Factor XIII: recommended terms and abbreviations

The History of Rare Bleeding Disorders

Deficiencies in coagulation factors I (FI), FII, FV, combined FV and FVIII (CF5F8) and vitamin K-dependent coagulation factors FVII, FX, FXI, and FXIII have been referred to as rare bleeding disorders (RBDs), rare coagulation factor deficiencies (RCFDs), or recessively inherited coagulation disorders. Fibrinogen was most likely the first member of this group to be identified, with reports of its discovery spanning from 1859 to 1966. If not, then the first coagulation factor to be identified was prothrombin in 1894, and the last coagulation factor to be found was FX in 1956, about 60 years later. The first patient to be diagnosed with an RBD was a 9-year-old boy with afibrinogenemia in 1920 and the vitamin K-dependent coagulation factors deficiency was the most recent RBD in this group to be identified in a 3-month-old child in 1966. The initial therapeutic option for nearly all patients with RBDs was whole blood transfusion; this was replaced in 1941 by fresh frozen plasma (FFP), and then in later years by cryoprecipitate and coagulation factor concentrates. Fibrinogen concentrate was the first coagulation factor concentrate produced in 1956. Coagulation factor concentrate is now available for FI, FVII, FX, FXI, and FXIII; however, FFP and/or platelet transfusion are the only treatments available for FV deficiency. The only recombinant concentrates available for RBDs are for FVII and FXIII, which date from 1988 and the 2000s, respectively. Even though the clinical presentations, diagnosis, and management of lesser-known bleeding disorders have improved significantly in recent decades, more studies are needed to reveal the hidden aspects of these disorders in order to overcome diagnostic and therapeutic challenges and ultimately improve the quality of life for those who are affected.

Clinical Profile of Congenital Factor XIII Deficiency in Children

OBJECTIVES

Congenital Factor 13 Deficiency (FXIIID) is a rare bleeding disorder (RBD) of autosomal recessive inheritance, with an incidence of 1 in 3-5 million. The clinical symptomatology, diagnosis, and management of FXIIID are described.

METHODS

A retrospective chart review of children with FXIIID was performed from January 2000 through October 2021 at a tertiary care center in Southern India. The diagnosis was performed by the Urea clot solubility test (UCST) and Factor XIII antigen assay.

RESULTS

Twenty children (representing 16 families) were included. Male: Female ratio was 1.5:1. The median age of symptom onset was 6 mo, and the median age of diagnosis was 1 y, demonstrating a delay in diagnosis. Consanguinity was present in 15 (75%) with 4 children having affected siblings. Clinical symptomatology ranged from mucosal bleeds to intracranial bleeds and hemarthrosis, with many children having a history of prolonged umbilical bleeding in their neonatal period. Fourteen children were on cryoprecipitate prophylaxis. Four children had breakthrough bleeds due to irregular prophylaxis, including one intracranial bleed due to a delay in cryoprecipitate prophylaxis during the covid pandemic.

CONCLUSIONS

Congenital FXIIID presents with a wide range of bleeding manifestations. The high prevalence of consanguinity in Southern India can be a cause of FXIIID's high prevalence in this region. There is a propensity for intracranial bleeding with a significant number having this at first presentation. Regular prophylaxis is required and feasible to prevent potentially fatal bleeds.

Potential Biomarkers of Acute Ischemic Stroke Etiology Revealed by Mass Spectrometry-Based Proteomic Characterization of Formalin-Fixed Paraffin-Embedded Blood Clots

Background and Aims

Besides the crucial role in the treatment of acute ischemic stroke (AIS), mechanical thrombectomy represents a unique opportunity for researchers to study the retrieved clots, with the possibility of unveiling biological patterns linked to stroke pathophysiology and etiology. We aimed to develop a shotgun proteomic approach to study and compare the proteome of formalin-fixed paraffin-embedded (FFPE) cardioembolic and large artery atherosclerotic (LAA) clots.

