Abstract
Signaling by mTOR is a well-recognized component of the pathway through which mechanical signals regulate protein synthesis and muscle mass. However, the mechanisms involved in the mechanical regulation of mTOR signaling have not been defined. Nevertheless, recent studies suggest that a mechanically-induced increase in phosphatidic acid (PA) may be involved. There is also evidence which suggests that mechanical stimuli, and PA, utilize ERK to induce mTOR signaling. Hence, we reasoned that a mechanically-induced increase in PA might promote mTOR signaling via an ERK-dependent mechanism. To test this, we subjected mouse skeletal muscles to mechanical stimulation in the presence or absence of a MEK/ERK inhibitor, and then measured several commonly used markers of mTOR signaling. Transgenic mice expressing a rapamycin-resistant mutant of mTOR were also used to confirm the validity of these markers. The results demonstrated that mechanically-induced increases in p70s6k T389 and 4E-BP1 S64 phosphorylation, and unexpectedly, a loss in total 4E-BP1, were fully mTOR-dependent signaling events. Furthermore, we determined that mechanical stimulation induced these mTOR-dependent events, and protein synthesis, through an ERK-independent mechanism. Similar to mechanical stimulation, exogenous PA also induced mTOR-dependent signaling via an ERK-independent mechanism. Moreover, PA was able to directly activate mTOR signaling in vitro. Combined, these results demonstrate that mechanical stimulation induces mTOR signaling, and protein synthesis, via an ERK-independent mechanism that potentially involves a direct interaction of PA with mTOR. Furthermore, it appears that a decrease in total 4E-BP1 may be part of the mTOR-dependent mechanism through which mechanical stimuli activate protein synthesis.
Highlights
It is well recognized that mechanical signals play a critical role in the regulation of skeletal muscle mass and the maintenance of muscle mass contributes significantly to health and issues associated with the quality of life [1,2,3]
These results demonstrated that our ex-vivo system would enable us to define the role of extracellular regulated kinase (ERK) in the mechanical activation of mammalian target of rapamycin (mTOR) signaling
In this study we employed an ex-vivo passive stretch model of mechanical stimulation to determine if signaling through ERK is necessary for the mechanical activation of mTOR signaling and protein synthesis
Summary
It is well recognized that mechanical signals play a critical role in the regulation of skeletal muscle mass and the maintenance of muscle mass contributes significantly to health and issues associated with the quality of life [1,2,3]. Advances in our knowledge are being made and it is becoming increasingly evident that mechanically-induced changes in muscle mass are largely driven by changes in the rate of protein synthesis [4,5,6]. Identifying the molecular mechanisms that control mechanically-induced changes in protein synthesis should provide fundamental insight into how mechanical stimuli regulate muscle mass. One mechanism that has been widely implicated in the regulation of protein synthesis involves signaling through the mammalian target of rapamycin (mTOR) [7]. MTOR can phosphorylate, and activate, the p70 ribosomal protein S6 kinase (p70s6k). Active p70s6k can, in-turn, promote an increase in the helicase activity of eIF4A, and provide an additional stimulus for the initiation of protein synthesis [10]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
