Thrombospondin-induced adhesion of human platelets

Platelet-targeted gene therapy induces immune tolerance in hemophilia and beyond

Blood platelets have unique storage and delivery capabilities. Platelets play fundamental roles in hemostasis, inflammatory reactions, and immune responses. Beyond their functions, platelets have been used as a target for gene therapy. Platelet-targeted gene therapy aims to deliver a sustained expression of neo-protein in vivo by genetically modifying the target cells, resulting in a cure for the disease. Even though there has been substantial progress in the field of gene therapy, the potential development of immune responses to transgene products or vectors remains a significant concern. Of note, multiple preclinical studies using platelet-specific lentiviral gene delivery to hematopoietic stem cells in hemophilia have demonstrated promising results with therapeutic levels of neo-protein that rescue the hemorrhagic bleeding phenotype and induce antigen-specific immune tolerance. Further studies using ovalbumin as a surrogate protein for platelet gene therapy have shown robust antigen-specific immune tolerance induced via peripheral clonal deletions of antigen-specific CD4- and CD8-T effector cells and induction of antigen-specific regulatory T (Treg) cells. This review discusses platelet-targeted gene therapy, focusing on immune tolerance induction.

Multiscale modelling of the extracellular matrix

The extracellular matrix is a complex three-dimensional network of molecules that provides cells with a complex microenvironment. The major constituents of the extracellular matrix such as collagen, elastin and associated proteins form supramolecular assemblies contributing to its physicochemical properties and organization. The structure of proteins and their supramolecular assemblies such as fibrils have been studied at the atomic level (e.g., by X-ray crystallography, Nuclear Magnetic Resonance and cryo-Electron Microscopy) or at the microscopic scale. However, many protein complexes are too large to be studied at the atomic level and too small to be studied by microscopy. Most extracellular matrix components fall into this intermediate scale, so-called the mesoscopic scale, preventing their detailed characterization. Simulation and modelling are some of the few powerful and promising approaches that can deepen our understanding of mesoscale systems. We have developed a set of modelling tools to study the self-organization of the extracellular matrix and large motion of macromolecules at the mesoscale level by taking advantage of the dynamics of articulated rigid bodies as a mean to study a larger range of motions at the cost of atomic resolution.

A novel mouse model of type 2N VWD was developed by CRISPR/Cas9 gene editing and recapitulates human type 2N VWD

A novel type 2N VWD mouse model was established in which VWF is incapable of binding FVIII but is otherwise fully functional.

VWF^2N/2N mice exhibited a severe bleeding phenotype after tail tip amputation but not in lateral tail vein or ventral artery injury models.

Type 2N von Willebrand disease is caused by mutations in the factor VIII (FVIII) binding site of von Willebrand factor (VWF), resulting in dysfunctional VWF with defective binding capacity for FVIII. We developed a novel type 2N mouse model using CRISPR/Cas9 technology. In homozygous VWF^2N/2N mice, plasma VWF levels were normal (1167 ± 257 mU/mL), but the VWF was completely incapable of binding FVIII, resulting in 53 ± 23 mU/mL of plasma FVIII levels that were similar to those in VWF-deficient (VWF^−/−) mice. When wild-type human or mouse VWF was infused into VWF^2N/2N mice, endogenous plasma FVIII was restored, peaking at 4 to 6 hours post-infusion, demonstrating that FVIII expressed in VWF^2N mice is viable but short-lived unprotected in plasma due to dysfunctional 2N VWF. The whole blood clotting time and thrombin generation were impaired in VWF^2N/2N but not in VWF^−/− mice. Bleeding time and blood loss in VWF^2N/2N mice were similar to wild-type mice in the lateral tail vein or ventral artery injury model. However, VWF^2N/2N mice, but not VWF^−/− mice, lost a significant amount of blood during the primary bleeding phase after a tail tip amputation injury model, indicating that alternative pathways can at least partially restore hemostasis when VWF is absent. In summary, we have developed a novel mouse model by gene editing with both the pathophysiology and clinical phenotype found in severe type 2N patients. This unique model can be used to investigate the biological properties of VWF/FVIII association in hemostasis and beyond.

The impact of GPIbα on platelet-targeted FVIII gene therapy in hemophilia A mice with pre-existing anti-FVIII immunity

Essentials Platelet-specific FVIII gene therapy is effective in hemophilia A mice even with inhibitors. The impact of platelet adherence via VWF/GPIbα binding on platelet gene therapy was investigated. GPIbα does not significantly affect platelet gene therapy of hemophilia A with inhibitors. Platelet gene therapy induces immune tolerance in hemophilia A mice with pre-existing immunity. SUMMARY: Background We have previously demonstrated that von Willebrand factor (VWF) is essential in platelet-specific FVIII (2bF8) gene therapy of hemophilia A (HA) with inhibitory antibodies (inhibitors). At the site of injury, platelet adherence is initiated by VWF binding to the platelet GPIb complex. Objective To investigate the impact of GPIbα on platelet gene therapy of HA with inhibitors. Methods Platelet-FVIII expression was introduced by 2bF8 lentivirus (2bF8LV) transduction of hematopoietic stem cells (HSCs) from GPIbαnull (Ibnull ) mice or rhF8-primed FVIIInull (F8null ) mice followed by transplantation into lethally irradiated rhF8-primed F8null recipients. Animals were analyzed by flow cytometry, FVIII assays and the tail bleeding test. Results After transplantation, 99% of platelets were derived from donors. The macrothrombocytopenia phenotype was maintained in F8null mice that received 2bF8LV-transduced Ibnull HSCs (2bF8-Ibnull /F8null ). The platelet-FVIII expression level in 2bF8-Ibnull /F8null recipients was similar to that obtained from F8null mice that received 2bF8LV-transduced F8null HSCs (2bF8-F8null /F8null ). The tail bleeding test showed that the remaining hemoglobin level in the 2bF8-Ibnull /F8null group was significantly higher than in the F8null control group, but there was no significant difference between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The half-life of inhibitor disappearance time was comparable between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The rhF8 re-challenge did not elicit a memory immune response once inhibitor titers dropped to undetectable levels after 2bF8 gene therapy. Conclusion GPIbα does not significantly impact platelet gene therapy of HA with inhibitors. 2bF8 gene therapy restores hemostasis and promotes immune tolerance in HA mice with pre-existing immunity.

