The midbrain-hindbrain phenotype of Wnt-1-/Wnt-1- mice results from stepwise deletion of engrailed-expressing cells by 9.5 days postcoitum

AP-2δ is a crucial transcriptional regulator of the posterior midbrain

Ap-2 transcription factors comprise a family of 5 closely related sequence-specific DNA binding proteins that play pivotal and non-redundant roles in embryonic organogenesis. To investigate the function of Ap-2δ, wδe analyzed its expression during embryogenesis and generated Ap-2δ-deficient mice. In line with the specific expression pattern of Ap-2δ in the mesencephalic tectum and the dorsal midbrain, Ap-2δ-deficient mice failed to maintain the colliculus inferior, a derivative of the dorsal midbrain, as a consequence of increased apoptotic cell death. To identify specific Ap-2δ target genes in cells of the developing dorsal midbrain, we performed whole genome analysis of cDNA expression levels. This approach identified a set of 12 putative target genes being expressed in the developing midbrain, including the transcription factors Pitx2, Mef2c, Bhlhb4 and Pou4f3. Using chromatin immunoprecipitation (CHIP) we showed that some of these genes are direct targets of Ap-2δ. Consistently, we demonstrate that Ap-2δ occupies and activates the Pou4f3 and Bhlhb4 promoters. In addition, known Pou4f3 target genes were downregulated in the posterior midbrain of Ap-2δ-deficient mice. Despite the absence of a central part of the auditory pathway, the presence of neuronal responses to sounds in the neocortex of Ap-2δ-deficient mice indicates that auditory information from the brainstem still reaches the neocortex. In summary, our data define Ap-2δ as an important transcription factor, specifying gene expression patterns required for the development of the posterior midbrain.

Wnt/β-catenin signalling is active in a highly dynamic pattern during development of the mouse cerebellum

The adult cerebellum is composed of several distinct cell types with well defined developmental origins. However, the molecular mechanisms that govern the generation of these cell types are only partially resolved. Wnt/β-catenin signalling has a wide variety of roles in generation of the central nervous system, though the specific activity of this pathway during cerebellum development is not well understood. Here, we present data that delineate the spatio-temporal specific pattern of Wnt/β-catenin signaling during mouse cerebellum development between E12.5 and P21. Using the BAT-gal Wnt/β-catenin reporter mouse, we found that Wnt/β-catenin activity is present transiently at the embryonic rhombic lip but not at later stages during the expansion of cell populations that arise from there. At late embryonic and early postnatal stages, Wnt/β-catenin activity shifts to the cerebellar ventricular zone and to cells arising from this germinal centre. Subsequently, the expression pattern becomes progressively restricted to Bergmann glial cells, which show expression of the reporter at P21. These results indicate a variety of potential functions for Wnt/β-catenin activity during cerebellum development.

Phenotypes in mTERT⁺/⁻ and mTERT⁻/⁻ mice are due to short telomeres, not telomere-independent functions of telomerase reverse transcriptase

Telomerase is essential for telomere length maintenance. Mutations in either of the two core components of telomerase, telomerase RNA (TR) or the catalytic protein component telomerase reverse transcriptase (TERT), cause the genetic disorders dyskeratosis congenita, pulmonary fibrosis, and other degenerative diseases. Overexpression of the TERT protein has been reported to have telomere length-independent roles, including regulation of the Wnt signaling pathway. To examine the phenotypes of TERT haploinsufficiency and determine whether loss of function of TERT has effects other than those associated with telomere shortening, we characterized both mTERT⁺/⁻ and mTERT⁻/⁻ mice on the CAST/EiJ genetic background. Phenotypic analysis showed a loss of tissue renewal capacity with progressive breeding of heterozygous mice that was indistinguishable from that of mTR-deficient mice. mTERT⁻/⁻ mice, from heterozygous mTERT⁺/⁻ mouse crosses, were born at the expected Mendelian ratio (26.5%; n = 1,080 pups), indicating no embryonic lethality of this genotype. We looked for, and failed to find, hallmarks of Wnt deficiency in various adult and embryonic tissues, including those of the lungs, kidneys, brain, and skeleton. Finally, mTERT⁻/⁻ cells showed wild-type levels of Wnt signaling in vitro. Thus, while TERT overexpression in some settings may activate the Wnt pathway, loss of function in a physiological setting has no apparent effects on Wnt signaling. Our results indicate that both TERT and TR are haploinsufficient and that their deficiency leads to telomere shortening, which limits tissue renewal. Our studies imply that hypomorphic loss-of-function alleles of hTERT and hTR should cause a similar disease spectrum in humans.

En1 and Wnt signaling in midbrain dopaminergic neuronal development

Dopaminergic neurons of the ventral mesodiencephalon are affected in significant health disorders such as Parkinson's disease, schizophrenia, and addiction. The ultimate goal of current research endeavors is to improve the clinical treatment of such disorders, such as providing a protocol for cell replacement therapy in Parkinson's disease that will successfully promote the specific differentiation of a stem cell into a dopaminergic neuronal phenotype. Decades of research on the developmental mechanisms of the mesodiencephalic dopaminergic (mdDA) system have led to the identification of many signaling pathways and transcription factors critical in its development. The unraveling of these pathways will help fill in the pieces of the puzzle that today dominates neurodevelopment research: how to make and maintain a mdDA neuron. In the present review, we provide an overview of the mdDA system, the processes and signaling molecules involved in its genesis, with a focus on the transcription factor En1 and the canonical Wnt pathway, highlighting recent findings on their relevance--and interplay--in the development and maintenance of the mdDA system.

Transmembrane protein 198 promotes LRP6 phosphorylation and Wnt signaling activation

Wnt/β-catenin signaling is fundamental in embryogenesis and tissue homeostasis in metazoans. Upon Wnt stimulation, cognate coreceptors LRP5 and LRP6 ([LRP5/6] low-density lipoprotein receptor-related proteins 5 and 6) are activated via phosphorylation at key residues. Although several kinases have been implicated, the LRP5/6 activation mechanism remains unclear. Here, we report that transmembrane protein 198 (TMEM198), a previously uncharacterized seven-transmembrane protein, is able to specifically activate LRP6 in transducing Wnt signaling. TMEM198 associates with LRP6 and recruits casein kinase family proteins, via the cytoplasmic domain, to phosphorylate key residues important for LRP6 activation. In mammalian cells, TMEM198 is required for Wnt signaling and casein kinase 1-induced LRP6 phosphorylation. During Xenopus embryogenesis, maternal and zygotic tmem198 mRNAs are widely distributed in the ectoderm and mesoderm. TMEM198 is required for Wnt-mediated neural crest formation, antero-posterior patterning, and particularly engrailed-2 expression in Xenopus embryos. Thus, our results identified TMEM198 as a membrane scaffold protein that promotes LRP6 phosphorylation and Wnt signaling activation.

