Events In Skeletal Muscle Research Articles

Comparing changes to the skeletal muscle transcriptome of young and aged mice to young and aged humans in response to acute eccentric contractions

Resistance exercise alters several molecular events in skeletal muscle that change muscle size and function, including changes to the transcriptome. The signals altering the muscle transcriptome in response to resistance exercise remain ill defined. Defining those regulatory signals in muscle requires non-human models where manipulations can assess cause and effect. The electrically induced, unilateral eccentric contraction model of resistance exercise in rodents is widely used to mimic resistance exercise. However, the transcriptional changes that occur in response to eccentric contractions in the rodent model and the continuity of the rodent model to resistance exercise in humans are not well known. The purpose of this study was to define the transcriptional changes in skeletal muscle that occur in response to acute eccentric contractions in young and aged mice and define their continuity to transcriptional changes in the muscle of young and aged humans. Four-month-old and 24-month-old male C57BL/6 mice were subjected to a bout of electrically induced, unilateral eccentric contractions of the tibialis anterior (TA) muscle. RNA was isolated from the TA and subjected to RNA Sequencing. Lists of differentially expressed genes (DEGs) from the TA of young and aged mice were compared to RNA Sequencing datasets generated from the vastus lateralis muscle of young and aged humans following acute eccentric contractions. L andscape I n S ilico deletion A nalysis (Lisa) was used to predict the magnitude of transcription factor regulation of the DEGs. DEGs for mice or humans were categorized into age specific DEGs (specific to young or aged) or DEGs shared by both ages. The number of DEGs for mice or humans were distributed equally to each category, showing an age specific distribution that was conserved in both mice and humans. However, the DEGs in young and aged mice had very little overlap to the DEGs in young and aged humans. Only 113 DEGs were common across all ages and species. Interestingly, the relationship of transcription factors predicted to regulate DEGs between young and aged mice or young and aged humans was very strong. However, the relationship of transcription factors predicted to regulate DEGs of young mice and young humans or aged mice and aged humans was rather weak. In all, these data highlight the consistencies and differences in the muscle transcriptomic response to eccentric contractions. This work was supported in part by NIH R03AG073445 (BSG). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Comparative analysis of alternative splicing events in skeletal muscle of different sheep

This paper aims to investigate the relationship between genes with alternative splicing (AS) events and breed-specific differences in muscle development in two breeds of sheep. RNA-seq was utilized to identify genes with AS between Small-tailed Han sheep and Dorset sheep. The gene lists of differentially spliced genes were identified, and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were conducted on these genes. In this study, 299 genes with 356 AS indicated significant differences between two diffrerent breeds. There are differences in 31 genes with 35 AS. Cassette, alt5′ and alt3’ exhibited the highest levels of enrichment across various significant levels. GO and KEGG enrichment analysis demonstrated a significant correlation between Wnt, TGF-beta, Notch and MAPK signaling pathways and the development of muscle in sheep. These findings indicate that genes with AS are linked to variations in muscle development in sheep. These results offer significant scientific and practical implications for improving the quality of sheep meat.

Leptin Enhances Nitric Oxide Production and Decreases Blood Flow in Rat Skeletal Muscle

Objective: There are a few studies related to nitric oxide (NO) and leptin interaction in the regulation of physiological events in skeletal muscle. Therefore, the aim of the present study is to investigate the interaction of leptin and NO in response to blood flow and nitric oxide synthase (NOS) distribution on rat skeletal muscle. Materials and Methods: Twenty-four adult-male Wistar albino rats were divided into 4 groups: control (C), Leptin (LP) (50 μg/kg), L-NG-nitroarginine methyl ester (LN) (a non-specific nitric oxide synthase inhibitor, 10 mg/kg,) and L-NAME+Leptin (LN+LP) administrated groups. Drugs were administered via the right jugular vein. Hemodynamic parameters: mean arterial pressure, heart rate and blood flow were recorded during the experiment and at the end blood samples for biochemical analyses of leptin and nitrite/nitrate levels and gastrocnemius muscle tissues from the right hindlimb for NOS distribution were taken. Results: Leptin infusion after L-NAME administration significantly decreased heart rate and blood flow (p<0.05, p<0.001). In addition, there was no change in the mean arterial pressure in the leptin group and leptin-administered L-NAME group. Leptin levels were the highest in the LP+LN group (p<0.01). Decreased nitrate/nitrate levels with L-NAME administration (p<0.01 vs C) reached control values by leptin infusion. Both endothelial NOS (eNOS) and neuronal NOS (nNOS) distributions were observed in the skeletal muscle cells of the leptin group. Conclusion: Although NO synthesis is inhibited by L-NAME, it is concluded that leptin partially enhances NO production and leptin uses NO as a mediator in its physiological effects.

