Mitochondrial division and fusion in metabolism

Bioactive food and exercise in chronic kidney disease: Targeting the mitochondria

Chronic kidney disease (CKD), which affects 10%-15% of the population, associates with a range of complications-such as cardiovascular disease, frailty, infections, muscle and bone disorders and premature ageing-that could be related to alterations of mitochondrial number, distribution, structure and function. As mitochondrial biogenesis, bioenergetics and the dynamic mitochondrial networks directly or indirectly regulate numerous intra- and extracellular functions, the mitochondria have emerged as an important target for interventions aiming at preventing or improving the treatment of complications in CKD. In this review, we discuss the possible role of bioactive food compounds and exercise in the modulation of the disturbed mitochondrial function in a uraemic milieu.

Why don't mice lacking the mitochondrial Ca2+ uniporter experience an energy crisis?

Current dogma holds that the heart balances energy demand and supply effectively and sustainably by sequestering enough Ca²⁺ into mitochondria during heartbeats to stimulate metabolic enzymes in the tricarboxylic acid (TCA) cycle and electron transport chain (ETC). This process is called excitation-contraction-bioenergetics (ECB) coupling. Recent breakthroughs in identifying the mitochondrial Ca²⁺ uniporter (MCU) and its associated proteins have opened up new windows for interrogating the molecular mechanisms of mitochondrial Ca²⁺ homeostasis regulation and its role in ECB coupling. Despite remarkable progress made in the past 7 years, it has been surprising, almost disappointing, that germline MCU deficiency in mice with certain genetic background yields viable pups, and knockout of the MCU in adult heart does not cause lethality. Moreover, MCU deficiency results in few adverse phenotypes, normal performance, and preserved bioenergetics in the heart at baseline. In this review, we briefly assess the existing literature on mitochondrial Ca²⁺ homeostasis regulation and then we consider possible explanations for why MCU-deficient mice are spared from energy crises under physiological conditions. We propose that MCU and/or mitochondrial Ca²⁺ may have limited ability to set ECB coupling, that other mitochondrial Ca²⁺ handling mechanisms may play a role, and that extra-mitochondrial Ca²⁺ may regulate ECB coupling. Since the heart needs to regenerate a significant amount of ATP to assure the perpetuation of heartbeats, multiple mechanisms are likely to work in concert to match energy supply with demand.

Mitochondrial Stasis Reveals p62-Mediated Ubiquitination in Parkin-Independent Mitophagy and Mitigates Nonalcoholic Fatty Liver Disease

It is unknown what occurs if both mitochondrial division and fusion are completely blocked. Here, we introduced mitochondrial stasis by deleting two dynamin-related GTPases for division (Drp1) and fusion (Opa1) in livers. Mitochondrial stasis rescues liver damage and hypotrophy caused by the single knockout (KO). At the cellular level, mitochondrial stasis re-establishes mitochondrial size and rescues mitophagy defects caused by division deficiency. Using Drp1KO livers, we found that the autophagy adaptor protein p62/sequestosome-1-which is thought to function downstream of ubiquitination-promotes mitochondrial ubiquitination. p62 recruits two subunits of a cullin-RING ubiquitin E3 ligase complex, Keap1 and Rbx1, to mitochondria. Resembling Drp1KO, diet-induced nonalcoholic fatty livers enlarge mitochondria and accumulate mitophagy intermediates. Resembling Drp1Opa1KO, Opa1KO rescues liver damage in this disease model. Our data provide a new concept that mitochondrial stasis leads the spatial dimension of mitochondria to a stationary equilibrium and a new mechanism for mitochondrial ubiquitination in mitophagy.

Reinventing an Organelle: The Reduced Mitochondrion in Parasitic Protists

Mitochondria originated from the endosymbiotic event commencing from the engulfment of an ancestral α-proteobacterium by the first eukaryotic ancestor. Establishment of niches has led to various adaptations among eukaryotes. In anaerobic parasitic protists, the mitochondria have undergone modifications by combining features shared from the aerobic mitochondria with lineage-specific components and mechanisms; a diversified class of organelles emerged and are generally called mitochondrion-related organelles (MROs). In this review we summarize and discuss the recent advances in the knowledge of MROs from parasitic protists, particularly the themes such as metabolic functions, contribution to parasitism, dynamics, protein targeting, and novel lineage- specific proteins, with emphasis on the diversity among these organelles.

DNM1L, a key prognostic predictor for gastric adenocarcinoma, is involved in cell proliferation, invasion, and apoptosis

Dynamin-1-like protein (DNM1L) encodes a member of the dynamin superfamily of GTPases. It mediates mitochondrial and peroxisomal division and is involved in the regulation of apoptosis. However, its role in gastric cancer remains unclear. MKN-45 gastric cancer cells were transfected with short hairpin RNA (shRNA) to suppress DNM1L expression. MTT, flow cytometry, and Transwell assays were used to detect the changes in cell proliferation, apoptosis, and invasion, respectively. Immunohistochemistry was used to detect DNM1L expression in gastric adenocarcinoma specimens, and the association of DNM1L expression with clinicopathological features and prognosis was analyzed. After the suppression of endogenous DNM1L expression in MKN-45 cells with shRNA, cell proliferation and invasion rates were significantly reduced, whereas apoptosis was significantly increased (all P<0.01). The expression of DNM1L was significantly higher in gastric adenocarcinoma specimens compared with that in pericarcinoma tissues (P<0.001). The expression of DNM1L increased with increasing infiltration depth, lymphatic metastasis, and higher tumor node metastasis stage (P<0.05). The expression of DNM1L associated negatively with prognosis (P<0.01). DNM1L plays a critical role in the proliferation, invasion and apoptosis of human gastric adenocarcinoma. DNM1L expression has prognostic significance for the survival of patients with gastric adenocarcinoma.

p62/sequestosome-1 knockout delays neurodegeneration induced by Drp1 loss

Purkinje neurons, one of the largest neurons in the brain, are critical for controlling body movements, and the dysfunction and degeneration of these cells cause ataxia. Purkinje neurons require a very efficient energy supply from mitochondria because of their large size and extensive dendritic arbors. We have previously shown that mitochondrial division mediated by dynamin-related protein 1 (Drp1) is critical for the development and survival of Purkinje neurons. Drp1 deficiency has been associated with one of the major types of ataxia: autosomal recessive spastic ataxia of Charlevoix Saguenay. Using post-mitotic Purkinje neuron-specific Drp1 knockout (KO) in mice, we investigated the molecular mechanisms that mediate the progressive degeneration of Drp1-KO Purkinje neurons in vivo. In these Purkinje neurons, p62/sequestosome-1, a multi-functional adaptor protein that balances apoptotic cell death and cell survival, was recruited to large mitochondria resulting from unopposed fusion in the absence of mitochondrial division. To test the role of p62 in Drp1-deficient neurodegeneration, we created mice lacking both Drp1 and p62 and found that the additional loss of p62 significantly extended the survival of Purkinje neurons lacking Drp1. These results provide insights into the neurodegenerative mechanisms of mitochondrial ataxia and a critical foundation for therapeutic interventions for this disease.

