Fibroblast growth factors as multifunctional signaling factors

The functional genomic response of developing embryonic submandibular glands to NF-kappa B inhibition

BACKGROUND

The proper balance between epithelial cell proliferation, quiescence, and apoptosis during development is mediated by the specific temporal and spatial appearance of transcription factors, growth factors, cytokines, caspases, etc. Since our prior studies suggest the importance of transcription factor NF-kappaB during embryonic submandibular salivary gland (SMG) development, we attempted to delineate the emergent dynamics of a cognate signaling network by studying the molecular patterns and phenotypic outcomes of interrupted NF-kappaB signaling in embryonic SMG explants.

RESULTS

SN50-mediated inhibition of NF-kappaB nuclear translocation in E15 SMG explants cultured for 2 days results in a highly significant increase in apoptosis and decrease in cell proliferation. Probabilistic Neural Network (PNN) analyses of transcriptomic and proteomic assays identify specific transcripts and proteins with altered expression that best discriminate control from SN50-treated SMGs. These include PCNA, GR, BMP1, BMP3b, Chk1, Caspase 6, E2F1, c-Raf, ERK1/2 and JNK-1, as well as several others of lesser importance. Increased expression of signaling pathway components is not necessarily probative of pathway activity; however, as confirmation we found a significant increase in activated (phosphorylated/cleaved) ERK 1/2, Caspase 3, and PARP in SN50-treated explants. This increased activity of proapoptotic (caspase3/PARP) and compensatory antiapoptotic (ERK1/2) pathways is consistent with the dramatic cell death seen in SN50-treated SMGs.

CONCLUSIONS

Our morphological and functional genomic analyses indicate that the primary and secondary effects of NF-kappaB-mediated transcription are critical to embryonic SMG developmental homeostasis. Relative to understanding complex genetic networks and organogenesis, our results illustrate the importance of evaluating the gene, protein, and activated protein expression of multiple components from multiple pathways within broad functional categories.

Development of an evolutionarily novel structure: fibroblast growth factor expression in the carapacial ridge of turtle embryos

The turtle shell, an evolutionarily novel structure, contains a bony exoskeleton that includes a dorsal carapace and a ventral plastron. The development of the carapace is dependent on the carapacial ridge (CR), a bulge in the dorsal flank that contains an ectodermal structure analogous to the apical ectodermal ridge (AER) of the developing limb (Burke. 1989a. J Morphol 199:363-378; Burke. 1989b. Fortschr Zool 35:206-209). Although the CR is thought to mediate the initiation and outgrowth of the carapace, the mechanisms of shell development have not been studied on the molecular level. Here, we present data suggesting that carapace formation is initiated by co-opting genes that had other functions in the ancestral embryo, specifically those of limb outgrowth. However, there is divergence in the signaling repertoire from that involved in limb initiation and outgrowth. In situ hybridizations with antisense riboprobes derived from Trionyx spiniferous fibroblast growth factor-10 (tfgf10) and Trachemys scripta (T. scripta) fibroblast-growth factor 8 (tfgf8) cDNAs were performed on sections of early T. scripta embryos (< 30 days). Expression of tfgf10 was localized to the mesenchyme subjacent to the ectoderm of the CR. In the chick limb bud, FGF10 is known to be expressed in the early limb-forming mesenchyme and is capable of inducing FGF8 in the AER to initiate the outgrowth of the limb bud. Although the expression of tfgf8 was found in the AER of the developing turtle limb, it was not seen in the CR. Thus, the initiation of the carapace is in agreement with FGF10 expression in the CR, but FGF8 does not appear to have a role in mediating early carapace outgrowth.

Novel fibroblast growth factor 2 transcripts are expressed in mouse embryos

We have discovered two new exons in the mouse fibroblast growth factor 2 (FGF-2 or bFGF) gene that can be alternatively spliced to the second coding exon of the gene. The newly identified exons 1b and 1c are located at, respectively, approximately 19 and 32 kb downstream of the canonical exon 1a. Using RT-PCR analysis, mRNAs containing exon 1c and canonical exons 2 and 3 were identified in embryonic limb, placenta, face, carcass and ocular tissues. A 3.7-kb transcript present in placenta and embryonic limb hybridizes with an exon 1c-derived probe in Northern blot analysis. Alternative splicing of exon 1c to exon 2 creates a transcript for which the predicted alternative FGF-2 (altFGF-2) polypeptide contains a novel N-terminal domain. Our data indicate that in mouse embryos multiple novel mRNA variants are transcribed from the FGF-2 locus using alternative splicing. These data suggest that proteins arising from these alternative transcripts may play a role in mouse embryogenesis.

Basic fibroblast growth factor maintains calcium homeostasis and granulosa cell viability by stimulating calcium efflux via a PKC delta-dependent pathway

Previous studies have demonstrated that basic fibroblast growth factor prevents granulosa cell apoptosis. The following six observations provide insight into the mechanism by which basic fibroblast growth factor mediates its antiapoptotic action. First, loading granulosa cells with 1,2 bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, an intracellular calcium chelator, prevented apoptosis when granulosa cells were deprived of basic fibroblast growth factor. Second, treatment with thapsigargin, an agent known to increase intracellular free calcium, induced granulosa cell apoptosis even in the presence of basic fibroblast growth factor. Third, an activator of PKC mimicked, whereas PKC inhibitors blocked, basic fibroblast growth factor's antiapoptotic action. Fourth, continuous basic fibroblast growth factor exposure maintained relatively constant levels of intracellular free calcium, and a PKC inhibitor induced a sustained 2- to 3-fold increase in intracellular free calcium. Fifth, granulosa cells, as well as spontaneously immortalized granulosa cells, were shown to express PKC delta, -lambda, and -zeta. Finally, the PKC delta-specific inhibitor, rottlerin, blocked basic fibroblast growth factor's antiapoptotic action in granulosa cells and spontaneously immortalized granulosa cells. These studies suggest that basic fibroblast growth factor regulates intracellular free calcium through a PKC delta-dependent mechanism and that a sustained increase in intracellular free calcium is sufficient to induce and is required for granulosa cell apoptosis. Additional studies demonstrated that in spontaneously immortalized granulosa cells, basic fibroblast growth factor increased PKC delta activity by 60% within 2.5 min compared with serum-free control levels. Rottlerin attenuated basic fibroblast growth factor's ability to stimulate PKC delta activity and to maintain intracellular free calcium. Further, intracellular free calcium levels in spontaneously immortalized granulosa cells transfected with a PKC delta antibody in the presence of basic fibroblast growth factor were 2-fold higher than those spontaneously immortalized granulosa cells transfected with IgG. Similarly, transfecting spontaneously immortalized granulosa cells with a specific PKC delta-substrate increased intracellular free calcium compared with spontaneously immortalized granulosa cells transfected with a specific substrate for PKC epsilon. Moreover, basic fibroblast growth factor increased and rottlerin attenuated (45)Ca efflux by 50% compared with that in basic fibroblast growth factor-treated cells. Finally, an inhibitor of the plasma membrane calciumadenosine triphosphatase pump suppressed (45)Ca efflux, elevated intracellular free calcium, and induced apoptosis. Collectively, these studies demonstrate that basic fibroblast growth factor activates PKC delta, which, in turn, stimulates calcium efflux, accounting in part for basic fibroblast growth factor's ability to maintain calcium homeostasis and, ultimately, granulosa cell viability.

