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Tissue transglutaminase

Tissue transglutaminase (tTG), also known as transglutaminase 2 (TG2) or protein-glutamine γ-glutamyltransferase 2, is a multifunctional calcium-dependent enzyme that catalyzes the formation of isopeptide bonds between the γ-carboxamide group of glutamine residues and the ε-amino group of lysine residues in proteins, resulting in irreversible protein crosslinking and stabilization of the extracellular matrix. This enzymatic activity, classified under EC 2.3.2.13, also enables deamidation of glutamine to glutamic acid under certain conditions, such as in the presence of water, and the enzyme exhibits additional GTP-binding and GTPase activities that regulate its conformation and non-enzymatic functions. Structurally, tTG consists of four distinct domains: an N-terminal β-sandwich domain, a central catalytic core containing the cysteine residue, and two C-terminal β-barrel domains, allowing it to adopt either a compact "closed" conformation when bound to GTP (which inhibits transglutaminase activity) or an extended "open" conformation activated by calcium ions that promotes crosslinking. This conformational flexibility underlies its dual roles as both an and a signaling protein, with widespread expression in tissues including the liver, , and , where it was first identified in 1957 through studies on liver extracts demonstrating calcium-dependent incorporation of amines into proteins. Beyond its enzymatic functions, tTG plays critical roles in cellular processes such as cell adhesion, migration, differentiation, wound healing, and apoptosis regulation by interacting with integrins, fibronectin, and other extracellular matrix components, thereby influencing tissue remodeling and homeostasis. Dysregulation of tTG activity or expression is implicated in numerous pathologies; in celiac disease, it serves as the primary autoantigen by deamidating gluten peptides to enhance their immunogenicity, leading to an autoimmune response with high diagnostic sensitivity (98-100%) via anti-tTG IgA antibodies. In neurodegenerative disorders like Alzheimer's and Parkinson's diseases, tTG promotes protein aggregation by crosslinking amyloid-β and α-synuclein, contributing to plaque and Lewy body formation. Furthermore, elevated tTG levels are associated with cancer progression, including enhanced tumor cell migration, invasion, and chemoresistance in malignancies such as ovarian and pancreatic cancers, as well as fibrosis in organs like the liver and heart through excessive matrix stabilization.

Structure

Gene

The TGM2 gene, which encodes tissue transglutaminase (also known as transglutaminase 2), is located on the long arm of human chromosome 20 at the cytogenetic band 20q11.23. This positioning places it within a region that includes other genes involved in cellular signaling and structural processes. The gene spans approximately 41 kb of genomic DNA and consists of 15 exons in its canonical transcript, encoding a primary protein isoform of 687 amino acids. Alternative splicing produces additional isoforms, but the main form is widely studied for its multifunctional roles. The promoter region of TGM2 contains regulatory elements responsive to , mediated by GC box motifs that facilitate transcriptional activation through interactions with transcription factors like Sp1. It also includes a β1 (TGF-β1) response element approximately 0.9 kb upstream of the transcription start site, enabling upregulation in response to inflammatory signals such as interleukin-1, interleukin-6, tumor necrosis factor-α, and TGF-β. These elements contribute to stress-induced expression, including during and stress, highlighting TGM2's role as a stress-responsive gene. TGM2 is ubiquitously expressed across tissues, with particularly high levels in endothelial cells, fibroblasts, and cells. Expression is detected in , , and fibromuscular tissues, often showing cytoplasmic localization. Under inflammatory conditions, such as in chronic wounds or autoimmune responses, TGM2 transcription is upregulated, promoting adaptive cellular responses. Regarding genetic variants, studies have investigated single nucleotide polymorphisms (SNPs) in TGM2 for associations with susceptibility, particularly , where the encoded protein serves as a key autoantigen. However, altering the protein sequence do not significantly contribute to risk. Some polymorphisms, such as those identified in regions, have been detected in patients and may modulate or expression levels, potentially influencing markers, though they are not primary susceptibility factors.

