Dissecting the interaction between transglutaminase 2 and fibronectin

Pathogenetic Contributions and Therapeutic Implications of Transglutaminase 2 in Neurodegenerative Diseases

Neurodegenerative diseases encompass a heterogeneous group of disorders that afflict millions of people worldwide. Characteristic protein aggregates are histopathological hallmark features of these disorders, including Amyloid β (Aβ)-containing plaques and tau-containing neurofibrillary tangles in Alzheimer's disease, α-Synuclein (α-Syn)-containing Lewy bodies and Lewy neurites in Parkinson's disease and dementia with Lewy bodies, and mutant huntingtin (mHTT) in nuclear inclusions in Huntington's disease. These various aggregates are found in specific brain regions that are impacted by neurodegeneration and associated with clinical manifestations. Transglutaminase (TG2) (also known as tissue transglutaminase) is the most ubiquitously expressed member of the transglutaminase family with protein crosslinking activity. To date, Aβ, tau, α-Syn, and mHTT have been determined to be substrates of TG2, leading to their aggregation and implicating the involvement of TG2 in several pathophysiological events in neurodegenerative disorders. In this review, we summarize the biochemistry and physiologic functions of TG2 and describe recent advances in the pathogenetic role of TG2 in these diseases. We also review TG2 inhibitors tested in clinical trials and discuss recent TG2-targeting approaches, which offer new perspectives for the design of future highly potent and selective drugs with improved brain delivery as a disease-modifying treatment for neurodegenerative disorders.

Transglutaminase 2 has higher affinity for relaxed than for stretched fibronectin fibers

Transglutaminase 2 (TG2) plays a vital role in stabilizing extracellular matrix (ECM) proteins through enzymatic crosslinking during tissue growth, repair, and inflammation. TG2 also binds non-covalently to fibronectin (FN), an essential component of the ECM, facilitating cell adhesion, migration, proliferation, and survival. However, the interaction between TG2 and fibrillar FN remains poorly understood, as most studies have focused on soluble or surface-adsorbed FN or FN fragments, which differ in their conformations from insoluble FN fibers. Using a well-established in vitro FN fiber stretch assay, we discovered that the binding of a crosslinking enzyme to ECM fibers is mechano-regulated. TG2 binding to FN is tuned by the mechanical tension of FN fibers, whereby TG2 predominantly co-localizes to low-tension FN fibers, while fiber stretching reduces their affinity for TG2. This mechano-regulated binding relies on the proximity between the N-terminal β-sandwich and C-terminal β-barrels of TG2. Crosslinking mass spectrometry (XL-MS) revealed a novel TG2-FN synergy site within TG2's C-terminal β-barrels that interacts with FN regions located outside of the canonical gelatin binding domain, specifically FNI2 and FNIII14-15. Combining XL-MS distance restraints with molecular docking revealed the mechano-regulated binding mechanism between TG2 and modules FNI7-9 by which mechanical forces regulate TG2-FN interactions. This highlights a previously unrecognized role of TG2 as a tension sensor for FN fibers. This novel interaction mechanism has significant implications in physiology and mechanobiology, including how forces regulate cell adhesion, spreading, migration, phenotype modulation, depending on the tensional state of ECM fibers. Data are available via ProteomeXchange with identifier PXD043976.

Design and Evolution of Enhanced Peptide-Peptide Ligation for Modular Transglutaminase Assembly

Robust and precise tools are needed to enhance the functionality and resilience of synthetic nanoarchitectures. Here, we have employed directed evolution and rational design to build a fast-acting molecular superglue from a bacterial adhesion protein. We have generated the SnoopLigase2 coupling system, a genetically encoded route for efficient transamidation between SnoopTag2 and DogTag2 peptides. Each peptide was selected for rapid reaction by phage display screening. The optimized set allows more than 99% completion and is compatible with diverse buffers, pH values, and temperatures, accelerating the reaction over 1000-fold. SnoopLigase2 directs a specific reaction in the mammalian secretory pathway, allowing covalent display on the plasma membrane. Transglutaminase 2 (TG2) has a network of interactions and substrates amidst the mammalian cell surface and extracellular matrix. We expressed a modified TG2 with resistance to oxidative inactivation and minimal self-reactivity. SnoopLigase2 enables TG2 functionalization with transforming growth factor alpha (TGFα) in routes that would be impossible through genetic fusion. The TG2:TGFα conjugate retained transamidase activity, stably anchored TGFα for signal activation in the extracellular environment, and reprogrammed cell behavior. This modular toolbox should create new opportunities for molecular assembly, both for novel biomaterials and complex cellular environments.

