Fgf8 and Fgf3 are required for zebrafish ear placode induction, maintenance and inner ear patterning

Mechanisms of otoconia and otolith development

BACKGROUND

Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years.

RESULTS

To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development.

CONCLUSIONS

This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

Compensatory regulation of the size of the inner ear in response to excess induction of otic progenitors by fibroblast growth factor signaling

BACKGROUND

The otic placode comprises the progenitors of the inner ear and the neurons that convey hearing and balance information to the brain. Transplantation studies in birds and amphibians demonstrate that when the otic placode is morphologically visible as a thickened patch of ectoderm, it is first committed to an otic fate. Fibroblast growth factor (FGF) signaling initiates induction of the otic placode, and levels of FGF signaling are fine-tuned by the Sprouty family of antagonists of receptor tyrosine kinase signaling.

RESULTS

Here, we examined the size of the otic placode and cup by combinatorial inactivation of the Sprouty1 and Sprouty2 genes. Interestingly, in a Sprouty gene dosage series, early enlargement of the otic placode was progressively restored to normal. Restoration of otic size was preceded by normal levels of FGF signaling, reduced cell proliferation and reduced cell death.

CONCLUSIONS

Our study demonstrates that excess otic placode cells, which form in response to increased FGF signaling, are not maintained in mammals. This suggests that growth plasticity exists in the mammalian otic placode and cup, and that FGF signaling may not be sufficient to induce the genetic program that maintains otic fate.

Fate map of the chicken otic placode

The inner ear is an intricate three-dimensional sensory organ that arises from a flat, thickened portion of the ectoderm termed the otic placode. There is evidence that the ontogenetic steps involved in the progressive specification of the highly specialized inner ear of vertebrates involve the concerted actions of diverse patterning signals that originate from nearby tissues, providing positional identity and instructive context. The topology of the prospective inner ear portions at placode stages when such patterning begins has remained largely unknown. The chick-quail model was used to perform a comprehensive fate mapping study of the chick otic placode, shedding light on the precise topological position of each presumptive inner ear component relative to the dorsoventral and anteroposterior axes of the otic placode and, implicitly, to the possible sources of inducing signals. The findings reveal the existence of three dorsoventrally arranged anteroposterior domains from which the endolymphatic system, the maculae and basilar papilla, and the cristae develop. This study provides new bases for the interpretation of earlier and future descriptive and experimental studies that aim to understand the molecular genetic mechanisms involved in otic placode patterning.

Foxi3 is necessary for the induction of the chick otic placode in response to FGF signaling

Vertebrate cranial sensory organs are derived from region at the border of the anterior neural plate called the pre-placodal region (PPR). The otic placode, the anlagen of the inner ear, is induced from PPR ectoderm by FGF signaling. We have previously shown that competence of embryonic ectoderm to respond to FGF signaling during otic placode induction correlates with the expression of PPR genes, but the molecular basis of this competence is poorly understood. Here, we characterize the function of a transcription factor, Foxi3 that is expressed at very early stages in the non-neural ectoderm and later in the PPR of chick embryos. Ablation experiments showed that the underlying hypoblast is necessary for the initiation of Foxi3 expression. Mis-expression of Foxi3 was sufficient to induce markers of non-neural ectoderm such as Dlx5, and the PPR such as Six1 and Eya2. Electroporation of Dlx5, or Six1 together with Eya1 also induced Foxi3, suggesting direct or indirect positive regulation between non-neural ectoderm genes and PPR genes. Knockdown of Foxi3 in chick embryos prevented the induction of otic placode markers, and was able to prevent competent cranial ectoderm from expressing otic markers in response to FGF2. In contrast, Foxi3 expression alone was not sufficient to confer competence to respond to FGF on embryonic ectoderm. Our analysis of PPR and FGF-responsive genes after Foxi3 knockdown at gastrula stages suggests it is not necessary for the expression of PPR genes at these stages, nor for the transduction of FGF signals. The early expression but late requirement for Foxi3 in ear induction suggests it may have some of the properties associated with pioneer transcription factors.

Establishing the pre-placodal region and breaking it into placodes with distinct identities

Specialized sensory organs in the vertebrate head originate from thickenings in the embryonic ectoderm called cranial sensory placodes. These placodes, as well as the neural crest, arise from a zone of ectoderm that borders the neural plate. This zone separates into a precursor field for the neural crest that lies adjacent to the neural plate, and a precursor field for the placodes, called the pre-placodal region (PPR), that lies lateral to the neural crest. The neural crest domain and the PPR are established in response to signaling events mediated by BMPs, FGFs and Wnts, which differentially activate transcription factors in these territories. In the PPR, members of the Six and Eya families, act in part to repress neural crest specific transcription factors, thus solidifying a placode developmental program. Subsequently, in response to environmental cues the PPR is further subdivided into placodal territories with distinct characteristics, each expressing a specific repertoire of transcription factors that provide the necessary information for their progression to mature sensory organs. In this review we summarize recent advances in the characterization of the signaling molecules and transcriptional effectors that regulate PPR specification and its subdivision into placodal domains with distinct identities.

Sensational placodes: neurogenesis in the otic and olfactory systems

For both the intricate morphogenetic layout of the sensory cells in the ear and the elegantly radial arrangement of the sensory neurons in the nose, numerous signaling molecules and genetic determinants are required in concert to generate these specialized neuronal populations that help connect us to our environment. In this review, we outline many of the proteins and pathways that play essential roles in the differentiation of otic and olfactory neurons and their integration into their non-neuronal support structures. In both cases, well-known signaling pathways together with region-specific factors transform thickened ectodermal placodes into complex sense organs containing numerous, diverse neuronal subtypes. Olfactory and otic placodes, in combination with migratory neural crest stem cells, generate highly specialized subtypes of neuronal cells that sense sound, position and movement in space, odors and pheromones throughout our lives.

