Opposing FGF and Retinoid Pathways Control Ventral Neural Pattern, Neuronal Differentiation, and Segmentation during Body Axis Extension

A clock and wavefront mechanism for somite formation

Somitogenesis, the sequential formation of a periodic pattern along the antero-posterior axis of vertebrate embryos, is one of the most obvious examples of the segmental patterning processes that take place during embryogenesis and also one of the major unresolved events in developmental biology. In this article, we develop a mathematical formulation of a new version of the Clock and Wavefront model proposed by Pourquié and co-workers (Dubrulle, J., McGrew, M.J., Pourquié, O., 2001. FGF signalling controls somite boundary position and regulates segmentation clock control of spatiotemporal Hox gene activation. Cell 106, 219-232). Dynamic expression of FGF8 in the presomitic mesoderm constitutes the wavefront of determination which sweeps along the body axis interacting as it moves with the segmentation clock to gate cells into somites. We also show that the model can mimic the anomalies formed when progression of the wavefront is disturbed and make some experimental predictions that can be used to test the hypotheses underlying the model.

Involvement of SIP1 in positioning of somite boundaries in the mouse embryo

Periodical production of somites provides an excellent model system for understanding genesis of metameric structures underlying embryonic development. This study reports production of somites with roughly half rostro-caudal length in homozygous Sip1 (Smad-interacting protein 1) knockout mouse embryos. This altered periodicity of somitogenesis is caused by the rostral expansion of the expression domain of genes involved in the maintenance of unsegmented state of paraxial mesoderm, e.g., Fgf8, Wnt3a, Dll3, and Tbx6. This is accompanied by the rostral extension of oscillatory gene expression such as L-fng, Hes7, and Dll1, and the rostrally shifted termination of Raldh2 expression that continues from the anterior embryonic side. The phenotype of Sip1-/- embryo introduces a new molecular component SIP1 in positioning of somite boundaries, and provides support for the current "clock and wavefront" model.

Retinoic-acid signalling in node ectoderm and posterior neural plate directs left-right patterning of somitic mesoderm

Somitogenesis requires bilateral rhythmic segmentation of paraxial mesoderm along the antero-posterior axis. The location of somite segmentation depends on opposing signalling gradients of retinoic acid (generated by retinaldehyde dehydrogenase-2; Raldh2) anteriorly and fibroblast growth factor (FGF; generated by Fgf8) posteriorly. Retinoic-acid-deficient embryos exhibit somite left-right asymmetry, but it remains unclear how retinoic acid mediates left-right patterning. Here, we demonstrate that retinoic-acid signalling is uniform across the left-right axis and occurs in node ectoderm but not node mesoderm. In Raldh2(-/-) mouse embryos, ectodermal Fgf8 expression encroaches anteriorly into node ectoderm and neural plate, but its expression in presomitic mesoderm is initially unchanged. The late stages of somitogenesis were rescued in Raldh2(-/-) mouse embryos when the maternal diet was supplemented with retinoic acid until only the 6-somite stage, demonstrating that retinoic acid is only needed during node stages. A retinoic-acid-reporter transgene marking the action of maternal retinoic acid in rescued Raldh2(-/-) embryos revealed that the targets of retinoic-acid signalling during somitogenesis are the node ectoderm and the posterior neural plate, not the presomitic mesoderm. Our findings suggest that antagonism of Fgf8 expression by retinoic acid occurs in the ectoderm and that failure of this mechanism generates excessive FGF8 signalling to adjacent mesoderm, resulting initially in smaller somites and then left-right asymmetry.

Retinoic acid signaling is essential for formation of the heart tube in Xenopus

Retinoic acid is clearly important for the development of the heart. In this paper, we provide evidence that retinoic acid is essential for multiple aspects of cardiogenesis in Xenopus by examining embryos that have been exposed to retinoic acid receptor antagonists. Early in cardiogenesis, retinoic acid alters the expression of key genes in the lateral plate mesoderm including Nkx2.5 and HAND1, indicating that early patterning of the lateral plate mesoderm is, in part, controlled by retinoic acid. We found that, in Xenopus, the transition of the heart from a sheet of cells to a tube required retinoic acid signaling. The requirement for retinoic acid signaling was determined to take place during a narrow window of time between embryonic stages 14 and 18, well before heart tube closure. At the highest doses used, the lateral fields of myocardium fail to fuse, intermediate doses lead to a fusion of the two sides but failure to form a tube, and embryos exposed to lower concentrations of antagonist form a heart tube that failed to complete all the landmark changes that characterize looping. The myocardial phenotypes observed when exposed to the retinoic acid antagonist resemble the myocardium from earlier stages of cardiogenesis, although precocious expression of cardiac differentiation markers was not seen. The morphology of individual cells within the myocardium appeared immature, closely resembling the shape and size of cells at earlier stages of development. However, the failures in morphogenesis are not merely a slowing of development because, even when allowed to develop through stage 40, the heart tubes did not close when embryos were exposed to high levels of antagonist. Indeed, some aspects of left-right asymmetry also remained even in hearts that never formed a tube. These results demonstrate that components of the retinoic acid signaling pathway are necessary for the progression of cardiac morphogenesis in Xenopus.

