Tissue transglutaminase contributes to interstitial renal fibrosis by favoring accumulation of fibrillar collagen through TGF-beta activation and cell infiltration

Transglutaminase-mediated stiffening of the glomerular basement membrane mitigates pressure-induced reductions in molecular sieving coefficient by reducing compression

Proteinuria, the presence of high molecular weight proteins in the urine, is a primary indicator of chronic kidney disease. Proteinuria results from increased molecular permeability of the glomerular filtration barrier combined with saturation or defects in tubular protein reabsorption. Any solute that passes into the glomerular filtrate traverses the glomerular endothelium, the glomerular basement membrane, and the podocyte slit diaphragm. Damage to any layer of the filter has reciprocal effects on other layers to increase glomerular permeability. The GBM is thought to act as a compressible ultrafilter that has increased molecular selectivity with increased pressure due to compression that reduced the porosity of the GBM with increased pressure. In multiple forms of chronic kidney disease, crosslinking enzymes are upregulated and may act to increase GBM stiffness. Here we show that enzymatically crosslinking porcine GBM with transglutaminase increases the stiffness of the GBM and mitigates pressure-dependent reductions in molecular sieving coefficient. This was modeled mathematically using a modified membrane transport model accounting for GBM compression. Changes in the mechanical properties of the GBM may contribute to proteinuria through pressure-dependent effects on GBM porosity.

The effect of TG2-inhibitory monoclonal antibody zampilimab on tissue fibrosis in human in vitro and primate in vivo models of chronic kidney disease

Fibrotic remodeling is the primary driver of functional loss in chronic kidney disease, with no specific anti-fibrotic agent available for clinical use. Transglutaminase 2 (TG2), a wound response enzyme that irreversibly crosslinks extracellular matrix proteins causing dysregulation of extracellular matrix turnover, is a well-characterized anti-fibrotic target in the kidney. We describe the humanization and characterization of two anti-TG2 monoclonal antibodies (zampilimab [hDC1/UCB7858] and BB7) that inhibit crosslinking by TG2 in human in vitro and rabbit/cynomolgus monkey in vivo models of chronic kidney disease. Determination of zampilimab half-maximal inhibitory concentration (IC50) against recombinant human TG2 was undertaken using the KxD assay and determination of dissociation constant (Kd) by surface plasmon resonance. Efficacy in vitro was established using a primary human renal epithelial cell model of tubulointerstitial fibrosis, to assess mature deposited extracellular matrix proteins. Proof of concept in vivo used a cynomolgus monkey unilateral ureteral obstruction model of chronic kidney disease. Zampilimab inhibited TG2 crosslinking transamidation activity with an IC50 of 0.25 nM and Kd of <50 pM. In cell culture, zampilimab inhibited extracellular TG2 activity (IC50 119 nM) and dramatically reduced transforming growth factor-β1-driven accumulation of multiple extracellular matrix proteins including collagens I, III, IV, V, and fibronectin. Intravenous administration of BB7 in rabbits resulted in a 68% reduction in fibrotic index at Day 25 post-unilateral ureteral obstruction. Weekly intravenous administration of zampilimab in cynomolgus monkeys with unilateral ureteral obstruction reduced fibrosis at 4 weeks by >50%, with no safety signals. Our data support the clinical investigation of zampilimab for the treatment of kidney fibrosis.

Conformational Modulation of Tissue Transglutaminase via Active Site Thiol Alkylating Agents: Size Does Not Matter

TG2 is a unique member of the transglutaminase family as it undergoes a dramatic conformational change, allowing its mutually exclusive function as either a cross-linking enzyme or a G-protein. The enzyme's dysregulated activity has been implicated in a variety of pathologies (e.g., celiac disease, fibrosis, cancer), leading to the development of a wide range of inhibitors. Our group has primarily focused on the development of peptidomimetic targeted covalent inhibitors, the nature and size of which were thought to be important features to abolish TG2's conformational dynamism and ultimately inhibit both its activities. However, we recently demonstrated that the enzyme was unable to bind guanosine triphosphate (GTP) when catalytically inactivated by small molecule inhibitors. In this study, we designed a library of models targeting covalent inhibitors of progressively smaller sizes (15 to 4 atoms in length). We evaluated their ability to inactivate TG2 by measuring their respective kinetic parameters kinact and KI. Their impact on the enzyme's ability to bind GTP was then evaluated and subsequently correlated to the conformational state of the enzyme, as determined via native PAGE and capillary electrophoresis. All irreversible inhibitors evaluated herein locked TG2 in its open conformation and precluded GTP binding. Therefore, we conclude that steric bulk and structural complexity are not necessary factors to consider when designing TG2 inhibitors to abolish G-protein activity.

Transglutaminase 2 in diabetes mellitus: Unraveling its multifaceted role and therapeutic implications for vascular complications

Diabetes, a severe metabolic disease characterized by chronic hypoglycemia, poses debilitating and life-threatening risks of microvascular and macrovascular complications, including blindness, kidney failure, heart attacks, and limb amputation. Addressing these complications is paramount, urging the development of interventions targeting diabetes-associated vascular dysfunctions. To effectively combat diabetes, a comprehensive understanding of the pathological mechanisms underlying complications and identification of precise therapeutic targets are imperative. Transglutaminase 2 (TGase2) is a multifunctional enzyme implicated in the pathogenesis of diverse diseases such as neurodegenerative disorders, fibrosis, and inflammatory conditions. TGase2 has recently emerged as a key player in both the pathogenesis and therapeutic intervention of diabetic complications. This review highlights TGase2 as a therapeutic target for diabetic complications and explores TGase2 inhibition as a promising therapeutic approach in their treatment.

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

High-Affinity Fluorogenic Substrate for Tissue Transglutaminase Reveals Enzymatic Hysteresis

Transglutaminases (TGases) are a family of calcium-dependent enzymes primarily known for their ability to cross-link proteins. Transglutaminase 2 (TG2) is one isozyme in this family whose role is multifaceted. TG2 can act not only as a typical transamidase through its catalytic core but also as a G-protein via its GTP binding site. These two discrete activities are tightly regulated by both environmental stimuli and redox reactions. Ubiquitously expressed in humans, TG2 has been implicated in numerous disease pathologies that require extensive investigation. The catalytic activity of TG2 can be monitored through various mechanisms, including hydrolysis, transamidation, or cleavage of isopeptide bonds. Activity assays are required to monitor the activity of this isozyme not only for studying its transamidation reaction but also for validation of therapeutics designed to abolish this activity. Herein, we present the design, synthesis, and evaluation of a new TG2 activity substrate based on a previously optimized inhibitor scaffold. The substrate APH7 exhibits excellent affinity, selectivity, and reactivity with TG2 (KM = 3.0 μM). Furthermore, its application also allowed the discovery of unique hysteresis at play within the catalytic activity and inhibition reactivity of TG2.

