Genetic and pharmacological intervention for treatment/prevention of hearing loss

The effect of photobiomodulation on hearing loss: A systematic review

OBJECTIVES

To assess outcomes associated with photobiomodulation therapy (PBMT) for hearing loss in human and animal studies.

DESIGN

Systematic review and narrative synthesis in accordance with PRISMA guidelines.

SETTING

Data bases searched: MEDLINE, EMBASE, CENTRAL, ClinicalTrials.gov and Web of Science. No limits were placed on language or year of publication. Review conducted in accordance with the PRISMA 2020 statement.

PARTICIPANTS

All human and animal subjects treated with PBMT for hearing loss.

MAIN OUTCOME MEASURES

Pre- and post-PBMT audio metric outcomes.

RESULTS

Searches identified 122 abstracts and 49 full text articles. Of these, 17 studies met the inclusion criteria, reporting outcomes in 327 animals (11 studies), 30 humans (1 study), and 40 animal specimens (5 studies). PBMT parameters included 6 different wavelengths: 908 nm (1 study), 810 nm (1 study), 532 & 635 nm (1 study), 830 nm (3 studies), 808 nm (11 studies). The duration ranged from 4 to 60 minutes in a session, and the follow-up ranged from 5-28 days. Outcomes improved significantly when wavelengths within the range of 800-830 nm were used, and with greater duration of PBMT exposure. Included studies predominantly consisted of non-randomized controlled trials (10 studies).

CONCLUSIONS

Hearing outcomes following PBMT appear to be superior to no PBMT for subjects with hearing loss, although higher level evidence is required to verify this. PBMT enables concentrated, focused delivery of light therapy to the inner ear through a non-invasive manner with minimal side effects. As a result of heterogeneity in reporting PBMT parameters and outcomes across the included studies, direct comparison is challenging.

Aminoglycosides-Related Ototoxicity: Mechanisms, Risk Factors, and Prevention in Pediatric Patients

Aminoglycosides are broad-spectrum antibiotics largely used in children, but they have potential toxic side effects, including ototoxicity. Ototoxicity from aminoglycosides is permanent and is a consequence of its action on the inner ear cells via multiple mechanisms. Both uncontrollable risk factors and controllable risk factors are involved in the pathogenesis of aminoglycoside-related ototoxicity and, because of the irreversibility of ototoxicity, an important undertaking for preventing ototoxicity includes antibiotic stewardship to limit the use of aminoglycosides. Aminoglycosides are fundamental in the treatment of numerous infectious conditions at neonatal and pediatric age. In childhood, normal auditory function ensures adequate neurocognitive and social development. Hearing damage from aminoglycosides can therefore strongly affect the normal growth of the child. This review describes the molecular mechanisms of aminoglycoside-related ototoxicity and analyzes the risk factors and the potential otoprotective strategies in pediatric patients.

Limited Link of Common Blood Parameters with Tinnitus

Background: Tinnitus severity is generally assessed by psychometric and audiological instruments. However, no objective measure exists to evaluate the subjective discomfort and suffering caused by this hearing phenomenon. The objective of this work was to determine the possible blood parameters for diagnostics and therapy. Methods: We measured tinnitus distress by using the Tinnitus Questionnaire (TQ) and collected tinnitus-related audiological measures, namely the hearing threshold (HT), tinnitus loudness (TL), and sensation level (SL, i.e., the tinnitus loudness/hearing threshold at a tinnitus frequency). Blood samples were taken from 200 outpatients of the Tinnitus Centre of the Charité, and 46 routine blood count parameters were examined. The possible interactions were determined by (robust) linear models. Results: Tinnitus distress and audiological measurements were largely uncorrelated but could partly be predicted by selected blood parameters. First, the erythrocyte counts predicted tinnitus distress to a small extent. Second, the levels of vitamin D3 explained about 6% of tinnitus loudness and, age-dependently, the hearing threshold variability. Last, the levels of uric acid explained about 5% of the sensation level variability. Conclusions: Tinnitus is a multidimensional phenomenon. The marginal influences of blood markers suggest the possible roles of inflammation and oxidative stress produced by psychological or somatic burdens. Clinically, a vitamin D substitution (in older patients) might have a hearing-protective effect.

