InteractiveFly: GeneBrief

ATP synthase, subunit d: Biological Overview | References

Diet composition is a critical determinant of lifespan, and nutrient imbalance is detrimental to health. However, how nutrients interact with genetic factors to modulate lifespan remains elusive. This study investigated how diet composition influences mitochondrial ATP synthase subunit d (ATPsyn-d) in modulating lifespan in Drosophila. ATPsyn-d knockdown extends lifespan in females fed low carbohydrate-to-protein (C:P) diets but not the high C:P ratio diet. This extension is associated with increased resistance to oxidative stress; transcriptional changes in metabolism, proteostasis, and immune genes; reduced protein damage and aggregation, and reduced phosphorylation of S6K and ERK in TOR and mitogen-activated protein kinase (MAPK) signaling, respectively. ATPsyn-d knockdown did not extend lifespan in females with reduced TOR signaling induced genetically by Tsc2 overexpression or pharmacologically by rapamycin. These data reveal a link among diet, mitochondria, and MAPK and TOR signaling in aging and stresses the importance of considering genetic background and diet composition in implementing interventions for promoting healthy aging (Sun, 2014).

Dietary nutrients are among the most critical environmental factors that modulate healthspan and lifespan. Nutrient imbalance is a major risk factor to human health and common among old people. Dietary restriction (DR), by reducing the amount of all or specific nutrients, is a potent nongenetic intervention that promotes longevity in many species. In general, protein restriction is more effective in influencing lifespan than sugar or calorie restriction in Drosophila. However, increasing evidence indicates that the composition of dietary nutrients, such as carbohydrate-to-protein (C:P) ratio, is more critical than individual nutrients in affecting health and lifespan. Optimal lifespan peaks at the C:P ratio 16:1 in Drosophila and 9:1 in Mexican fruit fly (Carey, 2008; Lee, 2008). A recent study in mice shows that lifespan is primarily regulated by the C:P ratio in the diet and tends to be longer with higher C:P ratios (Solon-Biet, 2014). Diet composition is also critical for DR to promote longevity in nonhuman primate rhesus monkeys. Two major nutrient-sensing pathways are known to modulate lifespan. One is target-of-rapamycin (TOR) signaling that mostly senses cellular amino acid content and the other is insulin/insulin-like signaling that primarily responds to circulating glucose and energy levels. Excessive carbohydrate and protein intake both contribute to development of insulin resistance and diabetes in animal models and humans. Dietary macronutrients, such as sugar, protein, and fat, may interact with each other to influence nutrient-sensing pathways and consequently health outcome. It is thus critical to take into account diet composition in elucidating molecular mechanisms of aging and in developing effective interventions for promoting healthy aging (Sun, 2014).

Aging is associated with transcriptional and translational changes in many genes and proteins. Some age-related changes are evolutionarily conserved, and many function in nutrient metabolism, such as mitochondrial electron transfer chain (ETC) genes, many of which are downregulated with age in worms, flies, rodents, and humans. Knocking down ETC genes affects lifespan in yeast, worms, and flies (Copeland, 2009; Kaeberlein, 2005; Lee, 2003; Zid, 2009). Mitochondrial genes also play a key role in numerous age-related diseases, such as Parkinson's and Alzheimer's disease. However, how mitochondrial genes interact with nutrients to modulate lifespan and health-span remains incompletely elucidated. Understanding gene-environment interactions will be a key to tackle aging and age-related diseases (Sun, 2014).

ATP synthase subunit d (ATPsyn-d) is a component of ATP synthase, ETC complex V (Wallace, 2005), and is known to modulate lifespan in C. elegans (Hansen, 2005). How ATPsyn-d modulates lifespan and whether it functions in modulating lifespan in other species remain to be determined (Sun, 2014).

Given the importance of nutrients as environmental factors in modulating lifespan, this study has investigated whether and how ATPsyn-d interacts with dietary macronutrients to modulate lifespan in Drosophila. ATPsyn-d was found to interact with dietary macronutrients to influence accumulation of oxidative damage and protein aggregates; resistance to oxidative stress; and expression of numerous genes involved in metabolism, proteolysis, and innate immune response and more importantly to modulate lifespan. Moreover, ATPsyn-d affects mitogen-activated protein (MAP) kinase (MAPK) signaling and genetically interacts with TOR signaling to influence lifespan of flies in a diet-composition-dependent manner. This study reveals the critical interaction between mitochondrial genes and nutritional factors and the underlying mechanisms involving TOR signaling in modulating lifespan (Sun, 2014).

