Stem cell transplantation via the cochlear lateral wall for replacement of degenerated spiral ganglion neurons

Intratympanic injection of MSC-derived small extracellular vesicles protects spiral ganglion neurons from degeneration

Sensorineural hearing loss is one of the most prevalent sensory deficits. Spiral ganglion neurons (SGNs) exhibit very limited regeneration capacity and their degeneration leads to profound hearing loss. Mesenchymal stem cell-derived small extracellular vesicles (MSC-sEV) have been demonstrated to repair tissue damage in various degenerative diseases. However, the effects of MSC-sEV on SGN degeneration remain unclear. In this study, we investigated the efficacy of MSC-sEV for protection against ouabain-induced SGN degeneration. MSC-sEV were derived from rat bone marrow and their components related to neuron growth were determined by proteomic analysis. In primary culture SGNs, MSC-sEV significantly promoted neurite growth and growth cone development. The RNA-Seq analysis of SGNs showed that enriched pathways include neuron development and axon regeneration, consistent with proteomics. In ouabain induced SGN degeneration rat model, MSC-sEV administration via intratympanic injection significantly enhanced SGN survival and mitigated hearing loss. Furthermore, after ouabain treatment, SGNs displayed evident signs of apoptosis, including nuclei condensation and fragmentation, with numerous cells exhibiting TUNEL-positive. However, administration of MSC-sEV effectively decreased the number of TUNEL-positive cells and reduced caspase-3 activation. In conclusion, our findings demonstrate the potential of MSC-sEV in preventing SGN degeneration and promoting neural growth, suggesting intratympanic injection of MSC-sEV is a specific and efficient strategy for neural hearing loss.

Tissue engineering strategies for spiral ganglion neuron protection and regeneration

Cochlear implants can directly activate the auditory system's primary sensory neurons, the spiral ganglion neurons (SGNs), via circumvention of defective cochlear hair cells. This bypass restores auditory input to the brainstem. SGN loss etiologies are complex, with limited mammalian regeneration. Protecting and revitalizing SGN is critical. Tissue engineering offers a novel therapeutic strategy, utilizing seed cells, biomolecules, and scaffold materials to create a cellular environment and regulate molecular cues. This review encapsulates the spectrum of both human and animal research, collating the factors contributing to SGN loss, the latest advancements in the utilization of exogenous stem cells for auditory nerve repair and preservation, the taxonomy and mechanism of action of standard biomolecules, and the architectural components of scaffold materials tailored for the inner ear. Furthermore, we delineate the potential and benefits of the biohybrid neural interface, an incipient technology in the realm of implantable devices. Nonetheless, tissue engineering requires refined cell selection and differentiation protocols for consistent SGN quality. In addition, strategies to improve stem cell survival, scaffold biocompatibility, and molecular cue timing are essential for biohybrid neural interface integration.

Surface Functional Modification by Ti3 C2 Tx MXene on PLLA Nanofibers for Optimizing Neural Stem Cell Engineering

Optimizing cell substrates by surface modification of neural stem cells (NSCs), for efficient and oriented neurogenesis, represents a promising strategy for treating neurological diseases. However, developing substrates with the advanced surface functionality, conductivity, and biocompatibility required for practical application is still challenging. Here, Ti3 C2 Tx MXene is introduced as a coating nanomaterial for aligned poly(l-lactide) (PLLA) nanofibers (M-ANF) to enhance NSC neurogenesis and simultaneously tailor the cell growth direction. Ti3 C2 Tx MXene treatment provides a superior conductivity substrate with a surface rich in functional groups, hydrophilicity, and roughness, which can provide biochemical and physical cues to support NSC adhesion and proliferation. Moreover, Ti3 C2 Tx MXene coating significantly promotes NSC differentiation into both neurons and astrocytes. Interestingly, Ti3 C2 Tx MXene acts synergistically with the alignment of nanofibers to promote the growth of neurites, indicating enhanced maturation of these neurons. RNA sequencing analysis further reveals the molecular mechanism by which Ti3 C2 Tx MXene modulates the fate of NSCs. Notably, surface modification by Ti3 C2 Tx MXene mitigates the in vivo foreign body response to implanted PLLA nanofibers. This study confirms that Ti3 C2 Tx MXene provides multiple advantages for decorating the aligned PLLA nanofibers to cooperatively improve neural regeneration.

