Complete sequence of a human receptor for acidic and basic fibroblast growth factors

Craniosynostosis: current conceptions and misconceptions

Cranial bones articulate in areas called sutures that must remain patent until skull growth is complete. Craniosynostosis is the condition that results from premature closure of one or more of the cranial vault sutures, generating facial deformities and more importantly, skull growth restrictions with the ability to severely affect brain growth. Typically, craniosynostosis can be expressed as an isolated event, or as part of syndromic phenotypes. Multiple signaling mechanisms interact during developmental stages to ensure proper and timely suture fusion. Clinical outcome is often a product of craniosynostosis subtypes, number of affected sutures and timing of premature suture fusion. The present work aimed to review the different aspects involved in the establishment of craniosynostosis, providing a close view of the cellular, molecular and genetic background of these malformations.

Constitutive and Regulated Shedding of Soluble FGF Receptors Releases Biologically Active Inhibitors of FGF-2

The identification of soluble fibroblast growth factor (FGF) receptors in blood and the extracellular matrix has led to the prediction that these proteins modulate the diverse biological activities of the FGF family of ligands in vivo. A recent structural characterization of the soluble FGF receptors revealed that they are primarily generated by proteolytic cleavage of the FGFR-1 ectodomain. Efforts to examine their biological properties are now focused on understanding the functional consequences of FGFR-1 ectodomain shedding and how the shedding event is regulated. We have purified an FGFR-1 ectodomain that is constitutively cleaved from the full-length FGFR-1(IIIc) receptor and released into conditioned media. This shed receptor binds FGF-2; inhibits FGF-2-induced cellular proliferation; and competes with high affinity, cell surface FGF receptors for ligand binding. FGFR-1 ectodomain shedding downregulates the number of high affinity receptors from the cell surface. The shedding mechanism is regulated by ligand binding and by activators of PKC, and the two signaling pathways appear to be independent of each other. Deletions and substitutions at the proposed cleavage site of FGFR-1 do not prevent ectodomain shedding. Broad spectrum inhibitors of matrix metalloproteases decrease FGFR-1 ectodomain shedding, suggesting that the enzyme responsible for constitutive, ligand-activated, and protein kinase C-activated shedding is a matrix metalloprotease. In summary, shedding of the FGFR-1 ectodomain is a highly regulated event, sharing many features with a common system that governs the release of diverse membrane proteins from the cell surface. Most importantly, the FGFR ectodomains are biologically active after shedding and are capable of functioning as inhibitors of FGF-2.

A specific survival response in dopamine neurons at most risk in Parkinson's disease

The specific expression of fibroblast growth factor 20 (FGF-20) in the adult substantia nigra and the association between FGF-20 mutations and Parkinson's disease provoked exploration of the function of this growth factor. We show by gain- and loss-of-function in vitro experiments that FGF-20 promotes survival and stimulates dopamine (DA) release in a calbindin-negative subset of cells that are preferentially lost in Parkinson's disease. FGF-20 selectively activates tyrosine hydroxylase in calbindin-negative neurons. In the adult substantia nigra, calbindin-negative neurons specifically express high levels of FGFR1 (FGF receptor 1). These data show that FGF signals to elevate DA levels and protect the specific midbrain neuron type at most risk in Parkinson's patients.

Structural basis for fibroblast growth factor receptor activation

FGF signaling plays a ubiquitous role in human biology as a regulator of embryonic development, homeostasis and regenerative processes. In addition, aberrant FGF signaling leads to diverse human pathologies including skeletal, olfactory, and metabolic disorders as well as cancer. FGFs execute their pleiotropic biological actions by binding, dimerizing and activating cell surface FGF receptors (FGFRs). Proper regulation of FGF-FGFR binding specificity is essential for the regulation of FGF signaling and is achieved through primary sequence variations among the 18 FGFs and seven FGFRs. The severity of human skeletal syndromes arising from mutations that violate FGF-FGFR specificity is a testament to the importance of maintaining precision in FGF-FGFR specificity. The discovery that heparin/heparan sulfate (HS) proteoglycans are required for FGF signaling led to numerous models for FGFR dimerization and heralded one of the most controversial issues in FGF signaling. Recent crystallographic analyses have led to two fundamentally different models for FGFR dimerization. These models differ in both the stoichiometry and minimal length of heparin required for dimerization, the quaternary arrangement of FGF, FGFR and heparin in the dimer, and in the mechanism of 1:1 FGF-FGFR recognition and specificity. In this review, we provide an overview of recent structural and biochemical studies used to differentiate between the two crystallographic models. Interestingly, the structural and biophysical analyses of naturally occurring pathogenic FGFR mutations have provided the most compelling and unbiased evidences for the correct mechanisms for FGF-FGFR dimerization and binding specificity. The structural analyses of different FGF-FGFR complexes have also shed light on the intricate mechanisms determining FGF-FGFR binding specificity and promiscuity and also provide a plausible explanation for the molecular basis of a large number craniosynostosis mutations.

Chronic obstructive pulmonary disease is associated with enhanced bronchial expression of FGF-1, FGF-2, and FGFR-1

An important feature of chronic obstructive pulmonary disease (COPD) is airway remodelling, the molecular mechanisms of which are poorly understood. In this study, the role of fibroblast growth factors (FGF-1 and FGF-2) and their receptor, FGFR-1, was assessed in bronchial airway wall remodelling in patients with COPD (FEV1 < 75%; n = 15) and without COPD (FEV1 > 85%; n = 16). FGF-1 and FGFR-1 were immunolocalized in bronchial epithelium, airway smooth muscle (ASM), submucosal glandular epithelium, and vascular smooth muscle. Quantitative digital image analysis revealed increased cytoplasmic expression of FGF-2 in bronchial epithelium (0.35 +/- 0.03 vs 0.20 +/- 0.04, p < 0.008) and nuclear localization in ASM (p < 0.0001) in COPD patients compared with controls. Elevated levels of FGFR-1 in ASM (p < 0.005) and of FGF-1 (p < 0.04) and FGFR-1 (p < 0.001) in bronchial epithelium were observed. In cultured human ASM cells, FGF-1 and/or FGF-2 (10 ng/ml) induced cellular proliferation, as shown by [3H]thymidine incorporation and by cell number counts. Steady-state mRNA levels of FGFR-1 were elevated in human ASM cells treated with either FGF-1 or FGF-2. The increased bronchial expression of fibroblast growth factors and their receptor in patients with COPD, and the mitogenic response of human ASM cells to FGFs in vitro suggest a potential role for the FGF/FGFR-1 system in the remodelling of bronchial airways in COPD.

