Mechanisms of life span extension by rapamycin in the fruit fly Drosophila melanogaster

Abstract

The target of rapamycin (TOR) pathway is a major nutrient-sensing pathway that, when genetically downregulated, increases life span in evolutionarily diverse organisms including mammals. The central component of this pathway, TOR kinase, is the target of the inhibitory drug rapamycin, a highly specific and well-described drug approved for human use. We show here that feeding rapamycin to adult Drosophila produces the life span extension seen in some TOR mutants. Increase in life span by rapamycin was associated with increased resistance to both starvation and paraquat. Analysis of the underlying mechanisms revealed that rapamycin increased longevity specifically through the TORC1 branch of the TOR pathway, through alterations to both autophagy and translation. Rapamycin could increase life span of weak insulin/Igf signaling (IIS) pathway mutants and of flies with life span maximized by dietary restriction, indicating additional mechanisms.

Figure 1

Rapamycin Treatment of Adult Drosophila Downregulates TOR Activity (A) Western blot analysis of phospho-S6K on whole-fly protein extracts. Flies were sampled after 1, 3, 5, or 12 days of rapamycin treatment at concentrations of 200 or 400 μM. A dose-dependent reduction in phospho-T398-S6K levels was observed and the degree of inhibition increased with longer treatment time. For all western blots, relative band intensity was estimated using Image J. (B) Western blot analysis of phospho-S6K in different body parts of w^Dah flies. Flies were maintained with or without 200 μM rapamycin for 2 weeks prior to preparation of protein extracts from heads, thoraces, and abdominal segments. Rapamycin was found to efficiently reduce levels of phospho-T398-S6K in all fly body parts.

Figure 2

Effects of Rapamycin on Life Span and Fecundity in Various Standard Laboratory Strains (A) Rapamycin treatment extends the life span of w^Dah females. Compared to flies on control food (0 μM rapamycin), flies on 50 and 200 μM rapamycin food have increased median life spans (p = 0.0005 and p < 0.0001, log-rank test). (B) Rapamycin treatment at 50, 200, and 400 μM extends the life span of w^Dah females (p = 0.0428, p < 0.0001, p = 0.0013, log-rank test compared to control). (C) Rapamycin extends life span in w^Dah males (p = 0.0241, log-rank test). (D) Reduced fecundity of females on 50, 200, and 400 μM rapamycin food. Data are given as mean number of eggs laid per female per day ± SEM. T test p values between egg laying under different conditions are p < 0.005. For each time point and each condition, eggs from ten vials containing ten flies per vial were counted. (E) Rapamycin enhances starvation resistance (p < 0.0001, log-rank test). For all stress assays, w^Dah females were pretreated for 2 weeks with 200 μM rapamycin, and initial population was 100 flies per condition. (F) Rapamycin pretreated w^Dah females have improved survival on paraquat (p < 0.0001, log-rank test). (G) Rapamycin treatment increases TAG levels. TAG levels in w^Dah flies were measured after 14 days of rapamycin treatment from heads and thoraces of five females. Twelve samples were measured per treatment. Data are shown as mean ± SEM. ^∗ indicates statistical significance of difference between untreated and rapamycin-treated flies (t test, p < 0.05).

Figure 3

Rapamycin Treatment Does Not Affect IIS Signaling or AMPK Activity Western blot analyses of downstream components of IIS signaling and of AMPK phosphorylation. Protein extracts were made from whole flies after 14 days of rapamycin treatment. No changes in the levels of phospho-Ser505-Akt (A), phospho-Ser21/S9-GSK3 (B), or phospho-T172-AMPK (C) were observed. The presence of two bands for AKT is due to two isoforms present in flies (A). GSK3 has several isoforms in flies, and two of them are detected in this blot (B).

Figure 4

Rapamycin Increases Autophagy and Decreases Protein Translation (A) Levels of ³⁵S-methionine incorporation in 7-day-old flies normalized to total protein content (±SEM). Rapamycin significantly reduces ³⁵S-methionine incorporation (^∗∗p = 0.0033, Student's t test). (B) Significantly more autolysosomes and lysosomes in rapamycin-treated flies (^∗∗∗p < 0.0001, Student's t test). Average number of LysoTracker-stained puncta in fly midguts isolated from rapamycin pretreated or control flies is presented (±SEM). (C) Representative confocal fluorescence images of fly midgut stained with LysoTracker Red. Midgut of control flies (upper image) and rapamycin pretreated flies (lower image) are shown. L indicates lumen of the gut; scale bar is 10 μM.

