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The nucleus accumbens (NAc) is a brain hub regulating motivated behaviors, including social competitiveness. Mitochondrial function in the NAc links anxiety with social competitiveness, and the mitochondrial fusion protein mitofusin 2 (Mfn2) in NAc neurons regulates anxiety-related behaviors. However, it remains unexplored whether accumbal Mfn2 levels also affect social behavior and whether Mfn2 actions in the emotional and social domain are driven by distinct cell types. Here, we found that subordinate-prone highly anxious rats show decreased accumbal Mfn2 levels and that Mfn2 overexpression promotes dominant behavior. In mice, selective Mfn2 downregulation in NAc dopamine D2 receptor-expressing medium spiny neurons (D2-MSNs) induced social subordination, accompanied by decreased accumbal mitochondrial functions and decreased excitability in D2-MSNs. Instead, D1-MSN-targeted Mfn2 downregulation affected competitive ability only transiently and likely because of an increase in anxiety-like behaviors. Our results assign dissociable cell-type specific roles to Mfn2 in the NAc in modulating social dominance and anxiety., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

In eukaryotic cells, different organelles interact at membrane contact sites stabilized by tethers. Mitochondrial mitofusin 2 (MFN2) acts as a membrane tether that interacts with an unknown partner on the endoplasmic reticulum (ER). In this work, we identified the MFN2 splice variant ERMIT2 as the ER tethering partner of MFN2. Splicing of MFN2 produced ERMIT2 and ERMIN2, two ER-specific variants. ERMIN2 regulated ER morphology, whereas ERMIT2 localized at the ER-mitochondria interface and interacted with mitochondrial mitofusins to tether ER and mitochondria. This tethering allowed efficient mitochondrial calcium ion uptake and phospholipid transfer. Expression of ERMIT2 ameliorated the ER stress, inflammation, and fibrosis typical of liver-specific Mfn2 knockout mice. Thus, ER-specific MFN2 variants display entirely extramitochondrial MFN2 functions involved in interorganellar tethering and liver metabolic activities.

Non-alcoholic fatty liver is the most common liver disease worldwide. Here, we show that the mitochondrial protein mitofusin 2 (Mfn2) protects against liver disease. Reduced Mfn2 expression was detected in liver biopsies from patients with non-alcoholic steatohepatitis (NASH). Moreover, reduced Mfn2 levels were detected in mouse models of steatosis or NASH, and its re-expression in a NASH mouse model ameliorated the disease. Liver-specific ablation of Mfn2 in mice provoked inflammation, triglyceride accumulation, fibrosis, and liver cancer. We demonstrate that Mfn2 binds phosphatidylserine (PS) and can specifically extract PS into membrane domains, favoring PS transfer to mitochondria and mitochondrial phosphatidylethanolamine (PE) synthesis. Consequently, hepatic Mfn2 deficiency reduces PS transfer and phospholipid synthesis, leading to endoplasmic reticulum (ER) stress and the development of a NASH-like phenotype and liver cancer. Ablation of Mfn2 in liver reveals that disruption of ER-mitochondrial PS transfer is a new mechanism involved in the development of liver disease., (Copyright © 2019. Published by Elsevier Inc.)

Acetaminophen (APAP) is the active component of many medications used to treat pain and fever worldwide. Its overuse provokes liver injury and it is the second most common cause of liver failure. Mitochondrial dysfunction contributes to APAP-induced liver injury but the mechanism by which APAP causes hepatocyte toxicity is not completely understood. Therefore, we lack efficient therapeutic strategies to treat this pathology. Here we show that APAP interferes with the formation of mitochondrial respiratory supercomplexes via the mitochondrial negative regulator MCJ, and leads to decreased production of ATP and increased generation of ROS. In vivo treatment with an inhibitor of MCJ expression protects liver from acetaminophen-induced liver injury at a time when N-acetylcysteine, the standard therapy, has no efficacy. We also show elevated levels of MCJ in the liver of patients with acetaminophen overdose. We suggest that MCJ may represent a therapeutic target to prevent and rescue liver injury caused by acetaminophen.

