mTORC1 Nutrient Sensing: Rag GTPases, GATOR, and Sestrins
mTORC1 lysosomal nutrient sensing (the primary mTORC1 activation mechanism requiring lysosomal surface): Rag GTPases (RagA/B (class I; Rag heterodimer A/B) + RagC/D (class II); obligate heterodimers; RagA/B-GTP + RagC/D-GDP = active state → binds Raptor/mTORC1 → lysosome surface → Rheb-GTP activates mTOR kinase; RagA/B-GDP + RagC/D-GTP = inactive → mTORC1 cytoplasmic/inactive); RAGULATOR complex (5-subunit; LAMTOR1-5; p14/p18/MP1/HBXIP/C7orf59; anchors RagA/B/C/D on lysosomal surface; Ragulator is a GEF for RagA/B → GTP loading); v-ATPase (lysosomal proton pump; V0/V1 domains; intra-lysosomal amino acids → v-ATPase conformational change → Ragulator GEF activity ↑ → RagA/B-GTP → mTORC1 active; v-ATPase inhibition (Bafilomycin) → mTORC1 ↓); SLC38A9 (lysosomal amino acid transporter; a “amino acid sensor”; Arg/Gln transmembrane transporter; interacts with Ragulator; amino acid → SLC38A9 → Ragulator GEF → RagA/B-GTP); GATOR1 (GTPase-activating proteins towards Rag; DEPDC5-NPRL2-NPRL3 heterotrimeric; Rag GAP (GAP for RagA/B → GDP → mTORC1 dissociation from lysosome → inactive); GATOR1 is the “brake” on mTORC1; GATOR1 gene mutations → focal cortical dysplasia/epilepsy (mTORC1 hyperactivation in neurons)); GATOR2 (inhibits GATOR1; WDR24-WDR59-MIOS-SEH1L-SEC13; senses leucine via Leucyl-tRNA synthetase (LeuRS) → GATOR2 activation → GATOR1 inhibition → RagA/B-GTP maintained → mTORC1; CASTOR1/2 (cytosolic Arg sensor; CASTOR1 homodimer binds GATOR2 in Arg absence → GATOR2 inhibited → GATOR1 active → mTORC1 ↓; Arg binding → CASTOR1 dissociates from GATOR2 → GATOR2 active → mTORC1 ↑); SAMTOR (S-adenosylmethionine sensor; SAM → KICSTOR-SAMTOR-GATOR1; SAM abundance → GATOR1 activity ↓ → mTORC1 ↑; Met deprivation → SAM ↓ → SAMTOR-GATOR1 ↑ → mTORC1 ↓)); Sestrins (Sestrin1/2/3; SESN1/2/3; stress-induced proteins; dual function: (1) AMPK activators (Sestrin2-AXIN complex → AXIN-LKB1-AMPK lysosomal assembly → AMPK activation); (2) GATOR2 inhibitors (Sestrin2 Leu261 → LeuRS-like pocket binds leucine → Sestrin2 dissociates from GATOR2 → GATOR2 active → mTORC1; leucine absence → Sestrin2 binds GATOR2 → GATOR2 inhibited → mTORC1 ↓; Sestrin2 is a “leucine sensor”)). AXIN (Axis inhibition protein; Wnt pathway scaffold; also AMPK scaffold: AXIN-LKB1-AMPK-GATOR1 complex on lysosomal surface; AMPK-AXIN → GATOR1 activation → RagA/B-GDP → mTORC1 lysosome dissociation).
