Intrinsically Self-renewing Neuroprogenitors From the A/J Mouse Spiral Ganglion as Virtually Unlimited Source of Mature Auditory Neurons

The caspase-inhibitor Emricasan efficiently counteracts cisplatin- and neomycin-induced cytotoxicity in cochlear cells

Cisplatin is a chemotherapeutic agent widely used to treat solid tumors. However, it can also be highly ototoxic, resulting in high-frequency hearing loss. Cisplatin causes degeneration of hair cells (HCs) and spiral ganglion neurons (SGNs) in the inner ear, which are essential components of the hearing process and cannot be regenerated in mammals. As the affected cells primarily die by apoptosis, we tested several anti-apoptotic small molecules to protect these cells from drug-induced toxicity. We found that the general caspase inhibitor Emricasan could significantly counteract the toxic effects of cisplatin in House Ear Institute-Organ of Corti 1 (HEI-OC1) cells, phoenix auditory cells, and primary SGNs. Importantly, the anti-cytotoxic effect in neuronal cells was even more pronounced than the effect of sodium thiosulfate (STS), which is currently the only approved prevention option for cisplatin-induced ototoxicity. Finally, we tested the protective effect of Emricasan treatment in the context of another ototoxic drug, i.e., the aminoglycoside antibiotic neomycin, and again found a significant increase in cell viability when the cultures were co-treated with Emricasan. These results suggest a promising strategy to prevent ototoxicity in patients by temporarily blocking the apoptotic pathway when applying cisplatin or aminoglycoside antibiotics. KEY MESSAGES: Anti-apoptotic small molecules can reduce cisplatin-induced toxicity. Emricasan can effectively exert its anti-apoptotic effect on cochlear cells. Strong protection from cisplatin- and neomycin-induced cytotoxicity with Emricasan. Sodium thiosulfate and Emricasan provide similar protective effects to cisplatin-treated cells. Emricasan is more potent than sodium thiosulfate in reducing neomycin-induced cytotoxicity.

Protection from cisplatin-induced hearing loss with lentiviral vector-mediated ectopic expression of the anti-apoptotic protein BCL-XL

Cisplatin is a highly effective chemotherapeutic agent, but it can cause sensorineural hearing loss (SNHL) in patients. Cisplatin-induced ototoxicity is closely related to the accumulation of reactive oxygen species (ROS) and subsequent death of hair cells (HCs) and spiral ganglion neurons (SGNs). Despite various strategies to combat ototoxicity, only one therapeutic agent has thus far been clinically approved. Therefore, we have developed a gene therapy concept to protect cochlear cells from cisplatin-induced toxicity. Self-inactivating lentiviral (LV) vectors were used to ectopically express various antioxidant enzymes or anti-apoptotic proteins to enhance the cellular ROS scavenging or prevent apoptosis in affected cell types. In direct comparison, anti-apoptotic proteins mediated a stronger reduction in cytotoxicity than antioxidant enzymes. Importantly, overexpression of the most promising candidate, Bcl-xl, achieved an up to 2.5-fold reduction in cisplatin-induced cytotoxicity in HEI-OC1 cells, phoenix auditory neurons, and primary SGN cultures. BCL-XL protected against cisplatin-mediated tissue destruction in cochlear explants. Strikingly, in vivo application of the LV BCL-XL vector improved hearing and increased HC survival in cisplatin-treated mice. In conclusion, we have established a preclinical gene therapy approach to protect mice from cisplatin-induced ototoxicity that has the potential to be translated to clinical use in cancer patients.

NADPH Oxidase 3: Beyond the Inner Ear

Reactive oxygen species (ROS) were formerly known as mere byproducts of metabolism with damaging effects on cellular structures. The discovery and description of NADPH oxidases (Nox) as a whole enzyme family that only produce this harmful group of molecules was surprising. After intensive research, seven Nox isoforms were discovered, described and extensively studied. Among them, the NADPH oxidase 3 is the perhaps most underrated Nox isoform, since it was firstly discovered in the inner ear. This stigma of Nox3 as "being only expressed in the inner ear" was also used by me several times. Therefore, the question arose whether this sentence is still valid or even usable. To this end, this review solely focuses on Nox3 and summarizes its discovery, the structural components, the activating and regulating factors, the expression in cells, tissues and organs, as well as the beneficial and detrimental effects of Nox3-mediated ROS production on body functions. Furthermore, the involvement of Nox3-derived ROS in diseases progression and, accordingly, as a potential target for disease treatment, will be discussed.

Candidate Key Proteins in Tinnitus: A Bioinformatic Study of Synaptic Transmission in Spiral Ganglion Neurons

To study key proteins associated with changes in synaptic transmission in the spiral ganglion in tinnitus, we build three gene lists from the GeneCard database: 1. Perception of sound (PoS), 2. Acoustic stimulation (AcouStim), and 3. Tinnitus (Tin). Enrichment analysis by the DAVID database resulted in similar Gene Ontology (GO) terms for cellular components in all gene lists, reflecting synaptic structures known to be involved in auditory processing. The STRING protein-protein interaction (PPI) network and the Cytoscape data analyzer were used to identify the top two high-degree proteins (HDPs) and their high-score interaction proteins (HSIPs) identified by the combined score (CS) of the corresponding edges. The top two protein pairs (key proteins) for the PoS are BDNF-GDNF and OTOF-CACNA1D and for the AcouStim process BDNF-NTRK2 and TH-CALB1. The Tin process showed BDNF and NGF as HDPs, with high-score interactions with NTRK1 and NGFR at a comparable level. Compared to the PoS and AcouStim process, the number of HSIPs of key proteins (CS > 90. percentile) increases strongly in Tin. In the PoS and AcouStim networks, BDNF receptor signaling is the dominant pathway, and in the Tin network, the NGF-signaling pathway is of similar importance. Key proteins and their HSIPs are good indicators of biological processes and of signaling pathways characteristic for the normal hearing on the one hand and tinnitus on the other.

