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

Inhibition of the NLRP3 inflammasome attenuates spiral ganglion neuron degeneration in aminoglycoside-induced hearing loss

JOURNAL/nrgr/04.03/01300535-202510000-00031/figure1/v/2024-11-26T163120Z/r/image-tiff Aminoglycosides are a widely used class of antibacterials renowned for their effectiveness and broad antimicrobial spectrum. However, their use leads to irreversible hearing damage by causing apoptosis of hair cells as their direct target. In addition, the hearing damage caused by aminoglycosides involves damage of spiral ganglion neurons upon exposure. To investigate the mechanisms underlying spiral ganglion neuron degeneration induced by aminoglycosides, we used a C57BL/6J mouse model treated with kanamycin. We found that the mice exhibited auditory deficits following the acute loss of outer hair cells. Spiral ganglion neurons displayed hallmarks of pyroptosis and exhibited progressive degeneration over time. Transcriptomic profiling of these neurons showed significant upregulation of genes associated with inflammation and immune response, particularly those related to the NLRP3 inflammasome. Activation of the canonical pyroptotic pathway in spiral ganglion neurons was observed, accompanied by infiltration of macrophages and the release of proinflammatory cytokines. Pharmacological intervention targeting NLRP3 using Mcc950 and genetic intervention using NLRP3 knockout ameliorated spiral ganglion neuron degeneration in the injury model. These findings suggest that NLRP3 inflammasome-mediated pyroptosis plays a role in aminoglycoside-induced spiral ganglion neuron degeneration. Inhibition of this pathway may offer a potential therapeutic strategy for treating sensorineural hearing loss by reducing spiral ganglion neuron degeneration.

Targeted spiral ganglion neuron degeneration in parvalbumin-Cre neonatal mice

The spiral ganglion neurons (SGNs) are the primary afferent neurons in the cochlea; damage to the SGNs leads to irreversible hearing impairment. Mouse models that allow selective SGN degeneration while sparing other cell types in the cochlea are lacking. Here, we investigated a genetic ablation method of the SGN using a Cre-responsive adeno-associated virus (AAV) vector expressing diphtheria toxin subunit-A (DTA). We microinjected AAV2-retro-FLEX-DTA-mCherry driven by the EF1a or hSYN promoter in neonatal parvalbumin-Cre (PVCre) and wild-type strains via the posterior semicircular canal. Apoptotic markers were observed in the degenerating SGNs as early as 3 days. After 1 week, we assessed the SGN cell density, revealing an average degeneration of 60% for AAV-DTA driven by the EF1a promoter and 61% for that driven by the hSYN promoter. By 1 month, injected ears demonstrated a nearly complete loss of SGN, while hair cell morphology was intact. The auditory brain stem response result showed significantly elevated threshold shifts at 1 month, while the distortion-product otoacoustic emissions function remained intact. Furthermore, we show that our method did not effectively ablate SGN in adult PVCre mice. We generated a neonatal mouse model with primary SGN degeneration in PVCre mice, mimicking auditory neuropathy phenotype using an AAV Cre-dependent expression of DTA.

Targeting Programmed Cell Death in Acquired Sensorineural Hearing Loss: Ferroptosis, Necroptosis, and Pyroptosis

Sensorineural hearing loss (SNHL), the most commonly-occurring form of hearing loss, is caused mainly by injury to or the loss of hair cells and spiral ganglion neurons in the cochlea. Numerous environmental and physiological factors have been shown to cause acquired SNHL, such as ototoxic drugs, noise exposure, aging, infections, and diseases. Several programmed cell death (PCD) pathways have been reported to be involved in SNHL, especially some novel PCD pathways that have only recently been reported, such as ferroptosis, necroptosis, and pyroptosis. Here we summarize these PCD pathways and their roles and mechanisms in SNHL, aiming to provide new insights and potential therapeutic strategies for SNHL by targeting these PCD pathways.

