Role of tissue transglutaminase in age-associated ventricular stiffness

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.

Targeting gut microbiota in aging-related cardiovascular dysfunction: focus on the mechanisms

The global population is aging and age-related cardiovascular disease is increasing. Even after controlling for cardiovascular risk factors, readmission and mortality rates remain high. In recent years, more and more in-depth studies have found that the composition of the gut microbiota and its metabolites, such as trimethylamine N-oxide (TMAO), bile acids (BAs), and short-chain fatty acids (SCFAs), affect the occurrence and development of age-related cardiovascular diseases through a variety of molecular pathways, providing a new target for therapy. In this review, we discuss the relationship between the gut microbiota and age-related cardiovascular diseases, and propose that the gut microbiota could be a new therapeutic target for preventing and treating cardiovascular diseases.

LOXL2 inhibition ameliorates pulmonary artery remodeling in pulmonary hypertension

Background

Conduit pulmonary arterial stiffening and the resultant increase in pulmonary vascular impedance has emerged as an important underlying driver of pulmonary arterial hypertension (PAH). Given that matrix deposition is central to vascular remodeling, we evaluated the role of the collagen crosslinking enzyme lysyl oxidase like 2 (LOXL2) in this study.

Methods and Results

Human pulmonary artery smooth muscle cells (PASMCs) subjected to hypoxia showed increased LOXL2 secretion. LOXL2 activity and expression were markedly higher in primary PASMCs isolated from pulmonary arteries of the rat Sugen 5416 + hypoxia (SuHx) model of severe PH. Similarly, LOXL2 protein and mRNA levels were increased in pulmonary arteries (PA) and lungs of rats with PH (SuHx and monocrotaline (MCT) models). Pulmonary arteries (PAs) isolated from rats with PH exhibited hypercontractility to phenylephrine and attenuated vasorelaxation elicited by acetylcholine, indicating severe endothelial dysfunction. Tensile testing revealed a a significant increase in PA stiffness in PH. Treatment with PAT-1251, a novel small-molecule LOXL2 inhibitor, improved active and passive properties of the PA ex vivo. There was an improvement in right heart function as measured by right ventricular pressure volume loops in-vivo with PAT-1251. Importantly PAT-1251 treatment ameliorated PH, resulting in improved pulmonary artery pressures, right ventricular remodeling, and survival.

Conclusion

Hypoxia induced LOXL2 activation is a causal mechanism in pulmonary artery stiffening in PH, as well as pulmonary artery mechanical and functional decline. LOXL2 inhibition with PAT-1251 is a promising approach to improve pulmonary artery pressures, right ventricular elastance, cardiac relaxation, and survival in PAH.

New & Noteworthy

Pulmonary arterial stiffening contributes to the progression of PAH and the deterioration of right heart function. This study shows that LOXL2 is upregulated in rat models of PH. LOXL2 inhibition halts pulmonary vascular remodeling and improves PA contractility, endothelial function and improves PA pressure, resulting in prolonged survival. Thus, LOXL2 is an important mediator of PA remodeling and stiffening in PH and a promising target to improve PA pressures and survival in PH.

Neonatal exposure to hypoxia induces early arterial stiffening via activation of lysyl oxidases

Hypoxia in the neonatal period is associated with early manifestations of adverse cardiovascular health in adulthood including higher risk of hypertension and atherosclerosis. We hypothesize that this occurs due to activation of lysyl oxidases (LOXs) and the remodeling of the large conduit vessels, leading to early arterial stiffening. Newborn C57Bl/6 mice were exposed to hypoxia (FiO2  = 11.5%) from postnatal day 1 (P1) to postnatal day 11 (P11), followed by resumption of normoxia. Controls were maintained in normoxia. Using in vivo (pulse wave velocity; PWV) and ex vivo (tensile testing) arterial stiffness indexes, we determined that mice exposed to neonatal hypoxia had significantly higher arterial stiffness compared with normoxia controls by young adulthood (P60), and it increased further by P120. Echocardiography performed at P60 showed that mice exposed to hypoxia displayed a compensated dilated cardiomyopathy. Western blotting revelated that neonatal hypoxia accelerated age-related increase in LOXL2 protein expression in the aorta and elevated LOXL2 expression in the PA at P11 with a delayed decay toward normoxic controls. In the heart and lung, gene and protein expression of LOX/LOXL2 were upregulated at P11, with a delayed decay when compared to normoxic controls. Neonatal hypoxia results in a significant increase in arterial stiffness in early adulthood due to aberrant LOX/LOXL2 expression. This suggests an acceleration in the mechanical decline of the cardiovascular system, that contributes to increased risk of hypertension in young adults exposed to neonatal hypoxia that may increase susceptibility to further insults.

