Early development of the cranial sensory nervous system: from a common field to individual placodes

Ildr1b is essential for semicircular canal development, migration of the posterior lateral line primordium and hearing ability in zebrafish: implications for a role in the recessive hearing impairment DFNB42

Immunoglobulin-like domain containing receptor 1 (ILDR1) is a poorly characterized gene that was first identified in lymphoma cells. Recently, ILDR1 has been found to be responsible for autosomal recessive hearing impairment DFNB42. Patients with ILDR1 mutations cause bilateral non-progressive moderate-to-profound sensorineural hearing impairment. However, the etiology and mechanism of ILDR1-related hearing loss remains to be elucidated. In order to uncover the pathology of DFNB42 deafness, we used the morpholino injection technique to establish an ildr1b-morphant zebrafish model. Ildr1b-morphant zebrafish displayed defective hearing and imbalanced swimming, and developmental delays were seen in the semicircular canals of the inner ear. The gene expression profile and real-time PCR revealed down-regulation of atp1b2b (encoding Na(+)/K(+) transporting, beta 2b polypeptide) in ildr1b-morphant zebrafish. We found that injection of atp1b2b mRNA into ildr1b-knockdown zebrafish could rescue the phenotype of developmental delay of the semicircular canals. Moreover, ildr1b-morphant zebrafish had reduced numbers of lateral line neuromasts due to the disruption of lateral line primordium migration. In situ hybridization showed the involvement of attenuated FGF signaling and the chemokine receptor 4b (cxcr4b) and chemokine receptor 7b (cxcr7b) in posterior lateral line primordium of ildr1b-morphant zebrafish. We concluded that Ildr1b is crucial for the development of the inner ear and the lateral line system. This study provides the first evidence for the mechanism of Ildr1b on hearing in vivo and sheds light on the pathology of DFNB42.

Early embryonic specification of vertebrate cranial placodes

Cranial placodes contribute to many sensory organs and ganglia of the vertebrate head. The olfactory, otic, and lateral line placodes form the sensory receptor cells and neurons of the nose, ear, and lateral line system; the lens placode develops into the lens of the eye; epibranchial, profundal, and trigeminal placodes contribute sensory neurons to cranial nerve ganglia; and the adenohypophyseal placode gives rise to the anterior pituitary, a major endocrine control organ. Despite these differences in fate, all placodes are now known to originate from a common precursor, the preplacodal ectoderm (PPE). The latter is a horseshoe-shaped domain of ectoderm surrounding the anterior neural plate and neural crest and is defined by expression of transcription factor Six1, its cofactor Eya1, and other members of the Six and Eya families. Studies in zebrafish, Xenopus, and chick reveal that the PPE is specified together with other ectodermal territories (epidermis, neural crest, and neural plate) during early embryogenesis. During gastrulation, domains of ventrally (e.g., Dlx3/Dlx5, GATA2/GATA3, AP2, Msx1, FoxI1, and Vent1/Vent2) and dorsally (e.g., Zic1, Sox3, and Geminin) restricted transcription factors are established in response to a gradient of BMP and help to define non-neural and neural competence territories, respectively. At neural plate stages, the PPE is then induced in the non-neural competence territory by signals from the adjacent neural plate and mesoderm including FGF, BMP inhibitors, and Wnt inhibitors. Subsequently, signals from more localized signaling centers induce restricted expression domains of various transcription factors within the PPE, which specify multiplacodal areas and ultimately individual placodes. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The author has declared no conflicts of interest for this article.

Vertebrate neurogenic placode development: historical highlights that have shaped our current understanding

With the flood of published research encountered today, it is important to occasionally reflect upon how we arrived at our current understanding in a particular scientific discipline, thereby positioning new discoveries into proper context with long-established models. This historical review highlights some of the important scientific contributions in the field of neurogenic placode development. By viewing cumulatively the rich historical data, we can more fully appreciate and apply what has been accomplished. Early descriptive work in fish and experimental approaches in amphibians and chick yielded important conceptual models of placode induction and cellular differentiation. Current efforts to discover genes and their molecular functions continue to expand our understanding of the placodes. Carefully considering the body of work may improve current models and help focus modern experimental design.

Hedgehog-PKA signaling and gnrh3 regulate the development of zebrafish gnrh3 neurons

GnRH neurons secrete GnRH that controls the development of the reproduction system. Despite many studies, the signals controlling the development of GnRH neurons from its progenitors have not been fully established. To understand the development of GnRH neurons, we examined the development of gnrh3-expressing cells using a transgenic zebrafish line that expresses green fluorescent protein (GFP) and LacZ driven by the gnrh3 promoter. GFP and LacZ expression recapitulated that of gnrh3 in the olfactory region, olfactory bulb and telencephalon. Depletion of gnrh3 by morpholinos led to a reduction of GFP- and gnrh3-expressing cells, while over-expression of gnrh3 mRNA increased the number of these cells. This result indicates a positive feed-forward regulation of gnrh3 cells by gnrh3. The gnrh3 cells were absent in embryos that lack Hedgehog signaling, but their numbers were increased in embryos overexpressing shhb. We manipulated the amounts of kinase that antagonizes the Hedgehog signaling pathway, protein kinase A (PKA), by treating embryos with PKA activator forskolin or by injecting mRNAs encoding its constitutively active catalytic subunit (PKA*) and dominant negative regulatory subunit (PKI) into zebrafish embryos. PKA* misexpression or forskolin treatment decreased GFP cell numbers, while PKI misexpression led to ectopic production of GFP cells. Our data indicate that the Hedgehog-PKA pathway participates in the development of gnrh3-expressing neurons during embryogenesis.

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.

