Decreased expression of mitochondrial aminoacyl-tRNA synthetases causes downregulation of OXPHOS subunits in type 2 diabetic muscle.

Type 2 diabetes mellitus (T2D) affects millions of people worldwide and is one of the leading causes of morbidity and mortality. The skeletal muscle (SKM) is one of the most important tissues involved in maintaining glucose homeostasis and substrate oxidation, and it undergoes insulin resistance in T2D. In this study, we identify the existence of alterations in the expression of mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs) in skeletal muscle from two different forms of T2D: early-onset type 2 diabetes (YT2) (onset of the disease before 30 years of age) and the classical form of the disease (OT2). GSEA analysis from microarray studies revealed the repression of mitochondrial mt-aaRSs independently of age, which was validated by real-time PCR assays. In agreement with this, a reduced expression of several encoding mt-aaRSs was also detected in skeletal muscle from diabetic (db/db) mice but not in obese ob/ob mice. In addition, the expression of the mt-aaRSs proteins most relevant in the synthesis of mitochondrial proteins, threonyl-tRNA, and leucyl-tRNA synthetases (TARS2 and LARS2) were also repressed in muscle from db/db mice. It is likely that these alterations participate in the reduced expression of proteins synthesized in the mitochondria detected in db/db mice. We also document an increased iNOS abundance in mitochondrial-enriched muscle fractions from diabetic mice that may inhibit aminoacylation of TARS2 and LARS2 by nitrosative stress. Our results indicate a reduced expression of mt-aaRSs in skeletal muscle from T2D patients, which may participate in the reduced expression of proteins synthesized in mitochondria. An enhanced mitochondrial iNOS could play a regulatory role in diabetes.

CBL/CAP Is Essential for Mitochondria Respiration Complex I Assembly and Bioenergetics Efficiency in Muscle Cells.

CBL is rapidly phosphorylated upon insulin receptor activation. Mice whole body CBL depletion improved insulin sensitivity and glucose clearance; however, the precise mechanisms remain unknown. We depleted either CBL or its associated protein SORBS1/CAP independently in myocytes and assessed mitochondrial function and metabolism compared to control cells. CBL- and CAP-depleted cells showed increased mitochondrial mass with greater proton leak. Mitochondrial respiratory complex I activity and assembly into respirasomes were reduced. Proteome profiling revealed alterations in proteins involved in glycolysis and fatty acid degradation. Our findings demonstrate CBL/CAP pathway couples insulin signaling to efficient mitochondrial respiratory function and metabolism in muscle.

Adipose tissue mitochondrial dysfunction in human obesity is linked to a specific DNA methylation signature in adipose-derived stem cells.

BACKGROUND: A functional population of adipocyte precursors, termed adipose-derived stromal/stem cells (ASCs), is crucial for proper adipose tissue (AT) expansion, lipid handling, and prevention of lipotoxicity in response to chronic positive energy balance. We previously showed that obese human subjects contain a dysfunctional pool of ASCs. Elucidation of the mechanisms underlying abnormal ASC function might lead to therapeutic interventions for prevention of lipotoxicity by improving the adipogenic capacity of ASCs. METHODS: Using epigenome-wide association studies, we explored the impact of obesity on the methylation signature of human ASCs and their differentiated counterparts. Mitochondrial phenotyping of lean and obese ASCs was performed. TBX15 loss- and gain-of-function experiments were carried out and western blotting and electron microscopy studies of mitochondria were performed in white AT biopsies from lean and obese individuals. RESULTS: We found that DNA methylation in adipocyte precursors is significantly modified by the obese environment, and adipogenesis, inflammation, and immunosuppression were the most affected pathways. Also, we identified TBX15 as one of the most differentially hypomethylated genes in obese ASCs, and genetic experiments revealed that TBX15 is a regulator of mitochondrial mass in obese adipocytes. Accordingly, morphological analysis of AT from obese subjects showed an alteration of the mitochondrial network, with changes in mitochondrial shape and number. CONCLUSIONS: We identified a DNA methylation signature in adipocyte precursors associated with obesity, which has a significant impact on the metabolic phenotype of mature adipocytes.

Mitochondrial DNA and TLR9 drive muscle inflammation upon Opa1 deficiency.

