bims-misrem Biomed News
on Mitochondria and sarcoplasmic reticulum in muscle mass
Issue of 2020‒03‒22
forty-two papers selected by
Rafael Antonio Casuso Pérez
University of Granada

  1. Redox Biol. 2020 Feb 26. pii: S2213-2317(20)30083-5. [Epub ahead of print] 101480
    Cheng AJ, Jude B, Lanner JT.
      Strenuous exercise is a potent stimulus to induce beneficial skeletal muscle adaptations, ranging from increased endurance due to mitochondrial biogenesis and angiogenesis, to increased strength from hypertrophy. While exercise is necessary to trigger and stimulate muscle adaptations, the post-exercise recovery period is equally critical in providing sufficient time for metabolic and structural adaptations to occur within skeletal muscle. These cyclical periods between exhausting exercise and recovery form the basis of any effective exercise training prescription to improve muscle endurance and strength. However, imbalance between the fatigue induced from intense training/competitions, and inadequate post-exercise/competition recovery periods can lead to a decline in physical performance. In fact, prolonged periods of this imbalance may eventually lead to extended periods of performance impairment, referred to as the state of overreaching that may progress into overtraining syndrome (OTS). OTS may have devastating implications on an athlete's career and the purpose of this review is to discuss potential underlying mechanisms that may contribute to exercise-induced OTS in skeletal muscle. First, we discuss the conditions that lead to OTS, and their potential contributions to impaired skeletal muscle function. Then we assess the evidence to support or refute the major proposed mechanisms underlying skeletal muscle weakness in OTS: 1) glycogen depletion hypothesis, 2) muscle damage hypothesis, 3) inflammation hypothesis, and 4) the oxidative stress hypothesis. Current data implicates reactive oxygen and nitrogen species (ROS) and inflammatory pathways as the most likely mechanisms contributing to OTS in skeletal muscle. Finally, we allude to potential interventions that can mitigate OTS in skeletal muscle.
  2. J Gerontol A Biol Sci Med Sci. 2020 Mar 16. pii: glaa059. [Epub ahead of print]
    Cordeiro AV, Brícola RS, Braga RR, Lenhare L, Silva VRR, Anaruma CP, Katashima CK, Crisol BM, Simabuco FM, Silva ASR, Cintra DE, Moura LP, Pauli JR, Ropelle ER.
      The impairment of the mitochondrial functions is a hallmark of aging. During aging, there is a downregulation of two mechanisms strictly associated with mitochondrial integrity, including the mitonuclear imbalance (e.g. imbalance in mitochondrial- versus nuclear-encoded mitochondrial proteins) and the mitochondrial Unfolded Protein Response (UPRmt). Here, we evaluated the effects of aerobic exercise in the mitonuclear imbalance and UPRmt markers in the skeletal muscle of old mice. We combined the physiological tests, molecular and bioinformatic analyzes to evaluate the effects of 4 weeks of Aerobic Exercise Training on mitonuclear imbalance and UPRmt markers in the skeletal muscle of young (2 mo.) and aged (24 mo.) C57BL/6J mice. Initially, we found that aging reduced several mitochondrial genes in the gastrocnemius muscle, and it was accompanied by the low levels of UPRmt markers, including Yme1l1 and Clpp mRNA. As expected, physical training improved the whole-body metabolism and physical performance of aged mice. The aerobic exercise increased key proteins involved in the mitochondrial biogenesis/functions (VDAC and SIRT1) along with mitochondrial-encoded genes (mtNd1, mtCytB, and mtD-Loop) in the skeletal muscle of old mice. Interestingly, aerobic exercise induced the mitonuclear imbalance, increasing MTCO1/ATP5a ratio and UPRmt markers in the skeletal muscle, including HSP60, Lonp1, and Yme1L1 protein levels in the gastrocnemius muscle of aged mice. These data demonstrate that aerobic exercise training induced mitonuclear imbalance and UPRmt in the skeletal muscle during aging. These phenomena could be involved in the improvement of the mitochondrial metabolism and oxidative capacity in aged individuals.
    Keywords:  Aging; Mitonuclear Imbalance; Physical Exercise; Skeletal Muscle; UPRmt
  3. Exp Physiol. 2020 Mar 19.
    Lavorato M, Formenti F, Franzini-Armstrong C.
      NEW FINDINGS: What is the topic for this review? This review summarizes recent discoveries in mitochondrial development and morphology studied with electron microscopy. What advances does it highlight? Although mitochondria are generally considered to be isolated from each other, this review highlights recently discovered evidence for the presence of intermitochondrial communication structures in cardiac and skeletal muscle, in animal models and humans. Within striated muscles, the means of intermitochondrial exchange and the reaction of mitochondria to external stimuli are uniquely dependent on the tissue, and we clearly differentiate between nanotunnels, the active protrusion of cardiac mitochondria, and the connecting ducts of skeletal muscle derived from fusion-fission and elongation events.ABSTRACT: This review focuses on recent discoveries in skeletal and cardiac muscles indicating that mitochondria behave as an interactive cohort with inter-organelle communication and specific reactions to stress signals. Our new finding is that intermitochondrial communications in cardiac and skeletal muscles rely on two distinct methods. In cardiac muscle, mitochondria are discrete entities and are fairly well immobilized in a structural context. The organelles have developed a unique method of communication, via nanotunnels, which allow temporary connection from one mitochondrion to another over distances of up to several micrometres, without overall movement of the individual organelles and loss of their identity. Skeletal muscle mitochondria, in contrast, are dynamic. Through fusion, fission and elongation, they form connections that include constrictions and connecting ducts (distinct from nanotunnels) and lose individual identity in the formation of extensive networks. Connecting elements in skeletal muscle are distinct from nanotunnels in cardiac muscle.
  4. Methods Cell Biol. 2020 ;pii: S0091-679X(19)30121-9. [Epub ahead of print]155 369-379
    Carraro M, Bernardi P.
      The mitochondrial permeability transition (PT) is an increase in the inner membrane permeability caused by the opening of a Ca2+-activated high-conductance channel, the so-called PT pore (PTP) or mitochondrial megachannel (MMC). Recent data indicate that F-ATP synthase contributes substantially to the generation of the PTP, yet this hypothesis is the matter of controversy. In this chapter, we will describe an approach to study the pore, i.e., the evaluation of mitochondrial swelling by means of a decrease in the absorbance at 540nm. This method should be useful to resolve apparent discrepancies in the literature and help solve emerging issues on the identity of mitochondrial pores.
    Keywords:  Absorbance; Mitochondria; Permeability transition; Swelling
  5. PLoS Genet. 2020 Mar 19. 16(3): e1008638
    Haeussler S, Köhler F, Witting M, Premm MF, Rolland SG, Fischer C, Chauve L, Casanueva O, Conradt B.
      Compromising mitochondrial fusion or fission disrupts cellular homeostasis; however, the underlying mechanism(s) are not fully understood. The loss of C. elegans fzo-1MFN results in mitochondrial fragmentation, decreased mitochondrial membrane potential and the induction of the mitochondrial unfolded protein response (UPRmt). We performed a genome-wide RNAi screen for genes that when knocked-down suppress fzo-1MFN(lf)-induced UPRmt. Of the 299 genes identified, 143 encode negative regulators of autophagy, many of which have previously not been implicated in this cellular quality control mechanism. We present evidence that increased autophagic flux suppresses fzo-1MFN(lf)-induced UPRmt by increasing mitochondrial membrane potential rather than restoring mitochondrial morphology. Furthermore, we demonstrate that increased autophagic flux also suppresses UPRmt induction in response to a block in mitochondrial fission, but not in response to the loss of spg-7, which encodes a mitochondrial metalloprotease. Finally, we found that blocking mitochondrial fusion or fission leads to increased levels of certain types of triacylglycerols and that this is at least partially reverted by the induction of autophagy. We propose that the breakdown of these triacylglycerols through autophagy leads to elevated metabolic activity, thereby increasing mitochondrial membrane potential and restoring mitochondrial and cellular homeostasis.
