The presence of fibroblast growth factor in the frog egg: its role as a natural mesoderm inducer

Mesoderm induction and patterning: Insights from neuromesodermal progenitors

The discovery of mesoderm inducing signals helped usher in the era of molecular developmental biology, and today the mechanisms of mesoderm induction and patterning are still intensely studied. Mesoderm induction begins during gastrulation, but recent evidence in vertebrates shows that this process continues after gastrulation in a group of posteriorly localized cells called neuromesodermal progenitors (NMPs). NMPs reside within the post-gastrulation embryonic structure called the tailbud, where they make a lineage decision between ectoderm (spinal cord) and mesoderm. The majority of NMP-derived mesoderm generates somites, but also contributes to lateral mesoderm fates such as endothelium. The discovery of NMPs provides a new paradigm in which to study vertebrate mesoderm induction. This review will discuss mechanisms of mesoderm induction within NMPs, and how they have informed our understanding of mesoderm induction more broadly within vertebrates as well as animal species outside of the vertebrate lineage. Special focus will be given to the signaling networks underlying NMP-derived mesoderm induction and patterning, as well as emerging work on the significance of partial epithelial-mesenchymal states in coordinating cell fate and morphogenesis.

Constitutive and Regulated Shedding of Soluble FGF Receptors Releases Biologically Active Inhibitors of FGF-2

The identification of soluble fibroblast growth factor (FGF) receptors in blood and the extracellular matrix has led to the prediction that these proteins modulate the diverse biological activities of the FGF family of ligands in vivo. A recent structural characterization of the soluble FGF receptors revealed that they are primarily generated by proteolytic cleavage of the FGFR-1 ectodomain. Efforts to examine their biological properties are now focused on understanding the functional consequences of FGFR-1 ectodomain shedding and how the shedding event is regulated. We have purified an FGFR-1 ectodomain that is constitutively cleaved from the full-length FGFR-1(IIIc) receptor and released into conditioned media. This shed receptor binds FGF-2; inhibits FGF-2-induced cellular proliferation; and competes with high affinity, cell surface FGF receptors for ligand binding. FGFR-1 ectodomain shedding downregulates the number of high affinity receptors from the cell surface. The shedding mechanism is regulated by ligand binding and by activators of PKC, and the two signaling pathways appear to be independent of each other. Deletions and substitutions at the proposed cleavage site of FGFR-1 do not prevent ectodomain shedding. Broad spectrum inhibitors of matrix metalloproteases decrease FGFR-1 ectodomain shedding, suggesting that the enzyme responsible for constitutive, ligand-activated, and protein kinase C-activated shedding is a matrix metalloprotease. In summary, shedding of the FGFR-1 ectodomain is a highly regulated event, sharing many features with a common system that governs the release of diverse membrane proteins from the cell surface. Most importantly, the FGFR ectodomains are biologically active after shedding and are capable of functioning as inhibitors of FGF-2.

A YWHAZ Variant Associated With Cardiofaciocutaneous Syndrome Activates the RAF-ERK Pathway

Cardiofaciocutaneous (CFC) syndrome is a genetic disorder characterized by distinctive facial features, congenital heart defects, and skin abnormalities. Several germline gain-of-function mutations in the RAS/RAF/MEK/ERK pathway are associated with the disease, including KRAS, BRAF, MEK1, and MEK2. CFC syndrome thus belongs to a group of disorders known as RASopathies, which are all caused by pathogenic mutations in various genes encoding components of the RAS pathway. We recently identified novel variants in YWHAZ, a 14-3-3 family member, in individuals with a phenotype consistent with CFC that may potentially be deleterious and disease-causing. In the current study, we take advantage of the vertebrate model Xenopus laevis to analyze the functional consequence of a particular YWHAZ variant, S230W, and investigate the molecular mechanisms underlying its activity. We show that compared with wild type YWHAZ, the S230W variant induces severe embryonic defects when ectopically expressed in early Xenopus embryos. The S230W variant also rescues the defects induced by a dominant negative FGF receptor more efficiently and enhances Raf-stimulated Erk phosphorylation to a higher level than wild type YWHAZ. Although neither YWHAZ nor the variant promotes membrane recruitment of Raf proteins, the variant binds to more Raf and escapes phosphorylation by casein kinase 1a. Our data provide strong support to the hypothesis that the S230W variant of YWHAZ is a gain-of-function mutation in the RAS-ERK pathway and may underlie a CFC phenotype.

FGF and canonical Wnt signaling cooperate to induce paraxial mesoderm from tailbud neuromesodermal progenitors through regulation of a two-step epithelial to mesenchymal transition

Mesoderm induction begins during gastrulation. Recent evidence from several vertebrate species indicates that mesoderm induction continues after gastrulation in neuromesodermal progenitors (NMPs) within the posteriormost embryonic structure, the tailbud. It is unclear to what extent the molecular mechanisms of mesoderm induction are conserved between gastrula and post-gastrula stages of development. Fibroblast growth factor (FGF) signaling is required for mesoderm induction during gastrulation through positive transcriptional regulation of the T-box transcription factor brachyury We find in zebrafish that FGF is continuously required for paraxial mesoderm (PM) induction in post-gastrula NMPs. FGF signaling represses the NMP markers brachyury (ntla) and sox2 through regulation of tbx16 and msgn1, thereby committing cells to a PM fate. FGF-mediated PM induction in NMPs functions in tight coordination with canonical Wnt signaling during the epithelial to mesenchymal transition (EMT) from NMP to mesodermal progenitor. Wnt signaling initiates EMT, whereas FGF signaling terminates this event. Our results indicate that germ layer induction in the zebrafish tailbud is not a simple continuation of gastrulation events.

Molecular specification of germ layers in vertebrate embryos

In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. Classic experiments in amphibian embryos revealed the tissue interactions involved in germ layer formation and provided the groundwork for the identification of secreted and intracellular factors involved in this process. We will begin this review by summarising the key findings of those studies. We will then evaluate them in the light of more recent genetic studies that helped clarify which of the previously identified factors are required for germ layer formation in vivo, and to what extent the mechanisms identified in amphibians are conserved across other vertebrate species. Collectively, these studies have started to reveal the gene regulatory network (GRN) underlying vertebrate germ layer specification and we will conclude our review by providing examples how our understanding of this GRN can be employed to differentiate stem cells in a targeted fashion for therapeutic purposes.

Maintenance of multipotency in human dermal fibroblasts treated with Xenopus laevis egg extract requires exogenous fibroblast growth factor-2

Direct reprogramming of a differentiated somatic cell into a developmentally more plastic cell would offer an alternative to applications in regenerative medicine that currently depend on either embryonic stem cells (ESCs), adult stem cells, or induced pluripotent stem cells (iPSCs). Here we report the potential of select Xenopus laevis egg extract fractions, in combination with exogenous fibroblast growth factor-2 (FGF2), to affect life span, morphology, gene expression, protein translation, and cellular localization of OCT4 and NANOG transcription factors, and the developmental potential of human dermal fibroblasts in vitro. A gradual change in morphology is accompanied by translation of embryonic transcription factors and their nuclear localization and a life span exceeding 60 population doublings. Cells acquire the ability to follow adipogenic, neuronal, and osteogenic differentiation under appropriate induction conditions in vitro. Analysis of active extract fractions reveals that Xenopus egg protein and RNAs as well as exogenously supplemented FGF2 are required and sufficient for induction and maintenance of this phenotypic change. Factors so far identified in the active fractions include FGF2 itself, transforming growth factor-β, maskin, and nucleoplasmin. Identification of critical factors needed for reprogramming may allow for nonviral, chemically defined derivation of human-induced multipotent cells that can be maintained by exogenous FGF2.

