Regulation of the fibroblast growth factor receptor in early Xenopus embryos

bFGF as a possible morphogen for the anteroposterior axis of the central nervous system in Xenopus

Vertebrate neural development is initiated during gastrulation by the inductive action of the dorsal mesoderm (Spemann's organizer in amphibians) on neighbouring ectoderm, which eventually gives rise to the central nervous system from forebrain to spinal cord. Here we present evidence that bFGF can mimic the organizer action by inducing Xenopus ectoderm cells in culture to express four position-specific neural markers (XeNK-2, En-2, XIHbox1 and XIHbox6) along the anteroposterior axis. bFGF also induced the expression of a general neural marker NCAM but not the expression of immediate-early mesoderm markers (goosecoid, noggin, Xbra and Xwnt-8), suggesting that bFGF directly neuralized ectoderm cells without forming mesodermal cells. The bFGF dose required to induce the position-specific markers was correlated with the anteroposterior location of their expression in vivo, with lower doses eliciting more anterior markers and higher doses more posterior markers. These data indicate that bFGF or its homologue is a promising candidate for a neural morphogen for anteroposterior patterning in Xenopus. Further, we showed that the ability of ectoderm cells to express the anterior markers in response to bFGF was lost by mid-gastrula, before the organizer mesoderm completely underlies the anterior dorsal ectoderm. Thus, an endogenous FGF-like molecule released from the involuting organizer may initiate the formation of the anteroposterior axis of the central nervous system during the early stages of gastrulation by forming a concentration gradient within the plane of dorsal ectoderm.

A type 1 serine/threonine kinase receptor that can dorsalize mesoderm in Xenopus

We have cloned a type I serine/threonine kinase receptor, XTrR-I, from Xenopus. XTrR-I (Xenopus transforming growth factor beta-related receptor type I) is expressed in all regions of embryos throughout early development. Overexpression of this receptor does not affect ectoderm or endoderm but dorsalizes the mesoderm such that muscle appears in ventral mesoderm and notochord appears in lateral mesoderm normally fated to become muscle. In addition, overexpression of XTrR-I in UV-treated embryos is able to cause formation of a partial dorsal axis. These results suggest that XTrR-I encodes a receptor which responds in normal development to a transforming growth factor beta-like ligand so as to promote dorsalization. Its function would therefore be to direct mesodermalized tissue into muscle or notochord.

Molecular cloning of tyrosine kinases in the early Xenopus embryo: identification of Eck-related genes expressed in cranial neural crest cells of the second (hyoid) arch

Growth factors and their receptors play an important role in controlling cellular proliferation, migration, and differentiation during vertebrate embryogenesis. We have used the reverse transcription-polymerase chain reaction to survey the repertoire of receptor tyrosine kinases (TK) expressed during early embryogenesis of Xenopus laevis. Twelve distinct Xenopus TK cDNA classes were identified among a total of 352 cDNAs screened. A single TK cDNA class has been described previously and encodes the fibroblast growth factor receptor FGFR-A1. The remaining 11 TK cDNA classes appear to encode novel genes of the FGFR, platelet-derived growth factor receptor (PDGFR), Eph, Csk, Tyk2, and Klg subfamilies. By RNase protection assays, Xenopus TK mRNAs are rare transcripts (< 10(7) mRNA molecules/embryo), and are usually found to be expressed also maternally in the embryo. Most Xenopus TK genes examined by whole-mount in situ hybridization were expressed widely in tissues derived from multiple germ layers. Two Eck-related genes, however, were found to be restricted in their expression to neural crest of the second (hyoid) arch. Our findings are consistent with the proposed function of TKs in the regulation of specification and differentiation of embryonic tissues.

Temporal regulation of the Xenopus FGF receptor in development: a translation inhibitory element in the 3′ untranslated region

Early frog embryogenesis depends on a maternal pool of mRNA to execute critical intercellular signalling events. FGF receptor-1, which is required for normal development, is stored as a stable, untranslated maternal mRNA transcript in the fully grown immature oocyte, but is translationally activated at meiotic maturation. We have identified a short cis-acting element in the FGF receptor 3′ untranslated region that inhibits translation of synthetic mRNA. This inhibitory element is sufficient to inhibit translation of heterologous reporter mRNA in the immature oocyte without changing RNA stability. Deletion of the poly(A) tract or polyadenylation signal sequences does not affect translational inhibition by this element. At meiotic maturation, we observe the reversal of translational repression mediated by the inhibitory element, mimicking that seen with endogenous maternal FGF receptor mRNA at meiosis. In addition, the activation of synthetic transcripts at maturation does not appear to require poly(A) lengthening. We also show that an oocyte cytoplasmic protein specifically binds the 3′ inhibitory element, suggesting that translational repression of Xenopus FGF receptor-1 maternal mRNA in the oocytes is mediated by RNA-protein interactions. These data describe a mechanism of translational control that appears to be independent of poly(A) changes.