  6. Nutrients. 2020 Mar 12. pii: E755. [Epub ahead of print]12(3):
    Walowski CO, Braun W, Maisch MJ, Jensen B, Peine S, Norman K, Müller MJ, Bosy-Westphal A.
      Assessment of a low skeletal muscle mass (SM) is important for diagnosis of ageing and disease-associated sarcopenia and is hindered by heterogeneous methods and terminologies that lead to differences in diagnostic criteria among studies and even among consensus definitions. The aim of this review was to analyze and summarize previously published cut-offs for SM applied in clinical and research settings and to facilitate comparison of results between studies. Multiple published reference values for discrepant parameters of SM were identified from 64 studies and the underlying methodological assumptions and limitations are compared including different concepts for normalization of SM for body size and fat mass (FM). Single computed tomography or magnetic resonance imaging images and appendicular lean soft tissue by dual X-ray absorptiometry (DXA) or bioelectrical impedance analysis (BIA) are taken as a valid substitute of total SM because they show a high correlation with results from whole body imaging in cross-sectional and longitudinal analyses. However, the random error of these methods limits the applicability of these substitutes in the assessment of individual cases and together with the systematic error limits the accurate detection of changes in SM. Adverse effects of obesity on muscle quality and function may lead to an underestimation of sarcopenia in obesity and may justify normalization of SM for FM. In conclusion, results for SM can only be compared with reference values using the same method, BIA- or DXA-device and an appropriate reference population. Limitations of proxies for total SM as well as normalization of SM for FM are important content-related issues that need to be considered in longitudinal studies, populations with obesity or older subjects.
    Keywords:  appendicular skeletal muscle mass index; fat-free mass index; sarcopenia; sarcopenic obesity; skeletal muscle area; skeletal muscle mass; skeletal muscle mass index
  7. Curr Opin Clin Nutr Metab Care. 2020 Mar 13.
    Brook MS, Wilkinson DJ, Atherton PJ.
      PURPOSE OF REVIEW: Skeletal muscle has many essential roles in maintaining human health, not only being crucial for locomotion, but further as a metabolically important organ. Muscle wasting in disease (cachexia) is highly prevalent, associated with poor clinical outcomes and is not fully reversible with nutritional interventions. Understanding proteostasis in diseased states is of great importance to design novel, effective nutritional/nutraceutical strategies aimed at alleviating muscle wasting. In this review, we will provide an update on muscle kinetics in disease and the effects of nutritional interventions.RECENT FINDINGS: Whole body and skeletal muscle kinetics are commonly shown to be imbalanced in disease, promoting overall catabolism that underlies the development of cachexia. However, recent advancements in defining the effectiveness of nutritional interventions on muscle anabolism are clouded by heterogenous patient populations and a lack of direct incorporation stable isotope techniques. Current recommendations are focused on combating malnutrition, with increased protein intake (high in EAA) demonstrating promise.
    SUMMARY: Recent progress in understanding catabolic states in cachexia across disease is minimal. Further, studies investigating muscle-specific protein turnover along with nutritional interventions are scarce. As such, there is a significant requirement for strong RCT's investigating both acute and chronic nutritional interventions and their impact on skeletal muscle in individual disease states.
  8. Methods Cell Biol. 2020 ;pii: S0091-679X(19)30131-1. [Epub ahead of print]155 337-368
    Greotti E, De Stefani D.
      Calcium (Ca2+) is a universal intracellular messenger capable of governing a plethora of different biological functions. Its versatility is guaranteed on the one hand by a cell type-specific Ca2+ signaling toolkit. On the other hand, the fine compartmentalization of changes in Ca2+ concentration ([Ca2+]) into specific subcellular domains adds a level of complexity, thus generating a variety of signals that can be differentially decoded into specific cellular events. In this context, mitochondrial Ca2+ dynamics plays a central role, by regulating both specific organelle functions (e.g., regulation of substrate oxidation, release of caspase cofactors) and global cellular events (e.g., shaping of cytoplasmic Ca2+ waves). Here we describe a general method for the detection of intramitochondrial [Ca2+] using bioluminescent and fluorescent genetically-encoded Ca2+ indicators (GECIs). We will discuss the characteristics of different GECIs, as well as their strengths, limitations and applications.
    Keywords:  Aequorin; Calcium; Cameleon; Genetically-encoded calcium indicators; Mitochondria
  9. Methods Cell Biol. 2020 ;pii: S0091-679X(19)30137-2. [Epub ahead of print]155 121-156
    Frazier AE, Vincent AE, Turnbull DM, Thorburn DR, Taylor RW.
      Measurement of the individual enzymes involved in mitochondrial oxidative phosphorylation (OXPHOS) forms a key part of diagnostic investigations in patients with suspected mitochondrial disease, and can provide crucial information on mitochondrial OXPHOS function in a variety of cells and tissues that are applicable to many research investigations. In this chapter, we present methods for analysis of mitochondrial respiratory chain enzymes in cells and tissues based on assays performed in two geographically separate diagnostic referral centers, as part of clinical diagnostic investigations. Techniques for sample preparation from cells and tissues, and spectrophotometric assays for measurement of the activities of OXPHOS complexes I-V, the combined activity of complexes II+III, and the mitochondrial matrix enzyme citrate synthase, are provided. The activities of mitochondrial respiratory chain enzymes are often expressed relative to citrate synthase activity, since these ratios may be more robust in accounting for variability that may arise due to tissue quality, handling and storage, cell growth conditions, or any mitochondrial proliferation that may be present in tissues from patients with mitochondrial disease. Considerations for adaption of these techniques to other cells, tissues, and organisms are presented, as well as comparisons to alternate methods for analysis of respiratory chain function. In this context, a quantitative immunofluorescence protocol is also provided that is suitable for measurement of the amount of multiple respiratory chain complexes in small diagnostic tissue samples. The analysis and interpretation of OXPHOS enzyme activities are then placed in the context of mitochondrial disease tissue pathology and diagnosis.
    Keywords:  Citrate synthase; Enzymology; Mitochondria; Mitochondrial disease; Oxidative phosphorylation; Quantitative immunofluorescence; Respiratory chain
  10. Methods Cell Biol. 2020 ;pii: S0091-679X(19)30140-2. [Epub ahead of print]155 491-518
    Hoppins S, Lackner LL, Lee JE, Mears JA.
      Mitochondria are required for cell survival and are best known for their role in energy production. These organelles also participate in many other biological processes that are critical for cellular function, and thus, play a central role in cellular life and death decisions. In a majority of cell types, mitochondria form highly dynamic, reticular networks. Maintaining the shape of these complex, ever-changing networks is critical for mitochondrial and cellular function, and requires the conserved activities of mitochondrial fission and fusion. Great advances in our knowledge about the molecular machines that mediate these dynamic activities have been made over the past 2 decades. These advances have been driven by the use of highly complementary in vitro and in vivo approaches that have proven extremely powerful for studying the complex membrane remodeling processes that drive fission and fusion of the organelle. In this chapter, we detail current methods used to examine the mechanisms and regulation of mitochondrial fission and fusion in vitro and in vivo.
