Polyanions in Coagulation and Thrombosis: Focus on Polyphosphate and Neutrophils Extracellular Traps

Factor XII Structure-Function Relationships

Factor XII (FXII), the zymogen of the protease FXIIa, contributes to pathologic processes such as bradykinin-dependent angioedema and thrombosis through its capacity to convert the homologs prekallikrein and factor XI to the proteases plasma kallikrein and factor XIa. FXII activation and FXIIa activity are enhanced when the protein binds to a surface. Here, we review recent work on the structure and enzymology of FXII with an emphasis on how they relate to pathology. FXII is a homolog of pro-hepatocyte growth factor activator (pro-HGFA). We prepared a panel of FXII molecules in which individual domains were replaced with corresponding pro-HGFA domains and tested them in FXII activation and activity assays. When in fluid phase (not surface bound), FXII and prekallikrein undergo reciprocal activation. The FXII heavy chain restricts reciprocal activation, setting limits on the rate of this process. Pro-HGFA replacements for the FXII fibronectin type 2 or kringle domains markedly accelerate reciprocal activation, indicating disruption of the normal regulatory function of the heavy chain. Surface binding also enhances FXII activation and activity. This effect is lost if the FXII first epidermal growth factor (EGF1) domain is replaced with pro-HGFA EGF1. These results suggest that FXII circulates in blood in a "closed" form that is resistant to activation. Intramolecular interactions involving the fibronectin type 2 and kringle domains maintain the closed form. FXII binding to a surface through the EGF1 domain disrupts these interactions, resulting in an open conformation that facilitates FXII activation. These observations have implications for understanding FXII contributions to diseases such as hereditary angioedema and surface-triggered thrombosis, and for developing treatments for thrombo-inflammatory disorders.

Cardiovascular disease and thrombosis: Intersections with the immune system, inflammation, and the coagulation system

The coagulation and immune system, both essential physiological systems in the human body, are intricately interconnected and play a critical role in determining the overall health of patients. These systems collaborate via various shared regulatory pathways, such as the Tissue Factor (TF) Pathway. Immunological cells that express TF and generate pro-inflammatory cytokines have the ability to affect coagulation. Conversely, coagulation factors and processes have a reciprocal effect on immunological responses by stimulating immune cells and regulating their functions. These interconnected pathways play a role in both preserving well-being and contributing to a range of pathological disorders. The close relationship between blood clotting and inflammation in the development of vascular disease has become a central focus of clinical study. This research specifically examines the crucial elements of this interaction within the contexts of cardiovascular disease and acute coronary syndrome. Tissue factor, the primary trigger of the extrinsic coagulation pathway, has a crucial function by inducing a proinflammatory reaction through the activation of coagulation factors. This, in turn, initiates coagulation and subsequent cellular signalling pathways. Protease-activated receptors establish the molecular connection between coagulation and inflammation by interacting with activated clotting factors II, X, and VII. Thrombosis, a condition characterised by the formation of blood clots, is the most dreaded consequence of cardiovascular disorders and a leading cause of death globally. Consequently, it poses a significant challenge to healthcare systems. Antithrombotic treatments efficiently target platelets and the coagulation cascade, but they come with the inherent danger of causing bleeding. Furthermore, antithrombotics are unable to fully eliminate thrombotic events, highlighting a treatment deficiency caused by a third mechanism that has not yet been sufficiently addressed, namely inflammation. Understanding these connections may aid in the development of novel approaches to mitigate the harmful mutual exacerbation of inflammation and coagulation. Gaining a comprehensive understanding of the intricate interaction among these systems is crucial for the management of diseases and the creation of efficacious remedies. Through the examination of these prevalent regulatory systems, we can discover novel therapeutic approaches that specifically target these complex illnesses. This paper provides a thorough examination of the reciprocal relationship between the coagulation and immune systems, emphasising its importance in maintaining health and understanding disease processes. This review examines the interplay between inflammation and thrombosis and its role in the development of thrombotic disorders.

