TBK1 Inhibitors as ALS Therapeutics: Targeting Downstream STING Signaling

The cGAS-STING pathway drives neuroinflammation in ALS through aberrant cytoplasmic mitochondrial DNA recognition, but therapeutic intervention may be more effectively achieved by targeting the downstream kinase TBK1 rather than STING itself. Following STING activation by cGAMP, the pathway’s inflammatory output critically depends on TBK1 (TANK-binding kinase 1), a 729-amino acid serine/threonine kinase that serves as the obligate signal transducer for both type I interferon and pro-inflammatory cytokine production. TBK1 contains an N-terminal kinase domain, a central ubiquitin-like domain (ULD), and a C-terminal adaptor-binding domain that facilitates STING interaction. Upon recruitment to activated STING at the Golgi, TBK1 undergoes trans-autophosphorylation at serine 172 within its activation loop, dramatically enhancing its catalytic activity. Activated TBK1 then phosphorylates STING at C-terminal serines 365 and 366, creating recruitment platforms for IRF3 and IRF7. TBK1-mediated phosphorylation of IRF3 at serines 396 and 398 promotes IRF3 dimerization and nuclear translocation, driving transcription of type I interferons (IFN-α/β) and interferon-stimulated genes (ISGs). Simultaneously, TBK1 activates NF-κB signaling through phosphorylation of IKKε and direct phosphorylation of p65/RelA at serine 536. This dual transcriptional program results in sustained production of inflammatory mediators including TNF-α, IL-1β, IL-6, and chemokines that perpetuate microglial activation and motor neuron toxicity. Targeting TBK1 offers several advantages over direct STING inhibition: (1) TBK1 represents a bottleneck where multiple innate immune pathways converge, potentially providing broader anti-inflammatory effects; (2) existing TBK1 inhibitors developed for oncology applications (such as BX795, MRT67307, and GSK8612) could be repurposed; (3) TBK1 inhibition preserves upstream cGAS-STING sensing while specifically blocking the inflammatory output, potentially maintaining some beneficial innate immune functions while reducing pathological neuroinflammation in ALS.