Making a vertebrate limb: new players enter from the wings

What initiates vertebrate limb development and induces limbs to form where they do? For several years the answer to this intriguing question has been framed in terms of a working model that limb induction depends on a dialogue between two members of the Fibroblast Growth Factor (FGF) family of intercellular signaling molecules, FGF8 and FGF10. Now, a recent paper has written roles for signals encoded by WNT genes, the vertebrate relatives of the Drosophila wingless gene, into the script.

A role for the perlecan protein core in the activation of the keratinocyte growth factor receptor

Perlecan, a widespread heparan sulphate (HS) proteoglycan, is directly involved in the storing of angiogenic growth factors, mostly members of the fibroblast growth factor (FGF) gene family. We have previously shown that antisense targeting of the perlecan gene causes a reduced growth and responsiveness to FGF7 [also known as keratinocyte growth factor (KGF)] in human cancer cells, and that the perlecan protein core interacts specifically with FGF7. In the present paper, we have investigated human colon carcinoma cells in which the perlecan gene was disrupted by targeted homologous recombination. After screening over 1000 clones, we obtained two clones heterozygous for the null mutation with no detectable perlecan, indicating that the other allele was non-functioning. The perlecan-deficient cells grew more slowly, did not respond to FGF7 with or without the addition of heparin, and were less tumorigenic than control cells. Paradoxically, the perlecan-deficient cells displayed increased FGF7 surface binding. However, the perlecan protein core was required for functional activation of the KGF receptor and downstream signalling. Because heparin could not substitute for perlecan, the HS chains are not critical for FGF7-mediated signalling in this cell system. These results provide the first genetic evidence that the perlecan protein core is a molecular entity implicated in FGF7 binding and activation of its receptor.

The mouse Fgfrl1 gene coding for a novel FGF receptor-like protein

The mouse Fgfrl1 gene codes for a novel cell surface protein that is closely related to the family of the FGF receptors. It contains three extracellular Ig C2 loops and an acidic box, which share 29-33% sequence identity (48-50% similarity) with FGF receptors 1-4. The intracellular portion of the novel protein, however, lacks a tyrosine kinase domain required for signal transduction by transphosphorylation. The gene for Fgfrl1 comprises six exons and is located on mouse chromosome 5 in close proximity to the Idua gene for L-iduronidase.

The FGFR pathway is required for the trunk-inducing functions of Spemann's organizer

Xenopus laevis embryogenesis is controlled by the inducing activities of Spemann's organizer. These inducing activities are separated into two distinct suborganizers: a trunk organizer and a head organizer. The trunk organizer induces the formation of posterior structures by emitting signals and directing morphogenesis. Here, we report that the fibroblast growth factor receptor (FGFR) signaling pathway, also known to regulate posterior development, performs critical functions within the cells of Spemann's organizer. Specifically, the FGFR pathway was required in the organizer cells in order for those cells to induce the formation of somitic muscle and the pronephros. Since the organizer influences the differentiation of these tissues by emitting signals that pattern the mesodermal germ layer, our data indicate that the FGFR regulates the production of these signals. In addition, the FGFR pathway was required for the expression of chordin, an organizer-specific protein required for the trunk-inducing activities of Spemann's organizer. Significantly, the FGFR pathway had a minimal effect on the function of the head organizer. We propose that the FGFR pathway is a defining molecular component that distinguishes the trunk organizer from the head organizer by controlling the expression of organizer-specific genes required to induce the formation of posterior structures and somitic muscle in neighboring cells. The implications of our findings for the evolutionarily conserved role of the FGFR pathway in the functions of Spemann's organizer and other vertebrate-signaling centers are discussed.

Cyclic AMP modulates the response of central nervous system glia to fibroblast growth factor-2 by redirecting signalling pathways

Fibroblast growth factor-2 (FGF-2) acts as both a potent mitogen and differentiation factor for CNS glia. In the present study, we provide evidence that intracellular cAMP determines the proliferation-differentiation decision of astroglia to FGF-2 by either facilitating FGF-2 signalling to extracellular signal-related protein kinase (ERK) or cAMP response element binding protein (CREB). Pharmacologically increasing intracellular cAMP levels in cultured cortical astroglia by treatment with dbcAMP or forskolin attenuated FGF-2-induced ERK phosphorylation and glial cell proliferation. Similarly, FGF-2-induced glial proliferation was attenuated in the presence of the MEK inhibitor, PD98059, thus, confirming a direct correlation between FGF-2-induced ERK activation and glial cell proliferation. On the other hand, increases in intracellular cAMP levels in cortical astroglia prolonged FGF-2-induced CREB phosphorylation and subsequently potentiated the cAMP response element-dependent transcription of the immediate early gene, c-fos. Moreover, the effects of cAMP on the time-course of FGF-2-dependent CREB phosphorylation were mimicked by PD98059, suggesting that the cAMP-induced redirection of FGF-2-signalling is linked to the RAF-MEK-ERK signalling pathway.

The evolution of controlled multitasked gene networks: the role of introns and other noncoding RNAs in the development of complex organisms

Eukaryotic phenotypic diversity arises from multitasking of a core proteome of limited size. Multitasking is routine in computers, as well as in other sophisticated information systems, and requires multiple inputs and outputs to control and integrate network activity. Higher eukaryotes have a mosaic gene structure with a dual output, mRNA (protein-coding) sequences and introns, which are released from the pre-mRNA by posttranscriptional processing. Introns have been enormously successful as a class of sequences and comprise up to 95% of the primary transcripts of protein-coding genes in mammals. In addition, many other transcripts (perhaps more than half) do not encode proteins at all, but appear both to be developmentally regulated and to have genetic function. We suggest that these RNAs (eRNAs) have evolved to function as endogenous network control molecules which enable direct gene-gene communication and multitasking of eukaryotic genomes. Analysis of a range of complex genetic phenomena in which RNA is involved or implicated, including co-suppression, transgene silencing, RNA interference, imprinting, methylation, and transvection, suggests that a higher-order regulatory system based on RNA signals operates in the higher eukaryotes and involves chromatin remodeling as well as other RNA-DNA, RNA-RNA, and RNA-protein interactions. The evolution of densely connected gene networks would be expected to result in a relatively stable core proteome due to the multiple reuse of components, implying that cellular differentiation and phenotypic variation in the higher eukaryotes results primarily from variation in the control architecture. Thus, network integration and multitasking using trans-acting RNA molecules produced in parallel with protein-coding sequences may underpin both the evolution of developmentally sophisticated multicellular organisms and the rapid expansion of phenotypic complexity into uncontested environments such as those initiated in the Cambrian radiation and those seen after major extinction events.

Apoptosis of granulosa cells and female infertility in achondroplastic mice expressing mutant fibroblast growth factor receptor 3G374R

