Early steps in the development of the forebrain

Zebrafish as a systems toxicology model for developmental neurotoxicity testing

The developing brain is extremely sensitive to many chemicals. Exposure to neurotoxicants during development has been implicated in various neuropsychiatric and neurological disorders, including autism spectrum disorder, attention deficit hyperactive disorder, schizophrenia, Parkinson's disease, and Alzheimer's disease. Although rodents have been widely used for developmental neurotoxicity testing, experiments using large numbers of rodents are time-consuming, expensive, and raise ethical concerns. Using alternative non-mammalian animal models may relieve some of these pressures by allowing testing of large numbers of subjects while reducing expenses and minimizing the use of mammalian subjects. In this review, we discuss some of the advantages of using zebrafish in developmental neurotoxicity testing, focusing on central nervous system development, neurobehavior, toxicokinetics, and toxicodynamics in this species. We also describe some important examples of developmental neurotoxicity testing using zebrafish combined with gene expression profiling, neuroimaging, or neurobehavioral assessment. Zebrafish may be a systems toxicology model that has the potential to reveal the pathways of developmental neurotoxicity and to provide a sound basis for human risk assessments.

Early development of the neural plate: new roles for apoptosis and for one of its main effectors caspase-3

Despite its tremendous complexity, the vertebrate nervous system emerges from a homogenous layer of neuroepithelial cells, the neural plate. Its formation relies on the time- and space-controlled progression of developmental programs. Apoptosis is a biological process that removes superfluous and potentially dangerous cells and is implemented through the activation of a molecular pathway conserved during evolution. Apoptosis and an unconventional function of one of its main effectors, caspase-3, contribute to the patterning and growth of the neuroepithelium. Little is known about the intrinsic and extrinsic cues controlling activities of the apoptotic machinery during development. The BarH-like (Barhl) proteins are homeodomain-containing transcription factors. The observations in Caenorhabditis elegans, Xenopus, and mice document that Barhl proteins act in cell survival and as cell type-specific regulators of a caspase-3 function that limits neural progenitor proliferation. In this review, we discuss the roles and regulatory modes of the apoptotic machinery in the development of the neural plate. We focus on the Barhl2, the Sonic Hedgehog, and the Wnt pathways and their activities in neural progenitor survival and proliferation.

Tolerance to seizure induced by kainic acid is produced in a specific period of zebrafish development

During brain development, the electrical disturbance promoted by a seizure can have several consequences, because it can disturb a set of steps extremely regulated needed to the correct brain maturation. Animal modeling of seizure is invaluable to contribute to the mechanistic understanding of punctual seizure event, and those that triggered in an immature neural network could alter the mature brain physiology. In the present study we observed that the exposure to kainic acid diluted directly in water of zebrafish decreased the locomotor activity at 7 days post-fertilization (dpf) animals and increased at 15 dpf, despite the absence of more specific seizure features. Pre-exposure to kainic acid (500 μM) diluted in water at 7 dpf animals reduced the susceptibility to a second exposure 2 months later by intraperitoneal injection. The current data suggest that these different responses are associated with neuronal maturation process and open a question about the window of development that are crucial to long lasting effects related to seizure in this animal model.

A conserved role for non-neural ectoderm cells in early neural development

During the early steps of head development, ectodermal patterning leads to the emergence of distinct non-neural and neural progenitor cells. The induction of the preplacodal ectoderm and the neural crest depends on well-studied signalling interactions between the non-neural ectoderm fated to become epidermis and the prospective neural plate. By contrast, the involvement of the non-neural ectoderm in the morphogenetic events leading to the development and patterning of the central nervous system has been studied less extensively. Here, we show that the removal of the rostral non-neural ectoderm abutting the prospective neural plate at late gastrulation stage leads, in mouse and chick embryos, to morphological defects in forebrain and craniofacial tissues. In particular, this ablation compromises the development of the telencephalon without affecting that of the diencephalon. Further investigations of ablated mouse embryos established that signalling centres crucial for forebrain regionalization, namely the axial mesendoderm and the anterior neural ridge, form normally. Moreover, changes in cell death or cell proliferation could not explain the specific loss of telencephalic tissue. Finally, we provide evidence that the removal of rostral tissues triggers misregulation of the BMP, WNT and FGF signalling pathways that may affect telencephalon development. This study opens new perspectives on the role of the neural/non-neural interface and reveals its functional relevance across higher vertebrates.

Fgf16 is required for specification of GABAergic neurons and oligodendrocytes in the zebrafish forebrain

Fibroblast growth factor (Fgf) signaling plays crucial roles in various developmental processes including those in the brain. We examined the role of Fgf16 in the formation of the zebrafish brain. The knockdown of fgf16 decreased cell proliferation in the forebrain and midbrain. fgf16 was also essential for development of the ventral telencephalon and diencephalon, whereas fgf16 was not required for dorsoventral patterning in the midbrain. fgf16 was additionally required for the specification and differentiation of γ-aminobutyric acid (GABA)ergic interneurons and oligodendrocytes, but not for those of glutamatergic neurons in the forebrain. Cross talk between Fgf and Hedgehog (Hh) signaling was critical for the specification of GABAergic interneurons and oligodendrocytes. The expression of fgf16 in the forebrain was down-regulated by the inhibition of Hh and Fgf19 signaling, but not by that of Fgf3/Fgf8 signaling. The fgf16 morphant phenotype was similar to that of the fgf19 morphant and embryos blocked Hh signaling. The results of the present study indicate that Fgf16 signaling, which is regulated by the downstream pathways of Hh-Fgf19 in the forebrain, is involved in forebrain development.

FoxA4 favours notochord formation by inhibiting contiguous mesodermal fates and restricts anterior neural development in Xenopus embryos

In vertebrates, the embryonic dorsal midline is a crucial signalling centre that patterns the surrounding tissues during development. Members of the FoxA subfamily of transcription factors are expressed in the structures that compose this centre. Foxa2 is essential for dorsal midline development in mammals, since knock-out mouse embryos lack a definitive node, notochord and floor plate. The related gene foxA4 is only present in amphibians. Expression begins in the blastula -chordin and -noggin expressing centre (BCNE) and is later restricted to the dorsal midline derivatives of the Spemann's organiser. It was suggested that the early functions of mammalian foxa2 are carried out by foxA4 in frogs, but functional experiments were needed to test this hypothesis. Here, we show that some important dorsal midline functions of mammalian foxa2 are exerted by foxA4 in Xenopus. We provide new evidence that the latter prevents the respecification of dorsal midline precursors towards contiguous fates, inhibiting prechordal and paraxial mesoderm development in favour of the notochord. In addition, we show that foxA4 is required for the correct regionalisation and maintenance of the central nervous system. FoxA4 participates in constraining the prospective rostral forebrain territory during neural specification and is necessary for the correct segregation of the most anterior ectodermal derivatives, such as the cement gland and the pituitary anlagen. Moreover, the early expression of foxA4 in the BCNE (which contains precursors of the whole forebrain and most of the midbrain and hindbrain) is directly required to restrict anterior neural development.

