Regulation of arterial thrombolysis by plasminogen activator inhibitor-1 in mice

Overexpression of the cell cycle inhibitor p16INK4a promotes a prothrombotic phenotype following vascular injury in mice

OBJECTIVE

Age-associated cellular senescence is thought to promote vascular dysfunction. p16(INK4a) is a cell cycle inhibitor that promotes senescence and is upregulated during normal aging. In this study, we examine the contribution of p16(INK4a) overexpression to venous thrombosis.

METHODS AND RESULTS

Mice overexpressing p16(INK4a) were studied with 4 different vascular injury models: (1) ferric chloride (FeCl(3)) and (2) Rose Bengal to induce saphenous vein thrombus formation; (3) FeCl(3) and vascular ligation to examine thrombus resolution; and (4) lipopolysaccharide administration to initiate inflammation-induced vascular dysfunction. p16(INK4a) transgenic mice had accelerated occlusion times (13.1 ± 0.4 minutes) compared with normal controls (19.7 ± 1.1 minutes) in the FeCl(3) model and 12.7 ± 2.0 and 18.6 ± 1.9 minutes, respectively in the Rose Bengal model. Moreover, overexpression of p16(INK4a) delayed thrombus resolution compared with normal controls. In response to lipopolysaccharide treatment, the p16(INK4a) transgenic mice showed enhanced thrombin generation in plasma-based calibrated automated thrombography assays. Finally, bone marrow transplantation studies suggested increased p16(INK4a) expression in hematopoietic cells contributes to thrombosis, demonstrating a role for p16(INK4a) expression in venous thrombosis.

CONCLUSIONS

Venous thrombosis is augmented by overexpression of the cellular senescence protein p16(INK4a).

Use of mouse models to study plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the main inhibitor of tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA) and therefore plays an important role in the plasminogen/plasmin system. PAI-1 is involved in a variety of cardiovascular diseases (mainly through inhibition of t-PA) as well as in cell migration and tumor development (mainly through inhibition of u-PA and interaction with vitronectin). PAI-1 is a unique member of the serpin superfamily, exhibiting particular unique conformational and functional properties. Since its involvement in various biological and pathophysiological processes PAI-1 has been the subject of many in vivo studies in mouse models. We briefly discuss structural and physiological differences between human and mouse PAI-1 that should be taken into account prior to extrapolation of data obtained in mouse models to the human situation. The current review provides an overview of the various models, with a focus on cardiovascular disease and cancer, using wild-type mice or genetically modified mice, either deficient in PAI-1 or overexpressing different variants of PAI-1.

Feedback regulation of endothelial cell surface plasmin generation by PKC-dependent phosphorylation of annexin A2

In response to blood vessel injury, hemostasis is initiated by platelet activation, advanced by thrombin generation, and tempered by fibrinolysis. The primary fibrinolytic protease, plasmin, can be activated either on a fibrin-containing thrombus or on cells. Annexin A2 (A2) heterotetramer (A2·p11)(2) is a key profibrinolytic complex that assembles plasminogen and tissue plasminogen activator and promotes plasmin generation. We now report that, in endothelial cells, plasmin specifically induces activation of conventional PKC, which phosphorylates serine 11 and serine 25 of A2, triggering dissociation of the (A2·p11)(2) tetramer. The resulting free p11 undergoes ubiquitin-mediated proteasomal degradation, thus preventing further translocation of A2 to the cell surface. In vivo, pretreatment of A2(+/+) but not A2(-/-) mice with a conventional PKC inhibitor significantly reduced thrombosis in a carotid artery injury model. These results indicate that augmentation of fibrinolytic vascular surveillance by blockade of serine phosphorylation is A2-dependent. We also demonstrate that plasmin-induced phosphorylation of A2 requires both cleavage of A2 and activation of Toll-like receptor 4 on the cell surface. We propose that plasmin can limit its own generation by triggering a finely tuned "feedback" mechanism whereby A2 becomes serine-phosphorylated, dissociates from p11, and fails to translocate to the cell surface.

Tissue factor and thrombosis: The clot starts here

Thrombosis, or complications from thrombosis, currently occupies the top three positions in the cardiovascular causes of morbidity and mortality in the developed world. There are a limited number of safe and effective drugs to prevent and treat thrombosis. Animal models of thrombosis are necessary to better understand the complex components and interactions involved in the formation of a clot. Tissue factor (TF) is required for the initiation of blood coagulation and likely plays a key role in both arterial and venous thrombosis. Understanding the role of TF in thrombosis may permit the development of new antithrombotic drugs. This review will focus on the role of TF in in vivo models of thrombosis.

Optimization, refinement and reduction of murine in vivo experiments to assess therapeutic approaches for haemophilia A

The tail cut bleeding model (CUT) is routinely used in factor VIII-deficient mice to assess pharmacodynamic effects of therapeutic strategies for haemophilia A. Results from this model are highly variable, many modifications to the model are reported and at times the animals' wellbeing may be compromised by recording survival as an endpoint. We therefore investigated if the ferric chloride carotid occlusion model (COM) used for thrombosis research can be applied to enhance data quality and animal welfare in haemophilia A research. Relative dose effects and relative dose variations were calculated for the CUT and COM. The requisite sample sizes were estimated and the importance of survival rates to assess rebleeds during recovery was evaluated by correlating initial blood loss to mortality. Relative dose effects increased with higher doses in both models. The COM was more sensitive at lower doses than the CUT, had up to 82% less variation across doses and clearly showed superior accuracy. Only 5% of the sample size required for the CUT would be needed to establish non-inferiority between a specific therapeutic dose in haemophilia A mice and healthy wild-type animals. A strong statistically significant correlation was found between initial blood loss and mortality within 24 h. Our findings clearly suggest that the COM is a valid tool for assessing haemophilia A treatment in vivo. The highly reproducible data means that significantly fewer animals are required and a more humane endpoint can be used by directly assessing clot stability instead of survival rate.

Does plasmin have anticoagulant activity?

The coagulation and fibrinolytic pathways regulate hemostasis and thrombosis, and an imbalance in these pathways may result in pathologic hemophilia or thrombosis. The plasminogen system is the primary proteolytic pathway for fibrinolysis, but also has important proteolytic functions in cell migration, extracellular matrix degradation, metalloproteinase activation, and hormone processing. Several studies have demonstrated plasmin cleavage and inactivation of several coagulation factors, suggesting plasmin may be not only be the primary fibrinolytic enzyme, but may have anticoagulant properties as well. The objective of this review is to examine both in vitro and in vivo evidence for plasmin inactivation of coagulation, and to consider whether plasmin may act as a physiological regulator of coagulation. While several studies have demonstrated strong evidence for plasmin cleavage and inactivation of coagulation factors FV, FVIII, FIX, and FX in vitro, in vivo evidence is lacking for a physiologic role for plasmin as an anticoagulant. However, inactivation of coagulation factors by plasmin may be useful as a localized anticoagulant therapy or as a combined thrombolytic and anticoagulant therapy.

