Radiation Therapy Oncology Group. Research Plan 2002-2006. Research Associates Committee.
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Organizational Objectives↗
Biomedical subjects
K Symes
Publications and source records attributed to K Symes.
Small-molecule control of insulin and PDGF receptor signaling and the role of membrane attachment.
BACKGROUND: Receptor tyrosine kinases (RTKs) regulate the proliferation, differentiation and metabolism of cells, and play key roles in tissue repair, tumorigenesis and development. To facilitate the study of RTKs, we have made conditional alleles that encode monomeric forms of the normally heterotetrameric insulin receptor and monomeric platelet-derived growth factor (PDGF) beta receptors fused to the FK506-binding protein 12 (FKBP12). The chimeric receptors can be induced to undergo dimerization or oligomerization by a small synthetic molecule called FK1012, and the consequences were studied in cells and embryonic tissues. RESULTS: When equipped with an amino-terminal plasma membrane localization sequence and expressed in HEK293 cells, these chimeric receptors could signal to downstream targets as indicated by the FK1012-dependent activation of p70 S6 kinase (p70(S6k)) and mitogen-activated protein (MAP) kinase. In Xenopus embryos, the engineered PDGF receptor protein induced the formation of mesoderm from animal-pole explants in an FK1012-dependent manner. A cytosolic variant of the protein underwent efficient transphosphorylation, yet failed to activate appreciably either p70(S6k) or MAP kinase following treatment with FK1012. These results provide evidence of a requirement for membrane localization of RTKs, consistent with current models of RTK signaling. CONCLUSION: We have developed an approach using the small molecule FK1012 to conditionally activate chimeric proteins containing FKBP fused to the insulin receptor or to the PDGF beta receptor. Using this system, we were able to induce mesoderm formation in Xenopus animal-cap tissue and to demonstrate that membrane localization is required for RTK signaling in transfected cells. This system should allow the further dissection of RTK-mediated pathways.
Alleles↗
Embryonic mesoderm cells spread in response to platelet-derived growth factor and signaling by phosphatidylinositol 3-kinase.
Abnormal mesoderm movement, leading to defects in axial organization, is observed in mouse and Xenopus laevis embryos deprived of platelet-derived growth factor (PDGF) AA signaling. However, neither the cellular response to PDGF nor the signaling pathways involved are understood. Herein we describe an in vitro assay to examine the direct effect of PDGF AA on aggregates of Xenopus embryonic mesoderm cells. We find that PDGF AA stimulates aggregates to spread on fibronectin. This behavior is similar to that of migrating mesoderm cells in vivo that spread and form lamellipodia and filipodia on contact with fibronectin-rich extracellular matrix. We go on to show two lines of evidence that implicate phosphatidylinositol 3-kinase (PI3K) as an important component of PDGF-induced mesoderm cell spreading. (i) The fungal metabolite wortmannin, which inhibits signaling by PI3K, blocks mesoderm spreading in response to PDGF AA. (ii) Activation of a series of receptors with specific tyrosine-to-phenylalanine mutations revealed PDGF-induced spreading of mesoderm cells depends on PI3K but not on other signaling molecules that interact with PDGF receptors including phospholipase C gamma, Ras GTPase-activating protein, and phosphotyrosine phosphatase SHPTP2. These results indicate that a PDGF signal, medicated by PI3K, can facilitate embryonic mesoderm cell spreading on fibronectin. We propose that PDGF, produced by the ectoderm, influences the adhesive properties of the adjacent mesoderm cells during gastrulation.
Androstadienes↗
PDGF signalling is required for gastrulation of Xenopus laevis.
During Xenopus gastrulation, platelet-derived growth factor (PDGF) receptor-alpha is expressed in involuting marginal zone cells which migrate over ectodermal cells expressing PDGF-A. To investigate the role of PDGF signalling during this process, we have generated a novel point mutant of PDGF receptor-alpha analogous to the W37 mutation of c-kit. This molecule is a specific, potent, dominant inhibitor of PDGF signalling in vivo. Injection of RNA encoding this protein into Xenopus embryos prevents closure of the blastopore, leads to abnormal gastrulation and a loss of anterior structures. Convergent extension is not inhibited in these embryos, but rather, involuting mesodermal cells fail to adhere to the overlying ectoderm. PDGF may therefore be required for mesodermal cell-substratum interaction.
