Transglutaminase type 2 in the regulation of proteostasis

Epigenetic modulation of RIPK3 by transglutaminase 2-dependent serotonylation of H3K4me3 affects necroptosis

The receptor interacting protein kinase 3 (RIPK3) is the main player in the activation of necroptosis, a pro-inflammatory regulated cell death modality induced by many different stimuli. RIPK3 is epigenetically regulated by DNA methylation and can be expressed when its promoter is associated with H3K4me3 histone. In this study, we show that Transglutaminase 2 protein (TG2) is necessary to induce necroptosis pathway allowing the expression of Ripk3 gene. Indeed, cells lacking TG2 show a strong downregulation of Ripk3 gene and are resistant to necroptotic stimuli. TG2 is known to promote the serotonylation of H3K4me3 histone (H3K4me3Q5ser) regulating in this way the target gene expression. Interestingly, we find that TG2 interacts with both histones H3K4me3 and H3K4me3Q5ser and these post-translational modifications are associated with the Ripk3 promoter only in presence of TG2. In addition, the absence of the H3K4me3Q5ser, in cells lacking TG2, is correlated with Ripk3 gene methylation. Altogether, these results indicate that RIPK3 expression requires TG2 mediated serotonylation of H3K4me3 to prevent Ripk3 promoter methylation, thus favouring its expression and necroptosis induction.

Transglutaminase Type 2-MITF axis regulates phenotype switching in skin cutaneous melanoma

Skin cutaneous melanoma (SKCM) is the deadliest form of skin cancer due to its high heterogeneity that drives tumor aggressiveness. Melanoma plasticity consists of two distinct phenotypic states that co-exist in the tumor niche, the proliferative and the invasive, respectively associated with a high and low expression of MITF, the master regulator of melanocyte lineage. However, despite efforts, melanoma research is still far from exhaustively dissecting this phenomenon. Here, we discovered a key function of Transglutaminase Type-2 (TG2) in regulating melanogenesis by modulating MITF transcription factor expression and its transcriptional activity. Importantly, we demonstrated that TG2 expression affects melanoma invasiveness, highlighting its positive value in SKCM. These results suggest that TG2 may have implications in the regulation of the phenotype switching by promoting melanoma differentiation and impairing its metastatic potential. Our findings offer potential perspectives to unravel melanoma vulnerabilities via tuning intra-tumor heterogeneity.

Lysyl Oxidases as Targets for Cancer Therapy and Diagnostic Imaging

The understanding of the contribution of the tumour microenvironment to cancer progression and metastasis, in particular the interplay between tumour cells, fibroblasts and the extracellular matrix has grown tremendously over the last years. Lysyl oxidases are increasingly recognised as key players in this context, in addition to their function as drivers of fibrotic diseases. These insights have considerably stimulated drug discovery efforts towards lysyl oxidases as targets over the last decade. This review article summarises the biochemical and structural properties of theses enzymes. Their involvement in tumour progression and metastasis is highlighted from a biochemical point of view, taking into consideration both the extracellular and intracellular action of lysyl oxidases. More recently reported inhibitor compounds are discussed with an emphasis on their discovery, structure-activity relationships and the results of their biological characterisation. Molecular probes developed for imaging of lysyl oxidase activity are reviewed from the perspective of their detection principles, performance and biomedical applications.

Peculiar Ca2+ Homeostasis, ER Stress, Autophagy, and TG2 Modulation in Celiac Disease Patient-Derived Cells

Celiac disease (CD) is an inflammatory intestinal disease caused by the ingestion of gluten-containing cereals by genetically predisposed individuals. Constitutive differences between cells from CD patients and control subjects, including levels of protein phosphorylation, alterations of vesicular trafficking, and regulation of type 2 transglutaminase (TG2), have been reported. In the present work, we investigated how skin-derived fibroblasts from CD and control subjects responded to thapsigargin, an endoplasmic reticulum ER stress inducer, in an attempt to contribute to the comprehension of molecular features of the CD cellular phenotype. We analyzed Ca²⁺ levels by single-cell video-imaging and TG2 activity by a microplate assay. Western blots and PCR analyses were employed to monitor TG2 levels and markers of ER stress and autophagy. We found that the cytosolic and ER Ca²⁺ level of CD cells was lower than in control cells. Treatments with thapsigargin differently activated TG2 in control and CD cells, as well as caused slightly different responses regarding the activation of ER stress and the expression of autophagic markers. On the whole, our findings identified further molecular features of the celiac cellular phenotype and highlighted that CD cells appeared less capable of adapting to a stress condition and responding in a physiological way.

