Why is Qi-Invigorating Therapy in Chinese Medicine Suitable for Mitochondrial Diseases? A Bioenergetic Perspective

1.1. Mendelian paradigm of genetics and mitochondrial diseases

Western biomedical science has stood on two philosophical pillars: the anatomical paradigm of medicine and the Mendelian paradigm of genetics. The Mendelian paradigm of genetics argues that genetic traits are transmitted across generations according to the laws of Gregor Mendel. However, Mendelian genetics is specific for nuclear DNA (nDNA) genes and have been powerful predictors of medical relationships for the past century, but fails to direct us toward answers for mitochondrial diseases. Since mitochondrial DNA (mtDNA) encodes genes for proteins of energy metabolism, human diseases affect many organs that could originate from systemic energy metabolism defects and could result from mutations in mtDNA [4]. The classical Mendelian genetic perspective has failed to explain the aetiology of mitochondrial diseases, because they are primarily systemic bioenergetic diseases, and the most important energy genes are located in mtDNA [5].

1.2. mtDNA and mitochondrial bioenergetics

In many animal species, mtDNA is a circular intron-free genome consisting of 14,000–17,000 base pairs (16,569 in humans), although in some species it is much larger, noncircular, and contains introns [6–7]. It is typically maternally inherited and present in multiple copies within each cell; the mtDNAs are grouped in DNA-protein complexes referred to as nucleoids [8–9] that are anchored to the inner mitochondrial membrane. Most somatic cells in mammals have 10³–10⁴ mtDNAs [10], but this number varies significantly by cell type and developmental stage [11].

Mitochondria are at the centre of bioenergetics and generate about 90% of cellular energy, regulate cellular redox status, and produce reactive oxygen species (ROS). The mitochondrial genome consists of thousands of copies of mtDNA, plus between 1,000 and 2,000 nDNA genes. mtDNA codes for 13 OXPHOS polypeptides, the 22 tRNAs, and the 12S and 16S rRNAs. mtDNA polypeptides encompass seven of OXPHOS complex I (ND1, ND2, ND3, ND4, ND4L, ND5, and ND6), one of complex III (cytochrome b), three of complex IV (COI, COII, and COIII), and two of complex V(ATPase6 and ATPase8). Consequently, mitochondrial biology and genetics become excellent candidates for expanding the anatomical and Mendelian paradigms to address the complexities of age-related diseases, ageing, and cancer [12], and life involves the interplay between structure and energy [4].

Both variation in the mtDNA and nDNA-coded mitochondrial genes sequences can perturb mitochondrial bioenergetics and result in disease. Since different tissues depend on mitochondrial energy to different extents, partial systemic energy deficiency can result in tissue-specific symptoms. Tissues with high energy demands include the brain, heart, muscles, kidneys, and the endocrine system: the organs commonly affected in metabolic and degenerative diseases [5]. A decline in energy is common in ageing, and the restoration of mitochondrial bioenergetics may offer a common approach for the treatment of numerous age-associated diseases [13].

The basic role of mitochondria in sustaining the normal cellular function has made dysfunction of this organelle a central feature of numerous diseases in any organ system at any stage of life [14–15]. ‘‘Mitochondrial medicine’’ has emerged as an active field of research. However, the mystery remains as to how tissue or cell type specificity occurs, and how a systemic disorder of one mitochondrial complex can cause a selective disease phenotype while leaving other tissues intact. It is believed that many of the mitochondrial diseases might actually be an expression of progressive organ system failure due to disruption of specific aspects of mitochondrial function. Even though dysfunctional mitochondria appear to be a common underlying problem for all these diseases, they each exhibit unique triggers and symptoms.

1.3. Mitochondrial dynamics and bioenergetics

Mitochondria are highly dynamic organelles that show constant movement, fusion, and fission [16–17]. The role of mitochondrial dynamics is enabling content mixing of mitochondrial matrix and membranous components between mitochondria [18]. Disruption of mitochondrial dynamics causes a series of diseases, including neurodegenerative disease, cardiovascular disease [19], and even cancer [20]. Benard et al. describe the fundamental mechanisms by which mammalian cells regulate energy production, and put emphasis on the importance of mitochondrial dynamics for the regulation of bioenergetics. In addition, most neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and Hereditary Spastic Paraplegia are associated with defects in mitochondrial dynamics and bioenergetics [21]. Therefore, to unravel the fundamental mechanisms by which mitochondrial form interacts with mitochondrial function could permit us to increase our basic knowledge on the regulation of energy metabolism and to decipher the pathophysiology of a group of rare neuronal diseases.

Pathogenic mutations in genes essentials for mitochondrial fusion and fission have complicated the mechanisms of these diseases [22]. Thus, the mitochondrial network organization is a new parameter for physiopathological analyses of mitochondrial diseases [21]. The possible interaction between mitochondrial dynamics and energy production could offer an opportunity for the discovery of drugs that target mitochondrial fusion or fission with a stimulatory effect on energy metabolism, to the benefit of patients suffering from mitochondrial diseases [23–24].

