ADAR1 and ZBP1 in innate immunity, cell death, and disease

Abstract

ADAR1 and ZBP1 are the only two mammalian proteins that contain Zα domains, which are thought to bind to nucleic acids in the Z-conformation. These two molecules are crucial in regulating diverse biological processes. While ADAR1-mediated RNA editing supports host survival and development, ZBP1-mediated immune responses provide host defense against infection and disease. Recent studies have expanded our understanding of the functions of ADAR1 and ZBP1 beyond their classical roles and established their fundamental regulation of innate immune responses, including NLRP3 inflammasome activation, inflammation, and cell death. Their roles in these processes have physiological impacts across development, infectious and inflammatory diseases, and cancer. In this review, we discuss the functions of ADAR1 and ZBP1 in regulating innate immune responses in development and disease.

Keywords: ADAR1; NLRP3; PANoptosis; PANoptosome; RIPK1; RIPK3; ZBP1; Zα domain; apoptosis; cancer; caspase; cell death; development; homeostasis; infection; inflammasome; inflammation; innate immunity; necroptosis; pyroptosis; tumorigenesis.

Figure 1:. Possible mechanisms of ADAR1 suppression of ZBP1-mediated PANoptosis

Two plausible mechanisms of ADAR1 regulation of ZBP1-mediated PANoptosis in mammalian cells may exist: 1) competition between ADAR1^p150 and RIPK3 for ZBP1 or 2) competition between ADAR1^p150 and ZBP1 for Z-NAs. ZBP1 has both Zα and RHIM domains. With its Zα domains, ZBP1 interacts with the Zα domain of ADAR1^p150; with its RHIM domains, ZBP1 interacts with RIPK3. Competition between ADAR1^p150 and RIPK3 in binding ZBP1 may exist. While the ZBP1-ADAR1^p150 complex does not induce cell death, the ZBP1-RIPK3 complex induces PANoptosis. For ZBP1 to interact with RIPK3, it should sense Z-NAs via its Zα domain. Since ADAR1^p150 also senses Z-NAs, there may be competition between ADAR1 and ZBP1 for binding to Z-NAs. Binding of Z-ΝAs by ADAR1 leads to the formation of modified NAs, which may fail to be efficiently sensed by NA sensors including ZBP1. ADAR, adenosine deaminase acting on RNA; NAs, nucleic acids; RIPK3, receptor-interacting serine/threonine-protein kinase 3; RHIM, receptor-interacting protein homotypic interaction motif; ZBP1, Z-DNA binding protein 1; Z-NAs, Z-nucleic acids.

Figure 2:. Schematic showing cancer-associated mutations in human ADAR1 and ZBP1

Cancer-associated mutations in the human proteins A) ADAR1 (https://pecan.stjude.cloud/proteinpaint/ADAR1) and B) ZBP1 (https://pecan.stjude.cloud/proteinpaint/ZBP1) as per the Catalogue of Somatic Mutations in Cancer (COSMIC) database and the Pediatric Cancer Genome Project depicted using ProteinPaint (https://pecan.stjude.cloud/proteinpaint). Each color-coded circle depicts a mutation, and the larger circles with numbers represent mutations found in more than one sample. This interactive tool allows for visualization of major attributes of the mutation, including the details of the sample from which the mutation was identified. ADAR, adenosine deaminase acting on RNA; dsRBD, double-stranded RNA binding domain; UTR, untranslated region; ZBP1, Z-DNA binding protein 1.

Figure 3:. Model of ADAR1 suppression of ZBP1-mediated PANoptosis and its therapeutic modulation for cancer therapy

In mammalian macrophages, IFN or IFN agonists induce expression of both ADAR1^p150 and ZBP1, which can interact via their Zα domains to induce cell survival and, in the context of cancers, tumor progression. During treatment with NEIs (i.e., KPT-330) and IFNs or their agonists, ADAR1^p150 is sequestered in the nucleus, causing dsRNA accumulation in the cytosol. The dsRNA is then recognized by the Zα domain of ZBP1, which is followed by the interaction of ZBP1 with RIPK3 via their RHIM domains, leading to the formation of a PANoptosome complex that also contains caspase-8, ASC, NLRP3, and other proteins. The multiprotein PANoptosome executes PANoptosis, which inhibits tumorigenesis ADAR1, adenosine deaminase acting on RNA1; ASC, apoptosis-associated speck-like protein containing a caspase activation and recruitment domain; IFN, interferon; NEI, nuclear export inhibitor; NLRP3, nucleotide-binding oligomerization domain-like receptor (NLR) family pyrin domain-containing 3; RIPK3, receptor-interacting serine/threonine-protein kinase 3; RHIM, receptor-interacting protein homotypic interaction motif; ZBP1, Z-DNA binding protein 1.

Figure I in Box 1:. Organization of domains in human ADAR family proteins

ADAR1, ADAR2, and ADAR3 are three members of the human ADAR family of proteins. All three proteins contain a dsRNA binding domain and a deaminase domain. While the deaminase domains of ADAR1 and ADAR2 are active, the deaminase domain of ADAR3 lacks catalytic activity. Alternative promoters and splicing give rise to two forms of ADAR1, the interferon (IFN)-inducible ADAR1^p150 isoform and the ADAR1^p110 isoform. The N-terminal region of the p150 isoform possesses two Z-DNA binding domains (Zα and Zβ). While ADAR1^p150 contains both NES and NLS signals, all other ADAR members contain only an NLS signal. ADAR, adenosine deaminase acting on RNA; dsRNA, double-stranded RNA; NES, nuclear export signal; NLS, nuclear localization sequence.

Figure II in Box 2:. Domain organization of Zα-containing proteins

ADAR1^p150, ZBP1, E3L, PKZ ORF112, and RBP7910 are Zα domain-containing proteins. While ADAR1^p150, E3L, and ORF112 contain a single Zα domain, ZBP1, PKZ, and RBP7910 contain two Zα domains. ADAR1 and E3L also possess dsRNA binding domains. ZBP1 contains two RHIM domains. The species where each protein can be found is designated in blue. AA, amino acid; ADAR1, adenosine deaminase acting on RNA 1; dsRBD, double-stranded RNA binding domain; dsRNA, double-stranded RNA; PKZ, protein kinase containing Z-DNA binding domains; RHIM, receptor-interacting protein homotypic interaction motif; ZBP1, Z-DNA binding protein 1.