Activation and Immune Regulation Mechanisms of PYHIN Family During Microbial Infection

Abstract

The innate immune system defenses against pathogen infections via patten-recognition receptors (PRRs). PRRs initiate immune responses by recognizing pathogen-associated molecular patterns (PAMPs), including peptidoglycan, lipopolysaccharide, and nucleic acids. Several nucleic acid sensors or families have been identified, such as RIG-I-like receptors (RLRs), Toll-like receptors (TLRs), cyclic GMP-AMP synthase (cGAS), and PYHIN family receptors. In recent years, the PYHIN family cytosolic DNA receptors have increased attention because of their important roles in initiating innate immune responses. The family members in humans include Absent in melanoma 2 (AIM2), IFN-γ inducible protein 16 (IFI16), interferon-inducible protein X (IFIX), and myeloid cell nuclear differentiation antigen (MNDA). The PYHIN family members are also identified in mice, including AIM2, p202, p203, p204, and p205. Herein, we summarize recent advances in understanding the activation and immune regulation mechanisms of the PYHIN family during microbial infection. Furthermore, structural characterizations of AIM2, IFI16, p202, and p204 provide more accurate insights into the signaling mechanisms of PYHIN family receptors. Overall, the molecular details will facilitate the development of reagents to defense against viral infections.

Keywords: AIM2; IFI16; PAMP; PRR; PYHIN family; innate immunity; p202; p204.

FIGURE 1

Domain organization of PYHIN family proteins. PYHIN family members possess an N-terminal pyrin domain (PYD) and one or two C-terminal HIN domains, classified as three subtypes; HIN A, HIN B, or HIN C. p202 from mouse lacks PYD and POP3 from human lacks of HIN domains.

FIGURE 2

Structures of PYHIN family proteins. (A) Structural superposition of PYD of AIM2, MNDA, and p205. The structure of human AIM2 PYD with MBP (PDB: 3VD8) and its mutant (F27G, PDB: 4O7Q) are shown in cyan and pale green. The structure of mouse AIM2 PYD (PDB: 2N00) is shown in slate. Structures of human MNDA PYD without MBP (PDB: 5WPZ) and with MBP (PDB: 5H7Q, 5WQ6) are shown in pink, orange, and wheat. The solution structure of p205 PYD (PDB: 2YU0) is shown in gray. (B) Structural superposition of HIN domains of IFI16, p202, and p204. Structures of p204 HINa (PDB: 5YZP) and HINb (PDB: 5YZW) are shown in wheat and cyan. Structures of IFI16 HINa (PDB: 2OQ0) and HINb (PDB: 3B6Y) are shown in orange and slate. Structures of p202 HINa (PDB: 4JBJ) and HINb (PDB: 4L5T) are shown in pink and pale green. (C) Crystal structure of p204 HINab: dsDNA complex. Structures of p204 HINa and HINb are shown in wheat and cyan. dsDNA and the linker between HINa and HINb are shown in gray. (D) Similar DNA-binding mode of AIM2, IFI16 HINb, and p204 HINa and HINb. Structures of p204 HINa: DNA (wheat) and HINb: DNA (cyan) are from HINab: dsDNA complex (PDB: 5Z7D). Structures of human AIM2 HIN (PDB: 3RN2), mouse AIM2 HIN (PDB: 4JBM), and IFI16 HINb (PDB: 3RNU) are shown in pink, pale green, and slate, respectively. (E) Similar DNA-binding mode of IFI16 HINa and p202 HINa. Structures of p202 HINa (PDB: 4L5R) and IFI16 HINa (PDB: 4QGU) are shown in gray and orange, respectively.

FIGURE 3

PYHIN family proteins in innate immunity recognition of viral dsDNA. Cytosolic dsDNA from invading virus activates AIM2, which presents an auto-inhibition state before recognizing viral dsDNA. AIM2 HIN domain binds to dsDNA in the cytosol and subsequently binds to the adapter ASC through PYD: PYD interaction. AIM2 inflammasome is formed through AIM2-ASC-procaspase-1 oligomerization. Activated caspase-1 can directly cleave pro-IL-1β and pro-IL-18 to IL-1β and IL-18, which can respond to infection. p202 exists as a tetramer in the cytosol and can also bind to dsDNA from invading viruses. p202 can inhibit the formation of AIM2 inflammasome. IFI16 presents an extended state before recognizing dsDNA. In the nucleus, viral dsDNA binds to IFI16 HIN domains and induces IFI16 oligomerization with the assistant of ASC and pro-caspase-1, and the oligomer migrates to the cytoplasm to cleave pro-IL-1β to IL-1β. In the cytosol, viral dsDNA can also directly initiate the activation of IFI16/p204 independent of the inflammasome. Multiple IFI16/p204 HINab domains synergistically bind to the long dsDNA, resulting in the adjacent PYD aggregate to activate STING and TBK1. Activated TBK1 phosphorylates IRF3 and induces the production of IFNβ to defend against the virus infection.