Molecular Mechanisms of Muscle Fatigue Constantin-Teodosiu, Dumitru; Constantin, Despina
International journal of molecular sciences,
11/2021, Volume:
22, Issue:
21
Journal Article
Peer reviewed
Open access
Muscle fatigue (MF) declines the capacity of muscles to complete a task over time at a constant load. MF is usually short-lasting, reversible, and is experienced as a feeling of tiredness or lack of ...energy. The leading causes of short-lasting fatigue are related to overtraining, undertraining/deconditioning, or physical injury. Conversely, MF can be persistent and more serious when associated with pathological states or following chronic exposure to certain medication or toxic composites. In conjunction with chronic fatigue, the muscle feels floppy, and the force generated by muscles is always low, causing the individual to feel frail constantly. The leading cause underpinning the development of chronic fatigue is related to muscle wasting mediated by aging, immobilization, insulin resistance (through high-fat dietary intake or pharmacologically mediated Peroxisome Proliferator-Activated Receptor (PPAR) agonism), diseases associated with systemic inflammation (arthritis, sepsis, infections, trauma, cardiovascular and respiratory disorders (heart failure, chronic obstructive pulmonary disease (COPD))), chronic kidney failure, muscle dystrophies, muscle myopathies, multiple sclerosis, and, more recently, coronavirus disease 2019 (COVID-19). The primary outcome of displaying chronic muscle fatigue is a poor quality of life. This type of fatigue represents a significant daily challenge for those affected and for the national health authorities through the financial burden attached to patient support. Although the origin of chronic fatigue is multifactorial, the MF in illness conditions is intrinsically linked to the occurrence of muscle loss. The sequence of events leading to chronic fatigue can be schematically denoted as: trigger (genetic or pathological) -> molecular outcome within the muscle cell -> muscle wasting -> loss of muscle function -> occurrence of chronic muscle fatigue. The present review will only highlight and discuss current knowledge on the molecular mechanisms that contribute to the upregulation of muscle wasting, thereby helping us understand how we could prevent or treat this debilitating condition.
ObjectivesTo characterise the sketetal muscle metabolic phenotype during early critical illness.MethodsVastus lateralis muscle biopsies and serum samples (days 1 and 7) were obtained from 63 ...intensive care patients (59% male, 54.7±18.0 years, Acute Physiology and Chronic Health Evaluation II score 23.5±6.5).Measurements and main resultsFrom day 1 to 7, there was a reduction in mitochondrial beta-oxidation enzyme concentrations, mitochondrial biogenesis markers (PGC1α messenger mRNA expression (−27.4CN (95% CI −123.9 to 14.3); n=23; p=0.025) and mitochondrial DNA copy number (−1859CN (IQR −5557–1325); n=35; p=0.032). Intramuscular ATP content was reduced compared tocompared with controls on day 1 (17.7mmol/kg /dry weight (dw) (95% CI 15.3 to 20.0) vs. 21.7 mmol/kg /dw (95% CI 20.4 to 22.9); p<0.001) and decreased over 7 days (−4.8 mmol/kg dw (IQR −8.0–1.2); n=33; p=0.001). In addition, the ratio of phosphorylated:total AMP-K (the bioenergetic sensor) increased (0.52 (IQR −0.09–2.6); n=31; p<0.001). There was an increase in intramuscular phosphocholine (847.2AU (IQR 232.5–1672); n=15; p=0.022), intramuscular tumour necrosis factor receptor 1 (0.66 µg (IQR −0.44–3.33); n=29; p=0.041) and IL-10 (13.6 ng (IQR 3.4–39.0); n=29; p=0.004). Serum adiponectin (10.3 µg (95% CI 6.8 to 13.7); p<0.001) and ghrelin (16.0 ng/mL (IQR −7–100); p=0.028) increased. Network analysis revealed a close and direct relationship between bioenergetic impairment and reduction in muscle mass and between intramuscular inflammation and impaired anabolic signaling. ATP content and muscle mass were unrelated to lipids delivered.ConclusionsDecreased mitochondrial biogenesis and dysregulated lipid oxidation contribute to compromised skeletal muscle bioenergetic status. In addition, intramuscular inflammation was associated with impaired anabolic recovery with lipid delivery observed as bioenergetically inert. Future clinical work will focus on these key areas to ameliorate acute skeletal muscle wasting.Trial registration number NCT01106300.
