Packaging zinc, fibrinogen, and factor XIII in platelet alpha-granules

An environmentally sensitive zinc-selective two-photon NIR fluorescent turn-on probe and zinc sensing in stroke

A two-photon near infrared (NIR) fluorescence turn-on sensor with high selectivity and sensitivity for Zn2+ detection has been developed. This sensor exhibits a large Stokes' shift (∼300 nm) and can be excited from 900 to 1000 nm, with an emission wavelength of ∼785 nm, making it ideal for imaging in biological tissues. The sensor's high selectivity for Zn2+ over other structurally similar cations, such as Cd2+, makes it a promising tool for monitoring zinc ion levels in biological systems. Given the high concentration of zinc in thrombi, this sensor could provide a useful tool for in vivo thrombus imaging.

Fluorescent peptide for detecting factor XIIIa activity and fibrin in whole blood clots forming under flow

Background

During clotting, thrombin generates fibrin monomers and activates plasma-derived transglutaminase factor (F) XIIIa; collagen and thrombin-activated platelets offer thrombin-independent cellular FXIIIa (cFXIIIa) for clotting. Detecting fibrin on collagen and tissue factor surfaces in whole blood clotting typically uses complex reagents like fluorescent fibrinogen or antifibrin antibody.

Objectives

We want to test whether the peptide using the α2- antiplasmin crosslinking mechanism by FXIIIa is a useful tool in both monitoring FXIIIa activity, and visualize and monitor fibrin formation, deposition, and extent of crosslinking within fibrin structures in whole blood clots formed under flow.

Methods

We tested a fluorescent peptide derived from α2-antiplasmin sequence (Ac-GNQEQVSPLTLLKWC-fluorescein) to monitor the location of transglutaminase activity and fibrin during whole blood clotting under microfluidic flow (wall shear rate, 100 s-1).

Results

The peptide rapidly colocated with accumulating fibrin due to transglutaminase activity, confirmed by Phe-Pro-Arg-chloromethylketone inhibiting fibrin and peptide labeling. The FXIIIa inhibitor T101 had no effect on fibrin generation but ablated the labeling of fibrin by the peptide. Similarly, Gly-Pro-Arg-Pro abated fibrin formation and thus strongly attenuated the peptide signal. At arterial wall shear rate (1000 s-1), less fibrin was formed, and consequently, less peptide labeling of fibrin was detected compared with venous conditions. The addition of tissue plasminogen activator caused a reduction of both fibrin and peptide signals. Also, the peptide strongly colocalized with fibrin (but not platelets) in clots from laser-injured mouse cremaster arterioles. For clotting under flow, FXIIIa activity was most likely plasma-derived since a RhoA inhibitor did not block α2-antiplasmin fragment cross-linking to fibrin.

Conclusion

Under flow, the majority of FXIIIa-dependent fibrin labeling with peptide during clotting was distal of thrombin activity. The synthetic peptide provided a strong and sustained labeling of fibrin as it formed under flow.

Delineating Zinc Influx Mechanisms during Platelet Activation

Zinc (Zn2+) is released by platelets during a hemostatic response to injury. Extracellular zinc ([Zn2+]o) initiates platelet activation following influx into the platelet cytosol. However, the mechanisms that permit Zn2+ influx are unknown. Fluctuations in intracellular zinc ([Zn2+]i) were measured in fluozin-3-loaded platelets using fluorometry and flow cytometry. Platelet activation was assessed using light transmission aggregometry. The detection of phosphoproteins was performed by Western blotting. [Zn2+]o influx and subsequent platelet activation were abrogated by blocking the sodium/calcium exchanged, TRP channels, and ZIP7. Cation store depletion regulated Zn2+ influx. [Zn2+]o stimulation resulted in the phosphorylation of PKC substates, MLC, and β3 integrin. Platelet activation via GPVI or Zn2+ resulted in ZIP7 phosphorylation in a casein kinase 2-dependent manner and initiated elevations of [Zn2+]i that were sensitive to the inhibition of Orai1, ZIP7, or IP3R-mediated pathways. These data indicate that platelets detect and respond to changes in [Zn2+]o via influx into the cytosol through TRP channels and the NCX exchanger. Platelet activation results in the externalization of ZIP7, which further regulates Zn2+ influx. Increases in [Zn2+]i contribute to the activation of cation-dependent enzymes. Sensitivity of Zn2+ influx to thapsigargin indicates a store-operated pathway that we term store-operated Zn2+ entry (SOZE). These mechanisms may affect platelet behavior during thrombosis and hemostasis.

Trace Metals and The Hemostatic System

In this narrative review we report the main relationships between trace metals and the hemostatic system since this aspect has seldom attracted the attention of the scientific community. A basic aspect to be considered is the importance of maintaining the fine control of all trace metals' levels since they have an important impact on the pathophysiology of the hemostatic system. It is worth noting that poor diet habits are responsible for most trace metal deficiencies, while pollution is responsible for dangerous exposure to them with a consequent negative impact on the general population. This appears of paramount importance in planning the implementation of food and nutrient support to ameliorate the hidden hunger and the quality of life of people especially in developing countries and limiting poisons both in the air and food. As it often happens, when damage to certain mechanisms takes a very long time to appear, no attention is paid to the importance of a systematic prevention to avoid late negative outcomes.

Loss of zinc transporters ZIP1 and ZIP3 augments platelet reactivity in response to thrombin and accelerates thrombus formation in vivo

Zinc (Zn2+) is considered as important mediator of immune cell function, thrombosis and haemostasis. However, our understanding of the transport mechanisms that regulate Zn2+ homeostasis in platelets is limited. Zn2+ transporters, ZIPs and ZnTs, are widely expressed in eukaryotic cells. Using mice globally lacking ZIP1 and ZIP3 (ZIP1/3 DKO), our aim was to explore the potential role of these Zn2+ transporters in maintaining platelet Zn2+ homeostasis and in the regulation of platelet function. While ICP-MS measurements indicated unaltered overall Zn2+ concentrations in platelets of ZIP1/3 DKO mice, we observed a significantly increased content of FluoZin3-stainable free Zn2+, which, however, appears to be released less efficiently upon thrombin-stimulated platelet activation. On the functional level, ZIP1/3 DKO platelets exhibited a hyperactive response towards threshold concentrations of G protein-coupled receptor (GPCR) agonists, while immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptor agonist signalling was unaffected. This resulted in enhanced platelet aggregation towards thrombin, bigger thrombus volume under flow ex vivo and faster in vivo thrombus formation in ZIP1/3 DKO mice. Molecularly, augmented GPCR responses were accompanied by enhanced Ca2+ and PKC, CamKII and ERK1/2 signalling. The current study thereby identifies ZIP1 and ZIP3 as important regulators for the maintenance of platelet Zn2+ homeostasis and function.

