Nuclear translocation of fibroblast growth factor during Xenopus mesoderm induction

Gene profiling of cells expressing different FGF-2 forms

Fibroblast Growth Factor-2 (FGF-2) induces cell proliferation, cell migration, embryonic development, cell differentiation, angiogenesis and malignant transformation. The four forms of FGF-2 (Low Molecular Weight) and (High Molecular Weights) are alternative translation products, and have a different subcellular localization: the high molecular weight (HMWFGF-2) forms are nuclear while the low molecular weight form, (LMWFGF-2) is mainly cytoplasmic. Our previous work demonstrated NIH 3T3 cells expressing different FGF-2 forms, displayed a different phenotype, suggesting that nuclear and cytoplasmic forms of FGF-2 may have different functions. Here we report a cDNA microarray-based study in NIH 3T3 fibroblasts expressing different FGF-2 forms. Several candidate genes that affect cell-cycle, tumor suppression, adhesion and transcription were identified as possible mediators of the HMWFGF-2 phenotype and signaling pattern. These results demonstrated that HMWFGF-2 and LMWFGF-2 target the expression of different genes. Particularly, our data suggest that HMWFGF-2 forms may function as inducers of growth inhibition and tumor suppression activities.

Immunocytochemical study of activin type IB receptor (XALK4) in Xenopus oocytes

Studies have shown that the activin type IB receptor is specific for activin/nodal signaling. Activin is produced by follicle cells in the ovary, and is incorporated into the oocytes. Antisera against three peptides were prepared, encompassing the extracellular, intracellular and serine/threonine kinase domains of the Xenopus type IB activin receptor (XALK4). Immunocytochemistry was done using these antisera to investigate the distribution of XALK4 in the Xenopus ovary. All three antisera stained the mitochondrial cloud of Xenopus previtellogenic oocytes. Purified antibody against the intracellular domain also recognized the mitochondrial cloud. Immunoelectron microscopy localized XALK4 on the endoplasmic reticulum of the mitochondrial cloud, although not on mitochondria.

Freeze-substitution: origins and applications

Freeze-substitution is a physicochemical process in which biological specimens are immobilized and stabilized for microscopy. Water frozen within cells is replaced by organic solvents at subzero temperatures. Freeze-substitution is widely used for ultrastructural and immunocytochemical analyses of cells by transmission and scanning electron microscopy. Less well recognized is its superiority over conventional chemical fixation in preserving labile and rare tissue antigens for immunocytochemistry by light microscopy. In the postgenome era, the focus of molecular genetics will shift from analyzing DNA sequence structure to elucidating the function of gene networks, the intercellular effects of polygenetic diseases, and the conformational rearrangements of proteins in situ. Novel strategies will be needed to integrate knowledge of chemical structures of normal and abnormal macromolecules with the physiology and developmental biology of cells and tissues from whole organisms. This review summarizes the progress and future prospects of freeze-substitution for such explorations.

Meiotic maturation induces animal-vegetal asymmetric distribution of aPKC and ASIP/PAR-3 in Xenopus oocytes

The asymmetric distribution of cellular components is an important clue for understanding cell fate decision during embryonic patterning and cell functioning after differentiation. In C. elegans embryos, PAR-3 and aPKC form a complex that colocalizes to the anterior periphery of the one-cell embryo, and are indispensable for anterior-posterior polarity that is formed prior to asymmetric cell division. In mammals, ASIP (PAR-3 homologue) and aPKCgamma form a complex and colocalize to the epithelial tight junctions, which play critical roles in epithelial cell polarity. Although the mechanism by which PAR-3/ASIP and aPKC regulate cell polarization remains to be clarified, evolutionary conservation of the PAR-3/ASIP-aPKC complex suggests their general role in cell polarity organization. Here, we show the presence of the protein complex in Xenopus laevis. In epithelial cells, XASIP and XaPKC colocalize to the cell-cell contact region. To our surprise, they also colocalize to the animal hemisphere of mature oocytes, whereas they localize uniformly in immature oocytes. Moreover, hormonal stimulation of immature oocytes results in a change in the distribution of XaPKC 2–3 hours after the completion of germinal vesicle breakdown, which requires the kinase activity of aPKC. These results suggest that meiotic maturation induces the animal-vegetal asymmetry of aPKC.

