Mitochondrial function as a therapeutic target in heart failure

Heart failure is a pressing worldwide public-health problem with millions of patients having worsening heart failure. Despite all the available therapies, the condition carries a very poor prognosis. Existing therapies provide symptomatic and clinical benefit, but do not fully address molecular abnormalities that occur in cardiomyocytes. This shortcoming is particularly important given that most patients with heart failure have viable dysfunctional myocardium, in which an improvement or normalization of function might be possible. Although the pathophysiology of heart failure is complex, mitochondrial dysfunction seems to be an important target for therapy to improve cardiac function directly. Mitochondrial abnormalities include impaired mitochondrial electron transport chain activity, increased formation of reactive oxygen species, shifted metabolic substrate utilization, aberrant mitochondrial dynamics, and altered ion homeostasis. In this Consensus Statement, insights into the mechanisms of mitochondrial dysfunction in heart failure are presented, along with an overview of emerging treatments with the potential to improve the function of the failing heart by targeting mitochondria.

Mitochondria are primarily located within subsarcolemmal, perinuclear, and intrafibrillar regions of the cardiomyocyte. Although they are symbiotic partners with the other cellular compartments, mitochondria are in many ways discrete entities. Mitochondrial dynamics in the form of fission, fusion, and autophagy are highly regulated processes that are essential for energy production and structural integrity of the organelles 22 – 29 . Altered mitochondrial biogenesis, fragmentation, and hyperplasia have been observed in studies of human 30 and animal models 31 , 32 of HF. These effects seem to be caused by altered expression of proteins that regulate mitochondrial dynamics 33 . As many of these factors are ‘master regulators’ of mitochondrial metabolism, these changes might be directly related to the decreased capacity to oxidize fatty acid substrates often seen in HF 34 , 35 . Mitochondria have their own DNA (mtDNA) and a genetic code that is distinct from the host-cell nuclear DNA. mtDNA is circular in shape, analogous to DNA found in lower organisms, and a primitive fingerprint leftover from bacterial origin. Evolutionary selection pressures have led to mitochondria ‘outsourcing’ almost all their protein-making needs to their cellular hosts. The overwhelming majority (>99%) of mitochondrial proteins come from nuclear-encoded DNA. These proteins are synthesized via cellular protein synthesis machinery, and are actively imported into mitochondria through mitochondrial membrane transporters 36 . mtDNA encodes 13 protein subunits found within three of the electron transport protein complexes, and a handful of ribosomal and transfer RNAs 37 . These proteins are made in specialized ribosomes or ‘mitoribosomes’, which are physically attached to the mitochondrial inner membrane 38 . Many inherited familial cardiomyopathies (both adult and paediatric) are associated with mtDNA mutations 39 . In humans, mitochondria are maternally inherited 40 , owing to high mitochondrial density in the egg and the active degradation of mitochondria in the sperm during fertilization 41 . The proximity of mtDNA to sites of mitochondrial ROS generation, poor repair mechanisms, and a lack of protective histones combine to make mtDNA particularly susceptible to oxidative injury and mutation. Mitochondrial genetics contribute to cardiomyopathies by expressing mutant proteins that influence energy homeostasis. With 1,000–10,000 genes per mitochondria (polyploidy), mitochondrial genetics operate on population-based (instead of Mendelian) principles 37 . Mutated mtDNA is found alongside nonmutated copies, leading to mitochondrial ‘heteroplasmy’. The extent of heteroplasmy in mutated mtDNA influences the susceptibility to inherited mitochondrial disease 42 . Mutated mtDNA can be found in 1 in 200 individuals, a frequency that is 20-fold higher than the incidence of mitochondrial disease. This mismatch indicates that healthy individuals often harbour mutated mtDNA that has no observable phenotypic consequences until a certain mutation threshold is reached 37 . Although very early in preclinical development, various innovative approaches to reduce the extent of heteroplasmy using genome editing might ultimately lead to effective therapy for HF caused by genetic mitochondrial disease 43 – 45 . Given that mitochondrial abnormalities, such as increased ROS production, altered mitochondrial energetics, and impaired mitochondrial ion homeostasis, are observed in genetic mitochondrial diseases as well as HF, innovative approaches that target mitochondrial dysfunction might share efficacy across these diseases.