Emerging therapeutic strategies targeting extracellular histones for critical and inflammatory diseases: an updated narrative review

Extracellular histones are crucial damage-associated molecular patterns involved in the development and progression of multiple critical and inflammatory diseases, such as sepsis, pancreatitis, trauma, acute liver failure, acute respiratory distress syndrome, vasculitis and arthritis. During the past decade, the physiopathologic mechanisms of histone-mediated hyperinflammation, endothelial dysfunction, coagulation activation, neuroimmune injury and organ dysfunction in diseases have been systematically elucidated. Emerging preclinical evidence further shows that anti-histone strategies with either their neutralizers (heparin, heparinoids, nature plasma proteins, small anion molecules and nanomedicines, etc.) or extracorporeal blood purification techniques can significantly alleviate histone-induced deleterious effects, and thus improve the outcomes of histone-related critical and inflammatory animal models. However, a systemic evaluation of the efficacy and safety of these histone-targeting therapeutic strategies is currently lacking. In this review, we first update our latest understanding of the underlying molecular mechanisms of histone-induced hyperinflammation, endothelial dysfunction, coagulopathy, and organ dysfunction. Then, we summarize the latest advances in histone-targeting therapy strategies with heparin, anti-histone antibodies, histone-binding proteins or molecules, and histone-affinity hemoadsorption in pre-clinical studies. Finally, challenges and future perspectives for improving the clinical translation of histone-targeting therapeutic strategies are also discussed to promote better management of patients with histone-related diseases.

Neutrophil extracellular traps mediate neuro-immunothrombosis

Neutrophil extracellular traps are primarily composed of DNA and histones and are released by neutrophils to promote inflammation and thrombosis when stimulated by various inflammatory reactions. Neutrophil extracellular trap formation occurs through lytic and non-lytic pathways that can be further classified by formation mechanisms. Histones, von Willebrand factor, fibrin, and many other factors participate in the interplay between inflammation and thrombosis. Neuro-immunothrombosis summarizes the intricate interplay between inflammation and thrombosis during neural development and the pathogenesis of neurological diseases, providing cutting-edge insights into post-neurotrauma thrombotic events. The blood-brain barrier defends the brain and spinal cord against external assaults, and neutrophil extracellular trap involvement in blood-brain barrier disruption and immunothrombosis contributes substantially to secondary injuries in neurological diseases. Further research is needed to understand how neutrophil extracellular traps promote blood-brain barrier disruption and immunothrombosis, but recent studies have demonstrated that neutrophil extracellular traps play a crucial role in immunothrombosis, and identified modulators of neuro-immunothrombosis. However, these neurological diseases occur in blood vessels, and the mechanisms are unclear by which neutrophil extracellular traps penetrate the blood-brain barrier to participate in immunothrombosis in traumatic brain injury. This review discusses the role of neutrophil extracellular traps in neuro-immunothrombosis and explores potential therapeutic interventions to modulate neutrophil extracellular traps that may reduce immunothrombosis and improve traumatic brain injury outcomes.

The ISTH DIC-score predicts early mortality in patients with non-promyelocitic acute myeloid leukemia

Coagulation disorders frequently complicate the clinical course of acute myeloid leukemia (AML) patients. This study examined the frequency and prognostic significance, with regards of early mortality, of the presence of overt disseminated intravascular coagulation (DIC) at AML diagnosis and its correlation with clinical and biological characteristics. A retrospective analysis of 351 newly diagnosed non-promyelocytic AML patients was conducted, utilizing the 2018 ISTH DIC-Score criteria to evaluate the presence of overt DIC at AML onset. The study cohort had a median age of 65 years with a predominance of male gender (59 %). Overt DIC was present in 21 % of cases and was associated with advanced age, comorbidities, poor performance status, hyperleukocytosis, LDH levels, NPM1 mutations, expression of CD33 and CD4, and lack of expression of CD34. With a median follow-up of 72 months (3-147 months), the 6-year overall survival (OS) was 17.4 %, with patients having overt DIC showing significantly poorer outcomes (7.2 % compared to 20.3 % of those without DIC, p < 0.001). Patients with overt DIC showed markedly high early mortality rates at 30 (42.5 % vs 8 %), 60 (49.3 % vs 16.9 %), and 120 days (64.4 % vs 25.6 %) from disease onset. In multivariate analysis overt DIC retained its independent prognostic value for early mortality. In conclusion, the prevalence and clinical relevance of DIC in non-promyelocytic AML is not negligible, underlining its potential as an unfavorable prognostic marker. In newly diagnosed patients with AML, early recognition and measure to counteract coagulation disturbances might help mitigate the elevated mortality risk associated with DIC.