Ventral mesodermal patterning in Xenopus embryos: expression patterns and activities of BMP-2 and BMP-4

We provide a comparative analysis of the expression patterns and ventral mesoderm-inducing properties of Xenopus BMP-2 and BMP-4. Transcripts for BMP-2 and BMP-4 are maternally stored in eggs, and zygotic expression of these genes is uniform in the ectoderm and mesoderm in late blastulae. During gastrulation, BMP-2 is expressed at a low level throughout the ectoderm and marginal zone, but at early neurula stages a patch of dorso-anterior cells displays enhanced expression. In contrast, BMP-4 transcripts are restricted to the ventrolateral marginal zone during gastrulation, and in late gastrula and early neurula BMP-4 is expressed in the epidermis but not the neural plate. At post-neurula stages, BMP-2 and BMP-4 transcripts are associated with a variety of mesodermal structures, including the pharyngeal pouches, heart, blood island, and blastopore. At tailbud stages, BMP-2 and BMP-4 are expressed in neural tissues including the neural tube and brain. In mesoderm induction assays, BMP-2 and BMP-4 induce Xhox3, an early ventral-posterior mesoderm marker, and larval alpha Tl globin, a marker for red blood cells. Induction of red blood cells in response to BMP-4 was demonstrated by staining with a hemoglobin-specific reagent. Little is known about factors that induce hematopoietic lineages in vertebrates, and these results provide evidence linking BMP activity and blood differentiation. Globin induction by BMP-2 and BMP-4 is blocked by co-expression of a dominant-negative activin receptor, suggesting that either endogenous activin signals are required for BMP-mediated induction, or that the truncated activin receptor interferes with signaling by BMP receptors. In assays on marginal zone explants, we demonstrate that BMP-4 respecifies dorsal mesoderm to form ventral mesoderm, consistent with its ability to induce blood and to ventralize embryos. BMP-2, however, does not display such activity. The findings extend and support evidence that BMP-2 and BMP-4 function in ventral mesoderm induction and patterning in Xenopus. Our data furthermore highlight the multiple functions these factors fulfill during early vertebrate embryogenesis.

Vertebrate embryonic induction: mesodermal and neural patterning

Within the fertilized egg lies the information necessary to generate a diversity of cell types in the precise pattern of tissues and organs that comprises the vertebrate body. Seminal embryological experiments established the importance of induction, or cell interactions, in the formation of embryonic tissues and provided a foundation for molecular studies. In recent years, secreted gene products capable of inducing or patterning embryonic tissues have been identified. Despite these advances, embryologists remain challenged by fundamental questions: What are the endogenous inducing molecules? How is the action of an inducer spatially and temporally restricted? How does a limited group of inducers give rise to diversity of tissues? In this review, the focus is on the induction and patterning of mesodermal and neural tissues in the frog Xenopus laevis, with an emphasis on families of secreted molecules that appear to underlie inductive events throughout vertebrate embryogenesis.

An analysis of Xenopus tyrosine kinase genes and their expression in early development

Xenopus laevis and X. borealis were screened for tyrosine kinase genes using the polymerase chain reaction (PCR) and reverse transcriptase (RT)-PCR and 34 X. laevis and 23 X. borealis tyrosine genes were identified. Eighteen of the genes represented novel tyrosine kinase family members. The rest could be classified into known tyrosine kinase subfamilies, of which however only three have been previously identified in Xenopus. Eight clones, including bFGFR (xFGFR1) and potential Trk, Ins R., Fak, Fyn, and Abl homologs, were used to probe temporal and spatial gene expression in early development. Quantitative RT-PCR and whole-mount and in situ hybridization showed that most of these mRNAs were present throughout development and were broadly distributed, mainly in ectodermal and mesodermal derived tissues. At the blastula stage, bFGFR mRNA was detected within the ectoderm and a gradient of expression was noted within the invaginating mesoderm. The unexpected promiscuous expression of many tyrosine kinase genes in early development is discussed.

A role for cytoplasmic determinants in mesoderm patterning: cell-autonomous activation of the goosecoid and Xwnt-8 genes along the dorsoventral axis of early Xenopus embryos

Although an induction event is required for the formation of mesoderm in Xenopus embryos, it is not clear that this induction is wholly sufficient to give rise to a correctly patterned mesodermal layer. We have studied the expression of the two genes, goosecoid and Xwnt-8, in Xenopus gastrulae in which cell-cell communication, and therefore mesoderm induction, has been prevented by frequent cell dispersion. Although neither the early panmesodermal marker Xbra nor the muscle-specific alpha-actin gene were activated under these conditions, goosecoid and Xwnt-8 were activated in cells of dorsal and ventrolateral origin respectively, thus correctly reflecting their distribution during normal development. We also show that the spatial pattern of expression of these two genes along the animal-vegetal axis is similar in normal and in dissociated early gastrulae: goosecoid is mainly expressed in future mesoderm while the domain of expression of Xwnt-8 spans the mesoderm-endoderm boundary. These results show that, during the blastula and early gastrula stages, gene activation can be controlled cell-autonomously along both the animal-vegetal and dorsoventral embryo axes. This suggests that the inheritance of localised maternal cytoplasmic determinants is a key event for the patterning of mesoderm. We present a modified model of mesoderm formation in which the different mesoderm cell types are produced as a result of cooperation between induction-dependent and induction-independent immediate-early genes.

