Transglutaminase 3 crosslinks the secreted gel-forming mucus component Mucin-2 and stabilizes the colonic mucus layer

The colonic mucus layer is organized as a two-layered system providing a physical barrier against pathogens and simultaneously harboring the commensal flora. The factors contributing to the organization of this gel network are not well understood. In this study, the impact of transglutaminase activity on this architecture was analyzed. Here, we show that transglutaminase TGM3 is the major transglutaminase-isoform expressed and synthesized in the colon. Furthermore, intrinsic extracellular transglutaminase activity in the secreted mucus was demonstrated in vitro and ex vivo. Absence of this acyl-transferase activity resulted in faster degradation of the major mucus component the MUC2 mucin and changed the biochemical properties of mucus. Finally, TGM3-deficient mice showed an early increased susceptibility to Dextran Sodium Sulfate-induced colitis. Here, we report that natural isopeptide cross-linking by TGM3 is important for mucus homeostasis and protection of the colon from inflammation, reducing the risk of colitis.

Identification of Chicken Transglutaminase 1 and In Situ Localization of Transglutaminase Activity in Avian Skin and Esophagus

Transglutaminase 1 (TGM1) is a membrane-anchored enzyme that cross-links proteins during terminal differentiation of epidermal and esophageal keratinocytes in mammals. The current genome assembly of the chicken, which is a major model for avian skin biology, does not include an annotated region corresponding to TGM1. To close this gap of knowledge about the genetic control of avian cornification, we analyzed RNA-sequencing reads from organotypic chicken skin and identified TGM1 mRNA. By RT-PCR, we demonstrated that TGM1 is expressed in the skin and esophagus of chickens. The cysteine-rich sequence motif required for palmitoylation and membrane anchorage is conserved in the chicken TGM1 protein, and differentiated chicken keratinocytes display membrane-associated transglutaminase activity. Expression of TGM1 and prominent transglutaminase activity in the esophageal epithelium was also demonstrated in the zebra finch. Altogether, the results of this study indicate that TGM1 is conserved among birds and suggest that chicken keratinocytes may be a useful model for the study of TGM1 in non-mammalian cornification.

A Novel Assay Using Enzyme Capture - ELISA for Accurate Determination of Factor XIII Activity

Background: The currently available, clinically approved factor XIII (fXIII) activity assays rely on indirect measurement of activated fXIII transglutaminase activity by detecting "ammonia-release" during an intermediate step of the reaction. Unfortunately, this assay is also sensitive to fXIII-independent ammonia-producing reactions that may take place physiologically in plasma, rendering it prone to overestimate fXIII activity, especially when levels are <10% (0.1 IU/mL). High quality quantitative fXIII activity assays are required for the accurate and timely diagnosis of fXIII deficiency. fXIII circulates in complex with fibrinogen, and activated fXIII retards fibrinolysis by incorporating α2-antiplasmin (α2AP), a potent antifibrinolytic, into the forming clot via intermolecular α2AP:fibrin crosslinks. We have thus developed a new fXIII transglutaminase activity assay that directly measures α2-AP incorporation onto captured fibrin(ogen):fXIII complexes. We hypothesized that this novel assay has high sensitivity in detecting FXIII activity, especially at the low end of the dynamic range (<10%; <0.01 IU/mL). Methods: A standard curve was established by diluting known concentrations of recombinant factor XIII-A (rFXIII-A; Tretten) into (1) fXIII-A congenitally deficient and (2) fXIII immunodepleted plasma to determine if our assay is able to detect variable fXIII concentrations from 1 to 200% (0.01 - 2 IU/mL). Results: We demonstrated that α2-AP incorporation is dependent upon rfXIII-A concentration in the starting sample and was linear from 1-12% (0.01 - 0.12 IU/mL; Figure). Further expansion of the standard curve to include all concentrations within the normal physiologic range demonstrated that the assay remains strongly linear from 1-20% (0.01 - 0.2 IU/mL; R2 0.966). Moderate linearity (R2 0.908) extended to 50% (0.5 IU/mL). The curve plateaued between 50%-200% (0.5 IU/mL- 2 IU/mL). Thus, the dynamic range of the Enzyme Capture-ELISA is expected within 1-50% (0.01 - 0.5 IU/mL). Conclusion: We conclude that Enzyme Capture-ELISA is a promising novel method for accurate detection of FXIII activity and is expected to improve sensitivity at low levels (<20%; <0.02 IU/mL), which are most clinically relevant. Further experiments are needed to refine the upper limit of assay linearity and thereby refine the dynamic range of the assay. With further optimization and validation, the EC-ELISA method may provide an improved diagnostic assay. Disclosures No relevant conflicts of interest to declare.

