Human blood platelet adhesion to artery subendothelium is mediated by factor VIII-Von Willebrand factor bound to the subendothelium

Humanizing thrombi in mice

Antiplatelet therapies form the cornerstone of atherothrombosis prevention, reducing the morbidity and mortality associated with cardiovascular disease. Despite these benefits, there is still an unmet need for more effective and safer pharmacological agents. To expedite this process, biological platforms that better reflect the intravascular environment in humans will be required in order to shorten drug development time, enable better determination of dosing regimes, and aid in the design of clinical studies. This article focuses on a unique genetically modified animal model that predicts the in vivo response of antiplatelet agents in humans more accurately than is currently possible using conventional murine models of thrombosis.

In vivo gene transfer strategies to achieve partial correction of von Willebrand disease

von Willebrand disease (VWD), the most common hereditary coagulation disorder, results from mutations in the 52-exon gene for von Willebrand factor (VWF), which encodes an 8.4-kB cDNA. Studies with VWF cDNA plasmids have demonstrated that in vivo gene transfer to the liver will correct the coagulation dysfunction in VWF(-/-) mice, but the correction is transient. To develop gene therapy for VWF that would mediate long-term expression of the VWF cDNA in liver, we first evaluated segmental pre-mRNA trans-splicing (SPTS) with two adeno-associated virus (AAV) serotype 8 vectors, each delivering one-half of the VWF cDNA. However, although the two vectors functioned well to generate VWF multimers after infection of cells in vitro, the efficiency of SPTS was insufficient to correct the VWF(-/-) mouse in vivo. As an alternative, we assessed the ability of a lentiviral vector to transfer the intact murine VWF cDNA in vivo directly to the neonatal liver of VWF(-/-) mice, using generation of VWF multimers, bleeding time, and bleeding volume as efficacy parameters. The VWF lentivirus generated VWF multimers and partially or completely corrected the coagulation defect on a persistent basis in 33% of the treated VWF-deficient mice. On the basis of the concept that partial persistent correction with gene transfer could be beneficial in VWD patients, these observations suggest that lentiviral delivery of VWF cDNA should be explored as a candidate for gene therapy in patients with a severe form of VWD.

In vivo analysis of the role of O-glycosylations of von Willebrand factor

The objective of this project was to study the function of O-glycosylations in von Willebrand factor (VWF) life cycle. In total, 14 different murine Vwf cDNAs mutated on one or several O-glycosylations sites were generated: 9 individual mutants, 2 doublets, 2 clusters and 1 mutant with all 9 murine glycosylation sites mutated (Del-O-Gly). We expressed each mutated cDNA in VWF deficient-mice by hydrodynamic injection. An immunosorbent assay with Peanut Agglutinin (PNA) was used to verify the O-glycosylation status. Wild-type (WT) VWF expressed by hepatocytes after hydrodynamic injection was able to bind PNA with slightly higher affinity than endothelial-derived VWF. In contrast, the Del-O-Gly VWF mutant did not bind PNA, demonstrating removal of O-linked glycans. All mutants displayed a normal multimeric pattern. Two mutants, Del-O-Gly and T1255A/T1256A, led to expression levels 50% lower than those induced by WT VWF and their half-life in vivo was significantly reduced. When testing the capacity of each mutant to correct the bleeding time of VWF-deficient mice, we found that S1486A, T1255A, T1256A and the doublet T1255A/T1256A were unable to do so. In conclusion we have shown that O-glycosylations are dispensable for normal VWF multimerization and biosynthesis. It also appears that some O-glycosylation sites, particularly the T1255 and T1256 residues, are involved in the maintenance of VWF plasma levels and are essential for normal haemostasis. As for the S1486 residue, it seems to be important for platelet binding as demonstrated in vitro using perfusion experiments.

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.

Mechanobiology of platelets: techniques to study the role of fluid flow and platelet retraction forces at the micro- and nano-scale

Coagulation involves a complex set of events that are important in maintaining hemostasis. Biochemical interactions are classically known to regulate the hemostatic process, but recent evidence has revealed that mechanical interactions between platelets and their surroundings can also play a substantial role. Investigations into platelet mechanobiology have been challenging however, due to the small dimensions of platelets and their glycoprotein receptors. Platelet researchers have recently turned to microfabricated devices to control these physical, nanometer-scale interactions with a higher degree of precision. These approaches have enabled exciting, new insights into the molecular and biomechanical factors that affect platelets in clot formation. In this review, we highlight the new tools used to understand platelet mechanobiology and the roles of adhesion, shear flow, and retraction forces in clot formation.

Platelet activation in heart failure patients supported by the HeartMate II ventricular assist device

OBJECTIVE

Thromboembolic events have been observed in heart failure (HF) patients supported by long-term mechanical circulatory support (MCS) devices. It has been hypothesized that these adverse events may be the result of platelet activation associated with high rotational speeds common to axial flow pumps. In this study, markers of platelet activation were investigated in HF patients supported by a HeartMate II left ventricular assist device (LVAD).

METHODS

The study group consisted of 34 HF patients supported by a HeartMate II axial flow LVAD implanted for destination therapy (DT). This patient population was 94% male (31 M, 3 F), supported by LVAD for 30 to 723 days (average 268 days), and with an anticoagulation regimen of Coumadin (0-8 mg daily dose) and aspirin (0-325 mg daily dose). Platelet adhesion markers (soluble P-selectin and solube CD40 ligand), platelet count (PC), hematocrit (Hct), and creatinine (Cr) were measured.

