The role of mitochondrial dysfunction in cardiovascular disease: a brief review

Abstract

Cardiovascular disease (CVD) is a leading cause of mortality worldwide. Proper mitochondrial function is necessary in tissues and organs that are of high energy demand, including the heart. Mitochondria are very sensitive to nutrient and oxygen supply and undergo metabolic adaptation to the changing environment. In CVD, such an adaptation is impaired, which, in turn, leads to a progressive decline of the mitochondrial function associated with abnormalities in the respiratory chain and ATP synthesis, increased oxidative stress, and loss of the structural integrity of mitochondria. Uncoupling of the electron transport chain in dysfunctional mitochondria results in enhanced production of reactive oxygen species, depletion of cell ATP pool, extensive cell damage, and apoptosis of cardiomyocytes. Mitophagy is a process, during which cells clear themselves from dysfunctional and damaged mitochondria using autophagic mechanism. Deregulation of this process in the failing heart, accumulation of dysfunctional mitochondria makes the situation even more adverse. In cardiac pathology, aberrations of the activity of the respiratory chain and ATP production may be considered as a core of mitochondrial dysfunction. Indeed, therapeutic restoration of these key functional properties can be considered as a primary goal for improvement of mitochondrial dysfunction in CVD.

Key messages

• Mitochondrial dysfunction plays a crucial role in cardiovascular disease pathogenesis.

• Cardiovascular disease is associated with altered mithochondrial biogenesis and clearance.

• In cardiovascular disease, impaired mitochondrial function results in decreased ATP production and enhanced ROS formation.

Keywords:

• Mitochondrial dysfunction
• cardiovascular disease
• ATP depletion
• apoptosis
• autophagy
• oxidative stress

Introduction

Cardiovascular disease (CVD) is widely distributed in the world and represents a major cause of mortality in humans. According to the annual (2017) report of the American Heart Association (AHA), 92.1 million of American adults were affected with some form of CVD. As an underlining cause of death, CVD contributes to around 801,000 deaths annually, hence for almost 1/3 (31%) of the global mortality in the US. Coronary artery disease is the most frequent cause of CVD-induced mortality accounting for 45% of cases [1]. In Europe, CVD is responsible for 3.9 million deaths per year, which accounts for 45% of all fatal cases [2]. In the US, rise of CVD prevalence is primarily attributed to the population ageing and obesity.

Cardiovascular disease has a complex aetiology. Multiple risk factors and pathological mechanisms contribute to this disease. In cells, various aberrations such as metabolic abnormalities, excessive production of reactive species (ROS), energy deficit, deregulation of autophagy, endoplasmic reticulum (ER) stress, and activation of apoptosis contribute to CVD pathogenesis. Mitochondrial dysfunction or malfunction plays a key role in these cellular perturbations.

Moreover, mitochondria may regulate cell stress response through so called retrograde signalling, when deprivation of the mitochondrial membrane potential leads to activation of a number of signalling proteins, which, in turn, up-regulates many stress-responsive genes in the nucleus [3]. This can explain why mitochondrial disorders exhibit complex clinical manifestations generally occurred in tissues that are highly dependent on the energy supply, such skeletal muscles, heart, and kidney [4]. In this review, we consider how alterations of the mitochondrial function can influence CVD pathogenesis, with emphasis to specific CVD forms.

Mitochondrial morphology in steady-state myocardium

Mitochondrion is a semi-autonomous organelle surrounded by a double membrane. Mitochondrial size generally varies from 0.75 to 8 μm [5]. In cardiomyocytes, there are two mitochondrial populations presented by interfibrillar and subsarcolemmal mitochondria. Interfibrillar mitochondria form parallel lengthwise series and have mostly tubular orientation of their cristae. Subsarcolemmal mitochondria are clustered below the sarcolemma and display lamellar cristae orientation [6]. In cardiac muscle cells, both mitochondrial populations are electrically coupled to each other providing electric conduction from one mitochondrion to another [7].

Mitochondrial biogenesis and morphology is controlled by fusion and fission proteins, which are sensitive to changes in cardiac muscle cells [8]. In fact, mitochondrial morphology is dynamic and responsive to cardiac changes including those that occur in heart pathology [9].

Mitochondrial morphology in cardiac pathology

Cardiac pathologies are generally associated with structural changes of mitochondria in the tissue, including formation of giant mitochondria (megamitochondria) that have a size up to 30 μm [10]. Giant mitochondria are generated by fusion of adjacent organelles due to overexpression of fusion proteins, such as mitofusin 1 (Mfn1) and Mfn2 [11] or by enlargement of an individual mitochondria. Megamitochondria are usually dysfunctional and eliminated through the mechanism of mitophagy [9]. Other pathological changes in mitochondrial shape involve loss or reorientation of cristae, formation of intramitochondrial rods and crystalloids mainly composed by creatine kinase crystals [12].

