By Increasing the Affinity of Heparin for Fibrin, Zn2+ Promotes the Formation of a Ternary Heparin–Thrombin–Fibrin Complex That Protects Thrombin from Inhibition by Antithrombin

Contributing factors to heparin resistance during cardiopulmonary bypass

Since the risk factors for heparin resistance (HR) before cardiopulmonary bypass (CPB) have not been fully clarified, this study investigated the contributing factors for HR after the initial unfractionated heparin (UFH) dose of 500 IU/kg. We retrospectively analyzed the data of 371 patients who underwent CPB surgery, with the initial UFH dose of 500 IU/kg, between May 2017 and December 2021. We defined HR as the failure to achieve activated clotting time (ACT) of > 480 s after the initial UFH dose of 500 IU/kg. HR was observed in 36 patients (9.7%) (HR group), while HR was not observed in 335 patients (control group). The HR group included significantly more patients with preoperative use of UFH, with significantly higher white blood cell counts, fibrinogen, fibrinogen degradation products, D-dimer, and C-reactive protein, and lower hemoglobin and albumin. The multivariable logistic regression analysis identified albumin (OR: 3.09, 95% CI 1.3504-7.0845, p = 0.0075) and fibrinogen (OR: 0.99, 95% CI 0.9869-0.9963, p = 0.0003) as independent predictors for HR. Using the Youden index, the cutoffs of albumin and fibrinogen were calculated as 3.8 g/dL and 303 mg/dL, respectively. The receiver operating characteristic curves showed the predictive performance of albumin (area under the curve (AUC): 0.78, sensitivity: 65%, specificity: 81%) and fibrinogen (AUC: 0.77, sensitivity: 56%, specificity: 88%). The incidence of HR after the initial UFH dose of 500 IU/kg was 9.7%. The preoperative albumin < 3.8 g/dL and fibrinogen > 303 mg/dL were independent predictors for HR.

Zn2+ Differentially Influences the Neutralisation of Heparins by HRG, Fibrinogen, and Fibronectin

For coagulation to be initiated, anticoagulant glycosaminoglycans (GAGs) such as heparins need to be neutralised to allow fibrin clot formation. Platelet activation triggers the release of several proteins that bind GAGs, including histidine-rich glycoprotein (HRG), fibrinogen, and fibronectin. Zn2+ ions are also released and have been shown to enhance the binding of HRG to heparins of a high molecular weight (HMWH) but not to those of low molecular weight (LMWH). The effect of Zn2+ on fibrinogen and fibronectin binding to GAGs is unknown. Here, chromogenic assays were used to measure the anti-factor Xa and anti-thrombin activities of heparins of different molecular weights and to assess the effects of HRG, fibrinogen, fibronectin, and Zn2+. Surface plasmon resonance was also used to examine the influence of Zn2+ on the binding of fibrinogen to heparins of different molecular weights. Zn2+ had no effect on the neutralisation of anti-factor Xa (FXa) or anti-thrombin activities of heparin by fibronectin, whereas it enhanced the neutralisation of unfractionated heparin (UFH) and HMWH by both fibrinogen and HRG. Zn2+ also increased neutralisation of the anti-FXa activity of LMWH by fibrinogen but not HRG. SPR showed that Zn2+ increased fibrinogen binding to both UFH and LMWH in a concentration-dependent manner. The presented results reveal that an increase in Zn2+ concentration has differential effects upon anticoagulant GAG neutralisation by HRG and fibrinogen, with implications for modulating anti-coagulant activity in plasma.

Tissue Engineering Scaffold Slowly Releasing Neurotrophic Factors to Bridge Long Peripheral Nerve Defect

Consistent application of neurotropic factors is necessary in peripheral nerve regeneration, yet challenging to achieve. Here we used a novel neurotropic factor controlled release system consisted of fibrin, fibronectin and hydrogel to slowly release two neurotrophic factors. At the same time, physiological saline and reverse nerve suturing were used as negative and positive control. A year after surgery, animals which were treated by neurotrophic factor slow release system achieved far better neural regeneration and myelination, as well as superior recovery of hindfoot than the negative control group. In the meanwhile, the results in the experimental group are still inferior to the nerve allograft group. In can be concluded from those results that, consistent releasing of neurotrophic factors can significantly promote long peripheral nerve regeneration, but still short of achieving the results same as the gold standard of autologous nerve grafting.

Influence of zinc on glycosaminoglycan neutralisation during coagulation

Heparan sulfate (HS), dermatan sulfate (DS) and heparin are glycosaminoglycans (GAGs) that serve as key natural and pharmacological anticoagulants. During normal clotting such agents require to be inactivated or neutralised. Several proteins have been reported to facilitate their neutralisation, which reside in platelet α-granules and are released following platelet activation. These include histidine-rich-glycoprotein (HRG), fibrinogen and high-molecular-weight kininogen (HMWK). Zinc ions (Zn2+) are also present in α-granules at a high concentration and participate in the propagation of coagulation by influencing the binding of neutralising proteins to GAGs. Zn2+ in many cases increases the affinity of these proteins to GAGs, and is thus an important regulator of GAG neutralisation and haemostasis. Binding of Zn2+ to HRG, HMWK and fibrinogen is mediated predominantly through coordination to histidine residues but the mechanisms by which Zn2+ increases the affinity of the proteins for GAGs are not yet completely clear. Here we will review current knowledge of how Zn2+ binds to and influences the neutralisation of GAGs and describe the importance of this process in both normal and pathogenic clotting.

