1 Introduction

Mitochondria are double-membrane-bound organelles exclusively found in eukaryotic cells and best known for its role in the generation of adenosine triphosphate (ATP) [1]. The endosymbiotic hypothesis proposes that mitochondria arise from the integration of a free-living aerobic bacterium into a host cell over a billion of years ago. In this relationship, the host cell provided a safe and nutrient-rich environment for the aerobic bacterium. It also acquired a new source of oxygen dependent-energy [2]. More recently, it has been suggested that the phagocytosed bacterium may have provided defense molecules for the host cell, also connecting the advantages of this endosymbiotic relationship to immunity [3]. Throughout evolution, a massive transfer of genes to the host cell allowed the evolvement of the endosymbiotic bacterium as a permanent organelle—the mitochondrion (mitochondria for plural) [4].

Derived from two Greek words: “mitos”—thread and “chondros”—granule, the organelle displays two lipid bilayer-membranes enclosing two aqueous compartments. The outer mitochondrial membrane surrounds the intermembrane space, while the inner mitochondrial membrane, which contains invaginations denominated cristae, encloses the matrix compartment [5, 6]. The inner membrane accommodates the oxidative phosphorylation system (OXPHOS)—a five multimeric protein complexes (Complex I–V) that uses redox reactions to generates ATP. At the expense of oxygen as a final electron acceptor, a sequential transfer of electrons from Complex I to Complex IV generates a proton electrochemical gradient across the inner membrane, also known as membrane potential, that is used by Complex V to drive ATP synthesis [1]. The mitochondrial membrane potential also helps metabolites transport and ion homeostasis.

Often referred as the powerhouse of the eukaryotic cells, the role of mitochondria goes beyond ATP production. The organelle orchestrates a myriad of other processes including reactive oxygen species (ROS) formation [7, 8], aldehyde metabolism [9], heat production [10], ion homeostasis [11] and programmed cell death [12] that ultimately dictate cell fate. This functional versatility is intimately linked to the content, size and number of mitochondria. Their morphological complexity is controlled by the processes termed mitochondrial biogenesis and dynamics. While mitochondrial biogenesis increases the number and content of the organelles in a coordinated effort with the nucleus [13], mitochondrial dynamics drives the formation of larger or smaller organelles through the antagonist activities of fusion and fission [14].

Mitochondria are recently recognized by their dynamic nature. In order to meet the cellular requirements for ATP, the mitochondrial network are under constantly remodeling. Indeed, metabolic cues (i.e. starvation, exercise) trigger not only fusion and fission machineries in order to create elongated or fragmented mitochondria [15], but also drive transcription factors activation to increase mitochondrial mass and boost oxidative metabolism [16]. Moreover, this dynamism helps impaired mitochondria to be rescued or eliminated. In the first case, fusion events allow damaged components to be diluted throughout the network, thereby avoiding the propagation of stress that might cause mitochondrial dysfunction or collapse [17]. On the opposite way, the fission process segregates part of dysfunctional mitochondria that now can be addressed for degradation in the lysosome, a process termed mitophagy [18].

Exciting new findings have revealed mitochondria as a major intracellular signaling platform regulating immune cell function. Indeed, the cellular metabolic plasticity provided by mitochondria not only allows immune cells to grow, but it is also required during transition from a metabolically quiescent stage to a highly active state [19]. Moreover, proteins located on the outer mitochondrial membrane, as well as mitochondrial DNA can dictate immune cell activation [20, 21]. Finally, due to the reciprocal crosstalk between mitochondrial metabolism and morphology, fluctuations in shape, size and position of the organelle within the cell likey affect both phenotype and activity of immune cells [22].

Considering the extensive knowledge highlighting mitochondria as the powerhouse of the cells as well as emerging evidence placing mitochondria at the heart of immunity, this chapter reviews the general processes regulating mitochondrial biogenesis and dynamics, and discuss the critical role of these processes in health and disease.

