Progressive restriction of otic fate: the role of FGF and Wnt in resolving inner ear potential

Modern In Vitro Techniques for Modeling Hearing Loss

Sensorineural hearing loss (SNHL) is a prevalent and growing global health concern, especially within operational medicine, with limited therapeutic options available. This review article explores the emerging field of in vitro otic organoids as a promising platform for modeling hearing loss and developing novel therapeutic strategies. SNHL primarily results from the irreversible loss or dysfunction of cochlear mechanosensory hair cells (HCs) and spiral ganglion neurons (SGNs), emphasizing the need for innovative solutions. Current interventions offer symptomatic relief but do not address the root causes. Otic organoids, three-dimensional multicellular constructs that mimic the inner ear's architecture, have shown immense potential in several critical areas. They enable the testing of gene therapies, drug discovery for sensory cell regeneration, and the study of inner ear development and pathology. Unlike traditional animal models, otic organoids closely replicate human inner ear pathophysiology, making them invaluable for translational research. This review discusses methodological advances in otic organoid generation, emphasizing the use of human pluripotent stem cells (hPSCs) to replicate inner ear development. Cellular and molecular characterization efforts have identified key markers and pathways essential for otic organoid development, shedding light on their potential in modeling inner ear disorders. Technological innovations, such as 3D bioprinting and microfluidics, have further enhanced the fidelity of these models. Despite challenges and limitations, including the need for standardized protocols and ethical considerations, otic organoids offer a transformative approach to understanding and treating auditory dysfunctions. As this field matures, it holds the potential to revolutionize the treatment landscape for hearing and balance disorders, moving us closer to personalized medicine for inner ear conditions.

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.

In vitro modeling of cranial placode differentiation: Recent advances, challenges, and perspectives

Cranial placodes are transient ectodermal thickenings that contribute to a diverse array of organs in the vertebrate head. They develop from a common territory, the pre-placodal region that over time segregates along the antero-posterior axis into individual placodal domains: the adenohypophyseal, olfactory, lens, trigeminal, otic, and epibranchial placodes. These placodes terminally differentiate into the anterior pituitary, the lens, and contribute to sensory organs including the olfactory epithelium, and inner ear, as well as several cranial ganglia. To study cranial placodes and their derivatives and generate cells for therapeutic purposes, several groups have turned to in vitro derivation of placodal cells from human embryonic stem cells (hESCs) or induced pluripotent stem cells (hiPSCs). In this review, we summarize the signaling cues and mechanisms involved in cranial placode induction, specification, and differentiation in vivo, and discuss how this knowledge has informed protocols to derive cranial placodes in vitro. We also discuss the benefits and limitations of these protocols, and the potential of in vitro cranial placode modeling in regenerative medicine to treat cranial placode-related pathologies.

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

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

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.

Pax2a, Sp5a and Sp5l act downstream of Fgf and Wnt to coordinate sensory-neural patterning in the inner ear

In zebrafish, sensory epithelia and neuroblasts of the inner ear form simultaneously in abutting medial and lateral domains, respectively, in the floor of the otic vesicle. Previous studies support regulatory roles for Fgf and Wnt, but how signaling is coordinated is poorly understood. We investigated this problem using pharmacological and transgenic methods to alter Fgf or Wnt signaling from early placodal stages to evaluate later changes in growth and patterning. Blocking Fgf at any stage reduces proliferation of otic tissue and terminates both sensory and neural specification. Wnt promotes proliferation in the otic vesicle but is not required for sensory or neural development. However, sustained overactivation of Wnt laterally expands sensory epithelia and blocks neurogenesis. pax2a, sp5a and sp5l are coregulated by Fgf and Wnt and show overlapping expression in the otic placode and vesicle. Gain- and loss-of-function studies show that these genes are together required for Wnt's suppression of neurogenesis, as well as some aspects of sensory development. Thus, pax2a, sp5a and sp5l are critical for mediating Fgf and Wnt signaling to promote spatially localized sensory and neural development.

Human stem cell models to study placode development, function and pathology

Placodes are embryonic structures originating from the rostral ectoderm that give rise to highly diverse organs and tissues, comprising the anterior pituitary gland, paired sense organs and cranial sensory ganglia. Their development, including the underlying gene regulatory networks and signalling pathways, have been for the most part characterised in animal models. In this Review, we describe how placode development can be recapitulated by the differentiation of human pluripotent stem cells towards placode progenitors and their derivatives, highlighting the value of this highly scalable platform as an optimal in vitro tool to study the development of human placodes, and identify human-specific mechanisms in their development, function and pathology.

Sox8 remodels the cranial ectoderm to generate the ear

The vertebrate inner ear arises from a pool of progenitors with the potential to contribute to all the sense organs and cranial ganglia in the head. Here, we explore the molecular mechanisms that control ear specification from these precursors. Using a multiomics approach combined with loss-of-function experiments, we identify a core transcriptional circuit that imparts ear identity, along with a genome-wide characterization of noncoding elements that integrate this information. This analysis places the transcription factor Sox8 at the top of the ear determination network. Introducing Sox8 into the cranial ectoderm not only converts non-ear cells into ear progenitors but also activates the cellular programs for ear morphogenesis and neurogenesis. Thus, Sox8 has the unique ability to remodel transcriptional networks in the cranial ectoderm toward ear identity.

Making a head: Neural crest and ectodermal placodes in cranial sensory development

During development of the vertebrate sensory system, many important components like the sense organs and cranial sensory ganglia arise within the head and neck. Two progenitor populations, the neural crest, and cranial ectodermal placodes, contribute to these developing vertebrate peripheral sensory structures. The interactions and contributions of these cell populations to the development of the lens, olfactory, otic, pituitary gland, and cranial ganglia are vital for appropriate peripheral nervous system development. Here, we review the origins of both neural crest and placode cells at the neural plate border of the early vertebrate embryo and investigate the molecular and environmental signals that influence specification of different sensory regions. Finally, we discuss the underlying molecular pathways contributing to the complex vertebrate sensory system from an evolutionary perspective, from basal vertebrates to amniotes.

Critical roles of FGF, RA, and WNT signalling in the development of the human otic placode and subsequent lineages in a dish

Introduction

Efficient induction of the otic placode, the developmental origin of the inner ear from human pluripotent stem cells (hPSCs), provides a robust platform for otic development and sensorineural hearing loss modelling. Nevertheless, there remains a limited capacity of otic lineage specification from hPSCs by stepwise differentiation methods, since the critical factors for successful otic cell differentiation have not been thoroughly investigated. In this study, we developed a novel differentiation system involving the use of a three-dimensional (3D) floating culture with signalling factors for generating otic cell lineages via stepwise differentiation of hPSCs.

Methods

We differentiated hPSCs into preplacodal cells under a two-dimensional (2D) monolayer culture. Then, we transferred the induced preplacodal cells into a 3D floating culture under the control of the fibroblast growth factor (FGF), bone morphogenetic protein (BMP), retinoic acid (RA) and WNT signalling pathways. We evaluated the characteristics of the induced cells using immunocytochemistry, quantitative PCR (qPCR), population averaging, and single-cell RNA-seq (RNA-seq) analysis. We further investigated the methods for differentiating otic progenitors towards hair cells by overexpression of defined transcription factors.