Biological effects of fucoidan isolated from Fucus vesiculosus on thrombosis and vascular cells

Background

Fucoidan is a highly sulfated glycosaminoglycan, which has a molecular structure similar to that of heparin. The antithrombotic effects of fucoidan in vitro have been widely reported, but its antithrombotic effects in vivo as well as its other biological properties in vitro have not been well investigated.

Methods

This study investigated the effects and mechanism of fucoidan from Fucus vesiculosus on thrombosis both in vitro and in vivo. A ferric chloride-induced mouse carotid artery thrombosis model was used to determine the antithrombotic effects of fucoidan in vivo. Additionally, changes in the levels of proinflammatory cytokines and chemokines were examined in vascular cells treated with fucoidan.

Results

In vivo studies employing a ferric chloride-induced mouse carotid artery thrombosis model indicated that fucoidan had a stronger antithrombotic activity than heparin. Further, vascular cells treated with fucoidan demonstrated a decrease in proinflammatory cytokine and chemokine production as well as inhibition of proliferation.

Conclusion

The major findings of this study showed that fucoidan has a stronger antithrombotic effect than heparin in vivo and that fucoidan has an inhibitory effect on proinflammatory cytokine production and proliferation of vascular cells.

Plasminogen activator inhibitor-1 regulates myoendothelial junction formation

RATIONALE

Plasminogen activator inhibitor-1 (PAI-1) is a biomarker for several vascular disease states; however, its target of action within the vessel wall is undefined.

OBJECTIVE

Determine the ability of PAI-1 to regulate myoendothelial junction (MEJ) formation.

METHODS AND RESULTS

MEJs are found throughout the vasculature linking endothelial cells (ECs) and vascular smooth muscle cells. Using a vascular cell coculture we isolated MEJ fractions and performed two-dimensional differential gel electrophoresis. Mass spectrometry identified PAI-1 as being enriched within MEJ fractions, which we confirmed in vivo. In the vascular cell coculture, recombinant PAI-1 added to the EC monolayer significantly increased MEJs. Conversely, addition of a PAI-1 monoclonal antibody to the EC monolayer reduced the number of MEJs. This was also observed in vivo where mice fed a high fat diet had increased PAI-1 and MEJs and the number of MEJs in coronary arterioles of PAI-1(-/-) mice was significantly reduced when compared to C57Bl/6 mice. The presence of MEJs in PAI-1(-/-) coronary arterioles was restored when their hearts were transplanted into and exposed to the circulation of C57Bl/6 mice. Application of biotin-conjugated PAI-1 to the EC monolayer in vitro confirmed the ability of luminal PAI-1 to translocate to the MEJ. Functionally, phenylephrine-induced heterocellular calcium communication in the vascular cell coculture was temporally enhanced when recombinant PAI-1 was present, and prolonged when PAI-1 was absent.

CONCLUSION

Our data implicate circulating PAI-1 as a key regulator of MEJ formation and a potential target for pharmacological intervention in diseases with vascular abnormalities (eg, diabetes mellitus).

Homocysteine inhibits neoangiogenesis in mice through blockade of annexin A2-dependent fibrinolysis

When plasma levels of homocysteine (HC), a thiol amino acid formed upon methionine demethylation, exceed 12 muM, individuals are at increased risk of developing large vessel atherothrombosis and small vessel dysfunction. The annexin A2 complex (termed "A2") is the cell surface coreceptor for plasminogen and TPA and accelerates the catalytic activation of plasmin, the major fibrinolytic agent in mammals. We previously showed that HC prevents A2-mediated, TPA-dependent activation of plasminogen in vitro by disulfide derivatization of the "tail" domain of A2. We also demonstrated that fibrinolysis and angiogenesis are severely impaired in A2-deficient mice. We now report here that, although hyperhomocysteinemic mice had a normal coagulation profile and normal platelet function, fibrin accumulated in their tissues due to reduced perivascular fibrinolytic activity and angiogenesis was impaired. A2 isolated from hyperhomocysteinemic mice failed to fully support TPA-dependent plasmin activation. However, infusion of hyperhomocysteinemic mice with fresh recombinant A2, which localized to neoangiogenic endothelial cells, resulted in normalization of angiogenesis and disappearance of peri- and intravascular fibrin. We therefore conclude that hyperhomocysteinemia impairs postnatal angiogenesis by derivatizing A2, preventing perivascular fibrinolysis, and inhibiting directed endothelial cell migration. These findings provide a mechanistic explanation for microvascular dysfunction and macrovascular occlusion in individuals with hyperhomocysteinemia.

In vivo veritas: Thrombosis mechanisms in animal models

Experimental models have enhanced our understanding of atherothrombosis pathophysiology and have played a major role in the search for adequate therapeutic interventions. Various animal models have been developed to simulate thrombosis and to study in vivo parameters related to hemodynamics and rheology that lead to thrombogenesis. Although no model completely mimics the human condition, much can be learned from existing models about specific biologic processes in disease causation and therapeutic intervention. In general, large animals such as pigs and monkeys have been better suited to study atherosclerosis and arterial and venous thrombosis than smaller species such as rats, rabbits, and dogs. On the other hand, mouse models of arterial and venous thrombosis have attracted increasing interest over the past two decades, owing to direct availability of a growing number of genetically modified mice, improved technical feasibility, standardization of new models of local thrombosis, and low maintenance costs. To simulate rupture of an atherosclerotic plaque, models of arterial thrombosis often involve vascular injury, which can be achieved by several means. There is no animal model that is sufficiently tall, that can mimic the ability of humans to walk upright, and that possesses the calf muscle pump that plays an important role in human venous hemodynamics. A number of spontaneous or genetically engineered animals with overexpression or deletion of various elements in the coagulation, platelet, and fibrinolysis pathways are now available. These animal models can replicate important aspects of thrombosis in humans, and provide a valuable resource in the development of novel concepts of disease mechanisms in human patients.