A new active peptide from Neptunea arthritica cumingii exerts protective effects against gentamicin-induced sensory-hair cell injury in zebrafish

Gentamicin is commonly used for effective treatment of severe Gram-negative bacterial infections. However, its use is being increasingly restricted owing to the ototoxic effects attributed to it. Gentamicin-induced ototoxicity is thought to be related with apoptosis induced by reactive oxygen species (ROS). In this study, we found a novel active peptide from Neptunea arthritica cumingii with otoprotective effects and no significant embryotoxic effects. The combined application of gentamicin and this novel active peptide helped sensory-hair cells to protect themselves from lethal ROS accumulation. This, in turn, reduced the expression of three genes (caspase-3, caspase-9, Bax), and thereby, the sensory-hair cell apoptosis promoted by ROS accumulation upon gentamicin administration. Our findings provided new insights into the prevention of gentamicin-induced hearing loss.

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.

Fenofibrate exerts protective effects against gentamicin-induced toxicity in cochlear hair cells by activating antioxidant enzymes

Fenofibrate, an activator of peroxisome proliferator-activated receptors (PPARs), has been shown to protect the kidneys and brain cells from oxidative stress; however, its role in preventing hearing loss has not been reported to date, at least to the best of our knowledge. In this study, we demonstrated the protective effects of fenofibrate against gentamicin (GM)-induced ototoxicity. We found that the auditory brainstem response threshold which was increased by GM was significantly reduced by pre-treatment with fenofibrate in rats. In cochlear explants, the disruption of hair cell layers by GM was also markedly attenuated by pre-treatment with fenofibrate. In addition, fenofibrate almost completely abolished GM-induced reactive oxygen species generation, which seemed to be mediated at least in part by the restoration of the expression of PPAR-α-dependent antioxidant enzymes, including catalase and superoxide dismutase (SOD)-1. Of note, fenofibrate markedly increased the expression of heme oxygenase-1 (HO-1) which was also induced to a certain degree by GM alone. The induced expression of HO-1 by fenofibrate appeared to be essential for mediating the protective effects of fenofibrate, as the inhibition of HO-1 activity significantly diminished the protective effects of fenofibrate against the GM-mediated death of sensory hair cells in cochlea explant culture, as well as in zebrafish neuromasts. These results suggest that fenofibrate protects sensory hair cells from GM-induced toxicity by upregulating PPAR-α-dependent antioxidant enzymes, including HO-1. Our results provide insight into the preventive therapy for hearing loss caused by aminoglycoside antibiotics.

Functional development of mechanosensitive hair cells in stem cell-derived organoids parallels native vestibular hair cells

Inner ear sensory epithelia contain mechanosensitive hair cells that transmit information to the brain through innervation with bipolar neurons. Mammalian hair cells do not regenerate and are limited in number. Here we investigate the potential to generate mechanosensitive hair cells from mouse embryonic stem cells in a three-dimensional (3D) culture system. The system faithfully recapitulates mouse inner ear induction followed by self-guided development into organoids that morphologically resemble inner ear vestibular organs. We find that organoid hair cells acquire mechanosensitivity equivalent to functionally mature hair cells in postnatal mice. The organoid hair cells also progress through a similar dynamic developmental pattern of ion channel expression, reminiscent of two subtypes of native vestibular hair cells. We conclude that our 3D culture system can generate large numbers of fully functional sensory cells which could be used to investigate mechanisms of inner ear development and disease as well as regenerative mechanisms for inner ear repair.

Sensory hair cells from the mammalian inner ear do not regenerate. Here, the authors induce direct hair cell formation from mouse embryonic stem cells using a three-dimensional culture system and observe differentiation of Type I and Type II vestibular hair cells and establishment of synapses with neurons.

