Precision Medicine for More Oxygen (P4O2)-Study Design and First Results of the Long COVID-19 Extension

Introduction: The coronavirus disease 2019 (COVID-19) pandemic has led to the death of almost 7 million people, however, with a cumulative incidence of 0.76 billion, most people survive COVID-19. Several studies indicate that the acute phase of COVID-19 may be followed by persistent symptoms including fatigue, dyspnea, headache, musculoskeletal symptoms, and pulmonary functional-and radiological abnormalities. However, the impact of COVID-19 on long-term health outcomes remains to be elucidated. Aims: The Precision Medicine for more Oxygen (P4O2) consortium COVID-19 extension aims to identify long COVID patients that are at risk for developing chronic lung disease and furthermore, to identify treatable traits and innovative personalized therapeutic strategies for prevention and treatment. This study aims to describe the study design and first results of the P4O2 COVID-19 cohort. Methods: The P4O2 COVID-19 study is a prospective multicenter cohort study that includes nested personalized counseling intervention trial. Patients, aged 40-65 years, were recruited from outpatient post-COVID clinics from five hospitals in The Netherlands. During study visits at 3-6 and 12-18 months post-COVID-19, data from medical records, pulmonary function tests, chest computed tomography scans and biological samples were collected and questionnaires were administered. Furthermore, exposome data was collected at the patient's home and state-of-the-art imaging techniques as well as multi-omics analyses will be performed on collected data. Results: 95 long COVID patients were enrolled between May 2021 and September 2022. The current study showed persistence of clinical symptoms and signs of pulmonary function test/radiological abnormalities in post-COVID patients at 3-6 months post-COVID. The most commonly reported symptoms included respiratory symptoms (78.9%), neurological symptoms (68.4%) and fatigue (67.4%). Female sex and infection with the Delta, compared with the Beta, SARS-CoV-2 variant were significantly associated with more persisting symptom categories. Conclusions: The P4O2 COVID-19 study contributes to our understanding of the long-term health impacts of COVID-19. Furthermore, P4O2 COVID-19 can lead to the identification of different phenotypes of long COVID patients, for example those that are at risk for developing chronic lung disease. Understanding the mechanisms behind the different phenotypes and identifying these patients at an early stage can help to develop and optimize prevention and treatment strategies.

An optimization and refinement of the whole-gut transit assay in mice

BACKGROUND

Gastrointestinal motility measurements in mice are currently performed under suboptimal conditions, as these nocturnal animals are measured during light conditions. In addition, other stressors, like individual housing, placement in a new cage during observation, and lack of bedding and cage enrichment cause animal discomfort and might contribute to higher variability. Here we aimed to develop a refined method of the widely-used whole-gut transit assay.

METHODS

Wildtype mice (N = 24) were subjected to the standard or refined whole-gut transit assay, either with or without a standardized slowing in gastrointestinal motility induced by loperamide. The standard assay consisted of a gavage with carmine red, observation during the light period and individual housing in a new cage without cage enrichment. For the refined whole-gut transit assay, mice were gavaged with UV-fluorescent DETEX®, observed during the dark period, while pairwise housed in their home cage with cage enrichment. Time until excretion of the first colored fecal pellet was assessed, and pellets were collected to assess number, weight, and water content.

KEY RESULTS

The DETEX®-containing pellets were UV-detectable, allowing to measure the mice in their active period in the dark. The refined method caused less variation (20.8% and 16.0%) compared to the standard method (29.0% and 21.7%). Fecal pellet number, weight, and water content was significantly different between the standard and refined method.

CONCLUSIONS & INFERENCES

This refined whole-gut transit assay provides a reliable approach to measure whole-gut transit time in mice in a more physiological context, with reduced variability compared to the standard method.

11β-HSD1 determines the extent of muscle atrophy in a model of acute exacerbation of COPD

Muscle atrophy is an extrapulmonary complication of acute exacerbations (AE) in chronic obstructive pulmonary disease (COPD). The endogenous production and therapeutic application of glucocorticoids (GCs) have been implicated as drivers of muscle loss in AE-COPD. The enzyme 11 β-hydroxysteroid dehydrogenase 1 (11β-HSD1) activates GCs and contributes toward GC-induced muscle wasting. To explore the potential of 11βHSD1 inhibition to prevent muscle wasting here, the objective of this study was to ascertain the contribution of endogenous GC activation and amplification by 11βHSD1 in skeletal muscle wasting during AE-COPD. Emphysema was induced by intratracheal (IT) instillation of elastase to model COPD in WT and 11βHSD1/KO mice, followed by vehicle or IT-LPS administration to mimic AE. µCT scans were obtained prior and at study endpoint 48 h following IT-LPS, to assess emphysema development and muscle mass changes, respectively. Plasma cytokine and GC profiles were determined by ELISA. In vitro, myonuclear accretion and cellular response to plasma and GCs were determined in C2C12 and human primary myotubes. Muscle wasting was exacerbated in LPS-11βHSD1/KO animals compared with WT controls. RT-qPCR and western blot analysis showed elevated catabolic and suppressed anabolic pathways in muscle of LPS-11βHSD1/KO animals relative to WTs. Plasma corticosterone levels were higher in LPS-11βHSD1/KO animals, whereas C2C12 myotubes treated with LPS-11βHSD1/KO plasma or exogenous GCs displayed reduced myonuclear accretion relative to WT counterparts. This study reveals that 11β-HSD1 inhibition aggravates muscle wasting in a model of AE-COPD, suggesting that therapeutic inhibition of 11β-HSD1 may not be appropriate to prevent muscle wasting in this setting.

Effects of a nutritional intervention on impaired behavior and cognitive function in an emphysematous murine model of COPD with endotoxin-induced lung inflammation

One cluster of the extrapulmonary manifestations in chronic obstructive pulmonary disease (COPD) is related to the brain, which includes anxiety, depression and cognitive impairment. Brain-related comorbidities are related to worsening of symptoms and increased mortality in COPD patients. In this study, a murine model of COPD was used to examine the effects of emphysema and repetitive pulmonary inflammatory events on systemic inflammatory outcomes and brain function. In addition, the effect of a dietary intervention on brain-related parameters was assessed. Adult male C57Bl/6J mice were exposed to elastase or vehicle intratracheally (i.t.) once a week on three consecutive weeks. Two weeks after the final administration, mice were i.t. exposed to lipopolysaccharide (LPS) or vehicle for three times with a 10 day interval. A dietary intervention enriched with omega-3 PUFAs, prebiotic fibers, tryptophan and vitamin D was administered from the first LPS exposure onward. Behavior and cognitive function, the degree of emphysema and both pulmonary and systemic inflammation as well as blood-brain barrier (BBB) integrity and neuroinflammation in the brain were assessed. A lower score in the cognitive test was observed in elastase-exposed mice. Mice exposed to elastase plus LPS showed less locomotion in the behavior test. The enriched diet seemed to reduce anxiety-like behavior over time and cognitive impairments associated with the presented COPD model, without affecting locomotion. In addition, the enriched diet restored the disbalance in splenic T-helper 1 (Th1) and Th2 cells. There was a trend toward recovering elastase plus LPS-induced decreased expression of occludin in brain microvessels, a measure of BBB integrity, as well as improving expression levels of kynurenine pathway markers in the brain by the enriched diet. The findings of this study demonstrate brain-associated comorbidities - including cognitive and behavioral impairments - in this murine model for COPD. Although no changes in lung parameters were observed, exposure to the specific enriched diet in this model appeared to improve systemic immune disbalance, BBB integrity and derailed kynurenine pathway which may lead to reduction of anxiety-like behavior and improved cognition.

Effects of Cigarette Smoke on Adipose and Skeletal Muscle Tissue: In Vivo and In Vitro Studies

Chronic obstructive pulmonary disease (COPD), often caused by smoking, is a chronic lung disease with systemic manifestations including metabolic comorbidities. This study investigates adaptive and pathological alterations in adipose and skeletal muscle tissue following cigarette smoke exposure using in vivo and in vitro models. Mice were exposed to cigarette smoke or air for 72 days and the pre-adipose cell line 3T3-L1 was utilized as an in vitro model. Cigarette smoke exposure decreased body weight, and the proportional loss in fat mass was more pronounced than the lean mass loss. Cigarette smoke exposure reduced adipocyte size and increased adipocyte numbers. Adipose macrophage numbers and associated cytokine levels, including interleukin-1β, interleukine-6 and tumor necrosis factor-α were elevated in smoke-exposed mice. Muscle strength and protein synthesis signaling were decreased after smoke exposure; however, muscle mass was not changed. In vitro studies demonstrated that lipolysis and fatty acid oxidation were upregulated in cigarette smoke-exposed pre-adipocytes. In conclusion, cigarette smoke exposure induces a loss of whole-body fat mass and adipose atrophy, which is likely due to enhanced lipolysis.

Fatty Acids as a Tool to Boost Cancer Immunotherapy Efficacy

Although immunotherapy represents one of the most potent therapeutic anti-cancer approaches, only a limited number of patients shows clinical benefit. Recent evidence suggests that patients' nutritional status plays a major role in immunotherapy outcome. Fatty acids are essential in a balanced diet and well-known to influence the immune response. Moreover, short-chain fatty acids (SCFAs) show beneficial effects in metabolic disorders as well as in cancer and polyunsaturated fatty acids (PUFAs) contribute to body weight and fat free mass preservation in cancer patients. In line with these data, several studies imply a role for SCFAs and PUFAs in boosting the outcome of immunotherapy. In this review, we specifically focus on mechanistic data showing that SCFAs modulate the immunogenicity of tumor cells and we discuss the direct effects of SCFAs and PUFAs on the immune system in the context of cancer. We provide preclinical and clinical evidence indicating that SCFAs and PUFAs may have the potential to boost immunotherapy efficacy. Finally, we describe the challenges and address opportunities for successful application of nutritional interventions focusing on SCFAs and PUFAs to increase the therapeutic potential of immunotherapeutic approaches for cancer.

