The surface-dependent autoactivation mechanism of factor XII

A comprehensive insight into the role of zinc deficiency in the renin-angiotensin and kinin-kallikrein system dysfunctions in COVID-19 patients

Hypozincemia is prevalent in severe acute respiratory syndrome coronavirus-2 (SARS-COV-2)-infected patients and has been considered as a risk factor in severe coronavirus disease-2019 (COVID-19). Whereas zinc might affect SARS-COV-2 replication and cell entry, the link between zinc deficiency and COVID-19 severity could also be attributed to the effects of COVID-19 on the body metabolism and immune response. Zinc deficiency is more prevalent in the elderly and patients with underlying chronic diseases, with established deleterious consequences such as the increased risk of respiratory infection. We reviewed the expected effects of zinc deficiency on COVID-19-related pathophysiological mechanisms focusing on both the renin-angiotensin and kinin-kallikrein systems. Mechanisms and effects were extrapolated from the available scientific literature. Zinc deficiency alters angiotensin-converting enzyme-2 (ACE2) function, leading to the accumulation of angiotensin II, des-Arg9-bradykinin and Lys-des-Arg9-bradykinin, which results in an exaggerated pro-inflammatory response, vasoconstriction and pro-thrombotic effects. Additionally, zinc deficiency blocks the activation of the plasma contact system, a protease cascade initiated by factor VII activation. Suggested mechanisms include the inhibition of Factor XII activation and limitation of high-molecular-weight kininogen, prekallikrein and Factor XII to bind to endothelial cells. The subsequent accumulation of Factor XII and deficiency in bradykinin are responsible for increased production of inflammatory mediators and marked hypercoagulability, as typically observed in COVID-19 patients. To conclude, zinc deficiency may affect both the renin-angiotensin and kinin-kallikrein systems, leading to the exaggerated inflammatory manifestations characteristic of severe COVID-19.

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.

Contact activation-induced complex formation between complement factor H and coagulation factor XIIa

BACKGROUND

The complement and coagulation systems share an evolutionary origin with many components showing structural homology. Certain components, including complement factor H (FH) and coagulation factor XII (FXII), have separately been shown to have auxiliary activities across the two systems.

OBJECTIVES

The interaction between FXII and FH was investigated.

METHODS

Using enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR) complex formation between different FXII forms and FH was investigated. The presence of α-FXIIa:FH complexes upon contact activation in plasma was evaluated by ELISA and immunoprecipitation.

RESULTS

We identified and characterized a direct interaction between the components and demonstrated that among different forms of FXII, only the activated α-FXIIa formed complexes with FH, with an apparent binding strength Kd of 34 ± 9 nmol/L. The complex formation involved the kringle domain of the heavy chain of FXII. C1-inhibitor induced inhibition of α-FXIIa did not alter the binding of α-FXIIa toward FH. We further demonstrated the presence of α-FXIIa:FH complexes in normal human plasma upon contact activation, indicating formation of α-FXIIa:FH complexes as a consequence of α-FXIIa generation. Complex formation between α-FXIIa and FH was also assessed in hereditary angioedema (HAE) patients with C1-inhibitor deficiency as well as rheumatoid arthritis (RA) patients with high levels of anti-cyclic citrullinated peptide (anti-CCP) upon contact activation. We observed elevated levels of α-FXIIa:FH complexes in HAE patients, and equal levels of complexes in RA patients and healthy individuals upon contact activation.

CONCLUSION

A direct interaction between α-FXIIa and FH is demonstrated. Our findings represent a new crosstalk between these systems, potentially important in the onset and pathology of inflammatory vascular diseases.

Polyphosphate, Zn2+ and high molecular weight kininogen modulate individual reactions of the contact pathway of blood clotting

BACKGROUND

Inorganic polyphosphate modulates the contact pathway of blood clotting, which is implicated in thrombosis and inflammation. Polyphosphate polymer lengths are highly variable, with shorter polymers (approximately 60-100 phosphates) secreted from human platelets, and longer polymers (up to thousands of phosphates) in microbes. We previously reported that optimal triggering of clotting via the contact pathway requires very long polyphosphates, although the impact of shorter polyphosphate polymers on individual proteolytic reactions of the contact pathway was not interrogated.