Intracellular and extracellular leukemia inhibitory factor proteins have different cellular activities that are mediated by distinct protein motifs

Although many growth factors and cytokines have been shown to be localized within the cell and nucleus, the mechanism by which these molecules elicit a biological response is not well understood. The cytokine leukemia inhibitory factor (LIF) provides a tractable experimental system to investigate this problem, because translation of alternatively spliced transcripts results in the production of differentially localized LIF proteins, one secreted from the cell and acting via cell surface receptors and the other localized within the cell. We have used overexpression analysis to demonstrate that extracellular and intracellular LIF proteins can have distinct cellular activities. Intracellular LIF protein is localized to both nucleus and cytoplasm and when overexpressed induces apoptosis that is inhibited by CrmA but not Bcl-2 expression. Mutational analysis revealed that the intracellular activity was independent of receptor interaction and activation and reliant on a conserved leucine-rich motif that was not required for activation of cell surface receptors by extracellular protein. This provides the first report of alternate intracellular and extracellular cytokine activities that result from differential cellular localization of the protein and are mediated by spatially distinct motifs.

Role of activin and other peptide growth factors in body patterning in the early amphibian embryo

The amphibian body plan is established as the result of a series of inductive interactions. During early cleavage stages cells in the vegetal hemisphere induce overlying animal hemisphere cells to form mesoderm. The interaction represents the first major body-patterning event and is mediated by peptide growth factors. Various peptide growth factors have been implicated in mesoderm development, including most notably members of the transforming growth factor-beta superfamily. Identification of the so-called "natural" inducer from among the several candidate peptide growth factors is being achieved by employing several experimental strategies, including the use of a tissue explant assay for testing potential inducers, cloning of marker genes as indices of early induction events, and microinjection of altered peptide growth factor receptors to disrupt normal embryonic inductions. Activin emerges as the most likely choice for assignment of the role of endogenous mesoderm inducer, because it currently best fulfills the rigorous set of criteria expected of such an important embryonic signaling molecule. Activin, however, may not act alone in mesoderm induction. Other peptide growth factors such as fibroblast growth factor might be involved, especially in the regional patterning of the mesoderm. In addition, several genes (e.g., Wnt and noggin), which are expressed after the mesoderm is initially induced, probably assist in further definition of the mesoderm pattern. Following mesoderm induction, the primary embryonic organizer tissue (first described in 1924 by Spemann) develops and contributes further to body patterning by its action as a neural inducer. Peptide growth factors such as activin may also be involved in the inductive event, either directly (by facilitating gene expression) or indirectly (by serving to constrain pathways).

Identification of a nuclear localization signal in activin/inhibin betaA subunit; intranuclear betaA in rat spermatogenic cells

Activin is a dimeric glycoprotein hormone that was initially characterized by its ability to stimulate pituitary FSH secretion and was subsequently recognized as a growth factor with diverse biological functions in a large variety of tissues. In the testis, activin has been implicated in the auto/paracrine regulation of spermatogenesis through its cognate cell membrane receptors on Sertoli and germ cells. In this study we provide evidence for intranuclear activin/inhibin betaA subunit and show its distribution in the rat seminiferous epithelium. We have shown by transient expression in HeLa cells of beta-galactosidase fusion proteins that the betaA subunit precursor contains a functional nuclear localization signal within the lysine-rich sequence corresponding to amino acids 231-244. In all stages of the rat seminiferous epithelial cycle, an intense immunohistochemical staining of nuclear betaA was demonstrated in intermediate or type B spermatogonia or primary spermatocytes in their initial stages of the first meiotic prophase, as well as in pachytene spermatocytes and elongating spermatids primarily in stages IX-XII. In some pachytene spermatocytes, the pattern of betaA immunoreactivity was consistent with the characteristic distribution of pachytene chromosomes. In the nuclei of round spermatids, betaA immunoreactivity was less intense, and in late spermatids it was localized in the residual cytoplasm, suggesting disposal of betaA before spermatozoal maturation. Immunoblot analysis of a protein extract from isolated testicular nuclei revealed a nuclear betaA species with a molecular mass of approximately 24 kDa, which is more than 1.5 times that of the mature activin betaA subunit present in activin dimers. These results suggest that activin/inhibin betaA may elicit its biological functions through two parallel signal transduction pathways, one involving the dimeric molecule and cell surface receptors and the other an alternately processed betaA sequence acting directly within the nucleus. According to our immunohistochemical data, betaA may play a significant role in the regulation of nuclear functions during meiosis and spermiogenesis.

