Mitochondrial Contact Site And Cristae Organizing System Complex Research Articles

MICOS Complex Loss Governs Age-Associated Murine Mitochondrial Architecture and Metabolism in the Liver, While Sam50 Dictates Diet Changes.

The liver, the largest internal organ and a metabolic hub, undergoes significant declines due to aging, affecting mitochondrial function and increasing the risk of systemic liver diseases. How the mitochondrial three-dimensional (3D) structure changes in the liver across aging, and the biological mechanisms regulating such changes confers remain unclear. In this study, we employed Serial Block Face-Scanning Electron Microscopy (SBF-SEM) to achieve high-resolution 3D reconstructions of murine liver mitochondria to observe diverse phenotypes and structural alterations that occur with age, marked by a reduction in size and complexity. We also show concomitant metabolomic and lipidomic changes in aged samples. Aged human samples reflected altered disease risk. To find potential regulators of this change, we examined the Mitochondrial Contact Site and Cristae Organizing System (MICOS) complex, which plays a crucial role in maintaining mitochondrial architecture. We observe that the MICOS complex is lost during aging, but not Sam50. Sam50 is a component of the sorting and assembly machinery (SAM) complex that acts in tandem with the MICOS complex to modulate cristae morphology. In murine models subjected to a high-fat diet, there is a marked depletion of the mitochondrial protein SAM50. This reduction in Sam50 expression may heighten the susceptibility to liver disease, as our human biobank studies corroborate that Sam50 plays a genetically regulated role in the predisposition to multiple liver diseases. We further show that changes in mitochondrial calcium dysregulation and oxidative stress accompany the disruption of the MICOS complex. Together, we establish that a decrease in mitochondrial complexity and dysregulated metabolism occur with murine liver aging. While these changes are partially be regulated by age-related loss of the MICOS complex, the confluence of a murine high-fat diet can also cause loss of Sam50, which contributes to liver diseases. In summary, our study reveals potential regulators that affect age-related changes in mitochondrial structure and metabolism, which can be targeted in future therapeutic techniques.

Population Studies Revealed Sex-Specific Roles of MIC25/CHCHD6 in Promoting NAFLD

One of the major complications of metabolic syndrome affecting the liver, in the absence of excessive alcohol, is non-alcoholic fatty liver disease (NAFLD). It is estimated that 20-30% of the population worldwide are affected by NAFLD. Recent studies have revealed that NAFLD is more prevalent in men than women, with men exhibiting severe NAFLD symptoms. As of now, there are no prevention and/or treatment options for NAFLD other than reducing the risk factors such as weight reduction. To this end, we used our ~100 diverse inbred strains of mice and identified MIC25 or CHCHD6, a core component of MICOS (mitochondrial contact site and cristae organizing system) complex, as a causal gene in mediating NAFLD. We also observed dramatic differences in the regulation and functions of MIC25 between sexes. Specifically, MIC25 expression was downregulated in females via estrogen and is strongly associated with NAFLD severity. We followed up by investigating the mechanistic roles of MIC25 using loss- and gain-of-function mouse models. In a steatosis mouse model, MIC25 silencing ameliorates plasma cholesterol levels, insulin resistance, and intrahepatic triglyceride accumulation, while MIC25 overexpression exacerbates them. Stable isotope tracing analyses revealed that MIC25 also regulated carbon flux through mitochondrial pyruvate carboxylase and glucose anaplerosis, thereby affecting de novo lipogenesis, fat and glutamine oxidation, and gluconeogenesis accompanied by mitochondrial dysfunction/maladaptation. Given the need for a promising NAFLD drug candidate with personalized therapeutic potential, we feel that our novel findings will be of considerable interest to the liver scientific community. R00DK120875. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A dynamin superfamily-like pseudoenzyme coordinates with MICOS to promote cristae architecture