1.4. Qi-invigoration and mitochondrial bioenergetics improvement

In TCM, “Qi” is expressed as the basic material that constitutes the body, maintains human life activities, and is its basic driving force. Qi, as the master of the brain, regulates Yin-Yang and five elements, tonifies the five vital organs of the human body, and keep the six hollow viscera unobstructed, and is in charge of chemical and biological transformation and defences, Qi dominate the whole vital activities. Qi is the centre in TCM from basic theory to clinical practice. According to TCM, “Qi is the root of the human”; “All the diseases originate from Qi”; “Both life and death depend on Qi, if Qi gets together, it will result to the birth; if Qi is strong, then the human body is healthy; if Qi is waning, the body will be weak; if Qi is disordered, the human will be sick; if Qi is depleted, the human will die”. This shows that Qi is closely related to health, disease, and life.

Qi can make the human body work in an orderly fashion by promoting a variety of physiological activities; “Qi-deficiency” will lead to a decline of physiological functions. As one part of a central medical classic The Yellow Emperor’s Inner Canon, ‘Plain Questions’ pointed out that consumption of the vital essence Qi leads to deficiency, ‘Miraculous Pivot’ says that Qi-deficiency in the heart, liver, spleen, lung, or kidneys becomes worse with increasing age, and ultimately all five internal organs of the body become deficient, spirit and Qi leave the body, and eventually the human will die. The basic idea of TCM to prevent and cure diseases is strengthening vital Qi to eliminate pathogenic factors. Strengthening vital Qi can improve body’s resistance to disease, in order to eliminate weakness syndromes, ward off illnesses, and be physically strong. On the understanding of the aetiology, TCM theory emphasizes the cause of disease — “vital Qi-deficiency”, especially.

Due to the fact that both Qi and bioenergy are at the centre of life activity, the author suggests that Qi and bioenergy are closely related, and proposes a scientific hypothesis that “Qi” is “bioenergy” [25]; that Qi-deficiency can lead to mitochondrial energy metabolism obstacles, which can be improved by Qi-invigoration; and demonstrates that Qi-invigoration can be achieved through improved mitochondrial bioenergetics [26]. Qi-deficiency is the common cause of a variety of diseases (including mitochondrial diseases) and Qi-invigoration is the basic principle for the treatment of Qi-deficiency. Doctors of TCM usually compose prescriptions made up of Qi-invigorating herbal medicines (QIHM) for Qi-deficiency, and have accumulated abundant clinical experience for a long time. QIHM is a group of herbal medicines which can invigorate Qi and treat syndromes of Qi-deficiency.

1.5. The central premise of the chapter

This chapter will examine the role of mitochondrial bioenergetics in the aetiology and progression of several mitochondrial diseases and explore potential therapeutic benefits of improving mitochondrial bioenergetics in mitigating the disease processes by Qi-invigorating therapy. The chapter aims to provide a comprehensive overview of why Qi-invigorating therapy in TCM is suitable for mitochondrial diseases from a bioenergetic perspective, try to find its underlying mechanism, and provide a novel way of thinking and scientific evidence in guiding Qi-invigorating therapy for the prevention and treatment of mitochondrial diseases.

2.1. A Primer on mitochondrial bioenergetics

The standard model of mammalian cellular bioenergetics is based on the pioneering work of Krebs and Mitchell over half a century ago [27]. In 1961, Peter Mitchell published a unique hypothesis regarding cellular bioenergetic conservation, to elucidate the role of mitochondria in energy generation — Mitchell theory, namely the chemiosmotic theory of OXPHOS, is one of the most fascinating discoveries in the history of science. ATP synthase was driven to produce ATP by the transmembrane proton gradient induced when electrons transfer along a series of respiratory enzyme complexes in the mitochondrial inner membrane, an insight for which Mitchell was awarded the Nobel Prize in Chemistry in 1978 [28]. Studies on bioenergetics have occupied an important position in the life sciences, the generation mechanism of ATP (the most important energy molecule in life activity) was elucidated by an academic of the National Academy of Sciences — Boyer — who proposed binding change and rotational catalysis mechanism of ATP synthase, and won the 1997 Nobel Prize in Chemistry. Products of the Krebs cycle, such as nicotinamide adenine dinucleotide (NADH) and succinate, are the obligatory factors that drive mitochondrial electron transport and maintain OXPHOS and ATP production (Figure 2) [27]. Mitochondrial bioenergetic processes are central to the production of cellular energy, and a decrease in the expression or activity of enzyme complexes responsible for these processes can result in an energy deficit that correlates with many metabolic diseases and ageing [29].