In vitro generation of spiral ganglion neurons from embryonic stem cells

Spiral ganglion neurons (SGNs) are crucial for transmitting auditory signals from the inner ear to the brainstem, playing a pivotal role in the peripheral hearing process. However, SGNs are usually damaged by a variety of insults, which causes permanent hearing loss. Generating SGNs from stem cells represents a promising strategy for advancing cell-replacement therapies to treat sensorineural hearing loss. SGNs comprise two subtypes of neurons (types 1 and 2); however, it remains a challenge to regenerate SGN subtypes. This study aimed to investigate the generation and characterization of SGN subtype neurons induced from embryonic stem cells (ESCs) in vitro. ESCs were cultured and treated with retinoic acid, followed by neuronal induction. The differentiated cells showed protein expressions of multiple neuronal markers, suggesting the generation of neuron-like cells. Protein expressions of vGlut-1 and GATA-3 indicate the generation of glutamatergic otic neuron-like cells. ESC-derived neuron-like cells cultured for 6 days showed co-expressions of calretinin, calbindin, and POU4F1 antibodies, suggesting an early stage of SGN subtype induction. However, 14-day in vitro induction generated cells showing two distinct SGN subtypes: a group of cells expressed calretinin (subtype 1a/2 precursor), and the other group expressed calbindin and POU4F1 (subtype 1b/c). These results suggest that in vitro generation of SGN subtypes from ESCs is culture time dependent.

Wnt signalling facilitates neuronal differentiation of cochlear Frizzled10-positive cells in mouse cochlea via glypican 6 modulation

Degeneration of cochlear spiral ganglion neurons (SGNs) leads to irreversible sensorineural hearing loss (SNHL), as SGNs lack regenerative capacity. Although cochlear glial cells (GCs) have some neuronal differentiation potential, their specific identities remain unclear. This study identifies a distinct subpopulation, Frizzled10 positive (FZD10+) cells, as an important type of GC responsible for neuronal differentiation in mouse cochlea. FZD10 + cells can differentiate into various SGN subtypes in vivo, adhering to natural proportions. Wnt signaling enhances the ability of FZD10 + cells to function as neural progenitors and increases the neuronal excitability of the FZD10-derived neurons. Single-cell RNA sequencing analysis characterizes FZD10-derived differentiating cell populations, while crosstalk network analysis identifies multiple signaling pathways and target genes influenced by Wnt signaling that contribute to the function of FZD10 + cells as neural progenitors. Pseudotime analysis maps the differentiation trajectory from proliferated GCs to differentiating neurons. Further experiments indicate that glypican 6 (GPC6) may regulate this neuronal lineage, while GPC6 deficiency diminishes the effects of Wnt signaling on FZD10-derived neuronal differentiation and synapse formation. These findings suggest the critical role of Wnt signaling in the neuronal differentiation derived from cochlear FZD10 + cells and provide insights into the mechanisms potentially involved in this process.

Morphological phenotyping of the aging cochlea in inbred C57BL/6N and outbred CD1 mouse strains

Morphological mouse phenotyping plays a pivotal role in the translational setting and even more in the area of auditory research, where mouse is a central model organism due to the evolutionary genetic relationship and morpho-functional analogies with the human auditory system. However, some results obtained in murine models cannot be translated to humans due to the inadequate description of experimental conditions underlying poor reproducibility. We approach the characterization of the aging process of the mouse cochlea in animals up to 18 months of age belonging to two of the most used outbred (CD1) and inbred (C57BL/6N) strains. Striving to reduce any environmental variable we performed our study compliantly to the ARRIVE guidelines. We integrated instrumental data (auditory brainstem response test), with morphological analyses to correlate functional discrepancies to morphological changes and track the differences in the evolution of sensorineural hearing loss in the two strains. We featured the localization of Gipc3, Myosin VIIa, and TMC1 in hair cells of the Corti organ as well as NF 200 and the density of type I neuron in the spiral ganglion. We outlined age-related hearing loss (ARHL) in both strains, and a clear drop in the selected marker localization. However, in CD1 we detected a different trend allowing the identification of potential strain-specific mechanisms, namely an increase in myosin VIIa in 6 months aging mice in comparison to 2 months old animals. Our findings represent an asset to investigate the strain-dependent physiological trigger of ARHL providing new insights in the translational area.

FOXO1-NCOA4 Axis Contributes to Cisplatin-Induced Cochlea Spiral Ganglion Neuron Ferroptosis via Ferritinophagy

Mammalian cochlea spiral ganglion neurons (SGNs) are crucial for sound transmission, they can be damaged by chemotherapy drug cisplatin and lead to irreversible sensorineural hearing loss (SNHL), while such damage can also render cochlear implants ineffective. However, the mechanisms underlying cisplatin-induced SGNs damage and subsequent SNHL are still under debate and there is no currently effective clinical treatment. Here, this study demonstrates that ferroptosis is triggered in SGNs following exposure to cisplatin. Inhibiting ferroptosis protects against cisplatin-induced SGNs damage and hearing loss, while inducing ferroptosis intensifies these effects. Furthermore, cisplatin prompts nuclear receptor coactivator 4 (NCOA4)-mediated ferritinophagy in SGNs, while knocking down NCOA4 mitigates cisplatin-induced ferroptosis and hearing loss. Notably, the upstream regulator of NCOA4 is identified and transcription factor forkhead box O1 (FOXO1) is shown to directly suppress NCOA4 expression in SGNs. The knocking down of FOXO1 amplifies NCOA4-mediated ferritinophagy, increases ferroptosis and lipid peroxidation, while disrupting the interaction between FOXO1 and NCOA4 in NCOA4 knock out mice prevents the cisplatin-induced SGN ferroptosis and hearing loss. Collectively, this study highlights the critical role of the FOXO1-NCOA4 axis in regulating ferritinophagy and ferroptosis in cisplatin-induced SGNs damage, offering promising therapeutic targets for SNHL mitigation.