Transplantation and survival of mouse inner ear progenitor/stem cells in the organ of Corti after cochleostomy of hearing-impaired guinea pigs: preliminary results

In mammals, damage to sensory receptor cells (hair cells) of the inner ear results in permanent sensorineural hearing loss. Here, we investigated whether postnatal mouse inner ear progenitor/stem cells (mIESCs) are viable after transplantation into the basal turns of neomycin-injured guinea pig cochleas. We also examined the effects of mIESC transplantation on auditory functions. Eight adult female Cavia porcellus guinea pigs (250-350g) were deafened by intratympanic neomycin delivery. After 7 days, the animals were randomly divided in two groups. The study group (n=4) received transplantation of LacZ-positive mIESCs in culture medium into the scala tympani. The control group (n=4) received culture medium only. At 2 weeks after transplantation, functional analyses were performed by auditory brainstem response measurement, and the animals were sacrificed. The presence of mIESCs was evaluated by immunohistochemistry of sections of the cochlea from the study group. Non-parametric tests were used for statistical analysis of the data. Intratympanic neomycin delivery damaged hair cells and increased auditory thresholds prior to cell transplantation. There were no significant differences between auditory brainstem thresholds before and after transplantation in individual guinea pigs. Some mIESCs were observed in all scalae of the basal turns of the injured cochleas, and a proportion of these cells expressed the hair cell marker myosin VIIa. Some transplanted mIESCs engrafted in the cochlear basilar membrane. Our study demonstrates that transplanted cells survived and engrafted in the organ of Corti after cochleostomy.

Minocycline protection of neomycin induced hearing loss in gerbils

This animal study was designed to determine if minocycline ameliorates cochlear damage is caused by intratympanic injection of the ototoxic aminoglycoside antibiotic neomycin. Baseline auditory-evoked brainstem responses were measured in gerbils that received 40 mM intratympanic neomycin either with 0, 1.2, or 1.5 mg/kg intraperitoneal minocycline. Four weeks later auditory-evoked brainstem responses were measured and compared to the baseline measurements. Minocycline treatments of 1.2 mg/kg and 1.5 mg/kg resulted in significantly lower threshold increases compared to 0 mg/kg, indicating protection of hearing loss between 6 kHz and 19 kHz. Cochleae were processed for histology and sectioned to allow quantification of the spiral ganglion neurons and histological evaluation of organ of Corti. Significant reduction of spiral ganglion neuron density was demonstrated in animals that did not receive minocycline, indicating that those receiving minocycline demonstrated enhanced survival of spiral ganglion neurons, enhanced survival of sensory hairs cells and spiral ganglion neurons, and reduced hearing threshold elevation correlates with minocycline treatment demonstrating that neomycin induced hearing loss can be reduced by the simultaneous application of minocycline.

Morphological and morphometric characterization of direct transdifferentiation of support cells into hair cells in ototoxin-exposed neonatal utricular explants

We have studied aminoglycoside-induced vestibular hair-cell renewal using long-term culture of utricular macula explants from 4-day-old rats. Explanted utricles were exposed to 1 mM of gentamicin for 48 h, during 2nd and 3rd days in vitro (DIV), and then recovering in unsupplemented medium. Utricles were harvested at specified time points from the 2nd through the 28th DIV. The cellular events that occurred within hair cell epithelia during the culture period were documented from serial sectioned specimens. Vestibular hair cells (HCs) and supporting cells (SCs) were systematically counted using light microscopy (LM) with the assistance of morphometric software. Ultrastructural observations were made from selected specimens with transmission electron microscopy (TEM). After 7 DIV, i.e. four days after gentamicin exposure, the density of HCs was 11% of the number of HCs observed in non-gentamicin-exposed control explants. At 28 DIV the HC density was 61% of the number of HCs observed in the control group explant specimens. Simultaneously with this increase in HCs there was a corresponding decline in the number of SCs within the epithelium. The proportion of HCs in relation to SCs increased significantly in the gentamicin-exposed explant group during the 5th to the 28th DIV period of culture. There were no significant differences in the volume estimations of the gentamicin-exposed and the control group explants during the observed period of culture. Morphological observations showed that gentamicin exposure induced extensive loss of HCs within the epithelial layer, which retained their intact apical and basal linings. At 7 to 14 DIV (i.e. 3-11 days after gentamicin exposure) a pseudostratified epithelium with multiple layers of disorganized cells was observed. At 21 DIV new HCs were observed that also possessed features resembling SCs. After 28 DIV a new luminal layer of HCs with several layers of SCs located more basally characterized the gentamicin-exposed epithelium. No mitoses were observed within the epithelial layer of any explants. Our conclusion is that direct transdifferentiation of SCs into HCs was the only process contributing to the renewal of HCs after gentamicin exposure in these explants of vestibular inner ear epithelia obtained from the labyrinths of 4-day-old rats.

