Celiac disease patient IgA antibodies induce endothelial adhesion and cell polarization defects via extracellular transglutaminase 2

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

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

Celiac disease (CD) is a common intestinal inflammatory disease involving both a genetic background and environmental triggers. The ingestion of gluten, a proteic component of several cereals, represents the main hexogen factor implied in CD onset that involves concomitant innate and adaptive immune responses to gluten. Immunogenicity of some gluten sequences are strongly enhanced as the consequence of the deamidation of specific glutamine residues by type 2 transglutaminase (TG2), a ubiquitous enzyme whose expression is up-regulated in the intestine of CD patients. A short gluten sequence resistant to intestinal proteases, the α-gliadin peptide 31-43, seems to modulate TG2 function in the gut; on the other hand, the enzyme can affect the biological activity of this peptide. In addition, an intense auto-immune response towards TG2 is a hallmark of CD. Auto-antibodies exert a range of biological effects on several cells, effects that in part overlap with those induced by peptide 31-43. In this review, we delineate a scenario in which TG2, anti-TG2 antibodies and peptide 31-43 closely relate to each other, thus synergistically participating in CD starting and progression.

Anti-type 2 transglutaminase antibodies as modulators of type 2 transglutaminase functions: a possible pathological role in celiac disease

Auto-antibodies to the ubiquitous enzyme type-2 transglutaminase (TG2) are a specific hallmark of celiac disease (CD), a widely diffused, multi-factorial disease, affecting genetically predisposed subjects. In CD an inflammatory response, at the intestinal level, is triggered by diet consumption of gluten-containing cereals. Intestinal mucosa displays various degrees of atrophy and hyperplasia, with consequent global intestinal dysfunction and other relevant extra-intestinal symptoms. Through deamidation of specific glutamines of gluten-derived gliadin peptides, TG2 strongly enhances gliadin immunogenicity. In addition, TG2 cross-linking activity may generate complexes between TG2 itself and gliadin peptides, and these complexes seem to cause the auto-immune response by means of an apten-carrier-like mechanism of antigen presentation. Anti-TG2 antibodies can be early detected in the intestinal mucosa of celiac patients and are also abundantly present into the serum, thus potentially reaching other organs and tissues by blood circulation. Recently, the possible pathogenetic role of auto-antibodies to TG2 in CD has been investigated. Here, we report an overview about the genesis of these antibodies, their specificity, their modulating ability toward TG2 enzymatic or non-enzymatic activities and their biological effects exerted by interacting with extracellular TG2 or with cell-surface TG2. We also discuss the auto-immune response occurring in CD against other TG members (i.e. type 3 and type 6) and analyze the occurrence of anti-TG2 antibodies in other auto-immune CD-related diseases. Data now available let us to suppose that, even if antibodies to TG2 do not represent the triggering molecules in CD, they could be important players in disease progression and manifestations.

Cardiac fibrosis can be attenuated by blocking the activity of transglutaminase 2 using a selective small-molecule inhibitor

Cardiac fibrosis is implicit in all forms of heart disease but there are no effective treatments. In this report, we investigate the role of the multi-functional enzyme Transglutaminase 2 (TG2) in cardiac fibrosis and assess its potential as a therapeutic target. Here we describe the use a highly selective TG2 small-molecule inhibitor to test the efficacy of TG2 inhibition as an anti-fibrotic therapy for heart failure employing two different in vivo models of cardiac fibrosis: Progressively induced interstitial cardiac fibrosis by pressure overload using angiotensin II infusion: Acutely induced focal cardiac fibrosis through myocardial infarction by ligation of the left anterior descending coronary artery (AMI model). In the AMI model, in vivo MRI showed that the TG2 inhibitor 1–155 significantly reduced infarct size by over 50% and reduced post-infarct remodelling at 20 days post insult. In both models, Sirius red staining for collagen deposition and levels of the TG2-mediated protein crosslink ε(γ-glutamyl)lysine were significantly reduced. No cardiac rupture or obvious signs of toxicity were observed. To provide a molecular mechanism for TG2 involvement in cardiac fibrosis, we show that both TGFβ1-induced transition of cardiofibroblasts into myofibroblast-like cells and TGFβ1-induced EndMT, together with matrix deposition, can be attenuated by the TG2 selective inhibitor 1–155, suggesting a new role for TG2 in regulating TGFβ1 signalling in addition to its role in latent TGFβ1 activation. In conclusion, TG2 has a role in cardiac fibrosis through activation of myofibroblasts and matrix deposition. TG2 inhibition using a selective small-molecule inhibitor can attenuate cardiac fibrosis.

