Current Biology Vol 24 No 5 R206
effects of visual content on eye guidance and lexical access during reading. PLoS One 7, e41766. 18. Sheridan, H., and Reingold, E.M. (2013). A further examination of the lexical-processing stages hypothesized by the E-Z Reader model. Attent. Percept. Psychophys. 75, 407–414.
19. Rayner, K. (1975). The perceptual span and peripheral cues during reading. Cogn. Psychol. 7, 65–81. 20. Kanonidou, E., Proudlock, F.A., and Gottlob, I. (2010). Reading strategies in mild to moderate strabismic amblyopia: an eye movement investigation. Invest. Ophthalmol. Vis. 51, 3502–3508.
Aging: It’s SIRTainly Possible to Restore Mitochondrial Dysfunction Mitochondrial dysfunction is strongly associated with aging. A recent study shows that reduced nuclear SIRT1 activity initiates age-related mitochondrial decline through a signaling pathway that perturbs expression of genes encoded by mitochondrial DNA. This reversible pathway has potential anti-aging therapeutic value. Brooke E. Christian1 and Gerald S. Shadel1,2,* Mitochondria are complex cellular organelles that produce ATP through the process of oxidative phosphorylation (OXPHOS), and there is strong evidence supporting a causative role for dysfunctional mitochondria in aging [1,2]. However, the underlying mechanism of this age-dependent mitochondrial demise remains unclear. One explanation that derives from the marriage of the so-called ‘mitochondrial’ and ‘free radical’ theories of aging [3,4] is that reactive oxygen species (ROS) produced aberrantly during mitochondrial electron transport damage mitochondrial components, including the resident mitochondrial DNA (mtDNA). Because mtDNA encodes essential OXPHOS protein subunits [5], ROS-mediated mtDNA damage and mutagenesis would therefore disrupt OXPHOS complex assembly and, in turn, lead to enhanced ROS production, which underlies increased mitochondrial damage, cellular oxidative stress, and tissue dysfunction that promote aging [6] (Figure 1). However, this theory is likely an oversimplification, and the critical question of which age-dependent cellular changes precipitate the initial decline in mitochondrial OXPHOS remains unanswered. Now, a recent study by Sinclair and colleagues [7] has shed new light on this issue by showing that alterations in nuclear gene expression due to reduced activity
of the deacetylase SIRT1 may be the culprit. Importantly, they also show that, by activating SIRT1 enzymatic activity (via increasing NAD+ levels), age-dependent mitochondrial dysfunction, and the pathology it promotes, could be reversed. To understand how SIRT1-dependent changes in nuclear gene expression can affect mitochondria, it is important to recall that a vast majority of the w1,500 mitochondrial proteins are encoded by nuclear genes and imported after synthesis in the cytoplasm. That is, only 13 (of the w80) essential OXPHOS subunits are encoded by the maternally inherited mammalian mtDNA, along with 2 rRNAs and 22 tRNAs needed to translate the mtDNA-encoded mRNAs on dedicated mitochondrial ribosomes [5]. Furthermore, all protein factors needed for expression and maintenance of mtDNA are nuclear gene products [8]. This situation dictates the necessity for signaling back-and-forth between mitochondria and the nucleus to ensure proper mitochondrial function and homeostasis [9]. In fact, it is miscommunication in one such pathway that Gomes et al. [7] propose is at the heart of mitochondrial OXPHOS failure with age. Specifically, these authors show that, in aging mouse skeletal muscle, the expression of mtDNA-encoded, but not nucleus-encoded, OXPHOS subunits is initially reduced. Most cells contain many copies of the circular 16.5 kb mtDNA molecule and these authors found that aging muscle has significantly reduced mtDNA copy
College of Medicine, Biological Sciences and Psychology, Henry Wellcome Building, University of Leicester, LE1 9HN, UK. E-mail: [email protected] http://dx.doi.org/10.1016/j.cub.2014.01.045