Prevalence of FXII-Deficiency and Its Relevance to Monitoring Anticoagulation in Adults Receiving Extracorporeal Membrane Oxygenation

During extracorporeal membrane oxygenation (ECMO) blood is exposed to artificial surfaces, resulting in contact activation of the intrinsic coagulation pathway initiated by coagulation factor XII (FXII). Little is known about the prevalence of acquired FXII-deficiency, especially during ECMO. The primary outcome was the prevalence of acquired FXII-deficiency (FXII activity <60%) during ECMO. Secondary outcomes included differences in hemorrhagic/thromboembolic complications, doses of unfractionated heparin administered, and time points of anticoagulation within target ranges between patients with and without FXII-deficiency. Of 193 adults receiving ECMO therapy between 2013 and 2021, FXII testing was performed in 64 (33%) patients. Of these, 89% ( n = 57) had an acquired FXII-deficiency. Median complication-free intervals were not different between patients with and without acquired FXII-deficiency (bleeding: 28 days [6-145] vs. 12 days [11-not available], p = 0.85; thromboembolism: 16 days [8-54] vs. 13 days [3-15], p = 0.053). Patients with acquired FXII-deficiency received less heparin (16,554 IU/day vs. 25,839 IU/day; p = 0.009) and were less likely to be within aPTT-target ranges (23.1% [14.3%-36.4%] vs. 37.8% [33.7%-58.3%], p = 0.005). Acquired FXII-deficiency is common during ECMO and may affect monitoring of anticoagulation. The impact of FXII-activity on complications needs to be determined in future studies.

The Concept of Thromboinflammation

Inflammation and thrombosis are intricate and closely interconnected biological processes that are not yet fully understood and lack effective targeted therapeutic approaches. Thrombosis initiated by inflammatory responses, known as immunothrombosis, can confer advantages to the host by constraining the spread of pathogens within the bloodstream. Conversely, platelets and the coagulation cascade can influence inflammatory responses through interactions with immune cells, endothelium, or complement system. These interactions can lead to a state of heightened inflammation resulting from thrombotic processes, termed as thromboinflammation. This review aims to comprehensively summarize the existing knowledge of thromboinflammation and addressing its significance as a challenging clinical issue.

Uncovering the role of cytoskeleton proteins in the formation of neutrophil extracellular traps

Neutrophils are a type of lymphocyte involved in innate immune defense. In response to specific stimuli, these phagocytic cells undergo a unique form of cell death, NETosis, during which they release neutrophil extracellular traps (NETs) composed of modified chromatin structures decorated with cytoplasmic and granular proteins. Multiple proteins and pathways have been implicated in the formation of NETs. The cytoskeleton, an interconnected network of filamentous polymers and regulatory proteins, plays a crucial role in resisting deformation, transporting intracellular cargo, and changing shape during movement of eukaryotic cells. It may also have evolved to defend eukaryotic organisms against infection. Recent research focuses on understanding the mechanisms underlying NETs formation and how cytoskeletal networks contribute to this process, by identifying enzymes that trigger NETosis or interact with NETs and influence cellular behavior through cytoskeletal dynamics. An enhanced understanding of the complex relationship between the cytoskeleton and NET formation will provide a framework for future research and the development of targeted therapeutic strategies, and supports the notion that the long-lived cytoskeleton structures may have a lasting impact on this area of research.

Polyphosphate as an antithrombotic target and hemostatic agent

Polyphosphate (PolyP) is a polymer comprised of linear phosphate units connected by phosphate anhydride bonds. PolyP exists in a diverse range of eukaryotes and prokaryotes with varied chain lengths ranging from six to thousands of phosphate units. Upon activation, human platelets and neutrophils release short-chain PolyP, along with other components, to initiate the coagulation pathway. Long-chain PolyP derived from cellular or bacterial organelles exhibits higher proinflammatory and procoagulant effects compared to short-chain PolyP. Notably, PolyP has been identified as a low-hemorrhagic antithrombotic target since neutralizing plasma PolyP suppresses the thrombotic process without impairing the hemostatic functions. As an inorganic polymer without uniform steric configuration, PolyP is typically targeted by cationic polymers or recombinant polyphosphatases rather than conventional antibodies, small-molecule compounds, or peptides. Additionally, because of its procoagulant property, PolyP has been incorporated in wound-dressing materials to facilitate blood hemostasis. This review summarizes current studies on PolyP as a low-hemorrhagic antithrombotic target and the development of hemostatic materials based on PolyP.

The Crossroads of the Coagulation System and the Immune System: Interactions and Connections

The coagulation and immune systems, two vital systems in the human body, share intimate connections that fundamentally determine patient health. These systems work together through several common regulatory pathways, including the Tissue Factor (TF) Pathway. Immune cells expressing TF and producing pro-inflammatory cytokines can influence coagulation, while coagulation factors and processes reciprocally impact immune responses by activating immune cells and controlling their functions. These shared pathways contribute to maintaining health and are also involved in various pathological conditions. Dysregulated coagulation, triggered by infection, inflammation, or tissue damage, can result in conditions such as disseminated intravascular coagulation (DIC). Concurrently, immune dysregulation may lead to coagulation disorders and thrombotic complications. This review elucidates these intricate interactions, emphasizing their roles in the pathogenesis of autoimmune diseases and cancer. Understanding the complex interplay between these systems is critical for disease management and the development of effective treatments. By exploring these common regulatory mechanisms, we can uncover innovative therapeutic strategies targeting these intricate disorders. Thus, this paper presents a comprehensive overview of the mutual interaction between the coagulation and immune systems, highlighting its significance in health maintenance and disease pathology.

