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Péladeau C, Ahmed A, Amirouche A, Crawford Parks TE, Bronicki LM, Ljubicic V, Renaud JM, Jasmin BJ. Combinatorial therapeutic activation with heparin and AICAR stimulates additive effects on utrophin A expression in dystrophic muscles. Hum Mol Genet 2015; 25:24-43. [PMID: 26494902 DOI: 10.1093/hmg/ddv444] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2015] [Accepted: 10/19/2015] [Indexed: 01/13/2023] Open
Abstract
Upregulation of utrophin A is an attractive therapeutic strategy for treating Duchenne muscular dystrophy (DMD). Over the years, several studies revealed that utrophin A is regulated by multiple transcriptional and post-transcriptional mechanisms, and that pharmacological modulation of these pathways stimulates utrophin A expression in dystrophic muscle. In particular, we recently showed that activation of p38 signaling causes an increase in the levels of utrophin A mRNAs and protein by decreasing the functional availability of the destabilizing RNA-binding protein called K-homology splicing regulatory protein, thereby resulting in increases in the stability of existing mRNAs. Here, we treated 6-week-old mdx mice for 4 weeks with the clinically used anticoagulant drug heparin known to activate p38 mitogen-activated protein kinase, and determined the impact of this pharmacological intervention on the dystrophic phenotype. Our results show that heparin treatment of mdx mice caused a significant ∼1.5- to 3-fold increase in utrophin A expression in diaphragm, extensor digitorum longus and tibialis anterior (TA) muscles. In agreement with these findings, heparin-treated diaphragm and TA muscle fibers showed an accumulation of utrophin A and β-dystroglycan along their sarcolemma and displayed improved morphology and structural integrity. Moreover, combinatorial drug treatment using both heparin and 5-amino-4-imidazolecarboxamide riboside (AICAR), the latter targeting 5' adenosine monophosphate-activated protein kinase and the transcriptional activation of utrophin A, caused an additive effect on utrophin A expression in dystrophic muscle. These findings establish that heparin is a relevant therapeutic agent for treating DMD, and illustrate that combinatorial treatment of heparin with AICAR may serve as an effective strategy to further increase utrophin A expression in dystrophic muscle via activation of distinct signaling pathways.
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Affiliation(s)
- Christine Péladeau
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
| | - Aatika Ahmed
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
| | - Adel Amirouche
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
| | - Tara E Crawford Parks
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
| | - Lucas M Bronicki
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
| | - Vladimir Ljubicic
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
| | - Jean-Marc Renaud
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
| | - Bernard J Jasmin
- Department of Cellular and Molecular Medicine and Centre for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
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RNA sequencing reveals a slow to fast muscle fiber type transition after olanzapine infusion in rats. PLoS One 2015; 10:e0123966. [PMID: 25893406 PMCID: PMC4404103 DOI: 10.1371/journal.pone.0123966] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2014] [Accepted: 03/02/2015] [Indexed: 11/19/2022] Open
Abstract
Second generation antipsychotics (SGAs), like olanzapine, exhibit acute metabolic side effects leading to metabolic inflexibility, hyperglycemia, adiposity and diabetes. Understanding how SGAs affect the skeletal muscle transcriptome could elucidate approaches for mitigating these side effects. Male Sprague-Dawley rats were infused intravenously with vehicle or olanzapine for 24h using a dose leading to a mild hyperglycemia. RNA-Seq was performed on gastrocnemius muscle, followed by alignment of the data with the Rat Genome Assembly 5.0. Olanzapine altered expression of 1347 out of 26407 genes. Genes encoding skeletal muscle fiber-type specific sarcomeric, ion channel, glycolytic, O2- and Ca2+-handling, TCA cycle, vascularization and lipid oxidation proteins and pathways, along with NADH shuttles and LDH isoforms were affected. Bioinformatics analyses indicate that olanzapine decreased the expression of slower and more oxidative fiber type genes (e.g., type 1), while up regulating those for the most glycolytic and least metabolically flexible, fast twitch fiber type, IIb. Protein turnover genes, necessary to bring about transition, were also up regulated. Potential upstream regulators were also identified. Olanzapine appears to be rapidly affecting the muscle transcriptome to bring about a change to a fast-glycolytic fiber type. Such fiber types are more susceptible than slow muscle to atrophy, and such transitions are observed in chronic metabolic diseases. Thus these effects could contribute to the altered body composition and metabolic disease olanzapine causes. A potential interventional strategy is implicated because aerobic exercise, in contrast to resistance exercise, can oppose such slow to fast fiber transitions.