Phosphorylation of the smooth muscle master splicing regulator RBPMS regulates its splicing activity

We previously identified RBPMS as a master regulator of alternative splicing in differentiated smooth muscle cells (SMCs). RBPMS is transcriptionally downregulated during SMC dedifferentiation, but we hypothesized that RBPMS protein activity might be acutely downregulated by post-translational modifications. Publicly available phosphoproteomic datasets reveal that Thr113 and Thr118 immediately adjacent to the RRM domain are commonly both phosphorylated. An RBPMS T113/118 phosphomimetic T/E mutant showed decreased splicing regulatory activity both in transfected cells and in a cell-free in vitro assay, while a non-phosphorylatable T/A mutant retained full activity. Loss of splicing activity was associated with a modest reduction in RNA affinity but significantly reduced RNA binding in nuclear extract. A lower degree of oligomerization of the T/E mutant might cause lower avidity of multivalent RNA binding. However, NMR analysis also revealed that the T113/118E peptide acts as an RNA mimic which can loop back and antagonize RNA-binding by the RRM domain. Finally, we identified ERK2 as the most likely kinase responsible for phosphorylation at Thr113 and Thr118. Collectively, our data identify a potential mechanism for rapid modulation of the SMC splicing program in response to external signals during the vascular injury response and atherogenesis.

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.

Platelet Activation via Glycoprotein VI Initiates Thrombin Generation: A Potential Role for Platelet-Derived Factor IX?

Collagen triggers coagulation via activation of factor (F) XII. In a platelet-rich environment, collagen can also trigger coagulation independently of FXII. We studied a novel mechanism of coagulation initiation via collagen-dependent platelet activation using thrombin generation (TG) in platelet-rich plasma. Collagen-induced coagulation is minimally affected by active-site inactivated FVIIa, anti-FVII antibodies, or FXIIa inhibition (corn trypsin inhibitor). Activation of platelets via specific glycoprotein (GP) VI agonists initiates TG, FX activation, and fibrin formation. To determine the platelet-derived trigger of coagulation, we systematically reconstituted factor-deficient plasmas with washed platelets. TG triggered by GPVI-activated platelets was significantly affected in FIX- and FVIII-deficient plasma but not in FVII- and FXII-deficient plasma. In a purified system composed of FX and FVIII, we observed that absence of FIX was compensated by GPVI-activated platelets, which could be inhibited by an anti-FIX antibody, suggesting FIXa activity from activated platelets. Furthermore, with the addition of FVIII in FIX-deficient plasma, TG induced by GPVI-activated platelets was restored, and was inhibited by the anti-FIX antibody. In conclusion, GPVI-activated platelets initiate TG, probably via platelet-derived FIXa activity.

Metal ion chelation enhances tissue plasminogen activator (tPA)-induced thrombolysis: an in vitro and in vivo study

Stroke is the third leading cause of death in the United States and the leading cause of adult disability. Despite enormous research efforts including many clinical trials, tissue plasminogen activator (tPA) remains the only FDA-approved treatment for acute ischemic stroke. Unfortunately, only 1-3% of stroke patients in the US receive this therapy because of the narrow time window and severe side effects for using tPA. The most deadly and damaging side effect is the risk of intracranial bleeding or hemorrhage. For that reason, the dose of tPA and its overall administration are under tight control, which may compromise the effect of thrombolysis. Studies have been focused on improving the effectiveness of tPA for higher rate of reperfusion, and the safety for less adverse bleeding episode. We studied how metal ions (zinc & iron) affect tPA-induced thrombolysis in vitro and in vivo, and proposed a method to improve the rate of thrombolysis. The amount of hemoglobin in the blood clot lysis was measured by a spectrophotometer. The tPA-induced thrombolysis was measured in vivo in femoral artery. Our results showed that Zn²⁺, Fe³⁺ and Fe²⁺ inhibited tPA-induced thrombolysis, with Zn²⁺ and Fe²⁺ being the most effective. Metal ion chelating agent EDTA when it was co-applied with tPA significantly enhanced the tPA-induced thrombolysis. The chelation alone did not have noticeable thrombolytic effect. In in vivo study of tPA-induced thrombosis following femoral artery thrombosis, the co-application of tPA and EDTA achieved significant higher rate of reperfusion than that by tPA treatment alone, suggesting that ion chelation facilitates tPA-induced thrombolysis and potentially improves the safety of tPA application by reducing the necessary dose of tPA application. Our results suggest that the co-application of a chelator and tPA improves the efficacy and, potentially, safety of tPA application, by reducing the necessary dose of tPA for thrombolysis.

The Interplay between Non-Esterified Fatty Acids and Plasma Zinc and Its Influence on Thrombotic Risk in Obesity and Type 2 Diabetes

Thrombosis is a major comorbidity of obesity and type-2 diabetes mellitus (T2DM). Despite the development of numerous effective treatments and preventative strategies to address thrombotic disease in such individuals, the incidence of thrombotic complications remains high. This suggests that not all the pathophysiological mechanisms underlying these events have been identified or targeted. Non-esterified fatty acids (NEFAs) are increasingly regarded as a nexus between obesity, insulin resistance, and vascular disease. Notably, plasma NEFA levels are consistently elevated in obesity and T2DM and may impact hemostasis in several ways. A potentially unrecognized route of NEFA-mediated thrombotic activity is their ability to disturb Zn²⁺ speciation in the plasma. Zn²⁺ is a potent regulator of coagulation and its availability in the plasma is monitored carefully through buffering by human serum albumin (HSA). The binding of long-chain NEFAs such as palmitate and stearate, however, trigger a conformational change in HSA that reduces its ability to bind Zn²⁺, thus increasing the ion’s availability to bind and activate coagulation proteins. NEFA-mediated perturbation of HSA-Zn²⁺ binding is thus predicted to contribute to the prothrombotic milieu in obesity and T2DM, representing a novel targetable disease mechanism in these disorders.

