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

Stomach development is dependent on fibroblast growth factor 10/fibroblast growth factor receptor 2b-mediated signaling

BACKGROUND & AIMS

Fibroblast growth factors (Fgfs) and their receptors (Fgfrs) are important intercellular signaling molecules that are essential to mammalian embryonic development. The signaling pathways between endoderm-derived gastric epithelium and the surrounding mesenchyme are largely unknown; however, the developmental expression profile of the IIIb isoform of Fgfr2 (Fgfr2b) and its main ligand, Fgf10, suggest that they may be strong candidates. Mice lacking either component (Fgfr2b-/- or Fgf10-/-) were examined to determine the role of Fgfr2b-mediated signaling during gastric organogenesis.

METHODS

Stomachs from embryonic day 13.5-18.5 Fgfr2b-/-, Fgf10-/-, and wild-type littermates were collected and analyzed by conventional histology, immunohistochemistry, in situ hybridization, and electron microscopy.

RESULTS

Fgfr2b-/- and Fgf10-/- fetuses had stomachs smaller than wild-type, consisting of relatively proportionate forestomach but disproportionately reduced glandular stomach, the mucosa of which has low cytoarchitectural complexity with a spiral arrangement of large mucosal folds. During mid to late fetal stages (embryonic day 15.5-18.5), epithelial differentiation to mucous and chief cell lineages was rudimentary, with no expression of several early cytodifferentiation markers including GATA4, GATA6, and H+/K+-adenosine triphosphatase and abnormal expression of members of the hedgehog family of signaling molecules.

CONCLUSIONS

Fgfr2b and Fgf10 are part of a signaling network with Sonic hedgehog and Indian hedgehog that are essential to anterior-posterior and radial patterning in gastric development.

Crkl deficiency disrupts Fgf8 signaling in a mouse model of 22q11 deletion syndromes

Deletions on chromosome 22q11.21 disrupt pharyngeal and cardiac development and cause DiGeorge and related human syndromes. CRKL (CRK-Like) lies within 22q11.21, and Crkl-/- mice have phenotypic features of 22q11 deletion (del22q11) syndromes. While human FGF8 does not localize to 22q11, deficiency of Fgf8 also generates many features of del22q11 syndrome in mice. Since Fgf8 signals via receptor-type tyrosine kinases, and Crk family adaptor proteins transduce intracellular signals downstream of tyrosine kinases, we investigated whether Crkl mediates Fgf8 signaling. In addition to discovering genetic interactions between Crkl and Fgf8 during morphogenesis of structures affected in del22q11 syndrome, we found that Fgf8 induces tyrosine phosphorylation of FgfRs 1 and 2 and their binding to Crkl. Crkl is required for normal cellular responses to Fgf8, including survival and migration, Erk activation, and target gene expression. These findings provide mechanistic insight into disrupted intercellular interactions in the pathogenesis of malformations seen in del22q11 syndrome.

Role of fibroblast growth factor receptors 1 and 2 in the metanephric mesenchyme

To determine the importance of fibroblast growth factor receptors (fgfrs) 1 and 2 in the metanephric mesenchyme, we generated conditional knockout mice (fgfr(Mes-/-)). Fgfr1(Mes-/-) and fgfr2(Mes-/-) mice develop normal-appearing kidneys. Deletion of both receptors (fgfr1/2(Mes-/-)) results in renal aplasia. Fgfr1/2(Mes-/-) mice develop a ureteric bud (and occasionally an ectopic bud) that does not elongate or branch, and the mice do not develop an obvious metanephric mesenchyme. By in situ hybridization, regions of mutant mesenchyme near the ureteric bud(s) express Eya1 and Six1, but not Six2, Sall1, or Pax2, while the ureteric bud expresses Ret and Pax2 normally. Abnormally high rates of apoptosis and relatively low rates of proliferation are present in mutant mesenchyme dorsal to the mutant ureteric bud at embryonic day (E) 10.5, while mutant ureteric bud tissues undergo high rates of apoptosis by E11.5. Thus, fgfr1 and fgfr2 together are critical for normal formation of metanephric mesenchyme. While the ureteric bud(s) initiates, it does not elongate or branch infgfr1/2(Mes-/-) mice. In metanephric mesenchymal rudiments, fgfr1 and fgfr2 appear to function downstream of Eya1 and Six1, but upstream of Six2, Sall1, and Pax2. Finally, this is the first example of renal aplasia in a conditional knockout model.

Transient exposure of fibroblast growth factor-2 induced proliferative but not destructive changes in mouse knee joints

Fibroblast growth factor-2 (FGF-2) has the potential to regenerate damaged articular cartilage tissue due to its exerting anabolic effects on chondrocytes. However, FGF-2 is involved in pathogenesis of rheumatoid arthritis, where the joint is destructed. The study aims at clarifying the effects of FGF-2 on joints. When radiolabeled FGF-2 was injected into knee joints of C57Bl/10 mice, a transient binding was observed in the superficial and intermediate zones of the articular cartilage as well as in the synovium and perichondrium. An FGF-2 injection (5 microg) caused synovial hyperplasia adjacent to the articular cartilage on day 7, cartilage formation adjacent to the articular cartilage on day 14, and osteophyte on day 21. The intensity of safranin-O staining of the articular cartilage increased on day 14. These changes were dose-dependent. No destructive changes in the joints were observed. In a joint, transient exposure of FGF-2 caused proliferative changes, but not destructive changes.

Enhancement by growth factors of cardiac myocyte differentiation from embryonic stem cells: A promising foundation for cardiac regeneration

Cell transplantation is a promising, still novel, potentially therapeutic approach for the treatment of heart diseases. Clinical applications require generation of large number of donor cells. Embryonic stem (ES) cells are capable of self-renewal apparently in an unlimited fashion, in vitro. Theoretically, they can differentiate into any cell type required for cell transplantation, including cardiac myocytes. Diverse growth factors have been implicated in programming diverse cellular processes, including development of the embryonic heart, ES cell self-renewal, and cardiac myocyte differentiation from ES cells. This review addresses the current understanding of the role of growth factors in the differentiation of cardiac myocytes from ES-embryoid body cell systems in vitro as well as cardiac regeneration in vivo.

Neural adhesion molecules L1 and CHL1 are survival factors for motoneurons

Many neurotrophic factors with survival activity for motoneurons in vivo were first identified using cultures of purified embryonic motoneurons. The L1 neural cell adhesion molecule has multiple roles in brain development. We showed by in situ hybridization and RT-PCR that L1 mRNA was expressed at significant levels in motoneurons of embryonic and postnatal spinal cord. We therefore cultured purified motoneurons from E14 rat embryos in the absence of trophic factors but with L1-Fc and CHL1-Fc fusion proteins. L1-Fc prevented the death of approximately half of the motoneurons that were saved by BDNF in a dose-dependent manner (EC50 = 10 pM). CHL1-Fc saved the same number of motoneurons as did L1-Fc, whereas P0-Fc had little neurotrophic activity at the same concentrations. Survival induced by L1 and CHL1 was completely inhibited by 20 microM LY294002 and PD98059, indicating that both MEK and PI3K pathways are required for signaling by these molecules. L1 can signal in other cell types through the FGF receptor FGFR1. In cultures of motoneurons, effects of suboptimal concentrations of L1 and suboptimal concentrations of FGF-2 were additive, but the effects of optimal concentrations of FGF-2 (50 ng/ml) were not further increased in the presence of L1-Fc. Thus, in this system, too, FGF and L1 may use similar signaling pathways.

