Interplay between transglutaminases and heparan sulphate in progressive renal scarring

S-Nitrosylation of Tissue Transglutaminase in Modulating Glycolysis, Oxidative Stress, and Inflammatory Responses in Normal and Indoxyl-Sulfate-Induced Endothelial Cells

Circulating uremic toxin indoxyl sulfate (IS), endothelial cell (EC) dysfunction, and decreased nitric oxide (NO) bioavailability are found in chronic kidney disease patients. NO nitrosylates/denitrosylates a specific protein's cysteine residue(s), forming S-nitrosothios (SNOs), and the decreased NO bioavailability could interfere with NO-mediated signaling events. We were interested in investigating the underlying mechanism(s) of the reduced NO and how it would regulate the S-nitrosylation of tissue transglutaminase (TG2) and its substrates on glycolytic, redox and inflammatory responses in normal and IS-induced EC injury. TG2, a therapeutic target for fibrosis, has a Ca²⁺-dependent transamidase (TGase) that is modulated by S-nitrosylation. We found IS increased oxidative stress, reduced NADPH and GSH levels, and uncoupled eNOS to generate NO. Immunoblot analysis demonstrated the upregulation of an angiotensin-converting enzyme (ACE) and significant downregulation of the beneficial ACE2 isoform that could contribute to oxidative stress in IS-induced injury. An in situ TGase assay demonstrated IS-activated TG2/TGase aminylated eNOS, NFkB, IkBα, PKM2, G6PD, GAPDH, and fibronectin (FN), leading to caspases activation. Except for FN, TGase substrates were all differentially S-nitrosylated either with or without IS but were denitrosylated in the presence of a specific, irreversible TG2/TGase inhibitor ZDON, suggesting ZDON-bound TG2 was not effectively transnitrosylating to TG2/TGase substrates. The data suggest novel roles of TG2 in the aminylation of its substrates and could also potentially function as a Cys-to-Cys S-nitrosylase to exert NO's bioactivity to its substrates and modulate glycolysis, redox, and inflammation in normal and IS-induced EC injury.

Tissue transglutaminase exacerbates renal fibrosis via alternative activation of monocyte-derived macrophages

Macrophages are important components in modulating homeostatic and inflammatory responses and are generally categorized into two broad but distinct subsets: classical activated (M1) and alternatively activated (M2) depending on the microenvironment. Fibrosis is a chronic inflammatory disease exacerbated by M2 macrophages, although the detailed mechanism by which M2 macrophage polarization is regulated remains unclear. These polarization mechanisms have little in common between mice and humans, making it difficult to adapt research results obtained in mice to human diseases. Tissue transglutaminase (TG2) is a known marker common to mouse and human M2 macrophages and is a multifunctional enzyme responsible for crosslinking reactions. Here we sought to identify the role of TG2 in macrophage polarization and fibrosis. In IL-4-treated macrophages derived from mouse bone marrow and human monocyte cells, the expression of TG2 was increased with enhancement of M2 macrophage markers, whereas knockout or inhibitor treatment of TG2 markedly suppressed M2 macrophage polarization. In the renal fibrosis model, accumulation of M2 macrophages in fibrotic kidney was significantly reduced in TG2 knockout or inhibitor-administrated mice, along with the resolution of fibrosis. Bone marrow transplantation using TG2-knockout mice revealed that TG2 is involved in M2 polarization of infiltrating macrophages derived from circulating monocytes and exacerbates renal fibrosis. Furthermore, the suppression of renal fibrosis in TG2-knockout mice was abolished by transplantation of wild-type bone marrow or by renal subcapsular injection of IL4-treated macrophages derived from bone marrow of wild-type, but not TG2 knockout. Transcriptome analysis of downstream targets involved in M2 macrophages polarization revealed that ALOX15 expression was enhanced by TG2 activation and promoted M2 macrophage polarization. Furthermore, the increase in the abundance of ALOX15-expressing macrophages in fibrotic kidney was dramatically suppressed in TG2-knockout mice. These findings demonstrated that TG2 activity exacerbates renal fibrosis by polarization of M2 macrophages from monocytes via ALOX15.