News at XI: moving beyond factor Xa inhibitors

Oral anticoagulants are a mainstay for the prevention and treatment of arterial and venous thrombosis. Direct oral anticoagulants (DOACs) have replaced vitamin K antagonists for many indications. Currently available DOACs include dabigatran, which inhibits thrombin, and apixaban, edoxaban, and rivaroxaban, which inhibit factor (F) Xa. A new class of DOACs is under development. These new DOACs, which include asundexian and milvexian, inhibit FXIa, which is positioned in the intrinsic pathway of coagulation. Anticoagulants that target FXIa have the potential to be safer than the current DOACs because there is emerging evidence that FXI is essential for thrombosis but mostly dispensable for hemostasis. In addition to the oral inhibitors of FXIa, parenteral inhibitors are also under development. These include fesomersen, an antisense oligonucleotide that reduces the hepatic synthesis of FXI; abelacimab, an antibody that binds to FXI and blocks its activation; and osocimab, an FXIa inhibitory antibody. Focusing on these new agents, this article describes the unmet needs in oral anticoagulation therapy, explains why FXI is a promising target for new oral anticoagulants, reviews phase 2 clinical data on new agents, describes ongoing phase 3 trials, and provides a perspective on the opportunities and challenges for FXI inhibitors.

Simplified predictive scores for thrombosis and bleeding complications in newly diagnosed acute leukemia patients

BACKGROUND

Bleeding and thrombotic complications are the leading causes of death in acute leukemia patients. The Conventional International Society of Thrombosis and Haemostasis Disseminated Intravascular Coagulation (ISTH DIC) scoring system is utilized to assess DIC diagnoses in various conditions. Nevertheless, limited studies have tested the system's accuracy in predicting thrombo-hemorrhagic events in individuals with acute leukemia. This study aimed to (1) validate the ISTH DIC scoring system and (2) propose a new Siriraj Acute Myeloid/Lymphoblastic Leukemia (SiAML) bleeding and thrombosis scoring system for thrombohemorrhagic risk assessment in acute leukemia.

METHODS

We conducted a retro-prospective observational study of newly diagnosed acute leukemia patients between March 2014 and December 2019. We recorded thrombohemorrhagic episodes within 30 days postdiagnosis and DIC profiles, including prothrombin time, platelet level, D-dimer, and fibrinogen. The sensitivities, specificities, positive and negative predictive values, and areas under receiver operating characteristic curves for the ISTH DIC and SiAML scoring systems were calculated.

RESULTS

In all, 261 acute leukemia patients were identified: 64% with acute myeloid leukemia, 27% with acute lymphoblastic leukemia, and 9% with acute promyelocytic leukemia. Overall bleeding and thrombotic events were 16.8% and 6.1%, respectively. With a cutoff of 5 for the ISTH DIC score, the sensitivity and specificity for bleeding prediction were 43.5% and 74.4%, respectively, while the corresponding values for thrombotic prediction were 37.5% and 71.8%, respectively. D-dimer > 5000 µg FEU/L and fibrinogen ≤ 150 mg/dL were significantly associated with bleeding. A SiAML-bleeding score was calculated using these factors, with a sensitivity and specificity of 65.2% and 65.6%, respectively. Conversely, D-dimer > 7000 µg FEU/L, platelet > 40 × 10⁹/L, and white blood cell level > 15 × 10⁹/L were significant variables related to thrombosis. Using these variables, we established a SiAML-thrombosis score with a sensitivity and specificity of 93.8% and 66.1%, respectively.

CONCLUSIONS

The proposed SiAML scoring system might be valuable for prognosticating individuals at risk for bleeding and thrombotic complications. Prospective validation studies are needed to verify its usefulness.

A site on factor XII required for productive interactions with polyphosphate

BACKGROUND

During plasma contact activation, factor XII (FXII) binds to surfaces through its heavy chain and undergoes conversion to the protease FXIIa. FXIIa activates prekallikrein and factor XI (FXI). Recently, we showed that the FXII first epidermal growth factor-1 (EGF1) domain is required for normal activity when polyphosphate is used as a surface.

