High-fat diet (HFD) plays a central role in the initiation of mitochondrial dysfunction that significantly contributes to skeletal muscle metabolic disorders in obesity. However, the mechanism by which HFD weakens skeletal muscle metabolism by altering mitochondrial function and biogenesis is unknown. Given the emerging roles of microRNAs (miRNAs) in the regulation of skeletal muscle metabolism, we sought to determine whether activation of a specific miRNA pathway would rescue the HFD-induced mitochondrial dysfunction via the sirtuin-1 (SIRT-1)/ peroxisome proliferator–activated receptor γ coactivator-1α (PGC-1α) pathway, a pathway that governs genes necessary for mitochondrial function. We here report that miR-149 strongly controls SIRT-1 expression and activity. Interestingly, miR-149 inhibits poly(ADP-ribose) polymerase-2 (PARP-2) and so increased cellular NAD+ levels and SIRT-1 activity that subsequently increases mitochondrial function and biogenesis via PGC-1α activation. In addition, skeletal muscles from HFD-fed obese mice exhibit low levels of miR-149 and high levels of PARP-2, and they show reduced mitochondrial function and biogenesis due to a decreased activation of the SIRT-1/PGC-1α pathway, suggesting that mitochondrial dysfunction in the skeletal muscle of obese mice may be because of, at least in part, miR-149 dysregulation. Overall, miR-149 may be therapeutically useful for treating HFD-induced skeletal muscle metabolic disorders in such pathophysiological conditions as obesity and type 2 diabetes.

MRG15 is a novel chromodomain protein that is a member of a family of genes related to MORF4. MORF4 (mortality factor on chromosome 4) induces senescence in a subset of human tumor cell lines. Our previous results indicated that MRG15 (MORF-related gene on chromosome 15) could derepress the B-myb promoter by association with Rb. In this study, sucrose gradient analysis demonstrated that MRG15 was present in two distinct nuclear protein complexes, MAF1 (MRG15-associated factor 1) and MAF2. Rb was associated with MRG15 and PAM14 (a novel coil-coil protein) in MAF1, and a histone acetyl transferase, hMOF, was an MRG15 partner in MAF2. Analysis of deletion mutants of MRG15 indicated that the leucine zipper at the C-terminal region of MRG15 was important for the protein associations in MAF1 and that the N-terminal chromodomain was required for the assembly of the MAF2 protein complex. Consistent with these data was the fact that a histone acetyltransferase activity associated with MRG15 was lost when the chromodomain was deleted and that both mutant MRG15 proteins failed to activate the B-myb promoter. The various mechanisms by which MRG15 could activate gene transcription are discussed.Replicative senescence, the final non-proliferative state reached by normal cells in culture, is considered a model for aging at the cellular level. Senescence is dominant over the phenotype of indefinite division exhibited by tumor cell lines (1). In an effort to identify senescence-related genes, MORF4, 1 (MORtality Factor on chromosome 4) was cloned as a gene capable of inducing a senescent phenotype in a subset of immortal human cell lines (2). MORF4 is a member of a family of genes, and two of these, MRG15 and MRGX (MORF4-related genes on chromosomes 15 and X, respectively), which have a high degree of homology to MORF4, are expressed in human cells. Analysis of the deduced amino acid sequences of the three predicted proteins has indicated the existence of helix-loophelix and leucine zipper domains, features typical of those found in transcriptional regulators, as well as regions thought to be involved in protein-protein interaction. MRG15 has a 96% similarity to MORF4 in amino acid sequence but fails to induce senescence upon introduction into immortal cells. The most striking structural difference between the two proteins is the presence of an N-terminal extension in MRG15 that includes a chromodomain. This region and/or nine single amino acid changes could therefore be responsible for the differential behavior of the two proteins. The fact that MORF4 is a truncated protein raises the hypothesis that it could have a dominant negative effect when transfected into cells, resulting in loss of cell proliferation.Proteins containing a chromodomain characterized to date have been found to be chromatin remodeling factors involved in causing conformational changes in chromatin by ATP-dependent movement of nucleosomes and modification of histones (3). Although the function of the chromodomain is not completely understood, some evidence ...

