Fibroblast growth factor receptor-1 (FGFR-1) is essential for normal neural tube and limb development

Caudal Fgfr1 disruption produces localised spinal mis-patterning and a terminal myelocystocele-like phenotype in mice

Closed spinal dysraphisms are poorly understood malformations classified as neural tube (NT) defects. Several, including terminal myelocystocele, affect the distal spine. We have previously identified a NT closure-initiating point, Closure 5, in the distal spine of mice. Here, we document equivalent morphology of the caudal-most closing posterior neuropore (PNP) in mice and humans. Closure 5 forms in a region of active FGF signalling, and pharmacological FGF receptor blockade impairs its formation in cultured mouse embryos. Conditional genetic deletion of Fgfr1 in caudal embryonic tissues with Cdx2Cre diminishes neuroepithelial proliferation, impairs Closure 5 formation and delays PNP closure. After closure, the distal NT of Fgfr1-disrupted embryos dilates to form a fluid-filled sac overlying ventrally flattened spinal cord. This phenotype resembles terminal myelocystocele. Histological analysis reveals regional and progressive loss of SHH- and FOXA2-positive ventral NT domains, resulting in OLIG2 labelling of the ventral-most NT. The OLIG2 domain is also subsequently lost, eventually producing a NT that is entirely positive for the dorsal marker PAX3. Thus, a terminal myelocystocele-like phenotype can arise after completion of NT closure with localised spinal mis-patterning caused by disruption of FGFR1 signalling.

Patterning of the antero-ventral mammalian brain: Lessons from holoprosencephaly comparative biology in man and mouse

Adult form and function are dependent upon the activity of specialized signaling centers that act early in development at the embryonic midline. These centers instruct the surrounding cells to adopt a positional fate and to form the patterned structures of the phylotypic embryo. Abnormalities in these processes have devastating consequences for the individual, as exemplified by holoprosencephaly in which anterior midline development fails, leading to structural defects of the brain and/or face. In the 25 years since the first association between human holoprosencephaly and the sonic hedgehog gene, a combination of human and animal genetic studies have enhanced our understanding of the genetic and embryonic causation of this congenital defect. Comparative biology has extended the holoprosencephaly network via the inclusion of gene mutations from multiple signaling pathways known to be required for anterior midline formation. It has also clarified aspects of holoprosencephaly causation, showing that it arises when a deleterious variant is present within a permissive genome, and that environmental factors, as well as embryonic stochasticity, influence the phenotypic outcome of the variant. More than two decades of research can now be distilled into a framework of embryonic and genetic causation. This framework means we are poised to move beyond our current understanding of variants in signaling pathway molecules. The challenges now at the forefront of holoprosencephaly research include deciphering how the mutation of genes involved in basic cell processes can also cause holoprosencephaly, determining the important constituents of the holoprosencephaly permissive genome, and identifying environmental compounds that promote holoprosencephaly. This article is categorized under: Congenital Diseases > Stem Cells and Development Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Molecular and Cellular Physiology Congenital Diseases > Environmental Factors.

Mechanisms of bone development and repair

Bone development occurs through a series of synchronous events that result in the formation of the body scaffold. The repair potential of bone and its surrounding microenvironment - including inflammatory, endothelial and Schwann cells - persists throughout adulthood, enabling restoration of tissue to its homeostatic functional state. The isolation of a single skeletal stem cell population through cell surface markers and the development of single-cell technologies are enabling precise elucidation of cellular activity and fate during bone repair by providing key insights into the mechanisms that maintain and regenerate bone during homeostasis and repair. Increased understanding of bone development, as well as normal and aberrant bone repair, has important therapeutic implications for the treatment of bone disease and ageing-related degeneration.

Endocrinological Features of Hartsfield Syndrome in an Adult Patient With a Novel Mutation of FGFR1

Hartsfield syndrome (HS: OMIM 615465) is a rare congenital disease associated with a mutation of the fibroblast growth factor receptor 1 gene (FGFR1) with the main features of holoprosencephaly and ectrodactyly. Patients with HS also present with endocrinological deficits, such as isolated hypogonadotropic hypogonadism and central diabetes insipidus. Although there are several studies on infancy/childhood history, there is no study of infant/childhood/adolescent/young adult HS natural history and endocrinological findings. Here, we report a male patient with HS associated with a novel de novo FGFR1 mutation (c. 1868A > C). The endocrinological profile was evaluated at ages 1 and 31 years. This long-term follow-up study highlights functional changes in the posterior pituitary gland and features of bone metabolism disorder. We also describe the anterior pituitary function. To our knowledge this is the first description of the natural history of an HS patient through birth to young adult age. Although the HS infants reported in the literature develop central diabetes insipidus, little is known about the serial changes in pituitary gland function during growth in HS patients. In this study we describe an adult patient with HS who showed improvement of hypernatremia during early adulthood. In addition, we emphasize the importance of prevention and treatment of osteoporosis in HS.

Signaling regulation during gastrulation: Insights from mouse embryos and in vitro systems

Gastrulation is the process whereby cells exit pluripotency and concomitantly acquire and pattern distinct cell fates. This is driven by the convergence of WNT, BMP, Nodal and FGF signals, which are tightly spatially and temporally controlled, resulting in regional and stage-specific signaling environments. The combination, level and duration of signals that a cell is exposed to, according its position within the embryo and the developmental time window, dictates the fate it will adopt. The key pathways driving gastrulation exhibit complex interactions, which are difficult to disentangle in vivo due to the complexity of manipulating multiple signals in parallel with high spatiotemporal resolution. Thus, our current understanding of the signaling dynamics regulating gastrulation is limited. In vitro stem cell models have been established, which undergo organized cellular differentiation and patterning. These provide amenable, simplified, deconstructed and scalable models of gastrulation. While the foundation of our understanding of gastrulation stems from experiments in embryos, in vitro systems are now beginning to reveal the intricate details of signaling regulation. Here we discuss the current state of knowledge of the role, regulation and dynamic interaction of signaling pathways that drive mouse gastrulation.

Posterior axis formation requires Dlx5/Dlx6 expression at the neural plate border

Neural tube defects (NTDs), one of the most common birth defects in human, present a multifactorial etiology with a poorly defined genetic component. The Dlx5 and Dlx6 bigenic cluster encodes two evolutionary conserved homeodomain transcription factors, which are necessary for proper vertebrate development. It has been shown that Dlx5/6 genes are essential for anterior neural tube closure, however their role in the formation of the posterior structures has never been described. Here, we show that Dlx5/6 expression is required during vertebrate posterior axis formation. Dlx5 presents a similar expression pattern in neural plate border cells during posterior neurulation of zebrafish and mouse. Dlx5/6-inactivation in the mouse results in a phenotype reminiscent of NTDs characterized by open thoracic and lumbar vertebral arches and failure of epaxial muscle formation at the dorsal midline. The dlx5a/6a zebrafish morphants present posterior NTDs associated with abnormal delamination of neural crest cells showing altered expression of cell adhesion molecules and defects of motoneuronal development. Our findings provide new molecular leads to decipher the mechanisms of vertebrate posterior neurulation and might help to gather a better understanding of human congenital NTDs etiology.

Epithelium morphogenesis and oviduct development are regulated by significant increase of expression of genes after long-term in vitro primary culture – a microarray assays

The correct oviductal development and morphogenesis of its epithelium are crucial factors influencing female fertility. Oviduct is involved in maintaining an optimal environment for gametes and preimplantation embryo development; secretory oviductal epithelial cells (OECs) synthesize components of oviductal fluid. Oviductal epithelium also participates in sperm binding and its hyperactivation. For better understanding of the genetic bases that underlay porcine oviductal development, OECs were isolated from porcine oviducts and established long-term primary culture. A microarray approach was utilized to determine the differentially expressed genes during specific time periods. Cells were harvested on day 7, 15 and 30 of in vitro primary culture and their RNA was isolated. Gene expression was analyzed and statistical analysis was performed. 48 differentially expressed genes belonging to “tube morphogenesis”, “tube development”, “morphogenesis of an epithelium”, “morphogenesis of branching structure” and “morphogenesis of branching epithelium” GO BP terms were selected, of which 10 most upregulated include BMP4, ARG1, SLIT2, FGFR1, DAB2, TNC, EPAS1, HHEX, ITGB3 and LOX. The results help to shed light on the porcine oviductal development and its epithelial morphogenesis, and show that after long-term culture the OECs still proliferate and maintain their tube forming properties.

