Although both DNA DSBs and DNA PK activity
Although both DNA DSBs and DNA-PK activity increase with aging in skeletal muscle (Figure 1), we cannot conclude that aging-associated increase in DNA-PK activity is solely mediated by chromosomal breaks, as DNA-PK is also activated independently of chromosomal breaks by ROS (Li et al., 2014), which increase with aging (Lee et al., 2010). In light of our finding that DNA-PK activity decreases mitochondrial function in skeletal muscle of older mice, activation of DNA-PK by ROS is an intriguing possibility, as mitochondria are the main source of ROS production in metabolic tissues and CR decreases ROS (Sohal and Weindruch, 1996).
The most prominent phenotype of SCID mice is lymphocyte deficiency. However, mice deficient in Rag1, which functions in the same pathway as DNA-PK in VDJ rearrangement in lymphocytes (Oettinger et al., 1990), had similar mitochondrial gene expressions and physical fitness to WT mice (Figure S5). Consistent with this, Liu et al. (2015) reported that Rag1−/− mice are not leaner than WT mice on HFD. Taken together, these findings indicate that the lymphocyte deficiency does not play a significant role in the metabolic phenotype of SCID mice. Nevertheless, we acknowledge that some of the changes seen in SCID tissues may be due to tissue cross-talk. For example, we cannot rule out the possibility that some phenotype in SCID skeletal muscle may have been caused by decreased adiposity.
Our finding that SCID mutation or treatment with DNA-PK inhibitor protects against aging-associated decline in metabolism and physical fitness without increasing DSBs is at odds with the observation that DNA-PK−/− mice undergo accelerated aging (Espejel et al., 2004). The most likely explanation for this paradox is that the SCID mutation is leaky and the DNA-PK inhibitor, like most inhibitor drugs, is a partial inhibitor at physiological doses. The residual DNA-PK activity may be sufficient to protect against the level of DNA DSBs generated naturally. It is interesting that old SCID muscle actually has lower γ-H2AX signal than WT muscle (Figure S3). One possibility is that higher AMPK activity and PGC-1α expression in DNA-PK-deficient p2x receptors reduce ROS, a source of DNA breaks. In the non-lymphoid cells, the Mre11-Rad50-Nbs1 complex may further contribute to NHEJ (Rass et al., 2009) in a redundant manner.
In addition to the increase in DNA DSBs, another change that occurs with aging is the decline of NAD+, the cofactor for Sirt1 (Imai and Guarente, 2014). As Sirt1 plays an important role in mitochondrial function and energy metabolism, the decline in NAD+ levels may contribute to mitochondrial and metabolic dysfunction. The decline during aging may be caused by DNA-PK-mediated inhibition of AMPK because AMPK promotes the synthesis of NAD+ by regulating the NAD+ biosynthetic enzyme Nampt (Fulco et al., 2008).
The biochemical function of HSP90 is to fold metastable proteins. One of the important concepts to come out of the HSP90 field is that it is a capacitor of phenotypic variation (Queitsch et al., 2002). That is, by acting as a buffer, HSP90 allows mutations or polymorphisms (inherited or acquired) to accumulate unseen, free from the pressures of natural selection. As demand for HSP90 function increases with stress, HSP90 is no longer able to shield these mutations, leading to a diverse array of defects. Adapting this concept to our work, HSP90α T5,7 phosphorylation, combined with the aging-associated stress, might decrease the ability of HSP90α to provide the necessary buffering at older age, at least in skeletal muscle (Figure 7C). As a consequence, chaperone function may be decreased, and mutations or polymorphisms may be exposed at older age. In this scenario, aging and aging-associated diseases may partly be a manifestation of the exposure of these mutations or polymorphisms.
Acknowledgments This work was supported by the Intramural Research Program of the National Heart Lung and Blood Institute in the National Institutes of Health. E.D.A. was supported by NIH grant HL73167. The LCR-HCR rat model system was funded by Office of Research Infrastructure Programs grant P40OD021331 (to L.G.K. and S.L.B.) from the NIH. This research was also supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), which is funded by the Ministry of Health & Welfare of the Republic of Korea (grant number HI14C1176). We acknowledge the expert care of the rat colony provided by Molly Kalahar and Lori Heckenkamp. L.G.K. ([email protected]) or S.L.B. ([email protected]) may be contacted for information on the LCR and HCR rats; these rat models are maintained as an international resource with support from the Department of Anesthesiology at the University of Michigan in Ann Arbor, Michigan. We thank Benoit Viollet for AMPKα2-KO mice, Dalton Saunders for his help with drug studies, and Zu-Xi Yu for assistance with electron microscopy. We also thank Adam Weidenhammer and Yiying Tsai for their technical assistance.