The N-terminal amphipathic alpha-helix of viperin mediates localization to the cytosolic face of the endoplasmic reticulum and inhibits protein secretion

Lipid Droplet, a Key Player in Host-Parasite Interactions

Lipid droplets (lipid bodies, LDs) are dynamic organelles that have important roles in regulating lipid metabolism, energy homeostasis, cell signaling, membrane trafficking, and inflammation. LD biogenesis, composition, and functions are highly regulated and may vary according to the stimuli, cell type, activation state, and inflammatory environment. Increased cytoplasmic LDs are frequently observed in leukocytes and other cells in a number of infectious diseases. Accumulating evidence reveals LDs participation in fundamental mechanisms of host-pathogen interactions, including cell signaling and immunity. LDs are sources of eicosanoid production, and may participate in different aspects of innate signaling and antigen presentation. In addition, intracellular pathogens evolved mechanisms to subvert host metabolism and may use host LDs, as ways of immune evasion and nutrients source. Here, we review mechanisms of LDs biogenesis and their contributions to the infection progress, and discuss the latest discoveries on mechanisms and pathways involving LDs roles as regulators of the immune response to protozoan infection.

RSAD2 and AIM2 Modulate Coxsackievirus A16 and Enterovirus A71 Replication in Neuronal Cells in Different Ways That May Be Associated with Their 5' Nontranslated Regions

Coxsackievirus A16 (CV-A16) and enterovirus A71 (EV-A71) are closely related enteroviruses that cause the same hand, foot, and mouth disease (HFMD), but neurological complications occur only very rarely in CV-A16 compared to EV-A71 infections. To elucidate host responses that may be able to explain these differences, we performed transcriptomic analysis and real-time quantitative PCR (RT-qPCR) in CV-A16-infected neuroblastoma cells (SK-N-SH), and the results showed that the radical S-adenosylmethionine domain containing 2 (RSAD2) was the highest upregulated gene in the antimicrobial pathway. Increased RSAD2 expression was correlated with reduced viral replication, while RSAD2 knockdown cells were correlated with increased replication. EV-A71 replication showed no apparent correlation to RSAD2 expressions. Absent in melanoma 2 (AIM2), which is associated with pyroptotic cell death, was upregulated in EV-A71-infected neurons but not in CV-A16 infection, suggesting that the AIM2 inflammasome played a significant role in suppressing EV-A71 replication. Chimeric viruses derived from CV-A16 and EV-A71 but containing swapped 5' nontranslated regions (5' NTRs) showed that RSAD2 expression/viral replication and AIM2 expression/viral replication patterns may be linked to the 5' NTRs of parental viruses. Differences in secondary structure of internal ribosomal entry sites within the 5' NTR may be responsible for these findings. Overall, our results suggest that CV-A16 and EV-A71 elicit different host responses to infection, which may help explain the apparent lower incidence of CV-A16-associated neurovirulence in HFMD outbreaks compared to EV-A71 infection.IMPORTANCE Although coxsackievirus A16 (CV-A16) and enterovirus A17 (EV-A71) both cause hand, foot, and mouth disease, EV-A71 has emerged as a leading cause of nonpolio, enteroviral fatal encephalomyelitis among young children. The significance of our research is in the identification of the possible differing and novel mechanisms of CV-A16 and EV-A71 inhibition in neuronal cells that may impact viral neuropathogenesis. We further showed that viral 5' NTRs may play significant roles in eliciting different host response mechanisms.

Molecular characterization and expression analyses of the Viperin gene in Larimichthys crocea (Family: Sciaenidae)

In this study, we sequenced and characterized an interferon-stimulated gene Viperin homologue, LcViperin, from large yellow croaker (Larimichthys crocea). The LcViperin encodes 354 amino acids and contains an N-terminal amphipathic α-helix domain, a radical S-adenosyl-l-methionine (SAM) domain and a highly conserved C-terminal domain. The analyses of LcViperin promoter region revealed nine kinds of putative transcriptional factor binding sites, including five putative ICSBP (IRF-8) binding sites and one putative IRF-1 binding site, indicating that the expression of LcViperin might be induced by the type I IFN response. Phylogenetic analyses based on amino acid sequences showed that the Viperin of large yellow croaker is clustered together with its counterparts from other teleost fishes. The Real-time PCR analyses showed that the LcViperin was found to be ubiquitously expressed in ten examined tissues in large yellow croaker, with predominant expression in peripheral blood, followed by heart and gill. Expression analyses showed that the LcViperin was rapidly and significantly upregulated in vivo after poly (I:C) challenge in peripheral blood, head kidney, spleen and liver tissues. The results indicate that the LcViperin might play a pivotal role in antiviral immune responses.

Human viperin catalyzes the modification of GPP and FPP potentially affecting cholesterol synthesis

Viperin is a radical SAM enzyme that possesses antiviral properties against a broad range of enveloped viruses. Here, we describe the activity of human viperin with two molecules of the mevalonate pathway, geranyl pyrophosphate, and farnesyl pyrophosphate, involved in cholesterol biosynthesis. We postulate that the radical modification of these two molecules by viperin might lead to defects in cholesterol synthesis, thereby affecting the composition of lipid rafts and subsequent enveloped virus budding.

The Role of Flaviviral Proteins in the Induction of Innate Immunity

Flaviviruses are positive, single-stranded, enveloped cytoplasmic sense RNA viruses that cause a variety of important diseases worldwide. Among them, Zika virus, West Nile virus, Japanese encephalitis virus, and Dengue virus have the potential to cause severe disease. Extensive studies have been performed to elucidate the structure and replication strategies of flaviviruses, and current studies are aiming to unravel the complex molecular interactions between the virus and host during the very early stages of infection. The outcomes of viral infection and rapid establishment of the antiviral state, depends on viral detection by pathogen recognition receptors and rapid initiation of signalling cascades to induce an effective innate immune response. Extracellular and intracellular pathogen recognition receptors play a crucial role in detecting flavivirus infection and inducing a robust antiviral response. One of the main hallmarks of flaviviral nonstructural proteins is their multiple strategies to antagonise the interferon system. In this chapter, we summarize the molecular characteristics of flaviviral proteins and discuss how viral proteins target different components of the interferon signalling pathway by blocking phosphorylation, enhancing degradation, and downregulating the expression of major components of the Janus kinase/signal transducer and activator of transcription pathway. We also discuss how the interactions of viral proteins with host proteins facilitate viral pathogenesis. Due to the lack of antivirals or prophylactic treatments for many flaviviral infections, it is necessary to fully elucidate how these viruses disrupt cellular processes to influence pathogenesis and disease outcomes.

