Platelet-coagulant protein interactions in contact activation

Cationic zinc is required for factor XII recruitment and activation by stimulated platelets and for thrombus formation in vivo

BACKGROUND

Although divalent zinc (Zn2+ ) is known to bind factor (F)XII and affect its sensitivity to autoactivation, little is known about the role of Zn2+ in the binding of FXII to platelets, where FXII activation is thought to occur in vivo, and the function of Zn2+ during thrombus formation following vascular injury remains poorly understood.

OBJECTIVES

To evaluate the role of Zn2+ in platelet-dependent FXIIa generation.

METHODS

FXII binding to platelets and FXII activation by stimulated platelets were assessed using flow cytometry and a platelet-dependent thrombin generation assay. The mouse cremaster laser injury model was used to evaluate the impact of Zn2+ chelation on thrombus formation in vivo.

RESULTS

Our data demonstrate that stimulated platelets support FXII-dependent thrombin generation and that FXII activation by platelets requires the presence of Zn2+ . By contrast, thrombin generation by stimulated endothelial cells occurred independently of FXII and Zn2+ . Using flow cytometry, we found that FXII-fluorescein-5-isothiocyanate binds to the surfaces of stimulated platelets in a specific and Zn2+ -dependent manner, whereas resting platelets demonstrated minimal binding. Other physiologically-relevant divalent cations are unable to support this interaction. Consistent with these findings, the Zn2+ -specific chelator ethylenediaminetetraacetic acid calcium disodium salt confers thromboprotection in the mouse cremaster laser injury model without causing increased bleeding. We observed an identical phenotype in FXII null mice tested in the same system.

CONCLUSIONS

Our results suggest a novel role for Zn2+ in the binding and activation of FXII at the platelet surface, an interaction that appears crucial to FXII-dependent thrombin generation but dispensable for hemostasis.

Cell Receptor and Cofactor Interactions of the Contact Activation System and Factor XI

The contact activation system (CAS) or contact pathway is central to the crosstalk between coagulation and inflammation and contributes to diverse disorders affecting the cardiovascular system. CAS initiation contributes to thrombosis but is not required for hemostasis and can trigger plasma coagulation via the intrinsic pathway [through factor XI (FXI)] and inflammation via bradykinin release. Activation of factor XII (FXII) is the principal starting point for the cascade of proteolytic cleavages involving FXI, prekallikrein (PK), and cofactor high molecular weight kininogen (HK) but the precise location and cell receptor interactions controlling these reactions remains unclear. FXII, PK, FXI, and HK utilize key protein domains to mediate binding interactions to cognate cell receptors and diverse ligands, which regulates protease activation. The assembly of contact factors has been demonstrated on the cell membranes of a variety of cell types and microorganisms. The cooperation between the contact factors and endothelial cells, platelets, and leukocytes contributes to pathways driving thrombosis yet the basis of these interactions and the relationship with activation of the contact factors remains undefined. This review focuses on cell receptor interactions of contact proteins and FXI to develop a cell-based model for the regulation of contact activation.

Factor XII-Driven Inflammatory Reactions with Implications for Anaphylaxis

Anaphylaxis is a life-threatening allergic reaction. It is triggered by the release of pro-inflammatory cytokines and mediators from mast cells and basophils in response to immunologic or non-immunologic mechanisms. Mediators that are released upon mast cell activation include the highly sulfated polysaccharide and inorganic polymer heparin and polyphosphate (polyP), respectively. Heparin and polyP supply a negative surface for factor XII (FXII) activation, a serine protease that drives contact system-mediated coagulation and inflammation. Activation of the FXII substrate plasma kallikrein leads to further activation of zymogen FXII and triggers the pro-inflammatory kallikrein-kinin system that results in the release of the mediator bradykinin (BK). The severity of anaphylaxis is correlated with the intensity of contact system activation, the magnitude of mast cell activation, and BK formation. The main inhibitor of the complement system, C1 esterase inhibitor, potently interferes with FXII activity, indicating a meaningful cross-link between complement and kallikrein-kinin systems. Deficiency in a functional C1 esterase inhibitor leads to a severe swelling disorder called hereditary angioedema (HAE). The significance of FXII in these disorders highlights the importance of studying how these processes are integrated and can be therapeutically targeted. In this review, we focus on how FXII integrates with inflammation and the complement system to cause anaphylaxis and HAE as well as highlight current diagnosis and treatments of BK-related diseases.

Polyphosphate nanoparticles on the platelet surface trigger contact system activation

Polyphosphate is an inorganic polymer that can potentiate several interactions in the blood coagulation system. Blood platelets contain polyphosphate, and the secretion of platelet-derived polyphosphate has been associated with increased thrombus formation and activation of coagulation factor XII. However, the small polymer size of secreted platelet polyphosphate limits its capacity to activate factor XII in vitro. Thus, the mechanism by which platelet polyphosphate contributes to thrombus formation remains unclear. Using live-cell imaging, confocal and electron microscopy, we show that activated platelets retain polyphosphate on their cell surface. The apparent polymer size of membrane-associated polyphosphate largely exceeds that of secreted polyphosphate. Ultracentrifugation fractionation experiments revealed that membrane-associated platelet polyphosphate is condensed into insoluble spherical nanoparticles with divalent metal ions. In contrast to soluble polyphosphate, membrane-associated polyphosphate nanoparticles potently activate factor XII. Our findings identify membrane-associated polyphosphate in a nanoparticle state on the surface of activated platelets. We propose that these polyphosphate nanoparticles mechanistically link the procoagulant activity of platelets with the activation of coagulation factor XII.

