Small Artery Remodeling and Erythrocyte Deformability in L-NAME-Induced Hypertension: Role of Transglutaminases

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Effect of long-term antihypertensive treatment on cerebrovascular structure and function in hypertensive rats

Midlife hypertension is an important risk factor for cognitive impairment and dementia, including Alzheimer's disease. We investigated the effects of long-term treatment with two classes of antihypertensive drugs to determine whether diverging mechanisms of blood pressure lowering impact the brain differently. Spontaneously hypertensive rats (SHR) were either left untreated or treated with a calcium channel blocker (amlodipine) or beta blocker (atenolol) until one year of age. The normotensive Wistar Kyoto rat (WKY) was used as a reference group. Both drugs lowered blood pressure equally, while only atenolol decreased heart rate. Cerebrovascular resistance was increased in SHR, which was prevented by amlodipine but not atenolol. SHR showed a larger carotid artery diameter with impaired pulsatility, which was prevented by atenolol. Cerebral arteries demonstrated inward remodelling, stiffening and endothelial dysfunction in SHR. Both treatments similarly improved these parameters. MRI revealed that SHR have smaller brains with enlarged ventricles. In addition, neurofilament light levels were increased in cerebrospinal fluid of SHR. However, neither treatment affected these parameters. In conclusion, amlodipine and atenolol both lower blood pressure, but elicit a different hemodynamic profile. Both medications improve cerebral artery structure and function, but neither drug prevented indices of brain damage in this model of hypertension.

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.

Mechanobiology of Microvascular Function and Structure in Health and Disease: Focus on the Coronary Circulation

The coronary microvasculature plays a key role in regulating the tight coupling between myocardial perfusion and myocardial oxygen demand across a wide range of cardiac activity. Short-term regulation of coronary blood flow in response to metabolic stimuli is achieved via adjustment of vascular diameter in different segments of the microvasculature in conjunction with mechanical forces eliciting myogenic and flow-mediated vasodilation. In contrast, chronic adjustments in flow regulation also involve microvascular structural modifications, termed remodeling. Vascular remodeling encompasses changes in microvascular diameter and/or density being largely modulated by mechanical forces acting on the endothelium and vascular smooth muscle cells. Whereas in recent years, substantial knowledge has been gathered regarding the molecular mechanisms controlling microvascular tone and how these are altered in various diseases, the structural adaptations in response to pathologic situations are less well understood. In this article, we review the factors involved in coronary microvascular functional and structural alterations in obstructive and non-obstructive coronary artery disease and the molecular mechanisms involved therein with a focus on mechanobiology. Cardiovascular risk factors including metabolic dysregulation, hypercholesterolemia, hypertension and aging have been shown to induce microvascular (endothelial) dysfunction and vascular remodeling. Additionally, alterations in biomechanical forces produced by a coronary artery stenosis are associated with microvascular functional and structural alterations. Future studies should be directed at further unraveling the mechanisms underlying the coronary microvascular functional and structural alterations in disease; a deeper understanding of these mechanisms is critical for the identification of potential new targets for the treatment of ischemic heart disease.

Thioredoxin deficiency exacerbates vascular dysfunction during diet-induced obesity in small mesenteric artery in mice

OBJECTIVE

Thioredoxin (Trx) is a small cellular redox protein with established antioxidant and disulfide reductase properties. We hypothesized that Trx deficiency in mice would cause increased oxidative stress with consequent redox imbalance that would exacerbate obesity-induced vascular dysfunction.

METHODS

Non-transgenic (NT, C57BL/6) and dominant-negative Trx (dnTrx-Tg, low levels of redox-active protein) mice were either fed a normal diet (NC) or high fat diet plus sucrose (HFS) diet for 4 months (3-month HFD+ 1-month HFS). Weight gain, glucose tolerance test (GTT), insulin tolerance test (ITT), and other metabolic parameters were performed following NC or HFS diet. Arterial structural remodeling and functional parameters were assessed by myography.

RESULTS

Our study found that dnTrx mice with lower levels of active Trx exacerbated myogenic tone, inward arterial remodeling, arterial stiffening, phenylephrine-induced contraction, and endothelial dysfunction of MA. Additionally, FeTMPyP, a peroxynitrite decomposition catalyst, acutely decreased myogenic tone and contraction and normalized endothelial function in MA from dnTrx-Tg mice on HFS via increasing nitric oxide (NO)-mediated relaxation.

CONCLUSIONS

Our results indicate that deficiency of active Trx exacerbates MA contractile and relaxing properties during diet-induced obesity demonstrating that loss of redox balance in obesity is a key mechanism of vascular endothelial dysfunction.

Hyperglycemia-induced transcriptional regulation of ROCK1 and TGM2 expression is involved in small artery remodeling in obese diabetic Göttingen Minipigs