Induction of neuronal differentiation of adult human olfactory neuroepithelial-derived progenitors

Neurosphere forming cells (NSFCs) have been established from cultures of adult olfactory neuroepithelium obtained from patients and cadavers as described previously. They remained undifferentiated in serum or defined media with or without neurotrophic factors. Many factors affect the differentiation of stem cells along a neuronal pathway. Retinoic acid (RA), forskolin (FN), and sonic hedgehog (Shh) have been reported to act as growth promoters during neurogenesis of embryonic CNS in vivo. The effect of RA, FN, and Shh on NSFCs' neuronal lineage restriction has not been described. The application of RA, FN, and Shh to NSFCs induced the expression of motoneuronal transcription factors, tyrosine hydroxylase, an indicator of dopamine production, and neurite formation. These studies further heighten the potential for using olfactory neuroepithelial progenitors for future autologous cell replacement strategies in neurodegenerative conditions and trauma as well as for use in diagnostic evaluation.

Role of Transcription Factors in Motoneuron Differentiation of Adult Human Olfactory Neuroepithelial-Derived Progenitors

Neurosphereforming cell (NSFC) lines have been established from cultures of human adult olfactory neuroepithelium. Few of these cells ever express mature neuronal or glial markers in minimal essential medium supplemented with 10% fetal bovine serum or defined medium. However, these neural progenitors have the potential to differentiate along glial or neuronal lineages. To evaluate the potential of NSFCs to form motoneurons, transcription factors Olig2, Ngn2, and HB9 were introduced into NSFCs to determine if their expression is sufficient for motoneuron specification and differentiation, as has been shown in the early development of the avian and murine central nervous systems in vivo. NSFCs transfected with Olig2, Ngn2, and HB9 alone exhibited no phenotypic lineage restriction. In contrast, simultaneous transfection of Ngn2 and HB9 cDNA increased the expression of Isl1/2, a motoneuron marker, when the cells were maintained in medium supplemented with retinoic acid, forskolin, and sonic hedgehog. Furthermore, a population of Olig2-expressing NSFCs also expressed Ngn2. Cotransfection of NSFCs with Olig2 and HB9, but not Olig2 and Ngn2, increased Isl1/2 expression. Coculture of NSFCs trans-fected with Ngn2-HB92 or Olig2 and HB9 with purified chicken skeletal muscle demonstrated frequent contacts that resembled neuromuscular junctions. These studies demonstrate that transcription factors governing the early development of chick and mouse motoneuron formation are able to drive human adult olfactory neuroepithelial progenitors to differentiate into motoneurons in vitro. Our long-term goal is to develop cell populations for future studies of the therapeutic utility of these olfactory-derived NSFCs for autologous cell replacement strategies for central nervous system trauma and neurodegenerative diseases.

Retinoic acid activates myogenesis in vivo through Fgf8 signalling

Retinoic acid (RA) has been shown to regulate muscle differentiation in vitro. Here, we have investigated the role of RA signalling during embryonic myogenesis in zebrafish. We have altered RA signalling from gastrulation stages onwards by either inhibiting endogenous RA synthesis using an inhibitor of retinaldehyde dehydrogenases (DEAB) or by addition of exogenous RA. DEAB reduces expression of the myogenic markers myoD and myogenin in somites, whereas RA induces increased expression of these genes and strongly induces premature myoD expression in the presomitic mesoderm (psm). The expression dynamics of myf5 in presomitic and somitic mesoderm suggest that RA promotes muscle differentiation, a role supported by the fact that RA activates expression of fast myosin, while DEAB represses it. We identify Fgf8 as a major relay factor in RA-mediated activation of myogenesis. We show that fgf8 expression in somites and anterior psm is regulated by RA, and find that in the absence of Fgf8 signalling in the acerebellar mutant RA fails to promote myoD expression. We propose that, in the developing embryo, localised synthesis of RA by Raldh2 in the anterior psm and in somites activates fgf8 expression which in turn induces the expression of myogenic genes and fast muscle differentiation.

Retinoic acid and hindbrain patterning

Retinoid signaling plays an important role in the developmental patterning of the hindbrain. Studies of the teratogenic effects of retinoids showed early on that the hindbrain suffered patterning defects in cases of retinoid excess or deficiency. Closer examination of these effects in animal models suggested that retinoids might play a physiological role in specifying the antero-posterior axis of the hindbrain. This idea was supported by the localization of retinoid synthetic and degradative enzymes, binding proteins, and receptors to the hindbrain and neighboring regions of the neuroepithelium and the mesoderm. In parallel, it became clear that the molecular patterning of the hindbrain, in terms of the regionalized expression of Hox genes and other developmental regulatory genes, is profoundly influenced by retinoid signaling.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

Dorsal-ventral patterning: a view from the top

The generation of dorsal interneurons in the spinal cord is dependent upon specific signaling pathways and the subsequent establishment of progenitor domains mediated by cross-repressive interactions of different groups of transcription factors. These events lead to the implementation of specific differentiation programs that direct the development of distinct dorsal interneuron subtypes. Recent studies have taken advantage of complementary gain and loss-of-function studies in the chick and mouse to clarify the in vivo roles of transforming growth factor beta signaling, basic helix-loop-helix and homeodomain transcription factors in dorsal interneuron development. The challenge now lies in identifying the precise molecular mechanisms involved and applying these insights to understanding how more ventrally located dorsal interneurons are specified.