Fibroblast growth factor 3, a protein with dual subcellular localization, is targeted to the nucleus and nucleolus by the concerted action of two nuclear localization signals and a nucleolar retention signal

The major isoform of fibroblast growth factor 3 (FGF3) is initiated from a CUG codon, and the resultant product is distributed to the nucleus/nucleolus and secretory pathway. This dual subcellular localization is achieved in part by the competing effects of two classical intracellular targeting signals located near the amino terminus. At the extreme amino terminus is a short stretch of 29 amino acids before a signal peptide necessary for translocation into the endoplasmic reticulum, which is next to an adjacent bipartite nuclear localization signal. The carboxyl-terminal region of FGF3 is also implicated in nuclear/nucleolar localization. We describe here the characterization of carboxyl-terminal signals by showing they are capable of directing a heterologous protein, beta-galactosidase, to the nucleus. Furthermore, appending both the amino- and carboxyl-terminal domains onto beta-galactosidase, reproduces the dual subcellular localization properties of FGF3. Nuclear uptake of FGF3 appears to be signal-mediated since it binds to karyopherin alpha, the nuclear localization signal binding subunit of a heterodimeric receptor of the nuclear import machinery. The import of FGF3 into the nucleus is energy-dependent, and the inhibition of this process has demonstrated the importance of the nucleolar retention signal in nucleoplasmic and nucleolar accumulation.

Of worms and men: an evolutionary perspective on the fibroblast growth factor (FGF) and FGF receptor families

FGFs (fibroblast growth factors) play major roles in a number of developmental processes. Recent studies of several human disorders, and concurrent analysis of gene knock-out and properties of the corresponding recombinant proteins have shown that FGFs and their receptors are prominently involved in the development of the skeletal system in mammals. We have compared the sequences of the nine known mammalian FGFs, FGFs from other vertebrates, and three additional sequences that we extracted from existing databases: two human FGF sequences that we tentatively designated FGF10 and FGF11, and an FGF sequence from Caenorhabditis elegans. Similarly, we have compared the sequences of the four FGF receptor paralogs found in chordates with four non-chordate FGF receptors, including one recently identified in C. elegans. The comparison of FGF and FGF receptor sequences in vertebrates and nonvertebrates shows that the FGF and FGF receptor families have evolved through phases of gene duplications, one of which may have coincided with the emergence of vertebrates, in relation with their new system of body scaffold.

Distribution of fibroblast growth factor (FGF)-2 and FGF receptor-1 messenger RNA expression and protein presence in the mid-trimester human fetus

Fibroblast growth factors (FGF) are known to have key roles in embryonic growth and morphogenesis, but their presence and contributions to fetal development are unclear. In particular, little information exists as to the relevance of FGF and their specific receptors to human fetal development. We studied the anatomical distribution of messenger RNA encoding FGF-2 and one of its high affinity receptors, FGFR1, using in situ hybridization in a variety of human fetal tissues in early second trimester. Corresponding protein distributions were determined by immunohistochemistry. Both FGF-2 and FGFR1 mRNA and proteins were found to be present in every organ and tissue examined, but with defined cellular localizations. In skeletal muscle, both FGF-2 and FGFR1 mRNA and peptides were present in differentiated fibers, and both co-localized to proliferating chondrocytes of the epiphyseal growth plate. FGF-2 and FGFR1 mRNA and peptides were also present within cardiac or gastrointestinal smooth muscle. Within the gastrointestinal tract FGF-2 mRNA and peptide were located in the submucosal tissue, whereas FGFR1 was expressed within the overlying mucosa. Similarly, in skin, FGF-2 was expressed within the dermis whereas FGFR1 mRNA and peptide were most apparent in the stratum germinativum of the epidermis. In kidney and lung, FGFR1 mRNA was located in the tubular and alveolar epithelia respectively, whereas FGF-2 was expressed in both epithelial and mesenchymal cell populations. Both growth factor and receptor were widespread in both neuroblasts and glioblasts in the cerebral cortex of the brain. Immunoreactivity for FGF-2 and FGFR1 was seen in all vascular endothelial cells of major vessels and capillaries. Within the skin, kidney, lung, and intestine FGF-2 immunoreactivity was found in basement membranes underlying epithelia, and was associated with the extracellular matrix and plasma membranes of many cell types. The results show that FGF-2 and one of its receptors are widely expressed anatomically in the mid-trimester human fetus.

