A novel mitochondrial outer membrane protein, MOMA-1, that affects cristae morphology in Caenorhabditis elegans

Molecular machineries shaping the mitochondrial inner membrane

Mitochondria display intricately shaped deep invaginations of the mitochondrial inner membrane (MIM) termed cristae. This peculiar membrane architecture is essential for diverse mitochondrial functions, such as oxidative phosphorylation or the biosynthesis of cellular building blocks. Conserved protein nano-machineries such as F1Fo-ATP synthase oligomers and the mitochondrial contact site and cristae organizing system (MICOS) act as adaptable protein-lipid scaffolds controlling MIM biogenesis and its dynamic remodelling. Signal-dependent rearrangements of cristae architecture and MIM fusion events are governed by the dynamin-like GTPase optic atrophy 1 (OPA1). Recent groundbreaking structural insights into these nano-machineries have considerably advanced our understanding of the functional architecture of mitochondria. In this Review, we discuss how the MIM-shaping machineries cooperate to control cristae and crista junction dynamics, including MIM fusion, in response to cellular signalling pathways. We also explore how mutations affecting MIM-shaping machineries compromise mitochondrial functions.

The mechanisms and roles of mitochondrial dynamics in C. elegans

If mitochondria are the powerhouses of the cell, then mitochondrial dynamics are the power grid that regulates how that energy output is directed and maintained in response to unique physiological demands. Fission and fusion dynamics are highly regulated processes that fine-tune the mitochondrial networks of cells to enable appropriate responses to intrinsic and extrinsic stimuli, thereby maintaining cellular and organismal homeostasis. These dynamics shape many aspects of an organism's healthspan including development, longevity, stress resistance, immunity, and response to disease. In this review, we discuss the latest findings regarding the mechanisms and roles of mitochondrial dynamics by focussing on the nematode Caenorhabditis elegans. Whole live-animal studies in C. elegans have enabled a true organismal-level understanding of the impact that mitochondrial dynamics play in homeostasis over a lifetime.

Distinct designer diamines promote mitophagy, and thereby enhance healthspan in C. elegans and protect human cells against oxidative damage

Impaired mitophagy is a primary pathogenic event underlying diverse aging-associated diseases such as Alzheimer and Parkinson diseases and sarcopenia. Therefore, augmentation of mitophagy, the process by which defective mitochondria are removed, then replaced by new ones, is an emerging strategy for preventing the evolvement of multiple morbidities in the elderly population. Based on the scaffold of spermidine (Spd), a known mitophagy-promoting agent, we designed and tested a family of structurally related compounds. A prototypic member, 1,8-diaminooctane (VL-004), exceeds Spd in its ability to induce mitophagy and protect against oxidative stress. VL-004 activity is mediated by canonical aging genes and promotes lifespan and healthspan in C. elegans. Moreover, it enhances mitophagy and protects against oxidative injury in rodent and human cells. Initial structural characterization suggests simple rules for the design of compounds with improved bioactivity, opening the way for a new generation of agents with a potential to promote healthy aging.

Exercise preserves physical fitness during aging through AMPK and mitochondrial dynamics

Exercise is a nonpharmacological intervention that improves health during aging and a valuable tool in the diagnostics of aging-related diseases. In muscle, exercise transiently alters mitochondrial functionality and metabolism. Mitochondrial fission and fusion are critical effectors of mitochondrial plasticity, which allows a fine-tuned regulation of organelle connectiveness, size, and function. Here we have investigated the role of mitochondrial dynamics during exercise in the model organism Caenorhabditis elegans. We show that in body-wall muscle, a single exercise session induces a cycle of mitochondrial fragmentation followed by fusion after a recovery period, and that daily exercise sessions delay the mitochondrial fragmentation and physical fitness decline that occur with aging. Maintenance of proper mitochondrial dynamics is essential for physical fitness, its enhancement by exercise training, and exercise-induced remodeling of the proteome. Surprisingly, among the long-lived genotypes we analyzed (isp-1,nuo-6, daf-2, eat-2, and CA-AAK-2), constitutive activation of AMP-activated protein kinase (AMPK) uniquely preserves physical fitness during aging, a benefit that is abolished by impairment of mitochondrial fission or fusion. AMPK is also required for physical fitness to be enhanced by exercise, with our findings together suggesting that exercise may enhance muscle function through AMPK regulation of mitochondrial dynamics. Our results indicate that mitochondrial connectivity and the mitochondrial dynamics cycle are essential for maintaining physical fitness and exercise responsiveness during aging and suggest that AMPK activation may recapitulate some exercise benefits. Targeting mechanisms to optimize mitochondrial fission and fusion, as well as AMPK activation, may represent promising strategies for promoting muscle function during aging.

Novel putative interactors of FZO-1/mitofusin 2 identified using large-scale yeast two-hybrid screening in C. elegans

Mitochondria are energy-converting organelles that shift between fusion and fission states in order to perform a variety of essential functions. Disruption of these dynamics is detrimental to cellular health and is associated with a range of human diseases. Mitofusin 2 is an essential large GTPase protein that orchestrates fusion of outer mitochondria membranes, and mutations in the encoding gene are causative for Charcot-Marie-Tooth disease. In order to gain further insights into the function of this crucial protein, we have performed large-scale yeast two-hybrid screening to identify interactors of the orthologous protein in Caenorhabditis elegans (FZO-1) . From this screening, we identified 12 novel interactors of FZO-1/mitofusin 2 that, based on their known functions, are strong candidates for further study.

Mitochonic Acid 5 Improves Duchenne Muscular Dystrophy and Parkinson's Disease Model of Caenorhabditis elegans

Mitochonic Acid 5 (MA-5) enhances mitochondrial ATP production, restores fibroblasts from mitochondrial disease patients and extends the lifespan of the disease model "Mitomouse". Additionally, MA-5 interacts with mitofilin and modulates the mitochondrial inner membrane organizing system (MINOS) in mammalian cultured cells. Here, we used the nematode Caenorhabditis elegans to investigate whether MA-5 improves the Duchenne muscular dystrophy (DMD) model. Firstly, we confirmed the efficient penetration of MA-5 in the mitochondria of C. elegans. MA-5 also alleviated symptoms such as movement decline, muscular tone, mitochondrial fragmentation and Ca2+ accumulation of the DMD model. To assess the effect of MA-5 on mitochondria perturbation, we employed a low concentration of rotenone with or without MA-5. MA-5 significantly suppressed rotenone-induced mitochondria reactive oxygen species (ROS) increase, mitochondrial network fragmentation and nuclear destruction in body wall muscles as well as endogenous ATP levels decline. In addition, MA-5 suppressed rotenone-induced degeneration of dopaminergic cephalic (CEP) neurons seen in the Parkinson's disease (PD) model. Furthermore, the application of MA-5 reduced mitochondrial swelling due to the immt-1 null mutation. These results indicate that MA-5 has broad mitochondrial homing and MINOS stabilizing activity in metazoans and may be a therapeutic agent for these by ameliorating mitochondrial dysfunction in DMD and PD.

