Glucose homeostasis in mice is transglutaminase 2 independent

Transglutaminase 2 Facilitates Murine Wound Healing in a Strain-Dependent Manner

Transglutaminase 2 (TG2) plays a role in cellular processes that are relevant to wound healing, but to date no studies of wound healing in TG2 knockout mice have been reported. Here, using 129T2/SvEmsJ (129)- or C57BL/6 (B6)-backcrossed TG2 knockout mice, we show that TG2 facilitates murine wound healing in a strain-dependent manner. Early healing of in vivo cutaneous wounds and closure of in vitro scratch wounds in murine embryonic fibroblast (MEF) monolayers were delayed in 129, but not B6, TG2 knockouts, relative to their wild-type counterparts, with wound closure in 129 being faster than in B6 wild-types. A single dose of exogenous recombinant wild-type TG2 to 129 TG2-/- mice or MEFs immediately post-wounding accelerated wound closure. Neutrophil and monocyte recruitment to 129 cutaneous wounds was not affected by Tgm2 deletion up to 5 days post-wounding. Tgm2 mRNA and TG2 protein abundance were higher in 129 than in B6 wild-types and increased in abundance following cutaneous and scratch wounding. Tgm1 and factor XIIA (F13A) mRNA abundance increased post-wounding, but there was no compensation by TG family members in TG2-/- relative to TG2+/+ mice in either strain before or after wounding. 129 TG2+/+ MEF adhesion was greater and spreading was faster than that of B6 TG2+/+ MEFs, and was dependent on syndecan binding in the presence, but not absence, of RGD inhibition of integrin binding. Adhesion and spreading of 129, but not B6, TG2-/- MEFs was impaired relative to their wild-type counterparts and was accelerated by exogenous addition or transfection of TG2 protein or cDNA, respectively, and was independent of the transamidase or GTP-binding activity of TG2. Rho-family GTPase activation, central to cytoskeletal organization, was altered in 129 TG2-/- MEFs, with delayed RhoA and earlier Rac1 activation than in TG2+/+ MEFs. These findings indicate that the rate of wound healing is different between 129 and B6 mouse strains, correlating with TG2 abundance, and although not essential for wound healing, TG2 facilitates integrin- and syndecan-mediated RhoA- and Rac1-activation in fibroblasts to promote efficient wound contraction.

A long non-coding RNA that harbors a SNP associated with type 2 diabetes regulates the expression of TGM2 gene in pancreatic beta cells

INTRODUCTION

Most of the disease-associated single nucleotide polymorphisms (SNPs) lie in non- coding regions of the human genome. Many of these variants have been predicted to impact the expression and function of long non-coding RNAs (lncRNA), but the contribution of these molecules to the development of complex diseases remains to be clarified.

METHODS

Here, we performed a genetic association study between a SNP located in a lncRNA known as LncTGM2 and the risk of developing type 2 diabetes (T2D), and analyzed its implication in disease pathogenesis at pancreatic beta cell level. Genetic association study was performed on human samples linking the rs2076380 polymorphism with T2D and glycemic traits. The pancreatic beta cell line EndoC-bH1 was employed for functional studies based on LncTGM2 silencing and overexpression experiments. Human pancreatic islets were used for eQTL analysis.

RESULTS

We have identified a genetic association between LncTGM2 and T2D risk. Functional characterization of the LncTGM2 revealed its implication in the transcriptional regulation of TGM2, coding for a transglutaminase. The T2Dassociated risk allele in LncTGM2 disrupts the secondary structure of this lncRNA, affecting its stability and the expression of TGM2 in pancreatic beta cells. Diminished LncTGM2 in human beta cells impairs glucose-stimulated insulin release.

CONCLUSIONS

These findings provide novel information on the molecular mechanisms by which T2D-associated SNPs in lncRNAs may contribute to disease, paving the way for the development of new therapies based on the modulation of lncRNAs.

