Mesoderm induction in early Xenopus embryos by heparin-binding growth 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.

Selective expression of PDGF A and its receptor during early mouse embryogenesis

Murine homologs of the PDGF A, PDGF B, and PDGF receptor alpha subunit genes were cloned. These were used, together with a mouse PDGF receptor beta subunit cDNA clone, to monitor gene expression in early postimplantation mouse embryos and in F9 embryonal carcinoma cells. RNAse protection analysis shows that PDGF A chain, but not B chain, mRNA is expressed in 6.5- to 8.5-day embryonic and extraembryonic tissues. Both alpha and beta receptor subunit mRNAs are expressed in early embryos, however, alpha subunit mRNA appears earlier and is more abundant than beta subunit mRNA. Undifferentiated F9 embryonal carcinoma stem cells express abundant levels of A chain, but not B chain, mRNA. Neither of the PDGF receptor genes is expressed in stem cells. Treatment with retinoic acid stimulates expression of both PDGF receptor genes. As in postimplantation mouse embryos, alpha receptor subunit mRNA appears earlier and is substantially more abundant than beta subunit mRNA. Collectively, these data demonstrate that the genes encoding the two chains of PDGF and their receptors are regulated independently during development and suggest that the two systems have some nonoverlapping functions in vivo. PDGF A, but not PDGF B, may be particularly important in modulating early events in mouse embryonic development.

XK endo B is preferentially expressed in several induced embryonic tissues during the development of Xenopus laevis

XK endo B is a type I keratin that was originally identified by its preferential expression in the embryonic notochord of the amphibian Xenopus laevis. A peptide identical to a short region of its predicted amino acid sequence was used to generate antibodies against the XK endo B protein. This paper reports an immunocytochemical study of the spatial expression pattern of XK endo B during development. The protein was observed in the notochord and endoderm as predicted from previous RNA analysis. In addition, XK endo B was detected in the cement gland, in the pituitary, olfactory and pharyngeal pouch rudiments, and in a nonuniform distribution in the neural tube as well as the inner sensorial layer of the ectoderm. XK endo B expression is not limited to any germ layer or any particular cell type, but is nevertheless highly restricted in its distribution in the embryo. Its expression in several different embryonic tissues requiring inductive interactions for differentiation makes XK endo B a valuable tool with which to study the regulation of induced gene expression during embryogenesis.

Purification and characterization of heparin-binding growth factors from porcine uterus

Heparin-binding growth factors present in pig uterine tissue were purified by approx. 50,000-fold using a combination of ammonium sulphate precipitation, ion-exchange chromatography and heparin-affinity chromatography. Purification of the uterus-derived growth factors (UDGFs) was monitored by the stimulation of [3H]thymidine incorporation into Swiss 3T3 cells and by a radioreceptor assay using 125I-labelled epidermal growth factor (EGF) as the ligand. The latter was shown to be a novel, rapid and reliable assay for heparin-binding growth factors which utilizes their trans-modulation of EGF receptor affinity. UDGFs exhibit strong affinity for immobilized heparin and two forms, named alpha UDGF and beta UDGF, were distinguished by salt gradient elution from heparin-agarose affinity columns. beta UDGF activity was eluted from heparin-agarose between 1.5 M- and 1.8 M-NaCl, and was correlated with the elution of a protein doublet of 17.2 kDa and 17.7 kDa. Immunoblotting of heparin-purified beta UDGF indicated that the beta UDGF doublet is immunologically related to the 146-amino-acid form of bovine basic fibroblast growth factor (bFGF), and that the 17.2 kDa component is an N-terminally truncated form of the 17.7 kDa component. After purification by C4 reversed-phase h.p.l.c., this doublet was biologically active and greater than 95% pure as assessed by silver-stained SDS/PAGE. Amino acid composition and sequence analysis confirmed that these beta UDGF polypeptides were microheterogeneous forms of bFGF. Fractions containing alpha UDGF activity were eluted from heparin-agarose in 1.3 M-NaCl. These fractions contained a 16.5 kDa protein which co-migrated on SDS/polyacrylamide gels with recombinant human acidic FGF (aFGF) and which which cross-reacted with an antiserum raised against aFGF. The identification of heparin-binding growth factors in porcine uterus at the time of implantation raises the possibility that they function in the reproductive tract during early pregnancy.

Expression pattern of the mouse T gene and its role in mesoderm formation

Formation of mesoderm is a crucial event in vertebrate development, establishing many of the important features of the body. Recent studies have implicated molecules that are similar to growth factors in mesoderm formation in Xenopus, but other gene products involved in this process have yet to be identified. Genetic evidence indicates that in the mouse the T gene (Brachyury) has a role in the formation and organization of mesoderm. Mice homozygous for mutant alleles of the T gene do not generate enough mesoderm, and show severe disruption in morphogenesis of mesoderm-derived structures, in particular the notochord. The cloning of the T gene has now allowed us to examine its expression pattern. We report that T-gene expression occurs in both early stage mesoderm and its epithelial progenitor, and then becomes restricted to the notochord. This expression pattern correlates with the tissues affected in the T-gene mutant, and indicates that the T gene has a direct role in the early events of mesoderm formation and in the morphogenesis of the notochord.

