Distinct roles of the two isoforms of the dynamin-like GTPase Mgm1 in mitochondrial fusion

Severe mitochondrial encephalomyopathy caused by de novo variants in OPA1 gene

Background

Mitochondria adjust their shape in response to the different energetic and metabolic requirements of the cell, through extremely dynamic fusion and fission events. Several highly conserved dynamin-like GTPases are involved in these processes and, among those, the OPA1 protein is a key player in the fusion of inner mitochondrial membranes. Hundreds of monoallelic or biallelic pathogenic gene variants have been described in OPA1, all associated with a plethora of clinical phenotypes without a straightforward genotype-phenotype correlation.

Methods

Here we report two patients harboring novel de novo variants in OPA1. DNA of two patients was analyzed using NGS technology and the pathogenicity has been evaluated through biochemical and morphological studies in patient's derived fibroblasts and in yeast model.

Results

The two patients here reported manifest with neurological signs resembling Leigh syndrome, thus further expanding the clinical spectrum associated with variants in OPA1. In cultured skin fibroblasts we observed a reduced amount of mitochondrial DNA (mtDNA) and altered mitochondrial network characterized by more fragmented mitochondria. Modeling in yeast allowed to define the deleterious mechanism and the pathogenicity of the identified gene mutations.

Conclusion

We have described two novel-single OPA1 mutations in two patients characterized by early-onset neurological signs, never documented, thus expanding the clinical spectrum of this complex syndrome. Moreover, both yeast model and patients derived fibroblasts showed mitochondrial defects, including decreased mtDNA maintenance, correlating with patients' clinical phenotypes.

OPA1, a molecular regulator of dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a disease with no specific treatment, poor prognosis and high mortality. During DCM development, there is apoptosis, mitochondrial dynamics imbalance and changes in cristae structure. Optic atrophy 1 (OPA1) appears at high frequency in these three aspects. DCM LMNA (LaminA/C) gene mutation can activate TP53, and the study of P53 shows that P53 affects OPA1 through Bak/Bax and OMA1 (a metalloprotease). OPA1 can be considered the missing link between DCMp53 and DCM apoptosis, mitochondrial dynamics imbalance and changes in cristae structure. OPA1 regulates apoptosis by regulating the release of cytochrome c from the mitochondrial matrix through CJs (crisp linkages, located in the inner mitochondrial membrane) and unbalances mitochondrial fusion and fission by affecting mitochondrial inner membrane (IM) fusion. OPA1 is also associated with the formation and maintenance of mitochondrial cristae. OPA1 is not the root cause of DCM, but it is an essential mediator in P53 mediating the occurrence and development of DCM, so OPA1 also becomes a molecular regulator of DCM. This review discusses the implication of OPA1 for DCM from three aspects: apoptosis, mitochondrial dynamics and ridge structure.

Mitochondrial dynamics involves molecular and mechanical events in motility, fusion and fission

Mitochondria are cell organelles that play pivotal roles in maintaining cell survival, cellular metabolic homeostasis, and cell death. Mitochondria are highly dynamic entities which undergo fusion and fission, and have been shown to be very motile in vivo in neurons and in vitro in multiple cell lines. Fusion and fission are essential for maintaining mitochondrial homeostasis through control of morphology, content exchange, inheritance of mitochondria, maintenance of mitochondrial DNA, and removal of damaged mitochondria by autophagy. Mitochondrial motility occurs through mechanical and molecular mechanisms which translocate mitochondria to sites of high energy demand. Motility also plays an important role in intracellular signaling. Here, we review key features that mediate mitochondrial dynamics and explore methods to advance the study of mitochondrial motility as well as mitochondrial dynamics-related diseases and mitochondrial-targeted therapeutics.

Mitochondrial quality control in health and in Parkinson's disease

As a central hub for cellular metabolism and intracellular signalling, the mitochondrion is a pivotal organelle, dysfunction of which has been linked to several human diseases including neurodegenerative disorders, and in particular Parkinson's disease. An inherent challenge that mitochondria face is the continuous exposure to diverse stresses which increase their likelihood of dysregulation. In response, eukaryotic cells have evolved sophisticated quality control mechanisms to monitor, identify, repair and/or eliminate abnormal or misfolded proteins within the mitochondrion and/or the dysfunctional mitochondrion itself. Chaperones identify unstable or otherwise abnormal conformations in mitochondrial proteins and can promote their refolding to recover their correct conformation and stability. However, if repair is not possible, the abnormal protein is selectively degraded to prevent potentially damaging interactions with other proteins or its oligomerization into toxic multimeric complexes. The autophagic-lysosomal system and the ubiquitin-proteasome system mediate the selective and targeted degradation of such abnormal or misfolded protein species. Mitophagy (a specific kind of autophagy) mediates the selective elimination of dysfunctional mitochondria, in order to prevent the deleterious effects the dysfunctional organelles within the cell. Despite our increasing understanding of the molecular responses toward dysfunctional mitochondria, many key aspects remain relatively poorly understood. Herein, we review the emerging mechanisms of mitochondrial quality control including quality control strategies coupled to mitochondrial import mechanisms. In addition, we review the molecular mechanisms regulating mitophagy with an emphasis on the regulation of PINK1/PARKIN-mediated mitophagy in cellular physiology and in the context of Parkinson's disease cell biology.

Pathways shaping the mitochondrial inner membrane

Mitochondria are complex organelles with two membranes. Their architecture is determined by characteristic folds of the inner membrane, termed cristae. Recent studies in yeast and other organisms led to the identification of four major pathways that cooperate to shape cristae membranes. These include dimer formation of the mitochondrial ATP synthase, assembly of the mitochondrial contact site and cristae organizing system (MICOS), inner membrane remodelling by a dynamin-related GTPase (Mgm1/OPA1), and modulation of the mitochondrial lipid composition. In this review, we describe the function of the evolutionarily conserved machineries involved in mitochondrial cristae biogenesis with a focus on yeast and present current models to explain how their coordinated activities establish mitochondrial membrane architecture.

