Mitochondrial complex I deficiency enhances skeletal myogenesis but impairs insulin signaling through SIRT1 inactivation

The clinical value of serum sirtuin-1 concentration in the diagnosis of metabolic dysfunction-associated steatotic liver disease

BACKGROUND

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most prevalent chronic liver disease and can affect individuals without producing any symptoms. We aimed to explore the value of serum sirtuin-1 (Sirt-1) in the diagnosis of MASLD.

METHODS

This case-control study analyzed data collected from 190 individuals aged 20 to 60 years. Anthropometric parameters, demographic information, and serum biochemical variables-including glycemic parameters, lipid profiles, liver enzymes, and Sirt-1 levels-were assessed. The correlation between serum Sirt-1 and biochemical variables was examined using Pearson's correlation coefficient. Receiver operating characteristic (ROC) curve analysis was employed to evaluate the diagnostic value of serum Sirt-1 in the context of MASLD.

RESULTS

Serum Sirt-1 levels was significantly lower in the MASLD group (p < 0.001) and was inversely correlated with serum insulin (r = -0.163, p = 0.025), HOMA-IR (r = -0.169, p = 0.020) and triglyceride (r = -0.190, p = 0.009) and positively correlated with serum levels of high-density lipoprotein cholesterol (HDL-C) (r = 0.214, p = 0.003). The area under the curve (AUC) of Sirt-1 to predict the presence of MASLD was 0.76 (p < 0.001, 95% CI: 0.69, 0.82) with a sensitivity of 78.9, specificity of 61.1, positive predictive value (PPV) of 67.0%, and negative predictive value (NPV) of 74.0%. The optimal cutoff, determined using Youden's index, was 23.75 ng/mL. This indicates that serum Sirt-1 levels below 23.75 ng/mL may be indicative of MASLD.

CONCLUSIONS

The present study demonstrated that serum Sirt-1 levels were significantly lower in patients with MASLD. Furthermore, these levels were correlated with various metabolic parameters, including insulin resistance and the serum lipid profile. A serum Sirt-1 level below the cutoff of 23.75 ng/mL exhibited a significant association with the presence of MASLD, suggesting its potential utility in identifying patients with this condition.

13C stable isotope tracing reveals distinct fatty acid oxidation pathways in proliferative versus oxidative cells

The TCA cycle serves as a central hub to balance catabolic and anabolic needs of the cell, where carbon moieties can either contribute to oxidative metabolism or support biosynthetic reactions. This differential TCA cycle engagement for glucose-derived carbon has been extensively studied in cultured cells, but the fate of fatty acid (FA)-derived carbons is poorly understood. To fill the knowledge gap, we have developed a strategy to culture cells with long-chain FAs without altering cell viability. By tracing 13C-FA, we show that FA oxidation (FAO) is robust in both proliferating and oxidative cells while the metabolic pathway after citrate formation is distinct. In proliferating cells, a significant portion of carbon derived from FAO exits canonical TCA cycle as citrate and converts to unlabeled malate in cytosol. Increasing FA supply or β-oxidation does not change the partition of FA-derived carbon between cytosol and mitochondria. Oxidation of glucose competes with FA-derived carbon for the canonical TCA pathway thus promoting FA carbon flowing into the alternative TCA pathway. Moreover, the coupling between FAO and the canonical TCA pathway changes with the state of oxidative energy metabolism.NEW & NOTEWORTHY By using 13C stable isotope-resolved metabolomics and FA-driven oxygen consumption rate analysis, our study provides novel insights into the fate of FA carbon through β-oxidation and downstream TCA cycle in proliferative and oxidative cells. Although both proliferative and oxidative cells demonstrate robust β-oxidation, they demonstrate distinct metabolic carbon fate downstream of citrate during TCA cycle oxidation. This differential TCA cycle engagement is likely to be important to balance catabolic and anabolic demands of the cell.

Skeletal muscle feature of different populations in large yellow croaker (Larimichthys crocea): from an epigenetic point of view

Fish skeletal muscle is composed of well-defined fiber types. In order to identify potential candidate genes affecting muscle growth and development under epigenetic regulation. Bisulfite sequencing was utilized to analyze and compare the muscle DNA methylation profiles of Larimichthys crocea inhabiting different environments. The results revealed that DNA methylation in L. crocea was predominantly CG methylation, with 2,396 differentially methylated regions (DMRs) identified through comparisons among different populations. The largest difference in methylation was observed between the ZhouShan and JinMen wild populations, suggesting that L. crocea may have undergone selection and domestication. Additionally, GO and KEGG enrichment analysis of differentially methylated genes (DMGs) revealed 626 enriched GO functional categories, including various muscle-related genes such as myh10, myf5, myf6, ndufv1, klhl31, map3k4, syn2b, sostdc1a, bag4, and hsp90ab. However, significant enrichment in KEGG pathways was observed only in the JinMen and XiangShan populations of L. crocea. Therefore, this study provides a theoretical foundation for a better understanding of the epigenetic regulation of skeletal muscle growth and development in L. crocea under different environmental conditions.

