Activation of factor XIII is accompanied by a change in oligomerization state

Urinary Protein-Biomarkers Reliably Indicate Very Early Kidney Damage in Children With Alport Syndrome Independently of Albuminuria and Inflammation

Introduction

Alport syndrome (AS) is a hereditary type IV collagen disease. It starts shortly after birth, without clinical symptoms, and progresses to end-stage kidney disease early in life. The earlier therapy starts, the more effectively end-stage kidney disease can be delayed. Clearly then, to ensure preemptive therapy, early diagnosis is an essential prerequisite.

Methods

To provide early diagnosis, we searched for protein biomarkers (BMs) by mass spectrometry in dogs with AS stage 0. At this very early stage, we identified 74 candidate BMs. Of these, using commercial enzyme-linked immunosorbent assays (ELISAs), we evaluated 27 in dogs and 28 in children, 50 with AS and 104 healthy controls.

Results

Most BMs from blood appeared as fractions of multiple variants of the same protein, as shown by their chromatographic distribution before mass spectrometry. Blood samples showed only minor differences because ELISAs rarely detect disease-specific variants. However, in urine , several proteins, individually or in combination, were promising indicators of very early and preclinical kidney injury. The BMs with the highest sensitivity and specificity were collagen type XIII, hyaluronan binding protein 2 (HABP2), and complement C4 binding protein (C4BP).

Conclusion

We generated very strong candidate BMs by our approach of first examining preclinical AS in dogs and then validating these BMs in children at early stages of disease. These BMs might serve for screening purposes for AS before the onset of kidney damage and therefore allow preemptive therapy.

Fbg αC 389-402 Enhances Factor XIII Cross-Linking in the Fibrinogen αC Region Via Electrostatic and Hydrophobic Interactions

Coagulation Factor XIII (FXIII) stabilizes blood clots by cross-linking glutamines and lysines in fibrin and other proteins. FXIII activity in the fibrinogen αC region (Fbg αC 221-610) is critical for clot stability and growth. Fbg αC 389-402 is a binding site for thrombin-activated FXIII, (FXIII-A*), with αC E396 promoting FXIII-A* binding and activity in αC. The current study aimed to discover additional residues within Fbg αC 389-402 that accelerate transglutaminase activity toward αC. Electrostatic αC residues (E395, E396, and D390), hydrophobic αC residues (W391 and F394), and residues αC 328-425 were studied by mutations to recombinant Fbg αC 233-425. FXIII activity was monitored through MS-based glycine ethyl ester (GEE) cross-linking and gel-based fluorescence monodansylcadaverine (MDC) cross-linking assays. Truncation mutations 403 Stop (Fbg αC 233-402), 389 Stop (Fbg αC 233-388), and 328 Stop (Fbg αC 233-327) reduced Q237-GEE and MDC cross-linking compared to wild-type (WT). Comparable cross-linking between 389 Stop and 328 Stop showed that FXIII is mainly affected by the loss of Fbg αC 389-402. Substitution mutations E396A, D390A, W391A, and F394A decreased cross-linking relative to WT, whereas E395A, E395S, E395K, and E396D had no effect. Similar FXIII-A* activities were observed for double mutants (D390A, E396A) and (W391A, E396A), relative to D390A and W391A, respectively. In contrast, cross-linking was reduced in (F394A, E396A), relative to F394A. In conclusion, Fbg αC 389-402 boosts FXIII activity in Fbg αC, with D390, W391, and F394 identified as key contributors in enhancing αC cross-linking.

Transglutaminase Activities of Blood Coagulant Factor XIII Are Dependent on the Activation Pathways and on the Substrates