Protocol for engineering and validating a synthetic mitochondrial intermembrane bridge in mammalian cells

Membrane contact sites are recognized as critical means of intercompartmental communication. Here, we describe a protocol for engineering and validating a synthetic bridge between the inner and outer mitochondrial membranes to support functioning of the endogenous mitochondrial contact site and cristae organizing system (MICOS). A chimeric protein, MitoT, is stably expressed in cultured mammalian cells to bridge the mitochondrial membranes. This approach can be a valuable tool to study the function of the MICOS complex and associated proteins. For complete details on the use and execution of this protocol, please refer to Viana et al. (2021).

SGPL1 stimulates VPS39 recruitment to the mitochondria in MICU1 deficient cells

Objective

Mitochondrial “retrograde” signaling may stimulate organelle biogenesis as a compensatory adaptation to aberrant activity of the oxidative phosphorylation (OXPHOS) system. To maintain energy-consuming processes in OXPHOS deficient cells, alternative metabolic pathways are functionally coupled to the degradation, recycling and redistribution of biomolecules across distinct intracellular compartments. While transcriptional regulation of mitochondrial network expansion has been the focus of many studies, the molecular mechanisms promoting mitochondrial maintenance in energy-deprived cells remain poorly investigated.

Methods

We performed transcriptomics, quantitative proteomics and lifespan assays to identify pathways that are mechanistically linked to mitochondrial network expansion and homeostasis in Caenorhabditis elegans lacking the mitochondrial calcium uptake protein 1 (MICU-1/MICU1). To support our findings, we carried out biochemical and image analyses in mammalian cells and mouse-derived tissues.

Results

We report that micu-1(null) mutations impair the OXPHOS system and promote C. elegans longevity through a transcriptional program that is independent of the mitochondrial calcium uniporter MCU-1/MCU and the essential MCU regulator EMRE-1/EMRE. We identify sphingosine phosphate lyase SPL-1/SGPL1 and the ATFS-1-target HOPS complex subunit VPS-39/VPS39 as critical lifespan modulators of micu-1(null) mutant animals. Cross-species investigation indicates that SGPL1 upregulation stimulates VPS39 recruitment to the mitochondria, thereby enhancing mitochondria-lysosome contacts. Consistently, VPS39 downregulation compromises mitochondrial network maintenance and basal autophagic flux in MICU1 deficient cells. In mouse-derived muscles, we show that VPS39 recruitment to the mitochondria may represent a common signature associated with altered OXPHOS system.

Conclusions

Our findings reveal a previously unrecognized SGPL1/VPS39 axis that stimulates intracellular organelle interactions and sustains autophagy and mitochondrial homeostasis in OXPHOS deficient cells.

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micu-1(null) nematodes are long-lived mitochondrial mutants.

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MICU-1/MICU1 deficiency stimulates VPS-39/VPS39 and SPL-1/SGPL1 upregulation.

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VPS-39 sustains mitochondrial network expansion in micu-1(null) nematodes.

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VPS39 and SGPL1 expression influences mitochondria-lysosome contact sites in MICU1 deficient cells.

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VPS39/SGPL1 signaling may be a common signature of mitochondrial deficient cells.

A feedback loop engaging propionate catabolism intermediates controls mitochondrial morphology

D-2-Hydroxyglutarate (D-2HG) is an α-ketoglutarate-derived mitochondrial metabolite that causes D-2-hydroxyglutaric aciduria, a devastating developmental disorder. How D-2HG adversely affects mitochondria is largely unknown. Here, we report that in Caenorhabditis elegans, loss of the D-2HG dehydrogenase DHGD-1 causes D-2HG accumulation and mitochondrial damage. The excess D-2HG leads to a build-up of 3-hydroxypropionate (3-HP), a toxic metabolite in mitochondrial propionate oxidation, by inhibiting the 3-HP dehydrogenase HPHD-1. We demonstrate that 3-HP binds the MICOS subunit MIC60 (encoded by immt-1) and inhibits its membrane-binding and membrane-shaping activities. We further reveal that dietary and gut bacteria affect mitochondrial health by modulating the host production of 3-HP. These findings identify a feedback loop that links the toxic effects of D-2HG and 3-HP on mitochondria, thus providing important mechanistic insights into human diseases related to D-2HG and 3-HP.

Mitochondrial Dysfunction in Rabies Virus-Infected Human and Canine Brains

Rabies is a fatal encephalitis caused by the Rabies lyssavirus (RABV). The presence of minimal neuropathological changes observed in rabies indicates that neuronal dysfunction, rather than neuronal death contributes to the fatal outcome. The role of mitochondrial changes has been suggested as a possible mechanism for neuronal dysfunction in rabies. However, these findings are mostly based on studies that have employed experimental models and laboratory-adapted virus. Studies on brain tissues from naturally infected human and animal hosts are lacking. The current study investigated the role of mitochondrial changes in rabies by morphological, biochemical and proteomic analysis of RABV-infected human and canine brains. Morphological analysis showed minimal inflammation with preserved neuronal and disrupted mitochondrial structure in both human and canine brains. Proteomic analysis revealed involvement of mitochondrial processes (oxidative phosphorylation, cristae formation, homeostasis and transport), synaptic proteins and autophagic pathways, with over-expression of subunits of mitochondrial respiratory complexes. Consistent with these findings, human and canine brains displayed elevated activities of complexes I (p < 0.05), IV (p < 0.05) and V (p < 0.05). However, this did not result in elevated ATP production (p < 0.0001), probably due to lowered mitochondrial membrane potential as noted in RABV-infected cells in culture. These could lead to mitochondrial dysfunction and mitophagy as indicated by expression of FKBP8 (p < 0.05) and PINK1 (p < 0.001)/PARKIN (p > 0.05) and ensuing autophagy, as shown by the status of LCIII (p < 0.05), LAMP1 (p < 0.001) and pertinent ultrastructural markers. We propose that altered mitochondrial bioenergetics and cristae architecture probably induce mitophagy, leading to autophagy and consequent neuronal dysfunction in rabies.