2 Mitochondrial Biogenesis

Mitochondrial biogenesis is a simplified term used to describe a complex process involving the increase in mass of pre-existing mitochondria. Due to their bacterial origin, mitochondria possess their own genetic material, which includes DNA and the translational/transcriptional system. The mitochondrial DNA (mtDNA) is a circular double strand DNA molecule containing ~16.5 kb that encodes only 37 genes: 22 transfer RNA and 2 ribosomal RNA (12S and 16S) required for translating 13 messenger RNA. Moreover, maternal inheritance, lack of introns (non-coding sections of a gene) and several copies per cell (1–10 copies per mitochondrion) are among the unique features that differ mtDNA from the nuclear DNA [23].

The entire protein-coding capacity of mtDNA relies on 13 essential subunits of the electron transport chain (ETC) that are replicated and transcribed within the mitochondrial matrix: 7 subunits of NADH: Ubiquinone oxidoreductase (Complex I), 1 subunit of Ubiquinone: Cytochrome c oxidoreductase (Complex III), 3 subunits of Cytochrome c Oxidase (Complex IV) and 2 subunits of ATP synthase (Complex V) [24]. The ~1100 remaining mitochondrial proteins [25, 26] have to be transcribed in the nucleus, translated in cytosolic ribosomes and imported into the organelle (Fig. 1). Therefore, mitochondrial biogenesis faces several challenges before promoting an increase in the mitochondrial content.

Fig. 1
figure 1

Summary of the transcriptional regulation of mitochondrial biogenesis. The expression of mitochondrial genes encoded by both nDNA (nuclear DNA) and mtDNA (mitochondrial DNA) is mainly regulated by a family of transcriptional coactivators named PGC-1 [peroxisome proliferator-activated receptor (PPAR) gamma coactivator 1]. PGC-1 members bind to and coactivate NRFs (nuclear respiratory factors) to induce the expression of multiple components of the OXPHOS (oxidative phosphorylation system), ETC (electron transport chain) and mtDNA replication. NRFs also regulate the levels of TFAM and TFB (mitochondrial transcription factors A and B, respectively) involved in the expression of genes encoded by the mtDNA. Interaction between PGC-1 and specific transcription factors such as PPARs and EERs (estrogen-related receptors) control the expression of many genes involved in FAO (fatty acid oxidation), TCA cycle (tricarboxylic acid cycle), glucose and lipid metabolism, and detoxifying enzymes. Nuclear-encoded mitochondrial proteins are translated in cytosolic ribosomes and imported into the organelle

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The first challenge relies on coordinating the gene expression between two genomes located into distinct subcellular compartments. Indeed, to ensure a proper OXPHOS, the number of ETC subunits must be stoichiometrically balanced [27]. mtDNA occurs in the ratio of ~1000:1 copies relative to nuclear DNA [23]. Second, the majority of mitochondrial proteins are translated in the cytosol; thus, demanding a synchronized cellular machinery to properly target, import and assemble these nuclear-encoded proteins [28, 29]. Failure in addressing these proteins to mitochondria not only impairs ETC subunitse stoichiometry, but also compromises mtDNA replication, which is orchestrated by the nuclear-encoded protein DNA polymerase gamma (POLG) [30]. For a complete description about how mitochondrial genome is replicated, transcribed and translated, please see reviews [28, 31, 32]. Finally, mitochondrial dynamics, which will be discussed above, must also be coordinated.

2.1 Transcription Factors Regulating Mitochondrial Biogenesis

The transcription of both nuclear and mitochondrial genomes is coordinated by specific proteins termed transcription factors. Transcription factors are able to modulate the rate of gene expression by binding to specific regulatory regions of DNA. These proteins contain effector domains that allow the interaction not only with other proteins essential for transcription, including the RNA polymerase, but also with other transcription factors; thereby regulating the amount of messenger RNA produced per gene [33].

