Activated protein synthesis and suppressed protein breakdown signaling in skeletal muscle of critically ill patients

Effect of an enteral amino acid blend on muscle and gut functionality in critically ill patients: a proof-of-concept randomized controlled trial

Background

A defining feature of prolonged critical illness is muscle wasting, leading to impaired recovery. Supplementation with a tailored blend of amino acids may bolster the innate gut defence, promote intestinal mucosa repair and limit muscle loss.

Methods

This was a monocentric, randomized, double-blind, placebo-controlled study that included patients with sepsis or acute respiratory distress syndrome. Patients received a specific combination of five amino acids or placebo mixed with enteral feeding for 21 days. Markers of renal function, gut barrier structure and functionality were collected at baseline and 1, 2, 3 and 8 weeks after randomization. Muscle structure and function were assessed through MRI measurements of the anterior quadriceps volume and by twitch airway pressure. Data were compared between groups relative to the baseline.

Results

Thirty-five critically ill patients were randomized. The amino acid blend did not impair urine output, blood creatinine levels or creatinine clearance. Plasma citrulline levels increased significantly along the treatment period in the amino acid group (difference in means [95% CI] 5.86 [1.72; 10.00] nmol/mL P = 0.007). Alanine aminotransferase and alkaline phosphatase concentrations were lower in the amino acid group than in the placebo group at one week (ratio of means 0.5 [0.29; 0.86] (P = 0.015) and 0.73 [0.57; 0.94] (P = 0.015), respectively). Twitch airway pressure and volume of the anterior quadriceps were greater in the amino acid group than in the placebo group 3 weeks after randomization (difference in means 10.6 [0.99; 20.20] cmH₂0 (P = 0.035) and 3.12 [0.5; 5.73] cm³/kg (P = 0.022), respectively).

Conclusions

Amino acid supplementation increased plasma citrulline levels, reduced alanine aminotransferase and alkaline phosphatase levels, and improved twitch airway pressure and anterior quadriceps volume.

Trial registration ClinicalTrials.gov, NCT02968836. Registered November 21, 2016.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-022-04232-5.

Variability in Skeletal Muscle Protein Synthesis Rates in Critically Ill Patients

(1) Background: Muscle protein synthesis in critically ill patients is, on average, normal despite dramatic muscle loss, but the variation is much larger than in controls. Here, we evaluate if this variation is due to 1) heterogeneity in synthesis rates, 2) morphological variation or infiltrating cells, or 3) heterogeneity in the synthesis of different protein fractions. (2) Methods: Muscle biopsies were taken from both legs of critically ill patients (n = 17). Mixed and mitochondrial protein synthesis rates and morphologies were evaluated in both legs. Synthesis rates of myosin and actin were determined in combined biopsies and compared with controls. (3) Results: Muscle protein synthesis rates had a large variability in the patients (1.4-10.8%/day). No differences in mixed and mitochondrial protein synthesis rates between both legs were observed. A microscopic examination revealed no morphological differences between the two legs or any infiltrating inflammatory cells. The synthesis rates for myosin were lower and for actin they were higher in the muscles of critically ill patients, compared with the controls. (4) Conclusions: The large variation in muscle protein synthesis rates in critically ill patients is not the result of heterogeneity in synthesis rates, nor due to infiltrating cells. There are differences in the synthesis rates of different proteins, but these do not explain the larger variations.

Hidden Agenda - The Involvement of Endoplasmic Reticulum Stress and Unfolded Protein Response in Inflammation-Induced Muscle Wasting

Critically ill patients at the intensive care unit (ICU) often develop a generalized weakness, called ICU-acquired weakness (ICUAW). A major contributor to ICUAW is muscle atrophy, a loss of skeletal muscle mass and function. Skeletal muscle assures almost all of the vital functions of our body. It adapts rapidly in response to physiological as well as pathological stress, such as inactivity, immobilization, and inflammation. In response to a reduced workload or inflammation muscle atrophy develops. Recent work suggests that adaptive or maladaptive processes in the endoplasmic reticulum (ER), also known as sarcoplasmic reticulum, contributes to this process. In muscle cells, the ER is a highly specialized cellular organelle that assures calcium homeostasis and therefore muscle contraction. The ER also assures correct folding of proteins that are secreted or localized to the cell membrane. Protein folding is a highly error prone process and accumulation of misfolded or unfolded proteins can cause ER stress, which is counteracted by the activation of a signaling network known as the unfolded protein response (UPR). Three ER membrane residing molecules, protein kinase R-like endoplasmic reticulum kinase (PERK), inositol requiring protein 1a (IRE1a), and activating transcription factor 6 (ATF6) initiate the UPR. The UPR aims to restore ER homeostasis by reducing overall protein synthesis and increasing gene expression of various ER chaperone proteins. If ER stress persists or cannot be resolved cell death pathways are activated. Although, ER stress-induced UPR pathways are known to be important for regulation of skeletal muscle mass and function as well as for inflammation and immune response its function in ICUAW is still elusive. Given recent advances in the development of ER stress modifying molecules for neurodegenerative diseases and cancer, it is important to know whether or not therapeutic interventions in ER stress pathways have favorable effects and these compounds can be used to prevent or treat ICUAW. In this review, we focus on the role of ER stress-induced UPR in skeletal muscle during critical illness and in response to predisposing risk factors such as immobilization, starvation and inflammation as well as ICUAW treatment to foster research for this devastating clinical problem.

Molecular Mechanisms of Muscle Fatigue

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.

Impact of prolonged sepsis on neural and muscular components of muscle contractions in a mouse model

BACKGROUND

Prolonged critically ill patients frequently develop debilitating muscle weakness that can affect both peripheral nerves and skeletal muscle. In-depth knowledge on the temporal contribution of neural and muscular components to muscle weakness is currently incomplete.

