Mitochondria are intra-cellular organelles producing more than 95% of ATP. Their normal structure, integrity, function and homeostatic properties are highly important since their unsuitable anatomy and abnormal-altered function are responsible for myocardial cell injury and death and consequently for cardiac disease genesis and progression [
1,
7,
8]. They have their own DNA (mtDNA), circular in shape, encode 13 subunits of protein whereas the majority of the mitochondrial proteins are governed by nuclear DNA, transported within mitochondria via mitochondrial membrane. The mtDNA, because of their own low protective mechanisms, is subjected to mutations responsible for many inherited cardiomyopathies [
1]. However, the amount of mutated mtDNA presented in each individual is higher than the incidence of myocardial diseases, serving thus as dormant source for subsequent emerging diseases when mitochondria mutations have reached a certain threshold [
9]. On the other hand, gene–gene and gene–environment interactions do not affect proportionally cardiac mitochondria, thanks to the powerful compensatory mechanism that they possess, showing a resistance to outsider harmful events, protecting thus mitochondria malfunction and hence the manifestation of heart diseases [
10]. However, when they are severely affected, they face a non-viable situation leading to detrimental effects. Usually are modestly affected, having thus the time to compensate, to rearrange their homeostatic status and change their metabolic actions. If the compensate process fails, then heart diseases emerge and within time progress [
11,
12,
13]. When the heart failure syndrome begins the fission and fragmentation processes are involved and as the syndrome deteriorates there is a decrease in mitochondrial cristae density, areas of cluster mitochondria are observed, vacuolar degeneration and calcium overload is present [
13,
14,
15,
16] leading to myocardial cell apoptosis and necrosis [
17,
18]. As a response, mitochondrial defensive mechanisms are increased, especially mitophagy, trying to protect the myocardial cells and the heart as a whole. This has been reported in both preserved and reduced ejection fraction being more actively involved in the latest [
14,
19]. However, although the protective mechanisms are beneficial, within time as the syndrome aggravates the protective mechanisms are over whelmed unable to protect and to maintain mitochondria normal function [
19,
20]. Moreover, the autophago-lysosomal system is dysfunctional [
21,
22] making thus the mitochondria more vulnerable. The heart failure syndrome of any cause deteriorates, the fort fell,
Figure 1.
There are different mitochondrial shapes across the human body depended on the tissue and the adjacent cell environment that can differentiate mitochondria structure-function-behavior. Thus, different mitochondria subpopulations are present demonstrating different response to metabolic–energetic status, underlying thus their complexity heterogeneity and diversity. In the myocardium, the most metabolically active and thus most rich organ in mitochondria, their main role is to regulate biogenesis, ion transport, and to protect themselves using their defensive mechanisms, fusion, fission, mitophagy etc. [
23,
24],
Figure 2. This is accomplished due to the presence of three different mitochondrial subtypes within cardio myocytes, a) interfibrillar, b) subsarcolemmal and c) perinuclear mitochondria, each subtype demonstrating different shape, function – response to metabolic and pathophysiologic changes. Indeed, the interfibrillar are oval, lying in longitudinal rows within myofibrils and exhibit higher rate of oxidation. Subsarcolemmal, are responsible for electrolyte and metabolite transport and offer the higher myocardial protection. Perinuclear, are of spherical shape, control nuclear function, and regulate mostly mitochondria fusion and fission process [
23]. Of interest, products of the association of mitochondrial respiratory chain complexes (I–IV) harbored in the inner mitochondria membrane create the mitochondrial super complexes. Their aim is to provide incremental capabilities in the electron transfer process. They constitute the respirasome (I+III
2+IV
1), responsible for mitochondrial and phospholipids (cardiolipin etc.) function, satisfying in a better way the energetic needs of the heart. Additionally, their presence and function reduce the amount of free radical production serving thus as a preventer of a possible mitochondrial dysfunction [
25,