An increase in the slippage of complex I and III of the electron transport chain (ETC) [30–32] can lead to overproduction of ROS, which can further damage the mtDNA and create a vicious feed-forward loop of energetic decline. The consequence of such an energetic decline is potentially grave, which might be manifested as either the normal progression of ageing, or as the onset of major diseases such as diabetes, AD, PD, and cancer [33–34]. Interest in the roles that mitochondria play in health and disease has grown markedly over the past two decades [35]. Mitochondria are involved both in male and female germ-line formation, which may also be affected by sexual antagonism of mitochondrial-nuclear gene interaction. Mitochondrial energy metabolism is also involved in tissue and organism differentiation. The mitochondrial bioenergetics system should be recognized as a candidate key driver of speciation events, at least in animals [36].

Cardiolipin is a unique phospholipid which is almost exclusively located in the inner mitochondrial membrane, where it is biosynthesized. Cardiolipin plays the functional role in several reactions and processes involved in mitochondrial bioenergetics [37]. Cardiolipin contains unsaturated fatty acyl chains which are readily oxidizable targets. Peroxidation of cardiolipin is now considered an important event in mitochondrial dysfunction in cellular pathophysiology and also an early step in apoptotic cell death. Abnormalities in cardiolipin content, fatty acyl chain composition, and remodelling appear to be, at least in part, responsible for mitochondrial dysfunction associated with several pathophysiological situations, including hypo- and hyperthyroid states, heart ischemia/reperfusion, heart failure, diabetes, Barth syndrome, as well as ageing and age-related cardiovascular and neurodegenerative disorders. Pharmacological strategies designed to prevent cardiolipin oxidation may open new perspectives for treatment of these disorders [37].

High-fat feeding is associated with reduced whole body respiration and, over sufficient duration, results in reduced mitochondrial respiration and ATP production. High-fat feeding also results in more ROS per unit respiration and per unit ATP being produced. The impaired high-fat-induced bioenergetics is, in part, mitigated by partial substitution of n-3 fatty acids in the diet [38]. Most of the beneficial effects of melatonin in a number of physiopathological situations, which may depend on its effect on mitochondrial bioenergetics, involve mitochondrial dysfunction as a primary cause of disease [39].

2.2. Mitochondrial bioenergetic dysfunction

Bioenergetic dysfunction is emerging as a cornerstone for understanding the pathophysiology of cardiovascular disease, diabetes, cancer, and neurodegeneration. A substantial bioenergetic reserve capacity of cells is a prospective index of ‘healthy’ mitochondrial populations, which appears to be essential for cell survival under conditions of pathological stress [40].

It has long been known that mitochondria show a diminished functional activity when isolated from a broad range of tissues subjected to pathological stress. Well-established examples are alcohol-dependent hepatotoxicity, cardiac ischemia/reperfusion, and neurodegenerative diseases [41–43]. In all of these cases, a role for reactive oxygen or nitrogen species (ROS/RNS) in causing mitochondrial dysfunction has been proposed. Typically, mitochondrial damage is defined as a relative decrease in respiratory parameters, such as a change in oxygen consumption in the presence of substrates and ADP (state 3 respiration), in the presence of substrates alone (state 4), or the ratio of these parameters (i.e., respiratory control ratio (RCR)). Other parameters include decreased activity of specific enzymes, deletions in mtDNA, and increases in oxidative markers such as protein oxidation. The challenge with these data is in integrating them into an overall model of cellular and tissue bioenergetic function. It is becoming apparent that mitochondria under pathological stress exhibit multiple defects that overall can be viewed as a decrease in mitochondrial quality [43–44].

An existing mitochondrial population is examined that was subjected to pathological stress with the formation of reactive oxygen, nitrogen, and lipid species (ROS/RNS/RLS). This oxidative stress damages mtDNA, impairing the ability of the organelle to replace damaged electron transport proteins and decreasing bioenergetic reserve capacity; the resulting increased mitochondrial ROS then oxidatively damage previously unmodified mitochondria. The damaged mitochondria are turned over by a mitophagic mechanism that then suppresses this vicious cycle. The mitochondrial population is now renewed through mitochondrial biogenesis. The bioenergetic reserve capacity is essential for resistance to oxidative stress and supplying ATP demand. Once the bioenergetic reserve is depleted, bioenergetic failure occurs and the cell is programmed for cell death [40].

A unique feature of the molecular machinery controlling cellular bioenergetics is that the proteins necessary for electron transport and OXPHOS are encoded by both nuclear and mitochondrial genomes. An emerging concept in the energetics field is that the reserve capacity can be used as an index of mitochondrial health [45–47]. It appears that a higher bioenergetic reserve capacity results in a greater ability to withstand oxidative stress [40].

