Mutations uncouple human fibroblast growth factor (FGF)-7 biological activity and receptor binding and support broad specificity in the secondary receptor binding site of FGFs

Understanding Folding of bFGF and Potential Cellular Protective Mechanisms of Neural Cells

Traumatic brain injury (TBI) is a serious health condition that affects an increasing number of people, especially veterans and athletes. TBI causes serious consequences because of its long-lasting impact on the brain and its alarming frequency of occurrence. Although the brain has some natural protective mechanisms, the processes that trigger them are poorly understood. Fibroblast growth factor (FGF) proteins interact with receptor proteins to protect cells. Gaps in the literature include how basic-FGF (bFGF) is activated by heparin, can heparin-like molecules induce neural protection, and the effect of allosteric binding on bFGF activity. To fill the gap in our understanding, we applied temperature replica exchange to study the influence of heparin binding to bFGF and how mutations in bFGF influence stability. A new favorable binding site was identified by comparing free energies computed from the potential of mean force (PMF). Although the varied sugars studied resulted in different interactions with bFGF compared to heparin, they each produced structural effects similar to those of bFGF that likely facilitate receptor binding and signaling. Our results also demonstrate how point mutations can trigger the same conformational change that is believed to promote favorable interactions with the receptor. A deeper atomic-level understanding of how chemicals are released during TBI is needed to improve the development of new treatments for TBI and could contribute to a better understanding of other diseases.

Construction and characterization of a functional variant hFGF7 with enhanced properties by circular permutation

Human fibroblast growth factor 7 (hFGF7) is a member of the paracrine-acting FGF family and mediates various reactions such as wound healing, tissue homeostasis, and liver regeneration. These activities make it a plausible candidate for pharmaceutical applications as a drug. However, the low expression level and stability of the recombinant hFGF7 were known to be major hurdles for further applications. Here, the expression level and stability of hFGF7 were attempted to improve by changing the order of amino acids through circular permutation (CP), thereby expecting an alternative fate according to the N-end rule. CP-hFGF7 variants were constructed systematically by using putative amino acid residues in the loop region that avoided the disruption of the structural integrity especially in the functional motif. Among them, cp-hFGF7115-114 revealed a relatively higher expression level in the soluble fraction than the wild-type hFGF7 and was efficiently purified (7 mg L-1) to apparent homogeneity. The activity and stability of the purified variant cp-hFGF7115-114 were comparable or superior to that of the wild-type hFGF7, thereby strongly suggesting that CP could be an alternative tool for the functional expression of hFGF7 in Escherichia coli.

The heparan sulfate co-receptor and the concentration of fibroblast growth factor-2 independently elicit different signalling patterns from the fibroblast growth factor receptor

Background

The fibroblast growth factor receptor (FGFR) interprets concentration gradients of FGF ligands and structural changes in the heparan sulfate (HS) co-receptor to generate different cellular responses. However, whether the FGFR generates different signals is not known.

Results

We have previously shown in rat mammary fibroblasts that in cells deficient in sulfation, and so in HS co-receptor, FGF-2 can only stimulate a transient phosphorylation of p42/44 ^MAPK and so cannot stimulate DNA synthesis. Here we demonstrate that this is because in the absence of HS, FGF-2 fails to stimulate the phosphorylation of the adaptor FGFR substrate 2 (FRS2). In cells possessing the HS co-receptor, FGF-2 elicits a bell-shaped dose response: optimal concentrations stimulate DNA synthesis, but supramaximal concentrations (≥ 100 ng/mL) have little effect. At optimal concentrations (300 pg/mL) FGF-2 stimulates a sustained dual phosphorylation of p42/44 ^MAPK and tyrosine phosphorylation of FRS2. In contrast, 100 ng/mL FGF-2 only stimulates a transient early peak of p42/44 ^MAPK phosphorylation and fails to stimulate appreciably the phosphorylation of FRS2 on tyrosine.

Conclusions

These results suggest that the nature of the FGFR signal produced is determined by a combination of the HS co-receptor and the concentration of FGF ligand. Both the phosphorylation of the adaptor FRS2, the kinetics (sustained or transient) of phosphorylation of p42/44(MAPK) are varied, and so differing cellular responses are produced.