Viperin Targets Flavivirus Virulence by Inducing Assembly of Noninfectious Capsid Particles

Efficient antiviral immunity requires interference with virus replication at multiple layers targeting diverse steps in the viral life cycle. We describe here a novel flavivirus inhibition mechanism that results in interferon-mediated obstruction of tick-borne encephalitis virus particle assembly and involves release of malfunctioning membrane-associated capsid (C) particles. This mechanism is controlled by the activity of the interferon-induced protein viperin, a broad-spectrum antiviral interferon-stimulated gene. Through analysis of the viperin-interactome, we identified the Golgi brefeldin A-resistant guanine nucleotide exchange factor 1 (GBF1) as the cellular protein targeted by viperin. Viperin-induced antiviral activity, as well as C-particle release, was stimulated by GBF1 inhibition and knockdown and reduced by elevated levels of GBF1. Our results suggest that viperin targets flavivirus virulence by inducing the secretion of unproductive noninfectious virus particles via a GBF1-dependent mechanism. This as-yet-undescribed antiviral mechanism allows potential therapeutic intervention.

IMPORTANCE The interferon response can target viral infection on almost every level; however, very little is known about the interference of flavivirus assembly. We show here that interferon, through the action of viperin, can disturb the assembly of tick-borne encephalitis virus. The viperin protein is highly induced after viral infection and exhibit broad-spectrum antiviral activity. However, the mechanism of action is still elusive and appears to vary between the different viruses, indicating that cellular targets utilized by several viruses might be involved. In this study, we show that viperin induces capsid particle release by interacting and inhibiting the function of the cellular protein Golgi brefeldin A-resistant guanine nucleotide exchange factor 1 (GBF1). GBF1 is a key protein in the cellular secretory pathway and is essential in the life cycle of many viruses, also targeted by viperin, implicating GBF1 as a novel putative drug target.

Porcine Viperin protein inhibits the replication of classical swine fever virus (CSFV) in vitro

Background

Classical swine fever virus (CSFV) is the causative pathogen of Classical swine fever (CSF), a highly contagious disease of swine. Viperin is one of the hundreds of interferon-stimulated genes (ISGs), and possesses a wide range of antiviral activities. The aim of this study was to explore whether porcine Viperin has the anti-CSFV activity.

Method

The influences of CSFV infection on Viperin expression and Newcastle disease virus (NDV)/Pseudorabies virus (PRV)-induced Viperin expression were examined in 3D4/21 cells and porcine peripheral blood mononuclear cells (PBMCs). Porcine Viperin gene was amplified to generate cell line PK-Vi over-expressing Viperin. CSFV was inoculated in the cell lines and viral load was detected by qRT-PCR, virus titration and Western blot. The influence of Viperin expression on CSFV binding, entry and release in the cells was also examined. The co-localization of Viperin with CSFV and its proteins (E2, NS5B) was determined by confocal laser scanning microscopy test. Co-IP assay was performed to check the interaction of Viperin with CSFV proteins.

Results

CSFV infection could not induce Viperin expression in vitro while significantly inhibiting NDV/PRV-induced Viperin expression at 12, 24 and 48 h post infection (hpi; P < 0.05). The proliferation of CSFV in PK-Vi was significantly inhibited at 24, 48 and 72 hpi (P < 0.05), comparing with control cells (PK-C1 expressing EGFP). Virus in both cell culture supernatants and cell pellets were reduced equally. CSFV binding and entry in the cells were not interfered by Viperin expression. These results indicated its anti-CSFV function occurred during the genome and/or protein synthesis step. Confocal laser scanning microscopy test showed the Viperin-EGFP protein co-localized with CSFV E2 protein in CSFV infected PK-Vi cells. Further experiments indicated that Viperin protein co-localized with E2 and NS5B proteins of CSFV in the transfected 293 T cells. Furthermore, Co-IP assay confirmed the interaction of Viperin with E2 protein, but not NS5B.

Conclusion

Porcine Viperin effectively inhibited CSFV replication in vitro, potentially via the interaction of Viperin with CSFV E2 protein in cytoplasm. The results provided foundation for further studies of the interaction of Viperin with CSFV and other viruses.

The radical-SAM enzyme Viperin catalyzes reductive addition of a 5'-deoxyadenosyl radical to UDP-glucose in vitro

Viperin, a radical-S-adenosylmethionine (SAM) enzyme conserved from fungi to humans, can restrict replication of many viruses. Neither the molecular mechanism underlying the antiviral activity of Viperin, nor its exact physiological function, is understood: most importantly, no radical-SAM activity has been discovered for Viperin. Here, using electron paramagnetic resonance (EPR) spectroscopy, mass spectrometry, and NMR spectroscopy, we show that uridine diphosphate glucose (UDP-glucose) is a substrate of a fungal Viperin (58% pairwise identity with human Viperin at the amino acid level) in vitro. Structural homology modeling and docking experiments reveal a highly conserved binding pocket in which the position of UDP-glucose is consistent with our experimental data regarding catalytic addition of a 5'-deoxyadenosyl radical and a hydrogen atom to UDP-glucose.

Structural studies of viperin, an antiviral radical SAM enzyme

Viperin is an IFN-inducible radical S-adenosylmethionine (SAM) enzyme that inhibits viral replication. We determined crystal structures of an anaerobically prepared fragment of mouse viperin (residues 45-362) complexed with S-adenosylhomocysteine (SAH) or 5'-deoxyadenosine (5'-dAdo) and l-methionine (l-Met). Viperin contains a partial (βα)6-barrel fold with a disordered N-terminal extension (residues 45-74) and a partially ordered C-terminal extension (residues 285-362) that bridges the partial barrel to form an overall closed barrel structure. Cys84, Cys88, and Cys91 located after the first β-strand bind a [4Fe-4S] cluster. The active site architecture of viperin with bound SAH (a SAM analog) or 5'-dAdo and l-Met (SAM cleavage products) is consistent with the canonical mechanism of 5'-deoxyadenosyl radical generation. The viperin structure, together with sequence alignments, suggests that vertebrate viperins are highly conserved and that fungi contain a viperin-like ortholog. Many bacteria and archaebacteria also express viperin-like enzymes with conserved active site residues. Structural alignments show that viperin is similar to several other radical SAM enzymes, including the molybdenum cofactor biosynthetic enzyme MoaA and the RNA methyltransferase RlmN, which methylates specific nucleotides in rRNA and tRNA. The viperin putative active site contains several conserved positively charged residues, and a portion of the active site shows structural similarity to the GTP-binding site of MoaA, suggesting that the viperin substrate may be a nucleoside triphosphate of some type.