METHODS

We used a fluid-resuscitated, antibiotic-treated, parenterally fed murine model of prolonged (5 days) sepsis-induced muscle weakness (caecal ligation and puncture; n = 148). Electromyography (EMG) measurements were performed in two nerve-muscle complexes, combined with histological analysis of neuromuscular junction denervation, axonal degeneration, and demyelination. In situ muscle force measurements distinguished neural from muscular contribution to reduced muscle force generation. In myofibres, imaging and biomechanics were combined to evaluate myofibrillar contractile calcium sensitivity, sarcomere organization, and fibre structural properties. Myosin and actin protein content and titin gene expression were measured on the whole muscle.

RESULTS

Five days of sepsis resulted in increased EMG latency (P = 0.006) and decreased EMG amplitude (P < 0.0001) in the dorsal caudal tail nerve-tail complex, whereas only EMG amplitude was affected in the sciatic nerve-gastrocnemius muscle complex (P < 0.0001). Myelin sheath abnormalities (P = 0.2), axonal degeneration (number of axons; P = 0.4), and neuromuscular junction denervation (P = 0.09) were largely absent in response to sepsis, but signs of axonal swelling [higher axon area (P < 0.0001) and g-ratio (P = 0.03)] were observed. A reduction in maximal muscle force was present after indirect nerve stimulation (P = 0.007) and after direct muscle stimulation (P = 0.03). The degree of force reduction was similar with both stimulations (P = 0.2), identifying skeletal muscle, but not peripheral nerves, as the main contributor to muscle weakness. Myofibrillar calcium sensitivity of the contractile apparatus was unaffected by sepsis (P ≥ 0.6), whereas septic myofibres displayed disorganized sarcomeres (P < 0.0001) and altered myofibre axial elasticity (P < 0.0001). Septic myofibres suffered from increased rupturing in a passive stretching protocol (25% more than control myofibres; P = 0.04), which was associated with impaired myofibre active force generation (P = 0.04), linking altered myofibre integrity to function. Sepsis also caused a reduction in muscle titin gene expression (P = 0.04) and myosin and actin protein content (P = 0.05), but not the myosin-to-actin ratio (P = 0.7).

CONCLUSIONS

Prolonged sepsis-induced muscle weakness may predominantly be related to a disruption in myofibrillar cytoarchitectural structure, rather than to neural abnormalities.

Acute effect of passive cycling exercise on serum levels of interleukin-8 and interleukin-10 in mechanically ventilated critically ill patients

Background/aims

A growing body of evidence has demonstrated that early mobilisation is beneficial for patients in the intensive care unit. The aim of this prospective study was to investigate the acute effect of an early passive cycling exercise session on serum interleukin-8 and interleukin-10 levels in critically ill patients.

Methods

A total of 11 haemodynamically stable and deeply sedated (Richmond Agitation and Sedation Scale-4) adult patients within the first 48 hours of mechanical ventilation received a single 20-minute session of passive cycle ergometer. Serum interleukin-8 and interleukin-10 levels were measured at baseline, immediately after the intervention and 60 minutes after the intervention.

Findings

Interleukin-8 levels decreased significantly 60 minutes after the passive cycle ergometer session compared to baseline (P=0.001). The serum levels of interleukin-10 increased immediately after the end of the 20 minutes and at 60 minutes after the intervention (P<0.001).

Conclusions

A single 20-minute session of passive cycle ergometer seems to have a positive effect on the inflammatory response in critically ill patients.

Hydrocortisone mitigates ICU-AW by fine-tuning of muscle atrophic and hypertrophic signaling pathways in a sepsis model with limb immobilization

AIMS

Intensive care unit-acquired weakness (ICU-AW) is a complex spectrum of disability that delays recovery of critically ill-immobilized patients with sepsis. Much discrepancy remain on the use of corticosteroids and their impact on muscle regeneration in critical illness management. Therefore, the aim of this study is to investigate whether hydrocortisone (HCT) modulates muscle mass turnover in ICU-AW induced by sepsis with limb immobilization (SI).

MAIN METHODS

Sepsis by cecal ligation puncture (CLP) with forelimb-immobilization were performed in rats. The study consisted of four groups: Sham (left forelimb-immobilization), Sham HCT (left forelimb-immobilization + HCT), SI (CLP + left forelimb-immobilization) and SI HCT (CLP + left forelimb-immobilization + HCT). Motor force, blood and muscle sampling were assessed.

KEY FINDINGS

HCT prevented body weight loss associated with SI and attenuated systemic and muscular inflammation. Besides, myosin was restituted in SI HCT group in conjunction to muscle mass and strength restoration. Pro-hypertrophic calcineurin (PP2B-Aβ) and nuclear factor of activated T-cells C3 (NFATc3) but not protein kinase B (Akt) were re-activated by HCT. Finally, pro-atrophic extracellular signal-regulated kinases (ERK1/2) and p38 mitogen-activated protein kinases (p38) but not nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) were inhibited in SI HCT group.

SIGNIFICANCE

This study unravels new molecular events thought to control muscle protein synthesis in ICU-AW induced by sepsis and limb immobilization. HCT has a potential to fine-tune muscle-signaling pathways and to reduce the negative outcomes of ICU-AW.

Protein delivery in intermittent and continuous enteral nutrition with a protein-rich formula in critically ill patients-a protocol for the prospective randomized controlled proof-of-concept Protein Bolus Nutrition (Pro BoNo) study

Background

Critically ill patients rapidly develop muscle wasting resulting in sarcopenia, long-term disability and higher mortality. Bolus nutrition (30–60 min period), whilst having a similar incidence of aspiration as continuous feeding, seems to provide metabolic benefits through increased muscle protein synthesis due to higher leucine peaks.

