DNA repair in cardiomyocytes is critical for maintaining cardiac function in mice

Heart failure has reached epidemic proportions in a progressively ageing population. The molecular mechanisms underlying heart failure remain elusive, but evidence indicates that DNA damage is enhanced in failing hearts. Here, we tested the hypothesis that endogenous DNA repair in cardiomyocytes is critical for maintaining normal cardiac function, so that perturbed repair of spontaneous DNA damage drives early onset of heart failure. To increase the burden of spontaneous DNA damage, we knocked out the DNA repair endonucleases xeroderma pigmentosum complementation group G (XPG) and excision repair cross-complementation group 1 (ERCC1), either systemically or cardiomyocyte-restricted, and studied the effects on cardiac function and structure. Loss of DNA repair permitted normal heart development but subsequently caused progressive deterioration of cardiac function, resulting in overt congestive heart failure and premature death within 6 months. Cardiac biopsies revealed increased oxidative stress associated with increased fibrosis and apoptosis. Moreover, gene set enrichment analysis showed enrichment of pathways associated with impaired DNA repair and apoptosis, and identified TP53 as one of the top active upstream transcription regulators. In support of the observed cardiac phenotype in mutant mice, several genetic variants in the ERCC1 and XPG gene in human GWAS data were found to be associated with cardiac remodelling and dysfunction. In conclusion, unrepaired spontaneous DNA damage in differentiated cardiomyocytes drives early onset of cardiac failure. These observations implicate DNA damage as a potential novel therapeutic target and highlight systemic and cardiomyocyte-restricted DNA repair-deficient mouse mutants as bona fide models of heart failure.

Lentiviral Hematopoietic Stem Cell Gene Therapy Corrects Murine Pompe Disease

Pompe disease is an autosomal recessive lysosomal storage disorder characterized by progressive muscle weakness. The disease is caused by mutations in the acid α-glucosidase (GAA) gene. Despite the currently available enzyme replacement therapy (ERT), roughly half of the infants with Pompe disease die before the age of 3 years. Limitations of ERT are immune responses to the recombinant enzyme, incomplete correction of the disease phenotype, lifelong administration, and inability of the enzyme to cross the blood-brain barrier. We previously reported normalization of glycogen in heart tissue and partial correction of the skeletal muscle phenotype by ex vivo hematopoietic stem cell gene therapy. In the present study, using a codon-optimized GAA (GAAco), the enzyme levels resulted in close to normalization of glycogen in heart, muscles, and brain, and in complete normalization of motor function. A large proportion of microglia in the brain was shown to be GAA positive. All astrocytes contained the enzyme, which is in line with mannose-6-phosphate receptor expression and the key role in glycogen storage and glucose metabolism. The lentiviral vector insertion site analysis confirmed no preference for integration near proto-oncogenes. This correction of murine Pompe disease warrants further development toward a cure of the human condition.

In vitro model to study the effects of matrix stiffening on Ca2+ handling and myofilament function in isolated adult rat cardiomyocytes

Key points

This paper describes a novel model that allows exploration of matrix‐induced cardiomyocyte adaptations independent of the passive effect of matrix rigidity on cardiomyocyte function.

Detachment of adult cardiomyocytes from the matrix enables the study of matrix effects on cell shortening, Ca²⁺ handling and myofilament function.

Cell shortening and Ca²⁺ handling are altered in cardiomyocytes cultured for 24 h on a stiff matrix.

Matrix stiffness‐impaired cardiomyocyte contractility is reversed upon normalization of extracellular stiffness.

Matrix stiffness‐induced reduction in unloaded shortening is more pronounced in cardiomyocytes isolated from obese ZSF1 rats with heart failure with preserved ejection fraction compared to lean ZSF1 rats.

Abstract

Extracellular matrix (ECM) stiffening is a key element of cardiac disease. Increased rigidity of the ECM passively inhibits cardiac contraction, but if and how matrix stiffening also actively alters cardiomyocyte contractility is incompletely understood. In vitro models designed to study cardiomyocyte–matrix interaction lack the possibility to separate passive inhibition by a stiff matrix from active matrix‐induced alterations of cardiomyocyte properties. Here we introduce a novel experimental model that allows exploration of cardiomyocyte functional alterations in response to matrix stiffening. Adult rat cardiomyocytes were cultured for 24 h on matrices of tuneable stiffness representing the healthy and the diseased heart and detached from their matrix before functional measurements. We demonstrate that matrix stiffening, independent of passive inhibition, reduces cell shortening and Ca²⁺ handling but does not alter myofilament‐generated force. Additionally, detachment of adult cultured cardiomyocytes allowed the transfer of cells from one matrix to another. This revealed that stiffness‐induced cardiomyocyte changes are reversed when matrix stiffness is normalized. These matrix stiffness‐induced changes in cardiomyocyte function could not be explained by adaptation in the microtubules. Additionally, cardiomyocytes isolated from stiff hearts of the obese ZSF1 rat model of heart failure with preserved ejection fraction show more pronounced reduction in unloaded shortening in response to matrix stiffening. Taken together, we introduce a method that allows evaluation of the influence of ECM properties on cardiomyocyte function separate from the passive inhibitory component of a stiff matrix. As such, it adds an important and physiologically relevant tool to investigate the functional consequences of cardiomyocyte–matrix interactions.