Decoupling of amniote gastrulation and streak formation reveals a morphogenetic unity in vertebrate mesoderm induction

Mesoderm is formed during gastrulation. This process takes place at the blastopore in lower vertebrates and in the primitive streak (streak) in amniotes. The evolutionary relationship between the blastopore and the streak is unresolved, and the morphogenetic and molecular changes leading to this shift in mesoderm formation during early amniote evolution are not well understood. Using the chick model, we present evidence that the streak is dispensable for mesoderm formation in amniotes. An anamniote-like circumblastoporal mode of gastrulation can be induced in chick and three other amniote species. The induction requires cooperative activation of the FGF and Wnt pathways, and the induced mesoderm field retains anamniote-like dorsoventral patterning. We propose that the amniote streak is homologous to the blastopore in lower vertebrates and evolved from the latter in two distinct steps: an initial pan-amniote posterior restriction of mesoderm-inducing signals; and a subsequent lineage-specific morphogenetic modification of the pre-ingression epiblast.

In vitro culture of human chondrocytes (1): A novel enhancement action of ferrous sulfate on the differentiation of human chondrocytes

Chondrogenic differentiation of mesenchymal cells is generally thought to be initiated by the inductive action of specific growth factors and depends on intimate cell-cell interactions. The aim of our investigation was to characterize the influences of basic fibroblast growth factor (bFGF) and ferroussulfate (FeSO(4)) on proliferation and differentiation of human articular chondrocytes (HAC). This is the first report of the effects of FeSO(4) on chondrogenesis of HAC. Multiplied chondrocytes of hip and shoulder joints were cultured in chondrocyte growth medium supplemented with bFGF, FeSO(4), or both bFGF + FeSO(4) for4weeks. A 20 mul aliquot of a cell suspension containing2 x 10(7) cells ml(-1) was delivered onto each well of 24-well tissue culture plates. Cells cultured with the growth medium only was used as a control. Alamar blue and alcian blue staining were done to determine the chondrocyte proliferation and differentiation, respectively, after 4 weeks. The samples exposed to bFGF, FeSO(4), and combination of both indicated sufficient cell proliferation similar to the control level. Differentiations of the HAC exposed to bFGF, FeSO(4),and bFGF + FeSO(4) were 1.2-, 2.0-, and 2.2-fold of the control, respectively. Therefore, chondrocyte differentiation was significantly enhanced by the addition of FeSO(4) andbFGF + FeSO(4). The combined effects of bFGF and FeSO(4) were additive, rather than synergistic. These results suggest that treatment with ferrous sulfate alone or in combination with basic fibroblast growth factor etc, is a powerful tool to promote the differentiation of HAC for the clinical application.

Germ layer induction in ESC--following the vertebrate roadmap

Controlled differentiation of pluripotential cells takes place routinely and with great success in developing vertebrate embryos. It therefore makes sense to take note of how this is achieved and use this knowledge to control the differentiation of embryonic stem cells (ESCs). An added advantage is that the differentiated cells resulting from this process in embryos have proven functionality and longevity. This unit reviews what is known about the embryonic signals that drive differentiation in one of the most informative of the vertebrate animal models of development, the amphibian Xenopus laevis. It summarizes their identities and the extent to which their activities are dose-dependent. The unit details what is known about the transcription factor responses to these signals, describing the networks of interactions that they generate. It then discusses the target genes of these transcription factors, the effectors of the differentiated state. Finally, how these same developmental programs operate during germ layer formation in the context of ESC differentiation is summarized.

Development of a multipotent clonal human periodontal ligament cell line

The periodontal ligament (PDL) that anchors the tooth root to the alveolar bone influences the lifespan of the tooth, and PDL lost through periodontitis is difficult to regenerate. The development of new PDL-regenerative therapies requires the isolation of PDL stem cells. However, their characteristics are unclear due to the absence of somatic PDL stem cell lines and because PDL is composed of heterogeneous cell populations. Recently, we succeeded in immortalizing human PDL fibroblasts that retained the properties of the primary cells. Therefore, we aimed to establish a human PDL-committed stem cell line and investigate the effects of basic fibroblast growth factor (bFGF) on the osteoblastic differentiation of the cells. Here, we report the development of cell line 1-17, a multipotent clonal human PDL cell line that expresses the embryonic stem cell-related pluripotency genes Oct3/4 and Nanog, as well as the PDL-related molecules periostin and scleraxis. Continuous treatment of cell line 1-17 with bFGF in osteoblastic induction medium inhibited its calcification, with down-regulated expression of FGF-Receptor 1 (FGF-R1), whereas later addition of bFGF potentiated its calcification. Furthermore, bFGF induced calcification of cell line 1-17 when it was co-cultured with osteoblastic cells. These results suggest that cell line 1-17 is a PDL-committed stem cell line and that bFGF exerts dualistic (i.e., promoting and inhibitory) effects on the osteoblastic differentiation of cell line 1-17 based on its differentiation stage.

Calcium signaling in vertebrate embryonic patterning and morphogenesis

Signaling pathways that rely on the controlled release and/or accumulation of calcium ions are important in a variety of developmental events in the vertebrate embryo, affecting cell fate specification and morphogenesis. One such major developmentally important pathway is the Wnt/calcium signaling pathway, which, through its antagonism of Wnt/beta-catenin signaling, appears to regulate the formation of the early embryonic organizer. In addition, the Wnt/calcium pathway shares components with another non-canonical Wnt pathway involved in planar cell polarity, suggesting that these two pathways form a loose network involved in polarized cell migratory movements that fashion the vertebrate body plan. Furthermore, left-right axis determination, neural induction and somite formation also display dynamic calcium release, which may be critical in these patterning events. Finally, we summarize recent evidence that propose a role for calcium signaling in stem cell biology and human developmental disorders.

Xenopus POU factors of subclass V inhibit activin/nodal signaling during gastrulation

Three POU factors of subclass V, Oct-25, Oct-60 and Oct-91 are expressed in Xenopus oocytes and early embryos. We here demonstrate that vegetal overexpression of Oct-25, Oct-60, Oct-91 or mammalian Oct-3/4 suppresses mesendoderm formation in Xenopus embryos. Oct-25 and Oct-60 are shown to inhibit activin/nodal and FGF signaling pathways. Loss of Oct-25 and Oct-60 function results in elevated transcription of mesendodermal marker genes and ectopic formation of endoderm in the equatorial region of gastrula stage embryos. Within the ectoderm, Oct-25 promotes neural fate by upregulating neuroectodermal genes, such as Xsox2, which prevent differentiation of neural progenitors into neurons. We also show that mouse Oct-3/4 and Xenopus Oct-25 or Oct-60 behave as functional homologues. We conclude that Xenopus Oct proteins are required to control the levels of embryonic signaling pathways, thereby ensuring the correct specification of germ layers.