To date, clinical evidence on achievement of nutritional goals and influence of bolus nutrition on skeletal muscle metabolism in ICU patients is lacking. The aim of the Pro BoNo study (Protein Bolus Nutrition) is to compare intermittent and continuous enteral feeding with a specific high-protein formula. We hypothesise that target quantity of protein is reached earlier (within 36 h) by an intermittent feeding protocol with a favourable influence on muscle protein synthesis.

Methods

Pro BoNo is a prospective randomised controlled study aiming to compare the impact of intermittent and continuous enteral feeding on preventing muscle wasting in 60 critically ill patients recruited during the first 48 h after ICU admission. The primary outcome measure is the time until the daily protein target (≥ 1.5 g protein/kg bodyweight/24 h) is achieved. Secondary outcome measures include tolerance of enteral feeding and evolution of glucose, urea and IGF-1. Ultrasound and muscle biopsy of the quadriceps will be performed.

Discussion

The Basel Pro BoNo study aims to collect innovative data on the effect of intermittent enteral feeding of critically ill patients on muscle wasting.

Trial registration

ClinicalTrials.gov NCT03587870. Registered on July 16, 2018. Swiss National Clinical Trials Portal SNCTP000003234. Last updated on July 24, 2019.

Insulin Modulates Myogenesis and Muscle Atrophy Resulting From Skin Scald Burn in Young Male Rats

BACKGROUND

Burn injuries (BIs) due to scalding are one of the most common accidents among children. BIs greater than 40% of total body surface area are considered extensive and result in local and systemic response. We sought to assess morphological and myogenic mechanisms through both short- and long-term intensive insulin therapies that affect the skeletal muscle after extensive skin BI in young rats.

MATERIALS AND METHODS

Wistar rats aged 21 d were distributed into four groups: control (C), control with insulin (C + I), scald burn injury (SI), and SI with insulin (SI + I). The SI groups were submitted to a 45% total body surface area burn, and the C + I and SI + I groups received insulin (5 UI/Kg/d) for 4 or 14 d. Glucose tolerance and the homeostatic model assessment of insulin resistance index were determined. Gastrocnemius muscles were analyzed for histopathological, morphometric, and immunohistochemical myogenic parameters (Pax7, MyoD, and MyoG); in addition, the expression of genes related to muscle atrophy (MuRF1 and MAFbx) and its regulation (IGF-1) were also assessed.

RESULTS

Short-term treatment with insulin favored muscle regeneration by primary myogenesis and decreased muscle atrophy in animals with BIs, whereas the long-term treatment modulated myogenesis by increasing the MyoD protein. Both treatments improved histopathological parameters and secondary myogenesis by increasing the MyoG protein.

CONCLUSIONS

Treatment with insulin benefits myogenic parameters during regeneration and modulates MuRF1, an important mediator of muscle atrophy.

Bioinformatics Analysis Identifies Hub Genes and Molecular Pathways Involved in Sepsis-Induced Myopathy

BACKGROUND Sepsis-induced myopathy (SIM) is a complication of sepsis that results in prolonged mechanical ventilation, long-term functional disability, and increased patient mortality. This study aimed to use bioinformatics analysis to identify hub genes and molecular pathways involved in SIM, to identify potential diagnostic or therapeutic biomarkers. MATERIAL AND METHODS The Gene Expression Omnibus (GEO) database was used to acquire the GSE13205 expression profile. The differentially expressed genes (DEGs) in cases of SIM and healthy controls, and the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were performed using the limma R/Bioconductor software package and clusterProfiler package in R, respectively. The protein-protein interaction (PPI) network data of DEGs was retrieved using the STRING database and analyzed using the Molecular Complex Detection (MCODE) Cytoscape software plugin. RESULTS A total of 196 DEGs were obtained in SIM samples compared with healthy samples, including 93 upregulated genes. The DEGs were significantly upregulated in mineral absorption, and the interleukin-17 (IL-17) signaling pathway and 103 down-regulated genes were associated with control of the bile secretion signaling pathway. A protein-protein interaction (PPI) network was constructed with 106 nodes and 192 edges. The top two important clusters were selected from the PPI by MCODE analysis. There were 16 hub genes with a high degree of connectivity in the PPI network that were selected, including heme oxygenase 1 (HMOX1), nicotinamide adenine dinucleotide phosphate quinone dehydrogenase 1 (NQO1), and metallothionein (MT)-1E. CONCLUSIONS Bioinformatics network analysis identified key hub genes and molecular mechanisms in SIM.

Impact of Very Early Physical Therapy During Septic Shock on Skeletal Muscle: A Randomized Controlled Trial

Supplemental Digital Content is available in the text.

Objectives:

As the catabolic state induced by septic shock together with the physical inactivity of patients lead to the rapid loss of muscle mass and impaired function, the purpose of this study was to test whether an early physical therapy during the onset of septic shock regulates catabolic signals and preserves skeletal muscle mass.

Design:

Randomized controlled trial.

Setting:

Tertiary mixed ICU.

Patients:

Adult patients admitted for septic shock within the first 72 hours.

Interventions:

Patients were assigned randomly into two groups. The control group benefited from manual mobilization once a day. The intervention group had twice daily sessions of both manual mobilization and 30-minute passive/active cycling therapy.