number, but no obvious changes in the amount of mitochondria. The ultimate explanation for this result was that TFAM, a crucial nucleus-encoded transcription factor required for activation of mitochondrial transcription and packaging of mtDNA [5], is misregulated. Intriguingly, these age-related changes in mitochondrial homeostasis were largely recapitulated in SIRT1-deficient muscle and in cultured myoblast cells in which nuclear pools of NAD+ (an enzymatic co-factor for SIRT1) were reduced. Thus, mitochondrial dysfunction is perhaps not initiated by ROS damage, but rather by a defect in nuclear–mitochondrial signaling (i.e. decreased expression of the nuclear TFAM gene due to loss of nuclear SIRT1 activity; Figure 1). As discussed by Gomes et al. [7], an established mechanism by which SIRT1 regulates mitochondria is through deacetylation of PGC-1a, a nuclear transcriptional co-activator that stimulates expression of nuclear genes involved in mitochondrial biogenesis, including TFAM [10]. Perhaps surprisingly, this is not the pathway that is disturbed initially in aging skeletal muscle. Instead, Gomes et al. [7] provide strong evidence that the SIRT1-dependent disruption of TFAM gene expression is mediated by stabilization of hypoxia-inducible factor 1 alpha (HIF-1a), which in turn inhibits the ability of c-Myc to activate the TFAM promoter. Interestingly, this did not involve modification of the acetylation status of HIF-1a, but rather downregulation of its negative regulator, the von Hippel-Landau protein (VHL). Control of mitochondrial biogenesis, TFAM, and other critical components of the mitochondrial transcription machinery (e.g. mitochondrial RNA polymerase and the mitochondrial ribosomal protein L12) by c-Myc has been documented in previous studies [5,11,12]. Thus, it is likely that TFAM is not the only target of the proposed age-dependent SIRT1–HIF-1a –c-Myc pathway that
Dispatch R207
NAD+ SIRT1 activity HIF-1 c-Myc TFAM
Phase I: Age-related disruption in nucleus–mitochondria communication Mitochondrion
TFAM
TFAM Mitochondrial gene expression OXPHOS activity
Nucleus
Phase II: Progressive OXPHOS decline Nuclear OXPHOS gene expression
Global disruption in OXPHOS assembly ROS production
Phase III: Vicious cycle of oxidative damage
ROS
OXPHOS damage
ROS
Phase IV: Cellular oxidative stress and age-related pathology Current Biology
Figure 1. Hypothesized time course of progression toward age-related mitochondrial dysfunction, oxidative stress and tissue pathology. The image shows the nucleus and a mitochondrion of a muscle cell undergoing age-related changes. Based on the study by Gomes et al. [7], the initial decline in nuclear NAD+ leads to a reduction of SIRT1 deacetylase activity, stabilization of HIF-1a, inactivation of c-Myc, and decreased expression of the TFAM gene (Phase I). Fewer TFAM molecules (and likely other mitochondrial regulatory factors as well) in mitochondria cause reduced mitochondrial gene expression and OXPHOS activity (Phase I). Following this initial OXPHOS decline, reduced expression of nuclear-encoded OXPHOS subunits ensues, leading to a more severe disruption in OXPHOS complex assembly and increased ROS production (Phase II). This sets into play a vicious cycle in which ROS damage mtDNA and OXPHOS complexes, resulting in yet more mitochondrial ROS production (Phase III). Eventually this situation escalates to cause global cellular oxidative stress that promotes age-related tissue dysfunction and pathology (Phase IV).
is causing the observed decline in mtDNA and mtDNA-encoded OXPHOS subunits. In fact, Gomes et al. [7] did not directly address whether TFAM protein is downregulated in aging skeletal muscle as predicted. In addition, it remains unclear whether it is mtDNA depletion per se, the loss of transcriptional functions of TFAM, or both of these mechanisms that underlies the observed decrease in mtDNA-encoded OXPHOS subunits. This is important when considering previous conflicting results on
age-dependent changes to TFAM levels and mtDNA copy number in mammalian tissues [13,14]. Thus, while the Gomes et al. [7] study reports mtDNA depletion with age in muscle (driven by TFAM reduction) and clearly describes how disruption of a newly identified signaling pathway can account for age-dependent mitochondrial dysfunction, there remains room for additional verification of this provocative model and delineation of the precise mechanisms involved.