FGF-dependent Notch signaling maintains the spinal cord stem zone

Generation of the spinal cord relies on proliferation of undifferentiated cells located in a caudal stem zone. Although fibroblast growth factor (FGF) signaling is required to maintain this cell group, we do not know how it controls cell behavior in this context. Here we characterize an overlooked expression domain of the Notch ligand, Delta1, in the stem zone and demonstrate that this constitutes a proliferative cell group in which Notch signaling is active. We show that FGF signaling is required for expression of the proneural gene cash4 in the stem zone, which in turn induces Delta1. We further demonstrate that Notch signaling is required for cell proliferation within the stem zone; however, it does not regulate cell movement out of this region, nor is loss of Notch signaling sufficient to drive neuronal differentiation within this tissue. These data identify a novel role for the Notch pathway during vertebrate neurogenesis in which signaling between high Delta1-expressing cells maintains the neural precursor pool that generates the spinal cord. Our findings also suggest a mechanism for the establishment of the cell selection process, lateral inhibition: Mutual inhibition between Delta/Notch-expressing stem zone cells switches to single Delta1-presenting neurons as FGF activity declines in the newly formed neuroepithelium.

Phosphorylation of the Brn-3a transcription factor is modulated during differentiation and regulates its functional activity

Brn-3a is a transcription factor expressed in a subset of neurons of the peripheral nervous system. Its role encompasses the activation of genes involved in neuronal differentiation and survival. While a lot of data have been produced on Brn-3a target promoters, very little is known about the upstream regulatory signals that mediate its activation in response to differentiation. In this work, we describe for the first time that Brn-3a is phosphorylated in IMR-32 neuroblastoma cells in response to differentiation induced by retinoic acid treatment and that its post-translational modification is potentially mediated by the activation of the MAPK/ERK pathway. Furthermore, we show that the mutation of a putative phosphorylated amino acid strongly reduces the ability of Brn-3a to mediate the differentiation of IMR-32 cells.

Morphogens in motion: growth control of the neural tube

The entire vertebrate nervous system develops from a simple epithelial sheet, the neural plate which, along development, acquires the large number and wide variety of neuronal cell types required for the construction of a functional mature nervous system. These include processes of growth and pattern formation of the neural tube that are achieved through complicated and tightly regulated genetic interactions. Pattern formation, particularly in the vertebrate central nervous system, is one of the best examples of a morphogen-type of function. Cell cycle progression, however, is generally accepted to be dependent on cell-intrinsic factors. Recent studies have demonstrated that proliferation of neural precursors is also somehow controlled by secreted signaling molecules, well-known by their role as morphogens, such as fibroblast growth factor (FGF), vertebrate orthologs of the Drosophila wingless (Wnt), hedgehog (Hh), and transforming growth factor beta (TGF-beta) families, that in turn regulate the activity of factors controlling cell cycle progression. In this review we will summarize the experimental data that support the idea that classical morphogens can be reused to regulate proliferation of neural precursors.

tortuga refines Notch pathway gene expression in the zebrafish presomitic mesoderm at the post-transcriptional level

We have identified the zebrafish tortuga (tor) gene by an ENU-induced mutation that disrupts the presomitic mesoderm (PSM) expression of Notch pathway genes. In tor mutants, Notch pathway gene expression persists in regions of the PSM where expression is normally off in wild type embryos. The expression of hairy/Enhancer of split-related 1 (her1) is affected first, followed by the delta genes deltaC and deltaD, and finally, by another hairy/Enhancer of split-related gene, her7. In situ hybridization with intron-specific probes for her1 and deltaC indicates that transcriptional bursts of expression are normal in tor mutants, suggesting that tor normally functions to refine her1 and deltaC message levels downstream of transcription. Despite the striking defects in Notch pathway gene expression, somite boundaries form normally in tor mutant embryos, although somitic mesoderm defects are apparent later, when cells mature to form muscle fibers. Thus, while the function of Notch pathway genes is required for proper somite formation, the tor mutant phenotype suggests that precise oscillations of Notch pathway transcripts are not essential for establishing segmental pattern in the presomitic mesoderm.

Somite formation: where left meets right

Somites are the bilaterally symmetric embryonic precursors of the vertebrate skeleton and axial muscle. Three recent studies reveal that somites form asymmetrically in the absence of retinoic acid signaling. These results uncover an unexpected relationship between somitogenesis and left-right patterning, and suggest that bilateral somite formation is regulated along the left-right axis.

Transmembrane Protein GDE2 Induces Motor Neuron Differentiation in Vivo

During neural development, coordinate regulation of cell-cycle exit and differentiation is essential for cell-fate specification, cell survival, and proper wiring of neuronal circuits. However, the molecules that direct these events remain poorly defined. In the developing spinal cord, the differentiation of motor neuron progenitors into postmitotic motor neurons is regulated by retinoid signaling. Here, we identify a retinoid-inducible gene, GDE2 (glycerophosphodiester phosphodiesterase 2), encoding a six-transmembrane protein that is necessary and sufficient to drive spinal motor neuron differentiation in vivo. A single amino acid mutation in the extracellular catalytic domain abolishes protein function. This reveals a critical role for glycerophosphodiester metabolism in motor neuron differentiation.

