Building inner ears: recent advances and future challenges for in vitro organoid systems

Organoids-the key to novel therapies for the inner ear?

The sensitivity and the complexity of the human inner ear in conjunction with the lack of regenerative capacity are the main reasons for hearing loss and tinnitus. Progress in the development of protective and regenerative therapies for the inner ear often failed in the past not least due to the fact that no suitable model systems for cell biological and pharmacological in vitro studies were available. A novel technology for creating "mini-organs", so-called organoids, could solve this problem and has now also reached inner ear research. It makes it possible to produce inner ear organoids from cochlear stem/progenitor cells, embryonic and induced pluripotent stem cells that mimic the structural characteristics and functional properties of the natural inner ear. This review focuses on the biological basis of these inner ear organoids, the current state of research and the promising prospects that are now opening up for basic and translational inner ear research.

From "self-differentiation" to organoids-the quest for the units of development

As proposed by Wilhelm Roux in 1885, the key goal of experimental embryology ("Entwicklungsmechanik") was to elucidate whether organisms or their parts develop autonomously ("self-differentiation") or require interactions with other parts or the environment. However, experimental embryologists soon realized that concepts like "self-differentiation" only make sense when applied to particular parts or units of the developing embryo as defined both in time and space. Whereas the formation of tissues or organs may initially depend on interactions with surrounding tissues, they later become independent of such interactions or "determined." Moreover, the determination of a particular tissue or organ primordium has to be distinguished from the spatially coordinated determination of its parts-what we now refer to as "patterning." While some primordia depend on extrinsic influences (e.g., signals from adjacent tissues) for proper patterning, others rely on intrinsic mechanisms. Such intrinsically patterned units may behave as "morphogenetic fields" that can compensate for lost parts and regulate their size and proper patterning. While these insights were won by experimental embryologists more than 100 years ago, they retain their relevance today. To enable the generation of more life-like organoids in vitro for studying developmental processes and diseases in a dish, questions about the spatiotemporal units of development (when and how tissues and organs are determined and patterned) need to be increasingly considered. This review briefly sketches this conceptual history and its continued relevance by focusing on the determination and patterning of the inner ear with a specific emphasis on some studies published in this journal.

Cellular diversity of human inner ear organoids revealed by single-cell transcriptomics

Human inner ear organoids are three-dimensional tissular structures grown in vitro that recapitulate some aspects of the fetal inner ear and allow the differentiation of inner ear cell types. These organoids offer a system in which to study human inner ear development, mutations causing hearing loss and vertigo, and new therapeutic drugs. However, the extent to which such organoids mimic in vivo human inner ear development and cellular composition remains unclear. Several recent studies have performed single-cell transcriptomics on human inner ear organoids to interrogate cellular heterogeneity, reveal the developmental trajectories of sensory lineages and compare organoid-derived vesicles to the developing human inner ear. Here, we discuss the new insights provided by these analyses that help to define new paths of investigation to understand inner ear development.

The Role of Pericytes in Inner Ear Disorders: A Comprehensive Review

Inner ear disorders, including sensorineural hearing loss, Meniere's disease, and vestibular neuritis, are prevalent conditions that significantly impact the quality of life. Despite their high incidence, the underlying pathophysiology of these disorders remains elusive, and current treatment options are often inadequate. Emerging evidence suggests that pericytes, a type of vascular mural cell specialized to maintain the integrity and function of the microvasculature, may play a crucial role in the development and progression of inner ear disorders. The pericytes are present in the microvasculature of both the cochlea and the vestibular system, where they regulate blood flow, maintain the blood-labyrinth barrier, facilitate angiogenesis, and provide trophic support to neurons. Understanding their role in inner ear disorders may provide valuable insights into the pathophysiology of these conditions and lead to the development of novel diagnostic and therapeutic strategies, improving the standard of living. This comprehensive review aims to provide a detailed overview of the role of pericytes in inner ear disorders, highlighting the anatomy and physiology in the microvasculature, and analyzing the mechanisms that contribute to the development of the disorders. Furthermore, we explore the potential pericyte-targeted therapies, including antioxidant, anti-inflammatory, and angiogenic approaches, as well as gene therapy strategies.

Recent Progress in Generation of Inner Ear Organoid

Inner ear organoids play a crucial role in hearing research. In comparison to other animal models and 2D cell culture systems, inner ear organoids offer significant advantages for studying the mechanisms of inner ear development and exploring novel approaches to disease treatment. Inner ear organoids derived from human cells are more closely resemble normal human organs in development and function. The 3D culture system of the inner ear organoid enhances cell-cell interactions and mimics the internal environment. In this review, the progress and limitations of organoid culture methods derived from tissue-specific progenitors and pluripotent stem cells (PSCs) are summarized, which may offer new insights into generating organoids that closely resemble the inner ear in terms of morphology and function.

