Incorporation of thrombospondin into fibrin clots

Reviewing the Rich History of Fibrin Clot Research with a Focus on Clinical Relevance

Fibrin, described on a single-lens microscopy for the first time by Malpighi in 1666 and named by de Fourcroy, has been extensively studied by biochemists, biophysicists, and more recently by clinicians who recognized that fibrin is the major component of most thrombi. Elucidation of key reactions leading to fibrin clot formation in the 1950s and 1960s grew interest in the clinical relevance of altered fibrin characteristics. Implementation of scanning electron microscopy to image fibrin clots in 1947 and clot permeation studies in the 1970s to evaluate an average pore size enabled plasma clot characterization in cohorts of patients. Unfavorably altered fibrin clot structure was demonstrated by Blombäck's group in coronary artery disease in 1992 and in diabetes in 1996. Fifteen years ago, similar plasma fibrin clot alterations were reported in patients following venous thromboembolism. Multiple myeloma was the first malignant disease to be found to lead to abnormal fibrin clot phenotype in the 1970s. Apart from anticoagulant agents, in 1998, aspirin was first shown to increase fibrin clot permeability in cardiovascular patients. The current review presents key data on the rich history of fibrin research, in particular, those that first documented abnormal fibrin clot properties in a variety of human disease states, as well as factors affecting fibrin phenotype.

The matricellular protein thrombospondin-1 in lung inflammation and injury

Matricellular proteins comprise a diverse group of molecular entities secreted into the extracellular space. They interact with the extracellular matrix (ECM), integrins, and other cell-surface receptors, and can alter matrix strength, cell attachment to the matrix, and cell-cell adhesion. A founding member of this group is thrombospondin-1 (TSP-1), a high molecular-mass homotrimeric glycoprotein. Given the importance of the matrix and ECM remodeling in the lung following injury, TSP-1 has been implicated in a number of lung pathologies. This review examines the role of TSP-1 as a damage controller in the context of lung inflammation, injury resolution, and repair in noninfectious and infectious models. This review also discusses the potential role of TSP-1 in human diseases as it relates to lung inflammation and injury.

Thrombospondin-1 Restricts Interleukin-36γ-Mediated Neutrophilic Inflammation during Pseudomonas aeruginosa Pulmonary Infection

Interleukin-36γ (IL-36γ), a member of the IL-1 cytokine superfamily, amplifies lung inflammation and impairs host defense during acute pulmonary Pseudomonas aeruginosa infection. To be fully active, IL-36γ is cleaved at its N-terminal region by proteases such as neutrophil elastase (NE) and cathepsin S (CatS). However, it remains unclear whether limiting extracellular proteolysis restrains the inflammatory cascade triggered by IL-36γ during P. aeruginosa infection. Thrombospondin-1 (TSP-1) is a matricellular protein with inhibitory activity against NE and the pathogen-secreted Pseudomonas elastase LasB-both proteases implicated in amplifying inflammation. We hypothesized that TSP-1 tempers the inflammatory response during lung P. aeruginosa infection by inhibiting the proteolytic environment required for IL-36γ activation. Compared to wild-type (WT) mice, TSP-1-deficient (Thbs1-/-) mice exhibited a hyperinflammatory response in the lungs during P. aeruginosa infection, with increased cytokine production and an unrestrained extracellular proteolytic environment characterized by higher free NE and LasB, but not CatS activity. LasB cleaved IL-36γ proximally to M19 at a cleavage site distinct from those generated by NE and CatS, which cleave IL-36γ proximally to Y16 and S18, respectively. N-terminal truncation experiments in silico predicted that the M19 and the S18 isoforms bind the IL-36R complex almost identically. IL-36γ neutralization ameliorated the hyperinflammatory response and improved lung immunity in Thbs1-/- mice during P. aeruginosa infection. Moreover, administration of cleaved IL-36γ induced cytokine production and neutrophil recruitment and activation that was accentuated in Thbs1-/- mice lungs. Collectively, our data show that TSP-1 regulates lung neutrophilic inflammation and facilitates host defense by restraining the extracellular proteolytic environment required for IL-36γ activation.IMPORTANCEPseudomonas aeruginosa pulmonary infection can lead to exaggerated neutrophilic inflammation and tissue destruction, yet host factors that regulate the neutrophilic response are not fully known. IL-36γ is a proinflammatory cytokine that dramatically increases in bioactivity following N-terminal processing by proteases. Here, we demonstrate that thrombospondin-1, a host matricellular protein, limits N-terminal processing of IL-36γ by neutrophil elastase and the Pseudomonas aeruginosa-secreted protease LasB. Thrombospondin-1-deficient mice (Thbs1-/-) exhibit a hyperinflammatory response following infection. Whereas IL-36γ neutralization reduces inflammatory cytokine production, limits neutrophil activation, and improves host defense in Thbs1-/- mice, cleaved IL-36γ administration amplifies neutrophilic inflammation in Thbs1-/- mice. Our findings indicate that thrombospondin-1 guards against feed-forward neutrophilic inflammation mediated by IL-36γ in the lung by restraining the extracellular proteolytic environment.

Thrombospondins: Purification of human platelet thrombospondin-1

Thrombospondins are a family of five secreted proteins that have diverse roles in modulating cellular function. Thrombospondins-1 and 2 were identified as matricellular proteins based on their functional roles combined with their transient appearance or accumulation in extracellular matrix at specific times during development and in response to injury or stress in mature tissues. Thrombospondin-1 is a major component of platelet α-granules, which provides a convenient source for purification of the protein. Methods are described to prepare thrombospondin-1 from human platelets in a biologically active form with minimal degradation or contamination with other platelet proteins. A nondenaturing method is described for removing bound transforming growth factor-β1.

THBS1 is induced by TGFB1 in the cancer stroma and promotes invasion of oral squamous cell carcinoma

BACKGROUND

THBS1 (thrombospondin-1) is the extracellular matrix (ECM) protein that affects diverse cellular activities. It constitutes the tumor stroma, but the role of THBS1 in oral squamous cell carcinoma (OSCC) development is unclear. The aim of this study was to clarify the relevance of THBS1 in the pathogenesis of OSCC.

MATERIALS AND METHODS

The expression of THBS1 was examined in 44 OSCC by immunohistochemical analysis and in 43 OSCC by cDNA microarray analysis. Cell culture experiments were conducted using human OSCC cell lines HSC3 and HO1N1 and mouse fibroblast ST2 cells to examine the effect of TGFB1 on THBS1 expression, and the effect of THBS1 on cellular behaviors.

RESULTS

THBS1 was specifically induced in the tumor microenvironment of OSCC. THBS1 appeared to be produced mainly by the stromal cells, but also by OSCC cells. TGFB1 stimulated THBS1 expression in ST2, primary fibroblasts, and the OSCC cells. THBS1 promoted migration and invasion of HSC3 and HO1N1 in transwell migration assays. THBS1 stimulated the expression of MMP3 (matrix metalloprotease 3), MMP9, MMP11, and MMP13 in ST2 cells and MMP3, MMP11, and MMP13 in HO1N1 cells. The RGD peptide suppressed the THBS1-stimulated migration and upregulation of MMP11 and MMP13.

CONCLUSIONS

THBS1 is a tumor-specific ECM protein that is induced by TGFB1 and promotes migration of cancer cells and stimulates the expression of MMPs partly through the integrin signaling, thereby favoring OSCC invasion.