Animals↗
Localization of PDGF A and PDGFR alpha mRNA in Xenopus embryos suggests signalling from neural ectoderm and pharyngeal endoderm to neural crest cells.
In situ hybridization analysis of Xenopus laevis embryos reveals that mRNA encoding the platelet-derived growth factor alpha receptor (PDGFR alpha) is expressed in cephalic neural crest masses prior to migration from the future neural tube and during their migration into the visceral arches. The analysis of fluorescently labeled neural crest tissue transplanted to unlabeled host embryos demonstrates that neural crest cells are the only detectable source of PDGFR alpha mRNA within visceral arches. Transcripts encoding PDGF A are present in neural ectoderm, otic vesicle and pharyngeal endoderm. Their location suggests that PDGF A provides a signal, first from the neural epithelium and later from the otic vesicle and pharyngeal endoderm, to cephalic neural crest cells during their migration in the arch region.
Animals↗
Morphological differences in Xenopus embryonic mesodermal cells are specified as an early response to distinct threshold concentrations of activin.
The involution of presumptive mesoderm that occurs during amphibian gastrulation is a complex process requiring the coordinated action of a diverse range of cells. We show that cells with distinct morphologies, resembling each of those normally found in the involuting tissue of the Xenopus embryo, are induced in dispersed animal pole cells by different doses of the potent mesoderm-inducing factor activin. Each cell type is induced within a restricted dose range of activin concentrations, the boundaries of which are well demarcated shortly after activin treatment. In contrast, Brachyury and goosecoid, two genes thought to pattern the presumptive mesoderm, and the gene encoding platelet-derived growth factor receptor alpha, which is expressed in the mesoderm of gastrula stage embryos, are induced by broad, overlapping ranges of high activin concentrations at such early times. Similarly, the response of the gene encoding platelet-derived growth factor A, which is expressed normally in ectoderm of gastrula stage embryos, diminishes gradually as the activin concentration increases. Dose windows for the expression of these four genes narrow and become distinct from one another in cell aggregates after several hours in culture, suggesting that activin prompts a dynamic program of gene expression in induced mesoderm.
Activins↗
An investigation of the specification of unequal cleavages in leech embryos.
Leech embryos develop via stereotyped cell divisions, many of which are unequal. The first division generates identifiable cells, blastomeres AB and CD, which normally follow distinct developmental pathways. When these two cells are dissociated and cultured in isolation, their fates remain distinct and are reminiscent of normal development, but their typical cleavage patterns are disrupted; cell AB undergoes relatively few cell divisions, giving rise to a variable number of macromeres and micromeres, while cell CD cleaves many times, usually forming a poorly organized set of macromeres, embryonic stem cells (teloblasts), and micromeres. We have investigated the hypothesis that the abnormal cleavage pattern of isolated CD blastomeres is due to removal of mechanical constraints normally imposed by cell AB. We find that when cell CD is constrained in vitro to mimic its in vivo shape, it cleaves more normally.
Animals↗
The biological effects of XTC-MIF: quantitative comparison with Xenopus bFGF.