Nuclear Respiratory Factors 1 (NRF-1) and 2 (NRF-2) are considered critical players in mitochondrial biogenesis. Together, these transcription factors display DNA-binding sites for most of the genes encoding respiratory subunits. First identified in 1989 as a transcriptional activator of the cytochrome c gene [34], NRF-1 targeted genes are now branded for encoding subunits of all five respiratory complexes of the ETC [35]. A couple of years later, NRF-2 was discovered by its specific binding to the cytochrome oxidase subunit IV promoter [36]. Although often recognized by their power of binding to antioxidant response element (ARE) and promoting gene expression of detoxifying enzymes [37], functional NRF-2 sites have been implicated in the expression of subunits of Complex II, IV and V of the OXPHOS [38].

The regulatory network of NRFs also targets other nuclear genes whose products function in the mitochondria, including components for assembling and importing mitochondrial proteins [39], and constituents of the mtDNA transcription and replication machinery [40]. Indeed, NRF-1 is able to stimulate the expression of mitochondrial transcription factors A (TFAM) [40] and B (TFB) [41]—two nuclear‐encoded transcription factors essential for replication, maintenance, and transcription of mtDNA [42, 43]. Moreover, not only TFAM and TFB have been recently added to the list of genes controlled by NRF-2 [38], but NRF-2 indirectly regulates mitochondrial biogenesis by driving the gene expression of NRF-1 [44]. Due to this essential role in coordinating bi-genomic respiratory subunits, deficiency of NRF can lead to a severe impairment of mitochondrial biogenesis [45, 46].

Members of the nuclear receptor superfamily also control the transcription of respiratory apparatus. The peroxisome proliferator-activated receptor (PPAR) family is composed by three isoforms: PPARα [47], PPAR β/δ [48], PPARγ [49]. The expression of PPAR isoforms differs among tissues and these transcription factors regulate metabolic pathways at different levels. While PPARγ is involved in glucose metabolism and regulation of fatty acid storage, PPARα and PPAR β/δ promote changes in cellular lipid metabolism by upregulating genes involved in mitochondrial fatty acid oxidation [50]. Estrogen-related receptors α (ERRα) and γ (ERRγ) represent another class of nuclear receptors targeting ~700 nuclear-encoded mitochondrial genes. The controlling of these transcription factors, expressed in mitochondrion-enriched tissues such as skeletal muscle and heart [51], is attached to all aspects of energy homeostasis, including mtDNA replication, OXPHOS, ion homeostasis and mitochondrial detoxifying mechanisms (reviewed in [52]). Moreover, ERRα can regulate the levels of PPARα transcripts [53], therefore magnifying the control over mitochondrial fatty acid oxidation pathway.

Finally, a relative small number of other transcription factors have been shown to activate or repress nuclear genes encoding mitochondrial proteins, including stimulatory protein 1 (Sp1), ying yang 1 transcription factor (YY1), cAMP-responsive element-binding protein (CREB) and myocyte enhancer factor 2 (MEF-2), and a detailed consideration of those is covered elsewhere [54].

2.2 The Role of Transcriptional Coactivators in Mitochondrial Biogenesis: PGC-1 Family

As described above, mitochondrial biogenesis requires the coordination of several transcription factors to proper ensure the expression of both nuclear and mitochondrial genes. Adding complexity to this process, mitochondrial metabolism and content differs widely among cells, tissues and organs; thereby demanding an extra layer of regulation. While transcription factors bind to DNA in a sequence-dependent manner, transcriptional coactivators interact with them and amplify the activity of the transcriptional machinery by recruiting multi-protein complexes to modify chromatin folding, interact with the RNA polymerase II complex and process messenger RNA [38]. Although the fundamental mechanisms of how mitochondrial biogenesis is orchestrated are still elusive, a major breakthrough came with the discovery of a family of transcriptional coactivators termed PPARγ coactivator 1 (PGC-1) [55]. PGC-1 proteins have emerged as major players in the transcriptional regulatory circuits controlling mitochondrial biogenesis and function.