Evaluation of the Sensitivity of Metabolic Profiling by Rapid Evaporative Ionization Mass Spectrometry: Toward More Radical Oral Cavity Cancer Resections

Radical resection for patients with oral cavity cancer remains challenging. Rapid evaporative ionization mass spectrometry (REIMS) of electrosurgical vapors has been reported for real-time classification of normal and tumor tissues for numerous surgical applications. However, the infiltrative pattern of invasion of oral squamous cell carcinomas (OSCC) challenges the ability of REIMS to detect low amounts of tumor cells. We evaluate REIMS sensitivity to determine the minimal amount of detected tumors cells during oral cavity cancer surgery. A total of 11 OSCC patients were included in this study. The tissue classification based on 185 REIMS ex vivo metabolic profiles from five patients was compared to histopathology classification using multivariate analysis and leave-one-patient-out cross-validation. Vapors were analyzed in vivo by REIMS during four glossectomies. Complementary desorption electrospray ionization–mass spectrometry imaging (DESI-MSI) was employed to map tissue heterogeneity on six oral cavity sections to support REIMS findings. REIMS sensitivity was assessed with a new cell-based assay consisting of mixtures of cell lines (tumor, myoblasts, keratinocytes). Our results depict REIMS classified tumor and soft tissues with 96.8% accuracy. In vivo REIMS generated intense mass spectrometric signals. REIMS detected 10% of tumor cells mixed with 90% myoblasts with 83% sensitivity and 82% specificity. DESI-MSI underlined distinct metabolic profiles of nerve features and a metabolic shift phosphatidylethanolamine PE(O-16:1/18:2))/cholesterol sulfate common to both mucosal maturation and OSCC differentiation. In conclusion, the assessment of tissue heterogeneity with DESI-MSI and REIMS sensitivity with cell mixtures characterized sensitive metabolic profiles toward in vivo tissue recognition during oral cavity cancer surgeries.

Physical exercise at the crossroad between muscle wasting and the immune system: implications for lung cancer cachexia

Cachexia is a syndrome characterized by involuntary weight loss and wasting of skeletal muscle mass. It is associated with worse overall survival and quality of life. The cancer-induced systemic inflammation and the consequent host derived catabolic stimuli, trigger cachexia by inhibiting muscle protein synthesis and enhancing muscle catabolism. The muscle itself may further promote chronic inflammation, introducing a vicious catabolic circle. Nutritional support alone plays a limited role in the treatment of cancer cachexia and should be combined with other interventions. Physical exercise lowers systemic inflammation and promotes muscle anabolism. It also attenuates the age-related physical decline in elderly and it might counteract the muscle wasting induced by the cancer cachexia syndrome. This review describes how cancer-induced systemic inflammation promotes muscle wasting and whether physical exercise may represent a suitable treatment for cancer-induced cachexia, particularly in patients with non-small cell lung cancer. We summarized pre-clinical and clinical studies investigating whether physical exercise would improve muscle performance and whether this improvement would translate in a clinically meaningful benefit for patients with cancer, in terms of survival and quality of life. Moreover, this review describes the results of studies investigating the interplay between physical exercise and the immune system, including the role of the intestinal microbiota.

Nutritional Interventions in Cancer Cachexia: Evidence and Perspectives From Experimental Models

Cancer cachexia is a complex metabolic syndrome characterized by involuntary skeletal muscle loss and is associated with poor clinical outcome, decreased survival and negatively influences cancer therapy. No curative treatments are available for cancer cachexia, but nutritional intervention is recommended as a cornerstone of multimodal therapy. Optimal nutritional care is pivotal in the treatment of cancer cachexia, and the effects of nutrients may extend beyond provision of adequate energy uptake, targeting different mechanisms or metabolic pathways that are affected or deregulated by cachexia. The evidence to support this notion derived from nutritional intervention studies in experimental models of cancer cachexia is systematically discussed in this review. Moreover, experimental variables and readout parameters to determine skeletal muscle wasting and cachexia are methodologically evaluated to allow critical comparison of similar studies. Single- and multinutrient intervention studies including qualitative modulation of dietary protein, dietary fat, and supplementation with specific nutrients, such as carnitine and creatine, were reviewed for their efficacy to counteract muscle mass loss and its underlying mechanisms in experimental cancer cachexia. Numerous studies showed favorable effects on impaired protein turnover and related metabolic abnormalities of nutritional supplementation in parallel with a beneficial impact on cancer-induced muscle wasting. The combination of high quality nutrients in a multitargeted, multinutrient approach appears specifically promising, preferentially as a multimodal intervention, although more studies investigating the optimal quantity and combination of nutrients are needed. During the review process, a wide variation in timing, duration, dosing, and route of supplementation, as well as a wide variation in animal models were observed. Better standardization in dietary design, and the development of experimental models that better recapitulate the etiology of human cachexia, will further facilitate successful translation of experimentally-based multinutrient, multimodal interventions into clinical practice.

Inflammation and Skeletal Muscle Wasting During Cachexia

Cachexia is the involuntary loss of muscle and adipose tissue that strongly affects mortality and treatment efficacy in patients with cancer or chronic inflammatory disease. Currently, no specific treatments or interventions are available for patients developing this disorder. Given the well-documented involvement of pro-inflammatory cytokines in muscle and fat metabolism in physiological responses and in the pathophysiology of chronic inflammatory disease and cancer, considerable interest has revolved around their role in mediating cachexia. This has been supported by association studies that report increased levels of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) in some, but not all, cancers and in chronic inflammatory diseases such as chronic obstructive pulmonary disease (COPD) and rheumatoid arthritis (RA). In addition, preclinical studies including animal disease models have provided a substantial body of evidence implicating a causal contribution of systemic inflammation to cachexia. The presence of inflammatory cytokines can affect skeletal muscle through several direct mechanisms, relying on activation of the corresponding receptor expressed by muscle, and resulting in inhibition of muscle protein synthesis (MPS), elevation of catabolic activity through the ubiquitin-proteasomal system (UPS) and autophagy, and impairment of myogenesis. Additionally, systemic inflammatory mediators indirectly contribute to muscle wasting through dysregulation of tissue and organ systems, including GCs via the hypothalamus-pituitary-adrenal (HPA) axis, the digestive system leading to anorexia-cachexia, and alterations in liver and adipocyte behavior, which subsequently impact on muscle. Finally, myokines secreted by skeletal muscle itself in response to inflammation have been implicated as autocrine and endocrine mediators of cachexia, as well as potential modulators of this debilitating condition. While inflammation has been shown to play a pivotal role in cachexia development, further understanding how these cytokines contribute to disease progression is required to reveal biomarkers or diagnostic tools to help identify at risk patients, or enable the design of targeted therapies to prevent or delay the progression of cachexia.

Identification of microRNAs in skeletal muscle associated with lung cancer cachexia

BACKGROUND

Cachexia, highly prevalent in patients with non-small cell lung cancer (NSCLC), impairs quality of life and is associated with reduced tolerance and responsiveness to cancer therapy and decreased survival. MicroRNAs (miRNAs) are small non-coding RNAs that play a central role in post-transcriptional gene regulation. Changes in intramuscular levels of miRNAs have been implicated in muscle wasting conditions. Here, we aimed to identify miRNAs that are differentially expressed in skeletal muscle of cachectic lung cancer patients to increase our understanding of cachexia and to allow us to probe their potential as therapeutic targets.

METHODS

A total of 754 unique miRNAs were profiled and analysed in vastus lateralis muscle biopsies of newly diagnosed treatment-naïve NSCLC patients with cachexia (n = 8) and age-matched and sex-matched healthy controls (n = 8). miRNA expression analysis was performed using a TaqMan MicroRNA Array. In silico network analysis was performed on all significant differentially expressed miRNAs. Differential expression of the top-ranked miRNAs was confirmed using reverse transcription-quantitative real-time PCR in an extended group (n = 48) consisting of NSCLC patients with (n = 15) and without cachexia (n = 11) and healthy controls (n = 22). Finally, these miRNAs were subjected to univariate and multivariate Cox proportional hazard analysis using overall survival and treatment-induced toxicity data obtained during the follow-up of this group of patients.

RESULTS

We identified 28 significant differentially expressed miRNAs, of which five miRNAs were up-regulated and 23 were down-regulated. In silico miRNA-target prediction analysis showed 158 functional gene targets, and pathway analysis identified 22 pathways related to the degenerative or regenerative processes of muscle tissue. Subsequently, the expression of six top-ranked miRNAs was measured in muscle biopsies of the entire patient group. Five miRNAs were detectable with reverse transcription-quantitative real-time PCR analysis, and their altered expression (expressed as fold change, FC) was confirmed in muscle of cachectic NSCLC patients compared with healthy control subjects: miR-424-5p (FC = 4.5), miR-424-3p (FC = 12), miR-450a-5p (FC = 8.6), miR-144-5p (FC = 0.59), and miR-451a (FC = 0.57). In non-cachectic NSCLC patients, only miR-424-3p was significantly increased (FC = 5.6) compared with control. Although the statistical support was not sufficient to imply these miRNAs as individual predictors of overall survival or treatment-induced toxicity, when combined in multivariate analysis, miR-450-5p and miR-451a resulted in a significant stratification between short-term and long-term survival.

