Transglutaminase 2 contributes to the blood clotting cascade

Technical Summary: Transglutaminase 2 Contributes to the Blood Clotting Cascade

Problem Statement
The stabilization of blood clots via the cross-linking of fibrin and associated proteins through Nε-(γ-glutamyl)lysine bonds is a critical component of the hemostatic mechanism, providing clot rigidity and resistance to fibrinolysis. Historically, this function has been ascribed exclusively to Factor XIIIa (plasma transglutaminase), which is activated by thrombin. While genetic defects in Factor XIIIa result in bleeding disorders, the phenotype is often milder than defects in other coagulation factors (e.g., Factor VIII or IX). This discrepancy has led to the hypothesis that another transglutaminase (TG) might partially compensate for Factor XIIIa deficiency. Tissue transglutaminase (TG2) is a ubiquitous enzyme, highly abundant in erythrocytes and endothelial cells, but its role in the blood clotting cascade has remained uncharacterized, particularly regarding its activation and contribution to clot stability in whole blood.

Methodology
The study employed a multi-faceted approach combining genetic models, biochemical assays, and clinical sample analysis:

1. Genetic Mouse Models: Thrombi were generated using whole blood from wild-type (WT) mice and TG2-deficient (TG2-/-) mice.

   • Clot Lysis Assays: Thrombi were exposed to tissue plasminogen activator (tPA) to measure the rate of lysis (hemolysis).
   • Cross-linking Analysis: Clots were digested and analyzed via HPLC to quantify Nε-(γ-glutamyl)lysine cross-links.
   • Inhibitor Studies: TG-specific inhibitors (R283) were added to WT clot mixtures to assess the impact of TG activity on stability.

2. Human Model Thrombi: Thrombi were prepared from blood of six healthy human donors.

   • Specific Inhibition: Clots were incubated with GTPγS (a non-hydrolysable GTP analogue that inhibits Ca2+-activated TG2 but not Factor XIIIa) and the specific TG active-site inhibitor R283.
   • Lysis Monitoring: The rate of lysis was monitored to determine the contribution of TG2 to stability compared to controls.

3. Clinical Cohort Analysis: Ten thrombi were collected from patients with acute ischemic stroke undergoing mechanical thrombectomy.

   • Immunofluorescence and Immunohistochemistry: Samples were stained with a specific anti-TG2 antibody (clone CUB 7402) to localize TG2 within the extracellular fibrin-rich regions and cellular components (specifically monocyte/macrophage lineages).
   • Scoring: A semi-quantitative pathological scoring system (0–3) was used to assess TG2 positivity in both extracellular and cellular compartments.

Key Results

• TG2 Deficiency Reduces Clot Stability: Thrombi from TG2-/- mice exhibited a 2–3 fold increase in the rate of hemolysis compared to WT mice. This instability correlated with a ~50% reduction in Nε-(γ-glutamyl)lysine cross-link levels in TG2-/- clots.
• Inhibition Confirms TG2 Role in Humans: In human model thrombi, the inclusion of GTPγS (inhibiting TG2) accelerated lysis by 1.3-fold compared to controls. The addition of the broad TG inhibitor R283 accelerated lysis by 1.7-fold, indicating that TG2 contributes significantly to human clot stability.
• Cellular Source: PCR analysis indicated that leukocytes do not express TG2 mRNA and platelets contain minimal TG2. The study suggests erythrocytes are the primary source of TG2 in the clot.
• Clinical Presence: TG2 was detected in all 10 human ischemic stroke thrombi analyzed. Immunofluorescence and immunohistochemistry confirmed TG2 localization in extracellular fibrin-rich regions and within blood cells (predominantly monocyte/macrophage lineage). While expression was heterogeneous among patients, 5 of 10 thrombi showed TG2 positivity in >20% of the fibrin-rich area.
• Absence in Cell-Free Plasma: Consistent with previous beliefs, no role for TG2 was observed in plasma clots formed in the absence of blood cells ("white clots"), where Factor XIIIa remains the dominant stabilizing factor.

Key Contributions

• Identification of a Secondary Stabilizer: The paper provides definitive evidence that TG2, distinct from Factor XIIIa, participates in the cross-linking and stabilization of the fibrin network during the initial formation of the hemostatic plug.
• Mechanism of Activation: The study highlights a distinct activation mechanism: while Factor XIIIa is proteolytically activated by thrombin, TG2 is activated by Ca2+ released following red blood cell lysis within the clot.
• Structural Specificity: The authors note that TG2 acts rapidly to form α-polymers and intermolecular cross-links between α and γ chains (bridging Aα and γ chains), whereas Factor XIIIa primarily forms γ-γ dimers initially. This difference suggests TG2 plays a crucial role in the initial rigidity of the clot.
• Clinical Correlation: The detection of TG2 in human stroke thrombi establishes a direct link between the enzyme and pathological clot formation in humans, moving beyond theoretical models.

Significance and Claims
The authors propose that TG2 should be recognized as an additional factor in the blood coagulation cascade, potentially acting as a "Factor XIV" that supports Factor XIIIa. The study suggests that the presence of TG2 in the blood cascade may explain the relatively mild phenotype observed in patients with Factor XIIIa deficiency, as TG2 can partially compensate for the lack of Factor XIIIa.

The paper posits that the evolutionary conservation of multiple transglutaminases (one proteolytically activated, the other Ca2+ activated) provides a robust, redundant protective mechanism for hemostasis and wound healing. Furthermore, the involvement of TG2 in clot formation suggests a causal role in cardiovascular events, where the release of TG2 from lysed erythrocytes may expand red clots into overt thrombi. The authors conclude that TG2 acts as a stress-responsive regulator at the interface of coagulation, inflammation, and endothelial survival, reinforcing the importance of this enzyme in maintaining organismal homeostasis.