The search for DAF-16/FOXO transcriptional targets: approaches and discoveries

The insulin/IGF-1 receptor (IIR)/FOXO pathway is remarkably conserved in worms, flies, and mammals, and downregulation of signaling in this pathway has been shown to extend lifespan in all of these animals. FOXO-mediated transcription is required for the long lifespan of IIR mutants; thus, there is great interest in identifying FOXO target genes, as they may carry out the biochemical activities that extend longevity. A number of approaches have been used to identify the transcriptional targets of FOXO. Thus far, the best data available on the components downstream of this pathway are from experiments involving the Caenorhabditis elegans FOXO transcription factor, DAF-16; some of these targets have been tested for their contributions to longevity, dauer formation, and fat storage. Here, I examine and compare the approaches used to identify DAF-16/FOXO targets, review the genes regulated by DAF-16, and discuss the processes that may be at work to extend lifespan in IIR mutants. Rather than upregulating every possible beneficial gene, DAF-16 appears to selectively upregulate genes that contribute to specific protective mechanisms, while simultaneously downregulating potentially deleterious genes. In addition to genes that carry out expected roles in stress protection, many previously unknown targets have been identified in these studies, suggesting that some mechanisms of lifespan extension still await discovery. These mechanisms may act cooperatively or cumulatively to increase longevity, and are likely to be at least partially conserved in higher organisms.