Role of platelets and platelet receptors in cancer metastasis

The interaction of tumor cells with platelets is a prerequisite for successful hematogenous metastatic dissemination. Upon tumor cell arrival in the blood, tumor cells immediately activate platelets to form a permissive microenvironment. Platelets protect tumor cells from shear forces and assault of NK cells, recruit myeloid cells by secretion of chemokines, and mediate an arrest of the tumor cell platelet embolus at the vascular wall. Subsequently, platelet-derived growth factors confer a mesenchymal-like phenotype to tumor cells and open the capillary endothelium to expedite extravasation in distant organs. Finally, platelet-secreted growth factors stimulate tumor cell proliferation to micrometastatic foci. This review provides a synopsis on the current literature on platelet-mediated effects in cancer metastasis and particularly focuses on platelet adhesion receptors and their role in metastasis. Immunoreceptor tyrosine-based activation motif (ITAM) and hemi ITAM (hemITAM) comprising receptors, especially, glycoprotein VI (GPVI), FcγRIIa, and C-type lectin-like-2 receptor (CLEC-2) are turned in the spotlight since several new mechanisms and contributions to metastasis have been attributed to this family of platelet receptors in the last years.

Peptide-Mediated Platelet Capture at Gold Micropore Arrays

Ordered spherical cap gold cavity arrays with 5.4, 1.6, and 0.98 μm diameter apertures were explored as capture surfaces for human blood platelets to investigate the impact of surface geometry and chemical modification on platelet capture efficiency and their potential as platforms for surface enhanced Raman spectroscopy of single platelets. The substrates were chemically modified with single-constituent self-assembled monolayers (SAM) or mixed SAMs comprised of thiol-functionalized arginine-glycine-aspartic acid (RGD, a platelet integrin target) with or without 1-octanethiol (adhesion inhibitor). As expected, platelet adhesion was promoted and inhibited at RGD and alkanethiol modified surfaces, respectively. Platelet adhesion was reversible, and binding efficiency at the peptide modified substrates correlated inversely with pore diameter. Captured platelets underwent morphological change on capture, the extent of which depended on the topology of the underlying substrate. Regioselective capture of the platelets enabled study for the first time of the surface enhanced Raman spectroscopy of single blood platelets, yielding high quality Raman spectroscopy of individual platelets at 1.6 μm diameter pore arrays. Given the medical importance of blood platelets across a range of diseases from cancer to psychiatric illness, such approaches to platelet capture may provide a useful route to Raman spectroscopy for platelet related diagnostics.

Thrombospondin 1 requires von Willebrand factor to modulate arterial thrombosis in mice

Thrombospondin 1 (TSP1) has been suggested as a counter receptor to platelet glycoprotein Ibα that supports initial platelet adhesion in absence of von Willebrand factor (VWF). Conversely, several other studies have shown that TSP1 interacts with VWF and may play a mechanistic role in modulating thrombosis. However, the in vivo evidence to support this mechanism remains unclear. Using intravital microscopy, in a 10% FeCl3-induced thrombosis model, we report similar platelet adhesion in Tsp1(-/-)/Vwf(-/-) mice compared with littermate Vwf(-/-) mice, suggesting that TSP1 does not mediate initial platelet adhesion in the absence of VWF. Tsp1(-/-) mice exhibited prolonged occlusion time and a significant decrease in the rate of thrombus growth (P < .05 vs wild-type), but not in the initial platelet adhesion. Complete deficiency of VWF abrogated the rate of thrombus growth in Tsp1(-/-) mice; therefore, we generated Tsp1(-/-)/Vwf(+/-) mice to determine whether TSP1 modulates thrombus growth under conditions of partial VWF deficiency. Tsp1(-/-)/Vwf(+/-) mice exhibited delayed thrombus growth kinetics and prolonged occlusion time (P < .05 vs Vwf(+/-)). Finally, we demonstrate that platelet-derived TSP1 modulates arterial thrombosis in vivo. We conclude that TSP1 released from platelets plays a mechanistic role in modulating thrombosis in the presence of VWF.

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.

Ligand capture and activation of human platelets at monolayer modified gold surfaces

The effect of RGD peptides, alkane and PEG in self assembled mixed monolayers on gold on platelet adhesion and activation is explored.

Transforming growth factor-beta-induced protein (TGFBIp/beta ig-h3) activates platelets and promotes thrombogenesis

Transforming growth factor-beta-induced protein (TGFBIp)/beta ig-h3 is a 68-kDa extracellular matrix protein that is functionally associated with the adhesion, migration, proliferation, and differentiation of various cells. The presence of TGFBIp in platelets led us to study the role of this protein in the regulation of platelet functions. Upon activation, platelet TGFBIp was released and associated with the platelets. TGFBIp mediates not only the adhesion and spread of platelets but also activates them, resulting in phosphatidylserine exposure, alpha-granule secretion, and increased integrin affinity. The fasciclin 1 domains of TGFBIp are mainly responsible for the activation of platelets. TGFBIp promotes thrombus formation on type I fibrillar collagen under flow conditions in vitro and induces pulmonary embolism in mice. Moreover, transgenic mice, which have approximately a 1.7-fold greater blood TGFBIp concentration, are significantly more susceptible to collagen- and epinephrine-induced pulmonary embolism than wild-type mice. These results suggest that TGFBIp, a human platelet protein, plays important roles in platelet activation and thrombus formation. Our findings will increase our understanding of the novel mechanism of platelet activation, contributing to a better understanding of thrombotic pathways and the development of new antithrombotic therapies.

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.

Adhesion of human platelets to serum amyloid A

Serum amyloid A (SAA) is an acute phase reactant, and its level in the blood is elevated to 1000-fold in response of the body to trauma, infection, inflammation, and neoplasia. SAA was reported to inhibit platelet aggregation and to induce adhesion of leukocytes. This study looked at adhesion of human platelets to SAA. Immobilized SAA supported the adhesion of human washed platelets; level of adhesion to SAA was comparable to fibronectin and lower than to fibrinogen. Adhesion to SAA was further enhanced by Mn(2+) and the physiological agonist, thrombin. Platelet adhesion to SAA was completely abolished by anti-SAA antibody. SAA-induced adhesion was inhibited by antibodies against the integrin receptor alphaIIbbeta3, by the peptide GRGDSP and by SAA-derived peptide containing YIGSR-like and RGD-like adhesion motifs (amino acids 29 to 42). Adhesion was not inhibited by control immunoglobulin G, by antibody against the integrin receptor alphaVbeta3, by the peptide GRGESP, and by SAA-derived peptide that includes incomplete RGD motif. SAA-derived peptide 29 to 42 also inhibited platelet adhesion to fibronectin. Transfected human melanoma cells expressing alphaIIbbeta3 adhered to SAA, whereas transfected cells expressing alphaVbeta3 did not. By using flow cytometry, the alphaIIbbeta3 cells displayed significantly higher levels of binding of soluble SAA than the alphaVbeta3 cells. These data indicate that human platelets specifically adhere to SAA in an RGD- and alphaIIbbeta3-dependent manner. Thus, SAA may play a role in modulating platelet adhesion at vascular injury sites by sharing platelet receptors with other platelet-adhesive proteins.