FGF signaling is required for brain left-right asymmetry and brain midline formation

Early disruption of FGF signaling alters left-right (LR) asymmetry throughout the embryo. Here we uncover a role for FGF signaling that specifically disrupts brain asymmetry, independent of normal lateral plate mesoderm (LPM) asymmetry. When FGF signaling is inhibited during mid-somitogenesis, asymmetrically expressed LPM markers southpaw and lefty2 are not affected. However, asymmetrically expressed brain markers lefty1 and cyclops become bilateral. We show that FGF signaling controls expression of six3b and six7, two transcription factors required for repression of asymmetric lefty1 in the brain. We found that Z0-1, atypical PKC (aPKC) and β-catenin protein distribution revealed a midline structure in the forebrain that is dependent on a balance of FGF signaling. Ectopic activation of FGF signaling leads to overexpression of six3b, loss of organized midline adherins junctions and bilateral loss of lefty1 expression. Reducing FGF signaling leads to a reduction in six3b and six7 expression, an increase in cell boundary formation in the brain midline, and bilateral expression of lefty1. Together, these results suggest a novel role for FGF signaling in the brain to control LR asymmetry, six transcription factor expressions, and a midline barrier structure.

Fgf3 and Fgf10a work in concert to promote maturation of the epibranchial placodes in zebrafish

Essential cellular components of the paired sensory organs of the vertebrate head are derived from transient thickenings of embryonic ectoderm known as cranial placodes. The epibranchial (EB) placodes give rise to sensory neurons of the EB ganglia that are responsible for relaying visceral sensations form the periphery to the central nervous system. Development of EB placodes and subsequent formation of EB ganglia is a multistep process regulated by various extrinsic factors, including fibroblast growth factors (Fgfs). We discovered that two Fgf ligands, Fgf3 and Fgf10a, cooperate to promote EB placode development. Whereas EB placodes are induced in the absence of Fgf3 and Fgf10a, they fail to express placode specific markers Pax2a and Sox3. Expression analysis and mosaic rescue experiments demonstrate that Fgf3 signal is derived from the endoderm, whereas Fgf10a is emitted from the lateral line system and the otic placode. Further analyses revealed that Fgf3 and Fgf10a activities are not required for cell proliferation or survival, but are required for placodal cells to undergo neurogenesis. Based on these data, we conclude that a combined loss of these Fgf factors results in a failure of the EB placode precursors to initiate a transcriptional program needed for maturation and subsequent neurogenesis. These findings highlight the importance and complexity of reiterated Fgf signaling during cranial placode formation and subsequent sensory organ development.

Gene miles-apart is required for formation of otic vesicle and hair cells in zebrafish

Hearing loss is a serious burden to physical and mental health worldwide. Aberrant development and damage of hearing organs are recognized as the causes of hearing loss, the molecular mechanisms underlining these pathological processes remain elusive. Investigation of new molecular mechanisms involved in proliferation, differentiation, migration and maintenance of neuromast primordium and hair cells will contribute to better understanding of hearing loss pathology. This knowledge will enable the development of protective agents and mechanism study of drug ototoxicity. In this study, we demonstrate that the zebrafish gene miles-apart, a homolog of sphingosine-1-phosphate receptor 2 (s1pr2) in mammals, has an important role in the development of otic vesicle, neuromasts and survival of hair cells. Whole-mount in situ hybridization of embryos showed that miles-apart expression occurred mainly in the encephalic region and the somites at 24 h.p.f. (hour post fertilization), in the midbrain/hindbrain boundary, the brainstem and the pre-neuromast of lateral line at 48 h.p.f. in a strict spatiotemporal regulation. Both up- and downregulation of miles-apart led to abnormal otoliths and semicircular canals, excess or few hair cells and neuromasts, and their disarranged depositions in the lateral lines. Miles-apart (Mil) dysregulation also caused abnormal expression of hearing-associated genes, including hmx2, fgf3, fgf8a, foxi1, otop1, pax2.1 and tmieb during zebrafish organogenesis. Moreover, in larvae miles-apart gene knockdown significantly upregulated proapoptotic gene zBax2 and downregulated prosurvival gene zMcl1b; in contrast, the level of zBax2 was decreased and of zMcl1b enhanced by miles-apart overexpression. Collectively, Mil activity is linked to organization and number decision of hair cells within a neuromast, also to deposition of neuromasts and formation of otic vesicle during zebrafish organogenesis. At the larva stage, Mil as an upstream regulator of bcl-2 gene family has a role in protection of hair cells against apoptosis by promoting expression of prosurvival gene zMcl1b and suppressing proapoptotic gene zBax2.

Role of fibroblast growth factor 8 in neurite outgrowth from spiral ganglion neurons in vitro

Many neurons degenerate after injuries resulting from overstimulation, drugs, genetic mutations, and aging. Although several growth factors and neurotrophins delay degeneration and promote regrowth of neural processes, the role of fibroblast growth factor 8 (FGF8) in mammalian spiral ganglion neurons (SGN) neurite outgrowth has not been examined. This study develops and uses SGN cell cultures suitable for experimental analysis, it investigates whether FGF8a and FGF8b isoforms affect the neurite outgrowth from SGN cultured in vitro. We found that both FGF8a and FGF8b promoted the outgrowth of neurites from cultured SGN. This response is mediated by FGF receptors and involves the activation of IκBα-mediated NFκB signaling pathway. These findings suggest that, besides its morphogenetic role during development, FGF8 may have trophic functions in the adult which are relevant to regeneration.

Genome-wide copy number variation analysis of a Branchio-oto-renal syndrome cohort identifies a recombination hotspot and implicates new candidate genes

Branchio-oto-renal (BOR) syndrome is an autosomal dominant disorder characterized by branchial arch anomalies, hearing loss and renal dysmorphology. Although haploinsufficiency of EYA1 and SIX1 are known to cause BOR, copy number variation analysis has only been performed on a limited number of BOR patients. In this study, we used high-resolution array-based comparative genomic hybridization on 32 BOR probands negative for coding-sequence and splice-site mutations in known BOR-causing genes to identify potential disease-causing genomic rearrangements. Of the >1,000 rare and novel copy number variants we identified, four were heterozygous deletions of EYA1 and several downstream genes that had nearly identical breakpoints associated with retroviral sequence blocks, suggesting that non-allelic homologous recombination seeded by this recombination hotspot is important in the pathogenesis of BOR. A different heterozygous deletion removing the last exon of EYA1 was identified in an additional proband. Thus, in total five probands (14 %) had deletions of all or part of EYA1. Using a novel disease-gene prioritization strategy that includes network analysis of genes associated with other deletions suggests that SHARPIN (Sipl1), FGF3 and the HOXA gene cluster may contribute to the pathogenesis of BOR.

