Ferdinand von Walden

Skeletal muscle proteolysis in response to short-term unloading in humans

Skeletal muscle atrophy is evident after muscle disuse, unloading, or spaceflight and results from decreased protein content as a consequence of decreased protein synthesis, increased protein breakdown or both. At this time, there are essentially no human data describing proteolysis in skeletal muscle undergoing atrophy on Earth or in space, primarily due to lack of valid and accurate methodology. This particular study aimed at assessing the effects of short-term unloading on the muscle contractile proteolysis rate. Eight men were subjected to 72-h unilateral lower limb suspension (ULLS) and intramuscular interstitial levels of the naturally occurring proteolytic tracer 3-methylhistidine (3MH) were measured by means of microdialysis before and on completion of this intervention. The 3MH concentration following 72-h ULLS (2.01 +/- 0.22 nmol/ml) was 44% higher (P < 0.05) than before ULLS (1.56 +/- 0.20 nmol/ml). The present experimental model and the employed method determining 3MH in microdialysates present a promising tool for monitoring skeletal muscle proteolysis or metabolism of specific muscles during conditions resulting in atrophy caused by, e.g., disuse and real or simulated microgravity. This study provides evidence that the atrophic processes are evoked rapidly and within 72 h of unloading and suggests that countermeasures should be employed in the early stages of space missions to offset or prevent muscle loss during the period when the rate of muscle atrophy is the highest.

Strength, power, fiber types, and mRNA expression in trained men and women with different ACTN3 R577X genotypes

Alpha-actinins are structural proteins of the Z-line. Human skeletal muscle expresses two alpha-actinin isoforms, alpha-actinin-2 and alpha-actinin-3, encoded by their respective genes ACTN2 and ACTN3. ACTN2 is expressed in all muscle fiber types, while only type II fibers, and particularly the type IIb fibers, express ACTN3. ACTN3 (R577X) polymorphism results in loss of alpha-actinin-3 and has been suggested to influence skeletal muscle function. The X allele is less common in elite sprint and power athletes than in the general population and has been suggested to be detrimental for performance requiring high power. The present study investigated the association of ACTN3 genotype with muscle power during 30-s Wingate cycling in 120 moderately to well-trained men and women and with knee extensor strength and fatigability in a subset of 21 men performing isokinetic exercise. Muscle biopsies were obtained from the vastus lateralis muscle to determine fiber-type composition and ACTN2 and ACTN3 mRNA levels. Peak and mean power and the torque-velocity relationship and fatigability output showed no difference across ACTN3 genotypes. Thus this study suggests that R577X polymorphism in ACTN3 is not associated with differences in power output, fatigability, or force-velocity characteristics in moderately trained individuals. However, repeated exercise bouts prompted an increase in peak torque in RR but not in XX genotypes, suggesting that ACTN3 genotype may modulate responsiveness to training. Our data further suggest that alpha-actinins do not play a significant role in determining muscle fiber-type composition. Finally, we show that ACTN2 expression is affected by the content of alpha-actinin-3, which implies that alpha-actinin-2 may compensate for the lack of alpha-actinin-3 and hence counteract the phenotypic consequences of the deficiency.

Muscle wasting in end-stage renal disease promulgates premature death: established, emerging and potential novel treatment strategies

Muscle wasting (or sarcopenia) is a common feature of the uremic phenotype and predisposes this vulnerable patient population to increased risk of comorbid complications, poor quality of life, frailty and premature death. The old age of dialysis patients is in addition a likely contributor to loss of muscle mass. As recent evidence suggests that assessment of muscle strength (i.e. function) is a better predictor of outcome and comorbidities than muscle mass, this opens new screening, assessment and therapeutic opportunities. Among established treatment strategies, the benefit of resistance exercise and endurance training are increasingly recognized among nephrologists as being effective and should be promoted in sedentary chronic kidney disease patients. Testosterone and growth hormone replacement appear as the most promising among emerging treatments strategies for muscle wasting. As treatment of muscle wasting is difficult and seldom successful in this often old, frail, sedentary and exercise-hesitant patient group, novel treatment strategies are urgently needed. In this review, we summarize recent studies on stimulation of mitochondrial biogenesis, myogenic stem (satellite) cells and manipulation of transforming growth factor family members, all of which hold promise for more effective therapies to target muscle mass loss and function in the future.

Mechanical loading induces the expression of a Pol I regulon at the onset of skeletal muscle hypertrophy

The main goal of the present study was to investigate the regulation of ribosomal DNA (rDNA) gene transcription at the onset of skeletal muscle hypertrophy. Mice were subjected to functional overload of the plantaris by bilateral removal of the synergist muscles. Mechanical loading resulted in muscle hypertrophy with an increase in rRNA content. rDNA transcription, as determined by 45S pre-rRNA abundance, paralleled the increase in rRNA content and was consistent with the onset of the hypertrophic response. Increased transcription and protein expression of c-Myc and its downstream polymerase I (Pol I) regulon (POL1RB, TIF-1A, PAF53, TTF1, TAF1C) was also consistent with the increase in rRNA. Similarly, factors involved in rDNA transcription, such as the upstream binding factor and the Williams syndrome transcription factor, were induced by mechanical loading in a corresponding temporal fashion. Chromatin immunoprecipitation revealed that these factors, together with Pol I, were enriched at the rDNA promoter. This, in addition to an increase in histone H3 lysine 9 acetylation, demonstrates that mechanical loading regulates rRNA synthesis by inducing a gene expression program consisting of a Pol I regulon, together with accessory factors involved in transcription and chromatin remodeling at the rDNA promoter. Altogether, these data indicate that transcriptional and epigenetic mechanisms take place in the regulation of ribosome production at the onset of muscle hypertrophy.

mTOR signaling regulates myotube hypertrophy by modulating protein synthesis, rDNA transcription and chromatin remodeling.