The conserved barH-like homeobox-2 gene barhl2 acts downstream of orthodentricle-2 and together with iroquois-3 in establishment of the caudal forebrain signaling center induced by Sonic Hedgehog

In this study, we investigated the gene regulatory network that governs formation of the Zona limitans intrathalamica (ZLI), a signaling center that secretes Sonic Hedgehog (Shh) to control the growth and regionalization of the caudal forebrain. Using loss- and gain-of-function, explants and grafting experiments in amphibians, we demonstrate that barhl2 acts downstream of otx2 and together with the iroquois (irx)-3 gene in establishment of the ZLI compartment initiated by Shh influence. We find that the presumptive (pre)-ZLI domain expresses barhl2, otx2 and irx3, whereas the thalamus territory caudally bordering the pre-ZLI expresses barhl2, otx2 and irx1/2 and early on irx3. We demonstrate that Barhl2 activity is required for determination of the ZLI and thalamus fates and that within the p2 alar plate the ratio of Irx3 to Irx1/2 contributes to ZLI specification and size determination. We show that when continuously exposed to Shh, neuroepithelial cells coexpressing barhl2, otx2 and irx3 acquire two characteristics of the ZLI compartment-the competence to express shh and the ability to segregate from anterior neural plate cells. In contrast, neuroepithelial cells expressing barhl2, otx2 and irx1/2, are not competent to express shh. Noteworthy in explants, under Shh influence, ZLI-like cells segregate from thalamic-like cells. Our study establishes that Barhl2 activity plays a key role in p2 alar plate patterning, specifically ZLI formation, and provides new insights on establishment of the signaling center of the caudal forebrain.

Genes and signaling networks regulated during zebrafish optic vesicle morphogenesis

BACKGROUND

The genetic cascades underpinning vertebrate early eye morphogenesis are poorly understood. One gene family essential for eye morphogenesis encodes the retinal homeobox (Rx) transcription factors. Mutations in the human retinal homeobox gene (RAX) can lead to gross morphological phenotypes ranging from microphthalmia to anophthalmia. Zebrafish rx3 null mutants produce a similar striking eyeless phenotype with an associated expanded forebrain. Thus, we used zebrafish rx3-/- mutants as a model to uncover an Rx3-regulated gene network during early eye morphogenesis.

RESULTS

Rx3-regulated genes were identified using whole transcriptomic sequencing (RNA-seq) of rx3-/- mutants and morphologically wild-type siblings during optic vesicle morphogenesis. A gene co-expression network was then constructed for the Rx3-regulated genes, identifying gene cross-talk during early eye development. Genes highly connected in the network are hub genes, which tend to exhibit higher expression changes between rx3-/- mutants and normal phenotype siblings. Hub genes down-regulated in rx3-/- mutants encompass homeodomain transcription factors and mediators of retinoid-signaling, both associated with eye development and known human eye disorders. In contrast, genes up-regulated in rx3-/- mutants are centered on Wnt signaling pathways, associated with brain development and disorders. The temporal expression pattern of Rx3-regulated genes was further profiled during early development from maternal stage until visual function is fully mature. Rx3-regulated genes exhibited synchronized expression patterns, and a transition of gene expression during the early segmentation stage when Rx3 was highly expressed. Furthermore, most of these deregulated genes are enriched with multiple RAX-binding motif sequences on the gene promoter.

CONCLUSIONS

Here, we assembled a comprehensive model of Rx3-regulated genes during early eye morphogenesis. Rx3 promotes optic vesicle morphogenesis and represses brain development through a highly correlated and modulated network, exhibiting repression of genes mediating Wnt signaling and concomitant enhanced expression of homeodomain transcription factors and retinoid-signaling genes.

Xenopus mutant reveals necessity of rax for specifying the eye field which otherwise forms tissue with telencephalic and diencephalic character

The retinal anterior homeobox (rax) gene encodes a transcription factor necessary for vertebrate eye development. rax transcription is initiated at the end of gastrulation in Xenopus, and is a key part of the regulatory network specifying anterior neural plate and retina. We describe here a Xenopus tropicalis rax mutant, the first mutant analyzed in detail from a reverse genetic screen. As in other vertebrates, this nonsense mutation results in eyeless animals, and is lethal peri-metamorphosis. Tissue normally fated to form retina in these mutants instead forms tissue with characteristics of diencephalon and telencephalon. This implies that a key role of rax, in addition to defining the eye field, is in preventing alternative forebrain identities. Our data highlight that brain and retina regions are not determined by the mid-gastrula stage but are by the neural plate stage. An RNA-Seq analysis and in situ hybridization assays for early gene expression in the mutant revealed that several key eye field transcription factors (e.g. pax6, lhx2 and six6) are not dependent on rax activity through neurulation. However, these analyses identified other genes either up- or down-regulated in mutant presumptive retinal tissue. Two neural patterning genes of particular interest that appear up-regulated in the rax mutant RNA-seq analysis are hesx1 and fezf2. These genes were not previously known to be regulated by rax. The normal function of rax is to partially repress their expression by an indirect mechanism in the presumptive retina region in wildtype embryos, thus accounting for the apparent up-regulation in the rax mutant. Knock-down experiments using antisense morpholino oligonucleotides directed against hesx1 and fezf2 show that failure to repress these two genes contributes to transformation of presumptive retinal tissue into non-retinal forebrain identities in the rax mutant.

Maternal Vsx1 plays an essential role in regulating prechordal mesendoderm and forebrain formation in zebrafish

Prechordal mesendoderm (PME) is a derivative of gastrula organizer underlying the anterior neural plate of vertebrate embryos. It has been firmly established that PME is critical for head induction and anterior-posterior patterning. Therefore, the establishment of PME in a desired shape and size at a correct position during early embryogenesis is crucial for normal head patterning. However, it remains largely unclear how the desired form and size of PME is generated at a predestined position during early embryogenesis. Here we show that in zebrafish a maternal transcription repressor Vsx1 is essential for this early developmental regulation. Knocking down maternal vsx1 resulted in impaired PME formation and progression associated with a deficient and posteriorized forebrain. Loss- and gain-of-function experiments showed that maternal Vsx1 is essential for repressing ntl ectopic expression in more animal region at early gastrula stages. Chromatin immunoprecipitation assay in combination with core consensus sequence mutation analysis further revealed that maternal Vsx1 can directly repress ntl transcription by binding to the proximal promoter at a specific site. Simultaneous inhibition of ntl function could successfully suppress the defects of both PME and forebrain formation in maternal Vsx1 knockdown embryos. Our results reveal a pivotal role for maternal Vsx1 as a direct transcriptional repressor of ntl expression at the margin of the zebrafish gastrula to ensure directional cell polarization and migration of PME cells.

Sema6a and Plxna2 mediate spatially regulated repulsion within the developing eye to promote eye vesicle cohesion

Organs are generated from collections of cells that coalesce and remain together as they undergo a series of choreographed movements to give the organ its final shape. We know little about the cellular and molecular mechanisms that regulate tissue cohesion during morphogenesis. Extensive cell movements underlie eye development, starting with the eye field separating to form bilateral vesicles that go on to evaginate from the forebrain. What keeps eye cells together as they undergo morphogenesis and extensive proliferation is unknown. Here, we show that plexina2 (Plxna2), a member of a receptor family best known for its roles in axon and cell guidance, is required alongside the repellent semaphorin 6a (Sema6a) to keep cells integrated within the zebrafish eye vesicle epithelium. sema6a is expressed throughout the eye vesicle, whereas plxna2 is restricted to the ventral vesicle. Knockdown of Plxna2 or Sema6a results in a loss of vesicle integrity, with time-lapse microscopy showing that eye progenitors either fail to enter the evaginating vesicles or delaminate from the eye epithelium. Explant experiments, and rescue of eye vesicle integrity with simultaneous knockdown of sema6a and plxna2, point to an eye-autonomous requirement for Sema6a/Plxna2. We propose a novel, tissue-autonomous mechanism of organ cohesion, with neutralization of repulsion suggested as a means to promote interactions between cells within a tissue domain.