OBJECTIVES

The aim of this study was to identify amino acids in the FXII EGF1 domain required for polyphosphate-dependent FXII functions.

METHODS

FXII with alanine substitutions for basic residues in the EGF1 domain were expressed in HEK293 fibroblasts. Wild-type FXII (FXII-WT) and FXII containing the EGF1 domain from the related protein Pro-HGFA (FXII-EGF1) were positive and negative controls. Proteins were tested for their capacity to be activated, and to activate prekallikrein and FXI, with or without polyphosphate, and to replace FXII-WT in plasma clotting assays and a mouse thrombosis model.

RESULTS

FXII and all FXII variants were activated similarly by kallikrein in the absence of polyphosphate. However, FXII with alanine replacing Lys73, Lys74, and Lys76 (FXII-Ala73,74,76) or Lys76, His78, and Lys81 (FXII-Ala76,78,81) were activated poorly in the presence of polyphosphate. Both have <5% of normal FXII activity in silica-triggered plasma clotting assays and have reduced binding affinity for polyphosphate. Activated FXIIa-Ala73,74,76 displayed profound defects in surface-dependent FXI activation in purified and plasma systems. FXIIa-Ala73,74,76 reconstituted FXII-deficient mice poorly in an arterial thrombosis model.

CONCLUSION

FXII Lys73, Lys74, Lys76, and Lys81 form a binding site for polyanionic substances such as polyphosphate that is required for surface-dependent FXII function.

Targeting NETs using dual-active DNase1 variants

Background

Neutrophil Extracellular Traps (NETs) are key mediators of immunothrombotic mechanisms and defective clearance of NETs from the circulation underlies an array of thrombotic, inflammatory, infectious, and autoimmune diseases. Efficient NET degradation depends on the combined activity of two distinct DNases, DNase1 and DNase1-like 3 (DNase1L3) that preferentially digest double-stranded DNA (dsDNA) and chromatin, respectively.

Methods

Here, we engineered a dual-active DNase with combined DNase1 and DNase1L3 activities and characterized the enzyme for its NET degrading potential in vitro. Furthermore, we produced a mouse model with transgenic expression of the dual-active DNase and analyzed body fluids of these animals for DNase1 and DNase 1L3 activities. We systematically substituted 20 amino acid stretches in DNase1 that were not conserved among DNase1 and DNase1L3 with homologous DNase1L3 sequences.

Results

We found that the ability of DNase1L3 to degrade chromatin is embedded into three discrete areas of the enzyme's core body, not the C-terminal domain as suggested by the state-of-the-art. Further, combined transfer of the aforementioned areas of DNase1L3 to DNase1 generated a dual-active DNase1 enzyme with additional chromatin degrading activity. The dual-active DNase1 mutant was superior to native DNase1 and DNase1L3 in degrading dsDNA and chromatin, respectively. Transgenic expression of the dual-active DNase1 mutant in hepatocytes of mice lacking endogenous DNases revealed that the engineered enzyme was stable in the circulation, released into serum and filtered to the bile but not into the urine.

Conclusion

Therefore, the dual-active DNase1 mutant is a promising tool for neutralization of DNA and NETs with potential therapeutic applications for interference with thromboinflammatory disease states.

Smart thrombosis inhibitors without bleeding side effects via charge tunable ligand design

Current treatments to prevent thrombosis, namely anticoagulants and platelets antagonists, remain complicated by the persistent risk of bleeding. Improved therapeutic strategies that diminish this risk would have a huge clinical impact. Antithrombotic agents that neutralize and inhibit polyphosphate (polyP) can be a powerful approach towards such a goal. Here, we report a design concept towards polyP inhibition, termed macromolecular polyanion inhibitors (MPI), with high binding affinity and specificity. Lead antithrombotic candidates are identified through a library screening of molecules which possess low charge density at physiological pH but which increase their charge upon binding to polyP, providing a smart way to enhance their activity and selectivity. The lead MPI candidates demonstrates antithrombotic activity in mouse models of thrombosis, does not give rise to bleeding, and is well tolerated in mice even at very high doses. The developed inhibitor is anticipated to open avenues in thrombosis prevention without bleeding risk, a challenge not addressed by current therapies.

Neutrophil extracellular traps are increased after extracorporeal membrane oxygenation support initiation and present in thrombus: A preclinical study using sheep as an animal model

BACKGROUND

The balance between thrombosis and hemostasis is a difficult issue during extracorporeal membrane oxygenation (ECMO) support. The pathogenesis leading to thrombotic complications during ECMO support is not well understood. Neutrophil extracellular traps (NETs) were reported to participate in thrombosis and have a key role in inflammation. This study aimed to explore the role of NETs in thrombosis during ECMO support and investigate NETs as a predictive biomarker for thrombotic complications during ECMO assistance.

