Metformin attenuates an increase of calcium-dependent and ubiquitin-proteasome markers in unloaded muscle

Current study tested a hypothesis that during skeletal muscle unloading, calcium-dependent signaling pathways, markers of protein synthesis and expression of E3 ubiquitin ligases can be regulated by metformin. Thirty-two male Wistar rats were randomly assigned into one of four groups: non-treated control (3C), control rats treated with metformin (3CM), 3 days of unloading/hindlimb suspension with placebo (3HS), and 3 days of unloading treated with metformin (3HSM). In soleus muscle of HS group level of phospho-AMPK (p-AMPK) was decreased by 46% while ATP content was increased by 49% when compared with the control group. There was an increase of the level of phospho-CaMK II (483%) and an up-regulation of CaN, SERCA2a, and Calpain-1 mRNA expression (87%, 41% and 62%, respectively, p<0.05) in the HS group relative to the control. HS group also had increased mRNA expression of MuRF1, MAFbx, and ubiquitin (167%, 146% and 191%, respectively, p<0.05) when compared to the control soleus muscle. Metformin treatment impeded unloading-induced changes in soleus muscle. Conclusions: metformin treatment during three days of soleus muscle unloading: 1) prevented the decrease of p-AMPK and increase of ATP content; 2) affected regulation of calcium-dependent signaling pathways via level of CaMK II phosphorylation or CaMK II, CaN, SERCA2a, and Calpain-1 mRNA expression; 3) attenuated an increase in the expression of critical markers of ubiquitin-proteasome pathways MuRF1, MAFbx, and ubiquitin while not affecting the unloading-induced increase of ULK-1 marker of autophagic/lysosomal pathway.

Effects of Various Muscle Disuse States and Countermeasures on Muscle Molecular Signaling

Skeletal muscle is capable of changing its structural parameters, metabolic rate and functional characteristics within a wide range when adapting to various loading regimens and states of the organism. Prolonged muscle inactivation leads to serious negative consequences that affect the quality of life and work capacity of people. This review examines various conditions that lead to decreased levels of muscle loading and activity and describes the key molecular mechanisms of muscle responses to these conditions. It also details the theoretical foundations of various methods preventing adverse muscle changes caused by decreased motor activity and describes these methods. A number of recent studies presented in this review make it possible to determine the molecular basis of the countermeasure methods used in rehabilitation and space medicine for many years, as well as to identify promising new approaches to rehabilitation and to form a holistic understanding of the mechanisms of gravity force control over the muscular system.

Plantar mechanical stimulation attenuates protein synthesis decline in disused skeletal muscle via modulation of nitric oxide level

Both research conducted under microgravity conditions and ground-based space analog studies have shown that air pump-based plantar mechanical stimulation (PMS) of cutaneous mechanoreceptors of the sole of the foot is able to increase neuromuscular activity in the musculature of the lower limbs. This type of stimulation is able to attenuate unloading-induced skeletal muscle atrophy and impaired muscle function. The aim of the present study was to evaluate the effects of PMS on anabolic signaling pathways in rat soleus muscle following 7-day hindlimb suspension (HS) and to elucidate if the effects of PMS on anabolic processes would be NO-dependent. The soles of the feet were stimulated with a frequency of 1-s inflation/1-s deflation with a total of 20 min followed by 10 min rest. This cycle was repeated for 4 h each day. We observed a decrease in the soleus muscle mass after 7-day HS, which was not prevented by PMS. We also observed a decrease in slow-type fiber cross-sectional area (CSA) by 56%, which significantly exceeded a decrease (-22%) in fast-type fiber CSA. PMS prevented a reduction in slow-twitch fiber CSA, but had no effect on fast-twitch fiber CSA. PMS prevented a 63% decrease in protein synthesis after 7-day HS as well as changes in several key anabolic signaling regulators, such as p70S6k, 4E-BP1, GSK3β, eEF-2, p90RSK. PMS also prevented a decrease in the markers of translational capacity (18S and 28S rRNA, c-myc, 45S pre-rRNA). Some effects of PMS on anabolic signaling were altered due to NO-synthase inhibitor (L-NAME) administration. Thus, PMS is able to partially prevent atrophic processes in rat soleus muscle during 7-day HS, affecting slow-type muscle fibers. This effect is mediated by alterations in anabolic signaling pathways and may depend on NO-synthase activity.

Plantar mechanical stimulation prevents calcineurin-NFATc1 inactivation and slow-to-fast fiber type shift in rat soleus muscle under hindlimb unloading

The prevailing myosin isoform [myosin heavy chain (MyHC)] in a skeletal muscle determines contractile properties of the muscle. Under actual or simulated microgravity conditions such as human bed rest or rat hindlimb unloading, decrease in expression of MyHC of the slow type [MyHC I(β)] has been observed. It was demonstrated that increasing sensory input by performing plantar mechanical stimulation (PMS) on the soles of the feet results in an increase in neuromuscular activation of the lower limb muscles and may prevent slow-to-fast fiber type shift. The calcineurin-nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) signaling pathway is the main cascade regulating MyHC I(β) expression. The present study was aimed to analyze the states of the calcineurin-NFATc1 signaling cascade under conditions of PMS during rat hindlimb unloading. Male Wistar rats were randomly assigned to vivarium control groups and 1-day unloading (1HS), 3-day unloading (3HS), 1HS+PMS, and 3HS+PMS groups. We found that both 1 day and 3 days of unloading caused decrease in MyHC I(β) mRNA expression and decrease in glycogen synthase kinase-3β phosphorylation (Ser 9) that brought about the kinase activation, and these effects of unloading were prevented by PMS. Three days of unloading also caused increase in expression of calsarcin-2 (myozenin-I), which was found to be the endogenous calcineurin inhibitor. The level of calsarcin-2 expression in the 3HS+PMS group did not differ from the control group. Therefore, we conclude that PMS upregulates the calcineurin-NFATc1 signaling pathway and prevents unloading-induced MyHC I(β) decrease. NEW & NOTEWORTHY It is widely accepted that changes in the myosin phenotype during functional unloading (disuse) are determined by a decreased expression of the myosin heavy chain (MyHC) I(β) gene, and this decrease leads to changes of contractile and fatigue characteristics of soleus muscle. The calcineurin-nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) pathway is one of the most important signaling cascades regulating slow MyHC isoform expression. The present study is the first to show that plantar mechanical stimulation upregulates calcineurin-NFATc1 signaling in soleus muscles of hindlimb-unloaded rats.