The Protective Effects of Human Embryonic Stem Cell-Derived Mesenchymal Stem Cells in Noise-Induced Hearing Loss of Rats

A few prior animal studies have suggested the transplantation or protective effects of mesenchymal stem cells (MSCs) in noise-induced hearing loss. This study intended to evaluate the fates of administered MSCs in the inner ears and the otoprotective effects of MSCs in the noise-induced hearing loss of rats. Human embryonic stem cell-derived MSCs (ES-MSCs) were systematically administered via the tail vein in adult rats. Eight-week-old Sprague-Dawley rats were randomly allocated to the control (n = 8), ES-MSC (n = 4), noise (n = 8), and ES-MSC+noise (n = 10) groups. In ES-MSC and ES-MSC+noise rats, 5 × 105 ES-MSCs were injected via the tail vein. In noise and ES-MSC+noise rats, broadband noise with 115 dB SPL was exposed for 3 h daily for 5 days. The hearing levels were measured using auditory brainstem response (ABR) at 4, 8, 16, and 32 kHz. Cochlear histology was examined using H&E staining and cochlear whole mount immunofluorescence. The presence of human DNA was examined using Sry PCR, and the presence of human cytoplasmic protein was examined using STEM121 immunofluorescence staining. The protein expression levels of heat shock protein 70 (HSP70), apoptosis-inducing factor (AIF), poly (ADP-ribose) (PAR), PAR polymerase (PARP), caspase 3, and cleaved caspase 3 were estimated. The ES-MSC rats did not show changes in ABR thresholds following the administration of ES-MSCs. The ES-MSC+ noise rats demonstrated lower ABR thresholds at 4, 8, and 16 kHz than the noise rats. Cochlear spiral ganglial cells and outer hair cells were more preserved in the ES-MSC+ noise rats than in the noise rats. The Sry PCR bands were highly detected in lung tissue and less in cochlear tissue of ES-MSC+noise rats. Only a few STEM121-positivities were observed in the spiral ganglial cell area of ES-MSC and ES-MSC+noise rats. The protein levels of AIF, PAR, PARP, caspase 3, and cleaved caspase 3 were lower in the ES-MSC+noise rats than in the noise rats. The systemic injection of ES-MSCs preserved hearing levels and attenuated parthanatos and apoptosis in rats with noise-induced hearing loss. In addition, a tiny number of transplanted ES-MSCs were observed in the spiral ganglial areas.

Regulation of Spiral Ganglion Neuron Regeneration as a Therapeutic Strategy in Sensorineural Hearing Loss

In the mammalian cochlea, spiral ganglion neurons (SGNs) are the primary neurons on the auditory conduction pathway that relay sound signals from the inner ear to the brainstem. However, because the SGNs lack the regeneration ability, degeneration and loss of SGNs cause irreversible sensorineural hearing loss (SNHL). Besides, the effectiveness of cochlear implant therapy, which is the major treatment of SNHL currently, relies on healthy and adequate numbers of intact SGNs. Therefore, it is of great clinical significance to explore how to regenerate the SGNs. In recent years, a number of researches have been performed to improve the SGNs regeneration strategy, and some of them have shown promising results, including the progress of SGN regeneration from exogenous stem cells transplantation and endogenous glial cells’ reprogramming. Yet, there are challenges faced in the effectiveness of SGNs regeneration, the maturation and function of newly generated neurons as well as auditory function recovery. In this review, we describe recent advances in researches in SGNs regeneration. In the coming years, regenerating SGNs in the cochleae should become one of the leading biological strategies to recover hearing loss.

Mechanism and Prevention of Spiral Ganglion Neuron Degeneration in the Cochlea

Sensorineural hearing loss (SNHL) is one of the most prevalent sensory deficits in humans, and approximately 360 million people worldwide are affected. The current treatment option for severe to profound hearing loss is cochlear implantation (CI), but its treatment efficacy is related to the survival of spiral ganglion neurons (SGNs). SGNs are the primary sensory neurons, transmitting complex acoustic information from hair cells to second-order sensory neurons in the cochlear nucleus. In mammals, SGNs have very limited regeneration ability, and SGN loss causes irreversible hearing loss. In most cases of SNHL, SGN damage is the dominant pathogenesis, and it could be caused by noise exposure, ototoxic drugs, hereditary defects, presbycusis, etc. Tremendous efforts have been made to identify novel treatments to prevent or reverse the damage to SGNs, including gene therapy and stem cell therapy. This review summarizes the major causes and the corresponding mechanisms of SGN loss and the current protection strategies, especially gene therapy and stem cell therapy, to promote the development of new therapeutic methods.