A long-standing quest is to define the mechanisms responsible for the mitochondrial dysfunction and accumulation of damaged mitochondria that occur during aging. Indeed, those defects are considered major contributors to the aging process. We have analyzed the effect of aging on the muscle expression of Mfn2 and the impact of Mfn2 ablation on muscle function. Our findings reveal that Mfn2 is repressed in muscle during aging, and that is a determinant for the inhibition of autophagy, and mitochondrial quality control, which lead to the accumulation of damaged mitochondria.

Sporadic inclusion body myositis (sIBM) is one of the most common myopathies in elderly people. Mitochondrial abnormalities at the histological level are present in these patients. We hypothesize that mitochondrial dysfunction may play a role in disease aetiology. We took the following measurements of muscle and peripheral blood mononuclear cells (PBMCs) from 30 sIBM patients and 38 age- and gender-paired controls: mitochondrial DNA (mtDNA) deletions, amount of mtDNA and mtRNA, mitochondrial protein synthesis, mitochondrial respiratory chain (MRC) complex I and IV enzymatic activity, mitochondrial mass, oxidative stress and mitochondrial dynamics (mitofusin 2 and optic atrophy 1 levels). Depletion of mtDNA was present in muscle from sIBM patients and PBMCs showed deregulated expression of mitochondrial proteins in oxidative phosphorylation. MRC complex IV/citrate synthase activity was significantly decreased in both tissues and mitochondrial dynamics were affected in muscle. Depletion of mtDNA was significantly more severe in patients with mtDNA deletions, which also presented deregulation of mitochondrial fusion proteins. Imbalance in mitochondrial dynamics in muscle was associated with increased mitochondrial genetic disturbances (both depletion and deletions), demonstrating that proper mitochondrial turnover is essential for mitochondrial homoeostasis and muscle function in these patients., (© 2016 The Author(s). published by Portland Press Limited on behalf of the Biochemical Society.)

Significance: Mitochondrial dynamics is a complex process that impacts on mitochondrial biology., Recent Advances: Recent evidence indicates that proteins participating in mitochondrial dynamics have additional cellular roles. Mitofusin 2 (Mfn2) is a potent modulator of mitochondrial metabolism with an impact on energy metabolism in muscle, liver, and hypothalamic neurons. In addition, Mfn2 is subjected to tight regulation. Hence, factors such as proinflammatory cytokines, lipid availability, or glucocorticoids block its expression, whereas exercise and increased energy expenditure promote its upregulation., Critical Issues: Importantly, Mfn2 controls cell metabolism and insulin signaling by limiting reactive oxygen species production and by modulation of endoplasmic reticulum stress. In this connection, it is critical to understand precisely the molecular mechanisms involved in the global actions of Mfn2., Future Directions: Future directions should concentrate into the analysis of those mechanisms, and to fully demonstrate that Mfn2 represents a cellular hub that senses the metabolic and hormonal milieu and drives the control of metabolic homeostasis.

Recent discoveries linking mitochondrial dynamics to various pathologies have generated interest in the design of robust methods to screen proteins involved in mitochondrial dynamics and small molecules that modulate mitochondrial dynamics. Here, we describe functional screening protocols to analyze mitochondrial parameters such as mitochondrial morphology, reactive oxygen species (ROS) levels, mitochondrial calcium, and oxygen consumption rate in cultured cells. Novel proteins that participate in the regulation of mitochondrial dynamics and function can be identified using such an approach.

