Unravelling the Interplay between Cardiac Metabolism and Heart Regeneration.

Ischemic heart disease (IHD) is the leading cause of heart failure (HF) and is a significant cause of morbidity and mortality globally. An ischemic event induces cardiomyocyte death, and the ability for the adult heart to repair itself is challenged by the limited proliferative capacity of resident cardiomyocytes. Intriguingly, changes in metabolic substrate utilisation at birth coincide with the terminal differentiation and reduced proliferation of cardiomyocytes, which argues for a role of cardiac metabolism in heart regeneration. As such, strategies aimed at modulating this metabolism-proliferation axis could, in theory, promote heart regeneration in the setting of IHD. However, the lack of mechanistic understanding of these cellular processes has made it challenging to develop therapeutic modalities that can effectively promote regeneration. Here, we review the role of metabolic substrates and mitochondria in heart regeneration, and discuss potential targets aimed at promoting cardiomyocyte cell cycle re-entry. While advances in cardiovascular therapies have reduced IHD-related deaths, this has resulted in a substantial increase in HF cases. A comprehensive understanding of the interplay between cardiac metabolism and heart regeneration could facilitate the discovery of novel therapeutic targets to repair the damaged heart and reduce risk of HF in patients with IHD.

Role of cardiac mitofusins in cardiac conduction following simulated ischemia-reperfusion.

Mitochondrial dysfunction induced by acute cardiac ischemia-reperfusion (IR), may increase susceptibility to arrhythmias by perturbing energetics, oxidative stress production and calcium homeostasis. Although changes in mitochondrial morphology are known to impact on mitochondrial function, their role in cardiac arrhythmogenesis is not known. To assess action potential duration (APD) in cardiomyocytes from the Mitofusins-1/2 (Mfn1/Mfn2)-double-knockout (Mfn-DKO) compared to wild-type (WT) mice, optical-electrophysiology was conducted. To measure conduction velocity (CV) in atrial and ventricular tissue from the Mfn-DKO and WT mice, at both baseline and following simulated acute IR, multi-electrode array (MEA) was employed. Intracellular localization of connexin-43 (Cx43) at baseline was evaluated by immunohistochemistry, while Cx-43 phosphorylation was assessed by Western-blotting. Mfn-DKO cardiomyocytes demonstrated an increased APD. At baseline, CV was significantly lower in the left ventricle of the Mfn-DKO mice. CV decreased with simulated-ischemia and returned to baseline levels during simulated-reperfusion in WT but not in atria of Mfn-DKO mice. Mfn-DKO hearts displayed increased Cx43 lateralization, although phosphorylation of Cx43 at Ser-368 did not differ. In summary, Mfn-DKO mice have increased APD and reduced CV at baseline and impaired alterations in CV following cardiac IR. These findings were associated with increased Cx43 lateralization, suggesting that the mitofusins may impact on post-MI cardiac-arrhythmogenesis.

Targeting the Metabolic-Inflammatory Circuit in Heart Failure With Preserved Ejection Fraction.

PURPOSE OF REVIEW: Heart failure with preserved ejection fraction (HFpEF) is a leading cause of morbidity and mortality. The current mechanistic paradigm supports a comorbidity-driven systemic proinflammatory state that evokes microvascular and myocardial dysfunction. Crucially, diabetes and obesity are frequently prevalent in HFpEF patients; as such, we review the involvement of a metabolic-inflammatory circuit in disease pathogenesis. RECENT FINDINGS: Experimental models of diastolic dysfunction and genuine models of HFpEF have facilitated discovery of underlying drivers of HFpEF, where metabolic derangement and systemic inflammation appear to be central components of disease pathophysiology. Despite a shared phenotype among these models, molecular signatures differ depending on type and combination of comorbidities present. Inflammation, oxidative stress, hypertension, and metabolic derangements have been positioned as therapeutic targets to suppress the metabolic-inflammatory circuit in HFpEF. However, the stratification of unique patient phenogroups within the collective HFpEF subgroup argues for specific interventions for distinct phenogroups.

Hydralazine protects the heart against acute ischaemia/reperfusion injury by inhibiting Drp1-mediated mitochondrial fission.

AIMS: Genetic and pharmacological inhibition of mitochondrial fission induced by acute myocardial ischaemia/reperfusion injury (IRI) has been shown to reduce myocardial infarct size. The clinically used anti-hypertensive and heart failure medication, hydralazine, is known to have anti-oxidant and anti-apoptotic effects. Here, we investigated whether hydralazine confers acute cardioprotection by inhibiting Drp1-mediated mitochondrial fission. METHODS AND RESULTS: Pre-treatment with hydralazine was shown to inhibit both mitochondrial fission and mitochondrial membrane depolarisation induced by oxidative stress in HeLa cells. In mouse embryonic fibroblasts (MEFs), pre-treatment with hydralazine attenuated mitochondrial fission and cell death induced by oxidative stress, but this effect was absent in MEFs deficient in the mitochondrial fission protein, Drp1. Molecular docking and surface plasmon resonance studies demonstrated binding of hydralazine to the GTPase domain of the mitochondrial fission protein, Drp1 (KD 8.6±1.0 µM), and inhibition of Drp1 GTPase activity in a dose-dependent manner. In isolated adult murine cardiomyocytes subjected to simulated IRI, hydralazine inhibited mitochondrial fission, preserved mitochondrial fusion events, and reduced cardiomyocyte death (hydralazine 24.7±2.5% vs. control 34.1±1.5%, P=0.0012). In ex vivo perfused murine hearts subjected to acute IRI, pre-treatment with hydralazine reduced myocardial infarct size (as % left ventricle: hydralazine 29.6±6.5% vs. vehicle control 54.1±4.9%, P=0.0083), and in the murine heart subjected to in vivo IRI, the administration of hydralazine at reperfusion, decreased myocardial infarct size (as % area-at-risk: hydralazine 28.9±3.0% vs. vehicle control 58.2±3.8%, P<0.001). CONCLUSION: We show that, in addition to its antioxidant and anti-apoptotic effects, hydralazine, confers acute cardioprotection by inhibiting IRI-induced mitochondrial fission, raising the possibility of repurposing hydralazine as a novel cardioprotective therapy for improving post-infarction outcomes.

