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

The human fibroblast growth factor receptor genes: a common structural arrangement underlies the mechanisms for generating receptor forms that differ in their third immunoglobulin domain

To determine the mechanisms by which multiple forms of fibroblast growth factor (FGF) receptors are generated, we have mapped the arrangement of exons and introns in the human FGF receptor 1 (FGFR 1) gene (flg). We found three alternative exons encoding a portion of the third immunoglobulin (Ig)-like domain of the receptor. One of these alternatives encodes a sequence that is part of a secreted form of FGFR 1. The other two encode sequences that are likely part of transmembrane forms of FGFR 1. One of these forms has not been previously reported in published cDNAs. Also, we have determined the structural organization of a portion of the human FGFR 2 gene (bek) and found a similar arrangement of alternative exons for the third Ig-like domain. The arrangement of these genes suggests that there are conserved mechanisms governing the expression of secreted FGF receptors as well as the expression of at least two distinct membrane-spanning forms of the FGF receptors. The diverse forms appear to be generated by alternative splicing of mRNA and selective use of polyadenylation signals.

Temporal, differential and regional expression of mRNA for basic fibroblast growth factor in the developing and adult rat brain

The expression of basic fibroblast growth factor (bFGF) mRNA and bFGF receptor mRNA was investigated in developing rat brain. In embryonic rat brain days 13-21 (E13-E21), an abundant 1.8 kb bFGF mRNA was detected. Expression of 1.8 kb bFGF mRNA was the highest at E17 to E19 and was relatively undetectable 20 days after birth. However, very little mitogenic activity was associated with prenatal brain. On the other hand, multiple bFGF mRNA species of 6.0, 3.7, 2.5, 1.8, 1.6, 1.4 and 1.0 kb were detected in total adult rat brain and a significant amount of mitogenic activity was present. Differential and spatial bFGF mRNA expression was found in different parts of developing rat brain. Embryonic hypothalamus was found to contain the 1.8 kb bFGF mRNA while the 6.0 kb bFGF mRNA transcript was predominant in adult hypothalamus. Adult pituitary and cortex transcribed the lower molecular weight mRNAs but not the 6.0 kb mRNA. Expression of high-affinity bFGF receptor (flg) mRNA was found to be temporally regulated. flg 4.3 kb mRNA expression was high in embryonic rat brain (E13-E19). There appears to be coordinate expression between the 1.8 kb bFGF mRNA and flg. These results suggest that the expression of basic FGF mRNA is complex since it is both temporally and differentially regulated with different species being expressed at different times in development.

Basic fibroblast growth factor is a beta-rich protein

The conformation of the 153-residue form of human basic fibroblast growth factor (bFGF) was studied with circular dichroism (CD) and sequence prediction methods. The far-UV CD spectrum with a minimum at 202 nm resembled that of an unordered polypeptide/protein or a protein rich in distorted antiparallel beta-sheets. Analysis of the CD spectrum by the least-squares method of Chang et al. (1978) and the CONTIN program of Provencher and Glöckner (1981) suggested that about one half of the molecule consisted of beta-sheet and there was no alpha-helix. These estimates agreed with the prediction by the sequence method of Garnier et al. (1978) using decision constants based on CD results. bFGF had an unusual CD band at 187 nm, which disappeared upon ionization of Tyr side chains at pH 11.7. It also had another unusual property of irreversibly converting the CD spectrum to a helix-like one with a double minimum at 205 and 215 and a maximum at 189 nm upon heating the solution to above 55 degrees C. The helicity was also enhanced in trifluoroethanol and in sodium dodecyl sulfate. The mutant bFGF in which cysteines 76 and 94 were replaced by serine residues had essentially the same properties as the wild-type.

Presence of activin (erythroid differentiation factor) in unfertilized eggs and blastulae of Xenopus laevis

Activin A, a member of the transforming growth factor beta superfamily, has recently been found to have potent mesoderm-inducing activity on isolated early Xenopus animal-cap cells. We measured the activin activity of the Xenopus egg extract by using an erythroid-differentiating test with Friend leukemia cells. The results showed that an activin homologue is, indeed, contained in unfertilized eggs and blastulae of Xenopus laevis in a considerable amount. This activity was eluted at the same retention time as human activin A when fractionated by reversed-phase HPLC. Furthermore, the fraction containing erythroid-differentiating factor activity had mesoderm-inducing activity on Xenopus animal-cap cells. The mesoderm-inducing activity of this fraction was suppressed when coincubated with follistatin, an activin-binding protein. These results suggest that an endogenous activin may be a natural mesoderm-inducing factor acting in Xenopus embryogenesis.

Interaction of bride of sevenless membrane-bound ligand and the sevenless tyrosine-kinase receptor

During development of the Drosophila retina, the R8 photoreceptor neuron induces a neighbouring cell to assume an R7 cell fate. Genetic data suggest that the induction is mediated by two transmembrane proteins encoded by bride of sevenless and sevenless. A direct interaction between these two proteins was demonstrated by the heterotypic aggregation of cell lines expressing them. In the developing eye the sevenless-dependent internalization of bride of sevenless by the R7 precursor cell provides evidence for a direct interaction between these two proteins in vivo.