Thinking clockwise

Throughout the Animal Kingdom, the time of embryonic development is maintained and strictly controlled. Each step of the process is successful only when it occurs at the right time and place. This raises the question: how is time controlled during embryonic development? Time control is particularly crucial during embryo segmentation processes, where the number of generated segments, as well as the time of formation of each segment, is extraordinarily constant and specific for each species. Somitogenesis is the process through which the vertebrate presomitic mesoderm is segmented along its anterior-posterior axis into round-shaped masses of epithelial cells, named somites. In the chick embryo, a new pair of somites is formed every 90 min. The discovery that this clock-like precision is dictated by the somitogenesis molecular clock constituted a landmark in the Developmental Biology field. Several genes exhibit cyclic gene expression in the embryo presomitic mesoderm from which the somites arise, presenting a 90 min oscillation period, the time required to form a pair of somites. The combined levels of dynamic gene expression throughout the presomitic mesoderm enable cells to acquire positional information, thus giving them a notion of time. Anterior-posterior patterning of the vertebrate nervous system also involves partition into discrete territories. This is particularly evident in the hindbrain where overt segmentation occurs. Nevertheless, little is known about the segmentation genes and mechanisms that may be involved. This paper intends to describe the molecular clock associated with vertebrate somitogenesis, suggesting that it may be operating in many other patterning processes.

Specification and maintenance of the spinal cord stem zone

Epiblast cells adjacent to the regressing primitive streak behave as a stem zone that progressively generates the entire spinal cord and also contributes to paraxial mesoderm. Despite this fundamental task, this cell population is poorly characterised, and the tissue interactions and signalling pathways that specify this unique region are unknown. Fibroblast growth factor (FGF) is implicated but it is unclear whether it is sufficient and/or directly required for stem zone specification. It is also not understood how establishment of the stem zone relates to the acquisition of spinal cord identity as indicated by expression of caudal Hox genes. Here, we show that many cells in the chick stem zone express both early neural and mesodermal genes; however, stem zone-specific gene expression can be induced by signals from underlying paraxial mesoderm without concomitant induction of an ambivalent neural/mesodermal cell state. The stem zone is a site of FGF/MAPK signalling and we show that although FGF alone does not mimic paraxial mesoderm signals, it is directly required in epiblast cells for stem zone specification and maintenance. We further demonstrate that caudal Hox gene expression in the stem zone also depends on FGF and that neither stem zone specification nor caudal Hox gene onset requires retinoid signalling. These findings thus support a two step model for spinal cord generation - FGF-dependent establishment of the stem zone in which progressively more caudal Hox genes are expressed, followed by the retinoid-dependent assignment of spinal cord identity.

The mechanisms of dorsoventral patterning in the vertebrate neural tube

We describe the essential features of and the molecules involved in dorsoventral (DV) patterning in the neural tube. The neural tube is, from its very outset, patterned in this axis as there is a roof plate, floor plate, and differing numbers and types of neuroblasts. These neuroblasts develop into different types of neurons which express a different range of marker genes. Early embryological experiments identified the notochord and the somites as being responsible for the DV patterning of the neural tube and we now know that 4 signaling molecules are involved and are generated by these surrounding structures. Fibroblast growth factors (FGFs) are produced by the caudal mesoderm and must be down-regulated before neural differentiation can occur. Retinoic acid (RA) is produced by the paraxial mesoderm and is an inducer of neural differentiation and patterning and is responsible for down-regulating FGF. Sonic hedgehog (Shh) is produced by the notochord and floor plate and is responsible for inducing ventral neural cell types in a concentration-dependent manner. Bone morphogenetic proteins (BMPs) are produced by the roof plate and are responsible for inducing dorsal neural cell types in a concentration-dependent manner. Subsequently, RA is used twice more. Once from the somites for motor neuron differentiation and secondly RA is used to define the motor neuron subtypes, but in the latter case it is generated within the neural tube from differentiating motor neurons rather than from outside. These 4 signaling molecules also interact with each other, generally in a repressive fashion, and DV patterning shows how complex these interactions can be.

Control of the segmentation process by graded MAPK/ERK activation in the chick embryo

The regular spacing of somites during vertebrate embryogenesis involves a dynamic gradient of FGF signaling that controls the timing of maturation of cells in the presomitic mesoderm (PSM). How the FGF signal is transduced by PSM cells is unclear. Here, we first show that the FGF gradient is translated into graded activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway along the PSM in the chicken embryo. Using in ovo electroporation of PSM cells, we demonstrate that constitutive activation of ERK signaling in the PSM blocks segmentation by preventing maturation of PSM cells, thus phenocopying the overexpression of FGF8. Conversely, inhibition of ERK phosphorylation mimics a loss of function of FGF signaling in the PSM. Interestingly, video microscopy analysis of cell movements shows that ERK regulates the motility of PSM cells, suggesting that the decrease of cell movements along the PSM enables mesenchymal PSM cells to undergo proper segmentation. Together, our data demonstrate that ERK is the effector of the gradient of FGF in the PSM that controls the segmentation process.

Requirement of mesodermal retinoic acid generated by Raldh2 for posterior neural transformation

Studies in amphibian embryos have suggested that retinoic acid (RA) may function as a signal that stimulates posterior differentiation of the nervous system as postulated by the activation-transformation model for anteroposterior patterning of the nervous system. We have tested this hypothesis in retinaldehyde dehydrogenase-2 (Raldh2) null mutant mice lacking RA synthesis in the somitic mesoderm. Raldh2(-/-) embryos exhibited neural induction (activation) as evidenced by expression of Sox1 and Sox2 along the neural plate, but differentiation of spinal cord neuroectodermal progenitor cells (posterior transformation) did not occur as demonstrated by a loss of Pax6 and Olig2 expression along the posterior neural plate. Spinal cord differentiation in Raldh2(-/-) embryos was rescued by maternal RA administration, and during the rescue RA was found to act directly in the neuroectoderm but not the somitic mesoderm. RA generated by Raldh2 in the somitic mesoderm was found to normally travel as a signal throughout the mesoderm and neuroectoderm of the trunk and into tailbud neuroectoderm, but not into tailbud mesoderm. Raldh2(-/-) embryos also exhibited increased Fgf8 expression in the tailbud, and decreased cell proliferation in tailbud neuroectoderm. Our findings demonstrate that RA synthesized in the somitic mesoderm is necessary for posterior neural transformation in the mouse and that Raldh2 provides the only source of RA for posterior development. An important concept to emerge from our studies is that the somitic mesodermal RA signal acts in the neuroectoderm but not mesoderm to generate a spinal cord fate.

