Decreased Plasma Fibronectin Leads to Delayed Thrombus Growth in Injured Arterioles

Myocardial ischemia-reperfusion injury released cellular fibronectin containing domain A (CFN-EDA): A destructive positive loop amplifying arterial thrombosis formation and exacerbating myocardial reperfusion injury

Previous research has identified intravascular platelet thrombi in regions affected by myocardial ischemia-reperfusion (MI/R) injury and neighbouring areas. However, the occurrence of arterial thrombosis in the context of MI/R injury remains unexplored. This study utilizes intravital microscopy to investigate carotid artery thrombosis during MI/R injury in rats, establishing a connection with the presence of prothrombotic cellular fibronectin containing extra domain A (CFN-EDA) protein. Additionally, the study examines samples from patients with coronary artery disease (CAD) both before and after coronary artery bypass grafting (CABG). Levels of CFN-EDA significantly increase following MI with further elevation observed following reperfusion of the ischemic myocardium. Thrombotic events, such as thrombus formation and growth, show a significant increase, while the time to complete cessation of blood flow in the carotid artery significantly decreases following MI/R injury induced by ferric chloride. The acute infusion of purified CFN-EDA protein accelerates in-vivo thrombotic events in healthy rats and significantly enhances in-vitro adenosine diphosphate and collagen-induced platelet aggregation. Treatment with anti-CFN-EDA antibodies protected the rat against MI/R injury and significantly improved cardiac function as evidenced by increased end-systolic pressure-volume relationship slope and preload recruitable stroke work compared to control. Similarly, in a human study, plasma CFN-EDA levels were notably elevated in CAD patients undergoing CABG. Post-surgery, these levels continued to rise over time, alongside cardiac injury biomarkers such as cardiac troponin and B-type natriuretic peptide. The study highlights that increased CFN-EDA due to CAD or MI initiates a destructive positive feedback loop by amplifying arterial thrombus formation, potentially exacerbating MI/R injury.

Fibronectin extra domain a limits liver dysfunction and protects mice during acute inflammation

Background and aim

The primary transcript of fibronectin (FN) undergoes alternative splicing to generate different isoforms, including FN containing the Extra Domain A (FN_EDA+), whose expression is regulated spatially and temporarily during developmental and disease conditions including acute inflammation. The role of FN_EDA+ during sepsis, however, remains elusive.

Methods

Mice constitutively express the EDA domain of fibronectin (EDA^+/+); lacking the FN EDA domain (EDA^-/-) or with a conditional ablation of EDA + inclusion only in liver produced FN (alb-CRE⁺EDA floxed mice) thus expressing normal plasma FN were used. Systemic inflammation and sepsis were induced by either LPS injection (70 mg/kg) or by cecal ligation and puncture (CLP) Neutrophils isolated from septic patients were tested for neutrophil binding ability.

Results

We observed that EDA^+/+ were protected toward sepsis as compared to EDA^-/- mice. Also alb-CRE⁺EDA floxed mice presented reduced survival, thus indicating a key role for EDA in protecting toward sepsis. This phenotype was associated with improved liver and spleen inflammatory profile. Ex vivo experiments showed that neutrophils bind to a larger extent to an FN_EDA + coated surface as compared to FN, thus potentially limiting their over-reactivity.

Conclusions

Our study demonstrates that the inclusion of the EDA domain in fibronectin dampens the nflammatoryi consequences of sepsis.

Fluorine-containing bio-inert polymers: Roles of intermediate water

Fluorine-containing polymers are used not only in industrial processes but also in medical applications, because they exhibit excellent heat, weather, and chemical resistance. As these polymers are not easily degraded in our body, it is difficult to use them in applications that require antithrombotic properties, such as artificial blood vessels. The material used for medical applications should not only be stable in vivo, but it should also be inert to biomolecules such as proteins or cells. In this review, this property is defined as "bio-inert," and previous studies in this field are summarized. Bio-inert materials are less recognized as foreign substances by proteins or cells in the living body, and they must be covered at interfaces designed with the concept of intermediate water (IW). On the basis of this concept, we present here the current understanding of bio-inertness and unusual blood compatibility found in fluoropolymers used in biomedical applications. IW is the water that interacts with materials with moderate strength and has been quantified by a variety of analytical methods and simulations. For example, by using differential scanning calorimetry (DSC) measurements, IW was defined as water frozen at around -40°C. To consider the role of the IW, quantification methods of the hydration state of polymers are also summarized. These investigations have been conducted independently because of the conflict between hydrophobic fluorine and bio-inert properties that require hydrophilicity. In recent years, not many materials have been developed that incorporate the good points of both aspects, and their properties have seldom been linked to the hydration state. This has been critically performed now. Furthermore, fluorine-containing polymers in medical use are reviewed. Finally, this review also describes the molecular design of the recently reported fluorine-containing bio-inert polymers for controlling their hydration state. STATEMENT OF SIGNIFICANCE: A material covered with a hydration layer known as intermediate water that interacts moderately with other objects is difficult to be recognized as a foreign substance and exhibits bio-inert properties. Fluoropolymers show high durability, but conflict with bio-inert characteristics requiring hydrophilicity as these research studies have been conducted independently. On the other hand, materials that combine the advantages of both hydrophobic and hydrophilic features have been developed recently. Here, we summarize the molecular architecture and analysis methods that control intermediate water and provide a guideline for designing novel fluorine-containing bio-inert materials.