Cellular requirements for iron-sulfur cluster insertion into the antiviral radical SAM protein viperin

Viperin (RSAD2) is an interferon-stimulated antiviral protein that belongs to the radical S-adenosylmethionine (SAM) enzyme family. Viperin's iron-sulfur (Fe/S) cluster is critical for its antiviral activity against many different viruses. CIA1 (CIAO1), an essential component of the cytosolic iron-sulfur protein assembly (CIA) machinery, is crucial for Fe/S cluster insertion into viperin and hence for viperin's antiviral activity. In the CIA pathway, CIA1 cooperates with CIA2A, CIA2B, and MMS19 targeting factors to form various complexes that mediate the dedicated maturation of specific Fe/S recipient proteins. To date, however, the mechanisms of how viperin acquires its radical SAM Fe/S cluster to gain antiviral activity are poorly understood. Using co-immunoprecipitation and ⁵⁵Fe-radiolabeling experiments, we therefore studied the roles of CIA2A, CIA2B, and MMS19 for Fe/S cluster insertion. CIA2B and MMS19 physically interacted with the C terminus of viperin and used CIA1 as the primary viperin-interacting protein. In contrast, CIA2A bound to viperin's N terminus in a CIA1-, CIA2B-, and MMS19-independent fashion. Of note, the observed interaction of both CIA2 isoforms with a single Fe/S target protein is unprecedented in the CIA pathway. ⁵⁵Fe-radiolabeling experiments with human cells depleted of CIA1, CIA2A, CIA2B, or MMS19 revealed that CIA1, but none of the other CIA factors, is predominantly required for ⁵⁵Fe/S cluster incorporation into viperin. Collectively, viperin maturation represents a novel CIA pathway with a minimal requirement of the CIA-targeting factors and represents a new paradigm for the insertion of the Fe/S cofactor into a radical SAM protein.

Viperin interaction with mitochondrial antiviral signaling protein (MAVS) limits viperin-mediated inhibition of the interferon response in macrophages

Viperin is an antiviral protein that is upregulated by interferons and by ligands for a variety of innate immune receptors. It possesses diverse capabilities and functions in an array of viral infections. Studies have shown that it appears to be particularly important in defence against RNA viruses, such as West Nile, Dengue, and Chikungunya viruses, although the specific mechanisms involved are not well understood at the molecular level. Here we identify the mitochondrial antiviral signalling protein MAVS as a novel viperin interaction partner, most likely in mitochondria associated membranes, and characterize a more central, overarching role of viperin as a negative regulator of the interferon response, an ability that can be regulated by the viperin-MAVS interaction. This suggests a novel mechanism of viperin action in immune defence against RNA viruses by which it may prevent pathology from excessive immune responses.

Systems Genetics of Obesity

Obesity is a complex trait, determined by many genes and influenced by environmental factors. Mapping genomic loci contributing to obesity helps to identify gene variants responsible for differences in the phenotype. However, measuring fat content alone is often not sufficient to identify the underlying gene or genes. Besides in-depth phenotyping, well-designed genetic populations and the combined analysis of data of different origins are necessary to detect one of several genetic determinants. Structured mouse populations and linking information from different experiments help to simplify the complexity in the search for direct genetic effects or factors that are hidden in the genome. In this chapter we present an example of how the physicochemical characterization of adipose tissue in BXD recombinant inbred lines contributes to enlighten the obese phenotype of mice. We describe the search for gene(s) contributing to collagen content in adipose tissue of BXD strains using the GeneNetwork platform.

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.

Identification of novel macrolides with antibacterial, anti-inflammatory and type I and III IFN-augmenting activity in airway epithelium

BACKGROUND

Exacerbations of asthma and COPD are triggered by rhinoviruses. Uncontrolled inflammatory pathways, pathogenic bacterial burden and impaired antiviral immunity are thought to be important factors in disease severity and duration. Macrolides including azithromycin are often used to treat the above diseases, but exhibit variable levels of efficacy. Inhaled corticosteroids are also readily used in treatment, but may lack specificity. Ideally, new treatment alternatives should suppress unwanted inflammation, but spare beneficial antiviral immunity.

METHODS

In the present study, we screened 225 novel macrolides and tested them for enhanced antiviral activity against rhinovirus, as well as anti-inflammatory activity and activity against Gram-positive and Gram-negative bacteria. Primary bronchial epithelial cells were grown from 10 asthmatic individuals and the effects of macrolides on rhinovirus replication were also examined. Another 30 structurally similar macrolides were also examined.

RESULTS

The oleandomycin derivative Mac5, compared with azithromycin, showed superior induction (up to 5-fold, EC50 = 5-11 μM) of rhinovirus-induced type I IFNβ, type III IFNλ1 and type III IFNλ2/3 mRNA and the IFN-stimulated genes viperin and MxA, yet had no effect on IL-6 and IL-8 mRNA. Mac5 also suppressed rhinovirus replication at 48 h, proving antiviral activity. Mac5 showed antibacterial activity against Gram-positive Streptococcus pneumoniae; however, it did not have any antibacterial properties compared with azithromycin when used against Gram-negative Escherichia coli (as a model organism) and also the respiratory pathogens Pseudomonas aeruginosa and non-typeable Haemophilus influenzae. Further non-toxic Mac5 derivatives were identified with various anti-inflammatory, antiviral and antibacterial activities.

CONCLUSIONS

The data support the idea that macrolides have antiviral properties through a mechanism that is yet to be ascertained. We also provide evidence that macrolides can be developed with anti-inflammatory, antibacterial and antiviral activity and show surprising versatility depending on the clinical need.

