The electromyographic activity of the soleus muscle is a reliable indicator of its functional status. Support unloading causes an immediate cessation of electrical activity of the soleus muscle, which resumes upon restoration of the support load. Prolonged support unloading, however, results in the emergence of spontaneous electrical activity of the soleus muscle. Previous research has established a correlation between this activity and the presence of the potassium-chloride cotransporter (KCC2) on the membranes of spinal cord motor neurons. It has also been demonstrated that the administration of the KCC2 activator prochlorperazine can eliminate spontaneous soleus muscle activity. Here, we aimed to investigate the effect of CLP290, an alternative KCC2 activator, on the spontaneous tonic activity of the rat soleus muscle. The results indicated that daily administration of CLP290 to rats during a 14-day period of hindlimb suspension prevented the reduction in KCC2 levels in lumbar spinal cord motor neurons and the increase in soleus muscle spontaneous tonic activity. Notably, there were no significant differences in the cross-sectional area of slow-type fibers between the antiorthostatic suspension groups with and without CLP290 administration.

The electromyographic activity of the soleus muscle is a reliable indicator of its functional status. Unloading of support causes an immediate cessation of electrical activity in the soleus muscle, which resumes upon restoration of the support load. Prolonged support unloading, however, results in the emergence of spontaneous electrical activity in the soleus muscle. Previous research has established a correlation between this activity and the presence of the potassium-chloride cotransporter (KCC2) on the membranes of spinal cord motor neurons. Additionally, it has been demonstrated that the introduction of the KCC2 activator prochlorperazine can eliminate spontaneous muscle activity. This study aimed to investigate the impact of CLP290, an alternative KCC2 activator, on the spontaneous tonic activity of the rat soleus muscle. The results indicated that daily administration of CLP290 to rats during a 14-day period of hindlimb suspension prevented the reduction in KCC2 levels in the motor neurons of the lumbar spinal cord and the increase in spontaneous tonic activity in the soleus muscle. Notably, there were no significant differences in the cross-sectional area of slow-type fibers between the antiorthostatic suspension groups with and without CLP290 administration.

Critical illness myopathy (CIM) is a primary myopathy that develops in critically ill patients. Histological features of CIM include a general reduction in muscle fiber cross-sectional area and a predominant loss of the motor protein myosin in the absence of inflammatory infiltrates but with detectable cytokine activation. This study was aimed to examine the state of the soleus muscle extracellular matrix in patients with CIM caused by chronic disorders of consciousness. Incisional needle biopsies of the soleus muscle were taken from 6 patients with a chronic (≥ 2 months) disorder of consciousness, undergoing treatment at the Polenov Neurosurgical Institute (Almazov National Medical Research Center, St. Petersburg), and healthy men (control). Histological staining of soleus muscle sections in patients with CIM revealed a significantly increased collagen area that exceeded control values by 82 along with type I and III collagen content, were increased. No changes were found in fibronectin and extracellular tissue growth factor mRNA levels. However, integrin α7 mRNA levels were elevated. Our results indicate significant skeletal muscle fibrosis in CIM, requiring further studies on the signaling pathways that regulate this process.

The soleus is one of the key muscles for stability of the majority mammals in Earth's gravity. It is well known that as soon as a laboratory animal (rat) is put in a real or modeled weightlessness (loss of the hindlimb contact with substrate due to tail-suspension) the electrical activity in m. soleus decreases sharply. However, starting on day 3 of the functional unloading this activity renews and grows to the level characteristic of control animals (approximately by day 14 of suspension). The phenomenon was termed "the spontaneous activity of unloaded postural muscle". The review discusses spinal mechanisms of the spontaneous postural muscle activity, the input of ion co-transporters in marrow motoneurons specifically, and effect of this activity on intra-cell signaling in fibers of unloaded m. soleus.

Functional unloading of skeletal muscles enhances proteasome degradation in which the key role is played by Е3-ligases atrogin-1/MAFbx and MuRF-1. Their expression starts growing on the first day of unloading. There is actually little literature on expression of ubiquitin ligases in dry immersion or bedrest. Our purpose was to investigate MuRF-1 and MAFbx expression and control in postural m. soleus and locomotor m. vastus lateralis during a 21-day bedrest study. The observed reduction of cross-section areas in slow fibers of m. soleus and m. vastus lateralis was accompanied by activation of MuRF-1 and MAFbx in m. soleus and MAFbx in m. vastus lateralis, respectively. The investigation of E3 ligases signal pathways showed an increased expression of myogenin and IL-6 receptors in both muscles, whereas transcriptional activation of FoxO3 was seen in m. soleus only. These results suggest elevated expression of Е3-ubiquitin ligases MuRF-1 and MAFbx in m. soleus and MAFbx in m. vastus lateralis, as well as their key signal pathways in bedrested human subjects. Also, our findings argue against the previously stated notion that degradation of protein has no influence on skeletal muscles atrophy due to hypokinesia.