Characterization of the Role of Integrin α5β1 in Platelet Function, Hemostasis, and Experimental Thrombosis

Objective  Integrins are key regulators of various platelet functions. The pathophysiological importance of most platelet integrins has been investigated, with the exception of α5β1, a receptor for fibronectin. The aim of this study was to characterize the role of α5β1 in megakaryopoiesis, platelet function, and to determine its importance in hemostasis and arterial thrombosis.

Approach and Results  We generated a mouse strain deficient for integrin α5β1 on megakaryocytes and platelets (PF4Cre-α5 ^−/− ). PF4Cre-α5 ^−/− mice were viable, fertile, and presented no apparent signs of abnormality. Megakaryopoiesis appears unaltered as evidence by a normal megakaryocyte morphology and development, which is in agreement with a normal platelet count. Expression of the main platelet receptors and the response of PF4Cre-α5 ^−/− platelets to a series of agonists were all completely normal. Adhesion and aggregation of PF4Cre-α5 ^−/− platelets under shear flow on fibrinogen, laminin, or von Willebrand factor were unimpaired. In contrast, PF4Cre-α5 ^−/− platelets displayed a marked decrease in adhesion, activation, and aggregation on fibrillar cellular fibronectin and collagen. PF4Cre-α5 ^−/− mice presented no defect in a tail-bleeding time assay and no increase in inflammatory bleeding in a reverse passive Arthus model and a lipopolysaccharide pulmonary inflammation model. Finally, no defects were observed in three distinct experimental models of arterial thrombosis based on ferric chloride-induced injury of the carotid artery, mechanical injury of the abdominal aorta, or laser-induced injury of mesenteric vessels.

Conclusion  In summary, this study shows that platelet integrin α5β1 is a key receptor for fibrillar cellular fibronectin but is dispensable in hemostasis and arterial thrombosis.

Increased blood levels of cellular fibronectin in asthma: Relation to the asthma severity, inflammation, and prothrombotic blood alterations

BACKGROUND

Recently, we have reported that asthma is characterized by prothrombotic blood alterations, which were related to the low-grade inflammatory state. Inflammation, however, may also lead to vascular dysfunction. The aim of this study was to evaluate plasma levels of cellular fibronectin (cFN), a marker of vascular injury in asthmatics, and to analyze their impact on described previously prothrombotic blood alterations.

METHODS

In a cross-sectional study, we investigated 164 adult stable asthmatics and 72 matched controls. Plasma cFN was measured using an ELISA. Its relations to inflammation, thrombin generation, fibrinolytic capacity, expressed as clot lysis time (CLT), and platelet markers were evaluated.

RESULTS

Asthma was associated with 50.1% higher plasma cFN levels as compared with controls (p < 0.001, after adjustment for potential confounders). There was a positive association of cFN with asthma severity and inverse with the FEV₁/VC index (β = 0.2 [95%CI:0.13-0.28] and β = -0.15 [95%CI: -0.23 to -0.07], respectively). In asthmatics cFN positively correlated with high-sensitivity C-reactive protein (β = 0.24 [95%CI:0.16-0.32]), fibrinogen (β = 0.13 [95%CI:0.04-0.21]), interleukin-6 (β = 0.23 [95%CI:0.15-0.3]), platelet factor 4 (β = 0.14 [95%CI:0.06-0.21]), plasminogen (β = 0.11 [95%CI:0.04-0.19]) and CLT (β = 0.35 [95%CI:0.28-0.42]). In both groups cFN was related to the endogenous thrombin potential (ETP) (β = 0.51 [95%CI:0.44-0.57], and β = 0.17 [95%CI:0.07-0.27], respectively). Multiple regression models showed that cFN was the most potent independent predictor of both ETP and CLT in asthmatics.

CONCLUSION

Presented study is the first to show increased plasma cellular fibronectin in asthma, which is associated with disease severity, inflammation, and prothrombotic blood alterations. This novel observation suggests a previously unknown modulator of prothrombotic plasma properties in asthmatics.

Interplay between elevated cellular fibronectin and plasma fibrin clot properties in type 2 diabetes

Type 2 diabetes is associated with faster formation of poorly lysable, denser fibrin clots and elevated cellular fibronectin (cFn), a marker of vascular injury. We investigated whether cFn affects clot properties in type 2 diabetes. In 200 consecutive patients with type 2 diabetes and 100 control subjects matched for age and sex, we determined plasma cFn along with clot formation and degradation using turbidimetric and permeability assays. Diabetic patients had elevated cFn (median, 3.99 [interquartile range, 2.87-4.81] µg/ml]), increased clot density (MaxAbsC) and prolonged lysis time (LysT) compared with those without type 2 diabetes (all p<0.01). Diabetic patients with documented cardiovascular disease (CVD, n=127, 63.5 %) had increased cFn (4.53 [3.68-4.95] µg/ml), decreased clot permeability (Ks) and increased MaxAbsC compared with those without CVD (all p<0.001). Diabetic patients with cFn in the top quartile (>4.81 µg/ml) were two times more likely to have CVD compared with those in the lowest quartile (odds ratio 1.80, 95 % confidence interval 1.41-2.46, p<0.001). No differences in cFn were observed in relation to microvascular complications. After adjustment for potential confounders, cFn accounted for 10.2 % of variance in Ks, 18.2 % of variance in clot density and 10.2 % of variance in AUC in diabetic patients. This study shows that elevated cFn is associated with unfavourably modified clot properties in type 2 diabetes, especially with concomitant CVD, which indicates novel links between vascular injury and prothrombotic alterations in diabetes. Coagulation, cellular fibronectin, type 2 diabetes, cardiovascular disease.

