Proteomics identification of acyl-acceptor and acyl-donor substrates for transglutaminase in a human intestinal epithelial cell line. Implications for celiac disease

Aberrant expression of transglutaminase 2 in pancreas and thymus of NOD mice underscores the importance of deamidation in neoantigen generation

Post-translational modifications can lead to a break in immune tolerance in autoimmune diseases such as type 1 diabetes (T1D). Deamidation, the conversion of glutamine to glutamic acid by transglutaminase (TGM) enzymes, is a post-translational modification of interest, with deamidated peptides being reported as autoantigens in T1D. However, little is known about how Tgm2, the most ubiquitously expressed Tgm isoform, is regulated and how tolerance against deamidated peptides is lost. Here, we report on the aberrant expression and regulation of Tgm2 in the pancreas and thymus of NOD mice. We demonstrate that Tgm2 expression is induced by the inflammatory cytokines IL1β and IFNγ in a synergistic manner and that murine pancreatic islets of NOD mice have higher Tgm2 levels, while Tgm2 levels in medullary thymic epithelial cells are reduced. We thus provide the first direct evidence to our knowledge that central tolerance establishment against deamidated peptides might be impaired due to lower Tgm2 expression in NOD medullary thymic epithelial cells, which together with the aberrantly high levels of deamidated peptides in NOD β-cells underscores the role of deamidation in amplifying T-cell reactivity.

Prohibitin ligands: a growing armamentarium to tackle cancers, osteoporosis, inflammatory, cardiac and neurological diseases

Over the last three decades, the scaffold proteins prohibitins-1 and -2 (PHB1/2) have emerged as key signaling proteins regulating a myriad of signaling pathways in health and diseases. Small molecules targeting PHBs display promising effects against cancers, osteoporosis, inflammatory, cardiac and neurodegenerative diseases. This review provides an updated overview of the various classes of PHB ligands, with an emphasis on their mechanism of action and therapeutic potential. We also describe how these ligands have been used to explore PHB signaling in different physiological and pathological settings.

Infection-driven activation of transglutaminase 2 boosts glucose uptake and hexosamine biosynthesis in epithelial cells

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme with transamidating activity. We report here that both expression and activity of TG2 are enhanced in mammalian epithelial cells infected with the obligate intracellular bacteria Chlamydia trachomatis. Genetic or pharmacological inhibition of TG2 impairs bacterial development. We show that TG2 increases glucose import by up-regulating the transcription of the glucose transporter genes GLUT-1 and GLUT-3. Furthermore, TG2 activation drives one specific glucose-dependent pathway in the host, i.e., hexosamine biosynthesis. Mechanistically, we identify the glucosamine:fructose-6-phosphate amidotransferase (GFPT) among the substrates of TG2. GFPT modification by TG2 increases its enzymatic activity, resulting in higher levels of UDP-N-acetylglucosamine biosynthesis and protein O-GlcNAcylation. The correlation between TG2 transamidating activity and O-GlcNAcylation is disrupted in infected cells because host hexosamine biosynthesis is being exploited by the bacteria, in particular to assist their division. In conclusion, our work establishes TG2 as a key player in controlling glucose-derived metabolic pathways in mammalian cells, themselves hijacked by C. trachomatis to sustain their own metabolic needs.

Isozyme-specific comprehensive characterization of transglutaminase-crosslinked substrates in kidney fibrosis

Chronic kidney disease is characterized by prolonged decline in renal function, excessive accumulation of ECM, and progressive tissue fibrosis. Transglutaminase (TG) is a crosslinking enzyme that catalyzes the formation of covalent bonds between glutamine and lysine residues, and is involved in the induction of renal fibrosis via the stabilization of ECM and the activation of TGF-β1. Despite the accumulating evidences indicating that TG2 is a key enzyme in fibrosis, genetic knockout of TG2 reduced by only 50% the elevated protein crosslinking and fibrous protein in renal fibrosis model, whereas treatment with TG inhibitor almost completely reduced these levels. Here, we also clarified the distributions of TG isozymes and their in situ activities and identified the isozyme-specific crosslinked substrates for both TG1 and TG2 in fibrotic kidney. We found that TG1 activity was markedly enhanced in renal tubular epithelium and interstitial areas, whereas TG2 activity increased only in the extracellular space. In total, 47 and 67 possible candidates were identified as TG1 and TG2 substrates, respectively, only in fibrotic kidney. Among them, several possible substrates related to renal disease and fibrosis were identified. These findings provide novel insights into the mechanisms of renal fibrosis through the targeting of isozyme-specific TG substrates.

The toxic alpha-gliadin peptide 31-43 enters cells without a surface membrane receptor

Alpha-gliadin peptide 31-43 is considered to be the main peptide responsible for the innate immune response in celiac disease patients. Recent evidence indicates that peptide 31-43 rapidly enters cells and interacts with the early endocytic vesicular compartment. However, the mechanism of its uptake is not completely understood. Our aim is to characterize, isolate and identify possible cell surface proteins involved in peptide 31-43 internalization by Caco-2 cells. In this study, we used a chemical cross-linker to block peptide 31-43 on cell surface proteins, and pulled-down peptide-proteins complexes using antibodies raised against peptide 31-43. Through this experimental approach, we did not observe any specific complex between cell proteins and peptide 31-43 in Coomassie-stained denaturating gels or by Western blotting. We also found that type 2 transglutaminase was not necessary for peptide 31-43 internalization, even though it had a regulatory role in the process. Finally, we demonstrated that peptide 31-43 did not behave as a classical ligand, indeed the labeled peptide did not displace the unlabeled peptide in a competitive binding assay. On the basis of these findings and of previous evidence demonstrating that peptide 31-43 is able to interact with a membrane-like environment in vitro, we conclude that membrane composition and organization, rather than a specific receptor protein, may have a major role in peptide 31-43 internalization by cells.

Global identification and analysis of isozyme-specific possible substrates crosslinked by transglutaminases using substrate peptides in mouse liver fibrosis

The transglutaminase (TG) family comprises eight isozymes that form the isopeptide bonds between glutamine and lysine residues and contribute to the fibrotic diseases via crosslinking-mediated stabilization of ECM and the activation of TGF-β in several tissues. However, despite a growing body of evidence implicating TG2 as a key enzyme in fibrosis, the causative role of TG2 and the involvement of the other isozymes have not yet been fully elucidated. Therefore, here we clarified the distributions of TG isozymes and their in situ activities and identified the isozyme-specific possible substrates for both TG1 and TG2 using their substrate peptides in mouse fibrotic liver. We found that TG1 activity was markedly enhanced intracellularly over a widespread area, whereas TG2 activity increased in the extracellular space. In total, 43 and 42 possible substrates were identified for TG1 and TG2, respectively, as involved in chromatin organization and cellular component morphogenesis. These included keratin 18, a biomarker for hepatic injury, which was accumulated in the fibrotic liver and showed the partly similar distribution with TG1 activity. These findings suggest that TG1 activity may be involved in the functional modification of intracellular proteins, whereas TG2 activity contributes to the stabilization of extracellular proteins during liver fibrosis.

