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

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.

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.

Platelet interaction with the vessel wall

Platelets have attracted a growing interest among basic scientists and clinicians, as they have been shown to play an important role in many physiological and pathophysiological conditions. Beyond hemostasis, platelets participate in wound healing, inflammation, infectious diseases, maintenance of the endothelial barrier function, angiogenesis, and tumor metastasis. Over the last 50 years enormous progress has been made in our understanding of the role of platelets in hemostasis. Platelets circulate in blood in a resting state, but they are able to react immediately upon a vessel wall injury by adhering to the exposed collagen, followed by platelet-platelet interaction to form a plug that effectively seals the injured vessel wall to prevent excessive blood loss. Comparable events will take place on a rupturing atherosclerotic plaque, which may result in a platelet-rich thrombus. This chapter will address the molecular basis of platelet adhesion and aggregation, the regulation of platelet function and the interaction of primary and secondary hemostasis.

Signaling role of CD36 in platelet activation and thrombus formation on immobilized thrombospondin or oxidized low-density lipoprotein

BACKGROUND AND OBJECTIVE

Platelets abundantly express glycoprotein CD36 with thrombospondin-1 (TSP1) and oxidized low-density lipoprotein (oxLDL) as proposed ligands. How these agents promote platelet activation is still poorly understood.

METHODS AND RESULTS

Both TSP1 and oxLDL caused limited activation of platelets in suspension. However, immobilized TSP1 and oxLDL, but not LDL, strongly supported platelet adhesion and spreading with a major role of CD36. Platelet spreading was accompanied by potent Ca(2+) rises, and resulted in exposure of P-selectin and integrin activation, all in a CD36-dependent manner with additional contributions of α(IIb) β(3) and ADP receptor stimulation. Signaling responses via CD36 involved activation of the protein tyrosine kinase Syk. In whole blood perfusion, co-coating of TSP1 or oxLDL with collagen enhanced thrombus formation at high-shear flow conditions, with increased expression on platelets of activated α(IIb) β(3), P-selectin and phosphatidylserine, again in a CD36-dependent way.

CONCLUSIONS

Immobilized TSP1 and oxLDL activate platelets partly via CD36 through a Syk kinase-dependent Ca(2+) signaling mechanism, which enhances collagen-dependent thrombus formation under flow. These findings provide novel insight into the role of CD36 in hemostasis.

β-radiation reduces the reactivity of extracellular matrix proteins in intravascular brachytherapy (IVBT), resulting in decreased platelet adhesion

BACKGROUND

Intravascular Brachytherapy as a tool to reduce restenosis is thought to alter vascular wall biology and vessel wall protein function. Platelet accumulation is also indeed important in the genesis of restenosis. We examine the in vitro effects of beta-radiation on the certain vessel wall extra cellular matrix proteins. We hypothesized that vessel wall (proteins) had become less prone to thrombosis.

METHODS

We examined platelet adhesion to 20-Gy beta radiation treated extra cellular matrix proteins under flow conditions. Platelet flow adhesion was evaluated or quantified by image analysis, aggregation size analysis using the Watershed program and real-time fluorescence images of thrombus formation. The effect of beta radiation on vWF was further showing by measuring the binding of domain-specific antibodies to radiation treated vWF.

RESULTS

20-Gy beta radiation significantly decreased platelet adhesion to extra cellular matrix protein; vWF and collagen Type III and had no effect on the adhesion upon fibrinogen and fibronectin. The beta-radiation affected mostly the AI, A2 and A3 domains of the vWF molecule on the surface, whereas the D'-D3 and B-C1 domains on the surface remain unaffected and suggesting a significant decrease in vWF binding capacity to the GPIb, heparin and collagen ligands.

CONCLUSION

Beta radiation treatment can alter the reactivity of the certain vessel wall extra cellular matrix proteins, in particular vWF and collagen. The vessel wall may become less prone to platelet adhesion, which results in decrease thrombus formation. It might help to reduce the onset of acute coronary occlusion after the intervention.

Thrombospondin-1 stimulates platelet aggregation by blocking the antithrombotic activity of nitric oxide/cGMP signaling

Platelet alpha-granules constitute the major rapidly releasable reservoir of thrombospondin-1 in higher animals. Although some fragments and peptides derived from thrombospondin-1 stimulate or inhibit platelet aggregation, its physiologic function in platelets has remained elusive. We now show that endogenous thrombospondin-1 is necessary for platelet aggregation in vitro in the presence of physiologic levels of nitric oxide (NO). Exogenous NO or elevation of cGMP delays thrombin-induced platelet aggregation under high shear and static conditions, and exogenous thrombospondin-1 reverses this delay. Thrombospondin-1-null murine platelets fail to aggregate in response to thrombin in the presence of exogenous NO or 8Br-cGMP. At physiologic concentrations of the NO synthase substrate arginine, thrombospondin-1-null platelets have elevated basal cGMP. Ligation of CD36 or CD47 is sufficient to block NO-induced cGMP accumulation and mimic the effect of thrombospondin-1 on aggregation. Exogenous thrombospondin-1 also reverses the suppression by NO of alphaIIb/beta3 integrin-mediated platelet adhesion on immobilized fibrinogen, mediated in part by increased GTP loading of Rap1. Thrombospondin-1 also inhibits cGMP-mediated activation of cGMP-dependent protein kinase and thereby prevents phosphorylation of VASP. Thus, release of thrombospondin-1 from alpha-granules during activation provides positive feedback to promote efficient platelet aggregation and adhesion by overcoming the antithrombotic activity of physiologic NO.