RESULTS

The soluble P-selectin marker was within normal platelet activity limits for all end points. The soluble CD40 ligand marker indicated platelet inactivity for all end points. Despite high shear stresses associated with a high-speed axial flow pump, the HeartMate II had no discernable effect on platelet activation. Current clinical doses of aspirin also appear to have little effect on platelet activation. Platelet count, hematocrit, and creatinine were normal in these patients over duration of support.

CONCLUSIONS

There were no discernable changes in platelet activation markers soluble P-selectin and soluble CD40 ligand in HF patients support by HeartMate II LVAD independently of length of support, anti-platelet, and anti-coagulation regimens.

Humanized mouse model of thrombosis is predictive of the clinical efficacy of antiplatelet agents

BACKGROUND

In vivo testing of novel antiplatelet agents requires informative biomarkers. By genetically modifying mouse von Willebrand factor (VWF(R1326H)), we have developed a small animal model that supports human but not mouse platelet-mediated thrombosis. Here, we evaluate the use of this biological platform as a pharmacodynamic biomarker for antithrombotic therapies.

METHODS AND RESULTS

The antithrombotic effects of several αIIbβ3 inhibitors were determined in VWF(R1326H) mutant mice infused with human platelets. Administration of abciximab, eptifibatide, or tirofiban at doses recommended for percutaneous coronary intervention (per 1 kg of body weight) significantly reduced human platelet-mediated thrombus formation in laser-injured arterioles by > 75% (P < 0.001). In contrast, clot size in wild-type control animals remained essentially unchanged (P > 0.05), results consistent with observed species differences in IC₅₀ values obtained by aggregometry. To further demonstrate that our biological platform is unique among standard mouse models, we evaluated the thrombogenic potential of platelets from healthy volunteers before and after clopidogrel therapy. Consistent with the antithrombotic effect of this agent, platelets postdrug administration formed smaller thrombi than cells before therapy and were less responsive to ADP-induced aggregation (P < 0.001).

CONCLUSIONS

The ability of αIIbβ3 and P2Y₁₂ inhibitors to limit human platelet clot formation at doses recommended by the American College of Cardiology/American Heart Association suggests that VWF(R1326H) mutant mice can serve as both a pharmacodynamic and a functional response biomarker, attributes essential for not only expediting drug development but also designing clinical studies.

The effect of shear stress on protein conformation: Physical forces operating on biochemical systems: The case of von Willebrand factor

Macromolecules and cells exposed to blood flow in the circulatory tree experience hydrodynamic forces that affect their structure and function. After introducing the general theory of the effects of shear forces on protein conformation, selected examples are presented in this review for biological macromolecules sensitive to shear stress. In particular, the biochemical effects of shear stress in controlling the von Willebrand Factor (VWF) conformation are extensively described. This protein, together with blood platelets, is the main actor of the early steps of primary haemostasis. Under the effect of shear forces >30 dyn/cm², VWF unfolding occurs and the protein exhibits an extended chain conformation oriented in the general direction of the shear stress field. The stretched VWF conformation favors also a process of self aggregation, responsible for the formation of a spider web network, particularly efficient in the trapping process of flowing platelets. Thus, the effect of shear stress on conformational changes in VWF shows a close structure-function relationship in VWF for platelet adhesion and thrombus formation in arterial circulation, where high shear stress is present. The investigation of biophysical effects of shear forces on VWF conformation contributes to unraveling the molecular interaction mechanisms involved in arterial thrombosis.

Proliferation, behavior, and cytokine gene expression of human umbilical vascular endothelial cells in response to different titanium surfaces

Success of dental implantation is initially affected by wound healing of both, hard and soft tissues. Endothelial cells (ECs) are involved as crucial cells in the angiogenesis and inflammation process of wound healing. In the present study, proliferation, mobility, cluster formation, and gene expression of angiogenesis-related molecules of human umbilical vascular endothelial cells (HUVECs) were investigated on titanium surfaces with different roughnesses: acid-etched (A), coarse-grit-blasted and acid-etched (SLA) surfaces, as well as on hydrophilic modified modA and modSLA surfaces. Cell behaviors were analyzed by proliferation assay and time-lapse microscopy, gene expression was analyzed by real time PCR. Results showed that cell proliferation, mobility, and cluster formation were highest on modA surfaces compared with all other surfaces. HUVECs moved slowly and exhibited seldom cell aggregation on SLA and modSLA surfaces during the whole observing period of 120 h. The gene expressions of the angiogenesis-related factors von Willebrand factor, thrombomodulin, endothelial cell protein C receptor, and adhesion molecules intercellular adhesion molecule-1 and E-selectin were most enhanced on modSLA surfaces. These results suggest that modA surface is optimal for proliferation and angiogenic behavior of ECs. However, modSLA surface seems to promote ECs to express angiogenesis-related factor genes, which play essential roles in controlling inflammation and revascularization of wound healing.