Type 2 transglutaminase in the nucleus: the new epigenetic face of a cytoplasmic enzyme

One of the major mysteries in science is how it is possible to pack the cellular chromatin with a total length of over 1 m, into a small sphere with a diameter of 5 mm "the nucleus", and even more difficult to envisage how to make it functional. Although we know that compaction is achieved through the histones, however, the DNA needs to be accessible to the transcription machinery and this is allowed thanks to a variety of very complex epigenetic mechanisms. Either DNA (methylation) or post-translational modifications of histone proteins (acetylation, methylation, ubiquitination and sumoylation) play a crucial role in chromatin remodelling and consequently on gene expression. Recently the serotonylation and dopaminylation of the histone 3, catalyzed by the Transglutaminase type 2 (TG2), has been reported. These novel post-translational modifications catalyzed by a predominantly cytoplasmic enzyme opens a new avenue for future investigations on the enzyme function itself and for the possibility that other biological amines, substrate of TG2, can influence the genome regulation under peculiar cellular conditions. In this review we analyzed the nuclear TG2's biology by discussing both its post-translational modification of various transcription factors and the implications of its epigenetic new face. Finally, we will focus on the potential impact of these events in human diseases.

Transglutaminase type 2-dependent crosslinking of IRF3 in dying melanoma cells

cGAS/STING axis is the major executor of cytosolic dsDNA sensing that leads to the production of type I interferon (IFNI) not only upon bacterial infection, but also in cancer cells, upon DNA damage. In fact, DNA damage caused by ionizing radiations and/or topoisomerase inhibitors leads to a release of free DNA into the cytosol, which activates the cGAS/STING pathway and the induction of IFNI expression. Doxorubicin-induced apoptotic cancer cells release damage-associated molecular patterns (DAMPs), including IFNI, which are able to stimulate the immune system. Our results indicate that Transglutaminase type 2 (TG2) is directly involved in the formation of a covalent cross-linked IRF3 (Interferon regulatory factor 3) dimers, thereby limiting the production of IFNI. Indeed, we demonstrated that upon doxorubicin treatment TG2 translocates into the nucleus of apoptotic melanoma cells interacting with IRF3 dimers. Interestingly, we show that both the knockdown of the enzyme as well as the inhibition of its transamidating activity lead to a decrease in the dimerization of IRF3 correlated with an increase in the IFNI mRNA levels. Taken together, these data demonstrate that TG2 negatively regulates the IRF3 pathway in human melanoma cells suggesting a so far unknown TG2-dependent mechanism by which cancer cells reduce the IFNI production after DNA damage to limit the immune system response.

Synergy of cystamine and pyraclostrobin against Fusarium graminearum involves membrane permeability mitigation and autophagy enhancement

The application of fungicide mixture is one of the most important measures to extend the service life of highly selective fungicides. Pyraclostrobin (PYR), which has been extensively used to control plant diseases by inhibiting mitochondrial respiration of pathogenic fungi, is at a high risk of resistance development. In this study, the potential of PYR alone or in combination with cystamine, an inhibitor of microbial transglutaminase, to suppress Fusarium graminearum was tested in vitro and in vivo. A synergistic effect of PYR/CYS mixture was observed both in vitro and when applied to etiolated wheat coleoptile. The control effect of PYR/CYS mixture on F. graminearum was better than that of PYR alone, which was reflected by the increased protection effect. The discrepancies of membrane permeability and the redox-physiological state were observed between PYR and PYR/CYS treatments, suggesting that an increased PYR availability in F. graminearum mycelia could be related with the observed synergistic action. Moreover, a synergistic profile was observed between PYR and CYS in regard of massive autophagosomes in mycelia, indicating that enhanced autophagy could be involved in the mode of action of PYR/CYS mixture. The differential content of mitochondrial metabolites between PYR and PYR/CYS treatments also provided evidence for CYS contribution to the fungicidal action of PYR/CYS mixture. The results provide insight into the synergistic mechanism of action of PYR/CYS mixture and an effective way to enhance the efficiency of PYR to combat F. graminearum.

