Hypothesis

Lysosomal cholesterol accumulation locks mTORC1 on the lysosomal surface, rendering it insensitive to starvation and actively suppressing autophagy in aged cells.

Mechanistic reasoning

1. In young cells, amino acid starvation triggers lysosomal cholesterol efflux via NPC1, reducing mTORC1 recruitment and allowing ULK1 dephosphorylation.
2. With age, lysosomal cholesterol builds up due to declined NPC1 activity and increased ACAT1-mediated esterification, creating a rigid lipid environment that sustains Rag GTPase‑mTORC1 interaction even when cytosolic amino acids are low.
3. This cholesterol‑rich platform explains the persistent mTORC1‑Ser758‑ULK1 phosphorylation observed in aged fibroblasts [2] and renders autophagy refractory to rapamycin unless cholesterol is removed.
4. Transient expression of OSK (OCT4, SOX2, KLF4) during reprogramming upregulates LXRα and NPC1 transcription, promoting cholesterol efflux from lysosomes, thereby resetting mTORC1 nutrient sensitivity and enabling the autophagy burst required for pluripotency [4].
5. Consequently, autophagy suppression in aging is not merely a passive decline but an active lipid‑mediated safeguard that preserves a damaged proteome by preventing catastrophic self‑digestion.

Testable predictions

• Prediction 1: Aged human fibroblasts will show a 2‑3‑fold increase in lysosomal free cholesterol (measured by filipin staining) compared with young cells, correlating with persistent mTORC1 signaling under EBSS starvation.
• Prediction 2: Acute depletion of lysosomal cholesterol using methyl‑β‑cyclodextrin (MβCD) will restore starvation‑induced LC3‑II conversion and reduce p‑ULK1‑Ser758 levels to young‑cell levels, even without OSK or rapamycin.
• Prediction 3: Transient OSK expression (2‑day doxycycline pulse) will decrease lysosomal cholesterol and increase NPC1 protein levels; this effect will be blocked by LXRα antagonist GSK2033, abrogating the autophagy burst.
• Prediction 4: In vivo, aged mice treated with a liver‑targeted LXRα agonist will exhibit improved starvation‑induced autophagy markers and reduced tissue p‑S6K levels, mimicking the effect of transient OSK.

Falsification

If lysosomal cholesterol levels do not differ between young and aged cells, or if MβCD fails to rescue autophagy despite confirmed cholesterol depletion, the lipid‑gate hypothesis is refuted. Similarly, if OSK does not alter lysosomal cholesterol or NPC1 expression, the link between reprogramming and lipid remodeling would be unsupported.

Broader implications

This model connects nutrient‑sensing, lipid homeostasis, and epigenetic rejuvenation, suggesting that modalities targeting lysosomal cholesterol (e.g., LXR activators, NPC1 gene therapy) could mimic the autophagy‑resetting effects of transient Yamanaka factors without exposing cells to tumorigenic risk.

References

[1] https://doi.org/10.1083/jcb.201610113 [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC6398527/ [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC11352966/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC4207015/ [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC10373966/