Structure-Activity Relationships of Potent, Targeted Covalent Inhibitors That Abolish Both the Transamidation and GTP Binding Activities of Human Tissue Transglutaminase

Human tissue transglutaminase (hTG2) is a multifunctional enzyme. It is primarily known for its calcium-dependent transamidation activity that leads to formation of an isopeptide bond between glutamine and lysine residues found on the surface of proteins, but it is also a GTP binding protein. Overexpression and unregulated hTG2 activity have been associated with numerous human diseases, including cancer stem cell survival and metastatic phenotype. Herein, we present a series of targeted covalent inhibitors (TCIs) based on our previously reported Cbz-Lys scaffold. From this structure-activity relationship (SAR) study, novel irreversible inhibitors were identified that block the transamidation activity of hTG2 and allosterically abolish its GTP binding ability with a high degree of selectivity and efficiency (k_inact/K_I > 10⁵ M^-1 min^-1). One optimized inhibitor (VA4) was also shown to inhibit epidermal cancer stem cell invasion with an EC₅₀ of 3.9 μM, representing a significant improvement over our previously reported "hit" NC9.

Associations of tissue transglutaminase antibody seropositivity with coronary heart disease: Findings from a prospective cohort study

BACKGROUND AND AIMS

Clinical experience and observational studies suggest that individuals with coeliac disease are at increased risk of coronary heart disease (CHD), but the precise mechanism for this is unclear. Laboratory studies suggest that it may relate to tissue transglutaminase antibodies (tTGAs). Our aim was to examine whether seropositivity for tTGA and endomysial antibodies (EMAs) are associated with incident CHD in humans.

METHODS AND RESULTS

We used data from Mini-Finland Health Survey, a prospective cohort study of Finnish men and women aged 35-80 at study baseline 1978-80. TTGA and EMA seropositivities were ascertained from baseline blood samples and incident CHD events were identified from national hospitalisation and death registers. Cox regression was used to examine the associations between antibody seropositivity and incident CHD. Of 6887 men and women, 562 were seropositive for tTGAs and 72 for EMAs. During a median follow-up of 26 years, 2367 individuals experienced a CHD event. We found no clear evidence for an association between tTGA positivity (hazard ratio, HR: 1.04, 95% confidence interval, CI: 0.83, 1.30) or EMA positivity (HR: 1.16, 95% CI: 0.77, 1.74) and incident CHD, once pre-existing CVD and known CHD risk factors had been adjusted for.

CONCLUSION

We found no clear evidence for an association of tTGA or EMA seropositivity with incident CHD outcomes, suggesting that tTG autoimmunity is unlikely to be the biological link between coeliac disease and CHD.

Transglutaminase 2 in human diseases

Transglutaminase 2 (TG2) is an inducible transamidating acyltransferase that catalyzes Ca(2+)-dependent protein modifications. In addition to being an enzyme, TG2 also serves as a G protein for several seven transmembrane receptors and acts as a co-receptor for integrin β1 and β3 integrins distinguishing it from other members of the transglutaminase family. TG2 is ubiquitously expressed in almost all cell types and all cell compartments, and is also present on the cell surface and gets secreted to the extracellular matrix via non-classical mechanisms. TG2 has been associated with various human diseases including inflammation, cancer, fibrosis, cardiovascular disease, neurodegenerative diseases, celiac disease in which it plays either a protective role, or contributes to the pathogenesis. Thus modulating the biological activities of TG2 in these diseases will have a therapeutic value.

Novel genetic risk variants for pediatric celiac disease

BACKGROUND

Celiac disease is a complex chronic immune-mediated disorder of the small intestine. Today, the pathobiology of the disease is unclear, perplexing differential diagnosis, patient stratification, and decision-making in the clinic.

METHODS

Herein, we adopted a next-generation sequencing approach in a celiac disease trio of Greek descent to identify all genomic variants with the potential of celiac disease predisposition.

RESULTS

Analysis revealed six genomic variants of prime interest: SLC9A4 c.1919G>A, KIAA1109 c.2933T>C and c.4268_4269delCCinsTA, HoxB6 c.668C>A, HoxD12 c.418G>A, and NCK2 c.745_746delAAinsG, from which NCK2 c.745_746delAAinsG is novel. Data validation in pediatric celiac disease patients of Greek (n = 109) and Serbian (n = 73) descent and their healthy counterparts (n = 111 and n = 32, respectively) indicated that HoxD12 c.418G>A is more prevalent in celiac disease patients in the Serbian population (P < 0.01), while NCK2 c.745_746delAAinsG is less prevalent in celiac disease patients rather than healthy individuals of Greek descent (P = 0.03). SLC9A4 c.1919G>A and KIAA1109 c.2933T>C and c.4268_4269delCCinsTA were more abundant in patients; nevertheless, they failed to show statistical significance.

CONCLUSIONS

The next-generation sequencing-based family genomics approach described herein may serve as a paradigm towards the identification of novel functional variants with the aim of understanding complex disease pathobiology.

