Avian auditory hair cell regeneration is accompanied by JAK/STAT-dependent expression of immune-related genes in supporting cells

Anatomical and Molecular Insights into Avian Inner Ear Sensory Hair Cell Regeneration

Inner ear sensory hair cells are essential for auditory and vestibular functions. In mammals, loss of these cells leads to permanent hearing loss due to the inability of supporting cells to regenerate hair cells. In contrast, avian species exhibit a remarkable capacity for hair cell regeneration, primarily through the activation and proliferation of supporting cells. This review provides a comprehensive examination of the anatomical and molecular mechanisms underlying sensory hair cell regeneration in two critical avian inner ear structures: the basilar papilla and the utricle. We describe the structural and functional differences between avian and mammalian inner ear epithelia and highlight how these distinctions correlate with regenerative capabilities. Specifically, we discuss two distinct regenerative mechanisms - mitotic regeneration and direct transdifferentiation - employed by avian supporting cells in response to hair cell loss. We also explore how epithelial organization influences regenerative responses, including cellular density, cytoskeletal dynamics such as circumferential filamentous actin bands, and mechanical properties like tissue jamming and unjamming states. Additionally, we examine molecular pathways such as Hippo signaling, which mediates mechanical cues critical for regulating supporting cell proliferation and differentiation during regeneration. Recent advancements in single-cell -omics technologies have further elucidated molecular signatures and signaling pathways involved in these processes, offering novel insights that may inform therapeutic strategies aimed at inducing hair cell regeneration in mammals. This review highlights key anatomical and molecular concepts derived from avian models that hold promise for overcoming regenerative limitations in mammalian inner ears, paving the way for innovative treatments for hearing loss.

Shh agonist enhances maturation in homotypic Lgr5-positive inner ear organoids

Background: The regeneration of functional hair cells (HCs) remains a critical challenge in addressing sensorineural hearing loss. This study aimed to investigate the molecular and functional mechanisms driving stereocilia maturation within inner ear organoids (IEO) derived from homogenic Lgr5-positive progenitor cells (LPCs) and to compare outcomes with traditional heterotypic cultures. Methods: Mouse cochlear LPCs were isolated via magnetic-activated cell sorting (MACS) to establish homotypic cultures, ensuring purity and eliminating the heterotypic influences present in traditional manual isolation (MI) methods. Differentiation into HCs was induced through Wnt and Notch signaling modulation. Transcriptomic profiling using bulk and single-cell RNA sequencing (scRNA-seq) identified gene expression changes linked to stereocilia development. A Sonic Hedgehog (Shh) agonist was applied to enhance structural maturation of HCs. Functional assessment included electron microscopy, FM1-43 uptake assays, and microelectrode array recordings in assembloids of IEO with primary spiral ganglion neurons (SGN) co-cultures. Results: While homotypic LPC-derived IEOs successfully differentiated into HC-like cells, initial morphological assessment revealed immature stereocilia structures. Bulk RNA-seq analysis highlighted a downregulation of morphogenesis-related genes in these organoids. The application of a Shh agonist, acting as a key morphogen, promoted stereocilia development, as evidenced by enhanced ultrastructural features and increased expression of cuticular plate-associated genes (Pls1, Lmo7 and Lrba). Single-cell RNA sequencing (scRNA-seq) further identified distinct cell clusters, which exhibited robust expression of stereocilia-related genes (Espn, Lhfpl5, Loxhd1 and Tmc1), indicative of advanced HC maturation. Electrophysiological assessments of IEO-SGN assembloids using microelectrode arrays confirmed functional mechanoelectrical transduction between cells. Conclusion: This integrated approach elucidates critical pathways and cellular dynamics underpinning stereocilia maturation and functional HC development in EIOs. These findings provide new insights into the molecular regulation of HC maturation and support the utility of Shh-modulated IEOs as a promising platform for inner ear regeneration and therapeutic development for inner ear regenerative therapies.

Gfi1 in the inner ear: A retrospective review

Gfi1 plays an important role in the development of hair cells (HCs), as indicated by its ability to regulate the expression of HC-related genes while the organ of Corti is developing. Given that the HCs and the supporting cells (SCs) are coming from a common stem/progenitor cell pool, it is conceivable to regenerate HCs from SCs that ectopically express Gfi1. The focus of this review was to elucidate the role of Gfi1 in controlling the development of HCs by dissecting the phenotypes of the inner ear in Gfi1-mutated mouse lines. In addition, we reviewed studies of regeneration in the mammalian inner ear, by which we discussed the novel function of Gfi1 as an essential factor in guiding non-HCs toward an HC destiny in coordination with Atoh1 and Pou4f3. Finally, we summarized the known Gfi1-specific Cre/CreER/reporter mouse lines and highlighted the pros and cons of each line, with the aim of providing insights for use in future studies. In summary, a better understanding of Gfi1 and its diverse roles is beneficial for advancing studies of HC regeneration in the inner ear.