The plasma von Willebrand factor O-glycome comprises a surprising variety of structures including ABH antigens and disialosyl motifs

BACKGROUND

von Willebrand factor (VWF) is a key component for maintenance of normal hemostasis. Its glycan moieties, accounting for about 20% of its molecular weight, have been shown to affect many of its properties. Previous studies reported correlations between VWF secretion, half-life and the nature or presence of its N-glycans, and more importantly between VWF plasma level and the type of N-linked ABH antigens. Despite the presence of 10 predicted O-glycosylation sites, the O-glycome remains poorly characterized, impairing the complete elucidation of its influence on VWF functions. So far only a single glycan structure, a disialyl core 1 glycan, has been identified.

OBJECTIVES

To define an exhaustive profile of the VWF O-glycan structures to help the understanding of their role in VWF regulation and properties.

METHODS

Plasma-derived VWF O-linked sugars were isolated and analyzed using state-of-the-art mass spectrometry methodologies.

RESULTS AND CONCLUSIONS

We provide here a detailed analysis of the human plasma-derived VWF O-glycome. Eighteen O-glycan structures including both core 1 and core 2 structures are now demonstrated to be present on VWF. Amongst the newly determined structures are unusual tetra-sialylated core 1 O-glycans and ABH antigen-containing core 2 O-glycans. In conjunction with current models explaining VWF activity, knowledge of the complete O-glycome will facilitate research aimed at providing a better understanding of the influence of glycosylation on VWF functions.

Nitric oxide inhibits von Willebrand factor-mediated platelet adhesion and spreading through regulation of integrin alpha(IIb)beta(3) and myosin light chain

BACKGROUND

von Willebrand factor (VWF)-mediated platelet adhesion and spreading at sites of vascular injury is a critical step in hemostasis. This process requires two individual receptors: glycoprotein Ib (GPIb)-V-IX and integrin alpha(IIb)beta(3). However, little is known about the negative regulation of these events.

OBJECTIVES

To examine if the endogenous platelet inhibitor nitric oxide (NO) has differential effects on adhesion, spreading and aggregation induced by immobilized VWF.

RESULTS

S-nitrosoglutathione (GSNO) inhibited platelet aggregation on immobilized VWF under static and flow conditions, but had no effect on platelet adhesion. Primary signaling events underpinning the actions of NO required cyclic GMP but not protein kinase A. Dissecting the roles of GPIb and integrin alpha(IIb)beta(3) demonstrated that NO targeted alpha(IIb)beta(3)-mediated aggregation and spreading, but did not significantly influence GPIb-mediated adhesion. To understand the relationship between the effects of NO on adhesion and subsequent aggregation, we evaluated the activation of alpha(IIb)beta(3) on adherent platelets. NO reduced the phosphorylation of extracellular stimuli-responsive kinase (ERK) and p38, required for integrin activation resulting in reduced binding of the activated alpha(IIb)beta(3)-specific antibody PAC-1 on adherent platelets. Detailed analysis of platelet spreading initiated by VWF demonstrated key roles for integrin alpha(IIb)beta(3) and myosin light chain (MLC) phosphorylation. NO targeted both of these pathways by directly modulating integrin affinity and activating MLC phosphatase.

CONCLUSION

These data demonstrate that initial activation-independent platelet adhesion to VWF via GPIb is resistant to NO, however, NO inhibits GPIb-mediated activation of alpha(IIb)beta(3) and MLC leading to reduced platelet spreading and aggregation.

Evaluation of lyophilized platelets as an infusible hemostatic agent in experimental non-compressible hemorrhage in swine

INTRODUCTION

Human lyophilized platelets hold promise as a novel hemostatic infusion agent for the control of traumatic hemorrhage. Rehydrated, lyophilized platelets (Stasix) were investigated as an infusible hemostatic agent in experimental non-compressible hemorrhage, using a porcine liver injury model.

METHODS

Yorkshire swine underwent a grade III liver injury and uncontrolled bleeding. After 15 min, animals were infused with Stasix (n = 10) or normal saline vehicle (n = 10). At 2 h, the liver was repaired, and the animals were monitored for another4 h. Resuscitation, including blood transfusion, was administered during the hospital phase. Laboratory data, including arterial blood gas, complete blood count, thromboelastography (TEG), and coagulation parameters, were collected. All animals underwent necropsy with complete histopathologic examination.

RESULTS

Overall survival in the Stasix group [8/10 (80%)] was significantly higher than in the control group [2/10 (20%)] (P = 0.023). Mean total blood loss index (g kg(-1)) was lower in Stasix-treated animals (22.2 +/- 3.5) than in control animals (34.7 +/- 3.4) (P = 0.019). Hemodynamic parameters were improved in the Stasix group, and a trend towards higher hemoglobin and lower lactate was observed. Coagulation and TEG parameters were not different between the groups. One surviving animal in the Stasix group had evidence of thrombi on necropsy.

CONCLUSIONS

This is the first reported study to evaluate rehydrated, lyophilized platelets as an infusible hemostatic agent for non-compressible hemorrhage. Stasix improved survival and reduced blood loss in a liver injury porcine model. However, evidence of thrombotic complications warrants further investigation prior to human use in the setting of traumatic hemorrhage.