Neurogenesis of the spiral ganglion cells in the cochlea requires the transcriptional cofactor TIS21

The molecular mechanisms controlling the proliferation and differentiation of spiral ganglion cells (SGCs) in the inner ear are still largely unknown. TIS21 is a transcriptional cofactor that shows antiproliferative, antiapoptotic, and prodifferentiative effects on neural progenitor cells. To investigate the function of TIS21 during SGC development, we analyzed SGC neurogenesis from embryonic day 13.5 (E13.5) to postnatal day 4 (P4) in Tis21-GFP knock-in mice, in which the protein-encoding exon of the Tis21 gene was replaced by EGFP. Through E13.5 to P4, we found fewer SGCs in homozygous Tis21-GFP knock-in mice than in wild-type mice. Our results suggest that TIS21 is required for development of SGCs. Deleting Tis21 may affect progenitor cells or neuroblasts at the beginning of cochlear-vestibular ganglion formation and would consequently lead to a decrease in the number of SGCs.

Mild maternal iron deficiency anemia induces DPOAE suppression and cochlear hair cell apoptosis by caspase activation in young guinea pigs

Iron deficiency (ID) anemia (IDA) alters auditory neural normal development in the mammalian cochlea. Previous results suggest that mild maternal IDA during pregnancy and lactation altered the hearing and nervous system development of the young offspring, but the mechanisms underlying the association are incompletely understood. The objective of this study was to evaluate the role of apoptosis in the development of sensory hair cells following mild maternal IDA during pregnancy and lactation. We established a maternal anemia model in female guinea pigs by using a mild iron deficient diet. The offspring were weaned on postnatal day (PND) 9 and then was given the iron sufficient diet. Maternal blood samples were collected on gestational day (GD) 21, GD 42, GD 63 and PND 9, serum level of iron (SI) or hemoglobin (Hb) was measured. Blood samples of pups were collected on PND 9 for SI measurement. On PND 24, pups were examined the distortion product otoacoustic emission (DPOAE) task, and then the cochleae were harvested for assessment of apoptosis by immunohistochemistry of cysteine-aspartic acid proteases 3/9 (caspase-3/9) and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assay, and by double immunofluorescence for the colocalization of TUNEL and caspase-3. Blood samples of pups were collected on PND 24 for SI and Hb measurements. Here we show that mild maternal IDA during pregnancy and lactation resulted in hearing impairment, decreased hair cell number, caspase-3/9 activation and increased apoptotic cell number of young guinea pigs. These results indicate a key role for apoptosis in inhibition of hair cell development, caused by mild maternal IDA during pregnancy and lactation.

Tinnitus

Tinnitus is a common medical symptom that can be debilitating. Risk factors include hearing loss, ototoxic medication, head injury, and depression. At presentation, the possibilities of otological disease, anxiety, and depression should be considered. No effective drug treatments are available, although much research is underway into mechanisms and possible treatments. Surgical intervention for any otological pathology associated with tinnitus might be effective for that condition, but the tinnitus can persist. Available treatments include hearing aids when hearing loss is identified (even mild or unilateral), wide-band sound therapy, and counselling. Cognitive behavioural therapy (CBT) is indicated for some patients, but availability of tinnitus-specific CBT in the UK is poor. The evidence base is strongest for a combination of sound therapy and CBT-based counselling, although clinical trials are constrained by the heterogeneity of patients with tinnitus.

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.

Biotechnology in the treatment of sensorineural hearing loss: foundations and future of hair cell regeneration

PURPOSE

To provide an overview of the methodologies involved in the field of hair cell regeneration. First, the author provides a tutorial on the biotechnological foundations of this field to assist the reader in the comprehension and interpretation of the research involved in hair cell regeneration. Next, the author presents a review of stem cell and gene therapy and provides a critical appraisal of their application to hair cell regeneration. The methodologies used in these approaches are highlighted.

