The aging phenotype arises when nuclear-encoded mitophagy fails to restrict mitochondrial DNA (mtDNA) heteroplasmy below a functional threshold, not because mtDNA mutations accumulate to cause irreversible bioenergetic collapse. While mtDNA mutates rapidly and lacks histone protection, naturally occurring heteroplasmy levels in aged tissues remain low (≈0.1 %). Yet even these modest loads can impair metabolism when specific sites are hit, suggesting that the decisive factor is the efficiency of quality‑control mechanisms that remove damaged mitochondria before mutant genomes clonally expand.

Mechanistic insight Nuclear genes govern every step of mitophagy: initiation (ULK1 complex), receptor activation (BNIP3L/NIX, FUNDC1), autophagosome formation (LC3 lipidation), and lysosomal degradation (TFEB‑driven lysosomal biogenesis). Age‑dependent decline in the activity of these nuclear regulators shifts the balance toward retention of deleterious mtDNA variants. Moreover, mtDNA stress triggers retrograde signaling—via ATM, NF‑κB, and ROS‑sensitive kinases—that suppresses transcription factors such as TFEB and PGC‑1α, creating a vicious loop where damaged mitochondria further impair nuclear quality‑control capacity.

Hypothesis Enhancing nuclear‑driven mitophagy will extend healthspan independent of mtDNA mutation rate, whereas impairing mitophagy will accelerate aging even when mtDNA mutation load is minimal.

Testable predictions

1. Gain‑of‑function – Overexpressing TFEB or BNIP3L/NIX in murine skeletal muscle or brain will:
   • Increase mitophagic flux (measured by mt‑Keima or LC3‑II turnover)
   • Reduce heteroplasmy at pathogenic mtDNA sites without altering overall mutation frequency
   • Improve respiratory capacity, lower ROS, and delay age‑related functional decline (grip strength, cognitive tests)
   • Extend median lifespan in wild‑type mice
2. Loss‑of‑function – Muscle‑specific knockout of Atg7 or FUNDC1 in young mice will:
   • Cause accumulation of swollen, membrane‑damaged mitochondria
   • Elevate heteroplasmy at specific mtDNA loci (detected by duplex sequencing)
   • Prematurely induce sarcopenia, insulin resistance, and reduced survival, despite total mtDNA mutation burden remaining below 0.2 %
3. Feedback loop disruption – Pharmacological inhibition of NF‑κB in aged animals will:
   • Reactivate TFEB nuclear translocation and lysosomal gene expression
   • Rescue mitophagy markers
   • Lower heteroplasmy levels and improve metabolic health, even without altering mtDNA replication or repair enzymes

Experimental approach

• Generate inducible, tissue‑specific transgenic lines for TFEB, BNIP3L/NIX, and CRISPR‑mediated knockouts of Atg7/FUNDC1.
• Quantify mtDNA heteroplasmy using duplex sequencing to distinguish low‑level variants from clonal expansions.
• Assess mitophagy flux via mt‑Keima microscopy and Western blot for LC3‑II/p62.
• Measure physiological outcomes (metabolic cages, grip strength, rotarod, glucose tolerance) and survival.
• Use NF‑κB inhibitors (e.g., IKKβ antagonist) to test retrograde signaling interruption.

Falsifiability If overexpression of nuclear mitophagy regulators fails to reduce heteroplasmy or improve healthspan, or if mitophagy deficiency does not accelerate aging despite low mtDNA mutation burden, the hypothesis would be refuted. Conversely, confirmation would shift focus from mutational load to nuclear control of mitochondrial quality as the central lever of aging.