The simultaneous occurrence of both hypercoagulability and hypofibrinolysis in blood and serum during systemic inflammation, and the roles of iron and fibrin(ogen)

Although the two phenomena are usually studied separately, we summarise a considerable body of literature to the effect that a great many diseases involve (or are accompanied by) both an increased tendency for blood to clot (hypercoagulability) and the resistance of the clots so formed (hypofibrinolysis) to the typical, 'healthy' or physiological lysis. We concentrate here on the terminal stages of fibrin formation from fibrinogen, as catalysed by thrombin. Hypercoagulability goes hand in hand with inflammation, and is strongly influenced by the fibrinogen concentration (and vice versa); this can be mediated via interleukin-6. Poorly liganded iron is a significant feature of inflammatory diseases, and hypofibrinolysis may change as a result of changes in the structure and morphology of the clot, which may be mimicked in vitro, and may be caused in vivo, by the presence of unliganded iron interacting with fibrin(ogen) during clot formation. Many of these phenomena are probably caused by electrostatic changes in the iron-fibrinogen system, though hydroxyl radical (OH˙) formation can also contribute under both acute and (more especially) chronic conditions. Many substances are known to affect the nature of fibrin polymerised from fibrinogen, such that this might be seen as a kind of bellwether for human or plasma health. Overall, our analysis demonstrates the commonalities underpinning a variety of pathologies as seen in both hypercoagulability and hypofibrinolysis, and offers opportunities for both diagnostics and therapies.

Procoagulant platelets form an α-granule protein-covered "cap" on their surface that promotes their attachment to aggregates

Strongly activated "coated" platelets are characterized by increased phosphatidylserine (PS) surface expression, α-granule protein retention, and lack of active integrin αIIbβ3. To study how they are incorporated into thrombi despite a lack of free activated integrin, we investigated the structure, function, and formation of the α-granule protein "coat." Confocal microscopy revealed that fibrin(ogen) and thrombospondin colocalized as "cap," a single patch on the PS-positive platelet surface. In aggregates, the cap was located at the point of attachment of the PS-positive platelets. Without fibrin(ogen) retention, their ability to be incorporated in aggregates was drastically reduced. The surface fibrin(ogen) was strongly decreased in the presence of a fibrin polymerization inhibitor GPRP and also in platelets from a patient with dysfibrinogenemia and a fibrinogen polymerization defect. In contrast, a fibrinogen-clotting protease ancistron increased the amount of fibrin(ogen) and thrombospondin on the surface of the PS-positive platelets stimulated with collagen-related peptide. Transglutaminases are also involved in fibrin(ogen) retention. However, platelets from patients with factor XIII deficiency had normal retention, and a pan-transglutaminase inhibitor T101 had only a modest inhibitory effect. Fibrin(ogen) retention was normal in Bernard-Soulier syndrome and kindlin-3 deficiency, but not in Glanzmann thrombasthenia lacking the platelet pool of fibrinogen and αIIbβ3. These data show that the fibrin(ogen)-covered cap, predominantly formed as a result of fibrin polymerization, is a critical mechanism that allows coated (or rather "capped") platelets to become incorporated into thrombi despite their lack of active integrins.

The activation peptide cleft exposed by thrombin cleavage of FXIII-A(2) contains a recognition site for the fibrinogen α chain

Formation of a stable fibrin clot is dependent on interactions between factor XIII and fibrin. We have previously identified a key residue on the αC of fibrin(ogen) (Glu396) involved in binding activated factor XIII-A(2) (FXIII-A(2)*); however, the functional role of this interaction and binding site(s) on FXIII-A(2)* remains unknown. Here we (1) characterized the functional implications of this interaction; (2) identified by liquid-chromatography-tandem mass spectrometry the interacting residues on FXIII-A(2)* following chemical cross-linking of fibrin(ogen) αC389-402 peptides to FXIII-A(2)*; and (3) carried out molecular modeling of the FXIII-A(2)*/peptide complex to identify contact site(s) involved. Results demonstrated that inhibition of the FXIII-A(2)*/αC interaction using αC389-402 peptide (Pep1) significantly decreased incorporation of biotinamido-pentylamine and α2-antiplasmin to fibrin, and fibrin cross-linking, in contrast to Pep1-E396A and scrambled peptide controls. Pep1 did not inhibit transglutaminase-2 activity, and incorporation of biotinyl-TVQQEL to fibrin was only weakly inhibited. Molecular modeling predicted that Pep1 binds the activation peptide cleft (AP-cleft) within the β-sandwich domain of FXIII-A(2)* localizing αC cross-linking Q366 to the FXIII-A(2)* active site. Our findings demonstrate that binding of fibrin αC389-402 to the AP-cleft is fundamental to clot stabilization and presents this region of FXIII-A(2)* as a potential site involved in glutamine-donor substrate recognition.

Substrates of Factor XIII-A: roles in thrombosis and wound healing

FXIII (Factor XIII) is a Ca2+-dependent enzyme which forms covalent ϵ-(γ-glutamyl)lysine cross-links between the γ-carboxy-amine group of a glutamine residue and the ϵ-amino group of a lysine residue. FXIII was originally identified as a protein involved in fibrin clot stabilization; however, additional extracellular and intracellular roles for FXIII have been identified which influence thrombus resolution and tissue repair. The present review discusses the substrates of FXIIIa (activated FXIII) involved in thrombosis and wound healing with a particular focus on: (i) the influence of plasma FXIIIa on the formation of stable fibrin clots able to withstand mechanical and enzymatic breakdown through fibrin–fibrin cross-linking and cross-linking of fibrinolysis inhibitors, in particular α2-antiplasmin; (ii) the role of intracellular FXIIIa in clot retraction through cross-linking of platelet cytoskeleton proteins, including actin, myosin, filamin and vinculin; (iii) the role of intracellular FXIIIa in cross-linking the cytoplasmic tails of monocyte AT1Rs (angiotensin type 1 receptors) and potential effects on the development of atherosclerosis; and (iv) the role of FXIIIa on matrix deposition and tissue repair, including cross-linking of extracellular matrix proteins, such as fibronectin, collagen and von Willebrand factor, and the effects on matrix deposition and cell–matrix interactions. The review highlights the central role of FXIIIa in the regulation of thrombus stability, thrombus regulation, cell–matrix interactions and wound healing, which is supported by observations in FXIII-deficient humans and animals.

Role of fibrin structure in thrombosis and vascular disease

Fibrin clot formation is a key event in the development of thrombotic disease and is the final step in a multifactor coagulation cascade. Fibrinogen is a large glycoprotein that forms the basis of a fibrin clot. Each fibrinogen molecule is comprised of two sets of Aα, Bβ, and γ polypeptide chains that form a protein containing two distal D regions connected to a central E region by a coiled-coil segment. Fibrin is produced upon cleavage of the fibrinopeptides by thrombin, which can then form double-stranded half staggered oligomers that lengthen into protofibrils. The protofibrils then aggregate and branch, yielding a three-dimensional clot network. Factor XIII, a transglutaminase, cross-links the fibrin stabilizing the clot protecting it from mechanical stress and proteolytic attack. The mechanical properties of the fibrin clot are essential for its function as it must prevent bleeding but still allow the penetration of cells. This viscoelastic property is generated at the level of each individual fiber up to the complete clot. Fibrinolysis is the mechanism of clot removal, and involves a cascade of interacting zymogens and enzymes that act in concert with clot formation to maintain blood flow. Clots vary significantly in structure between individuals due to both genetic and environmental factors and this has an effect on clot stability and susceptibility to lysis. There is increasing evidence that clot structure is a determinant for the development of disease and this review will discuss the determinants for clot structure and the association with thrombosis and vascular disease.

