Interplay between Type 2 Transglutaminase (TG2), Gliadin Peptide 31-43 and Anti-TG2 Antibodies in Celiac Disease

Allosteric Activation of Transglutaminase 2 via Inducing an "Open" Conformation for Osteoblast Differentiation

Osteoblasts play an important role in the regulation of bone homeostasis throughout life. Thus, the damage of osteoblasts can lead to serious skeletal diseases, highlighting the urgent need for novel pharmacological targets. This study introduces chemical genetics strategy by using small molecule forskolin (FSK) as a probe to explore the druggable targets for osteoporosis. Here, this work reveals that transglutaminase 2 (TGM2) served as a major cellular target of FSK to obviously induce osteoblast differentiation. Then, this work identifies a previously undisclosed allosteric site in the catalytic core of TGM2. In particular, FSK formed multiple hydrogen bonds in a saddle-like domain to induce an "open" conformation of the β-sandwich domain in TGM2, thereby promoting the substrate protein crosslinks by incorporating polyamine. Furthermore, this work finds that TGM2 interacted with several mitochondrial homeostasis-associated proteins to improve mitochondrial dynamics and ATP production for osteoblast differentiation. Finally, this work observes that FSK effectively ameliorated osteoporosis in the ovariectomy mice model. Taken together, these findings show a previously undescribed pharmacological allosteric site on TGM2 for osteoporosis treatment, and also provide an available chemical tool for interrogating TGM2 biology and developing bone anabolic agent.

Inhibition of Transglutaminase 2 as a Therapeutic Strategy in Celiac Disease-In Vitro Studies in Intestinal Cells and Duodenal Biopsies

Enzymatic modification of gliadin peptides by human transglutaminase 2 (TG2) is a key mechanism in the pathogenesis of celiac disease (CD) and represents a potential therapeutic target. Recently, we have identified the small oxidative molecule PX-12 as an effective inhibitor of TG2 in vitro. In this study, we further investigated the effect of PX-12 and the established active-site directed inhibitor ERW1041 on TG2 activity and epithelial transport of gliadin peptides. We analyzed TG2 activity using immobilized TG2, Caco-2 cell lysates, confluent Caco-2 cell monolayers and duodenal biopsies from CD patients. TG2-mediated cross-linking of pepsin-/trypsin-digested gliadin (PTG) and 5BP (5-biotinamidopentylamine) was quantified by colorimetry, fluorometry and confocal microscopy. Cell viability was tested with a resazurin-based fluorometric assay. Epithelial transport of promofluor-conjugated gliadin peptides P31-43 and P56-88 was analyzed by fluorometry and confocal microscopy. PX-12 reduced TG2-mediated cross-linking of PTG and was significantly more effective than ERW1041 (10 µM, 15 ± 3 vs. 48 ± 8%, p < 0.001). In addition, PX-12 inhibited TG2 in cell lysates obtained from Caco-2 cells more than ERW1041 (10 µM; 12 ± 7% vs. 45 ± 19%, p < 0.05). Both substances inhibited TG2 comparably in the intestinal lamina propria of duodenal biopsies (100 µM, 25 ± 13% vs. 22 ± 11%). However, PX-12 did not inhibit TG2 in confluent Caco-2 cells, whereas ERW1041 showed a dose-dependent effect. Similarly, epithelial transport of P56-88 was inhibited by ERW1041, but not by PX-12. Cell viability was not negatively affected by either substance at concentrations up to 100 µM. PX-12 did not reduce TG2 activity or gliadin peptide transport in confluent Caco-2 cells. This could be caused by rapid inactivation or degradation of the substance in the Caco-2 cell culture. Still, our in vitro data underline the potential of the oxidative inhibition of TG2. The fact that the TG2-specific inhibitor ERW1041 reduced the epithelial uptake of P56-88 in Caco-2 cells further strengthens the therapeutic potential of TG2 inhibitors in CD.

Type 2 Transglutaminase in Coeliac Disease: A Key Player in Pathogenesis, Diagnosis and Therapy

Type 2 transglutaminase (TG2) is the main autoantigen in coeliac disease (CD), a widespread inflammatory enteropathy caused by the ingestion of gluten-containing cereals in genetically predisposed individuals. As a consequence, serum antibodies to TG2 represent a very useful marker in CD diagnosis. However, TG2 is also an important player in CD pathogenesis, for its ability to deamidate some Gln residues of gluten peptides, which become more immunogenic in CD intestinal mucosa. Given the importance of TG2 enzymatic activities in CD, several studies have sought to discover specific and potent inhibitors that could be employed in new therapeutical approaches for CD, as alternatives to a lifelong gluten-free diet. In this review, we summarise all the aspects regarding TG2 involvement in CD, including its enzymatic reactions in pathogenesis, the role of anti-TG2 antibodies in disease management, and the exploration of recent strategies to reduce deamidation or to use transamidation to detoxify gluten.