The mitochondrial genome encompasses in the order of one to two thousand nDNA genes and thousands of copies of the mtDNA. Hence, a large number of mitochondrial gene targets can mutate and have significant effects on cellular bioenergetics. Given that a human has in the order of 10¹⁷ mitochondrial capacitors, this is a great deal of potential energy, the vital force that animates our life. When breathing stops, the membrane potential collapses, energy transduction ceases, and death ensues [48–51]. However, there is a clear need to determine whether the contribution of mitochondrial dysfunction to different age-related diseases is explained by a cellular bioenergetic deficiency or by changes in mitochondrial ROS production affecting oxidative damage and signalling [52].

2.3. Bioenergetics and disease

2.3.2. Bioenergetic deficiency for mitochondrial diseases

The role of mitochondria in mitochondrial diseases aroused people’s concern due to the central role of mitochondria in producing chemical energy (ATP) to meet cellular requirements. Old mitochondria appear morphologically altered and functionally produce more ROS and less ATP. Mitochondrial bioenergetic processes are central to the production of cellular energy, and energetic deficit correlates with many metabolic diseases and ageing. As the mutant mtDNAs accumulate, they progressively erode the individual cell’s energetic capacity. Ultimately, cellular energetics drops below the threshold necessary for normal cellular and tissue function, and leads to a decline in organ function, tissue failure, and an ageing phenotype [12,60].

While mitochondrial defects are systemic, the clinical manifestations are often organ specific. This is because different organs and tissues have different needs and roles in energy homeostasis. Certain tissues require high levels of mitochondrial ATP, such as the central nervous system, the heart, and the muscles. These organs are preferentially affected as mitochondrial energy production declines. Other tissues store energy in fat. The liver is an energy homeostatic tissue, maintaining the serum glucose level within acceptable limits [49].

Mitochondrial disease or dysfunction (a cellular energy deficiency problem) is at the root of all these diseases. It is generally accepted that minimizing mitochondrial dysfunction is of central importance in maintaining cellular and body’s fitness and also in counteracting mitochondrial diseases. The chapter may hold an important key to understanding mitochondrial disease (i.e., the body cannot make enough energy, or ATP). Mitochondrial diseases primarily affect the brain, heart, and muscle tissues at varying levels of severity. Almost all cells in the body have mitochondria, which are tiny “power plants” that produce the body’s essential energy. When someone has mitochondrial disease, their power plants do not work properly and some functions in the body (e.g., muscles and neurological pathways) do not work normally. The body can have a power failure similar to a “brown out” or a “black out”.

Mitochondria are highly dynamic organelles which play a central role in cellular homeostasis. Mitochondrial dysfunction leads to life-threatening disorders and accelerates the ageing process. A common denominator of these lifespan-modulating interventions seems to be defined by their ability to impact on cellular energy metabolism and stress response [61].

Alterations in mitochondrial structure and function, nDNA, and/or mtDNA mutations can impair OXPHOS, which in turn can reduce energy production, alter the cellular redox state, increase ROS production, deregulate Ca²⁺ homeostasis, and ultimately activate the mitochondrial permeability transition pore (MPTP), leading to apoptosis. The consequences of OXPHOS perturbation result in decline of mitochondrial function, and energy output declines, which accounts for ageing, degenerative diseases, metabolic deregulation, endocrine dysfunction, and symptoms such as diabetes, obesity, and cardiovascular disease. As inflammatory response initiates, this will contribute to autoimmune diseases. Finally, cancer cells must manage energy resources to permit rapid replication [5] (Figure 1).

The milder mtDNA variants can also affect caloric metabolism and result in metabolic diseases, such as diabetes and obesity, and degenerative diseases, such as psychiatric disorders, Parkinson’s disease (PD), and Alzheimer’s disease (AD). The more severe mtDNA mutations, like MERRF (myoclonus epilepsy with ragged red fibres) and MELAS (myopathy, encephalopathy, lactic acidosis, stroke-like episodes), cause progressive multisystem diseases, frequently resulting in premature death. The most severe mtDNA mutations can lead to lethal childhood diseases, such as Leigh syndrome [62].

While mtDNA mutations have permitted humans to adapt to stable regional environmental energetic differences, many energy resources and demands fluctuate cyclically, for example, seasonal changes in temperature and food supply. As these mitochondrial bioenergetic parameters fluctuate with the environment, they drive posttranslational modification of the proteins of the epigenome and the signal transduction pathways. In this way, the expression of nDNA-coded bioenergetic genes is coupled to environmental changes through mitochondrial energy flux [63–64]. This new bioenergetic perspective not only provides a framework within which the genetics and pathophysiology of “complex” diseases, such as PD, AD, autism spectrum disorders (ASDs), and psychiatric disorders [5] can be re-evaluated, it also provides a coherent theory for the aetiology of the “complex” metabolic and degenerative diseases, suggesting powerful new approaches for their presymptomatic diagnoses, reliable prognosis, and effective treatment and prevention.