Potential Protective Mechanism in the Cardiac Microvascular Injury

Cardiac microvascular injury often occurs in patients with type 2 diabetes mellitus (T2DM) who develop hyperglycemia and hyperlipidemia. However, besides reported contradictory roles in cardiac diseases, the function of TRPV1 (transient receptor potential vanilloid 1) in cardiac microvessels is not well defined. This study was performed to determine the detailed role of TRPV1 in cardiac microvascular endothelial cells (CMECs) in T2DM. T2DM mice were established by multiple injections of low-dose streptozotocin and high-fat feeding. CMECs were cultured separately in mediums of normal glucose, high glucose (HG), high fatty acid (HF), and HG plus HF (HG-HF). HG-HF inhibited TRPV1 expression in CMECs, reducing cellular Ca2+ content ([Ca2+]i). T2DM impaired cardiac function, disturbed glucose uptake, and damaged microvascular barrier, which were further aggravated by TRPV1-/- Exposure to HG-HF, particularly in TRPV1-/- CMECs, led to a higher level of apoptosis and a lower level of nitric oxide production in viable CMECs. HG-HF markedly enhanced generation of reactive oxygen species and nitrotyrosine, especially in the absence of TRPV1. H2O2 administration reduced TRPV1 expression in CMECs. HG-HF significantly depressed expression of PGC-1α (peroxisome proliferator-activated receptor-γ coactivator-1α) and OPA1 (optic atrophy 1) by reducing [Ca2+]i, whereas OPA1 supplementation partly reversed those detrimental effects induced by TRPV1-/- Furthermore, capsaicin treatment not only attenuated CMECs injury induced by HG-HF but also mitigated cardiac microvascular injury induced by T2DM. Collectively, T2DM leads to cardiac microvascular injury by exacerbating the vicious circle of TRPV1 blockage and reactive oxygen species overload. Long-term capsaicin can protect cardiac microvessels against T2DM via suppressing oxidative/nitrative stress mediated by TRPV1/Ca2+/PGC-1α/OPA1 pathway in CMECs.

Structural basis of mitochondrial receptor binding and constriction by DRP1

Mitochondrial inheritance, genome maintenance, and metabolic adaptation depend on organelle fission by Dynamin-Related Protein 1 (DRP1) and its mitochondrial receptors. DRP1 receptors include the paralogs Mitochondrial Dynamics 49 and 51 (MID49/MID51) and Mitochondrial Fission Factor (MFF), but the mechanisms by which these proteins recruit and regulate DRP1 are unknown. Here we present a cryoEM structure of human, full-length DRP1 coassembled with MID49 and an analysis of structure- and disease-based mutations. We report that GTP induces a remarkable elongation and rotation of the G-domain, Bundle-Signaling Element (BSE) and connecting hinge loops of DRP1. In this conformation, a network of multivalent interactions promotes polymerization of a linear DRP1 filament with MID49/MID51. Following coassembly, GTP hydrolysis and exchange lead to MID receptor dissociation, filament shortening and curling of DRP1 oligomers into constricted and closed rings. Together, these views of full-length, receptor- and nucleotide-bound conformations reveal how DRP1 performs mechanical work through nucleotide-driven allostery.

A brain-enriched Drp1 isoform associates with lysosomes, late endosomes, and the plasma membrane

Dynamin-related protein 1 (Drp1) constricts mitochondria as a mechanochemical GTPase during mitochondrial division. The Drp1 gene contains several alternative exons and produces multiple isoforms through RNA splicing. Here we performed a systematic analysis of Drp1 transcripts in different mouse tissues and identified a previously uncharacterized isoform that is highly enriched in the brain. This Drp1 isoform is termed Drp1_ABCD because it contains four alterative exons: A, B, C, and D. Remarkably, Drp1_ABCD is located at lysosomes, late endosomes, and the plasma membrane in addition to mitochondria. Furthermore, Drp1_ABCD is concentrated at the interorganelle interface between mitochondria and lysosomes/late endosomes. The localizations of Drp1_ABCD at lysosomes, late endosomes, and the plasma membrane require two exons, A and B, that are present in the GTPase domain. Drp1_ABCD assembles onto these membranes in a manner that is regulated by its oligomerization and GTP hydrolysis. Experiments using lysosomal inhibitors show that the association of Drp1_ABCD with lysosomes/late endosomes depends on lysosomal pH but not their protease activities. Thus, Drp1 may connect mitochondria to endosomal-lysosomal pathways in addition to mitochondrial division.

The regulation of tumor cell physiology by mitochondrial dynamics

Mitochondrial dynamics are increasingly recognized to play an important role in regulating mitochondrial function in response to diverse stimuli. Given the overlap in the physiological processes influenced by mitochondria and the physiological processes disrupted in tumor cells, we speculate that tumor cells alter mitochondrial shape to promote the tumorigenic phenotype. Here, we briefly review the evidence linking changes in mitochondrial fusion and fission to a number of key tumorigenic processes, including metabolic rewiring, inhibition of cell death, cell migration, cell proliferation and self-renewal capacity. The role of mitochondrial dynamics in tumor growth is an important emerging area of research, a better understanding of which may lead to promising new therapeutic options for the treatment of cancer.

Tumour microenvironment on mitochondrial dynamics and chemoresistance in cancer

Mitochondria, evolutionally acquired symbionts of eukaryotic cells, are essential cytoplasmic organelles. They are structurally dynamic organelles that continually go through fission and fusion processes in response to various stimuli. Tumour tissue is composed of not just cancer cells but also various cell types like fibroblasts, mesenchymal stem and immune cells. Mitochondrial dynamics of cancer cells has been shown to be significantly affected by features of tumour microenvironment such as hypoxia, inflammation and energy deprivation. The interactions of cancer cells with tumour microenvironment like hypoxia give rise to the inter- and intratumoural heterogeneity, causing chemoresistance. In this review, we will focus on the chemoresistance by tumoural heterogeneity in relation to mitochondrial dynamics of cancer cells. Recent findings in molecular mechanisms involved in the control of mitochondrial dynamics as well as the impact of mitochondrial dynamics on drug sensitivity in cancer are highlighted in the current review.

A Transcriptomic Analysis of Saccharomyces cerevisiae Under the Stress of 2-Phenylethanol

2-Phenylethanol (2-PE) is a kind of advanced aromatic alcohol with rose fragrance, which is wildly used for the deployment of flavors and fragrances. Microbial transformation is the most feasible method for the production of natural 2-PE. But a bottleneck problem is the toxicity of 2-PE on the cells. The molecular mechanisms of the toxic effect of 2-PE to Saccharomyces cerevisiae are not well studied. In this study, we analyzed the transcriptomes of S. cerevisiae in the media with and without 2-PE, respectively, using Illumina RNA-Seq technology. We identified 580 differentially expressed genes between S. cerevisiae in two different treatments. GO and KEGG enrichment analyses of these genes suggested that most genes encoding mitochondrial proteins, cytoplasmic, and plasma membrane proteins were significantly up-regulated, whereas the enzymes related to amino acid metabolism were down-regulated. These results indicated that 2-PE suppressed the synthesis of plasma membrane proteins, which suppressed the transport of nutrients required for growth. The findings in this study will provide insight into the inhibitory mechanism of 2-PE to yeast and other microbes.