Saccharomyces cerevisiae as a Tool for Studying Mutations in Nuclear Genes Involved in Diseases Caused by Mitochondrial DNA Instability

Mitochondrial DNA (mtDNA) maintenance is critical for oxidative phosphorylation (OXPHOS) since some subunits of the respiratory chain complexes are mitochondrially encoded. Pathological mutations in nuclear genes involved in the mtDNA metabolism may result in a quantitative decrease in mtDNA levels, referred to as mtDNA depletion, or in qualitative defects in mtDNA, especially in multiple deletions. Since, in the last decade, most of the novel mutations have been identified through whole-exome sequencing, it is crucial to confirm the pathogenicity by functional analysis in the appropriate model systems. Among these, the yeast Saccharomyces cerevisiae has proved to be a good model for studying mutations associated with mtDNA instability. This review focuses on the use of yeast for evaluating the pathogenicity of mutations in six genes, MPV17/SYM1, MRM2/MRM2, OPA1/MGM1, POLG/MIP1, RRM2B/RNR2, and SLC25A4/AAC2, all associated with mtDNA depletion or multiple deletions. We highlight the techniques used to construct a specific model and to measure the mtDNA instability as well as the main results obtained. We then report the contribution that yeast has given in understanding the pathogenic mechanisms of the mutant variants, in finding the genetic suppressors of the mitochondrial defects and in the discovery of molecules able to improve the mtDNA stability.

MICOS and the mitochondrial inner membrane morphology - when things get out of shape

Mitochondria play a key role in cellular signalling, metabolism and energetics. Proper architecture and remodelling of the inner mitochondrial membrane are essential for efficient respiration, apoptosis and quality control in the cell. Several protein complexes including mitochondrial contact site and cristae organizing system (MICOS), F₁ F_O -ATP synthase, and Optic Atrophy 1 (OPA1), facilitate formation, maintenance and stability of cristae membranes. MICOS, the F₁ F_O -ATP synthase, OPA1 and inner membrane phospholipids such as cardiolipin and phosphatidylethanolamine interact with each other to organize the inner membrane ultra-structure and remodel cristae in response to the cell's demands. Functional alterations in these proteins or in the biosynthesis pathway of cardiolipin and phosphatidylethanolamine result in an aberrant inner membrane architecture and impair mitochondrial function. Mitochondrial dysfunction and abnormalities hallmark several human conditions and diseases including neurodegeneration, cardiomyopathies and diabetes mellitus. Yet, they have long been regarded as secondary pathological effects. This review discusses emerging evidence of a direct relationship between protein- and lipid-dependent regulation of the inner mitochondrial membrane morphology and diseases such as fatal encephalopathy, Leigh syndrome, Parkinson's disease, and cancer.

Dominant Optic Atrophy (DOA): Modeling the Kaleidoscopic Roles of OPA1 in Mitochondrial Homeostasis

In the year 2000, the discovery of OPA1 mutations as causative for dominant optic atrophy (DOA) was pivotal to rapidly expand the field of mitochondrial dynamics and describe the complex machinery governing this pathway, with a multitude of other genes and encoded proteins involved in neurodegenerative disorders of the optic nerve. OPA1 turned out to be a much more complex protein than initially envisaged, connecting multiple pathways beyond its strict role in mitochondrial fusion, such as sensing of OXPHOS needs and mitochondrial DNA maintenance. As a consequence, an increasing need to investigate OPA1 functions at multiple levels has imposed the development of multiple tools and models that are here reviewed. Translational mitochondrial medicine, with the ultimate objective of translating basic science necessary to understand pathogenic mechanisms into therapeutic strategies, requires disease modeling at multiple levels: from the simplest, like in yeast, to cell models, including the increasing use of reprogrammed stem cells (iPSCs) from patients, to animal models. In the present review, we thoroughly examine and provide the state of the art of all these approaches.

The Power of Yeast in Modelling Human Nuclear Mutations Associated with Mitochondrial Diseases

The increasing application of next generation sequencing approaches to the analysis of human exome and whole genome data has enabled the identification of novel variants and new genes involved in mitochondrial diseases. The ability of surviving in the absence of oxidative phosphorylation (OXPHOS) and mitochondrial genome makes the yeast Saccharomyces cerevisiae an excellent model system for investigating the role of these new variants in mitochondrial-related conditions and dissecting the molecular mechanisms associated with these diseases. The aim of this review was to highlight the main advantages offered by this model for the study of mitochondrial diseases, from the validation and characterisation of novel mutations to the dissection of the role played by genes in mitochondrial functionality and the discovery of potential therapeutic molecules. The review also provides a summary of the main contributions to the understanding of mitochondrial diseases emerged from the study of this simple eukaryotic organism.

The Effects of Regulatory Lipids on Intracellular Membrane Fusion Mediated by Dynamin-Like GTPases

Membrane fusion mediates a number of fundamental biological processes such as intracellular membrane trafficking, fertilization, and viral infection. Biological membranes are composed of lipids and proteins; while lipids generally play a structural role, proteins mediate specific functions in the membrane. Likewise, although proteins are key players in the fusion of biological membranes, there is emerging evidence supporting a functional role of lipids in various membrane fusion events. Intracellular membrane fusion is mediated by two protein families: SNAREs and membrane-bound GTPases. SNARE proteins are involved in membrane fusion between transport vesicles and their target compartments, as well as in homotypic fusion between organelles of the same type. Membrane-bound GTPases mediate mitochondrial fusion and homotypic endoplasmic reticulum fusion. Certain membrane lipids, known as regulatory lipids, regulate these membrane fusion events by directly affecting the function of membrane-bound GTPases, instead of simply changing the biophysical and biochemical properties of lipid bilayers. In this review, we provide a summary of the current understanding of how regulatory lipids affect GTPase-mediated intracellular membrane fusion by focusing on the functions of regulatory lipids that directly affect fusogenic GTPases.

Abnormal Mitochondrial Quality Control in Neurodegenerative Diseases

Neurodegenerative diseases, including Alzheimer’s, Parkinson’s, Huntington’s, and amyotrophic lateral sclerosis, are characterized by a progressive loss of selective neuron subtypes in the central nervous system (CNS). Although various factors account for the initiation and development of these diseases, accumulating evidence shows that impaired mitochondrial function is a prominent and common mechanism. Mitochondria play a critical role in neurons and are involved in energy production, cellular metabolism regulation, intracellular calcium homeostasis, immune responses, and cell fate. Thus, cells in the CNS heavily rely on mitochondrial integrity. Many aspects of mitochondrial dysfunction are manifested in neurodegenerative diseases, including aberrant mitochondrial quality control (mitoQC), mitochondrial-driven inflammation, and bioenergetic defects. Herein, we briefly summarize the molecular basis of mitoQC, including mitochondrial proteostasis, biogenesis, dynamics, and organelle degradation. We also focus on the research, to date, regarding aberrant mitoQC and mitochondrial-driven inflammation in several common neurodegenerative diseases. In addition, we outline novel therapeutic strategies that target aberrant mitoQC in neurodegenerative diseases.