RhoA vesicle trafficking-mediated transglutaminase 2 membrane translocation promotes IgA1 mesangial deposition in IgA nephropathy

Transglutaminase 2 (TGase2) has been shown to contribute to the mesangial IgA1 deposition in a humanized mouse model of IgA nephropathy (IgAN), but the mechanism is not fully understood. In this study, we found that inhibition of TGase2 activity could dramatically decrease the amount of polymeric IgA1 (pIgA1) isolated from patients with IgAN that interacts with human mesangial cells (HMC). TGase2 was expressed both in the cytosol and on the membrane of HMC. Upon treatment with pIgA1, there were more TGase2 recruited to the membrane. Using a cell model of mesangial deposition of pIgA1, we identified 253 potential TGase2-associated proteins in the cytosolic fraction and observed a higher concentration of cellular vesicles and increased expression of Ras homolog family member A (RhoA) in HMC after pIgA1 stimulation. Both the amount of pIgA1 deposited on HMC and membrane TGase2 level were decreased by inhibition of the vesicle trafficking pathway. Mechanistically, TGase2 was found to be coprecipitated with RhoA in the cellular vesicles. Membrane TGase2 expression was greatly increased by overexpression of RhoA, while it was reduced by knockdown of RhoA. Our in vitro approach demonstrated that TGase2 was transported from the cytosol to the membrane through a RhoA-mediated vesicle-trafficking pathway that can facilitate pIgA1 interaction with mesangium in IgAN.

Cell-Impermeable Inhibitors Confirm That Intracellular Human Transglutaminase 2 Is Responsible for the Transglutaminase-Associated Cancer Phenotype

Transglutaminase 2 (TG2) is a multifunctional enzyme primarily responsible for crosslinking proteins. Ubiquitously expressed in humans, TG2 can act either as a transamidase by crosslinking two substrates through formation of an Nε(ɣ-glutaminyl)lysine bond or as an intracellular G-protein. These discrete roles are tightly regulated by both allosteric and environmental stimuli and are associated with dramatic changes in the conformation of the enzyme. The pleiotropic nature of TG2 and multi-faceted activities have resulted in TG2 being implicated in numerous disease pathologies including celiac disease, fibrosis, and cancer. Targeted TG2 therapies have not been selective for subcellular localization, such that currently no tools exist to selectively target extracellular over intracellular TG2. Herein, we have designed novel TG2-selective inhibitors that are not only highly potent and irreversible, but also cell impermeable, targeting only extracellular TG2. We have also further derivatized the scaffold to develop probes that are intrinsically fluorescent or bear an alkyne handle, which target both intra- and extracellular TG2, in order to facilitate cellular labelling and pull-down assays. The fluorescent probes were internalized and imaged in cellulo, and provide the first implicit experimental evidence that by comparison with their cell-impermeable analogues, it is specifically intracellular TG2, and presumably its G-protein activity, that contributes to transglutaminase-associated cancer progression.

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.

Novel irreversible peptidic inhibitors of transglutaminase 2

Transglutaminase 2 (TG2), also referred to as tissue transglutaminase, plays crucial roles in both protein crosslinking and cell signalling. It is capable of both catalysing transamidation and acting as a G-protein, these activities being conformation-dependent, mutually exclusive, and tightly regulated. The dysregulation of both activities has been implicated in numerous pathologies. TG2 is expressed ubiquitously in humans and is localized both intracellularly and extracellularly. Targeted TG2 therapies have been developed but have faced numerous hurdles including decreased efficacy in vivo. Our latest efforts in inhibitor optimization involve the modification of a previous lead compound's scaffold by insertion of various amino acid residues into the peptidomimetic backbone, and derivatization of the N-terminus with substituted phenylacetic acids, resulting in 28 novel irreversible inhibitors. These inhibitors were evaluated for their ability to inhibit TG2 in vitro and their pharmacokinetic properties, and the most promising candidate 35 (k inact/K I = 760 × 103 M-1 min-1) was tested in a cancer stem cell model. Although these inhibitors display exceptional potency versus TG2, with k inact/K I ratios nearly ten-fold higher than their parent compound, their pharmacokinetic properties and cellular activity limit their therapeutic potential. However, they do serve as a scaffold for the development of potent research tools.

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.

Tissue mechanics coevolves with fibrillar matrisomes in healthy and fibrotic tissues

Fibrillar proteins are principal components of extracellular matrix (ECM) that confer mechanical properties to tissues. Fibrosis can result from wound repair in nearly every tissue in adults, and it associates with increased ECM density and crosslinking as well as increased tissue stiffness. Such fibrotic tissues are a major biomedical challenge, and an emerging view posits that the altered mechanical environment supports both synthetic and contractile myofibroblasts in a state of persistent activation. Here, we review the matrisome in several fibrotic diseases, as well as normal tissues, with a focus on physicochemical properties. Stiffness generally increases with the abundance of fibrillar collagens, the major constituent of ECM, with similar mathematical trends for fibrosis as well as adult tissues from soft brain to stiff bone and heart development. Changes in expression of other core matrisome and matrisome-associated proteins or proteoglycans contribute to tissue stiffening in fibrosis by organizing collagen, crosslinking ECM, and facilitating adhesion of myofibroblasts. Understanding how ECM composition and mechanics coevolve during fibrosis can lead to better models and help with antifibrotic therapies.

Structure-activity relationships of hydrophobic alkyl acrylamides as tissue transglutaminase inhibitors

Tissue transglutaminase (TG2) is a multifunctional protein that plays biological roles based on its ability to catalyse protein cross-linking and to function as a non-canonical G-protein known as Ghα. The non-regulated activity of TG2 has been implicated in fibrosis, celiac disease and the survival of cancer stem cells, underpinning the therapeutic potential of cell permeable small molecule inhibitors of TG2. In the current study, we designed a small library of inhibitors to explore the importance of a terminal hydrophobic moiety, as well as the length of the tether to the irreversible acrylamide warhead. Subsequent kinetic evaluation using an in vitro activity assay provided values for the k _inact and K _I parameters for each of these irreversible inhibitors. The resulting structure-activity relationship (SAR) clearly indicated the affinity conferred by dansyl and adamantyl moieties, as well as the efficiency provided by the shortest warhead tether. We also provide the first direct evidence of the capability of these inhibitors to suppress the GTP binding ability of TG2, at least partially. However, it is intriguing to note that the SAR trends observed herein are opposite to those predicted by molecular modelling - namely that longer tether groups should improve binding affinity by allowing for deeper insertion of the hydrophobic moiety into a hydrophobic pocket on the enzyme. This discrepancy leads us to question whether the existing crystallographic structures of TG2 are appropriate for docking non-peptidic inhibitors. In the absence of a more relevant crystallographic structure, the data from rigorous kinetic studies, such as those provided herein, are critically important for the development of future small molecule TG2 inhibitors.