Mitochondrial cristae architecture is crucial for optimal respiratory function of the organelle. Cristae shape is maintained in part by the mitochondrial contact site and cristae organizing system (MICOS) complex. While MICOS is required for normal cristae morphology, the precise mechanistic role of each of the seven human MICOS subunits, and how the complex coordinates with other cristae-shaping factors, has not been fully determined. Here, we examine the MICOS complex in Schizosaccharomyces pombe, a minimal model whose genome only encodes for four core subunits. Using an unbiased proteomics approach, we identify a poorly characterized inner mitochondrial membrane protein that interacts with MICOS and is required to maintain cristae morphology, which we name Mmc1. We demonstrate that Mmc1 works in concert with MICOS to promote normal mitochondrial morphology and respiratory function. Mmc1 is a distant relative of the dynamin superfamily of proteins (DSPs), GTPases, which are well established to shape and remodel membranes. Similar to DSPs, Mmc1 self-associates and forms high-molecular-weight assemblies. Interestingly, however, Mmc1 is a pseudoenzyme that lacks key residues required for GTP binding and hydrolysis, suggesting that it does not dynamically remodel membranes. These data are consistent with the model that Mmc1 stabilizes cristae architecture by acting as a scaffold to support cristae ultrastructure on the matrix side of the inner membrane. Our study reveals a new class of proteins that evolved early in fungal phylogeny and is required for the maintenance of cristae architecture. This highlights the possibility that functionally analogous proteins work with MICOS to establish cristae morphology in metazoans.

Genetic ablation of Immt induces a lethal disruption of the MICOS complex.

The mitochondrial contact site and cristae organizing system (MICOS) is important for crista junction formation and for maintaining inner mitochondrial membrane architecture. A key component of the MICOS complex is MIC60, which has been well studied in yeast and cell culture models. However, only one recent study has demonstrated the embryonic lethality of losing Immt (the gene encoding MIC60) expression. Tamoxifen-inducible ROSA-CreERT2-mediated deletion of Immt in adult mice disrupted the MICOS complex, increased mitochondria size, altered cristae morphology, and was lethal within 12 d. Pathologically, these mice displayed defective intestinal muscle function (paralytic ileus) culminating in dehydration. We also identified bone marrow (BM) hypocellularity in Immt-deleted mice, although BM transplants from wild-type mice did not improve survival. Altogether, this inducible mouse model demonstrates the importance of MIC60 in vivo, in both hematopoietic and non-hematopoietic tissues, and provides a valuable resource for future mechanistic investigations into the MICOS complex.

Mic19 depletion impairs endoplasmic reticulum-mitochondrial contacts and mitochondrial lipid metabolism and triggers liver disease

Endoplasmic reticulum (ER)-mitochondria contacts are critical for the regulation of lipid transport, synthesis, and metabolism. However, the molecular mechanism and physiological function of endoplasmic reticulum-mitochondrial contacts remain unclear. Here, we show that Mic19, a key subunit of MICOS (mitochondrial contact site and cristae organizing system) complex, regulates ER-mitochondria contacts by the EMC2-SLC25A46-Mic19 axis. Mic19 liver specific knockout (LKO) leads to the reduction of ER-mitochondrial contacts, mitochondrial lipid metabolism disorder, disorganization of mitochondrial cristae and mitochondrial unfolded protein stress response in mouse hepatocytes, impairing liver mitochondrial fatty acid β-oxidation and lipid metabolism, which may spontaneously trigger nonalcoholic steatohepatitis (NASH) and liver fibrosis in mice. Whereas, the re-expression of Mic19 in Mic19 LKO hepatocytes blocks the development of liver disease in mice. In addition, Mic19 overexpression suppresses MCD-induced fatty liver disease. Thus, our findings uncover the EMC2-SLC25A46-Mic19 axis as a pathway regulating ER-mitochondria contacts, and reveal that impairment of ER-mitochondria contacts may be a mechanism associated with the development of NASH and liver fibrosis.

Three-dimensional mitochondria reconstructions of murine cardiac muscle changes in size across aging.