Fibroblast Growth Factor (FGF) 18 Signals through FGF Receptor 3 to Promote Chondrogenesis

Signaling by fibroblast growth factor (FGF) 18 and FGF receptor 3 (FGFR3) have been shown to regulate proliferation, differentiation, and matrix production of articular and growth plate chondrocytes in vivo and in vitro. Notably, the congenital absence of either FGF18 or FGFR3 resulted in similar expansion of the growth plates of fetal mice and the addition of FGF18 to human articular chondrocytes in culture enhanced proliferation and matrix production. Based on these and other experiments it has been proposed that FGF18 signals through FGFR3 to promote cartilage production by chondrocytes. Its role in chondrogenesis remains to be defined. In the current work we used the limb buds of FGFR3(+/+) and FGFR3(-/-) embryonic mice as a source of mesenchymal cells to determine how FGF18 signaling affects chondrogenesis. Confocal laser-scanning microscopy demonstrated impaired cartilage nodule formation in the FGFR3(-/-) cultures. Potential contributing factors to the phenotype were identified as impaired mitogenic response to FGF18, decreased production of type II collagen and proteoglycan in response to FGF18 stimulation, impaired interactions with the extracellular matrix resulting from altered integrin receptor expression, and altered expression of FGFR1 and FGFR2. The data identified FGF18 as a selective ligand for FGFR3 in limb bud mesenchymal cells, which suppressed proliferation and promoted their differentiation and production of cartilage matrix. This work, thus, identifies FGF18 and FGFR3 as potential molecular targets for intervention in tissue engineering aimed at cartilage repair and regeneration of damaged cartilage.

Regulation of VEGF and bFGF mRNA expression and other proliferative compounds in skeletal muscle cells

The role of muscle contraction, prostanoids, nitric oxide and adenosine in the regulation of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF) and endothelial cell proliferative compounds in skeletal muscle cell cultures was examined. VEGF and bFGF mRNA, protein release as well as the proliferative effect of extracellular medium was determined in non-stimulated and electro-stimulated rat and human skeletal muscle cells. In rat skeletal muscle cells these aspects were also determined after treatment with inhibitors and/or donors of nitric oxide (NO), prostanoids and adenosine. Electro-stimulation caused an elevation in the VEGF and bFGF mRNA levels of rat muscle cells by 33% and 43% (P < 0.05), respectively, and in human muscle cells VEGF mRNA was elevated by 24%. Medium from electro-stimulated human, but not rat muscle cells induced a 126% higher (P < 0.05) endothelial cell proliferation than medium from non-stimulated cells. Cyclooxygenase inhibition of rat muscle cells induced a 172% increase (P < 0.05) in VEGF mRNA and a 104% increase in the basal VEGF release. Treatment with the NO donor SNAP (0.5 microM) decreased (P < 0.05) VEGF and bFGF mRNA by 42 and 38%, respectively. Medium from SNAP treated muscle cells induced a 45% lower (P < 0.05) proliferation of endothelial cells than control medium. Adenosine enhanced the basal VEGF release from muscle cells by 75% compared to control. The present data demonstrate that contractile activity, NO, adenosine and products of cyclooxygenase regulate the expression of VEGF and bFGF mRNA in skeletal muscle cells and that contractile activity and NO regulate endothelial cell proliferative compounds in muscle extracellular fluid.

Pathological angiogenesis is reduced by targeting pericytes via the NG2 proteoglycan

The NG2 proteoglycan is expressed by nascent pericytes during the early stages of angiogenesis. To investigate the functional role of NG2 in neovascularization, we have compared pathological retinal and corneal angiogenesis in wild type and NG2 null mice. During ischemic retinal neovascularization, ectopic vessels protruding into the vitreous occur twice as frequently in wild type retinas as in NG2 null retinas. In the NG2 knock-out retina, proliferation of both pericytes and endothelial cells is significantly reduced, and the pericyte:endothelial cell ratio falls to 0.24 from the wild type value of 0.86. Similarly, bFGF-induced angiogenesis is reduced more than four-fold in the NG2 null cornea compared to that seen in the wild type retina. Significantly, NG2 antibody is effective in reducing angiogenesis in the wild type cornea, suggesting that the proteoglycan can be an effective target for anti-angiogenic therapy. These experiments therefore demonstrate both the functional importance of NG2 in pericyte development and the feasibility of using pericytes as anti-angiogenic targets.

FGF signaling in the developing endochondral skeleton

Mutations in fibroblast growth factor receptors (Fgfrs) are the etiology of many craniosynostosis and chondrodysplasia syndromes in humans. The phenotypes associated with these human syndromes and the phenotypes resulting from targeted mutagenesis in the mouse have defined essential roles for FGF signaling in both endochondral and intramembranous bone development. In this review, I will focus on the role of FGF signaling in chondrocytes and osteoblasts and how FGFs regulate the growth and development of endochondral bone.

Role of fibroblast growth factor receptors 1 and 2 in the ureteric bud

Fibroblast growth receptors (FGFRs) consist of four signaling family members. Mice with deletions of fgfr1 or fgfr2 are embryonic lethal prior to the onset of kidney development. To determine roles of FGFR1 and FGFR2 in the ureteric bud, we used a conditional targeting approach. First, we generated transgenic mice using the Hoxb7 promoter to drive cre recombinase and green fluorescent protein expression throughout ureteric bud tissue. We crossed Hoxb7creEGFP mice with mice carrying lox-p sites flanking critical regions of fgfr1 and/or fgfr2. Absence of fgfr1 from the ureteric bud (fgfr1(UB-/-)) results in no apparent renal abnormalities. In contrast, fgfr2(UB-/-) mice have very aberrant ureteric bud branching, thin ureteric bud stalks, and fewer ureteric bud tips. Fgfr2(UB-/-) ureteric bud tips also demonstrate inappropriate regions of apoptosis and reduced proliferation. The nephrogenic mesenchymal lineage in fgfr2(UB-/-) mice develops normal-appearing glomeruli and tubules, and only slightly fewer nephrons than controls. In contrast, fgfr2(UB-/-) kidneys have abnormally thickened subcapsular cortical stromal mesenchyme. Ultimately, fgfr2(UB-/-) adult kidneys are small and abnormally shaped or are hydronephrotic. Finally, there are no additional abnormalities in the fgfr1/2(UB-/-) kidneys versus the fgfr2(UB-/-) kidneys. In conclusion, FGFR2, but not FGFR1, appears crucial for ureteric bud branching morphogenesis and stromal mesenchyme patterning.

Immunolocalization of fibroblast growth factor-2 (FGF-2) in the developing root and supporting structures of the murine tooth

Epithelio-mesenchymal interactions are active during the development of the root of the tooth and are regulated by a variety of growth factors, such as fibroblast growth factors. FGF-2, 3, 4, and 8 have all been shown to play a role in the development of the crown of the tooth, but less is known about the factors that govern root formation, particularly FGF-2. The aim of this study was thus to elucidate the spatial and temporal expression of FGF-2 in the root of the developing tooth, as this growth factor is believed to be a mediator of epithelio-mesenchymal interactions. Parasagittal sections of the maxillary and mandibular arches of post-natal mice were utilized and the roots of the molar teeth were studied. Immunocytochemistry utilizing an antibody to FGF-2 was performed on sections of teeth at various stages of development. Intense immunostaining for FGF-2 was observed in differentiating odontoblasts at the apical end of the tooth and in the furcation zone of the developing root at all the stages examined. FGF-2 localization was also observed in cementoblasts on post-natal days 16, 20 and 24. The pattern of localization of FGF-2 in the developing root suggests that this growth factor may participate in the signaling network associated with root development.

