Super-resolution live imaging reveals spatiotemporal uncoupling of mitochondria-ER contacts and fission in stem cell aging

Cellular senescence drives tissue aging and functional decline, and while mitochondrial dysfunction is a classic senescence hallmark, how inter-organelle communication regulates mitochondrial dynamics in aging—especially how altered mitochondria-ER contact sites (MERCs) impact mitochondrial fission and stem cell senescence—remains unclear. Here, we first capture the nanoscale dynamics of MERCs and mitochondrial fission via live-cell super-resolution GI-SIM imaging in two human mesenchymal stem cell (hMSC) aging models: replicative senescence and Hutchinson-Gilford progeria syndrome (HGPS). We show senescent hMSCs exhibit reduced mitochondrial fission, spatiotemporally uncoupled from concurrent MERC loss. Mechanistically, ER tethering protein VAPB downregulation underlies age-related MERC reduction; VAPB ablation in young hMSCs recapitulates aging defects (impaired MERCs/fission, abnormal organelle morphology, premature senescence), whereas endogenous VAPB restoration in senescent hMSCs re-establishes MERCs, rescues fission and attenuates senescence markers. Our study identifies VAPB-mediated tethering as a critical regulator of mitochondrial homeostasis in stem cell aging, establishing a direct causal link between MERC integrity, mitochondrial dynamics and cellular senescence, and highlighting MERCs as a potential target for ameliorating age-related stem cell dysfunction.