Hypothesis

Aging‑associated decline of cystatin B does more than unleash cathepsin proteases; it destabilizes lysosomal membranes, triggering v‑ATPase loss and lysosomal alkalinization. This alkaline shift further impairs cystatin B binding and fuels mTORC1 hyperactivity in senescent cells, creating a self‑reinforcing loop that trades cellular specialization for survival. Restoring cystatin B levels should reseal lysosomes, reacidify the lumen, dampen mTORC1 signaling, and rescue lysosomal function without globally suppressing growth pathways.

Mechanistic Rationale

1. Cystatin B as a membrane stabilizer – Beyond inhibiting cysteine proteases, cystatin B binds LAMP2 and PI(3,5)P2 on the lysosomal inner membrane, increasing resistance to permeabilizing stimuli. Its age‑related drop removes this brace, making lysosomes prone to LMP.
2. LMP drives v‑ATPase dysfunction – Leaked cathepsins cleave subunits of the v‑ATPase V0 complex, reducing proton pumping. The resulting luminal alkalinization diminishes cystatin B affinity (its inhibition is pH‑dependent) and impairs autophagosome‑lysosome fusion.
3. Alkaline lysosomes activate mTORC1 – Lysosomal amino‑acid sensing (via Rag GTPases) requires an acidic environment; alkalinization skews Ragulator toward an active state, sustaining mTORC1 signaling even when nutrients are scarce.
4. mTORC1 represses cystatin B transcription – S6K1‑mediated phosphorylation of FOXO3 reduces its nuclear localization, lowering cystatin B gene expression. Thus, mTORC1 activity further depletes the very inhibitor that could seal the lysosome.

This loop explains why mTOR inhibition extends lifespan: it breaks the feedback but also curtails anabolic programs. Directly reinforcing the lysosomal membrane should uncouple longevity from growth suppression.

Testable Predictions

• Prediction 1: In aged microglia, viral overexpression of cystatin B will reduce cytosolic cathepsin B levels by >50% (measured by immunoblot of cytosolic fractions) and decrease mitochondrial ROS (MitoSOX fluorescence) compared with GFP control.
• Prediction 2: Cystatin B‑overexpressing aged microglia will show restored lysosomal pH (LysoSensor Yellow/Blue ratio) to youthful levels (~4.5) and increased v‑ATPase V0 subunit a1 abundance.
• Prediction 3: Downstream mTORC1 activity (p‑S6K/S6K ratio) will drop by ~30% in cystatin B‑rescued cells, despite unchanged extracellular amino‑acid concentrations.
• Prediction 4: In vivo, aged mice receiving intracerebroventricular cystatin B‑AAV will exhibit lower IL‑1β and TNF‑α levels, improved performance in the Y‑maze spontaneous alternation task, and no reduction in somatic growth rate or food intake relative to controls.

Falsification Criteria

If cystatin B overexpression fails to (a) raise lysosomal acidity, (b) diminish cytosolic cathepsin leakage, or (c) lower mTORC1 signaling in aged microglia, the proposed membrane‑stabilizing feedback loop is unlikely to be a primary driver of the mTOR‑lysosome trade‑off. Conversely, a rescue of lysosomal function without mTORC1 suppression would support the idea that longevity can be achieved by fixing the organelle rather than dialing down cellular civilization.