Axial patterning and the establishment of polarity in the frog embryo

We review the early development of the frog Xenopus laevis, concentrating on events that lead to the establishment of the body plan. Recent molecular and embryological experiments suggest that peptide growth factors and homeobox genes interact to specify cell fates and pattern during early development of this vertebrate.

Isolation, characterization, and localization of heparin-binding growth factors in the heart

Acidic and basic fibroblast growth factors (aFGF and bFGF) are angiogenic polypeptide mitogens for cells of mesodermal and neuroectodermal origin. In this report we describe the purification from several normal human hearts (including a very fresh, nonischemic sample) of heparin-binding, acid-, heat- and trypsin-sensitive 14-18-kD peptides that crossreact with antisera against aFGF and bFGF. Further evidence includes (a) prevention of mitogenicity by protamine and by anti-bFGF, (b) displacement of 125I-bFGF from cell membranes, and (c) stimulation of capillary endothelial cell migration. Specific immunohistochemistry localized bFGF to endothelial cells and, surprisingly, to cardiac myocytes, with almost no immunoreactivity in smooth muscle cells. These peptides may function in cardiac embryogenesis, hypertrophy, atherogenesis, angiogenesis, and wound healing, and may also have endocrine, neurotropic, or vasomotor functions.

Fibroblast growth factor receptor levels decrease during chick embryogenesis

Two putative receptors for fibroblast growth factor (FGF) of approximately 150 and 200 kD were identified in membrane preparations from chick embryos. Specific binding (femtomoles/milligram) of 125I-aFGF to whole chick embryonic membranes was relatively constant from day 2 to 7, then decreased fivefold between days 7 and 13. Day-19 chick embryos retained 125I-aFGF binding at low levels to brain, eye, and liver tissues but not to skeletal muscle or cardiac tissues. The 200-kD FGF receptor began to decline between day 4.5 and 7 and was barely detectable by day 9, whereas the 150-kD FGF receptor began to decline by day 7 but was still detectable in day-9 embryonic membranes. It is not known whether the two FGF-binding proteins represent altered forms of one polypeptide, but it is clear that their levels undergo differential changes during development. Because endogenous chick FGF may remain bound to FGF receptor in membrane preparations, membranes were treated with acidic (pH 4.0) buffers to release bound FGF; such treatment did not affect 125I-aFGF binding and moderately increased the number of binding sites in day-7 and -19 embryos. Consequently, the observed loss of high affinity 125I-aFGF binding sites and FGF-binding polypeptides most likely represents a loss of FGF receptor protein. These experiments provide in vivo evidence to support the hypothesis that regulation of FGF receptor levels may function as a mechanism for controlling FGF-dependent processes during embryonic development.

Comparison of the ability of basic and acidic fibroblast growth factor to stimulate the proliferation of an established keratinocyte cell line: modulation of their biological effects by heparin, transforming growth factor beta (TGF beta), and epidermal growth factor (EGF)

The bioactivity of both bFGF and aFGF in the BALB/MK-1 cell line has been compared to that of EGF. Our results indicate that, for that cell type, aFGF was far more potent than bFGF in inducing cell proliferation. In the presence of heparin, aFGF was as potent as EGF. In addition, excess bFGF has an inhibitory effect on the proliferation of MK cells exposed to a saturating concentration of aFGF, therefore acting as a partial agonist of aFGF. Surprisingly, bFGF, although it had low biological activity, was capable of synergizing the effect of EGF. In its presence, cultures exposed to saturating concentration of EGF have a final cell density 3- to 4-fold higher than that of counterpart cultures exposed to EGF alone. TGF beta, which in previous studies has been shown to inhibit the growth of keratinocytes, also inhibited the growth of BALB/MK-1 cells in response to either bFGF or aFGF. These studies suggest a role for FGF in regulating BALB/MK proliferation. aFGF provides positive growth signals which can be negatively modulated by excess bFGF or TGF beta, while bFGF, although a poor mitogen, could act by potentiating the effect of subsaturating concentrations of EGF.

Acidic fibroblast growth factor is a modulator of epithelial plasticity in a rat bladder carcinoma cell line

During normal embryogenesis and neoplastic transformation epithelia change their state of differentiation and degree of cohesiveness. It is thus essential to identify the signals modulating these transitions. We report here that acidic fibroblast growth factor (FGF) induces cells derived from a rat bladder carcinoma to lose their epithelial character and to acquire some properties typical of mesenchymal cells. The structurally related basic FGF did not have such an effect; both factors, however, had a mitogenic activity for these cells. Two distinct populations of receptors for acidic FGF and basic FGF were distinguished by their ligand-binding characteristics. The observations that both acidic and basic FGFs had a mitogenic effect on NBT-II cells and that only acidic FGF caused cell dissociation and dispersion strongly suggest that these two biological activities could be medicated through distinct signaling pathways.