Apoptosis and its control during differentiation of primary myoblasts isolated from rat soleus muscle after 7-d tail suspension were investigated. Primary myoblasts were first subjected to myogenic differentiation. TUNEL labeling of double-strand DNA breaks was applied to detect apoptotic cells during myoblast differentiation. Western blot was used to determine the apoptosis markers and a number of signaling molecules, i.e. protein BAX, activated caspase-3, phospho-AMPK (Thr172), phospho-AKT (Ser 437), phospho-p27Kip1 (Thr198), phospho-p27Kip1 (Thr157). PCR analysis was used to investigate the expression of pro-apoptotic markers in myoblasts. The investigation showed enhancement of apoptosis as well as АМРК and p27Kip1 dephosphorylation in the process of differentiation of myoblasts taken from m. soleus of tail-suspended rats.

It is well known that the inactivity of mammalian skeletal muscles leads to the cessation of their electrical activity and is accompanied by atrophic changes in muscle fibers. However, it has been repeatedly noted that starting from the 3rd day of functional unloading, spontaneous rhythmic neuromuscular activity appears, which is the result of a decrease in the expression of the potassium chloride co-transporter KCC-2 in neurons of the lumbar spinal cord. A decrease in the expression of KCC-2 and the onset of autonomous electrical activity of the unloaded muscle can be prevented by the administration of the neuroleptic prochlorperazine. Thus, the aim of this study was to evaluate the structural and signaling effects of the reduced spontaneous activity of the unloaded m.soleus. It was found that daily administration of prochlorperazine to rats under conditions of 7-day simulated gravitational unloading prevented a decrease in the content of the main markers of ribosome biogenesis (c-Myc, 18S rRNA and 28S rRNA), and also partially prevented a decrease in the cross-sectional area of fast and slow muscle fibers in the m.soleus. Morphofunctional changes caused by a decrease of spontaneous activity of the unloaded muscle were accompanied by complete or partial prevention of activation of key proteolytic markers expression (MuRF-1, MAFbx/atrogin-1, ubiquitin). Thus, we assume that spontaneous neuromuscular activity may be a factor that augments muscle atrophy during the first week of functional unloading.

The main effect of chronic alcoholic myopathy is atrophy of fast muscle fibers (18 years of alcohol abuse), followed by slow fibers (31 years of alcohol abuse); plasma IGF-1 levels decrease and muscle IRS-1 and p-p70S6k decrease, which is evidence for suppression of the functioning of the mTORC1 signal pathway and reductions in protein synthesis. Patients show increases in the expression of mRNA encoding HSP90/70, which can lead to increases in the protection of proteins from degradation. All patients showed increased expression of E3 ligase, which is evidence for an increase in the operation of the ubiquitin-proteasomal signal pathway for protein degradation.

In the 1930—1940s R. G. Leibson and A. G. Ginеtsinsky with colleagues carried out the experiments were it was shown that the changes in tenotomized rabbit soleus muscle are completely eliminated if the nerve is cut at the same time. The authors concluded that atrophy from inactivity is not a passive process, but it is an active result of neural influences which are carried out by the ventral roots of sciatic nerve. The nature of these influences has not been studied and nervous control is not clear. The present study was carried out on 2-month old Californian rabbits to confirm the described phenomenon and to investigate the assumed neurotrophic mechanisms. The morphological signs of atrophy and degeneration of tenotomized rabbit muscles, muscle fiber death of and growth of connective tissue were eliminated on day 11 of exposure by simultaneous Achilles tenotomy. The effect of colchicine application on the tenotomized muscle was almost the same as denervation (10 mM solution was applicated at the sciatic nerve for 10 min), but in the case of colchicine the degenerative changes were more pronounced. Denervation of the tenotomized muscle or colchicine application maintained the fast fiber cross-sectional area at the level of intact control. Sciatic nerve denervation or colchicine application prevented the rise in soleus muscle TrkB and TrkС expression and altered some neurotrophin expression. Denervation with tenotomy decreased TrkB mRNA expression by 3.4 times (p < 0.05), TrkC — by 3.2 times (p < 0.05), NT3-by 3.4 times (p < 0.05) compared with the tentotomized contralateral limb, colchicine with tenotomy — by 3.6 and 2.9 times respectively (p < 0.05).