Expression and function of transcription factor cMyb during cranial neural crest development

The transcription factor cMyb has well known functions in vertebrate hematopoiesis, but little was known about its distribution or function at early developmental stages. Here, we show that cMyb transcripts are present at the neural plate during gastrulation in chick embryos. cMyb expression then resolves to the cranial neural folds and is maintained in early migrating cranial neural crest cells during and after neurulation. Morpholino-mediated knock-down of cMyb reduces expression of Pax7 and Twist at the neural plate border, as well as reducing expression of neural crest specifier gene Slug/Snail2 and completely eliminating expression of Ets1. On the other hand, its loss results in abnormal maintenance of Zic1, but little or no effect on other neural crest specifier genes like FoxD3 or Sox9. These results place cMyb in a critical hierarchical position within the cranial neural crest cell gene regulatory network, likely directly inhibiting Zic1 and upstream of Ets1 and some, but not all, neural crest specifier genes.

The evolutionary history of vertebrate cranial placodes--I: cell type evolution

Vertebrate cranial placodes are crucial contributors to the vertebrate cranial sensory apparatus. Their evolutionary origin has attracted much attention from evolutionary and developmental biologists, yielding speculation and hypotheses concerning their putative homologues in other lineages and the developmental and genetic innovations that might have underlain their origin and diversification. In this article we first briefly review our current understanding of placode development and the cell types and structures they form. We next summarise previous hypotheses of placode evolution, discussing their strengths and caveats, before considering the evolutionary history of the various cell types that develop from placodes. In an accompanying review, we also further consider the evolution of ectodermal patterning. Drawing on data from vertebrates, tunicates, amphioxus, other bilaterians and cnidarians, we build these strands into a scenario of placode evolutionary history and of the genes, cells and developmental processes that underlie placode evolution and development.

Pax3 isoforms in sensory neurogenesis: expression and function in the ophthalmic trigeminal placode

BACKGROUND

In the trigeminal placode, Pax3 is classified as necessary but not sufficient for sensory neuron differentiation. One hypothesis is that different Pax3 isoforms regulate cellular differentiation uniquely. Pax3 is known to sometimes activate and sometimes repress gene transcription, and its activity can be dependent on the isoforms present. Pax3 isoforms had not previously been characterized in chick sensory neurogenesis.

RESULTS

Reverse transcriptase-polymerase chain reaction (PCR) analysis revealed three well-expressed Pax3 splice variants: full-length (flPax3), Pax3V1, and Pax3V2. Each was characterized for its effect on neurogenesis by misexpression in placodal ectoderm. The differences observed were more apparent under conditions of enhanced neurogenesis (by means of Notch inhibition), where flPax3 and Pax3V1 caused failed differentiation, while Pax3V2 misexpression resembled the neuronal differentiation seen in controls. Quantitative PCR analysis revealed a progressive increase in Pax3 expression, but no significant change in relative isoform expression. Of interest, Notch inhibition led to a significant increase in Pax3 expression.

CONCLUSIONS

We can conclude that: (1) flPax3 and Pax3V1 inhibit neuronal differentiation; (2) Pax3V2 is permissive for neuronal differentiation; (3) while absolute levels change over time, relative splice form expression levels are largely maintained in the trigeminal placode domain; and (4) Pax3 expression generally increases in response to Notch inhibition.

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.

Identification and characterization of FGF2-dependent mRNA: microRNA networks during lens fiber cell differentiation

MicroRNAs (miRNAs) and fibroblast growth factor (FGF) signaling regulate a wide range of cellular functions, including cell specification, proliferation, migration, differentiation, and survival. In lens, both these systems control lens fiber cell differentiation; however, a possible link between these processes remains to be examined. Herein, the functional requirement for miRNAs in differentiating lens fiber cells was demonstrated via conditional inactivation of Dicer1 in mouse (Mus musculus) lens. To dissect the miRNA-dependent pathways during lens differentiation, we used a rat (Rattus norvegicus) lens epithelial explant system, induced by FGF2 to differentiate, followed by mRNA and miRNA expression profiling. Transcriptome and miRNome analysis identified extensive FGF2-regulated cellular responses that were both independent and dependent on miRNAs. We identified 131 FGF2-regulated miRNAs. Seventy-six of these miRNAs had at least two in silico predicted and inversely regulated target mRNAs. Genes modulated by the greatest number of FGF-regulated miRNAs include DNA-binding transcription factors Nfib, Nfat5/OREBP, c-Maf, Ets1, and N-Myc. Activated FGF signaling influenced bone morphogenetic factor/transforming growth factor-β, Notch, and Wnt signaling cascades implicated earlier in lens differentiation. Specific miRNA:mRNA interaction networks were predicted for c-Maf, N-Myc, and Nfib (DNA-binding transcription factors); Cnot6, Cpsf6, Dicer1, and Tnrc6b (RNA to miRNA processing); and Ash1l, Med1/PBP, and Kdm5b/Jarid1b/Plu1 (chromatin remodeling). Three miRNAs, including miR-143, miR-155, and miR-301a, down-regulated expression of c-Maf in the 3′-UTR luciferase reporter assays. These present studies demonstrate for the first time global impact of activated FGF signaling in lens cell culture system and predicted novel gene regulatory networks connected by multiple miRNAs that regulate lens differentiation.