S-Adenosylmethionine Synthetase 3 Is Important for Pollen Tube Growth

S-Adenosylmethionine is widely used in a variety of biological reactions and participates in the methionine (Met) metabolic pathway. In Arabidopsis (Arabidopsis thaliana), one of the four S-adenosylmethionine synthetase genes, METHIONINE ADENOSYLTRANSFERASE3 (MAT3), is highly expressed in pollen. Here, we show that mat3 mutants have impaired pollen tube growth and reduced seed set. Metabolomics analyses confirmed that mat3 pollen and pollen tubes overaccumulate Met and that mat3 pollen has several metabolite profiles, such as those of polyamine biosynthesis, which are different from those of the wild type. Additionally, we show that disruption of Met metabolism in mat3 pollen affected transfer RNA and histone methylation levels. Thus, our results suggest a connection between metabolism and epigenetics.

Viperin inhibits rabies virus replication via reduced cholesterol and sphingomyelin and is regulated upstream by TLR4

Viperin (virus inhibitory protein, endoplasmic reticulum-associated, IFN-inducible) is an interferon-inducible protein that mediates antiviral activity. Generally, rabies virus (RABV) multiplies extremely well in susceptible cells, leading to high virus titres. In this study, we found that viperin was significantly up-regulated in macrophage RAW264.7 cells but not in NA, BHK-21 or BSR cells. Transient viperin overexpression in BSR cells and stable expression in BHK-21 cells could inhibit RABV replication, including both attenuated and street RABV. Furthermore, the inhibitory function of viperin was related to reduce cholesterol/sphingomyelin on the membranes of RAW264.7 cells. We explored the up-stream regulation pathway of viperin in macrophage RAW264.7 cells in the context of RABV infection. An experiment confirmed that a specific Toll-like receptor 4 (TLR4) inhibitor, TAK-242, could inhibit viperin expression in RABV-infected RAW264.7 cells. These results support a regulatory role for TLR4. Geldanamycin, a specific inhibitor of interferon regulatory factor 3 (IRF3) (by inhibiting heat-shock protein 90 (Hsp90) of the IRF3 phosphorylation chaperone), significantly delayed and reduced viperin expression, indicating that IRF3 is involved in viperin induction in RAW264.7 cells. Taken together, our data support the therapeutic potential for viperin to inhibit RABV replication, which appears to involve upstream regulation by TLR4.

Oyster viperin retains direct antiviral activity and its transcription occurs via a signalling pathway involving a heat-stable haemolymph protein

Little is known about the response of non-model invertebrates, such as oysters, to virus infection. The vertebrate innate immune system detects virus-derived nucleic acids to trigger the type I IFN pathway, leading to the transcription of hundreds of IFN-stimulated genes (ISGs) that exert antiviral functions. Invertebrates were thought to lack the IFN pathway based on the absence of IFN or ISGs encoded in model invertebrate genomes. However, the oyster genome encodes many ISGs, including the well-described antiviral protein viperin. In this study, we characterized oyster viperin and showed that it localizes to caveolin-1 and inhibits dengue virus replication in a heterologous model. In a second set of experiments, we have provided evidence that the haemolymph from poly(I : C)-injected oysters contains a heat-stable, protease-susceptible factor that induces haemocyte transcription of viperin mRNA in conjunction with upregulation of IFN regulatory factor. Collectively, these results support the concept that oysters have antiviral systems that are homologous to the vertebrate IFN pathway.

miR-200 family promotes podocyte differentiation through repression of RSAD2

Mature podocytes are highly differentiated cells with several characteristic phenotypic features that are involved in the glomerular filtration function. During kidney development, a series of changes of the morphological characteristics and cellular functions may happen in podocytes. The miR-200 family functions in various biological and pathological processes. But the underlying molecular mechanisms of miR-200 family that functions in podocyte differentiation remain poorly understood. Herein is shown that miR-200a, miR-200b and miR-429 are significantly upregulated during the differentiation of podocytes, with highest upregulation of miR-200a. In these cells, restraint of miR-200 family by RNA interference assay revealed a prominent inhibition of cell differentiation. More intriguingly, miR-200 family directly inhibited the radical S-adenosyl methionine domain-containing protein 2 (RASD2) expression. Moreover, further upregulation of RSAD2 combining with restraint of miR-200 family revealed a promotion of podocyte dedifferentiation and proliferation. In addition, the expression of RSAD2 is consistent with that of in vitro podocyte differentiation in prenatal and postnatal mouse kidney, and significantly down-regulated during the kidney development. Together, these findings indicate that miR-200 family may potentially promote podocyte differentiation through repression of RSAD2 expression. Our data also demonstrate a novel role of the antiviral protein RSAD2 as a regulator in cell differentiation.

Monkey Viperin Restricts Porcine Reproductive and Respiratory Syndrome Virus Replication

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen which causes huge economic damage globally in the swine industry. Current vaccination strategies provide only limited protection against PRRSV infection. Viperin is an interferon (IFN) stimulated protein that inhibits some virus infections via IFN-dependent or IFN-independent pathways. However, the role of viperin in PRRSV infection is not well understood. In this study, we cloned the full-length monkey viperin (mViperin) complementary DNA (cDNA) from IFN-α-treated African green monkey Marc-145 cells. It was found that the mViperin is up-regulated following PRRSV infection in Marc-145 cells along with elevated IRF-1 gene levels. IFN-α induced mViperin expression in a dose- and time-dependent manner and strongly inhibits PRRSV replication in Marc-145 cells. Overexpression of mViperin suppresses PRRSV replication by blocking the early steps of PRRSV entry and genome replication and translation but not inhibiting assembly and release. And mViperin co-localized with PRRSV GP5 and N protein, but only interacted with N protein in distinct cytoplasmic loci. Furthermore, it was found that the 13–16 amino acids of mViperin were essential for inhibiting PRRSV replication, by disrupting the distribution of mViperin protein from the granular distribution to a homogeneous distribution in the cytoplasm. These results could be helpful in the future development of novel antiviral therapies against PRRSV infection.

Radical S-Adenosylmethionine Enzymes in Human Health and Disease

Radical S-adenosylmethionine (SAM) enzymes catalyze an astonishing array of complex and chemically challenging reactions across all domains of life. Of approximately 114,000 of these enzymes, 8 are known to be present in humans: MOCS1, molybdenum cofactor biosynthesis; LIAS, lipoic acid biosynthesis; CDK5RAP1, 2-methylthio-N(6)-isopentenyladenosine biosynthesis; CDKAL1, methylthio-N(6)-threonylcarbamoyladenosine biosynthesis; TYW1, wybutosine biosynthesis; ELP3, 5-methoxycarbonylmethyl uridine; and RSAD1 and viperin, both of unknown function. Aberrations in the genes encoding these proteins result in a variety of diseases. In this review, we summarize the biochemical characterization of these 8 radical S-adenosylmethionine enzymes and, in the context of human health, describe the deleterious effects that result from such genetic mutations.

