Organ of Corti size is governed by Yap/Tead-mediated progenitor self-renewal

Human cytomegalovirus infection coopts chromatin organization to diminish TEAD1 transcription factor activity

Human cytomegalovirus (HCMV) infects up to 80% of the world's population. Here, we show that HCMV infection leads to widespread changes in human chromatin accessibility and chromatin looping, with hundreds of thousands of genomic regions affected 48 hours after infection. Integrative analyses reveal HCMV-induced perturbation of Hippo signaling through drastic reduction of TEAD1 transcription factor activity. We confirm extensive concordant loss of TEAD1 binding, active H3K27ac histone marks, and chromatin looping interactions upon infection. Our data position TEAD1 at the top of a hierarchy involving multiple altered important developmental pathways. HCMV infection reduces TEAD1 activity through four distinct mechanisms: closing of TEAD1-bound chromatin, reduction of YAP1 and phosphorylated YAP1 levels, reduction of TEAD1 transcript and protein levels, and alteration of TEAD1 exon-6 usage. Altered TEAD1-based mechanisms are highly enriched at genetic risk loci associated with eye and ear development, providing mechanistic insight into HCMV's established roles in these processes.

Ptch1 is essential for cochlear marginal cell differentiation and stria vascularis formation

A common cause of deafness in humans is dysregulation of the endocochlear potential generated by the stria vascularis (SV). Thus, proper formation of the SV is critical for hearing. Using single-cell transcriptomics and a series of Shh signaling mutants, we discovered that the Shh receptor Patched1 (Ptch1) is essential for marginal cell (MC) differentiation and SV formation. Single-cell RNA sequencing analyses revealed that the cochlear roof epithelium is already specified into discrete domains with distinctive gene expression profiles at embryonic day 14, with Gsc as a marker gene of the MC lineage. Ptch1 deficiency leads to defective specification of MC precursors along the cochlear basal-apical regions. We demonstrated that elevated Gli2 levels impede MC differentiation through sustaining Otx2 expression and maintaining the progenitor state of MC precursors. Our results uncover an early specification of cochlear non-sensory epithelial cells and establish a crucial role of the Ptch1-Gli2 axis in regulating the development of SV.

Spherical harmonics analysis reveals cell shape-fate relationships in zebrafish lateral line neuromasts

Cell shape is a powerful readout of cell state, fate and function. We describe a custom workflow to perform semi-automated, 3D cell and nucleus segmentation, and spherical harmonics and principal components analysis to distill cell and nuclear shape variation into discrete biologically meaningful parameters. We apply these methods to analyze shape in the neuromast cells of the zebrafish lateral line system, finding that shapes vary with cell location and identity. The distinction between hair cells and support cells accounted for much of the variation, which allowed us to train classifiers to predict cell identity from shape features. Using transgenic markers for support cell subpopulations, we found that subtypes had different shapes from each other. To investigate how loss of a neuromast cell type altered cell shape distributions, we examined atoh1a mutants that lack hair cells. We found that mutant neuromasts lacked the cell shape phenotype associated with hair cells, but did not exhibit a mutant-specific cell shape. Our results demonstrate the utility of using 3D cell shape features to characterize, compare and classify cells in a living developing organism.

Stem cells as potential therapeutics for hearing loss

Hearing impairment is a global health problem. Stem cell therapy has become a cutting-edge approach to tissue regeneration. In this review, the recent advances in stem cell therapy for hearing loss have been discussed. Nanomaterials can modulate the stem cell microenvironment to augment the therapeutic effects further. The potential of combining nanomaterials with stem cells for repairing and regenerating damaged inner ear hair cells (HCs) and spiral ganglion neurons (SGNs) has also been discussed. Stem cell-derived exosomes can contribute to the repair and regeneration of damaged tissue, and the research progress on exosome-based hearing loss treatment has been summarized as well. Despite stem cell therapy's technical and practical limitations, the findings reported so far are promising and warrant further investigation for eventual clinical translation.