Histone posttranslational modifications and cell fate determination: lens induction requires the lysine acetyltransferases CBP and p300

Lens induction is a classical embryologic model to study cell fate determination. It has been proposed earlier that specific changes in core histone modifications accompany the process of cell fate specification and determination. The lysine acetyltransferases CBP and p300 function as principal enzymes that modify core histones to facilitate specific gene expression. Herein, we performed conditional inactivation of both CBP and p300 in the ectodermal cells that give rise to the lens placode. Inactivation of both CBP and p300 resulted in the dramatic discontinuation of all aspects of lens specification and organogenesis, resulting in aphakia. The CBP/p300(-/-) ectodermal cells are viable and not prone to apoptosis. These cells showed reduced expression of Six3 and Sox2, while expression of Pax6 was not upregulated, indicating discontinuation of lens induction. Consequently, expression of αB- and αA-crystallins was not initiated. Mutant ectoderm exhibited markedly reduced levels of histone H3 K18 and K27 acetylation, subtly increased H3 K27me3 and unaltered overall levels of H3 K9ac and H3 K4me3. Our data demonstrate that CBP and p300 are required to establish lens cell-type identity during lens induction, and suggest that posttranslational histone modifications are integral to normal cell fate determination in the mammalian lens.

BMP-Smad 1/5/8 signalling in the development of the nervous system

The transcription factors, Smad1, Smad5 and Smad8, are the pivotal intracellular effectors of the bone morphogenetic protein (BMP) family of proteins. BMPs and their receptors are expressed in the nervous system (NS) throughout its development. This review focuses on the actions of Smad 1/5/8 in the developing NS. The mechanisms by which these Smad proteins regulate the induction of the neuroectoderm, the central nervous system (CNS) primordium, and finally the neural crest, which gives rise to the peripheral nervous system (PNS), are reviewed herein. We describe how, following neural tube closure, the most dorsal aspect of the tube becomes a signalling centre for BMPs, which directs the pattern of the development of the dorsal spinal cord (SC), through the action of Smad1, Smad5 and Smad8. The direct effects of Smad 1/5/8 signalling on the development of neuronal and non-neuronal cells from various neural progenitor cell populations are then described. Finally, this review discusses the neurodevelopmental abnormalities associated with the knockdown of Smad 1/5/8.

Sma- and Mad-related protein 7 (Smad7) is required for embryonic eye development in the mouse

Smad7 is an intracellular inhibitory protein that antagonizes the signaling of TGF-β family members. Deletion of Smad7 in the mouse leads to an abnormality in heart development. However, whether Smad7 has a functional role in the development of other organs has been elusive. Here we present evidence that Smad7 imparts a role to eye development in the mouse. Smad7 is expressed in both the lens and retina in the developing embryonic eye. Depletion of Smad7 caused various degrees of coloboma and microphthalmia with alterations in cell apoptosis and proliferation in eyes. Smad7 was implicated in lens differentiation but was not required for the induction of the lens placode. The development of the periocular mesenchyme was retarded with the down-regulation of Bmp7 and Pitx2 in mutant mice. Retinal spatial patterning was affected by Smad7 deletion and was accompanied by altered bone morphogenetic protein (BMP) signaling. At late gestation stages, TGF-β signaling was up-regulated in the differentiating retina. Smad7 mutant mice displayed an expanded optic disc with increasing of sonic hedgehog (SHH) signaling. Furthermore, loss of Smad7 led to a temporal change in retinal neurogenesis. In conclusion, our study suggests that Smad7 is essential for eye development. In addition, our data indicate that alterations in the signaling of BMP, TGF-β, and SHH likely underlie the defects in eye development caused by Smad7 deletion.

The corneal epithelium and lens develop independently from a common pool of precursors

BACKGROUND

The corneal epithelium (CE) overlays a stroma, which is derived from neural crest cells, and appears to be committed during chick development, but appears still labile in adult rabbit. Its specification was hitherto regarded as resolved and dependent upon the lens, although without experimental support. Here, we challenged CE fate by changing its environment at different stages.

RESULTS

Recombination with a dermis showed that CE commitment is linked to stroma formation, which results in Pax6 stabilization in both species. Surgical ablation shows that CE specification has already taken place when the lens placode invaginates, while removal of the early lens placode led to lens renewal. To block lens formation, bone morphogenetic protein (BMP) signaling, one of its last inducing factors, was inhibited by over-expression of Gremlin in the ocular ectoderm. This resulted in lens-less embryos which formed a corneal epithelium if they survived 2 weeks.

CONCLUSION

The corneal epithelium and lens share a common pool of precursors. The adoption of the CE fate might be dependent on the loss of a lens placode favoring environment. The corneal fate is definitively stabilized by the migration of Gremlin-expressing neural crest cells in the lens peripheral ectoderm.

Development of cranial placodes: insights from studies in chick

This review focuses on how research, using chick as a model system, has contributed to our knowledge regarding the development of cranial placodes. This review highlights when and how molecular signaling events regulate early specification of placodal progenitor cells, as well as the development of individual placodes including morphological movements. In addition, we briefly describe various techniques used in chick that are important for studies in cell and developmental biology.

Regenerative strategies for craniofacial disorders

Craniofacial disorders present markedly complicated problems in reconstruction because of the complex interactions of the multiple, simultaneously affected tissues. Regenerative medicine holds promise for new strategies to improve treatment of these disorders. This review addresses current areas of unmet need in craniofacial reconstruction and emphasizes how craniofacial tissues differ from their analogs elsewhere in the body. We present a problem-based approach to illustrate current treatment strategies for various craniofacial disorders, to highlight areas of need, and to suggest regenerative strategies for craniofacial bone, fat, muscle, nerve, and skin. For some tissues, current approaches offer excellent reconstructive solutions using autologous tissue or prosthetic materials. Thus, new “regenerative” approaches would need to offer major advantages in order to be adopted. In other tissues, the unmet need is great, and we suggest the greatest regenerative need is for muscle, skin, and nerve. The advent of composite facial tissue transplantation and the development of regenerative medicine are each likely to add important new paradigms to our treatment of craniofacial disorders.