With sparse treatment options, cardiac disease remains a significant cause of death among humans. As a person ages, mitochondria breakdown and the heart becomes less efficient. Heart failure is linked to many mitochondria-associated processes, including endoplasmic reticulum stress, mitochondrial bioenergetics, insulin signaling, autophagy, and oxidative stress. The roles of key mitochondrial complexes that dictate the ultrastructure, such as the mitochondrial contact site and cristae organizing system (MICOS), in aging cardiac muscle are poorly understood. To better understand the cause of age-related alteration in mitochondrial structure in cardiac muscle, we used transmission electron microscopy (TEM) and serial block facing-scanning electron microscopy (SBF-SEM) to quantitatively analyze the three-dimensional (3-D) networks in cardiac muscle samples of male mice at aging intervals of 3 mo, 1 yr, and 2 yr. Here, we present the loss of cristae morphology, the inner folds of the mitochondria, across age. In conjunction with this, the three-dimensional (3-D) volume of mitochondria decreased. These findings mimicked observed phenotypes in murine cardiac fibroblasts with CRISPR/Cas9 knockout of Mitofilin, Chchd3, Chchd6 (some members of the MICOS complex), and Opa1, which showed poorer oxidative consumption rate and mitochondria with decreased mitochondrial length and volume. In combination, these data show the need to explore if loss of the MICOS complex in the heart may be involved in age-associated mitochondrial and cristae structural changes.NEW & NOTEWORTHY This article shows how mitochondria in murine cardiac changes, importantly elucidating age-related changes. It also is the first to show that the MICOS complex may play a role in outer membrane mitochondrial structure.

MICU1 regulates mitochondrial cristae structure and function independently of the mitochondrial Ca2+ uniporter channel.

MICU1 is a calcium (Ca2+)-binding protein that regulates the mitochondrial Ca2+ uniporter channel complex (mtCU) and mitochondrial Ca2+ uptake. MICU1 knockout mice display disorganized mitochondrial architecture, a phenotype that is distinct from that of mice with deficiencies in other mtCU subunits and, thus, is likely not explained by changes in mitochondrial matrix Ca2+ content. Using proteomic and cellular imaging techniques, we found that MICU1 localized to the mitochondrial contact site and cristae organizing system (MICOS) and directly interacted with the MICOS components MIC60 and CHCHD2 independently of the mtCU. We demonstrated that MICU1 was essential for MICOS complex formation and that MICU1 ablation resulted in altered cristae organization, mitochondrial ultrastructure, mitochondrial membrane dynamics, and cell death signaling. Together, our results suggest that MICU1 is an intermembrane space Ca2+ sensor that modulates mitochondrial membrane dynamics independently of matrix Ca2+ uptake. This system enables distinct Ca2+ signaling in the mitochondrial matrix and at the intermembrane space to modulate cellular energetics and cell death in a concerted manner.

3D reconstruction shows mouse skeletal muscle may decline in function across aging due to the MICOS complex

Mitochondria are active organelles, undergoing harmonized cycles of fission and fusion, driven by ‘mitochondrial dynamics,’ to retain their morphology, distribution, and size. Mitochondrial dynamics has emerged as a critical process in maintaining cellular homeostasis. Interestingly, it has been long respected that a decrease in mitochondrial function escorts aging in specific tissues. For example, skeletal muscle gradually loses mass, strength, endurance, and oxidative capacity during aging. This decrease might, in turn, contribute to the observed age-dependent decline in organ function by mutations or alterations in mitochondrial fusion and fission proteins associated with several diseases. However, new players in regulating mitochondrial shape and cristae shape have been recently studied, such as the mitochondrial contact site and cristae organizing system (MICOS) complex. The MICOS complex is a multi-protein interaction hub that helps define cristate and mitochondria architecture. However, the MICOS complex has not been implicated in regulating organelle structure changes during aging. Thus, we hypothesized that loss of the MICOS complex during aging may increase mitochondrial fragmentation, decrease nanotunnels, and alter cristae morphology. To do this, we examined the three-dimensional morphology of mitochondria networks in young (3-month) and aged (2-year) murine gastrocnemius muscle via serial block face-scanning electron microscopy and the Amira program for segmentation, analysis, and quantification. We found differences in mitochondrial network configuration, nanotunneling, size, shape, number, contact sites, MICOS dynamics, and gene expression in skeletal muscle during aging. We also found an association between OPA-1 and the MICOS complex in the gastrocnemius with mitochondrial aging. Furthermore, the loss of the MICOS complex was linked with decreased oxidative capacity and altered mitochondrial metabolism. Potentially, this suggests a novel relationship between the MICOS complex and aging in skeletal muscle.