Basic fibroblast growth factor and fibroblast growth factor receptor-1 in the human hippocampal formation

Basic fibroblast growth factor (bFGF) is an important mitogen and neurotrophic factor that binds and signals through the high-affinity receptor, fibroblast growth factor receptor 1 (FGFR1). However, only a limited amount of information is available concerning the molecular forms and anatomical distribution of fibroblast growth factors (FGFs) in the normal human brain. We found multiple bFGF and FGFR1 mRNA transcripts which vary in expression pattern across human brain regions. Using in situ hybridization and immunohistochemistry, we localized bFGF and FGFR1 mRNA and protein to cells in the normal adult human hippocampus and caudal entorhinal cortex (ERC). The majority of pyramidal neurons contained FGFR1 mRNA and protein in the mesial temporal lobe, with neurons in the CA2/CA3 region demonstrating the highest levels of FGFR1 mRNA. In contrast to FGFR1, bFGF mRNA expression was detected at very low levels in a small fraction of the neurons in the human hippocampus and caudal ERC. While bFGF mRNA may be expressed at low levels in neurons, bFGF-immunopositive cells with astrocytic features were detected throughout the mesial temporal lobe in rats, monkeys and humans. bFGF immunoreactive processes are found traversing the dentate gyrus, and bFGF immunoreactive cells are found in the neurogenic subgranular zone in all three mammalian species studied. The anatomical distribution of these two FGF family members suggests that bFGF is endogenously positioned to be involved in ongoing neurogenesis in the adult hippocampus, and that FGF trophic signaling to differentiated neurons could involve the release of astrocytic bFGF acting on neuronal FGFR1 in the normal adult human hippocampus.

Sorting polyclonal antibodies into functionally distinct fractions using peptide phage display: 'a library on top of a library'

A general approach for sorting antibodies (Abs) to a restricted protein domain was developed using phage-displayed peptide libraries. The method is demonstrated by fractionating polyclonal antibodies (pAbs), raised against a short peptide derived from the extracellular, juxtamembrane region of fibroblast growth factor receptor 1 (FGFR1) into fractions with distinct chemical and biological characteristics. Screening two combinatorial peptide libraries, with the pAb, several sequences, homologous to different regions within the original peptide, were identified. Four of the corresponding peptides were synthesized and used as peptide-conjugated affinity columns for the fractionation of the pAbs. The fractions obtained were unique in their recognition patterns and in their capacity to immunoprecipitate and immunoblot, as well as to modulate the activity of FGFR1. This technique is, therefore, highly sufficient in separating pAbs to monospecific fractions and may also be used for fine mapping of different, even overlapping, sequences within a restricted peptide or protein domain.

Prevalent expression of fibroblast growth factor (FGF) receptors and FGF2 in human tumor cell lines

Basic fibroblast growth factor (FGF2) has potent mitogenic and angiogenic activities that have been implicated in tumor development and malignant progression. The biological effects of FGF2 and other members of the FGF ligand family are mediated by 4 transmembrane tyrosine kinase receptors (FGFRs). To better understand the roles of FGFRs in cancer, the expression of FGF2 and each of the 4 FGFRs was assessed by RNase protection analysis of 60 human tumor cell lines, representing 9 tumor types. Expression of at least one FGFR isoform was detected in 90% and FGF2 mRNA in 35% of the cell lines. Our comprehensive analysis of FGF2 and FGFR expression in human tumor cell lines provides evidence that FGF signaling pathways are active in a majority of human tumor cell lines, and lends support to the development of anti-tumor strategies that target FGFRs.

Regulation of vascular endothelial growth factor receptor-2 (Flk-1) expression in vascular endothelial cells

We have previously reported the existence of a synergistic interaction between vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) in the induction of angiogenesis in vitro. Here we demonstrate that bFGF increases VEGF receptor-2 (VEGFR-2/Flk-1) expression: mRNA levels were increased by 4.5- to 8.0-fold and total protein by 2.0- to 3.5-fold, in bovine microvascular endothelial (BME), aortic endothelial (BAE), and transformed fetal aortic (GM7373) endothelial cells. VEGF itself did not affect VEGFR-2 expression, and neither bFGF nor VEGF altered expression of FGF receptor-1. We also show that synergism occurs at the level of proliferation when this is measured in a three-dimensional but not in a conventional two-dimensional assay. Differences in the level of VEGFR-2 expression were also observed when cells were grown on or within collagen gels under different conditions: mRNA levels were lowest under sparse conditions, increased 20- to 26-fold at confluence, and increased even further (57-fold) when cells were cultured in suspension in three-dimensional collagen gels. Finally, a synergistic increase was seen in the level of expression of urokinase and urokinase receptor mRNAs when cells were exposed to bFGF and VEGF for 4 days. These findings demonstrate that the level of VEGFR-2 expression can be modulated by environmental factors including cytokines and the geometry of the culture conditions and provide some insight into the mechanisms of synergism between bFGF and VEGF in the induction of angiogenesis in vitro.