Retinaldehyde dehydrogenase 2 and Hoxc8 are required in the murine brachial spinal cord for the specification of Lim1+ motoneurons and the correct distribution of Islet1+ motoneurons

Retinoic acid (RA) activity plays sequential roles during the development of the ventral spinal cord. Here, we have investigated the functions of local RA synthesis in the process of motoneuron specification and early differentiation using a conditional knockout strategy that ablates the function of the retinaldehyde dehydrogenase 2 (Raldh2) synthesizing enzyme essentially in brachial motoneurons, and later in mesenchymal cells at the base of the forelimb. Mutant (Raldh2L-/-) embryos display an early embryonic loss of a subset of Lim1+ brachial motoneurons, a mispositioning of Islet1+ neurons and inappropriate axonal projections of one of the nerves innervating extensor limb muscles, which lead to an adult forepaw neuromuscular defect. The molecular basis of the Raldh2L-/- phenotype relies in part on the deregulation of Hoxc8, which in turn regulates the RA receptor RARbeta. We further show that Hoxc8 mutant mice, which exhibit a similar congenital forepaw defect, display at embryonic stages molecular defects that phenocopy the Raldh2L-/- motoneuron abnormalities. Thus, interdependent RA signaling and Hox gene functions are required for the specification of brachial motoneurons in the mouse.

Retinoic acid coordinates somitogenesis and left–right patterning in vertebrate embryos

A striking feature of the body plan of a majority of animals is bilateral symmetry. Almost nothing is known about the mechanisms controlling the symmetrical arrangement of the left and right body sides during development. Here we report that blocking the production of retinoic acid (RA) in chicken embryos leads to a desynchronization of somite formation between the two embryonic sides, demonstrated by a shortened left segmented region. This defect is linked to a loss of coordination of the segmentation clock oscillations. The lateralization of this defect led us to investigate the relation between somitogenesis and the left-right asymmetry machinery in RA-deficient embryos. Reversal of the situs in chick or mouse embryos lacking RA results in a reversal of the somitogenesis laterality defect. Our data indicate that RA is important in buffering the lateralizing influence of the left-right machinery, thus permitting synchronization of the development of the two embryonic sides.

Retinoic Acid Controls the Bilateral Symmetry of Somite Formation in the Mouse Embryo

A striking characteristic of vertebrate embryos is their bilaterally symmetric body plan, which is particularly obvious at the level of the somites and their derivatives such as the vertebral column. Segmentation of the presomitic mesoderm must therefore be tightly coordinated along the left and right embryonic sides. We show that mutant mice defective for retinoic acid synthesis exhibit delayed somite formation on the right side. Asymmetric somite formation correlates with a left-right desynchronization of the segmentation clock oscillations. These data implicate retinoic acid as an endogenous signal that maintains the bilateral synchrony of mesoderm segmentation, and therefore controls bilateral symmetry, in vertebrate embryos.

A gradient of Gli activity mediates graded Sonic Hedgehog signaling in the neural tube

During development, many signaling factors behave as morphogens, long-range signals eliciting different cellular responses according to their concentration. In ventral regions of the spinal cord, Sonic Hedgehog (Shh) is such a signal and controls the emergence, in precise spatial order, of distinct neuronal subtypes. The Gli family of transcription factors plays a central role in this process. Here we demonstrate that a gradient of Gli activity is sufficient to mediate, cell-autonomously, the full range of Shh responses in the neural tube. The incremental two- to threefold changes in Shh concentration, which determine alternative neuronal subtypes, are mimicked by similar small changes in the level of Gli activity, indicating that a gradient of Gli activity represents the intracellular correlate of graded Shh signaling. Moreover, our analysis suggests that cells integrate the level of signaling over time, consistent with the idea that signal duration, in addition to signal strength, is an important parameter controlling dorsal-ventral patterning. Together, these data indicate that Shh signaling is transduced, without amplification, into a gradient of Gli activity that orchestrates patterning of the ventral neural tube.

The contribution of chicken embryology to the understanding of vertebrate limb development

The chicken is an excellent model organism for studying vertebrate limb development, mainly because of the ease of manipulating the developing limb in vivo. Classical chicken embryology has provided fate maps and elucidated the cell-cell interactions that specify limb pattern. The first defined chemical that can mimic one of these interactions was discovered by experiments on developing chick limbs and, over the last 15 years or so, the role of an increasing number of developmentally important genes has been uncovered. The principles that underlie limb development in chickens are applicable to other vertebrates and there are growing links with clinical genetics. The sequence of the chicken genome, together with other recently assembled chicken genomic resources, will present new opportunities for exploiting the ease of manipulating the limb.