Antithrombotic and antiplatelet activities of fenofibrate, a lipid-lowering drug

Fenofibrate, a lipid-lowering drug, inhibits hydroxyl-methylglutaryl coenzyme A (HMG-CoA)-reductase activity, thus reducing cholesterol synthesis and increasing the clearance of circulating LDL-cholesterol via the high affinity receptor system. In addition, fenofibrate has beneficial effects such as the inhibition of tissue factor expression, antithrombotic effect and anti-inflammatory effect. The aim of this study was to investigate the effects of fenofibrate on thrombus formation in vivo and platelet activation in vitro and ex vivo. The carotid arteries of male Sprague-Dawley rats were subjected to chemical injury by FeCl(3), and then blood flow was measured with a blood flowmeter. Fenofibrate (200 and 400mg/kg/day for 1 week) delayed the time to occlusion by 61.3% (p<0.05, n=10) and 90.7% (p<0.01, n=10), respectively. Fenofibrate also significantly inhibited ex vivo platelet aggregations induced by collagen (7.5microg/ml) (p<0.01, n=11) and ADP (10microM) (p<0.01, n=11), respectively, but did not affect coagulation times following activated partial thromboplastin and prothrombin activation, indicating the antithrombotic effect was mediated by its inhibition on platelet activation rather than coagulation system. This antiplatelet activity was revealed to be mediated by the suppression of thromboxane A(2) receptor, cytosolic calcium mobilization, and cyclooxygenase (COX)-1 activity. Taken together, we demonstrate that fenofibrate can significantly inhibit artery thrombus formation in vivo, which may be due to antiplatelet activity via the inhibition of thromboxane A(2) receptor, cytosolic calcium mobilization and COX-1 activity, and the beneficial effect of fenofibrate on cardiovascular system may be also due to its modulation of platelet activation.

Delta6-desaturase (FADS2) deficiency unveils the role of omega3- and omega6-polyunsaturated fatty acids

Mammalian cell viability is dependent on the supply of the essential fatty acids (EFAs) linoleic and alpha-linolenic acid. EFAs are converted into omega3- and omega6-polyunsaturated fatty acids (PUFAs), which are essential constituents of membrane phospholipids and precursors of eicosanoids, anandamide and docosanoids. Whether EFAs, PUFAs and eicosanoids are essential for cell viability has remained elusive. Here, we show that deletion of delta6-fatty acid desaturase (FADS2) gene expression in the mouse abolishes the initial step in the enzymatic cascade of PUFA synthesis. The lack of PUFAs and eicosanoids does not impair the normal viability and lifespan of male and female fads2 -/- mice, but causes sterility. We further provide the molecular evidence for a pivotal role of PUFA-substituted membrane phospholipids in Sertoli cell polarity and blood-testis barrier, and the gap junction network between granulosa cells of ovarian follicles. The fads2 -/- mouse is an auxotrophic mutant. It is anticipated that FADS2 will become a major focus in membrane, haemostasis, inflammation and atherosclerosis research.

Small-molecule inhibitors of integrin alpha2beta1 that prevent pathological thrombus formation via an allosteric mechanism

There is a grave need for safer antiplatelet therapeutics to prevent heart attack and stroke. Agents targeting the interaction of platelets with the diseased vessel wall could impact vascular disease with minimal effects on normal hemostasis. We targeted integrin alpha(2)beta(1), a collagen receptor, because its overexpression is associated with pathological clot formation whereas its absence does not cause severe bleeding. Structure-activity studies led to highly potent and selective small-molecule inhibitors. Responses of integrin alpha(2)beta(1) mutants to these compounds are consistent with a computational model of their mode of inhibition and shed light on the activation mechanism of I-domain-containing integrins. A potent compound was proven efficacious in an animal model of arterial thrombosis, which demonstrates in vivo efficacy for inhibition of this platelet receptor. These results suggest that targeting integrin alpha(2)beta(1) could be a potentially safe, effective approach to long-term therapy for cardiovascular disease.

Overview of murine thrombosis models

There are a myriad of options on where and how to perform thrombosis studies in mice. Models have been developed for systemic thrombosis, larger and smaller vessels of both the arterial and venous systems as well as several different microvascular beds. However, there are important differences between the models and investigators need to be careful and thoughtful when they choose which model to use.

Evaluation of the physiological significance of botrocetin/ von Willebrand factor in vitro signaling

BACKGROUND

A signaling pathway is difficult, if not impossible, to elucidate in platelets using only in vivo studies. Likewise, the physiological significance of signaling information obtained exclusively from in vitro observations is unknown. Therefore, both in vitro and in vivo experiments are required to establish the physiological significance of a signaling pathway.

OBJECTIVE

To evaluate the physiological significance of signaling data obtained from botrocetin (bt)/von Willebrand factor (VWF)-stimulated washed platelets.

METHOD

Stable thrombus formation in response to FeCl(3)-induced injury of the mouse carotid artery was used to evaluate the physiological significance of signaling data obtained from bt/VWF-stimulated washed platelets.

RESULTS

Syk, PLCgamma2, Galphaq and P2Y12, but not LAT, were found either to be required for or to affect stable thrombus formation. Prior in vitro studies had demonstrated that LAT is not required for bt/VWF-induced platelet aggregation in the presence of exogenous fibrinogen. These data provide the first demonstration of the in vivo role for these signaling molecules in GPIb-dependent/initiated signal transduction and are consistent with the signaling pathway deduced from in vitro studies of bt/VWF-stimulated washed platelets using metabolic inhibitors and knockout mice.

CONCLUSION

The broad agreement between the in vitro and the in vivo results establish that bt/VWF stimulation of washed platelets can provide physiologically significant glycoprotein Ib-dependent/initiated signaling data.

The effects of pharmacological PAI-1 inhibition on thrombus formation and neointima formation after arterial injury

OBJECTIVE

Plasminogen activator inhibitor (PAI)-1 plays a role in neointimal formation after percutaneous coronary intervention (PCI), the effect of overexpression or lack of PAI-1 is controversial. Murine arterial injury models develop neointimal hyperplasia similar to that observed in clinical coronary arterial restenosis after PCI.

METHODS AND RESULTS

To clarify the role of PAI-1 in thrombus formation and neointimal formation after arterial injury, we used a specific PAI-1 inhibitor (IMD-1622) in a rat aorta-vein shunt model and a mouse arterial injury model. While the non-treated shunt model showed massive thrombus formation, IMD-1622 administration suppressed this. Injured arteries with vehicles showed significant neointimal formation with enhancement of adhesion molecules, fibrinogen accumulation and cell proliferation on day 28 after injury. However, intimal thickening and expression of these factors were suppressed in PAI-1 recipients.

CONCLUSION

A specific PAI-1 inhibitor prevents thrombus formation and arterial neointimal formation after arterial injury. Thus, PAI-1 plays a critical role in arterial remodeling after mechanical injury. PAI-1 regulation may be useful to prevent thrombus and neointimal formation after PCI.