Transglutaminase 2 has opposing roles in the regulation of cellular functions as well as cell growth and death

Transglutaminase 2 (TG2) is primarily known as the most ubiquitously expressed member of the transglutaminase family with Ca²⁺-dependent protein crosslinking activity; however, this enzyme exhibits multiple additional functions through GTPase, cell adhesion, protein disulfide isomerase, kinase, and scaffold activities and is associated with cell growth, differentiation, and apoptosis. TG2 is found in the extracellular matrix, plasma membrane, cytosol, mitochondria, recycling endosomes, and nucleus, and its subcellular localization is an important determinant of its function. Depending upon the cell type and stimuli, TG2 changes its subcellular localization and biological activities, playing both anti- and pro-apoptotic roles. Increasing evidence indicates that the GTP-bound form of the enzyme (in its closed form) protects cells from apoptosis but that the transamidation activity of TG2 (in its open form) participates in both facilitating and inhibiting apoptosis. A difficulty in the study and understanding of this enigmatic protein is that opposing effects have been reported regarding its roles in the same physiological and/or pathological systems. These include neuroprotective or neurodegenerative effects, hepatic cell growth-promoting or hepatic cell death-inducing effects, exacerbating or having no effect on liver fibrosis, and anti- and pro-apoptotic effects on cancer cells. The reasons for these discrepancies have been ascribed to TG2's multifunctional activities, genetic variants, conformational changes induced by the immediate environment, and differences in the genetic background of the mice used in each of the experiments. In this article, we first report that TG2 has opposing roles like the protagonist in the novel Dr. Jekyll and Mr. Hyde, followed by a summary of the controversies reported, and finally discuss the possible reasons for these discrepancies.

Associations of coeliac disease with coronary heart disease and cerebrovascular disease: A systematic review and meta-analysis

AIMS

Clinical experience suggests that atherosclerotic disease is common in individuals with coeliac disease, but epidemiological studies have had contradicting findings. To summarise the currently available evidence, we systematically reviewed and analysed observational studies of the association of coeliac disease or dermatitis herpetiformis with coronary heart disease (CHD) or stroke.

DATA SYNTHESIS

We searched for studies comparing CHD or stroke outcomes with individuals with and without coeliac disease or dermatitis herpetiformis. Three investigators independently searched electronic databases, identified relevant studies and extracted data. Study-specific results were combined in random-effects meta-analyses, and heterogeneity was quantified using the I(2) statistic and meta-regression. Twenty-one studies were included in our systematic review and 18 in the meta-analyses. For CHD, the pooled hazard ratio for incident disease was 1.05 (95% confidence interval (CI): 0.93, 1.19) and the overall standardised mortality ratio was 1.21 (0.99, 1.49). For stroke and brain haemorrhage, the corresponding estimates were 1.10 (95% CI: 1.00, 1.21) and 1.43 (0.97, 2.10), respectively. There was moderate to considerable heterogeneity among the study-specific estimates. In addition, many estimates were based on small numbers of outcomes and they had limitations in terms of adjustment for potential confounders.

CONCLUSION

Our meta-analyses lend some support to an association between coeliac disease and CHD or cerebrovascular disease, but the evidence base was heterogeneous and had limitations. Our systematic review highlighted a need in this area for adequately powered prospective studies with appropriate adjustment for potentially confounding factors.

Celiac Disease and Type 1 Diabetes in Adults: Is This a High-Risk Group for Screening?

The association between celiac disease (CD), an autoimmune condition involving intestinal inflammation related to gluten ingestion, and type 1 diabetes has long been recognized. CD prevalence rates 4 to 6 times greater in adults with type 1 diabetes than in the general population. Much of the existing literature focuses on important implications related to the impact of a gluten-free diet on short-term outcomes in metabolic control and quality of life. Canadian Diabetes Association guidelines recommend targeted CD screening in patients with type 1 diabetes who have classic symptoms, such as abdominal pain, bloating, diarrhea, unexplained weight loss or labile metabolic control; however, a significant proportion (40% to 60%) of patients may have mild or absent symptoms. Recent evidence suggests that adult patients with both conditions are at higher risk for diabetes microvascular comorbidities, increased mortality and impaired bone health if the CD is untreated. The purpose of this review is to describe the association between CD and type 1 diabetes and to summarize recent literature that evaluates risks in patients with both conditions.

Inhibitors of tissue transglutaminase

Tissue transglutaminase (TG2) catalyzes the cross-linking of proteins by the formation of isopeptide bonds between glutamine (Gln) and lysine (Lys) side chains. Although TG2 is essential for the stabilization of the extracellular matrix, its unregulated activity has been implicated in celiac disease, fibrosis, and cancer metastasis, among other disorders. Given the importance and range of TG2-related pathologies, recent work has focused on the development of potent and selective inhibitors against TG2. In this review, we present the latest and most noteworthy irreversible and reversible inhibitors of TG2, and offer perspectives for the design of future inhibitors, in the hope that lead compounds with therapeutic potential may soon be discovered.