Stem Cell-Based Therapies in Hearing Loss

In recent years, neural stem cell transplantation has received widespread attention as a new treatment method for supplementing specific cells damaged by disease, such as neurodegenerative diseases. A number of studies have proved that the transplantation of neural stem cells in multiple organs has an important therapeutic effect on activation and regeneration of cells, and restore damaged neurons. This article describes the methods for inducing the differentiation of endogenous and exogenous stem cells, the implantation operation and regulation of exogenous stem cells after implanted into the inner ear, and it elaborates the relevant signal pathways of stem cells in the inner ear, as well as the clinical application of various new materials. At present, stem cell therapy still has limitations, but the role of this technology in the treatment of hearing diseases has been widely recognized. With the development of related research, stem cell therapy will play a greater role in the treatment of diseases related to the inner ear.

Cochlear implant-based electric-acoustic stimulation modulates neural stem cell-derived neural regeneration

Cochlear implantation is considered to be the best therapeutic method for profound sensorineural hearing loss, but insufficient numbers of functional spiral ganglion neurons hinder the clinical effects of cochlear implantation. Stem cell transplantation has the potential to provide novel strategies for spiral ganglion neuron regeneration after injury. However, some obstacles still need to be overcome, such as low survival and uncontrolled differentiation. Several novel technologies show promise for modulating neural stem cell behaviors to address these issues. Here, a device capable of electrical stimulation was designed by combining a cochlear implant with a graphene substrate. Neural stem cells (NSCs) were cultured on the graphene substrate and subjected to electrical stimulation transduced from sound waves detected by the cochlear implant. Cell behaviors were studied, and this device showed good biocompatibility for NSCs. More importantly, electric-acoustic stimulation with higher frequencies and amplitudes induced NSC death and apoptosis, and electric-acoustic stimulation could promote NSCs to proliferate and differentiate into neurons only when low-frequency stimulation was supplied. The present study provides experimental evidence for understanding the regulatory role of electric-acoustic stimulation on NSCs and highlights the potentials of the above-mentioned device in stem cell therapy for hearing loss treatment.

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.

Biodistribution of poly clustered superparamagnetic iron oxide nanoparticle labeled mesenchymal stem cells in aminoglycoside induced ototoxic mouse model

Recently, application of stem cell therapy in regenerative medicine has become an active field of study. Mesenchymal stem cells (MSCs) are known to have a strong ability for homing. MSCs labeled with superparamagnetic iron oxide nanoparticles (SPIONs) exhibit enhanced homing due to magnetic attraction. We have designed a SPION that has a cluster core of iron oxide-based nanoparticles coated with PLGA-Cy5.5. We optimized the nanoparticles for internalization to enable the transport of PCS nanoparticles through endocytosis into MSCs. The migration of magnetized MSCs with SPION by static magnets was seen in vitro. The auditory hair cells do not regenerate once damaged, ototoxic mouse model was generated by administration of kanamycin and furosemide. SPION labeled MSC's were administered through different injection routes in the ototoxic animal model. As result, the intratympanic administration group with magnet had the highest number of cells in the brain followed by the liver, cochlea, and kidney as compared to those in the control groups. The synthesized PCS (poly clustered superparamagnetic iron oxide) nanoparticles, together with MSCs, by magnetic attraction, could synergistically enhance stem cell delivery. The poly clustered superparamagnetic iron oxide nanoparticle labeled in the mesenchymal stem cells have increased the efficacy of homing of the MSC's to the target area by synergetic effect of magnetic attraction and chemotaxis (SDF-1/CXCR4 axis). This technique allows delivery of the stem cells to the areas with limited vasculatures. The nanoparticle in the biomedicine allows drug delivery, thus, the combination of nanomedicince together with the regenerative medicine will provide highly effective therapy.