Environmental Factors Contribute to β Cell Endoplasmic Reticulum Stress and Neo-Antigen Formation in Type 1 Diabetes

Type 1 diabetes (T1D) is an autoimmune disease in which immune-mediated targeting and destruction of insulin-producing pancreatic islet β cells leads to chronic hyperglycemia. There are many β cell proteins that are targeted by autoreactive T cells in their native state. However, recent studies have demonstrated that many β cell proteins are recognized as neo-antigens following posttranslational modification (PTM). Although modified neo-antigens are well-established targets of pathology in other autoimmune diseases, the effects of neo-antigens in T1D progression and the mechanisms by which they are generated are not well understood. We have demonstrated that PTM occurs during endoplasmic reticulum (ER) stress, a process to which β cells are uniquely susceptible due to the high rate of insulin production in response to dynamic glucose sensing. In the context of genetic susceptibility to autoimmunity, presentation of these modified neo-antigens may activate autoreactive T cells and cause pathology. However, inherent β cell ER stress and protein PTM do not cause T1D in every genetically susceptible individual, suggesting the contribution of additional factors. Indeed, many environmental factors, such as viral infection, chemicals, or inflammatory cytokines, are associated with T1D onset, but the mechanisms by which these factors lead to disease onset remain unknown. Since these environmental factors also cause ER stress, exposure to these factors may enhance production of neo-antigens, therefore boosting β cell recognition by autoreactive T cells and exacerbating T1D pathogenesis. Therefore, the combined effects of physiological ER stress and the stress that is induced by environmental factors may lead to breaks in peripheral tolerance, contribute to antigen spread, and hasten disease onset. This Hypothesis and Theory article summarizes what is currently known about ER stress and protein PTM in autoimmune diseases including T1D and proposes a role for environmental factors in breaking immune tolerance to β cell antigens through neo-antigen formation.

Inherent ER stress in pancreatic islet β cells causes self-recognition by autoreactive T cells in type 1 diabetes

Type 1 diabetes (T1D) is an autoimmune disease characterized by pancreatic β cell destruction induced by islet reactive T cells that have escaped central tolerance. Many physiological and environmental triggers associated with T1D result in β cell endoplasmic reticulum (ER) stress and dysfunction, increasing the potential for abnormal post-translational modification (PTM) of proteins. We hypothesized that β cell ER stress induced by environmental and physiological conditions generates abnormally-modified proteins for the T1D autoimmune response. To test this hypothesis we exposed the murine CD4(+) diabetogenic BDC2.5 T cell clone to murine islets in which ER stress had been induced chemically (Thapsigargin). The BDC2.5 T cell IFNγ response to these cells was significantly increased compared to non-treated islets. This β cell ER stress increased activity of the calcium (Ca(2+))-dependent PTM enzyme tissue transglutaminase 2 (Tgase2), which was necessary for full stress-dependent immunogenicity. Indeed, BDC2.5 T cells responded more strongly to their antigen after its modification by Tgase2. Finally, exposure of non-antigenic murine insulinomas to chemical ER stress in vitro or physiological ER stress in vivo caused increased ER stress and Tgase2 activity, culminating in higher BDC2.5 responses. Thus, β cell ER stress induced by chemical and physiological triggers leads to β cell immunogenicity through Ca(2+)-dependent PTM. These findings elucidate a mechanism of how β cell proteins are modified and become immunogenic, and reveal a novel opportunity for preventing β cell recognition by autoreactive T cells.

The transglutaminase type 2 and pyruvate kinase isoenzyme M2 interplay in autophagy regulation

Autophagy is a self-degradative physiological process by which the cell removes worn-out or damaged components. Constant at basal level it may become highly active in response to cellular stress. The type 2 transglutaminase (TG2), which accumulates under stressful cell conditions, plays an important role in the regulation of autophagy and cells lacking this enzyme display impaired autophagy/mitophagy and a consequent shift their metabolism to glycolysis. To further define the molecular partners of TG2 involved in these cellular processes, we analysed the TG2 interactome under normal and starved conditions discovering that TG2 interacts with various proteins belonging to different functional categories. Herein we show that TG2 interacts with pyruvate kinase M2 (PKM2), a rate limiting enzyme of glycolysis which is responsible for maintaining a glycolytic phenotype in malignant cells and displays non metabolic functions, including transcriptional co-activation and protein kinase activity. Interestingly, the ablation of PKM2 led to the decrease of intracellular TG2's transamidating activity paralleled by an increase of its tyrosine phosphorylation. Along with this, a significant decrease of ULK1 and Beclin1 was also recorded, thus suggesting a block in the upstream regulation of autophagosome formation. These data suggest that the PKM2/TG2 interplay plays an important role in the regulation of autophagy in particular under cellular stressful conditions such as those displayed by cancer cells.

β cell ER stress and the implications for immunogenicity in type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by hyperglycemia due to progressive immune-mediated destruction of insulin-producing pancreatic islet β cells. Although many elegant studies have identified β cell autoantigens that are targeted by the autoimmune response, the mechanisms by which these autoantigens are generated remain poorly understood. Normal β cell physiology includes a high demand for insulin production and secretion in response to dynamic glucose sensing. This secretory function predisposes β cells to significantly higher levels of endoplasmic reticulum (ER) stress compared to nonsecretory cells. In addition, many environmental triggers associated with T1D onset further augment this inherent ER stress in β cells. ER stress may increase abnormal post-translational modification (PTM) of endogenous β cell proteins. Indeed, in other autoimmune disorders such as celiac disease, systemic lupus erythematosus, multiple sclerosis, and rheumatoid arthritis, abnormally modified neo-antigens are presented by antigen presenting cells (APCs) in draining lymph nodes. In the context of genetic susceptibility to autoimmunity, presentation of neo-antigens activates auto-reactive T cells and pathology ensues. Therefore, the ER stress induced by normal β cell secretory physiology and environmental triggers may be sufficient to generate neo-antigens for the autoimmune response in T1D. This review summarizes what is currently known about ER stress and protein PTM in target organs of other autoimmune disease models, as well as the data supporting a role for ER stress-induced neo-antigen formation in β cells in T1D.

Glutathionylspermidine in the modification of protein SH groups: the enzymology and its application to study protein glutathionylation

Cysteine is very susceptible to reactive oxygen species. In response; posttranslational thiol modifications such as reversible disulfide bond formation have arisen as protective mechanisms against undesired in vivo cysteine oxidation. In Gram-negative bacteria a major defense mechanism against cysteine overoxidation is the formation of mixed protein disulfides with low molecular weight thiols such as glutathione and glutathionylspermidine. In this review we discuss some of the mechanistic aspects of glutathionylspermidine in prokaryotes and extend its potential use to eukaryotes in proteomics and biochemical applications through an example with tissue transglutaminase and its S-glutathionylation.

