Platelet factor 4 neutralizes heparan sulfate-enhanced antithrombin inactivation of factor Xa by preventing interaction(s) of enzyme with polysaccharide

Chemokines, molecular drivers of thromboinflammation and immunothrombosis

Blood clotting is a finely regulated process that is essential for hemostasis. However, when dysregulated or spontaneous, it promotes thrombotic disorders. The fact that these are triggered, accompanied and amplified by inflammation is reflected in the term thromboinflammation that includes chemokines. The role of chemokines in thrombosis is therefore illuminated from a cellular perspective, where endothelial cells, platelets, red blood cells, and leukocytes may be both the source and target of chemokines. Chemokine-dependent prothrombotic processes may thereby occur independently of chemokine receptors or be mediated by chemokine receptors, although the binding and activation of classical G protein-coupled receptors and their signaling pathways differ from those of atypical chemokine receptors, which do not function via cell activation and recruitment. Regardless of binding to their receptors, chemokines can induce thrombosis by forming platelet-activating immune complexes with heparin or other polyanions that are pathognomonic for HIT and VITT. In addition, chemokines can bind to NETs and alter their structure. They also change the electrical charge of the cell surface of platelets and interact with coagulation factors, thereby modulating the balance of fibrinolysis and coagulation. Moreover, CXCL12 activates CXCR4 on platelets independently of classical migratory chemokine activity and causes aggregation and thrombosis via the PI3Kβ and Btk signaling pathways. In contrast, typical chemokine-chemokine receptor interactions are involved in the processes that contribute to the adhesiveness of the endothelium in the initial phase of venous thrombosis, where neutrophils and monocytes subsequently accumulate in massive numbers. Later, the reorganization and resolution of a thrombus require coordinated cell migration and invasion of the thrombus, and, as such, indeed, chemokines recruit leukocytes to existing thrombi. Therefore, chemokines contribute in many independent ways to thrombosis.

The Glycosaminoglycan Side Chains and Modular Core Proteins of Heparan Sulphate Proteoglycans and the Varied Ways They Provide Tissue Protection by Regulating Physiological Processes and Cellular Behaviour

This review examines the roles of HS-proteoglycans (HS-PGs) in general, and, in particular, perlecan and syndecan as representative examples and their interactive ligands, which regulate physiological processes and cellular behavior in health and disease. HS-PGs are essential for the functional properties of tissues both in development and in the extracellular matrix (ECM) remodeling that occurs in response to trauma or disease. HS-PGs interact with a biodiverse range of chemokines, chemokine receptors, protease inhibitors, and growth factors in immune regulation, inflammation, ECM stabilization, and tissue protection. Some cell regulatory proteoglycan receptors are dually modified hybrid HS/CS proteoglycans (betaglycan, CD47). Neurexins provide synaptic stabilization, plasticity, and specificity of interaction, promoting neurotransduction, neurogenesis, and differentiation. Ternary complexes of glypican-1 and Robbo-Slit neuroregulatory proteins direct axonogenesis and neural network formation. Specific neurexin-neuroligin complexes stabilize synaptic interactions and neural activity. Disruption in these interactions leads to neurological deficits in disorders of functional cognitive decline. Interactions with HS-PGs also promote or inhibit tumor development. Thus, HS-PGs have complex and diverse regulatory roles in the physiological processes that regulate cellular behavior and the functional properties of normal and pathological tissues. Specialized HS-PGs, such as the neurexins, pikachurin, and Eyes-shut, provide synaptic stabilization and specificity of neural transduction and also stabilize the axenome primary cilium of phototoreceptors and ribbon synapse interactions with bipolar neurons of retinal neural networks, which are essential in ocular vision. Pikachurin and Eyes-Shut interactions with an α-dystroglycan stabilize the photoreceptor synapse. Novel regulatory roles for HS-PGs controlling cell behavior and tissue function are expected to continue to be uncovered in this fascinating class of proteoglycan.