    Keywords:  Mitochondrial division; Mitochondrial dynamics; Mitochondrial fission; Mitochondrial fusion
  11. Redox Biol. 2020 Mar 10. pii: S2213-2317(19)31613-1. [Epub ahead of print] 101499
    Margaritelis NV, Paschalis V, Theodorou AA, Kyparos A, Nikolaidis MG.
      Redox reactions control fundamental processes of human biology. Therefore, it is safe to assume that the responses and adaptations to exercise are, at least in part, mediated by redox reactions. In this review, we are trying to show that redox reactions are the basis of exercise physiology by outlining the redox signaling pathways that regulate four characteristic acute exercise-induced responses (muscle contractile function, glucose uptake, blood flow and bioenergetics) and four chronic exercise-induced adaptations (mitochondrial biogenesis, muscle hypertrophy, angiogenesis and redox homeostasis). Based on our analysis, we argue that redox regulation should be acknowledged as central to exercise physiology.
    Keywords:  Adaptations; Antioxidants; Exercise; Redox biology; Responses; Signaling
  12. Aging (Albany NY). 2020 Mar 17. 12
    D'Souza RF, Woodhead JST, Hedges CP, Zeng N, Wan J, Kumagai H, Lee C, Cohen P, Cameron-Smith D, Mitchell CJ, Merry TL.
      Mitochondria putatively regulate the aging process, in part, through the small regulatory peptide, mitochondrial open reading frame of the 12S rRNA-c (MOTS-c) that is encoded by the mitochondrial genome. Here we investigated the regulation of MOTS-c in the plasma and skeletal muscle of healthy aging men. Circulating MOTS-c reduced with age, but older (70-81 y) and middle-aged (45-55 y) men had ~1.5-fold higher skeletal muscle MOTS-c expression than young (18-30 y). Plasma MOTS-c levels only correlated with plasma in young men, was associated with markers of slow-type muscle, and associated with improved muscle quality in the older group (maximal leg-press load relative to thigh cross-sectional area). Using small mRNA assays we provide evidence that MOTS-c transcription may be regulated independently of the full length 12S rRNA gene in which it is encoded, and expression is not associated with antioxidant response element (ARE)-related genes as previously seen in culture. Our results suggest that plasma and muscle MOTS-c are differentially regulated with aging, and the increase in muscle MOTS-c expression with age is consistent with fast-to-slow type muscle fiber transition. Further research is required to determine the molecular targets of endogenous MOTS-c in human muscle but they may relate to factors that maintain muscle quality.
    Keywords:  MOTS-c; aging; mitochondria; mitochondrial derived peptides; muscle
  13. Acta Physiol (Oxf). 2020 Mar 16.
    Patten D, Harper ME, Boardman N.
      The mitochondrial GTPase, OPA1 (Optic Atrophy 1) plays a major role in mitochondrial structure and function, regulating both the fusion of membranes and cristae ultrastructure 1 . OPA1 is associated with a wide range of pathologies in which mitochondrial dysfunction occurs, including neurodegenerative and cardiac diseases, as well as primary mitochondrial diseases such as dominant optic atrophy, Behr syndrome and Leigh-like syndrome. In this issue of Acta Physiologica, Ding et al 2 report that OPA1 plays a role in mitigating the progression of diabetic cardiomyopathy. In their study, Ding et al demonstrate that increased mitochondrial fragmentation due to OPA1 deficiency may be an underlying cause of cardiac dysfunction in hearts of streptozotocin (STZ)-induced diabetic rats.
  14. Biochem Biophys Res Commun. 2020 Mar 12. pii: S0006-291X(20)30443-5. [Epub ahead of print]
    Sakushima K, Yoshikawa M, Osaki T, Miyamoto N, Hashimoto T.
      Although several studies have implied that a hypoxic environment may be a factor that influences muscle hypertrophy, scant attention has been paid to the effect of oxygen molecules on the morphological characteristics of muscle. The purpose of the present study was to examine the effect of semisevere (i.e., 5%) to moderate (i.e., 10% or 15%) hypoxic environments on the morphological characteristics of skeletal muscle and the associated mechanisms. C2C12 skeletal muscle cells were divided into various groups, namely, the normoxia group (20.9% O2) and hypoxia groups (5% O2, 10% O2, and 15% O2), and cell growth and the expression of associated proteins in the hypoxia groups were compared with those in the normoxia group. The myotube diameter and cell differentiation index were determined on day 6 by immunocytochemical analyses. The expression of proteins associated with muscle cell differentiation (MyoD and myogenin) and muscle hypertrophy (mTOR and p70s6K) were analyzed by Western blotting. We found that compared with normoxia, a 5% oxygen environment inhibited differentiation and caused muscle atrophy. However, compared with normoxia, a 10% oxygen environment promoted muscle differentiation, and 10% oxygen and 15% oxygen environments induced muscle hypertrophy. Compared with normoxia, a 10% oxygen environment promoted myogenin and the expression of mTOR, p70s6K, and the metabolic signal AMPK. We concluded that a hypoxic environment, if not too severe, may promote muscle differentiation and hypertrophy by increasing the expression of proteins associated with muscle cell differentiation and hypertrophy.
    Keywords:  Muscle cell differentiation; Myogenesis; Protein expressions; Resistance exercise; Sarcopenia
  15. Aging Cell. 2020 Mar 20. e13124
    Adelnia F, Ubaida-Mohien C, Moaddel R, Shardell M, Lyashkov A, Fishbein KW, Aon MA, Spencer RG, Ferrucci L.
      Adequate support of energy for biological activities and during fluctuation of energetic demand is crucial for healthy aging; however, mechanisms for energy decline as well as compensatory mechanisms that counteract such decline remain unclear. We conducted a discovery proteomic study of skeletal muscle in 57 healthy adults (22 women and 35 men; aged 23-87 years) to identify proteins overrepresented and underrepresented with better muscle oxidative capacity, a robust measure of in vivo mitochondrial function, independent of age, sex, and physical activity. Muscle oxidative capacity was assessed by 31 P magnetic resonance spectroscopy postexercise phosphocreatine (PCr) recovery time (τPCr ) in the vastus lateralis muscle, with smaller τPCr values reflecting better oxidative capacity. Of the 4,300 proteins quantified by LC-MS in muscle biopsies, 253 were significantly overrepresented with better muscle oxidative capacity. Enrichment analysis revealed three major protein clusters: (a) proteins involved in key energetic mitochondrial functions especially complex I of the electron transport chain, tricarboxylic acid (TCA) cycle, fatty acid oxidation, and mitochondrial ABC transporters; (b) spliceosome proteins that regulate mRNA alternative splicing machinery, and (c) proteins involved in translation within mitochondria. Our findings suggest that alternative splicing and mechanisms that modulate mitochondrial protein synthesis are central features of the molecular mechanisms aimed at maintaining mitochondrial function in the face of impairment. Whether these mechanisms are compensatory attempt to counteract the effect of aging on mitochondrial function should be further tested in longitudinal studies.
    Keywords:  31P MRS; bioenergetic; mitochondria; proteomic; skeletal muscle
  16. J Appl Physiol (1985). 2020 Mar 19.
    Arentson-Lantz EJ, Fiebig KN, Anderson-Catania KJ, Deer RR, Wacher A, Fry CS, Lamon S, Paddon-Jones D.