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.

The essential cysteines in the CIPC motif of the thioredoxin-like Trypanosoma brucei MICOS subunit TbMic20 do not form an intramolecular disulfide bridge in vivo

The mitochondrial protein import machinery of trypanosomatids is highly divergent from that of the well-studied models such as baker’s yeast. A notable example is that the central catalyst of the mitochondrial intermembrane space import and assembly pathway (MIA), named Mia40, is missing in trypanosomatids. Mia40 works in a two-step process. First it recognizes by direct binding reduced MIA substrate proteins and then catalyzes their oxidative folding to produce intramolecular disulfide bridges. It was recently proposed that a thioredoxin-like subunit of the trypanosomal mitochondrial contact site and cristae organizing system (MICOS) called TbMic20 may be the Mia40 replacement. Our study performed on procyclic stage of the parasite revealed that each of the two cysteines in TbMic20′s active site is essential for the stability of MIA substrate proteins although they do not form a disulfide bridge in vivo. The two cysteines of Mia40′s active site form an intramolecular disulfide bridge at steady state, which is a prerequisite for its oxidative folding of MIA substrates. Thus, we conclude that TbMic20 is unlikely to represent a bona fide Mia40 replacement and plays a still unresolved role in the stability and/or import of MIA substrates in trypanosomatids. Despite this, the effect of TbMic20 depletion and mutation indicates that the trypanosomal MICOS complex still plays a vital role in the maturation and/or stability of proteins imported by the MIA pathway.

SAMM50 acts with p62 in piecemeal basal- and OXPHOS-induced mitophagy of SAM and MICOS components.

Mitophagy is the degradation of surplus or damaged mitochondria by autophagy. In addition to programmed and stress-induced mitophagy, basal mitophagy processes exert organelle quality control. Here, we show that the sorting and assembly machinery (SAM) complex protein SAMM50 interacts directly with ATG8 family proteins and p62/SQSTM1 to act as a receptor for a basal mitophagy of components of the SAM and mitochondrial contact site and cristae organizing system (MICOS) complexes. SAMM50 regulates mitochondrial architecture by controlling formation and assembly of the MICOS complex decisive for normal cristae morphology and exerts quality control of MICOS components. To this end, SAMM50 recruits ATG8 family proteins through a canonical LIR motif and interacts with p62/SQSTM1 to mediate basal mitophagy of SAM and MICOS components. Upon metabolic switch to oxidative phosphorylation, SAMM50 and p62 cooperate to mediate efficient mitophagy.

Abstract 15070: Micu1 Regulates Mitochondrial Cristae Structure and Function Independent of the Mitochondrial Calcium Uniporter Channel