Mitochondrial Molecular Abnormalities Revealed by Proteomic Analysis of Hippocampal Organelles of Mice Triple Transgenic for Alzheimer Disease

Mitochondrial dysfunction is implicated in the pathogenesis of Alzheimer's disease (AD). However, the precise mitochondrial molecular deficits in AD remain poorly understood. Mitochondrial and nuclear proteomic analysis in mature male triple transgenic AD mice (PS1M146V/APPSwe/TauP301L) by two-dimensional fluorescence difference gel electrophoresis (2D-DIGE) coupled with MALDI-TOF-MS/MS, bio-informatics analysis and immunofluorescent staining were performed in this study. In addition to impaired spatial memory impairment and intracellular accumulation of amyloid 1-42 (Aβ1-42) in the 3xTg-AD mice, a well-accepted mouse model of the human disease, we also found significantly increased DNA oxidative damage in entorhinal cortex, hippocampal CA1, CA3 and dental gyrus (DG), as evidenced by the positive staining of 8-hydroxyguanosine, a biomarker of mild cognitive impairment early in AD. We identified significant differences in 27 hippocampal mitochondrial proteins (11 increased and 16 decreased), and 37 hippocampal nuclear proteins (12 increased and 25 decreased) in 3xTg-AD mice compared with the wild-type (WT) mice. Differentially expressed mitochondrial and nuclear proteins were mainly involved in energy metabolism (>55%), synapses, DNA damage, apoptosis and oxidative stress. Two proteins were differentially expressed in both hippocampal mitochondria and nuclei, namely electron transport chain (ETC)-related protein ATP synthase subunit d (ATP5H) was significantly decreased, and apoptosis-related dynamin-1 (DYN1), a pre-synaptic and mitochondrial division-regulated protein that was significantly increased. In sum, perturbations of hippocampus mitochondrial energy metabolism-related proteins responsible for ATP generation via oxidation phosphorylation (OXPHOS), especially nuclear-encoded OXPHOS proteins, correlated with the amyloid-associated cognitive deficits of this murine AD model. The molecular changes in respiratory chain-related proteins and DYN1 may represent novel biomarkers of AD.

Insights into the Mechanisms of Chloroplast Division

The endosymbiosis of a free-living cyanobacterium into an ancestral eukaryote led to the evolution of the chloroplast (plastid) more than one billion years ago. Given their independent origins, plastid proliferation is restricted to the binary fission of pre-existing plastids within a cell. In the last 25 years, the structure of the supramolecular machinery regulating plastid division has been discovered, and some of its component proteins identified. More recently, isolated plastid-division machineries have been examined to elucidate their structural and mechanistic details. Furthermore, complex studies have revealed how the plastid-division machinery morphologically transforms during plastid division, and which of its component proteins play a critical role in generating the contractile force. Identifying the three-dimensional structures and putative functional domains of the component proteins has given us hints about the mechanisms driving the machinery. Surprisingly, the mechanisms driving plastid division resemble those of mitochondrial division, indicating that these division machineries likely developed from the same evolutionary origin, providing a key insight into how endosymbiotic organelles were established. These findings have opened new avenues of research into organelle proliferation mechanisms and the evolution of organelles.

Protein kinase D activation induces mitochondrial fragmentation and dysfunction in cardiomyocytes

KEY POINTS

Abnormal mitochondrial morphology and function in cardiomyocytes are frequently observed under persistent Gq protein-coupled receptor (Gq PCR) stimulation. Cardiac signalling mechanisms for regulating mitochondrial morphology and function under pathophysiological conditions in the heart are still poorly understood. We demonstrate that a downstream kinase of Gq PCR, protein kinase D (PKD) induces mitochondrial fragmentation via phosphorylation of dynamin-like protein 1 (DLP1), a mitochondrial fission protein. The fragmented mitochondria enhance reactive oxygen species generation and permeability transition pore opening in mitochondria, which initiate apoptotic signalling activation. This study identifies a novel PKD-specific substrate in cardiac mitochondria and uncovers the role of PKD on cardiac mitochondria, with special emphasis on the molecular mechanism(s) underlying mitochondrial injury with abnormal mitochondrial morphology under persistent Gq PCR stimulation. These findings provide new insights into the molecular basis of cardiac mitochondrial physiology and pathophysiology, linking Gq PCR signalling with the regulation of mitochondrial morphology and function.

ABSTRACT

Regulation of mitochondrial morphology is crucial for the maintenance of physiological functions in many cell types including cardiomyocytes. Small and fragmented mitochondria are frequently observed in pathological conditions, but it is still unclear which cardiac signalling pathway is responsible for regulating the abnormal mitochondrial morphology in cardiomyocytes. Here we demonstrate that a downstream kinase of Gq protein-coupled receptor (Gq PCR) signalling, protein kinase D (PKD), mediates pathophysiological modifications in mitochondrial morphology and function, which consequently contribute to the activation of apoptotic signalling. We show that Gq PCR stimulation induced by α1 -adrenergic stimulation mediates mitochondrial fragmentation in a fission- and PKD-dependent manner in H9c2 cardiac myoblasts and rat neonatal cardiomyocytes. Upon Gq PCR stimulation, PKD translocates from the cytoplasm to the outer mitochondrial membrane (OMM) and phosphorylates a mitochondrial fission protein, dynamin-like protein 1 (DLP1), at S637. PKD-dependent phosphorylation of DLP1 initiates DLP1 association with the OMM, which then enhances mitochondrial fragmentation, mitochondrial superoxide generation, mitochondrial permeability transition pore opening and apoptotic signalling. Finally, we demonstrate that DLP1 phosphorylation at S637 by PKD occurs in vivo using ventricular tissues from transgenic mice with cardiac-specific overexpression of constitutively active Gαq protein. In conclusion, Gq PCR-PKD signalling induces mitochondrial fragmentation and dysfunction via PKD-dependent DLP1 phosphorylation in cardiomyocytes. This study is the first to identify a novel PKD-specific substrate, DLP1 in mitochondria, as well as the functional role of PKD in cardiac mitochondria. Elucidation of these molecular mechanisms by which PKD-dependent enhanced fission mediates cardiac mitochondrial injury will provide novel insight into the relationship among mitochondrial form, function and Gq PCR signalling.

