Novel aspects of platelet factor XIII function

RUNX1 Isoforms Regulate RUNX1 and Target-Genes Differentially in Platelets-Megakaryocytes: Association with Clinical Cardiovascular Events

Background

Hematopoietic transcription factor RUNX1 is expressed from proximal P2 and distal P1 promoter to yield isoforms RUNX1 B and C, respectively. The roles of these isoforms in RUNX1 autoregulation and downstream-gene regulation in megakaryocytes and platelets are unknown.

Objectives

To understand the regulation of RUNX1 and its target genes by RUNX1 isoforms.

Methods

We performed studies on RUNX1 isoforms in megakaryocytic HEL cells and HeLa cells (lack endogenous RUNX1), in platelets from 85 healthy volunteers administered aspirin or ticagrelor, and on the association of RUNX1 target genes with acute events in 587 patients with cardiovascular disease (CVD).

Results

In chromatin immunoprecipitation and luciferase promoter assays, RUNX1 isoforms B and C bound and regulated P1 and P2 promoters. In HeLa cells RUNX1B decreased and RUNX1C increased P1 and P2 activities, respectively. In HEL cells, RUNX1B overexpression decreased RUNX1C and RUNX1A expression; RUNX1C increased RUNX1B and RUNX1A. RUNX1B and RUNX1C regulated target genes (MYL9, F13A1, PCTP, PDE5A and others) differentially in HEL cells. In platelets RUNX1B transcripts (by RNAseq) correlated negatively with RUNX1C and RUNX1A; RUNX1C correlated positively with RUNX1A. RUNX1B correlated positively with F13A1, PCTP, PDE5A, RAB1B, and others, and negatively with MYL9. In our previous studies, RUNX1C transcripts in whole blood were protective against acute events in CVD patients. We found that higher expression of RUNX1 targets F13A1 and RAB31 associated with acute events.

Conclusions

RUNX1 isoforms B and C autoregulate RUNX1 and regulate downstream genes in a differential manner and this associates with acute events in CVD.

Novel function of transglutaminase 2 in extracellular histone-induced acute lung injury

Extracellular histones induce endothelial damage, resulting in lung haemorrhage; however, the underlying mechanism remains unclear. Factor XIII, as a Ca2+-dependent cross-linking enzyme in blood, mediates fibrin deposition. As another isozyme, transglutaminase 2 (TG2) has a catalytic activity distributing in most tissues. Herein, we investigated whether TG2 promotes fibrin deposition and mediates the adhesion of platelets to ECs in histone-induced acute lung injury (ALI). We evaluated the lung histology and the adhesion of platelets to endothelial cells (ECs) after injecting histones to wild-type (WT) C57BL/6J and TG2 knockout (TG2-/-) mice, and administered a TG2 inhibitor (NC9) to WT mice. Pulmonary haemorrhage was more severe in TG2-/- mice than that in WT mice. The area of fibrin deposition and the proportion of CD41+CD31+ cells were lower in TG2-/- mice than in WT mice. Pre-treatment of NC9 decreased the area of fibrin deposition and the proportion of CD41+CD31+ cells in WT mice. These results suggest that TG2 prevents from pulmonary haemorrhage in ALI by promoting the adhesion of platelets to ECs and the fibrin deposition.

The role of tumor-educated platelets in ovarian cancer: A comprehensive review and update

Platelets have recently surfaced as critical players in cancer metastasis and the local and systemic responses to tumor growth. The emerging concept of "Tumor-educated platelets (TEPs)" comprises the exchange of biomolecules between tumor cells and platelets, thereby leading to the "education" of platelets. Increased platelet numbers have long been associated with cancer patients' tumor metastasis and poor clinical prognosis. However, it is very recently that researchers have delved deeper into the tumor-microenvironment and probed the mechanism of interactions between tumor cells and platelets. Designing strategies to target the TEPs and the communications between platelets and tumor cells can prove to be a promising breakthrough in cancer therapy. Through this review, we aim to analyze the recent developments in this field and discuss the characteristics of TEPs, focusing on ovarian cancer-associated TEPs and their characteristics, the interplay between ovarian cancer-associated TEPs and cancer cells, and the purview of TEP-targeted cancer diagnosis and therapy, including platelet biomarkers and inhibitors.

Fibrinogen and Factor XIII in Venous Thrombosis and Thrombus Stability

As the third most common vascular disease, venous thromboembolism is associated with significant mortality and morbidity. Pathogenesis underlying venous thrombosis is still not fully understood. Accumulating data suggest fibrin network structure and factor XIII-mediated crosslinking are major determinants of venous thrombus mass, composition, and stability. Understanding the cellular and molecular mechanisms mediating fibrin(ogen) and factor XIII production and function and their ability to influence venous thrombogenesis and resolution may inspire new anticoagulant strategies that target these proteins to reduce or prevent venous thrombosis in certain at-risk patients. This article summarizes fibrinogen and factor XIII biology and current knowledge of their function during venous thromboembolism.