Heart ischemia leads to fragmentation of mitochondria due to up-regulation of dynamin-related protein 1 (Dnp1), a fission protein [13]. This protein can form a pro-apoptotic complex with Mfn2 and BAX on the outer mitochondrial membrane that is accompanied with opening of the mitochondrial permeability transition pore (MPTP), release of cytochrome c, mitochondrial fragmentation, and induction of cardiomyocyte apoptosis [14]. Inhibition of Dnp1 leads to protection of cardiac mitochondria from fragmentation and prevents apoptosis [13].

Functional mitochondrial abnormalities and CVD

Functional abnormalities of cardiac mitochondria in CVD lead to enhanced oxidative stress, diminished ATP production and energy supply, increased cell apoptosis, and impaired autophagic mechanisms [15].

Excessive ROS production in CVD

The mitochondrial respiratory chain is the main pathway of energy production stored in ATP molecules. Under normal conditions, the respiratory chain works efficiently using over 98% of the electron transport for ATP synthesis. Only 1–2% of electrons are released to generate superoxide radical that, in turn, is decomposed by superoxide dismutase [16]. Regulated ROS production is crucially involved in the induction of protective signalling mechanisms in ischemic preconditioning [17]. Uncoupling of the mitochondrial electron transport chain from ATP production results in ROS overproduction, which then leads to oxidation of lipids and proteins and extensive cell damage. Enhanced ROS generation promotes atherogenesis at all stages by inducing endothelial dysfunction, vessel inflammation, accumulation of oxidized low dense lipoprotein (oxLDL) in the arterial wall, formation of the initial lesion and its maturation to the advanced plaque with possible progression to plaque rupture [18].

Reduced ATP production in CVD

Cellular ATP is mainly synthesized by the mitochondria through the OXPHOS mechanism performed by the electron transport chain, which is localized in the internal membrane of mitochondria [19]. The heart contains relatively low ATP levels due to intensive ATP intake and fast ATP turnover [20]. Mitochondrial creatinine kinase is involved in controlling of the respiratory chain activity and integrating the high-energy phosphate metabolism. Phosphocreatine produced from creatine by this enzyme plays a major role in maintaining the ATP buffer content of the myocardium [21]. The enzyme exists in two forms: highly reactive octameric and less active dimeric that exist in dynamic balance. In heart disease, equilibrium between both forms is shifted towards the dimer due to increased dissociation of the octamer [22] and formation of inactive crystalloids from the octameric creatine kinase [12]. This, in turn, impairs the respiratory control and compensation of high cardiac ATP consumption by phosphocreatine hydrolysis. Compared with the normal heart, failing heart has impaired OXPHOS and decreased ATP levels that results in reduced cardiac performance [23].

The adult heart obtains 50–70% of its energy through mitochondrial β-oxidation of fatty acids [24]. Depending on the nutrient availability, the myocardium can dynamically switch from the preferential use of lipids to glucose as the energy resource in order to keep the stable production of ATP. In cardiomyocytes, this substrate selection flexibility is regulated by insulin-dependent signalling [25]. However, in heart failure, insulin signalling is down-regulated or disrupted, which in turn leads to the loss of metabolic flexibility. As a result, cardiac ATP is progressively depleted, since the reduction of fatty acid oxidation is not compensated by an increase in glucose oxidation as an alternative source of energy [26].

Increased apoptosis in CVD

Dysfunctional mitochondria can initiate intrinsic mechanism of apoptosis of cardiomyocytes in case of impaired activity of the electron transfer chain, ATP depletion and increased oxidative stress [27]. Induction of apoptosis occurs as a response to irreversible cell damage induced by acute or chronic ischemia and metabolic stress. As mentioned above, ischemia/reperfusion injury is related to the apoptotic death of cardiac muscle cells [28] by initiating mitochondrial fragmentation, formation of the pro-apoptotic assembly between Dnp1, Mfn2, and BAX, increased mitochondrial permeability and the release of cytochrome c to the cytosol [14]. Cytochrome c forms a multiprotein complex with apoptotic protease-activating factor 1 (APAF1), which then activates procaspase-9. Once activated, this initiator caspase proteolytically activates the effector caspases and trigger a cascade of irreversible events leading to apoptosis [29]. In cardiac ischemia, high temperature requirement A2 (HtrA2), a mitochondrial serine protease, is also released from the mitochondria to the cytoplasm, where it contributes to caspase activation, thereby promoting apoptosis [30].