Thrombosis and thrombocytopenia in COVID-19 and after COVID-19 vaccination

Thrombosis that occurs in coronavirus disease 19 (COVID-19) is a serious complication and a critical aspect of pathogenesis in the disease progression. Although thrombocytopenia is uncommon in the initial presentation, it may also reflect disease severity due to the ability of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to activate platelets. This occurs directly through the spike protein-angiotensin converting enzyme 2 (ACE2) interaction and indirectly by coagulation and inflammation activation. Dysregulation in both innate and adaptive immune systems is another critical factor that causes thrombosis and thrombocytopenia in COVID-19.

Vaccination is the most potent and effective tool to mitigate COVID-19; however, rare side effects, namely vaccine-induced immune thrombotic thrombocytopenia (VITT)/thrombosis with thrombocytopenia syndrome (TTS) can occur following adenovirus-vectored vaccine administration. VITT/TTS is rare, and thrombocytopenia can be the clue to detect this serious complication. It is important to consider that thrombocytopenia and/or thromboembolism are not events limited to post-vaccination with vectored vaccine, but are also seen rarely after vaccination with other vaccines.

Various conditions mimic VITT/TTS, and it is vital to achieving the correct diagnosis at an earlier stage. Antiplatelet factor 4 (PF4) antibody detection by the enzyme-linked immunosorbent assay (ELISA) is used for diagnosing VITT/TTS. However, false-positive rates also occur in vaccinated people, who do not show any thrombosis or thrombocytopenia. Vaccinated people with messenger RNA vaccine can show positive but low density and non-functional in terms of platelet aggregation, it is vital to check the optical density. If anti-PF4 ELISA is not available, discriminating other conditions such as antiphospholipid syndrome, thrombotic thrombocytopenic purpura, immune thrombocytopenic purpura, systemic lupus erythematosus, and hemophagocytic syndrome/hemophagocytic lymphohistiocytosis is critical when the patients show thrombosis with thrombocytopenia after COVID-19 vaccination.

Prothrombotic Phenotype in COVID-19: Focus on Platelets

COVID-19 infection is associated with a broad spectrum of presentations, but alveolar capillary microthrombi have been described as a common finding in COVID-19 patients, appearing as a consequence of a severe endothelial injury with endothelial cell membrane disruption. These observations clearly point to the identification of a COVID-19-associated coagulopathy, which may contribute to thrombosis, multi-organ damage, and cause of severity and fatality. One significant finding that emerges in prothrombotic abnormalities observed in COVID-19 patients is that the coagulation alterations are mainly mediated by the activation of platelets and intrinsically related to viral-mediated endothelial inflammation. Beyond the well-known role in hemostasis, the ability of platelets to also release various potent cytokines and chemokines has elevated these small cells from simple cell fragments to crucial modulators in the blood, including their inflammatory functions, that have a large influence on the immune response during infectious disease. Indeed, platelets are involved in the pathogenesis of acute lung injury also by promoting NET formation and affecting vascular permeability. Specifically, the deposition by activated platelets of the chemokine platelet factor 4 at sites of inflammation promotes adhesion of neutrophils on endothelial cells and thrombogenesis, and it seems deeply involved in the phenomenon of vaccine-induced thrombocytopenia and thrombosis. Importantly, the hyperactivated platelet phenotype along with evidence of cytokine storm, high levels of P-selectin, D-dimer, and, on the other hand, decreased levels of fibrinogen, von Willebrand factor, and thrombocytopenia may be considered suitable biomarkers that distinguish the late stage of COVID-19 progression in critically ill patients.

The roles of platelets in COVID-19-associated coagulopathy and vaccine-induced immune thrombotic thrombocytopenia

In coronavirus disease 2019 (COVID-19), multiple thromboinflammatory events contribute to the pathophysiology, including coagulation system activation, suppressed fibrinolysis, vascular endothelial cell injury, and prothrombotic alterations in immune cells such as macrophages and neutrophils. Although thrombocytopenia is not an initial presentation as an infectious coagulopathy, recent studies have demonstrated the vital role of platelets in COVID-19-associated coagulopathy SARS-CoV-2 and its spike protein have been known to directly or indirectly promote release of prothrombotic and inflammatory mediators that lead to COVID-19-associated coagulopathy. Although clinical features of vaccine-induced immune thrombotic thrombocytopenia include uncommon locations of thrombosis, including cerebral venous sinus, we speculate coronavirus spike-protein-initiated prothrombotic pathways are involved in the pathogenesis of vaccine-induced immune thrombotic thrombocytopenia, as current evidence suggests that the spike protein is the promotor and other cofactors such as perturbed immune response and inflammatory reaction enhance the production of anti-platelet factor 4 antibody.