Background: MICU1 is an EF-hand domain containing Ca 2+ -sensor regulating the mitochondrial Ca 2+ uniporter channel and mitochondrial Ca 2+ uptake. MICU1-null mouse and fly models display perinatal lethality with disorganized mitochondrial architecture. Interestingly, these phenotypes are distinct from other mtCU loss-of-function models ( MCU, MICU2, EMRE, MCUR1 ) and thus are likely not explained solely by changes in matrix Ca 2+ content. Using size-exclusion proteomics and co-immunofluorescence, we found that MICU1 localizes to mitochondrial complexes lacking MCU. These observations suggest that MICU1 may have additional cellular functions independent of the MCU. Methods: Biotin-based proximity labeling and proteomics, protein biochemistry, live-cell Ca 2+ imaging, electron microscopy, confocal and super-resolution imaging were utilized to identify and validate MICU1 novel functions. Results: The expression of a MICU1-BioID2 fusion protein in MCU +/+ and MCU -/- cells allowed the identification of the total vs. MCU-independent MICU1 interactome. LC-MS analysis of purified biotinylated proteins identified the mitochondrial contact site and cristae organizing system (MICOS) components Mitofilin (MIC60) and Coiled-coil-helix-coiled-coil helix domain containing 2 (CHCHD2) as MCU independent novel MICU1 interactors. We demonstrate that MICU1 is essential for proper organization of the MICOS complex and that MICU1 ablation results in altered cristae organization, mitochondrial ultrastructure, mitochondrial membrane dynamics, membrane potential, and cell death signaling. We hypothesize that MICU1 is a MICOS Ca 2+ - sensor since perturbing MICU1 is sufficient to modulate cytochrome c release independent of Ca 2+ uptake across the inner mitochondrial membrane. Conclusions: Here, we provide the first experimental evidence of an intermembrane space Ca 2+ - sensor regulating mitochondrial membrane dynamics, independent of changes in matrix Ca 2+ content. This study provides a novel paradigm to understand Ca 2+ -dependent regulation of mitochondrial structure and function and may help explain the mitochondrial remodeling reported to occur in numerous disease states.

Cristae Membrane Dynamics - A Paradigm Change.

Mitochondria are dynamic organelles that have essential metabolic and regulatory functions. Earlier studies using electron microscopy (EM) revealed an immense diversity in the architecture of cristae - infoldings of the mitochondrial inner membrane (IM) - in different cells, tissues, bioenergetic and metabolic conditions, and during apoptosis. However, cristae were considered to be largely static entities. Recently, advanced super-resolution techniques have revealed that cristae are independent bioenergetic units that are highly dynamic and remodel on a timescale of seconds. These advances, coupled with mechanistic and structural studies on key molecular players, such as the MICOS (mitochondrial contact site and cristae organizing system) complex and the dynamin-like GTPase OPA1, have changed our view on mitochondria in a fundamental way. We summarize these recent findings and discuss their functional implications.

An antibody toolbox to track complex I assembly defines AIF's mitochondrial function.

An ability to comprehensively track the assembly intermediates (AIs) of complex I (CI) biogenesis in Drosophila will enable the characterization of the precise mechanism(s) by which various CI regulators modulate CI assembly. Accordingly, we generated 21 novel antibodies to various mitochondrial proteins and used this resource to characterize the mechanism by which apoptosis-inducing factor (AIF) regulates CI biogenesis by tracking the AI profile observed when AIF expression is impaired. We find that when the AIF-Mia40 translocation complex is disrupted, the part of CI that transfers electrons to ubiquinone is synthesized but fails to progress in the CI biosynthetic pathway. This is associated with a reduction in intramitochondrial accumulation of the Mia40 substrate, MIC19. Importantly, knockdown of either MIC19 or MIC60, components of the mitochondrial contact site and cristae organizing system (MICOS), fully recapitulates the AI profile observed when AIF is inhibited. Thus, AIF's effect on CI assembly is principally due to compromised intramitochondrial transport of the MICOS complex.

CARD19 interacts with MICOS complex proteins and protects against mitochondrial dysfunction in macrophages

Abstract CARD19 is a mitochondrial protein of unknown function; gene expression databases indicate that CARD19 is highly expressed in myeloid cells. We have observed that Card19 −/− mice injected with LPS display elevated levels of some pro-inflammatory cytokines relative to Card19 +/+ mice. Because CARD19 is a mitochondrial protein and mitochondrial dysfunction is frequently associated with immune dysregulation and chronic inflammation, we sought to identify the function of CARD19 in macrophages. We demonstrated via super resolution microscopy that endogenous CARD19 colocalizes with mitochondrial markers in bone marrow derived macrophages (BMDMs) and has a punctate distribution. Through immunoprecipitation and mass spectrometry analyses, we determined that CARD19 interacts with MIC19 and other components of the mitochondrial contact site and cristae organizing system (MICOS) complex. We further demonstrated that CARD19 colocalizes with MIC19 in mitochondrial sub-domains via super resolution microscopy. We then measured outcomes of mitochondrial stress in BMDMs. Consistent with previous reports of MICOS deficiencies, we found that Card19 −/− BMDMs have a modestly decreased oxygen consumption rate (OCR) measured by a Seahorse extracellular flux analyzer relative to Card19 +/+ BMDMs. Additionally, Card19 −/− BMDMs display elevated levels of mitochondrial reaction oxygen species (mROS) and a population of mitochondria with a decreased mitochondrial membrane potential, as measured by flow cytometry. Based on these data, we propose that CARD19 may be a previously unknown regulator of MICOS function which potentially links this complex to other mitochondrial and/or non-mitochondrial CARD proteins in macrophages.