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

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

Role of Transglutaminase 2 in Cell Death, Survival, and Fibrosis

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme catalyzing the crosslinking between Gln and Lys residues and involved in various pathophysiological events. Besides this crosslinking activity, TG2 functions as a deamidase, GTPase, isopeptidase, adapter/scaffold, protein disulfide isomerase, and kinase. It also plays a role in the regulation of hypusination and serotonylation. Through these activities, TG2 is involved in cell growth, differentiation, cell death, inflammation, tissue repair, and fibrosis. Depending on the cell type and stimulus, TG2 changes its subcellular localization and biological activity, leading to cell death or survival. In normal unstressed cells, intracellular TG2 exhibits a GTP-bound closed conformation, exerting prosurvival functions. However, upon cell stimulation with Ca²⁺ or other factors, TG2 adopts a Ca²⁺-bound open conformation, demonstrating a transamidase activity involved in cell death or survival. These functional discrepancies of TG2 open form might be caused by its multifunctional nature, the existence of splicing variants, the cell type and stimulus, and the genetic backgrounds and variations of the mouse models used. TG2 is also involved in the phagocytosis of dead cells by macrophages and in fibrosis during tissue repair. Here, we summarize and discuss the multifunctional and controversial roles of TG2, focusing on cell death/survival and fibrosis.

The Multifaceted Role of HSF1 in Pathophysiology: Focus on Its Interplay with TG2

The cellular environment needs to be strongly regulated and the maintenance of protein homeostasis is crucial for cell function and survival. HSF1 is the main regulator of the heat shock response (HSR), the master pathway required to maintain proteostasis, as involved in the expression of the heat shock proteins (HSPs). HSF1 plays numerous physiological functions; however, the main role concerns the modulation of HSPs synthesis in response to stress. Alterations in HSF1 function impact protein homeostasis and are strongly linked to diseases, such as neurodegenerative disorders, metabolic diseases, and different types of cancers. In this context, type 2 Transglutaminase (TG2), a ubiquitous enzyme activated during stress condition has been shown to promote HSF1 activation. HSF1-TG2 axis regulates the HSR and its function is evolutionary conserved and implicated in pathological conditions. In this review, we discuss the role of HSF1 in the maintenance of proteostasis with regard to the HSF1-TG2 axis and we dissect the stress response pathways implicated in physiological and pathological conditions.

Transglutaminase 2 Regulates Innate Immunity by Modulating the STING/TBK1/IRF3 Axis

We have recently shown that type 2 transglutaminase (TG2) plays a key role in the host's inflammatory response during bacterial infections. In this study, we investigated whether the enzyme is involved in the regulation of the STING pathway, which is the main signaling activated in the presence of both self- and pathogen DNA in the cytoplasm, leading to type I IFN (IFN I) production. In this study, we demonstrated that TG2 negatively regulates STING signaling by impairing IRF3 phosphorylation in bone marrow-derived macrophages, isolated from wild-type and TG2 knockout mice. In the absence of TG2, we found an increase in the IFN-β production and in the downstream JAK/STAT pathway activation. Interestingly, proteomic analysis revealed that TG2 interacts with TBK1, affecting its interactome composition. Indeed, TG2 ablation facilitates the TBK1-IRF3 interaction, thus indicating that the enzyme plays a negative regulatory effect on IRF3 recruitment in the STING/TBK1 complex. In keeping with these findings, we observed an increase in the IFNβ production in bronchoalveolar lavage fluids from COVID-19-positive dead patients paralleled by a dramatic decrease of the TG2 expression in the lung pneumocytes. Taken together, these results suggest that TG2 plays a negative regulation on the IFN-β production associated with the innate immunity response to the cytosolic presence of both self- and pathogen DNA.

