Mic10, a Core Subunit of the Mitochondrial Contact Site and Cristae Organizing System, Interacts with the Dimeric F1Fo-ATP Synthase

Mitochondrial complexome and import network

Mitochondria perform crucial functions in cellular metabolism, protein and lipid biogenesis, quality control, and signaling. The systematic analysis of protein complexes and interaction networks provided exciting insights into the structural and functional organization of mitochondria. Most mitochondrial proteins do not act as independent units, but are interconnected by stable or dynamic protein-protein interactions. Protein translocases are responsible for importing precursor proteins into mitochondria and form central elements of several protein interaction networks. These networks include molecular chaperones and quality control factors, metabolite channels and respiratory chain complexes, and membrane and organellar contact sites. Protein translocases link the distinct networks into an overarching network, the mitochondrial import network (MitimNet), to coordinate biogenesis, membrane organization and function of mitochondria.

Mitochondrial Cristae Morphology Reflecting Metabolism, Superoxide Formation, Redox Homeostasis, and Pathology

Significance: Mitochondrial (mt) reticulum network in the cell possesses amazing ultramorphology of parallel lamellar cristae, formed by the invaginated inner mitochondrial membrane. Its non-invaginated part, the inner boundary membrane (IBM) forms a cylindrical sandwich with the outer mitochondrial membrane (OMM). Crista membranes (CMs) meet IBM at crista junctions (CJs) of mt cristae organizing system (MICOS) complexes connected to OMM sorting and assembly machinery (SAM). Cristae dimensions, shape, and CJs have characteristic patterns for different metabolic regimes, physiological and pathological situations. Recent Advances: Cristae-shaping proteins were characterized, namely rows of ATP-synthase dimers forming the crista lamella edges, MICOS subunits, optic atrophy 1 (OPA1) isoforms and mitochondrial genome maintenance 1 (MGM1) filaments, prohibitins, and others. Detailed cristae ultramorphology changes were imaged by focused-ion beam/scanning electron microscopy. Dynamics of crista lamellae and mobile CJs were demonstrated by nanoscopy in living cells. With tBID-induced apoptosis a single entirely fused cristae reticulum was observed in a mitochondrial spheroid. Critical Issues: The mobility and composition of MICOS, OPA1, and ATP-synthase dimeric rows regulated by post-translational modifications might be exclusively responsible for cristae morphology changes, but ion fluxes across CM and resulting osmotic forces might be also involved. Inevitably, cristae ultramorphology should reflect also mitochondrial redox homeostasis, but details are unknown. Disordered cristae typically reflect higher superoxide formation. Future Directions: To link redox homeostasis to cristae ultramorphology and define markers, recent progress will help in uncovering mechanisms involved in proton-coupled electron transfer via the respiratory chain and in regulation of cristae architecture, leading to structural determination of superoxide formation sites and cristae ultramorphology changes in diseases. Antioxid. Redox Signal. 39, 635-683.

Effect of Tamoxifen on Proteome Expression during In Vitro Myogenesis in Murine Skeletal Muscle C2C12 Cells

Tamoxifen (TMX), a selective estrogen receptor modulator, is commonly used in the treatment of hormone-responsive cancers. However, the effects of TMX in anabolic tissues harboring estrogen receptors, such as skeletal muscle, are poorly understood. We report a tandem mass-tag approach to TMX-treated myogenesis in C2C12 cells, a well-characterized model of in vitro murine skeletal muscle differentiation. A longitudinal analysis of >10,000 proteins identified in untreated C2C12 myogenesis revealed a novel subset of 1,062 myogenically regulated proteins. These proteins clustered into five distinct longitudinal expression trends which significantly overlap those obtained in similar analyses performed in human myocytes. We document a specific functional enrichment for adiponectin-signaling unique to TMX-treated myogenesis, as well as a subset of 198 proteins that are differentially expressed in TMX-treated cells relative to controls at one or more stages of myogenesis, the majority of which were involved in steroid and lipid metabolism. Further analysis highlights metallothionein-1 as a novel target of TMX treatment at each stage of C2C12 myogenesis. Finally, we present a powerful, self-validating pipeline for analyzing the total proteomic response to in vitro treatment across every stage of muscle cell development which can be easily adapted to study the effects of other drugs on myogenesis.

Glaucomatous optic neuropathy: Mitochondrial dynamics, dysfunction and protection in retinal ganglion cells

Glaucoma is a leading cause of irreversible blindness worldwide and is characterized by a slow, progressive, and multifactorial degeneration of retinal ganglion cells (RGCs) and their axons, resulting in vision loss. Despite its high prevalence in individuals 60 years of age and older, the causing factors contributing to glaucoma progression are currently not well characterized. Intraocular pressure (IOP) is the only proven treatable risk factor. However, lowering IOP is insufficient for preventing disease progression. One of the significant interests in glaucoma pathogenesis is understanding the structural and functional impairment of mitochondria in RGCs and their axons and synapses. Glaucomatous risk factors such as IOP elevation, aging, genetic variation, neuroinflammation, neurotrophic factor deprivation, and vascular dysregulation, are potential inducers for mitochondrial dysfunction in glaucoma. Because oxidative phosphorylation stress-mediated mitochondrial dysfunction is associated with structural and functional impairment of mitochondria in glaucomatous RGCs, understanding the underlying mechanisms and relationship between structural and functional alterations in mitochondria would be beneficial to developing mitochondria-related neuroprotection in RGCs and their axons and synapses against glaucomatous neurodegeneration. Here, we review the current studies focusing on mitochondrial dynamics-based structural and functional alterations in the mitochondria of glaucomatous RGCs and therapeutic strategies to protect RGCs against glaucomatous neurodegeneration.