Thr-114 is an important functional residue of fibroblast growth factor 10 identified by structure-based mutational analysis

Fibroblast growth factor 10 (FGF10) plays important roles in vertebrate limb development, lung branching morphogenesis, and epidermis regeneration. The receptor (FGFR2b) binding specificity is an essential element in regulating the diverse functions of FGF10. Analyzing the FGF10:FGFR2b complex we found that Thr-114 in beta4 of FGF10 could form specific interactions with D3 of FGFR2b. To investigate the role of Thr-114 played on functions of FGF10, two mutants of FGF10 were constructed, named TA (Thr-114-->Ala) and TR (Thr-114-->Arg), respectively. The biological activity assays showed that the receptor-binding affinity, the stimulating growth effect on rat tracheal epithelium (RTE) cells, and the inducing ability in receptor phosphorylation of both mutants were decreased, which were consistent with the interaction analysis of the TA:FGFR2b and TR:FGFR2b complexes. These results suggested that Thr-114 is a crucial functional residue for FGF10, and mutating Thr-114 to Ala or Arg would lead to great decrease in receptor-binding affinity and biological activity of FGF10.

Construction and characterization of a high activity mutant of human keratinocyte growth factor-2

Keratinocyte growth factor-2 (KGF-2) plays an important role in vertebrate limb development, lung branching morphogenesis, regeneration and reconstruction of the epidermis. Previous studies have used the wild type factor. Here, we have constructed a double-site mutant of human KGF-2, named STEA. STEA possesses higher receptor binding affinity and promotes better proliferation activity on rat tracheal epithelium (RTE) cells than recombinant human KGF-2. These results suggest that the simultaneous mutation of Ser115 to Thr and Glu117 to Ala improves the biological activity of KGF-2.

Molecular insights into the klotho-dependent, endocrine mode of action of fibroblast growth factor 19 subfamily members

Unique among fibroblast growth factors (FGFs), FGF19, -21, and -23 act in an endocrine fashion to regulate energy, bile acid, glucose, lipid, phosphate, and vitamin D homeostasis. These FGFs require the presence of Klotho/betaKlotho in their target tissues. Here, we present the crystal structures of FGF19 alone and FGF23 in complex with sucrose octasulfate, a disaccharide chemically related to heparin. The conformation of the heparin-binding region between beta strands 10 and 12 in FGF19 and FGF23 diverges completely from the common conformation adopted by paracrine-acting FGFs. A cleft between this region and the beta1-beta2 loop, the other heparin-binding region, precludes direct interaction between heparin/heparan sulfate and backbone atoms of FGF19/23. This reduces the heparin-binding affinity of these ligands and confers endocrine function. Klotho/betaKlotho have evolved as a compensatory mechanism for the poor ability of heparin/heparan sulfate to promote binding of FGF19, -21, and -23 to their cognate receptors.

Fibroblast growth factors share binding sites in heparan sulphate

HS (heparan sulphate) proteoglycans bind secreted signalling proteins, including FGFs (fibroblast growth factors) through their HS side chains. Such chains contain a wealth of differentially sulphated saccharide epitopes. Whereas specific HS structures are commonly believed to modulate FGF-binding and activity, selective binding of defined HS epitopes to FGFs has generally not been demonstrated. In the present paper, we have identified a series of sulphated HS octasaccharide epitopes, derived from authentic HS or from biosynthetic libraries that bind with graded affinities to FGF4, FGF7 and FGF8b. These HS species, along with previously identified oligosaccharides that interact with FGF1 and FGF2, constitute the first comprehensive survey of FGF-binding HS epitopes based on carbohydrate sequence analysis. Unexpectedly, our results demonstrate that selective modulation of FGF activity cannot be explained in terms of binding of individual FGFs to specific HS target epitopes. Instead, different FGFs bind to identical HS epitopes with similar relative affinities and low selectivity, such that the strength of these interactions increases with increasing saccharide charge density. We conclude that FGFs show extensive sharing of binding sites in HS. This conclusion challenges the current notion of specificity in HS-FGF interactions, and instead suggests that a set of common HS motifs mediates cellular targeting of different FGFs.

Targeting perlecan in human keratinocytes reveals novel roles for perlecan in epidermal formation

Heparin-binding growth factors are crucial for the formation of human epidermis, but little is known about the role of heparan sulfate proteoglycans in this process. Here we investigated the role of the heparan sulfate proteoglycan, perlecan, in the formation of human epidermis, by utilizing in vitro engineered human skin. By disrupting perlecan expression either in the dermis or the epidermis, we found that epidermally derived perlecan is essential for epidermal formation. Perlecan-deficient keratinocytes formed a strikingly thin and poorly organized epidermis because of premature apoptosis and failure to complete their stratification program. Exogenous perlecan fully restored epidermal formation. Perlecan deposition in the basement membrane zone correlated with formation of multilayered epidermis. Perlecan deficiency, however, had no effect on the lining and deposition of major basement membrane components as was evident by a continuous linear staining of laminin and collagen IV. Similarly, perlecan deficiency did not affect the distribution of beta1 integrin. Addition of the perlecan ligand, fibroblast growth factor 7, protected perlecan-deficient keratinocytes from cell death and improved the thickness of the epidermis. Taken together, our results revealed novel roles for perlecan in epidermal formation. Perlecan regulates both the survival and terminal differentiation steps of keratinocytes. Our results suggested a model whereby perlecan regulates these processes via controlling the bioavailability of perlecan-binding soluble factors involved in epidermal morphogenesis.