Role of platelets in the development of atherosclerotic disease and possible interference with platelet inhibitor drugs

During the last two decades, significant advances have been made in the understanding of atherosclerotic disease. The pathogenesis of atherosclerosis appears to depend on a precise sequence of critical events based on the interaction of blood elements and lipids with the arterial wall. The major critical events and their sequence appears to be as follows: hemodynamic stress and endothelial injury; arterial wall-platelet interaction; smooth muscle cell proliferation; lipid entry and accumulation; significant arterial narrowing with fibrosis and development of thrombi; and complications in the form of calcification, ulceration, aneurysm, acute thrombotic occlusion and embolization. This sequence of critical events starts at a young age and in all geographic racial groups. Their evolution into advanced symptomatic lesions takes many years and varies in incidence and extent among different geographic and ethnic groups. It appears that in promoting and accelerating this process into the advanced stage of the disease, the presence at a young age of the so-called risk factors of atherosclerotic disease is most important. The recent advances in the understanding of the atherosclerotic process will be highlighted in this chapter with particular attention being focused on the role of platelets and thrombosis in the development of the disease and the possible role of platelet inhibitor drugs on the prevention of coronary atherosclerotic disease.

Factor VIII and von Willebrand factor interaction: biological, clinical and therapeutic importance

The interaction of factor VIII (FVIII) with von Willebrand Factor (VWF) is of direct clinical significance in the diagnosis and treatment of patients with haemophilia A and von Willebrand disease (VWD). A normal haemostatic response to vascular injury requires both FVIII and VWF. It is well-established that in addition to its role in mediating platelet to platelet and platelet to matrix binding, VWF has a direct role in thrombin and fibrin generation by acting as a carrier molecule for the cofactor FVIII. Recent studies show that the interaction affects not only the biology of both FVIII and VWF, and the pathology of haemophilia and VWD, but also presents opportunities in the treatment of haemophilia. This review details the mechanisms and the molecular determinants of FVIII interaction with VWF, and the role of FVIII-VWF interaction in modulating FVIII interactions with other proteases, cell types and cellular receptors. The effect of defective interaction of FVIII with VWF as a result of mutations in either protein is discussed.

Platelet adhesion under flow

Platelet-adhesive mechanisms play a well-defined role in hemostasis and thrombosis, but evidence continues to emerge for a relevant contribution to other pathophysiological processes, including inflammation, immune-mediated responses to microbial and viral pathogens, and cancer metastasis. Hemostasis and thrombosis are related aspects of the response to vascular injury, but the former protects from bleeding after trauma, while the latter is a disease mechanism. In either situation, adhesive interactions mediated by specific membrane receptors support the initial attachment of single platelets to cellular and extracellular matrix constituents of the vessel wall and tissues. In the subsequent steps of thrombus growth and stabilization, adhesive interactions mediate platelet-to-platelet cohesion (i.e., aggregation) and anchoring to the fibrin clot. A key functional aspect of platelets is their ability to circulate in a quiescent state surveying the integrity of the inner vascular surface, coupled to a prompt reaction wherever alterations are detected. In many respects, therefore, platelet adhesion to vascular wall structures, to one another, or to other blood cells are facets of the same fundamental biological process. The adaptation of platelet-adhesive functions to the effects of blood flow is the main focus of this review.

Platelet adhesive dynamics. Part II: high shear-induced transient aggregation via GPIbalpha-vWF-GPIbalpha bridging

A three-dimensional multiscale computational model, platelet adhesive dynamics (PAD), is developed and applied in Part I and Part II articles to characterize and quantify key biophysical aspects of GPIbalpha-von-Willebrand-factor (vWF)-mediated interplatelet binding at high shear rates, a necessary and enabling step that initiates shear-induced platelet aggregation. In this article, an adhesive dynamics model of the transient aggregation of two unactivated platelets via GPIbalpha-vWF-GPIbalpha bridging is developed and integrated with the three-dimensional hydrodynamic flow model discussed in Part I. Platelet binding efficiencies predicted by PAD are in good agreement with platelet aggregation behavior observed experimentally, as documented in the literature. Deviations from average vWF ligand size or healthy GPIbalpha-vWF-A1 binding kinetics are observed in simulations to have significant effects on the dynamics of transient platelet aggregation, i.e., the efficiency of platelet aggregation and characteristics of bond failure, in ways that typify diseased conditions. The GPIbalpha-vWF-A1 bond formation rate is predicted to have piecewise linear dependence on the prevailing fluid shear rate, with a sharp transition in fluid shear dependency at 7200 s(-1). Interplatelet bond force-loading is found to be complex and highly nonlinear. These results demonstrate PAD as a powerful predictive modeling tool for elucidating platelet adhesive phenomena under flow.

L1503R is a member of group I mutation and has dominant-negative effect on secretion of full-length VWF multimers: an analysis of two patients with type 2A von Willebrand disease

Type 2A von Willebrand disease (VWD) is characterized by decreased platelet-dependent function of von Willebrand factor (VWF); this in turn is associated with an absence of high-molecular-weight multimers. Sequence analysis of the VWF gene from two unrelated type 2A VWD patients showed an identical, novel, heterozygous T-->G transversion at nucleotide 4508, resulting in the substitution of L1503R in the VWF A2 domain. This substitution, which was not found in 60 unrelated normal individuals, was introduced into a full-length VWF cDNA and subsequently expressed in 293T cells. Only trace amount of the mutant VWF protein was secreted but most of the same was retained in 293T cells. Co-transfection experiment of both wild-type and mutant plasmids indicated the dominant-negative mechanism of disease development; as more of mutant DNA was transfected, VWF secretion was impaired in the media, whereas more of VWF was stored in the cell lysates. Molecular dynamic simulations of structural changes induced by L1503R indicated that the mean value of all-atom root-mean-squared-deviation was shifted from those with wild type or another mutation L1503Q that has been reported to be a group II mutation, which is susceptible to ADAMTS13 proteolysis. Protein instability of L1503R may be responsible for its intracellular retention and perhaps the larger VWF multimers, containing more mutant VWF subunits, are likely to be mal-processed and retained within the cell.