Protein Structure

Tissue (TG2), also known as 2, is a 78 kDa multifunctional characterized by a modular consisting of four distinct . The N-terminal β-sandwich domain (residues 1–139) forms a compact structure stabilized by hydrogen bonds and hydrophobic interactions, serving as a scaffold for interdomain contacts. Adjacent to this is the central catalytic core domain (residues 147–460), which adopts an α/β fold typical of papain-like proteases and houses the . The C-terminal region comprises two β-barrel domains: β-barrel 1 (residues 472–583) and β-barrel 2 (residues 591–687), each featuring antiparallel β-sheets that contribute to binding and conformational dynamics. The catalytic site within the core domain centers on Cys277, the nucleophilic that initiates the reaction by forming a transient (thiol-acyl) intermediate with the γ-carboxamide group of residues in substrate proteins. This residue is part of a conserved (Cys277-His335-Asp358), where His335 acts as a general base to deprotonate Cys277, enhancing its nucleophilicity, and Asp358 stabilizes the through hydrogen bonding, facilitating proton transfer during . The cleft is lined by residues such as Trp279 and Tyr306, which position substrates for efficient acyl transfer. GTP and GDP binding occurs primarily at a site located at the interface between the catalytic core and β-barrel 1 domain, involving key residues such as Arg476, Val479, and Tyr583, which coordinate the nucleotide's phosphate groups and guanine base via hydrogen bonds and van der Waals interactions. A secondary binding mode may involve β-barrel 2, contributing to cooperative allosteric effects that inhibit transglutaminase activity by stabilizing the closed conformation. These sites enable TG2's dual role as a GTPase, distinct from its cross-linking function. TG2 adopts distinct conformational states regulated by ligand binding: the "closed" state, promoted by GTP binding, compacts the structure with the β-barrel domains folding over the , rendering it inaccessible and inhibiting ; in contrast, the "open" state, induced by Ca²⁺ binding to sites in the catalytic domain (e.g., involving Asp252 and Glu254), repositions the barrels away from the core, exposing the for access. This interconversion relies on flexibility in the regions (residues approximately 140–146, 461–471, and 584–590), linker segments connecting the domains, which undergo secondary structure changes, including α-helix to transitions, to accommodate the ~90° domain rotations observed in crystal structures. Regarding post-translational modifications, TG2 possesses several predicted N-glycosylation sites (e.g., Asn247 and Asn367) based on , but experimental evidence indicates no occurs in the native protein, preserving its compact structure and activity. Other modifications, such as at Ser216, may influence conformation but are not integral to the core architecture.

Cross-linking Activity

Tissue transglutaminase (TG2), also known as 2, primarily exerts its cross-linking activity through a calcium-dependent transamidation reaction that forms covalent bonds between proteins. In this process, a residue on one protein serves as the acyl donor, while the ε-amino group of a residue on another protein acts as the acyl acceptor, resulting in the formation of an ε-(γ-glutamyl) and the release of . This reaction is catalyzed within the of TG2, where calcium ions induce a conformational change from a closed, inactive state to an open, catalytically active form, enabling substrate binding. The detailed mechanism begins with the of the active-site residue (Cys277 in TG2), which performs a nucleophilic attack on the γ-carboxamide group of the side chain, forming a acyl-enzyme intermediate and liberating . Subsequently, the ε-amino group of a residue attacks this intermediate, displacing the and forming the stable . The general reaction can be represented as: Protein₁-Gln + Protein₂-Lys → Protein₁-(γ-glutamyl)-ε-lysine-Protein₂ + NH₃ This transamidation is favored under conditions where primary amine donors are available, distinguishing it from reactions that occur with as the acceptor. Common substrates for TG2 cross-linking include (ECM) components such as , , and , which contain accessible and residues. For instance, TG2 cross-links to itself and other ECM proteins, enhancing matrix assembly, while it also incorporates into networks during . The biological outcomes of this cross-linking activity include the stabilization of the , which provides structural integrity to s, and the promotion of clot formation during by reinforcing scaffolds. These modifications contribute to remodeling and repair by creating insoluble, mechanically robust protein networks resistant to . Kinetic parameters for TG2 cross-linking indicate a requirement for millimolar concentrations of Ca²⁺, with an EC₅₀ around 0.27 mM and full activation typically observed at 5-10 mM, reflecting its regulation by intracellular calcium levels. The operates optimally at a of approximately 7-8, aligning with physiological conditions in extracellular environments.