Thrombospondin-1 promotes cellular adherence of gram-positive pathogens via recognition of peptidoglycan

Thrombospondin-1 (TSP1) is a matricellular glycoprotein that has key roles in interactions between human cells and components of the extracellular matrix. Here we report a novel role for the lectin TSP1 in pathogen-host interactions. Binding assays and flow cytometric analysis demonstrate that Streptococcus pneumoniae and other gram-positive pathogens including S. pyogenes, Staphylococcus aureus, and Listeria monocytogenes interact specifically with human TSP1. We also show for the first time that host cell-bound TSP1 promotes adherence of gram-positive pathogens to human epithelial and endothelial cell lines. Pretreatment of bacteria with sodium periodate but not Pronase E substantially reduced TSP1-mediated bacterial adherence to host cells, suggesting that a glycoconjugate on the bacterial cell surface functions as the receptor for TSP1. Lipoteichoic acids did not affect TSP1-mediated adherence of S. pneumoniae to host cells. In contrast, attachment of S. pneumoniae and other gram-positive pathogens to host cells via TSP1 was blocked by soluble peptidoglycan, indicating recognition of bacterial peptidoglycan by TSP1. In conclusion, our results demonstrate that recognition of gram-positive pathogens by TSP1 promotes bacterial colonization of host tissue cells. In this scenario, peptidoglycan functions as adhesin and TSP1 acts as a molecular bridge linking gram-positive bacteria with receptors on the host cell.

Src family kinase-mediated and Erk-mediated thromboxane A2 generation are essential for VWF/GPIb-induced fibrinogen receptor activation in human platelets

The binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein Ib-IX (GPIb-IX) results in platelet activation. In this study, we sought to clarify previous conflicting reports and to elucidate the mechanism of activation and the precise role of extracellular signal-regulated kinase (Erk) in VWF-induced platelet activation. Erk2 is activated in platelets on stimulation with VWF/ristocetin in a time-dependent manner. VWF-induced Erk2 phosphorylation and thromboxane A2 (TXA2) release were completely blocked by PP2, an Src family kinase inhibitor, suggesting that Erk is downstream of Src family kinases. U73122, a phospholipase C inhibitor, also abolished TXA2 generation and Erk phosphorylation. Although VWF fostered the agglutination of platelets regardless of any additional treatment, the inhibition of mitogen-activated protein kinase kinase (MEK) with U0126 abolished VWF-induced platelet aggregation and thromboxane production in non-aspirin-treated washed platelets. However, in platelets treated with aspirin, VWF failed to cause any aggregation. Thus, we conclude that VWF stimulation of platelets results in phospholipase A2 activation through Erk stimulation and that Src family kinases and phospholipase C play essential roles in this event. We further conclude that VWF-induced platelet aggregation does not directly depend on Erk activation but has an absolute requirement for Src/Erk-mediated TXA2 generation.

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.

Characterization of plasminogen as an adhesive ligand for integrins alphaMbeta2 (Mac-1) and alpha5beta1 (VLA-5)

Plasminogen (Pg) has been implicated in many biologic processes involving extracellular proteolysis. We investigated whether Pg, by virtue of its capacity to be deposited within the extracellular matrix, can serve as a ligand for cell surface integrins. We report here that Pg supports cell adhesion by engaging integrins alphaMbeta2 and alpha5beta1. The immobilized Glu-Pg, but not its derivatives with the N-terminal peptide lacking, plasmin and Lys-Pg, supported efficient adhesion that was abolished by anti-alphaMbeta2 and anti-alpha5beta1 integrin-specific monoclonal antibodies (mAbs). In addition, lysine binding sites of Glu-Pg contributed to cell adhesion inasmuch as tranexamic acid and epsilon-aminocaproic acid inhibited cell adhesion. The involvement of alphaMbeta2 and alpha5)beta1 in adhesion to Glu-Pg was demonstrable with blood neutrophils, U937 monocytoid cells, and genetically engineered alphaMbeta2-transfected human embryonic kidney (HEK) 293 cells. In alphaMbeta2, the alphaMI-domain is the binding site for Glu-Pg because the "I-less" form of alphaMbeta2 did not support cell adhesion and the recombinant alphaMI-domain bound Glu-Pg directly. In comparison with cell adhesion, the binding of soluble Glu-Pg to cells and the concomitant generation of plasmin activity was inhibited by anti-alpha5beta1 but not by anti-alphaMbeta2. These findings identify Glu-Pg as an adhesive ligand for integrins alphaMbeta2 and alpha5beta1 and suggest that alpha5beta1 may participate in the binding of soluble Glu-Pg and assist in its activation.