Methods

We used 16 cardioembolic and 15 LAA FFPE thrombi from 31 AIS patients. The thrombus proteome was analyzed by label-free quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS). MaxQuant v1.5.2.8 and Perseus v.1.6.15.0 were used for bioinformatics analysis. Protein classes were identified using the PANTHER database and the STRING database was used to predict protein interactions.

Results

We identified 1,581 protein groups as part of the AIS thrombus proteome. Fourteen significantly differentially abundant proteins across the two etiologies were identified. Four proteins involved in the ubiquitin-proteasome pathway, blood coagulation or plasminogen activating cascade were identified as significantly abundant in LAA clots. Ten proteins involved in the ubiquitin proteasome-pathway, cytoskeletal remodeling of platelets, platelet adhesion or blood coagulation were identified as significantly abundant in cardioembolic clots.

Conclusion

Our results outlined a set of 14 proteins for a proof-of-principle characterization of cardioembolic and LAA FFPE clots, advancing the proteome profile of AIS human thrombi and understanding the pathophysiology of ischemic stroke.

Measuring Factor XIII Inhibitors in Patients with Factor XIII Deficiency: A Case Report and Systematic Review of Current Practices in Japan

Factor XIII (FXIII) deficiency is a rare but serious coagulopathy. FXIII is critical in blood coagulation, and FXIII deficiencies can lead to uncontrolled or spontaneous bleeding. FXIII deficiencies can be congenital or acquired; acquired FXIII deficiency can be categorized as autoimmune and non-autoimmune. Immunological tests to measure FXIII inhibitors are required to diagnose acquired FXIII deficiency; however, appropriate test facilities are limited, which increases the turnaround time of these tests. In the case of critical bleeding, delayed test results may worsen prognosis due to delayed treatment. Here, we report a case of acquired FXIII deficiency, followed by a review of FXIII deficiency cases in Japan. We performed a systematic review to investigate the present conditions of the diagnosis and treatment of FXIII deficiency, including the measurement of FXIII inhibitors in Japan. FXIII inhibitor testing was only performed in 29.7 of acquired FXIII deficiency cases. Clinical departments other than internal medicine and pediatrics were often involved in medical treatment at the time of onset. Therefore, it is important for doctors in clinical departments other than internal medicine and pediatrics to consider FXIII deficiency and perform FXIII inhibitor testing when examining patients with prolonged bleeding of unknown cause or persistent bleeding after trauma.

Feasibility of an automated coagulation factor XIIIa test using its isopeptidase activity

Plasma transglutaminase FXIII provides mechanical and biochemical stability to blood clots. Congenital or acquired deficiency may be associated with bleeding diathesis and requires therefore careful monitoring. The precise automated measurement of a large number of plasma samples can provide new insights regarding the clinical relevance of certain FXIII levels. There is still the unmet diagnostic need for a reliable high-throughput method. Here we report the development and feasibility study of a promising prototype, adapting the precise FXIIIa isopeptidase assay principle on the optimized automated Ceveron s100 platform.

Novel inhibitor ZED3197 as potential drug candidate in anticoagulation targeting coagulation FXIIIa (F13a)

BACKGROUND

Factor XIII (FXIII) is the final enzyme of the coagulation cascade. While the other enzymatic coagulation factors are proteases, FXIII belongs to the transglutaminase family. FXIIIa covalently crosslinks the fibrin clot and represents a promising target for drug development to facilitate fibrinolysis. However, no FXIII-inhibiting compound has entered clinical trials. Here, we introduce the features of a peptidomimetic inhibitor of FXIIIa (ZED3197) as a potential drug candidate.

METHODS

The potency of ZED3197 against FXIIIa and the selectivity against other human transglutaminases were characterized using transamidation and isopeptidase assays. The inhibition of fibrin crosslinking was evaluated by biochemical methods and thromboelastometry. Further, the pharmacology of the compound was explored in a rabbit model of venous stasis and reperfusion.