Zinc regulates reactive oxygen species generation in platelets

Vascular complications resulting from atherosclerosis development are a major cause of death. Reactive oxygen species (ROS) are produced by platelets during activation, and have been demonstrated to positively regulate platelet activatory responses. Zn2+ is also an important hemostatic cofactor in platelets, acting both as a platelet agonist and second messenger. Whilst the effect of Zn2+-dependent signaling mechanisms on ROS production in nucleated cells has been demonstrated, comparable roles in platelets have yet to be investigated. In this study we investigated the relationship between fluctuations in cytosolic Zn2 [Zn2+]i and platelet ROS production. Agonist-evoked ROS production, GSH levels and GPx activity are abrogated in platelets treated with the Zn2+-chelator, TPEN. Conversely, increasing platelet [Zn2+]i using Zn2+ ionophores potentiated ROS generation and decreased GSH levels and GPx activity. Zn2+-dependent ROS production was sensitive to pretreatment with DPI or mitoTEMPO, NADPH oxidase and mitochondria inhibitors respectively. Increasing [Zn2+]i resulted in increases of Erk1/2 and JNK phosphorylation. Our data are consistent with a functional association between [Zn2+]i and ROS production in platelets that could influence thrombus formation in a clinical context.

Factor XIII-A: An Indispensable "Factor" in Haemostasis and Wound Healing

Factor XIII (FXIII) is a transglutaminase enzyme that catalyses the formation of ε-(γ-glutamyl)lysyl isopeptide bonds into protein substrates. The plasma form, FXIIIA₂B₂, has an established function in haemostasis, with fibrin being its principal substrate. A deficiency in FXIII manifests as a severe bleeding diathesis emphasising its crucial role in this pathway. The FXIII-A gene (F13A1) is expressed in cells of bone marrow and mesenchymal lineage. The cellular form, a homodimer of the A subunits denoted FXIII-A, was perceived to remain intracellular, due to the lack of a classical signal peptide for its release. It is now apparent that FXIII-A can be externalised from cells, by an as yet unknown mechanism. Thus, three pools of FXIII-A exist within the circulation: plasma where it circulates in complex with the inhibitory FXIII-B subunits, and the cellular form encased within platelets and monocytes/macrophages. The abundance of this transglutaminase in different forms and locations in the vasculature reflect the complex and crucial roles of this enzyme in physiological processes. Herein, we examine the significance of these pools of FXIII-A in different settings and the evidence to date to support their function in haemostasis and wound healing.

Anomalous mechanics of Zn2+-modified fibrin networks

Fibrin is the main component of blood clots. The mechanical properties of fibrin are therefore of critical importance in successful hemostasis. One of the divalent cations released by platelets during hemostasis is Zn2+; however, its effect on the network structure of fibrin gels and on the resultant mechanical properties remains poorly understood. Here, by combining mechanical measurements with three-dimensional confocal microscopy imaging, we show that Zn2+ can tune the fibrin network structure and alter its mechanical properties. In the presence of Zn2+, fibrin protofibrils form large bundles that cause a coarsening of the fibrin network due to an increase in fiber diameter and reduction of the total fiber length. We further show that the protofibrils in these bundles are loosely coupled to one another, which results in a decrease of the elastic modulus with increasing Zn2+ concentrations. We explore the elastic properties of these networks at both low and high stress: At low stress, the elasticity originates from pulling the thermal slack out of the network, and this is consistent with the thermal bending of the fibers. By contrast, at high stress, the elasticity exhibits a common master curve consistent with the stretching of individual protofibrils. These results show that the mechanics of a fibrin network are closely correlated with its microscopic structure and inform our understanding of the structure and physical mechanisms leading to defective or excessive clot stiffness.

Albumin-mediated alteration of plasma zinc speciation by fatty acids modulates blood clotting in type-2 diabetes

Zn2+ is an essential regulator of coagulation and is released from activated platelets. In plasma, the free Zn2+ concentration is fine-tuned through buffering by human serum albumin (HSA). Importantly, the ability of HSA to bind/buffer Zn2+ is compromised by co-transported non-esterified fatty acids (NEFAs). Given the role of Zn2+ in blood clot formation, we hypothesise that Zn2+ displacement from HSA by NEFAs in certain conditions (such as type 2 diabetes mellitus, T2DM) impacts on the cellular and protein arms of coagulation. To test this hypothesis, we assessed the extent to which increasing concentrations of a range of medium- and long-chain NEFAs reduced Zn2+-binding ability of HSA. Amongst the NEFAs tested, palmitate (16 : 0) and stearate (18 : 0) were the most effective at suppressing zinc-binding, whilst the mono-unsaturated palmitoleate (16 : 1c9) was markedly less effective. Assessment of platelet aggregation and fibrin clotting parameters in purified systems and in pooled plasma suggested that the HSA-mediated impact of the model NEFA myristate on zinc speciation intensified the effects of Zn2+ alone. The effects of elevated Zn2+ alone on fibrin clot density and fibre thickness in a purified protein system were mirrored in samples from T2DM patients, who have derranged NEFA metabolism. Crucially, T2DM individuals had increased total plasma NEFAs compared to controls, with the concentrations of key saturated (myristate, palmitate, stearate) and mono-unsaturated (oleate, cis-vaccenate) NEFAs positively correlating with clot density. Collectively, these data strongly support the concept that elevated NEFA levels contribute to altered coagulation in T2DM through dysregulation of plasma zinc speciation.

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.

Efficient synthesis of NIR emitting bis[2-(2'-hydroxylphenyl)benzoxazole] derivative and its potential for imaging applications

Unassymetric bis[2-(2'-hydroxyphenylbenzoxole)] bis(HBO) derivatives with a DPA functionality for zinc binding have been developed with an efficient synthetic route, using the retrosynthetic analysis. Comparison of bis(HBO) derivatives with different substitution patterns allows us to verify and optimize their unique fluorescence properties. Upon binding zinc cation, bis(HBO) derivatives give a large fluorescence turn-on in both visible (λ_em ≈ 536 nm) and near-infrared (NIR) window (λ_em ≈ 746 nm). The probes are readily excitable by a 488 nm laser, making this series of compounds a suitable imaging tool for in vitro and in vivo study on a confocal microscope. The application of zinc binding-induced fluorescence turn-on is successfully demonstrated in cellular environments and thrombus imaging.

