Evaluating the Effects of Fibrinogen αC Mutations on the Ability of Factor XIII to Crosslink the Reactive αC Glutamines (Q237, Q328, Q366)

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.

Functional and Structural Characterization of Nucleic Acid Ligands That Bind to Activated Coagulation Factor XIII

Coagulation factor XIII (FXIII) is a protransglutaminase which plays an important role in clot stabilization and composition by cross-linking the α- and γ-chains of fibrin and increasing the resistance of the clot to mechanical and proteolytic challenges. In this study, we selected six DNA aptamers specific for activated FXIII (FXIIIa) and investigated the functional characterization of FXIIIa after aptamer binding. One of these aptamers, named FA12, efficiently captures FXIIIa even in the presence of zymogenic FXIII subunits. Furthermore, this aptamer inhibits the incorporation of FXIII and α2-antiplasmin (α2AP) into fibrin(ogen) with IC₅₀-values of 38 nM and 17 nM, respectively. In addition to FA12, also another aptamer, FA2, demonstrated significant effects in plasma-based thromboelastometry (rotational thromboelastometry analysis, ROTEM)-analysis where spiking of the aptamers into plasma decreased clot stiffness and elasticity (p < 0.0001). The structure–function correlations determined by combining modeling/docking strategies with quantitative in vitro assays revealed spatial overlap of the FA12 binding site with the binding sites of two FXIII substrates, fibrinogen and α2AP, while FA2 binding sites only overlap those of fibrinogen. Taken together, these features especially render the aptamer FA12 as an interesting candidate molecule for the development of FXIIIa-targeting therapeutic strategies and diagnostic assays.