Mutations that cause osteoglophonic dysplasia define novel roles for FGFR1 in bone elongation

Activating mutations in the genes for fibroblast growth factor receptors 1-3 (FGFR1-3) are responsible for a diverse group of skeletal disorders. In general, mutations in FGFR1 and FGFR2 cause the majority of syndromes involving craniosynostosis, whereas the dwarfing syndromes are largely associated with FGFR3 mutations. Osteoglophonic dysplasia (OD) is a "crossover" disorder that has skeletal phenotypes associated with FGFR1, FGFR2, and FGFR3 mutations. Indeed, patients with OD present with craniosynostosis, prominent supraorbital ridge, and depressed nasal bridge, as well as the rhizomelic dwarfism and nonossifying bone lesions that are characteristic of the disorder. We demonstrate here that OD is caused by missense mutations in highly conserved residues comprising the ligand-binding and transmembrane domains of FGFR1, thus defining novel roles for this receptor as a negative regulator of long-bone growth.

The pre-Mendelian, pre-Darwinian world: Shifting relations between genetic and epigenetic mechanisms in early multicellular evolution

The reliable dependence of many features of contemporary organisms on changes in gene content and activity is tied to the processes of Mendelian inheritance and Darwinian evolution. With regard to morphological characters, however, Mendelian inheritance is the exception rather than the rule, and neo-Darwinian mechanisms in any case do not account for the origination (as opposed to the inherited variation) of such characters. It is proposed, therefore, that multicellular organisms passed through a pre-Mendelian, pre-Darwinian phase, whereby cells, genes and gene products constituted complex systems with context-dependent, self-organizing morphogenetic capabilities. An example is provided of a plausible 'core' mechanism for the development of the vertebrate limb that is both inherently pattern forming and morphogenetically plastic. It is suggested that most complex multicellular structures originated from such systems. The notion that genes are privileged determinants of biological characters can only be sustained by neglecting questions of evolutionary origination and the evolution of developmental mechanisms.

Unraveling the molecular pathways that regulate early telencephalon development

The telencephalon, at the rostral end of the developing central nervous system, starts off as a sheet of neuroepithelial cells. During development, this sheet of cells becomes patterned and morphologically partitioned into areas that give rise to the adult cerebral hemispheres. How does this happen? How are telencephalic precursor cells instructed to generate myriad neural cell types in different areas and at different times as well as to change their rates of cell proliferation, differentiation, and death? The molecular pathways required for patterning the telencephalic neuroepithelium and forming the cerebral hemispheres are beginning to be unraveled.

Origination and innovation in the vertebrate limb skeleton: an epigenetic perspective

The vertebrate limb has provided evolutionary and developmental biologists with grist for theory and experiment for at least a century. Its most salient features are its pattern of discrete skeletal elements, the general proximodistal increase in element number as development proceeds, and the individualization of size and shape of the elements in line with functional requirements. Despite increased knowledge of molecular changes during limb development, however, the mechanisms for origination and innovation of the vertebrate limb pattern are still uncertain. We suggest that the bauplan of the limb is based on an interplay of genetic and epigenetic processes; in particular, the self-organizing properties of precartilage mesenchymal tissue are proposed to provide the basis for its ability to generate regularly spaced nodules and rods of cartilage. We provide an experimentally based "core" set of cellular and molecular processes in limb mesenchyme that, under realistic conditions, exhibit the requisite self-organizing behavior for pattern origination. We describe simulations that show that under limb bud-like geometries the core mechanism gives rise to skeletons with authentic proximodistal spatiotemporal organization. Finally, we propose that evolution refines skeletal templates generated by this process by mobilizing accessory molecular and biomechanical regulatory processes to shape the developing limb and its individual elements. Morphological innovation may take place when such modulatory processes exceed a threshold defined by the dynamics of the skeletogenic system and elements are added or lost.

Co-ordination of TGF-beta and FGF signaling pathways in bone organ cultures

Transforming growth factor-beta (TGF-beta) is known to regulate chondrocyte proliferation and hypertrophic differentiation in embryonic bone cultures by a perichondrium dependent mechanism. To begin to determine which factors in the perichondrium mediate the effects of TGF-beta, we studied the effect of Insulin-like Growth Factor-1 (IGF-I) and Fibroblast Growth Factors-2 and -18 (FGF2, FGF18) on metatarsal organ cultures. An increase in chondrocyte proliferation and hypertrophic differentiation was observed after treatment with IGF-I. A similar effect was seen after the perichondrium was stripped from the metatarsals suggesting IGF-I acts directly on the chondrocytes. Treatment with FGF-2 or FGF-18 resulted in a decrease in bone elongation as well as hypertrophic differentiation. Treatment also resulted in a decrease in BrdU incorporation into chondrocytes and an increase in BrdU incorporation in perichondrial cells, similar to what is seen after treatment with TGF-beta1. A similar effect was seen with FGF2 after the perichondrium was stripped suggesting that, unlike TGF-beta, FGF2 acts directly on chondrocytes to regulate proliferation and hypertrophic differentiation. To test the hypothesis that TGF-beta regulates IGF or FGF signaling, activation of the receptors was characterized after treatment with TGF-beta. Activation was measured as the level of tyrosine phosphorylation on the receptor. Treatment with TGF-beta for 24h did not alter the level of IGFR-I tyrosine phosphorylation. In contrast, treatment with TGF-beta resulted in and increase in tyrosine phosphorylation on FGFR3 without alterations in total FGFR3 levels. TGF-beta also stimulated expression of FGF18 mRNA in the cultures and the effects of TGF-beta on metatarsal development were blocked or partially blocked by pretreatment with FGF signaling inhibitors. The results suggest a model in which FGF through FGFR3 mediates some of the effects of TGF-beta on embryonic bone formation.

Distinct transcriptional control and action of fibroblast growth factor receptor 4 in differentiating skeletal muscle cells

Although FGF signaling promotes myoblast proliferation and represses myogenic differentiation, one of the FGF receptors (FGFR), FGFR4, is expressed mainly in mature skeletal muscle. Disruption of FGFR4 signaling interrupts chick limb muscle formation. To determine the developmental regulation of FGFR4 expression, we compared the transcriptional control and action of FGFR4 in myoblasts and myotubes. We identified higher FGFR4 expression in differentiated myotubes than precursor myoblasts. FGFR4 promoter activity was localized within a region 115 bp upstream of the transcription start site. Overlapping fragments of this promoter displayed a distinct difference when compared by electromobility shift assay (EMSA) using nuclear extracts from myoblasts and myotubes. While fragments B (-95/-56) and C (-65/-26) formed specific complexes in both cell types, these complexes were consistently more intense in myotubes than myoblasts. These complexes were efficiently competed by an Sp-type oligonucleotide and were supershifted by Sp1 and by Sp3 antibodies. Deletions of the Sp-binding sites in fragment B (-95/-56) confirmed their critical contribution to promoter activity. Moreover, Sp1 expression correlated with FGFR4-expression in myotubes. To determine whether FGFR4 expression regulates myoblast differentiation, we infected a soluble dominant-negative FGFR4-containing adenovirus into these cells. This significantly impeded Erk1/2 phosphorylation and differentiation of myoblasts into MHC-expressing myotubes. Our findings point to distinct transcriptional regulation and action for FGFR4 in differentiating skeletal muscle cells.