Establishment of epitope-defined monoclonal antibodies with specificity for fibroblast growth factor receptor types 1 and 2

The development of specific antibody probes for characterizing the expression of the family of 4 fibroblast growth factor receptor (FGFR) types has been difficult because of their close homology to each other and high degree of evolutionary conservation. Of the existing anti-FGFR monoclonal antibodies (MAbs), there are few that are useful for staining paraffin-embedded tissues. We have raised MAbs against human FGFR1 and FGFR2 in both rats and mice using bacterial recombinant receptor fusion proteins as immunogens. We used peptide epitope mapping to characterize the immune sera and the selected MAbs. Immunized animals were selected that displayed the broadest reactivity against epitopes unique to the immunizing receptor type. We produced FGFR1 specific MAbs that bind epitopes in immunoglobulin domain I (Ig-I) and FGFR2 specific MAbs that bind epitopes in Ig-I, Ig-II, and the acid box. The specificity of the antibodies was demonstrated by ELISA and immunoblot analysis of purified recombinant FGFR1 and FGFR2 extracellular domains produced both in E. coli and in eucaryotic cells. Based on the lack of epitope homology, these MAbs would not be expected to cross-react with FGFR3 or FGFR4. We isolated MAbs that bound to paraffin embedded tissue and immunoblots of recombinant receptor. These epitope-defined MAbs can distinguish between members of the FGF receptor family and should be useful as tools for assessing FGF receptor expression in a variety of normal and diseased tissues.

Differential expression of the keratinocyte growth factor (KGF) and KGF receptor genes in human vascular smooth muscle cells and arteries

Keratinocyte growth factor (KGF) is a secreted member of the fibroblast growth factor (FGF) family of heparin-binding proteins. Studies reported to date indicate that it functions primarily as an important paracrine mediator of epithelial cell growth and differentiation. KGF appears to act via binding to a specific FGF receptor-2 isoform generated by an alternative splicing mechanism. To determine whether KGF may play a role in vascular smooth muscle cell (SMC) biology, we investigated KGF and KGF receptor gene expression in human SMC cultured in vitro as well as in several human nonatherosclerotic artery and atheroma specimens. KGF mRNA but not KGF receptor mRNA was expressed by SMCs, as determined by Northern blot hybridization analysis or reverse transcription-polymerase chain reaction assays, respectively. Additional experiments demonstrated that 1) human SMCs produce and secrete mitogenically active KGF and that 2) the cytokine interleukin-1 increases KGF mRNA and protein levels in human SMCs. We also found that KGF transcripts but not KGF receptor transcripts were expressed in control and atherosclerotic human arteries. Taken together, these results indicate that KGF is unlikely to be involved in SMC growth regulation unless it can function intracellularly or interact with a presently unidentified KGF receptor.

Abnormal FGFR 3 expression in cartilage of thanatophoric dysplasia fetuses

Thanatophoric dysplasia (TD), the commonest lethal skeletal dysplasia in humans, is accounted for by recurrent mutations in the fibroblast growth factor receptor 3 gene (FGFR 3), causing its constitutive activation in vitro. Taking advantage of medical abortion of 18 TD fetuses, cartilage sections were studied for FGFR 3 gene expression by in situ hybridization and immunohistochemistry. Specific antibodies revealed high amounts of FGFR 3 in cartilage of TD fetuses with no increased level of the corresponding mRNA. The specific signal was mainly detected in the nucleus of proliferative and hypertrophic chondrocytes. Based on this observation and the abnormal expression of collagen type X in hypertrophic TD chondrocytes, we suggest that constitutive activation of the receptor through formation of a stable dimer increases its stability and promotes its translocation into the nucleus, where it might interfere with terminal chondrocyte differentiation.

Fibroblast growth factor-1 induction of delayed-early mRNA expression in NIH 3T3 cells is prolonged by heparin addition

Fibroblast growth factor (FGF)-1, also known as acidic FGF, is a multifunctional heparin-binding protein that is mitogenic for a wide variety of cell types cultured in vitro and a potent angiogenic agent in vivo. These cellular responses are mediated via high-affinity binding to a family of four membrane-spanning tyrosine kinase receptors. FGF-1-stimulated mitogenesis is potentiated by heparin, a sulfated glycosaminoglycan. In this study, we examined the effect of exogenous heparin on FGF-1-inducible gene expression in murine NIH 3T3 cells using both wild-type FGF-1 and FGF-1/glu132, an FGF-1 mutant with a reduced apparent affinity for heparin. The induction levels and temporal expression kinetics of two immediate-early response mRNAs (early growth response gene-1, thrombospondin-1) as well as two delayed-early response mRNAs (proliferin, ornithine decarboxylase) were monitored by Northern blot hybridization analysis. We found that although FGF-1 alone can promote the initial induction of these four mRNAs, heparin coaddition is necessary for prolonged delayed-early mRNA expression. This heparin effect occurs when cells are stimulated with wild-type FGF-1 but not with FGF-1/glu132. Furthermore, FGF-1 and heparin must be added together at the initial time of mitogen stimulation and they must remain present in the cell culture medium for a minimum period of 8 h to promote sustained delayed-early mRNA expression. These findings are consistent with the proposal that heparin promotes a long-term FGF-1:FGFR interaction which is required for sustained delayed-early gene expression and a full mitogenic response.

Basic fibroblast growth factor (FGF-2) affects development of acoustico-vestibular neurons in the chick embryo brain in vitro

The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.