Ototoxicity: a high risk to auditory function that needs to be monitored in drug development

Hearing loss constitutes a major global health concern impacting approximately 1.5 billion people worldwide. Its incidence is undergoing a substantial surge with some projecting that by 2050, a quarter of the global population will experience varying degrees of hearing deficiency. Environmental factors such as aging, exposure to loud noise, and the intake of ototoxic medications are implicated in the onset of acquired hearing loss. Ototoxicity resulting in inner ear damage is a leading cause of acquired hearing loss worldwide. This could be minimized or avoided by early testing of hearing functions in the preclinical phase of drug development. While the assessment of ototoxicity is well defined for drug candidates in the hearing field - required for drugs that are administered by the otic route and expected to reach the middle or inner ear during clinical use - ototoxicity testing is not required for all other therapeutic areas. Unfortunately, this has resulted in more than 200 ototoxic marketed medications. The aim of this publication is to raise awareness of drug-induced ototoxicity and to formulate some recommendations based on available guidelines and own experience. Ototoxicity testing programs should be adapted to the type of therapy, its indication (targeting the ear or part of other medications classes being potentially ototoxic), and the number of assets to test. For multiple molecules and/or multiple doses, screening options are available: in vitro (otic cell assays), ex vivo (cochlear explant), and in vivo (in zebrafish). In assessing the ototoxicity of a candidate drug, it is good practice to compare its ototoxicity to that of a well-known control drug of a similar class. Screening assays provide a streamlined and rapid method to know whether a drug is generally safe for inner ear structures. Mammalian animal models provide a more detailed characterization of drug ototoxicity, with a possibility to localize and quantify the damage using functional, behavioral, and morphological read-outs. Complementary histological measures are routinely conducted notably to quantify hair cells loss with cochleogram. Ototoxicity studies can be performed in rodents (mice, rats), guinea pigs and large species. However, in undertaking, or at the very least attempting, all preclinical investigations within the same species, is crucial. This encompasses starting with pharmacokinetics and pharmacology efficacy studies and extending through to toxicity studies. In life read-outs include Auditory Brainstem Response (ABR) and Distortion Product OtoAcoustic Emissions (DPOAE) measurements that assess the activity and integrity of sensory cells and the auditory nerve, reflecting sensorineural hearing loss. Accurate, reproducible, and high throughput ABR measures are fundamental to the quality and success of these preclinical trials. As in humans, in vivo otoscopic evaluations are routinely carried out to observe the tympanic membrane and auditory canal. This is often done to detect signs of inflammation. The cochlea is a tonotopic structure. Hair cell responsiveness is position and frequency dependent, with hair cells located close to the cochlea apex transducing low frequencies and those at the base transducing high frequencies. The cochleogram aims to quantify hair cells all along the cochlea and consequently determine hair cell loss related to specific frequencies. This measure is then correlated with the ABR & DPOAE results. Ototoxicity assessments evaluate the impact of drug candidates on the auditory and vestibular systems, de-risk hearing loss and balance disorders, define a safe dose, and optimize therapeutic benefits. These types of studies can be initiated during early development of a therapeutic solution, with ABR and otoscopic evaluations. Depending on the mechanism of action of the compound, studies can include DPOAE and cochleogram. Later in the development, a GLP (Good Laboratory Practice) ototoxicity study may be required based on otic related route of administration, target, or known potential otic toxicity.

Application of organoids in otolaryngology: head and neck surgery

PURPOSE

The purpose of this review is to systematically summarize the application of organoids in the field of otolaryngology and head and neck surgery. It aims to shed light on the current advancements and future potential of organoid technology in these areas, particularly in addressing challenges like hearing loss, cancer research, and organ regeneration.

METHODS

Review of current literature regrading organoids in the field of otolaryngology and head and neck surgery.

RESULTS

The review highlights several advancements in the field. In otology, the development of organoid replacement therapies offers new avenues for treating hearing loss. In nasal science, the creation of specific organoid models aids in studying nasopharyngeal carcinoma and respiratory viruses. In head and neck surgery, innovative approaches for squamous cell carcinoma prediction and thyroid regeneration using organoids have been developed.

CONCLUSION

Organoid research in otolaryngology-head and neck surgery is still at an early stage. This review underscores the potential of this technology in advancing our understanding and treatment of various conditions, predicting a transformative impact on future medical practices in these fields.

Mapping the developmental potential of mouse inner ear organoids at single-cell resolution

Inner ear organoids recapitulate development and are intended to generate cell types of the otic lineage for applications such as basic science research and cell replacement strategies. Here, we use single-cell sequencing to study the cellular heterogeneity of late-stage mouse inner ear organoid sensory epithelia, which we validated by comparison with datasets of the mouse cochlea and vestibular epithelia. We resolved supporting cell sub-types, cochlear-like hair cells, and vestibular type I and type II-like hair cells. While cochlear-like hair cells aligned best with an outer hair cell trajectory, vestibular-like hair cells followed developmental trajectories similar to in vivo programs branching into type II and then type I extrastriolar hair cells. These results highlight the transcriptional accuracy of the organoid developmental program but will also inform future strategies to improve synaptic connectivity and regional specification.

Blood-Labyrinth Barrier in Health and Diseases: Effect of Hormetic Nutrients

Significance: The stria vascularis, located in the inner ear, consists of three layers, one of which is the blood-labyrinth barrier (BLB). It is formed by endothelial cells, sealed together to prevent the passage of toxic substances from the blood to the inner ear, by pericytes and perivascular-resident macrophage-like melanocyte. Recent Advances: There are various causes that lead to hearing loss, and among these are noise-induced and autoimmune hearing loss, ear disorders related to ototoxic medication, Ménière's disease, and age-related hearing loss. For all of these, major therapeutic interventions include drug-loaded nanoparticles, via intratympanic or intracochlear delivery. Critical Issues: Since many pathologies associated with hearing loss are characterized by a weakening of the BLB, in this review, the molecular mechanisms underlying the response to damage of BLB cellular components have been discussed. In addition, insight into the role of hormetic nutrients against hearing loss pathology is proposed. Future Directions: BLB cellular components of neurovascular cochlear unit play important physiological roles, owing to their impermeable function against all ototoxic substances that can induce damage. Studies are needed to investigate the cross talk occurring between these cellular components to exploit their possible role as novel targets for therapeutic interventions that may unravel future path based on the use of hormetic nutrients. Antioxid. Redox Signal. 40, 542-563.

