Spirulina and RIG-I/MAVS: Cytosolic RNA Sensing, Type I Interferon Induction, and Mitochondrial Signalling

RIG-I and MDA5: Cytosolic dsRNA Sensors with Distinct Specificity

Retinoic acid-inducible gene I (RIG-I, also DDX58) and melanoma differentiation- associated gene 5 (MDA5, IFIH1) are DExD/H-box RNA helicases that serve as the primary cytoplasmic surveillance system for foreign RNA. Both proteins contain an N-terminal tandem CARD (caspase activation and recruitment domain) followed by a central helicase domain and a C-terminal domain (CTD). In the resting state, RIG-I adopts an auto-inhibited conformation in which the CARD domains are masked by an intramolecular interaction with the CTD. Upon binding to viral RNA, the helicase domain wraps around the RNA duplex in an ATP-hydrolysis-coupled conformational change that exposes the CARDs.

RIG-I and MDA5 are not redundant—they recognise distinct viral RNA features. RIG-I preferentially detects short dsRNA (under ~300 bp) with a 5′-triphosphate (5′-ppp) or 5′-diphosphate overhang, features found on the genomic RNA of negative-sense RNA viruses (influenza, Sendai, rabies) and replication intermediates of positive-sense viruses (dengue, hepatitis C). The 5′-ppp is the critical pathogen-associated molecular pattern (PAMP); self-mRNA is capped with 7-methylguanosine and lacks free 5′-ppp, providing the self/non-self discrimination. MDA5 preferentially detects long dsRNA (kilobase-length), forming filamentous assemblies along the RNA duplex—a structure characteristic of picornavirus (poliovirus, encephalomyocarditis virus) and coronavirus replication complexes. LGP2, a third family member lacking CARDs, acts as a positive co-regulator of MDA5 and a context-dependent modulator of RIG-I.

MAVS on the Outer Mitochondrial Membrane: The Prion-Like Signalosome

Once RIG-I or MDA5 are activated, their exposed CARD2 domains interact with the CARD of MAVS (mitochondrial antiviral signalling protein; also called IPS-1, VISA, or CARDIF) through homotypic CARD–CARD electrostatic interactions. This is not simply a binary binding event—MAVS forms functional prion-like aggregates on the outer mitochondrial membrane (OMM), propagating through a positive-feedback self-templating mechanism that amplifies the initial RIG-I/MDA5 signal into a robust, switch-like activation. The MAVS N-terminal CARD is followed by a proline- rich region (scaffold for adaptor recruitment) and a C-terminal transmembrane domain that anchors it to the OMM; deletion of the TM domain ablates signalling even if protein expression is intact, confirming that mitochondrial localisation is mechanistically essential rather than incidental.

MAVS is also present, at lower levels, on peroxisomes and mitochondria-associated membranes (MAMs). Peroxisomal MAVS preferentially drives IRF1/IRF3-dependent expression of interferon-stimulated genes (ISGs) with faster but more transient kinetics; mitochondrial MAVS drives the full type I IFN (IFNα/β) induction with later peak kinetics. This spatial distribution means that peroxisome integrity and mitochondrial health are both relevant to the overall antiviral response amplitude.

TBK1–IRF3 and IKKε–IRF7: Downstream Kinase Cascades

Active MAVS aggregates recruit TRAF3 (TNF receptor-associated factor 3) and TRAF6 through their TRAF-binding motifs in the proline-rich region. TRAF3 bridges MAVS to the kinase complex containing TBK1 (TANK-binding kinase 1) and IKKε (also IKKi/IKBKE). TBK1 phosphorylates IRF3 (interferon regulatory factor 3) at the C-terminal cluster (Ser386 and Ser396 are the key activating sites), triggering IRF3 dimerisation, nuclear translocation, and transcription of IFNB1 (IFN-β) and a subset of early ISGs. In plasmacytoid dendritic cells (pDCs), which are the primary IFNα producers, IKKε preferentially phosphorylates IRF7 (constitutively expressed in pDCs), driving the full IFNα repertoire. In most other cell types, IRF7 requires IFN-β-driven (IFNAR → JAK1/TYK2 → STAT1/2 → ISGF3 → IRF7 induction) amplification before the IRF7-IFNα axis is engaged.

TRAF6 downstream of MAVS activates IKKα/β, inducing NF-κB and driving expression of pro-inflammatory cytokines (TNF-α, IL-6, IL-12) alongside the interferon response. This dual output—type I IFN plus NF-κB-driven cytokines—is appropriate for a bona fide viral infection but must be tightly regulated to avoid immunopathology. Multiple negative regulators constrain the pathway: NLRX1 (an NLR protein on the OMM) competitively binds MAVS to attenuate CARD–CARD interactions; RNF125 and RNF5 are E3 ligases that ubiquitinate RIG-I and MAVS for proteasomal degradation; PCBP2 and AIP4 (ITCH) form a complex that ubiquitinates MAVS at Lys7/Lys10/Lys500 with K48-linked chains, routing it to proteasomal clearance after signal termination.

