Repeated bouts of resistance exercise in rats alter mechanistic target of rapamycin complex 1 activity and ribosomal capacity but not muscle protein synthesis

Transcriptional and morphological responses following distinct muscle contraction protocols for Snell dwarf (Pit1dw/dw) mice

The Snell dwarf mouse (Pit1dw/dw), an animal model of congenital combined pituitary hormone deficiency, displays skeletal muscle weakness. While enhanced responsivity to repeated exposures of muscle contractions have been documented for Snell dwarf mice, the response following single exposure to distinct contraction protocols remained uncharacterized. The purpose of this study was to investigate the muscle recovery of Snell dwarf and control littermate mice following a single exposure to two separate protocols-an intermittent slow velocity (30°/s) contraction protocol or a continuous rapid velocity (500°/s) contraction protocol. Following both protocols for control mice, torque values were 30% and 80% of pre-protocol values at 5 min and 3 days, respectively. At 10 days, performance returned to baseline for the 30°/s protocol and were depressed for the 500°/s protocol. For Snell dwarf mice following both protocols, torques were depressed to 5% of pre-protocol values at 5 min and returned to baseline by 3 days. Recovery following the 30°/s protocol for control mice and both protocols for Snell dwarf mice coincided with increased transcriptional output, upregulation of cytokine-mediated signaling genes, and a distribution shift to smaller muscle fibers with reduced area per nucleus. These features represent efficacious remodeling ubiquitous across distinct contraction paradigms in the context of the Pit1 mutation.

Belt electrode tetanus muscle stimulation reduces denervation-induced atrophy of rat multiple skeletal muscle groups

Belt electrode-skeletal muscle electrical stimulation (B-SES) involves the use of belt-shaped electrodes to contract multiple muscle groups simultaneously. Twitch contractions have been demonstrated to protect against denervation-induced muscle atrophy in rats, possibly through mitochondrial biosynthesis. This study examined whether inducing tetanus contractions with B-SES suppresses muscle atrophy and identified the underlying molecular mechanisms. We evaluated the effects of acute (60 Hz, 5 min) and chronic (60 Hz, 5 min, every alternate day for one week) B-SES on the tibialis anterior (TA) and gastrocnemius (GAS) muscles in Sprague-Dawley rats using belt electrodes attached to both ankle joints. After acute stimulation, a significant decrease in the glycogen content was observed in the left and right TA and GAS, suggesting that B-SES causes simultaneous contractions in multiple muscle groups. B-SES enhanced p70S6K phosphorylation, an indicator of the mechanistic target of rapamycin complex 1 activity. During chronic stimulations, rats were divided into control (CONT), denervation-induced atrophy (DEN), and DEN + electrically stimulated with B-SES (DEN + ES) groups. After seven days of treatment, the wet weight (n = 8-11 for each group) and muscle fiber cross-sectional area (CSA, n = 6 for each group) of the TA and GAS muscles were reduced in the DEN and DEN + ES groups compared with that in the CON group. The DEN + ES group showed significantly higher muscle weight and CSA than those in the DEN group. Although RNA-seq and pathway analysis suggested that mitochondrial biogenesis is a critical event in this phenomenon, mitochondrial content showed no difference. In contrast, ribosomal RNA 28S and 18S (n = 6) levels in the DEN + ES group were higher than those in the DEN group, even though RNA-seq showed that the ribosome biogenesis pathway was reduced by electrical stimulation. The mRNA levels of the muscle proteolytic molecules atrogin-1 and MuRF1 were significantly higher in DEN than those in CONT. However, they were more suppressed in DEN + ES than those in DEN. In conclusion, tetanic electrical stimulation of both ankles using belt electrodes effectively reduced denervation-induced atrophy in multiple muscle groups. Furthermore, ribosomal biosynthesis plays a vital role in this phenomenon.

The Plateau in Muscle Growth with Resistance Training: An Exploration of Possible Mechanisms

It is hypothesized that there is likely a finite ability for muscular adaptation. While it is difficult to distinguish between a true plateau following a long-term training period and short-term stalling in muscle growth, a plateau in muscle growth has been attributed to reaching a genetic potential, with limited discussion on what might physiologically contribute to this muscle growth plateau. The present paper explores potential physiological factors that may drive the decline in muscle growth after prolonged resistance training. Overall, with chronic training, the anabolic signaling pathways may become more refractory to loading. While measures of anabolic markers may have some predictive capabilities regarding muscle growth adaptation, they do not always demonstrate a clear connection. Catabolic processes may also constrain the ability to achieve further muscle growth, which is influenced by energy balance. Although speculative, muscle cells may also possess cell scaling mechanisms that sense and regulate their own size, along with molecular brakes that hinder growth rate over time. When considering muscle growth over the lifespan, there comes a point when the anabolic response is attenuated by aging, regardless of whether or not individuals approach their muscle growth potential. Our goal is that the current review opens avenues for future experimental studies to further elucidate potential mechanisms to explain why muscle growth may plateau.

