Background: Senescent cells transfer damaged mitochondria to neighboring stem cells through tunneling nanotubes (TNTs), propagating mitochondrial dysfunction and accelerating biological aging. Caloric restriction (CR) enhances mitophagy and mitochondrial quality control, but its effect on intercellular mitochondrial transfer is unexplored.

Hypothesis: Caloric restriction mimetics (CRMs) — specifically rapamycin and spermidine — reduce transfer of dysfunctional mitochondria from senescent cells to mesenchymal stem cells (MSCs) via two mechanisms: (1) upregulating PINK1/Parkin-mediated mitophagy within senescent cells, degrading depolarized mitochondria before transfer, and (2) downregulating Miro1/TRAK2-dependent mitochondrial docking on TNTs. This dual mechanism restores recipient MSC oxidative phosphorylation (Complex I/III activity), reduces mtROS-driven NLRP3 inflammasome activation, and decelerates DunedinPACE by ≥0.15 pace-of-aging units in treated co-cultures.

Testable Predictions:

1. Co-culture of senescent fibroblasts with MSCs shows ≥40% increase in TNT-mediated mitochondrial transfer (MitoTracker-labeled) vs. non-senescent controls.
2. Rapamycin (10 nM) or spermidine (100 μM) pre-treatment of senescent cells reduces mitochondrial transfer by ≥50% and restores recipient MSC oxygen consumption rate to ≥80% of baseline.
3. Miro1 knockdown in senescent cells phenocopies CRM effects on transfer reduction.
4. DunedinPACE in recipient MSCs decelerates proportionally to transfer reduction (r ≥ 0.7).

Clinical Relevance: In rheumatoid arthritis, senescent synovial fibroblasts accumulate in inflamed joints. If CRMs block dysfunctional mitochondrial transfer to joint-resident MSCs, this preserves regenerative capacity and reduces inflammaging-driven joint destruction — orthogonal to senolytic strategies that kill rather than modulate senescent cells.

Proposed Model: In vitro co-culture (IMR-90 senescent + BM-MSCs), Transwell vs. direct contact, ± CRMs, 14-day longitudinal with epigenetic clock at days 0, 7, 14.