Expression of a dominant negative mutant of the FGF receptor disrupts mesoderm formation in Xenopus embryos

Point mutation in FGF receptor eliminates phosphatidylinositol hydrolysis without affecting mitogenesis

Stimulation of growth factor receptors with tyrosine kinase activity is followed by rapid receptor dimerization, tyrosine autophosphorylation and phosphorylation of signalling molecules such as phospholipase C gamma (PLC gamma) and the ras GTPase-activating protein. PLC gamma and GTPase-activating protein bind to specific tyrosine-phosphorylated regions in growth factor receptors through their src-homologous SH2 domains. Growth factor-induced tyrosine phosphorylation of PLC gamma is essential for stimulation of phosphatidylinositol hydrolysis in vitro and in vivo. We have shown that a short phosphorylated peptide containing tyrosine at position 766 from a conserved region of the fibroblast growth factor (FGF) receptor is a binding site for the SH2 domain of PLC gamma (ref. 8). Here we show that an FGF receptor point mutant in which Tyr 766 is replaced by a phenylalanine residue (Y766F) is unable to associate with and tyrosine-phosphorylate PLC gamma or to stimulate hydrolysis of phosphatidylinositol. Nevertheless, the Y766F FGF receptor mutant can be autophosphorylated, and can phosphorylate several cellular proteins and stimulate DNA synthesis. Our data show that phosphorylation of the conserved Tyr 766 of the FGF receptor is essential for phosphorylation of PLC gamma and for hydrolysis of phosphatidylinositol, but that elimination of this hydrolysis does not affect FGF-induced mitogenesis.

The role of Wnt genes in vertebrate development

Over the past decade, many potential candidates for molecules involved in pattern formation in the vertebrate embryo have been identified. Manipulation of the expression of some of these factors has generated fascinating results that have allowed investigators to address their roles in embryogenesis. One such family consists of a group of putative cell signaling molecules related to the proto-oncogene Wnt-1. An accumulating body of evidence suggests that the Wnt-family plays a major role in several aspects of vertebrate development.

Fibroblast growth factor receptors contain a conserved HAV region common to cadherins and influenza strain A hemagglutinins: a role in protein-protein interactions?

The first extracellular domain of the cadherins has been shown to exhibit extensive sequence homology with the amino termini of the HA1 chains of influenza strain A hemagglutinins. These regions of homology are known to be functionally important in both the cadherins and the hemagglutinins. The homologous regions harbor the tripeptide HAV, which has been identified as being the cadherin cell adhesion recognition sequence. Here we report that members of the rapidly expanding family of fibroblast growth factor receptors also possess HAV-containing regions. These regions are homologous to the HAV-containing regions present within both the hemagglutinins and the cadherins and appear to be involved in regulating the function of the fibroblast growth factor receptors. We speculate that the HAV motif may represent an evolutionarily conserved amino acid sequence that will prove to be functionally important in a wide variety of proteins.

Vertebrate gastrulation

Our understanding of the mechanisms that control gastrulation is still in its infancy. One problem is that gastrulation is a complex set of coordinated behaviours involving directional cell movements, several types of cell interactions, changes in cell fate and gene expression. Therefore, the successful analysis of its control mechanisms requires simultaneous analysis of more than one of these, or at least some way of separating them. Although progress has been slow, some recent studies have made significant advances in the field and we can probably look forward to some major breakthroughs in the near future.

