Heparin binding by fibronectin module III-13 involves six discontinuous basic residues brought together to form a cationic cradle

Shaping Oncogenic Microenvironments: Contribution of Fibronectin

The extracellular matrix (ECM) is a complex network of proteins and glycans, dynamically remodeled and specifically tailored to the structure/function of each organ. The malignant transformation of cancer cells is determined by both cell intrinsic properties, such as mutations, and extrinsic variables, such as the mixture of surrounding cells in the tumor microenvironment and the biophysics of the ECM. During cancer progression, the ECM undergoes extensive remodeling, characterized by disruption of the basal lamina, vascular endothelial cell invasion, and development of fibrosis in and around the tumor cells resulting in increased tissue stiffness. This enhanced rigidity leads to aberrant mechanotransduction and further malignant transformation potentiating the de-differentiation, proliferation and invasion of tumor cells. Interestingly, this fibrotic microenvironment is primarily secreted and assembled by non-cancerous cells. Among them, the cancer-associated fibroblasts (CAFs) play a central role. CAFs massively produce fibronectin together with type I collagen. This review delves into the primary interactions and signaling pathways through which fibronectin can support tumorigenesis and metastasis, aiming to provide critical molecular insights for better therapy response prediction.

Heparin-binding motif mutations of human diamine oxidase allow the development of a first-in-class histamine-degrading biopharmaceutical

Background

Excessive plasma histamine concentrations cause symptoms in mast cell activation syndrome, mastocytosis, or anaphylaxis. Anti-histamines are often insufficiently efficacious. Human diamine oxidase (hDAO) can rapidly degrade histamine and therefore represents a promising new treatment strategy for conditions with pathological histamine concentrations.

Methods

Positively charged amino acids of the heparin-binding motif of hDAO were replaced with polar serine or threonine residues. Binding to heparin and heparan sulfate, cellular internalization and clearance in rodents were examined.

Results

Recombinant hDAO is rapidly cleared from the circulation in rats and mice. After mutation of the heparin-binding motif, binding to heparin and heparan sulfate was strongly reduced. The double mutant rhDAO-R568S/R571T showed minimal cellular uptake. The short α-distribution half-life of the wildtype protein was eliminated, and the clearance was significantly reduced in rodents.

Conclusions

The successful decrease in plasma clearance of rhDAO by mutations of the heparin-binding motif with unchanged histamine-degrading activity represents the first step towards the development of rhDAO as a first-in-class biopharmaceutical to effectively treat diseases characterized by excessive histamine concentrations in plasma and tissues.

Funding

Austrian Science Fund (FWF) Hertha Firnberg program grant T1135 (EG); Sigrid Juselius Foundation, Medicinska Understödsförening Liv och Hälsa rft (TAS and SeV).

The Molecular Dance of Fibronectin: Conformational Flexibility Leads to Functional Versatility

Fibronectin, a large multimodular protein and one of the major fibrillar components of the extracellular matrix, has been the subject of study for many decades and plays critical roles in embryonic development and tissue homeostasis. Moreover, fibronectin has been implicated in the pathology of many diseases, including cancer, and abnormal depositions of fibronectin have been identified in a number of amyloid and nonamyloid lesions. The ability of fibronectin to carry all these diverse functionalities depends on interactions with a large number of molecules, including adhesive and signaling cell surface receptors, other components of the extracellular matrix, and growth factors and cytokines. The regulation and integration of such large number of interactions depends on the modular architecture of fibronectin, which allows a large number of conformations, exposing or destroying different binding sites. In this Review, we summarize the current knowledge regarding the conformational flexibility of fibronectin, with an emphasis on how it regulates the ability of fibronectin to interact with various signaling molecules and cell-surface receptors and to form supramolecular assemblies and fibrillar structures.

Heparin-fibronectin interactions in the development of extracellular matrix insolubility

During extracellular matrix (ECM) assembly, fibronectin (FN) fibrils are irreversibly converted into a detergent-insoluble form which, through FN's multi-domain structure, can interact with collagens, matricellular proteins, and growth factors to build a definitive matrix. FN also has heparin/heparan sulfate (HS) binding sites. Using HS-deficient CHO cells, we show that the addition of soluble heparin significantly increased the amount of FN matrix that these cells assemble. Sulfated HS glycosaminoglycan (GAG) mimetics similarly increased FN assembly and demonstrated a dependence on GAG sulfation. The length of the heparin chains also plays a role in assembly. Chains of sufficient length to bind to two FN molecules gave maximal stimulation of assembly whereas shorter heparin had less of an effect. Using a decellularized fibroblast matrix for proteolysis, detergent fractionation, and mass spectrometry, we found that the predominant domain within insoluble fibril fragments is FN's major heparin-binding domain HepII (modules III_12-14). Multiple HepII domains bind simultaneously to a single heparin chain in size exclusion chromatography analyses. We propose a model in which heparin/HS binding to the HepII domain connects multiple FNs together to facilitate the formation of protein interactions for insoluble fibril assembly.

Human melanoma inhibitory protein binds to the FN12-14 Hep II domain of fibronectin

The heparin binding site (Hep II) of fibronectin plays a major role in tumor cell metastasis. Its interaction with heparan sulfate proteoglycans occurs in a variety of physiological processes including focal adhesion and migration. The melanoma inhibitory activity (MIA) is an important protein that is functionally involved in melanoma development, progression, and tumor cell invasion. After its secretion by malignant melanoma cells, MIA interacts with fibronectin and thereby actively facilitates focal cell detachment from surrounding structures and strongly promotes tumor cell invasion and the formation of metastases. In this report, the authors have determined the molecular basis of the interaction of MIA with the Hep II domain of fibronectin based on nuclear magnetic resonance spectroscopic binding assays. The authors have identified the type III modules 12 to 14 of fibronectin's Hep II as the major MIA binding sites. These results now provide a new target protein-protein binding interface for the discovery of novel antimetastatic agents against malignant melanoma in the future.

A clamp-like orientation of basic residues set in a parallelogram is essential for heparin binding

While the majority of studies have focused on the biological roles of heparin-binding proteins, relatively little is known about their key residues and structural elements responsible for heparin interaction. In this study, we employed the IgG-binding domain B1 of Streptococcal protein G as a miniature scaffold to investigate how certain positively charged residues within the β-sheet conformation become favorable for heparin binding. By performing a series of arginine substitution mutations followed by gain-of-heparin-binding analysis, we deduced that a clamp-like orientation with discontinuous basic residues separated by ~ 5 Å with ~ 100° interior angle is advantageous for high heparin affinity.

The fibronectin III-1 domain activates a PI3-Kinase/Akt signaling pathway leading to αvβ5 integrin activation and TRAIL resistance in human lung cancer cells

Background

Fibronectin is a mechanically sensitive protein which is organized in the extracellular matrix as a network of interacting fibrils. The lung tumor stroma is enriched for fibronectin which is thought to contribute to metastasis and drug resistance. Fibronectin is an elastic, multi-modular protein made up of individually folded domains, some of which can stretch in response to increased mechanical tension. Very little is known about the relationship of fibronectin’s unfolded domains to lung cancer resistance to chemotherapy. In the present study, we evaluated the impact of unfolding the first Type III domain of fibronectin (FnIII-1c) on TNF-related apoptosis inducing ligand (TRAIL) resistance.