A basement membrane extract-based three-dimensional culture system promotes the neuronal differentiation of cochlear Sox10-positive glial cells in vitro

Spiral ganglion neurons (SGNs) in the mammalian cochleae are essential for the delivery of acoustic information, and damage to SGNs can lead to permanent sensorineural hearing loss as SGNs are not capable of regeneration. Cochlear glial cells (GCs) might be a potential source for SGN regeneration, but the neuronal differentiation ability of GCs is limited and its properties are not clear yet. Here, we characterized the cochlear Sox10-positive (Sox10+) GCs as a neural progenitor population and developed a basement membrane extract-based three-dimensional (BME-3D) culture system to promote its neuronal generation capacity in vitro. Firstly, the purified Sox10+ GCs, isolated from Sox10-creER/tdTomato mice via flow cytometry, were able to form neurospheres after being cultured in the traditional suspension culture system, while significantly more neurospheres were found and the expression of stem cell-related genes was upregulated in the BME-3D culture group. Next, the BME-3D culture system promoted the neuronal differentiation ability of Sox10+ GCs, as evidenced by the increased number, neurite outgrowth, area of growth cones, and synapse density as well as the promoted excitability of newly induced neurons. Notably, the BME-3D culture system also intensified the reinnervation of newly generated neurons with HCs and protected the neurospheres and derived-neurons against cisplatin-induced damage. Finally, transcriptome sequencing analysis was performed to identify the characteristics of the differentiated neurons. These findings suggest that the BME-3D culture system considerably promotes the proliferation capacity and neuronal differentiation efficiency of Sox10+ GCs in vitro, thus providing a possible strategy for the SGN regeneration study.

Current advances in biomaterials for inner ear cell regeneration

Inner ear cell regeneration from stem/progenitor cells provides potential therapeutic strategies for the restoration of sensorineural hearing loss (SNHL), however, the efficiency of regeneration is low and the functions of differentiated cells are not yet mature. Biomaterials have been used in inner ear cell regeneration to construct a more physiologically relevant 3D culture system which mimics the stem cell microenvironment and facilitates cellular interactions. Currently, these biomaterials include hydrogel, conductive materials, magneto-responsive materials, photo-responsive materials, etc. We analyzed the characteristics and described the advantages and limitations of these materials. Furthermore, we reviewed the mechanisms by which biomaterials with different physicochemical properties act on the inner ear cell regeneration and depicted the current status of the material selection based on their characteristics to achieve the reconstruction of the auditory circuits. The application of biomaterials in inner ear cell regeneration offers promising opportunities for the reconstruction of the auditory circuits and the restoration of hearing, yet biomaterials should be strategically explored and combined according to the obstacles to be solved in the inner ear cell regeneration research.

Photobiomodulation Can Enhance Stem Cell Viability in Cochlea with Auditory Neuropathy but Does Not Restore Hearing

Sensorineural hearing loss is very difficult to treat. Currently, one of the techniques used for hearing rehabilitation is a cochlear implant that can transform sound into electrical signals instead of inner ear hair cells. However, the prognosis remains very poor if sufficient auditory nerve cells are not secured. In this study, the effect of mouse embryonic stem cells (mESC) and photobiomodulation (PBM) combined treatment on auditory function and auditory nerve cells in a secondary neuropathy animal model was investigated. To confirm the engraftment of stem cells in vitro, cochlear explants were treated with kanamycin (KM) to mimic nerve damage and then cocultured with GFP-mESC. GFP-mESCs were observed to have attached and integrated into the explanted samples. An animal model for secondary neurodegeneration was achieved by KM treatment and was treated by a combination therapy of GFP-mESC and NIR-PBM at 8 weeks of KM treatment. Hearing recovery by functional testing using auditory brain stem response (ABR) and eABR was measured as well as morphological changes and epifluorescence analysis were conducted after 2 weeks of combination therapy. KM treatment elevated the hearing threshold at 70-80 dB and even after the combination treatment with GFP-mESC and PBM was applied, the auditory function was not restored. In addition, the stem cells transplanted into cochlea has exponentially increased due to PBM treatment although did not produce any malignancy. This study confirmed that the combined treatment with mESC and PBM could not improve hearing or increase the response of the auditory nerve. Nevertheless, it is noteworthy in this study that the cells are distributed in most cochlear tissues and the proliferation of stem cells was very active in animals irradiated with PBM compared to other groups wherein the stem cells had disappeared immediately after transplantation or existed for only a short period of time.