SoxC transcription factors shape the epigenetic landscape to establish competence for sensory differentiation in the mammalian organ of Corti

The auditory organ of Corti is comprised of only two major cell types-the mechanosensory hair cells and their associated supporting cells-both specified from a single pool of prosensory progenitors in the cochlear duct. Here, we show that competence to respond to Atoh1, a transcriptional master regulator necessary and sufficient for induction of mechanosensory hair cells, is established in the prosensory progenitors between E12.0 and 13.5. The transition to the competent state is rapid and is associated with extensive remodeling of the epigenetic landscape controlled by the SoxC group of transcription factors. Conditional loss of Sox4 and Sox11-the two homologous family members transiently expressed in the inner ear at the time of competence establishment-blocks the ability of prosensory progenitors to differentiate as hair cells. Mechanistically, we show that Sox4 binds to and establishes accessibility of early sensory lineage-specific regulatory elements, including ones associated with Atoh1 and its direct downstream targets. Consistent with these observations, overexpression of Sox4 or Sox11 prior to developmental establishment of competence precociously induces hair cell differentiation in the cochlear progenitors. Further, reintroducing Sox4 or Sox11 expression restores the ability of postnatal supporting cells to differentiate as hair cells in vitro and in vivo. Our findings demonstrate the pivotal role of SoxC family members as agents of epigenetic and transcriptional changes necessary for establishing competence for sensory receptor differentiation in the inner ear.

Experimental drugs for the prevention or treatment of sensorineural hearing loss

INTRODUCTION

Sensorineural hearing loss results in irreversible loss of inner ear hair cells and spiral ganglion neurons. Reduced sound detection and speech discrimination can span all ages, and sensorineural hearing rehabilitation is limited to amplification with hearing aids or cochlear implants. Recent insights into experimental drug treatments for inner ear regeneration and otoprotection have paved the way for clinical trials in order to restore a more physiological hearing experience. Paired with the development of innovative minimally invasive approaches for drug delivery to the inner ear, new, emerging treatments for hearing protection and restoration are within reach.

AREAS COVERED

This expert opinion provides an overview of the latest experimental drug therapies to protect from and to restore sensorineural hearing loss.

EXPERT OPINION

The degree and type of cellular damage to the cochlea, the responsiveness of remaining, endogenous cells to regenerative treatments, and the duration of drug availability within cochlear fluids will determine the success of hearing protection or restoration.

Transcriptional dynamics of delaminating neuroblasts in the mouse otic vesicle

An abundance of research has recently highlighted the susceptibility of cochleovestibular ganglion (CVG) neurons to noise damage and aging in the adult cochlea, resulting in hearing deficits. Furthering our understanding of the transcriptional cascades that contribute to CVG development may provide insight into how these cells can be regenerated to treat inner ear dysfunction. Here we perform a high-depth single-cell RNA sequencing analysis of the E10.5 otic vesicle and its surrounding tissues, including CVG precursor neuroblasts and emerging CVG neurons. Clustering and trajectory analysis of otic-lineage cells reveals otic markers and the changes in gene expression that occur from neuroblast delamination toward the development of the CVG. This dataset provides a valuable resource for further identifying the mechanisms associated with CVG development from neurosensory competent cells within the otic vesicle.