Unraveling the Mechanisms of Vestibular Neuron Formation from Human Induced Pluripotent Stem Cells

The development of in vitro models that accurately recapitulate the complex cellular and molecular interactions of the inner ear is crucial for understanding inner ear development, function, and disease. In this study, we utilized a customized microfluidic platform to generate human induced pluripotent stem cell (hiPSC)-derived three-dimensional otic sensory neurons (OSNs). hiPSC-derived otic neuronal progenitors (ONPs) were cultured in hydrogel-embedded microfluidic channels over a 40-day period. Careful modulation of Wnt and Shh signaling pathways was used to influence dorsoventral patterning and direct differentiation toward a vestibular neuron lineage. After validating the microfluidic platform, OSN spheroid transcription factor and protein expression were assessed using real-time quantitative polymerase chain reaction (RT-qPCR), immunocytochemistry, and flow cytometry. The results demonstrated the successful differentiation of hiPSCs into ONPs and subsequent divergent differentiation into vestibular neuronal lineages, as evidenced by the expression of characteristic markers. Overall, our microfluidic platform provides a physiologically relevant environment for the culture and differentiation of hiPSCs, offering a valuable tool for studying inner ear development, disease and drug screening, and regenerative medicine applications.

Shared features in ear and kidney development - implications for oto-renal syndromes

The association between ear and kidney anomalies has long been recognized. However, little is known about the underlying mechanisms. In the last two decades, embryonic development of the inner ear and kidney has been studied extensively. Here, we describe the developmental pathways shared between both organs with particular emphasis on the genes that regulate signalling cross talk and the specification of progenitor cells and specialised cell types. We relate this to the clinical features of oto-renal syndromes and explore links to developmental mechanisms.

Modeling, applications and challenges of inner ear organoid

More than 6% of the world's population is suffering from hearing loss and balance disorders. The inner ear is the organ that senses sound and balance. Although inner ear disorders are common, there are limited ways to intervene and restore its sensory and balance functions. The development and establishment of biologically therapeutic interventions for auditory disorders require clarification of the basics of signaling pathways that control inner ear development and the establishment of endogenous or exogenous cell-based therapeutic methods. In vitro models of the inner ear, such as organoid systems, can help identify new protective or regenerative drugs, develop new gene therapies, and be considered as potential tools for future clinical applications. Advances in stem cell technology and organoid culture offer unique opportunities for modeling inner ear diseases and developing personalized therapies for hearing loss. Here, we review and discuss the mechanisms for the establishment and the potential applications of inner ear organoids.

Inner Ear Organoids: Strengths and Limitations

Inner ear organoids derived from differentiation of human pluripotent stem cells have recently gained momentum as tools to study inner ear development and developmental defects. An additional exciting aspect about this technology is represented by its translational potential, specifically, the use of organoids to validate therapeutics for hearing and balance restoration on human/patient-specific cells. This latter aspect will be briefly discussed here including opportunities and current limitations.

Stem Cell-Based Therapies for Auditory Hair Cell Regeneration in the Treatment of Hearing Loss

The incidence and prevalence of hearing loss is increasing globally at an accelerated pace. Hair cells represent the sensory receptors of auditory and vestibular systems. Hair cell absence, loss or degeneration due to congenital diseases, trauma, toxicity, infection or advancing age, results in disabling hearing loss. Regenerative medicine approaches consisting in stem cell-based hair cell rescue or regeneration, gene therapy, as well as cell and tissue engineering are expected to dramatically improve the therapeutic arsenal available for addressing hearing loss. Current strategies that are using different stem cell types to rescue or to induce hair cell proliferation and regeneration are presented. Gene and cell therapy methods that modulates transdifferentiation of surrounding cell types into hair cells are presented, together with their specific advantages and limitations. Several modalities for improving therapeutic targeting to the inner ear such as nanoparticle-mediated cell and gene delivery are introduced. Further steps in building more relevant high-throughput models for testing novel drugs and advanced therapies are proposed as a modality to accelerate translation to clinical settings.

Stem Cell-Based Hair Cell Regeneration and Therapy in the Inner Ear

Hearing loss has become increasingly prevalent and causes considerable disability, thus gravely burdening the global economy. Irreversible loss of hair cells is a main cause of sensorineural hearing loss, and currently, the only relatively effective clinical treatments are limited to digital hearing equipment like cochlear implants and hearing aids, but these are of limited benefit in patients. It is therefore urgent to understand the mechanisms of damage repair in order to develop new neuroprotective strategies. At present, how to promote the regeneration of functional hair cells is a key scientific question in the field of hearing research. Multiple signaling pathways and transcriptional factors trigger the activation of hair cell progenitors and ensure the maturation of newborn hair cells, and in this article, we first review the principal mechanisms underlying hair cell reproduction. We then further discuss therapeutic strategies involving the co-regulation of multiple signaling pathways in order to induce effective functional hair cell regeneration after degeneration, and we summarize current achievements in hair cell regeneration. Lastly, we discuss potential future approaches, such as small molecule drugs and gene therapy, which might be applied for regenerating functional hair cells in the clinic.