The Immunobiology and Pathogenesis of Celiac Disease

Among human leukocyte antigen (HLA)-associated disorders, celiac disease has an immunopathogenesis that is particularly well understood. The condition is characterized by hypersensitivity to cereal gluten proteins, and the disease lesion is localized in the gut. Still, the diagnosis can be made by detection of highly disease-specific autoantibodies to transglutaminase 2 in the blood. We now have mechanistic insights into how the disease-predisposing HLA-DQ molecules, via presentation of posttranslationally modified gluten peptides, are connected to the generation of these autoantibodies. This review presents our current understanding of the immunobiology of this common disorder that is positioned in the border zone between food hypersensitivity and autoimmunity.

A Multidisciplinary Approach Establishes a Link between Transglutaminase 2 and the Kv10.1 Voltage-Dependent K+ Channel in Breast Cancer

Since the multifunctionality of transglutaminase 2 (TG2) includes extra- and intracellular functions, we investigated the effects of intracellular administration of TG2 inhibitors in three breast cancer cell lines, MDA-MB-231, MDA-MB-436 and MDA-MB-468, which are representative of different triple-negative phenotypes, using a patch-clamp technique. The first cell line has a highly voltage-dependent a membrane current, which is low in the second and almost absent in the third one. While applying a voltage protocol to responsive single cells, injection of TG2 inhibitors triggered a significant decrease of the current in MDA-MB-231 that we attributed to voltage-dependent K⁺ channels using the specific inhibitors 4-aminopyridine and astemizole. Since the Kv10.1 channel plays a dominant role as a marker of cell migration and survival in breast cancer, we investigated its relationship with TG2 by immunoprecipitation. Our data reveal their physical interaction affects membrane currents in MDA-MB-231 but not in the less sensitive MDA-MB-436 cells. We further correlated the efficacy of TG2 inhibition with metabolic changes in the supernatants of treated cells, resulting in increased concentration of methyl- and dimethylamines, representing possible response markers. In conclusion, our findings highlight the interference of TG2 inhibitors with the Kv10.1 channel as a potential therapeutic tool depending on the specific features of cancer cells.

Hypoxia and inflammation conditions differentially affect the expression of tissue transglutaminase spliced variants and functional properties of extravillous trophoblast cells

PROBLEM

Persistent hypoxia and inflammation beyond early pregnancy are involved in a bad outcome because of defective trophoblast invasiveness. Tissue transglutaminase (TG2) coregulates several cell functions. An aberrant expression and/or transamidation activity could contribute to placental dysfunction.

METHOD OF STUDY

The first-trimester trophoblast cell line (Swan-71) was used to study TG2 expression and cell functions in the absence or presence of inflammatory cytokines (TNF-α, IL-1β) or chemical hypoxia (CoCl₂ ). We analyzed The concentration of cytokines in the supernatant by ELISA; Cell migration by scratch assay; NF-κB activation by detection of nuclear p65 by immunofluorescence or flow cytometry using a Swan-71 NF-κB-hrGFP reporter cell line. Tissue transglutaminase expression was analyzed by immunoblot and confocal microscopy. Expression of spliced mRNA variants of tissue transglutaminase was analyzed by RT-PCR. Transamidation activity was assessed by flow cytometry using 5-(biotinamido)-pentylamine substrate.