The generation and diversification of spinal motor neurons: signals and responses

Motor neurons are probably the best characterised neuronal class in the vertebrate central nervous system and have become a paradigm for understanding the mechanisms that control the development of vertebrate neurons. For many investigators working on this problem the chick embryo is the model system of choice and from these studies a picture of the steps involved in motor neuron generation has begun to emerge. These findings suggest that motor neuron generation is shaped by extracellular signals that regulate intrinsic, cell-autonomous determinants at sequential steps during development. The chick embryo has played a prominent role in identifying the sources of these signals, defining their molecular identities and determining the cell intrinsic programs they regulate.

Sonic Hedgehog and Retinoic Acid are not sufficient to induce motoneuron generation in the avian caudal neural tube

The caudal neural tube (CNT) of the avian embryo is devoid of both dorsal and ventral roots. We show that the lack of ventral roots in the CNT, from somite 48 caudalwards, is due to an absence of post-mitotic motoneurons (MNs). The absence of MNs is not due to a defective notochordal induction since Sonic Hedgehog (SHH) signaling is intact and the caudal notochord is able to induce ectopic MNs when grafted laterally to a host neural tube. The transcription factors involved in MN specification (Pax6, Nkx6.1, and Olig2) are all expressed in the CNT, despite the lower expression level of Pax6, but an overlap between Olig2 and the ventrally expressed transcription factor Nkx2.2 is observed in the CNT. Grafting a quail CNT into the cervical level of a chick host rescues MN generation, demonstrating both the CNT potential for MN generation and the key role of the caudal environment in the MN differentiation blockade. The transplantation of the CNT-flanking somites into the cervical level does not inhibit MN generation. Furthermore, implantation of a retinoic-acid-soaked bead laterally to the CNT does not rescue MN generation. Together, these data indicate that the rostral environment contains a signal different from both SHH and Retinoic Acid that acts on MN differentiation.

The somite segmentation clock: It takes a licking and keeps on ticking

This primer describes molecular mechanisms critical for somite segmentation clock function. In addition, two investigators of the segmentation clock, Olivier Pourquie, and Yun-Jin Jiang, give their perspectives on current research and on the future of the field.

All-trans retinoic acid-responsive genes identified in the human SH-SY5Y neuroblastoma cell line and their regulated expression in the nervous system of early embryos

The vitamin A metabolite, all-trans retinoic acid (atRA), is required for embryonic development. atRA binds to the nuclear retinoic acid receptors and regulates the transcription of specific target genes. In order to identify atRA-induced genes that play a role in neural development, a subtractive library was created from SH-SY5Y neuroblastoma cells, a human cell line that exhibits changes in cell adhesion and neurite outgrowth after exposure to the vitamin A acid. We report here the identification of 14 genes that are rapidly induced by atRA (retinoic acid induced in neuroblastoma or RAINB), eight of which were previously not known to be atRA responsive (BTBD11, calmin, cyclin M2, ephrin B2, HOXD10, NEDD9, RAINB6 and tenascin R). mRNA regulation by atRA was confirmed in SH-SY5Y cells by Northern blotting, and gene regulation was studied in additional human cell lines using the quantitative polymerase chain reaction. The majority of the atRA-responsive clones revealed in this screen are highly expressed in the nervous system of developing rat embryos. Further, the expression of several of these genes is perturbed in developing rat embryos exposed to excess atRA or conversely, deprived of sufficient retinoid during early development. We propose that a subset of these genes lie downstream of atRA and its receptors in the regulation of neurite outgrowth and cell adhesion in both neural and non-neural tissues within the developing embryo.

Non-neural ectoderm is really neural: evolution of developmental patterning mechanisms in the non-neural ectoderm of chordates and the problem of sensory cell homologies

In chordates, the ectoderm is divided into the neuroectoderm and the so-called non-neural ectoderm. In spite of its name, however, the non-neural ectoderm contains numerous sensory cells. Therefore, the term "non-neural" ectoderm should be replaced by "general ectoderm." At least in amphioxus and tunicates and possibly in vertebrates as well, both the neuroectoderm and the general ectoderm are patterned anterior/posteriorly by mechanisms involving retinoic acid and Hox genes. In amphioxus and tunicates the ectodermal sensory cells, which have a wide range of ciliary and microvillar configurations, are mostly primary neurons sending axons to the CNS, although a minority lack axons. In contrast, vertebrate mechanosensory cells, called hair cells, are all secondary neurons that lack axons and have a characteristic eccentric cilium adjacent to a group of microvilli of graded lengths. It has been highly controversial whether the ectodermal sensory cells in the oral siphons of adult tunicates are homologous to vertebrate hair cells. In some species of tunicates, these cells appear to be secondary neurons, and microvillar and ciliary configurations of some of these cells approach those of vertebrate hair cells. However, none of the tunicate cells has all the characteristics of a hair cell, and there is a high degree of variation among ectodermal sensory cells within and between different species. Thus, similarities between the ectodermal sensory cells of any one species of tunicate and craniate hair cells may well represent convergent evolution rather than homology.

Retinoic-acid-concentration-dependent acquisition of neural cell identity during in vitro differentiation of mouse embryonic stem cells

Retinoic acid (RA) is one of the most important morphogens, and its embryonic distribution correlates with neural differentiation and positional specification in the developing central nervous system. To investigate the concentration-dependent effects of RA on neural differentiation of mouse embryonic stem cells (ES cells), we investigated the precise expression profiles of neural and regional specific genes by ES cells aggregated into embryoid bodies (EBs) exposed to various concentrations of RA or the BMP antagonist Noggin. RA promoted both neural differentiation and caudalization in a concentration-dependent manner, and the concentration of RA was found to regulate dorso-ventral identity, i.e., higher concentrations of RA induced a dorsal phenotype, and lower concentrations of RA induced a more ventral phenotype. The induction of the more ventral phenotype was due to the higher expression level of the N-terminus of sonic hedgehog protein (Shh-N) when treated with low concentration RA, as it was abrogated by an inhibitor of Shh signaling, cyclopamine. These findings suggest that the concentration of RA strictly and simultaneously regulates the neuralization and positional specification during differentiation of mouse ES cells and that it may be possible to use it to establish a strategy for controlling the identity of ES-cell-derived neural cells.