Deamidation Activity

Tissue transglutaminase (TG2), in the absence of primary amines, catalyzes the of residues in peptides and proteins, where serves as the acyl acceptor to convert to while releasing . This reaction introduces negative charges into the substrate, altering its properties without forming covalent cross-links between molecules. The mechanism begins with the nucleophilic attack by the (Cys277) on the δ-carbon of the side chain, forming a transient acyl-enzyme intermediate and liberating . In the subsequent step, acts as the to this intermediate, yielding the deamidated residue. This process shares the initial acyl-enzyme formation with the cross-linking pathway but diverges by proceeding via rather than amine . Key substrates for this activity include gluten-derived peptides such as , where TG2 selectively deamidates specific residues (e.g., in the α- peptide PFPQPQLPYPR), enhancing their binding affinity to /DQ8 molecules and thereby increasing immunogenicity in celiac disease. Other examples encompass intracellular proteins like glyceraldehyde-3-phosphate (GAPDH), where of multiple glutaminyl residues modulates enzymatic function. Physiologically, TG2-mediated modifies proteins to influence signaling pathways, such as stabilizing the inhibitor p21 by deamidating residues, which promotes arrest. It also facilitates protein degradation or alters aggregation propensity, as seen in the deamidation of β-amyloid peptides, which reduces their fibrillization. Although less prevalent than cross-linking under typical cellular conditions, this activity holds critical importance in targeted pathologies like disease. In contrast to transamidation, predominates at low concentrations of primary amines and active TG2 enzyme, as well as under high dilution, favoring over amine incorporation. Both reactions are calcium-dependent, but deamidation can proceed efficiently in environments with limited amine availability, such as endolysosomal compartments.

Tissue (tTG), also known as 2 (TG2), is allosterically regulated by nucleotides such as GTP and GDP, which bind to sites within its two β-barrel domains (β1 and β2). This stabilizes a compact, closed conformation that masks the catalytic core, thereby inhibiting activity. The (Kd) for GTP is approximately 1.6 μM for wild-type TG2, while mutants like Y516F exhibit even higher affinity (Kd ≈ 0.8 μM), indicating tight interaction under physiological conditions. GTP and GDP compete with Ca²⁺ for conformational control, as to residues such as Arg-579 in the β-barrel domain prevents the from adopting an active state even in the presence of moderate Ca²⁺ levels. In contrast, Ca²⁺ serves as a positive allosteric effector by binding to five or six non-canonical EF-hand-like motifs primarily in the catalytic and β-barrel domains, inducing an open, extended conformation that exposes the Cys-277-His-335-Asp-358 triad. Activation requires Ca²⁺ concentrations in the millimolar range (apparent ≈ 3 mM), far exceeding typical intracellular levels, which ensures tTG remains dormant inside cells under normal conditions. High Ca²⁺ binding (e.g., 4–6 ions per molecule) not only promotes structural opening but also reduces GTP affinity, thereby overriding nucleotide inhibition. The allosteric regulation follows a reciprocal model where GTP binding and hydrolysis via tTG's intrinsic GTPase activity modulate transglutaminase function, while Ca²⁺ binding reciprocally influences nucleotide affinity and GTPase efficiency. This bidirectional control allows tTG to switch between GTPase/scaffolding roles in low-Ca²⁺ environments and cross-linking/deamidation activities in high-Ca²⁺ settings. A recent discovery (as of 2024) reveals that TG2 also functions as an RNA-binding protein, with high affinity (K_D = 88 nM) for structured RNAs and poly(dG) sequences, preferentially in its open conformation. This RNA binding, mediated by superficial residues in the catalytic core (e.g., amino acids 173–177), may represent an additional layer of allosteric regulation influencing TG2's multifunctionality, though its precise physiological impacts require further investigation. Physiologically, intracellular GTP levels (≈ 100–500 μM) maintain tTG in its inactive closed state, preventing unwanted protein cross-linking within the . Upon or cellular stress, exposure to extracellular Ca²⁺ (1–2 mM) activates tTG, enabling extracellular matrix stabilization and other functions. Experimental evidence from crystal structures confirms these conformational shifts: the GDP-bound closed form (PDB: 1KV3) shows nucleotide-induced compaction with blocked , while Ca²⁺-bound open structures (PDB: 6KZB) reveal domain separation and accessibility. GTP-complexed crystals (PDB: 4PYG) further demonstrate how nucleotide docking in the β-barrel cleft stabilizes inhibitory interactions, such as hydrogen bonding between Cys-277 and Tyr-516.