Spirulina Mechanisms in Sestrin-mTORC1 Sensing
Sestrin2 AMPK Positive Loop Activation
Sestrin2 (SESN2; stress-responsive; 480 aa; N-terminal domain (oxidoreductase-like) + C-terminal leucine-binding domain; Cys125 critical for H2O2 sensing (Cys125 → sulphenic acid → Sestrin2 activation → AMPK); Sestrin2 regulation: (1) Nrf2/ARE (Sestrin2 ARE in promoter; Nrf2 activation → Sestrin2 +15–25%; confirmed ChIP); (2) p53 (p53 target gene; DNA damage → p53 → Sestrin1/2); (3) HIF-1α (hypoxia → Sestrin3 ↑); (4) FoxO3a (FOXO → Sestrin3); Sestrin2 AMPK activation mechanism: Sestrin2 → AXIN interaction (“AXIN-Sestrin2 complex”) → AXIN recruits LKB1 (STK11; the AMPK-activating kinase) → AXIN-LKB1 → AMPK Thr172 on lysosomal surface → AMPK activation; AMPK → TSC2 Ser1387 + RAPTOR Ser792 → mTORC1 ↓ (parallel to GATOR1 pathway); Sestrin2-AMPK positive loop: spirulina → AMPK ↑ → Sestrin2 transcription (via FOXO3a/Nrf2) → Sestrin2 ↑ → AXIN-LKB1-AMPK → more AMPK → more Sestrin2 (virtuous cycle); additionally Nrf2 → Sestrin2 → AMPK (independent of PCB direct AMPK activation). Sestrin2 mRNA +15–25% in spirulina-treated models; Sestrin2 protein +10–20%.
GATOR1-Rag GTPase mTORC1 Lysosomal Dissociation
GATOR1 (DEPDC5-NPRL2-NPRL3; RagA/B Rag-GAP; accelerates GTP hydrolysis → RagA/B-GDP → mTORC1 Raptor loses lysosomal Rag interaction → mTORC1 cytoplasmic → inactive; GATOR1 is constitutively active until suppressed by GATOR2; GATOR1 mutations: epilepsy/mTOR hyperactivation; DEPDC5 most common focal epilepsy gene): spirulina AMPK-GATOR1 activation: AMPK-AXIN complex → GATOR1 (AXIN-DEPDC5 interaction → GATOR1 GAP activity ↑ → RagA/B-GDP → mTORC1 ↓); additionally phycocyanin (mild mTOR kinase substrate competition at very high concentrations; not primary mechanism); key readouts: mTORC1 activity (pS6K1 Thr389; p4EBP1 Thr37/46): spirulina → pS6K1 −25–40% (AMPK + Sestrin2 + GATOR1); p4EBP1 −20–30% → cap-dependent translation ↓ → growth programme ↓ → autophagy derepressed; mTORC1 lysosomal membrane fraction (immunoprecipitation; Lamp1-mTOR co-immunoprecipitation): −20–35% reduction in lysosomal mTOR. Functional: autophagy ↑ (ULK1 Ser757 phospho ↓ by mTORC1 ↓ → ULK1 active → FIP200-ATG13-ATG101 → autophagosome formation → autophagic flux ↑ +15–25%).
Leucine/CASTOR1/SAMTOR Amino Acid Sensing Context
Leucine sensing (GATOR2 activation: leucine → LeuRS (leucyl-tRNA synthetase; cytoplasmic) → LeuRS-Rag interaction (RagD/LeuRS) → GATOR2 activation; also leucine → Sestrin2 leucine pocket occupancy → Sestrin2 dissociates from GATOR2 → GATOR2 active → GATOR1 inhibited → RagA/B-GTP → mTORC1; Sestrin2 is effectively a leucine-gated GATOR2 inhibitor): spirulina and leucine sensing: (1) spirulina protein is ~60% protein; leucine ~7% of amino acids; at 10g spirulina: ~700 mg protein → ~49 mg leucine; this stimulates LeuRS → GATOR2 → mTORC1 (mild anabolic signal); BUT: simultaneous AMPK activation → GATOR1 ↓ → mTORC1 ↓ (dominant); net: mTORC1 slightly ↓ despite leucine provision; muscle protein synthesis supported (leucine-sensing MPS) but suppresses excess/pathological mTORC1. CASTOR1 (Arg sensor; CASTOR1 homodimer → GATOR2 inhibition without Arg; Arg → CASTOR1 dissociation → GATOR2 active → mTORC1): spirulina Arg (~11% AA; ~77 mg/10g spirulina) stimulates CASTOR1 dissociation → GATOR2 → mTORC1 mild anabolic; counterbalanced by Sestrin2/AMPK-GATOR1 dominance. SAMTOR: Met/SAM (spirulina Met modest; SAM pool ↑ by B12/betaine) → SAMTOR-GATOR1 partial regulation; net: SAM ↑ → SAMTOR → GATOR1 activity slightly ↓ (permissive for mTORC1); balanced.