The double inhibition of endogenously produced BMP and Wnt factors synergistically triggers dorsal telencephalic differentiation of mouse ES cells

Embryonic stem (ES) cells are becoming a popular model of in vitro neurogenesis, as they display intrinsic capability to generate neural progenitors that undergo the known steps of in vivo neural development. These include the acquisition of distinct regional fates, which depend on growth factors and signals that are present in the culture medium. The control of the intracellular signaling that is active at different steps of ES cell neuralization, even when cells are cultured in chemically defined medium, is complicated by the endogenous production of growth factors. However, this endogenous production has been poorly investigated so far. To address this point, we performed a high-throughput analysis of the expression of morphogens during mouse ES cell neuralization in minimal medium. We found that during their neuralization, ES cells increased the expression of members of Wnt, Fibroblast Growth Factor (FGF), and BMP families. Conversely, the expression of Activin/Nodal and Shh ligands was low in early steps of neuralization. In this experimental condition, neural progenitors and neurons generated by ES cells expressed a gene expression profile that was consistent with a midbrain identity. We found that endogenous BMP and Wnt signaling, but not FGF signaling, synergistically affected ES cell neural patterning, by turning off a profile of dorsal/telencephalic gene expression. Double BMP and Wnt inhibition allowed neuralized ES cells to sequentially activate key genes of cortical differentiation. Our findings are consistent with a novel synergistic effect of Wnt and BMP endogenous signaling of ES cells in inhibiting a cortical differentiation program.

Evolution and development of interhemispheric connections in the vertebrate forebrain

Axonal connections between the left and right sides of the brain are crucial for bilateral integration of lateralized sensory, motor, and associative functions. Throughout vertebrate species, forebrain commissures share a conserved developmental plan, a similar position relative to each other within the brain and similar patterns of connectivity. However, major events in the evolution of the vertebrate brain, such as the expansion of the telencephalon in tetrapods and the origin of the six-layered isocortex in mammals, resulted in the emergence and diversification of new commissural routes. These new interhemispheric connections include the pallial commissure, which appeared in the ancestors of tetrapods and connects the left and right sides of the medial pallium (hippocampus in mammals), and the corpus callosum, which is exclusive to eutherian (placental) mammals and connects both isocortical hemispheres. A comparative analysis of commissural systems in vertebrates reveals that the emergence of new commissural routes may have involved co-option of developmental mechanisms and anatomical substrates of preexistent commissural pathways. One of the embryonic regions of interest for studying these processes is the commissural plate, a portion of the early telencephalic midline that provides molecular specification and a cellular scaffold for the development of commissural axons. Further investigations into these embryonic processes in carefully selected species will provide insights not only into the mechanisms driving commissural evolution, but also regarding more general biological problems such as the role of developmental plasticity in evolutionary change.

COUP-TFs and eye development

Recent studies reveal that COUP-TF genes are essential for neural development, cardiovascular development, energy metabolism and adipogenesis, as well as for organogenesis of multiple systems. In this review, we mainly describe the COUP-TF genes, molecular mechanisms of COUP-TF action, and their crucial functions in the morphogenesis of the murine eye. Mutations of COUP-TF genes lead to the congenital coloboma and/or optic atrophy in both mouse and human, indicating that the study on COUP-TFs and the eye will benefit our understanding of the etiology of human ocular diseases. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Wnt signaling in form deprivation myopia of the mice retina

BACKGROUND

The canonical Wnt signaling pathway plays important roles in cellular proliferation and differentiation, axonal outgrowth, cellular maintenance in retinas. Here we test the hypothesis that elements of the Wnt signaling pathway are involved in the regulation of eye growth and prevention of myopia, in the mouse form-deprivation myopia model.

METHODOLOGY/PRINCIPAL FINDINGS

(1) One hundred twenty-five C57BL/6 mice were randomly distributed into form-deprivation myopia and control groups. Form-deprivation myopia (FDM) was induced by suturing the right eyelid, while the control group received no treatment. After 1, 2, and 4 weeks of treatment, eyes were assessed in vivo by cycloplegic retinoscopic refraction and axial length measurement by photography or A-scan ultrasonography. Levels of retinal Wnt2b, Fzd5 and β-catenin mRNA and protein were evaluated using RT-PCR and western blotting, respectively. (2) Another 96 mice were divided into three groups: control, drugs-only, and drugs+FDM (by diffuser). Experimentally treated eyes in the last two groups received intravitreal injections of vehicle or the proteins, DKK-1 (Wnt-pathway antagonist) or Norrin (Wnt-pathway agonist), once every three days, for 4 injections total. Axial length and retinoscopic refraction were measured on the 14th day of form deprivation. Following form-deprivation for 1, 2, and 4 weeks, FDM eyes had a relatively myopic refractive error, compared with contralateral eyes. There were no significant differences in refractive error between right and left eye in control group. The amounts of Wnt2b, Fzd5 and β-catenin mRNA and protein were significantly greater in form-deprived myopia eyes than in control eyes.DKK-1 (antagonist) reduced the myopic shift in refractive error and increase in axial elongation, whereas Norrin had the opposite effect in FDM eyes.

CONCLUSIONS/SIGNIFICANCE

Our studies provide the first evidence that the Wnt2b signaling pathway may play a role in the development and progression of form-deprivation myopia, in a mammalian model.

Cortical neurogenesis from pluripotent stem cells: complexity emerging from simplicity

The cerebral cortex contains dozens of neuronal subtypes grouped in specific layers and areas. Recent studies have revealed how embryonic and induced pluripotent stem cells (PSC) can differentiate into a wide diversity of cortical neurons in vitro, while recapitulating many of the temporal and spatial features that characterize corticogenesis. PSC-derived neurons can integrate into the brain following in vivo transplantation and display patterns of morphology and connectivity specific of cortical neurons. PSC-corticogenesis thus emerges as a robust model that provides new ways to link cortical development, evolution, and disease.

Thinking out of the dish: what to learn about cortical development using pluripotent stem cells

The development of the cerebral cortex requires the tightly coordinated generation of dozens of neuronal subtypes that will populate specific layers and areas. Recent studies have revealed how pluripotent stem cells (PSC), whether of mouse or human origin, can differentiate into a wide range of cortical neurons in vitro, which can integrate appropriately into the brain following in vivo transplantation. These models are largely artificial but recapitulate a substantial fraction of the complex temporal and regional patterning events that occur during in vivo corticogenesis. Here, we review these findings with emphasis on the new perspectives that they have brought for understanding of cortical development, evolution, and diseases.