[Organoids-the key to novel therapies for the inner ear? German version]

The sensitivity and the complexity of the human inner ear in conjunction with the lack of regenerative capacity are the main reasons for hearing loss and tinnitus. Progress in the development of protective and regenerative therapies for the inner ear often failed in the past not least due to the fact that no suitable model systems for cell biological and pharmacological in vitro studies were available. A novel technology for creating "mini-organs", so-called organoids, could solve this problem and has now also reached inner ear research. It makes it possible to produce inner ear organoids from cochlear stem/progenitor cells, embryonic and induced pluripotent stem cells that mimic the structural characteristics and functional properties of the natural inner ear. This review focuses on the biological basis of these inner ear organoids, the current state of research and the promising prospects that are now opening up for basic and translational inner ear research.

Spatially distinct otic mesenchyme cells show molecular and functional heterogeneity patterns before hearing onset

The cochlea consists of diverse cellular populations working in harmony to convert mechanical stimuli into electrical signals for the perception of sound. Otic mesenchyme cells (OMCs), often considered a homogeneous cell type, are essential for normal cochlear development and hearing. Despite being the most numerous cell type in the developing cochlea, OMCs are poorly understood. OMCs are known to differentiate into spatially and functionally distinct cell types, including fibrocytes of the lateral wall and spiral limbus, modiolar osteoblasts, and specialized tympanic border cells of the basilar membrane. Here, we show that OMCs are transcriptionally and functionally heterogeneous and can be divided into four distinct populations that spatially correspond to OMC-derived cochlear structures. We also show that this heterogeneity and complexity of OMCs commences during early phases of cochlear development. Finally, we describe the cell-cell communication network of the developing cochlea, inferring a major role for OMC in outgoing signaling.

Human pluripotent stem cell-derived inner ear organoids recapitulate otic development in vitro

Our molecular understanding of the early stages of human inner ear development has been limited by the difficulty in accessing fetal samples at early gestational stages. As an alternative, previous studies have shown that inner ear morphogenesis can be partially recapitulated using induced pluripotent stem cells directed to differentiate into inner ear organoids (IEOs). Once validated and benchmarked, these systems could represent unique tools to complement and refine our understanding of human otic differentiation and model developmental defects. Here, we provide the first direct comparisons of the early human embryonic otocyst and fetal sensory organs with human IEOs. We use multiplexed immunostaining and single-cell RNA-sequencing to characterize IEOs at three key developmental steps, providing a new and unique signature of in vitro-derived otic placode, epithelium, neuroblasts and sensory epithelia. In parallel, we evaluate the expression and localization of crucial markers at these equivalent stages in human embryos. Together, our data indicate that the current state-of-the-art protocol enables the specification of bona fide otic tissue, supporting the further application of IEOs to inform inner ear biology and disease.

Mapping oto-pharyngeal development in a human inner ear organoid model

Inner ear development requires the coordination of cell types from distinct epithelial, mesenchymal and neuronal lineages. Although we have learned much from animal models, many details about human inner ear development remain elusive. We recently developed an in vitro model of human inner ear organogenesis using pluripotent stem cells in a 3D culture, fostering the growth of a sensorineural circuit, including hair cells and neurons. Despite previously characterizing some cell types, many remain undefined. This study aimed to chart the in vitro development timeline of the inner ear organoid to understand the mechanisms at play. Using single-cell RNA sequencing at ten stages during the first 36 days of differentiation, we tracked the evolution from pluripotency to various ear cell types after exposure to specific signaling modulators. Our findings showcase gene expression that influences differentiation, identifying a plethora of ectodermal and mesenchymal cell types. We also discern aspects of the organoid model consistent with in vivo development, while highlighting potential discrepancies. Our study establishes the Inner Ear Organoid Developmental Atlas (IODA), offering deeper insights into human biology and improving inner ear tissue differentiation.

A single-cell level comparison of human inner ear organoids with the human cochlea and vestibular organs

Inner ear disorders are among the most common congenital abnormalities; however, current tissue culture models lack the cell type diversity to study these disorders and normal otic development. Here, we demonstrate the robustness of human pluripotent stem cell-derived inner ear organoids (IEOs) and evaluate cell type heterogeneity by single-cell transcriptomics. To validate our findings, we construct a single-cell atlas of human fetal and adult inner ear tissue. Our study identifies various cell types in the IEOs including periotic mesenchyme, type I and type II vestibular hair cells, and developing vestibular and cochlear epithelium. Many genes linked to congenital inner ear dysfunction are confirmed to be expressed in these cell types. Additional cell-cell communication analysis within IEOs and fetal tissue highlights the role of endothelial cells on the developing sensory epithelium. These findings provide insights into this organoid model and its potential applications in studying inner ear development and disorders.