Opa1 participates in inner mitochondrial membrane fusion and cristae morphogenesis. Here, we show that muscle-specific Opa1 ablation causes reduced muscle fiber size, dysfunctional mitochondria, enhanced Fgf21, and muscle inflammation characterized by NF-κB activation, and enhanced expression of pro-inflammatory genes. Chronic sodium salicylate treatment ameliorated muscle alterations and reduced the muscle expression of Fgf21. Muscle inflammation was an early event during the progression of the disease and occurred before macrophage infiltration, indicating that it is a primary response to Opa1 deficiency. Moreover, Opa1 repression in muscle cells also resulted in NF-κB activation and inflammation in the absence of necrosis and/or apoptosis, thereby revealing that the activation is a cell-autonomous process and independent of cell death. The effects of Opa1 deficiency on the expression NF-κB target genes and inflammation were absent upon mitochondrial DNA depletion. Under Opa1 deficiency, blockage or repression of TLR9 prevented NF-κB activation and inflammation. Taken together, our results reveal that Opa1 deficiency in muscle causes initial mitochondrial alterations that lead to TLR9 activation, and inflammation, which contributes to enhanced Fgf21 expression and to growth impairment.

TP53INP2 regulates adiposity by activating ß-catenin through autophagy-dependent sequestration of GSK3ß.

Excessive fat accumulation is a major risk factor for the development of type 2 diabetes mellitus and other common conditions, including cardiovascular disease and certain types of cancer. Here, we identify a mechanism that regulates adiposity based on the activator of autophagy TP53INP2. We report that TP53INP2 is a negative regulator of adipogenesis in human and mouse preadipocytes. In keeping with this, TP53INP2 ablation in mice caused enhanced adiposity, which was characterized by greater cellularity of subcutaneous adipose tissue and increased expression of master adipogenic genes. TP53INP2 modulates adipogenesis through autophagy-dependent sequestration of GSK3ß into late endosomes. GSK3ß sequestration was also dependent on ESCRT activity. As a result, TP53INP2 promotes greater ß-catenin levels and induces the transcriptional activity of TCF/LEF transcription factors. These results demonstrate a link between autophagy, sequestration of GSK3ß into late endosomes and inhibition of adipogenesis in vivo.

Early-onset and classical forms of type 2 diabetes show impaired expression of genes involved in muscle branched-chain amino acids metabolism.

The molecular mechanisms responsible for the pathophysiological traits of type 2 diabetes are incompletely understood. Here we have performed transcriptomic analysis in skeletal muscle, and plasma metabolomics from subjects with classical and early-onset forms of type 2 diabetes (T2D). Focused studies were also performed in tissues from ob/ob and db/db mice. We document that T2D, both early and late onset, are characterized by reduced muscle expression of genes involved in branched-chain amino acids (BCAA) metabolism. Weighted Co-expression Networks Analysis provided support to idea that the BCAA genes are relevant in the pathophysiology of type 2 diabetes, and that mitochondrial BCAA management is impaired in skeletal muscle from T2D patients. In diabetic mice model we detected alterations in skeletal muscle proteins involved in BCAA metabolism but not in obese mice. Metabolomic analysis revealed increased levels of branched-chain keto acids (BCKA), and BCAA in plasma of T2D patients, which may result from the disruption of muscle BCAA management. Our data support the view that inhibition of genes involved in BCAA handling in skeletal muscle takes place as part of the pathophysiology of type 2 diabetes, and this occurs both in early-onset and in classical type 2 diabetes.

Mitochondrial DNA disturbances and deregulated expression of oxidative phosphorylation and mitochondrial fusion proteins in sporadic inclusion body myositis.

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.

Mfn2 modulates the UPR and mitochondrial function via repression of PERK.

Mitofusin 2 (Mfn2) is a key protein in mitochondrial fusion and it participates in the bridging of mitochondria to the endoplasmic reticulum (ER). Recent data indicate that Mfn2 ablation leads to ER stress. Here we report on the mechanisms by which Mfn2 modulates cellular responses to ER stress. Induction of ER stress in Mfn2-deficient cells caused massive ER expansion and excessive activation of all three Unfolded Protein Response (UPR) branches (PERK, XBP-1, and ATF6). In spite of an enhanced UPR, these cells showed reduced activation of apoptosis and autophagy during ER stress. Silencing of PERK increased the apoptosis of Mfn2-ablated cells in response to ER stress. XBP-1 loss-of-function ameliorated autophagic activity of these cells upon ER stress. Mfn2 physically interacts with PERK, and Mfn2-ablated cells showed sustained activation of this protein kinase under basal conditions. Unexpectedly, PERK silencing in these cells reduced ROS production, normalized mitochondrial calcium, and improved mitochondrial morphology. In summary, our data indicate that Mfn2 is an upstream modulator of PERK. Furthermore, Mfn2 loss-of-function reveals that PERK is a key regulator of mitochondrial morphology and function.

