regenerative approaches. Moreover, optogenetics, i.e. controlling the activity of cells by light, offers a more general alternative strategy. Over little more than a decade, optogenetic approaches have been developed and applied to better understand the function of biological systems, while protein engineers have identified and designed new opsin variants with desired physiological features. Considering potential clinical applications of optogenetics, the spotlight is on the sensory systems. Multiple efforts have been undertaken to restore lost or hampered function in eye and ear. Optogenetic stimulation promises to overcome fundamental shortcomings of electrical stimulation, namely poor spatial resolution and cellular specificity, and accordingly to deliver more detailed sensory information. This review aims at providing a comprehensive reference on current gene therapeutic and optogenetic research relevant to the restoration of hearing and vision. We will introduce gene-therapeutic approaches and discuss the biotechnological and optoelectronic aspects of optogenetic hearing and vision restoration.

Targeting dysregulation of redox homeostasis in noise-induced hearing loss: Oxidative stress and ROS signaling

Hearing loss caused by exposure to recreational and occupational noise remains a worldwide disabling condition and dysregulation of redox homeostasis is the hallmark of cochlear damage induced by noise exposure. In this review we discuss the dual function of ROS to both promote cell damage (oxidative stress) and cell adaptive responses (ROS signaling) in the cochlea undergoing a stressful condition such as noise exposure. We focus on animal models of noise-induced hearing loss (NIHL) and on the function of exogenous antioxidants to maintaining a physiological role of ROS signaling by distinguishing the effect of exogenous "direct" antioxidants (i.e. CoQ₁₀, NAC), that react with ROS to decrease oxidative stress, from the exogenous "indirect" antioxidants (i.e. nutraceutics and phenolic compounds) that can activate cellular redox enzymes through the Keap1-Nrf2-ARE pathway. The anti-inflammatory properties of Nrf2 signaling are discussed in relation to the ROS/inflammation interplay in noise exposure. Unveiling the mechanisms of ROS regulating redox-associated signaling pathways is essential in providing relevant targets for innovative and effective therapeutic strategies against NIHL.