Graphene Hybrid Inner Ear Organoid with Enhanced Maturity

Inner ear organoids (IEOs) are 3D structures grown in vitro, which can mimic the complex cellular structure and function of the inner ear. IEOs are potential solutions to problems related to inner ear development, disease modeling, and drug delivery. However, current approaches in generating IEOs using chemical factors have a few limitations, resulting in unpredictable outcomes. In this study, we propose the use of nanomaterial-based approaches, specifically by using graphene oxide (GO). GO's unique properties promote cell-extracellular matrix interactions and cell-cell gap junctions, thereby enhancing hair cell formation, which is an essential part of IEO development. We also investigated the potential applications for drug testing. Our findings suggest that GO is a promising candidate for enhancing the functionality of IEOs and advancing our understanding of the problems underlying inner ear development. The use of nanomaterial-based approaches may provide a more reliable and effective method for building better IEOs in the future.

Human pluripotent stem cells-derived inner ear organoids recapitulate otic development in vitro

Our molecular understanding of the early stages of human inner ear development has been limited by the difficulty in accessing fetal samples at early gestational stages. As an alternative, previous studies have shown that inner ear morphogenesis can be partially recapitulated using induced pluripotent stem cells (iPSCs) directed to differentiate into Inner Ear Organoids (IEOs). Once validated and benchmarked, these systems could represent unique tools to complement and refine our understanding of human otic differentiation and model developmental defects. Here, we provide the first direct comparisons of the early human embryonic otocyst and human iPSC-derived IEOs. We use multiplexed immunostaining, and single-cell RNA sequencing to characterize IEOs at three key developmental steps, providing a new and unique signature of in vitro derived otic -placode, -epithelium, -neuroblasts, and -sensory epithelia. In parallel, we evaluate the expression and localization of critical markers at these equivalent stages in human embryos. We show that the placode derived in vitro (days 8-12) has similar marker expression to the developing otic placode of Carnegie Stage (CS) 11 embryos and subsequently (days 20-40) this gives rise to otic epithelia and neuroblasts comparable to the CS13 embryonic stage. Differentiation of sensory epithelia, including supporting cells and hair cells starts in vitro at days 50-60 of culture. The maturity of these cells is equivalent to vestibular sensory epithelia at week 10 or cochlear tissue at week 12 of development, before functional onset. Together, our data indicate that the current state-of-the-art protocol enables the specification of bona fide otic tissue, supporting the further application of IEOs to inform inner ear biology and disease.

Transcriptomic profile comparison reveals conservation of ionocytes across multiple organs

Single-cell RNA sequencing has recently led to the identification of a flurry of rare, new cell types, such as the CFTR-high ionocytes in the airway epithelium. Ionocytes appear to be specifically responsible for fluid osmolarity and pH regulation. Similar cells exist in multiple other organs and have received various names, including intercalated cell in the kidney, mitochondria-rich cell in the inner ear, clear cell in the epididymis, and ionocyte in the salivary gland. Here, we compare the previously published transcriptomic profile of cells expressing FOXI1, the signature transcription factor expressed in airway ionocytes. Such FOXI1+ cells were found in datasets representing human and/or murine kidney, airway, epididymis, thymus, skin, inner ear, salivary gland, and prostate. This allowed us to assess the similarities between these cells and identify the core transcriptomic signature of this ionocyte 'family'. Our results demonstrate that, across all these organs, ionocytes maintain the expression of a characteristic set of genes, including FOXI1, KRT7, and ATP6V1B1. We conclude that the ionocyte signature defines a class of closely related cell types across multiple mammalian organs.

Modeling nervous system tumors with human stem cells and organoids

Nervous system cancers are the 10th leading cause of death worldwide, many of which are difficult to diagnose and exhibit varying degrees of treatment resistance. The limitations of existing cancer models, such as patient-derived xenograft (PDX) models and genetically engineered mouse (GEM) models, call for the development of novel preclinical cancer models to more faithfully mimic the patient's cancer and offer additional insights. Recent advances in human stem cell biology, organoid, and genome-editing techniques allow us to model nervous system tumors in three types of next-generation tumor models: cell-of-origin models, tumor organoids, and 3D multicellular coculture models. In this review, we introduced and compared different human stem cell/organoid-derived models, and comprehensively summarized and discussed the recently developed models for various primary tumors in the central and peripheral nervous systems, including glioblastoma (GBM), H3K27M-mutant Diffuse Midline Glioma (DMG) and H3G34R-mutant High-grade Glioma (HGG), Low-grade Glioma (LGG), Neurofibromatosis Type 1 (NF1), Neurofibromatosis Type 2 (NF2), Medulloblastoma (MB), Atypical Teratoid/rhabdoid Tumor (AT/RT), and meningioma. We further compared these models with PDX and GEM models, and discussed the opportunities and challenges of precision nervous cancer modeling with human stem cells and organoids.

Mitochondrial form and function in hair cells

Hair cells (HCs) are specialised sensory receptors residing in the neurosensory epithelia of inner ear sense organs. The precise morphological and physiological properties of HCs allow us to perceive sound and interact with the world around us. Mitochondria play a significant role in normal HC function and are also intricately involved in HC death. They generate ATP essential for sustaining the activity of ion pumps, Ca²⁺ transporters and the integrity of the stereociliary bundle during transduction as well as regulating cytosolic calcium homoeostasis during synaptic transmission. Advances in imaging techniques have allowed us to study mitochondrial populations throughout the HC, and how they interact with other organelles. These analyses have identified distinct mitochondrial populations between the apical and basolateral portions of the HC, in which mitochondrial morphology appears determined by the physiological processes in the different cellular compartments. Studies in HCs across species show that ototoxic agents, ageing and noise damage directly impact mitochondrial structure and function resulting in HC death. Deciphering the molecular mechanisms underlying this mitochondrial sensitivity, and how their morphology relates to their function during HC death, requires that we first understand this relationship in the context of normal HC function.