Isolation and developmental expression of the amphibian homolog of the fibroblast growth factor receptor 3

Recent observations suggest that fibroblast growth factors (FGFs) and their receptors are involved in the control of embryogenesis. Several FGF receptor genes have been identified so far and their expression is differentially regulated. As part of a continuing effort to analyse the differential expression of FGF receptors and their potential role during amphibian development, we have isolated a Pleurodeles homolog of FGF receptor 3 (FGFR-3), which we designated PFR-3 because of its highest homology to human FGFR-3 (75% overall identity). PFR-3 is a maternally derived mRNA. While a low level of expression persists during the cleavage and gastrula stages, a significant increase in the mRNA was observed at the end of the gastrula stage. RNase protection analysis on dissected tissues showed that PFR-3 mRNA was mainly localized to the ectoderm at the early gastrula stage and then shifted to the embryonic neural tissues, whereas adult brain had decreased levels of PFR-3 mRNA expression. Consistent with the loss of FGF receptors during skeletal muscle terminal differentiation, PFR-3 as well as other FGF receptor mRNAs were undetectable in the adult skeletal muscle. However, highest levels of PFR-3 mRNA expression were found in the testis. In situ hybridization revealed strong expression in the germinal epithelium of the embryonic brain (especially the diencephalon and rhombencephalon) and neural tube, in the lens and the cranial ganglia. The epithelium of the developing gut, like the pharynx and esophagus, also prominently expressed PFR-3 mRNA. Other sites of expression were found in the liver and in the mesenchymal condensation sites of branchial arches.(ABSTRACT TRUNCATED AT 250 WORDS)

Inducing factors in Xenopus early embryos

Recent results make it possible to postulate credible candidates for each of the known inducing signals that act to determine cell fate during Xenopus early development. Experiments on biological activity, expression patterns and inhibition of function suggest that Vg-1 and Wnt-11 may act as the primary mesoderm-inducing signals, FGF and activin may serve to relay their effects, and noggin may be a major component of the dorsalizing and neural-inducing signals from the organizer.

Basic fibroblast growth factor induces differentiation of neural tube and neural crest lineages of cultured ectoderm cells from Xenopus gastrula

The vertebrate nervous system is initially induced from a section of dorsal ectoderm by signal(s) from the underlying dorsal mesoderm during gastrulation. In an effort to identify the neural inducing factor(s) emanating from the dorsal mesoderm, we have examined the inductive action of various growth factors by applying them to ectoderm cells from Xenopus gastrulae (8- to 12.5-hour age; embryonic stage 9+ to 11 1/2) in a microculture system. Monoclonal antibodies that specifically recognize cellular differentiation antigens from three distinct ectoderm lineages (N1 for CNS neurons from neural tube, Me1 for melanophores from neural crest and E3 for skin epidermal cells from epidermal lineages, respectively) and a mesoderm lineage (Mu1 for muscle cells) were used as markers to monitor the differentiation of cultured ectoderm cells. We found that basic fibroblast growth factor (bFGF) was capable of specifically and reproducibly inducing gastrula ectoderm cells to produce CNS neurons and melanophores at concentrations as low as 5 pM, a value about 50-fold lower than that required to induce the formation of muscle cells from blastula animal cap cells (6-hour age; stage 8+). The induction of neural lineages by bFGF was correlated with a suppression of epidermal differentiation in a dose-dependent manner. bFGF never induced the formation of muscle cells from gastrula ectoderm cells even at concentrations as high as 5 nM. The response of ectoderm cells to bFGF changed dramatically during gastrulation. Ectoderm cells from early (8- to 9-hour age; stage 9+ to 10) gastrula gave rise to CNS neurons, but yielded few melanophores. As ectoderm cells were prepared from gastrulae of increasing age, their competence to form neurons was gradually lost, whereas the production of melanophores was enhanced and peaked in 11-hour gastrula (stage 10 1/2). The ability to form both neurons and melanophores was substantially reduced in 12.5-hour gastrula (stage 11 1/2). By examining ectoderm cells from the ventral and dorsal sides independently, it was also shown that during gastrulation the change in response to bFGF of the ventral ectoderm preceded that of the dorsal ectoderm. The state of competence of the ectoderm changed primarily due to intrinsic factors rather than by instruction from other parts of the gastrula embryo. This was shown by adding bFGF to cultures of ectoderm cells that were isolated at 9-hour (stage 10) and cultured for increasing periods to allow their autonomous development. The time course of both loss of neuronal competence and gain and loss of melanophore competence closely paralleled that observed in vivo during gastrulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Refinement of the structure of human basic fibroblast growth factor at 1.6 A resolution and analysis of presumed heparin binding sites by selenate substitution