Mitochondrial Aspects of Synaptic Dysfunction in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by brain deposition of amyloid plaques and tau neurofibrillary tangles along with steady cognitive decline. Synaptic damage, an early pathological event, correlates strongly with cognitive deficits and memory loss. Mitochondria are essential organelles for synaptic function. Neurons utilize specialized mechanisms to drive mitochondrial trafficking to synapses in which mitochondria buffer Ca2+ and serve as local energy sources by supplying ATP to sustain neurotransmitter release. Mitochondrial abnormalities are one of the earliest and prominent features in AD patient brains. Amyloid-β (Aβ) and tau both trigger mitochondrial alterations. Accumulating evidence suggests that mitochondrial perturbation acts as a key factor that is involved in synaptic failure and degeneration in AD. The importance of mitochondria in supporting synaptic function has made them a promising target of new therapeutic strategies for AD. Here, we review the molecular mechanisms regulating mitochondrial function at synapses, highlight recent findings on the disturbance of mitochondrial dynamics and transport in AD, and discuss how these alterations impact synaptic vesicle release and thus contribute to synaptic pathology associated with AD.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

Glycosyltransferase MDR1 assembles a dividing ring for mitochondrial proliferation comprising polyglucan nanofilaments

Mitochondria, which evolved from a free-living bacterial ancestor, contain their own genomes and genetic systems and are produced from preexisting mitochondria by binary division. The mitochondrion-dividing (MD) ring is the main skeletal structure of the mitochondrial division machinery. However, the assembly mechanism and molecular identity of the MD ring are unknown. Multi-omics analysis of isolated mitochondrial division machinery from the unicellular alga Cyanidioschyzon merolae revealed an uncharacterized glycosyltransferase, MITOCHONDRION-DIVIDING RING1 (MDR1), which is specifically expressed during mitochondrial division and forms a single ring at the mitochondrial division site. Nanoscale imaging using immunoelectron microscopy and componential analysis demonstrated that MDR1 is involved in MD ring formation and that the MD ring filaments are composed of glycosylated MDR1 and polymeric glucose nanofilaments. Down-regulation of MDR1 strongly interrupted mitochondrial division and obstructed MD ring assembly. Taken together, our results suggest that MDR1 mediates the synthesis of polyglucan nanofilaments that assemble to form the MD ring. Given that a homolog of MDR1 performs similar functions in chloroplast division, the establishment of MDR1 family proteins appears to have been a singular, crucial event for the emergence of endosymbiotic organelles.

Engineered Ferritin for Magnetogenetic Manipulation of Proteins and Organelles Inside Living Cells

Magnetogenetics is emerging as a novel approach for remote-controlled manipulation of cellular functions in tissues and organisms with high spatial and temporal resolution. A critical, still challenging issue for these techniques is to conjugate target proteins with magnetic probes that can satisfy multiple colloidal and biofunctional constraints. Here, semisynthetic magnetic nanoparticles are tailored based on human ferritin coupled to monomeric enhanced green fluorescent protein (mEGFP) for magnetic manipulation of proteins inside living cells. This study demonstrates efficient delivery, intracellular stealth properties, and rapid subcellular targeting of those magnetic nanoparticles via GFP-nanobody interactions. By means of magnetic field gradients, rapid spatial reorganization in the cytosol of proteins captured to the nanoparticle surface is achieved. Moreover, exploiting efficient nanoparticle targeting to intracellular membranes, remote-controlled arrest of mitochondrial dynamics using magnetic fields is demonstrated. The studies establish subcellular control of proteins and organelles with unprecedented spatial and temporal resolution, thus opening new prospects for magnetogenetic applications in fundamental cell biology and nanomedicine.

Implications of mitochondrial network organization in mitochondrial stress signalling in NARP cybrid and Rho0 cells

Mitochondrial dysfunctions lead to the generation of signalling mediators that influence the fate of that organelle. Mitochondrial dynamics and their positioning within the cell are important elements of mitochondria-nucleus communication. The aim of this project was to examine whether mitochondrial shape, distribution and fusion/fission proteins are involved in the mitochondrial stress response in a cellular model subjected to specifically designed chronic mitochondrial stress: WT human osteosarcoma cells as controls, NARP cybrid cells as mild chronic stress and Rho0 as severe chronic stress. We characterized mitochondrial distribution in these cells using confocal microscopy and evaluated the level of proteins directly involved in the mitochondrial dynamics and their regulation. We found that the organization of mitochondria within the cell is correlated with changes in the levels of proteins involved in mitochondrial dynamics and proteins responsible for regulation of this process. Induction of the autophagy/mitophagy process, which is crucial for cellular homeostasis under stress conditions was also shown. It seems that mitochondrial shape and organization within the cell are implicated in retrograde signalling in chronic mitochondrial stress.

Hetero-oligomer of dynamin-related proteins participates in the fission of highly divergent mitochondria from Entamoeba histolytica

Entamoeba histolytica is an anaerobic parasitic protist and possesses mitosomes, one of the most highly divergent mitochondrion-related organelles (MROs). Although unique metabolism and protein/metabolite transport machinery have been demonstrated in Entamoeba mitosomes, the mechanism of mitosomal fusion and fission remains to be elucidated. In this study, we demonstrate that two dynamin-related proteins (DRPs) are cooperatively involved in the fission of Entamoeba mitosomes. Expression of a dominant negative form of EhDrpA and EhDrpB, and alternatively, repression of gene expression of EhDrpA and EhDrpB genes, caused elongation of mitosomes, reflecting inhibition of mitosomal fission. Moreover, EhDrpA and EhDrpB formed an unprecedented hetero-oligomeric complex with an approximate 1:2 to 1:3 ratio, suggesting that the observed elongation of mitosomes is likely caused by the disruption and instability of the complex caused by an imbalance in the two DRPs. Altogether, this is the first report of a hetero-oligomeric DRP complex which participates in the fission of mitochondria and MROs.

Phosphatidic Acid and Cardiolipin Coordinate Mitochondrial Dynamics

Membrane organelles comprise both proteins and lipids. Remodeling of these membrane structures is controlled by interactions between specific proteins and lipids. Mitochondrial structure and function depend on regulated fusion and the division of both the outer and inner membranes. Here we discuss recent advances in the regulation of mitochondrial dynamics by two critical phospholipids, phosphatidic acid (PA) and cardiolipin (CL). These two lipids interact with the core components of mitochondrial fusion and division (Opa1, mitofusin, and Drp1) to activate and inhibit these dynamin-related GTPases. Moreover, lipid-modifying enzymes such as phospholipases and lipid phosphatases may organize local lipid composition to spatially and temporarily coordinate a balance between fusion and division to establish mitochondrial morphology.

