Hypothesis

mTOR activity does not only decide whether an arrested cell becomes senescent or quiescent; it also governs the rhythmic release of lysosomal exosomes that carry SASP factors, microRNAs, and metabolic enzymes. We propose that oscillatory mTORC1 signaling creates pulses of exosome secretion that synchronize neighboring cells’ stress responses, thereby establishing a tissue-level 'civilization' signal. When mTORC1 is chronically suppressed (e.g., by constant rapamycin), this oscillatory exosome flow dampens, decoupling cellular longevity from coordinated tissue function and leading to a loss of adaptive repair despite extended cell lifespan.

Mechanistic Reasoning

1. mTORC1-lysosome coupling – Active mTORC1 phosphorylates and activates the lysosomal V-ATPase-Ragulator complex, promoting lysosomal exocytosis (Persistent mTORC1 signaling in cell senescence). In senescent cells, constitutive mTORC1 drives a baseline exosome release rich in IL-6, IL-8, and miR-34a.
2. Nutrient-oscillation driven pulses – During refeeding after starvation, transient mTORC1 spikes trigger a burst of lysosomal exosome release that carries anti-SASP cargo (e.g., TFEB-dependent cathepsins and NAD+-boosting enzymes). This pulse can reset neighboring cells to a quiescent, repair-competent state.
3. Feedback via exosome-borne miRNAs – Exosomal miR-21 and miR-146a, whose loading is mTORC1-sensitive, modulate PTEN and IRAK1 in recipient cells, fine-tuning their own mTOR activity and creating a community-level feedback loop (p53-mTOR crosstalk determines cell fate).

Testable Predictions

• Prediction 1: In human fibroblasts induced to senescence by irradiation, pharmacological inhibition of nSMase2 (GW4869) will reduce exosome release without altering mTORC1 activity, and will diminish paracrine SASP spread in 3D organoids while leaving intracellular senescence markers (p16, SA-β-gal) unchanged.
• Prediction 2: Mice treated with intermittent rapamycin (2-day on/5-day off) will show higher amplitude lysosomal exosome oscillations in plasma (measured by exosomal TFEB and phospho-S6K) compared with continuous rapamycin dosing, correlating with improved treadmill endurance and lower tissue IL-6 despite similar leukocyte telomere length.
• Prediction 3: Artificial restoration of exosome oscillations in continuously rapamycin-treated mice via intravenous injection of TFEB-loaded exosomes will rescue tissue-level regeneration (e.g., muscle fiber cross-sectional area after injury) without affecting the extended lifespan of individual cells.

Falsification

If exosome secretion proves insensitive to mTORC1 oscillations (i.e., lysosomal exosome flux remains constant across fed/fasted states) or if blocking exosome release does not alter paracrine SASP or tissue-level function despite changes in cellular senescence, the hypothesis would be refuted.

By framing mTOR as a dial that tunes not only cell-intrinsic survival versus growth but also the timing of intercellular exosome signaling, we bridge the molecular senescence-quiescence dichotomy with the emergent property of tissue civilization versus mere survival.