Mesoderm in Xenopus and other amphibian embryos is induced by signals from the vegetal hemisphere acting on equatorial or animal hemisphere cells. These signals are diffusible and two classes of candidate signal molecule have been identified: the fibroblast growth factor (FGF) and transforming growth factor beta (TGF-beta) types. In this paper, we compare the effects of cloned Xenopus basic FGF (XbFGF) and electophoretically homogeneous XTC-MIF (a TGF-beta-like factor obtained from a Xenopus cell line) on animal pole explants. We find that they have a similar minimum active concentration (0.1-0.2 ng ml-1) but that, nonetheless, XTC-MIF is at least 40 times more active in inducing muscle. In general, we find that the two factors cause inductions of significantly different characters in terms of tissue type, morphology, gene expression and timing. At low concentrations (0.1-1.0 ng ml-1) both factors induce the differentiation of 'mesenchyme' and 'mesothelium' as well as blood-like cells. These latter cells do not, however, react with an antibody to Xenopus globin. This raised the possibility that the identification of red blood cells in other studies on mesoderm induction might have been mistaken, but combinations of animal pole regions with ventral vegetal pole regions confirmed that genuine erythrocytes are formed. The identity of the blood-like cells formed in response to the inducing factors remains unknown. At higher concentrations XTC-MIF induces neural tissue, notochord, pronephros and substantial and often segmented muscle. By contrast, XbFGF only induces significant amounts of muscle above 24 ng ml-1 and even then this is much less than that induced by XTC-MIF. For both factors an exposure of less than 30 min is effective. Competence of animal pole cells to respond to XbFGF is completely lost by the beginning of gastrulation (stage 10) while competence to XTC-MIF is detectable until somewhat later (stage 11). Since animal pole tissue is known to be able to respond to the natural inducer at least until stage 10, and perhaps until stage 10.5, this suggests that bFGF cannot be the sole inducer of mesoderm in vivo. Taken together, these results are consistent with XTC-MIF being a dorsoanterior inducer and XbFGF a ventroposterior inducer, suggesting that body pattern is established by the interaction of two types of inducing signal. This model is discussed in view of the qualitative and quantitative differences between the factors.
Animals↗
Mesoderm induction and the control of gastrulation in Xenopus laevis: the roles of fibronectin and integrins.
Exposure of isolated Xenopus animal pole ectoderm to the XTC mesoderm-inducing factor (XTC-MIF) causes the tissue to undergo gastrulation-like movements. In this paper, we take advantage of this observation to investigate the control of various aspects of gastrulation in Xenopus. Blastomeres derived from induced animal pole regions are able, like marginal zone cells, but unlike control animal pole blastomeres, to spread and migrate on a fibronectin-coated surface. Dispersed animal pole cells are also able to respond to XTC-MIF in this way; this is one of the few mesoderm-specific responses to induction that has been observed in single cells. The ability of induced animal pole cells to spread on fibronectin is abolished by the peptide GRGDSP. However, the elongation of intact explants is unaffected by this peptide. This may indicate that fibronectin-mediated cell migration is not required for convergent extension. We have investigated the molecular basis of XTC-MIF-induced gastrulation-like movements by measuring rates of synthesis of fibronectin and of the integrin beta 1 chain in induced and control explants. No significant differences were observed, and this suggests that gastrulation is not initiated simply by control of synthesis of these molecules. In future work, we intend to investigate synthesis of other integrin subunits and to examine possible post-translational modifications to fibronectin and the integrins.
Animals↗
Potentiation by the lithium ion of morphogenetic responses to a Xenopus inducing factor.
We have cultured explants of Xenopus blastular animal cap tissue from embryos that had received an earlier treatment with LiCl and from their untreated siblings, in various concentrations of XTC-cell-derived mesoderm-inducing factor (XTC-MIF, Smith, 1987; Smith et al. 1988). The pretreatment with lithium that we used transforms later morphogenesis in the whole embryo to give radialized body forms with anterior/dorsal levels of structure grossly over-represented. In addition, animal caps from 'Li+' embryos were allowed to develop without exposure to in vitro MIF (Li+ controls) and compared with normal uninduced control explants, and explants were made from normal early blastulae but given various initial treatments with LiCl in culture. The results confirm that the lithium ion itself will not induce mesoderm in competent, animal cap tissue of Xenopus. It does, however, enhance the responsiveness of this tissue to XTC-MIF, in a way that parallels its recently reported effect in the case of another mesoderm inducer of different character, bFGF (Slack et al. 1988). The effects observed are sufficient to imply that the altered body pattern that follows lithium treatment, in whole embryos, could be caused by modulation of the responses to an unaltered pattern of in situ inductive stimuli. We also observe evidence that appreciable inductive signals reach animal pole tissue beyond the limits of mesoderm formation in normal development. Relatively low concentrations of MIF prevent the development of an epidermis-specific marker in dissociated blastular animal cap cells (Symes et al. 1988). When such experiments are repeated in relation to the lithium pretreatment of embryos, such treatment is seen to have sensitized the cell population, so that the MIF concentration range that assures complete suppression of the marker is reduced. The results are discussed in relation to induction considered as pattern formation.