OBJECTIVES AND METHODS

We conducted in vitro measurements of enzyme kinetics to investigate the ability of varying polyphosphate sizes, together with high molecular weight kininogen and Zn2+ , to mediate four individual proteolytic reactions of the contact pathway: factor XII autoactivation, factor XII activation by kallikrein, prekallikrein activation by factor XIIa, and prekallikrein autoactivation.

RESULTS

The individual contact pathway reactions were differentially dependent on polyphosphate length. Very long-chain polyphosphate was required to support factor XII autoactivation, whereas platelet-size polyphosphate significantly accelerated the activation of factor XII by kallikrein, and the activation of prekallikrein by factor XIIa. Intriguingly, polyphosphate did not support prekallikrein autoactivation. We also report that high molecular weight kininogen was required only when kallikrein was the enzyme (ie, FXII activation by kallikrein), whereas Zn2+ was required only when FXII was the substrate (ie, FXII activation by either kallikrein or FXIIa). Activation of prekallikrein by FXIIa required neither Zn2+ nor high molecular weight kininogen.

CONCLUSIONS

Platelet polyphosphate and Zn2+ can promote subsets of the reactions of the contact pathway, with implications for a variety of disease states.

Zinc: An important cofactor in haemostasis and thrombosis

There is mounting evidence that zinc, the second most abundant transition metal in blood, is an important mediator of haemostasis and thrombosis. Prompted by the observation that zinc deficiency is associated with bleeding and clotting abnormalities, there now is evidence that zinc serves as an effector of coagulation, anticoagulation and fibrinolysis. Zinc binds numerous plasma proteins and modulates their structure and function. Because activated platelets secrete zinc into the local microenvironment, the concentration of zinc increases in the vicinity of a thrombus. Consequently, the role of zinc varies depending on the microenvironment; a feature that endows zinc with the capacity to spatially and temporally regulate haemostasis and thrombosis. This paper reviews the mechanisms by which zinc regulates coagulation, platelet aggregation, anticoagulation and fibrinolysis and outlines how zinc serves as a ubiquitous modulator of haemostasis and thrombosis.

The initiation and effects of plasma contact activation: an overview

The plasma contact system sits atop the intrinsic coagulation cascade and plasma kallikrein-kinin pathway, and in vivo its activation contributes, respectively, to coagulation and inflammation mainly via two downstream pathways. This system has been widely investigated, its activation mechanisms by negatively charged surfaces and the interactions within its components, factor XII, prekallikrein and high molecular weight kininogen are well understood at the biochemical level. However, as most of the activators that have been discovered by in vitro experiments are exogenous, the physiological activators and roles of the contact system have remained unclear and controversial. In the last two decades, several physiological activators have been identified, and a better understanding of its roles and its connection with other signaling pathways has been obtained from in vivo studies. In this article, we present an overview of the contact pathway with a focus on the activation mechanisms, natural stimuli, possible physiological roles, potential risks of its excessive activation, remaining questions and future prospects.

Inhibition of thrombin generation in human plasma by phospholipid transfer protein

Background

Plasma phospholipid transfer protein (PLTP) transfers lipids between donors and acceptors (e.g., from HDL to VLDL) and modulates lipoprotein composition, size, and levels. No study has reported an assessment of the effects of PLTP on blood clotting reactions, such as reflected in thrombin generation assays, or on the association of venous thrombosis (VTE) risk with PLTP activity.

Methods

The in vitro effects of PLTP on blood coagulation reactions and the correlations between plasma PLTP activity levels and VTE were studied.

Results

Recombinant (r) PLTP concentration-dependently inhibited plasma thrombin generation and factor XII-dependent kallikrein generation when sulfatide was used to stimulate factor XII autoactivation in plasma. However, rPLTP did not inhibit thrombin generation in plasma induced by factor XIa or tissue factor, implicating an effect of PLTP on contact activation reactions. In purified systems, rPLTP inhibited factor XII autoactivation stimulated by sulfatide in the presence of VLDL. In surface plasmon resonance studies, purified factor XII bound to immobilized rPLTP, implying that rPLTP inhibits factor XII-dependent contact activation by binding factor XII in the presence of lipoproteins. Analysis of plasmas from 40 male patients with unprovoked VTE and 40 matched controls indicated that low PLTP lipid transfer activity (≤25th percentile) was associated with an increased risk of VTE after adjustment for body mass index, plasma lipids, and two known thrombophilic genetic risk factors.