Optimized protocol for quantification of extracellular nicotinamide adenine dinucleotide: evaluating clinical parameters and pre-analytical factors for translational research

Nicotinamide adenine dinucleotide (NAD+), a coenzyme for more than 500 enzymes, plays a central role in energy production, metabolism, cellular signaling, and DNA repair. Until recently, NAD+ was primarily considered to be an intracellular molecule (iNAD+), however, its extracellular species (eNAD+) has recently been discovered and has since been associated with a multitude of pathological conditions. Therefore, accurate quantification of eNAD+ in bodily fluids such as plasma is paramount to answer important research questions. In order to create a clinically meaningful and reliable quantitation method, we analyzed the relationship of cell lysis, routine clinical laboratory parameters, blood collection techniques, and pre-analytical processing steps with measured plasma eNAD+ concentrations. Initially, NAD+ levels were assessed both intracellularly and extracellularly. Intriguingly, the concentration of eNAD+ in plasma was found to be approximately 500 times lower than iNAD+ in peripheral blood mononuclear cells (0.253 ± 0.02 μM vs. 131.8 ± 27.4 μM, p = 0.007, respectively). This stark contrast suggests that cellular damage or cell lysis could potentially affect the levels of eNAD+ in plasma. However, systemic lactate dehydrogenase in patient plasma, a marker of cell damage, did not significantly correlate with eNAD+ (n = 33; r = -0.397; p = 0.102). Furthermore, eNAD+ was negatively correlated with increasing c-reactive protein (CRP, n = 33; r = -0.451; p = 0.020), while eNAD+ was positively correlated with increasing hemoglobin (n = 33; r = 0.482; p = 0.005). Next, variations in blood drawing, sample handling and pre-analytical processes were examined. Sample storage durations at 4°C (0-120 min), temperature (0° to 25°C), cannula sizes for blood collection and tourniquet times (0 - 120 s) had no statistically significant effect on eNAD+ (p > 0.05). On the other hand, prolonged centrifugation (> 5 min) and a faster braking mode of the centrifuge rotor (< 4 min) resulted in a significant decrease in eNAD+ levels (p < 0.05). Taken together, CRP and hemoglobin appeared to be mildly correlated with eNAD+ levels whereas cell damage was not correlated significantly to eNAD+ levels. The blood drawing trial did not show any influence on eNAD+, in contrast, the preanalytical steps need to be standardized for accurate eNAD+ measurement. This work paves the way towards robust eNAD+ measurements, for use in future clinical and translational research, and provides an optimized hands-on protocol for reliable eNAD+ quantification in plasma.

High-fat diet during pregnancy promotes fetal skeletal muscle fatty acid oxidation and insulin resistance in an ovine model

Maternal diet during pregnancy is associated with offspring metabolic risk trajectory in humans and animal models, but the prenatal origins of these effects are less clear. We examined the effects of a high-fat diet (HFD) during pregnancy on fetal skeletal muscle metabolism and metabolic risk parameters using an ovine model. White-faced ewes were fed a standardized diet containing 5% fat wt/wt (CON), or the same diet supplemented with 6% rumen-protected fats (11% total fat wt/wt; HFD) beginning 2 wk before mating until midgestation (GD75). Maternal HFD increased maternal weight gain, fetal body weight, and low-density lipoprotein levels in the uterine and umbilical circulation but had no significant effects on circulating glucose, triglycerides, or placental fatty acid transporters. Fatty acid (palmitoylcarnitine) oxidation capacity of permeabilized hindlimb muscle fibers was >50% higher in fetuses from HFD pregnancies, whereas pyruvate and maximal (mixed substrate) oxidation capacities were similar to CON. This corresponded to greater triacylglycerol content and protein expression of fatty acid transport and oxidation enzymes in fetal muscle but no significant effect on respiratory chain complexes or pyruvate dehydrogenase expression. However, serine-308 phosphorylation of insulin receptor substrate-1 was greater in fetal muscle from HFD pregnancies along with c-jun-NH2 terminal kinase activation, consistent with prenatal inhibition of skeletal muscle insulin signaling. These results indicate that maternal high-fat feeding shifts fetal skeletal muscle metabolism toward a greater capacity for fatty acid over glucose utilization and favors prenatal development of insulin resistance, which may predispose offspring to metabolic syndrome later in life.NEW & NOTEWORTHY Maternal diet during pregnancy is associated with offspring metabolic risk trajectory in humans and animal models, but the prenatal origins of these effects are less clear. This study examined the effects of a high-fat diet during pregnancy on metabolic risk parameters using a new sheep model. Results align with findings previously reported in nonhuman primates, demonstrating changes in fetal skeletal muscle metabolism that may predispose offspring to metabolic syndrome later in life.

Metformin suppresses cardiac fibroblast proliferation under high-glucose conditions via regulating the mitochondrial complex I protein Grim-19 involved in the Sirt1/Stat3 signaling pathway

