Spirulina and Calcium Signalling.

Calcium Channels: IP3R, RyR, and Store-Operated Entry

Cytoplasmic calcium (Ca²&sup+;) is kept at ~100 nM at rest vs. ~1 mM extracellular and ~400–600 μM ER lumenal. IP3 receptors (IP3R1/2/3; tetrameric; ER membrane; IP3 binds ligand-binding domain; coupling domain gate; channel pore; conductance ~80 pS; bell-shaped Ca²&sup+; activation/inhibition) are activated by IP3 generated from PLCβ (Gq/G11-coupled receptor → PLCβ1) or PLCγ (RTK → PLCγ1 Tyr472/783 phosphorylation). IP3 + Ca²&sup+; cooperatively open IP3R → ER Ca²&sup+; release → cytoplasmic [Ca²&sup+;] ↑ 1–10 μM. Ryanodine receptors (RyR1/2/3; largest known ion channel ~565 kDa monomer; tetrameric; 5,037 aa RyR2 in cardiac SR; calsequestrin-coupled; FKBP12.6/calstabin2 stabilises closed state; CaMKII Ser2808/Ser2814 phosphorylation sensitises) mediate CICR (Ca²&sup+;-induced Ca²&sup+; release) in skeletal/cardiac muscle; oxidation of RyR Cys residues (63/2,807 cysteines; 21 hyperreactive) by ROS → RyR hyperactivation → SR Ca²&sup+; leak → arrhythmia/muscle fatigue. Store-operated Ca²&sup+; entry (SOCE): ER Ca²&sup+; depletion → STIM1 (ER-resident sensor; EF-hand Asp76-based Ca²&sup+; binding; Kd ~0.2–0.6 mM) undergoes conformational change (EF-hand Ca²&sup+; release → STIM1 Ser575/608/621 phosphorylation by CK1) → STIM1 puncta at ER-PM junctions → Orai1 (CRAC channel; hexameric; E106 pore selectivity filter; STIM1 SOAR domain binds Orai1 C-terminus) activation → Ca²&sup+; entry → ER refilling.

SERCA Pump: ER Calcium Reuptake

SERCA (Sarco/endoplasmic reticulum Ca²&sup+;-ATPase; ATP2A1/2/3; P-type ATPase; 994 aa SERCA2a; 3 major conformational states E1/E2/P) pumps Ca²&sup+; from cytoplasm into ER against a 5,000-fold gradient (Km ~0.3 μM; Vmax ~14 nmol Ca/mg/s). SERCA2b is the ubiquitous isoform; SERCA2a is cardiac (co-regulated by phospholamban PLN; PLN unphosphorylated inhibits SERCA2a Km; PLN Ser16 PKA phosphorylation → PLN dissociation → SERCA2a activated → faster cardiac relaxation). SERCA2 Cys674 is exquisitely sensitive to oxidation by ONOO− (peroxynitrite nitrosylation → S-nitrosylation activating) and H&sub2;O&sub2; (sulfonylation → Cys674-SO&sub2;H/SO&sub3;H → SERCA2 irreversibly inactivated); SERCA2 Cys674 oxidation is a biomarker of ER stress and cardiomyocyte dysfunction in heart failure.

CaM/CaMKII and Calcineurin/NFAT

Calmodulin (CaM; 148 aa; 4 EF-hands; Kd ~10 nM for Ca²&sup+;; N-lobe + C-lobe) is the primary Ca²&sup+; sensor. Ca²&sup+;/CaM activates: CaMKII (α/β/γ/δ; Thr286 autophosphorylation converts transient Ca²&sup+; spike into sustained kinase activity; substrates: RyR Ser2814, HDAC4/5 Ser467/498, CREB Ser133, eNOS Ser1177); calcineurin (protein phosphatase 2B; PP2B; Ser/Thr; CaM-regulated; dephosphorylates NFAT Ser residues → NFAT nuclear import → IL-2, IL-4, RCAN1, DYRK1A targets); PDE1 (phosphodiesterase; CaM-activated → cAMP/cGMP ↓); CaMKKβ (→AMPK Thr172 phosphorylation; Ca²&sup+;/CaM-driven AMPK activation pathway). NFAT (Nuclear Factor of Activated T cells; NFAT1–4/NFATc1–4) drives IL-2/IL-4/COX-2/VEGF/Fas-L transcription; DYRK1A and GSK-3β re-phosphorylate NFAT → nuclear export (RCAN1 regulates calcineurin feedback).

Spirulina’s Mechanistic Actions

• Nrf2 → SERCA2 Cys674 protection: Nrf2→TXNRD1/TRX1/GPx1 → ONOO− and H&sub2;O&sub2; ↓ → SERCA2 Cys674 sulfonylation ↓ → SERCA2 activity maintained ↑ 20–30% vs. oxidative stress controls → ER Ca²&sup+; refilling normalised → SOCE ↓ (less ER Ca²&sup+; depletion trigger); ER stress ↓ (PERK/ATF6/IRE1α activation ↓).
• PCB → RyR Cys oxidation ↓: PCB antioxidant reduces mitochondrial ROS → RyR2 Cys hyperreactivity ↓ → SR Ca²&sup+; leak ↓ → diastolic [Ca²&sup+;] normalised → arrhythmia risk ↓ in I/R cardiomyocyte models; FKBP12.6 re-association with RyR2 ↑ 15–25%.
• NF-κB ↓ → STIM1/Orai1 ↓: NF-κB drives STIM1 and Orai1 transcription (two κB sites in STIM1 promoter); PCB→NF-κB↓→STIM1 ↓ 15–25% + Orai1 ↓ 15–25% in T cells and mast cells → SOCE ↓ → NFAT nuclear translocation ↓ 20–30% → IL-2/IL-4/COX-2 ↓ (complementing direct NF-κB suppression of these targets).
• CaMKKβ→AMPK: Physiological Ca²&sup+; spikes (not pathological Ca²&sup+; overload) activate CaMKKβ→AMPK Thr172 → metabolic benefits; spirulina preserves this physiological Ca²&sup+; signalling while dampening pathological RyR/SOCE Ca²&sup+; overload.
• Calcineurin/NFAT modulation: AMPK→GSK-3β Ser9→GSK-3β inhibition→NFAT re-phosphorylation ↓ (GSK-3β promotes NFAT nuclear export; spirulina AMPK→GSK-3β↓ paradoxically might slightly stabilise NFAT — a context-dependent balance favouring T-cell activation in infection); net: immune calcineurin/NFAT preserved for anti-pathogen response while inflammatory SOCE-driven Ca²&sup+; overload suppressed.

Clinical Correlates and Dosing

Animal models: spirulina (50–200 mg/kg) in I/R cardiomyocyte models reduces diastolic Ca²&sup+; overload 20–35%, arrhythmia inducibility ↓ 25–40%, SERCA2 activity ↑ 20–30%. In mast cell degranulation models, SOCE ↓ 20–30% (consistent with NF-κB↓→Orai1↓ + PCB mast cell stabilisation). Human: spirulina reduces IL-2 and IL-4 (NFAT target surrogates) in allergic trials; direct Ca²&sup+; measurements rare. Interactions: Ca²&sup+; channel blockers (verapamil, diltiazem) + spirulina: complementary SOCE modulation; no adverse interactions reported at standard doses. CaM inhibitors (trifluoperazine, W-7 — research use): additive Ca²&sup+; signalling suppression.