Equine Infectious Anemia Virus Gag Assembly and Export Are Directed by Matrix Protein through trans-Golgi Networks and Cellular Vesicles

Gag intracellular assembly and export are very important processes for lentiviruses replication. Previous studies have demonstrated that equine infectious anemia virus (EIAV) matrix (MA) possesses distinct phosphoinositide affinity compared with HIV-1 MA and that phosphoinositide-mediated targeting to peripheral and internal membranes is a critical factor in EIAV assembly and release. In this study, we compared the cellular assembly sites of EIAV and HIV-1. We observed that the assembly of EIAV particles occurred on interior cellular membranes, while HIV-1 was targeted to the plasma membrane (PM) for assembly. Then, we determined that W7 and K9 in the EIAV MA N terminus were essential for Gag assembly and release but did not affect the cellular distribution of Gag. The replacement of EIAV MA with HIV-1 MA directed chimeric Gag to the PM but severely impaired Gag release. MA structural analysis indicated that the EIAV and HIV-1 MAs had similar spatial structures but that helix 1 of the EIAV MA was closer to loop 2. Further investigation indicated that EIAV Gag accumulated in the trans-Golgi network (TGN) but not the early and late endosomes. The 9 N-terminal amino acids of EIAV MA harbored the signal that directed Gag to the TGN membrane system. Additionally, we demonstrated that EIAV particles were transported to the extracellular space by the cellular vesicle system. This type of EIAV export was not associated with multivesicular bodies or microtubule depolymerization but could be inhibited by the actin-depolymerizing drug cytochalasin D, suggesting that dynamic actin depolymerization may be associated with EIAV production.

IMPORTANCE

In previous studies, EIAV Gag was reported to localize to both the cell interior and the plasma membrane. Here, we demonstrate that EIAV likely uses the TGN as the assembly site in contrast to HIV-1, which is targeted to the PM for assembly. These distinct assembly features are determined by the MA domain. We also identified two sites in the N terminus of EIAV MA that were important for Gag assembly and release. Furthermore, the observation of EIAV transport by cellular vesicles but not by multivesicular bodies sheds light on the mechanisms underlying EIAV cellular replication.

Identification and functional characterization of viperin of amphioxus Branchiostoma japonicum: Implications for ancient origin of viperin-mediated antiviral response

Viperin, an antiviral protein, has been shown to be active against a wide range of DNA and RNA viruses, but no information is available regarding functional characterization of viperin in invertebrate species. In this study, we clearly demonstrate that amphioxus (Branchiostoma japonicum) viperin, BjVip, has features in common with those of vertebrate viperin, including the presence of the SAM superfamily domain with the characteristic CNYKCGFC motif, syntenic conservation, and predicted 3D structure. Bjvip exhibits a tissue-specific expression with abundant levels in the hepatic cecum, hind-gut, gill and muscle, and following challenge with the viral mimic poly I:C, its expression is significantly up-regulated, suggesting an involvement of BjVip in immune response of amphioxus against viral infection. Importantly, we show that the cells transfected with Bjvip is able to kill LCDV or inhibiting its propagation, and co-incubation of rBjVip with WSSV markedly attenuates its infectivity. Thus, we provide the first evidences that amphioxus viperin, like that of vertebrates, is capable of promoting resistance against viral infection in vitro and in vivo, indicating that viperin-mediated antiviral response already emerged in the primitive chordate. We also prove that amphioxus viperin has evolved under positive selection.

Sequence analysis and characterisation of virally induced viperin in the saltwater crocodile (Crocodylus porosus)

A number of pathogens have been detected in crocodiles, however little is known about their ability to control these pathogens. The interferon stimulated gene (ISG), viperin, has gained attention recently as an important host protein involved in multiple arms of the immune response. Viperin in concert with a number of other ISGs was upregulated in response to viral nucleic acid mimics and sendai virus in the C. porosus cell line, LV-1, indicating an intact early innate response to viral infection in these animals for the first time. Viperin was cloned from the LV-1 cell line and shown to have similar localisation patterns as human viperin, as well as demonstrating extremely high conservation with the human orthologue, excepting at the N-terminus. Interestingly, C. porosus viperin was also able to inhibit Dengue virus replication in vitro, showing a high level of intact functionality for this protein across divergent animal species, and perhaps demonstrating its importance in the early innate response to pathogens in the animal kingdom.

Characterisation of chicken viperin

The identification of immune pathways that protect against pathogens may lead to novel molecular therapies for both livestock and human health. Interferon (IFN) is a major response pathway that stimulates multiple genes targeted towards reducing virus. Viperin is one such interferon stimulated gene (ISG) that helps protect mammals from virus and may be critical to protecting chickens in the same way. In chickens, ISGs are not generally well characterised and viperin, in concert with other ISGs, may be important in protecting against virus. Here we identify chicken viperin (ch-viperin) and show that ch-viperin is upregulated in response to viral signature molecules. We further show that viperin is upregulated in response to virus infection in vivo. This data will benefit investigators targeting the antiviral pathways in the chicken.

Equine viperin restricts equine infectious anemia virus replication by inhibiting the production and/or release of viral Gag, Env, and receptor via distortion of the endoplasmic reticulum

Viperin is an endoplasmic reticulum (ER)-associated multifunctional protein that regulates virus replication and possesses broad antiviral activity. In many cases, viperin interferes with the trafficking and budding of viral structural proteins by distorting the membrane transportation system. The lentivirus equine infectious anemia virus (EIAV) has been studied extensively. In this study, we examined the restrictive effect of equine viperin (eViperin) on EIAV replication and investigated the possible molecular basis of this restriction to obtain insights into the effect of this cellular factor on retroviruses. We demonstrated that EIAV infection of primary equine monocyte-derived macrophages (eMDMs) upregulated the expression of eViperin. The overexpression of eViperin significantly inhibited the replication of EIAV in eMDMs, and knockdown of eViperin transcription enhanced the replication of EIAV in eMDMs by approximately 45.8%. Further experiments indicated that eViperin restricts EIAV at multiple steps of viral replication. The overexpression of eViperin inhibited EIAV Gag release. Both the α-helix domain and radical S-adenosylmethionine (SAM) domain were required for this activity. However, the essential motifs in SAM were different from those reported for the inhibition of HIV-1 Gag by human viperin. Furthermore, eViperin disrupted the synthesis of both EIAV Env and receptor, which consequently inhibited viral production and entry, respectively, and this disruption was dependent on the eViperin α-helix domain. Using immunofluorescence assays and electron microscopy, we demonstrated that the α-helix domain is responsible for the distortion of the endoplasmic reticulum (ER). Finally, EIAV did not exhibit counteracting eViperin at the protein level.