Hyperglycemia associated with myocardial oxidative stress and fibrosis is the main cause of diabetic cardiomyopathy. Currently, no approved drug is available for preventing or treating diabetes-induced cardiac fibrosis. Metformin has been reported to improve glycemic control and ameliorate diabetic cardiomyopathy. This study aimed to investigate the effects and mechanism of metformin on diabetes-induced cardiac fibrosis and high glucose-induced proliferation of cardiac fibroblasts (CFs). In this study, db/db mice were treated with metformin [250 mg/kg⋅d, gavage]. CFs were cultured in high-glucose medium to mimic an in vitro diabetes model and then subjected to treatment with or without metformin. Cardiac fibrosis was analyzed using immunohistochemistry, Masson's trichrome staining, and Western blot analysis. Cell Counting Kit-8 (CCK-8) assays and cell colony formation assays were used to examine cell proliferation capacity. Transwell and scratch-wound assays were used to detect the migration ability of CFs. Retinoid-interferon-induced mortality-19 (Grim-19), sirtuin1 (Sirt1), and signal transducer and activator of transcription 3 (Stat3) were detected using Western blot analysis. The genes downstream of the Stat3 pathway were detected using quantitative reverse transcription PCR (qRT‒PCR). Metformin treatment markedly attenuated cardiac fibrosis in db/db mice and the proliferation and migration of CFs under high-glucose conditions. Mechanistically, we found an intersection between metformin and Grim-19 using bioinformatics. Metformin was found to suppress the expression of p-Stat3 and elevate the expression of mitochondrial complex I protein Grim-19 and Sirt1, thus inhibiting the proliferation and migration of CFs under high-glucose conditions. Our data suggested that metformin inhibited the proliferation and migration of CFs by regulating the expression of mitochondrial complex I Grim-19 protein involved in the Sirt1/Stat3 signaling pathway under high-glucose conditions, thus providing new ideas for treating diabetes-induced cardiac fibrosis.

Polymorphisms of CRELD1 and DNAJC30 and their relationship with chicken carcass traits

Carcass traits play important roles in the broiler industry and single nucleotide polymorphism (SNP) can be efficient molecular markers for marker-assisted breeding of chicken carcass traits. Based on our previous RNA-seq data (accession number GSE58755), cysteine rich with epidermal growth factor like domains 1 (CRELD1) and DnaJ heat shock protein family member C30 (DNAJC30) are differentially expressed in breast muscle between white recessive rock chicken (WRR) and Xinghua chicken (XH). In this study, we further characterize the potential function and SNP mutation of CRELD1 and DNAJC30 in chicken for the first time. According to protein interaction network and enrichment analysis, CRELD1 and DNAJC30 may play some roles in chicken muscle development and fat deposition. In WRR and XH, the results of the relative tissue expression pattern demonstrated that CRELD1 and DNAJC30 are not only differentially expressed in breast muscle but also leg muscle and abdominal fat. Therefore, we identified 5 SNP sites of CRELD1 and 7 SNP sites of DNAJC30 and genotyped them in an F₂ chicken population. There are 4 sites of CRELD1 and 3 sites of DNAJC30 are associated with chicken carcass traits like breast muscle weight, body weight, dressed weight, leg weight percentage, eviscerated weight with giblet percentage, intermuscular adipose width, shank length, and girth. These results suggest that the SNP sites of CRELD1 and DNAJC30 can be potential molecular markers to improve the chicken carcass traits and lay the foundation for marker-assisted selection.

Preventing c2c12 muscular cells damage combining magnesium and potassium with vitamin D3 and curcumin

Background and aim

Physical activity is defined as any bodily movement produced by skeletal muscles which causes energy consumption; moderate and constant physical activity is known to be beneficial and to slow the muscle loss process associated with aging. The aim of the present study was to test, in an in vitro exercise model, the biological effects of a new formulation composed of magnesium and potassium combined with vitamin D and curcumin created to support muscle activity and to prevent hypercontraction damage.

Experimental procedure

C2C12 cells were treated with vitamin D, buffered magnesium bisglycinate, curcumin, and potassium citrate. Cell viability, morpho-functional changes, calcium and magnesium movements, and the main kinases involved in glucose uptake were analyzed. The glycogen level and lactate were also evaluated.

Results and conclusion

Important results about a positive effect on mitochondrial activity, ATP production, oxygen consumption and in the physiological differentiation of C2C12 cells were obtained. Further experiments were performed under conditions that mimic the biological aspects of strenuous exercise. The combination of magnesium, vitamin D3, curcumin, and potassium citrate revealed beneficial effects on skeletal muscle cells under physiological conditions as well as while mimicking intense activity. In particular, in an in vitro model, they were able to control the hypercontraction, restoring ion fluxes, reducing inflammation signaling and supporting the main mechanism involved on aerobic activity. Our results have indicated for the first time that this new combination could be considered as a new nutraceutical formulation to improve physical performance and muscle recovery.

Genetic basis of negative heterosis for growth traits in chickens revealed by genome-wide gene expression pattern analysis

Background

Heterosis is an important biological phenomenon that has been extensively utilized in agricultural breeding. However, negative heterosis is also pervasively observed in nature, which can cause unfavorable impacts on production performance. Compared with systematic studies of positive heterosis, the phenomenon of negative heterosis has been largely ignored in genetic studies and breeding programs, and the genetic mechanism of this phenomenon has not been thoroughly elucidated to date. Here, we used chickens, the most common agricultural animals worldwide, to determine the genetic and molecular mechanisms of negative heterosis.

Results

We performed reciprocal crossing experiments with two distinct chicken lines and found that the body weight presented widely negative heterosis in the early growth of chickens. Negative heterosis of carcass traits was more common than positive heterosis, especially breast muscle mass, which was over − 40% in reciprocal progenies. Genome-wide gene expression pattern analyses of breast muscle tissues revealed that nonadditivity, including dominance and overdominace, was the major gene inheritance pattern. Nonadditive genes, including a substantial number of genes encoding ATPase and NADH dehydrogenase, accounted for more than 68% of differentially expressed genes in reciprocal crosses (4257 of 5587 and 3617 of 5243, respectively). Moreover, nonadditive genes were significantly associated with the biological process of oxidative phosphorylation, which is the major metabolic pathway for energy release and animal growth and development. The detection of ATP content and ATPase activity for purebred and crossbred progenies further confirmed that chickens with lower muscle yield had lower ATP concentrations but higher hydrolysis activity, which supported the important role of oxidative phosphorylation in negative heterosis for growth traits in chickens.