Assessing the Dynamic Mitochondrial Fission and Fusion Events in Skeletal Muscle in vivo

Skeletal muscle plays an important role in systemic metabolism and overall health. Within skeletal muscle, there is a decline in mitochondrial function with age. Under normal healthy conditions, mitochondria remodel via two dynamic processes, mitochondrial protein turnover and mitochondrial dynamics, which includes mitochondrial fission and fusion events. While capturing the dynamic nature of mitochondrial protein turnover in skeletal muscle is now possible, assessing mitochondrial fission and fusion relies on snapshots of protein markers from muscle homogenate at distinct timepoints. Recent technical advancements allow for measurement of mitochondrial dynamics in skeletal muscle in vitro; however, rates of mitochondrial dynamics in other tissues are significantly slower in vivo compared to in vitro conditions. Therefore, the purpose of this study was to develop novel imaging methods to assess mitochondrial dynamics in vivo. Our hypothesis was that we could capture and assess in vivo mitochondrial fission/fusion events using a novel imaging approach. Using multiphoton microscopy, we activated a mitochondrial-targeted photo-activated GFP that was electroporated into the tibialis anterior (TA) muscle of 5 adult C57Bl6 mice to assess the rates of mitochondrial fission and fusion events. In vivo mitochondrial fission and fusion rates were about 50 times slower in comparison to previously reported rates in in vitro models. Due to the slower than expected rates of mitochondrial fission/fusion events in skeletal muscle in vivo, we used an additional imaging approach to explore changes mitochondrial morphology over more prolonged periods of time. To do so, we imaged control and denervated TA muscles of 5 adult C57Bl6 mice using an electroporated mitochondrial-targeted GFP (mitochondrial matrix) and TOM20_tdTomato (outer mitochondrial matrix). We determined that two weeks of denervation decreased the cross-sectional area that was co-stained in the denervated limb compared to a control limb, suggesting a decrease in cristae structure. In addition, two weeks of denervation decreased the cross-sectional area of the population of subsarcolemmal mitochondria in the denervated limb compared to a control limb. Together, our data emphasizes the importance of assessing mitochondrial dynamics in vivo. Additionally, changes to mitochondrial morphology in skeletal muscle fibers are regional, which stresses the importance of assessing mitochondrial subpopulations.

Autophagy in the mesh of collagen VI

Autophagy is a very versatile process through which the cell degrades damaged long-lived proteins, entire organelles, or pathogens, by engulfing them in characteristic double-membrane vesicles and conveying the cargo to lysosomes. It is a dynamic pathway tunable at multiple levels and responsive to nutrient and stress stimuli, also coming from the extracellular microenvironment and its remodeling. In the extracellular matrix, collagen type VI forms a distinctive set of beaded microfilaments that assemble into an intricate and multimodular meshwork of tightly linked proteins and surface receptors. When missing or defective, collagen VI triggers a series of pathological events in skeletal muscle and other tissues, with a remarkable impact on key cell processes, such as apoptosis and autophagy. In this review, we discuss the current knowledge about collagen VI regulation of autophagy in the different experimental models and human pathologies where it was studied, and provide some hints for future directions aimed at the fine dissection of this intriguing relationship, as well as its prospective translational impact for disease and therapy.