Divergent early mesoderm specification underlies distinct head and trunk muscle programmes in vertebrates

Head and trunk muscles have discrete embryological origins and are governed by distinct regulatory programmes. Whereas the developmental route of trunk muscles from mesoderm is well studied, that of head muscles is ill defined. Here, we show that, unlike the myogenic trunk paraxial mesoderm, head mesoderm development is independent of the T/Tbx6 network in mouse. We reveal that, in contrast to Wnt and FGF-driven trunk mesoderm, dual inhibition of Wnt/β-catenin and Nodal specifies head mesoderm. Remarkably, the progenitors derived from embryonic stem cells by dual inhibition efficiently differentiate into cardiac and skeletal muscle cells. This twin potential is the defining feature of cardiopharyngeal mesoderm: the head subtype giving rise to heart and branchiomeric head muscles. Therefore, our findings provide compelling evidence that dual inhibition specifies head mesoderm and unravel the mechanism that diversifies head and trunk muscle programmes during early mesoderm fate commitment. Significantly, this is the first report of directed differentiation of pluripotent stem cells, without transgenes, into progenitors with muscle/heart dual potential. Ability to generate branchiomeric muscle in vitro could catalyse efforts in modelling myopathies that selectively involve head muscles.

Identification of transcripts potentially involved in neural tube closure using RNA sequencing

Anencephaly is a fatal human neural tube defect (NTD) in which the anterior neural tube remains open. Zebrafish embryos with reduced Nodal signaling display an open anterior neural tube phenotype that is analogous to anencephaly. Previous work from our laboratory suggests that Nodal signaling acts through induction of the head mesendoderm and mesoderm. Head mesendoderm/mesoderm then, through an unknown mechanism, promotes formation of the polarized neuroepithelium that is capable of undergoing the movements required for closure. We compared the transcriptome of embryos treated with a Nodal signaling inhibitor at sphere stage, which causes NTDs, to embryos treated at 30% epiboly, which does not cause NTDs. This screen identified over 3,000 transcripts with potential roles in anterior neurulation. Expression of several genes encoding components of tight and adherens junctions was significantly reduced, supporting the model that Nodal signaling regulates formation of the neuroepithelium. mRNAs involved in Wnt, FGF, and BMP signaling were also differentially expressed, suggesting these pathways might regulate anterior neurulation. In support of this, we found that pharmacological inhibition of FGF-receptor function causes an open anterior NTD as well as loss of mesodermal derivatives. This suggests that Nodal and FGF signaling both promote anterior neurulation through induction of head mesoderm.

Spina Bifida: Pathogenesis, Mechanisms, and Genes in Mice and Humans

Spina bifida is among the phenotypes of the larger condition known as neural tube defects (NTDs). It is the most common central nervous system malformation compatible with life and the second leading cause of birth defects after congenital heart defects. In this review paper, we define spina bifida and discuss the phenotypes seen in humans as described by both surgeons and embryologists in order to compare and ultimately contrast it to the leading animal model, the mouse. Our understanding of spina bifida is currently limited to the observations we make in mouse models, which reflect complete or targeted knockouts of genes, which perturb the whole gene(s) without taking into account the issue of haploinsufficiency, which is most prominent in the human spina bifida condition. We thus conclude that the need to study spina bifida in all its forms, both aperta and occulta, is more indicative of the spina bifida in surviving humans and that the measure of deterioration arising from caudal neural tube defects, more commonly known as spina bifida, must be determined by the level of the lesion both in mouse and in man.

An FGF3-BMP Signaling Axis Regulates Caudal Neural Tube Closure, Neural Crest Specification and Anterior-Posterior Axis Extension

During vertebrate axis extension, adjacent tissue layers undergo profound morphological changes: within the neuroepithelium, neural tube closure and neural crest formation are occurring, while within the paraxial mesoderm somites are segmenting from the presomitic mesoderm (PSM). Little is known about the signals between these tissues that regulate their coordinated morphogenesis. Here, we analyze the posterior axis truncation of mouse Fgf3 null homozygotes and demonstrate that the earliest role of PSM-derived FGF3 is to regulate BMP signals in the adjacent neuroepithelium. FGF3 loss causes elevated BMP signals leading to increased neuroepithelium proliferation, delay in neural tube closure and premature neural crest specification. We demonstrate that elevated BMP4 depletes PSM progenitors in vitro, phenocopying the Fgf3 mutant, suggesting that excessive BMP signals cause the Fgf3 axis defect. To test this in vivo we increased BMP signaling in Fgf3 mutants by removing one copy of Noggin, which encodes a BMP antagonist. In such mutants, all parameters of the Fgf3 phenotype were exacerbated: neural tube closure delay, premature neural crest specification, and premature axis termination. Conversely, genetically decreasing BMP signaling in Fgf3 mutants, via loss of BMP receptor activity, alleviates morphological defects. Aberrant apoptosis is observed in the Fgf3 mutant tailbud. However, we demonstrate that cell death does not cause the Fgf3 phenotype: blocking apoptosis via deletion of pro-apoptotic genes surprisingly increases all Fgf3 defects including causing spina bifida. We demonstrate that this counterintuitive consequence of blocking apoptosis is caused by the increased survival of BMP-producing cells in the neuroepithelium. Thus, we show that FGF3 in the caudal vertebrate embryo regulates BMP signaling in the neuroepithelium, which in turn regulates neural tube closure, neural crest specification and axis termination. Uncovering this FGF3-BMP signaling axis is a major advance toward understanding how these tissue layers interact during axis extension with important implications in human disease.

During embryological development, the vertebrate embryo undergoes profound growth in a head-to-tail direction. During this process, formation of different structures within adjacent tissue layers must occur in a coordinated fashion. Insights into how these adjacent tissues molecularly communicate with each other is essential to understanding both basic embryology and the underlying causes of human birth defects. Mice lacking Fgf3, which encodes a secreted signaling factor, have long been known to have premature axis termination, but the underlying mechanism has not been studied until now. Through a series of complex genetic experiments, we show that FGF3 is an essential factor for coordination of neural tube development and axis extension. FGF3 is secreted from the mesodermal layer, which is the major driver of extending the axis, and negatively regulates expression of another class of secreted signaling molecules in the neuroepithelium, BMPs. In the absence of FGF3, excessive BMP signals cause a delay in neural tube closure, premature specification of neural crest cells and negatively affect the mesoderm, causing a premature termination of the embryological axis. Our work suggests that FGF3 may be a player in the complex etiology of the human birth defect, spina bifida, the failure of posterior neural tube closure.