Intracellular signalling pathways involved in mesoderm induction by FGF

We have examined the possible role of two signal transducing mechanisms, tyrosine phosphorylation and activation of protein kinase C (PKC), during fibroblast growth factor (FGF)-induced mesoderm induction in Xenopus. Tyrosine phosphorylation was examined through the use of a monoclonal anti-phosphotyrosine antibody. This antibody was shown to recognize the FGF receptor crosslinked to radioiodinated FGF. We also studied the response of Xenopus ectodermal explants to sodium orthovanadate, a compound that has been shown to elevate intracellular phosphotyrosine levels. Thirty percent of explants cultured in 100 microM vanadate were induced. In addition, vanadate synergized with FGF to give inductions that were more dorsal in nature than either vanadate or FGF alone. The role of PKC was evaluated by measuring PKC activity during mesoderm induction by FGF and by examining the effect of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) on explants. TPA did not induce mesoderm, however, activation of PKC was detected in FGF-treated explants. Therefore, activation of the PKC pathway alone is not sufficient for mesoderm induction. Simultaneous treatment with TPA and FGF resulted in a significant inhibition of mesoderm induction by FGF, suggesting that activation of PKC could be part of a negative feedback mechanism. In contrast, TPA had no effect on induction by activin A.

Ventrolateral regionalization of Xenopus laevis mesoderm is characterized by the expression of alpha-smooth muscle actin

Mesodermal patterning in the amphibian embryo has been extensively studied in its dorsal aspects, whereas little is known regarding its ventrolateral regionalization due to a lack of specific molecular markers for derivatives of this type of mesoderm. Since smooth muscles (SM) are thought to arise from lateral plate mesoderm, we have analyzed the expression of an alpha-actin isoform specific for SM with regard to mesoderm patterning. Using an antibody directed against alpha-SM actin that recognized specifically this actin isoform in Xenopus, we have found that the expression of alpha-SM actin is restricted to visceral and vascular SM with a transient expression in the heart. The overall expression of the alpha-SM actin appears restricted to the ventral aspects of the differentiating embryo. alpha-SM actin expression appears to be activated following mesoderm induction in animal cap derivatives. Moreover, at the gastrula stage, SM precursor cells are regionalized since they will only differentiate from ventrolateral marginal zone explants. Using the animal cap assay, we have found that alpha-SM actin expression is specifically induced in treated animal cap with bFGF or a low concentration of XTC-MIF, which induce ventral structures, but not with a high concentration of XTC-MIF, which induces dorsal structures. Altogether, these results establish that alpha-SM actin is a reliable marker for ventrolateral mesoderm. We discuss the importance of this novel marker in studying mesoderm regionalization.

The location and expression of fibroblast growth factor (FGF) in F9 visceral and parietal embryonic cells after retinoic acid-induced differentiation

It is well-established that fibroblast growth factors (FGFs) participate in mesoderm formation and patterning in the developing embryo. To identify cells in mammalian embryos that produce and/or respond to FGFs, we utilized the F9 teratocarcinoma cell system. Undifferentiated F9 cells resemble inner cell mass (ICM) cells of the mouse blastocyst by several criteria including having a characteristic high nuclear to cytoplasmic ratio and by their expression of stage-specific embryonic antigens. F9 stem cells differ from ICM cells by their low spontaneous rate of differentiation and their differentiation potential. ICM cells are heterogeneous with a proportion of the cells maintaining totipotency. In contrast, F9 stem cells appear capable of forming only endodermal derivatives. Retinoic acid (RA) treatment of F9 stem cells is required for them to differentiate, and under different culturing conditions the F9 cells will form either extraembryonic parietal or visceral endoderm. We have previously shown that FGF is synthesized by F9 parietal endoderm, but not by F9 stem cells. Our present study demonstrates that F9 aggregate cultures that contain visceral endoderm cells produce cell-associated-heparin-binding mitogens for 3T3 and endothelial cells, factors with characteristics of FGFs. Furthermore, our studies detect endothelial cell-mitogens within the extracellular matrix (ECM) of F9 parietal endoderm cells, not detected within F9 stem cell 'matrices'. Parietal endoderm cell matrix mitogens could be removed by prior treatment of the ECM with buffers containing heparin or 2 M NaCl, and could be neutralized by basic FGF antibodies.