Methods

NCI-H460 non-small cell lung cancer cells were treated with FnIII-1c then assessed for TRAIL-induced apoptosis. Subsequent analysis of FnIII-1c-mediated signaling pathways was also completed. Human non-small cell lung cancer tissue sections were assessed for the expression of vitronectin by immunohistochemistry.

Results

FnIII-1c inhibited TRAIL-induced activation of caspase 8 and subsequent apoptosis in NCI-H460 lung cancer cells. FnIII-1c treatment was associated with the activation of the phosphatidylinositol-3-kinase/alpha serine/threonine kinase (PI3K/Akt) pathway and the αvβ5 integrin receptor for vitronectin, both of which were required for TRAIL resistance. Immunohistochemical staining of sections from non-small cell lung cancers showed that vitronectin was localized around blood vessels and in the tumor-stroma interface.

Conclusions

Unfolding of Type III domains within the fibronectin matrix may promote TRAIL resistance through the activation of a PI3K/Akt/αvβ5 signaling axis and point to a novel mechanism by which changes in secondary structure of fibronectin contribute to cancer cell resistance to apoptosis.

Stimulatory effects of advanced glycation endproducts (AGEs) on fibronectin matrix assembly

Advanced glycation endproducts (AGEs) are a heterogeneous group of compounds that form via non-enzymatic glycation of proteins throughout our lifespan and at a higher rate in certain chronic diseases such as diabetes. AGEs contribute to the progression of fibrosis, in part by stimulating cellular pathways that affect gene expression. Long-lived ECM proteins are targets for non-enzymatic glycation but the question of whether the AGE-modified ECM leads to excess ECM accumulation and fibrosis remains unanswered. In this study, cellular changes due to AGE accretion in the ECM were investigated. Non-enzymatic glycation of proteins in a decellularized fibroblast ECM was achieved by incubating the ECM in a solution of methylglyoxal (MGO). Mass spectrometry of fibronectin (FN) isolated from the glycated matrix identified twenty-eight previously unidentified MGO-derived AGE modification sites including functional sites such as the RGD integrin-binding sequence. Mesangial cells grown on the glycated, decellularized matrix assembled increased amounts of FN matrix. Soluble AGE-modified bovine serum albumin (BSA) also stimulated FN matrix assembly and this effect was reduced by function-blocking antibodies against the receptor for AGE (RAGE). These results indicate that cells respond to AGEs by increasing matrix assembly and that RAGE is involved in this response. This raises the possibility that the accumulation of ECM during the progression of fibrosis may be enhanced by cell interactions with AGEs on a glycated ECM.

Bordetella filamentous hemagglutinin and fimbriae: critical adhesins with unrealized vaccine potential

Pertussis, or whooping cough, is a highly contagious respiratory disease that is caused by the Gram-negative bacterium Bordetella pertussis, which is transmitted exclusively from human to human. While vaccination against B. pertussis has been successful, replacement of the whole cell vaccine with an acellular component vaccine has correlated with reemergence of the disease, especially in adolescents and infants. Based on their presumed importance in mediating adherence to host tissues, filamentous hemagglutinin (FHA) and fimbria (FIM) were selected as components of most acellular pertussis vaccines. In this review, we describe the biogenesis of FHA and FIM, recent data that show that these factors do, in fact, play critical roles in adherence to respiratory epithelium, and evidence that they also contribute to persistence in the lower respiratory tract by modulating the host immune response. We also discuss shortcomings of whole cell and acellular pertussis vaccines and the possibility that FHA and FIM could serve as effective protective antigens in next-generation vaccines.

Filamentous hemagglutinin of Bordetella pertussis: a key adhesin with immunomodulatory properties?

The filamentous hemagglutinin of pathogenic Bordetellae is a prototype of a large two-partner-system-secreted and β-structure-rich bacterial adhesin. It exhibits several binding activities that may facilitate bacterial adherence to airway mucosa and host phagocytes in the initial phases of infection. Despite three decades of research on filamentous hemagglutinin, there remain many questions on its structure-function relationships, integrin interactions and possible immunomodulatory signaling capacity. Here we review the state of knowledge on this important virulence factor and acellular pertussis vaccine component. Specific emphasis is placed on outstanding questions that are yet to be answered.

Fell-Muir Lecture: Heparan sulphate and the art of cell regulation: a polymer chain conducts the protein orchestra

Heparan sulphate (HS) sits at the interface of the cell and the extracellular matrix. It is a member of the glycosaminoglycan family of anionic polysaccharides with unique structural features designed for protein interaction and regulation. Its client proteins include soluble effectors (e.g. growth factors, morphogens, chemokines), membrane receptors and cell adhesion proteins such as fibronectin, fibrillin and various types of collagen. The protein-binding properties of HS, together with its strategic positioning in the pericellular domain, are indicative of key roles in mediating the flow of regulatory signals between cells and their microenvironment. The control of transmembrane signalling is a fundamental element in the complex biology of HS. It seems likely that, in some way, HS orchestrates diverse signalling pathways to facilitate information processing inside the cell. A dictionary definition of an orchestra is 'a large group of musicians who play together on various instruments …' to paraphrase, the HS orchestra is 'a large group of proteins that play together on various receptors'. HS conducts this orchestra to ensure that proteins hit the right notes on their receptors but, in the manner of a true conductor, does it also set 'the musical pulse' and create rhythm and harmony attractive to the cell? This is too big a question to answer but fun to think about as you read this review.

The interplay of fibronectin functionalization and TGF-β1 presence on fibroblast proliferation, differentiation and migration in 3D matrices

TGF-β1 dependent fibroblast behaviour in a wound healing context is mimicked by topologically and mechanically defined collagen matrices with fibronectin functionalization.

Regulation of the innate immune response by fibronectin: synergism between the III-1 and EDA domains

Fibronectin is a critical component of the extracellular matrix and alterations to its structure will influence cellular behavior. Matrix fibronectin is subjected to both mechanical and biochemical regulation. The Type III domains of fibronectin can be unfolded in response to increased cellular contractility, included or excluded from the molecule by alternative splicing mechanisms, or released from the matrix by proteolysis. Using Inflammatory Cytokine microarrays we found that the alternatively spliced fibronectin Type III domain, FnEDA, and the partially unfolded III-1 domain, FnIII-1c, induced the expression of a multitude of pro-inflammatory cytokines in human dermal fibroblasts, most notably CXCL1-3, IL-8 and TNF-α. FnIII-1c, a peptide representing an unfolded intermediate structure of the first Type III domain has been shown to initiate the toll-like receptor-4 (TLR4)-NFκB-dependent release of cytokines from human dermal fibroblasts (You, et al., J. Biol. Chem., 2010). Here we demonstrate that FnIII-1c and the alternatively spliced FnEDA domain induce a TLR4 dependent activation of p38 MAP kinase and its downstream effector, MAPKAP Kinase-2 (MK-2), to regulate cytokine expression in fibroblasts. RT-qPCR analysis indicated that the p38-MK-2 pathway regulates IL-8 mRNA stability. Interestingly, addition of FnIII-1c and FnEDA synergistically enhanced TLR4-dependent IL-8 release. These data indicate that Fn contains two Type III domains which can activate TLR signaling to induce an inflammatory response in fibroblasts. Furthermore, our data identifies the NF-κB and p38/MK2 signaling pathways as transducers of signals initiated in response to structural changes in fibronectin.