Conditional Deletion of Fgfr1 in the Proximal and Distal Tubule Identifies Distinct Roles in Phosphate and Calcium Transport

A postnatal role of fibroblast growth factor receptor-1 (FGFR1) in the kidney is suggested by its binding to α-Klotho to form an obligate receptor for the hormone fibroblast growth factor-23 (FGF-23). FGFR1 is expressed in both the proximal and distal renal tubular segments, but its tubular specific functions are unclear. In this study, we crossed Fgfr1^flox/flox mice with either gamma-glutamyltransferase-Cre (γGT-Cre) or kidney specific-Cre (Ksp-Cre) mice to selectively create proximal tubule (PT) and distal tubule (DT) Fgfr1 conditional knockout mice (designated Fgfr1^PT-cKOand Fgfr1^DT-cKO, respectively). Fgfr1^PT-cKO mice exhibited an increase in sodium-dependent phosphate co-transporter expression, hyperphosphatemia, and refractoriness to the phosphaturic actions of FGF-23, consistent with a direct role of FGFR1 in mediating the proximal tubular phosphate responses to FGF-23. In contrast, Fgfr1^DT-cKO mice unexpectedly developed hypercalciuria, secondary elevations of parathyroid hormone (PTH), hypophosphatemia and enhanced urinary phosphate excretion. Fgfr1^PT-cKO mice also developed a curly tail/spina bifida-like skeletal phenotype, whereas Fgfr1^DT-cKO mice developed renal tubular micro-calcifications and reductions in cortical bone thickness. Thus, FGFR1 has dual functions to directly regulate proximal and distal tubule phosphate and calcium reabsorption, indicating a physiological role of FGFR1 signaling in both phosphate and calcium homeostasis.

Identification and characterization of secondary neural tube-derived embryonic neural stem cells in vitro

Secondary neurulation is an embryonic progress that gives rise to the secondary neural tube, the precursor of the lower spinal cord region. The secondary neural tube is derived from aggregated Sox2-expressing neural cells at the dorsal region of the tail bud, which eventually forms rosette or tube-like structures to give rise to neural tissues in the tail bud. We addressed whether the embryonic tail contains neural stem cells (NSCs), namely secondary NSCs (sNSCs), with the potential for self-renewal in vitro. Using in vitro neurosphere assays, neurospheres readily formed at the rosette and neural-tube levels, but less frequently at the tail bud tip level. Furthermore, we identified that sNSC-generated neurospheres were significantly smaller in size compared with cortical neurospheres. Interestingly, various cell cycle analyses revealed that this difference was not due to a reduction in the proliferation rate of NSCs, but rather the neuronal commitment of sNSCs, as sNSC-derived neurospheres contain more committed neuronal progenitor cells, even in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). These results suggest that the higher tendency for sNSCs to spontaneously differentiate into progenitor cells may explain the limited expansion of the secondary neural tube during embryonic development.

In Vitro Differentiation of Pluripotent Stem Cells into Functional β Islets Under 2D and 3D Culture Conditions and In Vivo Preclinical Validation of 3D Islets

Since the advent of pluripotent stem cells, (embryonic and induced pluripotent stem cells), applications of such pluripotent stem cells are of prime importance. Indeed, scientists are involved in studying the basic biology of pluripotent stem cells, but equal impetus is there to direct the pluripotent stem cells into multiple lineages for cell therapy applications. Scientists across the globe have been successful, to a certain extent, in obtaining cells of definitive endoderm and also pancreatic β islets by differentiating human pluripotent stem cells. Pluripotent stem cell differentiation protocols aim at mimicking in vivo embryonic development. As in vivo embryonic development is a complex process and involves interplay of multiple cytokines, the differentiation protocols also involve a stepwise use of multiple cytokines. Indeed the novel markers for pancreas organogenesis serve as the roadmaps to develop new protocols for pancreatic differentiation from pluripotent stem cells. Earliest developed protocols for pancreas differentiation involved "Nestin selection pathway," a pathway common for both neuronal and pancreatic differentiation lead to the generation of cells that were a combination of cells from neuronal lineage. Eventually with the discovery of hierarchy of β cell transcription factors like Pdx1, Pax4, and Nkx2.2, forced expression of such transcription factors proved successful in converting a pluripotent stem cell into a β cell. Protocols developed almost half a decade ago to the recent ones rather involve stepwise differentiations involving various cytokines and could generate as high as 25 % functional insulin-positive cells in vitro. Most advanced protocols for β islet differentiations from human pluripotent stem cells focused on 3D culture conditions, which reportedly produced 60-65 % functional β islet cells. Here, we describe the protocol for differentiation of human pluripotent stem cells into functional β cells under both 2D and 3D culture conditions.

Fibroblast growth factor (FGF) signaling in development and skeletal diseases

Fibroblast growth factors (FGF) and their receptors serve many functions in both the developing and adult organism. Humans contain 18 FGF ligands and four FGF receptors (FGFR). FGF ligands are polypeptide growth factors that regulate several developmental processes including cellular proliferation, differentiation, and migration, morphogenesis, and patterning. FGF-FGFR signaling is also critical to the developing axial and craniofacial skeleton. In particular, the signaling cascade has been implicated in intramembranous ossification of cranial bones as well as cranial suture homeostasis. In the adult, FGFs and FGFRs are crucial for tissue repair. FGF signaling generally follows one of three transduction pathways: RAS/MAP kinase, PI3/AKT, or PLCγ. Each pathway likely regulates specific cellular behaviors. Inappropriate expression of FGF and improper activation of FGFRs are associated with various pathologic conditions, unregulated cell growth, and tumorigenesis. Additionally, aberrant signaling has been implicated in many skeletal abnormalities including achondroplasia and craniosynostosis. The biology and mechanisms of the FGF family have been the subject of significant research over the past 30 years. Recently, work has focused on the therapeutic targeting and potential of FGF ligands and their associated receptors. The majority of FGF-related therapy is aimed at age-related disorders. Increased understanding of FGF signaling and biology may reveal additional therapeutic roles, both in utero and postnatally. This review discusses the role of FGF signaling in general physiologic and pathologic embryogenesis and further explores it within the context of skeletal development.

Role of FGF/FGFR signaling in skeletal development and homeostasis: learning from mouse models

Fibroblast growth factor (FGF)/fibroblast growth factor receptor (FGFR) signaling plays essential roles in bone development and diseases. Missense mutations in FGFs and FGFRs in humans can cause various congenital bone diseases, including chondrodysplasia syndromes, craniosynostosis syndromes and syndromes with dysregulated phosphate metabolism. FGF/FGFR signaling is also an important pathway involved in the maintenance of adult bone homeostasis. Multiple kinds of mouse models, mimicking human skeleton diseases caused by missense mutations in FGFs and FGFRs, have been established by knock-in/out and transgenic technologies. These genetically modified mice provide good models for studying the role of FGF/FGFR signaling in skeleton development and homeostasis. In this review, we summarize the mouse models of FGF signaling-related skeleton diseases and recent progresses regarding the molecular mechanisms, underlying the role of FGFs/FGFRs in the regulation of bone development and homeostasis. This review also provides a perspective view on future works to explore the roles of FGF signaling in skeletal development and homeostasis.

The expression of fibroblast growth factor receptors during early bovine conceptus development and pharmacological analysis of their actions on trophoblast growth in vitro

The overall aim of this work was to examine the expression profiles for fibroblast growth factor receptors (FGFRs) and describe their biological importance during bovine pre- and peri-implantation conceptus development. FGFR1 and FGFR2 mRNAs were detected at 1-, 2-, 8-cell, morula and blastocyst stages whereas FGFR3 and FGFR4 mRNAs were detected after the 8-cell stage but not earlier. The abundance of FGFR1, FGFR3, and FGFR4 mRNAs increased at the morula and blastocyst stages. Immunofluorescence microscopy detected FGFR2 and FGFR4 exclusively in trophoblast cells whereas FGFR1 and FGFR3 were detected in both trophoblast cells and inner cell mass in blastocysts. Neither transcripts for FGF10 nor its receptor (FGFR2b) were temporally related to interferon τ (IFNT) transcript profile during peri- and postimplantation bovine conceptus development. A series of studies used a chemical inhibitor of FGFR kinase function (PD173074) to examine FGFR activation requirements during bovine embryo development. Exposing embryos to the inhibitor (1 μM) beginning on day 5 post-fertilization did not alter the percentage of embryos that developed into blastocysts or blastocyst cell numbers. The inhibitor did not alter the abundance of CDX2 mRNA but decreased (P<0.05) the relative abundance of IFNT mRNA in blastocysts. Exposing blastocysts to the inhibitor from days 8 to 11 post-fertilization reduced (P<0.05) the percentage of blastocysts that formed outgrowths after transfer to Matrigel-coated plates. In conclusion, each FGFR was detected in bovine embryos, and FGFR activation is needed to maximize IFNT expression and permit outgrowth formation.

