Andrew J. Murton

The involvement of the ubiquitin proteasome system in human skeletal muscle remodelling and atrophy

Skeletal muscle exhibits great plasticity in response to altered activity levels, ultimately resulting in tissue remodelling and substantial changes in mass. Animal research would suggest that the ubiquitin proteasome system, in particular the ubiquitin ligases MAFbx/atrogin-1 and MuRF1, are instrumental to the processes underlying these changes. This review article therefore examines the role of proteasomal-mediated protein degradation in human skeletal muscle in health and disease. Specifically, the effects of exercise, disuse and inflammatory disease states on the ubiquitin proteasome system in human skeletal muscle are examined. The article also identifies several inconsistencies between published human studies and data obtained from animal models of muscle atrophy, highlighting the need for a more comprehensive examination of the molecular events responsible for modulating muscle mass in humans.

Mechanisms regulating muscle mass during disuse atrophy and rehabilitation in humans

Muscle mass loss accompanies periods of bedrest and limb immobilization in humans and requires rehabilitation exercise to effectively restore mass and function. Although recent evidence points to an early and transient rise in muscle protein breakdown contributing to this decline in muscle mass, the driving factor seems to be a reduction in muscle protein synthesis, not least in part due to the development of anabolic resistance to amino acid provision. Although the AKT signaling pathway has been identified in small animals as central to the regulation of muscle protein synthesis, several studies in humans have now demonstrated a disassociation between AKT signaling and muscle protein synthesis during feeding, exercise, and immobilization, suggesting that the mechanisms regulating protein synthesis in human skeletal muscle are more complex than initially thought (at least in non-inflammatory states). During rehabilitation, exercise-induced myogenesis may in part be responsible for the recovery of muscle mass. Rapid and sustained exercise-induced suppression of myostatin mRNA expression, that precedes any gain in muscle mass, points to this, along with other myogenic proteins, as being potential regulators of muscle regeneration during exercise rehabilitation in humans.

Temporal changes in the involvement of pyruvate dehydrogenase complex in muscle lactate accumulation during lipopolysaccharide infusion in rats

A characteristic manifestation of sepsis is muscle lactate accumulation. This study examined any putative (causative) association between pyruvate dehydrogenase complex (PDC) inhibition and lactate accumulation in the extensor digitorum longus (EDL) muscle of rats infused with lipopolysaccharide (LPS), and explored the involvement of increased transcription of muscle‐specific pyruvate dehydrogenase kinase (PDK) isoenzymes. Conscious, male Sprague–Dawley rats were infused i.v. with saline (0.4 ml h−1, control) or LPS (150 μg kg−1 h−1) for 2 h, 6 h or 24 h (n= 6–8). Muscle lactate concentration was elevated after 2, 6 and 24 h LPS infusion. Muscle PDC activity was the same at 2 h and 6 h, but was 65% lower after 24 h of LPS infusion (P < 0.01), when there was a 47% decrease in acetylcarnitine concentration (P < 0.05), and a 24‐fold increase in PDK4 mRNA expression (P < 0.001). These changes were preceded by marked increases in tumour necrosis factor‐α and interleukin‐6 mRNA expression at 2 h. The findings indicate that the early (2 and 6 h) elevation in muscle lactate concentration during LPS infusion was not attributable to limited muscle oxygen availability or ATP production (evidenced by unchanged ATP and phosphocreatine (PCr) concentrations) or to PDC inhibition, whereas after 24 h, muscle lactate accumulation appears to have resulted from PDC activation status limiting pyruvate flux, most probably due to cytokine‐mediated up‐regulation of PDK4 transcription.