Double-Stranded RNA Enhances Matrix Metalloproteinase-1 and -13 Expressions through TLR3-Dependent Activation of Transglutaminase 2 in Dermal Fibroblasts

UV-irradiation induces the secretion of double-stranded RNA (dsRNA) derived from damaged noncoding RNAs in keratinocytes, which enhance the expression of matrix metalloproteinases (MMP) in non-irradiated dermal fibroblasts, leading to dysregulation of extracellular matrix homeostasis. However, the signaling pathway responsible for dsRNA-induced MMP expression has not been fully understood. Transglutaminase 2 (TG2) is an enzyme that modifies substrate proteins by incorporating polyamine or crosslinking of proteins, thereby regulating their functions. In this study, we showed that TG2 mediates dsRNA-induced MMP-1 expression through NF-κB activation. Treatment of poly(I:C), a synthetic dsRNA analogue binding to toll-like receptor 3 (TLR3), generates ROS, which in turn activates TG2 in dermal fibroblast. Subsequently, TG2 activity enhances translocation of p65 into the nucleus, where it augments transcription of MMP. We confirmed these results by assessing the level of MMP expression in Tlr3^−/−, TG2-knockdowned and Tgm2^−/− dermal fibroblasts after poly(I:C)-treatment. Moreover, treatment with quercetin showed dose-dependent suppression of poly(I:C)-induced MMP expression. Furthermore, ex vivo cultured skin from Tgm2^−/− mice exhibited a significantly reduced level of MMP mRNA compared with those from wild-type mice. Our results indicate that TG2 is a critical regulator in dsRNA-induced MMP expression, providing a new target and molecular basis for antioxidant therapy in preventing collagen degradation.

Urinary levels of pro-fibrotic transglutaminase 2 (TG2) may help predict progression of chronic kidney disease

Renal clinical chemistry only detects kidney dysfunction after considerable damage has occurred and is imperfect in predicting long term outcomes. Consequently, more sensitive markers of early damage and better predictors of progression are being urgently sought, to better support clinical decisions and support shorter clinical trials. Transglutaminase 2 (TG2) is strongly implicated in the fibrotic remodeling that drives chronic kidney disease (CKD). We hypothesized that urinary TG2 and its ε-(γ-glutamyl)-lysine crosslink product could be useful biomarkers of kidney fibrosis and progression. Animal models: a rat 4-month 5/6th subtotal nephrectomy model of CKD and a rat 8-month streptozotocin model of diabetic kidney disease had 24-hour collection of urine, made using a metabolic cage, at regular periods throughout disease development. Patients: Urine samples from patients with CKD (n = 290) and healthy volunteers (n = 33) were collected prospectively, and progression tracked for 3 years. An estimated glomerular filtration rate (eGFR) loss of 2–5 mL/min/year was considered progressive, with rapid progression defined as > 5 mL/min/year. Assays: TG2 was measured in human and rat urine samples by enzyme-linked immunosorbent assay (ELISA) and ε-(γ-glutamyl)-lysine by exhaustive proteolytic digestion and amino acid analysis. Urinary TG2 and ε-(γ-glutamyl)-lysine increased with the development of fibrosis in both animal model systems. Urinary TG2 was 41-fold higher in patients with CKD than HVs, with levels elevated 17-fold by CKD stage 2. The urinary TG2:creatinine ratio (UTCR) was 9 ng/mmol in HV compared with 114 ng/mmol in non-progressive CKD, 1244 ng/mmol in progressive CKD and 1898 ng/mmol in rapidly progressive CKD. Both urinary TG2 and ε-(γ-glutamyl)-lysine were significantly associated with speed of progression in univariate logistic regression models. In a multivariate model adjusted for urinary TG2, ε-(γ-glutamyl)-lysine, age, sex, urinary albumin:creatinine ratio (UACR), urinary protein:creatinine ratio (UPCR), and CKD stage, only TG2 remained statistically significant. Receiver operating characteristic (ROC) curve analysis determined an 86.4% accuracy of prediction of progression for UTCR compared with 73.5% for UACR. Urinary TG2 and ε-(γ-glutamyl)-lysine are increased in CKD. In this pilot investigation, UTCR was a better predictor of progression in patients with CKD than UACR. Larger studies are now warranted to fully evaluate UTCR value in predicting patient outcomes.

Respiratory syncytial virus infection induces the release of transglutaminase 2 from human airway epithelial cells

Respiratory syncytial virus (RSV) is an important human pathogen that causes severe lower respiratory tract infections in young children, the elderly, and the immunocompromised, yet no effective treatments or vaccines are available. The precise mechanism underlying RSV-induced acute airway disease and associated sequelae are not fully understood; however, early lung inflammatory and immune events are thought to play a major role in the outcome of the disease. Moreover, oxidative stress responses in the airways play a key role in the pathogenesis of RSV. Oxidative stress has been shown to elevate cytosolic calcium (Ca²⁺) levels, which in turn activate Ca²⁺-dependent enzymes, including transglutaminase 2 (TG2). Transglutaminase 2 is a multifunctional cross-linking enzyme implicated in various physiological and pathological conditions; however, its involvement in respiratory virus-induced airway inflammation is largely unknown. In this study, we demonstrated that RSV-induced oxidative stress promotes enhanced activation and release of TG2 from human lung epithelial cells as a result of its translocation from the cytoplasm and subsequent release into the extracellular space, which was mediated by Toll-like receptor (TLR)-4 and NF-κB pathways. Antioxidant treatment significantly inhibited RSV-induced TG2 extracellular release and activation via blocking viral replication. Also, treatment of RSV-infected lung epithelial cells with TG2 inhibitor significantly reduced RSV-induced matrix metalloprotease activities. These results suggested that RSV-induced oxidative stress activates innate immune receptors in the airways, such as TLRs, that can activate TG2 via the NF-κB pathway to promote cross-linking of extracellular matrix proteins, resulting in enhanced inflammation.

Dual Role of p73 in Cancer Microenvironment and DNA Damage Response

Understanding the mechanisms that regulate cancer progression is pivotal for the development of new therapies. Although p53 is mutated in half of human cancers, its family member p73 is not. At the same time, isoforms of p73 are often overexpressed in cancers and p73 can overtake many p53 functions to kill abnormal cells. According to the latest studies, while p73 represses epithelial–mesenchymal transition and metastasis, it can also promote tumour growth by modulating crosstalk between cancer and immune cells in the tumor microenvironment, M2 macrophage polarisation, Th2 T-cell differentiation, and angiogenesis. Thus, p73 likely plays a dual role as a tumor suppressor by regulating apoptosis in response to genotoxic stress or as an oncoprotein by promoting the immunosuppressive environment and immune cell differentiation.