Fibroblast growth factors play an important role in the control of ovarian folliculogenesis, but the complete repertoire of ovarian receptors which can transduce the fibroblast growth factor signals and their precise localization in the ovary have not yet been characterized. The most common form of inherited human dwarfism results from a point mutation in the transmembrane region of fibroblast growth factor receptor 3. A mouse model for achondroplasia was generated by introducing the human mutation (glycine 380-arginine) into the mouse fibroblast growth factor receptor 3 (G374R) by a "knock-in" approach using gene targeting leading to a constitutively active receptor. This resulted in the development of dwarf mice that share many features with human achondroplasia. Here we report that female (fibroblast growth factor receptor 3 G374R) dwarf mice become infertile. While no significant changes were observed in the anatomical and histological appearance of ovaries of 3-wk-old dwarf mice, a dramatic difference was observed in ovaries of 3-month-old mice. The normal ovary consists mainly of healthy corpora lutea and follicles at different stages of development, whereas the ovaries of the dwarf mice remain small and contain mainly follicles with a progressive apoptosis in the granulosa cells, and no corpora lutea could be observed. The levels of LH, FSH, and progesterone were lower by 72.3%, 38.0%, and 40.0%, respectively, in the blood of the dwarf mice compared with normal mice, and the total bioactivity of pituitary FSH and LH was lower by 65.6% and 79.6%, respectively, in the dwarf mice compared with normal mice. However treatment with PMSG and human CG of the dwarf mice led to rapid follicular development and formation of corpora lutea. Interestingly, the expression of the tumor suppressor gene p53 was increased dramatically in ovaries of the dwarf mice. The presence of the fibroblast growth factor receptor 3 cellular receptors in both normal and dwarf animals was demonstrated by Western blot and immunostaining. However, the distribution of the fibroblast growth factor receptors in the two strains shows significant differences. In the normal ovaries fibroblast growth factor receptor 3 was homogeneously distributed on the cell membrane of the granulosa cells and was absent in theca as well as corpora lutea cells, whereas in dwarf mice ovaries it was highly clustered on granulosa cells and very often appears in endocytic vesicles. Aged oocytes were more frequently observed in preantral follicles of ovaries of the dwarf mice. Nevertheless, oocytes isolated from antral follicles resume their meiotic division at a high percentage, similar to oocytes obtained from normal ovaries. The results imply fibroblast growth factor receptor 3 involvement in the control of follicular development through regulation of granulosa cell growth and differentiation, and that unovulation in the dwarf mice could be overcome in part by administration of exogenous gonadotropins. Moreover, it is suggested that the infertile phenotype is partially due to defects in the pituitary-gonadal axis.

Molecular cloning of silkworm paralytic peptide and its developmental regulation

The silkworm paralytic peptide (PP) is a member of the ENF peptide family that exerts multiple biological activities involved in defense reaction and growth regulation. We isolated its cDNA and examined mRNA expression profiles. cDNA encoded 131 amino acids from which the 23-residue PP sequence was found at the C-terminal portion. Immunoblot analysis and paralytic activity assay indicated that inactive pro-protein in larval hemolymph was processed into active peptide immediately after bleeding. In the last larval instar, 0.6-kb PP mRNA was expressed in various tissues, of which the fat body was predominant. Its expression in the fat body decreased during the feeding period and then increased during metamorphic process. Juvenile hormone and 20-hydroxyecdysone upregulated its expression. At the embryonic stage, 1.5-kb mRNA, in addition to 0.6-kb mRNA, was expressed from 1 day after oviposition to hatching. PP was thus expressed stage-specifically under hormonal control.

Fibroblast growth factor inhibits chondrocytic growth through induction of p21 and subsequent inactivation of cyclin E-Cdk2

Fibroblast growth factor (FGF) and its receptor (FGFR) are thought to be negative regulators of chondrocytic growth, as exemplified by achondroplasia and related chondrodysplasias, which are caused by constitutively active mutations in FGFR3. To understand the growth-inhibitory mechanisms of FGF, we analyzed the effects of FGF2 on cell cycle-regulating molecules in chondrocytes. FGF2 dramatically inhibited proliferation of rat chondrosarcoma (RCS) cells and arrested their cell cycle at the G(1) phase. FGF2 increased p21 expression in RCS cells, which assembled with the cyclin E-Cdk2 complexes, although the expression of neither cyclin E nor Cdk2 increased. In addition, the kinase activity of immunoprecipitated cyclin E or Cdk2, assessed with retinoblastoma protein (pRb) as substrate, was dramatically reduced by FGF-2. Moreover, FGF2 shifted pRb to its underphosphorylated, active form in RCS cells. FGF2 not only induced p21 protein expression in proliferating chondrocytes in mouse fetal limbs cultured in vitro but also decreased their proliferation as assessed by the expression of histone H4 mRNA, a marker for cells in S phase. Furthermore, inhibitory effects of FGF2 on chondrocytic proliferation were partially reduced in p21-null limbs, compared with those in wild-type limbs in vitro. Taken together, FGF's growth inhibitory effects of chondrocytes appear to be mediated at least partially through p21 induction and the subsequent inactivation of cyclin E-Cdk2 and activation of pRb.

NoBP, a nuclear fibroblast growth factor 3 binding protein, is cell cycle regulated and promotes cell growth

Secreted and nuclear forms of fibroblast growth factor 3 (FGF3) have opposing effects on cells. The secreted form stimulates cell growth and transformation, while the nuclear form inhibits DNA synthesis and cell proliferation. By using the yeast two-hybrid system we have identified a nucleolar FGF3 binding protein (NoBP) which coimmunoprecipitated and colocalized with FGF3 in transfected COS-1 cells. Characterization of the NoBP binding domain of FGF3 exactly matched the sequence requirements of FGF3 for its translocation into the nucleoli, suggesting that NoBP might be the nucleolar binding partner of FGF3 essential for its nucleolus localization. Carboxyl-terminal domains of NoBP contain linear nuclear and nucleolar targeting motifs which are capable of directing a heterologous protein beta-galactosidase to the nucleus and the nucleoli. While NoBP expression was detected in all analyzed proliferating established cell lines, NoBP transcription was rapidly downregulated in the promyelocytic leukemia cell line HL60 when induced to differentiate. Analysis on the expression pattern of NoBP mRNA throughout the cell cycle in HeLa cells synchronized by lovastatin demonstrated a substantial upregulation during the late G(1)/early S phase. NoBP overexpression conferred a proliferating effect onto NIH 3T3 cells and can counteract the inhibitory effect of nuclear FGF3, suggesting a role of NoBP in controlling proliferation in cells. We propose that NoBP is the functional target of nuclear FGF3 action.

FGF signaling controls somite boundary position and regulates segmentation clock control of spatiotemporal Hox gene activation

Vertebrate segmentation requires a molecular oscillator, the segmentation clock, acting in presomitic mesoderm (PSM) cells to set the pace at which segmental boundaries are laid down. However, the signals that position each boundary remain unclear. Here, we report that FGF8 which is expressed in the posterior PSM, generates a moving wavefront at which level both segment boundary position and axial identity become determined. Furthermore, by manipulating boundary position in the chick embryo, we show that Hox gene expression is maintained in the appropriately numbered somite rather than at an absolute axial position. These results implicate FGF8 in ensuring tight coordination of the segmentation process and spatiotemporal Hox gene activation.

Platelet-derived growth factor C (PDGF-C), a novel growth factor that binds to PDGF alpha and beta receptor

We have characterized platelet-derived growth factor (PDGF) C, a novel growth factor belonging to the PDGF family. PDGF-C is a multidomain protein with the N-terminal region homologous to the extracellular CUB domain of neuropilin-1, and the C-terminal region consists of a growth factor domain (GFD) with homology to vascular endothelial growth factor (25%) and PDGF A-chain (23%). A serum-sensitive cleavage site between the two domains allows release of the GFD from the CUB domain. Competition binding and immunoprecipitation studies on cells bearing both PDGF alpha and beta receptors reveal a high affinity binding of recombinant GFD (PDGF-CC) to PDGF receptor-alpha homodimers and PDGF receptor-alpha/beta heterodimers. PDGF-CC exhibits greater mitogenic potency than PDGF-AA and comparable or greater mitogenic activity than PDGF-AB and PDGF-BB on several mesenchymal cell types. Analysis of PDGF-CC in vivo in a diabetic mouse model of delayed wound healing showed that PDGF-CC significantly enhanced repair of a full-thickness skin excision. Together, these studies describe a third member of the PDGF family (PDGF-C) as a potent mitogen for cells of mesenchymal origin in in vitro and in vivo systems with a binding pattern similar to PDGF-AB.