Loss of FGF-dependent mesoderm identity and rise of endogenous retinoid signalling determine cessation of body axis elongation

By analyzing cellular and molecular changes in key cell populations in the tailbud during embryogenesis, this work uncovers critical signaling events that determine vertebrate body length.

The endogenous mechanism that determines vertebrate body length is unknown but must involve loss of chordo-neural-hinge (CNH)/axial stem cells and mesoderm progenitors in the tailbud. In early embryos, Fibroblast growth factor (FGF) maintains a cell pool that progressively generates the body and differentiation onset is driven by retinoid repression of FGF signalling. This raises the possibility that FGF maintains key tailbud cell populations and that rising retinoid activity underlies cessation of body axis elongation. Here we show that sudden loss of the mesodermal gene (Brachyury) from CNH and the mesoderm progenitor domain correlates with FGF signalling decline in the late chick tailbud. This is accompanied by expansion of neural gene expression and a similar change in cell fate markers is apparent in the human tailbud. Fate mapping of chick tailbud further revealed that spread of neural gene expression results from continued ingression of CNH-derived cells into the position of the mesoderm progenitor domain. Using gain and loss of function approaches in vitro and in vivo, we then show that attenuation of FGF/Erk signalling mediates this loss of Brachyury upstream of Wnt signalling, while high-level FGF maintains Brachyury and can induce ectopic CNH-like cell foci. We further demonstrate a rise in endogenous retinoid signalling in the tailbud and show that here FGF no longer opposes retinoid synthesis and activity. Furthermore, reduction of retinoid signalling at late stages elevated FGF activity and ectopically maintained mesodermal gene expression, implicating endogenous retinoid signalling in loss of mesoderm identity. Finally, axis termination is concluded by local cell death, which is reduced by blocking retinoid signalling, but involves an FGFR-independent mechanism. We propose that cessation of body elongation involves loss of FGF-dependent mesoderm identity in late stage tailbud and provide evidence that rising endogenous retinoid activity mediates this step and ultimately promotes cell death in chick tailbud.

The mechanism that determines body length is unknown but likely operates at the elongating tail end of vertebrate embryos. In the early embryo, fibroblast growth factor (FGF) signalling maintains a proliferative pool of cells in the tailbud that progressively generates the body. It also protects these cells from the differentiating influence of retinoic acid, which is produced by the maturing mesoderm tissues of the extending body. We show here, in the chick embryo, that the “endgame”—that is, the termination of body axis elongation—comes when the mesodermal gene brachyury is suddenly lost from axial stem cell population and presumptive mesoderm cells in the tailbud late in development. Using gain- and loss-of-function approaches, we demonstrate that this step is mediated by loss of FGF signalling. We present evidence that this is due to rising retinoid signalling in the tailbud and that FGF signalling in the tailbud no longer opposes retinoid synthesis and activity. Finally, we reveal that these events are followed by local cell death in the tailbud, which can be reduced by the attenuation of retinoid signalling but involves a mechanism that is independent of FGF signalling via its usual receptor. We propose that cessation of body elongation involves loss of FGF-dependent mesoderm identity in the late tailbud and that this is mediated by rising endogenous retinoid activity, which ultimately promotes cell death in the chick tailbud.

Ascorbic acid rescues cardiomyocyte development in Fgfr1(-/-) murine embryonic stem cells

Fibroblast growth factor receptor 1 (Fgfr1) gene knockout impairs cardiomyocyte differentiation in murine embryonic stem cells (mESC). Here, various chemical compounds able to enhance cardiomyocyte differentiation in mESC [including dimethylsulfoxide, ascorbic acid (vitC), free radicals and reactive oxygen species] were tested for their ability to rescue the cardiomyogenic potential of Fgfr1(-/-) mESC. Among them, only the reduced form of vitC, l-ascorbic acid, was able to recover beating cell differentiation in Fgfr1(-/-) mESC. The appearance of contracting cells was paralleled by the expression of early and late cardiac gene markers, thus suggesting their identity as cardiomyocytes. In the attempt to elucidate the mechanism of action of vitC on Fgfr1(-/-) mESC, we analyzed several parameters related to the intracellular redox state, such as reactive oxygen species content, Nox4 expression, and superoxide dismutase activity. The results did not show any relationship between the antioxidant capacity of vitC and cardiomyocyte differentiation in Fgfr1(-/-) mESC. No correlation was found also for the ability of vitC to modulate the expression of pluripotency genes. Then, we tested the hypothesis that vitC was acting as a prolyl hydroxylase cofactor by maintaining iron in a reduced state. We first analyze hypoxia inducible factor (HIF)-1α mRNA and protein levels that were found to be slightly upregulated in Fgfr1(-/-) cells. We treated mESC with Fe(2+) or the HIF inhibitor CAY10585 during the first phases of the differentiation process and, similar to vitC, the two compounds were able to rescue cardiomyocyte formation in Fgfr1(-/-) mESC, thus implicating HIF-1α modulation in Fgfr1-dependent cardiomyogenesis.

The ubiquitin ligase mLin41 temporally promotes neural progenitor cell maintenance through FGF signaling

How self-renewal versus differentiation of neural progenitor cells is temporally controlled during early development remains ill-defined. We show that mouse Lin41 (mLin41) is highly expressed in neural progenitor cells and its expression declines during neural differentiation. Loss of mLin41 function in mice causes reduced proliferation and premature differentiation of embryonic neural progenitor cells. mLin41 was recently implicated as the E3 ubiquitin ligase that mediates degradation of Argonaute 2 (AGO2), a key effector of the microRNA pathway. However, our mechanistic studies of neural progenitor cells indicate mLin41 is not required for AGO2 ubiquitination or stability. Instead, mLin41-deficient neural progenitors exhibit hyposensitivity for fibroblast growth factor (FGF) signaling. We show that mLin41 promotes FGF signaling by directly binding to and enhancing the stability of Shc SH2-binding protein 1 (SHCBP1) and that SHCBP1 is an important component of FGF signaling in neural progenitor cells. Thus, mLin41 acts as a temporal regulator to promote neural progenitor cell maintenance, not via the regulation of AGO2 stability, but through FGF signaling.

Evolutionarily conserved requirement of Cdx for post-occipital tissue emergence

Mouse Cdx genes are involved in axial patterning and partial Cdx mutants exhibit posterior embryonic defects. We found that mouse embryos in which all three Cdx genes are inactivated fail to generate any axial tissue beyond the cephalic and occipital primordia. Anterior axial tissues are laid down and well patterned in Cdx null embryos, and a 3' Hox gene is initially transcribed and expressed in the hindbrain normally. Axial elongation stops abruptly at the post-occipital level in the absence of Cdx, as the posterior growth zone loses its progenitor activity. Exogenous Fgf8 rescues the posterior truncation of Cdx mutants, and the spectrum of defects of Cdx null embryos matches that resulting from loss of posterior Fgfr1 signaling. Our data argue for a main function of Cdx in enforcing trunk emergence beyond the Cdx-independent cephalo-occipital region, and for a downstream role of Fgfr1 signaling in this function. Cdx requirement for the post-head section of the axis is ancestral as it takes place in arthropods as well.