Access to the Apical Cochlear Modiolus for Possible Stem Cell-based and Gene Therapy of the Auditory Nerve

OBJECTIVE

Loss of spiral ganglion neurons (SGN) is permanent and responsible for a substantial number of patients suffering from hearing impairment. It can derive from the degeneration of SGNs due to the death of sensory hair cells as well as from auditory neuropathy. Utilizing stem cells to recover lost SGNs increasingly emerges as a possible therapeutic option, but access to human SGNs is difficult due to their protected location within the bony impacted cochlea. Aim of this study was to establish a reliable and practicable approach to access SGNs in the human temporal bone for possible stem cell and gene therapies.

METHODS

In seven human temporal bone specimen a transcanal approach was used to carefully drill a cochleostomy in the lateral second turn followed by insertion of a tungsten needle into the apical modiolus to indicate the spot for intramodiolar injections. Subsequent cone beam computed tomography (CBCT) served as evaluation for positioning of the marker and cochleostomy size.

RESULTS

The apical modiolus could be exposed in all cases by a cochleostomy (1.6 mm2, standard deviation ±0.23 mm2) in the lateral second turn. 3D reconstructions and analysis of CBCT revealed reliable positioning of the marker in the apical modiolus, deviating on average 0.9 mm (standard deviation ±0.49 mm) from the targeted center of the second cochlear turn.

CONCLUSION

We established a reliable, minimally invasive, transcanal surgical approach to the apical cochlear modiolus in the human temporal bone in foresight to stem cell-based and gene therapy of the auditory nerve.

Cell Transplantation to Restore Lost Auditory Nerve Function is a Realistic Clinical Opportunity

Hearing is one of our most important means of communication. Disabling hearing loss (DHL) is a long-standing, unmet problem in medicine, and in many elderly people, it leads to social isolation, depression, and even dementia. Traditionally, major efforts to cure DHL have focused on hair cells (HCs). However, the auditory nerve is also important because it transmits electrical signals generated by HCs to the brainstem. Its function is critical for the success of cochlear implants as well as for future therapies for HC regeneration. Over the past two decades, cell transplantation has emerged as a promising therapeutic option for restoring lost auditory nerve function, and two independent studies on animal models show that cell transplantation can lead to functional recovery. In this article, we consider the approaches most likely to achieve success in the clinic. We conclude that the structure and biochemical integrity of the auditory nerve is critical and that it is important to preserve the remaining neural scaffold, and in particular the glial scar, for the functional integration of donor cells. To exploit the natural, autologous cell scaffold and to minimize the deleterious effects of surgery, donor cells can be placed relatively easily on the surface of the nerve endoscopically. In this context, the selection of donor cells is a critical issue. Nevertheless, there is now a very realistic possibility for clinical application of cell transplantation for several different types of hearing loss.

Neural Tissue Degeneration in Rosenthal's Canal and Its Impact on Electrical Stimulation of the Auditory Nerve by Cochlear Implants: An Image-Based Modeling Study

Sensorineural deafness is caused by the loss of peripheral neural input to the auditory nerve, which may result from peripheral neural degeneration and/or a loss of inner hair cells. Provided spiral ganglion cells and their central processes are patent, cochlear implants can be used to electrically stimulate the auditory nerve to facilitate hearing in the deaf or severely hard-of-hearing. Neural degeneration is a crucial impediment to the functional success of a cochlear implant. The present, first-of-its-kind two-dimensional finite-element model investigates how the depletion of neural tissues might alter the electrically induced transmembrane potential of spiral ganglion neurons. The study suggests that even as little as 10% of neural tissue degeneration could lead to a disproportionate change in the stimulation profile of the auditory nerve. This result implies that apart from encapsulation layer formation around the cochlear implant electrode, tissue degeneration could also be an essential reason for the apparent inconsistencies in the functionality of cochlear implants.