Considering the essential role of mitochondrial function in metabolism and aging, this study investigated how diet composition influences the function of ATPsynd, a component of mitochondrial ATP synthase, in aging and the underlying mechanisms. ATPsyn-d knockdown extends lifespan in Drosophila under low sugar-high protein diets, but not under a high sugar-low protein diet. Lifespan extension induced by ATPsyn-d knockdown is associated with increased resistance to oxidative stress and improved protein homeostasis. Furthermore, evidence is provided suggesting ATPsyn-d modulates lifespan through genetically interacting with TOR signaling. Knocking down of atp-5, the worm ATPsynd, extends lifespan in C. elegans, along with the current data suggesting a conserved role of ATPsyn-d in modulating lifespan (Hansen, 2005). Altogether, these findings reveal a connection among diet, mitochondrial ATP synthase, and MAPK and TOR signaling in modulating lifespan and shed light on the molecular mechanisms underlying the impact of diet composition on lifespan (Sun, 2014).

The following model is proposed to explain how ATPsyn-d interacts with dietary macronutrients to modulate lifespan, considering the genetic interaction between ATPsyn-d and TOR signaling and the fact that suppression of TOR signaling by altering expression of its components, such as Tsc1/Tsc2, S6K, and 4E-BP, activates autophagy, improves proteostasis, and promotes longevity in high-protein diets, but not necessarily low-protein diets (Kapahi, 2004; Zid, 2009). It is postulated that TOR signaling is regulated by ATPsyn-d and perhaps other mitochondrial proteins. ATPsyn-d knockdown reduces MAPK signaling and probably affects other signaling pathways, which may consequently decrease TOR signaling to extend lifespan in Drosophila fed high-protein diets, such as SY1:9 and SY1:1, but not low protein diets. It is possible that diet-dependent response is due to knockdown of ATPsyn-d protein to different extent by RNAi under different dietary conditions. This is not likely the case. The amount of ATPsyn-d knockdown is not much different between flies on sugar (S) and yeast (Y) SY1:9 and SY9:1, although lifespan is not increased by ATPsyn-d knockdown for flies under SY9:1. Therefore, variations in ATPsyn-d knockdown under current experimental conditions unlikely contribute significantly to diet-dependent lifespan extension. Consistent with this model, ATPsyn-d knockdown increases resistance to acute oxidative stress, reduces cellular oxidative damage, and improves proteostasis in Drosophila. Reduced oxidative damage by ATPsynd knockdown may lead to decreased MAPK signaling, which in turn modulates TOR signaling and proteostasis (Sun, 2014).

Another likely scenario would be that ATPsyn-d and TOR signaling form a positive but vicious feedback loop through MAPK signaling to induce molecular, metabolic, and physiological changes detrimental to lifespan. This vicious cycle can be disrupted by high-C:P diet, knockdown of mitochondrial genes, or suppression of TOR signaling pharmacologically by rapamycin or genetically by Tsc2 overexpression. Consistent with this possibility is that ATPsyn-d knockdown reduces phosphorylation of S6K, a component of TOR signaling, and increases expression of genes involved in maintaining proteostasis and possibly autophagy, which are regulated by TOR signaling. The level of pS6K reflects the strength of TOR signaling, and reduction- of-function mutants of S6K are known to extend lifespan in several species (Zoncu, 2011). ATPsyn-d may genetically interact with TOR signaling to modulate lifespan by influencing protein levels of both S6K and pS6K, although it does not necessarily affect the pS6K/S6K ratio, which may not be a reliable indicator for the strength of TOR signaling under the three SY diets due to the change of S6K level. Furthermore, ATPsyn-d knockdown reduces oxidative damage and polyubiquitinated protein aggregates, which are biomarkers of aging. Rapamycin reduces lifespan extension induced by ATPsyn-d knockdown, which may be due to exacerbation of some detrimental effects of reduced TOR signaling (Zoncu, 2011). However, this observation further supports the connection between ATPsyn-d and TOR signaling. Although both rapamycin and ATPsyn-d knockdown reduce pS6K level, ATPsyn-d knockdown, but not rapamycin, decreases S6K level, suggesting ATPsyn-d knockdown and rapamycin affect TOR signaling in different manners. Further studies are warranted to clarify the epistatic relationship between ATPsyn-d and TOR signaling (Sun, 2014).