Early Steps towards Hearing: Placodes and Sensory Development

Sensorineural hearing loss is the most prevalent sensory deficit in humans. Most cases of hearing loss are due to the degeneration of key structures of the sensory pathway in the cochlea, such as the sensory hair cells, the primary auditory neurons, and their synaptic connection to the hair cells. Different cell-based strategies to replace damaged inner ear neurosensory tissue aiming at the restoration of regeneration or functional recovery are currently the subject of intensive research. Most of these cell-based treatment approaches require experimental in vitro models that rely on a fine understanding of the earliest morphogenetic steps that underlie the in vivo development of the inner ear since its initial induction from a common otic-epibranchial territory. This knowledge will be applied to various proposed experimental cell replacement strategies to either address the feasibility or identify novel therapeutic options for sensorineural hearing loss. In this review, we describe how ear and epibranchial placode development can be recapitulated by focusing on the cellular transformations that occur as the inner ear is converted from a thickening of the surface ectoderm next to the hindbrain known as the otic placode to an otocyst embedded in the head mesenchyme. Finally, we will highlight otic and epibranchial placode development and morphogenetic events towards progenitors of the inner ear and their neurosensory cell derivatives.

Craniofacial Development Is Fine-Tuned by Sox2

The precise control of neural crest stem cell delamination, migration and differentiation ensures proper craniofacial and head development. Sox2 shapes the ontogeny of the cranial neural crest to ensure precision of the cell flow in the developing head. Here, we review how Sox2 orchestrates signals that control these complex developmental processes.

Generation of innervated cochlear organoid recapitulates early development of auditory unit

Functional cochlear hair cells (HCs) innervated by spiral ganglion neurons (SGNs) are essential for hearing, whereas robust models that recapitulate the peripheral auditory circuity are still lacking. Here, we developed cochlear organoids with functional peripheral auditory circuity in a staging three-dimensional (3D) co-culture system by initially reprogramming cochlear progenitor cells (CPCs) with increased proliferative potency that could be long-term expanded, then stepwise inducing the differentiation of cochlear HCs, as well as the outgrowth of neurites from SGNs. The function of HCs and synapses within organoids was confirmed by a series of morphological and electrophysiological evaluations. Single-cell mRNA sequencing revealed the differentiation trajectories of CPCs toward the major cochlear cell types and the dynamic gene expression during organoid HC development, which resembled the pattern of native HCs. We established the cochlear organoids with functional synapses for the first time, which provides a platform for deciphering the mechanisms of sensorineural hearing loss.

Temporal and regulatory dynamics of the inner ear transcriptome during development in mice

The inner ear controls hearing and balance, while the temporal molecular signatures and transcriptional regulatory dynamics underlying its development are still unclear. In this study, we investigated time-series transcriptome in the mouse inner ear from embryonic day 11.5 (E11.5) to postnatal day 7 (P7) using bulk RNA-Seq. A total of 10,822 differentially expressed genes were identified between pairwise stages. We identified nine significant temporal expression profiles using time-series expression analysis. The constantly down-regulated profiles throughout the development are related to DNA activity and neurosensory development, while the constantly upregulated profiles are related to collagen and extracellular matrix. Further co-expression network analysis revealed that several hub genes, such as Pnoc, Cd9, and Krt27, are related to the neurosensory development, cell adhesion, and keratinization. We uncovered three important transcription regulatory paths during mice inner ear development. Transcription factors related to Hippo/TGFβ signaling induced decreased expressions of genes related to the neurosensory and inner ear development, while a series of INF genes activated the expressions of genes in immunoregulation. In addition to deepening our understanding of the temporal and regulatory mechanisms of inner ear development, our transcriptomic data could fuel future multi-species comparative studies and elucidate the evolutionary trajectory of auditory development.

Loss of Mst1/2 activity promotes non-mitotic hair cell generation in the neonatal organ of Corti

Mammalian sensory hair cells (HCs) have limited capacity for regeneration, which leads to permanent hearing loss after HC death. Here, we used in vitro RNA-sequencing to show that the Hippo signaling pathway is involved in HC damage and self-repair processes. Turning off Hippo signaling through Mst1/2 inhibition or Yap overexpression induces YAP nuclear accumulation, especially in supporting cells, which induces supernumerary HC production and HC regeneration after injury. Mechanistically, these effects of Hippo signaling work synergistically with the Notch pathway. Importantly, the supernumerary HCs not only express HC markers, but also have cilia structures that are able to form neural connections to auditory regions in vivo. Taken together, regulating Hippo suggests new strategies for promoting cochlear supporting cell proliferation, HC regeneration, and reconnection with neurons in mammals.