RESULTS

Chemical hypoxia and TGase inhibition, but not inflammatory stimuli, decreased Swan-71 migration. IL-6 production was also decreased by chemical hypoxia, but increased by inflammation. Intracellular TGase activity was increased by all stimuli, but NF-κB activation was observed only in the presence of proinflammatory cytokines. TG2 expression was decreased by CoCl₂ and TNF-α. Translocation of TG2 and p65 to nuclei was observed only with TNF-α, without colocalization. Differential relative expression of spliced variants of mRNA was observed between CoCl₂ and inflammatory stimuli.

CONCLUSION

The observed decrease in total TG2 expression and relative increase in short variants under hypoxia conditions could contribute to impaired trophoblast invasion and impact on pregnancy outcome.

The Outside-In Journey of Tissue Transglutaminase in Cancer

Tissue transglutaminase (TG2) is a member of the transglutaminase family that catalyzes Ca²⁺-dependent protein crosslinks and hydrolyzes guanosine 5′-triphosphate (GTP). The conformation and functions of TG2 are regulated by Ca²⁺ and GTP levels; the TG2 enzymatically active open conformation is modulated by high Ca²⁺ concentrations, while high intracellular GTP promotes the closed conformation, with inhibition of the TG-ase activity. TG2’s unique characteristics and its ubiquitous distribution in the intracellular compartment, coupled with its secretion in the extracellular matrix, contribute to modulate the functions of the protein. Its aberrant expression has been observed in several cancer types where it was linked to metastatic progression, resistance to chemotherapy, stemness, and worse clinical outcomes. The N-terminal domain of TG2 binds to the 42 kDa gelatin-binding domain of fibronectin with high affinity, facilitating the formation of a complex with β-integrins, essential for cellular adhesion to the matrix. This mechanism allows TG2 to interact with key matrix proteins and to regulate epithelial to mesenchymal transition and stemness. Here, we highlight the current knowledge on TG2 involvement in cancer, focusing on its roles translating extracellular cues into activation of oncogenic programs. Improved understanding of these mechanisms could lead to new therapeutic strategies targeting this multi-functional protein.

Comparative Profiling of TG2 and Its Effectors in Human Relapsing Remitting and Progressive Multiple Sclerosis

Multiple Sclerosis (MS) is a chronic CNS autoimmune disease characterized by immune-mediated demyelination, axon loss, and disability. Dysregulation of transglutaminase-2 (TG2) has been implicated in disease initiation and progression. Herein, TG2 expression in post-mortem human brain tissue from Relapsing Remitting MS (RRMS) or Progressive MS (PMS) individuals were examined and correlated with the presence of TG2 binding partners and effectors implicated in the processes of inflammation, scar formation, and the antagonism of repair. Tissues from Relapsing-Remitting Multiple Sclerosis (RRMS; n = 6), Progressive Multiple Sclerosis (PMS; n = 5), and non-MS control (n = 6) patients underwent immunohistochemistry for TG2, PLA2, COX-2, FN, CSPG, and HSPG. TG2 was strongly upregulated in active RRMS and PMS lesions, within blood vessels and the perivascular tissue of sclerotic plaques. TG2 colocalization was observed with GFAP+ astrocytes and ECM, including FN, HSPG, and CSPG, which also increased in either RRMS or PMS lesions. Although TG2 was not colocalized with inflammatory mediators COX-2 and PLA2, or the macrophage-microglia marker Iba1, its increased expression correlated with their elevation in active RRMS and PMS lesions. In summary, the correlation of strong TG2 induction in either RRMS or PMS with some of its binding partners but not others implicates potentially different roles for TG2 in disparate MS forms that may warrant further investigation.