METHOD

The author conducted a narrative review of the fields of cellular, molecular, and developmental biology, tissue engineering, and stem cell and gene therapy using the PubMed database.

RESULTS

The use of biotechnological approaches to the treatment of hearing loss--approaches such as stem cell and gene therapy-has led to new methods of regenerating cochlear hair cells in mammals.

CONCLUSIONS

Incredible strides have been made in assembling important pieces of the puzzle that comprise hair cell regeneration. However, mammalian hair cell regeneration using stem cell and gene therapy are years--if not decades--away from being clinically feasible. If the goals of the biological approaches are met, these therapies may represent future treatments for hearing loss.

Morphological and morphometric characteristics of vestibular hair cells and support cells in long term cultures of rat utricle explants

A method for long term culture of utricular macula explants is demonstrated to be stable and reproducible over a period of 28 days in vitro (DIV). This culture system for four-day-old rat utricular maculae is potentially suitable for studies of hair cell loss, repair and regeneration processes as they occur in post-natal mammalian inner ear sensory epithelia. The cellular events that occur within utricular macula hair cell epithelia during 28 days of culture are documented from serial sections. Vestibular hair cells (HCs) and supporting cells (SCs) were systematically counted using light microscopy (LM) and the assistance of morphometric computer software. Ultrastructural observations were made with transmission electron microscopy (TEM) for describing the changes in the fine detailed morphological characteristics that occurred in the explants related to time in vitro. After 2 DIV the density of HCs was 77%, at 21 DIV it was 69%, and at 28 DIV it was 52% of HCs present at explantation. Between 2 DIV and 28 DIV there was a 1.7% decrease of the vestibular macula HC density per DIV. The corresponding decrease of SC density within the utricular explants was less than 1% per DIV. The overall morphology of the epithelia, i.e. relationship of HCs to SCs, was well preserved during the first two weeks in culture. After this time a slight deterioration of the epithelia was observed and although type I and type II HCs were identified by TEM observations, these two HC types could no longer be distinguished from one another by LM observations. In preparations cultured for 21 DIV, SC nuclei were located more apical and further away from the basal membrane compared to their position in macula explants fixed immediately after dissection. The loss of cells that occurred was probably due to expulsion from the apical (i.e. luminal) surface of the sensory epithelia, but no lesions of the apical lining or ruptures of the basal membrane were observed. There were no significant changes in the volume of the vestibular HC comprising macular epithelium during the observation period of 28 DIV.

Towards gene therapy for deafness

Many hearing disorders are associated with the damage or loss of sensory hair cells (HC) which can produce a profound and irreversible deafness. Apoptosis pathway is reported to play an important role leading to rapid expansion of the HC lesion after exposure to intense noise. Furthermore, progress made over the last year in understanding molecular mechanisms involved in the proliferative and regenerative capacity of sensory cells in the mammalian inner ear has raised the possibility that targeted therapies might prevent the loss of these cells and preserve the patient's hearing. A first step towards the successful therapeutic exploitation is a better understanding of the different pathways that control survival and proliferation of sensory cells. In this review, we provide an overview of recent findings concerning the possibility to prevent apoptosis in auditory cells. We also show the current knowledge on the molecular mechanisms involved in the potential regenerative behavior of these cells and the progress of gene therapy to prevent deafness noise-induced.

Efficient cochlear gene transfection in guinea-pigs with adeno-associated viral vectors by partial digestion of round window membrane

The auditory portion of the inner ear, the cochlea, is an ideal organ for local gene transfection owing to its relative isolation. Various carriers have been tested for cochlear gene transfection. To date, viral vectors appear to have much higher transfection efficacy than non-viral mechanisms. Among these vectors, recombinant adeno-associated virus (rAAV) vectors have several advantages such as being non-pathogenic and the ability to produce prolonged gene expression in various cell types. However, rAAV vectors cannot pass through the intact round window membrane (RWM), otherwise a very attractive approach to access the human inner ear. In this study, performed in guinea-pigs, we describe a method to increase the permeability of RWM to rAAV vectors by partial digestion with collagenase solution. Elevated delivery of rAAV across the partially digested RWM increased transfection efficacy to a satisfactory level, even though it was still lower than that achieved by direct cochleostomy injection. Functional tests (auditory brainstem responses) showed that this enzymatic manipulation did not cause permanent hearing loss if applied appropriately. Morphological observations suggested that the damage to RWM caused by partial digestion healed within four weeks. Taken together, these findings suggest that partial digestion of the RWM is a safe and effective method for increasing the transfection of cochlear sensory cells with rAAV.