Long non‑coding RNA EBLN3P promotes the recovery of the function of impaired spiral ganglion neurons by competitively binding to miR‑204‑5p and regulating TMPRSS3 expression

Sensorineural hearing loss (SNHL) is one of the major leading causes of hearing impairment, and is typically characterized by the degeneration of spiral ganglion neurons (SGNs). In previous studies by the authors, it was demonstrated that microRNA (miRNA or miR)-204-5p decreased the viability of SGNs by inhibiting the expression of transmembrane protease, serine 3 (TMPRSS3), which was closely associated with the development of SGNs. However, the upstream regulatory mechanism of miR-204-5p was not fully elucidated. The present study found that an important upstream regulatory factor of miR-204-5p, long non-coding RNA (lncRNA) EBLN3P, was expressed at low levels in impaired SGNs, whereas it was expressed at high levels in normal SGNs. Mechanistic analyses demonstrated that lncRNA EBLN3P functioned as a competing endogenous RNA (ceRNA) when regulating miR-204-5p in normal SGNs. In addition, lncRNA EBLN3P regulated TMPRSS3 expression via the regulation of miR-204-5p in normal SGNs. In vitro functional analysis revealed that lncRNA EBLN3P promoted the recovery of the viability of normal SGNs and inhibited the apoptosis of normal SGNs. Finally, the results revealed a recovery-promoting effect of lncRNA EBLN3P on the structure and function of impaired SGNs in models of deafness. On the whole, the findings of the present study demonstrate that lncRNA EBLN3P promotes the recovery of the function of impaired SGNs by competitively binding to miR-204-5p and regulating TMPRSS3 expression. This suggests that lncRNA EBLN3P may be a potential therapeutic target for diseases involving SNHL.

The biological strategies for hearing re-establishment based on the stem/progenitor cells

The cochlea is the essential organ for hearing and includes both auditory sensory hair cells and spiral ganglion neurons. The discovery of inner ear stem cell brings hope to the regeneration of hair cell and spiral ganglion neuron as well as the followed hearing re-establishment. Thus the investigation on characteristics of inner ear stem/progenitor cells and related regulating clue is important to make such regeneration a reality. In addition, attempts have also been made to transplant exogenous stem cells into the inner ear to restore hearing function. In this review, we describe recent advances in the characterization of mammalian inner ear progenitor/stem cells and the mechanisms of regulating their proliferation and differentiation, and summarize studies that have used exogenous stem cells to repair damaged hair cells and neurons in the inner ear.

Hearing regeneration and regenerative medicine: present and future approaches

More than 5% of the world population lives with a hearing impairment. The main factors responsible for hearing degeneration are ototoxic drugs, aging, continued exposure to excessive noise and infections. The pool of adult stem cells in the inner ear drops dramatically after birth, and therefore an endogenous cellular source for regeneration is absent. Hearing loss can emerge after the degeneration of different cochlear components, so there are multiple targets to be reached, such as hair cells (HCs), spiral ganglion neurons (SGNs), supporting cells (SCs) and ribbon synapses. Important discoveries in the hearing regeneration field have been reported regarding stem cell transplantation, migration and survival; genetic systems for cell fate monitoring; and stem cell differentiation to HCs, SGNs and SCs using adult stem cells, embryonic stem cells and induced pluripotent stem cells. Moreover, some molecular mediators that affect the establishment of functional synapses have been identified. In this review, we will focus on reporting the state of the art in the regenerative medicine field for hearing recovery. Stem cell research has enabled remarkable advances in regeneration, particularly in neuronal cells and synapses. Despite the progress achieved, there are certain issues that need a deeper development to improve the results already obtained, or to develop new approaches aiming for the clinical application.

Transplantation of human limbus-derived mesenchymal stromal cells via occipital approach improves hearing in animal auditory neuropathy

OBJECTIVE

To develop a surgical approach for cell transplantation into mouse cochlear nerves via an intracranial route and investigate whether transplantation of human limbus-derived mesenchymal stromal cells (HL-MSCs) can improve hearing in this model of auditory neuropathy.

METHODS

We used 8-week-old CBA/CaJ male mice and created ouabain-induced auditory neuropathy. The surgical approach passed through the cerebellum to reveal the superior semicircular canal and brainstem, allowing access to the auditory nerve. Then HL-MSCs were injected around the cochlear nerve trunk using a micropipette driven by a micropump. Hearing thresholds in the mice were determined by auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs).