RESULTS

ZED3197 proved to be a potent and selective inhibitor of human FXIIIa. Further, the compound showed broad inhibitory activity against cellular FXIIIA from various animal species. Rotational thromboelastometry in whole human blood indicated that the inhibitor, in a dose-dependent manner, prolonged clot formation, reduced clot firmness, and facilitated clot lysis without affecting the clotting time, indicating minimal impact on hemostasis. In vivo, the novel FXIIIa inhibitor effectively decreased the weight of clots and facilitated flow restoration without prolongation of the bleeding time.

CONCLUSIONS

ZED3197 is the first drug-like potent compound targeting FXIIIa, a yet untapped target in anticoagulation. Due to the function of FXIII downstream of thrombin the approach provides minimal impact on hemostasis. In vivo data imply that the inhibitor dissociates an antithrombotic effect from increased bleeding tendency.

Structure functional insights into calcium binding during the activation of coagulation factor XIII A

The dimeric FXIII-A₂, a pro-transglutaminase is the catalytic part of the heterotetrameric coagulation FXIII-A₂B₂ complex that upon activation by calcium binding/thrombin cleavage covalently cross-links preformed fibrin clots protecting them from premature fibrinolysis. Our study characterizes the recently disclosed three calcium binding sites of FXIII-A concerning evolution, mutual crosstalk, thermodynamic activation profile, substrate binding, and interaction with other similarly charged ions. We demonstrate unique structural aspects within FXIII-A calcium binding sites that give rise to functional differences making FXIII unique from other transglutaminases. The first calcium binding site showed an antagonistic relationship towards the other two. The thermodynamic profile of calcium/thrombin-induced FXIII-A activation explains the role of bulk solvent in transitioning its zymogenic dimeric form to an activated monomeric form. We also explain the indirect effect of solvent ion concentration on FXIII-A activation. Our study suggests FXIII-A calcium binding sites could be putative pharmacologically targetable regions.

On the early events of the calcium-induced activation of coagulation factor XIII-A2 and tissue transglutaminase: an in silico study

Both coagulation factor XIII-A₂ (FXIII-A₂) and tissue transglutaminase (TG2) play distinctive and important roles in homeostasis by crosslinking proteins or peptides via isopeptide bonds. In this present study, a series of microsecond-long all-atom molecular dynamics (MD) simulations were carried out in order to reveal the dynamic, atomic-level events which may contribute to the activation of these proteins via the binding of calcium ions. In addition to previously conducted in vitro and crystallographic studies, further suggestions have been made concerning the calcium binding features of these enzymes. The different systems used for running the simulations were based on the zymogen, computationally cleaved and even the activation peptide (AP-FXIII) free FXIII-A₂' homodimer form. The effects of various ionic environments have also been explored in the simulations of FXIII-A₂. Our results suggest that the presence of calcium ions can cause increased AP fluctuations, which ultimately could lead to their relocation on the homodimer surface. The release of these APs seems to be crucial for rotation of the A subunits based on equilibrium MD simulations. The primary evidence for this assumption comes from the predicted principal component eigenvector which is considered as the first, large-scale event of the overall activation process. To get a more accurate atomic-level description, the calcium binding sites of TG2 have also been investigated in our extensive in silico experiments, which suggests the presence of previously unidentified binding sites as well.Communicated by Ramaswamy H. Sarma.

State of the art in factor XIII laboratory assessment

Factor XIII, a heterotetrameric proenzyme, is converted to an activated transglutaminase by thrombin and calcium in the final phases of coagulation. Factor XIII catalyzes the formation of crosslinks between fibrin monomers and between α2-antiplasmin and fibrin. These crosslinks mechanically stabilize fibrin against shear stress, enable red cell retention within the clot, and protect the clot from premature degradation. Congenital factor XIII deficiency is caused by autosomal recessive mutations, presenting early in life with a severe bleeding diathesis. Acquired deficiency may be caused by autoimmunity. Currently available assays for laboratory diagnosis of factor XIII deficiency include clot solubility assays, quantitative factor XIII activity assays, factor XIII antigen assays, and genetic testing. The International Society on Thrombosis and Haemostasis Scientific and Standardization Committee has recommended an algorithm for the laboratory diagnosis and differentiation of the different forms of factor XIII deficiency. However, implementation of this algorithm has been limited by technical and budgetary challenges associated with the currently available factor XIII-specific assays. The purpose of this review is to discuss the advantages and limitations of the currently available assays employed for the laboratory diagnosis of factor XIII deficiency.