Measurements and Main Results:

Skeletal muscle biopsies and electrophysiology testing were performed at day 1 and day 7. Muscle biopsies were analyzed for histology and molecular components of signaling pathways regulating protein synthesis and degradation as well as inflammation markers. Hemodynamic values and patient perception were collected during each session. Twenty-one patients were included. Three died before the second muscle biopsy. Ten patients in the control and eight in the intervention group were analyzed. Markers of the catabolic ubiquitin-proteasome pathway, muscle atrophy F-box and muscle ring finger-1 messenger RNA, were reduced at day 7 only in the intervention group, but without difference between groups (muscle atrophy F-box: –7.3% ± 138.4% in control vs –56.4% ± 37.4% in intervention group; p = 0.23 and muscle ring finger-1: –30.8% ± 66.9% in control vs –62.7% ± 45.5% in intervention group; p = 0.15). Muscle fiber cross-sectional area (µm²) was preserved by exercise (–25.8% ± 21.6% in control vs 12.4% ± 22.5% in intervention group; p = 0.005). Molecular regulations suggest that the excessive activation of autophagy due to septic shock was lower in the intervention group, without being suppressed. Markers of anabolism and inflammation were not modified by the intervention, which was well tolerated by the patients.

Conclusions:

Early physical therapy during the first week of septic shock is safe and preserves muscle fiber cross-sectional area.

Adipose tissue protects against sepsis-induced muscle weakness in mice: from lipolysis to ketones

Background

ICU-acquired weakness is a debilitating consequence of prolonged critical illness that is associated with poor outcome. Recently, premorbid obesity has been shown to protect against such illness-induced muscle wasting and weakness. Here, we hypothesized that this protection was due to increased lipid and ketone availability.

Methods

In a centrally catheterized, fluid-resuscitated, antibiotic-treated mouse model of prolonged sepsis, we compared markers of lipolysis and fatty acid oxidation in lean and obese septic mice (n = 117). Next, we compared markers of muscle wasting and weakness in septic obese wild-type and adipose tissue-specific ATGL knockout (AAKO) mice (n = 73), in lean septic mice receiving either intravenous infusion of lipids or standard parenteral nutrition (PN) (n = 70), and in lean septic mice receiving standard PN supplemented with either the ketone body 3-hydroxybutyrate or isocaloric glucose (n = 49).

Results

Obese septic mice had more pronounced lipolysis (p ≤ 0.05), peripheral fatty acid oxidation (p ≤ 0.05), and ketogenesis (p ≤ 0.05) than lean mice. Blocking lipolysis in obese septic mice caused severely reduced muscle mass (32% loss vs. 15% in wild-type, p < 0.001) and specific maximal muscle force (59% loss vs. 0% in wild-type; p < 0.001). In contrast, intravenous infusion of lipids in lean septic mice maintained specific maximal muscle force up to healthy control levels (p = 0.6), whereas this was reduced with 28% in septic mice receiving standard PN (p = 0.006). Muscle mass was evenly reduced with 29% in both lean septic groups (p < 0.001). Lipid administration enhanced fatty acid oxidation (p ≤ 0.05) and ketogenesis (p < 0.001), but caused unfavorable liver steatosis (p = 0.01) and a deranged lipid profile (p ≤ 0.01). Supplementation of standard PN with 3-hydroxybutyrate also attenuated specific maximal muscle force up to healthy control levels (p = 0.1), but loss of muscle mass could not be prevented (25% loss in both septic groups; p < 0.001). Importantly, this intervention improved muscle regeneration markers (p ≤ 0.05) without the unfavorable side effects seen with lipid infusion.

Conclusions

Obesity-induced muscle protection during sepsis is partly mediated by elevated mobilization and metabolism of endogenous fatty acids. Furthermore, increased availability of ketone bodies, either through ketogenesis or through parenteral infusion, appears to protect against sepsis-induced muscle weakness also in the lean.

Electronic supplementary material

The online version of this article (10.1186/s13054-019-2506-6) contains supplementary material, which is available to authorized users.

RNA-sequencing reveals altered skeletal muscle contraction, E3 ligases, autophagy, apoptosis, and chaperone expression in patients with critical illness myopathy

Background

Critical illness myopathy (CIM) is associated with severe skeletal muscle wasting and impaired function in intensive care unit (ICU) patients. The mechanisms underlying CIM remain incompletely understood. To elucidate the biological activities occurring at the transcriptional level in the skeletal muscle of ICU patients with CIM, the gene expression profiles, potential upstream regulators, and enrichment pathways were characterized using RNA sequencing (RNA-seq). We also compared the skeletal muscle gene signatures in ICU patients with CIM and genes perturbed by mechanical loading in one leg of the ICU patients, with an aim of reducing the loss of muscle function.

Methods

RNA-seq was used to assess gene expression changes in tibialis anterior skeletal muscle samples from seven critically ill, immobilized, and mechanically ventilated ICU patients with CIM and matched control subjects. We also examined skeletal muscle gene expression for both legs of six ICU patients with CIM, where one leg was mechanically loaded for 10 h/day for an average of 9 days.

Results

In total, 6257 of 17,221 detected genes were differentially expressed (84% upregulated; p < 0.05 and fold change ≥ 1.5) in skeletal muscle from ICU patients with CIM when compared to control subjects. The differentially expressed genes were highly associated with gene changes identified in patients with myopathy, sepsis, long-term inactivity, polymyositis, tumor, and repeat exercise resistance. Upstream regulator analysis revealed that the CIM signature could be a result of the activation of MYOD1, p38 MAPK, or treatment with dexamethasone. Passive mechanical loading only reversed expression of 0.74% of the affected genes (46 of 6257 genes).