Requirement of plasminogen binding to its cell-surface receptor α-enolase for efficient regeneration of normal and dystrophic skeletal muscle.

Adult regenerative myogenesis is central for restoring normal tissue structure and function after muscle damage. In muscle repair after injury, as in severe myopathies, damaged and necrotic fibers are removed by infiltrating inflammatory cells and then replaced by muscle stem cells or satellite cells, which will fuse to form new myofibers. Extracellular proteolysis mediated by uPA-generated plasmin plays a critical role in controlling inflammation and satellite-cell-dependent myogenesis. α-enolase has been described as plasminogen receptor in several cell types, where it acts concentrating plasmin proteolytic activity on the cell surface. In this study, we investigated whether α-enolase plasminogen receptor plays a regulatory role during the muscular repair process. Inhibitors of α-enolase/plasminogen binding: MAb11G1 (a monoclonal antibody against α-enolase) and ε-aminocaproic acid, EACA (a lysine analogue) inhibited the myogenic abilities of satellite cells-derived myoblasts. Furthermore, knockdown of α-enolase decreased myogenic fusion of myoblasts. Injured wild-type mice and dystrophic mdx mice were also treated with MAb11G1 and EACA. These treatments had negative impacts on muscle repair impairing satellite cell functions in vitro in agreement with blunted growth of new myofibers in vivo. Furthermore, both MAb11G1 and EACA treatments impaired adequate inflammatory cell infiltration and promoted extracellular matrix deposition in vivo, which resulted in persistent degeneration. These results demonstrate the novel requirement of α-enolase for restoring homeostasis of injured muscle tissue, by controlling the pericellular localization of plasmin activity.

α-Enolase, a multifunctional protein: its role on pathophysiological situations.

α-Enolase is a key glycolytic enzyme in the cytoplasm of prokaryotic and eukaryotic cells and is considered a multifunctional protein. α-enolase is expressed on the surface of several cell types, where it acts as a plasminogen receptor, concentrating proteolytic plasmin activity on the cell surface. In addition to glycolytic enzyme and plasminogen receptor functions, α-Enolase appears to have other cellular functions and subcellular localizations that are distinct from its well-established function in glycolysis. Furthermore, differential expression of α-enolase has been related to several pathologies, such as cancer, Alzheimer's disease, and rheumatoid arthritis, among others. We have identified α-enolase as a plasminogen receptor in several cell types. In particular, we have analyzed its role in myogenesis, as an example of extracellular remodelling process. We have shown that α-enolase is expressed on the cell surface of differentiating myocytes, and that inhibitors of α-enolase/plasminogen binding block myogenic fusion in vitro and skeletal muscle regeneration in mice. α-Enolase could be considered as a marker of pathological stress in a high number of diseases, performing several of its multiple functions, mainly as plasminogen receptor. This paper is focused on the multiple roles of the α-enolase/plasminogen axis, related to several pathologies.

Mitochondrial dysfunction in insulin resistance: differential contributions of chronic insulin and saturated fatty acid exposure in muscle cells.

Mitochondrial dysfunction has been associated with insulin resistance, obesity and diabetes. Hyperinsulinaemia and hyperlipidaemia are hallmarks of the insulin-resistant state. We sought to determine the contributions of high insulin and saturated fatty acid exposure to mitochondrial function and biogenesis in cultured myocytes. Differentiated C2C12 myotubes were left untreated or exposed to chronic high insulin or high palmitate. Mitochondrial function was determined assessing: oxygen consumption, mitochondrial membrane potential, ATP content and ROS (reactive oxygen species) production. We also determined the expression of several mitochondrial genes. Chronic insulin treatment of myotubes caused insulin resistance with reduced PI3K (phosphoinositide 3-kinase) and ERK (extracellular-signal-regulated kinase) signalling. Insulin treatment increased oxygen consumption but reduced mitochondrial membrane potential and ROS production. ATP cellular levels were maintained through an increased glycolytic rate. The expression of mitochondrial OXPHOS (oxidative phosphorylation) subunits or Mfn-2 (mitofusin 2) were not significantly altered in comparison with untreated cells, whereas expression of PGC-1α (peroxisome-proliferator-activated receptor γ co-activator-1α) and UCPs (uncoupling proteins) were reduced. In contrast, saturated fatty acid exposure caused insulin resistance, reducing PI3K (phosphoinositide 3-kinase) and ERK (extracellular-signal-regulated kinase) activation while increasing activation of stress kinases JNK (c-Jun N-terminal kinase) and p38. Fatty acids reduced oxygen consumption and mitochondrial membrane potential while up-regulating the expression of mitochondrial ETC (electron chain complex) protein subunits and UCP proteins. Mfn-2 expression was not modified by palmitate. Palmitate-treated cells also showed a reduced glycolytic rate. Taken together, our findings indicate that chronic insulin and fatty acid-induced insulin resistance differentially affect mitochondrial function. In both conditions, cells were able to maintain ATP levels despite the loss of membrane potential; however, different protein expression suggests different adaptation mechanisms.

uPA deficiency exacerbates muscular dystrophy in MDX mice.