Molecular profiling of urinary extracellular vesicles in chronic kidney disease and renal fibrosis

Chronic kidney disease (CKD) is a long-term kidney damage caused by gradual loss of essential kidney functions. A global health issue, CKD affects up to 16% of the population worldwide. Symptoms are often not apparent in the early stages, and if left untreated, CKD can progress to end-stage kidney disease (ESKD), also known as kidney failure, when the only possible treatments are dialysis and kidney transplantation. The end point of nearly all forms of CKD is kidney fibrosis, a process of unsuccessful wound-healing of kidney tissue. Detection of kidney fibrosis, therefore, often means detection of CKD. Renal biopsy remains the best test for renal scarring, despite being intrinsically limited by its invasiveness and sampling bias. Urine is a desirable source of fibrosis biomarkers as it can be easily obtained in a non-invasive way and in large volumes. Besides, urine contains biomolecules filtered through the glomeruli, mirroring the pathological state. There is, however, a problem of highly abundant urinary proteins that can mask rare disease biomarkers. Urinary extracellular vesicles (uEVs), which originate from renal cells and carry proteins, nucleic acids, and lipids, are an attractive source of potential rare CKD biomarkers. Their cargo consists of low-abundant proteins but highly concentrated in a nanosize-volume, as well as molecules too large to be filtered from plasma. Combining molecular profiling data (protein and miRNAs) of uEVs, isolated from patients affected by various forms of CKD, this review considers the possible diagnostic and prognostic value of uEVs biomarkers and their potential application in the translation of new experimental antifibrotic therapeutics.

Inhibition of Transglutaminase 2 Reduces Peritoneal Injury in a Chlorhexidine-Induced Peritoneal Fibrosis Model

Long-term peritoneal dialysis (PD) is often associated with peritoneal dysfunction leading to withdrawal from PD. The characteristic pathologic features of peritoneal dysfunction are widely attributed to peritoneal fibrosis and angiogenesis. The detailed mechanisms remain unclear, and treatment targets in clinical settings have yet to be identified. We investigated transglutaminase 2 (TG2) as a possible novel therapeutic target for peritoneal injury. TG2 and fibrosis, inflammation, and angiogenesis were investigated in a chlorhexidine gluconate (CG)-induced model of peritoneal inflammation and fibrosis, representing a noninfectious model of PD-related peritonitis. Transforming growth factor (TGF)-β type I receptor (TGFβR-I) inhibitor and TG2-knockout mice were used for TGF-β and TG2 inhibition studies, respectively. Double immunostaining was performed to identify cells expressing TG2 and endothelial-mesenchymal transition (EndMT). In the rat CG model of peritoneal fibrosis, in situ TG2 activity and protein expression increased during the development of peritoneal fibrosis, as well as increases in peritoneal thickness and numbers of blood vessels and macrophages. TGFβR-I inhibitor suppressed TG2 activity and protein expression, as well as peritoneal fibrosis and angiogenesis. TGF-β1 expression, peritoneal fibrosis, and angiogenesis were suppressed in TG2-knockout mice. TG2 activity was detected by α-smooth muscle actin-positive myofibroblasts, CD31-positive endothelial cells, and ED-1-positive macrophages. CD31-positive endothelial cells in the CG model were α-smooth muscle actin-positive, vimentin-positive, and vascular endothelial-cadherin-negative, suggesting EndMT. In the CG model, EndMT was suppressed in TG2-knockout mice. TG2 was involved in the interactive regulation of TGF-β. As inhibition of TG2 reduced peritoneal fibrosis, angiogenesis, and inflammation associated with TGF-β and vascular endothelial growth factor-A suppression, TG2 may provide a new therapeutic target for ameliorating peritoneal injuries in PD.