Obesity and diabetes in humans are associated with hypertrophic remodeling and increased media:lumen ratio of small resistance arteries, which is an independent predictor of cardiovascular events. In order to minimize increases in media:lumen ratio, hypertrophic remodeling should be accompanied by outward remodeling. We aimed to investigate the mechanisms of structural remodeling in small pial arteries (PAs) and terminal mesenteric arteries (TMAs) from obese Göttingen Minipigs with or without diabetes. Göttingen Minipigs received either control diet (lean control (LC)), high fat/high fructose/high cholesterol diet (FFC), or FFC diet with streptozotocin (STZ)-induced diabetes (FFC/STZ) for 13 months. At the end of the study (20 months), we assessed body weight, fasting plasma biochemistry, passive vessel dimensions, mRNA expression (matrix metallopeptidases 2/9 (MMP2, MMP9), tissue inhibitor of metallopeptidase 1 (TIMP1), transglutaminase 2 (TGM2), Rho-kinase 1 (ROCK1), TGFβ-receptor 2 (TGFBR2), and IGF1-receptor (IGFR1) genes), and immunofluorescence in PAs and TMAs. We performed multiple linear correlation analyses using plasma values, structural data, and gene expression data. We detected outward hypertrophic remodeling in TMAs and hypertrophic remodeling in PAs from FFC/STZ animals. ROCK1 and TGM2 genes were up-regulated in PAs and TMAs from the FFC/STZ group. Passive lumen diameter (PLD) of TMAs was correlated with plasma values of glucose (GLU), fructosamine (FRA), total cholesterol (TC), and triglycerides (TGs). ROCK1 and TGM2 expressions in TMAs were correlated with PLD, plasma GLU, fructosamine, and TC. ROCK1 and TGM2 proteins were immunolocalized in the media of PAs and TMAs, and their fluorescence levels were increased in the FFC/STZ group. Hyperglycemia/hyperlipidemia is involved in regulation of ROCK1 and TGM2 expression leading to outward remodeling of small resistance arteries in obese diabetic Göttingen Minipigs.

Animal Models of Hypertension: A Scientific Statement From the American Heart Association

Hypertension is the most common chronic disease in the world, yet the precise cause of elevated blood pressure often cannot be determined. Animal models have been useful for unraveling the pathogenesis of hypertension and for testing novel therapeutic strategies. The utility of animal models for improving the understanding of the pathogenesis, prevention, and treatment of hypertension and its comorbidities depends on their validity for representing human forms of hypertension, including responses to therapy, and on the quality of studies in those models (such as reproducibility and experimental design). Important unmet needs in this field include the development of models that mimic the discrete hypertensive syndromes that now populate the clinic, resolution of ongoing controversies in the pathogenesis of hypertension, and the development of new avenues for preventing and treating hypertension and its complications. Animal models may indeed be useful for addressing these unmet needs.

Shear Stress Attenuates Inward Remodeling in Cultured Mouse Thoracodorsal Arteries in an eNOS-Dependent, but Not Hemodynamic Manner, and Increases Cx37 Expression

BACKGROUND

Arteries chronically constricted in culture remodel to smaller diameters. Conversely, elevated luminal shear stress (SS) promotes outward remodeling of arteries in vivo and prevents inward remodeling in culture in a nitric oxide synthase (NOS)-dependent manner.

OBJECTIVES

To determine whether SS-induced prevention of inward remodeling in cultured arteries is specifically eNOS-dependent and requires dilation, and whether SS alters the expression of eNOS and other genes potentially involved in remodeling.

METHODS

Female mouse thoracodorsal arteries were cannulated, pressurized to 80 mm Hg, and cultured for 2 days with low SS (<7 dyn/cm2), high SS (≥15 dyn/cm2), high SS + L-NAME (NOS inhibitor, 10-4 M), or high SS in arteries from eNOS-/- mice. In separate arteries cultured 1 day with low or high SS, eNOS and connexin (Cx) 37, Cx40, and Cx43 mRNA were assessed with real-time PCR.

RESULTS

High SS caused little change in passive diameters after culture (-4.7 ± 2.0%), which was less than low SS (-18.9 ± 1.4%; p < 0.0001), high SS eNOS-/- (-18.0 ± 1.5; p < 0.001), or high SS + L-NAME (-12.0 ± 0.6%; nonsignificant) despite similar constriction during culture. Cx37 mRNA expression was increased (p < 0.05) with high SS, but other gene levels were not different.

CONCLUSIONS

eNOS is involved in SS-induced prevention of inward remodeling in cultured small arteries. This effect does not require NO-mediated dilation. SS increased Cx37.

Knockdown of transglutaminase-2 prevents early age-induced vascular changes in mice1

PURPOSE

To determine whether the absence of transglutaminase 2 enzyme (TG2) in TG2 knockout mice (TG2-/-) protect them against early age-related functional and histological arterial changes.

METHODS

Pulse wave velocity (PWV) was measured using non-invasive Doppler and mean arterial pressure (MAP) was measured in awake mice using tail-cuff system. Thoracic aortas were excised for evaluation of endothelial dependent vasodilation (EDV) by wire myography, as well as histological analyses.

RESULTS

PWV and MAP were similar in TG2-/-mice to age-matched wild type (WT) control mice. Old WT mice exhibited a markedly attenuated EDV as compared to young WT animals. The TG2-/-young and old mice had enhanced EDV responses (p<0.01) as compared to WT mice. There was a significant increase in TG2 crosslinks by IHC in WT old group compared to Young, with no stain in the TG2-/-animals. Optical microscopy examination of Old WT mice aorta showed thinning and fragmentation of elastic laminae. Young WT mice, old and young TG2-/-mice presented regularly arranged and parallel elastic laminae of the tunica media.

CONCLUSION

The genetic suppression of TG2 delays the age-induced endothelial dysfunction and histological modifications.

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.