Application of a Fluorescence Anisotropy-Based Assay to Quantify Transglutaminase 2 Activity in Cell Lysates

Transglutaminase 2 (TGase 2) is a multifunctional protein which is involved in various physiological and pathophysiological processes. The latter also include its participation in the development and progression of malignant neoplasms, which are often accompanied by increased protein synthesis. In addition to the elucidation of the molecular functions of TGase 2 in tumor cells, knowledge of its concentration that is available for targeting by theranostic agents is a valuable information. Herein, we describe the application of a recently developed fluorescence anisotropy (FA)-based assay for the quantitative expression profiling of TGase 2 by means of transamidase-active enzyme in cell lysates. This assay is based on the incorporation of rhodamine B-isonipecotyl-cadaverine (R-I-Cad) into N,N-dimethylated casein (DMC), which results in an increase in the FA signal over time. It was shown that this reaction is not only catalyzed by TGase 2 but also by TGases 1, 3, and 6 and factor XIIIa using recombinant proteins. Therefore, control measurements in the presence of a selective irreversible TGase 2 inhibitor were mandatory to ascertain the specific contribution of TGase 2 to the overall FA rate. To validate the assay regarding the quality of quantification, spike/recovery and linearity of dilution experiments were performed. A total of 25 cancer and 5 noncancer cell lines were characterized with this assay method in terms of their activatable TGase 2 concentration (fmol/µg protein lysate) and the results were compared to protein synthesis data obtained by Western blotting. Moreover, complementary protein quantification methods using a biotinylated irreversible TGase 2 inhibitor as an activity-based probe and a commercially available ELISA were applied to selected cell lines to further validate the results obtained by the FA-based assay. Overall, the present study demonstrates that the FA-based assay using the substrate pair R-I-Cad and DMC represents a facile, homogenous and continuous method for quantifying TGase 2 activity in cell lysates.

Nucleotide binding kinetics and conformational change analysis of tissue transglutaminase with switchSENSE

Function, activity, and interactions of proteins crucially depend on their three-dimensional structure and are often regulated by effector binding and environmental changes. Tissue transglutaminase (Transglutaminase 2, TG2) is a multifunctional protein, allosterically regulated by nucleotides and Ca²⁺ ions, which trigger opposing conformational changes. Here we introduce switchSENSE as a versatile tool for TG2 characterization and provide novel insights into protein conformation as well as analyte binding kinetics. For the first time, we succeeded in measuring the kinetic rate constants and affinities (k_on, k_off, K_D) for guanosine nucleotides (GMP, GDP, GTP, GTPγS). Further, the conformational changes induced by GDP, Ca²⁺ and the covalent inhibitor Z-DON were observed by changes in TG2's hydrodynamic diameter. We confirmed the well-known compaction by guanosine nucleotides and extension by Ca²⁺, and provide evidence for TG2 conformations so far not described by structural analysis. Moreover, we analyze the influence of the peptidic Z-DON inhibitor and the R580A mutation on the conformational responsiveness of TG2 to its natural effectors. In summary, this work shows how the combination of structural and kinetic information obtained by switchSENSE opens new perspectives for the characterization of conformationally active proteins and their interactions with ligands, e.g. potential drug candidates.

Transglutaminase 2-Mediated p53 Depletion Promotes Angiogenesis by Increasing HIF-1α-p300 Binding in Renal Cell Carcinoma

Angiogenesis and the expression of vascular endothelial growth factor (VEGF) are increased in renal cell carcinoma (RCC). Transglutaminase 2 (TGase 2), which promotes angiogenesis in endothelial cells during wound healing, is upregulated in RCC. Tumor angiogenesis involves three domains: cancer cells, the extracellular matrix, and endothelial cells. TGase 2 stabilizes VEGF in the extracellular matrix and promotes VEGFR-2 nuclear translocation in endothelial cells. However, the role of TGase 2 in angiogenesis in the cancer cell domain remains unclear. Hypoxia-inducible factor (HIF)-1α-mediated VEGF production underlies the induction of angiogenesis in cancer cells. In this study, we show that p53 downregulated HIF-1α in RCC, and p53 overexpression decreased VEGF production. Increased TGase 2 promoted angiogenesis by inducing p53 degradation, leading to the activation of HIF-1α. The interaction of HIF-1α and p53 with the cofactor p300 is required for stable transcriptional activation. We found that TGase 2-mediated p53 depletion increased the availability of p300 for HIF-1α-p300 binding. A preclinical xenograft model suggested that TGase 2 inhibition can reverse angiogenesis in RCC.