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 Homeostasis in Platelet-Related Diseases

Zn2+ deficiency in the human population is frequent in underdeveloped countries. Worldwide, approximatively 2 billion people consume Zn2+-deficient diets, accounting for 1-4% of deaths each year, mainly in infants with a compromised immune system. Depending on the severity of Zn2+ deficiency, clinical symptoms are associated with impaired wound healing, alopecia, diarrhea, poor growth, dysfunction of the immune and nervous system with congenital abnormalities and bleeding disorders. Poor nutritional Zn2+ status in patients with metastatic squamous cell carcinoma or with advanced non-Hodgkin lymphoma, was accompanied by cutaneous bleeding and platelet dysfunction. Forcing Zn2+ uptake in the gut using different nutritional supplementation of Zn2+ could ameliorate many of these pathological symptoms in humans. Feeding adult rodents with a low Zn2+ diet caused poor platelet aggregation and increased bleeding tendency, thereby attracting great scientific interest in investigating the role of Zn2+ in hemostasis. Storage protein metallothionein maintains or releases Zn2+ in the cytoplasm, and the dynamic change of this cytoplasmic Zn2+ pool is regulated by the redox status of the cell. An increase of labile Zn2+ pool can be toxic for the cells, and therefore cytoplasmic Zn2+ levels are tightly regulated by several Zn2+ transporters located on the cell surface and also on the intracellular membrane of Zn2+ storage organelles, such as secretory vesicles, endoplasmic reticulum or Golgi apparatus. Although Zn2+ is a critical cofactor for more than 2000 transcription factors and 300 enzymes, regulating cell differentiation, proliferation, and basic metabolic functions of the cells, the molecular mechanisms of Zn2+ transport and the physiological role of Zn2+ store in megakaryocyte and platelet function remain elusive. In this review, we summarize the contribution of extracellular or intracellular Zn2+ to megakaryocyte and platelet function and discuss the consequences of dysregulated Zn2+ homeostasis in platelet-related diseases by focusing on thrombosis, ischemic stroke and storage pool diseases.

Defective Zn2+ homeostasis in mouse and human platelets with α- and δ-storage pool diseases

Zinc (Zn²⁺) can modulate platelet and coagulation activation pathways, including fibrin formation. Here, we studied the (patho)physiological consequences of abnormal platelet Zn²⁺ storage and release. To visualize Zn²⁺ storage in human and mouse platelets, the Zn²⁺ specific fluorescent dye FluoZin3 was used. In resting platelets, the dye transiently accumulated into distinct cytosolic puncta, which were lost upon platelet activation. Platelets isolated from Unc13d^−/− mice, characterized by combined defects of α/δ granular release, showed a markedly impaired Zn²⁺ release upon activation. Platelets from Nbeal2^−/− mice mimicking Gray platelet syndrome (GPS), characterized by primarily loss of the α-granule content, had strongly reduced Zn²⁺ levels, which was also confirmed in primary megakaryocytes. In human platelets isolated from patients with GPS, Hermansky-Pudlak Syndrome (HPS) and Storage Pool Disease (SPD) altered Zn²⁺ homeostasis was detected. In turbidity and flow based assays, platelet-dependent fibrin formation was impaired in both Nbeal2^−/− and Unc13d^−/− mice, and the impairment could be partially restored by extracellular Zn²⁺. Altogether, we conclude that the release of ionic Zn²⁺ store from secretory granules upon platelet activation contributes to the procoagulant role of Zn²⁺ in platelet-dependent fibrin formation.

Let's cross-link: diverse functions of the promiscuous cellular transglutaminase factor XIII-A

Essentials Plasma Factor XIII, a heterodimer of A and B subunits FXIIIA2 B2 , is a transglutaminase enzyme with a well-established role in haemostasis. Cells of bone marrow and mesenchymal lineage express the FXIII-A gene (F13A1) that encodes the cellular form of the transglutaminase, a homodimer of the A subunits, FXIII-A. FXIII-A was presumed to function intracellularly, however, several lines of evidence now indicate that FXIII-A is externalised by an as yet unknown mechanism This review describes the mounting evidence that FXIII-A is a diverse transglutaminase with many intracellular and extracellular substrates that can participate in an array of biological processes SUMMARY: Factor XIII is a tranglutaminase enzyme that catalyzes the formation of ε-(γ-glutamyl)lysyl isopeptide bonds in protein substrates. The plasma form, FXIII-A2 B2 , has an established function in hemostasis, where its primary substrate is fibrin. A deficiency in FXIII manifests as a severe bleeding diathesis, underscoring its importance in this pathway. The cellular form of the enzyme, a homodimer of the A-subunits, denoted FXIII-A, has not been studied in as extensive detail. FXIII-A was generally perceived to remain intracellular, owing to the lack of a classical signal peptide for its release. In the last decade, emerging evidence has revealed that this diverse transglutaminase can be externalized from cells, by an as yet unknown mechanism, and can cross-link extracellular substrates and participate in a number of diverse pathways. The FXIII-A gene (F13A1) is expressed in cells of bone marrow and mesenchymal lineage, notably megakaryocytes, monocytes/macrophages, dendritic cells, chrondrocytes, osteoblasts, and preadipocytes. The biological processes that FXIII-A is coupled with, such as wound healing, phagocytosis, and bone and matrix remodeling, reflect its expression in these cell types. This review describes the mounting evidence that this cellular transglutaminase can be externalized, usually in response to stimuli, and participate in extracellular cross-linking reactions. A corollary of being involved in these biological pathways is the participation of FXIII-A in pathological processes. In conclusion, the functions of this transglutaminase extend far beyond its role in hemostasis, and our understanding of this enzyme in terms of its secretion, regulation and substrates is in its infancy.

Agonist-Evoked Increases in Intra-Platelet Zinc Couple to Functional Responses

Background  Zinc (Zn ²⁺ ) is an essential trace element that regulates intracellular processes in multiple cell types. While the role of Zn ²⁺ as a platelet agonist is known, its secondary messenger activity in platelets has not been demonstrated.

Objectives  This article determines whether cytosolic Zn ²⁺ concentrations ([Zn ²⁺ ] _i ) change in platelets in response to agonist stimulation, in a manner consistent with a secondary messenger, and correlates the effects of [Zn ²⁺ ] _i changes on activation markers.

Methods  Changes in [Zn ²⁺ ] _i were quantified in Fluozin-3 (Fz-3)-loaded washed, human platelets using fluorometry. Increases in [Zn ²⁺ ] _i were modelled using Zn ²⁺ -specific chelators and ionophores. The influence of [Zn ²⁺ ] _i on platelet function was assessed using platelet aggregometry, flow cytometry and Western blotting.

Results  Increases of intra-platelet Fluozin-3 (Fz-3) fluorescence occurred in response to stimulation by cross-linked collagen-related peptide (CRP-XL) or U46619, consistent with a rise of [Zn ²⁺ ] _i . Fluoresence increases were blocked by Zn ²⁺ chelators and modulators of the platelet redox state, and were distinct from agonist-evoked [Ca ²⁺ ] _i signals. Stimulation of platelets with the Zn ²⁺ ionophores clioquinol (Cq) or pyrithione (Py) caused sustained increases of [Zn ²⁺ ] _i , resulting in myosin light chain phosphorylation, and cytoskeletal re-arrangements which were sensitive to cytochalasin-D treatment. Cq stimulation resulted in integrin α _IIb β ₃ activation and release of dense, but not α, granules. Furthermore, Zn ²⁺ -ionophores induced externalization of phosphatidylserine.