Resolving sub-molecular binding and electrical switching mechanisms of single proteins at electroactive conducting polymers

Polymer-based electrodes for interfacing biological tissues are becoming increasingly sophisticated. Their many functions place them at the cross-roads of electromaterials, biomaterials, and drug-delivery systems. For conducting polymers, the mechanism of conductivity requires doping with anionic molecules such as extracellular matrix molecules, a process that distinguishes them as biomaterials and provides a means to control interactions at the cellular-electrode interface. However, due to their complex structure, directly observing the selective binding of target molecules or proteins has so far eluded researchers. This situation is compounded by the polymer's ability to adopt different electronic states that alter the polymer-dopant interactions. Here, the ability to resolve sub-molecular binding specificity between sulfate and carboxyl groups of dopants and heparin binding domains of human plasma fibronectin is demonstrated. The interaction exploits a form of biological 'charge complementarity' to enable specificity. When an electrical signal is applied to the polymer, the specific interaction is switched to a non-specific, high-affinity binding state that can be reversibly controlled using electrochemical processes. Both the specific and non-specific interactions are integral for controlling protein conformation and dynamics. These details, which represent the first direct measurement of biomolecular recognition between a single protein and any type of organic conductor, give new molecular insight into controlling cellular interactions on these polymer surfaces.

Analysis of stromal cell secretomes reveals a critical role for stromal cell-derived hepatocyte growth factor and fibronectin in angiogenesis

OBJECTIVE

Angiogenesis requires tightly coordinated crosstalk between endothelial cells (ECs) and stromal cells, such as fibroblasts and smooth muscle cells. The specific molecular mechanisms moderating this process are still poorly understood.

METHODS AND RESULTS

Stromal cell-derived factors are essential for EC sprouting and lumen formation. We therefore compared the abilities of 2 primary fibroblast isolates and a primary smooth muscle cell isolate to promote in vitro angiogenesis, and analyzed their secretomes using a combination of nano liquid chromatography-mass spectrometry/mass spectrometry, quantitative PCR, and ELISA. Each isolate exhibited a different level of angiogenic ability. Using quantitative MS, we then compared the secretomes of a fibroblast isolate exhibiting low angiogenic activity, a fibroblast isolate exhibiting high angiogenic activity, and human umbilical vein ECs. High angiogenic fibroblast supernatants exhibited an overabundance of proteins associated with extracellular matrix constituents compared with low angiogenic fibroblasts or ECs. Finally, small interfering RNA technology and purified protein were used to confirm a role for stromal cell-derived hepatocyte growth factor and fibronectin in inducing EC sprouting.

CONCLUSIONS

Differences in stromal cell ability to induce angiogenesis are a result of differences in the secreted proteomes of both extracellular matrix proteins and proangiogenic growth factors.

Plasma and cellular fibronectin: distinct and independent functions during tissue repair

Fibronectin (FN) is a ubiquitous extracellular matrix (ECM) glycoprotein that plays vital roles during tissue repair. The plasma form of FN circulates in the blood, and upon tissue injury, is incorporated into fibrin clots to exert effects on platelet function and to mediate hemostasis. Cellular FN is then synthesized and assembled by cells as they migrate into the clot to reconstitute damaged tissue. The assembly of FN into a complex three-dimensional matrix during physiological repair plays a key role not only as a structural scaffold, but also as a regulator of cell function during this stage of tissue repair. FN fibrillogenesis is a complex, stepwise process that is strictly regulated by a multitude of factors. During fibrosis, there is excessive deposition of ECM, of which FN is one of the major components. Aberrant FN-matrix assembly is a major contributing factor to the switch from normal tissue repair to misregulated fibrosis. Understanding the mechanisms involved in FN assembly and how these interplay with cellular, fibrotic and immune responses may reveal targets for the future development of therapies to regulate aberrant tissue-repair processes.

Correlation of in situ mechanosensitive responses of the Moraxella catarrhalis adhesin UspA1 with fibronectin and receptor CEACAM1 binding

Bacterial cell surfaces are commonly decorated with a layer formed from multiple copies of adhesin proteins whose binding interactions initiate colonization and infection processes. In this study, we investigate the physical deformability of the UspA1 adhesin protein from Moraxella catarrhalis, a causative agent of middle-ear infections in humans. UspA1 binds a range of extracellular proteins including fibronectin, and the epithelial cellular receptor carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1). Electron microscopy indicates that unliganded UspA1 is densely packed at, and extends about 800 Å from, the Moraxella surface. Using a modified atomic force microscope, we show that the adhesive properties and thickness of the UspA1 layer at the cell surface varies on addition of either fibronectin or CEACAM1. This in situ analysis is then correlated with the molecular structure of UspA1. To provide an overall model for UspA1, we have determined crystal structures for two N-terminal fragments which are then combined with a previous structure of the CEACAM1-binding site. We show that the UspA1-fibronectin complex is formed between UspA1 head region and the 13th type-III domain of fibronectin and, using X-ray scattering, that the complex involves an angular association between these two proteins. In combination with a previous study, which showed that the CEACAM1-UspA1 complex is distinctively bent in solution, we correlate these observations on isolated fragments of UspA1 with its in situ response on the cell surface. This study therefore provides a rare direct demonstration of protein conformational change at the cell surface.

Identification of novel and distinct binding sites within tenascin-C for soluble and fibrillar fibronectin

Interactions between fibronectin and tenascin-C within the extracellular matrix provide specific environmental cues that dictate tissue structure and cell function. The major binding site for fibronectin lies within the fibronectin type III-like repeats (TNfn) of tenascin-C. Here, we systematically screened TNfn domains for their ability to bind to both soluble and fibrillar fibronectin. All TNfn domains containing the TNfn3 module interact with soluble fibronectin. However, TNfn domains bind differentially to fibrillar fibronectin. This distinct binding pattern is dictated by the fibrillar conformation of FN. TNfn1-3, but not TNfn3-5, binds to immature fibronectin fibrils, and additional TNfn domains are required for binding to mature fibrils. Multiple binding sites for distinct regions of fibronectin exist within tenascin-C. TNfn domains comprise a binding site for the N-terminal 70-kDa domain of fibronectin that is freely available and a binding site for the central binding domain of fibronectin that is cryptic in full-length tenascin-C. The 70-kDa and central binding domain regions are key for fibronectin matrix assembly; accordingly, binding of several TNfn domains to these regions inhibits fibronectin fibrillogenesis. These data highlight the complexity of protein-protein binding, the importance of protein conformation on these interactions, and the implications for the physiological assembly of complex three-dimensional matrices.