Efficient cochlear gene transfection in guinea-pigs with adeno-associated viral vectors by partial digestion of round window membrane

The auditory portion of the inner ear, the cochlea, is an ideal organ for local gene transfection owing to its relative isolation. Various carriers have been tested for cochlear gene transfection. To date, viral vectors appear to have much higher transfection efficacy than non-viral mechanisms. Among these vectors, recombinant adeno-associated virus (rAAV) vectors have several advantages such as being non-pathogenic and the ability to produce prolonged gene expression in various cell types. However, rAAV vectors cannot pass through the intact round window membrane (RWM), otherwise a very attractive approach to access the human inner ear. In this study, performed in guinea-pigs, we describe a method to increase the permeability of RWM to rAAV vectors by partial digestion with collagenase solution. Elevated delivery of rAAV across the partially digested RWM increased transfection efficacy to a satisfactory level, even though it was still lower than that achieved by direct cochleostomy injection. Functional tests (auditory brainstem responses) showed that this enzymatic manipulation did not cause permanent hearing loss if applied appropriately. Morphological observations suggested that the damage to RWM caused by partial digestion healed within four weeks. Taken together, these findings suggest that partial digestion of the RWM is a safe and effective method for increasing the transfection of cochlear sensory cells with rAAV.

Regeneration of Hair Cells: Making Sense of All the Noise

Hearing loss affects hundreds of millions of people worldwide by dampening or cutting off their auditory connection to the world. Current treatments for sensorineural hearing loss (SNHL) with cochlear implants are not perfect, leaving regenerative medicine as the logical avenue to a perfect cure. Multiple routes to regeneration of damaged hair cells have been proposed and are actively pursued. Each route not only requires a keen understanding of the molecular basis of ear development but also faces the practical limitations of stem cell regulation in the delicate inner ear where topology of cell distribution is essential. Improvements in our molecular understanding of the minimal essential genes necessary for hair cell formation and recent advances in stem cell manipulation, such as seen with inducible pluripotent stem cells (iPSCs) and epidermal neural crest stem cells (EPI-NCSCs), have opened new possibilities to advance research in translational stem cell therapies for individuals with hearing loss. Despite this, more detailed network maps of gene expression are needed, including an appreciation for the roles of microRNAs (miRs), key regulators of transcriptional gene networks. To harness the true potential of stem cells for hair cell regeneration, basic science and clinical medicine must work together to expedite the transition from bench to bedside by elucidating the full mechanisms of inner ear hair cell development, including a focus on the role of miRs, and adapting this knowledge safely and efficiently to stem cell technologies.

Concentration-dependent effect of nerve growth factor on cell fate determination of neural progenitors

Stem cell-based spiral ganglion neuron (SGN) replacement therapy has been proposed to be a promising strategy to restore hearing either via replacing degenerated neurons or by improving the efficacy of cochlear implants which rely on functional neurons. However, lack of suitable donor cells and low survival rate of implanted cells are the major obstacles to successful implementation of therapeutic transplantation. The present study investigated the potential of mouse inner ear statoacoustic ganglion (SAG)-derived neural progenitors (NPs) to differentiate toward SGN-like glutamatergic cells and the influence to cell survival and differentiation when nerve growth factor (NGF) was supplied. We found that SAG-NPs could form neurospheres, proliferate, and differentiate into cells expressing neuronal protein neurofilament and β-III tubulin. NGF affected the cell fate of SAG-NP in a concentration-dependent manner in vitro. Low concentration of NGF (2-5 ng/mL) promoted cell proliferation. Medium concentration of NGF (20-40 ng/mL) stimulated cells to differentiate into bi-polar SGN-like cells expressing glutamatergic proteins. High concentration of NGF (100 ng/mL) could rescue cells from induced apoptosis. In the in vivo study, NGF (100 ng/mL) dramatically enhanced SAG-NP survival rate after implantation into adult mammalian inner ear. This finding raises the possibility to further induce these NPs to differentiate into SGN-like neurons in future in vivo study. In conclusion, given the capability of proliferation and differentiation into SGN-like cells with the supplement of NGF in vitro, SAG-NPs can serve as donor cells in stem cell-based SGN replacement therapy. NGF improved the survival of SAG-NPs not only in vitro but also in vivo.

