Mitochondrial Heteroplasmy as a Tunable Rheostat Governing UPR Arm Balance and Cellular Senescence

Evidence shows that aged tissues exhibit exaggerated UPR flux coupled with mitochondrial DNA heteroplasmy, yet no study has directly lowered heteroplasmy to test whether UPR arm balance improves.

We hypothesize that mitochondrial heteroplasmy acts as a tunable rheostat that skews UPR signaling toward chronic PERK/IRE1 activation and suppresses ATF6‑mediated adaptive transcription, driving senescence.

Mechanistic Rationale

• mtDNA mutations disrupt ER‑mitochondria associated membranes (MAMs), altering Ca²⁺ flux and elevating ROS, which directly modulates the oligomerization state of UPR sensors embedded at MAMs [4].
• Heteroplasmy shifts mitochondrial metabolite ratios (ATP/ADP, NAD⁺/NADH, acetyl‑CoA) that serve as allosteric regulators of PERK autophosphorylation and IRE1 RNase activity, biasing the stress response toward pro‑apoptotic branches.
• Specific mtDNA‑encoded tRNA defects impair mitochondrial translation, creating a mito‑nuclear protein stoichiometry imbalance. This imbalance favors ATF4‑dependent transcription of CHOP while sequestering co‑activators such as CBP/p300 away from ATF6 target promoters, thereby suppressing adaptive UPR output.
• Released mtDNA fragments activate cytosolic cGAS‑STING signaling, which can amplify PERK‑eIF2α‑ATF4 signaling through IRF‑dependent transcriptional cross‑talk, further tilting the UPR arm balance.

Together, these mechanisms position heteroplasmy load as a quantitative dial that sets the set‑point of UPR arm activation.

Experimental Design

1. Model: Use 24‑month‑old C57BL/6J mice showing elevated liver mtDNA heteroplasmy (≈40‑60% for a predefined pathogenic mutation).
2. Intervention: Deliver mitochondria‑targeted DdCBE base editors via AAV8 to reduce the target mutation load; include a sham‑edited AAV8‑GFP control.
3. Verification: Quantify heteroplasmy by droplet digital PCR at 2, 4, and 8 weeks post‑treatment; confirm absence of nuclear off‑target edits by whole‑genome sequencing.
4. Readouts:
   • UPR arm activation: western blot for phospho‑PERK, phospho‑IRE1, cleaved ATF6; reporter assays for CHOP (PERK), XBP1 splicing (IRE1), and GRP94 (ATF6).
   • Proteostasis: luciferase‑based protein aggregation assay and BiP ELISA.
   • Senescence: SA‑β‑gal staining, p16^Ink4a mRNA, and SASP cytokine profiling.
   • Physiological: serum ALT/AST, liver histology, and frailty index.
5. Timeline: Assess at 4‑week intervals up to 24 weeks.

Expected Outcomes and Falsifiability

• If hypothesis is correct: Mice with heteroplasmy reduced below ~20 % will show a normalized UPR arm ratio (phospho‑PERK ≈ phospho‑IRE1 ≈ cleaved ATF6), reduced CHOP and BiP overexpression, restored proteostatic capacity, and a significant decline in senescence markers compared with sham‑treated aged controls.
• If hypothesis is false: Despite verified heteroplasmy lowering, UPR arm activation remains skewed toward PERK/IRE1, proteostasis does not improve, and senescence metrics remain unchanged relative to controls. This outcome would falsify the claim that mtDNA heteroplasmy directly tunes UPR arm balance and drives senescence.

Potential confounds such as inflammatory responses to AAV delivery will be controlled by including AAV8‑GFP sham groups and monitoring innate immune activation. Rescue experiments using mitochondria‑targeted catalase to scavenge ROS without altering heteroplasmy can further dissect the contribution of oxidative stress versus metabolite signaling.

By directly testing whether modulating mtDNA heteroplasmy reshapes UPR signaling, this work will clarify whether the mitochondrial genome operates as a master regulator of the aging proteostatic program or merely as a correlated bystander.