[Molecular mechanisms of the polymerization of fibrin and the formation of its three-dimensional network]

The results of biochemical, immunochemical, and X-ray studies of the structures of fibrinogen and fibrin molecules were analyzed. The mechanisms of the successive formation of the fibrin three-dimensional network were described: the polymerization of monomeric molecules with the formation of bifilar protofibrils, the lateral association of protofibrils, and the embranchment of the forming fibrils. Data on the electron and confocal microscopy of the polymeric fibrin were considered. The role of the known polymerization centers of fibrin which participated in the formation of protofibrils and their lateral association was discussed. Data on the existence of the previously unknown polymerization centers were given. In particular, the experimental results demonstrated that one of such centers which participated in the formation of protofibrils was located in the Bbeta12-46 fragment, and did not require the cleavage of fibrinopeptide B for its functioning. The results of the computer modeling of the spatial structure of the fibrin(ogen) molecule and the intermolecular interactions in the course of the fibrin polymerization were presented. The location of the alphaC domains in the fibrin(ogen) molecule and their role in the polymerization process were discussed. Information on the structure of the calcium-binding sites of fibrin(ogen) and the functional role of Ca2+ in fibrin polymerization was published. The structure of factor XIII(a) and the mechanisms of fibrin stabilization by this factor were briefly described.

Preclinical toxicity biomarkers for combination treatment with clotting factors rFXIII and rFVIIa

Combination treatment with the clotting factors recombinant activated factor VII (rFVIIa), serine protease, and recombinant factor XIII (rFXIII), protransglutaminase, is being explored for haemostatic therapy. We performed a single-dose toxicology study in the cynomolgus monkey, with four dose groups receiving 0.1 + 0.34 mg kg(-1) (group 1), 0.33 + 1.12 mg kg(-1) (group 2), 1.67 + 5.60 mg kg(-1) (group 3) and 5.00 + 16.80 mg kg(-1) (group 4) of a rFVIIa + rFXIII combination. In the three lower dose groups, no clinical, histopathological or blood chemistry changes were observed. In group 4, the animals died at 4 h post-dosing, with histopathology revealing a systemic coagulopathy resembling, but distinct from, disseminated intravascular coagulation. Due to the absence of toxicity warning signs, toxicity biomarkers were identified by a Western blot-based screening of approximately 20 plasma proteins known to be involved in the clotting cascade. Three of the examined proteins were specifically affected by rFVIIa + rFXIII treatment. Fibronectin and fibrinogen exhibited dose-dependent reductions from less than 10% reduction (group 2) to more than 90% reduction (group 4). These reductions were reversible, and specific. For vitronectin, a dose-dependent conversion to the 65-kDa form was found to occur in groups 3 and 4. Thus, fibrinogen, fibronectin and vitronectin represent the first biomarkers for clotting factor toxicity.

Platelets, thrombospondin-1 and human dermal fibroblasts cooperate for stimulation of endothelial cell tubulogenesis through VEGF and PAI-1 regulation

During cutaneous wound repair, platelets, dermal fibroblasts (DF) and endothelial cells all cooperate. We have presently investigated the regulation of endothelial cell tubulogenesis by human platelet thrombospondin-1 (TSP-1), in comparison to transforming growth factor-beta1 (TGF-beta1) and total platelet lysates (PL), in a fibrin matrix cell culture system incorporating DF. TSP-1, TGF-beta1 and PL all stimulated VEGF expression in DF dose dependently at mRNA and protein level. TSP-1- and PL-treated DF supernatants significantly stimulated capillary-like structure formation (tubulogenesis) by dermal microvascular endothelial cells (HMEC-1 and HDMEC), in part via VEGF, as confirmed with neutralizing anti-VEGF antibodies. In contrast, TGF-beta1-treated DF supernatants did not induce tubulogenesis. This apparent discrepancy could be explained by the differential expression regulation in HMEC-1 of fibrinolysis and metalloproteinase mediators by TSP-1 and TGF-beta1. TSP-1 potently reduced the expression of plasminogen activator inhibitor-1 (PAI-1) (mRNA and protein), whereas TGF-beta1 enhanced it. The crucial role of PAI-1 in tubulogenesis was confirmed via the addition of active recombinant PAI-1, which abrogated tubulogenesis. In contrast, neutralizing PAI-1 antibodies enhanced tubulogenesis. Our results suggest that platelet TSP-1 released in a wound stimulates endothelial cell tubulogenesis through an upregulation of DF VEGF expression and a downregulation of endothelial cell PAI-1 expression.

Do the isolated fibrinogen αC-domains form ordered oligomers?

Previous electron microscopy (EM) studies revealed that the proteolytically prepared, truncated, bovine fibrinogen alphaC-domain (Aalpha223-539 fragment) upon transfer from acidic to neutral pH formed ordered oligomers which could mimic alpha polymers of cross-linked fibrin. In this study, we demonstrated that although its recombinant analog, bAalpha224-538, as well as the full-length version of the alphaC-domain (bAalpha224-568), upon similar treatment also formed oligomers with ordered structure, both were monomeric when kept in neutral pH buffer. To search further for conditions for their oligomerization, we treated bAalpha224-568 with factor XIIIa, purified the cross-linked soluble fraction, and confirmed that it consisted of oligomers. Similar cross-linked oligomers were obtained with the recombinant human alphaC-domain (residues Aalpha221-610). In a cell adhesion assay, the adhesion of human umbilical vein endothelial cells (HUVEC) to the alphaC-domains substantially increased upon oligomerization. These results demonstrate that the recombinant alphaC-domains can form stable oligomers which may mimic properties of the alphaC-domains in cross-linked fibrin.

Role of factor XIII in fibrin clot formation and effects of genetic polymorphisms

Factor XIII and fibrinogen are unusual among clotting factors in that neither is a serine protease. Fibrin is the main protein constituent of the blood clot, which is stabilized by factor XIIIa through an amide or isopeptide bond that ligates adjacent fibrin monomers. Many of the structural and functional features of factor XIII and fibrin(ogen) have been elucidated by protein and gene analysis, site-directed mutagenesis, and x-ray crystallography. However, some of the molecular aspects involved in the complex processes of insoluble fibrin formation in vivo and in vitro remain unresolved. The findings of a relationship between fibrinogen, factor XIII, and cardiovascular or other thrombotic disorders have focused much attention on these 2 proteins. Of particular interest are associations between common variations in the genes of factor XIII and altered risk profiles for thrombosis. Although there is much debate regarding these observations, the implications for our understanding of clot formation and therapeutic intervention may be of major importance. In this review, we have summarized recent findings on the structure and function of factor XIII. This is followed by a review of the effects of genetic polymorphisms on protein structure/function and their relationship to disease.

Incorporation of vitronectin into fibrin clots. Evidence for a binding interaction between vitronectin and gamma A/gamma' fibrinogen

Vitronectin is an abundant plasma protein that regulates coagulation, fibrinolysis, complement activation, and cell adhesion. Recently, we demonstrated that plasma vitronectin inhibits fibrinolysis by mediating the interaction of type 1 plasminogen activator inhibitor with fibrin (Podor, T. J., Peterson, C. B., Lawrence, D. A., Stefansson, S., Shaughnessy, S. G., Foulon, D. M., Butcher, M., and Weitz, J. I. (2000) J. Biol. Chem. 275, 19788-19794). The current studies were undertaken to further examine the interactions between vitronectin and fibrin(ogen). Comparison of vitronectin levels in plasma with those in serum indicates that approximately 20% of plasma vitronectin is incorporated into the clot. When the time course of biotinylated-vitronectin incorporation into clots formed from (125)I-fibrinogen is monitored, vitronectin incorporation into the clot parallels that of fibrinogen in the absence or presence of activated factor XIII. Vitronectin binds specifically to fibrin matrices with an estimated K(d) of approximately 0.6 microm. Additional vitronectin subunits are assembled on fibrin-bound vitronectin multimers through self-association. Confocal microscopy of fibrin clots reveals the globular vitronectin aggregates anchored at intervals along the fibrin fibrils. This periodicity raised the possibility that vitronectin interacts with the gamma A/gamma' variant of fibrin(ogen) that represents about 10% of total fibrinogen. In support of this concept, the vitronectin which contaminates fibrinogen preparations co-purifies with the gamma A/gamma' fibrinogen fraction, and clots formed from gamma A/gamma' fibrinogen preferentially bind vitronectin. These studies reveal that vitronectin associates with fibrin during coagulation, and may thereby modulate hemostasis and inflammation.