Conclusions

These findings revealed that nonadditive genes and their related oxidative phosphorylation were the major genetic and molecular factors in the negative heterosis of growth in chickens, which would be beneficial to future breeding strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-021-00574-2.

Opening of Intermediate Conductance Ca2+-Activated K+ Channels in C2C12 Skeletal Muscle Cells Increases the Myotube Diameter via the Akt/Mammalian Target of Rapamycin Pathway

The activation of potassium channels and the ensuing hyperpolarization in skeletal myoblasts are essential for myogenic differentiation. However, the effects of K+ channel opening in myoblasts on skeletal muscle mass are unclear. Our previous study revealed that pharmacological activation of intermediate conductance Ca2+-activated K+ channels (IKCa channels) increases myotube formation. In this study, we investigated the effects of 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (DCEBIO), a Ca2+-activated K+ channel opener, on the mass of skeletal muscle. Application of DCEBIO to C2C12 cells during myogenesis increased the diameter of C2C12 myotubes in a concentration-dependent manner. This DCEBIO-induced hypertrophy was abolished by gene silencing of IKCa channels. However, it was resistant to 1 µM but sensitive to 10 µM TRAM-34, a specific IKCa channel blocker. Furthermore, DCEBIO reduced the mitochondrial membrane potential by opening IKCa channels. Therefore, DCEBIO should increase myotube mass by opening of IKCa channels distributed in mitochondria. Pharmacological studies revealed that mitochondrial reactive oxygen species (mitoROS), Akt, and mammalian target of rapamycin (mTOR) are involved in DCEBIO-induced myotube hypertrophy. An additional study demonstrated that DCEBIO-induced muscle hypertrophic effects are only observed when applied in the early stage of myogenic differentiation. In an in vitro myotube inflammatory atrophy experiment, DCEBIO attenuated the reduction of myotube diameter induced by endotoxin. Thus, we concluded that DCEBIO increases muscle mass by activating the IKCa channel/mitoROS/Akt/mTOR pathway. Our study suggests the potential of DCEBIO in the treatment of muscle wasting diseases. SIGNIFICANCE STATEMENT: Our study shows that 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (DCEBIO), a small molecule opener of Ca2+-activated K+ channel, increased muscle diameter via the mitochondrial reactive oxygen species/Akt/mammalian target of rapamycin pathway. And DCEBIO overwhelms C2C12 myotube atrophy induced by endotoxin challenge. Our report should inform novel role of K+ channel in muscle development and novel usage of K+ channel opener such as for the treatment of muscle wasting diseases.

Apoptosome-dependent myotube formation involves activation of caspase-3 in differentiating myoblasts

Caspase-2, -9, and -3 are reported to control myoblast differentiation into myotubes. This had been previously explained by phosphatidylserine exposure on apoptotic myoblasts inducing differentiation in neighboring cells. Here we show for the first time that caspase-3 is activated in the myoblasts undergoing differentiation. Using RNAi, we also demonstrate that differentiation requires both cytochrome c and Apaf-1, and by using a new pharmacological approach, we show that apoptosome formation is required. We also show that Bid, whose cleavage links caspase-2 to the mitochondrial death pathway, was required for differentiation, and that the caspase cleavage product, tBid, was generated during differentiation. Taken together, these data suggest that myoblast differentiation requires caspase-2 activation of the mitochondrial death pathway, and that this occurs in the cells that differentiate. Our data also reveal a hierarchy of caspases in differentiation with caspase-2 upstream of apoptosome activation, and exerting a more profound control of differentiation, while caspases downstream of the apoptosome primarily control cell fusion.

The Lifespan Extension Ability of Nicotinic Acid Depends on Whether the Intracellular NAD+ Level Is Lower than the Sirtuin-Saturating Concentrations

Calorie restriction can extend lifespan by increasing intracellular nicotinamide adenine dinucleotide (NAD⁺), thereby upregulating the activity of sirtuins (Caenorhabditis elegans Sir-2.1; human SIRT1). Nicotinic acid (NA) can be metabolized to NAD⁺; however, the calorie restriction mimetic (CRM) potential of NA is unclear. This study explored the ability and mechanism of NA to extend the lifespan of human Hs68 cells and C. elegans. We found that NA can efficiently increase the intracellular NAD⁺ levels in Hs68 cells and C. elegans; however, NA was only able to extend the lifespan of C. elegans. The steady-state NAD⁺ level in C. elegans was approximately 55 μM. When intracellular NAD⁺ was increased by a mutation of pme-1 (poly (ADP-ribose) metabolism enzyme 1) or by pretreatment with NAD⁺ in the medium, the lifespan extension ability of NA disappeared. Additionally, the saturating concentration of NAD⁺ required by SIRT1 was approximately 200 μM; however, the steady-state concentration of NAD⁺ in Hs68 cells reached up to 460 μM. These results demonstrate that the lifespan extension ability of NA depends on whether the intracellular level of NAD⁺ is lower than the sirtuin-saturating concentration in Hs68 cells and in C. elegans. Thus, the CRM potential of NA should be limited to individuals with lower intracellular NAD⁺.

