Spirulina and Mitochondrial Dynamics: Fusion, Fission, DRP1, and Mitophagy

Mitochondrial Fusion: MFN1/2 and OPA1

Mitochondrial fusion is mediated by: (1) outer membrane: mitofusin 1 and 2 (MFN1/2, dynamin-related GTPases forming trans-GTP-hydrolysis tethers between adjacent mitochondria); (2) inner membrane: OPA1 (long and short forms; long forms anchored by MFN1/2-assembled outer membranes, short forms generated by stress proteases YME1L/OMA1). Fusion dilutes damaged proteins and mtDNA mutations within the mitochondrial network, maintaining function. MFN2 also forms ER-mitochondria contact sites (MAMs) important for calcium and phospholipid transfer. SIRT1/3 deacetylate and stabilise MFN1/2; AMPK promotes MFN1 stability. Spirulina supports MFN1/2 via SIRT1/SIRT3 and AMPK activation, promoting network connectivity.

Mitochondrial Fission: DRP1 and MiD49/MiD51/FIS1

Dynamin-related protein 1 (DRP1/DNM1L) is a cytosolic GTPase that oligomerises into rings constricting the outer membrane. Receptors recruiting DRP1 include MiD49/MiD51 (MIEF1/MIEF2) and FIS1. DRP1 activity is regulated by phosphorylation: CDK1/5 Ser616 activates DRP1 (pro-fission); PKA Ser637 inhibits DRP1 (anti-fission). Excessive DRP1-driven fission fragments the mitochondrial network, reduces OXPHOS efficiency, and releases cytochrome c (apoptotic signal). AMPK phosphorylates and inhibits DRP1 Ser637 (same as PKA), opposing fission. PCB reduces iNOS/peroxynitrite, preventing oxidative DRP1 S-nitrosylation (Cys644, pro-fission modification active in neurodegenerative contexts).

PINK1-Parkin Mitophagy: Quality Control

The PINK1-Parkin pathway selectively targets damaged (depolarised) mitochondria for autophagic clearance. In healthy mitochondria, PINK1 (PTEN-induced kinase 1) is imported and cleaved by PARL/MPP; in depolarised mitochondria, PINK1 accumulates on the outer membrane, phosphorylates ubiquitin Ser65 and MFN2/parkin itself (activating parkin E3 ligase). Activated Parkin ubiquitinates outer membrane proteins (VDAC1, MFN1/2, TOM20/CISD2), recruiting autophagy adaptors (p62/SQSTM1, optineurin/OPTN, TAX1BP1, NDP52) via ubiquitin-binding domains. AMPK directly phosphorylates ULK1 Ser317/777 to initiate autophagosome formation, coordinating PINK1-Parkin ubiquitin signals with autophagy machinery. Spirulina's AMPK and SIRT3 (maintaining mitochondrial membrane potential, reducing PINK1 accumulation on healthy mitochondria) axes collectively orchestrate mitophagy.

BNIP3L/NIX and FUNDC1: Receptor-Mediated Mitophagy

In addition to PINK1-Parkin, mitophagy can occur through mitophagy receptors: BNIP3L (NIX) is induced by HIF-1alpha during hypoxia and drives reticulocyte mitochondrial clearance during red blood cell maturation; FUNDC1 is activated by dephosphorylation at Ser17 by PGAM5 phosphatase under hypoxia; BCNL3 (BNIP3) is induced by FOXO3a during starvation. These receptors interact with LC3-II on autophagosome membranes via LIR (LC3-interacting region) motifs. Spirulina's HIF-1alpha attenuation (via Nrf2/NF-kB) reduces BNIP3L/HIF-driven mitophagy while AMPK-ULK1 supports starvation-type receptor-mediated mitophagy.

Cristae Remodelling: OPA1 and Cytochrome c Release

Inner membrane cristae architecture determines cytochrome c retention (cytochrome c is concentrated in cristae, accessed by the cristae junctions). OPA1 oligomers maintain tight cristae junctions via GTPase-dependent curvature. Apoptotic stimuli (tBid, Ca2+ overload) activate OMA1 to cleave long-OPA1 to short-OPA1, opening cristae junctions and releasing cytochrome c even before outer membrane permeabilisation. Spirulina's SIRT3-Complex I/III activity preservation (reducing mitochondrial ROS that activates OMA1), calcium homeostasis support (SERCA2), and BCL-2 family modulation (Nrf2-NQO1-mediated ROS reduction stabilises anti-apoptotic BCL-2) all protect OPA1 oligomers and cristae integrity.

Spirulina and Mitochondrial Morphology Evidence

Electron microscopy studies in CCl4 hepatotoxicity and doxorubicin cardiotoxicity models show spirulina supplementation preserves mitochondrial morphology (intact cristae, regular inner/outer membrane structure, reduced vacuolation) compared to damaged controls. These ultrastructural findings are consistent with SIRT3 activation, AMPK-DRP1 fission inhibition, and PINK1-Parkin quality control, providing morphological validation of the described biochemical mechanisms.