ArfGAP2 deficiency ameliorates autoinflammation by regulating STING signaling and proton channel activity

Abstract

The cGAS-STING pathway is a critical regulator of innate immunity. When cyclic GMP-AMP synthase (cGAS) detects aberrant cytosolic DNA, it synthesizes the second messenger 2′3′-cGAMP, which binds and activates stimulator of interferon genes (STING) on the endoplasmic reticulum (ER). Activated STING then translocates to the Golgi apparatus, where it recruits and mutually phosphorylates TANK-binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3). Phosphorylated IRF3 dimerizes and translocates to the nucleus, where it drives the production of type 1 interferons (IFN-1). In addition to being activated by IFN-1, STING also engages the nuclear factor kappa-B (NF-κB) pathway to induce the production of proinflammatory cytokines and chemokines. Moreover, IFN-1 signaling upregulates the expressions of interferon-stimulated genes (ISGs) through the IFN-α receptor (IFNAR). STING-associated vasculopathy with onset in infancy (SAVI) is a severe pediatric autoimmune disease caused by heterozygous gain-of-function mutations in STING, such as the N154S variant in humans and N153S in mice. Initially, classified as an interferonopathy due to constitutive activation of the STING pathway, SAVI is characterized by systemic inflammation, vasculopathy, interstitial lung disease, T-cell cytopenia, skin ulcerations, and premature death. However, recent studies challenge this paradigm, demonstrating that SAVI pathology develops independently of IFNAR-1 and IRFs (IRF3 and IRF7) [1,2], suggesting that alternative mechanisms drive disease progression. Emerging evidence indicates that STING restricts microbial infection through noncanonical autophagy and cell death pathways [3,4]. These functions may depend on its recently identified role as a proton channel in the Golgi apparatus [4,5]. Xun et al. [4] demonstrated that ligand-bound STING forms an ion channel in its transmembrane domain, facilitating proton efflux from post-Golgi vesicles and inducing Golgi deacidification. This raises a critical question: could STING-mediated Golgi deacidification be a potential mechanism underlying SAVI pathogenesis? A recent study by Poddar et al. [6] identified ADP ribosylation factor GTPase-activating protein 2 (ArfGAP2), which is involved in coatomer protein-1 (COP-1) coating in Golgi vesicles, as a key regulator of both STING signaling and proton channel activity, offering novel therapeutic insights for SAVI. First, to elucidate the role of STING in SAVI pathogenesis, they conducted a genome-wide CRISPR-Cas9 screen in T cells resistant to chronic STING activation and identified ArfGAP2 as a critical STING modulator among multiple Golgi-related proteins. Further investigation revealed that genetic ablation of ArfGAP2 in Jurkat T cells significantly attenuated STING-mediated ISG induction. Compelling evidence indicates that STING activation is associated with its subcellular location [7]. While ArfGAP family proteins typically regulate Golgi membrane trafficking, vesicle transport, and cargo sorting [8], Poddar et al. [6] surprisingly reported that ArfGAP2 enhances STING-mediated ISG induction and promotes LC3 lipidation without altering STING palmitoylation or its Golgi localization. Further experiments revealed that ArfGAP2 enhances STING signaling and IFN-β secretion in mouse bone marrow-derived macrophages (BMDMs). In addition to promoting IFN-1 induction, ArfGAP2 promotes the secretion of NF-κB-dependent proinflammatory cytokines activated by STING in THP-1 monocytes [6]. In addition to inducing ISGs and NF-κB signaling, activated STING acts as a proton channel triggering Golgi deacidification [4]. Given the well-established importance of the Golgi pH in regulating enzyme activity, protein modification, and membrane trafficking [9], researchers have further explored how STING and ArfGAP2 modulate cargo transport and secretion. They reported that the loss of ArfGAP2 impairs STING-mediated proton channel activity in the Golgi, leading to a lower luminal pH. Moreover, ArfGAP2-deficient cells presented significant alterations in the cell surface proteome upon STING activation, accompanied by altered sorting, secretion and trafficking rates of specific protein cargos in the Golgi [6].

1. Introduction

The cGAS-STING pathway is a critical regulator of innate immunity. When cyclic GMP-AMP synthase (cGAS) detects aberrant cytosolic DNA, it synthesizes the second messenger 2′3′-cGAMP, which binds and activates stimulator of interferon genes (STING) on the endoplasmic reticulum (ER). Activated STING then translocates to the Golgi apparatus, where it recruits and mutually phosphorylates TANK-binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3). Phosphorylated IRF3 dimerizes and translocates to the nucleus, where it drives the production of type 1 interferons (IFN-1). In addition to being activated by IFN-1, STING also engages the nuclear factor kappa-B (NF-κB) pathway to induce the production of proinflammatory cytokines and chemokines. Moreover, IFN-1 signaling upregulates the expressions of interferon-stimulated genes (ISGs) through the IFN-α receptor (IFNAR).

STING-associated vasculopathy with onset in infancy (SAVI) is a severe pediatric autoimmune disease caused by heterozygous gain-of-function mutations in STING, such as the N154S variant in humans and N153S in mice. Initially, classified as an interferonopathy due to constitutive activation of the STING pathway, SAVI is characterized by systemic inflammation, vasculopathy, interstitial lung disease, T-cell cytopenia, skin ulcerations, and premature death. However, recent studies challenge this paradigm, demonstrating that SAVI pathology develops independently of IFNAR-1 and IRFs (IRF3 and IRF7) [1,2], suggesting that alternative mechanisms drive disease progression.

Emerging evidence indicates that STING restricts microbial infection through noncanonical autophagy and cell death pathways [3,4]. These functions may depend on its recently identified role as a proton channel in the Golgi apparatus [4,5]. Xun et al. [4] demonstrated that ligand-bound STING forms an ion channel in its transmembrane domain, facilitating proton efflux from post-Golgi vesicles and inducing Golgi deacidification. This raises a critical question: could STING-mediated Golgi deacidification be a potential mechanism underlying SAVI pathogenesis?