Long-term expression of murine activated factor VII is safe, but elevated levels cause premature mortality

Intravenous infusion of recombinant human activated Factor VII (FVIIa) has been used for over a decade in the successful management of bleeding episodes in patients with inhibitory antibodies to Factor VIII or Factor IX. Previously, we showed that expression of murine FVIIa (mFVIIa) from an adeno-associated viral (AAV) vector corrected abnormal hemostatic parameters in hemophilia B mice. To pursue this as a therapeutic approach, we sought to define safe and effective levels of FVIIa for continuous expression. In mice transgenic for mFVIIa or injected with AAV-mFVIIa, we analyzed survival, expression levels, in vitro and in vivo coagulation tests, and histopathology for up to 16 months after birth/mFVIIa expression. We found that continuous expression of mFVIIa at levels at or below 1.5 microg/ml was safe, effective, and compatible with a normal lifespan. However, expression levels of 2 microg/ml or higher were associated with thrombosis and early mortality, with pathologic findings in the heart and lungs that were rescued in a low-factor X (low-FX) mouse background, suggesting a FX-mediated effect. The findings from these mouse models of continuous FVIIa expression have implications for the development of a safe gene transfer approach for hemophilia and are consistent with the possibility of thromboembolic risk of continuously elevated FVIIa levels.

Arterial thrombosis in mice with factor V Leiden mutation

The factor V Leiden (FVL) mutation has been demonstrated to be associated with the development of venous thrombosis in humans. Whether such a propensity also exists in the arterial circulation remains controversial. In an effort to minimize the variability that clouds the clinical study of arterial thrombosis, we studied FVL-associated arterial thrombosis in an experimental model of homozygous, heterozygous, and wild-type mice. Heterozygous FVL mice were crossbred to C57BL/6J mice over several generations. The genotypes of the resulting three genotype groups (wild type, heterozygous FVL, and homozygous FVL) were blinded to the investigators. Arterial injury was produced with the injection of ferric chloride into an isolated segment of carotid artery. Arterial thrombosis was assessed with an ultrasonic flow probe and the time to occlusion (TTO) was recorded. The carotid artery occluded within 60 minutes of injury in 72 of the animals studied (97.3%). The carotid artery remained patent at 60 minutes in the remaining two animals, both of whom were subsequently found to be genotypically wild type. There was a statistically significant relationship between TTO and genotype (p = .002). TTO was greatest in the wild-type mice (p < .001 vs heterozygous, < .001 vs homozygous) and least in the homozygotes (p < .001 vs heterozygotes). Increased thrombogenicity is present in mice with the FVL mutation and is more prolonged in homozygotes than heterozygotes. These findings provide some corroboration to the clinical studies that suggest an increased risk of arterial events in patients with the FVL mutation.

Hyperglycemia accelerates arterial thrombus formation and attenuates the antithrombotic response to endotoxin in mice

Recent human studies reveal that hyperglycemia induces procoagulant and antifibrinolytic effects in blood that may contribute to a greater risk of arterial thrombosis, but the direct relationship between high blood glucose levels and thrombosis has not yet been investigated. We performed a number of experiments to clarify whether hyperglycemia was causally related to arterial thrombosis and whether the combined stimulus of hyperglycemia and inflammation would enhance the thrombotic effect. In a model of ferric-chloride-induced carotid artery thrombosis, hyperglycemia did not influence the time to occlusion in mice pretreated with streptozotocin, but the rate of thrombus formation was accelerated. This effect was associated with increased thrombin generation and could not be explained by changes in vessel-wall tissue factor activity. The prothrombotic effect of hyperglycemia was assessed in a separate experiment, showing that collagen/thrombin-induced platelet procoagulant activity was increased in hyperglycemic mice. The effect of inflammation was studied by injecting a low dose of endotoxin that caused a systemic inflammatory state after 24 h (increased plasma levels of tumor necrosis factor alpha, interleukin-6 and monocyte chemotactic protein 1 in diabetic and nondiabetic mice) associated with a mild delay in thrombus formation. This reduced rate of thrombus formation was attenuated by hyperglycemia. Together, these data establish a discrete but clear contribution of hyperglycemia in experimental arterial thrombosis.

In vivo thrombus formation

Thrombus formation, including platelet adhesion, activation, secretion and aggregation as well as tissue factor-initiated thrombin generation and fibrin formation, has been studied in the past using in vitro systems, often with isolated components. Given the complexity of hemostasis and thrombosis, many of the concepts that have been developed to explain these processes are being revisited by studying thrombus formation in live animals using intravital microscopy and genetically altered mice. Although much of the dogma that has evolved has been confirmed by in vivo studies of thrombus formation, there have also been conflicts between old concepts and new direct observations. In vivo studies of the initiation of thrombus formation, platelet accumulation and thrombin generation have provided evidence for the participation of novel proteins and identified new pathways and mechanisms.

Double knockouts reveal that protein tyrosine phosphatase 1B is a physiological target of calpain-1 in platelets

Calpains are ubiquitous calcium-regulated cysteine proteases that have been implicated in cytoskeletal organization, cell proliferation, apoptosis, cell motility, and hemostasis. Gene targeting was used to evaluate the physiological function of mouse calpain-1 and establish that its inactivation results in reduced platelet aggregation and clot retraction potentially by causing dephosphorylation of platelet proteins. Here, we report that calpain-1 null (Capn1-/-) platelets accumulate protein tyrosine phosphatase 1B (PTP1B), which correlates with enhanced tyrosine phosphatase activity and dephosphorylation of multiple substrates. Treatment of Capn1-/- platelets with bis(N,N-dimethylhydroxamido)hydroxooxovanadate, an inhibitor of tyrosine phosphatases, corrected the aggregation defect and recovered impaired clot retraction. More importantly, platelet aggregation, clot retraction, and tyrosine dephosphorylation defects were rescued in the double knockout mice lacking both calpain-1 and PTP1B. Further evaluation of mutant mice by the ferric chloride-induced arterial injury model suggests that the Capn1-/- mice are relatively resistant to thrombosis in vivo. Together, our results demonstrate that PTP1B is a physiological target of calpain-1 and suggest that a similar mechanism may regulate calpain-1-mediated tyrosine dephosphorylation in other cells.

Cancer-associated thrombosis

There is strong evidence linking venous thromboembolic events and malignancy. Laboratory markers of coagulation activation such as thrombin-antithrombin complex or prothrombin fragments 1+2 support the premise that malignancy is a hypercoagulable state. Inflammatory cytokines (e.g. tumor necrosis factor and interferon-gamma), coagulation proteins (e.g. tissue factor and factor VIII), and procoagulant microparticles may be elevated in patients with malignancy. However, the molecular basis for cancer associated thrombosis remains unknown and the relative contribution of chemotherapeutics, tumor cells, endothelium, and circulating procoagulants in promoting thrombus formation continues to be investigated.