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Brandauer J, Andersen MA, Kellezi H, Risis S, Frøsig C, Vienberg SG, Treebak JT. AMP-activated protein kinase controls exercise training- and AICAR-induced increases in SIRT3 and MnSOD. Front Physiol 2015; 6:85. [PMID: 25852572 PMCID: PMC4371692 DOI: 10.3389/fphys.2015.00085] [Citation(s) in RCA: 60] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2014] [Accepted: 03/04/2015] [Indexed: 12/13/2022] Open
Abstract
The mitochondrial protein deacetylase sirtuin (SIRT) 3 may mediate exercise training-induced increases in mitochondrial biogenesis and improvements in reactive oxygen species (ROS) handling. We determined the requirement of AMP-activated protein kinase (AMPK) for exercise training-induced increases in skeletal muscle abundance of SIRT3 and other mitochondrial proteins. Exercise training for 6.5 weeks increased SIRT3 (p < 0.01) and superoxide dismutase 2 (MnSOD; p < 0.05) protein abundance in quadriceps muscle of wild-type (WT; n = 13–15), but not AMPK α2 kinase dead (KD; n = 12–13) mice. We also observed a strong trend for increased MnSOD abundance in exercise-trained skeletal muscle of healthy humans (p = 0.051; n = 6). To further elucidate a role for AMPK in mediating these effects, we treated WT (n = 7–8) and AMPK α2 KD (n = 7–9) mice with 5-amino-1-β-D-ribofuranosyl-imidazole-4-carboxamide (AICAR). Four weeks of daily AICAR injections (500 mg/kg) resulted in AMPK-dependent increases in SIRT3 (p < 0.05) and MnSOD (p < 0.01) in WT, but not AMPK α2 KD mice. We also tested the effect of repeated AICAR treatment on mitochondrial protein levels in mice lacking the transcriptional coactivator peroxisome proliferator-activated receptor γ-coactivator 1α (PGC-1α KO; n = 9–10). Skeletal muscle SIRT3 and MnSOD protein abundance was reduced in sedentary PGC-1α KO mice (p < 0.01) and AICAR-induced increases in SIRT3 and MnSOD protein abundance was only observed in WT mice (p < 0.05). Finally, the acetylation status of SIRT3 target lysine residues on MnSOD (K122) or oligomycin-sensitivity conferring protein (OSCP; K139) was not altered in either mouse or human skeletal muscle in response to acute exercise. We propose an important role for AMPK in regulating mitochondrial function and ROS handling in skeletal muscle in response to exercise training.
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Affiliation(s)
- Josef Brandauer
- Section of Integrative Physiology, The Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen Copenhagen, Denmark ; Department of Health Sciences, Gettysburg College Gettysburg, PA, USA
| | - Marianne A Andersen
- Section of Integrative Physiology, The Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen Copenhagen, Denmark
| | - Holti Kellezi
- Section of Integrative Physiology, The Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen Copenhagen, Denmark
| | - Steve Risis
- Section of Integrative Physiology, The Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen Copenhagen, Denmark
| | - Christian Frøsig
- Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, The August Krogh Centre, University of Copenhagen Copenhagen, Denmark
| | - Sara G Vienberg
- Section of Integrative Physiology, The Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen Copenhagen, Denmark
| | - Jonas T Treebak
- Section of Integrative Physiology, The Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen Copenhagen, Denmark
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Mizunoya W, Iwamoto Y, Sato Y, Tatsumi R, Ikeuchi Y. Cold exposure increases slow-type myosin heavy chain 1 (MyHC1) composition of soleus muscle in rats. Anim Sci J 2013; 85:293-304. [PMID: 24206444 DOI: 10.1111/asj.12143] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2013] [Accepted: 08/05/2013] [Indexed: 01/09/2023]
Abstract
The aim of this study was to examine the effects of cold exposure on rat skeletal muscle fiber type, according to myosin heavy chain (MyHC) isoform and metabolism-related factors. Male Wistar rats (7 weeks old) were housed individually at 4 ± 2°C as a cold-exposed group or at room temperature (22 ± 2°C) as a control group for 4 weeks. We found that cold exposure significantly increased the slow-type MyHC1 content in the soleus muscle (a typical slow-type fiber), while the intermediate-type MyHC2A content was significantly decreased. In contrast to soleus, MyHC composition of extensor digitorum longus (EDL, a typical fast-type fiber) and gastrocnemius (a mix of slow-type and fast-type fibers) muscle did not change from cold exposure. Cold exposure increased mRNA expression of mitochondrial uncoupling protein 3 (UCP3) in both the soleus and EDL. Cold exposure also increased mRNA expression of myoglobin, peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α) and forkhead box O1 (FOXO1) in the soleus. Upregulation of UCP3 and PGC1α proteins were observed with Western blotting in the gastrocnemius. Thus, cold exposure increased metabolism-related factors in all muscle types that were tested, but MyHC isoforms changed only in the soleus.
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Affiliation(s)
- Wataru Mizunoya
- Department of Bioresource Sciences, Graduate School of Agriculture, Kyushu University, Fukuoka, Japan
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Kristensen JM, Larsen S, Helge JW, Dela F, Wojtaszewski JFP. Two weeks of metformin treatment enhances mitochondrial respiration in skeletal muscle of AMPK kinase dead but not wild type mice. PLoS One 2013; 8:e53533. [PMID: 23341947 PMCID: PMC3544921 DOI: 10.1371/journal.pone.0053533] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2012] [Accepted: 12/03/2012] [Indexed: 12/25/2022] Open
Abstract
Metformin is used as an anti-diabetic drug. Metformin ameliorates insulin resistance by improving insulin sensitivity in liver and skeletal muscle. Reduced mitochondrial content has been reported in type 2 diabetic muscles and it may contribute to decreased insulin sensitivity characteristic for diabetic muscles. The molecular mechanism behind the effect of metformin is not fully clarified but inhibition of complex I in the mitochondria and also activation of the 5'AMP activated protein kinase (AMPK) has been reported in muscle. Furthermore, both AMPK activation and metformin treatment have been associated with stimulation of mitochondrial function and biogenesis. However, a causal relationship in skeletal muscle has not been investigated. We hypothesized that potential effects of in vivo metformin treatment on mitochondrial function and protein expressions in skeletal muscle are dependent upon AMPK signaling. We investigated this by two weeks of oral metformin treatment of muscle specific kinase dead α(2) (KD) AMPK mice and wild type (WT) littermates. We measured mitochondrial respiration and protein activity and expressions of key enzymes involved in mitochondrial carbohydrate and fat metabolism and oxidative phosphorylation. Mitochondrial respiration, HAD and CS activity, PDH and complex I-V and cytochrome c protein expression were all reduced in AMPK KD compared to WT tibialis anterior muscles. Surprisingly, metformin treatment only enhanced respiration in AMPK KD mice and thereby rescued the respiration defect compared to the WT mice. Metformin did not influence protein activities or expressions in either WT or AMPK KD mice.We conclude that two weeks of in vivo metformin treatment enhances mitochondrial respiration in the mitochondrial deficient AMPK KD but not WT mice. The improvement seems to be unrelated to AMPK, and does not involve changes in key mitochondrial proteins.