Maintenance of complex I and its supercomplexes by NDUF-11 is essential for mitochondrial structure, function and health

Mitochondrial supercomplexes form around a conserved core of monomeric complex I and dimeric complex III; wherein a subunit of the former, NDUFA11, is conspicuously situated at the interface. We identified nduf-11 (B0491.5) as encoding the Caenorhabditis elegans homologue of NDUFA11. Animals homozygous for a CRISPR-Cas9-generated knockout allele of nduf-11 arrested at the second larval (L2) development stage. Reducing (but not eliminating) expression using RNAi allowed development to adulthood, enabling characterisation of the consequences: destabilisation of complex I and its supercomplexes and perturbation of respiratory function. The loss of NADH dehydrogenase activity was compensated by enhanced complex II activity, with the potential for detrimental reactive oxygen species (ROS) production. Cryo-electron tomography highlighted aberrant morphology of cristae and widening of both cristae junctions and the intermembrane space. The requirement of NDUF-11 for balanced respiration, mitochondrial morphology and development presumably arises due to its involvement in complex I and supercomplex maintenance. This highlights the importance of respiratory complex integrity for health and the potential for its perturbation to cause mitochondrial disease. This article has an associated First Person interview with Amber Knapp-Wilson, joint first author of the paper.

Miro proteins connect mitochondrial function and intercellular transport

Mitochondria are organelles present in most eukaryotic cells, where they play major and multifaceted roles. The classical notion of the main mitochondrial function as the powerhouse of the cell per se has been complemented by recent discoveries pointing to mitochondria as organelles affecting a number of other auxiliary processes. They go beyond the classical energy provision via acting as a relay point of many catabolic and anabolic processes, to signaling pathways critically affecting cell growth by their implication in de novo pyrimidine synthesis. These additional roles further underscore the importance of mitochondrial homeostasis in various tissues, where its deregulation promotes a number of pathologies. While it has long been known that mitochondria can move within a cell to sites where they are needed, recent research has uncovered that mitochondria can also move between cells. While this intriguing field of research is only emerging, it is clear that mobilization of mitochondria requires a complex apparatus that critically involves mitochondrial proteins of the Miro family, whose role goes beyond the mitochondrial transfer, as will be covered in this review.

Effects of Aging on Expression of Mic60 and OPA1 and Mitochondrial Morphology in Myocardium of Tibetan Sheep

Simple Summary

Mitochondria play a crucial role in the adaptation to high altitude hypoxia environment in Tibetan sheep, and the changes of its morphology and structure directly affect its function. OPA1 and Mic60 are important mitochondria-shaping proteins that work together to regulate the morphology of mitochondrial inner membrane and cristae. It has been shown that aging affects the expression of OPA1 and Mic60 in mice, but it has not been investigated in sheep and hence it is not known whether it might affect the ultrastructure of mitochondria. In this context, reverse transcription-quantitative PCR (RT-qPCR), enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry method were used to measure the expression of Mic60 and OPA1 genes and proteins in myocardium of adult and aged Tibetan sheep, and the ultrastructure of mitochondria were compared by transmission electron microscope. The results suggest that aging can reduces the expression of Mic60 and OPA1 genes and OPA1 protein, which can affect the mitochondrial function.

Abstract

In order to investigate the effects of aging on the expression of Mic60 and OPA1 and mitochondrial morphology in plateau animals, the expression of Mic60 and OPA1 genes and proteins, and the morphology of mitochondria in the myocardium of adult and aged Tibetan sheep were investigated. The expression of Mic60 and OPA1 genes and OPA1 protein were higher (p < 0.05) in the myocardium of adult Tibetan sheep than in those of the aged ones. The number of mitochondrial cristae in the myocardium of adult was higher than that in aged (p < 0.05). The density of mitochondria in the myocardium of adult was higher than that in aged (p < 0.01). Compared with the adult Tibetan sheep, the mitochondrial crista of aged were relatively sparse, the crista membrane was wide, and the mitochondria were not closely linked, showing fragmentation. These results suggest that the myocardial mitochondria of the adult have better energy supply ability, indicating that aging can lead to the weakening of oxygen supply in the myocardial mitochondria of Tibetan sheep.

Shaping the mitochondrial inner membrane in health and disease

Mitochondria play central roles in cellular energetics, metabolism and signalling. Efficient respiration, mitochondrial quality control, apoptosis and inheritance of mitochondrial DNA depend on the proper architecture of the mitochondrial membranes and a dynamic remodelling of inner membrane cristae. Defects in mitochondrial architecture can result in severe human diseases affecting predominantly the nervous system and the heart. Inner membrane morphology is generated and maintained in particular by the mitochondrial contact site and cristae organizing system (MICOS), the F₁ F_o -ATP synthase, the fusion protein OPA1/Mgm1 and the nonbilayer-forming phospholipids cardiolipin and phosphatidylethanolamine. These protein complexes and phospholipids are embedded in a network of functional interactions. They communicate with each other and additional factors, enabling them to balance different aspects of cristae biogenesis and to dynamically remodel the inner mitochondrial membrane. Genetic alterations disturbing these membrane-shaping factors can lead to human pathologies including fatal encephalopathy, dominant optic atrophy, Leigh syndrome, Parkinson's disease and Barth syndrome.

The MICOS complex, a structural element of mitochondria with versatile functions

Mitochondria perform a plethora of functions in various cells of different tissues. Their architecture differs remarkably, for instance in neurons versus steroidogenic cells. Furthermore, aberrant mitochondrial architecture results in mitochondrial dysfunction. This indicates strongly that mitochondrial architecture and function are intimately linked. Therefore, a deep knowledge about the determinants of mitochondrial architecture and their function on a molecular level is of utmost importance. In the past decades, various proteins and protein complexes essential for formation of mitochondrial architecture have been identified. Here we will review the current knowledge of the MICOS complex, one of the major structural elements of mitochondria. MICOS is a multi-subunit complex present in the inner mitochondrial membrane. Multiple interaction partners in the inner and outer mitochondrial membrane point to participation in a multitude of important processes, such as generation of mitochondrial architecture, lipid metabolism, and protein import into mitochondria. Since the MICOS complex is highly conserved in form and function throughout evolution, we will highlight the importance of MICOS for mammals. We will emphasize in particular the current knowledge of the association of MICOS with severe human diseases, including Charcot–Marie–Tooth disease type 2, Alzheimer's disease, Parkinson's disease, Frontotemporal Dementia and Amyotrophic Lateral Sclerosis.