Zebrafish Foxi1 provides a neuronal ground state during inner ear induction preceding the Dlx3b/4b-regulated sensory lineage

Vertebrate inner ear development is a complex process that involves the induction of a common territory for otic and epibranchial precursors and their subsequent segregation into otic and epibranchial cell fates. In zebrafish, the otic-epibranchial progenitor domain (OEPD) is induced by Fgf signaling in a Foxi1- and Dlx3b/4b-dependent manner, but the functional differences of Foxi1 and Dlx3b/4b in subsequent cell fate specifications within the developing inner ear are poorly understood. Based on pioneer tracking (PioTrack), a novel Cre-dependent genetic lineage tracing method, and genetic data, we show that the competence to embark on a neuronal or sensory fate is provided sequentially and very early during otic placode induction. Loss of Foxi1 prevents neuronal precursor formation without affecting hair cell specification, whereas loss of Dlx3b/4b inhibits hair cell but not neuronal precursor formation. Consistently, in Dlx3b/4b- and Sox9a-deficient b380 mutants almost all otic epithelial fates are absent, including sensory hair cells, and the remaining otic cells adopt a neuronal fate. Furthermore, the progenitors of the anterior lateral line ganglia also arise from the OEPD in a Foxi1-dependent manner but are unaffected in the absence of Dlx3b/4b or in b380 mutants. Thus, in addition to otic fate Foxi1 provides neuronal competence during OEPD induction prior to and independently of the Dlx3b/4b-mediated sensory fate of the developing inner ear.

Analysis of FGF-dependent and FGF-independent pathways in otic placode induction

The inner ear develops from a patch of thickened cranial ectoderm adjacent to the hindbrain called the otic placode. Studies in a number of vertebrate species suggest that the initial steps in induction of the otic placode are regulated by members of the Fibroblast Growth Factor (FGF) family, and that inhibition of FGF signaling can prevent otic placode formation. To better understand the genetic pathways activated by FGF signaling during otic placode induction, we performed microarray experiments to estimate the proportion of chicken otic placode genes that can be up-regulated by the FGF pathway in a simple culture model of otic placode induction. Surprisingly, we find that FGF is only sufficient to induce about 15% of chick otic placode-specific genes in our experimental system. However, pharmacological blockade of the FGF pathway in cultured chick embryos showed that although FGF signaling was not sufficient to induce the majority of otic placode-specific genes, it was still necessary for their expression in vivo. These inhibitor experiments further suggest that the early steps in otic placode induction regulated by FGF signaling occur through the MAP kinase pathway. Although our work suggests that FGF signaling is necessary for otic placode induction, it demonstrates that other unidentified signaling pathways are required to co-operate with FGF signaling to induce the full otic placode program.

Induction of the inner ear: stepwise specification of otic fate from multipotent progenitors

Despite its complexity in the adult, during development the inner ear arises from a simple epithelium, the otic placode. Placode specification is a multistep process that involves the integration of various signalling pathways and downstream transcription factors in time and space. Here we review the molecular events that successively commit multipotent ectodermal precursors to the otic lineage. The first step in this hierarchy is the specification of sensory progenitor cells, which can contribute to all sensory placodes, followed by the induction of a common otic-epibranchial field and finally the establishment the otic territory. In recent years, some of the molecular components that control this process have been identified, and begin to reveal complex interactions. Future studies will need to unravel how this information is integrated and encoded in the genome. This will form the blueprint for stem cell differentiation towards otic fates and generate a predictive gene regulatory network that models the earliest steps of otic specification.

A spatial and temporal gradient of Fgf differentially regulates distinct stages of neural development in the zebrafish inner ear

Neuroblasts of the statoacoustic ganglion (SAG) initially form in the floor of the otic vesicle during a relatively brief developmental window. They soon delaminate and undergo a protracted phase of proliferation and migration (transit-amplification). Neuroblasts eventually differentiate and extend processes bi-directionally to synapse with hair cells in the inner ear and various targets in the hindbrain. Our studies in zebrafish have shown that Fgf signaling controls multiple phases of this complex developmental process. Moderate levels of Fgf in a gradient emanating from the nascent utricular macula specify SAG neuroblasts in laterally adjacent otic epithelium. At a later stage, differentiating SAG neurons express Fgf5, which serves two functions: First, as SAG neurons accumulate, increasing levels of Fgf exceed an upper threshold that terminates the initial phase of neuroblast specification. Second, elevated Fgf delays differentiation of transit-amplifying cells, balancing the rate of progenitor renewal with neuronal differentiation. Laser-ablation of mature SAG neurons abolishes feedback-inhibition and causes precocious neuronal differentiation. Similar effects are obtained by Fgf5-knockdown or global impairment of Fgf signaling, whereas Fgf misexpression has the opposite effect. Thus Fgf signaling renders SAG development self-regulating, ensuring steady production of an appropriate number of neurons as the larva grows.

Neurons of the statoacoustic ganglion (SAG), which innervate the inner ear, are derived from neuroblasts originating from the floor of the otic vesicle. Neuroblasts quickly delaminate from the otic vesicle to form dividing progenitors, which eventually differentiate into mature neurons of the SAG. Fgf has been implicated in initial neuroblast specification in multiple vertebrate species. However, the role of Fgf at later stages remains uncertain, because previous studies have not been able to evaluate the effects of changing levels of Fgf, nor have they been able to clearly distinguish the effects of Fgf at different stages of SAG development. We have combined conditional loss of function, misexpression, and laser-ablation studies in zebrafish to elucidate how graded Fgf coordinates distinct steps in SAG development. Initially moderate Fgf in a spatial gradient specifies neuroblasts within the otic vesicle. Later, mature SAG neurons express Fgf5 and, as additional neurons accumulate outside the otic vesicle, rising levels of Fgf terminate further specification. Elevated Fgf also slows maturation of progenitors, maintaining a stable progenitor pool in which growth and differentiation are evenly balanced. This feedback facilitates steady production of new neurons as the animal grows through larval and adults stages.

