The Charcot-Marie-Tooth type 2A gene product, Mfn2, up-regulates fuel oxidation through expression of OXPHOS system

All the little pieces. -Regulation of mitochondrial fusion and fission by ubiquitin and small ubiquitin-like modifer and their potential relevance in the heart.-

Mitochondria are dynamic organelles that undergo a constant cycle of division and fusion to maintain their function. The process of mitochondrial fusion has the effect of mixing their content, allowing complementation of protein components, mtDNA repair, and distribution of metabolic intermediates. Fission, on the other hand, enables mitochondria to increase in number and capacity, and to segregate mitochondria for autophagy by the lysosome ("mitophagy"). Disruption of these protein quality control mechanisms has recently been identified in multiple cardiac diseases, including cardiac hypertrophy, heart failure, dilated cardiomyopathy, and ischemic heart disease, and is intimately tied to mitochondrial control of apoptosis. Proteins that regulate mitochondrial fusion and fission have been discovered, including Mfn1, Mfn2, and Opa1 (fusion) and Drp1 and Fis1 (fission). In this review, we discuss how these proteins are regulated by post-translational modification with ubiquitin and SUMO (small ubiquitin-like modifier). We then present what is known about the ubiquitin and SUMO ligases that regulate these post-translational modifications and regulation of mitochondrial fusion and fission, exploring their potential as therapeutic targets of cardiac disease.

Charcot-Marie-Tooth disease CMT4A: GDAP1 increases cellular glutathione and the mitochondrial membrane potential

Mutations in GDAP1 lead to recessively or dominantly inherited peripheral neuropathies (Charcot-Marie-Tooth disease, CMT), indicating that GDAP1 is essential for the viability of cells in the peripheral nervous system. GDAP1 contains domains characteristic of glutathione-S-transferases (GSTs), is located in the outer mitochondrial membrane and induces fragmentation of mitochondria. We found GDAP1 upregulated in neuronal HT22 cells selected for resistance against oxidative stress. GDAP1 over-expression protected against oxidative stress caused by depletion of the intracellular antioxidant glutathione (GHS) and against effectors of GHS depletion that affect the mitochondrial membrane integrity like truncated BH3-interacting domain death agonist and 12/15-lipoxygenase. Gdap1 knockdown, in contrast, increased the susceptibility of motor neuron-like NSC34 cells against GHS depletion. Over-expression of wild-type GDAP1, but not of GDAP1 with recessively inherited mutations that cause disease and reduce fission activity, increased the total cellular GHS content and the mitochondrial membrane potential up to a level where it apparently limits mitochondrial respiration, leading to reduced mitochondrial Ca(2+) uptake and superoxide production. Fibroblasts from autosomal-recessive CMT4A patients had reduced GDAP1 levels, reduced GHS concentration and a reduced mitochondrial membrane potential. Thus, our results suggest that the potential GST GDAP1 is implicated in the control of the cellular GHS content and mitochondrial activity, suggesting an involvement of oxidative stress in the pathogenesis of CMT4A.

Expression of nuclear-encoded genes involved in mitochondrial biogenesis and dynamics in experimentally denervated muscle

The abundance, morphology, and functional properties of mitochondria become altered in response to denervation. To gain insight into the regulation of this process, mitochondrial enzyme activities and gene expression involved in mitochondrial biogenesis and dynamics in mouse gastrocnemius muscle was investigated. Sciatic nerve transactions were performed on mice, and then gastrocnemius muscles were isolated at days 5 and 30 after surgery. Muscle weight was decreased significantly by 15% and 62% at days 5 and 30 after surgery, respectively. The activity of citrate synthase, a marker of oxidative enzyme, was reduced significantly by 31% and 53% at days 5 and 30, respectively. Enzyme histochemical analysis revealed that subsarcolemmal mitochondria were largely lost than intermyofibrillar mitochondria at day 5, and this trend was further progressed at day 30 after surgery. Expression levels of peroxisome proliferator-activated receptor, γ coactivator 1 (PGC-1)α, estrogen-related receptor α (ERRα), and mitofusin 2 were down-regulated throughout the experimental period, whereas those of PGC-1β, PRC, nuclear respiratory factor (NRF)-1, NRF-2, TFAM, and Lon protease were down-regulated at day 30 after surgery. These results suggest that PGC-1α, ERRα, and mitofusin 2 may be important factors in the process of denervation-induced mitochondrial adaptation. In addition, other PGC-1 family of transcriptional coactivators and DNA binding transcription factors may also contribute to mitochondrial adaptation after early response to denervation.

[Mitochondrial dynamics: a potential new therapeutic target for heart failure]

Mitochondria are dynamic organelles able to vary their morphology between elongated interconnected mitochondrial networks and fragmented disconnected arrays, through events of mitochondrial fusion and fission, respectively. These events allow the transmission of signaling messengers and exchange of metabolites within the cell. They have also been implicated in a variety of biological processes including embryonic development, metabolism, apoptosis, and autophagy. Although the majority of these studies have been confined to noncardiac cells, emerging evidence suggests that changes in mitochondrial morphology could participate in cardiac development, the response to ischemia-reperfusion injury, heart failure, and diabetes mellitus. In this article, we review how the mitochondrial dynamics are altered in different cardiac pathologies, with special emphasis on heart failure, and how this knowledge may provide new therapeutic targets for treating cardiovascular diseases.

Mitofusins are required for angiogenic function and modulate different signaling pathways in cultured endothelial cells

The mitofusin proteins MFN1 and MFN2 function to maintain mitochondrial networks by binding one another and initiating outer mitochondrial membrane fusion. While it has recently been recognized that vascular endothelial cells rely upon mitochondria as signaling rather than energy-producing moieties, the role of mitochondrial dynamics in endothelial cell function has not been addressed. To begin to understand what role mitochondrial dynamics play in this context, we examined the regulation of MFN1 and MFN2 and the consequences of siRNA-mediated knockdown of these proteins in cultured endothelial cells. Treatment with VEGF-A led to the upregulation of MFN2 and, to a lesser extent, MFN1. Knockdown of either MFN led to disrupted mitochondrial networks and diminished mitochondrial membrane potential. Knockdown of either MFN decreased VEGF-mediated migration and differentiation into network structures. MFN ablation also diminished endothelial cell viability and increased apoptosis under low mitogen conditions. Knockdown of MFN2 uniquely resulted in a decrease in the generation of reactive oxygen species as well as the blunting of the gene expression of components of the respiratory chain and transcription factors associated with oxidative metabolism. In contrast, ablation of MFN1 led to the selective reduction of VEGF-stimulated Akt-eNOS signaling. Taken together, our data indicate that mitochondrial dynamics, particularly those mediated by the mitofusins, play a role in endothelial cell function and viability.