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.

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

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.

Functional development of the olfactory system in zebrafish

The olfactory system has become a popular model to study the function of neuronal circuits and the molecular and cellular mechanisms underlying the development of neurons and their connections. An excellent model to combine studies of function and development is the zebrafish because it not only permits sophisticated molecular and genetic analyses of development, but also functional measurements of neuronal activity patterns in the intact brain. This article reviews insights into the functional development of the olfactory system that have been obtained in zebrafish. The focus is on the specification of olfactory sensory neurons (OSNs), the mechanisms controlling odorant receptor expression and OSN identity, the pathfinding of OSN axons towards target glomeruli in the olfactory bulb (OB), the development of glomeruli and functional topographic maps in the OB, and the development of inhibitory interneurons in the OB.

Graded levels of Pax2a and Pax8 regulate cell differentiation during sensory placode formation

Pax gene haploinsufficiency causes a variety of congenital defects. Renal-coloboma syndrome, resulting from mutations in Pax2, is characterized by kidney hypoplasia, optic nerve malformation, and hearing loss. Although this underscores the importance of Pax gene dosage in normal development, how differential levels of these transcriptional regulators affect cell differentiation and tissue morphogenesis is still poorly understood. We show that differential levels of zebrafish Pax2a and Pax8 modulate commitment and behavior in cells that eventually contribute to the otic vesicle and epibranchial placodes. Initially, a subset of epibranchial placode precursors lie lateral to otic precursors within a single Pax2a/8-positive domain; these cells subsequently move to segregate into distinct placodes. Using lineage-tracing and ablation analyses, we show that cells in the Pax2a/8+ domain become biased towards certain fates at the beginning of somitogenesis. Experiments involving either Pax2a overexpression or partial, combinatorial Pax2a and Pax8 loss of function reveal that high levels of Pax favor otic differentiation whereas low levels increase cell numbers in epibranchial ganglia. In addition, the Fgf and Wnt signaling pathways control Pax2a expression: Fgf is necessary to induce Pax2a, whereas Wnt instructs the high levels of Pax2a that favor otic differentiation. Our studies reveal the importance of Pax levels during sensory placode formation and provide a mechanism by which these levels are controlled.

Regulation of Six1 expression by evolutionarily conserved enhancers in tetrapods

The Six1 homeobox gene plays critical roles in vertebrate organogenesis. Mice deficient for Six1 show severe defects in organs such as skeletal muscle, kidney, thymus, sensory organs and ganglia derived from cranial placodes, and mutations in human SIX1 cause branchio-oto-renal syndrome, an autosomal dominant developmental disorder characterized by hearing loss and branchial defects. The present study was designed to identify enhancers responsible for the dynamic expression pattern of Six1 during mouse embryogenesis. The results showed distinct enhancer activities of seven conserved non-coding sequences (CNSs) retained in tetrapod Six1 loci. The activities were detected in all cranial placodes (excluding the lens placode), dorsal root ganglia, somites, nephrogenic cord, notochord and cranial mesoderm. The major Six1-expression domains during development were covered by the sum of activities of these enhancers, together with the previously identified enhancer for the pre-placodal region and foregut endoderm. Thus, the eight CNSs identified in a series of our study represent major evolutionarily conserved enhancers responsible for the expression of Six1 in tetrapods. The results also confirmed that chick electroporation is a robust means to decipher regulatory information stored in vertebrate genomes. Mutational analysis of the most conserved placode-specific enhancer, Six1-21, indicated that the enhancer integrates a variety of inputs from Sox, Pax, Fox, Six, Wnt/Lef1 and basic helix-loop-helix proteins. Positive autoregulation of Six1 is achieved through the regulation of Six protein-binding sites. The identified Six1 enhancers provide valuable tools to understand the mechanism of Six1 regulation and to manipulate gene expression in the developing embryo, particularly in the sensory organs.

Apoptosis contributes to placode morphogenesis in the posterior placodal area of mice

In the embryonic head of vertebrates, neurogenic and non-neurogenic ectodermal placodes arise from the panplacodal primordium. Whether and how growth processes of the ectodermal layer, changes in the transcriptional precursor cell profile, or positional changes among precursor cells contribute to interplacodal boundary formation is subject to intense investigation. We demonstrate that large scale apoptosis in the multiplacodal posterior placodal area (PPA) of C57BL/6 mice assists in the segregation of otic and epibranchial placodes. Complex patterns of interplacodal apoptosis precede and parallel the structural individualization of high-grade thickened placodes, with the fundamental separation between otic and epibranchial precursor cells being seemingly prevalent. Interplacodal apoptosis between the emerging epibranchial placodes, which express Neurogenin2 prior to their complete structural individualization, comes out most strongly between the epibranchial placodes 1 and 2. Apoptosis then moves from interplacodal to intraplacodal positions in dorsal and, with a delay, ventral parts of the epibranchial placodes. Intraplacodal apoptosis appears to exert corrective actions among premigratory neuroblasts, and helps to eliminate the epibranchial placodes. The present findings confirm and extend earlier observations in Tupaia belangeri (Washausen et al. in Dev Biol 278:86-102, 2005), regarded as an intermediate between primates and other eutherian orders. Having now available maps of apoptosis in the PPA of embryonic mice, further investigations into the functions of inter- and intraplacodal apoptosis can be carried out in an experimentally and genetically more accessible mammalian model organism.

