MOSAIC ATOH1 DELETION IN THE CHICK AUDITORY EPITHELIUM REVEALS A HOMEOSTATIC MECHANISM TO RESTORE HAIR CELL NUMBER.

The mechanosensory hair cell of the vertebrate inner ear responds to the mechanical deflections that result from hearing or change in the acceleration due to gravity, to allow us to perceive and interpret sounds, maintain balance and spatial orientation. In mammals, ototoxic compounds, disease, and acoustic trauma can result in damage and extrusion of hair cells, without replacement, resulting in hearing loss. In contrast, non-mammalian vertebrates can regenerate sensory hair cells. Upon damage, hair cells are extruded and an associated cell type, the supporting cell is transformed into a hair cell. The mechanisms that can trigger regeneration are not known. Using mosaic deletion of the hair cell master gene, Atoh1, in the embryonic avian inner ear, we find that despite hair cells depletion at E9, by E12, hair cell number is restored in sensory epithelium. Our study suggests a homeostatic mechanism can restores hair cell number in the basilar papilla, that is activated when juxtracrine signalling is disrupted. Restoration of hair cell numbers during development may mirror regenerative processes, and our work provides insights into the mechanisms that trigger regeneration.

FGFR1-mediated protocadherin-15 loading mediates cargo specificity during intraflagellar transport in inner ear hair-cell kinocilia.

The mechanosensory hair cells of the inner ear are required for hearing and balance and have a distinctive apical structure, the hair bundle, that converts mechanical stimuli into electrical signals. This structure comprises a single cilium, the kinocilium, lying adjacent to an ensemble of actin-based projections known as stereocilia. Hair bundle polarity depends on kinociliary protocadherin-15 (Pcdh15) localization. Protocadherin-15 is found only in hair-cell kinocilia, and is not localized to the primary cilia of adjacent supporting cells. Thus, Pcdh15 must be specifically targeted and trafficked into the hair-cell kinocilium. Here we show that kinocilial Pcdh15 trafficking relies on cell type-specific coupling to the generic intraflagellar transport (IFT) transport mechanism. We uncover a role for fibroblast growth factor receptor 1 (FGFR1) in loading Pcdh15 onto kinociliary transport particles in hair cells. We find that on activation, FGFR1 binds and phosphorylates Pcdh15. Moreover, we find a previously uncharacterized role for clathrin in coupling this kinocilia-specific cargo with the anterograde IFT-B complex through the adaptor, DAB2. Our results identify a modified ciliary transport pathway used for Pcdh15 transport into the cilium of the inner ear hair cell and coordinated by FGFR1 activity.

Changing shape and shaping change: Inducing the inner ear.

The inner ear arises from non-neural ectoderm as a result of instructions sent by surrounding tissues. These interactions progressively restrict the potential of the ectoderm, resulting in the formation of the otic placode, a disk of thickened ectoderm that will give rise to all of the inner ear derivatives and its neurons. While otic placode is a surface structure, the inner ear is internalised, embedded within the cranial mesenchyme. Here, the cellular and molecular interactions that restrict the lineage of non-neural ectoderm in its transition to otic placode are reviewed, and how these interactions impinge on the coordination of otic placodal cell shape that drive the dramatic morphogenesis of the placode, as it becomes the otocyst.

FGFR1-Frs2/3 signalling maintains sensory progenitors during inner ear hair cell formation.

Inner ear mechanosensory hair cells transduce sound and balance information. Auditory hair cells emerge from a Sox2-positive sensory patch in the inner ear epithelium, which is progressively restricted during development. This restriction depends on the action of signaling molecules. Fibroblast growth factor (FGF) signalling is important during sensory specification: attenuation of Fgfr1 disrupts cochlear hair cell formation; however, the underlying mechanisms remain unknown. Here we report that in the absence of FGFR1 signaling, the expression of Sox2 within the sensory patch is not maintained. Despite the down-regulation of the prosensory domain markers, p27(Kip1), Hey2, and Hes5, progenitors can still exit the cell cycle to form the zone of non-proliferating cells (ZNPC), however the number of cells that form sensory cells is reduced. Analysis of a mutant Fgfr1 allele, unable to bind to the adaptor protein, Frs2/3, indicates that Sox2 maintenance can be regulated by MAP kinase. We suggest that FGF signaling, through the activation of MAP kinase, is necessary for the maintenance of sensory progenitors and commits precursors to sensory cell differentiation in the mammalian cochlea.