Conserved across many species, PGC-1 family is formed by three members that share similar domain structures to interact with nuclear receptors [56]. The first and most studied member of this family is PGC-1α. First identified in brown adipose tissue during adaptive thermogenesis—a process that regulates heat production in response to cold and diet, PGC-1α is considered the master regulator of mitochondrial biogenesis in mammals [55]. Similar to PGC-1α, PGC-1β is predominantly expressed in tissues with abundant mitochondria (e.g. heart and skeletal muscle [57]). However, it is not upregulated upon cold exposure [56]. The third member of this family is the PGC-1 related coactivator (PRC). Despite the relatively low homology with the other two isoforms, PRC is ubiquitously expressed and supports mitochondrial biogenesis during early embryogenesis [58, 59]. Together, they bind to and coactivate most of the transcription factors regulating expression of mitochondrial proteins encoded by the nucleus.

Several studies have shown that PGC-1α is capable of regulating virtually every aspect of mitochondrial content [60]. Indeed, by binding to and coactivating NRF-1 and NRF-2, PGC-1α promotes not only a powerful induction of nuclear-encoded mitochondrial respiratory chain subunits, but also leads to the transcription of the mitochondrial genome through the induction of TFAM [61]. Moreover, the interaction between PGC-1α and transcription factors such as PPARα [62], PPARδ [63], ERRα [64], EERγ [64, 65], thyroid hormone receptor [66] and estrogen receptor [57, 67], controls fat and glucose metabolism. And since PGC-1α and PGC-1β share similar molecular structures and functions, it is not surprising that the mitochondrial gene expression driven by these two coactivators overlaps [68, 69]. Interestingly, their work results in mitochondria with different metabolic features [70]; thereby suggesting that distinct upstream pathways modulate PGC-1α and PGC-1β.

The pioneering work of Puigserver and coworkers first showed in 1998 that PGC-1α is dramatically induced (up to 50-fold) upon cold exposure in brown fat and skeletal muscle [55]. Since then, many studies have determined that the expression of PGC-1 family members are controlled by a variety of external stimuli, such as exercise, cold and nutrient deprivation, in a tissue-dependent manner (reviewed in [71]). Among the transcription factors regulating PGC1-α levels, CREB is responsible for integrating multiple signaling pathways in different cell types to boost mitochondrial function. For example, CREB-dependent induction of PGC1-α occurs in fasted liver [72], in exercised skeletal muscle [73], as well as in brown adipose tissue during cold [55]. Moreover, in a positive autoregulatory loop, PGC-1α regulates its own expression when binding to some of its transcription factors targets such as MEF2 [73] and ERRγ [74]. With equal importance of transcriptional levels, posttranslational modifications of PGC-1α also control mitochondrial biogenesis.

Posttranslational modifications refer to biochemical modifications of a protein (e.g. phosphorylation, acetylation, methylation) capable of influencing not only its structure, but also its activity [75]. The fine-tuning of PGC-1α activity occurs via posttranslational mechanisms. First, phosphorylation of PGC-1α protein is able to triple its half-life, which is relatively short (~2.3 h) [76]. Second, posttranslational modifications interfere with PGC-1α signal transduction by either increasing or inhibiting its activity. In response to bioenergetics imbalance, PGC-1α displays increased activity when phosphorylated by AMP-activated protein kinase (AMPK) [77] and deacetylated by Sirtuin 1 (SIRT1) [78]. On the contrary, PGC-1α can be phosphorylated by glycogen synthase kinase 3β (GSK3β) leading to its inhibition and degradation [79]. Third, most of the signaling pathways conducting these protein modifications have their gene expression regulated by PGC-1 family; thus, reinforcing the feed forward loop [80].

Finally, although the molecular mechanisms are not fully elucidated, it has been demonstrated that posttranslational modifications of PGC-1α result in a preferential induction of biogenesis in a time-, tissue- and subset of mitochondrial genes-dependent manner [78, 81]. This can be explained, at least in part, by the discovery of different splicing variants of PGC-1α: novel truncated PGC-1α (NT-PGC-1α), PGC-1α-β and PGC-1α4. While NT-PGC-1α [82] and PGC-1α-β [83] specifically affect energy metabolism by promoting mitochondrial biogenesis in brown adipose tissue and skeletal muscle, respectively, PGC-1α4 leads to skeletal muscle hypertrophy by regulating a non-mitochondrial gene program [84]. Interestingly, exercise is able to induce and activate all these variants [84, 85].