Substrate utilization in the failing heart has been extensively reviewed previously 57 – 60 . Overall, altered substrate metabolism seems to be centrally involved in HF, although the direction of the metabolic alterations is complex and is likely to depend on the particular stage of HF progression and differences in the availability of substrate (whether the heart is in a ‘fed’ or ‘fasted’ state) 58 , 59 . The heart utilizes different substrates simultaneously to produce energy. Mitochondrial fatty acid oxidation (FAO) is the predominate substrate used in the healthy adult human heart, being responsible for 60–80% of cardiac ATP production, followed by lesser contributions from glucose, lactate, and ketone bodies 61 . However, the heart can shift the relative contribution of these substrates in an effort to adapt to varying physiological conditions. Under conditions of low oxygen content, such as ischaemia and HF, ATP content is thought to decrease by as much as 40% 3 . In HF, fatty acid oxidation and the oxidative capacity of the mitochondria decline, and can no longer maintain sufficient levels of ATP, especially during conditions of increased cardiac workload such as exercise. The failing heart shifts its predominant fuel source from mitochondrial FAO toward glycolytic pathways. This switch is most apparent in late and end-stage HF 57 , and is 30% more energetically efficient in the failing heart, because more ATP is produced per mole of oxygen during carbohydrate oxidation 62 . Numerous studies investigating FAO, glucose oxidation, and (to a lesser extent) ketone body oxidation have aimed to establish a metabolic phenotype, underlying molecular mechanisms, and potential therapeutic targets of the failing heart. The reduction in fatty acid uptake and FAO that occurs during HF might be owing to dysregulated molecular mechanisms responsible for fatty acid metabolism. For example, the level of peroxisome proliferator-activated receptor-α (PPARα), a transcription factor highly expressed in the heart and responsible for fatty acid transport into the mitochondria and peroxisomes, has been reported to be downregulated in both animal models and humans with HF 63 , 64 . Similarly, tissue from animals and humans with HF has reduced activity of the transcription factor responsible for mitochondrial biogenesis, PPAR-γ co-activator (PGC)-1α 64 , 65 . Because these transcription factors have a critical role in the regulation of cardiac mitochondrial energy production, these data suggest that decreased PPARα and PGC-1α activity might be an important precursor leading to impaired FAO during HF. Therefore, further inhibition of FAO to increase glycolytic flux via PPARα and/or PGC-1α is a plausible therapeutic target. Small-molecule regulators of PGC-1α are needed, and animal models overexpressing the transcription factor are inherently problematic, ostensibly owing to increased mitochondrial biogenesis-induced cardiomyopathy 66 . Similarly, PPARα antagonists in animal models of HF have yielded inconclusive data 67 , whereas clinical PPARα ligands are reportedly safe, but their efficacy in a HF population is currently unknown 61 . Although the safety of PPARα ligands is promising, further evidence demonstrating their efficacy in both animal models and humans with HF is needed. Levels of circulating free fatty acids might be higher in the failing heart than under healthy conditions owing to hormonal stimulation. The rise in serum catecholamine levels increases plasma free fatty acid concentrations, and subsequently stimulates FAO 68 . As a result, reducing the availability of circulating free fatty acids via transient adrenergic antagonists might be a viable therapy to inhibit FAO and increase glycolytic ATP production. Traditionally, β-adrenergic receptor antagonists are used in HF owing to their negative ionotropic effects that reduce cardiac workload and spare oxygen by decreasing sympathetic activity 68 . Many, such as carvedilol, have been clinically shown to lessen infarct size after ischaemia by decreasing sympathetic activity, followed by inhibition of mitochondrial fatty acid uptake and increased glucose oxidation 69 . Malonyl-CoA endogenously regulates fatty acid concentrations by controlling the activity of carnitine O -palmitoyltransferase (CPT) 1, a rate-limiting enzyme in mitochondrial fatty acid uptake 68 . When intracellular levels of malonyl-CoA are increased, CPT1 is inhibited and mitochondrial fatty acid uptake is stopped 70 . The intracellular concentration of malonyl-CoA is dependent on the balance between its synthesis via acetyl-CoA carboxylase and degradation via malonyl-CoA decarboxylase. Therefore, the upregulation of acetyl-CoA carboxylase or inhibition of malonyl-CoA decarboxylase would increase intracellular malonyl-CoA levels, and prevent mitochondrial uptake of free fatty acids to reduce FAO. As expected, inhibiting malonyl-CoA decarboxylase in animal models has reportedly improved cardiac fu