Peptide growth factors can provoke "fetal" contractile protein gene expression in rat cardiac myocytes

Cardiac-specific gene expression is intricately regulated in response to developmental, hormonal, and hemodynamic stimuli. To test whether cardiac muscle might be a target for regulation by peptide growth factors, the effect of three growth factors on the actin and myosin gene families was investigated by Northern blot analysis in cultured neonatal rat cardiac myocytes. Transforming growth factor-beta 1 (TGF beta 1, 1 ng/ml) and basic fibroblast growth factor (FGF, 25 ng/ml) elicited changes corresponding to those induced by hemodynamic load. The "fetal" beta-myosin heavy chain (MHC) was up-regulated about four-fold, whereas the "adult" alpha MHC was inhibited greater than 50-60%; expression of alpha-skeletal actin increased approximately two-fold, with little or no change in alpha-cardiac actin. Thus, peptide growth factors alter the program of differentiated gene expression in cardiac myocytes, and are sufficient to provoke fetal contractile protein gene expression, characteristic of pressure-overload hypertrophy. Acidic FGF (25 ng/ml) produced seven- to eightfold reciprocal changes in MHC expression but, unlike either TGF-beta 1 or basic FGF, inhibited both striated alpha-actin genes by 70-90%. Expression of vascular smooth muscle alpha-actin, the earliest alpha-actin induced during cardiac myogenesis, was increased by all three growth factors. Thus, three alpha-actin genes demonstrate distinct responses to acidic vs. basic FGF.

Activin as a cell differentiation factor

Activin, originally discovered as a polypeptide hormone that is capable of stimulating follicle-stimulating hormone secretion from pituitary cells in vitro, has recently been found to have a much wider range of biological activities. There are a number of reports of activin action as a cell differentiation factor on various types of cells rather than as a modulator of hormone secretion, as predicted initially, based on its structural similarity to transforming growth factor-beta. Studies of the distribution of activin and its receptor in a variety of tissues and its wide-ranging actions clearly illustrates its multifunctional properties. In particular, activin has been shown to be a potential regulator of early development of Xenopus laevis. Observation of activin effect in embryogenesis is of general importance to our understanding of the role of the family of growth factors in developmental processes.

Turnover of functional basic fibroblast growth factor receptors on the surface of BHK and NIH 3T3 cells

The recovery of functional cell-surface bFGF receptors after trypsin treatment was studied in BHK cells and NIH 3T3 cells. Restoration of functional bFGF receptors occurred at an approximately linear rate with 50% of the high-affinity binding capacity restored after 4 hr. Restoration of functional receptors required protein synthesis but not RNA synthesis. Upon exposure of BHK cells to bFGF, cell-surface receptors were rapidly lost, with only 25% remaining after 1 hr. When the bFGF was removed, down-regulated BHK and NIH 3T3 cells recovered cell-surface receptors at about the same rate observed in trypsin-treated cells. The recovery of receptors after down-regulation was inhibited by protein synthesis inhibitors. Addition of the protein synthesis inhibitor cycloheximide to unperturbed cultures of BHK or NIH 3T3 cells resulted in a time-dependent loss of cell-surface bFGF receptors, demonstrating that the receptors turn over constantly in the absence of ligand. These results suggest that bFGF receptors do not recycle and must be continuously synthesized.

Mesoderm induction in Xenopus laevis distinguishes between the various TGF-beta isoforms

Induction of mesoderm in ectodermal explants of Xenopus laevis blastula embryos had previously been shown to respond selectively to TGF-beta 2, with TGF-beta s 1 and 5 having no activity in this assay. As TGF-beta s 1, 2, and 3 are frequently coexpressed in tissues, we wished to examine the activity of TGF-beta 3 relative to that of TGF-beta s 1 and 2 in this assay as well as in other in vitro assays. We report here that when the activity of recombinant TGF-beta 3 is normalized to that of TGF-beta 1 in the assay for growth inhibition in CCL-64 cells, it is also equal to that of TGF-beta 1 in assays for stimulation of both anchorage-independent growth of rat NRK cells and chemotaxis of human monocytes. In contrast, in the assay for mesoderm induction, recombinant TGF-beta 3 is 10-fold more active than TGF-beta 2, inducing expression of muscle specific alpha-actin at concentrations as low as 1 ng/ml. These results suggest that more complex systems, in contrast to individual cell types, may respond selectively to the various TGF-beta isoforms and that there might be biological consequences of TGF-beta isoform switching in vivo.