Hyperglycemia in acute ischemic stroke: a vascular perspective

Admission hyperglycemia complicates approximately one-third of acute ischemic strokes and is associated with a worse clinical outcome. Both human and animal studies have showed that hyperglycemia is particularly detrimental in ischemia/reperfusion. Decreased reperfusion blood flow has been observed after middle cerebral artery occlusion in acutely hyperglycemic animals, suggesting the vasculature as an important site of hyperglycemic reperfusion injury. This paper reviews biochemical and molecular pathways in the vasculature that are rapidly affected by hyperglycemia and concludes that these changes result in a pro-vasoconstrictive, pro-thrombotic and pro-inflammatory phenotype that renders the vasculature vulnerable to reperfusion injury. Understanding these pathways should lead to the development of rational therapies that reduce hyperglycemic reperfusion injury and thus improve outcome in this large subset of acute ischemic stroke patients.

Murine models of myocardial and limb ischemia: Diagnostic end-points and relevance to clinical problems

Ischemic disease represents the new epidemic worldwide. Animal models of ischemic disease are useful because they can help us to understand the underlying pathogenetic mechanisms and develop new therapies. The present review article summarizes the results of a consensus conference on the status and future development of experimentation in the field of cardiovascular medicine using murine models of peripheral and myocardial ischemia. The starting point was to recognize the limits of the approach, which mainly derive from species- and disease-related differences in cardiovascular physiology. For instance, the mouse heart beats at a rate 10 times faster than the human heart. Furthermore, healing processes are more rapid in animals, as they rely on mechanisms that may have lost relevance in man. The main objective of the authors was to propose general guidelines, diagnostic end points and relevance to clinical problems.

Genetic background determines response to hemostasis and thrombosis

Background

Thrombosis is the fatal and disabling consequence of cardiovascular diseases, the leading cause of mortality and morbidity in Western countries. Two inbred mouse strains, C57BL/6J and A/J, have marked differences in susceptibility to obesity, atherosclerosis, and vessel remodeling. However, it is unclear how these diverse genetic backgrounds influence pathways known to regulate thrombosis and hemostasis. The objective of this study was to evaluate thrombosis and hemostasis in these two inbred strains and determine the phenotypic response of A/J chromosomes in the C57BL/6J background.

Methods

A/J and C57Bl/6J mice were evaluated for differences in thrombosis and hemostasis. A thrombus was induced in the carotid artery by application of the exposed carotid to ferric chloride and blood flow measured until the vessel occluded. Bleeding and rebleeding times, as surrogate markers for thrombosis and hemostasis, were determined after clipping the tail and placing in warm saline. Twenty-one chromosome substitution strains, A/J chromosomes in a C57BL/6J background, were screened for response to the tail bleeding assay.

Results

Thrombus occlusion time was markedly decreased in the A/J mice compared to C57BL/6J mice. Tail bleeding time was similar in the two strains, but rebleeding time was markedly increased in the A/J mice compared to C57BL/6J mice. Coagulation times and tail morphology were similar, but tail collagen content was higher in A/J than C57BL/6J mice. Three chromosome substitution strains, B6-Chr5^A/J, B6-Chr11^A/J, and B6-Chr17^A/J, were identified with increased rebleeding time, a phenotype similar to A/J mice. Mice heterosomic for chromosomes 5 or 17 had rebleeding times similar to C57BL/6J mice, but when these two chromosome substitution strains, B6-Chr5^A/J and B6-Chr17^A/J, were crossed, the A/J phenotype was restored in these doubly heterosomic progeny.

Conclusion

These results indicate that susceptibility to arterial thrombosis and haemostasis is remarkably different in C57BL/and A/J mice. Three A/J chromosome substitution strains were identified that expressed a phenotype similar to A/J for rebleeding, the C57Bl/6J background could modify the A/J phenotype, and the combination of two A/J QTL could restore the phenotype. The diverse genetic backgrounds and differences in response to vascular injury induced thrombosis and the tail bleeding assay, suggest the potential for identifying novel genetic determinants of thrombotic risk.

Plasminogen activator inhibitor-1 is associated with impaired endothelial function in women with systemic lupus erythematosus

Endothelial function, measured noninvasively by brachial artery flow-mediated dilatation (FMD), has been shown to be impaired in patients with systemic lupus erythematosus (SLE). We hypothesized that depressed FMD in SLE patients is associated with increased levels of plasminogen activator inhibitor-1 (PAI-1), an inhibitor of fibrinolysis and regulator of vasoactivity. In this cross-sectional study of female SLE patients under the age of 55, putative markers of cardiovascular disease (CVD) such as PAI-1 were measured in addition to lupus-related disease activity (SLEDAI). The primary outcome, FMD, was measured using high-resolution ultrasound of the brachial artery gated to the R wave to determine endothelial-dependent vasomotion. Endothelial-independent vasomotion was measured in response to nitroglycerin (NMD). Seventy-six female SLE patients, mean age 38.3 +/- 9.4 years, were included. All patients demonstrated normal NMD responses, indicating that depression of FMD was related to decreased endothelial nitric oxide production. Increased PAI-1 was related to depressed FMD by univariate regression (P = 0.004). In a multivariable regression model adjusting for t-PA (tissue plasminogen activator)/PAI-1 ratio, SLEDAI, age at visit, family history of cardiovascular disease, SLE disease duration and body mass index, every 1 ng/mL increase in PAI-1 was associated with a reduction of 0.07 units FMD (P = 0.039). PAI-1 was associated with impaired endothelial dysfunction, after controlling for several potential confounders. Given the high incidence of cardiovascular disease in SLE, further investigation of the role of subclinical markers of CVD is needed.

Increased venous versus arterial thrombosis in the Factor V Leiden mouse

BACKGROUND

Deep vein thrombosis (DVT) occurs with high prevalence in association with the Factor V Leiden (R506Q) mutation, whereas most evidence suggests no correlation with clinical arterial thrombosis.

OBJECTIVE

This study compared arterial to venous thrombosis in the mutationally analogous Factor V Leiden mouse.

METHODS

Three separate vascular thrombosis models were evaluated in Fv(+/+) (wild-type), Fv(Q/+) (heterozygous) and Fv(Q/Q) (homozygous) Factor V Leiden mice.

RESULTS

In a FeCl(3)-induced arterial thrombosis model, no statistical differences among the three genotypes were found in the time to thrombotic occlusion. In contrast, Fv(Q/+) and Fv(Q/Q) mice demonstrated larger femoral vein thrombi at 30 and 60 min compared to wild-types, with Fv(Q/Q) mice having statistically larger thrombi than both wild-type and Fv(Q/+) mice at 10 and 60 min and 24 h (p<0.05). In a model of thrombotic occlusion following arterial and venous anastomotic repair, both Fv(Q/+) and Fv(Q/Q) mice had higher rates of venous thrombosis than wild-types, but only Fv(Q/Q) homozygotes showed a statistically greater arterial occlusion rate than wild-types.