3 Mitochondrial Dynamics

The high-resolution electron microscopy images of mitochondria, published by Palade [5] and Sjostrand [6] in the 1950s, revealed for the first time the unique ultrastructure of these organelles. Those images also showed a lack of physical connection between mitochondria; thus, suggesting that the organelle was stationary and working independently. Two decades later, descriptions of mega-mitochondria formation in tissues such as liver [86] and skeletal muscle [87] started to question this independency. In the 1990s, advances in electron microscopy along with the development of mitochondrial-targeted fluorescent proteins allowed the observations that mitochondrial can dynamically rearrange their structure over time [88, 89]. Since then, a complete set of genes driving these morphological changes was discovered (reviewed in [90]) and mitochondrial dynamics has been consolidated as a new area of study in mitochondrial biology.

Mitochondrial dynamics refers to a set of processes including the regulation of mitochondrial morphology and connectivity, as well as their position inside the cells. The mitochondrial ability to reshape, rebuild and redistribute itself is orchestrated by the opposite role of fusion and fission processes [90]. Members of a large family of dynamin guanosine triphosphatases (GTPases) use the hydrolysis of guanosine triphosphate (GTP) to create conformational changes in the mitochondrial membrane that will lead to either the union between two organelles or the division of one mitochondrion in two organelles [91] (Fig. 2).

Fig. 2
figure 2

Simplified model for mitochondrial fusion and fission. The OMM (outer mitochondrial membrane) fuses through interaction of  homo- or hereto-oligomers Mfn1 (Mitofusin 1) and Mfn2 (Mitofusin 2) of two opposing mitochondria. Following OMM fusion, OPA1 (Optic atrophy 1) drives IMM (inner mitochondrial membrane) fusion. Please note that, as membrane-bound proteins, Mitofusins and OPA1 are still present in the new fused membranes, but are now disassembled. Mitochondrial fragmentation requires activation of cytosolic Drp1 (Dynamin-related protein 1) and recruitment to the organelle via OMM-bound receptors (R). At these sites, the Drp1 oligomerizes in a ring-like structure and constricts the mitochondria into 2 daughters. Of interest, asymmetrical fission of a damaged or senescent mitochondrion produces 1 dysfunctional organelle that can either be eliminated by mitophagy or re-enter the mitochondrial network and regenerate by fusing with other healthy organelles

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Despite often viewed as a separate phenomenon, the recycling of mitochondria through mitophagy—a specific form of autophagy, is influenced by mitochondrial fission and therefore directly interferes with the dynamic nature of the organelle. To a detailed description of mitophagy, readers are referred to excellent reviews on this topic [92, 93]. Together, mitochondrial fusion-fission machinery, mitochondrial biogenesis and mitophagy comprise a well-conserved quality control axis capable of controlling the function of the organelle, and as consequence, interfering with cellular physiology [94].

3.1 Mitochondrial Fusion

Mitochondrial fusion is an evolutionary conserved process that merges two neighboring mitochondria. By allowing the exchange of mitochondrial proteins, metabolites and mtDNA, mitochondrial fusion maximizes cellular respiration [95]. It is also required for the maintenance of mtDNA integrity [96]. Moreover, the newly elongated fused organelle prevents erroneous degradation of mitochondria [17]. Considering that mitochondria have outer and inner membranes, the fusion process requires bringing together four membranes in separated events. First, mitofusin 1 (Mfn1) and mitofusin 2 (Mfn2) proteins are responsible for fusing the outer mitochondrial membrane. Later, optic atrophy factor 1 (OPA1) governs the union of the inner mitochondrial membrane [97]. As nuclear-encoded mitochondrial proteins, they need to be synthesized in the cytosol and imported into mitochondria, therefore reinforcing the connection between mitochondrial biogenesis and dynamics. Additionally, these GTPases contain a transmembrane domain that anchors part of them to the lipid bilayer, whereas their free part can physically interact with other GTPases to promote the tethering [98].