Glycosaminoglycan Neutralization in Coagulation Control

The glycosaminoglycans (GAGs) heparan sulfate, dermatan sulfate, and heparin are important anticoagulants that inhibit clot formation through interactions with antithrombin and heparin cofactor II. Unfractionated heparin, low-molecular-weight heparin, and heparin-derived drugs are often the main treatments used clinically to handle coagulatory disorders. A wide range of proteins have been reported to bind and neutralize these GAGs to promote clot formation. Such neutralizing proteins are involved in a variety of other physiological processes, including inflammation, transport, and signaling. It is clear that these interactions are important for the control of normal coagulation and influence the efficacy of heparin and heparin-based therapeutics. In addition to neutralization, the anticoagulant activities of GAGs may also be regulated through reduced synthesis or by degradation. In this review, we describe GAG neutralization, the proteins involved, and the molecular processes that contribute to the regulation of anticoagulant GAG activity.

Heparin-induced thrombocytopenia in cardiovascular patients: pathophysiology, diagnosis, and treatment

Heparin-induced thrombocytopenia is a life-threatening immune-mediated platelet activation condition that can cause arterial and venous thromboembolism. The triggering complex, platelet factor 4/heparin antibody, has several unique immunologic characteristics that have not been well elucidated until recently. In patients undergoing cardiovascular procedures such as percutaneous coronary intervention and coronary artery bypass graft surgery, the prevalence of platelet factor 4/heparin antibody is significantly higher than that in the general population. The acuity and graveness of the thromboembolic phenomenon requires early diagnosis and empirical initiation of treatment, even before confirmatory test results are available. Also, although multiple therapeutic modalities exist, the safety and efficacy of each option depends upon the clinical setting. Therefore, this review will focus on the updated pathophysiology of heparin-induced thrombocytopenia, new diagnostic criteria, and the various treatment options for cardiovascular patients with different conditions.

CXCL4L1 inhibits angiogenesis and induces undirected endothelial cell migration without affecting endothelial cell proliferation and monocyte recruitment

BACKGROUND AND OBJECTIVES

The non-allelic variant of CXCL4/PF4, CXCL4L1/PF4alt, differs from CXCL4 in three amino acids of the C-terminal α-helix and has been characterized as a potent anti-angiogenic regulator. Although CXCL4 structurally belongs to the chemokine family, it does not behave like a 'classical' chemokine, lacking significant chemotactic properties. Specific hallmarks are its angiostatic, anti-proliferative activities, and proinflammatory functions, which can be conferred by heteromer-formation with CCL5/RANTES enhancing monocyte recruitment.

METHODS AND RESULTS

Here we show that tube formation of endothelial cells was inhibited by CXCL4L1 and CXCL4, while only CXCL4L1 triggered chemokinesis of endothelial cells. The chemotactic response towards VEGF and bFGF was attenuated by both variants and CXCL4L1-induced chemokinesis was blocked by bFGF or VEGF. Endothelial cell proliferation was inhibited by CXCL4 (IC(50) 6.9 μg mL(-1)) but not by CXCL4L1, while both chemokines bound directly to VEGF and bFGF. Moreover, CXCL4 enhanced CCL5-induced monocyte arrest in flow adhesion experiments and monocyte recruitment into the mouse peritoneal cavity in vivo, whereas CXCL4L1 had no effect. CXCL4L1 revealed lower affinity to CCL5 than CXCL4, as quantified by isothermal fluorescence titration. As evidenced by the reduction of the activated partial thromboplastin time, CXCL4L1 showed a tendency towards less heparin-neutralizing activity than CXCL4 (IC(50) 2.45 vs 0.98 μg mL(-1)).