This paper describes a novel model that allows exploration of matrix‐induced cardiomyocyte adaptations independent of the passive effect of matrix rigidity on cardiomyocyte function.

Detachment of adult cardiomyocytes from the matrix enables the study of matrix effects on cell shortening, Ca²⁺ handling and myofilament function.

Cell shortening and Ca²⁺ handling are altered in cardiomyocytes cultured for 24 h on a stiff matrix.

Matrix stiffness‐impaired cardiomyocyte contractility is reversed upon normalization of extracellular stiffness.

Matrix stiffness‐induced reduction in unloaded shortening is more pronounced in cardiomyocytes isolated from obese ZSF1 rats with heart failure with preserved ejection fraction compared to lean ZSF1 rats.

Pregnancy mitigates cardiac pathology in a mouse model of left ventricular pressure overload

In Western countries heart disease is the leading cause of maternal death during pregnancy. The effect of pregnancy on the heart is difficult to study in patients with preexisting heart disease. Since experimental studies are scarce, we investigated the effect of pressure overload, produced by transverse aortic constriction (TAC) in mice, on the ability to conceive, pregnancy outcome, and maternal cardiac structure and function. Four weeks of TAC produced left ventricular (LV) hypertrophy and dysfunction with marked interstitial fibrosis, decreased capillary density, and induced pathological cardiac gene expression. Pregnancy increased relative LV and right ventricular weight without affecting the deterioration of LV function following TAC. Surprisingly, the TAC-induced increase in relative heart and lung weight was mitigated by pregnancy, which was accompanied by a trend towards normalization of capillary density and natriuretic peptide type A expression. Additionally, the combination of pregnancy and TAC increased the cardiac phosphorylation of c-Jun, and STAT1, but reduced phosphoinositide 3-kinase phosphorylation. Finally, TAC did not significantly affect conception rate, pregnancy duration, uterus size, litter size, and pup weight. In conclusion, we found that, rather than exacerbating the changes associated with cardiac pressure overload, pregnancy actually attenuated pathological LV remodeling and mitigated pulmonary congestion, and pathological gene expression produced by TAC, suggesting a positive effect of pregnancy on the pressure-overloaded heart.

Normal and high eNOS levels are detrimental in both mild and severe cardiac pressure-overload

Nitric oxide (NO) produced by endothelial NO synthase (eNOS) exerts beneficial effects in a variety of cardiovascular disease states. Studies on the benefit of eNOS activity in pressure-overload cardiac hypertrophy and dysfunction produced by aortic stenosis are equivocal, which may be due to different expression levels of eNOS or different severities of pressure-overload. Consequently, we investigated the effects of eNOS-expression level on cardiac hypertrophy and dysfunction produced by mild or severe pressure-overload. To unravel the impact of eNOS on pressure-overload cardiac dysfunction we subjected eNOS deficient, wildtype and eNOS overexpressing transgenic (eNOS-Tg) mice to 8weeks of mild or severe transverse aortic constriction (TAC) and studied cardiac geometry and function at the whole organ and tissue level. In both mild and severe TAC, lack of eNOS ameliorated, whereas eNOS overexpression aggravated, TAC-induced cardiac remodeling and dysfunction. Moreover, the detrimental effects of eNOS in severe TAC were associated with aggravation of TAC-induced NOS-dependent oxidative stress and by further elevation of eNOS monomer levels, consistent with enhanced eNOS uncoupling. In the presence of TAC, scavenging of reactive oxygen species with N-acetylcysteine reduced eNOS S-glutathionylation, eNOS monomer and NOS-dependent superoxide levels in eNOS-Tg mice to wildtype levels. Accordingly, N-acetylcysteine improved cardiac function in eNOS-Tg but not in wildtype mice with TAC. In conclusion, independent of the severity of TAC, eNOS aggravates cardiac remodeling and dysfunction, which appears due to TAC-induced eNOS uncoupling and superoxide production.

