Hypothesis: Mitochondrial microproteins as epigenetic rheostats of aging

Recent work shows that mitochondrial DNA (mtDNA) mutations do not necessarily impair oxidative phosphorylation yet still correlate with accelerated epigenetic aging and tissue‑specific phenotypes [2, 3]. Moreover, age‑associated large mtDNA deletions preferentially remove microprotein‑encoding regions rather than respiratory chain genes [4]. This disconnects the classic bioenergetic view of mtDNA aging from a emerging model in which mtDNA‑derived microproteins act as retrograde signals that re‑program nuclear gene expression and chromatin states.

Core proposition – Specific mtDNA‑encoded microproteins (e.g., SHMOOSE, MOTS‑c) are released into the cytosol, translocate to the nucleus, and directly modulate the activity of histone acetyltransferases (HATs) such as p300/CBP, leading to site‑specific acetylation of histone H3 lysine 27 (H3K27ac) at promoters of longevity‑associated pathways (e.g., FOXO3, SIRT1). Altered H3K27ac reshapes the epigenome, driving the epigenetic aging clock and functional decline, while mitochondrial respiration remains largely unchanged.

Mechanistic rationale – Microproteins are small, hydrophobic peptides that can cross mitochondrial membranes via transient pores or vesicles. Their nascent N‑terminal mitochondrial targeting sequences are cleaved upon import, exposing cytosolic motifs that bind the KIX domain of p300, enhancing its HAT activity. This interaction is sensitive to the redox state of the intermembrane space; increased ROS (a byproduct of mtDNA damage) oxidizes cysteine residues on the microprotein, increasing its affinity for p300. Consequently, even modest mtDNA heteroplasmy that does not impair electron transport can amplify nuclear acetylation signaling.

Testable predictions

1. Genetic manipulation – Mito‑targeted base editing to introduce or delete the SHMOOSE open reading frame in mouse mtDNA will alter nuclear H3K27ac levels at FOXO3/SIRT1 promoters without changing basal oxygen consumption rates (Seahorse assay).
2. Pharmacological blockade – Cell‑permeable peptides that compete for the p300 KIX domain will rescue the acetylation phenotype and delay epigenetic aging (DNAm GrimAge) in cells harboring high SHMOOSE heteroplasmy.
3. In vivo longevity – Mice with neuron‑specific overexpression of a nuclear‑localized, acetylation‑defective SHMOOSE mutant will exhibit unchanged ATP production but extended median lifespan and reduced cortical epigenetic age versus wild‑type controls.
4. Falsification – If precise mtDNA microprotein editing fails to shift nuclear H3K27ac, epigenetic age, or lifespan despite confirmed changes in microprotein abundance, the hypothesis is refuted, supporting the view that mtDNA influences aging through other pathways.

Experimental outline

• Generate mito‑CRISPR‑Cas9 derived mouse lines with precise SHMOOSE knock‑in/knock‑out (validate by ddPCR heteroplasmy quantification).
• Isolate cortical neurons and fibroblasts; perform Western blot for H3K27ac, ChIP‑seq for p300 binding, and RNA‑seq for FOXO3/SIRT1 targets.
• Measure mitochondrial respiration (OCR) and ATP production to confirm bioenergetic neutrality.
• Track epigenetic age using the mouse Horvath clock over 12‑month longitudinal sampling.
• Conduct survival assays; apply log‑rank test for lifespan differences.

By positioning mtDNA‑encoded microproteins as direct modulators of nuclear histone acetylation, this hypothesis bridges the observed dissociation between mtDNA damage and respiratory decline with robust epigenetic aging signals. It shifts the focus from preserving oxidative phosphorylation to editing microprotein‑encoding mtDNA loci or intercepting their nuclear signaling—a falsifiable, experimentally tractable refinement of the mitochondrial aging paradigm.