T. M. Mirzoev

Key Markers of mTORC1-Dependent and mTORC1-Independent Signaling Pathways Regulating Protein Synthesis in Rat Soleus Muscle During Early Stages of Hindlimb Unloading.

Background/Aims: The purpose of the study was to assess the amount of rRNA and phosphorylation status of the key markers of mTORC1-dependent (70s6k, 4E-BP1) and mTORC1-independent (GSK-3β, AMPK) signaling pathways controlling protein synthesis in rat soleus during early stages of mechanical unloading (hindlimb suspension (HS) for 1-, 3- and 7 days). Methods: The content of the key signaling molecules of various anabolic signaling pathways was determined by Western-blotting. The amount of 28S rRNA was evaluated by RT-PCR. The rate of protein synthesis was assessed using in-vivo SUnSET technique. Results: HS for 3 and 7 days induced a significant (p<0.05) decrease in the rate of global protein synthesis in soleus muscle in comparison with control. HS within 24 hours resulted in a significant (p<0.05) decrease in p-4E-BP1 content, p-AMPK content and increase in p-p70s6k content in rat soleus muscle. Following three days of HS the content of p-AKT was decreased (p<0.05). After 7 days of HS the phosphorylation level of AKT and GSK-3beta was significantly reduced (p<0.05) compared to control. We also observed a significant decrease in the amount of 28S rRNA in rat soleus following 1, 3 and 7 days of HS. Conclusion: Taken together, the results of our study suggest that a decline in the global rate of protein synthesis in rat soleus during early stages of simulated microgravity is associated with impaired ribosome biogenesis as well as reduced activity of mTORC1-independent signaling pathways.

Rapid decline in MyHC I(β) mRNA expression in rat soleus during hindlimb unloading is associated with AMPK dephosphorylation

Inactivation of a skeletal muscle results in slow to fast myosin heavy chain (MyHC) shift. AMP‐activated protein kinase (AMPK) can be implicated in the regulation of genes encoding the slow MyHC isoform. Here we report that AMPK dephosphorylation after 24 h of mechanical unloading can contribute to histone deacetylase (HDAC) nuclear translocation; activation of AMPK prevents HDAC4 nuclear accumulation after 24 h of unloading and AMPK dephosphorylation inhibits slow MyHC expression following 24 h of unloading. Our data indicate that AMPK dephosphorylation during the first 24 h of mechanical unloading has a significant impact on the expression of MyHC isoforms in rat soleus causing a decrease in MyHC I(β) pre‐mRNA and mRNA expression as well as MyHC IIa mRNA expression.

Akt-dependent and Akt-independent pathways are involved in protein synthesis activation during reloading of disused soleus muscle.

Introduction: The purpose of our study was to assess the contribution of insulin growth factor‐1–dependent and phosphatidic acid‐dependent signaling pathways to activation of protein synthesis (PS) in rat soleus muscle during early recovery from unloading. Methods: Wistar rats were divided into: Control, 14HS [14‐day hindlimb suspension (HS)], 3R+placebo (3‐day reloading + saline administration), 3R+Wort (3‐day reloading + wortmannin administration), 3R+But (3‐day reloading + 1‐butanol administration). SUnSET and Western blot analyses were used in this study. Results: Wortmannin and 1‐butanol induced a decrease in protein kinase B (phospho‐Akt) and the rate of PS (P < 0.05) versus Control. In 3R+placebo and 3R+Wort, phosphorylation of glycogen synthase kinase 3 beta (phospho‐GSK‐3β) was increased versus Control (P < 0.05). Wortmannin administration during reloading did not alter phospho‐p70S6K (70 kDa ribosomal protein S6 kinase) versus 3R+placebo. In 3R+But, there was a decline in phospho‐GSK‐3β versus 3R+placebo and Control. In 3R+But, there was a decrease in phopho‐p70S6K (P < 0.05) versus 3R+placebo. Conclusions: These results suggest that PS activation during 3‐day reloading following 14HS involves both Akt‐dependent and Akt‐independent pathways. Muscle Nerve 55: 393–399, 2017