Transforming growth factor beta 1 down-regulates vascular endothelial growth factor receptor 2/flk-1 expression in vascular endothelial cells

Although the importance of the vascular endothelial growth factor (VEGF)/VEGF tyrosine kinase receptor (VEGFR) system in angiogenesis is well established, very little is known about the regulation of VEGFR expression in vascular endothelial cells. We have cloned partial cDNAs encoding bovine VEGFR-1 (flt) and -2 (flk-1) and used them to study VEGFR expression by bovine microvascular- and large vessel-derived endothelial cells. Both cell lines express flk-1, but not flt. Transforming growth factor beta 1 (TGF-beta 1) reduced the high affinity 125I-VEGF binding capacity of both cell types in a dose-dependent manner, with a 2.0-2.7-fold decrease at 1-10 ng/ml. Cross-linking experiments revealed a decrease in 125I-VEGF binding to a cell surface monomeric protein corresponding to Flk-1 on the basis of its affinity for VEGF, molecular mass (185-190 kDa), and apparent internalization after VEGF binding. Immunoprecipitation and Western blot experiments demonstrated a decrease in Flk-1 protein expression, and TGF-beta 1 reduced flk-1 mRNA levels in a dose-dependent manner. These results imply that TGF-beta 1 is a major regulator of the VEGF/Flk-1 signal transduction pathway in endothelial cells.

Basic fibroblast growth factor (FGF-2) affects development of acoustico-vestibular neurons in the chick embryo brain in vitro

The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.

Autocrine growth regulation of human glomerular mesangial cells is primarily mediated by basic fibroblast growth factor

For various forms of human glomerulonephritis a close relationship between inflammatory injury and a local mesangial proliferative response has been described. Herein, we used primary cultures of human glomerular mesangial cells (HMCs) from five different donors to determine the autocrine growth-inducing capacity of their supernatants after stimulation with different cytokines and lipopolysaccharide (LPS) to determine whether this effect is due to basic fibroblast growth factor (bFGF). The basal growth-inducing capacity of supernatants collected from serum-free cultured HMC and concentrated 100-fold above a cut-off size of 10 kd was significantly increased by interleukin (IL)-1 beta, platelet-derived growth factor (PDGF), and LPS up to 15-fold, but not by IL-1 alpha, IL-6, or bFGF. An anti-human bFGF antibody blocked the majority of IL-1 or LPS-induced proliferative effect of supernatants; complete inhibition was achieved by a combination of anti-human bFGF- and anti-human platelet-derived growth factor antibodies. HMCs express different isoforms of bFGF (18, 21.5, and 24 kd) in membrane, cytosolic, and nuclear fractions. All isoforms of bFGF were found in the nuclear fraction of HMC, whether stimulated or not. Immunoblots for bFGF protein of HMC demonstrated that only a approximate to 16 kd bFGF protein was released into HMC supernatants after stimulation with IL-1 beta, platelet-derived growth factor-BB, and LPS. The 18 kd isoform of bFGF accumulated in the membranes but was not released after stimulation with IL-1 alpha, IL-6, and bFGF, suggesting that its release was a prerequisite for autocrine growth stimulation. By means of reverse transcription polymerase chain reaction controlled by Southern blots, bFGF-mRNA expression of HMC was enhanced by IL-1 alpha, IL-1 beta, and LPS. Finally, we were able to show that HMCs are expressing bFGF receptors. In summary, our data demonstrate for the first time that the autocrine proliferative response of HMC to major inflammatory factors may primarily be mediated by bFGF.

High affinity immunoreactive FGF receptors in the extracellular matrix of vascular endothelial cells--implications for the modulation of FGF-2

We recently characterized three FGF-binding proteins (FGF-BPs) which are soluble forms of the extracellular domains of the high affinity FGF receptors (Hanneken, A. M., W. Ying, N. Ling, and A. Baird. Proc. Natl. Acad. Sci. USA. 1994. 91:9170-9174). These proteins circulate in blood and have been proposed to modulate the biological activity of the FGF family of proteins. Immunohistochemical studies now demonstrate that these soluble, truncated FGF receptors are also present in the basement membranes of retinal vascular endothelial cells. These immunoreactive proteins can be detected with antibodies raised to the extracellular domain of FGFR-1 but not with antibodies raised to either the juxtamembrane domain or the cytoplasmic domain of FGFR-1. Western blotting of human retinal extracts with the antibody raised to the extracellular domain of FGFR-1 detects specific, low molecular mass proteins at 85 kD and 55 kD, corresponding in size to the FGF-BPs, which are not detected with antibodies against the cytoplasmic domain of the receptor. The interaction of this receptor with the extracellular matrix is not dependent on the presence of FGF-2. Immunoreactive receptors are still detected in vascular basement membranes after the removal of FGF-2 with heparitinase. In addition, the recombinant extracellular domain of FGFR-1 continues to bind to corneal endothelial cell matrix after endogenous FGF-2 has been removed with 2 M NaCl. Acid treatment, which has been shown to disrupt protein interactions with the extracellular matrix, leads to a significant reduction in the presence of the matrix form of the FGF receptor. This loss can be restored with exogenous incubations of the recombinant extracellular domain of FGFR-1. This report is the first demonstration that a truncated form of a high affinity growth factor receptor can be localized to the extracellular matrix. These findings add to the list of binding proteins associated with the extracellular matrix (IGFBP-5) and suggest a potentially new regulatory mechanism for controlling the biological availability of FGF, and other peptide growth factors, in the extracellular matrix.

A common mutation in the fibroblast growth factor receptor 1 gene in Pfeiffer syndrome

Pfeiffer syndrome (PS) is one of the classic autosomal dominant craniosynostosis syndromes with craniofacial anomalies and characteristic broad thumbs and big toes. We have previously mapped one of the genes for PS to the centromeric region of chromosome 8 by linkage analysis. Here we present evidence that mutations in the fibroblast growth factor receptor-1 (FGFR1) gene, which maps to 8p, cause one form of familial Pfeiffer syndrome. A C to G transversion in exon 5, predicting a proline to arginine substitution in the putative extracellular domain, was identified in all affected members of five unrelated PS families but not in any unaffected individuals. FGFR1 therefore becomes the third fibroblast growth factor receptor to be associated with an autosomal dominant skeletal disorder.

