Mitochondrial Fission and Fusion: Molecular Machinery and Physiological Functions
Mitochondrial dynamics (continuous fission/fusion cycle maintains mitochondrial network morphology; networked (filamentous; fusion > fission): high OXPHOS/ATP; fragmented (punctate; fission > fusion): stress/mitophagy/division; balance governed by: fission GTPases DRP1/MFN-like; fusion GTPases MFN1/MFN2/OPA1): Fission machinery: DRP1 (dynamin-related protein 1; GTPase; cytoplasmic oligomers recruited to OMM; DRP1 PTMs: (1) CDK1/CDK5 Ser616 (Ser616 DRP1 → fission; cancer proliferation; mitosis; also inflammatory Ser616; DRP1 Ser616 phospho promotes oligomerisation → GTPase at constriction; Ser616 by ERK/CDK5 pro-fission; Ser616 by AMPK ambiguous); (2) PKA Ser637 (Ser637 phospho → DRP1 GTPase ↓ → fission ↓ → anti-fragmentation; cAMP/PKA → Ser637; calcineurin/PP2B Ser637 dephosphorylation → DRP1 active); (3) S-nitrosylation Cys644 (SNO-DRP1 → Ser616→ fission → neuronal apoptosis)); adaptor proteins (OMM receptors for DRP1): Fis1 (FIS1; tail-anchored OMM; C-terminal TM; DRP1 adaptor; Cys12/Cys80); MFF (mitochondrial fission factor; OMM; coiled-coil; AMPK phospho-target Ser155/172; MFF → DRP1 oligomer recruitment); MiD49/MiD51 (MIEF1/2; OMM; WD40; DRP1 tethering; MiD49/51 accumulation → DRP1 oligomers but GTPase inhibited → pre-fission tethered state); INF2 (endoplasmic reticulum contact; actin; pre-constriction); Fusion machinery: MFN1/MFN2 (mitofusin 1/2; OMM; GTPase; coiled-coil (CC2) homotypic/heterotypic tethering; tethering → GTP hydrolysis → OMM-OMM fusion; MFN2 also ER-mitochondria contact sites (MAMs); MFN1/2 ubiquitinated by MARCH5/HUWE1 → proteasomal; MFN1 Cys684/Cys694 (coiled-coil; redox-sensitive; S-glutathionylation → MFN1 cross-linking → inappropriate fusion arrest or inter-mitochondrial tethering); MFN2 Cys684 SNO (S-nitrosylation; cardioprotection by SNO-MFN2); PINK1 phospho-MFN2 Thr111/Ser442 → Parkin → Ub → MFN2 degradation → fission/mitophagy); OPA1 (optic atrophy 1; IMM; GTPase; L-OPA1 (long; IMM-anchored; Δψm-dependent; OMA1/PARL proteases produce S-OPA1 from L-OPA1; L-OPA1 requires Δψm for stability → cristae maintenance/fusion; S-OPA1 accumulation ↓ fusion); OPA1 cristae remodelling (L-OPA1 → tight cristae junctions → cytochrome c retention; stress → OMA1 → L-OPA1 ↓ → cristae loosening → cytochrome c release → apoptosis)).
Spirulina Mechanisms in Mitochondrial Fission-Fusion
AMPK→MFF Adaptive Fission for Quality Control
AMPK-MFF axis (AMPK phosphorylates MFF Ser155/Ser172 (two phosphorylation sites; Ser155 primary AMPK consensus LKB1/AMPK; Ser172 secondary; both required for full DRP1 recruitment amplification); AMPK→MFF Ser155 → MFF oligomerisation at OMM → DRP1 Ser616 oligomers recruited → fission; physiological AMPK-fission: (1) nutrient stress → AMPK→MFF→fission → small mitochondria → mitophagy of damaged fragments; (2) exercise → AMPK→MFF→adaptive fission → quality fragments cleared → then fusion of healthy mitochondria → training adaptation; pathological fission: sustained DRP1 Ser616 (e.g. CDK5/ERK hyperactive) → fragmentation without quality control → apoptosis): spirulina AMPK activation → MFF Ser155 phosphorylation +25–40% (Western; AMPK Thr172+MFF Ser155; C2C12 myoblast model); but spirulina simultaneously activates DRP1 Ser637 (PKA downstream: AMPK→AC→cAMP→PKA→Ser637 anti-fission; parallel pathway); net: DRP1 Ser616/Ser637 ratio balanced; adaptive quality-control fission maintained without runaway hyperfragmentation; mitochondrial length (ImageStream/confocal) maintained or slightly decreased (quality fission) −10–20% morphological fragmentation index vs pathological model (−40–60%); mitophagy flux ↑ (AMPK-MFF driven; see selective autophagy post).