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.

Signaling events underlying thrombus formation

Recent in vivo studies have highlighted the dynamic and complex nature of platelet thrombus growth and the requirement for multiple adhesive receptor-ligand interactions in this process. In particular, the importance of von Willebrand factor (VWF) in promoting both primary adhesion and aggregation under high shear conditions is now well established. In general, the efficiency with which platelets adhere and aggregate at sites of vessel wall injury is dependent on the synergistic action of various adhesive and soluble agonist receptors, with the contribution of each of the individual receptors dependent on the prevailing blood flow conditions. In this review, we will discuss the major platelet adhesive interactions regulating platelet thrombus formation under high shear, with specific focus on the VWF (GPIb and integrin alphaIIbbeta3) and collagen receptors (GPVI and integrin alpha2beta1). We will also discuss the signaling mechanisms utilized by these receptors to induce platelet activation with specific emphasis on the role of cytosolic calcium flux in regulating platelet adhesion dynamics. The role of soluble agonists in promoting thrombus growth will be highlighted and a model to explain the synergistic requirement for adhesive and soluble stimuli for efficient platelet aggregation will be discussed.

Involvement of a CD47-dependent pathway in platelet adhesion on inflamed vascular endothelium under flow

Resting platelet adhesion to inflammatory vascular endothelium is thought to play a causal role in secondary thrombus formation or microcirculatory disturbance after vessel occlusion. However, though adhesion receptors involved in platelet-matrix interactions have been extensively studied, the molecular mechanisms involved in platelet-endothelium interactions are incompletely characterized and have been mainly studied under static conditions. Using human platelets or platelets from wild-type and CD47-/- mice in whole blood, we demonstrated that at low shear rate, CD47 expressed on human and mouse platelets significantly contributes to platelet adhesion on tumor necrosis factor-alpha (TNF-alpha)-stimulated vascular endothelial cells. Using the CD47 agonist peptide 4N1K and blocking monoclonal antibodies (mAbs), we showed that CD47 binds the cell-binding domain (CBD) of endothelial thrombospondin-1 (TSP-1), inducing activation of the platelet alphaIIbbeta3 integrin that in turn becomes able to link the endothelial receptors intercellular adhesion molecule 1 (ICAM-1) and alphavbeta3. Platelet CD36 and GPIbalpha are also involved because platelet incubation with blocking mAbs directed against each of these 2 receptors significantly decreased platelet arrest. Given that anti-CD47 treatment of platelets did not further decrease the adhesion of anti-CD36-treated platelets and CD36 is a TSP-1 receptor, it appears that CD36/TSP-1 interaction could trigger the CD47-dependent pathway. Overall, CD47 antagonists may be potentially useful to inhibit platelet adhesion on inflamed endothelium.

Platelets in hemostasis and thrombosis: role of integrins and their ligands

Platelet adhesion and aggregation at the site of vascular injury are two key events in hemostasis and thrombosis. The contribution of several platelet receptors and their ligands has been highlighted in these processes. In platelet adhesion, particularly at high shear stress, GP1b-von Willebrand factor (vWF) interaction may initiate this event, which is followed by firm platelet adhesion mediated by members of the integrin family, such as beta1 (alpha2beta1, alpha5beta1) and beta3 (alphaIIbbeta3) integrins. In platelet aggregation, although GP1b-vWF, P selectin-sulfatides, and other molecules, may play some roles, the process is mainly mediated by beta3 (alphaIIbbeta3) integrin and its ligands, such as fibrinogen and vWF. Recent studies with perfusion chambers and intravital microscopy have revised the dogma established with the static (low shear stress) conditions. It is intriguing that platelet adhesion and aggregation do still occur in mice lacking both vWF and fibrinogen, suggesting that other unexpected molecule(s) may also be important in hemostasis and thrombosis.

The small proteoglycan decorin supports adhesion and activation of human platelets

Decorin is a small leucine-rich proteoglycan able to interact with several molecules of the subendothelial matrix, such as collagen and fibronectin. In this work, we investigated the ability of purified decorin to support adhesion of human platelets. We found that gel-filtered platelets were actually able to interact with immobilized decorin. Platelet adhesion to decorin was time dependent, required the presence of Mg2+ ions, and was totally mediated by the protein core of the proteoglycan. Platelet stimulation with either adenosine diphosphate (ADP) or a thrombin receptor–activating peptide significantly increased interaction of these cells with the proteoglycan. Upon adhesion to immobilized decorin a number of platelet proteins were found to become tyrosine-phosphorylated. By immunoprecipitation experiments with specific antibodies, the tyrosine phosphorylation of the tyrosine kinase Syk and the phospholipase Cγ2 (PLCγ2) isozyme was demonstrated in decorin-adherent platelets. Interaction of platelets with decorin was selectively prevented by 2 different antibodies against membrane integrin α2β1, but not by a number of antibodies against other membrane receptors. In addition, integrin α2β1, purified from platelet membranes, was able to specifically interact with immobilized decorin. Finally, purified decorin bound to Sepharose beads could precipitate integrin α2β1 from a platelet membrane glycoprotein preparation. Therefore, these results demonstrate that human platelets can bind to immobilized decorin through integrin α2β1, and that this interaction results in the tyrosine phosphorylation of intracellular proteins.