CONCLUSIONS

 CXCL4L1 may act angiostatically by causing random endothelial cell locomotion, disturbing directed migration towards angiogenic chemokines, serving as a homeostatic chemokine with a moderate structural distinction yet different functional profile from CXCL4.

Dual effect of Platelet Factor 4 on the activities of Factor Xa

Platelet Factor 4 (PF4) prevents inhibition of blood coagulation proteases by heparin via formation of a putative enzyme-PF4 complex. To investigate the contribution of the latter, the activity of factor Xa (fXa) was determined in chromogenic assays measuring hydrolysis of a peptide substrate S2765 or cleavage of the macromolecular substrate prothrombin in the activating complex, prothrombinase. Upon preincubation with fXa and heparin, PF4 at about 250 nM decreased the k(cat) of S2765 hydrolysis about fivefold and that of prothrombin activation about 25-fold. In the presence of saturating fVa, inhibition of fXa by PF4 was abolished, while in the presence of limiting fVa, PF4 altered the interaction of fXa with fVa. Interestingly, high concentrations of PF4 restored fXa activity toward S2765 and prothrombin, indicating a dual effect of PF4 on fXa activities. These findings suggest that PF4 in the presence of heparin is an allosteric effector of the prothrombinase complex.

Novel thrombo-resistant coating based on iron-polysaccharide complex multilayers

The biocompatibility of iron-polysaccharide complexes has been well-documented. Herein, a stable thrombo-resistant coating was fabricated by consecutive adsorption of Fe (III) and polysaccharides including heparin (Hep) and dextran sulfate (DS) onto various surface by layer-by-layer self-assembly technique via both electrostatic interaction and chemical complexation process. The absorbance at 350 nm increased linearly with the number of Fe3+/Hep multilayer, indicating the formation of multilayer structure and the uniform coating. Compared with (Fe3+/Hep)10, the (Fe3+/DS/Fe3+/Hep)5 coating was more hydrophilic and stable due to the incorporation of DS. The activated partial thromboplastin time (APTT) and platelet adhesion assays showed that both (Fe3+/Hep)10 and (Fe3+/DS/Fe3+/Hep)5 coated surfaces were anticoagulant. The complexing with ferric ions did not compromise the catalytic capacity of heparin to promote antithrombin(III)-mediated thrombin inactivation. Chromogenic assays for heparin activity proved definitively that the inhibition of locally produced thrombin was contributed to the thromboresistance of the surface-bound heparin. The surface with Hep or DS as the outmost layer showed stronger anticoagulant activity than Fe3+, indicating that the outermost layer of the coating played a key role in anticoagulant activity. The utilization of dextran sulfate/heparin surfaces was more advantageous than merely the heparin surface for improving blood-contacting medical devices for long-term usage.

Novel hollow microcapsules based on iron-heparin complex multilayers

Iron-polysaccharide complex have been extensively utilized in the treatment of iron deficiency anemia for parenteral administration. Herein, a novel iron-heparin complexed hollow capsules with nanoscaled wall thickness have been fabricated by means of alternating deposition of ferric ions (III) (Fe+) and heparin (Hep) onto the surface of submicroscaled (488 nm) and microscaled (10.55 microm) polystyrene latex particles via both electrostatic interaction and chemical complexation processes, followed by dissolution of the cores using tetrahydrofuran. Confocal micrographs and atomic force microscopy (AFM) images prove that iron-heparin complexed submicroscaled hollow capsules keep spherical shapes in solution and even after drying. The activated partial thromboplastin time (APTT) assay shows that complexing with ferric ions do not compromise the catalytic capacity of heparin to promote antithrombin III-mediated thrombin inactivation. The anticoagulant activity value of (Fe3+/Hep)8 capsules is evaluated to be about 95.7 U/mg, indicating that approximately 0.55 mg heparin was in 1 mg powder of submicroscaled (Fe3+/Hep)8 hollow capsules. Compared with the same dosage of heparin, iron-heparin complexed hollow capsules display a more prolonged anticoagulant duration than heparin. All these results reveal that such submicroscaled iron-heparin complexed hollow capsules have application potential as an injectable anticoagulant vehicle.