Fragmentation of fibronectin by inherent autolytic and matrix metalloproteinase activities

Fibronectin (FN) purified by gelatin affinity chromatography is unstable and undergoes fragmentation. The cleavage has been ascribed to inherent autolytic protease activities as well as co-purified matrix metalloproteinases (MMP). Understanding the mechanism by which the proteolysis of FN occurs is important, because the FN fragments have biological activities that differ from those of intact FN. Having excluded contributions of other plasma-derived proteases, the present experiments demonstrated that cleavage of FN by MMP-2 to distinct fragments occurred in synergy with inherent FN activities. Limited heat treatment of FN at 56°C for 30 min inactivated the inherent protease activities sharply reducing autolysis of FN in a manner similar to that seen in the presence of serine proteinase inhibitors. Heat treatment did not alter cell attachment to FN, but significantly increased the susceptibility of FN to enzymatic cleavage by MMP-2. The carboxyl-terminal hemopexin-like domain (PEX) of MMP-2 was shown to possess critical exodomain properties required for the interactions of MMP-2 with FN, and FN was cleaved at a significantly reduced rate by an MMP-2 variant with deletion of PEX. Verifying the specificity of interactions, isolated PEX competed FN cleavage by MMP-2 in a concentration-dependent manner. These results have further elucidated the synergistic contributions of inherent autolytic serine protease-like activities and MMP-2 to fragmentation of FN and provide the rationale and basis for modified preparation and handling of FN used in biological research.

Regulation of arterial remodeling and angiogenesis by urokinase-type plasminogen activatorThis article is one of a selection of papers from the NATO Advanced Research Workshop on Translational Knowledge for Heart Health (published in part 2 of a 2-part Special Issue)

A wide variety of disorders are associated with an imbalance in the plasminogen activator system, including inflammatory diseases, atherosclerosis, intimal hyperplasia, the response mechanism to vascular injury, and restenosis. Urokinase-type plasminogen activator (uPA) is a multifunctional protein that in addition to its fibrinolytic and matrix degradation capabilities also affects growth factor bioavailability, cytokine modulation, receptor shedding, cell migration and proliferation, phenotypic modulation, protein expression, and cascade activation of proteases, inhibitors, receptors, and modulators. uPA is the crucial protein for neointimal growth and vascular remodeling. Moreover, it was recently shown to be implicated in the stimulation of angiogenesis, which makes it a promising multipurpose therapeutic target. This review is focused on the mechanisms by which uPA can regulate arterial remodeling, angiogenesis, and cell migration and proliferation after arterial injury and the means by which it modulates gene expression in vascular cells. The role of domain specificity of urokinase in these processes is also discussed.

Matrix metalloproteinase processing of CXCL11/I-TAC results in loss of chemoattractant activity and altered glycosaminoglycan binding

The CXCR3 chemokine receptor regulates the migration of Th1 lymphocytes and responds to three ligands: CXCL9/MIG, CXCL10/IP-10, and CXCL11/I-TAC. We screened for potential regulation of T cell responses by matrix metalloproteinase (MMP) processing of these important chemokines. The most potent of the CXCR3 ligands, CXCL11, was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry as a substrate of the PMN-specific MMP-8, macrophage-specific MMP-12, and the general leukocyte MMP-9. The 73-amino acid residue CXCL11 is processed at both the amino and carboxyl termini to generate CXCL11-(5-73), -(5-63), and -(5-58) forms. NH2-terminal truncation results in loss of agonistic properties, as shown in calcium mobilization and chemotaxis experiments using CXCR3 transfectants and human T lymphocytes. Moreover, CXCL11-(5-73) is a CXCR3 antagonist and interestingly shows enhanced affinity to heparin. However, upon COOH-terminal truncation to position 58 there is loss of antagonist activity and heparin binding. Together this highlights an unexpected site for receptor interaction and that the carboxyl terminus is critical for glycosaminoglycan binding, an essential function for the formation of chemokine gradients in vivo. Hence, MMP activity might regulate CXCL11 tissue gradients in two ways. First, the potential of CXCL11-(5-73) to compete active CXCL11 from glycosaminoglycans might lead to the formation of an antagonistic haptotactic chemokine gradient. Second, upon further truncation, MMPs disperse the CXCL11 gradients in a novel way by proteolytic loss of a COOH-terminal GAG binding site. Hence, these results reveal potential new roles in down-regulating Th1 lymphocyte chemoattraction through MMP processing of CXCL11.

Requirements for sulfate transport and the diastrophic dysplasia sulfate transporter in fibronectin matrix assembly

Diastrophic dysplasia sulfate transporter (DTDST) is a sulfate/chloride antiporter whose function is impaired in several human chondrodysplasias. We show that DTDST is upregulated by dexamethasone stimulation of HT1080 fibrosarcoma cells and is required for fibronectin (FN) extracellular matrix deposition by these cells. DTDST imports sulfate for the modification of glycosaminoglycans. We find that N-sulfation of these chains is important for FN matrix assembly and that sulfation of cell surface proteoglycans is reduced in the absence of DTDST. Of the candidate HT1080 cell surface proteoglycans, only loss of syndecan-2 compromises FN assembly, as shown by syndecan-2 small interfering RNA knockdown. DTDST is both necessary and sufficient to induce FN matrix assembly in HT1080 cells. Knockdown of DTDST ablates FN matrix, whereas its overexpression increases assembly without dexamethasone stimulation. These results identify a previously unrecognized regulatory pathway for matrix assembly via modulation of a sulfate transporter and proteoglycan sulfation. These data raise the possibility that FN assembly defects contribute to chondrodysplasias.

Cellular Adhesion Responses to the Heparin-binding (HepII) Domain of Fibronectin Require Heparan Sulfate with Specific Properties

Cell surface heparan sulfate (HS) proteoglycans are required in development and postnatal repair. Important classes of ligands for HS include growth factors and extracellular matrix macromolecules. For example, the focal adhesion component syndecan-4 interacts with the III(12-14) region of fibronectin (HepII domain) through its HS chains. The fine structure of HS is critical to growth factor responses, and whether this extends to matrix ligands is unknown but is suggested from in vitro experiments. Cell attachment to HepII showed that heparin oligosaccharides of >or=14 sugar residues were required for optimal inhibition. The presence of N-sulfated glucosamine in the HS was essential, whereas 2-O-sulfation of uronic acid or 6-O-sulfation of glucosamine had marginal effects. In the more complex response of focal adhesion formation through syndecan-4, N-sulfates were again required and also glucosamine 6-O-sulfate. The significance of polymer N-sulfation and sulfated domains in HS was confirmed by studies with mutant Chinese hamster ovary cells where heparan sulfation was compromised. Finally, focal adhesion formation was absent in fibroblasts synthesizing short HS chains resulting from a gene trap mutation in one of the two major glucosaminoglycan polymerases (EXT1). Several separate, specific properties of cell surface HS are therefore required in cell adhesion responses to the fibronectin HepII domain.