Fibronectin maintains the balance between hemostasis and thrombosis

Fibronectin is a dimeric protein widely distributed in solid tissues and blood. This major extracellular matrix protein is indispensable for embryogenesis and plays crucial roles in many physiological and pathological processes. Fibronectin pre-mRNA undergoes alternative splicing to generate over 20 splicing variants, which are categorized as either plasma fibronectin (pFn) or cellular fibronectin (cFn). All fibronectin variants contain integrin binding motifs, as well as N-terminus collagen and fibrin binding motifs. With motifs that can be recognized by platelet integrins and coagulation factors, fibronectin, especially pFn, has long been suspected to be involved in hemostasis and thrombosis, but the exact function of fibronectin in these processes is controversial. The advances made using intravital microscopy models and fibronectin deficient and mutant mice have greatly facilitated the direct investigation of fibronectin function in vivo. Recent studies revealed that pFn is a vital hemostatic factor that is especially crucial for hemostasis in both genetic and anticoagulant-induced deficiencies of fibrin formation. pFn may also be an important self-limiting regulator to prevent hemorrhage as well as excessive thrombus formation and vessel occlusion. In addition to pFn, cFn is found to be prothrombotic and may contribute to thrombotic complications in various diseases. Further investigations of the role of pFn and cFn in thrombotic and hemorrhagic diseases may provide insights into development of novel therapeutic strategies (e.g., pFn transfusion) for the maintenance of the fine balance between hemostasis and thrombosis.

Enhanced Megakaryopoiesis and Platelet Activity in Hypercholesterolemic, B6-Ldlr-/-, Cdkn2a-Deficient Mice

BACKGROUND

Genome-wide association studies for coronary artery disease/myocardial infarction revealed a 58 kb risk locus on 9p21.3. Refined genetic analyses revealed unique haplotype blocks conferring susceptibility to atherosclerosis per se versus risk for acute complications in the presence of underlying coronary artery disease. The cell proliferation inhibitor locus, CDKN2A, maps just upstream of the myocardial infarction risk block, is at least partly regulated by the noncoding RNA, ANRIL, overlapping the risk block, and has been associated with platelet counts in humans. Thus, we tested the hypothesis that CDKN2A deficiency predisposes to increased platelet production, leading to increased platelet activation in the setting of hypercholesterolemia.

METHODS AND RESULTS

Platelet production and activation were measured in B6-Ldlr(-/-)Cdkn2a(+/-) mice and a congenic strain carrying the region of homology with the human 9p21.3/CDKN2A locus. The strains exhibit decreased expression of CDKN2A (both p16(INK4a) and p19(ARF)) but not CDKN2B (p15(INK4b)). Compared with B6-Ldlr(-/-) controls, both Cdkn2a-deficient strains exhibited increased platelet counts and bone marrow megakaryopoiesis. The platelet overproduction phenotype was reversed by treatment with cyclin-dependent kinase 4/6 inhibitor, PD0332991/palbociclib, that mimics the endogenous effect of p16(INK4a). Western diet feeding resulted in increased platelet activation, increased thrombin/antithrombin complex, and decreased bleeding times in Cdkn2a-deficient mice compared with controls.

CONCLUSIONS

Together, the data suggest that one or more Cdkn2a transcripts modulate platelet production and activity in the setting of hypercholesterolemia, amenable to pharmaceutical intervention. Enhanced platelet production and activation may predispose to arterial thrombosis, suggesting an explanation, at least in part, for the association of 9p21.3 and myocardial infarction.

Fibrillar cellular fibronectin supports efficient platelet aggregation and procoagulant activity

The ability of cellular fibronectin, found in the vessel wall in a fibrillar conformation, to regulate platelet functions and trigger thrombus formation remains largely unknown. In this study, we evaluated how parietal cellular fibronectin can modulate platelet responses under flow conditions. A fibrillar network was formed by mechanically stretching immobilised dimeric cellular fibronectin. Perfusion of anticoagulated whole blood over this surface resulted in efficient platelet adhesion and thrombus growth. The initial steps of platelet adhesion and activation, as evidenced by filopodia extension and an increase in intracellular calcium levels (419 ± 29 nmol/l), were dependent on integrins α5β1 and αIIbβ3. Subsequent thrombus growth was mediated by these integrins together with the GPIb-V-IX complex, GPVI and Toll-like receptor 4. The involvement of Toll-like receptor 4 could be conveyed via its binding to the EDA region of cellular fibronectin. Upon thrombus formation, the platelets became procoagulant and generated fibrin as revealed by video-microscopy. This work provides evidence that fibrillar cellular fibronectin is a strong thrombogenic surface which supports efficient platelet adhesion, activation, aggregation and procoagulant activity through the interplay of a series of receptors including integrins α5β1 and αIIbβ3, the GPIb-V-IX complex, GPVI and Toll-like receptor 4.

