Calcium activation of tissue transglutaminase in radioligand binding and enzyme-linked autoantibody immunoassays in childhood celiac disease

Zinc as a modulator of transglutaminase activity - Laboratory and pathophysiological aspects

For a whole century, citrate has been used as an in vitro anticoagulant via chelation of calcium. Later, also EDTA was introduced as an anticoagulant. An often overlooked fact is that zinc is bound to citrate and EDTA with affinities much greater than that for calcium, imposing problems in biomedical research. In vivo, proteins of the S100 family are released from leukocytes and known to bind calcium. Some of them, e.g., calprotectin, also chelate zinc. Thus, at an inflamed site, the ratio between Ca²⁺ and Zn²⁺ is changed. This mechanism is of importance for the modulation of the activation of a fascinating family of post-translationally acting calcium-dependent thiol enzymes, the transglutaminases, which are inhibited by zinc. This presentation illustrates the complexity of in vitro studies with zinc. Moreover, it exemplifies the role of Zn²⁺ in pathophysiological situations such as celiac disease and neurodegeneration.

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Citrate, EDTA and DTT bind zinc as well as calcium.

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At inflammation, calprotectin binds Zn²⁺, which leads to low concentrations of the ion.

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Zn²⁺ inhibits the activation of transglutaminases and peptidylarginine deiminases.

Open conformation tissue transglutaminase testing for celiac dietary assessment

OBJECTIVES

Anti tissue-transglutaminase antibody is the mainstay of celiac disease serologic testing. Whilst it has high sensitivity in patients on an unrestricted diet, sensitivity is poor for evaluation of gluten free diet adherence.

AIM

To assess the utility of a novel assay measuring Immunoglobulin-A antibodies to catalytically active open conformation tissue-transglutaminase in assessment of ongoing gluten exposure in celiac disease patients on an alleged gluten free diet.

METHODS

Through prospective assessment, 147 patients with celiac disease were divided into good and poor adherence. Open and closed (conventional) tissue-transglutaminase titres were measured using standard enzyme linked immunosorbent assay. 50 patients with inflammatory bowel disease served as disease controls.

RESULTS

Overall 128 patients had been on gluten free diet for more than six months and 19 were found to be poorly adherent on dietary review. Within this group 13 (68.4%) and 10 (52.6%) patients respectively were positive for the open conformation and conventional assay (p=0.51). Two and one control patients tested positive for closed and open assays respectively.

CONCLUSIONS

Compared to native assays open conformation tissue-transglutaminase may have higher sensitivity in the poor gluten free diet adherence group and higher specificity in the control population. Larger population studies are warranted to assess whether the open conformation tissue-transglutaminase assay may be superior to the conventional assay.

Peptidylarginine deiminases and the pathogenesis of rheumatoid arthritis: a reflection of the involvement of transglutaminase in coeliac disease

Post-translational modifications are associated with certain autoimmune diseases. For example, in the initial steps of coeliac disease (CD), transglutaminase type 2 (TG2) catalyzes a post-translational deamidation of specific glutamine residues in dietary gluten, resulting in antibodies against both modified gliadin and against TG2. Anti-TG2 has become a specific biomarker for CD. In rheumatoid arthritis (RA), the presence of antibodies against citrullinated peptides (ACPA) characterizes a distinct subset of this inflammatory disorder. Moreover, antibodies against the enzyme that catalyzes the citrullination (peptidylarginine deiminase; PAD) are found in RA. Their relation to disease severity indicates a possible pathogenetic role. Thus, in two major autoimmune diseases (CD and RA), antibodies are present against a post-translationally modified substrate and against the calcium-dependent thiol-enzyme (TG2 and PAD, respectively) responsible for the modification. This review highlights the similarities between the TGs and the PADs and their putative pathogenetic roles in autoimmune diseases. Possible mechanisms of the effects of cigarette smoking and alcohol consumption on RA are discussed. By reflecting the progress in CD, the pathogenesis of ACPA-positive RA can be hypothesized where expression and regulation of PADs play significant roles. Indeed, autoimmune diseases should be studied collectively as well as individually. The new insight may lead towards innovative pharmacotherapeutic principles.

Transglutaminase and the pathogenesis of coeliac disease

In 1997, a German group demonstrated that the antigen of the biomarker EMA (endomysial antibodies) in coeliac disease is a calcium-dependent thiol enzyme, transglutaminase type 2 (TG2). This most important discovery opened up an exciting field of research aimed at a better understanding of the pathogenesis of coeliac disease, a T-cell-driven autoimmune disorder with a prevalence of about 1%. The accidental activation of TG2, possibly caused by a stress-induced local deficiency of zinc in the intestinal wall, might play a key role where the enzyme catalyzes an atypical deamidation of specific glutamine residues of food gliadins. The genetic contribution is HLA DQ2 or DQ8, which can form a complex with the TG2-modified gliadin residues, resulting in an immune response with the formation of antibodies against both gliadin and the enzyme. Indeed, the immunopathogenesis of coeliac disease can now be recognized partly at the molecular level. Progress has already improved the opportunities for laboratory diagnostics and, hopefully, new ways of treating and preventing coeliac disease will become available. These exciting developments might stimulate research within other fields of autoimmune disorders. With its focus on TG2, this review highlights some of the intriguing mechanisms of the pathogenesis of coeliac disease, such as the structure of the neo-antigen, the involvement of calcium and zinc, and the effects of coeliac antibodies on TG2 activity. Moreover, the many pitfalls due to dubious laboratory practice are addressed, as is the potential when a fundamental biological mechanism is understood at the molecular level.