Ectopic mesoderm formation in Xenopus embryos caused by widespread expression of a Brachyury homologue

The Brachyrury (T) gene is required cell-autonomously for mesoderm formation in the posterior of the mouse embryo, and both is complementary DNA sequence and expression pattern closely resemble those of a Xenopus homologue (Xbra), suggesting that these genes have an evolutionarily conserved function in vertebrate development. Strong expression of Xbra messenger RNA is found in the ring of involuting mesoderm during Xenopus gastrulation, and the expression of Xbra is an immediate-early response of animal pole blastomeres to mesoderm-inducing factors. To assess the role of Xbra in mesoderm formation, we increased its domain of expression in the embryo by microinjection of Xbra transcripts into the animal pole of Xenopus embryos at the one-cell stage. We show that expression of Xbra by cells of the early embryo is sufficient to direct their development into differentiated mesodermal tissues. At the molecular level this response shows a sharp threshold of sensitivity to the dose of Xbra RNA delivered, and we suggest that Xbra may act as a genetic switch initiating posterior mesodermal specification during embryogenesis.

Developmental expression of the Xenopus int-2 (FGF-3) gene: activation by mesodermal and neural induction

We have used a probe specific for the Xenopus homologue of the mammalian proto-oncogene int-2 (FGF-3) to examine the temporal and spatial expression pattern of the gene during Xenopus development. int-2 is expressed from just before the onset of gastrulation through to prelarval stages. In the early gastrula, it is expressed around the blastopore lip. This is maintained in the posterior third of the prospective mesoderm and neuroectoderm in the neurula. A second expression domain in the anterior third of the neuroectoderm alone appears in the late gastrula, which later resolves into the optic vesicles, hypothalamus and midbrain-hindbrain junction region. Further domains of expression arise in tailbud to prelarval embryos, including the stomodeal mesenchyme, the endoderm of the pharyngeal pouches and the cranial ganglia flanking the otocyst. It is shown, by treatment of blastula ectoderm with bFGF and activin, that int-2 can be expressed in response to mesoderm induction. By heterotypic grafting of gastrula ectoderm into axolotl neural plate, we have also demonstrated that int-2 can be expressed in response to neural induction. These results suggest that int-2 has multiple functions in development, including an early role in patterning of the anteroposterior body axis and a later role in the development of the tail, brain-derived structures and other epithelia.

Expression of the fibroblast growth factor receptor FGFR-1/flg during gastrulation and segmentation in the mouse embryo

Recent evidence from studies in both amphibians and mammals suggest that the fibroblast growth factor (FGF) family of signaling molecules and their receptors may play regulatory roles during early embryogenesis. We have used both standard and whole-mount in situ hybridization techniques to analyze the temporal and spatial expression patterns of the murine fibroblast growth factor receptor-1 (FGFR-1) in order to help define the role of FGFs in the processes of gastrulation and segmentation. FGFR-1 transcripts were detected in the primitive ectoderm of the egg cylinder embryo but not in the primitive endoderm or ectoplacental cone. During gastrulation, FGFR-1 mRNA were expressed at high levels in the migrating embryonic mesoderm of the mid-streak-stage embryo. Late-streak-stage embryos displayed strong expression in both the embryonic ectoderm and mesoderm. Within the ectodermal lineage, FGFR-1 mRNA later became localized to the neural ectoderm during its formation and continued to be expressed at high levels throughout neural development. In the mesodermal lineage, FGFR-1 transcripts became concentrated in the posterior medial mesoderm of the embryo as it condensed to form paraxial mesoderm. The most striking expression patterns were observed before and during segmentation where FGFR-1 was strongly expressed in the presomitic mesoderm and the rostral half of the newly formed somites. The patterns of expression are consistent with a role for FGFR-1 in posterior mesoderm formation. FGFR-1 may also play significant roles in the formation of neural ectoderm and the early events that establish compartments within the developing somites.

The argos gene encodes a diffusible factor that regulates cell fate decisions in the Drosophila eye

The argos gene encodes a protein that is required for viability and that regulates the determination of cells in the Drosophila eye. A developmental analysis of argos mutant eyes indicates that the mystery cells, which are usually nonneuronal, are transformed into extra photoreceptors, and that supernumerary cone cells and pigment cells are also recruited. Clonal analysis indicates that argos acts nonautonomously and can diffuse over the range of several cell diameters. Conceptual translation of the argos gene suggests that it encodes a secreted protein.