Transglutaminase 2 mediates UVB-induced matrix metalloproteinase-1 expression by inhibiting nuclear p65 degradation in dermal fibroblasts

Matrix metalloproteinases (MMPs) play a key role in tissue remodelling by cleaving extracellular matrix (ECM) components. In the skin, UV irradiation increases expression of MMPs that causes dysregulation of ECM homeostasis in dermis, leading to acceleration of skin aging. However, the mediator(s) that links UV irradiation to the upregulation of MMPs have not been fully defined. Previously, we showed that UVB irradiation activated transglutaminase 2 (TG2) in keratinocytes, eliciting an inflammatory response by activating NF-κB signalling. In this study, we reported the role of TG2 in mediating the UVB-induced expression of MMP-1. In human dermal fibroblasts, UVB irradiation enhanced the expression and activity of TG2, which in turn promotes the expression of MMP-1. Analyses of MMP-1 promoter showed that activation of the NF-κB signalling pathway, rather than AP-1, was responsible for the TG2-mediated upregulation of MMP-1. Moreover, Western blot analysis revealed that TG2 increased the activity of NF-κB by inhibiting degradation of p65 in the nucleus. Furthermore, ex vivo skin from TG2-knockout mice exhibited significantly reduced levels of MMP-1 compared to that from wild-type mice. These results indicate that TG2 functions as a mediator for the UVB-induced expression of MMP-1 in dermal fibroblasts, providing a new target for preventing skin photodamage.

Adaptation to exercise-induced stress is not dependent on cardiomyocyte α1A-adrenergic receptors

The 'fight or flight' response to physiological stress involves sympathetic nervous system activation, catecholamine release and adrenergic receptor stimulation. In the heart, this induces positive inotropy, previously attributed to the β₁-adrenergic receptor subtype. However, the role of the α_1A-adrenergic receptor, which has been suggested to be protective in cardiac pathology, has not been investigated in the setting of physiological stress. To explore this, we developed a tamoxifen-inducible, cardiomyocyte-specific α_1A-adrenergic receptor knock-down mouse model, challenged mice to four weeks of endurance swim training and assessed cardiac outcomes. With 4-OH tamoxifen treatment, expression of the α_1A-adrenergic receptor was knocked down by 80-89%, without any compensatory changes in the expression of other adrenergic receptors, or changes to baseline cardiac structure and function. Swim training caused eccentric hypertrophy, regardless of genotype, demonstrated by an increase in heart weight/tibia length ratio (30% and 22% in vehicle- and tamoxifen-treated animals, respectively) and an increase in left ventricular end diastolic volume (30% and 24% in vehicle- and tamoxifen-treated animals, respectively) without any change in the wall thickness/chamber radius ratio. Consistent with physiological hypertrophy, there was no increase in fetal gene program (Myh7, Nppa, Nppb or Acta1) expression. In response to exercise-induced volume overload, stroke volume (39% and 30% in vehicle- and tamoxifen-treated animals, respectively), cardiac output/tibia length ratio (41% in vehicle-treated animals) and stroke work (61% and 33% in vehicle- and tamoxifen-treated animals, respectively) increased, regardless of genotype. These findings demonstrate that cardiomyocyte α_1A-adrenergic receptors are not necessary for cardiac adaptation to endurance exercise stress and their acute ablation is not deleterious.

Transglutaminase 2 as a novel target in chronic kidney disease - Methods, mechanisms and pharmacological inhibition