In Utero Exposure to Maternal Diabetes Is Associated With Early Abnormal Vascular Structure in Offspring

Aim/hypothesis:In utero exposure to maternal diabetes increases the risk of developing hypertension and cardiovascular disorders during adulthood. We have previously shown that this is associated with changes in vascular tone in favor of a vasoconstrictor profile, which is involved in the development of hypertension. This excessive constrictor tone has also a strong impact on vascular structure. Our objective was to study the impact of in utero exposure to maternal diabetes on vascular structure and remodeling induced by chronic changes in hemodynamic parameters. Methods and Results: We used an animal model of rats exposed in utero to maternal hyperglycemia (DMO), which developed hypertension at 6 months of age. At a pre-hypertensive stage (3 months of age), we observed deep structural modifications of the vascular wall without any hemodynamic perturbations. Indeed, in basal conditions, resistance arteries of DMO rats are smaller than those of control mother offspring (CMO) rats; in addition, large arteries like thoracic aorta of DMO rats have an increase of smooth muscle cell attachments to elastic lamellae. In an isolated perfused kidney, we also observed a leftward shift of the flow/pressure relationship, suggesting a rise in renal peripheral vascular resistance in DMO compared to CMO rats. In this context, we studied vascular remodeling in response to reduced blood flow by in vivo mesenteric arteries ligation. In DMO rats, inward remodeling induced by a chronic reduction in blood flow (1 or 3 weeks after ligation) did not occur by contrast to CMO rats in which arterial diameter decreased from 428 ± 17 μm to 331 ± 20 μm (at 125 mmHg, p = 0.001). In these animals, the transglutaminase 2 (TG2) pathway, essential for inward remodeling development in case of flow perturbations, was not activated in low-flow (LF) mesenteric arteries. Finally, in old hypertensive DMO rats (18 months of age), we were not able to detect a pressure-induced remodeling in thoracic aorta. Conclusions: Our results demonstrate for the first time that in utero exposure to maternal diabetes induces deep changes in the vascular structure. Indeed, the early narrowing of the microvasculature and the structural modifications of conductance arteries could be a pre-emptive adaptation to fetal programming of hypertension.

Update on novel targets and potential treatment avenues in pulmonary hypertension

Pulmonary hypertension (PH) is a condition marked by a combination of constriction and remodeling within the pulmonary vasculature. It remains a disease without a cure, as current treatments were developed with a focus on vasodilatory properties but do not reverse the remodeling component. Numerous recent advances have been made in the understanding of cellular processes that drive pathologic remodeling in each layer of the vessel wall as well as the accompanying maladaptive changes in the right ventricle. In particular, the past few years have yielded much improved insight into the pathways that contribute to altered metabolism, mitochondrial function, and reactive oxygen species signaling and how these pathways promote the proproliferative, promigratory, and antiapoptotic phenotype of the vasculature during PH. Additionally, there have been significant advances in numerous other pathways linked to PH pathogenesis, such as sex hormones and perivascular inflammation. Novel insights into cellular pathology have suggested new avenues for the development of both biomarkers and therapies that will hopefully bring us closer to the elusive goal: a therapy leading to reversal of disease.

Brief serotonin exposure initiates arteriolar inward remodeling processes in vivo that involve transglutaminase activation and actin cytoskeleton reorganization

Inward remodeling of the resistance vasculature is strongly associated with life-threatening cardiovascular events. Previous studies have demonstrated that both actin polymerization and the activation of transglutaminases mediate early stages of the transition from a structurally normal vessel to an inwardly remodeled one. Ex vivo studies further suggest that a few hours of exposure to vasoconstrictor agonists induces inward remodeling in the absence of changes in intraluminal pressure. Here we report that a short, 10-min, topical exposure to serotonin (5-HT) + N(ω)-nitro-l-arginine methyl ester hydrochloride (l-NAME) was sufficient to initiate inward remodeling processes in rat cremasteric feed arterioles (100-200 μm lumen diameter), in vivo. Addition of the transglutaminase inhibitor, cystamine, blocked the in vivo remodeling. We further demonstrate that, in isolated arterioles, 5-HT + l-NAME activates transglutaminases and modulates the phosphorylation state of cofilin, a regulator of actin depolymerization. The 5-HT + l-NAME-induced remodeling process in isolated arterioles was also inhibited by an inhibitor of Lim Kinase, the kinase that phosphorylates and inactivates cofilin. Therefore, our results indicate that a brief vasoconstriction induced by 5-HT + l-NAME is able to reduce the passive structural diameter of arterioles through processes that are dependent on the activation of transglutaminases and Lim kinase, and the subsequent phosphorylation of cofilin.

Monocytic Tissue Transglutaminase in a Rat Model for Reversible Acute Rejection and Chronic Renal Allograft Injury

Acute rejection is a major risk factor for chronic allograft injury (CAI). Blood leukocytes interacting with allograft endothelial cells during acute rejection were suggested to contribute to the still enigmatic pathogenesis of CAI. We hypothesize that tissue transglutaminase (Tgm2), a multifunctional protein and established marker of M2 macrophages, is involved in acute and chronic graft rejection. We focus on leukocytes accumulating in blood vessels of rat renal allografts (Fischer-344 to Lewis), an established model for reversible acute rejection and CAI. Monocytes in graft blood vessels overexpress Tgm2 when acute rejection peaks on day 9 after transplantation. Concomitantly, caspase-3 is activated, suggesting that Tgm2 expression is linked to apoptosis. After resolution of acute rejection on day 42, leukocytic Tgm2 levels are lower and activated caspase-3 does not differ among isografts and allografts. Cystamine was applied for 4 weeks after transplantation to inhibit extracellular transglutaminase activity, which did, however, not reduce CAI in the long run. In conclusion, this is the first report on Tgm2 expression by monocytes in vivo. Tgm2 may be involved in leukocytic apoptosis and thus in reversion of acute rejection. However, our data do not support a role of extracellular transglutaminase activity as a factor triggering CAI during self-limiting acute rejection.

Transglutaminase 2 promotes PDGF-mediated activation of PDGFR/Akt1 and β-catenin signaling in vascular smooth muscle cells and supports neointima formation

BACKGROUND

Phenotypic switch of vascular smooth muscle cells (VSMCs) accompanies neointima formation and associates with vascular diseases. Platelet-derived growth factor (PDGF)-induced activation of PDGFR/Akt1 and β-catenin signaling pathways in VSMCs has been implicated in vessel occlusion. Transglutaminase 2 (TG2) regulates these pathways and its levels are increased in the neointima.

