Two FGF receptor genes are differentially expressed in epithelial and mesenchymal tissues during limb formation and organogenesis in the mouse

Region- and stage-specific effects of FGFs and BMPs in chick mandibular morphogenesis

The mandibular processes are specified as at least two independent functional regions: two large lateral regions where morphogenesis is dependent on fibroblast growth factor (FGF)-8 signaling, and a small medial region where morphogenesis is independent of FGF-8 signaling. To gain insight into signaling pathways that may be involved in morphogenesis of the medial region, we have examined the roles of pathways regulated by FGFs and bone morphogenetic proteins (BMPs) in morphogenesis of the medial and lateral regions of the developing chick mandible. Our results show that, unlike in the lateral region, the proliferation and growth of the mesenchyme in the medial region is dependent on signals derived from the overlying epithelium. We also show that medial and lateral mandibular mesenchyme respond differently to exogenous FGFs and BMPs. FGF-2 and FGF-4 can mimic many of the effects of mandibular epithelium from the medial region, including supporting the expression of Msx genes, outgrowth of the mandibular processes and elongation of Meckel's cartilage. On the other hand, laterally placed FGF beads did not induce ectopic expression of Msx genes and did not affect the growth of the mandibular processes. These functional studies, together with our tissue distribution studies, suggest that FGF-mediated signaling (other than FGF-8), through interactions with FGF receptor-2 and downstream target genes including Msx genes, is part of the signaling pathway that mediates the growth-promoting interactions in the medial region of the developing mandible. Our observations also suggest that BMPs play multiple stage- and region-specific roles in mandibular morphogenesis. In this study, we show that exogenous BMP-7 applied to the lateral region at early stages of development (stage 20) caused apoptosis, ectopic expression of Msx genes, and inhibited outgrowth of the mandibular processes and the formation of Meckel's cartilage. Our additional experiments suggest that the differences between the effects of BMP-7 on lateral mandibular mesenchyme at stage 20 and previously reported results at stage 23 (Wang et al., [1999] Dev. Dyn. 216:320-335) are related to differences in stages of differentiation in that BMP-7 promotes apoptosis in undifferentiated lateral mandibular mesenchyme, whereas it promotes chondrogenesis at later stages of development. We also showed that, unlike mandibular epithelium and medially placed FGF beads, medially placed BMP-7 did not support outgrowth of the isolated mesenchyme and at stage 20 induced the formation of a duplicated rod of cartilage extending from the body of Meckel's cartilage. These observations suggest that BMPs do not play essential roles in growth-promoting interactions in the medial region of the developing mandible. However, BMP-mediated signaling is a part of the signaling pathways regulating chondrogenesis of the mandibular mesenchyme.

Regulation of mandibular growth and morphogenesis

The development of the vertebrate face is a dynamic process that starts with the formation of facial processes/prominences. Facial processes are small buds made up of mesenchymal masses enclosed by an epithelial layer that surround the primitive mouth. The 2 maxillary processes, the 2 lateral nasal processes, and the frontonasal processes form the upper jaw. The lower jaw is formed by the 2 mandibular processes. Although the question of the embryonic origin of facial structures has received considerable attention, the mechanisms that control differential growth of the facial processes and patterning of skeletal tissues within these structures have been difficult to study and still are not well-understood. This has been partially due to the lack of readily identifiable morphologically discrete regions in the developing face that regulate patterning of the face. Nonetheless, in recent years there has been significant progress in the understanding of the signaling network controlling the patterning and development of the face (for review, see Richman et al., 1991; Francis-West et al., 1998). This review focuses on current understanding of the processes and signaling molecules that are involved in the formation of the mandibular arch.

Regulation of endochondral cartilage growth in the developing avian limb: cooperative involvement of perichondrium and periosteum

The perichondrium and periosteum have recently been suggested to be involved in the regulation of limb growth, serving as potential sources of signaling molecules that are involved in chondrocyte proliferation, maturation, and hypertrophy. Previously, we observed that removal of the perichondrium and periosteum from tibiotarsi in organ culture resulted in an overall increase in longitudinal cartilage growth, suggesting negative regulation originating from these tissues. To determine if the perichondrium and periosteum regulate growth through the production of diffusible factors, we have tested various conditioned media from these tissues for the ability to modify cartilage growth in tibiotarsal organ cultures from which these tissues have been removed. Both negative and positive regulatory activities were detected. Negative regulation was observed with conditioned medium from (1) cell cultures of the region bordering both the perichondrium and the periosteum, (2) co-cultures of perichondrial and periosteal cells, and (3) a mixture of conditioned media from perichondrial cell cultures and periosteal cell cultures. The requirement for regulatory factors from both the perichondrium and periosteum suggests a novel mechanism of regulation. Positive regulation was observed with conditioned media from several cell types, with the most potent activity being from articular perichondrial cells and hypertrophic chondrocytes.

Fibroblast growth factor receptor 2 (Fgfr2) plays an important role in eyelid and skin formation and patterning

Initiating as protruding ridges above and below the optic vesicle, the eyelids of mice grow across the eye and temporarily fuse in fetal life. Mutations of a number of genes disrupt this developmental process and result in a birth defect, "open-eyelids at birth." Here we show that a critical event for eyelid induction occurs at embryonic day 11.5 (E11.5) when the single cell-layered ectoderm in the presumptive eyelid territory increases proliferation and undergoes morphologic transition to form cube-shaped epithelial cells. Using embryos lacking the Fgfr2 Ig domain III (Fgfr2(DeltaIII/DeltaIII)) generated by tetraploid rescue and chimeric embryo formation approaches, we demonstrate that this event is controlled by Fgfr2 signals as the Fgfr2(DeltaIII/DeltaIII) mutation blocks these changes and results in embryos without eyelids. Fgfr2 and its ligands are differentially expressed in the ectoderm and underlying mesenchyme and function in a reciprocal interacting loop that specifies eyelid development. We also demonstrate that similar defects account for failure of skin formation at early stages. Interestingly, Fgfr2-independent skin formation occurs at E14.5 mutant embryos, resulting in much thinner, yet well-differentiated epidermis. Notably, mutant skin remains thin with decreased hair density after transplantation to wild-type recipients. These data demonstrate an essential role of Fgfr2 in eyelid and skin formation and patterning.

Early anterior/posterior patterning of the midbrain and cerebellum

Transplantation studies performed in chicken embryos indicated that early anterior/posterior patterning of the vertebrate midbrain and cerebellum might be regulated by an organizing center at the junction between the midbrain and hindbrain. More than a decade of molecular and genetic studies have shown that such an organizer is indeed central to development of the midbrain and anterior hindbrain. Furthermore, a complicated molecular network that includes multiple positive and negative feedback loops underlies the establishment and refinement of a mid/hindbrain organizer, as well as the subsequent function of the organizer. In this review, we first introduce the expression patterns of the genes known to be involved in this patterning process and the quail-chick transplantation experiments that have provided the foundation for understanding the genetic pathways regulating mid/hindbrain patterning. Subsequently, we discuss the molecular genetic studies that have revealed the roles for many genes in normal early patterning of this region. Finally, some of the remaining questions and future directions are discussed.

A soluble dominant negative fibroblast growth factor receptor 4 isoform in human MCF-7 breast cancer cells

Fibroblast growth factor receptors (FGFRs) are receptor tyrosine kinases encoded by four closely related genes. FGFR 1, 2, and 3 have a number of isoforms derived by alternative splicing, alternative initiation and exon switching; however, FGFR4 has been reported to encode a single intact receptor with three extracellular immunoglobulin (Ig)-like domains, a transmembrane domain, and a split intracellular kinase. Here we describe a novel C-terminally truncated soluble isoform of FGFR4 expressed by human epithelial breast cancer MCF-7 cells. This isoform results from failure of splicing of intron 4 resulting in an mRNA species that encodes an in-frame premature stop codon. Cells transfected with the corresponding cDNA containing intron 4 express a truncated releasable protein that is identified in conditioned media. This soluble FGFR4 isoform (sFGFR4) abrogates the effect of FGF-1-induced MAPK phosphorylation and PRL gene activation. These findings represent the first description of an endogenous soluble C-terminally truncated FGFR4 isoform with FGF modulatory properties.

