Human and Rodent Skeletal Muscles Express Angiotensin II Type 1 Receptors

Abundant evidence reveals that activation of the renin-angiotensin system promotes skeletal muscle atrophy in several conditions including congestive heart failure, chronic kidney disease, and prolonged mechanical ventilation. However, controversy exists about whether circulating angiotensin II (AngII) promotes skeletal muscle atrophy by direct or indirect effects; the centerpiece of this debate is the issue of whether skeletal muscle fibers express AngII type 1 receptors (AT1Rs). While some investigators assert that skeletal muscle expresses AT1Rs, others argue that skeletal muscle fibers do not contain AT1Rs. These discordant findings in the literature are likely the result of study design flaws and additional research using a rigorous experimental approach is required to resolve this issue. We tested the hypothesis that AT1Rs are expressed in both human and rat skeletal muscle fibers. Our premise was tested using a rigorous, multi-technique experimental design. First, we established both the location and abundance of AT1Rs on human and rat skeletal muscle fibers by means of an AngII ligand-binding assay. Second, using a new and highly selective AT1R antibody, we carried out Western blotting and determined the abundance of AT1R protein within isolated single muscle fibers from humans and rats. Finally, we confirmed the presence of AT1R mRNA in isolated single muscle fibers from rats. Our results support the hypothesis that AT1Rs are present in both human and rat skeletal muscle fibers. Moreover, our experiments provide the first evidence that AT1Rs are more abundant in fast, type II muscle fibers as compared with slow, type I fibers. Together, these discoveries provide the foundation for an improved understanding of the mechanism(s) responsible for AngII-induced skeletal muscle atrophy.

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Contractile function of single muscle fibers after hindlimb unweighting in aged rats

Journal of Applied Physiology ◽

10.1152/jappl.1998.84.1.229 ◽

1998 ◽

Vol 84 (1) ◽

pp. 229-235 ◽

Cited By ~ 14

Author(s):

L. V. Thompson ◽

J. A. Shoeman

Keyword(s):

Skeletal Muscle ◽

Functional Properties ◽

Contractile Function ◽

Muscle Fibers ◽

Type I ◽

Single Muscle ◽

Aged Rats ◽

Active Force ◽

Hindlimb Unweighting ◽

Skeletal Muscle Fibers

Thompson, L. V., and J. A. Shoeman. Contractile function of single muscle fibers after hindlimb unweighting in aged rats. J. Appl. Physiol. 84(1): 229–235, 1998.—This investigation determined how muscle atrophy produced by hindlimb unweighting (HU) alters the contractile function of single muscle fibers from older animals (30 mo). After 1 wk of HU, small bundles of fibers were isolated from the soleus muscles and the deep region of the lateral head of the gastrocnemius muscles. Single glycerinated fibers were suspended between a motor lever and force transducer, functional properties were studied, and the myosin heavy chain (MHC) composition was determined electrophoretically. After HU, the diameter of type I MHC fibers of the soleus declined (88 ± 2 vs. 80 ± 4 μm) and reductions were observed in peak active force (47 ± 3 vs. 28 ± 3 mg) and peak specific tension (Po; 80 ± 5 vs. 56 ± 5 kN/m2). The maximal unloaded shortening velocity increased. The type I MHC fibers from the gastrocnemius showed reductions in diameter (14%), peak active force (41%), and Po (24%), whereas the type IIa MHC fibers showed reductions in peak active force and Po. Thus 1 wk of inactivity has a significant effect on the force-generating capacity of single skeletal muscle fibers from older animals in a fiber type-specific manner (type I MHC > type IIa MHC > type I-IIa MHC). The decline in the functional properties of single skeletal muscle fibers in the older animals appears to be more pronounced than what has been reported in younger animal populations.

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Elevated nuclear Foxo1 suppresses excitability of skeletal muscle fibers

AJP Cell Physiology ◽

10.1152/ajpcell.00003.2013 ◽

2013 ◽

Vol 305 (6) ◽

pp. C643-C653 ◽

Cited By ~ 5

Author(s):

Erick O. Hernández-Ochoa ◽

Tova Neustadt Schachter ◽

Martin F. Schneider

Keyword(s):

