Structural Analysis of dsRNA Binding to Anti-viral Pattern Recognition Receptors LGP2 and MDA5

Molecular Mechanisms of Foot-and-Mouth Disease Virus Targeting the Host Antiviral Response

Foot-and-mouth disease virus (FMDV) is the causative agent of an acute vesicular disease affecting pigs, cattle and other domestic, and wild animals worldwide. The aim of the host interferon (IFN) response is to limit viral replication and spread. Detection of the viral genome and products by specialized cellular sensors initiates a signaling cascade that leads to a rapid antiviral response involving the secretion of type I- and type III-IFNs and other antiviral cytokines with antiproliferative and immunomodulatory functions. During co-evolution with their hosts, viruses have acquired strategies to actively counteract host antiviral responses and the balance between innate response and viral antagonism may determine the outcome of disease and pathogenesis. FMDV proteases Lpro and 3C have been found to antagonize the host IFN response by a repertoire of mechanisms. Moreover, the putative role of other viral proteins in IFN antagonism is being recently unveiled, uncovering sophisticated immune evasion strategies different to those reported to date for other members of the Picornaviridae family. Here, we review the interplay between antiviral responses induced by FMDV infection and viral countermeasures to block them. Research on strategies used by viruses to modulate immunity will provide insights into the function of host pathways involved in defense against pathogens and will also lead to development of new therapeutic strategies to fight virus infections.

Discrimination of cytosolic self and non-self RNA by RIG-I-like receptors

RIG-I-like receptors (RLRs) are cytosolic innate immune sensors that detect pathogenic RNA and induce a systemic antiviral response. During the last decade, many studies focused on their molecular characterization and the identification of RNA agonists. Therefore, it became more and more clear that RLR activation needs to be carefully regulated, because constitutive signaling or detection of endogenous RNA through loss of specificity is detrimental. Here, we review the current understanding of RLR activation and selectivity. We specifically focus upon recent findings on the function of the helicase domain in discriminating between different RNAs, and whose malfunctioning causes serious autoimmune diseases.

Quantitative microspectroscopic imaging reveals viral and cellular RNA helicase interactions in live cells

Human cells detect RNA viruses through a set of helicases called RIG-I-like receptors (RLRs) that initiate the interferon response via a mitochondrial signaling complex. Many RNA viruses also encode helicases, which are sometimes covalently linked to proteases that cleave signaling proteins. One unresolved question is how RLRs interact with each other and with viral proteins in cells. This study examined the interactions among the hepatitis C virus (HCV) helicase and RLR helicases in live cells with quantitative microspectroscopic imaging (Q-MSI), a technique that determines FRET efficiency and subcellular donor and acceptor concentrations. HEK293T cells were transfected with various vector combinations to express cyan fluorescent protein (CFP) or YFP fused to either biologically active HCV helicase or one RLR (i.e. RIG-I, MDA5, or LGP2), expressed in the presence or absence of polyinosinic-polycytidylic acid (poly(I:C)), which elicits RLR accumulation at mitochondria. Q-MSI confirmed previously reported RLR interactions and revealed an interaction between HCV helicase and LGP2. Mitochondria in CFP-RIG-I:YFP-RIG-I cells, CFP-MDA5:YFP-MDA5 cells, and CFP-MDA5:YFP-LGP2 cells had higher FRET efficiencies in the presence of poly(I:C), indicating that RNA causes these proteins to accumulate at mitochondria in higher-order complexes than those formed in the absence of poly(I:C). However, mitochondria in CFP-LGP2:YFP-LGP2 cells had lower FRET signal in the presence of poly(I:C), suggesting that LGP2 oligomers disperse so that LGP2 can bind MDA5. Data support a new model where an LGP2-MDA5 oligomer shuttles NS3 to the mitochondria to block antiviral signaling.

Avian Interferons and Their Antiviral Effectors

Interferon (IFN) responses, mediated by a myriad of IFN-stimulated genes (ISGs), are the most profound innate immune responses against viruses. Cumulatively, these IFN effectors establish a multilayered antiviral state to safeguard the host against invading viral pathogens. Considerable genetic and functional characterizations of mammalian IFNs and their effectors have been made, and our understanding on the avian IFNs has started to expand. Similar to mammalian counterparts, three types of IFNs have been genetically characterized in most avian species with available annotated genomes. Intriguingly, chickens are capable of mounting potent innate immune responses upon various stimuli in the absence of essential components of IFN pathways including retinoic acid-inducible gene I, IFN regulatory factor 3 (IRF3), and possibility IRF9. Understanding these unique properties of the chicken IFN system would propose valuable targets for the development of potential therapeutics for a broader range of viruses of both veterinary and zoonotic importance. This review outlines recent developments in the roles of avian IFNs and ISGs against viruses and highlights important areas of research toward our understanding of the antiviral functions of IFN effectors against viral infections in birds.