Molecular cloning and functional characterisation of chicken Atonal homologue 1: a comparison with human Atoh1.

BACKGROUND INFORMATION: The vertebrate basic helix-loop-helix transcription factor Atoh1 is essential for maturation and survival of mechanosensory hair cells of the inner ear, neurogenesis, differentiation of the intestine, homeostasis of the colon and is implicated in cancer progression. Given that mutations in Atoh1 are detected in malignant tumours, study of functionally different Atoh1 alleles and homologues might yield useful avenues for investigation. The predicted sequence of chicken Atoh1 (cAtoh1) has large regions of dissimilarity to that of mammalian Atoh1 homologues. We hypothesise that cAtoh1 might have intrinsic functional differences to mammalian Atoh1. RESULTS: In this study, we cloned and sequenced the full open reading frame of cAtoh1. In overexpression experiments, we show that this sequence is sufficient to generate a cAtoh1 protein capable of inducing hair cell markers when expressed in nonsensory regions of the developing inner ear, and that morpholino-mediated knock-down using a section of the sequence 5' to the start codon inhibits differentiation of hair cells in the chicken basilar papilla. Furthermore, we compare the behaviour of cAtoh1 and human Atoh1 (hAtoh1) in embryonic mouse cochlear explants, showing that cAtoh1 is a potent inducer of hair cell differentiation and that it can overcome Sox2-mediated repression of hair cell differentiation more effectively than hAtoh1. CONCLUSIONS: cAtoh1 is both necessary and sufficient for avian mechanosensory hair cell differentiation. The non-conserved regions of the cAtoh1 coding region have functional consequences on its behaviour.

Expression of the heparan sulfate 6-O-endosulfatases, Sulf1 and Sulf2, in the avian and mammalian inner ear suggests a role for sulfation during inner ear development.

BACKGROUND: Inner ear morphogenesis is tightly regulated by the temporally and spatially coordinated action of signaling ligands and their receptors. Ligand-receptor interactions are influenced by heparan sulfate proteoglycans (HSPGs), cell surface molecules that consist of glycosaminoglycan chains bound to a protein core. Diversity in the sulfation pattern within glycosaminoglycan chains creates binding sites for numerous cell signaling factors, whose activities and distribution are modified by their association with HSPGs. RESULTS: Here we describe the expression patterns of two extracellular 6-O-endosulfatases, Sulf1 and Sulf2, whose activity modifies the 6-O-sulfation pattern of HSPGs. We use in situ hybridization to determine the temporal and spatial distribution of transcripts during the development of the chick and mouse inner ear. We also use immunocytochemistry to determine the cellular localization of Sulf1 and Sulf2 within the sensory epithelia. Furthermore, we analyze the organ of Corti in Sulf1/Sulf2 double knockout mice and describe an increase in the number of mechanosensory hair cells. CONCLUSIONS: Our results suggest that the tuning of intracellular signaling, mediated by Sulf activity, plays an important role in the development of the inner ear.

Zebra finch as a developmental model.

The domesticated zebra finch (Taeniopygia guttata) is a well-established animal model for studying vocal learning. It is also a tractable model for developmental analyses. The finch genome has been sequenced and methods for its transgenesis have been reported. Hatching and sexual maturation in this species takes only two weeks and three months, respectively. Finch colonies can be established relatively easily and its eggs are laid at a stage earlier than in other common avian experimental models, facilitating the analysis of very early avian development. Representing the Neoaves to which 95% of all bird species belong, the finch can potentially complement two existing, Galloanserae developmental models, the chick, and quail. Here, we provide a step-by-step guide for how to set up a finch colony in a conventional laboratory environment. Technical tips are offered to optimize hens' productivity and ensure a constant supply of fertilized finch eggs. Methods of handling finch eggs and embryos for subsequent embryological, cellular, or molecular analyses are also discussed. We conclude by emphasizing scientific values and cost effectiveness of maintaining a finch colony for avian developmental studies. genesis 53:669-677, 2015. © 2015 Wiley Periodicals, Inc.