Dynamical mechanisms for skeletal pattern formation in the vertebrate limb

We describe a 'reactor-diffusion' mechanism for precartilage condensation based on recent experiments on chondrogenesis in the early vertebrate limb and additional hypotheses. Cellular differentiation of mesenchymal cells into subtypes with different fibroblast growth factor (FGF) receptors occurs in the presence of spatio-temporal variations of FGFs and transforming growth factor-betas (TGF-betas). One class of differentiated cells produces elevated quantities of the extracellular matrix protein fibronectin, which initiates adhesion-mediated preskeletal mesenchymal condensation. The same class of cells also produces an FGF-dependent laterally acting inhibitor that keeps condensations from expanding beyond a critical size. We show that this 'reactor-diffusion' mechanism leads naturally to patterning consistent with skeletal form, and describe simulations of spatio-temporal distribution of these differentiated cell types and the TGF-beta and inhibitor concentrations in the developing limb bud.

Zebrafish fgfr1 is a member of the fgf8 synexpression group and is required for fgf8 signalling at the midbrain-hindbrain boundary

FGFR1 is an important signalling molecule during embryogenesis and in adulthood. FGFR1 mutations in human may lead to developmental defects and pathological conditions, including cancer and Alzheimer's disease. Here, we describe cloning and expression analysis of the zebrafish fibroblast growth factor receptor 1 ( fgfr1). Initially, fgfr1 is expressed in the adaxial mesoderm with transcripts distinctly localised to the anterior portion of each half-somite. Hereupon, fgfr1 is also strongly expressed in the otic vesicles, branchial arches and the brain, especially at the midbrain-hindbrain boundary (MHB). The expression patterns of fgfr1 and fgf8 are strikingly similar and knock-down of fgfr1 phenocopies many aspects observed in the fgf8 mutant acerebellar, suggesting that Fgf8 exerts its function mainly by binding to FgfR1.

Control and regulation of pulmonary hypoplasia associated with congenital diaphragmatic hernia

Control of fetal lung growth and development is exquisitely orchestrated and regulated. Branching morphogenesis is carefully choreographed with cell growth, proliferation, differentiation, and apoptosis in a spatially and temporally dependent manner. Some of the signals and pathways mediating these events have recently been uncovered, but much remains unknown. The precise etiologic derangements that give rise to pulmonary hypoplasia in congenital diaphragmatic hernia remain elusive. Some clues have been discovered in developmental and signaling pathways that include receptor tyrosine kinase growth factors, homeobox genes, transcription factors, airway distension, retinoid signaling, and oxidation-reduction.

Perlecan and tumor angiogenesis

Perlecan is a major heparan sulfate proteoglycan (HSPG) of basement membranes (BMs) and connective tissues. The core protein of perlecan is divided into five domains based on sequence homology to other known proteins. Commonly, the N-terminal domain I of mammalian perlecan is substituted with three HS chains that can bind a number of matrix molecules, cytokines, and growth factors. Perlecan is essential for metazoan life, as shown by genetic manipulations of nematodes, insects, and mice. There are also known human mutations that can be lethal. In vertebrates, new functions of perlecan emerged with the acquisition of a closed vascular system and skeletal connective tissues. Many of perlecan's functions may be related to the binding and presentation of growth factors to high-affinity tyrosine kinase (TK) receptors. Data are accumulating, as discussed here, that similar growth factor-mediated processes may have unwanted promoting effects on tumor cell proliferation and tumor angiogenesis. Understanding of these attributes at the molecular level may offer opportunities for therapeutic intervention.

Roles of FGF receptors in mammalian development and congenital diseases

Four fibroblast growth factor receptors (FGFR1-4) constitute a family of transmembrane tyrosine kinases that serve as high affinity receptors for at least 22 FGF ligands. Gene targeting in mice has yielded valuable insights into the functions of this important gene family in multiple biological processes. These include mesoderm induction and patterning; cell growth, migration, and differentiation; organ formation and maintenance; neuronal differentiation and survival; wound healing; and malignant transformation. Furthermore, discoveries that mutations in three of the four receptors result in more than a dozen human congenital diseases highlight the importance of these genes in skeletal development. In this review, we will discuss recent progress on the roles of FGF receptors in mammalian development and congenital diseases, with an emphasis on signal transduction pathways.

Systemic and local regulation of the growth plate

The growth plate is the final target organ for longitudinal growth and results from chondrocyte proliferation and differentiation. During the first year of life, longitudinal growth rates are high, followed by a decade of modest longitudinal growth. The age at onset of puberty and the growth rate during the pubertal growth spurt (which occurs under the influence of estrogens and GH) contribute to sex difference in final height between boys and girls. At the end of puberty, growth plates fuse, thereby ceasing longitudinal growth. It has been recognized that receptors for many hormones such as estrogen, GH, and glucocorticoids are present in or on growth plate chondrocytes, suggesting that these hormones may influence processes in the growth plate directly. Moreover, many growth factors, i.e., IGF-I, Indian hedgehog, PTHrP, fibroblast growth factors, bone morphogenetic proteins, and vascular endothelial growth factor, are now considered as crucial regulators of chondrocyte proliferation and differentiation. In this review, we present an update on the present perception of growth plate function and the regulation of chondrocyte proliferation and differentiation by systemic and local regulators of which most are now related to human growth disorders.

Expression of fibroblast growth factor receptor-3 (FGFR3), signal transducer and activator of transcription-1, and cyclin-dependent kinase inhibitor p21 during endochondral ossification: differential role of FGFR3 in skeletal development and fracture repair

Increasing evidence suggests that fibroblast growth factor receptor-3 (FGFR3) is a negative regulator of endochondral bone growth; however, its role during skeletal repair is unknown. Using a rat model of closed femoral fracture healing, we analyzed the spatial and temporal expression of FGFR3. To assess a possible role for FGFR3 during healing, we also analyzed the spatial and temporal expression of signal transducer and activator of transcription-1 (STAT1) and cyclin-dependent kinase inhibitor p21, important mediators of FGFR3 signaling. Before these experiments, we studied the spatial expression of FGFR3 during skeletal development in mouse embryos. At 16.5 and 19.5 d post coitum, FGFR3 mRNA was strongly expressed in resting and proliferating chondrocytes but weakly in hypertrophic chondrocytes and not in osteoblasts. In contrast, during fracture repair, it was strongly expressed in prehypertrophic chondrocytes, and the expression level reached a maximum on d 14. Immunoreactivity for STAT1 was detected in the cytoplasm of chondrocytes on d 4 and 7 and both in the cytoplasm and nucleus of hypertrophic chondrocytes on d 14. Furthermore, FGFR3, STAT1, and p21 exhibited a similar temporal expression profile, suggesting that FGFR3-mediated STAT1-p21 signaling plays a role in fracture repair. These results indicate a differential role of FGFR3 in skeletal development and fracture repair.