Platelet aggregation and thrombosis in xenotransplantation between pigs and humans

Allografts are currently short for clinical potential recipients. Organs and tissues from pigs could be a potential alternative source for clinical transplantation because of their high similarity in anatomical and physiological aspects. Thrombosis could be a consequence of the immunological response or the physiological incompatibilities in cell and molecular levels across species. Platelets play an essential role in haemostasis and the incompatibility of platelets between pigs and humans could be related to rejection and dysfunction of xenografts. Pig blood components, including plasma, leukocytes, red blood cells and platelets, could induce aggregation of human platelets directly, which then resulted in severe thrombosis after xenotransplantation. On the other hand, the existence of potential incompatibilities in coagulation and fibrinolytic system between pigs and humans in the context of xenotransplantation is an important consideration. Here we reviewed platelet incompatibility between pigs and humans related to thrombosis after xenotransplantation, and contribution of immunosuppressive agents to minimizing thrombosis and rejection.

Modifying murine von Willebrand factor A1 domain for in vivo assessment of human platelet therapies

The A1 domain of von Willebrand factor (VWF-A1) plays a crucial role in hemostasis and thrombosis by initiating platelet adhesion at sites of arterial injury through interactions with the platelet receptor glycoprotein Ib alpha (GPIbalpha). Here we report that murine VWF-A1 supports limited binding of human platelets. However, atomic models of GPIbalpha-VWF-A1 complexes identified an electrostatic 'hot-spot' that, when mutated in murine VWF-A1, switches its binding specificity from mouse to human GPIbalpha. Furthermore, mice expressing this mutant VWF-A1 display a bleeding phenotype that can be corrected by infusion of human platelets. Mechanistically, human platelets correct the phenotype by forming occlusive thrombi, an event that can be abrogated by blockade of GPIbalpha or by the preadministration of inhibitors of platelet activation or adhesion (clopidogrel (Plavix) and abciximab (ReoPro), respectively). Thus, by modifying a protein interface, we have generated a potential biological platform for preclinical screening of antithrombotics that specifically target human platelets.

Two novel monoclonal antibodies to VWFA3 inhibit VWF-collagen and VWF-platelet interactions

BACKGROUND

The interaction of collagen-von Willebrand factor (VWF)-GPIb is essential for platelet adhesion, especially under high shear conditions. VWF, which acts as a bridge between platelets and exposed subendothelium, interacts with collagen through its A3 domain, which is a new target for the antithrombotic agent.

OBJECTIVE

To develop functional blockers that specifically inhibit VWF-dependent adhesion of platelets to collagen under high shear stress.

METHODS

To develop murine antihuman VWF A3 monoclonal antibodies (mAbs) by standard hybridoma technology, and characterize their abilities to block interactions between VWF A3 and collagen as well as platelet function.

RESULTS

Thirty anti-VWF-A3 mAbs were obtained. Among them, two mAbs, designated as SZ-123 and SZ-125, were found to inhibit VWF-collagen type III interaction. SZ-123 and SZ-125 inhibited the binding of purified human VWF (1.5 or 3 mug mL(-1)) to human placenta collagen type III (IC(50) = 0.07 +/- 0.02 and 0.15 +/- 0.03 mug mL(-1), respectively) or to calf skin collagen type III (IC(50) = 0.48 +/- 0.06 and 0.51 +/- 0.07 mug mL(-1), respectively) coated on plates. Under flow shear condition (1000 s(-1)), SZ-123 and SZ-125 inhibited platelet adhesion on human placenta collagen- or calf skin collagen-coated surfaces. Both mAbs also inhibited platelet aggregation induced by ristocetin, botrocetin or bovine plasma.

CONCLUSIONS

SZ-123 and SZ-125 inhibited VWF-collagen and VWF-platelet interactions.

Adhesion mechanisms in platelet function

Platelet adhesion is an essential function in response to vascular injury and is generally viewed as the first step during which single platelets bind through specific membrane receptors to cellular and extracellular matrix constituents of the vessel wall and tissues. This response initiates thrombus formation that arrests hemorrhage and permits wound healing. Pathological conditions that cause vascular alterations and blood flow disturbances may turn this beneficial process into a disease mechanism that results in arterial occlusion, most frequently in atherosclerotic vessels of the heart and brain. Besides their relevant role in hemostasis and thrombosis, platelet adhesive properties are central to a variety of pathophysiological processes that extend from inflammation to immune-mediated host defense and pathogenic mechanisms as well as cancer metastasis. All of these activities depend on the ability of platelets to circulate in blood as sentinels of vascular integrity, adhere where alterations are detected, and signal the abnormality to other platelets and blood cells. In this respect, therefore, platelet adhesion to vascular wall structures, to one another (aggregation), or to other blood cells, represent different aspects of the same fundamental biological process. Detailed studies by many investigators over the past several years have been aimed to dissect the complexity of these functions, and the results obtained now permit an attempt to integrate all the available information into a picture that highlights the balanced diversity and synergy of distinct platelet adhesive interactions.