Single-cell atlas comparison across vertebrates reveals auditory cell evolution and mechanisms for hair cell regeneration

Mammals suffer permanent hearing impairment from the loss of auditory hair cells due to their inability to regenerate. In contrast, lower vertebrates exhibit extraordinary capacity for hair cell regeneration and hearing restoration, but the mechanisms remain unclear. Here we characterize the single-cell atlas of Xenopus laevis inner ear and perform a comprehensive comparison with mouse model. An exceptionally conserved inner ear neuronal cell type is discovered. The results reveal that the outer hair cells (OHCs) exist exclusively in mammals. Importantly, our analyses reveal an orchestrated gene expression program in Xenopus, characterized by upregulation of hair cell regeneration-related genes, coupled with downregulation of proliferation inhibitory genes. These findings unveil a natural feature of regenerative capacity in Xenopus, and provide molecular and evolutionary evidences for differential regenerative capacities across vertebrates. This work offers insights from amphibians into developing strategies to solve the challenges of hair cell regeneration in humans.

Oncostatin M promotes osteogenic differentiation of tendon-derived stem cells through the JAK2/STAT3 signalling pathway

PURPOSE

Oncostatin M (OSM) is involved in the regulation of osteogenic differentiation and has a major role in the development of heterotopic ossification. The role of OSM in osteogenic differentiation of tendon-derived stem cells (TDSCs) and its mechanism have not been reported. This study aim to investigate the role of OSM in osteogenic differentiation of TDSCs and study the mechanism.

METHODS

TDSCs were differentiated in osteogenic differentiation medium for 7 days. Recombinant OSM was added to the osteogenic differentiation medium for 7 and 14 days. The effect of Janus kinase 2 (JAK2) inhibitor AZD1480 and signal transducer and activator of transcription 3 (STAT3) inhibitor stattic in the presence of recombinant OSM on osteogenic differentiation of TDSCs was examined after differentiation for 7 and 14 days. Alkaline phosphatase and alizarin red staining were used to assess the effects on early and mid-stage osteogenic differentiation, respectively. Western blotting and qPCR were used to assess the expression of receptor and signalling pathway-related proteins and osteogenic marker genes, respectively.

RESULTS

TDSCs were successfully induced to differentiate into osteoblasts. Recombinant OSM promoted osteogenic differentiation of TDSCs to early and mid-stages. After addition of AZD1480 or stattic, decreased alkaline phosphatase and alizarin red staining were observed in the early and mid-stages of osteogenic differentiation. Additionally, decreased expression of receptor and pathway-related proteins, and osteogenic genes was found by western blotting and qPCR, respectively.

CONCLUSION

OSM promotes osteogenic differentiation of TDSCs and the JAK2/STAT3 signalling pathway plays an important role.

Hyperosmotic sisomicin infusion: a mouse model for hearing loss

Losing either type of cochlear sensory hair cells leads to hearing impairment. Inner hair cells act as primary mechanoelectrical transducers, while outer hair cells enhance sound-induced vibrations within the organ of Corti. Established inner ear damage models, such as systemic administration of ototoxic aminoglycosides, yield inconsistent and variable hair cell death in mice. Overcoming this limitation, we developed a method involving surgical delivery of a hyperosmotic sisomicin solution into the posterior semicircular canal of adult mice. This procedure induced rapid and synchronous apoptotic demise of outer hair cells within 14 h, leading to irreversible hearing loss. The combination of sisomicin and hyperosmotic stress caused consistent and synergistic ototoxic damage. Inner hair cells remained until three days post-treatment, after which deterioration in structure and number was observed, culminating in a complete hair cell loss by day seven. This robust animal model provides a valuable tool for otoregenerative research, facilitating single-cell and omics-based studies toward exploring preclinical therapeutic strategies.