Chronic kidney disease (CKD) is a global health problem with a prevalence of 10-15%. Progressive fibrosis of the renal tissue is a main feature of CKD, but current treatment strategies are relatively unspecific and delay, but do not prevent, CKD. Exploration of novel pharmacological targets to inhibit fibrosis development are therefore important. Transglutaminase 2 (TG2) is known to be central for extracellular collagenous matrix formation, but TG2 is a multifunctional enzyme and novel research has broadened our view on its extra- and intracellular actions. TG2 exists in two conformational states with different catalytic properties as determined by substrate availability and local calcium concentrations. The open conformation of TG2 depends on calcium and has transamidase activity, central for protein modification and cross-linking of extracellular protein components, while the closed conformation is a GTPase involved in transmembrane signaling processes. We first describe different methodologies to assess TG2 activity in renal tissue and cell cultures such as biotin cadaverine incorporation. Then we systematically review animal CKD models and preliminary studies in humans (with diabetic, IgA- and chronic allograft nephropathy) to reveal the role of TG2 in renal fibrosis. Mechanisms behind TG2 activation, TG2 externalization dependent on Syndecan-4 and interactions between TG and profibrotic molecules including transforming growth factor β and the angiotensin II receptor are discussed. Pharmacological TG2 inhibition shows antifibrotic effects in CKD. However, the translation of TG2 inhibition to treat CKD in patients is a challenge as clinical information is limited, and further studies on pharmacokinetics and efficacy of the individual compounds are required.

Transglutaminase 2 limits the extravasation and the resultant myocardial fibrosis associated with factor XIII-A deficiency

Background and aims

Transglutaminase (TG) 2 and Factor (F) XIII-A have both been implicated in cardiovascular protection and repair. This study was designed to differentiate between two competing hypotheses: that TG2 and FXIII-A mediate these functions in mice by fulfilling separate roles, or that they act redundantly in this respect.

Methods

Atherosclerosis was assessed in brachiocephalic artery plaques of fat-fed mixed strain apolipoprotein (Apo)e deficient mice that lacked either or both transglutaminases. Cardiac fibrosis was assessed both in the mixed strain mice and also in C57BL/6J Apoe expressing mice lacking either or both transglutaminases.

Results

No difference was found in the density of buried fibrous caps within brachiocephalic plaques from mice expressing or lacking these transglutaminases. Cardiac fibrosis developed in both Apoe/F13a1 double knockout and F13a1 single knockout mice, but not in Tgm2 knockout mice. However, concomitant Tgm2 knockout markedly increased fibrosis, as apparent in both Apoe/Tgm2/F13a1 knockout and Tgm2/F13a1 knockout mice. Amongst F13a1 knockout and Tgm2/F13a1 knockout mice, the extent of fibrosis correlated with hemosiderin deposition, suggesting that TG2 limits the extravasation of blood in the myocardium, which in turn reduces the pro-fibrotic stimulus. The resulting fibrosis was interstitial in nature and caused only minor changes in cardiac function.

Conclusions

These studies confirm that FXIII-A and TG2 fulfil different roles in the mouse myocardium. FXIII-A protects against vascular leakage while TG2 contributes to the stability or repair of the vasculature. The protective function of TG2 must be considered when designing clinical anti-fibrotic therapies based upon FXIII-A or TG2 inhibition.

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Double transglutaminase 2 and Factor XIII-A knockout exacerbates cardiac fibrosis.

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Double knockout does not promote the growth of, or destabilise, brachiocephalic plaques.

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FXIII-A in resident cardiac macrophages does not protect against cardiac fibrosis.

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FXIII-A in inflammatory macrophages may contribute to protection against fibrosis.

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Transglutaminase 2 and Factor XIII-A protect against extravasation of blood.

Adhesion G-protein coupled receptors: Implications for metabolic function

Adhesion G-protein coupled receptors (aGPCRs) are emerging as important actors in energy homeostasis. Recent biochemical and functional studies using transgenic mice indicate that aGPCRs play important roles in endocrine and metabolic functions including β-cell differentiation, insulin secretion, adipogenesis and whole body fuel homeostasis. Most aGPCRs are orphans, for which endogenous ligands have not yet been identified, and many of the endogenous ligands of the already de-orphanised aGPCRs are components of the extracellular matrix (ECM). In this review we focus on aGPCR expression in metabolically active tissues, their activation by ECM proteins, and current knowledge of their potential roles in islet development, insulin secretion, adipogenesis and muscle function.