Isolation from mouse fibroblasts of a cDNA encoding a new form of the fibroblast growth factor receptor (flg)

Structural definition of the receptors for neurotropic and angiogenic modulators such as fibroblast growth factors and related polypeptides will yield insight into the mechanisms that control early development, embryogenesis, organogenesis, wound repair and neovessel formation. We isolated 3 murine cDNAs encoding different binding domains of these receptors (flg). Comparison of these ectoplasmic portions showed that two of the forms corresponded to previously described murine molecules whereas the third one had a different ectoplasmic portion generated by specific changes in two regions. Interestingly, expression of this third form seems to be restricted in its tissue distribution. Such modifications could influence the ligand specificity of the different receptors and/or their binding affinity.

Basic fibroblast growth factor is efficiently released from a cytolsolic storage site through plasma membrane disruptions of endothelial cells

Cells of gut and skin frequently suffer mechanically-induced plasma membrane disruptions in vivo, and bioactive molecules, including basic fibroblast growth factor (bFGF), could enter and leave cytoplasm through these disruptions. We here provide three lines of evidence that bFGF is released with surprising efficiency through plasma membrane disruptions, resembling those known to occur in vivo, produced by scraping endothelial cells from their culturing substratum. First, 41% of the total of bFGF extractable in 1 M NaCl by freeze-thaw and sonication was released simply by scraping the endothelial cells. Second, relative to release of lactate dehydrogenase, cells wounded by scraping under conditions promoting greater than 60% cell survival released a significantly larger amount (up to twofold more) of growth promoting activity than did cells uniformly killed and irreversibly permeabilized by scraping in the cold or by freezing and thawing. Last, cells that survived membrane disruptions released, and contained, less bFGF on each subsequent wounding, consistent with release of bFGF through transient (i.e., survivable) membrane disruptions. A polyclonal antibody against bFGF completely neutralized the growth promoting activity released by scraping, confirming that bFGF is released through endothelial cell plasma membrane disruptions. Cell fractionation and immunolocalization, including a novel permeabilization technique for electron microscope immunolocalization, demonstrated a cytosolic location of bFGF. We conclude that many characteristics of bFGF--its broad spectrum of producing and target cell types, cytosolic location, efficient release through biologically and pathologically relevant plasma membrane wounds, and its release from cells that survive membrane wounds--make it a strong candidate as a "wound hormone" for rapidly initiating the cell growth required for routine maintenance of tissue integrity and/or repair after injury.

Localization of bFGF-like proteins as punctate inclusions in the preseptation myocardium of the chicken embryo

Immunocytochemistry has been employed to map the appearance of bFGF-like proteins in precardiac and preseptation myocardial cells between stages 6 and 15 of chicken embryogenesis. Stage 6 embryos exhibited no staining, with the exception of a subtle signal in endoderm cells. At subsequent stages, staining was observed only in cells of the developing myocardium, first appearing at the time of heart tube fusion (stage 9+) as punctate cytoplasmic aggregates. While the expression of bFGF-like antigen was temporally similar to that of myosin heavy chain, their staining patterns differed in that bFGF-like proteins were nonsarcomeric and did not extend into the inflow or outflow tracts. Western blotting of heparin agarose affinity-isolated proteins from stage 15 hearts revealed an antigen migrating at approximately 19 kDa. In contrast with the unique localization of bFGF-like proteins in myocardial cells, FGF receptor (FGFR) staining was widely distributed in the embryo; however, concentrated deposits of FGFR were detected in endothelial and myocardial cells, which diminished in the myocardium but not in the endothelium by stage 15. These results suggest that FGF-like proteins may have autocrine and/or paracrine functions during early cardiac morphogenesis.

Biotinylated basic fibroblast growth factor is biologically active

Basic fibroblast growth factor (bFGF) was modified by biotinylation via amino group substitution, using biotin-N-hydroxysuccinimide ester at molar reaction ratios of 20, 200, and 2000 per bFGF molecule (respectively named bio-bFGF.20, bio-bFGF.200, and bio-bFGF.2000). The biotinylated bFGF derivatives, bio-bFGF.20 and bio-bFGF.200, conserved the same affinity for heparin as native bFGF, in contrast to bio-FGF.2000 which lost this property. Bio-bFGF.20 and bio-bFGF.200 were as effective as native bFGF in their capacity to compete with 125I-bFGF for binding to bFGF receptor on bovine brain membranes. The biological activity of these bFGF derivatives was tested on CCL39 cells; bio-bFGF.20 and bio-bFGF.200 were as able as native bFGF to promote growth of CCL39.

