Folate Acts in E. coli to Accelerate C. elegans Aging Independently of Bacterial Biosynthesis

摘要

class="head no_bottom_margin" id="sec1title">IntroductionBoth nutrition and the host-associated microbiota are thought to impact longevity (, ). Diet influences the metabolism of gut microbes, which in turn can synthesize nutrients for the host. These interactions make it difficult to unravel the contributions of diet and the gut microbiota to long-term health (). This complexity can be addressed with model systems such as the nematode Caenorhabditis elegans (). Yet even here, there are numerous interactions between the nutrient agar medium, the Escherichia coli bacterial lawn, and the worm. Chemical manipulations of the medium and genetic manipulations of both E. coli and C. elegans provide tools to understand these interactions ().Folates in their reduced tetrahydrofolate (THF) form are required as enzymatic cofactors in the folate cycle; a series of metabolic steps found in all cells (including both bacteria and animals) required for cell biosynthesis. Products include purines, pyrimidines, glycine, and methionine, which are required to generate the methyl donor molecule S-adenosyl methionine (SAM) (). Animals cannot synthesize folates and so obtain folates from their diets and associated microbes (, ). Our previous research showed that C. elegans lifespan is increased when E. coli folate synthesis is disrupted either by a mutation in the gene aroD, which is needed to make aromatic compounds including the folate precursor para-aminobenzoic acid (PABA), or by sulfamethoxazole (SMX), a sulfonamide drug that competes with PABA for the active site of the enzyme dihydropteroate synthase (). This enzyme is a key step in folate biosynthesis and is absent from animals. C. elegans obtains folates from E. coli and thus several possible mechanisms might explain why E. coli folate synthesis affects C. elegans lifespan. Distinguishing the effects of folates in bacteria and folates in their animal hosts is important because folate supplementation is beneficial to human health and any intervention would need to maintain healthy levels of serum folate.Dietary, or caloric, restriction has been shown to extend the lifespan of C. elegans (, ). SMX does not slow E. coli growth and therefore has no effect on food availability. Furthermore, C. elegans grow and reproduce normally (). Thus, a limitation of macronutrients is an unlikely explanation. Alternatively, inhibition of E. coli folate synthesis may influence C. elegans lifespan by limiting dietary folate and/or a specific change in folate-dependent nutrients (). For example, restriction of methionine increases lifespan in rodents and influences lifespan in Drosophila (, ). Mutation of C. elegans sams-1, the gene encoding SAM synthase, extends lifespan (href="#bib24" rid="bib24" class=" bibr popnode">Hansen et al., 2005). The diabetes drug metformin increases C. elegans lifespan in a manner dependent on the E. coli strain and changes in C. elegans folate and methionine metabolism are implicated in mediating the lifespan extension (href="#bib9" rid="bib9" class=" bibr popnode">Cabreiro et al., 2013).Another possible explanation is that folate synthesis inhibition increases C. elegans lifespan by altering E. coli physiology. E. coli can accumulate in the intestine of older C. elegans adults and because treatment of E. coli with antibiotics or UV increases worm lifespan, this accumulation is widely thought to accelerate C. elegans aging (href="#bib17" rid="bib17" class=" bibr popnode">Garigan et al., 2002, href="#bib18" rid="bib18" class=" bibr popnode">Gems and Riddle, 2000, href="#bib36" rid="bib36" class=" bibr popnode">McGee et al., 2011). More subtly, changes in bacterial toxicity caused by changes in bacterial metabolism might influence C. elegans aging. The E. coli ubiG mutant, which cannot synthesis coenzyme Q/ubiquinone, increases worm lifespan by influencing bacterial respiration rather than dietary intake of Q (href="#bib43" rid="bib43" class=" bibr popnode">Saiki et al., 2008).Here, we show that modulating folate uptake or the folate cycle in C. elegans does not affect lifespan, suggesting E. coli folate influences C. elegans lifespan by acting on E. coli physiology. Apart from the Q synthesis genes and aroD (href="#bib43" rid="bib43" class=" bibr popnode">Saiki et al., 2008, href="#bib49" rid="bib49" class=" bibr popnode">Virk et al., 2012), little is known about how E. coli genetics influences C. elegans lifespan. A genetic screen of over 1,000 E. coli mutants shows that bacterial growth does not correlate with C. elegans survival and only a few specific interventions increase C. elegans lifespan, including the mutation of genes involved in E. coli folate synthesis, but not in the E. coli folate cycle. In addition to its role in bacterial growth, we propose that folate acts to change E. coli physiology in a way that accelerates C. elegans aging.