Transglutaminase regulation of cell function

Transglutaminases (TGs) are multifunctional proteins having enzymatic and scaffolding functions that participate in regulation of cell fate in a wide range of cellular systems and are implicated to have roles in development of disease. This review highlights the mechanism of action of these proteins with respect to their structure, impact on cell differentiation and survival, role in cancer development and progression, and function in signal transduction. We also discuss the mechanisms whereby TG level is controlled and how TGs control downstream targets. The studies described herein begin to clarify the physiological roles of TGs in both normal biology and disease states.

Effects of pH-Shift Processing and Microbial Transglutaminase on the Gel and Emulsion Characteristics of Porcine Myofibrillar System

This study investigated the effects of microbial transglutaminase (MTGase) and pH-shift processing on the functional properties of porcine myofibrillar proteins (MP). The pH-shift processing was carried out by decreasing the pH of MP suspension to 3.0, followed by re-adjustment to pH 6.2. The native (CM) and pH-shifted MP (PM) was reacted with and without MTGase, and the gelling and emulsion characteristics were compared. To compare the pH-shifted MTGase-treated MP (PT), deamidation (DM) was conducted by reacting MTGase with MP at pH 3.0. Rigid thermal gel was produced by MTGase-treated native MP (CT) and PT. PM and DM showed the lowest storage modulus (G') at the end of thermal scanning. Both MTGase and pH-shifting produced harder MP gel, and the highest gel strength was obtained in PT. All treatments yielded lower than CM, and CT showed significantly higher yield than PM and DM treatments. For emulsion characteristics, pH-shifting improved the emulsifying ability of MP-stabilized emulsion, while the treatments had lower emulsion stability. PM-stabilized emulsion exhibited the lowest creaming stability among all treatments. The emulsion stability could be improved by the usage of MTGase. The results indicated that pH-shifting combined with MTGase had a potential application to modify or improve functional properties of MP in manufacturing of meat products.

Biological functionalities of transglutaminase 2 and the possibility of its compensation by other members of the transglutaminase family

Transglutaminase 2 (TG2) is the most widely distributed and most abundantly expressed member of the transglutaminase family of enzymes, a group of intracellular and extracellular proteins that catalyze the Ca²⁺-dependent posttranslational modification of proteins. It is a unique member of the transglutaminase family owing to its specialized biochemical, structural and functional elements, ubiquitous tissue distribution and subcellular localization, and substrate specificity. The broad substrate specificity of TG2 and its flexible interaction with numerous other gene products may account for its multiple biological functions. In addition to the classic Ca²⁺-dependent transamidation of proteins, which is a hallmark of transglutaminase enzymes, additional Ca²⁺-independent enzymatic and nonenzymatic activities of TG2 have been identified. Many such activities have been directly or indirectly implicated in diverse cellular physiological events, including cell growth and differentiation, cell adhesion and morphology, extracellular matrix stabilization, wound healing, cellular development, receptor-mediated endocytosis, apoptosis, and disease pathology. Given the wide range of activities of the transglutaminase gene family it has been suggested that, in the absence of active versions of TG2, its function could be compensated for by other members of the transglutaminase family. It is in the light of this assertion that we review, herein, TG2 activities and the possibilities and premises for compensation for its absence.

Celiac anti-type 2 transglutaminase antibodies induce phosphoproteome modification in intestinal epithelial Caco-2 cells

BACKGROUND

Celiac disease is an inflammatory condition of the small intestine that affects genetically predisposed individuals after dietary wheat gliadin ingestion. Type 2-transglutaminase (TG2) activity seems to be responsible for a strong autoimmune response in celiac disease, TG2 being the main autoantigen. Several studies support the concept that celiac anti-TG2 antibodies may contribute to disease pathogenesis. Our recent findings on the ability of anti-TG2 antibodies to induce a rapid intracellular mobilization of calcium ions, as well as extracellular signal-regulated kinase phosphorylation, suggest that they potentially act as signaling molecules. In line with this concept, we have investigated whether anti-TG2 antibodies can induce phosphoproteome modification in an intestinal epithelial cell line.

METHODS AND PRINCIPAL FINDINGS

We studied phosphoproteome modification in Caco-2 cells treated with recombinant celiac anti-TG2 antibodies. We performed a two-dimensional electrophoresis followed by specific staining of phosphoproteins and mass spectrometry analysis of differentially phosphorylated proteins. Of 14 identified proteins (excluding two uncharacterized proteins), three were hypophosphorylated and nine were hyperphosphorylated. Bioinformatics analyses confirmed the presence of phosphorylation sites in all the identified proteins and highlighted their involvement in several fundamental biological processes, such as cell cycle progression, cell stress response, cytoskeletal organization and apoptosis.

CONCLUSIONS

Identification of differentially phosphorylated proteins downstream of TG2-antibody stimulation suggests that in Caco-2 cells these antibodies perturb cell homeostasis by behaving as signaling molecules. We hypothesize that anti-TG2 autoantibodies may destabilize the integrity of the intestinal mucosa in celiac individuals, thus contributing to celiac disease establishment and progression. Since several proteins here identified in this study were already known as TG2 substrates, we can also suppose that transamidating activity and differential phosphorylation of the same targets may represent a novel regulatory mechanism whose relevance in celiac disease pathogenesis is still unexplored.

Transglutaminase 2 transamidation activity during first-phase insulin secretion: natural substrates in INS-1E

Transglutaminase 2 (TG2) is a multifunctional protein with Ca(2+)-dependent transamidating and G protein activity. Previously, we reported that tgm2 -/- mice have an impaired insulin secretion and that naturally occurring TG2 mutations associated with familial, early-onset type 2 diabetes, show a defective transamidating activity. Aim of this study was to get a better insight into the role of TG2 in insulin secretion by identifying substrates of TG2 transamidating activity in the pancreatic beta cell line INS-1E. To this end, we labeled INS-1E that are capable of secreting insulin upon glucose stimulation in the physiologic range, with an artificial acyl acceptor (biotinamido-pentylamine) or donor (biotinylated peptide), in basal condition and after stimulus with glucose for 2, 5, and 8 min. Biotinylated proteins were analyzed by two-dimensional electrophoresis and mass spectrometry. In addition, subcellular localization of TG2 in human endocrine pancreas was studied by electron microscopy. Among several TG2's transamidating substrates in INS-1E, mass spectrometry identified cytoplasmic actin (a result confirmed in human pancreatic islet), tropomyosin, and molecules that participate in insulin granule structure (e.g., GAPDH), glucose metabolism, or [Ca(2+)] sensing (e.g., calreticulin). Physical interaction between TG2 and cytoplasmic actin during glucose-stimulated first-phase insulin secretion was confirmed by co-immunoprecipitation. Electron microscopy revealed that TG2 is localized close to insulin and glucagon granules in human pancreatic islet. We propose that TG2's role in insulin secretion may involve cytoplasmic actin remodeling and may have a regulative action on other proteins during granule movement. A similar role of TG2 in glucagon secretion is also suggested.