Isolation and characterization of TGF-beta 2 and TGF-beta 5 from medium conditioned by Xenopus XTC cells

TGF-beta 2 and -beta 5 have been purified from medium conditioned by Xenopus cultured cells (XTC) and identified based on their N-terminal amino acid sequence analysis and biological activity. When applied in high concentrations, Xenopus TGF-beta 2, like porcine TGF-beta 2, induces expression of mesodermal markers from cultured Xenopus ectodermal explants, whereas TGF-beta 5 is inactive in this assay. However, the TGF-beta 's could be separated from the major mesoderm-inducing activity present in XTC medium. Xenopus TGF-beta 2 and -beta 5 are approximately equivalent to TGF-beta 1 in their abilities to inhibit the growth of mink lung CCL-64 cells, induce anchorage-independent growth of rat NRK cells, inhibit the proliferation and antibody secretion of human B-lymphocytes, and stimulate chemotaxis of human monocytes. These data establish the functional activity of TGF-beta 5 and suggest that more complex multicellular systems, in contrast to most isolated cells, discriminate between the different TGF-beta s.

The role of fibroblast growth factors in the central nervous system

The fibroblast growth factors are well-characterized mitogens that are found in the central nervous system (CNS). Their physiological roles are not yet known, but increasing evidence suggests their involvement in CNS development, injury responses, and possibly oncogenesis.

Growth factors in the eye

In this review we report the distribution and functional significance of growth factors in the eye. Representatives of the major growth factor families are found in the eye: fibroblast growth factor, insulin and insulin-like growth factor, transforming growth factor-beta, platelet-derived growth factor, nerve growth factor, epidermal growth factor and colony-stimulating factor. There are numerous examples of their actions on ocular tissues in vitro and in some cases in vivo. The findings presented clearly illustrate that a growth factor can elicit different responses depending on the context of its action; the cell type involved, the concentration of the growth factor and the presence or absence of other growth factors can all influence the cellular response both quantitatively and qualitatively. The results of these studies in the eye are of general significance to our understanding of the role of growth factors in biological processes.

A Xenopus mRNA related to Drosophila twist is expressed in response to induction in the mesoderm and the neural crest

We have cloned a Xenopus cDNA related to the twist gene, which is required for mesodermal differentiation in Drosophila. Northern blots of dissected embryos and in situ hybridization show that the corresponding mRNA, called Xtwi, first appears in early gastrulae, and is present only in mesodermal cells. Within the mesoderm, Xtwi is expressed in the notochord and lateral plate, but not in the myotome; therefore there is a complementary pattern of Xtwi and muscle-specific gene expression in the mesoderm. Xtwi expression therefore marks the subdivision of the mesoderm. Xtwi is also activated a few hours later in the early development of the neural crest. This gene is thus expressed in response to two sequential early inductions in frog development.

Expression and developmental regulation of the k-FGF oncogene in human and murine embryonal carcinoma cells

Embryonal carcinoma (EC) cells provide an effective model system for studying growth factor production and regulation during mammalian embryogenesis. Our earlier data indicated that the mouse EC cell lines F9 and PC-13 and the human EC cell line NT2/D1 produce a factor with properties similar to those ascribed to members of the fibroblast growth factor (FGF) family and that production of this FGF-related factor is suppressed when all three EC cell lines are induced to differentiate. Subsequent studies suggested that NT2/D1 EC cells express transcripts for basic FGF (bFGF). The current study confirms and extends these findings using a combination of reverse transcription and polymerase chain reaction (RT-PCR). In this study, the expression of bFGF and other members of the FGF family have been examined in F9 and PC-13 cells in addition to NT2/D1 EC cells. In contrast to NT2/D1 EC cells, bFGF expression could not be detected in F9 and PC-13 EC cells. Additionally, expression of four other members of the FGF family (acidic FGF, int-2, FGF-5, and FGF-6) were not detected in NT2/D1, F9, or PC-13 EC cells. However, expression of another member of the FGF family, the k-FGF oncogene, was detected in NT2/D1, F9, and PC-13 EC cells. Moreover, the expression of this transcript is reduced dramatically when each of the three EC cell lines is induced to differentiate. Taken together, our findings argue that expression of the k-FGF oncogene is predominantly responsible for the FGF-related activity detected in EC cells and that differentiation of these EC cells results in suppression of this oncogene.

An antisense mRNA directs the covalent modification of the transcript encoding fibroblast growth factor in Xenopus oocytes

The Xenopus oocyte contains three maternal basic fibroblast growth factor (bFGF) transcripts, the largest of which encodes the complete bFGF polypeptide. We show that the smallest transcript is transcribed in the opposite direction of the largest transcript and overlaps part of the coding sequence of bFGF. This antisense transcript encodes a highly conserved 25 kd polypeptide. In addition, the antisense transcript causes modification of the mRNA encoding bFGF during maturation of the oocyte, converting half of the adenine residues to inosine in the region of overlap between the sense and antisense transcripts. As this activity acts only on double-stranded RNA, the two mRNAs must be hybridized in the oocyte. We discuss a possible role for the antisense transcript in regulating the stability of the bFGF mRNA.