      Older adults are at increased risk of being bedridden and experiencing negative health outcomes including the loss of muscle tissue and functional capacity. We hypothesized that supplementing daily meals with a small quantity (3-4 g/meal) of leucine would partially preserve lean leg mass and function of older adults during bed rest. During a 7 day bed rest protocol, followed by 5 days of inpatient rehabilitation, healthy older men and women (67.8 ± 1.1 y, 14 men; 6 women) were randomized to receive isoenergentic meals supplemented with leucine (LEU, 0.06 g/kg/meal; n=10) or an alanine control, (CON, 0.06g/kg/ meal; n=10). Outcomes were assessed at baseline, following bed rest and after rehabilitation. Body composition was measured using dual energy x-ray absorptiometry. Functional capacity was assessed using knee extensor isokinetic and isometric dynamometry, peak aerobic capacity and the short physical performance battery. Muscle fiber type, cross-sectional area, signaling protein expression levels and single fiber characteristics were determined from biopsies of the vastus lateralis. Leucine supplementation reduced the loss of leg lean mass during bed rest (LEU vs. CON: -423 vs. -1035 ± 143 g; p=0.008) but had limited impact on strength or endurance based functional outcomes. Similarly, leucine had no effect on markers of anabolic signaling and protein degradation during bed rest or rehabilitation. In conclusion, providing older adults with supplemental leucine has minimal impact on total energy or protein consumption, and has the potential to partially counter some, but not all of the negative effects of inactivity on muscle health.
    Keywords:  aging; bed rest; dietary supplementation; nutrition
  17. Science. 2020 Mar 19. pii: eaay2494. [Epub ahead of print]
    Zhou Z, Torres M, Sha H, Halbrook CJ, Van den Bergh F, Reinert RB, Yamada T, Wang S, Luo Y, Hunter AH, Wang C, Sanderson TH, Liu M, Taylor A, Sesaki H, Lyssiotis CA, Wu J, Kersten S, Beard DA, Qi L.
      The endoplasmic reticulum (ER) engages mitochondria at specialized ER domains known as mitochondria-associated membranes (MAMs). Here, we used three-dimensional high-resolution imaging to investigate the formation of pleomorphic "megamitochondria" with altered MAMs in brown adipocytes lacking the Sel1L-Hrd1 protein complex of ER-associated protein degradation (ERAD). Mice with ERAD deficiency in brown adipocytes were cold sensitive and exhibited mitochondrial dysfunction. ERAD deficiency affected ER-mitochondria contacts and mitochondrial dynamics, at least in part, by regulating the turnover of the MAM protein, sigma receptor 1 (SigmaR1). Thus, our study provides molecular insights into ER-mitochondrial crosstalk and expands our understanding of the physiological importance of Sel1L-Hrd1 ERAD.
  18. Acta Physiol (Oxf). 2020 Mar 16.
    Snijders T, Aussieker T, Holwerda A, Parise G, van Loon L, Verdijk LB.
      Within the current paradigm of the myonuclear domain theory, it is postulated that a linear relationship exists between muscle fiber size and myonuclear content. The myonuclear domain is kept (relatively) constant by adding additional nuclei (supplied by muscle satellite cells) during muscle fiber hypertrophy and nuclear loss (by apoptosis) during muscle fiber atrophy. However, data from recent animal studies suggest that myonuclei that are added to support muscle fiber hypertrophy are not lost within various muscle atrophy models. Such myonuclear permanence has been suggested to constitute a mechanism allowing the muscle fiber to (re)grow more efficiently during retraining, a phenomenon referred to as 'muscle memory'. The concept of 'muscle memory by myonuclear permanence' has mainly been based on data attained from rodent experimental models. Whether the postulated mechanism also holds true in humans remains largely ambiguous. Nevertheless, there are several studies in humans that provide evidence to potentially support or contradict (parts of) the muscle memory hypothesis. The goal of the present review is to discuss the evidence for the existence of 'muscle memory' in both animal and human models of muscle fiber hypertrophy as well as atrophy. Furthermore, to provide additional insight in the potential presence of muscle memory by myonuclear permanence in humans, we present new data on previously performed exercise training studies. Finally, suggestions for future research are provided to establish whether muscle memory really exists in humans.
    Keywords:  Muscle adaptation; Muscle memory; Myonuclear domain size; Myonuclei; Satellite cell
  19. Methods Cell Biol. 2020 ;pii: S0091-679X(19)30136-0. [Epub ahead of print]155 45-79
    Priesnitz C, Pfanner N, Becker T.
      Mitochondria are deeply integrated into crucial functions of eukaryotic cells, including ATP production via oxidative phosphorylation, biosynthesis of iron-sulfur clusters, amino acids, lipids and heme, signaling pathways, and programmed cell death. The import of about 1000 different proteins that are produced as precursors on cytosolic ribosomes is essential for mitochondrial functions and biogenesis. The translocase of the outer mitochondrial membrane (TOM) forms the entry gate for the vast majority of mitochondrial proteins. Research of the last years has uncovered a complicated network of protein translocases and pathways that sort proteins into the mitochondrial subcompartments: outer and inner membranes, intermembrane space, and matrix. The in vitro import of a large number of different precursor proteins into mitochondria has been a pivotal experimental assay to identify these protein-sorting routes. This experimental set-up enables studies on the kinetics of protein transport into isolated mitochondria, on the processing of precursor proteins, and on their assembly into functional protein machineries. In vitro protein import assays are widely used and are indispensable for research on mitochondrial protein biogenesis.
    Keywords:  Blue native electrophoresis; Mitochondria; Protein assembly; Protein import; Protein sorting; TIM23 complex; TOM complex
  20. J Appl Physiol (1985). 2020 Mar 19.
    Horwath O, Apró W, Moberg M, Godhe M, Helge T, Ekblom MM, Lindén Hirschberg A, Ekblom B.
      It is well established that testosterone administration induces muscle fiber hypertrophy and myonuclear addition in men, however, it remains to be determined whether similar morphological adaptations can be achieved in women. The aim of the present study was therefore to investigate whether exogenously administered testosterone alters muscle fiber morphology in skeletal muscle of young healthy, physically active women. Thirty-five young (20-35 years), recreationally trained women were randomly assigned to either 10-week testosterone administration (10 mg daily) or placebo. Before and after the intervention, hormone concentrations and body composition were assessed, and muscle biopsies were obtained from the vastus lateralis. Fiber type composition, fiber size, satellite cell- and myonuclei content, as well as muscle capillarization were assessed in a fiber type-specific manner using immunohistochemistry. Following the intervention, testosterone administration elevated serum testosterone concentration (5.1-fold increase, P=0.001), and induced significant accretion of total lean mass (+1.9%, P=0.002) and leg lean mass (+2.4%, P=0.001). On the muscle fiber level, testosterone increased mixed fiber cross-sectional area (+8.2%, P=0.001), an effect primarily driven by increases in type II fiber size (9.2%, P=0.006). Whereas myonuclei content remained unchanged, a numerical increase (+30.8%) was found for satellite cells associated with type II fibers in the Testosterone group. In parallel with fiber hypertrophy, testosterone significantly increased capillary contacts (+7.5%, P=0.015) and capillary-to-fiber ratio (+9.2%, P=0.001) in type II muscle fibers. The current study provides novel insight into fiber type-specific adaptations present already after 10 weeks of only moderately elevated testosterone levels in women.
    Keywords:  androgens; capillarization; myonuclear domain; myonuclei; satellite cells
  21. Trends Endocrinol Metab. 2020 Apr;pii: S1043-2760(20)30032-1. [Epub ahead of print]31(4): 269-271
    Martín-Rodríguez S, de Pablos-Velasco P, Calbet JAL.