Extrinsic tension results in FGF-2 release, membrane permeability change, and intracellular Ca++ increase in immature cranial sutures

There are numerous studies cataloging the temporal profiles of the various growth factors during the morphogenesis of cranial sutures. There are also many clearly documented mutations of the receptors of some of these growth factors such as fibroblast growth factor (FGF)R-2 and FGFR-3 in clinical craniosynostosis. It is obvious, and often concluded, that growth factors play a role or are involved in craniofacial development. However, precisely what that role is, what causes the changes in the growth factor levels, and why these changes occur in the particular temporal and spatial patterns observed remains elusive. Using simple physics, we applied a plasma membrane disruption model and the principles of complex adaptive systems to arrive at a conjecture of calvarial morphogenesis. The purpose of this article is to introduce the concept of complex adaptive systems, to propose our conjecture, and to provide experimental proof of some key steps in this conjecture: tension induces rapid and demonstrable physiological responses in some cells within the immature cranial sutures. These responses include increases of intracellular Ca++, plasma membrane permeability, and the release of growth factors, e.g., FGF-2. Paired coronal sutures from 1-week-old Sprague-Dawley rat pups were subjected to either 0.59 N of tensile force or no force for 5 minutes in a protein-free medium. FGF-2 levels in the media were measured by slot blot analysis. Western blot analysis was used to determine FGF-2 levels in the sutures. To determine cell membrane permeability changes, fluorescein-conjugated dextran, with a molecular weight of 10 kd, was added to the media during the 5 minutes with or without tensile force. Laser confocal microscopy was used to compare the amount of entry of this impermeant tracer and the pattern of permeability change at the tissue level. To determine the intracellular pCa++, the sutures were first loaded with a calcium indictor, FURA-2 AM, and then subjected isotonically to 0.059 N of tension. The intracellular pCa++ was expressed as ratio of Ca++-bound FURA-2 to Ca++-free FURA-2. The experimental findings were as follows: 1) Sutures, in response to tension, release FGF-2. 2) Sutures contain higher levels of FGF-2 when strained. 3) There is an increase in the sutural cell membrane permeability as a result of tensile strain. 4) The cells along the leading edges of the ossification fronts (at the insertion sites of Sharpey's fibers) demonstrated the maximum permeability increase. 5) There was an immediate (within seconds) increase in intracellular Ca++. and 6) This increase in intracellular Ca++ caused by tension was reversible and independent of the extracellular Ca++ ion availability. In summary, these data support, in part, the conjecture that growth of the brain places strain on the cells within the immature sutures, which causes the iteration of a set of cellular subroutines. These subroutines integrate to generate the emergent property of directed cranial expansion with dissipation of the initiating strains.

Differential regulation of the fibroblast growth factor (FGF) family by alpha(2)-macroglobulin: evidence for selective modulation of FGF-2-induced angiogenesis

The fibroblast growth factor (FGF) family has an important role in processes such as angiogenesis, wound healing, and development in which precise control of proteinase activity is important. The human plasma proteinase inhibitor alpha(2)-macroglobulin (alpha(2)M) regulates cellular growth by binding and modulating the activity of many cytokines and growth factors. These studies investigate the ability of native and activated alpha(2)M (alpha(2)M*) to bind to members of the FGF family. Both alpha(2)M and alpha(2)M* bind specifically and saturably to FGF-1, -2, -4, and -6, although the binding to alpha(2)M* is of significantly higher affinity. Neither alpha(2)M nor alpha(2)M* bind to FGF-5, -7, -9, or -10. FGF-2 was chosen for more extensive study in view of its important role in angiogenesis. It was demonstrated that FGF-2 binds to the previously identified TGF-beta binding site. The alpha(2)M* inhibits FGF-2-dependent fetal bovine heart endothelial cell proliferation in a dose-dependent manner. Unexpectedly, alpha(2)M* does not affect FGF-2-induced vascular tubule formation on Matrigel basement membrane matrix or collagen gels. Further studies demonstrate that FGF-2 partitions between fluid-phase alpha(2)M* and solid-phase Matrigel or collagen. These studies suggest that the ability of alpha(2)M* to modulate the activity of FGF-2 is dependent on an interplay with extracellular matrix components. (Blood. 2001;97:3450-3457)

Identification of a new fibroblast growth factor receptor, FGFR5

A novel fibroblast growth factor receptor (FGFR), designated FGFR5, was identified from an EST database of a murine lymph node stromal cell cDNA library. The EST has approximately 32% identity to the extracellular domain of FGFR1-4. Library screening with this EST identified two full-length alternative transcripts which we designated as FGFR5 beta and FGFR5 gamma. The main difference between these transcripts is that FGFR5 beta contains three extracellular Ig domains whereas FGFR5 gamma contains only two. A unique feature of FGFR5 is that it does not contain an intracellular tyrosine kinase domain. Predictive structural modelling of the extracellular domain of FGFR5 gamma suggested that it was a member of the I-set subgroup of the Ig-superfamily, consistent with the known FGFRs. Northern analysis of mouse and human FGFR5 showed detectable mRNA in a broad range of tissues, including kidney, brain and lung. Genomic sequencing identified four introns but identified no alternative transcripts containing a tyrosine kinase domain. Extracellular regions of FGFR5 beta and 5 gamma were cloned in-frame with the Fc fragment of human IgG(1) to generate recombinant non-membrane bound protein. Recombinant FGFR5 beta Fc and R5 gamma Fc demonstrated specific binding to the ligand FGF-2, but not FGF-7 or EGF. However, biological data suggest that FGF-2 binding to these proteins is with lower affinity than its cognate receptor FGFR2C. The above data indicate that this receptor should be considered as the fifth member of the FGFR family.

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.

Fibroblast growth factor-2 promotes axon branching of cortical neurons by influencing morphology and behavior of the primary growth cone

Interstitial branching is an important mechanism for target innervation in the developing CNS. A previous study of cortical neurons in vitro showed that the terminal growth cone pauses and enlarges in regions from which interstitial axon branches later develop (Szebenyi et al., 1998). In the present study, we investigated how target-derived signals affect the morphology and behaviors of growth cones leading to development of axon branches. We used bath and local application of a target-derived growth factor, FGF-2, on embryonic pyramidal neurons from the sensorimotor cortex and used time-lapse digital imaging to monitor effects of FGF-2 on axon branching. Observations of developing neurons over periods of several days showed that bath-applied FGF-2 significantly increased growth cone size and slowed growth cone advance, leading to a threefold increase in axon branching. FGF-2 also had acute effects on growth cone morphology, promoting rapid growth of filopodia within minutes. Application of FGF-2-coated beads promoted local axon branching in close proximity to the beads. Branching was more likely to occur when the FGF-2 bead was on or near the growth cone, suggesting that distal regions of the axon are more responsive to FGF-2 than other regions of the axon shaft. Together, these results show that interstitial axon branches can be induced locally through the action of a target-derived growth factor that preferentially exerts effects on the growth cone. We suggest that, in target regions, growth factors such as FGF-2 and other branching factors may induce formation of collateral axon branches by enhancing the pausing and enlargement of primary growth cones that determine future branch points.