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Affiliation(s)
- Jonas M Kristensen
- Section of Molecular Physiology Group, August Krogh Centre, Department of Nutrition, Exercise and Sport Sciences, University of Copenhagen, Denmark.
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Park S, Scheffler TL, Gerrard DE. Chronic high cytosolic calcium decreases AICAR-induced AMPK activity via calcium/calmodulin activated protein kinase II signaling cascade. Cell Calcium 2011; 50:73-83. [PMID: 21641034 DOI: 10.1016/j.ceca.2011.05.009] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2011] [Revised: 05/06/2011] [Accepted: 05/07/2011] [Indexed: 11/19/2022]
Abstract
Calcium is important for muscle contraction and controls many cellular processes. Although there is evidence that calcium-mediated signals regulate AMP-activated protein kinase (AMPK) activity, the molecular mechanisms by which calcium regulates AMPK are poorly understood. To compare the function of sustained vs. intermittent calcium oscillations on AMPK activity and define specific signals in this pathway, we administered mice with aminoimidazole-carboxamide-ribonucleotide (AICAR) and caffeine with or without dantrolene. AMPK activity was increased by 10 d AICAR treatment (P < 0.01). Ten day caffeine treatment decreased AICAR-induced AMPK activity to control level. This repressed AMPK activity was blocked by dantrolene. Different calcium frequencies were simulated in C2C12 myotubes by alternating media containing caffeine and dantrolene. Intermittent calcium oscillation increased AMPK activity compared to control (P < 0.05), whereas sustained calcium oscillation decreases AICAR-induced AMPK activity to control level. This result suggests a biphasic control of AMPK activity by calcium. Knockdown of CaMKII expression by short-hairpin RNA resulted in increased AMPK phosphorylation by AICAR even in the presence of caffeine. These data show different calcium oscillations elicit distinct responses in muscle cells suggesting that the negative effects of chronic calcium treatment on AMPK activity is partly mediated through the CaMKII signals.
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Affiliation(s)
- S Park
- Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, VA 24061, USA
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Ljubicic V, Khogali S, Renaud JM, Jasmin BJ. Chronic AMPK stimulation attenuates adaptive signaling in dystrophic skeletal muscle. Am J Physiol Cell Physiol 2011; 302:C110-21. [PMID: 21940670 DOI: 10.1152/ajpcell.00183.2011] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
In the present study, we evaluated how a pharmacologically induced phenotype shift in dystrophic skeletal muscle would affect subsequent intracellular signaling in response to a complementary, adaptive physiological stimulus. mdx mice were treated with the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR; 500 mg·kg(-1)·day(-1)) for 30 days, and then one-half of the animals were subjected to a bout of treadmill running to induce acute AMPK and p38 MAPK signaling. The mRNA levels of phenotypic modifiers, including peroxisome proliferator-activated receptor-δ (PPARδ), PPARγ coactivator-1α (PGC-1α), receptor interacting protein 140 (RIP 140), and silent information regulator two ortholog 1 (SIRT1) were assessed in skeletal muscle, as well as the expression of the protein arginine methyltransferase genes PRMT1 and CARM1. We found unique AMPK and p38 phosphorylation and expression signatures between dystrophic and healthy muscle. In dystrophic skeletal muscle, treadmill running induced PPARδ, PGC-1α, and SIRT1 mRNAs, three molecules that promote the slow, oxidative myogenic program. In the mdx animals that received the chronic AICAR treatment, running-elicited AMPK and p38 phosphorylation was attenuated compared with vehicle-treated mice. Similarly, acute stress-evoked expression of PPARδ, PGC-1α, and SIRT1 was also blunted by chronic pharmacological AMPK stimulation. Skeletal muscle PRMT1 and CARM1 protein contents were higher in mdx mice compared with wild-type littermates. The acute running-evoked induction of PRMT1 and CARM1 mRNAs was also attenuated by the AICAR treatment. Our data demonstrate that prior pharmacological conditioning is a salient determinant in how dystrophic muscle adapts to subsequent complementary, acute physiological stress stimuli. These results provide insight into possible therapeutic applications of synthetic agonists in neuromuscular diseases, such as during chronic administration to Duchenne muscular dystrophy patients.
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Affiliation(s)
- Vladimir Ljubicic
- Department of Cellular and Molecular Medicine, Faculty of Medicine, and Center for Neuromuscular Disease, University of Ottawa, Ottawa, Ontario, Canada.