Inhibition of transglutaminase 2 (TG2) ameliorates ventricular fibrosis in isoproterenol-induced heart failure in rats

AIMS

Transglutaminase (TG) inhibitors represent promising therapeutic interventions in cardiac fibrosis and related dysfunctions. However, it remains unknown how TG inhibition, TG2 in particular, affects the signaling systems that drive pathological fibrosis. This study aimed to examine the effect TG inhibition by cystamine on the progression of isoproterenol (ISO)-induced cardiac fibrosis and dysfunction in rats.

MATERIALS AND METHODS

Cardiac fibrosis was established by intraperitoneal injection of ISO to rats (ISO group), followed by 6 weeks of cystamine injection (ISO + Cys group). The control groups were administered normal saline alone or with cystamine. Hemodynamics, lipid profile, liver enzymes, urea, and creatinine were assessed in conjunction with heart failure markers (serum NT-proANP and cTnI). Left ventricular (LV) and atrial (LA) fibrosis, total collagen content, and mRNA expression of profibrotic markers including TG2 were quantified by Masson's trichrome staining, LC-MS/MS and quantitative PCR, respectively.

KEY FINDINGS

Cystamine administration to ISO rats significantly decreased diastolic and mean arterial pressures, total cholesterol, triglycerides, LDL, liver enzymes, urea, and creatinine levels, while increasing HDL. NT-proANP and cTnI serum levels remained unchanged. In LV tissues, significant reductions in ISO-induced fibrosis and elevated total collagen content were achieved after cystamine treatment, together with a reduction in TG2 concentration. Reduced mRNA expression of several profibrotic genes (COL1A1, FN1, MMP-2, CTGF, periostin, CX43) was also evidenced in LV tissues of ISO rats upon cystamine administration, whereas TGF-β1 expression was depressed in LA tissues. Cystamine decreased TG2 mRNA expression in the LV of control rats, while LV expression of TG2 was relatively low in ISO rats irrespective of cystamine treatment.

SIGNIFICANCE

TG2 inhibition by cystamine in vivo exerted cardioprotective effects against ISO-induced cardiac fibrosis in rats decreasing the LV abundance of several profibrotic markers and the content of TG2 and collagen, suggesting that TG2 pharmacological inhibition could be beneficial to alleviate cardiac fibrosis.

Involvement and possible role of transglutaminases 1 and 2 in mediating fibrotic signalling, collagen cross-linking and cell proliferation in neonatal rat ventricular fibroblasts

Transglutaminase (TG) isoforms control diverse normal and pathophysiologic processes through their capacity to cross-link extracellular matrix (ECM) proteins. Their functional and signalling roles in cardiac fibrosis remain poorly understood, despite some evidence of TG2 involvement in abnormal ECM remodelling in heart diseases. In this study, we investigated the role of TG1 and TG2 in mediating fibrotic signalling, collagen cross-linking, and cell proliferation in healthy fibroblasts by siRNA-mediated knockdown. siRNA for TG1, TG2 or negative control was transfected into cultured neonatal rat ventricular fibroblasts and cardiomyocytes. mRNA expression of TGs and profibrotic, proliferation and apoptotic markers was assessed by qPCR. Cell proliferation and soluble and insoluble collagen were determined by ELISA and LC-MS/MS, respectively. TG1 and TG2 were both expressed in neonatal rat cardiomyocytes and fibroblasts before transfection. Other TGs were not detected before and after transfection. TG2 was predominantly expressed and more effectively silenced than TG1. Knocking down TG1 or TG2 significantly modified profibrotic markers mRNA expression in fibroblasts, decreasing connective tissue growth factor (CTGF) and increasing transforming growth factor-β1 compared to the negative siRNA control. Reduced expression of collagen 3A1 was found upon TG1 knockdown, while TG2 knockdown raised α-smooth muscle actin expression. TG2 knockdown further increased fibroblast proliferation and the expression of proliferation marker cyclin D1. Lower insoluble collagen content and collagen cross-linking were evidenced upon silencing TG1 or TG2. Transcript levels of collagen 1A1, fibronectin 1, matrix metalloproteinase-2, cyclin E2, and BCL-2-associated X protein/B-cell lymphoma 2 ratio were strongly correlated with TG1 mRNA expression, whereas TG2 expression correlated strongly with CTGF mRNA abundance. These findings support a functional and signalling role for TG1 and TG2 from fibroblasts in regulating key processes underlying myocardial ECM homeostasis and dysregulation, suggesting that these isoforms could be potential and promising targets for the development of cardiac fibrosis therapies.