Xenopus blastulae show regional differences in competence for mesoderm induction: correlation with endogenous basic fibroblast growth factor levels

Single cells from the animal cap and marginal zone (MZ) of mid-blastula stage embryos can undergo mesodermal or ectodermal differentiation as small clones under defined conditions in culture. Here we report that cells treated with Xenopus basic fibroblast growth factor (XbFGF), a mesoderm-inducing factor, usually differentiated into muscle. MZ cells, which normally give rise to most of the mesoderm, responded to lower concentrations of XbFGF than animal pole (AP) presumptive ectoderm cells. This difference in sensitivity correlated with immunocytochemical staining patterns that showed much greater levels of endogenous bFGF within MZ than AP cells in early embryos. At the mid-late blastula stage, nuclei of MZ cells were strongly immunoreactive. Nuclear staining persisted during gastrula and neurula stages, and extracellular bFGF also became apparent. Subsequently in somites, immunoreactivity of nuclei declined while that of the extracellular matrix was retained during tailbud stages. Nuclear localization of bFGF appeared to be temporally correlated with new transcription of muscle-specific genes, and extracellular bFGF was present during morphological differentiation. The results suggest that a cell's competence for mesoderm induction is related to its position in the embryo.

Embryonic expression and functional analysis of a Xenopus activin receptor

We report the isolation and characterization of a Xenopus activin receptor (XAR1). The amino acid sequence of this protein shows extensive homology with a murine activin receptor. The mRNA is expressed maternally and is ubiquitously distributed during the early stages of embryogenesis. Consistent with a possible role in mesoderm induction and patterning, interference with the normal expression of the receptor by overexpression in the early embryo results in the formation of ectopic dorsal axial structures. During neurulation the XAR1 mRNA is expressed predominantly in the presumptive brain and spinal cord, suggesting an additional function for XAR1 in neurogenesis.

Dominant negative mutants of transforming growth factor-beta 1 inhibit the secretion of different transforming growth factor-beta isoforms

Transforming growth factor-beta (TGF-beta) is a secreted polypeptide factor that is thought to play a major role in the regulation of proliferation of many cell types and various differentiation processes. Several related isoforms have been structurally characterized, three of which, TGF-beta 1, -beta 2, and -beta 3, have been detected in mammalian cells and tissues. Each TGF-beta form is a homodimer of a 112-amino-acid polypeptide which is encoded as a larger polypeptide precursor. We have introduced several mutations in the TGF-beta 1 precursor domain, resulting in an inhibition of TGF-beta 1 secretion. Coexpression of these mutants with wild-type TGF-beta 1, -beta 2, and -beta 3 results in a competitive and specific inhibition of the secretion of different TFG-beta forms, indicating that these mutated versions act as dominant negative mutants for TGF-beta secretion. Overexpression of dominant negative mutants can thus be used to abolish endogenous secretion of TGF-beta and structurally related family members, both in vitro and in vivo, and to probe in this way the physiological functions of the members of the TGF-beta superfamily.

Lithium-sensitive production of inositol phosphates during amphibian embryonic mesoderm induction

Mesoderm induction and body axis determination in frog (Xenopus) embryos are thought to involve growth factor-mediated cell-cell signaling, but the signal transduction pathways are unknown. Li+, which inhibits the polyphosphoinositide (PI) cycle signal transduction pathway in many cells, also disrupts axis determination and mesoderm induction. Amounts of the PI cycle-derived second messenger, inositol 1,4,5-trisphosphate, increased during mesoderm induction in normal embryos; addition of Li+ inhibited the embryonic inositol monophosphatase and reversed this increase. Embryonic PI cycle activity thus shows characteristics that indicate it may function in mesoderm induction and axis determination.