Ribosome production is an early event during skeletal muscle hypertrophy and precedes muscle protein accretion. Signaling via mTOR is crucial for ribosome production and hypertrophy; however, the mechanisms by which it regulates these processes remain to be identified. Herein, we investigated the activation of mTOR signaling in hypertrophying myotubes and determined that mTOR coordinates various aspects of gene expression important for ribosome production. First, inhibition of translation with cycloheximide had a more potent effect on protein synthesis than rapamycin indicating that mTOR function during hypertrophy is not on general, but rather on specific protein synthesis. Second, blocking Pol II transcription had a similar effect as Rapamycin and, unexpectedly, revealed the necessity of Pol II transcription for Pol I transcription, suggesting that mTOR may regulate ribosome production also by controlling Class II genes at the transcriptional level. Third, Pol I activity is essential for rDNA transcription and, surprisingly, for protein synthesis as selective Pol I inhibition blunted rDNA transcription, protein synthesis, and the hypertrophic response of myotubes. Finally, mTOR has nuclear localization in muscle, which is not sensitive to rapamycin. Inhibition of mTOR signaling by rapamycin disrupted mTOR-rDNA promoter interaction and resulted in altered histone marks indicative of repressed transcription and formation of higher-order chromatin structure. Thus mTOR signaling appears to regulate muscle hypertrophy by affecting protein synthesis, Class I and II gene expression, and chromatin remodeling.

Altered autophagy gene expression and persistent atrophy suggest impaired remodeling in chronic hemiplegic human skeletal muscle

Upper motor neuron lesions after stroke are a major cause of disability. We aimed to determine whether skeletal muscles from these patients display typical molecular signatures of inflammation, growth arrest, and atrophy.

Humanin skeletal muscle protein levels increase after resistance training in men with impaired glucose metabolism

Humanin (HN) is a mitochondrially encoded and secreted peptide linked to glucose metabolism and tissue protecting mechanisms. Whether skeletal muscle HN gene or protein expression is influenced by exercise remains unknown. In this intervention study we show, for the first time, that HN protein levels increase in human skeletal muscle following 12 weeks of resistance training in persons with prediabetes. Male subjects (n = 55) with impaired glucose regulation (IGR) were recruited and randomly assigned to resistance training, Nordic walking or a control group. The exercise interventions were performed three times per week for 12 weeks with progressively increased intensity during the intervention period. Biopsies from the vastus lateralis muscle and venous blood samples were taken before and after the intervention. Skeletal muscle and serum protein levels of HN were analyzed as well as skeletal muscle gene expression of the mitochondrially encoded gene MT‐RNR2, containing the open reading frame for HN. To elucidate mitochondrial training adaptation, mtDNA, and nuclear DNA as well as Citrate synthase were measured. Skeletal muscle HN protein levels increased by 35% after 12 weeks of resistance training. No change in humanin protein levels was seen in serum in any of the intervention groups. There was a significant correlation between humanin levels in serum and the improvements in the 2 h glucose loading test in the resistance training group. The increase in HN protein levels in skeletal muscle after regular resistance training in prediabetic males may suggest a role for HN in the regulation of glucose metabolism. Given the preventative effect of exercise on diabetes type 2, the role of HN as a mitochondrially derived peptide and an exercise‐responsive mitokine warrants further investigation.

Forearm Flexor Muscles in Children with Cerebral Palsy Are Weak, Thin and Stiff

Children with cerebral palsy (CP) often develop reduced passive range of motion with age. The determining factor underlying this process is believed to be progressive development of contracture in skeletal muscle that likely changes the biomechanics of the joints. Consequently, to identify the underlying mechanisms, we modeled the mechanical characteristics of the forearm flexors acting across the wrist joint. We investigated skeletal muscle strength (Grippit®) and passive stiffness and viscosity of the forearm flexors in 15 typically developing (TD) children (10 boys/5 girls, mean age 12 years, range 8–18 yrs) and nine children with CP Nine children (6 boys/3 girls, mean age 11 ± 3 years (yrs), range 7–15 yrs) using the NeuroFlexor® apparatus. The muscle stiffness we estimate and report is the instantaneous mechanical response of the tissue that is independent of reflex activity. Furthermore, we assessed cross-sectional area of the flexor carpi radialis (FCR) muscle using ultrasound. Age and body weight did not differ significantly between the two groups. Children with CP had a significantly weaker (−65%, p < 0.01) grip and had smaller cross-sectional area (−43%, p < 0.01) of the FCR muscle. Passive stiffness of the forearm muscles in children with CP was increased 2-fold (p < 0.05) whereas viscosity did not differ significantly between CP and TD children. FCR cross-sectional area correlated to age (R2 = 0.58, p < 0.01), body weight (R2 = 0.92, p < 0.0001) and grip strength (R2 = 0.82, p < 0.0001) in TD children but only to grip strength (R2 = 0.60, p < 0.05) in children with CP. We conclude that children with CP have weaker, thinner, and stiffer forearm flexors as compared to typically developing children.

Bioactive food and exercise in chronic kidney disease: Targeting the mitochondria

Chronic kidney disease (CKD), which affects 10%‐15% of the population, associates with a range of complications—such as cardiovascular disease, frailty, infections, muscle and bone disorders and premature ageing—that could be related to alterations of mitochondrial number, distribution, structure and function. As mitochondrial biogenesis, bioenergetics and the dynamic mitochondrial networks directly or indirectly regulate numerous intra‐ and extracellular functions, the mitochondria have emerged as an important target for interventions aiming at preventing or improving the treatment of complications in CKD. In this review, we discuss the possible role of bioactive food compounds and exercise in the modulation of the disturbed mitochondrial function in a uraemic milieu.