Increased risk of type 2 diabetes mellitus (T2DM) is linked to impaired muscle mitochondrial function and reduced mitochondrial DNA copy number (mtDNA
). However, studies have failed to control for ...habitual physical activity levels, which directly influences both mtDNA copy number and insulin sensitivity. We, therefore, examined whether physical conditioning status (maximal oxygen uptake, V̇O
) was associated with skeletal muscle mitochondrial volume and mtDNA
, and was predictive of T2DM in overweight, middle-aged men.
Whole-body physiological (ISI-insulin sensitivity index, HOMA-IR, V̇O
) and muscle biochemical/molecular (vastus lateralis; mtDNA
, mitochondrial and glycolytic enzymes activity, lipid content and markers of lipid peroxidation) measurements were performed in three groups of overweight, middle-aged male volunteers (n = 10 per group): sedentary T2DM (ST2DM); sedentary control (SC) and non-sedentary control (NSC), who differed in aerobic capacity (ST2DM < SC < NSC).
mtDNA
was greater in NSC versus SC and ST2DM (P < 0.001; P < 0.001), and less in ST2DM versus SC (P < 0.01). Across all groups, mtDNA
positively correlated with ISI (P < 0.001; r = 0.688) and V̇O
(normalised to free fat mass; r = 0.684, P < 0.001), and negatively correlated to HOMA-IR (r = -0.544, P < 0.01). The activity of mitochondrial enzymes (GluDH, CS and β-HAD) was greater in NSC than ST2DM (P < 0.01, P < 0.001 and P < 0.05) and SC (P < 0.05, P < 0.01 and P < 0.05), but similar between ST2DM and SC. Intramuscular-free fatty acids, triglycerides and malondialdehyde contents were similar between ST2DM and SC.
Body composition and indices of muscle mitochondrial volume/function were similar between SC and ST2DM. However, mtDNA
differed and was positively associated with ISI, HOMA-IR and V̇O
across all groups. Collectively, the findings support the contention that habitual physical activity is a key component of T2DM development, possibly by influencing mtDNA
.
Sepsis causes muscle atrophy and insulin resistance, but the underlying mechanisms are unclear. Therefore, the present study
examined the effects of lipopolysaccharide (LPS)-induced endotoxaemia on ...the expression of Akt, Forkhead Box O (FOXO) and
its downstream targets, to identify any associations between changes in FOXO-dependent processes influencing muscle atrophy
and insulin resistance during sepsis. Chronically instrumented male SpragueâDawley rats received a continuous intravenous
infusion of LPS (15 μg kg â1 h â1 ) or saline for 24 h at 0.4 ml h â1 . Animals were terminally anaesthetized and the extensor digitorum longus muscles from both hindlimbs were removed and snap-frozen.
Measurements were made of mRNA and protein expression of selected signalling molecules associated with pathways regulating
protein synthesis and degradation and carbohydrate metabolism. LPS infusion induced increases in muscle tumour necrosis factor-α
(8.9-fold, P < 0.001) and interleukin-6 (8.4-fold, P < 0.01), paralleled by reduced insulin receptor substrate-1 mRNA expression (â0.7-fold, P < 0.01), and decreased Akt1 protein and cytosolic FOXO1 and FOXO3 phosphorylation. These changes were accompanied by significant
increases in muscle atrophy F-box mRNA (5.5-fold, P < 0.001) and protein (2-fold, P < 0.05) expression, and pyruvate dehydrogenase kinase 4 mRNA (15-fold, P < 0.001) and protein (1.6-fold, P < 0.05) expression. There was a 29% reduction in the muscle protein: DNA ratio, a 56% reduction in pyruvate dehydrogenase
complex (PDC) activity ( P < 0.05), and increased glycogen degradation and lactate accumulation. The findings of this study suggest a potential role
for Akt/FOXO in the simultaneous impairment of carbohydrate oxidation, at the level of PDC, and up-regulation of muscle protein
degradation, in LPS-induced endotoxaemia.