Coincident Phosphatidic Acid Interaction Restrains Drp1 in Mitochondrial Division

Mitochondria divide to control their size, distribution, turnover, and function. Dynamin-related protein 1 (Drp1) is a critical mechanochemical GTPase that drives constriction during mitochondrial division. It is generally believed that mitochondrial division is regulated during recruitment of Drp1 to mitochondria and its oligomerization into a division apparatus. Here, we report an unforeseen mechanism that regulates mitochondrial division by coincident interactions of Drp1 with the head group and acyl chains of phospholipids. Drp1 recognizes the head group of phosphatidic acid (PA) and two saturated acyl chains of another phospholipid by penetrating into the hydrophobic core of the membrane. The dual phospholipid interactions restrain Drp1 via inhibition of oligomerization-stimulated GTP hydrolysis that promotes membrane constriction. Moreover, a PA-producing phospholipase, MitoPLD, binds Drp1, creating a PA-rich microenvironment in the vicinity of a division apparatus. Thus, PA controls the activation of Drp1 after the formation of the division apparatus.

The constriction and scission machineries involved in mitochondrial fission

A key event in the evolution of eukaryotic cells was the engulfment of an aerobic bacterium by a larger anaerobic archaebacterium, leading to a close relationship between the host and the newly formed endosymbiont. Mitochondria, originating from this event, have evolved to be the main place of cellular ATP production. Maintaining elements of their independence, mitochondria undergo growth and division in the cell, thereby ensuring that new daughter cells inherit a mitochondrial complement. Mitochondrial division is also important for other processes, including quality control, mitochondrial (mt)DNA inheritance, transport and cell death. However, unlike bacterial fission, which uses a dynamin-related protein to constrict the membrane at its inner face, mitochondria use dynamin and dynamin-related proteins to constrict the outer membrane from the cytosolic face. In this Review, we summarize the role of proteins from the dynamin superfamily in mitochondrial division. This includes recent findings highlighting that dynamin-2 (Dnm2) is involved in mitochondrial scission, which led to the reappraisal of the role of dynamin-related protein 1 (Drp1; also known as Dnm1l) and its outer membrane adaptors as components of the mitochondrial constriction machinery along with ER components and actin.

Mitochondrial dynamics: The dynamin superfamily and execution by collusion

Distinct dynamin superfamily GTPases catalyze the constant fission and fusion of the elaborate mitochondrial networks that navigate the eukaryotic cytoplasm. Long believed to be the singular handiwork of dynamin-related protein 1 (Drp1), a cytosolic family member that transiently localizes to the mitochondrial surface, the execution of mitochondrial fission is now arguably believed to entail membrane remodeling events that are initiated upstream of Drp1 by ER-associated cytoskeletal networks and completed downstream by the prototypical dynamin, dynamin 2 (Dyn2). Recent developments in the field have also placed a sharp focus on the membrane microenvironment around the division apparatus and the potential facilitatory role of specific lipids in mitochondrial fission. Here, I will review current progress, as well as highlight the most visible gaps in knowledge, in elucidating the varied functions of the dynamin superfamily in the coordinated events of mitochondrial fission and fusion. The essential roles of protein and lipid cofactors are also highlighted.

An unstructured loop that is critical for interactions of the stalk domain of Drp1 with saturated phosphatidic acid

Dynamin-related protein 1 (Drp1) is a dynamin superfamily GTPase, which drives membrane constriction during mitochondrial division. To mediate mitochondrial division, Drp1 is recruited to the mitochondrial outer membrane and is assembled into the division machinery. We previously showed that Drp1 interacts with phosphatidic acid (PA) and saturated phospholipids in the mitochondrial membrane, and this interaction restrains Drp1 in initiating the constriction of mitochondria. Here, we show that the role of saturated acyl chains of phospholipids is independent of their contribution to the membrane curvature or lipid packing suggesting their direct interaction with Drp1. We further show that an unstructured loop in the stalk domain of Drp1 is critical for interaction with unsaturated PA. Our data significantly advance our understanding of this unique protein-lipid interaction involved in mitochondrial division.

The Impact of Environmental Factors in Influencing Epigenetics Related to Oxidative States in the Cardiovascular System

Oxidative states exert a significant influence on a wide range of biological and molecular processes and functions. When their balance is shifted towards enhanced amounts of free radicals, pathological phenomena can occur, as the generation of reactive oxygen species (ROS) in tissue microenvironment or in the systemic circulation can be detrimental. Epidemic chronic diseases of western societies, such as cardiovascular disease, obesity, and diabetes correlate with the imbalance of redox homeostasis. Current advances in our understanding of epigenetics have revealed a parallel scenario showing the influence of oxidative stress as a major regulator of epigenetic gene regulation via modification of DNA methylation, histones, and microRNAs. This has provided both the biological link and a potential molecular explanation between oxidative stress and cardiovascular/metabolic phenomena. Accordingly, in this review, we will provide current insights on the physiological and pathological impact of changes in oxidative states on cardiovascular disorders, by specifically focusing on the influence of epigenetic regulation. A special emphasis will highlight the effect on epigenetic regulation of human's current life habits, external and environmental factors, including food intake, tobacco, air pollution, and antioxidant-based approaches. Additionally, the strategy to quantify oxidative states in humans in order to determine which biological marker could best match a subject's profile will be discussed.

Mitochondrial dysfunction and oxidative stress in metabolic disorders - A step towards mitochondria based therapeutic strategies

Mitochondria are the powerhouses of the cell and are involved in essential functions of the cell, including ATP production, intracellular Ca2+ regulation, reactive oxygen species production & scavenging, regulation of apoptotic cell death and activation of the caspase family of proteases. Mitochondrial dysfunction and oxidative stress are largely involved in aging, cancer, age-related neurodegenerative and metabolic syndrome. In the last decade, tremendous progress has been made in understanding mitochondrial structure, function and their physiology in metabolic syndromes such as diabetes, obesity, stroke and hypertension, and heart disease. Further, progress has also been made in developing therapeutic strategies, including lifestyle interventions (healthy diet and regular exercise), pharmacological strategies and mitochondria-targeted approaches. These strategies were mainly focused to reduce mitochondrial dysfunction and oxidative stress and to maintain mitochondrial quality in metabolic syndromes. The purpose of our article is to highlight the recent progress on the mitochondrial role in metabolic syndromes and also summarize the progress of mitochondria-targeted molecules as therapeutic targets to treat metabolic syndromes. This article is part of a Special Issue entitled: Oxidative Stress and Mitochondrial Quality in Diabetes/Obesity and Critical Illness Spectrum of Diseases - edited by P. Hemachandra Reddy.