Abnormal Mitochondrial Quality Control in Neurodegenerative Diseases

Neurodegenerative diseases, including Alzheimer's, Parkinson's, Huntington's, and amyotrophic lateral sclerosis, are characterized by a progressive loss of selective neuron subtypes in the central nervous system (CNS). Although various factors account for the initiation and development of these diseases, accumulating evidence shows that impaired mitochondrial function is a prominent and common mechanism. Mitochondria play a critical role in neurons and are involved in energy production, cellular metabolism regulation, intracellular calcium homeostasis, immune responses, and cell fate. Thus, cells in the CNS heavily rely on mitochondrial integrity. Many aspects of mitochondrial dysfunction are manifested in neurodegenerative diseases, including aberrant mitochondrial quality control (mitoQC), mitochondrial-driven inflammation, and bioenergetic defects. Herein, we briefly summarize the molecular basis of mitoQC, including mitochondrial proteostasis, biogenesis, dynamics, and organelle degradation. We also focus on the research, to date, regarding aberrant mitoQC and mitochondrial-driven inflammation in several common neurodegenerative diseases. In addition, we outline novel therapeutic strategies that target aberrant mitoQC in neurodegenerative diseases.

Mitochondrial-nuclear heme trafficking in budding yeast is regulated by GTPases that control mitochondrial dynamics and ER contact sites

Heme is a cofactor and signaling molecule that is essential for much of aerobic life. All heme-dependent processes in eukaryotes require that heme is trafficked from its site of synthesis in the mitochondria to hemoproteins located throughout the cell. However, the mechanisms governing the mobilization of heme out of the mitochondria, and the spatio-temporal dynamics of these processes, are poorly understood. Here, using genetically encoded fluorescent heme sensors, we developed a live-cell assay to monitor heme distribution dynamics between the mitochondrial inner membrane, where heme is synthesized, and the mitochondrial matrix, cytosol and nucleus. Surprisingly, heme trafficking to the nucleus is ∼25% faster than to the cytosol or mitochondrial matrix, which have nearly identical heme trafficking dynamics, potentially supporting a role for heme as a mitochondrial-nuclear retrograde signal. Moreover, we discovered that the heme synthetic enzyme 5-aminolevulinic acid synthase (ALAS, also known as Hem1 in yeast), and GTPases in control of the mitochondrial dynamics machinery (Mgm1 and Dnm1) and ER contact sites (Gem1), regulate the flow of heme between the mitochondria and nucleus. Overall, our results indicate that there are parallel pathways for the distribution of bioavailable heme.This article has an associated First Person interview with the first author of the paper.

Cryo-EM structures of S-OPA1 reveal its interactions with membrane and changes upon nucleotide binding

Mammalian mitochondrial inner membrane fusion is mediated by optic atrophy 1 (OPA1). Under physiological conditions, OPA1 undergoes proteolytic processing to form a membrane-anchored long isoform (L-OPA1) and a soluble short isoform (S-OPA1). A combination of L-OPA1 and S-OPA1 is essential for efficient membrane fusion; however, the relevant mechanism is not well understood. In this study, we investigate the cryo-electron microscopic structures of S-OPA1–coated liposomes in nucleotide-free and GTPγS-bound states. S-OPA1 exhibits a general dynamin-like structure and can assemble onto membranes in a helical array with a dimer building block. We reveal that hydrophobic residues in its extended membrane-binding domain are critical for its tubulation activity. The binding of GTPγS triggers a conformational change and results in a rearrangement of the helical lattice and tube expansion similar to that of S-Mgm1. These observations indicate that S-OPA1 adopts a dynamin-like power stroke membrane remodeling mechanism during mitochondrial inner membrane fusion.

Two forms of Opa1 cooperate to complete fusion of the mitochondrial inner-membrane

Mitochondrial membrane dynamics is a cellular rheostat that relates metabolic function and organelle morphology. Using an in vitro reconstitution system, we describe a mechanism for how mitochondrial inner-membrane fusion is regulated by the ratio of two forms of Opa1. We found that the long-form of Opa1 (l-Opa1) is sufficient for membrane docking, hemifusion and low levels of content release. However, stoichiometric levels of the processed, short form of Opa1 (s-Opa1) work together with l-Opa1 to mediate efficient and fast membrane pore opening. Additionally, we found that excess levels of s-Opa1 inhibit fusion activity, as seen under conditions of altered proteostasis. These observations describe a mechanism for gating membrane fusion.

Single amino acid mutations in the Saccharomyces cerevisiae rhomboid peptidase, Pcp1p, alter mitochondrial morphology

Key to mitochondrial activities is the maintenance of mitochondrial morphology, specifically cristae structures formed by the invagination of the inner membrane that are enriched in proteins of the electron transport chain. In Saccharomyces cerevisiae , these cristae folds are a result of the membrane fusion activities of Mgm1p and the membrane-bending properties of adenosine triphosphate (ATP) synthase oligomerization. An additional protein linked to mitochondrial morphology is Pcp1p, a serine protease responsible for the proteolytic processing of Mgm1p. Here, we have used hydroxylamine-based random mutagenesis to identify amino acids important for Pcp1p peptidase activity. Using this approach we have isolated five single amino acid mutants that exhibit respiratory growth defects that correlate with loss of mitochondrial genome stability. Reduced Pcp1p protease activity was confirmed by immunoblotting with the accumulation of improperly processed Mgm1p. Ultra-structural analysis of mitochondrial morphology in these mutants found a varying degree of defects in cristae organization. However, not all of the mutants presented with decreased ATP synthase complex assembly as determined by blue native polyacrylamide gel electrophoresis. Together, these data suggest that there is a threshold level of processed Mgm1p required to maintain ATP synthase super-complex assembly and mitochondrial cristae organization.