Cellular ROS production occurs when ROS formation outpaces or exhausts compensatory signals and overwhelms endogenous scavenging systems 89 – 91 . ROS are produced at several different sites within cells, both within and outside of mitochondria (reviewed in detail previously 92 – 95 ). Mitochondrial ROS production occurs at various sites along the inner mitochondrial membrane as well as in the mitochondrial matrix by components of the ETC and the Krebs cycle, respectively 96 ( FIG. 4 ). ROS production is typically low under normal physiological conditions 93 , and is kept in check by intracellular and intramitochondrial scavenging systems. Pathological ROS levels in the heart typically occur when ROS production outpaces endogenous scavenging capacity. ROS (and other associated reactive intermediates) can damage proteins and lipids, trigger cell-death cascades, and evoke synchronized collapses in the cellular energy grid 97 , 98 . Heightened mitochondrial ROS production and downstream ROS-mediated damage has been reported in patients with HF as well as in preclinical models of the disease 31 , 99 – 101 . Although ROS are typically associated with pathological states, ROS levels in the heart per se are best characterized by the term ‘hormesis’: small amounts can evoke adaptive signalling and create beneficial, compensatory responses. Modest production of ROS has been shown to mediate beneficial myocardial signalling involved in physiological responses such as (transient) sympathetic drive 102 , many preconditioning paradigms 103 , cardiac mitochondrial quality control 104 , and exercise 105 . Exercise training is known to augment endogenous ROS-scavenging mechanisms in the heart 105 – 107 , restore bio-energetic efficiency in porcine models of HFpEF 108 , and improve symptoms and quality of life in trials involving patients with HFrEF 109 , 110 or HFpEF 111 . Consistent with the ROS hormesis concept, several studies have noted that administration of high doses of ROS scavengers can abolish the beneficial effects of exercise 112 , 113 , including humans taking oral vitamin C or E supplements 114 . Mitochondrial production of ROS depends on the mitochondrial membrane potential. Increased expression of mitochondrial uncoupling proteins in HF 115 might be a compensatory mechanism to reduce ROS by ‘uncoupling to survive’ 116 , whereby a reduction in mitochondrial membrane potential is postulated to lower ROS emission from mitochondria. This view is popular and almost dogmatic, but the decrease in ROS production by uncoupling is a prominent effect during mitochondrial state 4 respiration (no ADP). Heart mitochondria, however, are never respiring in state 4. Pathological ROS production in cardio-myocytes is likely to be more closely linked to decreased or collapsed membrane potential and/or depletion of the NADPH pool 117 – 119 , whereby ROS production overwhelms endogenous scavenging through mitochondrial membrane-dependent mechanisms 89 . The repeated lack of benefits of ROS scavenging compounds in clinical trials of patients with HF 11 , 120 , 121 continues to plague cardiovascular drug development, suggesting that oxygen radical scavenging per se is not a plausible mechanism of action for long-term improvements in HF. Lack of tissue permeability, poor intra-cellular targeting, and ineffective therapeutic doses might contribute to the poor translation of benefits of anti oxidants to date. This approach to therapy, however, might ultimately succeed when novel scavenging compounds that overcome permeability and targeting problems, such as XJB-5–131 (REFS 122 , 123 ), mitoTEMPO 124 , 125 , and EUK8/EUK134 (REFS 126 – 128 ), are tested in humans.

The mitochondrial permeability transition pore (MPTP) is a nonspecific pore that opens in response to increased calcium levels and oxidative challenge, and is associated with ROS production, apoptotic cell death, and mitochondrial dysfunction. Increased proclivity of MPTP opening occurs in both acute and chronic heart disease, and numerous preclinical studies have demonstrated efficacy in cardiac pathology with MPTP blockers, such as cyclosporin, NIM811, and TRO40303 (reviewed previously 175 – 179 ). Although the opening of the MPTP has historically been thought of as a pathological event leading to cell death, studies now suggest that transient MPTP opening might be a physiological ‘reset’ mechanism to prevent mitochondrial calcium overload. Rare, transient openings of the MPTP have been observed in individual mitochondria of primary cardiomyocytes 180 . Small, brief MPTP openings were found to be more frequent in HF cardiomyocytes, and were associated with transient mitochondrial depolarization and mitochondrial calcium release. If opening of these pores might be a normal compensatory mechanism akin to ‘pressure release valves’, the concept of treating HF by blocking them becomes increasingly difficult. Ongoing uncertainty regarding the molecular identity of the MPTP further complicates the development of novel therapies that act on the pore 176 , 181 – 185 . The MPTP seems to be comprised of ATP synthase (complex V) dimers and to be gated by mitochondrial matrix calcium content via cyclophillin D 186 , 187 . Clinical studies have failed to demonstrate efficacy in most 188 , 189 , but not all 190 , 191 , studies; however, most of these studies focused on reducing acute cardiac ischaemia– reperfusion injury and not in limiting left ventricular dysfunction in HF. Chronic administration of cyclosporin has been linked with renal pathology and immunosuppresfsion 192 , 193 , and cyclosporin was found to evoke systemic hypertension in porcine models of HFpEF 194 . Accordingly, cyclosporin is not an appropriate approach for the long-term management of HF. Further work with alternative MPTP blockers is needed to determine whether inhibiting or delaying MPTP opening is a clinically plausible approach to alter the progression of HF.

The vast majority of HF trials over the past decade have been neutral, and event rates remain unacceptably high. Perhaps most alarming, no proven therapies exist for patients with worsening chronic HF or HFpEF — populations that collectively comprise the majority of the total HF population. Moreover, although systemic blockade of maladaptive neurohormonal responses has improved outcomes in HFrEF, these agents also lower blood pressure and/or heart rate, and development of new haemodynamically active drugs for stepwise addition to existing therapies raises safety and tolerability concerns. Therefore, an ideal novel therapy would be haemodynamically neutral and target the myocardium as the centrepiece of the therapeutic mechanism. In this context, overwhelming evidence from both preclinical and clinical studies indicates bioenergetic insufficiency in HF. Studies using preclinical models of the disease continue to advance our understanding of the cellular and molecular mechanisms that contribute to poor bioenergetics of the failing heart. Considerable potential exists to fill this unmet need, mitigate the economic burdens, and reduce symptoms in patients with HF by focusing on the development of new therapeutic modalities that target mitochondrial abnormalities in HF.