Alexander Nickel

Reversal of Mitochondrial Transhydrogenase Causes Oxidative Stress in Heart Failure

Mitochondrial reactive oxygen species (ROS) play a central role in most aging-related diseases. ROS are produced at the respiratory chain that demands NADH for electron transport and are eliminated by enzymes that require NADPH. The nicotinamide nucleotide transhydrogenase (Nnt) is considered a key antioxidative enzyme based on its ability to regenerate NADPH from NADH. Here, we show that pathological metabolic demand reverses the direction of the Nnt, consuming NADPH to support NADH and ATP production, but at the cost of NADPH-linked antioxidative capacity. In heart, reverse-mode Nnt is the dominant source for ROS during pressure overload. Due to a mutation of the Nnt gene, the inbred mouse strain C57BL/6J is protected from oxidative stress, heart failure, and death, making its use in cardiovascular research problematic. Targeting Nnt-mediated ROS with the tetrapeptide SS-31 rescued mortality in pressure overload-induced heart failure and could therefore have therapeutic potential in patients with this syndrome.

Myocardial energetics in heart failure

It has become common sense that the failing heart is an “engine out of fuel”. However, undisputable evidence that, indeed, the failing heart is limited by insufficient ATP supply is currently lacking. Over the last couple of years, an increasingly complex picture of mechanisms evolved that suggests that potentially metabolic intermediates and redox state could play the more dominant roles for signaling that eventually results in left ventricular remodeling and contractile dysfunction. In the pathophysiology of heart failure, mitochondria emerge in the crossfire of defective excitation–contraction coupling and increased energetic demand, which may provoke oxidative stress as an important upstream mediator of cardiac remodeling and cell death. Thus, future therapies may be guided towards restoring defective ion homeostasis and mitochondrial redox shifts rather than aiming solely at improving the generation of ATP.

Effect of Renal Denervation on Neurohumoral Activation Triggering Atrial Fibrillation in Obstructive Sleep Apnea

Obstructive sleep apnea is characterized by repetitive collapses of the upper airway, negative thoracic pressure periods, and intermittent hypoxia, stimulating the autonomic nervous system. The increased sympathetic drive during obstructive sleep apnea results in postapneic blood pressure rises and neurohumoral activation potentially involved in the initiation and progression to permanent atrial fibrillation (AF). In a pig model mimicking obstructive sleep apnea, we studied the effects of repetitive obstructive respiratory events for 4 hours on the occurrence of spontaneous AF episodes, postapneic blood pressure rises, and neurohumoral activation. In addition, renal sympathetic denervation was performed to investigate the impact of the sympathetic nervous system. Repetitive obstructive respiratory events caused pronounced postapneic blood pressure rises, prolonged duration of spontaneous AF episodes triggered by spontaneous atrial beats, and increased plasma renin activity and aldosterone concentrations. This was associated with increased nicotinamide adenine dinucleotide phosphate-oxidase activity, reduced antioxidative capacity, and elevated expression of connective tissue growth factor, a redox-sensitive mediator of fibrosis. Renal sympathetic denervation inhibited postapneic blood pressure rises and decreased plasma renin activity and aldosterone concentrations. The occurrence and duration of spontaneous AF were reduced comparable with a combined pharmacological blockade of angiotensin receptor and &bgr;-adrenoceptor. Increased atrial oxidative stress, together with the activation of profibrotic pathways and intermittent hypoxia, was not attenuated after renal sympathetic denervation. Repetitive obstructive respiratory events triggered spontaneous AF, increased atrial oxidative stress, and activated profibrotic pathways in the atrium. Renal sympathetic denervation reduced spontaneous AF and postapneic blood pressure rises by combined reduction of sympathetic drive and components of the circulating renin–angiotensin system. However, the generation of atrial oxidative stress was not modulated.