Conclusion

These data imply that PLTP may be an antithrombotic plasma protein by inhibiting generation of prothrombotic factor XIIa in the presence of VLDL. This newly discovered anticoagulant activity of PLTP merits further clinical and biochemical studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12959-015-0054-0) contains supplementary material, which is available to authorized users.

New agents for thromboprotection. A role for factor XII and XIIa inhibition

Blood coagulation is essential for hemostasis, however excessive coagulation can lead to thrombosis. Factor XII starts the intrinsic coagulation pathway and contact-induced factor XII activation provides the mechanistic basis for the diagnostic aPTT clotting assay. Despite its function for fibrin formation in test tubes, patients and animals lacking factor XII have a completely normal hemostasis. The lack of a bleeding tendency observed in factor XII deficiency states is in sharp contrast to deficiencies of other components of the coagulation cascade and factor XII has been considered to have no function for coagulation in vivo. Recently, experimental animal models showed that factor XII is activated by an inorganic polymer, polyphosphate, which is released from procoagulant platelets and that polyphosphate-driven factor XII activation has an essential role in pathologic thrombus formation. Cumulatively, the data suggest to target polyphosphate, factor XII, or its activated form factor XIIa for anticoagulation. As the factor XII pathway specifically contributes to thrombosis but not to hemostasis, interference with this pathway provides a unique opportunity for safe anticoagulation that is not associated with excess bleeding. The review summarizes current knowledge on factor XII functions, activators and inhibitors.

Single-chain factor XII exhibits activity when complexed to polyphosphate

BACKGROUND

The mechanism underpinning factor XII autoactivation was originally characterized with non-physiological surfaces, such as dextran sulfate (DS), ellagic acid, and kaolin. Several 'natural' anionic activating surfaces, such as platelet polyphosphate (polyP), have now been identified.

OBJECTIVE

To analyze the autoactivation of FXII by polyP of a similar length to that found in platelets (polyP70 ).

METHODS AND RESULTS

PolyP70 showed similar efficacy to DS in stimulating autoactivation of FXII, as detected with amidolytic substrate. Western blotting revealed different forms of FXII with the two activating surfaces: two-chain αFXIIa was formed with DS, whereas single-chain FXII (scFXII; 80 kDa) was formed with polyP70 . Dissociation of scFXII from polyP70 abrogated amidolytic activity, suggesting reversible exposure of the active site. Activity of scFXII-polyP70 was enhanced by Zn(2+) and was sensitive to NaCl concentration. A bell-shaped concentration response to polyP70 was evident, as is typical of surface-mediated reactions. Reaction of scFXII-polyP70 with various concentrations of S2302 generated a sigmoidal curve, in contrast to a hyperbolic curve for αFXIIa, from which a Hill coefficient of 3.67 was derived, indicative of positive cooperative binding. scFXII-polyP70 was more sensitive to inhibition by H-d-Pro-Phe-Arg-chloromethylketone and corn trypsin inhibitor than αFXIIa, but inhibition profiles for C1-inhibitor were similar. Active scFXII-polyP70 was also able to cleave its physiological targets FXI and prekallikrein to their active forms.

CONCLUSIONS

Autoactivation of FXII by polyP, of the size found in platelets, proceeds via an active single-chain intermediate. scFXII-polyP70 shows activity towards physiological substrates, and may represent the primary event in initiating contact activation in vivo.

Dynamics of pathologic clot formation: a mathematical model

Recent studies have provided evidence of a significant role of the Hageman factor in pathologic clot formation. Since auto-activation of the Hageman factor triggers the intrinsic coagulation pathway, we study the dynamics of pathologic clot formation considering the intrinsic pathway as the predominant mechanism of this process. Our methodological approach to studying the dynamics of clot formation is based on mathematical modelling. Activation of the blood coagulation cascade, particularly its intrinsic pathway, is known to involve platelets. Therefore, equations accounting for the effects of activated platelets on the intrinsic pathway activation are included in our model. This brings about a considerable increase in the values of kinetic constants involved in the model of the principal biochemical processes resulting in clot formation. The purpose of this study is to elucidate the mechanism of pathologic clot formation. Since the time window of thrombolysis is 3-6h, we hypothesize that in many cases the rate of pathologic clot formation is much lower than that of haemostatic clot. This assumption is used to simplify the mathematical model and to estimate kinetic constants of biochemical reactions that initiate pathologic clot formation. The insights we gained from our mathematical model may lead to new approaches to the prophylaxis of pathologic clot formation. We believe that one of the most efficient ways to prevent pathologic clot formation is simultaneous inhibition of activated factors ХII and ХI.