Early development of the vertebrate inner ear

This is a review of the biological processes and the main signaling pathways required to generate the different otic cell types, with particular emphasis on the actions of insulin-like growth factor I. The sensory organs responsible of hearing and balance have a common embryonic origin in the otic placode. Lineages of neural, sensory, and support cells are generated from common otic neuroepithelial progenitors. The sequential generation of the cell types that will form the adult inner ear requires the coordination of cell proliferation with cell differentiation programs, the strict regulation of cell survival, and the metabolic homeostasis of otic precursors. A network of intracellular signals operates to coordinate the transcriptional response to the extracellular input. Understanding the molecular clues that direct otic development is fundamental for the design of novel treatments for the protection and repair of hearing loss and balance disorders.

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.

The peripheral sensory nervous system in the vertebrate head: a gene regulatory perspective

In the vertebrate head, crucial parts of the sense organs and sensory ganglia develop from special regions, the cranial placodes. Despite their cellular and functional diversity, they arise from a common field of multipotent progenitors and acquire distinct identity later under the influence of local signalling. Here we present the gene regulatory network that summarises our current understanding of how sensory cells are specified, how they become different from other ectodermal derivatives and how they begin to diversify to generate placodes with different identities. This analysis reveals how sequential activation of sets of transcription factors subdivides the ectoderm over time into smaller domains of progenitors for the central nervous system, neural crest, epidermis and sensory placodes. Within this hierarchy the timing of signalling and developmental history of each cell population is of critical importance to determine the ultimate outcome. A reoccurring theme is that local signals set up broad gene expression domains, which are further refined by mutual repression between different transcription factors. The Six and Eya network lies at the heart of sensory progenitor specification. In a positive feedback loop these factors perpetuate their own expression thus stabilising pre-placodal fate, while simultaneously repressing neural and neural crest specific factors. Downstream of the Six and Eya cassette, Pax genes in combination with other factors begin to impart regional identity to placode progenitors. While our review highlights the wealth of information available, it also points to the lack information on the cis-regulatory mechanisms that control placode specification and of how the repeated use of signalling input is integrated.

Cloning and expression analysis of Fgf5, 6 and 7 during early chick development

FGFs with similar sequences can play different roles depending on the model organisms examined. Determining these roles requires knowledge of spatio-temporal Fgf gene expression patterns. In this study, we report the cloning of chick Fgf5, 6 and 7, and examine their gene expression patterns by whole mount in situ hybridization. We show that Fgf5's spatio-temporally restricted expression pattern indicates a potentially novel role during inner ear development. Fgf6 and Fgf7, although belonging to different subfamilies with diverged sequences, are expressed in similar patterns within the mesoderm. Alignment of protein sequences and phylogenetic analysis demonstrate that FGF5 and FGF6 are highly conserved between chick, human, mouse and zebrafish. FGF7 is similarly conserved except for the zebrafish, which has considerably diverged.

Conditions that influence the response to Fgf during otic placode induction

Despite the vital importance of Fgf for otic induction, previous attempts to study otic induction through Fgf misexpression have yielded widely varying and contradictory results. There are also discrepancies regarding the ability of Fgf to induce otic tissue in ectopic locations, raising questions about the sufficiency of Fgf and the degree to which other local factors enhance or restrict otic potential. Using heat shock-inducible transgenes to misexpress Fgf3 or Fgf8 in zebrafish, we found that the stage, distribution and level of misexpression strongly influence the response to Fgf. Fgf misexpression during gastrulation can inhibit or promote otic development, depending on context, whereas misexpression after gastrulation leads to expansion of otic markers throughout preplacodal ectoderm surrounding the head. Elevated Fgf also expands expression of the putative competence factor Foxi1, which is required for Fgf to expand other otic markers. Misexpression of downstream factors Pax2a or Pax8 also expands otic markers but cannot bypass the requirement for Fgf or Foxi1. Co-misexpression of Pax2/8 with Fgf8 potentiates formation of ectopic otic vesicles expressing a full range of otic markers. These findings document the variables critically affecting the response to Fgf and clarify the roles of foxi1 and pax2/8 in the otic response.

Fgf and Hh signalling act on a symmetrical pre-pattern to specify anterior and posterior identity in the zebrafish otic placode and vesicle

Specification of the otic anteroposterior axis is one of the earliest patterning events during inner ear development. In zebrafish, Hedgehog signalling is necessary and sufficient to specify posterior otic identity between the 10 somite (otic placode) and 20 somite (early otic vesicle) stages. We now show that Fgf signalling is both necessary and sufficient for anterior otic specification during a similar period, a function that is completely separable from its earlier role in otic placode induction. In lia(-/-) (fgf3(-/-)) mutants, anterior otic character is reduced, but not lost altogether. Blocking all Fgf signalling at 10-20 somites, however, using the pan-Fgf inhibitor SU5402, results in the loss of anterior otic structures and a mirror image duplication of posterior regions. Conversely, overexpression of fgf3 during a similar period, using a heat-shock inducible transgenic line, results in the loss of posterior otic structures and a duplication of anterior domains. These phenotypes are opposite to those observed when Hedgehog signalling is altered. Loss of both Fgf and Hedgehog function between 10 and 20 somites results in symmetrical otic vesicles with neither anterior nor posterior identity, which, nevertheless, retain defined poles at the anterior and posterior ends of the ear. These data suggest that Fgf and Hedgehog act on a symmetrical otic pre-pattern to specify anterior and posterior otic identity, respectively. Each signalling pathway has instructive activity: neither acts simply to repress activity of the other, and, together, they appear to be key players in the specification of anteroposterior asymmetries in the zebrafish ear.

Her9 represses neurogenic fate downstream of Tbx1 and retinoic acid signaling in the inner ear

Proper spatial control of neurogenesis in the inner ear ensures the precise innervation of mechanotransducing cells and the propagation of auditory and equilibrium stimuli to the brain. Members of the Hairy and enhancer of split (Hes) gene family regulate neurogenesis by inhibiting neuronal differentiation and maintaining neural stem cell pools in non-neurogenic zones. Remarkably, their role in the spatial control of neurogenesis in the ear is unknown. In this study, we identify her9, a zebrafish ortholog of Hes1, as a key gene in regulating otic neurogenesis through the definition of the posterolateral non-neurogenic field. First, her9 emerges as a novel otic patterning gene that represses proneural function and regulates the extent of the neurogenic domain. Second, we place Her9 downstream of Tbx1, linking these two families of transcription factors for the first time in the inner ear and suggesting that the reported role of Tbx1 in repressing neurogenesis is in part mediated by the bHLH transcriptional repressor Her9. Third, we have identified retinoic acid (RA) signaling as the upstream patterning signal of otic posterolateral genes such as tbx1 and her9. Finally, we show that at the level of the cranial otic field, opposing RA and Hedgehog signaling position the boundary between the neurogenic and non-neurogenic compartments. These findings permit modeling of the complex genetic cascade that underlies neural patterning of the otic vesicle.