Pro-apoptotic BH3 domain-only factors such as heart-expressed BNip3 and Nix (also known as BNip3L) control cardiomyocyte apoptosis. These proteins form heterodimers with anti-apoptotic factors such as Bcl2 and BclX1 thereby permitting activation of pro-apoptotic proteins BAX and BAK. Nix can also induce programmed necrosis of cardiac muscle cells by stimulating Ca²⁺ release from the sarcoplasmic reticulum. Increase in cytosolic Ca²⁺ induces MPTP opening followed by ATP depletion and cell death [31]. While BNip3 is essential for the induction of apoptosis in cardiomyocytes in the ischemic myocardium, Nix mediates cardiomyocyte death in hypertrophied myocardium [32]. In fact, both factors play important roles in ischemia- and non-ischemia-induced cardiac remodelling, when apoptotic and necrotic cardiomyocytes are replaced by fibroblasts in process of heart scarring and cardiac fibrosis [33]. Thus, mitochondrial dysfunction induced by cardiac stress plays a key role in triggering subsequent apoptosis of cardiomyocytes.

Deregulated autophagy in CVD

Dysfunctional mitochondria are cleared by mitophagy, a kind of autophagy that is permanently performed in normal cardiomyocytes in order to support cell survival through removing long-lived organelles and proteins [34]. At low intense cardiac stress, dysfunctional mitochondria can be efficiently removed by mitophagy. Mechanistically, stress induces accumulation of PTEN-induced putative kinase 1 (PINK1) on the mitochondrial surface. PINK1 phosphorylates Mfn2, which, in turn, interacts with Parkin, a E3 ubiquitin ligase [35]. Parkin ubiquitinates key mitochondria-associated proteins that can be then sensed by autophagy adaptor proteins (Figure 1). These autophagy adaptors are involved in recruiting of autophagosomes to targeted mitochondria followed by autophagosomal– mitochondrial fusion and degradation [36].

Figure 1. Molecular mechanism of Parkin-mediated mitophagy. Depolarization of the mitochondrial membrane causes stabilization of PTEN-induced putative kinase 1 (PINK1) and its translocation to the outer membrane where this enzyme phosphorylates (activates) mitofusin 2 (Mfn2), a fusion protein resided on the mitochondrial surface. Parkin, a E3 ubiquitin ligase. Activated Parkin then polyubiquitinates major mitochondrial proteins located on the outer membrane thereby creating targets recognized by autophagy adaptor proteins p62 that mediates interaction with the microtubule-associated protein 1 light chain 3 (LC3), which is involved in elongation of the phagophore membrane. As a result, the mitochondrion is enveloped by the phagophore forming the autophagosome in which the mitochondrion is digested.

Damaging events such as acute cardiac ischemia-reperfusion injury lead to the reduction of the autophagy flux. As a consequence, damaged mitochondria accumulate in cardiomyocytes that leads to severe oxidative stress and causes cardiomyocyte apoptotic death [37]. Loss of a proper control of autophagy was shown to be involved in the pathogenesis of various CVDs, including ischemic heart disease, cardiac hypertrophy, heart failure, and dilated cardiomyopathy.

Mitochondrial dysfunction in specific CVD forms

Atherosclerosis

Mitochondrial dysfunction is a key contributor to oxidative stress, an important pro-atherogenic molecular mechanism. However, oxidative metabolism does not seem to be of great significance for vascular endothelial cells (ECs), in which glycolytic process is a major energy resource [38]. Yet, mitochondria are crucially involved in EC functioning as triggers of nitric acid (NO) production, apoptosis, intracellular signalling, etc. [39]. ROS overproduction was found to promote EC senescence [40], which can, in turn, induce apoptosis and accelerate atherogenesis [41].

In VSMCs, oxLDL inhibits the mitochondrial function by down-regulating the respiratory activity and ATP production and stimulates VSMC hyperplasia, migration, and proliferation, i.e. promotes the pro-atherogenic neointima formation [42]. Interestingly, hyperplasia suppressor gene (HSG), a rodent orthologue of human fusion protein Mfn2, blocks proliferation of VSMCs. HSG expression is greatly reduced in affected arteries of apolipoprotein E (apoE)-deficient hypercholesterolemic mice, an atherosclerotic model [43]. Anti-atherogenic properties of human Mfn2 were demonstrated in a rabbit model of atherosclerosis, when overproduction of Mfn2 resulted in reduced VSMC proliferation/hyperplasia and diminished plaque progression [44]. Thus, in atherosclerosis, mitochondrial dysfunction exerts opposite effects on vascular cells by inducing senescence and death of ECs while stimulating phenotypic switch in VSMCs from the quiescent contractile to proliferative “synthetic” phenotype.

