A genetic model of methionine restriction extends Drosophila health- and lifespan

Loss of metabolic homeostasis is a hallmark of aging and is characterized by dramatic metabolic reprogramming. To analyze how the fate of labeled methionine is altered during aging, we applied ¹³C5-Methionine labeling to Drosophila and demonstrated significant changes in the activity of different branches of the methionine metabolism as flies age. We further tested whether targeted degradation of methionine metabolism components would “reset” methionine metabolism flux and extend the fly lifespan. Specifically, we created transgenic flies with inducible expression of Methioninase, a bacterial enzyme capable of degrading methionine and revealed methionine requirements for normal maintenance of lifespan. We also demonstrated that microbiota-derived methionine is an alternative and important source in addition to food-derived methionine. In this genetic model of methionine restriction (MetR), we also demonstrate that either whole-body or tissue-specific Methioninase expression can dramatically extend Drosophila health- and lifespan and exerts physiological effects associated with MetR. Interestingly, while previous dietary MetR extended lifespan in flies only in low amino acid conditions, MetR from Methioninase expression extends lifespan independently of amino acid levels in the food. Finally, because impairment of the methionine metabolism has been previously associated with the development of Alzheimer’s disease, we compared methionine metabolism reprogramming between aging flies and a Drosophila model relevant to Alzheimer’s disease, and found that overexpression of human Tau caused methionine metabolism flux reprogramming similar to the changes found in aged flies. Altogether, our study highlights Methioninase as a potential agent for health- and lifespan extension.

Aging is the primary risk factor for many major human pathologies (1). Age-dependent metabolic reprogramming has been noted in different organisms (2), including worms (3), mice (4), and humans (5–7). Moreover, we have previously demonstrated that metabolism in general, and methionine metabolism in particular, is perturbed during aging in Drosophila (8). In addition, methionine metabolism is altered in the tissues of several long-lived species, such as naked mole rats (9), in long-lived mutants, such as flies selected for delayed reproductive senescence (8, 10), and in long-lived Ames mice (11).Although alterations in levels of specific metabolites suggest that the activity of the methionine metabolism pathway is affected, it is difficult to determine if the flux via methionine metabolism is up- or down-regulated. For example, deficiencies of enzymes involved in methionine metabolism (MAT, CBS, GNMT, AHCY) lead to methionine and homocysteine elevation (hypermethioninemias and hyperhomocysteinemias), and elevated levels of methionine and homocysteine reflect a disruption of flux in methionine metabolism (12). Additionally, metabolite changes do not explain how the flux is reprogrammed between different branches of methionine metabolism, as it is possible that the absolute levels of metabolites do not change if metabolites get processed via different routes.Impairment of methionine metabolism flux results in the accumulation of detrimental metabolites belonging to the methionine cycle, such as S-adenosylhomocysteine (SAH) and homocysteine, and leads to different pathological manifestations (13). Lifespan can be extended by up-regulating the clearance of these metabolites via activation of methionine metabolism flux. In fact, methionine restriction (MetR) extends lifespan in yeast, flies, rodents, and human diploid fibroblasts (14–17) and exerts beneficial effects on metabolic health and inflammatory responses (18–20). Moreover, lifespan in worms and flies can be extended through manipulation of different enzymes, either belonging to the methionine metabolism pathway or those that affect the levels of methionine metabolism metabolites. Some of these manipulations include the overexpression of Cbs (21) and Gnmt (22, 23), and the down-regulation of SamS (24) and AhcyL1/AhcyL2 (8). In addition, manipulation of methionine metabolism in just one tissue is sufficient for lifespan extension (8, 21, 22).Restoring age-reprogrammed activity of methionine metabolism can be an attractive option for the extension of health- and lifespan. However, studies relating to MetR have the following challenges: 1) MetR does not decrease levels of methionine equally across different organs; 2) it is impossible to study tissue-specific effects of MetR via manipulations of methionine levels in the food; and 3) the activation of methionine metabolism enzymes is difficult to achieve due to a lack of small-molecule activators. To solve these problems, we created transgenic flies carrying the enzyme Methioninase, which allows for the rapid, inducible, and tissue-specific degradation of methionine.l-Methionine α-deamino-γ-mercaptomethane-lyase (Methioninase) is a bacterial enzyme that is capable of degrading methionine to ammonia, α-ketobutyrate, and methanthiol. Because methionine dependency was attributed to various cancers, recombinant Methioninase (rMetase) has been tested in various cancer models in vitro and in vivo (25, 26). Methioninase has also entered several clinical trials in humans (26–28). Based on this, Methioninase may provide an alternative option to dietary MetR, since it can be expressed in a tissue-specific manner, and its recombinant form can be used in humans.To understand how methionine metabolism is reprogrammed with age or neurodegeneration, we supplemented flies with a labeled ¹³C5-methionine tracer and estimated its fate between different branches of methionine metabolism. We further created transgenic flies with inducible expression of Methioninase (genetic MetR) and performed metabolomics profiling to demonstrate that the effect was similar to the effects of MetR caused by the depletion of methionine from fly food (dietary MetR). It has been previously shown that dietary MetR extends the lifespan in flies only in low amino acid conditions (15). We demonstrate that either whole-body or tissue-specific expression of Methioninase (genetic MetR) can extend Drosophila lifespan without lowering levels of amino acids in the food. Altogether, our studies offer a strategy for restoring age-dependent defects related to impaired methionine metabolism that has strong potential for lifespan extension and treatment of neurodegenerative diseases in humans.