The regenerative capacity of neonatal tissues

It is well established that humans and other mammals are minimally regenerative compared with organisms such as zebrafish, salamander or amphibians. In recent years, however, the identification of regenerative potential in neonatal mouse tissues that normally heal poorly in adults has transformed our understanding of regenerative capacity in mammals. In this Review, we survey the mammalian tissues for which regenerative or improved neonatal healing has been established, including the heart, cochlear hair cells, the brain and spinal cord, and dense connective tissues. We also highlight common and/or tissue-specific mechanisms of neonatal regeneration, which involve cells, signaling pathways, extracellular matrix, immune cells and other factors. The identification of such common features across neonatal tissues may direct therapeutic strategies that will be broadly applicable to multiple adult tissues.

Hearing loss: The final frontier of pharmacology

Despite a prevalence greater than cancer or diabetes, there are no currently approved drugs for the treatment of hearing loss. Research over the past two decades has led to a vastly improved understanding of the cellular and molecular mechanisms in the cochlea that lead to hearing deficits and the advent of novel strategies to combat them. Combined with innovative methods that enable local drug delivery to the inner ear, these insights have paved the way for promising therapies that are now under clinical investigation. In this review, we will outline this renaissance of cochlear biology and drug development, focusing on noise, age-related, and chemotherapy-induced hearing dysfunction.

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.

Emerging therapies for human hearing loss

INTRODUCTION

More than 5% of the world's population have a disabling hearing loss which can be managed by hearing aids or implanted electrical devices. However, outcomes are highly variable, and the sound perceived by recipients is far from perfect. Sparked by the discovery of progenitor cells in the cochlea and rapid progress in drug delivery to the cochlea, biological and pharmaceutical therapies are currently in development to improve the function of the cochlear implant or eliminate the need for it altogether.

AREAS COVERED

This review highlights progress in emerging regenerative strategies to restore hearing and adjunct therapies to augment the cochlear implant. Novel approaches include the reprogramming of progenitor cells to restore the sensory hair cell population in the cochlea, gene therapy and gene editing to treat hereditary and acquired hearing loss. A detailed review of optogenetics is also presented as a technique that could enable optical stimulation of the spiral ganglion neurons, replacing or complementing electrical stimulation.

EXPERT OPINION

Increasing evidence of substantial reversal of hearing loss in animal models, alongside rapid advances in delivery strategies to the cochlea and learnings from clinical trials will amalgamate into a biological or pharmaceutical therapy to replace or complement the cochlear implant.

The noncoding genome and hearing loss

The age of sequencing has provided unprecedented insights into the human genome. The coding region of the genome comprises nearly 20,000 genes, of which approximately 4000 are associated with human disease. Beyond the protein-coding genome, which accounts for only 3% of the genome, lies a vast pool of regulatory elements in the form of promoters, enhancers, RNA species, and other intricate elements. These features undoubtably influence human health and disease, and as a result, a great deal of effort is currently being invested in deciphering their identity and mechanism. While a paucity of material has caused a lag in identifying these elements in the inner ear, the emergence of technologies for dealing with a minimal number of cells now has the field working overtime to catch up. Studies on microRNAs (miRNAs), long non-coding RNAs (lncRNAs), methylation, histone modifications, and more are ongoing. A number of microRNAs and other noncoding elements are known to be associated with hearing impairment and there is promise that regulatory elements will serve as future tools and targets of therapeutics and diagnostics. This review covers the current state of the field and considers future directions for the noncoding genome and implications for hearing loss.