Xenopus ADAMTS1 negatively modulates FGF signaling independent of its metalloprotease activity

We have isolated the Xenopus ortholog of ADAMTS1 (a disintegrin and metalloprotease with thrombospondin motifs), XADAMTS1, which is expressed in the presumptive ectoderm, then the Spemann organizer, and later in the trunk organizer region and posterior ectoderm in the Xenopus embryo. We show that, when overexpressed in the dorsal marginal zone or in the anterior ectoderm by mRNA injection, XADAMTS1 inhibits gastrulation or generates embryos with an enlarged cement gland, respectively. XADAMTS1 also reduces the expression of Xbra in both whole embryos and FGF-treated animal caps. These effects of XADAMTS1 are likely to be due to its inhibition of the Ras-MAPK cascade because XADAMTS1 inhibits the phosphorylation of ERK by FGF4 in animal caps. Deletion analysis of XADAMTS1 revealed that a combination of the signal peptide and the C-terminal region containing the thrombospondin type 1 repeats is necessary and sufficient for this function, whereas the metalloprotease domain is dispensable. In addition, loss-of-function analysis with antisense morpholino oligos showed that knockdown of XADAMTS1 sensitizes animal caps to Xbra induction by FGF2. These data suggest that secreted XADAMTS1 negatively modulates FGF signaling in the Xenopus embryo.

B-Raf and C-Raf are required for Ras-stimulated p42 MAP kinase activation in Xenopus egg extracts

During mitosis, a select pool of MEK1 and p42/p44 MAPK becomes activated at the kinetochores and spindle poles, without substantial activation of the bulk of the cytoplasmic p42/p44 MAPK. Recently, we set out to identify the MAP kinase kinase kinase (MAPKKK) responsible for this mitotic activation, using cyclin-treated Xenopus egg extracts as a model system, and presented evidence that Mos was the relevant MAPKKK . However, a second MAPKKK distinct from Mos was readily detectable as well. Here, we partially purify this second MAPKKK and identify it as B-Raf. No changes in the activity of B-Raf were detectable during progesterone-induced oocyte maturation, after egg fertilization, or during the early embryonic cell cycle, arguing against a role for B-Raf in the mitotic activation of MEK1 and p42 MAPK. Ras proteins can bring about activation of MEK1 and p42 MAPK in extracts, and Ras may contribute to signaling from the classical progesterone receptor during oocyte maturation and from receptor tyrosine kinases during early embryogenesis. We found that both B-Raf and C-Raf, but not Mos, are required for Ras-induced MEK1 and p42 MAPK activation. These data indicate that two upstream stimuli, active Ras and active Cdc2, utilize different MAPKKKs to activate MEK1 and p42 MAPK.

Functions and regulations of fibroblast growth factor signaling during embryonic development

Fibroblast growth factors (FGF) are secreted molecules which function through the activation of specific tyrosine kinases receptors, the FGF receptors that transduce the signal by activating different pathways including the Ras/MAP kinase and the phospholipase-C gamma pathways. FGFs are involved in the regulation of many developmental processes including patterning, morphogenesis, differentiation, cell proliferation or migration. Such a diverse set of activities requires a tight control of the transduction signal which is achieved through the induction of different feedback inhibitors such as the Sproutys, Sef and MAP kinase phosphatase 3 which are responsible for the attenuation of FGF signals, limiting FGF activities in time and space.

Fibroblast growth factor signaling during early vertebrate development

Fibroblast growth factors (FGFs) have been implicated in diverse cellular processes including apoptosis, cell survival, chemotaxis, cell adhesion, migration, differentiation, and proliferation. This review presents our current understanding on the roles of FGF signaling, the pathways employed, and its regulation. We focus on FGF signaling during early embryonic processes in vertebrates, such as induction and patterning of the three germ layers as well as its function in the control of morphogenetic movements.

Cellular and molecular adaptations to injurious mechanical stress

Transient disruptions of plasma membrane integrity--'wounds'--are frequently suffered by cells of gut, skin, muscle and the aorta, organs that are normally subjected to mechanical stress in vivo. As a protection against such potentially fatal mechanically induced injuries, cells may employ specialized submembranous proteins that mechanically reinforce the plasma membrane and thus prevent wounding or, should wounding occur, they may assemble a cytoskeletal structure to aid wound healing. Membrane wounds may provide a route out of the cytoplasm for basic fibroblast growth factor, explaining how a growth factor that lacks a conventional signal peptide sequence can act extracellularly.

An early Fgf signal required for gene expression in the zebrafish hindbrain primordium

We have explored the role of fibroblast growth factor (Fgf) signaling in regulating gene expression in the early zebrafish hindbrain primordium. We demonstrate that a dominant negative Fgf receptor (FgfR) construct disrupts gene expression along the entire rostrocaudal axis of the hindbrain primordium and, using an FgfR antagonist, we find that this Fgf signal is required at early gastrula stages. This effect cannot be mimicked by morpholino antisense oligos to Fgf3, Fgf8 or Fgf24--three Fgf family members known to be secreted from signaling centers at the midbrain-hindbrain boundary (MHB), in rhombomere 4 and in caudal mesoderm at gastrula stages. We propose that an Fgf signal is required in the early gastrula to initiate hindbrain gene expression and that this is distinct from the later roles of Fgfs in patterning the hindbrain during late gastrula/early segmentation stages. We also find that blocking either retinoic acid (RA) or Fgf signaling disrupts hindbrain gene expression at gastrula stages, suggesting that both pathways are essential at this stage. However, both pathways must be blocked simultaneously to disrupt hindbrain gene expression at segmentation stages, indicating that these signaling pathways become redundant at later stages. Furthermore, exogenous application of RA or Fgf alone is sufficient to induce hindbrain genes in gastrula stage tissues, suggesting that the two-signal requirement can be overcome under some conditions. Our results demonstrate an early role for Fgf signaling and reveal a dynamic relationship between the RA and Fgf signaling pathways during hindbrain development.

RNA editing and regulation of Drosophila 4f-rnp expression by sas-10 antisense readthrough mRNA transcripts

We have previously described an example of extensively A-to-G edited cDNA derived from adult heads of the fruitfly Drosophila melanogaster. In that study, the source of the predicted antisense RNA pairing strand for template recognition by dADAR editase was not identified, and the biological significance of the observed hyperediting was not known. Here, we address each of these questions. 4f-rnp and sas-10 are closely adjacent X-linked genes located on opposite DNA strands that produce convergent transcripts. We show that developmentally regulated antisense sas-10 readthrough mRNA arises by activation of an upstream promoter P2 during the late embryo stage of fly development. The sas-10 readthrough transcripts pair with 4f-rnp mRNA to form double-stranded molecules, as indicated by A-to-G editing observed in both RNA strands. It would be predicted that perfect RNA duplexes would be targeted for modification/degradation by enzyme pathways that recognize double-stranded RNAs, leading to decline in 4f-rnp mRNA levels, and this is what we observe. The observation using quantitative RT-PCR that sas-10 readthrough and 4f-rnp transcript levels are inversely related suggests a role for the antisense RNA in posttranscriptional regulation of 4f-rnp gene expression during development. Potential molecular mechanisms that could lead to this result are discussed, one of which is targeted transcript degradation via the RNAi pathway. Insofar as the dADAR editase and RNAi pathways are known to be constitutive in this system, it is likely that control of antisense RNA transcription is the rate-limiting factor. The results provide insight into roles of naturally occurring antisense RNAs in regulation of eukaryotic gene expression.