Conclusions

RNA-seq analysis revealed that the marked muscle atrophy and weakness observed in ICU patients with CIM were associated with the altered expression of genes involved in muscle contraction, newly identified E3 ligases, autophagy and calpain systems, apoptosis, and chaperone expression. In addition, MYOD1, p38 MAPK, and dexamethasone were identified as potential upstream regulators of skeletal muscle gene expression in ICU patients with CIM. Mechanical loading only marginally affected the skeletal muscle transcriptome profiling of ICU patients diagnosed with CIM.

Electronic supplementary material

The online version of this article (10.1186/s13395-019-0194-1) contains supplementary material, which is available to authorized users.

Muscle wasting in the presence of disease, why is it so variable?

Skeletal muscle wasting is a common clinical feature of many chronic diseases and also occurs in response to single acute events. The accompanying loss of strength can lead to significant disability, increased care needs and have profound negative effects on quality of life. As muscle is the most abundant source of amino acids in the body, it appears to function as a buffer for fuel and substrates that can be used to repair damage elsewhere and to feed the immune system. In essence, the fundamentals of muscle wasting are simple: less muscle is made than is broken down. However, although well-described mechanisms modulate muscle protein turnover, significant individual differences in the amount of muscle lost in the presence of a given severity of disease complicate the understanding of underlying mechanisms and suggest that individuals have different sensitivities to signals for muscle loss. Furthermore, the rate at which muscle protein is turned over under normal conditions means that clinically significant muscle loss can occur with changes in the rate of protein synthesis and/or breakdown that are too small to be measurable. Consequently, the changes in expression of factors regulating muscle turnover required to cause a decline in muscle mass are small and, except in cases of rapid wasting, there is no consistent pattern of change in the expression of factors that regulate muscle mass. MicroRNAs are fine tuners of cell phenotype and are therefore ideally suited to cause the subtle changes in proteome required to tilt the balance between synthesis and degradation in a way that causes clinically significant wasting. Herein we present a model in which muscle loss as a consequence of disease in non-muscle tissue is modulated by a set of microRNAs, the muscle expression of which is associated with severity of disease in the non-muscle tissue. These microRNAs alter fundamental biological processes including the synthesis of ribosomes and mitochondria leading to reduced protein synthesis and increased protein breakdown, thereby freeing amino acids from the muscle. We argue that the variability in muscle loss observed in the human population arises from at least two sources. The first is from pre-existing or disease-induced variation in the expression of microRNAs controlling the sensitivity of muscle to the atrophic signal and the second is from the expression of microRNAs from imprinted loci (i.e. only expressed from the maternally or paternally inherited allele) and may control the rate of myonuclear recruitment. In the absence of disease, these factors do not correlate with muscle mass, since there is no challenge to the established balance. However, in the presence of such a challenge, these microRNAs determine the rate of decline for a given disease severity. Together these mechanisms provide novel insight into the loss of muscle mass and its variation in the human population. The involvement of imprinted loci also suggests that genes that regulate early development also contribute to the ability of individuals to resist muscle loss in response to disease.

A conceptual framework: the early and late phases of skeletal muscle dysfunction in the acute respiratory distress syndrome

Patients with acute respiratory distress syndrome (ARDS) often develop severe diaphragmatic and limb skeletal muscle dysfunction. Impaired muscle function in ARDS is associated with increased mortality, increased duration of mechanical ventilation, and functional disability in survivors. In this review, we propose that muscle dysfunction in ARDS can be categorized into an early and a late phase. These early and late phases are based on the timing in relationship to lung injury and the underlying mechanisms. The early phase occurs temporally with the onset of lung injury, is driven by inflammation and disuse, and is marked predominantly by muscle atrophy from increased protein degradation. The ubiquitin-proteasome, autophagy, and calpain-caspase pathways have all been implicated in early-phase muscle dysfunction. Late-phase muscle weakness persists in many patients despite resolution of lung injury and cessation of ongoing acute inflammation-driven muscle atrophy. The clinical characteristics and mechanisms underlying late-phase muscle dysfunction do not involve the massive protein degradation and atrophy of the early phase and may reflect a failure of the musculoskeletal system to regain homeostatic balance. Owing to these underlying mechanistic differences, therapeutic interventions for treating muscle dysfunction in ARDS may differ during the early and late phases. Here, we review clinical and translational investigations of muscle dysfunction in ARDS, placing them in the conceptual framework of the early and late phases. We hypothesize that this conceptual model will aid in the design of future mechanistic and clinical investigations of the skeletal muscle system in ARDS and other critical illnesses.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

Neuromuscular electrical stimulation prevents muscle wasting in critically ill comatose patients

Fully sedated patients, being treated in the intensive care unit (ICU), experience substantial skeletal muscle loss. Consequently, survival rate is reduced and full recovery after awakening is compromised. Neuromuscular electrical stimulation (NMES) represents an effective method to stimulate muscle protein synthesis and alleviate muscle disuse atrophy in healthy subjects. We investigated the efficacy of twice-daily NMES to alleviate muscle loss in six fully sedated ICU patients admitted for acute critical illness [n=3 males, n=3 females; age 63 ± 6 y; APACHE II (Acute Physiology and Chronic Health Evaluation II) disease-severity-score: 29 ± 2]. One leg was subjected to twice-daily NMES of the quadriceps muscle for a period of 7 ± 1 day whereas the other leg acted as a non-stimulated control (CON). Directly before the first and on the morning after the final NMES session, quadriceps muscle biopsies were collected from both legs to assess muscle fibre-type-specific cross-sectional area (CSA). Furthermore, phosphorylation status of the key proteins involved in the regulation of muscle protein synthesis was assessed and mRNA expression of selected genes was measured. In the CON leg, type 1 and type 2 muscle-fibre-CSA decreased by 16 ± 9% and 24 ± 7% respectively (P<0.05). No muscle atrophy was observed in the stimulated leg. NMES increased mammalian target of rapamycin (mTOR) phosphorylation by 19 ± 5% when compared with baseline (P<0.05), with no changes in the CON leg. Furthermore, mRNA expression of key genes involved in muscle protein breakdown either declined [forkhead box protein O1 (FOXO1); P<0.05] or remained unchanged [muscle atrophy F-box (MAFBx) and muscle RING-finger protein-1 (MuRF1)], with no differences between the legs. In conclusion, NMES represents an effective and feasible interventional strategy to prevent skeletal muscle atrophy in critically ill comatose patients.