Proteolytic regulation of mitochondrial dynamics

Spatiotemporal changes in the abundance, shape, and cellular localization of the mitochondrial network, also known as mitochondrial dynamics, are now widely recognized to play a key role in mitochondrial and cellular physiology as well as disease states. This process involves coordinated remodeling of the outer and inner mitochondrial membranes by conserved dynamin-like guanosine triphosphatases and their partner molecules in response to various physiological and stress stimuli. Although the core machineries that mediate fusion and partitioning of the mitochondrial network have been extensively characterized, many aspects of their function and regulation are incompletely understood and only beginning to emerge. In the present review we briefly summarize current knowledge about how the key mitochondrial dynamics-mediating factors are regulated via selective proteolysis by mitochondrial and cellular proteolytic machineries.

Structural basis for membrane tethering by a bacterial dynamin-like pair

Dynamin-like proteins (DLPs) are large GTPases that restructure membrane. DLPs such as the mitofusins form heterotypic oligomers between isoform pairs that bridge and fuse opposing membranes. In bacteria, heterotypic oligomerisation may also be important for membrane remodelling as most DLP genes are paired within operons. How DLPs tether opposing membranes is unknown. Here we show the crystal structure of a DLP heterotypic pair from the pathogen Campylobacter jejuni. A 2:2 stoichiometric tetramer is observed where heterodimers, conjoined by a random coil linker, assemble back-to-back to form a tripartite DLP chain with extreme flexibility. In vitro, tetramerisation triggers GTPase activity and induces lipid binding. Liposomes are readily tethered and form tubes at high tetramer concentration. Our results provide a direct mechanism for the long-range binding and bridging of opposing membranes by a bacterial DLP pair. They also provide broad mechanistic and structural insights that are relevant to other heterotypic DLP complexes.

Dynamin-like proteins (DLPs) such as the mitofusins form homotypic and heterotypic oligomers that bridge and fuse opposing membranes. Here, Liu, Noel and Low present the crystal structure of a bacterial DLP heterotypic pair, providing insights into the mechanism behind long-range binding of opposing membranes.

Deciphering OPA1 mutations pathogenicity by combined analysis of human, mouse and yeast cell models

OPA1 is the major gene responsible for Dominant Optic Atrophy (DOA) and the syndromic form DOA "plus". Over 370 OPA1 mutations have been identified so far, although their pathogenicity is not always clear. We have analyzed one novel and a set of known OPA1 mutations to investigate their impact on protein functions in primary skin fibroblasts and in two "ad hoc" generated cell systems: the MGM1/OPA1 chimera yeast model and the Opa1-/- MEFs model expressing the mutated human OPA1 isoform 1. The yeast model allowed us to confirm the deleterious effects of these mutations and to gain information on their dominance/recessivity. The MEFs model enhanced the phenotypic alteration caused by mutations, nicely correlating with the clinical severity observed in patients, and suggested that the DOA "plus" phenotype could be induced by the combinatorial effect of mitochondrial network fragmentation with variable degrees of mtDNA depletion. Overall, the two models proved to be valuable tools to functionally assess and define the deleterious mechanism and the pathogenicity of novel OPA1 mutations, and useful to testing new therapeutic interventions.

Mitochondrial quality control in amyotrophic lateral sclerosis: towards a common pathway?

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by loss of upper and lower motor neurons. Different mechanisms contribute to the disease initiation and progression, including mitochondrial dysfunction which has been proposed to be a central determinant in ALS pathogenesis. Indeed, while mitochondrial defects have been mainly described in ALS-linked SOD1 mutants, it is now well established that mitochondria become also dysfunctional in other ALS conditions. In such context, the mitochondrial quality control system allows to restore normal functioning of mitochondria and to prevent cell death, by both eliminating and replacing damaged mitochondrial components or by degrading the entire organelle through mitophagy. Recent evidence shows that ALS-related genes interfere with the mitochondrial quality control system. This review highlights how ineffective mitochondrial quality control may render motor neurons defenseless towards the accumulating mitochondrial damage in ALS.

OPA1 and cardiolipin team up for mitochondrial fusion

Fusion between the inner membranes of two mitochondria requires the GTPase optic atrophy 1 (OPA1), but the molecular mechanism is poorly understood. A study now shows that fusion of two liposomes can be performed by OPA1 tethered to just one liposome, through an interaction with the phospholipid cardiolipin on the opposing liposome.

An evidence based hypothesis on the existence of two pathways of mitochondrial crista formation

Metabolic function and architecture of mitochondria are intimately linked. More than 60 years ago, cristae were discovered as characteristic elements of mitochondria that harbor the protein complexes of oxidative phosphorylation, but how cristae are formed, remained an open question. Here we present experimental results obtained with yeast that support a novel hypothesis on the existence of two molecular pathways that lead to the generation of lamellar and tubular cristae. Formation of lamellar cristae depends on the mitochondrial fusion machinery through a pathway that is required also for homeostasis of mitochondria and mitochondrial DNA. Tubular cristae are formed via invaginations of the inner boundary membrane by a pathway independent of the fusion machinery. Dimerization of the F₁F_O-ATP synthase and the presence of the MICOS complex are necessary for both pathways. The proposed hypothesis is suggested to apply also to higher eukaryotes, since the key components are conserved in structure and function throughout evolution.

DOI:http://dx.doi.org/10.7554/eLife.18853.001

Cells contain compartments called mitochondria, which are often called the powerhouses of the cell because they provide energy that drives vital cellular processes. Mitochondria have two membranes: an outer and an inner membrane. The outer membrane separates the mitochondria from the rest of the cell. The inner membrane is elaborately folded and the folds – called cristae – create a larger space to accommodate all of the protein machinery involved in producing energy. The cristae can be shaped as flat sac-like structures called lamellar cristae or as tubes known as tubular cristae.

Mitochondria are dynamic and are constantly fusing with other mitochondria and splitting up. Even though the internal architecture of mitochondria was first revealed around 60 years ago, it is still not clear how the cristae form. Harner et al. now address this question in yeast cells by combining imaging, biochemistry and genetic approaches.

The experiments show that lamellar cristae form when two mitochondria fuse with each other. The outer membranes merge and then the inner membranes start to fuse around their edges to generate the sac-like structure of lamellar cristae. A yeast protein called Mgm1 (known as Opa1 in mammals) drives the fusion of the inner membranes, but this process only takes place when enzymes called F₁F_O-ATP synthases on the inner membrane form pairs with one another. These F₁F_O-ATP synthase pairs stabilize the cristae membranes as they curve to form the sac-like structure. Later on, the formation of a group of proteins called the MICOS complex halts the fusion process to prevent the lamellar cristae from completely separating from the rest of the inner membrane.