The effect of dimethylbiguanide on thrombin activity, FXIII activation, fibrin polymerization, and fibrin clot formation

The antihyperglycemic drug dimethylbiguanide (DMB, also known as metformin) reduces the risk of cardiovascular complications in type 2 diabetes, although the mechanism(s) involved are unclear. DMB reduces glycosylation-related protein cross-linking, a process similar to fibrin cross-linking catalyzed by activated factor XIII (FXIII). To investigate whether the cardioprotective effect of DMB could be related to effects on clot stabilization, we studied the effects of DMB on FXIII, thrombin activity, and cleavage of fibrin(ogen). Activity of purified and plasma FXIII was inhibited by DMB. Analysis by mass spectrometry and FXIII-coupled magnetic particles excluded binding of DMB to FXIII. Thrombin-induced cleavage of the activation peptide from FXIII was inhibited in a dose-dependent manner, as was fibrinopeptide cleavage from fibrinogen. Ancrod-induced cleavage of fibrinopeptide A was not affected. DMB prolonged clotting time of normal plasma. Fiber thickness and pore size of fibrin clots, measured by permeation experiments and visualized by scanning electron microscopy, decreased significantly with DMB. No interactions between DMB and the active site of thrombin were found. Turbidity experiments demonstrated that DMB changed polymerization and lateral aggregation of protofibrils. These results suggest that DMB interferes with FXIII activation and fibrin polymerization, but not only by binding to thrombin on a different location than the active site. In patients on DMB therapy, FXIII antigen and activity levels in vivo were reduced over a 12-week period. These findings indicate that part of the cardioprotective effect of DMB in patients with type 2 diabetes may be attributed to alterations in fibrin structure/function.

Thrombospondin-1 as an endogenous inhibitor of angiogenesis and tumor growth

Thrombospondin-1 (TSP-1) is a matricellular glycoprotein that influences cellular phenotype and the structure of the extracellular matrix. These effects are important components of the tissue remodeling that is associated with angiogenesis and neoplasia. The genetic mutations in oncogenes and tumor suppressor genes that occur within tumor cells are frequently associated with decreased expression of TSP-1. However, the TSP-1 that is produced by stromal fibroblasts, endothelial cells and immune cells suppresses tumor progression. TSP-1 inhibits angiogenesis through direct effects on endothelial cell migration and survival and through indirect effects on growth factor mobilization. TSP-1 that is present in the tumor microenvironment also acts to suppress tumor cell growth through activation of transforming growth factor beta in those tumor cells that are responsive to TGF beta. In this review, the molecular basis for the role of TSP-1 in the inhibition of tumor growth and angiogenesis is summarized.

Conformational changes upon conversion of fibrinogen into fibrin. The mechanisms of exposure of cryptic sites

Conformational changes upon conversion of fibrinogen to fibrin result in the exposure of multiple binding sites that provide its interaction with various proteins and cells and, thus, its participation in a number of physiological and pathological processes. Here we focus on conformational changes in the fibrinogen D regions (domains) and alpha C-domains that are directly involved in intermolecular interactions upon fibrin assembly. According to the current view, two alpha C-domains that interact intramolecularly in fibrinogen undergo an intra- to intermolecular switch to form alpha C-polymers in fibrin. The availability of recombinant fragments that correspond to the alpha C-domain made it possible to further clarify this mechanism and to reveal novel cryptic sites in this domain for plasminogen and its activator tPA, whose exposure may play an important role in the regulation of fibrinolysis. To elucidate the mechanism of exposure of cryptic sites in the D regions, we tested the accessibility of their fibrin specific epitopes (A alpha 148-160 and gamma 312-324) that are also involved in binding of plasminogen and tPA, in several fragments derived from fibrinogen (fragment D), and crosslinked fibrin (fragment D-D and its non-covalent complex with the E1 fragment, D-D:E1). Neither D nor D-D bound tPA, plasminogen, or anti-A alpha 148-160 and anti-gamma 312-324 monoclonal antibodies. At the same time both epitopes became accessible in the D-D:E1 complex. Melting of D and D-D revealed that their domains have the same stability while in the D-D:E1 complex they became more stable. These results indicate that upon fibrin assembly, driven primarily by the interaction between complementary binding sites of the E and two D regions, the latter undergo conformational changes that cause the exposure of their cryptic sites. They also suggest that the fibrin specific conformation of the D regions is preserved in the D-D:E1 complex.

Modulators of intraplatelet calcium concentration affect the binding of thrombospondin to blood platelets in healthy donors and patients with type 2 diabetes mellitus

Thrombospondin (TSP), which is secreted from alpha-granules of activated platelets, binds to its surface receptor (CD36) in the presence of Ca2+.

OBJECTIVES

We monitored how the modulation of intraplatelet Ca2+ affects TSP binding to CD36 on platelets from healthy donors and patients with type 2 diabetes mellitus. We also aimed to verify whether the impaired Ca2+ mobilisation in diabetes influences TSP binding upon the pharmacological modulation of calcium transport.

METHODS

Whole blood cytometry was used to monitor TSP release/binding and CD36 presentation in platelets from 28 type 2 patients and 33 healthy donors.

RESULTS

No significant changes in TSP and CD36 levels were revealed between the groups in circulating platelets and TRAP-, collagen- or thrombin-activated platelets. In healthy donors, 1 microM thapsigargin (TG) elevated the TRAP-activated TSP binding (by up to 50%, p<0.001), 5 mM EGTA reversed the effect (by up to 85%, p<0.001), and overcame the effect of TG when used together. Less profoundly expressed effects occurred in the NIDDM group. In both groups TG increased the presentation of CD36 in TRAP-stimulated platelets (p<0.05), whereas EGTA lowered the TRAP-stimulated increase in CD36 (p<0.001). The inhibition of CD36 by EGTA was stronger in healthy volunteers (41% vs. 32%, respectively, p<0.05), whereas the activation by TG was higher in the NIDDM group (11% vs. 27%, p<0.05). When acting together the suppressive effects of EGTA on TG-dependent Ca2+ mobilisation were much attenuated in diabetic subjects (p<0.05).

CONCLUSION

Both the release of TSP and CD36 presentation are under the influence of agents modulating intracellular Ca2+. Diabetic platelets seem more vulnerable to the releasers of cytosolic [Ca2+] and more resistant to the blockers of cytosolic [Ca2+] mobilisation.