Muscle wasting and early mobilization in acute respiratory distress syndrome

Survivors of acute respiratory distress syndrome often sustain muscle wasting and functional impairment related to intensive care unit (ICU)-acquired weakness (ICUAW) and this disability may persist for years after ICU discharge. Early diagnosis in cooperative patients by physical examination is recommended to identify patients at risk for weaning failure and to minimize prolongation of risk factors for ICUAW. When possible, early rehabilitation in critically ill patients improves functional outcomes, likely by reducing disuse atrophy. Interventions designed to correct the functional impairment are lacking and further research to delineate the molecular pathways that give rise to ICUAW are needed.

Alterations in molecular muscle mass regulators after 8 days immobilizing Special Forces mission

In military operations, declined physical capacity can endanger the life of soldiers. During special support and reconnaissance (SSR) missions, Special Forces soldiers sustain 1-2 weeks full-body horizontal immobilization, which impairs muscle strength and performance. Adequate muscle mass and strength are necessary in combat or evacuation situations, which prompt for improved understanding of muscle mass modulation during SSR missions. To explore the molecular regulation of myofiber size during a simulated SSR operation, nine male Special Forces soldiers were biopsied in m. vastus lateralis pre and post 8 days immobilizing restricted prone position. After immobilization, total mammalian target of rapamycin protein was reduced by 42% (P < 0.05), whereas total and phosphorylated protein levels of Akt, ribosomal protein S6k, 4E-BP1, and glycogen synthase kinase3β were unchanged. Messenger RNA (mRNA) levels of the atrogenes forkhead box O3 (FoxO3), atrogin1, and muscle ring finger protein1 (MuRF1) increased by 36%, 53%, and 71% (P < 0.01), MuRF1 protein by 51% (P = 0.05), whereas FoxO1 and peroxisome proliferator-activated receptor γ coactivator-1 β mRNAs decreased by 29% and 40% (P < 0.01). In conclusion, occupational immobilization in Special Forces soldiers led to modulations in molecular muscle mass regulators during 8 days prone SSR mission, which likely contribute to muscle loss observed in such operations. The present data expand our knowledge of human muscle mass regulation during short-term immobilization.

Regulation of muscle protein synthesis and the effects of catabolic states

Protein synthesis and degradation are dynamically regulated processes that act in concert to control the accretion or loss of muscle mass. The present article focuses on the mechanisms involved in the impairment of protein synthesis that are associated with skeletal muscle atrophy. The vast majority of mechanisms known to regulate protein synthesis involve modulation of the initiation phase of mRNA translation, which comprises a series of reactions that result in the binding of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. The function of the proteins involved in both events has been shown to be repressed under atrophic conditions such as sepsis, cachexia, chronic kidney disease, sarcopenia, and disuse atrophy. The basis for the inhibition of protein synthesis under such conditions is likely to be multifactorial and includes insulin/insulin-like growth factor 1 resistance, pro-inflammatory cytokine expression, malnutrition, corticosteroids, and/or physical inactivity. The present article provides an overview of the existing literature regarding mechanisms and signaling pathways involved in the regulation of mRNA translation as they apply to skeletal muscle wasting, as well as the efficacy of potential clinical interventions such as nutrition and exercise in the maintenance of skeletal muscle protein synthesis under atrophic conditions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Short-term bed rest increases TLR4 and IL-6 expression in skeletal muscle of older adults

Bed rest induces significant loss of leg lean mass in older adults. Systemic and tissue inflammation also accelerates skeletal muscle loss, but it is unknown whether inflammation is associated to inactivity-induced muscle atrophy in healthy older adults. We determined if short-term bed rest increases toll-like receptor 4 (TLR4) signaling and pro-inflammatory markers in older adult skeletal muscle biopsy samples. Six healthy, older adults underwent seven consecutive days of bed rest. Muscle biopsies (vastus lateralis) were taken after an overnight fast before and at the end of bed rest. Serum cytokine expression was measured before and during bed rest. TLR4 signaling and cytokine mRNAs associated with pro- and anti-inflammation and anabolism were measured in muscle biopsy samples using Western blot analysis and qPCR. Participants lost ∼4% leg lean mass with bed rest. We found that after bed rest, muscle levels of TLR4 protein expression and interleukin-6 (IL-6), nuclear factor-κB1, interleukin-10, and 15 mRNA expression were increased after bed rest (P < 0.05). Additionally, the cytokines interferon-γ, and macrophage inflammatory protein-1β, were elevated in serum samples following bed rest (P < 0.05). We conclude that short-term bed rest in older adults modestly increased some pro- and anti-inflammatory cytokines in muscle samples while systemic changes in pro-inflammatory cytokines were mostly absent. Upregulation of TLR4 protein content suggests that bed rest in older adults increases the capacity to mount an exaggerated, and perhaps unnecessary, inflammatory response in the presence of specific TLR4 ligands, e.g., during acute illness.