Oxidative stress in cardiac hypertrophy: From molecular mechanisms to novel therapeutic targets.

When faced with increased workload the heart undergoes remodelling, where it increases its muscle mass in an attempt to preserve normal function. This is referred to as cardiac hypertrophy and if sustained, can lead to impaired contractile function. Experimental evidence supports oxidative stress as a critical inducer of both genetic and acquired forms of cardiac hypertrophy, a finding which is reinforced by elevated levels of circulating oxidative stress markers in patients with cardiac hypertrophy. These observations formed the basis for using antioxidants as a therapeutic means to attenuate cardiac hypertrophy and improve clinical outcomes. However, the use of antioxidant therapies in the clinical setting has been associated with inconsistent results, despite antioxidants having been shown to exert protection in several animal models of cardiac hypertrophy. This has forced us to revaluate the mechanisms, both upstream and downstream of oxidative stress, where recent studies demonstrate that apart from conventional mediators of oxidative stress, metabolic disturbances, mitochondrial dysfunction and inflammation as well as dysregulated autophagy and protein homeostasis contribute to disease pathophysiology through mechanisms involving oxidative stress. Importantly, novel therapeutic targets have been identified to counteract oxidative stress and attenuate cardiac hypertrophy but more interestingly, the repurposing of drugs commonly used to treat metabolic disorders, hypertension, peripheral vascular disease, sleep disorders and arthritis have also been shown to improve cardiac function through suppression of oxidative stress. Here, we review the latest literature on these novel mechanisms and intervention strategies with the aim of better understanding the complexities of oxidative stress for more precise targeted therapeutic approaches to prevent cardiac hypertrophy.

Targeting Mitochondrial Fission Using Mdivi-1 in A Clinically Relevant Large Animal Model of Acute Myocardial Infarction: A Pilot Study.

Background: New treatments are needed to reduce myocardial infarct size (MI) and prevent heart failure (HF) following acute myocardial infarction (AMI), which are the leading causes of death and disability worldwide. Studies in rodent AMI models showed that genetic and pharmacological inhibition of mitochondrial fission, induced by acute ischemia and reperfusion, reduced MI size. Whether targeting mitochondrial fission at the onset of reperfusion is also cardioprotective in a clinically-relevant large animal AMI model remains to be determined. Methods: Adult pigs (30-40 kg) were subjected to closed-chest 90-min left anterior descending artery ischemia followed by 72 h of reperfusion and were randomized to receive an intracoronary bolus of either mdivi-1 (1.2 mg/kg, a small molecule inhibitor of the mitochondrial fission protein, Drp1) or vehicle control, 10-min prior to reperfusion. The left ventricular (LV) size and function were both assessed by transthoracic echocardiography prior to AMI and after 72 h of reperfusion. MI size and the area-at-risk (AAR) were determined using dual staining with Tetrazolium and Evans blue. Heart samples were collected for histological determination of fibrosis and for electron microscopic analysis of mitochondrial morphology. Results: A total of 14 pigs underwent the treatment protocols (eight control and six mdivi-1). Administration of mdivi-1 immediately prior to the onset of reperfusion did not reduce MI size (MI size as % of AAR: Control 49.2 ± 8.6 vs. mdivi-1 50.5 ± 11.4; p = 0.815) or preserve LV systolic function (LV ejection fraction %: Control 67.5 ± 0.4 vs. mdivi-1 59.6 ± 0.6; p = 0.420), when compared to vehicle control. Similarly, there were no differences in mitochondrial morphology or myocardial fibrosis between mdivi-1 and vehicle control groups. Conclusion: Our pilot study has shown that treatment with mdivi-1 (1.2 mg/kg) at the onset of reperfusion did not reduce MI size or preserve LV function in the clinically-relevant closed-chest pig AMI model. A larger study, testing different doses of mdivi-1 or using a more specific Drp1 inhibitor are required to confirm these findings.

Remote ischemic conditioning in ST-segment elevation myocardial infarction - an update.

Acute myocardial infarction (AMI) and the heart failure (HF) that often results are among the leading causes of death and disability in the world. As such, novel strategies are required to protect the heart against the detrimental effects of acute ischemia/reperfusion injury (IRI), in order to reduce myocardial infarct (MI) size and prevent the onset of HF. The endogenous cardioprotective strategy of remote ischemic conditioning (RIC), in which cycles of brief ischemia and reperfusion are applied to a tissue or organ away from the heart, has been reported in experimental studies to reduce MI size in animal models of acute IRI. In the clinical setting, RIC can be induced by simply inflating and deflating a cuff placed on the upper arm or thigh to induce brief cycles of ischemia and reperfusion, a strategy which has been shown to reduce MI size in ST-segment elevation myocardial infarction (STEMI) patients undergoing primary percutaneous coronary intervention (PPCI). The results of the ongoing CONDI2/ERIC-PPCI trial are eagerly awaited, and will provide definitive answers with regards to the cardioprotective effect and clinical outcome benefits of RIC in STEMI.