IF intramuscular AAV9-mitoTALEN (targeting the common large-scale mtDNA deletion breakpoint region, ~1×10¹¹ vg per hindlimb, delivered bilaterally via intramuscular injection) is administered in combination with a structured eccentric downhill treadmill exercise protocol (−20° decline, 17 m/min, 60 min/day, 5 days/week for 4 weeks, initiating 2 weeks post-AAV injection to allow initial nuclease expression) to 22–24-month-old male and female C57BL/6J-NZB heteroplasmic mice (carrying the NZB/BALB mtDNA polymorphism as a tractable model of heteroplasmy, or the mtDNA deletor mouse expressing a dominant-negative Twinkle helicase),

THEN skeletal muscle heteroplasmy measured by droplet digital PCR (ddPCR) for mutant:WT mtDNA ratio will shift significantly further toward WT—by an estimated additional 25–40 percentage points above the mitoTALEN-only or exercise-only controls—within 8 weeks of treatment initiation, accompanied by restoration of complex I/IV enzymatic activity, increased fiber cross-sectional area, improved grip strength and voluntary wheel running distance, and expansion of Pax7⁺ satellite cells bearing predominantly WT mtDNA genomes within the treated muscle,

BECAUSE the following mechanistic chain operates synergistically:

1. mitoTALEN creates a "deletion sink" in post-mitotic fibers: AAV9-encoded mitoTALENs, whose mitochondrial targeting sequences direct the nuclease dimers to the matrix, introduce targeted double-strand breaks exclusively at the deletion junction breakpoints in mutant mtDNA. Because mammalian mitochondria lack canonical non-homologous end joining or homologous recombination repair, the cleaved circular mutant genomes are rapidly degraded by endogenous exonucleases, reducing mutant load within mature myofibers. (AAV9-mitoTALEN achieves significant heteroplasmy shift in skeletal muscle in vivo)[https://pubmed.ncbi.nlm.nih.gov/30013188/]

2. Mutant mtDNA depletion creates selective pressure for WT replication in fibers: Following nuclease-mediated mutant genome degradation, total mtDNA copy number initially falls. Compensatory mitochondrial biogenesis (via retrograde PGC-1α/NRF1/TFAM signaling) drives preferential replication of the surviving WT templates to restore copy number homeostasis, consolidating the heteroplasmy shift in the post-mitotic compartment. (mitoTALEN reduces mutant load and drives compensatory WT repopulation)[https://pubmed.ncbi.nlm.nih.gov/30013188/]

3. Eccentric exercise generates mechanical damage that activates quiescent satellite cells: Eccentric downhill running imposes high-force eccentric contractions that cause sarcomeric disruption, Z-disc streaming, and localized inflammatory signaling (MyoD, IL-6, HGF release), which break satellite cell quiescence. In aged muscle, eccentric loading is particularly effective at activating the satellite cell pool, which otherwise remains deeply quiescent and contributes minimally to fiber maintenance. (Exercise-induced satellite cell act...

SENS category: GlycoSENS

Key references: • PMID: 30013188 • PMID: 34417588 • PMID: 10580071 • PMID: 11157564