3.1. Classic mitochondrial disorders

Classic mitochondrial disorders result from mutations in mtDNA or nuclear genes that disrupt mitochondrial respiratory function. These diseases typically have brain and skeletal muscle manifestations, and therefore they are often referred to as mitochondrial encephalomyopathies. Based on the tissues typically affected in mtDNA diseases, it is generally thought that tissues such as brain, skeletal muscle, heart muscle, and endocrine glands are particularly dependent on respiratory function and have a lower bioenergetic threshold [70].

Leber’s hereditary optical neuropathy (LHON) and mitochondrial myopathies were in 1988 the first diseases demonstrated to be caused by mtDNA mutations [71–72]. There are now more than 200 such diseases, including MELAS, MERRF, and many others [73]. Importantly, cellular dysfunction and clinical disease do not occur until a threshold proportion of mtDNAs carrying mutations (heteroplasmy) is reached [74–75]. Together, these diseases affect at least 1/10,000 people clinically [76], establishing mtDNA mutations as a major cause of disease.

Most patients with mtDNA diseases caused by deletions that cross two or more gene boundaries (intergenic mutation) generally do not reproduce. Therefore, most intergenic mutations arise de novo, resulting in sporadic disease [71]. Common presentations of this class of diseases include chronic progressive external ophthalmoplegia (CPEO) and the Kearns-Sayre syndrome (KSS) [77]. In contrast, most maternally inherited mtDNA diseases are the result of mutations confined within the gene commonly involving one or few base changes (intragenic mutations). Examples of this latter class of mtDNA mutations include LHON [72], MERRF syndrome [78–79], and neurogenic muscle weakness, ataxia, retinitis pigmentosum (NARP), and Leigh syndrome [80]. Over 200 pathogenic mtDNA base substitution mutations associated with a broad spectrum of clinical phenotypes have been identified, including encephalomyopathy, mitochondrial myopathy, exercise intolerance, gastrointestinal syndromes, dystonia, diabetes, deafness, cardiomyopathy, AD, and PD, etc [81].

In addition to recent deleterious mutations and ancient adaptive mtDNA variants, somatic mtDNA mutations play an important role in human health. Moreover, somatic mtDNA intergenic and intragenic mutation levels are elevated in the organs of patients with degenerative diseases. The accumulation of somatic mtDNA mutations causes an age-related decline in mitochondrial function, thus exacerbating partial inborn defects in mitochondrial function leading to the delayed onset and progression course of age-related diseases [53]. Experimental results from mtDNA mutator mice suggest that mtDNA mutations in somatic stem cells may drive progeroid phenotypes without increasing oxidative stress, thus indicating that mtDNA mutations that lead to a bioenergetic deficiency may drive the ageing process [52]. Thus, somatic mtDNA mutations provide the ageing clock. Large-scale deletions in some cell types can attain a threshold to directly affect OXPHOS. Single-cell studies have shown that deletions can occur in ~32 to 80% of mtDNA molecules in substantia nigra neurons of brains from patients with PD [82].

Since most mitochondrial proteins are nuclear encoded, it is not surprising that mutations in many nuclear genes also cause mitochondrial diseases. For example, mutations in DNA polymerase γ — the only mtDNA polymerase and thus responsible for all mtDNA replication and repair — cause a wide range of mitochondrial diseases including progressive external ophthalmoplegia, and Leigh’s syndrome, among others [83]. Friedreich’s ataxia is caused by mutations in the gene encoding frataxin which is involved in iron-sulphur cluster assembly [84]. Diseases caused by mtDNA and nDNA mutations are estimated to collectively have an incidence of ~1/4000 individuals [85–86].

These and related studies will permit determination of exactly how important the mitochondrial and mtDNAs are in determining the complex physiology and inheritance of the common ‘‘complex’’ metabolic and degenerative diseases that demand much of the healthcare resources in both developing and developed countries.

3.2. Mitochondria and cardiovascular diseases

Cardiovascular disease remains the commonest form of mortality and morbidity in the Western world. There remains an urgency to identify and translate therapies to reduce the effects of this disease and its associated co-morbidities. Vascular smooth muscle cells are the primary source of extracellular matrix and collagen, and it has been suggested that loss of viability and vitality of these cells contributes to plaque vulnerability and rupture. It is the loss of ATP that may ultimately be more detrimental. Finding alternative sources of ATP synthesis by energetic reconfiguration may also provide a vital link in delaying the kinetics of plaque rupture [87].

Excitation–contraction coupling in cardiac myocytes consumes vast amounts of ATP that need to be replenished by OXPHOS. During a single heartbeat, ~2% of the cellular ATP is consumed, and the whole ATP pool of cardiac myocytes is turned over within one minute. To orchestrate OXPHOS in response to constantly changing workloads of the heart, constant availability of ATP must be secured [88].