Eukaryotic elongation factor 2 kinase activation in M. soleus under 14-day hindlimb unloading of rats

Functional unloading of m. soleus of male Wistar rats was found to cause a reduction in protein synthesis. The level of phosphorylation of the translation elongation factor 2 (eEF2) and the eEF2 kinase (eEF2k) activity in m. soleus after 14 days of unloading were assessed. Rats were divided into the control group (C) and the group with hindlimb unloading for 14 days (HU14). The level of eEF2 phosphorylation in group HU14 was 80%, whereas in the control is was 40%. The indices of eEF2k expression and protein content in group HU14 increased compared to group C.

The response of skeletal muscle to alcohol abuse: Gender differences

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.

Divergent Anabolic Signalling responses of Murine Soleus and Tibialis Anterior Muscles to Chronic 2G Hypergravity

The purpose of the study was to assess the rate of protein synthesis (PS) and elucidate signalling pathways regulating PS in mouse soleus (Sol) and tibialis anterior (TA) muscles following chronic hypergravity (30-day centrifugation at 2G). The content of the key signalling proteins of the various anabolic signalling pathways was determined by Western-blotting. The rate of PS was assessed using in-vivo SUnSET technique. An exposure to 2G centrifugation did not induce any significant changes in the rate of PS as well as phosphorylation status of the key anabolic markers (AKT, p70s6k, 4E-BP1, GSK-3beta, eEF2) in Sol. On the contrary, a significant 55% increase in PS (p < 0.05) was found in TA. The cause of such a rise in PS could be associated with an increase in AKT (+72%, p < 0.05), GSK-3beta (+60%, p < 0.05) and p70s6k (+40%, p < 0.05) phosphorylation, as well as a decrease in eEF2 phosphorylation (−46%, p < 0.05) as compared to control values. Thus, the results of our study indicate that 30-day 2G centrifugation induces a distinct anabolic response in mouse Sol and TA muscles. The activation of the PS rate in TA could be linked to an up-regulation of both mTORC1-dependent and mTORC1-independent signalling pathways.

Signaling targets of alcoholic intoxication in human skeletal muscle

Intracellular signaling pathways were investigated in skeletal muscle cells at the early stages of alcohol addiction manifestations. No muscle fiber atrophy was observed in m. vastus lateralis of male patients. No significant changes in the signaling mechanisms that control protein degradation were detected as well. However, the concentration of the insulin-like growth factor (IGF-1) in blood plasma as well as the content of markers of intracellular signaling pathways regulating protein synthesis were significantly reduced compared to the control group.

The realization of a mechanical signal during gravitational unloading: The response of mTORC1 targets to eccentric contractions

The aim of this study was to assess the response of key mTORC1 substrates to a bout of contractile stimuli under different times of functional unloading. Functional unloading of hind-limb muscles was carried out by the method of antiorthostatic suspension. Twenty-eight Wistar rats were divided into four groups: control, and hindlimb suspension for 1, 3, and 7 days. After hindlimb suspension, isolated soleus muscles of rats were subjected to a bout of ex vivo eccentric contractions. The contents of phosphorylated forms of p70s6k and 4E-BP1 were then determined using western blotting. It was found that an eccentric load resulted in a significant increase in p70s6k phosphorylation and reduced 4E-BP1 phosphorylation both in control and suspended rats, but in the case of suspension the response was dramatically reduced. Thus, it can be concluded that a bout of eccentric contractions of isolated rat soleus muscle during functional unloading causes a weaker activation of the Akt-mTORC1-p70s6k signaling pathway compared with the control. This may indicate that it is important to maintain muscle tone for a more efficient muscle perception of an external mechanical signal and subsequent activation of anabolic signaling pathways.