The negative regulation of expression of genes involved in various metabolic pathways in a skeletal muscle is the main function of histone deacetylases 4 and 5 (HDAC4/HDAC5). HDAC4 and HDAC5 seem to be the targets of the AMP-activated protein kinase (AMPK). Earlier, an essential decrease in the level of Thr172-phosphorylated-AMPK in a rat soleus muscle at the first day of gravitational unloading was shown. Possibility of a protein kinase D (PKD) to phosphorylate histone deacetylases 4/5 has been shown, too. We supposed that under the conditions of gravitational unloading, alterations in AMPK phosphorylation level can affect regulation of nuclear-cytoplasmic traffic of class II histone deacetylases and of various skeletal muscle genes expression. To verify the hypothesis, we used administration of an AMPK activator, AICAR, before and during a day-long hindlimb suspension. It was shown that at an early stage of gravitational unloading, HDAC4 is not a PKD target, and its nuclear import is realized due to decrease in AMPK activity. We were the first to show reciprocal relations between AMPK and PKD in a skeletal muscle at early gravitational unloading.

This is the first study to observe a 25% atrophy of m. vastus lateralis and a decrease in the size of type I and II muscle fibers (by 35 and 44%, respectively) using magnetic resonance imaging (MRI) in women after five to seven years of alcohol intoxication. The decrease in muscle volume is due to the predominant destruction of contractile apparatus as compared with other components.

Skeletal muscle myosin phenotype (i.e., the predominance in the muscle of a particular isoform or isoforms of myosin heavy chains (MyHC)) determines the properties of muscle, such as contraction speed and fatigue. The aim of this study was to identify the functional relationship between the decrease of the nitric oxide (NO) content, the GSK-3β phosphorylation (leading to the GSK-3β activation), the NFATc1 amount in the muscle nuclei, and the MyHC I(β) isoform expression in the rat soleus muscle under gravitational unloading. Male Wistar rats were divided into five groups: the vivarium control group; the group of animals with a 7-day hind limb suspension receiving placebo; the group of animals with a hind limb suspension receiving a NO donor (L-arginine); the group of animals with a hind limb suspension receiving a NO donor and a NO-synthase inhibitor (L-NAME); and the group of animals with a hind limb suspension receiving a GSK-3β inhibitor. We have shown that a 7-day unloading leads to a NO content decrease in the soleus muscle, and this effect is prevented by L-arginine administration. In addition, administration of L-arginine blocks the GSK-3β phosphorylation decrease, NFATc1 export from the muscle nuclei, and MyHC I(β) expression decrease caused by unloading. The L-arginine effect in each case can be blocked by the NO-synthase inhibitor. Administration of the GSK-3β inhibitor prevents the unloading-induced NFATc1 export from the muscle nuclei and a decrease of the MyHC I(β) expression. The prevention of the MyHC I(β) expression decrease and the NFATc1 export from the nucleus by the selective GSK-3β inhibition confirms the hypothesis on the NO influence on the MyHC I(β) expression and the NFATc1 export from the nucleus via the GSK-3β phosphorylation decrease. Thus, the NO level decrease in the rat soleus muscle in unloading leads to the GSK-3β activation, which in turn, promotes the NFATc1 export from the nucleus and stabilization of the fast myosin phenotype.

A gender analysis has been carried out to analyze changes in intracellular signaling pathways that lead to the development of chronic alcoholic myopathy. It is known that acute or chronic alcohol intoxication can result in alcohol-induced lesions in skeletal muscles. Chronic alcoholic myopathy occurs much more frequently and can develop either independently or in combination with other forms of alcoholic disease (liver and heart lesions, malabsorption syndrome, or alcohol polyneuropathy). This disease is manifested by atrophy of skeletal muscles and a performance decrement. Most of the studies on the pathogenesis of chronic alcoholic myopathy have been carried out on male patients. Studies on alcoholic myopathy-induced muscle damage in females have not been previously reported.

Ubiquitin-proteasomal proteolytic pathway is one of the key signaling pathways determining protein degradation in muscle fibers. Among the E3 ubiquitin ligases, rate limiting enzymes of the ubiquitin-proteasomal pathway, the most interesting ones are the MuRF isoforms: MuRF-1 and MuRF-2. There are some pieces of evidence that these enzymes are also involved in the regulation of gene expression in skeletal muscle under some specific conditions (i. e. muscle disuse). We supposed that it was disuse that brought about to altered localization of MuRFs in postural muscle fibers and their translocation to nuclei. In the study using the conventional simulation model of the gravitational unloading (rat hindlimb suspension according to Ilyin and Novikov modified by Morey-Holton) we found that from the 3rd day till 14th day of unloading the content of MuRF-1 and MuRF-2 in the nuclear fraction 4-5 fold increased in unloaded soleus as compared to the control values. These data obtained by means of electrophoresis and western blot of the nuclear fraction of rat soleus were confirmed in the immunohistochemical study of co-localization of MuRF-1 and MuRF-2 antibodies and DAPI nuclear stain on transverse frozen sections of soleus muscle. Thus in the present study we observed the phenomenon of MuRF isoforms accumulation in nuclei of soleus muscle fibers during simulated gravitational unloading.