Mitochondrial fusion and fission is a dynamic process critical for the maintenance of mitochondrial function and cell viability. During excitotoxicity neuronal mitochondria are fragmented, but the mechanism underlying this process is poorly understood. Here, we show that Mfn2 is the only member of the mitochondrial fusion/fission machinery whose expression is reduced in in vitro and in vivo models of excitotoxicity. Whereas in cortical primary cultures, Drp1 recruitment to mitochondria plays a primordial role in mitochondrial fragmentation in an early phase that can be reversed once the insult has ceased, Mfn2 downregulation intervenes in a delayed mitochondrial fragmentation phase that progresses even when the insult has ceased. Downregulation of Mfn2 causes mitochondrial dysfunction, altered calcium homeostasis, and enhanced Bax translocation to mitochondria, resulting in delayed neuronal death. We found that transcription factor MEF2 regulates basal Mfn2 expression in neurons and that excitotoxicity-dependent degradation of MEF2 causes Mfn2 downregulation. Thus, Mfn2 reduction is a late event in excitotoxicity and its targeting may help to reduce excitotoxic damage and increase the currently short therapeutic window in stroke., (© 2014 The Authors.)

Mitofusin 2 (Mfn2), a protein that participates in mitochondrial fusion, is required to maintain normal mitochondrial metabolism in skeletal muscle and liver. Given that muscle Mfn2 is repressed in obese or type 2 diabetic subjects, this protein may have a potential pathophysiological role in these conditions. To evaluate whether the metabolic effects of Mfn2 can be dissociated from its function in mitochondrial dynamics, we studied a form of human Mfn2, lacking the two transmembrane domains and the COOH-terminal coiled coil (ΔMfn2). This form localized in mitochondria but did not alter mitochondrial morphology in cells or in skeletal muscle fibers. The expression of ΔMfn2 in mouse skeletal muscle stimulated glucose oxidation and enhanced respiratory control ratio, which occurred in the absence of changes in mitochondrial mass. ΔMfn2 did not stimulate mitochondrial respiration in Mfn2-deficient muscle cells. The expression of ΔMfn2 in mouse liver or in hepatoma cells stimulated gluconeogenesis. In addition, ΔMfn2 activated basal and maximal respiration both in muscle and liver cells. In all, we show that a form of Mfn2 lacking mitochondrial fusion activity stimulates mitochondrial function and enhances glucose metabolism in muscle and liver tissues. This study suggests that Mfn2 regulates metabolism independently of changes in mitochondrial morphology.

Mitochondria are dynamic subcellular organelles that convert nutrient intermediates into readily available energy equivalents. Optimal mitochondrial function is ensured by a highly evolved quality control system, coordinated by protein machinery that regulates a process of continual fusion and fission. In this work, we provide in vivo evidence that the ATP-independent metalloprotease OMA1 plays an essential role in the proteolytic inactivation of the dynamin-related GTPase OPA1 (optic atrophy 1). We also show that OMA1 deficiency causes a profound perturbation of the mitochondrial fusion-fission equilibrium that has important implications for metabolic homeostasis. Thus, ablation of OMA1 in mice results in marked transcriptional changes in genes of lipid and glucose metabolic pathways and substantial alterations in circulating blood parameters. Additionally, Oma1-mutant mice exhibit an increase in body weight due to increased adipose mass, hepatic steatosis, decreased energy expenditure and impaired thermogenenesis. These alterations are especially significant under metabolic stress conditions, indicating that an intact OMA1-OPA1 system is essential for developing the appropriate adaptive response to different metabolic stressors such as a high-fat diet or cold-shock. This study provides the first description of an unexpected role in energy metabolism for the metalloprotease OMA1 and reinforces the importance of mitochondrial quality control for normal metabolic function.

