Akhilesh Kumar, H. Kumar
Cellular & Molecular Immunology, 2019
Viruses are obligate intracellular entities that require a living host and its machinery for replication. During replication, viral components known as pathogen-associated molecular patterns (PAMPs) are sensed by a family of innate immune sensors or pattern recognition receptors (PRRs) and promote an antiviral state. Viral nucleic acids are one of the key PAMPs sensed by cytosolic RIG-I-like receptors (RLRs), Cyclic GMP-AMP Synthase (cGAS), Absent In Melanoma 2 (AIM2), Interferon Gamma Inducible Protein 16 (IFI16), and Z-DNA Binding Protein 1 (ZBP1 or DAI) and endosome-localized Toll-like receptors (TLR3, 7, 8, and 9). Recognition of viral RNA by Retinoic acid-inducible gene I (RIG-I), Melanoma differentiation associated gene 5 (MDA5), and TLR3, 7, and 8 or viral DNA by TLR9, cGAS, AIM2, IFI16, and DAI triggers a cascade of signaling events to recruit transcription factors, interferon regulatory factors and NF-κB. Activated IRF3/7 and NF-κB translocate to the nucleus and induce expression of type I and III interferons and proinflammatory cytokines, respectively. The production of these interferons is further enhanced in an autocrine and paracrine manner through the JAK-STAT signaling pathway. Type I and III interferons further induce interferoninducible genes, and together with proinflammatory cytokines, they develop an antiviral state. The antiviral state is characterized by by apoptosis of virally infected cells and inhibition of the cellular protein synthesis machinery, making the uninfected cells resistant to viral infection and initiates virus-specific adaptive immune responses (Fig. 1a). The cytosolic sensors RLRs consist of three members, (RIG-I, MDA5, and Laboratory of genetics and physiology 2 (LGP2), which recognize viral RNA. These sensors contain a DExD/H-box RNA helicase domain at the C-terminus with an ATP binding motif. RIG-I and MDA5 additionally possess two caspase activation and recruitment domains (CARDs) at the N-terminus, which are required for homotypic CARD-CARD interactions and downstream signaling. LGP2 lacks the CARD and therefore is considered to acts as a negative regulator of the RLR pathway by sequestering RNA away from RIG-I and MDA5. However, in vivo genetic studies revealed that LGP2 potentiates RIG-I and MDA5-mediated responses under physiological conditions. RIG-I and MDA5 are specialized to recognize viral RNAs with different molecular weights, and MDA5 preferentially binds to long (>1 kb) dsRNA molecules. In contrast, RIG-I participates in the recognition of short (<300 bp) RNA molecules, 5′ppp dsRNA, 5′pp dsRNA or ssRNA. In the inactive state, RIG-I and MDA5 adopt a closed structure with the CARDs concealed by its C-terminal domain (CTD). The interaction of RNA with CTD activates RIG-I by exposing the CARDs, promoting oligomerization and finally interacting with a mitochondrial membrane-associated CARD containing adapter protein interferon-β promoter stimulator-1 (IPS-1), also known as mitochondrial antiviral signaling protein, virusinduced signaling adapter or CARD adapter inducing interferon-β. Activation of IPS-1 leads to the recruitment of signaling proteins such as TRAF2, TRAF6, and TRADD. These changes recruit TRAF3 and TANK to induce the activation of TBK1 and IKKε and finally leads to the production of type I and III interferons and proinflammatory cytokines through the transcription factors IRF3/IRF7 and NF-κB, respectively (Fig. 1a). The RLR pathway is vital for viral defense, and its dysregulation can cause autoimmune diseases; therefore, the RLR signaling pathway is tightly regulated at the posttranslational level. Regulation of the RLR pathway by ubiquitination is pivotal for antiviral responses and immune homeostasis and occurs at various levels, such as sensing, signaling and transcription factor activation. Ubiquitin is a small peptide consisting of 76 amino acids, and multiple ubiquitin molecules can be sequentially added to one of the seven lysine (K) residues of ubiquitin (K6, K11, K27, K29, K33, K48, and K63), resulting in a variety of ubiquitin chains that can then be covalently linked to a target protein by E3 ubiquitin ligase. This posttranslational modification called ubiquitination can alter the function or stability of a protein depending on the type of ubiquitination. RIG-I is regulated by a number of E3 ubiquitin ligases, K27 and K48-linked ubiquitination by TRIM40 and K48-linked ubiquitination by RNF122, and STUB1 promotes proteasomal degradation, while K63-linked ubiquitination promotes oligomerization and RIG-I stabilization. In addition, multiple ubiquitin chains can be added to different lysine residues of RIG-I, and this multisite RIG-I ubiquitination is essential for its activation and helps in fine-tuning of type-I interferon production during virus infection. Binding of RNA ligands by RIG-I CTD exposes its CARDs for K63-linked ubiquitination at distinct sites by TRIM25, TRIM4, MEX3c and Riplet. Ubiquitination stabilizes RIG-I oligomerization and promotes the interaction with the CARD of IPS-1, resulting in IPS-1 aggregation and dimerization in the mitochondrial membrane, required for downstream signaling. TRIM25 is an interferon-inducible E3 ubiquitin ligase that interacts with RIG-I through its C-terminal SPla and the RYanodine receptor (SPRY) domain and ubiquitinates the second CARD of RIG-I at the lysine residues K99, K169, K172, K181, K190, and K193. The K172R mutation severely reduced RIG-I ubiquitination, suggesting that K172 is essential for TRIM25-mediated RIG-I ubiquitination and subsequent binding to IPS-1. Being a pivotal molecule in antiviral signaling, TRIM25 is tightly regulated through several different mechanisms. Linear ubiquitin assembly complex adds K48-linked ubiquitin molecules and marks TRIM25 for degradation, while USP15 removes these marks and prevents proteasomal degradation during viral infection. Noncoding RNAs
Cited by 1 publication.
Field of study: Biology