Two forms of the basic fibroblast growth factor receptor-like mRNA are expressed in the developing mouse brain

Biological Significance and Targeting of the FGFR Axis in Cancer

The pleiotropic effects of fibroblast growth factors (FGFs), the widespread expression of all seven signalling FGF receptors (FGFRs) throughout the body, and the dramatic phenotypes shown by many FGF/R knockout mice, highlight the diversity, complexity and functional importance of FGFR signalling. The FGF/R axis is critical during normal tissue development, homeostasis and repair. Therefore, it is not surprising that substantial evidence also pinpoints the involvement of aberrant FGFR signalling in disease, including tumourigenesis. FGFR aberrations in cancer include mutations, gene fusions, and amplifications as well as corrupted autocrine/paracrine loops. Indeed, many clinical trials on cancer are focusing on targeting the FGF/FGFR axis, using selective FGFR inhibitors, nonselective FGFR tyrosine kinase inhibitors, ligand traps, and monoclonal antibodies and some have already been approved for the treatment of cancer patients. The heterogeneous tumour microenvironment and complexity of FGFR signalling may be some of the factors responsible for the resistance or poor response to therapy with FGFR axis-directed therapeutic agents. In the present review we will focus on the structure and function of FGF(R)s, their common irregularities in cancer and the therapeutic value of targeting their function in cancer.

Functional Roles of FGF Signaling in Early Development of Vertebrate Embryos

Fibroblast growth factors (FGFs) comprise a large family of growth factors, regulating diverse biological processes including cell proliferation, migration, and differentiation. Each FGF binds to a set of FGF receptors to initiate certain intracellular signaling molecules. Accumulated evidence suggests that in early development and adult state of vertebrates, FGFs also play exclusive and context dependent roles. Although FGFs have been the focus of research for therapeutic approaches in cancer, cardiovascular disease, and metabolic syndrome, in this review, we mainly focused on their role in germ layer specification and axis patterning during early vertebrate embryogenesis. We discussed the functional roles of FGFs and their interacting partners as part of the gene regulatory network for germ layer specification, dorsal-ventral (DV), and anterior-posterior (AP) patterning. Finally, we briefly reviewed the regulatory molecules and pharmacological agents discovered that may allow modulation of FGF signaling in research.

FGF signaling repertoire of the indirect developing hemichordate Ptychodera flava

Fibroblast growth factors (FGFs) are a group of ligands that play multiple roles during development by transducing signals through FGF receptors (FGFRs) to downstream factors. At least 22 FGF ligands and 4 receptors have been identified in vertebrates, while six to eight FGF ligands and a single FGFR are present in invertebrate chordates, such as tunicates and amphioxus. The chordate FGFs can be categorized into at least seven subfamilies, and the members of which expanded during the evolution of early vertebrates. In contrast, only one FGF and two FGFRs have been found in sea urchins. Thus, it is unclear whether the FGF subfamilies duplicated in the lineage leading to the chordates, or sea urchins lost several fgf genes. Analyses of the FGF signaling repertoire in hemichordates, which together with echinoderms form the closest group to the chordates, may provide insights into the evolution of FGF signaling in deuterostomes. In this study, we identified five FGFs and three FGFRs from Ptychodera flava, an indirect-developing hemichordate acorn worm. Phylogenetic analyses revealed that hemichordates possess a conserved FGF8/17/18 in addition to several putative hemichordate-specific FGFs. Analyses of sequence similarity and protein domain organizations suggested that the sea urchin and hemichordate FGFRs arose from independent lineage-specific duplications. Furthermore, the acorn worm fgf and fgfr genes were demonstrated to be expressed during P. flava embryogenesis. These results set the foundations for further functional studies of FGF signaling in hemichordates and provided insights into the evolutionary history of the FGF repertoire.

Functional roles of fibroblast growth factor receptors (FGFRs) signaling in human cancers

The fibroblast growth factor receptors (FGFRs) regulate important biological processes including cell proliferation and differentiation during development and tissue repair. Over the past decades, numerous pathological conditions and developmental syndromes have emerged as a consequence of deregulation in the FGFRs signaling network. This review aims to provide an overview of FGFR family, their complex signaling pathways in tumorigenesis, and the current development and application of therapeutics targeting the FGFRs signaling for treatment of refractory human cancers.

Cooperation of nuclear fibroblast growth factor receptor 1 and Nurr1 offers new interactive mechanism in postmitotic development of mesencephalic dopaminergic neurons

Experiments in mice deficient for Nurr1 or expressing the dominant-negative FGF receptor (FGFR) identified orphan nuclear receptor Nurr1 and FGFR1 as essential factors in development of mesencephalic dopaminergic (mDA) neurons. FGFR1 affects brain cell development by two distinct mechanisms. Activation of cell surface FGFR1 by secreted FGFs stimulates proliferation of neural progenitor cells, whereas direct integrative nuclear FGFR1 signaling (INFS) is associated with an exit from the cell cycle and neuronal differentiation. Both Nurr1 and INFS activate expression of neuronal genes, such as tyrosine hydroxylase (TH), which is the rate-limiting enzyme in dopamine synthesis. Here, we show that nuclear FGFR1 and Nurr1 are expressed in the nuclei of developing TH-positive cells in the embryonic ventral midbrain. Both nuclear receptors were effectively co-immunoprecipitated from the ventral midbrain of FGF-2-deficient embryonic mice, which previously showed an increase of mDA neurons and enhanced nuclear FGFR1 accumulation. Immunoprecipitation and co-localization experiments showed the presence of Nurr1 and FGFR1 in common nuclear protein complexes. Fluorescence recovery after photobleaching and chromatin immunoprecipitation experiments demonstrated the Nurr1-mediated shift of nuclear FGFR1-EGFP mobility toward a transcriptionally active population and that both Nurr1 and FGFR1 bind to a common region in the TH gene promoter. Furthermore, nuclear FGFR1 or its 23-kDa FGF-2 ligand (FGF-2(23)) enhances Nurr1-dependent activation of the TH gene promoter. Transcriptional cooperation of FGFR1 with Nurr1 was confirmed on isolated Nurr1-binding elements. The proposed INFS/Nurr1 nuclear partnership provides a novel mechanism for TH gene regulation in mDA neurons and a potential therapeutic target in neurodevelopmental and neurodegenerative disorders.