Keratinocyte growth factor is up-regulated by estrogen in the porcine uterine endometrium and functions in trophectoderm cell proliferation and differentiation

Keratinocyte growth factor (KGF) is expressed by uterine endometrial epithelial cells during the estrous cycle and during pregnancy in pigs, whereas KGF receptor is expressed in conceptus trophectoderm and endometrial epithelia. In particular, KGF expression in the endometrium is highest on day 12 of pregnancy. This corresponds to the period of maternal recognition of pregnancy in pigs, which is signaled by large amounts of estrogen secreted by conceptus trophectoderm acting on the endometrium. Our hypothesis is that estrogens of conceptus origin stimulate endometrial epithelial KGF expression, and, in turn, secreted KGF stimulates proliferation and differentiation of conceptus trophectoderm. To determine the factors affecting KGF expression in the uterus, endometrial explants from gilts on day 9 of the estrous cycle were cultured in the presence of 17beta-estradiol, catechol estrogens, or progesterone. 17beta-Estradiol stimulated the expression of KGF (P < 0.05), whereas catechol estrogens had no effect (P > 0.05). Between days 9 and 15 of pregnancy, proliferating cell nuclear antigen was abundant in conceptuses, but was barely detectable in uterine endometrial epithelia. To determine the effects of KGF on conceptus trophectoderm, porcine trophectoderm (pTr) cells were treated with recombinant rat KGF (rKGF). rKGF increased the proliferation of pTr cells (P < 0.01) as measured by [(3)H]thymidine incorporation. rKGF elicited phosphorylation of KGF receptor and activated the mitogen-activated protein kinase (ERK1/2) cascade in pTr cells. pTr cell differentiation was affected by rKGF, because it increased expression of urokinase-type plasminogen activator, a marker for differentiation in pTr cells. Collectively, these results indicate that estrogen, the pregnancy recognition signal from the conceptus in pigs, increases uterine epithelial KGF expression, and, in turn, KGF stimulates the proliferation and differentiation of conceptus trophectoderm.

FGF-8b increases angiogenic capacity and tumor growth of androgen-regulated S115 breast cancer cells

Fibroblast growth factor 8 (FGF-8) is a secreted heparin-binding protein, which has transforming potential. Alternative splicing of the mouse Fgf-8 gene potentially codes for eight protein isoforms (a-h) which differ in their transforming capacity in transfected cells. S115 mouse mammary tumor cells express a transformed phenotype and secrete FGF-8 in an androgen-dependent manner. In order to study the role of FGF-8 isoforms in the induction of transformed phenotype of breast cancer cells, we over-expressed FGF-8 isoforms a, b and e in S115 cells. Over-expression of FGF-8b, but not FGF-8a or FGF-8e, induced androgen and anchorage independent growth of S115 cells. FGF-8b-transfected S115 cells formed rapidly growing tumors with increased vascularization when injected s.c. into nude mice. FGF-8a also slightly increased tumor growth and probably tumor vascularization but FGF-8e was not found to have any effects. The angiogenic activity of FGF-8b and heparin-binding growth factor fraction (HBGF) of S115 cell conditioned media was tested in in vitro and in vivo models for angiogenesis using immortomouse brain capillary endothelial cells (IBEC) and chorion allantoic membrane (CAM) assays. Recombinant FGF-8b protein was able to stimulate proliferation, migration, and vessel-like tube formation of IBECs. In addition, stimulatory effect of S115-HBGF on IBE cell proliferation was evident. A positive angiogenic response to FGF-8b was also seen in CAM assay. The results demonstrate that the expression of Fgf-8b is able to promote vessel formation. Angiogenic capacity probably markedly contributes to the ability of FGF-8b to increase tumor growth of androgen-regulated S115 mouse breast cancer cells.

Secreted FGFR3, but not FGFR1, inhibits lens fiber differentiation

The vertebrate lens has a distinct polarity with cuboidal epithelial cells on the anterior side and differentiated fiber cells on the posterior side. It has been proposed that the anterior-posterior polarity of the lens is imposed by factors present in the ocular media surrounding the lens (aqueous and vitreous humor). The differentiation factors have been hypothesized to be members of the fibroblast growth factor (FGF) family. Though FGFs have been shown to be sufficient for induction of lens differentiation both in vivo and in vitro, they have not been demonstrated to be necessary for endogenous initiation of fiber cell differentiation. To test this possibility, we have generated transgenic mice with ocular expression of secreted self-dimerizing versions of FGFR1 (FR1) and FGFR3 (FR3). Expression of FR3, but not FR1, leads to an expansion of proliferating epithelial cells from the anterior to the posterior side of the lens due to a delay in the initiation of fiber cell differentiation. This delay is most apparent postnatally and correlates with appropriate changes in expression of marker genes including p57(KIP2), Maf and Prox1. Phosphorylation of Erk1 and Erk2 was reduced in the lenses of FR3 mice compared with nontransgenic mice. Though differentiation was delayed in FR3 mice, the lens epithelial cells still retained their intrinsic ability to respond to FGF stimulation. Based on these results we propose that the initiation of lens fiber cell differentiation in mice requires FGF receptor signaling and that one of the lens differentiation signals in the vitreous humor is a ligand for FR3, and is therefore likely to be an FGF or FGF-like factor.

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.

Identification of tyrosine residues in constitutively activated fibroblast growth factor receptor 3 involved in mitogenesis, Stat activation, and phosphatidylinositol 3-kinase activation

Fibroblast growth factor receptor 3 (FGFR3) mutations are frequently involved in human developmental disorders and cancer. Activation of FGFR3, through mutation or ligand stimulation, results in autophosphorylation of multiple tyrosine residues within the intracellular domain. To assess the importance of the six conserved tyrosine residues within the intracellular domain of FGFR3 for signaling, derivatives were constructed containing an N-terminal myristylation signal for plasma membrane localization and a point mutation (K650E) that confers constitutive kinase activation. A derivative containing all conserved tyrosine residues stimulates cellular transformation and activation of several FGFR3 signaling pathways. Substitution of all nonactivation loop tyrosine residues with phenylalanine rendered this FGFR3 construct inactive, despite the presence of the activating K650E mutation. Addition of a single tyrosine residue, Y724, restored its ability to stimulate cellular transformation, phosphatidylinositol 3-kinase activation, and phosphorylation of Shp2, MAPK, Stat1, and Stat3. These results demonstrate a critical role for Y724 in the activation of multiple signaling pathways by constitutively activated mutants of FGFR3.

FGF and VEGF function in angiogenesis: signalling pathways, biological responses and therapeutic inhibition

Angiogenic growth factors such as fibroblast growth factors (FGFs) and vascular endothelial growth factors (VEGFs) are currently targets of intense efforts to inhibit deregulated blood vessel formation in diseases such as cancer. FGFs and VEGFs exert their effects via specific binding to cell surface-expressed receptors equipped with tyrosine kinase activity. Activation of the receptor kinase activity allows coupling to downstream signal transduction pathways that regulate proliferation, migration and differentiation of endothelial cells. Inhibitors of FGF and VEGF signalling are currently in clinical trials. In this article, the current knowledge of FGF- and VEGF-induced signal transduction that leads to specific biological responses will be summarized. Furthermore, the manner in which this knowledge is being exploited to regulate angiogenesis will be discussed.

WNT signals control FGF-dependent limb initiation and AER induction in the chick embryo

A regulatory loop between the fibroblast growth factors FGF-8 and FGF-10 plays a key role in limb initiation and AER induction in vertebrate embryos. Here, we show that three WNT factors signaling through beta-catenin act as key regulators of the FGF-8/FGF-10 loop. The Wnt-2b gene is expressed in the intermediate mesoderm and the lateral plate mesoderm in the presumptive chick forelimb region. Cells expressing Wnt-2b are able to induce Fgf-10 and generate an extra limb when implanted into the flank. In the presumptive hindlimb region, another Wnt gene, Wnt-8c, controls Fgf-10 expression, and is also capable of inducing ectopic limb formation in the flank. Finally, we also show that the induction of Fgf-8 in the limb ectoderm by FGF-10 is mediated by the induction of Wnt-3a. Thus, three WNT signals mediated by beta-catenin control both limb initiation and AER induction in the vertebrate embryo.