Pontocerebellar hypoplasia: review of classification and genetics, and exclusion of several genes known to be important for cerebellar development

The pontocerebellar hypoplasias are a heterogeneous group of rare and devastating conditions characterized by multiple structural abnormalities of the ventral pons, inferior olive, and cerebellum. Here, we briefly review these conditions and discuss genes recently discovered to be involved in pontocerebellar hypoplasia pathogenesis. We then present data that exclude several genes important for cerebellar development as causes of pontocerebellar hypoplasia-4 and pontocerebellar hypoplasia-5, and we demonstrate that not all cases of clinically defined pontocerebellar hypoplasia-4 result from mutations in TSEN54. We conclude that classification based on clinical, imaging, and neuropathological findings does not differentiate between pontocerebellar hypoplasia subtypes with different genetic causes.

Gpr177/mouse Wntless is essential for Wnt-mediated craniofacial and brain development

We have previously demonstrated that Gpr177, the mouse orthologue of Drosophila Wls/Evi/Srt, is required for establishment of the anterior-posterior axis. The Gpr177 null phenotype is highly reminiscent to the loss of Wnt3, the earliest abnormality among all Wnt knockouts in mice. The expression of Gpr177 in various cell types and tissues lead us to hypothesize that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt-dependent developmental and pathogenic processes. Here, we create a new mouse strain permitting conditional inactivation of Gpr177. The loss of Gpr177 in the Wnt1-expressing cells causes mid/hindbrain and craniofacial defects which are far more severe than the Wnt1 knockout, but resemble the double knockout of Wnt1 and Wnt3a as well as β-catenin deletion in the Wnt1-expressing cells. Our findings demonstrate the importance of Gpr177 in Wnt1-mediated development of the mouse embryo, suggesting an overlapping function of Wnt family members in the Wnt1-expressing cells.

Stem cell potential in Parkinson's disease and molecular factors for the generation of dopamine neurons

Parkinson's disease (PD) involves the loss of dopamine (DA) neurons, making it the most expected neurodegenerative disease to be treated by cell replacement therapy. Stem cells are a promising source for cell replacement therapy due to their ability to self-renew and their pluripotency/multipotency that allows them to generate various types of cells. However, it is challenging to derive midbrain DA neurons from stem cells. Thus, in this review, I will discuss the molecular factors that are known to play critical roles in the generation and survival of DA neurons. The developmental process of DA neurons and functions of extrinsic soluble factors and homeodomain proteins, forkhead box proteins, proneural genes, Nurr1 and genes involved in epigenetic control are discussed. In addition, different types of stem cells that have potential for future cell replacement therapy are reviewed.

Wnt1-lmx1a forms a novel autoregulatory loop and controls midbrain dopaminergic differentiation synergistically with the SHH-FoxA2 pathway

Selective degeneration of midbrain dopaminergic (mDA) neurons is associated with Parkinson's disease (PD), and thus an in-depth understanding of molecular pathways underlying mDA development will be crucial for optimal bioassays and cell replacement therapy for PD. In this study, we identified a novel Wnt1-Lmx1a autoregulatory loop during mDA differentiation of ESCs and confirmed its in vivo presence during embryonic development. We found that the Wnt1-Lmx1a autoregulatory loop directly regulates Otx2 through the beta-catenin complex and Nurr1 and Pitx3 through Lmx1a. We also found that Lmx1a and Lmx1b cooperatively regulate mDA differentiation with overlapping and cross-regulatory functions. Furthermore, coactivation of both Wnt1 and SHH pathways by exogenous expression of Lmx1a, Otx2, and FoxA2 synergistically enhanced the differentiation of ESCs to mDA neurons. Together with previous works, this study shows that two regulatory loops (Wnt1-Lmx1a and SHH-FoxA2) critically link extrinsic signals to cell-intrinsic factors and cooperatively regulate mDA neuron development.

The neuropathology of autism: defects of neurogenesis and neuronal migration, and dysplastic changes

Autism is characterized by a broad spectrum of clinical manifestations including qualitative impairments in social interactions and communication, and repetitive and stereotyped patterns of behavior. Abnormal acceleration of brain growth in early childhood, signs of slower growth of neurons, and minicolumn developmental abnormalities suggest multiregional alterations. The aim of this study was to detect the patterns of focal qualitative developmental defects and to identify brain regions that are prone to developmental alterations in autism. Formalin-fixed brain hemispheres of 13 autistic (4–60 years of age) and 14 age-matched control subjects were embedded in celloidin and cut into 200-μm-thick coronal sections, which were stained with cresyl violet and used for neuropathological evaluation. Thickening of the subependymal cell layer in two brains and subependymal nodular dysplasia in one brain is indicative of active neurogenesis in two autistic children. Subcortical, periventricular, hippocampal and cerebellar heterotopias detected in the brains of four autistic subjects (31%) reflect abnormal neuronal migration. Multifocal cerebral dysplasia resulted in local distortion of the cytoarchitecture of the neocortex in four brains (31%), of the entorhinal cortex in two brains (15%), of the cornu Ammonis in four brains and of the dentate gyrus in two brains. Cerebellar flocculonodular dysplasia detected in six subjects (46%), focal dysplasia in the vermis in one case, and hypoplasia in one subject indicate local failure of cerebellar development in 62% of autistic subjects. Detection of flocculonodular dysplasia in only one control subject and of a broad spectrum of focal qualitative neuropathological developmental changes in 12 of 13 examined brains of autistic subjects (92%) reflects multiregional dysregulation of neurogenesis, neuronal migration and maturation in autism, which may contribute to the heterogeneity of the clinical phenotype.

