Functional changes in the conformation of thrombospondin-1 during complexation with fibronectin or heparin

Controlled extracellular proteolysis of thrombospondins

Limited proteolysis of thrombospondins is a powerful mechanism to ensure dynamic tuning of their activities in the extracellular space. Thrombospondins are multifunctional matricellular proteins composed of multiple domains, each with a specific pattern of interactions with cell receptors, matrix components and soluble factors (growth factors, cytokines and proteases), thus with different effects on cell behavior and responses to changes in the microenvironment. Therefore, the proteolytic degradation of thrombospondins has multiple functional consequences, reflecting the local release of active fragments and isolated domains, exposure or disruption of active sequences, altered protein location, and changes in the composition and function of TSP-based pericellular interaction networks. In this review current data from the literature and databases is employed to provide an overview of cleavage of mammalian thrombospondins by different proteases. The roles of the fragments generated in specific pathological settings, with particular focus on cancer and the tumor microenvironment, are discussed.

Contribution of Human Thrombospondin-1 to the Pathogenesis of Gram-Positive Bacteria

A successful colonization of different compartments of the human host requires multifactorial contacts between bacterial surface proteins and host factors. Extracellular matrix proteins and matricellular proteins such as thrombospondin-1 play a pivotal role as adhesive substrates to ensure a strong interaction with pathobionts like the Gram-positive Streptococcus pneumoniae and Staphylococcus aureus. The human glycoprotein thrombospondin-1 is a component of the extracellular matrix and is highly abundant in the bloodstream during bacteremia. Human platelets secrete thrombospondin-1, which is then acquired by invading pathogens to facilitate colonization and immune evasion. Gram-positive bacteria express a broad spectrum of surface-exposed proteins, some of which also recognize thrombospondin-1. This review highlights the importance of thrombospondin-1 as an adhesion substrate to facilitate colonization, and we summarize the variety of thrombospondin-1-binding proteins of S. pneumoniae and S. aureus.

Plasma degradome affected by variable storage of human blood

Background

The successful application of—omics technologies in the discovery of novel biomarkers and targets of therapeutic interventions is facilitated by large collections of well curated clinical samples stored in bio banks. Mining the plasma proteome holds promise to improve our understanding of disease mechanisms and may represent a source of biomarkers. However, a major confounding factor for defining disease-specific proteomic signatures in plasma is the variation in handling and processing of clinical samples leading to protein degradation. To address this, we defined a plasma proteolytic signature (degradome) reflecting pre-analytical variability in blood samples that remained at ambient temperature for different time periods after collection and prior to processing.

Methods

We obtained EDTA blood samples from five healthy volunteers (n = 5), and blood tubes remained at ambient temperature for 30 min, 8, 24 and 48 h prior to centrifugation and isolation of plasma. Naturally occurred peptides derived from plasma samples were compared by label-free quantitative LC–MS/MS. To profile protein degradation, we analysed pooled plasma samples at T = 30 min and 48 h using PROTOMAP analysis. The proteolytic pattern of selected protein candidates was further validated by immunoblotting.

Results

A total of 820 plasma proteins were surveyed by PROTOMAP, and for 4 % of these, marked degradation was observed. We show distinct proteolysis patterns for talin-1, coagulation factor XI, complement protein C1r, C3, C4 and thrombospondin, and several proteins including S100A8, A9, annexin A1, profiling-1 and platelet glycoprotein V are enriched after 48 h blood storage at ambient temperature. In particular, thrombospondin protein levels increased after 8 h and proteolytic fragments appeared after 24 h storage time.

Conclusions

The overall impact of blood storage at ambient temperature for variable times on the plasma proteome and degradome is relatively minor, but in some cases can cause a potential bias in identifying and assigning relevant proteomic markers. The observed effects on the plasma proteome and degradome are predominantly triggered by limited leucocyte and platelet cell activation due to blood handling and storage. The baseline plasma degradome signature presented here can help filtering candidate protein markers relevant for clinical biomarker studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12014-016-9126-9) contains supplementary material, which is available to authorized users.

