4E-BP extends lifespan upon dietary restriction by enhancing mitochondrial activity in Drosophila

Abstract

Dietary restriction (DR) extends lifespan in multiple species. To examine the mechanisms of lifespan extension upon DR, we assayed genome-wide translational changes in Drosophila. A number of nuclear encoded mitochondrial genes, including those in Complex I and IV of the electron transport chain, showed increased ribosomal loading and enhanced overall activity upon DR. We found that various mitochondrial genes possessed shorter and less structured 5'UTRs, which were important for their enhanced mRNA translation. The translational repressor 4E-BP, the eukaryotic translation initiation factor 4E binding protein, was upregulated upon DR and mediated DR dependent changes in mitochondrial activity and lifespan extension. Inhibition of individual mitochondrial subunits from Complex I and IV diminished the lifespan extension obtained upon DR, reflecting the importance of enhanced mitochondrial function during DR. Our results imply that translational regulation of nuclear-encoded mitochondrial gene expression by 4E-BP plays an important role in lifespan extension upon DR. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

Figure 1. mRNA Translation Associated Changes Upon DR

(A) Polysomal distribution of mRNAs of young adult male flies on 4% YE and 0.25% YE for 6 days showing the individual ribosomal subunits and the polysome peaks, normalized to body weight of flies. RNA was prepared from the low (Lo) and high (Hi) translation fractions for microarray analysis performed in triplicate. (B) Levels of ³⁵S-methionine incorporation in 7 day old control flies normalized to protein content on 4% YE or 0.25% YE. (±SEM) (N=3) (*p<0.05, ** p<0.01). (C) 4E-BP protein is induced on 0.25% YE (DR) as measured by western blot using 30μg of protein probed with a polyclonal anti-d4E-BP (Miron et al., 2001). This induction is not dependent on dFOXO, as dFOXO null mutant flies show a normal response. β-tubulin levels were measured as a loading control. The blot is representative of three replicates. (D) Functional grouping of differentially translated genes according to GO terminology using DAVID analysis software.

Figure 2. 4E-BP Dependent Induction of Mitochondrial Processes upon DR

(A) Mitochondrial protein density was calculated by measuring the protein concentration from crude homogenates and from isolated mitochondrial fractions and normalized to citrate synthase activity in control (4E-BP +/+) and d4E-BP null mutant (4E-BP-/-) flies. (±SEM) (n≥7). (B) Complex I activity measured on isolated mitochondria from whole male flies normalized to mitochondrial protein content in control and d4E-BP null mutant flies. (±SEM) (Control n=3, Null n=4). (C) Cytochrome C oxidase (COX) activity measured on crude homogenates from whole male control and d4E-BP null mutant flies upon DR, normalized to total protein content (±SEM) (n=7). (*p<0.05, ** p<0.01, *** p<0.001).

Figure 3. Reduction of Mitochondrial ETC Subunits Shortens Lifespan upon DR

(A-B) Mean lifespan for Complex I (CG9762) knockdown using UAS-RNAi and Act5C-Gal4 P [Switch] (+RU486) compared to genetically identical control flies (-RU486) on various yeast YE concentrations. (C-D) Mean lifespan for Complex IV (CG11015) knockdown using UAS-RNAi and Act5C-Gal4 P [Switch] (+RU486) compared to genetically identical control flies (-RU486) on various YE concentrations. (Statistical analyses are shown in Table S3).

Figure 4. The role of 5′UTR Structural Properties of Differentially Translated mRNAs

(A-C) 5′UTR secondary structure analysis of differentially translated mRNAs upon DR and mitochondrial genes was calculated using DAMBE. (A) Distribution of theoretical 5′UTR folding free energies, ΔG, for mRNAs which showed higher or lower translation ratios upon DR and the whole genome. (B) Distribution of theoretical 5′UTR folding free energies of genes in Complex I, Complex IV and ribosomal proteins of the mitochondria. (C) Mean ΔG, length and GC content (±SEM) are given the whole genome, translationally up and downregulated genes and all Complexes of the mitochondrial ETC. P-values were calculated by generating sampling distributions for each experimental distribution, as described in the methods. (D, E) Analysis of 5′UTR dependent translational control. (D) Structural properties of 5′UTRs tested. (E) 5′UTRs were cloned into a mono-cistronic firefly luciferase plasmid, downstream of the Act5C promoter and upstream of FLuc. DNA transfection of various constructs in S2 cells, with data presented as the ratio of FLuc activity in the presence of activated d4E-BP, (d4E-BP^LLAA) versus the presence of GFP, a nonspecific control, with all data normalized to a RLuc transfection control plasmid (±SEM) (n≥6) (*p<0.05, ** p<0.01, *** p<0.001)

Figure 5. 4E-BP is Required for Maximal Lifespan Extension upon DR in Drosophila

(A, B) Lifespan of male and female revertant and d4E-BP null mutant flies on various YE concentrations. (A) Male control and d4E-BP null mutant flies (B) Female control and d4E-BP null mutant flies. (C, D) Rescue of DR effect by ubiquitously expressing d4E-BP using daughterless (da)-Gal4 in the 4E-BP null mutant background. Solid symbols DR (0.25% YE), empty symbols control (5% YE). (C) Male 4E-BP null mutant flies have an 8% lifespan extension upon DR. Control flies have a lifespan extension of 35% and putting 4E-BP back in the null mutant line gives a lifespan extension of 37%. (D) Female 4E-BP null mutant flies have a 13% lifespan extension upon DR. Control flies have a lifespan extension of 42% and putting 4E-BP back in the null mutant background gives a lifespan extension of 48%. (Statistical analyses with replicates are shown in Tables S4 and S5)

Figure 6. Overexpression of Activated d4E-BP Extends Lifespan in a Nutrient Dependent Manner in Drosophila

(A-D) Male and female flies overexpressing d4E-BP^LLs (4E-BPs) using armadillo (arm) GAL4 strain extends lifespan on high nutrition (5%YE) but not under DR (0.25% YE). (A) Survival of male flies on high nutrition. (B) Female flies on high nutrition. (C) Male flies under DR. (D) Female flies under DR. ‘+’ is the Benzer Lab w1118 strain, into which each of these lines was outcrossed 6 times. P values were obtained by comparing the survival curves with GraphPad Prism Software using the longest lived control. (Statistical analyses with replicates are shown in Table S7)