Miro clusters regulate ER-mitochondria contact sites and link cristae organization to the mitochondrial transport machinery

Mitochondrial Rho (Miro) GTPases localize to the outer mitochondrial membrane and are essential machinery for the regulated trafficking of mitochondria to defined subcellular locations. However, their sub-mitochondrial localization and relationship with other critical mitochondrial complexes remains poorly understood. Here, using super-resolution fluorescence microscopy, we report that Miro proteins form nanometer-sized clusters along the mitochondrial outer membrane in association with the Mitochondrial Contact Site and Cristae Organizing System (MICOS). Using knockout mouse embryonic fibroblasts we show that Miro1 and Miro2 are required for normal mitochondrial cristae architecture and Endoplasmic Reticulum-Mitochondria Contacts Sites (ERMCS). Further, we show that Miro couples MICOS to TRAK motor protein adaptors to ensure the concerted transport of the two mitochondrial membranes and the correct distribution of cristae on the mitochondrial membrane. The Miro nanoscale organization, association with MICOS complex and regulation of ERMCS reveal new levels of control of the Miro GTPases on mitochondrial functionality.

Regulation of mitochondrial cristae remodelling by acetylcholine alleviates palmitate-induced cardiomyocyte hypertrophy

Mitochondrial dysfunction is associated with obesity-induced cardiac remodelling. Recent research suggests that the cristae are the true bioenergetic components of cells. Acetylcholine (ACh), the major neurotransmitter of the vagus nerve, exerts cardio-protective effects against ischaemia. This study investigated the role of cristae remodelling in palmitate (PA)-induced neonatal rat cardiomyocyte hypertrophy and explored the beneficial effects of ACh. We found loose, fragmented and even lysed cristae in PA-treated neonatal cardiomyocytes along with declines in mitochondrial network and complex expression and overproduction of mitochondrial reactive oxygen species (ROS); these changes ultimately resulted in increased myocardial size. Overexpression of mitofilin by adenoviral infection partly improved cristae shape, mitochondrial network, and ATP content and attenuated cell hypertrophy. Interestingly, siRNA-mediated AMP-activated protein kinase (AMPK) silencing increased the number of cristae with a balloon-like morphology without disturbing mitofilin expression. Furthermore, AMPK knockdown abolished the effects of mitofilin overexpression on cristae remodelling and inhibited the interaction of mitofilin with sorting and assembly machinery 50 (Sam50) and coiled-coil helix coiled-coil helix domain-containing protein 3 (CHCHD3), two core components of the mitochondrial contact site and cristae organizing system (MICOS) complex. Intriguingly, ACh upregulated mitofilin expression and AMPK phosphorylation via the muscarinic ACh receptor (MAChR). Moreover, ACh enhanced protein-protein interactions between mitofilin and other components of the MICOS complex, thereby preventing PA-induced mitochondrial dysfunction and cardiomyocyte hypertrophy; however, these effects were abolished by AMPK silencing. Taken together, our data suggest that ACh improves cristae remodelling to defend against PA-induced myocardial hypertrophy, presumably by increasing mitofilin expression and activating AMPK to form the MICOS complex through MAChR. These results suggest new and promising therapeutic approaches targeting mitochondria to prevent lipotoxic cardiomyopathy.