Current understanding of the thrombospondin-1 interactome

The multifaceted action of thrombospondin-1 (TSP-1) depends on its ability to physically interact with different ligands, including structural components of the extracellular matrix, other matricellular proteins, cell receptors, growth factors, cytokines and proteases. Through this network, TSP-1 regulates the ligand activity, availability and structure, ultimately tuning the cell response to environmental stimuli in a context-dependent manner, contributing to physiological and pathological processes. Complete mapping of the TSP-1 interactome is needed to understand its diverse functions and to lay the basis for the rational design of TSP-1-based therapeutic approaches. So far, large-scale approaches to identify TSP-1 ligands have been rarely used, but many interactions have been identified in small-scale studies in defined biological systems. This review, based on information from protein interaction databases and the literature, illustrates current knowledge of the TSP-1 interactome map.

Thrombospondin-1: a unique marker to identify in vitro platelet activation when monitoring in vivo processes

BACKGROUND

Measuring platelet activation in patients has become a potent method to investigate pathophysiological processes. However, the commonly applied markers are sensitive to detrimental influences by in vitro platelet activation during blood analysis.

OBJECTIVES

Protein isoforms of platelet-derived thrombospondin-1 (TSP-1) were investigated for their potential to identify in vitro platelet activation when monitoring in vivo processes.

METHODS

TSP-1 was determined in plasma, serum or supernatant of purified platelets by ELISA and immunoblotting and was compared with standard markers of platelet activation. A collective of 20 healthy individuals and 30 cancer patients was analyzed.

RESULTS

 While in vitro platelet degranulation led to a selective increase in the 200-kDa full-length molecule, an in vivo process involving platelet activation such as wound healing resulted in the predominant rise of the 140-kDa TSP-1 protein. The physiological ratio of circulating TSP-1 variants was determined and a cut-off level at 1.0 was defined to identify plasma samples with artificial in vitro platelet activation exceeding the cut-off level. In contrast, cancer patients known to frequently exhibit increased in vivo activation of platelets presented with a significantly decreased ratio of TSP-1 variants as compared with healthy volunteers.

CONCLUSIONS

In comparison to standard platelet markers, TSP-1 constitutes a sensitive and stable parameter suited to monitor in vitro platelet activation. The analysis of TSP-1 protein isoforms further offers a valuable tool to reliably discriminate between in vitro and in vivo effects, to exclude variability introduced during blood processing and improve clinical monitoring.

The interaction of Thrombospondins with extracellular matrix proteins

The thrombospondins (TSPs) are a family of five matricellular proteins that appear to function as adapter molecules to guide extracellular matrix synthesis and tissue remodeling in a variety of normal and disease settings. Various TSPs have been shown to bind to fibronectin, laminin, matrilins, collagens and other extracellular matrix (ECM) proteins. The importance of TSP-1 in this context is underscored by the fact that it is rapidly deposited at the sites of tissue damage by platelets. An association of TSPs with collagens has been known for over 25 years. The observation that the disruption of the TSP-2 gene in mice leads to collagen fibril abnormalities provided important in vivo evidence that these interactions are physiologically important. Recent biochemical studies have shown that TSP-5 promotes collagen fibril assembly and structural studies suggest that TSPs may interact with collagens through a highly conserved potential metal ion dependent adhesion site (MIDAS). These interactions are critical for normal tissue homeostasis, tumor progression and the etiology of skeletal dysplasias.