Changes in expression of serine biosynthesis and integrated stress response genes during myogenic differentiation of C2C12 cells

Skeletal muscle is a highly metabolic and dynamic tissue that is formed through the complex and well-organised process of myogenesis. Although there is a good understanding about the role of the Muscle Regulatory Factors during myogenesis, little is known about the potential interplay of other metabolic proteins. The aim of this study was to determine the endogenous mRNA expression profile for a novel group of genes, recently associated with β2-adrenergic agonist (BA) induced muscle hypertrophy in pigs [1], during myogenic differentiation in C2C12 cells and their response to dibutyryl cyclic-AMP (dbcAMP). These genes included mitochondrial phosphoenolpyruvate carboxykinase (PCK2/PEPCK-M), genes involved in serine biosynthesis (Phosphoglycerate dehydrogenase, PHGDH; Phosphoserine aminotransferase-1, PSAT1; Phosphoserine phosphatase, PSPH) and those involved in an integrated stress response (Asparagine synthetase, ASNS; Sestrin-2, SESN2; and Activating transcription factor-5, ATF5). A coordinated peak in endogenous PCK2, PHGDH, PSAT1, PSPH, ASNS, ATF5 and SESN2 mRNA expression was observed at day 2 of differentiation (P < 0.001) in C2C12 cells, which coincided with the peak in myogenin mRNA. Myotube hypertrophy was induced with dbcAMP (1 mM) treatment from day 0, thereby mimicking the in vivo BA response. Although dbcAMP treatment from day 0 induced larger myotubes and increased both myosin heavy chain-IIB (MyHC-IIB) and pyruvate carboxylase (PC) mRNA, the expression of PCK2, PHGDH, PSAT1 and ASNS mRNA were all unaffected. Treatment with dbcAMP from day 4 increased MyHC-IIB mRNA, however this was less dramatic compared to the response observed following treatment from day 0, but there was no effect on PC mRNA. There was also no effect of dbcAMP treatment from day 4 on PCK2, PHGDH, PSAT1 and ASNS mRNA. To conclude, the coordinated day 2 peak in endogenous expression of PCK2, PHGDH, PSAT1, PSPH, ASNS, ATF5 and SESN2 mRNA may relate to a shift in biosynthetic demand required to initiate myogenic differentiation. However, dbcAMP had no effect on the expression of these genes in vitro suggesting that the effects observed in BA-treated pigs might be via other signalling pathways from the activation of the β2-adrenergic receptor, but independent of cAMP, or that there are species differences in the response.

Glucose metabolism during in vitro maturation of mouse oocytes: An study using RNA interference

In previous studies on glucose metabolism during in vitro maturation, intact cumulus-oocyte complexes (COCs) were treated with enzyme inhibitors/activators. Because inhibitors/activators may have non-specificity and/or toxicity, and culture of COCs cannot differentiate whether glucose metabolism of cumulus cells (CCs) or that of the oocyte supports oocyte maturation, results from the previous studies must be verified by silencing genes in either CCs or cumulus-denuded oocytes (DOs). In this study, RNAi was adopted to specify the effects of glucose metabolism in CCs or DOs on oocyte maturation. Although silencing either glyceraldehyde 3-phosphate dehydrogenase (GAPDH) or glucose-6-phosphate dehydrogenase (G6PD) genes in CCs significantly decreased competence of the cocultured DOs, silencing G6PD impaired competence to a greater extent. While silencing G6PD or GAPDH of CCs decreased glutathione and ATP contents of cocultured DOs to similar extents, silencing G6PD increased oxidative stress as well. Analysis on metabolite contents and oxidative stress index and culture of DOs in medium conditioned with gene-silenced CCs indicated that CCs supported oocyte maturation by releasing glucose metabolites. Silencing mitochondrial pyruvate carrier 1 or NADH dehydrogenase (ubiquintone) flavoprotein 1 of DOs significantly impaired their maturation. The results have unequivocally confirmed that CCs promote oocyte maturation by releasing glucose metabolites from both pentose phosphate pathway (PPP) and glycolysis. Pyruvate is transferred into DOs by mitochondrial pyruvate carrier (MPC) and utilized through mitochondrial electron transport to support maturation.

Metformin Is a Direct SIRT1-Activating Compound: Computational Modeling and Experimental Validation

Metformin has been proposed to operate as an agonist of SIRT1, a nicotinamide adenine dinucleotide (NAD⁺)-dependent deacetylase that mimics most of the metabolic responses to calorie restriction. Herein, we present an in silico analysis focusing on the molecular docking and dynamic simulation of the putative interactions between metformin and SIRT1. Using eight different crystal structures of human SIRT1 protein, our computational approach was able to delineate the putative binding modes of metformin to several pockets inside and outside the central deacetylase catalytic domain. First, metformin was predicted to interact with the very same allosteric site occupied by resveratrol and other sirtuin-activating compounds (STATCs) at the amino-terminal activation domain of SIRT1. Second, metformin was predicted to interact with the NAD⁺ binding site in a manner slightly different to that of SIRT1 inhibitors containing an indole ring. Third, metformin was predicted to interact with the C-terminal regulatory segment of SIRT1 bound to the NAD⁺ hydrolysis product ADP-ribose, a "C-pocket"-related mechanism that appears to be essential for mechanism-based activation of SIRT1. Enzymatic assays confirmed that the net biochemical effect of metformin and other biguanides such as a phenformin was to improve the catalytic efficiency of SIRT1 operating in conditions of low NAD⁺ in vitro. Forthcoming studies should confirm the mechanistic relevance of our computational insights into how the putative binding modes of metformin to SIRT1 could explain its ability to operate as a direct SIRT1-activating compound. These findings might have important implications for understanding how metformin might confer health benefits via maintenance of SIRT1 activity during the aging process when NAD⁺ levels decline.