Expression of mouse fibroblast growth factor and fibroblast growth factor receptor genes during early inner ear development

The inner ear, which mediates hearing and equilibrium, develops from an ectodermal placode located adjacent to the developing hindbrain. Induction of the placode and its subsequent morphogenesis and differentiation into the inner ear epithelium and its sensory neurons, involves signalling interactions within and between otic and non-otic tissues. Several members of the fibroblast growth factor (FGF) family play important roles at various stages of otic development; however, there are additional family members that have not been evaluated. In this study, we surveyed the expression patterns of 18 mouse Fgf and 3 Fgf receptor (Fgfr) genes during early otic development. Two members of the Fgf family, Fgf4 and Fgf16, and all three tested members of the Fgfr family, Fgfr2c, Fgfr3c, and Fgfr4, were expressed in tissues relevant to inner ear development. Fgf4 transcripts were expressed in the preplacodal and placodal ectoderm, suggesting potential roles in placode induction and/or maintenance. Fgf16 was expressed in the posterior otic cup and vesicle, suggesting roles in otic cell fate decisions and/or axis formation.

Fibroblast growth factor receptor-1 is essential for in vitro cardiomyocyte development

Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling plays a crucial role in mesoderm formation and patterning. Heartless mutant studies in Drosophila suggest that FGFR1, among the different FGFRs, may play a role in cardiogenesis. However, fgfr1-/- mice die during gastrulation before heart formation. To establish the contribution of FGFR1 in cardiac development, we investigated the capacity of murine fgfr1+/- and fgfr1-/- embryonic stem (ES) cells to differentiate to cardiomyocytes in vitro. Clusters of pulsating cardiomyocytes were observed in >90% of 3-dimensional embryoid bodies (EBs) originated from fgfr1+/- ES cells at day 9 to 10 of differentiation. In contrast, 10% or less of fgfr1-/- EBs showed beating foci at day 16. Accordingly, fgfr1-/- EBs were characterized by impaired expression of early cardiac transcription factors Nkx2.5 and d-Hand and of late structural cardiac genes myosin heavy chain (MHC)-alpha, MHC-beta, and ventricular myosin light chain. Homozygous fgfr1 mutation resulted also in alterations of the expression of mesoderm-related early genes, including nodal, BMP2, BMP4, T(bra), and sonic hedgehog. Nevertheless, fgfr1+/- and fgfr1-/- EBs similarly express cardiogenic precursor, endothelial, hematopoietic, and skeletal muscle markers, indicating that fgfr1-null mutation exerts a selective effect on cardiomyocyte development in differentiating ES cells. Accordingly, inhibitors of FGFR signaling, including the FGFR1 tyrosine kinase inhibitor SU 5402, the MEK1/2 inhibitor U0126, and the protein kinase C inhibitor GF109 all prevented cardiomyocyte differentiation in fgfr1+/- EBs without affecting the expression of the hematopoietic/endothelial marker flk-1. In conclusion, the data point to a nonredundant role for FGFR1-mediated signaling in cardiomyocyte development.

Fgfr mRNA isoforms in craniofacial bone development

Mutations in genes encoding for fibroblast growth factor receptors (FGFRs) have been identified as causes of both chondrodysplasias and craniosynostoses, both of which cause abnormalities in the growth and development of the craniofacial region. FGFRs form mRNA splicing isoforms, each with distinct ligand binding specificity and tissue distribution. These confer specific biological functions on these isoforms. Although it is known that FGFRs are expressed at numerous locations during early mouse development, including the craniofacial area, relatively little is known about the expression of the splicing isoforms during craniofacial bone development. To address this, we have performed a detailed survey to detect these genes in the developing mouse craniofacial region. We have analyzed the developing mouse mandible, calvaria, and cranial base, in particular the spheno-occipital synchondrosis, a key centre of craniofacial growth. Fgfr1c was detected weakly in osteoblastic cells in both the developing calvarial and mandibular bones. Fgfr3b and Fgfr3c were found chiefly in proliferating chondrocytes of the cranial base synchondroses and the mandibular condyle. Fgfr2b transcripts were most notably detected in the perichondria of the mandibular condyle and the cranial base. Fgfr2c transcripts were detected with high intensity in differentiating osteoblasts at the sutural osteogenic fronts of the calvarial bones. In addition, Fgfr2c was also expressed in the perichondria of the mandibular condyle and the cranial base. These expression patterns suggest both differing and similar functions for -b and -c isoforms. The former is exemplified by Fgfr1 transcripts, which show distinct differences in their distribution, being mutually exclusive. Similar functions are suggested by the overlapping expression patterns of the -b and -c isoforms of both Fgfr2 and Fgfr3. Fgfr4 transcripts were found in developing muscles. These data help to explain the disturbances in craniofacial growth exhibited by both patients and the growing number of transgenic mice carrying mutations in genes encoding FGFRs/Fgfrs.

Tissue interactions pattern the mesenchyme of the embryonic mouse lung

The mechanisms that control proliferation and differentiation of embryonic lung mesenchyme are largely unknown. We describe an explant system in which exogenous recombinant N-Sonic Hedgehog (N-Shh) protein sustains the survival and proliferation of lung mesenchyme in a dose-dependent manner. In addition, Shh upregulates several mesenchymal cell markers, including its target gene Patched (Ptc), intercellular signaling genes Bone Morphogenetic Protein-4 (Bmp4) and Noggin (Nog), and smooth muscle actin and myosin. In explants exposed to N-Shh in the medium, these products are upregulated throughout the mesenchyme, but not in the periphery. This exclusion zone correlates with the presence of an overlying mesothelial layer, which, as in vivo, expresses Fibroblast Growth Factor 9 (Fgf9). Recombinant Fgf9 protein inhibits the differentiation response of the mesenchyme to N-Shh, but does not affect proliferation. We propose a model for how factors made by two epithelial cell populations, the inner endoderm and the outer jacket of mesothelium, coordinately regulate the proliferation and differentiation of the lung mesoderm.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Fibroblast growth factors as regulators of central nervous system development and function

Fibroblast growth factors (FGFs) are multifunctional signaling proteins that regulate developmental processes and adult physiology. Over the last few years, important progress has been made in understanding the function of FGFs in the embryonic and adult central nervous system. In this review, I will first discuss studies showing that FGF signaling is already required during formation of the neural plate. Next, I will describe how FGF signaling centers control growth and patterning of specific brain structures. Finally, I will focus on the function of FGF signaling in the adult brain and in regulating maintenance and repair of damaged neural tissues.

Fibroblast growth factors lead to increased Msx2 expression and fusion in calvarial sutures

Craniosynostosis, the premature fusion of the skull bones at the sutures, represents a disruption to the coordinated growth and development of the expanding brain and calvarial vault and is the second most common birth defect that affects the craniofacial complex. Mutations in the human homeobox-containing gene, Msx2, have been shown to cause Boston type craniosynostosis, and we have shown that overexpression of Msx2 leads to craniosynostosis in mice. Activating mutations in fibroblast growth factor (FGF) receptors are thought to cause craniosynostosis in Crouzon, Apert, Jackson-Weiss, Beare-Stevenson, and Muenke syndromes. To mimic activated signaling by mutated FGF receptors, we used heparin acrylic beads to deliver FGF ligands to mouse calvaria and demonstrated increased Msx2, Runx2, Bsp, and Osteocalcin gene expression, decreased cell proliferation, and suture obliteration and fusion. FGF2 elicited the greatest increase in Msx2 expression, and FGF1 was most likely to cause suture obliteration and fusion. Of the three sutures studied, the coronal suture exhibited the greatest increase in Msx2 expression and was the most likely to undergo obliteration and fusion. These results are intriguing because the coronal suture is the most commonly affected suture in syndromic craniosynostosis. These results suggest that Msx2 is a downstream target of FGF receptor signaling and that increased FGF signaling leads to osteogenic differentiation by sutural mesenchyme in mouse calvaria. These results are consistent with the hypotheses that increased Msx2 expression and activated signaling by mutated FGF receptors lead to craniosynostosis.