Manipulating gene activity in Wnt1-expressing precursors of neural epithelial and neural crest cells

Targeted gene disruption or expression often encounters lethality. Conditional approaches, permitting manipulation at desired stages, are required to overcome this problem in order to analyze gene function in later developmental processes. Wnt1 has been shown to be expressed in neural crest precursors at the dorsal midline region. However, its expression was not detected in emigrated neural crest cells, the descendants of Wnt1-expressing precursors. We have developed mouse transgenic systems to manipulate gene activity in the Wnt1-expressing precursors and their derivatives by integrating the tetracycline-dependent activation and Cre-mediated recombination methods. A new Wnt1-rtTA strain, carrying rtTA under control of Wnt1 regulatory elements, has been created for gene manipulation in a spatiotemporal-specific fashion. Together with our previously developed Wnt1-Cre;R26STOPrtTA model, these systems permit conditional gene expression and ablation in pre-migratory and/or post-migratory neural crest cells. This study demonstrated the versatility of our mouse models to achieve gene manipulation in early neural development.

Requirement of prorenin receptor and vacuolar H+-ATPase-mediated acidification for Wnt signaling

Wnt/beta-catenin signaling is important in stem cell biology, embryonic development, and disease, including cancer. However, the mechanism of Wnt signal transmission, notably how the receptors are activated, remains incompletely understood. We found that the prorenin receptor (PRR) is a component of the Wnt receptor complex. PRR functions in a renin-independent manner as an adaptor between Wnt receptors and the vacuolar H+-adenosine triphosphatase (V-ATPase) complex. Moreover, PRR and V-ATPase were required to mediate Wnt signaling during antero-posterior patterning of Xenopus early central nervous system development. The results reveal an unsuspected role for the prorenin receptor, V-ATPase activity, and acidification during Wnt/beta-catenin signaling.

Spatial analysis of expression patterns predicts genetic interactions at the mid-hindbrain boundary

The isthmic organizer mediating differentiation of mid- and hindbrain during vertebrate development is characterized by a well-defined pattern of locally restricted gene expression domains around the mid-hindbrain boundary (MHB). This pattern is established and maintained by a regulatory network between several transcription and secreted factors that is not yet understood in full detail. In this contribution we show that a Boolean analysis of the characteristic spatial gene expression patterns at the murine MHB reveals key regulatory interactions in this network. Our analysis employs techniques from computational logic for the minimization of Boolean functions. This approach allows us to predict also the interplay of the various regulatory interactions. In particular, we predict a maintaining, rather than inducing, effect of Fgf8 on Wnt1 expression, an issue that remained unclear from published data. Using mouse anterior neural plate/tube explant cultures, we provide experimental evidence that Fgf8 in fact only maintains but does not induce ectopic Wnt1 expression in these explants. In combination with previously validated interactions, this finding allows for the construction of a regulatory network between key transcription and secreted factors at the MHB. Analyses of Boolean, differential equation and reaction-diffusion models of this network confirm that it is indeed able to explain the stable maintenance of the MHB as well as time-courses of expression patterns both under wild-type and various knock-out conditions. In conclusion, we demonstrate that similar to temporal also spatial expression patterns can be used to gain information about the structure of regulatory networks. We show, in particular, that the spatial gene expression patterns around the MHB help us to understand the maintenance of this boundary on a systems level.

Understanding brain formation during development is a tantalizing challenge. It is also essential for the fight against neurodegenerative diseases. In vertebrates, the central nervous system arises from a structure called the neural plate. This tissue is divided into four regions, which continue to develop into forebrain, midbrain, hindbrain and spinal cord. Interactions between locally expressed genes and signaling molecules are responsible for this patterning. Two key signaling molecules in this process are Fgf8 and Wnt1 proteins. They are secreted from a signaling center located at the boundary between prospective mid- and hindbrain (mid-hindbrain boundary, MHB) and mediate development of these two brain regions. Here, we logically analyze the spatial gene expression patterns at the MHB and predict interactions involved in the differentiation of mid- and hindbrain. In particular, our analysis indicates that Wnt1 depends on Fgf8 for stable maintenance. A time-course analysis of Wnt1 expression after implantation of Fgf8-coated beads in mouse neural plate/tube explants experimentally validates our prediction about the interactions between these two key patterning molecules. Subsequently, we demonstrate that available data allows construction of a mathematical model able to explain the maintenance of the signaling center at the MHB. We begin to understand this small aspect of brain formation on a systems level.

The development of the thalamic motor learning area is regulated by Fgf8 expression

Habenular nuclei play a key role in the control of motor and cognitive behavior, processing emotion, motivation, and reward values in the brain. Thus, analysis of the molecular and cellular mechanisms underlying the development and evolution of this region will contribute to a better understanding of brain function. The Fgf8 gene is expressed in the dorsal midline of the diencephalon, close to the area in which the habenular region will develop. Given that Fgf8 is an important morphogenetic signal, we decided to investigate the role of Fgf8 signaling in diencephalic development. To this end, we analyzed the effects of altered Fgf8 expression in the mouse embryo, using molecular and cellular markers. Decreasing Fgf8 activity in the diencephalon was found to be associated with dosage-dependent alterations in the epithalamus: the habenular region and pineal gland are reduced or lacking in Fgf8 hypomorphic mice. Actually, our findings indicate that Fgf8 may be the master gene for these diencephalic domains, acting as an inductive and morphogenetic regulator. Therefore, the emergence of the habenular region in vertebrates could be understood in terms of a phylogenetic territorial addition caused by de novo expression of Fgf8 in the diencephalic alar plate. This region specializes to permit the development of adaptive control of the motor function in the vertebrate brain.