Immobilized transition metal ions stimulate contact activation and drive factor XII-mediated coagulation

BACKGROUND

Upon contact with an appropriate surface, factor XII (FXII) undergoes autoactivation or cleavage by kallikrein. Zn(2+) is known to facilitate binding of FXII and the cofactor, high molecular weight kininogen (HK), to anionic surfaces.

OBJECTIVES

To investigate whether transition metal ions immobilized on liposome surfaces can initiate coagulation via the contact pathway.

METHODS AND RESULTS

Liposomes containing a metal ion-chelating lipid, 1,2-dioleoyl-sn-glycero-3-{(N[5-amino-1-carboxypentyl]iminodiacetic acid)succinyl} ammonium salt (DOGS-NTA), were prepared by membrane extrusion (20% DOGS-NTA, 40% phosphatidylcholine, 10% phosphatidylserine, and 30% phosphatidylethanolamine). Ni(2+) immobilized on such liposomes accelerated clotting in normal plasma, but not factor XI (FXI)-deficient or FXII-deficient plasma. The results were similar to those obtained with a commercial activated partial thromboplastin time reagent. Charging such liposomes with other transition metal ions revealed differences in their procoagulant capacity, with Ni(2+) > Cu(2+) > Co(2+) and Zn(2+). Plasma could be depleted of FXI, FXII and HK by adsorption with Ni(2+) -containing beads, resulting in longer clot times. Consistent with this, FXI, FXII and HK bound to immobilized Ni(2+) or Cu(2+) with high affinity as determined by surface plasmon resonance. In the presence of Ni(2+) -bearing liposomes, K(m) and k(cat) values derived for autoactivation of FXII and prekallikrein, as well as for activation of FXII by kallikrein or prekallikrein by FXIIa, were similar to literature values obtained in the presence of dextran sulfate.

CONCLUSIONS

Immobilized Ni(2+) and Cu(2+) bind FXII, FXI and HK with high affinity and stimulate activation of the contact pathway, driving FXII-mediated coagulation. Activation of the contact system by immobilized transition metal ions may have implications during pathogenic infection or in individuals exposed to high levels of pollution.

Stimulation of blood mononuclear cells with bacterial virulence factors leads to the release of pro-coagulant and pro-inflammatory microparticles

Severe infectious diseases remain a major and life-threatening health problem. In serious cases a systemic activation of the coagulation cascade and hypovolemic shock are critical complications that are associated with high mortality rates. Here we report that blood mononuclear cells, stimulated with M1 protein of Streptococcus pyogenes or other bacterial virulence factors, produce not only pro-coagulant, but also pro-inflammatory microparticles (MPs). Our results also show that activation of the contact system on MPs contributes to these two effects. Phosphatidylserine (PS) plays an important role in these processes as its upregulation on MPs allows an assembly and activation of the contact system. This in turn results in stabilization of the tissue factor-induced clot and a processing of high-molecular-weight kininogen by plasma kallikrein followed by the release of bradykinin, a potent vascular mediator. Pro-coagulant monocyte-derived MPs were identified in plasma samples from septic patients and further analysis of MPs from these patients revealed that their pro-coagulant activity is dependent on the tissue factor- and contact system-driven pathway.

Histidine-rich glycoprotein binds factor XIIa with high affinity and inhibits contact-initiated coagulation

Histidine-rich glycoprotein (HRG) circulates in plasma at a concentration of 2μM and binds plasminogen, fibrinogen, and thrombospondin. Despite these interactions, the physiologic role of HRG is unknown. Previous studies have shown that mice and humans deficient in HRG have shortened plasma clotting times. To better understand this phenomenon, we examined the effect of HRG on clotting tests. HRG prolongs the activated partial thromboplastin time in a concentration-dependent fashion but has no effect on tissue factor–induced clotting, localizing its effect to the contact pathway. Plasma immunodepleted of HRG exhibits a shortened activated partial thromboplastin time that is restored to baseline with HRG replenishment. To explore how HRG affects the contact pathway, we examined its binding to factors XII, XIIa, XI, and XIa. HRG binds factor XIIa with high affinity, an interaction that is enhanced in the presence of Zn2+, but does not bind factors XII, XI, or XIa. In addition, HRG inhibits autoactivation of factor XII and factor XIIa–mediated activation of factor XI. These results suggest that, by binding to factor XIIa, HRG modulates the intrinsic pathway of coagulation, particularly in the vicinity of a thrombus where platelet release of HRG and Zn2+ will promote this interaction.