CONCLUSION

The Factor V Leiden mouse demonstrated a greater propensity for venous vs. arterial thrombosis, paralleling clinical epidemiologic findings and supporting its use for research on deep vein thrombosis.

Bruton tyrosine kinase is essential for botrocetin/VWF-induced signaling and GPIb-dependent thrombus formation in vivo

Botrocetin (bt)-facilitated binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex on platelets in suspension initiates a signaling cascade that causes alphaIIbbeta3 activation and platelet aggregation. Previous work has demonstrated that bt/VWF-mediated agglutination activates alphaIIbbeta3 and elicits ATP secretion in a thromboxane A2 (TxA2)-dependent manner. The signaling that results in TxA2 production was shown to be initiated by Lyn, enhanced by Src, and propagated through Syk, SLP-76, PI3K, PLCgamma2, and PKC. Here, we demonstrate that the signaling elicited by GPIb-mediated agglutination that results in TxA2 production is dependent on Bruton tyrosine kinase (Btk). The results demonstrate that Btk is downstream of Lyn, Syk, SLP-76, and PI3K; upstream of ERK1/2, PLCgamma2, and PKC; and greatly enhances Akt phosphorylation. The relationship(s), if any, between ERK1/2, PLCgamma2, and PKC were not elucidated. The requirement for Btk and TxA2 receptor function in GPIb-dependent arterial thrombosis was confirmed in vivo by characterizing blood flow in ferric chloride-treated mouse carotid arteries. These results demonstrate that the Btk family kinase, Tec, cannot provide the function(s) missing because of the absence of Btk and that Btk is essential for both bt/VWF-mediated agglutination-induced TxA2 production and GPIb-dependent stable arterial thrombus formation in vivo.

Carbon Monoxide-Induced Early Thrombolysis Contributes to Heme Oxygenase-1-Mediated Inhibition of Neointimal Growth after Vascular Injury in Hypercholesterolemic Mice

Arterial thrombosis is a critical event in the pathogenesis of lesion development. In this study, we evaluated the effect of heme oxygenase-1 (HO-1), a stress-inducible enzyme with vasoprotective functions, on arterial thrombosis following vascular mechanical injury. The carotid arteries of apoE-deficient mice were subjected to angioplasty with a modified beaded-needle. Arterial thrombosis occurred at 12 h after injury. Treatment of the injured vessels with an adenovirus bearing HO-1 gene (Adv-HO-1) (1 x 10(8) pfu), but not saline or empty adenovirus (Adv), immediately after angioplasty resulted in earlier thrombolysis and restoration of blood flow detected at 24 h. Immunohistochemistry revealed that the arterial plasminogen activator inhibitor-1 (PAI-1) expression was markedly reduced in Adv-HO-1-treated injured arteries as compared to control counterparts. The thrombolytic effect was also observed by exposing animals with existing arterial thrombosis to carbon monoxide (CO) (250 ppm, 2 h), a byproduct derived from heme degradation by HO-1. In parallel with less fibrin(ogen) deposition, the macrophage infiltration, monocyte chemoattractant protein-1 expression and neointimal formation assessed at 2 weeks after angioplasty were substantially reduced in injured arteries treated with Adv-HO-1. These results support a role of early thrombolysis induced by CO in HO-1-mediated protection against intimal hyperplasia after vascular injury.

Human complement receptor type 1-directed loading of tissue plasminogen activator on circulating erythrocytes for prophylactic fibrinolysis

Plasminogen activators (PAs) are not used for thromboprophylaxis due to rapid clearance, bleeding, and extravascular toxicity. We describe a novel strategy that overcomes these limitations. We conjugated tissue-type PA (tPA) to a monoclonal antibody (mAb) against complement receptor type 1 (CR1) expressed primarily on human RBCs. Anti-CR1/tPA conjugate, but not control conjugate (mIgG/tPA), bound to human RBCs (1.2 x 10(3) tPA molecules/cell at saturation), endowing them with fibrinolytic activity. In vitro, RBC-bound anti-CR1/tPA caused 90% clot lysis versus 20% by naive RBCs. In vivo, more than 40% of anti-CR1/(125)I-tPA remained within the circulation ( approximately 90% bound to RBCs) 3 hours after injection in transgenic mice expressing human CR1 (TgN-hCR1) versus less than 10% in wild-type (WT) mice, without RBC damage; approximately 90% of mIgG/(125)I-tPA was cleared from the circulation within 30 minutes in both WT and TgN-hCR1 mice. Anti-CR1/tPA accelerated lysis of pulmonary emboli and prevented stable occlusive carotid arterial thrombi from forming after injection in TgN-hCR1 mice, but not in WT mice, whereas soluble tPA and mIgG/tPA were ineffective. Anti-CR1/tPA caused 20-fold less rebleeding in TgN-hCR1 mice than the same dose of tPA. CR1-directed immunotargeting of PAs to circulating RBCs provides a safe and practical means to deliver fibrinolytics for thromboprophylaxis in settings characterized by a high imminent risk of thrombosis.

Thrombus formation in vivo

To examine thrombus formation in a living mouse, new technologies involving intravital videomicroscopy have been applied to the analysis of vascular windows to directly visualize arterioles and venules. After vessel wall injury in the microcirculation, thrombus development can be imaged in real time. These systems have been used to explore the role of platelets, blood coagulation proteins, endothelium, and the vessel wall during thrombus formation. The study of biochemistry and cell biology in a living animal offers new understanding of physiology and pathology in complex biologic systems.

Transendothelial migration of ferric ion in FeCl3 injured murine common carotid artery

INTRODUCTION/OBJECTIVES

Adventitial application of FeCl(3) causes endothelial injury, platelet aggregation, and a rapid onset of thrombus formation. The transmigration pathway of the ferric ion has not been definitively identified. Using a combination of TEM and X-ray elemental analysis, this study aims to elucidate the endothelial pathway of ferric ion migration in carotid artery.

METHODS AND RESULTS

Vascular injury was induced by placing a Whatman #1 filter paper strip saturated with 10% FeCl(3) over the common carotid artery in male C57BL/6 mice for 3 min. After rinsing in saline, the mice were terminated at 10 or 30 min. The FeCL(3) exposed segments of the common carotid artery were dissected, and processed for TEM. Thrombus formation was observed in all cases. Endothelial and smooth muscle injuries were observed in segments of the vessel in direct contact with the oxidant. The endothelial injury ranged from minimal damage to total denudation. The basal endothelial surface adjacent to the internal elastic lamina showed accumulation of electron opaque vesicles. The membrane enclosed particles transmigrated across the endothelium and exocytosed into the lumen. The nature of the particles shown by STEM/EDS was rich in ferric ion. Elemental analysis also showed that some ferric oxide aggregates formed near the developing thrombus in the vascular lumen.