Induction of FGF receptor 2-IIIb expression and response to its ligands in epithelial ovarian cancer

Epithelial ovarian cancers (EOCs) arise in the Ovarian Surface Epithelium (OSE). This tissue is a simple, poorly committed mesothelium which exhibits characteristics of epithelial and mesenchymal cells when grown in culture. In contrast, EOCs frequently exhibit properties of complex epithelial tissues of the female reproductive tract, such as oviductal, endometrial and cervical epithelia, and show induction of expression of epithelial markers such as E-cadherin. Fibroblast Growth Factor Receptor 2 isoform IIIb (FGF receptor 2-IIIb) is a spliced variant of FGF receptor 2 that binds the ligands FGF-1 and FGF-7 with high affinity, and is expressed exclusively by epithelial cells. We have studied the expression of FGF receptor 2-IIIb and its ligands in primary cultures of normal human OSE, EOC cell lines and snap frozen tissue from EOCs. Expression of FGF receptor 2-IIIb mRNA is undetectable in normal OSE, but is expressed in 16/20 (80%) of EOCs. FGFs 1 and 7 mRNAs are expressed in normal OSE, whilst only 4/20 (20%) and 12/20 (60%) of EOCs demonstrated expression for these ligands respectively. However, FGF-7 protein was detected in 70% (mean level=0.7 ng/ml) of ascitic fluids obtained from patients with EOC. FGFs 1 and 7 stimulate DNA synthesis in EOC cell lines that express FGF receptor 2-IIIb. Moreover, DNA synthesis in these cell lines can be partially blocked by blocking antisera to FGFs 1 and 7. It is suggested that induction of expression of FGF receptor 2-IIIb may play a role in the development of EOCs by rendering the OSE susceptible to paracrine and/or autocrine stimulation by its requisite FGF ligands.

Development of the thymus requires signaling through the fibroblast growth factor receptor R2-IIIb

Mice deficient for fibroblast growth factor (Fgf)R2-IIIb show a block in thymic growth after embryonic day 12.5, a stage that just precedes its detection in thymic epithelial cells. Fgf7 and Fgf10, the main ligands for FgfR2-IIIb, are expressed in the mesenchyme surrounding the thymic epithelial primordium, and Fgf10-deficient mice also exhibit impaired thymic growth. Hence, Fgf signaling is essential for thymic epithelial proliferation. In addition to the proliferative block, most thymic epithelial cells fail to progress from an immature cytokeratin 5-positive to a cytokeratin 5-negative phenotype. Nevertheless, sufficient epithelial cell differentiation occurs in the severely hypoplastic thymus to allow the development of CD4/CD8-double-positive thymocytes and a very small number of single-positive thymocytes expressing TCRs.

Differential response of Shh expression between chick forelimb and hindlimb buds by FGF-4

The interactions of Sonic hedgehog (Shh) and fibroblast growth factor (FGF) play important roles in vertebrate limb pattern formation. In the posterior region of the chick limb bud, Shh and FGF-4 each maintain expression in a positive feedback loop. In the anterior region, Shh can also induce Fgf-4 expression in the anterior apical ectodermal ridge. However, the possibility of Shh induction by FGF protein is unclear. Because many experiments to analyze gene expression have been carried out by using the forelimb bud of the chick embryo, we investigated gene expression of the cells in the anterior region of the chick hindlimb bud after FGF-4 application and compared the results with those for the forelimb bud. When an FGF-4-containing bead was implanted into the anterior region of a stage 20 hindlimb bud, ectopic expression of Shh was induced in the mesenchyme beneath the anterior end of the apical ectodermal ridge at 36 hr after implantation. Subsequent to Shh activation, Hoxd13 was also observed in the anterior-distal region of the limb bud. Furthermore, FGF-4 implantation to the hindlimb bud caused additional digit formation accompanying respecification of positional value in the anterior tissue. Ectopic Shh was induced in cells located distal to the FGF-4 bead, and the cells of the flank region did not contribute to ectopic Shh induction. On the other hand, no ectopic Shh and Hoxd13 expression was detected by grafting an FGF-4 bead into the forelimb bud. Although FGF-4 implantation to the forelimb bud occasionally induced extra digit 2 formation, no embryos had an extra digit 3 or digit 4, and many specimens exhibited normal skeletal pattern. These results demonstrate the difference between the fore- and hindlimb buds in the cell competence of Shh induction in response to FGF-4, suggesting the possibility that the responsiveness of mesenchymal cells in signaling molecules is not the same in the fore- and hindlimb buds.

FGFs-1 and -2, and TGF beta 1 as inductive signals modulating in vitro odontoblast differentiation

We have studied the expression of FGF1 and FGF2 during mouse odontogenesis by immunohistochemistry. FGF1 was detected in differentiated odontoblasts and at the secretory pole of ameloblasts. Localization of FGF2 was mainly observed within the basement membrane interposed between dental epithelium and dental mesenchyme. These findings indicate that FGF1 and FGF2 may participate in the control of odontoblast and ameloblast differentiation. Thereafter, we studied the ability of FGF1 and FGF2, alone or in combination with TGF beta 1, to induce polarization and/or functional differentiation of preodontoblasts. Dental papillae (DP) obtained from first lower molars of 17-day-old mouse embryo were cultured in the presence or the absence of growth factors. DP cultured with FGF1 + TGF beta 1 showed gradients of odontoblast-like cell differentiation, which displayed alkaline phosphatase reactivity. DP treated with FGF2 + TGF beta 1 exhibited pre-odontoblast cell polarization, and the cell bodies displayed long cytoplasm processes. However, following this treatment we did not observe extracellular matrix secretion, and alkaline phosphatase activity was completely inhibited. In summary, our results show that exogenous addition of FGF1 to pre-odontoblasts induces their terminal differentiation, by synergistically acting with TGF beta 1. In contrast, FGF2 may regulate the effect of TGF beta 1, permitting cell polarization but restraining pre-odontoblast functions.

Fibroblast growth factor receptors and their ligands in the adult rat kidney

BACKGROUND

Fibroblast growth factors (FGFs) are a family of at least 21 heparin-binding proteins involved in many biological processes, both during development and in the adult, including cell proliferation, differentiation, and angiogenesis. FGFs mediate their effects through high-affinity tyrosine kinase receptors (FGFRs), which are encoded by four genes. The aims of the present study were to localize FGFR-1 through FGFR-3 in the normal adult rat kidney and to determine which functional FGFR variants and FGFs were expressed.

METHODS

Avidin-biotin-enhanced horseradish peroxidase immunohistochemistry was used on paraffin sections of rat kidney to localize FGFR-1 through FGFR-3, whereas reverse transcriptase-polymerase chain reaction was used to examine expression of the receptor variants and also of FGF-1 through FGF-10 in cortex, outer medulla, and inner medulla.

RESULTS

By immunohistochemistry, each receptor was localized to distinct and overlapping nephron segments, such that one or more FGFRs were localized to all nephron and collecting duct epithelia. FGFR-1 and FGFR-3 were localized to glomeruli, FGFR-3 to proximal tubules and FGFR-1 to thin limbs. FGFR-1 through FGFR-3 were localized to distal straight tubules, with FGFR-1 and FGFR-3 localized to distal convoluted tubules. FGFR-1 and FGFR-3 were localized to medullary collecting ducts. In addition, FGFR-1 was localized to the smooth muscle of renal arteries. All seven FGFR variants were expressed in the cortex and outer medulla, with fewer FGFRs in the inner medulla. FGF-1, FGF-2, FGF-7, FGF-8, and FGF-9 were expressed in the kidney, with FGF-10 expression found only in the cortex.