P.293Novel splicing events in skeletal muscle revealed by RNA sequencing

Most human genes consist of more than one coding exon and therefore undergo splicing to remove the intronic areas. Alternative splicing gives rise to alternative isoforms. Many alternative splicing events (ASEs) have been shown to be developmental stage and/or tissue specific. Hence a comprehensive understanding of ASEs is of great importance for understanding diseases showing tissue and/or developmental stage specificity, but also for discovery of aberrant splicing. RNA sequencing (RNA-seq) enables detailed studies of the transcriptome and also reveals the detailed ASE landscape in the cells at the time of sampling. As such RNA-seq is an invaluable tool for splicing detection and direct discovery of aberrant splicing when samples of relevant tissues are available. Using RNA-seq we have studied ASEs in 42 samples from muscles of the lower leg from patients without muscle disorders. A large amount of unexpected splicing events were detected. This also included hitherto undescribed exons even in identified genes causing neuromuscular disorders. The findings were validated by other sequencing methods (PCR and the long-read technology PacBio) and also by comparison with public RNA-seq data from the ENCODE project. Our results are important for understanding of the complex splicing observed in skeletal muscle. Further, for correct interpretation and annotation of mutations affecting splicing in these genes, a comprehensive understanding of the ASEs is crucial.

Early energy metabolism-related molecular events in skeletal muscle of diabetic rats: The effects of l-arginine and SOD mimic

Considering the vital role of skeletal muscle in control of whole-body metabolism and the severity of long-term diabetic complications, we aimed to reveal the molecular pattern of early diabetes-related skeletal muscle phenotype in terms of energy metabolism, focusing on regulatory mechanisms, and the possibility to improve it using two redox modulators, l-arginine and superoxide dismutase (SOD) mimic. Alloxan-induced diabetic rats (120 mg/kg) were treated with l-arginine or the highly specific SOD mimic, M40403, for 7 days. As appropriate controls, non-diabetic rats received the same treatments. We found that l-arginine and M40403 restored diabetes-induced impairment of phospho-5′-AMP-activated protein kinase α (AMPKα) signaling by upregulating AMPKα protein itself and its downstream effectors, peroxisome proliferator-activated receptor-γ coactivator-1α and nuclear respiratory factor 1. Also, there was a restitution of the protein levels of oxidative phosphorylation components (complex I, complex II and complex IV) and mitofusin 2. Furthermore, l-arginine and M40403 induced translocation of glucose transporter 4 to the membrane and upregulation of protein of phosphofructokinase and acyl coenzyme A dehydrogenase, diminishing negative diabetic effects on limiting factors of glucose and lipid metabolism. Both treatments abolished diabetes-induced downregulation of sarcoplasmic reticulum calcium-ATPase proteins (SERCA 1 and 2). Similar effects of l-arginine and SOD mimic treatments suggest that disturbances in the superoxide/nitric oxide ratio may be responsible for skeletal muscle mitochondrial and metabolic impairment in early diabetes. Our results provide evidence that l-arginine and SOD mimics have potential in preventing and treating metabolic disturbances accompanying this widespread metabolic disease.

Effects of chronic high-fat feeding on skeletal muscle mass and function in middle-aged mice.

Increased adipose tissue may promote catabolic events in skeletal muscle. The aim of this study was to test whether high-fat diet (HFD)-induced obesity would accelerate the onset of muscle wasting in middle-aged mice. Muscle was collected from C57BL/6 mice at 9months of age (baseline) and 14months of age after consuming a control (C) or HFD. Mice in C and HFD were also subjected to evaluations of body composition and function before and after their respective diets. HFD demonstrated significant (p<0.05) losses of grip strength (-15%) and sensorimotor coordination (-11%), whereas C did not. Lean mass decreased to a greater degree in HFD although not significantly (C: -20.69±7.94 vs. HFD: -31.14±5.49%, p>0.05). Gastrocnemius, quadriceps, and hamstrings mass in C and HFD were significantly reduced from baseline (-27 to 43 and -39 to 47%, respectively, p<0.05) with no differences between the two; however, soleus mass was lower only in HFD (-24%, p=0.03). Myofiber area, satellite cells, and myonuclei of the gastrocnemius were lower only in HFD (-23, -19, and -16%, respectively, p<0.05) compared to baseline. HFD-induced obesity adversely affected function in middle-aged mice. Atrophy of the soleus in HFD but not C suggests sensitivity of oxidative muscle to HFD-dependent catabolism more so than aging. In the muscles containing fast/mixed fibers, aging effects may have concealed the catabolic nature of HFD; however, morphological changes in the gastrocnemius including decreased fiber area, satellite cells, and myonuclei are consistent with an atrophic phenotype related to HFD.

