Transposable elements (TEs) are mobile genetic elements, highly enriched in heterochromatin, that constitute a large percentage of the DNA content of eukaryotic genomes. Aging in Drosophila melanogaster is characterized by loss of repressive heterochromatin structure and loss of silencing of reporter genes in constitutive heterochromatin regions. Using next-generation sequencing, we found that transcripts of many genes native to heterochromatic regions and TEs increased with age in fly heads and fat bodies. A dietary restriction regimen, known to extend life span, repressed the age-related increased expression of genes located in heterochromatin, as well as TEs. We also observed a corresponding age-associated increase in TE transposition in fly fat body cells that was delayed by dietary restriction. Furthermore, we found that manipulating genes known to affect heterochromatin structure, including overexpression of Sir2, Su(var)3-9, and Dicer-2, as well as decreased expression of Adar, mitigated age-related increases in expression of TEs. Increasing expression of either Su(var)3-9 or Dicer-2 also led to an increase in life span. Mutation of Dicer-2 led to an increase in DNA double-strand breaks. Treatment with the reverse transcriptase inhibitor 3TC resulted in decreased TE transposition as well as increased life span in TE-sensitized Dicer-2 mutants. Together, these data support the retrotransposon theory of aging, which hypothesizes that epigenetically silenced TEs become deleteriously activated as cellular defense and surveillance mechanisms break down with age. Furthermore, interventions that maintain repressive heterochromatin and preserve TE silencing may prove key to preventing damage caused by TE activation and extending healthy life span.aging | heterochromatin | transposable elements | dietary restriction | silencing

Sirtuins are an evolutionarily conserved family of NAD-dependent deacylases that control metabolism, stress response, genomic stability, and longevity. Here, we show the sole mitochondrial sirtuin in , Sirt4, regulates energy homeostasis and longevity. knockout flies have a short lifespan, with increased sensitivity to starvation and decreased fertility and activity. In contrast, flies overexpressing either ubiquitously or specifically in the fat body are long-lived. Despite rapid starvation, knockout flies paradoxically maintain elevated levels of energy reserves, including lipids, glycogen, and trehalose, while fasting, suggesting an inability to properly catabolize stored energy. Metabolomic analysis indicates several specific pathways are affected in knockout flies, including glycolysis, branched-chain amino acid metabolism, and impaired catabolism of fatty acids with chain length C18 or greater. Together, these phenotypes point to a role for Sirt4 in mediating the organismal response to fasting, and ensuring metabolic homeostasis and longevity.