Inner ear organoids: progress and outlook, with a focus on the vascularization

The inner ear is a complex organ that encodes sound, motion, and orientation in space. Given the complexity of the inner ear, it is not surprising that treatments are relatively limited despite the fact that, in 2015, hearing loss was the fourth leading cause of years lived with disability worldwide. Inner ear organoid models are a promising tool to advance the study of multiple aspects of the inner ear to aid the development of new treatments and validate drug-based therapies. The blood supply of the inner ear plays a pivotal role in growth, maturation, and survival of inner ear tissues and their physiological functions. This vasculature cannot be ignored in order to achieve a truly in vivo-like model that mimics the microenvironment and niches of organ development. However, this aspect of organoid development has remained largely absent in the generation of inner ear organoids. The current review focuses on three-dimensional inner ear organoid and how recent technical progress in generating in vitro vasculature can enhance the next generation of these models.

Modelling inner ear development and disease using pluripotent stem cells - a pathway to new therapeutic strategies

The sensory epithelia of the mammalian inner ear enable sound and movement to be perceived. Damage to these epithelia can cause irreversible sensorineural hearing loss and vestibular dysfunction because they lack regenerative capacity. The human inner ear cannot be biopsied without causing permanent damage, significantly limiting the tissue samples available for research. Investigating disease pathology and therapeutic developments have therefore traditionally relied on animal models, which often cannot completely recapitulate the human otic systems. These challenges are now being partly addressed using induced pluripotent stem cell-derived cultures, which generate the sensory epithelial-like tissues of the inner ear. Here, we review how pluripotent stem cells have been used to produce two-dimensional and three-dimensional otic cultures, the strengths and limitations of these new approaches, and how they have been employed to investigate genetic and acquired forms of audiovestibular dysfunction. This Review provides an overview of the progress in pluripotent stem cell-derived otic cultures thus far, focusing on their applications in disease modelling and therapeutic trials. We survey their current limitations and future directions, highlighting their prospective utility for high-throughput drug screening and developing personalised medicine approaches.

Induced Pluripotent Stem Cells, a Stepping Stone to In Vitro Human Models of Hearing Loss

Hearing loss is the most prevalent sensorineural impairment in humans. Yet despite very active research, no effective therapy other than the cochlear implant has reached the clinic. Main reasons for this failure are the multifactorial nature of the disorder, its heterogeneity, and a late onset that hinders the identification of etiological factors. Another problem is the lack of human samples such that practically all the work has been conducted on animals. Although highly valuable data have been obtained from such models, there is the risk that inter-species differences exist that may compromise the relevance of the gathered data. Human-based models are therefore direly needed. The irruption of human induced pluripotent stem cell technologies in the field of hearing research offers the possibility to generate an array of otic cell models of human origin; these may enable the identification of guiding signalling cues during inner ear development and of the mechanisms that lead from genetic alterations to pathology. These models will also be extremely valuable when conducting ototoxicity analyses and when exploring new avenues towards regeneration in the inner ear. This review summarises some of the work that has already been conducted with these cells and contemplates future possibilities.

G protein-coupled receptors in cochlea: Potential therapeutic targets for hearing loss

The prevalence of hearing loss-related diseases caused by different factors is increasing worldwide year by year. Currently, however, the patient’s hearing loss has not been effectively improved. Therefore, there is an urgent need to adopt new treatment measures and treatment techniques to help improve the therapeutic effect of hearing loss. G protein-coupled receptors (GPCRs), as crucial cell surface receptors, can widely participate in different physiological and pathological processes, particularly play an essential role in many disease occurrences and be served as promising therapeutic targets. However, no specific drugs on the market have been found to target the GPCRs of the cochlea. Interestingly, many recent studies have demonstrated that GPCRs can participate in various pathogenic process related to hearing loss in the cochlea including heredity, noise, ototoxic drugs, cochlear structure, and so on. In this review, we comprehensively summarize the functions of 53 GPCRs known in the cochlea and their relationships with hearing loss, and highlight the recent advances of new techniques used in cochlear study including cryo-EM, AI, GPCR drug screening, gene therapy vectors, and CRISPR editing technology, as well as discuss in depth the future direction of novel GPCR-based drug development and gene therapy for cochlear hearing loss. Collectively, this review is to facilitate basic and (pre-) clinical research in this area, and provide beneficial help for emerging GPCR-based cochlear therapies.

Advancements in inner ear development, regeneration, and repair through otic organoids

The vertebrate inner ear contains a diversity of unique cell types arranged in a particularly complex 3D cytoarchitecture. Both of these features are integral to the proper development, function, and maintenance of hearing and balance. Since the elucidation of the timing and delivery of signaling molecules to produce inner ear sensory cells, supporting cells, and neurons from human induced pluripotent stem cells, we have entered a revolution using organ-like 'otic organoid' cultures to explore inner ear specific genetic programs, developmental rules, and potential therapeutics. This review aims to highlight a selection of reviews and primary research papers from the past two years of particular merit that use otic organoids to investigate the broadly defined topics of cell reprogramming, regeneration, and repair.