Linking mitochondrial dynamics, cristae remodeling and supercomplex formation: How mitochondrial structure can regulate bioenergetics

The dynamic and fluid nature of mitochondria allows for modifications in mitochondrial shape, connectivity and cristae architecture. The precise balance of mitochondrial dynamics is among the most critical features in the control of mitochondrial function. In the past few years, mitochondrial shape has emerged as a key regulatory factor in the determination of the bioenergetic capacity of cells. This is mostly due to the recent discoveries linking changes in cristae organization with supercomplex assembly of the electron transport chain (ETC), also defined as the formation of respirosomes. Here we will review the most current advances demonstrating the impact of mitochondrial dynamics and cristae shape on oxidative metabolism, respiratory efficiency, and redox state. Furthermore, we will discuss the implications of mitochondrial dynamics and supercomplex assembly under physiological and pathological conditions.

Mitochondrial Subtype Identification and Characterization

Healthy, functional mitochondria are central to many cellular and physiological phenomena, including aging, metabolism, and stress resistance. A key feature of healthy mitochondria is a high membrane potential (Δψ) or charge differential (i.e., proton gradient) between the matrix and inner mitochondrial membrane. Mitochondrial Δψ has been extensively characterized via flow cytometry of intact cells, which measures the average membrane potential within a cell. However, the characteristics of individual mitochondria differ dramatically even within a single cell, and thus interrogation of mitochondrial features at the organelle level is necessary to better understand and accurately measure heterogeneity. Here we describe a new flow cytometric methodology that enables the quantification and classification of mitochondrial subtypes (via their Δψ, size, and substructure) using the small animal model C. elegans. Future application of this methodology should allow research to discern the bioenergetic and mitochondrial component in a number of human disease and aging models, including, C. elegans, cultured cells, small animal models, and human biopsy samples. © 2018 by John Wiley & Sons, Inc.

Cell Biology of the Mitochondrion

Mitochondria are best known for harboring pathways involved in ATP synthesis through the tricarboxylic acid cycle and oxidative phosphorylation. Major advances in understanding these roles were made with Caenorhabditiselegans mutants affecting key components of the metabolic pathways. These mutants have not only helped elucidate some of the intricacies of metabolism pathways, but they have also served as jumping off points for pharmacology, toxicology, and aging studies. The field of mitochondria research has also undergone a renaissance, with the increased appreciation of the role of mitochondria in cell processes other than energy production. Here, we focus on discoveries that were made using C. elegans, with a few excursions into areas that were studied more thoroughly in other organisms, like mitochondrial protein import in yeast. Advances in mitochondrial biogenesis and membrane dynamics were made through the discoveries of novel functions in mitochondrial fission and fusion proteins. Some of these functions were only apparent through the use of diverse model systems, such as C. elegans Studies of stress responses, exemplified by mitophagy and the mitochondrial unfolded protein response, have also benefitted greatly from the use of model organisms. Recent developments include the discoveries in C. elegans of cell autonomous and nonautonomous pathways controlling the mitochondrial unfolded protein response, as well as mechanisms for degradation of paternal mitochondria after fertilization. The evolutionary conservation of many, if not all, of these pathways ensures that results obtained with C. elegans are equally applicable to studies of human mitochondria in health and disease.

Integrative functions of the mitochondrial contact site and cristae organizing system

Mitochondria are complex double-membrane-bound organelles of eukaryotic cells that function as energy-converting powerhouses, metabolic factories and signaling centers. The outer membrane controls the exchange of material and information with other cellular compartments. The inner membrane provides an extended, highly folded surface for selective transport and energy-coupling reactions. It can be divided into an inner boundary membrane and tubular or lamellar cristae membranes that accommodate the oxidative phosphorylation units. Outer membrane, inner boundary membrane and cristae come together at crista junctions, where the mitochondrial contact site and cristae organizing system (MICOS) acts as a membrane-shaping and -connecting scaffold. This peculiar architecture is of pivotal importance for multiple mitochondrial functions. Many elaborate studies in the past years have shed light on the subunit composition and organization of MICOS. In this review article, we summarize these insights and then move on to discuss exciting recent discoveries on the integrative functions of MICOS. Multi-faceted connections to other major players of mitochondrial biogenesis and physiology, like the protein import machineries, the oxidative phosphorylation system, carrier proteins and phospholipid biosynthesis enzymes, are currently emerging. Therefore, we propose that MICOS acts as a central hub in mitochondrial membrane architecture and functionality.

Regulated membrane remodeling by Mic60 controls formation of mitochondrial crista junctions

The mitochondrial contact site and cristae organizing system (MICOS) is crucial for the formation of crista junctions and mitochondrial inner membrane architecture. MICOS contains two core components. Mic10 shows membrane-bending activity, whereas Mic60 (mitofilin) forms contact sites between inner and outer membranes. Here we report that Mic60 deforms liposomes into thin membrane tubules and thus displays membrane-shaping activity. We identify a membrane-binding site in the soluble intermembrane space-exposed part of Mic60. This membrane-binding site is formed by a predicted amphipathic helix between the conserved coiled-coil and mitofilin domains. The mitofilin domain negatively regulates the membrane-shaping activity of Mic60. Binding of Mic19 to the mitofilin domain modulates this activity. Membrane binding and shaping by the conserved Mic60–Mic19 complex is crucial for crista junction formation, mitochondrial membrane architecture and efficient respiratory activity. Mic60 thus plays a dual role by shaping inner membrane crista junctions and forming contact sites with the outer membrane.

The MICOS complex has an essential role in crista junction formation and mitochondrial inner membrane morphology. Here, the authors show that one of its components, Mic60, known to form contact sites between inner and outer membranes, also displays membrane-shaping activity.

Mitochondrial contact site and cristae organizing system: A central player in membrane shaping and crosstalk

Mitochondria are multifunctional metabolic factories and integrative signaling organelles of eukaryotic cells. The structural basis for their numerous functions is a complex and dynamic double-membrane architecture. The outer membrane connects mitochondria to the cytosol and other organelles. The inner membrane is composed of a boundary region and highly folded cristae membranes. The evolutionarily conserved mitochondrial contact site and cristae organizing system (MICOS) connects the two inner membrane domains via formation and stabilization of crista junction structures. Moreover, MICOS establishes contact sites between inner and outer mitochondrial membranes by interacting with outer membrane protein complexes. MICOS deficiency leads to a grossly altered inner membrane architecture resulting in far-reaching functional perturbations in mitochondria. Consequently, mutations affecting the function of MICOS are responsible for a diverse spectrum of human diseases. In this article, we summarize recent insights and concepts on the role of MICOS in the organization of mitochondrial membranes. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

A Select Subset of Electron Transport Chain Genes Associated with Optic Atrophy Link Mitochondria to Axon Regeneration in Caenorhabditis elegans

