Allotopic Expression of mtDNA-Encoded OXPHOS Subunits Rescues Cardiac Progenitor Cell Function by Bypassing Mitochondrial Genome Instability

Hypothesis: Restoring oxidative phosphorylation in aged c-Kit+ cardiac progenitor cells (CPCs) through nuclear expression of mitochondrially targeted, allotopic versions of essential mtDNA-encoded OXPHOS subunits will reverse senescence and restore regenerative capacity, independent of nuclear genome alterations.

Mechanistic rationale: Aged CPCs accumulate mtDNA mutations that impair complex I and IV assembly, lowering mitochondrial membrane potential and increasing ROS, which triggers a p53‑dependent senescence program and forces reliance on glycolysis. This metabolic block prevents the metabolic shift to OXPHOS required for differentiation and proliferation. Allotopic expression bypasses the mutation‑prone mitochondrial genome by delivering nuclear‑encoded, mitochondrially imported copies of key subunits (e.g., MT‑ND4 for complex I and MT‑CO1 for complex IV). Imported subunits can assemble with endogenous mtDNA‑encoded partners to form functional respiratory complexes, thereby restoring ATP production, reducing ROS, and attenuating retrograde stress signals that sustain senescence. Notably, this approach does not require correcting the heterogeneous mtDNA mutational load; it supplies a functional "bypass" that can compensate for multiple defects simultaneously.

Experimental design: Use AAV9 vectors carrying a cardiac‑specific promoter (cTnT) driving expression of mitochondrially targeted MT‑ND4 and MT‑CO1, each fused to a mitochondrial targeting sequence (MTS) proven to import proteins into the mitochondrion. Inject vectors into senescent mice (24‑month‑old) that exhibit c‑Kit+ CPC accumulation in hypoxic niches but fail to differentiate after injury. Control groups receive AAV9‑GFP or empty vector. Four weeks post‑injection, isolate CPCs and assess: (1) mitochondrial respiration (Seahorse OCR), (2) ROS levels (MitoSOX), (3) senescence markers (p16^INK4a, SA‑β‑gal), (4) proliferation (Ki‑67) and differentiation (cTnT, α‑SMA) after in‑vitro cardiac induction, and (5) in‑vivo cardiac function (echocardiographic EF, fractional shortening) after myocardial infarction.

Predicted outcomes: Allotopic expression will increase OCR and ATP production in CPCs, lower ROS, reduce senescence marker expression, and restore the ability to proliferate and differentiate into cardiomyocytes. Functionally, treated mice will show improved post‑MI EF and reduced scar size compared with controls. If mtDNA mutations are merely a passenger, rescuing OXPHOS via allotopic expression will not improve CPC behavior or cardiac recovery.

Falsifiability: The hypothesis is falsifiable because a null result—no significant improvement in mitochondrial function, senescence markers, differentiation, or cardiac performance despite confirmed mitochondrial import of the allotopic subunits—would refute the claim that mtDNA‑encoded OXPHOS deficiency is a rate‑limiting driver of CPC senescence. Conversely, a positive result would support the notion that the mitochondrial genome, not the nuclear genome, is the primary regulator of aging in this progenitor pool and validate allotopic expression as a therapeutic strategy to bypass mtDNA instability.