Determinants and outcomes of mitochondrial dynamics

Mitochondria are not only central organelles in metabolism and energy conversion but are also platforms for cellular signaling cascades. Classically, the shape and ultrastructure of mitochondria were depicted as static. The discovery of morphological transitions during cell death and of conserved genes controlling mitochondrial fusion and fission contributed to establishing the concept that mitochondrial morphology and ultrastructure are dynamically regulated by mitochondria-shaping proteins. These finely tuned, dynamic changes in mitochondrial shape can in turn control mitochondrial function, and their alterations in human diseases suggest that this space can be explored for drug discovery. Here, we review the basic tenets and molecular mechanisms of mitochondrial morphology and ultrastructure, describing how they can coordinately define mitochondrial function.

ATP synthase interactome analysis identifies a new subunit l as a modulator of permeability transition pore in yeast

The mitochondrial ATP synthase, an enzyme that synthesizes ATP and is involved in the formation of the mitochondrial mega-channel and permeability transition, is a multi-subunit complex. In S. cerevisiae, the uncharacterized protein Mco10 was previously found to be associated with ATP synthase and referred as a new 'subunit l'. However, recent cryo-EM structures could not ascertain Mco10 with the enzyme making questionable its role as a structural subunit. The N-terminal part of Mco10 is very similar to k/Atp19 subunit, which along with subunits g/Atp20 and e/Atp21 plays a major role in stabilization of the ATP synthase dimers. In our effort to confidently define the small protein interactome of ATP synthase we found Mco10. We herein investigate the impact of Mco10 on ATP synthase functioning. Biochemical analysis reveal in spite of similarity in sequence and evolutionary lineage, that Mco10 and Atp19 differ significantly in function. The Mco10 is an auxiliary ATP synthase subunit that only functions in permeability transition.

Beyond being an energy supplier, ATP synthase is a sculptor of mitochondrial cristae

Mitochondrial F₁F_O-ATP synthase plays a key role in cellular bioenergetics; this enzyme is present in all eukaryotic linages except in amitochondriate organisms. Despite its ancestral origin, traceable to the alpha proteobacterial endosymbiotic event, the actual structural diversity of these complexes, due to large differences in their polypeptide composition, reflects an important evolutionary divergence between eukaryotic lineages. We discuss the effect of these structural differences on the oligomerization of the complex and the shape of mitochondrial cristae.

CHCHD2 maintains mitochondrial contact site and cristae organizing system stability and protects against mitochondrial dysfunction in an experimental model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's dementia. Mitochondrial dysfunction is involved in the pathology of PD. Coiled-coil-helix-coiled-coil-helix domain-containing 2 (CHCHD2) was identified as associated with autosomal dominant PD. However, the mechanism of CHCHD2 in PD remains unclear.

METHODS

Short hairpin RNA (ShRNA)-mediated CHCHD2 knockdown or lentivirus-mediated CHCHD2 overexpression was performed to investigate the impact of CHCHD2 on mitochondrial morphology and function in neuronal tumor cell lines represented with human neuroblastoma (SHSY5Y) and HeLa cells. Blue-native polyacrylamide gel electrophoresis (PAGE) and two-dimensional sodium dodecyl sulfate-PAGE analysis were used to illustrate the role of CHCHD2 in mitochondrial contact site and cristae organizing system (MICOS). Co-immunoprecipitation and immunoblotting were used to address the interaction between CHCHD2 and Mic10. Serotype injection of adeno-associated vector-mediated CHCHD2 and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration were used to examine the influence of CHCHD2 in vivo.

RESULTS

We found that the overexpression of CHCHD2 can protect against methyl-4-phenylpyridinium (MPP+)-induced mitochondrial dysfunction and inhibit the loss of dopaminergic neurons in the MPTP-induced mouse model. Furthermore, we identified that CHCHD2 interacted with Mic10, and overexpression of CHCHD2 can protect against MPP+-induced MICOS impairment, while knockdown of CHCHD2 impaired the stability of MICOS.

CONCLUSION

This study indicated that CHCHD2 could interact with Mic10 and maintain the stability of the MICOS complex, which contributes to protecting mitochondrial function in PD.

Mitochondrial Membrane Remodeling

Mitochondria are key regulators of many important cellular processes and their dysfunction has been implicated in a large number of human disorders. Importantly, mitochondrial function is tightly linked to their ultrastructure, which possesses an intricate membrane architecture defining specific submitochondrial compartments. In particular, the mitochondrial inner membrane is highly folded into membrane invaginations that are essential for oxidative phosphorylation. Furthermore, mitochondrial membranes are highly dynamic and undergo constant membrane remodeling during mitochondrial fusion and fission. It has remained enigmatic how these membrane curvatures are generated and maintained, and specific factors involved in these processes are largely unknown. This review focuses on the current understanding of the molecular mechanism of mitochondrial membrane architectural organization and factors critical for mitochondrial morphogenesis, as well as their functional link to human diseases.