Pax2/8 proteins coordinate sequential induction of otic and epibranchial placodes through differential regulation of foxi1, sox3 and fgf24

Vertebrate cranial placodes contribute vitally to development of sensory structures of the head. Amongst posterior placodes, the otic placode forms the inner ear whereas nearby epibranchial placodes produce sensory ganglia within branchial clefts. Though diverse in fate, these placodes show striking similarities in their early regulation. In zebrafish, both are initiated by localized Fgf signaling plus the ubiquitous competence factor Foxi1, and both express pax8 and sox3 in response. It has been suggested that Fgf initially induces a common otic/epibranchial field, which later subdivides in response to other signals. However, we find that otic and epibranchial placodes form at different times and by distinct mechanisms. Initially, Fgf from surrounding tissues induces otic expression of pax8 and sox3, which cooperate synergistically to establish otic fate. Subsequently, pax8 works with related genes pax2a/pax2b to downregulate otic expression of foxi1, a necessary step for further otic development. Additionally, pax2/8 activate otic expression of fgf24, which induces epibranchial expression of sox3. Knockdown of fgf24 or sox3 causes severe epibranchial deficiencies but has little effect on otic development. These findings clarify the roles of pax8 and sox3 and support a model whereby the otic placode forms first and induces epibranchial placodes through an Fgf-relay.

Characterization of new otic enhancers of the pou3f4 gene reveal distinct signaling pathway regulation and spatio-temporal patterns

POU3F4 is a member of the POU-homedomain transcription factor family with a prominent role in inner ear development. Mutations in the human POU3F4 coding unit leads to X-linked deafness type 3 (DFN3), characterized by conductive hearing loss and progressive sensorineural deafness. Microdeletions found 1 Mb 5' upstream of the coding region also displayed the same phenotype, suggesting that cis-regulatory elements might be present in that region. Indeed, we and others have recently identified several enhancers at the 1 Mb 5' upstream interval of the pou3f4 locus. Here we characterize the spatio-temporal patterns of these regulatory elements in zebrafish transgenic lines. We show that the most distal enhancer (HCNR 81675) is activated earlier and drives GFP reporter expression initially to a broad ear domain to progressively restrict to the sensory patches. The proximal enhancer (HCNR 82478) is switched later during development and promotes expression, among in other tissues, in sensory patches from its onset. The third enhancer (HCNR 81728) is also active at later stages in the otic mesenchyme and in the otic epithelium. We also characterize the signaling pathways regulating these enhancers. While HCNR 81675 is regulated by very early signals of retinoic acid, HCNR 82478 is regulated by Fgf activity at a later stage and the HCNR 81728 enhancer is under the control of Hh signaling. Finally, we show that Sox2 and Pax2 transcription factors are bound to HCNR 81675 genomic region during otic development and specific mutations to these transcription factor binding sites abrogates HCNR 81675 enhancer activity. Altogether, our results suggest that pou3f4 expression in inner ear might be under the control of distinct regulatory elements that fine-tune the spatio-temporal activity of this gene and provides novel data on the signaling mechanisms controlling pou3f4 function.

Identification of early requirements for preplacodal ectoderm and sensory organ development

Preplacodal ectoderm arises near the end of gastrulation as a narrow band of cells surrounding the anterior neural plate. This domain later resolves into discrete cranial placodes that, together with neural crest, produce paired sensory structures of the head. Unlike the better-characterized neural crest, little is known about early regulation of preplacodal development. Classical models of ectodermal patterning posit that preplacodal identity is specified by readout of a discrete level of Bmp signaling along a DV gradient. More recent studies indicate that Bmp-antagonists are critical for promoting preplacodal development. However, it is unclear whether Bmp-antagonists establish the proper level of Bmp signaling within a morphogen gradient or, alternatively, block Bmp altogether. To begin addressing these issues, we treated zebrafish embryos with a pharmacological inhibitor of Bmp, sometimes combined with heat shock-induction of Chordin and dominant-negative Bmp receptor, to fully block Bmp signaling at various developmental stages. We find that preplacodal development occurs in two phases with opposing Bmp requirements. Initially, Bmp is required before gastrulation to co-induce four transcription factors, Tfap2a, Tfap2c, Foxi1, and Gata3, which establish preplacodal competence throughout the nonneural ectoderm. Subsequently, Bmp must be fully blocked in late gastrulation by dorsally expressed Bmp-antagonists, together with dorsally expressed Fgf and Pdgf, to specify preplacodal identity within competent cells abutting the neural plate. Localized ventral misexpression of Fgf8 and Chordin can activate ectopic preplacodal development anywhere within the zone of competence, whereas dorsal misexpression of one or more competence factors can activate ectopic preplacodal development in the neural plate. Conversely, morpholino-knockdown of competence factors specifically ablates preplacodal development. Our work supports a relatively simple two-step model that traces regulation of preplacodal development to late blastula stage, resolves two distinct phases of Bmp dependence, and identifies the main factors required for preplacodal competence and specification.

Cranial placodes, which produce sensory structures in the head, arise from a contiguous band of preplacodal ectoderm surrounding the anterior neural plate during gastrulation. Little is known about early regulation of preplacodal ectoderm, but modulation of signaling through Bone Morphogenetic Protein (Bmp) is clearly involved. Recent studies show that dorsally expressed Bmp-antagonists help establish preplacodal ectoderm, but it is not clear whether antagonists titrate Bmp to a discrete low level that actively induces preplacodal fate or, alternatively, whether Bmp must be fully blocked to permit preplacodal development. We show that in zebrafish preplacodal development occurs in distinct phases with differing Bmp requirements. Initially, Bmp is required before gastrulation to render all ventral ectoderm competent to form preplacodal tissue. We further show that four transcription factors, Foxi1, Gata3, Tfap2a, and Tfap2c, specifically mediate preplacodal competence. Once induced, these factors no longer require Bmp. Thereafter, Bmp must be fully blocked by dorsally expressed Bmp-antagonists to permit preplacodal development. In addition, dorsally expressed Fgf and/or Pdgf are also required, activating preplacodal development in competent cells abutting the neural plate. Thus, we have resolved the role of Bmp and traced the regulation of preplacodal development to pre-gastrula stage.