Connecting mitochondrial dynamics and life-or-death events via Bcl-2 family proteins

The morphology of a population of mitochondria is the result of several interacting dynamical phenomena, including fission, fusion, movement, elimination and biogenesis. Each of these phenomena is controlled by underlying molecular machinery, and when defective can cause disease. New understanding of the relationships between form and function of mitochondria in health and disease is beginning to be unraveled on several fronts. Studies in mammals and model organisms have revealed that mitochondrial morphology, dynamics and function appear to be subject to regulation by the same proteins that regulate apoptotic cell death. One protein family that influences mitochondrial dynamics in both healthy and dying cells is the Bcl-2 protein family. Connecting mitochondrial dynamics with life-death pathway forks may arise from the intersection of Bcl-2 family proteins with the proteins and lipids that determine mitochondrial shape and function. Bcl-2 family proteins also have multifaceted influences on cells and mitochondria, including calcium handling, autophagy and energetics, as well as the subcellular localization of mitochondrial organelles to neuronal synapses. The remarkable range of physical or functional interactions by Bcl-2 family proteins is challenging to assimilate into a cohesive understanding. Most of their effects may be distinct from their direct roles in apoptotic cell death and are particularly apparent in the nervous system. Dual roles in mitochondrial dynamics and cell death extend beyond BCL-2 family proteins. In this review, we discuss many processes that govern mitochondrial structure and function in health and disease, and how Bcl-2 family proteins integrate into some of these processes.

Bcl-2 family proteins: master regulators of cell survival

The most prominent function of proteins of the Bcl-2 family is regulation of the initiation of intrinsic (mitochondrial) pathways of apoptosis. However, recent research has revealed that in addition to regulation of mitochondrial apoptosis, proteins of the Bcl-2 family play important roles in regulating other cellular pathways with a strong impact on cell survival like autophagy, endoplasmic reticulum (ER) stress response, intracellular calcium dynamics, cell cycle progression, mitochondrial dynamics and energy metabolism. This review summarizes the recent knowledge about functions of Bcl-2 family proteins that are related to cell survival.

Therapeutic Strategies for Mitochondrial Dysfunction and Oxidative Stress in Age-Related Metabolic Disorders

Mitochondria are complex, intercellular organelles present in the cells and are involved in multiple roles including ATP formation, free radicals generation and scavenging, calcium homeostasis, cellular differentiation, and cell death. Many studies depicted the involvement of mitochondrial dysfunction and oxidative damage in aging and pathogenesis of age-related metabolic disorders and neurodegenerative diseases. Remarkable advancements have been made in understanding the structure, function, and physiology of mitochondria in metabolic disorders such as diabetes, obesity, cardiovascular diseases, and stroke. Further, much progress has been done in the improvement of therapeutic strategies, including lifestyle interventions, pharmacological, and mitochondria-targeted therapeutic approaches. These strategies were mainly focused to reduce the mitochondrial dysfunction caused by oxidative stress and to retain the mitochondrial health in various diseases. In this chapter, we have highlighted the involvement of mitochondrial dysfunction in the pathophysiology of various disorders and recent progress in the development of mitochondria-targeted molecules as therapeutic measures for metabolic disorders.

A Drosophila model of GDAP1 function reveals the involvement of insulin signalling in the mitochondria-dependent neuromuscular degeneration

Charcot-Marie-Tooth disease is a rare peripheral neuropathy for which there is no specific treatment. Some forms of Charcot-Marie-Tooth are due to mutations in the GDAP1 gene. A striking feature of mutations in GDAP1 is that they have a variable clinical manifestation, according to disease onset and progression, histology and mode of inheritance. Studies in cellular and animal models have revealed a role of GDAP1 in mitochondrial morphology and distribution, calcium homeostasis and oxidative stress. To get a better understanding of the disease mechanism we have generated models of over-expression and RNA interference of the Drosophila Gdap1 gene. In order to get an overview about the changes that Gdap1 mutations cause in our disease model, we have combined a comprehensive determination of the metabolic profile in the flies by nuclear magnetic resonance spectroscopy with gene expression analyses and biophysical tests. Our results revealed that both up- and down-regulation of Gdap1 results in an early systemic inactivation of the insulin pathway before the onset of neuromuscular degeneration, followed by an accumulation of carbohydrates and an increase in the β-oxidation of lipids. Our findings are in line with emerging reports of energy metabolism impairments linked to different types of neural pathologies caused by defective mitochondrial function, which is not surprising given the central role of mitochondria in the control of energy metabolism. The relationship of mitochondrial dynamics with metabolism during neurodegeneration opens new avenues to understand the cause of the disease, and for the discovery of new biomarkers and treatments.

Eat and Death: Chronic Over-Eating

Obesity-related co-morbidities decrease life quality, reduce working ability and lead to early death. The total amount of dietary fat consumption may be the most potent food-related risk factor for weight gain. In this respect, dietary intake of high-caloric, high-fat diets due to chronic over-eating and sedentary lifestyle lead to increased storage of triglycerides not only in adipose tissue but also ectopically in other tissues . Increased plasma concentrations of non-esterified free fatty acids and lipid-overloaded hypertrophic adipocytes may cause insulin resistance in an inflammation-independent manner. Even in the absence of metabolic disorders, mismatch between fatty acid uptake and utilization leads to the accumulation of toxic lipid species resulting in organ dysfunction. Lipid-induced apoptosis, ceramide accumulation, reactive oxygen species overproduction, endoplasmic reticulum stress, and mitochondrial dysfunction may play role in the pathogenesis of lipotoxicity. The hypothalamus senses availability of circulating levels of glucose, lipids and amino acids, thereby modifies feeding according to the levels of those molecules. However, the hypothalamus is also similarly vulnerable to lipotoxicity as the other ectopic lipid accumulated tissues. Chronic overnutrition most likely provides repetitive and persistent signals that up-regulate inhibitor of nuclear factor kappa B kinase beta subunit/nuclear factor kappa B (IKKβ/NF-κB) in the hypothalamus before the onset of obesity. However, the mechanisms by which high-fat diet induced peripheral signals affect the hypothalamic arcuate nucleus remain largely unknown. In this chapter, besides lipids and leptin, the role of glucose and insulin on specialized fuel-sensing neurons of hypothalamic neuronal circuits has been debated.

Mitochondrial lipids in neurodegeneration

Mitochondrial dysfunction is a common feature of many neurodegenerative diseases, including proteinopathies such as Alzheimer's or Parkinson's disease, which are characterized by the deposition of aggregated proteins in the form of insoluble fibrils or plaques. The distinct molecular processes that eventually result in mitochondrial dysfunction during neurodegeneration are well studied but still not fully understood. However, defects in mitochondrial fission and fusion, mitophagy, oxidative phosphorylation and mitochondrial bioenergetics have been linked to cellular demise. These processes are influenced by the lipid environment within mitochondrial membranes as, besides membrane structure and curvature, recruitment and activity of different proteins also largely depend on the respective lipid composition. Hence, the interaction of neurotoxic proteins with certain lipids and the modification of lipid composition in different cell compartments, in particular mitochondria, decisively impact cell death associated with neurodegeneration. Here, we discuss the relevance of mitochondrial lipids in the pathological alterations that result in neuronal demise, focussing on proteinopathies.