Dual role of Mic10 in mitochondrial cristae organization and ATP synthase-linked metabolic adaptation and respiratory growth

Invaginations of the mitochondrial inner membrane, termed cristae, are hubs for oxidative phosphorylation. The mitochondrial contact site and cristae organizing system (MICOS) and the dimeric F₁F_o-ATP synthase play important roles in controlling cristae architecture. A fraction of the MICOS core subunit Mic10 is found in association with the ATP synthase, yet it is unknown whether this interaction is of relevance for mitochondrial or cellular functions. Here, we established conditions to selectively study the role of Mic10 at the ATP synthase. Mic10 variants impaired in MICOS functions stimulate ATP synthase oligomerization like wild-type Mic10 and promote efficient inner membrane energization, adaptation to non-fermentable carbon sources, and respiratory growth. Mic10's functions in respiratory growth largely depend on Mic10^ATPsynthase, not on Mic10^MICOS. We conclude that Mic10 plays a dual role as core subunit of MICOS and as partner of the F₁F_o-ATP synthase, serving distinct functions in cristae shaping and respiratory adaptation and growth.

•

Dual role of Mic10 of mitochondrial contact site and cristae organizing system (MICOS)

•

Mic10 binds to mitochondrial ATP synthase and stabilizes higher order assemblies

•

Oligomerization of Mic10 is required for its function in MICOS, not at ATP synthase

•

Mic10 binding to ATP synthase supports metabolic adaptation and respiratory growth

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.

Mitochondrial compartmentalization: emerging themes in structure and function

Within cellular structures, compartmentalization is the concept of spatial segregation of macromolecules, metabolites, and biochemical pathways. Therefore, this concept bridges organellar structure and function. Mitochondria are morphologically complex, partitioned into several subcompartments by a topologically elaborate two-membrane system. They are also dynamically polymorphic, undergoing morphogenesis events with an extent and frequency that is only now being appreciated. Thus, mitochondrial compartmentalization is something that must be considered both spatially and temporally. Here, we review new developments in how mitochondrial structure is established and regulated, the factors that underpin the distribution of lipids and proteins, and how they spatially demarcate locations of myriad mitochondrial processes. Consistent with its pre-eminence, disturbed mitochondrial compartmentalization contributes to the dysfunction associated with heritable and aging-related diseases.

Cristae-dependent quality control of the mitochondrial genome

Mitochondrial genomes (mtDNA) encode essential subunits of the mitochondrial respiratory chain. Mutations in mtDNA can cause a shortage in cellular energy supply, which can lead to numerous mitochondrial diseases. How cells secure mtDNA integrity over generations has remained unanswered. Here, we show that the single-celled yeast Saccharomyces cerevisiae can intracellularly distinguish between functional and defective mtDNA and promote generation of daughter cells with increasingly healthy mtDNA content. Purifying selection for functional mtDNA occurs in a continuous mitochondrial network and does not require mitochondrial fission but necessitates stable mitochondrial subdomains that depend on intact cristae morphology. Our findings support a model in which cristae-dependent proximity between mtDNA and the proteins it encodes creates a spatial “sphere of influence,” which links a lack of functional fitness to clearance of defective mtDNA.

Returning to the Fold for Lessons in Mitochondrial Crista Diversity and Evolution

Cristae are infoldings of the mitochondrial inner membrane jutting into the organelle's innermost compartment from narrow stems at their base called crista junctions. They are emblematic of aerobic mitochondria, being the fabric for the molecular machinery driving cellular respiration. Electron microscopy revealed that diverse eukaryotes possess cristae of different shapes. Yet, crista diversity has not been systematically examined in light of our current knowledge about eukaryotic evolution. Since crista form and function are intricately linked, we take a holistic view of factors that may underlie both crista diversity and the adherence of cristae to a recognizable form. Based on electron micrographs of 226 species from all major lineages, we propose a rational crista classification system that postulates cristae as variations of two general morphotypes: flat and tubulo-vesicular. The latter is most prevalent and likely ancestral, but both morphotypes are found interspersed throughout the eukaryotic tree. In contrast, crista junctions are remarkably conserved, supporting their proposed role as diffusion barriers that sequester cristae contents. Since cardiolipin, ATP synthase dimers, the MICOS complex, and dynamin-like Opa1/Mgm1 are known to be involved in shaping cristae, we examined their variation in the context of crista diversity. Moreover, we have identified both commonalities and differences that may collectively be manifested as diverse variations of crista form and function.

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.