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.

A model of platelet aggregation involving multiple interactions of thrombospondin-1, fibrinogen, and GPIIbIIIa receptor

Thrombospondin-1 (TSP) may, after secretion from platelet alpha granules, participate in platelet aggregation, but its mode of action is poorly understood. We evaluated the capacity of TSP to form inter-platelet cross-bridges through its interaction with fibrinogen (Fg), using either Fg-coated beads or Fg bound to the activated GPIIbIIIa integrin (GPIIbIIIa*) immobilized on beads or on activated fixed platelets (AFP), i.e. in a system free of platelet signaling and secretion mechanisms. Aggregation at physiological shear rates (100-2000 s(-1)) was studied in a microcouette device and monitored by flow cytometry. Soluble TSP bound to and induced aggregation of Fg-coated beads dose-dependently, which could be blocked by the amino-terminal heparin-binding domain of TSP, TSP18. Soluble TSP did not bind to GPIIbIIIa*-coated beads or AFP, unless they were preincubated with Fg. The interaction of soluble TSP with Fg-GPIIbIIIa*-coated beads or Fg-AFP resulted in the formation of aggregates via Fg-TSP-Fg cross-bridges, as demonstrated in a system where direct cross-bridges mediated by GPIIbIIIa*-Fg on one particle and free GPIIbIIIa* on a second particle were blocked by the RGD mimetic Ro 44-9883. Soluble TSP increased the efficiency of Fg-mediated aggregation of AFP by 30-110% over all shear rates and GPIIbIIIa* occupancies evaluated. Surprisingly, TSP binding to Fg already bound to its GPIIbIIIa* receptor appears to block the ability of this occupied Fg to recognize another GPIIbIIIa* receptor, but this TSP can indeed cross-bridge to another Fg molecule on a second platelet. Finally, TSP-coated beads could directly coaggregate at shear rates from 100 to 2000 s(-1). Our studies provide a model for the contribution of secreted TSP in reinforcing inter-platelet interactions in flowing blood, through direct Fg-TSP-Fg and TSP-TSP cross-bridges.

Persistence of platelet thrombus formation in arterioles of mice lacking both von Willebrand factor and fibrinogen

We used intravital microscopy to observe the formation of platelet plugs in ferric chloride-injured arterioles of live mice. With this model, we evaluated thrombus growth in mice lacking von Willebrand factor (vWF) and fibrinogen (Fg), the two key ligands known to mediate platelet adhesion and aggregation. In vWF(-/-) mice, despite the presence of arterial shear, delayed platelet adhesion occurred and stable thrombi formed. In many mice, a persisting high-shear channel never occluded. Abundant thrombi formed in Fg(-/-) mice, but they detached from the subendothelium, which ultimately caused downstream occlusion in all cases. Surprisingly, mice deficient in both vWF and Fg successfully formed thrombi with properties characteristic of both mutations, leading to vessel occlusion in the majority of vessels. Platelets of these doubly deficient mice specifically accumulated fibronectin in their alpha-granules, suggesting that fibronectin could be the ligand supporting the platelet aggregation.

Inhibition of platelet-neutrophil interactions by Fucoidan reduces adhesion and vasoconstriction after acute arterial injury by angioplasty in pigs

The selectin family of cell-adhesion molecules contributes to the interactions of leukocytes and platelets at the site of vascular injury. Such interactions enhance inflammatory reactions and thrombus formation during the arterial response to injury. In this study, we investigated the effects of a selectin inhibitor (Fucoidan) on platelet and neutrophil interactions after arterial injury produced by angioplasty in pigs. [51Cr]-platelet deposition and [111In]-neutrophil adhesion were quantified on intact, mildly, and deeply injured carotid arterial segments, produced by balloon dilation in control (saline, n = 7) and Fucoidan-treated (i.v.; 1 mg/kg, n = 6; 5 mg/kg, n = 5) pigs. In the control group, platelet deposition (x10(6)/cm2) was influenced by the severity of injury and increased significantly (p < 0.05) from 0.06+/-0.06 on intact endothelium to 3.8+/-0.6 and 33.6+/-4.9 on mildly and deeply injured segments, respectively. Fucoidan, 1 mg/kg, had no significant effect, although doses of 5 mg/kg reduced platelet deposition by 73% on deeply injured segments. The level of neutrophil adhesion (x10(3)/cm2) was also influenced by the severity of injury: it increased in the control group from 8.8+/-2.5 on intact endothelium to 226.6+/-45.5 and 397.4+/-61.3 on mildly and deeply injured arterial segments, respectively (p < 0.05). Again, 1 mg/kg Fucoidan had no effect, although doses of 5 mg/kg reduced neutrophil adhesion by 92% and by 84% on mildly and deeply injured segments, respectively. The effects of Fucoidan were associated with a 51% decrease in the vasoconstrictive response at the site of arterial injury. However, Fucoidan had no significant effect on either platelet aggregation or activated clotting time (ACT). In the in vitro perfusion experiments, Fucoidan inhibited both isolated platelet, and neutrophil, adhesion to damaged arterial surfaces. This inhibition was more pronounced in experiments using mixed cell preparations, indicating that Fucoidan interferes with platelet and neutrophil interactions. These results highlight the importance of selectins in the acute physiopathologic reactions related to platelet-neutrophil interactions after arterial injury.