Pluripotent stem cell studies elucidate the underlying mechanisms of early embryonic development

Early embryonic development is a multi-step process that is intensively regulated by various signaling pathways. Because of the complexity of the embryo and the interactions between the germ layers, it is very difficult to fully understand how these signals regulate embryo patterning. Recently, pluripotent stem cell lines derived from different developmental stages have provided an in vitro system for investigating molecular mechanisms regulating cell fate decisions. In this review, we summarize the major functions of the BMP, FGF, Nodal and Wnt signaling pathways, which have well-established roles in vertebrate embryogenesis. Then, we highlight recent studies in pluripotent stem cells that have revealed the stage-specific roles of BMP，FGF and Nodal pathways during neural differentiation. These findings enhance our understanding of the stepwise regulation of embryo patterning by particular signaling pathways and provide new insight into the mechanisms underlying early embryonic development.

Interplay of cadherin-mediated cell adhesion and canonical Wnt signaling

The epithelial-mesenchymal transition is essential in both embryonic development and the progression of carcinomas. Wnt signaling and cadherin-mediated adhesion have been implicated in both processes; clarifying their role will depend on linking them to rearrangements of cellular structure and behavior. beta-Catenin is an essential molecule both in cadherin-mediated cell adhesion and in canonical Wnt signaling. Numerous experiments have shown that the loss of cadherin-mediated cell adhesion can promote beta-catenin release and signaling; this is accomplished by proteases, protein kinases and other molecules. Cadherin loss can also signal to several other regulatory pathways. Additionally, many target genes of Wnt signaling influence cadherin adhesion. The most conspicuous of these Wnt target genes encode the transcription factors Twist and Slug, which directly inhibit the E-cadherin gene promoter. Other Wnt/beta-catenin target genes encode metalloproteases or the cell adhesion molecule L1, which favor the degradation of E-cadherin. These factors provide a mechanism whereby cadherin loss and increased Wnt signaling induce epithelial-mesenchymal transition in both carcinomas and development.

Directing traffic in neural cells: determinants of receptor tyrosine kinase localization and cellular responses

The trafficking of receptor tyrosine kinases (RTKs) to distinct subcellular locations is essential for the specificity and fidelity of signal transduction and biological responses. This is particularly important in the PNS and CNS in which RTKs mediate key events in the development and maintenance of neurons and glia through a wide range of neural processes, including survival, proliferation, differentiation, neurite outgrowth, and synaptogenesis. The mechanisms that regulate the targeting of RTKs to their subcellular destinations for appropriate signal transduction, however, are still elusive. In this review, we discuss evidence for the spatial organization of signaling machinery into distinct subcellular compartments, as well as the role for ligand specificity, receptor sorting signals, and lipid raft microdomains in RTK targeting and the resultant cellular responses in neural cells.

Differential involvement of the extracellular 6-O-endosulfatases Sulf1 and Sulf2 in brain development and neuronal and behavioural plasticity

The extracellular sulfatases Sulf1 and Sulf2 remove specific 6-O-sulfate groups from heparan sulfate, thereby modulating numerous signalling pathways underlying development and homeostasis. In vitro data have suggested that the two enzymes show functional redundancy. To elucidate their in vivo functions and to further address the question of a putative redundancy, we have generated Sulf1- and Sulf2-deficient mice. Phenotypic analysis of these animals revealed higher embryonic lethality of Sulf2 knockout mice, which can be associated with neuroanatomical malformations during embryogenesis. Sulf1 seems not to be essential for developmental or postnatal viability, as mice deficient in this sulfatase show no overt phenotype. However, neurite outgrowth deficits were observed in hippocampal and cerebellar neurons of both mutant mouse lines, suggesting that not only Sulf2 but also Sulf1 function plays a role in the developing nervous system. Behavioural analysis revealed differential deficits with regard to cage activity and spatial learning for Sulf1- and Sulf2-deficient mouse lines. In addition, Sulf1-specific deficits were shown for synaptic plasticity in the CA1 region of the hippocampus, associated with a reduced spine density. These results reveal that Sulf1 and Sulf2 fulfil non-redundant functions in vivo in the development and maintenance of the murine nervous system.

Stem cells, signals and vertebrate body axis extension

The progressive generation of chick and mouse axial tissues - the spinal cord, skeleton and musculature of the body - has long been proposed to depend on the activity of multipotent stem cells. Here, we evaluate evidence for the existence and multipotency of axial stem cells. We show that although the data strongly support their existence, there is little definitive information about their multipotency or extent of contribution to the axis. We also review the location and molecular characteristics of these putative stem cells, along with their evolutionary conservation in vertebrates and the signalling mechanisms that regulate and arrest axis extension.

The role of fibroblast growth factors in tumor growth

Biological processes that drive cell growth are exciting targets for cancer therapy. The fibroblast growth factor (FGF) signaling network plays a ubiquitous role in normal cell growth, survival, differentiation, and angiogenesis, but has also been implicated in tumor development. Elucidation of the roles and relationships within the diverse FGF family and of their links to tumor growth and progression will be critical in designing new drug therapies to target FGF receptor (FGFR) pathways. Recent studies have shown that FGF can act synergistically with vascular endothelial growth factor (VEGF) to amplify tumor angiogenesis, highlighting that targeting of both the FGF and VEGF pathways may be more efficient in suppressing tumor growth and angiogenesis than targeting either factor alone. In addition, through inducing tumor cell survival, FGF has the potential to overcome chemotherapy resistance highlighting that chemotherapy may be more effective when used in combination with FGF inhibitor therapy. Furthermore, FGFRs have variable activity in promoting angiogenesis, with the FGFR-1 subgroup being associated with tumor progression and the FGFR-2 subgroup being associated with either early tumor development or decreased tumor progression. This review highlights the growing knowledge of FGFs in tumor cell growth and survival, including an overview of FGF intracellular signaling pathways, the role of FGFs in angiogenesis, patterns of FGF and FGFR expression in various tumor types, and the role of FGFs in tumor progression.

Fibroblast growth factor signaling in development of the cerebral cortex

Despite substantial and exciting recent progress in our understanding of developmental processes in the cerebral cortex, there is still much to be learned about the molecular and cellular mechanisms that account for formation of the cortical structures, and in turn, how the regulation of these mechanisms is linked to cortical functions and behaviors in animals and humans. Fibroblast growth factors (FGFs) are a classic family of growth factors that are important in neural development and whose structures and signaling have been well-studied molecularly and biochemically. Recent advances have revealed their diverse but specific functions in patterning and neurogenesis during cortical development, as evidenced by multiple experimental approaches using in vivo models. Importantly, changes in FGF signaling during development have been shown to influence structure and function of the cerebral cortex as well as animal behavior, and have been implicated in disorders of nervous system function and intellectual development in humans. For example, disturbance of FGF pathways during development has been implicated in the pathogenesis of autism spectrum disorders. Experimental models with altered cortical structure due to perturbations of FGF signaling present a unique opportunity whereby molecular and cellular mechanisms that underlie cortical function and animal behavior can be directly studied and linked to each other.