Assessing and counteracting fibrosis is a cornerstone of the treatment of CKD secondary to systemic and renal limited autoimmune disorders

Chronic kidney disease (CKD) is an increasing cause of morbidity and mortality worldwide. Besides the higher prevalence of diabetes, hypertension and aging worldwide, immune mediated disorders remain an important cause of kidney disease and are especially prevalent in young adults. Regardless of the initial insult, final pathway to CKD and kidney failure is always the loss of normal tissue and fibrosis development, in which the dynamic equilibrium between extracellular matrix synthesis and degradation is disturbed, leading to excessive production and accumulation. During fibrosis, a multitude of cell types intervene at different levels, but myofibroblasts and inflammatory cells are considered critical in the process. They exert their effects through different molecular pathways, of which transforming growth factor β (TGF-β) has demonstrated to be of particular importance. Additionally, CKD itself promotes fibrosis due to the accumulation of toxins and hormonal changes, and proteinuria is simultaneously a manifestation of CKD and a specific driver of renal fibrosis. Pathways involved in renal fibrosis and CKD are closely interrelated, and although important advances have been made in our knowledge of them, it is still necessary to translate them into clinical practice. Given the complexity of this process, it is highly likely that its treatment will require a multi-target strategy to control the origin of the damage but also the mechanisms that perpetuate it. Fortunately, rapid technology development over the last years and new available drugs in the nephrologist's armamentarium give reasons for optimism that more personalized assistance for CKD and renal fibrosis will appear in the future.

Transglutaminase 2: Development of therapeutic antibodies reveals four inhibitory epitopes and confirms extracellular function in fibrotic remodelling

BACKGROUND AND PURPOSE

Transglutaminase type 2 (TG2) catalyses formation of ε-(γ-glutamyl)-lysine bonds between proteins, including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlies tissue scarring and fibrosis. Many transglutaminase inhibitors exist and allowed for proof-of-concept studies in disease models, but their lack of specificity for the TG2 isoform, and/or poor pharmacokinetic/pharmacodynamic properties have limited their clinical application. We hypothesised that a high affinity TG2-specific antibody could be developed to specifically inhibit extracellular TG2 activity, with characteristics suitable for therapeutic development.

EXPERIMENTAL APPROACH

Individual human TG2 domains were used to immunise mice and generate hybridomas. Supernatants were screened for inhibition of recombinant human TG2 activity, with TG2 specificity determined by ELISA.

KEY RESULTS

Thirteen TG2-specific supernatants inhibited human transamidation activity. Each hybridoma was cloned and antibody mapped to an epitope in the TG2 core domain, using phage display panning of a TG2 fragment library. Four distinct inhibitory epitopes were determined. The most effective antibodies (AB1, DC1 and BB7) bound to amino acids 313-327 (catalytic core), with an IC₅₀ of approximately 10 nM. The antibodies inhibit TG2 in human cells and block ECM accumulation in a primary human proximal tubular epithelial cell model of fibrosis, only 7 antibodies inhibited rat TG2, and all with higher IC₅₀ values.

CONCLUSIONS AND IMPLICATIONS

We identified a preferred inhibitory epitope in human TG2, developed antibodies with required characteristics for clinical development, and established that targeted inhibition of extracellular TG2 transamidation activity is sufficient to modify fibrotic remodelling.

Fighting the Fiber: Targeting Collagen in Lung Fibrosis

Organ fibrosis is characterized by epithelial injury and aberrant tissue repair, where activated effector cells, mostly fibroblasts and myofibroblasts, excessively deposit collagen into the extracellular matrix. Fibrosis frequently results in organ failure and has been estimated to contribute to at least one third of all global deaths. Also lung fibrosis, in particular idiopathic pulmonary fibrosis (IPF), is a fatal disease with rising incidence worldwide. As current treatment options targeting fibrogenesis are insufficient, there is an urgent need for novel therapeutic strategies. During the last decade, several studies have proposed to target intra- and extracellular components of the collagen biosynthesis, maturation, and degradation machinery. This includes intra- and extracellular targets directly acting on collagen gene products, but also such that anabolize essential building blocks of collagen, in particular glycine and proline biosynthetic enzymes. Collagen, however, is a ubiquitous molecule in the body and fulfils essential functions as a macromolecular scaffold, growth factor reservoir, and receptor binding site in virtually every tissue. This review summarizes recent advances and future directions in this field. Evidence for the proposed therapeutic targets and where they currently stand in terms of clinical drug development for treatment of fibrotic disease is provided. The drug targets are furthermore discussed in light of (1) specificity for collagen biosynthesis, maturation and degradation, and (2) specificity for disease-associated collagen. As therapeutic success and safety of these drugs may largely depend on targeted delivery, different strategies for specific delivery to the main effector cells and to the extracellular matrix are discussed.

Transglutaminase 2 mediates lung inflammation and remodeling by transforming growth factor beta 1 via alveolar macrophage modulation

Transforming growth factor beta 1 (TGF-β1) induces pulmonary fibrosis by enhancing epithelial apoptosis and affects the enzymatic activity of transglutaminase 2 (TG2). The aim of this study was to determine the role of TG2 in TGF-β1-induced lung remodeling and alveolar macrophage modulation. We characterized the in vivo effects of TGF-β1 and TG2 on lung inflammation, fibrosis, and macrophage activity using transgenic C57BL/6 mice with wild and null TG2 loci. The effect of TG2 inhibition on in vitro TGF-β1-stimulated alveolar macrophages was assessed through mRNA analysis. TG2 was remarkably upregulated in the lungs of TGF-β1 transgenic (TGF-β1 Tg) mice, especially in alveolar macrophages and epithelial cells. In the absence of TG2, TGF-β1-induced inflammation was suppressed, decreasing the number of macrophages in the bronchoalveolar lavage fluid. In addition, the alveolar destruction and peribronchial fibrosis induced by TGF-β1 overexpression were significantly reduced, which correlated with decreases in the expression of fibroblast growth factor and matrix metallopeptidase 12, respectively. However, TG2 deficiency did not compromise the phagocytic activity of alveolar macrophages in TGF-β1 Tg mice. At the same time, TG2 contributed to the regulation of TGF-β1-induced macrophage activation. Inhibition of TG2 did not affect the TGF-β1-induced expression of CD86, an M1 marker, in macrophages, but it did reverse the TGF-β1-induced expression of CD206. This result suggests that TG2 mediates TGF-β1-induced M2-like polarization but does not contribute to TGF-β1-induced M1 polarization. In conclusion, TG2 regulates macrophage modulation and plays an important role in TGF-β1-induced lung inflammation, destruction, and fibrosis.