Aims: Mitofusin-2 (Mfn2) expression is dysregulated in vascular proliferative disorders and its overexpression attenuates the proliferation of vascular smooth muscle cells (VSMCs) and neointimal lesion development after balloon angioplasty. We sought to gain insight into the mechanisms that control Mfn2 expression in VSMCs., Methods and Results: We cloned and characterized 2 kb of the 5'-flanking region of the human Mfn2 gene. Its TATA-less promoter contains a CpG island. In keeping with this, 5'-rapid amplification of cDNA ends revealed six transcriptional start sites (TSSs), of which TSS2 and TSS5 were the most frequently used. The strong CpG island was found to be non-methylated under conditions characterized by large differences in Mfn2 gene expression. The proximal Mfn2 promoter contains six putative Sp1 motifs. Sp1 binds to the Mfn2 promoter and its overexpression activates the Mfn2 promoter in VSMCs. Chemical inhibition of Sp1 reduced Mfn2 expression, and Sp1 silencing reduced transcriptional activity of the Mfn2 promoter. In keeping with this view, Sp1 and Mfn2 mRNA levels were down-regulated in the aorta early after an atherogenic diet in apolipoprotein E-knockout mice or in VSMCs cultured in the presence of low serum., Conclusion: Sp1 is a key factor in maintaining basal Mfn2 transcription in VSMCs. Given the anti-proliferative actions of Mfn2, Sp1-induced Mfn2 transcription may represent a mechanism for prevention of VSMC proliferation and neointimal lesion and development.

Mitochondria in mammalian cells are visualized as a network or as filaments that undergo continuous changes in shape and in localization within the cells. These changes are a consequence of the activity of different processes such as mitochondrial fusion and fission, and mitochondrial remodelling. In all, these processes are referred to as mitochondrial dynamics, and relevant questions, still unexplained, are why cells require such an active dynamics, or why mitochondria move to specific cellular regions. In this review we will summarize some of the biological functions assigned to the proteins identified as participating in mitochondrial fusion, namely mitofusin 1, mitofusin 2 and OPA1. In addition to the capacity of these proteins to promote fusion, mitofusin 2 or OPA1 regulate mitochondrial metabolism and loss-of-function reduces oxygen consumption and the capacity to oxidize substrates. We propose that mitochondrial fusion proteins operate as integrators of signals so they regulate both mitochondrial fusion and metabolism., (Copyright 2010 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2010 Elsevier B.V. All rights reserved.)

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.

Mitochondrial dysfunction has been reported in skeletal muscle of obese subjects and of type 2 diabetic patients. Reduced mitochondrial mass and defective activity have been proposed to explain this dysfunction. Alterations in mitochondrial function may be crucial to explain the metabolic changes and insulin resistance that characterize both obesity and type 2 diabetes. Consequently, the identification of the primary mechanisms involved is of great relevance. Mitochondrial dynamics refers to the movement of mitochondria along the cytoskeleton and also to the regulation of mitochondrial morphology and distribution, which depend on fusion and fission events. In recent years, some of the proteins that participate in mitochondrial fusion and fission have been identified in mammalian cells. Recent evidence indicates that proteins participating in these processes are also involved in metabolism. The mitochondrial fusion protein mitofusin 2 stimulates respiration, substrate oxidation and the expression of subunits that participate in respiratory complexes in cultured cells. In this regard, skeletal muscle of obese subjects and of type 2 diabetic patients shows reduced mitofusin 2 expression. Therefore, alterations in the activity of the proteins involved in mitochondrial dynamics, and particularly mitofusin 2, may participate in the reduced mitochondrial function present in skeletal muscle in obesity and in type 2 diabetes.

The mitochondria form dynamic organized networks or filaments that are controlled by fusion and fission events. A balance between mitochondrial fusion and fission is crucial for the correct function of these organelles. Thus, a detailed characterization of the proteins involved in mitochondrial fusion and fission, and the study of the mechanisms that regulate these two processes, would contribute to elucidating how this balance is achieved. In this regard, alterations in some of the proteins that participate in mitochondrial dynamics are linked to human pathology. In addition, experimental data support the view that mitochondrial metabolism is regulated via the manipulation of the proteins involved in mitochondrial dynamics, particularly the mitofusin 2 protein. This review evaluates the potential of mitochondrial fusion and fission proteins as targets for drug discovery, with an emphasis on mitofusin 2. The pharmacological modulation of mitochondrial dynamics may be beneficial for the treatment of specific disorders of mitochondrial dynamics and mitochondrial-inherited diseases, as well as in complex diseases with a central mitochondrial dysfunction.