Male-to-female sex reversal in mice lacking fibroblast growth factor 9

Fgfs direct embryogenesis of several organs, including the lung, limb, and anterior pituitary. Here we report male-to-female sex reversal in mice lacking Fibroblast growth factor 9 (Fgf9), demonstrating a novel role for FGF signaling in testicular embryogenesis. Fgf9(-/-) mice also exhibit lung hypoplasia and die at birth. Reproductive system phenotypes range from testicular hypoplasia to complete sex reversal, with most Fgf9(-/-) XY reproductive systems appearing grossly female at birth. Fgf9 appears to act downstream of Sry to stimulate mesenchymal proliferation, mesonephric cell migration, and Sertoli cell differentiation in the embryonic testis. While Sry is found only in some mammals, Fgfs are highly conserved. Thus, Fgfs may function in sex determination and reproductive system development in many species.

Expression and purification of human recombinant GST-FGF receptor-1

When tumors undergo the angiogenic switch, cell growth and tissue invasion is facilitated by the formation of new capillaries from preexisting blood vessels, a process known as angiogenesis. Growth factors such as vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) and fibroblast growth factor (FGF) trigger the process of angiogenesis. Here we describe a protocol for the expression and one-step purification of human recombinant GST-FGF receptor type 1 (FGFR-1) from Sf9 cells. This protocol allows generating an active kinase as indicated by its reactivity with a monoclonal antibody to phosphorylated tyrosine. The purified enzyme displays a specific activity of 1.2 x 10(4) pmol mg(-1) min(-1), which is in the range of activities reported for homogeneously purified recombinant kinases. We have employed a number of compounds to show that the GST-FGFR-1 preparation is suitable to the identification of tyrosine kinase inhibitors. Considering that inhibitors of angiogenesis may represent an attractive tool in therapeutic strategies targeting invasive metastatic tumors the results presented here, along with available data on the structure of the ATP-binding pocket of FGFR-1, should facilitate the rational design of specific FGFR-1 inhibitory compounds.

Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution

The turtle shell is an evolutionary novelty that is synapomorphic for chelonians. The carapace is initiated by the entrapment of the ribs by the carapacial ridge (CR), a lateral bulge of the dorsal ectoderm and dermal mesoderm. The mechanisms by which the CR is initiated, the ribs entrapped and the dorsal dermis ossified, remains unknown. Similarly, the formation of the plastron remains unexplained. Here, we present a series of anatomical investigations into plastron and carapace formation in the red-eared slider, Trachemys scripta, and the snapping turtle, Chelydra serpentina. We document the entrapment of the ribs by the CR and the formation of the plastron and carapacial bones by intramembranous ossification. We note the formation of the ossification centers around each rib, which suggest that the rib is organizing dermal ossification by secreting paracrine factors. The nuchal ossification center is complex and appears to involve multiple bone-forming regions. Individual ossification centers at the periphery of the carapace form the peripheral and pygial bones. The intramembranous ossification of the plastron proceeds from nine distinct ossification centers, and there appear to be interactions between the spicules of apposing centers as they draw near each other.

Involvement of growth factors in the regulation of pubertal mammary growth in cattle

Pubertal mammary growth in heifers is dependent on interactions of many hormones and growth factors of which some are stimulatory while others are inhibitory. Although estrogen and growth hormone (GH) are of primary importance, more recent studies have suggested a role for both systemic and mammary tissue-specific growth factors. Growth factors may act as mediators of estrogen and GH or through specific effects of their own. These growth factors include insulin (INS), IGFs (IGF-I and IGF-II), epidermal growth factor (EGF), FGFs (FGF-1 and FGF-2), TGFs (TGF-alpha and TGF-beta's, amphiregulin (AR), platelets derived growth factor (PDGF), and mammary derived growth factor-1 (MDGF-1). Using mammary epithelial cells derived from prepubertal heifers and cultured in three-dimensional collagen gels as an in vitro model, we have investigated the mitogenic effects of a number of different growth factors (IGF-I, des(1-3) IGF-I, IGF-II, INS, EGF, TGF-alpha, AR, FGF-1, FGF-2, and TGF-beta 1). As expected, IGF-I, des(1-3)IGF-I, IGF-II and INS all stimulated proliferation of mammary cells with des(1-3)IGF-I being the most potent and INS the least potent. The mitogenic effect of IGF-I could be inhibited by both IGFBP-2 and IGFBP-3 showing that these binding proteins modulate the bioactivity of IGF-I in the mammary gland at the cellular level. Regulation of IGF availability by IGFBPs in the extracellular environment therefore is critical for IGF action in the mammary gland. Proliferation of mammary epithelial cells was also stimulated by growth factors of the EGF family, i.e. EGF, TGF-alpha and AR, however, not as much as growth factors from the IGF family. Members of the fibroblast growth factor family showed various mitogenic activities. FGF-1 stimulated DNA synthesis while FGF-2 in concentrations above 10 ng/ml inhibited DNA synthesis. TGF-beta 1 at very low concentrations stimulated proliferation slightly whereas higher concentrations strongly inhibited proliferation of mammary epithelial cells and inhibited mitogenesis induced by growth factors of both the EGF- and IGF family. This shows that TGF-beta 1 is a very potent regulator of pubertal mammary growth.

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.

Fibroblast growth factors

SUMMARY

Fibroblast growth factors (FGFs) make up a large family of polypeptide growth factors that are found in organisms ranging from nematodes to humans. In vertebrates, the 22 members of the FGF family range in molecular mass from 17 to 34 kDa and share 13-71% amino acid identity. Between vertebrate species, FGFs are highly conserved in both gene structure and amino-acid sequence. FGFs have a high affinity for heparan sulfate proteoglycans and require heparan sulfate to activate one of four cell-surface FGF receptors. During embryonic development, FGFs have diverse roles in regulating cell proliferation, migration and differentiation. In the adult organism, FGFs are homeostatic factors and function in tissue repair and response to injury. When inappropriately expressed, some FGFs can contribute to the pathogenesis of cancer. A subset of the FGF family, expressed in adult tissue, is important for neuronal signal transduction in the central and peripheral nervous systems.

Attenuation of FGF signalling in mouse beta-cells leads to diabetes

Fibroblast growth factor (FGF) signalling has been implicated in patterning, proliferation and cell differentiation in many organs, including the developing pancreas. Here we show that the FGF receptors (FGFRs) 1 and 2, together with the ligands FGF1, FGF2, FGF4, FGF5, FGF7 and FGF10, are expressed in adult mouse beta-cells, indicating that FGF signalling may have a role in differentiated beta-cells. When we perturbed signalling by expressing dominant-negative forms of the receptors, FGFR1c and FGFR2b, in the pancreas, we found that that mice with attenuated FGFR1c signalling, but not those with reduced FGFR2b signalling, develop diabetes with age and exhibit a decreased number of beta-cells, impaired expression of glucose transporter 2 and increased proinsulin content in beta-cells owing to impaired expression of prohormone convertases 1/3 and 2. These defects are all characteristic of patients with type-2 diabetes. Mutations in the homeobox gene Ipf1/Pdx1 are linked to diabetes in both mouse and human. We also show that Ipf1/Pdx1 is required for the expression of FGFR1 signalling components in beta-cells, indicating that Ipf1/Pdx1 acts upstream of FGFR1 signalling in beta-cells to maintain proper glucose sensing, insulin processing and glucose homeostasis.