RESULTS

We produced ouabain-induced neuropathy in mice with an elevated hearing threshold but normal DPOAE. Using immunohistological staining, we detected HL-MSCs were localized in the cochlear nerve trunk 2 days after cell transplantation via this occipital approach. More spiral ganglion neurons were detected in ouabain-treated cochleae 3 months after HL-MSCs transplantation compared to those without HL-MSCs transplantation. The ABR showed significant hearing improvement 3 months after HL-MSCs transplantation.

CONCLUSIONS

We successfully established a mouse model for cell transplantation into the intracranial cochlear nerve trunk and showed that HL-MSCs potentially can be applied as cell therapy to treat sensorineural hearing loss.

Stem Cells: A New Hope for Hearing Loss Therapy

Permanent hearing loss was considered which cannot be cured since cochlear hair cells and primary afferent neurons cannot be regenerated. In recent years, due to the in-depth study of stem cell and its therapeutic potential, regenerating auditory sensory cells is made possible. By using two strategies of endogenous stem cell activation and exogenous stem cell transplantation, researchers hope to find methods to restore hearing function. However, there are complex factors that need to be considered in the in vivo application of stem cell therapy, such as stem cell-type choice, signaling pathway regulations, transplantation approaches, internal environment of the cochlea, and external stimulation. After years of investigations, some theoretic progress has been made in the treatment of hearing loss using stem cells, but there are also many problems which limited its application that need to be solved. Understanding the future perspective of stem cell therapy in hearing loss, solving the encountered problems, and promoting its development are the common goals of audiological researchers. In this review, we present critical experimental findings of stem cell therapy on treatment of hearing loss and intend to bring hope to researchers and patients.

The olfactory mucosa: a potential source of stem cells for hearing regeneration

The olfactory mucosa contains cells that enable it to generate new neurons and other supporting cells throughout life, allowing it to replace cells of the mucosa that have been damaged by exposure to various insults. In this article, we discuss the different types of stem cell found within the olfactory mucosa and their properties. In particular, the mesenchymal-like cells found within the lamina propria will be reviewed in detail. In addition, we discuss potential applications of olfactory-derived stem cells toward hearing regeneration secondary to either inner hair cell loss or primary or secondary auditory nerve degeneration.

Ouabain Does Not Induce Selective Spiral Ganglion Cell Degeneration in Guinea Pigs

Round window membrane (RWM) application of ouabain is known to selectively destroy type I spiral ganglion cells (SGCs) in cochleas of several rodent species, while leaving hair cells intact. This protocol has been used in rats and Mongolian gerbils, but observations in the guinea pig are conflicting. This is why we reinvestigated the effect of ouabain on the guinea pig cochlea. Ouabain solutions of different concentrations were placed, in a piece of gelfoam, upon the RWM of the right cochleas. Auditory function was assessed using acoustically evoked auditory brainstem responses (aABR). Finally, cochleas were fixed and processed for histological examination. Due to variability within treatment groups, histological data was pooled and three categories based upon general histological observations were defined: cochleas without outer hair cell (OHC) and SGC loss (Category 1), cochleas with OHC loss only (Category 2), and cochleas with OHC and SGC loss (Category 3). Animals treated with 1 mM or 10 mM ouabain showed shifts in hearing thresholds, corresponding with varying histological changes in their cochleas. Most cochleas exhibited complete outer hair cell loss in the basal and middle turns, while some had no changes, together with either moderate or near-complete loss of SGCs. Neither loss of inner hair cells nor histological changes of the stria vascularis were observed in any of the animals. Cochleas in Category 1 had normal aABRs and morphology. On average, in Category 2 OHC loss was 46.0±5.7%, SGC loss was below threshold, ABR threshold shift was 44.9±2.7 dB, and ABR wave II amplitude was decreased by 17.1±3.8 dB. In Category 3 OHC loss was 68.3±6.9%, SGC loss was 49.4±4.3%, ABR threshold shift was 39.0±2.4 dB, and ABR amplitude was decreased by 15.8±1.6 dB. Our results show that ouabain does not solely destroy type I SGCs in the guinea pig cochlea.

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

Hearing loss in mammals is an irreversible process caused by degeneration of the hair cells of the inner ear. Current therapies for hearing loss include hearing aids and cochlear implants that provide substantial benefits to most patients, but also have several shortcomings. There is great interest in the development of regenerative therapies to treat deafness in the future. Cell-based therapies, based either on adult, multipotent stem, or other types of pluripotent cells, offer promise for generating differentiated cell types to replace lost or damaged hair cells of the inner ear. In this review, we focus on the methods proposed and avenues for research that seem the most promising for stem cell-based auditory sensory cell regeneration, from work collected over the past 15 years.