Results

We demonstrated that hPSC-derived preplacodal cells acquired the potential to differentiate into posterior placodal cells in 3D floating culture with FGF2 and RA. Subsequent activation of WNT signalling induced otic placodal cell formation. By single-cell RNA-seq (scRNA-seq) analysis, we identified multiple clusters of otic placode- and otocyst marker-positive cells in the induced spheres. Moreover, the induced otic cells showed the potential to generate hair cell-like cells by overexpression of the transcription factors ATOH1, POU4F3 and GFI1.

Conclusions

We demonstrated the critical role of FGF2, RA and WNT signalling in a 3D environment for the in vitro differentiation of otic lineage cells from hPSCs. The induced otic cells had the capacity to differentiate into inner ear hair cells with stereociliary bundles and tip link-like structures. The protocol will be useful for in vitro disease modelling of sensorineural hearing loss and human inner ear development and thus contribute to drug screening and stem cell-based regenerative medicine.

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A 3D floating culture condition is critical for inducing otic placodal cells from hPSCs-derived preplacodal cells.

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Activation of FGF, RA, WNT signalling pathways is indispensable for differentiating otic lineage under the 3D condition.

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Overexpression of defined transcription factors facilitated the generation of hair cells from hPSCs-derived otic cells.

OUP accepted manuscript

The spiral ganglion of the cochlea is essential for hearing and contains primary bipolar neurons that relay action potentials generated by mechanosensory hair cells. Injury to spiral ganglion neurons (SGNs) causes permanent hearing loss because these cells have limited regenerative capacity. Establishment of human cell-derived inner ear tissue in vitro could facilitate the development of treatments for hearing loss. Here, we report a stepwise protocol for differentiating human-induced pluripotent stem cells (hiPSCs) into otic organoids that contain SGN-like cells and demonstrate that otic organoids have potential for use as an experimental model of drug-induced neuropathy. Otic progenitor cells (OPCs) were created by 2D culture of hiPSCs for 9 days. Otic spheroids were formed after 2D culture of OPCs for 2 days in a hypoxic environment. Otic organoids were generated by 3D culture of otic spheroids under hypoxic conditions for 5 days and normoxic conditions for a further 30 days or more. The protein expression profile, morphological characteristics, and electrophysiological properties of SGN-like cells in otic organoids were similar to those of primary SGNs. Live-cell imaging of AAV-syn-EGFP-labeled neurons demonstrated temporal changes in cell morphology and revealed the toxic effects of ouabain (which causes SGN-specific damage in animal experiments) and cisplatin (a chemotherapeutic drug with ototoxic adverse effects). Furthermore, a cyclin-dependent kinase-2 inhibitor suppressed the toxic actions of cisplatin on SGN-like cells in otic organoids. The otic organoid described here is a candidate novel drug screening system and could be used to identify drugs for the prevention of cisplatin-induced neuropathy.

Directed Differentiation of Human Pluripotent Stem Cells into Inner Ear Organoids

The sensory epithelia of the inner ear contain mechanosensitive hair cells that detect sound and head acceleration. This protocol details a 3D differentiation method to generate inner ear organoids containing sensory epithelia with hair cells. Human pluripotent stem cells are aggregated in low-binding 96-well plates and treated in chemically defined media with extracellular matrix to promote epithelialization. Small molecules and recombinant proteins are applied in a stepwise manner to recapitulate the morphogenic cues (BMP, TGF-β, FGF, and WNT) present during inner ear development in vivo. These treatments induce the sequential formation of nonneural ectoderm, otic-epibranchial progenitor domain, and otic placodes. The derived otic placodes then undergo self-guided morphogenesis to form otic vesicles, which eventually give rise to sensory epithelia containing hair cells and supporting cells, as well as neurons with synaptic formations to hair cells. This human stem cell-derived inner ear organoid system provides an ideal platform to study human inner ear development and disease in vitro.

Comparative assessment of Fgf's diverse roles in inner ear development: A zebrafish perspective

Progress in understanding mechanisms of inner ear development has been remarkably rapid in recent years. The research community has benefited from the availability of several diverse model organisms, including zebrafish, chick, and mouse. The complexity of the inner ear has proven to be a challenge, and the complexity of the mammalian cochlea in particular has been the subject of intense scrutiny. Zebrafish lack a cochlea and exhibit a number of other differences from amniote species, hence they are sometimes seen as less relevant for inner ear studies. However, accumulating evidence shows that underlying cellular and molecular mechanisms are often highly conserved. As a case in point, consideration of the diverse functions of Fgf and its downstream effectors reveals many similarities between vertebrate species, allowing meaningful comparisons the can benefit the entire research community. In this review, I will discuss mechanisms by which Fgf controls key events in early otic development in zebrafish and provide direct comparisons with chick and mouse.

Development and evolution of the vestibular apparatuses of the inner ear

The vertebrate inner ear is a labyrinthine sensory organ responsible for perceiving sound and body motion. While a great deal of research has been invested in understanding the auditory system, a growing body of work has begun to delineate the complex developmental program behind the apparatuses of the inner ear involved with vestibular function. These animal studies have helped identify genes involved in inner ear development and model syndromes known to include vestibular dysfunction, paving the way for generating treatments for people suffering from these disorders. This review will provide an overview of known inner ear anatomy and function and summarize the exciting discoveries behind inner ear development and the evolution of its vestibular apparatuses.

Responses of Epibranchial Placodes to Disruptions of the FGF and BMP Signaling Pathways in Embryonic Mice

Placodes are ectodermal thickenings of the embryonic vertebrate head. Their descendants contribute to sensory organ development, but also give rise to sensory neurons of the cranial nerves. In mammals, the signaling pathways which regulate the morphogenesis and neurogenesis of epibranchial placodes, localized dorsocaudally to the pharyngeal clefts, are poorly understood. Therefore, we performed mouse whole embryo culture experiments to assess the impact of pan-fibroblast growth factor receptor (FGFR) inhibitors, anti-FGFR3 neutralizing antibodies or the pan-bone morphogenetic protein receptor (BMPR) inhibitor LDN193189 on epibranchial development. We demonstrate that each of the three paired epibranchial placodes is regulated by a unique combination of FGF and/or bone morphogenetic protein (BMP) signaling. Thus, neurogenesis depends on fibroblast growth factor (FGF) signals, albeit to different degrees, in all epibranchial placodes (EP), whereas only EP1 and EP3 significantly rely on neurogenic BMP signals. Furthermore, individual epibranchial placodes vary in the extent to which FGF and/or BMP signals (1) have access to certain receptor subtypes, (2) affect the production of Neurogenin (Ngn)2⁺ and/or Ngn1⁺ neuroblasts, and (3) regulate either neurogenesis alone or together with structural maintenance. In EP2 and EP3, all FGF-dependent production of Ngn2⁺ neuroblasts is mediated via FGFR3 whereas, in EP1, it depends on FGFR1 and FGFR3. Differently, production of FGF-dependent Ngn1⁺ neuroblasts almost completely depends on FGFR3 in EP1 and EP2, but not in EP3. Finally, FGF signals turned out to be responsible for the maintenance of both placodal thickening and neurogenesis in all epibranchial placodes, whereas administration of the pan-BMPR inhibitor, apart from its negative neurogenic effects in EP1 and EP3, causes only decreases in the thickness of EP3. Experimentally applied inhibitors most probably not only blocked receptors in the epibranchial placodes, but also endodermal receptors in the pharyngeal pouches, which act as epibranchial signaling centers. While high doses of pan-FGFR inhibitors impaired the development of all pharyngeal pouches, high doses of the pan-BMPR inhibitor negatively affected only the pharyngeal pouches 3 and 4. In combination with partly concordant, partly divergent findings in other vertebrate classes our observations open up new approaches for research into the complex regulation of neurogenic placode development.