Implications of enigmatic transglutaminase 2 (TG2) in cardiac diseases and therapeutic developments

Cardiac diseases are the leading cause of mortality and morbidity worldwide. Mounting evidence suggests that transglutaminases (TGs), tissue TG (TG2) in particular, are involved in numerous molecular responses underlying the pathogenesis of cardiac diseases. The TG family has several intra- and extracellular functions in the human body, including collagen cross-linking, angiogenesis, cell growth, differentiation, migration, adhesion as well as survival. TGs are thiol- and calcium-dependent acyl transferases that catalyze the formation of a covalent bond between the γ-carboxamide group of a glutamine residue and an amine group, thus increasing the stability, rigidity, and stiffness of the myocardial extracellular matrix (ECM). Excessive accumulation of cross-linked collagen leads to increase myocardial stiffness and fibrosis. Beyond TG2 extracellular protein cross-linking action, mounting evidence suggests that this pleiotropic TG isozyme may also promote fibrotic diseases through cell survival and profibrotic pathway activation at the signaling, transcriptional and translational levels. Due to its multiple functions and localizations, TG2 fulfils critical yet incompletely understood roles in myocardial fibrosis and associated heart diseases, such as cardiac hypertrophy, heart failure, and age-related myocardial stiffness under several conditions. This review summarizes current knowledge and existing gaps regarding the ECM-dependent and ECM-independent roles of TG2 and highlights the therapeutic prospects of targeting TG2 to treat cardiac diseases.

Roles and potential clinical implications of tissue transglutaminase in cardiovascular diseases

Cardiovascular disease (CVD)-related mortality and morbidity are among the most critical disease burdens worldwide. CVDs encompass many diseases and involve complex pathogenesis and pathological changes. While research on these diseases has advanced significantly, treatments and their efficacy remain rather limited. New therapeutic strategies and targets must, therefore, be explored. Tissue transglutaminase (TG2) is pivotal to the pathological development of CVDs, including participating in the cross-linking of extracellular proteins, activation of fibroblasts, hypertrophy and apoptosis of cardiomyocytes, proliferation and migration of smooth muscle cells (SMCs), and inflammatory reactions. Regulating TG2 activity and expression could ensure remarkable improvements in disorders like heart failure (HF), pulmonary hypertension (PH), hypertension, and coronary atherosclerosis. In this review, we summarize recent advances in TG2: we discuss its role and mechanisms in the progression of various CVDs and its potential as a diagnostic and therapeutic target.

Aging influences the cardiac macrophage phenotype and function during steady state and during inflammation

Aging‐mediated immune dysregulation affects the normal cardiac immune cell phenotypes and functions, resulting in cardiac distress. During cardiac inflammation, immune activation is critical for mounting the regenerative responses to maintain normal heart function. We investigated the impact of aging on myeloid cell phenotype and function during cardiac inflammation induced by a sub‐lethal dose of LPS. Our data show that hearts of old mice contain more myeloid cells than the hearts of young mice. However, while the number of monocytic‐derived suppressor cells did not differ between young and old mice, monocytic‐derived suppressor cells from old mice were less able to suppress T‐cell proliferation. Since cardiac resident macrophages (CRMs) are important for immune surveillance, clearance of dead cells, and tissue repair, we focused our studies on CRMs phenotype and function during steady state and LPS treatment. In the steady state, we observed significantly more MHC‐II^low and MHC‐II^high CRMs in the hearts of old mice; however, these populations were decreased in both young and aged mice upon LPS treatment and the decrease in CRM populations correlated with defects in cardiac electrical activity. Notably, mice treated with a liver X receptor (LXR) agonist showed an increase in MerTK expression in CRMs of both young and old mice, which resulted in the reversal of cardiac electrical dysfunction caused by lipopolysaccharide (LPS). We conclude that aging alters the phenotype of CRMs, which contributes to the dysregulation of cardiac electrical dysfunction during infection in aged mice.