Growth factors and tyrosine protein kinases in normal and malignant melanocytes

Melanomas are highly variable with respect to aberrant gene expression and chromosomal lesions but share a common characteristic of an acquired independence from environmental growth factors that are needed for proliferation of normal melanocytes. Receptors with tyrosine kinase activity play a critical role in normal melanocyte proliferation and in the uncontrolled growth of melanomas. Normal human melanocytes depend on exogenous peptide growth factors such as basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), or mast cell growth factor (MGF), all of which stimulate receptors with tyrosine kinase activity. In contrast, human melanoma cells from primary nodular and metastatic lesions grow autonomously partially because of inappropriate production of bFGF and continuous activation of the bFGF-receptor kinase. Animal models also provide evidence for the importance of receptor-tyrosine kinases in normal melanocyte proliferation and in malignant transformation. In the mouse, genes residing in three loci in which inactivation mutations lead to piebaldism, the dominant spotting (W), patch (Ph), and Sl encode, respectively, the receptor-kinases c-kit and platelet derived growth factor receptor, and the ligand for c-kit: MGF. In vivo transformation of mouse melanocytes to melanoma, due to constitutive expression of a transmembrane tyrosine kinase, the oncogene ret, was recently demonstrated in transgenic mice. Studies on a fish model, Xiphophorus, in which melanoma is inherited, showed that the dominant tumor inducing gene, Tu, encodes an EGF-receptor related tyrosine kinase which is expressed only in melanomas and not in normal tissues. Taken together, the results suggest that the uncontrolled growth of melanomas is due, in large part, to constitutive activation of receptors with tyrosine kinase activity.

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.

Comparative aspects of conceptus signals for maternal recognition of pregnancy

Maintenance of corpus luteum (CL) function is essential for establishment of pregnancy in mammals. Estrogens from pig conceptuses (embryo and associated membranes) initiate events that, with prolactin, redirect secretion of the uterine luteolytic hormone prostaglandin F2 alpha (PGF) from an endocrine (to uterine veins) to an exocrine (to uterine lumen) direction to prevent luteolysis. Ovine conceptuses secrete ovine trophoblast protein-1 (oTP-1), which exhibits high amino acid sequence relatedness with alpha II interferons (IFN alpha II) and inhibits synthesis of endometrial receptors for oxytocin and uterine production of luteolytic pulses of PGF. Estrogens and oTP-1 are local antiluteolytic signals to endometrium, whereas human chorionic gonadotrophin (hCG) appears to have a direct luteotrophic effect on CL. A progestational endometrium secretes proteins that serve as growth factors, transport proteins, regulatory proteins and enzymes, as well as transporting nutrients into the uterine lumen to support conceptus development.

The NH2-terminal extension of high molecular weight bFGF is a nuclear targeting signal

Basic fibroblast growth factor (bFGF) is a member of the heparin-binding growth factor (HBGF) family that includes at least seven species. These proteins are potent regulators of a number of cellular processes, including cell division and angiogenesis. Multiple forms of bFGF exist differing only in the length of their NH2-terminal extensions. These species of bFGF also have unique subcellular distributions. The smallest form (18 kD) occurs predominantly in the cytosol, while the higher molecular weight forms (22, 22.5, 24 kD) are associated with the nucleus and ribosomes. Here we report that the nuclear localization of the higher molecular weight forms of bFGF derives specifically from the amino acid sequences within the NH2-terminal extension. This has been demonstrated by constructing a chimeric protein containing the NH2-terminal extension of the highest molecular weight form of bFGF fused to beta-galactosidase (beta-gal). After transfection in a transient expression system, the chimeric protein accumulated in the nuclei of transfected cells, while the wild-type beta-gal was found predominantly in the cytoplasm.

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.

Pattern formation during animal development

At the beginning of this century, embryologists defined the central problems of developmental biology that remain today. These questions include how differentiated cells arise and form tissues and organs and how pattern is generated. In short, how does an egg give rise to an adult? In recent years, the application of molecular biology to embryological problems has led to significant advances and recast old problems in molecular and cellular terms. Although not necessarily comprehensive, this idiosyncratic review is intended to highlight selected findings and indicate where there are important gaps in our knowledge for those less than familiar with developmental biology.

Preliminary X-ray crystallographic analysis of a modified basic fibroblast growth factor

Human basic fibroblast growth factor (hbFGF) has been modified, with Ala3 and Ser5 substituted by glutamic acid, and the purified recombinant protein has been crystallized. The crystals are triclinic (space group P1) with unit cell parameters a = 31.0 A, b = 33.6 A, c = 34.7 A, alpha = 88 degrees, beta = 85 degrees, gamma = 76 degrees, and they diffract to at least 2 A.