The mechanical forces that shape our senses

Developing organs are shaped, in part, by physical interaction with their environment in the embryo. In recent years, technical advances in live-cell imaging and material science have greatly expanded our understanding of the mechanical forces driving organ formation. Here, we provide a broad overview of the types of forces generated during embryonic development and then focus on a subset of organs underlying our senses: the eyes, inner ears, nose and skin. The epithelia in these organs emerge from a common origin: the ectoderm germ layer; yet, they arrive at unique and complex forms over developmental time. We discuss exciting recent animal studies that show a crucial role for mechanical forces in, for example, the thickening of sensory placodes, the coiling of the cochlea and the lengthening of hair. Finally, we discuss how microfabricated organoid systems can now provide unprecedented insights into the physical principles of human development.

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.

Follistatin promotes LIN28B-mediated supporting cell reprogramming and hair cell regeneration in the murine cochlea

Hair cell (HC) loss within the inner ear cochlea is a leading cause for deafness in humans. Before the onset of hearing, immature supporting cells (SCs) in neonatal mice have some limited capacity for HC regeneration. Here, we show that in organoid culture, transient activation of the progenitor-specific RNA binding protein LIN28B and Activin antagonist follistatin (FST) enhances regenerative competence of maturing/mature cochlear SCs by reprogramming them into progenitor-like cells. Transcriptome profiling and mechanistic studies reveal that LIN28B drives SC reprogramming, while FST is required to counterbalance hyperactivation of transforming growth factor-β-type signaling by LIN28B. Last, we show that LIN28B and FST coactivation enhances spontaneous cochlear HC regeneration in neonatal mice and that LIN28B may be part of an endogenous repair mechanism that primes SCs for HC regeneration. These findings indicate that SC dedifferentiation is critical for HC regeneration and identify LIN28B and FST as main regulators.

Hippo/YAP signaling pathway protects against neomycin-induced hair cell damage in the mouse cochlea

The Hippo/Yes-associated protein (YAP) signaling pathway has been shown to be able to maintain organ size and homeostasis by regulating cell proliferation, differentiation, and apoptosis. The abuse of aminoglycosides is one of the main causes of sensorineural hearing loss (SSNHL). However, the role of the Hippo/YAP signaling pathway in cochlear hair cell (HC) damage protection in the auditory field is still unclear. In this study, we used the YAP agonist XMU-MP-1 (XMU) and the inhibitor Verteporfin (VP) to regulate the Hippo/YAP signaling pathway in vitro. We showed that YAP overexpression reduced neomycin-induced HC loss, while downregulated YAP expression increased HC vulnerability after neomycin exposure in vitro. We next found that activation of YAP expression inhibited C-Abl-mediated cell apoptosis, which led to reduced HC loss. Many previous studies have reported that the level of reactive oxygen species (ROS) is significantly increased in cochlear HCs after neomycin exposure. In our study, we also found that YAP overexpression significantly decreased ROS accumulation, while downregulation of YAP expression increased ROS accumulation. In summary, our results demonstrate that the Hippo/YAP signaling pathway plays an important role in reducing HC injury and maintaining auditory function after aminoglycoside exposure. YAP overexpression could protect against neomycin-induced HC loss by inhibiting C-Abl-mediated cell apoptosis and decreasing ROS accumulation, suggesting that YAP could be a novel therapeutic target for aminoglycosides-induced sensorineural hearing loss in the clinic.

Interplay Between Notch and YAP/TAZ Pathways in the Regulation of Cell Fate During Embryo Development