Fibroblast growth factor enhances type beta 1 transforming growth factor gene expression in osteoblast-like cells

Fibroblast growth factor (FGF) and type beta transforming growth factor (TGF beta) are potent modulators of proliferation and differentiation in many types of cells. TGF beta acts in an autocrine manner, and the regulation of TGF beta gene expression is one of the crucial events in the control of cellular functions. This study examines FGF regulation of TGF beta 1 gene expression in osteoblast-like cells. Bovine basic FGF (bFGF) increased the steady-state level of 2.5-kb TGF beta 1 mRNA two- to threefold in rat osteosarcoma (ROS17/2.8) cells in a dose-dependent manner, starting at 0.1 ng/ml. The increase of the message was detectable within 3 h after the addition of bFGF, peaked at 6 h, and lasted at least up to 48 h. This effect was blocked by a protein kinase inhibitor, K252a, indicating the involvement of phosphorylation. bFGF increased the rate of TGF beta 1 gene transcription estimated by nuclear run-on assay, while the stability of TGF beta 1 mRNA was not altered. bFGF increased the TGF beta activity in the conditioned media, estimated by DNA synthesis inhibition assay using mink lung epithelial (CCL-64) cells. Parathyroid hormone reduced the abundance of TGF beta 1 mRNA in ROsS17/2.8 cells and opposed the bFGF effect on TGF beta 1 mRNA. bFGF also increased the steady-state level of TGF beta 1 mRNA in mouse calvaria-derived MC3T3E1 and human osteosarcoma SaOS-2 cells. These findings indicate that FGF enhances the expression of TGF beta 1 gene in osteoblast-like cells and point to the tight relationship of the two growth factors involved in the control of cellular functions.

Basic fibroblast growth factor in atria and ventricles of the vertebrate heart

Extracts from atrial and ventricular heart tissue of several species (chicken, rat, sheep, and cow) are strongly mitogenic for chicken skeletal myoblasts, with the highest apparent concentration of biological activity in the atrial extracts. Using several approaches (biological activity assay and biochemical and immunological analyses), we have established that (a) all cardiac extracts contain an 18,000-D peptide which is identified as basic fibroblast growth factor (bFGF) since it elutes from heparin-Sepharose columns at salt concentrations greater than 1.4 M and is recognized by bFGF-specific affinity-purified antibodies; (b) bFGF is more abundant in the atrial extracts in all species so examined; (c) avian cardiac tissue extracts contain the highest concentration of immunoreactive bFGF; and (d) avian ventricles contain a higher relative molecular mass (23,000-D) bFGF-like peptide which is absent from atrial extracts. Examination of frozen bovine cardiac tissue sections by indirect immunofluorescence using anti-bFGF antibodies shows bFGF-like reactivity associated with nuclei and intercalated discs of muscle fibers. There is substantial accumulation of bFGF around atrial but not ventricular myofibers, resulting most likely from more extensive endomysium in the atria. Blood vessels and single, nonmuscle, connective tissue cells react strongly with the anti-bFGF antibodies. Higher bFGF content and pericellular distribution in atrial muscles suggest a correlation with increased regenerative potential in this tissue. Distribution within the myofibers is intriguing, raising the possibility for an intimate and continuous involvement of bFGF-like components with normal myocardial function.

Cultured human retinal pigment epithelial cells express basic fibroblast growth factor and its receptor

Basic fibroblast growth factor (bFGF) has been implicated in the maintenance of neuronal differentiation, the induction of neovascularization and intravitreal proliferative diseases. We have found that human retinal pigment epithelial (RPE) cells grown in vitro transcribe the bFGF gene and synthesize a peptide that crossreacts with anti-bFGF antibodies. In culture, these cells appear to release activity with biological and biochemical properties similar to bFGF. RPE cells have specific bFGF receptors and proliferate in response to bFGF. Thus, it is possible that the RPE cell is an important source of retinal bFGF and may respond to bFGF in an autocrine manner.

Promotion of embryonic chick limb cartilage differentiation by transforming growth factor-beta

This study represents a first step in investigating the possible involvement of transforming growth factor-beta (TGF-beta) in the regulation of embryonic chick limb cartilage differentiation. TGF-beta 1 and 2 (1-10 ng/ml) elicit a striking increase in the accumulation of Alcian blue, pH 1-positive cartilage matrix, and a corresponding twofold to threefold increase in the accumulation of 35S-sulfate- or 3H-glucosamine-labeled sulfated glycosaminoglycans (GAG) by high density micromass cultures prepared from the cells of whole stage 23/24 limb buds or the homogeneous population of chondrogenic precursor cells comprising the distal subridge mesenchyme of stage 25 wing buds. Moreover, TGF-beta causes a striking (threefold to sixfold) increase in the steady-state cytoplasmic levels of mRNAs for cartilage-characteristic type II collagen and the core protein of cartilage-specific proteoglycan. Only a brief (2 hr) exposure to TGF-beta at the initiation of culture is sufficient to stimulate chondrogenesis, indicating that the growth factor is acting at an early step in the process. Furthermore, TGF-beta promotes the formation of cartilage matrix and cartilage-specific gene expression in low density subconfluent spot cultures of limb mesenchymal cells, which are situations in which little, or no chondrogenic differentiation normally occurs. These results provide strong incentive for considering and further investigating the role of TGF-beta in the control of limb cartilage differentiation.