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Ljubicic V, Miura P, Burt M, Boudreault L, Khogali S, Lunde JA, Renaud JM, Jasmin BJ. Chronic AMPK activation evokes the slow, oxidative myogenic program and triggers beneficial adaptations in mdx mouse skeletal muscle. Hum Mol Genet 2011; 20:3478-93. [PMID: 21659335 DOI: 10.1093/hmg/ddr265] [Citation(s) in RCA: 130] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
A therapeutic approach for Duchenne muscular dystrophy (DMD) is to up-regulate utrophin in skeletal muscle in an effort to compensate for the lack of dystrophin. We previously hypothesized that promotion of the slow, oxidative myogenic program, which triggers utrophin up-regulation, can attenuate the dystrophic pathology in mdx animals. Since treatment of healthy mice with the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) enhances oxidative capacity and elicits a fast-to-slow fiber-type transition, we evaluated the effects of chronic AMPK stimulation on skeletal muscle phenotype and utrophin expression in mdx mice. Daily AICAR administration (500 mg/kg/day, 30 days) of 5-7-week-old mdx animals induced an elevation in mitochondrial cytochrome c oxidase enzyme activity, an increase in myosin heavy-chain type IIa-positive fibers and slower twitch contraction kinetics in the fast, glycolytic extensor digitorum longus muscle. Utrophin expression was significantly enhanced in response to AICAR, which occurred coincident with an elevated β-dystroglycan expression along the sarcolemma. These adaptations were associated with an increase in sarcolemmal structural integrity under basal conditions, as well as during damaging eccentric contractions ex vivo. Notably, peroxisome proliferator-activated receptor γ co-activator-1α (PGC-1α) and silent information regulator two ortholog 1 protein contents were significantly higher in muscle from mdx mice compared with wild-type littermates and AICAR further increased PGC-1α expression. Our data show that AICAR-evoked muscle plasticity results in beneficial phenotypic adaptations in mdx mice and suggest that the contextually novel application of this compound for muscular dystrophy warrants further study.
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Affiliation(s)
- Vladimir Ljubicic
- Department of Cellular and Molecular Medicine, Faculty of Medicine, Centre for Neuromuscular Disease, University of Ottawa, Ottawa, Ontario, Canada K1H 8M5
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Brown JD, Hancock CR, Mongillo AD, Benjamin Barton J, DiGiovanni RA, Parcell AC, Winder WW, Thomson DM. Effect of LKB1 deficiency on mitochondrial content, fibre type and muscle performance in the mouse diaphragm. Acta Physiol (Oxf) 2011; 201:457-66. [PMID: 21073663 DOI: 10.1111/j.1748-1716.2010.02226.x] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
AIM The liver kinase B1 (LKB1)/AMP-activated protein kinase (AMPK) signalling pathway is a major regulator of skeletal muscle metabolic processes. During exercise, LKB1-mediated phosphorylation of AMPK leads to its activation, promoting mitochondrial biogenesis and glucose transport, among other effects. The roles of LKB1 and AMPK have not been fully characterized in the diaphragm. METHODS Two methods of AMPK activation were used to characterize LKB1/AMPK signalling in diaphragms from muscle-specific LKB1 knockout (KO) and littermate control mice: (1) acute injection of 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) and (2) 5-min direct electrical stimulation of the diaphragm. Diaphragms were excised 60 min post-AICAR injection and immediately after electrical stimulation. RESULTS AMPK phosphorylation increased with AICAR and electrical stimulation in control but not KO mice. Acetyl CoA carboxylase phosphorylation increased with AICAR in control but not KO mice, but increased in both genotypes with electrical stimulation. While the majority of mitochondrial protein levels were lower in KO diaphragms, uncoupling protein 3, complex I and cytochrome oxidase IV protein levels were not different between genotypes. KO diaphragms have a lower percentage of IIx fibres and an elevated percentage of IIb fibres when compared with control diaphragms. While in vitro peak force generation was similar between genotypes, KO diaphragms fatigued more quickly and had an impaired ability to recover. CONCLUSION LKB1 regulates AMPK phosphorylation, mitochondrial protein expression, fibre type distribution, as well as recovery of the diaphragm from fatigue.
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Affiliation(s)
- J D Brown
- Department of Physiology and Developmental Biology, Brigham Young University, Provo, UT, USA
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Williams DB, Sutherland LN, Bomhof MR, Basaraba SAU, Thrush AB, Dyck DJ, Field CJ, Wright DC. Muscle-specific differences in the response of mitochondrial proteins to beta-GPA feeding: an evaluation of potential mechanisms. Am J Physiol Endocrinol Metab 2009; 296:E1400-8. [PMID: 19318515 DOI: 10.1152/ajpendo.90913.2008] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Beta-Guanadinopropionic acid (beta-GPA) feeding leads to reductions in skeletal muscle phosphagen concentrations and has been used as a tool with which to study the effects of energy charge on skeletal muscle metabolism. Supplementing standard rodent diets with beta-GPA leads to increases in mitochondrial enzyme content in fast but not slow-twitch muscles from male rats. Given this apparent discrepancy between muscle types we used beta-GPA feeding as a model to study signaling pathways involved in mitochondrial biogenesis. We hypothesized that beta-GPA feeding would result in a preferential activation of p38 MAPK and AMPK signaling and reductions in RIP140 protein content in triceps but not soleus muscle. Despite similar reductions in high-energy phosphate concentrations, 6 wk of beta-GPA feeding led to increases in mitochondrial proteins in triceps but not soleus muscles. Differences in the response of mitochondrial proteins to beta-GPA feeding did not seem to be related to a differential activation of p38 MAPK and AMPK signaling pathways or discrepancies in the induction of PPARgamma coactivator (PGC)-1alpha and -1beta. The protein content and expression of the nuclear corepressor RIP140 was reduced in triceps but not soleus muscle. Collectively our results indicate that chronic reductions in high-energy phosphates lead to the activation of p38 MAPK and AMPK signaling and increases in the expression of PGC-1alpha and -1beta in both soleus and triceps muscles. The lack of an effect of beta-GPA feeding on mitochondrial proteins in the soleus muscles could be related to a fiber type-specific effect of beta-GPA on RIP140 protein content.