A key finding by Gomes et al. [7] that is of potential anti-aging therapeutic value is that the reduction in mitochondrial OXPHOS function and several features of muscle pathology of 22-month-old mice are reversed by a one-week treatment with nicotinamide mononucleotide (NMN), which promotes NAD+ biosynthesis. NMN has been previously shown to be effective in reversing age-related type 2 diabetes [15]. Together, these studies suggest that decreased mitochondrial OXPHOS may underlie multiple age-related pathologies and that upregulating SIRT1 activity may help reverse this through the newly discovered pathway. Interestingly, caloric restriction also reversed the age-dependent increase in HIF-1a and the decline in VHL function, consistent with the ability of caloric restriction to increase SIRT1 and NAD+ in mammals and increase lifespan in many organisms [16]. While Gomes et al. [7] demonstrate that many of the effects of NMN supplementation and caloric restriction in muscle are likely mediated through this nuclear SIRT1-dependent pathway, NMN supplementation would presumably increase other subcellular pools of NAD+ as well. Several sirtuins are located in mitochondria and SIRT3, in particular, has been shown to mediate some beneficial effects of caloric restriction and regulate TFAM acetylation [17]. Thus, an intriguing possibility is that NMN supplementation and/or caloric restriction are working through the modulation of both mitochondrial and nuclear NAD+ pools to achieve maximal benefit. Conversely, if modulation of only the nuclear NAD+ pool is beneficial, global NMN supplementation could have unintended side effects. For example, altering the mitochondrial NAD+:NADH ratio can directly affect OXPHOS activity and mitochondrial ROS formation [18], which may impact ROS signaling or promote oxidative stress. Placing the Gomes et al. [7] study in the context of the mitochondrial and free radical theories of aging provides a nice explanation of how mitochondrial dysfunction in aging, at least in muscle, is initiated. That is, reduced NAD+ levels in the nucleus promote a decline in expression of mtDNA-encoded subunits by the proposed SIRT1–HIF-1a–c-Myc–TFAM pathway. One might predict that this would cause one of several well-documented
Current Biology Vol 24 No 5 R208
mitochondrial stress responses (e.g. the mitochondrial unfolded protein response) or an increase in ROS production. However, Gomes et al. [7] conclude that neither of these is involved in the muscle responses they observe. Nonetheless, they do report a later significant increase in ROS after AMP kinase is activated and when both mtDNA-encoded and nuclear-encoded OXPHOS subunits are declining (the so-called second, PGC-1a-dependent phase of the response). We offer an extended interpretation of their results that highlights a potential involvement of ROS (Figure 1). That is, we agree that nuclear NAD+ depletion could initiate OXPHOS dysfunction by the SIRT1–HIF-1a–c-Myc–TFAM pathway proposed, but posit that this eventually leads to increased ROS production when the second phase of the response commences, characterized by a global disruption of OXPHOS gene expression caused by a reduction in both mtDNA- and nuclear-encoded OXPHOS subunits. This, coupled with the pseudo-hypoxic state (i.e. upregulation of HIF-1a) as discussed by Gomes et al. [7], likely promotes increased ROS production, oxidative stress, and age-related pathology. Clearly, the role of mitochondria in aging requires much more effort to resolve. We believe the Gomes et al. [7] study provides strong evidence for a novel initiating event for eventual mitochondrial decline that falls in line with the mitochondrial free radical theory of aging (Figure 1). But, at the same time, there is substantial evidence that mitochondrial stress (e.g. ROS and electron transport