Dominant negative mutants of transforming growth factor-beta 1 inhibit the secretion of different transforming growth factor-beta isoforms

Transforming growth factor-beta (TGF-beta) is a secreted polypeptide factor that is thought to play a major role in the regulation of proliferation of many cell types and various differentiation processes. Several related isoforms have been structurally characterized, three of which, TGF-beta 1, -beta 2, and -beta 3, have been detected in mammalian cells and tissues. Each TGF-beta form is a homodimer of a 112-amino-acid polypeptide which is encoded as a larger polypeptide precursor. We have introduced several mutations in the TGF-beta 1 precursor domain, resulting in an inhibition of TGF-beta 1 secretion. Coexpression of these mutants with wild-type TGF-beta 1, -beta 2, and -beta 3 results in a competitive and specific inhibition of the secretion of different TFG-beta forms, indicating that these mutated versions act as dominant negative mutants for TGF-beta secretion. Overexpression of dominant negative mutants can thus be used to abolish endogenous secretion of TGF-beta and structurally related family members, both in vitro and in vivo, and to probe in this way the physiological functions of the members of the TGF-beta superfamily.

Inhibition of African cassava mosaic virus systemic infection by a movement protein from the related geminivirus tomato golden mosaic virus

Plant viruses encode proteins that mediate their movement through the host plant leading to the establishment of a systemic infection. We have analyzed the effect of tomato golden mosaic virus (TGMV) genes BL1 and BR1, which are thought to be involved in the process of virus movement, on the infectivity of African cassava mosaic virus (ACMV) in Nicotiana benthamiana. Recombinant genomes were constructed by replacing the ACMV coat protein coding sequence with those of either BL1 or BR1. Replication of recombinants containing BL1 and BR1 coding sequences in the sense orientation with respect to the coat protein promoter was detected in the inoculated leaves only when the constructs were co-inoculated, suggesting that both genes are being expressed and act in a cooperative manner. Co-inoculated recombinants induced localized symptoms on inoculated leaves but did not spread systematically, either because of a defect in BL1 and/or BR1 expression or due to the inability of the TGMV gene products to functionally complement their ACMV counterparts. Systemic spread of ACMV was inhibited when the recombinant containing the BL1 coding sequence in the sense, but not in the antisense, orientation was co-inoculated with ACMV DNA B. Disruption of the BL1 coding sequence by a frameshift mutation restored the ability of the recombinant to spread systemically, suggesting that the gene product is responsible for the inhibitory effect. The inhibitory phenotype was mimicked by a chimera containing amino-terminal sequences of TGMV BL1 and carboxy-terminal sequences of its ACMV homologue, BC1. The chimera has characteristics of a dominant negative mutant. We suggest that dominant negative mutants of virus movement genes may provide a novel source for virus resistance genes.

Cloning of a second type of activin receptor and functional characterization in Xenopus embryos

A complementary DNA coding for a second type of activin receptor (ActRIIB) has been cloned from Xenopus laevis that fulfills the structural criteria of a transmembrane protein serine kinase. Ectodermal explants from embryos injected with activin receptor RNA show increased sensitivity to activin, as measured by the induction of muscle actin RNA. In addition, injected embryos display developmental defects characterized by inappropriate formation of dorsal mesodermal tissue. These results demonstrate that this receptor is involved in signal transduction and are consistent with the proposed role of activin in the induction and patterning of mesoderm in Xenopus embryos.

Protein kinase C isozymes have distinct roles in neural induction and competence in Xenopus

The restricted ability of embryonic tissue to respond to inductive signals is controlled by a poorly understood phenomenon, termed competence. In Xenopus, dorsal ectoderm is more competent than ventral ectoderm to become induced to neural tissue. We tested whether the Xenopus protein kinase C (PKC) isozymes alpha and beta have a role in neural induction and competence. We found that PKC alpha is predominantly localized in dorsal ectoderm, whereas PKC beta is uniformly distributed. Overexpression of PKC beta conveys a higher propensity for neural differentiation to both dorsal and ventral ectoderm, but their difference in competence remains. However, ectopic expression of PKC alpha elevates the level of neural competence of ventral ectoderm to that of dorsal ectoderm. These data indicate that different PKC isozymes have distinct roles in mediating both neural induction and competence.