Wnt signaling is regulated by endoplasmic reticulum retention

Precise regulation of Wnt signaling is important in many contexts, as in development of the vertebrate forebrain, where excessive or ectopic Wnt signaling leads to severe brain defects. Mutation of the widely expressed oto gene causes loss of the anterior forebrain during mouse embryogenesis. Here we report that oto is the mouse ortholog of the gpi deacylase gene pgap1, and that the endoplasmic reticulum (ER)-resident Oto protein has a novel and deacylase-independent function during Wnt maturation. Oto increases the hydrophobicities of Wnt3a and Wnt1 by promoting the addition of glycophosphatidylinositol (gpi)-like anchors to these Wnts, which results in their retention in the ER. We also report that oto-deficient embryos exhibit prematurely robust Wnt activity in the Wnt1 domain of the early neural plate. We examine the effect of low oto expression on Wnt1 in vitro by knocking down endogenous oto expression in 293 and M14 melanoma cells using shRNA. Knockdown of oto results in increased Wnt1 secretion which is correlated with greatly enhanced canonical Wnt activity. These data indicate that oto deficiency increases Wnt signaling in vivo and in vitro. Finally, we address the mechanism of Oto-mediated Wnt retention under oto-abundant conditions, by cotransfecting Wnt1 with gpi-specific phospholipase D (GPI-PLD). The presence of GPI-PLD in the secretory pathway results in increased secretion of soluble Wnt1, suggesting that the gpi-like anchor lipids on Wnt1 mediate its retention in the ER. These data now provide a mechanistic framework for understanding the forebrain defects in oto mice, and support a role for Oto-mediated Wnt regulation during early brain development. Our work highlights a critical role for ER retention in regulating Wnt signaling in the mouse embryo, and gives insight into the notoriously inefficient secretion of Wnts.

Ontogeny of substantia nigra dopamine neurons

Understanding the ontogeny of A9 dopamine (DA) neurons is critical not only to determining basic developmental events that facilitate the emergence of the substantia nigra pars compacta (SNc) but also to the extraction and de novo generation of DA neurons as a potential cell therapy for Parkinson's disease. Recent research has identified a precise window for DA cell birth (differentiation) in the ventral mesencephalon (VM) as well as a number of factors that may facilitate this process. However, application of these factors in vitro has had limited success in specifying a dopaminergic cell fate from undifferentiated cells, suggesting that other cell/molecular signals may as yet remain undiscovered. To resolve this, current work seeks to identify particularly potent and novel DA neuron differentiation factors within the developing VM specifically at the moment of ontogeny. Through such (past and present) studies, a catalog of proteins that play a pivotal role in the generation of nigral DA neurons during normal CNS development has begun to emerge. In the future, it will be crucial to continue to evaluate the critical developmental window where DA neuron ontogeny occurs, not only to facilitate our potential to protect these cells from degeneration in the adult brain but also to mimic the developmental environment in a way that enhances our ability to generate these cells anew either in vitro or in vivo. Here we review our present understanding of factors that are thought to be involved in the emergence of the A9 dopamine neuron group from the ventral mesencephalon.

Wnt-mediated self-renewal of neural stem/progenitor cells

In this work we have uncovered a role for Wnt signaling as an important regulator of stem cell self-renewal in the developing brain. We identified Wnt-responsive cells in the subventricular zone of the developing E14.5 mouse brain. Responding cell populations were enriched for self-renewing stem cells in primary culture, suggesting that Wnt signaling is a hallmark of self-renewing activity in vivo. We also tested whether Wnt signals directly influence neural stem cells. Using inhibitors of the Wnt pathway, we found that Wnt signaling is required for the efficient cloning and expansion of single-cell derived populations that are able to generate new stem cells as well as neurons, astrocytes, and oligodendrocytes. The addition of exogenous Wnt3a protein enhances clonal outgrowth, demonstrating not only a critical role for the Wnt pathway for the regulation of neurogenesis but also its use for the expansion of neural stem cells in cell culture and in tissue engineering.

Intraflagellar transport protein 172 is essential for primary cilia formation and plays a vital role in patterning the mammalian brain

IFT172, also known as Selective Lim-domain Binding protein (SLB), is a component of the intraflagellar transport (IFT) complex. In order to evaluate the biological role of the Ift172 gene, we generated a loss-of-function mutation in the mouse. The resulting Slb mutant embryos die between E12.5 and 13.0, and exhibit severe cranio-facial malformations, failure to close the cranial neural tube, holoprosencephaly, heart edema and extensive hemorrhages. Cilia outgrowth in cells of the neuroepithelium is initiated but the axonemes are severely truncated and do not contain visible microtubules. Morphological and molecular analyses revealed a global brain-patterning defect along the dorsal-ventral (DV) and anterior-posterior (AP) axes. We demonstrate that Ift172 gene function is required for early regulation of Fgf8 at the midbrain-hindbrain boundary and maintenance of the isthmic organizer. In addition, Ift172 is required for proper function of the embryonic node, the early embryonic organizer and for formation of the head organizing center (the anterior mesendoderm, or AME). We propose a model suggesting that forebrain and mid-hindbrain growth and AP patterning depends on the early function of Ift172 at gastrulation. Our data suggest that the formation and function of the node and AME in the mouse embryo relies on an indispensable role of Ift172 in cilia morphogenesis and cilia-mediated signaling.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

A role for receptor protein tyrosine phosphatase lambda in midbrain development

The mid-hindbrain boundary (MHB) harbors an important organizing center for the adjacent brain regions. Here, we present evidence that the receptor protein tyrosine phosphatase lambda (RPTPlambda) is part of the complex molecular network that maintains and shapes the MHB region. RPTPlambda is expressed in a tight band of cells in the caudal midbrain, anterior to the transverse ring of Wnt1 expression. Forced expression of RPTPlambda across the mid-hindbrain region repressed expression of Wnt1, whereas RNA interference-mediated knock-down of RPTPlambda resulted in expansion and distortion of the Wnt1 domain. When ectopically expressed in the mesencephalon, RPTPlambda specifically inhibited the induction of Wnt1 expression after subsequent stimulation with Fgf8. Reduced Wnt1 expression after RPTPlambda transfection correlated with a decrease in Ras- mitogen-activated protein kinase activity at the MHB. We further show that in the embryonic midbrain, RPTPlambda can bind to beta-catenin, a central component of the canonical Wnt signaling pathway. Overexpression of RPTPlambda suppressed the activity of a beta-catenin responsive promoter in the midbrain and reduced progenitor cell proliferation. Cotransfection of Wnt1 or of a stabilized form of beta-catenin together with RPTPlambda partially rescued the RPTPlambda-mediated proliferation defect. Together, these data suggest that RPTPlambda may play a dual role in the control of midbrain development: as a negative modulator of Fgf8-induced Wnt1 expression at the MHB, which may help to confine the Wnt1 domain to it characteristic tight ring at the MHB; and as an inhibitor of canonical Wnt signaling through interaction with and presumably sequestration of beta-catenin.