The role of mitochondria within injured neurons is an area of active interest since these organelles are vital for the production of cellular energy in the form of ATP. Using mechanosensory neurons of the nematode Caenorhabditis elegans to test regeneration after neuronal injury in vivo, we surveyed genes related to mitochondrial function for effects on axon regrowth after laser axotomy. Genes involved in mitochondrial transport, calcium uptake, mitophagy, or fission and fusion were largely dispensable for axon regrowth, with the exception of eat-3/Opa1. Surprisingly, many genes encoding components of the electron transport chain were dispensable for regrowth, except for the iron-sulfur proteins gas-1, nduf-2.2, nduf-7, and isp-1, and the putative oxidoreductase rad-8. In these mutants, axonal development was essentially normal and axons responded normally to injury by forming regenerative growth cones, but were impaired in subsequent axon extension. Overexpression of nduf-2.2 or isp-1 was sufficient to enhance regrowth, suggesting that mitochondrial function is rate-limiting in axon regeneration. Moreover, loss of function in isp-1 reduced the enhanced regeneration caused by either a gain-of-function mutation in the calcium channel EGL-19 or overexpression of the MAP kinase DLK-1. While the cellular function of RAD-8 remains unclear, our genetic analyses place rad-8 in the same pathway as other electron transport genes in axon regeneration. Unexpectedly, rad-8 regrowth defects were suppressed by altered function in the ubiquinone biosynthesis gene clk-1. Furthermore, we found that inhibition of the mitochondrial unfolded protein response via deletion of atfs-1 suppressed the defective regrowth in nduf-2.2 mutants. Together, our data indicate that while axon regeneration is not significantly affected by general dysfunction of cellular respiration, it is sensitive to the proper functioning of a select subset of electron transport chain genes, or to the cellular adaptations used by neurons under conditions of injury.

New Imaging Tools to Analyze Mitochondrial Morphology in Caenorhabditis elegans

Mitochondria are highly dynamic organelles that constantly fuse and divide. This process is essential as several neurodegenerative diseases have been associated with defects in mitochondrial fusion or fission. Several tools have been developed over the years to visualize mitochondria in organisms such as Caenorhabditis elegans. Combining these tools with the powerful genetics of C. elegans has led to the discovery of new regulators of mitochondrial morphology. In this chapter, we present additional tools to further characterize mitochondrial morphology as well as regulators of mitochondrial morphology. Specifically, we introduce a photoactivatable mitoGFP (PAmitoGFP) that allows to investigate the connectivity of complex mitochondrial networks. In addition, we describe an immunostaining protocol that enables localization studies of these newly identified regulators of mitochondrial morphology.

High-Density Lipoprotein Regulation of Mitochondrial Function

Lipoproteins play a key role in regulating plasma and tissue levels of cholesterol. Apolipoprotein B (apoB)-containing lipoproteins, including chylomicrons, very-low density lipoprotein (VLDL) and low-density lipoprotein (LDL), serve as carriers of triglycerides and cholesterol and deliver these metabolites to peripheral tissues. In contrast, high-density lipoprotein (HDL) mediates Reverse Cholesterol Transport (RCT), a process by which excess cholesterol is removed from the periphery and taken up by hepatocytes where it is metabolized and excreted. Anti-atherogenic properties of HDL have been largely ascribed to apoA-I, the major protein component of the lipoprotein particle. The inflammatory response associated with atherosclerosis and ischemia-reperfusion (I-R) injury has been linked to the development of mitochondrial dysfunction. Under these conditions, an increase in reactive oxygen species (ROS) formation induces damage to mitochondrial structural elements, leading to a reduction in ATP synthesis and initiation of the apoptotic program. Recent studies suggest that HDL-associated apoA-I and lysosphingolipids attenuate mitochondrial injury by multiple mechanisms, including the suppression of ROS formation and induction of autophagy. Other apolipoproteins, however, present in lower abundance in HDL particles may exert opposing effects on mitochondrial function. This chapter examines the role of HDL-associated apolipoproteins and lipids in the regulation of mitochondrial function and bioenergetics.

Developmentally programmed germ cell remodelling by endodermal cell cannibalism

Primordial germ cells (PGCs) in many species associate intimately with endodermal cells, but the significance of such interactions is largely unexplored. Here, we show that Caenorhabditis elegans PGCs form lobes that are removed and digested by endodermal cells, dramatically altering PGC size and mitochondrial content. We demonstrate that endodermal cells do not scavenge lobes PGCs shed, but rather, actively remove lobes from the cell body. CED-10 (Rac)-induced actin, DYN-1 (dynamin) and LST-4 (SNX9) transiently surround lobe necks and are required within endodermal cells for lobe scission, suggesting that scission occurs through a mechanism resembling vesicle endocytosis. These findings reveal an unexpected role for endoderm in altering the contents of embryonic PGCs, and define a form of developmentally programmed cell remodelling involving intercellular cannibalism. Active roles for engulfing cells have been proposed in several neuronal remodelling events, suggesting that intercellular cannibalism may be a more widespread method used to shape cells than previously thought.

Identification and Characterization of Mitochondrial Subtypes in Caenorhabditis elegans via Analysis of Individual Mitochondria by Flow Cytometry

Mitochondrial bioenergetics has been implicated in a number of vital cellular and physiological phenomena, including aging, metabolism, and stress resistance. Heterogeneity of the mitochondrial membrane potential (Δψ), which is central to organismal bioenergetics, has been successfully measured via flow cytometry in whole cells but rarely in isolated mitochondria from large animal models. Similar studies in small animal models, such as Caenorhabditis elegans (C. elegans), are critical to our understanding of human health and disease but lack analytical methodologies. Here we report on new methodological developments that make it possible to investigate the heterogeneity of Δψ in C. elegans during development and in tissue-specific studies. The flow cytometry methodology described here required an improved collagenase-3-based mitochondrial isolation procedure and labeling of mitochondria with the ratiometric fluorescent probe JC-9. To demonstrate feasibility of tissue-specific studies, we used C. elegans strains expressing blue-fluorescent muscle-specific proteins, which enabled identification of muscle mitochondria among mitochondria from other tissues. This methodology made it possible to observe, for the first time, critical changes in Δψ during C. elegans larval development and provided direct evidence of the elevated bioenergetic status of muscle mitochondria relative to their counterparts in the rest of the organism. Further application of these methodologies can help tease apart bioenergetics and other biological complexities in C. elegans and other small animal models used to investigate human disease and aging.