Harner et al. also found that tubular cristae form using a different mechanism when the inner membrane of the mitochondria grows inwards. This process also requires pairs of F₁F_O-ATP synthases and the MICOS complex, but does not involve Mgm1/Opa1. Together, these findings show that lamellar and tubular cristae in yeast form using two different mechanisms. Since the key components of these mechanisms are also found in virtually all other eukaryotes, the findings of Harner et al. are also likely to apply to many other organisms including animals.

DOI:http://dx.doi.org/10.7554/eLife.18853.002

Regulation of Mitofusin1 by Mahogunin Ring Finger-1 and the proteasome modulates mitochondrial fusion

Health and homoeostasis are maintained by a dynamic balance between mitochondrial fission and fusion. Mitochondrial fusion machinery is largely unknown in mammals. Only a few reports have illustrated the role of Fzo1 in mitochondrial fusion known in Saccharomyces cerevisiae. We demonstrate that the ubiquitin ligase Mahogunin Ring Finger-1 (MGRN1) interacts with and constitutively ubiquitinates the mammalian homolog, Mitofusin1 (Mfn1) via K63 linkages. In mice models, loss of Mgrn1 function leads to severe developmental defects and adult-onset spongiform neurodegeneration, similar to prion diseases. The tethering of mitochondria to form the ~180kDa Mfn1 complex is independent of MGRN1-mediated ubiquitination. However, successful mitochondrial fusion requires formation of higher oligomers of Mfn1 which in turn needs GTPase activity, intact heptad repeats of Mfn1 and ubiquitination by MGRN1. Following ubiquitination, proteasomal processing of Mfn1 completes the mitochondrial fusion process. This step requires functional p97 activity. These findings suggest a sequence of events where GTPase activity of Mfn1 and tethering of adjacent mitochondria precedes its MGRN1-mediated ubiquitination and proteasomal degradation culminating in mitochondrial fusion.

Invited review: Mechanisms of GTP hydrolysis and conformational transitions in the dynamin superfamily

Dynamin superfamily proteins are multidomain mechano-chemical GTPases which are implicated in nucleotide-dependent membrane remodeling events. A prominent feature of these proteins is their assembly- stimulated mechanism of GTP hydrolysis. The molecular basis for this reaction has been initially clarified for the dynamin-related guanylate binding protein 1 (GBP1) and involves the transient dimerization of the GTPase domains in a parallel head-to-head fashion. A catalytic arginine finger from the phosphate binding (P-) loop is repositioned toward the nucleotide of the same molecule to stabilize the transition state of GTP hydrolysis. Dynamin uses a related dimerization-dependent mechanism, but instead of the catalytic arginine, a monovalent cation is involved in catalysis. Still another variation of the GTP hydrolysis mechanism has been revealed for the dynamin-like Irga6 which bears a glycine at the corresponding position in the P-loop. Here, we highlight conserved and divergent features of GTP hydrolysis in dynamin superfamily proteins and show how nucleotide binding and hydrolysis are converted into mechano-chemical movements. We also describe models how the energy of GTP hydrolysis can be harnessed for diverse membrane remodeling events, such as membrane fission or fusion. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 580-593, 2016.

Effects of lipids on mitochondrial functions

Mitochondria contain two membranes: the outer and inner membrane. Whereas the outer membrane is particularly enriched in phospholipids, the inner membrane has an unusual high protein content and forms large invaginations termed cristae. The proper phospholipid composition of the membranes is crucial for mitochondrial functions. Phospholipids affect activity, biogenesis and stability of protein complexes including protein translocases and respiratory chain supercomplexes. Negatively charged phospholipids such as cardiolipin are important for the architecture of the membranes and recruit soluble factors to the membranes to support mitochondrial dynamics. Thus, phospholipids not only form the hydrophobic core of biological membranes that surround mitochondria, but also create a specific environment to promote functions of various protein machineries. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

OPA1 processing in cell death and disease - the long and short of it

The regulation of mitochondrial dynamics by the GTPase OPA1, which is located at the inner mitochondrial membrane, is crucial for adapting mitochondrial function and preserving cellular health. OPA1 governs the delicate balance between fusion and fission in the dynamic mitochondrial network. A disturbance of this balance, often observed under stress and pathologic conditions, causes mitochondrial fragmentation and can ultimately result in cell death. As discussed in this Commentary, these morphological changes are regulated by proteolytic processing of OPA1 by the inner-membrane peptidases YME1L (also known as YME1L1) and OMA1. Long, membrane-bound forms of OPA1 are required for mitochondrial fusion, but their processing to short, soluble forms limits fusion and can facilitate mitochondrial fission. Excessive OPA1 processing by the stress-activated protease OMA1 promotes mitochondrial fragmentation and, if persistent, triggers cell death and tissue degeneration in vivo The prevention of OMA1-mediated OPA1 processing and mitochondrial fragmentation might thus offer exciting therapeutic potential for human diseases associated with mitochondrial dysfunction.

Mito-Morphosis: Mitochondrial Fusion, Fission, and Cristae Remodeling as Key Mediators of Cellular Function

Permanent residency in the eukaryotic cell pressured the prokaryotic mitochondrial ancestor to strategize for intracellular living. Mitochondria are able to autonomously integrate and respond to cellular cues and demands by remodeling their morphology. These processes define mitochondrial dynamics and inextricably link the fate of the mitochondrion and that of the host eukaryote, as exemplified by the human diseases that result from mutations in mitochondrial dynamics proteins. In this review, we delineate the architecture of mitochondria and define the mechanisms by which they modify their shape. Key players in these mechanisms are discussed, along with their role in manipulating mitochondrial morphology during cellular action and development. Throughout, we highlight the evolutionary context in which mitochondrial dynamics emerged and consider unanswered questions whose dissection might lead to mitochondrial morphology-based therapies.