Gene targeting of CK2 catalytic subunits

Protein kinase CK2 is a highly conserved and ubiquitous serine-threonine kinase. It is a tetrameric enzyme that is made up of two regulatory CK2beta subunits and two catalytic subunits, either CK2alpha/CK2alpha, CK2alpha/CK2alpha', or CK2alpha'/CK2alpha'. Although the two catalytic subunits diverge in their C termini, their enzymatic activities are similar. To identify the specific function of the two catalytic subunits in development, we have deleted them individually from the mouse genome by homologous recombination. We have previously reported that CK2alpha' is essential for male germ cell development, and we now demonstrate that CK2alpha has an essential role in embryogenesis, as mice lacking CK2alpha die in mid-embryogenesis, with cardiac and neural tube defects.

Variation in WNT7A is unlikely to be a cause of familial congenital talipes equinovarus

Background

Genetic factors make an important contribution to the aetiology of congenital talipes equinovarus (CTEV), the most common developmental disorder of the lower limb. WNT7A was suggested as a candidate gene for CTEV on the basis of a genome-wide scan for linkage in a large multi-case family. WNT7A is a plausible candidate gene for CTEV as it provides a signal for pattern formation during limb development, and mutation in WNT7A has been reported in a number of limb malformation syndromes.

Methods

We investigated the role of WNT7A using a family-based linkage approach in our large series of European multi-case CTEV families. Three microsatellite markers were used, of which one (D3S2385) is intragenic, and the other two (D3S2403, D3S1252) are 700 kb 5' to the start and 20 kb from the 3' end of the gene, respectively. Ninety-one CTEV families, comprising 476 individuals of whom 211 were affected, were genotyped. LOD scores using recessive and incomplete-dominant inheritance models, and non-parametric linkage scores, excluded linkage.

Results

No significant evidence for linkage was observed using either parametric or non-parametric models. LOD scores for the parametric models remained strongly negative in the regions between the markers, and in the 0.5 cM intervals outside the marker map. No significant lod scores were obtained when the data were analysed allowing for heterogeneity.

Conclusion

Our evidence suggests that the WNT7A gene is unlikely to be a major contributor to the aetiology of familial CTEV.

Desire, disease, and the origins of the dopaminergic system

The dopaminergic neurons in the midbrain region of the central nervous system project an extensive network of connections throughout the forebrain, including the neocortex. The midbrain-forebrain dopaminergic circuits are thought to regulate a diverse set of behaviors, from the control of movement to modulation of cognition and desire--because they relate to mood, attention, reward, and addiction. Defects in these pathways, including neurodegeneration, are implicated in a variety of psychiatric and neurological diseases, such as schizophrenia, attention-deficit/hyperactivity disorder, drug addiction, and Parkinson disease. Based on the importance of the midbrain dopaminergic neurons to normal and pathological brain function, there is considerable interest in the molecular mechanisms that regulate their development. The goal of this short review is to outline new methods and recent advances in identifying the molecular networks that regulate midbrain dopaminergic neuron differentiation and fate. Midbrain dopaminergic neurons are descended from progenitor cells located near the ventral midline of the neural tube floor plate around the cephalic flexure. It is now clear that their initial formation is dependent on interactions between the signaling molecules Sonic hedgehog, WINGLESS 1, and FIBROBLAST growth factor 8, but there is still an extensive wider network of molecular interactions that must be resolved before the complete picture of dopaminergic neuron development can be described.

Six3 inactivation causes progressive caudalization and aberrant patterning of the mammalian diencephalon

The homeobox gene Six3 represses Wnt1 transcription. It is also required in the anterior neural plate for the development of the mammalian rostral forebrain. We have now determined that at the 15- to 17-somite stage, the prospective diencephalon is the most-anterior structure in the Six3-null brain, and Wnt1 expression is anteriorly expanded. Consequently, the brain caudalizes, and at the 22- to 24-somite stage, the prospective thalamic territory is the most-anterior structure. At around E11.0, the pretectum replaces this structure. Analysis of Six3;Wnt1 double-null mice revealed that Six3-mediated repression of Wnt1 is necessary for the formation of the rostral diencephalon and that Six3 activity is required for the formation of the telencephalon. These results provide insight into the mechanisms that establish anteroposterior identity in the developing mammalian brain.

Morphology, molecular codes, and circuitry produce the three-dimensional complexity of the cerebellum

The most noticeable morphological feature of the cerebellum is its folded appearance, whereby fissures separate its anterior-posterior extent into lobules. Each lobule is molecularly coded along the medial-lateral axis by parasagittal stripes of gene expression in one cell type, the Purkinje cells (PCs). Additionally, within each lobule distinct combinations of afferents terminate and supply the cerebellum with synchronized sensory and motor information. Strikingly, afferent terminal fields are organized into parasagittal domains, and this pattern bears a close relationship to PC molecular coding. Thus, cerebellum three-dimensional complexity obeys a basic coordinate system that can be broken down into morphology and molecular coding. In this review, we summarize the sequential stages of cerebellum development that produce its laminar structure, foliation, and molecular organization. We also introduce genes that regulate morphology and molecular coding, and discuss the establishment of topographical circuits within the context of the two coordinate systems. Finally, we discuss how abnormal cerebellar organization may result in neurological disorders like autism.

The derivatives of the Wnt3a lineage in the central nervous system

The dorsal midline of the vertebrate neural tube is a source of signals that direct cell fate specification and proliferation. Using genetic fate mapping in the mouse and a previously generated Wnt3aCre line, we report here that genetically labeled cells of the Wnt3a lineage migrate widely from the dorsal midline into the dorsal half of the adult brain and spinal cord, contributing to diverse structures in the diencephalon, midbrain, and brainstem and extensively populating the rostral spinal cord. Conspicuously, many of these structures are linked in specific functional networks. Wnt3a lineage cells populate nuclei of the central auditory system from the medulla to thalamus, and the trigeminal sensory system from the cervical spinal cord to the midbrain. Our findings reveal the rich contributions of the Wnt3a lineage to a variety of brain structures and show that functionally integrated nuclei can share a molecular identity, provided by transient gene expression early in their development.

