ATM is a kinase and major master regulator of the DNA damage response cascade. Ataxia telangiectasia (AT) patients have mutations in ATM and suffer from telangiectasia, radiosensitivity, cancer, and neurologic features. 1 Some features of AT are readily explained by the loss of DNA repair capacity, ie radiosensitivity and cancer, but not the neurologic deficits. Therefore, we initiated studies to explore an alternative hypothesis to explain the neurologic findings. Prior literature revealed mitochondrial deficits in AT. 2 Since there are well-established links between DNA repair defects, neurodegeneration and mitochondrial dysfunction, 3, 4 we focused our attention on mitochondria. In 2 other DNA repair-deficient syndromes, Cockayne Syndrome (CS), 4, 5 and Xeroderma Pigmentosum group A (XPA), 3 we reported that persistent nuclear DNA damage created a signal to mitochondria, which we called nuclear to mitochondrial (NM) signaling. 6 Nicotinamide adenine dinucleotide (NAD+), an important metabolite in all human cells, was the signaling molecule. Emerging evidence strongly suggest NAD+ is a key player in aging, neurodegeneration and metabolic diseases. In NM signaling, we proposed that persistent activation of poly (ADP-ribose) polymerase1 (PARP1), an NAD+-dependent enzyme that poly (ADP-ribosylates)(PARylates) proteins, induces a loss of intracellular NAD+ whereby other NAD+-dependent enzymes, like the sirtuins, experience a loss of activity. 6 PARP1 is a major abundant nuclear protein, which serves as a sensor of DNA damage. In response to DNA damage, the rate of PAR synthesis increases rapidly up to 500-fold which can consume a significant amount of NAD+. 7 In the NM signaling cascade, the pathways critically affected by PARP1-dependent depletion of intracellular NAD+ are SIRT1-dependent protein deacetylation, mitochondrial oxidative phosphorylation and mitophagy, the selective clearance of damaged or defective mitochondria. SIRT1 is a multifunctional protein deacetylase which plays important roles in cellular pathways including: longevity, metabolism, cellular senescence, genome maintenance, DNA repair, and inflammation. Thus, loss of NAD+ has direct and indirect consequences on multiple cellular endpoints. Ultimately, depletion of intracellular NAD+ alters the NAD+/SIRT1 axis and leads to defects in mitochondrial homeostasis, ROS production, DNA repair and cell survival. 6

To test whether increased NAD+ might ameliorate some of the features noted above we activate the NAD+/SIRT1 pathway using 3 strategies: NAD+ supplementation with an NAD+ precursor nicotinamide riboside (NR), PARP1 inhibition with olaparib or SIRT1 activation by SRT1720. Consistently, all 3 treatments improved the mitochondrial phenotypes, and decreased PARylation. We have consistently seen that loss of activity from the NAD+/SIRT1 axis also inhibits mitophagy. Thus, we interrogated whether NR could improve mitophagy in the ATM-knockdown cells, and as expected replenishment of NAD+ stimulated mitophagy and improved mitochondria morphology. Using a worm model of AT, atm-1, we corroborated our results and showed that the mitochondrial improvements were the result of increased mitophagy. Further, we demonstrated that NR-dependent enhancements in mitophagy were dependent upon expression of SIR-2.1 (the worm homolog of SIRT1), DAF16, PINK-1 and DCT-1 (homolog of mammalian NIX/BNIP3L). On a functional level, atm-1 worms treated with NR, SRT1720 or olaparib all experienced improved lifespan, mitochondrial morphology, swimming and memory. Our data suggest that stimulating the …