In Situ Investigation of Biomaterial Surfaces Using LDH and ELISA

Lactate dehydrogenase (LDH) and enzyme-linked immunosorbent assay (ELISA) have attracted much attention recently for the evaluation of blood compatibility of biomaterials due to their convenience and quantifiability. In this paper, the use of LDH and ELISA is described for in situ investigation of platelet behavior on biomaterial surfaces, including quantification of platelet adhesion and platelet activation, after suitable testing conditions have been established. The material samples investigated in these tests included low temperature isotropic carbon (LTIC), Ti-O films, and phosphorus- and aluminum-doped TiO2 films. The evaluation results show that the lowest platelet adhesion and activation are observed on phosphorus-doped TiO2 films while the highest platelet adhesion and activation are observed on LTIC. In addition, conventional platelet adhesion experiments were performed for comparison, and yielding similar evaluation results as LDH and ELISA. It is suggested that LDH and ELISA tests can be successfully applied to evaluate the blood compatibility of biomaterials and can show many advantages, such as quantification, reliability and objectivity, compared with conventional platelet adhesion test.

Pharmacological treatment of ischemic stroke

Current pharmacological strategies for acute ischemic stroke largely mirror those employed in acute coronary syndromes. However, important differences in the effectiveness and versatility of the principal agents have emerged between these 2 clinical settings. In general, the level of success achieved with drugs in acute coronary syndromes has not carried over to the same extent when the same drug types are used in stroke. The principal reason is that reperfusion or anticoagulant therapies in the setting of brain infarction run a significant risk of hemorrhagic transformation that has no direct equivalent in myocardial infarction. Consequently, a significant challenge in acute stroke therapeutics is the ability to select patients for drugs where only a narrow therapeutic margin exists and to identify methods that can minimize hemorrhage risk. Other brain-specific vascular factors also pertain in explaining differences in outcome of drugs generally regarded as having a broad cardiovascular remit. The relatively limited efficacy of antiplatelets in stroke might relate to the composition and heterogeneity of the cerebrovascular lesion, while the poor outcome associated with acute anti-hypertensive use is partly due to loss of cerebrovascular autoregulation. Finally, downstream consequences of arterial occlusion within the brain such as excitotoxicity and plasticity are organ specific and, as such, deserve their own pharmacological approaches. In this review, we describe the general mechanism of each drug class used in ischemic stroke and then report on the clinical experience and application for each.

Heterophilic interactions of platelet factor 4 and RANTES promote monocyte arrest on endothelium

The chemokines platelet factor 4 (PF4) and RANTES (regulated on activation normal T cell expressed and secreted) are secreted by activated platelets and influence multiple cell types and biologic processes. For instance, PF4 inhibits progenitor cell proliferation and angiogenesis, while platelet-derived RANTES is involved in vascular recruitment of monocytes. However, little is known about functional interactions of PF4 and RANTES. Here we show that the presence of PF4 enhanced the arrest of RANTES-stimulated monocytes and monocytic cells on activated endothelial cells under flow conditions, while binding of PF4 to the monocyte surface was increased by RANTES. Both RANTES-triggered arrest and PF4 binding involved monocytic chondroitin sulfate. Ligand blots and surface plasmon resonance revealed a robust heterophilic interaction of PF4 with RANTES but not with RANTES variants defective in higher order oligomerization. The tetrameric mutant E26A bound to the monocyte surface without increasing PF4 binding, and monocyte arrest induced by E26A-RANTES was not enhanced by PF4. Stimulation of monocytes with supernatants of activated platelets triggered arrest involving RANTES and PF4, as shown by inhibition studies. Our results suggest that heterophilic interactions with PF4 require structural motifs important in RANTES oligomerization and amplify RANTES-triggered effects on monocyte adhesion. This may have implications for the modulation of inflammatory recruitment by platelet-derived chemokines.