Cellular signaling by fibroblast growth factors (FGFs) and their receptors (FGFRs) in male reproduction

The major function of the reproductive system is to ensure the survival of the species by passing on hereditary traits from one generation to the next. This is accomplished through the production of gametes and the generation of hormones that function in the maturation and regulation of the reproductive system. It is well established that normal development and function of the male reproductive system is mediated by endocrine and paracrine signaling pathways. Fibroblast growth factors (FGFs), their receptors (FGFRs), and signaling cascades have been implicated in a diverse range of cellular processes including: proliferation, apoptosis, cell survival, chemotaxis, cell adhesion, motility, and differentiation. The maintenance and regulation of correct FGF signaling is evident from human and mouse genetic studies which demonstrate that mutations leading to disruption of FGF signaling cause a variety of developmental disorders including dominant skeletal diseases, infertility, and cancer. Over the course of this review, we will provide evidence for differential expression of FGFs/FGFRs in the testis, male germ cells, the epididymis, the seminal vesicle, and the prostate. We will show that this signaling cascade has an important role in sperm development and maturation. Furthermore, we will demonstrate that FGF/FGFR signaling is essential for normal epididymal function and prostate development. To this end, we will provide evidence for the involvement of the FGF signaling system in the regulation and maintenance of the male reproductive system.

Inhibition of tumor growth with a vaccine based on xenogeneic homologous fibroblast growth factor receptor-1 in mice

Angiogenesis is important for the growth of solid tumors. The breaking of the immune tolerance against the molecule associated with angiogenesis should be a useful approach for cancer therapy. However, the immunity to self-molecules is difficult to elicit by a vaccine based on autologous or syngeneic molecules due to immune tolerance. Basic fibroblast growth factor (bFGF) is a specific and potent angiogenic factor implicated in tumor growth. The biological activity of bFGF is mediated through interaction with its high-affinity receptor, fibroblast growth factor receptor-1 (FGFR-1). In this study, we selected Xenopus FGFR-1 as a model antigen by the breaking of immune tolerance to explore the feasibility of cancer therapy in murine tumor models. We show here that vaccination with Xenopus FGFR-1 (pxFR1) is effective at antitumor immunity in three murine models. FGFR-1-specific autoantibodies in sera of pxFR1-immunized mice could be found in Western blotting analysis. The purified immunoglobulins were effective at the inhibition of endothelial cell proliferation in vitro and at the antitumor activity in vivo. The antitumor activity and production of FGFR-1-specific autoantibodies could be abrogated by depletion of CD4+ T lymphocytes. Histological examination revealed that the autoantibody was deposited on the endothelial cells within tumor tissues from pxFR1-immunized mice, and intratumoral angiogenesis was significantly suppressed. Furthermore, the inhibition of angiogenesis could also be found in alginate-encapsulate tumor cell assay. These observations may provide a new vaccine strategy for cancer therapy through the induction of autoimmunity against FGFR-1 associated with angiogenesis in a cross-reaction.

Characterization of fibroblast growth factor receptors expressed in principal cells in the initial segment of the rat epididymis

Studies from our laboratory support a model in which growth factors produced in the testis reach the epididymis via the luminal system and play an important role in maintaining the function of epithelial cells, particularly in the initial segment. Previous work showed that gamma-glutamyl transpeptidase (GGT) mRNA IV, which is highly expressed in the rat initial segment, may be under the control of luminal fibroblast growth factor 2 (FGF-2) from the testis. The current studies were undertaken to identify which fibroblast growth factor receptors (FGFRs) are present in the principal cells of the rat initial segment and to identify other potential ligands for these receptors in rat rete testis fluid (RTF). Immunoblot analysis revealed that FGFRs 1-4 were present, and reverse transcription polymerase chain reaction (RT-PCR) analysis confirmed that both the IIIb and IIIc splice variants of FGFRs 1-3 were expressed. However, RT-PCR using RNA isolated from principal cells collected by laser capture microdissection revealed only FGFR-1 IIIc. Additional PCR analysis established that both the alpha and beta forms of FGFR-1 IIIc were expressed in principal cells. Both FGF-4 and FGF-8 were present in rat RTF, as determined by immunoblotting. Thus, FGF-2, -4, and -8, found in RTF, may act upon FGFR-1 IIIc in the principal cells of the initial segment to regulate GGT mRNA IV expression.

Paracrine action of FGF4 during periimplantation development maintains trophectoderm and primitive endoderm

FGF4, a member of the fibroblast growth factor (FGF) family, is absolutely required for periimplantation mouse development, although its precise role at this stage remains unknown. The nature of the defect leading to postimplantation lethality of embryos lacking zygotic FGF4 is unclear and little is known about downstream targets of FGF4-initiated signaling within the various cellular compartments of the blastocyst. Here we report that postimplantation lethality of Fgf4(-/-) embryos is unlikely to reflect strictly mitogenic requirements for FGF4. Rather, our results suggest that FGF4 is required to maintain trophectoderm and primitive endoderm identity at embryonic day 4.5. This result is consistent with the reported in vitro activity of FGF4 in maintaining trophoblast stem cells and with the requirement for receptor tyrosine kinase signaling in primitive endoderm formation. Thus, postimplantation lethality of Fgf4(-/-) embryos likely results from the failure of proper differentiation and function of extraembryonic cell types.

A sonic hedgehog-dependent signaling relay regulates growth of diencephalic and mesencephalic primordia in the early mouse embryo

Sonic hedgehog (Shh) is a key signal in the specification of ventral cell identities along the length of the developing vertebrate neural tube. In the presumptive hindbrain and spinal cord, dorsal development is largely Shh independent. By contrast, we show that Shh is required for cyclin D1 expression and the subsequent growth of both ventral and dorsal regions of the diencephalon and midbrain in early somite-stage mouse embryos. We propose that a Shh-dependent signaling relay regulates proliferation and survival of dorsal cell populations in the diencephalon and midbrain. We present evidence that Fgf15 shows Shh-dependent expression in the diencephalon and may participate in this interaction, at least in part, by regulating the ability of dorsal neural precursors to respond to dorsally secreted Wnt mitogens.

The antiangiogenic agents SU5416 and SU6668 increase the antitumor effects of fractionated irradiation