OBJECTIVE

The aim of this study was to evaluate the role of TG2 in PDGF/β-catenin signaling cross-talk and assess its contribution to neointima.

METHODS

Aortic VSMCs from wild-type and TG2 knockout mice were tested in vitro for levels of VSMC markers, proliferation, migration and PDGF-induced activation of PDGFR/Akt1 and β-catenin pathways. Neointima in these mice was studied ex vivo in coronary vessels using a heart slice model and in vivo using a carotid artery ligation model.

RESULTS

Genetic deletion of TG2 attenuated the PDGF-induced phenotypic switch of aortic VSMCs, reduced their proliferation and migration rates, and inhibited PDGF-induced activation of PDGFR/Akt1 and β-catenin pathways in both ex vivo and in vivo neointima models. Importantly, genetic deletion of TG2 also markedly attenuated vessel occlusion.

CONCLUSIONS

TG2 promotes neointima formation by mediating the PDGF-induced activation of the PDGFR/Akt1 and β-catenin pathways in VSMCs. This study identifies TG2 as a potential therapeutic target for blocking neointima in blood vessels.

Transglutaminase activity is decreased in large arteries from hypertensive rats compared with normotensive controls

Transglutaminases (TGs) catalyze the formation of covalent cross-links between glutamine residues and amine groups. This cross-linking activity has been implicated in arterial remodeling. Because hypertension is characterized by arterial remodeling, we hypothesized that TG activity, expression, and functionality would be increased in the aorta, but not in the vena cava (which does not undergo remodeling), from hypertensive rats relative to normotensive rats. Spontaneously hypertensive stroke-prone rats (SHRSP) and DOCA-salt rats as well as their respective normotensive Wistar-Kyoto or Sprague-Dawley counterparts were used. Immunohistochemistry and Western blot analysis measured the presence and expression of TG1 and TG2, in situ activity assays quantified active TGs, and isometric contractility was used to measure TG functionality. Contrary to our hypothesis, the activity (52% DOCA-salt vs. control rats and 56% SHRSP vs. control rats, P < 0.05), expression (TG1: 54% DOCA-salt vs. control rats, P > 0.05, and TG2: 77% DOCA-salt vs. control rats, P < 0.05), and functionality of TG1 and TG2 were decreased in the aorta, but not in the vena cava, from hypertensive rats. Mass spectrometry identified proteins uniquely amidated by TGs in the aorta that play roles in cytoskeletal regulation, redox regulation, and DNA/RNA/protein synthesis and regulation and in the vena cava that play roles in cytoskeletal regulation, coagulation regulation, and cell metabolism. Consistent with the idea that growing cells lose TG2 expression, vascular smooth muscle cells placed in culture lost TG2 expression. We conclude that the expression, activity, and functionality of TG1 and TG2 are decreased in the aorta, but not in the vena cava, from hypertensive rats compared with control rats.

Testosterone and β-oestradiol prevent inward remodelling of rat small mesenteric arteries: role of NO and transglutaminase

Increasing evidence shows that sex hormones exert a protective effect on the vasculature, especially in the regulation of the active vasomotor responses. However, whether sex hormones affect vascular remodelling is currently unclear. In the present study, we tested the hypothesis that testosterone in males and β-oestradiol in females prevent inward remodelling, possibly through inhibition of cross-linking activity induced by enzymes of the TG (transglutaminase) family. Small mesenteric arteries were isolated from male and female Wistar rats. Dose-dependent relaxation to testosterone and β-oestradiol was inhibited by the NO synthase inhibitor L-NAME (NG-nitro-L-arginine methyl ester), confirming that these hormones induce NO release. When arteries were cannulated, pressurized and kept in organ culture with ET-1 (endothelin-1) for 3 days we observed strong vasoconstriction and inward remodelling. Remodelling was significantly inhibited by testosterone in males, and by β-oestradiol in females. This preventive effect of sex hormones was not observed in the presence of L-NAME. Inward remodelling was also reduced by the inhibitor of TG L682.777, both in males and females. In arteries from female rats, ET-1 increased TG activity, and this effect was prevented by β-oestradiol. L-NAME induced a significant increase in TG activity in the presence of sex hormones in arteries from both genders. We conclude that testosterone and β-oestradiol prevent constriction-induced inward remodelling. Inward remodelling, both in males and females, depends on NO and TG activity. In females, inhibition of inward remodelling could be mediated by NO-mediated inhibition of TG activity.

Activation of Extracellular Transglutaminase 2 by Mechanical Force in the Arterial Wall

Inward remodeling of small arteries occurs after prolonged vasoconstriction, low blood flow, and in several models of hypertension. The cross-linking enzyme, transglutaminases 2 (TG2), is able to induce inward remodeling and stiffening of arteries. The activity of TG2 is dependent on its conformation, which can be open or closed, and on its redox state. Several factors have been shown to be involved in modulating TG2 activity, including Ca²⁺ and GTP/GDP concentrations, as well as the redox state of the environment. This review introduces the hypothesis that mechanical force could be involved in regulating the activity of TG2 during inward remodeling by promoting its open and reduced active state. Several aspects of TG2, such as its structure and localization, are assessed in order to provide arguments that support the hypothesis. We conclude that a direct activation of TG2 by mechanical force exerted by smooth muscle cells may explain the link between smooth muscle activation and inward remodeling, as observed in several physiological and pathological conditions.

Implication for transglutaminase 2-mediated activation of β-catenin signaling in neointimal vascular smooth muscle cells in chronic cardiac allograft rejection

BACKGROUND

Cardiac allograft vasculopathy (CAV) remains the main cause of long-term transplant rejection. CAV is characterized by hyperproliferation of vascular smooth muscle cells (VSMCs). Canonical β-catenin signaling is a critical regulator of VSMC proliferation in development; however, the role of this pathway and its regulation in CAV progression are obscure. We investigated the activity of β-catenin signaling and the role for a putative activating ligand, transglutaminase 2 (TG2), in chronic cardiac rejection.