Blood flow devices in medical research and clinical testing in humans: are we approaching personalized medicine?

This review focuses on studies of blood flow devices employed in man to unravel the mechanisms of bleeding and thrombotic disorders, and on the characterization of novel experimental antithrombotic entities and drug candidates in biopharmaceutical research and development. Clinical studies with drug candidates and new therapeutic strategies have also been performed, and the predictability of these experimental approaches to clinical situations is excellent. Based on the solid validation of these flow devices, miniature flow devices employing nonanticoagulated blood drawn directly from an antecubital vein should be developed for diagnostic purposes. It is anticipated that such a diagnostic flow device could develop into a personalized medicine approach.

Cav3.1 (alpha1G) controls von Willebrand factor secretion in rat pulmonary microvascular endothelial cells

The T-type Ca2+ channel Cav3.1 subunit is present in pulmonary microvascular endothelial cells (PMVECs), but not in pulmonary artery endothelial cells (PAECs). The present study sought to assess the role of Cav3.1 in thrombin-induced Weibel-Palade body exocytosis and consequent von Willebrand factor (VWF) release. In PMVECs and PAECs transduced with a green fluorescent protein (GFP)-tagged VWF chimera, we examined the real-time dynamics and secretory process of VWF-GFP-containing vesicles in response to thrombin and the cAMP-elevating agent isoproterenol. Whereas thrombin stimulated a progressive decrease in the number of VWF-GFP-containing vesicles in both cell types, isoproterenol only decreased the number of VWF-GFP-containing vesicles in PAECs. In PMVECs, thrombin-induced decrease in the number of VWF-GFP-containing vesicles was nearly abolished by the T-type Ca2+ channel blocker mibefradil as well as by Cav3.1 gene silencing with small hairpin RNA. Expression of recombinant Cav3.1 subunit in PAECs resulted in pronounced increase in thrombin-stimulated Ca2+ entry, which is sensitive to mibefradil. Together, these data indicate that VWF secretion from lung endothelial cells is regulated by two distinct pathways involving Ca2+ or cAMP, and support the hypothesis that activation of Cav3.1 T-type Ca2+ channels in PMVECs provides a unique cytosolic Ca2+ source important for Gq-linked agonist-induced VWF release.

ADAP is required for normal alphaIIbbeta3 activation by VWF/GP Ib-IX-V and other agonists

Interaction between von Willebrand factor (VWF) and platelet GP Ib-IX-V is required for hemostasis, in part because intracellular signals from VWF/GP Ib-IX-V activate the ligand-binding function of integrin alphaIIbbeta3. Because they also induce tyrosine phosphorylation of the ADAP adapter, we investigated ADAP's role in GP Ib-IX-V signal transduction. Fibrinogen or ligand-mimetic POW-2 Fab binding to alphaIIbbeta3 was stimulated by adhesion of ADAP+/+ murine platelets to dimeric VWF A1A2 but was significantly reduced in ADAP-/- platelets (P<.01). alphaIIbbeta3 activation by ADP or a Par4 thrombin receptor agonist was also decreased in ADAP-/- platelets. ADAP stabilized the expression of another adapter, SKAP-HOM, via interaction with the latter's SH3 domain. However, no abnormalities in alphaIIbbeta3 activation were observed in SKAP-HOM-/- platelets, which express normal ADAP levels, further implicating ADAP as a modulator of alphaIIbbeta3 function. Under shear flow conditions over a combined surface of VWF A1A2 and fibronectin to test interactions involving GP Ib-IX-V and alphaIIbbeta3, respectively, ADAP-/- platelets displayed reduced alphaIIbbeta3-dependent stable adhesion. Furthermore, ADAP-/- mice demonstrated increased rebleeding from tail wounds. These studies establish ADAP as a component of inside-out signaling pathways that couple GP Ib-IX-V and other platelet agonist receptors to alphaIIbbeta3 activation.

Identification of a 2-stage platelet aggregation process mediating shear-dependent thrombus formation

Disturbances of blood flow at sites of atherosclerotic plaque rupture are one of the key pathogenic events promoting platelet activation and arterial thrombus formation. Shear effects of platelets have been extensively investigated in vitro; however, the mechanisms by which shear promotes platelet aggregation in vivo remain incompletely understood. By employing high-resolution imaging techniques to in vitro and in vivo thrombosis models, we demonstrate a unique mechanism initiating shear-dependent platelet aggregation involving aggregate formation between discoid platelets. These discoid platelet aggregates are initially unstable and result from the development of membrane tethers between coadhering platelets. Tether formation involves the adhesive function of GPIb/V/IX and integrin alphaIIbbeta3, and conversion of discoid platelet aggregates into stable aggregates requires released ADP. The efficiency of this process is regulated by 3 independent variables, including the reactivity of the adhesive substrate, the level of shear flow, and the platelet density at the adhesive surface. These studies identify a new mechanism initiating platelet aggregation that is critically influenced by shear, physical proximity between translocating platelets, and membrane tether formation. Moreover, they provide a model to explain how the discoid morphology of platelets facilitates the maintenance of adhesive interactions with thrombogenic surfaces under high shear stress conditions.