Post-translational Modifications

Tissue transglutaminase (TG2) undergoes several post-translational modifications that modulate its enzymatic activity, stability, and cellular localization, primarily in response to and signaling cues. Oxidation of the catalytic site and adjacent cysteines is a key regulatory mechanism, where exposure to leads to the formation of bonds that inactivate TG2. Specifically, a redox-sensitive cysteine triad involving Cys230, Cys370, and Cys371 facilitates intramolecular bond formation, with Cys370 pairing preferentially with Cys230 under mild oxidation or with Cys371 under stronger oxidative conditions, resulting in conformational changes that inhibit transamidation activity. This oxidation is reversible; thioredoxin-1 (TRX-1) reduces the Cys370-Cys371 bond, restoring TG2 activity with a catalytic of 1.6 μM⁻¹ min⁻¹. Additionally, the endoplasmic reticulum-resident protein ERp57 accelerates TG2 oxidation by promoting the Cys370-Cys371 , inactivating it up to 2000-fold faster than small-molecule oxidants like H₂O₂, with an IC₅₀ of 60 nM. Phosphorylation at specific serine and threonine residues further fine-tunes TG2 function. Protein kinase A () phosphorylates TG2 at Ser216, which inhibits its crosslinking activity while enhancing interactions that activate nuclear factor-κB (NF-κB) and downregulate phosphatase and tensin homolog (PTEN), promoting and S-phase progression in cancer cells by up to 54% in models. Ser212 is another potential PKA site, and tyrosine residues such as Tyr219 and Tyr369 have been identified in cancer contexts, though their precise roles in localization or modulation remain under investigation. Ubiquitination targets TG2 for proteasomal degradation, particularly under conditions of high and elevated calcium levels. Calcium influx, triggered by stressors like UV irradiation or H₂O₂, induces polyubiquitination of TG2, leading to its rapid degradation, which is blocked by proteasome inhibitors such as MG132. This process is mediated by E3 ligases like , which ubiquitinates TG2 to control its levels and prevent excessive accumulation in pathological states. These modifications collectively respond to the cellular environment, enhancing TG2 inhibition during and to prevent aberrant protein crosslinking, while allowing reactivation or as needed for . For instance, ERp57 knockdown in endothelial cells increases TG2 activity fourfold, underscoring the regulatory balance in inflammatory contexts.

Biological Functions

Intracellular Roles

Tissue transglutaminase (tTG), also known as transglutaminase 2 (TG2), is primarily localized in the and in its inactive state, where it exists in a compact, GTP-bound conformation that suppresses its enzymatic activities. For example, in human SH-SY5Y cells, approximately 93% of tTG is found in the , with about 7% in the , often associated with or the nuclear matrix; this distribution allows tTG to participate in intracellular signaling without immediate cross-linking. Upon cellular stress or calcium influx, tTG can translocate between these compartments, modulating its interactions and functions while remaining distinct from its secreted extracellular forms. As a GTPase, tTG hydrolyzes GTP to GDP, enabling its role in signal transduction pathways, particularly as a Gα subunit in G-protein-coupled receptor signaling. This activity is crucial for coupling receptors such as α1-adrenergic and oxytocin receptors to phospholipase C (PLC), facilitating the hydrolysis of phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate and diacylglycerol. In its GTP-bound state, tTG inhibits its own transamidation activity, prioritizing signaling; GTP-binding defective mutants enhance cross-linking but disrupt this regulatory balance, underscoring the GTPase function's dominance in non-stressed cells. Additionally, tTG exhibits kinase-like activity independent of GTP, phosphorylating substrates such as insulin-like growth factor-binding protein-3, which mimics kinase signaling to promote cell survival pathways. Through GTP-independent binding, tTG also activates PLCδ1 by releasing it from inhibitory complexes, contributing to calcium mobilization without relying on canonical G-protein hydrolysis. tTG possesses (PDI)-like activity that aids in within the reducing cytosolic environment, distinct from its transamidation domain and independent of calcium or . This function involves catalyzing bond rearrangements in nascent or misfolded proteins, similar to classical PDIs, and is enhanced by oxidized while inhibited by bacitracin. In , tTG plays dual roles: it promotes cell death by cross-linking intracellular proteins, stabilizing apoptotic bodies and preventing leakage of cellular contents, as seen in elevated expression during early stages. Conversely, tTG inhibits by cross-linking and inactivating , such as caspase-3, thereby stabilizing anti-apoptotic factors and enhancing cell survival under certain stresses. In cytoskeletal regulation, tTG modifies key components like and through transamidation or polyamination, influencing and dynamics essential for cell structure and migration. For instance, polyamination of by tTG stabilizes neuronal , promoting their and resistance to depolymerizing agents, which supports axonal integrity. Similarly, cross-linking of filaments enhances cytoskeletal rigidity, facilitating directed cell motility in processes like , without altering overall rates but improving filament stability. These modifications occur intracellularly in response to calcium signals, linking tTG to mechanotransduction and intracellular transport.