The microRNA-15 family inhibits the TGFβ-pathway in the heart

AIMS

The overloaded heart remodels by cardiomyocyte hypertrophy and interstitial fibrosis, which contributes to the development of heart failure. Signalling via the TGFβ-pathway is crucial for this remodelling. Here we tested the hypothesis that microRNAs in the overloaded heart regulate this remodelling process via inhibition of the TGFβ-pathway.

METHODS AND RESULTS

We show that the miRNA-15 family, which we found to be up-regulated in the overloaded heart in multiple species, inhibits the TGFβ-pathway by targeting of TGFBR1 and several other genes within this pathway directly or indirectly, including p38, SMAD3, SMAD7, and endoglin. Inhibition of miR-15b by subcutaneous injections of LNA-based antimiRs in C57BL/6 mice subjected to transverse aorta constriction aggravated fibrosis and to a lesser extent also hypertrophy.

CONCLUSION

We identified the miR-15 family as a novel regulator of cardiac hypertrophy and fibrosis acting by inhibition of the TGFβ-pathway.

Sexual dimorphic response to exercise in hypertrophic cardiomyopathy-associated MYBPC3-targeted knock-in mice

Hypertrophic cardiomyopathy (HCM), the most common genetic cardiac disorder, is frequently caused by mutations in MYBPC3, encoding cardiac myosin-binding protein C (cMyBP-C). Moreover, HCM is the leading cause of sudden cardiac death (SCD) in young athletes. Interestingly, SCD is more likely to occur in male than in female athletes. However, the pathophysiological mechanisms leading to sex-specific differences are poorly understood. Therefore, we studied the effect of sex and exercise on functional properties of the heart and sarcomeres in mice carrying a MYBPC3 point mutation (G > A transition in exon 6) associated with human HCM. Echocardiography followed by isometric force measurements in left ventricular (LV) membrane-permeabilized cardiomyocytes was performed in wild-type (WT) and heterozygous (HET) knock-in mice of both sex (N = 5 per group) in sedentary mice and mice that underwent an 8-week voluntary wheel-running exercise protocol. Isometric force measurements in single cardiomyocytes revealed a lower maximal force generation (F max) of the sarcomeres in male sedentary HET (13.0 ± 1.1 kN/m(2)) compared to corresponding WT (18.4 ± 1.8 kN/m(2)) male mice. Exercise induced a higher F max in HET male mice, while it did not affect HET females. Interestingly, a low cardiac troponin I bisphosphorylation, increased myofilament Ca(2+)-sensitivity, and LV hypertrophy were particularly observed in exercised HET females. In conclusion, in sedentary animals, contractile differences are seen between male and female HET mice. Male and female HET hearts adapted differently to a voluntary exercise protocol, indicating that physiological stimuli elicit a sexually dimorphic cardiac response in heterozygous MYBPC3-targeted knock-in mice.

Exercise training in adverse cardiac remodeling

Cardiac remodeling in response to a myocardial infarction or chronic pressure-overload is an independent risk factor for the development of heart failure. In contrast, cardiac remodeling produced by regular physical exercise is associated with a decreased risk for heart failure. There is evidence that exercise training has a beneficial effect on disease progression and survival in patients with cardiac remodeling and dysfunction, but concern has also been expressed that exercise training may aggravate pathological remodeling and dysfunction. Here we present studies from our laboratory into the effects of exercise training on pathological cardiac remodeling and dysfunction in mice. The results indicate that even in the presence of a large infarct, exercise training exerts beneficial effects on the heart. These effects were mimicked in part by endothelial nitric oxide synthase (eNOS) overexpression and abrogated by eNOS deficiency, demonstrating the importance of nitric oxide signaling in mediating the cardiac effects of exercise. Exercise prior to a myocardial infarction was also cardioprotective. In contrast, exercise tended to aggravate pathological cardiac remodeling and dysfunction in the setting of pressure-overload produced by an aortic stenosis. These observations emphasize the critical importance of the underlying pathological stimulus for cardiac hypertrophy and remodeling, in determining the effects of exercise training. Future studies are needed to define the influence of exercise type, intensity and duration in different models and severities of pathological cardiac remodeling. Together such studies will aid in optimizing the therapy of exercise training in the setting of cardiovascular disease.

