IF AAV9-mediated allotopic expression of a codon-universalized MT-ND6 construct (six mitochondria-specific codons converted to universal equivalents, N-terminally fused to the COX8A mitochondrial targeting sequence, delivered IV at 1×10¹² vg/mouse) is co-administered with mitochondria-targeted ubiquinone (MitoQ10, 500 nmol/kg/day oral, replacing unformulated CoQ10 to exploit the triphenylphosphonium-driven ~500-fold mitochondrial accumulation advantage) beginning at 18 months of age in male C57BL/6J mice,

THEN at 6–9 months post-treatment, cardiac mitochondria will show: (1) ≥40% restoration of rotenone-sensitive NADH:ubiquinone oxidoreductase (Complex I) enzymatic activity by spectrophotometric assay; (2) ≥30% reduction in 4-HNE adduct burden by ELISA and immunoblot; (3) quantifiable re-emergence of CI/CIII respiratory supercomplex bands by Blue Native PAGE (BN-PAGE) with in-gel CI activity staining; and (4) ND6 protein co-localization with TOM20 and COX IV in ≥50% of cardiomyocytes by confocal immunofluorescence — outcomes that demonstrably exceed those of either allotopic ND6 alone or MitoQ10 alone (each predicted <20% CI restoration) — and absent cytoplasmic ND6 aggregates, confirming successful MTS-directed mitochondrial import,

BECAUSE the following stepwise causal chain operates:

1. Codon universalization is the rate-limiting prerequisite for nuclear-expressed mtDNA subunit import. In the cybrid rescue model, optimized oND1 constructs (universalized codons + MTS) yielded stable ~37 kDa mature/processed protein and functionally restored Complex I — but only when codon optimization was applied; non-optimized constructs failed to produce detectable protein or CI rescue. Extending this exact strategy to MT-ND6, whose unusually high mitochondria-specific codon density makes it especially vulnerable to ribosome stalling in the cytoplasm, is mechanistically justified (Codon optimization is essential for allotopic expression)[https://doi.org/10.1016/j.redox.2020.101429].

2. MTS-directed import produces two protein populations: a processed mature form (functional) and an unprocessed precursor (potentially aggregation-prone). The ND1 allotopic system showed a predominant ~37 kDa band (MPP-cleaved, matrix-localized) and a minor ~60 kDa band (precursor/uncleaved) — demonstrating that mitochondrial processing peptidase (MPP) cleavage efficiency is a quantitative bottleneck for functional subunit yield (Two ND1 protein forms detected by denaturing gel)[https://doi.org/10.1016/j.redox.2020.101429]. In 18–22 month hearts, where MPP activity declines with age [SPECULATIVE — based on reported general mitochondrial protease deterioration in cardiac aging, not directly cited in evidence set], the precursor:mature ratio may be unfavorable, limiting how much allotopic ND6 successfully integrates into assembling CI.

3. **Successful allotopic subunit integration into CI is necessary but not sufficient for respiratory chain function: CI/CIII supercomp...

SENS category: LysoSENS

Key references: • doi.org/10.1016/j.redox.2020.101429]. • doi.org/10.1016/j.redox.2020.101429]