Active Autophagy Suppression in Aging Is Reinforced by Lysosomal Calcium Leakage via TRPML1 Dysregulation

Aged cells keep the autophagy machinery intact but locked down by persistent mTORC1 signaling and declining AMPK activity [1][2]. This view explains why restoring core components such as WIPI2B can reactivate autophagosome formation even when upstream inhibitors accumulate [1]. Yet the persistence of the block suggests that additional layers enforce the suppressed state beyond simple kinase imbalances. We hypothesize that age‑dependent leakage of calcium from lysosomes through the TRPML1 channel creates a cytosolic calcium microdomain that activates calcineurin, which in turn dephosphorylates and activates the transcription factor NFAT. NFAT drives expression of genes that sustain mTORC1 activity, including Rag GTPases and lysosomal v‑ATPase subunits, thereby closing a positive feedback loop that keeps autophagy off.

Evidence linking lysosomal calcium to mTORC1 comes from studies showing that lysosomal Ca2+ release via TRPML1 activates mTORC1 on the lysosomal surface through a calmodulin‑dependent pathway [3]. In young cells, autophagosome‑lysosome fusion triggers a transient Ca2+ flux that temporarily inhibits mTORC1, allowing a reset after nutrient stress. With age, oxidative damage alters TRPML1 gating, causing a chronic low‑level leak that pre‑activates mTORC1 and blocks the inhibitory phospho‑ULK1‑Ser757 signal that normally follows AMPK activation [2]. Consequently, even when AMPK rises during starvation, ULK1 remains phosphorylated at the inhibitory site, and TFEB stays cytoplasmic because sustained mTORC1 activity prevents its nuclear translocation.

This model predicts three testable outcomes. First, pharmacological agonists of TRPML1 (e.g., ML‑SA1) should restore lysosomal calcium transients, reduce basal mTORC1 signaling (measured by p‑S6K), and increase autophagic flux (LC3‑II turnover) in fibroblasts from old mice compared with vehicle. Second, genetic deletion of Trpml1 in progeroid mice will exacerbate autophagy suppression, leading to higher p62 accumulation and accelerated senescence markers (SA‑β‑gal, p16^INK4a). Third, rescuing NFAT activity with a calcineurin inhibitor (FK506) in aged TRPML1‑deficient cells should uncouple the lysosomal calcium leak from mTORC1 hyperactivity, partially rescuing autophagy without altering TRPML1 expression.

These experiments directly challenge the notion that autophagy failure in aging is solely a consequence of kinase dysregulation. Instead, they position lysosomal calcium homeostasis as a upstream gatekeeper that, when disrupted, locks the cleanup machinery in a suppressed state to preserve a damaged but viable cytosol.