An alternatively processed mRNA from the avian c-erbB gene encodes a soluble, truncated form of the receptor that can block ligand-dependent transformation

At least four major transcripts are produced by the avian c-erbB/epidermal growth factor receptor gene. cDNAs corresponding to the smallest one, a 2.6-kb transcript, were isolated from an adult chicken liver cDNA library. Sequence analysis revealed that the 3' end of one cDNA clone diverged from the known sequence of the extracellular ligand-binding domain (LBD) of the full-length receptor. A genomic DNA subfragment that contained this unique 3' divergent end was isolated. Sequence analysis of this genomic DNA fragment revealed that the 2.6-kb c-erbB transcript is produced by alternative processing. Translation of this 2.6-kb transcript would produce a secreted, truncated receptor molecule which contains the amino-terminal three-fourths of the extracellular LBD of the native receptor. COS1 cells and primary chicken embryo fibroblast cells were transfected with expression vectors that contained the 2.6-kb c-erbB cDNA. Conditioned medium from these transfected cells contained a 70-kDa protein that was specifically immunoprecipitated by a polyclonal antiserum directed against the LBD of the avian c-erbB gene product. The 70-kDa truncated receptor could be coimmunoprecipitated from conditioned medium of transfected COS1 cells that was supplemented with recombinant human transforming growth factor alpha (TGF alpha) by a monoclonal antibody against human TGF alpha. Additionally, transfected chicken embryo fibroblast cells that overexpressed the 70-kDa truncated receptor were blocked in their ability to form TGF alpha-dependent colonies in soft agar. These data suggest that the secreted, truncated receptor encoded by the 2.6-kb c-erbB transcript can bind to TGF alpha and may play an important growth-regulatory function in vitro.

An alternatively processed mRNA from the avian c-erbB gene encodes a soluble, truncated form of the receptor that can block ligand-dependent transformation

At least four major transcripts are produced by the avian c-erbB/epidermal growth factor receptor gene. cDNAs corresponding to the smallest one, a 2.6-kb transcript, were isolated from an adult chicken liver cDNA library. Sequence analysis revealed that the 3' end of one cDNA clone diverged from the known sequence of the extracellular ligand-binding domain (LBD) of the full-length receptor. A genomic DNA subfragment that contained this unique 3' divergent end was isolated. Sequence analysis of this genomic DNA fragment revealed that the 2.6-kb c-erbB transcript is produced by alternative processing. Translation of this 2.6-kb transcript would produce a secreted, truncated receptor molecule which contains the amino-terminal three-fourths of the extracellular LBD of the native receptor. COS1 cells and primary chicken embryo fibroblast cells were transfected with expression vectors that contained the 2.6-kb c-erbB cDNA. Conditioned medium from these transfected cells contained a 70-kDa protein that was specifically immunoprecipitated by a polyclonal antiserum directed against the LBD of the avian c-erbB gene product. The 70-kDa truncated receptor could be coimmunoprecipitated from conditioned medium of transfected COS1 cells that was supplemented with recombinant human transforming growth factor alpha (TGF alpha) by a monoclonal antibody against human TGF alpha. Additionally, transfected chicken embryo fibroblast cells that overexpressed the 70-kDa truncated receptor were blocked in their ability to form TGF alpha-dependent colonies in soft agar. These data suggest that the secreted, truncated receptor encoded by the 2.6-kb c-erbB transcript can bind to TGF alpha and may play an important growth-regulatory function in vitro.