Transglutaminases Are Active in Perivascular Adipose Tissue

Transglutaminases (TGs) are crosslinking enzymes best known for their vascular remodeling in hypertension. They require calcium to form an isopeptide bond, connecting a glutamine to a protein bound lysine residue or a free amine donor such as norepinephrine (NE) or serotonin (5-HT). We discovered that perivascular adipose tissue (PVAT) contains significant amounts of these amines, making PVAT an ideal model to test interactions of amines and TGs. We hypothesized that transglutaminases are active in PVAT. Real time RT-PCR determined that Sprague Dawley rat aortic, superior mesenteric artery (SMA), and mesenteric resistance vessel (MR) PVATs express TG2 and blood coagulation Factor-XIII (FXIII) mRNA. Consistent with this, immunohistochemical analyses support that these PVATs all express TG2 and FXIII protein. The activity of TG2 and FXIII was investigated in tissue sections using substrate peptides that label active TGs when in a catalyzing calcium solution. Both TG2 and FXIII were active in rat aortic PVAT, SMAPVAT, and MRPVAT. Western blot analysis determined that the known TG inhibitor cystamine reduced incorporation of experimentally added amine donor 5-(biotinamido)pentylamine (BAP) into MRPVAT. Finally, experimentally added NE competitively inhibited incorporation of BAP into MRPVAT adipocytes. Further studies to determine the identity of amidated proteins will give insight into how these enzymes contribute to functions of PVAT and, ultimately, blood pressure.

Transglutaminase 2 Inhibitor LDN 27219 Age-Dependently Lowers Blood Pressure and Improves Endothelium-Dependent Vasodilation in Resistance Arteries

Transglutaminase 2 (TG2) is an enzyme which in the open conformation exerts transamidase activity, leading to protein cross-linking and fibrosis. In the closed conformation, TG2 participates in transmembrane signaling as a G protein. The unspecific transglutaminase inhibitor cystamine causes vasorelaxation in rat resistance arteries. However, the role of TG2 conformation in vascular function is unknown. We investigated the vascular effects of selective TG2 inhibitors by myography in isolated rat mesenteric and human subcutaneous resistance arteries, patch-clamp studies on vascular smooth muscle cells, and blood pressure measurements in rats and mice. LDN 27219 promoted the closed TG2 conformation and inhibited transamidase activity in mesenteric arteries. In contrast to TG2 inhibitors promoting the open conformation (Z-DON, VA5), LDN 27219 concentration-dependently relaxed rat and resistance human arteries by a mechanism dependent on nitric oxide, large-conductance calcium-activated and voltage-gated potassium channels 7, lowering blood pressure. LDN 27219 also potentiated acetylcholine-induced relaxation by opening potassium channels in the smooth muscle; these effects were abolished by membrane-permeable TG2 inhibitors promoting the open conformation. In isolated arteries from 35- to 40-week-old rats, transamidase activity was increased, and LDN 27219 improved acetylcholine-induced relaxation more than in younger rats. Infusion of LDN 27219 decreased blood pressure more effectively in 35- to 40-week than 12- to 14-week-old anesthetized rats. In summary, pharmacological modulation of TG2 to the closed conformation age-dependently lowers blood pressure and, by opening potassium channels, potentiates endothelium-dependent vasorelaxation. Our findings suggest that promoting the closed conformation of TG2 is a potential strategy to treat age-related vascular dysfunction and lowers blood pressure.

Increased Transglutaminase 2 Expression and Activity in Rodent Models of Obesity/Metabolic Syndrome and Aging

Diastolic dysfunction of the heart and decreased compliance of the vasculature and lungs (i.e., increased organ tissue stiffness) are known features of obesity and the metabolic syndrome. Similarly, cardiac diastolic dysfunction is associated with aging. Elevation of the enzyme transglutaminase 2 (TG2) leads to protein cross-linking and enhanced collagen synthesis and participates as a candidate pathway for development of tissue stiffness. With these observations in mind we hypothesized that TG2 may be elevated in tissues of a rat model of obesity/metabolic syndrome (the ZSF 1 rat) and a mouse model of aging, i.e., the senescent SAMP8 mouse. In the experiments reported here, TG2 expression and activity were found for the first time to be spontaneously elevated in organs from both the ZSF1 rat and the SAMP8 mouse. These observations are consistent with a hypothesis that a TG2-related pathway may participate in the known tissue stiffness associated with cardiac diastolic dysfunction in these two rodent models. The potential TG2 pathway needs better correlation with physiologic dysfunction and may eventually provide novel therapeutic insights to improve tissue compliance.