Extracellular reactive oxygen species are generated by a plasma membrane oxidative phosphorylation system

Although the oxidative phosphorylation (OXPHOS) system has been found in mitochondria and the plasma membrane of various mammalian cell lines, understanding the physiological functions of the plasma membrane OXPHOS system is challenging. Here, we demonstrated that OXPHOS I, II, III, IV and V subunits were expressed in the plasma membrane of HepG2 cells and primary mouse hepatocytes, as determined by non-permeabilized immunofluorescence, total internal reflection fluorescence (TIRF) microscopy, cell surface-biotin labeling and plasma membrane and lipid raft isolation. Next, we demonstrated that NADH administration generated extracellular superoxide and improved insulin signaling in HepG2 cells and primary mouse hepatocytes. The NADH-dependent generation of extracellular superoxide was prevented by knockdown of NDUFV-1, the first subunit of OXPHOS I receiving electrons from NADH and the NADH-improved insulin signaling was abolished by extracellular catalase. Thus, we conclude that the OXPHOS system in the plasma membrane may be required for the generation of extracellular ROS and the regulation of insulin signaling.

Caveolin-1 deficiency induces premature senescence with mitochondrial dysfunction

Paradoxical observations have been made regarding the role of caveolin-1 (Cav-1) during cellular senescence. For example, caveolin-1 deficiency prevents reactive oxygen species-induced cellular senescence despite mitochondrial dysfunction, which leads to senescence. To resolve this paradox, we re-addressed the role of caveolin-1 in cellular senescence in human diploid fibroblasts, A549, HCT116, and Cav-1^-/- mouse embryonic fibroblasts. Cav-1 deficiency (knockout or knockdown) induced cellular senescence via a p53-p21-dependent pathway, downregulating the expression level of the cardiolipin biosynthesis enzymes and then reducing the content of cardiolipin, a critical lipid for mitochondrial respiration. Our results showed that Cav-1 deficiency decreased mitochondrial respiration, reduced the activity of oxidative phosphorylation complex I (CI), inactivated SIRT1, and decreased the NAD⁺ /NADH ratio. From these results, we concluded that Cav-1 deficiency induces premature senescence via mitochondrial dysfunction and silent information regulator 2 homologue 1 (SIRT1) inactivation.

Mitochondrial oxidative phosphorylation complexes exist in the sarcolemma of skeletal muscle

Although proteomic analyses have revealed the presence of mitochondrial oxidative phosphorylation (OXPHOS) proteins in the plasma membrane, there have been no in-depth evaluations of the presence or function of OXPHOS I-V in the plasma membrane. Here, we demonstrate the in situ localization of OXPHOS I-V complexes to the sarcolemma of skeletal muscle by immunofluorescence and immunohistochemistry. A portion of the OXPHOS I-V complex proteins was not co-stained with MitoTracker but co-localized with caveolin-3 in the sarcolemma of mouse gastrocnemius. Mitochondrial matrix-facing OXPHOS complex subunits were ectopically expressed in the sarcolemma of the non-permeabilized muscle fibers and C2C12 myotubes. The sarcolemmal localization of cytochrome c was also observed from mouse gastrocnemius muscles and C2C12 myotubes, as determined by confocal and total internal resonance fluorescence (TIRF) microscopy. Based on these data, we conclude that a portion of OXPHOS complexes is localized in the sarcolemma of skeletal muscle and may have non-canonical functions. [BMB Reports 2016; 49(2): 116-121]

Mitochondrial biogenesis is decreased in skeletal muscle of pig fetuses exposed to maternal high-energy diets

Mitochondria plays an important role in the regulation of energy homeostasis. Moreover, mitochondrial biogenesis accompanies skeletal myogenesis, and we previously reported that maternal high-energy diet repressed skeletal myogenesis in pig fetuses. Therefore, the aim of this study was to evaluate the effects of moderately increased maternal energy intake on skeletal muscle mitochondrial biogenesis and function of the pig fetuses. Primiparous purebred Large White sows were allocated to a normal energy intake group (NE) as recommended by the National Research Council (NRC) and a high energy intake group (HE, 110% of NRC recommendations). On day 90 of gestation, fetal umbilical vein blood and longissimus (LM) muscle were collected. Results showed that the weight gain of sows fed HE diet was higher than NE sows on day 90 of gestation (P<0.05). Maternal HE diet increased fetal umbilical vein serum triglyceride and insulin concentrations (P<0.05), and tended to increase the homeostasis model assessment index (P=0.08). Furthermore, HE fetuses exhibited increased malondialdehyde concentration (P<0.05), and decreased activities of antioxidative enzymes (P<0.05) and intracellular NAD+ level (P<0.05) in LM muscle. These alterations in metabolic traits of HE fetuses were accompanied by reduced mitochondrial DNA amount (P<0.05) and down-regulated messenger RNA expression levels of genes responsible for mitochondrial biogenesis and function (P<0.05). Our results suggest that moderately increased energy supply during gestation decreases mitochondrial biogenesis, function and antioxidative capacity in skeletal muscle of pig fetuses.