Specification and regionalisation of the neural plate border

During early vertebrate development, the embryonic ectoderm becomes subdivided into neural, neural plate border (border) and epidermal regions. The nervous system is derived from the neural and border domains which, respectively, give rise to the central and peripheral nervous systems. To better understand the functional nervous system we need to know how individual neurons are specified and connected. Our understanding of the early development of the peripheral nervous system has been lagging compared to knowledge regarding central nervous system and epidermal cell lineage decision. Recent advances have shown when and how the specification of border cells is initiated. One important insight is that border specification is already initiated at blastula stages, and can be molecularly and temporally distinguished from rostrocaudal regionalisation of the border. From findings in several species, it is clear that Wnt, Bone Morphogenetic Protein and Fibroblast Growth Factor signals play important roles during the specification and regionalisation of the border. In this review, we highlight the individual roles of these signals and compare models of border specification, including a new model that describes how temporal coordination and epistatic interactions of extracellular signals result in the specification and regionalisation of border cells.

Efficient generation of lens progenitor cells from cataract patient-specific induced pluripotent stem cells

The development of a technique to induce the transformation of somatic cells to a pluripotent state via the ectopic expression of defined transcription factors was a transformational event in the field of regenerative medicine. The development of this technique also impacted ophthalmology, as patient-specific induced pluripotent stemcells (iPSCs) may be useful resources for some ophthalmological diseases. The lens is a key refractive element in the eye that focuses images of the visual world onto the retina. To establish a new model for drug screening to treat lens diseases and investigating lens aging and development, we examined whether human lens epithelial cells (HLECs) could be induced into iPSCs and if lens-specific differentiation of these cells could be achieved under defined chemical conditions. We first efficiently reprogrammed HLECs from age-related cataract patients to iPSCs with OCT-4, SOX-2, and KLF-4. The resulting HLEC-derived iPS (HLE-iPS) colonies were indistinguishable from human ES cells with respect to morphology, gene expression, pluripotent marker expression and their ability to generate all embryonic germ-cell layers. Next, we performed a 3-step induction procedure: HLE-iPS cells were differentiated into large numbers of lens progenitor-like cells with defined factors (Noggin, BMP and FGF2), and we determined that these cells expressed lens-specific markers (PAX6, SOX2, SIX3, CRYAB, CRYAA, BFSP1, and MIP). In addition, HLE-iPS-derived lens cells exhibited reduced expression of epithelial mesenchymal transition (EMT) markers compared with human embryonic stem cells (hESCs) and fibroblast-derived iPSCs. Our study describes a highly efficient procedure for generating lens progenitor cells from cataract patient HLEC-derived iPSCs. These patient-derived pluripotent cells provide a valuable model for studying the developmental and molecular biological mechanisms that underlie cell determination in lens development and cataract pathophysiology.

Mutual repression between Pax3 and Pax6 is involved in the positioning of ophthalmic trigeminal placode in avian embryo

Cranial sense organs and a subset of cranial sensory neurons are generated from placodes, thickenings of the ectoderm. Pax3 has been known as a marker for ophthalmic trigeminal placode specification, and also an important regulator of trigeminal placode neuron differentiation. In this study, I show that Pax6 is initially expressed in the preplacodal region at the level of ophthalmic trigeminal placode, and that this expression gradually regresses in a medial-to-lateral direction as Pax3 expression expands in the same direction. Misexpression studies revealed that Pax6 represses Pax3 expression indirectly as a transcriptional activator in a cell-autonomous manner. Pax3-misexpression represses Pax6 expression in an indirect fashion, suggesting that unknown factor(s) downstream of Pax3 may repress Pax6 expression, and thereby allow an expansion of Pax3-positive ophthalmic trigeminal placode region. These results indicate that the mutual repression between Pax3 and Pax6 has important roles in the specification and the positioning of the ophthalmic trigeminal placode.

The orchestration of mammalian tissue morphogenesis through a series of coherent feed-forward loops

Tissue morphogenesis requires intricate temporal and spatial control of gene expression that is executed through specific gene regulatory networks (GRNs). GRNs are comprised from individual subcircuits of different levels of complexity. An important question is to elucidate the mutual relationship between those genes encoding DNA-binding factors that trigger the subcircuit with those that play major "later" roles during terminal differentiation via expression of specific genes that constitute the phenotype of individual tissues. The ocular lens is a classical model system to study tissue morphogenesis. Pax6 is essential for both lens placode formation and subsequent stages of lens morphogenesis, whereas c-Maf controls terminal differentiation of lens fibers, including regulation of crystallins, key lens structural proteins required for its transparency and refraction. Here, we show that Pax6 directly regulates c-Maf expression during lens development. A 1.3-kb c-Maf promoter with a 1.6-kb upstream enhancer (CR1) recapitulated the endogenous c-Maf expression pattern in lens and retinal pigmented epithelium. ChIP assays revealed binding of Pax6 and c-Maf to multiple regions of the c-Maf locus in lens chromatin. To predict functional Pax6-binding sites, nine novel variants of Pax6 DNA-binding motifs were identified and characterized. Two of these motifs predicted a pair of Pax6-binding sites in the CR1. Mutagenesis of these Pax6-binding sites inactivated transgenic expression in the lens but not in retinal pigmented epithelium. These data establish a novel regulatory role for Pax6 during lens development, link together the Pax6/c-Maf/crystallin regulatory network, and suggest a novel type of GRN subcircuit that controls a major part of embryonic lens development.