Interaction between peptide growth factors and homoeobox genes in the establishment of antero-posterior polarity in frog embryos

The expression of the Xenopus homoeobox gene xhox3 is an early response to mesoderm induction by peptide growth factors and the level of xhox3 expression marks the antero-posterior character of the induced mesoderm. Different peptide growth factors specify different antero-posterior mesodermal cell fates as seen by the level of xhox3 expression and the capacity to induce specific secondary neural/epidermal structures. These factors and homoeobox genes thus form part of the mechanism necessary for establishing antero-posterior polarity in the frog embryo.

Expression of nicotinic acetylcholine receptors in aneural Xenopus embryos

During gastrulation in vertebrate embryos, the mesoderm moves inward and under the ectoderm and these two cell layers subsequently differentiate in close proximity to each other, providing an opportunity for the exchange of inductive signals. This study examines whether the activation of muscle nicotinic acetylcholine receptor (AChR) genes and the subsequent expression of receptors in Xenopus myotomal muscle are dependent on interaction between the ectoderm and the mesoderm, or their derivatives, after the onset of gastrulation. We eliminated such interaction by inducing total exogastrulation of Xenopus embryos. During exogastrulation, the mesoderm moves away from the ectoderm, and the nervous system fails to develop. Single channel recordings from the myotomal muscle of exogastrulated embryos revealed the presence of two major classes of AChRs, which could be distinguished on the basis of channel conductance. The current amplitudes, conductances, reversal potentials, and open times of these channels closely resembled those reported for the two major classes of AChR channels normally expressed in vivo. We conclude that interaction between ectoderm and mesoderm following the onset of gastrulation is not required for the future expression of the major classes of AChRs in myotomal muscle.

Mesoderm induction and blood island formation by angiogenic growth factors and embryonic inducing factors

Factors which induce mesoderm, including endothelium lined cavities and primitive blood cells in omnipotent amphibian ectoderm, have been isolated from different sources. Recently it was shown that angiogenic factors, which belong to the protein families of the heparin binding growth factors (acidic and basic fibroblast growth factor) and the transforming growth factors (TGF-beta 1 and -beta 2), also induce mesodermal tissues in amphibian ectoderm. In triturus ectoderm, capillary like endothelial networks are induced preferentially by the transforming growth factors. The relationship between growth factors and inducing factors is discussed.

Neural induction is mediated by cross-talk between the protein kinase C and cyclic AMP pathways

Embryonic inductions appear to be mediated by the concerted action of different inducing factors that modulate one another's activity. Such modulation is likely to reflect interactions between the signal transduction pathways through which the inducing factors act. We tested this idea for the induction of neural tissue. We report that both adenylate cyclase activity and cAMP concentration increase substantially in induced neuroectoderm during neural induction. The enhancement of adenylate cyclase activity requires protein kinase C (PKC) activation, indicating cross-talk between these two signal transduction pathways. This cross-talk appears to be essential for neural induction. Whereas cAMP analogs alone were not neural inducers, they had a synergistic inducing effect if ectoderm was first incubated with TPA (12-O-tetradecanoylphorbol 13-acetate), a PKC activator. These results strongly suggest that at least two signals mediate neural induction. The first signal activates PKC and the second signal then activates the cAMP pathway effectively.

Thyroid hormone induces constitutive keratin gene expression during Xenopus laevis development

We have used in vitro explant cultures of Xenopus laevis skin to investigate the role that the thyroid hormone triiodothyronine (T3) plays in activating the 63-kilodalton (kDa) keratin genes. The activation of these genes in vivo requires two distinct steps, one independent of T3 and one dependent on T3. In this report we have shown that the same two steps are required to fully activate the 63-kDa keratin genes in skin explant cultures, and we have characterized the T3-mediated step in greater detail. Unlike the induction of transcription by T3 or steroid hormones in adult tissues, there was a long latent period of approximately 2 days between the addition of T3 to skin cultures and an increase in concentration of keratin mRNA. While the T3 induction of 63-kDa keratin gene transcription cannot occur until age 48, a short transient exposure of stage 40 skin cultures to T3 resulted in high-level expression of these genes 5 days later, when normal siblings had reached stage 48. This result indicates that T3 induces a stable change in epidermal cells which can be expressed much later, after extensive cell proliferation has occurred in the absence of T3. Once the 63-kDa keratin genes were induced, they were stably expressed, and by the end of metamorphosis T3 had no further effect on their expression. The results suggest that T3 induces constitutive expression of the 63-kDa keratin genes during metamorphosis.