Mechanism Underlying a Role for Factor XIII (FXIII) Polymorphism in Sickle Cell Disease-Associated Priapism

Background: The pathophysiology of priapism in sickle cell disease (SCD) is poorly understood. While blood stasis is essential to tumescence, most research in SCD-associated priapism to date has focused on the potential role of abnormal signaling pathways. We have previously observed that a coding sequence single nucleotide polymorphism (rs5988) of the transglutaminase factor XIII gene (FXIII) is strongly associated with SCD priapism, with an odds ratio of 2.52 [C.I. 1.27 -5.03] for the risk genotype (G/G, expressing only FXIII E652) vs the most common non-risk genotype (G/C, expressing both FXIII E652 and FXIII Q652). We therefore explored the effect of the rs5988 polymorphism on various aspects of FXIII function. Methods: Recombinant FXIII (rFXIII) E652 and rFXIII Q652 were expressed by 293 kidney cells and isolated from serum-free tissue culture supernatant. Before use in assays, activation of rFXIII by thrombin was confirmed by generating activated rFXIII (rFXIIIa) with thrombin (10 U/ml), followed SDS-PAGE, western blotting for FXIII, and densitometry for quantitation. Whole blood samples and plasmas were obtained from previously genotyped subjects with SCD under an IRB-approved protocol. Plasma FXIII and rFXIII transglutaminase activity was measured by the ability to catalyze 5-(biotinamido)pentylamine incorporation into a suitable substrate. Plasma FXIII antigen was assayed by ELISA. Clot contraction was measured after tissue factor-initiated clotting of recalcified whole blood. Clot resistance to lysis was measured after exposure to tissue plasminogen activator (tPA). Results: Transglutaminase activity of each rFXIII was measured using fibrinogen and fibronectin as substrates. At 20 minutes, rFXIIIa E652 showed 1.44-fold more transglutaminase (crosslinking) activity toward fibrin(ogen) than rFXIIIa Q652 (p=0.027), and a nonsignificant trend toward more activity (1.32-fold, p=0.079) toward fibronectin. Kinetic assays also showed that rFXIII E652 had significantly greater activity toward both matrices (p=0.006 and 0.012, respectively), suggesting the risk genotype (homozygosity for the G allele) enhances fibrin(ogen) and/or fibronectin crosslinking. FXIII activity in the plasma of 18 genotyped SCD patients (3 CC, 7 GC, 8 GG) demonstrated a consistent, but not significant, trend toward increased FXIII activity with increasing presence of G alleles (80.01% CC, 98.90% CG, and 107.20% GG). Although results were not adjusted for genotype at other loci reported to affect FXIII expression, there was no significant difference in FXIII antigen among genotypes. Compared to contracted whole blood clots from patients with only one or no risk alleles, clots from SCD patients with two copies of the risk allele (GG, expressing only FXIII E652) did not differ in either RBC retention within clots or clot mass (weight). Moreover, inhibition of transglutaminase activity with T101 significantly increased RBC release and decreased clot weight to a similar degree in contracted clots from patients with either the GC or GG genotype. In clots formed from FXIII-deficient plasma supplemented with rFXIIIa E652 or rFXIIIa Q652 and ABO-compatible donor RBCs, clots containing rFXIIIa E652 were 14% more resistant to lysis than clots containing rFXIIIa Q652 (p=0.016). Parallel studies with SCD patient plasma samples also showed that clots containing only FXIII E652 were more resistant to lysis than clots containing both FXIII E652 and FXIII Q652(p=0.0001). Conclusions: These data suggest the FXIII rs5988 polymorphism does not alter protein expression or clot contraction but may regulate clot stability via slightly increased transglutaminase activity and enhanced resistance to lysis. These effects may predispose patients to formation of microclots during tumescence, thus impairing blood egress and increasing risk of priapism. Further studies should be conducted to determine if anticoagulation or fibrinolytic treatments are viable preventative or treatment strategies for SCD patients with the risk FXIIIGG genotype and recurrent priapism. Disclosures Telen: Pfizer, Inc.: Consultancy, Research Funding. Wolberg:GlaxoSmithKline: Employment; Novo Nordisk: Research Funding.