Lipid induced insulin resistance is associated with an impaired skeletal muscle protein synthetic response to amino acid ingestion in healthy young men

The ability to maintain skeletal muscle mass appears to be impaired in insulin-resistant conditions, such as type 2 diabetes, that are characterized by muscle lipid accumulation. The current study investigated the effect of acutely increasing lipid availability on muscle protein synthesis. Seven healthy young male volunteers underwent a 7-h intravenous infusion of l-[ring-2H5]phenylalanine on two randomized occasions combined with 0.9% saline or 10% Intralipid at 100 mL/h. After a 4-h “basal” period, a 21-g bolus of amino acids was administered and a 3-h hyperinsulinemic-euglycemic clamp was commenced (“fed” period). Muscle biopsy specimens were obtained from the vastus lateralis at 1.5, 4, and 7 h. Lipid infusion reduced fed whole-body glucose disposal by 20%. Furthermore, whereas the mixed muscle fractional synthetic rate increased from the basal to the fed period during saline infusion by 2.2-fold, no change occurred during lipid infusion, despite similar circulating insulin and leucine concentrations. This “anabolic resistance” to insulin and amino acids with lipid infusion was associated with a complete suppression of muscle 4E-BP1 phosphorylation. We propose that increased muscle lipid availability may contribute to anabolic resistance in insulin-resistant conditions by impairing translation initiation.

Improving muscle mass and function in cachexia: non-drug approaches

Purpose of reviewTherapeutic exercise may help maintain or slow down the rate of decline in muscle mass and physical function that occurs with cachexia. This review considers recent evidence in relation to patients with cachexia as regards the rationale for the use of exercise, the challenges in its clinical application and future developments. Recent findingsExercise may attenuate the effects of cachexia by modulating muscle metabolism, insulin sensitivity and levels of inflammation. Studies targeting cachectic patients have demonstrated that even in advanced disease peripheral muscle has the capacity to respond to exercise training. Nonetheless, there are challenges in implementing the use of exercise, particularly once cachexia is established in which tolerance to even low levels of exercise is poor. Strategies to make exercise a more accessible therapy are required and could include offering exercise earlier on in the course of the disease, at lower intensities and in various forms, including more novel approaches. SummaryThe use of therapeutic exercise has a sound rationale, even in patients with advanced disease and cachexia. Because of practical issues with its application, further study is required to examine if benefits achieved in small studies can be translated to a wider clinical population.

Obesity appears to be associated with altered muscle protein synthetic and breakdown responses to increased nutrient delivery in older men, but not reduced muscle mass or contractile function.

Obesity is increasing, yet despite the necessity of maintaining muscle mass and function with age, the effect of obesity on muscle protein turnover in older adults remains unknown. Eleven obese (BMI 31.9 ± 1.1 kg · m−2) and 15 healthy-weight (BMI 23.4 ± 0.3 kg · m−2) older men (55–75 years old) participated in a study that determined muscle protein synthesis (MPS) and leg protein breakdown (LPB) under postabsorptive (hypoinsulinemic-euglycemic clamp) and postprandial (hyperinsulinemic hyperaminoacidemic-euglycemic clamp) conditions. Obesity was associated with systemic inflammation, greater leg fat mass, and patterns of mRNA expression consistent with muscle deconditioning, whereas leg lean mass, strength, and work done during maximal exercise were no different. Under postabsorptive conditions, MPS and LPB were equivalent between groups, whereas insulin and amino acid administration increased MPS in only healthy-weight subjects and was associated with lower leg glucose disposal (LGD) (63%) in obese men. Blunting of MPS in the obese men was offset by an apparent decline in LPB, which was absent in healthy-weight subjects. Lower postprandial LGD in obese subjects and blunting of MPS responses to amino acids suggest that obesity in older adults is associated with diminished muscle metabolic quality. This does not, however, appear to be associated with lower leg lean mass or strength.

Physiological control of muscle mass in humans during resistance exercise, disuse and rehabilitation.

Purpose of reviewThe preservation of skeletal muscle mass is central to maintaining mobility and quality of life with aging and also impacts on our capacity to recover from illness. However, our understanding of the processes that regulate muscle mass in humans during exercise, chronic disuse and rehabilitation remains unclear. This brief review aims to highlight some of the more recent and important findings concerning these physiological stimuli. Recent findingsAlthough several studies have detailed the molecular events that occur following an acute bout of resistance exercise, a paucity of data appears to remain concerning the molecular and signaling events that underpin resistance exercise training. Reports of increased transcripts for inflammatory proteins following eccentric but not concentric exercise could represent the stimulus for the instigation of structural adaptations that occur following intense muscle lengthening contractions. Studies investigating processes underlying disuse-induced muscle atrophy provide initial evidence to support the notion that transient increases in muscle protein degradation occur following the onset of muscle disuse in humans. SummaryThe need for further studies to improve our basic understanding of muscle-associated processes in humans remains, particularly in relation to the temporal changes in muscle processes that occur during resistance training.