Cellular autophagy, the compelling roles in hearing function and dysfunction

Sensorineural hearing loss (SNHL) is currently a major health issue. As one of the most common neurodegenerative diseases, SNHL is associated with the degradation of hair cells (HCs), spiral ganglion neurons (SGNs), the stria vascularis, supporting cells and central auditory system cells. Autophagy is a highly integrated cellular system that eliminates impaired components and replenishes energy to benefit cellular homeostasis. Etiological links between autophagy alterations and neurodegenerative diseases, such as SNHL, have been established. The hearing pathway is complex and depends on the comprehensive functions of many types of tissues and cells in auditory system. In this review, we discuss the roles of autophagy in promoting and inhibiting hearing, paying particular attention to specific cells in the auditory system, as discerned through research. Hence, our review provides enlightening ideas for the role of autophagy in hearing development and impairment.

An In Vitro Model for Characterization of Drug Permeability across the Tympanic Membrane

Otic disorders, such as otitis media and hearing loss, affect a substantial portion of the global population. Despite this, oto-therapeutics, in particular those intended to treat hearing loss, have seen limited development and innovation. A significant factor to this is likely a result of the inherent costs and complexities of drug discovery and development. With in vitro 3D tissue models seeing increased utility for the rapid, high-throughput screening of drug candidates, it stands to reason that the field of otology could greatly benefit from such innovations. In this study, we propose and describe an in vitro 3D model, designed using a physiologically based approach, which we suggest can be used to estimate drug permeability across human tympanic membranes (TM). We characterize the permeability properties of several template drugs in this model under various growth and storage conditions. The availability of such cost-effective, rapid, high-throughput screening tools should allow for increased innovation and the discovery of novel drug candidates over the currently used animal models. In the context of this TM permeation model, it may promote the development of topical drugs and formulations that can non-invasively traverse the TM and provide tissue-targeted drug delivery as an alternative to systemic treatment, an objective which has seen limited study until present.

Development and Characterization of an In Vitro Round Window Membrane Model for Drug Permeability Evaluations

Hearing loss and balance disorders are highly common disorders, and the development of effective oto-therapeutics remains an area of intense research. Drug development and screening in the hearing research field heavily rely on the use of preclinical models with often ambiguous translational relevance. This often leads to failed advancement in the market of effective therapeutics. In this context, especially for inner ear-specific pathologies, the availability of an in vitro, physiologically relevant, round window membrane (RWM) model could enable rapid, high-throughput screening of potential topical drugs for inner ear and cochlear dysfunctions and could help accelerate the advancement to clinic and market of more viable drug candidates. In this study, we report the development and evaluation of an in vitro model that mimics the native RWM tissue morphology and microenvironment as shown via immunostaining and histological analyses. The developed three-dimensional (3D) in vitro model was additionally assessed for barrier integrity by transepithelial electrical resistance, and the permeability of lipophilic and hydrophilic drugs was determined. Our collective findings suggest that this in vitro model could serve as a tool for rapid development and screening of topically deliverable oto-therapeutics.

Vestibular Deficits in Deafness: Clinical Presentation, Animal Modeling, and Treatment Solutions

The inner ear is responsible for both hearing and balance. These functions are dependent on the correct functioning of mechanosensitive hair cells, which convert sound- and motion-induced stimuli into electrical signals conveyed to the brain. During evolution of the inner ear, the major changes occurred in the hearing organ, whereas the structure of the vestibular organs remained constant in all vertebrates over the same period. Vestibular deficits are highly prevalent in humans, due to multiple intersecting causes: genetics, environmental factors, ototoxic drugs, infections and aging. Studies of deafness genes associated with balance deficits and their corresponding animal models have shed light on the development and function of these two sensory systems. Bilateral vestibular deficits often impair individual postural control, gaze stabilization, locomotion and spatial orientation. The resulting dizziness, vertigo, and/or falls (frequent in elderly populations) greatly affect patient quality of life. In the absence of treatment, prosthetic devices, such as vestibular implants, providing information about the direction, amplitude and velocity of body movements, are being developed and have given promising results in animal models and humans. Novel methods and techniques have led to major progress in gene therapies targeting the inner ear (gene supplementation and gene editing), 3D inner ear organoids and reprograming protocols for generating hair cell-like cells. These rapid advances in multiscale approaches covering basic research, clinical diagnostics and therapies are fostering interdisciplinary research to develop personalized treatments for vestibular disorders.

Regeneration of Hair Cells in the Human Vestibular System

The vestibular system is a critical part of the human balance system, malfunction of this system will lead to balance disorders, such as vertigo. Mammalian vestibular hair cells, the mechanical receptors for vestibular function, are sensitive to ototoxic drugs and virus infection, and have a limited restorative capacity after damage. Considering that no artificial device can be used to replace vestibular hair cells, promoting vestibular hair cell regeneration is an ideal way for vestibular function recovery. In this manuscript, the development of human vestibular hair cells during the whole embryonic stage and the latest research on human vestibular hair cell regeneration is summarized. The limitations of current studies are emphasized and future directions are discussed.

