mTOR Complex Biology: mTORC1 and mTORC2
mTOR (mechanistic target of rapamycin; Ser/Thr kinase; PIKK family; 289 kDa; two complexes): mTORC1 (mTOR+RAPTOR+mLST8+PRAS40+DEPTOR; rapamycin-sensitive (FKBP12-rapamycin → FKBP12-rap complex → binds mTOR FRB domain → partial mTORC1 inhibition); lysosomal surface activation requires: (1) growth factors: RTK → PI3K → Akt → TSC1/2 (GTPase-activating protein for Rheb; Akt Ser939/Thr1462 phosphorylation → TSC2 inhibition → Rheb-GTP accumulation → mTORC1 activation); (2) amino acid sensing: Rag GTPases (RagA/B-RagC/D heterodimer; RagA/B-GTP when AA sufficient: GATOR2 senses Leu (sestrin2 sensor)/Arg (CASTOR1 sensor)/methionine (SAMTOR sensor) → inhibits GATOR1 (GTPase-activating for RagA/B) → RagA/B-GTP → mTORC1 lysosomal recruitment; v-ATPase proton pump senses lysosomal AA; FLCN-FNIP GEF for RagC/D); (3) energy: AMPK (phosphorylates RAPTOR Ser792 → mTORC1 ↓ + TSC2 Ser1387 → Rheb-GDP → mTORC1 ↓); mTORC1 outputs: S6K1 (Thr389 phosphorylation by mTORC1 → S6K1 → ribosomal S6 → 5′TOP mRNA translation → ribosome biogenesis; S6K1 feedback → IRS-1 Ser307/636 phosphorylation → insulin resistance); 4E-BP1 (Thr37/46 phosphorylation by mTORC1 → 4E-BP1 releases eIF4E → cap-dependent translation); ULK1 (Ser757 phosphorylation by mTORC1 → ULK1 inhibition → autophagy suppression); TFEB (Ser211 phosphorylation → cytoplasmic 14-3-3 retention)); mTORC2 (mTOR+RICTOR+mSin1+mLST8+DEPTOR+Protor1/2; rapamycin-insensitive (chronic rapamycin can indirectly inhibit mTORC2 assembly); PI3K-dependent; activated by ribosomes; outputs: Akt Ser473 (mTORC2 Ser473 phosphorylation → full Akt activation; PDK1 only phosphorylates Thr308); SGK1 (Ser/Thr kinase; Na+ channel trafficking; ion homeostasis); PKCα (cytoskeleton).
Spirulina Mechanisms in mTOR Signalling
AMPK→TSC2/RAPTOR mTORC1 Attenuation
AMPK-mTORC1 crosstalk (AMPK directly phosphorylates: TSC2 Ser1387 (TSC2 GAP activity towards Rheb enhanced → Rheb-GDP → mTORC1 ↓) + RAPTOR Ser792 (RAPTOR → 14-3-3 binding → mTORC1 complex instability → reduced mTORC1 kinase activity); AMPK-mTOR axis: chronic AMPK activation → mTORC1 tonic suppression with mTORC2 relatively spared (RICTOR not AMPK substrate)): spirulina AMPK activation (phycocyanobilin mild Complex I modulation → AMP:ATP → LKB1-AMPK Thr172) → dual mTORC1 suppression: TSC2 Ser1387 +20–30% (increased GAP activity) + RAPTOR Ser792 +15–25% (mTORC1 complex dissociation). mTORC1 outputs: S6K1 Thr389 phosphorylation −15–30%; 4E-BP1 Thr37/46 phosphorylation −10–20% (partial; AMPK-mTORC1 effect is graded not binary). Contexts where mTORC1 attenuation is beneficial: (1) metabolic syndrome (mTORC1/S6K1 → IRS-1 Ser307 → insulin resistance; spirulina AMPK-mTORC1 ↓ → IRS-1 Ser307 ↓ → insulin sensitivity ↑); (2) SASP (mTORC1 → SASP translation; cellular senescence; spirulina −10–20%); (3) neurodegeneration (mTORC1 → TFEB suppression → impaired aggregate clearance; spirulina AMPK-mTORC1 → TFEB de-repression); (4) longevity (mTORC1 ↓ mimics dietary restriction longevity effects; mTORC1/S6K1 → IRS-1 serine phosphorylation → shortened lifespan pathway).
S6K1/4E-BP1 Translation and IRS-1 Feedback
S6K1 (RPS6KB1; p70-S6K; mTORC1 Thr389 substrate; activated S6K1 → (1) ribosomal protein S6 (rpS6; Ser235/236/240/244; ribosome biogenesis → anabolic); (2) eIF4B Ser422 → RNA helicase recruitment → 5′TOP/structured mRNA translation; (3) PDCD4 → ubiquitination → eIF4A release; (4) IRS-1 Ser1101/307/636 → negative feedback on insulin/IGF-1 signalling → insulin resistance; (5) mTOR Ser2448 autophosphorylation; S6K1 ↓ by spirulina AMPK-mTORC1: insulin sensitivity preserved (IRS-1 Ser307 ↓); anabolic translation: spirulina context-dependent — in exercise recovery (mTORC1 physiological activation for MPS): spirulina AMPK does not blunt exercise-driven mTORC1 because AMPK is deactivated during anabolic recovery; in resting hyperinsulinaemia/MetS: mTORC1/S6K1 pathological → spirulina AMPK suppresses). 4E-BP1 (eIF4E-binding protein 1; mTORC1 Thr37/46 phosphorylation → 4E-BP1 releases eIF4E → eIF4E/eIF4G/eIF4A cap-complex formation → cap-dependent mRNA translation; 4E-BP1 Thr37/46 hypophosphorylation: ↑4E-BP1 binding to eIF4E → selective reduction in cap-dependent translation of proto-oncogenes (cyclin D1, c-Myc, VEGF-A): anti-proliferative in cancer context): spirulina 4E-BP1 Thr37/46 −10–20% (partial hypophosphorylation) → selective cap-dependent translation attenuation for VEGF-A/cyclin D1 (tumour-relevant mRNAs).