Mechanotransduction in fibrosis: Mechanisms and treatment targets

To perceive and integrate the environmental cues, cells and tissues sense and interpret various physical forces like shear, tensile, and compression stress. Mechanotransduction involves the sensing and translation of mechanical forces into biochemical and mechanical signals to guide cell fate and achieve tissue homeostasis. Disruption of this mechanical homeostasis by tissue injury elicits multiple cellular responses leading to pathological matrix deposition and tissue stiffening, and consequent evolution toward pro-inflammatory/pro-fibrotic phenotypes, leading to tissue/organ fibrosis. This review focuses on the molecular mechanisms linking mechanotransduction to fibrosis and uncovers the potential therapeutic targets to halt or resolve fibrosis.

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

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

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

The Plasticity of Nanofibrous Matrix Regulates Fibroblast Activation in Fibrosis

Natural extracellular matrix (ECM) mostly has a fibrous structure that supports and mechanically interacts with local residing cells to guide their behaviors. The effect of ECM elasticity on cell behaviors has been extensively investigated, while less attention has been paid to the effect of matrix fiber-network plasticity at microscale, although plastic remodeling of fibrous matrix is a common phenomenon in fibrosis. Here, a significant decrease is found in plasticity of native fibrotic tissues, which is associated with an increase in matrix crosslinking. To explore the role of plasticity in fibrosis development, a set of 3D collagen nanofibrous matrix with constant modulus but tunable plasticity is constructed by adjusting the crosslinking degree. Using plasticity-controlled 3D culture models, it is demonstrated that the decrease of matrix plasticity promotes fibroblast activation and spreading. Further, a coarse-grained molecular dynamic model is developed to simulate the cell-matrix interaction at microscale. Combining with molecular experiments, it is revealed that the enhanced fibroblast activation is mediated through cytoskeletal tension and nuclear translocation of Yes-associated protein. Taken together, the results clarify the effects of crosslinking-induced plasticity changes of nanofibrous matrix on the development of fibrotic diseases and highlight plasticity as an important mechanical cue in understanding cell-matrix interactions.

Tissue xanthine oxidoreductase activity in a mouse model of aristolochic acid nephropathy

Xanthine oxidoreductase (XOR) is a critical enzyme in purine metabolism and uric acid production, and its levels are reported to increase during stress, thereby promoting organ damage. Herein, we investigated the activity of XOR in a mouse model of aristolochic acid I (AA)-induced nephropathy, a type of nephrotoxic chronic kidney disease (CKD). A persistent decrease in renal function was observed in mice up to 4 weeks after 4 weeks of AA (2.5 mg kg^-1 ) administration. Renal histology revealed an increase in tubular interstitial fibrosis over time. Although AA administration did not change XOR activity in the plasma, heart, liver, or muscle, XOR activity was persistently increased in renal tissue. Our results suggest that the renal tissue-specific increase in XOR activity is involved in the progression of tubulo-interstitial disorders, specifically fibrosis.

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.

Role of the vascular endothelial sodium channel activation in the genesis of pathologically increased cardiovascular stiffness

Cardiovascular (CV) stiffening represents a complex series of events evolving from pathological changes in individual cells of the vasculature and heart which leads to overt tissue fibrosis. While vascular stiffening occurs naturally with ageing it is accelerated in states of insulin (INS) resistance, such as obesity and type 2 diabetes. CV stiffening is clinically manifested as increased arterial pulse wave velocity and myocardial fibrosis-induced diastolic dysfunction. A key question that remains is how are these events mechanistically linked. In this regard, heightened activation of vascular mineralocorticoid receptors (MR) and hyperinsulinaemia occur in obesity and INS resistance states. Further, a downstream mediator of MR and INS receptor activation, the endothelial cell Na+ channel (EnNaC), has recently been identified as a key molecular determinant of endothelial dysfunction and CV fibrosis and stiffening. Increased activity of the EnNaC results in a number of negative consequences including stiffening of the cortical actin cytoskeleton in endothelial cells, impaired endothelial NO release, increased oxidative stress-meditated NO destruction, increased vascular permeability, and stimulation of an inflammatory environment. Such endothelial alterations impact vascular function and stiffening through regulation of vascular tone and stimulation of tissue remodelling including fibrosis. In the case of the heart, obesity and INS resistance are associated with coronary vascular endothelial stiffening and associated reductions in bioavailable NO leading to heart failure with preserved systolic function (HFpEF). After a brief discussion on mechanisms leading to vascular stiffness per se, this review then focuses on recent findings regarding the role of INS and aldosterone to enhance EnNaC activity and associated CV stiffness in obesity/INS resistance states. Finally, we discuss how coronary artery-mediated EnNaC activation may lead to cardiac fibrosis and HFpEF, a condition that is especially pronounced in obese and diabetic females.

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.

Generating cell-derived matrices from human trabecular meshwork cell cultures for mechanistic studies

Ocular hypertension has been attributed to increased resistance to aqueous outflow often as a result of changes in trabecular meshwork (TM) extracellular matrix (ECM) using in vivo animal models (for example, by genetic manipulation) and ex vivo anterior segment perfusion organ cultures. These are, however, complex and difficult in dissecting molecular mechanisms and interactions. In vitro approaches to mimic the underlying substrate exist by manipulating either ECM topography, mechanics, or chemistry. These models best investigate the role of individual ECM protein(s) and/or substrate property, and thus do not recapitulate the multifactorial extracellular microenvironment; hence, mitigating its physiological relevance for mechanistic studies. Cell-derived matrices (CDMs), however, are capable of presenting a 3D-microenvironment rich in topography, chemistry, and whose mechanics can be tuned to better represent the network of native ECM constituents in vivo. Critically, the composition of CDMs may also be fine-tuned by addition of small molecules or relevant bioactive factors to mimic homeostasis or pathology. Here, we first provide a streamlined protocol for generating CDMs from TM cell cultures from normal or glaucomatous donor tissues. Second, we document how TM cells can be pharmacologically manipulated to obtain glucocorticoid-induced CDMs and how generated pristine CDMs can be manipulated with reagents like genipin. Finally, we summarize how CDMs may be used in mechanistic studies and discuss their probable application in future TM regenerative studies.

E3 ligase carboxyl-terminus of Hsp70-interacting protein (CHIP) suppresses fibrotic properties in oral mucosa

BACKGROUND/PURPOSE

Oral submucous fibrosis (OSF) represents a precancerous lesion of oral mucosa that may progress into oral cancer and its major etiological factor is areca nut chewing. Carboxyl-terminus of Hsp70-interacting protein (CHIP) functions as an ubiquitin E3 ligase and is associated with fibrosis diseases. In the current study, we sought to investigate whether CHIP participated in the areca nut-mediated OSF development.