Proteasome Impairment Induces Recovery of Mitochondrial Membrane Potential and an Alternative Pathway of Mitochondrial Fusion

Mitochondria are vital and highly dynamic organelles that continuously fuse and divide to maintain mitochondrial quality. Mitochondrial dysfunction impairs cellular integrity and is known to be associated with various human diseases. However, the mechanism by which the quality of mitochondria is maintained remains largely unexplored. Here we show that impaired proteasome function recovers the growth of yeast cells lacking Fzo1, a pivotal protein for mitochondrial fusion. Decreased proteasome activity increased the mitochondrial oxidoreductase protein Mia40 and the ratio of the short isoform of mitochondrial intermembrane protein Mgm1 (s-Mgm1) to the long isoform (l-Mgm1). The increase in Mia40 restored mitochondrial membrane potential, while the increase in the s-Mgm1/l-Mgm1 ratio promoted mitochondrial fusion in an Fzo1-independent manner. Our findings demonstrate a new pathway for mitochondrial quality control that is induced by proteasome impairment.

Phosphatidylethanolamine Metabolism in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic cells. The existence of four only partially redundant biochemical pathways that produce PE, highlights the importance of this essential phospholipid. The CDP-ethanolamine and phosphatidylserine decarboxylase pathways occur in different subcellular compartments and are the main sources of PE in cells. Mammalian development fails upon ablation of either pathway. Once made, PE has diverse cellular functions that include serving as a precursor for phosphatidylcholine and a substrate for important posttranslational modifications, influencing membrane topology, and promoting cell and organelle membrane fusion, oxidative phosphorylation, mitochondrial biogenesis, and autophagy. The importance of PE metabolism in mammalian health has recently emerged following its association with Alzheimer's disease, Parkinson's disease, nonalcoholic liver disease, and the virulence of certain pathogenic organisms.

Specific degradation of phosphatidylglycerol is necessary for proper mitochondrial morphology and function

In yeast, phosphatidylglycerol (PG) is a minor phospholipid under standard conditions; it can be utilized for cardiolipin (CL) biosynthesis by CL synthase, Crd1p, or alternatively degraded by the phospholipase Pgc1p. The Saccharomyces cerevisiae deletion mutants crd1Δ and pgc1Δ both accumulate PG. Based on analyses of the phospholipid content of pgc1Δ and crd1Δ yeast, we revealed that in yeast mitochondria, two separate pools of PG are present, which differ in their fatty acid composition and accessibility for Pgc1p-catalyzed degradation. In contrast to CL-deficient crd1Δ yeast, the pgc1Δ mutant contains normal levels of CL. This makes the pgc1Δ strain a suitable model to study the effect of accumulation of PG per se. Using fluorescence microscopy, we show that accumulation of PG with normal levels of CL resulted in increased fragmentation of mitochondria, while in the absence of CL, accumulation of PG led to the formation of large mitochondrial sheets. We also show that pgc1Δ mitochondria exhibited increased respiration rates due to increased activity of cytochrome c oxidase. Taken together, our results indicate that not only a lack of anionic phospholipids, but also excess PG, or unbalanced ratios of anionic phospholipids in mitochondrial membranes, have harmful consequences on mitochondrial morphology and function.

Mitochondrial fusion/fission dynamics in neurodegeneration and neuronal plasticity

Mitochondria are dynamic organelles that continually move, fuse and divide. The dynamic balance of fusion and fission of mitochondria determines their morphology and allows their immediate adaptation to energetic needs, keeps mitochondria in good health by restoring or removing damaged organelles or precipitates cells in apoptosis in cases of severe defects. Mitochondrial fusion and fission are essential in mammals and their disturbances are associated with several diseases. However, while mitochondrial fusion/fission dynamics, and the proteins that control these processes, are ubiquitous, associated diseases are primarily neurological disorders. Accordingly, inactivation of the main actors of mitochondrial fusion/fission dynamics is associated with defects in neuronal development, plasticity and functioning, both ex vivo and in vivo. Here, we present the central actors of mitochondrial fusion and fission and review the role of mitochondrial dynamics in neuronal physiology and pathophysiology. Particular emphasis is placed on the three main actors of these processes i.e. DRP1,MFN1-2, and OPA1 as well as on GDAP1, a protein of the mitochondrial outer membrane preferentially expressed in neurons. This article is part of a Special Issue entitled: Mitochondria & Brain.

Validation of a MGM1/OPA1 chimeric gene for functional analysis in yeast of mutations associated with dominant optic atrophy

Mutations in OPA1 are associated with DOA or DOA plus. Novel mutations in OPA1 are periodically identified, but often the causative effect of the mutation is not demonstrated. A chimeric protein containing the N-terminal region of Mgm1, the yeast orthologue of OPA1, and the C-terminal region of OPA1 was constructed. This chimeric construct can be exploited to evaluate the pathogenicity of most of the missense mutations in OPA1 as well as to determine whether the dominance of the mutation is due to haploinsufficiency or to gain of function.

Mitochondrial Genome Maintenance 1 (Mgm1) Protein Alters Membrane Topology and Promotes Local Membrane Bending

Large GTPases of the dynamin superfamily promote membrane fusion and division, processes that are crucial for intracellular trafficking and organellar dynamics. To promote membrane scission, dynamin proteins polymerize, wrap around, and constrict the membrane; however, the mechanism underlying their role in membrane fusion remains unclear. We previously reported that the mitochondrial dynamin-related protein mitochondrial genome maintenance 1 (Mgm1) mediates fusion by first tethering opposing membranes and then undergoing a nucleotide-dependent structural transition. However, it is still unclear how Mgm1 directly affects the membrane to drive fusion of tethered membranes. Here, we show that Mgm1 association with the membrane alters the topography of the membrane, promoting local membrane bending. We also demonstrate that Mgm1 creates membrane ruffles resulting in the formation of tubular structures on both supported lipid bilayers and liposomes. These data suggest that Mgm1 membrane interactions impose a mechanical force on the membrane to overcome the hydrophilic repulsion of the phospholipid head groups and initiate the fusion reaction. The work reported here provides new insights into a possible mechanism of Mgm1-driven mitochondrial membrane fusion and sheds light into how members of the dynamin superfamily function as fusion molecules.

Mitophagy and mitochondrial dynamics in Saccharomyces cerevisiae

Mitochondria fulfill central cellular functions including energy metabolism, iron-sulfur biogenesis, and regulation of apoptosis and calcium homeostasis. Accumulation of dysfunctional mitochondria is observed in ageing and many human diseases such as cancer and various neurodegenerative disorders. Appropriate quality control of mitochondria is important for cell survival in most eukaryotic cells. One important pathway in this respect is mitophagy, a selective form of autophagy which removes excess and dysfunctional mitochondria. In the past decades a series of essential factors for mitophagy have been identified and characterized. However, little is known about the molecular mechanisms regulating mitophagy. The role of mitochondrial dynamics in mitophagy is controversially discussed. Here we will review recent advances in this context promoting our understanding on the molecular regulation of mitophagy in Saccharomyces cerevisiae and on the role of mitochondrial dynamics in mitochondrial quality control.