Prostaglandins in Health and Disease: An Overview

OBJECTIVES

Prostaglandins are a group of biologically active compounds that play major roles in human physiology in both health and disease. They function in many different ways and in all major organs. This article reviews the basic physiology of prostaglandins and their application to specific effects on these systems in normal and abnormal clinical states. The critical therapeutic implications of the use of nonsteroidal antiinflammatory drugs in altering organ homeostasis are also examined.

METHODS

References were taken from Medline, Embase, and Index Medicus from 1966 to September 2005. A search was done with keywords, including prostaglandins, nonsteroidal antiinflammatory drugs, inflammation, arachidonic acid, Cox-1 (cyclooxygenase-1), and Cox-2 (cyclooxygenase-2).

RESULTS

There was a close correlation and predictability between basic prostaglandin physiology and the anticipated effects of these compounds on the heart, lungs, kidneys, gastrointestinal tract, bones and joints, brain, and male and female reproductive systems. These effects are organ and tissue specific. Despite these findings, unexplained and sometimes paradoxical physiologic responses were identified. A prime example of this is the role of prostaglandins in bone metabolism demonstrating both stimulatory and inhibitory effects. In addition all NSAIDs have the potential to impair the normal physiologic effects of prostaglandins depending primarily on the specific organ and the clinical setting.

CONCLUSIONS

Prostaglandins are regulatory compounds that play important roles in many physiologic processes in the human body. An understanding of the basic science of prostaglandins is valuable in anticipating the organ-specific biologic effects of these unique compounds in health and disease. However, at selected sites and under different physiologic conditions, unexplained and sometimes paradoxical effects are generated. Impairment of their regulatory functions can lead to significant short- or long-term organ dysfunction.

Activation-independent platelet adhesion and aggregation under elevated shear stress

Platelet aggregation, which contributes to bleeding arrest and also to thrombovascular disorders, is thought to initiate after signaling-induced activation. We found that this paradigm does not apply under blood flow conditions comparable to those existing in stenotic coronary arteries. Platelets interacting with immobilized von Willebrand factor (VWF) aggregate independently of activation when soluble VWF is present and the shear rate exceeds 10 000 s(-1) (shear stress = 400 dyn/cm(2)). Above this threshold, active A1 domains become exposed in soluble VWF multimers and can bind to glycoprotein Ibalpha, promoting additional platelet recruitment. Aggregates thus formed are unstable until the shear rate approaches 20 000 s(-1) (shear stress = 800 dyn/cm.(2)). Above this threshold, adherent platelets at the interface of surface-immobilized and membrane-bound VWF are stretched into elongated structures and become the core of aggregates that can persist on the surface for minutes. When isolated dimeric A1 domain is present instead of native VWF multimers, activation-independent platelet aggregation occurs without requiring shear stress above a threshold level, but aggregates never become firmly attached to the surface and progressively disaggregate as shear rate exceeds 6000 s(-1). Platelet and VWF modulation by hydrodynamic force is a mechanism for activation-independent aggregation that may support thrombotic arterial occlusion.

Von Willebrand factor and thrombosis

There is increasing evidence that von Willebrand factor (VWF), an adhesive multimeric protein that has an important function in primary hemostasis and as a carrier of factor VIII, has a pivotal role in thrombogenesis. In fact, while the presence in plasma of unusually large VWF multimers due to a congenital or acquired deficiency of a VWF-cleaving metalloprotease has been implicated in the pathogenesis of thrombotic thrombocytopenic purpura (TTP), high plasma levels of VWF have been associated with a slightly increased risk of arterial thrombosis. With regard to the association between VWF and venous thrombosis, clear conclusions cannot yet be drawn from the conflicting published data. Patients with von Willebrand disease, an inherited hemorrhagic disorder, may also paradoxically experience thrombotic events as a result of interactions among multiple prothrombotic risk factors. After a description of the structure and physiology of VWF, all these aspects are discussed in the present review.

Identification of amino acid residues essential for heparin binding by the A1 domain of human von Willebrand factor

Platelet adhesion is mediated by von Willebrand factor (VWF) that binds platelet glycoprotein Ib (GPIb). Previous observations suggested that heparin competitively inhibits the binding of VWF to GPIb and may down-regulate platelet adhesion. We performed charged-to-alanine scanning mutagenesis of domain A1 and studied dose-dependent binding to heparin-Sepharose beads. Mutations at Lys1362 and Arg1395, at which the GPIb binding was markedly decreased, showed 41% and 42% binding, respectively. Clustered mutations in the segments 1332KDRKR1336 and 1405KKKK1408, which have been proposed as heparin binding sequences, showed 72% and 52% binding, respectively. However, single alanine substitutions within these clusters showed normal binding. Our findings suggest that heparin may inhibit the binding of VWF to GPIb by interacting with GPIb binding and interpret why some hemorrhagic complications of heparin therapy are not predictable based on techniques for monitoring the conventional anticoagulant effects of heparin.