Differential binding of factor XII and activated factor XII to soluble and immobilized fibronectin--localization of the Hep-1/Fib-1 binding site for activated factor XII

Fibronectins (FNs) are dimeric glycoproteins that adopt a globular conformation when present in plasma and solution and an extended conformation in the extracellular matrix. Factor XII (FXII) is a zymogen of the proteolytically active FXIIa that plays a role in thrombus stabilization by enhancing clot formation and in inflammation by enhancing bradykinin formation. To investigate whether the extracellular matrix could play a role in these events, we have recently shown that FXIIa, but not FXII, binds to the extracellular matrix (ECM), and suggested that FN may be the target for the binding. Immunofluorescence microscopy has in the present investigation confirmed that FXIIa added to the ECM colocalizes with FN deposited during growth of human umbilical vein endothelial cells. The aim of the present study, therefore, was to further elucidate the interaction between FXIIa and FN by the use of a solid face binding assay. This showed, like the binding to the ECM, that FXIIa, but not FXII, binds in a Zn2+-independent manner to immobilized FN. The K(D) for the binding was 8.5 +/- 0.9 nM (n = 3). The binding was specific for the immobilized FN, as the binding could not be inhibited by soluble FN. Furthermore, soluble FN did not bind to immobilized FXIIa. However, soluble FN could bind to FXII, and this binding inhibited the surface-induced autoactivation of FXII and subsequent binding of the generated FXIIa to immobilized FN. The presence of FXII in an anti-FN immunoprecipitate of plasma indicated that some FXII in plasma circulates bound to FN. The binding of FXIIa to FN was inhibited by gelatine and fibrin but not by heparin, indicating that FXIIa binds to immobilized FN through the type I repeat modules. Accordingly, FXIIa was found to bind to immobilized fragments of FN containing the type I repeat modules in the N-terminal domain to which fibrin and gelatine bind.

Pharmacological regulation of factor XII activation may be a new target to control pathological coagulation

FXII was identified 50 years ago as a coagulation protein in the intrinsic pathway of blood coagulation as FXII deficient patients had marked prolongation of the in vitro surface-activated coagulation time. However, series of investigations have convincingly shown that FXII has no role in normal hemostasis. Recently, experimentally induced thrombosis in factor XII-knockout mice has provided evidence that factor XII (FXII) deficient mice are protected against ischemic brain injury after obstructive clot formation. Based on these experiments it has, therefore, been suggested, that blocking of FXII could be a unique target to prevent obstructive clot formation in arterial thrombosis without side effect of increased bleeding. FXII deficiency has, however, not convincingly been shown to protect against arterial thrombosis in humans. The target mentioned above may either be an inhibition of FXII activation or an inhibition of its proteolytic activity. FXII is a zymogen of the proteolytic enzyme, FXIIa, the substrates of which are factor XI and prekallikrein. Thus, FXIIa is not only involved in the activation of the coagulation system, but is also associated with the kallikrein/kinin system. The activation of the latter is deeply involved in inflammation and pain sensation. Furthermore, FXIIa binds to endothelial cells and to the extracellular matrix, indicating a role in vascular repair. Therefore, a complete evaluation of all these properties of FXII and FXIIa has to be considered when formulating a strategy for blocking FXII activation.