The role of thrombospondin-1 in tumor progression

The role of thrombospondin-1 (TSP-1) in tumor progression is both complex and controversial. It is clear from the literature that the function of TSP-1 in malignancy depends on the presence of other factors and the level of TSP-1 expression in the tumor tissue. High levels of TSP-1 secreted by tumors, which were engineered to overexpress TSP-1, inhibit tumor growth, while anti-sense inhibition of TSP-1 production in certain tumors also inhibits growth. Clearly, the presence of other factors in these experimental systems must be important. The role of TSP-1 in angiogenesis also depends on the levels of TSP-1, the presence and level of angiogenic stimulators such as basic fibroblast growth factor (bFGF), and the localization of TSP-1 in the tissue. Matrix-bound TSP-1 promotes capillary tube formation in the rat aorta model of angiogenesis, while TSP-1 inhibits bFGF- induced angiogenesis in the rat cornea model. The inhibitory effect also depends on the proteolytic state of TSP-1 since the amino terminus promotes angiogenesis in the cornea model, while the remaining 140-kDa fragment inhibits bFGF-induced angiogenesis. Both the stimulatory and inhibitory effects of TSP-1 are likely due to upregulation of matrix-degrading enzymes and their inhibitors. These enzymes are critical for maintaining optimal matrix turnover during angiogenesis. These varied TSP-1-dependent mechanisms offer new targets for the development of anti-angiogenic therapeutics for the treatment of a variety of cancers, as well as other pathologies involving inappropriate angiogenesis such as diabetic retinopathy.

Real-time analysis of the interaction of platelets with immobilized thrombospondin under flow conditions

The platelet granule protein (TS) is extracellularly secreted upon platelet activation and then binds to the platelet surface where it can interact with various adhesive proteins. Here, we have analyzed platelet interactions with a TS-coated surface under flow conditions, a model for platelet adhesion onto surface-bound TS under physiological conditions. Platelets exhibited temporary, very short-time adhesion on the TS surface, but no firm adhesion. This adhesion was inhibited by NNKY5-5 (anti-glycoprotein (GP) Ib antibody) and AJvW-2 (anti-von Willebrand factor (vWF)), indicating that both platelet GP Ib and plasma vWF contribute to this interaction. Antibodies against platelet collagen receptor integrin alpha(2)beta(1) had no significant effect. These results suggested that binding of vWF to TS is the first step in platelet interaction with the TS surface. By surface plasmon resonance spectroscopy, a dissociation constant (K(d)) of 3.97x10(-7) M was obtained for the binding reaction between immobilized TS and vWF. These results suggest the following model for platelet interaction with the TS surface under flow: plasma vWF first binds to the immobilized TS and then platelets interact with the TS-bound vWF. A low density of bound vWF would account for the observed weak interaction between TS and platelets under flow.

Pro-adhesive and chemotactic activities of thrombospondin-1 for breast carcinoma cells are mediated by alpha3beta1 integrin and regulated by insulin-like growth factor-1 and CD98

Thrombospondin-1 (TSP1) is a matricellular protein that displays both pro- and anti-adhesive activities. Binding to sulfated glycoconjugates mediates most high affinity binding of soluble TSP1 to MDA-MB-435 cells, but attachment and spreading of these cells on immobilized TSP1 is primarily beta1 integrin-dependent. The integrin alpha3beta1 is the major mediator of breast carcinoma cell adhesion and chemotaxis to TSP1. This integrin is partially active in MDA-MB-435 cells but is mostly inactive in MDA-MB-231 and MCF-7 cells, which require beta1 integrin activation to induce spreading on TSP1. Integrin-mediated cell spreading on TSP1 is accompanied by extension of filopodia containing beta1 integrins. TSP1 binding activity of the alpha3beta1 integrin is not stimulated by CD47-binding peptides from TSP1 or by protein kinase C activation, which activate alphavbeta3 integrin function in the same cells. In MDA-MB-231 but not MDA-MB-435 cells, this integrin is activated by pertussis toxin, whereas serum, insulin, insulin-like growth factor-1, and ligation of CD98 increase activity of this integrin in both cell lines. Serum stimulation is accompanied by increased surface expression of CD98, whereas insulin-like growth factor-1 does not increase CD98 expression. Thus, the pro-adhesive activity of TSP1 for breast carcinoma cells is controlled by several signals that regulate activity of the alpha3beta1 integrin.

Thrombospondins in early Xenopus embryos: dynamic patterns of expression suggest diverse roles in nervous system, notochord, and muscle development

The thrombospondins (TSPs) are a family of extracellular matrix (ECM) glycoproteins that modulate many cell behaviors including adhesion, migration, and proliferation. Here we report the molecular cloning of the Xenopus homologs of TSP-1 and TSP-3, and the developmental patterns of expression of Xenopus TSP-1, TSP-3, and TSP-4 mRNAs. Xenopus TSP-1 and TSP-3 protein sequences each share approximately 80% amino acid identity with their mammalian counterparts. TSP-1 mRNAs are detectable at low levels in fertilized eggs indicating that this TSP is a maternally deposited transcript. Zygotic expression of TSP-1, TSP-3, and TSP-4 begins at the end of gastrulation and transcripts encoding each protein accumulate through the tadpole stages of development. Whole mount in situ hybridizations reveal that each TSP mRNA is localized in the embryo with distinct, developmentally regulated patterns of expression. TSP-1 mRNAs are detected in a wide range of tissues including the floor plate of the neural tube, epidermis, somites, notochord and, most notably, alternating rhombomeres. Transcripts encoding TSP-3 are expressed in the notochord, floor plate, sensorial layer of the epidermis and sensory epithelia. TSP-4 mRNAs are restricted to somitic mesoderm and skeletal muscle. These data suggest that the TSPs represent a functionally diverse family of ECM proteins with tissue-specific functions during embryogenesis.

Effects of thrombospondin-1 on disease course and angiogenesis in rat adjuvant-induced arthritis

Leukocyte extravasation into the synovium is important in rheumatoid arthritis (RA). Thrombospondin (TSP)-1 mediates cell adhesion and migration and inhibits angiogenesis, and it has been implicated in RA. However, little information is available on the role of TSP-1 in arthritis-associated inflammation and neovascularization. Therefore, we analyzed the effects of TSP-1 in adjuvant-induced arthritis (AIA), a rat model for RA. Hydron pellets containing TSP-1 were implanted in one ankle of AIA rats post-adjuvant injection, while the contralateral ankle received sham implants. Body weight loss and joint swelling were determined in comparison to nonimplanted AIA controls. In addition, synovial vessel counts were obtained in TSP-1-versus sham-implanted ankles of the same rat. The implantation of TSP-1 pellets into one ankle resulted in an enhancement of swelling in both ankles. Furthermore, TSP-1 exhibited a biphasic modulatory effect on synovial vessel counts (P < 0.05). In conclusion, TSP-1 implanted into one ankle of AIA rats may augment the severity of the disease. One possible explanation, among others, for the modulating effect of TSP-1 on inflammation may be its effect on arthritis-related angiogenesis.