Lipofection strategy for the study of Xenopus retinal development

The analysis of gene function during retinal development can be addressed by perturbing gene expression either by inhibition or by overexpression in desired regions and at defined stages of development. An in vivo lipofection strategy has been applied for stage-specific and region-specific expression of genes in Xenopus retina. Due to colipofection efficiency, this strategy enables us to study functional interaction of genes by lipofecting multiple expression constructs. This lipofection technique also allows us to transfect morpholino oligonucleotides into retinoblasts to block gene function. We present here various aspects of this technique, including recent improvements and modifications.

Expression of fibroblast growth factors in rat dorsal root ganglion neurons and regulation after peripheral nerve injury

Using cDNA array, we observed the expression of eight members of the fibroblast growth factor (FGF) family, FGF 2, 5, 7, 9, 10, 13 and 14, in rat lumbar 4 and 5 dorsal root ganglia (DRGs). Over a period of 28 days after sciatic nerve transection, the array signals for FGF 2 and 7 were significantly increased in the DRGs, while FGF 13 decreased. Using the reverse transcription polymerase chain reaction (RT-PCR), we confirmed the axotomy-induced changes in the expression of FGF 7 and 13. hybridization showed that FGF 13 was expressed in 60% of DRG neurons under normal circumstance. Seven days after axotomy the number of FGF 13-positive neurons was decreased to 18%, but partially recovered to 40% after 28 days. FGF 13 immunoreactivity was also decreased. These data indicate that FGFs are important for DRG neurons under normal circumstance and after nerve injury.

Partial characterization of endothelial FGF receptor functional domain by monoclonal antibody VBS-1

Polypeptide growth factors mediate their cellular responses by binding to and activating specific cell surface receptors. Monoclonal antibody (MAb) VBS-1, produced against native fibroblast growth factor receptor-1 (FGFR-1), inhibited the binding of fibroblast growth factor-2 (FGF-2) to its receptor on coronary venular endothelial cells (CVECs) as determined by 125I-FGF-2 Scatchard analysis and [3H]thymidine uptake assays (ED50 = 80 ng/mL). Enzyme studies demonstrated that MAb VBS-1 binds to a protein epitope. Proteolytic mapping of the CVEC-FGFR established that a 52 kDa doublet contained the FGF binding site and the MAb VBS-1 antigenic epitope. N-glycanase digestion suggested the presence of a 50 kDa core protein for the CVEC-FGFR. Tunicamycin treatment resulted in the loss of expression of the core protein and the mature receptor, indicating the importance of CVEC-FGFR n-linked glycosylation. By Northern blot analysis, it was determined that CVECs express fgfr-1 and not fgfr-2. VBS-1 recognized FGFR-1 (140 kDa) and crossreacted weakly with FGFR-2 (135 kDa). Using a combination of affinity crosslinking, proteolytic mapping and Mab VBS-1 binding studies, we have located the FGF binding site near the NH2-terminal domain of the receptor close to the highly acidic box.

Planarian fibroblast growth factor receptor homologs expressed in stem cells and cephalic ganglions

The strong regenerative capacity of planarians is considered to reside in the totipotent somatic stem cell called the 'neoblast'. However, the signal systems regulating the differentiation/growth/migration of stem cells remain unclear. The fibroblast growth factor (FGF)/FGF receptor (FGFR) system is thought to mediate various developmental events in both vertebrates and invertebrates. We examined the molecular structures and expression of DjFGFR1 and DjFGFR2, two planarian genes closely related to other animal FGFR genes. DjFGFR1 and DjFGFR2 proteins contain three and two immunoglobulin-like domains, respectively, in the extracellular region and a split tyrosine kinase domain in the intracellular region. Expression of DjFGFR1 and DjFGFR2 was observed in the cephalic ganglion and mesenchymal space in intact planarians. In regenerating planarians, accumulation of DjFGFR1-expressing cells was observed in the blastema and in fragments regenerating either a pharynx or a brain. In X-ray-irradiated planarians, which had lost regenerative capacity, the number of DjFGFR1-expressing cells in the mesenchymal space decreased markedly. These results suggest that the DjFGFR1 protein may be involved in the signal systems controlling such aspects of planarian regeneration as differentiation/growth/migration of stem cells.

Pluripotent cells (stem cells) and their determination and differentiation in early vertebrate embryogenesis

Mammalian embryonic stem cells can be obtained from the inner cell mass of blastocysts or from primordial germ cells. These stem cells are pluripotent and can develop into all three germ cell layers of the embryo. Somatic mammalian stem cells, derived from adult or fetal tissues, are more restricted in their developmental potency. Amphibian ectodermal and endodermal cells lose their pluripotency at the early gastrula stage. The dorsal mesoderm of the marginal zone is determined before the mid-blastula transition by factors located after cortical rotation in the marginal zone, without induction by the endoderm. Secreted maternal factors (BMP, FGF and activins), maternal receptors and maternal nuclear factors (beta-catenin, Smad and Fast proteins), which form multiprotein transcriptional complexes, act together to initiate pattern formation. Following mid-blastula transition in Xenopus laevis (Daudin) embryos, secreted nodal-related (Xnr) factors become important for endoderm and mesoderm differentiation to maintain and enhance mesoderm induction. Endoderm can be induced by high concentrations of activin (vegetalizing factor) or nodal-related factors, especially Xnr5 and Xnr6, which depend on Wnt/beta-catenin signaling and on VegT, a vegetal maternal transcription factor. Together, these and other factors regulate the equilibrium between endoderm and mesoderm development. Many genes are activated and/or repressed by more than one signaling pathway and by regulatory loops to refine the tuning of gene expression. The nodal related factors, BMP, activins and Vg1 belong to the TGF-beta superfamily. The homeogenetic neural induction by the neural plate probably reinforces neural induction and differentiation. Medical and ethical problems of future stem cell therapy are briefly discussed.

eFGF and its mode of action in the community effect during Xenopus myogenesis

The community effect is an interaction among a group of many nearby precursor cells, necessary for them to maintain tissue-specific gene expression and differentiate co-ordinately. During Xenopus myogenesis, the muscle precursor cells must be in group contact throughout gastrulation in order to develop into terminally differentiated muscle. The molecular basis of this community interaction has not to date been elucidated. We have developed an assay for testing potential community factors, in which isolated muscle precursor cells are treated with a candidate protein and cultured in dispersion. We have tested a number of candidate factors and we find that only eFGF protein is able to mediate a community effect, stimulating stable muscle-specific gene expression in demonstrably single muscle precursor cells. In contrast, Xwnt8, bFGF, BMP4 and TGF(β)2 do not show this capacity. We show that eFGF is expressed in the muscle precursor cells at the right time to mediate the community effect. Moreover, the time when the muscle precursor cells are sensitive to eFGF corresponds to the period of the endogenous community effect. Finally, we demonstrate that FGF signalling is essential for endogenous community interactions. We conclude that eFGF is likely to mediate the community effect in Xenopus myogenesis.