Gα(h)/transglutaminase-2 activity is required for maximal activation of adenylylcyclase 8 in human and rat glioma cells

Gα(h) (or transglutaminase-2 (TG2)) is an atypical guanine nucleotide binding-protein that associates with G protein-coupled receptors. TG2 also exerts transglutaminase activity that catalyzes posttranslational protein cross-linking with the formation of ε-(γ-glutamyl) lysine or (γ-glutamyl) polyamine bonds. Here, the role of Gα(h)/TG2 in signal transduction in glial cells was examined in detail. In 1321N1 human astrocytoma cells that lack Gα(h)/TG2, overexpression of Gα(h)/TG2 caused an enhancement of cAMP accumulation stimulated with the β-adrenergic receptor agonist, isoproterenol, or the adenylylcyclase activator, forskolin. This cAMP-enhancement was reversed by the TG2 inhibitor, ERW1069. In rat C6 glioma cells that express endogenous Gα(h)/TG2, cAMP accumulation induced by isoproterenol or forskolin was significantly inhibited by overexpression of Gα(h)/TG2-C277V, a dominant-negative mutant that lacks transglutaminase activity, but was not inhibited by the Gα(h)/TG2-S171E mutant that cannot bind GTP/GDP. These results suggest Gα(h)/TG2 potentiates adenylylcyclase activity by its transglutaminase activity and not by its G-protein activity. Gα(h)/TG2 also increased the activities of the cAMP response element and interleukin-6 promoter, accompanied by an of cAMP in both glioma cells. Since adenylylcyclase 8 plays a major role in cAMP production, we focused on post-translational modification of adenylylcyclase 8 by Gα(h)/TG2. Adenylylcyclase 8 is expressed in both 1321N1 and C6 cells; however, Gα(h)/TG2 affected neither adenylylcyclase 8 expression levels, glycosylation, nor dimerization status. In contrast, pentylamine, a substrate of Gα(h)/TG2, was incorporated into adenylylcyclase 8 in a transglutaminase activity-dependent manner. Taking these results together, Gα(h)/TG2 promotes cAMP production accompanied by a modification of adenylylcyclase 8 in glioma cells.

Substrates of Factor XIII-A: roles in thrombosis and wound healing

FXIII (Factor XIII) is a Ca2+-dependent enzyme which forms covalent ϵ-(γ-glutamyl)lysine cross-links between the γ-carboxy-amine group of a glutamine residue and the ϵ-amino group of a lysine residue. FXIII was originally identified as a protein involved in fibrin clot stabilization; however, additional extracellular and intracellular roles for FXIII have been identified which influence thrombus resolution and tissue repair. The present review discusses the substrates of FXIIIa (activated FXIII) involved in thrombosis and wound healing with a particular focus on: (i) the influence of plasma FXIIIa on the formation of stable fibrin clots able to withstand mechanical and enzymatic breakdown through fibrin–fibrin cross-linking and cross-linking of fibrinolysis inhibitors, in particular α2-antiplasmin; (ii) the role of intracellular FXIIIa in clot retraction through cross-linking of platelet cytoskeleton proteins, including actin, myosin, filamin and vinculin; (iii) the role of intracellular FXIIIa in cross-linking the cytoplasmic tails of monocyte AT1Rs (angiotensin type 1 receptors) and potential effects on the development of atherosclerosis; and (iv) the role of FXIIIa on matrix deposition and tissue repair, including cross-linking of extracellular matrix proteins, such as fibronectin, collagen and von Willebrand factor, and the effects on matrix deposition and cell–matrix interactions. The review highlights the central role of FXIIIa in the regulation of thrombus stability, thrombus regulation, cell–matrix interactions and wound healing, which is supported by observations in FXIII-deficient humans and animals.

Gliadin peptides induce tissue transglutaminase activation and ER-stress through Ca2+ mobilization in Caco-2 cells

BACKGROUND

Celiac disease (CD) is an intestinal inflammatory condition that develops in genetically susceptible individuals after exposure to dietary wheat gliadin. The role of post-translational modifications of gliadin catalyzed by tissue transglutaminase (tTG) seems to play a crucial role in CD. However, it remains to be established how and where tTG is activated in vivo. We have investigated whether gliadin peptides modulate intracellular Ca(2+) homeostasis and tTG activity.

METHODS/PRINCIPAL FINDINGS

We studied Ca(2+) homeostasis in Caco-2 cells by single cell microfluorimetry. Under our conditions, A-gliadin peptides 31-43 and 57-68 rapidly mobilized Ca(2+) from intracellular stores. Specifically, peptide 31-43 mobilized Ca(2+) from the endoplasmic reticulum (ER) and mitochondria, whereas peptide 57-68 mobilized Ca(2+) only from mitochondria. We also found that gliadin peptide-induced Ca(2+) mobilization activates the enzymatic function of intracellular tTG as revealed by in situ tTG activity using the tTG substrate pentylamine-biotin. Moreover, we demonstrate that peptide 31-43, but not peptide 57-68, induces an increase of tTG expression. Finally, we monitored the expression of glucose-regulated protein-78 and of CCAAT/enhancer binding protein-homologous protein, which are two biochemical markers of ER-stress, by real-time RT-PCR and western blot. We found that chronic administration of peptide 31-43, but not of peptide 57-68, induces the expression of both genes.

CONCLUSIONS

By inducing Ca(2+) mobilization from the ER, peptide 31-43 could promote an ER-stress pathway that may be relevant in CD pathogenesis. Furthermore, peptides 31-43 and 57-68, by activating intracellular tTG, could alter inflammatory key regulators, and induce deamidation of immunogenic peptides and gliadin-tTG crosslinking in enterocytes and specialized antigen-presenting cells.

The function of tissue transglutaminase in celiac disease

Celiac disease is a chronic small bowel disorder caused by an abnormal immune response to an array of epitopes of wheat gluten and related proteins of rye and barley in genetically susceptible individuals who express the HLA-DQ2/-DQ8 haplotype. Gluten peptides are efficiently presented by celiac disease-specific HLA-DQ2- and HLA-DQ8-positive antigen presenting cells to CD4(+) T-cells that, once activated, drive a T helper cell type 1 response leading to the development of the typical celiac lesion-villous atrophy, crypt hyperplasia and intraepithelial and lamina propria infiltration of inflammatory cells. Tissue transglutaminase (tTG) is a calcium dependent ubiquitous enzyme which catalyses posttranslational modification of proteins and is released from cells during inflammation. tTG is suggested to exert at least two crucial roles in celiac disease: as a deamidating enzyme, that can enhance the immunostimulatory effect of gluten, and as a target autoantigen in the immune response. Since glutamine-rich gliadin peptides are excellent substrates for tTG, and the resulting deamidated and thus negatively charged peptides have much higher affinity for the HLA-DQ2 and HLA-DQ8 molecules, the action of tTG is believed to be a key step in the pathogenesis of celiac disease. This review is focused on the function of tTG in celiac disease, although it also deals with novel advances in tTG-based therapies.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