Differential expression of fibroblast growth factor receptors in the developing murine choroid plexus

The choroid plexuses (CPs) are specialised secretory organs situated within the ventricles of the brain involved in the production of cerebrospinal fluid (CSF) and the maintenance of the blood-CSF barrier. Abnormal function of the CPs can lead to hydrocephalus and raised intracranial pressure, pathologies frequently observed in certain craniofacial syndromes caused by single point mutations in fibroblast growth factor receptors (FGFRs). At present, relatively little is known about the embryonic CPs in terms of gene or protein expression, function as the brain develops or on the potential role of FGFRs within this context. Given the limited information available on the regulation of FGFRs during development of the CPs and periventricular tissues, we have carried out a detailed analysis of the localisation of FGFR1, 2, 3 and 4 proteins in these regions of the murine embryo from the time of formation of the CP in the third ventricle at E12.5 throughout the second half of gestation, and examined the expression of different FGFR isoforms at E12.5 by RT-PCR. We show here that FGFR1 and FGFR4 are expressed in murine CPs at E12.5 but not at E15.5 or E18.5, suggesting a role for the signaling pathways transduced by these receptors at early stages of CP development. In contrast, FGFR2 expression is maintained throughout CP development, indicating that this receptor may play a role in the function of immature and mature CP. Also FGFR3 is detected at each developmental stage studied, but surprisingly its expression appears confined to the nuclei of CP cells, suggesting that FGFR3 in the CP does not respond to extracellular FGFs but may act in intracrine fashion.

Multiple joint and skeletal patterning defects caused by single and double mutations in the mouse Gdf6 and Gdf5 genes

Growth/differentiation factors 5, 6, and 7 (GDF5/6/7) represent a distinct subgroup within the bone morphogenetic protein (BMP) family of secreted signaling molecules. Previous studies have shown that the Gdf5 gene is expressed in transverse stripes across developing skeletal elements and is one of the earliest known markers of joint formation during embryonic development. Although null mutations in this gene disrupt formation of some bones and joints in the skeleton, many sites are unaffected. Here, we show that the closely related family members Gdf6 and Gdf7 are expressed in different subsets of developing joints. Inactivation of the Gdf6 gene causes defects in joint, ligament, and cartilage formation at sites distinct from those seen in Gdf5 mutants, including the wrist and ankle, the middle ear, and the coronal suture between bones in the skull. Mice lacking both Gdf5 and Gdf6 show additional defects, including severe reduction or loss of some skeletal elements in the limb, additional fusions between skeletal structures, scoliosis, and altered cartilage in the intervertebral joints of the spinal column. These results show that members of the GDF5/6/7 subgroup are required for normal formation of bones and joints in the limbs, skull, and axial skeleton. The diverse effects on joint development and the different types of joints affected in the mutants suggest that members of the GDF family play a key role in establishing boundaries between many different skeletal elements during normal development. Some of the skeletal defects seen in single or double mutant mice resemble defects seen in human skeletal diseases, which suggests that these genes may be candidates that underlie some forms of carpal/tarsal coalition, conductive deafness, scoliosis, and craniosynostosis.

Inhibition of skeletal muscle development: less differentiation gives more muscle

The fact that stem cells have to be protected from premature differentiation is true for many organs in the developing embryo and the adult organism. However, there are several arguments that this is particularly important for (skeletal) muscle. There are some evolutionary arguments that muscle is a "default" pathway for mesodermal cells, which has to be actively prevented in order to allow cells to differentiate into other tissues. Myogenic cells originate from very small areas of the embryo where only a minor portion of these cells is supposed to differentiate. Differentiated muscle fibres are unconditionally post-mitotic, leaving undifferentiated stem cells as the only source of regeneration. The mechanical usage of muscle and its superficial location in the vertebrate body makes regeneration a frequently used mechanism. Looking at the different inhibitory mechanisms that have been found within the past 10 or so years, it appears as if evolution has taken this issue very serious. At all possible levels we find regulatory mechanisms that help to fine tune the differentiation of myogenic cells. Secreted molecules specifying different populations of somitic cells, diffusing or membrane-bound signals among fellow myoblasts, modulating molecules within the extracellular matrix and last, but not least, a changing set of activating and repressing cofactors. We have come a long way from the simple model of MyoD just to be turned on at the right time in the right cell.

Prenatal, but not postnatal, inhibition of fibroblast growth factor receptor signaling causes emphysema

Although fibroblast growth factor (FGF) signaling is required for the formation of the lung in the embryonic period, it is unclear whether FGF receptor activity influences lung morphogenesis later in development. We generated transgenic mice expressing a soluble FGF receptor (FGFR-HFc) under conditional control of the lung-specific surfactant protein C promoter (SP-C-rtTA), to inhibit FGF activity at various times in late gestation and postnatally. Although expression of FGFR-HFc early in development caused severe fetal lung hypoplasia, activation of the transgene in the postnatal period did not alter alveolarization, lung size, or histology. In contrast, expression of the transgene at post-conception day E14.5 decreased lung tubule formation before birth and caused severe emphysema at maturity. FGFR-HFc caused mild focal emphysema when expressed from E16.5 but did not alter alveolarization when expressed after birth. Although FGF signaling was required for branching morphogenesis early in lung development, postnatal alveolarization was not influenced by FGFR-HFc.

Expression of the FGF receptor 2 gene (fgfr2) during embryogenesis in the zebrafish Danio rerio

We isolated a full-length cDNA clone for the zebrafish homologue of fibroblast growth factor receptor (FGFR) 2. The deduced protein sequence is typical of vertebrate FGFRs in that it has three Ig-like domains in the extracellular region. The expression of fgfr2 is initiated during epiboly in the paraxial mesoderm. During early somitogenesis, fgfr2 expression was noted in the anterior neural plate as well as in newly formed somites. Whereas fgfr2 expression in somites is transient, it increases in the central nervous system (CNS), i.e. in the ventral telencephalon, anterior diencephalon, midbrain, and respective rhombomeres of the hindbrain, from the mid-somitogenesis stage. The dorsal telencephalon and the region around the midbrain-hindbrain boundary are devoid of fgfr2 expression. Essentially the same expression pattern is observed until 48 h post-fertilization in the CNS, although rhombomeric expression in the hindbrain is progressively confined to narrower stripes. After somitogenesis, fgfr2 expression was also observed in the lens, hypochord, endoderm, and fin mesenchyme. We compared the expression of the four fgfr genes (fgfr1-4) in the CNS of zebrafish embryos and show that fgfr1 is the only fgfr gene that is expressed in the dorsal telencephalon and isthmic region from which expression of fgfr2-4 is absent.