Conclusion  These data suggest that agonist-evoked fluctuations in intra-platelet Zn ²⁺ couple to functional responses, in a manner that is consistent with a role as a secondary messenger. Increased intra-platelet Zn ²⁺ regulates signalling processes, including shape change, α _IIb β ₃ up-regulation and dense granule release, in a redox-sensitive manner.

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.

Factor XIII in plasma, but not in platelets, mediates red blood cell retention in clots and venous thrombus size in mice

The transglutaminase factor XIII (FXIII) stabilizes clots against mechanical and biochemical disruption and is essential for hemostasis. In vitro and in vivo models of venous thrombosis demonstrate that FXIII mediates clot size by promoting red blood cell (RBC) retention. However, the key source of FXIII and whether FXIII activity can be reduced to suppress thrombosis without imposing deleterious hemostatic consequences are 2 critical unresolved questions. FXIII is present in multiple compartments, including plasma (FXIII_plasma) as a heterotetramer of A₂ and B₂ subunits and platelets (FXIII_plt) as an A₂ homodimer. We determined the role of the FXIII compartment and level in clot contraction, composition, and size in vitro and using in vivo models of hemostasis and venous thrombosis. Reducing overall FXIII levels decreased whole blood clot weight but did not alter thrombin generation or contraction of platelet-rich plasma clots. In reconstituted platelet-rich plasma and whole blood clot contraction assays, FXIII_plasma, but not FXIII_plt, produced high-molecular-weight fibrin crosslinks, promoted RBC retention, and increased clot weights. Genetically imposed reduction of FXIII delayed FXIII activation and fibrin crosslinking, suggesting FXIII levels mediate the kinetics of FXIII activation and activity and that the timing of these processes is a critical determinant of RBC retention during clot formation and contraction. A 50% reduction in FXIII_plasma produced significantly smaller venous thrombi but did not increase bleeding in tail transection or saphenous vein puncture models in vivo. Collectively, these findings suggest that partial FXIII reduction may be a therapeutic strategy for reducing venous thrombosis.

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.

Extracellular chloride is required for efficient platelet aggregation

Anion channels perform a diverse range of functions and have been implicated in ATP release, volume regulation, and phosphatidylserine exposure. Platelets have been shown to express several anion channels but their function is incompletely understood. Due to a paucity of specific pharmacological blockers, we investigated the effect of extracellular chloride substitution on platelet activation using aggregometry and flow cytometry. In the absence of extracellular chloride, we observed a modest reduction of the maximum aggregation response to thrombin or collagen-related peptide. However, the rate of aggregation was substantially reduced in a manner that was dependent on the extracellular chloride concentration and aggregation in the absence of chloride was noticeably biphasic, indicative of impaired secondary signaling. This was further investigated by targeting secondary agonists with aspirin and apyrase or by blockade of the ADP receptor P2Y12. Under these conditions, the rates of aggregation were comparable to those recorded in the absence of extracellular chloride. Finally, we assessed platelet granule release by flow cytometry and report a chloride-dependent element of alpha, but not dense, granule secretion. Taken together these data support a role for anion channels in the efficient induction of platelet activation, likely via enhancement of secondary signaling pathways.

Functional Regulation of the Plasma Protein Histidine-Rich Glycoprotein by Zn2+ in Settings of Tissue Injury

Divalent metal ions are essential nutrients for all living organisms and are commonly protein-bound where they perform important roles in protein structure and function. This regulatory control from metals is observed in the relatively abundant plasma protein histidine-rich glycoprotein (HRG), which displays preferential binding to the second most abundant transition element in human systems, Zinc (Zn²⁺). HRG has been proposed to interact with a large number of protein ligands and has been implicated in the regulation of various physiological and pathological processes including the formation of immune complexes, apoptotic/necrotic and pathogen clearance, cell adhesion, antimicrobial activity, angiogenesis, coagulation and fibrinolysis. Interestingly, these processes are often associated with sites of tissue injury or tumour growth, where the concentration and distribution of Zn²⁺ is known to vary. Changes in Zn²⁺ levels have been shown to modify HRG function by altering its affinity for certain ligands and/or providing protection against proteolytic disassembly by serine proteases. This review focuses on the molecular interplay between HRG and Zn²⁺, and how Zn²⁺ binding modifies HRG-ligand interactions to regulate function in different settings of tissue injury.

Alterations of the platelet proteome in type I Glanzmann thrombasthenia caused by different homozygous delG frameshift mutations in ITGA2B

Glanzmann thrombasthenia (GT) is one of the best characterised inherited platelet function disorders but global platelet proteome has not been determined in these patients. We investigated the proteome and function of platelets from two patients with type I GT, caused by different homozygous ITGA2b mutations, from family members and unrelated controls. The global proteome of highly purified washed platelets was quantified by liquid chromatography-mass spectrometry (LC-MS) and targeted MS-methods. Platelet function was analysed by flow cytometry, light transmission aggregometry and flow-based assays. Platelets from GT patients showed less than 5 % relative levels of the integrin subunit α_IIb and 5-9 % fibrinogen compared to controls. These patients demonstrated loss of α_IIbβ₃-dependent platelet function, but normal platelet granule secretion induced by physiological agonists. Platelets from heterozygous family members of a patient expressed 50-60 % of control α_IIb levels which were sufficient for normal α_IIbβ₃-dependent platelet function. Studying type I GT as model disease we established quantitative LC-MS to detect and clearly distinguish normal platelets, platelets from GT heterozygotes and platelets from GT patients. Diminished levels of factor XIIIB chain, plasminogen and carboxypeptidase 2B were identified in thrombasthenic platelets. Additionally, GT platelets showed up to 2.5-fold increased levels of FcγRIIA and laminin-α4 chain. Elevated levels of platelet FcγRIIA was associated with increased CD63-surface expression after FcγRIIA-crosslinking in one GT-patient which might present a compensatory mechanism of platelet activation in GT. We demonstrate that quantitative LC-MS based proteomics is suitable to validate known but also to identify previously unknown protein level changes of dysfunctional platelets.