Multivalent ligand-receptor binding interactions in the fibroblast growth factor system produce a cooperative growth factor and heparin mechanism for receptor dimerization

The binding interactions for the three primary reactants of the fibroblast growth factor (FGF) system, basic FGF (bFGF), an FGF receptor, FGFR1, and the cofactor heparin/heparan sulfate (HS), were explored by isothermal titrating calorimetry, ultracentrifugation, and molecular modeling. The binding reactions were first dissected into three binary reactions: (1) FGFR1 + bFGF<==>FGFR1/bFGF, K1 = 41 (+/- 12) nM; (2) FGFR1 + HS<==>FGFR1/HS, K2 = 104 (+/- 17) microM; and (3) bFGF + HS<==>bFGF/HS, K3 = 470 (+/- 20) nM, where HS = low MW heparin, approximately 3 kDa. The first, binding of bFGF to FGFR1 in the absence of HS, was found to be a simple binary binding reaction that is enthalpy dominated and characterized by a single equilibrium constant, K1. The conditional reactions of bFGF and FGFR1 in the presence of heparin were then examined under conditions that saturate only the bFGF heparin site (1.5 equiv of HS/bFGF) or saturate the HS binding sites of both bFGF and FGFR1 (1.0 mM HS). Both 3-and 5-kDa low MW heparins increased the affinity for FGFR1 binding to bFGF by approximately 10-fold (Kd = 4.9 +/- 2.0 nM), relative to the reaction with no HS. In addition, HS, at a minimum of 1.5 equiv/bFGF, induced a second FGFR1 molecule to bind to another lower affinity secondary site on bFGF (K4 = 1.9 +/- 0.7 microM) in an entropy-dominated reaction to yield a quaternary complex containing two FGFR1, one bFGF, and at least one HS. Molecular weight estimates by analytical ultracentrifugation of such fully bound complexes were consistent with this proposed composition. To understand these binding reactions in terms of structural components of FGFR1, a three-dimensional model of FGFR1 was constructed using segment match modeling. Electrostatic potential calculations confirmed that an elongated cluster, approximately 15 x 35 A, of nine cationic residues focused positive potential (+2kBT) to the solvent-exposed beta-sheet A, B, E, C' surface of the D(II) domain model, strongly implicating this locus as the HS binding region of FGFR1. Structural models for HS binding to FGFR1, and HS binding to bFGF, were built individually and then assembled to juxtapose adjacent binding sites for receptor and HS on bFGF, against matching proposed growth factor and HS binding sites on FGFR1. The calorimetric binding results and the molecular modeling exercises suggest that bFGF and HS participate in a concerted bridge mechanism for the dimerization of FGFR1 in vitro and presumably for mitogenic signal transduction in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential expression patterns of mRNAs for members of the fibroblast growth factor receptor family, FGFR-1-FGFR-4, in rat brain

We have examined the region-specific expression of mRNAs for four members of rat FGF receptor family, FGFR-1, FGFR-2 FGFR-3, and FGFR-4, in rat brain by in situ hybridization. The FGFR-1, FGFR-2, and FGFR-3 mRNAs were expressed widely but differentially in the brain. However, the FGFR-4 mRNA was not expressed in the brain. The FGFR-1 mRNA was strongly expressed in several regions including the hippocampus, cerebellum, and pedunculopotine tegmental nucleus. The FGFR-2 mRNA expression was high in the choroid plexus, and moderate in the fiber-rich regions (the corpus callosum, external capsule, and internal capsule) and the olfactory bulb. The FGFR-3 mRNA was expressed diffusely in the brain. We have also examined the cellular localization of these mRNAs in the brain. Although the FGFR-1 mRNA was expressed preferentially in neurons, the FGFR-2 and FGFR-3 mRNAs were expressed preferentially in glial cells. The present findings that the FGFR-1, FGFR-2, and FGFR-3 mRNAs were expressed widely but with region- and cell-specificity in the brain indicate that these receptors have different roles in the brain.

Rat FGF receptor-4 mRNA in the brain is expressed preferentially in the medial habenular nucleus

The fibroblast growth factor (FGF) receptor family consists of four members, FGFR-1, FGFR-2, FGFR-3 and FGFR-4, that are closely related receptor tyrosine kinases. We examined the expression of rat FGFR-4 mRNA in the brain by in situ hybridization and compared it with that of the mRNAs for other FGF receptors. In contrast with FGFR-1, FGFR-2 and FGFR-3 mRNAs which are expressed widely in the brain, the FGFR-4 mRNA in the brain is expressed preferentially in the medial habenular nucleus neurons. The present finding indicates that FGFR-4 has a function specific to the medial habenular nucleus.

Differential expression of two fibroblast growth factor-receptor genes is associated with malignant progression in human astrocytomas

Malignant astrocytomas, which are highly invasive, vascular neoplasms, compose the majority of nervous system tumors in humans. Elevated expression of fibroblast growth factors (FGFs) in astrocytomas has implicated the FGF family of mitogens in the initiation and progression of astrocyte-derived tumors. In this study, we demonstrated that human astrocytomas undergo parallel changes in FGF-receptor (FGFR) expression during their progression from a benign to a malignant phenotype. FGFR type 2 (BEK) expression was abundant in normal white matter and in all low-grade astrocytomas but was not seen in malignant astrocytomas. Conversely, FGFR type 1 (FLG) expression was absent or barely detectable in normal white matter but was significantly elevated in malignant astrocytomas. Malignant astrocytomas also expressed an alternatively spliced form of FGFR-1 (FGFR-1 beta) containing two immunoglobulin-like disulfide loops, whereas normal human adult and fetal brains expressed a receptor form (FGFR-1 alpha) containing three immunoglobulin-like disulfide loops. Intermediate grades of astrocytic tumors exhibited a gradual loss of FGFR-2 and a shift in expression from FGFR-1 alpha to FGFR-1 beta as they progressed from benign to malignant phenotype. These results suggest that differential expression and alternative splicing of FGFRs may be critical in the malignant progression of astrocytic tumors.