Non‐technical summary A clinical trait of critically ill patients following trauma, surgical complications and/or sepsis is the presence of a marked skeletal muscle wasting, which severely ...compromises muscle function, especially the respiratory muscles, and clinical outcomes such as morbis, mortality and the length of hospitalization. However, the molecular mechanisms responsible for this are largely unresolved. In this paper we provide novel and interesting translational data that show how the muscle mass is regulated during critical illness. The present observations also show that the muscle signalling pathways are not only important for the size of muscle mass, but could also play a significant role in the whole body glucose control. This is extremely important and relevant to the clinical setting as uncontrolled blood glucose concentrations in critically ill patients impact on clinical outcome. From a clinical perspective, the present data suggest therapeutic strategies to preserve muscle mass and metabolic function in critical illness.
Critically ill patients experience marked skeletal muscle atrophy, but the molecular mechanisms responsible for this are largely unresolved. Therefore, we investigated key genes and proteins, identified from cell and animal studies to control protein synthesis and breakdown, in vastus lateralis biopsy samples obtained from 10 patients and 10 age‐ and sex‐matched healthy controls. Muscle cytokines IL‐6 and TNF‐α mRNA were higher in patients than in controls (6.5‐fold; P < 0.001 and 2‐fold; P < 0.01). From the perspective of muscle protein breakdown, muscle‐specific E3‐ligases (MAFbx and MuRF1) were higher in patients at mRNA (4.5‐fold; P < 0.05 and 2.5‐fold; P < 0.05) and protein (5‐fold; P < 0.001 and 4.5‐fold; P < 0.001) level. Furthermore, 20S proteasome mRNA and protein were higher in patients (5‐fold; P < 0.001 and 2.5‐fold; P < 0.01). Cathepsin‐L mRNA was 2‐fold higher (P < 0.01), whilst calpain‐3 mRNA (2‐fold; P < 0.01) and protein (4‐fold; P < 0.01) were lower in patients. Another novel observation was the 3‐fold (P < 0.05) and 8.5‐fold (P < 0.001) higher expression of myostatin mRNA and protein in patients. Widespread dephosphorylation (inactivation) of proteins regulating translation initiation factor activation and protein synthesis (Akt1, GSK3α,β, mTOR, p70S6K and 4E‐BP1) was observed in patients, which was paralleled by increases in their mRNAs. Finally, PDK4 mRNA and protein was 2‐fold (P < 0.05) and 2.6‐fold (P < 0.01), respectively, higher in patients. In conclusion, we showed comprehensive alterations in molecular events thought to reduce muscle mass and carbohydrate (CHO) oxidation in critically ill patients. Nevertheless, these catabolic events were matched by a cellular programme of anabolic restoration at the transcriptional level. This shows a high molecular plasticity in the muscle of patients, and strategies to preserve muscle mass and metabolic function should focus on maintaining Akt phosphorylation and inhibiting myostatin expression.
Background
Bed rest (BR) reduces whole‐body insulin‐stimulated glucose disposal (GD) and alters muscle fuel metabolism, but little is known about metabolic adaptation from acute to chronic BR nor the ...mechanisms involved, particularly when volunteers are maintained in energy balance.
Methods
Healthy males (n = 10, 24.0 ± 1.3 years), maintained in energy balance, underwent 3‐day BR (acute BR). A second cohort matched for sex and body mass index (n = 20, 34.2 ± 1.8 years) underwent 56‐day BR (chronic BR). A hyperinsulinaemic euglycaemic clamp (60 mU/m2/min) was performed to determine rates of whole‐body insulin‐stimulated GD before and after BR (normalized to lean body mass). Indirect calorimetry was performed before and during steady state of each clamp to calculate rates of whole‐body fuel oxidation. Muscle biopsies were taken to determine muscle glycogen, metabolite and intramyocellular lipid (IMCL) contents, and the expression of 191 mRNA targets before and after BR. Two‐way repeated measures analysis of variance was used to detect differences in endpoint measures.
Results
Acute BR reduced insulin‐mediated GD (Pre 11.5 ± 0.7 vs. Post 9.3 ± 0.6 mg/kg/min, P < 0.001), which was unchanged in magnitude following chronic BR (Pre 10.2 ± 0.4 vs. Post 7.9 ± 0.3 mg/kg/min, P < 0.05). This reduction in GD was paralleled by the elimination of the 35% increase in insulin‐stimulated muscle glycogen storage following both acute and chronic BR. Acute BR had no impact on insulin‐stimulated carbohydrate (CHO; Pre 3.69 ± 0.39 vs. Post 4.34 ± 0.22 mg/kg/min) and lipid (Pre 1.13 ± 0.14 vs. Post 0.59 ± 0.11 mg/kg/min) oxidation, but chronic BR reduced CHO oxidation (Pre 3.34 ± 0.18 vs. Post 2.72 ± 0.13 mg/kg/min, P < 0.05) and blunted the magnitude of insulin‐mediated inhibition of lipid oxidation (Pre 0.60 ± 0.07 vs. Post 0.85 ± 0.06 mg/kg/min, P < 0.05). Neither acute nor chronic BR increased muscle IMCL content. Plentiful mRNA abundance changes were detected following acute BR, which waned following chronic BR and reflected changes in fuel oxidation and muscle glycogen storage at this time point.