Interactome mapping reveals important pathways in skeletal muscle development of pigs.

The regulatory relationship and connectivity among genes involved in myogenesis and hypertrophy of skeletal muscle in pigs still remain large challenges. Presentation of gene interactions is a potential way to understand the mechanisms of developmental events in skeletal muscle. In this study, genome-wide transcripts and miRNA profiling was determined for Landrace pigs at four time points using microarray chips. A comprehensive method integrating gene ontology annotation and interactome network mapping was conducted to analyze the biological patterns and interaction modules of muscle development events based on differentially expressed genes and miRNAs. Our results showed that in total 484 genes and 34 miRNAs were detected for the duration from embryonic stage to adult in pigs, which composed two linear expression patterns with consensus changes. Moreover, the gene ontology analysis also disclosed that there were three typical biological events i.e., microstructure assembly of sarcomere at early embryonic stage, myofibril formation at later embryonic stage and function establishments of myoblast cells at postnatal stage. The interactome mappings of different time points also found the down-regulated trend of gene expression existed across the whole duration, which brought a possibility to introduce the myogenesis related miRNAs into the interactome regulatory networks of skeletal muscle in pigs.

Obese mice incur greater myofiber membrane disruption in response to mechanical load compared with lean mice

Obesity is associated with modified transmembrane signaling events in skeletal muscle, such as insulin signaling and glucose transport. The underlying cause of these obesity-related effects on transmembrane signaling is still unknown. In general, the function of membrane proteins responsible for transmembrane signaling is modulated by the biochemical makeup of the membrane, such as lipid composition, in which they are embedded. Any obesity-related alterations in membrane composition would also be predicted to modify membrane biomechanical properties and membrane susceptibility to mechanical load-induced damage. The primary objective of this study was to investigate whether obesity influences myofiber membrane susceptibility to mechanical damage in skeletal muscle. Myofiber membrane damage was compared between 12-week-old obese, hypercholesterolemic (B6.V Lep(ob) /J) and isogenic, normocholesterolemic control (C57BL6/J) male mice following either normal cage activity or strenuous eccentric exercise (downhill running). Myofiber membrane damage was quantified in perfusion-fixed frozen sections of the gastrocnemius muscle via sarcoplasmic concentration of either albumin (cage activity experiment) or a fluorescent marker that had been injected immediately before activity (eccentric exercise experiment). Obese mice exhibited evidence of increased myofiber membrane damage compared with lean mice after both normal cage activity and eccentric exercise indicating that myofiber membranes of obese mice are more susceptible to mechanical damage in general and that eccentric exercise exacerbates this effect. These observations are consistent with the notion that obesity influences the biochemical and biomechanical properties of myofiber membranes.

Inositol 1,4,5 Trisphosphate Receptor Type 1 (IP3R1) Activate Ryanodine Receptor (RyR1) to Mediate Ca2+ Spark Signaling in Adult Mammalian Skeletal Muscle