CONCLUSION

Our results showed the ferric ion permeated the endothelial basal lamina before entering the arterial lumen via endocytic-exocytic pathway. This study provides an ultrastructural framework for future analysis of the adluminal and luminal injuries in this model.

Factor V Leiden improves in vivo hemostasis in murine hemophilia models

The role of factor V Leiden (FVL) as a modifier of the severe hemophilia phenotype is still unclear. We used mice with hemophilia A or B crossed with FVL to elucidate in vivo parameters of hemostasis. Real-time thrombus formation in the microcirculation was monitored by deposition of labeled platelets upon laser-induced endothelial injury using widefield microscopy in living animals. No thrombi formed in hemophilic A or B mice following vascular injuries. However, hemophilic mice, either heterozygous or homozygous for FVL, formed clots at all injured sites. Injection of purified activated FV into hemophilic A or B mice could mimic the in vivo effect of FVL. In contrast to these responses to a laser injury in a microvascular bed, FVL did not provide sustained hemostasis following damage of large vessels in a ferric chloride carotid artery injury model, despite of the improvement of clotting times and high circulating thrombin levels. Together these data provide evidence that FVL has the ability to improve the hemophilia A or B phenotype, but this effect is principally evident at the microcirculation level following a particular vascular injury. Our observations may partly explain the heterogeneous clinical evidence of the beneficial role of FVL in hemophilia.

Thrombotic phenotype of mice with a combined deficiency in plasminogen activator inhibitor 1 and vitronectin

OBJECTIVE

The role of vitronectin (VN) in thrombosis is not fully understood, primarily because this adhesive glycoprotein not only stabilizes plasminogen activator inhibitor 1 (PAI-1) and thus protects fibrin from premature lysis, but also because it binds to platelet integrins and may influence platelet aggregation. The absence of quantitative approaches to characterize the thrombi formed in animal models under different conditions further complicates this analysis.

METHODS

In this report, we describe a more comprehensive approach to assess the stability of thrombi formed in mice deficient in PAI-1 (PAI-1(-/-)), VN (VN(-/-)) or both (PAI-1(-/-)/VN(-/-)).

RESULTS

We observed that all deficient mice developed unstable thrombi compared with wild type (WT) mice. Thus, only 31% of the thrombi formed in WT mice were unstable compared with 74% of PAI-1(-/-), 80% of VN(-/-), and 87% of PAI-1(-/-)/VN(-/-) mice. In this regard, the average number of emboli per WT mouse was significantly lower (0.55) compared with VN(-/-) (2.66), PAI-1(-/-) (2.1), and VN(-/-)/PAI-1(-/-) (2.35) mice. Finally, the total duration of complete vascular occlusion was higher and the rate of vascular patency was lower in the WT mice compared with the deficient mice.

CONCLUSIONS

Taken together, these observations indicate that the thrombotic phenotype of mice with a combined deficiency in PAI-1 and VN does not differ significantly from the phenotype of mice with deficiencies in only PAI-1 or VN. This observation suggests that PAI-1 and VN may influence thrombus stability by regulating a common pathway.

Platelet PECAM-1 inhibits thrombus formation in vivo

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a cell surface glycoprotein receptor expressed on a range of blood cells, including platelets, and on vascular endothelial cells. PECAM-1 possesses adhesive and signaling properties, the latter being mediated by immunoreceptor tyrosine-based inhibitory motifs present on the cytoplasmic tail of the protein. Recent studies in vitro have demonstrated that PECAM-1 signaling inhibits the aggregation of platelets. In the present study we have used PECAM-1-deficient mice and radiation chimeras to investigate the function of this receptor in the regulation of thrombus formation. Using intravital microscopy and laser-induced injury to cremaster muscle arterioles, we show that thrombi formed in PECAM-1-deficient mice were larger, formed more rapidly than in control mice, and were more stable. Larger thrombi were also formed in control mice that received transplants of PECAM-1-deficient bone marrow, in comparison to mice that received control transplants. A ferric chloride model of thrombosis was used to investigate thrombus formation in carotid arteries. In PECAM-1-deficient mice the time to 75% vessel occlusion was significantly shorter than in control mice. These data provide evidence for the involvement of platelet PECAM-1 in the negative regulation of thrombus formation.

Platelets in atherothrombosis: lessons from mouse models

Platelets play a central role in hemostasis and thrombosis but also in the initiation of atherosclerosis, making platelet receptors and their intracellular signaling pathways important molecular targets for antithrombotic and anti-inflammatory therapy. Historically, much of the knowledge about hemostasis and thrombosis has been derived from patients suffering from bleeding and thrombotic disorders and the identification of the underlying molecular defects. In recent years, the availability of genetically modified mouse strains with defined defects in platelet function and the development of in vivo models to assess platelet-related physiologic and pathophysiologic processes have opened new ways to identify the individual roles and the interplay of platelet proteins in adhesion, activation, aggregation, secretion, and procoagulant activity in vitro and in vivo. This review will summarize key findings made by these approaches and discuss them in the context of human disease.

HMG-CoA reductase inhibitor protects against in vivo arterial thrombosis by augmenting platelet-derived nitric oxide release in rats

Acute coronary syndromes are caused by platelet-mediated thrombosis following rupture of a plaque. HMG-CoA reductase inhibitors (statins) reduce the incidence of events early after acute coronary syndromes, which are independent of its cholesterol-lowering effect. Accordingly, we investigated whether statins inhibit platelet-mediated arterial thrombus formation in vivo and, if so, the underlying mechanisms. Rats were divided into 4 groups. Group 1 was treated with the vehicle, whereas groups 2, 3, and 4 were treated with cerivastatin for 7 days (1, 2, and 5 mg/kg i.p., respectively). Cerivatatin did not change serum cholesterol levels. Carotid arterial thrombosis was created by perivascular FeCl3 delivery. Cerivastatin significantly prolonged the time to thrombotic occlusion of carotid artery. Cerivastatin significantly dose-dependently inhibited both ex vivo platelet P-selectin expression, a marker of platelet activation, and platelet aggregation. Cerivastatin significantly augmented platelet-derived nitric oxide (NO) release, and up-regulated platelet and endothelial nitric oxide synthase (NOS) mRNA expressions. N-nitro-L-arginine methylester abolished the effects of cerivastatin. This study demonstrates that in vivo administration of statin protects against platelet-mediated arterial thrombosis, possibly by augmenting platelet- and endothelium-derived NO releases via up-regulation of platelet and endothelial NOS.

