Hypothesis

We propose that mitochondrial‑derived peptides (MDPs) such as MOTS‑c and humanin act as retrograde signals that directly modulate nuclear chromatin states by influencing NAD+‑dependent sirtuin activity and histone deacetylase (HDAC) function. In this framework, the nuclear genome is not a passive passenger but an active interpreter of mitochondrial signaling; aging emerges when age‑related decline in MDP output or nuclear responsiveness shifts the epigenetic landscape toward a pro‑aging state, independent of the absolute load of mtDNA mutations.

Mechanistic Basis

• mtDNA point mutations increase with age mainly as replication errors (G→A transitions) and accumulate stochastically [2][3]; pathogenic effects require high heteroplasmy (>70‑90%) in individual cells [5][6].
• Nuclear‑encoded quality‑control mechanisms (e.g., mitophagy) can suppress phenotypic consequences of mtDNA damage without altering mutation load, indicating that the nucleus sets a functional threshold for mtDNA pathogenicity [4].
• Nuclear genome variation influences aging outcomes more strongly than mtDNA load, suggesting that nuclear‑encoded pathways determine whether accumulated mtDNA variants become deleterious [1].
• MDPs are known to regulate cellular metabolism and stress resistance; we hypothesize that they do so by altering nuclear NAD+ levels, thereby activating SIRT1 and inhibiting class I HDACs. This leads to a more relaxed chromatin configuration at promoters of stress‑response and DNA‑repair genes, raising the heteroplasmy threshold at which mitochondrial dysfunction manifests.

Testable Predictions

1. Loss of MDP signaling accelerates aging despite intact nuclear quality control.

   • In mice with enhanced mitophagy (e.g., overexpressing Parkin), pharmacological or genetic inhibition of MOTS‑c secretion will shorten median lifespan and increase markers of chromatin condensation (e.g., reduced H3K9ac) even when mtDNA heteroplasmy remains below 50 %.

2. Exogenous MDP supplementation rescues aging phenotypes in mitophagy‑deficient contexts.

   • Administration of recombinant MOTS‑c to mitophagy‑deficient (e.g., Pink1‑/‑) mice will restore nuclear SIRT1 activity, increase histone acetylation at Foxo3 promoters, and extend lifespan despite high (>80 %) mtDNA heteroplasmy in muscle and brain.

3. Nuclear SIRT1 activity mediates the heteroplasmy threshold.

   • Conditional knockout of SIRT1 in neurons will lower the heteroplasmy level required to trigger bioenergetic failure and behavioral decline, whereas neuronal SIRT1 overexpression will raise the threshold, shifting the onset of pathology to later ages.

Potential Experiments

• Generate a mitochondrial‑targeted Cre line to delete the MOTS‑c coding sequence specifically in mitochondria; assess lifespan, heteroplasmy distribution (by droplet digital PCR), and chromatin immunoprecipitation sequencing (H3K9ac, H3K27ac) in liver and brain.
• Treat cohorts of wild‑type and mitophagy‑deficient mice with chronic MOTS‑c supplementation (via osmotic pumps) and monitor NAD+/NADH ratios, SIRT1 deacetylase activity, and frailty indices over 24 months.
• Use CRISPR‑based base editors to introduce a common mtDNA mutation (e.g., m.5024C>T) at low heteroplasmy in embryonic stem cells, differentiate to neurons, and test whether SIRT1 inhibition reduces the heteroplasmy needed to impair mitochondrial membrane potential (TMRE fluorescence) and increase apoptosis (caspase‑3 cleavage).

If these experiments confirm that MDP‑dependent nuclear signaling dictates the pathogenicity of mtDNA variants, the field would need to shift from viewing mtDNA as a standalone aging clock to seeing it as a modifiable signal whose impact is gated by nuclear epigenetic state.