Expression of ras-like proteins in embryonic and adult cells of Xenopus laevis

A polyclonal antibody raised against v-Ha-ras p21 was purified and its specificity was checked on Ha-ras transformed cell lines. It was used to immunoprecipitate p21 from different Xenopus laevis cell types: brain cells, blood cells, and embryonic material. By one-dimensional Western blot analysis, we show that ras p21 is synthesized very early in oogenesis and accumulates throughout vitellogenesis. The ras p21 content, estimated to be 1.1 ng in the full-grown oocyte, remains constant during oocyte maturation and egg cleavage. Increase in the amount of ras p21 occurs at the beginning of neurulation. Two-dimensional Western blot patterns reveal the presence of multiple molecular forms of p21 in all Xenopus cell types studied. The numerous resolved polypeptides were ascribed to the expression of at least two different ras genes. Furthermore, specific charge modifications of the ras polypeptides are observed in brain, blood, and embryonic cells. During oogenesis and early embryonic development, differences in two-dimensional patterns mainly concern variations in the relative amounts of the different polypeptides. The results are discussed in relation to the well documented synthesis activities of the growing oocyte and of the early developing embryo.

Development and growth of mouse embryonic kidney in organ culture and modulation of development by soluble growth factor

Differentiation of the metanephrogenic mesenchyme is triggered by an inductive tissue interaction between an inducer tissue and the mesenchyme. It is generally believed that the epithelial ureter bud acts as an inducer during in vivo development. In response to the inductive stimulus most of the mesenchymal cells convert into epithelial cells, while a small fraction differentiates into stromal cells. In vitro, differentiation of isolated mesenchyme to epithelium can be induced by a variety of embryonic tissues, but nothing is known about the molecular nature of the inducing stimulus. In recent years, large numbers of polypeptide growth factors have been described, which in addition to proliferative effects were shown to exert effects on a variety of biological phenomena such as chemotaxis, inflammation, tissue repair, or induction of embryonic development. We therefore analyzed whether growth factors in the absence of inducer tissue can induce isolated kidney mesenchyme to differentiate into epithelium or interstitium. As expected, both growth and differentiation into epithelium were stimulated by an inducer tissue, the spinal cord. We found that none of the various growth factors tested (including epidermal growth factor, transforming growth factors alpha and beta, insulin-like growth factors I and II, fibroblast growth factor, platelet-derived growth factor, and retinoic acid) could mimick the effect of an inducer tissue, although we tested the factors over a wide concentration range. One of the tested factors, epidermal growth factor (EGF) stimulated the mesenchymal cells to become stromal cells, although it could not stimulate development into epithelium. EGF could stimulate stromal development both when the mesenchyme was cultured in isolation and when the mesenchyme was stimulated by an inducer tissue to become epithelium. The expansion of the stromal compartment in response to EGF treatment occurred at the expense of the epithelial cells, but EGF could not completely suppress the formation of epithelium. These data suggest the presence of EGF receptors in the developing kidney, but since application of soluble EGF leads to abnormal development, soluble EGF cannot be the natural ligand. We suggest that locally produced mitogens with an EGF-like structure may regulate the relative amounts of stroma (interstitium) and epithelium in the developing kidney.

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.

Bone morphogenetic protein 4 (BMP-4), a member of the TGF-beta family, in early embryos of Xenopus laevis: analysis of mesoderm inducing activity

We have screened a Xenopus ovary cDNA library using a synthetic oligonucleotide derived from that part of the inhibin beta A sequence, which is highly conserved within the TGF-beta family. Out of several clones yielding autoradiographic signals four turned out to represent Xenopus counterparts to the human bone morphogenetic protein 4 (BMP-4). Each two of the four sequences are nearly identical and probably account for different alleles whereas the two pairs showing 5% divergence may have arisen by genome duplication in this tetraploid species. The amino acid sequence of the Xenopus protein is 80% homologous to the human sequence showing no single exchange within the last 100 amino acids at the C-terminus. This region, which constitutes the main part of the mature, biologically active protein, also exhibits substantial homologies to other representatives of the TGF-beta family, especially to the Drosophila DPPC protein. Transfection of COS-1 cells with the Xenopus BMP-4 sequence under control of the CMV-promoter leads to the secretion of a protein which exhibits mesoderm inducing activity when tested with animal cap explants from Xenopus blastula stage embryos.

Differentiation of aggregated murine P19 embryonal carcinoma cells is induced by a novel visceral endoderm-specific FGF-like factor and inhibited by activin A

Aggregation of P19 embryonal carcinoma cells in the presence of a factor, secreted by the visceral endoderm-like cell line END-2, induces differentiation to cell types including visceral endoderm, mesoderm-derived muscle tissue and neurons. This factor is different from activin A, type beta transforming growth factors (TGF beta) and fibroblast growth factors (FGF) although its acid- and heat-lability and its stability in the presence of reducing agents resemble the properties of the FGFs. The END-2 factor is completely inhibited in its action by activin A. This inhibitory effect of activin A is not specific for the END-2 factor as retinoic acid (RA)-induced differentiation of aggregated P19 EC cells into neurons (10(-8) M RA) or mesoderm-derived muscle tissue (10(-9) M RA) is also completely inhibited by activin A. The results of this study suggest that the END-2 activity and activin A are intimately involved in the induction and regulation, respectively, of early differentiation processes in vertebrate embryogenesis.