Extracellular Roles

Tissue transglutaminase (TG2), also known as , is externalized to the through unconventional pathways that bypass the classical endoplasmic reticulum-Golgi route. This process involves delivery into recycling endosomes via phospholipid-dependent mechanisms, regulated by Rab11 GTPase and the N-terminal β-sandwich domain of TG2. Such enables TG2 to accumulate on the cell surface and within the (ECM) in various cell types, including fibroblasts and endothelial cells, where it exerts its enzymatic and non-enzymatic functions. In the ECM, TG2 enhances structural integrity by catalyzing the cross-linking of key components, including , IV, and , which increases matrix rigidity and resistance to degradation. This stabilization is critical for maintaining tissue architecture during remodeling, as TG2 forms ε-(γ-glutamyl) isopeptide bonds between and residues in these proteins. Additionally, TG2 supports by acting as a co-receptor for , such as α5β1, forming ternary complexes with that promote assembly and downstream signaling via pathways like RhoA/. TG2 contributes to by cross-linking to stabilize blood clots and facilitate into the injury site, thereby accelerating tissue repair. In this context, TG2 upregulation at sites enhances ECM deposition and cell-matrix interactions essential for provisional formation. Conversely, TG2 modulates by interacting with receptor (VEGFR) or modifying thrombospondin, which limits endothelial and vessel sprouting. However, dysregulated TG2 activity leads to excessive ECM cross-linking, promoting scar tissue accumulation and contributing to fibrotic processes through increased stiffness and reduced turnover.

Clinical Significance

Role in Celiac Disease

Tissue transglutaminase (TG2), also known as , plays a central pathological role in celiac disease by deamidating gluten-derived peptides. This enzymatic modification converts neutral residues into negatively charged residues, thereby enhancing the peptides' binding affinity to and molecules on antigen-presenting cells. The increased affinity facilitates presentation to CD4+ T cells, triggering a robust adaptive characterized by production and in the intestinal mucosa. TG2 also serves as the primary autoantigen in celiac disease, where (IgA) antibodies against TG2 (anti-tTG IgA) are a diagnostic hallmark. These autoantibodies exhibit high diagnostic accuracy, with of more than 90% and specificity of more than 95% in untreated patients. Their presence correlates with active disease and exposure, reflecting the autoimmune component driven by TG2's interaction with deamidated . In the celiac intestine, TG2 is upregulated in the , where its cross-linking activity contributes to remodeling and immune complex formation, exacerbating villous and chronic . This upregulation, often triggered by initial gluten-induced stress, amplifies tissue damage through stabilization of inflammatory mediators and promotion of mononuclear cell infiltration. Genetic variations in the TGM2 gene, such as specific single nucleotide polymorphisms (SNPs), have been associated with increased susceptibility to celiac disease by altering TG2 expression or function. Recent research up to 2025 has shown impaired (Treg) function in celiac disease, with TG2 inhibitors showing potential to improve to . Additionally, TG2 inhibitors like ZED1227 have shown promise in phase 2 clinical trials, where they reduced gluten-induced mucosal inflammation and injury in celiac patients.