Optimizing neuronal differentiation from induced pluripotent stem cells to model ASD

Autism spectrum disorder (ASD) is an early-onset neurodevelopmental disorder characterized by deficits in social communication, and restricted and repetitive patterns of behavior. Despite its high prevalence, discovery of pathophysiological mechanisms underlying ASD has lagged due to a lack of appropriate model systems. Recent advances in induced pluripotent stem cell (iPSC) technology and neural differentiation techniques allow for detailed functional analyses of neurons generated from living individuals with ASD. Refinement of cortical neuron differentiation methods from iPSCs will enable mechanistic studies of specific neuronal subpopulations that may be preferentially impaired in ASD. In this review, we summarize recent accomplishments in differentiation of cortical neurons from human pluripotent stems cells and efforts to establish in vitro model systems to study ASD using personalized neurons.

An essential role for LPA signalling in telencephalon development

Lysophosphatidic acid (LPA) has wide-ranging effects on many different cell types, acting through G-protein-coupled receptors such as LPAR6. We show that Xenopus lpar6 is expressed from late blastulae and is enriched in the mesoderm and dorsal ectoderm of early gastrulae. Expression in gastrulae is an early response to FGF signalling. Transcripts for lpar6 are enriched in the neural plate of Xenopus neurulae and loss of function caused forebrain defects, with reduced expression of telencephalic markers (foxg1, emx1 and nkx2-1). Midbrain (en2) and hindbrain (egr2) markers were unaffected. Foxg1 expression requires LPAR6 within ectoderm and not mesoderm. Head defects caused by LPAR6 loss of function were enhanced by co-inhibiting FGF signalling, with defects extending into the hindbrain (en2 and egr2 expression reduced). This is more severe than expected from simple summation of individual defects, suggesting that LPAR6 and FGF have overlapping or partially redundant functions in the anterior neural plate. We observed similar defects in forebrain development in loss-of-function experiments for ENPP2, an enzyme involved in the synthesis of extracellular LPA. Our study demonstrates a role for LPA in early forebrain development.

A zebrafish chemical suppressor screening identifies small molecule inhibitors of the Wnt/β-catenin pathway

Genetic screening for suppressor mutants has been successfully used to identify important signaling regulators. Using an analogy to genetic suppressor screening, we developed a chemical suppressor screening method to identify inhibitors of the Wnt/β-catenin signaling pathway. We used zebrafish embryos in which chemically induced β-catenin accumulation led to an "eyeless" phenotype and conducted a pilot screening for compounds that restored eye development. This approach allowed us to identify geranylgeranyltransferase inhibitor 286 (GGTI-286), a geranylgeranyltransferase (GGTase) inhibitor. Our follow-up studies showed that GGTI-286 reduces nuclear localization of β-catenin and transcription dependent on β-catenin/T cell factor in mammalian cells. In addition to pharmacological inhibition, GGTase gene knockdown also attenuates the nuclear function of β-catenin. Overall, we validate our chemical suppressor screening as a method for identifying Wnt/β-catenin pathway inhibitors and implicate GGTase as a potential therapeutic target for Wnt-activated cancers.

From pluripotency to forebrain patterning: an in vitro journey astride embryonic stem cells

Embryonic stem cells (ESCs) have been used extensively as in vitro models of neural development and disease, with special efforts towards their conversion into forebrain progenitors and neurons. The forebrain is the most complex brain region, giving rise to several fundamental structures, such as the cerebral cortex, the hypothalamus, and the retina. Due to the multiplicity of signaling pathways playing different roles at distinct times of embryonic development, the specification and patterning of forebrain has been difficult to study in vivo. Research performed on ESCs in vitro has provided a large body of evidence to complement work in model organisms, but these studies have often been focused more on cell type production than on cell fate regulation. In this review, we systematically reassess the current literature in the field of forebrain development in mouse and human ESCs with a focus on the molecular mechanisms of early cell fate decisions, taking into consideration the specific culture conditions, exogenous and endogenous molecular cues as described in the original studies. The resulting model of early forebrain induction and patterning provides a useful framework for further studies aimed at reconstructing forebrain development in vitro for basic research or therapy.

Specification and positioning of the anterior neuroectoderm in deuterostome embryos

The molecular mechanisms used by deuterostome embryos (vertebrates, urochordates, cephalochordates, hemichordates, and echinoderms) to specify and then position the anterior neuroectoderm (ANE) along the anterior-posterior axis are incompletely understood. Studies in several deuterostome embryos suggest that the ANE is initially specified by an early, broad regulatory state. Then, a posterior-to-anterior wave of respecification restricts this broad ANE potential to the anterior pole. In vertebrates, sea urchins and hemichordates a posterior-anterior gradient of Wnt/β-catenin signaling plays an essential and conserved role in this process. Recent data collected from the basal deuterostome sea urchin embryo suggests that positioning the ANE to the anterior pole involves more than the Wnt/β-catenin pathway, instead relying on the integration of information from the Wnt/β-catenin, Wnt/JNK, and Wnt/PKC pathways. Moreover, comparison of functional and expression data from the ambulacrarians, invertebrate chordates, and vertebrates strongly suggests that this Wnt network might be an ANE positioning mechanism shared by all deuterostomes.

Canonical Wnt/β-catenin signalling is essential for optic cup formation

A multitude of signalling pathways are involved in the process of forming an eye. Here we demonstrate that β-catenin is essential for eye development as inactivation of β-catenin prior to cellular specification in the optic vesicle caused anophthalmia in mice. By achieving this early and tissue-specific β-catenin inactivation we find that retinal pigment epithelium (RPE) commitment was blocked and eye development was arrested prior to optic cup formation due to a loss of canonical Wnt signalling in the dorsal optic vesicle. Thus, these results show that Wnt/β-catenin signalling is required earlier and play a more central role in eye development than previous studies have indicated. In our genetic model system a few RPE cells could escape β-catenin inactivation leading to the formation of a small optic rudiment. The optic rudiment contained several neural retinal cell classes surrounded by an RPE. Unlike the RPE cells, the neural retinal cells could be β-catenin-negative revealing that differentiation of the neural retinal cell classes is β-catenin-independent. Moreover, although dorsoventral patterning is initiated in the mutant optic vesicle, the neural retinal cells in the optic rudiment displayed almost exclusively ventral identity. Thus, β-catenin is required for optic cup formation, commitment to RPE cells and maintenance of dorsal identity of the retina.

Integration of anterior neural plate patterning and morphogenesis by the Wnt signaling pathway

Wnts are essential for a multitude of processes during embryonic development and adult homeostasis. The molecular structure of the Wnt pathway is extremely complex, and it keeps growing as new molecular components and novel interactions are uncovered. Recent studies have advanced our understanding on how the diverse molecular outcomes of the Wnt pathway are integrated during organ development, an integration that is also essential, although mechanistically poorly understood, during the formation of the anterior part of the nervous system, the forebrain. In this article, the author has summarized these findings and discussed their implications for forebrain development. A special emphasis has been put forth on studies performed in the zebrafish as this model system has been instrumental for our current understanding of forebrain patterning.