Coupling segmentation to axis formation

A characteristic feature of the vertebrate body is its segmentation along the anteroposterior axis, as illustrated by the repetition of vertebrae that form the vertebral column. The vertebrae and their associated muscles derive from metameric structures of mesodermal origin, the somites. The segmentation of the body is established by somitogenesis, during which somites form sequentially in a rhythmic fashion from the presomitic mesoderm. This review highlights recent findings that show how dynamic gradients of morphogens and retinoic acid, coupled to a molecular oscillator, drive the formation of somites and link somitogenesis to the elongation of the anteroposterior axis.

Specific regulation of cyclins D1 and D2 by FGF and Shh signaling coordinates cell cycle progression, patterning, and differentiation during early steps of spinal cord development

In the vertebrate embryo, spinal cord elongation requires FGF signaling that promotes the continuous development of the posterior nervous system by maintaining a stem zone of proliferating neural progenitors. Those escaping the caudal neural stem zone, which is expressed to Shh signal, initiate ventral patterning in the neural groove before starting neuronal differentiation in the neural tube. Here we investigated the integration of D-type cyclins, known to govern cell cycle progression under the control of extracellular signals, in the program of spinal cord maturation. In chicken embryo, we find that cyclin D2 is preferentially expressed in the posterior neural plate, whereas cyclin D1 appears in the neural groove. We demonstrated by loss- and gain-of-function experiments that FGF signaling maintains cyclin D2 in the immature caudal neural epithelium, while Shh activates cyclin D1 in the neural groove. Moreover, forced maintenance of cyclin D1 or D2 in the neural tube favors proliferation at the expense of neuronal differentiation. These results contribute to our understanding of how the cell cycle control can be linked to the patterning programs to influence the balance between proliferation and neuronal differentiation in discrete progenitors domains.

Temporal requirement for hedgehog signaling in ventral telencephalic patterning

Hedgehog signaling is required for multiple aspects of brain development, including growth, the establishment of both dorsal and ventral midline patterning and the generation of specific cell types such as oligodendrocytes and interneurons. To identify more precisely when during development hedgehog signaling mediates these events, we directed the removal of hedgehog signaling within the brain by embryonic day 9 of development, using a FoxG1(Cre) driver line to mediate the removal of a conditional smoothened null allele. We observed a loss of ventral telencephalic patterning that appears to result from an initial lack of specification of these structures rather than by changes in proliferation or cell death. A further consequence of the removal of smoothened in these mice is the near absence of both oligodendrocytes and interneurons. Surprisingly, the dorsal midline appears to be patterned normally in these mutants. Together with previous analyses, the present results demonstrate that hedgehog signaling in the period between E9.0 and E12 is essential for the patterning of ventral regions and the generation of cell types that are thought to largely arise from them.

Retinoic acid signalling specifies intermediate character in the developing telencephalon

The organisation of the telencephalon into its major structures depends on its early regionalisation along the dorsoventral axis. Previous studies have provided evidence that sonic hedgehog (SHH) is required for the generation of telencephalic cells of ventral character, and that sequential WNT and fibroblast growth factor (FGF) signalling specifies cells of dorsal telencephalic character. However, the signalling mechanisms that specify telencephalic cells of an intermediate character remain to be defined. We provide evidence here that retinoic acid has a crucial role in specifying telencephalic progenitor cells of intermediate character.

The chick embryo: a leading model in somitogenesis studies

The vertebrate body is built on a metameric organization which consists of a repetition of functionally equivalent units, each comprising a vertebra, its associated muscles, peripheral nerves and blood vessels. This periodic pattern is established during embryogenesis by the somitogenesis process. Somites are generated in a rhythmic fashion from the presomitic mesoderm and they subsequently differentiate to give rise to the vertebrae and skeletal muscles of the body. Somitogenesis has been very actively studied in the chick embryo since the 19th century and many of the landmark experiments that led to our current understanding of the vertebrate segmentation process have been performed in this organism. Somite formation involves an oscillator, the segmentation clock whose periodic signal is converted into the periodic array of somite boundaries by a spacing mechanism relying on a traveling threshold of FGF signaling regressing in concert with body axis extension.

Differentiation of mouse embryonic stem cells into a defined neuronal lineage

Although it has long been known that cultured embryonic stem cells can generate neurons, the lineage relationships with their immediate precursors remain unclear. We report here that selection of highly proliferative stem cells followed by treatment with retinoic acid generated essentially pure precursors that markers identified as Pax-6-positive radial glial cells. As they do in vivo, these cells went on to generate neurons with remarkably uniform biochemical and electrophysiological characteristics.