Sef is synexpressed with FGFs during chick embryogenesis and its expression is differentially regulated by FGFs in the developing limb

The signaling pathways leading to growth and patterning of various organs are tightly controlled during the development of any organism. These control mechanisms usually involve the utilization of feedback- and pathway-specific antagonists where the pathway induces the expression of its own antagonist. Sef is a feedback antagonist of fibroblast growth factor (FGF) signaling, which has been identified recently in zebrafish and mammals. Here, we report the isolation of chicken Sef (cSef) and demonstrate the conserved nature of the regulatory relationship with FGF signaling. In chick embryos, Sef is expressed in a pattern that coincides with many known sites of FGF signaling. In the developing limb, cSef is expressed in the mesoderm underlying the apical ectodermal ridge (AER) in the region known as the progress zone. cSef message first appeared after limb budding and AER formation. Expression was intense at stages of rapid limb outgrowth, and gradually decreased to almost undetectable levels when differentiation was clearly apparent. Gain- and loss-of-function experiments showed that FGFs differentially regulate the expression of cSef in various tissues. Thus, removal of the AER down-regulated cSef expression, and FGF2 but not FGF4 or FGF8 beads substituted for the AER in maintaining cSef expression. At sites where cSef is not normally expressed, FGF4 and FGF2, but not FGF8 beads, induced cSef expression. Our results demonstrate the complexity of cSef regulation by FGFs and point to FGF2 as a prime candidate in regulating cSef expression during normal limb development. The spatiotemporal pattern of cSef expression during limb development suggests a role for cSef in regulating limb outgrowth but not limb initiation.

Interactions of multiple heparin binding growth factors with neuropilin-1 and potentiation of the activity of fibroblast growth factor-2

The hypothesis that neuropilin-1 (Npn-1) may interact with heparin-binding proteins other than vascular endothelial growth factor has been tested using an optical biosensor-based binding assay. The results show that fibroblast growth factor (FGF) 1, 2, 4, and 7, FGF receptor 1, hepatocyte growth factor/scatter factor (HGF/SF), FGF-binding protein, normal protease sensitive form of prion protein, antithrombin III, and Npn-1 itself are all able to interact with Npn-1 immobilized on the sensor surface. FGF-2, FGF-4, and HGF/SF are also shown to interact with Npn-1 in a solution assay. Moreover, these protein-protein interactions are dependent on the ionic strength of the medium and are inhibited by heparin, and the kinetics of binding of FGF-2, FGF-4 and HGF/SF to Npn-1 are characterized by fast association rate constants (270,000-1,600,000 m(-1) s(-1)). These results suggest that Npn-1 possesses a "heparin" mimetic site that is able to interact at least in part through ionic bonding with the heparin binding site on many of the proteins studied. Npn-1 was also found to potentiate the growth stimulatory activity of FGF-2 on human umbilical vein endothelial cells, indicating that Npn-1 may not just bind but also regulate the activity of heparin-binding proteins.

Keratinocyte growth factor/fibroblast growth factor 7, a homeostatic factor with therapeutic potential for epithelial protection and repair

Keratinocyte growth factor (KGF) is a paracrine-acting, epithelial mitogen produced by cells of mesenchymal origin. It is a member of the fibroblast growth factor (FGF) family, and acts exclusively through a subset of FGF receptor isoforms (FGFR2b) expressed predominantly by epithelial cells. The upregulation of KGF after epithelial injury suggested it had an important role in tissue repair. This hypothesis was reinforced by evidence that intestinal damage was worse and healing impaired in KGF null mice. Preclinical data from several animal models demonstrated that recombinant human KGF could enhance the regenerative capacity of epithelial tissues and protect them from a variety of toxic exposures. These beneficial effects are attributed to multiple mechanisms that collectively act to strengthen the integrity of the epithelial barrier, and include the stimulation of cell proliferation, migration, differentiation, survival, DNA repair, and induction of enzymes involved in the detoxification of reactive oxygen species. KGF is currently being evaluated in clinical trials to test its ability to ameliorate severe oral mucositis (OM) that results from cancer chemoradiotherapy. In a phase 3 trial involving patients who were treated with myeloablative chemoradiotherapy before autologous peripheral blood progenitor cell transplantation for hematologic malignancies, KGF significantly reduced both the incidence and duration of severe OM. Similar investigations are underway in patients being treated for solid tumors. On the basis of its success in ameliorating chemoradiotherapy-induced OM in humans and tissue damage in a variety of animal models, additional clinical applications of KGF are worthy of investigation.