Role of alpha v integrins in mesangial cell adhesion to vitronectin and von Willebrand factor

This study demonstrates (by flow cytometry and immunoprecipitation after cell surface radiolabeling and by using monoclonal antibodies to alpha v, beta 3, and alpha v beta 3 and alpha v beta 5 complexes) that alpha v beta 3, the vitronectin receptor, and alpha v beta 5 are expressed in vitro on cultured human mesangial cells (HMC) of the 5th to 8th passages. Antibodies to alpha v, beta 3 and alpha v beta 3 respectively precipitated an alpha beta heterodimer with molecular weights of 140 and 97 kDa. We analyzed the role of the various integrins in HMC interactions with vitronectin, and with fibronectin and von Willebrand factor (vWf), which are synthetized respectively by mesangial and endothelial cells. Cell adhesion increased in a dose dependent manner with the concentration of plastic-coated matrix protein and vWf. Inhibition of cell attachment with monoclonal antibodies to integrins indicated that HMC adhesion to vWf primarily involves alpha v beta 3, and that alpha v beta 5 may also contribute to cell binding to vWf. Adhesion to vitronectin involves both alpha v beta 3 and alpha v beta 5 complexes. In contrast, adhesion to fibronectin was not affected by monoclonal antibodies to alpha v beta 3 and alpha v beta 5 complexes. We propose that integrins alpha v beta 3 and alpha v beta 5, present on HMC, could mediate an interaction between mesangial and endothelial cells by binding to vWf, released at the basal site of endothelial cells.

Heparin-binding domain, type 1 and type 2 repeats of thrombospondin mediate its interaction with human breast cancer cells

Thrombospondin is an adhesive glycoprotein that promotes breast cancer cell adhesion to human vascular endothelial cells (Incardona et al., 1995). In this study, we have identified the molecular domains of thrombospondin that mediate its binding to specific receptors on the human breast adenocarcinoma cell line, MDA-MB-231. Two recombinant fragments from the amino-terminus (TSPN18 and TSPN28), and the fusion proteins of the type 1 and type 2 repeats of human thrombospondin, inhibited binding of radiolabeled thrombospondin to MDA-MB-231 cells in suspension by 40-60% at 50 micrograms/ml whereas the type 3 repeat, carboxy-terminus and unfused glutathione-S-transferase as well as the synthetic peptide Gly-Arg-Gly-Asp-Ser (500 micrograms/ml) had little or no effect. Heparin and various glycosaminoglycans as heparan sulfate, chondroitin sulfates A, B or C, and fucoidan inhibited thrombospondin binding to MDA-MB-231 cells by more than 60% whereas dextran sulfate had only little effect. Treatment of cells with heparitinase, chondroitinase ABC, and hyaluronidase, but not with neuraminidase, induced 30-50% inhibition of thrombospondin binding suggesting the participation of both heparan sulfate and chondroitin sulfate cell surface-associated molecules. Inhibition of proteoglycan sulfation by chlorate or inhibition of glycosaminoglycan chain formation by two beta-D-xylosides also led to a substantial inhibition of thrombospondin binding. Our results indicate that several domains within the thrombospondin molecule, namely the amino-terminus, type 1 and type 2 repeats, participate in its binding to specific receptors bearing sulfated glycosaminoglycans on MDA-MB-231 cells. Biological assays have indicated that, in addition to these domains, the peptide Gly-Arg-Gly-Asp-Ser inhibited MDA-MB-231 cell attachment to thrombospondin suggesting that the last type 3 repeat of the molecule may also contribute to its cell adhesive activity.

Disturbance of plasma and platelet thrombospondin levels in sickle cell disease

Thrombospondin (TSP), a large protein found in platelet alpha-granules (as TSP-1), mediates adhesion of sickle reticulocytes to cultured vascular endothelium. To further explore the physiologic relevance of this observation, we have measured plasma TSP levels and platelet TSP-1 content in subjects with sickle cell disease. Plasma TSP levels were similar for normal controls (mean 491 ng/ml, range 331-723) and steady-state HbSS patients (mean 536, range 333-1107) and were significantly (P = 0.012) but variably elevated for HbSS patients presenting with acute painful crisis (mean 868, range 442-2780). Some of these elevated plasma TSP levels reached those previously observed to support maximal red cell adhesion to endothelium in vitro. Compared to normals, both steady-state and in-crisis HbSS patients had significantly (P < 0.001) depressed platelet TSP-1 content (82.6 +/- 11.9, 47.1 +/- 16.0 and 45.9 +/- 20.7 ng/l0(6) platelets, respectively, mean +/- SD). HbSC disease patients, all examined during steady state, had low-normal plasma levels of TSP and either normal or depressed platelet TSP-1 content. Serial observations on three sickle cell anemia subjects indicated a probable relationship between platelet TSP-1 release, elevated plasma TSP levels, and acute vaso-occlusive episodes. These results suggest a state of ongoing release and depletion of TSP-1 from activated platelets in patients with sickle cell disease.

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

Thrombospondin 1 is expressed by proliferating mesangial cells and is up-regulated by PDGF and bFGF in vivo

Thrombospondin 1 has been shown to be linked to PDGF-mediated mesangial cell proliferation and migration in vitro, but little is known regarding its expression or regulation in glomerular disease. Experimental mesangial proliferative nephritis was induced in rats by injection of anti-Thy1 antibody. Mesangial cell proliferation was associated with de novo expression of thrombospondin 1 mRNA (detected by Northern blot and in situ hybridization) and protein (by Western blot and immunostaining). Although some thrombospondin 1 was expressed by platelets and macrophages, double labeling showed that most thrombospondin 1 mRNA and protein were expressed by proliferating alpha-actin-positive mesangial cells. Thrombospondin 1 expression in anti-Thy1 nephritis was complement-dependent and could be reduced by treatment with anti-PDGF or anti-bFGF antibodies. Thrombospondin 1 could also be induced in normal rats by infusion of PDGF and in rats which were primed with low dose anti-Thy1 antibody by infusion of PDGF of bFGF. Thus, this study demonstrates that proliferating mesangial cells express thrombospondin 1 de novo in disease and that thrombospondin 1 expression in vivo is regulated by PDGF and bFGF.