Building the developmental oculome: systems biology in vertebrate eye development and disease

The vertebrate eye is a sophisticated multicomponent organ that has been actively studied for over a century, resulting in the identification of the major embryonic and molecular events involved in its complex developmental program. Data gathered so far provides sufficient information to construct a rudimentary network of the various signaling molecules, transcription factors, and their targets for several key stages of this process. With the advent of genomic technologies, there has been a rapid expansion in our ability to collect and process biological information, and the use of systems-level approaches to study specific aspects of vertebrate eye development has already commenced. This is beginning to result in the definition of the dynamic developmental networks that operate in ocular tissues, and the interactions of such networks between coordinately developing ocular tissues. Such an integrative understanding of the eye by a comprehensive systems-level analysis can be termed the 'oculome', and that of serial developmental stages of the eye as it transits from its initiation to a fully formed functional organ represents the 'developmental oculome'. Construction of the developmental oculome will allow novel mechanistic insights that are essential for organ regeneration-based therapeutic applications, and the generation of computational models for eye disease states to predict the effects of drugs. This review discusses our present understanding of two of the individual components of the developing vertebrate eye--the lens and retina--at both the molecular and systems levels, and outlines the directions and tools required for construction of the developmental oculome.

Developmental expression patterns of candidate cofactors for vertebrate six family transcription factors

Six family transcription factors play important roles in craniofacial development. Their transcriptional activity can be modified by cofactor proteins. Two Six genes and one cofactor gene (Eya1) are involved in the human Branchio-otic (BO) and Branchio-otic-renal (BOR) syndromes. However, mutations in Six and Eya genes only account for approximately half of these patients. To discover potential new causative genes, we searched the Xenopus genome for orthologues of Drosophila cofactor proteins that interact with the fly Six-related factor, SO. We identified 33 Xenopus genes with high sequence identity to 20 of the 25 fly SO-interacting proteins. We provide the developmental expression patterns of the Xenopus orthologues for 11 of the fly genes, and demonstrate that all are expressed in developing craniofacial tissues with at least partial overlap with Six1/Six2. We speculate that these genes may function as Six-interacting partners with important roles in vertebrate craniofacial development and perhaps congenital syndromes.

Insect Tc-six4 marks a unit with similarity to vertebrate placodes

Cranial placodes are specialized ectodermal regions in the developing vertebrate head that give rise to both neural and non-neural cell types of the neuroendocrine system and the sense organs of the visual, olfactory and acoustic systems. The cranial placodes develop from a panplacodal region which is specifically marked by genes of the eyes absent/eya and two "six homeobox" family members (sine oculis/six1 and six4). It had been believed that cranial placodes are evolutionary novelties of vertebrates. However, data from non-vertebrate chordates suggest that placode-like structures evolved in the chordate ancestor already. Here, we identify a morphological structure in the embryonic head of the beetle Tribolium castaneum with placode-like features. It is marked by the orthologs of the panplacodal markers Tc-six4, Tc-eya and Tc-sine oculis/six1 (Tc-six1) and expresses several genes known to be involved in adenohypophyseal placode development in vertebrates. Moreover, it contributes to both epidermal and neural tissues. We identify Tc-six4 as a specific marker for this structure that we term the insect head placode. Finally, we reveal the regulatory gene network of the panplacodal genes Tc-six4, Tc-eya and Tc-six1 and identify them as head epidermis patterning genes. Our finding of a placode-like structure in an insect suggests that a placode precursor was already present in the last common ancestor of bilaterian animals.

Epigenetic regulatory mechanisms in vertebrate eye development and disease

Eukaryotic DNA is organized as a nucleoprotein polymer termed chromatin with nucleosomes serving as its repetitive architectural units. Cellular differentiation is a dynamic process driven by activation and repression of specific sets of genes, partitioning the genome into transcriptionally active and inactive chromatin domains. Chromatin architecture at individual genes/loci may remain stable through cell divisions, from a single mother cell to its progeny during mitosis, and represents an example of epigenetic phenomena. Epigenetics refers to heritable changes caused by mechanisms distinct from the primary DNA sequence. Recent studies have shown a number of links between chromatin structure, gene expression, extracellular signaling, and cellular differentiation during eye development. This review summarizes recent advances in this field, and the relationship between sequence-specific DNA-binding transcription factors and their roles in recruitment of chromatin remodeling enzymes. In addition, lens and retinal differentiation is accompanied by specific changes in the nucleolar organization, expression of non-coding RNAs, and DNA methylation. Epigenetic regulatory mechanisms in ocular tissues represent exciting areas of research that have opened new avenues for understanding normal eye development, inherited eye diseases and eye diseases related to aging and the environment.

Conserved expression of mouse Six1 in the pre-placodal region (PPR) and identification of an enhancer for the rostral PPR

All cranial sensory organs and sensory neurons of vertebrates develop from cranial placodes. In chick, amphibians and zebrafish, all placodes originate from a common precursor domain, the pre-placodal region (PPR), marked by the expression of Six1/4 and Eya1/2. However, the PPR has never been described in mammals and the mechanism involved in the formation of PPR is poorly defined. Here, we report the expression of Six1 in the horseshoe-shaped mouse ectoderm surrounding the anterior neural plate in a pattern broadly similar to that of non-mammalian vertebrates. To elucidate the identity of Six1-positive mouse ectoderm, we searched for enhancers responsible for Six1 expression by in vivo enhancer assays. One conserved non-coding sequence, Six1-14, showed specific enhancer activity in the rostral PPR of chick and Xenopus and in the mouse ectoderm. These results strongly suggest the presence of PPR in mouse and that it is conserved in vertebrates. Moreover, we show the importance of the homeodomain protein-binding sites of Six1-14, the Six1 rostral PPR enhancer, for enhancer activity, and that Dlx5, Msx1 and Pax7 are candidate binding factors that regulate the level and area of Six1 expression, and thereby the location of the PPR. Our findings provide critical information and tools to elucidate the molecular mechanism of early sensory development and have implications for the development of sensory precursor/stem cells.