Modeling gap junction beta 2 gene-related deafness with human iPSC

There are >120 forms of non-syndromic deafness associated with identified genetic loci. In particular, mutation of the gap junction beta 2 gene (GJB2), which encodes connexin (CX)26 protein, is the most frequent cause of hereditary deafness worldwide. We previously described an induction method to develop functional CX26 gap junction-forming cells from mouse-induced pluripotent stem cells (iPSCs) and generated in vitro models for GJB2-related deafness. However, functional CX26 gap junction-forming cells derived from human iPSCs or embryonic stem cells (ESCs) have not yet been reported. In this study, we generated human iPSC-derived functional CX26 gap junction-forming cells (iCX26GJCs), which have the characteristics of cochlear supporting cells. These iCX26GJCs had gap junction plaque-like formations at cell-cell borders and co-expressed several markers that are expressed in cochlear supporting cells. Furthermore, we generated iCX26GJCs derived from iPSCs from two patients with the most common GJB2 mutation in Asia, and these cells reproduced the pathology of GJB2-related deafness. These in vitro models may be useful for establishing optimal therapies and drug screening for various mutations in GJB2-related deafness.

Activin/Nodal/TGF-β Pathway Inhibitor Accelerates BMP4-Induced Cochlear Gap Junction Formation During in vitro Differentiation of Embryonic Stem Cells

Mutations in gap junction beta-2 (GJB2), the gene that encodes connexin 26 (CX26), are the most frequent cause of hereditary deafness worldwide. We recently developed an in vitro model of GJB2-related deafness (induced CX26 gap junction-forming cells; iCX26GJCs) from mouse induced pluripotent stem cells (iPSCs) by using Bone morphogenetic protein 4 (BMP4) signaling-based floating cultures (serum-free culture of embryoid body-like aggregates with quick aggregation cultures; hereafter, SFEBq cultures) and adherent cultures. However, to use these cells as a disease model platform for high-throughput drug screening or regenerative therapy, cell yields must be substantially increased. In addition to BMP4, other factors may also induce CX26 gap junction formation. In the SFEBq cultures, the combination of BMP4 and the Activin/Nodal/TGF-β pathway inhibitor SB431542 (SB) resulted in greater production of isolatable CX26-expressing cell mass (CX26+ vesicles) and higher Gjb2 mRNA levels than BMP4 treatment alone, suggesting that SB may promote BMP4-mediated production of CX26+ vesicles in a dose-dependent manner, thereby increasing the yield of highly purified iCX26GJCs. This is the first study to demonstrate that SB accelerates BMP4-induced iCX26GJC differentiation during stem cell floating culture. By controlling the concentration of SB supplementation in combination with CX26+ vesicle purification, large-scale production of highly purified iCX26GJCs suitable for high-throughput drug screening or regenerative therapy for GJB2-related deafness may be possible.

A gradient of Wnt activity positions the neurosensory domains of the inner ear

The auditory and vestibular organs of the inner ear and the neurons that innervate them originate from Sox2-positive and Notch-active neurosensory domains specified at early stages of otic development. Sox2 is initially present throughout the otic placode and otocyst, and then it becomes progressively restricted to a ventro-medial domain. Using gain- and loss-of-function approaches in the chicken otocyst, we show that these early changes in Sox2 expression are regulated in a dose-dependent manner by Wnt/beta-catenin signalling. Both high and very low levels of Wnt activity repress Sox2 and neurosensory competence. However, intermediate levels allow the maintenance of Sox2 expression and sensory organ formation. We propose that a dorso-ventral (high-to-low) gradient and wave of Wnt activity initiated at the dorsal rim of the otic placode progressively restricts Sox2 and Notch activity to the ventral half of the otocyst, thereby positioning the neurosensory competent domains in the inner ear.

Directed differentiation and direct reprogramming: Applying stem cell technologies to hearing research

Hearing loss is the most widely spread sensory disorder in our society. In the majority of cases, it is caused by the loss or malfunctioning of cells in the cochlea: the mechanosensory hair cells, which act as primary sound receptors, and the connecting auditory neurons of the spiral ganglion, which relay the signal to upper brain centers. In contrast to other vertebrates, where damage to the hearing organ can be repaired through the activity of resident cells, acting as tissue progenitors, in mammals, sensory cell damage or loss is irreversible. The understanding of gene and cellular functions, through analysis of different animal models, has helped to identify causes of disease and possible targets for hearing restoration. Translation of these findings to novel therapeutics is, however, hindered by the lack of cellular assays, based on human sensory cells, to evaluate the conservation of molecular pathways across species and the efficacy of novel therapeutic strategies. In the last decade, stem cell technologies enabled to generate human sensory cell types in vitro, providing novel tools to study human inner ear biology, model disease, and validate therapeutics. This review focuses specifically on two technologies: directed differentiation of pluripotent stem cells and direct reprogramming of somatic cell types to sensory hair cells and neurons. Recent development in the field are discussed as well as how these tools could be implemented to become routinely adopted experimental models for hearing research.

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.

Development of the cochlea

The cochlea, a coiled structure located in the ventral region of the inner ear, acts as the primary structure for the perception of sound. Along the length of the cochlear spiral is the organ of Corti, a highly derived and rigorously patterned sensory epithelium that acts to convert auditory stimuli into neural impulses. The development of the organ of Corti requires a series of inductive events that specify unique cellular characteristics and axial identities along its three major axes. Here, we review recent studies of the cellular and molecular processes regulating several aspects of cochlear development, such as axial patterning, cochlear outgrowth and cellular differentiation. We highlight how the precise coordination of multiple signaling pathways is required for the successful formation of a complete organ of Corti.