Transglutaminase Type 2 is Involved in the Hematopoietic Stem Cells Homeostasis

Type 2 transglutaminase (TG2) is a multifunctional protein involved in various biological processes playing a key regulatory role in cell homeostasis such as cell death and autophagy. New evidence is emerging that support an important role of autophagy in regulating normal hematopoiesis. Prompted by these findings, in this study we investigated in vivo involvement of TG2 in mouse hematopoiesis under normal or nutrient deprivation conditions. We found that the number and rate of differentiation of bone marrow hematopoietic stem cell was decreased in the TG2 knockout mice. We present evidence showing that these effects on hematopoietic system are very likely due to the TG2-dependent impairment of autophagy. In fact, stimulation of autophagy by starvation is able to rescue the block of the differentiation of stem cells progenitors in the TG2 KO mice. It was also shown that the RhoA/ERK½ pathway, known to be essential for regulation of the bone marrow progenitor cells homeostasis, was significantly impaired in the absence of TG2. Hence, this study expanded our knowledge about TG2 discovering a role of this enzyme in regulation of hematopoiesis.

Transglutaminase Type 2 regulates the Wnt/β-catenin pathway in vertebrates

TG2 is a multifunctional enzyme involved in several cellular processes and has emerging as a potential regulator of gene expression. In this regard, we have recently shown that TG2 is able to activate HSF1, the master transcriptional regulator of the stress-responsive genes; however, its effect on the overall gene expression remains unclear. To address this point, we analyzed, by RNA-seq, the effect of TG2 on the overall transcriptome as well as we characterized the TG2 interactome in the nucleus. The data obtained from these omics approaches reveal that TG2 markedly influences the overall cellular transcriptome profile and specifically the Wnt and HSF1 pathways. In particular, its ablation leads to a drastic downregulation of many key members of these pathways. Interestingly, we found that key components of the Wnt/β-catenin pathway are also downregulated in cells lacking HSF1, thus confirming that TG2 regulates the HSF1 and this axis controls the Wnt signaling. Mechanistic studies revealed that TG2 can regulate the Wnt pathway by physically interacts with β-catenin and its nuclear interactome includes several proteins known to be involved in the regulation of the Wnt signaling. In order to verify whether this effect is playing a role in vivo, we ablated TG2 in Danio rerio. Our data show that the zebrafish lacking TG2 cannot complete the development and their death is associated with an evident downregulation of the Wnt pathway and a defective heat-shock response. Our findings show for the first time that TG2 is essential for the correct embryonal development of lower vertebrates, and its action is mediated by the Wnt/HSF1 axis.

Proteome-wide Changes in the mdx-4cv Spleen due to Pathophysiological Cross Talk with Dystrophin-Deficient Skeletal Muscle

Duchenne muscular dystrophy is primarily characterized by progressive muscle wasting due to deficiency in the membrane cytoskeletal protein dystrophin but is also associated with body-wide cellular disturbances in a variety of non-muscle tissues. In this study, we have focused on the comparative proteomic analysis of the spleen and established considerable changes in this crucial secondary lymphoid organ from the genetic mdx-4cv mouse model of dystrophinopathy. An apparent short isoform of dystrophin and associated glycoproteins were identified in spleen by mass spectrometry but appear not be affected in muscular dystrophy. In contrast, the mdx-4cv spleen showed significant proteome-wide changes in other protein species that are involved in metabolism, signaling, and cellular architecture. Since the spleen plays a key role in the immune response, these proteomic alterations may reflect pathophysiological cross talk between the lymphoid system and dystrophic muscles, which are affected by both fiber degeneration and inflammation.

