Mechanistic target of rapamycin complex 1 (mTORC1) activity occurs predominantly in the periphery of human skeletal muscle fibers, in close proximity to focal adhesion complexes, following anabolic stimuli

Abstract

Following anabolic stimuli (e.g. mechanical loading and/or amino acid provision) the mechanistic target of rapamycin complex 1 (mTORC1), a master regulator of protein synthesis, translocates toward the cell periphery. However, it is unknown if mTORC1 activity occurs prior to or following this translocation. We therefore aimed to determine the cellular location of mTORC1 activity in human skeletal muscle following anabolic stimuli. Fourteen young, healthy males either ingested a protein-carbohydrate beverage (0.25g/kg protein, 0.75g/kg carbohydrate) alone (n=7, 23{+/-}5yrs, 76.8{+/-}3.6kg, 13.6{+/-}3.8%BF, FED) or following a whole-body resistance exercise bout (n=7, 22{+/-}2yrs, 78.1{+/-}3.6kg, 12.2{+/-}4.9%BF, EXFED). Vastus lateralis muscle biopsies were obtained at rest (PRE) and 120 and 300min following anabolic stimuli. The spatial regulation of mTORC1 activity was assessed through immunofluorescent staining of p-RPS6Ser240/244, an mTORC1-specific phosphorylation event. p-RPS6Ser240/244 measured by immunofluorescent staining or immunoblot was positively correlated (r=0.76, p<0.001). Peripheral staining intensity of p-RPS6Ser240/244 increased above PRE in both FED and EXFED at 120min (~54% and ~138% respectively, p<0.05) but was greater in EXFED at both post-stimuli time points (p<0.05). The peripheral-central ratio of p-RPS6240/244 staining was displayed a similar pattern, suggesting mTORC1 activity occurs predominantly in the periphery of fibers. Moreover, p-RPS6Ser240/244 intensity within paxillin-positive regions, a marker of focal adhesion complexes, was elevated at 120min irrespective of stimulus (p=0.006) before returning to PRE at 300min. These data confirm that mTORC1 activity occurs in the region of human muscle fibers to which mTORC1 translocates following anabolic stimuli and identifies focal adhesion complexes as a potential site of mTORC1 activation in vivo.