Skeletal Muscle ◽

Electrical Stimulation ◽

Muscle Atrophy ◽

Action Potentials ◽

Muscle Fibers ◽

Calcium Transients ◽

Skeletal Muscle Atrophy ◽

Nuclear Activity ◽

Ec Coupling ◽

Skeletal Muscle Fibers

Forkhead box O 1 (Foxo1) controls the expression of proteins that carry out processes leading to skeletal muscle atrophy, making Foxo1 of therapeutic interest in conditions of muscle wasting. The transcription of Foxo1-regulated proteins is dependent on the translocation of Foxo1 to the nucleus, which can be repressed by insulin-like growth factor-1 (IGF-1) treatment. The role of Foxo1 in muscle atrophy has been explored at length, but whether Foxo1 nuclear activity affects skeletal muscle excitation-contraction (EC) coupling has not yet been examined. Here, we use cultured adult mouse skeletal muscle fibers to investigate the effects of Foxo1 overexpression on EC coupling. Fibers expressing Foxo1-green fluorescent protein (GFP) exhibit an inability to contract, impaired propagation of action potentials, and ablation of calcium transients in response to electrical stimulation compared with fibers expressing GFP alone. Evaluation of the transverse (T)-tubule system morphology, the membranous system involved in the radial propagation of the action potential, revealed an intact T-tubule network in fibers overexpressing Foxo1-GFP. Interestingly, long-term IGF-1 treatment of Foxo1-GFP fibers, which maintains Foxo1-GFP outside the nucleus, prevented the loss of normal calcium transients, indicating that Foxo1 translocation and the atrogenes it regulates affect the expression of proteins involved in the generation and/or propagation of action potentials. A reduction in the sodium channel Nav1.4 expression in fibers overexpressing Foxo1-GFP was also observed in the absence of IGF-1. We conclude that increased nuclear activity of Foxo1 prevents the normal muscle responses to electrical stimulation and that this indicates a novel capability of Foxo1 to disable the functional activity of skeletal muscle.

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Activating transcription factor 4 (ATF4) promotes skeletal muscle atrophy by forming a heterodimer with the transcriptional regulator C/EBPβ

Journal of Biological Chemistry ◽

10.1074/jbc.ra119.012095 ◽

2020 ◽

Vol 295 (9) ◽

pp. 2787-2803 ◽

Cited By ~ 8

Author(s):

Scott M. Ebert ◽

Steven A. Bullard ◽

Nathan Basisty ◽

George R. Marcotte ◽

Zachary P. Skopec ◽

...

Keyword(s):

Skeletal Muscle ◽

Transcription Factor ◽

Muscle Atrophy ◽

Molecular Level ◽

Muscle Fibers ◽

Skeletal Muscle Atrophy ◽

Skeletal Muscle Fibers ◽

Activating Transcription Factor 4 ◽

Activating Transcription Factor ◽

Transcription Factor 4

Skeletal muscle atrophy is a highly-prevalent and debilitating condition that remains poorly understood at the molecular level. Previous work found that aging, fasting, and immobilization promote skeletal muscle atrophy via expression of activating transcription factor 4 (ATF4) in skeletal muscle fibers. However, the direct biochemical mechanism by which ATF4 promotes muscle atrophy is unknown. ATF4 is a member of the basic leucine zipper transcription factor (bZIP) superfamily. Because bZIP transcription factors are obligate dimers, and because ATF4 is unable to form highly-stable homodimers, we hypothesized that ATF4 may promote muscle atrophy by forming a heterodimer with another bZIP family member. To test this hypothesis, we biochemically isolated skeletal muscle proteins that associate with the dimerization- and DNA-binding domain of ATF4 (the bZIP domain) in mouse skeletal muscle fibers in vivo. Interestingly, we found that ATF4 forms at least five distinct heterodimeric bZIP transcription factors in skeletal muscle fibers. Furthermore, one of these heterodimers, composed of ATF4 and CCAAT enhancer-binding protein β (C/EBPβ), mediates muscle atrophy. Within skeletal muscle fibers, the ATF4–C/EBPβ heterodimer interacts with a previously unrecognized and evolutionarily conserved ATF–C/EBP composite site in exon 4 of the Gadd45a gene. This three-way interaction between ATF4, C/EBPβ, and the ATF–C/EBP composite site activates the Gadd45a gene, which encodes a critical mediator of muscle atrophy. Together, these results identify a biochemical mechanism by which ATF4 induces skeletal muscle atrophy, providing molecular-level insights into the etiology of skeletal muscle atrophy.