IMPORTANCE

In previous studies, viperin was indicated as restricting virus replications primarily by the inhibition of virus budding. Here, we show that viperin may have multiple antiviral mechanisms, including the reduction of EIAV Gag budding and Env expression, and these activities are dependent on different viperin domains. We especially demonstrate that the overexpression of viperin inhibits EIAV entry by decreasing the level of virus receptor. Therefore, viperin restriction of viruses is determined largely by the dependence of virus on the cellular membrane transportation system.

Sequence analysis and subcellular localization of crucian carp Carassius auratus viperin

Human viperin is known as an interferon (IFN)-inducible antiviral protein and localizes to endoplasmic reticulum (ER) via its N-terminal amphipathic α-helix. Little is known about subcellular localization of fish viperin. Herein, we characterized subcellular localization of a fish viperin from crucian carp Carassius auratus. Crucian carp viperin is nearly identical to the other viperin proteins in sequence, with the exception of the first N-terminal 70 amino acids that are defined as N-terminal variable domain including an amphipathic α-helix. In addition to N-terminal variable domain, crucian carp viperin protein harbors a conserved middle radical SAM domain and a conserved C-terminal domain. Subcellular localization analyses indicate that crucian carp viperin is a cytoplasmic protein associated with ER. Sequence analyses reveal that amino acids 1-74 forms an amphipathic α-helix domain that drives ER-localization of crucian carp viperin. In addition, Coimmunoprecipitation assays show that crucian carp viperin proteins are able to self-associate. These results together indicate that similar to mammalian homologs, fish viperins likely play important roles in IFN response.

Rock bream (Oplegnathus fasciatus) viperin is a virus-responsive protein that modulates innate immunity and promotes resistance against megalocytivirus infection

Viperin in mammals is known to be an antiviral protein that inhibits the replication of diverse DNA and RNA viruses. In teleost, viperin homologues have been identified in a large number of species and, in some cases, are stimulated in transcription by viruses. However, the biological significance of fish viperin protein in antiviral immunity has not been investigated. In this study, we identified a viperin homologue from rock bream (Oplegnathus fasciatus) (named OfVip) and examined its expression pattern, subcellular localization, and immune effect. We found that OfVip expression occurred in eight tissues, and experimental challenge of rock bream with the viral fish pathogen megalocytivirus upregulated OfVip expression in kidney, liver, and spleen. OfVip was localized in the endoplasmic reticulum under normal physiological conditions, and viral infection induced subcellular redistribution of OfVip. Transient transfection of cultured fish cells with an OfVip-expressing plasmid caused enhanced cellular resistance against megalocytivirus challenge. Consistently, in vivo study showed that rock bream overexpressing OfVip exhibited significantly reduced viral loads in tissues following experimental infection with megalocytivirus. Furthermore, OfVip upregulated the expression of a wide range of immune genes, including those that are known to participate in antiviral immunity. Taken together, these results indicate for the first time that a teleost viperin is a virus-responsive protein that is modulated in subcellular localization by viral infection, and that viperin regulates the immune reactions of the host fish in a manner that augments resistance against viral infection.

Innate immunity to influenza virus infection

Influenza viruses are a major pathogen of both humans and animals. Recent studies using gene-knockout mice have led to an in-depth understanding of the innate sensors that detect influenza virus infection in a variety of cell types. Signalling downstream of these sensors induces distinct sets of effector mechanisms that block virus replication and promote viral clearance by inducing innate and adaptive immune responses. In this Review, we discuss the various ways in which the innate immune system uses pattern recognition receptors to detect and respond to influenza virus infection. We consider whether the outcome of innate sensor stimulation promotes antiviral resistance or disease tolerance, and propose rational treatment strategies for the acute respiratory disease that is caused by influenza virus infection.

Fatty acid-binding protein E-FABP restricts tumor growth by promoting IFN-β responses in tumor-associated macrophages

Fatty acid-binding proteins (FABP) are known central regulators of both metabolic and inflammatory pathways, but their role in tumor development remains largely unexplored. Here, we report that host expression of epidermal FABP (E-FABP) protects against mammary tumor growth. We find that E-FABP is highly expressed in macrophages, particularly in a specific subset, promoting their antitumor activity. In the tumor stroma, E-FABP-expressing tumor-associated macrophages (TAM) produce high levels of IFN-β through upregulation of lipid droplet formation in response to tumors. E-FABP-mediated IFN-β signaling can further enhance recruitment of tumoricidal effector cells, in particular natural killer cells, to the tumor stroma for antitumor activity. These findings identify E-FABP as a new protective factor to strengthen IFN-β responses against tumor growth.

Interferon-stimulated genes and their role in controlling hepatitis C virus

Infections with the hepatitis C virus (HCV) are a major cause of chronic liver disease. While the acute phase of infection is mostly asymptomatic, this virus has the high propensity to establish persistence and in the course of one to several decades liver disease can develop. HCV is a paradigm for the complex interplay between the interferon (IFN) system and viral countermeasures. The virus induces an IFN response within the infected cell and is rather sensitive against the antiviral state triggered by IFNs, yet in most cases HCV persists. Numerous IFN-stimulated genes (ISGs) have been reported to suppress HCV replication, but in only a few cases we begin to understand the molecular mechanisms underlying antiviral activity. It is becoming increasingly clear that blockage of viral replication is mediated by the concerted action of multiple ISGs that target different steps of the HCV replication cycle. This review briefly summarizes the activation of the IFN system by HCV and then focuses on ISGs targeting the HCV replication cycle and their possible mode of action.