Eph/Ephrin signalling maintains eye field segregation from adjacent neural plate territories during forebrain morphogenesis

During forebrain morphogenesis, there is extensive reorganisation of the cells destined to form the eyes, telencephalon and diencephalon. Little is known about the molecular mechanisms that regulate region-specific behaviours and that maintain the coherence of cell populations undergoing specific morphogenetic processes. In this study, we show that the activity of the Eph/Ephrin signalling pathway maintains segregation between the prospective eyes and adjacent regions of the anterior neural plate during the early stages of forebrain morphogenesis in zebrafish. Several Ephrins and Ephs are expressed in complementary domains in the prospective forebrain and combinatorial abrogation of their activity results in incomplete segregation of the eyes and telencephalon and in defective evagination of the optic vesicles. Conversely, expression of exogenous Ephs or Ephrins in regions of the prospective forebrain where they are not usually expressed changes the adhesion properties of the cells, resulting in segregation to the wrong domain without changing their regional fate. The failure of eye morphogenesis in rx3 mutants is accompanied by a loss of complementary expression of Ephs and Ephrins, suggesting that this pathway is activated downstream of the regional fate specification machinery to establish boundaries between domains undergoing different programmes of morphogenesis.

The bilaterian forebrain: an evolutionary chimaera

The insect, annelid and vertebrate forebrains harbour two major centres of output control, a sensory-neurosecretory centre releasing hormones and a primordial locomotor centre that controls the initiation of muscular body movements. In vertebrates, both reside in the hypothalamus. Here, we review recent comparative neurodevelopmental evidence indicating that these centres evolved from separate condensations of neurons on opposite body sides ('apical nervous system' versus 'blastoporal nervous system') and that their developmental specification involved distinct regulatory networks (apical six3 and rx versus mediolateral nk and pax gene-dependent patterning). In bilaterian ancestors, both systems approached each other and became closely intermingled, physically, functionally and developmentally. Our 'chimeric brain hypothesis' sheds new light on the vast success and rapid diversification of bilaterian animals in the Cambrian and revises our understanding of brain architecture.

Retinal pigment epithelium development, plasticity, and tissue homeostasis

The retinal pigment epithelium (RPE) is a simple epithelium interposed between the neural retina and the choroid. Although only 1 cell-layer in thickness, the RPE is a virtual workhorse, acting in several capacities that are essential for visual function and preserving the structural and physiological integrities of neighboring tissues. Defects in RPE function, whether through chronic dysfunction or age-related decline, are associated with retinal degenerative diseases including age-related macular degeneration. As such, investigations are focused on developing techniques to replace RPE through stem cell-based methods, motivated primarily because of the seemingly limited regeneration or self-repair properties of mature RPE. Despite this, RPE cells have an unusual capacity to transdifferentiate into various cell types, with the particular fate choices being highly context-dependent. In this review, we describe recent findings elucidating the mechanisms and steps of RPE development and propose a developmental framework for understanding the apparent contradiction in the capacity for low self-repair versus high transdifferentiation.

Neuropeptides: developmental signals in placode progenitor formation

Few families of signaling factors have been implicated in the control of development. Here, we identify the neuropeptides nociceptin and somatostatin, a neurotransmitter and neuroendocrine hormone, as a class of developmental signals in both chick and zebrafish. We show that signals from the anterior mesendoderm are required for the formation of anterior placode progenitors, with one of the signals being somatostatin. Somatostatin controls ectodermal expression of nociceptin, and both peptides regulate Pax6 in lens and olfactory progenitors. Consequently, loss of somatostatin and nociceptin signaling leads to severe reduction of lens formation. Our findings not only uncover these neuropeptides as developmental signals but also identify a long-sought-after mechanism that initiates Pax6 in placode progenitors and may explain the ancient evolutionary origin of neuropeptides, predating a complex nervous system.

Human embryonic stem cell applications for retinal degenerations

Loss of vision in severe retinal degenerations often is a result of photoreceptor cell or retinal pigment epithelial cell death or dysfunction. Cell replacement therapy has the potential to restore useful vision for these individuals especially after they have lost most or all of their light-sensing cells in the eye. A reliable, well-characterized source of retinal cells will be needed for replacement purposes. Human embryonic stem cells (ES cells) can provide an unlimited source of replacement retinal cells to take over the function of lost cells in the eye. The author's intent for this review is to provide an historical overview of the field of embryonic stem cells with relation to the retina. The review will provide a quick primer on key pathways involved in the development of the neural retina and RPE followed by a discussion of the various protocols out in the literature for generating these cells from non-human and human embryonic stem cells and end with in vivo application of ES cell-derived photoreceptors and RPE cells.

LHX2 is necessary for the maintenance of optic identity and for the progression of optic morphogenesis

Eye formation is regulated by a complex network of eye field transcription factors (EFTFs), including LIM-homeodomain gene LHX2. We disrupted LHX2 function at different stages during this process using a conditional knock-out strategy in mice. We find that LHX2 function is required in an ongoing fashion to maintain optic identity across multiple stages, from the formation of the optic vesicle to the differentiation of the neuroretina. At each stage, loss of Lhx2 led to upregulation of a set of molecular markers that are normally expressed in the thalamic eminence and in the anterodorsal hypothalamus in a portion of the optic vesicle or retina. Furthermore, the longer LHX2 function was maintained, the further optic morphogenesis progressed. Early loss of function caused profound mispatterning of the entire telencephalic-optic-hypothalamic field, such that the optic vesicle became mispositioned and appeared to arise from the diencephalic-telencephalic boundary. At subsequent stages, loss of Lhx2 did not affect optic vesicle position but caused arrest of optic cup formation. If Lhx2 was selectively disrupted in the neuroretina from E11.5, the neuroretina showed gross dysmorphology along with aberrant expression of markers specific to the thalamic eminence and anterodorsal hypothalamus. Our findings indicate a continual requirement for LHX2 throughout the early stages of optic development, not only to maintain optic identity by suppressing alternative fates but also to mediate multiple steps of optic morphogenesis. These findings provide new insight into the anophthalmic phenotype of the Lhx2 mutant and reveal novel roles for this transcription factor in eye development.

An eye on eye development

The vertebrate eye is composed of both surface ectodermal and neuroectodermal derivatives that evaginate laterally from an epithelial anlage of the forming diencephalon. The retina is composed of a limited number of neuronal and non-neuronal cell types and is seen as a model for the brain with reduced complexity. The eye develops in a stereotypic manner building on evolutionarily conserved molecular networks. Eye formation is initiated at the onset of gastrulation by the determination of the eye field in the anterior neuroectoderm. Homeobox transcription factors, in particular Six3 are crucially involved in the establishment and maintenance of retinal identity. The eye field expands by proliferation as gastrulation proceeds and is initially confined to a single retinal primordium by the differential activity of specifying transcription factors. This central field is subsequently split in response to secreted factors emanating from the ventral midline. Concomitant with medio-lateral patterning at the onset of neurulation, morphogenesis sets in and laterally evaginates the optic vesicle. Strikingly during this process the neuroectoderm in the eye field transiently loses epithelial features and cells migrate individually. In a second morphogenetic event, the vesicle is transformed into the optic cup, concomitant with onset and progression of retinal differentiation. Accompanying optic cup morphogenesis, neural differentiation is initiated from a retinal signalling centre in a stereotypic and species specific manner by secreted signalling factors. Here we will give an overview of key events during vertebrate eye formation and highlight key players in the respective processes.