Expression and localization of basic fibroblast growth factor (bFGF) in the repair process of rat liver injury

BACKGROUND/AIMS

To clarify the expression and localization of basic fibroblast growth factor in the repair process of liver injury, acute liver injury was induced by administration of carbon tetrachloride, D-glactosamine hydrochloride, or dimethylnitrosamine to rats.

METHODS

We measured basic fibroblast growth factor protein in the liver tissue by radioimmunoassay, evaluated the expression of basic fibroblast growth factor mRNA by the reverse transcriptase polymerase chain reaction, and identified basic fibroblast growth factor-positive cells by immunostaining.

RESULTS

In the carbon tetrachloride injured liver, the basic fibroblast growth factor protein contents began to increase 2 days after administration when liver injury was most marked, and reached a peak after 4 days, decreasing thereafter. In the carbon tetrachloride-injured liver, basic fibroblast growth factor mRNA expression was observed from 12 h after administration, prior to an increase in the protein content. In the D-galactosamine hydrochloride-injured liver, basic fibroblast growth factor protein also increased. On the other hand, in the dimethylnitrosamine-injured liver, the basic fibroblast growth factor protein content decreased 2 days after administration when liver injury was marked, but increased after 7 days. In the regenerating liver after partial hepatectomy, the basic fibroblast growth factor protein content did not increase. Among cell fractions, the Ito cell fraction obtained from the carbon tetrachloride-injured liver after 4 days showed expression of basic fibroblast growth factor mRNA. In cells cultured for 24 h, this fraction was immunopositive for basic fibroblast growth factor. Ito cells in the liver tissue markedly increased in the carbon tetrachloride-injured liver and increased after 7 days in the dimethylnitrosamine-injured liver.

CONCLUSIONS

This study confirmed basic fibroblast growth factor production in the liver tissue in the repair process of liver injury. Our results suggest that basic fibroblast growth factor is primarily produced in Ito cells, acts on sinusoidal wall cells including Ito cells by the autocrine and paracrine mechanisms, and promotes extracellular matrix production and vascularization, involving the repair process of liver injury.

Control of the embryonic body plan by activin during amphibian development

Embryonic induction plays an important role in establishing the fundamental body plan during early amphibian development. The factors mediating this embryonic induction have, however, only recently been discovered. In the mid-1980's, certain peptide growth factors belonging to the FGF and TGF-beta families were found to have a mesoderm-inducing effect on isolated Xenopus blastula ectoderm. The study of embryonic induction subsequently expanded rapidly and knowledge at the molecular level has gradually accumulated. One of these peptide growth factors, activin, a member of the TGF-beta superfamily, is present maternally in the Xenopus early embryo and induces various mesodermal and endodermal tissues in isolated presumptive ectoderm. After exposure of presumptive ectoderm to activin, many genes are expressed in the same manner as in normal embryogenesis. Ectoderm treated with activin can induce a complete secondary embryo, the same as the organizer does in transplantation experiments. These findings suggest that activin is one of the first induction signals responsible for establishing the embryonic body plan in early amphibian development. In this article we shall review to what extent we can control the embryonic body plan in vitro, referring to some significant findings in this field.

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

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

Expression of acidic FGF mRNA in rat auditory brainstem during postnatal maturation

In situ hybridization was used to investigate the mRNA distribution of acidic and basic fibroblast growth factor (aFGF and bFGF) in the auditory brainstem of neonatal and adult rats. bFGF mRNA was not detected at any age. In adult rats, aFGF mRNA was strongly expressed in the principal neurons of the anteroventral and posteroventral cochlear nuclei, but not in the octopus cells. In the dorsal cochlear nucleus, aFGF mRNA was seen only in scattered smaller vertical cells. aFGF was strongly expressed in the nucleus of the trapezoid body and in all periolivary cell groups, but not in the medial and lateral olivary nuclei. No expression was observed in the lemniscal nuclei or in the central nucleus of the inferior colliculus, but large neurons in the external zone of the colliculus were labeled. Developmentally, low levels of aFGF expression appeared in the cochlear nuclei and olivary nuclei between P0 and P6. This expression increased rapidly during the onset of hearing, between P10 and P14, and reached adult level by P14-P17. Labeling in collicular neurons appeared slightly later. The results suggest that the appearance of strong aFGF mRNA expression is related to the onset of function.