CONCLUSIONS

We identified differentially expressed miRNAs putatively involved in lung cancer cachexia. These findings call for further studies to investigate the causality of these miRNAs in muscle atrophy and the mechanisms underlying their differential expression in lung cancer cachexia.

Regulation of PGC-1α expression by a GSK-3β-TFEB signaling axis in skeletal muscle

OBJECTIVE

In muscle cells, the peroxisome proliferator-activated receptor γ co-activator 1 (PGC-1) signaling network, which has been shown to be disturbed in the skeletal muscle in several chronic diseases, tightly controls mitochondrial biogenesis and oxidative substrate metabolism. Previously, we showed that inactivation of glycogen synthase kinase (GSK)-3β potently increased Pgc-1α abundance and oxidative metabolism in skeletal muscle cells. The current study aims to unravel the molecular mechanism driving the increase in Pgc-1α mediated by GSK-3β inactivation.

METHODS

GSK-3β was inactivated genetically or pharmacologically in C2C12 myotubes and the requirement of transcription factors known to be involved in Pgc-1α transcription for increases in Pgc-1α abundance mediated by inactivation of GSK-3β was examined.

RESULTS

Enhanced PGC-1α promoter activation after GSK-3β inhibition suggested a transcriptionally-controlled mechanism. While myocyte enhancer factor (MEF)2 transcriptional activity was unaltered, GSK-3β inactivation increased the abundance and activity of the transcription factors estrogen-related receptor (ERR)α and ERRγ. Pharmacological inhibition or knock-down of ERRα and ERRγ however failed to prevent increases in Pgc-1α mRNA mediated by GSK-3β inactivation. Interestingly, GSK-3β inactivation activated transcription factor EB (TFEB), evidenced by decreased phosphorylation and enhanced nuclear localization of the TFEB protein. Moreover, knock-down of TFEB completely prevented increases in Pgc-1α gene expression, PGC-1α promoter activity and PGC-1α protein abundance induced by GSK-3β inactivation. Furthermore, mutation of a specific TFEB binding site on the PGC-1α promoter blocked promoter activation upon inhibition of GSK-3β.

CONCLUSIONS

In skeletal muscle, GSK-3β inactivation causes dephosphorylation and nuclear translocation of TFEB resulting in TFEB-dependent induction of Pgc-1α expression.

Pulmonary inflammation-induced alterations in key regulators of mitophagy and mitochondrial biogenesis in murine skeletal muscle

Background

Both mitophagy, a selective mechanism for clearance of mitochondria, and mitochondrial biogenesis are key processes determining mitochondrial content and oxidative capacity of the musculature. Abnormalities in these processes could therefore contribute to deterioration of peripheral muscle oxidative capacity as observed in e.g. chronic obstructive pulmonary disease. Although it has been suggested that inflammatory mediators can modulate both mitophagy and mitochondrial biogenesis, it is unknown whether acute pulmonary inflammation affects these processes in oxidative and glycolytic skeletal muscle in vivo. Therefore, we hypothesised that molecular signalling patterns of mitochondrial breakdown and biogenesis temporally shift towards increased breakdown and decreased biogenesis in the skeletal muscle of mice exposed to one single bolus of IT-LPS, as a model for acute lung injury and pulmonary inflammation.

Methods

We investigated multiple important constituents and molecular regulators of mitochondrial breakdown, biogenesis, dynamics, and mitochondrial content in skeletal muscle over time in a murine (FVB/N background) model of acute pulmonary- and systemic inflammation induced by a single bolus of intra-tracheally (IT)-instilled lipopolysaccharide (LPS). Moreover, we compared the expression of these constituents between gastrocnemius and soleus muscle.

Results

Both in soleus and gastrocnemius muscle, IT-LPS instillation resulted in molecular patterns indicative of activation of mitophagy. This coincided with modulation of mRNA transcript abundance of genes involved in mitochondrial fusion and fission as well as an initial decrease and subsequent recovery of transcript levels of key proteins involved in the molecular regulation of mitochondrial biogenesis. Moreover, no solid differences in markers for mitochondrial content were found.

Conclusions

These data suggest that one bolus of IT-LPS results in a temporal modulation of mitochondrial clearance and biogenesis in both oxidative and glycolytic skeletal muscle, which is insufficient to result in a reduction of mitochondrial content.

Automated CT-derived skeletal muscle mass determination in lower hind limbs of mice using a 3D U-Net deep learning network

The loss of skeletal muscle mass is recognized as a complication of several chronic diseases and is associated with increased mortality and a decreased quality of life. Relevant and reliable animal models in which muscle wasting can be monitored noninvasively over time are instrumental to investigate and develop new therapies. In this work, we developed a fully automatic deep learning algorithm for segmentation of micro cone beam computed tomography images of the lower limb muscle complex in mice and subsequent muscle mass calculation. A deep learning algorithm was trained on manually segmented data from 32 mice. Muscle wet mass measurements were obtained from 47 mice and served as a data set for model validation and reverse model validation. The automatic algorithm performance was ~150 times faster than manual segmentation. Reverse validation of the algorithm showed high quantitative metrics (i.e., a Dice similarity coefficient of 0.93, a Hausdorff distance of 0.4 mm, and a center of mass displacement of 0.1 mm), substantiating the robustness and accuracy of the model. A high correlation (R² = 0.92) was obtained between the computed tomography-derived muscle mass measurements and the muscle wet masses. Longitudinal follow-up revealed time-dependent changes in muscle mass that separated control from lung tumor-bearing mice, which was confirmed as cachexia. In conclusion, this deep learning model for automated assessment of the lower limb muscle complex provides highly accurate noninvasive longitudinal evaluation of skeletal muscle mass. Furthermore, it facilitates the workflow and increases the amount of data derived from mouse studies while reducing the animal numbers.NEW & NOTEWORTHY This deep learning application enables highly accurate noninvasive longitudinal evaluation of skeletal muscle mass changes in mice with minimal requirement for operator involvement in the data analysis. It provides a unique opportunity to increase and analyze the amount of data derived from animal studies automatically while reducing animal numbers and analytical workload.

Skeletal muscle unloading results in increased mitophagy and decreased mitochondrial biogenesis regulation

Introduction

Physical inactivity significantly contributes to loss of muscle mass and performance in bed‐bound patients. Loss of skeletal muscle mitochondrial content has been well‐established in muscle unloading models, but the underlying molecular mechanism remains unclear. We hypothesized that apparent unloading‐induced loss of muscle mitochondrial content is preceded by increased mitophagy‐ and decreased mitochondrial biogenesis‐signaling during the early stages of unloading.

Methods

We analyzed a comprehensive set of molecular markers involved in mitochondrial‐autophagy, −biogenesis, −dynamics, and ‐content, in the gastrocnemius muscle of C57BL/6J mice subjected to 0‐ and 3‐days hind limb suspension, and in biopsies from human vastus lateralis muscle obtained before and after 7 days of one‐leg immobilization.

Results

In both mice and men, short‐term skeletal muscle unloading results in molecular marker patterns indicative of increased receptor‐mediated mitophagy and decreased mitochondrial biogenesis regulation, before apparent loss of mitochondrial content.

Discussion

These results emphasize the early‐onset of skeletal muscle disuse‐induced mitochondrial remodeling.

Distinct skeletal muscle molecular responses to pulmonary rehabilitation in chronic obstructive pulmonary disease: a cluster analysis

BACKGROUND

Pulmonary rehabilitation (PR) is a cornerstone in the management of chronic obstructive pulmonary disease (COPD), targeting skeletal muscle to improve functional performance. However, there is substantial inter-individual variability in the effect of PR on functional performance, which cannot be fully accounted for by generic phenotypic factors. We performed an unbiased integrative analysis of the skeletal muscle molecular responses to PR in COPD patients and comprehensively characterized their baseline pulmonary and physical function, body composition, blood profile, comorbidities, and medication use.

METHODS

Musculus vastus lateralis biopsies were obtained from 51 COPD patients (age 64 ± 1 years, sex 73% men, FEV₁ , 34 (26-41) %pred.) before and after 4 weeks high-intensity supervised in-patient PR. Muscle molecular markers were grouped by network-constrained clustering, and their relative changes in expression values-assessed by qPCR and western blot-were reduced to process scores by principal component analysis. Patients were subsequently clustered based on these process scores. Pre-PR and post-PR functional performance was assessed by incremental cycle ergometry and 6 min walking test (6MWT).

RESULTS

Eight molecular processes were discerned by network-constrained hierarchical clustering of the skeletal muscle molecular rehabilitation responses. Based on the resulting process scores, four clusters of patients were identified by hierarchical cluster analysis. Two major patient clusters differed in PR-induced autophagy (P < 0.001), myogenesis (P = 0.014), glucocorticoid signalling (P < 0.001), and oxidative metabolism regulation (P < 0.001), with Cluster 1 (C1; n = 29) overall displaying a more pronounced change in marker expression than Cluster 2 (C2; n = 16). General baseline characteristics did not differ between clusters. Following PR, both 6 min walking distance (+26.5 ± 8.3 m, P = 0.003) and peak load on the cycle ergometer test (+9.7 ± 1.9 W, P < 0.001) were improved. However, the functional improvement was more pronounced in C1, as a higher percentage of patients exceeded the minimal clinically important difference in peak workload (61 vs. 21%, P = 0.022) and both peak workload and 6 min walking test (52 vs. 8%, P = 0.008) upon PR.