The three-dimensional structure of human basic fibroblast growth factor has been refined to a crystallographic residual of 16.1% at 1.6 A resolution. The structure has a Kunitz-type fold and is composed of 12 antiparallel beta-strands, 6 of which form a beta-barrel. One bound sulfate ion has been identified in the model, hydrogen bonded to the side chains of Asn 27, Arg 120, and Lys 125. The side chain of Arg 120 has two conformations, both of which permit hydrogen bonds to the sulfate. This sulfate binding site has been suggested as the binding site for heparin (Eriksson, A.E., Cousens, L.S., Weaver, L.H., & Matthews, B.W., 1991, Proc. Natl. Acad. Sci. USA 88, 3441-3445). Two beta-mercaptoethanol (BME) molecules are also included in the model, each forming a disulfide bond to the S gamma atoms of Cys 69 and Cys 92, respectively. The side chain of Cys 92 has two conformations of which only one can bind BME. Therefore the BME molecule is half occupied at this site. The locations of possible sulfate binding sites on the protein were examined by replacing the ammonium sulfate in the crystallization medium with ammonium selenate. Diffraction data were measured to 2.2 A resolution and the structure refined to an R-factor of 13.8%. The binding of the more electron-dense selenate ion was identified at two positions. One position was identical to the sulfate binding site identified previously. The second selenate binding site, which is of lower occupancy, is situated 5.6 A from the first. This ion is hydrogen bonded by the side chain of Lys 135 and Arg 120. Thus the side chain of Arg 120 binds two selenate ions simultaneously. It is suggested that the observed second selenate binding site should also be considered as a possible binding site for heparin, or that both selenate binding sites might simultaneously contribute to the binding of heparin.

Tyrosine phosphorylation of the focal adhesion kinase pp125FAK during development: relation to paxillin

Significant changes in the level of protein tyrosine phosphorylation accompany avian embryonic development. A comparison of different tissues reveals that a similar and remarkably restricted complement of proteins is modified in this manner. In each case the major proteins detected using anti-phosphotyrosine antibodies have molecular masses of approximately 170, 150, 125, 70 and 50 kDa. As a first step in determining the function of this protein modification in embryogenesis we have initiated a study to identify these phosphoproteins. We have previously reported that the 70 kDa band is paxillin, a component of actin-membrane attachment sites associated with regions of cell adhesion (Turner, C. E. (1991) J. Cell Biol. 115, 201–207). We report here that the 125 kDa phosphotyrosine-containing protein is the tyrosine kinase pp125FAK, a protein that co-localizes with paxillin at sites of adhesion (Schaller et al. (1992) Proc. Nat. Acad. Sci. USA 89, 5192–5196). Tyrosine phosphorylation of both pp125FAK and paxillin was detected at low levels as early as embryonic day 3 and increased steadily during the first half of development, reached a maximum between embryonic days eight and twelve, and declined to background levels prior to hatching. Paxillin protein expression also increased during the first half of embryogenesis, suggesting little change in the overall phosphorylation of this protein through embryonic day 8. In contrast, pp125FAK, following an initial increase, is expressed at a constant high level during these early embryonic stages, implying an increase in its overall phosphotyrosine content.(ABSTRACT TRUNCATED AT 250 WORDS)

The frog prince-ss: a molecular formula for dorsoventral patterning in Xenopus

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Spatially restricted expression of fibroblast growth factor receptor-2 during Xenopus development

The fibroblast growth factors (FGFs) play a role in Xenopus laevis embryonic development, particularly in the induction of ventral-type mesoderm. We have isolated a full-length cDNA from Xenopus that we have designated Xenopus fibroblast growth factor receptor-2 (XFGFR-2), with significant amino acid sequence similarity to the previously described bek gene (FGFR-2). We expressed the XFGFR-2 cDNA in COS1 cells and showed that it functions as an FGF receptor by binding radiolabeled FGF-2. RNA gel blot analysis demonstrates that unlike Xenopus fibroblast growth factor receptor-1 (XFGFR-1), XFGFR-2 mRNA expression begins during gastrulation and continues through early tadpole stages. Whole-mount in situ hybridization demonstrates that XFGFR-2 mRNA is localized to the anterior neural plate in early neurula stage embryos. Later in development, XFGFR-2 expression is found in the eye anlagen, midbrain-hindbrain boundary and the otic vesicle. In addition, XFGFR-2 transcripts are expressed in animal caps in a manner that is independent of mesoderm-inducing factors. These results indicate that XFGFR-2 may have a role in development that is distinct from that of XFGFR-1.