Genetic Mechanism Study of Auditory Phoenix Spheres and Transcription Factors Prediction for Direct Reprogramming by Bioinformatics

Background: Hearing loss is the most common irreversible sensory disorder. By delivering regenerative cells into the cochlea, cell-based therapy provides a novel strategy for hearing restoration. Recently, newly-identified phoenix cells have drawn attention due to their nearly unlimited self-renewal and neural differentiation capabilities. They are a promising cell source for cell therapy and a potential substitute for induced pluripotent stem cells (iPSCs) in many in vitro applications. However, the underlying genomic mechanism of their self-renewal capabilities is largely unknown. The aim of this study was to identify hub genes and potential molecular mechanisms between differentiated and undifferentiated phoenix cells and predict transcription factors (TFs) for direct reprogramming. Material and Methods: The datasets were downloaded from the ArrayExpress database. Samples of differentiated and undifferentiated phoenix cells with three biological replicates were utilised for bioinformatic analysis. Differentially expressed genes (DEGs) were screened and the Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment were investigated. The gene set enrichment analysis (GSEA) was conducted to verify the enrichment of four self-defined gene set collections, followed by protein-protein interaction (PPI) network construction and subcluster analysis. The prediction of TFs for direct reprogramming was performed based on the TRANSFAC database. Results: Ten hub genes were identified to be the key candidates for self-renewal. Ten TFs were predicted as the direct reprogramming factors. This study provides a theoretical foundation for understanding phoenix cells and clues for direct reprogramming, which would stimulate further experiments and clinical applications in hearing research and treatment.

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.

In vitro and in vivo models: What have we learnt about inner ear regeneration and treatment for hearing loss?

The sensory cells of the inner ear, called hair cells, do not regenerate spontaneously and therefore, hair cell loss and subsequent hearing loss are permanent in humans. Conversely, functional hair cell regeneration can be observed in non-mammalian vertebrate species like birds and fish. Also, during postnatal development in mice, limited regenerative capacity and the potential to isolate stem cells were reported. Together, these findings spurred the interest of current research aiming to investigate the endogenous regenerative potential in mammals. In this review, we summarize current in vitro based approaches and briefly introduce different in vivo model organisms utilized to study hair cell regeneration. Furthermore, we present an overview of the findings that were made synergistically using both, the in vitro and in vivo based tools.

NADPH Oxidase 3 Deficiency Protects From Noise-Induced Sensorineural Hearing Loss

The reactive oxygen species (ROS)-generating NADPH oxidase NOX3 isoform is highly and specifically expressed in the inner ear. NOX3 is needed for normal vestibular development but NOX-derived ROS have also been implicated in the pathophysiology of sensorineural hearing loss. The role of NOX-derived ROS in noise-induced hearing loss, however, remains unclear and was addressed with the present study. Two different mouse strains, deficient in NOX3 or its critical subunit p22phox, were subjected to a single noise exposure of 2 h using an 8-16 kHz band noise at an intensity of 116-120 decibel sound pressure level. In the hours following noise exposure, there was a significant increase in cochlear mRNA expression of NOX3 in wild type animals. By using RNAscope in situ hybridization, NOX3 expression was primarily found in the Rosenthal canal area, colocalizing with auditory neurons. One day after the noise trauma, we observed a high frequency hearing loss in both knock-out mice, as well as their wild type littermates. At day seven after noise trauma however, NOX3 and p22phox knockout mice showed a significantly improved hearing recovery and a marked preservation of neurosensory cochlear structures compared to their wild type littermates. Based on these findings, an active role of NOX3 in the pathophysiology of noise-induced hearing loss can be demonstrated, in line with recent evidence obtained in other forms of acquired hearing loss. The present data demonstrates that the absence of functional NOX3 enhances the hearing recovery phase following noise trauma. This opens an interesting clinical window for pharmacological or molecular intervention aiming at post prevention of noise-induced hearing loss.

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.

Phoenix auditory neurons as 3R cell model for high throughput screening of neurogenic compounds

Auditory neurons connect the sensory hair cells from the inner ear to the brainstem. These bipolar neurons are relevant targets for pharmacological intervention aiming at protecting or improving the hearing function in various forms of sensorineural hearing loss. In the research laboratory, neurotrophic compounds are commonly used to improve survival and to promote regeneration of auditory neurons. One important roadblock delaying eventual clinical applications of these strategies in humans is the lack of powerful in vitro models allowing high throughput screening of otoprotective and regenerative compounds. The recently discovered auditory neuroprogenitors (ANPGs) derived from the A/J mouse with an unprecedented capacity to self-renew and to provide mature auditory neurons offer the possibility to overcome this bottleneck. In the present study, we further characterized the new phoenix ANPGs model and compared it to the current gold-standard spiral ganglion organotypic explant (SGE) model to assay neurite outgrowth, neurite length and glutamate-induced Ca²⁺ response in response to neurotrophin-3 (NT-3) and brain derived neurotrophic factor (BDNF) treatment. Whereas both, SGEs and phoenix ANPGs exhibited a robust and sensitive response to neurotrophins, the phoenix ANPGs offer a considerable range of advantages including high throughput suitability, lower experimental variability, single cell resolution and an important reduction of animal numbers. The phoenix ANPGs in vitro model therefore provides a robust high-throughput platform to screen for otoprotective and regenerative neurotrophic compounds in line with 3R principles and is of interest for the field of auditory neuroscience.