Fibroblast growth factor receptor-1 phosphorylation requirement for cardiomyocyte differentiation in murine embryonic stem cells

Fibroblast growth factor receptor-1 (Fgfr1) gene knockout impairs cardiac and haematopoietic development in murine embryonic stem cells (mESC). In FGFR1, tyrosine residues Y653 and Y654 are responsible for its tyrosine kinase (TK) activity whereas phosphorylated Y463 and Y766 represent docking sites for intracellular substrates. Aim of this study was the characterization of FGFR1 signalling requirements necessary for cardiomyocyte differentiation in mESC. To this purpose, fgfr1(-/-) mESC were infected with lentiviral vectors harbouring human wild-type hFGFR1 or the Y653/654F, Y463F and Y766F hFGFR1 mutants. The resulting embryonic stem (ES) cell lines were differentiated as embryoid bodies (EBs) and beating foci formation was evaluated. In order to appraise the presence of cells belonging to cardiovascular and haematopoietic lineages, specific markers were analysed by quantitative PCR, whole mount in situ hybridization and immunofluorescence. Transduction with TK(+) hFGFR1 or the TK(+) Y766F-hFGFR1 mutant rescued cardiomyocyte beating foci formation in fgfr1(-/-) EBs whereas the TK(-) Y653/654F-hFGFR1 mutant and the TK(+) Y463F-hFGFR1 mutant were both ineffective. Analysis of the expression of early and late cardiac markers in differentiating EBs confirmed these observations. At variance with cardiomyocyte differentiation, all the transduced TK(+) FGFR1 forms were able to rescue haematopoietic differentiation in EBs originated by infected fgfr1(-/-) mESC, only the TK(-) Y653/654F-hFGFR1 mutant being ineffective. In keeping with these observations, treatment with different signalling pathway inhibitors indicates that protein kinase C and ERK activation are essential for cardiomyocyte but not for haematopoietic differentiation in EBs generated by fgfr1(+/-) approximately mESC. In conclusion, our results suggest that, although FGFR1 kinase activity is necessary for both cardiac and haematopoietic lineage maturation in mESC, phosphorylation of Y463 in the intracellular domain of the receptor is a specific requirement for cardiomyocyte differentiation.

Fgf4 is required for left-right patterning of visceral organs in zebrafish

Fgf signaling plays essential roles in many developmental events. To investigate the roles of Fgf4 signaling in zebrafish development, we generated Fgf4 knockdown embryos by injection with Fgf4 antisense morpholino oligonucleotides. Randomized LR patterning of visceral organs including the liver, pancreas, and heart was observed in the knockdown embryos. Prominent expression of Fgf4 was observed in the posterior notochord and Kupffer's vesicle region in the early stages of segmentation. Lefty1, lefty2, southpaw, and pitx2 are known to play crucial roles in LR patterning of visceral organs. Fgf4 was essential for the expression of lefty1, which is necessary for the asymmetric expression of southpaw and pitx2 in the lateral plate mesoderm, in the posterior notochord, and the expression of lefty2 and lefty1 in the left cardiac field. Fgf8 is also known to be crucial for the formation of Kupffer's vesicle, which is needed for the LR patterning of visceral organs. In contrast, Fgf4 was required for the formation of cilia in Kupffer's vesicle, indicating that the role of Fgf4 in the LR patterning is quite distinct from that of Fgf8. The present findings indicate that Fgf4 plays a unique role in the LR patterning of visceral organs in zebrafish.

Advances in the molecular pathogenesis of craniofacial conditions

The impact that the understanding of fibroblast growth factor receptor (FGFR) biology and its relevance to the pathogenesis of the craniosynostoses has made cannot be underestimated. As the genetic and molecular pathology of other conditions become increasingly understood, there is much hope that robust and relevant animal models of these conditions may be generated. From these models-and in conjunction with laboratory studies in vitro-comes a real hope of improved therapeutic strategies. The future lies in increased cooperation between clinicians working in high-volume centers and basic scientists. This article decribes the results of a decade of research in which the molecular pathology of the craniosynostoses was unravelled. The understanding of the importance of FGFR mutations to the genetic etiology of craniosynostosis opened up novel studies in developmental biology in various tissues. Such studies describe the functional effects of FGFR mutations. Investigations of FGFR expression in human craniofacial development have related functional molecular studies to human craniosynostosis syndromes, which provides a link between the gene mutation and the affected child.

Knockdown of the intraflagellar transport protein IFT46 stimulates selective gene expression in mouse chondrocytes and affects early development in zebrafish

Bone morphogenetic proteins (BMPs) act as multifunctional regulators in morphogenesis during development. In particular they play a determinant role in the formation of cartilage molds and their replacement by bone during endochondral ossification. In cell culture, BMP-2 favors chondrogenic expression and promotes hypertrophic maturation of chondrocytes. In mouse chondrocytes we have identified a BMP-2-sensitive gene encoding a protein of 301 amino acids. This protein, named mIFT46, is the mouse ortholog of recently identified Caenorhabditis elegans and Chlamydomonas reinhardtii intraflagellar transport (IFT) proteins. After generation of a polyclonal antibody against mIFT46, we showed for the first time that the endogenous protein is located in the primary cilium of chondrocytes. We also found that mIFT46 is preferentially expressed in early hypertrophic chondrocytes located in the growth plate. Additionally, mIFT46 knockdown by small interfering RNA oligonucleotides in cultured chondrocytes specifically stimulated the expression of several genes related to skeletogenesis. Furthermore, Northern blotting analysis indicated that mIFT46 is also expressed before chondrogenesis in embryonic mouse development, suggesting that the role of mIFT46 might not be restricted to cartilage. To explore the role of IFT46 during early development, we injected antisense morpholino oligonucleotides in Danio rerio embryos to reduce zebrafish IFT46 protein (zIFT46) synthesis. Dramatic defects in embryonic development such as a dorsalization and a tail duplication were observed. Thus our results taken together indicate that the ciliary protein IFT46 has a specific function in chondrocytes and is also essential for normal development of vertebrates.

Sp1 is required for transcriptional activation of the fibroblast growth factor receptor 1 gene in neonatal cardiomyocytes

Fibroblast growth factor receptor 1 (FGFR1) is the predominant FGFR in cardiac tissue and regulates proliferation, differentiation, and maintenance of normal myocardium. During development of cardiac tissue, FGFR1 gene expression regulates cardiomyocyte proliferation. The focus of this study was to determine the molecular mechanism of transcriptional activation of the FGFR1 gene in proliferating neonatal cardiomyocytes. Analysis of DNA sequence of the FGFR1 gene identified three potential Sp factor binding sites located at 49 bp, 68 bp, and 100 bp upstream from the 3' end of the promoter segment. Mutation of each of these sites resulted in a significant decline in FGFR1 promoter activity compared to wild type promoter activity, and combinatorial mutation of all three sites completely abrogated promoter activity to background levels. In addition, overexpression of Sp1 in neonatal cardiomyocytes resulted in a dose-dependent increase in wild type FGFR1 promoter activity. However, Sp1-mediated up-regulation of promoter activity was abrogated when all three Sp interacting sites were mutated. Chromatin immunoprecipitation (ChIP) assays were used to demonstrate direct interactions of Sp1 with the proximal promoter region of the FGFR1 gene in neonatal cardiomyocytes. ChIP assays using Drosophila Schneider Line 2 (SL2) cells transiently transfected with wild type or mutant FGFR1 promoter constructs verified the direct interaction between Sp1 and the three Sp1 interacting sites of the promoter. Western blot analyses indicated that Sp1 was present in cytoplasmic and nuclear extracts of neonatal myocardium. These results indicate that Sp1 is a necessary positive regulator of FGFR1 gene transcription in neonatal cardiomyocytes.

Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects

BACKGROUND

The number of mouse mutants and strains with neural tube closure defects (NTDs) now exceeds 190, including 155 involving known genes, 33 with unidentified genes, and eight "multifactorial" strains.

METHODS

The emerging patterns of mouse NTDs are considered in relation to the unknown genetics of the common human NTDs, anencephaly, and spina bifida aperta.

RESULTS

Of the 150 mouse mutants that survive past midgestation, 20% have risk of either exencephaly and spina bifida aperta or both, parallel to the majority of human NTDs, whereas 70% have only exencephaly, 5% have only spina bifida, and 5% have craniorachischisis. The primary defect in most mouse NTDs is failure of neural fold elevation. Most null mutations (>90%) produce syndromes of multiple affected structures with high penetrance in homozygotes, whereas the "multifactorial" strains and several null-mutant heterozygotes and mutants with partial gene function (hypomorphs) have low-penetrance nonsyndromic NTDs, like the majority of human NTDs. The normal functions of the mutated genes are diverse, with clusters in pathways of actin function, apoptosis, and chromatin methylation and structure. The female excess observed in human anencephaly is found in all mouse exencephaly mutants for which gender has been studied. Maternal agents, including folate, methionine, inositol, or alternative commercial diets, have specific preventative effects in eight mutants and strains.