Mitochondria: isolation, structure and function

Mitochondria are complex organelles constantly undergoing processes of fusion and fission, processes that not only modulate their morphology, but also their function. Yet the assessment of mitochondrial function in skeletal muscle often involves mechanical isolation of the mitochondria, a process which disrupts their normally heterogeneous branching structure and yields relatively homogeneous spherical organelles. Alternatively, methods have been used where the sarcolemma is permeabilized and mitochondrial morphology is preserved, but both methods face the downside that they remove potential influences of the intracellular milieu on mitochondrial function. Importantly, recent evidence shows that the fragmented mitochondrial morphology resulting from routine mitochondrial isolation procedures used with skeletal muscle alters key indices of function in a manner qualitatively similar to mitochondria undergoing fission in vivo. Although these results warrant caution when interpreting data obtained with mitochondria isolated from skeletal muscle, they also suggest that isolated mitochondrial preparations might present a useful way of interrogating the stress resistance of mitochondria. More importantly, these new findings underscore the empirical value of studying mitochondrial function in minimally disruptive experimental preparations. In this review, we briefly discuss several considerations and hypotheses emerging from this work.

The complex interplay between mitochondrial dynamics and cardiac metabolism

Mitochondria are highly dynamic organelles, capable of undergoing constant fission and fusion events, forming networks. These dynamic events allow the transmission of chemical and physical messengers and the exchange of metabolites within the cell. In this article we review the signaling mechanisms controlling mitochondrial fission and fusion, and its relationship with cell bioenergetics, especially in the heart. Furthermore we also discuss how defects in mitochondrial dynamics might be involved in the pathogenesis of metabolic cardiac diseases.

Mitochondrial adaptations in skeletal muscle to hindlimb unloading

To gain insight into the regulation of mitochondrial adaptations to hindlimb unloading (HU), the activity of mitochondrial enzymes and the expression of nuclear-encoded genes which control mitochondrial properties in mouse gastrocnemius muscle were investigated. Biochemical and enzyme histochemical analysis showed that subsarcolemmal mitochondria were lost largely than intermyofibrillar mitochondria after HU. Gene expression analysis revealed disturbed or diminished gene expression patterns. The three main results of this analysis are as follows. First, in contrast to peroxisome proliferator-activated receptor γ coactivator 1 β (PGC-1β) and PGC-1-related coactivator, which were down-regulated by HU, PGC-1α was up-regulated concomitant with decreased expression of its DNA binding transcription factors, PPARα, and estrogen-related receptor α (ERRα). Moreover, there was no alteration in expression of nuclear respiratory factor 1, but its downstream target gene, mitochondrial transcription factor A, was down-regulated. Second, both mitofusin 2 and fission 1, which control mitochondrial morphology, were down-regulated. Third, ATP-dependent Lon protease, which participates in mitochondrial-protein degradation, was also down-regulated. These findings suggest that HU may induce uncoordinated expression of PGC-1 family coactivators and DNA binding transcription factors, resulting in reducing ability of mitochondrial biogenesis. Furthermore, down-regulation of mitochondrial morphology-related genes associated with HU may be also involved in alterations in intracellular mitochondrial distribution.

MISC-1/OGC links mitochondrial metabolism, apoptosis and insulin secretion

We identified MISC-1 (Mitochondrial Solute Carrier) as the C. elegans orthologue of mammalian OGC (2-oxoglutarate carrier). OGC was originally identified for its ability to transfer α-ketoglutarate across the inner mitochondrial membrane. However, we found that MISC-1 and OGC are not solely involved in metabolic control. Our data show that these orthologous proteins participate in phylogenetically conserved cellular processes, like control of mitochondrial morphology and induction of apoptosis. We show that MISC-1/OGC is required for proper mitochondrial fusion and fission events in both C. elegans and human cells. Transmission electron microscopy reveals that loss of MISC-1 results in a decreased number of mitochondrial cristae, which have a blebbed appearance. Furthermore, our pull-down experiments show that MISC-1 and OGC interact with the anti-apoptotic proteins CED-9 and Bcl-x(L), respectively, and with the pro-apoptotic protein ANT. Knock-down of misc-1 in C. elegans and OGC in mouse cells induces apoptosis through the caspase cascade. Genetic analysis suggests that MISC-1 controls apoptosis through the physiological pathway mediated by the LIN-35/Rb-like protein. We provide genetic and molecular evidence that absence of MISC-1 increases insulin secretion and enhances germline stem cell proliferation in C. elegans. Our study suggests that the mitochondrial metabolic protein MISC-1/OGC integrates metabolic, apoptotic and insulin secretion functions. We propose a novel mechanism by which mitochondria integrate metabolic and cell survival signals. Our data suggest that MISC-1/OGC functions by sensing the metabolic status of mitochondria and directly activate the apoptotic program when required. Our results suggest that controlling MISC-1/OGC function allows regulation of mitochondrial morphology and cell survival decisions by the metabolic needs of the cell.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Mitofusin-2 maintains mitochondrial structure and contributes to stress-induced permeability transition in cardiac myocytes

Mitofusin-2 (Mfn-2) is a dynamin-like protein that is involved in the rearrangement of the outer mitochondrial membrane. Research using various experimental systems has shown that Mfn-2 is a mediator of mitochondrial fusion, an evolutionarily conserved process responsible for the surveillance of mitochondrial homeostasis. Here, we find that cardiac myocyte mitochondria lacking Mfn-2 are pleiomorphic and have the propensity to become enlarged. Consistent with an underlying mild mitochondrial dysfunction, Mfn-2-deficient mice display modest cardiac hypertrophy accompanied by slight functional deterioration. The absence of Mfn-2 is associated with a marked delay in mitochondrial permeability transition downstream of Ca(2+) stimulation or due to local generation of reactive oxygen species (ROS). Consequently, Mfn-2-deficient adult cardiomyocytes are protected from a number of cell death-inducing stimuli and Mfn-2 knockout hearts display better recovery following reperfusion injury. We conclude that in cardiac myocytes, Mfn-2 controls mitochondrial morphogenesis and serves to predispose cells to mitochondrial permeability transition and to trigger cell death.

Mitochondrial dynamics in diabetes

Mitochondria are at the center of cellular energy metabolism and regulate cell life and death. The cell biological aspect of mitochondria, especially mitochondrial dynamics, has drawn much attention through implications in human pathology, including neurological disorders and metabolic diseases. Mitochondrial fission and fusion are the main processes governing the morphological plasticity and are controlled by multiple factors, including mechanochemical enzymes and accessory proteins. Emerging evidence suggests that mitochondrial dynamics plays an important role in metabolism-secretion coupling in pancreatic β-cells as well as complications of diabetes. This review describes an overview of mechanistic and functional aspects of mitochondrial fission and fusion, and comments on the recent advances connecting mitochondrial dynamics with diabetes and diabetic complications.