A large body of work has shown that endothelial dysfunction contributes to the pathogenesis of cardiovascular disease, and considerable effort has been put into defining operative mechanisms. Recent work has emphasized the importance of mitochondria for endothelial function. Although they have long been recognized for their role in bioenergetics, mitochondria participate in a host of other cellular processes. It is now recognized that endothelial mitochondria play a prominent role in signalling cellular responses to environmental cues. An important mode of mitochondrial signalling is the regulated production of ROS. Blood markers of mitochondrial dysfunction, such as mtDNA damage and mitochondrial oxygen consumption in circulating leukocytes or platelets, may facilitate studies of systemic abnormalities of mitochondrial function and their relation to cardiovascular disease [89]. The presented studies suggest that mitochondria-directed therapies have potential for the prevention and management of cardiovascular disease, in part by improving mitochondrial function in the endothelium.

3.3. Mitochondria and metabolic diseases

Metabolic syndrome is a growing epidemic in the United States and worldwide. Currently, approximately 34% of the US population are living with this diagnosis [90–91] and it is expected that this number will continue to increase. One of the hallmarks of metabolic syndrome is central obesity, an increase in visceral white adipose tissue (WAT) around the midsection. Central obesity not only contributes to metabolic syndrome but also increases individual risk for type 2 diabetes mellitus (T2DM) and cardiovascular disease [92].

3.4. OXPHOS disorders

OXPHOS disorders can be classified genetically according to whether the primary defect is in the nuclear or mitochondrial genome, as well as by the pathway primarily affected. There remains a considerable lack of understanding of the pathogenic mechanisms involved in clinical symptoms and the deterioration. The central role of OXPHOS in metabolism suggests that many features are related to abnormal metabolic consequences of the defects.

In addition, OXPHOS defects can be tissue specific, due to the variable metabolic thresholds for the different OXPHOS enzyme complexes in each tissue [120]. Tissue specificity may limit the systemic effect of metabolic changes while still inducing marked abnormalities within the affected tissue. There are, however, likely to be many contributing factors including tissue-specific expression of nuclear OXPHOS genes, different metabolic needs of a tissue (for example, burst activity seen in some neurons versus cells with predominantly continuous biosynthetic functions), and tissue-dependent segregation of heteroplasmy [121].

The treatment of patients with OXPHOS disease remains very limited, but counteracting the most damaging metabolic changes may be beneficial in some patients, and is the mainstay of treatment at present. An inability of mitochondria to supply sufficient ATP to meet cellular needs is often assumed to be the primary effect of mitochondrial disease mutations [122].

3.5. Mitochondria and neurodegenerative diseases

The neurodegenerative diseases are a key health issue because they are profoundly debilitating [123]. In the past decade, the genetic causes underlying many familial neurodegenerative disorders, such as Huntington’s disease (HD), Parkinson’s disease (PD), Alzheimer’s disease (AD), dominant optic atrophy, Friedreich ataxia, amyotrophic lateral sclerosis, and Leber’s hereditary optic atrophy have been elucidated. However, the common pathogenic mechanisms of neuronal death are still largely unknown. Mitochondrial dysfunction has emerged as a potential ‘lowest common denominator’ linking these disorders [124]. Oxidative damage to mitochondrial membranes, enzymes, and the ETC components, culminate in impaired mitochondrial ATP production and facilitated MPTP opening [125]. Mitochondria play multiple roles in the maintenance of neuronal function under physiological and pathological conditions [126].

Brain function is almost totally dependent on a continuous supply of glucose and oxygen. Although the brain represents only 2% of the body weight, it receives 15% of the cardiac output, and consumes 20% of total body oxygen and 25% of total body glucose [127]. A high aerobic capacity of the brain is required for the mitochondria to generate sufficient ATP to maintain and restore ion gradients across the enormous area of plasma membrane, to maintain the compartmentation of neurotransmitters, and to drive exocytotic and endocytotic synaptic vesicle cycling [126]. Given the brain’s high energy requirements, any decline in brain respiratory chain enzyme complexes’ activity with ageing could have a significant impact on brain function, as well as on the aetiology and progression of age-associated neurodegenerative disorders [128–130].

An aged brain has a decreased capacity to produce ATP by OXPHOS and this becomes limiting under physiological conditions in aged individuals. The impairment of brain mitochondrial function in ageing is mainly due to decreased electron transfer rates in complex I and IV [131]. Previous results have shown an impairment of complex I activity in brain mitochondria of aged rats which was ascribed, in part, to oxidation of cardiolipin [132]. The age-associated alterations to brain mitochondrial cardiolipin were prevented by treatment of aged rats with melatonin [133]. It is reasonable to assume that melatonin’s ability to prevent the age-related alterations of mitochondrial bioenergetic parameters in rat brains, could be ascribed (in addition to other factors) to its protective effect against cardiolipin peroxidation. Thus, the effect of long-term administration of melatonin against the age-dependent mitochondrial oxidative damage could be accompanied by an improvement of mitochondrial bioenergetics and brain function, and therefore to health in general [128].