aFGF, bFGF and flg mRNAs show distinct patterns of induction in the hippocampus following kainate-induced seizures

We report that kainic acid-induced seizures lead to marked increases in mRNAs encoding basic and acidic fibroblast growth factors (bFGF and aFGF, respectively) and flg, one of their receptors, in the rat hippocampus. Anticonvulsant pretreatment inhibits the up-regulation of these mRNAs. The observed increase in flg mRNA levels involves the pyramidal cells of all hippocampal subfields and the granular cells of the dentate gyrus. The increased expression of aFGF and bFGF mRNAs is limited to neuron populations that are resistant to seizure-induced injury, the granular cells of dentate gyrus and pyramidal cells of CA1 region, respectively. The results suggest that the increase in the FGFs and flg may play pivotal roles in neuron survival and in long-term changes occurring in the hippocampus following seizure activity.

Distinct patterns of expression of fibroblast growth factors and their receptors in human atheroma and nonatherosclerotic arteries. Association of acidic FGF with plaque microvessels and macrophages

Because fibroblast growth factors (FGFs) modulate important functions of endothelial cells (EC) and smooth muscle cells (SMC), we studied FGF expression in human vascular cells and control or atherosclerotic arteries. All cells and arteries contained acidic (a) FGF and basic (b) FGF mRNA. Northern analysis detected aFGF mRNA only in one of five control arteries but in all five atheroma tested, while levels of bFGF mRNA did not differ among control (n = 3) vs. plaque specimens (n = 6). Immunolocalization revealed abundant bFGF protein in control vessels (n = 10), but little in plaques (n = 14). In contrast, atheroma (n = 14), but not control arteries (n = 10), consistently exhibited immunoreactive aFGF, notably in neovascularized and macrophage-rich regions of plaque. Because macrophages colocalized with aFGF, we tested human monocytoid THP-1 cells and demonstrated accumulation of aFGF mRNA during PMA-induced differentiation. We also examined the expression of mRNA encoding FGF receptors (FGFRs). All cells and arteries contained FGFR-1 mRNA. Only SMC and control vessels had FGFR-2 mRNA, while EC and some arteries contained FGFR-4 mRNA. The relative lack of bFGF in plaques vs. normal arteries suggests that this growth factor may not contribute to cell proliferation in advanced atherosclerosis. However, aFGF produced by plaque macrophages may stimulate the growth of microvessels during human atherogenesis.

Biochemical characterization of the molecular interaction between recombinant basic fibroblast growth factor and a recombinant soluble fibroblast growth factor receptor

The extracellular domain of human fibroblast growth factor receptor (XC-FGF-R) was expressed in Escherichia coli. The protein was purified to homogeneity and the interaction with basic fibroblast growth factor (bFGF), its physiological ligand, was examined. Using resins on which bFGF was reversibly bound, we analysed the characteristics of the binding between XC-FGF-R and immobilized bFGF. We also investigated the stoichiometry of the binding between XC-FGF-R and recombinant human bFGF (rhbFGF) applying non-denaturing gel electrophoresis, chemical cross-linking followed by SDS/PAGE, and gel-filtration chromatography. In cross-linking and gel-filtration chromatography experiments, a 1:1 complex between rhbFGF and XC-FGF-R was observed. The complex was separated from the non-complexed proteins using non-denaturing PAGE in the presence of 0.1% Triton X-100. The band corresponding to the complex was recognized by specific antibodies directed against bFGF and its receptor, blotted on poly(vinylidene difluoride) membranes and submitted to sequence and amino acid analysis. The data obtained from these determinations confirmed the formation of a 1:1 complex between rhbFGF and XC-FGF-R.

The structure and expression of the FGF receptor-1 mRNA isoforms in rat tissues

In this paper, we describe the structure of rat FGF receptor-1 mRNA isoforms and their expression in a variety of rat tissues. The rat FGFR-1 has the characteristics of FGFR-1 as well as mouse, human and chicken homologs. FGFR-1 mRNA was detectable in all the tissues examined by Northern analysis or polymerase chain reaction, indicating that FGFR-1 mRNA is widely expressed in rat tissues. The rat FGFR-1 mRNA has isoforms in both the extracellular and intracellular regions. The extracellular isoforms which have two or three immunoglobulin-like domains, are expressed almost equally in the tissues except for brain. However, the large form is a major form in the brain. Furthermore, in the brain, half of FGFR-1 mRNAs have the six nucleotides, which encode a potential serine-threonine kinase phosphorylation site in the intracellular juxta-membrane region, deleted. In contrast to the brain, the deletion isoform is a minor form in the other tissues. The tissue-specific expression of the isoforms indicates that they have different physiological functions. Although other isoforms of FGFR-1 mRNA in tumor cell lines have been reported, the isoforms were undetectable in all rat tissues examined, indicating the isoforms are products of abnormal alternative splicing in tumor cell lines.