A complex rearrangement mutation in the mouse titin gene leads to an in-frame 83-amino acid deletion in the N2A region of titin. Autosomal recessive inheritance of the titin muscular dystrophy with myositis (Ttn(mdm/mdm)) mutation leads to a severe early-onset muscular dystrophy and premature death. We hypothesized that the N2A deletion would negatively impact the force-generating capacity and passive mechanical properties of the mdm diaphragm. We measured in vitro active isometric contractile and passive length-tension properties to assess muscle function at 2 and 6 wk of age. Micro-CT, myosin heavy chain Western blotting, and histology were used to assess diaphragm structure. Marked chest wall distortions began at 2 wk and progressively worsened until 5 wk. The percentage of myofibers with centrally located nuclei in mdm mice was significantly (P < 0.01) increased at 2 and 6 wk by 4% and 17%, respectively, compared with controls. At 6 wk, mdm diaphragm twitch stress was significantly (P < 0.01) reduced by 71%, time to peak twitch was significantly (P < 0.05) reduced by 52%, and half-relaxation time was significantly (P < 0.05) reduced by 57%. Isometric tetanic stress was significantly (P < 0.05) depressed in 2- and 6-wk mdm diaphragms by as much as 64%. Length-tension relationships of the 2- and 6-wk mdm diaphragms showed significantly (P < 0.05) decreased extensibility and increased stiffness. Slow myosin heavy chain expression was aberrantly favored in the mdm diaphragm at 6 wk. Our data strongly support early contractile and passive mechanical aberrations of the respiratory pump in mdm mice.

Skeletal muscle aging is associated with increased inflammation and oxidative stress, a decrease in the ability to rebuild muscle after injury and in response to exercise. In this perspective, we discuss the mechanisms regulating Sirt1 activity and expression in skeletal muscles, emphasizing their implications in muscle physiology and the impairment of muscle function with age.

Mechanical loading of muscles by intrinsic muscle activity or passive stretch leads to an increase in the production of reactive oxygen species (1, 2). The NAD-dependent protein deacetylase SIRT1 is involved in the protection against oxidative stress by enhancing FOXO-driven Sod2 transcription (3-5). In this report, we unravel a mechanism triggered by mechanical stretch of skeletal muscle cells that leads to an EGR1-dependent transcriptional activation of the Sirt1 gene. The resulting transient increase in SIRT1 expression generates an antioxidative response that contributes to reactive oxygen species scavenging.Skeletal muscles show a rapid response to changes in mechanical environment. Mechanical stimuli are translated to biochemical signals that promote adaptations in muscle mass, structure, and function. This response is mediated by mechanosensitive signaling pathways that provoke changes in gene expression that promote cell growth and survival.Muscle contractility provokes an increase in ROS 2 production due to increased mitochondrial activity and through activation of NADPH oxidase at the membrane (6, 7). Passive mechanical stretch of the diaphragmatic muscle is also associated with increased ROS production (8). As a result, activation of the ROS-responsive MAPK signaling pathway and downstream effectors as the proinflammatory transcriptional factors AP-1 and NF B (9 -11) occurs.High levels of ROS lead to cell damage by lipid and protein oxidation and contributes to muscle fatigue and consequently loss of contractile force production (1, 8). The mechanisms by which muscles performing constant activity, i.e. the heart and diaphragm, are protected against excessive ROS generation remain elusive. SIRT1, the mammalian orthologue of Sir2, is an NAD-dependent protein deacetylase, which has been shown to increase lifespan in model organisms as yeast, worms, and flies (12)(13)(14). In mammals, SIRT1 is involved in a plethora of physiological processes such as metabolism (15-17), neurogenesis (18), and gametogenesis (19) and has a beneficial effect on age-associated pathologies (20). Particularly in the heart, it has been reported that SIRT1 participates in protection against age-associated oxidative stress by up-regulation of Mn-SOD (21).At the cellular level, SIRT1 has a prosurvival effect by deacetylating p53 and the FOXO (Forkhead box type O transcription factor) transcription factors. Deacetylation by SIRT1 inhibits p53 apoptotic response and switches FOXO transcription activity toward genes involved in cell cycle arrest, DNA repair and resistance to oxidative stress and away from proapoptotic genes (3,4,22,23).Mechanisms controlling Sirt1 expression at the transcriptional level have been reported to occur in response to nutrient availability and DNA damage. In actively growing cells, the cell cycle regulator E2F1 is responsible for cell cycle-dependent SIRT1 fluctuations, DNA damage stabilizes E2F1 and induces Sirt1 transcription; in turn, deacetylation of E2F1 by SIRT1 inhibits E2F1 transcriptional and apopt...