Genetic adaptation to diet preserves longevity

Abstract

Diet affects cellular metabolism and organismal life history traits. Pang & Curran (2014) use Caenorhabditis elegans and its bacterial diets to unveil genetic perturbations in proline catabolism that shorten lifespan, but, only on specific Escherichia coli diets, thereby unveiling a genome-encoded adaptive response to different nutritional states.

Our diet not only affects our daily functioning and tendency to get sick, it may also contribute to how long we live. The nematode C. elegans is a prime model organism to study the genetics of aging as it shares many key developmental, physiological and metabolic pathways with humans, and many genes have been identified that can prolong or shorten the animal’s lifespan (Tissenbaum, 2012).More recently, the worm has also emerged as a powerful system to gain insight into the effects different diets and nutrients exert on major life history traits such as developmental rate, fertility and aging. For instance, when fed a diet of Comamonas bacteria, wildtype C. elegans develop faster, have reduced fertility and die younger than when fed the standard laboratory diet of E. coli OP50 or HT115 (MacNeil et al., 2013). The response to Comamonas depends on intact metabolism of branched chain and other amino acidsin the worm, and reveals how different diets can have dramatic effects on an animal’s physiology. While E. coli diets elicit similar life history traits in wild type worms, important differences between them are unveiled in C. elegans mutants. For instance, animals harboring mutations in nhr-114,a nuclear hormone receptor transcription factor, aresterile when fed E. coli OP50 but not E. coli HT115, and this effect is rescued by supplementation of tryptophan (Gracida and Eckmann, 2013). Pang & Curran (2014) expand on this theme by identifying that alh-6 mutant C. elegans exhibit a diet-specific reduction in lifespan on the E. coli OP50 diet.

How do different bacteria elicit distinct and specific effects in either wild type or mutant C. elegans? Understanding this requires:(1) elucidating the mechanisms by which the worm responds to particular diets, and (2) identifying what the bacteria provide or lack.C. elegans genes responsible for this remarkable adaptive response to various diets can be identified by forward genetics, or by systematic, genome-scale RNAi perturbations (Watson et al., 2013). While searching for genetic perturbations that activate SKN-1, the worm NRF transcription factor that protects the animal from a variety of stresses (Pang and Curran, 2014), the authors identified alh-6 mutants and fortuitously discovered that these animals are much healthier when fed the E. coli HT115 strain than when fed the E. coli OP50 strain. In fact, alh-6 mutant animals exhibited a 40% reduced lifespan when fed the OP50 strain compared to wild type animals, as well as diminished egg laying and reduced fertility. However, developmental rate was unaffected. Remarkably, the life shortening effect of alh-6 mutations requires exposure to the E. coli OP50 diet between the L3 and L4 larval stages, as well as continued exposure during adulthood, suggesting that an unknown developmental component is involved. Altogether these results led the authors to propose that a wild type copy of alh-6 is required for an adaptive response to diet: it is necessary to prolong lifespan of C. elegans on a diet of E. coli OP50, but not HT115.

Yet the key question is how does alh-6 protect against the life shortening effect of the E. coli OP50 diet? alh-6 encodes a metabolic enzyme that is involved in the two-step breakdown of proline. Specifically, ALH-6 converts the intermediate metabolite 1-pyrroline-5-carboxylate (P5C) into glutamate. Antioxidants such as N-acetylcysteine and vitamin C reversed the accelerated aging effect of E. coli OP50 bacteria, suggesting that the effect occurs via the buildup of reactive oxygen species (ROS) and an associated impairment of mitochondrial function. Remarkably, the activation of skn-1 expression by alh-6 mutations is independent of the lifespan reduction caused by these mutations. Since ROS is known to activate skn-1 expression (Hoeven et al., 2011), this suggests that skn-1 activation is a secondary effect.

Insulin signaling and dietary restriction (DR) are well-known regulators of aging (Tissenbaum, 2012). Remarkably, as with Comamonas bacteria, the effect of E. coli OP50 bacteria is independent of insulin signaling (MacNeil et al., 2013). Rather, alh-6 acts via the DR pathway, as an eat-2 mutation, which causes reduced feeding, fails to increase lifespan in alh-6 mutant animals. The activation of the adaptive response to E. coli OP50 occurs, at least in part, at the level of gene expression because, although alh-6 levels are unaffected in animals fed this diet, they do express higher levels of prodh/B0513.5, which converts proline into P5C. Since the activation of SKN-1 by alh-6 mutations can be uncoupled from the accelerated aging on the E. coli OP50 diet, other transcription factors are likely involved in mediating this response.

The compounds provided by the bacteria that elicit the effects in the worm remain unknown. E. coli OP50 and HT115 do not exhibit large differences in most macronutrients, with the exception of carbohydrate levels, which are higher in HT115 bacteria (Brooks et al., 2009; MacNeil et al., 2013). Given the molecular function of ALH-6, it is tempting to speculate that E. coli OP50 bacteria provide larger quantities of proline, P5C or both. This is supported by the observation that alh-6 mutant animals fed E. coli HT115 supplemented with excess proline exhibited a reduced lifespan, whereas supplementing proline to E. coli OP50 had no additional adverse effects on longevity. However, it is likely that the intermediate P5C and not proline itself is toxic to the animal because the detrimental effect of alh-6 mutation is lost when the conversion of proline into this compound is prevented by a perturbation of PRODH/B0513.5. In the future, mass spectrometry of different bacteria may reveal whether they harbor different levels of proline or P5C. Since both are E. coli strains, it is unlikely that such differences would result from major differences in metabolic pathways. Rather, variations in metabolic pathway genes may underlie the observed effects. Exploring variation in proline metabolism and other gene activity or expression, bacterial genetic screens, or targeted mutagenesis of candidate genes will be useful to unravel the precise mechanism involved.

Altogether, this study provides an important step forward in our appreciation of the effect different amino acids can exert: whether it is the conversion of proline central to this study, tryptophan innhr-114 mutants on the E. coli OP50 diet (Gracida and Eckmann, 2013), or branched chain amino acid metabolism in response to the Comamonas diet(Watson et al., 2013). C. elegans and its bacterial diets will no doubt continue to provide a powerful interspecies paradigm to dissect the interactions between specific genes and nutrients, and their effects on development, fertility and aging. As with many other key findings from the nematode, results may illuminate how particular nutrients, such as amino acids, affect cellular and organismal physiology in health and disease in humans as well.