An AP-1/clathrin coat plays a novel and essential role in forming the Weibel-Palade bodies of endothelial cells

Clathrin provides an external scaffold to form small 50-100-nm transport vesicles. In contrast, formation of much larger dense-cored secretory granules is driven by selective aggregation of internal cargo at the trans-Golgi network; the only known role of clathrin in dense-cored secretory granules formation is to remove missorted proteins by small, coated vesicles during maturation of these spherical organelles. The formation of Weibel-Palade bodies (WPBs) is also cargo driven, but these are cigar-shaped organelles up to 5 mum long. We hypothesized that a cytoplasmic coat might be required to make these very different structures, and we found that new and forming WPBs are extensively, sometimes completely, coated. Overexpression of an AP-180 truncation mutant that prevents clathrin coat formation or reduced AP-1 expression by small interfering RNA both block WPB formation. We propose that, in contrast to other secretory granules, cargo aggregation alone is not sufficient to form immature WPBs and that an external scaffold that contains AP-1 and clathrin is essential.

von Willebrand factor and factor VIII as risk factors for arterial and venous thrombosis

Since the early 1990s attempts have been made to elucidate whether high concentrations of von Willebrand factor (VWF) and factor VIII (FVIII) in plasma are associated with an increased risk of thrombosis. Several prospective studies on the role of VWF in arterial thrombosis, mainly coronary heart disease, were performed in healthy individuals and patients with previous cardiovascular disease. Although the majority showed an association between high VWF levels and arterial thrombosis, others failed to confirm such findings. A smaller number of studies have evaluated FVIII, mainly for its association with venous thrombosis. Two prospective observations, together with several case-control studies, provided solid evidence of an association between high FVIII levels and a first or recurrent episode of venous thrombosis. On the whole, high levels of VWF and FVIII in plasma confer a moderately high risk of arterial and venous thrombosis, respectively. These findings have no therapeutic implication, but they should be taken into account in the assessment of the individual risk profile.

Effects of a new pathogen-reduction technology (Mirasol PRT) on functional aspects of platelet concentrates

BACKGROUND

Several strategies are being developed to reduce the risk of pathogen transmission associated with platelet (PLT) transfusion.

STUDY DESIGN AND METHODS

The impact of a new technology for pathogen reduction based on riboflavin plus illumination (Mirasol PRT, Navigant Biotechnologies, Inc.) at 6.2 and 12.3 J per mL on functional and biochemical characteristics of PLTs was evaluated. PLT concentrates (PCs) obtained by apheresis were treated with Mirasol PRT and stored at 22 degrees C. Modifications in major PLT glycoproteins (GPIbalpha, GPIV, and GPIIb-IIIa), adhesive ligands (von Willebrand factor [VWF], fibrinogen [Fg], and fibronectin), activation antigens (P-selectin and LIMP), and apoptotic markers (annexin V binding and factor [F]Va) were analyzed by flow cytometry. Adhesive and cohesive PLT functions were evaluated with well-established perfusion models. Studies were performed on the preparation day (Day 0) and during PCs storage (Days 3 and 5).

RESULTS

Levels of glycoproteins remained stable during storage in PCs treated with 6.2 J per mL pathogen reduction technology (PRT) and similar to those observed in nontreated PCs. When 12.3 J per mL PRT was applied, however, levels of GPIbalpha moderately decreased on Days 3 and 5. VWF, Fg, and FVa were not modified in their expression levels, either by treatment or by storage period. Fibronectin appeared more elevated in all PRT samples. A progressive increase in P-selectin and LIMP expression and in annexin V binding was observed during storage of PRT-treated PCs. Functional studies indicated that 6.2 J per mL Mirasol PRT-treated PLTs preserved adhesive and cohesive functions to levels compatible with those observed in the respective control PCs.

CONCLUSION

PLT function was well preserved in PCs treated with 6.2 J per mL Mirasol PRT and stored for 5 days.

The von Willebrand factor self-association is modulated by a multiple domain interaction

BACKGROUND

Platelet adhesion and aggregation at sites of vascular injury exposed to rapid blood flow require von Willebrand factor (VWF). VWF becomes immobilized by binding to subendothelial components or by a self-association at the interface of soluble and surface-bound VWF.

OBJECTIVES

As this self-association has been demonstrated only under shear conditions, our first goal was to determine whether the same interaction could be observed under static conditions. Furthermore, we wanted to identify VWF domain(s) important for this self-association.

RESULTS

Biotinylated VWF (b-VWF) interacted dose-dependently and specifically with immobilized VWF in an enzyme-linked immunosorbent assay (ELISA) assay, showing that shear is not necessary to induce the VWF self-association. Whereas anti-VWF monoclonal antibodies (mAbs) had no effect on the self-association, the proteolytic VWF-fragments SpII(1366-2050) and SpIII(1-1365) inhibited the b-VWF-VWF interaction by 70 and 80%, respectively. Moreover, a specific binding of b-VWF to immobilized Sp-fragments was demonstrated. Finally, both biotinylated SpII and SpIII were able to bind specifically to both immobilized SpII and SpIII. Similar results were observed under flow conditions, which confirmed the functional relevance of our ELISA system.

CONCLUSION

We have developed an ELISA binding assay in which a specific VWF self-association under static conditions can be demonstrated. Our results suggest a multiple domain interaction between immobilized and soluble VWF.