The MICOS complex of human mitochondria

Mitochondria are organelles of endosymbiotic origin, surrounded by two membranes. The inner membrane forms invaginations called cristae that enhance its surface and are important for mitochondrial function. A recently described mitochondrial contact site and cristae organizing system (MICOS) in the inner mitochondrial membrane is crucial for the formation and maintenance of cristae structure. The MICOS complex in human mitochondria exhibits specificities and greater complexity in comparison to the yeast system. Many subunits of this complex have been previously described, but several others and their function remain to be explored. This review will summarize our present knowledge about the human MICOS complex and its constituents, while discussing the future research perspectives in this exciting and important field.

The Oxidation Status of Mic19 Regulates MICOS Assembly

The function of mitochondria depends on the proper organization of mitochondrial membranes. The morphology of the inner membrane is regulated by the recently identified mitochondrial contact site and crista organizing system (MICOS) complex. MICOS mutants exhibit alterations in crista formation, leading to mitochondrial dysfunction. However, the mechanisms that underlie MICOS regulation remain poorly understood. MIC19, a peripheral protein of the inner membrane and component of the MICOS complex, was previously reported to be required for the proper function of MICOS in maintaining the architecture of the inner membrane. Here, we show that human and Saccharomyces cerevisiae MIC19 proteins undergo oxidation in mitochondria and require the mitochondrial intermembrane space assembly (MIA) pathway, which couples the oxidation and import of mitochondrial intermembrane space proteins for mitochondrial localization. Detailed analyses identified yeast Mic19 in two different redox forms. The form that contains an intramolecular disulfide bond is bound to Mic60 of the MICOS complex. Mic19 oxidation is not essential for its integration into the MICOS complex but plays a role in MICOS assembly and the maintenance of the proper inner membrane morphology. These findings suggest that Mic19 is a redox-dependent regulator of MICOS function.

QIL1 is a novel mitochondrial protein required for MICOS complex stability and cristae morphology

The mitochondrial contact site and cristae junction (CJ) organizing system (MICOS) dynamically regulate mitochondrial membrane architecture. Through systematic proteomic analysis of human MICOS, we identified QIL1 (C19orf70) as a novel conserved MICOS subunit. QIL1 depletion disrupted CJ structure in cultured human cells and in Drosophila muscle and neuronal cells in vivo. In human cells, mitochondrial disruption correlated with impaired respiration. Moreover, increased mitochondrial fragmentation was observed upon QIL1 depletion in flies. Using quantitative proteomics, we show that loss of QIL1 resulted in MICOS disassembly with the accumulation of a MIC60-MIC19-MIC25 sub-complex and degradation of MIC10, MIC26, and MIC27. Additionally, we demonstrated that in QIL1-depleted cells, overexpressed MIC10 fails to significantly restore its interaction with other MICOS subunits and SAMM50. Collectively, our work uncovers a previously unrecognized subunit of the MICOS complex, necessary for CJ integrity, cristae morphology, and mitochondrial function and provides a resource for further analysis of MICOS architecture.

Ancient homology of the mitochondrial contact site and cristae organizing system points to an endosymbiotic origin of mitochondrial cristae

Mitochondria are eukaryotic organelles that originated from an endosymbiotic α-proteobacterium. As an adaptation to maximize ATP production through oxidative phosphorylation, mitochondria contain inner membrane invaginations called cristae. Recent work has characterized a multi-protein complex in yeast and animal mitochondria called MICOS (mitochondrial contact site and cristae organizing system), responsible for the determination and maintenance of cristae [1-4]. However, the origin and evolution of these characteristic mitochondrial features remain obscure. We therefore conducted a comprehensive search for MICOS components across the major groups that encompass eukaryotic diversity to determine the extent of conservation of this complex. We detected homologs for the majority of MICOS components among opisthokonts (the group containing animals and fungi), but only Mic60 and Mic10 were consistently identified outside this group. The conservation of Mic60 and Mic10 in eukaryotes is consistent with their central role in MICOS function [5-7], indicating that the basic mechanism for cristae determination arose early in evolution and has remained relatively unchanged. We found that eukaryotes with ultrastructurally simplified anaerobic mitochondria that lack cristae have also lost MICOS. We then searched for a prokaryotic MICOS and identified a homolog of Mic60 present only in α-proteobacteria, providing evidence for the endosymbiotic origin of mitochondrial cristae. Our study clarifies the origins of mitochondrial cristae and their subsequent evolutionary history, provides evidence for a general mechanism of cristae formation and maintenance in eukaryotes, and points to a new potential factor involved in membrane differentiation in prokaryotes.

Central role of Mic10 in the mitochondrial contact site and cristae organizing system

The mitochondrial contact site and cristae organizing system (MICOS) is a conserved multi-subunit complex crucial for maintaining the characteristic architecture of mitochondria. Studies with deletion mutants identified Mic10 and Mic60 as core subunits of MICOS. Mic60 has been studied in detail; however, topogenesis and function of Mic10 are unknown. We report that targeting of Mic10 to the mitochondrial inner membrane requires a positively charged internal loop, but no cleavable presequence. Both transmembrane segments of Mic10 carry a characteristic four-glycine motif, which has been found in the ring-forming rotor subunit of F1Fo-ATP synthases. Overexpression of Mic10 profoundly alters the architecture of the inner membrane independently of other MICOS components. The four-glycine motifs are dispensable for interaction of Mic10 with other MICOS subunits but are crucial for the formation of large Mic10 oligomers. Our studies identify a unique role of Mic10 oligomers in promoting the formation of inner membrane crista junctions.

MICOS coordinates with respiratory complexes and lipids to establish mitochondrial inner membrane architecture

The conserved MICOS complex functions as a primary determinant of mitochondrial inner membrane structure. We address the organization and functional roles of MICOS and identify two independent MICOS subcomplexes: Mic27/Mic10/Mic12, whose assembly is dependent on respiratory complexes and the mitochondrial lipid cardiolipin, and Mic60/Mic19, which assembles independent of these factors. Our data suggest that MICOS subcomplexes independently localize to cristae junctions and are connected via Mic19, which functions to regulate subcomplex distribution, and thus, potentially also cristae junction copy number. MICOS subunits have non-redundant functions as the absence of both MICOS subcomplexes results in more severe morphological and respiratory growth defects than deletion of single MICOS subunits or subcomplexes. Mitochondrial defects resulting from MICOS loss are caused by misdistribution of respiratory complexes in the inner membrane. Together, our data are consistent with a model where MICOS, mitochondrial lipids and respiratory complexes coordinately build a functional and correctly shaped mitochondrial inner membrane.

DOI:http://dx.doi.org/10.7554/eLife.07739.001

Structures called mitochondria provide energy that cells need to live and grow. To do this, mitochondria convert energy stored within sugars and other carbon-rich compounds into the energy currency of cells, a molecule called adenosine triphosphate (called ATP for short). Defective mitochondria can cause cells to starve and also cause severe human diseases.