Reduced contribution of endothelin to the regulation of systemic and pulmonary vascular tone in severe familial hypercholesterolaemia

Vascular dysfunction has been associated with familial hypercholesterolaemia (FH), a severe form of hyperlipidaemia. We recently demonstrated that swine with FH exhibit reduced exercise-induced systemic, but not pulmonary, vasodilatation involving reduced nitric oxide (NO) bioavailability. Since NO normally limits endothelin (ET) action, we examined the hypothesis that reduced systemic vasodilatation during exercise in FH swine results from increased ET-mediated vasoconstriction. Systemic and pulmonary vascular responses to exercise were examined in chronically instrumented normal and FH swine in the absence and presence of the ETA/B receptor antagonist tezosentan. Intrinsic reactivity to ET was further assessed in skeletal muscle arterioles. FH swine exhibited ∼9-fold elevation in total plasma cholesterol versus normal swine. Similar to our recent findings, systemic, not pulmonary, vasodilatation during exercise was reduced in FH swine. Blockade of ET receptors caused marked systemic vasodilatation at rest and during exercise in normal swine that was significantly reduced in FH swine. The reduced role of ET in FH swine in vivo was not the result of decreased arteriolar ET responsiveness, as responsiveness was increased in isolated arterioles. Smooth muscle ET receptor protein content was unaltered by FH. However, circulating plasma ET levels were reduced in FH swine. ET receptor antagonism caused pulmonary vasodilatation at rest and during exercise in normal, but not FH, swine. Therefore, contrary to our hypothesis, FH swine exhibit a generalised reduction in the role of ET in regulating vascular tone in vivo probably resulting from reduced ET production. This may represent a unique vascular consequence of severe familial hypercholesterolaemia.

Phenotyping of left and right ventricular function in mouse models of compensated hypertrophy and heart failure with cardiac MRI

Background

Left ventricular (LV) and right ventricular (RV) function have an important impact on symptom occurrence, disease progression and exercise tolerance in pressure overload-induced heart failure, but particularly RV functional changes are not well described in the relevant aortic banding mouse model. Therefore, we quantified time-dependent alterations in the ventricular morphology and function in two models of hypertrophy and heart failure and we studied the relationship between RV and LV function during the transition from hypertrophy to heart failure.

Methods

MRI was used to quantify RV and LV function and morphology in healthy (n = 4) and sham operated (n = 3) C57BL/6 mice, and animals with a mild (n = 5) and a severe aortic constriction (n = 10).

Results

Mice subjected to a mild constriction showed increased LV mass (P<0.01) and depressed LV ejection fraction (EF) (P<0.05) as compared to controls, but had similar RVEF (P>0.05). Animals with a severe constriction progressively developed LV hypertrophy (P<0.001), depressed LVEF (P<0.001), followed by a declining RVEF (P<0.001) and the development of pulmonary remodeling, as compared to controls during a 10-week follow-up. Myocardial strain, as a measure for local cardiac function, decreased in mice with a severe constriction compared to controls (P<0.05).

Conclusions

Relevant changes in mouse RV and LV function following an aortic constriction could be quantified using MRI. The well-controlled models described here open opportunities to assess the added value of new MRI techniques for the diagnosis of heart failure and to study the impact of new therapeutic strategies on disease progression and symptom occurrence.

Abstract 28: In Vivo Knockdown of Mirna-15b Results in Increased Cardiac Hypertrophy and Fibrosis in Response to Pressure Overload of the Mouse Heart

MiRNAs play an important role in the control of diverse aspects of cardiac function. MiR-15b is highly expressed in the heart and is found consistently upregulated in hypertrophic and failing hearts. To investigate the function of miR-15b in the heart we set out two experiments. In the first experiment we generated two independent transgenic mouse lines that drive miR-15b expression under the αMHC-promotor and show a three and four fold overexpression of miR-15b. Strikingly, both lines show a decrease in heart weight/tibia length of 20% in adult and aged mice when compared to littermate controls. We investigated the response of these transgenic mice to thoracic aorta constriction (TAC) and found no differences in the hypertrophic response or in cardiac function measured by echocardiography between wild-type and transgenic mice. In a second experiment, we inhibited miR-15b using LNA-based antimiRs. In these mice, TAC resulted in an increased hypertrophic response and increased cardiac fibrosis when compared to a negative control antimiR.