New insights into the functions and localization of nuclear transglutaminase 2

Transglutaminase 2 (TG2; EC 2.3.2.13) is the most abundantly expressed member of the transglutaminase family and exerts opposing effects on cell growth, differentiation and apoptosis via multiple activities, including transamidase, GTPase, cell adhesion, protein disulfide isomerase, kinase and scaffold activities. It is distributed in and around various parts of a cell, including the extracellular matrix, plasma membrane, cytosol, mitochondria and nucleus. Generally, nuclear TG2 represents only 5-7% of the total TG2 in a cell, and various stimuli will increase nuclear TG2 via cellular stress and/or an increased intracellular Ca(2+) concentration. There is increasing evidence indicating the importance of nuclear TG2 in regulating gene expression via post-translational modification of (or interaction with) transcriptional factors and related proteins. These include E2F1, hypoxia inducible factor 1, Sp1 and histones. Through this mechanism, TG2 controls cell growth or survival, differentiation and apoptosis, and is involved in the pathogenesis and/or treatment of neurodegenerative diseases, liver diseases and cancers. The balance between import from the cytoplasm to the nucleus, and export from the nucleus to the cytoplasm, determines the level of TG2 in the nucleus. Selective regulation of the expression, activity or localization of nuclear TG2 will be important for basic research, as well as clinical applications, suggesting a new era for this long-studied enzyme.

Anti-tissue transglutaminase antibodies activate intracellular tissue transglutaminase by modulating cytosolic Ca2+ homeostasis

Anti-tissue transglutaminase (tTG) antibodies are specifically produced in the small-intestinal mucosa of celiac disease (CD) patients. It is now recognized that these antibodies, acting on cell-surface tTG, may play an active role in CD pathogenesis triggering an intracellular response via the activation of different signal transduction pathways. In this study, we report that anti-tTG antibodies, both commercial and from a CD patient, induce a rapid Ca(2+) mobilization from intracellular stores in Caco-2 cells. We characterized the mechanism of Ca(2+) release using thapsigargin and carbonylcyanide-p-trifluoromethoxyphenylhydrazone, which are able to deplete specifically endoplasmic reticulum and mitochondria of Ca(2+), respectively. Our data highlight that both pathways of calcium release were involved, thus indicating that the spectrum of cellular responses downstream can be very wide. In addition, we demonstrate that the increased Ca(2+) level in the cells evoked by anti-tTG antibodies was sufficient to activate tTG, which is normally present as a latent protein due to the presence of low Ca(2+) and to the inhibitory effect of GTP/GDP. Herein, we discuss the importance of intracellular tTG activation as central in the context of CD pathogenesis.

Self-assembly of tissue transglutaminase into amyloid-like fibrils using physiological concentration of Ca2+

Tissue transglutaminase (tTG or TG2) is a member of the transglutaminase family that catalyzes calcium dependent formation of isopeptide bonds. It has been shown that the expression of TG2 is elevated in neurodegenerative diseases such as Parkinson's, Huntington's, and Alzheimer's. We have investigated the self-assembly of TG2 in vitro. First, using software, hot spots, which are prone for aggregation, were identified in domain 2 of the enzyme. Next we expressed and purified recombinant TG2 and its truncated version that contains only the catalytic domain, and examined their amyloidogenic behavior in various conditions including different temperatures and pHs, in the presence of metal ions and Guanosine triphosphate (GTP). To analyze various stages leading to TG2 fibrillation, we employed various techniques including Thioflavin T (ThT) binding assay, Congo-Red, birefringence, Circular Dichroism (CD), 8-anilino-1-naphthalene sulfonic acid (ANS) binding, Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Our results indicated that using low concentrations of Ca(2+), TG2 self-assembled into amyloid-like fibrils; this self-assembly occurred at the physiological temperature (37 °C) and at a higher temperature (57 °C). The truncated version of TG2 (domain 2) also forms amyloid-like fibrils only in the presence of Ca(2+). Because amyloid formation has occurred with domain 2 alone where no enzymatic activity was shown, self-cross-linking by the enzyme was ruled out as a mechanism of amyloid induction. The self-assembly of TG2 was not significant with magnesium and zinc ions, indicating specificity of the self-assembly for calcium ions. The calcium role in self-assembly of TG2 into amyloid may be extended to other proteins with similar biophysical properties to produce novel biomaterials.

The role of Lamin A in cytoskeleton organization in colorectal cancer cells: a proteomic investigation

Up-regulated expression of lamin A has been implicated in increased cell invasiveness and mortality in colorectal cancer. Here we use quantitative proteomics to investigate lamin A regulated changes in the cytoskeleton that might underpin increased cell motility. Using siRNA knockdown of lamin A in a model cell line (SW480/lamA) we confirm that the presence of lamin A promotes cell motility. Using an enhanced technique to prepare cytoskeleton fractions in combination with 2D DiGE we were able to accurately and reproducibly detect changes in the representation of protein species within the cytoskeleton as low as 20%. In total 64 protein spots displayed either increased or decreased representation within the cytoskeleton of SW480/lamA cells compared to controls. Of these the identities of 29 spots were determined by mass spectrometry. A majority were multiple forms of three classes of proteins, including components of the actin and IF cytoskeletons, protein chaperones and translation initiation and elongation factors. In particular our data reveal that the representation of tissue transglutaminase 2, which is known to modify elements of the cytoskeleton and is associated with cancer progression, was highly over-represented in the cytoskeleton fraction of SW480/lamA cells. Overall, our data are consistent with changed protein cross-linking and folding that favours the formation of dynamic actin filaments over stress fibres accounting for the altered cell motility properties in SW480/lamA cells.

Transdifferentiation of epidermis to mucous epithelium by retinol accompanies increase in transglutaminase 2/Gh and decrease in transglutaminase 3

We showed previously that transdifferentiation of skin epidermis to mucous epithelium can be induced by treatment with 20 µM retinol for 1 d followed by culture for 4 d without retinol in chick embryonic tarsometatarsal skin. In mouse epidermal cells, 3 µM retinoic acid (an active metabolite of retinol) inhibits epidermal keratinization in consistent with an increase in transglutaminase (TG)2/Gh, while its physiological role in the skin is still unresolved. TG1, TG3 and TG5 are also found in mammalian keratinocytes and play an important role in the formation of the stratum corneum in the skin by the introduction of cross-links into proteins. The most characteristic enzyme function of TG family is calcium-dependent transamidation activity (transamidase) that introduces inter or intramolecular ε-(γ-glutamyl)lysine cross-links into the protein. TG2/Gh is a multifunctional protein and ubiquitously expressed member of transglutaminase family that has been implicated in a variety of biological processes. By in situ hybridization analysis, we showed that TG2/Gh mRNA expression started to increase throughout the skin during the culture for 1 d with retinol, while it was weak in the control skin. On the other hand, an expression of TG3 mRNA was increased in the keratinized epidermis of control skin but was decreased by retinol. In situ transamidase activity of transglutaminase was weak in retinol-pretreated skin. Therefore, it was indicated that functions other than transamidase of TG2/Gh protein might be important in retinol-induced epidermal mucous transdifferentiation.