Commonly used mouse strains have distinct vascular properties

Mice are the most common animal model to investigate human disease and explore physiology. Mice are practical, cost efficient, and easily used for genetic manipulations. Although variability in cardiac structure and function among mouse strains is well noted, the effect of mouse strain on vascular stiffness indices is not known. Here, we compared mouse strain-dependent differences in key vascular stiffness indices among frequently used inbred mouse strains-C57Bl/6J, 129S, and Bl6/129S. In young healthy animals, baseline blood pressure and heart rate were identical in all strains, and independent of gender. However, both active in vivo and passive ex vivo vascular stiffness indices exhibited distinct differences. Specifically, both male and female 129S animals demonstrated the highest tensile stiffness, were least responsive to acetylcholine-induced vasorelaxation, and showed the lowest pulse wave velocity (PWV), an index of in vivo stiffness. C57Bl/6J mice demonstrated the highest PWV, lowest tensile stiffness, and the highest response to acetylcholine-induced vasorelaxation. Interestingly, within each strain, female mice had more compliant aortas. C57Bl/6J mice had thinner vessel walls with fewer layers, whereas 129S mice had the thickest walls with the most layers. Values in the Bl6/129S mixed background mice fell between C57Bl/6J and 129S mice. In conclusion, we show that underlying vascular properties of different inbred wild-type mouse strains are distinct, despite superficial similarities in blood pressure. For each genetic modification, care should be taken to identify proper controls, and conclusions might need to be verified in more than one strain to minimize the risk of false positive studies.

Unveiling the Role of Inflammation and Oxidative Stress on Age-Related Cardiovascular Diseases

The global population above 60 years has been growing exponentially in the last decades, which is accompanied by an increase in the prevalence of age-related chronic diseases, highlighting cardiovascular diseases (CVDs), such as hypertension, atherosclerosis, and heart failure. Aging is the main risk factor for these diseases. Such susceptibility to disease is explained, at least in part, by the increase of oxidative stress, in which it damages cellular components such as proteins, DNA, and lipids. In addition, the chronic inflammatory process in aging “inflammaging” also contributes to cell damage, creating a stressful environment which drives to the development of CVDs. Taken together, it is possible to identify the molecular connection between oxidative stress and the inflammatory process, especially by the crosstalk between the transcription factors Nrf-2 and NF-κB which are mediated by redox signalling and are involved in aging. Therapies that control this process are key targets in the prevention/combat of age-related CVDs. In this review, we show the basics of inflammation and oxidative stress, including the crosstalk between them, and the implications on age-related CVDs.

Glycolysis regulated transglutaminase 2 activation in cardiopulmonary fibrogenic remodeling

The pathophysiology of pulmonary hypertension (PH) and heart failure (HF) includes fibrogenic remodeling associated with the loss of pulmonary arterial (PA) and cardiac compliance. We and others have previously identified transglutaminase 2 (TG2) as a participant in adverse fibrogenic remodeling. However, little is known about the biologic mechanisms that regulate TG2 function. We examined physiological mouse models of experimental PH, HF, and type 1 diabetes that are associated with altered glucose metabolism/glycolysis and report here that TG2 expression and activity are elevated in pulmonary and cardiac tissues under all these conditions. We additionally used PA adventitial fibroblasts to test the hypothesis that TG2 is an intermediary between enhanced tissue glycolysis and fibrogenesis. Our in vitro results show that glycolytic enzymes and TG2 are upregulated in fibroblasts exposed to high glucose, which stimulates cellular glycolysis as measured by Seahorse analysis. We examined the relationship of TG2 to a terminal glycolytic enzyme, pyruvate kinase M2 (PKM2), and found that PKM2 regulates glucose-induced TG2 expression and activity as well as fibrogenesis. Our studies further show that TG2 inhibition blocks glucose-induced fibrogenesis and cell proliferation. Our findings support a novel role for glycolysis-mediated TG2 induction and tissue fibrosis associated with experimental PH, HF, and hyperglycemia.

Cystamine and cysteamine as inhibitors of transglutaminase activity in vivo

Cystamine is commonly used as a transglutaminase inhibitor. This disulphide undergoes reduction in vivo to the aminothiol compound, cysteamine. Thus, the mechanism by which cystamine inhibits transglutaminase activity in vivo could be due to either cystamine or cysteamine, which depends on the local redox environment. Cystamine inactivates transglutaminases by promoting the oxidation of two vicinal cysteine residues on the enzyme to an allosteric disulphide, whereas cysteamine acts as a competitive inhibitor for transamidation reactions catalyzed by this enzyme. The latter mechanism is likely to result in the formation of a unique biomarker, N-(γ-glutamyl)cysteamine that could serve to indicate how cyst(e)amine acts to inhibit transglutaminases inside cells and the body.