Aggregation substance-mediated adherence of Enterococcus faecalis to immobilized extracellular matrix proteins

Aggregation substance (AS) of Enterococcus faecalis (E. faecalis), a sex pheromone plasmid encoded cell surface protein, mediates the formation of bacterial aggregates, thereby promoting plasmid transfer. The influence of pAD1-encoded AS, Asa1, on binding to immobilized extracellular matrix proteins was studied. The presence of AS increased enterococcal adherence to fibronectin more than eight-fold, to thrombospondin more than four-fold, to vitronectin more than three-fold, and to collagen type I more than two-fold (P<0.001). In contrast, binding to laminin and collagen type IV occurred independently of AS. Adherence of the constitutively AS expressing E. faecalis OG1X(pAM721) to immobilized fibronectin was found to be approximately five times higher than that of Staphylococcus aureus Cowan and approximately 30 times higher than that of Streptococcus bovis. Investigation of strains with various deletions within the structural gene of asa1 suggests that attachment to immobilized fibronectin is mainly mediated by amino acids within the variable region or by neighbouring residues. Thus, AS may promote adherence to injured epithelium and endothelium, where extracellular matrix proteins are exposed, thereby facilitating colonization and infection.

The factor XIII V34L polymorphism accelerates thrombin activation of factor XIII and affects cross-linked fibrin structure

Factor XIII on activation by thrombin cross-links fibrin. A common polymorphism Val to Leu at position 34 in the FXIII A subunit is under investigation as a risk determinant of thrombosis. Because Val34Leu is close to the thrombin cleavage site, the hypothesis that it would alter the function of FXIII was tested. Analysis of FXIII subunit proteolysis by thrombin using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography showed that FXIII 34Leu was cleaved by thrombin more rapidly and by lower doses than 34Val. Mass spectrometry of isolated activation peptides confirmed the predicted single methyl group difference and demonstrated that the thrombin cleavage site is unaltered by Val34Leu. Kinetic analysis of activation peptide release demonstrated that the catalytic efficiency (kcat/Km) of thrombin was 0.5 for FXIII 34Leu and 0.2 (μmol/L)−1× sec−1 for 34Val. Presence of fibrin increased the catalytic efficiency to 4.8 and 2.2 (μmol/L)−1 × sec−1, respectively. Although the 34Leu peptide was released at a similar rate as fibrinopeptide A, the 34Val peptide was released more slowly than fibrinopeptide A but more quickly than fibrinopeptide B generation. Cross-linking of γ- and -chains appeared earlier when fibrin was incubated with FXIII 34Leu than with 34Val. Fully activated 34Leu and 34Val FXIII showed similar cross-linking activity. Analysis of fibrin clots prepared using plasma from FXIII 34Leu subjects by turbidity and permeability measurements showed reduced fiber mass/length ratio and porosity compared to 34Val. The structural differences were confirmed by electron microscopy. These results demonstrate that Val34Leu accelerates activation of FXIII by thrombin and consequently affects the structure of the cross-linked fibrin clot.

The factor XIII V34L polymorphism accelerates thrombin activation of factor XIII and affects cross-linked fibrin structure

Factor XIII on activation by thrombin cross-links fibrin. A common polymorphism Val to Leu at position 34 in the FXIII A subunit is under investigation as a risk determinant of thrombosis. Because Val34Leu is close to the thrombin cleavage site, the hypothesis that it would alter the function of FXIII was tested. Analysis of FXIII subunit proteolysis by thrombin using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography showed that FXIII 34Leu was cleaved by thrombin more rapidly and by lower doses than 34Val. Mass spectrometry of isolated activation peptides confirmed the predicted single methyl group difference and demonstrated that the thrombin cleavage site is unaltered by Val34Leu. Kinetic analysis of activation peptide release demonstrated that the catalytic efficiency (kcat/Km) of thrombin was 0.5 for FXIII 34Leu and 0.2 (μmol/L)−1× sec−1 for 34Val. Presence of fibrin increased the catalytic efficiency to 4.8 and 2.2 (μmol/L)−1 × sec−1, respectively. Although the 34Leu peptide was released at a similar rate as fibrinopeptide A, the 34Val peptide was released more slowly than fibrinopeptide A but more quickly than fibrinopeptide B generation. Cross-linking of γ- and -chains appeared earlier when fibrin was incubated with FXIII 34Leu than with 34Val. Fully activated 34Leu and 34Val FXIII showed similar cross-linking activity. Analysis of fibrin clots prepared using plasma from FXIII 34Leu subjects by turbidity and permeability measurements showed reduced fiber mass/length ratio and porosity compared to 34Val. The structural differences were confirmed by electron microscopy. These results demonstrate that Val34Leu accelerates activation of FXIII by thrombin and consequently affects the structure of the cross-linked fibrin clot.

Thermal conformational changes of bovine fibrinogen by differential scanning calorimetry and circular dichroism

The thermal denaturation of bovine fibrinogen has been investigated using differential scanning calorimetry (DSC) and circular dichroism (CD) spectroscopy. Differential scanning calorimetry measurements were carried out while changing the scan-rate. The transition at 57 degrees C was found to be irreversible and highly scan-rate dependent, suggesting that the denaturation is, at least in part, under kinetic control. The secondary structural changes at various temperatures were monitored by far-ultraviolet CD spectroscopy. These results show that the DSC transition for the thermal denaturation of bovine fibrinogen can be interpreted in terms of a kinetic process, N --> F, where k is a first-order kinetic constant that changes with temperature according to the Arrhenius equation. An important transition peak was observed at 78.8 degrees C which is attributed to the C-terminal parts of the Aalpha chains of fibrinogen.

Subunit antigen and activity levels of blood coagulation factor XIII in healthy individuals. Relation to sex, age, smoking, and hypertension

Factor (F) XIII covalently cross-links and stabilizes the fibrin-clot. Recent evidence suggests a role for FXIII in atherothrombotic diseases, but no information is available regarding the association of FXIII with common risk factors. The aim of this study was to investigate the relationship of FXIII with age, sex, smoking, and hypertension. Plasma levels of FXIII A-subunit antigen, FXIII B-subunit antigen, and FXIII cross-linking activity were measured in 612 healthy individuals (250 men and 362 women). FXIII A- and B-subunit levels were correlated significantly with age in both men (r=0.21, P=0.001, and r=0.17, P=0.008, respectively) and women (r=0.20, P<0.0005, and r=0.13, P=0.011, respectively). FXIII B-subunit levels and activity were correlated significantly with FXIII A-subunit levels (r=0.60, P<0.0005, and r=0.14, P<0.0005, respectively) and fibrinogen (r=0.26, P<0.0005, and r=0.14, P=0.001, respectively). Women had higher levels of FXIII A-subunit (111.8% versus 105.2%, P<0.01) and B-subunit (109.5% versus 103.8%, P<0.01) than did men. FXIII A-subunit was significantly increased in smokers (117.0% versus 104.6%, P<0.0005) and in subjects with hypertension (114.9% versus 107.8%, P<0.05). In a multiple regression model, FXIII A-subunit was significantly increased by female sex (+6.4%, P<0.007), smoking (+12.3%, P<0.0005), and increasing age (+3.7% per 10 years, P<0.0005). FXIII B-subunit was significantly related to female sex and fibrinogen, and FXIII activity was significantly related to fibrinogen levels. In conclusion, the FXIII A-subunit level increases significantly with female sex, age, and smoking, whereas FXIII B-subunit and FXIII activity are associated with FXIII A-subunit level and fibrinogen. Although evidence for a causal relationship between FXIII A-subunit and vascular disease is not available, these results might suggest a role for elevated FXIII A-subunit levels in the pathogenesis of vascular disease.