Increasing evidence has demonstrated the importance of diet composition or carbohydrate to protein ratio in modulating lifespan and health. Nutrient geometry studies conducted in Drosophila have shown that C:P ratio in the diet is far more important in determining lifespan than calorie content or single macronutrient (Lee, 2008; Skorupa, 2008). A recent tour de force nutrient geometry study in mice has confirmed and expanded the view on the critical role of C:P ratio in regulating lifespan and cardiometabolic health to mammals (Solon-Biet, 2014). An important implication from nutrient geometric studies is that diet composition would have a significant impact on the effectiveness of inventions for promoting healthy aging by genetic, pharmaceutical, or nutraceutical approaches (Sun, 2014).

This indeed is the case, although evidence comes from only a handful of studies. Rapamycin feeding extends lifespan in yeast, worms, flies, and mice (Bjedov, 2010; Zoncu, 2011). Although rapamycin feeding has been shown to extend lifespan of flies under a broad range of diets (Bjedov, 2010), some studies have shown that rapamycin feeding does not extend lifespan in flies under high carbohydrate-low protein diets (Sun, 2012). Supplementation of a nutraceutical derived from cranberry extends lifespan in female flies under a high-C:P-ratio diet, but not a low-C:P-ratio diet (Wang, 2014). Suppression of TOR signaling by overexpression of Tsc1/Tsc2 extends lifespan in flies under relatively higher-protein diets, but not under low-protein diets, although those studies focused on the variation of protein concentration instead of C:P ratio (Kapahi, 2004). Consistent with the link between ATPsyn-d and TOR signaling, ATPsyn-d knockdown extends lifespan in female flies under low sugar-high protein diets, but not high sugar-low protein diet, likely due to the fact that TOR signaling is already low under the high sugar-low protein diet. It was further shown that rapamycin feeding extends lifespan in wild-type female flies, but not in ATPsyn-d knockdown flies (Sun, 2014).

Aging is associated with profound decline in protein homeostasis, and many longevity-related pathways, such as TOR and insulin-like signaling, modulate lifespan through improving proteostasis (Taylor, 2011). Suppression of TOR signaling extends lifespan through decreasing protein translation and increasing autophagy, key processes for maintaining proteostasis (Zoncu, 2011). This study found that ATPsyn-d knockdown reduces the level of 4-HNE (an α, β-unsaturated hydroxyalkenal that is produced by lipid peroxidation) protein adducts; a biomarker for lipid protein oxidation (Tsai, 1998); and the level and aggregation of polyubiquitinated protein, a biomarker for proteostasis and aging (Rana, 2013). ATPsyn-d is a key component of mitochondrial ATP synthase complex. Along with the link between ATPsyn-d and TOR signaling, these data suggest that mitochondrial ATP synthase is critical for maintaining proteostasis and modulating lifespan. This notion is further supported by a recent study showing that α-ketoglutarate, an intermediate in the TCA cycle, suppresses mitochondrial ATP synthase probably by binding to ATP synthase subunit b (ATPsyn-b) and also inhibits TOR signaling to extend lifespan in C. elegans (Chin, 2014). However, it remains to be determined whether suppression of ATP synthase by α-ketoglutarate results in inhibition of TOR signaling in C. elegans or any other species. It is also likely that ATPsyn-d and ATPsyn-b influences ATP synthase and TOR signaling through different mechanisms, because α-ketoglutarate reduces cellular ATP level in C. elegans, whereas ATPsyn-d knockdown does not significantly change or even increase ATP level in Drosophila. This also suggests that lifespan extension is not necessarily associated with decreased ATP level, which is supported by a study in Drosophila showing that any change of ATP level is not correlated with any change of lifespan induced by knockdown of a number of mitochondrial genes (Copeland, 2009). Nevertheless, these studies suggest that ATP synthase is a key and conserved player linking dietary nutrients from TOR signaling to proteostasis and lifespan (Sun, 2014).

