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Mitochondria not only synthesize energy required for cellular functions but are also involved in numerous cellular pathways including apoptosis, calcium homoeostasis, inflammation and immunity. Mitochondria are dynamic organelles that undergo cycles of fission and fusion, and these transitions between fragmented and hyperfused networks ensure mitochondrial function, enabling adaptations to metabolic changes or cellular stress. Defects in mitochondrial morphology have been associated with numerous diseases, highlighting the importance of elucidating the molecular mechanisms regulating mitochondrial morphology. Here, we discuss recent structural insights into the assembly and mechanism of action of the core mitochondrial dynamics proteins, such as the dynamin-related protein 1 (DRP1) that controls division, and the mitofusins (MFN1 and MFN2) and optic atrophy 1 (OPA1) driving membrane fusion. Furthermore, we provide an updated view of the complex interplay between different proteins, lipids and organelles during the processes of mitochondrial membrane fusion and fission. Overall, we aim to present a valuable framework reflecting current perspectives on how mitochondrial membrane remodelling is regulated. Mitochondrial fusion and fission events are tightly regulated by core mitochondrial dynamics proteins. Recent structural and functional findings have characterized how these proteins interact with each other, with the mitochondrial membrane and with other organelles to guide membrane remodelling.

Peroxisomes—tiny intracellular organelles that contain metabolic enzymes—are generated in mammalian cells by the fusion of structures that arise from both mitochondria and the endoplasmic reticulum. Mitochondria have a role in peroxisome formation Peroxisomes are home to a variety of metabolic enzymes. These membrane-bound organelles work closely with mitochondria, exchanging metabolites for completion of a number of enzymatic cascades. Peroxisomes can either appear anew or divide from pre-existing organelles, specifically the endoplasmic reticulum. Intrigued by the knowledge that in the absence of peroxisomes, several integral peroxisomal membrane proteins are imported into the mitochondria of mammalian cells, Heidi McBride and co-workers investigate the role of mitochondria in the formation of peroxisomes. They find that in mammalian cells, peroxisomes are the product of structures that arise from not just the endoplasmic reticulum, but also from mitochondria. Peroxisomes function together with mitochondria in a number of essential biochemical pathways, from bile acid synthesis to fatty acid oxidation 1 . Peroxisomes grow and divide from pre-existing organelles 2 , but can also emerge de novo in the cell 3 . The physiological regulation of de novo peroxisome biogenesis remains unclear, and it is thought that peroxisomes emerge from the endoplasmic reticulum in both mammalian and yeast cells 4 . However, in contrast to the yeast system 5 , 6 , 7 , 8 , a number of integral peroxisomal membrane proteins are imported into mitochondria in mammalian cells in the absence of peroxisomes, including Pex3, Pex12, Pex13, Pex14, Pex26, PMP34 and ALDP 9 , 10 , 11 , 12 , 13 , 14 , 15 . Overall, the mitochondrial localization of peroxisomal membrane proteins in mammalian cells has largely been considered a mis-targeting artefact in which de novo biogenesis occurs exclusively from endoplasmic reticulum-targeted peroxins 16 . Here, in following the generation of new peroxisomes within human patient fibroblasts lacking peroxisomes, we show that the essential import receptors Pex3 and Pex14 target mitochondria, where they are selectively released into vesicular pre-peroxisomal structures. Maturation of pre-peroxisomes containing Pex3 and Pex14 requires fusion with endoplasmic reticulum-derived vesicles carrying Pex16, thereby providing full import competence. These findings demonstrate the hybrid nature of newly born peroxisomes, expanding their functional links to mitochondria.