Angiogenesis is critical for tumor development, growth and metastasis. The vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF) and platelet-derived growth factor (PDGF) and their tyrosine kinase receptors are major regulators of angiogenesis. Radiation induces the production of VEGF, FGF and PDGF in many tumor cells. We hypothesized that inhibition of the function of these growth factors could inhibit tumor angiogenesis and thereby enhance the efficacy of radiation therapy. To test this hypothesis, we used the small molecule inhibitors SU5416 (an inhibitor for Vegf receptor) and SU6668 (an inhibitor for Vegf, Fgf and Pdgf receptors) alone and in combination with fractionated irradiation to treat C3H mice bearing SCC VII carcinomas. The SCC VII tumors express Vegf, Fgf2 (also known as bFGF), Pdgf and their associated receptors. Animals were given either SU5416 or SU6668 daily before or after irradiation (2 Gy per fraction per day for 5 days). The results from these experiments demonstrate that administration of either SU5416 or SU6668 without radiation delayed tumor growth. Administration of SU5416 at a dose of 25 mg/kg per day (the maximum tolerated effective dose) inhibited tumor growth by 17.9% on day 7 (P < 0.05 compared to untreated control mice) and produced an average tumor growth delay time of 0.5-2.0 days. When combined with fractionated irradiation, administration of SU5416 increased the inhibition of tumor growth to 50-53% on day 7 and the tumor growth delay time to 5.7-6.5 days (P < 0.001 compared with SU5416 alone; P < or = 0.05 compared with radiation alone). SU6668 alone inhibited tumor growth in a dose-dependent manner. Administration of SU6668 at a dose of 75 mg/kg per day (a suboptimal dose) inhibited tumor growth by 36% on day 7 and produced an average tumor growth delay time of 3.3 +/- 1.4 days. The combination of SU6668 with fractionated radiation increased inhibition of tumor growth to 66-70% and the tumor growth delay time from 3.3 days to 11.9 days (P < or = 0.001 compared with either radiation alone or SU6668 alone). Administration of these agents before or after irradiation produced similar results (P = 0.40 for SU5416; P = 0.98 for SU6668). SU5416 or SU6668 alone or in combination with radiation was very well tolerated with little or no toxicity. These results suggest that inhibition of Vegf, Fgf and Pdgf receptor function by SU5416 and SU6668 can enhance the efficacy of irradiation. The targeting of multiple tyrosine kinase receptors by SU6668 is more effective than inhibition of the Vegf receptor alone by SU5416 for the enhancement of tumor cell killing by fractionated irradiation.

Fgf receptor signaling plays a role in lens induction

We describe experiments showing that fibroblast growth factor receptor (Fgfr) signaling plays a role in lens induction. Three distinct experimental strategies were used: (1) using small-molecule inhibitors of Fgfr kinase activity, we showed that both the transcription level and protein expression of Pax6, a transcription factor critical for lens development, was diminished in the presumptive lens ectoderm; (2) transgenic mice (designated Tfr7) that expressed a dominant-negative Fgf receptor exclusively in the presumptive lens ectoderm showed defects in formation of the lens placode at E9.5 but in addition, showed reduced levels of expression for Pax6, Sox2 and Foxe3, all markers of lens induction; (3) by performing crosses between Tfr7 transgenic and Bmp7-null mice, we showed that there is a genetic interaction between Fgfr and Bmp7 signaling at the induction phases of lens development. This manifested as exacerbated lens development defects and lower levels of Pax6 and Foxe3 expression in Tfr7/Tfr7, Bmp7+/– mice when compared with Tfr7/Tfr7 mice alone. As Bmp7 is an established lens induction signal, this provides further evidence that Fgfr activity is important for lens induction. This analysis establishes a role for Fgfr signaling in lens induction and defines a genetic pathway in which Fgfr and Bmp7 signaling converge on Pax6 expression in the lens placode with the Foxe3 and Sox2 genes lying downstream.

Responsiveness of developing dental tissues to fibroblast growth factors: expression of splicing alternatives of FGFR1, -2, -3, and of FGFR4; and stimulation of cell proliferation by FGF-2, -4, -8, and -9

To elucidate the roles of fibroblast growth factors (FGF) in tooth development, we have analyzed the expression patterns of fibroblast growth factor receptors (FGFR) in mouse teeth by in situ hybridization and studied the effects of FGF-2, -4, -8, and -9 on cell proliferation in vitro by local application with beads on isolated dental mesenchymes. mRNAs of FGFR-1, -2, and -3 were localized by probes specific for the alternative splice variants IIIb and IIIc. The expression patterns of FGFR1 -2, and -3 were completely different, and the two splicing variants of FGFR1 and 2 exhibited different expression domains. FGFR4 was not expressed in the developing teeth. The IIIb splice forms of FGFR1 and -2 were expressed in the dental epithelium during morphogenesis. The IIIc splice form of FGFR1 was expressed both in epithelium and mesenchyme whereas FGFR2 IIIc was confined to the mesenchymal cells of the dental follicle. Both splice forms of FGFR3 were expressed in dental papilla mesenchyme. None of the FGF-receptors was detected in the primary enamel knot, the putative signaling center regulating tooth morphogenesis. This may explain the fact that enamel knot cells do not proliferate, although they express intensely mitogenic FGFs. Beads releasing FGF-2, -4, -8, or -9 proteins stimulated cell proliferation in cultured dental mesenchymes. These data, together with our earlier data on FGF expression [Kettunen and Thesleff (1998): Dev Dyn 211:256-268] suggest that FGF-8 and -9 mediate epithelial-mesenchymal interactions during tooth initiation. During advancing morphogenesis FGF-3, -4, and -9 may act both on mesenchyme and epithelium. Finally, the intense expression of FGFR1 in odontoblasts and ameloblasts and FGFR2 IIIb in ameloblasts suggests that FGFs participate in regulation of their differentiation and/or secretory functions.

A novel type I fibroblast growth factor receptor activates mitogenic signaling in the absence of detectable tyrosine phosphorylation of FRS2

A novel variant of the fibroblast growth factor receptor type 1 (FGFR-1) was identified in human placental RNA. In this receptor (FGFR-1L) portions of the second and third immunoglobulin-like (Ig-like) domains are deleted. To determine whether FGFR-1L was functional, full-length variant (pSV/FGFR-1L) and wild-type (pSV/FGFR-1) receptors were stably transfected into rat L6 myoblasts cells. Transfected L6 clones expressed respective proteins and bound (125)I-labeled FGF-2 with K(d) values of 99 pm (FGFR-1) and 26 pm (FGFR-1L). FGF-1 and FGF-2 competed efficiently with (125)I-FGF-2 for binding to FGFR-1 and FGFR-1L, whereas FGF-4 was less efficient. FGF-1, FGF-2, and FGF-4 enhanced mitogen-activated protein kinase (MAPK) activity, increased steady-state c-fos mRNA levels, and stimulated proliferation through either receptor, whereas KGF was without effect. FGFR-1 expressing clones exhibited ligand-induced tyrosine phosphorylation of fibroblast growth factor receptor substrate 2 (FRS2), a 90-kDa adaptor protein that links FGFR-1 activation to the MAPK cascade. In contrast, tyrosine phosphorylation of FRS2 was not evident with FGFR-1L. In addition, phospholipase C-gamma was not tyrosine phosphorylated via activated FGFR-1L. These findings indicate that FGFR-1L binds FGF-1 and FGF-2 with high affinity and is capable of mitogenic signaling, but may activate MAPK to occur via non-classical signaling intermediates.

Role of fibroblast growth factor-2 in human brain: a focus on development

Trophic factors have gained a great degree of attention as regulators of neural cells proliferation and differentiation as well as of brain maturation. Very little is known, however, about their effects on human immature nervous system. In this paper, data on expression of fibroblast-growth factor-2 and its receptors are reviewed and discussed in the light of its possible role in human brain development.