Expression of fibroblast growth factor receptors by embryonal carcinoma cells and early mouse embryos

We have previously shown that differentiation of embryonal carcinoma (EC) cells leads to both increased binding of FGF (fibroblast growth factor) and suppression of k-FGF expression. In the current study, we examined the expression of FGF receptors by EC cells, EC-derived differentiated cells and early mammalian embryos using the technique of reverse transcription-polymerase chain reaction (RT-PCR). We determined that both mouse, F9, and human, NT2/D1, EC cells as well as their differentiated counterparts express transcripts for two forms of FGF receptors, bek (bacterially expressed kinase) and flg (fms-like gene). In addition, we determined that mouse blastocysts express flg transcripts. The presence of FGF receptor transcripts in early embryos and the previous finding of FGF-related activity in medium conditioned by mouse blastocysts argue that the FGF family plays important roles during early mammalian development.

Xwnt-8 modifies the character of mesoderm induced by bFGF in isolated Xenopus ectoderm

In Xenopus, growth factors of the TGF-beta, FGF and Wnt oncogene families have been proposed to play a role in generating embryonic pattern. In this paper we examine potential interactions between the bFGF and Xwnt-8 signaling pathways in the induction and dorsal-ventral patterning of mesoderm. Injection of Xwnt-8 mRNA into 2-cell Xenopus embryos does not induce mesoderm formation in animal cap ectoderm isolated from these embryos at the blastula stage, but alters the response of this tissue to mesoderm induction by bFGF. While animal cap explants isolated from non-injected embryos differentiate to form ventral types of mesoderm and muscle in response to bFGF, explants from Xwnt-8 injected embryos form dorsal mesodermal and neural tissues in response to the same concentration of bFGF, even if the ectoderm is isolated from the prospective ventral sides of embryos or from UV-ventralized animals. Our results support a model whereby dorso-ventral mesodermal patterning can be attained by a single mesoderm inducing agent, possibly bFGF, which is uniformly distributed across the prospective dorsal-ventral axis, and which acts in concert with a dorsally localized signal, possibly a Wnt protein, which either alters the response of ectoderm to induction or modifies the character of mesoderm after its induction.

Diverse receptors for fibroblast growth factors

The development and maintenance of multicellular organisms requires a complex interplay between cells in different tissues. Many of the factors mediating cell-cell communication are polypeptides, which were originally identified because of their ability to stimulate cell growth. In addition to growth signalling several of these factors have been observed to modulate cell survival, chemotaxis and differentiation both in vitro and in vivo. Fibroblast growth factors are a good example of polypeptide mitogens eliciting a wide variety of responses depending on the target cell type. Our knowledge of the cell surface receptors mediating the effects of FGFs has recently expanded remarkably. Perhaps not surprisingly, the complexity of the FGF family and FGF induced responses is reflected as diversity and redundancy of the FGF receptors.

Of fin and fur: mutational analysis of vertebrate embryonic development

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Vertebrate axis formation

Molecular understanding of axis formation has recently taken a great leap forward with the identification and functional characterization of regulatory genes that appear to act at the top of the hierarchy leading to positional specification in the vertebrate. Analysis of these genes, which encode peptide growth factors and their receptors as well as transcription factors, is disclosing principles of early cell fate specification that are common to all vertebrates.

Peptide growth factors and their interactions during chondrogenesis

Peptide growth factors have been implicated in three aspects of cartilage growth and metabolism; the induction of mesoderm and differentiation of a cartilaginous skeleton in the early embryo, the growth and differentiation of chondrocytes within the epiphyseal growth plates leading to endochondral calcification, and the processes of articular cartilage damage and repair. Three peptide growth factor classes have been strongly implicated in these processes, the fibroblast growth factor family (FGF), the insulin-like growth factors (IGFs) including insulin, and transforming growth factor-beta (TGF-beta) and related molecules. Each of these peptide groups are expressed in the early embryo. Basic FGF, TGF-beta and the related activin have been shown to induce the appearance of mesoderm from primitive neuroectoderm. TGF-beta and related bone morphometric proteins can induce the differentiation of cartilage from primitive mesenchyme, and together with basic FGF and IGFs promote cartilage growth. Each class of growth factor is expressed within the epiphyseal growth plate where their autocrine/paracrine interactions regulate the rate of chondrocyte proliferation, matrix protein synthesis and terminal differentiation and mineralization. Basic FGF may prove useful in articular cartilage repair, while basic FGF, IGFs and TGF-beta are among a number of growth factors and cytokines that have been implicated in cartilage disease.