Potential Ototoxicity of Insulin-like Growth Factor 1 Receptor Signaling Inhibitors: An In Silico Drug Repurposing Study of the Regenerating Cochlear Neuron Transcriptome

Spiral ganglion neurons (SGNs) connect cochlear hair cells with higher auditory pathways and their degeneration due to drug toxicity (ototoxicity) contributes to hearing loss. This study aimed to identify drug classes that are negatively correlated with the transcriptome of regenerating SGNs. Human orthologs of differentially expressed genes within the regenerating neonatal mouse SGN transcriptome were entered into CMap and the LINCS unified environment and perturbation-driven gene expression was analyzed. The CMap connectivity scores ranged from 100 (positive correlation) to -100 (negative correlation). Insulin-like growth factor 1/receptor (IGF-1/R) inhibitors were highly negatively correlated with the regenerating SGN transcriptome (connectivity score: -98.87). A systematic literature review of clinical trials and observational studies reporting otologic adverse events (AEs) with IGF-1/R inhibitors identified 108 reports (6141 treated patients). Overall, 16.9% of the treated patients experienced any otologic AE; the rate was highest for teprotumumab (42.9%). In a meta-analysis of two randomized placebo-controlled trials of teprotumumab, there was a significantly higher risk of hearing-related (pooled Peto OR [95% CI]: 7.95 [1.57, 40.17]) and of any otologic AEs (3.56 [1.35, 9.43]) with teprotumumab vs. a placebo, whether or not dizziness/vertigo AEs were included. These results call for close audiological monitoring during IGF-1-targeted treatment, with prompt referral to an otolaryngologist should otologic AEs develop.

WNT Activation and TGFβ-Smad Inhibition Potentiate Stemness of Mammalian Auditory Neuroprogenitors for High-Throughput Generation of Functional Auditory Neurons In Vitro

Hearing loss affects over 460 million people worldwide and is a major socioeconomic burden. Both genetic and environmental factors (i.e., noise overexposure, ototoxic drug treatment and ageing), promote the irreversible degeneration of cochlear hair cells and associated auditory neurons, leading to sensorineural hearing loss. In contrast to birds, fish and amphibians, the mammalian inner ear is virtually unable to regenerate due to the limited stemness of auditory progenitors, and no causal treatment is able to prevent or reverse hearing loss. As of today, a main limitation for the development of otoprotective or otoregenerative therapies is the lack of efficient preclinical models compatible with high-throughput screening of drug candidates. Currently, the research field mainly relies on primary organotypic inner ear cultures, resulting in high variability, low throughput, high associated costs and ethical concerns. We previously identified and characterized the phoenix auditory neuroprogenitors (ANPGs) as highly proliferative progenitor cells isolated from the A/J mouse cochlea. In the present study, we aim at identifying the signaling pathways responsible for the intrinsic high stemness of phoenix ANPGs. A transcriptomic comparison of traditionally low-stemness ANPGs, isolated from C57Bl/6 and A/J mice at early passages, and high-stemness phoenix ANPGs was performed, allowing the identification of several differentially expressed pathways. Based on differentially regulated pathways, we developed a reprogramming protocol to induce high stemness in presenescent ANPGs (i.e., from C57Bl6 mouse). The pharmacological combination of the WNT agonist (CHIR99021) and TGFβ/Smad inhibitors (LDN193189 and SB431542) resulted in a dramatic increase in presenescent neurosphere growth, and the possibility to expand ANPGs is virtually limitless. As with the phoenix ANPGs, stemness-induced ANPGs could be frozen and thawed, enabling distribution to other laboratories. Importantly, even after 20 passages, stemness-induced ANPGs retained their ability to differentiate into electrophysiologically mature type I auditory neurons. Both stemness-induced and phoenix ANPGs resolve a main bottleneck in the field, allowing efficient, high-throughput, low-cost and 3R-compatible in vitro screening of otoprotective and otoregenerative drug candidates. This study may also add new perspectives to the field of inner ear regeneration.