Thrombospondin modulates melanoma--platelet interactions and melanoma tumour cell growth in vivo

In this study we have investigated the role of thrombospondin (TSP) as a possible ligand playing a key role in human M3Da. melanoma cell interaction with platelets and in tumour growth. TSP is secreted (80 +/- 6 ng TSP 10(-6) cells) and bound to the surface of M3Da. cells via receptors different from CD36, as shown by biosynthetic labelling and immunofluorescence studies. The levels of TSP binding to M3Da. cells evaluated by binding studies, using an anti-TSP monoclonal antibody (MAb) (LYP8), shows 367,000 +/- 58,000 (mean +/- s.d.) LYP8 binding sites per cell with a dissociation constant (Kd) of 67 nM. TSP binding to M3Da. cells shows 400,000 +/- 50,000 TSP binding sites per cell with a Kd of 10 nM. The capacity of anti-TSP MAb (LYP8) to inhibit M3Da.-platelet interactions was followed on an aggregometer and evaluated by electron microscopy studies. The biological role of TSP binding to M3Da. cells was investigated by implanting subcutaneously the M3Da. cell line in nude mice and following the size and time of in vivo tumour growth. Reducing the availability or the functional level of TSP by using an anti-TSP MAb (LYP8) resulted in a significant decrease in platelet aggregates interacting with M3Da. melanoma cells. Using an enzyme-linked immunosorbent assay, purified alpha nu beta 3 was shown to bind TSP. Moreover, LYP8-coated M3Da. cells showed a reduced capacity to form tumours in vivo. M3Da. cells were observed to attach and spread on human platelet TSP-coated plastic wells. This attachment by M3Da. cells was inhibited in a similar way by LYP8 and an anti-alpha nu beta 3 MAb (LYP18). The results obtained in this study show that TSP secreted and bound to the surface of a human melanoma cell line (M3Da.) acts as a link between aggregated platelets and the M3Da. cell surface. Moreover, these results shows that TSP can modulate tumour growth in vivo. Reagents such as MAbs directed against TSP and peptides derived from TSP could not only be used as a new therapeutic approach in the control of tumour metastasis of melanoma, but may also contribute to elucidation of the role of TSP in cancer biology.

Localization of thrombospondin, CD36 and CD51 during prenatal development of the human mammary gland

Thrombospondin (TSP) is a 450 kDa extracellular matrix glycoprotein expressed in normal, hyperplastic, and neoplastic human breast. In this study, the patterns of expression of TSP were determined during development of the human fetal mammary gland between the 15th and the 39th week of gestation. Using immunohistochemistry, TSP is found in the dense mesenchyme immediately adjacent to the mammary bud, and at the membrane of budding epithelial cells invading the surrounding mesenchyme. As formation of the ductal tree system occurs, TSP is deposited at the myoepithelial-stromal junction of mammary ducts. Such an immunolocalization of TSP in buds and ducts of the fetal mammary gland has been confirmed at the mRNA level using in situ hybridization. Presence of TSP transcripts in nascent breast tissue has been also demonstrated by polymerase chain reaction assay. Comparison of TSP immunolocalization with that of two known TSP cell surface receptors, CD36 and CD51, reveals no codistribution of TSP with these receptors during mammary gland development. As opposed to TSP, CD36 is strongly expressed at the membrane of preadipocytes present in the fat pad tissue, but absent from budding epithelial cells. CD51 is only weakly expressed by malpighian epithelial cells and does not colocalize with TSP. In lactating ducts of a newborn, TSP disappears from the myoepithelial-stromal junction of ducts and is synthesized at the apices of secretory epithelial cells of lactating ducts together with CD36. In conclusion, our findings support the existence of an important role for TSP during development of the human fetal mammary gland.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of activated GPIIb/IIIa receptors on the luminal surface of adherent platelets. Paradoxical loss of luminal receptors when platelets adhere to high density fibrinogen

The accessibility of activated GPIIb/IIIa receptors on the luminal surface of platelets adherent to damaged blood vessels or atherosclerotic plaques is likely to play a crucial role in subsequent platelet recruitment. To define better the factors involved in this process, we developed a functional assay to assess the presence of activated, luminal GPIIb/IIIa receptors, based on their ability to bind erythrocytes containing a high density of covalently coupled RGD-containing peptides (thromboerythrocytes). Platelets readily adhered to wells coated with purified type I rat skin collagen and the adherent platelets bound a dense lawn of thromboerythrocytes. With fibrinogen-coated wells, platelet adhesion increased as the fibrinogen-coating concentration increased, reaching a plateau at about 11 micrograms/ml. Thromboerythrocyte binding to the platelets adherent to fibrinogen showed a paradoxical response, increasing at fibrinogen coating concentrations up to approximately 4-6 micrograms/ml and then dramatically decreasing at higher fibrinogen-coating concentrations. Scanning electron microscopy demonstrated that the morphology of platelets adherent to collagen was similar to that of platelets adherent to low density fibrinogen, with extensive filopodia formation and ruffling. In contrast, platelets adherent to high density fibrinogen showed a bland, flattened appearance. Immunogold staining of GPIIb/IIIa receptors demonstrated concentration of the receptors on the filopodia, and depletion of receptors on the flattened portion of the platelets. Thus, there is a paradoxical loss of accessible, activated GPIIb/IIIa receptors on the luminal surface of platelets adherent to high density fibrinogen. Two factors may contribute to this result: engagement of GPIIb/IIIa receptors with fibrinogen on the abluminal surface leading to the loss of luminal receptors, and loss of luminal filopodia that interact with thromboerythrocytes. These data provide insight into the differences in thrombogenicity between surfaces, and may provide a mechanism for purposefully passivating platelet-reactive artificial surfaces.

Interaction of thrombospondin with platelet glycoproteins GPIa-IIa and GPIIb-IIIa

The interaction of human thrombospondin (TSP) with GPIa-IIa and GPIIb-IIIa was studied. The binding for both proteins became time-independent after 60 min. A 7-fold excess concentration of unlabelled GPIa-IIa added either initially, or after time-dependent binding, resulted in a 50% inhibition of GPIa-IIa bound to TSP. GPIa-IIa and GPIIb-IIIa specifically bound TSP since: (a) the binding of GPIIb-IIIa to TSP was dependent on the presence of 1 mM MgCl2 and 1 mM CaCl2, whereas binding of GPIa-IIa was ion-independent. (b) The binding was saturable, with dissociation constants of 0.69 +/- 0.17 microM and 3.77 +/- 1.02 microM for GPIa-IIa and GPIIb-IIIa respectively. (c) GPIIb-IIIa and GPIa-IIa did not significantly bind to BSA. (d) GPIIb-IIIa bound fibrinogen ion-specifically, whereas little or no binding of GPIa-IIa was detectable. (e) Both GPIIb-IIIa and GPIa-IIa bound collagen in an ion-independent manner. (f) GPIIb-IIIa did not compete with GPIa-IIa for binding to TSP. (g) Binding of GPIa-IIa to TSP was inhibited with anti-(GPIa-IIa) (6F1), whereas mouse IgG and anti-(GPIIb-IIIa) (AP-2) had no effect. (h) The interaction of GPIa-IIa with TSP is 5.5-fold more favourable than that of GPIIb-IIIa suggesting that GPIa-IIa may be a preferred binding protein for TSP-mediated platelet adhesion.