Mesoderm and jaw development in vertebrates: the role of growth factors

The head and neck arise during development as the result of a complex series of cellular and molecular interactions that begin in the fertilized egg. In this article, the role of an important class of molecules, growth factors, is examined in two main steps of the developmental sequence: the initial induction of mesoderm and the later induction of jaw cartilage and bone. The article focuses particularly on the roles of members of the transforming growth factor-beta (TGF-beta), fibroblast growth factor (FGF), and epithelial growth factor (EGF) families in these processes and current models of growth factor involvement. Possible experiments for the future are discussed.

Differential expression and regulation of two distinct fibroblast growth factor receptors during early development of the urodele amphibian Pleurodeles waltl

Fibroblast growth factor (FGF) has been shown to be involved in mesoderm induction during amphibian development. Its presence in the embryo suggests that FGF is an endogenous inducer. By polymerase chain reaction (PCR) methods and by screening a Pleurodeles waltl tail-bud cDNA library with a cDNA probe for human FGF receptor, we have isolated two full-length cDNA clones, which we designate PFR1 and PFR4 based on their homology to the human FGF receptors FGFR-1 and FGFR-4. Both cDNA clones encode Pleurodeles FGF receptors that share characteristics common to members of the FGF receptor superfamily. The deduced amino acid sequence of PFR1 is 85% identical overall with the human fms-like-gene (FLG). PFR4 is most closely related to the human FGFR-4 (66% overall identity). The tyrosine kinase catalytic domains of both receptors are remarkably conserved. The two receptors show distinct patterns of regulation during early development. PFR1 first appears as a maternally derived mRNA and mRNA levels remain constant during early developmental stages. However, PFR4 mRNA is first expressed at the late blastula stage, which suggests that its expression is a result of zygotic transcription. Furthermore, northern blot analysis indicates that PFR1 mRNA is distributed evenly in the early gastrula while PFR4 mRNA is predominantly localized to the presumptive ectoderm. At tail-bud stage, PFR1 transcripts are localized primarily to the neural and mesodermal tissues, PFR4 transcripts are most abundantly expressed in neural tissue, and more transcripts are detected in lateral plate mesoderm than in the somites. When animal cap explants of blastulae are cultured in the presence of mesoderm-inducing factors, PFR1 mRNA levels are maintained by bFGF and activin A. In contrast, PFR4 mRNA levels are significantly down-regulated. These observations suggest a differential expression and regulation of FGF receptors in early amphibian development.

Synergistic principles of development: overlapping patterning systems in Xenopus mesoderm induction

The first inductive event in Xenopus development establishes the mesoderm at the equator of the developing embryo. As part of this process, the dorsal-ventral and anterior-posterior axes of the embryo are initially established. A number of signalling molecules which may play a role in mesodermal induction and patterning have been identified in the last several years, including members of the FGF, TGF-beta and Wnt gene families. A variety of experiments, using either purified factors or injection of RNA encoding these factors, have added to the wealth of classical embryogical experimental data collected over the last century. We have synthesized some recent results with the classical data to provide a framework for examining the process of mesoderm induction, and to formulate putative roles for some of the different factors. We incorporate these ideas into a working model of mesoderm induction that provides a basis for future experimental directions. Finally, we suggest that mesoderm induction may not be a discrete set of well separated events, but instead may be a process involving partially overlapping signals that produce the same pattern.

The role of growth factors in embryonic induction in Xenopus laevis

Establishment of the body pattern in all animals, and especially in vertebrate embryos, depends on cell interactions. During the cleavage and blastula stages in amphibians, signal(s) from the vegetal region induce the equatorial region to become mesoderm. Two types of peptide growth factors have been shown by explant culture experiments to be active in mesoderm induction. First, there are several isoforms of fibroblast growth factor (FGF), including aFGF, bFGF, and hst/kFGF. FGF induces ventral, but not the most dorsal, levels of mesodermal tissue; bFGF and its mRNA, and an FGF receptor and its mRNA, are present in the embryo. Thus, FGF probably has a role in mesoderm induction, but is unlikely to be the sole inducing agent in vivo. Second, members of the transforming growth factor-beta (TGF-beta) family. TGF-beta 2 and TGF-beta 3 are active in induction, but the most powerful inducing factors are the distant relatives of TGF-beta named activin A and activin B, which are capable of inducing all types of mesoderm. An important question relates to the establishment of polarity during the induction of mesoderm. While all regions of the animal hemisphere of frog embryos are competent to respond to activins by mesoderm differentiation, only explants that include cells close to the equator form structures with some organization along dorsoventral and anteroposterior axes. These observations suggest that cells in the blastula animal hemisphere are already polarized to some extent, although inducers are required to make this polarity explicit.(ABSTRACT TRUNCATED AT 250 WORDS)

Elimination of smooth muscle cells in experimental restenosis: targeting of fibroblast growth factor receptors