Basic fibroblast growth factor utilizes both types of component subunits of Gs for dual signaling in human adipocytes. Stimulation of adenylyl cyclase via Galph(s) and inhibition of NADPH oxidase by Gbeta gamma(s)

Basic fibroblast growth factor (bFGF), a ligand of receptor protein-tyrosine kinases, promoted the dissociation of G(s) and had antagonistic stimulatory and inhibitory effects on adenylyl cyclase and NADPH oxidase in human fat cell plasma membranes. The bFGF-induced activation of adenylyl cyclase was blocked by COOH-terminal anti-Galpha(s), indicating that it was mediated by Galpha(s). The inhibitory action of bFGF was mimicked by exogenously supplied Gbetagamma-subunits and was reversed by anti-Gbeta(1/2), or betaARK-CT, a COOH-terminal beta-adrenergic receptor kinase fragment that specifically binds free Gbetagamma, indicating that it was transduced by Gbetagamma complexes. The bFGF-induced inhibition of NADPH-dependent H(2)O(2) generation was also reversed by peptide 100-119, an inhibitor of G(s) activation by ligand-occupied beta-adrenergic receptors, indicating that the Gbetagamma complexes mediating the inhibitory action of the growth factor are derived from G(s). The findings suggest a direct, non-kinase-dependent, coupling of bFGF receptor(s) to G(s) and provide the first example of a ligand of receptor protein-tyrosine kinases that is capable of utilizing both types of component subunits of a single heterotrimeric G protein for dual signaling in a single cell type.

Hindbrain patterning: FGFs regulate Krox20 and mafB/kr expression in the otic/preotic region

Krox20 and mafB/kr are regulatory genes involved in hindbrain segmentation and anteroposterior (AP) patterning. They are expressed in rhombomeres (r) r3/r5 and r5/r6 respectively, as well as in the r5/r6 neural crest. Since several members of the fibroblast growth factor (FGF) family are expressed in the otic/preotic region (r2-r6), we investigated their possible involvement in the regulation of Krox20 and mafB/kr. Application of exogenous FGFs to the neural tube of 4- to 7-somite chick embryos led to ectopic expression in the neural crest of the somitic hindbrain (r7 and r8) and to the extension of the Krox20- or mafB/kr-positive areas in the neuroepithelium. Application of an inhibitor of FGF signalling led to severe and specific downregulation of Krox20 and mafB/kr in the hindbrain neuroepithelium and neural crest. These data indicate that FGFs are involved in the control of regional induction and/or maintenance of Krox20 and mafB/kr expression, thus identifying a novel function for these factors in hindbrain development, besides their proposed more general role in early neural caudalisation.

FGF signalling controls the timing of Pax6 activation in the neural tube

We have recently demonstrated that Pax6 activation occurs in phase with somitogenesis in the spinal cord. Here we show that the presomitic mesoderm exerts an inhibitory activity on Pax6 expression. This repressive effect is mediated by the FGF signalling pathway. The presomitic mesoderm displays a decreasing caudorostral gradient of FGF8, and grafting FGF8-soaked beads at the level of the neural tube abolishes Pax6 activation. Conversely, when FGF signalling is disrupted, Pax6 is prematurely activated in the neural plate. We propose that the progression of Pax6 activation in the neural tube is controlled by the caudal regression of the anterior limit of FGF activity. Hence, as part of its posteriorising activity, FGF8 downregulation acts as a switch from early (posterior) to a later (anterior) state of neural epithelial development.

Translocation of FGF2 to the cell surface without release into conditioned media

Like most cells in culture, stably transfected COS-1 cells (CF18) that constitutively overexpress basic fibroblast growth factor (FGF2) do not release the growth factor into conditioned media. Yet, when cells were biotinylated, 30% of the total cell-associated immunoreactive FGF2 was detected on the cell surface. Under similar conditions, up to 70% of the total immunoreactive FGF2 in transfected endothelial cells (MAE ZIP) or untransfected rat (C6) and human (U87MG) glioblastoma cell lines was detected on their cell surface. When peripheral plasma membrane proteins were removed from the cell surface with 0.1 M sodium carbonate, the amount of exported FGF2 was significantly reduced, whereas cell viability was unaffected. FGF2 then reappeared on the cell surface in a time-dependent manner. Ouabain, a cardenolide previously shown to inhibit the export of FGF2 from transiently transfected COS-1 cells, blocked the appearance of FGF2 onto the surface of transfected CF18 cells and MAE ZIP cells but had no detectable effect on C6 and U87MG cells. The observation that the translocation of FGF2 onto the cell surface is dissociated from its release into conditioned medium is consistent with FGF2's being rarely found in biological fluids but always cell associated and in the extracellular matrix. The findings point to a role played by the protein export pathway in controlling FGF2 activity and the normal physiological function that this growth factor plays in cell growth and differentiation. The widely accepted presumption that the absence of FGF2 in conditioned media reflects its inability to exit the cell needs to be reevaluated.

Basic fibroblast growth factor inhibits apoptosis of spontaneously immortalized granulosa cells by regulating intracellular free calcium levels through a protein kinase Cdelta-dependent pathway

Previous studies have shown that basic fibroblast growth factor (bFGF) inhibits primary granulosa cells from undergoing apoptosis. The present studies were designed to determine whether spontaneously immortalized granulosa cells (SIGCs) undergo apoptosis when deprived of growth factors and whether bFGF prevents apoptosis. In the absence of serum, the SIGCs lost cell contact and underwent apoptosis as indicated by the presence of annexin V binding, DNA ladders, and nuclear fragmentation. Basic FGF maintained cell contact and reduced the percentage of apoptotic cells. This antiapoptotic action was not observed ifbFGF was added 30 min after serum withdrawal. Further, intracellular free calcium ([Ca2+]i) levels gradually increased 3- to 4-fold within 10 min of serum withdrawal. This increase was inhibited by bFGF. The intracellular calcium chelator, BAPTA, completely prevented the SIGCs from undergoing apoptosis in the absence of serum. These observations suggest that bFGF's ability to regulate [Ca2+]i is an essential component of its antiapoptotic action. The phorbol ester TPA, an activator of protein kinase C (PKC), blocked apoptosis due to serum deprivation. Conversely, bisindolylmaleimide II, an inhibitor of PKC, completely attenuated, whereas bisindolylmaleimide V, an inactive bisindolylmaleimide analog, did not influence bFGF's antiapoptotic action. Also, treatment with the PKC inhibitor, chelerythrine chloride, interfered with bFGF's ability to maintain calcium homeostasis. Western blot analysis revealed that SIGCs expressed PKCdelta, tau, lambda, and zeta. Of these PKC isoforms, only PKCdelta has been shown to be activated by TPA. In apoptotic SIGCs, PKCdelta levels were depleted. When PKCdelta levels were reduced by pretreatment with 500 nM TPA, neither bFGF nor 10 nM TPA suppressed apoptosis. Collectively, these observations suggest that bFGF maintains [Ca2+]i and thereby SIGC viability through a PKCdelta-dependent pathway.

Characterization of a novel protein (FGFRL1) from human cartilage related to FGF receptors

Utilizing a subtractive cDNA cloning approach we have identified a novel protein from human cartilage. This protein represents an integral membrane protein with 504 amino acids and a molecular mass of 55 kDa. It is composed of a signal peptide, three extracellular Ig-like modules, a transmembrane segment, and a short intracellular domain. The extracellular domain is closely related to the extracellular domain of FGF receptors. The intracellular domain, however, does not show any similarity to the protein tyrosine kinase domain of FGF receptors. The novel gene (FGFRL1) is located on human chromosome 4 band p16 in close proximity to the gene for FGFR3. Its mRNA is preferentially expressed in cartilaginous tissues. Owing to the structural similarity, it is conceivable that the novel protein plays a role in the modulation of FGF receptor activity.