METHODS

Hearts from Bm12 mice were transplanted into C57BL/6 mice (class II mismatch), and allografts were harvested 8 weeks after transplantation. Accumulation and sub-cellular distribution of β-catenin protein and expression of several components of β-catenin signaling were analyzed as hallmarks of pathway activation. In vitro, platelet-derived growth factor treatment was used to mimic the inflammatory milieu in VSMC and organotypic heart slice cultures.

RESULTS

Activation of β-catenin in allografts compared with isografts or naïve hearts was evidenced by the augmented expression of β-catenin target genes, as well as the accumulation and nuclear localization of the β-catenin protein in VSMCs of the occluded allograft vessels. Expression of TG2, an activator of β-catenin signaling in VSMCs, was dramatically increased in allografts. Further, our ex vivo data demonstrate that TG2 is required for VSMC proliferation and for β-catenin activation by platelet-derived growth factor in cardiac tissue.

CONCLUSIONS

β-Catenin signaling is activated in occluded vessels in murine cardiac allografts. TG2 is implicated as an endogenous activator of this signaling pathway and may therefore have a role in the pathogenesis of CAV during chronic allograft rejection.

Tissue transglutaminase promotes PDGF/PDGFR-mediated signaling and responses in vascular smooth muscle cells

Although the pivotal role of platelet derived growth factor (PDGF)-mediated signaling in vascular diseases was demonstrated, the pathophysiological mechanisms driving its over-activation remain incompletely understood. Tissue transglutaminase (tTG) is a multifunctional protein expressed in the vasculature, including smooth muscle cells (SMCs), and implicated in several vascular pathologies. The goal of this study is to define the regulation of PDGF-BB/PDGFRβ-induced signaling pathways and cell responses by tTG in vascular SMCs. We find that in human aortic SMCs, shRNA-mediated depletion and over-expression of tTG reveals its ability to down-regulate PDGFRβ levels and induce receptor clustering. In these cells, tTG specifically amplifies the activation of PDGFRβ and its multiple downstream signaling targets in response to PDGF-BB. Furthermore, tTG promotes dedifferentiation and increases survival, proliferation, and migration of human aortic SMCs mediated by this growth factor. Finally, PDGF-BB stimulates tTG expression in human aortic SMCs in culture and in the blood vessels in response to injury. Together, our results show that tTG in vascular SMCs acts as a principal enhancer within the PDGF-BB/PDGFRβ signaling axis involved in phenotypic modulation of these cells, thereby suggesting a novel role for this protein in the progression of vascular diseases.

Tissue-specific responses to loss of transglutaminase 2

Of the eight catalytic transglutaminases (TGs), transglutaminase 2 (TG2) has been the most comprehensively studied due to its ubiquitous expression in multiple cell types. Despite the observed critical role for this enzyme in multiple biological processes in vitro, TG2 knockout mouse models have shown no severe developmental phenotypes, suggesting compensation by other TGs. To begin characterization of the compensating mechanisms, we analyzed total transamidating activity and expression patterns of all catalytically active TGs in seven different tissues/organs from wild-type and TG2 knockout mice. Inhibitory analysis with TG2-specific inhibitor KCC-009 suggests that relative contribution of TG2 in total transamidating activity differs in various tissues. Accordingly, our data indicate tissue-specific mechanisms of compensation for the loss of TG2, including transcriptional compensation in heart and liver versus functional compensation in aorta, kidney and skeletal/cartiagenous tissues. On the contrary, no compensation has been detected in skeletal muscle, suggesting a limited role for the TG2-mediated transamidation in normal development of this tissue.

Nitric oxide regulates tissue transglutaminase localization and function in the vasculature

The multifunctional enzyme tissue transglutaminase (TG2) contributes to the development and progression of several cardiovascular diseases. Extracellular rather than intracellular TG2 is enzymatically active, however, the mechanism by which it is exported out of the cell remains unknown. Nitric oxide (NO) is shown to constrain TG2 externalization in endothelial and fibroblast cells. Here, we examined the role of both exogenous and endogenous (endothelial cell-derived) NO in regulating TG2 localization in vascular cells and tissue. NO synthase inhibition in endothelial cells (ECs) using N-nitro L-arginine methyl ester (L-NAME) led to a time-dependent decrease in S-nitrosation and increase in externalization of TG2. Laminar shear stress led to decreased extracellular TG2 in ECs. S-nitrosoglutathione treatment led to decreased activity and externalization of TG2 in human aortic smooth muscle and fibroblast (IMR90) cells. Co-culture of these cells with ECs resulted in increased S-nitrosation and decreased externalization and activity of TG2, which was reversed by L-NAME. Aged Fischer 344 rats had higher tissue scaffold-associated TG2 compared to young. NO regulates intracellular versus extracellular TG2 localization in vascular cells and tissue, likely via S-nitrosation. This in part, explains increased TG2 externalization and activity in aging aorta.