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

Mechanisms underlying mechanical overload-induced skeletal muscle hypertrophy have been extensively researched since the landmark report by Morpurgo (1897) of "work-induced hypertrophy" in dogs that were treadmill trained. Much of the preclinical rodent and human resistance training research to date supports that involved mechanisms include enhanced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling, an expansion in translational capacity through ribosome biogenesis, increased satellite cell abundance and myonuclear accretion, and postexercise elevations in muscle protein synthesis rates. However, several lines of past and emerging evidence suggest that additional mechanisms that feed into or are independent of these processes are also involved. This review first provides a historical account of how mechanistic research into skeletal muscle hypertrophy has progressed. A comprehensive list of mechanisms associated with skeletal muscle hypertrophy is then outlined, and areas of disagreement involving these mechanisms are presented. Finally, future research directions involving many of the discussed mechanisms are proposed.

Anti-osteoporosis mechanism of resistance exercise in ovariectomized rats based on transcriptome analysis: a pilot study

Postmenopausal osteoporosis is the main cause of fractures in women. Resistance exercise has a positive effect on bone mineral density in postmenopausal osteoporosis patients, but its mechanism is unclear. The purpose of this study was to explore the mechanism of resistance exercise in improving ovariectomized osteoporotic rats based on the transcriptome sequencing technique. Eighteen female Sprague-Dawley rats were randomly divided into the sham-operated group, the non-exercise group, and the resistance exercise group. The rat model of postmenopausal osteoporosis was established by bilateral ovariectomy. Ten weeks after the operation, the resistance exercise group received 2 weeks of adaptive training, and 12 weeks of resistance exercise began in the 13th week. The rats were trained 5 days per week, in 4 sets of 3 repetitions per day. After the intervention, all rats were sacrificed, and the body weight, bone mineral density, trabecular bone microarchitecture, and bone biomechanics were examined. At the same time, RNA-seq and enrichment analysis of gene ontology and Kyoto Encyclopedia of Genes and Genomes were performed on the left tibias, followed by Elisa and RT-qPCR verification. It had been found that resistance exercise can effectively counteract the weight gain of ovariectomized osteoporotic rats, and has a good effect on bone mineral density and trabecular bone microarchitecture. Enrichment analysis showed that regulation of gene expression and osteoclast differentiation is the most closely related biological process and signaling pathway shared by RE/Ovx and NE/Ovx groups. Our results revealed that resistance exercise can play a role in inhibiting osteoclast activation and preventing the enhancement of osteoclast bone resorption function in ovariectomized osteoporotic rats by inhibiting Fos/Fosb-regulated TRAP activation and relieving Calcr inhibition, which has important application value in preventing bone loss caused by estrogen deficiency.

Combined effects of functional overload and denervation on skeletal muscle mass and its regulatory proteins in mice

Skeletal muscle is a highly pliable tissue and various adaptations such as muscle hypertrophy or atrophy are induced by overloading or disuse, respectively. However, the combined effect of overloading and disuse on the quantitative adaptation of skeletal muscle is unknown. Thus, the aim of this study was to investigate the effects of the combined stimuli of overloading and disuse on mouse skeletal muscle mass and the expression of regulatory factors for muscle protein anabolism or catabolism. Male mice from the Institute Cancer Research were subjected to denervation concomitant with unilateral functional overload or functional overload concomitant with unilateral denervation. Disuse and functional overload were induced by sciatic nerve transection (denervation) and synergist ablation, respectively, and the plantaris muscle was harvested 14 days after the operation. Our results showed that denervation attenuated functional overload-induced muscle hypertrophy and functional overload partially ameliorated the denervation-induced muscle atrophy. P70S6K phosphorylation, an indicator of mechanistic target of rapamycin complex 1 (mTORC1) activation, was not increased by unilateral functional overload in denervated muscles or by unilateral denervation in functional overloaded muscles. Denervation did not affect the increase of LC3-II, a marker of autophagy activation, and ubiquitinated protein expression upon unilateral functional overload. Also, functional overload did not affect ubiquitinated protein expression during unilateral denervation. Thus, our findings suggest that functional overload-induced muscle hypertrophy or denervation-induced muscle atrophy was attenuated by the combined stimuli of overload and denervation.