Basic fibroblast growth factor promotes subplate cell survival in explant cultures of embryonic mouse cortex

Subplate neurons form a transient layer immediately below the embryonic cortex and die early in postnatal life. It has been suggested that trophic factors, perhaps coming from cortical afferents, maintain the initial survival of these cells. Later withdrawal of these factors may cause subplate cell death. We tested whether basic fibroblast growth factor (bFGF) has survival-promoting effects on subplate cells in organotypic cultures from the late embryonic mouse cortex. We found that the survival of subplate cells was promoted by adding bFGF to the cultures. By contrast, there was no effect of bFGF on the survival of overlying cortical neurons. These results indicate that bFGF may have a role in the regulation of subplate cell survival and death in vivo.

Fibroblast growth factor receptor 4 (FGFR4) is expressed in adult rat and human retinal photoreceptors and neurons

The fibroblast growth factor (FGF) family, with its prototype members acidic FGF (FGF-1) and basic FGF (FGF-2), binds to four related receptor tyrosine kinases, termed FGFR1, R2, R3, and R4, expressed on most types of cells in tissue culture. In many respects, the FGFR appear similar to other growth factor receptors; thus, dimerization of receptor monomers on ligand binding is likely to be a requisite for activation of the kinase domains, leading to receptor trans-phosphorylation. Within the central nervous system (CNS), including retina, FGFR1 and R2 have been widely described as the predominant forms. FGFR4 is reported to be strongly expressed only during early stages of development, and apart from one small region (the lateral habenular nucleus) is not detectable in adult CNS. Screening of different neural and nonneural tissues by reverse transcriptase-polymerase chain reaction (RT-PCR) revealed that whereas FGFR1 and R2 were strongly expressed in adult cortex, cerebellum, retina, and kidney, robust FGFR4 expression was only seen in retina and kidney. FGFR4 mRNA was present within fractions of the outer and inner nuclear layers isolated from adult rat retinas, and could also be detected in pure photoreceptor cultures prepared from young rat retinas. On the contrary, FGFR4 mRNA could not be detected in primary cultures of retinal Müller glia or pigment epithelium, indicating specific enrichment in retinal neurons. In situ hybridization studies of adult rat retina showed FGFR4 expression in all retinal cellular layers, especially prominent in the outer nuclear layer. FGFR4 protein was detected by immunoblotting of homogenates of rat retina, with specific antibody binding to bands at 115, 47, and 30 kDa. FGFR4 mRNA and protein were also reliably detected in postmortem adult human retina. The potential roles of these signal transduction molecules in FGF-induced biological responses in the retina are discussed.

Downregulation of c-kit (Stem Cell Factor Receptor) in Transformed Hematopoietic Precursor Cells by Stroma Cells

We show a dramatic downregulation of the stem cell factor (SCF) receptor in different hematopoietic cell lines by murine stroma. Growth of the human erythroid/macrophage progenitor cell line TF-1 is dependent on granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3). However, TF-1 cells clone and proliferate equally well on stroma. Independent stroma-dependent TF-1 clones (TF-1S) were generated on MS-5 stroma. Growth of TF-1S and TF-1 cells on stroma still requires interaction between c-kit (SCF receptor) and its ligand SCF, because antibodies against c-kit inhibit growth to less than 2%. Surprisingly, c-kit receptor expression (RNA and protein) was downregulated by 2 to 3 orders of magnitude in TF-1S and TF-1 cells grown on stroma. This stroma-dependent regulation of the kit receptor in TF-1 was also observed on exposure to kit ligand-negative stroma, thus indicating the need for heterologous receptor ligand interaction. Removal of stroma induced upregulation by 2 to 4 orders of magnitude. Downregulation and upregulation of c-kit expression could also be shown for the megakaryocytic progenitor cell line M-07e and was comparable to that of TF-1, indicating that stroma-dependent regulation of c-kit is a general mechanism. Downregulation may be an economic way to compensate for the increased sensitivity of the c-kit/ligand interaction on stroma. The stroma-dependent c-kit regulation most likely occurs at the transcriptional level, because mechanisms, such as splicing, attenuation, differential promoter usage, or mRNA stability, could be excluded.

Downregulation of c-kit (Stem Cell Factor Receptor) in Transformed Hematopoietic Precursor Cells by Stroma Cells

We show a dramatic downregulation of the stem cell factor (SCF) receptor in different hematopoietic cell lines by murine stroma. Growth of the human erythroid/macrophage progenitor cell line TF-1 is dependent on granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3). However, TF-1 cells clone and proliferate equally well on stroma. Independent stroma-dependent TF-1 clones (TF-1S) were generated on MS-5 stroma. Growth of TF-1S and TF-1 cells on stroma still requires interaction between c-kit (SCF receptor) and its ligand SCF, because antibodies against c-kit inhibit growth to less than 2%. Surprisingly, c-kit receptor expression (RNA and protein) was downregulated by 2 to 3 orders of magnitude in TF-1S and TF-1 cells grown on stroma. This stroma-dependent regulation of the kit receptor in TF-1 was also observed on exposure to kit ligand-negative stroma, thus indicating the need for heterologous receptor ligand interaction. Removal of stroma induced upregulation by 2 to 4 orders of magnitude. Downregulation and upregulation of c-kit expression could also be shown for the megakaryocytic progenitor cell line M-07e and was comparable to that of TF-1, indicating that stroma-dependent regulation of c-kit is a general mechanism. Downregulation may be an economic way to compensate for the increased sensitivity of the c-kit/ligand interaction on stroma. The stroma-dependent c-kit regulation most likely occurs at the transcriptional level, because mechanisms, such as splicing, attenuation, differential promoter usage, or mRNA stability, could be excluded.

Expression of fibroblast growth factor receptors in peri-implantation mouse embryos

FGF receptor (FGFR) function is essential during peri-implantation mouse development. To understand which receptors are functioning, we tested for the expression of all four FGF receptors in peri-implantation blastocysts. By RT-PCR, FGFR-3 and FGFR-4 were detected at high levels, FGFR-2 at lower levels, and FGFR-1 was detected at background levels compared to control tissues. Because FGFR-3 and FGFR-4 were detected at the highest levels, we studied these in detail. Between 3.5 days after fertilization (E3.5) and E6.0, FGFR-4 mRNA was detected ubiquitously in the peri-implantation embryo, restricted to the inner cell mass (ICM) and its derivatives and primitive endoderm by E6.0, and was not detected at E6.5. FGFR-3 mRNA was detected ubiquitously in the peri-implantation embryo with a tendency towards extraembryonic cells. We tested blastocyst outgrowths, a model for implantation, for FGFR-3 and FGFR-4 protein. FGFR-3 protein was detected in all cells early during the outgrowth. Later, FGFR-3 was detected in the extraembryonic endoderm and trophoblast giant cells (TGC), but not in the ICM. FGFR-4 protein was detected in all cells of the implanting embryo, but was restricted to the ICM/primitive endoderm in later stage outgrowths. The distribution of the receptor proteins in the blastocyst outgrowths is similar to the distribution of the mRNA detected by in situ hybridization of sections of embryos. The data suggest roles for FGFR-3 and FGFR-4 in peri-implantation development.