Efficient generation of lens progenitor cells and lentoid bodies from human embryonic stem cells in chemically defined conditions

The eye lens is an encapsulated avascular organ whose function is to focus light on the retina. Lens comprises a single progenitor cell lineage in multiple states of differentiation. Disruption of lens function leading to protein aggregation and opacity results in age-onset cataract. Cataract is a complex disease involving genetic and environmental factors. Here, we report the development of a new 3-stage system that differentiates human embryonic stem cells (hESCs) into large quantities of lens progenitor-like cells and differentiated 3-dimensional lentoid bodies. Inhibition of BMP signaling by noggin triggered differentiation of hESCs toward neuroectoderm. Subsequent reactivation of BMP and activation of FGF signaling stimulated formation of lens progenitor cells marked by the expression of PAX6 and alpha-crystallins. The formation of lentoid bodies was most efficient in the presence of FGF2 and Wnt-3a, yielding approximately 1000 lentoid bodies/30-mm well. Lentoid bodies expressed and accumulated lens-specific markers including alphaA-, alphaB-, beta-, and gamma-crystallins, filensin, CP49, and MIP/aquaporin 0. Collectively, these studies identify a novel procedure to generate lens cells from hESCs that can be applied for studies of lens differentiation and cataractogenesis using induced pluripotent stem (iPS) cells derived from various cataract patients.

Mapping the face in the somatosensory brainstem

The facial somatosensory map in the cortex is derived from facial representations that are first established at the brainstem level and then serially 'copied' at each stage of the somatosensory pathway. Recent studies have provided insights into the molecular mechanisms involved in the development of somatotopic maps of the face and whiskers in the trigeminal nuclei of the mouse brainstem. This work has revealed that early molecular regionalization and positional patterning of trigeminal ganglion and brainstem target neurons are established by homeodomain transcription factors, the expression of which is induced and maintained by signals from the brain and face. Such position-dependent information is fundamental in transforming the early spatial layout of sensory receptors into a topographic connectivity map that is conferred to higher brain levels.

Key steps in the morphogenesis of a cranial placode in an invertebrate chordate, the tunicate Ciona savignyi

Tunicates and vertebrates share a common ancestor that possessed cranial neurogenic placodes, thickenings in embryonic head epidermis giving rise to sensory structures. Though orthology assignments between vertebrate and tunicate placodes are not entirely resolved, vertebrate otic placodes and tunicate atrial siphon primordia are thought to be homologous based on morphology and position, gene expression, and a common signaling requirement during induction. Here, we probe key points in the morphogenesis of the tunicate atrial siphon. We show that the siphon primordium arises within a non-dividing field of lateral-dorsal epidermis. The initial steps of atrial primordium invagination are similar to otic placode invagination, but a placode-derived vesicle is never observed as for the otic vesicle of vertebrates. Rather, confocal imaging reveals an atrial opening through juvenile stages and beyond. We inject a photoactivatable lineage tracer to show that the early atrial siphon of the metamorphic juvenile, including its aperture and lining, derives from cells of the atrial placode itself. Finally, we perturb the routing of the gut to the left atrium by laser ablation and pharmacology to show that this adaptation to a sessile lifestyle depends on left-right patterning mechanisms present in the free-swimming chordate ancestor.

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.

Selective neuronal requirement for huntingtin in the developing zebrafish

Huntington's disease shares a common molecular basis with eight other neurodegenerative diseases, expansion of an existing polyglutamine tract. In each case, this repeat tract occurs within otherwise unrelated proteins. These proteins show widespread and overlapping patterns of expression in the brain and yet the diseases are distinguished by neurodegeneration in a specific subset of neurons that are most sensitive to the mutation. It has therefore been proposed that expansion of the polyglutamine region in these genes may result in perturbation of the normal function of the respective proteins, and that this perturbation in some way contributes to the neuronal specificity of these diseases. The normal functions of these proteins have therefore become a focus for investigation as potential pathogenic pathways. We have used synthetic antisense morpholinos to inhibit the translation of huntingtin mRNA during early zebrafish development and have previously reported the effects of huntingtin reduction on iron transport and homeostasis. Here we report an analysis of the effects of huntingtin loss-of-function on the developing nervous system, observing distinct defects in morphology of neuromasts, olfactory placode and branchial arches. The potential common origins of these defects were explored, revealing impaired formation of the anterior-most region of the neural plate as indicated by reduced pre-placodal and telencephalic gene expression with no effect on mid- or hindbrain formation. These investigations demonstrate a specific 'rate-limiting' role for huntingtin in formation of the telencephalon and the pre-placodal region, and differing levels of requirement for huntingtin function in specific nerve cell types.

New directions in craniofacial morphogenesis

The vertebrate head is an extremely complicated structure: development of the head requires tissue-tissue interactions between derivates of all the germ layers and coordinated morphogenetic movements in three dimensions. In this review, we highlight a number of recent embryological studies, using chicken, frog, zebrafish and mouse, which have identified crucial signaling centers in the embryonic face. These studies demonstrate how small variations in growth factor signaling can lead to a diversity of phenotypic outcomes. We also discuss novel genetic studies, in human, mouse and zebrafish, which describe cell biological mechanisms fundamental to the growth and morphogenesis of the craniofacial skeleton. Together, these findings underscore the complex interactions leading to species-specific morphology. These and future studies will improve our understanding of the genetic and environmental influences underlying human craniofacial anomalies.

