Evolution of the fibrinogen gamma' chain: implications for the binding of factor XIII, thrombin and platelets

Fibrinogen and fibrin: synthesis, structure, and function in health and disease

Fibrinogen is an extraordinary molecule by any estimation. It is large, structurally intricate, and circulates at high concentrations. Its biological end product, insoluble fibrin(ogen) or fibrin, can assume a diverse array of conformations with the ability to interact with numerous plasma proteins and cells and withstand biochemical and biomechanical disruption to facilitate wound healing. Quantitative and qualitative defects in fibrinogen or fibrin are associated with bleeding, thrombosis, inflammation, and diseases affected by these processes. Numerous studies investigating mechanisms by which fibrin(ogen) and fibrin contribute to health and disease have been published. This review for the 20th-anniversary series in the Journal of Thrombosis and Haemostasis summarizes interesting aspects of fibrin(ogen) biology, biochemistry, biophysics, and physiology and highlights exciting findings published in the past 2 decades.

Amikacin disaggregates platelet clumps in EDTA blood samples from cats and dogs when added postcollection

BACKGROUND

Pseudothrombocytopenia may lead to the erroneous diagnosis of thrombocytopenia, resulting in unnecessary testing and treatment. The addition of exogenous substances to blood samples prior to collection has been shown to mitigate platelet (PLT) clumps in blood samples. Postcollection additives aiming to disaggregate PLT clumps have been largely unexplored.

OBJECTIVES

We aimed to determine if the addition of amikacin to blood samples postcollection aids in the disaggregation of PLT clumps in cats and dogs.

METHODS

For this prospective study, EDTA-collected blood samples from 28 cats and 17 dogs were obtained from a hospital population at UC Davis Veterinary Medical Teaching Hospital. Samples had PLT clumps detected on blood smears and thrombocytopenia per analyzer count. Amikacin was added to samples postcollection, and an additional CBC was performed. Flow cytometry was performed to assess PLT-fibrinogen binding in amikacin-treated aliquots.

RESULTS

PLT-clumped samples treated with amikacin significantly increased PLT numbers by 134% and decreased mean platelet volume (MPV) values by 14% (P ≤ 0.0001) in cats, and increased PLT numbers by 32% (P = 0.04) and increased MPV values by 9% (P = 0.02) in dogs. Mean cell volume (MCV) slightly increased (<4%) for both species. No other CBC parameters were substantially affected by the addition of amikacin. Flow cytometry showed decreased PLT-fibrinogen binding in the majority of cats but was not significant (P > 0.05).

CONCLUSIONS

Adding amikacin to PLT-clumped blood samples postcollection may be a convenient solution for pseudothrombocytopenia in cats and dogs. Future studies are needed to elucidate the mechanism of amikacin and its effectiveness under different storage conditions. This is the first reported use of amikacin postcollection to disaggregate PLT clumps in blood samples from animals.

Identification and relative quantification of 3-nitrotyrosine residues in fibrinogen nitrated in vitro and fibrinogen from ischemic stroke patient plasma using LC-MS/MS

Ischemic stroke is one of the leading causes of death and disability worldwide. This acute vascular event interferes with blood supply to the brain and induces a burst of free radicals such as nitric oxide and superoxide, producing peroxynitrite, a precursor of strong nitrating agents. Fibrinogen is one of the most abundant plasma proteins; it plays a role in the hemostatic system, mediating clot formation, which can be affected by nitrotyrosine formation. We hypothesized that nitration of fibrinogen by ONOOH and ONOOCO₂^- radical products could be one of the early events of the ischemic stroke, and protein-bound 3-nitrotyrosine could be a potential biomarker for diagnosis and/or prognosis of this condition. A targeted mass spectrometry approach was developed to analyze the nitration of fibrinogen and its association with ischemic stroke. First, a comprehensive mapping of 3-nitrotyrosine locations and their relative quantification was performed by LC-MS/MS, using in vitro nitrated fibrinogen samples. Twenty different 3-nitrotyrosine residues were identified on fibrinogen nitrated in vitro, varying with the peroxynitrite tofibrinogen molar ratio used. Nine tyrosine residues that were consistently modified at different treatment ratios were chosen to perform a targeted LC-MS/MS analysis in clinical samples. Enriched fibrinogen fractions from clinical samples from 24 ischemic stroke and 12 patients with non-inflammatory conditions were analysed with this method. Three of the nine tyrosine residues analysed (βY452, βY475 and γY380) showed a significant difference between the ischemic stroke and non-inflammatory disease groups. ROC curve analysis suggested an association of these residues either individually or in combination with ischemic stroke. Different tyrosine nitration patterns were also observed in fibrinogen modified in vitro and in vivo, suggesting differences in the nitration process in these situations. This is the first study showing a putative association between the nitration profile of specific tyrosine residues in human fibrinogen and ischemic stroke.

