Daf-16 mediated repression of cytosolic ribosomal protein genes facilitates a hypoxia sensitive to hypoxia resistant transformation in long-lived germline mutants

In C. elegans, germline ablation leads to long life span and stress resistance. It has been reported that mutations that block oogenesis or an upstream step in germline development confer strong resistance to hypoxia. We demonstrate here that the hypoxia resistance of sterile mutants is dependent on developmental stage and age. In just a 12-hour period, sterile animals transform from hypoxia sensitive L4 larvae into hypoxia resistant adults. Since this transformation occurs in animals with no germline, the physiological programs that determine hypoxia sensitivity in germline mutants occur independently of germline signals and instead rely on signals from somatic tissues. Furthermore, we found two distinct mechanisms of hypoxia resistance in germline deficient animals. First, a DAF-16/FoxO independent mechanism that occurs in all hypoxia resistant sterile adults and, second, a DAF-16/FoxO dependent mechanism that confers an added layer of resistance, or super-resistance, to animals with no germline as they age past day 1 of adulthood. RNAseq data showed that genes involved in both cytosolic and mitochondrial protein translation are repressed in sterile adults and further repressed only in germline deficient mutants as they age. Importantly, mutation of daf-16 specifically blocked the repression of cytosolic ribosomal protein genes, but not mitochondrial ribosomal protein genes, implicating DAF-16/FoxO mediated repression of cytosolic ribosomal protein genes as a mechanism of hypoxia super-resistance. Consistent with this hypothesis, the hypoxia super-resistance of aging germline deficient adults was also suppressed by dual mutation of ncl-1 and larp-1, two regulators of protein translation and ribosomal protein abundance. These studies provide novel insight into a profound physiological transformation that takes place in germline mutants during development, showing that some of the unique physiological properties of these long-lived animals are derived from developmentally dependent DAF-16/FoxO mediated repression of genes involved in cytosolic protein translation. Author summaryIn addition to being extremely long lived, germline deficient animals have other extraordinary properties, such as robust resistance to oxygen deprivation. Here we provide new insight into the mechanisms of hypoxia resistance in germline deficient animals. We demonstrate that, in just a 12-hour period, germline mutants transform from hypoxia sensitive larvae into highly hypoxia resistant adults. Therefore, hypoxia resistance is not a universal property of germline ablated animals, but is instead switched on only in adult animals. We have found two distinct mechanisms of hypoxia resistance in germline deficient animals and both mechanisms are mediated by signals from somatic tissues and can operate independently of the germline. Using RNA sequencing, we show that daf-16 is required for the developmental repression of genes involved in cytosolic protein translation and this phenomenon is one of the mechanisms of hypoxia resistance in germline mutants. Consistent with this hypothesis, dual mutation of ncl-1 and larp-1, two regulators of protein translation and cytosolic ribosomal protein levels, also suppressed the hypoxia resistance of germline ablated animals. We conclude that the L4/adult developmental switch presents an excellent opportunity for investigating the mechanisms of longevity and hypoxia resistance in germline deficient animals.

Automated summary

In C. elegans, germline ablation leads to enhanced resistance to hypoxia. This resistance is dependent on developmental stage and age, and is mediated by signals from somatic tissues. Two mechanisms of hypoxia resistance in germline deficient animals were identified, with one being DAF-16/FoxO independent and the other being DAF-16/FoxO dependent. The repression of cytosolic ribosomal protein genes by DAF-16/FoxO was found to be a mechanism of hypoxia super-resistance.