mTOR and autophagy axis: nutrient sensing, longevity, and peptide modulators

Detailed explanation

Mechanistic target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase functioning as the principal nutrient-and-growth-factor sensor of eukaryotic cells. mTOR assembles into two functionally distinct multi-protein complexes: mTORC1 (comprising mTOR, Raptor, mLST8, PRAS40, and DEPTOR) which primarily regulates protein synthesis, ribosome biogenesis, lipogenesis, and autophagy suppression; and mTORC2 (comprising mTOR, Rictor, mLST8, mSIN1, and Protor) which primarily regulates cytoskeletal organisation, cell survival, and metabolism through Akt phosphorylation at Ser-473. mTORC1 is activated by amino acid availability (particularly leucine and arginine, sensed through the Rag GTPases and lysosomal amino-acid transporters), by growth factor signalling (insulin and IGF-1 through the PI3K/Akt/TSC1-TSC2/Rheb axis), and by cellular energy status (indirectly through AMPK-mediated inhibition when ATP is low). Active mTORC1 phosphorylates downstream substrates including S6K1 (ribosomal S6 kinase 1, driving protein synthesis initiation through eIF4B and eEF2K), 4E-BP1 (releasing eIF4E for cap-dependent translation), TFEB (a lysosomal-and-autophagy master transcription factor), and ULK1 (the autophagy-initiating kinase, phosphorylated at inhibitory Ser-757 by mTORC1 to suppress autophagy). This substrate profile positions mTORC1 as the master anabolic switch — active mTORC1 drives protein synthesis, ribosome production, and lipogenesis while suppressing the catabolic autophagy programme. Autophagy is the evolutionarily conserved cellular self-renewal process by which damaged organelles, aggregated proteins, and other cytoplasmic constituents are engulfed by double-membrane autophagosomes and delivered to lysosomes for degradation and recycling. Basal autophagy maintains cellular quality control; stress-induced autophagy is triggered by nutrient deprivation, oxidative stress, and hypoxia. Autophagy induction requires mTORC1 inactivation to release ULK1 and permit AMPK-mediated phosphorylation of ULK1 at Ser-317 and Ser-777. The ULK1-Beclin1-VPS34 initiation complex nucleates the phagophore; the ATG8/LC3 lipidation system extends and closes the autophagosomal membrane; and STX17-SNAP29-VAMP8 SNARE-mediated fusion delivers autophagosome contents to lysosomes for hydrolase-mediated degradation. The mTOR-autophagy axis is central to the caloric-restriction longevity framework — the observation across multiple model organisms that reduced caloric intake extends lifespan and healthspan. Mechanistically, caloric restriction lowers insulin/IGF-1 signalling and amino acid availability, reducing mTORC1 activity and elevating autophagy. Rapamycin (the natural-product mTOR inhibitor from which mTOR was named) pharmacologically recapitulates this axis and has been demonstrated to extend lifespan in yeast, C. elegans, Drosophila, and mice — the most rigorously validated longevity pharmacology across models. Metformin, resveratrol, and 2-deoxy-D-glucose have been characterised as partial caloric-restriction mimetics acting on overlapping AMPK-mTOR-autophagy axes. Peptide research on the mTOR-autophagy axis has emerged principally through the mitochondrial-derived peptide (MDP) framework. MOTS-c is the best-characterised MDP with mTOR-axis effects — MOTS-c activates AMPK through folate/AICAR-cycle intermediates, indirectly suppressing mTORC1 activity via AMPK-mediated TSC2 phosphorylation and Raptor phosphorylation. This AMPK-mTOR-autophagy connection is the mechanistic basis of MOTS-c's metabolic-regulatory and exercise-endurance effects. Humanin has been reported to modulate autophagy in some contexts, though the primary mechanism (Bax/Bak inhibition and cytoprotective STAT3 signalling) is distinct from AMPK-mTOR modulation. Epitalon (Epithalon) has been characterised as a longevity peptide with reported effects on telomerase activity and pineal-melatonin regulation; direct mTOR-axis engagement is less well-characterised but the longevity-framework overlap is substantial. AC-SDKP has been reported to enhance autophagy in some cardiac-injury and fibrosis contexts, though the mechanism remains incompletely characterised. Standard research assays for mTOR-autophagy pharmacology include Western blot for phospho-S6K1 (Thr389), phospho-4E-BP1 (Thr37/46), phospho-ULK1 (Ser757 for mTORC1-mediated inhibition, Ser317/Ser777 for AMPK-mediated activation), LC3-I to LC3-II conversion (with bafilomycin flux control), p62/SQSTM1 accumulation (autophagy substrate), and phospho-AMPK (Thr172). Autophagy flux is assessed by tandem-fluorescent RFP-GFP-LC3 reporter or by lysosomal-inhibitor accumulation assays. mTORC1 activity is functionally assessed by rapamycin sensitivity and cellular hyperphagic response.