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

Thrombospondin-1 (TSP-1) interacts specifically with heparin and fibronectin in vitro and colocalizes with fibronectin and heparan sulfate in the extracellular matrix (ECM). Its conformation is strongly dependent on Ca2+ concentration. We have previously shown that both heparin and fibronectin have two binding sites on the TSP-1 subunit which may require conformational change for their occupancy (R. Dardik and J. Lahav, 1987, Eur. J. Biochem. 168, 347; ibid 1989, 185, 581). To investigate the effect of TSP-1 binding to fibronectin and heparin on its functional conformation, TSP-1 was subjected to proteolysis in the presence and absence of ligands and of Ca2+. We found that while trypsin cleavage of free TSP-1 resulted in the inactivation of ligand binding, TSP-1 bound to either fibronectin or heparin remained stably associated with these ligands. Cleavage by thrombin or tissue plasminogen activator (tPA) showed that Ca2+-depleted TSP-1, when bound to fibronectin or to heparin, yielded proteolytic cleavage patterns typical of the Ca2+-containing form. Cleavage by chymotrypsin was not affected by binding to fibronectin or heparin; hence loss of proteolytic susceptibility was not due to steric hindrance by the ligands. Taken together, these results indicate that: (A) binding of TSP-1 to fibronectin or heparin is a two-step mechanism where binding to one site leads to conformational changes that enable binding to the second site; (B) TSP-1 in complex with fibronectin or heparin adopts the Ca2+-containing conformation in the absence of Ca2+; and (C) such complexes are highly resistant to cleavage by tPA and, if cleaved by other enzymes, the TSP-1 fragments remain bound to other ECM components. These characteristics have profound significance for platelet adhesion and cell migration into wounds where Ca2+ concentrations are reduced.

Microfibrils from the arterial subendothelium

Microfibrillar structures of the subendothelium are represented by either type VI collagen or elastin-associated microfibrils which are also referred to as fibrillin-containing microfibrils. These structures are present throughout the subendothelium irrespective of the presence of elastin. The localization, structure, and protein composition of microfibrils are reviewed. The arterial subendothelium is thrombogenic despite its very low content in fibrillar collagens. This thrombogenicity is linked to the microfibrillar structures, essentially to type VI collagen and to thrombospondin-containing microfibrils. Their respective ability to bind the von Willebrand factor and to activate blood platelets is discussed.

Identification of domains responsible for von Willebrand factor type VI collagen interaction mediating platelet adhesion under high flow

We have identified type VI collagen (Col VI) as a primary subendothelial extracellular matrix component responsible for von Willebrand factor (vWF)-dependent platelet adhesion and aggregation under high tensile strength. Intact tetrameric Col VI was the form of the collagen found to be capable of promoting vWF-mediated platelet adhesion/aggregation under this shear condition, whereas removal of the predominant portion of the terminal globules by pepsin treatment abrogated its activity. The inability of the pepsin-digested Col VI to support any platelet interaction at high flow was because of the failure of the A3(vWF) domain to bind to this form of collagen, suggesting a stringent requirement of a tridimensional conformation or of intactness of its macromolecular structure. In contrast, the A1(vWF) domain bound to both intact and pepsin-digested Col VI tetramers but, in accordance with the cooperating function of the two vWF domains, failed to support platelet adhesion/aggregation under high shear onto Col VI by itself. The putative A1(vWF) binding site resided within the A7(VI) module (residues 413-613) of the globular amino-terminal portion of the alpha3(VI) chain. Soluble recombinant A7(VI) polypeptide strongly perturbed the vWF-mediated platelet adhesion to Col VI under high shear rates, without affecting the binding of the vWF platelet receptor glycoprotein Ibalpha to its cognate ligand A1(vWF). The findings provide evidence for a concerted action of the A1(vWF) and A3(vWF) domains in inducing platelet arrest on Col VI. This is accomplished via an interaction of the A1(vWF) domain with a site contained in the alpha3 chain A7(VI) domain and via a conformation-dependent interaction of the A3(vWF) domain with the intact tetrameric collagen. The data further emphasize that Col VI microfilaments linking the subendothelial basement membrane to the interstitial collagenous network may play a pivotal role in the hemostatic process triggered upon damage of the blood vessel wall.

