mTORC1 Tunes Extracellular Vesicle Signaling to Balance Tissue Cooperation and Cellular Survival

Hypothesis: mTORC1 activity sets the composition and release rate of extracellular vesicles (EVs), thereby tuning the balance between tissue‑level cooperation and cell‑autonomous survival. When mTORC1 is active, EVs are enriched in growth‑promoting cargos such as miR‑21, cyclin D1 mRNA, and phosphorylated AKT, reinforcing paracrine signals that support proliferation, differentiation, and extracellular matrix production—functions we interpret as the 'civilizational' mode of multicellularity. When mTORC1 is inhibited, EV cargo shifts toward stress‑responsive molecules including miR‑125b, HSP70, and LC3‑II, reflecting autophagic components and danger‑associated signals that promote cell‑intrinsic maintenance but reduce cooperative signaling. This shift explains why chronic mTOR suppression extends lifespan yet impairs wound healing and vaccine response: the tissue loses its ability to broadcast growth cues while retaining internal repair mechanisms. Intermittent or low‑dose rapamycin, which preferentially attenuates mTORC1 while sparing mTORC2, would preserve a mixed EV phenotype—enough cooperative cargo to sustain regeneration, yet sufficient stress cargo to limit damage accumulation. The hypothesis is testable by comparing EV isolates from young, old, and rapamycin‑treated mice under continuous versus intermittent dosing. Predictions: (1) continuous rapamycin reduces EV‑associated miR‑21 and increases LC3‑II relative to controls; (2) intermittent rapamycin restores miR‑21 to near‑baseline levels while keeping LC3‑II elevated; (3) functional assays show that serum from intermittently treated animals stimulates fibroblast proliferation more than serum from continuously treated animals, yet both suppress ROS production in stressed cells. Falsification would occur if EV cargo composition does not track mTORC1 activity as described, or if intermittent dosing fails to improve regenerative readouts despite maintaining the predicted EV signature.

Mechanistically, mTORC1 phosphorylates the Rab GTPase‑activating protein TBC1D15, limiting Rab7‑active GTP and thereby biasing MVB formation toward exosomes rich in RNA‑binding proteins; inhibition leads to accumulation of active Rab7, promoting ILV loading of LC3‑II and HSP70. Additionally, mTORC1 suppresses neutral sphingomyelinase 2 (nSMase2) activity; its release upon inhibition raises ceramide, favoring ceramide‑rich microdomains that encapsulate stress‑laden cargos. These connections provide a direct biochemical route whereby the dial remodels EV content.