Wheat Amylase Trypsin Inhibitors Aggravate Intestinal Inflammation Associated with Celiac Disease Mediated by Gliadin in BALB/c Mice

Celiac disease (CD) is an autoimmune intestinal disorder caused by the ingestion of gluten in people who carry the susceptible gene. In current celiac disease research, wheat gluten is often the main target of attention, neglecting the role played by non-gluten proteins. This study aimed to describe the effects of wheat amylase trypsin inhibitors (ATI, non-gluten proteins) and gliadin in BALB/c mice while exploring the further role of relevant adjuvants (cholera toxin, polyinosinic: polycytidylic acid and dextran sulfate sodium) intervention. An ex vivo splenocyte and intestinal tissue were collected for analysis of the inflammatory profile. The consumption of gliadin and ATI caused intestinal inflammation in mice. Moreover, the histopathology staining of four intestinal sections (duodenum, jejunum, terminal ileum, and middle colon) indicated that adjuvants, especially polyinosinic: polycytidylic acid, enhanced the villi damage and crypt hyperplasia in co-stimulation with ATI and gliadin murine model. Immunohistochemical results showed that tissue transglutaminase and IL-15 expression were significantly increased in the jejunal tissue of mice treated with ATI and gliadin. Similarly, the expression of inflammatory factors (TNF-α, IL-1β, IL-4, IL-13) and Th1/Th2 balance also showed that the inflammation response was significantly increased after co-stimulation with ATI and gliadin. This study provided new evidence for the role of wheat amylase trypsin inhibitors in the pathogenesis of celiac disease.

A short update on new approaches to celiac disease

Celiac disease is an autoimmune enteropathy of the small intestine, related to gluten intolerance occurring in genetically predisposed patients. Currently, the only available treatment is a lifelong gluten-free diet. However, the total avoidance of gluten is difficult and poses a challenge to patients, nutritionists and treating physicians. For this reason, scientists have developed in recent years new therapeutic approaches complementary to dietary treatment, such as modification of gluten to make gliadin non-toxic, reduction of the inflammatory response with elafin and Lactococcus Lactis, degradation of gluten by endoproteolytic enzymes, and correction of nutritional deficiencies by adding pseudo-cereals to the diet of celiac patients. This literature review focuses on the different treatment strategies for celiac disease previously studied and summarizes the latest advances in this field.

Structural Perspective of Gliadin Peptides Active in Celiac Disease

Gluten fragments released in gut of celiac individuals activate the innate or adaptive immune systems. The molecular mechanisms associated with the adaptive response involve a series of immunodominant gluten peptides which are mainly recognized by human leucocyte antigen (HLA)-DQ2.5 and HLA-DQ8. Other peptides, such as A-gliadin P31-43, are not recognized by HLA and trigger innate responses by several routes not yet well detailed. Among the gluten fragments known to be active in Celiac disease, here we focus on the properties of all gluten peptides with known tri-dimensional structure either those locked into HLA-DQ complexes whose crystals were X-ray analyzed or characterized in solution as free forms. The aim of this work was to find the structural reasons why some gluten peptides prompt the adaptive immune systems while others do not, by apparently involving just the innate immune routes. We propose that P31-43 is a non-adaptive prompter because it is not a good ligand for HLA-DQ. Even sharing a similar ability to adopt polyproline II structure with the adaptive ones, the way in which the proline residues are located along the sequence disfavors a productive P31-43-HLA-DQ binding.

The gliadin p31-43 peptide: Inducer of multiple proinflammatory effects

Coeliac disease (CD) is the prototype of an inflammatory chronic disease induced by food. In this context, gliadin p31-43 peptide comes into the spotlight as an important player of the inflammatory/innate immune response to gliadin in CD. The p31-43 peptide is part of the p31-55 peptide from α-gliadins that remains undigested for a long time, and can be present in the small intestine after ingestion of a gluten-containing diet. Different biophysical methods and molecular dynamic simulations have shown that p31-43 spontaneously forms oligomeric nanostructures, whereas experimental approaches using in vitro assays, mouse models, and human duodenal tissues have shown that p31-43 is able to induce different forms of cellular stress by driving multiple inflammatory pathways. Increased proliferative activity of the epithelial cells in the crypts, enterocyte stress, activation of TG2, induction of Ca²⁺, IL-15, and NFκB signaling, inhibition of CFTR, alteration of vesicular trafficking, and activation of the inflammasome platform are some of the biological effects of p31-43, which, in the presence of appropriate genetic susceptibility and environmental factors, may act together to drive CD.