Platelet surface-associated activation and secretion-mediated inhibition of coagulation factor XII

Coagulation factor XII (fXII) is important for arterial thrombosis, but its physiological activation mechanisms are unclear. In this study, we elucidated the role of platelets and platelet-derived material in fXII activation. FXII activation was only observed upon potent platelet stimulation (with thrombin, collagen-related peptide, or calcium ionophore, but not ADP) accompanied by phosphatidylserine exposure and was localised to the platelet surface. Platelets from three patients with grey platelet syndrome did not activate fXII, which suggests that platelet-associated fXII-activating material might be released from α-granules. FXII was preferentially bound by phosphotidylserine-positive platelets and annexin V abrogated platelet-dependent fXII activation; however, artificial phosphotidylserine/phosphatidylcholine microvesicles did not support fXII activation under the conditions herein. Confocal microscopy using DAPI as a poly-phosphate marker did not reveal poly-phosphates associated with an activated platelet surface. Experimental data for fXII activation indicates an auto-inhibition mechanism (k_i/k_a = 180 molecules/platelet). Unlike surface-associated fXII activation, platelet secretion inhibited activated fXII (fXIIa), particularly due to a released C1-inhibitor. Platelet surface-associated fXIIa formation triggered contact pathway-dependent clotting in recalcified plasma. Computer modelling suggests that fXIIa inactivation was greatly decreased in thrombi under high blood flow due to inhibitor washout. Combined, the surface-associated fXII activation and its inhibition in solution herein may be regarded as a flow-sensitive regulator that can shift the balance between surface-associated clotting and plasma-dependent inhibition, which may explain the role of fXII at high shear and why fXII is important for thrombosis but negligible in haemostasis.

Hematological Change in Venous Blood of the Lower Leg during Prolonged Sitting in a Low Humidity and Hypobaric Environment

The present study examined the effects of low humidity and hypobaric conditions on hematological change in venous blood of the lower leg during quiet prolonged sitting. Ten healthy male students participated as the subjects after singing a consent form to participate in this study. Their diet and water intake were controlled from 19:00 on the day before the experiments. The subjects sat for 130 min in a climatic chamber. Four experimental conditions in the chamber were designed from a combination of relative humidity (20% or 60%) and air pressure (sea level or equivalent to an altitude of 2,000 m). Ambient temperature was maintained at 24 degrees C in every condition. Venous blood was sampled from the lower leg before and after exposure to the experimental conditions, and was analyzed for blood viscosity and hematological indices. Also, body weight and leg circumference were measured as indices of total water loss and edema, respectively. Regarding the results of ANOVA, significant interactions between humidity and time were observed in blood viscosity, red blood cell count and hematocrit (each p<0.05). However, there were no significant differences in these indices among the conditions. Significant increases were observed in leg circumference (p<0.01), platelet count (p<0.05) and total protein (p<0.05) after the exposure compared with those before the exposure. There were no noticeable effects of hypobaric conditions in every measurement. In conclusion, prolonged sitting seems to be a more hazardous factor for thrombogenesis low humidity and hypobaric conditions during a long-distance flight.

Hepatic and extrahepatic factors critical for liver injury during lipopolysaccharide exposure

Bacterial endotoxin [lipopolysaccharide (LPS)] causes liver injury in vivo that is dependent on platelets, neutrophils [polymorphonuclear leukocytes (PMNs)], and several inflammatory mediators, including thrombin. We tested the hypothesis that thrombin contributes to LPS-induced hepatocellular injury through direct interactions with platelets and/or PMNs in vitro. Perfusion of isolated livers from LPS-treated rats with buffer containing thrombin resulted in a significant increase in alanine aminotransferase (ALT) activity in the perfusion medium, indicating hepatocellular damage. This effect was completely abolished by prior depletion of PMNs from the LPS-treated donor rats but not by depletion of platelets, suggesting interaction between thrombin and PMNs in the pathogenesis. Thrombin did not, however, enhance degranulation of rat PMNs in vitro, and it was not directly toxic to isolated rat hepatocytes in the presence of PMNs even after LPS exposure, suggesting that hepatocellular killing by the PMN-thrombin combination requires the intervention of an additional factor(s) within the liver. In livers from naive donors perfused with buffer containing PMNs and LPS, no injury occurred in the absence of thrombin. Addition of thrombin (10 nM) to the medium caused pronounced ALT release. These results indicate that thrombin and PMNs are sufficient extrahepatic requirements for LPS-induced hepatocellular damage in intact liver.

Biomembrane surfaces as models for polymer design: the potential for haemocompatibility

A major restriction in the application of polymeric biomaterials is the propensity of their surfaces to support thrombosis. Theoretical approaches to the design of thromboresistant polymers have been inadequate because of the complexity of surface thrombosis. We have developed a new, practical approach to this problem--the design of polymers which mimic the thromboresistant surfaces of blood cell membranes. Haemostatic processes are mediated by reactions which occur at membrane-plasma interfaces. The extra-cellular surfaces of the plasma membranes of red blood cells and quiescent platelets are thromboresistant; in contrast, their cytoplasmic surfaces are thrombogenic. The simplest common feature among the blood-compatible cellular and model membranes is the high content of the electrically neutral phospholipids which contain the phosphorylcholine head group. We have developed model systems of biological membranes which utilize polymerizable phosphatidylcholines and which mimic nonreactive cell surfaces. Polymeri phospholipids represent a new class of hybrid biomaterials with characteristics both of biomembranes (polar surfaces, nonthrombogenic, low antigenic potential and low permeability) and of synthetic polymers (chemical and physical stability).