26]. As mentioned before the main aim of mitochondria is the production of energy. Almost 90% of produced ATP is used to support the contraction-relaxation coupling. The separation and the assembly of actin/myosin are both highly energetic depended procedures and therefore their normal behavior is closely related to the source of energy production. Additionally, ions exchange, mainly CA2+ release and sequestration, require high amount of energy, produced at the site of mitochondria. This factory of energy; mitochondria, must act as quick as possible in order to fulfill the needs of human body. Indeed, under high energetic requirements they have to adapt accordingly and to produce enough energy to satisfy the needs. To accomplish this issue, they possess the capability to protect their self, to interconnect, to change their shape, and to move within the cell since they ‘can cross cell boundaries’ [
27]. The need of change in shape and motion may occur under normal circumstances; high load exercise training or under different pathological clinical scenarios; myocardial ischemia, hypertension, cardiac hypertrophy, heart failure etc. In case of mutated or dysfunctional mitochondria etc., the production of energy is inadequate, whereas the accumulation of harmful substances; free radicals’ production, heat shock and unfolded/misfolded proteins, promote the beginning and the progress of cardiac diseases. Keeping with the above-mentioned reports it is clear that mitochondria have to have their own protective mechanisms in order to avoid their dysfunction-malfunction transformation and keep the energetic-metabolic-homeostatic status of the cell on the road. Indeed, mitochondria morphology and function adapt to the different environment, activating their self-protective actions, that is necessary for the cell survival, [
23]
Figure 2. These actions are under specific protein control; guanosine triphosphate hydrolase enzyme family, mitochondrial fission and fusion proteins, mitochondrial dynamics proteins 49 and 51, etc.
Figure 2, facilitate a continuous adaptation of mitochondria shape and function, promote genetic material exchange between the mitochondria, ensuring their ability of optimal function [
23,
28]. The mechanism through which mitochondria can receive genetic material from the mitochondria of other cells, although not fully elucidated, facilitate inter-cellular molecular crosstalk that represent an adaptive mechanism trying to avoid mitochondria malfunction [
27]. Three different modes of inter-cellular mitochondria transport have been proposed a) tunneling nanotubes (TNTs), b) membrane extracellular vesicles (EVs) and c) gap junctions (GJC) [
29]. TNTs represent the principal way of mitochondria transport, are formed rapidly by mitochondria membrane protrusions, and are composed of F-actin and transport proteins [
30]. Membrane micro-vesicles represent heterogeneous components released from intra to extra cellular environment and thus called extracellular vesicles (EVs). Smaller EVs contain exosomes, small RNAs, genomic DNA, mtDNA, while larger EVs can contain even entire mitochondria [
27,
31,
32]. Their principal role is to eliminate abnormal proteins and can serve as an additional mode for inter cellular communication (in nervous system) [
33,
34]. GJCs are transport gates for several substances’ transportation including nutrients, metabolites, mitochondria [
35] and it seems that play a role for the intercellular transportation of reactive oxygen substances [
24,
36]. Although, as it has been suggested, mitochondrial structure change is connected to several pathologies, this knowledge is not thoroughly investigated and not used in every day clinical practice [
24,
37]. The presence of different mitochondrial phenotypes; donut-like, ellipsoid shape etc., as well their side of action may represent defensive response to several harmful events [
37,
38,
39]. A response that can affect the main protective mitochondrial mechanism; fission and fusion. Taken into account the presence of different mitochondrial subpopulations, their altered shape and the role of specific drivers - proteins for fission; mitochondrial fission factor, mitochondrial division 49 and 51 etc., and fusion; mitofusin1, 2 etc.
Figure 2, may represent an early sign of a disease and their measurement might facilitate clinical diagnosis. Additionally, the study of mtDNA heteroplasmy (different alleles in one patient) [
40] may give us further knowledge about the mitochondrial abnormal status and to let us discover earlier the upcoming consequences.