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

Identification of Factor XIII β-Sandwich Residues Mediating Glutamine Substrate Binding and Activation Peptide Cleavage

BACKGROUND

 Factor XIII (FXIII) forms covalent crosslinks across plasma and cellular substrates and has roles in hemostasis, wound healing, and bone metabolism. FXIII activity is implicated in venous thromboembolism (VTE) and is a target for developing pharmaceuticals, which requires understanding FXIII - substrate interactions. Previous studies proposed the β-sandwich domain of the FXIII A subunit (FXIII-A) exhibits substrate recognition sites.

MATERIAL AND METHODS

 Recombinant FXIII-A proteins (WT, K156E, F157L, R158Q/E, R171Q, and R174E) were generated to identify FXIII-A residues mediating substrate recognition. Proteolytic (FXIII-A*) and non-proteolytic (FXIII-A°) forms were analyzed for activation and crosslinking activities toward physiological substrates using SDS-PAGE and MALDI-TOF MS.

RESULTS

 All FXIII-A* variants displayed reduced crosslinking abilities compared to WT for Fbg αC (233 - 425), fibrin, and actin. FXIII-A* WT activity was greater than A°, suggesting the binding site is more exposed in FXIII-A*. With Fbg αC (233 - 425), FXIII-A* variants R158Q/E, R171Q, and R174E exhibited decreased activities approaching those of FXIII-A°. However, with a peptide substrate, FXIII-A* WT and variants showed similar crosslinking suggesting the recognition site is distant from the catalytic site. Surprisingly, FXIII-A R158E and R171Q displayed slower thrombin activation than WT, potentially due to loss of crucial H-bonding with neighboring activation peptide (AP) residues.

CONCLUSION

 In conclusion, FXIII-A residues K156, F157, R158, R171, and R174 are part of a binding site for physiological substrates [fibrin (α and γ) and actin]. Moreover, R158 and R171 control AP cleavage during thrombin activation. These investigations provide new molecular details on FXIII - substrate interactions that control crosslinking abilities.

Reciprocal stabilization of coagulation factor XIII-A and -B subunits is a determinant of plasma FXIII concentration

ABSTRACT

Transglutaminase factor XIII (FXIII) is essential for hemostasis, wound healing, and pregnancy maintenance. Plasma FXIII is composed of A and B subunit dimers synthesized in cells of hematopoietic origin and hepatocytes, respectively. The subunits associate tightly in circulation as FXIII-A2B2. FXIII-B2 stabilizes the (pro)active site-containing FXIII-A subunits. Interestingly, people with genetic FXIII-A deficiency have decreased FXIII-B2, and therapeutic infusion of recombinant FXIII-A2 (rFXIII-A2) increases FXIII-B2, suggesting FXIII-A regulates FXIII-B secretion, production, and/or clearance. We analyzed humans and mice with genetic FXIII-A deficiency and developed a mouse model of rFXIII-A2 infusion to define mechanisms mediating plasma FXIII-B levels. Like humans with FXIII-A deficiency, mice with genetic FXIII-A deficiency had reduced circulating FXIII-B2, and infusion of FXIII-A2 increased FXIII-B2. FXIII-A-deficient mice had normal hepatic function and did not store FXIII-B in liver, indicating FXIII-A does not mediate FXIII-B secretion. Transcriptional analysis and polysome profiling indicated similar F13b levels and ribosome occupancy in FXIII-A-sufficient and -deficient mice and in FXIII-A-deficient mice infused with rFXIII-A2, indicating FXIII-A does not induce de novo FXIII-B synthesis. Unexpectedly, pharmacokinetic/pharmacodynamic modeling of FXIII-B antigen after rFXIII-A2 infusion in humans and mice suggested FXIII-A2 slows FXIII-B2 loss from plasma. Accordingly, comparison of free FXIII-B2 vs FXIII-A2-complexed FXIII-B2 (FXIII-A2B2) infused into mice revealed faster clearance of free FXIII-B2. These data show FXIII-A2 prevents FXIII-B2 loss from circulation and establish the mechanism underlying FXIII-B2 behavior in FXIII-A deficiency and during rFXIII-A2 therapy. Our findings reveal a unique, reciprocal relationship between independently synthesized subunits that mediate an essential hemostatic protein in circulation. This trial was registered at www.ClinicalTrials.com as #NCT00978380.

Thrombomodulin activation driven by LXR agonist attenuates renal injury in diabetic nephropathy

Objective

Inflammation and thrombosis are recognized as interrelated biological processes. Both thrombomodulin (TM) and factor XIII-A (FXIII-A) are involved in inflammation and coagulation process. However, their role in the pathogenesis of diabetic nephropathy (DN) remains unclear. In vitro study, the liver X receptor (LXR) agonist T0901317 can up-regulate the expression of TM in glomerular endothelial cells. Now we evaluated the interaction between TM activation and FXIII-A and their effects against renal injury.

Methods

We first evaluated the serum levels of FXIII-A and TM and the expression of TM, LXR-α and FXIII-A in renal tissues of patients with biopsy-proven DN. We then analyzed the expression of TM, LXR-α and FXIII-A in renal tissues of db/db DN mice after upregulating TM expression via T0901317 or downregulating its expression via transfection of TM shRNA-loaded adenovirus. We also investigated the serum levels of Tumor necrosis factor (TNF)-α, Interleukin (IL)-6, creatinine, and urinary microalbumin level in db/db mice.

Results

Our study showed that elevations in serum levels of FXIII-A positively correlated to the serum levels of TM and were also associated with end-stage kidney disease in patients with DN. The number of TM⁺ cells in the renal tissues of patients with DN negatively correlated with the number of FXIII-A⁺ cells and positively correlated with the number of LXR-α⁺ cells and estimated glomerular filtration rate (eGFR), whereas the number of FXIII-A⁺ cells negatively correlated with the eGFR.

Conclusion

Thrombomodulin activation with T0901317 downregulated FXIII-A expression in the kidney tissue and alleviated renal injury in db/db mice.

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.

Illustrated State-of-the-Art Capsules of the ISTH 2020 Congress

This year's Congress of the International Society of Thrombosis and Haemostasis (ISTH) was hosted virtually from Philadelphia July 17-21, 2021. The conference, now held annually, highlighted cutting-edge advances in basic, population and clinical sciences of relevance to the Society. Despite being held virtually, the 2021 congress was of the same scope and quality as an annual meeting held in person. An added feature of the program is that talks streamed at the designated times will then be available on-line for asynchronous viewing. The program included 77 State of the Art (SOA) talks, thematically grouped in 28 sessions, given by internationally recognized leaders in the field. The SOA speakers were invited to prepare brief illustrated reviews of their talks that were peer reviewed and are included in this article. The topics, across the main scientific themes of the congress, include Arterial Thromboembolism, Coagulation and Natural Anticoagulants, COVID-19 and Coagulation, Diagnostics and Omics, Fibrinogen, Fibrinolysis and Proteolysis, Hemophilia and Rare Bleeding Disorders, Hemostasis in Cancer, Inflammation and Immunity, Pediatrics, Platelet Disorders, von Willebrand Disease and Thrombotic Angiopathies, Platelets and Megakaryocytes, Vascular Biology, Venous Thromboembolism and Women's Health. These illustrated capsules highlight the major scientific advances with potential to impact clinical practice. Readers are invited to take advantage of the excellent educational resource provided by these illustrated capsules. They are also encouraged to use the image in social media to draw attention to the high quality and impact of the science presented at the congress.