Junctionally restricted RhoA activity is necessary for apical constriction during phase 2 inner ear placode invagination.

After induction, the inner ear is transformed from a superficially located otic placode into an epithelial vesicle embedded in the mesenchyme of the head. Invagination of this epithelium is biphasic: phase 1 involves the expansion of the basal aspect of the otic cells, and phase 2, the constriction of their apices. Apical constriction is important not only for otic invagination, but also the invagination of many other epithelia; however, its molecular basis is still poorly understood. Here we show that phase 2 otic morphogenesis, like phase 1 morphogenesis, results from the activation of myosin-II. However unlike the actin depolymerising activity observed basally, active myosin-II results in actomyosin contractility. Myosin-II activation is triggered by the accumulation of the planar cell polarity (PCP) core protein, Celsr1 in apical junctions (AJ). Apically polarized Celsr1 orients and recruits the Rho Guanine exchange factor (GEF) ArhGEF11 to apical junctions, thus restricting RhoA activity to the junctional membrane where it activates the Rho kinase ROCK. We suggest that myosin-II and RhoA activation results in actomyosin dependent constriction in an apically polarised manner driving otic epithelium invagination.

From placode to labyrinth: culture of the chicken inner ear.

The inner ear transduces the mechanical stimuli that are associated with sound and balance perception. Missteps during its formation often result in deafness, and thus understanding otic development has a profound clinical relevance. The intricate complexity of the inner ear is derived from a simple epithelial sheet during embryogenesis. Study of this process in vitro has provided insight into the mechanisms of otic induction, patterning and differentiation. This article details methods for the culture of otic placode, otocyst, and basilar papilla, providing a toolkit for the investigation of multiple facets of otic organogenesis, for regeneration studies and for setting up small molecule screens to identify possible therapeutic targets.

From shared lineage to distinct functions: the development of the inner ear and epibranchial placodes.

The inner ear and the epibranchial ganglia constitute much of the sensory system in the caudal vertebrate head. The inner ear consists of mechanosensory hair cells, their neurons, and structures necessary for sound and balance sensation. The epibranchial ganglia are knots of neurons that innervate and relay sensory signals from several visceral organs and the taste buds. Their development was once thought to be independent, in line with their independent functions. However, recent studies indicate that both systems arise from a morphologically distinct common precursor domain: the posterior placodal area. This review summarises recent studies into the induction, morphogenesis and innervation of these systems and discusses lineage restriction and cell specification in the context of their common origin.

Pax2 modulates proliferation during specification of the otic and epibranchial placodes.

BACKGROUND: The inner ear and epibranchial ganglia of vertebrates arise from a shared progenitor domain that is induced by FGF signalling, the posterior placodal area (PPA), before being segregated by Wnt signalling. One of the first genes activated in the PPA is the transcription factor Pax2. Loss-of- and gain-of function studies have defined a role for Pax2 in placodal morphogenesis and later inner ear development, but have not addressed the role Pax2 plays during the formation and maintenance of the PPA. RESULTS: To understand the role of Pax2 during the development of the PPA, we used over-expression and repression of Pax2. Both gave rise to a smaller otocyst and repressed the formation of epibranchial placodes. In addition, cell cycle analysis revealed that Pax2 suppression reduced proliferation of the PPA. CONCLUSIONS: Our results suggest that Pax2 functions in the maintenance but not the induction of the PPA. One role of Pax2 is to maintain proper cell cycle proliferation in the PPA.

Induction of the chick columella and its integration with the inner ear.