Lymphocyte locomotion in three-dimensional collagen gels. Comparison of three quantitative methods for analysing cell trajectories

We evaluated three different quantitative evaluation methods for lymphocyte locomotion in three-dimensional collagen gels: (1) the length of the two-dimensional migration path (distance migrated) was compared to (2) the resulting average displacement from the starting to the end point and (3) the displacement of the furthest migrating population (cells with high displacement). Locomotion of immunomagnetically isolated human CD4+ and CD8+ peripheral blood lymphocytes suspended in type I collagen gels was recorded using time-lapse videomicroscopy. Paths of randomly selected locomoting cells were digitized, reconstructed and quantitatively analysed. For spontaneously locomoting CD4+ and CD8+ lymphocytes (90 min observation period) the mean total distance migrated was 10.0 +/- 3.7 microns/min (CD4+; n = 114 cells) and 5.6 +/- 3.3 microns/min (CD8+; n = 90 cells). The mean displacement from the individual starting point amounted to 1.3 +/- 0.7 micron/min for CD4+ and 1.1 +/- 0.7 micron/min for CD8+ cells, thus representing only 5-25% of the total migration path (index range displacement/distance migrated: 0.13-50%). Incubation with interleukin-8 and/or receptor blocking by monoclonal antibodies against VLA-2 (Gi9) or VLA-4 (HP2/1) integrins significantly altered the mean length of the migration paths for six out of ten different experimental conditions. Average displacement or displacement of the most active cells detected significant changes in two and three out of ten samples. Whereas the interleukin-8 induced locomotory changes were correctly represented by end point determination, relatively slight but significant modulation in lymphocyte behaviour by anti-integrin antibodies was revealed solely by analysis of the complete cell trajectory. In conclusion, the cell trajectory may represent a sensitive method for evaluating induced subtle changes in lymphocyte locomotory characteristics.

Thrombospondin-platelet interactions. Role of divalent cations, wall shear rate, and platelet membrane glycoproteins

The role of thrombospondin, a multifunctional matrix glycoprotein, in platelet adhesion is controversial: both adhesive and antiadhesive properties have been attributed to this molecule. Because shear flow has a significant influence on platelet adhesion, we have assessed thrombospondin-platelet interactions both under static and flow conditions. The capacity of thrombospondin to support platelet adhesion depended upon its conformation. In a Ca(2+)-depleted conformation, such as in citrated plasma, thrombospondin was nonadhesive or antiadhesive as it inhibited platelet adhesion to fibrinogen, fibronectin, laminin, and von Willebrand factor by 30-70%. In a Ca(2+)-replete conformation, however, thrombospondin effectively supported platelet adhesion. Shear rate influenced this adhesion; percent surface coverage on thrombospondin increased from 5.4 +/- 0.3 at 0 s-1 to 41.5 +/- 6.7 at 1,600 s-1. In contrast to the extensive platelet spreading observed on fibronectin at all shear rates, platelet spreading on thrombospondin occurred only sporadically and at high shear rates. GPIa-IIa, GPIIb-IIIa, GPIV, and the vitronectin receptor, which are all proposed platelet receptors for thrombospondin, were not solely responsible for platelet adhesion to thrombospondin. These results suggest that thrombospondin may play a dual role in adhesive processes in vivo: (a) it may function in conjunction with other adhesive proteins to maintain optimal platelet adhesion at various shear rates; and (b) it may serve as a modulator of cellular adhesive functions under specific microenvironmental conditions.

Characterization of integrin receptors in normal and neoplastic human brain

We studied the immunohistochemical expression of integrin alpha and beta chains in the normal and neoplastic human brain. Normal astrocytes expressed alpha 2, alpha 3, alpha 6, beta 1, and beta 4 chains in some areas facing major interstitial tissues, but they were consistently negative for the other integrins examined (alpha 4, alpha 5, alpha V, alpha L, alpha M, alpha X, beta 2, beta 3). Neoplastic astrocytes in vivo and in vitro showed increased expression of alpha 3 and beta 1, and some also of alpha 5, alpha V, beta 3, and beta 4. Neoexpression of alpha 4 and reduced levels of beta 4 were detected in glioblastoma vascular proliferations compared with normal endothelial cells. Oligodendroglioma, ependymoma, choroid plexus papilloma, pituitary adenoma, and meningioma cells showed the same integrin pattern as their normal counterparts. Adhesion assays using the astrocytoma cell lines U-138 MG and U-373 MG revealed strong attachment to collagen types I to VI and undulin, which was inhibited by antibodies to beta 1, but not by those to alpha 2, alpha 3, alpha 6, and alpha V. We conclude that astrocytomas show increased levels or neoexpression of various integrins and strong attachment to various extracellular matrix components, which appears to be almost exclusively mediated by beta 1-integrins.

Role of thrombospondin in the adhesion of human endothelial cells in primary culture

The role of thrombospondin on the adhesion of endothelial cells in primary culture was studied using a serum-free defined medium or thrombospondin-depleted fetal bovine serum. Under these conditions, only 6% of the cells adhered to gelatin-coated dishes, whereas cells adhering to gelatin in the presence of normal fetal bovine serum were considered as 100% adhesion. The percentage of cells attached to fibronectin or thrombospondin-coated dishes in thrombospondin-depleted serum was 66 and 32%, respectively. The addition of purified platelet thrombospondin to thrombospondin-depleted serum increased the adhesion of endothelial cells to gelatin and to thrombospondin, up to 32 and 59%, respectively, and restored the attachment to fibronectin to the same extent as that observed in the presence of normal serum. In contrast to the attachment, the spreading of the adhering cells was not further influenced by the addition of soluble thrombospondin. Subcultured cells did not require any protein for adhering to gelatin substrata. These observations indicate that thrombospondin plays a major role in the adhesion of endothelial cells in primary culture.