Native electrospray mass spectrometry approaches to probe the interaction between zinc and an anti-angiogenic peptide from histidine-rich glycoprotein

Zinc modulates the biological function of histidine-rich glycoprotein (HRG) through binding to its His-rich region (HRR). The Zn2+-binding properties of a 35 amino-acid biologically-active peptide mimic of the HRR, HRGP330, were investigated using dissociative mass spectrometry approaches in addition to travelling-wave ion mobility mass spectrometry (TWIM-MS). Native mass spectrometry confirmed zinc binding to HRGP330; however, broadening of the 1H NMR resonances upon addition of Zn2+ ions precluded the attainment of structural information. A complementary approach employing TWIM-MS indicated that HRGP330 has a more compact structure in the presence of Zn2+ ions. Top-down MS/MS data supported a metal-binding-induced conformational change, as fewer fragments were observed for Zn2+-bound HRGP330. Zn2+-bound fragments of both N-terminal and C-terminal ends of the peptide were identified from collision-induced dissociation (CID) and electron transfer dissociation/proton transfer reaction (ETD/PTR) experiments, suggesting that multiple binding sites exist within this region of HRG. The combination of mass spectrometry and NMR approaches provides new insight into the highly dynamic interaction between zinc and this His-rich peptide.

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.

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.

Thrombin and fibrinogen γ' impact clot structure by marked effects on intrafibrillar structure and protofibril packing

Previous studies have shown effects of thrombin and fibrinogen γ' on clot structure. However, structural information was obtained using electron microscopy, which requires sample dehydration. Our aim was to investigate the role of thrombin and fibrinogen γ' in modulating fibrin structure under fully hydrated conditions. Fibrin fibers were studied using turbidimetry, atomic force microscopy, electron microscopy, and magnetic tweezers in purified and plasma solutions. Increased thrombin induced a pronounced decrease in average protofibril content per fiber, with a relatively minor decrease in fiber size, leading to the formation of less compact fiber structures. Atomic force microscopy under fully hydrated conditions confirmed that fiber diameter was only marginally decreased. Decreased protofibril content of the fibers produced by high thrombin resulted in weakened clot architecture as analyzed by magnetic tweezers in purified systems and by thromboelastometry in plasma and whole blood. Fibers produced with fibrinogen γ' showed reduced protofibril packing over a range of thrombin concentrations. High-magnification electron microscopy demonstrated reduced protofibril packing in γ' fibers and unraveling of fibers into separate protofibrils. Decreased protofibril packing was confirmed in plasma for high thrombin concentrations and fibrinogen-deficient plasma reconstituted with γ' fibrinogen. These findings demonstrate that, in fully hydrated conditions, thrombin and fibrinogen γ' have dramatic effects on protofibril content and that protein density within fibers correlates with strength of the fibrin network. We conclude that regulation of protofibril content of fibers is an important mechanism by which thrombin and fibrinogen γ' modulate fibrin clot structure and strength.

Reduced plasminogen binding and delayed activation render γ'-fibrin more resistant to lysis than γA-fibrin

Fibrin (Fn) clots formed from γ'-fibrinogen (γ'-Fg), a variant with an elongated γ-chain, are resistant to lysis when compared with clots formed from the predominant γA-Fg, a finding previously attributed to differences in clot structure due to delayed thrombin-mediated fibrinopeptide (FP) B release or impaired cross-linking by factor XIIIa. We investigated whether slower lysis of γ'-Fn reflects delayed plasminogen (Pg) binding and/or activation by tissue plasminogen activator (tPA), reduced plasmin-mediated proteolysis of γ'-Fn, and/or altered cross-linking. Clots formed from γ'-Fg lysed more slowly than those formed from γA-Fg when lysis was initiated with tPA/Pg when FPA and FPB were both released, but not when lysis was initiated with plasmin, or when only FPA was released. Pg bound to γ'-Fn with an association rate constant 22% lower than that to γA-Fn, and the lag time for initiation of Pg activation by tPA was longer with γ'-Fn than with γA-Fn. Once initiated, however, Pg activation kinetics were similar. Factor XIIIa had similar effects on clots formed from both Fg isoforms. Therefore, slower lysis of γ'-Fn clots reflects delayed FPB release, which results in delayed binding and activation of Pg. When clots were formed from Fg mixtures containing more than 20% γ'-Fg, the upper limit of the normal level, the delay in lysis was magnified. These data suggest that circulating levels of γ'-Fg modulate the susceptibility of clots to lysis by slowing Pg activation by tPA and provide another example of the intimate connections between coagulation and fibrinolysis.

Zn2+ mediates high affinity binding of heparin to the αC domain of fibrinogen

The nonspecific binding of heparin to plasma proteins compromises its anticoagulant activity by reducing the amount of heparin available to bind antithrombin. In addition, interaction of heparin with fibrin promotes formation of a ternary heparin-thrombin-fibrin complex that protects fibrin-bound thrombin from inhibition by the heparin-antithrombin complex. Previous studies have shown that heparin binds the E domain of fibrinogen. The current investigation examines the role of Zn(2+) in this interaction because Zn(2+) is released locally by platelets and both heparin and fibrinogen bind the cation, resulting in greater protection from inhibition by antithrombin. Zn(2+) promotes heparin binding to fibrinogen, as determined by chromatography, fluorescence, and surface plasmon resonance. Compared with intact fibrinogen, there is reduced heparin binding to fragment X, a clottable plasmin degradation product of fibrinogen. A monoclonal antibody directed against a portion of the fibrinogen αC domain removed by plasmin attenuates binding of heparin to fibrinogen and a peptide analog of this region binds heparin in a Zn(2+)-dependent fashion. These results indicate that the αC domain of fibrinogen harbors a Zn(2+)-dependent heparin binding site. As a consequence, heparin-catalyzed inhibition of factor Xa by antithrombin is compromised by fibrinogen to a greater extent when Zn(2+) is present. These results reveal the mechanism by which Zn(2+) augments the capacity of fibrinogen to impair the anticoagulant activity of heparin.