Chronic liver injury drives non-traditional intrahepatic fibrin(ogen) crosslinking via tissue transglutaminase

Essentials Fibrin clots are often implicated in the progression of liver fibrosis. Liver fibrosis was induced in transgenic mice with defects in clot formation or stabilization. Liver fibrosis and fibrin(ogen) deposition do not require fibrin polymerization or factor XIIIa. Fibrin(ogen) is an in vivo substrate of tissue transglutaminase in experimental liver fibrosis. SUMMARY: Background Intravascular fibrin clots and extravascular fibrin deposits are often implicated in the progression of liver fibrosis. However, evidence supporting a pathological role of fibrin in hepatic fibrosis is indirect and based largely on studies using anticoagulant drugs that inhibit activation of the coagulation protease thrombin, which has other downstream targets that promote fibrosis. Therefore, the goal of this study was to determine the precise role of fibrin deposits in experimental hepatic fibrosis. Methods Liver fibrosis was induced in mice expressing mutant fibrinogen insensitive to thrombin-mediated proteolysis (i.e. locked in the monomeric form), termed FibAEK mice, and factor XIII A2 subunit-deficient (FXIII-/- ) mice. Female wild-type mice, FXIII-/- mice and homozygous FibAEK mice were challenged with carbon tetrachloride (CCl4 ) twice weekly for 4 weeks or 6 weeks (1 mL kg-1 , intraperitoneal). Results Hepatic injury and fibrosis induced by CCl4 challenge were unaffected by FXIII deficiency or inhibition of thrombin-catalyzed fibrin polymer formation (in FibAEK mice). Surprisingly, hepatic deposition of crosslinked fibrin(ogen) was not reduced in CCl4 -challenged FXIII-/- mice or FibAEK mice as compared with wild-type mice. Rather, deposition of crosslinked hepatic fibrin(ogen) following CCl4 challenge was dramatically reduced in tissue transglutaminase-2 (TGM2)-deficient (TGM2-/- ) mice. However, the reduction in crosslinked fibrin(ogen) in TGM2-/- mice did not affect CCl4 -induced liver fibrosis. Conclusions These results indicate that neither traditional fibrin clots, formed by the thrombin-activated FXIII pathway nor atypical TGM2-crosslinked fibrin(ogen) contribute to experimental CCl4 -induced liver fibrosis. Collectively, the results indicate that liver fibrosis occurs independently of intrahepatic fibrin(ogen) deposition.

Transglutaminase in Receptor and Neurotransmitter-Regulated Functions

Transglutaminases (TGs) and especially TG2 play important roles in neurotransmitter and receptor signaling pathways. Three different mechanisms by which TG2 interacts with neurotransmitter and receptor signaling systems will be discussed in this review. The first way in which TG2 interacts with receptor signaling is via its function as a guanine nucleotide binding protein (G-protein) coupling to G-protein coupled receptors (GPCRs) to activate down-stream signaling pathways. TG2 can exist in a least two conformations, a closed GTP-bound conformation and an open calcium-bound conformation. In the closed GTP-bound conformation, TG2 is capable of functioning as a G-protein for GPCRs. In the open calcium-bound conformation, TG2 catalyzes a transamidation reaction cross-linking proteins or catalyzing the covalent binding of a mono- or polyamine to a protein. The second mechanism is regulation of the transamidation reaction catalyzed by TG2 via receptor stimulation which can increase local calcium concentrations and thereby increase transamidation reactions. The third way in which TG2 plays a role in neurotransmitter and receptor signaling systems is via its use of monoamine neurotransmitters as a substrate. Monoamine neurotransmitters including serotonin can be substrates for transamidation to a protein often a small G-protein (also known as a small GTPase) resulting in activation of the small G-protein. The transamidation of a monoamine neurotransmitter or serotonin has been designated as monoaminylation or more specifically serotonylation, respectively. Other proteins are also targets for monoaminylation such as fibronectin and cytoskeletal proteins. These receptor and neurotransmitter-regulated reactions by TG2 play roles in physiological and key pathophysiological processes.