Expression of the fibroblast growth factor receptor genes in fracture repair

The spatial and temporal expression domains of the fibroblast growth factor receptor genes were examined in the healing rat femur fracture by in situ hybridization. Fibroblast growth factor receptor gene expression was detected in diverse fracture tissues throughout healing. Fibroblast growth factor receptor 1 and 2 expression was present throughout fracture repair, in the early proliferating periosteal mesenchyme, in the osteoblasts during intramembranous bone formation, and in the chondrocytes and osteoblasts during endochondral bone formation. Fibroblast growth factor receptor 3 expression colocalized with fibroblast growth factor receptor 1 and 2 expression in the chondrocytes and osteoblasts beginning at 10 days of healing, and persisted throughout endochondral bone formation. Fibroblast growth factor receptor 3 recapitulated its expression in fetal skeletal development, suggesting that it has a similar function in the control of endochondral bone growth during fracture repair. Fibroblast growth factor receptor 4 expression was not observed at any time. The extensive colocalized expression of the fibroblast growth factor receptors in healing indicates that fibroblast growth factor regulation of fracture callus maturation is extensive, and accurate identification of the receptor isoforms is necessary to establish the functions of fibroblast growth factor family members in fracture repair.

Synergistic effect of fibroblast growth factor-4 in ectopic bone formation induced by bone morphogenetic protein-2

Bone morphogenetic protein family members (BMPs) are essential signaling molecules during limb development and, in this process, fibroblast growth factor family members (FGFs) cooperate with BMPs. FGFs also exert anabolic effects in bone when systemically or locally applied. Thus, it is likely that the cooperation with FGFs also occurs in BMP-induced ectopic bone formation and that the exogenous FGF application would promote this bone formation. In the present study, after subcutaneously implanting recombinant human BMP-2 (rhBMP-2) in rats, we examined the expression of FGF-4 and FGF receptors (FGFRs) mRNAs and the effect of exogenous recombinant human FGF-4 (rhFGF-4) on bone formation. Three days after implantation, the pellets containing rhBMP-2 were surrounded by fibroblastic mesenchymal cells; on day 7, cartilage tissue appeared; on day 10, hypertrophic chondrocytes and a small amount of mineralized tissue were observed; and, on day 14, the amount of mineralized tissue increased. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that FGF-4 expression appeared at early stages (days 3 and 7) and its expression decreased at later stages (days 10, 14, and 21), whereas FGFRs were expressed continuously. In situ hybridization revealed that, on days 3 and 7, FGF-4, and FGFR subtypes 1 and 2 (FGFR-1 and FGFR-2) were expressed in mesenchymal cells and chondrocytes, and in the area of alkaline phosphatase (ALP) expression. On day 10, FGF-4 was not detected, whereas the expression of FGFR-1 and FGFR-2 was detectable in the area of alkaline phosphatase (ALP) expression. Injection of rhFGF-4 on days 2, 3, and 4 enhanced the mineralized tissue formation induced by rhBMP-2; however, neither rhFGF-4 treatment on days 6, 7, and 8 nor rhFGF-4 treatment on days 9, 10, and 11 influenced the amount of rhBMP-2-induced mineralization. Our results indicate that FGF-4 and FGFR signals play important roles during rhBMP-2-induced bone formation. We further suggest that the combination of rhBMP-2 and rhFGF-4 would be useful for bone augmentation.

Ectodermal FGFs induce perinodular inhibition of limb chondrogenesis in vitro and in vivo via FGF receptor 2

The formation of cartilage elements in the developing vertebrate limb, where they serve as primordia for the appendicular skeleton, is preceded by the appearance of discrete cellular condensations. Control of the size and spacing of these condensations is a key aspect of skeletal pattern formation. Limb bud cell cultures grown in the absence of ectoderm formed continuous sheet-like masses of cartilage. With the inclusion of ectoderm, these cultures produced one or more cartilage nodules surrounded by zones of noncartilaginous mesenchyme. Ectodermal fibroblast growth factors (FGF2 and FGF8), but not a mesodermal FGF (FGF7), substituted for ectoderm in inhibiting chondrogenic gene expression, with some combinations of the two ectodermal factors leading to well-spaced cartilage nodules of relatively uniform size. Treatment of cultures with SU5402, an inhibitor FGF receptor tyrosine kinase activity, rendered FGFs ineffective in inducing perinodular inhibition. Inhibition of production of FGF receptor 2 (FGFR2) by transfection of wing and leg cell cultures with antisense oligodeoxynucleotides blocked appearance of ectoderm- or FGF-induced zones of perinodular inhibition of chondrogenesis and, when introduced into the limb buds of developing embryos, led to shorter, thicker, and fused cartilage elements. Because FGFR2 is expressed mainly at sites of precartilage condensation during limb development in vivo and in vitro, these results suggest that activation of FGFR2 by FGFs during development elicits a lateral inhibitor of chondrogenesis that limits the expansion of developing skeletal elements.

Altered whisker patterns induced by ectopic expression of Shh are topographically represented by barrels

Barrels in the somatosensory cortex are segregated columns, which somatotopically relate to facial whiskers. The barrel pattern is assumed to be determined by an extrinsic mechanism (the domino theory). This theory is based on whisker lesion experiments and developmental observations regarding the serial establishment of the somatotopic pattern in which pattern formations are relayed from the periphery to the central nervous system. However, the barrel pattern is possibly determined by an intrinsic mechanism, especially in its primitive form. In order to investigate the definitive mechanism, we established an experimental system in which the cortical barrel pattern can be altered, not by using a lesion paradigm, but by epigenetically changing the whisker pattern. Sonic hedgehog (Shh) plays a pivotal role in whisker development. We transfected an adenovirus harboring chicken Shh (Ad-cShh) to mouse embryos (E9.5-E11.5) using an in utero surgical technique. When Ad-cShh was expressed in the epidermis, Bmp4, Ptch, Ptch2 and Gli1 were induced ectopically in the interfollicular region. In contrast, the expression of Bmp2 and Shh itself was unaltered. At a suitable dose of Ad-cShh, some pups displayed supernumerary whiskers or a disordered whisker pattern. The barrel patterns of these mice after the critical period were topographic representations of the contralateral side of the new whisker patterns when visualized by a cytochrome oxidase or Nissle staining method, supporting the instructive role of the extrinsic mechanism.

Distribution of fibroblast growth factors (FGFR-1 and -3) and platelet-derived growth factor receptors (PDGFR) in the rat mandibular condyle during growth

OBJECTIVES

To elucidate the role of the fibroblast growth factors 1 and 3 (FGFR-1, -3) and the platelet derived growth factor (PDGFR) in the growth of the mandibular condylar cartilage in the rat.

SETTING AND SAMPLE POPULATION

Institute of Dentistry and Department of Biostatistics, University of Turku, Turku, Finland. The material consisted of 1- to 21-day-old Long-Evans/Turku rats (total of 24 animals, three in each age group).

DESIGN

An immunohistological in vivo study combined with histomorphometry and biostatistical analysis.

EXPERIMENTAL VARIABLE

The animals were killed with an overdose of carbon dioxide and thereafter decapitated. Heads were fixed in 4% paraformaldehyde, decalcified in 12.5% ethylenediaminetetraacetic acid, cut sagittally into two halves and sectioned sagittally at 6 microns. In order to detect FGFR-1, -3 and PDGFR the sections were treated with H2O2/methanol (1:100), after which FGFR-1 and PDGFR monoclonal and FGFR-3 polyclonal antibodies were applied. The reaction products were visualized by using the Vectastain ABC Elite Kit using peroxidase substrate kit DAB as substrate. Negative and positive controls were also prepared. The sections were counterstained with hematoxylin.