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MESH Headings
- Adaptor Proteins, Signal Transducing/genetics
- Adaptor Proteins, Signal Transducing/metabolism
- Adenylate Kinase/metabolism
- Animal Feed
- Animals
- Body Weight/drug effects
- Body Weight/physiology
- DNA, Mitochondrial/genetics
- Energy Metabolism/drug effects
- Energy Metabolism/physiology
- Guanidines/pharmacology
- Male
- Mitochondrial Proteins/genetics
- Mitochondrial Proteins/metabolism
- Muscle Fibers, Fast-Twitch/drug effects
- Muscle Fibers, Fast-Twitch/enzymology
- Muscle Fibers, Slow-Twitch/drug effects
- Muscle Fibers, Slow-Twitch/enzymology
- Muscle, Skeletal/cytology
- Muscle, Skeletal/enzymology
- Nuclear Proteins/genetics
- Nuclear Proteins/metabolism
- Nuclear Receptor Interacting Protein 1
- PPAR gamma/metabolism
- Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha
- Propionates/pharmacology
- RNA, Messenger/metabolism
- RNA-Binding Proteins/genetics
- RNA-Binding Proteins/metabolism
- Rats
- Rats, Wistar
- Signal Transduction/drug effects
- Signal Transduction/physiology
- Transcription Factors/genetics
- Transcription Factors/metabolism
- p38 Mitogen-Activated Protein Kinases/metabolism
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Affiliation(s)
- Deon B Williams
- Alberta Diabetes Institute, 4126C HRIF East, Univ. of Alberta, Edmonton, Alberta, Canada T6G 2E1
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Jensen TE, Wojtaszewski JFP, Richter EA. AMP-activated protein kinase in contraction regulation of skeletal muscle metabolism: necessary and/or sufficient? Acta Physiol (Oxf) 2009; 196:155-74. [PMID: 19243572 DOI: 10.1111/j.1748-1716.2009.01979.x] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
In skeletal muscle, the contraction-activated heterotrimeric 5'-AMP-activated protein kinase (AMPK) protein is proposed to regulate the balance between anabolic and catabolic processes by increasing substrate uptake and turnover in addition to regulating the transcription of proteins involved in mitochondrial biogenesis and other aspects of promoting an oxidative muscle phenotype. Here, the current knowledge on the expression of AMPK subunits in human quadriceps muscle and evidence from rodent studies suggesting distinct AMPK subunit expression pattern in different muscle types is reviewed. Then, the intensity and time dependence of AMPK activation in human quadriceps and rodent muscle are evaluated. Subsequently, a major part of this review critically examines the evidence supporting a necessary and/or sufficient role of AMPK in a broad spectrum of skeletal muscle contraction-relevant processes. These include glucose uptake, glycogen synthesis, post-exercise insulin sensitivity, fatty acid (FA) uptake, intramuscular triacylglyceride hydrolysis, FA oxidation, suppression of protein synthesis, proteolysis, autophagy and transcriptional regulation of genes relevant to promoting an oxidative phenotype.
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Affiliation(s)
- T E Jensen
- Molecular Physiology Group, Copenhagen Muscle Research Centre, Section of Human Physiology, Department of Exercise and Sport Sciences, University of Copenhagen, Copenhagen, Denmark
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Martins KJB, Murdoch GK, Shu Y, Harris RLW, Gallo M, Dixon WT, Foxcroft GR, Gordon T, Putman CT. Satellite cell ablation attenuates short-term fast-to-slow fibre type transformations in rat fast-twitch skeletal muscle. Pflugers Arch 2009; 458:325-35. [DOI: 10.1007/s00424-008-0625-z] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2008] [Accepted: 12/03/2008] [Indexed: 10/21/2022]
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Gallo M, MacLean I, Tyreman N, Martins KJB, Syrotuik D, Gordon T, Putman CT. Adaptive responses to creatine loading and exercise in fast-twitch rat skeletal muscle. Am J Physiol Regul Integr Comp Physiol 2008; 294:R1319-28. [PMID: 18216140 DOI: 10.1152/ajpregu.00631.2007] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We investigated the effects of chronic creatine loading and voluntary running (Run) on muscle fiber types, proteins that regulate intracellular Ca2+, and the metabolic profile in rat plantaris muscle to ascertain the bases for our previous observations that creatine loading results in a higher proportion of myosin heavy chain (MHC) IIb, without corresponding changes in contractile properties. Forty Sprague-Dawley rats were assigned to one of four groups: creatine-fed sedentary, creatine-fed run-trained, control-fed sedentary, and control-fed run-trained animals. Proportion and cross-sectional area increased 10% and 15% in type IIb fibers and the proportion of type IIa fibers decreased 11% in the creatine-fed run-trained compared with the control-fed run-trained group (P < 0.03). No differences were observed in fast Ca2+-ATPase isoform SERCA1 content (P > 0.49). Creatine feeding alone induced a 41% increase (P < 0.03) in slow Ca2+-ATPase (SERCA2) content, which was further elevated by 33% with running (P < 0.02). Run training alone reduced parvalbumin content by 50% (P < 0.05). By comparison, parvalbumin content was dramatically decreased by 75% (P < 0.01) by creatine feeding alone but was not further reduced by run training. These adaptive changes indicate that elevating the capacity for high-energy phosphate shuttling, through creatine loading, alleviates the need for intracellular Ca2+ buffering by parvalbumin and increases the efficiency of Ca2+ uptake by SERCAs. Citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities were elevated by run training (P < 0.003) but not by run training + creatine feeding. This indicates that creatine loading during run training supports a faster muscle phenotype that is adequately supported by the existing glycolytic potential, without changes in the capacity for terminal substrate oxidation.