Role of Transglutaminase 2 in Migration of Tumor Cells and How Mouse Models Fit

A search for the "magic bullet", a molecule, the targeting abilities of which could stop the migration of tumor cells, is currently underway, but remains in the early stages. There are still many unknowns regarding the cell migration. The main approach is the employment of mouse models, that are sources of valuable information, but still cannot answer all of the questions. One of the molecules of interest is Transglutaminase 2 (TG2). It is a well-described molecule involved in numerous pathways and elevated in metastatic tumors. The question remains whether mice and humans can give the same answer considering TG2.

Spotlight on the transglutaminase 2 gene: a focus on genomic and transcriptional aspects

The type 2 isoenzyme is the most widely expressed transglutaminase in mammals displaying several intra- and extracellular activities depending on its location (protein modification, modulation of gene expression, membrane signalling and stabilization of cellular interactions with the extracellular matrix) in relation to cell death, survival and differentiation. In contrast with the appreciable knowledge about the regulation of the enzymatic activities, much less is known concerning its inducible expression, which is altered in inflammatory and neoplastic diseases. In this context, we first summarize the gene's basic features including single-nucleotide polymorphism characterization, epigenetic DNA methylation and identification of regulatory regions and of transcription factor-binding sites at the gene promoter, which could concur to direct gene expression. Further aspects related to alternative splicing events and to ncRNAs (microRNAs and lncRNAs) are involved in the modulation of its expression. Notably, this important gene displays transcriptional variants relevant for the protein's function with the occurrence of at least seven transcripts which support the synthesis of five isoforms with modified catalytic activities. The different expression of the TG2 (type 2 transglutaminase) variants might be useful for dictating the multiple biological features of the protein and their alterations in pathology, as well as from a therapeutic perspective.

Cardiac hypertrophy limits infarct expansion after myocardial infarction in mice

We have previously demonstrated that adult transgenic C57BL/6J mice with CM-restricted overexpression of the dominant negative W v mutant protein (dn-c-kit-Tg) respond to pressure overload with robust cardiomyocyte (CM) cell cycle entry. Here, we tested if outcomes after myocardial infarction (MI) due to coronary artery ligation are improved in this transgenic model. Compared to non-transgenic littermates (NTLs), adult male dn-c-kit-Tg mice displayed CM hypertrophy and concentric left ventricular (LV) hypertrophy in the absence of an increase in workload. Stroke volume and cardiac output were preserved and LV wall stress was markedly lower than that in NTLs, leading to a more energy-efficient heart. In response to MI, infarct size in adult (16-week old) dn-c-kit-Tg hearts was similar to that of NTL after 24 h but was half that in NTL hearts 12 weeks post-MI. Cumulative CM cell cycle entry was only modestly increased in dn-c-kit-Tg hearts. However, dn-c-kit-Tg mice were more resistant to infarct expansion, adverse LV remodelling and contractile dysfunction, and suffered no early death from LV rupture, relative to NTL mice. Thus, pre-existing cardiac hypertrophy lowers wall stress in dn-c-kit-Tg hearts, limits infarct expansion and prevents death from myocardial rupture.

Transglutaminases in autoimmune and inherited skin diseases: The phenomena of epitope spreading and functional compensation

Transglutaminases (TGs) are structurally and functionally related enzymes that modify the post-translational structure and activity of proteins or peptides, and thus are able to turn on or switch off their function. Depending on location and activities, TGs are able to modify the signalling, the function and the fate of cells and extracellular connective tissues. Besides mouse models, human diseases enable us to appreciate the function of various TGs. In this study, skin diseases induced by genetic damages or autoimmune targeting of these enzymes will be discussed. TG1, TG3 and TG5 contribute to the cutaneous barrier and thus to the integrity and function of epidermis. TGM1 mutations related to autosomal recessive ichthyosis subtypes, TGM5 mutations to a mild epidermolysis bullosa phenotype and as novelty TGM3 mutation to uncombable hair syndrome will be discussed. Autoimmunity to TG2, TG3 and TG6 may develop in a few of those genetically determined individuals who lost tolerance to gluten, and manifest as coeliac disease, dermatitis herpetiformis or gluten-dependent neurological symptoms, respectively. These gluten responder diseases commonly occur in combination. In autoimmune diseases, the epitope spreading is remarkable, while in some inherited pathologies, a unique compensation of the lost enzyme function is noted.