Assay to Measure Interactions between Purified Drp1 and Synthetic Liposomes

A mitochondrion is a dynamic intracellular organelle that actively divides and fuses to control its size, number and shape in cells. A regulated balance between mitochondrial division and fusion is fundamental to the function, distribution and turnover of mitochondria (Roy et al., 2015). Mitochondrial division is mediated by dynamin-related protein 1 (Drp1), a mechano-chemical GTPase that constricts mitochondrial membranes (Tamura et al., 2011). Mitochondrial membrane lipids such as phosphatidic acid and cardiolipin bind Drp1, and Drp1-phospholipid interactions provide key regulatory mechanisms for mitochondrial division (Montessuit et al., 2010; Bustillo-Zabalbeitia et al., 2014; Macdonald et al., 2014; Stepanyants et al., 2015; Adachi et al., 2016). Here, we describe biochemical experiments that quantitatively measure interactions of Drp1 with lipids using purified recombinant Drp1 and synthetic liposomes with a defined set of phospholipids. This assay makes it possible to define the specificity of protein-lipid interaction and the role of the head group and acyl chains.

The Effects of Ascorbate, N-Acetylcysteine, and Resveratrol on Fibroblasts from Patients with Mitochondrial Disorders

Reactive oxygen species (ROS) are assumed to be implicated in the pathogenesis of inborn mitochondrial diseases affecting oxidative phosphorylation (OXPHOS). In the current study, we characterized the effects of three small molecules with antioxidant properties (N-acetylcysteine, ascorbate, and resveratrol) on ROS production and several OXPHOS parameters (growth in glucose free medium, ATP production, mitochondrial content and membrane potential (MMP)), in primary fibroblasts derived from seven patients with different molecularly defined and undefined mitochondrial diseases. N-acetylcysteine appeared to be the most beneficial compound, reducing ROS while increasing growth and ATP production in some patients' cells. Ascorbate showed a variable positive or negative effect on ROS, ATP production, and mitochondrial content, while incubation with resveratrol disclosed either no effect or detrimental effect on ATP production and MMP in some cells. The individual responses highlight the importance of investigating multiple parameters in addition to ROS to obtain a more balanced view of the overall effect on OXPHOS when evaluating antioxidant treatment options for mitochondrial diseases.

Mitochondrial networking in diabetic left ventricle cardiomyocytes

Cardiomyocyte mitochondria preserve "the quorum sensing" attribute of their aerobic bacterial ancestors, as shown by the transient physical connectivity and communication not only with each other, but also with other intracellular organelles and with cytosol, ensuing cellular homeostasis. In this review, we present original electron microscopy evidence on mitochondrial networking within diabetic left ventricular cardiomyocytes, focusing on: (i) the inter-mitochondrial communication, allowing electrochemical signals transfer and outer membrane components or matrix proteins exchange, (ii) the interplay between mitochondria and the cardiomyocyte nucleus, nucleolus, sarcoplasmic reticulum, lysosomes, and lipid droplets viewed as attributes of mitochondrial "quality control" and "retrograde signaling function", and (iii) the crosstalk between mitochondria and cardiomyocyte cytosol, as part of the adaptive responses that allow cells survival. Confirmation of such interactions in diabetic myocardium and identification of molecules involved are ongoing, foreseeing the alleviation of heart contractile dysfunction in cardiomyopathy.

Multiple Roles of Mitochondria in Aging Processes

Aging is a multifactorial process influenced by genetic factors, nutrition, and lifestyle. According to mitochondrial theory of aging, mitochondrial dysfunction is widely considered a major contributor to age-related processes. Mitochondria are both the main source and targets of detrimental reactions initiated in association with age-dependent deterioration of the cellular functions. Reactions leading to increased reactive oxygen species generation, mtDNA mutations, and oxidation of mitochondrial proteins result in subsequent induction of apoptotic events, impaired oxidative phosphorylation capacity, mitochondrial dynamics, biogenesis and autophagy. This review summarizes the major changes of mitochondria related to aging, with emphasis on mitochondrial DNA mutations, the role of the reactive oxygen species, and structural and functional changes of mitochondria.

Dynamin-Related Protein 1 Deficiency Leads to Receptor-Interacting Protein Kinase 3-Mediated Necroptotic Neurodegeneration

Mitochondria are dynamic organelles that divide and fuse to modulate their number and shape. We have previously reported that the loss of dynamin-related protein 1 (Drp1), which mediates mitochondrial division, leads to the degeneration of cerebellar Purkinje cells in mice. Because Drp1 has been shown to be important for apoptosis and necroptosis, it is puzzling how Purkinje neurons die in the absence of Drp1. In this study, we tested whether neurodegeneration involves necrotic cell death by generating Purkinje cell-specific Drp1-knockout (KO) mice that lack the receptor-interacting protein kinase 3 (Rip3), which regulates necroptosis. We found that the loss of Rip3 significantly delays the degeneration of Drp1-KO Purkinje neurons. In addition, before neurodegeneration, mitochondrial tubules elongate because of unopposed fusion and subsequently become large spheres as a result of oxidative damage. Surprisingly, Rip3 loss also helps Drp1-KO Purkinje cells maintain the elongated morphology of the mitochondrial tubules. These data suggest that Rip3 plays a role in neurodegeneration and mitochondrial morphology in the absence of mitochondrial division.

Modulation of antigen processing by haem-oxygenase 1. Implications on inflammation and tolerance

Haem-oxygenase-1 (HO-1) is an enzyme responsible for the degradation of haem that can suppress inflammation, through the production of carbon monoxide (CO). It has been shown in several experimental models that genetic and pharmacological induction of HO-1, as well as non-toxic administration of CO, can reduce inflammatory diseases, such as endotoxic shock, type 1 diabetes and graft rejection. Recently, it was shown that the HO-1/CO system can alter the function of antigen-presenting cells (APCs) and reduce T-cell priming, which can be beneficial during immune-driven inflammatory diseases. The molecular mechanisms by which the HO-1 and CO reduce both APC- and T-cell-driven immunity are just beginning to be elucidated. In this article we discuss recent findings related to the immune regulatory capacity of HO-1 and CO at the level of recognition of pathogen-associated molecular patterns and T-cell priming by APCs. Finally, we propose a possible regulatory role for HO-1 and CO over the recently described mitochondria-dependent immunity. These concepts could contribute to the design of new therapeutic tools for inflammation-based diseases.

Loss of OMA1 delays neurodegeneration by preventing stress-induced OPA1 processing in mitochondria

Loss of OMA1 in a mouse model of neurodegeneration stabilizes fusion-active L-OPA1, which supports neuronal survival by preventing apoptosis, independent of its effects on cristae morphogenesis.

Proteolytic cleavage of the dynamin-like guanosine triphosphatase OPA1 in mitochondria is emerging as a central regulatory hub that determines mitochondrial morphology under stress and in disease. Stress-induced OPA1 processing by OMA1 triggersmitochondrial fragmentation, which is associated with mitophagy and apoptosis in vitro. Here, we identify OMA1 as a critical regulator of neuronal survival in vivo and demonstrate that stress-induced OPA1 processing by OMA1 promotes neuronal death and neuroinflammatory responses. Using mice lacking prohibitin membrane scaffolds as a model of neurodegeneration, we demonstrate that additional ablation of Oma1 delays neuronal loss and prolongs lifespan. This is accompanied by the accumulation of fusion-active, long OPA1 forms, which stabilize the mitochondrial genome but do not preserve mitochondrial cristae or respiratory chain supercomplex assembly in prohibitin-depleted neurons. Thus, long OPA1 forms can promote neuronal survival independently of cristae shape, whereas stress-induced OMA1 activation and OPA1 cleavage limit mitochondrial fusion and promote neuronal death.

