Hypothesis

Chronic mTORC1 activation accelerates age‑related lysosomal membrane permeabilization (LMP) by inhibiting the transcription factor TFEB, thereby reducing expression of lysosomal membrane proteins (e.g., LAMP2) and the cysteine protease inhibitor cystatin B (CSTB). This creates a dual vulnerability: weakened lysosomal membranes and diminished cytosolic buffering of leaked cathepsins, shifting cellular resources from proteostatic maintenance to growth‑oriented "civilization" functions.

Mechanistic Rationale

• mTORC1 phosphorylates TFEB on serine residues, retaining it in the cytoplasm and blocking its nuclear translocation [2]. TFEB drives transcription of the CLEAR network, which includes LAMP2 (critical for lysosomal membrane stability) and CSTB (a potent cytosolic cathepsin inhibitor).
• S6K1, a downstream mTORC1 effector, phosphorylates lysosomal-associated proteins such as LAMP1/2, altering their glycosylation and rendering the membrane more susceptible to oxidative‑induced permeabilization [3].
• When mTORC1 is inhibited (e.g., by rapamycin or Torin1), TFEB dephosphorylates, translocates to the nucleus, and upregulates LAMP2 and CSTB, thereby reinforcing membrane integrity and boosting cytosolic cathepsin buffering capacity.
• This mechanism extends the "civilization‑versus‑survival" dial: active mTORC1 pushes cells toward anabolic growth at the expense of lysosomal self‑repair, while its inhibition reallocates resources to survival‑focused proteostasis.

Experimental Design

1. Cell model: Primary cortical neurons from young (3 mo) and aged (24 mo) rats, and human iPSC‑derived neurons.
2. Treatments: (a) Vehicle, (b) Rapamycin (100 nM, 24 h), (c) Torin1 (250 nM, 6 h), (d) Constitutively active TFEB (TFEB‑S121A) overexpression via AAV.
3. Readouts:
   • LMP quantified by galectin‑3 puncta formation (immunofluorescence) and cathepsin D leakage into cytosol (subcellular fractionation + western blot).
   • LAMP2 and CSTB mRNA (qPCR) and protein levels (western blot/ELISA).
   • TFEB nuclear/cytoplasmic ratio (immunostaining).
   • Downstream cathepsin activity assays using fluorogenic substrates in cytosolic extracts.
   • Rescue experiments: co‑treat rapamycin with TFEB siRNA to test dependence on TFEB.
4. Controls: Non‑targeting siRNA, DMSO vehicle, and lysosomal stabilizer (e.g., cysteamine) as positive control for LMP reduction.

Expected Outcomes

• Aged neurons will show elevated galectin‑3 puncta, increased cytosolic cathepsin D, reduced LAMP2 and CSTB, and cytoplasmic TFEB localisation.
• Rapamycin/Torin1 treatment will decrease LMP (fewer galectin‑3 puncta), lower cytosolic cathepsin D, and increase LAMP2, CSTB, and nuclear TFEB.
• Overexpression of TFEB‑S121A will mimic mTOR inhibition effects, rescuing LMP and CSTB levels even in the presence of active mTORC1.
• TFEB knock‑down will abolish the protective effects of rapamycin, confirming TFEB as the mediator.

Potential Caveats & Alternatives

• If mTOR inhibition fails to raise CSTB or reduce LMP, the hypothesis would be falsified, suggesting that mTOR influences lysosomal stability through TFEB‑independent pathways (e.g., via lipid biosynthesis or autophagy flux).
• Compensatory upregulation of other cystatins (CSTA, CSTC) could mask CSTB‑specific effects; measuring total cystatin activity would address this.
• Cell‑type specificity: microglia may respond differently due to distinct basal mTOR activity; parallel experiments in primary microglia would clarify scope.

By linking mTORC1 signalling to TFEB‑controlled lysosomal membrane repair and cytosolic cathepsin inhibition, this hypothesis provides a testable mechanistic bridge between the "civilization‑versus‑survival" dial and the observed age‑dependent lysosomal leakage, opening avenues for interventions that preserve proteostatic integrity without globally suppressing growth pathways.