Rapamycin extends lifespan by tuning BAG3-mediated chaperone-assisted selective autophagy via mechanical tension sensing

Hypothesis

Rapamycin's lifespan extension requires functional BAG3‑mediated chaperone‑assisted selective autophagy (CASA) that is activated by reduced cellular mechanical tension downstream of mTORC1 inhibition.

Mechanistic Rationale

• mTORC1 promotes actin polymerization and myosin II activity, raising intracellular tension.
• Inhibition of mTORC1 by rapamycin lowers RhoA‑ROCK signaling, decreasing stress‑fiber formation and overall tension.
• BAG3 contains a mechanical‑sensing domain that binds exposed hydrophobic patches on misfolded cytoskeletal proteins when tension drops, recruiting Hsp70 and LC3 to initiate CASA.
• Thus, rapamycin does not merely upregulate BAG3 expression; it changes the subcellular context that enables BAG3 to act as a mechanostat, shifting clearance from the proteasome to autophagy.
• If BAG3 cannot sense tension (e.g., mutation of its mechanosensing domain), rapamycin‑induced CASA fails, and lifespan extension is lost.

Experimental Plan

• Generate a knock‑in mouse line expressing BAG3 with a point mutation in its mechanosensing domain (e.g., L145P) that abolishes tension‑dependent activation but preserves basal expression.
• Treat wild‑type and mutant cohorts with rapamycin (14 ppm chow) starting at 12 months of age.
• Monitor survival, calculate median and maximal lifespan.
• In parallel, harvest skeletal muscle and heart at 6 months post‑treatment to assess:
  • CASA flux using a tandem fluorescent‑LC3 reporter coupled to a known BAG3 substrate (e.g., mutant desmin).
  • Levels of phosphorylated cofilin and F‑actin/G‑actin ratio as readouts of tension.
  • Autophagosome‑lysosome fusion via LysoTracker and LC3‑II turnover with bafilomycin A1.
• Include control groups receiving vehicle and, where possible, a genetic mTORC1 hypomorph (Raptor heterozygous) to confirm pathway specificity.

Predicted Outcomes

• If the hypothesis is correct, rapamycin will extend lifespan in wild‑type mice but not in the BAG3 mechanosensor mutants, despite comparable drug exposure and mTORC1 inhibition (measured by p‑S6K).
• Mutants will show blunted CASA activation (reduced LC3‑II accumulation on desmin aggregates) and unchanged actin dynamics, indicating uncoupling of mTORC1 inhibition from proteostatic remodeling.
• Conversely, Raptor heterozygotes should phenocopy wild‑type rapamycin response, confirming that the effect stems from mTORC1 down‑regulation rather than off‑target actions.

Potential Caveats

• Compensatory up‑regulation of other BAG family members could mask the phenotype; therefore, double‑knock‑out or siRNA approaches may be needed in vitro.
• Systemic effects of rapamycin on immunity or metabolism could independently influence longevity; using inducible, tissue‑specific BAG3 mutants (e.g., muscle‑ or cardiomyocyte‑restricted) will help isolate the proteostatic arm.
• The mechanosensing domain of BAG3 may also interact with integrin‑FAK signaling; measuring focal adhesion kinase phosphorylation will clarify whether the observed tension changes are upstream or downstream of BAG3 activation.

Key References

• Rapamycin extends lifespan in mice, yeast, worms, and flies by inhibiting mTORC1, which activates protective stress‑response pathways like autophagy, thus mimicking hormesis rather than directly repairing accumulated damage1.
• Rapamycin's benefits arise from mimicking caloric restriction through reduced translation errors, enhanced autophagy, and dampened inflammation2.
• Even transient, late-life rapamycin treatment (3 months in middle-aged mice) produces persistent lifespan extension up to 60%3.
• BAG3 acts as a mechanosensor upregulated during stress and aging, coordinating selective autophagy of mechanically damaged proteins4.
• Concerns about side effects and limited evidence for longevity benefits in healthy adults5.