Factors in plasma and platelets do not fully account for the proliferation of smooth muscle cells in vascular injury, implying that additional factors are involved. Recently, we and others have observed that vascular injury regulates basic fibroblast growth factor, suggesting a further role for this pleiotropic factor. We report here that injury of rat arteries leads to an increase in fibroblast growth factor receptors in vascular smooth muscle cells. This up-regulation makes smooth muscle cells susceptible, in vitro and in vivo, to the lethal effects of a conjugate of basic fibroblast growth factor with the ribosome inactivator saporin. Saporin alone has no effect, whereas the conjugate kills proliferating, but not quiescent, smooth muscle cells in vitro. In vivo, one to three doses inhibit neointimal proliferation but have no apparent effect on the uninjured artery. Thus, the up-regulation of fibroblast growth factor receptors in vascular injury suggests new therapeutic possibilities for such refractory conditions as restenosis following balloon angioplasty.

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.

Characterization of the murine BEK fibroblast growth factor (FGF) receptor: activation by three members of the FGF family and requirement for heparin

The bek gene encodes a member of the high-affinity fibroblast growth factor receptor family. The BEK/FGFR-2 receptor is a membrane-spanning tyrosine kinase with the typical features of FGF receptors. We have cloned a murine bek cDNA and expressed it in receptor-negative Chinese hamster ovary cells and in 32D myeloid cells. The BEK receptor expressed in Chinese hamster ovary cells binds acidic FGF, basic FGF, and Kaposi FGF equally well but does not bind keratinocyte growth factor or FGF-5 appreciably. Upon treatment with basic FGF or Kaposi FGF, the BEK receptor is phosphorylated and a mitogenic response is achieved. Heparan sulfate proteoglycans have been shown to play an obligate role in basic FGF binding to the high-affinity FLG receptor. Unlike the BEK-expressing Chinese hamster ovary cells, 32D cells expressing the BEK receptor require the addition of exogenous heparin in order to grow in the presence of basic FGF or Kaposi FGF. We show that the addition of heparin greatly enhances the binding of radio-labeled basic FGF to the receptor. Thus the BEK receptor, like FLG, also requires an interaction with heparan sulfate proteoglycans to facilitate binding to its ligands.

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.

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.

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.

The fms-like tyrosine kinase, a receptor for vascular endothelial growth factor

The fms-like tyrosine kinase (Flt) is a transmembrane receptor in the tyrosine kinase family. Expression of flt complementary DNA in COS cells conferred specific, high-affinity binding of vascular endothelial growth factor, also known as vascular permeability factor (VEGF-VPF), a factor that induces vascular permeability when injected in the guinea pig skin and stimulates endothelial cell proliferation. Expression of Flt in Xenopus laevis oocytes caused the oocytes to release calcium in response to VEGF-VPF. These findings show that flt encodes a receptor for VEGF-VPF.

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.

A tyrosine-phosphorylated carboxy-terminal peptide of the fibroblast growth factor receptor (Flg) is a binding site for the SH2 domain of phospholipase C-gamma 1

Phospholipase C-gamma (PLC-gamma) is a substrate of the fibroblast growth factor receptor (FGFR; encoded by the flg gene) and other receptors with tyrosine kinase activity. It has been demonstrated that the src homology region 2 (SH2 domain) of PLC-gamma and of other signalling molecules such as GTPase-activating protein and phosphatidylinositol 3-kinase-associated p85 direct their binding toward tyrosine-autophosphorylated regions of the epidermal growth factor or platelet-derived growth factor receptor. In this report, we describe the identification of Tyr-766 as an autophosphorylation site of flg-encoded FGFR by direct sequencing of a tyrosine-phosphorylated tryptic peptide isolated from the cytoplasmic domain of FGFR expressed in Escherichia coli. The same phosphopeptide was found in wild-type FGFR phosphorylated either in vitro or in living cells. Like other growth factor receptors, tyrosine-phosphorylated wild-type FGFR or its cytoplasmic domain becomes associated with intact PLC-gamma or with a fusion protein containing the SH2 domain of PLC-gamma. To delineate the site of association, we have examined the capacity of a 28-amino-acid tryptic peptide containing phosphorylated Tyr-766 to bind to various constructs containing SH2 and other domains of PLC-gamma. It is demonstrated that the tyrosine-phosphorylated peptide binds specifically to the SH2 domain but not to the SH3 domain or other regions of PLC-gamma. Hence, Tyr-766 and its flanking sequences represent a major binding site in FGFR for PLC-gamma. Alignment of the amino acid sequences surrounding Tyr-766 with corresponding regions of other FGFRs revealed conserved tyrosine residues in all known members of the FGFR family. We propose that homologous tyrosine-phosphorylated regions in other FGFRs also function as binding sites for PLC-gamma and therefore are involved in coupling to phosphatidylinositol breakdown.