Growth factors in development, transformation, and tumorigenesis

Mammalian tissue development and regeneration take place within a milieu of regulatory growth factors. These affect many parameters of cell development, such that survival, proliferation, differentiation, and certain aspects of cell behavior are all influenced by a balance between stimulatory and inhibitory signals. The precise effect of any given factor is determined by the responding cell type, the concentration of factor, and the presence of other stimuli, such that some growth factors may fulfill a variety of functions under different circumstances. Classically, growth factor stimuli are transmitted into the cell via activation of specific, transmembrane receptors that modify key regulatory proteins in the cytoplasm. These in turn affect the decisions controlling proliferation and differentiation, including changes in gene expression and reactivity to other factors. There are indications that some factors may function both extra- and intracellularly and that this characteristic is correlated with potential oncogenicity. The relatively low transforming ability of extracellular factors alone is probably attributable to the limitations imposed by down-regulation of their cell surface receptors. Aberrant production of secreted growth factors can, however, play decisive roles in tumorigenesis by increasing the proliferation rate and degree of cellular autonomy and extending the area available for tumor expansion.

cDNA cloning and expression of a human FGF receptor which binds acidic and basic FGF

We have isolated and characterized a cDNA clone, phFGFR, encoding a human fibroblast growth factor (FGF) receptor. phFGFR contains an open reading frame which encodes an 820 amino acid polypeptide with three immunoglobulin-like domains in the extracellular part and an intracellular split tyrosine kinase domain. Transient expression in COS-1 cells and immunoprecipitation using an antiserum raised against a C-terminal peptide, gave rise to two components, representing mature (130 kDa) and precursor (115 kDa) forms of the phFGFR encoded polypeptide, which was denoted hFGFR-1. Crosslinking of iodinated acidic FGF (aFGF) and basic FGF (bFGF) to transiently expressing COS-1 cells revealed a major band of 95 kDa, which was competed for by both aFGF and bFGF. From Scatchard analyses, the Kd:s for binding of aFGF and bFGF to hFGFR-1 were estimated to 25 pM and 41 pM, respectively. Thus, phFGFR encodes a human FGF receptor with high affinity for both aFGF and bFGF.

Functional characterization of the human basic fibroblast growth factor gene promoter

In order to understand the regulation of basic fibroblast growth factor (bFGF) gene expression, we have cloned and characterized the human bFGF gene and its regulatory elements. Using restriction endonuclease digestion, we have mapped the entire gene and sequenced all intron/exon boundaries to confirm authenticity and to determine organization. The data show that intron 1 is at least 16 kb long while intron 2 is 16 kb long. The human bFGF gene, including its three exons, is therefore at least 36 kb long. There are five GC boxes which may represent SP-1 binding sites and one potential AP-1 binding site within the core promoter region. Primer extension analysis indicates the presence of one bFGF-RNA transcription start site. We used a standard bacterial CAT gene expression system to identify the DNA sequence containing the functional bFGF gene promoter. Deletion analysis suggests the presence of two negative regulatory elements; one in the non-transcribed 5'-promoter region and the other within transcribed (but non-translated) sequences 3' of the promoter core.

Activins are expressed early in Xenopus embryogenesis and can induce axial mesoderm and anterior structures

We show that mammalian and Xenopus activins induce dorsal axial mesoderm and anterior structures in explants of Xenopus blastula cells that would otherwise form epidermis. The induced explants of animal cap cells can form notochord, muscle, neural tissue, and eyes all arranged in a rudimentary axial pattern. Activin A shares inductive properties and antigenic determinants with PIF, an inducing factor recently isolated from mouse macrophage culture supernatants. Genes encoding Xenopus activin beta A and beta B chains were cloned. Activin beta B transcripts are first detected in Xenopus blastula, whereas activin beta A transcripts do not appear until the late gastrula stage. Recombinant Xenopus activin beta B protein induces mesodermal and neural tissues similar to those induced by mammalian activin A and PIF. Furthermore, ectopic expression of Xenopus activin beta B produces a second body axis in embryos injected with synthetic mRNA. Our results suggest that early induction and axial patterning are accomplished by endogenous activin B, not activin A, in Xenopus.

Basic fibroblast growth factor accumulates in the nuclei of various bFGF-producing cell types

The intracellular localization of basic fibroblast growth factor (bFGF) was studied in BHK-21 cells transfected with an expression vector containing the complementary DNA (cDNA) of the human bFGF gene (pbFGF). The intracellular location of bFGF was determined using indirect immunofluorescence. The antibodies used were polyclonal antibodies directed against either recombinant human bFGF or recombinant Xenopus bFGF. The nuclei of transfected cells that produce bFGF, but not the nuclei of untransfected cells, were labeled strongly by the antibodies. The nuclear staining was totally abolished when anti-bFGF antibodies preadsorbed with bFGF were used. Several types of endothelial cells known to produce bFGF were also stained in their nuclei by the antibodies. Nuclear extracts prepared from transfected cells were found to contain bFGF as determined using heparin-sepharose affinity chromatography, followed by Western blot analysis of fractions, which stimulated the proliferation BHK-21 cells. The mitogenic activity associated with the nuclei was not destroyed when isolated cell nuclei were digested by trypsin. It is therefore likely that the nucleus associated bFGF is intranuclear. These findings suggest that some biological activities of bFGF may be mediated by nuclear bFGF binding proteins or by the direct binding of bFGF to DNA.