METHODS

The mRNA expression of CHIP in arecoline-stimulated buccal mucosal fibroblasts (BMFs) and OSF tissues was determined by qRT-PCR. Collagen gel contraction, migration and invasion assays were carried out to evaluate the myofibroblast activation. The protein expression levels of α-SMA and transglutaminase 2 (TGM2) were assessed by Western blot.

RESULTS

The expression level of CHIP was reduced in BMFs following arecoline treatment in a dose-dependent manner, which was consistent with the observation of lower CHIP expression in OSF specimen compared to the normal counterparts. Ectopic expression of CHIP mitigated the myofibroblast activities, including elevated collagen gel contractility and cell motility. In addition, we showed that overexpression of CHIP downregulated the α-SMA and TGM-2 expression, which may lead to less fibrosis alteration.

CONCLUSION

CHIP may not only function as a key regulator of protein quality control but also a critical deciding factor to oral fibrogenesis. Our findings suggested that CHIP possesses the anti-fibrotic effect, which may be mediated by TGM2 regulation. Restoration of CHIP could be a therapeutic direction to help OSF patients.

Urinary transglutaminase 2 as a potent biomarker to predict interstitial fibrosis and tubular atrophy of kidney allograft during early posttransplant period in deceased donor kidney transplantation

Purpose

Transglutaminase type 2 (TG2) is an extracellular matrix crosslinking enzyme with a pivotal role in kidney fibrosis. We tested whether quantification of urinary TG2 may represent a noninvasive method to estimate the severity of kidney allograft fibrosis.

Methods

We prospectively collected urine specimens from 18 deceased donor kidney transplant recipients at 1-day, 7-day, 1-month, 3-month, and 6-month posttransplant. In addition, kidney allograft tissue specimens at 0-day and 6-month posttransplant were sampled to analyze the correlation of urinary TG2 and kidney allograft fibrosis.

Results

Thirteen recipients had increased interstitial fibrosis and tubular atrophy (IFTA) scores at the 6-month protocol biopsy (IFTA group). The mean level of urinary TG2 in the IFTA group was higher compared to that of 5 other recipients without IFTA (no IFTA group). Conversely, the mean level of urinary syndecan-4 in the IFTA group was lower than levels in patients without IFTA. In the IFTA group, double immunofluorescent staining revealed that TG2 intensity was significantly upregulated and colocalizations of TG2/heparin sulfate proteoglycan and nuclear syndecan-4 were prominent, usually around tubular structures.

Conclusion

Urinary TG2 in early posttransplant periods is a potent biomarker for kidney allograft inflammation or fibrosis.

Extracellular Matrix in Kidney Fibrosis: More Than Just a Scaffold

Kidney fibrosis is the common histological end-point of progressive, chronic kidney diseases (CKDs) regardless of the underlying etiology. The hallmark of renal fibrosis, similar to all other organs, is pathological deposition of extracellular matrix (ECM). Renal ECM is a complex network of collagens, elastin, and several glycoproteins and proteoglycans forming basal membranes and interstitial space. Several ECM functions beyond providing a scaffold and organ stability are being increasingly recognized, for example, in inflammation. ECM composition is determined by the function of each of the histological compartments of the kidney, that is, glomeruli, tubulo-interstitium, and vessels. Renal ECM is a dynamic structure undergoing remodeling, particularly during fibrosis. From a clinical perspective, ECM proteins are directly involved in several rare renal diseases and indirectly in CKD progression during renal fibrosis. ECM proteins could serve as specific non-invasive biomarkers of fibrosis and scaffolds in regenerative medicine. The gold standard and currently only specific means to measure renal fibrosis is renal biopsy, but new diagnostic approaches are appearing. Here, we discuss the localization, function, and remodeling of major renal ECM components in healthy and diseased, fibrotic kidneys and the potential use of ECM in diagnostics of renal fibrosis and in tissue engineering.

Tissue Transglutaminase-Mediated AT1 Receptor Sensitization Underlies Pro-inflammatory Cytokine LIGHT-Induced Hypertension

BACKGROUND

Although numerous recent studies have shown a strong link between inflammation and hypertension, the underlying mechanisms by which inflammatory cytokines induce hypertension remain to be fully elucidated. Hypertensive disorders are also associated with elevated pressor sensitivity. Tissue transglutaminase (TG2), a potent cross-linking enzyme, is known to be transcriptionally activated by inflammatory cytokines and stabilize angiotensin II (Ang II) receptor AT₁ (AT₁R) via ubiquitination-preventing posttranslational modification. Here we sought to investigate the TG2-mediated AT₁R stabilization in inflammation-induced hypertension and its functional consequences with a focus on receptor abundance and Ang II responsiveness.

METHODS AND RESULTS

Using an experimental model of inflammation-induced hypertension established by introducing the pro-inflammatory tumor necrosis factor cytokine LIGHT, we provide pharmacologic and genetic evidence that TG2 is required for LIGHT-induced hypertension (systolic pressure on day 6: LIGHT = 152.3 ± 7.4 vs. LIGHT+ERW1041E [TG2 inhibitor] = 105.8 ± 13.1 or LIGHT+TG2^−/− = 114.3 ± 4.3 mm Hg, P < 0.05, n = 4–5) and renal compromise (urine albumin/creatinine: LIGHT = 0.17 ± 0.05 vs. LIGHT+ERW1041E = 0.03 ± 0.01 or LIGHT+TG2^−/− = 0.06 ± 0.01 mg/mg; plasma creatinine: LIGHT = 1.11 ± 0.04 vs. LIGHT+ERW1041E = 0.94 ± 0.04 or LIGHT+TG2^−/− = 0.88 ± 0.09 mg/dl; urine volume: LIGHT = 0.23 ± 0.1 vs. LIGHT+ERW1041E = 0.84 ± 0.13 or LIGHT+TG2^−/− = 1.02 ± 0.09 ml/24 hour on day 14, P < 0.05, n = 4–5). Our mechanistic studies showed that the TG2-mediated AT₁R modification and accumulation (relative renal AT₁R level: phosphate-buffered saline [PBS] = 1.23 ± 0.22, LIGHT = 3.49 ± 0.37, and LIGHT+ERW1041E = 1.77 ± 0.46, P < 0.05, n = 3; LIGHT+TG2^+/+ = 85.28 ± 36.11 vs. LIGHT+TG2^−/− = 7.01 ± 5.68, P < 0.05, n = 3) induced by LIGHT is associated with abrogated β-arrestin binding (AT₁R/associated β-arrestin ratio: PBS = 2.62 ± 1.07, LIGHT = 38.60 ± 13.91, and LIGHT+ERW1041E = 6.97 ± 2.91, P < 0.05, n = 3; LIGHT+TG2^+/+ = 66.43 ± 44.81 vs. LIGHT+TG2^−/− = 2.45 ± 1.78, P < 0.01, n = 3) and could be found in renal medulla tubules of kidneys (relative tubular AT₁R level: PBS = 5.91 ± 2.93, LIGHT = 92.82 ± 19.54, LIGHT+ERW1041E = 28.49 ± 11.65, and LIGHT+TG2^−/− = 0.14 ± 0.10, P < 0.01, n = 5) and the blood vasculature (relative vascular AT₁R level: PBS = 0.70 ± 0.30, LIGHT = 13.75 ± 2.49, and LIGHT+ERW1041E = 3.28 ± 0.87, P < 0.01, n = 3), 2 of the tissues highly related to the genesis of hypertension. Our in vitro cellular assays showed that LIGHT stimulation triggered a rapid TG2-dependent increase in the abundance of AT₁Rs (relative AT₁R level after 2-hour LIGHT treatment: AT₁R (WT)+TG2 = 2.21 ± 0.23, AT₁R (Q315A)+TG2 = 0.18 ± 0.23, P < 0.05 vs. starting point = 1, n = 2) and downstream calcium signaling (fold increase in NFAT-driven luciferase activity: Saline = 0.02 ± 0.03, Ang II = 0.17 ± 0.08, LIGHT = 0.05 ± 0.04, LIGHT+Ang II = 0.90 ± 0.04 (P < 0.01 vs. Ang II), and LIGHT+Ang II+ERW1041E = 0.15 ± 0.15 (P < 0.01 vs. LIGHT+Ang II), n = 3).