BMP/SMAD signaling regulates the cell behaviors that drive the initial dorsal-specific regional morphogenesis of the otocyst

During development of the otocyst, regional morphogenesis establishes a dorsal vestibular chamber and a ventral auditory chamber, which collectively constitute the membranous labyrinth of the inner ear. We identified the earliest morphogenetic event heralding the formation of the vestibular chamber, a rapid thinning and expansion of the dorsolateral wall of the otocyst, and showed that this process is generated by changes in otocyst cell shape from columnar to squamous, as opposed to changes in other cell behaviors, such as localized changes in cell proliferation or cell death. Moreover, we showed that thinning and expansion of the dorsolateral otocyst is regulated by BMP/SMAD signaling, which is both sufficient and necessary for localized thinning and expansion. Finally, we showed that BMP/SMAD signaling causes fragmentation of E-cadherin in the dorsolateral otocyst, occurring concomitantly with cell shape change, suggesting that BMP/SMAD signaling regulates cell-cell adhesion during the initial morphogenesis of the otocyst epithelium. Collectively, our results show that BMP signaling via SMADs regulates the cell behaviors that drive the initial dorsal-specific morphogenesis of the otocyst, providing new information about how regional morphogenesis of a complex organ rudiment, the developing membranous labyrinth, is initiated.

Analysis of X chromosome genomic DNA sequence copy number variation associated with premature ovarian failure (POF)

BACKGROUND

Premature ovarian failure (POF) is a heterogeneous disease defined as amenorrhoea for >6 months before age 40, with an FSH serum level >40 mIU/ml (menopausal levels). While there is a strong genetic association with POF, familial studies have also indicated that idiopathic POF may also be genetically linked. Conventional cytogenetic analyses have identified regions of the X chromosome that are strongly associated with ovarian function, as well as several POF candidate genes. Cryptic chromosome abnormalities that have been missed might be detected by array comparative genomic hybridization.

METHODS

In this study, samples from 42 idiopathic POF patients were subjected to a complete end-to-end X/Y chromosome tiling path array to achieve a detailed copy number variation (CNV) analysis of X chromosome involvement in POF. The arrays also contained a 1 Mb autosomal tiling path as a reference control. Quantitative PCR for selected genes contained within the CNVs was used to confirm the majority of the changes detected. The expression pattern of some of these genes in human tissue RNA was examined by reverse transcription (RT)-PCR.

RESULTS

A number of CNVs were identified on both Xp and Xq, with several being shared among the POF cases. Some CNVs fall within known polymorphic CNV regions, and others span previously identified POF candidate regions and genes.

CONCLUSIONS

The new data reported in this study reveal further discrete X chromosome intervals not previously associated with the disease and therefore implicate new clusters of candidate genes. Further studies will be required to elucidate their involvement in POF.

Pax2 coordinates epithelial morphogenesis and cell fate in the inner ear

Crucial components of the vertebrate eye, ear and nose develop from discrete patches of surface epithelium, called placodes, which fold into spheroids and undergo complex morphogenesis. Little is known about how the changes in cell and tissue shapes are coordinated with the acquisition of cell fates. Here we explore whether these processes are regulated by common transcriptional mechanisms in the developing ear. After specification, inner ear precursors elongate to form the placode, which invaginates and is transformed into the complex structure of the adult ear. We show that the transcription factor Pax2 plays a key role in coordinating otic fate and placode morphogenesis, but appears to regulate each process independently. In the absence of Pax2, otic progenitors not only lose otic marker expression, but also fail to elongate due to the loss of apically localised N-cadherin and N-CAM. In the absence of either N-cadherin or N-CAM otic cells lose apical cell–cell contact and their epithelial shape. While misexpression of Pax2 leads to ectopic activation of both adhesion molecules, it is not sufficient to confer otic identity. These observations suggest that Pax2 controls cell shape independently from cell identity and thus acts as coordinator for these processes.

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.

The PAX258 gene subfamily: a comparative perspective

Whole genome duplication events are thought to have substantially contributed to organismal complexity, largely via divergent transcriptional regulation. Members of the vertebrate PAX2, PAX5 and PAX8 gene subfamily derived from an ancient class of paired box genes and arose from such whole genome duplication events. These genes are critical in establishing the midbrain-hindbrain boundary, specifying interneuron populations and for eye, ear and kidney development. Also PAX2 has adopted a unique role in pancreas development, whilst PAX5 is essential for early B-cell differentiation. The contribution of PAX258 genes to their collective role has diverged across paralogues and the animal lineages, resulting in a complex wealth of literature. It is now timely to provide a comprehensive comparative overview of these genes and their ancient and divergent roles. We also discuss their fundamental place within gene regulatory networks and the likely influence of cis-regulatory elements over their differential roles during early animal development.

Characterization of harpy/Rca1/emi1 mutants: patterning in the absence of cell division

We have characterized mutations in the early arrest gene, harpy (hrp), and show that they introduce premature stops in the coding region of early mitotic inhibitor1 (Rca1/emi1). In harpy mutants, cells stop dividing during early gastrulation. Lineage analysis confirms that there is little change in cell number after approximately cycle-14. Gross patterning occurs relatively normally, and many organ primordia are produced on time but with smaller numbers of cells. Despite the lack of cell division, some organ systems continue to increase in cell number, suggesting recruitment from surrounding areas. Analysis of bromodeoxyuridine incorporation shows that endoreduplication continues in many cells well past the first day of development, but cells cease endoreduplication once they begin to differentiate and express cell-type markers. Despite relatively normal gross patterning, harpy mutants show several defects in morphogenesis, cell migration and differentiation resulting directly or indirectly from the arrest of cell division.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Specification of cell fate in the mammalian cochlea

Mammalian auditory sensation is mediated by the organ of Corti, a specialized sensory epithelium found in the cochlea of the inner ear. Proper auditory function requires that the many different cell types found in the sensory epithelium be precisely ordered within an exquisitely patterned cellular mosaic. The development of this mosaic depends on a series of cell fate decisions that transform the initially nearly uniform cochlear epithelium into the complex structure of the mature organ of Corti. The prosensory domain, which contains the progenitors of both the mechanosensory hair cells and their associated supporting cells, first becomes distinct from both the neural and the nonsensory domains. Further cell fate decisions subdivide prosensory cells into populations of inner and outer hair cells, and several different types of supporting cells. A number of different signaling pathways and transcription factors are known to be necessary for these developmental processes; in this review, we will summarize these results with an emphasis on recent findings.