The Effect of Activated FXIII, a Transglutaminase, on Vascular Smooth Muscle Cells

Plasma factor XIII (pFXIII) is a heterotetramer of FXIII-A and FXIII-B subunits. The cellular form (cFXIII), a dimer of FXIII-A, is present in a number of cell types. Activated FXIII (FXIIIa), a transglutaminase, plays an important role in clot stabilization, wound healing, angiogenesis and maintenance of pregnancy. It has a direct effect on vascular endothelial cells and fibroblasts, which have been implicated in the development of atherosclerotic plaques. Our aim was to explore the effect of FXIIIa on human aortic smooth muscle cells (HAoSMCs), another major cell type in the atherosclerotic plaque. Osteoblastic transformation induced by Pi and Ca²⁺ failed to elicit the expression of cFXIII in HAoSMCs. EZ4U, CCK-8 and CytoSelect Wound Healing assays were used to investigate cell proliferation and migration. The Sircol Collagen Assay Kit was used to monitor collagen secretion. Thrombospondin-1 (TSP-1) levels were measured by ELISA. Cell-associated TSP-1 was detected by the immunofluorescence technique. The TSP-1 mRNA level was estimated by RT-qPCR. Activated recombinant cFXIII (rFXIIIa) increased cell proliferation and collagen secretion. In parallel, a 67% decrease in TSP-1 concentration in the medium and a 2.5-fold increase in cells were observed. TSP-1 mRNA did not change significantly. These effects of FXIIIa might contribute to the pathogenesis of atherosclerotic plaques.

Factor XIII is a newly identified binding partner for platelet collagen receptor GPVI-dimer-An interaction that may modulate fibrin crosslinking

Background

In the fibrin-forming process, thrombin cleaves fibrinogen to fibrin, which form fibrils and then fibers, producing a gel-like clot. Thrombin also activates coagulation factor XIII (FXIII), which crosslinks fibrin γ-chains and α-chains, stabilizing the clot. Many proteins bind to fibrin, including FXIII, an established regulation of clot structure, and platelet glycoprotein VI (GPVI), whose contribution to clot function is largely unknown. FXIII is present in plasma, but the abundant FXIII in platelet cytosol becomes exposed to the surface of strongly activated platelets.

Objectives

We determined if GPVI interacts with FXIII and how this might modulate clot formation.

Methods

We measured interactions between recombinant proteins of the GPVI extracellular domain: GPVI-dimer (GPVI-Fc₂) or monomer (GPVI_ex) and FXIII proteins (nonactivated and thrombin-activated FXIII, FXIII subunits A and B) by ELISA. Binding to fibrin clots and fibrin γ-chain crosslinking were analyzed by immunoblotting.

Results

GPVI-dimer, but not GPVI-monomer, bound to FXIII. GPVI-dimer selectively bound to the FXIII A-subunit, but not to the B-subunit, an interaction that was decreased or abrogated by the GPVI-dimer-specific antibody mFab-F. The GPVI-dimer-FXIII interaction decreased the extent of γ-chain crosslinking, indicating a role in the regulation of clot formation.

Conclusions

This is the first report of the specific interaction between GPVI-dimer and the A-subunit of FXIII, as determined in an in vitro system with defined components. GPVI-dimer-FXIII binding was inhibitory toward FXIII-catalyzed crosslinking of fibrin γ-chains in fibrin clots. This raises the possibility that GPVI-dimer may negatively modulate fibrin crosslinking induced by FXIII, lessening clot stability.

Structure functional insights into calcium binding during the activation of coagulation factor XIII A

The dimeric FXIII-A₂, a pro-transglutaminase is the catalytic part of the heterotetrameric coagulation FXIII-A₂B₂ complex that upon activation by calcium binding/thrombin cleavage covalently cross-links preformed fibrin clots protecting them from premature fibrinolysis. Our study characterizes the recently disclosed three calcium binding sites of FXIII-A concerning evolution, mutual crosstalk, thermodynamic activation profile, substrate binding, and interaction with other similarly charged ions. We demonstrate unique structural aspects within FXIII-A calcium binding sites that give rise to functional differences making FXIII unique from other transglutaminases. The first calcium binding site showed an antagonistic relationship towards the other two. The thermodynamic profile of calcium/thrombin-induced FXIII-A activation explains the role of bulk solvent in transitioning its zymogenic dimeric form to an activated monomeric form. We also explain the indirect effect of solvent ion concentration on FXIII-A activation. Our study suggests FXIII-A calcium binding sites could be putative pharmacologically targetable regions.

Platelets loaded with liposome-encapsulated thrombin have increased coagulability

Essentials Platelet transfusions can have limited efficacy during hemorrhage associated with coagulopathy. Thrombin can be shielded by encapsulation into nanoliposomes and delivered to platelets ex vivo. Loading platelets with liposomal thrombin improved several aspects of platelet coagulability. Platelets loaded with liposomal thrombin can overcome some coagulopathic deficiencies in vitro.