Calcium indirectly regulates immunochemical reactivity and functional activities of the N-domain of thrombospondin-1

Conformational changes induced in thrombospondin-1 by removal of calcium regulate interactions with some ligands of its N-modules. Because calcium binds primarily to elements of the C-terminal signature domain of thrombospondin-1, which are distant from the N-modules, such regulation was unexpected. To clarify the mechanism for this regulation, we compared ligand binding to the N-modules of thrombospondin-1 in the full-length protein and recombinant trimeric thrombospondin-1 truncated prior to the signature domain. Three monoclonal antibodies were identified that recognize the N-modules, two of which exhibit calcium-dependent binding to native thrombospondin-1 but not to the truncated trimeric protein. These antibodies or calcium selectively modulate interactions of fibronectin, heparin, sulfatide, alpha3beta1 integrin, tumor necrosis factor-alpha-stimulated gene-6 protein, and, to a lesser extent, alpha4beta1 integrin with native thrombospondin-1 but not with the truncated protein. These results indicate connectivity between calcium binding sites in the C-terminal signature domain and the N-modules of thrombospondin-1 that regulates ligand binding and functional activities of the N-modules.

Fibroblast growth factor-2 binding to the thrombospondin-1 type III repeats, a novel antiangiogenic domain

Thrombospondin-1, an antiangiogenic matricellular protein, binds with high affinity to the angiogenic fibroblast growth factor-2, affecting its bioavailability and activity. The present work aimed at further locating the fibroblast growth factor-2 binding site of thrombospondin-1 and investigating its activity, using recombinant thrombospondin-1 proteins. Only recombinant constructs containing the thrombospondin-1 type III repeats bound fibroblast growth factor-2, whereas other domains, including the known anti-angiogenic type I repeats, were inactive. Binding was specific and inhibited by the anti thrombospondin-1 monoclonal antibody B5.2. Surface plasmon resonance analysis on BIAcore revealed a binding affinity (K(d)) of 310nM for the type III repeats and 11nM for intact thrombospondin-1. Since the type III repeats bind calcium, the effect of calcium on thrombospondin-1 binding to fibroblast growth factor-2 was investigated. Binding was modulated by calcium, as thrombospondin-1 or the type III repeats bound to fibroblast growth factor-2 only in calcium concentrations <0.3mM. The type III repeats inhibited binding of fibroblast growth factor-2 to endothelial cells, fibroblast growth factor-2-induced endothelial cell proliferation in vitro and angiogenesis in the chorioallantoic membrane assay in vivo, thus indicating the antiangiogenic activity of the domain. In conclusion, this study demonstrates that the fibroblast growth factor-2 binding site of thrombospondin-1 is located in the type III repeats. The finding that this domain is active in inhibiting angiogenesis indicates that the type III repeats represent a novel antiangiogenic domain of thrombospondin-1.

Minimal model for tumor angiogenesis

In this work, we show a mathematical model for the angiogenesis by endothelial cells. We present the model at the level of partial differential equations, describing the spatiotemporal evolution of the cell population, the extracellular matrix macromolecules, the proteases, the tumor angiogenic factors, and the possible presence of inhibitors. We mainly focus, however, on a complementary, more physiologically realistic, hybrid approach in which the cells are treated as individual particles. We examine the model numerically in two-dimensional settings, discussing its comparison with experimental results.

pH-specific sequestration of phosphoglucose isomerase/autocrine motility factor by fibronectin and heparan sulphate

Phosphoglucose isomerase (PGI) is a glycolytic enzyme that moonlights as a cytokine under the aliases autocrine motility factor (AMF), neuroleukin and maturation factor. The cytokine function of PGI/AMF targets multiple cell types however mechanisms that regulate and sequester this ubiquitous, circulating cytokine remain largely unidentified. PGI/AMF is shown here to exhibit fibronectin (FN)-dependent cell surface association at both neutral and acid pH. Direct PGI/AMF binding to FN and fluorescence resonance energy transfer (FRET) between PGI/AMF and FN were detected only at pH 5. At neutral pH, the interaction of PGI/AMF with FN is receptor-mediated requiring prior clathrin-dependent endocytosis. PGI/AMF and FN do not co-internalize and PGI/AMF undergoes a second round of endocytosis upon recycling to the plasma membrane indicating that recycling PGI/AMF receptor complexes associate with FN fibrils. Heparan sulphate does not affect cell association of PGI/AMF at neutral pH but enhances the FN-independent cell surface association of PGI/AMF at acid pH identifying two distinct mechanisms for PGI/AMF sequestration under acidic conditions. However, only PGI/AMF sequestration by FN at acid pH was able to stimulate cell motility upon pH neutralization identifying FN as a pH-dependent cytokine trap for PGI/AMF. The multiple ways of cellular association of PGI/AMF may represent acquired mechanisms to regulate and harness the cytokine function of PGI/AMF.