Advances in translational inner ear stem cell research

Stem cell research is expanding our understanding of developmental biology as well as promising the development of new therapies for a range of different diseases. Within hearing research, the use of stem cells has focused mainly on cell replacement. Stem cells however have a broad range of other potential applications that are just beginning to be explored in the ear. Mesenchymal stem cells are an adult derived stem cell population that have been shown to produce growth factors, modulate the immune system and can differentiate into a wide variety of tissue types. Potential advantages of mesenchymal/adult stem cells are that they have no ethical constraints on their use. However, appropriate regulatory oversight seems necessary in order to protect patients from side effects. Disadvantages may be the lack of efficacy in many preclinical studies. But if proven safe and efficacious, they are easily translatable to clinical trials. The current review will focus on the potential application on mesenchymal stem cells for the treatment of inner ear disorders.

Epithelial-mesenchymal transition, and collective and individual cell migration regulate epithelial changes in the amikacin-damaged organ of Corti

Characterizing the microenvironment of a damaged organ of Corti and identifying the basic mechanisms involved in subsequent epithelial reorganization are critical for improving the outcome of clinical therapies. In this context, we studied the expression of a variety of cell markers related to cell shape, cell adhesion and cell plasticity in the rat organ of Corti poisoned with amikacin. Our results indicate that, after severe outer hair cell losses, the cytoarchitectural reorganization of the organ of Corti implicates epithelial-mesenchymal transition mechanisms and involves both collective and individual cell migratory processes. The results also suggest that both root cells and infiltrated fibroblasts participate in the homeostasis of the damaged epithelium, and that the flat epithelium that may emerge offers biological opportunities for late regenerative therapies.

Genetic Effects on Sensorineural Hearing Loss and Evidence-based Treatment for Sensorineural Hearing Loss

In this article, the mechanism of inheritance behind inherited hearing loss and genetic susceptibility in noise-induced hearing loss are reviewed. Conventional treatments for sensorineural hearing loss (SNHL), i.e. hearing aid and cochlear implant, are effective for some cases, but not without limitations. For example, they provide little benefit for patients of profound SNHL or neural hearing loss, especially when the hearing loss is in poor dynamic range and with low frequency resolution. We emphasize the most recent evidence-based treatment in this field, which includes gene therapy and allotransplantation of stem cells. Their promising results have shown that they might be options of treatment for profound SNHL and neural hearing loss. Although some treatments are still at the experimental stage, it is helpful to be aware of the novel therapies and endeavour to explore the feasibility of their clinical application.

Neurotoxicity of trimethyltin in rat cochlear organotypic cultures

Trimethyltin (TMT), which has a variety of applications in industry and agricultural, is a neurotoxin that is known to affect the auditory system as well as central nervous system of humans and experimental animals. However, the mechanisms underlying TMT-induced auditory dysfunction are poorly understood. To gain insights into the neurotoxic effect of TMT on the peripheral auditory system, we treated cochlear organotypic cultures with concentrations of TMT ranging from 5 to 100 μM for 24 h. Interestingly, TMT preferentially damaged auditory nerve fibers and spiral ganglion neurons in a dose-dependent manner, but had no noticeable effects on the sensory hair cells at the doses employed. TMT-induced damage to auditory neurons was associated with significant soma shrinkage, nuclear condensation, and activation of caspase-3, biomarkers indicative of apoptotic cell death. Our findings show that TMT is exclusively neurotoxicity in rat cochlear organotypic culture and that TMT-induced auditory neuron death occurs through a caspase-mediated apoptotic pathway.

miR-204 suppresses cochlear spiral ganglion neuron survival in vitro by targeting TMPRSS3