The redox state of transglutaminase 2 controls arterial remodeling

While inward remodeling of small arteries in response to low blood flow, hypertension, and chronic vasoconstriction depends on type 2 transglutaminase (TG2), the mechanisms of action have remained unresolved. We studied the regulation of TG2 activity, its (sub) cellular localization, substrates, and its specific mode of action during small artery inward remodeling. We found that inward remodeling of isolated mouse mesenteric arteries by exogenous TG2 required the presence of a reducing agent. The effect of TG2 depended on its cross-linking activity, as indicated by the lack of effect of mutant TG2. The cell-permeable reducing agent DTT, but not the cell-impermeable reducing agent TCEP, induced translocation of endogenous TG2 and high membrane-bound transglutaminase activity. This coincided with inward remodeling, characterized by a stiffening of the artery. The remodeling could be inhibited by a TG2 inhibitor and by the nitric oxide donor, SNAP. Using a pull-down assay and mass spectrometry, 21 proteins were identified as TG2 cross-linking substrates, including fibronectin, collagen and nidogen. Inward remodeling induced by low blood flow was associated with the upregulation of several anti-oxidant proteins, notably glutathione-S-transferase, and selenoprotein P. In conclusion, these results show that a reduced state induces smooth muscle membrane-bound TG2 activity. Inward remodeling results from the cross-linking of vicinal matrix proteins, causing a stiffening of the arterial wall.

Small artery remodelling in hypertension

Increased blood pressure (essential hypertension) is associated with increased cardiovascular risk, and the condition is treated primarily with a view to reducing this parameter. However, in the early stages, the main pathological changes are increased peripheral resistance and altered cardiovascular structure. The aim of this MiniReview was to trace the endeavours over the past several decades to translate these findings into answering the question whether normalization of resistance vessel structure should be a target for therapy. This MiniReview describes first the altered structure of the resistance vasculature in essential hypertension, where the vessels show increased media/lumen ratio because of inward eutrophic remodelling. Secondly, evidence is presented that altered small artery structure appears to have prognostic consequences. Then, the cellular mechanisms that may be involved are discussed, where there is evidence that vasoconstriction in itself can cause inward remodelling and that this can be prevented by vasodilators. This leads to a discussion of the degree to which it may be possible to rectify the abnormal structure, where it appears that this may be achieved using a therapy that causes vasodilatation in the patient concerned. Finally, the consequences of these findings are considered as regards clues for strategies that may be able to improve the outcome of antihypertensive therapy. The MiniReview concludes that there is reasonably strong evidence that improvement in abnormal resistance vessel structure requires a treatment that reduces peripheral resistance in the individual patient.

Effect of cystamine on blood pressure and vascular characteristics in spontaneously hypertensive rats

BACKGROUND

Tissue transglutaminase (t-TG) has been implicated in small artery remodelling. The aim of this study was to determine if cystamine, an inhibitor of t-TG, could reduce blood pressure in spontaneously hypertensive rats (SHR) and if so to what extent this is mediated through small arteries.

METHODS

In vitro inhibition of t-TG, with cystamine, was studied in organ culture and wire myograph setups in small mesenteric arteries obtained from SHR. In vivo treatment with cystamine (80 mg/kg/day) or amlodipine (10 mg/kg/day) was performed with osmotic pumps in adult SHR, and hemodynamic parameters determined with telemetry. Plasma concentrations of cystamine were determined with a liquid chromatography setup. Small arteries were harvested following administration of cystamine, and structural as well as functional characteristics were determined.

RESULTS

SHR small arteries showed inward remodelling following in vitro activation. Administration of cystamine caused attenuation of the inward remodelling induced by activation. In vivo administration of cystamine caused a 9 ± 2 mm Hg reduction in blood pressure, but with no detectable alterations in small artery structure.

CONCLUSION

t-TG is potentially involved in vascular remodelling of SHR small arteries and results support a possible role for t-TG in blood pressure control.

Matrix metalloproteinases and small artery remodeling

Inward eutrophic remodeling is a common structural change found in small resistance arteries that has been associated with an increased risk for life threatening cardiovascular events, the number one cause of death in industrialized societies. Because inward eutrophic remodeling is the most prevalent small artery structural change found in hypertension, hypertensive animals are the most common in vivo models used to study this particular remodeling process. In vitro, the isolated artery, pressure myograph has also been used as a model to study the mechanisms responsible for the development of small artery remodeling. Compelling recent evidence indicates that the matrix metalloproteinases (MMPs), a family of endopeptidases whose primary function is the cleavage and degradation of extracellular matrix components, are involved in vasoconstriction and the pathogenesis of hypertension. In this review we provide an overview of the known and potential roles that MMPs have on vascular remodeling, paying particular attention to their role on the inward eutrophic remodeling process of small resistance arteries that occurs in hypertension.

Role of transglutaminases in cuff-induced atherosclerotic lesion formation in femoral arteries of ApoE3 Leiden mice

Transglutaminases play an important role in vascular remodeling, calcification, cell adhesion and endothelial barrier function. In this study we investigate the influence of combined inhibition of both tissue-type transglutaminase (TG2) and the plasma transglutaminase FXIIIA on early lesion development.

METHODS

A cuff was placed around the femoral arteries of ApoE3 Leiden mice while fed a Western type diet to induce atherosclerotic lesion development. An osmotic minipump was placed in the intraperitoneal cavity containing an irreversible inhibitor of TG2 and FXIIIA activity ((1,3,4,5-tetramethyl-2-[(2-oxopropyl)thio]imidazolium chloride, Zedira). Atherosclerotic lesion composition was analyzed using immunohistochemistry and RT-PCR.

RESULTS

Inhibition of transglutaminases did not influence lesion size or geometric remodeling of the vessels. However, systemic transglutaminase inhibition resulted in 41% less macrophage infiltrate in the media of the vessels. Additional in vitro experiments on HL60 cells confirmed a decreased migratory response during transglutaminase inhibition.

CONCLUSION

Inhibition of TG2 and FXIIIA during early development of lesions reduced the macrophage content in the media of atherosclerotic vessels, while not affecting lesion size or geometric remodeling.