The Diversity of the Mitochondrial Outer Membrane Protein Import Channels: Emerging Targets for Modulation

The functioning of mitochondria and their biogenesis are largely based on the proper function of the mitochondrial outer membrane channels, which selectively recognise and import proteins but also transport a wide range of other molecules, including metabolites, inorganic ions and nucleic acids. To date, nine channels have been identified in the mitochondrial outer membrane of which at least half represent the mitochondrial protein import apparatus. When compared to the mitochondrial inner membrane, the presented channels are mostly constitutively open and consequently may participate in transport of different molecules and contribute to relevant changes in the outer membrane permeability based on the channel conductance. In this review, we focus on the channel structure, properties and transported molecules as well as aspects important to their modulation. This information could be used for future studies of the cellular processes mediated by these channels, mitochondrial functioning and therapies for mitochondria-linked diseases.

Mitochondrial Contact Site and Cristae Organization System and F1FO-ATP Synthase Crosstalk Is a Fundamental Property of Mitochondrial Cristae

Mitochondrial cristae are polymorphic invaginations of the inner membrane that are the fabric of cellular respiration. Both the mitochondrial contact site and cristae organization system (MICOS) and the F1FO-ATP synthase are vital for sculpting cristae by opposing membrane-bending forces. While MICOS promotes negative curvature at crista junctions, dimeric F1FO-ATP synthase is crucial for positive curvature at crista rims. Crosstalk between these two complexes has been observed in baker's yeast, the model organism of the Opisthokonta supergroup. Here, we report that this property is conserved in Trypanosoma brucei, a member of the Discoba clade that separated from the Opisthokonta ∼2 billion years ago. Specifically, one of the paralogs of the core MICOS subunit Mic10 interacts with dimeric F1FO-ATP synthase, whereas the other core Mic60 subunit has a counteractive effect on F1FO-ATP synthase oligomerization. This is evocative of the nature of MICOS-F1FO-ATP synthase crosstalk in yeast, which is remarkable given the diversification that these two complexes have undergone during almost 2 eons of independent evolution. Furthermore, we identified a highly diverged, putative homolog of subunit e, which is essential for the stability of F1FO-ATP synthase dimers in yeast. Just like subunit e, it is preferentially associated with dimers and interacts with Mic10, and its silencing results in severe defects to cristae and the disintegration of F1FO-ATP synthase dimers. Our findings indicate that crosstalk between MICOS and dimeric F1FO-ATP synthase is a fundamental property impacting crista shape throughout eukaryotes. IMPORTANCE Mitochondria have undergone profound diversification in separate lineages that have radiated since the last common ancestor of eukaryotes some eons ago. Most eukaryotes are unicellular protists, including etiological agents of infectious diseases, like Trypanosoma brucei. Thus, the study of a broad range of protists can reveal fundamental features shared by all eukaryotes and lineage-specific innovations. Here, we report that two different protein complexes, MICOS and F1FO-ATP synthase, known to affect mitochondrial architecture, undergo crosstalk in T. brucei, just as in baker's yeast. This is remarkable considering that these complexes have otherwise undergone many changes during their almost 2 billion years of independent evolution. Thus, this crosstalk is a fundamental property needed to maintain proper mitochondrial structure even if the constituent players considerably diverged.

Inter-membrane association of the Sec and BAM translocons for bacterial outer-membrane biogenesis

The outer-membrane of Gram-negative bacteria is critical for surface adhesion, pathogenicity, antibiotic resistance and survival. The major constituent - hydrophobic β-barrel Outer-Membrane Proteins (OMPs) - are first secreted across the inner-membrane through the Sec-translocon for delivery to periplasmic chaperones, for example SurA, which prevent aggregation. OMPs are then offloaded to the β-Barrel Assembly Machinery (BAM) in the outer-membrane for insertion and folding. We show the Holo-TransLocon (HTL) - an assembly of the protein-channel core-complex SecYEG, the ancillary sub-complex SecDF, and the membrane 'insertase' YidC - contacts BAM through periplasmic domains of SecDF and YidC, ensuring efficient OMP maturation. Furthermore, the proton-motive force (PMF) across the inner-membrane acts at distinct stages of protein secretion: (1) SecA-driven translocation through SecYEG and (2) communication of conformational changes via SecDF across the periplasm to BAM. The latter presumably drives efficient passage of OMPs. These interactions provide insights of inter-membrane organisation and communication, the importance of which is becoming increasingly apparent.

Altered Outer Hair Cell Mitochondrial and Subsurface Cisternae Connectomics Are Candidate Mechanisms for Hearing Loss in Mice

Organelle crosstalk is vital for cellular functions. The propinquity of mitochondria, ER, and plasma membrane promote regulation of multiple functions, which include intracellular Ca2+ flux, and cellular biogenesis. Although the purposes of apposing mitochondria and ER have been described, an understanding of altered organelle connectomics related to disease states is emerging. Since inner ear outer hair cell (OHC) degeneration is a common trait of age-related hearing loss, the objective of this study was to investigate whether the structural and functional coupling of mitochondria with subsurface cisternae (SSC) was affected by aging. We applied functional and structural probes to equal numbers of male and female mice with a hearing phenotype akin to human aging. We discovered the polarization of cristae and crista junctions in mitochondria tethered to the SSC in OHCs. Aging was associated with SSC stress and decoupling of mitochondria with the SSC, mitochondrial fission/fusion imbalance, a remarkable reduction in mitochondrial and cytoplasmic Ca2+ levels, reduced K+-induced Ca2+ uptake, and marked plasticity of cristae membranes. A model of structure-based ATP production predicts profound energy stress in older OHCs. This report provides data suggesting that altered membrane organelle connectomics may result in progressive hearing loss.