Hair cell fate decisions in cochlear development and regeneration

The discovery of avian cochlear hair cell regeneration in the late 1980s and the concurrent development of new techniques in molecular and developmental biology generated a renewed interest in understanding the genetic mechanisms that regulate hair cell development in the embryonic avian and mammalian cochlea and regeneration in the mature avian cochlea. Research from many labs has demonstrated that the development of the inner ear utilizes a complex series of genetic signals and pathways to generate the endorgans, specify cell identities, and establish innervation patterns found in the inner ear. Recent studies have shown that the Notch signaling pathway, the Atoh1/Hes signaling cascade, the stem cell marker Sox2, and some of the unconventional myosin motor proteins are utilized to regulate distinct steps in inner ear development. While many of the individual genes involved in these pathways have been identified from studies of mutant and knockout mouse cochleae, the interplay of all these signals into a single systemic program that directs this process needs to be explored. We need to know not only what genes are involved, but understand how their gene products interact with one another in a structural and temporal framework to guide hair cell and supporting cell differentiation and maturation.

Perinatal cisplatin exposure induces cochlear apoptosis in newborn guinea pigs

The objective of this study was to evaluate the role of apoptosis in the development of the newborn cochlear structures and hearing loss caused by prenatal cis-diaminedichloroplatinum (cisplatin) exposure. Pregnant albino guinea pigs were intraperitoneally injected with 1.5 mg/kg body weight cisplatin once a day for seven consecutive days at gestational day (GD) 51 to GD 57. At postnatal day (PND) 14, pups were examined in the distortion product otoacoustic emission (DPOAE) task. The temporal bones were then removed and immunohistochemically stained for caspase 3, using the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method. Cisplatin used during pregnancy could induce hearing loss in newborn and cochlear hair cell apoptosis. In conclusion, apoptosis may play an important role in the development of hearing impairment, caused by perinatal cisplatin exposure.

Anti-apoptotic factor z-Val-Ala-Asp-fluoromethylketone promotes the survival of cochlear hair cells in a mouse model for human deafness

A major challenge in the inner ear research field is to restore hearing loss of both non-genetic and genetic origin. A large effort is being made to protect hair cells from cell death after exposure to noise or drugs that can cause hearing loss. Our research focused on protecting hair cells from cell death occurring in a genetic model for human deafness. POU4F3 is a transcription factor associated with human hearing impairment. Pou4f3 knockout mice (Pou4f3(-/-)) have no cochlear hair cells, resulting in complete deafness. Although the hair cells appear to form properly, they progressively degenerate via apoptosis. In order to rescue the hair cells in the knockout mice, we produced explant cultures from mouse cochleae at an early embryonic stage and treated the cells with z-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), a general caspase inhibitor. Hair cell numbers in the knockout mice treated with z-VAD-fmk were significantly higher than in the untreated mice. We found that the time window that z-VAD-fmk has a protective effect is between E14.5 (P=0.001) to E16.5 (P=0.03), but not after E18.5. The source of the surviving hair cells is not due to proliferation, as measured by 5-bromo-2-deoxyuridine (BrdU) labeling, or to supporting cell transdifferentiation to hair cells, since there was no change in supporting cell numbers. Instead, the survival appears to be a direct effect of the anti-apoptotic agent on the dying hair cells with an early developmental window. These results help towards providing a comprehensive understanding of the molecular mechanisms of hair cell death, which might lead to the development of new therapeutic anti-apoptotic agents to alleviate hereditary hearing loss (HL).