Revisiting the mechanism of coagulation factor XIII activation and regulation from a structure/functional perspective

The activation and regulation of coagulation Factor XIII (FXIII) protein has been the subject of active research for the past three decades. Although discrete evidence exists on various aspects of FXIII activation and regulation a combinatorial structure/functional view in this regard is lacking. In this study, we present results of a structure/function study of the functional chain of events for FXIII. Our study shows how subtle chronological submolecular changes within calcium binding sites can bring about the detailed transformation of the zymogenic FXIII to its activated form especially in the context of FXIIIA and FXIIIB subunit interactions. We demonstrate what aspects of FXIII are important for the stabilization (first calcium binding site) of its zymogenic form and the possible modes of deactivation (thrombin mediated secondary cleavage) of the activated form. Our study for the first time provides a structural outlook of the FXIIIA₂B₂ heterotetramer assembly, its association and dissociation. The FXIIIB subunits regulatory role in the overall process has also been elaborated upon. In summary, this study provides detailed structural insight into the mechanisms of FXIII activation and regulation that can be used as a template for the development of future highly specific therapeutic inhibitors targeting FXIII in pathological conditions like thrombosis.

Anti-factor XIII A subunit (FXIII-A) autoantibodies block FXIII-A2 B2 assembly and steal FXIII-A from native FXIII-A2 B2

BACKGROUND

Autoimmune hemophilia-like disease (hemorrha-philia or hemorrhagic disorder) caused by anti-factor XIII antibodies (termed AH13) or 'autoimmune FXIII deficiency' is a life-threatening bleeding disorder. AH13 was thought to be rare worldwide.

OBJECTIVES

Because the number of diagnosed AH13 cases has recently been increasing, at least in Japan, we conducted a nationwide survey supported by the Japanese Ministry of Health, Labor, and Welfare, and explored the pathologic mechanism(s) of AH13.

METHODS

We diagnosed AH13 cases during the last 11 years according to the presence of anti-FXIII autoantibodies confirmed by a dot blot assay and ELISA, and characterized 33 of these both immunologically and biochemically.

RESULTS

The AH13 cases were immunologically classified into three types, Aa, Ab, and B. Type Aa autoantibodies, observed in 27 cases, were directed against the native FXIII A subunit (FXIII-A), and blocked FXIII activation. The autoantibodies not only prevented assembly of new FXIII-A2 B2 heterotetramers, but also removed FXIII-A from native FXIII-A2 B2 heterotetramers by forming an FXIII-A-IgG complex. Type Ab autoantibodies, detected in three cases, preferentially bound to activated FXIII-A and inhibited its activity. Type Aa and Ab autoantibodies were 'neutralizing' FXIII antibodies (or FXIII inhibitors), and thus could be screened with functional assays. Type B antibodies, detected in two cases, were non-neutralizing anti-FXIII B subunit (FXIII-B) autoantibodies that possibly accelerated the clearance of FXIII, and thus could be diagnosed exclusively with immunologic methods.

CONCLUSION

There are three major types of anti-FXIII autoantibody, with distinct targets and mechanisms that cause AH13.

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.