Towards a resolution of the stoichiometry of the fibroblast growth factor (FGF)-FGF receptor-heparin complex

The 22 members of the fibroblast growth factor (FGF) family have been implicated in cell proliferation, differentiation, survival, and migration. They are required for both development and maintenance of vertebrates, demonstrating an exquisite pattern of affinities for both protein and proteoglycan receptors. Recent crystal structures have suggested two models for the complex between FGFs, FGF receptors (FGFRs) and the proteoglycan heparan sulphate that mediates signalling, and have provided insight into how FGFs show differing affinities for the range of FGFRs. However, the physiological relevance of the two different models has not been made clear. Here, we demonstrate that the two complexes can be prepared from the same protein components, confirming that neither complex is the product of misfolded protein samples. Analyses of the complexes with mass spectrometry and analytical ultracentrifugation show that the species observed are consistent with the crystal structures formed using the two preparation protocols. This analysis supports the contention that both of the crystal structures reflect the state of the molecules in solution. Mass spectrometry of the complexes suggests that the stoichiometry of the complexes is 2 FGF1:2 FGFR2:1 heparin, regardless of the method used to prepare the complexes. These observations suggest that the two proposed complex architectures may both have relevance to the formation of an in vivo signalling complex, with a combination of the two interactions contributing to the formation of a larger focal complex.

Alternative splicing generates an isoform of the human Sef gene with altered subcellular localization and specificity

Receptor tyrosine kinases (RTKs) control a multitude of biological processes and are therefore subjected to multiple levels of regulation. Negative feedback is one of the mechanisms that provide an effective means to control RTK-mediated signaling. Sef has recently been identified as a specific antagonist of fibroblast growth factor (FGF) signaling in zebrafish and subsequently in mouse and human. Sef encodes a putative type I transmembrane protein that antagonizes the Ras/mitogen-activated protein kinase pathway in all three species. Mouse Sef was also shown to inhibit the phosphatidylinositol 3-kinase pathway. We show here that an alternative splicing mechanism generates an isoform of human Sef, hSef-b, which unlike the previously reported Sef (hSef-a) is a cytosolic protein. Contrary to hSef-a, which is ubiquitously expressed, hSef-b transcripts display a restricted pattern of expression in human tissues. hSef-b inhibits FGF-induced cell proliferation and prevents the activation of mitogen-activated protein kinase without affecting the upstream component MAPK kinase. Furthermore, hSef-b does not antagonize FGF induction of the phosphatidylinositol 3-kinase pathway. In addition to the effects on FGF signaling, hSef-b inhibited cellular response to platelet-derived growth factor but not other RTK ligands. Therefore, alternative splicing of the hSef gene expands the Sef feedback inhibition repertoire of RTK signaling.

Structure-based mutational analyses in FGF7 identify new residues involved in specific interaction with FGFR2IIIb

Receptor binding specificity is an essential element in regulating the diverse activities of fibroblast growth factors (FGFs). FGF7 is ideal to study how this specificity is conferred at the structural level, as it interacts exclusively with one isoform of the FGF-receptor (FGFR) family, known as FGFR2IIIb. Previous mutational analysis suggested the importance of the beta4/beta5 loop of FGF7 in specific receptor recognition. Here a theoretical model of FGFR2IIIb/FGF7 complex showed that this loop interacts with the FGFR2IIIb unique exon. In addition, the model revealed new residues that either directly interact with the FGFR2IIIb unique exon (Asp63, Leu142) or facilitate this interaction (Arg65). Mutations in these residues reduced both receptor binding affinity and biological activity of FGF7. Altogether, these results provide the basis for understanding how receptor-binding specificity of FGF7 is conferred at the structural level.