Mitochondrial plasticity is a key regulator of cell fate decisions. Mitochondrial division involves Dynamin-related protein-1 (Drp1) oligomerization, which constricts membranes at endoplasmic reticulum (ER) contact sites. The mechanisms driving the final steps of mitochondrial division are still unclear. Here, we found that microdomains of phosphatidylinositol 4-phosphate [PI(4)P] on trans-Golgi network (TGN) vesicles were recruited to mitochondria–ER contact sites and could drive mitochondrial division downstream of Drp1. The loss of the small guanosine triphosphatase ADP-ribosylation factor 1 (Arf1) or its effector, phosphatidylinositol 4-kinase IIIβ [PI(4)KIIIβ], in different mammalian cell lines prevented PI(4)P generation and led to a hyperfused and branched mitochondrial network marked with extended mitochondrial constriction sites. Thus, recruitment of TGN-PI(4)P–containing vesicles at mitochondria–ER contact sites may trigger final events leading to mitochondrial scission.

Neuronal mitochondria play important roles beyond ATP generation, including Ca 2+ uptake, and therefore have instructive roles in synaptic function and neuronal response properties. Mitochondrial morphology differs significantly between the axon and dendrites of a given neuronal subtype, but in CA1 pyramidal neurons (PNs) of the hippocampus, mitochondria within the dendritic arbor also display a remarkable degree of subcellular, layer-specific compartmentalization. In the dendrites of these neurons, mitochondria morphology ranges from highly fused and elongated in the apical tuft, to more fragmented in the apical oblique and basal dendritic compartments, and thus occupy a smaller fraction of dendritic volume than in the apical tuft. However, the molecular mechanisms underlying this striking degree of subcellular compartmentalization of mitochondria morphology are unknown, precluding the assessment of its impact on neuronal function. Here, we demonstrate that this compartment-specific morphology of dendritic mitochondria requires activity-dependent, Ca 2+ and Camkk2-dependent activation of AMPK and its ability to phosphorylate two direct effectors: the pro-fission Drp1 receptor Mff and the recently identified anti-fusion, Opa1-inhibiting protein, Mtfr1l. Our study uncovers a signaling pathway underlying the subcellular compartmentalization of mitochondrial morphology in dendrites of neurons in vivo through spatially precise and activity-dependent regulation of mitochondria fission/fusion balance. The mechanisms regulating mitochondrial architecture in neurons remain unclear. The authors report that in dendrites, mitochondria structure is specified by the CAMKK2-AMPK pathway through compartment-specific and activity-dependent levels of fission.

Hepatic metabolism requires mitochondria to adapt their bioenergetic and biosynthetic output to accompany the ever-changing anabolic/catabolic state of the liver cell, but the wiring of this process is still largely unknown. Using a postprandial mouse liver model and quantitative cryo-EM analysis, we show that when the hepatic mammalian target of rapamycin complex 1 (mTORCI) signaling pathway disengages, the mitochondria network fragments, cristae density drops by 30%, and mitochondrial respiratory capacity decreases by 20%. Instead, mitochondria-ER contacts (MERCs), which mediate calcium and phospholipid fluxes between these organelles, double in length. These events are associated with the transient expression of two previously unidentified C-terminal fragments (CTFs) of Optic atrophy 1 (Opa1), a mitochondrial GTPase that regulates cristae biogenesis and mitochondria dynamics. Expression of Opa1 CTFs in the intermembrane space has no effect on mitochondria morphology, supporting a model in which they are intermediates of an Opa1 degradation program. Using an in vitro assay, we show that these CTFs indeed originate from the cleavage of Opa1 at two evolutionarily conserved consensus sites that map within critical folds of the GTPase. This processing of Opa1, termed Gcleavage, is mediated by the activity of a cysteine protease whose activity is independent from that of Oma1 and presenilin-associated rhomboid-like (PARL), two known Opal regulators. However, C-deavage requires Mitofusin-2 (Mfn2), a key factor in mitochondria-ER tethering, thereby linking cristae remodeling to MERC assembly. Thus, in vivo, mitochondria adapt to metabolic shifts through the parallel remodeling of the cristae and of the MERCs via a mechanism that degrades Opa1 in an Mfn2-dependent pathway.