Conclusions
Acute BR suppressed insulin‐stimulated GD and storage, but the extent of this suppression increased no further in chronic BR. However, insulin‐mediated inhibition of fat oxidation after chronic BR was less than acute BR and was accompanied by blunted CHO oxidation. The juxtaposition of these responses shows that the regulation of GD and storage can be dissociated from substrate oxidation. Additionally, the shift in substrate oxidation after chronic BR was not explained by IMCL accumulation but reflected by muscle mRNA and pyruvate dehydrogenase kinase 4 protein abundance changes, pointing to lack of muscle contraction per se as the primary signal for muscle adaptation.
Background
Bed‐rest (BR) of only a few days duration reduces muscle protein synthesis and induces skeletal muscle atrophy and insulin resistance, but the scale and juxtaposition of these events have ...not been investigated concurrently in the same individuals. Moreover, the impact of short‐term exercise‐supplemented remobilization (ESR) on muscle volume, protein turnover and leg glucose uptake (LGU) in humans is unknown.
Methods
Ten healthy males (24 ± 1 years, body mass index 22.7 ± 0.6 kg/m2) underwent 3 days of BR, followed immediately by 3 days of ESR consisting of 5 × 30 maximal voluntary single‐leg isokinetic knee extensions at 90°/s each day. An isoenergetic diet was maintained throughout the study (30% fat, 15% protein and 55% carbohydrate). Resting LGU was calculated from arterialized‐venous versus venous difference across the leg and leg blood flow during the steady‐state of a 3‐h hyperinsulinaemic–euglycaemic clamp (60 mU/m2/min) measured before BR, after BR and after remobilization. Glycogen content was measured in vastus lateralis muscle biopsy samples obtained before and after each clamp. Leg muscle volume (LMV) was measured using magnetic resonance imaging before BR, after BR and after remobilization. Cumulative myofibrillar protein fractional synthetic rate (FSR) and whole‐body muscle protein breakdown (MPB) were measured over the course of BR and remobilization using deuterium oxide and 3‐methylhistidine stable isotope tracers that were administered orally.
Results
Compared with before BR, there was a 45% decline in insulin‐stimulated LGU (P < 0.05) after BR, which was paralleled by a reduction in insulin‐stimulated leg blood flow (P < 0.01) and removal of insulin‐stimulated muscle glycogen storage. These events were accompanied by a 43% reduction in myofibrillar protein FSR (P < 0.05) and a 2.5% decrease in LMV (P < 0.01) during BR, along with a 30% decline in whole‐body MPB after 2 days of BR (P < 0.05). Myofibrillar protein FSR and LMV were restored by 3 days of ESR (P < 0.01 and P < 0.01, respectively) but not by ambulation alone. However, insulin‐stimulated LGU and muscle glycogen storage were not restored by ESR.
Conclusions
Three days of BR caused concurrent reductions in LMV, myofibrillar protein FSR, myofibrillar protein breakdown and insulin‐stimulated LGU, leg blood flow and muscle glycogen storage in healthy, young volunteers. Resistance ESR restored LMV and myofibrillar protein FSR, but LGU and muscle glycogen storage remained depressed, highlighting divergences in muscle fuel and protein metabolism. Furthermore, ambulation alone did not restore LMV and myofibrillar protein FSR in the non‐exercised contralateral limb, emphasizing the importance of exercise rehabilitation following even short‐term BR.