Cell fate decisions during the development of the peripheral nervous system in the vertebrate head

Sensory placodes and neural crest cells are among the key cell populations that facilitated the emergence and diversification of vertebrates throughout evolution. Together, they generate the sensory nervous system in the head: both form the cranial sensory ganglia, while placodal cells make major contributions to the sense organs-the eye, ear and olfactory epithelium. Both are instrumental for integrating craniofacial organs and have been key to drive the concentration of sensory structures in the vertebrate head allowing the emergence of active and predatory life forms. Whereas the gene regulatory networks that control neural crest cell development have been studied extensively, the signals and downstream transcriptional events that regulate placode formation and diversity are only beginning to be uncovered. Both cell populations are derived from the embryonic ectoderm, which also generates the central nervous system and the epidermis, and recent evidence suggests that their initial specification involves a common molecular mechanism before definitive neural, neural crest and placodal lineages are established. In this review, we will first discuss the transcriptional networks that pattern the embryonic ectoderm and establish these three cell fates with emphasis on sensory placodes. Second, we will focus on how sensory placode precursors diversify using the specification of otic-epibranchial progenitors and their segregation as an example.

Generation of inner ear organoids from human pluripotent stem cells

The sensory epithelia of the inner ear contain mechanosensitive hair cells that transmit sound, gravity and head motion signals. This protocol describes an in vitro 3D differentiation method, by which the inner ear sensory epithelium harboring hair cells are derived from human pluripotent stem cells (hPSCs). To begin the differentiation, hPSCs are aggregated in low-binding 96-well plates and treated with extracellular matrix proteins to promote epithelialization. By recapitulating signaling pathway activation and attenuation during in vivo inner ear development, the aggregates are treated with small molecules and recombinant proteins that modulate signaling pathways such as BMP, FGF and WNT in a stepwise manner. These treatments induce sequential formation of non-neural ectoderm (NNE), otic-epibranchial progenitor domain (OEPD), and otic placodes. The otic placodes subsequently undergo self-guided morphogenesis to form otic vesicles, which eventually give rise to sensory epithelia containing inner ear hair cells and supporting cells, as well as neurons forming synapses with the hair cells. These hPSC-derived inner ear sensory structures are designated human inner ear organoids. As human inner ear biopsies are nearly impossible to obtain without causing severe injuries to the auditory system of the patients, the human inner ear organoid system provides a powerful in vitro platform for studying human inner ear disease and development.

Notch signalling regulates epibranchial placode patterning and segregation

Epibranchial placodes are the geniculate, petrosal and nodose placodes that generate parts of cranial nerves VII, IX and X, respectively. How the three spatially separated placodes are derived from the common posterior placodal area is poorly understood. Here, we reveal that the broad posterior placode area is first patterned into a Vgll2⁺/Irx5⁺ rostral domain and a Sox2⁺/Fgf3⁺/Etv5⁺ caudal domain relative to the first pharyngeal cleft. This initial rostral and caudal patterning is then sequentially repeated along each pharyngeal cleft for each epibranchial placode. The caudal domains give rise to the neuronal and non-neuronal cells in the placode, whereas the rostral domains are previously unrecognized structures, serving as spacers between the final placodes. Notch signalling regulates the balance between the rostral and caudal domains: high levels of Notch signalling expand the caudal domain at the expense of the rostral domain, whereas loss of Notch signalling produces the converse phenotype. Collectively, these data unravel a new patterning principle for the early phases of epibranchial placode development and a role for Notch signalling in orchestrating epibranchial placode segregation and differentiation.

Enhancer activation by FGF signalling during otic induction

Vertebrate ear progenitors are induced by fibroblast growth factor signalling, however the molecular mechanisms leading to the coordinate activation of downstream targets are yet to be discovered. The ear, like other sensory placodes, arises from the pre-placodal region at the border of the neural plate. Using a multiplex NanoString approach, we determined the response of these progenitors to FGF signalling by examining the changes of more than 200 transcripts that define the otic and other placodes, neural crest and neural plate territories. This analysis identifies new direct and indirect FGF targets during otic induction. Investigating changes in histone marks by ChIP-seq reveals that FGF exposure of pre-placodal cells leads to rapid deposition of active chromatin marks H3K27ac near FGF-response genes, while H3K27ac is depleted in the vicinity of non-otic genes. Genomic regions that gain H3K27ac act as cis-regulatory elements controlling otic gene expression in time and space and define a unique transcription factor signature likely to control their activity. Finally, we show that in response to FGF signalling the transcription factor dimer AP1 recruits the histone acetyl transferase p300 to selected otic enhancers. Thus, during ear induction FGF signalling modifies the chromatin landscape to promote enhancer activation and chromatin accessibility.

Inner ear organoids: new tools to understand neurosensory cell development, degeneration and regeneration

The development of therapeutic interventions for hearing loss requires fundamental knowledge about the signaling pathways controlling tissue development as well as the establishment of human cell-based assays to validate therapeutic strategies ex vivo Recent advances in the field of stem cell biology and organoid culture systems allow the expansion and differentiation of tissue-specific progenitors and pluripotent stem cells in vitro into functional hair cells and otic-like neurons. We discuss how inner ear organoids have been developed and how they offer for the first time the opportunity to validate drug-based therapies, gene-targeting approaches and cell replacement strategies.

FGF- and SHH-based molecular signals regulate barbel and craniofacial development in catfish

BACKGROUND

Catfish (Siluriformes) are characterized by unique morphologies, including enlarged jaws with movable barbels and taste buds covering the entire body surface. Evolution of these characteristics was a crucial step in their adaptive radiation to freshwater environments. However, the developmental processes of the catfish craniofacial region and taste buds remain to be elucidated; moreover, little is known about the molecular mechanisms underlying the morphogenesis of these structures.

RESULTS

In Amur catfish (Silurus asotus), three pairs of barbel primordia are formed by 2 days post-fertilization (dpf). Innervation of the peripheral nerves and formation of muscle precursors are also established during early development. Taste buds from the oral region to the body trunk are formed by 4 dpf. We then isolated catfish cognates Shh (SaShh) and Fgf8 (SaFgf8), which are expressed in maxillary barbel primordium at 1-2 dpf. Further, SHH signal inhibition induces reduction of mandibular barbels with abnormal morphology of skeletal elements, whereas it causes no apparent abnormality in the trigeminal and facial nerve morphology. We also found that mandibular barbel lengths and number of taste buds are reduced by FGF inhibition, as seen in SHH signal inhibition. However, unlike with SHH inhibition, the abnormal morphology of the trigeminal and facial nerves was observed in FGF signal-inhibited embryos.

CONCLUSION

The developmental processes of Amur catfish are consistent with those reported for other catfish species. Thus, developmental aspects of craniofacial structures and taste buds may be conserved in Siluriformes. Our findings also suggest that SHH signaling plays a crucial role in the formation of barbels and taste buds, without affecting nerve projection, while FGF signaling is required for the development of barbels, taste buds, and branchial nerves. Thus, SHH and FGF signaling plays key roles in the ontogenesis and evolution of some catfish-specific characteristics.