Evaluation of Factor V G1691A, prothrombin G20210A, Factor XIII V34L, MTHFR A1298C, MTHFR C677T and PAI-1 4G/5G genotype frequencies of patients subjected to cardiovascular disease (CVD) panel in south-east region of Turkey

Cardiovascular disease (CVD) risk factors, such as arterial hypertension, obesity, dyslipidemia or diabetes mellitus, as well as CVDs, including myocardial infarction, coronary artery disease or stroke, are the most prevalent diseases and account for the major causes of death worldwide. In the present study, 4,709 unrelated patients subjected to CVD panel in south-east part of Turkey between the years 2010 and 2013 were enrolled and DNA was isolated from the blood samples of these patients. Mutation analyses were conducted using the real-time polymerase chain reaction method to screen six common mutations (Factor V G1691A, PT G20210A, Factor XIII V34L, MTHFR A1298C and C677T and PAI-1 -675 4G/5G) found in CVD panel. The prevalence of these mutations were 0.57, 0.25, 2.61, 13.78, 9.34 and 24.27 % in homozygous form, respectively. Similarly, the mutation percent of them in heterozygous form were 7.43, 3.44, 24.91, 44.94, 41.09 and 45.66%, respectively. No mutation was detected in 92 (1.95%) patients in total. Because of the fact that this is the first study to screen six common mutations in CVD panel in south-east region of Turkey, it has a considerable value on the diagnosis and treatment of these diseases. Upon the results of the present and previous studied a careful examination for these genetic variants should be carried out in thrombophilia screening programs, particularly in Turkish population.

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.

Synthesis and characterization of novel fluorogenic substrates of coagulation factor XIII-A

Further development of our recently published Glu(pNA)-containing peptides (Anal. Biochem. 428 (2012) 73-80) provided new fluorogenic substrates for the activated blood coagulation factor XIII. A first series was designed by incorporation of Glu(AMC) at the penultimate position from the N terminus. For the best derivative H-Tyr-Glu(AMC)-Val-Lys-Val-Ile-NH2, a moderate kcat/Km value of 34s(-1)M(-1) was determined, which is more than 100-fold reduced compared with the previously reported Glu(pNA) substrates. Furthermore, two fluorescence resonance energy transfer (FRET) substrates were prepared by incorporation of an N-methyl-anthraniloyl fluorophore and a 2,4-dinitrophenyl quencher. Both substrates were excellently cleaved by FXIII-A2(∗), which is generated from its zymogen by activation of thrombin in the presence of calcium ions. In the absence and presence of H-Gly-ethyl ester, kcat/Km values of 8010 and 8660s(-)(1)M(-)(1), respectively, were found for the conversion of H-Lys(N(Me)Abz)-Glu(NH-(CH2)4-NH-Dnp)-Val-Lys-Val-Ile-Gly-NH2 (substrate 8). These values are more than 200-fold improved compared with the Glu(AMC) substrates. Substrate 8 is suitable for the measurement of FXIII-A2(∗) activities in plasma samples as well as for in vitro measurements. Furthermore, it was used for the determination of the inhibitory potency of a newly synthesized chloromethyl ketone derivative, Cbz-Phe-Glu(CMK)-Val-Lys-Val-Ile-Gly-NH2, which was found to be a potent irreversible inhibitor of FXIII-A2(∗).

Differences in the inhibition of coagulation factor XIII-A from animal species revealed by Michael Acceptor- and thioimidazol based blockers

INTRODUCTION

The A-subunit of blood coagulation factor XIII is a pro-transglutaminase, which cross-links α- and γ-fibrin-chains in its activated form. Selective inhibitors against FXIII-A may be desirable drugs to prevent the development of thromboses. Animal models are generally used for proof of principle and for toxicological studies in drug development. The aim of the study was to investigate the specificity of a set of FXIII-A-blockers against FXIII-A from different species, i.e. human, dog, mouse, rat and pig. Thus the usefulness of different animal species for FXIII-A-blocker drug development should be evaluated.

MATERIALS AND METHODS

FXIII-A proteins were recombinantly produced in insect cells and purified to homogeneity. They were characterized by SDS- and native PAGE, a transamidase assay and isopeptidase assay. The inhibition second-order rate constants of different irreversible inhibitors were determined using the isopeptidase assay.