Decreased S-nitrosylation of tissue transglutaminase contributes to age-related increases in vascular stiffness

RATIONALE

Although an age-related decrease in NO bioavailability contributes to vascular stiffness, the underlying molecular mechanisms remain incompletely understood. We hypothesize that NO constrains the activity of the matrix crosslinking enzyme tissue transglutaminase (TG2) via S-nitrosylation in young vessels, a process that is reversed in aging.

OBJECTIVE

We sought to determine whether endothelium-dependent NO regulates TG2 activity by S-nitrosylation and whether this contributes to age-related vascular stiffness.

METHODS AND RESULTS

We first demonstrate that NO suppresses activity and increases S-nitrosylation of TG2 in cellular models. Next, we show that nitric oxide synthase (NOS) inhibition leads to increased surface and extracellular matrix-associated TG2. We then demonstrate that endothelium-derived bioactive NO primarily mediates its effects through TG2, using TG2(-/-) mice chronically treated with the NOS inhibitor l-N(G)-nitroarginine methyl ester (L-NAME). We confirm that TG2 activity is modulated by endothelium-derived bioactive NO in young rat aorta. In aging rat aorta, although TG2 expression remains unaltered, its activity increases and S-nitrosylation decreases. Furthermore, TG2 inhibition decreases vascular stiffness in aging rats. Finally, TG2 activity and matrix crosslinks are augmented with age in human aorta, whereas abundance remains unchanged.

CONCLUSIONS

Decreased S-nitrosylation of TG2 and increased TG activity lead to enhanced matrix crosslinking and contribute to vascular stiffening in aging. TG2 appears to be the member of the transglutaminase family primarily contributing to this phenotype. Inhibition of TG2 could thus represent a therapeutic target for age-associated vascular stiffness and isolated systolic hypertension.

Strain-dependent susceptibility for hypertension in mice resides in the natural killer gene complex

Hypertension is associated with chronic vascular inflammation. We tested the hypothesis that the sensitivity to develop hypertension and vascular remodeling depends on the immunological background. Blood pressure, vascular remodeling, endothelial function, vascular architecture (number of collateral arteries), and expression of inflammatory cytokines were determined in mice that received N(G)-nitro-l-arginine methyl ester (l-NAME) to inhibit nitric oxide synthesis. We studied C57BL/6, BALB/c, and BALB.B6-Cmv1r mice, a congenic strain where the natural killer (NK) gene complex of C57BL/6 mice is introduced in the BALB/c background. During a 4-wk treatment with l-NAME, blood pressure initially increased in both C57BL/6 and BALB/C mice, but after 4 wk, only C57BL/6 mice showed a significant increase in mean arterial blood pressure (+53 mmHg; P < 0.001) and small artery inward remodeling. Endothelial function and vascular design were significantly different between C57BL/6 mice and BALB/C mice. The inflammatory response was similar in C57BL/6 and BALB/C mice, except for the leukocyte marker CD11b. Cellular colocalization of CD11b with NK1.1 indicated the recruitment of NK cells in C57BL/6 mice. Congenic BALB.B6-Cmv1r mice showed the same endothelial response and vascular architecture as BALB/c mice. However, BALB.B6-Cmv1r mice displayed a similar sensitivity to hypertension and vascular remodeling as C57BL/6 mice. In conclusion, we have identified the NK gene complex as an important determinant in the genetically determined sensitivity to develop l-NAME-induced hypertension in mice.

Arginase inhibition restores NOS coupling and reverses endothelial dysfunction and vascular stiffness in old rats

There is increasing evidence that upregulation of arginase contributes to impaired endothelial function in aging. In this study, we demonstrate that arginase upregulation leads to endothelial nitric oxide synthase (eNOS) uncoupling and that in vivo chronic inhibition of arginase restores nitroso-redox balance, improves endothelial function, and increases vascular compliance in old rats. Arginase activity in old rats was significantly increased compared with that shown in young rats. Old rats had significantly lower nitric oxide (NO) and higher superoxide (O2(-)) production than young. Acute inhibition of both NOS, with N(G)-nitro-l-arginine methyl ester, and arginase, with 2S-amino- 6-boronohexanoic acid (ABH), significantly reduced O2(-) production in old rats but not in young. In addition, the ratio of eNOS dimer to monomer in old rats was significantly decreased compared with that shown in young rats. These results suggest that eNOS was uncoupled in old rats. Although the expression of arginase 1 and eNOS was similar in young and old rats, inducible NOS (iNOS) was significantly upregulated. Furthermore, S-nitrosylation of arginase 1 was significantly elevated in old rats. These findings support our previously published finding that iNOS nitrosylates and activates arginase 1 (Santhanam et al., Circ Res 101: 692-702, 2007). Chronic arginase inhibition in old rats preserved eNOS dimer-to-monomer ratio and significantly reduced O2(-) production and enhanced endothelial-dependent vasorelaxation to ACh. In addition, ABH significantly reduced vascular stiffness in old rats. These data indicate that iNOS-dependent S-nitrosylation of arginase 1 and the increase in arginase activity lead to eNOS uncoupling, contributing to the nitroso-redox imbalance, endothelial dysfunction, and vascular stiffness observed in vascular aging. We suggest that arginase is a viable target for therapy in age-dependent vascular stiffness.

Transglutaminases and disease: lessons from genetically engineered mouse models and inherited disorders

The human transglutaminase (TG) family consists of a structural protein, protein 4.2, that lacks catalytic activity, and eight zymogens/enzymes, designated factor XIII-A (FXIII-A) and TG1-7, that catalyze three types of posttranslational modification reactions: transamidation, esterification, and hydrolysis. These reactions are essential for biological processes such as blood coagulation, skin barrier formation, and extracellular matrix assembly but can also contribute to the pathophysiology of various inflammatory, autoimmune, and degenerative conditions. Some members of the TG family, for example, TG2, can participate in biological processes through actions unrelated to transamidase catalytic activity. We present here a comprehensive review of recent insights into the physiology and pathophysiology of TG family members that have come from studies of genetically engineered mouse models and/or inherited disorders. The review focuses on FXIII-A, TG1, TG2, TG5, and protein 4.2, as mice deficient in TG3, TG4, TG6, or TG7 have not yet been reported, nor have mutations in these proteins been linked to human disease.