The posteriorizing gene Gbx2 is a direct target of Wnt signalling and the earliest factor in neural crest induction

Wnt signalling is required for neural crest (NC) induction; however, the direct targets of the Wnt pathway during NC induction remain unknown. We show here that the homeobox gene Gbx2 is essential in this process and is directly activated by Wnt/beta-catenin signalling. By ChIP and transgenesis analysis we show that the Gbx2 regulatory elements that drive expression in the NC respond directly to Wnt/beta-catenin signalling. Gbx2 has previously been implicated in posteriorization of the neural plate. Here we unveil a new role for this gene in neural fold patterning. Loss-of-function experiments using antisense morpholinos against Gbx2 inhibit NC and expand the preplacodal domain, whereas Gbx2 overexpression leads to transformation of the preplacodal domain into NC cells. We show that the NC specifier activity of Gbx2 is dependent on the interaction with Zic1 and the inhibition of preplacodal genes such as Six1. In addition, we demonstrate that Gbx2 is upstream of the neural fold specifiers Pax3 and Msx1. Our results place Gbx2 as the earliest factor in the NC genetic cascade being directly regulated by the inductive molecules, and support the notion that posteriorization of the neural folds is an essential step in NC specification. We propose a new genetic cascade that operates in the distinction between anterior placodal and NC territories.

Activation of Six1 target genes is required for sensory placode formation

In vertebrates, cranial placodes form crucial parts of the sensory nervous system in the head. All cranial placodes arise from a common territory, the preplacodal region, and are identified by the expression of Six1/4 and Eya1/2 genes, which control different aspects of sensory development in invertebrates as well as vertebrates. While So and Eya can induce ectopic eyes in Drosophila, the ability of their vertebrate homologues to induce placodes in non-placodal ectoderm has not been explored. Here we show that Six1 and Eya2 are involved in ectodermal patterning and cooperate to induce preplacodal gene expression, while repressing neural plate and neural crest fates. However, they are not sufficient to induce ectopic sensory placodes in future epidermis. Activation of Six1 target genes is required for expression of preplacodal genes, for normal placode morphology and for placode-specific Pax protein expression. These findings suggest that unlike in the fly where the Pax6 homologue Eyeless acts upstream of Six and Eya, the regulatory relationships between these genes are reversed in early vertebrate placode development.

FGF signaling is essential for ophthalmic trigeminal placode cell delamination and differentiation

The ophthalmic trigeminal (opV) placode gives rise exclusively to sensory neurons of the peripheral nervous system, providing an advantageous model for understanding neurogenesis. The signaling pathways governing opV placode development have only recently begun to be elucidated. Here, we investigate the fibroblast growth factor receptor-4 (FGFR4), an opV expressed gene, to examine if and how FGF signaling regulates opV placode development. After inhibiting FGFR4, Pax3+ opV placode cells failed to delaminate from the ectoderm and did not contribute to the opV ganglion. Blocking FGF signaling also led to a loss of the early and late neuronal differentiation markers Ngn2, Islet-1, NeuN, and Neurofilament. In addition, without FGF signaling, cells that stalled in the ectoderm lost their opV placode-specific identity by down-regulating Pax3. We conclude that FGF signaling, through FGFR4, is necessary for delamination and differentiation of opV placode cells.

Retinoic acid signaling in mammalian eye development

Retinoic acid (RA) is a biologically active metabolite of vitamin A (retinol) that serves as a signaling molecule during a number of developmental and physiological processes. RA signaling plays multiple roles during embryonic eye development. RA signaling is initially required for reciprocal interactions between the optic vesicle and invaginating lens placode. RA signaling promotes normal development of the ventral retina and optic nerve through its activities in the neural crest cell-derived periocular mesenchyme. RA coordinates these processes by regulating biological activities of a family of non-steroid hormone receptors, RARalpha/beta/gamma, and RXRalpha/beta/gamma. These DNA-binding transcription factors recognize DNA as RAR/RXR heterodimers and recruit multiprotein transcriptional co-repressor complexes. RA-binding to RAR receptors induces a conformational change in the receptor, followed by the replacement of co-repressor with co-activator complexes. Inactivation of RARalpha/beta/gamma receptors in the periocular mesenchyme abrogates anterior eye segment formation. This review summarizes recent genetic studies of RA signaling and progress in understanding the molecular mechanism of transcriptional co-activators that function with RAR/RXR.

Activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the ophthalmic trigeminal placode

Vertebrate cranial neurogenic placodes are relatively simple model systems for investigating the control of sensory neurogenesis. The ophthalmic trigeminal (opV) placode, for which the earliest specific marker is the paired domain homeodomain transcription factor Pax3, forms cutaneous sensory neurons in the ophthalmic lobe of the trigeminal ganglion. We previously showed that Pax3 expression in avian opV placode cells correlates with specification and commitment to a Pax3+, cutaneous sensory neuron fate. Pax3 can act as a transcriptional activator or repressor, depending on the cellular context. We show using mouse Splotch(2H) mutants that Pax3 is necessary for the normal neuronal differentiation of opV placode cells. Using an electroporation construct encoding a Pax3-Engrailed fusion protein, which represses Pax3 target genes, we show that activation of Pax3 target genes is required cell-autonomously within chick opV placode cells for expression of the opV placode markers FGFR4 and Ngn2, maintenance of the preplacodal marker Eya2, expression of Pax3 itself (suggesting that Pax3 autoregulates), neuronal differentiation and delamination. Mis-expression of Pax3 in head ectoderm is sufficient to induce FGFR4 and Ngn2 expression, but neurons do not differentiate, suggesting that additional signals are necessary to enable Pax3+ cells to differentiate as neurons. Mis-expression of Pax3 in the Pax2+ otic and epibranchial placodes also downregulates Pax2 and disrupts otic vesicle closure, suggesting that Pax3 is sufficient to alter the identity of these cells. Overall, our results suggest that activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the opV placode.