HLA-DQ genotype and biochemical characterization of anti-transglutaminase 2 antibodies in patients with type 1 diabetes mellitus in Taiwan

Human Leukocyte Antigen (HLA)-DQ2 and HLA-DQ8 are genetic risk factors for Type 1 Diabetes Mellitus (T1DM) and Celiac disease (CD) in Caucasians, but their association with Taiwanese Han population is unknown. We screened 532 Taiwanese T1DM patients for CD biomarkers including anti-tissue transglutaminase (TGM2), anti-gliadin and anti-neoepitope antibodies (Abs), sequencing DQB1 genotypes, and characterized the TGM2 Abs. We report that 3.76% of Taiwanese patients had TGM2-Abs and all had no CD's symptoms. In contrast to Caucasian's CD patients, DQ2/DQ8 only constituted ~4/5 of TGM2-Abs positive patients, while the other ~1/5 patients belonged to different HLA genotypes. Either anti-gliadin or anti-neoepitope Abs coexisted with ~3/4 of TGM2-Abs positive patients that were likely due to gluten-ingestion, while the cause of TGM2-Abs production for other ~1/4 of patients was unknown. Purified anti-TGM2 IgA (TGA) and anti-TGM2 IgG (TGG) could bind on endothelial cells surface, recognized native better than denatured forms of TGM2, and TGA inhibited TGM2's transamidation activity by up to 80% but TGG had no effects. Epitope mapping of all TGM2-Abs positive sera demonstrated that TGM2-Abs had heterogeneity in specificities. This is the first study on the differences between Taiwanese Han group and Caucasian in HLA genotypes and properties of TGM2-Abs.

Small Molecules Target the Interaction between Tissue Transglutaminase and Fibronectin

Tissue transglutaminase (TG2) is a multifunctional protein with enzymatic, GTP-ase, and scaffold properties. TG2 interacts with fibronectin (FN) through its N-terminus domain, stabilizing integrin complexes, which regulate cell adhesion to the matrix. Through this mechanism, TG2 participates in key steps involved in metastasis in ovarian and other cancers. High-throughput screening identified several small molecule inhibitors (SMI) for the TG2/FN complex. Rational medicinal chemistry optimization of the hit compound (TG53) led to second-generation analogues (MT1-6). ELISA demonstrated that these analogues blocked TG2/FN interaction, and bio-layer interferometry (BLI) showed that the SMIs bound to TG2. The compounds also potently inhibited cancer cell adhesion to FN and decreased outside-in signaling mediated through the focal adhesion kinase. Blockade of TG2/FN interaction by the small molecules caused membrane ruffling, delaying the formation of stable focal contacts and mature adhesions points and disrupted organization of the actin cytoskeleton. In an in vivo model measuring intraperitoneal dissemination, MT4 and MT6 inhibited the adhesion of ovarian cancer cells to the peritoneum. Pretreatment with MT4 also sensitized ovarian cancer cells to paclitaxel. The data support continued optimization of the new class of SMIs that block the TG2/FN complex at the interface between cancer cells and the tumor niche.

Mapping the minimum domain of the fibronectin binding site on transglutaminase 2 (TG2) and its importance in mediating signaling, adhesion, and migration in TG2-expressing cells