A tyrosine-phosphorylated carboxy-terminal peptide of the fibroblast growth factor receptor (Flg) is a binding site for the SH2 domain of phospholipase C-gamma 1

Phospholipase C-gamma (PLC-gamma) is a substrate of the fibroblast growth factor receptor (FGFR; encoded by the flg gene) and other receptors with tyrosine kinase activity. It has been demonstrated that the src homology region 2 (SH2 domain) of PLC-gamma and of other signalling molecules such as GTPase-activating protein and phosphatidylinositol 3-kinase-associated p85 direct their binding toward tyrosine-autophosphorylated regions of the epidermal growth factor or platelet-derived growth factor receptor. In this report, we describe the identification of Tyr-766 as an autophosphorylation site of flg-encoded FGFR by direct sequencing of a tyrosine-phosphorylated tryptic peptide isolated from the cytoplasmic domain of FGFR expressed in Escherichia coli. The same phosphopeptide was found in wild-type FGFR phosphorylated either in vitro or in living cells. Like other growth factor receptors, tyrosine-phosphorylated wild-type FGFR or its cytoplasmic domain becomes associated with intact PLC-gamma or with a fusion protein containing the SH2 domain of PLC-gamma. To delineate the site of association, we have examined the capacity of a 28-amino-acid tryptic peptide containing phosphorylated Tyr-766 to bind to various constructs containing SH2 and other domains of PLC-gamma. It is demonstrated that the tyrosine-phosphorylated peptide binds specifically to the SH2 domain but not to the SH3 domain or other regions of PLC-gamma. Hence, Tyr-766 and its flanking sequences represent a major binding site in FGFR for PLC-gamma. Alignment of the amino acid sequences surrounding Tyr-766 with corresponding regions of other FGFRs revealed conserved tyrosine residues in all known members of the FGFR family. We propose that homologous tyrosine-phosphorylated regions in other FGFRs also function as binding sites for PLC-gamma and therefore are involved in coupling to phosphatidylinositol breakdown.

Identification of chicken embryo kinase 5, a developmentally regulated receptor-type tyrosine kinase of the Eph family

Chicken embryo kinase 5 (Cek5) is a transmembrane tyrosine kinase of the Eph family that was identified by screening a 10-d chicken embryo cDNA expression library with anti-phosphotyrosine antibodies. The extracellular region of Cek5 contains a cysteine rich N-terminal subdomain and a C-terminal subdomain mostly devoid of cysteines and comprising two repeats similar to fibronectin type III repeats. Immunoblotting experiments with anti-Cek5 polyclonal antibodies indicated that Cek5 is a membrane-associated 120-kDa protein containing intramolecular (but not intermolecular) disulfide bonds. Cek5 is already expressed in 2-d-old chicken embryos and is also expressed, at higher levels, later in development. In 10-d-old chicken embryos, Cek5 is expressed at substantial levels in nearly all the tissues examined, whereas in adult it is expressed predominantly in the brain. The expression of Cek5 in the brain gradually diminishes during embryonic development, whereas in the skeletal muscle of the thigh a sharp decrease in Cek5 expression was detected at the time of terminal muscle differentiation. Its wide tissue distribution throughout development and its sustained expression in adult brain suggest that Cek5 is an important component of signal transduction pathways, likely to interact with a widely distributed and important ligand, which is as yet unknown.

cDNA cloning and developmental expression of fibroblast growth factor receptors from Xenopus laevis

The heparin-binding growth factors constitute a family of homologous polypeptides including basic and acidic fibroblast growth factors (FGFs). These factors participate in a variety of processes, including wound healing, angiogenesis, neuronal survival, and inductive events in the early amphibian embryo. We have isolated three closely related species of cDNA clones for Xenopus FGF receptors. One of these, designated XFGFR-A1, encodes an open reading frame of 814 amino acids. A second class encodes an identical amino acid sequence with the exception of an 88-amino-acid deletion near the 5' end. This species probably arises through alternative splicing. A third class of cDNA corresponding to the shorter form of XFGFR-A1 was isolated and shown to be 95% homologous and is designated XFGFR-A2. Xenopus FGF receptors are similar to FGF receptors from other species in that they contain a transmembrane domain, a tyrosine kinase domain split by a 14-amino-acid insertion, and a unique conserved stretch of eight acidic residues in the extracellular domain. Overexpression of Xenopus FGF receptor protein by transfection of COS1 cells with the corresponding cDNA in a transient expression vector leads to the appearance of new FGF binding sites on transfected cells, consistent with these cDNAs encoding for FGF receptors. RNA gel blot analysis demonstrates that Xenopus FGF receptor mRNA is a maternal message and is expressed throughout early development. When blastula-stage ectoderm is cultured in control amphibian salt solutions, Xenopus FGF receptor mRNA declines to undetectable levels by late neurula stages. However, when cultured in the presence of FGF of XTC mesoderm-inducing factor, Xenopus FGF receptor RNA expression is maintained.