      Metformin has antidiabetic, anticancer, and prolongevity effects, but seems to interfere with aerobic training mitochondrial adaptations. The primary mechanism of action has been suggested to be the inhibition of mitochondrial complex I. Recent papers (Wang et al. and Cameron et al.), however, provide evidence to deny the hypothesis of a direct action of metformin on complex I.
    Keywords:  exercise; metformin; mitochondrial complex I
  22. Redox Biol. 2020 Feb 29. pii: S2213-2317(20)30114-2. [Epub ahead of print] 101484
    Jackson MJ, Stretton C, McArdle A.
      Hydrogen peroxide appears to be the key reactive oxygen species involved in redox signalling, but comparisons of the low concentrations of hydrogen peroxide that are calculated to exist within cells with those previously shown to activate common signalling events in vitro indicate that direct oxidation of key thiol groups on "redox-sensitive" signalling proteins is unlikely to occur. A number of potential mechanisms have been proposed to explain how cells overcome this block to hydrogen peroxide-stimulated redox signalling and these will be discussed in the context of the redox-stimulation of specific adaptations of skeletal muscle to contractile activity and exercise. It is argued that current data implicate a role for currently unidentified effector molecules (likely to be highly reactive peroxidases) in propagation of the redox signal from sites of hydrogen peroxide generation to common adaptive signalling pathways.
  23. Front Pharmacol. 2020 ;11 142
    Ahmad K, Shaikh S, Ahmad SS, Lee EJ, Choi I.
      Skeletal muscle (SM) comprises around 40% of total body weight and is among the most important plastic tissues, as it supports skeletal development, controls body temperature, and manages glucose levels. Extracellular matrix (ECM) maintains the integrity of SM, enables biochemical signaling, provides structural support, and plays a vital role during myogenesis. Several human diseases are coupled with dysfunctions of the ECM, and several ECM components are involved in disease pathologies that affect almost all organ systems. Thus, mutations in ECM genes that encode proteins and their transmembrane receptors can result in diverse SM diseases, a large proportion of which are types of fibrosis and muscular dystrophy. In this review, we present major ECM components of SMs related to muscle-associated diseases, and discuss two major ECM myopathies, namely, collagen myopathy and laminin myopathies, and their therapeutic managements. A comprehensive understanding of the mechanisms underlying these ECM-related myopathies would undoubtedly aid the discovery of novel treatments for these devastating diseases.
    Keywords:  collagen; extracellular matrix; laminin; myopathy; skeletal muscle
  24. Cells. 2020 Mar 12. pii: E695. [Epub ahead of print]9(3):
    Di Conza G, Ho PC.
      The endoplasmic reticulum (ER) is a critical organelle, storing the majority of calcium and governing protein translation. Thus, it is crucial to keep the homeostasis in all ER components and machineries. The ER stress sensor pathways, including IRE1/sXBP1, PERK/EIf2 and ATF6, orchestrate the major regulatory circuits to ensure ER homeostasis. The embryonic or postnatal lethality that occurs upon genetic depletion of these sensors reveals the essential role of the ER stress pathway in cell biology. In contrast, the impairment or excessive activation of ER stress has been reported to cause or aggravate several diseases such as atherosclerosis, diabetes, NAFDL/NASH, obesity and cancer. Being part of innate immunity, myeloid cells are the first immune cells entering the inflammation site. Upon entry into a metabolically stressed disease environment, activation of ER stress occurs within the myeloid compartment, leading to the modulation of their phenotype and functions. In this review, we discuss causes and consequences of ER stress activation in the myeloid compartment with a special focus on the crosstalk between ER, innate signaling and metabolic environments.
    Keywords:  ER stress; chronic diseases; infection; innate immunity
  25. Methods Cell Biol. 2020 ;pii: S0091-679X(19)30160-8. [Epub ahead of print]155 157-180
    Agrawal RR, Tamucci KA, Pera M, Larrea D.
      Mitochondria are responsible for the generation of ATP by oxidative phosphorylation; however, these multifaceted organelles regulate many other key cellular functions as well, such as calcium homeostasis, apoptosis, and biosynthesis of steroid hormones, heme and phospholipids. In order to carry out these functions, mitochondria establish physical and functional connections with other organelles such as the plasma membrane, lipid droplets/vesicles, peroxisomes, endosomes, and the endoplasmic reticulum. Dysregulation of any of the aforementioned processes or inter-organelle contacts can lead to mitochondrial dysfunction and subsequent changes in oxygen consumption and ATP production. Seahorse technology has become a critical tool for quantification of mitochondrial oxygen consumption and can help differentiate primary mitochondrial disorders from disorders where alterations in mitochondrial metabolism are consequences of a prior, upstream insult. In this chapter, we describe the application of Seahorse technology for assaying mitochondrial respiration in whole cells, permeabilized cells and isolated mitochondria. We leave it to the researcher's discretion to determine which of these approaches will generate the most physiologically relevant data based on the model system and research question at hand.
    Keywords:  Mitochondria; Oxygen consumption; Respiration
  26. Adv Exp Med Biol. 2020 ;1248 265-294
    Li C, Xia B, Wang S, Xu J.
      In consistent with other membrane-bound and secretory proteins, immune checkpoint proteins go through a set of modifications in the endoplasmic reticulum (ER) to acquire their native functional structures before they function at their destinations. There are various ER-resident chaperones and enzymes synergistically regulate and catalyze the glycosylation, folding and transporting of proteins. The whole processing is under the surveillance of ER quality control system which allows the correctly folded proteins to exit from the ER with the help of coat proteinII(COPII) coated vesicles, while retains the rest of terminally misfolded ones in the ER and then eliminates them via ER-associated degradation (ERAD) or ER-to-lysosomes-associated degradation (ERLAD). The dysfunction of the ER causes ER stress which triggers unfolded protein response (UPR) to restore ER proteostasis. Unsolvable prolonged ER stress ultimately results in cell death. This chapter reviews the process that proteins undergo in the ER, and the glycosylation, folding and degradation of immune checkpoint proteins as well as the associated potential immunotherapies to date.
    Keywords:  COPII; ER chaperones; ER enzymes; ER proteostasis; ER quality control; ER stress; ERAD; ERLAD; Endoplasmic reticulum; Glycosylation; Immune checkpoint proteins; Immunotherapy; Protein folding; UPR; Vesicle trafficking
  27. Biomolecules. 2020 Mar 12. pii: E441. [Epub ahead of print]10(3):
    Levitt DE, Yeh AY, Prendergast MJ, Jr RGB, Adler KA, Cook G, Molina PE, Simon L.
      Alcohol use and aging are risk factors for falls requiring immobilization and leading to skeletal muscle atrophy. Skeletal muscle regeneration is integral to post-immobilization recovery. This study aimed to elucidate the effects of alcohol and ovarian hormone loss on the expression of genes implicated in muscle regeneration. Three-month-old female rats received an ovariectomy or a sham surgery, consumed an alcohol-containing or control diet for 10 weeks, were subjected to unilateral hind limb immobilization for seven days, and finally were allowed a three (3d)- or 14 (14d)-day recovery. Immobilization decreased the quadriceps weight at 3d and 14d, and alcohol decreased the quadriceps weight at 14d in the nonimmobilized hind limb (NI). At 3d, alcohol decreased gene expression of myoblast determination protein (MyoD) in the immobilized hind limb (IMM) and myocyte enhancer factor (Mef)2C and tumor necrosis factor (TNF)α in NI, and ovariectomy increased MyoD and decreased TNFα expression in NI. At 14d, alcohol increased the gene expression of Mef2C, MyoD, TNFα, and transforming growth factor (TFG)β in IMM and decreased monocyte chemoattractant protein (MCP)1 expression in NI; ovariectomy increased TNFα expression in NI, and alcohol and ovariectomy together increased Mef2C expression in NI. Despite increased TGFβ expression, there was no concomitant alcohol-mediated increase in collagen in IMM at 14d. Overall, these data indicate that alcohol dysregulated the post-immobilization alteration in the expression of genes implicated in regeneration. Whether alcohol-mediated molecular changes correspond with post-immobilization functional alterations remains to be determined.