Inhibiting Transglutaminase 2 Mediates Kidney Fibrosis via Anti-Apoptosis

Transglutaminase 2 (TG2) is a calcium-dependent transamidating acyltransferase enzyme of the protein-glutamine γ-glutamyltransferase family implicated in kidney injury. In this study, we identified associations between TG2 and chronic kidney disease (CKD) identified by visualizing TG2 in kidney biopsy samples derived from CKD patients using immunohistochemistry and measuring the plasma TG2 concentrations. Our study revealed a connection between TG2 and the pathological markers of kidney disease. We showed high plasma TG2 levels in samples from patients with advanced CKD. In addition, we observed an increase in TG2 expression in tissues concomitant with advanced CKD in human samples. Moreover, we investigated the effect of TG2 inhibition on kidney injury using cystamine, a well-known competitive inhibitor of TG2. TG2 inhibition reduced apoptosis and accumulation of extracellular molecules (ECM) such as fibronectin and pro-inflammatory cytokine IL-8. Collectively, the increased expression of TG2 that was observed in advanced CKD, hence inhibiting TG2 activity, could protect kidney cells from ECM molecule accumulation, apoptosis, and inflammatory responses, thereby preventing kidney fibrosis.

The Transglutaminase-2 Interactome in the APP23 Mouse Model of Alzheimer's Disease

Amyloid-beta (Aβ) deposition in the brain is closely linked with the development of Alzheimer’s disease (AD). Unfortunately, therapies specifically targeting Aβ deposition have failed to reach their primary clinical endpoints, emphasizing the need to broaden the search strategy for alternative targets/mechanisms. Transglutaminase-2 (TG2) catalyzes post-translational modifications, is present in AD lesions and interacts with AD-associated proteins. However, an unbiased overview of TG2 interactors is lacking in both control and AD brain. Here we aimed to identify these interactors using a crossbreed of the AD-mimicking APP23 mouse model with wild type and TG2 knock-out (TG2^−/−) mice. We found that absence of TG2 had no (statistically) significant effect on Aβ pathology, soluble brain levels of Aβ_1–40 and Aβ_1–42, and mRNA levels of TG family members compared to APP23 mice at 18 months of age. Quantitative proteomics and network analysis revealed a large cluster of TG2 interactors involved in synaptic transmission/assembly and cell adhesion in the APP23 brain typical of AD. Comparative proteomics of wild type and TG2^−/− brains revealed a TG2-linked pathological proteome consistent with alterations in both pathways. Our data show that TG2 deletion leads to considerable network alterations consistent with a TG2 role in (dys)regulation of synaptic transmission and cell adhesion in APP23 brains.

Collagen crosslinking: effect on structure, mechanics and fibrosis progression

Biophysical properties of extracellular matrix (ECM), such as matrix stiffness, viscoelasticity and matrix fibrous structure, are emerging as important factors that regulate progression of fibrosis and other chronic diseases. The biophysical properties of the ECM can be rapidly and profoundly regulated by crosslinking reactions in enzymatic or non-enzymatic manners, which further alter the cellular responses and drive disease progression. In-depth understandings of crosslinking reactions will be helpful to reveal the underlying mechanisms of fibrosis progression and put forward new therapeutic targets, whereas related reviews are still devoid. Here, we focus on the main crosslinking mechanisms that commonly exist in a plethora of chronic diseases (e.g. fibrosis, cancer, osteoarthritis) and summarize current understandings including the biochemical reaction, the effect on ECM properties, the influence on cellular behaviors, and related studies in disease model establishment. Potential pharmaceutical interventions targeting the crosslinking process and relevant clinical studies are also introduced. Limitations of pharmaceutical development may be due to the lack of systemic investigations related to the influence on crosslinking mechanism from micro to macro level, which are discussed in the last section. We also propose the unclarified questions regarding crosslinking mechanisms and potential challenges in crosslinking-targeted therapeutics development.