Fibroblast growth factor receptor-1 expression in the cortex and hippocampus in Alzheimer's disease

Localization of fibroblast growth receptor (FGFR)-1 immunoreactivity was investigated immunochemically in postmortem brain tissue of Alzheimer's disease (AD) and age-matched control cases using a rabbit polyclonal antibody and a mouse monoclonal antibody specific for FGFR-1. In control cases, FGFR-1 immunoreactivity was identified in astrocytes in white matter and in hippocampal pyramidal neurons. In AD cases, the immunoreactivity in reactive astrocytes surrounding senile plaques was increased. The pattern of FGFR-1 immunoreactivity was confirmed in selected cases by in situ hybridization for FGFR-1 mRNA. Immunoreactivity using a monoclonal antibody demonstrated a similar distribution pattern. The localization of FGFR-1 is consistent with previous reports on the involvement of FGF-1 and FGF-2 in AD.

Correlation between the 1.6 A crystal structure and mutational analysis of keratinocyte growth factor

A comprehensive deletion, mutational, and structural analysis of the native recombinant keratinocyte growth factor (KGF) polypeptide has resulted in the identification of the amino acids responsible for its biological activity. One of these KGF mutants (delta23KGF-R144Q) has biological activity comparable to the native protein, and its crystal structure was determined by the multiple isomorphous replacement plus anomalous scattering method (MIRAS). The structure of KGF reveals that it folds into a beta-trefoil motif similar to other members of fibroblast growth factor (FGF) family whose structures have been resolved. This fold consists of 12 anti-parallel beta-strands in which three pairs of the strands form a six-stranded beta-barrel structure and the other three pairs of beta-strands cap the barrel with hairpin triplets forming a triangular array. KGF has 10 well-defined beta strands, which form five double-stranded anti-parallel beta-sheets. A sixth poorly defined beta-strand pair is in the loop between residues 133 and 144, and is defined by only a single hydrogen bond between the two strands. The KGF mutant has 10 additional ordered amino terminus residues (24-33) compared to the other FGF structures, which are important for biological activity. Based on mutagenesis, thermal stability, and structural data we postulate that residues TRP125, THR126, and His127 predominantly confer receptor binding specificity to KGF. Additionally, residues GLN152, GLN138, and THR42 are implicated in heparin binding. The increased thermal stability of delta23KGF-R144Q can structurally be explained by the additional formation of hydrogen bonds between the GLN side chain and a main-chain carbonyl on an adjoining loop. The correlation of the structure and biochemistry of KGF provides a framework for a rational design of this potentially important human therapeutic.

Paracrine effects of bFGF and KGF on the process of mouse blastocyst implantation

Implantation is a complex process that requires the interaction of the blastocyst, and subsequently, that of the developing embryos with the endometrium. Several growth factors and cytokines are involved in implantation, but the details of their actions as related to the regulation of blastocyst implantation remain unclear. In the present study, the RT-PCR method was used to determine the gene expression of basic fibroblast growth factor (bFGF), keratinocyte growth factor (KGF), FGF receptor 1 (FGFR1), FGF receptor 2 (FGFR2), and KGF receptor (KGFR) in mouse embryos and in the stromal and epithelial cells of the uterine endometrium. Basic FGF and KGF mRNA were expressed in the endometrial cells, but were not expressed in the embryos. The mRNAs of receptors for bFGF and KGF were expressed in the blastocysts and in the in vitro implanting embryos, suggesting that bFGF and KGF may exert paracrine effects on blastocyst implantation. In this mouse model of blastocyst implantation, it was found that transforming growth factor alpha (TGF-alpha) at the concentrations of 1 ng/ml and 10 ng/ml significantly enhanced the blastocyst attachment and trophoblast spreading and increased trophoblast surface area. Relatively high concentrations of bFGF (100-500 ng/ml) significantly enhanced the rates of blastocyst attachment and of trophoblast spreading and promoted the expansion of the surface area of the implanting embryos. Unlike the rates of blastocyst attachment and trophoblast spreading, the surface area of the spreading embryos was significantly increased by addition of KGF (1-100 ng/ml). These results suggest that the bFGF and KGF derived from the endometrial cells exert paracrine effects on the process of implantation by stimulating trophoblast outgrowth through their cognate receptors.

Cloning and interspecies comparisons of three newt (Notophthalmus viridescens) fibroblast growth factor receptor sequences

We report the nucleotide sequences of two fibroblast growth factor receptor (FGFR) cDNAs, FGFR1 and FGFR3, from the newt species Notophthalmus viridescens. These two cDNA sequences and a previously published newt FGFR cDNA, FGFR2, were used to derive the amino acid sequences which were then compared with their homologues from other species. This comparison shows that the intracellular tyrosine kinase domain is highly conserved across the species examined with the second half of the domain slightly more conserved than the first half. The 3' portion of the carboxyl terminal tail is not very highly conserved. The comparison of the extracellular portion of FGFR2 shows a high degree of conservation among the Ig-like domains and a low degree of conservation in the region that links the third Ig-like domain with the transmembrane domain.

Differential expression of the fibroblast growth factor receptor (FGFR) multigene family in normal human adult tissues

This report describes a systematic analysis of the expression of the fibroblast growth factor receptor (FGFR) multigene family (FGFR1, FGFR2, FGFR3, and FGFR4) in archival serial sections of normal human adult tissues representing the major organ systems, using immunohistochemical techniques. Polyclonal antisera specific for FGFR1, FGFR2, FGFR3, and FGFR4 and a three-stage immunoperoxidase technique were employed to determine the cellular distribution of these receptors at the protein level. The expression profiles for the tissue-specific cellular localization of the FGFR multigene family demonstrated wide-spread and striking differential patterns of expression of individual receptors in the epithelia and mesenchyme of multiple tissues (stomach, salivary glands, pancreas, thymus, ureter, and cornea) and co-expression of FGFR1-4 in the same cell types of other tissues. The wide-spread expression of FGFR1-4 in multiple organ systems suggests an important functional role in normal tissue homeostasis. Differences in the spatial patterns of FGFR gene expression may generate functional diversity in response to FGF-1 and FGF-2, both of which bind with equally high affinity to more than one receptor subtype. In vivo, this may lead to functional differences that are crucial for the regulation of normal physiological processes and are responsible for the pathological mechanisms that orchestrate various disease processes.