chain dysfunction) can extend lifespan by engaging retrograde signaling pathways that signal back to the nucleus [19,20], some of which involve HIF-1a. The excitement and challenge going forward will be to determine precise mechanisms through which mitochondrial dysfunction and signaling can have both beneficial and deleterious effects on aging, and whether these pathways can be augmented or prevented to extend human healthspan and longevity. References 1. Shigenaga, M.K., Hagen, T.M., and Ames, B.N. (1994). Oxidative damage and mitochondrial decay in aging. Proc. Natl. Acad. Sci. USA 91, 10771–10778. 2. Balaban, R.S., Nemoto, S., and Finkel, T. (2005). Mitochondria, oxidants, and aging. Cell 120, 483–495. 3. Harman, D. (1992). Free radical theory of aging. Mutat. Res. 275, 257–266. 4. Miquel, J., Economos, A.C., Fleming, J., and Johnson, J.E., Jr. (1980). Mitochondrial role in cell aging. Exp. Gerontol. 15, 575–591. 5. Bonawitz, N.D., Clayton, D.A., and Shadel, G.S. (2006). Initiation and beyond: multiple functions of the human mitochondrial transcription machinery. Mol. Cell 24, 813–825. 6. Mandavilli, B.S., Santos, J.H., and Van Houten, B. (2002). Mitochondrial DNA repair and aging. Mutat. Res. 509, 127–151. 7. Gomes, A.P., Price, N.L., Ling, A.J., Moslehi, J.J., Montgomery, M.K., Rajman, L., White, J.P., Teodoro, J.S., Wrann, C.D., Hubbard, B.P., et al. (2013). Declining NAD(+) induces a pseudohypoxic state disrupting nuclear-mitochondrial communication during aging. Cell 155, 1624–1638. 8. Bestwick, M.L., and Shadel, G.S. (2013). Accessorizing the human mitochondrial transcription machinery. Trends Biochem. Sci. 38, 283–291. 9. Butow, R.A., and Avadhani, N.G. (2004). Mitochondrial signaling: the retrograde response. Mol. Cell 14, 1–15. 10. Wu, Z., Puigserver, P., Andersson, U., Zhang, C., Adelmant, G., Mootha, V., Troy, A., Cinti, S., Lowell, B., Scarpulla, R.C., et al. (1999). Mechanisms controlling mitochondrial biogenesis and respiration through the thermogenic coactivator PGC-1. Cell 98, 115–124.
11. Li, F., Wang, Y., Zeller, K.I., Potter, J.J., Wonsey, D.R., O’Donnell, K.A., Kim, J.W., Yustein, J.T., Lee, L.A., and Dang, C.V. (2005). Myc stimulates nuclearly encoded mitochondrial genes and mitochondrial biogenesis. Mol. Cell Biol. 25, 6225–6234. 12. Surovtseva, Y.V., Shutt, T.E., Cotney, J., Cimen, H., Chen, S.Y., Koc, E.C., and Shadel, G.S. (2011). Mitochondrial ribosomal protein L12 selectively associates with human mitochondrial RNA polymerase to activate transcription. Proc. Natl. Acad. Sci. USA 108, 17921–17926. 13. Kazachkova, N., Ramos, A., Santos, C., and Lima, M. (2013). Mitochondrial DNA damage patterns and aging: revising the evidences for humans and mice. Aging Dis. 4, 337–350. 14. Masuyama, M., Iida, R., Takatsuka, H., Yasuda, T., and Matsuki, T. (2005). Quantitative change in mitochondrial DNA content in various mouse tissues during aging. Biochim. Biophys. Acta 1723, 302–308. 15. Yoshino, J., Mills, K.F., Yoon, M.J., and Imai, S. (2011). Nicotinamide mononucleotide, a key NAD(+) intermediate, treats the pathophysiology of diet- and age-induced diabetes in mice. Cell Metab. 14, 528–536. 16. Cohen, H.Y., Miller, C., Bitterman, K.J., Wall, N.R., Hekking, B., Kessler, B., Howitz, K.T., Gorospe, M., de Cabo, R., and Sinclair, D.A. (2004). Calorie restriction promotes mammalian cell survival by inducing the SIRT1 deacetylase. Science 305, 390–392. 17. Hebert, A.S., Dittenhafer-Reed, K.E., Yu, W., Bailey, D.J., Selen, E.S., Boersma, M.D., Carson, J.J., Tonelli, M., Balloon, A.J., Higbee, A.J., et al. (2013). Calorie restriction and SIRT3 trigger global reprogramming of the mitochondrial protein acetylome. Mol. Cell 49, 186–199. 18. Murphy, M.P. (2009). How mitochondria produce reactive oxygen species. Biochem. J. 417, 1–13. 19. Ristow, M., and Schmeisser, S. (2011). Extending life span by increasing oxidative stress. Free Radic. Biol. Med. 51, 327–336. 20. Schroeder, E.A., Raimundo, N., and Shadel, G.S. (2013). Epigenetic silencing mediates mitochondria stress-induced longevity. Cell Metab. 17, 954–964.