Gene disruption of medaka (Oryzias latipes) orthologue for mammalian tissue-type transglutaminase (TG2) causes movement retardation

Transglutaminases are an enzyme family that catalyses protein cross-linking essential for several biological functions. In the previous studies, we characterized the orthologues of the mammalian transglutaminase family in medaka (Oryzias latipes), an established fish model. Among the human isozymes, tissue-type transglutaminase (TG2) has multiple functions that are involved in several biological phenomena. In this study, we established medaka mutants deficient for the orthologue of human TG2 using the CRISPR/Cas9 and transcription activator-like effector nucleases systems. Although apparent morphological changes in the phenotype were not observed, movement retardation was found in the mutant fish when evaluated by a tank-diving test. Furthermore, comparative immunohistochemistry analysis using in this fish model revealed that orthologue of human TG2 was expressed at the periventricular layer of the optic tectum. Our findings provide novel insight for the relationship between tissue-type transglutaminase and the nervous system and the associated behaviour.

In situ detection of intracellular tissue transglutaminase based on aggregation-induced emission

Herein, a novel strategy for in situ imaging and real-time monitoring of intracellular tissue transglutaminase (TG2) is presented based on aggregation-induced emission (AIE). It has high sensitivity and specificity, minimal background signal and can also effectively distinguish different cell types (drug-resistant cancer cells, cancer cells and normal cells).

The role of transglutaminase 2 in mediating glial cell function and pathophysiology in the central nervous system

The ubiquitously expressed transglutaminase 2 (TG2) has diverse functions in virtually all cell types, with its role depending not only on cell type, but also on specific subcellular localization. In the central nervous system (CNS) different types of glial cells, such as astrocytes, microglia, and oligodendrocytes and their precursor cells (OPCs), play pivotal supportive functions. This review is focused on what is currently known about the role of TG2 in each type of glial cell, in the context of normal function and pathophysiology. For example, astrocytic TG2 facilitates their migration and proliferation, but hinders their ability to protect neurons after CNS injury. The review also examines the interactions between glial cell types, and how TG2 in one cell type may affect another, as well as implications for specific TG2 populations as therapeutic targets in CNS pathology.

Transglutaminase Type 2 Regulates ER-Mitochondria Contact Sites by Interacting with GRP75

Transglutaminase type 2 (TG2) is a multifunctional enzyme that plays a key role in mitochondria homeostasis under stressful cellular conditions. TG2 interactome analysis reveals an enzyme interaction with GRP75 (glucose-regulated protein 75). GRP75 localizes in mitochondria-associated membranes (MAMs) and acts as a bridging molecule between the two organelles by assembling the IP3R-GRP75-VDAC complex, which is involved in the transport of Ca²⁺ from the endoplasmic reticulum (ER) to mitochondria. We demonstrate that the TG2 and GRP75 interaction occurs in MAMs. The absence of the TG2-GRP75 interaction leads to an increase of the interaction between IP3R-3 and GRP75; a decrease of the number of ER-mitochondria contact sites; an impairment of the ER-mitochondrial Ca²⁺ flux; and an altered profile of the MAM proteome. These findings indicate TG2 is a key regulatory element of the MAMs.

The Role of Tissue Transglutaminase in Cancer Cell Initiation, Survival and Progression

Tissue transglutaminase (transglutaminase type 2; TG2) is the most ubiquitously expressed member of the transglutaminase family (EC 2.3.2.13) that catalyzes specific post-translational modifications of proteins through a calcium-dependent acyl-transfer reaction (transamidation). In addition, this enzyme displays multiple additional enzymatic activities, such as guanine nucleotide binding and hydrolysis, protein kinase, disulfide isomerase activities, and is involved in cell adhesion. Transglutaminase 2 has been reported as one of key enzymes that is involved in all stages of carcinogenesis; the molecular mechanisms of action and physiopathological effects depend on its expression or activities, cellular localization, and specific cancer model. Since it has been reported as both a potential tumor suppressor and a tumor-promoting factor, the role of this enzyme in cancer is still controversial. Indeed, TG2 overexpression has been frequently associated with cancer stem cells’ survival, inflammation, metastatic spread, and drug resistance. On the other hand, the use of inducers of TG2 transamidating activity seems to inhibit tumor cell plasticity and invasion. This review covers the extensive and rapidly growing field of the role of TG2 in cancer stem cells survival and epithelial–mesenchymal transition, apoptosis and differentiation, and formation of aggressive metastatic phenotypes.