Spirulina and Amino Acid Sensing: mTORC1, GCN2, and the Lysosomal Amino Acid Axis

Lysosomal Amino Acid Sensing: Ragulator and v-ATPase

mTORC1 is recruited to the lysosomal surface by the Ragulator complex (LAMTOR1-5), which anchors the RAG GTPase heterodimer (RagA/B:RagC/D) to the lysosome. Amino acids accumulate in the lysosomal lumen and signal through: (1) the v-ATPase (vacuolar H+-ATPase), which communicates luminal amino acid status to Ragulator via a V1/V0 interaction (not yet fully characterised); (2) SLC38A9 (a lysosomal arginine sensor/transporter); (3) the GATOR1/GATOR2 supercomplex: GATOR1 (DEPDC5-NPRL2-NPRL3, a GAP for RagA/B) inactivates RagA by converting RagA-GTP to RagA-GDP. GATOR2 (WDR24-WDR59-MIOS-SEH1L-SEC13) inhibits GATOR1 when amino acids are sufficient.

Leucine Sensing: Sestrin2 and CASTOR1

Leucine is sensed by Sestrin2 (SESN2): leucine binds Sestrin2 LBS (leucine-binding site, Trp444 critical), causing Sestrin2 dissociation from GATOR2, releasing GATOR2 to inhibit GATOR1, resulting in active RagA-GTP and mTORC1 recruitment. Arginine is sensed by CASTOR1 (cellular arginine sensor for mTORC1), which forms homodimers binding arginine (Asp364 binding site); arginine binding releases CASTOR1 from GATOR2. SLC38A9 is a low-affinity lysosomal arginine sensor/transporter that directly activates Ragulator. Spirulina is rich in leucine (~5.3 g/100g protein), arginine, and all essential amino acids, providing the substrate for robust Sestrin2/CASTOR1-mediated mTORC1 activation in muscle anabolism contexts.

FLCN-FNIP and RagC/D GAP Activity

Folliculin (FLCN) complexed with FNIP1/2 acts as a GAP for RagC/D (converting RagC/D-GTP to RagC/D-GDP, which is the active state for mTORC1 recruitment since active Rag heterodimer is RagA/B-GTP:RagC/D-GDP). FLCN is recruited to lysosomes by Ragulator when amino acids are present, completing the mTORC1 activation cycle. FLCN loss-of-function mutations cause Birt-Hogg-Dube syndrome (renal cell carcinoma, fibrofolliculomas, pulmonary cysts). Understanding the FLCN-RagC/D node clarifies why complete amino acid sufficiency (all EAAs from spirulina) activates mTORC1 more robustly than partial amino acid supplementation.

GCN2: eIF2alpha Kinase for Amino Acid Deficiency

GCN2 (EIF2AK4) is activated by uncharged tRNAs that accumulate during amino acid starvation (GCN2 HisRS-like domain binds uncharged tRNA with ~10 microM affinity). Activated GCN2 phosphorylates eIF2alpha Ser51, globally suppressing translation while activating ATF4 (which induces amino acid transport genes: SLC7A11, SLC7A5/ LAT1, SLC3A2, and asparagine synthetase ASNS; plus NRF2 target overlap). Spirulina's complete amino acid profile reduces GCN2 activation, maintaining translation efficiency. However, GCN2-ATF4-SLC7A11 induction provides cystine for GSH, so complete GCN2 suppression by adequate leucine/essential AAs could theoretically reduce this cystine import pathway.

GATOR1 Mutations and Cancer

DEPDC5, NPRL2, and NPRL3 (GATOR1 components) are frequently mutated or deleted in glioblastoma, hepatocellular carcinoma, and bladder cancer, constitutively activating mTORC1 even in amino acid-poor conditions. This is the amino acid sensing equivalent of TSC1/2 mutation (Akt-driven mTORC1 activation). Rapamycin targets this pathway. Spirulina's AMPK activation opposes mTORC1 via TSC2/RAPTOR phosphorylation, providing an amino acid-level GATOR1-independent brake on mTORC1, relevant in GATOR1-mutant tumour contexts.

Amino Acid Quality and Bioavailability from Spirulina

Spirulina's protein (60-70% DW) has a PDCAAS (protein digestibility-corrected amino acid score) of ~0.8-0.9, limited by methionine+cysteine (sulfur AAs). Leucine (5.3 g%), isoleucine (4.1%), and valine (5.5%) as branch-chain AAs (BCAAs) activate mTORC1-S6K1 signalling in muscle. The limiting sulfur AAs are supplemented by spirulina's B6/folate support of the transsulfuration pathway (endogenous cysteine from homocysteine). This amino acid completeness, combined with the AMPK-mTOR balancing effects, positions spirulina as a net mTORC1 activator in muscle (anabolic) while simultaneously limiting mTORC1 hyperactivation through AMPK in metabolically stressed tissues.