Regulation of the fibroblast growth factor receptor in early Xenopus embryos

Recent evidence suggests that fibroblast growth factor (FGF) is a primary mesoderm inducer in Xenopus development. We have isolated a full-length cDNA clone for the Xenopus FGF receptor. Like other FGF receptors, the Xenopus homolog is a membrane-spanning protein with a split intracellular tyrosine kinase domain. The Xenopus FGF receptor mRNA is present as a maternal message whose levels are constant through early development. There is no specific regional localization of the transcript by analysis of FGF receptor mRNA levels in microdissected embryonic tissue. In isolated animal-pole blastomeres, FGF receptor mRNA declines over 16 hr in culture and this loss can be prevented by incubation with FGF or activin. Despite the presence of the FGF receptor mRNA in the oocyte, oocytes in culture do not respond to added FGF. However, injection of exogenous Xenopus FGF receptor transcripts into oocytes does generate a functional response to FGF. Our data suggest that posttranscriptional response to FGF. Our data suggest that posttranscriptional mechanisms regulate the FGF receptor in the oocyte and early embryo and further suggest that mesoderm-inducing factors influence receptor mRNA levels during the time of early tissue formation.

A two-channel hypothesis for regulation of cell division and differentiation

We propose the hypothesis that extracellular regulation of cell division and differentiation acts through just two communications channels. These channels consist of a series of redundant components: extracellular messenger hormones; these hormones' receptors; cytoplasmic proteins activated by the hormone-receptor complex; and trans-activating nuclear regulatory proteins. One channel, here labeled "D" ("differentiate"), includes transforming growth factor-beta as one of its hormones; the other, labeled "G" ("growth") includes epidermal growth factor. We postulate that signal reception occurs as a result of competition between different actuating proteins for equilibrium-controlled binding to critical DNA sites. Stem cells commit to mitosis when some high proportion of critical sites is occupied by actuating proteins of the G class, and to terminal differentiation when a high proportion is occupied by "D" actuators. Intermediate occupancy can either lead to division into one differentiated and one stem cell, or to maintenance of cells in the reference state, quiescence. Equilibrium control of binding implies that critical site occupancy will be proportional to the relative concentrations of "D" and "G" actuating proteins in the nuclear fluid. These concentrations depend on the external hormone concentrations, the numbers of receptors on the cell membrane, and the coefficients of the rate-determining steps between internalization of the hormone-receptor complexes and activation of the actuating proteins. All of these quantities can be affected by various factors, including endocrine hormones. This model is consistent with most reported behavior of various growth factors, interferons, etc, toward a variety of cells in culture. It predicts that under otherwise constant conditions, high relative concentrations of a D-hormone (e.g. transforming growth factor-beta) will induce commitment to terminal differentiation, while high relative concentrations of a "G" hormone (e.g. epidermal growth factor) will induce mitosis. We have seen no report of an experiment which adequately tests this prediction. The model implies that cancer causing mutations are those which increase the relative intensity of the "G" signal; this can occur via changes in components of either channel. Such mutant cells should be both more likely to divide and less likely to differentiate than normal stem cells. Conversely, mutations which increase relative sensitivity to the "D" signal during embryonal development can lead to premature differentiation, cessation of growth, and structural abnormalities (terata).

"Pure" human hematopoietic progenitors: permissive action of basic fibroblast growth factor

Methodology has been developed that enables virtually complete purification and abundant recovery of early hematopoietic progenitors from normal human adult peripheral blood. A fraction of the pure progenitors is multipotent (generates mixed colonies) and exhibits self-renewal capacity (gives rise to blast cell colonies). This methodology provides a fundamental tool for basic and clinical studies on hematopoiesis. Optimal in vitro cloning of virtually pure progenitors requires not only the stimulatory effect of interleukin-3, granulocyte-macrophage colony-stimulating factor, and erythropoietin, but also the permissive action of basic fibroblast growth factor. These findings suggest a regulatory role for this growth factor in early hematopoiesis.

Diverse forms of a receptor for acidic and basic fibroblast growth factors

We recently reported the isolation of a chicken cDNA clone encoding a basic fibroblast growth factor (FGF) receptor that has three immunoglobulinlike domains in the extracellular region. We have now identified four unique human cDNA clones encoding previously unknown FGF receptor variants which contain only two immunoglobulinlike domains. Two of the human clones encode membrane-spanning receptors, and two encode putative secreted forms. Both the three- and two-immunoglobulinlike-domain forms mediate biological responsiveness to acidic and basic FGF. Thus, the first immunoglobulinlike domain of the three-domain form may have a function other than binding of acidic and basic FGF.