CONCLUSIONS

Mapping of these receptors is critical to the determination of the effects of FGF ligands in discrete regions of the kidney. The distributions of the FGFRs in the normal adult kidney and the restricted expression of FGF ligands suggest that specific FGFs have distinct and important roles in the maintenance of normal kidney structure and function.

Fibroblast growth factors in the developing central nervous system

1. It is now clear that members of the fibroblast growth factor (FGF) family have multiple roles during the formation of the central nervous system (CNS). 2. There are at least 23 members of the FGF family and, of these, 10 are expressed in the developing CNS, along with four FGF receptors (FGFR-1-4). 3. The present review discusses the roles of these FGFs, with emphasis on FGF-2, FGF-8, FGF-15 and FGF-17. Fibroblast growth factors-2 and -15 are generally expressed throughout the developing CNS, whereas FGF-8 and FGF-17 are tightly localized to specific regions of the developing brain and are only expressed in the embryo during the early phases of proliferation and neurogenesis. 4. Expression studies on FGFRs in the chick and mouse indicate that FGFR-1 is most generally expressed, whereas FGFR-2 and FGFR-3 show highly localized but changing patterns of expression throughout CNS development. The FGFR-4 has been localized to the developing CNS in fish but not at a detailed level, as yet, in chick or mouse. 5. A picture is emerging from these studies that particular FGFs signal through specific receptors in a highly localized manner to regulate the development of different regions of the brain. 6. This picture has been demonstrated so far for the developing cortex (FGF-2-/- mice), the forebrain and midbrain (FGF-8 hypomorphs) and the cerebellum (FGF-17/FGF-8 mutant mice). In addition, generation of mutant animals deleted for FGFR-1 and FGFR-2b IIIb demonstrate their importance in FGF signalling. 7. However, there are significant gaps in our knowledge of the localization of members of the FGF family and their receptors. More detailed information on the spatio-temporal mapping of FGFs and FGFR isoforms is required in order to understand the molecular mechanisms through which FGFs signal.

Somatic gene therapy in the cardiovascular system

This review surveys a range of approaches using plasmid DNA encoding the 165-amino-acid isoform of vascular endothelial growth factor (phVEGF165) to therapeutically modulate micro- or macrovascular endothelial cells, focusing on strategies to augment postnatal collateral circulation in arterial insufficiency or to accelerate re-endothelialization after balloon angioplasty to prevent restenosis. We focus on intra-arterial and intramuscular/intramyocardial gene transfer of the VEGF165 gene, the options that have been most thoroughly studied to date in patients. We review developmental and postnatal significance of the endothelial-cell-specific mitogen VEGF that has stimulated these studies and present limitations of current knowledge as well as challenges for the future.

Molecular regulation of lung development

There is increasing evidence suggesting that formation of the tracheobronchial tree and alveoli results from heterogeneity of the epithelial-mesenchymal interactions along the developing respiratory tract. Recent genetic data support this idea and show that this heterogeneity is likely the result of activation of distinct networks of signaling molecules along the proximal-distal axis. Among these signals, fibroblast growth factors, retinoids, Sonic hedgehog, and transforming growth factors appear to play prominent roles. We discuss how these and other pattern regulators may be involved in initiation, branching, and differentiation of the respiratory system.

A competitive PCR-based method to measure human fibroblast growth factor receptor 1-4 (FGFR1-4) gene expression

The four members of the fibroblast growth factor receptor (FGFR) family are cell-surface membrane-spanning tyrosine kinase receptors involved in a wide spectrum of biologic processes. Much evidence also indicates that mutations in FGFR genes result in several craniosynostotic disorders and chondrodysplasias, and that changes in qualitative and quantitative FGFR expression profiles are implicated in tumor induction or progression. Here, we describe a precise and reliable competitive PCR-based assay to evaluate human FGFR1-4 gene expression. A single multispecific synthetic competitive template was designed to amplify FGFR1-4 homologous stretches and constructed to contain FGFR1/FGFR2/FGFR3/FGFR4/GAPDH tandemly arranged forward and reverse primers that allow competition for cDNA-specific primer annealing. The housekeeping GAPDH transcript was utilized as a reference for comparing the expression profiles of different RNA pools. The assay herein described allows the comparison of relative FGFR expression levels, both within a single RNA pool and among multiple RNA pool samples. The major advantages of such a PCR-based approach are its ability to obtain unbiased FGFR mRNA expression patterns and to detect transcripts present in low copy number. Qualitative and semiquantitative analyses of the FGFR1-4 transcript repertoire in mesenchymal- and epithelial-derived primary cell cultures and cell lines demonstrated the utility of such a method to investigate the FGFR1-4 functional role in FGF signal transduction.

Two distal Sp1-binding cis-elements regulate fibroblast growth factor receptor 1 (FGFR1) gene expression in myoblasts

Skeletal myoblast cell proliferation and subsequent differentiation are dependent on developmentally regulated expression of the fibroblast growth factor receptor 1 (FGFR1) gene. We have previously reported the isolation and initial characterization of the chicken FGFR1 gene (cek1) promoter. Both distal and proximal regions of the promoter were identified as necessary for developmentally regulated transcriptional activity in proliferating myoblasts, including its down-regulation in differentiated muscle fibers in vitro. Here we report detailed characterization of the molecular mechanism regulating FGFR1 promoter activity via the distal promoter region in proliferating myoblasts. This region was identified as a 242 base pair segment located greater than 1 kilobase upstream from the start of transcription that conferred increased transcriptional activity to a minimal thymidine kinase promoter. This segment contains two Sp1 binding sites. Site directed mutagenesis and transfection studies indicated that both Sp1 sites are functional and both are required for FGFR1 promoter activity. Furthermore, Sp1 binding to the two sites was synergistic enhancing FGFR1 promoter activity. The specificity of Sp1 binding to the two distal promoter cis-elements was verified by electromobility shift and transfection assays employing an Sp1 expression construct. Differences in myoblast versus fibroblast-specific protein-DNA complex formation at these sites correlated with high promoter activity in myoblasts and significantly reduced promoter activity in fibroblasts. These studies for the first time establish a molecular mechanism regulating FGFR1 gene expression during myoblast proliferation.

Century of Jackson-Weiss syndrome: further definition of clinical and radiographic findings in "lost" descendants of the original kindred

Jackson-Weiss syndrome (JWS) is a condition consisting of craniosynostosis characterized by premature fusion of the cranial sutures and/or characteristic radiographic anomalies of the feet. The condition is inherited as an autosomal dominant trait with high penetrance and variable expressivity. Six different mutations in the fibroblast growth factor receptor 2 have been identified in patients with the clinical diagnosis of JWS. Jabs et al. [1994: Nat Genet 8:275-279] identified an Ala344Gly substitution in two branches of the family in which the clinical syndrome was originally described. This is the only publication to document this mutation in a family with the clinical diagnosis of JWS. In this study, we have identified a previously unrecognized branch of the original family with individuals that meet the clinical criteria for the diagnosis of JWS. We demonstrate that a mutation that produces the Ala344Gly substitution is present in affected members. This family illustrates the widely variable expression of the mutation, including a novel phenotype in the proband with a leg-length discrepancy and unilateral absence of the fifth digital ray in her right foot. We identify the clinical and detailed radiographic features of each affected individual and offer considerations when making the diagnosis of JWS.