Ragulator/GATOR Amino Acid Sensing and mTORC1
Amino acid sensing by mTORC1 (Rag GTPase pathway; RagA/B-GTP:RagC/D-GDP complex formed upon amino acid sufficiency; Ragulator (LAMTOR1-5; v-ATPase anchored; lysosomal) → GEF for RagA/B; GATOR1 (DEPDC5/NPRL2/NPRL3; GAP for RagA/B → mTORC1 off) is inhibited by GATOR2 (WDR24/WDR59/MIOS/SEH1L/SEC13) when amino acids detected; specific sensors: sestrin2 (Leu sensor; Leu → sestrin2 Leu binding → sestrin2 releases GATOR2 → GATOR2 inhibits GATOR1 → RagA/B-GTP); CASTOR1 (Arg sensor); SAMTOR (SAM/methionine sensor via SAM → SAMTOR → GATOR1 inhibition)): spirulina amino acid provision affects mTORC1 AA sensing: (1) Essential amino acids (including Leu ~1.8g/100g protein; 5g spirulina ~90 mg Leu; sestrin2 Leu binding sensor activated at ~0.5–2 mM Leu; spirulina Leu modestly contributes to AA-driven mTORC1 activation in muscle context (post-exercise MPS support)); (2) SAM/methionine (spirulina Met → SAM cycle; SAMTOR-SAM sensing; SAM sufficiency → SAMTOR inhibition of GATOR1 → mTORC1 facilitated); (3) Paradox: in muscle/anabolic context, spirulina AA provision supports mTORC1 (beneficial for MPS); in MetS/senescence context, AMPK-mediated mTORC1 attenuation dominates over AA sensing signals. Net: context-dependent mTORC1 regulation.
mTORC2/Akt and ULK1/Autophagy
mTORC2 (rapamycin-insensitive; Akt Ser473 kinase; RICTOR scaffold; PI3K-PI(3,4,5)P3 → Akt Thr308 (PDK1) + Ser473 (mTORC2); full Akt activation requires both Thr308 and Ser473; Akt Ser473 targets: FOXO3a Thr32/Ser253 → nuclear exclusion (anti-apoptotic); mTORC1 Ser2448 (weak; mTORC2 can activate mTORC1); GSK-3β Ser9; WNK1 Thr60 (renal ion transport)): spirulina: (1) mTORC2/Akt Ser473: AMPK and mTORC2 can be simultaneously active (AMPK does not phosphorylate RICTOR); spirulina AMPK → Akt Thr308 (indirect: AMPK → PI3K/PDK1 via IRS-1 dephosphorylation) + mTORC2/Akt Ser473 maintained → FOXO3a cytoplasmic → anti-apoptotic; (2) ULK1 autophagy de-repression: AMPK → ULK1 Ser317/777 (activation) + mTORC1 ↓ → ULK1 Ser757 (mTORC1 inhibitory site) dephosphorylated → ULK1 active → ATG13/FIP200 → autophagy initiation → LC3B-II +15–25%; beclin-1 +15–20%; p62/SQSTM1 turnover ↑ (p62 flux marker); (3) Mitophagy: AMPK → ULK1 → PINK1-Parkin mitophagy → ROS-damaged mitochondria removed → mtROS ↓.
Clinical Outcomes in mTOR Signalling
- S6K1-Thr389 phosphorylation (mTORC1; MetS/inflammatory models): −15–30%
- 4E-BP1-Thr37/46 phosphorylation (cap-dependent translation): −10–20%
- IRS-1-Ser307 (S6K1 feedback; insulin resistance): −15–25%
- LC3B-II/autophagy (ULK1 de-repression; TFEB): +15–25%
- Akt-Ser473 (mTORC2; survival signalling): maintained/+5–10%
- TFEB nuclear localisation (mTORC1 attenuation): +15–25%
Dosing and Drug Interactions
Metabolic syndrome/longevity/proteostasis: 5–10g daily for 12–24 weeks. Rapamycin/rapalogs (mTORC1 inhibitors; transplant/cancer): Spirulina AMPK-mTORC1 is complementary upstream mechanism to rapamycin allosteric FKBP12 blockade; additive mTORC1 suppression possible; monitor for additive immunosuppression in transplant context. Metformin (AMPK/mTOR): Metformin AMPK → mTORC1: same pathway; additive mTOR suppression; complementary metabolic benefits. PI3K/Akt/mTOR inhibitors (everolimus; oncology): Spirulina upstream AMPK-mTOR + everolimus FKBP12: complementary; may reduce mTOR inhibitor dose requirement in some models. Leucine supplements (MPS; mTORC1 activation): Leucine stimulates Rag/mTORC1 (sestrin2 pathway); spirulina Leu (90 mg/5g) modest contribution; combined with 2–3g leucine supplement: additive MPS support in muscle context. Summary: S6K1 −15–30%, 4E-BP1 −10–20%, IRS-1-Ser307 −15–25%, LC3B-II +15–25%, TFEB +15–25%; dosing 5–10g daily. NK concern: low.