Transglutaminase 2 mediates UV-induced skin inflammation by enhancing inflammatory cytokine production

UV irradiation elicits acute inflammation in the skin by increasing proinflammatory cytokine production in keratinocytes. However, the downstream protein target(s) that link UV radiation to the activation of signaling pathways responsible for cytokine expression have not been fully elucidated. In this study, we report a novel role of transglutaminase 2 (TG2), a member of the TG enzyme family whose activities are critical for cornified envelope formation, in mediating UV-induced inflammation. Our results showed that TG2-deficient mice exhibited reduced inflammatory responses to UV irradiation, including reduced erythema, edema, dilation of blood vessels, inflammatory cell infiltration, and levels of inflammatory cytokines. Using primary mouse keratinocytes and HaCaT cells, we found that UV irradiation-induced cytokine production by activating TG2, but not by upregulating TG2 expression, and that ER calcium release triggered by the UV-induced activation of phospholipase C was required for TG2 activation. Moreover, TG2 activity enhanced p65 phosphorylation, leading to an increase in NF-κB transcriptional activity. These results indicate that TG2 is a critical mediator of cytokine expression in the UV-induced inflammatory response of keratinocytes, and suggest that TG2 inhibition might be useful for preventing UV-related skin disorders, such as photoaging and skin cancer caused by chronic UV exposure.

Browning deficiency and low mobilization of fatty acids in gonadal white adipose tissue leads to decreased cold-tolerance of transglutaminase 2 knock-out mice

During cold-exposure 'beige' adipocytes with increased mitochondrial content are activated in white adipose tissue (WAT). These cells, similarly to brown adipose tissue (BAT), dissipate stored chemical energy in the form of heat with the help of uncoupling protein 1 (UCP1). We investigated the effect of tissue transglutaminase (TG2) ablation on the function of ATs in mice. Although TG2^+/+ and TG2^-/- mice had the same amount of WAT and BAT, we found that TG2^+/+ animals could tolerate acute cold exposure for 4h, whereas TG2^-/- mice only for 3h. Both TG2^-/- and TG2^+/+ animals used up half of the triacylglycerol content of subcutaneous WAT (SCAT) after 3h treatment; however, TG2^-/- mice still possessed markedly whiter and higher amount of gonadal WAT (GONAT) as reflected in the larger size of adipocytes and lower free fatty acid levels in serum. Furthermore, lower expression of 'beige' marker genes such as UCP1, TBX1 and TNFRFS9 was observed after cold exposure in GONAT of TG2^-/- mice, paralleled with a lower level of UCP1 protein and a decreased mitochondrial content. The detected changes in gene expression of Resistin and Adiponectin did not provoke glucose intolerance in the investigated TG2^-/- mice, and TG2 deletion did not influence adrenaline, noradrenaline, glucagon and insulin production. Our data suggest that TG2 has a tissue-specific role in GONAT function and browning, which becomes apparent under acute cold exposure.

Genomic variants reveal differential evolutionary constraints on human transglutaminases and point towards unrecognized significance of transglutaminase 2

Transglutaminases (TGMs) catalyze Ca2+-dependent transamidation of proteins with specified roles in blood clotting (F13a) and in cornification (TGM1, TGM3). The ubiquitous TGM2 has well described enzymatic and non-enzymatic functions but in-spite of numerous studies its physiological function in humans has not been defined. We compared data on non-synonymous single nucleotide variations (nsSNVs) and loss-of-function variants on TGM1-7 and F13a from the Exome aggregation consortium dataset, and used computational and biochemical analysis to reveal the roles of damaging nsSNVs of TGM2. TGM2 and F13a display rarer damaging nsSNV sites than other TGMs and sequence of TGM2, F13a and TGM1 are evolutionary constrained. TGM2 nsSNVs are predicted to destabilize protein structure, influence Ca2+ and GTP regulation, and non-enzymatic interactions, but none coincide with conserved functional sites. We have experimentally characterized six TGM2 allelic variants detected so far in homozygous form, out of which only one, p.Arg222Gln, has decreased activities. Published exome sequencing data from various populations have not uncovered individuals with homozygous loss-of-function variants for TGM2, TGM3 and TGM7. Thus it can be concluded that human transglutaminases differ in harboring damaging variants and TGM2 is under purifying selection suggesting that it may have so far not revealed physiological functions.