Origin of climbing fiber neurons and their developmental dependence on Ptf1a

Climbing fiber (CF) neurons in the inferior olivary nucleus (ION) extend their axons to Purkinje cells, playing a crucial role in regulating cerebellar function. However, little is known about their precise place of birth and developmental molecular machinery. Here, we describe the origin of the CF neuron lineage and the involvement of Ptf1a (pancreatic transcription factor 1a) in CF neuron development. Ptf1a protein was found to be expressed in a discrete dorsolateral region of the embryonic caudal hindbrain neuroepithelium. Because expression of Ptf1a is not overlapping other transcription factors such as Math1 (mouse atonal homolog 1) and Neurogenin1, which are suggested to define domains within caudal hindbrain neuroepithelium (Landsberg et al., 2005), we named the neuroepithelial region the Ptf1a domain. Analysis of mice that express beta-galactosidase from the Ptf1a locus revealed that CF neurons are derived from the Ptf1a domain. In contrast, retrograde labeling of precerebellar neurons indicated that mossy fiber neurons are not derived from Ptf1a-expressing progenitors. We could observe a detailed migratory path of CF neurons from the Ptf1a domain to the ION during embryogenesis. In Ptf1a null mutants, putative immature CF neurons produced from this domain were unable to migrate or differentiate appropriately, resulting in a failure of ION formation. Apoptotic cells were observed in the mutant hindbrain. Furthermore, the fate of some cells in the Ptf1a lineage were changed to mossy fiber neurons in Ptf1a null mutants. These findings clarify the precise origin of CF neurons and suggest that Ptf1a controls their fate, survival, differentiation, and migration during development.

The alpha catalytic subunit of protein kinase CK2 is required for mouse embryonic development

Protein kinase CK2 (formerly casein kinase II) is a highly conserved and ubiquitous serine/threonine kinase that is composed of two catalytic subunits (CK2alpha and/or CK2alpha') and two CK2beta regulatory subunits. CK2 has many substrates in cells, and key roles in yeast cell physiology have been uncovered by introducing subunit mutations. Gene-targeting experiments have demonstrated that in mice, the CK2beta gene is required for early embryonic development, while the CK2alpha' subunit appears to be essential only for normal spermatogenesis. We have used homologous recombination to disrupt the CK2alpha gene in the mouse germ line. Embryos lacking CK2alpha have a marked reduction in CK2 activity in spite of the presence of the CK2alpha' subunit. CK2alpha(-/-) embryos die in mid-gestation, with abnormalities including open neural tubes and reductions in the branchial arches. Defects in the formation of the heart lead to hydrops fetalis and are likely the cause of embryonic lethality. Thus, CK2alpha appears to play an essential and uncompensated role in mammalian development.

TCF4 deficiency expands ventral diencephalon signaling and increases induction of pituitary progenitors

The anterior and intermediate lobes of the pituitary gland are formed from Rathke's pouch. FGF, BMP and WNT signals emanating from the ventral diencephalon influence pouch growth and development. In order to examine the role of canonical WNT signaling during pituitary development we examined the pituitary expression of the TCF/LEF family of transcription factors, which mediate WNT signaling through the binding of beta-catenin. We report here the expression of several members of this family during pituitary development and the functional role of one member, TCF4 (TCF7L2), in the induction of the pituitary primordium. TCF4 is expressed in the ventral diencephalon early in pituitary development, rostral to a domain of BMP and FGF expression. Tcf4 deficient mice express Fgf10 and Bmp4; however, the Bmp and Fgf expression domains are expanded rostrally. As a result, additional pituitary progenitor cells are recruited into Rathke's pouch in Tcf4 mutants. Mutants also exhibit an expansion of the Six6 expression domain within Rathke's pouch, which may increase the number of proliferating pouch cells, resulting in a greatly enlarged anterior pituitary gland. This suggests that TCF4 negatively regulates pituitary growth through two mechanisms. The first mechanism is to restrict the domains of BMP and FGF signaling in the ventral diencephalon, and the second mechanism is the restriction of Six6 within Rathke's pouch. Thus, TCF4 is necessary both intrinsically and extrinsically to Rathke's pouch to ensure the proper growth of the pituitary gland.

Distinct but redundant expression of the Frizzled Wnt receptor genes at signaling centers of the developing mouse brain

The establishment of the regional subdivisions of the vertebrate CNS is accomplished through the activity of different neuroepithelial organizing centers. The wingless/int (Wnt) family of secreted glycoproteins, among other factors, plays a crucial role in signaling from these centers. Wnt1 secreted from the boundary between the mid- and hindbrain, for instance, controls the development of this brain region and of associated neuronal populations. Different Wnts secreted from the caudomedial pallium, the cortical hem, pattern the adjacent hippocampal field. The first step in Wnt signal transduction is binding of the Wnt ligand to its receptors, the seven-pass transmembrane Frizzled proteins. Inactivation of different Frizzled genes in mice have revealed an extensive functional redundancy between these receptors. In order to discriminate between a possible participation of different Frizzled receptors in the transduction of Wnt signals at the mid-/hindbrain boundary and the cortical hem, we have performed a detailed expression study of the 10 known murine Frizzled genes at crucial stages of mouse embryonic development. Our analysis reveals a highly dynamic yet distinct expression pattern of individual Frizzled genes in the anterior neural tube of the developing mouse embryo. The overlapping spatio-temporal expression of at least two and up to six Frizzled genes in any region of the developing mouse brain, however, also suggests a vast functional redundancy of the murine Frizzled receptors. This redundancy has to be taken into consideration for future analyses of Frizzled receptor function at these signaling centers in the mouse.

FGF signalling pathways in development of the midbrain and anterior hindbrain

Development of the central nervous system is coordinated by intercellular signalling centres established within the neural tube. The isthmic organizer (IsO), located between the midbrain and anterior hindbrain, is one such centre. Important signal molecules secreted by the IsO include members of the fibroblast growth factor and Wnt families. These signals are integrated with dorsally and ventrally derived signals to regulate development of the midbrain and rhombomere 1 of the hindbrain. The IsO is operational for a remarkably long period of time. Depending on the developmental stage, it controls a variety of processes such as cell survival, cell identity, neural precursor proliferation, neuronal differentiation and axon guidance. This review focuses on the fibroblast growth factor signalling, its novel molecular regulatory mechanisms and how this pathway regulates multiple aspects of cell behaviour in the developing midbrain and anterior hindbrain.

