Genetic Targeting of the dTORC1 Inhibitor, dSAMTOR, during Drosophila Aging: A Tissue-specific Pathology

mTORC1 regulates mammalian cell metabolism and growth, in response to diverse environmental stimuli. Nutrient signals control the localization of mTORC1 onto lysosome-surface scaffolds that are critically implicated in its amino acid-dependent activation. Arginine, Leucine and S-Adenosyl-Methionine (SAM) can serve as major mTORC1-signaling activators, with SAM binding to SAMTOR (SAM + TOR), a fundamental SAM sensor, abolishing protein’s (SAMTOR’s) inhibitory action(s) against mTORC1, thereby releasing its (mTORC1) kinase activity. Given the lack of knowledge regarding the role of SAMTOR in invertebrates, we have, in silico, identified the Drosophila SAMTOR homologue (dSAMTOR) and have, herein, genetically targeted it, through utilization of the GAL4/UAS transgenic tool. Survival profiles and negative geotaxis patterns were examined both in control and dSAMTOR-downregulated adult flies, during aging. One of the two gene-targeted schemes resulted in lethal phenotypes, whereas the other one caused rather moderate pathologies in most tissues. Screening of head-specific kinase activities, via PamGene technology application, unveiled the significant upregulation of several kinases, including the dTORC1 characteristic substrate, dp70S6K, in dSAMTOR-downregulated flies, thus strongly supporting the inhibitory dSAMTOR function(s) upon dTORC1/dp70S6K signaling axis in Drosophila-brain settings. Importantly, genetic targeting of the Drosophila BHMT -bioinformatics- counterpart (dBHMT), an enzyme that catabolizes Betaine to produce Methionine (the SAM precursor), led to severe compromises in fly longevities, with glia-, motor neuron- and muscle-specific dBHMT downregulations exhibiting the strongest effects. Abnormalities in wing-vein architectures were also detected in dBHMT-targeted flies, thereby justifying their notably reduced negative geotaxis capacities, herein, observed, mainly in the brain-(mid)gut axis. In vivo adult-fly exposure to clinically relevant doses of Methionine revealed the mechanistic synergism of decreased dSAMTOR and increased Methionine levels in pathogenic longevity, thus rendering (d)SAMTOR an important component in Methionine-associated disorders, including Homocystinuria(s).