Identification and characterization of plasma kallikrein-kinin system inhibitors from salivary glands of the blood-sucking insectTriatoma infestans

Two plasma kallikrein-kinin system inhibitors in the salivary glands of the kissing bug Triatoma infestans, designated triafestin-1 and triafestin-2, have been identified and characterized. Reconstitution experiments showed that triafestin-1 and triafestin-2 inhibit the activation of the kallikrein-kinin system by inhibiting the reciprocal activation of factor XII and prekallikrein, and subsequent release of bradykinin. Binding analyses showed that triafestin-1 and triafestin-2 specifically interact with factor XII and high molecular weight kininogen in a Zn2+-dependent manner, suggesting that they specifically recognize Zn2+-induced conformational changes in factor XII and high molecular weight kininogen. Triafestin-1 and triafestin-2 also inhibit factor XII and high molecular weight kininogen binding to negatively charged surfaces. Furthermore, they interact with both the N-terminus of factor XII and domain D5 of high molecular weight kininogen, which are the binding domains for biological activating surfaces. These results suggest that triafestin-1 and triafestin-2 inhibit activation of the kallikrein-kinin system by interfering with the association of factor XII and high molecular weight kininogen with biological activating surfaces, resulting in the inhibition of bradykinin release in an animal host during insect blood-feeding.

Mathematical modeling of material-induced blood plasma coagulation

Contact activation of the intrinsic pathway of the blood coagulation cascade is initiated when a procoagulant material interacts with coagulation factor XII, (FXII) yielding a proteolytic enzyme FXIIa. Procoagulant surface properties are thought to play an important role in activation. To study the mechanism of interaction between procoagulant materials and blood plasma, a mathematical model that is similar in form and in derivation to Michaelis-Menten enzyme kinetics was developed in order to yield tractable relationships between dose (surface area and energy) and response (coagulation time (CT)). The application of this model to experimental data suggests that CT is dependent on the FXIIa concentration and that the amount of FXIIa generated can be analyzed using a model that is linearly dependent on contact time. It is concluded from these experiments and modeling analysis that the primary mechanism for activation of coagulation involves autoactivation of FXII by the procoagulant surface or kallikrein-mediated reciprocal activation of FXII. FXIIa-induced self-amplification of FXII is insignificant.

Factor XII binding to endothelial cells depends on caveolae

It is now generally accepted that factor XII (FXII) binds to cellular surfaces in the vascular system. One of the suggested receptors of this binding is the glycosylphosphatidylinositol-anchored urokinase-like plasminogen activator (u-PAR) harbored in caveolae/lipid rafts. However, binding of FXII to human umbilical vein endothelial cells (HUVEC) has never been shown to be localized to these specialized membrane structures. Using microscopical techniques, we here report that FXII binds to specific patches of the HUVEC plasma membrane with a high density of caveolae. Further investigations of FXII binding to caveolae were performed by sucrose density-gradient centrifugations. This showed that the majority of FXII, chemically cross-linked to HUVEC, could be identified in the same fractions of the gradient as caveolin-1, a marker of caveolae, while the majority of u-PAR was identified in noncaveolae lipid rafts. Accordingly, cholesterol-depleted cells were found to bind significantly reduced amounts of FXII. These observations, combined with the presence of a minority of u-PAR in caveolae concomitant with FXII binding, indicate that FXII binding to u-PAR may be secondary and depends upon the structural elements within caveolae. Thus, FXII binding to HUVEC depends on intact caveolae on the cellular surface.

Expression, refolding, and activation of the catalytic domain of human blood coagulation factor XII

Human blood coagulation factor XII (FXII; 80 kDa) contains a C-terminal serine protease zymogen domain, which becomes activated upon contacting a negative surface. Activated FXII (alphaFXIIa) brings about reciprocal activation of FXII and kallikrein that by further hydrolysis produces the free catalytic domain (betaFXIIa; 28 kDa). Increased levels of alphaFXIIa are associated with coronary heart disease, sepsis, and diabetes. Biophysical investigation of the structural basis of activation, substrate specificity, and regulation of FXII requires an efficient bacterial system for producing the wild-type and mutant recombinant proteins. Here, the cDNA of the zymogen domain of FXII (betaFXII) was cloned into the pET-28a(+) vector and the plasmid was transformed into Escherichia coli strain BL21 (DE3) and overexpressed. The multi-disulfide, recombinant protein, His(6)-betaFXII (rbetaFXII), expressed as an inclusion body, was purified by means of a Ni(2+)-charged resin. The matrix-bound rbetaFXII was subjected to refolding with the glutathione redox system and activated by the in vivo activator, kallikrein. The active form, rbetaFXIIa, obtained in milligram quantities, exhibited similar structural and comparable functional properties relative to human betaFXIIa, as indicated by circular dichroism spectroscopy and kinetics of substrate hydrolysis. Thermodynamics of enzyme:inhibitor complex formation, including the expected 1:1 stoichiometry, was determined for rbetaFXIIa by isothermal calorimetric titration with a specific recombinant protein inhibitor, Cucurbita maxima trypsin inhibitor-V (rCMTI-V; 7kDa).