Sensorineural hearing loss (SNHL) is the most common cause of hearing impairment. One of the essential steps to prevent progressive hearing loss is to protect spiral ganglion neurons (SGNs) from ongoing degeneration. MicroRNAs and TMPRSS3 (transmembrane protease, serine 3) have been reported to be involved in development of SGNs and genesis of SNHL. The aim of this study was to investigate the role of miR-204 and TMPRSS3 in SGNs. Effect of miR-204 on cell viability of SGNs was first examined using MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay. Expression of TMPRSS3 in SGNs with or without addition of miR-204 was assessed by real-time PCR and western blot further. A luciferase reporter activity assay was conducted to confirm target association between miR-204 and 3'-UTR of TMPRSS3. Finally, role of TMPRSS3 on cell viability of SGNs was evaluated by transfection of TMPRSS3 siRNA. Cell viability of SGNs was suppressed by miR-204 in a concentration-dependent manner. Overexpression of miR-204 reduced expression of TMPRSS3 in SGNs at both mRNA and protein levels. Binding to the 3'-UTR of TMPRSS3 by miR-204 was identified by luciferase assay. Knockdown of TMPRSS3 by siRNA significantly inhibits cell viability of SGNs. miR-204 could be a potential therapeutic target in sensorineural hearing loss.

Generation of induced pluripotent stem cells from neonatal mouse cochlear cells

The sensory epithelium (SE) within the mammalian cochleae has a limited capacity for regeneration, and the loss of mammalian cochlear hair cells always lead to permanent hearing loss. Previous reports show that early postnatal cochlea harbors stem/progenitor-like cells nominated otospheres which have a limited regenerative/repair capacity, while these cell populations are progressively lost during the postnatal development. Induced pluripotent stem cells (iPS cells) directly reprogrammed from non-embryonic cells have captured great attentions in the scientific community. In the present study, we determine whether Yamanaka׳s factors can induce the reprogramming of cochlear cells into iPS cells. We introduce defined factors Oct3/4, Sox2 and Klf4 into otospheres derived from postnatal day-1 (P1) mouse SE, and analyze characteristics alterations in cochlear cells. After transduction, otospheres generated colonies exhibiting a normal karyotype and morphology similar to that of mouse embryonic stem cells (ESCs). Moreover, these cochlear iPS cells also express ESC-like markers. Importantly, the cochlear iPS cells show pluripotency in vitro and in vivo, as evidenced by differentiation into three germ layers by embryoid body formation, as well as high efficient formation of teratomas containing three germ layers in immunodeficient mice. Thus, pluripotent cochlear iPS cells can be generated from cochlear cells by using three Yamanaka׳s transcription factors. These attempts represent the first step toward generating fully pluripotent iPS cells from mammalian cochleae with defined exogenous genes.

Acoustical stimulus changes the expression of stromal cell-derived factor-1 in the spiral ganglion neurons of the rat cochlea

Neural stem cell (NSC) transplantation into the cochlea has been tested as a treatment for spiral ganglion neuron (SGN) degenerative disease and injury in various animal models. A recent study has shown evidence of functional recovery after transplantation of the stem cells into a degenerated-SGN model. Chemokine stromal cell-derived factor-1 (SDF-1, or known as CXC chemokine ligand-12, CXCL-12) signaling through CXCR4 has previously been identified as a key step in the homing of the stem cells within the injury areas; meanwhile, studies have revealed that the SDF-1/CXCR4 axis is also involved in axon guidance and pathfinding. A study found that transplanted neural precursor cells can migrate to the root of the auditory nerve when animals are subjected to an augmented acoustic environment (AAE). In accordance with these studies, we hypothesize that AAE will up-regulate the expression of SDF-1 in acoustic nerves. We tested our hypothesis by examining the expression of SDF-1 in different acoustic environments, and the results were confirmed by the auditory brainstem response (ABR), immunohistochemical and RT-PCR analyses. The results showed that SDF-1 was expressed at a relatively low level in the SGNs under normal animal unit acoustic conditions (40-50 dB). Moreover, it was significantly up-regulated in the SGNs under the 75 dB (augmented physiological process without hearing loss) and 90 dB AAE (pathological process with light hearing loss) conditions; however, under the 115 dB AAE (pathological process with severe hearing loss) condition, the expression of SDF-1 was not up-regulated. The results confirmed that appropriately augmented acoustical stimuli lead to the up-regulation of SDF-1, which may assist in the migration of the transplanted cells and the subsequent establishment of essential synaptic contacts between the exogenous cells and the host auditory pathway.