Hypothesis

In aged cells, autophagy is not merely blocked by mTORC1 hyperactivation or Rubicon accumulation; it is actively suppressed by a lysosomal calcium overload that stems from mitochondrial ROS‑driven SASP signaling. Elevated lysosomal Ca2+ inhibits the VPS34‑Beclin1 complex through calmodulin‑dependent kinase II (CaMKII) phosphorylation of Beclin1 at Ser90/93, thereby preventing autophagosome nucleation even when upstream inhibitors are removed.

Mechanistic Rationale

1. Mitochondrial ROS amplify SASP – Senescent mitochondria leak superoxide, which activates NF‑κB and GATA4, reinforcing SASP secretion (4).
2. SASP factors (IL‑6, IL‑8, MCP‑1) engage lysosomal Ca2+ channels – Cytokine binding to lysosomal TRPML1 and TPPC2 increases Ca2+ efflux from the lysosome into the cytosol, a process shown to be ROS‑sensitive in macrophages (2).
3. Lysosomal Ca2+ overload activates CaMKII – Cytosolic Ca2+ binds calmodulin, activating CaMKII, which phosphorylates Beclin1 at sites that disrupt its interaction with VPS34, directly inhibiting PI3P production needed for phagophore formation (5).
4. Feedback loop – Impaired autophagy leads to accumulation of damaged mitochondria, further raising ROS and SASP, locking the system in a self‑reinforcing suppressed state.

This model explains why rapamycin or Atg5 overexpression can restore flux: they bypass the calcium‑dependent block at the nucleation step, but do not correct the upstream calcium dysregulation, which is why flux restoration is often partial and transient in aged neurons where calcium buffering is weaker (6).

Testable Predictions

• Prediction 1: Lysosomal Ca2+ concentration, measured with Lyso‑Ca2+‑sensitive dyes (e.g., Fluo‑4 AM loaded into lysosomes), will be significantly higher in aged fibroblasts and muscle fibers compared with young controls, and will correlate inversely with LC3‑II turnover.
• Prediction 2: Pharmacological inhibition of lysosomal Ca2+ release (using ML-SI1 for TRPML1 or GPN for TPPC2) will rescue autophagy flux in aged cells even when mTORC1 remains active, as evidenced by increased p62 degradation and GFP‑LC3 puncta.
• Prediction 3: Genetic knockdown of CaMKIIδ in aged mice will reduce Beclin1 phosphorylation (Ser90/93) and restore autophagosome formation without altering mTORC1 activity, detectable by phospho‑specific Western blot.
• Prediction 4: Senescent cell conditioned media will elevate lysosomal Ca2+ in naïve young cells; neutralizing IL‑6 or IL‑8 in the media will block this effect.

Experimental Approaches

1. Live‑cell lysosomal Ca2+ imaging – Transfect cells with Lysosomal GCaMP6f; quantify basal and stimulus‑evoked Ca2+ fluxes in young vs. aged primary myoblasts.
2. Rescue experiments – Treat aged muscle explants with ML‑SI1 (TRPML1 blocker) or BAPTA‑AM (intracellular Ca2+ chelator) and measure autophagic flux via mCherry‑GFP‑LC3 reporter.
3. Beclin1 phospho‑mutants – Express Beclin1‑S90A/S93A (non‑phosphorylatable) in aged hepatocytes; assess whether autophagy genes (LC3, ATG5) are re‑engaged and whether senescence markers (p16, SA‑β‑gal) decline.
4. SASP neutralization – Co‑culture senescent fibroblasts with young myoblasts; add anti‑IL‑6/IL‑8 antibodies and track lysosomal Ca2+ and autophagy readouts.

Potential Falsification

If lysosomal Ca2+ levels are not elevated in aged cells, or if buffering lysosomal Ca2+ fails to improve autophagy flux despite mTORC1 inhibition, the calcium‑overload mechanism would be refuted. Likewise, if CaMKII inhibition does not decrease Beclin1 phosphorylation or rescue flux, the proposed kinase link would be invalid. Conversely, restoration of flux by calcium manipulation without changes in mTORC1 or Rubicon would strongly support the hypothesis.