A multiaxial computer-controlled organ culture and biomechanical device for mouse carotid arteries

Much of our understanding of vascular mechanotransduction has come from studies using either cell culture or in vivo animal models, but the recent success of organ culture systems offers an exciting alternative. In studying cell-mediated vascular adaptations to altered loading, organ culture allows one to impose well-controlled mechanical loads and to perform multiaxial mechanical tests on the same vessel throughout the culture period, and thereby to observe cell-mediated vascular adaptations independent of neural and hormonal effects. Here, we present a computer-controlled perfused organ culture and biomechanical testing device designed for small caliber (50-5000 micron) blood vessels. This device can control precisely the pulsatile pressure, luminal flow, and axial load (or stretch) and perform intermittent biaxial (pressure-diameter and axial load-length) and functional tests to quantify adaptations in mechanical behavior and cellular function, respectively. Device capabilities are demonstrated by culturing mouse carotid arteries for 4 days.

Evaluation of PAI-039 [{1-benzyl-5-[4-(trifluoromethoxy)phenyl]-1H-indol-3-yl}(oxo)acetic acid], a novel plasminogen activator inhibitor-1 inhibitor, in a canine model of coronary artery thrombosis

We tested a novel, orally active inhibitor of plasminogen activator inhibitor-1 (PAI-1) in a canine model of electrolytic injury. Dogs received by oral gavage either vehicle (control) or the PAI-1 inhibitor PAI-039 [{1-benzyl-5-[4-(trifluoromethoxy)phenyl]-1H-indol-3-yl}(oxo)acetic acid] (1, 3, and 10 mg/kg) and were subjected to electrolytic injury of the coronary artery. PAI-039 caused prolongation in time to coronary occlusion (control, 31.7 +/- 6.3 min; 3 mg/kg PAI-039, 66.0 +/- 6.4 min; 10 mg/kg, 56.7 +/- 7.4 min; n = 5-6; p < 0.05) and a reduced thrombus weight (control, 7.6 +/- 1.5 mg; 10 mg/kg PAI-039, 3.6 +/- 1.0 mg; p < 0.05). Although occlusive thrombosis was observed across all groups based upon the absence of measurable blood flow, a high incidence (>60%) of spontaneous reperfusion occurred only in those groups receiving PAI-039. Spontaneous reperfusion in the 10 mg/kg PAI-039 group accounted for total blood flow (area under the curve of coronary blood flow) of 99.6 +/- 11.7 ml after initial thrombotic occlusion (p < 0.05 compared with control). Plasma PAI-1 activity was reduced in all drug-treated groups (percentage of reduction in activity p < 0.05; 10 mg/kg PAI-039), whereas ADP-, 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2alpha) (U46619)-, and collagen-induced platelet aggregation, as well as template bleeding and prothrombin time, remained unaffected by PAI-039. Ex vivo clot lysis analysis revealed normal clot formation but accelerated clot lysis in PAI-039-treated groups. The pharmacokinetic profile of PAI-039 indicated an oral bioavailability of 43 +/- 15.3% and a plasma half-life of 6.2 +/- 1.3 h. In conclusion, PAI-039 is an orally active prothrombolytic drug that inhibits PAI-1 and accelerates fibrinolysis while maintaining normal coagulation in a model of coronary occlusion.

The rat ear vein model for investigating in vivo thrombogenicity of ultrafine particles (UFP)

Recent studies in rodents indicate that intravenous or intratracheal administration of ultrafine particles (UFP) increases thrombogenesis in a surgically exposed peripheral vein after photodynamic excitation of intravenously injected rose bengal (RB). We sought to adapt the invasive peripheral vein RB model to a noninvasive monitoring of ear veins under an inverted microscope. Animals received one of the following: an intraperitoneal, intravenous bolus, or intravenously infused dose of RB. An ear vein was illuminated by a green laser, and formation of a thrombus was captured with a digital camera. Only continuous intravenous infusion produced a steady-state RB plasma level and reproducible thrombus responses in different ear veins of the same rat. This system was then used to study the thrombogenic effects of iv-administered positively or negatively charged 60-nm ultrafine polystyrene particles (PSP). Significant dose-dependent enhancement of thrombus formation was found, as indicated by decreased laser illumination time to 33% of baseline values at 0.5 mg/kg. Negatively charged PSP of the same size failed to affect thrombus formation. We also studied the thrombogenic effect of PSP without the use of RB. The findings were the same as with RB, although the illumination time had to be increased. When 0.5 mg/kg was instilled intratracheally, the laser illumination time to form a thrombus was decreased to 42% of the baseline value, suggesting translocation of UFP into the bloodstream. These results are consistent with previous findings using the invasive model, and they validate the use of this non-invasive ear vein model to evaluate thrombogenic effects of UFP deposition in the respiratory tract.

Adipose Tissue and Atherothrombosis

Obesity is associated with increased cardiovascular mortality and morbidity mainly through insulin resistance. Dysregulation of protein secretion by adipose tissue is involved in obesity-related diseases. Adipose tissue contributes to create a subinflammatory status which could explain the disturbances in the haemostatic and fibrinolytic systems observed in obesity. Elevated plasma levels of PAI-1 demonstrated the strongest association with the degree of insulin resistance and could be an underlying mechanism for the thrombotic tendency and the progression of atherothrombosis during obesity. The effect of PAI-1 was examined on adipose tissue growth in several mouse models as well as on adipocyte differentiation in vitro. Most of the data indicate that PAI-1 can effectively modulate weight gain and may be a potential therapeutic target for controlling cardiovascular morbidity in obese subjects.

Application of “ATTEMPTS” for drug delivery

A novel enzyme drug delivery system, Antibody, Targeted, Triggered, Electrically, Modified, Prodrug, Type, Strategy ("ATTEMPTS"), was developed in our laboratory to attenuate the toxicity associated with drug activity at non-targeted tissues. Tissue plasminogen activator is a prime example of an enzyme drug that exhibits systemic toxicity due to its indiscriminate activation of both targeted (i.e., clot-bound) and non-targeted (i.e., systemic) plasminogen. In brief, tissue plasminogen activator (t-PA) was modified to contain positive surface charges and then rendered inactive upon electrostatic binding with a negatively charged heparin-antifibrin antibody conjugate. After targeting the complex to the clot site, t-PA activity was restored by administration of protamine, a clinical heparin antidote. Cation-modified t-PA (CM-tPA) was obtained by chemical conjugation of t-PA with a poly(Arg)7Cys peptide using the crosslinker N-succinimidyl 3-2-(pyridlydithio)propionate (SPDP). Anti-fibrin IgG was chemically conjugated with heparin via oxidation of the carbohydrate moiety on its Fc region. Both in vitro characterization and in vivo studies using a rat thrombosis model clearly demonstrated that heparin-IgG conjugate induced inhibition of CM-tPA could be effectively reversed upon addition of protamine. Overall, the ATTEMPTS system was proven to induce clot dissolution without causing t-PA associated systemic toxicity due to the degradation of critical plasma factors by systemic plasmin production.