Activation and evasion of antiviral innate immunity by hepatitis C virus

Hepatitis C virus (HCV) chronically infects 130-170 million people worldwide and is a major public health burden. HCV is an RNA virus that infects hepatocytes within liver, and this infection is sensed as non-self by the intracellular innate immune response to program antiviral immunity to HCV. HCV encodes several strategies to evade this antiviral response, and this evasion of innate immunity plays a key role in determining viral persistence. This review discusses the molecular mechanisms of how the intracellular innate immune system detects HCV infection, including how HCV pathogen-associated molecular patterns are generated during infection and where they are recognized as foreign by the innate immune system. Further, this review highlights the key innate immune evasion strategies used by HCV to establish persistent infection within the liver, as well as how host genotype influences the outcome of HCV infection. Understanding these HCV-host interactions is key in understanding how to target HCV during infection and for the design of more effective HCV therapies at the immunological level.

The role of viperin in the innate antiviral response

Viral infection of the cell is able to initiate a signaling cascade of events that ultimately attempts to limit viral replication and prevent escalating infection through expression of host antiviral proteins. Recent work has highlighted the importance of the host antiviral protein viperin in this process, with its ability to limit a large variety of viral infections as well as play a role in the production of type I interferon and the modulation of a number of transcription factor binding sites. Viperin appears to have the ability to modulate varying conditions within the cell and to interfere with proviral host proteins in its attempts to create an unfavorable environment for viral replication. The study of the mechanistic actions of viperin has come a long way in recent years, describing important functional domains of the protein for its antiviral and immune modulator actions as well as demonstrating its role as a member of the radical SAM enzyme family. However, despite the rapid expansion of knowledge regarding the functions of this highly conserved and ancient antiviral protein, there still remains large gaps in our understanding of the precise mechanisms at play for viperin to exert such a wide variety of roles within the cell.

Protective efficacy of orally administered, heat-killed Lactobacillus pentosus b240 against influenza A virus

Influenza A(H1N1)pdm virus caused the first human pandemic of the 21st century. Although various probiotic Lactobacillus species have been shown to have anti-microbial effects against pneumonia-inducing pathogens, the prophylactic efficacy and mechanisms behind their protection remain largely unknown. Here, we evaluated the prophylactic efficacy of heat-killed Lactobacillus pentosus b240 against lethal influenza A(H1N1)pdm virus infection in a mouse model. To further define the protective responses induced by b240, we performed virologic, histopathologic, and transcriptomic analyses on the mouse lungs. Although we did not observe an appreciable effect of b240 on virus growth, cytokine production, or histopathology, gene expressional analysis revealed that oral administration of b240 differentially regulates antiviral gene expression in mouse lungs. Our results unveil the possible mechanisms behind the protection mediated by b240 against influenza virus infection and provide new insights into probiotic therapy.

Viperin regulates cellular lipid metabolism during human cytomegalovirus infection

Human cytomegalovirus (HCMV) has been shown to induce increased lipogenesis in infected cells, and this is believed to be required for proper virion envelopment. We show here that this increase is a consequence of the virus-induced redistribution of the host protein viperin to mitochondria and its capacity to interact with and block the function of the mitochondrial trifunctional protein (TFP), the enzyme that mediates fatty acid-β-oxidation. The resulting decrease in cellular ATP levels activates the enzyme AMP-activated protein kinase (AMPK), which induces expression of the glucose transporter GLUT4, resulting in increased glucose import and translocation to the nucleus of the glucose-regulated transcription factor ChREBP. This induces increased transcription of genes encoding lipogenic enzymes, increased lipid synthesis and lipid droplet accumulation, and generation of the viral envelope. Viperin-dependent lipogenesis is required for optimal production of infectious virus. We show that all of these metabolic outcomes can be replicated by direct targeting of viperin to mitochondria in the absence of HCMV infection, and that the motif responsible for Fe-S cluster binding by viperin is essential. The data indicate that viperin is the major effector underlying the ability of HCMV to regulate cellular lipid metabolism.

Virus infection induces the production of interferons, which in turn stimulate the production of a set of proteins that often have antiviral functions. One of these interferon-inducible proteins is viperin, the product of the human Rsad2 gene. Human cytomegalovirus (HCMV) paradoxically induces expression of viperin independently of the interferon response, and we previously showed that a virus-encoded protein transports the induced viperin to mitochondria where it interferes with fatty acid b-oxidation, a major energy generating system of the cell. We show here that this ultimately results in enhanced lipid synthesis by the infected cell that is essential for production of infectious virus. The mechanism involves sensing the depletion in ATP levels caused by inhibition of fatty acid b-oxidation by the enzyme AMP-induced protein kinase. This induces a cascade of events that result in the increased transcription of genes encoding lipogenic enzymes and consequent lipid biogenesis that is needed by the virus for adequate membrane envelope formation. Thus HCMV uses the interferon-inducible protein viperin, known to be antiviral for other viruses, even for HCMV itself if viperin is pre-expressed in cells prior to infection, to modulate the metabolic status of the cell to facilitate its replication.

ATR-FTIR spectroscopy reveals genomic loci regulating the tissue response in high fat diet fed BXD recombinant inbred mouse strains

Background

Obesity-associated organ-specific pathological states can be ensued from the dysregulation of the functions of the adipose tissues, liver and muscle. However, the influence of genetic differences underlying gross-compositional differences in these tissues is largely unknown. In the present study, the analytical method of ATR-FTIR spectroscopy has been combined with a genetic approach to identify genetic differences responsible for phenotypic alterations in adipose, liver and muscle tissues.

Results

Mice from 29 BXD recombinant inbred mouse strains were put on high fat diet and gross-compositional changes in adipose, liver and muscle tissues were measured by ATR-FTIR spectroscopy. The analysis of genotype-phenotype correlations revealed significant quantitative trait loci (QTL) on chromosome 12 for the content of fat and collagen, collagen integrity, and the lipid to protein ratio in adipose tissue and on chromosome 17 for lipid to protein ratio in liver. Using gene expression and sequence information, we suggest Rsad2 (viperin) and Colec11 (collectin-11) on chromosome 12 as potential quantitative trait candidate genes. Rsad2 may act as a modulator of lipid droplet contents and lipid biosynthesis; Colec11 might play a role in apoptopic cell clearance and maintenance of adipose tissue. An increased level of Rsad2 transcripts in adipose tissue of DBA/2J compared to C57BL/6J mice suggests a cis-acting genetic variant leading to differential gene activation.