Thrombospondin 1: a multifunctional protein implicated in the regulation of tumor growth

Thrombospondins belong to a family of extracellular matrix (ECM) proteins widely found from embryonic to adult tissues. The modular structure of thrombospondins contains a series of peptide sequences implicated in a multiplicity of biological functions. Extracellular matrix undergoes important alterations under proteolysis that occurs in pathological processes like tumorigenesis. An elevated secretion of thrombospondin 1 (TSP1) is often observed in tumors and is sometimes considered as a predictive factor. However, the role of TSP1 in cancer progression remains controversial and must be carefully apprehended. The regulation of cell adhesion, proliferation, apoptosis by TSP1 is examined in the present review and it is clear from the literature and from our investigations that TSP1 presents both stimulatory and inhibitory effects. The exposition of cryptic sites upon conformational changes can partially explain this contradiction. More interestingly, the analysis of TSP1-directed intracellular signaling pathways activated through specific receptors or supramolecular receptors docking systems may be useful to discriminate the precise function of TSP1 in tumor progression. The central role played by TSP1 in the control of matrix-degrading enzyme activation and catabolism reveals attractive tracks of research and highlights the involvement of the lipoprotein receptor-related protein (LRP) receptor in these events. Therefore, TSP1-derived peptides constitute a source of potentially active matrikins which could provide essential tools in cancer therapy.

Thrombospondin-4 and its variants: expression and differential effects on endothelial cells

BACKGROUND

In a recent large-scale genetic association study, a single nucleotide polymorphism in the thrombospondin-4 (TSP-4) gene, resulting in a proline-for-alanine substitution at position 387, was associated with a significantly increased risk for premature atherosclerosis. TSP-4 had not previously been implicated in vascular pathology, and very little information is available on its expression and functions.

METHODS AND RESULTS

The goal of this study was to assess TSP-4 expression in vessel wall and to identify differences in functions of TSP-4 variants that could account for the proatherogenic effects of the (P387)TSP-4 variant. TSP-4 expression was demonstrated in human endothelial cells (ECs) and vascular smooth muscle cells from brain blood vessels and coronary arteries. (P387)TSP-4 and its fragment (residues 326 to 722), but not the A(387) forms, suppressed EC adhesion and proliferation. The (P387)TSP-4 was more active in inducing the phosphorylation of focal adhesion kinase, consistent with inhibition of proliferation. Both variant fragments increased the proliferation of human aortic smooth muscle cells.

CONCLUSIONS

TSP-4 is expressed by vascular cells and influences the vessel wall by modulating the proliferation of ECs and smooth muscle cells. The A387P substitution is a "gain-of-function" mutation, favoring a form of TSP-4 that interferes with EC adhesion and proliferation and may thereby be proatherogenic.