Binding areas of urokinase-type plasminogen activator?plasminogen activator inhibitor-1 complex for endocytosis receptors of the low-density lipoprotein receptor family, determined by site-directed mutagenesis

Some endocytosis receptors related to the low-density lipoprotein receptor, including low-density lipoprotein receptor-related protein-1A, very-low-density lipoprotein receptor, and sorting protein-related receptor, bind protease-inhibitor complexes, including urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1), and the uPA-PAI-1 complex. The unique capacity of these receptors for high-affinity binding of many structurally unrelated ligands renders mapping of receptor-binding surfaces of serpin and serine protease ligands a special challenge. We have mapped the receptor-binding area of the uPA-PAI-1 complex by site-directed mutagenesis. Substitution of a cluster of basic residues near the 37-loop and 60-loop of uPA reduced the receptor-binding affinity of the uPA-PAI-1 complex approximately twofold. Deletion of the N-terminal growth factor domain of uPA reduced the affinity 2-4-fold, depending on the receptor, and deletion of both the growth factor domain and the kringle reduced the affinity sevenfold. The binding affinity of the uPA-PAI-1 complex to the receptors was greatly reduced by substitution of basic and hydrophobic residues in alpha-helix D and alpha-helix E of PAI-1. The localization of the implicated residues in the 3D structures of uPA and PAI-1 shows that they form a continuous receptor-binding area spanning the serpin as well as the A-chain and the serine protease domain of uPA. Our results suggest that the 10-100-fold higher affinity of the uPA-PAI-1 complex compared with the free components depends on the bonus effect of bringing the binding areas on uPA and PAI-1 together on the same binding entity.

Bartonella henselae Pap31, an extracellular matrix adhesin, binds the fibronectin repeat III13 module

Bartonella henselae wound-associated infections suggest involvement of extracellular matrix molecules in adhesion and invasion. Pap31 was previously identified as a hemin-binding protein. Our recent studies suggest the protein is an adhesin that is recognized by the host's immune systems. In this study we examined the interactions of B. henselae Pap31 with fibronectin (Fn), heparin (Hep), and human umbilical vein endothelial cells (HUVECs). The cloned gene was expressed in Escherichia coli, and the purified Pap31 protein elicited strong antibody responses in mice and was reactive with rabbit anti-live B. henselae and mouse anti-Pap31 antibodies by Western blotting. Pap31 bound to immobilized Fn and to HUVECs in a dose-dependent manner and to Hep. Fn fragment-binding assays identified the Hep-1 and Hep-2 binding domains of human Fn and in particular the (12-13)FnIII repeat module as primary binding sites for this adhesin. Furthermore, Pap31 binding to the above Fn fragments could be inhibited by Hep, suggesting a common binding site involving the 13FnIII repeat module on the Hep-2 domain of Fn. Adherence of intact B. henselae to HUVECs was inhibited by increasing concentrations of anti-Pap31 antibodies. In addition, purified Pap31 coprecipitated effectively with Fn and anti-Fn antibodies. Taken together, these data suggest that Pap31 is an Fn-binding protein mediating the B. henselae-host interaction(s), and they implicate the 13FnIII repeat module as an important binding site for this adhesin on the Fn molecule. These interactions may be important initial steps leading to bacterial attachment and colonization that promote the establishment of B. henselae infections in vivo.

Human endothelial cell interaction with biomimetic surfactant polymers containing Peptide ligands from the heparin binding domain of fibronectin

Biomimetic materials that mimic the extracellular matrix (ECM) provide a means to control cellular functions such as adhesion and growth, which are vital to successful engineering of tissue-incorporated biomaterials. Novel "ECM-like" biomimetic surfactant polymers consisting of a poly(vinyl amine) backbone with pendant cell-adhesive peptides derived from one of the heparin-binding domains of fibronectin were developed to improve endothelial cell adhesion and growth on vascular biomaterials. Heparin-binding peptide (HBP) sequences, alone and in combination with RGD peptides, were examined for their ability to promote human pulmonary artery endothelial cell (HPAEC) adhesion and growth (HBP1, WQPPRARI; HBP2, SPPRRARVT; HBP1:RGD; and HBP2:RGD) and compared with cell adhesion and growth on fibronectin and on negative control polymer surfaces in which alanines were substituted for the positively charged arginine residues in the two peptides. The results showed that HPAECs adhered and spread equally well on all HBP-containing polymers and the positive fibronectin control, showing similar stress fiber and focal adhesion formation. However, the HBP alone was unable to support long-term HPAEC growth and survival, showing a loss of focal adhesions and cytoskeletal disorganization by 24 h after seeding. With the addition of RGD, the surfaces behaved similarly or better than fibronectin. The negative control polymers showed little to no initial cell attachment, and the addition of soluble heparin to the medium reduced initial cell adhesion on both the HBP2 and HBP2:RGD surfaces. These results indicate that the HBP surfaces promote initial HPAEC adhesion and spreading, but not long-term survival.

Cobra cardiotoxin and phospholipase A2 as GAG-binding toxins: on the path from structure to cardiotoxicity and inflammation

Glycosaminoglycans (GAGs) represent the sulfated carbohydrate moieties of proteoglycans which occur abundantly in tissues of the cardiovascular system. Many proteins bind specifically to GAGs and perform an important role in inflammation, cell proliferation, and blood coagulation processes. Recently, in vitro GAG-binding studies of cardiotoxins (CTXs) and basic phospholipase A(2) (PLA(2)) from cobra venom established the toxins as two new families of GAG-binding proteins. In particular, discontinuous basic residues in beta-sheet CTXs may form a cationic cradle suitable for heparin binding, as in the case of fibronectin module III-13. The binding specificity of beta-sheet proteins to different GAGs can be further enhanced by involving other cationic clusters near the flexible loop of the molecule. Since the three-dimensional structures of many CTXs and PLA(2) are available, these two toxins may serve as models for the elucidation of the molecular recognition of GAG-binding proteins and also as polypeptide templates for further improvement of the binding specificity suitable for future biomedical application. Research along the line of GAG-guided toxicity of cobra venom components may help us to understand the functional role of GAGs and the action mechanism of cobra venom components in the cardiovascular system.

Heparin II domain of fibronectin uses alpha4beta1 integrin to control focal adhesion and stress fiber formation, independent of syndecan-4

Co-signaling events between integrins and cell surface proteoglycans play a critical role in the organization of the cytoskeleton and adhesion forces of cells. These processes, which appear to be responsible for maintaining intraocular pressure in the human eye, involve a novel cooperative co-signaling pathway between alpha5beta1 and alpha4beta1 integrins and are independent of heparan sulfate proteoglycans. Human trabecular meshwork cells isolated from the eye were plated on type III 7-10 repeats of fibronectin (alpha5beta1 ligand) in the absence or presence of the heparin (Hep) II domain of fibronectin. In the absence of the Hep II domain, cells had a bipolar morphology with few focal adhesions and stress fibers. The addition of the Hep II domain increased cell spreading and the numbers of focal adhesions and stress fibers. Cell spreading and stress fiber formation were not mediated by heparan sulfate proteoglycans because treatment with chlorate, heparinase, or soluble heparin did not prevent Hep II domain-mediated cell spreading. Cell spreading and stress fiber formation were mediated by alpha4beta1 integrin because soluble anti-alpha4 integrin antibodies inhibited Hep II domain-mediated cell spreading and soluble vascular cell adhesion molecule-1 (alpha4beta1 ligand)-induced cell spreading. This is the first demonstration of the Hep II domain mediating cell spreading and stress fiber formation through alpha4beta1 integrin. This novel pathway demonstrates a cooperative, rather than antagonistic, role between alpha5beta1 and alpha4beta1 integrins and suggests that interactions between the Hep II domain and alpha4beta1 integrin could modulate the strength of cytoskeleton-mediated processes in the trabecular meshwork of the human eye.