cDNA cloning and developmental expression of fibroblast growth factor receptors from Xenopus laevis

The heparin-binding growth factors constitute a family of homologous polypeptides including basic and acidic fibroblast growth factors (FGFs). These factors participate in a variety of processes, including wound healing, angiogenesis, neuronal survival, and inductive events in the early amphibian embryo. We have isolated three closely related species of cDNA clones for Xenopus FGF receptors. One of these, designated XFGFR-A1, encodes an open reading frame of 814 amino acids. A second class encodes an identical amino acid sequence with the exception of an 88-amino-acid deletion near the 5' end. This species probably arises through alternative splicing. A third class of cDNA corresponding to the shorter form of XFGFR-A1 was isolated and shown to be 95% homologous and is designated XFGFR-A2. Xenopus FGF receptors are similar to FGF receptors from other species in that they contain a transmembrane domain, a tyrosine kinase domain split by a 14-amino-acid insertion, and a unique conserved stretch of eight acidic residues in the extracellular domain. Overexpression of Xenopus FGF receptor protein by transfection of COS1 cells with the corresponding cDNA in a transient expression vector leads to the appearance of new FGF binding sites on transfected cells, consistent with these cDNAs encoding for FGF receptors. RNA gel blot analysis demonstrates that Xenopus FGF receptor mRNA is a maternal message and is expressed throughout early development. When blastula-stage ectoderm is cultured in control amphibian salt solutions, Xenopus FGF receptor mRNA declines to undetectable levels by late neurula stages. However, when cultured in the presence of FGF of XTC mesoderm-inducing factor, Xenopus FGF receptor RNA expression is maintained.

Three-dimensional structure of human basic fibroblast growth factor

The three-dimensional structure of human basic fibroblast growth factor (bFGF) has been determined by x-ray crystallography and refined to a crystallographic residual of 17.4% at 2.2-A resolution. The structure was initially solved at a nominal resolution of 2.8 A by multiple isomorphous replacement using three heavy-atom derivatives. Although the map clearly showed the overall fold of the molecule, electron density was not observed for the first 19 amino-terminal and the last 3 carboxyl-terminal amino acids, suggesting that they are disordered. The bFGF crystals were grown from 2.0 M ammonium sulfate at pH 8.1 in space group P1 with cell dimensions a = 30.9 A, b = 33.4 A, c = 35.9 A, alpha = 59.5 degrees, beta = 72.0 degrees, and gamma = 75.6 degrees. There is one molecule per unit cell and the crystals diffract to spacings beyond 1.9 A. The overall structure of bFGF can be described as a trigonal pyramid with a fold very similar to that reported for interleukin 1 beta, interleukin 1 alpha, and soybean trypsin inhibitor. An apparent sulfate ion is bound within a basic region on the surface of the molecule and has a ligands the main-chain amide of Arg-120 and the side chains of Asn-27, Arg-120, and Lys-125. This is suggested as the presumed binding site for heparin. Residues 106-115, which are presumed to bind to the bFGF receptor [Baird, A., Schubert, D., Ling, N. & Guillemin, R. (1988) Proc. Natl. Acad. Sci. USA 85, 2324-2328], include an irregular loop that extends somewhat from the surface of the protein and is about 25 A from the presumed heparin binding site. The backbone structure of this putative receptor-binding loop is very similar, although not identical, to the corresponding region of interleukin 1 beta.

The Drosophila FGF-R homolog is expressed in the embryonic tracheal system and appears to be required for directed tracheal cell extension

The Drosophila homolog of the vertebrate fibroblast growth factor receptor (FGF-R) was isolated by low-stringency hybridization. In contrast to the diversity of this subclass of receptor tyrosine kinases in vertebrates, the Drosophila genome appears to encode only a single homolog. Nucleotide sequence analysis demonstrates that the Drosophila FGF-R homolog (DFGF-R) protein has a conserved sequence, size, and organization. The extracellular region encodes three immunoglobulin-like domains, and the cytoplasmic kinase domain exhibits a high degree of similarity to the vertebrate FGF-Rs with the typical split kinase and comparably sized juxtamembrane and carboxy-terminal regions. The DFGF-R was mapped to position 70C on the third chromosome, and two overlapping chromosomal deficiencies that remove the gene were identified. Developmental Northern blots show that the gene has a single transcript of 4.3 kb and is expressed at all stages of development. Localization of the transcript and protein in embryos has shown that the gene is predominantly expressed in a restricted set of tissues: the developing tracheal system and the delaminating midline glial and neural cells. In embryos homozygous for a deletion of several genes including the DFGF-R locus, the initial formation of the tracheal pits is not affected. However, the extension of tracheal cell processes leading to the formation of the elaborate tree structure is blocked. The DFGF-R protein may thus participate in receiving spatial cues that guide tracheal cell outgrowth.