Role in Cancer

Tissue transglutaminase (TG2), also known as , exhibits a dual role in cancer progression, acting as both a pro-tumorigenic and anti-tumorigenic factor depending on its localization, conformational state, and the tumor context. Intracellularly, TG2 functions as a that promotes survival signaling, such as activation of and PI3K/AKT pathways, which enhance proliferation, , and resistance to in various malignancies. Extracellularly, TG2's cross-linking activity stabilizes the () by forming rigid protein networks, which stiffens the and facilitates through increased and via like β1 and β5. Conversely, TG2 can exert anti-tumor effects in certain scenarios, such as inducing through stabilization of when its cross-linking activity is inhibited, leading to reduced cell viability and proliferation. In some models, TG2-mediated modifications inhibit by altering matrix composition, thereby limiting vascularization and tumor growth, as observed in where high TG2 expression correlates with reduced invasiveness. TG2 is overexpressed in several cancers, including breast, pancreatic, and glioblastoma (GBM), where it drives tumor progression. In breast cancer, elevated TG2 promotes EMT, metastasis, and chemoresistance to agents like doxorubicin via Wnt/β-catenin signaling. In pancreatic cancer, TG2 overexpression recruits tumor-associated macrophages and confers resistance to gemcitabine, associating with poor patient survival. In GBM, TG2 enhances invasion and radioresistance by nuclear translocation and p53 degradation; recent studies show that TG2 knockdown in GBM cell lines like U87 reduces proliferation, colony formation, and tumor growth in xenografts while increasing apoptosis and DNA damage sensitivity post-irradiation. This duality presents a therapeutic : TG2 inhibition sensitizes cancer cells to by overcoming resistance mechanisms, as seen with inhibitors like NC9 enhancing in resistant lines, yet it may paradoxically promote by disrupting integrity and reducing in some contexts, such as and colorectal cancers. High TG2 expression generally correlates with poor in solid tumors, predicting worse overall survival in pancreatic and GBM, though outcomes vary in subtypes.

Roles in Other Diseases

Tissue transglutaminase (TG2) plays a significant role in neurodegeneration by catalyzing the cross-linking of key proteins into toxic aggregates. In , TG2 promotes the formation of Lewy bodies through ε-(γ-glutamyl)lysine isopeptide bonds that link α-synuclein monomers into oligomers and , contributing to neuronal toxicity and disease progression. Similarly, in , TG2 facilitates amyloid-β plaque development by cross-linking peptides into insoluble and induces aggregation into neurofibrillary tangles via dimerization and helical filament formation, exacerbating synaptic dysfunction and cognitive decline. Recent studies have also implicated TG2 in , where it cross-links mutant into intraneuronal inclusions, driving striatal degeneration through mechanisms involving calcium dysregulation and . In fibrotic disorders, TG2 contributes to extracellular matrix () remodeling by enhancing protein cross-linking, leading to tissue stiffness and pathological scarring. In liver fibrosis, TG2-mediated cross-linking of and during the inflammatory phase stabilizes the , promoting progression to . Lung fibrosis involves upregulated TG2 activity in myofibroblasts, which increases deposition and rigidity, impairing respiratory function in conditions like . In cardiac fibrosis, TG2 drives ventricular stiffening through cross-linking in response to injury, contributing to diastolic dysfunction and . TG2 influences cardiovascular pathologies beyond fibrosis by modulating vascular structure and stability. In , TG2 is localized to plaque fibrous caps and shoulder regions, where its cross-linking activity stabilizes atheromatous lesions against rupture by forming resilient networks. Thrombin-induced upregulation of TG2 in endothelial cells further supports plaque integrity, potentially mitigating acute thrombotic events. In , TG2 mediates small artery inward remodeling and aortic stiffening, reducing vessel compliance and perpetuating elevated through cytoskeletal reorganization in vascular cells. TG2 exacerbates chronic inflammation in autoimmune and gastrointestinal disorders by promoting signaling. In , TG2 upregulation in synovial fibroblasts activates pathways, inducing pro-inflammatory s such as TNF-α and IL-6, which sustain joint inflammation and erosion. In (IBD), TG2 is elevated in colonic mucosa, enhancing TNF-α and IL-6 production via activation, thereby amplifying mucosal damage and in conditions like . Beyond these, TG2 is involved in wound healing disorders and diabetes-related complications, with emerging ties to broader . In hypertrophic scars and keloids, excessive TG2 activity leads to aberrant ECM cross-linking, resulting in over-deposition of and prolonged fibrotic responses that impair normal tissue repair. In , hyperglycemia activates TG2, promoting glomerular through TGF-β signaling and accumulation, which contributes to progressive renal decline. Recent investigations highlight TG2's role in regulatory T-cell (Treg) dysfunction, where its activity impairs Treg suppressive function in autoimmune contexts, potentially extending inflammatory dysregulation to diseases like and .