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 beta3-adrenergic agonists or exercise induce the expression of this gene in muscle. Estrogen-related receptor-alpha transcription factor is a key regulator of Mfn2 transcription and recruits peroxisome proliferator-activated receptor gamma coactivator (PGC)-1beta and PGC-1alpha. These 2 nuclear coactivators are potent, positive regulators of Mfn2 expression in muscle cells, and ablation of PGC-1beta causes Mfn2 downregulation in skeletal muscle and in the heart. We propose that PGC-1beta is a regulator of normal expression of Mfn2 in muscle, whereas PGC-1alpha participates in the stimulation of Mfn2 expression under a variety of conditions characterized by enhanced energy expenditure.

Mitofusin 2 (Mfn2) is a mitochondrial membrane protein that participates in mitochondrial fusion and regulates mitochondrial metabolism in mammalian cells. Here, we show that Mfn2 gene expression is induced in skeletal muscle and brown adipose tissue by conditions associated with enhanced energy expenditure, such as cold exposure or beta(3)-adrenergic agonist treatment. In keeping with the role of peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1 alpha on energy expenditure, we demonstrate a stimulatory effect of PGC-1 alpha on Mfn2 mRNA and protein expression in muscle cells. PGC-1 alpha also stimulated the activity of the Mfn2 promoter, which required the integrity of estrogen-related receptor-alpha (ERR alpha)-binding elements located at -413/-398. ERR alpha also activated the transcriptional activity of the Mfn2 promoter, and the effects were synergic with those of PGC-1 alpha. Mfn2 loss of function reduced the stimulatory effect of PGC-1 alpha on mitochondrial membrane potential. Exposure to cold substantially increased Mfn2 gene expression in skeletal muscle from heterozygous Mfn2 knock-out mice, which occurred in the presence of higher levels of PGC-1 alpha mRNA compared with control mice. Our results indicate the existence of a regulatory pathway involving PGC-1 alpha, ERR alpha, and Mfn2. Alterations in this regulatory pathway may participate in the pathophysiology of insulin-resistant conditions and type 2 diabetes.

The primary gene mutated in Charcot-Marie-Tooth type 2A is mitofusin-2 (Mfn2). Mfn2 encodes a mitochondrial protein that participates in the maintenance of the mitochondrial network and that regulates mitochondrial metabolism and intracellular signaling. The potential for regulation of human Mfn2 gene expression in vivo is largely unknown. Based on the presence of mitochondrial dysfunction in insulin-resistant conditions, we have examined whether Mfn2 expression is dysregulated in skeletal muscle from obese or nonobese type 2 diabetic subjects, whether muscle Mfn2 expression is regulated by body weight loss, and the potential regulatory role of tumor necrosis factor (TNF)alpha or interleukin-6. We show that mRNA concentration of Mfn2 is decreased in skeletal muscle from both male and female obese subjects. Muscle Mfn2 expression was also reduced in lean or in obese type 2 diabetic patients. There was a strong negative correlation between the Mfn2 expression and the BMI in nondiabetic and type 2 diabetic subjects. A positive correlation between the Mfn2 expression and the insulin sensitivity was also detected in nondiabetic and type 2 diabetic subjects. To determine the effect of weight loss on Mfn2 mRNA expression, six morbidly obese subjects were subjected to weight loss by bilio-pancreatic diversion. Mean expression of muscle Mfn2 mRNA increased threefold after reduction in body weight, and a positive correlation between muscle Mfn2 expression and insulin sensitivity was again detected. In vitro experiments revealed an inhibitory effect of TNFalpha or interleukin-6 on Mfn2 expression in cultured cells. We conclude that body weight loss upregulates the expression of Mfn2 mRNA in skeletal muscle of obese humans, type 2 diabetes downregulates the expression of Mfn2 mRNA in skeletal muscle, Mfn2 expression in skeletal muscle is directly proportional to insulin sensitivity and is inversely proportional to the BMI, TNFalpha and interleukin-6 downregulate Mfn2 expression and may participate in the dysregulation of Mfn2 expression in obesity or type 2 diabetes, and the in vivo modulation of Mfn2 mRNA levels is an additional level of regulation for the control of muscle metabolism and could provide a molecular mechanism for alterations in mitochondrial function in obesity or type 2 diabetes.