Resistance exercise and the mechanisms of muscle mass regulation in humans: Acute effects on muscle protein turnover and the gaps in our understanding of chronic resistance exercise training adaptation☆

Increasing muscle mass is important when attempting to maximize sports performance and achieve physique augmentation. However, the preservation of muscle mass is essential to maintaining mobility and quality of life with aging, and also impacts on our capacity to recover from illness. Nevertheless, our understanding of the processes that regulate muscle mass in humans during resistance exercise training, chronic disuse and rehabilitation training following atrophy remains very unclear. Here, we report on some of the recent developments in the study of those processes thought to be responsible for governing human muscle protein turnover in response to intense physical activity. Specifically, the effects of acute and chronic resistance exercise in healthy volunteers and also in response to rehabilitation resistance exercise training following muscle atrophy will be discussed, with discrepancies and gaps in our understanding highlighted. In particular, ubiquitin-proteasome mediated muscle proteolysis (Muscle Atrophy F-box/Atrogin-1 and Muscle RING Finger 1), translation initiation of muscle protein synthesis (mammalian target of rapamycin signaling), and satellite cell mediated myogenesis are highlighted as pathways of special relevance to muscle protein metabolism in response to acute resistance exercise. Furthermore, research focused on quantifying signaling and molecular events that modulate muscle protein synthesis and protein degradation under conditions of chronic resistance training is highlighted as being urgently needed to improve knowledge gaps. These studies need to include multiple time-point measurements over the course of any training intervention and must include dynamic measurements of muscle protein synthesis and degradation and sensitive measures of muscle mass. This article is part of a Directed Issue entitled Molecular basis of muscle wasting.

Transient transcriptional events in human skeletal muscle at the outset of concentric resistance exercise training

We sought to ascertain the time course of transcriptional events that occur in human skeletal muscle at the outset of resistance exercise (RE) training in RE naive individuals and determine whether the magnitude of response was associated with exercise-induced muscle damage. Sixteen RE naive men were recruited; eight underwent two sessions of 5 × 30 maximum isokinetic knee extensions (180°/s) separated by 48 h. Muscle biopsies of the vastus lateralis, obtained from different sites, were taken at baseline and 24 h after each exercise bout. Eight individuals acted as nonexercise controls with biopsies obtained at the same time intervals. Transcriptional changes were assessed by microarray and protein levels of heat shock protein (HSP) 27 and αB-crystallin in muscle cross sections by immunohistochemistry as a proxy measure of muscle damage. In control subjects, no probe sets were significantly altered (false discovery rate < 0.05), and HSP27 and αB-crystallin protein remained unchanged throughout the study. In exercised subjects, significant intersubject variability following the initial RE bout was observed in the muscle transcriptome, with greatest changes occurring in subjects with elevated HSP27 and αB-crystallin protein. Following the second bout, the transcriptome response was more consistent, revealing a cohort of probe sets associated with immune activation, the suppression of oxidative metabolism, and ubiquitination, as differentially regulated. The results reveal that the initial transcriptional response to RE is variable in RE naive volunteers, potentially associated with muscle damage and unlikely to reflect longer term adaptations to RE training. These results highlight the importance of considering multiple time points when determining the transcriptional response to RE and associated physiological adaptation.

Mechanisms responsible for disuse muscle atrophy: Potential role of protein provision and exercise as countermeasures

Muscle disuse is often observed after injury or during periods of illness, resulting in the loss of muscle mass and strength, with sometimes debilitating consequences. Although substantial advancements have been made in determining the mechanisms responsible for the etiology of muscle disuse atrophy in rodents, only in recent years have studies of any significant number focused on reaffirming these findings in humans. In this review, we discuss the processes responsible for disuse atrophy as based on current evidence and highlight where gaps in our knowledge persist. Furthermore, given the emphasis placed on resistance exercise and nutrition as potential therapeutic countermeasures, we consider recent advancements in the study of resistance exercise and nutrition in the stimulation of muscle protein synthesis and the associated implications when devising effective treatment strategies.