Contrast with cGAS-STING: DNA vs RNA Sensing

The cGAS (cyclic GMP-AMP synthase)–STING (stimulator of interferon genes) pathway is the cytosolic DNA surveillance counterpart to RIG-I/MAVS. cGAS binds double- stranded DNA (from viral genomes, mitochondrial DNA leaked from damaged organelles, nuclear chromatin extruded in senescent or genotoxically stressed cells) and synthesises the second messenger cGAMP (2′3′-cGAMP), which binds and activates STING on the ER membrane. Activated STING translocates to the Golgi and recruits TBK1, which then phosphorylates IRF3—the same kinase and transcription factor used downstream of MAVS. Both pathways thus converge on TBK1–IRF3 for type I IFN induction but diverge at the sensing and second-messenger level. Importantly, STING is activated by mitochondrial DNA (mtDNA) released from damaged mitochondria—a sterile, damage-associated pathway that is independent of viral infection. This distinction matters for understanding how spirulina's mitochondrial effects are relevant: protecting mitochondrial integrity reduces mtDNA-driven cGAS-STING activation in the context of oxidative stress, while the RIG-I/MAVS pathway itself is not directly triggered by mitochondrial damage.

Mitochondrial Membrane Integrity, ROS, and MAVS Function

The requirement for MAVS to reside on a functional OMM means that mitochondrial health is a permissive condition for antiviral signalling. Mitochondrial membrane potential (ΔΨm) supports MAVS prion-like aggregation; mitochondrial fusion— governed by MFN1/MFN2 (outer membrane) and OPA1 (inner membrane)—appears to facilitate MAVS signalosome assembly, while mitochondrial fragmentation (DRP1- mediated fission, which can be triggered by oxidative stress) disperses MAVS aggregates and attenuates signalling. Conversely, excessive mitochondrial ROS can suppress MAVS activity through oxidative modification of cysteine residues in MAVS itself (Cys79, within the CARD domain) or through oxidative inactivation of upstream RIG-I.

Spirulina phycocyanin is a potent free-radical scavenger; its biliverdin-derived tetrapyrrole chromophore quenches peroxyl radicals, superoxide, and hydroxyl radicals at rates that compare favourably with synthetic antioxidants. By reducing mitochondrial ROS burden, phycocyanin preserves the Cys residues of RIG-I and MAVS in their reduced, functional state, maintaining the capacity for proper CARD–CARD engagement. Additionally, spirulina activates the Nrf2–ARE pathway, inducing HO-1 (haeme oxygenase-1) and NQO1, which reduce cellular oxidative stress broadly and support glutathione regeneration—further protecting RIG-I and MAVS from oxidative inactivation. Through AMPK activation, spirulina also promotes mitochondrial biogenesis (via PGC-1α) and fusion over fission, supporting the elongated, networked mitochondria on which optimal MAVS signalosome assembly depends.

The immunomodulatory nuance is that these effects are supportive and homeostatic rather than amplifying. Spirulina is not an interferon inducer or a MAVS agonist— there is no evidence that dietary spirulina generates the 5′-ppp dsRNA structures that activate RIG-I, nor does spirulina trigger MAVS aggregation de novo. What it appears to do is preserve the structural and biochemical conditions under which the RIG-I/MAVS pathway can operate efficiently when needed—a physiological tuning function. This is meaningfully different from stating that spirulina directly activates antiviral immunity, a claim that would require activation of the actual sensing machinery.

Practical Takeaway

For individuals who supplement spirulina and want to understand its relationship to antiviral immunity, the RIG-I/MAVS pathway offers a coherent mechanistic framework. The pathway's dependence on mitochondrial integrity and redox state means that spirulina's well-documented effects on ROS quenching, mitochondrial biogenesis, and oxidative stress are genuinely relevant to antiviral signalling capacity—even though spirulina is not triggering the pathway directly. Think of it as maintaining the fire station rather than starting fires. Under conditions of high oxidative stress—persistent inflammation, poor diet, aging—mitochondrial dysfunction can impair MAVS signalosome assembly and blunt the type I IFN response to genuine viral challenge. Spirulina's contributions to mitochondrial health and redox homeostasis represent one plausible route through which broad antioxidant supplementation might support, rather than supplant, innate antiviral defences.