BACKGROUND: The auditory complex of the chick, like that of humans, is made of intimate and highly ordered connections between the inner ear, the middle ear, and the outer ear. Unlike mammals, the middle ear of chick has only one ossicle, known as the columella. The independent lineages of the two suggest that some mechanism must exist that ensures the connectivity between the inner ear and the columella; however, the basis of integration is not known. RESULTS: Using quail-chick chimeras, we demonstrate that columella development depends on signaling interactions. Specifically, both pharyngeal endoderm and cranial paraxial mesoderm can alter the morphology of the columella. Only a discrete region of pharyngeal endoderm exerts this patterning activity, and this region is specified by the overlying paraxial mesoderm. CONCLUSIONS: Paraxial mesoderm is also used in the induction of the inner ear, thus we propose that this overlapping source of signalling cues in both middle and inner ear development may underlie the integration of these structures.

In Ovo and Ex Ovo Methods to Study Avian Inner Ear Development.

The inner ear perceives sound and maintains balance using the cochlea and vestibule. It does this by using a dedicated mechanosensory cell type known as the hair cell. Basic research in the inner ear has led to a deep understanding of how the hair cell functions, and how dysregulation can lead to hearing loss and vertigo. For this research, the mouse has been the pre-eminent model system. However, mice, like all mammals, have lost the ability to replace hair cells. Thus, when trying to understand cellular therapies for restoring inner ear function, complementary studies in other vertebrate species could provide further insights. The auditory epithelium of birds, the basilar papilla (BP), is a sheet of epithelium composed of mechanosensory hair cells (HCs) intercalated by supporting cells (SCs). Although the anatomical architecture of the basilar papilla and the mammalian cochlea differ, the molecular mechanisms of inner ear development and hearing are similar. This makes the basilar papilla a useful system for not only comparative studies but also to understand regeneration. Here, we describe dissection and manipulation techniques for the chicken inner ear. The technique shows genetic and small molecule inhibition methods, which offer a potent tool for studying the molecular mechanisms of inner ear development. In this paper, we discuss in ovo electroporation techniques to genetically perturb the basilar papilla using CRIPSR-Cas9 deletions, followed by dissection of the basilar papilla. We also demonstrate the BP organ culture and optimal use of culture matrices, to observe the development of the epithelium and the hair cells.

FGF signaling regulates cytoskeletal remodeling during epithelial morphogenesis.

Changes in the cytoskeletal architecture underpin the dynamic changes in tissue shape that occur during development. It is clear that such changes must be coordinated so that individual cell behaviors are synchronized; however, the mechanisms by which morphogenesis is instructed and coordinated are unknown. After its induction in non-neural ectoderm, the inner ear undergoes morphogenesis, being transformed from a flat ectodermal disk on the surface of the embryo to a hollowed sphere embedded in the head. We provide evidence that this shape change relies on extrinsic signals subsequent to genetic specification. By using specific inhibitors, we find that local fibroblast growth factor (FGF) signaling triggers a phosphorylation cascade that activates basal myosin II through the activation of phospholipase Cgamma. Myosin II exhibits a noncanonical activity that results in the local depletion of actin filaments. Significantly, the resulting apical actin enrichment drives morphogenesis of the inner ear. Thus, FGF signaling directly exerts profound cytoskeletal effects on otic cells, coordinating the morphogenesis of the inner ear. The iteration of this morphogenetic signaling system suggests that it is a more generally applicable mechanism in other epithelial tissues undergoing shape change.

Rostral paraxial mesoderm regulates refinement of the eye field through the bone morphogenetic protein (BMP) pathway.

The eye field is initially a large single domain at the anterior end of the neural plate and is the first indication of optic potential in the vertebrate embryo. During the course of development, this domain is subject to interactions that shape and refine the organogenic field. The action of the prechordal mesoderm in bisecting this single region into two bilateral domains has been well described, however the role of signalling interactions in the further restriction and refinement of this domain has not been previously characterised. Here we describe a role for the rostral cephalic paraxial mesoderm in limiting the extent of the eye field. The anterior transposition of this mesoderm or its ablation disrupted normal development of the eye. Importantly, perturbation of optic vesicle development occurred in the absence of any detectable changes in the pattern of neighbouring regions of the neural tube. Furthermore, negative regulation of eye development is a property unique to the rostral paraxial mesoderm. The rostral paraxial mesoderm expresses members of the bone morphogenetic protein (BMP) family of signalling molecules and manipulation of endogenous BMP signalling resulted in abnormalities of the early optic primordia.