CONCLUSIONS

Our data indicate an essential and systemic role for TG2 in bridging inflammation to hypertension via its posttranslational modifications stabilizing AT₁ receptor and sensitizing Ang II. Our findings also suggest that TG2 inhibitors could be used as a novel group of cardiovascular agents.

Diet-Induced Obesity Promotes Kidney Endothelial Stiffening and Fibrosis Dependent on the Endothelial Mineralocorticoid Receptor

Obesity is characterized by enhanced MR (mineralocorticoid receptor) activation, vascular stiffness, and associated cardiovascular and kidney disease. Consumption of a Western-style diet (WD), high in saturated fat and refined carbohydrates, by female mice, leads to obesity and vascular stiffening. Use of ECMR (endothelial cell-specific MR) knockout mice supports that ECMR activation is critical for development of vascular and cardiac fibrosis and stiffening. However, the role of ECMR activation in kidney inflammation and fibrosis remains unknown. We hypothesized that cell-specific deletion of ECMR would prevent WD-induced central aortic stiffness and protect the kidney from endothelial dysfunction and vascular stiffening. Four-week-old female ECMR KO and wild-type mice were fed either mouse chow or WD for 16 weeks. WD feeding increased body weight and fat mass, proteinuria, as well as vascular stiffness indices (pulse wave velocity and kidney artery stiffening) and impaired endothelial-dependent vasodilatation without blood pressure changes. The WD-induced kidney arterial stiffening was associated with attenuated eNOS (endothelial NO synthase) activation, increased oxidative stress, proinflammatory immune responses, alterations in extracellular matrix degradation pathways, and fibrosis. ECMR deletion prevented these abnormalities by improving eNOS activation and reducing macrophage proinflammatory M1 polarization, expression of TG2 (transglutaminase 2), and MMP (matrix metalloproteinase)-9. Our data support the concept that ECMR activation contributes to endothelial dysfunction, increased kidney artery fibrosis/stiffening, and impaired NOS (NO synthase) activation, processes associated with macrophage infiltration and polarization, inflammation, and oxidative stress, collectively resulting in tubulointerstitial fibrosis in females consuming a WD.

Biopolymer-delivered vascular endothelial growth factor improves renal outcomes following revascularization

Renal angioplasty and stenting (PTRAs) resolves renal artery stenosis, but inconsistently improves renal function, possibly due to persistent parenchymal damage. We developed a bioengineered fusion of a drug delivery vector (elastin-like polypeptide, ELP) with vascular endothelial growth factor (VEGF), and showed its therapeutic efficacy. We tested the hypothesis that combined ELP-VEGF therapy with PTRAs improves renal recovery more efficiently than PTRAs alone, by protecting the stenotic renal parenchyma. Unilateral renovascular disease (RVD) was induced by renal artery stenosis in 14 pigs. Six weeks later, stenotic kidney blood flow (RBF) and glomerular filtration rate (GFR) were quantified in vivo using multidetector CT. Blood and urine were collected during in vivo studies. All pigs underwent PTRAs and then were randomized into single intrarenal ELP-VEGF administration or placebo (n = 7 each) groups. Pigs were observed for four additional weeks, in vivo CT studies were repeated, and then pigs were euthanized for ex vivo studies to quantify renal microvascular (MV) density, angiogenic factor expression, and morphometric analysis. Renal hemodynamics were similarly blunted in all RVD pigs. PTRAs resolved stenosis but modestly improved RBF and GFR. However, combined PTRAs+ ELP-VEGF improved RBF, GFR, regional perfusion, plasma creatinine, asymmetric dimethlyarginine (ADMA), and albuminuria compared with PTRAs alone, accompanied by improved angiogenic signaling, MV density, and renal fibrosis. Greater improvement of renal function via coadjuvant ELP-VEGF therapy may be driven by enhanced MV proliferation and repair, which ameliorates MV rarefaction and fibrogenic activity that PTRAs alone cannot offset. Thus, our study supports a novel strategy to boost renal recovery in RVD after PTRAs.

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.

Recent advances in the physiopathology of IgA nephropathy

Immunoglobulin A nephropathy or Berger's disease is the most common type of primary glomerulonephritis, which is characterized by IgA1-containing immune-deposits in the glomerular mesangium. Microscopic haematuria and proteinuria are the most common presentations. Mesangial cell proliferation with IgA deposition is found on renal biopsy. Mechanims of the disease implicate at least four key molecules have been implicated in immune complex formation: galactose-deficient IgA1, autoantibodies anti-galactose-deficient-IgA1, soluble CD89 (Fc receptor for IgA) and the CD71 mesangial IgA receptor (transferrin receptor). These factors associated with environmental factors (antigens, food and microbiota) are correlated with disease progression and recurrence after transplantation. This review exploits recent data on the role of these molecular players of the disease, which may improve future therapeutic management of immunoglobulin A nephropathy.