A role for Connexin43 during neurodevelopment

Connexin43 (Cx43) is the predominant gap junction protein expressed in premitotic radial glial cells and mature astrocytes. It is thought to play a role in many aspects of brain development and physiology, including intercellular communication, the release of neuroactive substances, and neural and glial proliferation and migration. To investigate the role of Cx43 in brain physiology, we generated a conditional knockout (cKO) mouse expressing Cre recombinase driven by the human GFAP promoter and a floxed Cx43 gene. The removal of Cx43 from GFAP-expressing cells affects the behavior of the mice and the development of several brain structures; however, the severity of the phenotype varies depending on the mouse background. One mouse subline, hereafter termed Shuffler, exhibits cellular disorganization of the cortex, hippocampus, and cerebellum, accompanied by ataxia and motor deficits. The Shuffler cerebellum is most affected and displays altered distribution and lamination of glia and neurons suggestive of cell migration defects. In all Shuffler mice by postnatal day two (P2), the hippocampus, cortex, and cerebellum are smaller. Disorganization of the ventricular and subventricular zone of the cortex is also evident. Given that these are sites of early progenitor cell proliferation, we suspect production and migration of neural progenitors may be altered. In conclusion, neurodevelopment of Shuffler/Cx43 cKO mice is abnormal, and the observed cellular phenotype may explain behavioral disturbances seen in these animals as well as in humans carrying Cx43 mutations.

Sustained interactive Wnt and FGF signaling is required to maintain isthmic identity

Fibroblast growth factor 8 (FGF8) is expressed at the mid-hindbrain boundary and is an important signal emanating from the isthmic organizer. Wnt1 is expressed in the caudal midbrain juxtaposed to Fgf8 expression and has been implicated in its regulation. In this study, we examine the requirement for continuous Wnt signaling in the maintenance of Fgf8 expression at the isthmus. We demonstrate that prior to HH10, ongoing Wnt signaling is required to maintain the normal pattern of isthmic Fgf8 expression in ovo. Similarly, in explant assays, sustained Wnt signaling is essential to maintain Fgf8 expression in rhombomere 1. The mechanism by which Wnt signaling regulates isthmic Fgf8 expression is likely to be a maintenance response rather than an inductive effect. Finally, we show that Wnt maintenance of Fgf8 expression is dependent upon positive feedback by FGF signaling itself, and that rhombomere 1 does not receive instructive cues from the posterior hindbrain. In summary, these findings establish that a sustained reciprocal interaction between Wnt and FGF signaling is essential to maintain isthmic identity.

Wnt signaling is required for antero-posterior patterning of the planarian brain

Wnt signaling functions in axis formation and morphogenesis in various animals and organs. Here we report that Wnt signaling is required for proper brain patterning during planarian brain regeneration. We showed here that one of the Wnt homologues in the planarian Dugesia japonica, DjwntA, was expressed in the posterior region of the brain. When DjwntA-knockdown planarians were produced by RNAi, they could regenerate their heads at the anterior ends of the fragments, but formed ectopic eyes with irregular posterior lateral branches and brain expansion. This suggests that the Wnt signal may be involved in antero-posterior (A-P) patterning of the planarian brain, as in vertebrates. We also investigated the relationship between the DjwntA and nou-darake/FGFR signal systems, as knockdown planarians of these genes showed similar phenotypes. Double-knockdown planarians of these genes did not show any synergistic effects, suggesting that the two signal systems function independently in the process of brain regeneration, which accords with the fact that nou-darake was expressed earlier than DjwntA during brain regeneration. These observations suggest that the nou-darake/FGFR signal may be involved in brain rudiment formation during the early stage of head regeneration, and subsequently the DjwntA signal may function in A-P patterning of the brain rudiment.

How to make a mesodiencephalic dopaminergic neuron

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

Wnt/β-catenin signaling regulates expression of PRDC, an antagonist of the BMP-4 signaling pathway

To identify novel target genes regulated by Wnt/beta-catenin signaling, microarray analysis was performed with L929 fibroblast cells. In this report, we show that the expression of PRDC (Protein Related to DAN and Cerberus), previously known as an antagonist of Nodal, BMP (bone morphogenetic protein), and Wnt signals, is specifically induced by Wnt/beta-catenin signaling, and also show that the transcriptional activation is mediated in a Tcf/LEF-independent manner. The PRDC induced by Wnt/beta-catenin does not inhibit Wnt signaling, but does inhibit BMP-4 signaling. The inhibition of BMP-4-induced reporter activity by the treatment of conditioned media from beta-catenin(S37A)-expressing cells suggests that the PRDC induced by beta-catenin is secreted outside of cells and antagonizes BMP-4 signaling. We propose that PRDC might serve as a mediator to antagonize BMP-4 signaling by Wnt.

Pbx proteins cooperate with Engrailed to pattern the midbrain-hindbrain and diencephalic-mesencephalic boundaries

Pbx proteins are a family of TALE-class transcription factors that are well characterized as Hox co-factors acting to impart segmental identity to the hindbrain rhombomeres. However, no role for Pbx in establishing more anterior neural compartments has been demonstrated. Studies done in Drosophila show that Engrailed requires Exd (Pbx orthologue) for its biological activity. Here, we present evidence that zebrafish Pbx proteins cooperate with Engrailed to compartmentalize the midbrain by regulating the maintenance of the midbrain-hindbrain boundary (MHB) and the diencephalic-mesencephalic boundary (DMB). Embryos lacking Pbx function correctly initiate midbrain patterning, but fail to maintain eng2a, pax2a, fgf8, gbx2, and wnt1 expression at the MHB. Formation of the DMB is also defective as shown by a caudal expansion of diencephalic epha4a and pax6a expression into midbrain territory. These phenotypes are similar to the phenotype of an Engrailed loss-of-function embryo, supporting the hypothesis that Pbx and Engrailed act together on a common genetic pathway. Consistent with this model, we demonstrate that zebrafish Engrailed and Pbx interact in vitro and that this interaction is required for both the eng2a overexpression phenotype and Engrailed's role in patterning the MHB. Our data support a novel model of midbrain development in which Pbx and Engrailed proteins cooperatively pattern the mesencephalic region of the neural tube.