Elevated muscle mass accompanied by transcriptional and nuclear alterations several months following cessation of resistance-type training in rats

Rodent studies investigating long-term effects following termination of hypertrophy-inducing loading have predominantly involved exposures such as synergist ablation and weighted wheel running or ladder climbing. This research yielded a spectrum of results regarding the extent of detraining in terms of muscle mass and myonuclei number. The studies were also limited in their lack of sensitive performance measures and indirect relatedness to resistance training. Our research group developed and validated a relevant rat model of resistance-type training that induces increased muscle mass and performance. The aim of the present study was to determine to what extent these features persist 3 months following the termination of this training. While performance returned to baseline, muscle mass remained elevated by 17% and a shift in distribution to larger muscle fibers persisted. A 16% greater total RNA and heightened mRNA levels of ribosomal protein S6 kinases implicated preserved transcriptional output and ribosomal content. Remodeling of muscle fiber nuclei was consistent with these findings - increased nuclear number and a distribution shift to a more circular nuclear shape. These findings indicate that muscle mass detrains at a slower rate than performance and implicates multiple forms of myonuclear remodeling in muscle memory.

An Evidence-Based Narrative Review of Mechanisms of Resistance Exercise-Induced Human Skeletal Muscle Hypertrophy

Skeletal muscle plays a critical role in physical function and metabolic health. Muscle is a highly adaptable tissue that responds to resistance exercise (RE; loading) by hypertrophying, or during muscle disuse, RE mitigates muscle loss. Resistance exercise training (RET)-induced skeletal muscle hypertrophy is a product of external (e.g., RE programming, diet, some supplements) and internal variables (e.g., mechanotransduction, ribosomes, gene expression, satellite cells activity). RE is undeniably the most potent nonpharmacological external variable to stimulate the activation/suppression of internal variables linked to muscular hypertrophy or countering disuse-induced muscle loss. Here, we posit that despite considerable research on the impact of external variables on RET and hypertrophy, internal variables (i.e., inherent skeletal muscle biology) are dominant in regulating the extent of hypertrophy in response to external stimuli. Thus, identifying the key internal skeletal muscle-derived variables that mediate the translation of external RE variables will be pivotal to determining the most effective strategies for skeletal muscle hypertrophy in healthy persons. Such work will aid in enhancing function in clinical populations, slowing functional decline, and promoting physical mobility. We provide up-to-date, evidence-based perspectives of the mechanisms regulating RET-induced skeletal muscle hypertrophy.

Percutaneous electrical stimulation-induced muscle contraction prevents the decrease in ribosome RNA and ribosome protein during pelvic hindlimb suspension

Skeletal muscle unloading leads to muscle atrophy. Ribosome synthesis has been implicated as an important skeletal muscle mass regulator owing to its translational capacity. Muscle unloading induces a reduction in ribosome synthesis and content, with muscle atrophy. Percutaneous electrical muscle stimulation (pEMS)-induced muscle contraction is widely used in clinics to improve muscle mass. However, its efficacy in rescuing the reduction in ribosomal synthesis has not been addressed thus far. We examined the effects of daily pEMS treatment on ribosome synthesis and content during mouse hindlimb unloading. Male C57BL/6J mice were randomly assigned to sedentary (SED) and hindlimb unloading by pelvic suspension (HU) groups. Muscle contraction was triggered by pEMS treatment of the right gastrocnemius muscle of a subset of the HU group (HU+pEMS). Hindlimb unloading for 6 days significantly lowered 28S rRNA, rpL10, and rpS3 expression, which was rescued by daily pEMS treatment. The protein expression of phospho-p70S6K and UBF was significantly higher in the HU+pEMS than in the HU group. The mRNA expression of ribophagy receptor Nufip1 increased in both the HU and HU+pEMS groups. Protein light chain 3 (LC3)-II expression and the LC3-II/LC3-I ratio were increased by HU, but pEMS attenuated this increase. Our findings indicate that during HU, daily pEMS treatment prevents the reduction in the levels of some proteins associated with ribosome synthesis. Additionally, the HU-induced activation of ribosome degradation may be attenuated. These data provide insights into ribosome content regulation and the mechanism of attenuation of muscle atrophy by pEMS treatment during muscle disuse.

Protein signaling in response to ex vivo dynamic contractions is independent of training status in rat skeletal muscle

New Findings

What is the central question of this study?