RESULTS

All FXIII-A species were able to assemble with recombinant human FXIII-B into a heterotetrameric complex. Kinetic parameters of FXIII-A species were determined. Second-order rate constants for FXIII-A inhibition by two irreversible inhibitors were determined and differed considerably. FXIII-A species of dog, mouse and rat were inhibited in a manner similar to human FXIII-A. Pig FXIII-A however was resistant to a previously described non-peptidic inhibitor. Furthermore, the results showed considerably better inhibition with the novel peptide-based inhibitor compared to the non-peptidic compound.

CONCLUSIONS

Our data shows that biochemical interspecies comparison studies are a prerequisite for animal studies. Peptide-derived inhibitors carrying a Michael Acceptor Pharmacophore (MAP) are a promising new class of FXIII-A-inhibitors.

Characterization of coagulation factor synthesis in nine human primary cell types

The coagulation/fibrinolysis system is essential for wound healing after vascular injury. According to the standard paradigm, the synthesis of most coagulation factors is restricted to liver, platelets and endothelium. We challenged this interpretation by measuring coagulation factors in nine human primary cell types. FX mRNA was expressed by fibroblasts, visceral preadipocytes/adipocytes and hepatocytes, but not in macrophages or other cells. All cells expressed FVIII except endothelial cells. Fibroblasts, endothelial cells and macrophages produced thrombomodulin but not FV. Interestingly, vascular-related cells (platelets/monocytes) that expressed FV did not express FX and vice versa. Monocytes expressed FV, FVIII and FXIIIA, which are positive regulators of clot formation, but these cells also contained thrombomodulin, a negative regulator of coagulation. Our data show that the expression of coagulation factors is much more complex than previously thought, and we speculate that this intricate regulation of coagulation factor expression is necessary for correct fine-tuning of fibrinogenesis versus fibrinolysis.

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.

Reversible activation of cellular factor XIII by calcium

Factor XIII (FXIII) is a pro-transglutaminase found in the plasma as well as intracellularly in platelets and macrophages. Plasma FXIII is activated by thrombin cleavage (FXIIIa*) and acts in the final stages of blood coagulation cascade. In contrast, the function and activation of cellular FXIII are less characterized. Cellular FXIII relies on a conformational activation of the protein. The nonproteolytic activation of FXIII to FXIIIa° induced by Ca(2+) alone is well known, but up until now it has been discussed under which conditions the process can be induced and whether it can be reversed. Here, we study the nature of the Ca(2+)-induced FXIII activation. Previously used methods to evaluate FXIII activity detect both FXIIIa* and FXIIIa° because they rely on occurrence of enzyme activity or on active site Cys-314 solvent accessibility. Therefore, an analytical HPLC method was developed that separates zymogen recombinant FXIII (rFXIII) from rFXIIIa°. The data demonstrate that nonproteolytic activation and deactivation are highly dependent on Ca(2+) concentration, buffer, and salt components. Moreover, it is established that Ca(2+) activation of rFXIII is fully reversible, and only 2-5 mm CaCl(2) is sufficient to retain full rFXIIIa° activity. However, below 2 mm CaCl(2) the rFXIIIa° molecule deactivates. The deactivated molecule can subsequently undergo a new activation round. Furthermore, it is demonstrated that thermal stress of freeze-dried rFXIII can induce a new predisposed form that activates faster than nonstressed rFXIII.