Rotavirus enterotoxin NSP4 binds to the extracellular matrix proteins laminin-beta3 and fibronectin

Rotavirus is the most important cause of viral gastroenteritis and dehydrating diarrhea in young children. Rotavirus nonstructural protein 4 (NSP4) is an enterotoxin that was identified as an important agent in symptomatic rotavirus infection. To identify cellular proteins that interact with NSP4, a two-hybrid technique with Saccharomyces cerevisiae was used. NSP4 cDNA, derived from the human rotavirus strain Wa, was cloned into the yeast shuttle vector pGBKT7. An intestinal cDNA library derived from Caco-2 cells cloned into the yeast shuttle vector pGAD10 was screened for proteins that interact with NSP4. Protein interactions were confirmed in vivo by coimmunoprecipitation and immunohistochemical colocalization. After two-hybrid library screening, we repeatedly isolated cDNAs encoding the extracellular matrix (ECM) protein laminin-beta3 (amino acids [aa] 274 to 878) and a cDNA encoding the ECM protein fibronectin (aa 1755 to 1884). Using deletion mutants of NSP4, we mapped the region of interaction with the ECM proteins between aa 87 and 145. Deletion analysis of laminin-beta3 indicated that the region comprising aa 726 to 875 of laminin-beta3 interacts with NSP4. Interaction of NSP4 with either laminin-beta3 or fibronectin was confirmed by coimmunoprecipitation. NSP4 was present in infected enterocytes and in the basement membrane (BM) of infected neonatal mice and colocalized with laminin-beta3, indicating a physiological interaction. In conclusion, two-hybrid screening with NSP4 yielded two potential target proteins, laminin-beta3 and fibronectin, interacting with the enterotoxin NSP4. The release of NSP4 from the basal side of infected epithelial cells and the subsequent binding to ECM proteins localized at the BM may signify a new mechanism by which rotavirus disease is established.

The ShdA adhesin binds to the cationic cradle of the fibronectin 13FnIII repeat module: evidence for molecular mimicry of heparin binding

Introduction of Salmonella enterica serotype Typhimurium into food products results from its ability to persist in the intestine of healthy livestock by mechanisms that are poorly understood. The non-fimbrial adhesin ShdA is a fibronectin binding protein required for persistent intestinal carriage of S. Typhimurium. We further investigated the molecular mechanism of ShdA-mediated intestinal persistence by determining the binding-site of this receptor in fibronectin. Analysis of ShdA binding to fibronectin proteolytic fragments and to recombinant fibronectin fusion proteins identified the (13)FnIII repeat module of the Hep-2 domain as the primary binding site for this adhesin. The (13)FnIII repeat module of fibronectin contains a cationic cradle formed by six basic residues (R6, R7, R9, R23, K25 and R54) that is a high affinity heparin-binding site conserved among fibronectin sequences from frogs to man. Binding of ShdA to the (13)FnIII repeat module of fibronectin and to a second extracellular matrix protein, Collagen I, could be inhibited by heparin. Furthermore, binding of ShdA to the Hep-2 domain was sensitive to the ionic buffer strength, suggesting that binding involved ionic interactions. We therefore determined whether amino acid substitutions of basic residues in the cationic cradle of the Hep-2 domain that inhibit heparin binding also abrogate binding of ShdA. Combined substitution of R6S and R7S strongly reduced ShdA binding to (13)FnIII. These data suggest that ShdA binds the Hep-2 domain of fibronectin by a mechanism that may mimic binding of the host polysaccharide heparin.

Tuning the Mechanical Stability of Fibronectin Type III Modules through Sequence Variations

Cells can switch the functional states of extracellular matrix proteins by stretching them while exerting mechanical force. Using steered molecular dynamics, we investigated how the mechanical stability of FnIII modules from the cell adhesion protein fibronectin is affected by natural variations in their amino acid sequences. Despite remarkably similar tertiary structures, FnIII modules share low sequence homology. Conversely, the sequence homology for the same FnIII module across multiple species is notably higher, suggesting that sequence variability is functionally significant. Our studies find that the mechanical stability of FnIII modules can be tuned through substitutions of just a few key amino acids by altering access of water molecules to hydrogen bonds that break early in the unfolding pathway. Furthermore, the FnIII hierarchy of mechanical unfolding can be changed by environmental conditions, such as pH for FnIII10, or by forming complexes with other molecules, such as heparin binding to FnIII13.

Human microvascular endothelial cell growth and migration on biomimetic surfactant polymers

Successful engineering of a tissue-incorporated vascular prosthesis requires cells to proliferate and migrate on the scaffold. Here, we report on a series of "ECM-like" biomimetic surfactant polymers that exhibit quantitative control over the proliferation and migrational properties of human microvascular endothelial cells (HMVEC). The biomimetic polymers consist of a poly(vinyl amine) (PVAm) backbone with hexanal branches and varying ratios of cell binding peptide (RGD) to carbohydrate (maltose). Proliferation and migration behavior of HMVEC was investigated using polymers containing RGD: maltose ratios of 100:0, 75:25 and 50:50, and compared with fibronectin (FN) coated glass (1 microg/cm2). A radial Teflon fence migration assay was used to examine HMVEC migration at 12 h intervals over a 48 h period. Migration was quantified using an inverted optical microscope, and HMVEC were examined by confocal microscopy for actin and focal adhesion organization/ arrangement. Over the range of RGD ligand density studied (approximately 0.19-0.6 peptides/nm2), our results show HMVEC migration decreases with increasing RGD density in the polymer. HMVEC were least motile on the 100% RGD polymer (approximately 0.38-0.6 peptides/nm2) with an average migration of 0.20 mm2/h in area covered, whereas HMVEC showed the fastest migration of 0.48+/-0.06 mm2/h on the 50% RGD surface ( approximately 0.19-0.30 peptides/nm2). In contrast, cell proliferation increased with increasing surface peptide density; proliferation on the 50% RGD surface was 1.5%+/-0.06/h compared with 2.2%+/-0.07/h on the 100% RGD surface. Our results show that surface peptide density affects cellular functions such as growth and migration, with the highest peptide density supporting the most proliferation but the slowest migration.

Identification and sequence analysis of mouse homologous gene coding for hepatitis C virus non-structural protein 5A-binding protein 37

AIM: To clone murine homologous gene to human NS5ABP37 gene, and to elucidate its biological functions and the possible effects in the pathogenesis of hepatitis C virus (HCV) infection.

METHODS: Yeast-two hybrid system was employed to screen the human liver cDNA library by using non-structural protein 5A (NS5A) of hepatitis C virus (HCV) as the bait. According to the homologous role between the species, a murine NS5ABP37 homologous to human NS5ABP37 was deduced by bioinformatics methods. The primary sequence of murine NS5ABP37 was searched for conserved domains by online tools of GeneBank.

RESULTS: Human NS5ABP37 was screened and cloned from human liver cDNA library by yeast-two hybrid system 3. The murine NS5ABP37 was deduced by bioinformatics methods. The open reading frame (ORF) of murine NS5ABP37 consisted of 1 488 nt encoding a protein of 495 amino acids. Calculated molecular weight was 54 583.67 dalton and predicted pI was 4.70. From the homologous protein search, murine NS5ABP37 was demonstrated homologous to leukocyte antigen related (LAR) protein precursor. The sequence of murine NS5ABP37 was deposited into GenBank, and the accession number was AY234860.