New insights into the role of mitochondria in aging: mitochondrial dynamics and more

A decline in mitochondrial function plays a key role in the aging process and increases the incidence of age-related disorders. A deeper understanding of the intricate nature of mitochondrial dynamics, which is described as the balance between mitochondrial fusion and fission, has revealed that functional and structural alterations in mitochondrial morphology are important factors in several key pathologies associated with aging. Indeed, a recent wave of studies has demonstrated the pleiotropic role of fusion and fission proteins in numerous cellular processes, including mitochondrial metabolism, redox signaling, the maintenance of mitochondrial DNA and cell death. Additionally, mitochondrial fusion and fission, together with autophagy, have been proposed to form a quality-maintenance mechanism that facilitates the removal of damaged mitochondria from the cell, a process that is particularly important to forestall aging. Thus, dysfunctional regulation of mitochondrial dynamics might be one of the intrinsic causes of mitochondrial dysfunction, which contributes to oxidative stress and cell death during the aging process. In this Commentary, we discuss recent studies that have converged at a consensus regarding the involvement of mitochondrial dynamics in key cellular processes, and introduce a possible link between abnormal mitochondrial dynamics and aging.

Dynamic regulation of mitochondrial function in preimplantation embryos and embryonic stem cells

Mitochondrial function is dependent upon regulation of biogenesis and dynamics. A number of studies have documented the importance of these organelles in both preimplantation embryos and embryonic stem cells (ESCs), however it remains unclear how mitochondria respond to their immediate microenvironment through modulation of morphology and movement, or whether perturbations in these processes will have a significant impact following differentiation/implantation. Here we review existing literature on two key aspects of nuclear-mitochondrial cross-talk and the dynamic processes involved in mediating mitochondrial function through regulation of mitochondrial biogenesis, morphology and movement, with particular emphasis on embryos and ESCs.

Muscle oxidative capacity during IL-6-dependent cancer cachexia

Many diseases are associated with catabolic conditions that induce skeletal muscle wasting. These various catabolic states may have similar and distinct mechanisms for inducing muscle protein loss. Mechanisms related to muscle wasting may also be related to muscle metabolism since glycolytic muscle fibers have greater wasting susceptibility with several diseases. The purpose of this study was to determine the relationship between muscle oxidative capacity and muscle mass loss in red and white hindlimb muscles during cancer cachexia development in the Apc(Min/+) mouse. Gastrocnemius and soleus muscles were excised from Apc(Min/+) mice at 20 wk of age. The gastrocnemius muscle was partitioned into red and white portions. Body mass (-20%), gastrocnemius muscle mass (-41%), soleus muscle mass (-34%), and epididymal fat pad (-100%) were significantly reduced in severely cachectic mice (n = 8) compared with mildly cachectic mice (n = 6). Circulating IL-6 was fivefold higher in severely cachectic mice. Cachexia significantly reduced the mitochondrial DNA-to-nuclear DNA ratio in both red and white portions of the gastrocnemius. Cytochrome c and cytochrome-c oxidase complex subunit IV (Cox IV) protein were reduced in all three muscles with severe cachexia. Changes in muscle oxidative capacity were not associated with altered myosin heavy chain expression. PGC-1α expression was suppressed by cachexia in the red and white gastrocnemius and soleus muscles. Cachexia reduced Mfn1 and Mfn2 mRNA expression and markers of oxidative stress, while Fis1 mRNA was increased by cachexia in all muscle types. Muscle oxidative capacity, mitochondria dynamics, and markers of oxidative stress are reduced in both oxidative and glycolytic muscle with severe wasting that is associated with increased circulating IL-6 levels.

Repeated transient mRNA bursts precede increases in transcriptional and mitochondrial proteins during training in human skeletal muscle

Exercise training induces mitochondrial biogenesis, but the time course of molecular sequelae that accompany repetitive training stimuli remains to be determined in human skeletal muscle. Therefore, throughout a seven-session, high-intensity interval training period that increased (12%), we examined the time course of responses of (a) mitochondrial biogenesis and fusion and fission proteins, and (b) selected transcriptional and mitochondrial mRNAs and proteins in human muscle. Muscle biopsies were obtained 4 and 24 h after the 1st, 3rd, 5th and 7th training session. PGC-1α mRNA was increased >10-fold 4 h after the 1st session and returned to control within 24 h. This 'saw-tooth' pattern continued until the 7th bout, with smaller increases after each bout. In contrast, PGC-1α protein was increased 24 h after the 1st bout (23%) and plateaued at +30-40% between the 3rd and 7th bout. Increases in PGC-1β mRNA and protein were more delayed and smaller, and did not persist. Distinct patterns of increases were observed in peroxisome proliferator-activated receptor (PPAR) α and γ protein (1 session), PPAR β/δ mRNA and protein (5 sessions) and nuclear respiratory factor-2 protein (3 sessions) while no changes occurred in mitochondrial transcription factor A protein. Citrate synthase (CS) and β-HAD mRNA were rapidly increased (1 session), followed 2 sessions later (session 3) by increases in CS and β-HAD activities, and mitochondrial DNA. Changes in COX-IV mRNA (session 3) and protein (session 5) were more delayed. Training also increased mitochondrial fission proteins (fission protein-1, >2-fold; dynamin-related protein-1, 47%) and the fusion protein mitofusin-1 (35%) but not mitofusin-2. This study has provided the following novel information: (a) the training-induced increases in transcriptional and mitochondrial proteins appear to result from the cumulative effects of transient bursts in their mRNAs, (b) training-induced mitochondrial biogenesis appears to involve re-modelling in addition to increased mitochondrial content, and (c) the 'transcriptional capacity' of human muscle is extremely sensitive, being activated by one training bout.

Krüppel-like factor 4 interacts with p300 to activate mitofusin 2 gene expression induced by all-trans retinoic acid in VSMCs

AIM

To elucidate how krüppel-like factor 4 (KLF4) activates mitofusin 2 (mfn-2) expression in all-trans retinoic acid (ATRA)-induced vascular smooth muscle cell (VSMC) differentiation.

METHODS

The mfn-2 promoter-reporter constructs and the KLF4 acetylation-deficient or phosphorylation-deficient mutants were constructed. Adenoviral vector of KLF4-mediated overexpression and Western blot analysis were used to determine the effect of KLF4 on mfn-2 expression. The luciferase assay and chromatin immunoprecipitation were used to detect the transactivation of KLF4 on mfn-2 gene expression. Co-immunoprecipitation and GST pull-down assays were used to determine the modification of KLF4 and interaction of KLF4 with p300 in VSMCs.

RESULTS

KLF4 mediated ATRA-induced mfn-2 expression in VSMCs. KLF4 bound directly to the mfn-2 promoter and activated its transcription. ATRA increased the interaction of KLF4 with p300 by inducing KLF4 phosphorylation via activation of JNK and p38 MAPK signaling. KLF4 acetylation by p300 increased its activity to transactivate the mfn-2 promoter.

CONCLUSION

ATRA induces KLF4 acetylation by p300 and increases the ability of KLF4 to transactivate the mfn-2 promoter in VSMCs.

