Cessation of electrically-induced muscle contraction activates autophagy in cultured myotubes.

Exercise is known to improve skeletal muscle function. The mechanism involves muscle contraction-induced activation of the mTOR pathway, which plays a central role in protein synthesis. However, mTOR activation blocks autophagy, a recycling mechanism with a critical role in cellular maintenance/homeostasis. These two responses to muscle contraction look contradictory to the functional improvement of exercise. Herein, we investigate these paradoxical muscle responses in a series of active-inactive phases in a cultured myotube model receiving electrical stimulation to induce intermittent muscle contraction. Our model shows that (1) contractile activity induces mTOR activation and muscle hypertrophy but blocks autophagy, resulting in the accumulation of damaged proteins, while (2) cessation of muscle contraction rapidly activates autophagy, removing damaged protein, yet a prolonged inactive state results in muscle atrophy. Our findings provide new insights into muscle biology and suggest that not only muscle contraction, but also the subsequent cessation of contraction plays a substantial role for the improvement of skeletal muscle function.

Post-injury stretch promotes recovery in a rat model of muscle damage induced by lengthening contractions.

We investigated the cellular mechanisms and therapeutic effect of post-injury stretch on the recovery process from muscle injury induced by lengthening contractions (LC). One day after LC, a single 15-min bout of muscle stretch was applied at an intensity of 3 mNm. The maximal isometric torque was measured before and at 2-21 days after LC. The myofiber size was analyzed at 21 days after LC. Developmental myosin heavy chain-immunoreactive (dMHC-ir) cells, a marker of regenerating myofibers, were observed in the early recovery stage (2-5 days after LC). We observed that LC-induced injury markedly decreased isometric torque and myofiber size, which recovered faster in rats that underwent stretch than in rats that did not. Regenerating myofiber with dMHC-ir cells was observed earlier in rats that underwent stretch. These results indicate that post-injury stretch may facilitate the regeneration and early formation of new myofibers, thereby promoting structural and functional recovery from LC-induced muscle injury.

Training at non-damaging intensities facilitates recovery from muscle atrophy.

INTRODUCTION: Resistance training promotes recovery from muscle atrophy, but optimum training programs have not been established. We aimed to determine the optimum training intensity for muscle atrophy. METHODS: Mice recovering from atrophied muscles after 2 weeks of tail suspension underwent repeated isometric training with varying joint torques 50 times per day. RESULTS: Muscle recovery assessed by maximal isometric contraction and myofiber cross-sectional areas (CSAs) were facilitated at 40% and 60% maximum contraction strength (MC), but at not at 10% and 90% MC. At 60% and 90% MC, damaged and contained smaller diameter fibers were observed. Activation of myogenic satellite cells and a marked increase in myonuclei were observed at 40%, 60%, and 90% MC. CONCLUSIONS: The increases in myofiber CSAs were likely caused by increased myonuclei formed through fusion of resistance-induced myofibers with myogenic satellite cells. These data indicate that resistance training without muscle damage facilitates efficient recovery from atrophy. Muscle Nerve 55: 243-253, 2017.

Stand-up exercise training facilitates muscle recovery from disuse atrophy by stimulating myogenic satellite cell proliferation in mice.

Determining the cellular and molecular recovery processes in inactivity - or unloading -induced atrophied muscles should improve rehabilitation strategies. We assessed the effects of stand-up exercise (SE) training on the recovery of atrophied skeletal muscles in male mice. Mice were trained to stand up and press an elevated lever in response to a light-tone cue preceding an electric foot shock and then subjected to tail suspension (TS) for 2 weeks to induce disuse atrophy in hind limb muscles. After release from TS, mice were divided into SE-trained (SE cues: 25 times per set, two sets per day) and non-SE-trained groups. Seven days after the training, average myofiber cross-sectional area (CSA) of the soleus muscle was significantly greater in the SE-trained group than in the non-SE-trained group (1843 ± 194 µm(2) vs. 1315 ± 153 µm(2)). Mean soleus muscle CSA in the SE trained group was not different from that in the CON group subjected to neither TS nor SE training (2005 ± 196 µm(2)), indicating that SE training caused nearly complete recovery from muscle atrophy. The number of myonuclei per myofiber was increased by ~60% in the SE-trained group compared with the non-SE-trained and CON groups (0.92 ± 0.03 vs. 0.57 ± 0.03 and 0.56 ± 0.11, respectively). The number of proliferating myonuclei, identified by 5-ethynyl-2'-deoxyuridine staining, increased within the first few days of SE training. Thus, it is highly likely that myogenic satellite cells proliferated rapidly in atrophied muscles in response to SE training and fused with existing myofibers to reestablish muscle mass.