Mgr2 functions as lateral gatekeeper for preprotein sorting in the mitochondrial inner membrane

The majority of preproteins destined for mitochondria carry N-terminal presequences. The presequence translocase of the inner mitochondrial membrane (TIM23 complex) plays a central role in protein sorting. Preproteins are either translocated through the TIM23 complex into the matrix or are laterally released into the inner membrane. We report that the small hydrophobic protein Mgr2 controls the lateral release of preproteins. Mgr2 interacts with preproteins in transit through the TIM23 complex. Overexpression of Mgr2 delays preprotein release, whereas a lack of Mgr2 promotes preprotein sorting into the inner membrane. Preproteins with a defective inner membrane sorting signal are translocated into the matrix in wild-type mitochondria but are released into the inner membrane in Mgr2-deficient mitochondria. We conclude that Mgr2 functions as a lateral gatekeeper of the mitochondrial presequence translocase, providing quality control for the membrane sorting of preproteins.

From membranes to organelles: emerging roles for dynamin-like proteins in diverse cellular processes

Dynamin is a GTPase mechanoenzyme most noted for its role in vesicle scission during endocytosis, and belongs to the dynamin family proteins. The dynamin family consists of classical dynamins and dynamin-like proteins (DLPs). Due to structural and functional similarities DLPs are thought to carry out membrane tubulation and scission in a similar manner to dynamin. Here, we discuss the newly emerging roles for DLPs, which include vacuole fission and fusion, peroxisome maintenance, endocytosis and intracellular trafficking. Specific focus is given to the role of DLPs in the budding yeast Saccharomyces cerevisiae because the diverse function of DLPs has been well characterized in this organism. Recent insights into DLPs may provide a better understanding of mammalian dynamin and its associated diseases.

The i-AAA protease YME1L and OMA1 cleave OPA1 to balance mitochondrial fusion and fission

OPA1 processing by YEM1L and OMA1 is dispensable for mitochondrial fusion and instead drives mitochondrial fragmentation, which is crucial for mitochondrial integrity and quality control.

Mitochondrial fusion and structure depend on the dynamin-like GTPase OPA1, whose activity is regulated by proteolytic processing. Constitutive OPA1 cleavage by YME1L and OMA1 at two distinct sites leads to the accumulation of both long and short forms of OPA1 and maintains mitochondrial fusion. Stress-induced OPA1 processing by OMA1 converts OPA1 completely into short isoforms, inhibits fusion, and triggers mitochondrial fragmentation. Here, we have analyzed the function of different OPA1 forms in cells lacking YME1L, OMA1, or both. Unexpectedly, deletion of Oma1 restored mitochondrial tubulation, cristae morphogenesis, and apoptotic resistance in cells lacking YME1L. Long OPA1 forms were sufficient to mediate mitochondrial fusion in these cells. Expression of short OPA1 forms promoted mitochondrial fragmentation, which indicates that they are associated with fission. Consistently, GTPase-inactive, short OPA1 forms partially colocalize with ER–mitochondria contact sites and the mitochondrial fission machinery. Thus, OPA1 processing is dispensable for fusion but coordinates the dynamic behavior of mitochondria and is crucial for mitochondrial integrity and quality control.

The mitochondrial rhomboid protease: its rise from obscurity to the pinnacle of disease-relevant genes

The Rhomboid proteases belong to a highly conserved family of proteins that are present in all branches of life. In Drosophila, the secretory pathway-localized rhomboid proteases are crucial for epidermal growth factor (EGF) signaling. The identification of a mitochondrial-localized rhomboid protease shed light on other functions of rhomboid proteases including the maintenance of mitochondrial morphology and the regulation of apoptosis. More recent work has revealed other functions of the mitochondrial rhomboid protease in mitochondrial and cellular biology, failure of which have been implicated in human diseases. In this review, we will summarize the current knowledge and disease relevance of the mitochondrial-localized rhomboid protease. This article is part of a Special Issue entitled: Intramembrane Proteases.

Mitochondrial DNA mutations provoke dominant inhibition of mitochondrial inner membrane fusion

Mitochondria are highly dynamic organelles that continuously move, fuse and divide. Mitochondrial dynamics modulate overall mitochondrial morphology and are essential for the proper function, maintenance and transmission of mitochondria and mitochondrial DNA (mtDNA). We have investigated mitochondrial fusion in yeast cells with severe defects in oxidative phosphorylation (OXPHOS) due to removal or various specific mutations of mtDNA. We find that, under fermentative conditions, OXPHOS deficient cells maintain normal levels of cellular ATP and ADP but display a reduced mitochondrial inner membrane potential. We demonstrate that, despite metabolic compensation by glycolysis, OXPHOS defects are associated to a selective inhibition of inner but not outer membrane fusion. Fusion inhibition was dominant and hampered the fusion of mutant mitochondria with wild-type mitochondria. Inhibition of inner membrane fusion was not systematically associated to changes of mitochondrial distribution and morphology, nor to changes in the isoform pattern of Mgm1, the major fusion factor of the inner membrane. However, inhibition of inner membrane fusion correlated with specific alterations of mitochondrial ultrastructure, notably with the presence of aligned and unfused inner membranes that are connected to two mitochondrial boundaries. The fusion inhibition observed upon deletion of OXPHOS related genes or upon removal of the entire mtDNA was similar to that observed upon introduction of point mutations in the mitochondrial ATP6 gene that are associated to neurogenic ataxia and retinitis pigmentosa (NARP) or to maternally inherited Leigh Syndrome (MILS) in humans. Our findings indicate that the consequences of mtDNA mutations may not be limited to OXPHOS defects but may also include alterations in mitochondrial fusion. Our results further imply that, in healthy cells, the dominant inhibition of fusion could mediate the exclusion of OXPHOS-deficient mitochondria from the network of functional, fusogenic mitochondria.