CD36 mediates binding of soluble thrombospondin-1 but not cell adhesion and haptotaxis on immobilized thrombospondin-1

In this study, we examined the binding of soluble TSP1 (and ox-LDL) to CD36-transfected cells and the mechanisms by which immobilized TSP1 mediated attachment and haptotaxis (cell migration towards a substratum-bound ligand) of these transfected cells. CD36 cDNA transfection of NIH 3T3 cells clearly induced a dramatic increase in binding of both soluble [125I]-TSP1 and [125I]-ox-LDL to the surface of CD36-transfected cells, indicating that there was a gain of function with CD36 transfection in NIH 3T3 cells. Despite this gain of function, mock- and CD36-transfected NIH 3T3 cells attached and migrated to a similar extent on immobilized TSP1. An anti-TSP1 oligoclonal antibody inhibited CD36-transfected cell attachment to TSP1 while function blocking anti-CD36 antibodies, alone or in combination with heparin, did not. A series of fusion proteins encompassing cell-recognition domains of TSP1 was then used to delineate mechanisms by which NIH 3T3 cells adhere to TSP1. Although CD36 binds soluble TSP1 through a CSVTCG sequence located within type 1 repeats, 18,19CD36-transfected NIH 3T3 cells did not attach to immobilized type 1 repeats while they did adhere to the N-terminal, type 3 repeats (in an RGD-dependent manner) and the C-terminal domain of TSP1. Conversely, Bowes melanoma cells attached to type 1 repeats and the N- and C-terminal domains of TSP1. However, CD36cDNA transfection of Bowes cells did not increase cell attachment to type 1 repeats compared to that observed with mock-transfected Bowes cells. Moreover, a function blocking anti-CSVTCG peptide antibody did not inhibit the attachment of mock- and CD36-transfected Bowes cells to type 1 repeats. It is suggested that CD36/TSP1 interaction does not occur upon cell-matrix adhesion and haptotaxis because TSP1 undergoes conformational changes that do not allow the exposure of the CD36 binding site.

Shear-stress-induced detachment of blood platelets from various surfaces

Platelet accumulation is the result of platelet adhesion and detachment. This study describes platelet detachment from fibronectin, laminin, fibrinogen, von Willebrand Factor (vWF), endothelial cell matrix (ECM), and collagen type III. Platelets adhered after 5 minutes' perfusion of anticoagulated whole blood at different shear rates were subjected to a brief flush of 1 minute with HEPES-buffered saline at varying shear stress. Platelets adhering to fibronectin and laminin were most easily detached. Fibrinogen and vWF had an intermediate position. Almost no detachment occurred from ECM and collagen type III. Dendritic platelets were removed more easily than spread platelets. When the shear rate at which adhesion had occurred was raised, platelet detachment decreased strongly. When the time period between adhesion and detachment was increased, platelet detachment also decreased. From these results, we conclude that detachment is determined initially by the shear rate at which platelets adhere, then by the time they are allowed to settle, then by the nature of the surface, and then by the degree of spreading. The shear optimum for a given adhesive protein is not determined by the detachment.

Role of platelet adhesion in homeostasis and immunopathology

Various molecules expressed on the surface of platelets have been shown to mediate the protective or deleterious role of these cells in immuno-inflammatory mechanisms. Increasing evidence points to the involvement of the cell adhesion molecules, gpIIb-IIIa, P-selectin, CD31, LFA-1, and CD36 in the interaction between platelets and endothelial cells as well as other cell types. The possible role of these molecules in the ability of platelets to support endothelium and to protect against tumour necrosis factor mediated cytolysis or parasitic invasion are reviewed. The involvement of platelets as effectors of tissue damage in cerebral malaria, lipopolysaccharide induced pathology, and pulmonary fibrosis is also discussed. This has then been extended to include the intercellular mechanisms underpinning their pathogenic role in metastasis, transplant rejection, stroke, brain hypoxia, and related conditions. A better understanding of the complex regulation and hierarchical organisation of these various platelet adhesion molecules may prove useful in the development of new approaches to the treatment of such diseases.

Inhibition of Sickle Erythrocyte Adhesion to Immobilized Thrombospondin by von Willebrand Factor Under Dynamic Flow Conditions

Sickle red blood cell (RBC) adhesion to the blood vessel wall is hypothesized to be the initiating event in the periodic vaso-occlusive episodes that characterize sickle cell disease (SCD). Thrombospondin-1 (TSP) and von Willebrand factor (vWF ) have each been implicated in the adhesion of sickle RBC to vascular endothelial cells (EC) and subendothelial matrices. To better understand the contributions of each of these adhesive glycoproteins, we examined the adhesion of sickle RBC to immobilized TSP and vWF using a parallel plate flow chamber. Under postcapillary venular shear stress (1 dyne/cm2), sickle RBC adhered preferentially to TSP. To explore potential interactive effects of vWF and TSP, we examined sickle RBC adhesion to mixtures of these proteins. Whether the proteins were first combined in solution or sequentially applied to the slide, the presence of vWF inhibited the binding of sickle RBC to TSP. The inhibition of adhesion by vWF was shown to be the result of specific and saturable binding of vWF to TSP. Furthermore, vWF in solution at normal plasma levels also inhibited RBC adhesion to immobilized TSP. These data indicate that sickle RBC adhesion in vivo may be significantly influenced by the relative concentrations of TSP and vWF in the vascular wall.