1Departments of Pathology, Yale School of Medicine, New Haven, CT 06520, USA. 2Genetics, Yale School of Medicine, New Haven, CT 06520, USA. *E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.01.027
effects of visual content on eye guidance and lexical access during reading. PLoS One 7, e41766. 18. Sheridan, H., and Reingold, E.M. (2013). A further examination of the lexical-processing stages hypothesized by the E-Z Reader model. Attent. Percept. Psychophys. 75, 407–414.
19. Rayner, K. (1975). The perceptual span and peripheral cues during reading. Cogn. Psychol. 7, 65–81. 20. Kanonidou, E., Proudlock, F.A., and Gottlob, I. (2010). Reading strategies in mild to moderate strabismic amblyopia: an eye movement investigation. Invest. Ophthalmol. Vis. 51, 3502–3508.
Aging: It’s SIRTainly Possible to Restore Mitochondrial Dysfunction Mitochondrial dysfunction is strongly associated with aging. A recent study shows that reduced nuclear SIRT1 activity initiates age-related mitochondrial decline through a signaling pathway that perturbs expression of genes encoded by mitochondrial DNA. This reversible pathway has potential anti-aging therapeutic value. Brooke E. Christian1 and Gerald S. Shadel1,2,* Mitochondria are complex cellular organelles that produce ATP through the process of oxidative phosphorylation (OXPHOS), and there is strong evidence supporting a causative role for dysfunctional mitochondria in aging [1,2]. However, the underlying mechanism of this age-dependent mitochondrial demise remains unclear. One explanation that derives from the marriage of the so-called ‘mitochondrial’ and ‘free radical’ theories of aging [3,4] is that reactive oxygen species (ROS) produced aberrantly during mitochondrial electron transport damage mitochondrial components, including the resident mitochondrial DNA (mtDNA). Because mtDNA encodes essential OXPHOS protein subunits [5], ROS-mediated mtDNA damage and mutagenesis would therefore disrupt OXPHOS complex assembly and, in turn, lead to enhanced ROS production, which underlies increased mitochondrial damage, cellular oxidative stress, and tissue dysfunction that promote aging [6] (Figure 1). However, this theory is likely an oversimplification, and the critical question of which age-dependent cellular changes precipitate the initial decline in mitochondrial OXPHOS remains unanswered. Now, a recent study by Sinclair and colleagues [7] has shed new light on this issue by showing that alterations in nuclear gene expression due to reduced activity
of the deacetylase SIRT1 may be the culprit. Importantly, they also show that, by activating SIRT1 enzymatic activity (via increasing NAD+ levels), age-dependent mitochondrial dysfunction, and the pathology it promotes, could be reversed. To understand how SIRT1-dependent changes in nuclear gene expression can affect mitochondria, it is important to recall that a vast majority of the w1,500 mitochondrial proteins are encoded by nuclear genes and imported after synthesis in the cytoplasm. That is, only 13 (of the w80) essential OXPHOS subunits are encoded by the maternally inherited mammalian mtDNA, along with 2 rRNAs and 22 tRNAs needed to translate the mtDNA-encoded mRNAs on dedicated mitochondrial ribosomes [5]. Furthermore, all protein factors needed for expression and maintenance of mtDNA are nuclear gene products [8]. This situation dictates the necessity for signaling back-and-forth between mitochondria and the nucleus to ensure proper mitochondrial function and homeostasis [9]. In fact, it is miscommunication in one such pathway that Gomes et al. [7] propose is at the heart of mitochondrial OXPHOS failure with age. Specifically, these authors show that, in aging mouse skeletal muscle, the expression of mtDNA-encoded, but not nucleus-encoded, OXPHOS subunits is initially reduced. Most cells contain many copies of the circular 16.5 kb mtDNA molecule and these authors found that aging muscle has significantly reduced mtDNA copy
College of Medicine, Biological Sciences and Psychology, Henry Wellcome Building, University of Leicester, LE1 9HN, UK. E-mail: [email protected] http://dx.doi.org/10.1016/j.cub.2014.01.045