Receptor specificity of the fibroblast growth factor family

Fibroblast growth factors (FGFs) are essential molecules for mammalian development. The nine known FGF ligands and the four signaling FGF receptors (and their alternatively spliced variants) are expressed in specific spatial and temporal patterns. The activity of this signaling pathway is regulated by ligand binding specificity, heparan sulfate proteoglycans, and the differential signaling capacity of individual FGF receptors. To determine potentially relevant ligand-receptor pairs we have engineered mitogenically responsive cell lines expressing the major splice variants of all the known FGF receptors. We have assayed the mitogenic activity of the nine known FGF ligands on these cell lines. These studies demonstrate that FGF 1 is the only FGF that can activate all FGF receptor splice variants. Using FGF 1 as an internal standard we have determined the relative activity of all the other members of the FGF family. These data should serve as a biochemical foundation for determining developmental, physiological, and pathophysiological processes that involve FGF signaling pathways.

Embryonic angiogenesis: a review

Supply with nutrients is essential from early embryonic stages onwards. Therefore, circulatory organs form the first functioning organ system. With the exception of the heart, this system is at first formed by only one cell type, the endothelial cell. Emergence, behavior, and differentiation of endothelial cells are discussed in this review. At first, endothelial cells develop from angioblasts (primary angiogenesis/angioblastic development), later they develop from preexisting endothelial cells (secondary angiogenesis/angiotrophic growth). The composition of the extracellular matrix may promote or inhibit angiogenesis. Various growth factors which can be bound to the extracellular matrix may have been found, but only two of them (VEGF, P1GF) seem to influence endothelial cell behavior directly. Heterogeneity and organ-typical differentiation of endothelial cells seem to be dependent on cell-cell signaling within each organ.

Morphological Differentiation of Astroglial Progenitor Cells from Egf-Responsive Neurospheres in Response to Fetal Calf Serum, Basic Fibroblast Growth Factor, and Retinol

Procurement of multipotential neuroglial stem cells is possible with the addition of epidermal growth factor (EGF). Stem cells will differentiate into neurons and glia upon the removal of EGF from the culture medium. We have previously characterized the neuronal differentiation of stem cells derived from long-term cultured nonpassage neurospheres. In the current study, we (1) characterize the morphological differentiation of the astroglial progenitor cell from 3-mo-old neurospheres, (2) examine whether the astroglial progenitor cells from neurospheres of different brain areas exhibit different differentiation responses to the same exogenous signals, and (3) test the effects of basic fibroblast growth factor (bFGF) and retinol on differentiation. Cerebral cortex, striatum, and mesencephalon cells were obtained from Embryonic Day 14 (E-14) rat fetuses and were dissociated for the procurement of neurospheres in chemically defined medium supplemented with EGF. After 3 mo in culture, the neurospheres, derived from each of the three brain areas, were subcultured into three groups on chamber slides: (1) basal medium, (2) the basal medium plus 20 ng/mL bFGF, and (3) the basal medium plus 10 μM retinol. Phenotypic expression of astroglial cells was examined after 14 days subculture. Our findings indicate that the 3-mo-old cultured nonpassage neurospheres contained numerous multipotential stem cells that stained positive with nestin, and that environmental factors played an important role in influencing the differentiation of astroglial progenitor cells. As detected by glial fibrillary acid protein (GFAP), astroglial progenitor cells turned into protoplasmic astrocytes in the FCS-containing basal medium, fibrous astrocytes in the presence of bFGF, and spindle-shaped astrocytes in the presence of retinol. There were no noticeable differences in differentiation among astroglial progenitor cells of the various brain region-derived neurospheres in any of the three medium conditions. Peculiar varicosity-and growth cone-like structures on the long slender GFAP-positive processes suggest that neuroblasts and glioblast may share common morphologies, features, or common progenitor cells during initial differentiation in vitro.

Outline structures for the extracellular domains of the fibroblast growth factor receptors

Fibroblast growth factor receptors (FGFRs) have three extracellular domains that belong to the immunoglobulin superfamily. We have determined the outline structures for these domains on the basis of their homology to the I set molecule telokin. The outline structures describe the relative positions of residues in each domain; their major secondary structures, and the extent to which residues are accessible to the solvent. They also provide the basis of a coherent description of the change in recognition properties that occur when the IIIb and IIIc exons are switched and of the effects of mutations in FGFRs that cause genetic diseases.

Conservation of ligand specificity between the mammalian and amphibian fibroblast growth factor receptors

We have previously cloned and sequenced a newt keratinocyte growth factor receptor (KGFR) cDNA which exhibited a unique spatial and temporal expression pattern in the regenerating newt limb. In this report, we further characterize the biochemical and functional properties of this newt KGFR. A stable Chinese hamster ovary transfectant overexpressing the newt KGFR was capable of binding both 125I-fibroblast growth factor-1 (FGF-1) and 125I-FGF-7 but not 125I-FGF-2, indistinguishable from the human KGFR. Scatchard analysis and cross-linking studies further support the conclusion that FGF-1 and FGF-7 are the ligands for the newt KGFR. In addition to their ability to bind to FGFs, both the human and the newt KGFR are also capable of repressing differentiation in mouse MM14 myoblasts. MM14 cells express FGFR1 and are repressed from differentiation by FGF-1, FGF-2, and FGF-4 but not FGF-7. Co-transfection of MM14 cells with either a human or newt KGFR expression construct conferred a response to FGF-7 as determined by a human alpha-cardiac actin/luciferase reporter construct. The response to FGF-7 was similar to the endogenous FGF response as FGF-7 prevented MM14 myoblasts from undergoing terminal differentiation. Thus, both the human and the newt KGFRs transduce signals similar to those transduced via the endogenous mouse FGFR1. Together these data indicate that this newly isolated newt KGFR is a functional receptor as it binds two FGF family members with high affinity and mediates signaling in skeletal muscle myoblasts. Because the binding pattern of the newt KGFR is similar to the pattern observed for its mammalian counterpart, it emphasizes the strict conservation that this ligand/receptor system has undergone through evolution.