Interaction of recombinant procollagen and properdin modules of thrombospondin-1 with heparin and fibrinogen/fibrin

Many properties have been assigned to the procollagen and properdin (Type I) modules of thrombospondin-1 (TSP1) based on activities of large proteolytic fragments of TSP1 or peptides containing TSP1-derived sequences. To examine the activities of the modules more exactly, we expressed the first properdin module (P1); the third properdin module (P3); the first and second properdin modules (P12); the first, second, and third properdin modules (P123); and the procollagen module with the first, second, and third properdin modules (CP123) in the GELEX expression vector (GE1) using the baculovirus system. GE1 encodes the pre-pro sequence, the transglutaminase cross-linking site(s), the protease-sensitive site, and the gelatin binding domain from the amino terminus of rat fibronectin. All five recombinant proteins were expressed by insect cells, secreted into the culture medium, and purified by gelatin-agarose affinity chromatography. P123 shared with TSP1 a resistance to trypsin unless reduced and alkylated. P12/GE1, P123/GE1, and CP123/GE1 bound poorly to heparin-agarose except in the absence of sodium chloride, whereas peptides based on P2 are known to bind to heparin in up to 150 mM sodium chloride. In cross-linking experiments employing activated recombinant factor XIII and the transglutaminase cross-linking site in the fibronectin-derived sequence, P12/GE1, P123/GE1, CP123/GE1, and P3/GE1 but not P1/GE1 became incorporated into a fibrin clot more than GE1 alone. Analysis of the complex indicated that cross-linking was to the portion of the fibrin alpha-chain remaining in the D-dimer of plasmin digests. P123 also cross-linked to the Aalpha-chain of unclotted fibrinogen. P123 competed for 125I-TSP1 incorporation into the fibrin clot. P123 did not cross-link to plasminogen, histidine-rich glycoprotein, fibronectin, or plasma globulins other than fibrinogen/fibrin. These results indicate that the properdin modules of TSP1 specifically interact with fibrinogen/fibrin but not with heparin under physiologic conditions.

Thrombspondin acts via integrin-associated protein to activate the platelet integrin alphaIIbbeta3

Integrin-associated protein (IAP or CD47) is a receptor for the cell/platelet-binding domain (CBD) of thrombospondin-1 (TS1), the most abundant protein of platelet alpha granules. Although it associates with alphaIIbbeta3, IAP has no known function in platelets. TS1, the CBD, and an IAP agonist peptide (4N1K) from the CBD of TS1 activate the platelet integrin alphaIIbbeta3, resulting in platelet spreading on immobilized fibrinogen, stimulation of platelet aggregation, and enhanced tyrosine phosphorylation of focal adhesion kinase. Furthermore, 4N1K peptide selectively stimulates the phosphorylation of LYN and SYK and their association with FAK. The phosphorylation of SYK is blocked by pertussis toxin, implicating a Gi-like heterotrimeric G protein. IAP solublized from membranes of unstimulated platelets binds specifically to an affinity column of 4N1K peptide. Both alphaIIb and beta3 integrin subunits and c-Src bind along with IAP. This complex of proteins is also detected with immunoprecipitation. Activation of platelets with the agonist peptide 4N1K results in the association of FAK with the IAP-alphaIIbbeta3 complex. Thus an important function of TS1 in platelets is that of a secreted costimulator of alphaIIbbeta3 whose unique properties result in its localization to the platelet surface and the fibrin clot.

Stimulation of platelet activation and aggregation by a carboxyl-terminal peptide from thrombospondin binding to the integrin-associated protein receptor

Thrombospondin, a major secretory product of the alpha-granules of activated platelets, is a large trimeric glycoprotein that plays an important role in platelet aggregation. On resting platelets, thrombospondin binds to a single receptor in a cation-independent manner, but upon platelet activation it binds at least two further, distinct receptors that are both dependent upon divalent cations. Each of these receptors on the platelet surface binds to different regions of the thrombospondin molecule, and such binding may be responsible for the multifunctional role of thrombospondin in aggregation. We show here that a peptide from the carboxyl terminus of thrombospondin, RFYVVMWK, directly and specifically induces the activation and aggregation of washed human platelets from different donors at concentrations of 5-25 microM. At lower concentrations the peptide synergizes with suboptimal concentrations of ADP to induce aggregation. Peptide affinity chromatography and immunoprecipitation with a monoclonal antibody were used to identify the receptor for the carboxyl-terminal peptide as the integrin-associated protein. The integrin-associated protein remained bound to the RFYVVMWK-containing peptide column when washed with a scrambled peptide in the presence of 5 mM EDTA, indicating a divalent cation-independent association. It is suggested that integrin-associated protein is the primary receptor for thrombospondin on the surface of resting platelets and is implicated in potentiating the platelet aggregation response.

Abnormal fibrin structure and inhibition of fibrinolysis in patients with multiple myeloma

Abnormal clot structures have been reported in patients with multiple myeloma, and purified immunoglobulin G (IgG) has been shown to influence fibrin assembly in purified systems. Recently fibrin structure has been demonstrated to be a major determinant of fibrinolytic rates. This study examined the effects of purified polyclonal and monoclonal myeloma IgG on fibrin structure and fibrinolysis in plasma clots. Clotting was initiated by the addition of thrombin (1.0 NIH units/ml) and calcium (10 mmol/L). Gelation was monitored as a time-dependent increase in optical density (633 nm). Fibrin fiber size (mu = mass-length ratio) was measured by scanning the gel from 800 to 400 nm. Two preparations of polyclonal IgG and plasma samples from 10 patients with myeloma were studied. Both Sandoglobulin (Sandoz Pharmaceuticals Corp.) and Gamimmune (Miles Inc., Cutter Biological) decreased final gel turbidity as the IgG concentration increased from 0 to 15 mg/ml. Because of its high maltose content, Gamimmune produced more-pronounced effects. Over a concentration range of 0 to 15 mg IgG per milliliter, mu decreased from 1.25 to 0.59 x 10(13) daltons/cm for Sandoglobulin and from 1.30 to 0.18 x 10(13) daltons/cm for Gamimmune. Polyclonal IgG at 15 mg/ml prolonged clot lysis induced by tissue-type plasminogen activator (tPA) from 800 seconds to > 12 hours. Similar effects were noted in myeloma clots. mu values in myeloma clots were significantly smaller than mu values in comparable normal clots. mu became smaller and lysis times became increasingly prolonged as the IgG level increased. High IgG concentrations induce thin fiber formation and impair fibrinolysis in plasma gels. These results demonstrate that fibrinolysis is inhibited in myeloma clots and that the degree of inhibition is correlated with IgG-mediated alterations in fibrin structure. Thin fibrin fibers may contribute to thrombotic risk in myeloma.

Novel aspects of blood coagulation factor XIII. I. Structure, distribution, activation, and function