number, but no obvious changes in the amount of mitochondria. The ultimate explanation for this result was that TFAM, a crucial nucleus-encoded transcription factor required for activation of mitochondrial transcription and packaging of mtDNA [5], is misregulated. Intriguingly, these age-related changes in mitochondrial homeostasis were largely recapitulated in SIRT1-deficient muscle and in cultured myoblast cells in which nuclear pools of NAD+ (an enzymatic co-factor for SIRT1) were reduced. Thus, mitochondrial dysfunction is perhaps not initiated by ROS damage, but rather by a defect in nuclear–mitochondrial signaling (i.e. decreased expression of the nuclear TFAM gene due to loss of nuclear SIRT1 activity; Figure 1). As discussed by Gomes et al. [7], an established mechanism by which SIRT1 regulates mitochondria is through deacetylation of PGC-1a, a nuclear transcriptional co-activator that stimulates expression of nuclear genes involved in mitochondrial biogenesis, including TFAM [10]. Perhaps surprisingly, this is not the pathway that is disturbed initially in aging skeletal muscle. Instead, Gomes et al. [7] provide strong evidence that the SIRT1-dependent disruption of TFAM gene expression is mediated by stabilization of hypoxia-inducible factor 1 alpha (HIF-1a), which in turn inhibits the ability of c-Myc to activate the TFAM promoter. Interestingly, this did not involve modification of the acetylation status of HIF-1a, but rather downregulation of its negative regulator, the von Hippel-Landau protein (VHL). Control of mitochondrial biogenesis, TFAM, and other critical components of the mitochondrial transcription machinery (e.g. mitochondrial RNA polymerase and the mitochondrial ribosomal protein L12) by c-Myc has been documented in previous studies [5,11,12]. Thus, it is likely that TFAM is not the only target of the proposed age-dependent SIRT1–HIF-1a –c-Myc pathway that
Dispatch R207
NAD+ SIRT1 activity HIF-1 c-Myc TFAM
Phase I: Age-related disruption in nucleus–mitochondria communication Mitochondrion
TFAM
TFAM Mitochondrial gene expression OXPHOS activity
Nucleus
Phase II: Progressive OXPHOS decline Nuclear OXPHOS gene expression
Global disruption in OXPHOS assembly ROS production
Phase III: Vicious cycle of oxidative damage
ROS
OXPHOS damage
ROS
Phase IV: Cellular oxidative stress and age-related pathology Current Biology
Figure 1. Hypothesized time course of progression toward age-related mitochondrial dysfunction, oxidative stress and tissue pathology. The image shows the nucleus and a mitochondrion of a muscle cell undergoing age-related changes. Based on the study by Gomes et al. [7], the initial decline in nuclear NAD+ leads to a reduction of SIRT1 deacetylase activity, stabilization of HIF-1a, inactivation of c-Myc, and decreased expression of the TFAM gene (Phase I). Fewer TFAM molecules (and likely other mitochondrial regulatory factors as well) in mitochondria cause reduced mitochondrial gene expression and OXPHOS activity (Phase I). Following this initial OXPHOS decline, reduced expression of nuclear-encoded OXPHOS subunits ensues, leading to a more severe disruption in OXPHOS complex assembly and increased ROS production (Phase II). This sets into play a vicious cycle in which ROS damage mtDNA and OXPHOS complexes, resulting in yet more mitochondrial ROS production (Phase III). Eventually this situation escalates to cause global cellular oxidative stress that promotes age-related tissue dysfunction and pathology (Phase IV).
is causing the observed decline in mtDNA and mtDNA-encoded OXPHOS subunits. In fact, Gomes et al. [7] did not directly address whether TFAM protein is downregulated in aging skeletal muscle as predicted. In addition, it remains unclear whether it is mtDNA depletion per se, the loss of transcriptional functions of TFAM, or both of these mechanisms that underlies the observed decrease in mtDNA-encoded OXPHOS subunits. This is important when considering previous conflicting results on
age-dependent changes to TFAM levels and mtDNA copy number in mammalian tissues [13,14]. Thus, while the Gomes et al. [7] study reports mtDNA depletion with age in muscle (driven by TFAM reduction) and clearly describes how disruption of a newly identified signaling pathway can account for age-dependent mitochondrial dysfunction, there remains room for additional verification of this provocative model and delineation of the precise mechanisms involved.