Advance and Application of Single-cell Transcriptomics in Auditory Research

Hearing loss and deafness, as a worldwide disability disease, have been troubling human beings. However, the auditory organ of the inner ear is highly heterogeneous and has a very limited number of cells, which are largely uncharacterized in depth. Recently, with the development and utilization of single-cell RNA sequencing (scRNA-seq), researchers have been able to unveil the complex and sophisticated biological mechanisms of various types of cells in the auditory organ at the single-cell level and address the challenges of cellular heterogeneity that are not resolved through by conventional bulk RNA sequencing (bulk RNA-seq). Herein, we reviewed the application of scRNA-seq technology in auditory research, with the aim of providing a reference for the development of auditory organs, the pathogenesis of hearing loss, and regenerative therapy. Prospects about spatial transcriptomic scRNA-seq, single-cell based genome, and Live-seq technology will also be discussed.

Murine cochlear damage models in the context of hair cell regeneration research

Understanding the complex pathologies associated with hearing loss is a significant motivation for conducting inner ear research. Lifelong exposure to loud noise, ototoxic drugs, genetic diversity, sex, and aging collectively contribute to human hearing loss. Replicating this pathology in research animals is challenging because hearing impairment has varied causes and different manifestations. A central aspect, however, is the loss of sensory hair cells and the inability of the mammalian cochlea to replace them. Researching therapeutic strategies to rekindle regenerative cochlear capacity, therefore, requires the generation of animal models in which cochlear hair cells are eliminated. This review discusses different approaches to ablate cochlear hair cells in adult mice. We inventoried the cochlear cyto- and histo-pathology caused by acoustic overstimulation, systemic and locally applied drugs, and various genetic tools. The focus is not to prescribe a perfect damage model but to highlight the limitations and advantages of existing approaches and identify areas for further refinement of damage models for use in regenerative studies.

Hyperosmotic Sisomicin Infusion: A Mouse Model for Hearing Loss

Hearing impairment arises from the loss of either type of cochlear sensory hair cells. Inner hair cells act as primary sound transducers, while outer hair cells enhance sound-induced vibrations within the organ of Corti. Established models, such as systemic administration of ototoxic aminoglycosides, yield inconsistent and variable hair cell death in mice. Overcoming this limitation, we developed a method involving surgical delivery of a hyperosmotic sisomicin solution into the posterior semicircular canal of adult mice. This procedure induced rapid and synchronous apoptotic demise of outer hair cells within 14 hours, leading to irreversible hearing loss. The combination of sisomicin and hyperosmotic stress caused consistent and synergistic ototoxic damage. Inner hair cells remained intact until three days post-treatment, after which deterioration in structure and number was observed, culminating in cell loss by day seven. This robust animal model provides a valuable tool for otoregenerative research, facilitating single-cell and omics-based studies toward exploring preclinical therapeutic strategies.

Hair cell regeneration, reinnervation, and restoration of hearing thresholds in the avian hearing organ

Hearing starts, at the cellular level, with mechanoelectrical transduction by sensory hair cells. Sound information is then transmitted via afferent synaptic connections with auditory neurons. Frequency information is encoded by the location of hair cells along the cochlear duct. Loss of hair cells, synapses, or auditory neurons leads to permanent hearing loss in mammals. Birds, in contrast, regenerate auditory hair cells and functionally recover from hearing loss. Here, we characterized regeneration and reinnervation in sisomicin-deafened chickens and found that afferent neurons contact regenerated hair cells at the tips of basal projections. In contrast to development, synaptic specializations are established at these locations distant from the hair cells' bodies. The protrusions then contracted as regenerated hair cells matured and became functional 2 weeks post-deafening. We found that auditory thresholds recovered after 4-5 weeks. We interpret the regeneration-specific synaptic reestablishment as a location-preserving process that might be needed to maintain tonotopic fidelity.

Manipulating Myc for reparative regeneration

The Myc family of proto-oncogenes is a key node for the signal transduction of external pro-proliferative signals to the cellular processes required for development, tissue homoeostasis maintenance, and regeneration across evolution. The tight regulation of Myc synthesis and activity is essential for restricting its oncogenic potential. In this review, we highlight the central role that Myc plays in regeneration across the animal kingdom (from Cnidaria to echinoderms to Chordata) and how Myc could be employed to unlock the regenerative potential of non-regenerative tissues in humans for therapeutic purposes. Mastering the fine balance of harnessing the ability of Myc to promote transcription without triggering oncogenesis may open the door to many exciting opportunities for therapeutic development across a wide array of diseases.