Nucleo-cytoplasmic translocation and secretion of fibroblast growth factor-2 during avian gastrulation

The expression and distribution of the fibroblast growth factor-2 (FGF-2 or bFGF) proteins during early avian embryogenesis has been analysed in detail. Three FGF-2 protein isoforms of 18.5, 20.0 and 21.5 kDa are expressed during gastrulation of chicken embryos. Using whole mount immunohistochemistry, these proteins were found to be predominantly nuclear in prestreak blastodiscs during mesoderm induction. Distribution of positive cells in the epiblast was mosaic, whereas all cells of the forming hypoblast expressed the FGF-2 proteins. During primitive streak formation, the proteins started to translocate to the cytoplasm in epiblast cells but remained nuclear in the hypoblast. The FGF-2 proteins became predominantly cytoplasmic in all cells during the subsequent developmental stages. Their highest levels were detected in endodermal cells underlying Hensen's node and the newly formed notochord, the dorsal apex of all epiblast cells and, most interestingly, in the extra-cellular basal lamina separating the epiblast from newly formed mesoderm. Heparin and suramin treatment of these advanced embryos (stage 4) revealed a dose-dependent inhibition on the regression of Hensen's node and formation of mesodermal derivatives such as somites. The results are discussed with respect to current models on FGF-mediated functions during vertebrate mesoderm induction and regionalization.

Nuclear/growth factors

The now classical model for cell-cell communication espouses that information travels between cells in the form of molecules that bind specific cell-surface receptors and trigger signal-transducing mechanisms that eventually lead to transcriptional modifications. Here we gather the available information suggesting that some growth factors may also act by interfering directly with gene transcription, following their internalization and nuclear translocation. Among these factors are bona fide growth factors such as Fibroblast Growth Factor-1 and -2 and Schwannoma Derived Growth Factor, for which internalization and nuclear translocation have been demonstrated. Conversely, we propose that some isoforms of nuclear factors of the homeoprotein family could pass from cell to cell. The implications of the model are presented in the context of the specificity of cellular interactions.

Molecular cloning of Xenopus HGF cDNA and its expression studies in Xenopus early embryogenesis

We isolated Xenopus HGF cDNA and examined its expression pattern in Xenopus early embryos and their dissected parts. Xenopus HGF consists of 710 amino acids and contains four kringle domains and serine protease-like structure just like mammalian HGF. Northern blot analysis showed that expression of Xenopus HGF mRNA starts at the late gastrula stage and its level increases during the period of later embryogenesis. Dissection experiments revealed that Xenopus HGF mRNA is expressed in the mesoderm region, especially in the ventral mesoderm, which for the most part gives rise to mesenchymal cells. Furthermore, HGF mRNA was expressed in response to activin A and basic FGF in blastula animal cap cells. Interestingly, a stronger activity was observed with bFGF than with activin and this finding corroborates the preferential expression of HGF mRNA in the ventral mesoderm. Based on these results, we conclude that the Xenopus homologue of HGF gene is transcribed during early embryogenesis preferentially in ventral mesodermal tissues, probably in response to the signals that induce ventral mesoderm.

Spatial and temporal expression of basic fibroblast growth factor (FGF-2) mRNA and protein in early Xenopus development

We have analyzed the expression pattern of bFGF (FGF-2) mRNA and protein in early Xenopus development using RNAse protections, in situ hybridization and immunocytochemical methods. We find that the maternal bFGF mRNA content is at its highest in the previtellogenic oocytes (stage I-II) but decreases during further oogenesis. This low maternal level persists into the early embryonic stages and is uniformly distributed on an RNA basis. Zygotic transcription is turned on both from anterior and posterior regions but not from the middle region in the mid-neurula stage, and the expression greatly increases during the late neurula and tailbud stages. In the tadpole stage, the expression is detected in the brain, eyes, ears and neural crest-derived mesenchyme of the head. Also, it is expressed along the mesodermal axis, the level falling as the myotomes differentiate. Immunocytochemical study shows abundant bFGF protein in early oocytes and much less in later oocytes. The localization is mainly to the nuclei of the early oocytes and to the cytoplasm of the later oocytes. Localization of maternal bFGF protein in the animal hemisphere is observed in the early embryonic stages and some reuptake into nuclei occurs by the early blastula stage. The zygotically synthesized protein starts to be expressed in the anterior region of the mid-neurula stage and soon also becomes detectable in the posterior region. By tailbud and pre-larval stages, this zygotic protein appears to be present along the entire neural and mesodermal axis. When these cases are sectioned it can be seen that bFGF protein is detected in most parts of the head. In the posterior domain, it is present in the mesoderm and clearly becomes localized in the muscle cells to the nuclei and to the cell termini adjoining the intermyotomal septa.