Factor XIII (FXIII) catalyzes formation of γ-glutamyl-ε-lysyl crosslinks between reactive glutamines (Q) and lysines (K). In plasma, FXIII is activated proteolytically (FXIII-A*) by the concerted action of thrombin and Ca2+. Cellular FXIII is activated nonproteolytically (FXIII-A°) by elevation of physiological Ca2+ concentrations. FXIII-A targets plasmatic and cellular substrates, but questions remain on correlating FXIII activation, resultant conformational changes, and crosslinking function to different physiological substrates. To address these issues, the characteristics of FXIII-A* versus FXIII-A° that contribute to transglutaminase activity and substrate specificities were investigated. Crosslinking of lysine mimics into a series of Q-containing substrates were measured using in-gel fluorescence, mass spectrometry, and UV-Vis spectroscopy. Covalent incorporation of fluorescent monodansylcadaverine revealed that FXIII-A* exhibits greater activity than FXIII-A° toward Q residues within Fbg αC (233-425 WT, Q328P Seoul II, and Q328PQ366N) and actin. FXIII-A* and FXIII-A° displayed similar activities toward α2-antiplasmin (α2AP), fibronectin, and Fbg αC (233-388, missing FXIII-binding site αC 389-402). Furthermore, the N-terminal α2AP peptide (1-15) exhibited similar kinetic properties for FXIII-A* and FXIII-A°. MALDI-TOF mass spectrometry assays with glycine ethyl ester and Fbg αC (233-425 WT, αC E396A, and truncated αC (233-388) further documented that FXIII-A* exerts greater benefit from the αC 389-402 binding site than FXIII-A°. Conformational properties of FXIII-A* versus A° are proposed to help promote transglutaminase function toward different substrates. A combination of protein substrate disorder and secondary FXIII-binding site exposure are utilized to control activity and specificity. From these studies, greater understandings of how FXIII-A targets different substrates are achieved.

Genetic landscape in coagulation factor XIII associated defects – Advances in coagulation and beyond

Coagulation factor XIII (FXIII) acts as a fine fulcrum in blood plasma that maintains the balance between bleeding and thrombosis by covalently crosslinking the pre-formed fibrin clot into an insoluble one that is resistant to premature fibrinolysis. In plasma, FXIII circulates as a pro-transglutaminase complex composed of the dimeric catalytic FXIII-A encoded by the F13A1 gene and dimeric carrier/regulatory FXIII-B subunits encoded by the F13B gene. Growing evidence accumulated over decades of exhaustive research shows that not only does FXIII play major roles in both pathological extremes of hemostasis i.e. bleeding and thrombosis, but that it is, in fact, a pleiotropic protein with physiological roles beyond coagulation. However, the current FXIII genetic-epidemiological literature is overwhelmingly derived from the bleeding pathology associated with its deficiency. In this article we review the current clinical, functional, and molecular understanding of this fascinating multifaceted protein, especially putting into the same perspective its genetic landscape.

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.

Self-Amplifying Nanotherapeutic Drugs Homing to Tumors in a Manner of Chain Reaction

Active tumor-targeting drug delivery has great potency in cancer therapy. However, the targeting efficiency of traditional active tumor-targeting nanotherapeutic drugs is limited by the scarcity of their accessible targets/receptors in tumors. Here, a novel self-amplifying tumor-targeting strategy with a chain reaction mechanism is developed. A coagulation targeting peptide (GNQEQVSPLTLLKXC, termed A15)-decorated poly(L-glutamic acid)-graft-maleimide poly(ethylene glycol)/combretastatin A4 conjugate (A15-PLG-CA4) is prepared to obtain a self-amplifying nanotherapeutic platform homing to tumors. After administration to tumor-bearing mice, A15-PLG-CA4 starts a chain reaction cycle consisting of intratumoral hemorrhage, target FXIIIa amplification, blood clot binding, and CA4 release in tumors. In this way, A15-PLG-CA4 increases the level of its accessible targets (FXIIIa) in a manner of chain reaction. The FXIIIa activity at 8 h is 4.1-fold more than the one at 0 h in the C26 tumors treated with A15-PLG-CA4. The total CA4 concentration at 24 h is 2.9-fold more than the control. A15-PLG-CA4 shows a significantly higher antitumor effect against large C26 tumors (≈500 mm3 ) thanks to the remarkable tumor-targeting ability compared with the control. Therefore, this report highlights the potential of the self-amplifying tumor-targeting strategy in the development of next generation active tumor-targeting nanotherapeutic drugs for tumor therapy.

The Structure of Blood Coagulation Factor XIII Is Adapted to Oxidation

The blood coagulation factor XIII (FXIII) plays a critical role in supporting coagulation and fibrinolysis due to both the covalent crosslinking of fibrin polymers, rendering them resistant to plasmin lysis, and the crosslinking of fibrin to proteins of the fibrinolytic system. The hypochlorite-mediated oxidation of the blood coagulation factor XIII (FXIII) at the different stages of its enzymatic activation is studied for the first time in this paper. The consolidated results obtained with the aid of MS/MS, electrophoresis, and colorimetry demonstrate that in the process of FXIII's conversion into FXIIIa, the vulnerability of FXIII to hypochlorite-induced oxidation increased as follows: native FXIII < FXIII + Ca2+ << FXIII + Ca2+/thrombin. The modification sites were detected among all the structural regions of the catalytic FXIII-A subunit, except for the activation peptide, and embraced several sushi domains of the FXIII-B subunit. Oxidized amino acid residues belonging to FXIII-A are surface-exposed residues and can perform an antioxidant role. The regulatory FXIII-B subunits additionally contribute to the antioxidant defense of the catalytic center of the FXIII-A subunits. Taken together, the present data along with the data from previous studies demonstrate that the FXIII proenzyme structure is adapted to oxidation.