Characterization of a biologically active extracellular domain of fibroblast growth factor receptor 1 expressed in Escherichia coli

The functional features of a recombinant fibroblast growth factor (FGF) receptor (FGF-R) were investigated by expressing at high level in Escherichia coli a soluble non-glycosylated form of FGF-R1. The extracellular domain of the mature protein (XC-FGF-R), comprising the first 356 amino acids, was purified from a large-scale fermentation. After cell lysis, the protein was quantitatively found in the pellet. XC-FGF-R was solubilized using guanidine/HCl and allowed to refold using two dialysis steps. The refolded protein was obtained in a homogeneous form after ammonium sulphate precipitation and gel-filtration chromatography. The soluble receptor had the ability to form a complex with recombinant human basic FGF (rhbFGF) in solution, as demonstrated by immunoprecipitation with anti-(FGF-R) serum. Formation of a rhbFGF/XC-FGF-R complex was visualized by cross-linking experiments. Quantitative binding experiments with the XC-FGF-R immobilized on Affi-Gel resin showed high binding affinity for 125I-bFGF (Kd = 5-10 nM). Purified XC-FGF-R inhibited binding of 125I-bFGF to its high-affinity receptors on baby hamster kidney cells. These data suggest that glycosylation of the FGF-R is not necessary for its ligand-binding activity. The use of an E. coli expression system resulted in the efficient production of a soluble receptor in a form suitable for ligand/receptor structural studies and screening of new potential agonists and antagonists of angiogenesis. These results indicate that E. coli can be used for the production of complex molecules such as Ig-like receptors.

Identification of four FGF receptor genes in Medaka fish (Oryzias latipes)

Four types of cDNA clones encoding tyrosine kinases highly homologous to mammalian fibroblast growth factor receptors (FGF-R) were isolated from Medaka fish (Oryzias latipes) by the reverse transcription-polymerase chain reaction. Comparison of the four deduced amino acid sequences with four known mammalian FGF-Rs indicated that four FGF-R species corresponding to mammalian FGF-Rs exist universally in vertebrates including fishes, although FGF-R4 might have diverged sequences between fishes and mammals. Each of four FGF-R genes is transcribed to various extents as multiple mRNAs possibly by alternative splicing in adult fishes.

Cloning, expression and tissue distribution of the gene encoding rat fibroblast growth factor receptor subtype 4

The rat homologue of the gene encoding the fibroblast growth factor receptor subtype 4 (FGFR4) was cloned from rat lung mRNA, and the cDNA sequence was found to be 95% similar and 92% identical to the human homologue. Northern blot analysis of adult rat tissues demonstrated that a 3.1-kb mRNA encoding FGFR4 is detectable only in the lung and kidney. The receptor variant described here encodes two potential immunoglobulin-like domains, 21 hydrophobic amino acids encoding a potential transmembrane domain, and a split tyrosine kinase motif. However, the acidic box and hydrophobic signal peptide domains are not present in this cDNA isolate.

Basic fibroblast growth factor in Dupuytren's contracture

Lesions excised from nine patients undergoing surgery for Dupuytren's contracture (DC) and three normal fascia were examined for the presence of the angiogenic protein basic fibroblast growth factor (basic FGF). Endothelial cell proliferation assays established basic FGF-like activity in extracts of DC. Western blotting confirmed the presence of an 18,000-dalton protein which was localized in the lesions by immunohistochemical staining. All of the cells implicated in the progression of the disease (endothelial cells, fibroblasts, and myofibroblasts) contain the growth factor. Endothelial cells within the narrowed or occluded vessels, as well as fibroblasts surrounding these vessels, stained intensely positive. In situ hybridization using an antisense probe for human basic FGF and its receptor's (FGFR-1) mRNA established the major difference between normal and DC tissues: their levels are significantly higher than in the normal tissues. Thus the cells in DC also express both basic FGF and FGFR-1, suggesting a potential autocrine/paracrine role for basic FGF in the pathogenesis of DC. This finding is thus the first description of a nontumoral proliferative disease that can be directly associated with increased basic FGF mRNA. The possibility that therapies can be developed on the basis that basic FGF and its receptor are expressed in DC is discussed.

Transient lesion-induced increase of basic fibroblast growth factor and its receptor in layer VIb (subplate cells) of the adult rat cerebral cortex

Basic fibroblast growth factor is a potent trophic factor with a wide spectrum of activity at various stages of neuronal development. In our studies on the effects of select lesions on the expression of growth factors, we observed that neurons of layer VIb of the rat cerebral cortex developed immunoreactivity for basic fibroblast growth factor and its receptor following injury. Recent evidence indicates that layer VIb of the rat cerebral cortex contains the subplate cell population, a group of neurons shown to participate in the development of the cerebral cortex. In this article, we examined the nature and time-course of the response to injury of the expression of basic fibroblast growth factor and its receptor in these cells. We used an anti-basic fibroblast growth factor monoclonal antibody that recognizes the active form of basic fibroblast growth factor, and a polyclonal antibody that recognizes the extracellular domain of the basic fibroblast growth factor receptor. The induction of basic fibroblast growth factor and its receptor in layer VIb cells occurred after entorhinal cortex lesion, fimbria-formix transection or aspiration of small segment of the frontoparietal cortex. The lesion-induced effect was transient, appearing by postlesion day 2 and having disappeared by postlesion day 7. These findings suggest that endogenous basic fibroblast growth factor may have a neuroprotective role on layer VIb neurons after trauma and/or may participate in cortical plasticity during adulthood.

Diverse receptors for fibroblast growth factors

The development and maintenance of multicellular organisms requires a complex interplay between cells in different tissues. Many of the factors mediating cell-cell communication are polypeptides, which were originally identified because of their ability to stimulate cell growth. In addition to growth signalling several of these factors have been observed to modulate cell survival, chemotaxis and differentiation both in vitro and in vivo. Fibroblast growth factors are a good example of polypeptide mitogens eliciting a wide variety of responses depending on the target cell type. Our knowledge of the cell surface receptors mediating the effects of FGFs has recently expanded remarkably. Perhaps not surprisingly, the complexity of the FGF family and FGF induced responses is reflected as diversity and redundancy of the FGF receptors.