Endocrinization of the early embryo: an emerging role for hormones and hormone-like factors

In the last decade, we witnessed the extension of endocrinologically based concepts and molecules to many other arenas of intercellular communication, e.g. immunology, hematology and cancer biology. At the start of the new decade we are witnessing the beginning of a similar transformation in our understanding of early embryogenesis, i.e. that hormones, growth factors and other hormone-like agents and their receptors, familiar to us in other contexts, may be the long-sought mediators of many key events in early embryogenesis. Why these agents were overlooked before and how they have started to emerge is the theme of this essay. The title 'Endocrinization of the Early Embryo' refers to both the biological and intellectual developments.

Cloning and expression of two distinct high-affinity receptors cross-reacting with acidic and basic fibroblast growth factors

The fibroblast growth factor (FGF) family consists of at least seven closely related polypeptide mitogens which exert their activities by binding and activation of specific cell surface receptors. Unanswered questions have been whether there are multiple FGF receptors and what factors determine binding specificity and biological response. We report the complete cDNA cloning of two human genes previously designated flg and bek. These genes encode two similar but distinct cell surface receptors comprised of an extracellular domain with three immunoglobulin-like regions, a single transmembrane domain, and a cytoplasmic portion containing a tyrosine kinase domain with a typical kinase insert. The expression of these two cDNAs in transfected NIH 3T3 cells led to the biosynthesis of proteins of 150 kd and 135 kd for flg and bek, respectively. Direct binding experiments with radiolabeled acidic FGF (aFGF) or basic FGF (bFGF), inhibition of binding with native growth factors, and Scatchard analysis of the binding data indicated that bek and flg bind either aFGF or bFGF with dissociation constants of (2-15) x 10(-11) M. The high affinity binding of two distinct growth factors to each of two different receptors represents a unique double redundancy without precedence among polypeptide growth factor-receptor interactions.

Diverse forms of a receptor for acidic and basic fibroblast growth factors

We recently reported the isolation of a chicken cDNA clone encoding a basic fibroblast growth factor (FGF) receptor that has three immunoglobulinlike domains in the extracellular region. We have now identified four unique human cDNA clones encoding previously unknown FGF receptor variants which contain only two immunoglobulinlike domains. Two of the human clones encode membrane-spanning receptors, and two encode putative secreted forms. Both the three- and two-immunoglobulinlike-domain forms mediate biological responsiveness to acidic and basic FGF. Thus, the first immunoglobulinlike domain of the three-domain form may have a function other than binding of acidic and basic FGF.

The gene for human acidic fibroblast growth factor encodes two upstream exons alternatively spliced to the first coding exon

We have isolated two cDNA clones encoding human acidic fibroblast growth factor (aFGF) which represent the utilization of alternative upstream exons in aFGF mRNA. Isolation and sequence analysis of genomic clones spanning the first coding exon and each of the upstream sequences confirms that the divergent 5' sequences are separate exons, spliced alternatively to the first coding exon 34 nucleotides upstream of the initiator AUG codon. Restriction mapping of the genomic clones provides a minimum size estimate of 45 kilobase pairs for the aFGF locus.

A mouse macrophage factor induces head structures and organizes a body axis in Xenopus

Soluble peptide factors have been implicated as the agents responsible for embryonic inductions in vertebrates. Here, a protein (PIF) secreted by a mouse macrophage cell line is shown to change the developmental fate of Xenopus embryonic cells. Exposure to PIF causes presumptive ectodermal explants to form anterior neural and mesodermal tissues, including brain and eye, instead of ciliated epidermis. In addition, the induced tissues are organized into a rudimentary embryonic axis. These results suggest that PIF or a closely related molecule is involved in inducing anterior structures and organizing the frog body plan.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

Effects of cell heterogeneity on production of polypeptide growth factors and mesoderm-inducing activity by Xenopus laevis XTC cells

The Xenopus laevis XTC cell line has been analyzed for the production of polypeptide growth factors and mesoderm-inducing activity. By the use of specific biological assays, it is shown that XTC cells produce a growth factor functionally related to the platelet-derived growth factor (PDGF) and two transforming growth factor (TGF) beta-like activities. Mesoderm-inducing activity, as measured on X. laevis ectodermal explants from stage 10 embryos, was found to coelute on a Bio-Gel P-100 column with one of the TGF beta-like activities at an apparent molecular weight of 6-10 kDa. Analysis of the DNA content from XTC cells by flow cytometry demonstrated that the cell line is heterogeneous and consists of both tetraploid and diploid cells. Cloning of the XTC cells and selecting single-cell colonies on the basis of their ability to grow in soft agar resulted in the isolation of several homogeneous, morphologically different clonal derivatives. Analysis of conditioned medium from these clonal derivatives showed that only one of them, the only diploid line among six investigated, produced a strong heat- and acid-stable mesoderm-inducing activity that induced notochord and muscle formation in stage 10 X. laevis ectodermal explants. The relation between this activity and a recently described TGF beta-like mesoderm-inducing factor obtained from XTC-conditioned medium will be discussed. In conclusion, a clonal cell line derived from X. laevis XTC cells which provides a good source for further characterization of mesoderm-inducing factors has been established.