Cellular fibronectin containing extra domain A promotes arterial thrombosis in mice through platelet Toll-like receptor 4

Cellular fibronectin containing extra domain A (Fn-EDA+), which is produced in response to tissue injury in several disease states, has prothrombotic activity and is known to interact with Toll-like-receptor 4 (TLR4). The underlying mechanism and cell types involved in mediating the prothrombotic effect of Fn-EDA+ still remain unknown. Using intravital microscopy, we evaluated susceptibility to carotid artery thrombosis after FeCl3-induced injury in mice expressing Fn lacking EDA (Fn-EDA(-/-) mice) or Fn containing EDA (Fn-EDA(+/+) mice). Fn-EDA(-/-) mice exhibited prolonged times to first thrombus formation and complete occlusion and a significant decrease in the rate of thrombus growth (P < .05 vs Fn-EDA(+/+) mice). Genetic deletion of TLR4 reversed the accelerated thrombosis in Fn-EDA(+/+) mice (P < .05) but had no effect in Fn-EDA(-/-) mice. Bone marrow transplantation experiments revealed that TLR4 expressed on hematopoietic cells contributes to accelerated thrombosis in Fn-EDA(+/+) mice. In vitro studies showed that cellular Fn-EDA+ interacts with platelet TLR4 and promotes agonist-induced platelet aggregation. Finally, Fn-EDA(+/+) mice specifically lacking platelet TLR4 exhibited prolonged times to first thrombus formation and complete occlusion (P < .05 vs Fn-EDA(+/+) mice containing platelet TLR4). We conclude that platelet TLR4 contributes to the prothrombotic effect of cellular Fn-EDA+, suggesting another link between thrombosis and innate immunity.

Vascular wall extracellular matrix proteins and vascular diseases

Extracellular matrix proteins form the basic structure of blood vessels. Along with providing basic structural support to blood vessels, matrix proteins interact with different sets of vascular cells via cell surface integrin or non-integrin receptors. Such interactions induce vascular cell de novo synthesis of new matrix proteins during blood vessel development or remodeling. Under pathological conditions, vascular matrix proteins undergo proteolytic processing, yielding bioactive fragments to influence vascular wall matrix remodeling. Vascular cells also produce alternatively spliced variants that induce vascular cell production of different matrix proteins to interrupt matrix homeostasis, leading to increased blood vessel stiffness; vascular cell migration, proliferation, or death; or vascular wall leakage and rupture. Destruction of vascular matrix proteins leads to vascular cell or blood-borne leukocyte accumulation, proliferation, and neointima formation within the vascular wall; blood vessels prone to uncontrolled enlargement during blood flow diastole; tortuous vein development; and neovascularization from existing pathological tissue microvessels. Here we summarize discoveries related to blood vessel matrix proteins within the past decade from basic and clinical studies in humans and animals - from expression to cross-linking, assembly, and degradation under physiological and vascular pathological conditions, including atherosclerosis, aortic aneurysms, varicose veins, and hypertension.

Thrombosis: tangled up in NETs

The contributions by blood cells to pathological venous thrombosis were only recently appreciated. Both platelets and neutrophils are now recognized as crucial for thrombus initiation and progression. Here we review the most recent findings regarding the role of neutrophil extracellular traps (NETs) in thrombosis. We describe the biological process of NET formation (NETosis) and how the extracellular release of DNA and protein components of NETs, such as histones and serine proteases, contributes to coagulation and platelet aggregation. Animal models have unveiled conditions in which NETs form and their relation to thrombogenesis. Genetically engineered mice enable further elucidation of the pathways contributing to NETosis at the molecular level. Peptidylarginine deiminase 4, an enzyme that mediates chromatin decondensation, was identified to regulate both NETosis and pathological thrombosis. A growing body of evidence reveals that NETs also form in human thrombosis and that NET biomarkers in plasma reflect disease activity. The cell biology of NETosis is still being actively characterized and may provide novel insights for the design of specific inhibitory therapeutics. After a review of the relevant literature, we propose new ways to approach thrombolysis and suggest potential prophylactic and therapeutic agents for thrombosis.

Platelet microRNAs: From platelet biology to possible disease biomarkers and therapeutic targets

Although anucleated, platelets contain megakaryocyte-derived messenger ribonucleic acid (mRNA) which can be translated to produce protein molecules. Recently, platelets have been found to contain small (∼23 base pair) non-coding microRNAs (miRNAs) derived from hairpin-like precursors. MiRNAs can specifically silence their mRNA targets regulating mRNA translation. Platelet miRNAs are reported to bind to important platelet target mRNAs involved in platelet reactivity including P2Y12 ADP receptor, GPIIb receptor, and cyclic AMP-dependent protein kinase A. They also regulate important functions such as platelet shape change, granules secretion, and platelet activation. Platelet miRNAs were also proposed as biomarkers of arteriosclerosis, although their role in vascular inflammation needs to be elucidated. Further, the possibility of using miRNAs as therapeutic tools has emerged. Using synthetic oligo-nucleotides that antagonize miRNAs binding to their mRNAs-targets or synthetic miRNAs mimics that enhance endogenous miRNAs function potentially will ultimately lead to the manipulation of platelet miRNAs expression and function with significant effects on specific protein levels and overall platelet reactivity.

Extracellular matrix proteins in hemostasis and thrombosis

The adhesion and aggregation of platelets during hemostasis and thrombosis represents one of the best-understood examples of cell-matrix adhesion. Platelets are exposed to a wide variety of extracellular matrix (ECM) proteins once blood vessels are damaged and basement membranes and interstitial ECM are exposed. Platelet adhesion to these ECM proteins involves ECM receptors familiar in other contexts, such as integrins. The major platelet-specific integrin, αIIbβ3, is the best-understood ECM receptor and exhibits the most tightly regulated switch between inactive and active states. Once activated, αIIbβ3 binds many different ECM proteins, including fibrinogen, its major ligand. In addition to αIIbβ3, there are other integrins expressed at lower levels on platelets and responsible for adhesion to additional ECM proteins. There are also some important nonintegrin ECM receptors, GPIb-V-IX and GPVI, which are specific to platelets. These receptors play major roles in platelet adhesion and in the activation of the integrins and of other platelet responses, such as cytoskeletal organization and exocytosis of additional ECM ligands and autoactivators of the platelets.