Heparitinase inhibition of mesoderm induction and gastrulation in Xenopus laevis embryos

We have examined the involvement of proteoglycan molecules in the induction of mesodermal tissue in Xenopus laevis embryos. Blastocoelic injections of the enzyme heparitinase at early blastula stages lead to gastrulation defects and to failures in the development of anterior embryonic structures. The period of sensitivity of embryos to this treatment suggests a possible role for these molecules during mesoderm induction. We show that heparan sulfate proteoglycans (HSPGs) and chondroitin sulfate proteoglycans are the predominant sulfated glycoconjugates synthesized in early Xenopus embryos and that HSPGs are degraded by blastocoelic injections of heparitinase. Further, bFGF induction of mesoderm in explants of Xenopus stage 8 embryonic animal cap tissue is blocked by heparitinase but not by Chondroitinase ABC, using three separate criteria of mesoderm induction. Since HSPGs present in blastula animal cap cells are digested by heparitinase under the culture conditions used in the mesoderm-induction assay, we suggest that cell-surface heparan sulfate proteoglycans are required for basic fibroblast growth factor-mediated mesoderm induction.

Fibroblast growth factors and their receptors: an information network controlling tissue growth, morphogenesis and repair

The stimulation of cellular metabolism by the nine fibroblast growth factors (FGFs) is mediated by a dual-receptor system. This comprises a family of four receptor tyrosine kinases (FGFR) and heparan sulphate proteoglycans (HSPG). The stimulation of cell division by FGFs has an obligate requirement for both partners of the dual-receptor system. The binding of the nine FGFs to the FGFRs is marked by a pattern of overlapping specificity despite alternative splicing events generating a large number of FGFR proteins. Thus many of the FGFR isoforms bind several FGFs. It is likely that each FGF requires a different pattern of sulphation within the heparan sulphate chains for binding. Therefore, the HSPG receptors may provide additional specificity, allowing a cell to fine tune its response to the FGFs present in the extracellular milieu. The HSPG receptors also control the availability of FGFs and hence regulate the transport of FGFs within a tissue. FGF-stimulated cell division would appear to have a mandatory requirement for the FGFs to be translocated to the nucleus via the cytosol after interacting with the dual-receptor system. The consequences of the potential direct action of FGFs in stimulating cell division are examined in the light of current models of signal transduction.

Analysis of gastrulation: different types of gastrulation movement are induced by different mesoderm-inducing factors in Xenopus laevis

In this paper we analyze the control of gastrulation in Xenopus laevis. Our approach takes advantage of the observation that mesoderm-inducing factors such as activin, FGF and BMP-4 induce presumptive ectodermal cells to undergo gastrulation-like movements. Activin, for example, makes intact animal pole regions undergo convergent extension and causes individual cells to spread and migrate on a fibronectin-coated substrate. By varying the concentrations of the growth factors to which animal pole cells are exposed, and by applying them in different combinations, we show how graded distributions of a combination of factors could establish the correct spatial and temporal patterns of gastrulation in the Xenopus embryo. The distributions we propose support and develop the model previously suggested by Green et al. (1992) to account for the spatial patterns of gene activation in the early embryo.

Processed Vg1 protein is an axial mesoderm inducer in Xenopus

Vg1 is a TGF beta-related growth factor encoded by a maternal mRNA localized to vegetal blastomeres in Xenopus embryos. Vg1 precursor protein is abundant in vegetal cells, but the processed mature form has not been readily detected and no activity has been demonstrated for the putative Vg1 mature protein. We have engineered a BMP2-Vg1 fusion (BVg1) that promotes formation of mature Vg1 protein in vivo. Injection of BVg1 mRNA induces dorsal mesoderm in animal cap cells, and BVg1 expression in ultraviolet-ventralized embryos fully restores a normal dorsal axis. Blastomeres expressing BVg1 act as a Nieuwkoop center, the region that induces the Spemann organizer. our results lead us to suggest that localized posttranslational processing of Vg1 precursor protein on the future dorsal side of the embryo is a key step in generating dorsal mesoderm and the body axis in Xenopus.

