1.A.140. The Stimulator of Interferon Genes Protein (STING) Family
STING is a facilitator of innate immune signaling that acts as a sensor of cytosolic DNA from bacteria and viruses and promotes the production of type I interferon (IFN-alpha and IFN-beta) (Ishikawa and Barber 2008, Shang et al. 2019). Moreover, an alternatively spliced STING isoform localizes in the cytoplasmic membrane and directly senses extracellular cGAMP (Li et al. 2022) (see 2.A.48.1.1). Zhang et al. 2022 presented a vivid panorama of STING biology taking into account the details of the biochemical assay and structural information, especially its versatile outputs and functions beyond IFN induction. They also summarized the roles of STING in the pathogenesis of various diseases and highlighted the development of small-molecular compounds targeting STING for disease treatment. A non-nucleotide STING agonist that does not target the cGAMP-binding pocket has been identified (Matsumoto et al. 2023).
2',3'-cGAMP, produced by the DNA sensor cGAS, activates STING and triggers an immune response during infection. Liu et al. 2023 found that apo-STING interacts with a bilayer with head-to-head as well as side-by-side packing, mediated by its ligand-binding domain (LBD). This type of assembly holds two endoplasmic reticulum (ER) membranes together not only to prevent STING ER exit but also to eliminate the recruitment of TBK1, representing the autoinhibited state of STING. The filament structure of the STING/2',3'-cGAMP complex, which adopts a bent monolayer assembly mediated by LBD and the transmembrane domain (TMD). The active, curved STING polymer could deform the ER membrane to support its ER exit and anterograde transport (Liu et al. 2023).
Increased STIM1 (see TC# 1.A.52.1.1) facilitates the aberrant proliferation and apoptosis of vascular smooth cells (VSMC) and macrophages which can promote the formation of rupture-prone plaques (Wan et al. 2022). While regulating the cytosolic Ca2+ concentration, STIM1 activates STING through altered intracellular Ca2+ concentrations; Ca2+ is a critical pro-inflammatory molecule. The cGAS-STING pathway is linked with cellular proliferation and phenotypic conversion of VSMC and enhances the progression of atherosclerosis plaques. Thus, STIM1/cGAS-STING is involved in the progression of atherosclerosis and plaque vulnerability (Wan et al. 2022). Human STING functions as a proton channel to promote LC3B lipidation and NLRP3 inflammasome activation (PMID# 37565720).
Proton leakage from organelles is a common signal for noncanonical light chain 3B (LC3B) lipidation and inflammasome activation, processes induced upon stimulator of interferon gene (STING) activation. On the basis of structural analysis, Liu et al. 2023 hypothesized that human STING is a proton channel. They found that STING activation induced a pH increase in the Golgi and that STING reconstituted in liposomes enabled transmembrane proton transport. Compound 53 (C53), a STING agonist that binds the putative channel interface, blocked STING-induced proton flux in the Golgi and in liposomes. STING-induced LC3B lipidation and inflammasome activation were also inhibited by C53, suggesting that STING's channel activity is critical for these two processes. STING's interferon-induction function can be decoupled from its roles in LC3B lipidation and inflammasome activation (Liu et al. 2023). Clathrin-associated AP-1 controls termination of STING signalling (Liu et al. 2022).
A conserved H+ channel function of STING mediates noncanonical autophagy and cell death (Xun et al. 2024). The cGAS/STING pathway triggers inflammation upon diverse cellular stresses such as infection, cellular damage, aging, and diseases. STING also triggers noncanonical autophagy, involving LC3 lipidation on STING vesicles through the V-ATPase-ATG16L1 axis, as well as induces cell death. The proton pump V-ATPase senses organelle deacidification in other contexts, but STING activates V-ATPase for noncanonical autophagy. A conserved channel function of STING in proton efflux and vesicle deacidification has been demonstrated (Xun et al. 2024). STING activation induces an electron-sparse pore in its transmembrane domain, which mediates proton flux in vitro and the deacidification of post-Golgi STING vesicles in cells. A chemical ligand of STING, C53, which binds to and blocks its channel, strongly inhibits STING-mediated proton flux in vitro. C53 fully blocks STING trafficking from the ER to the Golgi, but adding C53 after STING arrives at the Golgi allows for selective inhibition of STING-dependent vesicle deacidification, LC3 lipidation, and cell death, without affecting trafficking. The discovery of STING as a channel opens new opportunities for selective targeting of canonical and noncanonical STING functions (Xun et al. 2024).
The transport reaction believed to be catalyzed by STING is:
H+ (out) ⇋ H+ (in)