Identification of differentially expressed genes in early inner ear development

To understand the etiology of congenital hearing loss, a comprehensive understanding of the molecular genetic mechanisms underlying normal ear development is required. We are identifying genes involved in otogenesis, with the longer term goal of studying their mechanisms of action, leading to inner ear induction and patterning. Using Agilent microarrays, we compared the differential expression of a test domain (which consisted of the pre-otic placodal ectoderm with the adjacent hindbrain ectoderm and the underlying mesendodermal tissues) with a rostral control domain (which included tissue that is competent, but not specified, to express inner ear markers in explant assays). We identified 1261 transcripts differentially expressed between the two domains at a 2-fold or greater change: 463 were upregulated and 798 were downregulated in the test domain. We validated the differential expression of several signaling molecules and transcription factors identified in this array using in situ hybridization. Furthermore, the expression patterns of the validated group of genes from the test domain were explored in detail to determine how the timing of their expression relates to specific events of otic induction and development. In conclusion, we identified a number of novel candidate genes for otic placode induction.

Pax2 and Pea3 synergize to activate a novel regulatory enhancer for spalt4 in the developing ear

The transcription factor spalt4 is a key early-response gene in otic placode induction. Here, we characterize the cis-regulatory regions of spalt4 responsible for activation of its expression in the developing otic placode and report the isolation of a novel core enhancer. Identification and mutational analysis of putative transcription factor binding sites reveal that Pea3, a downstream effector of FGF signaling, and Pax2 directly activate spalt4 during ear development. Morpholino-mediated knock-down of each factor reduces or eliminates reporter expression. In contrast, combined over-expression of Pea3 and Pax2 drives ectopic reporter expression, suggesting that they function synergistically. These studies expand the gene regulatory network underlying early otic development by identifying direct inputs that mediate spalt4 expression.

Building the world's best hearing aid; regulation of cell fate in the cochlea

In mammals, auditory perception is initially mediated through sensory cells located in a rigorously patterned mosaic of unique cell types located within the coiled cochlea. Identification of the factors that direct multipotent progenitor cells to develop as each of these specialized cell types has the potential to enhance our understanding of the development of the auditory system and to identify potential targets for regenerative therapies. Recent results have identified specific signaling molecules and pathways, including Notch, Hedgehog, Sox2 and Fgfs, that guide progenitor cells to develop first as a sensory precursor, referred to as a prosensory cell, and subsequently as one of the specialized cell types within the sensory mosaic.

Molecular and tissue interactions governing induction of cranial ectodermal placodes

Whereas neural crest cells are the source of the peripheral nervous system in the trunk of vertebrates, the "ectodermal placodes," together with neural crest, form the peripheral nervous system of the head. Cranial ectodermal placodes are thickenings in the ectoderm that subsequently ingress or invaginate to make important contributions to cranial ganglia, including epibranchial and trigeminal ganglia, and sensory structures, the ear, nose, lens, and adenohypophysis. Recent studies have uncovered a number of molecular signals mediating induction and differentiation of placodal cells. Here, we described recent advances in understanding the tissue interactions and signals underlying induction and neurogenesis of placodes, with emphasis on the trigeminal and epibranchial. Important roles of Fibroblast Growth Factors, Platelet Derived Growth Factors, Sonic Hedgehog, TGFbeta superfamily members, and Wnts are discussed.

Mesendodermal signals required for otic induction: Bmp-antagonists cooperate with Fgf and can facilitate formation of ectopic otic tissue

Induction of otic placodes requires Fgf from surrounding tissues. We tested the hypothesis that mesendodermally derived Bmp-antagonists Chordin, Follistatin-a, and Crossveinless-2 cooperate in this process. Injecting morpholinos for all three genes, or treatment with the Nodal inhibitor SB431542 to block mesoderm-formation, reduces otic induction and strongly enhances the effects of disrupting fgf3 or fgf8. In contrast, using a lower dose of SB431542, combined with partial loss of Fgf, causes a dramatic medial expansion of otic tissue and formation of a single, large otic vesicle spanning the width of the hindbrain. Under these conditions, paraxial cephalic mesoderm forms ectopically at the midline, migrates into the head, and later transfates to form otic tissue beneath the hindbrain. Blocking expression of Bmp-antagonists blocks formation of medial otic tissue. These data show the importance of mesendodermal Bmp-antagonists for otic induction and that paraxial cephalic mesendoderm can facilitate its own otic differentiation under certain circumstances. Developmental Dynamics 238:1582-1594, 2009. (c) 2009 Wiley-Liss, Inc.

A zebrafish model for Waardenburg syndrome type IV reveals diverse roles for Sox10 in the otic vesicle

In humans, mutations in the SOX10 gene are a cause of the auditory-pigmentary disorder Waardenburg syndrome type IV (WS4) and related variants. SOX10 encodes an Sry-related HMG box protein essential for the development of the neural crest; deafness in WS4 and other Waardenburg syndromes is usually attributed to loss of neural-crest-derived melanocytes in the stria vascularis of the cochlea. However, SOX10 is strongly expressed in the developing otic vesicle and so direct roles for SOX10 in the otic epithelium might also be important. Here, we examine the otic phenotype of zebrafish sox10 mutants, a model for WS4. As a cochlea is not present in the fish ear, the severe otic phenotype in these mutants cannot be attributed to effects on this tissue. In zebrafish sox10 mutants, we see abnormalities in all otic placodal derivatives. Gene expression studies indicate deregulated expression of several otic genes, including fgf8, in sox10 mutants. Using a combination of mutant and morphant data, we show that the three sox genes belonging to group E (sox9a, sox9b and sox10) provide a link between otic induction pathways and subsequent otic patterning: they act redundantly to maintain sox10 expression throughout otic tissue and to restrict fgf8 expression to anterior macula regions. Single-cell labelling experiments indicate a small and transient neural crest contribution to the zebrafish ear during normal development, but this is unlikely to account for the strong defects seen in the sox10 mutant. We discuss the implication that the deafness in WS4 patients with SOX10 mutations might reflect a haploinsufficiency for SOX10 in the otic epithelium, resulting in patterning and functional abnormalities in the inner ear.