Lack of effect of 1-desamino-8-D-arginine vasopressin on direct adhesion of platelets to collagen

Administration of 1-desamino-8-D-arginine vasopressin (DDAVP), a synthetic vasopressin derivative, causes an increase in plasma factor VIII and von Willebrand factor (vWF). Recently, evidence has become available that intravenous infusion of DDAVP shortens the prolonged bleeding times in some patients with primary platelet defects even though their plasma levels of vWF and FVIII are normal prior to drug administration. The mechanism of this effect of DDAVP has not been well defined and it has been generally considered that the beneficial effect on the bleeding time is related to the rise in plasma vWF and its impact on platelet adhesion to subendothelial components including collagen, an important step in hemostasis. Thus, studies aimed at understanding the effect of DDAVP have focused on vWF-mediated adhesion of platelets to the subendothelium. For example, Sakariassen et al studied the platelet adherence to human arterial subendothelium and concluded that DDAVP improves hemostasis by causing enhanced vWF-mediated platelet adherence. This mechanism would explain the shortening of the bleeding time in patients with milder forms of vWD with subnormal levels of vWF. But patients with congenital platelets defects have normal plasma vWF raising the possibility that there may be other mechanisms contributing to the beneficial effect of DDAVP. It is clear that platelets can interact directly with collagen, mediated by specific platelet binding sites. Further, DDAVP binds to platelets even though by itself does not activate them. The present investigation was designed to elucidate whether DDAVP had any effect on the direct adhesion of platelets to collagen in the absence of mediation by vWF. Hashemi et al have suggested that the effect of DDAVP on endothelial cell release of vWF is mediated by an as yet uncharacterized intermediate factor(s) released from peripheral mononuclear cells. Therefore, we studied the effect also of plasma samples obtained from patients treated with DDAVP on adhesion of platelets to collagen.

Identification and characterization of thrombospondin-4, a new member of the thrombospondin gene family

A new member of the thrombospondin gene family, designated thrombospondin-4, has been identified in the Xenopus laevis genome. The predicted amino acid sequence indicates that the protein is similar to the other members of this gene family in the structure of the type 3 repeats and the COOH-terminal domain. Thrombospondin-4 contains four type 2 repeats and lacks the type 1 repeats that are found in thrombospondin-1 and 2. The amino-terminal domain of thrombospondin-4 has no significant homology with the other members of the thrombospondin gene family or with other proteins in the database. RNAse protection analysis establishes that the initial expression of Xenopus thrombospondin-4 is observed during neurulation. Levels of mRNA expression increase twofold during tailbud stages but decrease by the feeding tadpole stage. The size of the thrombospondin-4 message is 3.3 Kb and 3.4 Kb in the frog and human, respectively. Northern blot analysis of human tissues reveals high levels of thrombospondin-4 expression in heart and skeletal muscle, low levels in brain, lung and pancreas and undetectable levels in the placenta, liver and kidney. These data establish the existence of a new member of the thrombospondin gene family that may participate in the genesis and function of cardiac and skeletal muscle.

Alterations of integrin expression in human lung cancer

Integrins are cell-surface receptors which are involved in cell-matrix and/or cell-cell adhesion. They have been suggested to play a role in tumor invasion and metastasis. We examined the expression of various integrin subunits in normal and cancer cells of the lung using 33 human lung cancer cell lines as well as 6 lung cancer samples from which tumor cell lines could be established. This study clearly demonstrated that changes in the expression of certain integrins occur frequently in lung cancer, especially in small cell lung cancer. Loss of the alpha 1 subunit of the beta 1 integrin family appears to be the most prominent change, although loss of other integrin subunits such as alpha 2 or emergence of some integrin subunits such as alpha v can also be observed. These results suggest that changes in integrin expression may contribute to the invasive and/or metastatic behavior of lung cancer.

Identification and characterization of a tumor cell receptor for CSVTCG, a thrombospondin adhesive domain

We have previously shown that peptides derived from the thrombospondin sequence, CSVTCG, promoted tumor cell adhesion. To further investigate this observation, the CSVTCG-tumor cell adhesion receptor from A549 human lung adenocarcinoma cells was isolated and characterized. A single protein peak was isolated by CSVTCG affinity chromatography which also analyzed as a single peak by anion exchange chromatography. The purified protein had a pI of 4.7 and analyzed on SDS-gels as a single band of M(r) = 50,000 under nonreducing conditions and as two protein bands of M(r) = 50,000, and 60,000 under reducing conditions. Purified CSVTCG binding protein (CBP) bound either CSVTCG- or TSP-Sepharose but showed little interaction with either VCTGSC- or BSA-Sepharose. CBP was cell surface exposed. CSVTCG derivatized with [125I] Bolton-Hunter reagent was taken up by cells in a dose-dependent manner and the cell association was inhibited with a monospecific polyclonal anti-CBP antibody. Examination of the cell proteins crosslinked to labeled CSVTCG by SDS-gel electrophoresis revealed one band that comigrated with purified CPB. Using an in vitro binding assay, purified CBP bound mannose, galactose, and glucosamine-specific lectins. CBP bound TSP saturably and reversibly. The binding was Ca+2/Mg+2 ion dependent and inhibited with fluid phase TSP and anti-CBP. Little or no binding was observed on BSA, fibronectin, GRGES, and GRGDS. Heparin, but not lactose, inhibited binding. Anti-CBP IgG and anti-CSVTCG peptide IgG inhibited A549 cell spreading and adhesion on TSP but not on fibronectin and laminin. These results indicate that CBP and the CSVTCG peptide domain of TSP can mediate TSP-promoted tumor cell adhesion.

Thrombospondin as a mediator of cancer cell adhesion in metastasis

Thrombospondin (TSP) is a 450 kDa adhesive glycoprotein. It is present in high concentrations in the platelet alpha-granule and can readily be secreted following platelet activation where local concentrations can be increased by 3-4 orders of magnitude. TSP is also synthesized by a variety of other cells and is incorporated into their extracellular matrix. TSP is a homotrimer with a number of functional domains, at least four of which might serve as receptor recognizing regions. The amino-terminal heparin binding domain interacts with heparin, other glycosaminoglycans and glycolipids and likely recognizes specific cell surface proteoglycans. The central disulfide cross-linked region, 210 kDa non-reduced and 70 kDa reduced, contains a peptide motif CSVTCG which is apparently responsible for binding to glycoprotein IV (CD36) with high affinity. Immediately adjacent to the calcium binding region of TSP, which undergoes considerable molecular relaxation in the absence of calcium, is an RGDA sequence. TSP has been demonstrated to bind to integrins of the alpha v beta 3 and alpha IIb beta 3 class. The carboxy-terminal region of TSP also contains at least one binding epitope for a cell receptor. There are 2 well characterized genes for TSP and truncated forms of TSP have been detected which have inhibitory effects on angiogenesis. Finally, TSP can interact with fibrinogen and fibronectin, perhaps on cellular surfaces, which might serve as secondary receptor-like mechanisms for TSP binding and subsequent mediation of cell adhesion.

