Molecular bases of gluten toxicity and Relationship with the enzyme Transglutaminase in celiac disease

Abstract

Celiac disease (CD) is a life-long gluten-sensitive immune-mediated disorder that primarily affects the small intestine of genetically susceptible individuals worldwide. Type 2 transglutaminase (TG2) has two crucial roles in CD pathogenesis: as a deamidating enzyme, of crucial importance in enhancing gluten immunogenicity, and as a target autoantigen in the immune response. The presence of constitutive alterations in CD cells compared to non-CD ones has recently led to the definition of the so-called “celiac cellular phenotype”, that may represent a predisposing condition to the damaging effects of gluten. TG2, and particularly anti-TG2 autoantibodies, also contributes to this phenotype. Indeed, anti-TG2 autoantibodies, by forming complexes with cell-surface TG2, specifically derange the uptake of the toxic α-gliadin peptide 31-43 by non-CD cells but not by CD ones. In this PhD thesis work, differences in TG2 subcellular distribution in CD and non-CD cells were investigated to determine how TG2 may be able to contribute to the different handling of p31- 43 by the two groups of cells. Then, in the attempt to identify other constitutive differences regarding TG2 in CD and non-CD cells, it was investigated whether p31-43 differentially modulated TG2 expression and activity in fibroblasts from the two groups of subjects. The data obtained showed that TG2 was associated with the cell surface membrane, the early endosomal compartment and the autophagic compartment more in CD cells than in non-CD ones. The data also showed the p31-43 differently affected TG2 expression and activity in CD and non-CD cells, activating TG2 more in non-CD cells than in CD ones and inducing TG2 expression in CD cells but not in non-CD ones. These findings support the idea that TG2 localization inside CD cells contributes to defining the “celiac cellular phenotype”, thus having an important but still undefined role in CD pathogenesis. Subsequently, it was investigated whether regulation of intracellular Ca2+ homeostasis regarding the endoplasmic reticulum (ER) was different in CD and non-CD cells. Relatedly, it was analyzed how CD and non-CD cells responded to stimulation with thapsigargin (THP), an ER-stress and autophagy inducer, also focusing the attention on TG2 modulation. The data obtained showed that, in CD cells, intracellular Ca2+ homeostasis was deregulated, thus causing both a strong Unfolding Protein Response (UPR) and an engulfed autophagy in response to stimulation with THP. Interestingly, p31-43 mobilized Ca2+ from intracellular stores more in CD cells than in non-CD ones; however, the p31-43-induced increase in Ca2+ only partially activated TG2. Overall, this PhD thesis work adds a small piece of knowledge about the complex interplay between gluten peptides and TG2 in the pathogenetic mechanisms of CD. [edited by Author]