Role of mitofusin 2 in cardiovascular oxidative injury

Mitochondria are highly dynamic organelles with constant shape changes regulated by fusion and fission events. In addition to regulating mitochondrial morphology, mitochondrial fusion/fission is involved in fundamental mitochondrial biological processes, including mitochondrial metabolism, energization, respiration, mitochondrial membrane potential, and mtDNA stability. Dysfunction of mitochondrial dynamics has been implicated in various human diseases, especially in neurodegenerative diseases. Emerging evidence indicates that impaired expression of mitochondrial fusion proteins or their malfunction participates in oxidative stress-induced cardiovascular injury. This review will focus on recent advances of mitochondrial fusion in regulating various cellular processes in cardiovascular system.

Redefining the role of mitochondria in exercise: a dynamic remodeling

Exercise induced adaptations in muscle are highly specific and dependent upon the type of exercise, as well as its frequency, intensity, and duration. Mitochondria are highly dynamic organelles. Fusion and fission reactions lead to a continuous remodeling of the mitochondrial network, which range from reticulum of elongated and branched filaments to collections of individual organelles. Mitochondrial network dynamics are sensitive to various physiological and pathological stimuli, and mitochondrial morphological changes are no epiphenomena, but central to cell function and survival. There is a strong correlation between mitochondrial network morphology, dynamic-related protein, and energy metabolism. It is expected that alteration in cellular energy status during exercise can also be achieved through mitochondrial network dynamics. In this review, we describe mitochondrial network remodeling response to acute and endurance exercise, which is accompanied by bioenergetics and redox regulation. In addition, potential mechanisms for metabolic and redox signaling involved in mitochondrial dynamic regulation are also reviewed.

Coenzyme Q protects Caenorhabditis elegans GABA neurons from calcium-dependent degeneration

Mitochondria are key regulators of cell viability and provide essential functions that protect against neurodegenerative disease. To develop a model for mitochondrial-dependent neurodegeneration in Caenorhabditis elegans, we used RNA interference (RNAi) and genetic ablation to knock down expression of enzymes in the Coenzyme Q (CoQ) biosynthetic pathway. CoQ is a required component of the ATP-producing electron transport chain in mitochondria. We found that reduced levels of CoQ result in a progressive uncoordinated (Unc) phenotype that is correlated with the appearance of degenerating GABA neurons. Both the Unc and degenerative phenotypes emerge during late larval development and progress in adults. Neuron classes in motor and sensory circuits that use other neurotransmitters (dopamine, acetylcholine, glutamate, serotonin) and body muscle cells were less sensitive to CoQ depletion. Our results indicate that the mechanism of GABA neuron degeneration is calcium-dependent and requires activation of the apoptotic gene, ced-4 (Apaf-1). A molecular cascade involving mitochondrial-initiated cell death is also consistent with our finding that GABA neuron degeneration requires the mitochondrial fission gene, drp-1. We conclude that the cell selectivity and developmental progression of CoQ deficiency in C. elegans indicate that this model may be useful for delineating the role of mitochondrial dysfunction in neurodegenerative disease.

Mitochondrial morphology and cardiovascular disease

Mitochondria are dynamic and are able to interchange their morphology between elongated interconnected mitochondrial networks and a fragmented disconnected arrangement by the processes of mitochondrial fusion and fission, respectively. Changes in mitochondrial morphology are regulated by the mitochondrial fusion proteins (mitofusins 1 and 2, and optic atrophy 1) and the mitochondrial fission proteins (dynamin-related peptide 1 and mitochondrial fission protein 1) and have been implicated in a variety of biological processes including embryonic development, metabolism, apoptosis, and autophagy, although the majority of studies have been largely confined to non-cardiac cells. Despite the unique arrangement of mitochondria in the adult heart, emerging data suggest that changes in mitochondrial morphology may be relevant to various aspects of cardiovascular biology-these include cardiac development, the response to ischaemia-reperfusion injury, heart failure, diabetes mellitus, and apoptosis. Interestingly, the machinery required for altering mitochondrial shape in terms of the mitochondrial fusion and fission proteins are all present in the adult heart, but their physiological function remains unclear. In this article, we review the current developments in this exciting new field of mitochondrial biology, the implications for cardiovascular physiology, and the potential for discovering novel therapeutic strategies for treating cardiovascular disease.

Mitofusin-2 protects against cold stress-induced cell injury in HEK293 cells

Mitochondrial impairment is hypothesized to contribute to cell injury during cold stress. Mitochondria fission and fusion are closely related in the function of the mitochondria, but the precise mechanisms whereby these processes regulate cell injury during cold stress remain to be determined. HEK293 cells were cultured in a cold environment (4.0+/-0.1 degrees C) for 2, 4, 8, or 12h. Western blot analyses showed that these cells expressed decreased fission-related protein Drp1 and increased fusion-related protein Mfn2 at 4h; meanwhile, electron microscopy analysis revealed large and long mitochondrial morphology within these cells, indicating increased mitochondrial fusion. With silencing of Mfn2 but not of Mfn1 by siRNA promoted cold-stress-induced cell death with decreased ATP production in HEK293 cells. Our results show that increased expression of Mfn2 and mitochondrial fusion are important for mitochondrial function as well as cell survival during cold stress. These findings have important implications for understanding the mechanisms of mitochondrial fusion and fission in cold-stress-induced cell injury.

Biophysical properties of mitochondrial fusion events in pancreatic beta-cells and cardiac cells unravel potential control mechanisms of its selectivity

Studies in various types of cells find that, on average, each mitochondrion becomes involved in a fusion event every 15 min, depending on the cell type. As most contact events do not result in mitochondrial fusion, it is expected that properties of the individual mitochondrion determine the likelihood of a fusion event. However, apart from membrane potential, the properties that influence the likelihood of entering a fusion event are not known. Here, we tag and track individual mitochondria in H9c2, INS1, and primary beta-cells and determine the biophysical properties that increase the likelihood of a fusion event. We found that the probability for fusion is independent of contact duration and organelle dimensions, but it is influenced by organelle motility. Furthermore, the history of a previous fusion event of the individual mitochondrion influenced both the likelihood for a subsequent fusion event, as well as the site on the mitochondrion at which the fusion occurred. These observations unravel the specific properties that distinguish mitochondria that will enter fusion events from the ones that will not. Altogether, these properties may help to elucidate the molecular mechanisms that regulate fusion at the level of the single mitochondrion.

Inhibiting mitochondrial fission protects the heart against ischemia/reperfusion injury

BACKGROUND

Whether alterations in mitochondrial morphology affect the susceptibility of the heart to ischemia/reperfusion injury is unknown. We hypothesized that modulating mitochondrial morphology protects the heart against ischemia/reperfusion injury.