The molecular mechanisms by which free fatty acids (FFA) inhibit muscle glucose oxidation is still elusive. We recently showed that C2C12 myotubes treated with palmitate (PAL) presented with greater ...protein expression levels of PDK4 and transcription factors PPARα and PPARδ and lower p-FOXO/t-FOXO protein ratios when compared to control. This was complemented with the hallmarks of metabolic inflexibility (MI), i.e., reduced rates of glucose uptake, PDC activity and maximal pyruvate-derived ATP production rates (MAPR). However, the relative contribution of these transcription factors to the increase in PDK4 and reduced glucose oxidation could not be established. Therefore, by using a similar myotube model, a series of individual siRNA gene silencing experiments, validated at transcriptional and translation levels, were performed in conjunction with measurements of glucose uptake, PDC activity, MAPR and concentrations of metabolites reflecting PDC flux (lactate and acetylcarnitine). Gene silencing of PPARα, δ and FOXO1 individually reduced PAL-mediated inhibition of PDC activity and increased glucose uptake, albeit by different mechanisms as only PPARδ and FOXO1 silencing markedly reduced PDK4 protein content. Additionally, PPARα and FOXO1 silencing, but not PPARδ, increased MAPR with PAL. PPARδ silencing also decreased FOXO1 protein. Since FOXO1 silencing did not alter PPARδ protein, this suggests that FOXO1 might be a PPARδ downstream target. In summary, this study suggests that the molecular mechanisms by which PAL reduces PDC-mediated glucose-derived pyruvate oxidation in muscle occur primarily through increased PPARδ and FOXO1 mediated increases in PDK4 protein expression and secondarily through PPARα mediated allosteric inhibition of PDC flux. Furthermore, since PPARδ seems to control FOXO1 expression, this may reflect an important role for PPARδ in preventing glucose oxidation under conditions of increased lipid availability.
Previous studies suggest that progesterone may possess neuroprotective properties after traumatic insult but, with the exception of reduced formation of cerebral oedema, limited experimental evidence ...has been presented to support this claim. In the present study we focused on the effect of progesterone treatment on structural and functional deficits in an experimental model of traumatic brain injury. Female mice exhibited significantly (P = 0.0445) reduced lesion volumes compared with males after aseptic cryogenic cerebral injury (ACI), suggesting that female sex steroids provide protection against this injury. In male mice, progesterone treatment after injury (three intraperitoneal doses of 8 mg/kg) reduced lesion volume (P = 0.0429) and improved performance in a spatial cognitive task (Morris water maze; P = 0.0014). However, progesterone had no demonstrable effect on the formation of oedema as measured using T2‐weighted magnetic resonance imaging, nor did it affect brain water content. The pro‐inflammatory cytokines TNF‐α and IL‐1β, and growth factors BDNF and G‐CSF, were all strongly transcriptionally activated after ACI. However, progesterone administration did not affect expression of these genes. This study provides strong evidence that progesterone possesses neuroprotective properties in a mouse model of traumatic brain injury, but suggests that the steroid achieves this effect through mechanism(s) independent of the inflammatory response or growth factor up‐regulation.
High-fat feeding inhibits pyruvate dehydrogenase complex (PDC)-controlled carbohydrate (CHO) oxidation, which contributes to muscle insulin resistance. We aimed to reveal molecular changes ...underpinning this process in resting and exercising humans. We also tested whether pharmacological activation of PDC overrides these diet-induced changes. Healthy males consumed a control diet (CD) and on two further occasions an isocaloric high-fat diet (HFD). After each diet, subjects cycled for 60 min after intravenous infusion with saline (CD and HFD) or dichloroacetate (HFD+DCA). Quadriceps muscle biopsies obtained before and after 10 and 60 min of exercise were used to estimate CHO use, PDC activation, and mRNAs associated with insulin, fat, and CHO signaling. Compared with CD, HFD increased resting pyruvate dehydrogenase kinase 2 (PDK2), PDK4, forkhead box class O transcription factor 1 (FOXO1), and peroxisome proliferator-activated receptor transcription factor α (PPARα) mRNA and reduced PDC activation. Exercise increased PDC activation and whole-body CHO use in HFD, but to a lower extent than in CD. Meanwhile PDK4 and FOXO1, but not PPARα or PDK2, mRNA remained elevated. HFD+DCA activated PDC throughout and restored whole-body CHO use during exercise. FOXO1 appears to play a role in HFD-mediated muscle PDK4 upregulation and inhibition of PDC and CHO oxidation in humans. Also, pharmacological activation of PDC restores HFD-mediated inhibition of CHO oxidation during exercise.
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