Mitochondrial impairment is hypothesized to contribute to the pathogenesis of insulin resistance. Mitofusin (Mfn) proteins regulate the biogenesis and maintenance of the mitochondrial network, and when inactivated, cause a failure in the mitochondrial architecture and decreases in oxidative capacity and glucose oxidation. Exercise increases muscle mitochondrial content, size, oxidative capacity and aerobic glucose oxidation. To address if Mfn proteins are implicated in these exercise-induced responses, we measured Mfn1 and Mfn2 mRNA levels, pre-, post-, 2 and 24 h post-exercise. Additionally, we measured the expression levels of transcriptional regulators that control mitochondrial biogenesis and functions, including PGC-1alpha, NRF-1, NRF-2 and the recently implicated ERRalpha. We show that Mfn1, Mfn2, NRF-2 and COX IV mRNA were increased 24 h post-exercise, while PGC-1alpha and ERRalpha mRNA increased 2 h post-exercise. Finally, using in vitro cellular assays, we demonstrate that Mfn2 gene expression is driven by a PGC-1alpha programme dependent on ERRalpha. The PGC-1alpha/ERRalpha-mediated induction of Mfn2 suggests a role of these two factors in mitochondrial fusion. Our results provide evidence that PGC-1alpha not only mediates the increased expression of oxidative phosphorylation genes but also mediates alterations in mitochondrial architecture in response to aerobic exercise in humans.

Mitofusin-2 (Mfn2) is a mitochondrial membrane protein that participates in mitochondrial fusion in mammalian cells and mutations in the Mfn2 gene cause Charcot-Marie-Tooth neuropathy type 2A. Here, we show that Mfn2 loss-of-function inhibits pyruvate, glucose and fatty acid oxidation and reduces mitochondrial membrane potential, whereas Mfn2 gain-of-function increases glucose oxidation and mitochondrial membrane potential. As to the mechanisms involved, we have found that Mfn2 loss-of-function represses nuclear-encoded subunits of OXPHOS complexes I, II, III and V, whereas Mfn2 overexpression induced the subunits of complexes I, IV and V. Obesity-induced Mfn2 deficiency in rat skeletal muscle was also associated with a decrease in the subunits of complexes I, II, III and V. In addition, the effect of Mfn2 overexpression on mitochondrial metabolism was mimicked by a truncated Mfn2 mutant that is inactive as a mitochondrial fusion protein. Our results indicate that Mfn2 triggers mitochondrial energization, at least in part, by regulating OXPHOS expression through signals that are independent of its role as a mitochondrial fusion protein.

In many cells and specially in muscle, mitochondria form elongated filaments or a branched reticulum. We show that Mfn2 (mitofusin 2), a mitochondrial membrane protein that participates in mitochondrial fusion in mammalian cells, is induced during myogenesis and contributes to the maintenance and operation of the mitochondrial network. Repression of Mfn2 caused morphological and functional fragmentation of the mitochondrial network into independent clusters. Concomitantly, repression of Mfn2 reduced glucose oxidation, mitochondrial membrane potential, cell respiration, and mitochondrial proton leak. We also show that the Mfn2-dependent mechanism of mitochondrial control is disturbed in obesity by reduced Mfn2 expression. In all, our data indicate that Mfn2 expression is crucial in mitochondrial metabolism through the maintenance of the mitochondrial network architecture, and reduced Mfn2 expression may explain some of the metabolic alterations associated with obesity.