Increased TG2 expression can result in induction of transforming growth factor beta1, causing increased synthesis and deposition of matrix proteins, which can be regulated by nitric oxide

In fibrotic conditions increases in TG2 activity has been linked to an increase in the deposition of extracellular matrix proteins. Using TG2 transfected Swiss 3T3 fibroblasts expressing TG2 under the control of the tetracycline-regulated inducible promoter, we demonstrate that induction of TG2 not only stimulates an increase in collagen and fibronectin deposition but also an increase in the expression of these proteins. Increased TG2 expression in these fibroblasts led to NF-kappaB activation, resulting in the increased expression of transforming growth factor (TGF) beta(1). In addition, cells overexpressing TG2 demonstrated an increase in biologically active TGFbeta(1) in the extracellular environment. A specific site-directed inhibitor of TG abolished the NF-kappaB and TGFbeta1 activation and the subsequent elevation in the synthesis and deposition of extracellular matrix proteins, confirming that this process depends on the induction of transglutaminase activity. Treatment of TG2-induced fibroblasts with nontoxic doses of nitric oxide donor S-nitroso-N-acetylpenicillamine resulted in decreased TG2 activity and apprehension of the inactive enzyme on the cell surface. This was paralleled by a reduction in activation of NF-kappaB and TGFbeta(1) production with a subsequent decrease in collagen expression and deposition. These findings support a role for NO in the regulation of TG2 function in the extracellular environment.

Serotonylation of vascular proteins important to contraction

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) was named for its source (sero-) and ability to modify smooth muscle tone (tonin). The biological effects of 5-HT are believed to be carried out by stimulation of serotonin receptors at the plasma membrane. Serotonin has recently been shown to be synthesized in vascular smooth muscle and taken up from external sources, placing 5-HT inside the cell. The enzyme transglutaminase uses primary amines such as 5-HT to covalently modify proteins on glutamine residues. We tested the hypothesis that 5-HT is a substrate for transglutaminase in arterial vascular smooth muscle, with protein serotonylation having physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

The model was the rat aorta and cultured aortic smooth muscle cells. Western analysis demonstrated that transglutaminase II was present in vascular tissue, and transglutaminase activity was observed as a cystamine-inhibitable incorporation of the free amine pentylamine-biotin into arterial proteins. Serotonin-biotin was incorporated into alpha-actin, beta-actin, gamma-actin, myosin heavy chain and filamin A as shown through tandem mass spectrometry. Using antibodies directed against biotin or 5-HT, immunoprecipitation and immunocytochemistry confirmed serotonylation of smooth muscle alpha-actin. Importantly, the alpha-actin-dependent process of arterial isometric contraction to 5-HT was reduced by cystamine.

CONCLUSIONS

5-HT covalently modifies proteins integral to contractility and the cytoskeleton. These findings suggest new mechanisms of action for 5-HT in vascular smooth muscle and consideration for intracellular effects of primary amines.

The plastic nature of the vascular wall: a continuum of remodeling events contributing to control of arteriolar diameter and structure

The diameter of resistance arteries has a profound effect on the distribution of microvascular blood flow and the control of systemic blood pressure. Here, we review mechanisms that contribute to the regulation of resistance artery diameter, both acutely and chronically, their temporal characteristics, and their interdependence. Furthermore, we hypothesize the existence of a remodeling continuum that allows for the vascular wall to rapidly modify its structural characteristics, specifically through the re-positioning of vascular smooth muscle cells. Importantly, the concepts presented more closely link acute vasoregulatory responses with adaptive changes in vessel wall structure. These rapid structural adaptations provide resistance vessels the ability to maintain a desired diameter under presumed optimal energetic and mechanical conditions.

A mechanism for arteriolar remodeling based on maintenance of smooth muscle cell activation

Structural adaptation in arterioles is part of normal vascular physiology but is also seen in disease states such as hypertension. Smooth muscle cell (SMC) activation has been shown to be central to microvascular remodeling. We hypothesize that, in a remodeling process driven by SMC activation, stress sensitivity of the vascular wall is a key element in the process of achieving a stable vascular structure. We address whether the adaptive changes in arterioles under different conditions can arise through a common mechanism: remodeling in a stress-sensitive wall driven by a shift in SMC activation. We present a simple dynamic model and show that structural remodeling of the vessel radius by rearrangement of the wall material around a lumen of a different diameter and driven by differences in SMC activation can lead to vascular structures similar to those observed experimentally under various conditions. The change in structure simultaneously leads to uniform levels of circumferential wall stress and wall strain, despite differences in transmural pressure. A simulated vasoconstriction caused by increased SMC activation leads to inward remodeling, whereas outward remodeling follows relaxation of the vascular wall. The results are independent of the specific myogenic properties of the vessel. The simulated results are robust in the face of parameter changes and, hence, may be generalized to vessels from different vascular beds.

Transglutaminases in vascular biology: relevance for vascular remodeling and atherosclerosis

The transglutaminase (Tgase) family consists of nine known members of whom at least three are expressed in the vascular system: type 1 Tgase, type 2 Tgase and factor XIII. The cross-linking of proteins is a characteristic feature of Tgases, of well-known importance for stabilizing the blood clot and providing mechanical strength to tissues. However, recent data suggest that Tgases play a role in several other processes in vascular biology. These newly discovered areas include endothelial barrier function, small artery remodeling, and atherosclerosis.