CONCLUSIONS

We identified patient groups with distinct skeletal muscle molecular responses to rehabilitation, associated with differences in functional improvements upon PR.

Regulation of muscle atrophy by microRNAs: 'AtromiRs' as potential target in cachexia

PURPOSE OF REVIEW

To provide an overview and describe the mode of action of miRNAs recently implicated in muscle atrophy, and discuss the challenges to explore their potential as putative therapeutic targets in cachexia.

RECENT FINDINGS

Recent work showed differentially expressed miRNAs in skeletal muscle of patients with cachexia-associated diseases. Studies using experimental models revealed miRNA regulation of the anabolic IGF-1 and catabolic TGF- β/myostatin pathways, and downstream protein synthesis and proteolysis signaling in control of muscle mass.

SUMMARY

Cachexia is a complex metabolic condition associated with progressive body weight loss, wasting of skeletal muscle mass and decrease in muscle strength. MiRNAs play a central role in post-transcriptional gene regulation by targeting mRNAs, thereby coordinating and fine-tuning many cellular processes. MiRNA expression profiling studies of muscle biopsies have revealed differentially expressed miRNAs in patients with low muscle mass or cachexia. Evaluation in experimental models has revealed muscle atrophy, inhibition of protein synthesis and activation of proteolysis in response to modulation of specific miRNAs, termed 'atromiRs' in this review. These exciting findings call for further studies aimed at exploring the conservation of differentially expressed miRNAs across diseases accompanied by cachexia, identification of miRNA clusters and targets involved in muscle atrophy, and probing whether these miRNAs might be potential therapeutic targets for cachexia.

Pulmonary inflammation-induced loss and subsequent recovery of skeletal muscle mass require functional poly-ubiquitin conjugation

BACKGROUND

Pulmonary inflammation in response to respiratory infections can evoke muscle wasting. Increased activity of the ubiquitin (Ub)-proteasome system (UPS) and the autophagy lysosome pathway (ALP) have been implicated in inflammation-induced muscle atrophy. Since poly-Ub conjugation is required for UPS-mediated proteolysis and has been implicated in the ALP, we assessed the effect of impaired ubiquitin conjugation on muscle atrophy and recovery following pulmonary inflammation, and compared activation and suppression of these proteolytic systems to protein synthesis regulation.

METHODS

Pulmonary inflammation was induced in mice by an intratracheal instillation of LPS. Proteolysis (UPS and ALP) and synthesis signaling were examined in gastrocnemius muscle homogenates. Ub-conjugation-dependency of muscle atrophy and recovery was addressed using Ub-K48R (K48R) mice with attenuated poly-ubiquitin conjugation, and compared to UBWT control mice.

RESULTS

Pulmonary inflammation caused a decrease in skeletal muscle mass which was accompanied by a rapid increase in expression of UPS and ALP constituents and reduction in protein synthesis signaling acutely after LPS. Muscle atrophy was attenuated in K48R mice, while ALP and protein synthesis signaling were not affected. Muscle mass recovery starting 72 h post LPS, correlated with reduced expression of UPS and ALP constituents and restoration of protein synthesis signaling. K48R mice however displayed impaired recovery of muscle mass.

CONCLUSION

Pulmonary inflammation-induced muscle atrophy is in part attributable to UPS-mediated proteolysis, as activation of ALP- and suppression of protein synthesis signaling occur independently of poly-Ub conjugation during muscle atrophy. Recovery of muscle mass following pulmonary inflammation involves inverse regulation of proteolysis and protein synthesis signaling, and requires a functional poly-Ub conjugation.

Glucocorticoid Receptor Signaling Impairs Protein Turnover Regulation in Hypoxia-Induced Muscle Atrophy in Male Mice

Hypoxemia may contribute to muscle wasting in conditions such as chronic obstructive pulmonary disease. Muscle wasting develops when muscle proteolysis exceeds protein synthesis. Hypoxia induces skeletal muscle atrophy in mice, which can in part be attributed to reduced food intake. We hypothesized that hypoxia elevates circulating corticosterone concentrations by reduced food intake and enhances glucocorticoid receptor (GR) signaling in muscle, which causes elevated protein degradation signaling and dysregulates protein synthesis signaling during hypoxia-induced muscle atrophy. Muscle-specific GR knockout and control mice were subjected to normoxia, normobaric hypoxia (8% oxygen), or pair-feeding to the hypoxia group for 4 days. Plasma corticosterone and muscle GR signaling increased after hypoxia and pair-feeding. GR deficiency prevented muscle atrophy by pair-feeding but not by hypoxia. GR deficiency differentially affected activation of ubiquitin 26S-proteasome and autophagy proteolytic systems by pair-feeding and hypoxia. Reduced food intake suppressed mammalian target of rapamycin complex 1 (mTORC1) activity under normoxic but not hypoxic conditions, and this retained mTORC1 activity was mediated by GR. We conclude that GR signaling is required for muscle atrophy and increased expression of proteolysis-associated genes induced by decreased food intake under normoxic conditions. Under hypoxic conditions, muscle atrophy and elevated gene expression of the ubiquitin proteasomal system-associated E3 ligases Murf1 and Atrogin-1 are mostly independent of GR signaling. Furthermore, impaired inhibition of mTORC1 activity is GR-dependent in hypoxia-induced muscle atrophy.

Inactivation of glycogen synthase kinase-3β (GSK-3β) enhances skeletal muscle oxidative metabolism

BACKGROUND

Aberrant skeletal muscle mitochondrial oxidative metabolism is a debilitating feature of chronic diseases such as chronic obstructive pulmonary disease, type 2 diabetes and chronic heart failure. Evidence in non-muscle cells suggests that glycogen synthase kinase-3β (GSK-3β) represses mitochondrial biogenesis and inhibits PPAR-γ co-activator 1 (PGC-1), a master regulator of cellular oxidative metabolism. The role of GSK-3β in the regulation of skeletal muscle oxidative metabolism is unknown.

AIMS

We hypothesized that inactivation of GSK-3β stimulates muscle oxidative metabolism by activating PGC-1 signaling and explored if GSK-3β inactivation could protect against physical inactivity-induced alterations in skeletal muscle oxidative metabolism.

METHODS

GSK-3β was modulated genetically and pharmacologically in C2C12 myotubes in vitro and in skeletal muscle in vivo. Wild-type and muscle-specific GSK-3β knock-out (KO) mice were subjected to hind limb suspension for 14days. Key constituents of oxidative metabolism and PGC-1 signaling were investigated.

RESULTS

In vitro, knock-down of GSK-3β increased mitochondrial DNA copy number, protein and mRNA abundance of oxidative phosphorylation (OXPHOS) complexes and activity of oxidative metabolic enzymes but also enhanced protein and mRNA abundance of key PGC-1 signaling constituents. Similarly, pharmacological inhibition of GSK-3β increased transcript and protein abundance of key constituents and regulators of mitochondrial energy metabolism. Furthermore, GSK-3β KO animals were protected against unloading-induced decrements in expression levels of these constituents.

CONCLUSION

Inactivation of GSK-3β up-regulates skeletal muscle mitochondrial metabolism and increases expression levels of PGC-1 signaling constituents. In vivo, GSK-3β KO protects against inactivity-induced reductions in muscle metabolic gene expression.

Cachexia in chronic obstructive pulmonary disease: new insights and therapeutic perspective

Cachexia and muscle wasting are well recognized as common and partly reversible features of chronic obstructive pulmonary disease (COPD), adversely affecting disease progression and prognosis. This argues for integration of weight and muscle maintenance in patient care. In this review, recent insights are presented in the diagnosis of muscle wasting in COPD, the pathophysiology of muscle wasting, and putative mechanisms involved in a disturbed energy balance as cachexia driver. We discuss the therapeutic implications of these new insights for optimizing and personalizing management of COPD-induced cachexia.

Preserved muscle oxidative metabolic phenotype in newly diagnosed non-small cell lung cancer cachexia

BACKGROUND

Cachexia augments cancer-related mortality and has devastating effects on quality of life. Pre-clinical studies indicate that systemic inflammation-induced loss of muscle oxidative phenotype (OXPHEN) stimulates cancer-induced muscle wasting. The aim of the current proof of concept study is to validate the presence of muscle OXPHEN loss in newly diagnosed patients with lung cancer, especially in those with cachexia.

METHODS

Quadriceps muscle biopsies of comprehensively phenotyped pre-cachectic (n = 10) and cachectic (n = 16) patients with non-small cell lung cancer prior to treatment were compared with healthy age-matched controls (n = 22). OXPHEN was determined by assessing muscle fibre type distribution (immunohistochemistry), enzyme activity (spectrophotometry), and protein expression levels of mitochondrial complexes (western blot) as well as transcript levels of (regulatory) oxidative genes (quantitative real-time PCR). Additionally, muscle fibre cross-sectional area (immunohistochemistry) and systemic inflammation (multiplex analysis) were assessed.

RESULTS

Muscle fibre cross-sectional area was smaller, and plasma levels of interleukin 6 were significantly higher in cachectic patients compared with non-cachectic patients and healthy controls. No differences in muscle fibre type distribution or oxidative and glycolytic enzyme activities were observed between the groups. Mitochondrial protein expression and gene expression levels of their regulators were also not different.

CONCLUSION

Muscle OXPHEN is preserved in newly diagnosed non-small cell lung cancer and therefore not a primary trigger of cachexia in these patients.