METHODS

Ten ovine models of ECMO support were established. Animals were then randomly divided into 2 groups (5 sheep/group): venoarterial (VA) ECMO group and venovenous (VV) ECMO group. The venous blood samples were collected at different time points. Markers of NETs were detected in plasma, neutrophils, and thrombi from the vessels and membrane. Moreover, circulating NETs levels in 8 adults treated in the intensive care unit (ICU) who received VA-ECMO and 8 healthy controls were detected; patient survival was also recorded.

RESULTS

In vivo study showed that neutrophils and NETs markers (dsDNA and citH3) levels were significantly elevated 6 h after ECMO compared to baseline. Isolated neutrophils from fresh blood at 6 h could release more NETs. dsDNA and citH3 levels were significantly higher in the VA mode than in the VV mode. NETs were found in thrombi from the vessel and membrane. Clinical data further revealed that dsDNA, citH3, and nucleosomes were higher in patients who received ECMO than in healthy controls.

CONCLUSIONS

These data suggest NETs might be associated with thrombus during ECMO support, especially in the VA mode. These findings provide new insight into preventing thrombotic complications by targeting NETs. Also, NETs may potentially become an early warning biomarker for thrombosis under ECMO assistance.

Intrinsic coagulation pathway-mediated thrombin generation in mouse whole blood

Calibrated Automated Thrombography (CAT) is a versatile and sensitive method for analyzing coagulation reactions culminating in thrombin generation (TG). Here, we present a CAT method for analyzing TG in murine whole blood by adapting the CAT assay used for measuring TG in human plasma. The diagnostically used artificial and physiologic factor XII (FXII) contact activators kaolin, ellagic acid and polyphosphate (polyP) stimulated TG in murine blood in a dose-dependent manner resulting in a gradual increase in endogenous thrombin potential and peak thrombin, with shortened lag times and times to peak. The activated FXII inhibitor rHA-Infestin-4 and direct oral anticoagulants (DOACs) interfered with TG triggered by kaolin, ellagic acid and polyP and TG was completely attenuated in blood of FXII- (F12 -/-) and FXI-deficient (F11 -/-) mice. Moreover, reconstitution of blood from F12 -/- mice with human FXII restored impaired contact-stimulated TG. HEK293 cell-purified polyP also initiated FXII-driven TG in mouse whole blood and addition of the selective inhibitor PPX_Δ12 ablated natural polyP-stimulated TG. In conclusion, the data provide a method for analysis of contact activation-mediated TG in murine whole blood. As the FXII-driven intrinsic pathway of coagulation has emerged as novel target for antithrombotic agents that are validated in mouse thrombosis and bleeding models, our novel assay could expedite therapeutic drug development.

Recent advances in factor XII structure and function

PURPOSE OF REVIEW

Factor XII (FXII), the precursor of the protease FXIIa, contributes to pathologic processes including angioedema and thrombosis. Here, we review recent work on structure-function relationships for FXII based on studies using recombinant FXII variants.

RECENT FINDINGS

FXII is a homolog of pro-hepatocyte growth factor activator (Pro-HGFA). We prepared FXII in which domains are replaced by corresponding parts of Pro-HGA, and tested them in FXII activation and activity assays. In solution, FXII and prekallikrein undergo reciprocal activation to FXIIa and kallikrein. The rate of this process is restricted by the FXII fibronectin type-2 and kringle domains. Pro-HGA replacements for these domains accelerate FXII and prekallikrein activation. When FXII and prekallikrein bind to negatively charged surfaces, reciprocal activation is enhanced. The FXII EGF1 domain is required for surface binding.

SUMMARY

We propose a model in which FXII is normally maintained in a closed conformation resistant to activation by intramolecular interactions involving the fibronectin type-2 and kringle domains. These interactions are disrupted when FXII binds to a surface through EGF1, enhancing FXII activation and prekallikrein activation by FXIIa. These observations have important implications for understanding the contributions of FXII to disease, and for developing therapies to treat thrombo-inflammatory disorders.

Fibrin structure, viscoelasticity and lysis face the interplay of biorelevant polyions

PURPOSE OF REVIEW

In the past 5 decades, heparins have been widely used as anticoagulants in the prevention and treatment of thrombosis. Subsequent development of heparin variants of various size and charge facilitated the discovery of their multiple biological actions and nonanticoagulant benefits. Platelet-derived or microbial polyphosphates, as well as DNA released in the course of neutrophil extracellular trap-formation are additional polyanions, which can modulate the development and stability of thrombi associated with cancer or inflammation. In this review, we focus on the size-dependent and electric charge-dependent modulatory effects of the three polyanions of different chemical structure.