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.

Transglutaminase as a therapeutic target for celiac disease

INTRODUCTION

The only current treatment for celiac disease is a strict gluten-free diet. The ubiquitous presence of gluten in groceries, however, makes the diet burdensome and difficult to maintain, and alternative treatment options are thus needed. Here, the important role of transglutaminase 2 (TG2) in the pathogenesis of celiac disease makes it an attractive target for drug development.

AREAS COVERED

The present paper gives an overview of TG2 and addresses its significance in the pathogenesis of celiac disease. Moreover, the article summarizes preclinical studies performed with TG2 inhibitors and scrutinizes issues related to this therapeutic approach.

EXPERT OPINION

Activation of TG2 in the intestinal mucosa is central in celiac disease pathogenesis and researchers have therefore suggested TG2 inhibitors as a potential therapeutic approach. However, a prerequisite for such a drug is that it should be specific for TG2 and not affect the activity of other members of the transglutaminase family. Such compounds have already been introduced and tested in vitro, but a major obstacle to further development is the lack of a well-defined animal model for celiac disease. Nonetheless, with encouraging results in preclinical studies clinical trials with TG2 inhibitors are eagerly awaited.

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.

A possible role of transglutaminase 2 in the nucleus of INS-1E and of cells of human pancreatic islets

Transglutaminase 2 (TG2) is a multifunctional protein with Ca^2 +-dependent transamidating and G protein activity. Previously we reported that the role of TG2 in insulin secretion may involve cytoplasmic actin remodeling and a regulative action on other proteins during granule movement. The aim of this study was to gain a better insight into the role of TG2 transamidating activity in mitochondria and in the nucleus of INS-1E rat insulinoma cell line (INS-1E) during insulin secretion. To this end we labeled INS-1E with an artificial donor (biotinylated peptide), in basal condition and after stimulus with glucose for 2, 5, and 8 min. Biotinylated proteins of the nuclear/mitochondrial-enriched fraction were analyzed using two-dimensional electrophoresis and mass spectrometry. Many mitochondrial proteins involved in Ca^2 + homeostasis (e.g. voltage-dependent anion-selective channel protein, prohibitin and different ATP synthase subunits) and many nuclear proteins involved in gene regulation (e.g. histone H3, barrier to autointegration factor and various heterogeneous nuclear ribonucleoprotein) were identified among a number of transamidating substrates of TG2 in INS-1E. The combined results provide evidence that a temporal link exists between glucose-stimulation, first phase insulin secretion and the action of TG on histone H3 both in INS-1E and human pancreatic islets.

Biological significance

Research into the role of transglutaminase 2 during insulin secretion in INS-1E rat insulinoma cellular model is depicting a complex role for this enzyme. Transglutaminase 2 acts in the different INS-1E compartments in the same way: catalyzing a post-translational modification event of its substrates. In this work we identify some mitochondrial and nuclear substrates of INS-1E during first phase insulin secretion. The finding that TG2 interacts with nuclear proteins that include BAF and histone H3 immediately after (2–5 min) glucose stimulus of INS-1E suggests that TG2 may be involved not only in insulin secretion, as suggested by our previous studies in cytoplasmic INS-1E fraction, but also in the regulation of glucose-induced gene transcription.

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Transglutaminase 2 localizes in the nucleus and in the mitochondrion of INS-1E.

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TG2 acts as a modifying enzyme in both compartments during FPIS.

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TG2 may contribute to Ca^2 + sensing in mitochondrion through its substrates.

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TG2 may contribute to chromatin condensation in nucleus through its substrates.