Blood flow restriction (BFR) training involves the use of cuffs or bands to partially restrict blood flow to the working muscles during low-load resistance exercise. Mechanical tension is known to activate pathways such as mTOR, which is responsible for protein synthesis, a mechanism that directly contributes to muscle hypertrophy. Recently, a novel polypeptide encoded by the long non-coding RNA (lncRNA) LINC00961 was shown to regulate mTOR activation and muscle regeneration (Matsumoto et al., 2017), implying crosstalk between mTOR and non-coding RNAs in skeletal muscle.
Understanding of the role of mTOR in muscle growth and hypertrophy has progressed recently from the evidence of several loss-of-function animal models, although it is relatively well-understood as being similar to the mechanism of the function of mTOR in cell growth regulation. One of the most widely recognized major players in controlling muscle mass is mammalian target of rapamycin (mTOR). Hence, the maintenance of muscle mass has been recognized as a determinant which directly influences quality of life. This review will highlight the fundamental role of mTOR in skeletal muscle growth by summarizing the phenotype of skeletal-specific mTOR deficiency. They primarily affect skeletal muscles; the limb muscles close to the center of the body (proximal) as well as the muscle farther from the center of the body (distal). Hence, estradiol does not seem to be important for acutely regulating muscle mass in eumenorrheic women.
Therefore, we suggest this tool as a sensitive and specific diagnostic biomarker in patients with MFM and FLNC variants of unknown significance. In addition, we demonstrate a highly similar composition of protein aggregates in all MFM-filaminopathy subtypes, independent of the individual FLNC mutation causative for this disease. Because the introduced stop codon occurs in the last exon, NMD does not occur, as confirmed by our RT-PCR experiments, indicating that a misfolded and unstable FLNc variant is indeed expressed, which not only leads to protein aggregation but also results in the formation of numerous myofibrillar lesions. Both mutations result in the same mutation at the protein level (p.W2710X), indicating that this specific stretch of guanines indeed is a mutation hotspot. All patients with MFM-filaminopathy but only 1 of 96 patients with different MFM subtypes had an FLNc ratio above 5. Our analyses revealed that proteomic aggregate profiles in 7 patients with MFM-filaminopathy with 3 distinct FLNC mutations were highly similar. Another clinically relevant point is that proteomic analysis can be helpful in differential diagnostic workup of patients with MFM.
It is also noteworthy that similar strategies to identify disease genes by combination of laser microdissection and proteomic analysis were already successful in other protein aggregate myopathies (Refs. 34 and 35 and our unpublished data). Therefore, we recommend testing genetically unresolved patients with MFM for mutations in both genes. Our previous proteomic studies only identified 28 of these proteins.27 Several of these newly identified, less abundantly overrepresented proteins are involved in protein degradation and protein quality control, e.g., SQSTM1 (p62), HSPA8 and BAG3. To analyze effects of expression of mutant FLNc in C2C12 myoblasts, we transiently transfected these cells with constructs encoding full-length wild-type or mutant FLNc. Whereas analysis of the wild-type domains resulted in the typical ß-strand secondary structure previously reported for these domains,16,17 the truncated mutant construct resulted in lower signals (figure 6B) for ß-strand structure with a concomitant increase of signal for random coil, suggesting improper folding. Proportions of αB crystallin, Xin actin-binding repeat-containing protein 1 (Xin), obscurin, and nestin were also similar. FLNc was always the most abundant protein, followed by desmin and the FLNc binding partners Xin actin-binding repeat-containing protein 2 (XIRP2) and nebulin-related-anchoring protein (N-RAP).
Hence, glucocorticoids may regulate mTOR by modulating the level of both BCAT2 and myostatin to regulate catabolism in skeletal muscle. Glucocorticoids also elicit muscle atrophy via controlling transcription of myostatin, an inhibitory regulator of muscle growth, which we discussed in the previous section. Exogenous administration of glucocorticoids induces muscle atrophy and the blockage of GR; adrenalectomy or treatment with the GR antagonist RU486 diminishes muscle atrophy in sepsis, cachexia, starvation, and severe insulinopenia (Menconi et al., 2007; Schakman et al., 2008). In addition, follistatin, an inhibitor of myostatin, activates Akt/mTOR/p70S6K1/S6 signaling in muscle growth, which exists independently of myostatin-driven mechanisms (Winbanks et al., 2012), supporting the disconnection between myostatin and mTOR signaling. The injection of a myostatin antibody enhances phosphorylation of p70S6K1 and S6 in muscle, but does not change phosphorylation of Akt and 4EBP1 in the concomitant increase of myofibrillar synthesis (Welle et al., 2009). However, on the other hand, several studies suggested that mTOR signaling and myostatin signaling could separately regulate muscle growth. Hence, the studies suggested that myostatin attenuates protein synthesis in muscle by coordinating the crosstalk between myostatin-mediated and mTOR signaling.
This demonstrates the significant and critical role that testosterone has in early postnatal growth and development of skeletal muscle. Cells used in in-vitro are in a developmental and high growth state, and therefore are more similar to skeletal muscle fibers in early postnatal animals 28,29. This anabolic role of testosterone throughout postnatal growth may also be reflected in experiments conducted in-vitro, which routinely show positive effects of testosterone supplementation on myoblasts and myotubes . On the other hand, testosterone depletion halted natural growth in young juvenile male mice.