Epigenetics and nutrition-related epidemics of metabolic diseases: Current perspectives and challenges

We live in a world fascinated by the relationship between disease and nutritional disequilibrium. The subtle and slow effects of chronic nutrient toxicity are a major public health concern. Since food is potentially important for the development of "metabolic memory", there is a need for more information on the type of nutrients causing adverse or toxic effects. We now know that metabolic alterations produced by excessive intake of some nutrients, drugs and chemicals directly impact epigenetic regulation. We envision that understanding how metabolic pathways are coordinated by environmental and genetic factors will provide novel insights for the treatment of metabolic diseases. New methods will enable the assembly and analysis of large sets of complex molecular and clinical data for understanding how inflammation and mitochondria affect bioenergetics, epigenetics and health. Collectively, the observations we highlight indicate that energy utilization and disease are intimately connected by epigenetics. The challenge is to incorporate metabolo-epigenetic data in better interpretations of disease, to expedite therapeutic targeting of key pathways linking nutritional toxicity and metabolism. An additional concern is that changes in the parental phenotype are detectable in the methylome of subsequent offspring. The effect might create a menace to future generations and preconceptional considerations.

Motor neuron mitochondrial dysfunction in spinal muscular atrophy

Spinal muscular atrophy (SMA), the leading genetic cause of infant mortality, predominantly affects high metabolic tissues including motor neurons, skeletal muscles and the heart. Although the genetic cause of SMA has been identified, mechanisms underlying tissue-specific vulnerability are not well understood. To study these mechanisms, we carried out a deep sequencing analysis of the transcriptome of spinal motor neurons in an SMA mouse model, in which we unexpectedly found changes in many genes associated with mitochondrial bioenergetics. Importantly, functional measurement of mitochondrial activities showed decreased basal and maximal mitochondrial respiration in motor neurons from SMA mice. Using a reduction-oxidation sensitive GFP and fluorescence sensors specifically targeted to mitochondria, we found increased oxidative stress level and impaired mitochondrial membrane potential in motor neurons affected by SMA. In addition, mitochondrial mobility was impaired in SMA disease conditions, with decreased retrograde transport but no effect on anterograde transport. We also found significantly increased fragmentation of the mitochondrial network in primary motor neurons from SMA mice, with no change in mitochondria density. Electron microscopy study of SMA mouse spinal cord revealed mitochondria fragmentation, edema and concentric lamellar inclusions in motor neurons affected by the disease. Intriguingly, these functional and structural deficiencies in the SMA mouse model occur during the presymptomatic stage of disease, suggesting a role in initiating SMA. Altogether, our findings reveal a critical role for mitochondrial defects in SMA pathogenesis and suggest a novel target for improving tissue health in the disease.

Mitochondrial Dynamics Is Linked to Longevity and Protects from End-Organ Injury: The Emerging Role of Sirtuin 3

SIGNIFICANCE

Mitochondrial integrity is instrumental in protecting against damage associated with aging and a variety of chronic disease conditions. Mitochondrial silent information regulator 3 (Sirt3) plays pivotal roles in maintaining mitochondrial homeostasis by regulating different aspects of the organelle processes.

RECENT ADVANCES

Mitochondria are highly dynamic organelles that constantly fuse and divide to maintain normal cell function, and perturbation in mitochondrial dynamics is responsible for mitochondrial dysfunction. Improved knowledge of mitochondrial physiology has disclosed the pleiotropic role of Sirt3 in mitochondria and shows how alterations in protein expression and/or activity may have an important impact on aging-associated organ dysfunction.

CRITICAL ISSUES

This review describes updated experimental evidence on the role of mitochondrial dysfunction during aging and renal diseases and highlights the emerging role of Sirt3 as a crucial regulator of mitochondrial dynamics.

FUTURE DIRECTIONS

Strategies that activate Sirt3 may offer attractive therapies to achieve healthy longevity and preserve functional integrity of multiple organs. Antioxid. Redox Signal. 25, 185-199.

Mitochondrial Mg(2+) homeostasis decides cellular energy metabolism and vulnerability to stress

Cellular energy production processes are composed of many Mg²⁺ dependent enzymatic reactions. In fact, dysregulation of Mg²⁺ homeostasis is involved in various cellular malfunctions and diseases. Recently, mitochondria, energy-producing organelles, have been known as major intracellular Mg²⁺ stores. Several biological stimuli alter mitochondrial Mg²⁺ concentration by intracellular redistribution. However, in living cells, whether mitochondrial Mg²⁺ alteration affect cellular energy metabolism remains unclear. Mg²⁺ transporter of mitochondrial inner membrane MRS2 is an essential component of mitochondrial Mg²⁺ uptake system. Here, we comprehensively analyzed intracellular Mg²⁺ levels and energy metabolism in Mrs2 knockdown (KD) cells using fluorescence imaging and metabolome analysis. Dysregulation of mitochondrial Mg²⁺ homeostasis disrupted ATP production via shift of mitochondrial energy metabolism and morphology. Moreover, Mrs2 KD sensitized cellular tolerance against cellular stress. These results indicate regulation of mitochondrial Mg²⁺via MRS2 critically decides cellular energy status and cell vulnerability via regulation of mitochondrial Mg²⁺ level in response to physiological stimuli.

Parkin suppresses Drp1-independent mitochondrial division

The cycle of mitochondrial division and fusion disconnect and reconnect individual mitochondria in cells to remodel this energy-producing organelle. Although dynamin-related protein 1 (Drp1) plays a major role in mitochondrial division in cells, a reduced level of mitochondrial division still persists even in the absence of Drp1. It is unknown how much Drp1-mediated mitochondrial division accounts for the connectivity of mitochondria. The role of a Parkinson's disease-associated protein-parkin, which biochemically and genetically interacts with Drp1-in mitochondrial connectivity also remains poorly understood. Here, we quantified the number and connectivity of mitochondria using mitochondria-targeted photoactivatable GFP in cells. We show that the loss of Drp1 increases the connectivity of mitochondria by 15-fold in mouse embryonic fibroblasts (MEFs). While a single loss of parkin does not affect the connectivity of mitochondria, the connectivity of mitochondria significantly decreased compared with a single loss of Drp1 when parkin was lost in the absence of Drp1. Furthermore, the loss of parkin decreased the frequency of depolarization of the mitochondrial inner membrane that is caused by increased mitochondrial connectivity in Drp1-knockout MEFs. Therefore, our data suggest that parkin negatively regulates Drp1-indendent mitochondrial division.