Blood coagulation factor XIII (FXIII) is a protransglutaminase that becomes activated by the concerted action of thrombin and Ca2+ in the final stage of the clotting cascade. In addition to plasma, FXIII also occurs in platelets, monocytes, and monocyte-derived macrophages. While the plasma factor is a heterotetramer consisting of paired A and B subunits (A2B2), its cellular counterpart lacks the B subunits and is a homodimer of potentially active A subunits (A2). The gene coding for the A and B subunits has been localized to chromosomes 6p24-25 and 1q31-32.1, respectively. The genomic as well as the primary protein structure of both subunits has been established, and most recently the three-dimensional structure of recombinant cellular FXIII has also been revealed. Monocytes/macrophages synthesize their own FXIII, and very likely FXIII in platelets is synthesized by the megakaryocytes. Cells of bone marrow origin seem to be the primary site for the synthesis of subunit A in plasma FXIII, but hepatocytes might also contribute. The B subunit of plasma FXIII is synthesized in the liver. Plasma FXIII circulates in association with its substrate precursor, fibrinogen. Fibrin(ogen) has an important regulatory role in the activation of plasma FXIII. The most important steps of the activation of plasma FXIII are the proteolytic removal of activation peptide by thrombin, the dissociation of subunits A and B, and the exposure of the originally buried active site on the free A subunits. The end result of this process is the formation of an active transglutaminase, which cross-links peptide chains through epsilon(gamma-glutamyl)lysyl isopeptide bonds. Cellular FXIII in platelets becomes activated through a nonproteolytic process. When intracytoplasmic Ca2+ is raised during platelet activation, the zymogen--in the absence of subunit B--assumes an active configuration. The protein substrates of activated FXIII include components of the clotting-fibrinolytic system, adhesive and contractile proteins. The main physiological function of plasma FXIII is to cross-link fibrin and protect it from the fibrinolytic plasmin. The latter effect is achieved mainly by covalently linking alpha 2 antiplasmin, the most potent physiological inhibitor of plasmin, to fibrin. Plasma FXIII seems to be involved in wound healing and tissue repair, and it is essential to maintaining pregnancy. Cellular FXIII, if exposed to the surface of the cells, might support or perhaps take over the hemostatic functions of plasma FXIII; however, its intracellular role has remained mostly unexplored.

The interaction of fibulin-1 with fibrinogen. A potential role in hemostasis and thrombosis

The fibulins are an emerging family of extracellular matrix and blood proteins presently having two members designated fibulin-1 and -2. Fibulin-1 is the predominant fibulin in blood, present at a concentration of 30-40 micrograms/ml (approximately 1000-fold higher than fibulin-2). During the course of isolating fibulin-1 from plasma by immunoaffinity chromatography, a 340-kDa polypeptide was consistently found to co-purify. This protein was identified as fibrinogen (Fg) based on its electrophoretic behavior and reactivity with Fg monoclonal antibodies. Radioiodinated fibulin-1 was shown to bind to Fg transferred onto nitrocellulose filters after SDS-polyacrylamide gel electrophoresis. In enzyme-linked immunosorbent assay, fibulin-1 bound to Fg (and fibrin) adsorbed onto microtiter well plastic, and conversely, Fg bound to fibulin-1-coated wells. The binding of Fg to fibulin-1 was also observed in surface plasmon resonance assays, and a dissociation constant (Kd) of 2.9 +/- 1.6 microM was derived. In addition, fluorescence anisotropy experiments demonstrated that the interaction was also able to occur in fluid phase, which suggests that complexes of fibulin-1 and Fg could exist in the blood. To localize the portion of Fg that is responsible for interacting with fibulin-1, proteolytic fragments of Fg were evaluated for their ability to promote fibulin-1 binding. Fragments containing the carboxyl-terminal region of the Bbeta chain (residues 216-468) were able to bind to fibulin-1. In addition, it was found that fibulin-1 was able to incorporate into fibrin clots formed in vitro and was immunologically detected within newly formed fibrin-containing thrombi associated with human atherectomy specimens. The interaction between fibulin-1 and Fg highlights potential new roles for fibulin-1 in hemostasis as well as thrombosis.

At high heparin concentrations, protamine concentrations which reverse heparin anticoagulant effects are insufficient to reverse heparin anti-platelet effects

Combined effects of heparin and protamine on plasma clot structure and platelet function were studied. Anticoagulant effects were monitored as changes in aPTT. Clot structure was defined in terms of fibrin fiber mass/length ratio (mu) and clot elastic modulus (EM). Platelet function was studied utilizing platelet aggregation and platelet force development (PFD) measurements. Heparin (1 U/ml) prolonged the aPTT from 30 to > 300 seconds, reduced PFD from 5,100 to 0 dynes, decreased mu (in batroxobin-induced gels) from 1.36 to 1.08 x 10(13) daltons/cm and decreased clot EM from 9,600 to 2000 dynes/cm2. Varying amounts of protamine reversed these effects: 16 micrograms/ml normalized the aPTT, 20 micrograms/ml normalized PFD, 32 micrograms/ml corrected mu, and 20 micrograms/ml returned EM to baseline. At high heparin concentrations (4 U/ml), protamine concentrations which corrected anticoagulant effects were inadequate to reverse antiplatelet effects. A protamine concentration of 40 micrograms/ml normalized the aPTT and mu, but 140 micrograms/ml of protamine was required to reverse heparin suppression of force development and clot elastic modulus. Excess protamine inhibited clotting and platelet function. In plasma containing 1 u heparin/ml, 140 micrograms protamine/ml reduced PFD by 83%, prolonged the aPTT by 63%, and reduced clot EM by 75%. In heparin free plasma, > 75 micrograms protamine/ml prolonged the aPTT. Thus, platelet function and clot structure are sensitive to protamine during heparin neutralization, and anti-platelet effects of heparin may persist when the aPTT is completely corrected. Excess protamine inhibits platelet function and compromises clot structure.

Melanocytes freshly isolated from normal human skin express the cell membrane receptor for the adhesive glycoprotein thrombospondin

Thrombospondin (TSP) is an adhesive protein with multiple binding sites, which is able to mediate several cell-to-cell and cell-to-matrix interactions, particularly through its cell membrane receptor (TSP-R). Because human keratinocytes are able to synthesize and express TSP, and as TSP is also localized at the dermal-epidermal junction in normal human skin, we questioned whether epidermal cells are able to bind available TSP, that is, to express TSP-R. To investigate this, we employed gold immunoelectron microscopy on epidermal cells freshly isolated from normal human skin; the TSP-R was detected by OKM5 monoclonal antibody. Epidermal cells showing ultrastructural characteristics of melanocytes were gold-stained on their plasma membrane, whereas keratinocytes, Langerhans cells and lymphocytes were unstained. Although functional studies are clearly necessary to clarify the role(s) played by the TSP-R on the cell surface of melanocytes, it is tempting to speculate that the TSP-R may be important for melanocyte adhesion to the dermal-epidermal junction and to keratinocytes. Such adhesion may not only subserve the steric localization of melanocytes, but also have important implications for those functional activities of melanocytes which have been shown to require close contact between these cells and adjacent keratinocytes and/or basement membrane components.

Hemodynamic, mechanical, and metabolic determinants of thrombolytic efficacy: a theoretic framework for assessing the limitations of thrombolysis in patients with cardiogenic shock

Although thrombolytic therapy has been shown to limit infarct size, preserve left ventricular function, and improve survival in most subgroups of patients with acute MI, a benefit has not been demonstrated in patients with clinical left ventricular dysfunction or overt cardiogenic shock before treatment is initiated. The reason(s) for the lack of benefit derived from thrombolytic therapy in these settings is unclear. Left ventricular dysfunction and overt cardiogenic shock are the result of extensive myocardial necrosis, typically in excess of 30% of the left ventricle, which progresses over time. The available data suggest that thrombolytic efficacy is decreased because of either hemodynamic, mechanical, or metabolic factors. As a result coronary patency is rarely achieved in a timely fashion, and if patency is achieved it typically is not maintained. The ability of mechanical revascularization by means of balloon angioplasty to reduce mortality suggests that reperfusion is a key determinant of outcome even among patients with large infarctions and early signs of left ventricular dysfunction. Thrombolytic therapy, which is widely available and extensively tested, represents the standard of care for patients with acute MI. Its apparent lack of efficacy in patients with congestive heart failure and cardiogenic shock is poorly understood. Further investigation must therefore be undertaken.