Presence of tissue transglutaminase in granular endoplasmic reticulum is characteristic of melanized neurons in Parkinson's disease brain

Parkinson's disease (PD) is characterized by the accumulation of α-synuclein aggregates and degeneration of melanized neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross-linking. In PD brain, tTG-induced cross-links have been identified in α-synuclein monomers, oligomers and α-synuclein aggregates. However, whether tTG and α-synuclein occur together in PD affected neurons remains to be established. Interestingly, using immunohistochemistry, we observed a granular distribution pattern of tTG, characteristic of melanized neurons in PD brain. Apart from tTG, these granules were also positive for typical endoplasmic reticulum (ER)-resident chaperones, that is, protein disulphide isomerase, ERp57 and calreticulin, suggesting a direct link to the ER. Additionally, we observed the presence of phosphorylated pancreatic ER kinase (pPERK), a classical ER stress marker, in tTG granule positive neurons in PD brain, although no subcellular colocalization of tTG and pPERK was found. Our data therefore suggest that tTG localization to granular ER compartments is specific for stressed melanized neurons in PD brain. Moreover, as also α-synuclein aggregates were observed in tTG granule positive neurons, these results provide a clue to the cellular site of interaction between α-synuclein and tTG.

Proteomic analyses lead to a better understanding of celiac disease: focus on epitope recognition and autoantibodies

Proteomic technologies are being used with increasing frequency in the scientific community. In this review, we have highlighted their use in celiac disease (CD). The available techniques, which include two-dimensional (2D) gel electrophoresis, mass spectrometry, and antibody and tissue arrays, have been used to identify proteins or changes in protein expression specific to gut tissue from patients with CD. A number of studies have employed proteomic methodologies to determine the diagnostic biomarkers in body fluids or to examine changes in protein expression and posttranslational modifications during signaling. A fast technological development of these methods, along with the combination of classic techniques with proteomics, will lead to new discoveries, which will consent a better understanding of the pathogenesis of CD.

Type 2 transglutaminase in Huntington's disease: a double-edged sword with clinical potential

Huntington's disease (HD) is a dominant genetic neurodegenerative disorder. The pathology affects principally neurons in the basal ganglia circuits and terminates invariably in death. There is compelling necessity for safe and effective therapeutic strategies to arrest, or even retard the progression of the pathogenesis. Recent findings indicate the autophagy-lysosome systems as appealing targets for pharmacological intervention. Autophagy exerts a critical role in controlling neuronal protein homeostasis, which is perturbed in HD, and is compromised in the pathogenesis of several neurodegenerative diseases. Type 2 transglutaminase (TG2) plays an important role both in apoptosis and autophagy regulation, and accumulates at high levels in cells under stressful conditions. TG2 inhibition, achieved either via drug treatments or genetic approaches, has been shown to be beneficial for the treatment of HD in animal models. In this review we will discuss the relevance of TG2 to the pathogenesis of HD, in an effort to define novel therapeutic avenues.

Screening of substrate peptide sequences for tissue-type transglutaminase (TGase 2) using T7 phage cDNA library

Transglutaminase (TGase) is a family of enzymes that catalyzes cross-linking reaction between glutamine- and lysine residue of substrate proteins in several mammalian biological events. Substrate proteins for TGase and their physiological relevance have been still in research, continuously expanding. In this study, we have established a novel screening system that enables identification of cDNA sequence encoding favorable primary structure as a substrate for tissue-type transglutaminase (TGase 2), a multifunctional and ubiquitously expressing isozyme. By the screening, we identified several T7 phage clones that displayed substrate peptides for TGase 2 as a translated product from human brain cDNA library. Among the selected clones, the C-terminal region of IKAP, IkappaB kinase complex associated protein, appeared as a highly reactive substrate sequence for TGase 2. This system will open possibility of rapid identification of substrate sequences for transglutaminases at a genetic level.

Celiac anti-tissue transglutaminase antibodies interfere with the uptake of alpha gliadin peptide 31-43 but not of peptide 57-68 by epithelial cells

Celiac disease is characterized by the secretion of IgA-class autoantibodies that target tissue transglutaminase (tTG). It is now recognized that anti-tTG antibodies are functional and not mere bystanders in the pathogenesis of celiac disease. Here we report that interaction between anti-tTG antibodies and extracellular membrane-bound tTG inhibits peptide 31-43 (but not peptide 57-68) uptake by cells, thereby impairing the ability of p31-43 to drive Caco-2 cells into S-phase. This effect did not involve tTG catalytic activity. Because anti-tTG antibodies interfered with epidermal growth factor endocytosis, we assume that they exert their effect by reducing peptide 31-43 endocytosis. Our results suggest that cell-surface tTG plays a hitherto unknown role in the regulation of gliadin peptide uptake and endocytosis.

Transglutaminase 2: a multi-functional protein in multiple subcellular compartments

Transglutaminase 2 (TG2) is a multifunctional protein that can function as a transglutaminase, G protein, kinase, protein disulfide isomerase, and as an adaptor protein. These multiple biochemical activities of TG2 account for, at least in part, its involvement in a wide variety of cellular processes encompassing differentiation, cell death, inflammation, cell migration, and wound healing. The individual biochemical activities of TG2 are regulated by several cellular factors, including calcium, nucleotides, and redox potential, which vary depending on its subcellular location. Thus, the microenvironments of the subcellular compartments to which TG2 localizes, such as the cytosol, plasma membrane, nucleus, mitochondria, or extracellular space, are important determinants to switch on or off various TG2 biochemical activities. Furthermore, TG2 interacts with a distinct subset of proteins and/or substrates depending on its subcellular location. In this review, the biological functions and molecular interactions of TG2 will be discussed in the context of the unique environments of the subcellular compartments to which TG2 localizes.