Hair Cell Regeneration: From Animals to Humans

Cochlear hair cells convert sound into electrical signals that are relayed via the spiral ganglion neurons to the central auditory pathway. Hair cells are vulnerable to damage caused by excessive noise, aging, and ototoxic agents. Non-mammals can regenerate lost hair cells by mitotic regeneration and direct transdifferentiation of surrounding supporting cells. However, in mature mammals, damaged hair cells are not replaced, resulting in permanent hearing loss. Recent studies have uncovered mechanisms by which sensory organs in non-mammals and the neonatal mammalian cochlea regenerate hair cells, and outlined possible mechanisms why this ability declines rapidly with age in mammals. Here, we review similarities and differences between avian, zebrafish, and mammalian hair cell regeneration. Moreover, we discuss advances and limitations of hair cell regeneration in the mature cochlea and their potential applications to human hearing loss.

Integrated scRNAseq analyses of mouse cochlear supporting cells reveal the involvement of Ezh2 in hair cell regeneration

BACKGROUND

In lower vertebrates like fish, the inner ear and lateral line hair cells (HCs) can regenerate after being damaged by proliferation/differentiation of supporting cells (SCs). However, the HCs of mouse cochlear could only regenerate within one to two weeks after birth but not for adults.

METHODS AND RESULTS

To better understand the molecular foundations, we collected several public single-cell RNA sequencing (scRNAseq) data of mouse cochleae from E14 to P33 and extracted the prosensory and supporting cells specifically. Gene Set Enrichment Analysis (GSEA) results revealed a down-regulation of genes in Notch signaling pathway during postnatal stages (P7 and P33). We also identified 107 time-course co-expression genes correlated with developmental stage and predicated that EZH2 and KLF15 may be the key transcriptional regulators for these genes. Expressions of candidate target genes of EZH2 and KLF15 were also found in supporting cells of the auditory epithelia in chick and the neuromasts in zebrafish. Furthermore, inhibiting EZH2 suppressed regeneration of hair cells in zebrafish neuromasts and altered expressions of some developmental stage correlated genes.

CONCLUSIONS

Our results extended the understanding for molecular basis of hair cell regeneration ability and revealed the potential role of Ezh2 in it.

An essential signaling cascade for avian auditory hair cell regeneration

Hearing loss is a chronic disease affecting millions of people worldwide, yet no restorative treatment options are available. Although non-mammalian species can regenerate their auditory sensory hair cells, mammals cannot. Birds retain facultative stem cells known as supporting cells that engage in proliferative regeneration when surrounding hair cells die. Here, we investigated gene expression changes in chicken supporting cells during auditory hair cell death. This identified a pathway involving the receptor F2RL1, HBEGF, EGFR, and ERK signaling. We propose a cascade starting with the proteolytic activation of F2RL1, followed by matrix-metalloprotease-mediated HBEGF shedding, and culminating in EGFR-mediated ERK signaling. Each component of this cascade is essential for supporting cell S-phase entry in vivo and is integral for hair cell regeneration. Furthermore, STAT3-phosphorylation converges with this signaling toward upregulation of transcription factors ATF3, FOSL2, and CREM. Our findings could provide a basis for designing treatments for hearing and balance disorders.

Luteolin inhibits the JAK/STAT pathway to alleviate auditory cell apoptosis of acquired sensorineural hearing loss based on network pharmacology, molecular docking, molecular dynamics simulation, and experiments in vitro

PURPOSE

The study aimed to explore the mechanisms of luteolin in acquired sensorineural hearing loss (SNHL) through network pharmacology, molecular docking, molecular dynamics simulation, and experimental verification.

METHODS

First, the practices of network pharmacology were used to obtain the intersecting targets of luteolin and acquired SNHL, construct the PPI (Protein-Protein Interaction) network, conduct GO and KEGG enrichments, and establish luteolin-acquired SNHL-target-pathway network, aiming to gain the core targets and pathways. Then, the affinity between the core targets and luteolin was verified by molecular docking. Moreover, molecular dynamics (MD) simulation was applied to simulate the binding between targets and luteolin. Finally, with the HEI-OC1 cell line, some molecular biology techniques were adopted to verify the pharmacological actions of luteolin and the significance of the pathway from KEGG enrichment in luteolin-protecting auditory cell damage related to acquired SNHL.

RESULTS

14 intersecting targets were obtained, and the 10 core targets were further verified through molecular docking and MD simulation to get 5 core targets. The JAK/STAT was selected as the critical pathway through KEGG enrichment. Luteolin could dose-dependently alleviate auditory cell apoptosis by inhibiting the JAK/STAT pathway, confirmed by a series of experiments in vitro.