RECENT FINDINGS

The polycationic histones have been recognized as potential biomarkers and therapeutic targets in several diseases related to inflammation and thrombosis. Since combating histones with activated protein C or heparin could cause unwanted bleeding, the quest for nonanticoagulant histone-neutralizing agents is ongoing. Polyanions may neutralize or exaggerate certain histone-mediated effects depending on their electric charge, size and histone effects under investigation. Several prothrombotic effects of polyphosphates and DNA are also size-dependent.

SUMMARY

The efficiency of future therapeutics targeting prothrombotic polyanions or histones is not a simple matter of electric charge, but may rely on a delicate combination of size, charge and chemical composition.

Anionic Ultrasmall Gold Nanoparticles Bind to Coagulation Factors and Disturb Normal Hemostatic Balance

Ultrasmall gold nanoparticles (usNPs) and nanoclusters are an emerging class of nanomaterials exhibiting distinctive physicochemical properties and in vivo behaviors. Although understanding the interactions of usNPs with blood components is of fundamental importance to advance their clinical translation, currently, little is known about the way that usNPs interact with the hemostatic system. This study describes the effects of a model anionic p-mercaptobenzoic acid-coated usNP on the coagulation cascade, with particular emphasis on the contact pathway. It is found that in a purified system, the anionic usNPs bind to and activate factor XII (FXII). The formed usNP-FXII complexes are short-lived (residence time of ∼10 s) and characterized by an affinity constant of ∼200 nM. In human plasma, the anionic usNPs activate the contact pathway and promote coagulation. The usNPs also exhibit anticoagulant activity in plasma by interfering with the thrombin-mediated cleavage of fibrinogen. Taken together, these findings establish that anionic usNPs can disturb the normal hemostatic balance, which in turn may hinder their clinical translation. Finally, it is shown that usNPs can be designed to be nearly inert in plasma by surface coating with the natural peptide glutathione.

Coagulation and complement: Key innate defense participants in a seamless web

In 1969, Dr. Oscar Ratnoff, a pioneer in delineating the mechanisms by which coagulation is activated and complement is regulated, wrote, “In the study of biological processes, the accumulation of information is often accelerated by a narrow point of view. The fastest way to investigate the body’s defenses against injury is to look individually at such isolated questions as how the blood clots or how complement works. We must constantly remind ourselves that such distinctions are man-made. In life, as in the legal cliché, the devices through which the body protects itself form a seamless web, unwrinkled by our artificialities.” Our aim in this review, is to highlight the critical molecular and cellular interactions between coagulation and complement, and how these two major component proteolytic pathways contribute to the seamless web of innate mechanisms that the body uses to protect itself from injury, invading pathogens and foreign surfaces.

Polyphosphate nanoparticles enhance the fibrin stabilization by histones more efficiently than linear polyphosphates

Introduction

Beyond the three-dimensional fibrin network, the mechanical and lytic stability of thrombi is supported by the matrix of neutrophil extracellular traps (NETs) composed of polyanionic DNA meshwork with attached proteins including polycationic histones. Polyphosphates represent another type of polyanions, which in their linear form are known to enhance the fibrin stabilizing effects of DNA and histones. However, in vivo polyphosphates are also present in the form of nanoparticles (PolyP-NP), the interference of which with the fibrin/NET matrix is poorly characterized.

Aims

To compare the effects of linear and nanoparticulate polyphosphates, and their combinations with relevant NET components (DNA, histone H3) on fibrin formation, structure, and lysis in in vitro assays focusing on histone-polyphosphate interactions.

Methods

Transmission electron microscopy and dynamic light scattering for stability of the PolyP-NP preparations. Turbidimetry for kinetics of fibrinogen clotting by thrombin and fibrin dissolution by tissue-type plasminogen activator/plasminogen. Scanning electron microscopy for fibrin structure. Surface plasmon resonance for strength of histone-PolyP interactions.

Results

Both linear PolyP and PolyP-NP accelerated the fibrin formation and slowed down its dissolution and these effects were strongly dependent on the number of individual PolyP particles and not on their size. Addition of DNA did not modify significantly the PolyP-NP effects on fibrin formation and lysis. Both linear and nanoparticulate PolyP counteracted the effect of histone in the acceleration of fibrinogen clotting by thrombin. PolyP-NP, but not linear PolyP enhanced the prolongation of lysis time in fibrin containing histone and caused more pronounced thickening of the fibrin fibers than the linear form. Finally, PolyP-NP bound weaker to histone than the linear form.