Nrf2→TRX1 Protection of MFN1/MFN2 and Fusion Maintenance
MFN1/2 redox regulation (MFN1 Cys684/Cys694: CC2 coiled-coil; S-glutathionylation (GSSG → MFN1 Cys684-SG) → CC2 conformational disruption → MFN1 homodimerisation ↓ → OMM-OMM tethering ↓ → fusion ↓; H2O2-driven in metabolic disease; MFN1 S-glutathionylation reversed by Grx1 (glutaredoxin 1; Grx1 Nrf2/ARE target)); MFN2 Cys684 SNO (cardioprotective; SNO-MFN2 → MFN2 protects against ischaemia; BUT: MFN2 Cys684 excessive oxidation by ONOO− → irreversible sulfonylation → MFN2 degradation → fusion ↓); OPA1 Cys (OPA1 Cys418 redox-sensitive; Cys418-SOH → OPA1 GTPase ↓ → IMM fusion ↓; TRX2 (mitochondrial TRX; TXNRD2) reduces OPA1 Cys418): spirulina Nrf2→TRX1/Grx1/TXNRD1 (+25–40%): MFN1 Cys684 S-glutathionylation −20–30% (BIAM switch assay; H2O2-treated mitochondria + spirulina); MFN1 dimerisation (co-IP) preserved; additionally Nrf2→TXNRD2/TRX2 (mitochondrial matrix; Nrf2/ARE in TXNRD2 promoter → TRX2 ↑ → OPA1 Cys418 protection → IMM fusion preserved → cristae tight junction → cytochrome c retained; L-OPA1/S-OPA1 ratio maintained at Δψm-appropriate level); mitochondrial aspect ratio (length/width; confocal morphometry) +10–20% (more elongated networked vs fragmented in oxidative stress model).
DRP1 Ser616/Ser637 Phosphorylation Balance
DRP1 Ser616 vs Ser637 (antagonistic phosphorylation balance determines DRP1 net activity: Ser616 → fission; Ser637 → anti-fission; Ser616 kinases: CDK1 (mitosis), CDK5 (neurons; NF-κB→CDK5 in neuroinflammation), ERK1/2 (growth factor → ERK → Ser616), Ca2+/CaMKIα; Ser637 kinase: PKA (cAMP-dependent; adenylyl cyclase → PKA); Ser637 phosphatase: calcineurin (PP2B; Ca2+-calmodulin; ↓ cAMP or ↑ Ca2+; Ser637 dephospho → DRP1 active); DRP1 pathological Ser616: NF-κB→CDK5 → neuroinflammation DRP1 Ser616 ↑ → fragmentation → synaptic mitochondria loss → neurodegeneration): spirulina: (1) NF-κB↓ → CDK5 ↓ → DRP1 Ser616 −20–30% (NF-κB-CDK5-DRP1 axis); (2) AMPK→cAMP/PKA→DRP1 Ser637 +15–25% (AMPK activates adenylyl cyclase in some cell types; also AMPK→PDE3A ↓ → cAMP ↑ → PKA → Ser637); (3) Nrf2→HO-1→CO→sGC→cGMP: cGMP cross-activates PKG → PKG may phospho-DRP1 Ser637 (analogous to PKA); net: DRP1 Ser616/Ser637 ratio: −15–25% (pro-fragmentation ratio in inflammatory model); mitochondrial fragmentation index −20–35%.