The effect of fibrin(ogen) on thrombin generation and decay

Defibrination causes a ~30% decrease of thrombin generation (TG) which can be restored by adding native fibrinogen in its original concentration (3 mg/ml). The fibrinogen variant γA/γ′, which binds thrombin with high affinity, is over four times more efficient in this respect than the more common γA/γA form. By using high tissue factor concentrations we accelerated prothrombin conversion so as to obtain a descending part of the TG curve that was governed by thrombin decay only. From that part we calculated the antithrombin (AT)- and α2-macroglobulin- dependent decay constants at a series of concentrations of native, γA/γA and γA/γ′ fibrinogen. We found that the increase of TG in the presence of fibrinogen is primarily due to a dose-dependent decrease of thrombin inactivation by α2-macroglobulin, where the γA/γ′ form is much more active than the γA/γA form. AT-dependent decay is somewhat decreased by γA/γ′ fibrinogen but hardly by the γA/γA form. We assume that binding of thrombin to fibrin(ogen) interferes with its binding to inhibitors. Attenuation of decay only in part explains the stimulating effect of fibrinogen on TG, as fibrinogen stimulates prothrombin conversion, regardless of the fibrinogen variant.

Note: Part of this work was presented at the ISTH meeting in 2013.

Comparative genomic study of arachnid immune systems indicates loss of beta-1,3-glucanase-related proteins and the immune deficiency pathway

Analyses of arthropod genomes have shown that the genes in the different innate humoral immune responses are conserved. These genes encode proteins that are involved in immune signalling pathways that recognize pathogens and activate immune responses. These immune responses include phagocytosis, encapsulation of the pathogen and production of effector molecules for pathogen elimination. So far, most studies have focused on insects leaving other major arthropod groups largely unexplored. Here, we annotate the immune-related genes of six arachnid genomes and present evidence for a conserved pattern of some immune genes, but also evolutionary changes in the arachnid immune system. Specifically, our results suggest that the family of recognition molecules of beta-1,3-glucanase-related proteins (βGRPs) and the genes from the immune deficiency (IMD) signalling pathway have been lost in a common ancestor of arachnids. These findings are consistent with previous work suggesting that the humoral immune effector proteins are constitutively produced in arachnids in contrast to insects, where these have to be induced. Further functional studies are needed to verify this. We further show that the full haemolymph clotting cascade found in the horseshoe crab is retrieved in most arachnid genomes. Tetranychus lacks at least one major component, although it is possible that this cascade could still function through recruitment of a different protein. The gel-forming protein in horseshoe crabs, coagulogen, was not recovered in any of the arachnid genomes; however, it is possible that the arachnid clot consists of a related protein, spätzle, that is present in all of the genomes.

Coagulation factor XIII deficiency. Diagnosis, prevalence and management of inherited and acquired forms

The plasma circulating zymogenic coagulation factor XIII (FXIII) is a protransglutaminase, which upon activation by thrombin and calcium cross-links preformed fibrin clots/fibrinolytic inhibitors making them mechanically stable and less susceptible to fibrinolysis. The zymogenic plasma FXIII molecule is a heterotetramer composed of two catalytic FXIII-A and two protective FXIII-B subunits. Factor XIII deficiency resulting from inherited or acquired causes can result in pathological bleeding episodes. A diverse spectrum of mutations have been reported in the F13A1 and F13B genes which cause inherited severe FXIII deficiency. The inherited severe FXIII deficiency, which is a rare coagulation disorder with a prevalence of 1 in 4 million has been the prime focus of clinical and genetic investigations owing to the severity of the bleeding phenotype associated with it. Recently however, with a growing understanding into the pleiotropic roles of FXIII, the fairly frequent milder form of FXIII deficiency caused by heterozygous mutations has become one of the subjects of investigative research. The acquired form of FXIII deficiency is usually caused by generation of autoantibodies or hyperconsumption in other disease states such as disseminated intravascular coagulation. Here, we update the knowledge about the pathophysiology of factor XIII deficiency and its therapeutic options.

The pleiotropic role of the fibrinogen γ′ chain in hemostasis

A fraction of fibrinogen contains a differently spliced γ chain called γ′, which presents itself mainly as heterodimer with the common γA chain as γA/γ′ fibrinogen. The γ′ chain differs from the γA chain in its C-terminus and has important functional implications for fibrinogen. The presence of the γ′ chain modulates thrombin and FXIII activity, influences clot architecture, and eliminates a platelet-binding site. Associations of γA/γ′ fibrinogen levels with arterial and venous thrombosis have been reported, indicating that the functional effects of γA/γ′ fibrinogen may contribute to the pathology of thrombosis. This review summarizes the key biologic aspects of this interesting variant of fibrinogen and discusses inconsistencies in current reports.