A Novel Enzyme Capture - ELISA for the Direct Determination of Factor XIII Activity

Background: Activated factor XIII (fXIII) mechanically stabilizes fibrin clots by catalyzing the formation of intramolecular crosslinks and retards plasmin-mediated fibrinolysis by incorporating intermolecular α2-antiplasmin (α2AP):fibrin crosslinks. High quality quantitative fXIII activity assays are required for the accurate and timely diagnosis of fXIII deficiency. Currently available high-throughput assays indirectly measure the transglutaminase activity of fXIII via detection of ammonia that is elaborated upon formation of ε-N-(γ-glutamyl)-lysyl crosslinking bonds. Unfortunately, these assays can be confounded by fXIII-independent ammonia producing and consuming reactions present in plasma. We thus set out to develop an assay which directly quantifies the transglutaminase activity. We hypothesized that fXIII could be captured in a microtiter plate with anti-fibrinogen antibodies and, following an activation step, its activity determined in a kinetic fashion via the rate of α2AP incorporation (measured in a subsequent ELISA step). Methods: Co-immunoprecipitation and western blotting techniques were utilized to confirm fXIII/fibrinogen interactions and to determine the presence of basal interactions between fibrinogen and α2AP. Simultaneously, this approach enabled the determination of suitable antibody pairs for use in the enzyme capture-ELISA (EC-ELISA). In microtiter plates, fibrinogen-captured fXIII was activated with excess thrombin, physiologic calcium, and GPRP peptide (to block fibrin polymerization) in the presence of excess α2AP. FXIII activity was stopped with a potent fXIII-inhibitor (iodoacetamide) at predetermined time points. Subsequently, α2AP incorporation was determined with a primary anti-α2AP antibody and secondary HRP-conjugated antibody. Results: Co-immunoprecipitation and western blotting experiments confirmed that both fXIII and α2AP circulate in complex with fibrinogen, as previously reported by others. However, α2AP incorporation significantly increased following incubation with activated factor XIII. Using the EC-ELISA method, we were able to demonstrate that α2AP is incorporated in a manner consistent with first order kinetics. Discussion: These experiments demonstrate the feasibility of an EC-ELISA assay to directly measure fXIII activity. Further work will be required to determine the sensitivity of this assay in test plasmas with varying known quantities of fXIII and how its performance characteristics compare to currently available indirect activity assays. With further optimization, the EC-ELISA method may lead to an improved, high throughput diagnostic assay for clinical use. Disclosures No relevant conflicts of interest to declare.

Cystamine and cysteamine as inhibitors of transglutaminase activity in vivo

Cystamine is commonly used as a transglutaminase inhibitor. This disulphide undergoes reduction in vivo to the aminothiol compound, cysteamine. Thus, the mechanism by which cystamine inhibits transglutaminase activity in vivo could be due to either cystamine or cysteamine, which depends on the local redox environment. Cystamine inactivates transglutaminases by promoting the oxidation of two vicinal cysteine residues on the enzyme to an allosteric disulphide, whereas cysteamine acts as a competitive inhibitor for transamidation reactions catalyzed by this enzyme. The latter mechanism is likely to result in the formation of a unique biomarker, N-(γ-glutamyl)cysteamine that could serve to indicate how cyst(e)amine acts to inhibit transglutaminases inside cells and the body.