The role of the cell-matrix interface in aging and its interaction with the renin-angiotensin system in the aged vasculature

The extracellular matrix (ECM) is an intricate network that provides structural and anchoring support to cells in order to stabilize cell morphology and tissue architecture. The ECM also controls many aspects of the cell's dynamic behavior and fate through its ongoing, bidirectional interaction with cells. These interactions between the cell and components of the surrounding ECM are implicated in several biological processes, including development and adult tissue repair in response to injury, throughout the lifespan of multiple species. The present review gives an overview of the growing evidence that cell-matrix interactions play a pivotal role in the aging process. The focus of the first part of the article is on recent studies using cell-derived decellularized ECM, which strongly suggest that age-related changes in the ECM induce cellular senescence, a well-recognized hallmark of aging. This is followed by a review of findings from genetic studies indicating that changes in genes involved in cell-ECM adhesion and matrix-mediated intracellular signaling cascades affect longevity. Finally, mention is made of novel data proposing an intricate interplay between cell-matrix interactions and the renin-angiotensin system that may have a significant impact on mammalian arterial stiffness with age.

Pharmacologic inhibition of the enzymatic effects of tissue transglutaminase reduces cardiac fibrosis and attenuates cardiomyocyte hypertrophy following pressure overload

Tissue transglutaminase (tTG) is a multifunctional protein with a wide range of enzymatic and non-enzymatic functions. We have recently demonstrated that tTG expression is upregulated in the pressure-overloaded myocardium and exerts fibrogenic actions promoting diastolic dysfunction, while preventing chamber dilation. Our current investigation dissects the in vivo and in vitro roles of the enzymatic effects of tTG on fibrotic remodeling in pressure-overloaded myocardium. Using a mouse model of transverse aortic constriction, we demonstrated perivascular and interstitial tTG activation in the remodeling pressure-overloaded heart. tTG inhibition through administration of the selective small molecule tTG inhibitor ERW1041E attenuated left ventricular diastolic dysfunction and reduced cardiomyocyte hypertrophy and interstitial fibrosis in the pressure-overloaded heart, without affecting chamber dimensions and ejection fraction. In vivo, tTG inhibition markedly reduced myocardial collagen mRNA and protein levels and attenuated transcription of fibrosis-associated genes. In contrast, addition of exogenous recombinant tTG to fibroblast-populated collagen pads had no significant effects on collagen transcription, and instead increased synthesis of matrix metalloproteinase (MMP)3 and tissue inhibitor of metalloproteinases (TIMP)1 through transamidase-independent actions. However, enzymatic effects of matrix-bound tTG increased the thickness of pericellular collagen in fibroblast-populated pads. tTG exerts distinct enzymatic and non-enzymatic functions in the remodeling pressure-overloaded heart. The enzymatic effects of tTG are fibrogenic and promote diastolic dysfunction, but do not directly modulate the pro-fibrotic transcriptional program of fibroblasts. Targeting transamidase-dependent actions of tTG may be a promising therapeutic strategy in patients with heart failure and fibrosis-associated diastolic dysfunction.

Mimicking Cardiac Fibrosis in a Dish: Fibroblast Density Rather than Collagen Density Weakens Cardiomyocyte Function

Cardiac fibrosis is one of the most devastating effects of cardiac disease. Current in vitro models of cardiac fibrosis do not sufficiently mimic the complex in vivo environment of the cardiomyocyte. We determined the local composition and mechanical properties of the myocardium in established mouse models of genetic and acquired fibrosis and tested the effect of myocardial composition on cardiomyocyte contractility in vitro by systematically manipulating the number of fibroblasts and collagen concentration in a platform of engineered cardiac microtissues. The in vitro results showed that while increasing collagen content had little effect on microtissue contraction, increasing fibroblast density caused a significant reduction in contraction force. In addition, the beating frequency dropped significantly in tissues consisting of 50% cardiac fibroblasts or higher. Despite apparent dissimilarities between native and in vitro fibrosis, the latter allows for the independent analysis of local determinants of fibrosis, which is not possible in vivo.