Hair cell differentiation becomes tissue specific by E9.5 in mouse inner ear.

Tissue culture is a standard method to study tissue interactions during embryogenesis. The serum that is usually used in culture can, however, confound results as it includes unidentified factors. In this study, we used a serum-free otocyst culture to investigate the tissue interactions that determine hair cell fate in mice otocysts. Otocysts cultured with surrounding tissues have the ability to produce mature hair cells in serum-free otocyst culture. Although isolated otocysts from E9.5 mice produced hair cells, those from E9.0 mice could not. This indicates that the mouse otocyst gains the ability to generate hair cells between E9.0 and E9.5 and that this ability depends on signals from the surrounding mesenchyme and/or the hindbrain.

Early steps in inner ear development: induction and morphogenesis of the otic placode.

Various cellular replacement therapies using in vitro generated cells to replace damaged tissue have been proposed as strategies to alleviate hearing loss. All such therapies must involve a complete understanding of the earliest steps in inner ear development; its induction as a thickened plate of cells in the non-neural, surface ectoderm of the embryo, to its internalization as an otocyst embedded in the head mesenchyme of the embryo. Such knowledge informs researchers addressing the feasibility of the proposed strategy and present alternatives if needed. In this review we describe the mechanisms of inner ear induction, concentrating on the factors that steer the fate of ectoderm into precursors of the inner ear. Induction then leads to inner ear morphogenesis and we describe the cellular changes that occur as the inner ear is converted from a superficial placode to an internalized otocyst, and how they are coordinated with a particular emphasis on how the signaling environment surrounding the inner ear influences these processes.

Characterization of the finch embryo supports evolutionary conservation of the naive stage of development in amniotes.

Innate pluripotency of mouse embryos transits from naive to primed state as the inner cell mass differentiates into epiblast. In vitro, their counterparts are embryonic (ESCs) and epiblast stem cells (EpiSCs), respectively. Activation of the FGF signaling cascade results in mouse ESCs differentiating into mEpiSCs, indicative of its requirement in the shift between these states. However, only mouse ESCs correspond to the naive state; ESCs from other mammals and from chick show primed state characteristics. Thus, the significance of the naive state is unclear. In this study, we use zebra finch as a model for comparative ESC studies. The finch blastoderm has mESC-like properties, while chick blastoderm exhibits EpiSC features. In the absence of FGF signaling, finch cells retained expression of pluripotent markers, which were lost in cells from chick or aged finch epiblasts. Our data suggest that the naive state of pluripotency is evolutionarily conserved among amniotes.

Development of the sensory organs.

The sensory organs--the eye, ear, and nose- are formed, in part, from ectodermal thickenings: placodes. Their development is distinct from that of other regions of the developing body and they are essential for the development of other structures. For example, the olfactory placode which gives rise to the nose is essential for the functional development of the reproductive organs and hence fertility. Recently much progress has been made in the understanding of placode development, at both a molecular and embryological level. This is important as abnormal development of placodes occurs in a number of human syndromes. Furthermore, knowledge of placode development will give insight into therapeutic strategies to prevent degenerative change such as deafness. This review highlights the current knowledge of placode development and the future challenges in unravelling the cascades of signalling interactions that control development of these unique structures.

Comparison of the expression patterns of several fibroblast growth factors during chick gastrulation and neurulation.

The fibroblast growth factor family consists of a large number of secreted polypeptide growth factors. The developmental importance of the family has been highlighted in many studies, which serve to underscore their role as local instructive signals directing diverse processes in the developing embryo. We wished to characterize and compare the expression patterns of nine members of the fibroblast growth factor family in the chick embryo. In this study, we survey the expression patterns of fgf-2, -3, -4, -8, -10, -12, -13, -14 and -18 during gastrula and neurula stages (Hamburger and Hamilton stages: 4-13). As well as providing a comparison of the expression patterns of those fibroblast growth factors already published, we provide new data on the expression patterns of some of these genes at early stages.