CONCLUSIONS

If the human homologs of the mouse NTD mutants contribute to risk of common human NTDs, it seems likely to be in multifactorial combinations of hypomorphs and low-penetrance heterozygotes, as exemplified by mouse digenic mutants and the oligogenic SELH/Bc strain.

Distinct functions of the major Fgf8 spliceform, Fgf8b, before and during mouse gastrulation

The vertebrate Fgf8 gene produces multiple protein isoforms by alternative splicing. Two evolutionarily conserved spliceforms, Fgf8a and Fgf8b, exhibit distinct bioactivities, with Fgf8b having a more potent inductive activity due to higher affinity for Fgf receptors. To investigate the in vivo requirement for Fgf8b, we created a splice-site mutation abolishing Fgf8b expression in mice. Analysis of this mutant has uncovered a novel function of Fgf8 signaling before the onset of gastrulation. We show that the loss of Fgf8b disrupts the induction of the brachyury gene in the pregastrular embryo and, in addition, disrupts the proper alignment of the anteroposterior axis with the shape of the embryo and the uterine axes at embryonic day (E) 6.5. Importantly, Fgf8-null embryos display the same phenotype as Fgf8b-deficient embryos at E6.5, demonstrating that signaling by Fgf8b is specifically required for development of the pregastrular embryo. By contrast, during gastrulation, Fgf8a can partially compensate for the loss of Fgf8b in mesoderm specification. We show that an increased level of Fgf8a expression, which leads to Fgf4 expression in the primitive streak, can also promote mesoderm migration in the absence of Fgf8b. Therefore, different Fgf signals may have distinct requirements for the morphogenesis and gene regulation before and during gastrulation. Importantly, our findings implicate Fgf8 in the morphogenetic process that establishes the defined relationship between the axes of the embryo and the uterus at the beginning of gastrulation, a perplexing phenomenon discovered two decades ago.

FGF-mediated induction of ciliary body tissue in the chick eye

Upon morphogenesis, the simple neuroepithelium of the optic vesicle gives rise to four basic tissues in the vertebrate optic cup: pigmented epithelium, sensory neural retina, secretory ciliary body and muscular iris. Pigmented epithelium and neural retina are established through interactions with specific environments and signals: periocular mesenchyme/BMP specifies pigmented epithelium and surface ectoderm/FGF specifies neural retina. The anterior portions (iris and ciliary body) are specified through interactions with lens although the molecular mechanisms of induction have not been deciphered. As lens is a source of FGF, we examined whether this factor was involved in inducing ciliary body. We forced the pigmented epithelium of the embryonic chick eye to express FGF4. Infected cells and their immediate neighbors were transformed into neural retina. At a distance from the FGF signal, the tissue transitioned back into pigmented epithelium. Ciliary body tissue was found in the transitioning zone. The ectopic ciliary body was never in contact with the lens tissue. In order to assess the contribution of the lens on the specification of normal ciliary body, we created optic cups in which the lens had been removed while still pre-lens ectoderm. Ciliary body tissue was identified in the anterior portion of lens-less optic cups. We propose that the ciliary body may be specified at optic vesicle stages, at the same developmental stage when the neural retina and pigmented epithelium are specified and we present a model as to how this could be accomplished through overlapping BMP and FGF signals.

The distribution of the growth factors FGF-2 and VEGF, and their receptors, in growing red deer antler

The cellular distributions of the growth factors FGF-2 and VEGF, and their receptors FGFR1, FGFR2 and FGFR3, and VEGFR-2 respectively, were visualized by immunohistochemistry and light microscopy in sections of growing red deer antler. Both of these signalling systems were widely expressed in the integument and osteocartilaginous compartments. FGF-2 was found in the same cells as all three FGFRs, indicating that FGF signalling may be principally autocrine. The patterns of labelling for VEGF and its receptor were similar to those seen for FGF-2 and FGFR-3, in both compartments. Our data are consistent with the findings of others in suggesting that FGF-2 induces expression of VEGF, to stimulate and maintain high rates of neovascularisation and angiogenesis, thereby providing nutrients to both velvet and bone as they rapidly grow and develop. The presence of FGF and VEGF and their receptors in epithelial cells suggests that these signalling systems play a role in skin development, raising the possibility that one or both may be involved in the close coupling of the coordinated growth of the integument and osteocartilage of antler, a process which is poorly understood at present.

A spatial and temporal map of FGF/Erk1/2 activity and response repertoires in the early chick embryo

During early vertebrate development Fibroblast Growth Factor (FGF) signalling is required for multiple activities including specification of mesodermal, neural and heart tissue, as well as gastrulation movements and regulation of differentiation and pattern onset in the extending body axis. A current challenge is to understand how FGF signalling generates such diverse outcomes. A key FGF downstream pathway is the Ras-MAPK/Erk1/2 cascade, which culminates in the phosphorylation of target proteins, such as the Ets family of transcription factors. To begin to assess specificity downstream of FGF in the chick embryo we have characterised the patterns of Fgfr1-4 expression and Erk1/2 activation, as well as expression of the Erk1/2 specific phosphatase, Mkp3 and of three Ets factor genes (Erm, Pea3 and Er81) from early blastula to the 10 somite stage. We identify new sites of Fgfr expression and show that nearly all regions of Erk1/2 activity are within Fgfr expression domains and require FGF signalling. Differences in intensity, duration, distribution and sub-cellular localisation of activated Erk1/2 are observed in distinct cell populations within the embryo and during wound healing. With few exceptions, a tight correspondence between Erk1/2 activation and Mkp3 expression is found, while specific combinations of Ets factors are associated with distinct regions of Erk1/2 activation. These findings provide a comprehensive spatial and temporal map of FGF/Erk1/2 activity during early chick development and identify region and tissue specific differences in expression of Fgfrs as well as Erk1/2 phosphorylation and transcriptional targets which help to define response specificity.

FGF6 in myogenesis

Important functions in myogenesis have been proposed for FGF6, a member of the fibroblast growth factor family accumulating almost exclusively in the myogenic lineage. However, the analyses of Fgf6 (-/-) mutant mice gave contradictory results and the role of FGF6 during myogenesis remained largely unclear. Recent reports support the concept that FGF6 has a dual function in muscle regeneration, stimulating myoblast proliferation/migration and muscle differentiation/hypertrophy in a dose-dependent manner. The alternative use of distinct signaling pathways recruiting either FGFR1 or FGFR4 might explain the dual role of FGF6 in myogenesis. A role for FGF6 in the maintenance of a reserve pool of progenitor cells in the skeletal muscle has been also strongly suggested. The aim of this review is to summarize our knowledge on the involvement of FGF6 in myogenesis.

Fibroblast growth factor-2 and receptor-1alpha(IIIc) regulate postnatal rat lung cell apoptosis

RATIONALE

Fibroblast growth factor receptor-1alpha(IIIc) [FGF-R1alpha(IIIc)] regulates recovery of neonatal rat lung growth, after 95% oxygen-mediated growth arrest. Its role in normal postnatal alveologenesis is unknown.

OBJECTIVE

To determine if FGF-R1alpha(IIIc) regulates normal postnatal alveologenesis.

METHODS

Truncated soluble FGF-R1alpha(IIIc) or neutralizing antibodies to FGF-1 or FGF-2 were injected intraperitoneally into 3-d-old rats. The pups were killed at Day 7 for studies of alveolar development.