Fibronectin splice variants: understanding their multiple roles in health and disease using engineered mouse models

The extracellular matrix (ECM) is a highly dynamic network of proteins, glycoproteins, and proteoglycans. Numerous diseases result from mutation in genes coding for ECM proteins, but only recently it has been reported that mutations in the fibronectin (FN) gene were associated with a human disorder. FN is one of the main components of the ECM. It generates protein diversity through alternative splicing of a single pre-mRNA, having at least 20 different isoforms in humans. The precise function of these protein isoforms has remained obscure in most cases. Only in the recent few years, it was possible to shed light on the multiple roles of the alternatively spliced FN isoforms. This substantial progress was achieved basically with the knowledge derived from engineered mouse models bearing subtle mutations in specific FN domains. These data, together with a recent report associating mutations in the FN gene to a form of glomerulopathy, clearly show that mutations in constitutive exons or misregulation of alternatively spliced domains of the FN gene may have nonlethal pathological consequences. In this review, we focus on the pathological consequences of mutations in the FN gene, by connecting the function of alternatively spliced isoforms of fibronectin to human diseases.

Arterial thrombosis--insidious, unpredictable and deadly

The formation of blood clots--thrombosis--at sites of atherosclerotic plaque rupture is a major clinical problem despite ongoing improvements in antithrombotic therapy. Progress in identifying the pathogenic mechanisms regulating arterial thrombosis has led to the development of newer therapeutics, and there is general anticipation that these treatments will have greater efficacy and improved safety. However, major advances in this field require the identification of specific risk factors for arterial thrombosis in affected individuals and a rethink of the 'one size fits all' approach to antithrombotic therapy.

Plasma and cellular fibronectin: distinct and independent functions during tissue repair

Fibronectin (FN) is a ubiquitous extracellular matrix (ECM) glycoprotein that plays vital roles during tissue repair. The plasma form of FN circulates in the blood, and upon tissue injury, is incorporated into fibrin clots to exert effects on platelet function and to mediate hemostasis. Cellular FN is then synthesized and assembled by cells as they migrate into the clot to reconstitute damaged tissue. The assembly of FN into a complex three-dimensional matrix during physiological repair plays a key role not only as a structural scaffold, but also as a regulator of cell function during this stage of tissue repair. FN fibrillogenesis is a complex, stepwise process that is strictly regulated by a multitude of factors. During fibrosis, there is excessive deposition of ECM, of which FN is one of the major components. Aberrant FN-matrix assembly is a major contributing factor to the switch from normal tissue repair to misregulated fibrosis. Understanding the mechanisms involved in FN assembly and how these interplay with cellular, fibrotic and immune responses may reveal targets for the future development of therapies to regulate aberrant tissue-repair processes.

Fibronectin: Functional character and role in alcoholic liver disease

Fibronectins are adhesive glycoproteins that can be found in tissue matrices and circulating in various fluids of the body. The variable composition of fibronectin molecules facilitates a diversity of interactions with cell surface receptors that suggest a role for these proteins beyond the structural considerations of the extracellular matrix. These interactions implicate fibronectin in the regulation of mechanisms that also determine cell behavior and activity. The two major forms, plasma fibronectin (pFn) and cellular fibronectin (cFn), exist as balanced amounts under normal physiological conditions. However, during injury and/or disease, tissue and circulating levels of cFn become disproportionately elevated. The accumulating cFn, in addition to being a consequence of prolonged tissue damage, may in fact stimulate cellular events that promote further damage. In this review, we summarize what is known regarding such interactions between fibronectin and cells that may influence the biological response to injury. We elaborate on the effects of cFn in the liver, specifically under a condition of chronic alcohol-induced injury. Studies have revealed that chronic alcohol consumption stimulates excess production of cFn by sinusoidal endothelial cells and hepatic stellate cells while impairing its clearance by other cell types resulting in the build up of this glycoprotein throughout the liver and its consequent increased availability to influence cellular activity that could promote the development of alcoholic liver disease. We describe recent findings by our laboratory that support a plausible role for cFn in the promotion of liver injury under a condition of chronic alcohol abuse and the implications of cFn stimulation on the pathogenesis of alcoholic liver disease. These findings suggest an effect of cFn in regulating cell behavior in the alcohol-injured liver that is worth further characterizing not only to gain a more comprehensive understanding of the role this reactive glycoprotein plays in the progression of injury but also for the insight further studies could provide towards the development of novel therapies for alcoholic liver disease.

Platelets at work in primary hemostasis

When platelet numbers are low or when their function is disabled, the risk of bleeding is high, which on the one hand indicates that in normal life vascular damage is a rather common event and that hence the role of platelets in maintaining a normal hemostasis is a continuously ongoing physiological process. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a hemostatic plug. This self-amplifying mechanism nevertheless requires a tight control to prevent uncontrolled platelet aggregate formation that eventually would occlude the vessel. Therefore endothelial cells produce inhibitory compounds such as prostacyclin and nitric oxide that limit the growth of the platelet thrombus to the damaged area. With this review, we intend to give an integrated survey of the platelet response to vascular injury in normal hemostasis.