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Affiliation(s)
- Maria Gallo
- E-417 Van Vliet Centre, Univ. of Alberta, Edmonton, AB, Canada T6G 2H9. )
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Putman CT, Martins KJB, Gallo ME, Lopaschuk GD, Pearcey JA, MacLean IM, Saranchuk RJ, Pette D. Alpha-catalytic subunits of 5'AMP-activated protein kinase display fiber-specific expression and are upregulated by chronic low-frequency stimulation in rat muscle. Am J Physiol Regul Integr Comp Physiol 2007; 293:R1325-34. [PMID: 17553841 DOI: 10.1152/ajpregu.00609.2006] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
5'-AMP-activated protein kinase (AMPK) signaling initiates adaptive changes in skeletal muscle fibers that restore homeostatic energy balance. The purpose of this investigation was to examine, in rats, the fiber-type protein expression patterns of the alpha-catalytic subunit isoforms in various skeletal muscles, and changes in their respective contents within the tibialis anterior (TA) after chronic low-frequency electrical stimulation (CLFS; 10 Hz, 10 h daily), applied for 4 +/- 1.2 or 25 +/- 4.8 days. Immunocytochemical staining of soleus (SOL) and medial gastrocnemius (MG) showed that 86 +/- 4.1 to 97 +/- 1.4% of type IIA fibers stained for both the alpha1- and alpha2-isoforms progressively decreased to 63 +/- 12.2% of type IID/X and 9 +/- 2.4% of IIB fibers. 39 +/- 11.4% of IID/X and 83 +/- 7.9% of IIB fibers expressed only the alpha2 isoform in the MG, much of which was localized within nuclei. alpha1 and alpha2 contents, assessed by immunoblot, were lowest in the white gastrocnemius [WG; 80% myosin heavy chain (MHC) IIb; 20% MHCIId/x]. Compared with the WG, alpha1 content was 1.6 +/- 0.08 (P < 0.001) and 1.8 +/- 0.04 (P < 0.0001)-fold greater in the red gastrocnemius (RG: 13%, MHCIIa) and SOL (21%, MHCIIa), respectively, and increased in proportion to MHCIIa content. Similarly, alpha2 content was 1.4 +/- 0.10 (P < 0.02) and 1.5 +/- 0.07 (P < 0.001)-fold greater in RG and SOL compared with WG. CLFS induced 1.43 +/- 0.13 (P < 0.007) and 1.33 +/- 0.08 (P < 0.009)-fold increases in the alpha1 and alpha2 contents of the TA and coincided with the transition of faster type IIB and IID/X fibers toward IIA fibers. These findings indicate that fiber types differ with regard to their capacity for AMPK signaling and that this potential is increased by CLFS.
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MESH Headings
- AMP-Activated Protein Kinases
- Animals
- Blotting, Western
- Catalysis
- Cell Nucleus/metabolism
- Electric Stimulation
- Electrophoresis, Polyacrylamide Gel
- Gene Expression Regulation, Enzymologic/physiology
- Immunohistochemistry
- Isoenzymes/biosynthesis
- Isoenzymes/genetics
- Male
- Multienzyme Complexes/biosynthesis
- Multienzyme Complexes/genetics
- Multienzyme Complexes/metabolism
- Muscle Fibers, Fast-Twitch/enzymology
- Muscle Fibers, Fast-Twitch/physiology
- Muscle Fibers, Skeletal/enzymology
- Muscle Fibers, Skeletal/physiology
- Muscle Fibers, Slow-Twitch/enzymology
- Muscle Fibers, Slow-Twitch/physiology
- Muscle, Skeletal/enzymology
- Muscle, Skeletal/physiology
- Myosin Heavy Chains/metabolism
- Protein Serine-Threonine Kinases/biosynthesis
- Protein Serine-Threonine Kinases/genetics
- Protein Serine-Threonine Kinases/metabolism
- Rats
- Rats, Sprague-Dawley
- Up-Regulation/physiology
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Affiliation(s)
- Charles T Putman
- Faculty of Physical Education and Recreation and The Centre for Neuroscience, Faculty of Medicine & Dentistry, University of Alberta, E-417 Van Vliet Centre, Edmonton, Alberta, Canada.
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15
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Harris RLW, Putman CT, Rank M, Sanelli L, Bennett DJ. Spastic tail muscles recover from myofiber atrophy and myosin heavy chain transformations in chronic spinal rats. J Neurophysiol 2006; 97:1040-51. [PMID: 17122320 PMCID: PMC5759973 DOI: 10.1152/jn.00622.2006] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Without intervention after spinal cord injury (SCI), paralyzed skeletal muscles undergo myofiber atrophy and slow-to-fast myofiber type transformations. We hypothesized that chronic spasticity-associated neuromuscular activity after SCI would promote recovery from such deleterious changes. We examined segmental tail muscles of chronic spinal rats with long-standing tail spasticity (7 mo after sacral spinal cord transection; older chronic spinals), chronic spinal rats that experienced less spasticity early after injury (young chronic spinals), and rats without spasticity after transection and bilateral deafferentation (spinal isolated). These were compared with tail muscles of age-matched normal rats. Using immunohistochemistry, we observed myofiber distributions of 15.9 +/- 3.5% type I, 18.7 +/- 10.7% type IIA, 60.8 +/- 12.6% type IID(X), and 2.3 +/- 1.3% type IIB (means +/- SD) in young normals, which were not different in older normals. Young chronic spinals demonstrated transformations toward faster myofiber types with decreased type I and increased type IID(X) paralleled by atrophy of all myofiber types compared with young normals. Spinal isolated rats also demonstrated decreased type I myofiber proportions and increased type II myofiber proportions, and severe myofiber atrophy. After 4 mo of complete spasticity (older chronic spinals), myofiber type transformations were reversed, with no significant differences in type I, IIA, IID(X), or IIB proportions compared with age-matched normals. Moreover, after this prolonged spasticity, type I, IIA, and IIB myofibers recovered from atrophy, and type IID(X) myofibers partially recovered. Our results indicate that early after transection or after long-term spinal isolation, relatively inactive tail myofibers atrophy and transform toward faster myofiber types. However, long-term spasticity apparently produces neuromuscular activity that promotes recovery of myofiber types and myofiber sizes.