Involvement of PI3K/Akt/TOR pathway in stretch-induced hypertrophy of myotubes.

Skeletal muscle cells are hypertrophied by mechanical stresses, but the underlying molecular mechanisms are not fully understood. Two signaling pathways, phosphatidylinositol 3-kinase (PI3K)/Akt to target of rapamycin (TOR) and extracellular signal-regulated kinase kinase (MEK) to extracellular signal-regulated kinase (ERK), have been proposed to be involved in muscle hypertrophy. In this study we examined the involvement of these pathways in primary cultures of chick skeletal myotubes subjected to passive cyclic stretching for 72 hours, a time that was sufficient to induce significant hypertrophy in our preparations. Hypertrophy was largely suppressed by wortmannin or rapamycin, inhibitors of PI3K or mTOR, respectively. Furthermore, phosphorylation of Akt was enhanced by stretching and suppressed by wortmannin. The MEK inhibitor, U0126, exerted a minimal influence on stretch-induced hypertrophy. We found that cyclic stretching of myotubes activates the PI3K/Akt/TOR pathway, resulting in muscle hypertrophy. The MEK/ERK pathway may contribute negatively to spontaneous hypertrophy.

Repetitive stretch suppresses denervation-induced atrophy of soleus muscle in rats.

This study was conducted to examine whether stretch-related mechanical loading on skeletal muscle can suppress denervation-induced muscle atrophy, and if so, to depict the underlying molecular mechanism. Denervated rat soleus muscle was repetitively stretched (every 5 s for 15 min/day) for 2 weeks. Histochemical analysis showed that the cross-sectional area of denervated soleus muscle fibers with repetitive stretching was significantly larger than that of control denervated muscle (P<0.05). We then examined the involvement of the Akt/mammalian target of the rapamycin (mTOR) cascade in the suppressive effects of repetitive stretching on muscle atrophy. Repetitive stretching significantly increased the Akt, p70S6K, and 4E-BP1 phosphorylation in denervated soleus muscle compared to controls (P<0.05). Furthermore, repetitive stretching-induced suppression of muscle atrophy was fully inhibited by rapamycin, a potent inhibitor of mTOR. These results indicate that denervation-induced muscle atrophy is significantly suppressed by stretch-related mechanical loading of the muscle through upregulation of the Akt/mTOR signal pathway.

Uniaxial cyclic stretch-stimulated glucose transport is mediated by a ca-dependent mechanism in cultured skeletal muscle cells.

OBJECTIVE: Mechanical stimuli such as stretch increase glucose transport and glycogen metabolism in skeletal muscle. However, the molecular mechanisms involved in the mechanotransduction events are poorly understood. The present study was conducted in order to determine whether the signaling mechanism leading to mechanical stretch-stimulated glucose transport is similar to, or distinct from, the signaling mechanisms leading to insulin- and contraction-stimulated glucose transport in cultured muscle cells. METHODS: Cultured C2C12 myotubes were stretched, after which the 2-deoxy-D-glucose (2-DG) uptake was measured. RESULTS: Following cyclic stretch, C2C12 myotubes showed a significant increase in 2-DG uptake, and this effect was not prevented by inhibiting phosphatidylinositol 3-kinase or 5'-AMP-activated protein kinase and by extracellular Ca(2+) chelation. Conversely, the stretch-stimulated 2-DG uptake was completely prevented by dantrolene (an inhibitor of Ca(2+) release from sarcoplasmic reticulum). Furthermore, the stretch-stimulated 2-DG uptake was prevented by the Ca(2+)/calmodulin-dependent kinase inhibitor KN93 which did not prevent the insulin-stimulated 2-DG uptake. CONCLUSIONS: These results suggest that the effects of stretch-stimulated glucose transport are independent of the insulin-signaling pathway. By contrast, following mechanical stretch in skeletal muscle, the signal transduction pathway leading to glucose transport may require the participation of cytosolic Ca(2+) and Ca(2+)/calmodulin kinase, but not 5'-AMP-activated protein kinase.