Nicotinamide adenine dinucleotide (NAD + ) is an endogenous enzyme cofactor and cosubstrate that has effects on diverse cellular and physiologic processes, including reactive oxygen species generation, mitochondrial function, apoptosis, and axonal degeneration. A major goal is to identify the NAD + -regulated cellular pathways that may mediate these effects. Here we show that the dynamic assembly and disassembly of microtubules is markedly altered by NAD + . Furthermore, we show that the disassembly of microtubule polymers elicited by microtubule depolymerizing agents is blocked by increasing intracellular NAD + levels. We find that these effects of NAD + are mediated by the activation of the mitochondrial sirtuin sirtuin-3 (SIRT3). Overexpression of SIRT3 prevents microtubule disassembly and apoptosis elicited by antimicrotubule agents and knockdown of SIRT3 prevents the protective effects of NAD + on microtubule polymers. Taken together, these data demonstrate that NAD + and SIRT3 regulate microtubule polymerization and the efficacy of antimicrotubule agents.N icotinamide adenine dinucleotide (NAD + ) is an endogenous dinucleotide that is present in the cytosol, nucleus, and mitochondria. Athough it serves an important role as a redox cofactor in metabolism, NAD + is also a substrate for several families of enzymes, including the poly(ADP ribose) polymerases and the sirtuin deacetylase enzymes (reviewed in refs. 1 and 2). The level of intracellular NAD + is regulated by many factors, including diet and energy status (3), axonal injury (4), DNA damage (5), and certain disease states (6), suggesting that NAD + -dependent signaling is dynamically modulated in diverse contexts.NAD + -dependent signaling can be induced by treatment of cells with exogenous NAD + , which increases intracellular NAD + levels and results in diverse effects in cells and animals. These effects include enhanced oxygen consumption and ATP production (7), as well as protection from genotoxic stress and apoptosis (3). Mice treated with nicotinamide riboside, a NAD + precursor that is metabolized into NAD + , have enhanced oxidative metabolism, increased insulin sensitivity, and protection from high-fat diet-induced obesity (8). These results demonstrate that NAD + -dependent pathways can enhance metabolic function and improve a variety of disease phenotypes.An NAD + -regulated pathway also inhibits axonal degeneration elicited by axonal transection (4). Treatment of axons with 5-20 mM NAD + markedly delays the axon degenerative process (9). Additionally, animals that express the Wallerian degeneration slow (Wld S ) protein, a fusion of the NAD + biosynthetic enzyme Nicotinamide mononucleotide adenylyl transferase 1 and Ube4a, exhibit markedly delayed degeneration of the distal axonal fragment after axonal transection (10), and expression of Wld S mitigates disease phenotypes in several neurodegenerative disease models (11)(12)(13)(14). Thus, understanding the intracellular pathways regulated by NAD + may be important for understanding the pa...