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Affiliation(s)
- R Luke W Harris
- Centre for Neuroscience, 5-13 Heritage Medical Research Centre, University of Alberta, Edmonton, Alberta T6G 2S2, Canada
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16
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Abstract
The stress-signaling protein, adenosine monophosphate-activated protein kinase (AMPK), regulates a variety of pathways in cells that 1) increase the provision and utilization of energy-providing substrates such as glucose and fatty acids, 2) inhibit energy-requiring pathways such as cholesterol biosynthesis and protein synthesis, and 3) increase the transcription of genes involved in energy metabolism and mitochondrial biogenesis. In the heart, AMPK therefore becomes very important in protecting against ischemia-reperfusion injury and regulating substrate metabolism in the face of changes in workload. This review summarizes the regulation of AMPK activity in the heart and discusses the effects of AMPK activation.
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Affiliation(s)
- Raymond Russell
- Section of Cardiovascular Medicine, Yale University School of Medicine, 333 Cedar Street, FMP 3, New Haven, CT 06510, USA.
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17
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Martins KJB, Gordon T, Pette D, Dixon WT, Foxcroft GR, Maclean IM, Putman CT. Effect of satellite cell ablation on low-frequency-stimulated fast-to-slow fibre-type transitions in rat skeletal muscle. J Physiol 2006; 572:281-94. [PMID: 16439424 PMCID: PMC1779649 DOI: 10.1113/jphysiol.2005.103366] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
The purpose of this study was to determine whether satellite cell ablation within rat fast-twitch muscles exposed to chronic low-frequency stimulation (CLFS) would limit fast-to-slow fibre-type transitions. Twenty-nine male Wistar rats were randomly assigned to one of three groups. Satellite cells of the left tibialis anterior were ablated by weekly exposure to a 25 Gy dose of gamma-irradiation during 21 days of CLFS (IRR-Stim), whilst a second group received only 21 days of CLFS (Stim). A third group received weekly doses of gamma-irradiation (IRR). Non-irradiated right legs served as internal controls. Continuous infusion of 5-bromo-2'-deoxyuridine (BrdU) revealed that CLFS induced an 8.0-fold increase in satellite cell proliferation over control (mean +/-s.e.m.: 23.9 +/- 1.7 versus 3.0 +/- 0.5 mm(-2), P < 0.0001) that was abolished by gamma-irradiation. M-cadherin and myogenin staining were also elevated 7.7- and 3.8-fold (P < 0.0001), respectively, in Stim compared with control, indicating increases in quiescent and terminally differentiating satellite cells; these increases were abolished by gamma-irradiation. Myonuclear content was elevated 3.3-fold (P < 0.0001) in Stim, but remained unchanged in IRR-Stim. Immunohistochemical analyses revealed attenuation of fast-to-slow fibre-type transitions in IRR-Stim compared with Stim. Comparable changes were observed at the protein level by SDS-PAGE. It is concluded that although considerable adaptive potential exists within myonuclei, satellite cells play a role in facilitating fast-to-slow fibre-type transitions.
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MESH Headings
- Adaptation, Physiological/physiology
- Animals
- Cell Differentiation/physiology
- Cell Proliferation
- Cells, Cultured
- Electric Stimulation
- Male
- Muscle Fibers, Fast-Twitch/physiology
- Muscle Fibers, Fast-Twitch/ultrastructure
- Muscle Fibers, Slow-Twitch/physiology
- Muscle Fibers, Slow-Twitch/ultrastructure
- Muscle, Skeletal/physiology
- Muscle, Skeletal/ultrastructure
- Rats
- Rats, Wistar
- Satellite Cells, Skeletal Muscle/physiology
- Satellite Cells, Skeletal Muscle/ultrastructure
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Affiliation(s)
- Karen J B Martins
- E-417 Van Vliet Centre, University of Alberta, Edmonton, Alberta, Canada
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18
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Bamford JA, Putman CT, Mushahwar VK. Intraspinal microstimulation preferentially recruits fatigue-resistant muscle fibres and generates gradual force in rat. J Physiol 2005; 569:873-84. [PMID: 16239281 PMCID: PMC1464280 DOI: 10.1113/jphysiol.2005.094516] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
Intraspinal microstimulation (ISMS), a novel rehabilitative therapy consisting of stimulation through fine, hair-like microwires targeted at the ventral spinal cord, has been proposed for restoring standing and walking following spinal cord injury. This study compared muscle recruitment characteristics of ISMS with those produced by peripheral nerve cuff stimulation (NCS). Thirty-three minutes of either ISMS or NCS at 1, 20 or 50 s(-1) and 1.2 x threshold (T) amplitude depleted glycogen from muscle fibres of vastus lateralis and rectus femoris. ISMS and NCS were also carried out at 20 s(-1) and 3.0T. Muscle serial sections were stained for glycogen and for myosin heavy chain (MHC)-based fibre types using a panel of monoclonal antibodies. The results of this study show that ISMS recruited fatigue-resistant (FR) fibres at 2.9, 1.9, 1.7 and 2.5 times their relative MHC content at 1, 20 and 50 s(-1) 1.2T and 20 s(-1) 3.0T, respectively. In contrast, NCS recruited FR fibres at 1.2, 1.0, 2.1 and 0.0 times their MHC content at 1, 20 and 50 s(-1) 1.2T and 20 s(-1) 3.0T, respectively. The proportion of FR fibres recruited by ISMS and NCS was significantly different in the 20 s(-1) 3.0T condition (P < 0.0001). We also report that force recruitment curves were 4.9-fold less steep (P < 0.019) for ISMS than NCS. The findings of this study provide evidence for the efficacy of ISMS and further our understanding of muscle recruitment properties of this novel rehabilitative therapy.