Lmx1bis essential forFgf8andWnt1expression in the isthmic organizer during tectum and cerebellum development in mice

Secreted factors FGF8 and WNT1 are essential either for the inductive activity of the isthmus organizer or for the regionalization of the midbrain-hindbrain boundary (MHB). However, transcriptional regulation of these secreted factors during development remains to be elucidated. Here we show that the LIM homeobox gene Lmx1b is expressed in the anterior embryo as early as E7.5 and its expression becomes progressively restricted to the isthmus at E9.0. Analysis of gene expression in the MHB of the mutant embryos showed that many genes were lost by E9.5. In the MHB of Lmx1b-/- embryos, the expression of Fgf8, which normally occurs at the 4-somite stage, was completely absent, whereas Wnt1 was downregulated before the 4-somite stage. Moreover,transcription factors En1 and Pax2 were also downregulated prior to the 4-somite stage, whereas Gbx2 downregulation occurred at the 4-somite stage. By contrast, Otx2 and Pax6 expression was not affected in Lmx1b-/- embryos. The requirement of specific Lmx1b expression in the MHB was further confirmed by Wnt1-Cre-mediated region-specific conditional knockout of Lmx1b. As a result of these molecular defects, the development of the tectum and cerebellum was severely impaired in Lmx1b-/-mice. Taken together, our results indicate that Lmx1b plays an essential role in the development of the tectum and cerebellum by regulating expression of Fgf8, Wnt1 and several isthmus-related transcription factors in the MHB, and is a crucial component of a cross-regulatory network required for the induction activity of the isthmic organizer in the MHB.

The ups and downs of Wnt signaling in prevalent neurological disorders

In order to function properly, the brain must be wired correctly during critical periods in early development. Mistakes in this process are hypothesized to occur in disorders like autism and schizophrenia. Later in life, signaling pathways are essential in maintaining proper communication between neuronal and non-neuronal cells, and disrupting this balance may result in disorders like Alzheimer's disease. The Wnt/beta-catenin pathway has a well-established role in cancer. Here, we review recent evidence showing the involvement of Wnt/beta-catenin signaling in neurodevelopment as well as in neurodegenerative diseases. We suggest that the onset/development of such pathological conditions may involve the additive effect of genetic variation within Wnt signaling components and of molecules that modulate the activity of this signaling cascade.

Role of retinoic acid in the differentiation of embryonal carcinoma and embryonic stem cells

Retinoic acid (RA), the most potent natural form of vitamin A, plays an important role in many diverse biological processes such as embryogenesis and cellular differentiation. This chapter is a review of the mechanism of action of RA and the role of specific RA-regulated genes during the cellular differentiation of embryonal carcinoma (EC) and embryonic stem (ES) cells. RA acts by binding to its nuclear receptors and inducing transcription of specific target genes. The most studied mouse EC cell lines include F9 cells, which can be induced by RA to differentiate into primitive, parietal, and visceral endodermal cells; and P19 cells, which can differentiate to endodermal and neuronal cells upon RA treatment. ES cells can be induced to differentiate into a number of different cell types; many of which require RA treatment. Over the years, many RA-regulated genes have been discovered in EC and ES cells using a diverse set of techniques. Current research focuses on the elucidation how these genes affect differentiation in EC and ES cells using a variety of molecular biology approaches. However, the exact molecule events that lead from a pluripotent stem cell to a fully differentiated cell following RA treatment are yet to be determined.

Impaired neural development caused by inducible expression of Axin in transgenic mice

Ablations of the Axin family genes demonstrated that they modulate Wnt signaling in key processes of mammalian development. The ubiquitously expressed Axin1 plays an important role in formation of the embryonic neural axis, while Axin2 is essential for craniofacial skeletogenesis. Although Axin2 is also highly expressed during early neural development, including the neural tube and neural crest, it is not essential for these processes, apparently due to functional redundancy with Axin1. To further investigate the role of Wnt signaling during early neural development, and its potential regulation by Axins, we developed a mouse model for conditional gene activation in the Axin2-expressing domains. We show that gene expression can be successfully targeted to the Axin2-expressing cells in a spatially and temporally specific fashion. High levels of Axin in this domain induce a region-specific effect on the patterning of neural tube. In the mutant embryos, only the development of midbrain is severely impaired even though the transgene is expressed throughout the neural tube. Axin apparently regulates beta-catenin in coordinating cell cycle progression, cell adhesion and survival of neuroepithelial precursors during development of ventricles. Our data support the conclusion that the development of embryonic neural axis is highly sensitive to the level of Wnt signaling.

Neural subtype specification from embryonic stem cells

One of the keys to using embryonic stem cells (ESCs) in brain research and potential application in neurological diseases is directed differentiation of neuronal and glial subtypes. This may be achieved by application of developmental principles in guiding cell lineage specification from naïve stem cells. Establishment of defined ESC differentiation models that recapitulate in vivo development, especially from human ESCs, will most likely provide a dynamic tool for dissecting molecular mechanisms underlying early embryonic development that is otherwise not readily obtainable. This is also a rational and realistic way of producing enriched populations of functional neurons and glia for pathological analyses as well as possible therapeutic applications.

The cephalic neural crest exerts a critical effect on forebrain and midbrain development

Encephalisation is the most important characteristic in the evolutionary transition leading from protochordates to vertebrates. This event has coincided with the emergence of a transient and pluripotent structure, the neural crest (NC), which is absent in protochordates. In vertebrates, NC provides the rostral cephalic vesicles with skeletal protection and functional vascularization. The surgical extirpation of the cephalic NC, which is responsible for building up the craniofacial skeleton, results in the absence of facial skeleton together with severe defects of preotic brain development, leading to exencephaly. Here, we have analyzed the role of the NC in forebrain and midbrain development. We show that (i) NC cells (NCC) control Fgf8 expression in the anterior neural ridge, which is considered the prosencephalic organizer; (ii) the cephalic NCC are necessary for the closure of the neural tube; and (iii) NCC contribute to the proper patterning of genes that are expressed in the prosencephalic and mesencephalic alar plate. Along with the development of the roof plate, NCC also concur to the patterning of the pallial and subpallial structures. We show that the NC-dependent production of FGF8 in anterior neural ridge is able to restrict Shh expression to the ventral prosencephalon. All together, these findings support the notion that the cephalic NC controls the formation of craniofacial structures and the development of preotic brain.