The Trinity of cGAS, TLR9, and ALRs Guardians of the Cellular Galaxy Against Host-Derived Self-DNA

Abstract

The immune system has evolved to protect the host from the pathogens and allergens surrounding their environment. The immune system develops in such a way to recognize self and non-self and develops self-tolerance against self-proteins, nucleic acids, and other larger molecules. However, the broken immunological self-tolerance leads to the development of autoimmune or autoinflammatory diseases. Pattern-recognition receptors (PRRs) are expressed by immunological cells on their cell membrane and in the cytosol. Different Toll-like receptors (TLRs), Nod-like receptors (NLRs) and absent in melanoma-2 (AIM-2)-like receptors (ALRs) forming inflammasomes in the cytosol, RIG (retinoic acid-inducible gene)-1-like receptors (RLRs), and C-type lectin receptors (CLRs) are some of the PRRs. The DNA-sensing receptor cyclic GMP-AMP synthase (cGAS) is another PRR present in the cytosol and the nucleus. The present review describes the role of ALRs (AIM2), TLR9, and cGAS in recognizing the host cell DNA as a potent damage/danger-associated molecular pattern (DAMP), which moves out to the cytosol from its housing organelles (nucleus and mitochondria). The introduction opens with the concept that the immune system has evolved to recognize pathogens, the idea of horror autotoxicus, and its failure due to the emergence of autoimmune diseases (ADs), and the discovery of PRRs revolutionizing immunology. The second section describes the cGAS-STING signaling pathway mediated cytosolic self-DNA recognition, its evolution, characteristics of self-DNAs activating it, and its role in different inflammatory conditions. The third section describes the role of TLR9 in recognizing self-DNA in the endolysosomes during infections depending on the self-DNA characteristics and various inflammatory diseases. The fourth section discusses about AIM2 (an ALR), which also binds cytosolic self-DNA (with 80-300 base pairs or bp) that inhibits cGAS-STING-dependent type 1 IFN generation but induces inflammation and pyroptosis during different inflammatory conditions. Hence, this trinity of PRRs has evolved to recognize self-DNA as a potential DAMP and comes into action to guard the cellular galaxy. However, their dysregulation proves dangerous to the host and leads to several inflammatory conditions, including sterile-inflammatory conditions autoinflammatory and ADs.

Keywords: Toll-like receptor 9; absent in melanoma-2; absent in melanoma-2-like receptors; autoimmunity; cyclic GMP–AMP synthase; inflammation; stimulator of interferon genes.

Figure 1

Schematic representation of cGAS-STING and TLR9 signaling in response to the recognition of host-derived self-DNA as a DAMP. The entry of the self-DNA in the cytosol due to mitochondrial damage, nuclear damage, exosome-derived DNA, and the phagocytosis of dead cell does not remain as hidden from cytosolic PRRs. The cGAS identifies them as a potent DAMP in the cytosol and catalyze the cGAMP formation. The cellular exposure to the ionic radiations also induces cytosolic levels of chromatin DNA, which is also recognized by cGAS as a DAMP. The cGAMP is recognized by downstream signaling molecule called STING located in ER. STING activation phosphorylates TBK1 that further activates or phosphorylates IRF3. IRF3 stimulates IRF3 target genes, including ISGs, which also include type 1 IFN. Also, STING activation activates NF-κB target pro-inflammatory genes for cytokines and chemokines. Hence, cGAS-STING signaling plays a crucial role in inflammation, cancer, auto-inflammation and ADs. The cGAMP recognition by STING also induces its autophagosome-mediated degradation. The p62 is an endogenous negative regulator of the STING and induces its autophagic degradation. BECN1 is also an endogenous cGAS inhibitor and their interaction removes Rubicon (an autophagy inhibitor) from the BECN1 that induces autophagy to remove cytosolic DNA. Hence, autophagy serves to remove cytosolic DNA without inducing inflammatory damage. Failure of autophagy increases inflammatory recognition of cytosolic DNAs by different cytosolic PRRs. On the other hand TLR9 is present in the ER during resting stages as soon as cytosolic CpG DNAs or host DNA enter into the endosome or endolysosomes TLR9 also migrates there and recognizes them as a crucial DAMP. TLR9 activation induces MyD88-dependent downstream signaling pathway to activate IRF3-based type 1 IFN production and NF-κB-mediated pro-inflammatory cytokine generation to cause inflammation and inflammatory diseases. MyD88 has a TIR domain and death domain (DD). The TIR domain of MyD88 activates interleukin-1 receptor-associated kinase-4 (IRAK-4) and IRAK-1. IRAK-4 subsequently recruits TRAF6 to activate transforming growth factor-β associated kinase 1 (TAK1). TAK1 is linked to TRAF6 via TAB2 adaptor protein, whereas TAB1 adaptor protein interacts constitutively with TAK1 and induces TAK1 kinase activity. TAK1 then phosphorylates IκB kinase (IKK) complex through K63-linked ubiquitination of NEMO (NF-κB essential modulator), an IκB kinase regulatory subunit that is critical for the NF-κB, IRF3, and MAPK signaling.

Figure 2

Schematic representation of AIM2 in response to self-DNA. AIM2 is an ALR that becomes activated upon recognizing and binding to the self-DNA coming into cytosol due to cellular damage, including mitochondrial and nuclear damage, and exosome with host DNA. AIM2 activates efficiently in response to self-DNA with 80–300 bp. The HIN domain of the AIM2 recognizes cytosolic DNA and its PYD interacts with the PYD of the ASC to form inflammasome complex that activates procaspase 1 (pro-CASP1) into CASP1. The CASP 1 cleaves pro-IL-1α and -IL-18 into IL-1α and IL-18. CASP1 also cleaves the linker region of the GSDMD and frees GSDMD domains (GSDMD-N and GSDMD-C). The free GSDMD-N terminals interacts with phosphoinositides or other acidic lipids to oligomerize and form GSDMD pore. The GSDMD pore mediates the IL-1α and IL-18 release from the cells. Also, the K⁺ efflux from the GSDMD pore inhibits cGAS activity and the cGAS-STING mediated type 1 IFN release along with inducing pyroptosis. The AIM2-induced GSDMD acts as a negative regulator of cGAS-STING-mediated type 1 IFN production. Also, AIM2-ASC inflammasome inhibits STING-TBK1 interaction required for IRF3-dependent type 1 IFN release. The AIM2 remains inactive in the absence of specific cytosolic DNAs.