Low STAT3 expression sensitizes to toxic effects of β-adrenergic receptor stimulation in peripartum cardiomyopathy

Abstract Aims The benefit of the β1-adrenergic receptor (β1-AR) agonist dobutamine for treatment of acute heart failure in peripartum cardiomyopathy (PPCM) is controversial. Cardiac STAT3 expression is reduced in PPCM patients. Mice carrying a cardiomyocyte-restricted deletion of STAT3 (CKO) develop PPCM. We hypothesized that STAT3-dependent signalling networks may influence the response to β-AR agonist treatment in PPCM patients and analysed this hypothesis in CKO mice. Methods and results Follow-up analyses in 27 patients with severe PPCM (left ventricular ejection fraction ≤25%) revealed that 19 of 20 patients not obtaining dobutamine improved cardiac function. All seven patients obtaining dobutamine received heart transplantation (n = 4) or left ventricular assist devices (n = 3). They displayed diminished myocardial triglyceride, pyruvate, and lactate content compared with non-failing controls. The β-AR agonist isoproterenol (Iso) induced heart failure with high mortality in postpartum female, in non-pregnant female and in male CKO, but not in wild-type mice. Iso induced heart failure and high mortality in CKO mice by impairing fatty acid and glucose uptake, thereby generating a metabolic deficit. The latter was governed by disturbed STAT3-dependent signalling networks, microRNA-199a-5p, microRNA-7a-5p, insulin/glucose transporter-4, and neuregulin/ErbB signalling. The resulting cardiac energy depletion and oxidative stress promoted dysfunction and cardiomyocyte loss inducing irreversible heart failure, which could be attenuated by the β1-AR blocker metoprolol or glucose-uptake-promoting drugs perhexiline and etomoxir. Conclusions Iso impairs glucose uptake, induces energy depletion, oxidative stress, dysfunction, and death in STAT3-deficient cardiomyocytes mainly via β1-AR stimulation. These cellular alterations may underlie the dobutamine-induced irreversible heart failure progression in PPCM patients who frequently display reduced cardiac STAT3 expression.

CaM Kinase II mediates maladaptive post-infarct remodeling and pro-inflammatory chemoattractant signaling but not acute myocardial ischemia/reperfusion injury.

CaMKII was suggested to mediate ischemic myocardial injury and adverse cardiac remodeling. Here, we investigated the roles of different CaMKII isoforms and splice variants in ischemia/reperfusion (I/R) injury by the use of new genetic CaMKII mouse models. Although CaMKIIδC was upregulated 1 day after I/R injury, cardiac damage 1 day after I/R was neither affected in CaMKIIδ‐deficient mice, CaMKIIδ‐deficient mice in which the splice variants CaMKIIδB and C were re‐expressed, nor in cardiomyocyte‐specific CaMKIIδ/γ double knockout mice (DKO). In contrast, 5 weeks after I/R, DKO mice were protected against extensive scar formation and cardiac dysfunction, which was associated with reduced leukocyte infiltration and attenuated expression of members of the chemokine (C‐C motif) ligand family, in particular CCL3 (macrophage inflammatory protein‐1α, MIP‐1α). Intriguingly, CaMKII was sufficient and required to induce CCL3 expression in isolated cardiomyocytes, indicating a cardiomyocyte autonomous effect. We propose that CaMKII‐dependent chemoattractant signaling explains the effects on post‐I/R remodeling. Taken together, we demonstrate that CaMKII is not critically involved in acute I/R‐induced damage but in the process of post‐infarct remodeling and inflammatory processes.

Mitochondrial energetics and calcium coupling in the heart

Contraction and relaxation of the heart consume large amounts of energy that need to be replenished by oxidative phosphorylation in mitochondria, and matching energy supply to demand involves the complimentary control of respiration through ADP and Ca2+. In heart failure, an imbalance between ADP and Ca2+ leads to oxidation of mitochondrial pyridine nucleotides, where NADH oxidation may limit ATP production and contractile function, while NADPH oxidation can induce oxidative stress with consecutive maladaptive remodelling. Understanding the complex mechanisms that disturb this finely tuned equilibrium may aid the development of drugs that could ameliorate the progression of heart failure beyond the classical neuroendocrine inhibition.

A proteolytic fragment of histone deacetylase 4 protects the heart from failure by regulating the hexosamine biosynthetic pathway

The stress-responsive epigenetic repressor histone deacetylase 4 (HDAC4) regulates cardiac gene expression. Here we show that the levels of an N-terminal proteolytically derived fragment of HDAC4, termed HDAC4-NT, are lower in failing mouse hearts than in healthy control hearts. Virus-mediated transfer of the portion of the Hdac4 gene encoding HDAC4-NT into the mouse myocardium protected the heart from remodeling and failure; this was associated with decreased expression of Nr4a1, which encodes a nuclear orphan receptor, and decreased NR4A1-dependent activation of the hexosamine biosynthetic pathway (HBP). Conversely, exercise enhanced HDAC4-NT levels, and mice with a cardiomyocyte-specific deletion of Hdac4 show reduced exercise capacity, which was characterized by cardiac fatigue and increased expression of Nr4a1. Mechanistically, we found that NR4A1 negatively regulated contractile function in a manner that depended on the HBP and the calcium sensor STIM1. Our work describes a new regulatory axis in which epigenetic regulation of a metabolic pathway affects calcium handling. Activation of this axis during intermittent physiological stress promotes cardiac function, whereas its impairment in sustained pathological cardiac stress leads to heart failure.