FGF19 sustains the high proliferative ability of keratinocytes in psoriasis through the regulation of Wnt/GSK-3β/β-catenin signalling via FGFR4

Accumulating evidence has shown that fibroblast growth factor 19 (FGF19) plays an important role in regulating cell proliferation. Psoriasis is characterized by the hyperproliferation of keratinocytes in skin lesions. However, whether FGF19 regulates the proliferation of keratinocytes in psoriasis remains unknown. In this study, we aimed to explore the potential relevance of FGF19 in psoriasis. We found that FGF19 was highly expressed in psoriatic skin from psoriasis patients, as well as keratinocytes that were stimulated with a cocktail of cytokines (M5), which is an in vitro model of psoriasis. Functional experiments demonstrated that FGF19 overexpression promoted the growth and proliferation of keratinocytes, while FGF19 knockdown showed opposite effect. Moreover, we found that FGF19 increased the phosphorylation of glycogen synthase kinase (GSK)-3β and promoted the expression of β-catenin and the activation of T cell factor 4 (TCF4) transcriptional activity. Notably, blocking Wnt/β-catenin signalling by silencing β-catenin partially reversed FGF19-mediated promotional effects on keratinocyte proliferation. In addition, FGFR4 inhibition significantly blocked the promotional effect of FGF19 on keratinocyte proliferation and GSK-3β/β-catenin/TCF4 signalling. Taken together, our results demonstrated that FGF19 contributes to sustaining the high proliferative ability of keratinocytes through promoting Wnt/GSK-3β/β-catenin signalling via FGFR4, highlighting the importance of FGF19 in the pathogenesis of psoriasis. Our study suggests that FGF19 may serve as a novel and potential therapeutic target for psoriasis.

Chicken embryos share mammalian patterns of apoptosis in the posterior placodal area

In the posterior placodal area (PPA) of C57BL/6N mice and primate-related Tupaia belangeri (Scandentia), apoptosis helps to establish morphologically separated otic and epibranchial placodes. Here, we demonstrate that basically identical patterns of apoptosis pass rostrocaudally through the Pax2⁺ PPA of chicken embryos. Interplacodal apoptosis eliminates unneeded cells either between the otic anlage and the epibranchial placodes 1, 2 and/or 3, respectively (type A), or between neighbouring epibranchial placodes (type B). These observations support the idea that in chicken embryos, as in mammals, interplacodal apoptosis serves to remove vestigial lateral line placodes (Washausen & Knabe, 2018, Biol Open 7, bio031815). A special case represents the recently discovered Pax2^- /Sox2⁺ paratympanic organ (PTO) placode that has been postulated to be molecularly distinct from and developmentally independent of the ventrally adjacent first epibranchial (or 'geniculate') placode (O'Neill et al. 2012, Nat Commun 3, 1041). We show that Sox2⁺ (PTO placodal) cells seem to segregate from the Pax2⁺ geniculate placode, and that absence of Pax2 in the mature PTO placode is due to secondary loss. We further report that, between Hamburger-Hamilton (HH) stages HH14 and HH26, apoptosis in the combined anlage of the first epibranchial and PTO placodes is almost exclusively found within and/or immediately adjacent to the dorsally located PTO placode. Hence, apoptosis appears to support decision-making processes among precursor cells of the early developing PTO placode and, later, regression of the epibranchial placodes 2 and 3.

Pluripotent stem cell-derived cochlear cells: a challenge in constant progress

Hearing loss is a common affection mainly resulting from irreversible loss of the sensory hair cells of the cochlea; therefore, developing therapies to replace missing hair cells is essential. Understanding the mechanisms that drive their formation will not only help to unravel the molecular basis of deafness, but also give a roadmap for recapitulating hair cells development from cultured pluripotent stem cells. In this review, we provide an overview of the molecular mechanisms involved in hair cell production from both human and mouse embryonic stem cells. We then provide insights how this knowledge has been applied to differentiate induced pluripotent stem cells into otic progenitors and hair cells. Finally, we discuss the current limitations for properly obtaining functional hair cell in a Petri dish, as well as the difficulties that have to be overcome prior to consider stem cell therapy as a potential treatment for hearing loss.

Building an Artificial Stem Cell Niche: Prerequisites for Future 3D-Formation of Inner Ear Structures-Toward 3D Inner Ear Biotechnology

In recent years, there has been an increased interest in stem cells for the purpose of regenerative medicine to deliver a wide range of therapies to treat many diseases. However, two‐dimensional cultures of stem cells are of limited use when studying the mechanism of pathogenesis of diseases and the feasibility of a treatment. Therefore, research is focusing on the strengths of stem cells in the three‐dimensional (3D) structures mimicking organs, that is, organoids, or organ‐on‐chip, for modeling human biology and disease. As 3D technology advances, it is necessary to know which signals stem cells need to multiply and differentiate into complex structures. This holds especially true for the complex 3D structure of the inner ear. Recent work suggests that although other factors play a role, the extracellular matrix (ECM), including its topography, is crucial to mimic a stem cell niche in vitro and to drive stem cells toward the formation of the tissue of interest. Technological developments have led to the investigation of biomaterials that closely resemble the native ECM. In the fast forward moving research of organoids and organs‐on‐chip, the inner ear has hardly received attention. This review aims to provide an overview, by describing the general context in which cells, matrix and morphogens cooperate in order to build a tissue, to facilitate research in 3D inner ear technology. Anat Rec, 303:408–426, 2020. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Fibroblast Growth Factor 15/19: From Basic Functions to Therapeutic Perspectives

Discovered 20 years ago, fibroblast growth factor (FGF)19, and its mouse ortholog FGF15, were the first members of a new subfamily of FGFs able to act as hormones. During fetal life, FGF15/19 is involved in organogenesis, affecting the development of the ear, eye, heart, and brain. At adulthood, FGF15/19 is mainly produced by the ileum, acting on the liver to repress hepatic bile acid synthesis and promote postprandial nutrient partitioning. In rodents, pharmacologic doses of FGF19 induce the same antiobesity and antidiabetic actions as FGF21, with these metabolic effects being partly mediated by the brain. However, activation of hepatocyte proliferation by FGF19 has long been a challenge to its therapeutic use. Recently, genetic reengineering of the molecule has resolved this issue. Despite a global overlap in expression pattern and function, murine FGF15 and human FGF19 exhibit several differences in terms of regulation, molecular structure, signaling, and biological properties. As most of the knowledge originates from the use of FGF19 in murine models, differences between mice and humans in the biology of FGF15/19 have to be considered for a successful translation from bench to bedside. This review summarizes the basic knowledge concerning FGF15/19 in mice and humans, with a special focus on regulation of production, morphogenic properties, hepatocyte growth, bile acid homeostasis, as well as actions on glucose, lipid, and energy homeostasis. Moreover, implications and therapeutic perspectives concerning FGF19 in human diseases (including obesity, type 2 diabetes, hepatic steatosis, biliary disorders, and cancer) are also discussed.