Inactivating mutations in SMARCA4 and concurrent epigenetic silencing of SMARCA2 characterize subsets of ovarian and lung cancers. Concomitant loss of these key subunits of SWI/SNF chromatin remodeling complexes in both cancers is associated with chemotherapy resistance and poor prognosis. Here, we discover that SMARCA4/2 loss inhibits chemotherapy-induced apoptosis through disrupting intracellular organelle calcium ion (Ca 2+ ) release in these cancers. By restricting chromatin accessibility to ITPR3 , encoding Ca 2+ channel IP3R3, SMARCA4/2 deficiency causes reduced IP3R3 expression leading to impaired Ca 2+ transfer from the endoplasmic reticulum to mitochondria required for apoptosis induction. Reactivation of SMARCA2 by a histone deacetylase inhibitor rescues IP3R3 expression and enhances cisplatin response in SMARCA4/2-deficient cancer cells both in vitro and in vivo. Our findings elucidate the contribution of SMARCA4/2 to Ca 2+ -dependent apoptosis induction, which may be exploited to enhance chemotherapy response in SMARCA4/2-deficient cancers. SMARCA4/2 loss in ovarian and lung cancers is associated with chemotherapy resistance. Here, the authors show that SMARCA4/2 deficiency in cancer cells reduces the expression of the ER-Ca 2+ channel IP3R3 and subsequently calcium transfer to the mitochondria, which inhibits apoptotic cell death.

Mitochondria form a dynamic network that responds to physiological signals and metabolic stresses by altering the balance between fusion and fission. Mitochondrial fusion is orchestrated by conserved GTPases MFN1/2 and OPA1, a process coordinated in yeast by Ugo1, a mitochondrial metabolite carrier family protein. We uncovered a homozygous missense mutation in SLC25A46 , the mammalian orthologue of Ugo1 , in a subject with Leigh syndrome. SLC25A46 is an integral outer membrane protein that interacts with MFN2, OPA1, and the mitochondrial contact site and cristae organizing system (MICOS) complex. The subject mutation destabilizes the protein, leading to mitochondrial hyperfusion, alterations in endoplasmic reticulum (ER) morphology, impaired cellular respiration, and premature cellular senescence. The MICOS complex is disrupted in subject fibroblasts, resulting in strikingly abnormal mitochondrial architecture, with markedly shortened cristae. SLC25A46 also interacts with the ER membrane protein complex EMC, and phospholipid composition is altered in subject mitochondria. These results show that SLC25A46 plays a role in a mitochondrial/ER pathway that facilitates lipid transfer, and link altered mitochondrial dynamics to early‐onset neurodegenerative disease and cell fate decisions. Synopsis Whole‐exome sequencing in a Leigh syndrome patient identified mutations in SLC25A46, a degenerate member of the mitochondrial metabolite transport family, linking altered mitochondrial dynamics to early‐onset neurodegenerative disease. Loss of SLC25A46 results in mitochondrial hyperfusion and striking changes in mitochondrial architecture. SLC25A46 is an outer membrane protein that interacts with MFN2, OPA1, the MICOS complex, and the EMC complex in the ER. Loss of SLC25A46 results in altered ER morphology and marked changes in the phospholipid composition of the mitochondrial membranes. Loss of SLC25A46 results in premature cellular senescence in dividing cells. Graphical Abstract Whole‐exome sequencing in a Leigh syndrome patient identified mutations in SLC25A46, a degenerate member of the mitochondrial metabolite transport family, linking altered mitochondrial dynamics to early‐onset neurodegenerative disease.