Combinatorial Fgf and Bmp signalling patterns the gastrula ectoderm into prospective neural and epidermal domains

Studies in fish and amphibia have shown that graded Bmp signalling activity regulates dorsal-to-ventral (DV) patterning of the gastrula embryo. In the ectoderm, it is thought that high levels of Bmp activity promote epidermal development ventrally, whereas secreted Bmp antagonists emanating from the organiser induce neural tissue dorsally. However, in zebrafish embryos, the domain of cells destined to contribute to the spinal cord extends all the way to the ventral side of the gastrula, a long way from the organiser. We show that in vegetal (trunk and tail) regions of the zebrafish gastrula, neural specification is initiated at all DV positions of the ectoderm in a manner that is unaffected by levels of Bmp activity and independent of organiser-derived signals. Instead, we find that Fgf activity is required to induce vegetal prospective neural markers and can do so without suppressing Bmp activity. We further show that Bmp signalling does occur within the vegetal prospective neural domain and that Bmp activity promotes the adoption of caudal fate by this tissue.

The vertebrate segmentation clock

In vertebrate embryos, somite segmentation is controlled by a molecular clock, in the form of a transcriptional oscillator that operates in the presomitic mesoderm. Most of the genes implicated in the oscillator belong to the Notch pathway; a recently discovered exception is the Wnt pathway gene Axin2. Experiments have revealed several negative feedback loops that might generate oscillations, leading to at least four different theories. The simplest of these is based on direct autoinhibition of certain members of the hairy/E(spl) family of Notch target genes--Hes7 in the mouse, and her1 and her7 in the zebrafish. A mathematical account of this mechanism explains some surprising observations and suggests that the period of oscillation is chiefly determined by the transcriptional and translational delays--the times required to make a molecule of the mRNA and a molecule of the protein.

Regionalization and fate specification in neurospheres: the role of Olig2 and Pax6

Neurosphere cultures are widely used to propagate multipotent CNS precursors, but their differentiation into neurons or oligodendrocytes is rather poor. To elucidate fate determination in this system, we examined the expression and function of candidate transcription factors in neurospheres derived from different CNS regions during development and adulthood. We observed prominent down-regulation of most transcription factors present in telencephalic precursors upon growth factor exposure in neurosphere cultures while Olig1 and Olig2 expression was strongly up-regulated. Interference with Olig2 in neurospheres revealed its role in self-renewal during expansion and for the generation of neurons and oligodendrocytes during differentiation. We further show that neurogenesis becomes fully Pax6-dependent in the neurosphere culture system, independent of the region of origin, and that Pax6 overexpression is sufficient to direct almost all neurosphere-derived cells towards neurogenesis. Thus, a pathway combining transcription factors of dorsal and ventral regions is activated in the neurosphere culture model.

Regulation of Segmental Patterning by Retinoic Acid Signaling during Xenopus Somitogenesis

Somites, the segmented building blocks of the vertebrate embryo, arise one by one in a patterning process that passes wavelike along the anteroposterior axis of the presomitic mesoderm (PSM). We have studied this process in Xenopus embryos by analyzing the expression of the bHLH gene, Thylacine1, which is turned on in the PSM as cells mature and segment, in a pattern that marks both segment boundaries and polarity. Here, we show that this segmental gene expression involves a PSM enhancer that is regulated by retinoic acid (RA) signaling at two levels. RA activates Thylacine1 expression in rostral PSM directly. RA also activates Thylacine1 expression in the caudal PSM indirectly by inducing the expression of MKP3, an inhibitor of the FGF signaling pathway. RA signaling is therefore a major contributor to segmental patterning by promoting anterior segmental polarity and by interacting with the FGF signaling pathway to position segmental boundaries.

Many ways to make a gradient

A recent publication describes a novel mechanism by which a morphogen gradient might be established. These results concern a gradient of FGF8 expression along the longitudinal axis of the chick embryo with a high level of transcripts at the tail, fading off in an anterior direction. Assaying for intron transcripts, it is shown that fgf8 is transcribed only in the tail cells and that the gradient of fgf8 transcripts is produced by growth and mRNA degradation. This possible mechanism of gradient formation can operate only when growth is involved, as is the case in many examples including the longitudinal axis formation of vertebrates, but is not in some other systems.

Opposing FGF and retinoid pathways: a signalling switch that controls differentiation and patterning onset in the extending vertebrate body axis

Construction of the trunk/caudal region of the vertebrate embryo involves a set of distinct molecules and processes whose relationships are just coming into focus. In addition to the subdivision of the embryo into head and trunk domains, this "caudalisation" process requires the establishment and maintenance of a stem zone. This sequentially generates caudal tissues over a long period which then undergo differentiation and patterning in the extending body axis. Here we review recent studies that show that changes in the signalling properties of the paraxial mesoderm act as a switch that controls onset of differentiation and pattern in the spinal cord. These findings identify distinct roles for different caudalising factors; in particular, Fibroblast Growth Factor (FGF) inhibits differentiation in the caudal stem zone, while Retinoic acid (RA) provided rostrally by somitic mesoderm is required for neuronal differentiation and establishment of ventral neural pattern. Furthermore, the mutual opposition of FGF and RA pathways controls not only neural differentiation but also mesoderm segmentation and might also underlie the progressive assignment of rostrocaudal identity by regulating Hox gene availability and activation.

Retinoids Run Rampant

Learning how the incredible diversity of neurons in the vertebrate central nervous system (CNS) is generated is a central focus of developmental neuroscience. Three studies in the September 25, 2003, issue of Neuron bring us closer to this goal by revealing how the interplay between Fibroblast Growth Factor (FGF), retinoic acid (RA), and Sonic hedgehog (Shh) signaling regulate progression of spinal cord progenitor cells through various phases of development and specify particular types of spinal motor neurons (MNs).