Figure 5

Rapamycin-Mediated Life Span Extension Is Blocked by the Ubiquitous Overexpression of Constitutively Active S6K, the Absence of 4E-BP, or Downregulation of Atg5 (A) Ubiquitous overexpression of a constitutively active form of S6K (UAS-S6K^const.act.) by the daughterless-GAL4 (daGAL4) driver abolishes rapamycin-mediated life span extension. Rapamycin increases median life span of w^Dah females (p < 0.0001, log-rank test), but not daGAL4 > UAS-S6K^const.act· (p = 0.1083, log-rank test). On control 0 μM rapamycin food, overexpression of constitutively active S6K slightly decreases life span (p = 0.0463, log-rank test). (B) Flies overexpressing constitutively active S6K are more sensitive to starvation compared to w^Dah control flies (p < 0.0001, log-rank test), and their starvation resistance is improved by rapamycin (p < 0.0001, log-rank test). (C) Rapamycin does not extend the life span of 4E-BP null mutant female flies (p = 0.4027, log-rank test) but does increase the life span of control yw flies (p = 0.0033, log-rank test). (D) 4E-BP null flies are sensitive to starvation compared to control flies (p < 0.0001, log-rank test), but rapamycin improves their starvation resistance (p < 0.0001, log-rank test). (E) Downregulation of autophagy abolishes rapamycin-mediated life span extension (p = 0.5383, log-rank test). Autophagy was downregulated by ubiquitous overexpression of UAS-atg5-RNAi using daughterless-GAL4 (daGAL4 > UAS-atg5RNAi). Note that since the experiments were run in parallel, life span data for wild-type controls are the same as in Figure 4A. (F) Flies with ubiquitous overexpression of UAS-atg5-RNAi (daGAL4 > UAS-atg5RNAi) were more starvation sensitive compared to their controls (p < 0.0001, log-rank test), and their starvation resistance was improved by rapamycin treatment (p < 0.0001, log-rank test). Note that these experiments were run in parallel to those in Figures 4B and 4D, hence the life span data for controls are the same.

Figure 6

Effect of Rapamycin Treatment on Long-Lived IIS Mutants (A) Rapamycin extends the life span of long-lived chico¹ heterozygotes, (p = 0.0005) but decreases the life span of chico¹/chico¹ homozygotes (p < 0.0001). On control food, chico¹ heterozygotes (p = 0.039) and chico¹/chico¹ nulls (p < 0.0001) have increased median life span compared to control w^Dah flies. Log-rank test is used for analyses. (B) Rapamycin does not extend the life span of mNSC-ablated flies. mNSCs were partially ablated by driving expression of the proapoptotic factor reaper (UAS-rpr) in the mNSCs using the dilp2-GAL4 driver (d2GAL4). On control food, mNSC-ablated flies are long lived (p < 0.0001, log-rank test compared to control). Log-rank test p values for rapamycin-treated versus nontreated flies of the same genotype are p < 0.0001 for w^Dah and p = 0.97 for d2GAL4 > UAS-rpr. (C) Flies pretreated with 200 μM rapamycin survived longer under starvation conditions compared to untreated flies. Log-rank statistics for nontreated versus rapamycin-pretreated flies for all genotypes are p < 0.0001. Compared to wild-type controls, both chico¹ heterozygotes and mNSC-ablated flies were more resistant to starvation without rapamycin pretreatment (p = 0.006 and p = 0.001, respectively, log-rank tests). (D) Flies pretreated with 200 μM rapamycin survive longer when fed 20 mM paraquat. Log-rank statistics for nontreated versus rapamycin pretreated flies for all genotypes are p < 0.0001. Compared to wild-type controls, mNSC-ablated flies were more resistant to paraquat ingestion without rapamycin pretreatment (p < 0.0001, log-rank test).

Figure 7

Rapamycin Increases Life Span Irrespective of Food Concentration Rapamycin increases (A) median and (B) maximum life span across different yeast concentrations in SYA food. Plotted are median life span values for w^Dah against yeast concentration (0.1×, 0.5×, 1.0×, 1.5×, and 2.0× yeast) in SYA food (black line) and on the same food concentrations but supplemented with 200 μM rapamycin (red line). Values on each of the curves represent median (A) or maximum (B) life span. Flies on 200 μM rapamycin food had significantly increased life span at each yeast concentration (^∗∗∗p < 0.001, log-rank test). For detailed statistical analyses and complete survival curves, please see Figure S5. Maximum life span is defined as the median of the last 10% survivorship.