Cells in growing tissues receive both biochemical and physical cues from their microenvironment. Growing evidence has shown that mechanical signals are fundamental regulators of cell behavior. However, how physical properties of the microenvironment are transduced into critical cell behaviors, such as proliferation, progenitor maintenance, or differentiation during development, is still poorly understood. The transcriptional co-activators YAP/TAZ shuttle between the cytoplasm and the nucleus in response to multiple inputs and have emerged as important regulators of tissue growth and regeneration. YAP/TAZ sense and transduce physical cues, such as those from the extracellular matrix or the actomyosin cytoskeleton, to regulate gene expression, thus allowing them to function as gatekeepers of progenitor behavior in several developmental contexts. The Notch pathway is a key signaling pathway that controls binary cell fate decisions through cell-cell communication in a context-dependent manner. Recent reports now suggest that the crosstalk between these two pathways is critical for maintaining the balance between progenitor maintenance and cell differentiation in different tissues. How this crosstalk integrates with morphogenesis and changes in tissue architecture during development is still an open question. Here, we discuss how progenitor cell proliferation, specification, and differentiation are coordinated with morphogenesis to construct a functional organ. We will pay special attention to the interplay between YAP/TAZ and Notch signaling pathways in determining cell fate decisions and discuss whether this represents a general mechanism of regulating cell fate during development. We will focus on research carried out in vertebrate embryos that demonstrate the important roles of mechanical cues in stem cell biology and discuss future challenges.

Gene therapy knockdown of Hippo signaling induces cardiomyocyte renewal in pigs after myocardial infarction

Human heart failure, a leading cause of death worldwide, is a prominent example of a chronic disease that may result from poor cell renewal. The Hippo signaling pathway is an inhibitory kinase cascade that represses adult heart muscle cell (cardiomyocyte) proliferation and renewal after myocardial infarction in genetically modified mice. Here, we investigated an adeno-associated virus 9 (AAV9)-based gene therapy to locally knock down the Hippo pathway gene Salvador (Sav) in border zone cardiomyocytes in a pig model of ischemia/reperfusion-induced myocardial infarction. Two weeks after myocardial infarction, when pigs had left ventricular systolic dysfunction, we administered AAV9-Sav-short hairpin RNA (shRNA) or a control AAV9 viral vector carrying green fluorescent protein (GFP) directly into border zone cardiomyocytes via catheter-mediated subendocardial injection. Three months after injection, pig hearts treated with a high dose of AAV9-Sav-shRNA exhibited a 14.3% improvement in ejection fraction (a measure of left ventricular systolic function), evidence of cardiomyocyte division, and reduced scar sizes compared to pigs receiving AAV9-GFP. AAV9-Sav-shRNA-treated pig hearts also displayed increased capillary density and reduced cardiomyocyte ploidy. AAV9-Sav-shRNA gene therapy was well tolerated and did not induce mortality. In addition, liver and lung pathology revealed no tumor formation. Local delivery of AAV9-Sav-shRNA gene therapy to border zone cardiomyocytes in pig hearts after myocardial infarction resulted in tissue renewal and improved function and may have utility in treating heart failure.

Mechanical forces shaping the development of the inner ear

The inner ear is one of the most complex structures in the mammalian body. Embedded within it are the hearing and balance sensory organs that contain arrays of hair cells that serve as sensors of sound and acceleration. Within the sensory organs, these hair cells are prototypically arranged in regular mosaic patterns. The development of such complex, yet precise, patterns require the coordination of differentiation, growth, and morphogenesis, both at the tissue and cellular scales. In recent years, there is accumulating evidence that mechanical forces at the tissue, the cellular, and the subcellular scales coordinate the development and organization of this remarkable organ. Here, we review recent works that reveal how such mechanical forces shape the inner ear, control its size, and establish regular cellular patterns. The insights learned from studying how mechanical forces drive the inner ear development are relevant for many other developmental systems in which precise cellular patterns are essential for their function.

TEAD family transcription factors in development and disease

The balance between stem cell potency and lineage specification entails the integration of both extrinsic and intrinsic cues, which ultimately influence gene expression through the activity of transcription factors. One example of this is provided by the Hippo signalling pathway, which plays a central role in regulating organ size during development. Hippo pathway activity is mediated by the transcriptional co-factors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), which interact with TEA domain (TEAD) proteins to regulate gene expression. Although the roles of YAP and TAZ have been intensively studied, the roles played by TEAD proteins are less well understood. Recent studies have begun to address this, revealing that TEADs regulate the balance between progenitor self-renewal and differentiation throughout various stages of development. Furthermore, it is becoming apparent that TEAD proteins interact with other co-factors that influence stem cell biology. This Primer provides an overview of the role of TEAD proteins during development, focusing on their role in Hippo signalling as well as within other developmental, homeostatic and disease contexts.