Protect this house: cytosolic sensing of viruses

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Cells are equipped with pattern recognition receptors to sense invading viruses.

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Nucleic acids of RNA viruses are sensed by RIG-I like receptors in the cytosol.

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Foreign DNA is sensed by cGAS and other DNA sensors in the cytosol.

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These pattern recognition receptors activate adaptor proteins to initiate antiviral innate immune responses.

The ability to recognize invading viral pathogens and to distinguish their components from those of the host cell is critical to initiate the innate immune response. The efficiency of this detection is an important factor in determining the susceptibility of the cell to viral infection. Innate sensing of viruses is, therefore, an indispensable step in the line of defense for cells and organisms. Recent discoveries have uncovered novel sensors of viral components and hallmarks of infection, as well as mechanisms by which cells discriminate between self and non-self. This review highlights the mechanisms used by cells to detect viral pathogens in the cytosol, and recent advances in the field of cytosolic sensing of viruses.

Host-Pathogen Interactions in Measles Virus Replication and Anti-Viral Immunity

The measles virus (MeV) is a contagious pathogenic RNA virus of the family Paramyxoviridae, genus Morbillivirus, that can cause serious symptoms and even fetal complications. Here, we summarize current molecular advances in MeV research, and emphasize the connection between host cells and MeV replication. Although measles has reemerged recently, the potential for its eradication is promising with significant progress in our understanding of the molecular mechanisms of its replication and host-pathogen interactions.

Resurrecting ancestral structural dynamics of an antiviral immune receptor: adaptive binding pocket reorganization repeatedly shifts RNA preference

Background

Although resurrecting ancestral proteins is a powerful tool for understanding the molecular-functional evolution of gene families, nearly all studies have examined proteins functioning in relatively stable biological processes. The extent to which more dynamic systems obey the same ‘rules’ governing stable processes is unclear. Here we present the first detailed investigation of the functional evolution of the RIG-like receptors (RLRs), a family of innate immune receptors that detect viral RNA in the cytoplasm.

Results

Using kinetic binding assays and molecular dynamics simulations of ancestral proteins, we demonstrate how a small number of adaptive protein-coding changes repeatedly shifted the RNA preference of RLRs throughout animal evolution by reorganizing the shape and electrostatic distribution across the RNA binding pocket, altering the hydrogen bond network between the RLR and its RNA target. In contrast to observations of proteins involved in metabolism and development, we find that RLR-RNA preference ‘flip flopped’ between two functional states, and shifts in RNA preference were not always coupled to gene duplications or speciation events. We demonstrate at least one reversion of RLR-RNA preference from a derived to an ancestral function through a novel structural mechanism, indicating multiple structural implementations of similar functions.

Conclusions

Our results suggest a model in which frequent shifts in selection pressures imposed by an evolutionary arms race preclude the long-term functional optimization observed in stable biological systems. As a result, the evolutionary dynamics of immune receptors may be less constrained by structural epistasis and historical contingency.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0818-6) contains supplementary material, which is available to authorized users.

LGP2 of black carp plays an important role in the innate immune response against SVCV and GCRV

RIG-I like receptors (RLRs) detect cytosolic RNA virus components and initiate antiviral innate immune response through downstream signaling in vertebrates. In this paper, LGP2 of black carp (Mylopharyngodon piceus) has been cloned and characterized, which is a key member of RLR family. The full-length cDNA of black carp LGP2 (bcLGP2) comprises 2941 nucleotides and the predicted bcLGP2 protein contains 682 amino acids. bcLGP2 shares core homologous structural domains of RLRs, including a N-terminnal DExD/H helicase domain, a helicase superfamily c-terminal domain, and a C-terminal regulatory domain (CTD). bcLGP2 mRNA was constitutively detected in all selected tissues including heart, liver, spleen, kidney, intestine, muscle, skin, gill; and bcLGP2 mRNA level was increased in all the tissues except gill in response to GCRV or SVCV infection. Q-PCR of Mylopharyngodon piceus fin (MPF) cells demonstrated that bcLGP2 transcription was up-regulated by Poly (I:C) treatment, GCRV or SVCV infection, but not by LPS or PMA treatment. Western blot analysis demonstrated that the molecular weight of bcLGP2 was around 80 KDa; and the immunofluorescence staining of both HeLa cells and EPC cells showed that bcLGP2 was a cytosolic protein. EPC cells transfected with plasmid expressing bcLGP2 showed obviously improved antiviral ability against SVCV and GCRV. In general, these data support the conclusion that bcLGP2 functions importantly in the host antiviral innate immune response.