Bone growth factors

Osteoblastic culture models, experimental, and clinical models have revealed that bone growth factors influence cellular activity. Growth factors including bone morphogenetic proteins, transforming growth factor beta, platelet-derived growth factor, insulin-like growth factors I and II, and acidic and basic fibroblast growth factors, are powerful tools for fracture healing and bone grafting. Understanding the role that bone growth factors play in bone repair is necessary to apply these factors in a clinical setting.

The VT+ and VT- isoforms of the fibroblast growth factor receptor type 1 are differentially expressed in the presumptive mesoderm of Xenopus embryos and differ in their ability to mediate mesoderm formation

Previously, we cloned a variant form of the type 1 fibroblast growth factor receptor (FGFR1), FGFR-VT-, from Xenopus embryos (Gillespie, L. L., Chen, G., and Paterno, G. D. (1995) J. Biol. Chem. 270, 22758-22763). This isoform differed from the reported FGFR1 sequence (FGFR-VT+) by a 2-amino acid deletion, Val(423)-Thr(424), in the juxtamembrane region. This deletion arises from the use of an alternate 5' splice donor site, and the activity of the VT+ and VT- forms of the FGFR1 was regulated by phosphorylation at this site. We have now investigated the expression pattern and function of these two isoforms in mesoderm formation in Xenopus embryos. Cells within the marginal zone are induced to form mesoderm during blastula stages. RNase protection analysis of blastula stage embryos revealed that the VT+ isoform was expressed throughout the embryo but that the VT- isoform was expressed almost exclusively in the marginal zone. The ratio of VT+:VT- transcripts in the marginal zone indicated that the VT+ form was predominant throughout blastula stages except for a brief interval, coinciding with the start of zygotic transcription, when a dramatic increase in VT- expression levels was detected. This increase could be mimicked in part by treatment of animal cap explants with FGF-2. Overexpression of the VT+ isoform in Xenopus embryos resulted in development of tadpoles with severe reductions in trunk and tail structures, while embryos overexpressing the VT- isoform developed normally. A standard mesoderm induction assay revealed that a 10-fold higher concentration of FGF-2 was required to reach 50% induction in VT+-overexpressing animal cap explants compared with those overexpressing the VT- isoform. Furthermore, little or no expression of the panmesodermal marker Brachyury (Xbra) was detected in VT+-overexpressing embryos, while VT--overexpressing embryos showed normal staining. This demonstrates that VT+ overexpression had a negative effect on mesoderm formation in vivo. These data are consistent with a model in which mesoderm formation in vivo is regulated, at least in part, by the relative expression levels of the VT+ and VT- isoforms.

FGF signaling and the anterior neural induction in Xenopus

We previously showed that FGF was capable of inducing Xenopus gastrula ectoderm cells in culture to express position-specific neural markers along the anteroposterior axis in a dose-dependent manner. However, conflicting results have been obtained concerning involvement of FGF signaling in the anterior neural induction in vivo using the same dominant-negative construct of Xenopus FGF receptor type-1 (delta XFGFR-1 or XFD). We explored this issue by employing a similar construct of receptor type-4a (XFGFR-4a) in addition, since expression of XFGFR-4a was seen to peak between gastrula and neurula stages, when the neural induction and patterning take place, whereas expression of XFGFR-1 had not a distinct peak during that period. Further, these two FGFRs are most distantly related in amino acid sequence in the Xenopus FGFR family. When we injected mRNA of a dominant-negative version of XFGFR-4a (delta XFGFR-4a) into eight animal pole blastomeres at 32-cell stage, anterior defects including loss of normal structure in telencephalon and eye regions became prominent as examined morphologically or by in situ hybridization. Overexpression of delta XFGFR-1 appeared far less effective than that of delta XFGFR-4a. Requirement of FGF signaling in ectoderm for anterior neural development was further confirmed in culture: when ectoderm cells that were overexpressing delta XFGFR-4a were cocultured with intact organizer cells from either early or late gastrula embryos, expression of anterior and posterior neural markers was inhibited, respectively. We also showed that autonomous neuralization of the anterior-type observed in ectoderm cells that were subjected to prolonged dissociation was strongly suppressed by delta XFGFR-4a, but not as much by delta XFGFR-1. It is thus indicated that FGF signaling in ectoderm, mainly through XFGFR-4, is required for the anterior neural induction by organizer. We may reconcile our data to the current "neural default model," which features the central roles of BMP4 signaling in ectoderm and BMP4 antagonists from organizer, simply postulating that the neural default pathway in ectoderm includes constitutive FGF signaling step.

Regulation of avian fibroblast growth factor receptor 1 (FGFR-1) gene expression during skeletal muscle differentiation

Myogenic cell proliferation and differentiation are regulated by a fibroblast growth factor (FGF) signal transduction cascade mediated by a high-affinity fibroblast growth factor receptor (FGFR). Exogenous FGF added to myogenic cultures has a mitogenic effect promoting myoblast proliferation while repressing differentiation. We have examined the regulation of the FGFR-1 gene (cek-1) in avian myogenic cultures by immunocytochemistry and Northern blot analysis. FGFR-1 protein was readily detected in undifferentiated myoblast cultures and was significantly reduced in differentiated muscle fiber cultures. Similarly, FGFR-1 mRNA was 2.5-fold more abundant in myoblast cultures than in differentiated cultures. To define the molecular mechanism regulating FGFR-1 gene expression in proliferating myoblasts and post-mitotic muscle fibers, we have isolated and partially characterized the avian FGFR-1 gene promoter. Transfection of FGFR-1 promoter-chloramphenicol acetyltransferase gene constructs into myogenic cultures identified two regions regulating expression of this gene in myoblasts. A distal region of 2226 bp conferred a high level of expression in myoblasts. This region functioned in an orientation-dependent manner and interacted with a promoter element(s) in a proximal 1058 bp promoter region to direct transcription. Deletion analysis revealed a 78 bp region that confers a high level of cek1 promoter activity in myoblasts. This DNA segment also contains Spl binding sites and interacts with a component in myoblast nuclear protein extracts. The proximal promoter region alone demonstrated no activity in directing transcription in either myoblasts or muscle fibers. Using the full-length promoter, gene expression was significantly decreased in differentiated muscle fibers relative to undifferentiated myoblasts indicating that the promoter-reporter gene constructs contain elements regulating expression of the endogenous FGFR-1 gene in both myoblasts and muscle fibers.

Axis determination by inhibition of Wnt signaling in Xenopus

The Wnt family of secreted polypeptides participate in a variety of developmental processes in which embryonic polarity is established. To study a role for Wnt ligands in vertebrate axis determination, we interfered with Wnt signaling in the embryo using the extracellular domain of Xenopus Frizzled 8 (ECD8), which blocks Wnt-dependent activation of a target gene in Xenopus ectodermal explants. Expression of ECD8 in ventral blastomeres resulted in formation of secondary axes containing abundant notochord and head structures. These results suggest that Wnt signaling is required to maintain ventral cell fates and has to be suppressed for dorsal development to occur.