A double membrane surrounds each mitochondrion. The outer membrane allows proteins and other substances to enter, while the inner membrane is elaborately folded and contains several groups of proteins—or complexes—including the respiratory complexes that generate ATP. Proper inner membrane folding is critically important. The membrane folds are held in place by structures called cristae junctions, which may also help to restrict proteins to particular areas of the inner membrane.

A large inner membrane complex of proteins known as MICOS is important for organizing the inner membrane into folds, although exactly how it does so is not fully understood. MICOS consists of at least six different proteins, most of which are found across yeast and animal species. Friedman et al. have now analyzed how the MICOS complex assembles on the inner membrane in yeast cells using a combination of fluorescence and electron microscopy, proteomics and biochemistry. This revealed that in yeast, MICOS is made up of two independent sub-complexes bridged together by a protein called Mic19, which additional experiments suggest controls the number and positions of the cristae junctions that hold the folds of the inner membrane in place.

As part of the approach to understand MICOS complex organization, Friedman et al. removed the six MICOS proteins from yeast cells. Inside these cells, the inner mitochondrial membrane was misfolded. Furthermore, the respiratory complexes did not work normally and as a consequence the cells were unable to grow normally, suggesting that the correct distribution of respiratory complexes in the inner membrane is important for ATP production and depends on MICOS.

These results indicate that MICOS stabilizes the structure of the inner membrane and organizes it into an efficient energy-generating machine. In many human mitochondrial diseases, the inner membrane of mitochondria folds incorrectly, in similar ways to the misfolding seen in the yeast cells that did not contain the MICOS complex. Therefore, the MICOS complex may also influence how these diseases develop.

DOI:http://dx.doi.org/10.7554/eLife.07739.002

Detailed analysis of the human mitochondrial contact site complex indicate a hierarchy of subunits

Mitochondrial inner membrane folds into cristae, which significantly increase its surface and are important for mitochondrial function. The stability of cristae depends on the mitochondrial contact site (MICOS) complex. In human mitochondria, the inner membrane MICOS complex interacts with the outer membrane sorting and assembly machinery (SAM) complex, to form the mitochondrial intermembrane space bridging complex (MIB). We have created knockdown cell lines of most of the MICOS and MIB components and have used them to study the importance of the individual subunits for the cristae formation and complex stability. We show that the most important subunits of the MIB complex in human mitochondria are Mic60/Mitofilin, Mic19/CHCHD3 and an outer membrane component Sam50. We provide additional proof that ApoO indeed is a subunit of the MICOS and MIB complexes and propose the name Mic23 for this protein. According to our results, Mic25/CHCHD6, Mic27/ApoOL and Mic23/ApoO appear to be periphery subunits of the MICOS complex, because their depletion does not affect cristae morphology or stability of other components.

Role of membrane contact sites in protein import into mitochondria

Mitochondria import more than 1,000 different proteins from the cytosol. The proteins are synthesized as precursors on cytosolic ribosomes and are translocated by protein transport machineries of the mitochondrial membranes. Five main pathways for protein import into mitochondria have been identified. Most pathways use the translocase of the outer mitochondrial membrane (TOM) as the entry gate into mitochondria. Depending on specific signals contained in the precursors, the proteins are subsequently transferred to different intramitochondrial translocases. In this article, we discuss the connection between protein import and mitochondrial membrane architecture. Mitochondria possess two membranes. It is a long-standing question how contact sites between outer and inner membranes are formed and which role the contact sites play in the translocation of precursor proteins. A major translocation contact site is formed between the TOM complex and the presequence translocase of the inner membrane (TIM23 complex), promoting transfer of presequence-carrying preproteins to the mitochondrial inner membrane and matrix. Recent findings led to the identification of contact sites that involve the mitochondrial contact site and cristae organizing system (MICOS) of the inner membrane. MICOS plays a dual role. It is crucial for maintaining the inner membrane cristae architecture and forms contacts sites to the outer membrane that promote translocation of precursor proteins into the intermembrane space and outer membrane of mitochondria. The view is emerging that the mitochondrial protein translocases do not function as independent units, but are embedded in a network of interactions with machineries that control mitochondrial activity and architecture.

Novel intracellular functions of apolipoproteins: the ApoO protein family as constituents of the Mitofilin/MINOS complex determines cristae morphology in mitochondria

Mitochondria exist in a highly dynamic network that is constantly altered by fusion and fission events depending on various factors such as cellular bioenergetic state and cell cycle. Next to this dynamic nature of the organelle, its cristae membrane also undergoes drastic morphological changes upon physiological or pathological alterations. The Mitofilin/mitochondrial inner membrane organizing system (MINOS) complex was recently reported to ensure mitochondrial architecture and crista junction integrity. Several subunits of this complex are linked to a diverse set of neurological human disorders. Recently, two apolipoproteins, ApoO (APOO) and ApoO-like (APOOL) were suggested to represent constituents of the mammalian Mitofilin/MINOS complex. APOOL was shown to bind the mitochondrial phospholipid cardiolipin (CL) and to interact physically with this complex. In this review we highlight the current view on the mammalian Mitofilin/MINOS complex and focus on APOOL and the role of CL in determining cristae morphology. We will discuss possible functions of the Mitofilin/MINOS complex on lipid transport, on assembly of respiratory supercomplexes, on F1FO-ATP synthase organization, on contact site formation, and on trapping CL within the cristae subcompartment.

Assembly of β-barrel proteins in the mitochondrial outer membrane

Mitochondria evolved through endosymbiosis of a Gram-negative progenitor with a host cell to generate eukaryotes. Therefore, the outer membrane of mitochondria and Gram-negative bacteria contain pore proteins with β-barrel topology. After synthesis in the cytosol, β-barrel precursor proteins are first transported into the mitochondrial intermembrane space. Folding and membrane integration of β-barrel proteins depend on the mitochondrial sorting and assembly machinery (SAM) located in the outer membrane, which is related to the β-barrel assembly machinery (BAM) in bacteria. The SAM complex recognizes β-barrel proteins by a β-signal in the C-terminal β-strand that is required to initiate β-barrel protein insertion into the outer membrane. In addition, the SAM complex is crucial to form membrane contacts with the inner mitochondrial membrane by interacting with the mitochondrial contact site and cristae organizing system (MICOS) and shares a subunit with the endoplasmic reticulum-mitochondria encounter structure (ERMES) that links the outer mitochondrial membrane to the endoplasmic reticulum (ER).