Fibroblast growth factor-induced gene expression and cartilage pattern formation in chick limb bud recombinants

To clarify the roles of fibroblast growth factors (FGF) in limb cartilage pattern formation, the effects of various FGF on recombinant limbs that were composed of dissociated and reaggregated mesoderm and ectodermal jackets were examined. Fibroblast growth factor-soaked beads were inserted just under the apical ectodermal ridge (AER) of recombinant limbs and the recombinant limbs were grafted and allowed to develop. Control recombinant limbs without FGF beads formed one or two cartilage elements. Recombinants with FGF-4 beads formed up to five cartilage elements, which were aligned along the anteroposterior (AP) axis. Each cartilage element showed digit-like segmentation. In contrast, recombinants with FGF-2 beads showed formation of multiple thick and unsegmented cartilage rods, which elongated inside and outside the AP plane from the distal end of the recombinants. Recombinants with FGF-8 beads formed a truncated cartilage pattern and recombinants with FGF-10 beads formed a cartilage pattern similar to that of the control recombinants. The expression of the Fgf-8, Msx-1 and Hoxa-13 genes in the developing recombinant limbs were examined. FGF-4 induced extension of the length of the Fgf-8-positive epidermis, or AER, along the AP axis 5 days after grafting, at which time the digits are specified. FGF-2 induced expansion of the Msx-1-positive area, first in the proximal direction and then along the dorsoventral axis. The functions of these FGF in recombinant and normal limb patterning are discussed in this paper.

FGF-10 disrupts lung morphogenesis and causes pulmonary adenomas in vivo

Transgenic mice in which fibroblast growth factor (FGF)-10 was expressed in the lungs of fetal and postnatal mice were generated with a doxycycline-inducible system controlled by surfactant protein (SP) C or Clara cell secretory protein (CCSP) promoter elements. Expression of FGF-10 mRNA in the fetal lung caused adenomatous malformations, perturbed branching morphogenesis, and caused respiratory failure at birth. When expressed after birth, FGF-10 caused multifocal pulmonary tumors. FGF-10-induced tumors were highly differentiated papillary and lepidic pulmonary adenomas. Epithelial cells lining the tumors stained intensely for thyroid transcription factor (TTF)-1 and SP-C but not CCSP, indicating that FGF-10 enhanced differentiation of cells to a peripheral alveolar type II cell phenotype. Withdrawal from doxycycline caused rapid regression of the tumors associated with rapid loss of the differentiation markers TTF-1, SP-B, and proSP-C. FGF-10 disrupted lung morphogenesis and induced multifocal pulmonary tumors in vivo and caused reversible type II cell differentiation of the respiratory epithelium.

A splicing switch and gain-of-function mutation in FgfR2-IIIc hemizygotes causes Apert/Pfeiffer-syndrome-like phenotypes

Intercellular signaling by fibroblast growth factors plays vital roles during embryogenesis. Mice deficient for fibroblast growth factor receptors (FgfRs) show abnormalities in early gastrulation and implantation, disruptions in epithelial-mesenchymal interactions, as well as profound defects in membranous and endochondrial bone formation. Activating FGFR mutations are the underlying cause of several craniosynostoses and dwarfism syndromes in humans. Here we show that a heterozygotic abrogation of FgfR2-exon 9 (IIIc) in mice causes a splicing switch, resulting in a gain-of-function mutation. The consequences are neonatal growth retardation and death, coronal synostosis, ocular proptosis, precocious sternal fusion, and abnormalities in secondary branching in several organs that undergo branching morphogenesis. This phenotype has strong parallels to some Apert's and Pfeiffer's syndrome patients.

Fibroblast growth factor receptor 2-IIIb acts upstream of Shh and Fgf4 and is required for limb bud maintenance but not for the induction of Fgf8, Fgf10, Msx1, or Bmp4

Mice deficient for FgfR2-IIIb were generated by placing translational stop codons and an IRES-LacZ cassette into exon IIIb of FgfR2. Expression of the alternatively spliced receptor isoform, FgfR2-IIIc, was not affected in mice deficient for the IIIb isoform. FgfR2-IIIb(-/-) (lac)(Z) mice survive to term but show dysgenesis of the kidneys, salivary glands, adrenal glands, thymus, pancreas, skin, otic vesicles, glandular stomach, and hair follicles, and agenesis of the lungs, anterior pituitary, thyroid, teeth, and limbs. Detailed analysis of limb development revealed an essential role for FgfR2-IIIb in maintaining the AER. Its absence did not prevent expression of Fgf8, Fgf10, Bmp4, and Msx1, but did prevent induction of Shh and Fgf4, indicating that they are downstream targets of FgfR2-IIIb activation. In the absence of FgfR2-IIIb, extensive apoptosis of the limb bud ectoderm and mesenchyme occurs between E10 and E10.5, providing evidence that Fgfs act primarily as survival factors. We propose that FgfR2-IIIb is not required for limb bud initiation, but is essential for its maintenance and growth.

Fibroblast growth factor signalling and cyclin D1 function are necessary for normal mammary gland development during pregnancy. A transgenic mouse approach

A number of growth factors, growth factor receptors and cell cycle regulatory proteins have been implicated in the genesis of mammary carcinomas both in animal models as well as in human breast tumour samples. Studies on the development of the mammary gland has revealed that several of the proto-oncogenes, or their closely related gene-family members, have a function in the normal growth and differentiation of the gland. In this review the role of fibroblast growth factor signalling and the critical requirement for the cell cycle regulator, cyclin D1 is discussed with respect to their normal function in mammary gland development and abnormal role in mammary carcinogenesis.

Expression of members of the Fgf family and their receptors during midfacial development

Members of the FGF family play diverse roles in patterning, cell proliferation and differentiation during embryogenesis. To begin to address their function during craniofacial development we have analyzed the expression of 18 members of the Fgf family (Fgf1-15, -17, -18 and -20) and the four members of the FGF-receptor family in the prospective midfacial region between E9.5 and E11.5 by whole-mount in situ hybridization. We show that at E9.5, Fgf3, -8, -9, -10 and -17 are broadly expressed in midfacial ectoderm. Concomitant with the outgrowth of the nasal processes at E10.5, expression of Fgf3, -8, -9, -10, -15, -17 and -18 was detected in spatially restricted regions of ectoderm at the edge of the nasal pit and at the oral edge of the medial nasal process. Expression of Fgf8, Fgf9, Fgf10 and Fgf17 was still observed in these domains at E11.5. In contrast to the restricted expression patterns of the ligands, FgfR1 and FgfR2 were broadly expressed in facial mesenchyme and ectoderm, respectively, indicating a wide competence of midfacial tissue to respond to FGF signaling.

Expression patterns of Fgf-8 during development and limb regeneration of the axolotl

Fgf-8 is one of the key signaling molecules implicated in the initiation, outgrowth, and patterning of vertebrate limbs. However, it is not clear whether FGF-8 plays similar role in development and regeneration of urodele limbs. We isolated a Fgf-8 cDNA from the Mexican axolotl (Ambystoma mexicanum) through the screening of an embryo cDNA library. The cloned 1.26-kb cDNA contained an open reading frame encoding 212 amino acid residues with 84%, 86%, and 80% amino acid identities to those of Xenopus, chick, and mouse, respectively. By using the above clone as a probe, we examined the temporal and spatial expression patterns of Fgf-8 in developing embryos and in regenerating larval limbs. In developing embryos, Fgf-8 was expressed in the neural fold, midbrain-hindbrain junction, tail and limb buds, pharyngeal clefts, and primordia of maxilla and mandible. In the developing axolotl limb, Fgf-8 began to be expressed in the prospective forelimb region at pre-limb-bud and limb bud stages. Interestingly, strong expression was detected in the mesenchymal tissue of the limb bud before digit forming stages. In the regenerating limb, Fgf-8 expression was noted in the basal layer of the apical epithelial cap (AEC) and the underlying thin layer of mesenchymal tissue during blastema formation stages. These data suggest that Fgf-8 is involved in the organogenesis of various craniofacial structures, the initiation and outgrowth of limb development, and the blastema formation and outgrowth of regenerating limbs. In the developing limb of axolotl, unlike in Xenopus or in amniotes such as chick and mouse, the Fgf-8 expression domain was localized mainly in the mesenchyme rather than epidermis. The unique expression pattern of Fgf-8 in axolotl suggests that the regulatory mechanism of Fgf-8 expression is different between urodeles and other higher species. The expression of Fgf-8 in the deep layer of the AEC and the thin layer of underlying mesenchymal tissue in the regenerating limbs support the previous notion that the amphibian AEC is a functional equivalent of the AER in amniotes.