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

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

Insights into the heparan sulphate-dependent externalisation of transglutaminase-2 (TG2) in glucose-stimulated proximal-like tubular epithelial cells

The extracellular matrix crosslinking enzyme transglutaminase 2 (TG2) is highly implicated in tissue fibrosis that precedes end-stage kidney failure. TG2 is unconventionally secreted through extracellular vesicles in a way that depends on the heparan sulphate (HS) proteoglycan syndecan-4 (Sdc4), the deletion of which reduces experimental kidney fibrosis as a result of lower extracellular TG2 in the tubule-interstitium. Here we establish a model of TG2 externalisation in NRK-52E tubular epithelial cells subjected to glucose stress. HS-binding TG2 mutants had reduced extracellular TG2 in transfected NRK-52E, suggesting that TG2-externalisation depends on an intact TG2 heparin binding site. Inhibition of N-ethylmaleimide sensitive factor (NSF) vesicle-fusing ATPase, which was identified in the recently elucidated TG2 kidney membrane-interactome, led to significantly lower TG2-externalisation, thus validating the involvement of membrane fusion in TG2 secretion. As cyclin-G-associated kinase (GAK) had emerged as a further TG2-partner in the fibrotic kidney, we investigated whether glucose-induced TG2-externalisation was accompanied by TG2 phosphorylation in consensus sequences of cyclin-dependent kinase (CDK). Glucose stress led to intense TG2 phosphorylation in serine/threonine CDK-target. TG2 phosphorylation by tyrosine kinases was also increased by glucose. Although the precise role of glucose-induced TG2 phosphorylation is unknown, these novel data suggest that phosphorylation may be involved in TG2 membrane-trafficking.

Spotlight on the Transglutaminase 2-Heparan Sulfate Interaction

Heparan sulfate proteoglycans (HSPGs), syndecan-4 (Sdc4) especially, have been suggested as potential partners of transglutaminase-2 (TG2) in kidney and cardiac fibrosis, metastatic cancer, neurodegeneration and coeliac disease. The proposed role for HSPGs in the trafficking of TG2 at the cell surface and in the extracellular matrix (ECM) has been linked to the fibrogenic action of TG2 in experimental models of kidney fibrosis. As the TG2-HSPG interaction is largely mediated by the heparan sulfate (HS) chains of proteoglycans, in the past few years a number of studies have investigated the affinity of TG2 for HS, and the TG2 heparin binding site has been mapped with alternative outlooks. In this review, we aim to provide a compendium of the main literature available on the interaction of TG2 with HS, with reference to the pathological processes in which extracellular TG2 plays a role.

Biocatalysis by Transglutaminases: A Review of Biotechnological Applications

The biocatalytic activity of transglutaminases (TGs) leads to the synthesis of new covalent isopeptide bonds (crosslinks) between peptide-bound glutamine and lysine residues, but also the transamidation of primary amines to glutamine residues, which ultimately can result into protein polymerisation. Operating with a cysteine/histidine/aspartic acid (Cys/His/Asp) catalytic triad, TGs induce the post-translational modification of proteins at both physiological and pathological conditions (e.g., accumulation of matrices in tissue fibrosis). Because of the disparate biotechnological applications, this large family of protein-remodelling enzymes have stimulated an escalation of interest. In the past 50 years, both mammalian and microbial TGs polymerising activity has been exploited in the food industry for the improvement of aliments' quality, texture, and nutritive value, other than to enhance the food appearance and increased marketability. At the same time, the ability of TGs to crosslink extracellular matrix proteins, like collagen, as well as synthetic biopolymers, has led to multiple applications in biomedicine, such as the production of biocompatible scaffolds and hydrogels for tissue engineering and drug delivery, or DNA-protein bio-conjugation and antibody functionalisation. Here, we summarise the most recent advances in the field, focusing on the utilisation of TGs-mediated protein multimerisation in biotechnological and bioengineering applications.