Clonal analysis of mesoderm induction in Xenopus laevis

Acidic fibroblast growth factor (aFGF) has been used to induce mesoderm from single animal pole cells of midblastula stage Xenopus embryos. The cells are individually cultured in a completely defined medium and are able to differentiate as small clones in a high proportion of cases. FGF-treated cells can give rise to several mesodermal cell types, while untreated cells show only epidermal or neural differentiation. Mesodermal differentiation can occur in clones of as few as eight cells, indicating that any additional cell-cell interactions required for mesodermal differentiation can be met by the medium used.

Experimental reversal of the normal dorsal-ventral timing of blastopore formation does not reverse axis polarity in Xenopus laevis embryos

During gastrulation in Xenopus laevis, the dorsal lip of the blastopore normally appears before the ventral lip. Metabolic gradient models propose that the dorsal lip develops from the region of highest metabolic activity and somehow dominates other regions to prevent them from becoming dorsal. To test these ideas, I applied a temperature gradient of 12 degrees C across the embryo. Localized heating of the prospective ventral vegetal region from early in the first cleavage period until gastrulation causes the blastopore lip to form first by 2 hr at the prospective ventral meridian rather than at the prospective dorsal meridian. Despite this reversal of the timing of blastopore formation, gastrulation is completed, and the neural plate forms at its usual position on the prospective dorsal meridian. This demonstrates that the earliest gastrulating regions of the blastopore do not necessarily become dorsal, nor do they inhibit dorsal development by other regions. It is unlikely that axis polarity is based on regional differences in energy metabolism.

Expression and control of vascular endothelial cells: proliferation and differentiation by fibroblast growth factors

Evidence from in vitro studies supports the concept that growth factors could be involved in the development and function of the vascular tree. Among the growth factors known to influence vascular endothelial cells' proliferation and differentiation, in vitro, are the fibroblast growth factors. These consist of two closely, structurally, related polypeptides that differ by their isoelectric point and have been called basic and acidic fibroblast growth factor. These growth factors, in particular basic fibroblast growth factor, which is expressed and synthesized by vascular endothelial cells, could influence the development, remodeling, and function of the vascular tree through regulating mechanisms involving paracrine and autocrine control of cell proliferation and differentiation.

Retinoic acid causes an anteroposterior transformation in the developing central nervous system

All-trans retinoic acid (RA) is well known as a biologically active form of vitamin A and a teratogen. The identification of nuclear receptors for this ligand suggests strongly that it is an endogenous signal molecule, and measurements of RA and teratogenic manipulations suggest further that RA is a morphogen specifying the anteroposterior axis during limb development. Besides the limb, RA and other retinoids affect development of other organs, including the central nervous system (CNS). None of these other effects has been investigated in detail. Our purpose here was to begin analysing the effects of RA on CNS development in Xenopus laevis. We find that RA acts on the developing CNS, transforming anterior neural tissue to a posterior neural specification. These and other findings raise the possibility that RA mediates an inductive interaction regulating anteroposterior differentiation within the CNS. Following recent reports implicating transforming growth factor-beta 2-like and fibroblast growth factor-like factors in mesoderm induction, this indicates that a different type of signal molecule (working through a nuclear receptor, not a plasma membrane receptor) might mediate inductive cell interactions during early embryonic development.

Mix.1, a homeobox mRNA inducible by mesoderm inducers, is expressed mostly in the presumptive endodermal cells of Xenopus embryos

In frogs, mesoderm presumably derives from presumptive ectoderm by induction under the control of diffusible substances produced by the endoderm. To analyze the early phase of mesoderm induction, I have isolated cDNA copies of mRNAs induced in presumptive ectoderm by mesoderm inducing factor secreted by the XTC cell line. One of the inducible mRNAs encodes a homeodomain-containing protein that is likely to play a regulatory role in development. Mix.1 behaves as an immediate early response to induction, and its kinetics of expression suggest a major role for MBT in the control of inducible gene expression. Unexpectedly, Mix.1 is expressed mostly in the future endoderm, suggesting that endoderm may be formed by induction in a similar way as mesoderm.