TNF-α-induced NF-κB activation stimulates skeletal muscle glycolytic metabolism through activation of HIF-1α

A shift in quadriceps muscle metabolic profile toward decreased oxidative metabolism and increased glycolysis is a consistent finding in chronic obstructive pulmonary disease (COPD). Chronic inflammation has been proposed as a trigger of this pathological metabolic adaptation. Indeed, the proinflammatory cytokine TNF-α impairs muscle oxidative metabolism through activation of the nuclear factor-κB (NF-κB) pathway. Putative effects on muscle glycolysis, however, are unclear. We hypothesized that TNF-α-induced NF-κB signaling stimulates muscle glycolytic metabolism through activation of the glycolytic regulator hypoxia-inducible factor-1α (HIF-1α). Wild-type C2C12 and C2C12-IκBα-SR (blocked NF-κB signaling) myotubes were stimulated with TNF-α, and its effects on glycolytic metabolism and involvement of the HIF pathway herein were investigated. As proof of principle, expression of HIF signaling constituents was investigated in quadriceps muscle biopsies of a previously well-characterized cohort of clinically stable patients with severe COPD and healthy matched controls. TNF-α increased myotube glucose uptake and lactate production and enhanced the activity and expression levels of multiple effectors of muscle glycolytic metabolism in a NF-κB-dependent manner. In addition, TNF-α activated HIF signaling, which required classical NF-κB activation. Moreover, the knockdown of HIF-1α largely attenuated TNF-α-induced increases in glycolytic metabolism. Accordingly, the mRNA levels of HIF-1α and the HIF-1α target gene, vascular endothelial growth factor (VEGF), were increased in muscle biopsies of COPD patients compared with controls, which was most pronounced in the patients with high levels of muscle TNF-α. In conclusion, these data show that TNF-α-induced classical NF-κB activation enhances muscle glycolytic metabolism in a HIF-1α-dependent manner.

Differential sensitivity of oxidative and glycolytic muscles to hypoxia-induced muscle atrophy

Hypoxia as a consequence of acute and chronic respiratory disease has been associated with muscle atrophy. This study investigated the sensitivity of oxidative and glycolytic muscles to hypoxia-induced muscle atrophy. Male mice were exposed to 8% normobaric oxygen for up to 21 days. Oxidative soleus and glycolytic extensor digitorum longus (EDL) muscles were isolated, weighed, and assayed for expression profiles of the ubiquitin-proteasome system (UPS), the autophagy-lysosome pathway (ALP), and glucocorticoid receptor (GR) and hypoxia-inducible factor-1α (HIF1α) signaling. Fiber-type composition and the capillary network were investigated. Hypoxia-induced muscle atrophy was more prominent in the EDL than the soleus muscle. Although increased expression of HIF1α target genes showed that both muscle types sensed hypoxia, their adaptive responses differed. Atrophy consistently involved a hypoxia-specific effect (i.e., not attributable to a hypoxia-mediated reduction of food intake) in the EDL only. Hypoxia-specific activation of the UPS and ALP and increased expression of the glucocorticoid receptor (Gr) and its target genes were also mainly observed in the EDL. In the soleus, stimulation of gene expression of those pathways could be mimicked to a large extent by food restriction alone. Hypoxia increased the number of capillary contacts per fiber cross-sectional area in both muscles. In the EDL, this was due to type II fiber atrophy, whereas in the soleus the absolute number of capillary contacts increased. These responses represent two distinct modes to improve oxygen supply to muscle fibers, but may aggravate muscle atrophy in chronic obstructive pulmonary disease patients who have a predominance of type II fibers.

Activation of alternative NF-κB signaling during recovery of disuse-induced loss of muscle oxidative phenotype

Physical inactivity-induced loss of skeletal muscle oxidative phenotype (OXPHEN), often observed in chronic disease, adversely affects physical functioning and quality of life. Potential therapeutic targets remain to be identified, since the molecular mechanisms involved in reloading-induced recovery of muscle OXPHEN remain incompletely understood. We hypothesized a role for alternative NF-κB, as a recently identified positive regulator of muscle OXPHEN, in reloading-induced alterations in muscle OXPHEN. Markers and regulators (including alternative NF-κB signaling) of muscle OXPHEN were investigated in gastrocnemius muscle of mice subjected to a hindlimb suspension/reloading (HLS/RL) protocol. Expression levels of oxidative phosphorylation subunits and slow myosin heavy chain isoforms I and IIA increased rapidly upon RL. After an initial decrease upon HLS, mRNA levels of peroxisome proliferator-activated receptor (PPAR)-γ coactivator (PGC) molecules PGC-1α and PGC-1β and mRNA levels of mitochondrial transcription factor A (Tfam) and estrogen-related receptor α increased upon RL. PPAR-δ, nuclear respiratory factor 1 (NRF-1), NRF-2α, and sirtuin 1 mRNA levels increased during RL although expression levels were unaltered upon HLS. In addition, both Tfam and NRF-1 protein levels increased significantly during the RL period. Moreover, upon RL, IKK-α mRNA and protein levels increased, and phosphorylation of P100 and subsequent processing to P52 were elevated, reflecting alternative NF-κB activation. We conclude that RL-induced recovery of muscle OXPHEN is associated with activation of alternative NF-κB signaling.

Pharmacological inhibition of GSK-3 in a guinea pig model of LPS-induced pulmonary inflammation: II. Effects on skeletal muscle atrophy

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is accompanied by pulmonary inflammation and associated with extra-pulmonary manifestations, including skeletal muscle atrophy. Glycogen synthase kinase-3 (GSK-3) has been implicated in the regulation of muscle protein- and myonuclear turnover; two crucial processes that determine muscle mass. In the present study we investigated the effect of the selective GSK-3 inhibitor SB216763 on muscle mass in a guinea pig model of lipopolysaccharide (LPS)-induced pulmonary inflammation-associated muscle atrophy.

METHODS

Guinea pigs were pretreated with either intranasally instilled SB216763 or corresponding vehicle prior to each LPS/saline challenge twice weekly. Pulmonary inflammation was confirmed and indices of muscle mass were determined after 12 weeks. Additionally, cultured skeletal muscle cells were incubated with tumor necrosis factor α (TNF-α) or glucocorticoids (GCs) to model the systemic effects of pulmonary inflammation on myogenesis, in the presence or absence of GSK-3 inhibitors.

RESULTS

Repeated LPS instillation induced muscle atrophy based on muscle weight and muscle fiber cross sectional area. Intriguingly, GSK-3 inhibition using SB216763 prevented the LPS-induced muscle mass decreases and myofiber atrophy. Indices of protein turnover signaling were unaltered in guinea pig muscle. Interestingly, inhibition of myogenesis of cultured muscle cells by TNF-α or synthetic GCs was prevented by GSK-3 inhibitors.

CONCLUSIONS

In a guinea pig model of LPS-induced pulmonary inflammation, GSK-3 inhibition prevents skeletal muscle atrophy without affecting pulmonary inflammation. Resistance to inflammation- or GC-induced impairment of myogenic differentiation, imposed by GSK-3 inhibition, suggests that sustained myogenesis may contribute to muscle mass maintenance despite persistent pulmonary inflammation. Collectively, these results warrant further exploration of GSK-3 as a potential novel drug target to prevent or reverse muscle wasting in COPD.

Regulation of skeletal muscle plasticity by glycogen synthase kinase-3β: a potential target for the treatment of muscle wasting

Muscle wasting is a prevalent and disabling condition in chronic disease and cancer and has been associated with increased mortality and impaired efficacy of surgical and medical interventions. Pharmacological therapies to combat muscle wasting are currently limited but considered as an important unmet medical need. Muscle wasting has been attributed to increased muscle proteolysis, and in particular ubiquitin 26S-proteasome system (UPS)-dependent protein breakdown. However, rates of muscle protein synthesis are also subject to extensive (patho) physiological regulation, and the balance between synthesis and degradation ultimately determines net muscle protein turnover. As multinucleated muscle fibers accommodate threshold changes in muscle protein content by the accretion and loss of muscle nuclei, myonuclear turnover may additionally determine muscle mass. Current insights in the mechanisms dictating muscle mass plasticity not only reveal intricate interactions and crosstalk between these processes, but imply the existence of signaling molecules that act as molecular switchboards, which coordinate and integrate cellular responses upon conditions that evoke changes in muscle mass. These "master regulators" of skeletal muscle mass plasticity are preferred targets for pharmacological modulation of skeletal muscle wasting. In this review Glycogen synthase kinase-3β (GSK-3β) is highlighted as a master regulator of muscle mass plasticity since, in addition to its role in UPS-mediated muscle protein degradation, it also controls protein synthesis, and influences myonuclear accretion and cell death. Moreover, the regulation of GSK-3β activity as well as currently available pharmacological inhibitors are described and discussed in the context of multimodal treatment strategies aimed at the inhibition of GSK-3β, and optimal exploitation of its potential role as a central regulator of skeletal muscle mass plasticity for the treatment of muscle wasting.