Diagnostic Applications

Tissue transglutaminase (tTG) serves as a key autoantigen in serological diagnostics, particularly for disease, where anti-tTG antibodies are detected via (ELISAs). The IgA anti-tTG is the preferred initial screening test, exhibiting a of 90.7% (95% CI: 87.3%-93.2%) and specificity of 96.4% (95% CI: 95.1%-97.4%) in a of 119 studies involving over 20,000 patients. For patients with IgA deficiency, which affects approximately 2-3% of cases, IgG anti-tTG or anti-deamidated peptide (DGP) IgG/IgA assays are recommended as alternatives or complements, improving detection rates when combined with IgA anti-tTG, with overall sensitivities reaching 93-98% and specificities around 95% in pediatric and adult cohorts. In tissue biopsies from the duodenal mucosa, (IHC) can visualize tTG expression, which is upregulated in active disease, aiding in the confirmation of histopathological changes such as villous atrophy. Additionally, endomysial (EMA) tests, performed on biopsy sections or , indirectly target tTG as the autoantigen, offering high specificity (up to 99%) for diagnosis when positive. Point-of-care (POC) tests for anti-tTG IgA provide rapid results using finger-prick samples, with one novel assay demonstrating 98.31% sensitivity and 98.02% specificity in a study of 401 patients, facilitating immediate screening in clinical settings. Emerging applications include urinary tTG levels or cross-linked products as non-invasive biomarkers for monitoring renal fibrosis in , where elevated urinary tTG correlates with interstitial fibrosis severity in posttransplant patients. Diagnostic limitations include false negatives in selective IgA deficiency, necessitating total IgA measurement alongside testing, and reduced specificity in non-celiac autoimmune conditions. These applications are endorsed by the and European Society for Paediatric , and Nutrition (ESPGHAN) guidelines, which recommend anti-tTG IgA as the first-line test for screening, with biopsy-free diagnosis possible in symptomatic children if levels exceed 10 times the upper limit of normal and confirmed by .

Therapeutic Targeting

Tissue transglutaminase (TG2) has emerged as a promising therapeutic target due to its dysregulation in various diseases, prompting the development of inhibitors that modulate its enzymatic activity. Small-molecule inhibitors targeting the active site, particularly at Cys277 in the catalytic domain, have shown potential in preclinical and clinical studies. For instance, KCC009, a selective TG2 inhibitor, disrupts fibronectin assembly in the extracellular matrix and has been investigated for its role in sensitizing glioblastoma cells to chemotherapy by blocking TG2-mediated cell survival pathways. Similarly, ZED1227, another Cys277-directed covalent inhibitor, has advanced to phase 2b trials (CEC-004/CEL) as of 2025, demonstrating histologic improvements and reduced gluten-induced mucosal damage in patients with celiac disease when used as an adjunct to a gluten-free diet. Disease-specific approaches leverage TG2's context-dependent roles. In cancer, particularly glioblastoma multiforme (GBM), (RNAi) targeting TG2 has reduced tumor cell invasion and remodeling in preclinical models, enhancing efficacy. Anti-TG2 antibodies have also been developed to inhibit extracellular TG2 functions, showing antifibrotic effects in human cell models by blocking accumulation, with potential extension to where TG2 promotes . For neurodegeneration and , serves as an irreversible TG2 inhibitor that reduces protein cross-linking; it has alleviated symptoms in models by mitigating neuronal damage and shown antifibrotic benefits in liver and kidney models by decreasing deposition. Therapeutic challenges include achieving isoform specificity, as TG2 shares structural similarities with other transglutaminases (e.g., , TG3), necessitating selective inhibitors to avoid off-target effects. Additionally, TG2's dual roles—protective in some contexts (e.g., ) and pathological in others (e.g., )—require context-dependent targeting strategies to balance inhibition without disrupting beneficial functions. Recent developments from 2024–2025 highlight emerging modalities. Allosteric inhibitors binding the GTP site, such as LDN-27219, stabilize TG2's closed conformation to suppress transamidase activity, showing preclinical efficacy in reducing renal and vascular by promoting antifibrotic polarization. In celiac disease, TG2 inhibitors and Treg modulation have gained attention as separate or combined approaches to improve and reduce inflammation in preclinical models, as indicated by systematic reviews. Clinical translation remains preclinical for most indications, with no approved TG2-targeted drugs as of 2025. However, preclinical studies in cardiac models demonstrate that TG2 inhibition—via or selective small molecules—attenuates and improves function post-myocardial infarction by shifting phenotypes toward anti-inflammatory M2 states, positioning these agents as promising adjunct therapies, particularly for celiac disease.