Duchenne muscular dystrophy (DMD) is a fatal and incurable muscle degenerative disorder. We identify a function of the protease urokinase plasminogen activator (uPA) in mdx mice, a mouse model of DMD. The expression of uPA is induced in mdx dystrophic muscle, and the genetic loss of uPA in mdx mice exacerbated muscle dystrophy and reduced muscular function. Bone marrow (BM) transplantation experiments revealed a critical function for BM-derived uPA in mdx muscle repair via three mechanisms: (1) by promoting the infiltration of BM-derived inflammatory cells; (2) by preventing the excessive deposition of fibrin; and (3) by promoting myoblast migration. Interestingly, genetic loss of the uPA receptor in mdx mice did not exacerbate muscular dystrophy in mdx mice, suggesting that uPA exerts its effects independently of its receptor. These findings underscore the importance of uPA in muscular dystrophy.

The plasminogen activation system in skeletal muscle regeneration: antagonistic roles of urokinase-type plasminogen activator (uPA) and its inhibitor (PAI-1).

The plasminogen activation (PA) system is an extensively used mechanism for the generation of proteolytic activity in the extracellular matrix, where it contributes to tissue remodeling in a wide range of physiopathological processes. Despite the limited information available at present on plasminogen activators, their inhibitors and cognate receptors in skeletal muscle, increasing evidence is accumulating on their important roles in the homeostasis of muscle fibers and their surrounding extracellular matrix. The development of mice deficient for the individual components of the PA system has provided an incisive approach to test the proposed muscle functions in vivo. Skeletal muscle regeneration induced by injury has been analyzed in urokinase-type plasminogen activator (uPA)-, tissue-type plasminogen activator (tPA)-, plasminogen (Plg)- and plasminogen activator inhibitor-1 (PAI-1)-deficient mice and has demonstrated profound effects of these molecules on the fibrotic state and the inflammatory response, which contribute to muscle repair. In particular, the opposite roles of uPA and its inhibitor PAI-1 in this process are highlighted. Delineating the mechanisms by which the different plasminogen activation system components regulate tissue repair will be of potential therapeutic value for severe muscle disorders.

Alpha-enolase plasminogen receptor in myogenesis.

Plasmin is a potent extracellular protease specialized in the degradation of fibrin (fibrinolysis). Active plasmin is generated by proteolytic activation of the zymogen plasminogen (Plg) by urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA). Alpha-enolase, although traditionally considered a glycolytic enzyme, constitutes a receptor for plasminogen on several cell types, serving to localize and promote plasminogen activation pericellularly. Localization of plasmin activity on the cell surface plays a critical role in fibrinolysis and in physiopathological processes involving extracellular matrix remodelling. Previous studies have unambiguously demonstrated that uPA-dependent plasmin generation is necessary for myogenesis in vitro and for muscle regeneration in vivo. However, the implication of alpha-enolase plasminogen receptor in myogenesis had never been investigated. This review focuses on the recently reported expression and function of alpha-enolase plasminogen receptor during myogenesis. Skeletal myoblasts express alpha-enolase plasminogen receptor, being its expression greatly induced during the differentiation process in vitro. MAb 11G1, a monoclonal antibody against anti-alpha-enolase plasminogen receptor, that inhibits plasmin generation, was able to fully abrogate myoblast fusion and differentiation. Moreover, both plasmin activity and alpha-enolase plasminogen receptor expression were significantly augmented in injury-induced regenerating muscle of wild type mice and in the dystrophic muscle of mdx mice, an animal model of Duchenne muscular dystrophy (DMD). Altogether, these results indicate that the plasminogen activation (PA) system is an important component of skeletal myogenesis in vitro and in vivo. In particular, the expression of alpha-enolase plasminogen receptor may serve to concentrate and enhance plasmin generation on the cell surface of migratory myoblasts contributing to efficient muscle repair.