Growth factors and cutaneous wound repair

The healing of an adult skin lesion is a well studied but complex affair of some considerable clinical interest. Endogenous growth factors, including the EGF, FGF, PDGF and TGF beta families, are released at the wound site and presumed to be a necessary part of the natural wound healing machinery. Moreover, members of each of these families have been shown to enhance healing if added exogenously to a wound site. In this review we shall briefly discuss what is known about the mechanics and cell biology of adult wound healing, describe the normal cellular source of growth factors during the healing process and, with reference to their known capacities in tissue culture, speculate as to how particular growth factors might be able to enhance healing.

Molecular nature of Spemann's organizer: the role of the Xenopus homeobox gene goosecoid

This study analyzes the function of the homeobox gene goosecoid in Xenopus development. First, we find that goosecoid mRNA distribution closely mimics the expected localization of organizer tissue in normal embryos as well as in those treated with LiCl and UV light. Second, goosecoid mRNA accumulation is induced by activin, even in the absence of protein synthesis. It is not affected by bFGF and is repressed by retinoic acid. Lastly, microinjection of goosecoid mRNA into the ventral side of Xenopus embryos, where goosecoid is normally absent, leads to the formation of an additional complete body axis, including head structures and abundant notochordal tissue. The results suggest that the goosecoid homeodomain protein plays a central role in executing Spemann's organizer phenomenon.

Activin receptor mRNA is expressed early in Xenopus embryogenesis and the level of the expression affects the body axis formation

Activin is a member of the transforming growth factor beta (TGF-beta) and possesses various activities in cellular control phenomena. During Xenopus embryonic development, activin is thought to act as a natural mesoderm-inducing factor. We isolated here the Xenopus activin receptor cDNA from Xenopus tadpole cDNA library and examined the expression of the Xenopus activin receptor gene during the course of early embryonic development. The Xenopus activin receptor has an 87% homology at the level of deduced amino acid sequence with the mouse activin receptor, and using the cDNA obtained, three bands of mRNA with different lengths were detected in Xenopus embryos throughout early embryogenesis. We synthesized activin receptor mRNA in vitro and tested the effect of the injection of the mRNA into Xenopus fertilized eggs on subsequent development. When the synthetic mRNA was injected into uncleaved fertilized eggs, embryos with reduced trunk structure were formed. However, when the mRNA was injected into the ventral blastomeres at the 16-cell stage, embryos with a secondary body axis were formed. These results indicate the importance of the function of activin receptor in the regulatory mechanism for body axis formation.

Ectopic expression of Fgf-4 in chimeric mouse embryos induces the expression of early markers of limb development in the lateral ridge

The biological consequences of constitutive fibroblast growth factor-4 (fgf-4) expression were investigated in chimeric embryos prepared between wild-type host embryos and murine ES cells transfected with a construct in which expression of the murine fgf-4 gene was directed by the phosphoglycerate kinase (PGK-1) promoter. The embryos exhibit abnormalities of the limbs and the anterior central nervous system (CNS). The limb phenotype comprised the induction of outgrowth along the lateral ridge between the definitive fore and hind limbs resembling the early phases of limb development. The CNS defects comprised a complete absence, or marked reduction in forebrain and midbrain structures and rudimentary or absent eye development. Constitutive expression of fgf-4 was also accompanied by ectopic expression of the sonic hedgehog (shh) and msx-1 genes in the lateral ridge. These findings indicate that FGF exhibits multiple activities in early development which include the ability to induce the expression of early markers of limb development in the lateral ridge.