Are myofibre protein signalling responses to ex vivo dynamic contractions altered by accustomization to voluntary endurance training in rats?

What is the main finding and its importance?

In response to ex vivo dynamic muscle contractions, canonical myofibre protein signalling pertaining to metabolic transcriptional regulation, as well as translation initiation and elongation, was not influenced by prior accustomization to voluntary endurance training in rats. Accordingly, intrinsic myofibre protein signalling responses to standardized contractile activity may be independent of prior exercise training in rat skeletal muscle.

Abstract

Skeletal muscle training status may influence myofibre regulatory protein signalling in response to contractile activity. The current study employed a purpose‐designed ex vivo dynamic contractile protocol to evaluate the effect of exercise‐accustomization on canonical myofibre protein signalling for metabolic gene expression and for translation initiation and elongation. To this end, rats completed 8 weeks of in vivo voluntary running training versus no running control intervention, whereupon an ex vivo endurance‐type dynamic contraction stimulus was conducted in isolated soleus muscle preparations from both intervention groups. Protein signalling response by phosphorylation was evaluated by immunoblotting at 0 and 3 h following ex vivo stimulation. Phosphorylation of AMP‐activated protein kinase α‐isoforms and its downstream target, acetyl‐CoA carboxylase, as well as phosphorylation of eukaryotic elongation factor 2 (eEF2) was increased immediately following the dynamic contraction protocol (at 0 h). Signalling for translation initiation and elongation was evident at 3 h after dynamic contractile activity, as evidenced by increased phosphorylation of p70 S6 kinase and eukaryotic translation initiation factor 4E‐binding protein 1, as well as a decrease in phosphorylation of eEF2 back to resting control levels. However, prior exercise training did not alter phosphorylation responses of the investigated signalling proteins. Accordingly, protein signalling responses to standardized endurance‐type contractions may be independent of training status in rat muscle during ex vivo conditions. The present findings add to our current understanding of molecular regulatory events responsible for skeletal muscle plasticity.

Central Suppression of the GH/IGF Axis and Abrogation of Exercise-Related mTORC1/2 Activation in the Muscle of Phenotype-Selected Male Marathon Mice (DUhTP)

The somatotropic axis is required for a number of biological processes, including growth, metabolism, and aging. Due to its central effects on growth and metabolism and with respect to its positive effects on muscle mass, regulation of the GH/IGF-system during endurance exercise is of particular interest. In order to study the control of gene expression and adaptation related to physical performance, we used a non-inbred mouse model, phenotype-selected for high running performance (DUhTP). Gene expression of the GH/IGF-system and related signaling cascades were studied in the pituitary gland and muscle of sedentary males of marathon and unselected control mice. In addition, the effects of three weeks of endurance exercise were assessed in both genetic groups. In pituitary glands from DUhTP mice, reduced expression of Pou1f1 (p = 0.002) was accompanied by non-significant reductions of Gh mRNA (p = 0.066). In addition, mRNA expression of Ghsr and Sstr2 were significantly reduced in the pituitary glands from DUhTP mice (p ≤ 0.05). Central downregulation of Pou1f1 expression was accompanied by reduced serum concentrations of IGF1 and coordinated downregulation of multiple GH/IGF-signaling compounds in muscle (e.g., Ghr, Igf1, Igf1r, Igf2r, Irs1, Irs2, Akt3, Gskb, Pik3ca/b/a2, Pten, Rictor, Rptor, Tsc1, Mtor; p ≤ 0.05). In response to exercise, the expression of Igfbp3, Igfbp 4, and Igfbp 6 and Stc2 mRNA was increased in the muscle of DUhTP mice (p ≤ 0.05). Training-induced specific activation of AKT, S6K, and p38 MAPK was found in muscles from control mice but not in DUhTP mice (p ≤ 0.05), indicating a lack of mTORC1 and mTORC2 activation in marathon mice in response to physical exercise. While hormone-dependent mTORC1 and mTORC2 pathways in marathon mice were repressed, robust increases of Ragulator complex compounds (p ≤ 0.001) and elevated sirtuin 2 to 6 mRNA expression were observed in the DUhTP marathon mouse model (p ≤ 0.05). Activation of AMPK was not observed under the experimental conditions of the present study. Our results describe coordinated downregulation of the somatotropic pathway in long-term selected marathon mice (DUhTP), possibly via the pituitary gland and muscle interaction. Our results, for the first time, demonstrate that GH/IGF effects are repressed in a context of superior running performance in mice.