METHODS AND RESULTS

In response to ischemia, mitochondria in HL-1 cells (a cardiac-derived cell line) undergo fragmentation, a process that is dependent on the mitochondrial fission protein dynamin-related protein 1 (Drp1). Transfection of HL-1 cells with the mitochondrial fusion proteins mitofusin 1 or 2 or with Drp1(K38A), a dominant-negative mutant form of Drp1, increased the percentage of cells containing elongated mitochondria (65+/-4%, 69+/-5%, and 63+/-6%, respectively, versus 46+/-6% in control: n=80 cells per group; P<0.05), decreased mitochondrial permeability transition pore sensitivity (by 2.4+/-0.5-, 2.3+/-0.7-, and 2.4+/-0.3-fold, respectively; n=80 cells per group; P<0.05), and reduced cell death after simulated ischemia/reperfusion injury (11.6+/-3.9%, 16.2+/-3.9%, and 12.1+/-2.9%, respectively, versus 41.8+/-4.1% in control: n=320 cells per group; P<0.05). Treatment of HL-1 cells with mitochondrial division inhibitor-1, a pharmacological inhibitor of Drp1, replicated these beneficial effects. Interestingly, elongated interfibrillar mitochondria were identified in the adult rodent heart with confocal and electron microscopy, and in vivo treatment with mitochondrial division inhibitor-1 increased the percentage of elongated mitochondria from 3.6+/-0.5% to 14.5+/-2.8% (P=0.023). Finally, treatment of adult murine cardiomyocytes with mitochondrial division inhibitor-1 reduced cell death and inhibited mitochondrial permeability transition pore opening after simulated ischemia/reperfusion injury, and in vivo treatment with mitochondrial division inhibitor-1 reduced myocardial infarct size in mice subject to coronary artery occlusion and reperfusion (21.0+/-2.2% with mitochondrial division inhibitor-1 versus 48.0+/-4.5% in control; n=6 animals per group; P<0.05).

CONCLUSIONS

Inhibiting mitochondrial fission protects the heart against ischemia/reperfusion injury, suggesting a novel pharmacological strategy for cardioprotection.

Cryopreservation-induced alterations in boar spermatozoa mitochondrial function are related to changes in the expression and location of midpiece mitofusin-2 and actin network

The authors analyzed changes in mitochondrial activity of boar semen during a standard cryopreservation protocol. For this purpose, mitochondrial activity was evaluated simultaneously with the rhythm of mitochondrial formation of reactive oxygen species (mROS) through a double MitoTracker Red/proxylfluorescamine stain. Moreover, we analyzed changes in the expression and location of two key regulatory elements of mitochondrial function, namely mitofusin-2 (Mfn2) and actin, during the freezing-thawing protocol. Our results indicate that mitochondrial activity and mROS formation decreased during cyropreservation, with an initial decrease during the cooling phase of the protocol. This decrease was accompanied by an increase in the amount of solubilized Mfn2, which was concomitant with a progressive extension of Mfn2 location from the apical zone of the midpiece to the whole midpiece. Simultaneously, cryopreservation induced a decrease in solubilized actin, which was concurrent with significant changes in the midpiece actin location. The observed changes in the expression and location of both Mfn2 and actin were already present after the cooling phase of the cryopreservation protocol. Our results suggest that freezing-thawing impaired mitochondrial function. This impairment was concomitant with a decrease in the mitochondrial capacity to synthesize mROS. This impairment is attributed to changes in mitochondrial volume as a result of alterations in the expression and location of both Mfn-2 and the actin network. Finally, the alterations of mitochondrial function induced by the cryopreservation protocol were already apparent at the cooling phase. This observation indicates that the cooling phase is a crucial stage in which mitochondrial alterations occur during cryopreservation.

Mitochondrial turnover and aging of long-lived postmitotic cells: the mitochondrial-lysosomal axis theory of aging

It is now generally accepted that aging and eventual death of multicellular organisms is to a large extent related to macromolecular damage by mitochondrially produced reactive oxygen species, mostly affecting long-lived postmitotic cells, such as neurons and cardiac myocytes. These cells are rarely or not at all replaced during life and can be as old as the whole organism. The inherent inability of autophagy and other cellular-degradation mechanisms to remove damaged structures completely results in the progressive accumulation of garbage, including cytosolic protein aggregates, defective mitochondria, and lipofuscin, an intralysosomal indigestible material. In this review, we stress the importance of crosstalk between mitochondria and lysosomes in aging. The slow accumulation of lipofuscin within lysosomes seems to depress autophagy, resulting in reduced turnover of effective mitochondria. The latter not only are functionally deficient but also produce increased amounts of reactive oxygen species, prompting lipofuscinogenesis. Moreover, defective and enlarged mitochondria are poorly autophagocytosed and constitute a growing population of badly functioning organelles that do not fuse and exchange their contents with normal mitochondria. The progress of these changes seems to result in enhanced oxidative stress, decreased ATP production, and collapse of the cellular catabolic machinery, which eventually is incompatible with survival.

Mitofusin 2 is necessary for transport of axonal mitochondria and interacts with the Miro/Milton complex

Mitofusins (Mfn1 and Mfn2) are outer mitochondrial membrane proteins involved in regulating mitochondrial dynamics. Mutations in Mfn2 cause Charcot-Marie-Tooth disease (CMT) type 2A, an inherited disease characterized by degeneration of long peripheral axons, but the nature of this tissue selectivity remains unknown. Here, we present evidence that Mfn2 is directly involved in and required for axonal mitochondrial transport, distinct from its role in mitochondrial fusion. Live imaging of neurons cultured from Mfn2 knock-out mice or neurons expressing Mfn2 disease mutants shows that axonal mitochondria spend more time paused and undergo slower anterograde and retrograde movements, indicating an alteration in attachment to microtubule-based transport systems. Furthermore, Mfn2 disruption altered mitochondrial movement selectively, leaving transport of other organelles intact. Importantly, both Mfn1 and Mfn2 interact with mammalian Miro (Miro1/Miro2) and Milton (OIP106/GRIF1) proteins, members of the molecular complex that links mitochondria to kinesin motors. Knockdown of Miro2 in cultured neurons produced transport deficits identical to loss of Mfn2, indicating that both proteins must be present at the outer membrane to mediate axonal mitochondrial transport. In contrast, disruption of mitochondrial fusion via knockdown of the inner mitochondrial membrane protein Opa1 had no effect on mitochondrial motility, indicating that loss of fusion does not inherently alter mitochondrial transport. These experiments identify a role for mitofusins in directly regulating mitochondrial transport and offer important insight into the cell type specificity and molecular mechanisms of axonal degeneration in CMT2A and dominant optic atrophy.

Interdependence of AMPK and SIRT1 for metabolic adaptation to fasting and exercise in skeletal muscle

During fasting and after exercise, skeletal muscle efficiently switches from carbohydrate to lipid as the main energy source to preserve glycogen stores and blood glucose levels for glucose-dependent tissues. Skeletal muscle cells sense this limitation in glucose availability and transform this information into transcriptional and metabolic adaptations. Here we demonstrate that AMPK acts as the prime initial sensor that translates this information into SIRT1-dependent deacetylation of the transcriptional regulators PGC-1alpha and FOXO1, culminating in the transcriptional modulation of mitochondrial and lipid utilization genes. Deficient AMPK activity compromises SIRT1-dependent responses to exercise and fasting, resulting in impaired PGC-1alpha deacetylation and blunted induction of mitochondrial gene expression. Thus, we conclude that AMPK acts as the primordial trigger for fasting- and exercise-induced adaptations in skeletal muscle and that activation of SIRT1 and its downstream signaling pathways are improperly triggered in AMPK-deficient states.