OUTCOME MEASURE

In order to measure the depth of the cell layer labeled with FGF-1, -3 and PDGF receptors, the condylar head was divided into four regions: anterior, superior, posterosuperior and posterior. The measurements were made perpendicular to the articular surface using a computerized image analysis system, the images being acquired by means of a microscope connected to a CCD camera. The mean of five equally distributed measurements of each region was used to indicate the depth of the cell layers secreting the receptors. Regression analysis was used to evaluate the association between the depth of the labeled cell layer in relation to total depth of the condylar head, as a function of age.

RESULTS

Our results show that the depth of the cell layer labeled for FGFR-1, -3 and PDGFR increase significantly as a function of age in the mandibular condylar head of rats.

CONCLUSION

Increase in the cell layer labeled for FGFR-1, -3 and PDGFR occurs during the stage when the articular function of the mandibular condyle intensifies. FGFR-1, -3 and PDGFR evidently have an important role in the growth regulation of the condylar cartilage during the most rapid growth period in the rat.

Fibroblast growth factor 18 influences proximal programming during lung morphogenesis

The structure and functions of the airways of the lung change dramatically along their lengths. Large-diameter conducting airways are supported by cartilaginous rings and smooth muscle tissue and are lined by ciliated and secretory epithelial cells that are involved in mucociliary clearance. Smaller peripheral airways formed during branching morphogenesis are lined by cuboidal and squamous cells that facilitate gas exchange to a network of fine capillaries. The factors that mediate formation of these changing cell types and structures along the length of the airways are unknown. We report here that conditional expression of fibroblast growth factor (FGF)-18 in epithelial cells of the developing lung caused the airway to adopt structural features of proximal airways. Peripheral lung tubules were markedly diminished in numbers, whereas the size and extent of conducting airways were increased. Abnormal smooth muscle and cartilage were found in the walls of expanded distal airways, which were accompanied by atypically large pulmonary blood vessels. Expression of proteins normally expressed in peripheral lung tubules, including SP-B and pro-SP-C, was inhibited. FGF-18 mRNA was detected in normal mouse lung in stromal cells surrounding proximal airway cartilage and in peripheral lung mesenchyme. Effects were unique to FGF-18 because expression of other members of the FGF family had different consequences. These data show that FGF-18 is capable of enhancing proximal and inhibiting peripheral programs during lung morphogenesis.

Protection from thymic epithelial cell injury by keratinocyte growth factor: a new approach to improve thymic and peripheral T-cell reconstitution after bone marrow transplantation

Decreased thymopoietic capacity contributes to the severe and clinically significant immune deficiency seen after bone marrow transplantation (BMT). One mechanism for thymopoietic failure is damage to the interleukin 7 (IL-7)-producing thymic epithelial cells (TECs) by irradiation and chemotherapy, which can be partially treated by IL-7 administration. Pretreatment of BMT recipients with keratinocyte growth factor (KGF, or Fgf7), an epithelial cell-specific growth factor, protects mucosal, cutaneous, and pulmonary epithelial cells from cytotoxic therapy-induced damage in experimental murine models. Like other epithelial cells, TECs specifically express KGF receptors. Because KGF specifically protects KGF receptor-bearing epithelial cells and post-BMT immune deficiency is caused by loss of TECs, we hypothesized that KGF pretreatment would improve post-BMT thymic function. To test the hypothesis, BMT recipient mice were given KGF or placebo prior to congenic or allogeneic BMT. Administration of KGF before murine BMT significantly increased the capacity of the thymus to generate donor-derived thymocytes. KGF pretreatment also normalized the proportion of thymic subpopulations, increased the number of naive T cells in the periphery, and improved the response to neoantigen immunization. KGF treatment caused increased production of intrathymic IL-7, and the thymopoietic effects of KGF required an intact IL-7 signaling pathway. These results demonstrate that KGF may have immunomodulatory effects by a unique mechanism of protection of TECs. Furthermore, thymic injury and prolonged posttransplantation immune deficiency in BMT recipients can be prevented by KGF administration.

Mouse Twist is required for fibroblast growth factor-mediated epithelial-mesenchymal signalling and cell survival during limb morphogenesis

Mouse Twist is essential for cranial neural tube, limb and somite development. [Genes Dev. 9 (1995) 686]. To identify the molecular defects disrupting limb morphogenesis, we have analysed expression of mesenchymal transcription factors involved in patterning and the cell-cell signalling cascades controlling limb bud development. These studies establish that Twist is essential for maintenance and progression of limb bud morphogenesis. In particular, the SHH/FGF signalling feedback loop operating between the polarizing region and the apical ectodermal ridge (AER) is disrupted. These defects in epithelial-mesenchymal signalling are most likely a direct consequence of disrupted fibroblast growth factor (FGF) signalling in Twist-deficient limb buds. In early limb buds, down-regulation of Fgf receptor 1 and Fgf10 expression in the mesenchyme occurs concurrent with loss of Fgf4 and Fgf8 expression in the AER. Finally, Twist function, most likely by regulating FGF signalling, is required for cell survival as apoptotic cells are detected in posterior and distal limb bud mesenchyme.

Interaction of fibroblast growth factor receptor 3 and the adapter protein SH2-B. A role in STAT5 activation

Fibroblast growth factor receptor 3 (FGFR3) influences a diverse array of biological processes, including cell growth, differentiation, and migration. Activating mutations in FGFR3 are associated with multiple myeloma, cervical carcinoma, and bladder cancer. To identify proteins that interact with FGFR3 and which may mediate FGFR3-dependent signaling, a yeast two-hybrid screen was employed using the cytoplasmic kinase domain of FGFR3 as bait. We identified the adapter protein SH2-B as an FGFR3-interacting protein. Coimmunoprecipitation experiments demonstrate binding of the SH2-B beta isoform to FGFR3 in 293T cells. Tyrosine phosphorylation of SH2-B beta was observed when coexpressed with activated FGFR3 mutants such as the weakly activated mutant N540K or the strongly activated mutant K650E, both associated with human developmental syndromes. The extent of tyrosine phosphorylation of SH2-B beta correlates with receptor activation, suggesting that FGFR3 activation mediates tyrosine phosphorylation of SH2-B beta. Furthermore, two tyrosine phosphorylation sites of FGFR3, Tyr-724 and Tyr-760, are required for optimal binding of the Src homology-2 (SH2) domain of SH2-B beta. We also demonstrate the phosphorylation and nuclear translocation of Stat5 by activated FGFR3, which increases in response to overexpression of SH2-B beta. Taken together, our results identify SH2-B beta as a novel FGFR3 binding partner that mediates signal transduction.

Keratinocyte and hepatocyte growth factors in the lung: roles in lung development, inflammation, and repair

A growing body of evidence indicates that the epithelial-specific growth factors keratinocyte growth factor (KGF), fibroblast growth factor (FGF)-10, and hepatocyte growth factor (HGF) play important roles in lung development, lung inflammation, and repair. The therapeutic potential of these growth factors in lung disease has yet to be fully explored. KGF has been best studied and has impressive protective effects against a wide variety of injurious stimuli when given as a pretreatment in animal models. Whether this protective effect could translate to a treatment effect in humans with acute lung injury needs to be investigated. FGF-10 and HGF may also have therapeutic potential, but more extensive studies in animal models are needed. Because HGF lacks true epithelial specificity, it may have less potential than KGF and FGF-10 as a targeted therapy to facilitate lung epithelial repair. Regardless of their therapeutic potential, studies of the unique roles played by these growth factors in the pathogenesis and the resolution of acute lung injury and other lung diseases will continue to enhance our understanding of the complex pathophysiology of inflammation and repair in the lung.