Dynamic expression of MEIS1 homeoprotein in E14.5 forebrain and differentiated forebrain-derived neural stem cells

Central nervous system development is controlled by highly conserved homeoprotein transcription factors including HOX and TALE (Three Amino acid Loop Extension). TALE proteins are primarily known as HOX-cofactors and play key roles in cell proliferation, differentiation and organogenesis. MEIS1 is a TALE member with established expression in the developing central nervous system. MEIS1 is essential for embryonic development and Meis1 knockout mice dies at embryonic day (E) 14.5. However, Meis1/MEIS1 expression in the devolving forebrain, at this critical time-point has not been studied. Here, for the first time we characterize the region-specific expression of MEIS1 in E14.5 mouse forebrain, filling the gap of MEIS1 expression profile between E12.5 and E16.5. Previously, we reported MEIS1 transcriptional regulatory role in neuronal differentiation and established forebrain-derived neural stem cells (NSC) for gene therapy application of neuronal genes. Here, we show the dynamic expression of Meis1/MEIS1 during the differentiation of forebrain-derived NSC toward a glial lineage. Our results show that Meis1/MEIS1 expression is induced during NSC differentiation and is expressed in both differentiated neurons and astrocytes. Confirming these results, we detected MEIS1 expression in primary cultures of in vivo differentiated cortical neurons and astrocytes. We further demonstrate Meis1/MEIS1 expression relative to other TALE family members in the forebrain-derived NSC in the absence of Hox genes. Our data provide evidence that forebrain-derived NSC can be used as an accessible in vitro model to study the expression and function of TALE proteins, supporting their potential role in modulating NSC self-renewal and differentiation.

Only scratching the cell surface: extracellular signals in cerebrum development

Numerous roles have been identified for extracellular signals such as Fibroblast Growth Factors (FGFs), Transforming Growth Factor-βs (TGFβs), Wingless-Int proteins (WNTs), and Sonic Hedgehog (SHH) in assigning fates to cells during development of the cerebrum. However, several fundamental questions remain largely unexplored. First, how does the same extracellular signal instruct precursor cells in different locations or at different stages to adopt distinct fates? And second, how does a precursor cell integrate multiple signals to adopt a specific fate? Answers to these questions require knowing the mechanisms that underlie each cell type's competence to respond to certain extracellular signals. This brief review provides illustrative examples of potential mechanisms that begin to bridge the gap between cell surface and cell fate during cerebrum development.

Multiple roles of Activin/Nodal, bone morphogenetic protein, fibroblast growth factor and Wnt/β-catenin signalling in the anterior neural patterning of adherent human embryonic stem cell cultures

Several studies have successfully produced a variety of neural cell types from human embryonic stem cells (hESCs), but there has been limited systematic analysis of how different regional identities are established using well-defined differentiation conditions. We have used adherent, chemically defined cultures to analyse the roles of Activin/Nodal, bone morphogenetic protein (BMP), fibroblast growth factor (FGF) and Wnt/β-catenin signalling in neural induction, anteroposterior patterning and eye field specification in hESCs. We show that either BMP inhibition or activation of FGF signalling is required for effective neural induction, but these two pathways have distinct outcomes on rostrocaudal patterning. While BMP inhibition leads to specification of forebrain/midbrain positional identities, FGF-dependent neural induction is associated with strong posteriorization towards hindbrain/spinal cord fates. We also demonstrate that Wnt/β-catenin signalling is activated during neural induction and promotes acquisition of neural fates posterior to forebrain. Therefore, inhibition of this pathway is needed for efficient forebrain specification. Finally, we provide evidence that the levels of Activin/Nodal and BMP signalling have a marked influence on further forebrain patterning and that constitutive inhibition of these pathways represses expression of eye field genes. These results show that the key mechanisms controlling neural patterning in model vertebrate species are preserved in adherent, chemically defined hESC cultures and reveal new insights into the signals regulating eye field specification.

Pyramidal neurons derived from human pluripotent stem cells integrate efficiently into mouse brain circuits in vivo

The study of human cortical development has major implications for brain evolution and diseases but has remained elusive due to paucity of experimental models. Here we found that human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), cultured without added morphogens, recapitulate corticogenesis leading to the sequential generation of functional pyramidal neurons of all six layer identities. After transplantation into mouse neonatal brain, human ESC-derived cortical neurons integrated robustly and established specific axonal projections and dendritic patterns corresponding to native cortical neurons. The differentiation and connectivity of the transplanted human cortical neurons complexified progressively over several months in vivo, culminating in the establishment of functional synapses with the host circuitry. Our data demonstrate that human cortical neurons generated in vitro from ESC/iPSC can develop complex hodological properties characteristic of the cerebral cortex in vivo, thereby offering unprecedented opportunities for the modeling of human cortex diseases and brain repair.

Anteroposterior and dorsoventral patterning are coordinated by an identical patterning clock

Establishment of the body plan in vertebrates depends on the temporally coordinated patterning of tissues along the body axes. We have previously shown that dorsoventral (DV) tissues are temporally patterned progressively from anterior to posterior by a BMP signaling pathway. Here we report that DV patterning along the zebrafish anteroposterior (AP) axis is temporally coordinated with AP patterning by an identical patterning clock. We altered AP patterning by inhibiting or activating FGF, Wnt or retinoic acid signaling combined with inhibition of BMP signaling at a series of developmental time points, which revealed that the temporal progression of DV patterning is directly coordinated with AP patterning. We investigated how these signaling pathways are integrated and suggest a model for how DV and AP patterning are temporally coordinated. It has been shown that in Xenopus dorsal tissues FGF and Wnt signaling quell BMP signaling by degrading phosphorylated (P) Smad1/5, the BMP pathway signal transducer, via phosphorylation of the Smad1/5 linker region. We show that in zebrafish FGF/MAPK, but not Wnt/GSK3, phosphorylation of the Smad1/5 linker region localizes to a ventral vegetal gastrula region that could coordinate DV patterning with AP patterning ventrally without degrading P-Smad1/5. Furthermore, we demonstrate that alteration of the MAPK phosphorylation sites in the Smad5 linker causes precocious patterning of DV tissues along the AP axis during gastrulation. Thus, DV and AP patterning are intimately coordinated to allow cells to acquire both positional and temporal information simultaneously.

Chicken embryonic brain: an in vivo model for verifying neural stem cell potency

OBJECT

The multipotency of neural stem cells (NSCs) can be assessed in vitro by detection of stage-specific markers in response to a suitable differentiation signal. This test is frequently used because it is fast and affordable. However, it is not clear how the in vitro potential for multilineage differentiation and stem cell marker expression would reflect the ability of NSCs to engraft into the brain following transplantation. The authors undertook this study to directly compare the in vitro potency and in vivo migration of human NSCs (hNSCs) expanded under conditions of gradually increased concentration of fetal bovine serum (FBS) as a maturation factor.

METHODS

Human NSCs isolated from fetal brain were propagated in serum free media (SF-hNSCs) and in media containing 0.1% and 0.2% serum. At Passage 4 in tissue culture the NSCs were harvested and either differentiated in vitro or transplanted into the lateral ventricle of chicken embryonic brain at the late stage of its development (Hamburger and Hamilton Stage 26). The in vitro differentiation was evaluated by immunostaining with neural or glial specific markers, and the in vivo migration was assessed using immunohistology.

RESULTS

The authors found that SF-hNSCs successfully engrafted into the chicken embryonic brain, which correlated with their ability to differentiate in vitro. NSCs grown at as low as 0.1% concentration of FBS failed to demonstrate the robust in vivo migration pattern but still preserved the capability to differentiate in vitro. Furthermore, NSCs generated in media containing a higher concentration of FBS (0.2%) lost both the in vivo engraftment and in vitro differentiation potential.