Mfn1 and Mfn2 are the major players in promoting outer membrane shape-changes. These isoforms display high homology (~80%) and initiate the fusion of the outer mitochondrial membrane by the formation of homo- (Mfn1-Mfn1 or Mfn2-Mfn2) or hetero-oligomers (Mfn1-Mfn2) between adjacent organelles [99]. Despite widely expressed and essential for embryonic development [100], Mfn1 is more abundant in heart and liver, while Mfn2 predominates in skeletal muscle, brain and adipose tissue [101]. Interestingly, each mitofusin not only differently affects mitochondrial morphology, but also plays distinct roles in cellular physiology. The absence of Mfn1 leads to highly fragmented mitochondria when compared to Mfn2 downregulation [100]. Moreover, Mfn2 also participates in calcium regulation by tethering the mitochondria to the endoplasmic reticulum [102, 103].

In order to complete the fusion process, OPA1 drives the unification of the two inner mitochondrial membranes. This intermembrane space-localized GTPase not only suffers alternative splicing—a mechanism by which different messenger RNA are generated from the same gene, but its activity is also regulated by proteolytic processing [104]. Because of that, there are at least eight variants of OPA1 in humans containing one or two proteolytic sites [105]. The mitochondrial proteases OMA1 and YME1L1 are responsible for generating long and short OPA1 isoforms, in a membrane potential dependent manner [106, 107]. Despite the fact that both isoforms are required for full fusion events, an excessive processing of short OPA1 limits fusion; therefore, triggering mitochondrial fragmentation [108,109,110]. Finally, regardless governing the delicate balance between fusion and fission, OPA1 variants are able to control apoptosis by regulating the cristae morphology and consequent release of cytochrome c—a component of ETC that triggers programed cell death [109, 111].

3.2 Mitochondrial Fission

Mitochondrial fission process is responsible for the asymmetrical segregation of portions of the organelle. Whereas this new spherical and smaller organelle facilitates motility throughout the cell, it is also involved in mtDNA replication and inheritance during cellular proliferation [112]. Fragmentation of the mitochondrial network also permits the selective removal of damaged organelles by mitophagy [18]. Unlike mitochondrial fusion, the division of the outer and inner membranes of the organelle is catalyzed by a single GTPase effector—dynamin-related protein 1 (Drp1) [113]. Unlike mitochondrial fusion-related proteins, Drp1 is a nuclear-encoded protein that resides in the cytosol as a small oligomer, thereby demanding recruitment to the mitochondrial surface. The assembly of several Drp1 oligomers forms a ring-like structure around the outer mitochondrial membrane and cut mitochondria into two separate entities in a GTP hydrolysis-dependent manner [114].

A multi-step process is required before completing mitochondrial membrane remodeling. First, Drp1 needs to be activated in order to translocate from the cytosol to mitochondria. Among the posttranslational modifications regulating Drp1 activity, phosphorylation has been extensively studied and serves as an efficient way to synchronize intracellular signaling pathways and mitochondrial metabolism. For example, protein kinase A (PKA) phosphorylation of Drp1 at serine-637 blocks fission and protects mitochondrial from degradation during starvation [115]. Dephosphorylation of the same residue by the phosphatase calcineurin triggers fission in a calcium-induced mitochondrial dysfunction environment [116, 117]. Ubiquitination of Drp1 by E3 ligases can either induce mitochondrial fragmentation or inhibit fission by promoting Drp1 degradation [118, 119]. Additional posttranslational modifications of Drp1 (e.g. SUMOylation and S-nitrosylation) also dictate mitochondrial dynamics. These regulatory mechanisms are reviewed elsewhere [120, 121].