Comparative in vivo analysis of the atherosclerotic plaque targeting properties of eight human monoclonal antibodies

OBJECTIVE

The selective in vivo localization of antibody derivatives in atherosclerotic plaques may open novel diagnostic and therapeutic applications. Here, we present a comparative in vivo localization analysis of eight radioiodinated human monoclonal antibodies in apolipoprotein E-deficient (ApoE(-/-)) mice.

METHODS

Animals were fed with a cholesterol-rich diet, followed by harvesting of the aorta 24h after intravenous antibody injection and investigated by autoradiographic analysis. Localization of F8 antibody on atherosclerotic plaque structures was further studied in three-color fluorescence microscopy.

RESULTS

The study revealed that the F8 antibody, specific to the alternatively spliced EDA domain of fibronectin, exhibited the highest plaque-targeting potential among the antibodies analyzed in this study, with an ability to preferentially localize to all plaques within the aorta. Targeting results were confirmed by injection of fluorescein-labeled F8 antibody, followed by three-color fluorescence microscopy analysis.

CONCLUSION

These findings open novel biomolecular avenues for the in vivo imaging of atherosclerotic plaques and for pharmacodelivery applications, since F8 had previously been reported by our group to strongly stain atherosclerotic plaques in human carotid arteries.

Emerging roles of fibronectin in thrombosis

Fibronectin (FN) is a glycoprotein recognized originally in the 1940's as a contaminant of fibrinogen in Cohn fraction I of plasma. Decades of research demonstrated FN synthesis by a variety of cells and defined FN as an essential component of the extracellular matrix with roles in embryogenesis, development, and wound healing. More recently, FN has emerged as player in platelet thrombus formation and diseases associated with thrombosis including vascular remodeling, atherosclerosis, and cardiac repair following a myocardial infarct. We discuss the mechanisms by which this might occur and conclude that FN may have a unique role in thrombosis without affecting normal hemostasis and therefore may be a reasonable therapeutic target for the prevention of thrombotic diseases.

Human blood vessel-derived endothelial progenitors for endothelialization of small diameter vascular prosthesis

BACKGROUND

Coronary bypass graft failure as a result of acute thrombosis and intimal hyperplasia has been the major challenge in surgical procedures involving small-diameter vascular prosthesis. Coating synthetic grafts with patients' own endothelial cells has been suggested to improve the patency rate and overall success of bypass surgeries.

METHODOLOGY/PRINCIPAL FINDINGS

We isolated endothelial progenitor cells (EPCs) from leftover pieces of human saphenous vein/mammary artery. We demonstrate that EPCs can be expanded to generate millions of cells under low-density culture conditions. Exposure to high-density conditions induces differentiation to endothelial cell phenotype. EPC-derived endothelial cells show expression of CD144high, CD31, and vWF. We then assessed the ability of differentiated endothelial cells to adhere and grow on small diameter expanded polytetrafluoroethylene (ePTFE) tubings. Since ePTFE tubings are highly hydrophobic, we optimized protocols to introduce hydrophilic groups on luminal surface of ePTFE tubings. We demonstrate here a stepwise protocol that involves introduction of hydrophilic moieties and coating with defined ECM components that support adhesion of endothelial cells, but not of blood platelets.

CONCLUSION/SIGNIFICANCE

Our data confirms that endothelial progenitors obtained from adult human blood vessels can be expanded in vitro under xenoprotein-free conditions, for potential use in endothelialization of small diameter ePTFE grafts. These endothelialized grafts may represent a promising treatment strategy for improving the clinical outcome of small-caliber vascular grafts in cardiac bypass surgeries.

Plasma fibronectin depletion enhances platelet aggregation and thrombus formation in mice lacking fibrinogen and von Willebrand factor

We previously showed that platelet aggregation and thrombus formation occurred in mice lacking both fibrinogen (Fg) and von Willebrand factor (VWF) and that plasma fibronectin (pFn) promoted thrombus growth and stability in injured arterioles in wild-type mice. To examine whether pFn is required for Fg/VWF-independent thrombosis, we generated Fg/VWF/conditional pFn triple-deficient (TKO; Cre+, Fnflox/flox, Fg/VWF−/−) mice and littermate control (Cre−, Fnflox/flox, Fg/VWF−/−) mice. Surprisingly, TKO platelet aggregation was not abolished, but instead was enhanced in both heparinized platelet-rich plasma and gel-filtered platelets. This enhancement was diminished when TKO platelets were aggregated in pFn-positive control platelet-poor plasma (PPP), whereas aggregation was enhanced when control platelets were aggregated in pFn-depleted TKO PPP. The TKO platelet aggregation can be completely inhibited by our newly developed mouse anti–mouse β3 integrin antibodies but was not affected by anti–mouse GPIbα antibodies. Enhanced platelet aggregation was also observed when heparinized TKO blood was perfused in collagen-coated perfusion chambers. Using intravital microscopy, we further showed that thrombogenesis in TKO mice was enhanced in both FeCl3-injured mesenteric arterioles and laser-injured cremaster arterioles. Our data indicate that pFn is not essential for Fg/VWF-independent thrombosis and that soluble pFn is probably an important inhibitory factor for platelet aggregation.