Protein Interactions

General Interactions

Tissue transglutaminase (TG2), also known as , participates in a wide array of protein-protein interactions that underpin its multifunctional roles in cellular , signaling, and stabilization. Over 50 binding partners have been identified through techniques such as and co-immunoprecipitation (co-IP), with these interactions often categorized by function, including (ECM) components, cytoskeletal elements, and signaling effectors; a comprehensive database of transglutaminase substrates and interactions, including those of TG2, is available at TRANSDAB. Such associations enable TG2 to influence diverse processes without relying solely on its enzymatic activity, as many interactions occur via non-covalent binding or GTPase-independent mechanisms. Among its substrates, TG2 prominently cross-links fibronectin at specific sites within the fibronectin type III (FNIII) domains, such as FNIII14–15, where lysine residues like Lys1837 and Lys1862 serve as reactive partners, thereby stabilizing ECM fibrils and enhancing matrix rigidity. TG2 also binds directly to integrins, particularly the β1 subunit, to promote RGD-independent cell adhesion and focal contact formation, which activates downstream kinases like focal adhesion kinase (FAK) and Src. Furthermore, TG2 interacts with phospholipase C δ1 (PLCδ1), inhibiting its activity through direct binding, with GTP-dependent dissociation of the complex relieving this inhibition and thereby modulating intracellular calcium release and phosphoinositide signaling pathways. Regulatory interactions fine-tune TG2's activity through modifications; thioredoxin-1 (TRX-1) reduces oxidized residues on extracellular TG2, thereby restoring its transamidation function and preventing inactivation in oxidative environments. In contrast, (NO) inhibits TG2 by S-nitrosylating critical residues, suppressing cross-linking activity and contributing to age-related vascular stiffness. TG2 forms signaling complexes with transcription factors and adhesion molecules, such as p65 (), where it stabilizes the complex to upregulate genes like and hypoxia-inducible factor 1α (HIF-1α), promoting inflammation and epithelial-mesenchymal transition. It also associates with syndecan-4 in focal adhesions, recruiting protein kinase Cα (PKCα) to enhance β1-integrin co-signaling and fibril assembly. Collectively, these general interactions modulate by stiffening the and activating RhoA pathways, while supporting survival through anti-apoptotic signaling in stressed or tumorigenic contexts.

Interaction with Erp57

Tissue transglutaminase (tTG), also known as TG2, interacts with endoplasmic reticulum protein 57 (Erp57, or PDIA3) through a redox-based mechanism involving thiol-disulfide exchange. Erp57 forms a transient mixed disulfide intermediate with the active-site cysteine residue Cys370 of tTG, which facilitates isomerization to an intramolecular disulfide bond between Cys370 and Cys371. This process oxidatively inactivates tTG, switching it from its catalytically active reduced state to an inactive oxidized conformation, with a bimolecular rate constant of 162 mM⁻¹ min⁻¹—approximately 400-fold faster than oxidation by cystine. The Erp57-tTG complex forms primarily in the but can also occur extracellularly or in the under conditions, such as during or cellular secretion of Erp57. Although tTG activity is calcium-dependent, the Erp57-mediated oxidation occurs independently of Ca²⁺, allowing regulation even in low-calcium environments. has confirmed the mixed at Cys370, supporting direct . Functionally, this interaction serves as a regulatory switch to prevent excessive tTG cross-linking activity, which could lead to pathological accumulation; the oxidation is reversible by reductases like thioredoxin-1, maintaining tTG's . Erp57 thus acts as a chaperone-like modulator, ensuring proper status without promoting irreversible inactivation. assays demonstrate that exogenous Erp57 reduces tTG activity to near-background levels. Experimental evidence includes co-immunoprecipitation studies in human umbilical vein endothelial cells (HUVECs), where Erp57 colocalizes with and co-precipitates tTG, confirming formation. SiRNA knockdown of Erp57 in these cells increases tTG activity by up to fourfold, indicating enhanced and less oxidation of tTG. Additionally, cryo-immunogold electron microscopy in duodenal biopsies shows reduced Erp57-tTG association in inflammatory conditions like celiac disease. This interaction is particularly critical in oxidative environments, such as chronic , where dysregulated tTG contributes to and ; lower Erp57 levels correlate with heightened tTG activity and disease progression. Recent studies highlight its potential as a therapeutic target for modulation, including in cancer, where the Erp57-tTG complex influences stability and .