Subjects with early-onset type 2 diabetes show defective activation of the skeletal muscle PGC-1{alpha}/Mitofusin-2 regulatory pathway in response to physical activity

OBJECTIVE Type 2 diabetes is associated with insulin resistance and skeletal muscle mitochondrial dysfunction. We have found that subjects with early-onset type 2 diabetes show incapacity to increase Vo(2max) in response to chronic exercise. This suggests a defect in muscle mitochondrial response to exercise. Here, we have explored the nature of the mechanisms involved. RESEARCH DESIGN AND METHODS Muscle biopsies were collected from young type 2 diabetic subjects and obese control subjects before and after acute or chronic exercise protocols, and the expression of genes and/or proteins relevant to mitochondrial function was measured. In particular, the regulatory pathway peroxisome proliferator-activated receptor gamma coactivator (PGC)-1alpha/mitofusin-2 (Mfn2) was analyzed. RESULTS At baseline, subjects with diabetes showed reduced expression (by 26%) of the mitochondrial fusion protein Mfn2 and a 39% reduction of the alpha-subunit of ATP synthase. Porin expression was unchanged, consistent with normal mitochondrial mass. Chronic exercise led to a 2.8-fold increase in Mfn2, as well as increases in porin, and the alpha-subunit of ATP synthase in muscle from control subjects. However, Mfn2 was unchanged after chronic exercise in individuals with diabetes, whereas porin and alpha-subunit of ATP synthase were increased. Acute exercise caused a fourfold increase in PGC-1alpha expression in muscle from control subjects but not in subjects with diabetes. CONCLUSIONS Our results demonstrate alterations in the regulatory pathway that controls PGC-1alpha expression and induction of Mfn2 in muscle from patients with early-onset type 2 diabetes. Patients with early-onset type 2 diabetes display abnormalities in the exercise-dependent pathway that regulates the expression of PGC-1alpha and Mfn2.

Alterations in the mitochondrial regulatory pathways constituted by the nuclear co-factors PGC-1alpha or PGC-1beta and mitofusin 2 in skeletal muscle in type 2 diabetes

Muscle mitochondrial metabolism is regulated by a number of factors, many of which are responsible for the transcription of nuclear genes encoding mitochondrial proteins such as PPARdelta, PGC-1alpha or PGC-1beta. Recent evidence indicates that proteins participating in mitochondrial dynamics also regulate mitochondrial metabolism. Thus, in cultured cells the mitochondrial fusion protein mitofusin 2 (Mfn2) stimulates respiration, substrate oxidation and the expression of subunits involved in respiratory complexes. Mitochondrial dysfunction has been reported in skeletal muscle of type 2 diabetic patients. Reduced mitochondrial mass and defective activity has been proposed to explain this dysfunction. Alterations in mitochondrial metabolism may be crucial to account for some of the pathophysiological traits that characterize type 2 diabetes. Skeletal muscle of type 2 diabetic patients shows reduced expression of PGC-1alpha, PGC-1beta, and Mfn2. In addition, a differential response to bilio-pancreatic diversion-induced weight loss in non-diabetic and type 2 diabetic patients has been reported. While non-diabetic morbidly obese subjects showed an increased expression of genes encoding Mfn2, PGC-1alpha, PGC-1beta, PPARdelta or SIRT1 in response to bariatric surgery-induced weight loss, no effect was detected in type 2 diabetic patients. These observations suggest the existence of a heritable component responsible for the abnormal control of the expression of genes encoding for modulators of mitochondrial biogenesis/metabolism, and which may participate in the development of the disease.

Multi-system neurological disease is common in patients with OPA1 mutations

Additional neurological features have recently been described in seven families transmitting pathogenic mutations in OPA1, the most common cause of autosomal dominant optic atrophy. However, the frequency of these syndromal 'dominant optic atrophy plus' variants and the extent of neurological involvement have not been established. In this large multi-centre study of 104 patients from 45 independent families, including 60 new cases, we show that extra-ocular neurological complications are common in OPA1 disease, and affect up to 20% of all mutational carriers. Bilateral sensorineural deafness beginning in late childhood and early adulthood was a prominent manifestation, followed by a combination of ataxia, myopathy, peripheral neuropathy and progressive external ophthalmoplegia from the third decade of life onwards. We also identified novel clinical presentations with spastic paraparesis mimicking hereditary spastic paraplegia, and a multiple sclerosis-like illness. In contrast to initial reports, multi-system neurological disease was associated with all mutational subtypes, although there was an increased risk with missense mutations [odds ratio = 3.06, 95% confidence interval = 1.44-6.49; P = 0.0027], and mutations located within the guanosine triphosphate-ase region (odds ratio = 2.29, 95% confidence interval = 1.08-4.82; P = 0.0271). Histochemical and molecular characterization of skeletal muscle biopsies revealed the presence of cytochrome c oxidase-deficient fibres and multiple mitochondrial DNA deletions in the majority of patients harbouring OPA1 mutations, even in those with isolated optic nerve involvement. However, the cytochrome c oxidase-deficient load was over four times higher in the dominant optic atrophy + group compared to the pure optic neuropathy group, implicating a causal role for these secondary mitochondrial DNA defects in disease pathophysiology. Individuals with dominant optic atrophy plus phenotypes also had significantly worse visual outcomes, and careful surveillance is therefore mandatory to optimize the detection and management of neurological disability in a group of patients who already have significant visual impairment.

Mitochondria: joining forces to thwart cell death

Mitochondria are highly dynamic organelles that undergo constant cycles of fusion and fission. An additional level of regulation of mitochondrial function, which is particularly important in neurons, is their active transport along microtubules. Recent evidence suggests that the mitochondrial fusion/fission machinery as well as the molecular motors responsible for their movement constitute powerful regulatory control points that directly impact metabolism and regulation of cell death. This is true for not only apoptosis, but also for excitotoxicity where calcium overload is a major component of the cell death process. In this review, we will describe the molecular mechanisms regulating fusion and fission and how this impinges on cell survival in the context of acute neuronal injury.