Shaping the dynamic mitochondrial network

In a majority of cell types, mitochondria form highly dynamic, tubular networks. Maintaining the shape of this complex network is critical for both mitochondrial and cellular function and involves the activities of mitochondrial division, fusion, motility, and tethering. Recent studies have advanced our understanding of the molecular mechanisms underlying these conserved activities and their integration with cellular needs.

Uniform nomenclature for the mitochondrial contact site and cristae organizing system

The mitochondrial inner membrane contains a large protein complex that functions in inner membrane organization and formation of membrane contact sites. The complex was variably named the mitochondrial contact site complex, mitochondrial inner membrane organizing system, mitochondrial organizing structure, or Mitofilin/Fcj1 complex. To facilitate future studies, we propose to unify the nomenclature and term the complex “mitochondrial contact site and cristae organizing system” and its subunits Mic10 to Mic60.

Apolipoprotein O is mitochondrial and promotes lipotoxicity in heart

Diabetic cardiomyopathy is a secondary complication of diabetes with an unclear etiology. Based on a functional genomic evaluation of obesity-associated cardiac gene expression, we previously identified and cloned the gene encoding apolipoprotein O (APOO), which is overexpressed in hearts from diabetic patients. Here, we generated APOO-Tg mice, transgenic mouse lines that expresses physiological levels of human APOO in heart tissue. APOO-Tg mice fed a high-fat diet exhibited depressed ventricular function with reduced fractional shortening and ejection fraction, and myocardial sections from APOO-Tg mice revealed mitochondrial degenerative changes. In vivo fluorescent labeling and subcellular fractionation revealed that APOO localizes with mitochondria. Furthermore, APOO enhanced mitochondrial uncoupling and respiration, both of which were reduced by deletion of the N-terminus and by targeted knockdown of APOO. Consequently, fatty acid metabolism and ROS production were enhanced, leading to increased AMPK phosphorylation and Ppara and Pgc1a expression. Finally, we demonstrated that the APOO-induced cascade of events generates a mitochondrial metabolic sink whereby accumulation of lipotoxic byproducts leads to lipoapoptosis, loss of cardiac cells, and cardiomyopathy, mimicking the diabetic heart-associated metabolic phenotypes. Our data suggest that APOO represents a link between impaired mitochondrial function and cardiomyopathy onset, and targeting APOO-dependent metabolic remodeling has potential as a strategy to adjust heart metabolism and protect the myocardium from impaired contractility.

How to analyze mitochondrial morphology in healthy cells and apoptotic cells in Caenorhabditis elegans

Mitochondria constantly undergo fusion and fission events. A proper balance of fusion and fission is essential in healthy cells, as disrupting this balance is associated with several neurodegenerative diseases. Mitochondrial fission has also been shown to play an important role during apoptosis. Hence, the machineries that control mitochondrial morphology have both nonapoptotic and apoptotic functions. Seminal work in yeast has identified some of the key components of these machineries. However, the list is certainly not complete and new factors that are specific to metazoans are being identified every year. In this review, we describe methodologies to test whether a particular candidate gene plays a role in the control of mitochondrial morphology in healthy cells and apoptotic cells using Caenorhabditis elegans.

Aim24 and MICOS modulate respiratory function, tafazzin-related cardiolipin modification and mitochondrial architecture

Structure and function of mitochondria are intimately linked. In a search for components that participate in building the elaborate architecture of this complex organelle we have identified Aim24, an inner membrane protein. Aim24 interacts with the MICOS complex that is required for the formation of crista junctions and contact sites between inner and outer membranes. Aim24 is necessary for the integrity of the MICOS complex, for normal respiratory growth and mitochondrial ultrastructure. Modification of MICOS subunits Mic12 or Mic26 by His-tags in the absence of Aim24 leads to complete loss of cristae and respiratory complexes. In addition, the level of tafazzin, a cardiolipin transacylase, is drastically reduced and the composition of cardiolipin is modified like in mutants lacking tafazzin. In conclusion, Aim24 by interacting with the MICOS complex plays a key role in mitochondrial architecture, composition and function. DOI: http://dx.doi.org/10.7554/eLife.01684.001.

Mutations in Fis1 disrupt orderly disposal of defective mitochondria

The mitochondrial fission protein Drp1 binds to Mff on mitochondria, followed by entry into a complex with Fis1 at the ER–mitochondrial interface. Mutations in Fis1 disrupt disposal of defective mitochondria when fission is induced by stress. Fis1 thus acts in sequence with Mff to couple mitochondrial fission with downstream degradation processes.

Mitochondrial fission is mediated by the dynamin-related protein Drp1 in metazoans. Drp1 is recruited from the cytosol to mitochondria by the mitochondrial outer membrane protein Mff. A second mitochondrial outer membrane protein, named Fis1, was previously proposed as recruitment factor, but Fis1^−/− cells have mild or no mitochondrial fission defects. Here we show that Fis1 is nevertheless part of the mitochondrial fission complex in metazoan cells. During the fission cycle, Drp1 first binds to Mff on the surface of mitochondria, followed by entry into a complex that includes Fis1 and endoplasmic reticulum (ER) proteins at the ER–mitochondrial interface. Mutations in Fis1 do not normally affect fission, but they can disrupt downstream degradation events when specific mitochondrial toxins are used to induce fission. The disruptions caused by mutations in Fis1 lead to an accumulation of large LC3 aggregates. We conclude that Fis1 can act in sequence with Mff at the ER–mitochondrial interface to couple stress-induced mitochondrial fission with downstream degradation processes.

APOOL is a cardiolipin-binding constituent of the Mitofilin/MINOS protein complex determining cristae morphology in mammalian mitochondria

Mitochondrial cristae morphology is highly variable and altered under numerous pathological conditions. The protein complexes involved are largely unknown or only insufficiently characterized. Using complexome profiling we identified apolipoprotein O (APOO) and apolipoprotein O-like protein (APOOL) as putative components of the Mitofilin/MINOS protein complex which was recently implicated in determining cristae morphology. We show that APOOL is a mitochondrial membrane protein facing the intermembrane space. It specifically binds to cardiolipin in vitro but not to the precursor lipid phosphatidylglycerol. Overexpression of APOOL led to fragmentation of mitochondria, a reduced basal oxygen consumption rate, and altered cristae morphology. Downregulation of APOOL impaired mitochondrial respiration and caused major alterations in cristae morphology. We further show that APOOL physically interacts with several subunits of the MINOS complex, namely Mitofilin, MINOS1, and SAMM50. We conclude that APOOL is a cardiolipin-binding component of the Mitofilin/MINOS protein complex determining cristae morphology in mammalian mitochondria. Our findings further assign an intracellular role to a member of the apolipoprotein family in mammals.