Characterization of a novel member of the FGF family, XFGF-20, in Xenopus laevis

The cDNA for a novel member of the FGF family (XFGF-20) was isolated from a Xenopus cDNA library prepared at the tailbud stage using as a probe the product of degenerate PCR performed with primers based on mammalian FGF-9s. This cDNA was 1860 bp long, and contained a single open reading frame that encoded 208 amino acid residues. The deduced amino acid sequence contained a motif characteristic of the FGF family and it was similar (73.1% overall homology) to XFGF-9 but differed from XFGF-9 in its amino-terminal region (33.3% homology). XFGF-20 mRNA was expressed only zygotically in embryos at and after the blastula stage, but it was also specifically expressed in the stomach and testis of adults. By contrast, XFGF-9 mRNA was expressed maternally in eggs and in many adult tissues. When XFGF-20 mRNA was overexpressed in early embryos, gastrulation was abnormal and development of anterior structures was suppressed. In such embryos, the expression of the Xbra transcript was suppressed during gastrulation while the expression of the transcripts of cerberus, Siamois, dkk-1, chordin, and Xotx-2 genes was normal. These results suggest that correct expression of XFGF-20 during gastrulation is required for the formation of normal head structures in Xenopus laevis during embryogenesis and that expression of the Xbra gene mediates this phenomenon.

Role of activin and other peptide growth factors in body patterning in the early amphibian embryo

The amphibian body plan is established as the result of a series of inductive interactions. During early cleavage stages cells in the vegetal hemisphere induce overlying animal hemisphere cells to form mesoderm. The interaction represents the first major body-patterning event and is mediated by peptide growth factors. Various peptide growth factors have been implicated in mesoderm development, including most notably members of the transforming growth factor-beta superfamily. Identification of the so-called "natural" inducer from among the several candidate peptide growth factors is being achieved by employing several experimental strategies, including the use of a tissue explant assay for testing potential inducers, cloning of marker genes as indices of early induction events, and microinjection of altered peptide growth factor receptors to disrupt normal embryonic inductions. Activin emerges as the most likely choice for assignment of the role of endogenous mesoderm inducer, because it currently best fulfills the rigorous set of criteria expected of such an important embryonic signaling molecule. Activin, however, may not act alone in mesoderm induction. Other peptide growth factors such as fibroblast growth factor might be involved, especially in the regional patterning of the mesoderm. In addition, several genes (e.g., Wnt and noggin), which are expressed after the mesoderm is initially induced, probably assist in further definition of the mesoderm pattern. Following mesoderm induction, the primary embryonic organizer tissue (first described in 1924 by Spemann) develops and contributes further to body patterning by its action as a neural inducer. Peptide growth factors such as activin may also be involved in the inductive event, either directly (by facilitating gene expression) or indirectly (by serving to constrain pathways).

Human fibroblast growth factor-18 stimulates fibroblast cell proliferation and is mapped to chromosome 14p11

The fibroblast growth factors (FGFs) play crucial roles in controlling embryonic development, cell growth, morphogenesis, and tissue repair in animals. Furthermore, FGFs may have a role in angiogenesis and may be involved in tumor invasion and metastasis. Here, we present the cloning and sequence of human FGF-18, a novel member of the FGF family. Sequence comparison indicates that FGF-18 is conserved with the other FGFs and most homologous to FGF-8 among the FGF family members. We showed that human FGF-18 was expressed primarily in the heart, skeletal muscle, and pancreas, and at lower levels in the other tissues. FGF-18 was also expressed at low levels in certain cancer cell lines. FGF-18 contains a typical signal peptide and was secreted when it was transfected into mammalian cells. Recombinant FGF-18 protein stimulated proliferation in the fibroblast cell line NIH3T3 in a dose-dependent manner, suggesting that FGF-18 is a functional growth factor. Finally, the FGF-18 gene was evolutionarily conserved, and localized to human chromosome 14p11.

FGF is required for posterior neural patterning but not for neural induction

Fibroblast growth factor (FGF) has been implicated in a variety of developmental processes including posterior mesoderm and neural patterning. Previous work has led to contradictory roles for FGF in neural induction and anteroposterior neural patterning. Launay et al. (Development 122, 869-880, 1996) suggested a requirement for FGF in anterior neural induction. In contrast, Kroll and Amaya (Development 122, 3173-3183, 1996) and Bang et al. (Development 124, 2075-2085, 1997) proposed that FGF is not required for early neural patterning. Here we use a loss-of-function assay to examine whether FGF is required for neural patterning in three experimental situations: (i) in Xenopus early embryos, (ii) in embryonic explants consisting of presumptive dorsal mesoderm and neurectoderm (Keller explants), and (iii) in explants of dorsal ectoderm and posterior mesoderm in which FGF signaling is specifically blocked in the ectoderm. When cultured until tailbud stages, Keller explants develop neural tissue with normal anteroposterior pattern. Overexpression of the dominant-negative FGF receptor (XFD) in Keller explants inhibited the posterior neural markers En-2, Krox-20, and HoxB9, but not the panneural marker nrp-1 and the anterior neurectodermal markers XAG-1 and Xotx-2. Similar results were seen in whole embryos, but only when XFD RNA was targeted to both the dorsal and lateral regions. In contrast, addition of FGF to Keller explants resulted in a shift of the midbrain-hindbrain boundary marker En-2 to a more anterior position normally fated to become cement gland. We also determined whether FGF is required specifically by the neurectoderm for anteroposterior neural patterning. Recombinants of dorsal ectoderm and posterior mesoderm were made in which FGF was specifically blocked in the ectoderm. Spinal cord and hindbrain markers were inhibited in these recombinants, whereas anterior markers and cement gland development were enhanced. Our results demonstrate that FGF is important for posterior development in both mesoderm and neurectoderm and that neural induction and posteriorization represent separable developmental events.

FGF-18, a novel member of the fibroblast growth factor family, stimulates hepatic and intestinal proliferation

The fibroblast growth factors (FGFs) play key roles in controlling tissue growth, morphogenesis, and repair in animals. We have cloned a novel member of the FGF family, designated FGF-18, that is expressed primarily in the lungs and kidneys and at lower levels in the heart, testes, spleen, skeletal muscle, and brain. Sequence comparison indicates that FGF-18 is highly conserved between humans and mice and is most homologous to FGF-8 among the FGF family members. FGF-18 has a typical signal sequence and was glycosylated and secreted when it was transfected into 293-EBNA cells. Recombinant murine FGF-18 protein (rMuFGF-18) stimulated proliferation in the fibroblast cell line NIH 3T3 in vitro in a heparan sulfate-dependent manner. To examine its biological activity in vivo, rMuFGF-18 was injected into normal mice and ectopically overexpressed in transgenic mice by using a liver-specific promoter. Injection of rMuFGF-18 induced proliferation in a wide variety of tissues, including tissues of both epithelial and mesenchymal origin. The two tissues which appeared to be the primary targets of FGF-18 were the liver and small intestine, both of which exhibited histologic evidence of proliferation and showed significant gains in organ weight following 7 (sometimes 3) days of FGF-18 treatment. Transgenic mice that overexpressed FGF-18 in the liver also exhibited an increase in liver weight and hepatocellular proliferation. These results suggest that FGF-18 is a pleiotropic growth factor that stimulates proliferation in a number of tissues, most notably the liver and small intestine.