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Affiliation(s)
- J A Bamford
- Centre for Neuroscience and Department of Biomedical Engineering, Room 503, Heritage Medical Research Centre, University of Alberta, Edmonton, Alberta, Canada
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Hilder TL, Baer LA, Fuller PM, Fuller CA, Grindeland RE, Wade CE, Graves LM. Insulin-independent pathways mediating glucose uptake in hindlimb-suspended skeletal muscle. J Appl Physiol (1985) 2005; 99:2181-8. [PMID: 16099889 DOI: 10.1152/japplphysiol.00743.2005] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
Insulin resistance accompanies atrophy in slow-twitch skeletal muscles such as the soleus. Using a rat hindlimb suspension model of atrophy, we have previously shown that an upregulation of JNK occurs in atrophic muscles and correlates with the degradation of insulin receptor substrate-1 (IRS-1) (Hilder TL, Tou JC, Grindeland RF, Wade CE, and Graves LM. FEBS Lett 553: 63-67, 2003), suggesting that insulin-dependent glucose uptake may be impaired. However, during atrophy, these muscles preferentially use carbohydrates as a fuel source. To investigate this apparent dichotomy, we examined insulin-independent pathways involved in glucose uptake following a 2- to 13-wk hindlimb suspension regimen. JNK activity was elevated throughout the time course, and IRS-1 was degraded as early as 2 wk. AMP-activated protein kinase (AMPK) activity was significantly higher in atrophic soleus muscle, as were the activities of the ERK1/2 and p38 MAPKs. As a comparison, we examined the kinase activity in solei of rats exposed to hypergravity conditions (2 G). IRS-1 phosphorylation, protein, and AMPK activity were not affected by 2 G, demonstrating that these changes were only observed in soleus muscle from hindlimb-suspended animals. To further examine the effect of AMPK activation on glucose uptake, C2C12 myotubes were treated with the AMPK activator metformin and then challenged with the JNK activator anisomycin. While anisomycin reduced insulin-stimulated glucose uptake to control levels, metformin significantly increased glucose uptake in the presence of anisomycin and was independent of insulin. Taken together, these results suggest that AMPK may be an important mediator of insulin-independent glucose uptake in soleus during skeletal muscle atrophy.
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Affiliation(s)
- Thomas L Hilder
- Dept. of Pharmacology, University of North Carolina, Chapel Hill, NC 27599-7365, USA
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20
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Thomson DM, Gordon SE. Diminished overload-induced hypertrophy in aged fast-twitch skeletal muscle is associated with AMPK hyperphosphorylation. J Appl Physiol (1985) 2005; 98:557-64. [PMID: 15465886 DOI: 10.1152/japplphysiol.00811.2004] [Citation(s) in RCA: 77] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Skeletal muscle mass declines with age, as does the potential for overload-induced fast-twitch skeletal muscle hypertrophy. Because 5′-AMP-activated protein kinase (AMPK) activity is thought to inhibit skeletal muscle protein synthesis and may therefore modulate muscle mass and hypertrophy, the purpose of this investigation was to examine AMPK phosphorylation status (a marker of AMPK activity) and its potential association with the attenuated overload-induced hypertrophy observed in aged skeletal muscle. One-week overload of fast-twitch plantaris and slow-twitch soleus muscles was achieved in young adult (8 mo; n = 7) and old (30 mo; n = 7) Fischer344 × Brown Norway male rats via unilateral gastrocnemius ablation. Significant ( P ≤ 0.05) age-related atrophy (as measured by total protein content) was noted in plantaris and soleus control (sham-operated) muscles. In fast-twitch plantaris muscles, percent hypertrophy with overload was significantly attenuated with age, whereas AMPK phosphorylation status as determined by Western blotting [phospho-AMPK (Thr172)/total AMPK] was significantly elevated with age (regardless of loading status). There was also a main effect of loading on AMPK phosphorylation status in plantaris muscles (overload > control). Moreover, a strong and significant negative correlation ( r = −0.82) was observed between AMPK phosphorylation status and percent hypertrophy in the overloaded plantaris muscles of all animals. In contrast to the plantaris, overload-induced hypertrophy of the slow-twitch soleus muscle was similar between ages, and AMPK phosphorylation in this muscle was also unaffected by age or overload. These data support the possibility that an age-related elevation in AMPK phosphorylation may partly contribute to the attenuated hypertrophic response observed with age in overloaded fast-twitch plantaris muscle.
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Affiliation(s)
- David M Thomson
- Human Performance Laboratory, 363 Ward Sports Medicine Bldg., East Carolina Univ., Greenville, NC 27858, USA
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