New insights into form and function of fibronectin splice variants

The extracellular matrix (ECM) is a highly dynamic structure that not only provides a physical framework for cells within connective tissues, but also imparts instructive signals for development, tissue homeostasis and basic cell functions through its composition and ability to exert mechanical forces. The ECM of tissues is composed of, in addition to proteoglycans and hyaluronic acid, a number of proteins, most of which are generated after alternative splicing of their pre-mRNA. However, the precise function of these protein isoforms is still obscure in most cases. Fibronectin (FN), one of the main components of the ECM, is also one of the best-known examples of a family of proteins generated by alternative splicing, having at least 20 different isoforms in humans. Over the last few years, considerable progress on elucidating the functions of the alternatively spliced FN isoforms has been achieved with the essential development of key engineered mouse strains. Here we summarize the phenotypes of the mouse strains having targeted mutations in the FN gene, which may lead to novel insights linking function of alternatively spliced isoforms of fibronectin to human pathologies.

Platelet function in intravascular device implant-induced intimal injury

Platelets are involved in the rapid response to intimal injury in which the underlying thrombogenic subendothelial matrix is exposed, leading to platelet adhesion, secretion, aggregation, and initiation of arterial thrombus formation. The platelet activation pathway involves a multistep process of distinct receptors, adhesive ligands, release of mediators, receptor-ligand interactions, and recruitment of more platelets to the site of injury. The balance between blood fluidity and intimal injury-induced arterial thrombosis is maintained by an intact endothelium that controls vessel tone, synthesizes inhibitors and activators of platelet function, and thereby allows the free flow of blood cell elements. An intravascular device implant causes intimal injury, which is accompanied by decreased antithrombotic potential of the endothelial cells and increased release of prothrombotic substances. A trigger for the formation of intimal injury-induced thrombus formation may be due to endothelial dysfunction and/or the loss of endothelial cell barrier between the subendothelial matrix and flowing blood, which allows initiation of platelet activation. A thorough understanding of the platelet regulatory mechanisms is necessary to develop effective antiplatelet therapy to prevent the complications of thrombosis following revascularization procedures using percutaneous coronary intervention. This review summarizes the temporal events following intravascular device implants, including endothelial cell injury, platelet activation, receptor-mediated signaling events, platelet-rich thrombus formation, and the redundant platelet pathways, all of which may be potential therapeutic targets.

Prothrombotic Effects of Fibronectin Isoforms Containing the EDA Domain

Objective— Fibronectin (FN) plays an important role in the formation of stable arterial thrombi at the site of vascular injury. FN containing Extra Domain A (EDA + FN) is absent from normal plasma, but elevated plasma levels of EDA + FN are found in several pathological conditions. We hypothesized that EDA + FN plays a special role in thrombosis.

Methods and Results— We used mouse strains constitutively including (EDA +/+ ) or excluding (EDA −/− ) the EDA domain in all tissues and plasma. Using a flow chamber and the ferric-chloride injury model we found that EDA + FN accelerates thrombosis both in vitro and in vivo at arterial shear rates. In EDA +/+ mice thrombi (>30 μm) grew faster when compared with EDA WT/WT (6.6±0.2 minutes versus 8.3±0.6 minutes, P <0.05) and the mean vessel occlusion time was shorter (9.9±0.4 minutes versus 14.6±1.7 minutes, P <0.05). However, the presence of EDA + FN affected neither single platelet adhesion to subendothelium nor thrombosis in veins. In addition, the mortality rate of EDA +/+ mice after collagen/epinephrine infusion was twice that of EDA WT/WT or EDA −/− mice.

Conclusions— Our findings reveal that EDA + FN has prothrombotic activity, and its presence in plasma may worsen pathological conditions in which this form is elevated.

A Major Fraction of Fibronectin Present in the Extracellular Matrix of Tissues Is Plasma-derived

The origin of the fibronectin (FN) found in the extracellular matrix of tissues has not been defined experimentally. Previous studies suggest that there is contribution from both local tissue production and transfer from plasma, but the extent of this phenomenon has not been addressed. We have shown before that engineered mice constitutively expressing extra domain A-containing FN (EDA(+)FN) have a significant decrease of FN levels in plasma and most tissues. We showed that hepatocytes modified to produce EDA(+)FN have normal extracellular matrix-FN levels but secrete less soluble FN. When we performed a liver-specific EDA-exon deletion in these animals, FN levels were restored both in plasma and tissues. Therefore, an important fraction of tissue FN, approximately an equal amount of that produced by the tissue itself, is actually plasma-derived, suggesting that plasma is an important source of tissue FN. The present results have potential significance for understanding the contributions of plasma FN, and perhaps other plasma proteins, in the modulation of cellular activities and in the formation of the extracellular matrix of tissues.

Murine models of vascular thrombosis (Eitzman series)

Thrombotic complications of vascular disease are the leading cause of morbidity and mortality in most industrialized countries. Despite this, safe and effective drugs targeting these complications are limited, especially in the chronic setting. This is because of the complexity of thrombosis in both arteries and veins, which is becoming increasingly evident as numerous factors are now known to affect the fate of a forming thrombus. To fully characterize thrombus formation in these settings, in vivo models are necessary to study the various components and intricate interactions that are involved. Genetic manipulations in mice are greatly facilitating the dissection of relevant pro- and antithrombotic influences. Standardized models for the study of thrombosis in mice as well as evolving techniques that allow imaging of molecular events during thrombus formation are now available. This review will highlight some of the recent developments in the field of thrombosis using mouse models and how these studies are expanding our knowledge of thrombotic disease.