Divalent metal binding by histidine-rich glycoprotein differentially regulates higher order oligomerisation and proteolytic processing

The serum protein histidine-rich glycoprotein (HRG) has been implicated in tissue injury and tumour growth. Several HRG functions are regulated by the divalent metal Zn²⁺ , including ligand binding and proteolytic processing that releases active HRG fragments. Although HRG can bind divalent metals other than Zn²⁺ , the impact of these divalent metals on the biophysical properties of HRG remains poorly understood. We now show that HRG binds Zn²⁺ , Ni²⁺ , Cu²⁺ and Co²⁺ with micromolar affinities, but differing stoichiometries, and regulate the release of specific HRG fragments during proteolysis. Furthermore, HRG binding to Zn²⁺ promotes HRG dimer formation in a Zn²⁺ -concentration- and pH-dependent manner. Our data highlight the complex divalent metal-dependent regulatory mechanisms that govern HRG function.

The contribution of zinc to platelet behaviour during haemostasis and thrombosis

Platelets are the primary cellular determinants of haemostasis and pathological thrombus formation leading to myocardial infarction and stroke. Following vascular injury or atherosclerotic plaque rupture, platelets are recruited to sites of damage and undergo activation induced by a variety of soluble and/or insoluble agonists. Platelet activation is a multi-step process culminating in the formation of thrombi, which contribute to the haemostatic process. Zinc (Zn(2+)) is acknowledged as an important signalling molecule in a diverse range of cellular systems, however there is limited understanding of the influence of Zn(2+) on platelet behaviour during thrombus formation. This review evaluates the contributions of exogenous and intracellular Zn(2+) to platelet function and assesses the potential pathophysiological implications of Zn(2+) signalling. We also provide a speculative assessment of the mechanisms by which platelets could respond to changes in extracellular and intracellular Zn(2+) concentration.

Zinc is a transmembrane agonist that induces platelet activation in a tyrosine phosphorylation-dependent manner

Following platelet adhesion and primary activation at sites of vascular injury, secondary platelet activation is induced by soluble platelet agonists, such as ADP, ATP, thrombin and thromboxane. Zinc ions are also released from platelets and damaged cells and have been shown to act as a platelet agonist. However, the mechanism of zinc-induced platelet activation is not well understood. Here we show that exogenous zinc gains access to the platelet cytosol and induces full platelet aggregation that is dependent on platelet protein tyrosine phosphorylation, PKC and integrin αIIbβ3 activity and is mediated by granule release and secondary signalling. ZnSO4 increased the binding affinity of GpVI, but not integrin α2β1. Low concentrations of ZnSO4 potentiated platelet aggregation by collagen-related peptide (CRP-XL), thrombin and adrenaline. Chelation of intracellular zinc reduced platelet aggregation induced by a number of different agonists, inhibited zinc-induced tyrosine phosphorylation and inhibited platelet activation in whole blood under physiologically relevant flow conditions. Our data are consistent with a transmembrane signalling role for zinc in platelet activation during thrombus formation.

Polyphosphate colocalizes with factor XII on platelet-bound fibrin and augments its plasminogen activator activity

Activated factor XII (FXIIa) has plasminogen activator capacity but its relative contribution to fibrinolysis is considered marginal compared with urokinase and tissue plasminogen activator. Polyphosphate (polyP) is released from activated platelets and mediates FXII activation. Here, we investigate the contribution of polyP to the plasminogen activator function of αFXIIa. We show that both polyP₇₀, of the chain length found in platelets (60-100 mer), and platelet-derived polyP significantly augment the plasminogen activation capacity of αFXIIa. PolyP₇₀ stimulated the autoactivation of FXII and subsequent plasminogen activation, indicating that once activated, αFXIIa remains bound to polyP₇₀ Indeed, complex formation between polyP₇₀ and αFXIIa provides protection against autodegradation. Plasminogen activation by βFXIIa was minimal and not enhanced by polyP₇₀, highlighting the importance of the anion binding site. PolyP₇₀ did not modulate plasmin activity but stimulated activation of Glu and Lys forms of plasminogen by αFXIIa. Accordingly, polyP₇₀ was found to bind to FXII, αFXIIa, and plasminogen, but not βFXIIa. Fibrin and polyP₇₀ acted synergistically to enhance αFXIIa-mediated plasminogen activation. The plasminogen activator activity of the αFXIIa-polyP₇₀ complex was modulated by C1 inhibitor and histidine-rich glycoprotein, but not plasminogen activator inhibitors 1 and 2. Platelet polyP and FXII were found to colocalize on the activated platelet membrane in a fibrin-dependent manner and decorated fibrin strands extending from platelet aggregates. We show that in the presence of platelet polyP and the downstream substrate fibrin, αFXIIa is a highly efficient and favorable plasminogen activator. Our data are the first to document a profibrinolytic function of platelet polyP.

Zinc promotes clot stability by accelerating clot formation and modifying fibrin structure

Zinc released from activated platelets binds fibrin(ogen) and attenuates fibrinolysis. Although zinc also affects clot formation, the mechanism and consequences are poorly understood. To address these gaps, the effect of zinc on clot formation and structure was examined in the absence or presence of factor (F) XIII. Zinc accelerated a) plasma clotting by 1.4-fold, b) fibrinogen clotting by 3.5- and 2.3-fold in the absence or presence of FXIII, respectively, c) fragment X clotting by 1.3-fold, and d) polymerisation of fibrin monomers generated with thrombin or batroxobin by 2.5- and 1.8-fold, respectively. Whereas absorbance increased up to 3.3-fold when fibrinogen was clotted in the presence of zinc, absorbance of fragment X clots was unaffected by zinc, consistent with reports that zinc binds to the αC-domain of fibrin(ogen). Scanning electron microscopic analysis revealed a two-fold increase in fibre diameter in the presence of zinc and in permeability studies, zinc increased clot porosity by 30-fold with or without FXIII. Whereas FXIII increased clot stiffness from 128 ± 19 Pa to 415 ± 27 Pa in rheological analyses, zinc reduced clot stiffness by 10- and 8.5-fold in the absence and presence of FXIII, respectively. Clots formed in the presence of zinc were more stable and resisted rupture with or without FXIII. Therefore, zinc accelerates clotting and reduces fibrin clot stiffness in a FXIII-independent manner, suggesting that zinc may work in concert with FXIII to modulate clot strength and stability.

Histidine-rich glycoprotein binds DNA and RNA and attenuates their capacity to activate the intrinsic coagulation pathway

When triggered by factor (F) XII and nucleic acids, we showed that thrombosis in HRG-deficient mice is accelerated compared with that in wild-type mice. In this study, we set out to identify the mechanisms by which nucleic acids promote contact activation, and to determine whether HRG attenuates their effects. DNA or RNA addition to human plasma enhances thrombin generation via the intrinsic pathway and shortens the clotting time. Their effect on the clotting time is seven- to 14-fold greater in HRG-deficient plasma than in control plasma. Investigations into the mechanisms of activation reveal that nucleic acids a) promote FXII activation in the presence of prekallikrein- and high molecular weight kininogen (HK), and b) enhance thrombin-mediated FXI activation by 10- to 12-fold. Surface plasmon resonance studies show that DNA and RNA bind FXII, FXIIa, HK, FXI, FXIa and thrombin with high affinity. HRG attenuates DNA- and RNA-mediated FXII activation, and FXI activation by FXIIa or by thrombin, suggesting that HRG down regulates the capacity of DNA and RNA to activate the intrinsic pathway. Therefore, HRG attenuates the procoagulant activity of nucleic acids at multiple levels.

