Formation and Resolution of Pial Microvascular Thrombosis in a Mouse Model of Thrombotic Thrombocytopenic Purpura

Clot Accumulation in 3D Microfluidic Bifurcating Microvasculature Network

The microvasculature, which makes up the majority of the cardiovascular system, plays a crucial role in the process of thrombosis, with the pathological formation of blood clots inside blood vessels. Since blood microflow conditions significantly influence platelet activation and thrombosis, accurately mimicking the structure of bifurcating microvascular networks and emulating local physiological blood flow conditions are valuable for understanding blood clot formation. In this work, we present an in vitro model for blood clotting in microvessels, focusing on 3D bifurcations that align with Murray's law, which guides vascular networks by maintaining a constant wall shear rate throughout. Using these models, we demonstrate that microvascular bifurcations act as sites facilitating thrombus formation compared to straight models. Additionally, by culturing endothelial cells on the luminal surfaces of the models, we show the potential of using our in vitro platforms to recapitulate the initial clotting in diseases involving endothelial dysfunction, such as Thrombotic Thrombocytopenic Purpura.

John A, Ling M, Le J, Harris J, Rhoads N, Wang Y, Fu X, Chen J, Fazio S, Lindner JR, López JA. Low-density lipoprotein promotes microvascular thrombosis by enhancing von Willebrand factor self-association

von Willebrand factor (VWF) mediates primary hemostasis and thrombosis in response to hydrodynamic forces. We previously showed that high shear promoted self-association of VWF into hyperadhesive strands, which can be attenuated by high-density lipoprotein (HDL) and apolipoprotein A-I. In this study, we show that low-density lipoprotein (LDL) binds VWF under shear and enhances self-association. Vortexing VWF in tubes resulted in its loss from the solution and deposition onto tube surfaces, which was prevented by HDL. At a stabilizing HDL concentration of 1.2 mg/mL, increasing concentrations of LDL progressively increased VWF loss, the effect correlating with the LDL-to-HDL ratio and not the absolute concentration of the lipoproteins. Similarly, HDL diminished deposition of VWF in a post-in-channel microfluidic device, whereas LDL increased both the rate and extent of strand deposition, with both purified VWF and plasma. Hypercholesterolemic human plasma also displayed accelerated VWF accumulation in the microfluidic device. The initial rate of accumulation correlated linearly with the LDL-to-HDL ratio. In Adamts13-/- and Adamts13-/-LDLR-/- mice, high LDL levels enhanced VWF and platelet adhesion to the myocardial microvasculature, reducing cardiac perfusion, impairing systolic function, and producing early signs of cardiomyopathy. In wild-type mice, high plasma LDL concentrations also increased the size and persistence of VWF-platelet thrombi in ionophore-treated mesenteric microvessels, exceeding the accumulation seen in similarly treated ADAMTS13-deficient mice that did not receive LDL infusion. We propose that targeting the interaction of VWF with itself and with LDL may improve the course of thrombotic microangiopathies, atherosclerosis, and other disorders with defective microvascular circulation.

An update on the pathogenesis and diagnosis of thrombotic thrombocytopenic purpura

INTRODUCTION

Severe ADAMTS13 deficiency defines thrombotic thrombocytopenic purpura (TTP). ADAMTS13 is responsible for VWF cleavage. In the absence of this enzyme, widespread thrombi formation occurs, causing microangiopathic anemia and thrombocytopenia and leading to ischemic organ injury. Understanding ADAMTS13 function is crucial to diagnose and manage TTP, both in the immune and hereditary forms.

AREAS COVERED

The role of ADAMTS13 in coagulation homeostasis and the consequences of its deficiency are detailed. Other factors that modulate the consequences of ADAMTS13 deficiency are explained, such as complement system activation, genetic predisposition, or the presence of an inflammatory status. Clinical suspicion of TTP is crucial to start prompt treatment and avoid mortality and sequelae. Available techniques to diagnose this deficiency and detect autoantibodies or gene mutations are presented, as they have become faster and more available in recent years.