Thrombospondin-1 mediates platelet adhesion at high shear via glycoprotein Ib (GPIb): an alternative/backup mechanism to von Willebrand factor

Acute thrombotic arterial occlusion is the leading cause of morbidity and mortality in the Western world. Von Willebrand factor is thought to be the only indispensable adhesive substrate to promote thrombus formation in high shear environments. We found that thrombospondin-1, a glycoprotein enriched in arteriosclerotic plaques, might function as an alternative substrate for thrombus formation. Platelets adhered to thrombospondin-1 in a shear dependent manner with an optimum shear as found in stenosed arteries. Adhesion is extremely firm, with no detachment of platelets up to a shear rate of 4000 s(-1). Experiments using platelets from a patient completely lacking von Willebrand factor showed that von Willebrand factor is not involved in platelet binding to thrombospondin-1. Platelet adhesion to thrombospondin-1 is not mediated via beta3-integrins or GPIa. CD36 partially mediates the adhesion of pre-activated platelets. We identified GPIb as high shear adhesion-receptor for thrombospondin-1. Soluble GPIb, as well as antibodies against the GPIb, blocked platelet adhesion almost completely. The new discovered thrombospondin-1-GPIb adhesion axis under arterial shear conditions might be important, not only during thrombus formation but also for pathological processes where other cells bind to the endothelium or subendothelium, including arteriosclerosis, inflammation and tumor metastasis, and a promising therapeutic target.

One of two chondrocyte-expressed isoforms of cartilage intermediate-layer protein functions as an insulin-like growth factor 1 antagonist

OBJECTIVE

Aging and osteoarthritic (OA) cartilage commonly demonstrate enhanced expression of the large, transforming growth factor beta (TGFbeta)-inducible glycoprotein cartilage intermediate-layer protein (CILP) as well as enhanced extracellular inorganic pyrophosphate (PPi) that promotes the deposition of calcium pyrophosphate dihydrate crystals. In normal chondrocytes, TGFbeta induces elevated chondrocyte extracellular PPi. Insulin-like growth factor 1 (IGF-1) normally blocks this response and reduces extracellular PPi. However, chondrocyte resistance to IGF-1 is observed in OA and aging. Because CILP was reported to chromatographically fractionate with PPi-generating nucleotide pyrophosphatase phosphodiesterase (NPP) activity, it has been broadly assumed that CILP itself has NPP activity. Our objective was to directly define CILP functions and their relationship to IGF-1 in chondrocytes.

METHODS

Using primary cultures of articular chondrocytes from the knee, we defined the function of the previously described CILP (CILP-1) and of a recently described 50.6% identical protein that we designated the CILP-2 isoform.

RESULTS

Both CILP isoforms were constitutively expressed by primary cultured articular chondrocytes, but only CILP-1 expression was detectable in cultured knee meniscal cartilage cells. Neither CILP isoform had intrinsic NPP activity. But CILP-1 blocked the ability of IGF-1 to decrease extracellular PPi, an activity specific for the CILP-1 N-terminal domain. The CILP-1 N-terminal domain also suppressed IGF-1-induced (but not TGFbeta-induced) proliferation and sulfated proteoglycan synthesis, and it inhibited ligand-induced IGF-1 receptor autophosphorylation.

CONCLUSION

Two CILP isoforms are differentially expressed by chondrocytes. Neither CILP isoform exhibits PPi-generating NPP activity. But, increased expression of CILP-1, via N-terminal domain-mediated inhibitory effects of CILP-1 on chondrocyte IGF-1 responsiveness, could impair chondrocyte growth and matrix repair and indirectly promote PPi supersaturation in aging and OA cartilage.

C-terminal peptide of thrombospondin-1 induces platelet aggregation through the Fc receptor gamma-chain-associated signaling pathway and by agglutination