Induction of the Xenopus organizer: expression and regulation of Xnot, a novel FGF and activin-regulated homeo box gene

We have searched for homeo box-containing genes expressed during gastrulation in Xenopus embryos with the expectation that analysis of the spatial and temporal expression of these genes will lead to greater understanding of the regionalization of the mesoderm. We describe the cloning and expression of Xnot, a novel homeo box-containing gene expressed primarily in the gastrula organizing region. We have studied the regulation of Xnot by signaling molecules involved in mesoderm induction and regionalization. Surprisingly, we found that FGF signaling is required for expression of Xnot in the gastrula organizing region, clearly implicating FGF in the induction of dorsal mesoderm. Furthermore, we found that Xnot is initially expressed throughout the embryo and that progressive translation of an unknown protein restricts expression of Xnot to the organizing region. Our results provide experimental evidence supporting the proposed division of Spemann's organizer into independently regulated organizing centers.

Activins are expressed in preimplantation mouse embryos and in ES and EC cells and are regulated on their differentiation

Members of the activin family have been suggested to act as mesoderm-inducing factors during early amphibian development. Little is known, however, about mesoderm formation in the mammalian embryo, and as one approach to investigating this we have studied activin expression during early mouse development. Activins are homo- or heterodimers of the beta A or beta B subunits of inhibin, itself a heterodimer consisting of one of the beta subunits together with an alpha subunit. Our results indicate that the oocyte contains mRNA encoding all three subunits, and antibody staining demonstrates the presence of both alpha and beta protein chains. From the fertilized egg stage onwards, alpha subunit protein cannot be detected, so the presence of beta subunits reflects the presence of activin rather than inhibin. Maternal levels of activin protein decline during early cleavage stages but increase, presumably due to zygotic transcription (see below), in the compacted morula. By 3.5 days, only the inner cell mass (ICM) cells of the blastocyst express activin, but at 4.5 days the situation is reversed; activin expression is confined to the trophectoderm. Using reverse transcription-PCR, neither beta A nor beta B mRNA was detectable at the two-cell stage but transcripts encoding both subunits were detectable at the morula stage, with beta B mRNA persisting into the blastocyst. We have also analyzed activin and inhibin expression in ES and EC cells. Consistent with the observation that activins are expressed in the ICM of 3.5-day blastocysts, we find high levels of beta A and beta B mRNA in all eight ES cell lines tested. F9 EC cells express only activin beta B, together with low levels of the inhibin alpha chain. When ES and EC cells are induced to differentiate, levels of activin fall dramatically. These results are consistent with a role for activins in mesoderm formation and other steps of early mouse development.

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

Collapse

Release and subcellular localization of acidic fibroblast growth factor expressed to high levels in HeLa cells

Acidic fibroblast growth factor (aFGF) lacks a classical signal sequence for secretion via the exocytic pathway but yet has to be released from cells in order to interact with high affinity receptors on the cell surface. To study the release process, we have expressed human aFGF in HeLa cells using a T7 RNA polymerase-driven vaccinia virus system. The high level of expression in combination with an efficient antibody allowed us to analyze the release of aFGF by pulse-chase experiments, and to immunolocalize the protein in transfected cells. In the absence of heparin, only negligible amounts of aFGF were detected in the medium during a 15 hr chase period. However, if heparin was present during the chase, readily detectable amounts (about 10-20% of total) of aFGF were found in the medium during the 15 hr chase. Extracellular aFGF was first detected at 8 hr and increased during the chase. Concomitantly, only small amounts of lactate dehydrogenase activity, used as a cytoplasmic marker, was released from the cells. Further analyses indicated that heparin both stabilized the protein from degradation and prevented the binding of released aFGF to extracellular heparan-sulfate proteoglycans. Thus, both factors contributed to the increased recovery of aFGF in the presence of heparin. The slow and inefficient release of aFGF is consistent with our previous results obtained in insect cells expressing aFGF to a very high level, as well as with those obtained by others in cultured cells producing FGF. Immunolocalization using an affinity purified antibody made against native aFGF, showed strong fluorescence in the nuclei in most cells, while staining in the cytoplasm was usually weaker and varied between cells. The nuclear localization was confirmed by subcellular fractionation and immunoblot analysis. At an early time point following transfection (4 hr), aFGF was preferentially localized to the nuclei, while the distribution of the protein between cytoplasm and nuclei was about equal at later time points (12 hr). Thus, we conclude that aFGF is capable of efficiently entering the nucleus and apparently becoming trapped there.