Cellular and molecular mechanisms of kidney fibrosis

Renal fibrosis is the final pathological process common to any ongoing, chronic kidney injury or maladaptive repair. It is considered as the underlying pathological process of chronic kidney disease (CKD), which affects more than 10% of world population and for which treatment options are limited. Renal fibrosis is defined by excessive deposition of extracellular matrix, which disrupts and replaces the functional parenchyma that leads to organ failure. Kidney's histological structure can be divided into three main compartments, all of which can be affected by fibrosis, specifically termed glomerulosclerosis in glomeruli, interstitial fibrosis in tubulointerstitium and arteriosclerosis and perivascular fibrosis in vasculature. In this review, we summarized the different appearance, cellular origin and major emerging processes and mediators of fibrosis in each compartment. We also depicted and discussed the challenges in translation of anti-fibrotic treatment to clinical practice and discuss possible solutions and future directions.

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.

Liver fibrosis: Direct antifibrotic agents and targeted therapies

Liver fibrosis and in particular cirrhosis are the major causes of morbidity and mortality of patients with chronic liver disease. Their prevention or reversal have become major endpoints in clinical trials with novel liver specific drugs. Remarkable progress has been made with therapies that efficiently address the cause of the underlying liver disease, as in chronic hepatitis B and C. Highly effective antiviral therapy can prevent progression or even induce reversal in the majority of patients, but such treatment remains elusive for the majority of liver patients with advanced alcoholic or nonalcoholic steatohepatitis, genetic or autoimmune liver diseases. Moreover, drugs that would speed up fibrosis reversal are needed for patients with cirrhosis, since even with effective causal therapy reversal is slow or the disease may further progress. Therefore, highly efficient and specific antifibrotic agents are needed that can address advanced fibrosis, i.e., the detrimental downstream result of all chronic liver diseases. This review discusses targeted antifibrotic therapies that address molecules and mechanisms that are central to fibrogenesis or fibrolysis, including strategies that allow targeting of activated hepatic stellate cells and myofibroblasts and other fibrogenic effector cells. Focus is on collagen synthesis, integrins and cells and mechanisms specific including specific downregulation of TGFbeta signaling, major extracellular matrix (ECM) components, ECM-crosslinking, and ECM-receptors such as integrins and discoidin domain receptors, ECM-crosslinking and methods for targeted delivery of small interfering RNA, antisense oligonucleotides and small molecules to increase potency and reduce side effects. With an increased understanding of the biology of the ECM and liver fibrosis and an improved preclinical validation, the translation of these approaches to the clinic is currently ongoing. Application to patients with liver fibrosis and a personalized treatment is tightly linked to the development of noninvasive biomarkers of fibrosis, fibrogenesis and fibrolysis.

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.

Matrix Stiffness: the Conductor of Organ Fibrosis

PURPOSE OF REVIEW

Organ fibrosis is a lethal component of scleroderma. The hallmark of scleroderma fibrosis is extensive extracellular matrix (ECM) deposition by activated myofibroblasts, specialized hyper-contractile cells that promote ECM remodeling and matrix stiffening. The purpose of this review is to discuss novel mechanistic insight into myofibroblast activation in scleroderma.

RECENT FINDINGS

Matrix stiffness, traditionally viewed as an end point of organ fibrosis, is now recognized as a critical regulator of tissue fibrogenesis that hijacks the normal physiologic wound-healing program to promote organ fibrosis. Here, we discuss how matrix stiffness orchestrates fibrosis by controlling three fundamental pro-fibrotic mechanisms: (a) mechanoactivation of myofibroblasts, (b) integrin-mediated latent transforming growth factor beta 1 (TGF-β1) activation, and (c) activation of non-canonical TGF-β1 signaling pathways. We also summarize novel therapeutic targets for anti-fibrotic therapy based on the mechanobiology of scleroderma. Future research on mechanobiology of scleroderma may lead to important clinical applications such as improved diagnosis and treatment of patients with scleroderma and other fibrotic-related diseases.

Role of hypoxia-inducible factor-1 in the development of renal fibrosis in mouse obstructed kidney: Special references to HIF-1 dependent gene expression of profibrogenic molecules

The aim of the study is to clarify the role of hypoxia-inducible factor-1 (HIF-1) in the development of renal fibrosis in mouse obstructive nephropathy. We used mice with floxed HIF-1α alleles and tamoxifen-inducible Cre/ERT2 recombinase under ubiquitin C promoter to induce global HIF-1α deletion. Following tamoxifen administration, mice were subjected to unilateral ureteral obstruction (UUO). At 3, 7 and 14 days after UUO, renal gene expression profiles and interstitial fibrosis were assessed. HIF-1 dependent up-regulation of prolyl hydroxylase 3 and glucose transporter-1 was observed in the obstructed kidney at 3 and 7 days but not at 14 days after UUO. Various factors promoting fibrosis were up-regulated during the development of fibrosis. HIF-1 dependent gene expression of profibrotic molecules, plasminogen activator inhibitor 1, connective tissue growth factor, lysyl oxidase like 2 and transglutaminase 2 was observed in the obstructed kidney but such HIF-1 dependency was limited to the early onset of renal fibrosis. Global HIF-1 deletion tended to attenuate interstitial collagen I deposition at 3 days but had no effects thereafter. It is suggested that HIF-1 dependent profibrogenic mechanisms are operating at the early onset of renal fibrosis but its contribution declines with the progression in mouse UUO model.

Inhibiting aerobic glycolysis suppresses renal interstitial fibroblast activation and renal fibrosis

Chronic kidney diseases generally lead to renal fibrosis. Despite great progress having been made in identifying molecular mediators of fibrosis, the mechanism that governs renal fibrosis remains unclear, and so far no effective therapeutic antifibrosis strategy is available. Here we demonstrated that a switch of metabolism from oxidative phosphorylation to aerobic glycolysis (Warburg effect) in renal fibroblasts was the primary feature of fibroblast activation during renal fibrosis and that suppressing renal fibroblast aerobic glycolysis could significantly reduce renal fibrosis. Both gene and protein assay showed that the expression of glycolysis enzymes was upregulated in mouse kidneys with unilateral ureter obstruction (UUO) surgery or in transforming growth factor-β1 (TGF-β1)-treated renal interstitial fibroblasts. Aerobic glycolysis flux, indicated by glucose uptake and lactate production, was increased in mouse kidney with UUO nephropathy or TGF-β1-treated renal interstitial fibroblasts and positively correlated with fibrosis process. In line with this, we found that increasing aerobic glycolysis can remarkably induce myofibroblast activation while aerobic glycolysis inhibitors shikonin and 2-deoxyglucose attenuate UUO-induced mouse renal fibrosis and TGF-β1-stimulated myofibroblast activation. Furthermore, mechanistic study indicated that shikonin inhibits renal aerobic glycolysis via reducing phosphorylation of pyruvate kinase type M2, a rate-limiting glycolytic enzyme associated with cell reliance on aerobic glycolysis. In conclusion, our findings demonstrate the critical role of aerobic glycolysis in renal fibrosis and support treatment with aerobic glycolysis inhibitors as a potential antifibrotic strategy.