    Keywords:  ethanol; immobilization; inflammation; ovarian hormone loss; ovariectomy; recovery; regeneration
  28. J Nutr Biochem. 2020 Jan 08. pii: S0955-2863(19)30532-7. [Epub ahead of print]79 108330
    Liu R, Chen L, Wang Y, Zhang G, Cheng Y, Feng Z, Bai X, Liu J.
      Adjusting ω-3/ω-6 polyunsaturated fatty acids (PUFAs) ratio in high-fat diet is one potential mean to improve metabolic syndrome; however, underlying mechanisms remain unclear. Four groups of mice were fed 60% kcal diets with saturated fatty acids, three different ω-3/ω-6 PUFAs ratios (low, middle and high) for 12 weeks, respectively. Body weight, atherosclerosis marker, insulin signal index and level of lipid accumulation in liver were significantly lowered in High group compared with saturated fatty acids group and Low group at week 12. Expressions of p-mTOR and raptor were inhibited by high ω-3 PUFAs. Importantly, ω-3 PUFAs intake up-regulated mitochondrial electron transport chain and tricarboxylic acid cycle pathway through metabolomics analysis in liver. Mitochondrial complexes activities were raised, fumaric acid was reduced and oxidative stress was alleviated in High group. We conclude that consuming long-term high-fat diet with same calories but high ω-3/ω-6 PUFAs ratio relieves metabolic syndrome by regulating mTORC1 pathway to enhance mitochondrial function.
    Keywords:  Metabolic syndrome; Mitochondrial function; TCA cycle; mTORC1; ω-3 PUFAs
  29. Methods Cell Biol. 2020 ;pii: S0091-679X(19)30156-6. [Epub ahead of print]155 33-44
    Montesinos J, Area-Gomez E.
      Organelles within cells are interconnected by physical associations or contact sites. In the last decade, many reports have shown that these interactions are functional domains that maintain cellular homeostasis. One of the best studied interactions is between endoplasmic reticulum (ER) and mitochondria via mitochondria-associated membranes or MAMs. MAMs are lipid rafts in the ER in close apposition to mitochondria, where multiple enzymatic activities converge to coordinately regulate cellular functions such as: the import of phosphatidylserine into mitochondria from the ER for decarboxylation to phosphatidylethanolamine, cholesterol esterification, calcium signaling, mitochondrial shape and motility, autophagy and apoptosis. In this chapter, we describe and discuss some of the methods to isolate and assay this interesting cellular region.
    Keywords:  Endoplasmic reticulum; Lipid raft; MAM; Mitochondria; Mitochondria-associated ER membranes; Organelle contact sites
  30. Nutrients. 2020 Mar 14. pii: E770. [Epub ahead of print]12(3):
    Takahashi K, Kitaoka Y, Yamamoto K, Matsunaga Y, Hatta H.
      We tested the hypothesis that oral lactate supplementation increases mitochondrial enzyme activity given the potential role of lactate for inducing mitochondrial biogenesis. In this study, mice were assigned to a saline-ingested sedentary group (S+S; n = 8), a lactate-ingested sedentary group (L+S; n = 9), a saline-ingested training group (S+T; n = 8), and a lactate-ingested training group (L+T; n = 8). Mice in the S+S and S+T groups received saline, whereas mice in the L+S and L+T groups received sodium lactate (equivalent to 5 g/kg of body weight) via oral gavage 5 days a week for 4 weeks. At 30 min after the ingestion, mice in the S+T and L+T groups performed endurance training (treadmill running, 20 m/min, 30 min, 5 days/week). At 30 min after lactate ingestion, the blood lactate level reached peak value (5.8 ± 0.4 mmol/L) in the L+S group. Immediately after the exercise, blood lactate level was significantly higher in the L+T group (9.3 ± 0.9 mmol/L) than in the S+T group (2.7 ± 0.3 mmol/L) (p < 0.01). Following a 4-week training period, a main effect of endurance training was observed in maximal citrate synthase (CS) (p < 0.01; S+T: 117 ± 3% relative to S+S, L+T: 110 ± 3%) and cytochrome c oxidase (COX) activities (p < 0.01; S+T: 126 ± 4%, L+T: 121 ± 4%) in the plantaris muscle. Similarly, there was a main effect of endurance training in maximal CS (p < 0.01; S+T: 105 ± 3%, L+T: 115 ± 2%) and COX activities (p < 0.01; S+T: 113 ± 3%, L+T: 122 ± 3%) in the soleus muscle. In addition, a main effect of oral lactate ingestion was found in maximal COX activity in the soleus (p < 0.05; L+S: 109 ± 3%, L+T: 122 ± 3%) and heart muscles (p < 0.05; L+S: 107 ± 3%, L+T: 107 ± 2.0%), but not in the plantaris muscle. Our results suggest that lactate supplementation may be beneficial for increasing mitochondrial enzyme activity in oxidative phenotype muscle.
    Keywords:  lactate; mitochondria; monocarboxylate transporter; skeletal muscle
  31. Geriatr Gerontol Int. 2020 Mar 17.
    Esain I, Rodriguez-Larrad A, Bidaurrazaga-Letona I, Gil SM.
      AIM: To evaluate the effects of 3 months of exercise cessation in physically active older individuals on inflammatory biomarkers and adiponectin, and examine any association modifications in physical functioning.METHODS: We evaluated the effects of exercise cessation in 49 physically active older adults on inflammatory biomarkers, adiponectin and physical functioning. Participants (38 women, 11 men) were aged >65 years old and had attended a 9-month supervised exercise program. After 3 months of exercise cessation, we measured anthropometry, physical activity, strength, balance, endurance exercise, cholesterol, triglycerides, C-reactive protein, glucose, glycosylated hemoglobin A1c, interleukin-6 and adiponectin levels.
    RESULTS: Adiponectin was positively correlated with high-density lipoprotein and negatively correlated with triglyceride levels after exercise cessation. Higher adiponectin levels were correlated with worse dynamic balance, endurance exercise and lower limb strength (men only), which might be explained by the adiponectin resistance hypothesis.
    CONCLUSIONS: Adiponectin therefore might be a promising biomarker of physical dysfunction in older adults, although further research is necessary to understand the mechanisms involved in its double-sided effects. Geriatr Gerontol Int 2020; ••: ••-••.
    Keywords:  biomarkers; exercise cessation; functional fitness; inflammation; older adults
  32. Antioxid Redox Signal. 2020 Mar 15.
    Louzada RAN, Bouviere J, Matta LP, Werneck de Castro JPS, Dupuy C, de Carvalho DP, Fortunato RS.