The interaction between the enzyme transglutaminase 2 (TG2) and fibronectin (FN) is involved in the cell-matrix interactions that regulate cell signaling, adhesion, and migration and play central roles in pathologic conditions, particularly fibrosis and cancer. A precise definition of the exact interaction domains on both proteins could provide a tool to design novel molecules with potential therapeutic applications. Although specific residues involved in the interaction within TG2 have been analyzed, little is known regarding the TG2 binding site on FN. This site has been mapped to a large internal 45-kDa protein fragment coincident with the gelatin binding domain (GBD). With the goal of defining the minimal FN interacting domain for TG2, we produced several expression constructs encoding different portions or modules of the GBD and tested their binding and functional properties. The results demonstrate that the I₈ module is necessary and sufficient for TG2-binding in vitro, but does not have functional effects on TG2-expressing cells. Modules I₇ and I₉ increase the strength of the binding and are required for cell adhesion. A 15-kDa fragment encompassing modules I_7-9 behaves as the whole 45-kDa GBD and mediates signaling, adhesion, spreading, and migration of TG2⁺ cells. This study provides new insights into the mechanism for TG2 binding to FN.-Soluri, M. F., Boccafoschi, F., Cotella, D., Moro, L., Forestieri, G., Autiero, I., Cavallo, L., Oliva, R., Griffin, M., Wang, Z., Santoro, C., Sblattero, D. Mapping the minimum domain of the fibronectin binding site on transglutaminase 2 (TG2) and its importance in mediating signaling, adhesion, and migration in TG2-expressing cells.

Strong Clonal Relatedness between Serum and Gut IgA despite Different Plasma Cell Origins

Mucosal antigens induce generation of lamina propria plasma cells (PCs) that secrete dimeric immunoglobulin A (IgA) destined for transport across the epithelium. In addition, blood contains monomeric IgA. To study the relationship between mucosal and systemic antibody responses, we took advantage of celiac disease patient samples for isolation of gut PCs as well as serum IgA and IgG reactive with a gluten-derived peptide or the autoantigen transglutaminase 2. Proteomic analysis of serum IgA revealed antigen-specific V-gene preferences, which matched those found in gut PCs. Further, gut PC CDR-H3 sequences were abundant in serum IgA but also detectable in serum IgG. Our data indicate that the same B cell clones that give rise to gut PCs also contribute to the serum antibody pool. However, serum IgA antibodies had a molecular composition distinct from that of IgA antibodies secreted in the gut, suggesting that individual B cell clones give rise to different PC populations.

Tissue Tranglutaminase Regulates Interactions between Ovarian Cancer Stem Cells and the Tumor Niche

Cancer progression and recurrence are linked to a rare population of cancer stem cells (CSC). Here, we hypothesized that interactions with the extracellular matrix drive CSC proliferation and tumor-initiating capacity and investigated the functions of scaffold protein tissue transglutaminase (TG2) in ovarian CSC. Complexes formed by TG2, fibronectin (FN), and integrin β1 were enriched in ovarian CSC and detectable in tumors. A function-inhibiting antibody against the TG2 FN-binding domain suppressed complex formation, CSC proliferation as spheroids, tumor-initiating capacity, and stemness-associated Wnt/β-catenin signaling. Disruption of the interaction between TG2 and FN also blocked spheroid formation and the response to Wnt ligands. TG2 and the Wnt receptor Frizzled 7 (Fzd7) form a complex in cancer cells and tumors, leading to Wnt pathway activation. Protein docking and peptide inhibition demonstrate that the interaction between TG2 and Fzd7 overlaps with the FN-binding domain of TG2. These results support a new function of TG2 in ovarian CSC, linked to spheroid proliferation and tumor-initiating capacity and mediated through direct interactions with Fzd7. We propose this complex as a new stem cell target.Significance: These findings reveal a new mechanism by which ovarian CSCs interact with the tumor microenvironment, promoting cell proliferation and tumor initiation. Cancer Res; 78(11); 2990-3001. ©2018 AACR.

Compartment resolved proteomics reveals a dynamic matrisome in a biomechanically driven model of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a severe fibrotic component that compromises treatment, alters the immune cell profile and contributes to patient mortality. It has been shown that early on in this process, dynamic changes in tissue biomechanics play an integral role in supporting pancreatic cancer development and progression. Despite the acknowledgement of its importance, a granular view of how stromal composition changes during the course of PDAC progression remains largely unknown. To mimic the quasi-mesenchymal phenotype and pronounced desmoplastic response observed clinically, we utilized a genetically engineered mouse model of PDAC that is driven by a Kras^G12D mutation and loss of Tgfbr2 expression. Application of compartment resolved proteomics revealed that PDAC progression in this KTC model is associated with dynamic stromal alterations that are indicative of a wound healing program. We identified an early provisional matricellular fibrosis that was accompanied by markers of macrophage activation and infiltration, consistent with the inflammatory phase of wound healing. At 20 weeks a proliferative phenotype was observed with increased fibroblast markers, further collagen deposition and loss of basement membrane and native cell markers.

