Microtubule acetylation is a biomarker of cytoplasmic health during cellular senescence

Cellular senescence is marked by cytoskeletal dysfunction, yet the role of microtubule post-translational modifications (PTMs) remains unclear. We demonstrate that microtubule acetylation increases during drug-induced senescence in human cells and during natural aging in Drosophila. Elevating acetylation via HDAC6 inhibition or TAT1 overexpression in BEAS-2B cells disrupts anterograde Rab6A vesicle transport, but spares retrograde transport of Rab5 endosomes. Hyperacetylation results in slowed microtubule polymerization and decreased cytoplasmic fluidity, impeding diffusion of micron-sized condensates. These effects are distinct from enhanced detyrosination, and correlate with altered viscoelasticity and resistance to osmotic stress. Modulating cytoplasmic viscosity reciprocally perturbs microtubule dynamics, revealing bidirectional mechanical regulation. Senescent cells phenocopy hyperacetylated cells, exhibiting analogous effects on transport and microtubule polymerization. Our findings establish acetylation as a biomarker for cytoplasmic health and a potential driver of age-related cytoplasmic densification and organelle transport decline, linking microtubule PTMs to biomechanical feedback loops that exacerbate senescence. This work highlights the role of acetylation in bridging cytoskeletal changes to broader aging hallmarks.