FGF10 acts as a major ligand for FGF receptor 2 IIIb in mouse multi-organ development

FGF receptor 2 isoform IIIb (FGFR2b), originally discovered as a receptor for FGF7, is known to be an important receptor in vertebrate morphogenesis, because FGFR2b null mice exhibit agenesis or dysgenesis of various organs, which undergo budding and branching morphogenesis. Since FGF7 null mice do not exhibit marked defects in organogenesis, it has been considered that other FGF(s) than FGF7 might function as a major ligand for FGFR2b during organogenesis. One of the candidate ligands is FGF10, because FGF10 binds to FGFR2b with high affinity and the formation of the limb and lung is arrested in FGF10 null mice as found in FGFR2b-deficient mice. Previous analyses of FGF10 null mice revealed that FGF10 is required for limb and lung development. To elucidate the role of FGF10 in wide-range organogenesis, we further analyzed the phenotypes of the FGF10 knockout mice. We found diverse phenotypes closely related to those for FGFR2b-deficient mice, which includes the absence of thyroid, pituitary, and salivary glands, while minor defects were observed in the formation of teeth, kidneys, hair follicles, and digestive organs. These results suggest that FGF10 acts as a major ligand for FGFR2b in mouse multi-organ development.

Activation of myocardial angiogenesis and upregulation of fibroblast growth factor-2 in transmyocardial-revascularization-treated mice

OBJECTIVE

To evaluate the growth factor responses associated with myocardial angiogenesis.

DESIGN

Mice were treated with transmyocardial revascularization (TMR) and evaluated for angiogenic and growth factor responses.

METHODS

TMR was performed via thoractomy with a 27 g needle. At 2, 5, and 7 days post-treatment, hearts were removed from the TMR treated and control groups, then assayed for angiogenesis, fibroblast growth factor (FGF)-2 expression and vascular endothelial cell growth factor (VEGF) expression.

RESULTS

TMR caused an angiogenic reaction in the myocardial blood vessels at 7 days post-TMR treatment. Elevated FGF-2 corresponded with increased TMR related angiogenesis. VEGF levels only increased in hearts that were prewounded then TMR treated.

CONCLUSIONS

The data show that TMR stimulates myocardial angiogenesis. The angiogenic reaction is mediated by FGF-2 which increased in most experimental treatment groups. The VEGF response was more specific, requiring prewounding then TMR treatment for a VEGF increase.

Mesenchymal expression of nuclear factor-kappaB inhibits epithelial growth and branching in the embryonic chick lung

It is becoming increasingly recognized that the ubiquitous, inducible transcription factor nuclear factor-kappaB (NF-kappaB) is involved in developmental processes. For example, NF-kappaB acts as a mediator of epithelial-mesenchymal interactions in the developing chick limb. We investigated the role of NF-kappaB in directing the branching morphogenesis of the developing chick lung, a process which relies on epithelial-mesenchymal communication. High level expression of relA was found in the mesenchyme surrounding the nonbranching structures of the lung but was not detected either in the mesenchyme surrounding the branching structures of the distal lung or in the developing lung epithelium. Specific inhibition of mesenchymal NF-kappaB in lung cultures resulted in increased epithelial budding. Conversely, expression of a trans-dominant activator of NF-kappaB in the lung mesenchyme repressed budding. Ectopic expression of RelA was sufficient to inhibit the ability of the distal mesenchyme to induce epithelial bud formation. Cellular proliferation in the mesenchyme was inhibited by hyperactivation of NF-kappaB in the mesenchyme of lung cultures. Interestingly, increased NF-kappaB activity in the mesenchyme also decreased the proliferation of the associated epithelium, while inhibition of NF-kappaB activity increased cellular proliferation in lung cultures. Expression patterns of several genes which are known to influence lung branching morphogenesis were altered in response to changes in mesenchymal NF-kappaB activity, including fgf10, bmp-4, and tgf-beta1. Thus NF-kappaB represents the first transcription factor reported to function within the lung mesenchyme to limit growth and branching of the adjacent epithelium.

Bmp4 and Fgf10 play opposing roles during lung bud morphogenesis

Morphogenesis of the mouse lung involves reciprocal interactions between the epithelial endoderm and the surrounding mesenchyme, leading to an invariant early pattern of branching that forms the basis of the respiratory tree. There is evidence that Fibroblast growth factor 10 (Fgf10) and Bone Morphogenetic Protein 4 (Bmp4), expressed in the distal mesenchyme and endoderm, respectively, play important roles in branching morphogenesis. To examine these roles in more detail, we have exploited an in vitro culture system in which isolated endoderm is incubated in Matrigel(TM) substratum with Fgf-loaded beads. In addition, we have used a Bmp4(lacZ) line of mice in which lacZ faithfully reports Bmp4 expression. Analysis of lung endoderm in vivo shows a dynamic pattern of Bmp4(lacZ) expression during bud outgrowth, extension and branching. In vitro, Fgf10 induces both proliferation and chemotaxis of isolated endoderm, whether it is derived from the distal or proximal lung. Moreover, after 48 hours, Bmp4(lacZ) expression is upregulated in the endoderm closest to the bead. Addition of 30–50 ng/ml of exogenous purified Bmp4 to the culture medium inhibits Fgf-induced budding or chemotaxis, and inhibits overall proliferation. By contrast, the Bmp-binding protein Noggin enhances Fgf-induced morphogenesis. Based on these and other results, we propose a model for the combinatorial roles of Fgf10 and Bmp4 in branching morphogenesis of the lung.

Function of fibroblast growth factors and vascular endothelial growth factors and their receptors in angiogenesis

Angiogenesis, formation of new vessels from pre-existing ones, results from stimulation of endothelial cells, which line the vessel wall. These cells will leave their resting state and start to digest the basement membrane, proliferate, migrate and eventually differentiate to form a hollow tube. All these steps can be induced by growth factors and this review will focus on two important types of angiogenic growth factors, vascular endothelial growth factor (VEGF; also denoted vascular permeability factor, VPF) and fibroblast growth factor (FGF). Both types of factors bind to cell surface expressed receptors, which are ligand-stimulatable tyrosine kinases. Binding of the growth factors to their receptors leads to activation of the intrinsic tyrosine kinase and signal transduction to downstream signalling cascades. This results in transcriptional changes and biological responses. The molecular aspects of signalling cascades critical for endothelial cell proliferation and migration are beginning to be delineated. In contrast, signalling cascades leading to endothelial cell differentiation remain to be determined. Angiogenesis is essential for a number of physiological events such as embryonic development, ovulation, and wound healing. It has become increasingly clear that a number of diseases depend on angiogenesis. For future development of therapeutic tools, it is important to understand the molecular mechanisms that regulate angiogenesis.