Role of fibrin structure in thrombosis and vascular disease

Fibrin clot formation is a key event in the development of thrombotic disease and is the final step in a multifactor coagulation cascade. Fibrinogen is a large glycoprotein that forms the basis of a fibrin clot. Each fibrinogen molecule is comprised of two sets of Aα, Bβ, and γ polypeptide chains that form a protein containing two distal D regions connected to a central E region by a coiled-coil segment. Fibrin is produced upon cleavage of the fibrinopeptides by thrombin, which can then form double-stranded half staggered oligomers that lengthen into protofibrils. The protofibrils then aggregate and branch, yielding a three-dimensional clot network. Factor XIII, a transglutaminase, cross-links the fibrin stabilizing the clot protecting it from mechanical stress and proteolytic attack. The mechanical properties of the fibrin clot are essential for its function as it must prevent bleeding but still allow the penetration of cells. This viscoelastic property is generated at the level of each individual fiber up to the complete clot. Fibrinolysis is the mechanism of clot removal, and involves a cascade of interacting zymogens and enzymes that act in concert with clot formation to maintain blood flow. Clots vary significantly in structure between individuals due to both genetic and environmental factors and this has an effect on clot stability and susceptibility to lysis. There is increasing evidence that clot structure is a determinant for the development of disease and this review will discuss the determinants for clot structure and the association with thrombosis and vascular disease.

Factor XIII deficiency

Inherited factor XIII (FXIII) deficiency is a rare bleeding disorder that can present with umbilical bleeding during the neonatal period, delayed soft tissue bruising, mucosal bleeding and life-threatening intracranial haemorrhage. FXIII deficiency has also been associated with poor wound healing and recurrent miscarriages. FXIII plays an integral role in haemostasis by catalysing the cross-linking of fibrin, platelet membrane and matrix proteins throughout thrombus formation, thus stabilizing the blood clot. The molecular basis of FXIII deficiency is characterized by a high degree of heterogeneity, which contributes to the different clinical manifestations of the disease. There have been more than 60 FXIII mutations identified in the current literature. In addition, single nucleotide polymorphisms have been described, some of which have been shown to affect FXIII activity, contributing further to the heterogeneity in patient presentation and severity of clinical symptoms. Although there is a lifelong risk of bleeding, the prognosis is excellent when current prophylactic treatment is available using cryoprecipitate or plasma-derived FXIII concentrate.

Comparison of coagulation factor XIII content and concentration in cryoprecipitate and fresh-frozen plasma

BACKGROUND

For patients with plasma coagulation factor XIII (pFXIII) deficiency, recommended means of replacement include infusions of fresh-frozen plasma (FFP), cryoprecipitate, or (where available) factor (F)XIII concentrates. Quantitative differences in pFXIII concentration in FFP and cryoprecipitate are not well defined and were, therefore, the subject of this study.

STUDY DESIGN AND METHODS

FFP and cryoprecipitate (10 bags each from blood group O donors) were analyzed to quantify pFXIII activity and antigen. Coagulation FVIII, fibrinogen, and von Willebrand factor (VWF) were also quantitated.

RESULTS

Mean (+/-SD) pFXIII activity in cryoprecipitate and FFP bags was 60 +/- 30 and 288 +/- 77 U per bag, respectively, and pFXIII antigen and activity levels were concordant. Other comparisons (mean +/- SD) between cryoprecipitate and FFP, respectively, were as follows: coagulation FVIII activity, 133 +/- 37 and 265 +/- 83 U per bag; fibrinogen content (Clauss kinetic assay), 183 +/- 44 and 725 +/- 199 mg per bag; VWF antigen content, 181 +/- 53 and 218 +/- 70 U per bag; VWF ristocetin cofactor activity, 168 +/- 34 and 221 +/- 65 U per bag; VWF collagen-binding activity, 164 +/- 40 and 208 +/- 71 U per bag; and fluid (plasma) volumes per bag, 21.3 +/- 2.7 and 245 +/- 29 mL.

CONCLUSION

In contrast to other cryoprecipitable coagulation proteins, pFXIII is only mildly enriched in cryoprecipitate when compared with FFP (approx. two- to threefold). Although both products can provide effective pFXIII replacement, FFP may be preferred when infusion volume is not a major consideration and pFXIII concentrates are not available. VWF is substantially enriched in cryoprecipitate (approx. ninefold compared with its concentration in FFP), with VWF activity content exceeding that of FVIII by approximately 26 percent on average.