CONCLUSIONS

The present study suggests that marker expression and in vitro differentiation assays might not provide adequate information regarding the behavior of NSCs following their transplantation. The in vivo migration following injection into chicken embryonic brain may provide an important assay of the potency of NSCs.

Developmental mechanisms directing early anterior forebrain specification in vertebrates

Research from the last 15 years has provided a working model for how the anterior forebrain is induced and specified during the early stages of embryogenesis. This model relies on three basic processes: (1) induction of the neural plate from naive ectoderm requires the inhibition of BMP/TGFβ signaling; (2) induced neural tissue initially acquires an anterior identity (i.e., anterior forebrain); (3) maintenance and expansion of the anterior forebrain depends on the antagonism of posteriorizing signals that would otherwise transform this tissue into posterior neural fates. In this review, we present a historical perspective examining some of the significant experiments that have helped to delineate this molecular model. In addition, we discuss the function of the relevant tissues that act prior to and during gastrulation to ensure proper anterior forebrain formation. Finally, we elaborate data, mainly obtained from the analyses of mouse mutants, supporting a role for transcriptional repressors in the regulation of cell competence within the anterior forebrain. The aim of this review is to provide the reader with a general overview of the signals as well as the signaling centers that control the development of the anterior neural plate.

BMP signaling protects telencephalic fate by repressing eye identity and its Cxcr4-dependent morphogenesis

Depletion of Wnt signaling is a major requirement for the induction of the anterior prosencephalon. However, the molecular events driving the differential regionalization of this area into eye-field and telencephalon fates are still unknown. Here we show that the BMP pathway is active in the anterior neural ectoderm during late blastula to early gastrula stage in zebrafish. Bmp2b mutants and mosaic loss-of-function experiments reveal that BMP acts as a repressor of eye-field fate through inhibition of its key transcription factor Rx3, thereby protecting the future telencephalon from acquiring eye identity. This BMP-driven mechanism initiates the establishment of the telencephalon prior to the involvement of Wnt antagonists from the anterior neural border. Furthermore, we demonstrate that Rx3 and BMP are respectively required to maintain and restrict the chemokine receptor cxcr4a, which in turn contributes to the morphogenetic separation of eye-field and telencephalic cells during early neurulation.

Protein tyrosine phosphatase receptor type O (Ptpro) regulates cerebellar formation during zebrafish development through modulating Fgf signaling

Protein activities controlled by receptor protein tyrosine phosphatases (RPTPs) play comparably important roles in transducing cell surface signals into the cytoplasm by protein tyrosine kinases. Previous studies showed that several RPTPs are involved in neuronal generation, migration, and axon guidance in Drosophila, and the vertebrate hippocampus, retina, and developing limbs. However, whether the protein tyrosine phosphatase type O (ptpro), one kind of RPTP, participates in regulating vertebrate brain development is largely unknown. We isolated the zebrafish ptpro gene and found that its transcripts are primarily expressed in the embryonic and adult central nervous system. Depletion of zebrafish embryonic Ptpro by antisense morpholino oligonucleotide knockdown resulted in prominent defects in the forebrain and cerebellum, and the injected larvae died on the 4th day post-fertilization (dpf). We further investigated the function of ptpro in cerebellar development and found that the expression of ephrin-A5b (efnA5b), a Fgf signaling induced cerebellum patterning factor, was decreased while the expression of dusp6, a negative-feedback gene of Fgf signaling in the midbrain-hindbrain boundary region, was notably induced in ptpro morphants. Further analyses demonstrated that cerebellar defects of ptpro morphants were partially rescued by inhibiting Fgf signaling. Moreover, Ptpro physically interacted with the Fgf receptor 1a (Fgfr1a) and dephosphorylated Fgfr1a in a dose-dependant manner. Therefore, our findings demonstrate that Ptpro activity is required for patterning the zebrafish embryonic brain. Specifically, Ptpro regulates cerebellar formation during zebrafish development through modulating Fgf signaling.

Integration of canonical and noncanonical Wnt signaling pathways patterns the neuroectoderm along the anterior-posterior axis of sea urchin embryos

Three different Wnt signaling pathways function to restrict the anterior neuroectoderm state to the anterior end of the sea urchin embryo, a mechanism of anterior fate restriction that could be conserved among deuterostomes.

Patterning the neuroectoderm along the anterior–posterior (AP) axis is a critical event in the early development of deuterostome embryos. However, the mechanisms that regulate the specification and patterning of the neuroectoderm are incompletely understood. Remarkably, the anterior neuroectoderm (ANE) of the deuterostome sea urchin embryo expresses many of the same transcription factors and secreted modulators of Wnt signaling, as does the early vertebrate ANE (forebrain/eye field). Moreover, as is the case in vertebrate embryos, confining the ANE to the anterior end of the embryo requires a Wnt/β-catenin-dependent signaling mechanism. Here we use morpholino- or dominant negative-mediated interference to demonstrate that the early sea urchin embryo integrates information not only from Wnt/β-catenin but also from Wnt/Fzl5/8-JNK and Fzl1/2/7-PKC pathways to provide precise spatiotemporal control of neuroectoderm patterning along its AP axis. Together, through the Wnt1 and Wnt8 ligands, they orchestrate a progressive posterior-to-anterior wave of re-specification that restricts the initial, ubiquitous, maternally specified, ANE regulatory state to the most anterior blastomeres. There, the Wnt receptor antagonist, Dkk1, protects this state through a negative feedback mechanism. Because these different Wnt pathways converge on the same cell fate specification process, our data suggest they may function as integrated components of an interactive Wnt signaling network. Our findings provide strong support for the idea that the sea urchin ANE regulatory state and the mechanisms that position and define its borders represent an ancient regulatory patterning system that was present in the common echinoderm/vertebrate ancestor.

The initial regulatory state of most cells in many deuterostome embryos, including those of vertebrates and sea urchins, supports anterior neural fate specification. It is important to restrict this neurogenic potential to the anterior end of the embryo during early embryogenesis, but the molecular mechanisms by which this re-specification of posterior fate occurs are incompletely understood in any embryo. The sea urchin embryo is ideally suited to study this process because, in contrast to vertebrates, anterior–posterior neuroectoderm patterning occurs independently of dorsal-ventral axis patterning and takes place before the complex cell movements of gastrulation. In this study, we show that a linked, three-step process involving at least three different Wnt signaling pathways provides precise spatiotemporal restriction of the anterior neuroectoderm regulatory state to the anterior end of the sea urchin embryo. Because these three pathways impinge on the same developmental process, they could be functioning as an integrated Wnt signaling network. Moreover, striking parallels among gene expression patterns and functional studies suggest that this mechanism of anterior fate restriction could be highly conserved among deuterostomes.