Similar to many longevity-related mutants, lifespan extension induced by ATPsyn-d knockdown is associated with reduced oxidative damage and increased resistance to oxidative stress. ATPsyn-d knockdown increases lifespan and resistance to paraquat, an acute oxidative stress response, under SY1:9 or SY1:1. However, ATPsyn-d knockdown increases resistance to paraquat but does not extend lifespan in female flies under SY9:1. In addition, ATPsyn-d knockdown decreases 4-HNE level, an indicator of accumulated oxidative damage, under SY1:9, but not SY1:1. These indicate that the effect of ATPsyn-d knockdown on oxidative damage and lifespan depends on diet composition, suggesting that oxidative stress resistance is at most partially responsible for lifespan extension. This should not be surprising because it is consistent with numerous studies in the literature showing that stress resistance does not always result in lifespan extension despite the strong link between oxidative stress and aging (Salmon, 2010) (Sun, 2014).

The role of mitochondrial genes in modulating lifespan is complex. Knockdown of some electron transfer chain (ETC) genes increases lifespan whereas knockdown of others decreases or does not alter lifespan in C. elegans and Drosophila (Copeland, 2009; Lee, 2003; Zid, 2009). This study reveals another layer of complexity regarding the role of ETC genes in lifespan modulation, namely the impact of diet composition. These findings indicate that ATPsyn-d knockdown promotes longevity at least partially through TOR signaling. TOR signaling senses cellular amino acid content and regulates numerous biological processes, including translation, autophagy, and lifespan (Zoncu, 2011). 4E-BP, a translational repressor in TOR signaling, mediates lifespan extension induced by dietary restriction (Sun, 2014).

Activated 4E-BP suppresses general translation but selectively increases translation of some mitochondrial ETC genes (Zid, 2009), the latter of which results in increased mitochondrial biogenesis and potentially lifespan. Lifespan extension induced by dietary restriction is suppressed by knocking down ETC genes regulated by 4E-BP. The findings by Zid suggest that increased protein expression of some ETC genes is associated with lifespan extension induced by dietary restriction. However, unlike those ETC genes, ATPsyn-d knockdown extends instead of decreases lifespan under high-protein diets. Therefore, it is likely that ETC genes can be categorized into two groups: one selectively upregulated by activated 4E-BP and the other insensitive to activated 4E-BP, the latter of which may include ATPsyn-d. The two groups of ETC genes may interact with dietary macronutrients to modulate lifespan perhaps through different modes of action. Future studies are warranted to investigate the dichotomous role of translation of ETC genes in modulating lifespan (Sun, 2014).

Search PubMed for articles about Drosophila ATPsyn-d

Bjedov, I., Toivonen, J. M., Kerr, F., Slack, C., Jacobson, J., Foley, A. and Partridge, L. (2010). Mechanisms of life span extension by rapamycin in the fruit fly Drosophila melanogaster. Cell Metab 11: 35-46. PubMed ID: 20074526

Carey, J. R., Harshman, L. G., Liedo, P., Muller, H. G., Wang, J. L. and Zhang, Z. (2008). Longevity-fertility trade-offs in the tephritid fruit fly, Anastrepha ludens, across dietary-restriction gradients. Aging Cell 7: 470-477. PubMed ID: 18346215

Chin, R. M., et al. (2014). The metabolite alpha-ketoglutarate extends lifespan by inhibiting ATP synthase and TOR. Nature 510: 397-401. PubMed ID: 24828042

Copeland, J. M., Cho, J., Lo, T., Jr., Hur, J. H., Bahadorani, S., Arabyan, T., Rabie, J., Soh, J. and Walker, D. W. (2009). Extension of Drosophila life span by RNAi of the mitochondrial respiratory chain. Curr Biol 19: 1591-1598. PubMed ID: 19747824

Kaeberlein, M., Powers, R. W., 3rd, Steffen, K. K., Westman, E. A., Hu, D., Dang, N., Kerr, E. O., Kirkland, K. T., Fields, S. and Kennedy, B. K. (2005). Regulation of yeast replicative life span by TOR and Sch9 in response to nutrients. Science 310: 1193-1196. PubMed ID: 16293764

Hansen, M., Hsu, A. L., Dillin, A. and Kenyon, C. (2005). New genes tied to endocrine, metabolic, and dietary regulation of lifespan from a Caenorhabditis elegans genomic RNAi screen. PLoS Genet 1: 119-128. PubMed ID: 16103914