A peptide from the C-terminal domain of thrombospondin-1 (Arg-Phe-Tyr-Val-Val-Met-Trp-Lys; known as 4N1-1) has been reported to induce platelet aggregation and to bind to the integrin-associated protein (IAP), which is also known as CD47. In this study, it was discovered that 4N1-1 or its derivative peptide, 4N1K, induces rapid phosphorylation of the Fc receptor (FcR) gamma chain, Syk, SLP-76, and phospholipase C gamma2 in human platelets. A specific inhibitor of Src family kinases, 4-amino-4-(4-methylphenyl)-7-(t-butyl) pyrazola[3,4-d]pyrimidine, prevented phosphorylation of these proteins, abolished platelet secretion, and reduced aggregation by approximately 50%. A similar inhibition of aggregation to 4N1-1 was obtained in the presence of Arg-Gly-Asp-Ser in mouse platelets deficient in FcR gamma chain or SLP-76 and in patients with type I Glanzmann thrombasthenia. These results show that 4N1-1 signals through a pathway similar to that used by the collagen receptor glycoprotein (GP) VI. The alphaIIbbeta3-independent aggregation induced by 4N1-1 was also observed in fixed platelets and platelets from patients with Bernard-Soulier syndrome, which are deficient in GPIbalpha. Surprisingly, the ability of 4N1-1 to stimulate aggregation and tyrosine phosphorylation was not altered in platelets pretreated with anti-IAP antibodies and in IAP-deficient mice. These results show that the C-terminal peptide of thrombospondin induces platelet aggregation through the FcR gamma-chain signaling pathway and through agglutination. The latter pathway is independent of signaling events and does not use GPIbalpha or alphaIIbbeta3. Neither of these pathways is mediated by IAP.

Conformational regulation of the fibronectin binding and alpha 3beta 1 integrin-mediated adhesive activities of thrombospondin-1

The recognition of extracellular matrix components can be regulated by conformational changes that alter the activity of cell surface integrins. We now demonstrate that conformational regulation of the matrix glycoprotein thrombospondin-1 (TSP1) can also modulate its binding to an integrin receptor. F18 1G8 is a conformation-sensitive TSP1 antibody that binds weakly to soluble TSP1 in the presence of divalent cations. However, binding of the antibody to melanoma cells was strongly stimulated by adding exogenous TSP1 in the presence of calcium, suggesting that TSP1 undergoes a conformational change following its binding to the cell surface. This conformation was not induced by known cell surface TSP1 receptors, whereas binding of F18 was stimulated when TSP1 bound to fibronectin but not to heparin or fibrinogen. Conversely, binding of F18 to TSP1 enhanced TSP1 binding to fibronectin. Exogenous fibronectin also stimulated TSP1-dependent binding of F18 to melanoma cells. Binding of the fibronectin-TSP1 complex to melanoma cells was mediated by alpha4beta1 and alpha5beta1 integrins. Furthermore, binding to F18 or fibronectin strongly enhanced the adhesive activity of immobilized TSP1 for some cell types. This enhancement of adhesion was mediated by alpha3beta1 integrin and required that the alpha3beta1 integrin be in an active state. Fibronectin also enhanced TSP1 binding to purified alpha3beta1 integrin. Therefore, both fibronectin and the F18 antibody induce conformational changes in TSP1 that enhance the ability of TSP1 to be recognized by alpha3beta1 integrin. The conformational and functional regulation of TSP1 activity by fibronectin represents a novel mechanism for extracellular signal transduction.

Regulation of tissue injury responses by the exposure of matricryptic sites within extracellular matrix molecules

Extracellular matrix (ECM) is known to provide signals controlling cell shape, migration, proliferation, differentiation, morphogenesis, and survival. Recent data shows that some of these signals are derived from biologically active cryptic sites within matrix molecules (matricryptic sites) that are revealed after structural or conformational alteration of these molecules. We propose the name, matricryptins, for enzymatic fragments of ECM containing exposed matricryptic sites. Mechanisms regulating the exposure of matricryptic sites within ECM molecules include the major mechanism of enzymatic breakdown as well as others including ECM protein multimerization, adsorption to other molecules, cell-mediated mechanical forces, and ECM denaturation. Such matrix alterations occur during or as a result of tissue injury, and thus, the appearance of matricryptic sites within an injury site may provide important new signals to regulate the repair process. Here, we review the data supporting this concept and provide insight into why the increased exposure of matricryptic sites may be an important regulatory step in tissue responses to injury.