The functional coupling between IP3R and RyR1 in skeletal muscle have not been extensively studied. Ca2+ sparks, a RyR1-dependent Ca2+ release events are rarely observed during resting condition in adult mammalian skeletal muscle due to the inhibitory function of the EC-coupling machinery. Previously we showed that membrane deformation elicited by osmotic stress generated robust Ca2+ sparks that are restricted to the periphery of sarcolemma in skeletal muscle. Here we tested the hypothesis that lipid signaling mediated by membrane deformation activates Ca2+ release from IP3R that subsequently leads to activation of neighboring RyR, producing robust and localized Ca2+ spark events. Immunohistochemical staining showed that IP3R1 and IP3R2 were localized along the sub-sarcolemma region, coinciding with the distribution of Ca2+ sparks in skeletal muscle. It is known that transient exposure of cells to osmotic stress could elevate PI(4,5)P2 and IP3 levels inside the cell. Pharmacological interventions blocking the activation of IP3R suppress the osmotic-stress induced Ca2+ spark events in skeletal muscle. Using the IP3R2-/- mice, we showed that stress-induced Ca2+ sparks was not affected, suggesting that the type 2 IP3R is not involved in the onset of Ca2+ sparks in skeletal muscle. Specific silencing of the type 1 IP3R from the wild type or the IP3R2-/- mice led to complete ablation of the stress-induced Ca2+ sparks in skeletal muscle. Overall, our findings reveal a functional coupling between IP3R1and RyR1 in regulating intracellular Ca2+ signaling in skeletal muscle. The stress induced Ca2+ sparks likely involve a cross-talk between IP3R1 and RyR1, which must overcome the inhibitory action of the EC coupling machinery at resting condition that is essential for the tight control of muscle contraction.

An intervention study in obese mice with astaxanthin, a marine carotenoid – effects on insulin signaling and pro-inflammatory cytokines

Astaxanthin (ASX), a xanthophyll carotenoid from the marine algae Hematococcus pluvialis, has anti-obesity and insulin-sensitivity effects. The specific molecular mechanisms of its actions are not yet established. The present study was designed to investigate the mechanisms underlying the insulin sensitivity effects of ASX in a non-genetic insulin resistant animal model. A group of male Swiss albino mice was divided into two and fed either a starch-based control diet or a high fat-high fructose diet (HFFD). Fifteen days later, mice in each dietary group were divided into two and were treated with either ASX (6 mg kg(-1) per day) in olive oil or olive oil alone. At the end of 60 days, glucose, insulin and pro-inflammatory cytokines in plasma, lipids and oxidative stress markers in skeletal muscle and adipose tissue were assessed. Further, post-receptor insulin signaling events in skeletal muscle were analyzed. Increased body weight, hyperglycemia, hyperinsulinemia and increased plasma levels of tumor necrosis factor-α and interleukin-6 observed in HFFD-fed mice were significantly improved by ASX addition. ASX treatment also reduced lipid levels and oxidative stress in skeletal muscle and adipose tissue. ASX improved insulin signaling by enhancing the autophosphorylation of insulin receptor-β (IR-β), IRS-1 associated PI3-kinase step, phospho-Akt/Akt ratio and GLUT-4 translocation in skeletal muscle. This study demonstrates for the first time that chronic ASX administration improves insulin sensitivity by activating the post-receptor insulin signaling and by reducing oxidative stress, lipid accumulation and proinflammatory cytokines in obese mice.

Exercise-induced phospho-proteins in skeletal muscle

Efforts to identify exercise-induced signaling events in skeletal muscle have been influenced by ground-breaking discoveries in the insulin action field. Initial discoveries demonstrating that exercise enhances insulin sensitivity raised the possibility that contraction directly modulates insulin receptor signaling events. Although the acute effects of exercise on glucose metabolism are clearly insulin-independent, the canonical insulin signaling cascade has been used as a framework by investigators in an attempt to resolve the mechanisms by which muscle contraction governs glucose metabolism. This review focuses on recent advances in our understanding of exercise-induced signaling pathways governing glucose metabolism in skeletal muscle. Particular emphasis will be placed on the characterization of AS160, a novel Akt substrate that plays a role in the regulation of glucose transport.

Signaling mechanisms in skeletal muscle: Acute responses and chronic adaptations to exercise

Physical activity elicits physiological responses in skeletal muscle that result in a number of health benefits, in particular in disease states, such as type 2 diabetes. An acute bout of exercise/muscle contraction improves glucose homeostasis by increasing skeletal muscle glucose uptake, while chronic exercise training induces alterations in the expression of metabolic genes, such as those involved in muscle fiber type, mitochondrial biogenesis, or glucose transporter 4 (GLUT4) protein levels. A primary goal of exercise research is to elucidate the mechanisms that regulate these important metabolic and transcriptional events in skeletal muscle. In this review, we briefly summarize the current literature describing the molecular signals underlying skeletal muscle responses to acute and chronic exercise. The search for possible exercise/contraction-stimulated signaling proteins involved in glucose transport, muscle fiber type, and mitochondrial biogenesis is ongoing. Further research is needed because full elucidation of exercise-mediated signaling pathways would represent a significant step toward the development of new pharmacological targets for the treatment of metabolic diseases such as type 2 diabetes.