Once activated, the second step involves the recruitment of Drp1 to specific regions of the outer mitochondrial membrane. Four specific adaptor proteins, also termed Drp1 receptors, facilitate this anchoring process: mitochondrial fission 1 protein (Fis1), mitochondrial fission factor (Mff) and mitochondrial dynamics proteins of 49 and 51 kDa (MiD49 and MiD51, respectively) [113, 122, 123]. This receptor-mediated recruitment of Drp1 assists mitochondrial fragmentation by allowing the self-assemble of Drp1 into oligomeric complexes at specific sites of the outer membrane pre-constricted by the endoplasmic reticulum [124, 125]. Similar to Drp1, these receptors can be activated by posttranslational modifications. In particular, MFF can be phosphorylated by AMPK in response to nutrient excess, favoring mitochondrial fission [126, 127]. Finally, recent evidence place another GTPase—dynamin 2 (Dyn2), as a mechanoenzyme involved in terminating membrane scission. It has been proposed that Drp1-mediated constriction allows Dyn2 assembly to complete the fission event [128].

4 Mitochondrial Dynamics in Health and Disease

Given the fact that mitochondrial biogenesis and dynamics interfere with a variety of intracellular processes including ATP production, ROS release and apoptosis, it is not surprising that they are critical in the context of both physiological and pathological events [129]. Disruption of mitochondrial homeostasis follows the clinical progression of a variety of chronic degenerative diseases (e.g. Heart Failure and Diabetes) [130]. Moreover, mitochondrial dysfunction is a common feature of rare inherited mitochondrial diseases, which are driven by mutations in either nuclear or mitochondrial DNA (e.g. Leigh syndrome and Friedreich’s ataxia) [131]. Either way, the inability of maintaining a healthy mitochondrial population has been placed as a central determinant of several diseases. Here, we discuss mitochondrial biogenesis and dynamics in the context of cardiac, metabolic and neurodegenerative diseases, as well as in mitochondrial diseases.

Since heart contractility requires elevated and sustained levels of ATP, an overall failure of mitochondrial function has been placed as a hallmark of cardiac diseases [132, 133]. Disrupted mitochondrial morphology—characterized by increase number of smaller organelles [134], has been detected in cardiac patients suggesting imbalance between fusion and fission as critical factor for heart pathophysiology. Indeed, while absence of Mfn1 or Mfn2 [135,136,137], or excessive OPA1 cleavage [138] are sufficient to disrupt mitochondrial fusion leading to cardiomyopathy in mice, inactivation of Drp1 blunts excessive fission of the organelle, thus counteracting cardiac dysfunction [139]. Likewise, small molecules capable of blocking fission (i.e. Mdivi-1 and P110) [139, 140] or improving fusion (SAMβA) [141], as well as exercise [142], reestablish mitochondrial dynamics and improve clinical outcome in preclinical models of cardiac diseases. Of interest, failing hearts display loss of mtDNA along with reduced expression of mitochondrial biogenesis markers [143]. Moreover, cardiac specific ablation of PGC-1α leads to cardiac dysfunction in mice [144]. Because of that, activators of AMPK (i.e. Metformin, AICAR) capable of stimulating mitochondrial biogenesis [145], are emerging as promising therapies to treat cardiovascular diseases [146].

Metabolic disorders including type 2 Diabetes and obesity not only arises from a complex combination of genetic and environmental factors such as insulin resistance, dyslipidemia, erroneous food intake and physical inactivity [147], but also display mitochondrial dysfunction as a common feature [148]. Part of this phenotype is due to impaired mitochondrial biogenesis and dynamics in a wide spectrum of tissues. Reduced expression of PGC-1 members along with defective translation of genes encoding subunits of respiratory chain have been observed in skeletal muscle from diabetic patients [149] and adipose tissue from obese subjects [150]. Strengthening these results, mice lacking PGC-1α in adipose tissue develop insulin resistance and abnormal thermogenic response [151]. Moreover, mitochondrial biogenesis have been linked to the beneficial effects of agonists of AMPK [152, 153] and PPAR [154, 155]—widely used drugs for the treatment of type 2 Diabetes. The excessive nutrient environment observed in metabolic disorders also promotes disruption of mitochondrial dynamics. Consistent with reduction of Mfn2 levels [156, 157], mitochondrial fragmentation associated with insulin sensitivity and altered metabolism has been observed in obesity and type 2 Diabetes [15, 158].