Binding of activated Factor XII to endothelial cells affects its inactivation by the C1-esterase inhibitor

It is well known that activated Factor XII (FXIIa) and kallikrein are rapidly inactivated in plasma as a result of reaction with endogenous inhibitors. The purpose of this may be to prevent uncontrolled deleterious spreading and activation of target zymogens. Both FXII and the complex plasma prekallikrein/high molecular mass kininogen become activated when they bind, in a Zn2+-dependent manner, to receptors on human umbilical vein endothelial cells (HUVEC). The C1-esterase inhibitor (C1-INH) is by far the most efficient inhibitor of FXIIa. In the present study it has been investigated whether binding of FXIIa to HUVEC might offer protection against inactivation by C1-INH. It appeared that the relative amidolytic activity of purified FXIIa bound to the surface of HUVEC decreased according to the concentration of C1-INH in medium; however, the decrease was smaller than that measured for inactivation of FXIIa in solution. The secondary rate constant for the inactivation was 3-10-fold lower for cell-bound than for soluble FXIIa. The inactivation was found to be caused by C1-INH binding to cell-bound FXIIa. Accordingly, the amidolytic activity of saturated amounts of cell-bound FXIIa was reduced in the presence of C1-INH and was theoretically nonexistent at physiological C1-INH concentrations. Amidolytic activity was, however, present on HUVEC incubated with plasma indicating that the endogenous C1-INH did not completely abolish the activity of FXIIa generated during the incubation period. This supports the hypothesis that binding to endothelial cells protects the activated FXII against inactivation by its major endogenous inhibitor. Hence, the function of FXII may be localized at cellular surfaces.

A mosquito salivary protein inhibits activation of the plasma contact system by binding to factor XII and high molecular weight kininogen

The salivary glands of female mosquitoes contain a variety of bioactive substances that assist their blood-feeding behavior. Here, we report a salivary protein of the malarial vector mosquito, Anopheles stephensi, that inhibits activation of the plasma contact system. This factor, named hamadarin, is a 16-kDa protein and a major component of the saliva of this mosquito. Assays using human plasma showed that hamadarin dose-dependently inhibits activation of the plasma contact system and subsequent release of bradykinin, a primary mediator of inflammatory reactions. Reconstitution experiments showed that hamadarin inhibits activation of the plasma contact system by inhibition of the reciprocal activation of factor XII and kallikrein. Direct binding assays demonstrated that this inhibitory effect is due to hamadarin binding to both factor XII and high molecular weight kininogen and interference in their association with the activating surface. The assays also showed that hamadarin binding to these proteins depends on Zn(2+) ions, suggesting that hamadarin binds to these contact factors by recognizing their conformational change induced by Zn(2+) binding. We propose that hamadarin may attenuate the host's acute inflammatory responses to the mosquito's bites by inhibition of bradykinin release and thus enable mosquitoes to take a blood meal efficiently and safely.

Rapid and cooperative binding of factor XII to human umbilical vein endothelial cells

When activated, factor XII (FXII) has been shown to play a role in a series of proteolytic cascades including systems as the fibrinolytic, the coagulation, the kallikrein-kinin and the complement. How FXII is activated in vivo remains poorly understood as the concentration and density of surface bound negative charges known to trigger the activation in vitro is far from sufficient in vivo. Specific binding of FXII to cellular receptors in the blood stream may, however, solve this problem which may be a question of inter molecular vicinity enhanced by binding to any surface. Here we report that the Zn(2+)-dependent binding of FXII to endothelial cells is rapid, saturable, specific and cooperative. Each endothelial cell from the human umbilical veins was found to bind (417 +/- 202) x 10(3) molecules of FXII with a Kd of (65 +/- 23) nM and a Hill coefficient of 2.1. The binding was inhibited by alpha-FXIIa but not by beta-FXIIa. The Kd for binding alpha-FXIIa was (50 +/- 27) nM. The rate of association was found to be 1.9 x 10(5) M(-1). min(-1). A confirmed inhibition by HK increased the Kd without affecting the maximal number of binding sites and the Hill coefficient. The concentration of HK in serum did not prevent binding of FXII/FXIIa to cells incubated with serum supplemented with Zn2+. The optimal concentration of Zn(2+) was 15 microM for binding factor XII/FXIIa whether purified or in serum.