The dynein regulatory complex is required for ciliary motility and otolith biogenesis in the inner ear

In teleosts, proper balance and hearing depend on mechanical sensors in the inner ear. These sensors include actin-based microvilli and microtubule-based cilia that extend from the surface of sensory hair cells and attach to biomineralized 'ear stones' (or otoliths). Otolith number, size and placement are under strict developmental control, but the mechanisms that ensure otolith assembly atop specific cells of the sensory epithelium are unclear. Here we demonstrate that cilia motility is required for normal otolith assembly and localization. Using in vivo video microscopy, we show that motile tether cilia at opposite poles of the otic vesicle create fluid vortices that attract otolith precursor particles, thereby biasing an otherwise random distribution to direct localized otolith seeding on tether cilia. Independent knockdown of subunits for the dynein regulatory complex and outer-arm dynein disrupt cilia motility, leading to defective otolith biogenesis. These results demonstrate a requirement for the dynein regulatory complex in vertebrates and show that cilia-driven flow is a key epigenetic factor in controlling otolith biomineralization.

dlx3b/4b are required for the formation of the preplacodal region and otic placode through local modulation of BMP activity

The vertebrate inner ear arises from the otic placode, a transient thickening of ectodermal epithelium adjacent to neural crest domains in the presumptive head. During late gastrulation, cells fated to comprise the inner ear are part of a domain in cranial ectoderm that contain precursors of all sensory placodes, termed the preplacodal region (PPR). The combination of low levels of BMP activity coupled with high levels of FGF signaling are required to establish the PPR through induction of members of the six/eya/dach, iro, and dlx families of transcription factors. The zebrafish dlx3b/4b transcription factors are expressed at the neural plate border where they play partially redundant roles in the specification of the PPR, otic and olfactory placodes. We demonstrate that dlx3b/4b assist in establishing the PPR through the transcriptional regulation of the BMP antagonist cv2. Morpholino-mediated knockdown of Dlx3b/4b results in loss of cv2 expression in the PPR and a transient increase in Bmp4 activity that lasts throughout early somitogenesis. Through the cv2-mediated inhibition of BMP activity, dlx3b/4b create an environment where FGF activity is favorable for PPR and otic marker expression. Our results provide insight into the mechanisms of PPR specification as well as the role of dlx3b/4b function in PPR and otic placode induction.

Hindbrain-derived Wnt and Fgf signals cooperate to specify the otic placode in Xenopus

Induction of the otic placode, the rudiment of the inner ear, is believed to depend on signals derived from surrounding tissues, the head mesoderm and the prospective hindbrain. Here we report the first attempt to define the specific contribution of the neuroectoderm to this inductive process in Xenopus. To this end we tested the ability of segments of the neural plate (NP), isolated from different axial levels, to induce the otic marker Pax8 when recombined with blastula stage animal caps. We found that one single domain of the NP, corresponding to the prospective anterior hindbrain, had Pax8-inducing activity in this assay. Surprisingly, more than half of these recombinants formed otic vesicle-like structures. Lineage tracing experiments indicate that these vesicle-like structures are entirely derived from the animal cap and express several pan-otic markers. Pax8 activation in these recombinants requires active Fgf and canonical Wnt signaling, as interference with either pathway blocks Pax8 induction. Furthermore, we demonstrate that Fgf and canonical Wnt signaling cooperate to activate Pax8 expression in isolated animal caps. We propose that in the absence of mesoderm cues the combined activity of hindbrain-derived Wnt and Fgf signals specifies the otic placode in Xenopus, and promotes its morphogenesis into an otocyst.

Inhibition of BMPs by follistatin is required for FGF3 expression and segmental patterning of the hindbrain

A network of molecular interactions is required in the developing vertebrate hindbrain for the formation and anterior-posterior patterning of the rhombomeres. FGF signaling is required in this network to upregulate the expression of the Krox20 and Kreisler segmentation genes, but little is known of how FGF gene expression is regulated in the hindbrain. We show that the dynamic expression of FGF3 in chick hindbrain segments and boundaries is similar to that of the BMP antagonist, follistatin. Consistent with a regulatory relationship between BMP signaling and FGF3 expression, we find that an increase in BMP activity due to blocking of follistatin translation by morpholino antisense oligonucleotides or overexpression of BMP results in strong inhibition of FGF3 expression. Conversely, addition of follistatin leads to an increase in the level of FGF3 expression. Furthermore, the segmental inhibition of BMP activity by follistatin is required for the expression of Krox20, Hoxb1 and EphA4 in the hindbrain. In addition, we show that the maintenance of FGF3 gene expression requires FGF activity, suggestive of an autoregulatory loop. These results reveal an antagonistic relationship between BMP activity and FGF3 expression that is required for correct segmental gene expression in the chick hindbrain, in which follistatin enables FGF3 expression by inhibiting BMP activity.

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

The development of the vertebrate inner ear is an emergent process. Its progression from a relatively simple disk of thickened epithelium within head ectoderm into a complex organ capable of sensing sound and balance is controlled by sequential molecular and cellular interactions. Fibroblast growth factor (FGF) and Wnt signals emanating from mesoderm and neural ectoderm have been shown to direct inner ear fate. However, the role of these multiple signals during inner ear induction is unclear. We demonstrate that the action of the FGFs and Wnts is sequential, and that their roles support a model of hierarchical fate decisions that progressively restrict the developmental potential of the ectoderm until otic commitment. We show that signalling by Fgf3 and Fgf19 is required to initiate a proliferative progenitor region that is a precursor to both the inner ear and the neurogenic epibranchial placodes. Significantly, we find that only after FGF action is attenuated can the subsequent action of Wnt signalling allow otic differentiation to proceed. In addition, gain and loss of function of Wnt-signalling components show a role for this signalling in repressing epibranchial fate. This interplay of signalling factors ensures the correct and ordered differentiation of both inner ear and epibranchial systems.