Emergence of the mitochondrial reticulum from fission and fusion dynamics

Mitochondria form a dynamic tubular reticulum within eukaryotic cells. Currently, quantitative understanding of its morphological characteristics is largely absent, despite major progress in deciphering the molecular fission and fusion machineries shaping its structure. Here we address the principles of formation and the large-scale organization of the cell-wide network of mitochondria. On the basis of experimentally determined structural features we establish the tip-to-tip and tip-to-side fission and fusion events as dominant reactions in the motility of this organelle. Subsequently, we introduce a graph-based model of the chondriome able to encompass its inherent variability in a single framework. Using both mean-field deterministic and explicit stochastic mathematical methods we establish a relationship between the chondriome structural network characteristics and underlying kinetic rate parameters. The computational analysis indicates that mitochondrial networks exhibit a percolation threshold. Intrinsic morphological instability of the mitochondrial reticulum resulting from its vicinity to the percolation transition is proposed as a novel mechanism that can be utilized by cells for optimizing their functional competence via dynamic remodeling of the chondriome. The detailed size distribution of the network components predicted by the dynamic graph representation introduces a relationship between chondriome characteristics and cell function. It forms a basis for understanding the architecture of mitochondria as a cell-wide but inhomogeneous organelle. Analysis of the reticulum adaptive configuration offers a direct clarification for its impact on numerous physiological processes strongly dependent on mitochondrial dynamics and organization, such as efficiency of cellular metabolism, tissue differentiation and aging.

Phosphatidylserine decarboxylase 1 (Psd1) promotes mitochondrial fusion by regulating the biophysical properties of the mitochondrial membrane and alternative topogenesis of mitochondrial genome maintenance protein 1 (Mgm1)

BACKGROUND

Phosphatidylethanolamine is proposed to regulate mitochondrial fusion, but its mechanism of action is unknown.

RESULTS

Decreasing phosphatidylethanolamine reduces the rate of lipid mixing and the biogenesis of Mgm1, a mitochondrial fusion protein.

CONCLUSION

Psd1 regulates the lipid and protein machineries of mitochondrial fusion.

SIGNIFICANCE

Understanding how lipid metabolism regulates mitochondrial dynamics will reveal its role in cellular functions such as apoptosis and autophagy. Non-bilayer-forming lipids such as cardiolipin, phosphatidic acid, and phosphatidylethanolamine (PE) are proposed to generate negative membrane curvature, promoting membrane fusion. However, the mechanism by which lipids regulate mitochondrial fusion remains poorly understood. Here, we show that mitochondrial-localized Psd1, the key yeast enzyme that synthesizes PE, is required for proper mitochondrial morphology and fusion. Yeast cells lacking Psd1 exhibit fragmented and aggregated mitochondria with impaired mitochondrial fusion during mating. More importantly, we demonstrate that a reduction in PE reduces the rate of lipid mixing during fusion of liposomes with lipid compositions reflecting the mitochondrial membrane. This suggests that the mitochondrial fusion defect in the Δpsd1 strain could be due to the altered biophysical properties of the mitochondrial membrane, resulting in reduced fusion kinetics. The Δpsd1 strain also has impaired mitochondrial activity such as oxidative phosphorylation and reduced mitochondrial ATP levels which are due to a reduction in mitochondrial PE. The loss of Psd1 also impairs the biogenesis of s-Mgm1, a protein essential for mitochondrial fusion, further exacerbating the mitochondrial fusion defect of the Δpsd1 strain. Increasing s-Mgm1 levels in Δpsd1 cells markedly reduced mitochondrial aggregation. Our results demonstrate that mitochondrial PE regulates mitochondrial fusion by regulating the biophysical properties of the mitochondrial membrane and by enhancing the biogenesis of s-Mgm1. While several proteins are required to orchestrate the intricate process of membrane fusion, we propose that specific phospholipids of the mitochondrial membrane promote fusion by enhancing lipid mixing kinetics and by regulating the action of profusion proteins.

Membrane tethering and nucleotide-dependent conformational changes drive mitochondrial genome maintenance (Mgm1) protein-mediated membrane fusion

Cellular membrane remodeling events such as mitochondrial dynamics, vesicle budding, and cell division rely on the large GTPases of the dynamin superfamily. Dynamins have long been characterized as fission molecules; however, how they mediate membrane fusion is largely unknown. Here we have characterized by cryo-electron microscopy and in vitro liposome fusion assays how the mitochondrial dynamin Mgm1 may mediate membrane fusion. Using cryo-EM, we first demonstrate that the Mgm1 complex is able to tether opposing membranes to a gap of ∼15 nm, the size of mitochondrial cristae folds. We further show that the Mgm1 oligomer undergoes a dramatic GTP-dependent conformational change suggesting that s-Mgm1 interactions could overcome repelling forces at fusion sites and that ultrastructural changes could promote the fusion of opposing membranes. Together our findings provide mechanistic details of the two known in vivo functions of Mgm1, membrane fusion and cristae maintenance, and more generally shed light onto how dynamins may function as fusion proteins.

Mechanistic perspective of mitochondrial fusion: tubulation vs. fragmentation

Mitochondrial fusion is a fundamental process driven by dynamin related GTPase proteins (DRPs), in contrast to the general SNARE-dependence of most cellular fusion events. The DRPs Mfn1/Mfn2/Fzo1 and OPA1/Mgm1 are the key effectors for fusion of the mitochondrial outer and inner membranes, respectively. In order to promote fusion, these two DRPs require post-translational modifications and proteolysis. OPA1/Mgm1 undergoes partial proteolytic processing, which results in a combination between short and long isoforms. In turn, ubiquitylation of mitofusins, after oligomerization and GTP hydrolysis, promotes and positively regulates mitochondrial fusion. In contrast, under conditions of mitochondrial dysfunction, negative regulation by proteolysis on these DRPs results in mitochondrial fragmentation. This occurs by complete processing of OPA1 and via ubiquitylation and degradation of mitofusins. Mitochondrial fragmentation contributes to the elimination of damaged mitochondria by mitophagy, and may play a protective role against Parkinson's disease. Moreover, a link of Mfn2 to Alzheimer's disease is emerging and mutations in Mfn2 or OPA1 cause Charcot-Marie-Tooth type 2A neuropathy or autosomal-dominant optic atrophy. Here, we summarize our current understanding on the molecular mechanisms promoting or inhibiting fusion of mitochondrial membranes, which is essential for cellular survival and disease control. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.