Kapahi, P., Zid, B. M., Harper, T., Koslover, D., Sapin, V. and Benzer, S. (2004). Regulation of lifespan in Drosophila by modulation of genes in the TOR signaling pathway. Curr Biol 14: 885-890. PubMed ID: 15186745

Lee, C. K., Weindruch, R. and Prolla, T. A. (2000). Gene-expression profile of the ageing brain in mice. Nat Genet 25: 294-297. PubMed ID: 10888876

Lee, K. P., Simpson, S. J., Clissold, F. J., Brooks, R., Ballard, J. W., Taylor, P. W., Soran, N. and Raubenheimer, D. (2008). Lifespan and reproduction in Drosophila: New insights from nutritional geometry. Proc Natl Acad Sci U S A 105: 2498-2503. PubMed ID: 18268352

Lee, S. S., Lee, R. Y., Fraser, A. G., Kamath, R. S., Ahringer, J. and Ruvkun, G. (2003). A systematic RNAi screen identifies a critical role for mitochondria in C. elegans longevity. Nat Genet 33: 40-48. PubMed ID: 12447374

Rana, A., Rera, M. and Walker, D. W. (2013). Parkin overexpression during aging reduces proteotoxicity, alters mitochondrial dynamics, and extends lifespan. Proc Natl Acad Sci U S A 110: 8638-8643. PubMed ID: 23650379

Skorupa, D. A., Dervisefendic, A., Zwiener, J. and Pletcher, S. D. (2008). Dietary composition specifies consumption, obesity, and lifespan in Drosophila melanogaster. Aging Cell 7: 478-490. PubMed ID: 18485125

Solon-Biet, S. M., McMahon, A. C., Ballard, J. W., Ruohonen, K., Wu, L. E., Cogger, V. C., Warren, A., Huang, X., Pichaud, N., Melvin, R. G., Gokarn, R., Khalil, M., Turner, N., Cooney, G. J., Sinclair, D. A., Raubenheimer, D., Le Couteur, D. G. and Simpson, S. J. (2014). The ratio of macronutrients, not caloric intake, dictates cardiometabolic health, aging, and longevity in ad libitum-fed mice. Cell Metab 19: 418-430. PubMed ID: 24606899

Sun, X., Komatsu, T., Lim, J., Laslo, M., Yolitz, J., Wang, C., Poirier, L., Alberico, T. and Zou, S. (2012). Nutrient-dependent requirement for SOD1 in lifespan extension by protein restriction in Drosophila melanogaster. Aging Cell 11: 783-793. PubMed ID: 22672579

Sun, X., Wheeler, C. T., Yolitz, J., Laslo, M., Alberico, T., Sun, Y., Song, Q. and Zou, S. (2014). Mitochondrial ATP synthase subunit interacts with TOR signaling to modulate protein homeostasis and lifespan in Drosophila. Cell Rep 8(6): 1781-92. PubMed ID: 25220459

Taylor, R. C. and Dillin, A. (2011). Aging as an event of proteostasis collapse. Cold Spring Harb Perspect Biol 3. PubMed ID: 21441594

Wallace, D. C. (2005). A mitochondrial paradigm of metabolic and degenerative diseases, aging, and cancer: a dawn for evolutionary medicine. Annu Rev Genet 39: 359-407. PubMed ID: 16285865

Wang, C., Yolitz, J., Alberico, T., Laslo, M., Sun, Y., Wheeler, C. T., Sun, X. and Zou, S. (2014). Cranberry interacts with dietary macronutrients to promote healthy aging in Drosophila. J Gerontol A Biol Sci Med Sci 69: 945-954. PubMed ID: 24149429

Zid, B. M., Rogers, A. N., Katewa, S. D., Vargas, M. A., Kolipinski, M. C., Lu, T. A., Benzer, S. and Kapahi, P. (2009). 4E-BP extends lifespan upon dietary restriction by enhancing mitochondrial activity in Drosophila. Cell 139: 149-160. PubMed ID: 19804760

Zoncu, R., Bar-Peled, L., Efeyan, A., Wang, S., Sancak, Y. and Sabatini, D. M. (2011). mTORC1 senses lysosomal amino acids through an inside-out mechanism that requires the vacuolar H(+)-ATPase. Science 334: 678-683. PubMed ID: 22053050

date revised: 30 September 2014

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