Age-Related Hearing Loss: Sensory and Neural Etiology and Their Interdependence

Age-related hearing loss (ARHL) is a common, increasing problem for older adults, affecting about 1 billion people by 2050. We aim to correlate the different reductions of hearing from cochlear hair cells (HCs), spiral ganglion neurons (SGNs), cochlear nuclei (CN), and superior olivary complex (SOC) with the analysis of various reasons for each one on the sensory deficit profiles. Outer HCs show a progressive loss in a basal-to-apical gradient, and inner HCs show a loss in a apex-to-base progression that results in ARHL at high frequencies after 70 years of age. In early neonates, SGNs innervation of cochlear HCs is maintained. Loss of SGNs results in a considerable decrease (~50% or more) of cochlear nuclei in neonates, though the loss is milder in older mice and humans. The dorsal cochlear nuclei (fusiform neurons) project directly to the inferior colliculi while most anterior cochlear nuclei reach the SOC. Reducing the number of neurons in the medial nucleus of the trapezoid body (MNTB) affects the interactions with the lateral superior olive to fine-tune ipsi- and contralateral projections that may remain normal in mice, possibly humans. The inferior colliculi receive direct cochlear fibers and second-order fibers from the superior olivary complex. Loss of the second-order fibers leads to hearing loss in mice and humans. Although ARHL may arise from many complex causes, HC degeneration remains the more significant problem of hearing restoration that would replace the cochlear implant. The review presents recent findings of older humans and mice with hearing loss.

Recent advancements in cell-based models for auditory disorders

Introduction: Cell-based models play an important role in understanding the pathophysiology and etiology of auditory disorders. For the auditory system, models have primarily focused on restoring inner and outer hair cells. However, they have largely underrepresented the surrounding structures and cells that support the function of the hair cells.

Methods: In this article, we will review recent advancements in the evolution of cell-based models of auditory disorders in their progression towards three dimensional (3D) models and organoids that more closely mimic the pathophysiology in vivo.

Results: With the elucidation of the molecular targets and transcription factors required to generate diverse cell lines of the components of inner ear, research is starting to progress from two dimensional (2D) models to a greater 3D approach. Of note, the 3D models of the inner ear, including organoids, are relatively new and emerging in the field. As 3D models of the inner ear continue to evolve in complexity, their role in modeling disease will grow as they bridge the gap between cell culture and in vivo models.

Conclusion: Using 3D cell models to understand the etiology and molecular mechanisms underlying auditory disorders holds great potential for developing more targeted and effective novel therapeutics.

Understanding the differentiation and epigenetics of cochlear sensory progenitors in pursuit of regeneration

PURPOSE OF REVIEW

Sensory hair cells (HCs) of the inner ear are responsible for our ability to hear and balance. Loss of these cells results in hearing loss. Stem cell replacement and in situ regeneration have the potential to replace lost HCs. Newly discovered contributions of transcription factor regulatory networks and epigenetic mechanisms in regulating HC differentiation and regeneration are placed into context of the literature.

RECENT FINDINGS

A wealth of new data has helped to define cochlear sensory progenitors in their developmental trajectories. This includes transcription factor networks, epigenetic manipulations, and cochlear HC subtype specification.

SUMMARY

Understanding how sensory progenitors differ and how HC subtypes arise will substantially inform efforts in hearing restoration.

Deafness-in-a-dish: modeling hereditary deafness with inner ear organoids

Sensorineural hearing loss (SNHL) is a major cause of functional disability in both the developed and developing world. While hearing aids and cochlear implants provide significant benefit to many with SNHL, neither targets the cellular and molecular dysfunction that ultimately underlies SNHL. The successful development of more targeted approaches, such as growth factor, stem cell, and gene therapies, will require a yet deeper understanding of the underlying molecular mechanisms of human hearing and deafness. Unfortunately, the human inner ear cannot be biopsied without causing significant, irreversible damage to the hearing or balance organ. Thus, much of our current understanding of the cellular and molecular biology of human deafness, and of the human auditory system more broadly, has been inferred from observational and experimental studies in animal models, each of which has its own advantages and limitations. In 2013, researchers described a protocol for the generation of inner ear organoids from pluripotent stem cells (PSCs), which could serve as scalable, high-fidelity alternatives to animal models. Here, we discuss the advantages and limitations of conventional models of the human auditory system, describe the generation and characteristics of PSC-derived inner ear organoids, and discuss several strategies and recent attempts to model hereditary deafness in vitro. Finally, we suggest and discuss several focus areas for the further, intensive characterization of inner ear organoids and discuss the translational applications of these novel models of the human inner ear.

Towards maturation of human otic hair cell-like cells in pluripotent stem cell-derived organoid transplants

Human otic organoids generated from pluripotent stem cells (PSCs) provide a promising platform for modeling, drug testing, and cell-based therapies of inner ear diseases. However, providing the appropriate niche that resembles inner ear development and its vasculature to generate otic organoids is less conspicuous. Here, we devised a strategy to enhance maturation of otic progenitor cells toward human hair cell-like cells (HCLCs) by assembling three-dimensional (3D) otic organoids that contain human PSC-derived otic cells, endothelial cells, and mesenchymal stem cells (MSCs). Heterotopic implantation of otic organoids, designated as grafted otic organoids (GOs), in ex ovo chick embryo chorioallantoic membrane (CAM) stimulated maturation of the HCLCs. Functional analysis revealed the presence of voltage-gated potassium currents without detectable sodium currents in these cells in the GOs. Our results demonstrated that implantation of 3D heterotypic cell mixtures of otic organoids improved maturation of human HCLCs. This GO-derived HCLCs could be an attractive source for drug discovery and other biomedical applications.