Regulation of cell fate and patterning in the developing mammalian cochlea

PURPOSE OF REVIEW

A significant proportion of hearing loss and deafness is caused by defects in the structure or function of cells within the organ of Corti. Identification of the molecular factors that regulate the development of this structure should provide valuable insights regarding inner ear formation and the signaling pathways that underlie congenital auditory deficits. In addition, targeted modulation of these same factors could be developed as therapies for hair cell regeneration.

RECENT FINDINGS

Results from experiments using transgenic and mutant mice, as well as in-vitro techniques, have identified genes and signaling pathways that are required to either specify unique auditory cell types, such as hair cells or supporting cells, or to generate the highly ordered cellular pattern that is characteristic for the organ of Corti. In particular, the hedgehog and fibroblast growth factor signaling pathways modulate the formation of the progenitor cells that will give rise to the organ of Corti. SRY-box containing gene 2, a transcription factor that is required for the formation of the cochlear progenitor cell population, has paradoxically been shown to also act as an inhibitor of hair cell development. Finally, the motor protein myosin II regulates extension of the organ of Corti and the alignment of hair cells and supporting cells into ordered rows.

SUMMARY

A better understanding of the signaling pathways that direct different aspects of cochlear development, such as specific of cell fates or cellular patterning, offers the potential to identify new pathways or molecules that could be targeted for therapeutic interventions.

Geldanamycin induces production of heat shock protein 70 and partially attenuates ototoxicity caused by gentamicin in the organ of Corti explants

Background

Heat shock protein 70 (HSP70) protects inner ear cells from damage and death induced by e.g. heat or toxins. Benzoquinone ansamycin antibiotic geldanamycin (GA) was demonstrated to induce the expression of HSP70 in various animal cell types. The aim of our study was to investigate whether GA induces HSP70 in the organ of Corti (OC), which contains the auditory sensory cells, and whether GA can protect these cells from toxicity caused by a common aminoglycoside antibiotic gentamicin.

Methods

To address these questions, we used the OC explants isolated from p3-p5 rats. As a read-out, we used RT-PCR, ELISA and immunofluorescence.

Results

We found that GA at the concentration of 2 μM efficiently induced HSP70 expression on mRNA and protein level in the OC explants. Confocal microscopy revealed that HSP70 induced by GA is expressed by hair cells and interdental cells of spiral limbus. Preincubation of explants with 2 μM GA prior to adding gentamicin (500 μM) significantly reduced the loss of outer but not inner hair cells, suggesting different mechanisms of otoprotection needed for these two cell types.

Conclusion

GA induced HSP70 in the auditory sensory cells and partially protected them from toxicity of gentamicin. Understanding the molecular mechanisms of GA otoprotection may provide insights for preventative therapy of the hearing loss caused by aminoglycoside antibiotics.

Metabolic imaging of the organ of corti--a window on cochlea bioenergetics

Hair cell loss is a major cause of sensorineural hearing loss. We have developed a method to examine metabolic events in hair cells in response to stimuli known to cause hair cell loss, such as acoustic trauma and aminoglycoside administration. The method employs two-photon excitation of the metabolic intermediate, reduced nicotinamide adenine dinucleotide (NADH), in hair cell mitochondria in an explanted mouse cochlea. Using this method, we show evidence that the aminoglycoside gentamicin selectively affects the level of mitochondrial NADH in outer hair cells, but not inner hair cells, within minutes of administration.

Feathers and fins: non-mammalian models for hair cell regeneration

Death of mechanosensory cells in the inner ear results in two profound disabilities: hearing loss and balance disorders. Although mammals lack the capacity to regenerate hair cells, recent studies in mice and other rodents have offered valuable insight into strategies for stimulating hair cell regeneration in mammals. Investigations of model organisms that retain the ability to form new hair cells after embryogenesis, such as fish and birds, are equally important and have provided clues as to the cellular and molecular mechanisms that may block hair cell regeneration in mammals. Here, we summarize studies on hair cell regeneration in the chicken and the zebrafish, discuss specific advantages of each model, and propose future directions for the use of non-mammalian models in understanding hair cell regeneration.