A wide range of predicted targets of miR-15 belong to the pathways of the TGFβ-superfamily and using a smad-dependent reporter we show that miR-15b inhibits TGFβ-induced Smad activity in HepG2 cells. One of the predicted targets in the TGFβ pathway is TGFβ receptor 1 (TGFβR1), of which the 3’UTR contains six predicted miR-15 binding sites. This suggests that the phenotype in the transgenic mice and after knockdown of miR-15b may be (partly) mediated by repression of TGFβR1. Indeed, in the adult miR-15b transgenic hearts we found a downregulation of TGFβR1 mRNA and protein and we confirmed binding of miR-15 to the TGFβR1 3’UTR by luciferase assays.

In conclusion, miR-15b causes a cardiac hypotrophic phenotype at baseline in transgenic mice and inhibition of miR-15b leads to a stronger hypertrophic and fibrotic response after TAC. Furthermore miR-15b inhibits the TGFβ pathway by targeting the TGFβR1 and possibly other targets in this pathway.

This research is funded by the Dutch Heart Foundation (NHF grant #2007B077).

Apelin enhances cardiac neovascularization after myocardial infarction by recruiting aplnr+ circulating cells

RATIONALE

Neovascularization stimulated by local or recruited stem cells after ischemia is a key process that salvages damaged tissue and shows similarities with embryonic vascularization. Apelin receptor (Aplnr) and its endogenous ligand apelin play an important role in cardiovascular development. However, the role of apelin signaling in stem cell recruitment after ischemia is unknown.

OBJECTIVE

To investigate the role of apelin signaling in recruitment after ischemia.

METHODS AND RESULTS

Aplnr was specifically expressed in circulating cKit+/Flk1+ cells but not in circulating Sca1+/Flk1+ and Lin+ cells. cKit+/Flk1+/Aplnr+ cells increased significantly early after myocardial ischemia but not after hind limb ischemia, indicative of an important role for apelin/Aplnr in cell recruitment during the nascent biological repair response after myocardial damage. In line with this finding, apelin expression was upregulated in the infarcted myocardium. Injection of apelin into the ischemic myocardium resulted in accelerated and increased recruitment of cKit+/Flk1+/Aplnr+ cells to the heart. Recruited Aplnr+/cKit+/Flk1+ cells promoted neovascularization in the peri-infarct area by paracrine activity rather than active transdifferentiation, resulting into cardioprotection as indicated by diminished scar formation and improved residual cardiac function. Aplnr knockdown in the bone marrow resulted in aggravation of myocardial ischemia-associated damage, which could not be rescued by apelin.

CONCLUSIONS

We conclude that apelin functions as a new and potent chemoattractant for circulating cKit+/Flk1+/Aplnr+ cells during early myocardial repair, providing myocardial protection against ischemic damage by improving neovascularization via paracine action.

Nucleotide excision DNA repair is associated with age-related vascular dysfunction

BACKGROUND

Vascular dysfunction in atherosclerosis and diabetes mellitus, as observed in the aging population of developed societies, is associated with vascular DNA damage and cell senescence. We hypothesized that cumulative DNA damage during aging contributes to vascular dysfunction.

METHODS AND RESULTS

In mice with genomic instability resulting from the defective nucleotide excision repair genes ERCC1 and XPD (Ercc1(d/-) and Xpd(TTD) mice), we explored age-dependent vascular function compared with that in wild-type mice. Ercc1(d/-) mice showed increased vascular cell senescence, accelerated development of vasodilator dysfunction, increased vascular stiffness, and elevated blood pressure at a very young age. The vasodilator dysfunction was due to decreased endothelial nitric oxide synthase levels and impaired smooth muscle cell function, which involved phosphodiesterase activity. Similar to Ercc1(d/-) mice, age-related endothelium-dependent vasodilator dysfunction in Xpd(TTD) animals was increased. To investigate the implications for human vascular disease, we explored associations between single-nucleotide polymorphisms of selected nucleotide excision repair genes and arterial stiffness within the AortaGen Consortium and found a significant association of a single-nucleotide polymorphism (rs2029298) in the putative promoter region of DDB2 gene with carotid-femoral pulse wave velocity.

CONCLUSIONS

Mice with genomic instability recapitulate age-dependent vascular dysfunction as observed in animal models and in humans but with an accelerated progression compared with wild-type mice. In addition, we found associations between variations in human DNA repair genes and markers for vascular stiffness, which is associated with aging. Our study supports the concept that genomic instability contributes importantly to the development of cardiovascular disease.