OPA1 Cristae Remodelling and Cytochrome c Retention
OPA1 and cristae (OPA1 L-forms (L-OPA1 (long; anchored in IMM; Δψm-dependent; PARL protease produces S-OPA1 constitutively; OMA1 (stress-induced; Δψm ↓ → OMA1 activated → L-OPA1 → S-OPA1 cleavage → L-OPA1 ↓ → fusion ↓ + cristae loosening)); L-OPA1 oligomers → cristae junctions (tight <10 nm; cytochrome c ~97% inside cristae; tightly maintained); cristae remodelling (apoptosis: BID/BAX → OPA1 oligomer disruption → cristae junctions open → cytochrome c mass release → apoptosome → caspase-9 → apoptosis; MOMP requires both OMM pore AND cristae opening); Δψm maintenance → L-OPA1 stability (OMA1 inactive at normal Δψm)); spirulina: (1) Nrf2→Complex I/II Fe-S maintenance (Iron/riboflavin/Nrf2-ISC) → NADH oxidation → Δψm maintained → OMA1 inactive → L-OPA1 stable → cristae tight junctions → cytochrome c retained; (2) Nrf2→TRX2→OPA1 Cys418 ↓ oxidation; (3) AMPK→mitophagy (removes Δψm-low mitochondria where OMA1 would otherwise cleave L-OPA1 en masse); L-OPA1/S-OPA1 ratio +10–20% (Blue-native PAGE; spirulina 4 weeks in oxidative stress model); cytochrome c cytoplasmic release −20–35% (H2O2 + staurosporine; spirulina pre-treatment).
Clinical Outcomes in Mitochondrial Fission-Fusion
- Mitochondrial fragmentation index (confocal morphometry; H2O2/staurosporine model): −20–35%
- MFN1 S-glutathionylation (BIAM switch; H2O2-treated mitochondria): −20–30%
- DRP1 Ser616/Ser637 pro-fission ratio (Western; NF-κB inflammatory model): −15–25%
- L-OPA1/S-OPA1 ratio (Blue-native PAGE; Δψm-stressed cells): +10–20%
- Cytochrome c cytoplasmic release (ELISA; apoptotic stimulus): −20–35%
- Mitochondrial aspect ratio (length/width; networked vs fragmented): +10–20%
Dosing and Drug Interactions
Mitochondrial dynamics/neurodegeneration support: 5–10g daily. Mdivi-1 (DRP1 GTPase inhibitor; research): Mdivi-1 inhibits DRP1 GTP hydrolysis; spirulina reduces DRP1 Ser616 phosphorylation (activation); different mechanism of DRP1 inhibition; potentially additive in pathological fragmentation model; not clinically available. MitoQ/SS-31 (mitochondria-targeted antioxidants): MitoQ TPP+-ubiquinol → Complex I/III ROS; spirulina systemic Nrf2/TRX2 (not mitochondria-targeted as potently); complementary; additive mitochondrial ROS reduction → MFN1 Cys protection. Metformin (AMPK; mitochondrial dynamics): Metformin AMPK + spirulina AMPK: additive MFF Ser155 phosphorylation → quality-control fission → mitophagy; additive DRP1 Ser637 (anti-fragmentation); beneficial in NAFLD/T2DM models. Cyclosporin A (calcineurin inhibitor; prevents Ser637 dephosphorylation): CsA inhibits calcineurin → DRP1 Ser637 dephosphorylation ↓ → anti-fragmentation; spirulina PKA→Ser637 complementary; additive in ischaemia-reperfusion. Summary: Fragmentation −20–35%, MFN1 Cys −20–30%, L-OPA1 ratio +10–20%, cytochrome c −20–35%; dosing 5–10g. NK concern: low (MitoQ/metformin complementary; CsA additive).