Myosin heavy chain is stabilized by BCL-2 interacting cell death suppressor (BIS) in skeletal muscle

BCL-2 interacting cell death suppressor (BIS), which is ubiquitously expressed, has important roles in various cellular processes, such as apoptosis, the cellular stress response, migration and invasion and protein quality control. In particular, BIS is highly expressed in skeletal and cardiac muscles, and BIS gene mutations result in human myopathy. In this study, we show that mRNA and protein levels of BIS were markedly increased during skeletal myogenesis in C2C12 cells and mouse satellite cells. BIS knockdown did not prevent the early stage of skeletal myogenesis, but did induce muscle atrophy and a decrease in the diameter of myotubes. BIS knockdown significantly suppressed the expression level of myosin heavy chain (MyHC) without changing the expression levels of myogenic marker proteins, such as Mgn, Cav-3 and MG53. In addition, BIS endogenously interacted with MyHC, and BIS knockdown induced MyHC ubiquitination and degradation. From these data, we conclude that molecular association of MyHC and BIS is necessary for MyHC stabilization in skeletal muscle.

Sirt1 AS lncRNA interacts with its mRNA to inhibit muscle formation by attenuating function of miR-34a

Recent studies demonstrate the functions of long non-coding RNAs (lncRNAs) in mediating gene expression at the transcriptional or translational level. Our previous study identified a Sirt1 antisense (AS) lncRNA transcribed from the Sirt1 AS strand. However, its role and regulatory mechanism is still unknown in myogenesis. Here, functional analyses showed that Sirt1 AS lncRNA overexpression promoted myoblast proliferation, but inhibited differentiation. Mechanistically, Sirt1 AS lncRNA was found to activate its sense gene, Sirt1. The luciferase assay provided evidences that Sirt1 AS lncRNA interacted with Sirt1 3′ UTR and rescued Sirt1 transcriptional suppression by competing with miR-34a. In addition, RNA stability assay showed that Sirt1 AS lncRNA prolonged Sirt1 mRNA half-life from 2 to 10 h. Ribonuclease protection assay further indicated that it fully bound to Sirt1 mRNA in the myoblast cytoplasm. Moreover, Sirt1 AS overexpression led to less mouse weight than the control because of less lean mass and greater levels of Sirt1, whereas the fat mass and levels of miR-34a were not altered. Based on the findings, a novel regulatory mechanism was found that Sirt1 AS lncRNA preferably interacted with Sirt1 mRNA forming RNA duplex to promote Sirt1 translation by competing with miR-34a, inhibiting muscle formation.

SIRT1 and insulin resistance

Sirtuin 1 (SIRT1) is a prototype mammalian NAD(+)-dependent protein deacetylase that has emerged as a key metabolic sensor in various metabolic tissues. Growing evidence suggests that SIRT1 regulates glucose and lipid metabolism through its deacetylase activity. In this review, we have summarized the recent progress in SIRT1 research with a particular focus on the role of SIRT1 in insulin resistance at different metabolic tissues. Recent data indicate that activated SIRT1 improves the insulin sensitivity of liver, skeletal muscle and adipose tissues and protects the function and cell mass of pancreatic β-cells. These findings suggest that SIRT1 might be a new therapeutic target for the prevention of disease related to insulin resistance, such as metabolic syndrome and type 2 diabetes mellitus.

NAD(+) Metabolism and the Control of Energy Homeostasis: A Balancing Act between Mitochondria and the Nucleus

NAD(+) has emerged as a vital cofactor that can rewire metabolism, activate sirtuins, and maintain mitochondrial fitness through mechanisms such as the mitochondrial unfolded protein response. This improved understanding of NAD(+) metabolism revived interest in NAD(+)-boosting strategies to manage a wide spectrum of diseases, ranging from diabetes to cancer. In this review, we summarize how NAD(+) metabolism links energy status with adaptive cellular and organismal responses and how this knowledge can be therapeutically exploited.

Rotenone inhibits primary murine myotube formation via Raf-1 and ROCK2

Rotenone (ROT) is a widely used inhibitor of complex I (CI), the first complex of the mitochondrial oxidative phosphorylation (OXPHOS) system. However, particularly at high concentrations ROT was also described to display off-target effects. Here we studied how ROT affected in vitro primary murine myotube formation. We demonstrate that myotube formation is specifically inhibited by ROT (10-100nM), but not by piericidin A (PA; 100nM), another CI inhibitor. At 100nM, both ROT and PA fully blocked myoblast oxygen consumption. Knock-down of Rho-associated, coiled-coil containing protein kinase 2 (ROCK2) and, to a lesser extent ROCK1, prevented the ROT-induced inhibition of myotube formation. Moreover, the latter was reversed by inhibiting Raf-1 activity. In contrast, ROT-induced inhibition of myotube formation was not prevented by knock-down of RhoA. Taken together, our results support a model in which ROT reduces primary myotube formation independent of its inhibitory effect on CI-driven mitochondrial ATP production, but via a mechanism primarily involving the Raf-1/ROCK2 pathway.