ICU-acquired weakness: mechanisms of disability

PURPOSE OF REVIEW

ICU-acquired weakness (ICUAW) is now recognized as a major complication of critical illness. There is no doubt that ICUAW is prevalent - some might argue ubiquitous - after critical illness, but its true role, the interaction with preexisting nerve and muscle lesions as well as its contribution to long-term functional disability, remains to be elucidated.

RECENT FINDINGS

In this article, we review the current state-of-the-art of the basic pathophysiology of nerve and muscle weakness after critical illness and explore the current literature on ICUAW with a special emphasis on the most important mechanisms of weakness.

SUMMARY

Variable contributions of structural and functional changes likely contribute to both early and late myopathy and neuropathy, although the specifics of the temporality of both processes, and the influence patient comorbidities, age, and nature of the ICU insult have on them, remain to be determined.

Intensive care unit-acquired weakness: clinical phenotypes and molecular mechanisms

Intensive care unit-acquired weakness (ICUAW) begins within hours of mechanical ventilation and may not be completely reversible over time. It represents a major functional morbidity of critical illness and is an important patient-centered outcome with clear implications for quality of life and resumption of prior work and lifestyle. There is heterogeneity in functional outcome related to ICUAW across various patient populations after an episode of critical illness. This state-of-the art review argues that this observed heterogeneity may represent a clinical spectrum of disability in which there are recognizable clinical phenotypes for outcome according to age, burden of comorbid illness, and ICU length of stay. It further argues that these functional outcomes are modified by mood, cognition, and caregiver physical and mental health. This proposed construct of clinical phenotypes will be used as a framework for a review of the current literature on the molecular biology of muscle and nerve injury. This translational approach for the development of models pairing clinical phenotypes for different functional outcomes after critical illness with molecular mechanism of injury may offer unique insights into the diagnosis and treatment of muscle and nerve lesions.

Genomic and proteomic profiling reveals reduced mitochondrial function and disruption of the neuromuscular junction driving rat sarcopenia

Molecular mechanisms underlying sarcopenia, the age-related loss of skeletal muscle mass and function, remain unclear. To identify molecular changes that correlated best with sarcopenia and might contribute to its pathogenesis, we determined global gene expression profiles in muscles of rats aged 6, 12, 18, 21, 24, and 27 months. These rats exhibit sarcopenia beginning at 21 months. Correlation of the gene expression versus muscle mass or age changes, and functional annotation analysis identified gene signatures of sarcopenia distinct from gene signatures of aging. Specifically, mitochondrial energy metabolism (e.g., tricarboxylic acid cycle and oxidative phosphorylation) pathway genes were the most downregulated and most significantly correlated with sarcopenia. Also, perturbed were genes/pathways associated with neuromuscular junction patency (providing molecular evidence of sarcopenia-related functional denervation and neuromuscular junction remodeling), protein degradation, and inflammation. Proteomic analysis of samples at 6, 18, and 27 months confirmed the depletion of mitochondrial energy metabolism proteins and neuromuscular junction proteins. Together, these findings suggest that therapeutic approaches that simultaneously stimulate mitochondrogenesis and reduce muscle proteolysis and inflammation have potential for treating sarcopenia.

JAK/STAT3 pathway inhibition blocks skeletal muscle wasting downstream of IL-6 and in experimental cancer cachexia

Cachexia, the metabolic dysregulation leading to sustained loss of muscle and adipose tissue, is a devastating complication of cancer and other chronic diseases. Interleukin-6 and related cytokines are associated with muscle wasting in clinical and experimental cachexia, although the mechanisms by which they might induce muscle wasting are unknown. One pathway activated strongly by IL-6 family ligands is the JAK/STAT3 pathway, the function of which has not been evaluated in regulation of skeletal muscle mass. Recently, we showed that skeletal muscle STAT3 phosphorylation, nuclear localization, and target gene expression are activated in C26 cancer cachexia, a model with high IL-6 family ligands. Here, we report that STAT3 activation is a common feature of muscle wasting, activated in muscle by IL-6 in vivo and in vitro and by different types of cancer and sterile sepsis. Moreover, STAT3 activation proved both necessary and sufficient for muscle wasting. In C(2)C(12) myotubes and in mouse muscle, mutant constitutively activated STAT3-induced muscle fiber atrophy and exacerbated wasting in cachexia. Conversely, inhibiting STAT3 pharmacologically with JAK or STAT3 inhibitors or genetically with dominant negative STAT3 and short hairpin STAT3 reduced muscle atrophy downstream of IL-6 or cancer. These results indicate that STAT3 is a primary mediator of muscle wasting in cancer cachexia and other conditions of high IL-6 family signaling. Thus STAT3 could represent a novel therapeutic target for the preservation of skeletal muscle in cachexia.

Effects of 2 weeks lower limb immobilization and two separate rehabilitation regimens on gastrocnemius muscle protein turnover signaling and normalization genes

BACKGROUND

Limb immobilization causes a rapid loss of muscle mass and strength that requires appropriate rehabilitation to ensure restoration of normal function. Whereas the knowledge of muscle mass signaling with immobilization has increased in recent years, the molecular regulation in the rehabilitation of immobilization-induced muscle atrophy is only sparsely studied. To investigate the phosphorylation and expression of candidate key molecular muscle mass regulators after immobilization and subsequent rehabilitation we performed two separate studies.

METHODS

We immobilized the lower limb for 2 weeks followed by the in-house hospital standard physiotherapy rehabilitation (Study 1). Secondly, we conducted an intervention study using the same 2 weeks immobilization protocol during which protein/carbohydrate supplementation was given. This was followed by 6 weeks of rehabilitation in the form of resistance training and continued protein/carbohydrate supplementation (Study 2). We obtained muscle biopsies from the medial gastrocnemius prior to immobilization (PRE), post-immobilization (IMMO) and post-rehabilitation (REHAB) and measured protein expression and phosphorylation of Akt, mTOR, S6k, 4E-BP1, GSK3β, ubiquitin and MURF1 and mRNA expression of Atrogin-1, MURF1, FOXO1, 3 and 4 as well as appropriate housekeeping genes.