Zinc delays clot lysis by attenuating plasminogen activation and plasmin-mediated fibrin degradation

Zinc circulates free in plasma at a concentration of 0.1-2 µM, but its levels increase locally when it is released from activated platelets. Although zinc influences many processes in haemostasis, its effect on fibrinolysis has not been thoroughly investigated. Using a fluorescent zinc-binding probe, we demonstrated that zinc binds tissue-type plasminogen activator (tPA) and plasmin with high affinity (Kd values of 0.2 µM), and surface plasmon resonance studies revealed that zinc binds fibrin with a Kd of 12.8 µM. Zinc had no effect on the affinity of plasminogen or plasmin for fibrin, but increased the affinity of tPA by two-fold. In the presence of 5 µM zinc, the catalytic efficiency of plasminogen activation by tPA was reduced by approximately two-fold, both in the absence or presence of fibrin. Zinc attenuated plasmin-mediated degradation of the fibrinogen alpha-chain by 43 %, but had no effect on trypsin degradation. tPA-mediated fibrin clot lysis was prolonged 2.5-fold by zinc in a concentration-dependent fashion, and tPA-mediated plasma clot lysis was attenuated by 1.5-fold. Therefore, our data indicate that zinc modulates fibrinolysis by attenuating tPA-mediated plasminogen activation and plasmin-induced fibrin degradation. These findings suggest that local release of zinc by platelets attenuates fibrinolysis.

Arterial thrombosis is accelerated in mice deficient in histidine-rich glycoprotein

Factor (F) XII, a key component of the contact system, triggers clotting via the intrinsic pathway, and is implicated in propagating thrombosis. Although nucleic acids are potent activators, it is unclear how the contact system is regulated to prevent uncontrolled clotting. Previously, we showed that histidine-rich glycoprotein (HRG) binds FXIIa and attenuates its capacity to trigger coagulation. To investigate the role of HRG as a regulator of the intrinsic pathway, we compared RNA- and DNA-induced thrombin generation in plasma from HRG-deficient and wild-type mice. Thrombin generation was enhanced in plasma from HRG-deficient mice, and accelerated clotting was restored to normal with HRG reconstitution. Although blood loss after tail tip amputation was similar in HRG-deficient and wild-type mice, carotid artery occlusion after FeCl3 injury was accelerated in HRG-deficient mice, and HRG administration abrogated this effect. To confirm that HRG modulates the contact system, we used DNase, RNase, and antisense oligonucleotides to characterize the FeCl3 model. Whereas DNase or FVII knockdown had no effect, carotid occlusion was abrogated with RNase or FXII knockdown, confirming that FeCl3-induced thrombosis is triggered by RNA in a FXII-dependent fashion. Therefore, in a nucleic acid-driven model, HRG inhibits thrombosis by modulating the intrinsic pathway of coagulation.

Functional factor XIII-A is exposed on the stimulated platelet surface

Factor XIII (FXIII) stabilizes thrombi against fibrinolysis by cross-linking α2-antiplasmin (α2AP) to fibrin. Cellular FXIII (FXIII-A) is abundant in platelets, but the extracellular functions of this pool are unclear because it is not released by classical secretion mechanisms. We examined the function of platelet FXIII-A using Chandler model thrombi formed from FXIII-depleted plasma. Platelets stabilized FXIII-depleted thrombi in a transglutaminase-dependent manner. FXIII-A activity on activated platelets was unstable and was rapidly lost over 1 hour. Inhibiting platelet activation abrogated the ability of platelets to stabilize thrombi. Incorporating a neutralizing antibody to α2AP into FXIII-depleted thrombi revealed that the stabilizing effect of platelet FXIII-A on lysis was α2AP dependent. Platelet FXIII-A activity and antigen were associated with the cytoplasm and membrane fraction of unstimulated platelets, and these fractions were functional in stabilizing FXIII-depleted thrombi against lysis. Fluorescence confocal microscopy and flow cytometry revealed exposure of FXIII-A on activated membranes, with maximal signal detected with thrombin and collagen stimulation. FXIII-A was evident in protruding caps on the surface of phosphatidylserine-positive platelets. Our data show a functional role for platelet FXIII-A through exposure on the activated platelet membrane where it exerts antifibrinolytic function by cross-linking α2AP to fibrin.

Immune functions of platelets

This review collects evidence about immune and inflammatory functions of platelets from a clinician's point of view. A focus on clinically relevant immune functions aims at stimulating further research, because the complexity of platelet immunity is incompletely understood and not yet translated into patient care. Platelets promote chronic inflammatory reactions (e.g. in atherosclerosis), modulate acute inflammatory disorders such as sepsis and other infections (participating in the host defense against pathogens), and contribute to exacerbations of autoimmune conditions (like asthma or arthritis). It would hence be obsolete to restrict a description of platelet functions to thrombosis and haemostasis--platelets clearly are the most abundant cells with immune functions in the circulation.

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.

The molecular basis of memory. Part 2: chemistry of the tripartite mechanism

We propose a tripartite mechanism to describe the processing of cognitive information (cog-info), comprising the (1) neuron, (2) surrounding neural extracellular matrix (nECM), and (3) numerous "trace" metals distributed therein. The neuron is encased in a polyanionic nECM lattice doped with metals (>10), wherein it processes (computes) and stores cog-info. Each [nECM:metal] complex is the molecular correlate of a cognitive unit of information (cuinfo), similar to a computer "bit". These are induced/sensed by the neuron via surface iontophoretic and electroelastic (piezoelectric) sensors. The generic cuinfo are used by neurons to biochemically encode and store cog-info in a rapid, energy efficient, but computationally expansive manner. Here, we describe chemical reactions involved in various processes that underline the tripartite mechanism. In addition, we present novel iconographic representations of various types of cuinfo resulting from"tagging" and cross-linking reactions, essential for the indexing cuinfo for organized retrieval and storage of memory.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Histidine-rich glycoprotein binds fibrin(ogen) with high affinity and competes with thrombin for binding to the gamma'-chain