Differential effects of heparin saccharides on the formation of specific fibroblast growth factor (FGF) and FGF receptor complexes

Heparan sulfates (HS) play an important role in the control of cell growth and differentiation by virtue of their ability to modulate the activities of heparin-binding growth factors, an issue that is particularly well studied for fibroblast growth factors (FGFs). HS/heparin co-ordinate the interaction of FGFs with their receptors (FGFRs) and are thought to play a critical role in receptor dimerization. Biochemical and crystallographic studies, conducted mainly with FGF-2 or FGF-1 and FGF receptors 1 and 2, suggests that an octasaccharide is the minimal length required for FGF- and FGFR-induced dimerization and subsequent activation. In addition, 6-O-sulfate groups are thought to be essential for binding of HS to FGFR and for receptor dimerization. We show here that oligosaccharides shorter than 8 sugar units support activation of FGFR2 IIIb by FGF-1 and interaction of FGFR4 with FGF-1. In contrast, only relatively long oligosaccharides supported receptor binding and activation in the FGF-1.FGFR1 or FGF-7.FGFR2 IIIb setting. In addition, both 6-O- and 2-O-desulfated heparin activated FGF-1 signaling via FGFR2 IIIb, whereas neither one stimulated FGF-1 signaling via FGFR1 or FGF-7 via FGFR2 IIIb. These findings indicate that the structure of HS required for activating FGFs is dictated by the specific FGF and FGFR combination. These different requirements may reflect the differences in the mode by which a given FGFR interacts with the various FGFs.

Identification of residues important both for primary receptor binding and specificity in fibroblast growth factor-7

Fibroblast growth factors (FGFs) mediate a multitude of physiological and pathological processes by activating a family of tyrosine kinase receptors (FGFRs). Each FGFR binds to a unique subset of FGFs and ligand binding specificity is essential in regulating FGF activity. FGF-7 recognizes one FGFR isoform known as the FGFR2 IIIb isoform or keratinocyte growth factor receptor (KGFR), whereas FGF-2 binds well to FGFR1, FGFR2, and FGFR4 but interacts poorly with KGFR. Previously, mutations in FGF-2 identified a set of residues that are important for high affinity receptor binding, known as the primary receptor-binding site. FGF-7 contains this primary site as well as a region that restricts interaction with FGFR1. The sequences that confer on FGF-7 its specific binding to KGFR have not been identified. By utilizing domain swapping and site-directed mutagenesis we have found that the loop connecting the beta4-beta5 strands of FGF-7 contributes to high affinity receptor binding and is critical for KGFR recognition. Replacement of this loop with the homologous loop from FGF-2 dramatically reduced both the affinity of FGF-7 for KGFR and its biological potency but did not result in the ability to bind FGFR1. Point mutations in residues comprising this loop of FGF-7 reduced both binding affinity and biological potency. The reciprocal loop replacement mutant (FGF2-L4/7) retained FGF-2 like affinity for FGFR1 and for KGFR. Our results show that topologically similar regions in these two FGFs have different roles in regulating receptor binding specificity and suggest that specificity may require the concerted action of distinct regions of an FGF.

Similarities and differences between the effects of heparin and glypican-1 on the bioactivity of acidic fibroblast growth factor and the keratinocyte growth factor

The keratinocyte growth factor (KGF or FGF-7) is unique among its family members both in its target cell specificity and its inhibition by the addition of heparin and the native heparan-sulfate proteoglycan (HSPG), glypican-1 in cells expressing endogenous HSPGs. FGF-1, which binds the FGF-7 receptor with a similar affinity as FGF-7, is stimulated by both molecules. In the present study, we investigated the modulation of FGF-7 activities by heparin and glypican-1 in HS-free background utilizing either HS-deficient cells expressing the FGF-7 receptor (designated BaF/KGFR cells) or soluble extracellular domain of the receptor. At physiological concentrations of FGF-7, heparin was required for high affinity receptor binding and for signaling in BaF/KGFR cells. In contrast, binding of FGF-7 to the soluble form of the receptor did not require heparin. However, high concentrations of heparin inhibited the binding of FGF-7 to both the cell surface and the soluble receptor, similar to the reported effect of heparin in cells expressing endogenous HSPGs. The difference in heparin dependence for high affinity interaction between the cell surface and soluble receptor may be due to other molecule(s) present on cell surfaces. Glypican-1 differed from heparin in that it stimulated FGF-1 but not FGF-7 activities in BaF/KGFR cells. Glypican-1 abrogated the stimulatory effect of heparin, and heparin reversed the inhibitory effect of glypican-1, indicating that this HSPG inhibits FGF-7 activities by acting, most likely, as a competitive inhibitor of stimulatory HSPG species for FGF-7. The regulatory effect of glypican-1 is mediated at the level of interaction with the growth factor as glypican-1 did not bind the KGFR. The effect of heparin and glypican-1 on FGF-1 and FGF-7 oligomerization was studied employing high and physiological concentrations of growth factors. We did not find a correlation between the effects of these glycosaminoglycans on FGFs biological activity and oligomerization. Altogether, our findings argue against the heparin-linked dimer presentation model as key in FGFR activation, and support the notion that HSPGs primarily affect high affinity interaction of FGFs with their receptors.