Distinct 3-disulfide-bonded isomers of tridegin differentially inhibit coagulation factor XIIIa: The influence of structural stability on bioactivity

Tridegin is a 66mer cysteine-rich coagulation factor XIIIa (FXI-IIa) inhibitor from the giant amazon leech Haementeria ghilianii of yet unknown disulfide connectivity. This study covers the structural and functional characterization of five different 3-disulfide-bonded tridegin isomers. In addition to three previously identified isomers, one isomer containing the inhibitory cystine knot (ICK, knottin) motif, and one isomer with the leech antihemostatic protein (LAP) motif were synthesized in a regioselective manner. A fluorogenic enzyme activity assay revealed a positive correlation between the constriction of conformational flexibility in the N-terminal part of the peptide and the inhibitory potential towards FXI-IIa with clear differences between the isomers. This observation was supported by molecular dynamics (MD) simulations and subsequent molecular docking studies. The presented results provide detailed structure-activity relationship studies of different tridegin disulfide isomers towards FXI-IIa and reveal insights into the possibly existing native linkage compared to non-native disulfide tridegin species.

Inhibitors of blood coagulation factor XIII

The blood coagulation factor XIII (FXIII) plays an essential role in the stabilization of fibrin clots. This factor, belonging to the class of transglutaminases, catalyzes the final step of secondary hemostasis, i.e. the crosslinking of fibrin polymers. These crosslinks protect the clots against premature fibrinolysis. Consequently, FXIII is an interesting target for the therapeutic treatment of cardiovascular diseases. In this context, inhibitors can influence FXIII in the activation process of the enzyme itself or in its catalytic activity. To date, there is no FXIII inhibitor in medical application, but several studies have been conducted in the past. These studies provided a better understanding of FXIII and identified new lead structures for FXIII inhibitors. Next to small molecule inhibitors, the most promising candidates for the development of clinically applicable FXIII inhibitors are the peptide inhibitors tridegin and transglutaminase-inhibiting Michael acceptors (TIMAs) due to their selectivity towards activated FXIII (FXIIIa). In this review, select FXIII inhibitors and their pharmacological potential are discussed.

Identification of amino acid residues that are crucial for FXIII-A intersubunit interactions and stability

Coagulation factor XIII (FXIII) is the main stabilizer of the fibrin clot. It circulates in plasma as a tetramer of two A-subunits and two B-subunits. Under physiological conditions, FXIII-A exists as a dimer (FXIII-A2). The interactions between the FXIII-A-subunits that stabilize the FXIII-A2 dimer are not fully understood. We therefore designed a systematic approach to identify amino acid residues crucial for the expression and stability of FXIII-A2. Based on the available FXIII-A2 crystal structure, we identified 12 amino acid residues forming intersubunit salt bridges and 21 amino acid residues forming hydrogen bonds between the two A-subunits. We chose 10 amino acid residues that form 5 particularly strong interactions, performed site-directed mutagenesis, and expressed the mutants in CHO cells. Disruption of these interactions by single mutation of Lys257, Lys113, Asp343, Glu401, or Asp404 abolished the expression of properly folded, soluble, and functional FXIII-A in CHO cells. On the contrary, mutation of Glu111, Arg100, or Asn112 had no significant effect on FXIII-A expression. Our results suggest that 4 intersubunit interactions (Arg11-Asp343, Lys113-Asp367, Lys257-Glu401, and Arg260-Asp404) are essential for the stability of FXIII-A2. Our findings are supported by reported mutations at Lys257, Arg260, and Asp404 found in patients with congenital FXIII-A deficiency.

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.