A MICOS–TIM22 Association Promotes Carrier Import into Human Mitochondria

Mitochondrial membrane proteins with internal targeting signals are inserted into the inner membrane by the carrier translocase (TIM22 complex). For this, precursors have to be initially directed from the TOM complex in the outer mitochondrial membrane across the intermembrane space toward the TIM22 complex. How these two translocation processes are topologically coordinated is still unresolved. Using proteomic approaches, we find that the human TIM22 complex associates with the mitochondrial contact site and cristae organizing system (MICOS) complex. This association does not appear to be conserved in yeast, whereby the yeast MICOS complex instead interacts with the presequence translocase. Using a yeast mic10Δ strain and a HEK293T MIC10 knockout cell line, we characterize the role of MICOS for protein import into the mitochondrial inner membrane and matrix. We find that a physiological cristae organization promotes efficient import via the presequence pathway in yeast, while in human mitochondria, the MICOS complex is dispensable for protein import along the presequence pathway. However, in human mitochondria, the MICOS complex is required for the efficient import of carrier proteins into the mitochondrial inner membrane. Our analyses suggest that in human mitochondria, positioning of the carrier translocase at the crista junction, and potentially in vicinity to the TOM complex, is required for efficient transport into the inner membrane.

Sam50-Mic19-Mic60 axis determines mitochondrial cristae architecture by mediating mitochondrial outer and inner membrane contact.

Mitochondrial cristae are critical for efficient oxidative phosphorylation, however, how cristae architecture is precisely organized remains largely unknown. Here, we discovered that Mic19, a core component of MICOS (mitochondrial contact site and cristae organizing system) complex, can be cleaved at N-terminal by mitochondrial protease OMA1 under certain physiological stresses. Mic19 directly interacts with mitochondrial outer-membrane protein Sam50 (the key subunit of SAM complex) and inner-membrane protein Mic60 (the key component of MICOS complex) to form Sam50-Mic19-Mic60 axis, which dominantly connects SAM and MICOS complexes to assemble MIB (mitochondrial intermembrane space bridging) supercomplex for mediating mitochondrial outer- and inner-membrane contact. OMA1-mediated Mic19 cleavage causes Sam50-Mic19-Mic60 axis disruption, which separates SAM and MICOS and leads to MIB disassembly. Disrupted Sam50-Mic19-Mic60 axis, even in the presence of SAM and MICOS complexes, causes the abnormal mitochondrial morphology, loss of mitochondrial cristae junctions, abnormal cristae distribution and reduced ATP production. Importantly, Sam50 displays punctate distribution at mitochondrial outer membrane, and acts as an anchoring point to guide the formation of mitochondrial cristae junctions. Therefore, we propose that Sam50-Mic19-Mic60 axis-mediated SAM-MICOS complexes integration determines mitochondrial cristae architecture.

Mic60 exhibits a coordinated clustered distribution along and across yeast and mammalian mitochondria

Mitochondria are tubular double-membrane organelles essential for eukaryotic life. They form extended networks and exhibit an intricate inner membrane architecture. The MICOS (mitochondrial contact site and cristae organizing system) complex, crucial for proper architecture of the mitochondrial inner membrane, is localized primarily at crista junctions. Harnessing superresolution fluorescence microscopy, we demonstrate that Mic60, a subunit of the MICOS complex, as well as several of its interaction partners are arranged into intricate patterns in human and yeast mitochondria, suggesting an ordered distribution of the crista junctions. We show that Mic60 forms clusters that are preferentially localized in the inner membrane at two opposing sides of the mitochondrial tubules so that they form extended opposing distribution bands. These Mic60 distribution bands can be twisted, resulting in a helical arrangement. Focused ion beam milling-scanning electron microscopy showed that in yeast the twisting of the opposing distribution bands is echoed by the folding of the inner membrane. We show that establishment of the Mic60 distribution bands is largely independent of the cristae morphology. We suggest that Mic60 is part of an extended multiprotein interaction network that scaffolds mitochondria.