Synergistic effects of FGF and non-ridge ectoderm on gene expression involved in the formation of the anteroposterior axis of the chick limb bud in cell culture

Skeletal patterning of the vertebrate limb is controlled by the zone of polarizing activity (ZPA), apical ectodermal ridge (AER) and dorsal ectoderm. In the present study, to understand the involvement of fibroblast growth factor (FGF) and non-ridge ectoderm in anteroposterior (AP) axis formation, gene expression in chick limb bud mesenchymal cells in culture was investigated by reverse transcription-polymerase chain reaction and in situ hybridization. It was found that Shh expression was locally maintained in the mesenchymal cells underneath and near non-ridge ectoderm in coculture with the posterior mesenchymal cells and non-ridge ectoderm in the presence of FGF-4 by in situ hybridization. In Shh-expressing anterior limb bud mesenchymal cells cultured with non-ridge ectoderm, it was also discovered that Bmp-2 was activated in the presence of FGF-2, -4 and -8, while Hoxd-13 was activated in the presence of FGF-4 and that FGF-2 had a similar effect but FGF-8 did not. This result indicates that Hoxd-13 activation by SHH depends on non-ridge ectoderm and FGF-2 or FGF-4, and that there may be a difference in the effect on AP axis formation of the limb bud between FGF-2, -4 and -8. Possible roles of these genes and signal molecules in AP pattern formation are discussed.

Integration of FGF and TWIST in calvarial bone and suture development

Mutations in the FGFR1-FGFR3 and TWIST genes are known to cause craniosynostosis, the former by constitutive activation and the latter by haploinsufficiency. Although clinically achieving the same end result, the premature fusion of the calvarial bones, it is not known whether these genes lie in the same or independent pathways during calvarial bone development and later in suture closure. We have previously shown that Fgfr2c is expressed at the osteogenic fronts of the developing calvarial bones and that, when FGF is applied via beads to the osteogenic fronts, suture closure is accelerated (Kim, H.-J., Rice, D. P. C., Kettunen, P. J. and Thesleff, I. (1998) Development 125, 1241–1251). In order to investigate further the role of FGF signalling during mouse calvarial bone and suture development, we have performed detailed expression analysis of the splicing variants of Fgfr1-Fgfr3 and Fgfr4, as well as their potential ligand Fgf2. The IIIc splice variants of Fgfr1-Fgfr3 as well as the IIIb variant of Fgfr2 being expressed by differentiating osteoblasts at the osteogenic fronts (E15). In comparison to Fgf9, Fgf2 showed a more restricted expression pattern being primarily expressed in the sutural mesenchyme between the osteogenic fronts. We also carried out a detailed expression analysis of the helix-loop-helix factors (HLH) Twist and Id1 during calvaria and suture development (E10-P6). Twist and Id1 were expressed by early preosteoblasts, in patterns that overlapped those of the FGF ligands, but as these cells differentiated their expression dramatically decreased. Signalling pathways were further studied in vitro, in E15 mouse calvarial explants. Beads soaked in FGF2 induced Twist and inhibited Bsp, a marker of functioning osteoblasts. Meanwhile, BMP2 upregulated Id1. Id1 is a dominant negative HLH thought to inhibit basic HLH such as Twist. In Drosophila, the FGF receptor FR1 is known to be downstream of Twist. We demonstrated that in Twist(+/)(−) mice, FGFR2 protein expression was altered. We propose a model of osteoblast differentiation integrating Twist and FGF in the same pathway, in which FGF acts both at early and late stages. Disruption of this pathway may lead to craniosynostosis.

Conditional expression of fibroblast growth factor-7 in the developing and mature lung

Effects of fibroblast growth factor-7 (FGF-7) on lung morphogenesis, respiratory epithelial cell differentiation, and proliferation were assessed in transgenic mice in which the human FGF-7 cDNA was controlled by a conditional promoter under the direction of regulatory elements from either the human surfactant protein-C (SP-C) or rat Clara cell secretory protein (ccsp) genes. Expression of FGF-7 was induced in respiratory epithelial cells of the fetal lung by administration of doxycycline to the dam. Prenatally, doxycycline induced FGF-7 mRNA in respiratory epithelial cells in both Sp-c and Ccsp transgenic lines, increasing lung size and causing cystadenomatoid malformation. Postnatally, mice bearing both Ccsp-rtta and (Teto)(7)-cmv-fgf-7 transgenes survived, and lung morphology was normal. Induction of FGF-7 expression by doxycycline in the Ccsp-rtta x (Teto)(7)-cmv-fgf-7 mice caused marked epithelial cell proliferation, adenomatous hyperplasia, and pulmonary infiltration with mononuclear cells. Epithelial cell hyperplasia caused by FGF-7 was largely resolved after removal of doxycycline. Surfactant proteins, TTF-1, and aquaporin 5 expression were conditionally induced by doxycycline. The Sp-c-rtta and Ccsp-rtta activator mice provide models in which expression is conditionally controlled in respiratory epithelial cells in the developing and mature lung, altering lung morphogenesis, differentiation, and proliferation.

Programmed cell death in the developing heart: regulation by BMP4 and FGF2

Programmed cell death, or apoptosis, plays an important role in embryonic development. To provide new insights into the role of programmed cell death in cardiac development, we examined the hearts of the murine embryos from E9.5 to postnatal day 3. Using terminal transferase-mediated dUTP nick end-labeling assays, apoptosis was detected in the endocardial cushions and myocardium from E11.5 to postnatal day 3 (P3). In the ventricular myocardium, more apoptotic cells were observed in the left than right ventricles throughout embryonic and early postnatal development. Apoptosis was also present in the trabeculae and papillary muscles of the ventricles. In the outflow tract, cell death was present in the endocardial cushions before they fuse to form the conotruncal septum (E11.5-E12. 5) and reached a peak intensity when the conotruncal septum formed (E13.5). In the atrioventricular (AV) endocardial cushions, cell death was detected in the fusion seam of the cushion tissues at E12. 5 and E13.5 during AV septation. When the patterns of apoptosis were compared with patterns of cell division, we found that programmed cell death occurred in the areas in the endocardial cushions and trabeculae where rates of cell proliferation were low. We also found that programmed cell death was regulated by the growth factors, BMP4 and FGF2, in vitro. BMP4 induced, whereas FGF2 inhibited, apoptosis in both endocardial cushions and ventricular myocardium. Overall, our observations show that there is apoptosis in the regions where fusion or remodeling of tissues occurs. We also show that cardiac programmed cell death can be influenced by growth factors.

The molecular basis of lung morphogenesis

To form a diffusible interface large enough to conduct respiratory gas exchange with the circulation, the lung endoderm undergoes extensive branching morphogenesis and alveolization, coupled with angiogenesis and vasculogenesis. It is becoming clear that many of the key factors determining the process of branching morphogenesis, particularly of the respiratory organs, are highly conserved through evolution. Synthesis of information from null mutations in Drosophila and mouse indicates that members of the sonic hedgehog/patched/smoothened/Gli/FGF/FGFR/sprouty pathway are functionally conserved and extremely important in determining respiratory organogenesis through mesenchymal-epithelial inductive signaling, which induces epithelial proliferation, chemotaxis and organ-specific gene expression. Transcriptional factors including Nkx2.1, HNF family forkhead homologues, GATA family zinc finger factors, pou and hox, helix-loop-helix (HLH) factors, Id factors, glucocorticoid and retinoic acid receptors mediate and integrate the developmental genetic instruction of lung morphogenesis and cell lineage determination. Signaling by the IGF, EGF and TGF-beta/BMP pathways, extracellular matrix components and integrin signaling pathways also directs lung morphogenesis as well as proximo-distal lung epithelial cell lineage differentiation. Soluble factors secreted by lung mesenchyme comprise a 'compleat' inducer of lung morphogenesis. In general, peptide growth factors signaling through cognate receptors with tyrosine kinase intracellular signaling domains such as FGFR, EGFR, IGFR, PDGFR and c-met stimulate lung morphogenesis. On the other hand, cognate receptors with serine/threonine kinase intracellular signaling domains, such as the TGF-beta receptor family are inhibitory, although BMP4 and BMPR also play key inductive roles. Pulmonary neuroendocrine cells differentiate earliest in gestation from among multipotential lung epithelial cells. MASH1 null mutant mice do not develop PNE cells. Proximal and distal airway epithelial phenotypes differentiate under distinct transcriptional control mechanisms. It is becoming clear that angiogenesis and vasculogenesis of the pulmonary circulation and capillary network are closely linked with and may be necessary for lung epithelial morphogenesis. Like epithelial morphogenesis, pulmonary vascularization is subject to a fine balance between positive and negative factors. Angiogenic and vasculogenic factors include VEGF, which signals through cognate receptors flk and flt, while novel anti-angiogenic factors include EMAP II.