Heparan sulfates mediate the binding of basic fibroblast growth factor to a specific receptor on neural precursor cells

Heparan sulfate proteoglycans are thought to be obligatory for receptor binding and subsequent mitogenic activity of basic fibroblast growth factor (FGF-2). In a previous study (Nurcombe V., Ford, M. D., Wildschut, J., Bartlett, P. F. (1993) Science 260, 103-106) we have shown that primary cultures of mouse neuroepithelial cells and a cell line derived from then, 2.3D, secrete a heparan sulfate proteoglycan with a high affinity for FGF-2. In this study, a combination of affinity chromatography and gel chromatography was used to further isolate heparan sulfate side chains with high affinity for FGF-2. These active chains had an average molecular weight of 18,000-20,000. In order to determine whether heparan sulfate chains with specificity for FGF-2 also displayed selectivity for the different FGF receptors, peptides designed to the heparin-binding region of the receptors were used in competitive inhibition studies. The structure of the predicted heparin-binding domain of the FGF receptor 1 was modeled on the basis of its presumed secondary and tertiary structure homology with immunoglobulin loops. These results suggested that many of the basic residues within the second immunoglobulin loop of the FGF receptor 1 form a basic domain in the molecule and therefore form part of a heparin-binding site. Peptides homologous to this region of FGF receptor 1 were shown to inhibit mitogenesis in 2.3D cells, while those to FGF receptor types 2, 3, and 4 did not. A reverse transcriptase-polymerase chain reaction assay designed to detect expression of the four FGF receptors types demonstrated that FGF receptors 1 and 3 were present on the 2.3D cell line but that receptors 2 and 4 were not. These findings indicate that unique heparan sulfate domains interact with specific cell-surface receptors to direct cellular responses.

Cell-Cell Signaling in Early Cortical Development

The remarkable process of neuronal migration has long been viewed as the key event of corticogenesis. Before neurons complete their final cell division and begin to migrate out of the proliferative zones, however, many important decisions concerning their ultimate fate have already been made. These critical decisions are, in part, under local environmental control. The signals by which proliferating cortical precursor cells interact with each other and with their environment are still largely unknown, but, as these factors are dis covered, they will alter our view of corticogenesis and most likely provide new insights into causes of cerebral malformations. This review is intended to describe the repertoire of cellular interactions detected thus far among proliferating cortical cells and to discuss possible roles of cell-cell signaling pathways. The Neurosci entist 1:268-276, 1995

Tyrosine kinases in megakaryocytopoiesis

Protein-tyrosine kinases (PTKs) are of vital importance in a variety of cell functions. Recent studies have provided considerable insight into the binding of growth factors to tyrosine kinase receptors and the consequent induction of signal pathways that lead to a biologic response. Future studies will further delineate the signals that result in a proliferative response and those that induce a differentiation response. Current studies, reviewed here, indicate an important biologic role for PTKs in the regulation of megakaryocyte development and maturation. Whether PTKs function in megakaryocytes in signaling pathways that are similar to pathways in other cells will need to be examined in future studies.

Characterization of fibroblast growth factor receptor 1 RNA expression in the embryonic mouse heart

We used reverse transcriptase-polymerase chain reaction (RT-PCR) to clone fibroblast growth factor receptor (FGFR) 1 isoforms from embryonic mouse heart and as a more sensitive method to characterize FGFR1 RNA expression in embryonic and adult mouse hearts. We describe the cloning of both full-length short (2259 base pairs) and long (2526 base pairs) FGFR1 isoform cDNAs which generated 86 and 102 kilodalton proteins, respectively, following in vitro translation. An assessment of FGFR1 RNA indicates that FGFR1-IIIc is the major form in both the embryonic and adult heart but there is an approximately 8.5-fold decrease in RNA levels in the adult. Differential RNA blotting as well as RT-PCR analyses are consistent with a switch in the relative expression of the short versus long FGFR1 isoforms during heart development. The long isoforms are more abundant in the embryo and the short isoforms predominate in the adult. This may be important in the regulation of growth and development of the heart.

Fibroblast growth factor (FGF) 3 from Xenopus laevis (XFGF3) binds with high affinity to FGF receptor 2

We demonstrate that purified fibroblast growth factor (FGF) 3 from Xenopus laevis (XFGF3) activates the mitogen-activated protein kinase pathway and induces DNA synthesis in quiescent cells. To characterize the high affinity cell surface receptors that mediate these responses, the ligand binding domains of different FGF receptors (FGFR) were expressed on COS-1 cells, and their affinity for XFGF3 was determined. Unlabeled XFGF3 efficiently competed with 125I-FGF1 for binding to the IIIb and IIIc isoforms of FGFR2, giving 50% displacement (ID50) at 0.3-0.8 nM. Higher XFGF3 concentrations were needed to displace 125I-FGF1 from FGFR3 and FGFR1 (ID50 approximately 4 and 21 nM, respectively), indicating that XFGF3 has a lower affinity for these receptors. No association of XFGF3 with FGFR4 was found using this assay. FGFR2 isoforms isolated from both mouse and Xenopus showed similar high affinity binding of XFGF3 as determined by direct binding assays (Kd values in the range of 0.2-0.6 nM). These results indicate that the binding specificity of XFGF3 is different from that of other FGFs, and identifies FGFR2 as its high affinity receptor.

Quantitative export of FGF-2 occurs through an alternative, energy-dependent, non-ER/Golgi pathway

Although basic fibroblast growth factor (bFGF/FGF-2) is found outside cells, it lacks a conventional signal peptide sequence; the mechanism underlying its export from cells is therefore unknown. Using a transient COS-1 cell expression system, we have identified a novel membrane-associated transport pathway that mediates export of FGF-2. This export pathway is specific for the 18-kD isoform of FGF-2, is resistant to the anti-Golgi effects of Brefeldin A, and is energy-dependent. In FGF-2-transfected COS-1 cells, this ER/Golgi-independent pathway appears to be constitutively active and functions to quantitatively export metabolically-labeled 18-kD FGF-2. Co-transfection and co-immunoprecipitation experiments, using a vector encoding the cytoplasmic protein neomycin phosphotransferase, further demonstrated the selectivity of this export pathway for FGF-2. When neomycin phosphotransferase was appended to the COOH-terminus of 18-kD FGF-2, the chimera was exported. However, the transmembrane anchor sequence of the integral membrane glycoprotein (G protein) of vesicular stomatitis virus (VSV) blocked export. The chimeric protein localized to the plasma membrane with its FGF-2 domain extracellular and remained cell-associated following alkaline carbonate extraction. Taken together, the data suggest that FGF-2 is "exported" from cells via a unique cellular pathway, which is clearly distinct from classical signal peptide-mediated secretion. This model system provides a basis for the development and testing of therapeutic agents which may block FGF-2 export. Such an intervention may be of considerable use for the treatment of angiogenesis-dependent diseases involving FGF-2.

Evidence for fibroblast growth factor mediation of compensatory adrenocortical proliferation

The role of basic fibroblast growth factor (bFGF) in the neurally mediated control of compensatory adrenocortical cell proliferation which occurs in response to unilateral adrenalectomy has been investigated. Three isoforms of bFGF have been identified in the rat adrenal with Western blots and bFGF immunoreactivity is most concentrated in the glomerulosa cells. A high affinity binding site (Kd = 10 pM) was identified in primary cultures of rat glomerulosa cells. Using autoradiography of 125I-bFGF binding, in vivo bFGF binding sites were found concentrated in the glomerulosa as well as the capsule cells. The compensatory adrenocortical proliferation was blocked by suramin and bFGF receptor density appeared to be regulated during this proliferation. These results support a role for bFGF in autocrine and paracrine stimulation of proliferation in the adrenal cortex and capsule. To specifically block the receptor-mediated effect of bFGF in this response, we have developed an antisense strategy. Antisense oligodeoxynucleotide targeted against bFGF-receptor mRNA blocks the proliferative effect of bFGF in primary glomerulosa cell cultures by approximately 50%. These results indicate that this antisense strategy interferes with the expression of bFGF-receptors and is an effective technique to reduce the proliferative effect of bFGF via the effect on its receptor.