Eye development and retinogenesis

Three embryonic tissue sources-the neural ectoderm, the surface ectoderm, and the periocular mesenchyme-contribute to the formation of the mammalian eye. For this reason, the developing eye has presented an invaluable system for studying the interactions among cells and, more recently, genes, in specifying cell fate. This article describes how the eye primordium is specified in the anterior neural plate by four eye field transcription factors and how the optic vesicle becomes regionalized into three distinct tissue types. Specific attention is given to how cross talk between the optic vesicle and surface ectoderm contributes to lens and optic cup formation. This article also describes how signaling networks and cell movements set up axes in the optic cup and establish the multiple cell fates important for vision. How multipotent retinal progenitor cells give rise to the six neuronal and one glial cell type in the mature retina is also explained. Finally, the history and progress of cellular therapeutics for the treatment of degenerative eye disease is outlined. Throughout this article, special attention is given to how disruption of gene function causes ocular malformation in humans. Indeed, the accessibility of the eye has contributed much to our understanding of the basic processes involved in mammalian development.

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.

The logic of gene regulatory networks in early vertebrate forebrain patterning

The vertebrate forebrain or prosencephalon is patterned at the beginning of neurulation into four major domains: the telencephalic, hypothalamic, retinal and diencephalic anlagen. These domains will then give rise to the majority of the brain structures involved in sensory integration and the control of higher intellectual and homeostatic functions. Understanding how forebrain pattering arises has thus attracted the interest of developmental neurobiologists for decades. As a result, most of its regulators have been identified and their hierarchical relationship is now the object of active investigation. Here, we summarize the main morphogenetic pathways and transcription factors involved in forebrain specification and propose the backbone of a possible gene regulatory network (GRN) governing its specification, taking advantage of the GRN principles elaborated by pioneer studies in simpler organisms. We will also discuss this GRN and its operational logic in the context of the remarkable morphological and functional diversification that the forebrain has undergone during evolution.

The positional identity of mouse ES cell-generated neurons is affected by BMP signaling

We investigated the effects of bone morphogenetic proteins (BMPs) in determining the positional identity of neurons generated in vitro from mouse embryonic stem cells (ESCs), an aspect that has been neglected thus far. Classical embryological studies in lower vertebrates indicate that BMPs inhibit the default fate of pluripotent embryonic cells, which is both neural and anterior. Moreover, mammalian ESCs generate neurons more efficiently when cultured in a minimal medium containing BMP inhibitors. In this paper, we show that mouse ESCs produce, secrete, and respond to BMPs during in vitro neural differentiation. After neuralization in a minimal medium, differentiated ESCs show a gene expression profile consistent with a midbrain identity, as evaluated by the analysis of a number of markers of anterior-posterior and dorsoventral identity. We found that BMPs endogenously produced during neural differentiation mainly act by inhibiting the expression of a telencephalic gene profile, which was revealed by the treatment with Noggin or with other BMP inhibitors. To better characterize the effect of BMPs on positional fate, we compared the global gene expression profiles of differentiated ESCs with those of embryonic forebrain, midbrain, and hindbrain. Both Noggin and retinoic acid (RA) support neuronal differentiation of ESCs, but they show different effects on their positional identity: whereas RA supports the typical gene expression profile of hindbrain neurons, Noggin induces a profile characteristic of dorsal telencephalic neurons. Our findings show that endogenously produced BMPs affect the positional identity of the neurons that ESCs spontaneously generate when differentiating in vitro in a minimal medium. The data also support the existence of an intrinsic program of neuronal differentiation with dorsal telencephalic identity. Our method of ESC neuralization allows for fast differentiation of neural cells via the same signals found during in vivo embryonic development and for the acquisition of cortical identity by the inhibition of BMP alone.

The visual system of zebrafish and its use to model human ocular diseases

Free swimming zebrafish larvae depend mainly on their sense of vision to evade predation and to catch prey. Hence, there is strong selective pressure on the fast maturation of visual function and indeed the visual system already supports a number of visually driven behaviors in the newly hatched larvae.The ability to exploit the genetic and embryonic accessibility of the zebrafish in combination with a behavioral assessment of visual system function has made the zebrafish a popular model to study vision and its diseases.Here, we review the anatomy, physiology, and development of the zebrafish eye as the basis to relate the contributions of the zebrafish to our understanding of human ocular diseases.

Morphogenesis underlying the development of the everted teleost telencephalon

Background

Although the mechanisms underlying brain patterning and regionalization are very much conserved, the morphology of different brain regions is extraordinarily variable across vertebrate phylogeny. This is especially manifest in the telencephalon, where the most dramatic variation is seen between ray-finned fish, which have an everted telencephalon, and all other vertebrates, which have an evaginated telencephalon. The mechanisms that generate these distinct morphologies are not well understood.

Results

Here we study the morphogenesis of the zebrafish telencephalon from 12 hours post fertilization (hpf) to 5 days post fertilization (dpf) by analyzing forebrain ventricle formation, evolving patterns of gene and transgene expression, neuronal organization, and fate mapping. Our results highlight two key events in telencephalon morphogenesis. First, the formation of a deep ventricular recess between telencephalon and diencephalon, the anterior intraencephalic sulcus (AIS), effectively creates a posterior ventricular wall to the telencephalic lobes. This process displaces the most posterior neuroepithelial territory of the telencephalon laterally. Second, as telencephalic growth and neurogenesis proceed between days 2 and 5 of development, the pallial region of the posterior ventricular wall of the telencephalon bulges into the dorsal aspect of the AIS. This brings the ventricular zone (VZ) into close apposition with the roof of the AIS to generate a narrow ventricular space and the thin tela choroidea (tc). As the pallial VZ expands, the tc also expands over the upper surface of the telencephalon. During this period, the major axis of growth and extension of the pallial VZ is along the anteroposterior axis. This second step effectively generates an everted telencephalon by 5 dpf.

Conclusion

Our description of telencephalic morphogenesis challenges the conventional model that eversion is simply due to a laterally directed outfolding of the telencephalic neuroepithelium. This may have significant bearing on understanding the eventual organization of the adult fish telencephalon.

Evolutionary crossroads in developmental biology: hemichordates

Hemichordates are a deuterostome phylum, the sister group to echinoderms, and closely related to chordates. They have thus been used to gain insights into the origins of deuterostome and chordate body plans. Developmental studies of this group have a long and distinguished history. Recent improvements in animal husbandry, functional tool development and genomic resources have resulted in novel developmental data from several species in this group. In this Primer, we introduce representative hemichordate species with contrasting modes of development and summarize recent findings that are beginning to yield important insights into deuterostome developmental mechanisms.

A zebrafish Notum homolog specifically blocks the Wnt/β-catenin signaling pathway

Multiple developmental processes require tightly controlled Wnt signaling, and its misregulation leads to congenital abnormalities and diseases. Glypicans are extracellular proteins that modulate the Wnt pathway. In addition to interacting with Wnts, these glycosophosphotidylinositol (GPI)-anchored, heparan-sulfate proteoglycans bind ligands of several other signaling pathways in both vertebrates and invertebrates. In Drosophila, Notum, a secreted α/β-hydrolase, antagonizes the signaling of the prototypical Wnt Wingless (Wg), by releasing glypicans from the cell surface. Studies of mammalian Notum indicate promiscuous target specificity in cell culture, but the role of Notum in vertebrate development has not been studied. Our work shows that zebrafish Notum 1a, an ortholog of mammalian Notum, contributes to a self-regulatory loop that restricts Wnt/β-catenin signaling. Notum 1a does not interact with Glypican 4, an essential component of the Wnt/planar cell polarity (PCP) pathway. Our results suggest a surprising specific role of Notum in the developing vertebrate embryo.