Along with progressive loss of neuronal systems, disruption of mitochondrial homeostasis plays a role in the pathogenesis of neurodegenerative disorders such as Parkinson´s, Alzheimer´s and Huntington´s diseases [159, 160]. Analysis of human brains from Alzheimer´s patients revealed not only structurally abnormal mitochondria [161, 162], but also indicated a strong link between Drp1-mediated mitochondrial fission and neurodegeneration [163]. Indeed, blocking mitochondrial fragmentation exhibits beneficial effects in preclinical models of Huntington´s [164] and Parkinson’s [165] disease, and Amyotrophic lateral sclerosis [166]. On the contrary, loss-of-function Drp1 mutations leading to giant and aberrant mitochondria are often associated with lethal neurological disorders including microcephaly [167] and refractory epilepsy [168]; therefore, reinforcing the role of an exquisite balance of mitochondrial fusion and fission events in cellular physiology. In the context of mitochondrial biogenesis, deficiencies in the ETC are related to mtDNA mutations in Alzheimer’s patients, which suppress mitochondrial transcription and replication [169]. Similarly, studies in animals have shown that whereas impaired mitochondrial biogenesis leads to loss of neurons [170], PGC-1α upregulation protects neural cells against oxidative stress-induced death [171].

Mostly driven by loss-of-function mutations in mtDNA or nuclear DNA, mitochondrial diseases refer to a heterogeneous group of disorders triggered by mitochondrial dysfunction [172]. Regardless the disease etiology, there is an overall decrease in content and function of respiratory chain subunits [173,174,175]. In this context, PGC-1α overexpression can boost ATP production by increasing the amount of the organelle in Leigh syndrome [174]. Inducers of mitochondrial biogenesis (i.e. AICAR) also delay the progression of mitochondrial myopathies in mice [174, 176]. Despite the fact that most of these disorders arise from defects in OXPHOS components, progressive neuronal degeneration along with aberrant mitochondrial morphology are observed in preclinical models of Leigh syndrome [175]. Progressive loss of vision observed in autosomal dominant optic atrophy disease is associated with OPA1 mutations [177]. Moreover, impaired mitochondrial fusion or fission by mutations in Mfn2 [178] and Dyn2 genes [179] cause the inherited Charcot Marie Tooth disease. Of interest, due to its involvement in mtDNA replication [112, 180], disruption of mitochondrial dynamics may increase the susceptibility to these inborn errors. Finally, highlighting the dynamic nature of mitochondria, gene therapy is the latest and attractive strategy to restore mitochondrial function and counteract clinical progression of primary mitochondrial diseases [181,182,183,184].

5 Concluding Remarks

The dynamic behavior of mitochondria morphology, controlled by mitochondrial biogenesis and fission-fusion machineries, are determinant for the whole-body homeostasis. Fluctuations in mitochondrial quantity, size and cellular position occur in response to numerous stress and metabolic conditions, which will lead to divergent outcomes. If transient, perturbations of mitochondrial mass and morphology enable metabolic adaptations to meet energetic requirements. On the contrary, sustained stress-induced mitochondrial dysfunction often triggers mitochondrial fragmentation and induces cell death. Moreover, due to the dynamic nature of mitochondria, studying the physiological and pathological significance of mitochondrial network in a time-, tissue- and stress-dependent manner is a challenging task. The development of advanced techniques capable of tracking fusion and fission events in vivo, as well as the identification of new players controlling biogenesis and dynamics will be crucial not only to overcome these obstacles, but also to open up new avenues for pharmacological interventions.