Transglutaminase-2 promotes metastatic and stem-like phenotypes in osteosarcoma

Osteosarcoma (OS) is a highly aggressive mesenchymal malignancy and the most common primary bone tumor in the pediatric population. OS frequently presents with or develops distal metastases. Patients with metastatic disease have extremely poor survival rates, thus necessitating improved molecular insights into OS metastatic biology. Utilizing our previously characterized genetically engineered mouse model (GEMM) of metastatic OS, we identified enhanced differential expression of Transglutaminase-2 (TGM2) in metastatic OS. However, the role of TGM2 in sarcoma development and metastatic progression remains largely undefined. To further investigate the role of TGM2 in OS metastasis, we performed both gain- and loss-of-function studies for TGM2 in human and mouse OS cell lines. Our data provide evidence that enhanced expression of TGM2 in metastatic OS contributes to migratory and invasive phenotypes. Besides the effects on metastatic phenotypes, we also observed that TGM2 contributes to OS stem-like properties. In addition, treatment with transglutaminase inhibitors had analogous effects on proliferation and migration to TGM2 knockdown. Finally, in vivo xenograft studies demonstrated that TGM2 functionally alters metastatic potential and survival outcome. Together, these data highlight TGM2 as a pro-metastatic factor in OS and a potential avenue for future therapeutic intervention to inhibit metastatic disease.

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

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

Cystamine and Disulfiram Inhibit Human Transglutaminase 2 via an Oxidative Mechanism

The catalytic activity of transglutaminase 2 (TG2), a ubiquitously expressed mammalian enzyme, is regulated by multiple post-translational mechanisms. Because elevated activity of TG2 in the extracellular matrix is associated with organ-specific diseases such as celiac disease and renal fibrosis, there is growing therapeutic interest in inhibitors of this enzyme. Cystamine, a symmetric disulfide compound, is one of the earliest reported TG2 inhibitors. Despite its widespread use as a tool compound to block TG2 activity in vitro and in vivo, its mechanism of action has remained unclear. Here, we demonstrate that cystamine irreversibly inhibits human TG2 ( k_inh/ K_i = 1.2 mM^-1 min^-1) via a mechanism fundamentally distinct from those proposed previously. Through mass spectrometric disulfide mapping and site-directed mutagenesis, we show that cystamine promotes the formation of a physiologically relevant disulfide bond between Cys370 and Cys371 that allosterically abrogates the catalytic activity of human TG2. This discovery led us to evaluate clinically useful thiol → disulfide oxidants for TG2 inhibitory activity. It is demonstrated that disulfiram, a relatively safe oral thiuram disulfide, is a fairly potent TG2 inhibitor ( k_inh/ K_i = 8.3 mM^-1 min^-1) and may therefore provide a practical tool for clinically validating this emerging therapeutic target in intestinal disorders such as celiac disease.

Proteomic Profiling Reveals the Transglutaminase-2 Externalization Pathway in Kidneys after Unilateral Ureteric Obstruction

Increased export of transglutaminase-2 (TG2) by tubular epithelial cells (TECs) into the surrounding interstitium modifies the extracellular homeostatic balance, leading to fibrotic membrane expansion. Although silencing of extracellular TG2 ameliorates progressive kidney scarring in animal models of CKD, the pathway through which TG2 is secreted from TECs and contributes to disease progression has not been elucidated. In this study, we developed a global proteomic approach to identify binding partners of TG2 responsible for TG2 externalization in kidneys subjected to unilateral ureteric obstruction (UUO) using TG2 knockout kidneys as negative controls. We report a robust and unbiased analysis of the membrane interactome of TG2 in fibrotic kidneys relative to the entire proteome after UUO, detected by SWATH mass spectrometry. The data have been deposited to the ProteomeXchange with identifier PXD008173. Clusters of exosomal proteins in the TG2 interactome supported the hypothesis that TG2 is secreted by extracellular membrane vesicles during fibrosis progression. In established TEC lines, we found TG2 in vesicles of both endosomal (exosomes) and plasma membrane origin (microvesicles/ectosomes), and TGF-β1 stimulated TG2 secretion. Knockout of syndecan-4 (SDC4) greatly impaired TG2 exosomal secretion. TG2 coprecipitated with SDC4 from exosome lysate but not ectosome lysate. Ex vivo, EGFP-tagged TG2 accumulated in globular elements (blebs) protruding/retracting from the plasma membrane of primary cortical TECs, and SDC4 knockout impaired bleb formation, affecting TG2 release. Through this combined in vivo and in vitro approach, we have dissected the pathway through which TG2 is secreted from TECs in CKD.