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Physiology of a Microgravity Environment Invited Review: Microgravity and skeletal muscle

Journal of Applied Physiology ◽

10.1152/jappl.2000.89.2.823 ◽

2000 ◽

Vol 89 (2) ◽

pp. 823-839 ◽

Cited By ~ 332

Author(s):

Robert H. Fitts ◽

Danny R. Riley ◽

Jeffrey J. Widrick

Keyword(s):

Skeletal Muscle ◽

Muscle Atrophy ◽

Thin Filament ◽

Molecular Mechanisms ◽

Skeletal Muscle Atrophy ◽

Microgravity Environment ◽

Type I ◽

Fiber Types ◽

Maximal Velocity ◽

Cross Sectional

Spaceflight (SF) has been shown to cause skeletal muscle atrophy; a loss in force and power; and, in the first few weeks, a preferential atrophy of extensors over flexors. The atrophy primarily results from a reduced protein synthesis that is likely triggered by the removal of the antigravity load. Contractile proteins are lost out of proportion to other cellular proteins, and the actin thin filament is lost disproportionately to the myosin thick filament. The decline in contractile protein explains the decrease in force per cross-sectional area, whereas the thin-filament loss may explain the observed postflight increase in the maximal velocity of shortening in the type I and IIa fiber types. Importantly, the microgravity-induced decline in peak power is partially offset by the increased fiber velocity. Muscle velocity is further increased by the microgravity-induced expression of fast-type myosin isozymes in slow fibers (hybrid I/II fibers) and by the increased expression of fast type II fiber types. SF increases the susceptibility of skeletal muscle to damage, with the actual damage elicited during postflight reloading. Evidence in rats indicates that SF increases fatigability and reduces the capacity for fat oxidation in skeletal muscles. Future studies will be required to establish the cellular and molecular mechanisms of the SF-induced muscle atrophy and functional loss and to develop effective exercise countermeasures.

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Effects of Creatine Supplementation During Resistance Training on Myosin Heavy Chain (MHC) Expression in Rat Skeletal Muscle Fibers

The Journal of Strength and Conditioning Research ◽

10.1519/jsc.0b013e3181aeb103 ◽

2010 ◽

Vol 24 (1) ◽

pp. 88-96 ◽

Cited By ~ 9

Author(s):

Andreo F Aguiar ◽

Danilo H Aguiar ◽

Alan DS Felisberto ◽

Fernanda R Carani ◽

Rachel C Milanezi ◽

...

Keyword(s):

Skeletal Muscle ◽

Resistance Training ◽

Myosin Heavy Chain ◽

Heavy Chain ◽

Muscle Fibers ◽

Creatine Supplementation ◽

Rat Skeletal Muscle ◽

Skeletal Muscle Fibers

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Glycogen Depletion in Rat Skeletal Muscle Fibers During Exercise

Metabolic Adaptation to Prolonged Physical Exercise ◽

10.1007/978-3-0348-5523-5_47 ◽

1975 ◽

pp. 397-401 ◽

Cited By ~ 3

Author(s):

R. B. Armstrong ◽

C. W. Saubert ◽

W. L. Sembrowich ◽

R. E. Shepherd ◽

P. D. Gollnick

Keyword(s):

Skeletal Muscle ◽

Muscle Fibers ◽

Glycogen Depletion ◽

Rat Skeletal Muscle ◽

Skeletal Muscle Fibers

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Defective Acetylcholine Receptor Subunit Switch Precedes Atrophy of Slow-Twitch Skeletal Muscle Fibers Lacking ERK1/2 Kinases in Soleus Muscle

Scientific Reports ◽

10.1038/srep38745 ◽

2016 ◽

Vol 6 (1) ◽

Cited By ~ 5

Author(s):

Shuo Wang ◽

Bonnie Seaberg ◽

Ximena Paez-Colasante ◽

Mendell Rimer

Keyword(s):

Skeletal Muscle ◽

Acetylcholine Receptor ◽

Soleus Muscle ◽

Muscle Fibers ◽

Neuromuscular Synapse ◽

Fiber Morphology ◽

Skeletal Muscle Fibers ◽

Slow Twitch

Abstract To test the role of extracellular-signal regulated kinases 1 and 2 (ERK1/2) in slow-twitch, type 1 skeletal muscle fibers, we studied the soleus muscle in mice genetically deficient for myofiber ERK1/2. Young adult mutant soleus was drastically wasted, with highly atrophied type 1 fibers, denervation at most synaptic sites, induction of “fetal” acetylcholine receptor gamma subunit (AChRγ), reduction of “adult” AChRε, and impaired mitochondrial biogenesis and function. In weanlings, fiber morphology and mitochondrial markers were mostly normal, yet AChRγ upregulation and AChRε downregulation were observed. Synaptic sites with fetal AChRs in weanling muscle were ~3% in control and ~40% in mutants, with most of the latter on type 1 fibers. These results suggest that: (1) ERK1/2 are critical for slow-twitch fiber growth; (2) a defective γ/ε-AChR subunit switch, preferentially at synapses on slow fibers, precedes wasting of mutant soleus; (3) denervation is likely to drive this wasting, and (4) the neuromuscular synapse is a primary subcellular target for muscle ERK1/2 function in vivo.

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