Proteomics of host-bacterial interactions: new insights from dual perspectives

Mass-spectrometry (MS)-based proteomics is a powerful and robust platform for studying the interactions between biological systems during health and disease. Bacterial infections represent a significant threat to global health and drive the pursuit of novel therapeutic strategies to combat emerging and resistant pathogens. During infection, the interplay between a host and pathogen determines the ability of the microbe to survive in a hostile environment and promotes an immune response by the host as a protective measure. It is the protein-level changes from either biological system that define the outcome of infection, and MS-based proteomics provides a rapid and effective platform to identify such changes. In particular, proteomics detects alterations in protein abundance, quantifies protein secretion and (or) release, measures an array of post-translational modifications that influence signaling cascades, and profiles protein-protein interactions through protein complex and (or) network formation. Such information provides new insight into the role of known and novel bacterial effectors, as well as the outcome of host cell activation. In this Review, we highlight the diverse applications of MS-based proteomics in profiling the relationship between bacterial pathogens and the host. Our work identifies a plethora of strategies for exploring mechanisms of infection from dual perspectives (i.e., host and pathogen), and we suggest opportunities to extrapolate the current knowledgebase to other biological systems for applications in therapeutic discovery.

MIC26 and MIC27 cooperate to regulate cardiolipin levels and the landscape of OXPHOS complexes

Homologous apolipoproteins of MICOS complex, MIC26 and MIC27, show an antagonistic regulation of their protein levels, making it difficult to deduce their individual functions using a single gene deletion. We obtained single and double knockout (DKO) human cells of MIC26 and MIC27 and found that DKO show more concentric onion-like cristae with loss of CJs than any single deletion indicating overlapping roles in formation of CJs. Using a combination of complexome profiling, STED nanoscopy, and blue-native gel electrophoresis, we found that MIC26 and MIC27 are dispensable for the stability and integration of the remaining MICOS subunits into the complex suggesting that they assemble late into the MICOS complex. MIC26 and MIC27 are cooperatively required for the integrity of respiratory chain (super) complexes (RCs/SC) and the F1Fo-ATP synthase complex and integration of F1 subunits into the monomeric F1Fo-ATP synthase. While cardiolipin was reduced in DKO cells, overexpression of cardiolipin synthase in DKO restores the stability of RCs/SC. Overall, we propose that MIC26 and MIC27 are cooperatively required for global integrity and stability of multimeric OXPHOS complexes by modulating cardiolipin levels.

The new role of F1Fo ATP synthase in mitochondria-mediated neurodegeneration and neuroprotection

The mitochondrial F₁F_o ATP synthase is one of the most abundant proteins of the mitochondrial inner membrane, which catalyzes the final step of oxidative phosphorylation to synthesize ATP from ADP and Pi. ATP synthase uses the electrochemical gradient of protons (Δμ_H⁺) across the mitochondrial inner membrane to synthesize ATP. Under certain pathophysiological conditions, ATP synthase can run in reverse to hydrolyze ATP and build the necessary Δμ_H⁺ across the mitochondrial inner membrane. Tight coupling between these two processes, proton translocation and ATP synthesis, is achieved by the unique rotational mechanism of ATP synthase and is necessary for efficient cellular metabolism and cell survival. The uncoupling of these processes, dissipation of mitochondrial inner membrane potential, elevated levels of ROS, low matrix content of ATP in combination with other cellular malfunction trigger the opening of the mitochondrial permeability transition pore in the mitochondrial inner membrane. In this review we will discuss the new role of ATP synthase beyond oxidative phosphorylation. We will highlight its function as a unique regulator of cell life and death and as a key target in mitochondria-mediated neurodegeneration and neuroprotection.

Glucose-Induced Expression of DAPIT in Pancreatic β-Cells

Transcript levels for selected ATP synthase membrane F_O-subunits-including DAPIT-in INS-1E cells were found to be sensitive to lowering glucose down from 11 mM, in which these cells are routinely cultured. Depending on conditions, the diminished mRNA levels recovered when glucose was restored to 11 mM; or were elevated during further 120 min incubations with 20-mM glucose. Asking whether DAPIT expression may be elevated by hyperglycemia in vivo, we studied mice with hyaluronic acid implants delivering glucose for up to 14 days. Such continuous two-week glucose stimulations in mice increased DAPIT mRNA by >5-fold in isolated pancreatic islets (ATP synthase F₁α mRNA by 1.5-fold). In INS-1E cells, the glucose-induced ATP increment vanished with DAPIT silencing (6% of ATP rise), likewise a portion of the mtDNA-copy number increment. With 20 and 11-mM glucose the phosphorylating/non-phosphorylating respiration rate ratio diminished to ~70% and 96%, respectively, upon DAPIT silencing, whereas net GSIS rates accounted for 80% and 90% in USMG5/DAPIT-deficient cells. Consequently, the sufficient DAPIT expression and complete ATP synthase assembly is required for maximum ATP synthesis and mitochondrial biogenesis, but not for insulin secretion as such. Elevated DAPIT expression at high glucose further increases the ATP synthesis efficiency.