Left-ventricular remodeling after myocardial infarction is associated with a cardiomyocyte-specific hypothyroid condition

Similarities in cardiac gene expression in hypothyroidism and left ventricular (LV) pathological remodeling after myocardial infarction (MI) suggest a role for impaired cardiac thyroid hormone (TH) signaling in the development of heart failure. Increased ventricular activity of the TH-degrading enzyme type 3 deiodinase (D3) is recognized as a potential cause. In the present study, we investigated the cardiac expression and activity of D3 over an 8-wk period after MI in C57Bl/6J mice. Pathological remodeling of the noninfarcted part of the LV was evident from cardiomyocyte hypertrophy, interstitial fibrosis, and impairment of contractility. These changes were maximal and stable from the first week onward, as was the degree of LV dilation. A strong induction of D3 activity was found, which was similarly stable for the period examined. Plasma T(4) levels were transiently decreased at 1 wk after MI, but T(3) levels remained normal. The high D3 activity was associated with increased D3 mRNA expression at 1 but not at 4 and 8 wk after MI. Immunohistochemistry localized D3 protein to cardiomyocytes. In vivo measurement of TH-dependent transcription activity in cardiomyocytes using a luciferase reporter assay indicated a 48% decrease in post-MI mice relative to sham-operated animals, and this was associated with a 50% decrease in LV tissue T(3) concentration. In conclusion, pathological ventricular remodeling after MI in the mouse leads to high and stable induction of D3 activity in cardiomyocytes and a local hypothyroid condition.

Exercise training does not improve cardiac function in compensated or decompensated left ventricular hypertrophy induced by aortic stenosis

There is ample evidence that regular exercise exerts beneficial effects on left ventricular (LV) hypertrophy, remodeling and dysfunction produced by ischemic heart disease or systemic hypertension. In contrast, the effects of exercise on pathological LV hypertrophy and dysfunction produced by LV outflow obstruction have not been studied to date. Consequently, we evaluated the effects of 8 weeks of voluntary wheel running in mice (which mitigates post-infarct LV dysfunction) on LV hypertrophy and dysfunction produced by mild (mTAC) and severe (sTAC) transverse aortic constriction. mTAC produced ~40% LV hypertrophy and increased myocardial expression of hypertrophy marker genes but did not affect LV function, SERCA2a protein levels, apoptosis or capillary density. Exercise had no effect on global LV hypertrophy and function in mTAC but increased interstitial collagen, and ANP expression. sTAC produced ~80% LV hypertrophy and further increased ANP expression and interstitial fibrosis and, in contrast with mTAC, also produced LV dilation, systolic as well as diastolic dysfunction, pulmonary congestion, apoptosis and capillary rarefaction and decreased SERCA2a and ryanodine receptor (RyR) protein levels. LV diastolic dysfunction was likely aggravated by elevated passive isometric force and Ca(2+)-sensitivity of myofilaments. Exercise training failed to mitigate the sTAC-induced LV hypertrophy and capillary rarefaction or the decreases in SERCA2a and RyR. Exercise attenuated the sTAC-induced increase in passive isometric force but did not affect myofilament Ca(2+)-sensitivity and tended to aggravate interstitial fibrosis. In conclusion, exercise had no effect on LV function in compensated and decompensated cardiac hypertrophy produced by LV outflow obstruction, suggesting that the effect of exercise on pathologic LV hypertrophy and dysfunction depends critically on the underlying cause.

Dendritic Cell Function in Transplantation Arteriosclerosis Is Regulated by Heme Oxygenase 1

Rationale : Heme oxygenase (HO)1 is an important modulator of physiological function with cytoprotective properties. Although HO1 has previously been associated with an improved survival of the vascular allograft in rat models in response to pharmaceutical induction of HO1 the exact mechanism by which HO1 exerts it protective function remains to be elucidated.

Objective : We sought to define the role of HO1 in dendritic cells (DCs) function that governs the alloimmune response underlying the development of transplantation associated vasculopathy.

Methods and Results : Loss of HO1 in DCs or by small interfering RNA silencing resulted in major histocompatibility complex class II (MHCII) upregulation by CIITA- driven transcriptional regulation and by STAT1 (signal transducers and activators of transcription 1) phosphorylation. As a result, increased MHCII alloantigen presentation by HO1 −/− DCs directed the primary T-cell response preferentially toward a CD4 + T-cell, rather than a CD8 + T-cell reaction. In a murine model for transplantation arteriosclerosis, adoptive transfer of HO1 −/− DCs before allograft transplantation was indeed associated with pronounced intragraft CD4 + T-cell infiltration and increased IgG deposition, suggestive of an accelerated development of vasculopathy toward the chronic phase. The role of HO1 in DC-mediated T cell activation was further validated by inhibition of endogenous HO1 in allograft recipients. Inhibition of HO1 in DCs aggravated transplant arteriosclerosis development, by increasing intima hyperplasia, and by activation of a CD4 + T cells allograft response, mediated by MHCII upregulation.