The growing complexity of platelet aggregation

Platelet aggregation, the process by which platelets adhere to each other at sites of vascular injury, has long been recognized as critical for hemostatic plug formation and thrombosis. Until relatively recently, platelet aggregation was considered a straightforward process involving the noncovalent bridging of integrin alpha(IIb)beta(3) receptors on the platelet surface by the dimeric adhesive protein fibrinogen. However, with recent technical advances enabling real-time analysis of platelet aggregation in vivo, it has become apparent that this process is much more complex and dynamic than previously anticipated. Over the last decade, it has become clear that platelet aggregation represents a multistep adhesion process involving distinct receptors and adhesive ligands, with the contribution of individual receptor-ligand interactions to the aggregation process dependent on the prevailing blood flow conditions. It now appears that at least 3 distinct mechanisms can initiate platelet aggregation, with each of these mechanisms operating over a specific shear range in vivo. The identification of shear-dependent mechanisms of platelet aggregation has raised the possibility that vascular-bed-specific inhibitors of platelet aggregation may be developed in the future that are safer and more effective than existing antiplatelet agents.

Platelet adhesion receptors and their ligands in mouse models of thrombosis

Platelet adhesion and aggregation at sites of vascular injury are two key events in hemostasis and thrombosis. Because of exciting advances in genetic engineering, the mouse has become an important and frequently used model to unravel the molecular mechanisms underlying the multistep process leading to the formation of a stable platelet plug. In gene-targeted mice, the crucial importance of platelet adhesion receptors such as glycoprotein Ib alpha or the alphaIIb beta3 integrin has been confirmed and further clarified. Their absence leads to highly impaired thrombus formation, independent of the model used to induce vascular injury. In contrast, the relative contribution of other receptors, such as glycoprotein VI, or of various platelet ligands may be regulated by the severity of injury, the type of vessel injured, and the signaling pathways that are generated. Murine models have also helped improve understanding of the second wave of events that leads to stabilization of the platelet aggregate. Despite the current limitations due to lack of standardization and the virtual absence of thrombosis models in diseased vessels, there is no doubt that the mouse will play a key role in the discovery and characterization of the next generation of antithrombotic agents. This review focuses on key findings about the molecular mechanisms supporting hemostasis and thrombosis that have been obtained with genetically engineered mouse models deficient in various platelet adhesion receptors and ligands. Combination of these models with sophisticated methods allowing direct visualization of platelet-vessel wall interactions after injury greatly contributed to recent advances in the field.

von Willebrand factor and factor VIII are independently required to form stable occlusive thrombi in injured veins

von Willebrand factor (VWF) protects factor VIII (FVIII) from proteolysis and mediates the initial contact of platelets with the injured vessel wall, thus playing an important role in hemostasis and thrombosis. VWF is crucial for the formation of occlusive thrombi at arterial shear rates. However, with only a few conflicting studies published, the role of VWF in venous thrombosis is still unclear. Using gene-targeted mice, we show that in ferric chloride-injured veins platelet adhesion to subendothelium is decreased and thrombus growth is impaired in VWF(-/-) mice when compared with wild type (WT). We also observed increased embolization in the VWF(-/-) mice, which was due to lower FVIII levels in these mice as recombinant factor VIII (r-FVIII) restored thrombus stability. Despite normalization of blood clotting time and thrombus stability after r-FVIII infusion, the VWF(-/-) venules did not occlude. Transgenic platelets lacking the VWF receptor GPIbalpha extracellular domain showed decreased adhesion to injured veins. But, after a delay, all the injured venules occluded in these transgenic mice. Thus, VWF likely uses other adhesion receptors besides GPIbalpha in thrombus growth under venous shear conditions. Our studies document crucial roles for VWF and FVIII in experimental thrombosis under venous flow conditions in vivo.

The role of platelet adhesion receptor GPIbalpha far exceeds that of its main ligand, von Willebrand factor, in arterial thrombosis

GPIbalpha binding to von Willebrand factor (VWF) exposed at a site of vascular injury is thought to be the first step in the formation of a hemostatic plug. However, our previous studies in VWF-deficient mice demonstrated delayed but not absent arterial thrombus formation, suggesting that, under these conditions, GPIbalpha may bind other ligands or that a receptor other than GPIbalpha can mediate platelet adhesion. Here, we studied thrombus formation in transgenic mice expressing GPIbalpha in which the extracellular domain was replaced by that of the human IL-4 receptor (IL4Ralpha/GPIbalpha-tg mice). Platelet adhesion to ferric chloride-treated mesenteric arterioles in IL4Ralpha/GPIbalpha-tg mice was virtually absent in contrast to avid adhesion in WT mice. As a consequence, arterial thrombus formation was inhibited completely in the mutant mice. Our studies further show that, when infused into WT recipient mice, IL4Ralpha/GPIbalpha-tg platelets or WT platelets lacking the 45-kDa N-terminal domain of GPIbalpha failed to incorporate into growing arterial thrombi, even if the platelets were activated before infusion. Surprisingly, platelets lacking beta3 integrins, which are unable to form thrombi on their own, incorporated efficiently into WT thrombi. Our studies provide in vivo evidence that GPIbalpha absolutely is required for recruitment of platelets to both exposed subendothelium and thrombi under arterial flow conditions. Thus, GPIbalpha contributes to arterial thrombosis by important adhesion mechanisms independent of the binding to VWF.