NMN/NAD+ enhances SIRT2-modulated microtubule dynamics to improve mitochondrial and mitophagy functions in senescent cells

The effect of NAD⁺ in enhancing mitochondrial function and energy metabolism in human cells is closely linked to NAD⁺-dependent sirtuins (i.e. SIRT1 and SIRT3). SIRT2 primarily functions in the cytoplasm, where it can serve as a key deacetylase for tubulin and modulates stability of microtubules. Microtubule plays a pivotal role in regulating mitochondrial dynamics, including mitochondrial movement, fission/fusion, repair, and mitophagy-dependent clearance. However, the potential role of NAD⁺ in modulating SIRT2-related microtubule stability, and the potential involvement of the NAD⁺-SIRT2-microtubule axis in regulating mitochondrial and mitophagy functions remains unexplored. In this study, we demonstrate that senescent muscle cells exhibit microtubule hyper-stabilization and reduced dynamics, concomitant with SIRT2 inactivation and tubulin hyperacetylation. These alterations impair microtubule-dependent mitochondrial repair and mitophagy function, resulting in mtDNA leakage, CGAS-STING1 activation and subsequently accelerated senescence. Notably, treatment with nicotinamide mononucleotide (NMN) effectively reactivates SIRT2, restores microtubule dynamics, and enhances mitochondrial quality control by promoting repair and mitophagy. Consequently, NMN mitigates CGAS-STING1-driven senescence. Our findings reveal a novel mechanism by which NMN preserves mitochondrial health in senescent cells via a SIRT2-microtubule axis, highlighting its protective role beyond canonical NAD⁺-sirtuin pathways, and suggesting microtubule dynamics as a promising therapeutic target for improving cellular defects associated with mitochondrial and mitophagy dysfunctions. Abbreviations: D-gal: D-galactose; EdU: 5-ethynyl-20-deoxyuridine; HDAC6: histone deacetylase 6; LAMP1: lysosome associated membrane protein 1; MSCs: mesenchymal stem/stromal cells; mtDNA: mitochondrial DNA; NAD⁺: nicotinamide adenine dinucleotide; NMN: nicotinamide mononucleotide; PBS: phosphate-buffered saline; SA-GLB1/β-gal: senescence-associated galactosidase beta 1; SIRT2: sirtuin 2.