Conclusions : These findings demonstrate that HO1 plays an important role in the genetic regulation of the vascular alloimmune response elicited by DCs.

Extracellular superoxide dismutase protects the heart against oxidative stress and hypertrophy after myocardial infarction

Extracellular superoxide dismutase (EC-SOD) contributes only a small fraction to total SOD activity in the heart but is strategically located to scavenge free radicals in the extracellular compartment. EC-SOD expression is decreased in myocardial-infarction (MI)-induced heart failure, but whether EC-SOD can abrogate oxidative stress or modify MI-induced ventricular remodeling has not been previously studied. Consequently, the effects of EC-SOD gene deficiency (EC-SOD KO) on left ventricular (LV) oxidative stress, hypertrophy, and fibrosis were studied in EC-SOD KO and wild-type mice under control conditions, and at 4 and 8 weeks after permanent coronary artery ligation. EC-SOD KO had no detectable effect on LV function in normal hearts but caused small but significant increases of LV fibrosis. At 8 weeks after MI, EC-SOD KO mice developed significantly more LV hypertrophy (LV mass increased 1.64-fold in KO mice compared to 1.35-fold in wild-type mice; p<0.01) and more fibrosis and myocyte hypertrophy which was more prominent in the peri-infarct region than in the remote myocardium. EC-SOD KO mice had greater increases of nitrotyrosine in the peri-infarct myocardium, and this was associated with a greater reduction of LV ejection fraction, a greater decrease of sarcoplasmic or endoplasmic reticulum calcium2+ ATPase, and a greater increase of atrial natriuretic peptide in the peri-infarct zone compared to wild-type mice. EC-SOD KO was associated with more increases of phosphorylated p38 (p-p38(Thr180/Tyr182)), p42/44 extracellular signal-regulated kinase (p-Erk(Thr202/Tyr204)), and c-Jun N-terminal kinase (p-JNK(Thr183/Tyr185)) both under control conditions and after MI, indicating that EC-SOD KO increases activation of mitogen-activated protein kinase signaling pathways. These findings demonstrate that EC-SOD plays an important role in protecting the heart against oxidative stress and infarction-induced ventricular hypertrophy.

Extracellular superoxide dismutase deficiency exacerbates pressure overload-induced left ventricular hypertrophy and dysfunction

Extracellular superoxide dismutase (SOD) contributes only a small fraction to total SOD activity in the normal heart but is strategically located to scavenge free radicals in the extracellular compartment. To examine the physiological significance of extracellular SOD in the response of the heart to hemodynamic stress, we studied the effect of extracellular SOD deficiency on transverse aortic constriction (TAC)-induced left ventricular remodeling. Under unstressed conditions extracellular SOD deficiency had no effect on myocardial total SOD activity, the ratio of glutathione:glutathione disulfide, nitrotyrosine content, or superoxide anion production but resulted in small but significant increases in myocardial fibrosis and ventricular mass. In response to TAC for 6 weeks, extracellular SOD-deficient mice developed more severe left ventricular hypertrophy (heart weight increased 2.56-fold in extracellular SOD-deficient mice as compared with 1.99-fold in wild-type mice) and pulmonary congestion (lung weight increased 2.92-fold in extracellular SOD-deficient mice as compared with 1.84-fold in wild-type mice). Extracellular SOD-deficient mice also had more ventricular fibrosis, dilation, and a greater reduction of left ventricular fractional shortening and rate of pressure development after TAC. TAC resulted in greater increases of ventricular collagen I, collagen III, matrix metalloproteinase-2, matrix metalloproteinase-9, nitrotyrosine, and superoxide anion production. TAC also resulted in a greater decrease of the ratio of glutathione:glutathione disulfide in extracellular SOD-deficient mice. The finding that extracellular SOD deficiency had minimal impact on myocardial overall SOD activity but exacerbated TAC induced myocardial oxidative stress, hypertrophy, fibrosis, and dysfunction indicates that the distribution of extracellular SOD in the extracellular space is critically important in protecting the heart against pressure overload.