Positive Feedback Regulation between Transglutaminase 2 and Toll-Like Receptor 4 Signaling in Hepatic Stellate Cells Correlates with Liver Fibrosis Post Schistosoma japonicum Infection

Liver fibrosis induced by Schistosoma japonicum (Sj) infection is characterized by the accumulation of extracellular matrix (ECM). The activated and differentiated hepatic stellate cells (HSCs) are the predominant ECM-producing cell type in the liver. Toll-like receptor (TLR) 4 pathway activation plays a key role in mice liver fibrosis models induced by alcohol, biliary ligation, and carbon tetrachloride 4. In this work, we found that TLR4 pathway activation correlated with the severity of liver fibrosis post Sj infection. The TLR4 receptor inhibitor TAK242 reduced the extent of liver fibrosis. The increased expression of TLR4, α-smooth muscle actin (α-SMA), and cytoglobin was observed in the HSCs of mouse liver after Sj infection. In response to stimulation with either lipopolysaccharide or Sj's soluble egg antigen (SEA), high levels of TLR4 and α-SMA were induced in HSCs and were inhibited by TAK242 treatment. In previous work, we had reported that a high level of transglutaminase 2 (TGM2) is crucial for liver fibrosis post Sj infection. Herein, we found that TLR4 signaling also controlled Tgm2 expression. Inhibition of TGM2 activity by cystamine (CTM) in Sj-infected mice or in HSCs induced with all-trans-retinoic acid (ATRA) stimulation led to a lowered activation of TLR4 signaling and a reduced α-SMA expression. These results were confirmed by downregulating the Tgm2 gene by specific siRNA. These observations implied the presence of a positive feedback regulation between TGM2 and TLR4 signaling in HSCs that correlated with liver fibrosis post Sj infection. This novel connection between TGM2 and TLR4 pathway activation in liver fibrosis induced by Sj infection enhances our understanding of liver diseases.

Epitope-dependent Functional Effects of Celiac Disease Autoantibodies on Transglutaminase 2

Transglutaminase 2 (TG2) is a Ca²⁺-dependent cross-linking enzyme involved in the pathogenesis of CD. We have previously characterized a panel of anti-TG2 mAbs generated from gut plasma cells of celiac patients and identified four epitopes (epitopes 1-4) located in the N-terminal part of TG2. Binding of the mAbs induced allosteric changes in TG2. Thus, we aimed to determine whether these mAbs could influence enzymatic activity through modulation of TG2 susceptibility to oxidative inactivation and Ca²⁺ affinity. All tested epitope 1 mAbs, as well as 679-14-D04, which recognizes a previously uncharacterized epitope, prevented oxidative inactivation and increased Ca²⁺ sensitivity of TG2. We have identified crucial residues for binding of 679-14-D04 located within a Ca²⁺ binding site. Epitope 1 mAbs and 679-14-D04, although recognizing separate epitopes, behaved similarly when assessing their effect on TG2 conformation, suggesting that the shared effects on TG2 function can be explained by induction of the same conformational changes. None of the mAbs targeting other epitopes showed these effects, but epitope 2 mAbs reduced the rate of TG2-catalyzed reactions. Collectively, these effects could be relevant to the pathogenesis of CD. In A20 B cells transduced with TG2-specific B-cell receptor, epitope 2-expressing cells had poorer uptake of TG2-gluten complexes and were less efficient in gluten epitope presentation to T cells than cells expressing an epitope 1 receptor. Thus, the ability of epitope 1-targeting B cells to keep TG2 active and protected from oxidation might explain why generation of epitope 1-targeting plasma cells seems to be favored in celiac patients.