The proliferative and migratory activities of breast cancer cells can be differentially regulated by heparan sulfates

To explore how heparan sulfate (HS) controls the responsiveness of the breast cancer cell lines MCF-7 and MDA-MB-231 to fibroblast growth factors (FGFs), we have exposed them to HS preparations known to have specificity for FGF-1 (HS glycosaminoglycan (HSGAG A)) or FGF-2 (HSGAGB). Proliferation assays confirmed that MCF-7 cells were highly responsive to FGF-2 complexed with GAGB, whereas migration assays indicated that FGF-1/HSGAGA combinations were stimulatory for the highly invasive MDA-MB-231 cells. Quantitative polymerase chain reaction for the levels of FGF receptor (FGFR) isoforms revealed that MCF-7 cells have greater levels of FGFR1 and that MDA-MB-231 cells have greater relative levels of FGFR2. Cross-linking demonstrated that FGF-2/HSGAGB primarily activated FGFR1, which in turn up-regulated the activity of mitogen-activated protein kinase; in contrast, FGF-1/HSGAGA led to the phosphorylation of equal proportions of both FGFR1 and FGFR2, which in turn led to the up-regulation of Src and p125(FAK). MDA-MB-231 cells were particularly responsive to vitronectin substrates in the presence of FGF-1/HSGAGA, and blocking antibodies established that they used the alpha(v)beta(3) integrin to bind to it. These results suggest that the clustering of particular FGFR configurations on breast cancer cells induced by different HS chains leads to distinct phenotypic behaviors.

Identification and functional characterization of the human and murine fibroblast growth factor receptor 4 promoters

Fibroblast growth factor receptors (FGFRs) play crucial roles in signal transduction of adult tissues and during embryonic development. To study the transcriptional control, we isolated and characterized the promoter of human FGFR4. Two transcription initiation sites were identified. The deletion analysis in different cell types defined a core promoter reaching from -9 to -198, lacking TATA and CCAAT boxes but displaying high GC content (77%) in a stretch of 300 bp upstream of the major mRNA start. This region harbors multiple binding motifs for transcription factors. Moreover, the region between -1085 and -1140 contains a potential repressor element, which downregulates transcriptional activity. To identify conserved regulatory elements, we isolated and analyzed also the murine FGFR4 promoter. Only one transcription start was identified using RNase protection assays. Sequence alignment of human and mouse shows a striking similarity in the core promoter region of both genes, encompassing conserved transcription factor binding sites and a splice acceptor site. Furthermore, the region containing the putative repressor element is also conserved suggesting a functional role for gene expression.

FGF-2 Up-regulation and proliferation of neural progenitors in the regenerating amphibian spinal cord in vivo

Regeneration of the spinal cord occurs spontaneously in adult urodele amphibians. The key cells in this regenerative process appear to be the ependymal cells that following injury migrate and proliferate to form the ependymal tube from which the spinal cord regenerates. Very little is known about the signal(s) that initiates and maintains the proliferative response of these cells. Fibroblast growth factor 2 (FGF-2) has been shown to play a role in maintaining neural progenitor cell cycling in vitro and may be important for neuronal survival and axonal growth after injury. We have investigated its role in regeneration of the spinal cord in vivo following tail amputation in the adult salamander, Pleurodeles waltl. We show that only the low-molecular-weight form of FGF-2 is found in Pleurodeles and that in the normal cord it is expressed in a subset of neurons, but is hardly detectable in ependymal cells. Tail amputation results in induction of FGF-2 in the ependymal cells of the regenerating structure, and later in regeneration FGF-2 is up-regulated in some newborn neurons. FGF-2 pattern of expression in the ependymal tube parallels that of proliferation. Furthermore, exogenous FGF-2 significantly increases ependymal cell proliferation in vivo. Overall our results strongly support the view that one important role of FGF-2 during spinal cord regeneration in Pleurodeles is to induce proliferation of neural progenitor cells.

Gene expression patterns of the fibroblast growth factors and their receptors during myogenesis of rat satellite cells

Satellite cells are the myogenic precursors in postnatal muscle and are situated beneath the myofiber basement membrane. We previously showed that fibroblast growth factor 2 (FGF2, basic FGF) stimulates a greater number of satellite cells to enter the cell cycle but does not modify the overall schedule of a short proliferative phase and a rapid transition to the differentiated state as the satellite cells undergo myogenesis in isolated myofibers. In this study we investigated whether other members of the FGF family can maintain the proliferative state of the satellite cells in rat myofiber cultures. We show that FGF1, FGF4, and FGF6 (as well as hepatocyte growth factor, HGF) enhance satellite cell proliferation to a similar degree as that seen with FGF2, whereas FGF5 and FGF7 are ineffective. None of the growth factors prolongs the proliferative phase or delays the transition of the satellite cells to the differentiating, myogenin(+) state. However, FGF6 retards the rapid exit of the cells from the myogenin(+) state that routinely occurs in myofiber cultures. To determine which of the above growth factors might be involved in regulating satellite cells in vivo, we examined their mRNA expression patterns in cultured rat myofibers using RT-PCR. The expression of all growth factors, excluding FGF4, was confirmed. Only FGF6 was expressed at a higher level in the isolated myofibers and not in the connective tissue cells surrounding the myofibers or in satellite cells dissociated away from the muscle. By Western blot analysis, we also demonstrated the presence of FGF6 protein in the skeletal musle tissue. Our studies therefore suggest that the myofibers serve as the main source for the muscle FGF6 in vivo. We also used RT-PCR to analyze the expression patterns of the four tyrosine kinase FGF receptors (FGFR1-FGFR4) and of the HGF receptor (c-met) in the myofiber cultures. Depending on the time in culture, expression of all receptors was detected, with FGFR2 and FGFR3 expressed only at a low level. Only FGFR4 was expressed at a higher level in the myofibers but not the connective tissue cell cultures. FGFR4 was also expressed at a higher level in satellite cells compared to the nonmyogenic cells when the two cell populations were released from the muscle tissue and fractionated by Percoll density centrifugation. The unique localization patterns of FGF6 and FGFR4 may reflect specific roles for these members of the FGF signaling complex during myogenesis in adult skeletal muscle.

Fibroblast growth factor-9 modulates the expression of myelin related proteins and multiple fibroblast growth factor receptors in developing oligodendrocytes

The effect of fibroblast growth factor (FGF)-9 on the expression of FGF receptors (FGFR) and the major myelin proteins was examined in cultures of developing rat brain oligodendrocytes (OLs), using immunological techniques. FGFR-1, -3, and -4 were expressed at all developmental stages but were not present in isolated myelin fractions. By contrast, FGFR-2 protein was predominantly localized to differentiating cells and myelin. FGF-9 altered FGFR and myelin protein levels during OL differentiation; there was increased expression of FGFR-1 and decreased levels of both FGFR-2 and myelin proteins. Further, FGF-9 stimulated mitogen-associated protein kinase (MAPK) phosphorylation. The effect of FGF-9 on MAPK, however, was transient and less robust in progenitor cells than in differentiated oligodendrocytes. The effects of FGF-9 and FGF-2 on FGFR and myelin protein levels were comparable; both up-regulated FGFR-1, and down-regulated FGFR-2, CNP, PLP and MBP. These findings suggest that FGF-9 may be important for glial cell development.