Modulation of fibrinolysis by thrombospondin

Thrombospondin is a large, trimeric glycoprotein secreted by activated platelets and growing cells. Thrombospondin copolymerizes with fibrin during blood coagulation and deposits in extracellular matrix. We found that thrombospondin is a slow (rate constant approximately 6.3 x 10(3) M-1 sec-1), tight-binding (Kd < 10(-9) M) inhibitor of plasmin as determined by loss of amidolytic activity, loss of ability to degrade fibrinogen, and decreased lysis zones in fibrin plate assays (Biochemistry 31: 265-269, 1992). Thrombospondin also slowly inhibits urokinase plasminogen activator. The lysis zone when urokinase is put on fibrin plates made from whole plasma is less if thrombospondin is present. The stoichiometry of inhibition is approximately one mole plasmin:one mole thrombospondin trimer, a somewhat surprising result considering the trimeric nature of thrombospondin. These results indicate that thrombospondin is an important regulator of fibrinolysis and degradation of extracellular matrix, particularly when these processes are initiated by urokinase and even when other inhibitors of fibrinolysis are present.

Localization of two binding domains for thrombospondin within fibronectin

Thrombospondin is a major glycoprotein of the platelet alpha-granule and is secreted during platelet activation. Several protease-resistant domains of thrombospondin mediate its interactions with components of the extracellular matrix including fibronectin, collagen, heparin, laminin, and fibrinogen. Thrombospondin, as well as fibronectin, is composed of several discretely located biologically active domains. We have characterized the thrombospondin binding domains of plasma fibronectin and determined the binding affinities of the purified domains; fibronectin has at least two binding sites for thrombospondin. Thrombospondin bound specifically to the 29-kDa amino-terminal heparin binding domain of fibronectin as well as to the 31-kDa non-heparin binding domain located within the larger 40-kDa carboxy-terminal fibronectin domain generated by chymotrypsin proteolysis. Platelet thrombospondin interacted with plasma fibronectin in a specific and saturable manner in blot binding as well as solid-phase binding assays. These interactions were independent of divalent cations. Thrombospondin bound to the 29-kDa fibronectin heparin binding domain with a Kd of 1.35 x 10(-9) M. The Kd for the 31-kDa domain of fibronectin was 2.28 x 10(-8) M. The 40-kDa carboxy-terminal fragment bound with a Kd of 1.65 x 10(-8) M. Heparin, which binds to both proteins, inhibited thrombospondin binding to the amino-terminal domain of fibronectin by more than 70%. The heparin effect was less pronounced with the non-heparin binding carboxy-terminal domain of fibronectin. By contrast, the binding affinity of the thrombospondin 150-kDa domain, which itself lacked heparin binding, was not affected by the presence of heparin. Based on these data, we conclude that thrombospondin binds with different affinities to two distinct domains in the fibronectin molecule.

Thrombospondin II: partial cDNA sequence, chromosome location, and expression of a second member of the thrombospondin gene family in humans

A novel form of human thrombospondin was identified during the screening of a human fibroblast cDNA library. We report the cDNA sequence for 1.8 kb of the 3' end of the cDNA, plus an additional 937 bp of 3'-untranslated sequence. The translated sequence reveals a high degree of similarity to thrombospondin I. The homology ranges from 56 to 80% for different regions within the two proteins. The repeating segments of amino acid sequence identified in thrombospondin I were found to be conserved in thrombospondin II. The new form of thrombospondin hybridizes to a 7.5-kb message by Northern analysis. The THBS2 gene is located at the distal long arm of chromosome 6 at 6q27. The gene is transcribed in fibroblasts, smooth muscle cells, and an osteosarcoma cell line, at levels somewhat lower than that of thrombospondin I. Umbilical vein endothelial cells do not transcribe thrombospondin II under the conditions of this study. These findings suggest that previous studies of thrombospondin function need to be reassessed to identify the functions specific to each molecule.

Thrombospondin sequence motif (CSVTCG) is responsible for CD36 binding

To clarify the role of CD36 as a TSP receptor and to investigate the mechanisms of the TSP-CD36 interaction, transfection studies were performed using CD36-cDNA in a CDM8 plasmid. Jurkat cells transfected with CD36 cDNA express an 88kD membrane surface protein and acquire the ability to bind thrombospondin. The TSP amino acid sequence, CSVTCG, mediates the interaction of thrombospondin with CD36. CD36 transfectants but not control transfectants bind radiolabeled tyrosinated peptide (YCSVTCG). The hexapeptide inhibits thrombospondin expression on activated human platelets and results in diminished platelet aggregation. CSVTCG-albumin conjugates support CD36-dependent adhesion of tumor cells. We conclude that the CSVTCG repeat sequence is a crucial determinant of CD36 thrombospondin binding.

Inhibition of factor XIII activation by an anti-peptide monoclonal antibody

As the final enzyme in the coagulation cascade, activated fibrin stabilizing factor or factor XIII catalyzes the intermolecular cross-linking of fibrin chains. To study this enzyme in plasma, we derived a monoclonal antibody (MAb 309) against a peptide sequence (NH2-G-V-N-L-Q-E-F-C-COOH) in the thrombin activation site of factor XIII. Radioimmunoassays indicate that MAb 309 binds specifically to both platelet and plasma factor XIII. Peptide inhibition studies demonstrate that the MAb binds equally well to the factor XIII (FXIII) zymogen and the active form of FXIII (FXIIIa). In immunoblots of whole platelet lysates, MAb 309 binds only to FXIII and does not cross-react with other proteins. In saturation binding studies, the antibody shows a binding avidity of (1.75 +/- 0.35) x 10(9) M-1. MAb 309 also inhibited 99% of apparent FXIIIa activity in a standard transglutaminase assay. SDS-PAGE analysis of fibrin clots showed that MAb 309 inhibited fibrin gamma-gamma cross-linking. Moreover, MAb 309 accelerated the lysis of plasma clots, consistent with inhibition of fibrin-fibrin and fibrin-alpha 2-antiplasmin cross-linking. Immunoblotting experiments revealed that MAb 309 affected apparent FXIIIa activity by inhibiting the thrombin activation of the FXIII zymogen. In addition to its utility as a specific probe for the FXIII a-subunit, the strategy used to obtain MAb 309 may be used to generate MAbs that inhibit the activation of other coagulation factor zymogens.

Neutron and X-ray scattering studies on the human complement protein properdin provide an analysis of the thrombospondin repeat

Properdin is a regulatory glycoprotein of the alternative pathway of the complement system of immune defense. It is responsible for the stabilization of the C3 convertase complex formed between C3b and the Bb fragment of factor B. Neutron and X-ray solution scattering experiments were performed on the dimeric and trimeric forms of properdin. These have RG values of 9.1 and 10.7 nm, respectively. The scattering curves were compared with Debye sphere modeling simulations for properdin. Good agreements were obtained for models similar to published electron micrographs showing that the properdin trimer has a triangular structure with sides of 26 nm. Such a structure also accounted for sedimentation coefficient data on properdin. Primary structure analyses for mouse and human properdin have shown that this contains six homologous motifs known as the thrombospondin repeat (TSR), which is the second most abundant domain type found in the complement proteins. Sequences for these 12 TSRs were aligned with 19 others found in thrombospondin and the late complement components. Three distinct groups of TSRs were identified, namely, the TSRs found in thrombospondin and properdin, the TSRs mostly found at the N-terminus of the late complement components, and the TSRs found at the C-terminus of the late components. Averaged secondary structure predictions suggested that all three groups contain similar backbone structures with two amphipathic turn regions and one hydrophilic beta-strand region. The mean dimensions of the TSRs of properdin in solution were determined to be approximately 4 nm X 1.7 nm X 1.7 nm, showing that these are elongated in structure.