MEASUREMENTS AND MAIN RESULTS

Injected, truncated soluble FGF-R1alpha(IIIc) inhibited phosphorylation of the endogenous FGF-R1, and downstream pathway, and paradoxically increased lung DNA content and tissue fraction while inhibiting lung cell DNA synthesis. The increase in tissue thickness was due to reduced apoptosis, as indicated by reductions in cleaved effector caspases 3 and 7. Inhibition of the intrinsic apoptosis pathway was suggested by decreases in the proapoptotic protein Bax and mitochondrial cytochrome c release, and an increase in the antiapoptotic protein Bcl-x(L). Injected antibodies to FGF-1 and FGF-2 had no effect on DNA synthesis, but both increased Bcl-x(L) content and decreased cytochrome c release and cleaved caspase-7 protein expression. However, only injection of the antibody to FGF-2 replicated the increased tissue fraction and inhibited apoptosis observed with the injection of truncated soluble FGF-R1alpha(IIIc).

CONCLUSIONS

Inhibition of ligand binding, most likely of FGF-2, to the FGF-R1alpha(IIIc) inhibits normal postnatal lung cell apoptosis.

Crkl deficiency disrupts Fgf8 signaling in a mouse model of 22q11 deletion syndromes

Deletions on chromosome 22q11.21 disrupt pharyngeal and cardiac development and cause DiGeorge and related human syndromes. CRKL (CRK-Like) lies within 22q11.21, and Crkl-/- mice have phenotypic features of 22q11 deletion (del22q11) syndromes. While human FGF8 does not localize to 22q11, deficiency of Fgf8 also generates many features of del22q11 syndrome in mice. Since Fgf8 signals via receptor-type tyrosine kinases, and Crk family adaptor proteins transduce intracellular signals downstream of tyrosine kinases, we investigated whether Crkl mediates Fgf8 signaling. In addition to discovering genetic interactions between Crkl and Fgf8 during morphogenesis of structures affected in del22q11 syndrome, we found that Fgf8 induces tyrosine phosphorylation of FgfRs 1 and 2 and their binding to Crkl. Crkl is required for normal cellular responses to Fgf8, including survival and migration, Erk activation, and target gene expression. These findings provide mechanistic insight into disrupted intercellular interactions in the pathogenesis of malformations seen in del22q11 syndrome.

Neuronal vulnerability in transgenic mice expressing an inducible dominant-negative FGF receptor

Fibroblast Growth Factors (FGFs) and their receptors (FGFRs) are widely expressed in the mature nervous system and are thought to mediate plasticity and repair. We report the generation of transgenic mice that can be induced to express a dominant-negative FGFR (dnFGFR) in select neuronal populations. We show that a modified Thy1 promoter [Vidal, M., Morris, R., Grosveld, F., and Spanopoulou, E. 1990. Tissue-specific control elements of the Thy-1 gene. EMBO J 9 833-840] can be used to drive widespread neuronal expression of the reverse tetracycline transactivator M2 (rtTA-M2 [Urlinger, S., Baron, U., Thellmann, M., Hasan, M.T., Bujard, H., and Hillen, W., 2000. Exploring the sequence space for tetracycline-dependent transcriptional activators: novel mutations yield expanded range and sensitivity. Proc. Natl. Acad. Sci. U. S. A. 97, 7963-7968]), which after stimulation with doxycycline induces co-expression of dnFGFR in mosaic subpopulations of rtTA-M2-positive forebrain neurons, but not in hindbrain and spinal cord rtTA-M2-positive neurons. Expression of dnFGFR did not cause overt neurodegeneration, but led to increased neuronal vulnerability: four days after a stab injury, cell death was marked in the hippocampus of dnFGFR-expressing animals when compared to controls. The nuclear morphology of dying CA1 pyramidal cells suggested an apoptotic mechanism of cell death. These observations demonstrate the importance of endogenous FGFs in the maintenance of the nervous system.

Using genomewide mutagenesis screens to identify the genes required for neural tube closure in the mouse

BACKGROUND

Neural tube closure is a critical embryological process that requires the coordination of many molecular and cellular events. Only recently has the molecular basis of the cell movements that drive neural tube closure begun to be elucidated. This has been accomplished in part due to the analysis of a growing number of genetically targeted and naturally occurring mouse mutant strains that have neural tube defects (NTDs). Currently there are more than 100 genes that when mutated result in NTDs in the mouse. Yet only approximately 10% of genes in the mouse genome have been mutated and their gross phenotype analyzed, suggesting that only a small percentage of the genes that can cause NTDs have been identified.

METHODS

In order to more systematically and fully understand the genetic basis of neural tube closure and to begin to define the molecular pathways that direct this key embryonic event, our laboratories have undertaken a forward genetic screen in mice. From this we hope to gain a better understanding of the regulation of this complex morphogenic processes.

CONCLUSIONS

The mouse provides a good model for human neural tube closure, and therefore the information gained from generating novel mouse models of NTDs will help to predict the genes responsible for human NTDs and provide experimental evidence for how they function.

Conditional inactivation of Fgfr1 in mouse defines its role in limb bud establishment, outgrowth and digit patterning

Previous studies have implicated fibroblast growth factor receptor 1 (FGFR1) in limb development. However, the precise nature and complexity of its role have not been defined. Here, we dissect Fgfr1 function in mouse limb by conditional inactivation of Fgfr1 using two different Cre recombinase-expressing lines. Use of the T (brachyury)-cre line led to Fgfr1 inactivation in all limb bud mesenchyme (LBM) cells during limb initiation. This mutant reveals FGFR1 function in two phases of limb development. In a nascent limb bud, FGFR1 promotes the length of the proximodistal (PD) axis while restricting the dimensions of the other two axes. It also serves an unexpected role in limiting LBM cell number in this early phase. Later on during limb outgrowth, FGFR1 is essential for the expansion of skeletal precursor population by maintaining cell survival. Use of mice carrying the sonic hedgehog(cre) (Shh(cre)) allele led to Fgfr1 inactivation in posterior LBM cells. This mutant allows us to test the role of Fgfr1 in gene expression regulation without disturbing limb bud growth. Our data show that during autopod patterning, FGFR1 influences digit number and identity, probably through cell-autonomous regulation of Shh expression. Our study of these two Fgfr1 conditional mutants has elucidated the multiple roles of FGFR1 in limb bud establishment, growth and patterning.

Mechanisms underlying differential responses to FGF signaling

Fibroblast growth factors (FGFs) are key regulators of several developmental processes in which cell fate and differentiation to various tissue lineages are determined. The importance of the proper spatial and temporal regulation of FGF signals is evident from human and mouse genetic studies which show that mutations leading to the dysregulation of FGF signals cause a variety of developmental disorders including dominant skeletal diseases and cancer. The FGF ligands signal via a family of receptor tyrosine kinases and, depending on the cell type or stage of maturation, produce diverse biological responses that include proliferation, growth arrest, differentiation or apoptosis. A central issue in FGF biology is to understand how these diverse cellular responses are determined and how similar signaling inputs can generate distinct patterns of gene expression that govern the specificity of the cellular response. In this review we draw upon studies from the past fifteen years and attempt to construct a molecular picture of the different levels of regulation by which such specific cellular responses could be achieved by FGF signals. We discuss whether specificity could lie in the nature of the ligand, the particular receptor, the signal transduction pathways utilized, or the transcriptional regulation of specific genes. Finally, we also discuss how the interplay of FGF signals with other signaling systems could contribute to the cellular response. In particular we focus on the interaction with the Wnt pathway since FGF/Wnt cross-talk is emerging as an important nexus in regulating a variety of biological processes.

PDGF signaling in cells and mice

Since its discovery over three decades ago, platelet-derived growth factor (PDGF) has been a model system for learning how growth factors regulate biological processes. For the first several decades investigators used cells grown in tissue culture. More recently, PDGF signaling has also been investigated in mice. This review outlines the advances in these two systems, and highlights some of the directions for future investigation.