CONCLUSION

This study manifested that luteolin could reduce acquired SNHL-related auditory cell apoptosis through the JAK/STAT pathway, which provided a new idea for acquired SNHL pharmacological treatment.

Regeneration of Hair Cells from Endogenous Otic Progenitors in the Adult Mammalian Cochlea: Understanding Its Origins and Future Directions

Sensorineural hearing loss is caused by damage to sensory hair cells and/or spiral ganglion neurons. In non-mammalian species, hair cell regeneration after damage is observed, even in adulthood. Although the neonatal mammalian cochlea carries regenerative potential, the adult cochlea cannot regenerate lost hair cells. The survival of supporting cells with regenerative potential after cochlear trauma in adults is promising for promoting hair cell regeneration through therapeutic approaches. Targeting these cells by manipulating key signaling pathways that control mammalian cochlear development and non-mammalian hair cell regeneration could lead to regeneration of hair cells in the mammalian cochlea. This review discusses the pathways involved in the development of the cochlea and the impact that trauma has on the regenerative capacity of the endogenous progenitor cells. Furthermore, it discusses the effects of manipulating key signaling pathways targeting supporting cells with progenitor potential to promote hair cell regeneration and translates these findings to the human situation. To improve hearing recovery after hearing loss in adults, we propose a combined approach targeting (1) the endogenous progenitor cells by manipulating signaling pathways (Wnt, Notch, Shh, FGF and BMP/TGFβ signaling pathways), (2) by manipulating epigenetic control, and (3) by applying neurotrophic treatments to promote reinnervation.

Immune responses in the mammalian inner ear and their implications for AAV-mediated inner ear gene therapy

Adeno-associated virus (AAV)-mediated inner ear gene therapy is a promising treatment option for hearing loss and dizziness. Several studies have shown that AAV-mediated inner ear gene therapy can be applied to various mouse models of hereditary hearing loss to improve their auditory function. Despite the increase in AAV-based animal and clinical studies aiming to rescue auditory and vestibular functions, little is currently known about the host immune responses to AAV in the mammalian inner ear. It has been reported that the host immune response plays an important role in the safety and efficacy of viral-mediated gene therapy. Therefore, in order for AAV-mediated gene therapy to be successfully and safely translated into patients with hearing loss and dizziness, a better understanding of the host immune responses to AAV in the inner ear is critical. In this review, we summarize the current knowledge on host immune responses to AAV-mediated gene therapy in the mammalian inner ear and other organ systems. We also outline the areas of research that are critical for ensuring the safety and efficacy of AAV-mediated inner ear gene therapy in future clinical and translational studies.

Future Pharmacotherapy for Sensorineural Hearing Loss by Protection and Regeneration of Auditory Hair Cells

Sensorineural hearing loss has been a global burden of diseases for decades. However, according to recent progress in experimental studies on hair cell regeneration and protection, clinical trials of pharmacotherapy for sensorineural hearing loss have rapidly progressed. In this review, we focus on recent clinical trials for hair cell protection and regeneration and outline mechanisms based on associated experimental studies. Outcomes of recent clinical trials provided valuable data regarding the safety and tolerability of intra-cochlear and intra-tympanic applications as drug delivery methods. Recent findings in molecular mechanisms of hair cell regeneration suggested the realization of regenerative medicine for sensorineural hearing loss in the near future.

Stepwise fate conversion of supporting cells to sensory hair cells in the chick auditory epithelium

In contrast to mammals, the avian cochlea, specifically the basilar papilla, can regenerate sensory hair cells, which involves fate conversion of supporting cells to hair cells. To determine the mechanisms for converting supporting cells to hair cells, we used single-cell RNA sequencing during hair cell regeneration in explant cultures of chick basilar papillae. We identified dynamic changes in the gene expression of supporting cells, and the pseudotime trajectory analysis demonstrated the stepwise fate conversion from supporting cells to hair cells. Initially, supporting cell identity was erased and transition to the precursor state occurred. A subsequent gain in hair cell identity progressed together with downregulation of precursor-state genes. Transforming growth factor β receptor 1-mediated signaling was involved in induction of the initial step, and its inhibition resulted in suppression of hair cell regeneration. Our data provide new insights for understanding fate conversion from supporting cells to hair cells in avian basilar papillae.