The blockade of dihydropyridine channels prevents an increase in μ-calpain level under m. soleus unloading

Molecular Signaling Effects behind the Spontaneous Soleus Muscle Activity Induced by 7-Day Rat Hindlimb Suspension

The elimination of ground reaction force (support withdrawal) vastly affects slow postural muscles in terms of their regulation and structure. One of the effects of support withdrawal in this study was an immediate postural muscle inactivation, followed by the daily gradual development of spontaneous activity of the slow postural soleus muscle in response to rat hindlimb suspension to mimic space flight. The origin of this activity is somewhat akin to muscle spasticity after spinal cord injuries and is the result of KCC2 content decline in the spinal cord's motor neurons. However, the physiological consequences of unloading-induced spontaneous activity remain unexplored. We have conducted an experiment with the administration of a highly specific KCC2 activator during 7-day unloading. For this experiment, 32 male Wistar rats were divided into 4 groups: C+placebo, C+CLP-290 (100 mg/kg b w), 7HS+placebo, and 7HS+CLP-hindlimb-suspended group with CLP-290 administration (100 mg/kg b w). The soleus muscles of the animals were dissected and analyzed for several proteostasis- and metabolism-related parameters. CLP-290 administration to the unloaded animals led to the upregulation of AMPK downstream (p-ACC) and mTOR targets (p-p70S6k and p-4E-BP) and an enhanced PGC1alpha decrease vs. the 7HS group, but neither prevented nor enhanced atrophy of the soleus muscle or myofiber CSA.

Metformin Attenuates Slow-to-Fast Fiber Shift and Proteolysis Markers Increase in Rat Soleus after 7 Days of Rat Hindlimb Unloading

Muscle unloading leads to signaling alterations that cause muscle atrophy and weakness. The cellular energy sensor AMPK can regulate myofiber-type shift, calcium-dependent signaling and ubiquitin-proteasome system markers. We hypothesized that the prevention of p-AMPK downregulation during the first week of muscle unloading would impede atrophy development and the slow-to-fast shift of soleus muscle fibers, and the aim of the study was to test this hypothesis. Thirty-two male Wistar rats were randomly assigned to four groups: placebo control (C), control rats treated with metformin (C + M), 7 days of hindlimb suspension (HS) + placebo (7HS), and 7 days of HS + metformin administration (7HS + M). In the soleus of the 7HS rats, we detected a slow-to-fast fiber-type shift as well as a significant downregulation of MEF-2D and p300 in the nuclei. In the 7HS group, we also found decreases in p-ACC (AMPK target) protein level and in the expression of E3 ubiquitin ligases and p-CaMK II protein level vs. the C group. The 7-day metformin treatment for soleus muscle unloading (1) prevented slow-to-fast fiber-type shift; (2) counteracted changes in the p-ACC protein level; (3) hindered changes in the nuclear protein level of the slow myosin expression activators MEF-2D and p300, but did not affect NFATc1 signaling; and (4) attenuated the unloading-induced upregulation of MuRF-1, atrogin-1, ubiquitin and myostatin mRNA expression, but did not prevent soleus muscle atrophy. Thus, metformin treatment during muscle disuse could be useful to prevent the decrease in the percentage of slow-type fatigue-resistant muscle fibers.

Roles of ATP and SERCA in the Regulation of Calcium Turnover in Unloaded Skeletal Muscles: Current View and Future Directions

A decrease in skeletal muscle contractile activity or its complete cessation (muscle unloading or disuse) leads to muscle fibers’ atrophy and to alterations in muscle performance. These changes negatively affect the quality of life of people who, for one reason or another, are forced to face a limitation of physical activity. One of the key regulatory events leading to the muscle disuse-induced changes is an impairment of calcium homeostasis, which leads to the excessive accumulation of calcium ions in the sarcoplasm. This review aimed to analyze the triggering mechanisms of calcium homeostasis impairment (including those associated with the accumulation of high-energy phosphates) under various types of muscle unloading. Here we proposed a hypothesis about the regulatory mechanisms of SERCA and IP3 receptors activity during muscle unloading, and about the contribution of these mechanisms to the excessive calcium ion myoplasmic accumulation and gene transcription regulation via excitation–transcription coupling.

Molecular Mechanisms of Muscle Tone Impairment under Conditions of Real and Simulated Space Flight

Kozlovskaya et al. [1] and Grigoriev et al. [2] showed that enormous loss of muscle stiffness (atonia) develops in humans under true (space flight) and simulated microgravity conditions as early as after the first days of exposure. This phenomenon is attributed to the inactivation of slow motor units and called reflectory atonia. However, a lot of evidence indicating that even isolated muscle or a single fiber possesses substantial stiffness was published at the end of the 20th century. This intrinsic stiffness is determined by the active component, i.e. the ability to form actin-myosin cross-bridges during muscle stretch and contraction, as well as by cytoskeletal and extracellular matrix proteins, capable of resisting muscle stretch. The main facts on intrinsic muscle stiffness under conditions of gravitational unloading are considered in this review. The data obtained in studies of humans under dry immersion and rodent hindlimb suspension is analyzed. The results and hypotheses regarding reduced probability of cross-bridge formation in an atrophying muscle due to increased interfilament spacing are described. The evidence of cytoskeletal protein (titin, nebulin, etc.) degradation during gravitational unloading is also discussed. The possible mechanisms underlying structural changes in skeletal muscle collagen and its role in reducing intrinsic muscle stiffness are presented. The molecular mechanisms of changes in intrinsic stiffness during space flight and simulated microgravity are reviewed.

Muscle-specific activation of calpain system in hindlimb unloading rats and hibernating Daurian ground squirrels: a comparison between artificial and natural disuse

To determine whether the regulation of calpain system is involved in non-hibernators and hibernators in disused condition, the soleus (SOL) and extensor digitorum longus (EDL) muscles were used for investigating the muscle mass, the ratio of muscle wet weight/body weight (MWW/BW), fiber-type distribution, fiber cross-sectional area (CSA), and the protein expression of MuRF1, calpain-1, calpain-2, calpastatin, desmin, troponin T, and troponin C in hindlimb unloading rats and hibernating Daurian ground squirrels. The muscle mass, MWW/BW, and fiber CSA were found significantly decreased in SOL and EDL of hindlimb unloading rats, but unchanged in hibernating ground squirrels. The MuRF1 expression was increased in both SOL and EDL of unloading rats, while it was only increased in SOL, but maintained in EDL of hibernating ground squirrels. The expression levels of calpain-1 and calpain-2 were increased in different degrees in unloaded SOL and EDL in rats, while they were maintained in EDL and even reduced in SOL of hibernating ground squirrels. Besides, the expression of calpastatin was decreased in unloaded rats, but increased in hibernating ground squirrels. The desmin expression was decreased in unloaded rats, but maintained in hibernating squirrels. Interestingly, the levels of troponin T and troponin C were decreased in both SOL and EDL of unloaded rats, but increased in hibernating ground squirrels with muscle-type specificity. In conclusion, differential calpain activation and substrate-selective degradation in slow and fast muscles are involved in the mechanisms of muscle atrophy of unloaded rats and remarkable ability of muscle maintenance of hibernating ground squirrels.