Vasomotor control in mice overexpressing human endothelial nitric oxide synthase

Nitric oxide (NO) plays a key role in regulating vascular tone. Mice overexpressing endothelial NO synthase [eNOS-transgenic (Tg)] have a 20% lower systemic vascular resistance (SVR) than wild-type (WT) mice. However, because eNOS enzyme activity is 10 times higher in tissue homogenates from eNOS-Tg mice, this in vivo effect is relatively small. We hypothesized that the effect of eNOS overexpression is attenuated by alterations in NO signaling and/or altered contribution of other vasoregulatory pathways. In isoflurane-anesthetized open-chest mice, eNOS inhibition produced a significantly greater increase in SVR in eNOS-Tg mice compared with WT mice, consistent with increased NO synthesis. Vasodilation to sodium nitroprusside (SNP) was reduced, whereas the vasodilator responses to phosphodiesterase-5 blockade and 8-bromo-cGMP (8-Br-cGMP) were maintained in eNOS-Tg compared with WT mice, indicating blunted responsiveness of guanylyl cyclase to NO, which was supported by reduced guanylyl cyclase activity. There was no evidence of eNOS uncoupling, because scavenging of reactive oxygen species (ROS) produced even less vasodilation in eNOS-Tg mice, whereas after eNOS inhibition the vasodilator response to ROS scavenging was similar in WT and eNOS-Tg mice. Interestingly, inhibition of other modulators of vascular tone [including cyclooxygenase, cytochrome P-450 2C9, endothelin, adenosine, and Ca-activated K(+) channels] did not significantly affect SVR in either eNOS-Tg or WT mice, whereas the marked vasoconstrictor responses to ATP-sensitive K(+) and voltage-dependent K(+) channel blockade were similar in WT and eNOS-Tg mice. In conclusion, the vasodilator effects of eNOS overexpression are attenuated by a blunted NO responsiveness, likely at the level of guanylyl cyclase, without evidence of eNOS uncoupling or adaptations in other vasoregulatory pathways.

Inducible nitric oxide synthase deficiency protects the heart from systolic overload-induced ventricular hypertrophy and congestive heart failure

Inducible nitric oxide synthase (iNOS) protein is expressed in cardiac myocytes of patients and experimental animals with congestive heart failure (CHF). Here we show that iNOS expression plays a role in pressure overload-induced myocardial chamber dilation and hypertrophy. In wild-type mice, chronic transverse aortic constriction (TAC) resulted in myocardial iNOS expression, cardiac hypertrophy, ventricular dilation and dysfunction, and fibrosis, whereas iNOS-deficient mice displayed much less hypertrophy, dilation, fibrosis, and dysfunction. Consistent with these findings, TAC resulted in marked increases of myocardial atrial natriuretic peptide 4-hydroxy-2-nonenal (a marker of lipid peroxidation) and nitrotyrosine (a marker for peroxynitrite) in wild-type mice but not in iNOS-deficient mice. In response to TAC, myocardial endothelial NO synthase and iNOS was expressed as both monomer and dimer in wild-type mice, and this was associated with increased reactive oxygen species production, suggesting that iNOS monomer was a source for the increased oxidative stress. Moreover, systolic overload-induced Akt, mammalian target of rapamycin, and ribosomal protein S6 activation was significantly attenuated in iNOS-deficient mice. Furthermore, selective iNOS inhibition with 1400W (6 mg/kg per hour) significantly attenuated TAC induced myocardial hypertrophy and pulmonary congestion. These data implicate iNOS in the maladaptative response to systolic overload and suggest that selective iNOS inhibition or attenuation of iNOS monomer content might be effective for treatment of systolic overload-induced cardiac dysfunction.

Catheter-based impedance measurements in the right atrium for continuously monitoring hematocrit and estimating blood viscosity changes; an in vivo feasibility study in swine

Hematocrit is the most important determinant of whole blood viscosity and it affects thrombosis. As hematocrit can be measured accurately in vitro by using an electrical impedance technique, aim of the present study is to investigate the diagnostic potential of using this technique in vivo to continuously monitor hematocrit. Characteristics of a special catheter for in vivo measurement of electrical resistivity in blood in the right atrium are described. In five anesthetized swine hematocrit is monitored continuously with this catheter while different levels of hemoconcentration are induced. In addition, blood viscosity is increased by inducing 'acute phase' reaction the day before surgery, resulting in variable degree of elevated fibrinogen levels in the five swine. Good reproducibility of the resistivity measurements (S.D < 0.01) and excellent correlation between resistivity data in vivo and hematocrit levels in each swine are found (r2 = 0.95-0.99). Furthermore, stepwise regression analysis of data from all swine shows a highly significant contribution also of other important parameters of blood viscosity, such as fibrinogen, total protein and temperature (cumulative r2 = 0.97). Determining hematocrit continuously in vivo by electrical resistivity measurements with a catheter in the right atrium is feasible and these measurements correlate significantly also with other important parameters of blood viscosity.