Deficiency in complex IV activity may be a crucial factor in the aetiology, progression, and prevalence of several neurodegenerative diseases associated with ageing, in particular AD. The results may prove useful in elucidating the molecular mechanisms underlying mitochondrial dysfunction associated with the brain’s ageing process, and may have implications in the aetiopathology of age-associated neurodegenerative disorders, and in the development of potential treatment strategies. Melatonin treatment may represent a valid therapeutic strategy for combating brain ageing process and age-related neurodegenerative disorders, in which complex IV deficiency and oxidation/depletion of cardiolipin could play a critical role [128].

In recent years, the field of neurometabolism has been greatly amplified by the recognition that bioenergetic dysregulation may be a critical pathophysiological factor in diseases of the nervous system [130]. Indeed, there is increasing appreciation for the concept of energy failure — principally from mitochondrial dysfunction — as a key mechanism resulting in neuronal death seen in neurodegenerative diseases [134].

Previous studies have suggested that mitochondrial dysfunction plays a central role in the pathogenesis of neurodegenerative disorders, including AD [135]. Alzheimer’s pathology is accompanied by a decrease in expression and activity of enzymes involved in mitochondrial bioenergetics, which would be expected to lead to compromised ETC complex activity and reduced ATP synthesis [136]. Mitochondrial bioenergetic deficit precedes Alzheimer’s pathology; in addition to the lowered mitochondrial bioenergetic capacity, impairment of OXPHOS is associated with increased free radical production and the resultant oxidative damage. Overproduction of ROS and higher oxidative stress is characteristic of brains from AD [137]. Mitochondria and brain bioenergetics are increasingly thought to play an important role in AD [138]. In summary, mitochondrial dysfunction and deficits in bioenergetics occur early in pathogenesis and precede the development of observable plaque formation of AD. Mitochondrial dysfunction provides a plausible mechanistic rationale for the hypometabolism in the brain that precedes AD diagnosis and suggests therapeutic targets for prevention of AD. Further, the age of reproductive senescence markedly exacerbated mitochondrial and bioenergetic dysfunction, which is coincident with marked increases in AD pathology. In addition, one consequence of oxidative damage to mtDNA may be impairment of mitochondrial respiratory capacity. Mitochondrial DNA damage is associated with reduced mitochondrial bioenergetics in Huntington’s disease. HD cells may be more dependent on glycolysis than respiration to compensate for ATP production [139]. A number of studies have identified mitochondrial abnormalities within neuronal cell bodies in progressive multiple sclerosis, leading to a deficiency of mitochondrial respiratory chain complexes or enzymes [140].

3.6. Mitochondria and cancer

Cancer cells exhibit large varieties of metabolic changes which are associated with alterations in mitochondrial structure, dynamics, and function. Mitochondria can regulate tumour growth through modulation of the TCA cycle and OXPHOS, and are also crucial in controlling redox homeostasis in the cell, inducing them to be either resistant or sensitive to apoptosis. All these reasons locate mitochondria on centre stage in understanding the molecular basis of tumour growth [141]. A change in cellular bioenergetics is one of the key hallmarks of cancer. The Nobel prize-winning German physician and scientist, Otto Warburg, discovered that mitochondria in cancer cells do not efficiently generate energy; that almost all neoplastic cells demonstrate enhanced uptake and utilization of glucose for glycolysis to generate ATP. While increased glycolytic flux may provide some benefit to cancer cells by increasing ATP production, recent studies suggest that it is advantageous mainly because it generates chemical ‘‘building blocks’’ required for the anabolic processes that must occur prior to cell division [142]. The most glycolytic tumour cells were found to be most resistant to therapy, and most aggressive in metastasis [143–144].

Altered metabolism was among the first cancer biomarkers [145]. Nevertheless, the notion that glycolysis is a universal feature of aggressive tumour growth, and that OXPHOS is counter-productive to tumorigenesis still pervades the literature [146]. There is accumulating evidence that mitochondrial metabolism plays an essential role in tumour cell proliferation. Growing tumours alter their metabolic profiles to meet the bioenergetic and biosynthetic demands of increased cell growth and proliferation [147–149].

Warburg proposed that cancer could originate from the sole inhibition of respiration in human cells, but are the mitochondria of cancer cells dysfunctional? Different studies have indicated that OXPHOS is capable of synthesizing ATP in cancer cells, albeit with a low efficiency [150]. To survive under conditions of glucose limitation, tumour cells use OXPHOS to derive energy from the amino-acids, glutamine, and serine. This process (glutaminolysis) has been demonstrated to occur in multiple types of cancer cells [142,151–153]. Deregulated energetics is a hallmark of malignancy, but metabolic heterogeneity among individual tumours is unknown. A study by Caro et al. [154] demonstrates that a subset of lymphomas is defined by reliance on mitochondrial energy generation and is selectively killed when this pathway is impaired.