Inhibition of MicroRNA-146a and Overexpression of Its Target Dihydrolipoyl Succinyltransferase Protect Against Pressure Overload-Induced Cardiac Hypertrophy and Dysfunction

Background: Cardiovascular diseases remain the predominant cause of death worldwide, with the prevalence of heart failure continuing to increase. Despite increased knowledge of the metabolic alterations that occur in heart failure, novel therapies to treat the observed metabolic disturbances are still lacking. Methods: Mice were subjected to pressure overload by means of angiotensin-II infusion or transversal aortic constriction. MicroRNA-146a was either genetically or pharmacologically knocked out or genetically overexpressed in cardiomyocytes. Furthermore, overexpression of dihydrolipoyl succinyltransferase (DLST) in the murine heart was performed by means of an adeno-associated virus. Results: MicroRNA-146a was upregulated in whole heart tissue in multiple murine pressure overload models. Also, microRNA-146a levels were moderately increased in left ventricular biopsies of patients with aortic stenosis. Overexpression of microRNA-146a in cardiomyocytes provoked cardiac hypertrophy and left ventricular dysfunction in vivo, whereas genetic knockdown or pharmacological blockade of microRNA-146a blunted the hypertrophic response and attenuated cardiac dysfunction in vivo. Mechanistically, microRNA-146a reduced its target DLST—the E2 subcomponent of the &agr;-ketoglutarate dehydrogenase complex, a rate-controlling tricarboxylic acid cycle enzyme. DLST protein levels significantly decreased on pressure overload in wild-type mice, paralleling a decreased oxidative metabolism, whereas DLST protein levels and hence oxidative metabolism were partially maintained in microRNA-146a knockout mice. Moreover, overexpression of DLST in wild-type mice protected against cardiac hypertrophy and dysfunction in vivo. Conclusions: Altogether we show that the microRNA-146a and its target DLST are important metabolic players in left ventricular dysfunction.

Endogenous nitric oxide formation in cardiac myocytes does not control respiration during β‐adrenergic stimulation

In the heart, endothelial nitric oxide (NO) controls oxygen consumption in the working heart through paracrine mechanisms. While cardiac myocytes contain several isoforms of NO synthases, it is unclear whether these can control respiration in an intracrine fashion. A long‐standing controversy is whether a NOS exists within mitochondria. By combining fluorescence technologies with electrical field stimulation or the patch‐clamp technique in beating cardiac myocytes, we identified a neuronal NO synthase (nNOS) as the most relevant source of intracellular NO during β‐adrenergic stimulation, while no evidence for a mitochondria‐located NOS was obtained. The amounts of NO produced by non‐mitochondrial nNOS were insufficient to regulate respiration during β‐adrenergic stimulation, arguing against intracrine control of respiration by NO within cardiac myocytes.

Ferulic acid, a bioactive component of rice bran, improves oxidative stress and mitochondrial biogenesis and dynamics in mice and in human mononuclear cells

The aim of the study was to characterize the vascular effects of rice bran enzymatic extract (RBEE). ApoE-/- mice were fed a high-fat/cholesterol diet (HFD) or HFD supplemented with 5% RBEE for 21 weeks. RBEE prevented development of atherosclerotic plaques and oxidative stress in mouse aorta as well as the down-regulation of markers of mitochondrial biogenesis. Analysis of the bioactive components identified ferulic acid (FA) as responsible component. In healthy human volunteers, FA intake reduced NADPH oxidase activity, superoxide release, apoptosis and necrosis in peripheral blood mononuclear cells. Differentiation and proliferation of endothelial progenitor cells were improved. In summary, the study identifies FA as a major active component of rice bran, which improves expression of mitochondrial biogenesis and dynamics markers and reduces oxidative stress in a mouse model of vascular damage as well as in endothelial cells and human mononuclear cells.