      <b>Significance:</b> Exercise-induced ROS production activates multiple intracellular signaling pathways through genomic and non-genomic mechanisms that are responsible for the beneficial effects of exercise in muscle. Beyond the positive effect of exercise on skeletal muscle cells, other tissues such as white and brown adipose, liver, central nervous system, endothelial, heart and endocrine organ tissues are also responsive to exercise. <b>Recent Advances:</b> Crosstalk between different cells is essential to achieve homeostasis and to promote the benefits of exercise through paracrine or endocrine signaling. This crosstalk can be mediated by different effectors that include the secretion of metabolites of muscle contraction, myokines and exosomes. During the past 20 years, it has been demonstrated that contracting muscle cells produce and secrete different classes of myokines, which functionally link muscle with nearly all other cell types. <b>Critical Issues:</b> The redox signaling behind this exercise-induced crosstalk is now being decoded. Many of these widespread beneficial effects of exercise require not only a complex ROS-dependent intramuscular signaling cascade, but simultaneously, an integrated network with many remote tissues. <b>Future Directions:</b> Strong evidence suggests that the powerful beneficial effect of regular physical activity for preventing (or treating) a large range of disorders might also rely on ROS-mediated signaling. Within a contracting muscle, ROS signalling may control exosomes and myokines secretion. In remote tissues, exercise generates regular and synchronized ROS waves, creating a transient pro-oxidative environment in many cells. These new concepts integrate exercise, ROS-mediated signalling and the widespread health benefits of exercise.
  33. Endocr Rev. 2020 Mar 16. pii: bnaa005. [Epub ahead of print]
    Diaz-Vegas A, Sanchez-Aguilera P, Krycer JR, Morales PE, Monsalves-Alvarez M, Cifuentes M, Rothermel BA, Lavandero S.
      Mitochondrial damage is implicated as a major contributing factor for a number of noncommunicable chronic diseases such as cardiovascular diseases, cancer, obesity, and insulin resistance/type 2 diabetes. Here, we discuss the role of mitochondria in maintaining cellular and whole-organism homeostasis, the mechanisms that promote mitochondrial dysfunction and the role of this phenomenon in noncommunicable chronic diseases. We also review the state of the art regarding the pre-clinical evidence associated with the regulation of the mitochondrial function and the development of current mitochondrial-targeted therapeutics to treat non communicable chronic diseases. Finally; we give an integrated vision of how the mitochondrial damage is implicated in these metabolic diseases.
    Keywords:  Mitochondria; cancer; cardiovascular diseases; insulin resistance; obesity
  34. Aging Cell. 2020 Mar 18. e13134
    Fan G, Kaßmann M, Cui Y, Matthaeus C, Kunz S, Zhong C, Zhu S, Xie Y, Tsvetkov D, Daumke O, Huang Y, Gollasch M.
      Caveolae position CaV 3.2 (T-type Ca2+ channel encoded by the α-3.2 subunit) sufficiently close to RyR (ryanodine receptors) for extracellular Ca2+ influx to trigger Ca2+ sparks and large-conductance Ca2+ -activated K+ channel feedback in vascular smooth muscle. We hypothesize that this mechanism of Ca2+ spark generation is affected by age. Using smooth muscle cells (VSMCs) from mouse mesenteric arteries, we found that both Cav 3.2 channel inhibition by Ni2+ (50 µM) and caveolae disruption by methyl-ß-cyclodextrin or genetic abolition of Eps15 homology domain-containing protein (EHD2) inhibited Ca2+ sparks in cells from young (4 months) but not old (12 months) mice. In accordance, expression of Cav 3.2 channel was higher in mesenteric arteries from young than old mice. Similar effects were observed for caveolae density. Using SMAKO Cav 1.2-/- mice, caffeine (RyR activator) and thapsigargin (Ca2+ transport ATPase inhibitor), we found that sufficient SR Ca2+ load is a prerequisite for the CaV 3.2-RyR axis to generate Ca2+ sparks. We identified a fraction of Ca2+ sparks in aged VSMCs, which is sensitive to the TRP channel blocker Gd3+ (100 µM), but insensitive to CaV 1.2 and CaV 3.2 channel blockade. Our data demonstrate that the VSMC CaV 3.2-RyR axis is down-regulated by aging. This defective CaV 3.2-RyR coupling is counterbalanced by a Gd3+ sensitive Ca2+ pathway providing compensatory Ca2+ influx for triggering Ca2+ sparks in aged VSMCs.
    Keywords:  T-type calcium channels; aging; calcium sparks; caveolae; ryanodine receptors; vascular smooth muscle
  35. EMBO J. 2020 Mar 18. e103111
    Guo QQ, Wang SS, Zhang SS, Xu HD, Li XM, Guan Y, Yi F, Zhou TT, Jiang B, Bai N, Ma MT, Wang Z, Feng YL, Guo WD, Wu X, Zhao GF, Fan GJ, Zhang SP, Wang CG, Cao LY, O'Rourke BP, Liu SH, Wang PY, Han S, Song XY, Cao L.
      The homeostatic link between oxidative stress and autophagy plays an important role in cellular responses to a wide variety of physiological and pathological conditions. However, the regulatory pathway and outcomes remain incompletely understood. Here, we show that reactive oxygen species (ROS) function as signaling molecules that regulate autophagy through ataxia-telangiectasia mutated (ATM) and cell cycle checkpoint kinase 2 (CHK2), a DNA damage response (DDR) pathway activated during metabolic and hypoxic stress. We report that CHK2 binds to and phosphorylates Beclin 1 at Ser90/Ser93, thereby impairing Beclin 1-Bcl-2 autophagy-regulatory complex formation in a ROS-dependent fashion. We further demonstrate that CHK2-mediated autophagy has an unexpected role in reducing ROS levels via the removal of damaged mitochondria, which is required for cell survival under stress conditions. Finally, CHK2-/- mice display aggravated infarct phenotypes and reduced Beclin 1 p-Ser90/Ser93 in a cerebral stroke model, suggesting an in vivo role of CHK2-induced autophagy in cell survival. Taken together, these results indicate that the ROS-ATM-CHK2-Beclin 1-autophagy axis serves as a physiological adaptation pathway that protects cells exposed to pathological conditions from stress-induced tissue damage.
    Keywords:   ROS ; Beclin 1; CHK2; autophagy; oxidative stress
  36. Cells. 2020 Mar 14. pii: E712. [Epub ahead of print]9(3):
    Eldarov CM, Vangely IM, Vays VB, Sheval EV, Holtze S, Hildebrandt TB, Kolosova NG, Popkov VA, Plotnikov EY, Zorov DB, Bakeeva LE, Skulachev VP.
      Electron microscopic study of cardiomyocytes taken from healthy Wistar and OXYS rats and naked mole rats (Heterocephalus glaber) revealed mitochondria in nuclei that lacked part of the nuclear envelope. The direct interaction of mitochondria with nucleoplasm is shown. The statistical analysis of the occurrence of mitochondria in cardiomyocyte nuclei showed that the percentage of nuclei with mitochondria was roughly around 1%, and did not show age and species dependency. Confocal microscopy of normal rat cardiac myocytes revealed a branched mitochondrial network in the vicinity of nuclei with an organization different than that of interfibrillar mitochondria. This mitochondrial network was energetically functional because it carried the membrane potential that responded by oscillatory mode after photodynamic challenge. We suggest that the presence of functional mitochondria in the nucleus is not only a consequence of certain pathologies but rather represents a normal biological phenomenon involved in mitochondrial/nuclear interactions.
    Keywords:  electron and confocal microscopy; healthy cells; heart; intranuclear mitochondria
  37. Diabetologia. 2020 Mar 17.
    Bilet L, Phielix E, van de Weijer T, Gemmink A, Bosma M, Moonen-Kornips E, Jorgensen JA, Schaart G, Zhang D, Meijer K, Hopman M, Hesselink MKC, Ouwens DM, Shulman GI, Schrauwen-Hinderling VB, Schrauwen P.
