Hypothesis

We propose that specific point mutations in mitochondrial tRNA genes activate a CDK5‑dependent kinase cascade that phosphorylates lamin A/C at serine‑22, leading to nuclear envelope softening and chromatin leakage, independent of the classic ROS‑JNK retrograde pathway.

Mechanistic Rationale

1. mtDNA tRNA dysfunction reduces mitochondrial translation efficiency, causing a buildup of uncharged tRNAs that activate the mitochondrial unfolded protein response (mtUPR) and subsequently the cytosolic kinase CDK5 via calcium release from the ER‑mitochondria interface.
2. CDK5 phosphorylates lamin A/C, weakening its interaction with LAP1 and emerin, which destabilizes the lamina and promotes extrusion of chromatin fragments (CCFs) and cGAS‑STING activation.
3. This pathway can be triggered even when ROS levels are modest, explaining why low‑heteroplasmy mtDNA mutations shorten lifespan without overt oxidative stress.

Testable Predictions

• Prediction 1: In aged mice harboring the pathogenic mtDNA tRNA^Leu(UUR) m.3243A>G mutation (low heteroplasmy), pharmacological inhibition of CDK5 (e.g., roscovitine) will restore lamin A/C phosphorylation levels to youthful baselines and reduce CCF extrusion, despite persistent mtDNA mutation load.
• Prediction 2: Mitochondrial‑targeted expression of a wild‑type tRNA^Leu(UUR) gene (allotopic expression) will suppress CDK5 activation and lamin A/C pathology, rescuing nuclear morphology without altering ROS‑JNK signaling.
• Prediction 3: Cells expressing a phospho‑deficient lamin A/C S22A mutant will be resistant to mtDNA tRNA‑induced nuclear envelope defects, showing diminished SASP secretion even when mtDNA mutation load is high.

Experimental Approach

• Generate a knock‑in mouse model carrying the m.3243A>G mtDNA mutation at ~15 % heteroplasmy in heart and liver.
• Treat cohorts with either CDK5 inhibitor, vehicle, or allotopic tRNA^Leu(UUR) AAV9 for 3 months.
• Assess lamin A/C phosphorylation (Western blot with phospho‑specific S22 antibody), nuclear envelope morphology (EM), CCF flow cytometry, and SASP cytokine profiling.
• Include ROS measurements (MitoSOX) to confirm that phenotypic rescue occurs without significant ROS reduction.

Falsification

If CDK5 inhibition or tRNA allotopic expression fails to improve lamin A/C integrity or SASP despite correcting the mtDNA mutation, or if lamin A/C S22A does not rescue the phenotype, the hypothesis would be falsified, indicating that mtDNA tRNA dysfunction acts through alternative retrograde mechanisms.

Broader Impact

Demonstrating a direct mtDNA‑to‑lamina signaling axis would shift focus from global ROS scavenging to precise kinase‑targeted interventions, offering a mechanistic bridge between mitochondrial genome integrity and nuclear aging phenotypes.

References [1] https://pubmed.ncbi.nlm.nih.gov/32001510/ [2] https://www.science.org/doi/10.1126/science.adf2034 [3] https://doi.org/10.1038/s41598-018-24290-6 [4] https://academic.oup.com/nar/article/50/17/9948/6696852 [5] https://english.cas.cn/newsroom/research-news/202602/t20260210_1150422.shtml [6] https://www.fightaging.org/archives/2020/02/a-discussion-of-recent-work-on-allotopic-expression-of-mitochondrial-genes-at-the-sens-research-foundation/ [7] https://pubmed.ncbi.nlm.nih.gov/35203698/