SUMMARY

Background Platelets are integral to clot formation and are often transfused to stop or prevent bleeding. However, transfusions of platelets are not always effective, particularly in the most severe cases of hemorrhage. Nanoparticle systems have been developed to mimic platelets but inherently lack important aspects of platelet function, which limits their potential effectiveness. Objectives Increasing the natural coagulability of transfusable platelets could increase their efficacy during treatment of severe hemorrhage. Thrombin is a potent platelet agonist that currently cannot be used intravenously because of the risk of thrombosis. We hypothesized that delivery of thrombin to ex vivo platelets via liposomal encapsulation would enable transfusable platelets to become more coagulable in response to platelet agonists. Methods Thrombin was encapsulated into nanoliposomes and delivered to platelets ex vivo. Platelet coagulability was measured by monitoring platelet activation, clot contraction, clot time and clot stability in several in vitro assays. These parameters were also measured under conditions where coagulation is compromised, including during acidosis, antiplatelet drugs, hemophilia A and trauma-induced coagulopathy. Results Liposomal thrombin was endocytosed and used by platelets ex vivo but was not secreted upon activation. These modified platelets became more sensitive and responsive to agonists and improved clotting time even under conditions that normally cause platelet dysfunction or have impaired coagulation. Conclusions Several aspects of platelet function were enhanced by ex vivo delivery of liposomal thrombin.

Factor XIII Subunit A in the Skin: Applications in Diagnosis and Treatment

The role of factor XIII subunit A (FXIII-A) is not restricted to hemostasis. FXIII-A is also present intracellularly in several human cells and serves as a diagnostic marker in a wide range of dermatological diseases from inflammatory conditions to malignancies. In this review, we provide a guide on the still controversial interpretation of dermal cell types expressing FXIII-A and assess the previously described mechanisms behind their accumulation under physiological and pathological conditions of the human skin. We summarize the intracellular functions of FXIII-A as well as its possible sources in the extracellular space of the dermis with a focus on its relevance to skin homeostasis and disease pathogenesis. Finally, the potential role of FXIII-A in wound healing, as a field with long-term therapeutic implications, is also discussed.

Patterns and functional implications of platelets upon tumor "education"

While platelets are traditionally recognized to play a predominant role in hemostasis and thrombosis, increasing evidence verifies its involvement in malignancies. As a component of the tumor microenvironment, platelets influence carcinogenesis, tumor metastasis and chemotherapy efficiency. Platelets status is thus predictable as a hematological biomarker of cancer prognosis and a hot target for therapeutic intervention. On the other hand, the role of circulating tumor cells (CTCs) as an inducer of platelet activation and aggregation has been well acknowledged. The cross-talk between platelets and CTCs is reciprocal on that the CTCs activate platelets while platelets contribute to CTCs' survival and dissemination. This review covers some of the current issues related to the loop between platelets and tumor aggression, including the manners of tumor cells in "educating" platelets and biofunctional alterations of platelets upon tumor "education". We also highlight the potential clinical applications on the interplay between tumors and platelets. Further studies with well-designed prospective multicenter trials may contribute to clinical "liquid biopsy" diagnosis by evaluating the global changes of platelets.

Development and validation of a high throughput whole blood thrombolysis plate assay

The objective of this work was to develop a high throughput assay for testing in vitro the thrombolytic activity using citrated whole blood samples, and to overcome the limitations of currently available techniques. We successfully developed a method that involves forming halo shaped, tissue factor induced, whole blood clots in 96 well plates, and then precisely measuring the thrombolysis process with a spectrophotometer plate reader. We here describe the implementation of this novel method, which we refer to as halo assay, and its validation with plasmin, urokinase and tissue plasminogen activator at different doses. The resulting data is a highly detailed thrombolysis profile, allowing comparison of different fibrinolytic agents. The time point analysis allows kinetic data to be collected and calculated to determine key parameters such as the activation time and the rate of fibrinolysis. We also assessed the capacity of the model to study the effect of clot maturation time on the fibrinolytic rate, an aspect of thrombosis rather unexplored with currently available methods, but of increasing importance in drug development. This novel thrombolysis assay could be an extremely useful research tool; to study the complex process of thrombolysis, and a valuable translational clinical tool; as a screening device to rapidly identify hypo- or hyper-fibrinolysis.

Global assays of fibrinolysis

Fibrinolysis is an important and integral part of the hemostatic system. Acting as a balance to blood coagulation, the fibrinolytic system protects the body from unwanted thrombus formation and occlusion of blood vessels. As long as blood coagulation and fibrinolysis remain in equilibrium, response to injury, such as vessel damage, is appropriately regulated. However, alterations in this balance may lead to thrombosis or bleeding. A variety of methods have been proposed to assess fibrinolytic activity in blood or its components, but due to the complexity of the system, the design of a "gold standard" assay that reflects overall fibrinolysis has remained an elusive goal. In this review, we describe the most commonly used methods that have been described, such as thromboelastography (TEG and ROTEM), global fibrinolytic capacity in plasma and whole blood, plasma turbidity methods, simultaneous thrombin and plasmin generation assays, euglobulin clot lysis time and fibrin plate methods. All of these assays have strengths and limitations. We suggest that some methods may be preferable for detecting hypofibrinolytic conditions, whereas others may be better for detecting hyperfibrinolytic states.