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

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

Responses of embryonic Xenopus cells to activin and FGF are separated by multiple dose thresholds and correspond to distinct axes of the mesoderm

The potent mesoderm-inducing factors activin and FGF are present as maternally synthesized proteins in embryos of X. laevis. We show that activin can act on explanted blastomeres to induce at least five different cell states ranging from posterolateral mesoderm to dorsoanterior organizer mesoderm. Each state is induced in a narrow dose range bounded by sharp thresholds. By contrast, FGF induces only posterolateral markers and does so over relatively broad dose ranges. FGF can modulate the actions of activin, potentiating them and broadening the threshold-bounded dose windows. Our results indicate that orthogonal gradients of activin and FGF would be sufficient to specify the main elements of the body plan.

A truncated activin receptor inhibits mesoderm induction and formation of axial structures in Xenopus embryos

Activins can induce mesoderm in embryonic explants and have been proposed as the natural inducer in Xenopus. A mutant activin receptor that inhibits activin signalling is used to show that activin is required for the induction of mesoderm in vivo and the patterning of the embryonic body plan. Blocking the activin signal transduction pathway also reveals autonomous induction of a neural marker and unmasks a relationship between activin and fibroblast growth factor.

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

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

Over-expression of fibroblast growth factors in Xenopus embryos

A number of forms of fibroblast growth factor (FGF) were over-expressed within Xenopus embryos by injection of synthetic FGF mRNAs into fertilized eggs. Injected embryos showed abnormalities in development which were mainly secondary to a disruption of gastrulation movements. The effects observed after injection of bFGF mRNA, however, were much less severe than those observed after injection of an altered form of bFGF mRNA which differs only by the addition of a signal sequence for secretion, or of another member of the FGF family, kFGF, which is normally efficiently secreted. All forms of FGF caused the induction of mesoderm in animal cap explants isolated from blastulae, but the amount of bFGF mRNA required to induce the formation of significant levels of mesoderm was higher by a factor of over a hundred than that of the FGFs which contain a signal sequence for secretion. Over-expressed bFGF accumulated in the nuclei of blastulae but did not necessarily cause mesoderm formation. These results show that FGFs must be secreted from the cells in which they are synthesised in order to act efficiently as mesoderm inducing factors and suggest that bFGF itself, which does not contain a signal sequence for secretion, is unlikely to be directly involved in mesoderm induction during early embryonic development.

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

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

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

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

Secretory and inductive properties of Drosophila wingless protein in Xenopus oocytes and embryos

Like its vertebrate homologues, Xenopus wnt-8 and murine wnt-1, we find that Drosophila wingless (wg) protein causes axis duplication when overexpressed in embryos of Xenopus laevis after mRNA injection. In many cases, the secondary axes contain eyes and cement glands, which reflect the induction of the most dorsoanterior mesodermal type, prechordal mesoderm. We show that the extent of axis duplication is dependent on the embryonic site of expression, with ventral expression leading to a more posterior point of axis bifurcation. The observed duplications are due to de novo generation of new axes as shown by rescue of UV-irradiated embryos. The true dorsal mesoderm-inducing properties of wg protein are indicated by its ability to generate extensive duplications after mRNA injection into D-tier cells of 32-cell embryos. As revealed by lineage mapping, the majority of these D cell progeny populate the endoderm; injections into animal blastomeres at this stage are far less effective in inducing secondary axes. However, when expressed in isolated animal cap explants, wg protein induces only ventral mesoderm, unless basic fibroblast growth factor is added, whereupon induction of muscle and occasionally notochord is seen. We conclude that in intact embryos, wg acts in concert with other factors to cause axis duplication. Immunolocalisation studies in embryos indicate that wg protein remains localised to the blastomeres synthesizing it and has a patchy, often perinuclear distribution within these cells, although some gets to the surface. In oocytes, the pool of wg protein is entirely intracellular and relatively unstable. When the polyanion suramin is added, most of the intracellular material is recovered in the external medium.

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

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

Peptide growth factors and their interactions during chondrogenesis

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