Histidine-rich glycoprotein (HRG) is an abundant protein that binds fibrinogen and other plasma proteins in a Zn(2+)-dependent fashion but whose function is unclear. HRG has antimicrobial activity, and its incorporation into fibrin clots facilitates bacterial entrapment and killing and promotes inflammation. Although these findings suggest that HRG contributes to innate immunity and inflammation, little is known about the HRG-fibrin(ogen) interaction. By immunoassay, HRG-fibrinogen complexes were detected in Zn(2+)-supplemented human plasma, a finding consistent with a high affinity interaction. Surface plasmon resonance determinations support this concept and show that in the presence of Zn(2+), HRG binds the predominant γ(A)/γ(A)-fibrinogen and the γ-chain elongated isoform, γ(A)/γ'-fibrinogen, with K(d) values of 9 nm. Likewise, (125)I-labeled HRG binds γ(A)/γ(A)- or γ(A)/γ'-fibrin clots with similar K(d) values when Zn(2+) is present. There are multiple HRG binding sites on fibrin(ogen) because HRG binds immobilized fibrinogen fragment D or E and γ'-peptide, an analog of the COOH terminus of the γ'-chain that mediates the high affinity interaction of thrombin with γ(A)/γ'-fibrin. Thrombin competes with HRG for γ'-peptide binding and displaces (125)I-HRG from γ(A)/γ'-fibrin clots and vice versa. Taken together, these data suggest that (a) HRG circulates in complex with fibrinogen and that the complex persists upon fibrin formation, and (b) by competing with thrombin for γ(A)/γ'-fibrin binding, HRG may modulate coagulation. Therefore, the HRG-fibrin interaction may provide a novel link between coagulation, innate immunity, and inflammation.

Factor XIII supports platelet activation and enhances thrombus formation by matrix proteins under flow conditions

BACKGROUND

Activated coagulation factor XIII (FXIIIa) is a transglutaminase that crosslinks fibrin at sites of vascular injury. FXIIIa also associates with blood platelets, although its role in platelet function is unclear and requires clarification.

OBJECTIVES

To evaluate the ability of FXIIIa to support platelet adhesion and spreading under conditions of physiologic flow, and to identify the underpinning receptors and signaling events.

METHODS AND RESULTS

Platelet adhesion to immobilized FXIIIa was measured by fluorescence microscopy, and signaling events were characterized by immunoblotting. Immobilized FXIIIa supported platelet adhesion and spreading under static conditions through mechanisms that were dually and differentially dependent on integrins α(IIb)β(3) and α(v)β(3). Platelet adhesion was independent of FXIIIa transglutaminase or protein disulfide isomerase activity. Moreover, adhesion was abolished by antibodies that prevented interaction with FXIIIa, but maintained when potential interactions with fibrinogen were blocked. Platelet adhesion to FXIIIa was reduced significantly by either the specific α(IIb)β(3) antagonist tirofiban or the selective α(v)β(3)-blocking antibody LM609, and abolished when they were used in combination. Importantly, platelet adhesion was preserved under venous and arterial flow conditions in which both integrins played essential roles. In contrast, FXIIIa stimulated the formation of filopodia and lamellipodia in adherent platelets that was mediated exclusively by α(IIb)β(3) and eliminated by the Src-family inhibitor 4-amino-5-(4-methylphenyl-7-(t-butyl)pyrazolo(3,4-d)pyrimidine, indicating a tyrosine kinase-dependent mechanism. Crucially, under conditions of arterial shear, FXIIIa accentuated platelet recruitment by von Willebrand factor and collagen.

CONCLUSIONS

Our data demonstrate a potential role for FXIIIa in supporting platelet adhesion at sites of vascular damage, particularly in association with other thrombogenic matrix proteins.

Immune versus thrombotic stimulation of platelets differentially regulates signalling pathways, intracellular protein-protein interactions, and alpha-granule release

In addition to haemostasis, platelets mediate inflammation and clearance of bacteria from the bloodstream. As with platelet-platelet interactions, platelet-bacteria interactions involve cytoskeletal rearrangements and release of granular content. Stimulation of the immune Toll-like receptor 2 (TLR2) on the platelet surface, activates phosphoinositide-3-kinase (PI3K) and causes platelet activation and platelet-dependent thrombosis. It remains unknown if platelet activation by immune versus thrombotic pathways leads to the differential regulation of signal transduction, protein-protein interactions, and alpha-granule release, and the physiological relevance of these potential differences. We investigated these processes after immune versus thrombotic platelet stimulation. We examined selected signalling pathways and found that phosphorylation kinetics of Akt, ERK1/2 and p38 differed dramatically between agonists. Next, we investigated platelet protein-protein interactions by mass spectrometry (MS)-based proteomics specifically targeting cytosolic factor XIIIa (FXIIIa) because of its function as a cytoskeleton-crosslinking protein whose binding partners have limited characterisation. Four FXIIIa-binding proteins were identified, two of which are novel interactions: FXIIIa-focal adhesion kinase (FAK) and FXIIIa-gelsolin. The binding of FAK to FXIIIa was found to be altered differentially by immune versus thrombotic stimulation. Lastly, we studied the effect of thrombin versus Pam(3)CSK(4) stimulation on alpha-granule release and observed differential release patterns for selected granule proteins and decreased fibrin clot formation compared with thrombin. The inhibition of PI3K caused a decrease in protein release after Pam(3)CSK(4)- but not after thrombin-stimulation. In summary, stimulation of platelets by either thrombotic or immune receptors leads to markedly different signalling responses and granular protein release consistent with differential contribution to coagulation and thrombosis.

The use of fibrin based matrices and fibrin microbeads (FMB) for cell based tissue regeneration

BACKGROUND

Due to its good cell attachment capabilities and promotion of cell migration, fibrin serves as an interim cell-binding matrix in wounded tissues. Due to their fast degradation, unprocessed fibrin matrices have limited use in tissue engineering.

OBJECTIVE

To describe stable fibrin-based matrices for isolation, growth and delivery of stem cells for implantation to enhance tissue regeneration.

METHODS

Fibrin microbeads (FMB) were produced by moderate-heat condensation of fibrin particles in oil without compromising the cell binding capability of the fibrin.

RESULTS

Mesenchymal stem cells (MSC) were separated from different sources at much higher yields with FMB. They were further expanded on them in suspension without trypsinization and passages. Cells on FMB could be induced to differentiate into different phenotypes, such as bone and cartilage. This enabled implantation of the cells on FMB for cell-based tissue regeneration.

CONCLUSIONS

FMB technology provides a simple and effective method for cell separation, expansion in suspension and delivery for tissue regeneration.