New Genes that Act Downstream of a FoxO Transcription Factor to Regulate Diapause and Longevity

Abstract

Age related diseases are leading causes of mortality worldwide. Environmental interventions such as dietary restriction can both extend lifespan and delay or ameliorate age related pathologies. Dietary restriction and other environmental stresses activate conserved stress response mechanisms that can delay many aspects of biological aging. Understanding the regulation of these stress responses and characterizing their biological mechanisms could enable the design of therapeutics to treat or prevent age-related diseases by directly targeting the aging process. Reductions in the activity of the conserved insulin and insulin-like growth factor signaling pathway (IIS) extend longevity across taxa. IIS antagonizes FoxO transcription factors, and increased FoxO activity promotes increased longevity in multiple organisms. Polymorphisms in human FoxO genes are associated with longevity, and FoxO transcription factors likely modulate various age-related diseases. Thus, an improved understanding of the mechanisms regulating FoxO activity and the mechanisms through which FoxO activity increases lifespan may be relevant to human health. The short-lived nematode Caenorhabditis elegans provides a useful model for studying FoxO regulation and aging. In C. elegans, the sole FoxO family member DAF-16 is inhibited by signals from an insulin-like growth factor receptor (IGFr) family member, DAF-2, and by signals from the germline. Disinhibition of DAF-16/FoxO activity during development causes entry into a larval diapause called dauer. Increasing DAF-16/FoxO activity during adulthood via loss of function mutations in daf-2/IGFr or ablation of the germline dramatically increases lifespan. Thus, the dauer diapause represents a valuable system for understanding how DAF-16/FoxO activity is regulated, while studies of adult longevity can be used to understand how DAF-16/FoxO activation increases lifespan. Reductions in IIS increase DAF-16/FoxO activity in part by causing it to be localized to the nucleus, where it can alter transcription of downstream target genes. Our group has discovered the EAK pathway, which inhibits nuclear DAF-16/FoxO activity. Mutations in eak genes enhance the constitutive dauer arrest phenotypes caused by mutation of the conserved gene akt-1. To identify downstream components of the EAK pathway we performed a forward genetic screen for suppression of dauer entry in eak-7;akt-1 mutants. This screen unexpectedly identified a novel allele of lin-14, which encodes a transcription factor that acts to ensure the proper sequence of developmental events. This lin-14 allele [lin-14(dp69)] functions along with lin-14 RNAi to suppress dauer entry in both DAF-16/FoxO-dependent and DAF-16/FoxO-independent contexts, suggesting that LIN-14 acts downstream of, or in parallel to, DAF-16/FoxO to regulate dauer entry. Two groups of transcriptional isoforms of DAF-16/FoxO, DAF-16A and DAF-16F, are required for increased lifespan. To identify downstream mechanisms by which DAF-16/FoxO activation increases longevity, we used transcriptome profiling to identify transcriptional events that are influenced by DAF-16A and F in both daf-2/IGFrmutants and mutants lacking a germline. We discovered that CEST-1, a previously unstudied member of the carboxylesterase family, is required for full lifespan extension in daf-2/IGFr mutants. CEST-1 expression is induced in daf-2/IGFr mutants, and CEST-1 localizes to the luminal membrane of the intestine. A conserved catalytic residue in the luminal CEST-1 domain is required for it to promote longevity, and CEST-1 overexpression is sufficient to increase lifespan. This evidence suggests that CEST-1 may eliminate a toxic metabolite in the gut or improve digestion of beneficial dietary components. Mammalian carboxylesterase family members are key regulators of metabolism, suggesting that carboxylesterase activity could regulate health and longevity in a conserved manner.