EXPERT OPINION

A better knowledge of TTP pathophysiology is leading to an improvement in diagnosis and follow-up, as well as a customized treatment in patients with TTP. This scenario is necessary to define the role of new targeted therapies already available or coming soon and the need to better diagnose and monitor at the molecular level the evolution of the disease.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Updated Understanding of Platelets in Thrombosis and Hemostasis: The Roles of Integrin PSI Domains and their Potential as Therapeutic Targets

Platelets are small blood cells known primarily for their ability to adhere and aggregate at injured vessels to arrest bleeding. However, when triggered under pathological conditions, the same adaptive mechanism of platelet adhesion and aggregation may cause thrombosis, a primary cause of heart attack and stroke. Over recent decades, research has made considerable progress in uncovering the intricate and dynamic interactions that regulate these processes. Integrins are heterodimeric cell surface receptors expressed on all metazoan cells that facilitate cell adhesion, movement, and signaling, to drive biological and pathological processes such as thrombosis and hemostasis. Recently, our group discovered that the plexin-semaphorin-integrin (PSI) domains of the integrin β subunits exert endogenous thiol isomerase activity derived from their two highly conserved CXXC active site motifs. Given the importance of redox reactions in integrin activation and its location in the knee region, this PSI domain activity may be critically involved in facilitating the interconversions between integrin conformations. Our monoclonal antibodies against the β3 PSI domain inhibited its thiol isomerase activity and proportionally attenuated fibrinogen binding and platelet aggregation. Notably, these antibodies inhibited thrombosis without significantly impairing hemostasis or causing platelet clearance. In this review, we will update mechanisms of thrombosis and hemostasis, including platelet versatilities and immune-mediated thrombocytopenia, discuss critical contributions of the newly discovered PSI domain thiol isomerase activity, and its potential as a novel target for anti-thrombotic therapies and beyond.

Targeting von Willebrand factor in liver diseases: A novel therapeutic strategy?

Acute and chronic liver disease are associated with substantial alterations in the hemostatic system. Evidence from both experimental and clinical studies suggests that anticoagulants slow the progression of liver disease. Efficacy of those anticoagulant drugs is, in part, attributed to a reduction of microthrombi formation within the liver. Although anticoagulant drugs show promising results, bleeding risk associated with these drugs is an obvious drawback, particularly in patients with a complex coagulopathy driven by decreased liver function. Identifying therapies that reduce intrahepatic thrombosis with minimal bleeding risk would significantly advance the field. Among the hemostatic alterations observed in patients are substantially increased levels of the platelet-adhesive protein von Willebrand factor (VWF). In contrast, levels of A Disintegrin and Metalloproteinase with Thrombospondin motifs, the enzyme that regulates VWF activity, are significantly reduced in patients with liver disease. Highly elevated VWF levels are proposed to accelerate intrahepatic thrombus formation and thus be a driver of disease progression. Strong clinical evidence suggesting a link between liver disease and changes in VWF is now being matched by emerging mechanistic data showing a detrimental role for VWF in the progression of liver disease. This review focuses on clinical and experimental evidence supporting a connection between VWF function and the progression of acute and chronic liver diseases. Furthermore, with the recent anticipated approval of several novel therapies targeting VWF, we discuss potential strategies and benefits of targeting VWF as an innovative therapy for patients with liver disease.

[Recurrent thrombocytopenia with hemolytic anemia in a boy aged 3 years]