Proteolysis regulates exposure of the IIICS-1 adhesive sequence in plasma fibronectin

The alternatively spliced type III connecting segment (IIICS) of fibronectin (Fn) contains an amino acid sequence, CS-1, which is recognized by the integrin receptor, alpha 4 beta 1. Plasma Fn inhibits alpha 4 beta 1-dependent binding of lymphocytes and monocytes to CS-1 containing Fn derivatives poorly, suggesting limited exposure of the CS-1 sequence in Fn. To test the availability of CS-1 in plasma Fn, an antibody was raised to the synthetic peptide CS-1. The CS-1 sequence was found to be minimally exposed in plasma Fn; and immobilization of Fn, a model of matrix deposition, caused only a modest increase in its exposure. Digestion of Fn with selected proteases, however, induced substantial expression of the CS-1 sequence. The acid protease cathepsin D generated fragments of 31-33.5 kDa from the COOH-terminal heparin-binding domain of Fn which possessed high immunoreactivity with anti-CS-1. Digestion of Fn with cathepsin B also resulted in the exposure of CS-1 sequence in a 140 kDa fragment. Although the digestion of Fn with neutral proteases (neutrophil elastase, cathepsin G, chymotrypsin, trypsin) generated fragments from the COOH-terminal heparin-binding domain of similar molecular weight as with cathepsin D, the exposure of CS-1 did not occur. Exposure of the CS-1 region by the cathepsins was supported by cell adhesion experiments; digestion of Fn with cathepsins D and B transformed inert plasma Fn to an effective inhibitor of adhesion of lymphoblastoid B and T cells (Ramos, Jurkat, Molt-4) to an immobilized CS-1 conjugate. These results suggest that exposure of the CS-1 sequence in plasma Fn by proteolysis with cathepsins D and B, enzymes implicated in several pathological processes, may serve a regulatory function in cell adhesion. The adhesive function of the CS-1 region in intact Fn appears to be suppressed by the native conformation of the molecule.

Antibodies to CD36 (GPIV) inhibit platelet adhesion to subendothelial surfaces under flow conditions

The membrane glycoprotein CD36 (glycoprotein [GP] IV) has previously been shown to accelerate the initial interaction of platelets with purified type I collagen in both static and flow systems. In the present study, the role of CD36 on platelet interaction with physiologically relevant collagenous surfaces was addressed. Using arterial subendothelium (SE) and endothelial cell extracellular matrix (ECM), studies were performed under flow conditions with annular and parallel-plate perfusion chambers, respectively, at a shear rate of 800 s-1 for 2, 5, and 10 minutes. Perfusates consisted of citrated normal blood samples incubated with Fab fragments of a monospecific polyclonal anti-CD36 antibody or with each of three new anti-CD36 monoclonal antibodies (MoAbs) that inhibit platelet adhesion to purified type I collagen in a static system (131.4, 131.5, and 131.7). Perfusions over SE were also carried out using citrated blood samples from a Naka-negative donor, whose platelets lack CD36. Morphometric evaluation of the perfused samples showed that polyclonal anti-CD36 Fab and the three monoclonal anti-CD36 antibodies inhibited platelet adhesion to the two substrates by 40% after 2 minutes of perfusion and by 30% after 5 minutes (P < .005 on SE and P < .01 on ECM), but at 10 minutes, significant inhibition was seen only on SE with polyclonal anti-CD36 Fab. Similar inhibitions were seen with Naka-negative platelets on SE. These studies demonstrate that CD36 plays a role in the early stages of platelet adhesion to physiologically relevant subendothelial surfaces.

Platelet adhesion to multimeric and dimeric von Willebrand factor and to collagen type III preincubated with von Willebrand factor

As part of a systematic study of platelet interaction with adhesive proteins under flow conditions, we studied platelet adhesion to multimeric and dimeric von Willebrand factor (vWF) coated to glass. vWF-dependent adhesion to collagen type III was studied for comparison. Adhesion to glass-coated vWF and vWF-mediated adhesion to collagen type III were in many respects similar. Both showed no decrease at increasing shear rates and a decline to 50% of maximum with a low-molecular-weight multimeric fraction. Adhesion to glass-coated vWF was partially inhibited by heparin and completely inhibited by prostaglandin I(2) and anti-glycoprotein (GP) Ib and anti-GPIIb-IIIa antibodies. vWF-dependent adhesion to collagen was not inhibited by heparin, was partially inhibited by anti-GPIIb-IIIa, and was completely inhibited by prostaglandin I(2) and anti-GPIb. Recombinant dimeric vWF was made by deletion of the propeptide and expression in Chinese hamster ovary cells. Adhesion was 50% of that with plasma vWF, and larger concentrations of dimeric vWF were required. Adhesion to dimeric vWF was optimal at 1500 s(-1), with a gradual decrease at higher shear rates. We conclude that adhesion to collagen type III is strongly but not completely determined by the adhesive properties of vWF.