Mitochondrial dynamics in mammalian health and disease

The meaning of the word mitochondrion (from the Greek mitos, meaning thread, and chondros, grain) illustrates that the heterogeneity of mitochondrial morphology has been known since the first descriptions of this organelle. Such a heterogeneous morphology is explained by the dynamic nature of mitochondria. Mitochondrial dynamics is a concept that includes the movement of mitochondria along the cytoskeleton, the regulation of mitochondrial architecture (morphology and distribution), and connectivity mediated by tethering and fusion/fission events. The relevance of these events in mitochondrial and cell physiology has been partially unraveled after the identification of the genes responsible for mitochondrial fusion and fission. Furthermore, during the last decade, it has been identified that mutations in two mitochondrial fusion genes (MFN2 and OPA1) cause prevalent neurodegenerative diseases (Charcot-Marie Tooth type 2A and Kjer disease/autosomal dominant optic atrophy). In addition, other diseases such as type 2 diabetes or vascular proliferative disorders show impaired MFN2 expression. Altogether, these findings have established mitochondrial dynamics as a consolidated area in cellular physiology. Here we review the most significant findings in the field of mitochondrial dynamics in mammalian cells and their implication in human pathologies.

Mitofusin 2 inhibits mitochondrial antiviral signaling

The innate immune response to viral infection involves the activation of multiple signaling steps that culminate in the production of type I interferons (IFNs). Mitochondrial antiviral signaling (MAVS), a mitochondrial outer membrane adaptor protein, plays an important role in this process. Here, we report that mitofusin 2 (Mfn2), a mediator of mitochondrial fusion, interacts with MAVS to modulate antiviral immunity. Overexpression of Mfn2 resulted in the inhibition of retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA-5), two cytosolic sensors of viral RNA, as well as of MAVS-mediated activation of the transcription factors interferon regulatory factor 3 (IRF-3) and nuclear factor kappaB (NF-kappaB). In contrast, loss of endogenous Mfn2 enhanced virus-induced production of IFN-beta and thereby decreased viral replication. Structure-function analysis revealed that Mfn2 interacted with the carboxyl-terminal region of MAVS through a heptad repeat region, providing a structural perspective on the regulation of the mitochondrial antiviral response. Our results suggest that Mfn2 acts as an inhibitor of antiviral signaling, a function that may be distinct from its role in mitochondrial dynamics.

Genes involved in mitochondrial biogenesis/function are induced in response to bilio-pancreatic diversion in morbidly obese individuals with normal glucose tolerance but not in type 2 diabetic patients

AIMS/HYPOTHESIS

The mechanisms allowing normalisation of insulin sensitivity and reversal of type 2 diabetes after bilio-pancreatic diversion (BPD) have not been elucidated. We studied whether the expression of genes relevant to mitochondrial biogenesis/function is induced in response to BPD and whether the response differs between morbidly obese patients with normal glucose tolerance (NGT) and patients with type 2 diabetes.

METHODS

The effect of stable weight reduction after BPD on metabolic variables and expression of nuclear genes encoding for mitochondrial proteins or regulators of mitochondrial function was investigated in skeletal muscle. Insulin sensitivity was assessed by euglycaemic-hyperinsulinaemic clamp and substrate oxidation by indirect calorimetry.

RESULTS

Both NGT and type 2 diabetic patients showed a net improvement of insulin sensitivity, with the latter also showing blood glucose normalisation. NGT patients had a large increase in glucose oxidation and substantial reduction in lipid oxidation. In contrast, type 2 diabetic patients had a blunted response to BPD in terms of glucose oxidation. NGT patients showed increased expression of genes encoding mitofusin 2, porin or citrate synthase; no significant changes were detected in diabetic patients. The expression of genes regulating mitochondrial activity (PGC-1beta [also known as PPARGC1B], PGC-1alpha [also known as PPARGC1A], PPARdelta [also known as PPARD], SIRT1) was induced only in NGT patients.

CONCLUSIONS/INTERPRETATION

These findings indicate that weight loss after BPD exerts a beneficial effect on insulin sensitivity via mechanisms that are independent of the expression of genes involved in mitochondrial biogenesis/activity. Furthermore, the observation that gene expression is not altered with weight loss in type 2 diabetic patients while it is induced in NGT patients suggests a heritable component.

Adenine nucleotide translocase is involved in a mitochondrial coupling defect in MFN2-related Charcot-Marie-Tooth type 2A disease

Charcot-Marie-Tooth type 2A disease (CMT2A), a dominantly inherited peripheral neuropathy, is caused by mutations in MFN2, a mitochondrial fusion protein. Having previously demonstrated a mitochondrial coupling defect in CMT2A patients' fibroblasts, we here investigate mitochondrial oxygen consumption and the expression of adenine nucleotide translocase (ANT) and uncoupling proteins from eight other patients with the disease. The mitochondrial uncoupling was associated with a higher respiratory rate, essentially involving complex II proteins. Furthermore, a twofold increase in the expression of ANT led to the reduced efficiency of oxidative phosphorylation in CMT2A cells, suggesting that MFN2 plays a role in controlling ATP/ADP exchanges.

AMPK regulates energy expenditure by modulating NAD+ metabolism and SIRT1 activity

AMP-activated protein kinase (AMPK) is a metabolic fuel gauge conserved along the evolutionary scale in eukaryotes that senses changes in the intracellular AMP/ATP ratio. Recent evidence indicated an important role for AMPK in the therapeutic benefits of metformin, thiazolidinediones and exercise, which form the cornerstones of the clinical management of type 2 diabetes and associated metabolic disorders. In general, activation of AMPK acts to maintain cellular energy stores, switching on catabolic pathways that produce ATP, mostly by enhancing oxidative metabolism and mitochondrial biogenesis, while switching off anabolic pathways that consume ATP. This regulation can take place acutely, through the regulation of fast post-translational events, but also by transcriptionally reprogramming the cell to meet energetic needs. Here we demonstrate that AMPK controls the expression of genes involved in energy metabolism in mouse skeletal muscle by acting in coordination with another metabolic sensor, the NAD+-dependent type III deacetylase SIRT1. AMPK enhances SIRT1 activity by increasing cellular NAD+ levels, resulting in the deacetylation and modulation of the activity of downstream SIRT1 targets that include the peroxisome proliferator-activated receptor-gamma coactivator 1alpha and the forkhead box O1 (FOXO1) and O3 (FOXO3a) transcription factors. The AMPK-induced SIRT1-mediated deacetylation of these targets explains many of the convergent biological effects of AMPK and SIRT1 on energy metabolism.

Regulation of mitofusin-2 expression in skeletal muscleThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

Fusion and fission of mitochondria regulate their morphology and distribution. Mitofusin-2 (Mfn2) is a mitochondrial protein involved in such fusion. Recent observations indicate that Mfn2 is a multifunctional protein that participates in cell proliferation and metabolism and that it is required for normal endoplasmic reticulum morphology. In relation to the metabolic role of Mfn2, alterations in activity have been reported to modify cell respiration, substrate oxidation, and oxidative phosphorylation subunit expression in cultured nonmuscle and muscle cells. Mfn2 expression in skeletal muscle is subject to regulation and conditions characterized by reduced mitochondrial activity, such as obesity or type 2 diabetes, and are associated with repressed Mfn2 . In contrast, cold-exposure treatment with β3-adrenergic agonists or exercise induce the expression of this gene in muscle. Estrogen-related receptor-α transcription factor is a key regulator of Mfn2 transcription and recruits peroxisome proliferator-activated receptor γ coactivator (PGC)-1β and PGC-1α. These 2 nuclear coactivators are potent, positive regulators of Mfn2 expression in muscle cells, and ablation of PGC-1β causes Mfn2 downregulation in skeletal muscle and in the heart. We propose that PGC-1β is a regulator of normal expression of Mfn2 in muscle, whereas PGC-1α participates in the stimulation of Mfn2 expression under a variety of conditions characterized by enhanced energy expenditure.