A boy, aged 3 years and 8 months, had recurrent thrombocytopenia with hemolytic anemia for more than 3 years. The physical examination showed no enlargement of the liver, spleen, and lymph nodes or finger deformities. Laboratory results showed a negative result of the direct antiglobulin test, normal coagulation function, and increases in bilirubin, lactate dehydrogenase and reticulocytes. The results of von Willebrand factor-cleaving protease ADAMTS13 activity assay showed extreme deficiency, and antibody assay showed negative ADAMTS13 inhibitory autoantibodies. Next-generation sequence showed compound heterozygous mutation in the ADAMTS13 gene. The boy was diagnosed with congenital thrombotic thrombocytopenic purpura. This disease may be easily misdiagnosed as Evans syndrome and is difficult to diagnose in clinical practice. The child had developed the disease since birth, but it took 3 years to make a confirmed diagnosis. Therefore, congenital thrombotic thrombocytopenic purpura should be considered for children with jaundice at birth, recurrent thrombocytopenia with hemolytic anemia, and negative results of the direct antiglobulin test. The detection of ADAMTS13 activity and ADAMTS13 inhibitory autoantibodies should be performed as soon as possible for a definite diagnosis, and gene detection should be performed to make a confirmed diagnosis when necessary.

[Recurrent thrombocytopenia with hemolytic anemia in a boy aged 3 years]

A boy, aged 3 years and 8 months, had recurrent thrombocytopenia with hemolytic anemia for more than 3 years. The physical examination showed no enlargement of the liver, spleen, and lymph nodes or finger deformities. Laboratory results showed a negative result of the direct antiglobulin test, normal coagulation function, and increases in bilirubin, lactate dehydrogenase and reticulocytes. The results of von Willebrand factor-cleaving protease ADAMTS13 activity assay showed extreme deficiency, and antibody assay showed negative ADAMTS13 inhibitory autoantibodies. Next-generation sequence showed compound heterozygous mutation in the ADAMTS13 gene. The boy was diagnosed with congenital thrombotic thrombocytopenic purpura. This disease may be easily misdiagnosed as Evans syndrome and is difficult to diagnose in clinical practice. The child had developed the disease since birth, but it took 3 years to make a confirmed diagnosis. Therefore, congenital thrombotic thrombocytopenic purpura should be considered for children with jaundice at birth, recurrent thrombocytopenia with hemolytic anemia, and negative results of the direct antiglobulin test. The detection of ADAMTS13 activity and ADAMTS13 inhibitory autoantibodies should be performed as soon as possible for a definite diagnosis, and gene detection should be performed to make a confirmed diagnosis when necessary.

Platelet Imaging

The knowledge gained through imaging platelets has formed the backbone of our understanding of their biology in health and disease. Early investigators relied on conventional light microscopy with limited resolution and were primarily able to identify the presence and basic morphology of platelets. The advent of high resolution technologies, in particular, electron microscopy, accelerated our understanding of the dynamics of platelet ultrastructure dramatically. Further refinements and improvements in our ability to localize and reliably identify platelet structures have included the use of immune-labeling techniques, correlative-fluorescence light and electron microscopy, and super-resolution microscopies. More recently, the expanded development and application of intravital microscopy in animal models has enhanced our knowledge of platelet functions and thrombus formation in vivo, as these experimental systems most closely replicate native biological environments. Emerging improvements in our ability to characterize platelets at the ultrastructural and organelle levels include the use of platelet cryogenic electron tomography with quantitative, unbiased imaging analysis, and the ability to genetically label platelet features with electron dense markers for analysis by electron microscopy.

Microvascular thrombosis: experimental and clinical implications

A significant amount of clinical and research interest in thrombosis is focused on large vessels (eg, stroke, myocardial infarction, deep venous thrombosis, etc.); however, thrombosis is often present in the microcirculation in a variety of significant human diseases, such as disseminated intravascular coagulation, thrombotic microangiopathy, sickle cell disease, and others. Further, microvascular thrombosis has recently been demonstrated in patients with COVID-19, and has been proposed to mediate the pathogenesis of organ injury in this disease. In many of these conditions, microvascular thrombosis is accompanied by inflammation, an association referred to as thromboinflammation. In this review, we discuss endogenous regulatory mechanisms that prevent thrombosis in the microcirculation, experimental approaches to induce microvascular thrombi, and clinical conditions associated with microvascular thrombosis. A greater understanding of the links between inflammation and thrombosis in the microcirculation is anticipated to provide optimal therapeutic targets for patients with diseases accompanied by microvascular thrombosis.