Animals↗
Inducing factors and the control of mesodermal pattern in Xenopus laevis.
The mesoderm of Xenopus laevis and other amphibia is formed through an inductive interaction during which cells of the vegetal hemisphere act on cells of the animal hemisphere. Two groups of factors mimic the effects of the vegetal hemisphere. One group consists of members of the fibroblast growth factor (FGF) family, while the other is related to transforming growth factor type beta (TGF-beta). In this paper we discuss the evidence that the FGF family represents 'ventral' mesoderm-inducing signals, and the TGF-beta family 'dorsal' signals. The evidence includes a discussion of the cell types formed in response to each type of factor, the fact that only XTC-MIF (a member of the TGF-beta family) and not bFGF can induce animal pole ectoderm to become Spemann's organizer, and an analysis of the timing of the gastrulation movements induced by the factors.
Animals↗
Purification, partial characterization and biological effects of the XTC mesoderm-inducing factor.
The mesoderm of Xenopus laevis is formed through an inductive interaction in which a signal from the vegetal hemisphere of the blastula acts on overlying animal pole cells. We have recently reported that the Xenopus XTC cell line secretes a mesoderm-inducing factor (MIF) which may resemble the natural signal. In this paper, we describe the purification and biological effects of XTC-MIF. XTC-MIF is a hydrophobic protein with an isoelectric point of 7.8 and an apparent relative molecular mass (Mr) of 23,500. On reduction, XTC-MIF loses its biological activity and the protein dissociates into two inactive subunits with apparent Mr of about 15,000. These properties closely resemble those of transforming growth factor type beta (TGF-beta), and it is interesting that TGF-beta 2 has recently been shown to have mesoderm-inducing activity. The biological response to XTC-MIF is graded. After exposure to 0.2-1.0 ng ml-1 XTC-MIF, stage-8 animal pole explants form mesenchyme and mesothelium. At higher concentrations, up to about 5 ng ml-1, muscle is formed, occasionally with neural tissue. In response to concentrations of XTC-MIF greater than 5-10 ng ml-1, notochord and neural tissue are usually formed. The formation of notochord and neural tissue in response to XTC-MIF represents a qualitative difference between this inducing factor and the other known group of MIFs, the heparin-binding growth factors.
Animals↗
Mesoderm induction in Xenopus laevis: responding cells must be in contact for mesoderm formation but suppression of epidermal differentiation can occur in single cells.
When Xenopus embryos are cultured in calcium- and magnesium-free medium (CMFM), the blastomeres lose adhesion but continue dividing to form a loose heap of cells. If divalent cations are restored at the early gastrula stage the cells re-adhere and eventually form muscle (a mesodermal cell type) as well as epidermis. If, however, the cells are dispersed during culture in CMFM, muscle does not form following reaggregation although epidermis does. This suggests that culturing blastomeres in a heap allows the transmission of mesoderm-induction signals from cell to cell while dispersion effectively dilutes the signal. In this paper, we have attempted to substitute for cell proximity by culturing dispersed blastomeres in XTC mesoderm-inducing factor (MIF). We find that dispersed cells do not respond to XTC-MIF by forming mesodermal cell types after reaggregation, but the factor does inhibit epidermal differentiation. One interpretation of this observation is that an early stage in mesoderm induction is the suppression of epidermal differentiation and that formation of mesoderm may require contact-mediated signals that are produced in response to XTC-MIF. We have gone on to study the suppression of epidermal differentiation in more detail. We find that this is a dose-dependent phenomenon that can occur in single cells in the absence of cell division. Animal pole blastomeres become more difficult to divert from epidermal differentiation at later stages of development and by stage 12 they are 'determined' to this fate. Fibroblast growth factor (FGF) also suppresses epidermal differentiation in isolated animal pole blastomeres and transforming growth factor-beta 1 acts synergistically with FGF in doing so.
Animals↗
The Xenopus animal pole blastomere.
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Animals↗