mTORC1 activity is supported by spatial association with focal adhesions

Abstract

The mammalian target of rapamycin complex 1 (mTORC1) integrates mitogenic and stress signals to control growth and metabolism. Activation of mTORC1 by amino acids and growth factors involves recruitment of the complex to the lysosomal membrane and is further supported by lysosome distribution to the cell periphery. Here, we show that translocation of lysosomes toward the cell periphery brings mTORC1 into proximity with focal adhesions (FAs). We demonstrate that FAs constitute discrete plasma membrane hubs mediating growth factor signaling and amino acid input into the cell. FAs, as well as the translocation of lysosome-bound mTORC1 to their vicinity, contribute to both peripheral and intracellular mTORC1 activity. Conversely, lysosomal distribution to the cell periphery is dispensable for the activation of mTORC1 constitutively targeted to FAs. This study advances our understanding of spatial mTORC1 regulation by demonstrating that the localization of mTORC1 to FAs is both necessary and sufficient for its activation by growth-promoting stimuli.

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

“…Importantly, the synergistic effect of EXFED was apparent for RPS6 Ser240/244 peripheral-central ratio as this measure was greater in EXFED compared to FED at both 120 and 300 min timepoints. Although unexpected, an increase in central mTORC1 activity, in response to nutrients, has been observed in vitro (22). Intramyofibrillar ribosomes have also been identified in rat skeletal muscle, albeit at approximately 75-90% lower expression to subsarcolemmal, peripheral ribosomes (41) and, assuming similar ribosome localization in human muscle, it is therefore possible that mTORC1 activity also occurred in these regions following EXFED in order to stimulate myofibrillar protein turnover.…”

“…For example, after exposure to these anabolic agents mTORC1 activity in Hela cells was seen to localize with paxillin (22), a marker of focal adhesion complexes (23,57). Moreover, disruption of focal adhesion complexes by knockout of integral proteins or via pharmacological agents, mTORC1 could no longer be activated by growth factors or amino acids (22). Our results are consistent with an integral role for focal adhesion complexes in mTORC1 activity in human muscle as colocalization of RPS6 Ser240/244 and Paxillin and the intensity of RPS6 Ser240/244 staining within paxillin-positive regions were elevated at 120 min in both FED and EXFED before returning to baseline at 300 min.…”

“…These complexes are enriched with growth factor receptors, amino acid transporters and integrins (22) suggesting they could be a site where anabolic stimuli convene to regulate mTORC1 kinase activity. Indeed, the forced targeting of mTORC1 to focal adhesion complexes elevates mTORC1 activity irrespective of lysosomal positioning (22). In skeletal muscle paxillin, a commonly used marker of focal adhesion complexes, is observed to be colocalized with the microvasculature (23), a region which mTORC1 has been seen to translocate to in response to anabolic stimuli (17) and where amino acid transporters (e.g.…”

“…A recent in vitro investigation has also identified focal adhesion complexes as an important site in mTORC1 regulation. These complexes are enriched with growth factor receptors, amino acid transporters and integrins (22) suggesting they could be a site where anabolic stimuli convene to regulate mTORC1 kinase activity. Indeed, the forced targeting of mTORC1 to focal adhesion complexes elevates mTORC1 activity irrespective of lysosomal positioning (22).…”

Regulation of mTORC1 activity by the Golgi apparatus

“…On a theoretical level, a multi-hub model would provide a greater capacity to fine-tune the complex network of mTOR signalling pathways, as is the case for other signalling pathways such as Toll-like receptor 4 (TLR4) activation 11 , where the cell surface and endosomes mediate different downstream outputs from the same receptor. Indeed, recent reports have provided evidence for spatially and functionally distinct pools of TORC1 at the vacuoles and endosomes in yeast 12 and for activation of mTORC1 at the cell surface of mammalian cells independent of lysosomes 13 . Notably, there is also growing evidence for a role of the secretory pathway in the regulation of mTORC1 signalling, in particular the Golgi apparatus.…”