Depletion of nucleophosmin via transglutaminase 2 cross-linking increases drug resistance in cancer cells

It has been suggested that nucleophosmin has an anti-apoptotic function via Bax binding. We found that nucleophosmin is a substrate of transglutaminase 2 (TGase 2) in cancer cells. Increased expression of TGase 2 expression is highly associated with drug resistance, and polymerization of nucleophosmin by TGase 2 also can be correlated with the drug resistance of cancer cells. In the present study, an accumulation of nucleophosmin in cytosol was detected when doxorubicin was treated to cancer cells, and it was found, moreover, that an increase of cytosolic nucleophosmin can result in drug-induced apoptosis. Nucleophosmin was polymerized by TGase 2, and the polymerization was inhibited with the TGase 2 inhibitor, cystamine, in vitro. The nucleophosmin level in the cytosolic cell fraction was reduced when TGase 2 was expressed, and the reduced nucleophosmin level was rescued by cystamine treatment. Moreover, nucleophosmin cross-linked by TGase 2 was eradicated in MCF7 cells via the ubiquitin-proteasomal pathway. In parallel with this nucleophosmin-level restoration, the pro-apoptotic Bax protein level was increased. Therefore, depletion of cytosolic nucleophosmin by TGase 2 can decrease Bax protein stability and lead to anti-apoptosis. Drug-resistant cancer cells became sensitive to doxorubicin treatment when nucleophosmin was expressed in cytosol. Taking these results together, it can be concluded that TGase 2 inhibits accumulation of cytosolic nucleophosmin through polymerization, which results in drug resistance in cancer cells.

A role for tissue transglutaminase in stabilization of membrane-cytoskeletal particles shed from the human placenta

Tissue transglutaminase (TGM2; also known as TG2 or tTG) localizes to the syncytial microvillous membrane (MVM) of the human placenta, the primary interface between maternal and fetal tissue. To identify TGM2 substrates in the MVM, membrane vesicles were prepared and labeled with biotinylated acyl donor or acceptor probes. Biotinylated species were selected on an avidin affinity matrix and identified by mass spectrometry of tryptic peptides. The most abundant were cytoskeletal (actin, tubulin, and cytokeratin) and membrane-associated (annexins, integrins, and placental alkaline phosphatase) proteins. During pregnancy, apoptotic particulate material, the end product of the trophoblast life cycle, is shed from the MVM into maternal circulation. Shed material was isolated from primary trophoblast cultures in which syncytial-like masses develop by fusion. A substantial fraction of actin in the particles was in the form of covalent polymeric aggregates, in contrast to cellular actin, which dissociated completely into monomer in SDS-PAGE. When cells were cultured in the presence of transglutaminase inhibitors, actin in the shed particles remained exclusively in monomeric form, and a reduction in trophoblast intercellular fusion and differentiation was observed. These findings suggest that transglutaminase-mediated cross-linking stabilizes the particulate material shed from the placenta.

Extracellular quality control in the epididymis

The epididymal lumen represents a unique extracellular environment because of the active sperm maturation process that takes place within its confines. Although much focus has been placed on the interaction of epididymal secretory proteins with spermatozoa in the lumen, very little is known regarding how the complex epididymal milieu as a whole is maintained, including mechanisms to prevent or control proteins that may not stay in their native folded state following secretion. Because some misfolded proteins can form cytotoxic aggregate structures known as amyloid, it is likely that control/surveillance mechanisms exist within the epididymis to protect against this process and allow sperm maturation to occur. To study protein aggregation and to identify extracellular quality control mechanisms in the epididymis, we used the cystatin family of cysteine protease inhibitors, including cystatin-related epididymal spermatogenic and cystatin C as molecular models because both proteins have inherent properties to aggregate and form amyloid. In this chapter, we present a brief summary of protein aggregation by the amyloid pathway based on what is known from other organ systems and describe quality control mechanisms that exist intracellularly to control protein misfolding and aggregation. We then present a summary of our studies of cystatin-related epididymal spermatogenic (CRES) oligomerization within the epididymal lumen, including studies suggesting that transglutaminase cross-linking may be one mechanism of extracellular quality control within the epididymis.

Phage display selection of efficient glutamine-donor substrate peptides for transglutaminase 2

Understanding substrate specificity and identification of natural targets of transglutaminase 2 (TG2), the ubiquitous multifunctional cross-linking enzyme, which forms isopeptide bonds between protein-linked glutamine and lysine residues, is crucial in the elucidation of its physiological role. As a novel means of specificity analysis, we adapted the phage display technique to select glutamine-donor substrates from a random heptapeptide library via binding to recombinant TG2 and elution with a synthetic amine-donor substrate. Twenty-six Gln-containing sequences from the second and third biopanning rounds were susceptible for TG2-mediated incorporation of 5-(biotinamido)penthylamine, and the peptides GQQQTPY, GLQQASV, and WQTPMNS were modified most efficiently. A consensus around glutamines was established as pQX(P,T,S)l, which is consistent with identified substrates listed in the TRANSDAB database. Database searches showed that several proteins contain peptides similar to the phage-selected sequences, and the N-terminal glutamine-rich domain of SWI1/SNF1-related chromatin remodeling proteins was chosen for detailed analysis. MALDI/TOF and tandem mass spectrometry-based studies of a representative part of the domain, SGYGQQGQTPYYNQQSPHPQQQQPPYS (SnQ1), revealed that Q(6), Q(8), and Q(22) are modified by TG2. Kinetic parameters of SnQ1 transamidation (K(M)(app) = 250 microM, k(cat) = 18.3 sec(-1), and k(cat)/K(M)(app) = 73,200) classify it as an efficient TG2 substrate. Circular dichroism spectra indicated that SnQ1 has a random coil conformation, supporting its accessibility in the full-length parental protein. Added together, here we report a novel use of the phage display technology with great potential in transglutaminase research.

Analysis of the ribosomal protein S19 interactome

Ribosomal protein S19 (RPS19) is a 16-kDa protein found mainly as a component of the ribosomal 40 S subunit. Its mutations are responsible for Diamond Blackfan anemia, a congenital disease characterized by defective erythroid progenitor maturation. Dysregulation of RPS19 has therefore been implicated in this defective erythropoiesis, although the link between them is still unclear. Two not mutually exclusive hypotheses have been proposed: altered protein synthesis and loss of unknown functions not directly connected with the structural role of RPS19 in the ribosome. A role in rRNA processing has been surmised for the yeast ortholog, whereas the extracellular RPS19 dimer has a monocyte chemotactic activity. Three proteins are known to interact with RPS19: FGF2, complement component 5 receptor 1, and a nucleolar protein called RPS19-binding protein. We have used a yeast two-hybrid approach to identify a fourth protein: the serine-threonine kinase PIM1. The present study describes our use of proteomics strategies to look for proteins interacting with RPS19 to determine its functions. Proteins were isolated by affinity purification with a GST-RPS19 recombinant protein and identified using LCMS/MS analysis coupled to bioinformatics tools. We identified 159 proteins from the following Gene Ontology categories: NTPases (ATPases and GTPases; five proteins), hydrolases/helicases (19 proteins), isomerases (two proteins), kinases (three proteins), splicing factors (five proteins), structural constituents of ribosome (29 proteins), transcription factors (11 proteins), transferases (five proteins), transporters (nine proteins), DNA/RNA-binding protein species (53 proteins), other (one dehydrogenase protein, one ligase protein, one peptidase protein, one receptor protein, and one translation elongation factor), and 13 proteins of still unknown function. Proteomics results were validated by affinity purification and Western blotting. These interactions were further confirmed by co-immunoprecipitation using a monoclonal RPS19 antibody. Many interactors are nucleolar proteins and thus are expected to take part in the RPS19 interactome; however, some proteins suggest additional functional roles for RPS19.