C-Jun N-Terminal Kinase Mediates Tumor Necrosis Factor-α Suppression of Differentiation in Myoblasts

The stress kinase c-jun N-terminal kinase (JNK) was recently shown to be involved in the pathophysiology of major inflammatory conditions, including Alzheimer's disease, stroke, obesity, and type II diabetes. However, the role of JNK in regulating inflammatory events in skeletal muscle is only beginning to be explored. IGF-I is the major hormone that promotes muscle growth and development. Here we used a novel, JNK interacting protein (JIP)-derived JNK peptide inhibitor to establish that JNK suppresses the biological activity of IGF-I in skeletal muscle progenitor cells. In these myoblasts, TNFalpha and its downstream receptor substrates, neutral-sphingomyelinase (N-SMase) and N-acetyl-d-sphingosine (C2-ceramide), induce JNK kinase activity in a time-dependent manner. Consistent with these results, TNFalpha induces JNK binding to insulin receptor substrate 1 (IRS-1) but is unable to inhibit IGF-I-induced IRS-1 tyrosine phosphorylation in myoblasts that are treated with the JNK peptide inhibitor. More importantly, JNK activation induced by TNFalpha, C2-ceramide, and N-SMase is associated with reduced expression of the critical muscle transcription factor myogenin as well as the differentiation marker myosin heavy chain (MHC). The JNK peptide inhibitor, but not the control peptide, completely reverses this inhibition of both myogenin and MHC. In the absence of IGF-I, TNFalpha, C2-ceramide, N-SMase and the JNK inhibitor are inactive, as shown by their inability to affect IRS tyrosine phosphorylation and protein expression of myogenin and MHC. These results establish that the resistance of muscle progenitor cells to IGF-I, which is caused by inflammatory stimuli, is mediated by the JNK stress kinase pathway.

Effects of age and resistance exercise on skeletal muscle interstitial prostaglandin F 2α

Prostaglandin (PG) F 2α has been shown to contribute to the anabolic events in skeletal muscle. We measured the skeletal muscle interstitial concentration of PGF 2α at rest and following a standard bout of resistance exercise in eight young (27±2 year) and eight old (75±4 year) men. Interstitial PGF 2α concentration was determined from microdialysate samples obtained from two microdialysis probes placed in the vastus lateralis. Microdialysates were collected 1 h pre- and 5–6, 8–9, and 24–25 h postexercise. The exercise bout consisted of 4 exercises (3 sets of 8 replications at 80% 1 RM per exercise) emphasizing the quadriceps. Interstitial PGF 2α levels were not different ( P>0.05) between young and old at rest (1.50±0.35 vs. 1.52±0.30 ng ml −1) or at any time point following the resistance exercise bout. For the young and old combined there was a change ( P<0.05) in PGF 2α levels at 5–6 h (93%) and 8–9 h (95%), which had returned to preexercise levels by 24–25 h. These results show that PGF 2α is increased in skeletal muscle following a standard bout of resistance exercise and aging does not alter interstitial levels of this PG at rest or after exercise. These data, coupled with previous findings, suggest that the anabolic factor PGF 2α should be considered when discussing the complex processes that regulate muscle mass in young and old individuals.