Postgastrulation effects of fibroblast growth factor on Xenopus development

We have investigated postgastrulation functions of FGFs in Xenopus development by the implantation of heparin beads soaked in FGF2 to various positions at various stages. Anterior implantations show different effects depending on whether they are made to early neurulae or to later stages. At stage 13-14 there is a total or partial suppression of anterior structures including the forebrain, eyes, and midbrain. From stage 15 onwards there is no loss of anterior parts but there is a change in the structure of the eye such that the neural retina remains continuous with the wall of the diencephalon and the territories normally forming the optic stalk and pigment epithelium instead become neural retina. Posterior implantations cause a disruption of somite segmentation without affecting the differentiation of muscle cells. This is associated with a prolongation of the uniform expression of X-Delta-2 during the phase of segmental determination. There is also an induction of ectopic otocysts, which can lie either ipsilateral or contralateral to the FGF-bead. The results are discussed in terms of the known late expression domains of the various Xenopus FGFs, and of the late functions of FGFs in higher vertebrates. They provide new evidence for a role of endogenous FGFs in the development of the eye, somites, and otocysts.

Induction and early development of the hematopoietic and immune systems in Xenopus

This paper reviews several aspects of the development of the hematopoietic and lymphoid systems in Xenopus. The developmental biology of hematopoietic stem cells and the early development of the thymus and B-cells are discussed. Recent advances in the development of molecular indices of the hematopoietic program are also considered. Previous studies as well as new data demonstrate that the hematopoietic program is initiated at the time of gastrulation. Recent advances suggest that both positive and negative regulation is necessary for the appropriate spatial and temporal expression of the hematopoietic program during development.

Inhibition of FGF-1 receptor tyrosine kinase activity by PD 161570, a new protein-tyrosine kinase inhibitor

Through direct synthetic efforts we discovered a small molecule which is a 40 nanomolar inhibitor of the human FGF-1 receptor tyrosine kinase. 1-Tert-butyl-3-[6-(2,6-dichloro-phenyl)-2-(4-diethylamino-butylamino)-py rido[2,3-d]pyrimidin-7-yl]-urea (PD 161570) had about 5- and 100-fold greater selectivity toward the FGF-1 receptor (IC50 = 40 nM) compared with the PDGFbeta receptor (IC50 = 262 nM) or EGF receptor (IC50 = 3.7 microM) tyrosine kinases, respectively. In addition, PD 161570 suppressed constitutive phosphorylation of the FGF-1 receptor in both human ovarian carcinoma cells (A121(p)) and Sf9 insect cells overexpressing the human FGF-1 receptor and blocked the growth of A121(p) cells in culture. The results demonstrate a novel synthetic inhibitor with nanomolar potency and specificity towards the FGF-1 receptor tyrosine kinase.

Neural induction and patterning by fibroblast growth factor, notochord and somite tissue in Xenopus

Two natural neural inducing sources have been used, the notochord and the somites together with the growth factor bFGF, to investigate the anterior/posterior patterning of neural tissue in an animal cap explant model in Xenopus laevis. Notochord and somite tissue from stages 12.5/13 and 16, respectively, were manually isolated, and combined heterochronically with responding animal cap ectoderm aged to gastrula stages. Somite recombinants were also constructed with animal caps injected with noggin mRNA. The responses of the ectoderm were analyzed by reverse transcription polymerase chain reaction (RT-PCR) detection of marker gene expression, and in some cases by in situ hybridization. The requirement for FGF receptor function was analyzed using the dominant negative FGF receptor (XFD). The experiments showed that bFGF is capable of direct neural induction in caps aged to stage 10.5. It was also shown that notochords are capable of inducing anterior neural tissue in gastrula stage animal cap ectoderm, and this induction is sensitive to XFD in the responding tissue. Injection of noggin mRNA results in the induction of anterior neural differentiation, and it was demonstrated that this induction was insensitive to the expression of XFD in the responding tissue. It was also shown that somite tissue recombined with gastrula stage animal cap ectoderm, can induce both anterior and posterior nervous tissue and can also posteriorize noggin-induced anterior neural tissue when combined with noggin-injected animal cap ectoderm. This response is partially sensitive to XFD expression. The results shed light on the role of competence of animal cap ectoderm and the signals from postgastrulation axial and paraxial mesoderm in the patterning of the amphibian nervous system.

Formation and function of Spemann's organizer

The organizer is formed in an equatorial sector of the blastula stage amphibian embryo by cells that have responded to two maternal agents: a general mesoendoderm inducer (involving the TFG-beta signaling pathway) and a dorsal modifier (probably involving the Wnt signaling pathway). The meso-endoderm inducer is secreted by most vegetal cells, those containing maternal materials that had been localized in the vegetal hemisphere of the oocyte during oogenesis. As a consequence of the inducer's distribution and action, the competence domains of prospective ectoderm, mesoderm, and endoderm are established in an animal-to-vegetal order in the blastula. The dorsal modifier signal is secreted by a sector of cells of the animal and vegetal hemispheres on one side of the blastula. These cells contain maternal materials transported there in the first cell cycle from the vegetal pole of the egg along microtubules aligned by cortical rotation. The Nieuwkoop center is the region of blastula cells secreting both maternal signals, and hence specifying the organizer in an equatorial sector. Final steps of organizer formation at the late blastula or early gastrula stage may involve locally secreted zygotic signals as well. At the gastrula stage, the organizer secretes a variety of zygotic proteins that act as antagonists to various members of the BMP and Wnt families of ligands, which are secreted by cells of the competence domains surrounding the organizer. BMPs and Wnts favor ventral development, and cells near the organizer are protected from these agents by the organizer's inducers. The nearby cells are derepressed in their inherent capacity for dorsal development, which is apparent in the neural induction of the ectoderm, dorsalization of the mesoderm, and anteriorization of the endoderm. The organizer also engages in extensive specialized morphogenesis, which brings it within range of responsive cell groups. It also self-differentiates to a variety of axial tissues of the body.

Differential regulation of urokinase-type plasminogen activator expression by basic fibroblast growth factor and serum in myogenesis. Requirement of a common mitogen-activated protein kinase pathway

The broad spectrum protease urokinase-type plasminogen activator (uPA) has been implicated in muscle regeneration in vivo as well as in myogenic proliferation and differentiation in vitro. These processes are known to be modulated by basic fibroblast growth factor (FGF-2) and serum. We therefore investigated the mechanism(s) underlying the regulation of uPA expression by these two stimuli in proliferating and differentiating myoblasts. The expression of uPA mRNA and the activity of the uPA gene product were induced by FGF-2 and serum in proliferating myoblasts. uPA induction occurred at the level of transcription and required the uPA-PEA3/AP1 enhancer element, since deletion of this site in the full promoter abrogated induction by FGF-2 and serum. Using L6E9 skeletal myoblasts, devoid of endogenous FGF receptors, which have been engineered to express either FGF receptor-1 (FGFR1) or FGF receptor-4 (FGFR4), we have demonstrated that both receptors, known to be expressed in skeletal muscle cell precursors, were able to mediate uPA induction by FGF-2, whereas serum stimulation was FGF receptor-independent. The induction of uPA by FGF-2 and serum in FGFR1- and in FGFR4-expressing myoblasts required the mitogen-activated protein kinase pathway, since treatment of cells with a specific inhibitor of the mitogen-activated protein kinase/extracellular signal-regulated kinase-2 kinase, PD98059, blocked uPA promoter induction. Although FGF-2 and serum induced uPA in proliferating myoblasts, their actions on cell-cell contact-induced differentiating myoblasts differed dramatically. FGF-2, but not serum, repressed uPA expression in differentiation-committed myoblasts, and these effects were also shown to occur at the level of uPA transcription. Altogether, these results indicate a dual regulation of the uPA gene by FGF-2 and serum, which ensures uPA expression throughout the whole myogenic process in different myoblastic lineages. The effects of FGF-2 and serum on uPA expression may contribute to the proteolytic activity required during myoblast migration and fusion, as well as in muscle regeneration.