CONCLUSION: Successful identification and cloning of murine NS5ABP37, which is homologous to LAR protein precursor, paves a way for elucidating the biological function of murine NS5ABP37 and pathogenesis of hepatitis C virus infection.

Fibroblast migration on fibronectin requires three distinct functional domains

Mesenchymal cell movement is normally constrained; however, fibronectin can provide a pathway for stromal cell migration during embryogenesis, morphogenesis, and wound healing. Cells can adhere to fibronectin via integrin and nonintegrin receptors, which bind multiple unique peptide sequences. Synthetic peptides and recombinant proteins were used to delineate the functional domains needed for human fibroblast migration over fibronectin. The 9th and 10th fibronectin type III repeats, which contain RGD and PHSRN synergy cell attachment sequences, support almost maximal fibroblast attachment, but not migration of primary dermal fibroblasts. Specific sequences within the heparin domain and the IIICS region are also required for migration. These findings predict and additional data confirm the necessity for the cooperation of multiple integrin and nonintegrin receptors for fibroblast migration on fibronectin. Such stringency of migration most likely imposes an immense constraint on normal mesenchymal cell mobility in unperturbed tissue. Loss of such restraint may be critical for the migration cancer cells through the extracellular matrix.

A novel consensus motif in fibronectin mediates dipeptidyl peptidase IV adhesion and metastasis

Lung endothelial dipeptidyl peptidase IV (DPPIV/CD26) is a vascular address for cancer cells decorated with cell-surface polymeric fibronectin (poly-FN). Here, we identified the DPPIV-binding sites in FN and examined the effect of binding site peptides on DPPIV/poly-FN adhesion and metastasis. Using proteolytic fragments and maltose-binding protein fusion proteins that together span full-length FN, we found DPPIV-binding sites in type III repeats 13, 14, and 15 (FNIII13, -14, and -15, respectively). DPPIV binding was mediated by the consensus motif T(I/L)TGLX(P/R)G(T/V)X and was confirmed by swapping this motif in FNIII13, -14, and -15 with the corresponding region in FNIII12, which did not bind DPPIV. DPPIV binding was lost in swapped FNIII13, -14, and -15 and gained in swapped FNIII12 (FNIII12(14)). Peptides containing the DPPIV-binding domain of FNIII14 blocked DPPIV/poly-FN adhesion and impeded pulmonary metastasis. This study adds to the classes of cell-surface adhesion receptors for FN and will help in the further characterization of the functional implications of the DPPIV/poly-FN adhesion in metastasis and possibly in cell-mediated immunity involving DPPIV-expressing lymphocytes.

Diverse role of LDL receptor-related protein in the clearance of proteases and in signaling

The low density lipoprotein receptor-related protein (LRP) is a large endocytic receptor that participates in several biological pathways and plays prominent roles in lipoprotein metabolism and in the catabolism of proteinases involved in coagulation and fibrinolysis. LRP also mediates the cellular entry of certain viruses and toxins and facilitates the activation of various lysosomal enzymes. Deletion of the LRP gene in mice is lethal, confirming an important role for this receptor in development, although its exact function in development is still not known. In addition to its role in the endocytosis of numerous ligands, recent studies are emerging that describe a signaling role for this receptor as well.

Tenascin-C blocks cell-cycle progression of anchorage-dependent fibroblasts on fibronectin through inhibition of syndecan-4

Tenascin-C is an adhesion-modulatory extracellular matrix protein that is predominantly expressed during embryonic development, wound healing and in tumor stroma. Here we report that anchorage-dependent human, rat and mouse fibroblasts adhere poorly and fail to proliferate on pure tenascin-C. This was due to a significant reduction of cyclin-dependent kinase 2 (cdk2) activity, resulting from elevated expression and association of the cdk inhibitors (CKIs) p21Cip1 and p27Kip1. To analyse the effect of tenascin-C on fibronectin-mediated adhesion, cells were plated on a mixed fibronectin/tenascin-C substratum. Compared to fibronectin alone, cell spreading and adhesion signaling were compromised, as determined by delayed phosphorylation kinetics of focal adhesion kinase (FAK). Despite the presence of growth factors, these cells remained arrested in the G1 phase of the cell cycle. In contrast to cells plated on pure tenascin-C, cdk2 activity appeared to be inhibited independently of CKIs. Interestingly, overexpression of the transmembrane proteoglycan syndecan-4 restored cell spreading, adhesion signaling and DNA replication on the fibronectin/tenascin-C substratum. A similar rescue was observed using a recombinant peptide that spans the syndecan-4-binding site in fibronectin. This indicates that tenascin-C causes cell cycle arrest and cdk2 inactivation by interfering with fibronectin-syndecan-4 interactions. We therefore propose that syndecan-4 signaling plays a central role in the control of cellular proliferation of anchorage-dependent fibroblasts.

Fibronectin interactions with osteoblasts: Identification of a non-integrin-mediated binding mechanism using a real-time fluorescence binding assay

Fibronectin (Fn) is an extracellular matrix protein that interacts with specific integrins on the cell surface, initiating signal transduction processes that lead to a reorganization of the cytoskeleton and the assembly of focal adhesions. Cell surface proteoglycans or glycosaminoglycans (GAGs) such as heparan sulfate are also known to participate in the interaction of Fn with the cell surface by binding to two different heparin-binding domains. The influence of Fn and GAGs on the spreading and differentiation of human osteoblasts was also previously described. In the current work, a method developed in our laboratory is established to evaluate the interaction between Fn and human osteoblasts and the influence of GAGs on such interactions. This technique makes use of fluoresceinphosphatidylethanolamine (FPE) such that when inserted into the lipidic bilayer, it acts as a fluorescent indicator of membrane interactions. The results indicate that the binding profile of Fn with the osteoblast cell surface is best represented by a hyperbolic single binding site model with a membrane affinity of 120 nM. Removal of cell surface heparan sulfate by treatment with heparitinase indicates that the cell surface moiety is directly involved in the binding process. Studies directed to assess the influence of heparin on the interaction of Fn with osteoblasts reveal that although it does not hamper Fn binding to the cell surface, it blocks the initial attachment to Fn-coated surfaces, indicating that binding to the integrin receptor alone is not enough to promote cell attachment but that the participation of the cell-surface GAGs is also a necessary condition.

Cytoplasmic interactions of syndecan-4 orchestrate adhesion receptor and growth factor receptor signalling

Syndecan-4 is a ubiquitous transmembrane proteoglycan that localizes to the focal adhesions of adherent cells and binds to a range of extracellular ligands, including growth factors and extracellular-matrix proteins. Engagement of syndecan-4 is essential for adhesion formation in cells adhering via certain integrins, and for cell proliferation and migration in response to growth factors. The cytoplasmic domain of syndecan-4 interacts with a number of signalling and structural proteins, and both extracellular and cytoplasmic domains are necessary for regulated activation of associated transmembrane receptors. PDZ domain-containing scaffold proteins (syntenin and CASK) bind to the C-terminus of the syndecan-4 cytoplasmic domain and co-ordinate clustering of receptors and connection to the actin cytoskeleton. Syndecan-4 also binds and activates protein kinase Calpha in the presence of phosphatidylinositol 4,5-bisphosphate, and regulates signalling by Rho-family GTPases and focal adhesion kinase. This review discusses the cytoplasmic interactions of syndecan-4 and how they affect cell behaviour as a consequence of the interaction with extracellular ligands. These conclusions also offer an insight into the role of syndecan-4 in vivo, and are consistent with phenotypes generated as a consequence of abnormal syndecan-4 expression in pathologies and gene disruption studies.