Low-intensity contraction activates the α1-isoform of 5′-AMP-activated protein kinase in rat skeletal muscle

Skeletal muscle expresses two catalytic subunits, alpha1 and alpha2, of the 5'-AMP-activated protein kinase (AMPK), which has been implicated in contraction-stimulated glucose transport and fatty acid oxidation. Muscle contraction activates the alpha2-containing AMPK complex (AMPKalpha2), but this activation may occur with or without activation of the alpha1-containing AMPK complex (AMPKalpha1), suggesting that AMPKalpha2 is the major isoform responsible for contraction-induced metabolic events in skeletal muscle. We report for the first time that AMPKalpha1, but not AMPKalpha2, can be activated in contracting skeletal muscle. Rat epitrochlearis muscles were isolated and incubated in Krebs-Ringer bicarbonate buffer containing pyruvate. In muscles stimulated to contract at a frequency of 1 and 2 Hz during the last 2 min of incubation, AMPKalpha1 activity increased twofold and AMPKalpha2 activity remained unchanged. Muscle stimulation did not change the muscle AMP concentration or the AMP-to-ATP ratio. AMPK activation was associated with increased phosphorylation of Thr(172) of the alpha-subunit, the primary activation site. Muscle stimulation increased the phosphorylation of acetyl-CoA carboxylase (ACC), a downstream target of AMPK, and the rate of 3-O-methyl-d-glucose transport. In contrast, increasing the frequency (>or=5 Hz) or duration (>or=5 min) of contraction activated AMPKalpha1 and AMPKalpha2 and increased AMP concentration and the AMP/ATP ratio. These results suggest that 1) AMPKalpha1 is the predominant isoform activated by AMP-independent phosphorylation in low-intensity contracting muscle, 2) AMPKalpha2 is activated by an AMP-dependent mechanism in high-intensity contracting muscle, and 3) activation of each isoform enhances glucose transport and ACC phosphorylation in skeletal muscle.

Insulin signal transduction in human skeletal muscle: identifying the defects in Type II diabetes

Type II diabetes is characterized by defects in insulin action on peripheral tissues, such as skeletal muscle, adipose tissue and liver and pancreatic beta-cell defects. Since the skeletal muscle accounts for approx. 75% of whole body insulin-stimulated glucose uptake, defects in this tissue play a major role in the impaired glucose homoeostasis in Type II diabetic patients. Thus identifying defective steps in this process may reveal attractive targets for drug development to combat insulin resistance and Type II diabetes. This review will describe the effects of insulin on glucose transport and other metabolic events in skeletal muscle that are mediated by intracellular signalling cascades. Evidence for impaired activation of the insulin receptor signalling cascade and defective glucose transporter 4 translocation in the skeletal muscle from Type II diabetic patients will be presented. Through the identification of the intracellular defects in insulin action that control glucose homoeostasis, a better understanding of the disease pathogenesis can be gained and strategies for intervention may be developed.

Molecular events in skeletal muscle during disuse atrophy.

This review summarizes the current knowledge of the molecular processes underlying skeletal muscle atrophy due to disuse. Because the processes involved with muscle wasting due to illness are similar to disuse, this literature is used for comparison. Areas that are ripe for further study and that will advance our understanding of muscle atrophy are suggested.

Invited review: intracellular signaling in contracting skeletal muscle.

Physical exercise is a significant stimulus for the regulation of multiple metabolic and transcriptional processes in skeletal muscle. For example, exercise increases skeletal muscle glucose uptake, and, after exercise, there are increases in the rates of both glucose uptake and glycogen synthesis. A single bout of exercise can also induce transient changes in skeletal muscle gene transcription and can alter rates of protein metabolism, both of which may be mechanisms for chronic adaptations to repeated bouts of exercise. A central issue in exercise biology is to elucidate the underlying molecular signaling mechanisms that regulate these important metabolic and transcriptional events in skeletal muscle. In this review, we summarize research from the past several years that has demonstrated that physical exercise can regulate multiple intracellular signaling cascades in skeletal muscle. It is now well established that physical exercise or muscle contractile activity can activate three of the mitogen-activated protein kinase signaling pathways, including the extracellular signal-regulated kinase 1 and 2, the c-Jun NH(2)-terminal kinase, and the p38. Exercise can also robustly increase activity of the AMP-activated protein kinase, as well as several additional molecules, including glycogen synthase kinase 3, Akt, and the p70 S6 kinase. A fundamental goal of signaling research is to determine the biological consequences of exercise-induced signaling through these molecules, and this review also provides an update of progress in this area.