Target of Rapamycin drives unequal responses to essential amino acid depletion in egg laying

Abstract

Nutrition shapes a broad range of life history traits, ultimately impacting animal fitness. A key fitness-related trait, female fecundity is well known to change as a function of diet. In particular, the availability of dietary protein is one of the main drivers of egg production, and in the absence of essential amino acids egg laying declines. However, it is unclear whether all essential amino acids have the same impact on phenotypes like fecundity. Using a holidic diet, we fed adult female D. melanogaster diets that contain all necessary nutrients except one of the 10 essential amino acids and assessed the effects on egg production. For most essential amino acids, depleting a single amino acid induced as rapid a decline in egg production as when there were no amino acids in the diet. However, when either methionine or histidine were excluded from the diet, egg production declined more slowly. Next, we tested whether GCN2 and TOR were involved in this difference in response across amino acids. While mutations in GCN2 did not eliminate the differences in the rates of decline in egg laying among amino acid drop-out diets, we found that inhibiting TOR signalling caused egg laying to decline rapidly for all drop-out diets. TOR signalling does this by regulating the yolk-forming stages of egg chamber development. Our results suggest that amino acids differ in their ability to induce signalling via the TOR pathway. This is important because if phenotypes differ in sensitivity to individual amino acids, this generates the potential for mismatches between the output of a pathway and the animal’s true nutritional status.

Transiently restricting individual amino acids protectsD. melanogasteragainst multiple stressors

“…Similarly, consuming less protein by spending more time on a diet without protein also reduced fecundity by approximately 2 eggs per female over 24h for every additional day without protein (F4 = 76.79, P < 0.001). These decreases in fecundity are similar to what is observed when flies are deprived of a single essential amino acid (Alves et al 2022). When we exposed these cohorts of flies to nicotine after pretreatment, we found that, contrary to expectations, no level of protein restriction increased nicotine resistance, and reducing protein to 25 or 0% was detrimental (Figure 3.C; Table 2; Supplementary Figure 1.A).…”

“…New proteins would surely require isoleucine and this could possibly be made available by recycling amino acid stores via protein breakdown (Johnstone et al 2024). If this is the case, then these stored amino acids must somehow be reserved for somatic protection, rather than for use in egg production, which ceases when any essential amino acid is removed from the diet (Sang and King 1961;Alves et al 2022). Studies tracking the fate of labelled amino acids into protein during isoleucine deprivation could be revealing, both for understanding the systems that are triggered to enhance stress resistance as well as to identify protein synthesis that is required to sustain lifespan.…”

“…A; Table 3). Although methionine is an essential amino acid, this result was unsurprising because egg laying has previously been shown to exhibit a slower decline after methionine removal than that of the other amino acids (Alves et al 2022). When we removed leucine from the diet, we observed a proportional reduction in egg laying, where fecundity was reduced by approximately 2 eggs per female in the 24h measured for every additional day that flies spent on a leucine dropout (Figure 4.…”