Small-molecule inhibition of Lats kinases may promote Yap-dependent proliferation in postmitotic mammalian tissues

Hippo signaling is an evolutionarily conserved pathway that restricts growth and regeneration predominantly by suppressing the activity of the transcriptional coactivator Yap. Using a high-throughput phenotypic screen, we identified a potent and non-toxic activator of Yap. In vitro kinase assays show that the compound acts as an ATP-competitive inhibitor of Lats kinases-the core enzymes in Hippo signaling. The substance prevents Yap phosphorylation and induces proliferation of supporting cells in the murine inner ear, murine cardiomyocytes, and human Müller glia in retinal organoids. RNA sequencing indicates that the inhibitor reversibly activates the expression of transcriptional Yap targets: upon withdrawal, a subset of supporting-cell progeny exits the cell cycle and upregulates genes characteristic of sensory hair cells. Our results suggest that the pharmacological inhibition of Lats kinases may promote initial stages of the proliferative regeneration of hair cells, a process thought to be permanently suppressed in the adult mammalian inner ear.

Neurog1, Neurod1, and Atoh1 are essential for spiral ganglia, cochlear nuclei, and cochlear hair cell development

We review the molecular basis of three related basic helix–loop–helix (bHLH) genes (Neurog1, Neurod1, and Atoh1) and upstream regulators Eya1/Six1, Sox2, Pax2, Gata3, Fgfr2b, Foxg1, and Lmx1a/b during the development of spiral ganglia, cochlear nuclei, and cochlear hair cells. Neuronal development requires early expression of Neurog1, followed by its downstream target Neurod1, which downregulates Atoh1 expression. In contrast, hair cells and cochlear nuclei critically depend on Atoh1 and require Neurod1 and Neurog1 expression for various aspects of development. Several experiments show a partial uncoupling of Atoh1/Neurod1 (spiral ganglia and cochlea) and Atoh1/Neurog1/Neurod1 (cochlear nuclei). In this review, we integrate the cellular and molecular mechanisms that regulate the development of auditory system and provide novel insights into the restoration of hearing loss, beyond the limited generation of lost sensory neurons and hair cells.

Dynamic patterns of YAP1 expression and cellular localization in the developing and injured utricle

The Hippo signaling pathway is a key regulator of tissue development and regeneration. Activation of the Hippo pathway leads to nuclear translocation of the YAP1 transcriptional coactivator, resulting in changes in gene expression and cell cycle entry. Recent studies have demonstrated the nuclear translocation of YAP1 during the development of the sensory organs of the inner ear, but the possible role of YAP1 in sensory regeneration of the inner ear is unclear. The present study characterized the cellular localization of YAP1 in the utricles of mice and chicks, both under normal conditions and after HC injury. During neonatal development, YAP1 expression was observed in the cytoplasm of supporting cells, and was transiently expressed in the cytoplasm of some differentiating hair cells. We also observed temporary nuclear translocation of YAP1 in supporting cells of the mouse utricle after short periods in organotypic culture. However, little or no nuclear translocation of YAP1 was observed in the utricles of neonatal or mature mice after ototoxic injury. In contrast, substantial YAP1 nuclear translocation was observed in the chicken utricle after streptomycin treatment in vitro and in vivo. Together, these data suggest that differences in YAP1 signaling may partially account for the differing regenerative abilities of the avian vs. mammalian inner ear.