MICOS assembly controls mitochondrial inner membrane remodeling and crista junction redistribution to mediate cristae formation

Mitochondrial function is critically dependent on the folding of the mitochondrial inner membrane into cristae; indeed, numerous human diseases are associated with aberrant crista morphologies. With the MICOS complex, OPA1 and the F₁ F_o -ATP synthase, key players of cristae biogenesis have been identified, yet their interplay is poorly understood. Harnessing super-resolution light and 3D electron microscopy, we dissect the roles of these proteins in the formation of cristae in human mitochondria. We individually disrupted the genes of all seven MICOS subunits in human cells and re-expressed Mic10 or Mic60 in the respective knockout cell line. We demonstrate that assembly of the MICOS complex triggers remodeling of pre-existing unstructured cristae and de novo formation of crista junctions (CJs) on existing cristae. We show that the Mic60-subcomplex is sufficient for CJ formation, whereas the Mic10-subcomplex controls lamellar cristae biogenesis. OPA1 stabilizes tubular CJs and, along with the F₁ F_o -ATP synthase, fine-tunes the positioning of the MICOS complex and CJs. We propose a new model of cristae formation, involving the coordinated remodeling of an unstructured crista precursor into multiple lamellar cristae.

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.

Mitochondrial proteins: from biogenesis to functional networks

Mitochondria are essential for the viability of eukaryotic cells as they perform crucial functions in bioenergetics, metabolism and signalling and have been associated with numerous diseases. Recent functional and proteomic studies have revealed the remarkable complexity of mitochondrial protein organization. Protein machineries with diverse functions such as protein translocation, respiration, metabolite transport, protein quality control and the control of membrane architecture interact with each other in dynamic networks. In this Review, we discuss the emerging role of the mitochondrial protein import machinery as a key organizer of these mitochondrial protein networks. The preprotein translocases that reside on the mitochondrial membranes not only function during organelle biogenesis to deliver newly synthesized proteins to their final mitochondrial destination but also cooperate with numerous other mitochondrial protein complexes that perform a wide range of functions. Moreover, these protein networks form membrane contact sites, for example, with the endoplasmic reticulum, that are key for integration of mitochondria with cellular function, and defects in protein import can lead to diseases.

Cristae undergo continuous cycles of membrane remodelling in a MICOS-dependent manner

The mitochondrial inner membrane can reshape under different physiological conditions. How, at which frequency this occurs in living cells, and the molecular players involved are unknown. Here, we show using state-of-the-art live-cell stimulated emission depletion (STED) super-resolution nanoscopy that neighbouring crista junctions (CJs) dynamically appose and separate from each other in a reversible and balanced manner in human cells. Staining of cristae membranes (CM), using various protein markers or two lipophilic inner membrane-specific dyes, further revealed that cristae undergo continuous cycles of membrane remodelling. These events are accompanied by fluctuations of the membrane potential within distinct cristae over time. Both CJ and CM dynamics depended on MIC13 and occurred at similar timescales in the range of seconds. Our data further suggest that MIC60 acts as a docking platform promoting CJ and contact site formation. Overall, by employing advanced imaging techniques including fluorescence recovery after photobleaching (FRAP), single-particle tracking (SPT), live-cell STED and high-resolution Airyscan microscopy, we propose a model of CJ dynamics being mechanistically linked to CM remodelling representing cristae membrane fission and fusion events occurring within individual mitochondria.

Listeria monocytogenes Exploits Mitochondrial Contact Site and Cristae Organizing System Complex Subunit Mic10 To Promote Mitochondrial Fragmentation and Cellular Infection

Pathogenic bacteria can target host cell organelles to take control of key cellular processes and promote their intracellular survival, growth, and persistence. Mitochondria are essential, highly dynamic organelles with pivotal roles in a wide variety of cell functions. Mitochondrial dynamics and function are intimately linked. Our previous research showed that Listeria monocytogenes infection impairs mitochondrial function and triggers fission of the mitochondrial network at an early infection stage, in a process that is independent of the presence of the main mitochondrial fission protein Drp1. Here, we analyzed how mitochondrial proteins change in response to L. monocytogenes infection and found that infection raises the levels of Mic10, a mitochondrial inner membrane protein involved in formation of cristae. We show that Mic10 is important for L. monocytogenes-dependent mitochondrial fission and infection of host cells. Our findings thus offer new insight into the mechanisms used by L. monocytogenes to hijack mitochondria to optimize host infection.