Isolation of cDNAs encoding four mouse FGF family members and characterization of their expression patterns during embryogenesis

To initiate a study of the role of the fibroblast growth factor (FGF) family in mammalian development, we have isolated cDNAs encoding four mouse FGF family members, aFGF, bFGF, kFGF, and FGF-5. This was achieved by a process that circumvents the use of cDNA libraries: for each family member, a cDNA fragment containing the conserved portion of the coding region was amplified from a pool of embryonic and teratocarcinoma cell cDNAs using the polymerase chain reaction (PCR) and cloned; the remaining coding sequences 5' and 3' to the conserved region were cloned using the RACE method. The cDNA clones obtained were used as probes to analyze the expression of these genes at the RNA level in teratocarcinoma cells and embryos at 10.5 to 17.5 days of gestation. Fgfk appears to be specific to undifferentiated teratocarcinoma stem cells. Fgf5 transcripts were detected at every stage and in every tissue tested, but showed a dramatic 15-fold increase in abundance as teratocarcinoma stem cells differentiated to simple embryoid bodies. Fgfb expression showed the greatest tissue-specific variability in abundance, with the highest levels detected in the developing limbs and tail. Fgfa showed the least variable pattern of expression, with transcripts detected at roughly equivalent levels in almost all samples analyzed. On the basis of these data we speculate on some possible roles that the different FGF family members may play in the developing embryo.

Mesoderm induction by fibroblast growth factor in early Xenopus development

In early amphibian development the mesoderm is formed around the equator of the blastula in response to inductive signals from the endoderm. At the time of its formation the mesoderm consists of a large 'ventral type' zone and a small 'organizer' zone. A screen of candidate substances showed that a small group of heparin binding growth factors (HBGFs) were active as mesoderm inducing agents in vitro. The fibroblast growth factors (aFGF and bFGF) and embryonal carcinoma derived growth factor (ECDGF) all show similar potency and can produce ventral inductions at concentrations above about 100 pm. Single blastula ectoderm cells can be induced and will differentiate in a defined medium to form mesodermal tissues and all inner blastula cells are competent to respond to the factors. Inducing activity can be extracted from Xenopus blastulae and can be purified by heparin affinity chromatography. Antibody neutralization and Western blotting experiments identify this activity as bFGF. The amounts present are small but would be sufficient to evoke ventral inductions in vivo. It is not yet known whether the bFGF is localized to the endoderm, although it is known that inducing activity secreted by endodermal cells can be neutralized by heparin. The competence of ectoderm to respond to FGF rises from about the 128-cell-stage and falls again by the onset of gastrulation. This change is paralleled by a rise and fall of binding of 125I-labelled aFGF. Chemical cross-linking reveals that this binding is attributable to a receptor of molecular mass about 130 kilodaltons (kDa). The receptor is present both in the marginal zone, which responds to the signal in vivo, and in the animal pole region, which is not induced in vivo but which will respond to HBGFs in vitro. In intact embryos we believe that the ventral type mesoderm forms the somites, kidney and other intermediate structures as well as the blood islands of the ventral midline. These intermediate structures are induced as a function of distance from the organizer in a process called 'dorsalization'. Lithium salts have a dorsalizing effect on whole embryos and also on explants from the ventral marginal zone, causing them to form large blocks of muscle. Lithium will also cause large muscle blocks to form when applied to ectoderm explants together with FGF. It is difficult to extend these results directly to mammalian embryos, but we have shown that the products of the murine int-2 gene and of the human k-fgf genes are active as mesoderm inducing factors.

Distribution of basic fibroblast growth factor in the 18-day rat fetus: localization in the basement membranes of diverse tissues

Immunohistochemical methods were used to study the distribution of basic FGF in the 18-d rat fetus. The results reveal a pattern of widespread yet specific staining that is consistent with the wide distribution of basic FGF. Immunoreactive basic FGF is associated with mesenchymal structures, mesoderm- and neuroectoderm-derived cells, and their extracellular matrices. As an example, skeletal and smooth muscle cells are strongly positive. The basement membrane underlying the epithelia always contain basic FGF. In some tissues (i.e., cartilage and bone) the intensity of immunostaining is dependent on the stage of cell differentiation. Although the staining of tissues is primarily associated with the extracellular matrix, there is significant intracellular staining in various cell types. This is particularly evident in the endocrine cells of the adrenal cortex, testis, and ovary. The histochemical findings reported here support the notion that basic FGF has the characteristics required to mediate many of the effects of the mesenchyme on cell growth and differentiation. The significance of these findings in understanding the role of basic FGF in regulating cell proliferation and differentiation is discussed.