Atherosclerotic lesions show significant mitochondrial dysfunction but the underlying mechanisms and consequences remain unknown. Cardiolipin is a phospholipid found exclusively in the mitochondrial inner membrane, the site of oxidative phosphorylation. Tafazzin is a trans-acylase that acylates immature monolysocardiolipin to mature cardiolipin. Tafazzin mutations can result in Barth’s Syndrome, which is characterised by dilated cardiomyopathy, skeletal myopathy and impaired growth. However, a role for tafazzin in atherosclerosis development has not been previously identified. Here we show that tafazzin expression is decreased in atherosclerotic lesions and specifically in plaque vascular smooth muscle cells (VSMCs). MicroRNA 125a-5p expression is increased in plaques, downregulates tafazzin expression and is induced by oxidised low-density lipoprotein in a NFκB-dependent manner. Silencing tafazzin or overexpression of mutant tafazzin decreases VSMC cardiolipin content and mitochondrial respiration, and promotes apoptosis and atherosclerosis. In contrast tafazzin overexpression increases respiration, protects against apoptosis and increases features of plaque stability. Tafazzin therefore has important effects on VSMC mitochondrial function and atherosclerosis, and is a potential therapeutic target in atherosclerotic disease. Atherosclerosis is the leading cause of death worldwide. Here, the authors show that defective vascular smooth muscle cell tafazzin promotes mitochondrial dysfunction and atherosclerosis, highlighting tafazzin as a potential therapeutic target.

A biallelic missense mutation in mitofusin 2 (MFN2) causes multiple symmetric lipomatosis and partial lipodystrophy, implicating disruption of mitochondrial fusion or interaction with other organelles in adipocyte differentiation, growth and/or survival. In this study, we aimed to document the impact of loss of mitofusin 1 (Mfn1) or 2 (Mfn2) on adipogenesis in cultured cells. We characterised adipocyte differentiation of wildtype (WT), Mfn1-/- and Mfn2-/- mouse embryonic fibroblasts (MEFs) and 3T3-L1 preadipocytes in which Mfn1 or 2 levels were reduced using siRNA. Mfn1-/- MEFs displayed striking fragmentation of the mitochondrial network, with surprisingly enhanced propensity to differentiate into adipocytes, as assessed by lipid accumulation, expression of adipocyte markers (Plin1, Fabp4, Glut4, Adipoq), and insulin-stimulated glucose uptake. RNA sequencing revealed a corresponding pro-adipogenic transcriptional profile including Pparg upregulation. Mfn2-/- MEFs also had a disrupted mitochondrial morphology, but in contrast to Mfn1-/- MEFs they showed reduced expression of adipocyte markers. Mfn1 and Mfn2 siRNA mediated knockdown studies in 3T3-L1 adipocytes generally replicated these findings. Loss of Mfn1 but not Mfn2 in cultured pre-adipocyte models is pro-adipogenic. This suggests distinct, non-redundant roles for the two mitofusin orthologues in adipocyte differentiation.

Impaired mitochondrial bioenergetics in macrophages promotes hyperinflammatory cytokine responses, but whether inherited mtDNA mutations drive similar phenotypes is unknown. Here, we profiled macrophages harbouring a heteroplasmic mitochondrial tRNA Ala mutation ( m.5019A > G ) to address this question. These macrophages exhibit combined respiratory chain defects, reduced oxidative phosphorylation, disrupted cristae architecture, and compensatory metabolic adaptations in central carbon metabolism. Upon inflammatory activation, m.5019A > G macrophages produce elevated type I interferon (IFN), while exhibiting reduced pro-inflammatory cytokines and oxylipins. Mechanistically, suppression of pro-IL-1β and COX2 requires autocrine IFN-β signalling. IFN-β induction is biphasic: an early TLR4-IRF3 driven phase, and a later response involving mitochondrial nucleic acids and the cGAS-STING pathway. In vivo, lipopolysaccharide (LPS) challenge of m.5019A > G mice results in elevated type I IFN signalling and exacerbated sickness behaviour. These findings reveal that a pathogenic mtDNA mutation promotes an imbalanced innate immune response, which has potential implications for the progression of pathology in mtDNA disease patients. Inherited mitochondrial DNA mutations can result in diverse clinical phenotypes. Here, the authors characterise a heteroplasmic tRNA Ala mutation ( m.5019A > G ) in mice and demonstrate that macrophages carrying this mutation display altered function and metabolism in vitro, along with increased type I IFN release following LPS challenge in vivo.