Mapping the heparin-binding site on the 13-14F3 fragment of fibronectin

Fibronectin, a multifunctional glycoprotein of the extracellular matrix, plays a major role in cell adhesion. Various studies have revealed that the human 13th and 14th fibronectin type III domains (labeled (13)F3 and (14)F3 here) contain a heparin-binding site. Mapping of the heparin-binding sites of (13-14)F3, (13)F3, and (14)F3 by NMR chemical shift perturbation, isothermal titration calorimetry, and molecular modeling show that (13)F3 provides the dominant heparin-binding site and that the residues involved are within the first 29 amino acids of (13)F3. Predictions from earlier biochemical and modeling studies as well as the x-ray structure of (12-14)F3 were tested. It was shown that the positively charged residues that project into the solvent from the ABE face of the triple-stranded beta sheet on (13)F3 are involved in binding, but (14)F3 does not appear to contribute significantly to heparin binding.

Basic residues in azurocidin/HBP contribute to both heparin binding and antimicrobial activity

Azurocidin/CAP37/HBP is an antimicrobial and chemotactic protein that is part of the innate defenses of human neutrophils. In addition, azurocidin is an inactive serine protease homolog with binding sites for diverse ligands including heparin and the bovine pancreatic trypsin inhibitor (BPTI). The structure of the protein reveals a highly cationic domain concentrated on one side of the molecule and responsible for its strong polarity. To investigate the role of this highly basic region, we produced three recombinant azurocidin mutant proteins that were altered in either one or both of two clusters of 4 basic residues located symmetrically on each side of a central cleft in the cationic domain. Two of the mutant proteins (Loop 3: R5Q, K6Q, R8Q, and R10Q; Loop 4: R61Q, R62Q, R63Q, and R65Q) exhibited little or no change in heparin and BPTI binding or in antimicrobial function. In contrast, the Loop 3/Loop 4 mutant (R5Q, K6Q, R8Q, R10Q, R61Q, R62Q, R63Q, and R65Q) in which all 8 basic residues were replaced showed greatly decreased ability to bind heparin and to kill Escherichia coli and Candida albicans. Thus, we report that the 8 basic residues that were altered in the Loop 3/Loop 4 mutant contribute to the ability of the wild-type azurocidin molecule to bind heparin and to kill E. coli and C. albicans. Because BPTI binding was comparable in wild-type and Loop 3/Loop 4 mutant protein, we conclude that the same 8 basic residues are not involved in the binding of BPTI to azurocidin, supporting the notion that the binding site for BPTI is distinct from the site involved in heparin binding and antimicrobial activity. Finally, we show that removal of all 4 positively charged amino acids in the 20-44 azurocidin sequence (DMC1: R23Q,H24S,H32S,R34Q), a region previously thought to contain an antimicrobial domain, does not affect the activity of the protein against E. coli, Streptococcus faecalis, and C. albicans.

Alternative splicing of the IIICS domain in fibronectin governs the role of the heparin II domain in fibrillogenesis and cell spreading

The Heparin (Hep) II-binding domain of fibronectin regulates the formation of focal adhesions and actin stress fibers and hence plays an important role in cell spreading, migration, and fibronectin fibrillogenesis. Using human skin fibroblast cultures, we demonstrate that alternative splicing of the neighboring IIICS domain may regulate the activities of the Hep II domain in cell spreading and fibronectin fibrillogenesis. Recombinant Hep II domains, adjacent to either the IIICS domain or the H89 splice variant that contains the amino-terminal sequence of the IIICS domain, blocked fibronectin fibrillogenesis and required sulfated proteoglycans to mediate cell spreading. If the Hep II domain was adjacent to either the H0 or H95 splice variants, which both lack the amino terminus of the IIICS domain, fibrillogenesis was not inhibited and cell spreading was independent of a sulfated proteoglycan-mediated mechanism. The effect of the splice variants on the Hep II domain could be mimicked using a Hep II domain that contained only 6 amino acids from the III(15) repeat or 10 amino acids from the IIICS domain suggesting that sequences proximal to the III(14) repeat determined the role of the Hep II domain in these processes. We propose that alternative splicing of the IIICS domain modulates interactions between heparan sulfate proteoglycans and the Hep II domain and that this serves as a mechanism to control the biological activities of fibronectin.

Interaction of an uuter membrane protein of enterotoxigenic Escherichia coli with cell surface heparan sulfate proteoglycans

We have previously shown that enterotoxigenic invasion protein A (Tia), a 25-kDa outer membrane protein encoded on an apparent pathogenicity island of enterotoxigenic Escherichia coli (ETEC) strain H10407, mediates attachment to and invasion into cultured human gastrointestinal epithelial cells. The epithelial cell receptor(s) for Tia has not been identified. Here we show that Tia interacts with cell surface heparan sulfate proteoglycans. Recombinant E. coli expressing Tia mediated invasion into wild-type epithelial cell lines but not invasion into proteoglycan-deficient cells. Furthermore, wild-type eukaryotic cells, but not proteoglycan-deficient eukaryotic cells, attached to immobilized polyhistidine-tagged recombinant Tia (rTia). Binding of epithelial cells to immobilized rTia was inhibited by exogenous heparan sulfate glycosaminoglycans but not by hyaluronic acid, dermatan sulfate, or chondroitin sulfate. Similarly, pretreatment of eukaryotic cells with heparinase I, but not pretreatment of eukaryotic cells with chrondroitinase ABC, inhibited attachment to rTia. In addition, we also observed heparin binding to both immobilized rTia and recombinant E. coli expressing Tia. Heparin binding was inhibited by a synthetic peptide representing a surface loop of Tia, as well as by antibodies directed against this peptide. Additional studies indicated that Tia, as a prokaryotic heparin binding protein, may also interact via sulfated proteoglycan molecular bridges with a number of mammalian heparan sulfate binding proteins. These findings suggest that the binding of Tia to host epithelial cells is mediated at least in part through heparan sulfate proteoglycans and that ETEC belongs on the growing list of pathogens that utilize these ubiquitous cell surface molecules as receptors.

The urokinase plasminogen activator receptor in the regulation of the actin cytoskeleton and cell motility

Cell migration is a complex process requiring tight control of several mechanisms including dynamic reorganization of the actin cytoskeleton and adhesion to the extracellular matrix. The GPI-anchored urokinase plasminogen activator receptor (uPAR) has an important role in the regulation of cell motility in many cell types. This is partly due to the localization of proteolytic activity on the cell surface by binding of the serine protease uPA. Results accumulated over the last decade suggest that uPAR is also involved in motility control through other mechanisms. These include induction of signal transduction events after ligation with uPA, binding to the extracellular matrix molecule vitronectin (VN), and association with integrins and other transmembrane partners. In this review these mechanisms will be discussed with a special emphasis on how the GPI-linked receptor transmits signals to the intracellular milieu and how uPAR participates in the regulation of actin cytoskeleton reorganization and cell adhesion during cell migration.