Conclusions

The interaction of PolyP with histone was a stronger modulator of fibrin formation and lysis than its interaction with DNA. In addition, the PolyP nanoparticles enhanced the thrombus stabilizing effects of histone more effectively than linear PolyP.

An Update on Safe Anticoagulation

Blood coagulation is essential to maintain the integrity of a closed circulatory system (hemostasis), but also contributes to thromboembolic occlusion of vessels (thrombosis). Thrombosis may cause deep vein thrombosis, pulmonary embolism, myocardial infarction, peripheral artery disease, and ischemic stroke, collectively the most common causes of death and disability in the developed world. Treatment for the prevention of thromboembolic diseases using anticoagulants such as heparin, coumarins, thrombin inhibitors, or antiplatelet drugs increase the risk of bleeding and are associated with an increase in potentially life-threatening hemorrhage, partially offsetting the benefits of reduced coagulation. Thus, drug development aiming at novel targets is needed to provide efficient and safe anticoagulation. Within the last decade, experimental and preclinical data have shown that some coagulation mechanisms principally differ in thrombosis and hemostasis. The plasma contact system protein factors XII and XI, high-molecular-weight kininogen, and plasma kallikrein specifically contribute to thrombosis, however, have minor, if any, role in hemostatic coagulation mechanisms. Inherited deficiency in contact system proteins is not associated with increased bleeding in humans and animal models. Therefore, targeting contact system proteins provides the exciting opportunity to interfere specifically with thromboembolic diseases without increasing the bleeding risk. Recent studies that investigated pharmacologic inhibition of contact system proteins have shown that this approach provides efficient and safe thrombo-protection that in contrast to classical anticoagulants is not associated with increased bleeding risk. This review summarizes therapeutic and conceptual developments for selective interference with pathological thrombus formation, while sparing physiologic hemostasis, that enables safe anticoagulation treatment.

NETs in the infarct-related coronary artery - a marker or mediator of adverse outcome?

No Abstract.

Identification of the factor XII contact activation site enables sensitive coagulation diagnostics

Contact activation refers to the process of surface-induced activation of factor XII (FXII), which initiates blood coagulation and is captured by the activated partial thromboplastin time (aPTT) assay. Here, we show the mechanism and diagnostic implications of FXII contact activation. Screening of recombinant FXII mutants identified a continuous stretch of residues Gln317-Ser339 that was essential for FXII surface binding and activation, thrombin generation and coagulation. Peptides spanning these 23 residues competed with surface-induced FXII activation. Although FXII mutants lacking residues Gln317-Ser339 were susceptible to activation by plasmin and plasma kallikrein, they were ineffective in supporting arterial and venous thrombus formation in mice. Antibodies raised against the Gln317-Ser339 region induced FXII activation and triggered controllable contact activation in solution leading to thrombin generation by the intrinsic pathway of coagulation. The antibody-activated aPTT allows for standardization of particulate aPTT reagents and for sensitive monitoring of coagulation factors VIII, IX, XI.

Management of Disseminated Intravascular Coagulation in Acute Leukemias

Disseminated intravascular coagulation (DIC) is characterized by the intravascular activation of coagulation with loss of localization arising from different causes, and is diagnosed using scoring systems which rely upon the presence of an underlying disorder compatible with DIC alongside hemostatic derangements such as low platelet count, prolonged prothrombin time, and elevated fibrinogen degradation products. DIC is common in patients with acute leukemia, with prevalence ranging from 17 to 100% in acute promyelocytic leukemia (APL) and 8.5 to 25% in acute lymphoblastic leukemia (ALL) and non-APL acute myeloid leukemia (AML). The pathophysiology is complex and varies between the leukemia subtypes, and is not fully reflected by the laboratory markers currently used to classify DIC. Similarly, the clinical consequence of DIC in acute leukemia also varies across the types of leukemia. DIC is primarily associated with bleeding in APL, while thrombosis is the dominant phenotype in ALL and non-APL AML. The cornerstone of managing DIC is the treatment of the underlying disease, as exemplified by the important role of early administration of all-trans retinoic acid in APL. Other aspects of management focus on supportive care aimed at minimizing the risk of bleeding, via transfusion of blood products. The use of blood products is more liberal in APL, due to the hemorrhagic phenotype and unacceptably high rates of early hemorrhagic death. This review will focus on the pathophysiology, risk factors, clinical implications, and the management of DIC in patients across the spectrum of acute leukemias.