Mitochondrial function adapts to cellular demands and is affected by the ability of the organelle to undergo fusion and fission in response to physiological and nonphysiological cues. We previously showed that infection with the human bacterial pathogen Listeria monocytogenes elicits transient mitochondrial fission and a drop in mitochondrion-dependent energy production through a mechanism requiring the bacterial pore-forming toxin listeriolysin O (LLO). Here, we performed quantitative mitochondrial proteomics to search for host factors involved in L. monocytogenes-induced mitochondrial fission. We found that Mic10, a critical component of the mitochondrial contact site and cristae organizing system (MICOS) complex, is significantly enriched in mitochondria isolated from cells infected with wild-type but not with LLO-deficient L. monocytogenes. Increased mitochondrial Mic10 levels did not correlate with upregulated transcription, suggesting a posttranscriptional mechanism. We then showed that Mic10 is necessary for L. monocytogenes-induced mitochondrial network fragmentation and that it contributes to L. monocytogenes cellular infection independently of MICOS proteins Mic13, Mic26, and Mic27. In conclusion, investigation of L. monocytogenes infection allowed us to uncover a role for Mic10 in mitochondrial fission.

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.

The ‘mitochondrial contact site and cristae organising system’ (MICOS) in health and human disease

The ‘mitochondrial contact site and cristae organising system’ (MICOS) is an essential protein complex that promotes the formation, maintenance and stability of mitochondrial cristae. As such, loss of core MICOS components disrupts cristae structure and impairs mitochondrial function. Aberrant mitochondrial cristae morphology and diminished mitochondrial function is a pathological hallmark observed across many human diseases such as neurodegenerative conditions, obesity and diabetes mellitus, cardiomyopathy, and in muscular dystrophies and myopathies. While mitochondrial abnormalities are often an associated secondary effect to the pathological disease process, a direct role for the MICOS in health and human disease is emerging. This review describes the role of MICOS in the maintenance of mitochondrial architecture and summarizes both the direct and associated roles of the MICOS in human disease.

Functional Interplay between Cristae Biogenesis, Mitochondrial Dynamics and Mitochondrial DNA Integrity

Mitochondria are vital cellular organelles involved in a plethora of cellular processes such as energy conversion, calcium homeostasis, heme biogenesis, regulation of apoptosis and ROS reactive oxygen species (ROS) production. Although they are frequently depicted as static bean-shaped structures, our view has markedly changed over the past few decades as many studies have revealed a remarkable dynamicity of mitochondrial shapes and sizes both at the cellular and intra-mitochondrial levels. Aberrant changes in mitochondrial dynamics and cristae structure are associated with ageing and numerous human diseases (e.g., cancer, diabetes, various neurodegenerative diseases, types of neuro- and myopathies). Another unique feature of mitochondria is that they harbor their own genome, the mitochondrial DNA (mtDNA). MtDNA exists in several hundreds to thousands of copies per cell and is arranged and packaged in the mitochondrial matrix in structures termed mt-nucleoids. Many human diseases are mechanistically linked to mitochondrial dysfunction and alteration of the number and/or the integrity of mtDNA. In particular, several recent studies identified remarkable and partly unexpected links between mitochondrial structure, fusion and fission dynamics, and mtDNA. In this review, we will provide an overview about these recent insights and aim to clarify how mitochondrial dynamics, cristae ultrastructure and mtDNA structure influence each other and determine mitochondrial functions.

Myristoyl group-aided protein import into the mitochondrial intermembrane space

The MICOS complex mediates formation of the crista junctions in mitochondria. Here we analyzed the mitochondrial import pathways for the six yeast MICOS subunits as a step toward understanding of the assembly mechanisms of the MICOS complex. Mic10, Mic12, Mic26, Mic27, and Mic60 used the presequence pathway to reach the intermembrane space (IMS). In contrast, Mic19 took the TIM40/MIA pathway, through its CHCH domain, to reach the IMS. Unlike canonical TIM40/MIA substrates, presence of the N-terminal unfolded DUF domain impaired the import efficiency of Mic19, yet N-terminal myristoylation of Mic19 circumvented this effect. The myristoyl group of Mic19 binds to Tom20 of the TOM complex as well as the outer membrane, which may lead to "entropy pushing" of the DUF domain followed by the CHCH domain of Mic19 into the import channel, thereby achieving efficient import.

BANFF: ending of bilayer membranes by mphiphilic α-helices is ecessary for orm and unction of organelles 1

By binding to and inserting into the lipid bilayer, amphiphilic α-helices of proteins are involved in the curvature of biological membranes in all organisms. In particular, they are involved in establishing the complex membrane architecture of intracellular organelles like the endoplasmatic reticulum, Golgi apparatus, mitochondria, and chloroplasts. Thus, amphiphilic α-helices are essential for maintenance of cellular metabolism and fitness of organisms. Here we focus on the structure and function of membrane-intrinsic proteins, which are involved in membrane curvature by amphiphilic α-helices, in mitochondria and chloroplasts of the eukaryotic model organisms yeast and Arabidopsis thaliana. Further, we propose a model for transport of fatty acids and lipid compounds across the envelope of chloroplasts in which amphiphilic α-helices might play a role.