Mesenchyme with fgf-10 expression is responsible for regenerative capacity in Xenopus limb buds

A young tadpole of an anuran amphibian can completely regenerate an amputated limb, and it exhibits an ontogenetic decline in the ability to regenerate its limbs. However, whether mesenchymal or epidermal tissue is responsible for this decrease of the capacity remains unclear. Moreover, little is known about the molecular interactions between these two tissues during regeneration. The results of this study showed that fgf-10 expression in the limb mesenchymal cells clearly corresponds to the regenerative capacity and that fgf-10 and fgf-8 are synergistically reexpressed in regenerating blastemas. However, neither fgf-10 nor fgf-8 is reexpressed after amputation of a nonregenerative limb. Nevertheless, nonregenerative epidermal tissue can reexpress fgf-8 under the influence of regenerative mesenchyme, as was demonstrated by experiments using a recombinant limb composed of regenerative limb mesenchyme and nonregenerative limb epidermis. Taken together, our data demonstrate that the regenerative capacity depends on mesenchymal tissue and suggest that fgf-10 is likely to be involved in this capacity.

Abnormal timing in the prenatal ossification of vertebral column and hand in Crouzon syndrome

We report on a radiographically examined fetus (gestational age 13 weeks) with Crouzon syndrome caused by a mutation in the gene encoding the fibroblast growth factor 2 (FGFR2). We found an approximately 2-week delay in vertebral body and hand ossification with normal vertebral arch ossification, suggesting that regionally delayed skeletal maturation might be a manifestation of FGFR2 mutation syndromes. The findings support other studies indicating that different signaling pathways control skeletal maturation in vertebral bodies and vertebral arches.

Up-regulation of the chondrogenic Sox9 gene by fibroblast growth factors is mediated by the mitogen-activated protein kinase pathway

Recent experiments have established that Sox9 is required for chondrocyte differentiation. Here, we show that fibroblast growth factors (FGFs) markedly enhance Sox9 expression in mouse primary chondrocytes as well as in C3H10T1/2 cells that express low levels of Sox9. FGFs also strongly increase the activity of a Sox9-dependent chondrocyte-specific enhancer in the gene for collagen type II. Transient transfection experiments using constructs encoding FGF receptors strongly suggested that all FGF receptors, FGFR1-R4, can transduce signals that lead to the increase in Sox9 expression. The increase in Sox9 levels induced by FGF2 was inhibited by a specific mitogen-activated protein kinase kinase (MAPKK)/mitogen-activated protein kinase/ERK kinase (MEK) inhibitor U0126 in primary chondrocytes. In addition, coexpression of a dual-specificity phosphatase, CL100/MKP-1, that is able to dephosphorylate and inactivate mitogen-activated protein kinases (MAPKs) inhibited the FGF2-induced increase in activity of the Sox9-dependent enhancer. Furthermore, coexpression of a constitutively active mutant of MEK1 increased the activity of the Sox9-dependent enhancer in primary chondrocytes and C3H10T1/2 cells, mimicking the effects of FGFs. These results indicate that expression of the gene for the master chondrogenic factor Sox9 is stimulated by FGFs in chondrocytes as well as in undifferentiated mesenchymal cells and strongly suggest that this regulation is mediated by the MAPK pathway. Because Sox9 is essential for chondrocyte differentiation, we propose that FGFs and the MAPK pathway play an important role in chondrogenesis.

Inhibition of chondrocyte differentiation in vitro by constitutive and inducible overexpression of the c-fos proto-oncogene

We have investigated the role of c-Fos in chondrocyte differentiation in vitro using both constitutive and inducible overexpression approaches in ATDC5 chondrogenic cells, which undergo a well-defined sequence of differentiation from chondroprogenitors to fully differentiated hypertrophic chondrocytes. Initially, we constitutively overexpressed exogenous c-fos in ATDC5 cells. Several stable clones expressing high levels of exogenous c-fos were isolated and those also expressing the cartilage marker type II collagen showed a marked decrease in cartilage nodule formation. To investigate further whether c-Fos directly regulates cartilage differentiation independently of potential clonal variation, we generated additional clones in which exogenous c-fos expression was tightly controlled by a tetracycline-regulatable promoter. Two clones, DT7.1 and DT12.4 were capable of nodule formation in the absence of c-fos. However, upon induction of exogenous c-fos, differentiation was markedly reduced in DT7.1 cells and was virtually abolished in clone DT12.4. Pulse experiments indicated that induction of c-fos only at early stages of proliferation/differentiation inhibited nodule formation, and limiting dilution studies suggested that overexpression of c-fos decreased the frequency of chondroprogenitor cells within the clonal population. Interestingly, rates of proliferation and apoptosis were unaffected by c-fos overexpression under standard conditions, suggesting that these processes do not contribute to the observed inhibition of differentiation. Finally, gene expression analyses demonstrated that the expression of the cartilage markers type II collagen and PTH/PTHrP receptor were down-regulated in the presence of exogenous c-Fos and correlated well with the differentiation status. Moreover, induction of c-fos resulted in the concomitant increase in the expression of fra-1 and c-jun, further highlighting the importance of AP-1 transcription factors in chondrocyte differentiation. These data demonstrate that c-fos overexpression directly inhibits chondrocyte differentiation in vitro, and therefore these cell lines provide very useful tools for identifying novel c-Fos-responsive genes that regulate the differentiation and activity of chondrocytes.

An important role for the IIIb isoform of fibroblast growth factor receptor 2 (FGFR2) in mesenchymal-epithelial signalling during mouse organogenesis

The fibroblast growth factor receptor 2 gene is differentially spliced to encode two transmembrane tyrosine kinase receptor proteins that have different ligand-binding specificities and exclusive tissue distributions. We have used Cre-mediated excision to generate mice lacking the IIIb form of fibroblast growth factor receptor 2 whilst retaining expression of the IIIc form. Fibroblast growth factor receptor 2(IIIb) null mice are viable until birth, but have severe defects of the limbs, lung and anterior pituitary gland. The development of these structures appears to initiate, but then fails with the tissues undergoing extensive apoptosis. There are also developmental abnormalities of the salivary glands, inner ear, teeth and skin, as well as minor defects in skull formation. Our findings point to a key role for fibroblast growth factor receptor 2(IIIb) in mesenchymal-epithelial signalling during early organogenesis.

Induction and differentiation of the zebrafish heart requires fibroblast growth factor 8 (fgf8/acerebellar)

Vertebrate heart development is initiated from bilateral lateral plate mesoderm that expresses the Nkx2.5 and GATA4 transcription factors, but the extracellular signals specifying heart precursor gene expression are not known. We describe here that the secreted signaling factor Fgf8 is expressed in and required for development of the zebrafish heart precursors, particularly during initiation of cardiac gene expression. fgf8 is mutated in acerebellar (ace) mutants, and homozygous mutant embryos do not establish normal circulation, although vessel formation is only mildly affected. In contrast, heart development, in particular of the ventricle, is severely abnormal in acerebellar mutants. Several findings argue that Fgf8 has a direct function in development of cardiac precursor cells: fgf8 is expressed in cardiac precursors and later in the heart ventricle. Fgf8 is required for the earliest stages of nkx2.5 and gata4, but not gata6, expression in cardiac precursors. Cardiac gene expression is restored in acerebellar mutant embryos by injecting fgf8 RNA, or by implanting a Fgf8-coated bead into the heart primordium. Pharmacological inhibition of Fgf signalling during formation of the heart primordium phenocopies the acerebellar heart phenotype, confirming that Fgf signaling is required independently of earlier functions during gastrulation. These findings show that fgf8/acerebellar is required for induction and patterning of myocardial precursors.