Lsd1 interacts with cMyb to demethylate repressive histone marks and maintain inner ear progenitor identity

During development, multipotent progenitor cells must maintain their identity while retaining the competence to respond to new signalling cues that drive cell fate decisions. This depends on both DNA-bound transcription factors and surrounding histone modifications. Here, we identify the histone demethylase Lsd1 as a crucial component of the molecular machinery that preserves progenitor identity in the developing ear prior to lineage commitment. Although Lsd1 is mainly associated with repressive complexes, we show that, in ear precursors, it is required to maintain active transcription of otic genes. We reveal a novel interaction between Lsd1 and the transcription factor cMyb, which in turn recruits Lsd1 to the promoters of key ear transcription factors. Here, Lsd1 prevents the accumulation of repressive H3K9me2, while allowing H3K9 acetylation. Loss of Lsd1 function causes rapid silencing of active promoters and loss of ear progenitor genes, and shuts down the entire ear developmental programme. Our data suggest that Lsd1-cMyb acts as a co-activator complex that maintains a regulatory module at the top of the inner ear gene network.

sox2 and sox3 cooperate to regulate otic/epibranchial placode induction in zebrafish

Expression of sox3 is one of the earliest markers of Fgf-dependent otic/epibranchial placode induction. We report here that sox2 is also expressed in the early otic/epibranchial placode in zebrafish. To address functions of sox2 and sox3, we generated knockouts and heat shock-inducible transgenes. Mutant analysis, and low-level misexpression, showed that sox2 and sox3 act redundantly to establish a full complement of otic/epibranchial cells. Disruption of pax8, another early regulator, caused similar placodal deficiencies to sox3 mutants or pax8-sox3 double mutants, suggesting that sox3 and pax8 operate in the same pathway. High-level misexpression of sox2 or sox3 during early stages cell-autonomously blocked placode induction, whereas misexpression several hours later could not reverse placodal differentiation. In an assay for ectopic placode-induction, we previously showed that misexpression of fgf8 induces a high level of ectopic sox3, but not pax8. Partial knockdown of sox3 significantly enhanced ectopic induction of pax8, whereas full knockdown of sox3 inhibited this process. Together these findings show that sox2 and sox3 are together required for proper otic induction, but the level of expression must be tightly regulated to avoid suppression of differentiation and maintenance of pluripotency.

Regulation of continuous but complex expression pattern of Six1 during early sensory development

BACKGROUND

In vertebrates, cranial sensory placodes give rise to neurosensory and endocrine structures, such as the olfactory epithelium, inner ear, and anterior pituitary. We report here the establishment of a transgenic mouse line that expresses Cre recombinase under the control of Six1-21, a major placodal enhancer of the homeobox gene Six1.

RESULTS

In the new Cre-expressing line, mSix1-21-NLSCre, the earliest Cre-mediated recombination was induced at embryonic day 8.5 in the region overlapping with the otic-epibranchial progenitor domain (OEPD), a transient, common precursor domain for the otic and epibranchial placodes. Recombination was later observed in the OEPD-derived structures (the entire inner ear and the VIIth-Xth cranial sensory ganglia), olfactory epithelium, anterior pituitary, pharyngeal ectoderm and pouches. Other Six1-positive structures, such as salivary/lacrimal glands and limb buds, were also positive for recombination. Moreover, comparison with another mouse line expressing Cre under the control of the sensory neuron enhancer, Six1-8, indicated that the continuous and complex expression pattern of Six1 during sensory organ formation is pieced together by separate enhancers.

CONCLUSIONS

mSix1-21-NLSCre has several unique characteristics to make it suitable for analysis of cell lineage and gene function in sensory placodes as well as nonplacodal Six1-positive structures. Developmental Dynamics 247:250-261, 2018. © 2017 Wiley Periodicals, Inc.

Hearing crosstalk: the molecular conversation orchestrating inner ear dorsoventral patterning

The inner ear is a structurally and functionally complex organ that functions in balance and hearing. It originates during neurulation as a localized thickened region of rostral ectoderm termed the otic placode, which lies adjacent to the developing caudal hindbrain. Shortly after the otic placode forms, it invaginates to delineate the otic cup, which quickly pinches off of the surface ectoderm to form a hollow spherical vesicle called the otocyst; the latter gives rise dorsally to inner ear vestibular components and ventrally to its auditory component. Morphogenesis of the otocyst is regulated by secreted proteins, such as WNTs, BMPs, and SHH, which determine its dorsoventral polarity to define vestibular and cochlear structures and sensory and nonsensory cell fates. In this review, we focus on the crosstalk that occurs among three families of secreted molecules to progressively polarize and pattern the developing otocyst. WIREs Dev Biol 2018, 7:e302. doi: 10.1002/wdev.302 This article is categorized under: Establishment of Spatial and Temporal Patterns > Gradients Signaling Pathways > Cell Fate Signaling Vertebrate Organogenesis > From a Tubular Primordium: Non-Branched.

Dlx3b/4b is required for early-born but not later-forming sensory hair cells during zebrafish inner ear development

Morpholino-mediated knockdown has shown that the homeodomain transcription factors Dlx3b and Dlx4b are essential for proper induction of the otic-epibranchial progenitor domain (OEPD), as well as subsequent formation of sensory hair cells in the developing zebrafish inner ear. However, increasing use of reverse genetic approaches has revealed poor correlation between morpholino-induced and mutant phenotypes. Using CRISPR/Cas9-mediated mutagenesis, we generated a defined deletion eliminating the entire open reading frames of dlx3b and dlx4b (dlx3b/4b) and investigated a potential phenotypic difference between mutants and morpholino-mediated knockdown. Consistent with previous findings obtained by morpholino-mediated knockdown of Dlx3b and Dlx4b, dlx3b/4b mutants display compromised otic induction, the development of smaller otic vesicles and an elimination of all indications of otic specification when combined with loss of foxi1, a second known OEPD competence factor in zebrafish. Furthermore, sensorigenesis is also affected in dlx3b/4b mutants. However, we find that only early-born sensory hair cells (tether cells), that seed and anchor the formation of otoliths, are affected. Later-forming sensory hair cells are present, indicating that two genetically distinct pathways control the development of early-born and later-forming sensory hair cells. Finally, impairment of early-born sensory hair cell formation in dlx3b/4b mutant embryos reverses the common temporal sequence of neuronal and sensory hair cell specification in zebrafish, resembling the order of cell specification in amniotes; Neurog1 expression before Atoh1 expression. We conclude that the Dlx3b/4b-dependent pathway has been either acquired newly in the fish lineage or lost in other vertebrate species during evolution, and that the events during early inner ear development are remarkably similar in fish and amniotes in the absence of this pathway.

Summary: The transcription factors Dlx3b and Dlx4b control the formation of early-born sensory hair cells or tether cells in the developing zebrafish inner ear.

A gene network regulated by FGF signalling during ear development

During development cell commitment is regulated by inductive signals that are tightly controlled in time and space. In response, cells activate specific programmes, but the transcriptional circuits that maintain cell identity in a changing signalling environment are often poorly understood. Specification of inner ear progenitors is initiated by FGF signalling. Here, we establish the genetic hierarchy downstream of FGF by systematic analysis of many ear factors combined with a network inference approach. We show that FGF rapidly activates a small circuit of transcription factors forming positive feedback loops to stabilise otic progenitor identity. Our predictive network suggests that subsequently, transcriptional repressors ensure the transition of progenitors to mature otic cells, while simultaneously repressing alternative fates. Thus, we reveal the regulatory logic that initiates ear formation and highlight the hierarchical organisation of the otic gene network.