Angiogenic growth factors in neural embryogenesis and neoplasia

"Blood vessels have the power to increase within themselves which is according to the necessity whether natural or diseased. As a further proof that this is a general principle, we find that all growing parts are much more vascular than those that are come to their full growth; because growth is an operation beyond the simple support of the part. This is the reason why young animals are more vascular than those that are full grown. This is not peculiar to the natural operation of growth, but applies also to disease and restoration."

BK1: an FGF-responsive central nervous system-derived cell line

Fibroblast growth factors (FGF) are expressed at high levels in the central nervous system (CNS), however their function in the CNS is not well understood. The immortalized neuronal cell line (BK1), derived from a transgenic mouse central nervous system tumor, expresses high levels of FGF receptor 1 (FGFR1) and demonstrates both morphologic and biochemical changes when treated with basic FGF (FGF-2). We have derived subclones of BK1 cells with varying degrees of FGF responsiveness by transfecting either a wild type (FRW) or a truncated (FRX) form of FGFR1. Cells expressing high levels of FGFR1 rapidly and uniformly respond to FGF, while cells expressing FRX fail to respond to FGF, either morphologically or by the expression of molecular markers. These BK1 subclones will prove useful to study FGFR mediated signal transduction and FGFR responsive genes in a CNS derived cell. These studies also demonstrate that a dominant negative FGF receptor can be used as a tool to elucidate the function of FGF in the central nervous system.

Basic fibroblast growth factor and fibroblast growth factor receptor I are implicated in the growth of human astrocytomas

Malignant astrocytomas are highly invasive, vascular neoplasms that comprise the majority of nervous system tumors in humans. A strong association has previously been made between malignancy in human astrocytic tumors and increased expression of certain fibroblast growth factor (FGF) family members, including basic and acidic FGF. The influence of endogenous basic FGF on glioblastoma cell growth in vitro was evaluated using basic FGF-specific antisense oligonucleotides. These studies indicated that human glioblastoma cell growth in vitro, can be inhibited by suppressing basic FGF expression. Human astrocytomas also exhibited changes in FGF receptor (FGFR) expression during the course of their progression from a benign to a malignant phenotype. FGFR2 (bek) expression was abundant in normal white matter and in all low grade astrocytomas, but was not observed in glioblastomas. Conversely, FGFR1 (flg) expression was absent or barely detectable in normal white matter, but was significantly elevated in glioblastomas. Glioblastomas also expressed an alternatively spliced form of FGFR1 containing two immunoglobulin-like disulfide loops (FGFR1 beta), whereas normal human adult and fetal brain expressed a form of the receptor containing three immunoglobulin-like disulfide loops (FGFR1 alpha). Intermediate grades of astrocytic tumors exhibited a gradual loss of FGFR2 and a shift in expression from FGFR1 alpha to FGFR1 beta as they progressed from a benign to a malignant phenotype. The underlying cytogenetic changes that contribute to these alterations are not entirely understood, but abnormalities in the p53 tumor suppressor gene may influence expression of bFGF as well as the FGFR. These results suggest that alterations in FGFR signal transduction pathways may play a critical role in the malignant progression of astrocytic tumors.

Mutations in the fibroblast growth factor receptor 2 gene cause Crouzon syndrome

Crouzon syndrome is an autosomal dominant condition causing premature fusion of the cranial sutures (craniosynostosis) and maps to chromosome 10q25-q26. We now present evidence that mutations in the fibroblast growth factor receptor 2 gene (FGFR2) cause Crouzon syndrome. We found SSCP variations in the B exon of FGFR2 in nine unrelated affected individuals as well as complete cosegregation between SSCP variation and disease in three unrelated multigenerational families. In four sporadic cases, the normal parents did not have SSCP variation. Finally, direct sequencing has revealed specific mutations in the B exon in all nine sporadic and familial cases, including replacement of a cysteine in an immunoglobulin-like domain in five patients.

Alternative splicing in fibroblast growth factor receptor 2 is associated with induced epithelial-mesenchymal transition in rat bladder carcinoma cells

We described previously that acidic fibroblast growth factor (aFGF), but not basic fibroblast growth factor (bFGF), can induce the rat carcinoma cell line NBT-II to undergo a rapid and reversible transition from epithelial to mesenchymal phenotype (EMT). We now find that NBT-II EMT is stimulated by keratinocyte growth factor (KGF) in cells grown at low density. Accordingly, a high-affinity receptor showing 98% homology to mouse FGF receptor 2b/KGF receptor was cloned and sequenced from NBT-II cells. Northern analysis indicated that mRNA for FGF receptor 2b/KGF receptor was drastically down-regulated within 1 wk in aFGF-induced mesenchymal NBT-II cells. This decrease coincided with an up-regulation of FGF receptor 2c/Bek, a KGF-insensitive, alternatively spliced form of FGF receptor 2b/KGF receptor. Functional studies confirmed that KGF could not maintain EMT induction on mesenchymal NBT-II cells. FGF receptor 1 and FGF receptor 2c/Bek could also support EMT induction when transfected into NBT-II cells in response to aFGF or bFGF. Such transfected cells could bind bFGF as well as aFGF. Therefore, EMT can be induced through different FGF receptors, but EMT may also regulate FGF receptor expression itself.

Isolation of tyrosine kinase related genes expressed in the early hematopoietic system

Transmembrane tyrosine kinase receptors are involved in cellular interactions which promote proliferation and differentiation of many cell types. To identify receptor tyrosine kinases important in embryonic hematopoietic cell development we have utilized the polymerase chain reaction (PCR) and degenerate oligonucleotides for isolation of such genes from mouse yolk sac and fetal liver. Sequence analysis of PCR amplified cDNAs from these hematopoietic sites of day 8 and 14 embryos, resulted in the isolation of nine tyrosine kinase and three serine/threonine kinase related clones. Two of these receptors, tek and flk-1, are expressed in both yolk sac and fetal liver and have been shown previously to be important for endothelial cell development. Two other clones, 9B4 and 9A2 appeared novel upon isolation but have been recently described as ryk and SK2 (rat homologue). Here we describe the twelve isolated kinases, the specific expression patterns of flk-1, tek and ryk kinases and their potential relationship to the development of the hematopoietic system.