A key finding by Gomes et al. [7] that is of potential anti-aging therapeutic value is that the reduction in mitochondrial OXPHOS function and several features of muscle pathology of 22-month-old mice are reversed by a one-week treatment with nicotinamide mononucleotide (NMN), which promotes NAD+ biosynthesis. NMN has been previously shown to be effective in reversing age-related type 2 diabetes [15]. Together, these studies suggest that decreased mitochondrial OXPHOS may underlie multiple age-related pathologies and that upregulating SIRT1 activity may help reverse this through the newly discovered pathway. Interestingly, caloric restriction also reversed the age-dependent increase in HIF-1a and the decline in VHL function, consistent with the ability of caloric restriction to increase SIRT1 and NAD+ in mammals and increase lifespan in many organisms [16]. While Gomes et al. [7] demonstrate that many of the effects of NMN supplementation and caloric restriction in muscle are likely mediated through this nuclear SIRT1-dependent pathway, NMN supplementation would presumably increase other subcellular pools of NAD+ as well. Several sirtuins are located in mitochondria and SIRT3, in particular, has been shown to mediate some beneficial effects of caloric restriction and regulate TFAM acetylation [17]. Thus, an intriguing possibility is that NMN supplementation and/or caloric restriction are working through the modulation of both mitochondrial and nuclear NAD+ pools to achieve maximal benefit. Conversely, if modulation of only the nuclear NAD+ pool is beneficial, global NMN supplementation could have unintended side effects. For example, altering the mitochondrial NAD+:NADH ratio can directly affect OXPHOS activity and mitochondrial ROS formation [18], which may impact ROS signaling or promote oxidative stress. Placing the Gomes et al. [7] study in the context of the mitochondrial and free radical theories of aging provides a nice explanation of how mitochondrial dysfunction in aging, at least in muscle, is initiated. That is, reduced NAD+ levels in the nucleus promote a decline in expression of mtDNA-encoded subunits by the proposed SIRT1–HIF-1a–c-Myc–TFAM pathway. One might predict that this would cause one of several well-documented
Current Biology Vol 24 No 5 R208
mitochondrial stress responses (e.g. the mitochondrial unfolded protein response) or an increase in ROS production. However, Gomes et al. [7] conclude that neither of these is involved in the muscle responses they observe. Nonetheless, they do report a later significant increase in ROS after AMP kinase is activated and when both mtDNA-encoded and nuclear-encoded OXPHOS subunits are declining (the so-called second, PGC-1a-dependent phase of the response). We offer an extended interpretation of their results that highlights a potential involvement of ROS (Figure 1). That is, we agree that nuclear NAD+ depletion could initiate OXPHOS dysfunction by the SIRT1–HIF-1a–c-Myc–TFAM pathway proposed, but posit that this eventually leads to increased ROS production when the second phase of the response commences, characterized by a global disruption of OXPHOS gene expression caused by a reduction in both mtDNA- and nuclear-encoded OXPHOS subunits. This, coupled with the pseudo-hypoxic state (i.e. upregulation of HIF-1a) as discussed by Gomes et al. [7], likely promotes increased ROS production, oxidative stress, and age-related pathology. Clearly, the role of mitochondria in aging requires much more effort to resolve. We believe the Gomes et al. [7] study provides strong evidence for a novel initiating event for eventual mitochondrial decline that falls in line with the mitochondrial free radical theory of aging (Figure 1). But, at the same time, there is substantial evidence that mitochondrial stress (e.g. ROS and electron transport