Tyrosine kinase signalling in breast cancer: fibroblast growth factors and their receptors

The fibroblast growth factors [Fgfs (murine), FGFs (human)] constitute a large family of ligands that signal through a class of cell-surface tyrosine kinase receptors. Fgf signalling has been associated in vitro with cellular differentiation as well as mitogenic and motogenic responses. In vivo, Fgfs are critical for animal development, and some have potent angiogenic properties. Several Fgfs have been identified as oncogenes in murine mammary cancer, where their deregulation is associated with proviral insertions of the mouse mammary tumour virus (MMTV). Thus, in some mammary tumours of MMTV-infected mouse strains, integration of viral genomic DNA into the somatic DNA of mammary epithelial cells was found to have caused the inappropriate expression of members of this family of growth factors. Although examination of human breast cancers has shown an altered expression of FGFs or of their receptors in some tumours, their role in the causation of breast disease is unclear and remains controversial.

The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation

A point mutation, Gly380Arg, in the transmembrane domain of fibroblast growth factor receptor 3 (FGFR3) leads to achondroplasia, the most common form of genetic dwarfism in humans. This substitution was suggested to enhance mutant receptor dimerization, leading to constitutive, ligand-independent activation. We found that dimerization and activation of the G380R mutant receptor are predominantly ligand dependent. However, using both transient and stable transfections, we found significant overexpression only of the mutant receptor protein. Metabolic pulse-chase experiments, cell surface labeling, and kinetics of uptake of radiolabeled ligand demonstrated a selective delay in the down-regulation of the mutant receptor. Moreover, this receptor was now resistant to ligand-mediated internalization, even at saturating ligand concentrations. Finally, transgenic mice expressing the human G380R mutant receptor under the mouse receptor transcriptional control demonstrated a markedly expanded area of FGFR3 immunoreactivity within their epiphyseal growth plates, compatible with an in vivo defect in receptor down-regulation. We propose that the achondroplasia mutation G380R uncouples ligand-mediated receptor activation from down-regulation at a site where the levels and kinetics of FGFR3 signals are crucial for chondrocyte maturation and bone formation.

Mapping ligand binding domains in chimeric fibroblast growth factor receptor molecules. Multiple regions determine ligand binding specificity

Fibroblast growth factors (FGFs) mediate essential cellular functions by activating one of four alternatively spliced FGF receptors (FGFRs). To determine the mechanism regulating ligand binding affinity and specificity, soluble FGFR1 and FGFR3 binding domains were compared for activity. FGFR1 bound well to FGF2 but poorly to FGF8 and FGF9. In contrast, FGFR3 bound well to FGF8 and FGF9 but poorly to FGF2. The differential ligand binding specificity of these two receptors was exploited to map specific ligand binding regions in mutant and chimeric receptor molecules. Deletion of immunoglobulin-like (Ig) domain I did not effect ligand binding, thus localizing the binding region(s) to the distal two Ig domains. Mapping studies identified two regions that contribute to FGF binding. Additionally, FGF2 binding showed positive cooperativity, suggesting the presence of two binding sites on a single FGFR or two interacting sites on an FGFR dimer. Analysis of FGF8 and FGF9 binding to chimeric receptors showed that a broad region spanning Ig domain II and sequences further N-terminal determines binding specificity for these ligands. These data demonstrate that multiple regions of the FGFR regulate ligand binding specificity and that these regions are distinct with respect to different members of the FGF family.

A role for p75 neurotrophin receptor in the control of hair follicle morphogenesis

During hair follicle (HF) morphogenesis, p75 neurotrophin receptor (p75NTR) reportedly is the first growth factor receptor found to be expressed by those fibroblasts that later develop into the dermal papilla (DP) of the HF. However, the functional role of p75NTR in HF morphogenesis is still unknown. Studying HF development in fetal and neonatal C57BL/6 murine back skin, we show that p75NTR-immunoreactivity (IR) is prominently expressed by DP fibroblasts as well as by skin nerves during the early steps of HF development. In contrast, p75NTR-IR disappears from the DP in the fully developed HF and it is expressed only in the epithelial outer root sheath of the HF. Compared to age-matched wild-type animals, p75NTR knockout (-/-) mice show significant acceleration of HF morphogenesis, and DP fibroblasts of p75NTR knockout mice show reduced proliferative activity in situ, indicating alterations in their transition from proliferation to differentiation. Although no significant differences in the expression of adhesion molecules (NCAM), selected morphogens (TGFbeta-2, HGF/SF, FGF-2, KGF), or their receptors (TGFbetaR-II, m-met, FGFR-1) were seen between DP of p75NTR knockout and wild-type mice, p75NTR mutants showed a prominent upregulation of FGFR-2, a high-affinity receptor for KGF, in both follicular DP and epithelium. Furthermore, the administration of anti-KGF neutralizing antibody significantly inhibited acceleration of HF morphogenesis in p75NTR knockout mice in vivo. These observations suggest that p75NTR plays an important role during HF morphogenesis, functioning as a receptor that negatively controls HF development, most likely via alterations in DP fibroblast proliferation/differentiation and via downregulation of KGF/FGFR-2 signaling in the HF.

Expression and localization of fibroblast growth factors and fibroblast growth factor receptors in the developing rat kidney

Expression and localization of fibroblast growth factors and fibroblast growth factor receptors in the developing rat kidney.

BACKGROUND

The permanent kidney, or metanephros, develops through a complex series of reciprocal inductive events and involves branching morphogenesis, tubulogenesis, angiogenesis, and tissue remodeling. Fibroblast growth factors (FGFs) are a family of growth and differentiation factors that have been implicated in metanephric development. FGFs exert their actions through tyrosine kinase receptors, FGFRs, which are encoded by four FGFR genes (FGFR1 through FGFR4).

METHODS

Reverse transcriptase-polymerase chain reaction was used to detect the expression of FGFs and FGFRs in rat metanephroi from embryonic day (E) 14 to E21. Nonradioactive in situ hybridization was used to localize FGF1 mRNA in E20 rat metanephroi, and immunohistochemistry was used to localize FGFRs in E15 and E20 rat metanephroi.

RESULTS

We detected the expression of mRNAs for FGF1 through FGF5, FGF7 through FGF10, and FGFR1 through FGFR4 (IIIb and IIIc splice variants) in rat metanephroi from E14 to E21. By in situ hybridization, FGF1 mRNA was detected in the nephrogenic zone, ureteric epithelium, and developing nephron elements. FGFR proteins were localized in a distinct pattern that altered with maturation. FGFR1 was widely distributed in developing metanephric epithelia and mesenchyme, but not in developing interstitium. FGFR2 was also widely distributed in nephron epithelia, particularly in proximal convoluted tubules, but was not detected in metanephric mesenchyme, mesenchymal condensates, or developing interstitium. FGFR3 was localized to mesenchymal condensates, nephron elements, and medullary interstitium but not proximal convoluted tubules. FGFR4 was localized mostly to maturing nephron structures and was not detected in nephrogenic mesenchyme, mesenchymal condensates, or developing interstitium.

CONCLUSIONS

These results indicate that FGFs and FGFRs are expressed in the developing rat metanephros from at least E14 and that they likely play important roles in metanephric development and maturation.