RESULTS

In both studies, no changes in protein expression or phosphorylation for any measured protein were observed. In Study 1, FOXO3 and FOXO4 mRNA expression decreased after IMMO and REHAB compared to PRE, whereas other mRNAs remained unchanged. Interestingly, we found significant changes in expression of the putative housekeeping genes GAPDH, HADHA and S26 with immobilization in both studies.

CONCLUSIONS

In neither study, the changes in muscle mass associated with immobilization and rehabilitation were accompanied by expected changes in expression of atrophy-related genes or phosphorylation along the Akt axis. Unexpectedly, we observed significant changes in several of the so-called housekeeping genes GAPDH, HADHA and S26 with immobilization in both studies, thereby questioning the usefulness of these genes for normalization of RNA data purposes in muscle immobilization studies.

Muscle wasting and the temporal gene expression pattern in a novel rat intensive care unit model

BACKGROUND

Acute quadriplegic myopathy (AQM) or critical illness myopathy (CIM) is frequently observed in intensive care unit (ICU) patients. To elucidate duration-dependent effects of the ICU intervention on molecular and functional networks that control the muscle wasting and weakness associated with AQM, a gene expression profile was analyzed at time points varying from 6 hours to 14 days in a unique experimental rat model mimicking ICU conditions, i.e., post-synaptically paralyzed, mechanically ventilated and extensively monitored animals.

RESULTS

During the observation period, 1583 genes were significantly up- or down-regulated by factors of two or greater. A significant temporal gene expression pattern was constructed at short (6 h-4 days), intermediate (5-8 days) and long (9-14 days) durations. A striking early and maintained up-regulation (6 h-14d) of muscle atrogenes (muscle ring-finger 1/tripartite motif-containing 63 and F-box protein 32/atrogin-1) was observed, followed by an up-regulation of the proteolytic systems at intermediate and long durations (5-14d). Oxidative stress response genes and genes that take part in amino acid catabolism, cell cycle arrest, apoptosis, muscle development, and protein synthesis together with myogenic factors were significantly up-regulated from 5 to 14 days. At 9-14 d, genes involved in immune response and the caspase cascade were up-regulated. At 5-14d, genes related to contractile (myosin heavy chain and myosin binding protein C), regulatory (troponin, tropomyosin), developmental, caveolin-3, extracellular matrix, glycolysis/gluconeogenesis, cytoskeleton/sarcomere regulation and mitochondrial proteins were down-regulated. An activation of genes related to muscle growth and new muscle fiber formation (increase of myogenic factors and JunB and down-regulation of myostatin) and up-regulation of genes that code protein synthesis and translation factors were found from 5 to 14 days.

CONCLUSIONS

Novel temporal patterns of gene expression have been uncovered, suggesting a unique, coordinated and highly complex mechanism underlying the muscle wasting associated with AQM in ICU patients and providing new target genes and avenues for intervention studies.

Progressive resistance training and cancer testis (PROTRACT) - efficacy of resistance training on muscle function, morphology and inflammatory profile in testicular cancer patients undergoing chemotherapy: design of a randomized controlled trial

Background

Standard treatment for patients with disseminated germ cell tumors is combination chemotherapy with bleomycin, etoposide and cisplatin (BEP). This treatment is highly effective, but the majority of patients experience severe adverse effects during treatment and are at risk of developing considerable long-term morbidity, including second malignant neoplasms, cardiovascular disease, and pulmonary toxicity. One neglected side effect is the significant muscular fatigue mentioned by many patients with testicular cancer both during and after treatment. Very limited information exists concerning the patho-physiological effects of antineoplastic agents on skeletal muscle. The primary aim of this study is to investigate the effects of BEP-treatment on the skeletal musculature in testicular cancer patients, and to examine whether the expected treatment-induced muscular deterioration can be attenuated or even reversed by high intensity progressive resistance training (HIPRT).

Design/Methods

The PROTRACT study is a randomized controlled trial in 30 testicular cancer patients undergoing three cycles of BEP chemotherapy. Participants will be randomized to either a 9-week HIPRT program (STR) initiated at the onset of treatment, or to standard care (UNT). 15 healthy matched control subjects (CON) will complete the same HIPRT program. All participants will take part in 3 assessment rounds (baseline, 9 wks, 21 wks) including muscle biopsies, maximum muscle strength tests, whole body DXA scan and blood samples. Primary outcome: mean fiber area and fiber type composition measured by histochemical analyses, satellite cells and levels of protein and mRNA expression of intracellular mediators of protein turnover. Secondary outcomes: maximum muscle strength and muscle power measured by maximum voluntary contraction and leg-extensor-power tests, body composition assessed by DXA scan, and systemic inflammation analyzed by circulating inflammatory markers, lipid and glucose metabolism in blood samples. Health related Quality of Life (QoL) will be assessed by validated questionnaires (EORTC QLQ-C30, SF-36).

Discussion

This study investigates the muscular effects of antineoplastic agents in testicular cancer patients, and furthermore evaluates whether HIPRT has a positive influence on side effects related to chemotherapy. A more extensive knowledge of the interaction between cytotoxic-induced physiological impairment and exercise-induced improvement is imperative for the future development of optimal rehabilitation programs for cancer patients.

Trial Registration

Current Controlled Trials ISRCTN32132990.