Simple nutrients bypass the requirement for HLH-30 in coupling lysosomal nutrient sensing to survival.

Lysosomes are ubiquitous acidified organelles that degrade intracellular and extracellular material trafficked via multiple pathways. Lysosomes also sense cellular nutrient levels to regulate target of rapamycin (TOR) kinase, a signaling enzyme that drives growth and suppresses activity of the MiT/TFE family of transcription factors that control biogenesis of lysosomes. In this study, we subjected worms lacking basic helix-loop-helix transcription factor 30 (hlh-30), the Caenorhabditis elegans MiT/TFE ortholog, to starvation followed by refeeding to understand how this pathway regulates survival with variable nutrient supply. Loss of HLH-30 markedly impaired survival in starved larval worms and recovery upon refeeding bacteria. Remarkably, provision of simple nutrients in a completely defined medium (C. elegans maintenance medium [CeMM]), specifically glucose and linoleic acid, restored lysosomal acidification, TOR activation, and survival with refeeding despite the absence of HLH-30. Worms deficient in lysosomal lipase 2 (lipl-2), a lysosomal enzyme that is transcriptionally up-regulated in starvation in an HLH-30-dependent manner, also demonstrated increased mortality with starvation-refeeding that was partially rescued with glucose, suggesting a critical role for LIPL-2 in lipid metabolism under starvation. CeMM induced transcription of vacuolar proton pump subunits in hlh-30 mutant worms, and knockdown of vacuolar H+-ATPase 12 (vha-12) and its upstream regulator, nuclear hormone receptor 31 (nhr-31), abolished the rescue with CeMM. Loss of Ras-related GTP binding protein C homolog 1 RAGC-1, the ortholog for mammalian RagC/D GTPases, conferred starvation-refeeding lethality, and RAGC-1 overexpression was sufficient to rescue starved hlh-30 mutant worms, demonstrating a critical need for TOR activation with refeeding. These results show that HLH-30 activation is critical for sustaining survival during starvation-refeeding stress via regulating TOR. Glucose and linoleic acid bypass the requirement for HLH-30 in coupling lysosome nutrient sensing to survival.

C. elegans SIRT6/7 homolog SIR-2.4 promotes DAF-16 relocalization and function during stress.

FoxO transcription factors and sirtuin family deacetylases regulate diverse biological processes, including stress responses and longevity. Here we show that the Caenorhabditis elegans sirtuin SIR-2.4--homolog of mammalian SIRT6 and SIRT7 proteins--promotes DAF-16-dependent transcription and stress-induced DAF-16 nuclear localization. SIR-2.4 is required for resistance to multiple stressors: heat shock, oxidative insult, and proteotoxicity. By contrast, SIR-2.4 is largely dispensable for DAF-16 nuclear localization and function in response to reduced insulin/IGF-1-like signaling. Although acetylation is known to regulate localization and activity of mammalian FoxO proteins, this modification has not been previously described on DAF-16. We find that DAF-16 is hyperacetylated in sir-2.4 mutants. Conversely, DAF-16 is acetylated by the acetyltransferase CBP-1, and DAF-16 is hypoacetylated and constitutively nuclear in response to cbp-1 inhibition. Surprisingly, a SIR-2.4 catalytic mutant efficiently rescues the DAF-16 localization defect in sir-2.4 null animals. Acetylation of DAF-16 by CBP-1 in vitro is inhibited by either wild-type or mutant SIR-2.4, suggesting that SIR-2.4 regulates DAF-16 acetylation indirectly, by preventing CBP-1-mediated acetylation under stress conditions. Taken together, our results identify SIR-2.4 as a critical regulator of DAF-16 specifically in the context of stress responses. Furthermore, they reveal a novel role for acetylation, modulated by the antagonistic activities of CBP-1 and SIR-2.4, in modulating DAF-16 localization and function.

Autofluorescence as a measure of senescence in C. elegans: look to red, not blue or green.

In C. elegans, intestinal autofluorescence (sometimes referred to as lipofuscin or "age pigment") accumulates with age and is often used as a marker of health or the rate of aging. We show that this autofluorescent material is spectrally heterogeneous, and that materials that fluoresce under different excitation wavelengths have distinct biological properties. Red autofluorescence (visible with a TRITC filter set) correlates well with an individual's remaining days of life, and is therefore a candidate marker of health. In contrast, blue autofluorescence (via a DAPI filter set) is chiefly an indicator of an individual's incipient or recent demise. Thus, population averages of blue fluorescence essentially measure the fraction of dead or near-dead individuals. This is related to but distinct from the health of the living population. Green autofluorescence (via a FITC or GFP filter set) combines both properties, and is therefore ill suited as a marker of either death or health. Moreover, our results show that care must be taken to distinguish GFP expression near the time of death from full-body green autofluorescence. Finally, none of this autofluorescence increases after oxidative stress, suggesting that the material, or its biology in C. elegans, is distinct from lipofuscin as reported in the mammalian literature.