Neuregulin 4 (Nrg4) is an adipokine that belongs to the epidermal growth factor family and binds to ErbB4 tyrosine kinase receptors. In 3T3-L1 adipocytes, the downregulation of Nrg4 expression enhances inflammation and autophagy, resulting in insulin resistance. Here, we searched for the causes of this phenotype. Nrg4 knockdown (Nrg4 KD) adipocytes showed a significant reduction in mitochondrial content and elongation, along with a lower content of the mitochondria fusion protein mitofusin 2 (MFN2), and increased H2O2 production compared to the control scrambled cells (Scr). The antioxidant N-acetylcysteine reversed the oxidative stress and reduced the gene expression of the pro-inflammatory cytokine tumor necrosis factor α (TNFα). Nrg4 KD adipocytes showed enhanced lipolysis and reduced lipogenesis, in addition to a significant reduction in several intermediates of the Krebs cycle. In summary, Nrg4 downregulation in adipocytes affects mitochondrial content and functioning, causing impaired cellular metabolism, which in turn results in oxidative stress, inflammation, and insulin resistance. [ABSTRACT FROM AUTHOR]

Mitochondrial network architecture plays a critical role in cellular physiology. Indeed, alterations in the shape of mitochondria upon exposure to cellular stress can cause the dysfunction of these organelles. In this scenario, mitochondrial dynamics proteins and the phospholipid composition of the mitochondrial membrane are key for fine-tuning the modulation of mitochondrial architecture. In addition, several factors including post-translational modifications such as the phosphorylation, acetylation, SUMOylation, and o-GlcNAcylation of mitochondrial dynamics proteins contribute to shaping the plasticity of this architecture. In this regard, several studies have evidenced that, upon metabolic stress, mitochondrial dynamics proteins are post-translationally modified, leading to the alteration of mitochondrial architecture. Interestingly, several proteins that sustain the mitochondrial lipid composition also modulate mitochondrial morphology and organelle communication. In this context, pharmacological studies have revealed that the modulation of mitochondrial shape and function emerges as a potential therapeutic strategy for metabolic diseases. Here, we review the factors that modulate mitochondrial architecture. [ABSTRACT FROM AUTHOR]

Aging is associated with a decline in mitochondrial function and the accumulation of abnormal mitochondria. However, the precise mechanisms by which aging promotes these mitochondrial alterations and the role of the latter in aging are still not fully understood. Mitochondrial dynamics is a key process regulating mitochondrial function and quality. Altered expression of some mitochondrial dynamics proteins has been recently associated with aging and with age-related alterations in yeast, Caenorhabditis elegans , mice, and humans. Here, we review the link between alterations in mitochondrial dynamics, aging, and age-related impairment. We propose that the dysregulation of mitochondrial dynamics leads to age-induced accumulation of unhealthy mitochondria and contributes to alterations linked to aging, such as diabetes and neurodegeneration. [ABSTRACT FROM AUTHOR]

Abstract: Contrary to previous assumptions, G proteins do not permanently reside on the plasma membrane, but are constantly monitoring the cytoplasmic surfaces of the plasma membrane and endomembranes. Here, we report that the Gαq and Gα11 proteins locate at the mitochondria and play a role in a complex signaling pathway that regulates mitochondrial dynamics. Our results provide evidence for the presence of the heteromeric G protein (Gαq/11βγ) at the outer mitochondrial membrane and for Gαq at the inner membrane. Both localizations are necessary to maintain the proper equilibrium between fusion and fission; which is achieved by altering the activity of mitofusin proteins, Drp1, OPA1 and the membrane potential at both the outer and inner mitochondrial membranes. As a result of the absence of Gαq/11, there is a decrease in mitochondrial fusion rates and a decrease in overall respiratory capacity, ATP production and OXPHOS-dependent growth. These findings demonstrate that the presence of Gαq proteins at the mitochondria serves as a physiological function: stabilizing elongated mitochondria and regulating energy production in Drp1 and Opa1 dependent mechanisms. This thereby links organelle dynamics and physiology. [Copyright &y& Elsevier]