Characterization of red cell membrane proteins as a function of red cell density:

Fresh human blood samples were collected from healthy controls and splenectomized and unsplenectomized patients with hereditary spherocytosis due to band 3 or ankyrin and spectrin deficiency. The erythrocytes were separated into age-related fractions using self-forming Percoll density gradients. Membrane proteins were analysed by 2D electrophoresis and identified by mass spectrometry. Annexin VII was present in reticulocytes but was then lost as the cells matured. A different pattern was found in band 3-deficient samples: annexin VII was in fact present in both mature and immature red cell membranes. Cytoskeletal anomalies may then influence the turn-over of annexin VII during erythrocyte maturation.

Transglutaminase 2 ablation leads to defective function of mitochondrial respiratory complex I affecting neuronal vulnerability in experimental models of extrapyramidal disorders

Transglutaminase 2 (TG2) represents the most ubiquitous isoform belonging to the TG family, and has been implicated in the pathophysiology of basal ganglia disorders, such as Parkinson's disease and Huntington's disease. We show that ablation of TG2 in knockout mice causes a reduced activity of mitochondrial complex I associated with an increased activity of complex II in the whole forebrain and striatum. Interestingly, TG2-/- mice were protected against nigrostriatal damage induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, which is converted in vivo into the mitochondrial complex I inhibitor, 1-methyl-4-phenyl-pyridinium ion. In contrast, TG2-/- mice were more vulnerable to nigrostriatal damage induced by methamphetamine or by the complex II inhibitor, 3-nitropropionic acid. Proteomic analysis showed that proteins involved in the mitochondrial respiratory chain, such as prohibitin and the beta-chain of ATP synthase, are substrates for TG2. These data suggest that TG2 is involved in the regulation of the respiratory chain both in physiology and pathology, contributing to set the threshold for neuronal damage in extrapyramidal disorders.

"Tissue" transglutaminase contributes to the formation of disulphide bridges in proteins of mitochondrial respiratory complexes

In this study we provide the first in vivo evidences showing that, under physiological conditions, "tissue" transglutaminase (TG2) might acts as a protein disulphide isomerase (PDI) and through this activity contributes to the correct assembly of the respiratory chain complexes. Mice lacking TG2 exhibit mitochondrial energy production impairment, evidenced by decreased ATP levels after physical challenge. This defect is phenotypically reflected in a dramatic decrease of motor behaviour of the animals. We propose that the molecular mechanism, underlying such a phenotype, resides in a defective disulphide bonds formation in ATP synthase (complex V), NADH-ubiquinone oxidoreductase (complex I), succinate-ubiquinone oxidoreductase (complex II) and cytochrome c oxidase (complex IV). In addition, TG2-PDI might control the respiratory chain by modulating the formation of the prohibitin complexes. These data elucidate a new pathway that directly links the TG2-PDI enzymatic activity with the regulation of mitochondrial respiratory chain function.

Interaction proteomics

The term proteome is traditionally associated with the identification of a large number of proteins within complex mixtures originating from a given organelle, cell or even organism. Current proteome investigations are basically focused on two major areas, expression proteomics and functional proteomics. Both approaches rely on the fractionation of protein mixtures essentially by two-dimensional polyacrylamide gel electrophoresis (2D-gel) and the identification of individual protein bands by mass spectrometric techniques (2D-MS). Functional proteomics approaches are basically addressing two main targets, the elucidation of the biological function of unknown proteins and the definition of cellular mechanisms at the molecular level. In the cell many processes are governed not only by the relative abundance of proteins but also by rapid and transient regulation of activity, association and localization of proteins and protein complexes. The association of an unknown protein with partners belonging to a specific protein complex involved in a particular process would then be strongly suggestive of its biological function. The identification of interacting proteins in stable complexes in a cellular system is essentially achieved by affinity-based procedures. Different strategies relying on this simple concept have been developed and a brief overview of the main approaches presently used in functional proteomics studies is described.

Mammalian transglutaminases. Identification of substrates as a key to physiological function and physiopathological relevance

Transglutaminases form a large family of intracellular and extracellular enzymes that catalyse the Ca2+-dependent post-translational modification of proteins. Despite significant advances in our understanding of the biological role of most mammalian transglutaminase isoforms, recent findings suggest new scenarios, most notably for the ubiquitous tissue transglutaminase. It is becoming apparent that some transglutaminases, normally expressed at low levels in many tissue types, are activated and/or overexpressed in a variety of diseases, thereby resulting in enhanced concentrations of cross-linked proteins. As applies to all enzymes that exert their metabolic function by modifying the properties of target proteins, the identification and characterization of the modified proteins will cast light on the functions of transglutaminases and their involvement in human diseases. In this paper we review data on the properties of mammalian transglutaminases, particularly as regards their protein substrates and the relevance of transglutaminase-catalysed reactions in physiological and disease conditions.

Alpha1B-adrenoceptor signaling and cell motility: GTPase function of Gh/transglutaminase 2 inhibits cell migration through interaction with cytoplasmic tail of integrin alpha subunits

A multifunctional enzyme, G(h), is a GTP-binding protein that couples to the alpha(1B)-adrenoreceptor and stimulates phospholipase C-delta1 but also displays transglutaminase 2 (TG2) activity. G(h)/TG2 has been implicated to play a role in cell motility. In this study we have examined which function of G(h)/TG2 is involved in this cellular response and the molecular basis. Treatment of human aortic smooth muscle cell with epinephrine inhibits migration to fibronectin and vitronectin, and the inhibition is blocked by the alpha(1)-adrenoreceptor antagonist prazosin or chloroethylclonidine. Up-regulation or overexpression of G(h)/TG2 in human aortic smooth muscle cells, DDT1-MF2, or human embryonic kidney cells, HEK 293 cells, results in inhibition of the migratory activity, and stimulation of the alpha(1B)-adrenoreceptor with the alpha(1) agonist further augments the inhibition of migration of human aortic smooth muscle cells and DDT1-MF2. G(h)/TG2 is coimmunoprecipitated by an integrin alpha(5) antibody and binds to the cytoplasmic tail peptide of integrins alpha(5), alpha(v), and alpha(IIb) subunits in the presence of guanosine 5'-3-O-(thio)triphosphate (GTPgammaS). Mutation of Lys-Arg residues in the GFFKR motif, present in the alpha(5)-tail, significantly reduces the binding of GTPgammaS-G(h)/TG2. Moreover, the motif-containing integrin alpha(5)-tail peptides block G(h)/TG2 coimmunoprecipitation and reverse the inhibition of the migratory activity of HEK 293 cells caused by overexpression G(h)/TG2. These results provide evidence that G(h) function initiates the modulation of cell motility via association of GTP-bound G(h)/TG2 with the GFFKR motif located in integrin alpha subunits.