Restricting a single amino acid cross-protects D. melanogaster from nicotine poisoning through mTORC1 and GCN2 signalling.

Dietary interventions that restrict protein intake have repeatedly been shown to offer beneficial health outcomes to the consumer. Benefits such as increased stress tolerance can be observed in response to restricting individual amino acids, thus mimicking dietary protein restriction. Here, we sought to further understand the relationship between dietary amino acids and stress tolerance using Drosophila melanogaster. Utilising a chemically defined medium for Drosophila, we found that transiently restricting adult flies of a single essential amino acid generally protects against a lethal dose of the naturally occurring insecticide, nicotine. This protection was conferred during the pre-treatment window, was specific for individual amino acids and depended on the identity of the focal amino acid, as well as the duration and intensity of its restriction. For instance, complete isoleucine deprivation for 7 days maximised its protective effect - increasing survival during nicotine exposure by 100%. However, a dose of 25% threonine was required to maximise its protective effect (53% enhanced survival). To understand the molecular basis of these effects, we modified the signalling of two cellular sensors of amino acids, GCN2 (General control non-derepressible) and mTORC1 (mechanistic Target of Rapamycin Complex 1) in combination with amino acid restriction. We found that GCN2 was necessary for diets to protect against nicotine, whereas suppression of mTORC1 was sufficient to induce nicotine resistance. This finding implies that amino acid restriction acts via amino acid signalling to cross-protect against seemingly unrelated stressors. Altogether, our study offers new insights into the physiological responses to restriction of individual amino acids that confer stress tolerance. This has broad potential for application in animal and human health.