Collectively, these studies illustrate how differences in mitochondrial economy, bioenergetic reserve capacity, and autophagy regulate health and disease. It is now also becoming clear that physiological processes such as cell differentiation are associated with profound changes in cellular bioenergetics, including reserve capacity [155]. The challenge for us is to continue to strive to understand more fully how bioenergetics regulates cell and tissue function. Certainly, this would place us in the best position to develop better and more targeted therapies [40].

Numerous studies on cancer cell bioenergetics evidence a large variability in the relative contribution of glycolysis and OXPHOS to cellular ATP production. The corresponding differences in the capacities for glycolysis and OXPHOS demonstrate that there is a cancer-specific metabolic remodelling caused by a combination of genetic and environmental factors. The dogma that cancer cells use solely glycolysis is no longer valid; as such, a detailed bioenergetic characterization of each type of tumour must be performed in order to develop adequate treatments. The growing interest in the mitochondrion in cancer research could lead to the development of adapted metabolic therapies that might also serve to treat mitochondrial diseases [151].

The differences in mitochondrial function between normal cells and cancer cells may offer a unique potential for the design of anticancer agents that deliver mitochondrial targeting drugs to selectively kill cancer cells. The journal Science revealed that bioenergetics was the core of malignant transformation of tumour [156], and abnormal energy metabolism was concluded as one of the 10 most notable features of cancer cells by the journal Cell [157] — bioenergetics dysfunction is becoming the cornerstone for understanding the pathophysiology of mitochondrial diseases [40]. Here, we summarize the metabolic changes of cancer cells.

In cancer cells, anabolism is enhanced, both glucose and glutamine are important carbon sources which are metabolized for the generation of energy, and anabolic precursors, heavily consumed by cancer cells, are early precursors of non-essential amino acids. This process predominates in cancer cells rendering glutamine an essential amino acid and the source of tricarboxylic acid (TCA) cycle-derived anabolic metabolites. Mitochondrial dysfunction may lead to the activation of HIF-1, therefore triggering the hypoxia pathway in the tumourigenic process. The inability of mitochondria to provide enough ATP for cell survival under hypoxic conditions, results in up-regulation of the glycolytic pathway. This occurs by induction of HIF-1, which not only stimulates key steps of glycolysis but also suppresses mitochondrial respiration in cancer cells, therefore, modulating the reciprocal relationship between glycolysis and OXPHOS. The switch between glycolysis and OXPHOS is controlled by the relative activities of pyruvate dehydrogenase (PDH) and lactate dehydrogenase (LDH). HIF-1 inactivates PDH through pyruvate dehydrogenase kinase 1 (PDK1) induction, resulting in suppression of the Krebs cycle and mitochondrial respiration. In addition, HIF-1 stimulates the expression of lactate dehydrogenase A, which facilitates the conversion of pyruvate to lactate. As a result, mitochondrial contribution to ATP synthesis declines, although the mitochondria might remain functionally intact [158] (Figure 2).

Glucose is mostly phosphorylated by hexokinase II (HKII) in cancer cells, which is up-regulated as its gene promoter sensitive to typical tumour markers such as HIF-1 and has easy access to ATP being more strictly bound to the mitochondria. HKII plays a pivotal role in bioenergetic metabolism, phosphorylates glucose using ATP synthesized by OXPHOS, and releases the product ADP in close proximity of the ANT to favour ATP re-synthesis within the matrix. Its product, Glucose-6-P, is only in part oxidized to pyruvate. This, in turn, is mostly reduced to lactate being both LDH and PDK1 up-regulated. A significant part of Glucose-6-P is used to synthesize nucleotides that also require amino acids and glutamine. Glutaminolysis (breaking down glutamine into α-ketoglutarate (α-KG)) generates malate which, through the malic enzyme, will give rise to NADPH that can be used to fuel lipid biosynthesis, and oxaloacetate (OAA), which will generate citrate, which is necessary for lipid biosynthesis. Citrate in part is diverted from the TCA cycle to the cytosol, where it is a substrate of citrate lyase, which supplies acetyl-CoA for lipid and phospholipid synthesis that also requires NADPH. The reprogramming of mitochondrial metabolism in many cancer cells comprises reduced pyruvate oxidation by PDH followed by the TCA cycle, and increased anaplerotic feeding of the same cycle, mostly from glutamine. This also increases the free fatty acids’ uptake, therefore β-oxidation is pushed to produce acetyl-CoA. In cancer cells, many signals can converge on the mitochondrion to decrease the mitochondria permeability transition (MPT), with consequent enhancement of apoptosis resistance. ROS can enhance Bcl-2 and may induce mtDNA mutations. As indicated, ROS in many cancer cells are over produced. Interestingly, studies have also shown that the Δψ_m is approximately 60 mV higher in carcinomas as compared to their normal controls, which also contribute to the increased ROS [158] (Figure 2).