A systems-level approach reveals new gene regulatory modules in the developing ear

The inner ear is a complex vertebrate sense organ, yet it arises from a simple epithelium, the otic placode. Specification towards otic fate requires diverse signals and transcriptional inputs that act sequentially and/or in parallel. Using the chick embryo, we uncover novel genes in the gene regulatory network underlying otic commitment and reveal dynamic changes in gene expression. Functional analysis of selected transcription factors reveals the genetic hierarchy underlying the transition from progenitor to committed precursor, integrating known and novel molecular players. Our results not only characterize the otic transcriptome in unprecedented detail, but also identify new gene interactions responsible for inner ear development and for the segregation of the otic lineage from epibranchial progenitors. By recapitulating the embryonic programme, the genes and genetic sub-circuits discovered here might be useful for reprogramming naïve cells towards otic identity to restore hearing loss.

Summary: Transcriptome analysis and knock down of select transcription factors reveals a genetic hierarchy as cells become committed to inner ear fate.

Directed Differentiation of Human Embryonic Stem Cells Toward Placode-Derived Spiral Ganglion-Like Sensory Neurons

The ability to generate spiral ganglion neurons (SGNs) from stem cells is a necessary prerequisite for development of cell-replacement therapies for sensorineural hearing loss. We present a protocol that directs human embryonic stem cells (hESCs) toward a purified population of otic neuronal progenitors (ONPs) and SGN-like cells. Between 82% and 95% of these cells express SGN molecular markers, they preferentially extend neurites to the cochlear nucleus rather than nonauditory nuclei, and they generate action potentials. The protocol follows an in vitro stepwise recapitulation of developmental events inherent to normal differentiation of hESCs into SGNs, resulting in efficient sequential generation of nonneuronal ectoderm, preplacodal ectoderm, early prosensory ONPs, late ONPs, and cells with cellular and molecular characteristics of human SGNs. We thus describe the sequential signaling pathways that generate the early and later lineage species in the human SGN lineage, thereby better describing key developmental processes. The results indicate that our protocol generates cells that closely replicate the phenotypic characteristics of human SGNs, advancing the process of guiding hESCs to states serving inner-ear cell-replacement therapies and possible next-generation hybrid auditory prostheses. © Stem Cells Translational Medicine 2017;6:923-936.

Modulation of Wnt Signaling Enhances Inner Ear Organoid Development in 3D Culture

Stem cell-derived inner ear sensory epithelia are a promising source of tissues for treating patients with hearing loss and dizziness. We recently demonstrated how to generate inner ear sensory epithelia, designated as inner ear organoids, from mouse embryonic stem cells (ESCs) in a self-organizing 3D culture. Here we improve the efficiency of this culture system by elucidating how Wnt signaling activity can drive the induction of otic tissue. We found that a carefully timed treatment with the potent Wnt agonist CHIR99021 promotes induction of otic vesicles—a process that was previously self-organized by unknown mechanisms. The resulting otic-like vesicles have a larger lumen size and contain a greater number of Pax8/Pax2-positive otic progenitor cells than organoids derived without the Wnt agonist. Additionally, these otic-like vesicles give rise to large inner ear organoids with hair cells whose morphological, biochemical and functional properties are indistinguishable from those of vestibular hair cells in the postnatal mouse inner ear. We conclude that Wnt signaling plays a similar role during inner ear organoid formation as it does during inner ear development in the embryo.

Co-regulation of the Notch and Wnt signaling pathways promotes supporting cell proliferation and hair cell regeneration in mouse utricles

This work sought to determine the crosstalk between the Notch and Wnt signaling pathways in regulating supporting cell (SC) proliferation and hair cell (HC) regeneration in mouse utricles. We cultured postnatal day (P)3 and P60 mouse utricles, damaged the HCs with gentamicin, and treated the utricles with the γ-secretase inhibitor DAPT to inhibit the Notch pathway and with the Wnt agonist QS11 to active the Wnt pathway. We also used Sox2-CreER, Notch1-flox (exon 1), and Catnb-flox (exon 3) transgenic mice to knock out the Notch pathway and activate the Wnt pathway in Sox2+ SCs. Notch inhibition alone increased SC proliferation and HC number in both undamaged and damaged utricles. Wnt activation alone promoted SC proliferation, but the HC number was not significantly increased. Here we demonstrated the cumulative effects of Notch inhibition and Wnt activation in regulating SC proliferation and HC regeneration. Simultaneously inhibiting Notch and overexpressing Wnt led to significantly greater SC proliferation and greater numbers of HCs than manipulating either pathway alone. Similar results were observed in the transgenic mice. This study suggests that the combination of Notch inhibition and Wnt activation can significantly promote SC proliferation and increase the number of regenerated HCs in mouse utricle.

Fgf3 and Fgf16 expression patterns define spatial and temporal domains in the developing chick inner ear

The inner ear is a morphologically complex sensory structure with auditory and vestibular functions. The developing otic epithelium gives rise to neurosensory and non-sensory elements of the adult membranous labyrinth. Extrinsic and intrinsic signals manage the patterning and cell specification of the developing otic epithelium by establishing lineage-restricted compartments defined in turn by differential expression of regulatory genes. FGF3 and FGF16 are excellent candidates to govern these developmental events. Using the chick inner ear, we show that Fgf3 expression is present in the borders of all developing cristae. Strong Fgf16 expression was detected in a portion of the developing vertical and horizontal pouches, whereas the cristae show weaker or undetected Fgf16 expression at different developmental stages. Concerning the rest of the vestibular sensory elements, both the utricular and saccular maculae were Fgf3 positive. Interestingly, strong Fgf16 expression delimited these Fgf16-negative sensory patches. The Fgf3-negative macula neglecta and the Fgf3-positive macula lagena were included within weakly Fgf16-expressing areas. Therefore, different FGF-mediated mechanisms might regulate the specification of the anterior (utricular and saccular) and posterior (neglecta and lagena) maculae. In the developing cochlear duct, dynamic Fgf3 and Fgf16 expression suggests their cooperation in the early specification and later cell differentiation in the hearing system. The requirement of Fgf3 and Fgf16 genes in endolymphatic apparatus development and neurogenesis are discussed. Based on these observations, FGF3 and FGF16 seem to be key signaling pathways that control the inner ear plan by defining epithelial identities within the developing otic epithelium.

The molecular basis of craniofacial placode development

The sensory organs of the vertebrate head originate from simple ectodermal structures known as cranial placodes. All cranial placodes derive from a common domain adjacent to the neural plate, the preplacodal region, which is induced at the border of neural and non-neural ectoderm during gastrulation. Induction and specification of the preplacodal region is regulated by the fibroblast growth factor, bone morphogenetic protein, WNT, and retinoic acid signaling pathways, and characterized by expression of the EYA and SIX family of transcriptional regulators. Once the preplacodal region is specified, different combinations of local signaling molecules and placode-specific transcription factors, including competence factors, promote the induction of individual cranial placodes along the neural axis of the head region. In this review, we summarize the steps of cranial placode development and discuss the roles of the main signaling molecules and transcription factors that regulate these steps during placode induction, specification, and development. For further resources related to this article, please visit the WIREs website.