Two additional protein-tyrosine kinases expressed in human lung: fourth member of the fibroblast growth factor receptor family and an intracellular protein-tyrosine kinase

The expression of protein-tyrosine kinases (PTKs; ATP:protein-tyrosine O-phosphotransferase, EC 2.7.1.112) was studied in normal human lung and various tumors by PCR followed by molecular cloning and sequence analysis. Six known PTKs (YES, FGR, LYN, HCK, PDGFB-R, and CSF1-R), as well as two additional members of this enzyme family, were detected in lung. One of the newly discovered sequences appears to represent a group of cytosolic PTKs. The cDNA sequence of the second unknown PTK revealed that it is a fourth member of the fibroblast growth factor receptor family. It was therefore called TKF (tyrosine kinase related to fibroblast growth factor receptor). Among a wide variety of cells and tissues tested, including human lymphocytes and macrophages, TKF was only found expressed in lung. Apart from normal lung, TKF expression could be demonstrated in some tumors of lung origin, but also in malignancies not derived from lung tissues. As fibroblast growth factors are generally involved in a variety of functions such as mitogenesis, angiogenesis, and wound healing, the specific expression of a receptor-related gene in lung only may point to yet another special function of this group of proteins.

The molecular biology of heparan sulfate fibroblast growth factor receptors

Two distinct classes of cell surface FGF-binding proteins have been identified. These receptors differ in both mode of interaction and in affinity for the FGFs. cDNAs that encode the low-affinity receptor were isolated from a hamster kidney cell line cDNA library by expression cloning. Transfected cells that contained these heparan sulfate proteoglycan FGF receptor cDNAs were enriched for by panning on basic FGF-coated plates. The analogous human cDNA was isolated from a hepatoma cell line cDNA library. The homology of our hamster cDNAs to the previously described murine integral membrane proteoglycan syndecan, together with an exact amino acid sequence match of our human-cDNA-encoded product to human syndecan, clearly indicates the identity of these independently isolated proteoglycans. Further confirmation that the expressed molecule serves as a proteoglycan core protein was achieved by immunoprecipitation of 35SO4-labeled material from solubilized transfected cells. Nitrous acid treatment and chondroitinase digestion revealed that 77% of the label was associated with heparan sulfate chains and 22% with chondroitin sulfate chains. These heparan sulfate chains contributed to the fivefold increase in the total heparan sulfate found to be present on the surface of the transfected cells compared with cells transfected with a vector lacking the cDNA insert.

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

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

Two cDNAs encoding novel human FGF receptor

Two types of cDNAs encoding novel human FGF receptors were isolated. These two cDNAs were found to be closely related to the oncogene bek. Products from these genes were membrane-bound when their cDNAs were transiently expressed in COS cells, whereas products from the regions coding extracellular domains were free of membrane attachment and found in the culture medium.

The expression of two isoforms of the human fibroblast growth factor receptor (flg) is directed by alternative splicing

Structural analysis of the cDNA for the human fibroblast growth factor receptor (flg) revealed the existence of a larger and a shorter isoform of the receptor. The larger form has three extracellular immunoglobulin-like domains. On the other hand, the shorter form deletes the first (the most external) immunoglobulin-like domain region. Two consecutive amino acids (Arg Met) between the first and second immunoglobulin-like domains are sometimes deleted from the shorter form. In this paper, we isolated and analyzed the gene for the human fibroblast growth factor receptor. Organization of the gene revealed that the isoforms are produced by two different types of alternative splicing (the cassette and internal donor types) from the common gene. In human placenta, the shorter form is expressed as the major isoform.

Molecular cloning of a human basic fibroblast growth factor receptor cDNA and expression of a biologically active extracellular domain in a baculovirus system

A cDNA clone encoding a human fibroblast growth factor (FGF) receptor was isolated from a hepatoma cell line cDNA library. The cDNA encodes a three immunoglobulinlike-domain FGF receptor that is similar to a human placental FGF receptor cDNA but lacks two amino acids. The variation observed at these two amino acids, also seen in the two immunoglobulinlike-domain FGF-receptors, can be explained by an alternate splicing mechanism. We have used a baculovirus expression system to produce high levels of a soluble, extracellular domain form of the FGF receptor (EC-FGF receptor). Spodoptera frugiperda (Sf9) insect cells infected with recombinant EC-FGF receptor viruses synthesized and secreted an EC-FGF receptor of apparent Mr = 58,000. The EC-FGF receptor purified from conditioned media of infected Sf9 cells by lentil lectin affinity chromatography was shown to bind basic FGF with high affinity (Kd = 1-5 nM), to inhibit the binding of radioiodinated basic FGF to its high affinity receptor and to inhibit endothelial cell proliferation. Furthermore, binding of basic FGF to the EC-FGF receptor was shown to be significantly enhanced by heparin. The availability of biologically active FGF receptors will allow an analysis of their interaction with members of the FGF family of proteins and viruses of the herpes family that have been shown to use the FGF receptor system for cell entry.

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

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

Regulation of the fibroblast growth factor receptor in early Xenopus embryos

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

Related fibroblast growth factor receptor genes exist in the human genome

We have isolated, from a human tumor cDNA library, a gene encoding a putative receptor-like protein-tyrosine kinase that we call TK14. The amino acid sequence of the TK14 protein is closely related to the available partial sequence of the mouse protein bek, and more distantly related to the sequences of a chicken basic fibroblast growth factor receptor (73% sequence homology) and the apparent human equivalent of this receptor, the FLG protein (encoded by the fms-like tyrosine kinase gene). Overexpression of the TK14 protein by transfection of COS-1 cells with the corresponding cDNA in a simian virus 40-based expression vector leads to the appearance of new cell-surface binding sites for both acidic and basic fibroblast growth factors. This has been demonstrated by specific binding assays and chemical cross-linking experiments using 125I-labeled growth factors. It appears, therefore, that the human genome contains at least two distinct genes, for TK14 and FLG, that code for related fibroblast growth factor receptors.