Pharmacologic targeting of sirtuin and PPAR signaling improves longevity and mitochondrial physiology in respiratory chain complex I mutant Caenorhabditis elegans

Mitochondrial respiratory chain (RC) diseases are highly morbid multi-systemic conditions for which few effective therapies exist. Given the essential role of sirtuin and PPAR signaling in mediating both mitochondrial physiology and the cellular response to metabolic stress in RC complex I (CI) disease, we postulated that drugs that alter these signaling pathways either directly (resveratrol for sirtuin, rosiglitazone for PPARγ, fenofibrate for PPARα), or indirectly by increasing NAD(+) availability (nicotinic acid), might offer effective treatment strategies for primary RC disease. Integrated effects of targeting these cellular signaling pathways on animal lifespan and multi-dimensional in vivo parameters were studied in gas-1(fc21) relative to wild-type (N2 Bristol) worms. Specifically, animal lifespan, transcriptome profiles, mitochondrial oxidant burden, mitochondrial membrane potential, mitochondrial content, amino acid profiles, stable isotope-based intermediary metabolic flux, and total nematode NADH and NAD(+) concentrations were compared. Shortened gas-1(fc21) mutant lifespan was rescued with either resveratrol or nicotinic acid, regardless of whether treatments were begun at the early larval stage or in young adulthood. Rosiglitazone administration beginning in young adult stage animals also rescued lifespan. All drug treatments reversed the most significant transcriptome alterations at the biochemical pathway level relative to untreated gas-1(fc21) animals. Interestingly, increased mitochondrial oxidant burden in gas-1(fc21) was reduced with nicotinic acid but exacerbated significantly by resveratrol and modestly by fenofibrate, with little change by rosiglitazone treatment. In contrast, the reduced mitochondrial membrane potential of mutant worms was further decreased by nicotinic acid but restored by either resveratrol, rosiglitazone, or fenofibrate. Using a novel HPLC assay, we discovered that gas-1(fc21) worms have significant deficiencies of NAD(+) and NADH. Whereas resveratrol restored concentrations of both metabolites, nicotinic acid only restored NADH. Characteristic branched chain amino acid elevations in gas-1(fc21) animals were normalized completely by nicotinic acid and largely by resveratrol, but not by either rosiglitazone or fenofibrate. We developed a visualization system to enable objective integration of these multi-faceted physiologic endpoints, an approach that will likely be useful to apply in future drug treatment studies in human patients with mitochondrial disease. Overall, these data demonstrate that direct or indirect pharmacologic restoration of altered sirtuin and PPAR signaling can yield significant health and longevity benefits, although by divergent bioenergetic mechanism(s), in a nematode model of mitochondrial RC complex I disease. Thus, these animal model studies introduce important, integrated insights that may ultimately yield rational treatment strategies for human RC disease.

Growth hormone-releasing peptide-biotin conjugate stimulates myocytes differentiation through insulin-like growth factor-1 and collagen type I

Based on the potential beneficial effects of growth hormone releasing peptide (GHRP)-6 on muscle functions, a newly synthesized GHRP-6-biotin conjugate was tested on cultured myoblast cells. Increased expression of myogenic marker proteins was observed in GHRP-6-biotin conjugate-treated cells. Additionally, increased expression levels of insulin-like growth factor-1 and collagen type I were observed. Furthermore, GHRP-6-biotin conjugate-treated cells showed increased metabolic activity, as indicated by increased concentrations of energy metabolites, such as ATP and lactate, and increased enzymatic activity of lactate dehydrogenase and creatine kinase. Finally, binding protein analysis suggested few candidate proteins, including desmin, actin, and zinc finger protein 691 as potential targets for GHRP6-biotin conjugate action. These results suggest that the newly synthesized GHRP-6-biotin conjugate has myogenic stimulating activity through, at least in part, by stimulating collagen type I synthesis and several key proteins. Practical applications of the GHRP-6-biotin conjugate could include improving muscle condition. [BMB Reports 2015; 48(9): 501-506]

Energy metabolism disorders in rare and common diseases. Toward bioenergetic modulation therapy and the training of a new generation of European scientists

Energy metabolism alterations are found in a large number of rare and common diseases of genetic or environmental origin. The number of patients that could benefit from bioenergetic modulation therapy (BIOMET) is therefore very important and includes individuals with pathologies as diverse as mitochondrial diseases, acute coronary syndrome, chronic kidney disease, asthma or even cancer. Although, the alteration of energy metabolism is disease specific and sometimes patient specific, the strategies for BIOMET could be common and target a series of bioenergetic regulatory mechanisms discussed in this article. An excellent training of scientists in the field of energy metabolism, related human diseases and drug discovery is also crucial to form a young generation of MDs, PHDs and Pharma or CRO-group leaders who will discover novel personalized bioenergetic medicines, through pharmacology, genetics, nutrition or adapted exercise training. The Mitochondrial European Educational Training (MEET) consortium was created to pursue this goal, and we dedicated here a special issue of Organelle in Focus (OiF) to highlight their objectives. A total of 10 OiFs articles constitute this Directed Issue on Mitochondrial Medicine. As part of this editorial article, we asked timely questions to the PR. Jan W. Smeitink, professor of Mitochondrial Medicine and CEO of Khondrion, a mitochondrial medicine company. He shared with us his objectives and strategies for the study of mitochondrial diseases and the identification of future treatments. This article is part of a Directed Issue entitled: Energy Metabolism Disorders and Therapies.