Mesoderm induction by heterodimeric AP-1 (c-Jun and c-Fos) and its involvement in mesoderm formation through the embryonic fibroblast growth factor/Xbra autocatalytic loop during the early development of Xenopus embryos

We have previously demonstrated the involvement of AP-1/Jun in fibroblast growth factor (FGF) signaling by loss-of-function assay (Dong, Z., Xu, R.-H., Kim, J., Zhan, S.-N., Ma, W.-Y., Colburn, N. H., and Kung, H. (1996) J. Biol. Chem. 271, 9942-9946). Further investigations by gain-of-function are reported in this study. AP-1 transactivation activity was increased by the treatment of animal cap explants with FGF. Ectopic overexpression of two components of AP-1 (c-jun and c-fos together, but not alone) produced posteriorized embryos and induced mesoderm formation in animal cap explants, indicating that both AP-1 heterodimers are required for mesoderm induction. Since Ras/AP-1 functions downstream of FGF signaling, we then tested the involvement of Ras/AP-1 in mesoderm maintenance mediated by embryonic FGF/Xbra using dominant-negative mutants. Mesoderm maintenance mediated by embryonic FGF/Xbra was blocked by dominant-negative mutants of Ras/AP-1, and AP-1 enhanced the expression of Xbra. Further studies demonstrated the inhibition of Ras/AP-1-mediated mesoderm formation by dominant-negative mutants of the FGF receptor and Xbra. These results indicate that Ras/AP-1 and FGF/Xbra signals are involved in the mesoderm maintenance machinery and mesoderm formation through the synergistic action of the diversified signal pathways derived from the FGF/Xbra autocatalytic loop.

Basic fibroblast growth factor (FGF-2) in mouse tooth morphogenesis

This study describes the spatio-temporal expression of basic Fibroblast growth factor (FGF-2) during odontogenesis of mouse as revealed by immunohistology. Parasagittal sections of mouse embryo head (13-18 day of gestation) containing various stages of developing tooth were incubated with a polyclonal anti-FGF-2 antibody and positive binding was evidentiated by using Streptavidin-Biotin complex-HRP system and AEC staining. We observed no FGF-2 staining at the dental lamina stage. At the bud stage slight staining is seen, limited to some epithelial cells. The intensity of the staining increases at the cap stage. In the bell stage, the stellate reticulum cells stain intensely. Later, odontoblasts and the dentin matrix stain deeply; but the epithelial cells stain faint. The extra cellular matrix of the dentin and dental papilla stain very intense but the enamel matrix is found negative. These results indicate the participation of FGF-2 in differentiation rather than in proliferation of tooth-forming cells. In particular, it appears that FGF-2 participates in odontoblast differentiation and in dentin matrix deposition. The spatio-temporally specific distribution pattern of FGF-2 in developing mouse tooth reported here emphasizes the importance of FGF-2 in mammalian odontogenesis.

FGF-8 is associated with anteroposterior patterning and limb regeneration in Xenopus

FGF-8 has attracted attention particularly because of its importance for limb development in the chick and mouse, although it also has a number of earlier expression domains in these species. We have now cloned an FGF-8 homologue from Xenopus in which it is easier to do functional studies on early development. There is no maternal expression, while zygotic expression is highest in the gastrula and neurula stages. XFGF-8 is expressed as a ring around the blastopore and subsequently in the tail bud. There are several domains in the head including the hatching gland, the branchial clefts, and the midbrain-hindbrain border. At later stages there is a prominent band of expression in the limb bud epidermis. Although there is no morphological apical ridge, this band of expression suggests that the Xenopus limb bud contains a cryptic region with a similar ability to stimulate mesenchymal outgrowth. The mesoderm-inducing activity of XFGF-8 is somewhat lower than that of other FGFs, while the posteriorizing activity is similar. These differences are probably due to the different receptor specificity. The posterior expression and high posteriorizing activity suggest that XFGF-8 contributes to the patterning of the anterior-posterior axis by FGF family members during gastrulation. In contrast to the amniotes, Xenopus limb buds can regenerate following damage. We show that regeneration is correlated with the reexpression of XFGF-8 in the distal epidermis, suggesting that this ability is critical for successful limb regeneration.

Xenopus Pax-2 displays multiple splice forms during embryogenesis and pronephric kidney development

Kidney organogenesis is initiated with the formation of the pronephric kidney and requires Pax-2 gene function. We report here the cloning and characterization of Pax-2 cDNAs from the frog Xenopus laevis, a model system suitable for the study of early kidney organogenesis. We show that expression of Xenopus Pax-2 (XPax-2) genes was confined to the nervous system, sensory organs, the visceral arches, and the developing excretory system. DNA sequencing of XPax-2 cDNAs isolated from head and pronephric kidney libraries revealed seven novel alternatively spliced Pax-2 isoforms. They all retain DNA-binding domains, but can differ significantly in their C termini with some isoforms containing a novel Pax-2 exon. We investigated the spectrum of XPax-2 splice events in pronephric kidneys, animal cap cultures and in whole embryos. Splicing of XPax-2 transcripts was found to be extensive and temporally regulated during Xenopus embryogenesis. Since all investigated tissues expressed essentially the full spectrum of XPax-2 splice variants, we conclude that splicing of XPax-2 transcripts does not occur in a tissue-specific manner.

Effects of basic fibroblast growth factor on human periodontal ligament cells

In order to clarify the regulatory mechanisms of periodontal regeneration by basic fibroblast growth factor (bFGF), effects of bFGF on proliferation, alkaline phosphatase activity, calcified nodule formation and extracellular matrix synthesis of human periodontal ligament (PDL) cells were examined in this study. bFGF enhanced the proliferative responses of PDL cells in a dose-dependent manner. The maximum mitogenic effect of bFGF on PDL cells was observed at the concentration of 10 ng/ml. In contrast, bFGF inhibited the induction of alkaline phosphatase activity and the mineralized nodule formation by PDL cells. Moreover, employing the reverse transcription-polymerase chain reaction (RT-PCR) technique, we observed that the levels of laminin mRNA of human PDL cells was specifically upregulated by bFGF stimulation, but that of type I collagen mRNA was downregulated. On the other hand, the expression of type III collagen and fibronectin mRNA were not altered even when the cells were activated by bFGF. These results suggest that suppressing cytodifferentiation of PDL cells into mineralized tissue forming cells, bFGF may play a role in wound healing by inducing growth of immature PDL cells and that in turn accelerates periodontal regeneration.