Platelet adhesion to collagen-coated wells: analysis of this complex process and a comparison with the adhesion to matrigel-coated wells

The mechanisms of platelet adhesion to collagen type III-coated wells and Matrigel-coated wells were analyzed. The adhesion of 51Cr-labeled platelets to collagen-coated wells showed a biphasic pattern. The early stage of adhesion was inhibited by antibodies against platelet glycoprotein(GP)s Ia/IIa and VI. The later stage of platelet adhesion was inhibited by an antibody against the GPIIb/IIIa complex and a concomitant release of 14C-labeled serotonin was observed. The percentage of adhered platelets was increased when a higher platelet concentration was added in the reaction medium. These results indicated that the adhesion assay of platelets to collagen-coated wells was composed of two reactions: the first one is the platelet-collagen interaction that depends on GPIa/IIa and GPVI on the platelet surface; and the second reaction is the platelet-platelet interaction, platelet aggregation, which depends on GPIIb/IIIa. The adhesion of platelets to Matrigel-coated wells was indicated to involve platelet-Matrigel interactions that were partly dependent on the laminin in the Matrigel solution.

Disulfides modulate RGD-inhibitable cell adhesive activity of thrombospondin

Thrombospondin (TSP) contains the Arg-Gly-Asp (RGD) sequence that is thought to be important for cell adhesion mediated by several cell-surface integrin receptors. The RGD sequence is located in the type 3 repeat region of TSP that has multiple Ca2+ binding sites and is subject to a complex intramolecular thiol-disulfide isomerization. TSP that we isolated from thrombin-activated human platelets using buffers containing 0.1 mM Ca2+, in which Cys974 is the major labeled cysteine, did not have RGD-inhibitable adhesive activity. However, one of our preparations of TSP and TSP purified following alternative procedures using greater than or equal to 0.3 mM Ca2+ did have RGD-inhibitable adhesive activity. Reduction of TSP with DTT, either before or after adsorption to surfaces, enhanced its adhesive activity. Reduced TSP supported robust cell spreading when coated at concentrations as low as 1 micrograms/ml, whereas "adhesive" TSP not treated with DTT was active at coating concentration of greater than 20 micrograms/ml and supported only modest cell spreading. Lower DTT concentrations were required for enhancement of the adhesive activity of TSP if Ca2+ was chelated with EDTA. Cellular adhesion to DTT-treated TSP was inhibited by RGD-containing peptide and by mAb to a functional site of the alpha v beta 3 integrin. Cell blots of reduced proteolytic fragments of TSP localized the adhesive activity to the RGD-containing type 3 repeat region. These results suggest a novel mechanism for regulation of integrin-ligand interactions in which the ligand can isomerize between inactive and active forms.

Human platelet glycoprotein IIIb binds to thrombospondin fragments bearing the C-terminal region, and/or the type I repeats (CSVTCG motif), but not to the N-terminal heparin-binding region

Major blood membrane platelet glycoprotein IIIb (GPIIIb), also termed GPIV or CD365, has been identified as a receptor for thrombospondin (TSP), collagen and Plasmodium falciparum-infected erythrocytes. The aim of the present study was to identify region(s) of TSP involved in binding of GPIIIb. Proteolytic fragments of TSP (M(r) 140 kDa, 120-18 kDa and 27 kDa on SDS/PAGE under reducing conditions) were purified by f.p.l.c. and identified by N-terminal gas-phase sequencing, e.l.i.s.a. and Western blots using monoclonal antibodies directed against defined domains of TSP. The 140 kDa and 120-18 kDa fragments (C-terminal region), but not the 27 kDa fragment (N-terminal region), were shown to bind to GPIIIb by using e.l.i.s.a. and affinity-chromatography systems. TSP binding to a GPIIIb-affinity column was Ca(2+)-dependent and reduced by 45% in the presence of EDTA. Moreover, TSP was only partially eluted with EDTA from a Ca(2+)-equilibrated GPIIIb column. A fragment of 68 kDa, obtained by further digestion of the 140 kDa fragment, bound to the GPIIIb-Sepharose affinity column. This fragment, or stalk-like region, bears the TSP type I repeats that show sequence similarity to regions on properdin, Plasmodium falciparum proteins and antistasin. Peptides (CSVTCG or SVTCGGGV) representing these repeats bound isolated GPIIIb in a Ca(2+)-independent way, but did not completely inhibit the GPIIIb and TSP interaction. These studies indicate that GPIIIb binds to the TSP via the C-terminal region and/or the CSVTCG motif, but not to the N-terminal region. Interaction between GPIIIb and the TSP C-terminal region or the CSVTCG motif is respectively Ca(2+)-dependent and -independent.

Biological activities of peptides and peptide analogues derived from common sequences present in thrombospondin, properdin, and malarial proteins

Thrombospondin (TSP), a major platelet-secreted protein, has recently been shown to have activity in tumor cell metastasis, cell adhesion, and platelet aggregation. The type 1 repeats of TSP contain two copies of CSVTCG and one copy of CSTSCG, per each of the three polypeptide chains of TSP and show homology with peptide sequences found in a number of other proteins including properdin, malarial circumsporozoite, and a blood-stage antigen of Plasmodium falciparum. To investigate whether these common sequences functioned as a cell adhesive domain in TSP, we assessed the effect of peptides corresponding to these sequences and an antibody raised against one of these sequences, CSTSCG, in three biological assays which depend, in part, on the cell adhesive activity of TSP. These assays were TSP-dependent cell adhesion, platelet aggregation, and tumor cell metastasis. We found that a number of peptides homologous to CSVTCG promoted the adhesion of a variety of cells including mouse B16-F10 melanoma cells, inhibited platelet aggregation and tumor cell metastasis, whereas control peptides had no effect. Anti-CSTSCG, which specifically recognized TSP, inhibited TSP-dependent cell adhesion, platelet aggregation, and tumor cell metastasis, whereas control IgG had no effect. These results suggest that CSVTCG and CSTSCG present in the type I repeats function in the adhesive interactions of TSP that mediate cell adhesion, platelet aggregation, and tumor cell metastasis. Peptides, based on the structure of these repeats, may find wide application in the treatment of thrombosis and in the prevention of cancer spread.

Thrombospondin binds normally to glycoprotein IIIb deficient platelets

Glycoprotein IIIb (GPIV, CD36) has been proposed as the platelet receptor for thrombospondin (TSP). We found two healthy blood donors, whose platelets were shown to be GPIIIb deficient. These platelets expressed endogeneous TSP as control platelets and their binding capacity for exogeneous TSP was the same. These results indicate that GPIIIb is not the major TSP receptor on platelets. However, it is not yet possible to exclude that in GPIIIb-deficient platelets other proteins may substitute for GPIIIb in TSP binding.