Deficiency of large tumor suppressor kinase 1 causes congenital hearing loss associated with cochlear abnormalities in mice

Mammalian auditory hair cells are not spontaneously replaced. Their number and coordinated polarization are fairly well-maintained and both these factors might be essential for the cochlear amplifier. Cell cycle regulation has critical roles in regulating appropriate cell size and cell number. However, little is known about the physiological roles of the Hippo pathway, which is one of the most important signaling cascades that regulates cell growth, differentiation, and regenerative capacity in the cochlear sensory epithelium. Herein, we investigated the in vivo role of the large tumor suppressor 1 (LATS1), an essential kinase in the Hippo/yes-associated protein pathway, in the cochlea using the LATS1 knockout mice. LATS1 was expressed in hair cells and supporting cells. It was strongly expressed on the surface of the cuticular plate of the organ of Corti. We found that LATS1 knockout caused congenital hearing loss due to the irregular orientation and slightly reduced number of hair cells, whereas the number of supporting cells remained unchanged. On the surface of the hair cells, the kinocilium and stereocilia were dispersed during and after morphogenesis. However, the expression of the receptor-independent polarity regulators, such as Par3 or Gαi, was not affected. We concluded that LATS1 has an indispensable role in the maturation of mammalian auditory hair cells, but not in the development of the supporting cells, and thus, has a role in the hearing acquisition.

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

While inner ear disorders are common, our ability to intervene and recover their sensory function is limited. In vitro models of the inner ear, like the organoid system, could aid in identifying new regenerative drugs and gene therapies. Here, we provide a perspective on the status of in vitro inner ear models and guidance on how to improve their applicability in translational research. We highlight the generation of inner ear cell types from pluripotent stem cells as a particularly promising focus of research. Several exciting recent studies have shown how the developmental signaling cues of embryonic and fetal development can be mimicked to differentiate stem cells into “inner ear organoids” containing otic progenitor cells, hair cells, and neurons. However, current differentiation protocols and our knowledge of embryonic and fetal inner ear development in general, have a bias toward the sensory epithelia of the inner ear. We propose that a more holistic view is needed to better model the inner ear in vitro. Moving forward, attention should be made to the broader diversity of neuroglial and mesenchymal cell types of the inner ear, and how they interact in space or time during development. With improved control of epithelial, neuroglial, and mesenchymal cell fate specification, inner ear organoids would have the ability to truly recapitulate neurosensory function and dysfunction. We conclude by discussing how single-cell atlases of the developing inner ear and technical innovations will be critical tools to advance inner ear organoid platforms for future pre-clinical applications.

Initiation of Supporting Cell Activation for Hair Cell Regeneration in the Avian Auditory Epithelium: An Explant Culture Model

Sensorineural hearing loss is a common disability often caused by the loss of sensory hair cells in the cochlea. Hair cell (HCs) regeneration has long been the main target for the development of novel therapeutics for sensorineural hearing loss. In the mammalian cochlea, hair cell regeneration is limited, but the auditory epithelia of non-mammalian organisms retain the capacity for hair cell regeneration. In the avian basilar papilla (BP), supporting cells (SCs), which give rise to regenerated hair cells, are usually quiescent. Hair cell loss induces both direct transdifferentiation and mitotic division of supporting cells. Here, we established an explant culture model for hair cell regeneration in chick basilar papillae and validated it for investigating the initial phase of hair cell regeneration. The histological assessment demonstrated hair cell regeneration via direct transdifferentiation of supporting cells. Labeling with 5-ethynyl-2′-deoxyuridine (EdU) revealed the occurrence of mitotic division in the supporting cells at specific locations in the basilar papillae, while no EdU labeling was observed in newly generated hair cells. RNA sequencing indicated alterations in known signaling pathways associated with hair cell regeneration, consistent with previous findings. Also, unbiased analyses of RNA sequencing data revealed novel genes and signaling pathways that may be related to the induction of supporting cell activation in the chick basilar papillae. These results indicate the advantages of our explant culture model of the chick basilar papillae for exploring the molecular mechanisms of hair cell regeneration.