The Plasma Factor XIII Heterotetrameric Complex Structure: Unexpected Unequal Pairing within a Symmetric Complex

Factor XIII (FXIII) is a predominant determinant of clot stability, strength, and composition. Plasma FXIII circulates as a pro-transglutaminase with two catalytic A subunits and two carrier-protective B subunits in a heterotetramer (FXIII-A₂B₂). FXIII-A₂ and -B₂ subunits are synthesized separately and then assembled in plasma. Following proteolytic activation by thrombin and calcium-mediated dissociation of the B subunits, activated FXIII (FXIIIa) covalently cross links fibrin, promoting clot stability. The zymogen and active states of the FXIII-A subunits have been structurally characterized; however, the structure of FXIII-B subunits and the FXIII-A₂B₂ complex have remained elusive. Using integrative hybrid approaches including atomic force microscopy, cross-linking mass spectrometry, and computational approaches, we have constructed the first all-atom model of the FXIII-A₂B₂ complex. We also used molecular dynamics simulations in combination with isothermal titration calorimetry to characterize FXIII-A₂B₂ assembly, activation, and dissociation. Our data reveal unequal pairing of individual subunit monomers in an otherwise symmetric complex, and suggest this unusual structure is critical for both assembly and activation of this complex. Our findings enhance understanding of mechanisms associating FXIII-A₂B₂ mutations with disease and have important implications for the rational design of molecules to alter FXIII assembly or activity to reduce bleeding and thrombotic complications.

Proteolytic and nonproteolytic activation mechanisms result in conformationally and functionally different forms of coagulation factor XIII A

Factor XIIIA (FXIIIA) is a transglutaminase that cross-links intra- and extracellular protein substrates. FXIIIA is expressed as an inactive zymogen, and during blood coagulation, it is activated by removal of an activation peptide by the protease thrombin. No such proteolytic FXIIIA activation is known to occur in other tissues or the intracellular form of FXIIIA. For those locations, FXIIIA is assumed instead to undergo activation by Ca²⁺ ions. Previously, we demonstrated a monomeric state for active FXIIIA. Current analytical ultracentrifugation and kinetic experiments revealed that thrombin-activated FXIIIA has a higher conformational flexibility and a stronger affinity toward glutamine substrate than does nonproteolytically activated FXIIIA. The proteolytic activation of FXIIIA was further investigated in a context of fibrin clotting. In a series of fibrin cross-linking assays and scanning electron microscopy studies of plasma clots, the activation rates of FXIIIA V34X variants were correlated with the extent of fibrin cross-linking and incorporation of nonfibrous protein into the clot. Overall, the results suggest conformational and functional differences between active FXIIIA forms, thus expanding the understanding of FXIIIA function. Those differences may serve as a basis for developing therapeutic strategies to target FXIIIA in different physiological environments. ENZYMES: Factor XIIIA ( EC 2.3.2.13).

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.

Factor XIII topology: organization of B subunits and changes with activation studied with single-molecule atomic force microscopy

Essentials Factor XIII is a heterotetramer with 2 catalytic A subunits and 2 non-catalytic B subunits. Structure of active and inactive factor XIII was studied with atomic force microscopy. Inactive factor XIII is made of an A2 globule and 2 flexible B subunits extending from it. Activated factor XIII separates into a B2 homodimer and 2 monomeric active A subunits. SUMMARY: Background Factor XIII (FXIII) is a precursor of the blood plasma transglutaminase (FXIIIa) that is generated by thrombin and Ca2+ and covalently crosslinks fibrin to strengthen blood clots. Inactive plasma FXIII is a heterotetramer with two catalytic A subunits and two non-catalytic B subunits. Inactive A subunits have been characterized crystallographically, whereas the atomic structure of the entire FXIII and B subunits is unknown and the oligomerization state of activated A subunits remains controversial. Objectives Our goal was to characterize the (sub)molecular structure of inactive FXIII and changes upon activation. Methods Plasma FXIII, non-activated or activated with thrombin and Ca2+ , was studied by single-molecule atomic force microscopy. Additionally, recombinant separate A and B subunits were visualized and compared with their conformations and dimensions in FXIII and FXIIIa. Results and Conclusions We showed that heterotetrameric FXIII forms a globule composed of two catalytic A subunits with two flexible strands comprising individual non-catalytic B subunits that protrude on one side of the globule. Each strand corresponds to seven to eight out of 10 tandem repeats building each B subunit, called sushi domains. The remainder were not seen, presumably because they were tightly bound to the globular A2 dimer. Some FXIII molecules had one or no visible strands, suggesting dissociation of the B subunits from the globular core. After activation of FXIII with thrombin and Ca2+ , B subunits dissociated and formed B2 homodimers, whereas the activated globular A subunits dissociated into monomers. These results characterize the molecular organization of FXIII and changes with activation.