chain dysfunction) can extend lifespan by engaging retrograde signaling pathways that signal back to the nucleus [19,20], some of which involve HIF-1a. The excitement and challenge going forward will be to determine precise mechanisms through which mitochondrial dysfunction and signaling can have both beneficial and deleterious effects on aging, and whether these pathways can be augmented or prevented to extend human healthspan and longevity. References 1. Shigenaga, M.K., Hagen, T.M., and Ames, B.N. (1994). Oxidative damage and mitochondrial decay in aging. Proc. Natl. Acad. Sci. USA 91, 10771–10778. 2. Balaban, R.S., Nemoto, S., and Finkel, T. (2005). Mitochondria, oxidants, and aging. Cell 120, 483–495. 3. Harman, D. (1992). Free radical theory of aging. Mutat. Res. 275, 257–266. 4. Miquel, J., Economos, A.C., Fleming, J., and Johnson, J.E., Jr. (1980). Mitochondrial role in cell aging. Exp. Gerontol. 15, 575–591. 5. Bonawitz, N.D., Clayton, D.A., and Shadel, G.S. (2006). Initiation and beyond: multiple functions of the human mitochondrial transcription machinery. Mol. Cell 24, 813–825. 6. Mandavilli, B.S., Santos, J.H., and Van Houten, B. (2002). Mitochondrial DNA repair and aging. Mutat. Res. 509, 127–151. 7. Gomes, A.P., Price, N.L., Ling, A.J., Moslehi, J.J., Montgomery, M.K., Rajman, L., White, J.P., Teodoro, J.S., Wrann, C.D., Hubbard, B.P., et al. (2013). Declining NAD(+) induces a pseudohypoxic state disrupting nuclear-mitochondrial communication during aging. Cell 155, 1624–1638. 8. Bestwick, M.L., and Shadel, G.S. (2013). Accessorizing the human mitochondrial transcription machinery. Trends Biochem. Sci. 38, 283–291. 9. Butow, R.A., and Avadhani, N.G. (2004). Mitochondrial signaling: the retrograde response. Mol. Cell 14, 1–15. 10. Wu, Z., Puigserver, P., Andersson, U., Zhang, C., Adelmant, G., Mootha, V., Troy, A., Cinti, S., Lowell, B., Scarpulla, R.C., et al. (1999). Mechanisms controlling mitochondrial biogenesis and respiration through the thermogenic coactivator PGC-1. Cell 98, 115–124.
11. Li, F., Wang, Y., Zeller, K.I., Potter, J.J., Wonsey, D.R., O’Donnell, K.A., Kim, J.W., Yustein, J.T., Lee, L.A., and Dang, C.V. (2005). Myc stimulates nuclearly encoded mitochondrial genes and mitochondrial biogenesis. Mol. Cell Biol. 25, 6225–6234. 12. Surovtseva, Y.V., Shutt, T.E., Cotney, J., Cimen, H., Chen, S.Y., Koc, E.C., and Shadel, G.S. (2011). Mitochondrial ribosomal protein L12 selectively associates with human mitochondrial RNA polymerase to activate transcription. Proc. Natl. Acad. Sci. USA 108, 17921–17926. 13. Kazachkova, N., Ramos, A., Santos, C., and Lima, M. (2013). Mitochondrial DNA damage patterns and aging: revising the evidences for humans and mice. Aging Dis. 4, 337–350. 14. Masuyama, M., Iida, R., Takatsuka, H., Yasuda, T., and Matsuki, T. (2005). Quantitative change in mitochondrial DNA content in various mouse tissues during aging. Biochim. Biophys. Acta 1723, 302–308. 15. Yoshino, J., Mills, K.F., Yoon, M.J., and Imai, S. (2011). Nicotinamide mononucleotide, a key NAD(+) intermediate, treats the pathophysiology of diet- and age-induced diabetes in mice. Cell Metab. 14, 528–536. 16. Cohen, H.Y., Miller, C., Bitterman, K.J., Wall, N.R., Hekking, B., Kessler, B., Howitz, K.T., Gorospe, M., de Cabo, R., and Sinclair, D.A. (2004). Calorie restriction promotes mammalian cell survival by inducing the SIRT1 deacetylase. Science 305, 390–392. 17. Hebert, A.S., Dittenhafer-Reed, K.E., Yu, W., Bailey, D.J., Selen, E.S., Boersma, M.D., Carson, J.J., Tonelli, M., Balloon, A.J., Higbee, A.J., et al. (2013). Calorie restriction and SIRT3 trigger global reprogramming of the mitochondrial protein acetylome. Mol. Cell 49, 186–199. 18. Murphy, M.P. (2009). How mitochondria produce reactive oxygen species. Biochem. J. 417, 1–13. 19. Ristow, M., and Schmeisser, S. (2011). Extending life span by increasing oxidative stress. Free Radic. Biol. Med. 51, 327–336. 20. Schroeder, E.A., Raimundo, N., and Shadel, G.S. (2013). Epigenetic silencing mediates mitochondria stress-induced longevity. Cell Metab. 17, 954–964.
1Departments of Pathology, Yale School of Medicine, New Haven, CT 06520, USA. 2Genetics, Yale School of Medicine, New Haven, CT 06520, USA. *E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.01.027