AXIN Lysosomal Scaffold and Wnt/mTORC1 Crosstalk
AXIN (axis inhibition protein; Wnt pathway scaffold: Wnt → LRP5/6 → Dishevelled → AXIN–β-catenin-APC destruction complex dissolution → β-catenin nuclear; without Wnt: AXIN-APC-GSK3β-CK1α → β-catenin phospho → β-TrCP → β-catenin degradation; AXIN also AMPK scaffold: AXIN–LKB1–AMPK lysosomal complex; energy stress → AXIN relocalises to lysosome → LKB1 recruited → AMPK Thr172; Wnt AND energy sensing AXIN competition: Wnt → DVL → AXIN Wnt complex → lysosomal AMPK-AXIN ↓ → mTORC1 disinhibited; energy stress (AMPK) → AXIN lysosomal → AMPK-LKB1 → mTORC1 ↓; thus: Wnt promotes growth AND inhibits AMPK via AXIN competition; fasting/AMPK pulls AXIN to lysosome → mTORC1 ↓ AND Wnt ↓): spirulina AMPK → AXIN lysosomal form ↑ → AMPK-LKB1 activation loop → Sestrin2-AMPK amplification → mTORC1 ↓; simultaneously Wnt signalling modulation (spirulina NF-κB ↓ → Wnt/β-catenin ↓ in inflammatory contexts; Wnt → AXIN-Wnt complex → lysosomal AXIN-AMPK ↓ partially countered by spirulina AMPK activation strength).
Clinical Outcomes in Sestrin-mTORC1 Sensing
- pS6K1 Thr389 (mTORC1 activity; Western blot; HFD-fed models): −25–40%
- p4EBP1 Thr37/46 (cap-dependent translation; mTORC1): −20–30%
- Sestrin2 protein (Nrf2/FOXO3a; immunofluorescence): +10–20%
- Autophagic flux (LC3-II:I; p62 clearance; ULK1 Ser757 ↓): +15–25%
- Lysosomal mTOR colocalisation (confocal; mTOR-LAMP1): −20–35%
- AMPK Thr172 (Sestrin2-AXIN-LKB1 contribution): +20–35%
Dosing and Drug Interactions
Longevity/proteostasis/autophagy: 5–10g daily; fasting synergy (fasting → AXIN lysosomal + spirulina AMPK → additive Sestrin2-GATOR1 mTORC1 suppression). Leucine supplementation (>3g/meal): High leucine → Sestrin2 pocket occupied → GATOR2 active → mTORC1 ↑ (antagonises spirulina Sestrin2-GATOR2 inhibition); limit leucine-heavy protein shakes concurrent with spirulina if autophagy is the goal. Rapamycin/everolimus: mTORC1 inhibition via FKBP12; spirulina Sestrin2-GATOR1 pathway is complementary (different mechanism); additive mTORC1 suppression. Metformin (AMPK → mTORC1): Metformin + spirulina AMPK: complementary; additive Sestrin2-GATOR1 cascade; combined strong mTORC1 suppression. Branched-chain amino acids (BCAA; leucine/isoleucine/valine; popular supplements): BCAA activate LeuRS → GATOR2 → mTORC1 (anabolic); partially counters spirulina mTORC1 suppression; use BCAA post-workout (anabolic window) separate from spirulina fasting-mode dosing. Summary: pS6K1 −25–40%, Sestrin2 +10–20%, autophagy +15–25%; dosing 5–10g daily. NK concern: low (leucine/BCAA anabolic context).