Identification of molecular signatures specific for distinct cranial sensory ganglia in the developing chick

Background

The cranial sensory ganglia represent populations of neurons with distinct functions, or sensory modalities. The production of individual ganglia from distinct neurogenic placodes with different developmental pathways provides a powerful model to investigate the acquisition of specific sensory modalities. To date there is a limited range of gene markers available to examine the molecular pathways underlying this process.

Results

Transcriptional profiles were generated for populations of differentiated neurons purified from distinct cranial sensory ganglia using microdissection in embryonic chicken followed by FAC-sorting and RNAseq. Whole transcriptome analysis confirmed the division into somato- versus viscerosensory neurons, with additional evidence for subdivision of the somatic class into general and special somatosensory neurons. Cross-comparison of distinct ganglia transcriptomes identified a total of 134 markers, 113 of which are novel, which can be used to distinguish trigeminal, vestibulo-acoustic and epibranchial neuronal populations. In situ hybridisation analysis provided validation for 20/26 tested markers, and showed related expression in the target region of the hindbrain in many cases.

Conclusions

One hundred thirty-four high-confidence markers have been identified for placode-derived cranial sensory ganglia which can now be used to address the acquisition of specific cranial sensory modalities.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-016-0057-y) contains supplementary material, which is available to authorized users.

Induction of differentiation of human embryonic stem cells into functional hair-cell-like cells in the absence of stromal cells

Sensorineural hearing loss and vestibular dysfunction have become the most common forms of sensory defects. Stem cell-based therapeutic strategies for curing hearing loss are being developed. Several attempts to develop hair cells by using chicken utricle stromal cells as feeder cells have resulted in phenotypic conversion of stem cells into inner ear hair-cell-like cells. Here, we induced the differentiation of human embryonic stem cells (hESCs) into otic epithelial progenitors (OEPs), and further induced the differentiation of OEPs into hair-cell-like cells using different substrates. Our results showed that OEPs cultured on the chicken utricle stromal cells with the induction medium could differentiate into hair-cell-like cells with stereociliary bundles. Co-culture with stromal cells, however, may be problematic for subsequent examination of the induced hair-cell-like cells. In order to avoid the interference from stromal cells, we cultured OEPs on laminin with different induction media and examined the effects of the induction medium on the differentiation potentials of OEPs into hair-cell-like cells. The results revealed that the culture of OEPs on laminin with the conditioned medium from chicken utricle stromal cells supplemented with EGF and all-trans retinoic acid (RA) could promote the organization of cells into epithelial clusters displaying hair-cell-like cells with stereociliary bundles. These cells also displayed the expected electrophysiological properties.

The mouse Foxi3 transcription factor is necessary for the development of posterior placodes

The inner ear develops from the otic placode, one of the cranial placodes that arise from a region of ectoderm adjacent to the anterior neural plate called the pre-placodal domain. We have identified a Forkhead family transcription factor, Foxi3, that is expressed in the pre-placodal domain and down-regulated when the otic placode is induced. We now show that Foxi3 mutant mice do not form otic placodes as evidenced by expression changes in early molecular markers and the lack of thickened placodal ectoderm, an otic cup or otocyst. Some preplacodal genes downstream of Foxi3-Gata3, Six1 and Eya1-are not expressed in the ectoderm of Foxi3 mutant mice, and the ectoderm exhibits signs of increased apoptosis. We also show that Fgf signals from the hindbrain and cranial mesoderm, which are necessary for otic placode induction, are received by pre-placodal ectoderm in Foxi3 mutants, but do not initiate otic induction. Finally, we show that the epibranchial placodes that develop in close proximity to the otic placode and the mandibular division of the trigeminal ganglion are missing in Foxi3 mutants. Our data suggest that Foxi3 is necessary to prime pre-placodal ectoderm for the correct interpretation of inductive signals for the otic and epibranchial placodes.

The Wnt and Notch signalling pathways in the developing cochlea: Formation of hair cells and induction of regenerative potential

The Wnt and Notch signalling pathways control proliferation, specification, and cell fate choices during embryonic development and in adult life. Hence, there is much interest in both signalling pathways in the context of stem cell biology and tissue regeneration. In the developing ear, the Wnt and Notch signalling pathways specify otic cells and refine the ventral boundary of the otic placode. Since both signalling pathways control events essential for the formation of sensory cells, such as proliferation and hair cell differentiation, these pathways could hold promise for the regeneration of hair cells in adult mammalian cochlea. Indeed, modulating either the Wnt or Notch pathways can trigger the regenerative potential of supporting cells. In the neonatal mouse cochlea, Notch-mediated regeneration of hair cells partially depends on Wnt signalling, which implies an interaction between the pathways. This review presents how the Wnt and Notch signalling pathways regulate the formation of sensory hair cells and how modulating their activity induces regenerative potential in the mammalian cochlea.

Cooperative and independent functions of FGF and Wnt signaling during early inner ear development

Background

In multiple vertebrate organisms, including chick, Xenopus, and zebrafish, Fibroblast Growth Factor (FGF) and Wnt signaling cooperate during formation of the otic placode. However, in the mouse, although FGF signaling induces Wnt8a expression during induction of the otic placode, it is unclear whether these two signaling pathways functionally cooperate. Sprouty (Spry) genes encode intracellular antagonists of receptor tyrosine kinase signaling, including FGF signaling. We previously demonstrated that the Sprouty1 (Spry1) and Sprouty2 (Spry2) genes antagonize FGF signaling during induction of the otic placode. Here, we investigate cross talk between FGF/SPRY and Wnt signaling during otic placode induction and assess whether these two signaling pathways functionally cooperate during early inner ear development in the mouse.

Methods

Embryos were generated carrying combinations of a Spry1 null allele, Spry2 null allele, β-catenin null allele, or a Wnt reporter transgene. Otic phenotypes were assessed by in situ hybridization, semi-quantitative reverse transcriptase PCR, immunohistochemistry, and morphometric analysis of sectioned tissue.

Results

Comparison of Spry1, Spry2, and Wnt reporter expression in pre-otic and otic placode cells indicates that FGF signaling precedes and is active in more cells than Wnt signaling. We provide in vivo evidence that FGF signaling activates the Wnt signaling pathway upstream of TCF/Lef transcriptional activation. FGF regulation of Wnt signaling is functional, since early inner ear defects in Spry1 and Spry2 compound mutant embryos can be genetically rescued by reducing the activity of the Wnt signaling pathway. Interestingly, we find that although the entire otic placode increases in size in Spry1 and Spry2 compound mutant embryos, the size of the Wnt-reporter-positive domain does not increase to the same extent as the Wnt-reporter-negative domain.

Conclusions

This study provides genetic evidence that FGF and Wnt signaling cooperate during early inner ear development in the mouse. Furthermore, our data suggest that although specification of the otic placode may be globally regulated by FGF signaling, otic specification of cells in which both FGF and Wnt signaling are active may be more tightly regulated.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0083-8) contains supplementary material, which is available to authorized users.