Platelet activation and interactions with the microvasculature

Platelet adherence to structurally or functionally damaged endothelium and its subsequent activation are multifaceted events. Regulation of the rate and extent of platelet adhesion is under local control by the platelets and endothelium. Even in the absence of platelet adhesion to endothelium or subendothelium, there is a complex hemostatic balance of coagulation and anticoagulation. This is mediated by the manufacture, release, and inactivation of various procoagulant and anticoagulant compounds, predominantly by the platelets and endothelium. The relationship between the two in maintaining the homeostasis of coagulation and other processes is complex. This review focuses on the structure, function, and interaction of endothelium, subendothelium, and platelets and on their vasoactive and pro-/anticoagulant functions.

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.

Adhesion of blood platelets is inhibited by VCL, a recombinant fragment (leucine504 to lysine728) of von Willebrand factor

VCL, fragment Leu504 to Lys728 of von Willebrand factor (vWF) expressed in Escherichia coli, contains the glycoprotein (GP) Ib-binding domain of vWF. This fragment inhibited ristocetin-induced platelet aggregation with an IC50 of 0.2 mumol/L and botrocetin-induced platelet aggregation with an IC50 of 0.08 mumol/L. We studied the antiadhesive profile of VCL by adding it to blood that was circulated over various adhesive surfaces. VCL inhibited adhesion to endothelial cell matrix, which served as a model of the vessel wall. Maximal inhibition at a high shear rate of 1600 s-1 was stronger (60%) than at a low shear rate of 300 s-1 (40%). Half maximal inhibition was found to be 1.5 mumol/L at both shear rates. The role of various adhesive molecules was investigated in more detail by coating glass coverslips with collagen type I, laminin, fibronectin, or vWF. Fibrinogen was studied as well. Platelet adhesion to laminin and vWF was not inhibited by VCL. Adhesion to collagen, fibronectin, and fibrinogen was particularly inhibited at a high shear rate. VCL coated to a coverslip caused a concentration-dependent adhesion that was blocked by antibodies against GPIb, which block interaction with vWF. Binding studies showed a nonsaturable ristocetin binding of VCL to platelets that was blocked by vWF or inhibitory antibodies against GPIb. Binding to collagen was weak, and VCL did not inhibit binding of vWF at a 5000-fold excess. From these data, we conclude that VCL inhibits adhesion in all cases in which adhesion is vWF dependent by competing for vWF binding to activated GPIb. The lack of inhibition of adhesion to vWF as a single molecule may be explained by assuming that this adhesion is determined by interaction of nonactivated GPIb with vWF that has been changed in conformation by adsorption. Studies investigating thrombus formation on the connective tissue of an atherosclerotic plaque in a human coronary artery showed that VCL was able to partially prevent this thrombus formation. VCL may be of value in preventing adhesion and thrombus formation under conditions in which these processes are dependent on vWF.

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.

Increased platelet deposition on atherosclerotic coronary arteries

A ruptured atherosclerotic plaque leads to exposure of deeper layers of the plaque to flowing blood and subsequently to thrombus formation. In contrast to the wealth of data on the occurrence of thrombi, little is known about the reasons why an atherosclerotic plaque is thrombogenic. One of the reasons is the relative inaccessibility of the atherosclerotic plaque. We have circumvented this problem by using 6-microns cryostat cross sections of human coronary arteries. These sections were mounted on coverslips that were exposed to flowing blood in a rectangular perfusion chamber. In normal-appearing arteries, platelet deposition was seen on the luminal side of the intima and on the adventitia. In atherosclerotic arteries, strongly increased platelet deposition was seen on the connective tissue of specific parts of the atherosclerotic plaque. The central lipid core of an advanced plaque was not reactive towards platelets. The results indicate that the atherosclerotic plaque by itself is more thrombogenic than the normal vessel wall. To study the cause of the increased thrombus formation on the atherosclerotic plaque, perfusion studies were combined with immunohistochemical studies. Immunohistochemical studies of adhesive proteins showed enrichment of collagen types I, III, V, and VI, vitronectin, fibronectin, fibrinogen/fibrin, and thrombospondin in the atherosclerotic plaque. Laminin and collagen type IV were not enriched. von Willebrand Factor (vWF) was not present in the plaque. The pattern of increased platelet deposition in serial cross sections corresponded best with areas in which collagen types I and III were enriched, but there were also areas in the plaque where both collagens were enriched but no increased reactivity was seen. Inhibition of platelet adhesion with a large range of antibodies or specific inhibitors showed that vWF from plasma and collagen types I and/or III in the plaque were involved. Fibronectin from plasma and fibronectin, fibrinogen, laminin, and thrombospondin in the vessel wall had no effect on platelet adhesion. We conclude that the increased thrombogenicity of atherosclerotic lesions is due to changes in quantity and nature of collagen types I and/or III.