Distinct responses of protein turnover regulatory pathways in hypoxia- and semistarvation-induced muscle atrophy

The balance of muscle protein synthesis and degradation determines skeletal muscle mass. We hypothesized that hypoxia-induced muscle atrophy and alterations in the regulation of muscle protein turnover include a hypoxia-specific component, in addition to the observed effects of reduction in food intake in response to hypoxia. Mice were subjected to normoxic, hypoxic (8% oxygen), or pair-fed conditions for 2, 4, and 21 days. Cell-autonomous effects of hypoxia on skeletal muscle were also assessed in differentiated C2C12 myotubes. Hypoxia induced an initial rapid loss of body and muscle weight, which remained decreased during chronic hypoxia and could only in part be explained by the hypoxia-induced reduction of food intake (semistarvation). Regulatory steps of protein synthesis (unfolded protein response and mammal target of rapamycin signaling) remained active in response to acute and sustained hypoxia but not to semistarvation. Activation of regulatory signals for protein degradation, including increased expression of Murf1, Atrogin-1, Bnip3, and Map1lc3b mRNAs, was observed in response to acute hypoxia and to a lesser extent following semistarvation. Conversely, the sustained elevation of Atrogin-1, Bnip3, and Map1lc3b mRNAs and the increased activity of their upstream transcriptional regulator Forkhead box O1 were specific to chronic hypoxia because they were not observed in response to reduced food intake. In conclusion, altered regulation of protein turnover during hypoxia-induced muscle atrophy resulted from an interaction of semistarvation and a hypoxia-specific component. The finding that food restriction but not hypoxia-induced semistarvation inhibited regulatory steps in protein synthesis suggests a hypoxia-specific impairment of the coordination between protein-synthesis signaling and protein-degradation signaling in skeletal muscle.

Regulation of skeletal muscle oxidative phenotype by classical NF-κB signalling

BACKGROUND

Impairments in skeletal muscle oxidative phenotype (OXPHEN) have been linked to the development of insulin resistance, metabolic inflexibility and progression of the metabolic syndrome and have been associated with progressive disability in diseases associated with chronic systemic inflammation. We previously showed that the inflammatory cytokine tumour necrosis factor-α (TNF-α) directly impairs muscle OXPHEN but underlying molecular mechanisms remained unknown. Interestingly, the inflammatory signalling pathway classical nuclear factor-κB (NF-κB) is activated in muscle in abovementioned disorders. Therefore, we hypothesised that muscle activation of classical NF-κB signalling is sufficient and required for inflammation-induced impairment of muscle OXPHEN.

METHODS

Myotubes from mouse and human muscle cell lines were subjected to activation or blockade of the classical NF-κB pathway. In addition, wild-type and MISR (muscle-specific inhibition of classical NF-κB) mice were injected intra-muscularly with TNF-α. Markers and key regulators of muscle OXPHEN were investigated.

RESULTS

Classical NF-κB activation diminished expression of oxidative phosphorylation (OXPHOS) sub-units, slow myosin heavy chain expression, activity of mitochondrial enzymes and potently reduced intra-cellular ATP levels. Accordingly, PGC-1/PPAR/NRF-1/Tfam signalling, the main pathway controlling muscle OXPHEN, was impaired upon classical NF-κB activation which required intact p65 trans-activation domains and depended on de novo gene transcription. Unlike wild-type myotubes, IκBα-SR myotubes (blocked classical NF-κB signalling) were refractory to TNF-α-induced impairments in OXPHEN and its regulation by the PGC-1/PPAR/NRF-1/Tfam cascade. In line with in vitro data, NF-κB blockade in vivo abrogated TNF-α-induced reductions in PGC-1α expression.

CONCLUSION

Classical NF-κB activation impairs skeletal muscle OXPHEN.

NF-κB activation is required for the transition of pulmonary inflammation to muscle atrophy

Disease exacerbations and muscle wasting comprise negative prognostic factors of chronic obstructive pulmonary disease (COPD). Transient systemic inflammation and malnutrition have been implicated in skeletal muscle wasting after acute exacerbations of COPD. However, the interactions between systemic inflammation and malnutrition in their contributions to muscle atrophy, as well as the molecular basis underlying the transition of systemic inflammation to muscle atrophy, remain unresolved. Pulmonary inflammation was induced in mice by an intratracheal instillation of LPS to model acute disease exacerbation. Systemic inflammation, nutritional intake, and body and muscle weights were determined. Muscle inflammatory signaling and atrophy signaling were examined, and the effect of the muscle-specific inactivation of NF-κB on muscle atrophy was assessed in genetically modified mice. The intratracheal LPS instillation was followed by markedly elevated circulating cytokine concentrations and NF-κB activation in extrapulmonary tissues, including skeletal muscle. The administration of intratracheal LPS increased the expression of muscle E3 ubiquitin ligases, which govern muscle proteolysis, in particular MuRF1, and caused a rapid loss of muscle mass. Reduced food intake only partly accounted for the observed muscle atrophy, and did not activate NF-κB in muscle. Rather, plasma transfer experiments revealed the presence of NF-κB-signaling and atrophy-signaling properties in the circulation of intratracheal LPS-treated mice. The genetic inhibition of muscle NF-κB activity suppressed intratracheal LPS-induced MuRF1 expression and resulted in a significant sparing of muscle tissue. Systemic inflammation and malnutrition contribute to the muscle wasting induced by acute pulmonary inflammation via distinct mechanisms, and muscle NF-κB activation is required for the transition from inflammatory to muscle atrophy signaling.

Glycogen synthase kinase-3β is required for the induction of skeletal muscle atrophy

Skeletal muscle atrophy commonly occurs in acute and chronic disease. The expression of the muscle-specific E3 ligases atrogin-1 (MAFbx) and muscle RING finger 1 (MuRF1) is induced by atrophy stimuli such as glucocorticoids or absence of IGF-I/insulin and subsequent Akt signaling. We investigated whether glycogen synthase kinase-3β (GSK-3β), a downstream molecule in IGF-I/Akt signaling, is required for basal and atrophy stimulus-induced expression of atrogin-1 and MuRF1, and myofibrillar protein loss in C2C12 skeletal myotubes. Abrogation of basal IGF-I signaling, using LY294002, resulted in a prominent induction of atrogin-1 and MuRF1 mRNA and was accompanied by a loss of myosin heavy chain fast (MyHC-f) and myosin light chains 1 (MyLC-1) and -3 (MyLC-3). The synthetic glucocorticoid dexamethasone (Dex) also induced the expression of both atrogenes and likewise resulted in the loss of myosin protein abundance. Genetic ablation of GSK-3β using small interfering RNA resulted in specific sparing of MyHC-f, MyLC-1, and MyLC-3 protein levels after Dex treatment or impaired IGF-I/Akt signaling. Interestingly, loss of endogenous GSK-3β suppressed both basal and atrophy stimulus-induced atrogin-1 and MuRF1 expression, whereas pharmacological GSK-3β inhibition, using CHIR99021 or LiCl, only reduced atrogin-1 mRNA levels in response to LY294002 or Dex. In conclusion, our data reveal that myotube atrophy and myofibrillar protein loss are GSK-3β dependent, and demonstrate for the first time that basal and atrophy stimulus-induced atrogin-1 mRNA expression requires GSK-3β enzymatic activity, whereas MuRF1 expression depends solely on the physical presence of GSK-3β.

Trans-10, cis-12 conjugated linoleic acid inhibits skeletal muscle differentiation and GLUT4 expression independently from NF-κB activation

SCOPE

The capacity of skeletal muscle to contribute to glucose homeostasis depends on muscular insulin sensitivity. The expression of glucose transporter (GLUT)-4 is increased during myoblast differentiation, a process essential in maintenance of adult muscle. Therefore, processes that affect muscle differentiation may influence insulin dependent glucose homeostasis. Conjugated linoleic acids, and in particular trans-10, cis-12 CLA (t10, c12-CLA), are potent inducers of NF-kB in cultured skeletal myotubes, and NF-kB activation inhibits muscle differentiation. The aims of this study were to evaluate whether CLAs inhibit myogenic differentiation and lower GLUT4 mRNA expression and to address the involvement of NF-kB activation in potential effects of CLA on these processes.

METHODS AND RESULTS

Incubation of C2C12 cells with t10, c12-CLA blocked the formation of myotubes, which was accompanied by reduced expression of the muscle specific genes creatine kinase, myogenin, myosin heavy chain perinatal and myosin heavy chain IIB, as well as decreased GLUT4 mRNA levels. However, genetic blockade of NF-kB was not sufficient to restore reduced myosin heavy chain protein expression following t10, c12-CLA treatment. Surprisingly, in contrast to myotubes, t10, c12-CLA was not able to activate NF-kB transcriptional activity in myoblasts.

CONCLUSION

In conclusion, t10, c12-CLA inhibits myogenic differentiation and GLUT4 expression, independently from NF-kB activation.

Inflammatory signaling in skeletal muscle insulin resistance: green signal for nutritional intervention?

PURPOSE OF REVIEW

To review the evidence implying a role of inflammatory signaling pathways, specifically nuclear factor-κB and c-Jun NH2-terminal kinase, in fatty acid-induced skeletal muscle insulin resistance and to discuss the potential of dietary interventions to interfere with these processes.

RECENT FINDINGS

Fatty acids can induce skeletal muscle insulin resistance via inflammatory signaling after binding Toll-like receptors at the cell membrane of muscle cells or after accumulating as intramyocellular lipid metabolites. In both processes, activation of intracellular inflammatory signaling is involved. The majority of literature addressing the causality of muscle nuclear factor-κB activation in skeletal muscle insulin resistance suggests that insulin resistance does not require muscle nuclear factor-κB activation. Recently, strong evidence was given that c-Jun NH2-terminal kinase signaling is an important inflammatory pathway involved in skeletal muscle insulin resistance. Furthermore, it is well established that proinflammatory cytokines originating from the enlarged adipose tissue or from activated adipose tissue macrophages can cause muscle insulin resistance. Recently, also macrophages resided in the muscle have been proposed to play an important role in muscle insulin resistance. Because of their anti-inflammatory characteristics, several dietary components like polyphenols may be interesting candidates for manipulating skeletal muscle insulin resistance.