Ischemic heart disease

High cardiac energy demand is almost completely compensated via mitochondrial OXPHOS. Indeed, cardiomyocytes are very vulnerable to oxygen deprivation. In heart ischemia, hypoxic conditions induce mitochondrial dysfunction that, in turn, can contribute to hypoxia-induced cardiac injury. Ischemia/reperfusion has deleterious effects on mitochondrial function by inducing oxidative stress and increasing Ca²⁺ flux into mitochondria. This leads to failure of the electrochemical gradient across the inner mitochondrial membrane and disruption of the respiratory chain activity [45]. In experimental ischemia-reperfusion models, initial activation of autophagy as an adaptive response to cardiac injury was shown [46]. Up-regulation of autophagy markers may rather reflect either elevated or insufficient autophagy flux due to the loss of strict control [47]. In acute cardiac injury, excessive autophagy can have adverse consequences since it causes degradation of essential and fully functional organelles and proteins. However, at late stages of heart disease, autophagy is inhibited [48], and low numbers of autophagic vacuoles can be associated with poor prognosis for diseased patients [49].

Cardiac hypertrophy

In heart hypertrophy, cardiomyocyte growth requires more energy and therefore should be accompanied with increase in mitochondrial abundance. In hypertrophied rat hearts after aortic constriction, mitochondrial amplification was observed [50]. However, in cardiac hypertrophy, initial increase in counts of mitochondria is detected only at early disease stage and declines with disease progression that is frequently accompanied with contractile abnormalities [51]. This leads to reduction of ability of mitochondria to up-regulate ATP production and results in reduced cardiac relaxation followed by diastolic dysfunction and heart failure [52].

Heart failure

In heart failure, mitochondria are frequently damaged due to membrane rupture and matrix depletion [53]. These mitochondria exhibit limited ability for ATP synthesis because of the impaired activity of the respiratory chain and reduced OXPHOS capacity [54,55]. Mitochondrial damage leads to enhanced ER and oxidative stress [56]. In subjects with heart failure, aberrations in the respiratory chain were shown to be related to decreased activity of complexes I and IV [57]. In end-stage heart failure, activities of other redox enzyme, NADPH-transhydrogenase, and the Krebs cycle enzyme such as isocitrate dehydrogenase, malate dehydrogenase, and aconitase are dramatically reduced. Reduced activities of mitochondrial enzymes are in part due to the abundant chemical modifications [58]. In addition, in human failing hearts, mtDNA replication was shown to be severely impaired, which results in mtDNA exhaustion, marked reduction of mtDNA-encoded proteins, and finally disturbance of mitochondrial biogenesis [59]. These data suggest that mitochondrial dysfunction followed by energy deprivation may play a key role in the induction and pathogenesis of heart failure.

Conclusions

Mitochondria play an essential role in all human cells and tissues/organs especially in those that have higher energy requirements such the myocardium. In mitochondria, the performance of the electron transfer chain and ATP-producing machinery have to be strictly regulated. In addition, mitochondria are plastic organelles whose morphology and cellular count can be dynamically changed in response to outer signals. The mitochondrial biogenesis and the fission/fusion balance should be also precisely controlled. In CVD, deregulation of the mitochondrial biogenesis may result in adverse consequences. For example, in ischemic heart disease or ischemia/reperfusion injury, increased oxidative stress and Ca²⁺ flux shifts the balance towards fission, which in turn leads to mitochondrial fragmentation and activates apoptosis. Fission overactivation can be also deleterious in heart failure and diabetic hyperglycaemia. By contrast, in restrictive cardiomyopathy, prevalence of the mitochondrial fusion may lead to the formation of giant mitochondria [60].

Since mitochondria are the principal source of energy in the heart, impairment of the respiratory chain and ATP synthesis may be considered as a core of mitochondrial dysfunction that then lead to the induction of oxidative stress, apoptosis, abnormal autophagy and other pathophysiological perturbations observed in CVD. Therefore, restoring the respiratory activity and ATP-producing capacity of mitochondria can be suggested as a primary therapeutic target to improve the mitochondrial dysfunction in various CVD forms.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Russian Science Foundation [Grant # 15-15-10022].