Injury-induced release of basic fibroblast growth factor from bovine aortic endothelium

Although the basic fibroblast growth factor (bFGF) gene lacks a traditional consensus signal peptide domain indicative for secretion, many cell types have receptors for bFGF. Since endothelium is a rich source of cell-associated bFGF, we asked under what conditions could bFGF be released or secreted from confluent cultures of bovine aortic endothelial (BAE) cells. The level of bFGF in BAE cell lysates was compared with the level of heparin-releasable bFGF in intact BAE cell monolayers, intact cells with exposed extracellular matrix (nonlytic matrices), and extracellular matrices prepared by cell lysis (lytic matrices). Less than 10% of total cell-associated bFGF was released from intact cell monolayers and nonlytic matrices. In contrast, the levels of bFGF released from lytic matrices depended upon the conditions used to prepare the matrices. Cell lysis at neutral pH generated matrices that released the highest bFGF levels (approximately 50% of total cell-associated bFGF). These matrices were heavily contaminated by histones, indicating the cellular release and adsorption of intracellular proteins to the matrix. Matrices prepared by BAE cell exposure to basic pH (100 mM NH4OH) contained low bFGF content and minor histone contamination. These latter matrices were chosen to study bFGF sequestration, under physiological conditions, into the extracellular matrix of confluent BAE cell cultures. Incubation with endotoxin, an agent acutely toxic to BAE cells, resulted in cellular release and adsorption of endogenous bFGF to cells and matrices, accompanied by histone deposition in the matrices. These results suggested that one mechanism for bFGF release from BAE cell monolayers was passive release induced by severe cell injury and/or cell lysis with secondary adsorption to the matrix.

Expression of fibroblast growth factor by F9 teratocarcinoma cells as a function of differentiation

F9 teratocarcinoma stem cells treated with retinoic acid (RA) and dibutyryl cAMP (but2 cAMP) differentiate into embryonic parietal endoderm. Using heparin-affinity chromatography, endothelial cell proliferation assays, immunoprecipitation, and Western analysis with antibodies specific for acidic and basic fibroblast growth factors (FGFs), we detected biologically active FGF in F9 cells only after differentiation. A bovine basic FGF cDNA probe hybridized to 2.2-kb mRNAs in both F9 stem and parietal endoderm cells and to a 3.8-kb mRNA in F9 stem cells. A genomic DNA probe for acidic FGF hybridized to a 5.8-6.0-kb mRNA in both F9 stem and parietal endoderm cells, and to a 6.0-6.3-kb mRNA only in parietal endoderm cells. Although these FGF mRNAs were present in the stem cells, we could find no evidence that F9 stem cells synthesized FGFs, whereas differentiated F9 cells synthesized both acidic and basic FGF-like proteins. We conclude that biologically active factors with properties characteristic of acidic and basic FGF are expressed by F9 parietal endoderm cells after differentiation. Differentiating embryonic parietal endoderm thus may serve as a source of FGF molecules in the developing blastocyst, where these factors appear to play a central role in subsequent embryogenesis.

Basic fibroblast growth factor in the chick embryo: immunolocalization to striated muscle cells and their precursors

The identification of acidic and basic fibroblast growth factors (FGFs) in a number of embryonic tissue extracts has implicated these growth factors in the regulation of a variety of embryonic events including angiogenesis, eye development, and muscle differentiation. Lack of information concerning the cellular distribution of the growth factor within these tissues has made it extremely difficult to assign developmental roles to FGF. We have localized bFGF in the developing chick embryo using immunohistochemical techniques and our monospecific polyclonal rabbit anti-human bFGF IgG. The spatial pattern for bFGF localization was highly specific. The anti-human bFGF antibodies recognized striated muscle cells and their precursors in 2-6-d chick embryos. Myocardium, somite myotome, and limb bud muscle all stain positively for bFGF. In addition, the anti-human bFGF antibodies localized specifically to the cell, rather than to the extracellular matrix or nucleus of myotubes. The localization of bFGF demonstrated here provides further support for the hypothesis (Clegg et al., 1987; Seed et al., 1988) that this growth factor is involved in muscle development.

Induction of mesoderm by a viral oncogene in early Xenopus embryos

During frog embryogenesis, mesoderm is specified in the equatorial region of the early embryo by a signal from the vegetal hemisphere. Prospective ectodermal cells dissected from the animal hemisphere can be respecified to form mesodermal tissues by recombination with vegetal tissue or by treatment with any of several polypeptide growth factors or growth factor-like molecules. Together with the discovery that several developmental mutations in Drosophila are in genes with significant homology to mammalian mitogens and oncogenes, these observations suggest that early developmental signals may use similar transduction pathways to mitogenic signals characterized in cultured mammalian cells. Whether mesoderm can be induced by activation of intracellular signal transduction pathways implicated in mitogenesis and oncogenesis has been investigated with the viral oncogene polyoma middle T. Microinjection of middle T messenger RNA into early embryos results in the respecification of isolated prospective ectodermal tissue to form characteristic mesodermal structures. Middle T in frog blastomeres appears to associate with cellular activities similar to those observed in polyoma-transformed mouse cells, and transformation-defective middle T mutants fail to induce mesoderm. These results suggest that early inductive signals and mitogenic and oncogenic stimuli may share common signal transduction pathways.