SUMMARY

Several dietary components, like polyphenols, have been reported to interfere with inflammatory signaling. To test whether these compounds can be used to prevent or reverse insulin resistance, well controlled human intervention studies have to be designed.

Palmitate-induced skeletal muscle insulin resistance does not require NF-κB activation

Palmitate activates the NF-κB pathway, and induces accumulation of lipid metabolites and insulin resistance in skeletal muscle cells. Little information is available whether and how these processes are causally related. Therefore, the objectives were to investigate whether intra-cellular lipid metabolites are involved in FA-induced NF-κB activation and/or insulin resistance in skeletal muscle and to investigate whether FA-induced insulin resistance and NF-κB activation are causally related. Inhibiting DGAT or CPT-1 by using, respectively, amidepsine or etomoxir increased DAG accumulation and sensitized myotubes to palmitate-induced insulin resistance. While co-incubation of palmitate with etomoxir increased NF-κB transactivation, co-incubation with amidepsine did not, indicating that DAG accumulation is associated with insulin resistance but not with NF-κB activation. Furthermore, pharmacological or genetic inhibition of the NF-κB pathway could not prevent palmitate-induced insulin resistance. In conclusion, we have demonstrated that activation of the NF-κB pathway is not required for palmitate-induced insulin resistance in skeletal muscle cells.

Regulation of mitochondrial biogenesis during myogenesis

Pathways involved in mitochondrial biogenesis associated with myogenic differentiation are poorly defined. Therefore, C(2)C(12) myoblasts were differentiated into multi-nucleated myotubes and parameters/regulators of mitochondrial biogenesis were investigated. Mitochondrial respiration, citrate synthase- and beta-hydroxyacyl-CoA dehydrogenase activity as well as protein content of complexes I, II, III and V of the mitochondrial respiratory chain increased 4-8-fold during differentiation. Additionally, an increase in the ratio of myosin heavy chain (MyHC) slow vs MyHC fast protein content was observed. PPAR transcriptional activity and transcript levels of PPAR-alpha, the PPAR co-activator PGC-1alpha, mitochondrial transcription factor A and nuclear respiratory factor 1 increased during differentiation while expression levels of PPAR-gamma decreased. In conclusion, expression and activity levels of genes known for their regulatory role in skeletal muscle oxidative capabilities parallel the increase in oxidative parameters during the myogenic program. In particular, PGC-1alpha and PPAR-alpha may be involved in the regulation of mitochondrial biogenesis during myogenesis.

PPARgamma inhibits NF-kappaB-dependent transcriptional activation in skeletal muscle

Skeletal muscle pathology associated with a chronic inflammatory disease state (e.g., skeletal muscle atrophy and insulin resistance) is a potential consequence of chronic activation of NF-kappaB. It has been demonstrated that peroxisome proliferator-activated receptors (PPARs) can exert anti-inflammatory effects by interfering with transcriptional regulation of inflammatory responses. The goal of the present study, therefore, was to evaluate whether PPAR activation affects cytokine-induced NF-kappaB activity in skeletal muscle. Using C(2)C(12) myotubes as an in vitro model of myofibers, we demonstrate that PPAR, and specifically PPARgamma, activation potently inhibits inflammatory mediator-induced NF-kappaB transcriptional activity in a time- and dose-dependent manner. Furthermore, PPARgamma activation by rosiglitazone strongly suppresses cytokine-induced transcript levels of the NF-kappaB-dependent genes intracellular adhesion molecule 1 (ICAM-1) and CXCL1 (KC), the murine homolog of IL-8, in myotubes. To verify whether muscular NF-kappaB activity in human subjects is suppressed by PPARgamma activation, we examined the effect of 8 wk of rosiglitazone treatment on muscular gene expression of ICAM-1 and IL-8 in type 2 diabetes mellitus patients. In these subjects, we observed a trend toward decreased basal expression of ICAM-1 mRNA levels. Subsequent analyses in cultured myotubes revealed that the anti-inflammatory effect of PPARgamma activation is not due to decreased RelA translocation to the nucleus or reduced RelA DNA binding. These findings demonstrate that muscle-specific inhibition of NF-kappaB activation may be an interesting therapeutic avenue for treatment of several inflammation-associated skeletal muscle abnormalities.

Fatty acid-induced NF-kappaB activation and insulin resistance in skeletal muscle are chain length dependent

The saturated fatty acid (SFA) palmitate induces insulin resistance in cultured skeletal muscle cells, which may be related to NF-kappaB activation. The aim of this study was to evaluate whether other SFAs also exert these effects on skeletal muscle and whether these relate to chain length. Therefore, we incubated L6 and C(2)C(12) skeletal muscle cells with four different fatty acids, caprylate (C8:0), laurate (C12:0), palmitate (C16:0), and stearate (C18:0), to study effects on GLUT4 translocation, deoxyglucose uptake, and NF-kappaB activation. Incubation of L6 cells with the long-chain FAs C16:0 and C18:0 reduced insulin-stimulated GLUT4 translocation and deoxyglucose uptake, whereas L6 cells incubated with the medium-chain FAs C8:0 and C12:0 remained insulin sensitive. Besides increasing NF-kappaB DNA binding activity in both L6 and C(2)C(12) cells, C16:0 also induced NF-kappaB transcriptional activity. C18:0 showed comparable effects, whereas the SFAs with shorter chain lengths were not able to elevate NF-kappaB transcriptional activity. Collectively, these results demonstrate that SFA-induced NF-kappaB activation coincides with insulin resistance and depends on FA chain length.

Extrapulmonary manifestations of chronic obstructive pulmonary disease in a mouse model of chronic cigarette smoke exposure

Cigarette smoking is the most commonly encountered risk factor for chronic obstructive pulmonary disease (COPD), reflected by irreversible airflow limitation, frequently associated with airspace enlargement and pulmonary inflammation. In addition, COPD has systemic consequences, including systemic inflammation, muscle wasting, and loss of muscle oxidative phenotype. However, the role of smoking in the development of these extrapulmonary manifestations remains rather unexplored. Mice were exposed to cigarette smoke or control air for 6 months. Subsequently, emphysema was assessed by morphometry of lung tissue, and blood cytokine and chemokine levels were determined by a multiplex assay. Soleus, plantaris, gastrocnemius, and tibialis muscles were dissected and weighed. Muscle fiber typing was performed based on I, IIA, IIB, and IIX myosin heavy-chain isoform composition. Lungs of the smoke-exposed animals showed pulmonary inflammation and emphysema. Moreover, circulating levels of primarily proinflammatory proteins, especially TNF-alpha, were elevated after smoke exposure. Despite an attenuated body weight gain, only the soleus showed a tendency toward lower muscle weight after smoke exposure. Oxidative fiber type IIA proportion was significantly reduced in the soleus. Muscle oxidative enzyme activity was slightly reduced after smoke exposure, being most prominent for citrate synthase in the soleus and tibialis. In this mouse model, chronic cigarette smoke exposure resulted in systemic features that closely resemble the early signs of the extrapulmonary manifestations observed in patients with COPD.

Glycogen synthase kinase 3 suppresses myogenic differentiation through negative regulation of NFATc3

Skeletal muscle atrophy is a prominent and disabling feature in many chronic diseases. Prevention or reversal of muscle atrophy by stimulation of skeletal muscle growth could be an important therapeutic strategy. Glycogen synthase kinase 3beta (GSK-3beta) has been implicated in the negative regulation of skeletal muscle growth. Since myogenic differentiation is an essential part of muscle growth, we investigated if inhibition of GSK-3beta is sufficient to stimulate myogenic differentiation and whether this depended on regulation of the transcription factor nuclear factor of activated T-cells (NFAT). In both myogenically converted mouse embryonic fibroblasts and C2C12 myoblasts, deficiency of GSK-3beta protein (activity) resulted in enhanced myotube formation and muscle-specific gene expression during differentiation, which was reversed by reintroduction of wild type but not kinase-inactive (K85R) GSK-3beta. In addition, GSK-3beta inhibition restored myogenic differentiation following calcineurin blockade, which suggested the involvement of NFAT. GSK-3beta-deficient mouse embryonic fibroblasts or myoblasts displayed enhanced nuclear translocation of NFATc3 and elevated NFAT-sensitive promoter transactivation, which was reduced by reintroducing wild type, but not K85R GSK-3beta. Overexpression of NFATc3 increased muscle gene promoter transactivation, which was abolished by co-expression of wild type GSK-3beta. Finally, stimulation of muscle gene expression observed following GSK-3beta inhibition was strongly attenuated in NFATc3-deficient myoblasts, indicating that this response requires NFATc3. Collectively, our data demonstrate negative regulation of myogenic differentiation by GSK-3beta through a transcriptional mechanism that depends on NFATc3. Inhibition of GSK-3beta may be a potential strategy in prevention or treatment of muscle atrophy.