Cholesterol signaling in single cells: lessons from STAR and sm-FISH

Cholesterol is an important regulator of cell signaling, both through direct impacts on cell membranes and through oxy-metabolites that activate specific receptors (steroids, hydroxy-cholesterols, bile acids). Cholesterol moves slowly through and between cell membranes with the assistance of specific binding proteins and transfer processes. The prototype cholesterol regulator is the Steroidogenesis Acute Regulatory (STAR), which moves cholesterol into mitochondria, where steroid synthesis is initiated by cytochrome P450 11A1 in multiple endocrine cell types. CYP27A1 generates hydroxyl cholesterol metabolites that activate LXR nuclear receptors to control cholesterol homeostatic and transport mechanisms. LXR regulation of cholesterol transport and storage as cholesterol ester droplets is shared by both steroid-producing cells and macrophage. This cholesterol signaling is crucial to brain neuron regulation by astrocytes and microglial macrophage, mediated by ApoE and sensitive to disruption by β-amyloid plaques. sm-FISH delivers appreciable insights into signaling in single cells, by resolving single RNA molecules as mRNA and by quantifying pre-mRNA at gene loci. sm-FISH has been applied to problems in physiology, embryo development and cancer biology, where single cell features have critical impacts. sm-FISH identifies novel features of STAR transcription in adrenal and testis cells, including asymmetric expression at individual gene loci, delayed splicing and 1:1 association of mRNA with mitochondria. This may represent a functional unit for the translation-dependent cholesterol transfer directed by STAR, which integrates into mitochondrial fusion dynamics. Similar cholesterol dynamics repeat with different players in the cycling of cholesterol between astrocytes and neurons in the brain, which may be abnormal in neurodegenerative diseases.

Impact of F1Fo-ATP-synthase dimer assembly factors on mitochondrial function and organismic aging

In aerobic organisms, mitochondrial F₁F_o-ATP-synthase is the major site of ATP production. Beside this fundamental role, the protein complex is involved in shaping and maintenance of cristae. Previous electron microscopic studies identified the dissociation of F₁F_o-ATP-synthase dimers and oligomers during organismic aging correlating with a massive remodeling of the mitochondrial inner membrane. Here we report results aimed to experimentally proof this impact and to obtain further insights into the control of these processes. We focused on the role of the two dimer assembly factors PaATPE and PaATPG of the aging model Podospora anserina. Ablation of either protein strongly affects mitochondrial function and leads to an accumulation of senescence markers demonstrating that the inhibition of dimer formation negatively influences vital functions and accelerates organismic aging. Our data validate a model that links mitochondrial membrane remodeling to aging and identify specific molecular components triggering this process.

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.

The Yin & Yang of Mitochondrial Architecture - Interplay of MICOS and F1Fo-ATP synthase in cristae formation

Oxidative phosphorylation takes place at specialized compartments of the inner mitochondrial membrane, the cristae. The elaborate ultrastructure of cristae membranes enables efficient chemi-osmotic coupling of respiratory chain and F₁F_o-ATP synthase. Dynamic membrane remodeling allows mitochondria to adapt to changing physiological requirements. The mitochondrial contact site and cristae organizing system (MICOS) and the oligomeric ATP synthase have been known to govern distinct features of cristae architecture. A new study 1 on the crosstalk between these two machineries now sheds light on the mechanisms of cristae formation and maintenance.

Cristae architecture is determined by an interplay of the MICOS complex and the F1FO ATP synthase via Mic27 and Mic10

The inner boundary and the cristae membrane are connected by pore-like structures termed crista junctions (CJs). The MICOS complex is required for CJ formation and enriched at CJs. Here, we address the roles of the MICOS subunits Mic27 and Mic10. We observe a positive genetic interaction between Mic27 and Mic60 and deletion of Mic27 results in impaired formation of CJs and altered cristae membrane curvature. Mic27 acts in an antagonistic manner to Mic60 as it promotes oligomerization of the F₁F_O-ATP synthase and partially restores CJ formation in cells lacking Mic60. Mic10 impairs oligomerization of the F₁F_O-ATP synthase similar to Mic60. Applying complexome profiling, we observed that deletion of Mic27 destabilizes the MICOS complex but does not impair formation of a high molecular weight Mic10 subcomplex. Moreover, this Mic10 subcomplex comigrates with the dimeric F₁F_O-ATP synthase in a Mic27-independent manner. Further, we observed a chemical crosslink of Mic10 to Mic27 and of Mic10 to the F₁F_O-ATP synthase subunit e. We corroborate the physical interaction of the MICOS complex and the F₁F_O-ATP synthase. We propose a model in which part of the F₁F_O-ATP synthase is linked to the MICOS complex via Mic10 and Mic27 and by that is regulating CJ formation.

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.

Who and how in the regulation of mitochondrial cristae shape and function

Mitochondrial adaptation to different physiological conditions highly relies on the regulation of mitochondrial ultrastructure, particularly at the level of cristae compartment. Cristae represent the membrane hub where most of the respiratory complexes embed to account for OXPHOS and energy production in the form of adenosine triphosphate (ATP). Changes in cristae number and shape define the respiratory capacity as well as cell viability. The identification of key regulators of cristae morphology and the understanding of their contribution to the mitochondrial ultrastructure and function have become an strategic goal to understand mitochondrial disorders and to exploit as therapeutic targets. This review summarizes the known regulators of cristae ultrastructure and discusses their contribution and implications for mitochondrial dysfunction.