Novel role of FATP1 in mitochondrial fatty acid oxidation in skeletal muscle cells

Carnitine palmitoyltransferase 1 (CPT1) catalyzes the first step in long-chain fatty acid import into mitochondria, and it is believed to be rate limiting for β-oxidation of fatty acids. However, in muscle, other proteins may collaborate with CPT1. Fatty acid translocase/CD36 (FAT/CD36) may interact with CPT1 and contribute to fatty acid import into mitochondria in muscle. Here, we demonstrate that another membrane-bound fatty acid binding protein, fatty acid transport protein 1 (FATP1), collaborates with CPT1 for fatty acid import into mitochondria. Overexpression of FATP1 using adenovirus in L6E9 myotubes increased both fatty acid oxidation and palmitate esterification into triacylglycerides. Moreover, immunocytochemistry assays in transfected L6E9 myotubes showed that FATP1 was present in mitochondria and coimmunoprecipitated with CPT1 in L6E9 myotubes and rat skeletal muscle in vivo. The cooverexpression of FATP1 and CPT1 also enhanced mitochondrial fatty acid oxidation, similar to the cooverexpression of FAT/CD36 and CPT1. However, etomoxir, an irreversible inhibitor of CPT1, blocked all these effects. These data reveal that FATP1, like FAT/CD36, is associated with mitochondria and has a role in mitochondrial oxidation of fatty acids.

Role of mitofusin 2 mutations in the physiopathology of Charcot-Marie-Tooth disease type 2A

Charcot-Marie-Tooth disease (CMT) is the most common form of hereditary peripheral neuropathy. The main axonal form of CMT, CMT2A, preferentially affects peripheral neurons with the longest neurites. CMT2A has been recently linked to mutations in the mitofusin 2 (Mfn2) gene. Mfn2 participates in mitochondrial fusion a process that together with mitochondrial fission, contributes to mitochondrial morphology. Many hypotheses have been postulated to understand how mutations in Mfn2 lead to CMT2A. In this review, we will describe the physiological role of Mfn2, the pathophysiology of CMT2A and current hypotheses about the deleterious role of mutant Mfn2 in neuronal function.

The cell-type specificity of mitochondrial dynamics

Recent advances in mitochondrial imaging have revealed that in many cells mitochondria can be highly dynamic. They can undergo fission/fusion processes modulated by various mitochondria-associated proteins and also by conformational transitions in the inner mitochondrial membrane. Moreover, precise mitochondrial distribution can be achieved by their movement along the cytoskeleton, recruiting various connector and motor proteins. Such movement is evident in various cell types ranging from yeast to mammalian cells and serves to direct mitochondria to cellular regions of high ATP demand or to transport mitochondria destined for elimination. Existing data also demonstrate that many aspects of mitochondrial dynamics, morphology, regulation and intracellular organization can be cell type-/tissue-specific. In many cells like neurons, pancreatic cells, HL-1 cells, etc., complex dynamics of mitochondria include fission, fusion, small oscillatory movements of mitochondria, larger movements like filament extension, retraction, fast branching in the mitochondrial network and rapid long-distance intracellular translocation of single mitochondria. Alternatively, mitochondria can be rather fixed in other cells and tissues like adult cardiomyocytes or skeletal muscles with a very regular organelle organization between myofibrils, providing the bioenergetic basis for contraction. Adult cardiac cells show no displacement of mitochondria with only very small-amplitude rapid vibrations, demonstrating remarkable, cell type-dependent differences in the dynamics and spatial arrangement of mitochondria. These variations and the cell-type specificity of mitochondrial dynamics could be related to specific cellular functions and demands, also indicating a significant role of integrations of mitochondria with other intracellular systems like the cytoskeleton, nucleus and endoplasmic reticulum (ER).

Mitochondrial reticulum network dynamics in relation to oxidative stress, redox regulation, and hypoxia

A single mitochondrial network in the cell undergoes constant fission and fusion primarily depending on the local GTP gradients and the mitochondrial energetics. Here we overview the main properties and regulation of pro-fusion and pro-fission mitodynamins, i.e. dynamins-related GTPases responsible for mitochondrial shape-forming, such as pro-fusion mitofusins MFN1, MFN2, and the inner membrane-residing long OPA1 isoforms, and pro-fission mitodynamins FIS1, MFF, and DRP1 multimers required for scission. Notably, the OPA1 cleavage into non-functional short isoforms at a diminished ATP level (collapsed membrane potential) and the DRP1 recruitment upon phosphorylation by various kinases are overviewed. Possible responses of mitodynamins to the oxidative stress, hypoxia, and concomitant mtDNA mutations are also discussed. We hypothesize that the increased GTP formation within the Krebs cycle followed by the GTP export via the ADP/ATP carrier shift the balance between fission and fusion towards fusion by activating the GTPase domain of OPA1 located in the peripheral intermembrane space (PIMS). Since the protein milieu of PIMS is kept at the prevailing oxidized redox potential by the TOM, MIA40 and ALR/Erv1 import-redox trapping system, redox regulations shift the protein environment of PIMS to a more reduced state due to the higher substrate load and increased respiration. A higher cytochrome c turnover rate may prevent electron transfer from ALR/Erv1 to cytochrome c. Nevertheless, the putative links between the mitodynamin responses, mitochondrial morphology and the changes in the mitochondrial bioenergetics, superoxide production, and hypoxia are yet to be elucidated, including the precise basis for signaling by the mitochondrion-derived vesicles.

Mitochondrial dynamics as a bridge between mitochondrial dysfunction and insulin resistance

Muscle from obese subjects or from type 2 diabetic patients show mitochondrial dysfunction, and this may participate in the insulin resistance in those conditions. The mechanisms involved in mitochondrial dysfunction are not completely understood. Dynamic mitochondrial filaments or networks form by mitochondrial fusion and fission events. There is substantial evidence that proteins participating in mitochondrial fusion or fission also have a role in metabolism. Thus, mitofusin-2 (Mfn2) a mitochondrial fusion protein, stimulates respiration, substrate oxidation and OXPHOS subunits expression. In this regard, muscle from obese subjects, or from type 2 diabetic patients, show a reduced expression of Mfn2 and, amelioration of insulin sensitivity by bariatric surgery is associated with an increased Mfn2 expression in muscle. Here, we propose the hypothesis that mitochondrial dynamics proteins play a role in mitochondrial dysfunction in obesity or in type 2 diabetes and that it may also participate in the development of insulin resistance.