      AIMS/HYPOTHESIS: Physical inactivity, low mitochondrial function, increased intramyocellular lipid (IMCL) deposition and reduced insulin sensitivity are common denominators of chronic metabolic disorders, like obesity and type 2 diabetes. Yet, whether low mitochondrial function predisposes to insulin resistance in humans is still unknown.METHODS: Here we investigated, in an intervention study, whether muscle with low mitochondrial oxidative capacity, induced by one-legged physical inactivity, would feature stronger signs of lipid-induced insulin resistance. To this end, ten male participants (age 22.4 ± 4.2 years, BMI 21.3 ± 2.0 kg/m2) underwent a 12 day unilateral lower-limb suspension with the contralateral leg serving as an active internal control.
    RESULTS: In vivo, mitochondrial oxidative capacity, assessed by phosphocreatine (PCr)-recovery half-time, was lower in the inactive vs active leg. Ex vivo, palmitate oxidation to 14CO2 was lower in the suspended leg vs the active leg; however, this did not result in significantly higher [14C]palmitate incorporation into triacylglycerol. The reduced mitochondrial function in the suspended leg was, however, paralleled by augmented IMCL content in both musculus tibialis anterior and musculus vastus lateralis, and by increased membrane bound protein kinase C (PKC) θ. Finally, upon lipid infusion, insulin signalling was lower in the suspended vs active leg.
    CONCLUSIONS/INTERPRETATION: Together, these results demonstrate, in a unique human in vivo model, that a low mitochondrial oxidative capacity due to physical inactivity directly impacts IMCL accumulation and PKCθ translocation, resulting in impaired insulin signalling upon lipid infusion. This demonstrates the importance of mitochondrial oxidative capacity and muscle fat accumulation in the development of insulin resistance in humans.
    FUNDING: PS was supported by a 'VICI' Research Grant for innovative research from the Netherlands Organization for Scientific Research (Grant 918.96.618).
    Keywords:  Fat oxidation; Insulin resistance; Intramyocellular lipid content; Mitochondrial function; Mitochondrial oxidative capacity; Physical inactivity; Unilateral lower-limb suspension
  38. mBio. 2020 Mar 17. pii: e00268-20. [Epub ahead of print]11(2):
    Huang G, Docampo R.
      Mitochondrial Ca2+ transport mediated by the uniporter complex (MCUC) plays a key role in the regulation of cell bioenergetics in both trypanosomes and mammals. Here we report that Trypanosoma brucei MCU (TbMCU) subunits interact with subunit c of the mitochondrial ATP synthase (ATPc), as determined by coimmunoprecipitation and split-ubiquitin membrane-based yeast two-hybrid (MYTH) assays. Mutagenesis analysis in combination with MYTH assays suggested that transmembrane helices (TMHs) are determinants of this specific interaction. In situ tagging, followed by immunoprecipitation and immunofluorescence microscopy, revealed that T. brucei ATPc (TbATPc) coimmunoprecipitates with TbMCUC subunits and colocalizes with them to the mitochondria. Blue native PAGE and immunodetection analyses indicated that the TbMCUC is present together with the ATP synthase in a large protein complex with a molecular weight of approximately 900 kDa. Ablation of the TbMCUC subunits by RNA interference (RNAi) significantly increased the AMP/ATP ratio, revealing the downregulation of ATP production in the cells. Interestingly, the direct physical MCU-ATPc interaction is conserved in Trypanosoma cruzi and human cells. Specific interaction between human MCU (HsMCU) and human ATPc (HsATPc) was confirmed in vitro by mutagenesis and MYTH assays and in vivo by coimmunoprecipitation. In summary, our study has identified that MCU complex physically interacts with mitochondrial ATP synthase, possibly forming an MCUC-ATP megacomplex that couples ADP and Pi transport with ATP synthesis, a process that is stimulated by Ca2+ in trypanosomes and human cells.IMPORTANCE The mitochondrial calcium uniporter (MCU) is essential for the regulation of oxidative phosphorylation in mammalian cells, and we have shown that in Trypanosoma brucei, the etiologic agent of sleeping sickness, this channel is essential for its survival and infectivity. Here we reveal that that Trypanosoma brucei MCU subunits interact with subunit c of the mitochondrial ATP synthase (ATPc). Interestingly, the direct physical MCU-ATPc interaction is conserved in T. cruzi and human cells.
    Keywords:  ATP synthase; Trypanosoma ; c ring; mitochondrial calcium uniporter
  39. Nat Commun. 2020 Mar 17. 11(1): 1416
    Mutvei AP, Nagiec MJ, Hamann JC, Kim SG, Vincent CT, Blenis J.
      The kinase mTOR complex 1 (mTORC1) promotes cellular growth and is frequently dysregulated in cancers. In response to nutrients, mTORC1 is activated on lysosomes by Rag and Rheb guanosine triphosphatases (GTPases) and drives biosynthetic processes. How limitations in nutrients suppress mTORC1 activity remains poorly understood. We find that when amino acids are limited, the Rap1-GTPases confine lysosomes to the perinuclear region and reduce lysosome abundance, which suppresses mTORC1 signaling. Rap1 activation, which is independent of known amino acid signaling factors, limits the lysosomal surface available for mTORC1 activation. Conversely, Rap1 depletion expands the lysosome population, which markedly increases association between mTORC1 and its lysosome-borne activators, leading to mTORC1 hyperactivity. Taken together, we establish Rap1 as a critical coordinator of the lysosomal system, and propose that aberrant changes in lysosomal surface availability can impact mTORC1 signaling output.
  40. Int J Mol Med. 2020 Mar 12.
    Li J, Lu Y, Li N, Li P, Su J, Wang Z, Wang T, Yang Z, Yang Y, Chen H, Xiao L, Duan H, Wu W, Liu X.
      Decreased diaphragm function is a crucial factor leading to reduced ventilatory efficiency and worsening of quality of life in chronic obstructive pulmonary disease (COPD). Exercise training has been demonstrated to effectively improve the function of the diaphragm. However, the mechanism of this process has not been identified. The emergence of metabolomics has allowed the exploration of new ideas. The present study aimed to analyze the potential biomarkers of exercise‑dependent enhancement of diaphragm function in COPD using metabolomics. Sprague Dawley rats were divided into three groups: COPD + exercise group (CEG); COPD model group (CMG); and control group (CG). The first two groups were exposed to cigarette smoke for 16 weeks to establish a COPD model. Then, the rats in the CEG underwent aerobic exercise training for 9 weeks. Following confirmation that exercise effectively improved the diaphragm function, a gas chromatography tandem time‑of‑flight mass spectrometry analysis system was used to detect the differential metabolites and associated pathways in the diaphragm muscles of the different groups. Following exercise intervention, the pulmonary function and diaphragm contractility of the CEG rats were significantly improved compared with those of the CMG rats. A total of 36 different metabolites were identified in the comparison between the CMG and the CG. Pathway enrichment analysis indicated that these different metabolites were involved in 17 pathways. A total of 29 different metabolites were identified in the comparison between the CMG and the CEG, which are involved in 14 pathways. Candidate biomarkers were selected, and the pathways analysis of these metabolites demonstrated that 2 types of metabolic pathways, the nicotinic acid and nicotinamide metabolism and arginine and proline metabolism pathways, were associated with exercise‑induced pulmonary rehabilitation.