Activation of the plasma kallikrein/kinin system on endothelial cell membranes

For more than three decades, it has been known that the plasma kallikrein/kinin system becomes activated when exposed to artificial, negatively charged surfaces. The existence of an encompassing in vivo, negatively charged surface capable of activation of the plasma kallikrein/kinin system has, however, never been convincingly demonstrated. In this report, we describe current knowledge on how the proteins of the plasma kallikrein/kinin system assemble to become activated on cell membranes. On endothelial cells, the activation of the plasma kallikrein/kinin system is not initiated by factor XII autoactivation as seen on artificial surfaces. On endothelial cells, prekallikrein is activated by an antipain sensitive protease. Prekallikrein activation is dependent on the presence of high molecular weight kininogen and an optimal free Zn2+ concentration. Kallikrein generated on the surface of endothelial cell is capable of activating factor XII. Further, kallikrein formed on endothelial cell membranes is capable of cleaving its receptor and native substrate, high molecular weight kininogen, liberating bradykinin and the HK PK complex from the endothelial cell surface. Endothelial cell-associated kallikrein also is capable of kinetically favorable pro-urokinase and, subsequent, plasminogen activation.

Activation of the plasma kallikrein/kinin system on endothelial cells

For more than two decades, it has been known that activation of the plasma kallikrein/kinin system only occurs when it is exposed to artificial, negatively charged surfaces. The existence of physiological, negatively charged surfaces has, however, never been demonstrated in vivo. In this report, we describe current knowledge about how the proteins of the plasma kallikrein/kinin system interact with and become activated on cell membranes. In this model, activation of the plasma kallikrein/kinin system on endothelial cells is not initiated by factor XII autoactivation, as seen on artificial surfaces. On endothelial cells, plasma prekallikrein is activated by a membrane-associated cysteine protease. This activation is dependent on the presence of high molecular weight kininogen and an optimal zinc (Zn2+) concentration. Although the initiation of activation of plasma prekallikrein is independent of factor XII, kallikrein-mediated factor XIIa generation, in turn, accelerates the activation of the system. Further kallikrein formed on endothelial cell membranes is capable of cleaving its receptor and native substrate, high molecular weight kininogen, liberating bradykinin and terminating activation. In addition, the kallikrein formed on the surface of endothelial cells results in kinetically favorable activation of prourokinase and, subsequently, plasminogen. Activation of the plasma kallikrein/kinin system on endothelial cells proceeds by a physiological mechanism to initiate cellular fibrinolysis independent of plasmin, fibrin, and tissue-type plasminogen activator.

Partial identification of the Zn2+-binding sites in factor XII and its activation derivatives

With the purpose of identifying the Zn2+-binding sites in factor XII, the effect of chemical modification of His, Glu and Asp residues, amino acids known to participate in the catalytic coordination binding of Zn2+ in a number of Zn2+-binding proteins, was analysed. The number of modifiable His residues in factor XII and alpha-factor XIIa was 16.0+/-0.7 and 17.3+/-0.7, respectively. When factor XII/alpha-factor XIIa was incubated with saturating concentrations of Zn2+ before the diethylpyrocarbonate modification of the His residues, these numbers were reduced to 6.3+/-0.1 and 8.21+/-0.5, indicating that ten and nine His residues, respectively, are involved in the binding. Analysis of the Zn2+-binding capacity of factor XII, alpha-factor XIIa and beta-factor XIIa showed that while factor XII contains four Zn2+-binding sites, alpha-factor XIIa had only three and beta-factor XIIa had none. Modification of the His residues resulted in a complete loss of Zn2+-binding while Asp/Glu modification resulted in loss of two and one Zn2+-binding sites in factor XII and alpha-factor XIIa, respectively. This suggests that two of the four sites in factor XII contain His residues, exclusively, while the two others are comprised of two His residues and one Asp/Glu residue. One of the latter is lost when factor XII is activated to alpha-factor XIIa. Two of the sites are suggested to be located at positions His40-His44 and His78-His82. The location of the remaining two sites are reduced to four possible positions.