Myogenic differentiation during regrowth of atrophied skeletal muscle is associated with inactivation of GSK-3β

Muscle atrophy contributes to morbidity and mortality in aging and chronic disease, emphasizing the need to gain understanding of the mechanisms involved in muscle atrophy and (re)growth. We hypothesized that the magnitude of muscle regrowth during recovery from atrophy determines whether myonuclear accretion and myogenic differentiation are required and that insulin-like growth factor (IGF)-I/Akt/glycogen synthase kinase (GSK)-3β signaling differs between regrowth responses. To address this hypothesis we subjected mice to hindlimb suspension (HS) to induce atrophy of soleus (−40%) and plantaris (−27%) muscle. Reloading-induced muscle regrowth was complete after 14 days and involved an increase in IGF-IEa mRNA expression that coincided with Akt phosphorylation in both muscles. In contrast, phosphorylation and inactivation of GSK-3β were observed during soleus regrowth only. Furthermore, soleus but not plantaris regrowth involved muscle regeneration based on a transient increase in expression of histone 3.2 and myosin heavy chain-perinatal, which are markers of myoblast proliferation and differentiation, and a strong induction of muscle regulatory factor (MRF) expression. Experiments in cultured muscle cells showed that IGF-I-induced MRF expression is facilitated by inactivation of GSK-3β and selectively occurs in the myoblast population. This study suggests that induction of IGF-I expression and Akt phosphorylation during recovery from muscle atrophy is independent of the magnitude of muscle regrowth. Moreover, our data demonstrate for the first time that the regenerative response characterized by myoblast proliferation, differentiation, and increased MRF expression in recovering muscle is associated with the magnitude of regrowth and may be regulated by inactivation of GSK-3β.

Interruption of Wnt signaling attenuates the onset of pressure overload-induced cardiac hypertrophy

The hypertrophic response of the heart has been recognized recently as the net result of activation of prohypertrophic and antihypertrophic pathways. Here we report the involvement of the Wnt/Frizzled pathway in the onset of cardiac hypertrophy development. Stimulation of the Wnt/Frizzled pathway activates the disheveled (Dvl) protein. Disheveled subsequently can inhibit glycogen synthase kinase-3beta, a protein with potent antihypertrophic actions through diverse molecular mechanisms. In the Wnt/Frizzled pathway, inhibition of glycogen synthase kinase-3beta leads to an increased amount of beta-catenin, which can act as a transcription factor for several hypertrophy-associated target genes. In this study we subjected mice lacking the Dvl-1 gene and their wild-type littermates to thoracic aortic constriction for 7, 14, and 35 days. In mice lacking the Dvl-1 gene, 7 days of pressure overload-induced increases in left ventricular posterior wall thickness and expression of atrial natriuretic factor and brain natriuretic protein were attenuated compared with their wild-type littermates. Beta-catenin protein amount was reduced in the group lacking the Dvl-1 gene, and an increased glycogen synthase kinase-3beta activity was observed. Moreover, the increase in the amount of Ser(473)-phosphorylated Akt, a stimulator of cardiac hypertrophy, was lower in the group lacking the Dvl-1 gene. In conclusion, we have demonstrated that interruption of Wnt signaling in the mice lacking the Dvl-1 gene attenuates the onset of pressure overload-induced cardiac hypertrophy through mechanisms involving glycogen synthase kinase-3beta and Akt. Therefore, the Wnt/Frizzled pathway may provide novel therapeutic targets for antihypertrophic therapy.

Muscle wasting and impaired muscle regeneration in a murine model of chronic pulmonary inflammation

Muscle wasting and increased circulating levels of inflammatory cytokines, including TNF-alpha, are common features of chronic obstructive pulmonary disease. To investigate whether inflammation of the lung is responsible for systemic inflammation and muscle wasting, we adopted a mouse model of pulmonary inflammation resulting from directed overexpression of a TNF-alpha transgene controlled by the surfactant protein C (SP-C) promoter. Compared with wild-type mice, SP-C/TNF-alpha mice exhibited increased levels of TNF-alpha in the circulation and increased endogenous TNF-alpha expression in skeletal muscle, potentially reflecting an amplificatory response to circulating TNF-alpha. Decreased muscle and body weights observed in SP-C/TNF-alpha mice were indicative of muscle wasting. Further evaluation of the SP-C/TNF-alpha mouse musculature revealed a decreased muscle regenerative capacity, shown by attenuated myoblast proliferation and differentiation in response to reloading of disuse-atrophied muscle, which may contribute to skeletal muscle wasting. Importantly, incubation of cultured myoblasts with TNF-alpha also resulted in elevated TNF-alpha mRNA levels and inhibition of myoblast differentiation. Collectively, our results demonstrate that chronic pulmonary inflammation results in muscle wasting and impaired muscle regeneration in SP-C/TNF-alpha mice, possibly as a consequence of an amplificatory TNF-alpha expression circuit extending from the lung to skeletal muscle.

Inhibition of glycogen synthase kinase-3β activity is sufficient to stimulate myogenic differentiation

Skeletal muscle atrophy is a prominent and disabling feature of chronic wasting diseases. Prevention or reversal of muscle atrophy by administration of skeletal muscle growth (hypertrophy)-stimulating agents such as insulin-like growth factor I (IGF-I) could be an important therapeutic strategy in these diseases. To elucidate the IGF-I signal transduction responsible for muscle formation (myogenesis) during muscle growth and regeneration, we applied IGF-I to differentiating C2C12myoblasts and evaluated the effects on phosphatidylinositol 3-kinase (PI3K)/Akt/glycogen synthase kinase-3β (GSK-3β) signaling and myogenesis. IGF-I caused phosphorylation and inactivation of GSK-3β activity via signaling through the PI3K/Akt pathway. We assessed whether pharmacological inhibition of GSK-3β with lithium chloride (LiCl) was sufficient to stimulate myogenesis. Addition of IGF-I or LiCl stimulated myogenesis, evidenced by increased myotube formation, muscle creatine kinase (MCK) activity, and troponin I (TnI) promoter transactivation during differentiation. Moreover, mRNAs encoding MyoD, Myf-5, myogenin, TnI-slow, TnI-fast, MCK, and myoglobin were upregulated in myoblasts differentiated in the presence of IGF-I or LiCl. Importantly, blockade of GSK-3β inhibition abrogated IGF-I- but not LiCl-dependent stimulation of myogenic mRNA accumulation, suggesting that the promyogenic effects of IGF-I require GSK-3β inactivation and revealing an important negative regulatory role for GSK-3β in myogenesis. Therefore, this study identifies GSK-3β as a potential target for pharmacological stimulation of muscle growth.

Tumor necrosis factor-alpha inhibits myogenic differentiation through MyoD protein destabilization

Tumor necrosis factor alpha (TNFalpha) has been implicated as a mediator of muscle wasting through nuclear factor kappa B (NF-kappaB) -dependent inhibition of myogenic differentiation. The aim of the present study was to identify the regulatory molecule(s) of myogenesis targeted by TNFalpha/NF-kappaB signaling. TNFalpha interfered with cell cycle exit and repressed the accumulation of transcripts encoding muscle-specific genes in differentiating C2C12 myoblasts. Overexpression of a p65 (RelA) mutant lacking the transcriptional activation domain attenuated the TNFalpha-mediated inhibition of muscle-specific gene transcription. The ability of muscle regulatory factor MyoD to induce muscle-specific transcription in 10T1/2 fibroblasts was also disrupted by wild-type p65, demonstrating that NF-kappaB transcriptional activity interferes with the function of MyoD. Inhibition of muscle-specific gene expression by TNFalpha was restored by overexpression of MyoD, whereas endogenous MyoD protein abundance and stability were reduced by TNFalpha through increased proteolysis of MyoD by the ubiquitin proteasome pathway. Last, the inhibitory effects of TNFalpha on myogenic differentiation were demonstrated in a mouse model of skeletal muscle regeneration, in which TNFalpha caused a delay in myoblast cell cycle exit. These results implicate that TNFalpha inhibits myogenic differentiation through destabilizing MyoD protein in a NF-kappaB-dependent manner, which interferes with skeletal muscle regeneration and may contribute to muscle wasting.

ROS in the local and systemic pathogenesis of COPD

An imbalance between oxidants and antioxidants is proposed in the pathogenesis of COPD. Potential alterations responsible for an imbalance in oxidant production and intra- and extracellular antioxidant defense systems are discussed with respect to COPD-related changes in the pulmonary compartment. In line with the current view of COPD as a disease with multiple systemic consequences, there is increasing evidence that imbalances in the redox milieu extend beyond the diseased lung in COPD patients. Skeletal muscle dysfunction is often observed in COPD and may result from imbalances in the redox environment of skeletal muscle. Potential triggers of oxidative stress in the muscle compartment include inflammation and hypoxia, and local sources of reactive oxygen and nitrogen species are discussed, as well the mechanisms by which skeletal muscle trophical state, contractility and fatigability may be affected by oxidative stress, resulting in skeletal muscle dysfunction.

Enhanced myogenic differentiation by extracellular matrix is regulated at the early stages of myogenesis

Myogenic cell lines have been used extensively in the study of skeletal muscle development, regeneration, and homeostasis. To induce myogenic differentiation, culture media composed of a wide variety of growth factors and other additives have been used. Because the diversity in these components may modulate the differentiation process differentially, we describe a differentiation protocol that does not require the introduction of any factors to the differentiation media (DM) other than those present in the growth media. By culturing C2C12 skeletal myocytes on a coating of diluted Matrigel, a soluble basement membrane, consisting of collagen IV, laminin, heparan sulfate proteoglycans, and entactin, myogenic differentiation was accomplished by mere serum reduction. Assessment of myotube formation, creatine kinase activity, myosin heavy chain-fast, and myogenin demonstrated that the kinetics and extent of myogenic differentiation were superior using this protocol, compared with a commonly used differentiation protocol, in which an extracellular matrix is not provided and the DM contains horse serum. In addition, the elevated transactivation of a troponin-I promoter reporter construct suggested that myogenesis was enhanced at the transcriptional level. Finally, assessment of genomic deoxyribonucleic acid content revealed that the Matrigel differentiation protocol resulted in lowered proliferation. This protocol may aid studies aimed at elucidating mechanisms of myogenic differentiation, where a homogeneous population of myotubes is preferred.