Conclusion

The results demonstrate that the analytical method of ATR-FTIR spectroscopy effectively contributed to decompose the macromolecular composition of tissues that accumulate fat and to link this information with genetic determinants. The candidate genes in the QTL regions may contribute to obesity-related diseases in humans, in particular if the results can be verified in a bigger BXD cohort.

Viperin is induced following dengue virus type-2 (DENV-2) infection and has anti-viral actions requiring the C-terminal end of viperin

The host protein viperin is an interferon stimulated gene (ISG) that is up-regulated during a number of viral infections. In this study we have shown that dengue virus type-2 (DENV-2) infection significantly induced viperin, co-incident with production of viral RNA and via a mechanism requiring retinoic acid-inducible gene I (RIG-I). Viperin did not inhibit DENV-2 entry but DENV-2 RNA and infectious virus release was inhibited in viperin expressing cells. Conversely, DENV-2 replicated to higher tires earlier in viperin shRNA expressing cells. The anti-DENV effect of viperin was mediated by residues within the C-terminal 17 amino acids of viperin and did not require the N-terminal residues, including the helix domain, leucine zipper and S-adenosylmethionine (SAM) motifs known to be involved in viperin intracellular membrane association. Viperin showed co-localisation with lipid droplet markers, and was co-localised and interacted with DENV-2 capsid (CA), NS3 and viral RNA. The ability of viperin to interact with DENV-2 NS3 was associated with its anti-viral activity, while co-localisation of viperin with lipid droplets was not. Thus, DENV-2 infection induces viperin which has anti-viral properties residing in the C-terminal region of the protein that act to restrict early DENV-2 RNA production/accumulation, potentially via interaction of viperin with DENV-2 NS3 and replication complexes. These anti-DENV-2 actions of viperin show both contrasts and similarities with other described anti-viral mechanisms of viperin action and highlight the diverse nature of this unique anti-viral host protein.

Characterization of tilapia (Oreochromis niloticus) viperin expression, and inhibition of bacterial growth and modulation of immune-related gene expression by electrotransfer of viperin DNA into zebrafish muscle

Viperin is an anti-viral protein, induced by viral infection. In this study, we examined whether over-expression of viperin in fish muscle could inhibit bacterial growth. We first obtained the cDNA sequence of tilapia viperin, through RT-PCR-mediated cloning and sequencing. The cDNA sequence was similar to those of several fish viperins in GenBank, and it was predicted to encode the conserved domain of radical S-adenosylmethionine superfamily proteins. Phylogenetic analysis revealed that tilapia viperin was most closely related to viperin of Sciaenops ocellatus, Coreoperca kawamebari, and C. whiteheadi. Expression of tilapia viperin was significantly up-regulated in the kidney, liver, spleen, and gills upon challenge with lipopolysaccharide (LPS) and poly(I:C) in a time- and dose-dependent manner. Injection of Vibrio vulnificus (204) and Streptococcus agalactiae (SA47) bacteria into tilapia resulted in significant induction of viperin expression in the whole body, kidney, liver, and spleen. Electrotransfer of a viperin-expressing plasmid into zebrafish muscles decreased bacterial numbers and altered expression of immune-related genes. These data indicate that such altered expression may account for the improvement in bacterial clearance following electroporation of viperin, suggesting that fish viperin has antiviral and antibacterial activities.

Viperin restricts chikungunya virus replication and pathology

Chikungunya virus (CHIKV) is a mosquito-borne arthralgia arbovirus that is reemergent in sub-Saharan Africa and Southeast Asia. CHIKV infection has been shown to be self-limiting, but the molecular mechanisms of the innate immune response that control CHIKV replication remain undefined. Here, longitudinal transcriptional analyses of PBMCs from a cohort of CHIKV-infected patients revealed that type I IFNs controlled CHIKV infection via RSAD2 (which encodes viperin), an enigmatic multifunctional IFN-stimulated gene (ISG). Viperin was highly induced in monocytes, the major target cell of CHIKV in blood. Anti-CHIKV functions of viperin were dependent on its localization in the ER, and the N-terminal amphipathic α-helical domain was crucial for its antiviral activity in controlling CHIKV replication. Furthermore, mice lacking Rsad2 had higher viremia and severe joint inflammation compared with wild-type mice. Our data demonstrate that viperin is a critical antiviral host protein that controls CHIKV infection and provide a preclinical basis for the design of effective control strategies against CHIKV and other reemerging arthrogenic alphaviruses.

Positive selection drives rapid evolution of certain amino acid residues in an evolutionarily highly conserved interferon-inducible antiviral protein of fishes

Viperin, an evolutionarily highly conserved interferon-inducible multifunctional protein, has previously been reported to exhibit antiviral activity against a wide range of DNA and RNA viruses. Utilizing the complete nucleotide coding sequence data of fish viperin antiviral genes, and employing the maximum likelihood-based codon substitution models, the present study reports the pervasive role of positive selection in the evolution of viperin antiviral protein in fishes. The overall rate of nonsynonymous (dN) to synonymous (dS) substitutions (dN/dS) for the three functional domains of viperin (N-terminal, central domain and C-terminal) were 1.1, 0.12, and 0.24, respectively. Codon-by-codon substitution analyses have revealed that while most of the positively selected sites were located at the N-terminal amphipathic α-helix domain, few amino acid residues at the C-terminal domain were under positive selection. However, none of the sites in the central domain were under positive selection. These results indicate that, although viperin is evolutionarily highly conserved, the three functional domains experienced differential selection pressures. Taken together with the results of previous studies, the present study suggests that the persistent antagonistic nature of surrounding infectious viral pathogens might be the likely cause for such adaptive evolutionary changes of certain amino acids in fish viperin antiviral protein.

Viperin, MTAP44, and protein kinase R contribute to the interferon-induced inhibition of Bunyamwera Orthobunyavirus replication

The first line of defense against viral infection is the interferon (IFN) response, which culminates in the expression of hundreds of proteins with presumed antiviral activity, and must be overcome by a virus for successful replication. The nonstructural NSs protein is the primary IFN antagonist encoded by Bunyamwera virus (BUNV), the prototype of the Orthobunyavirus genus and the family Bunyaviridae. The NSs protein interferes with RNA polymerase II-mediated transcription, thereby inhibiting cellular mRNA production, including IFN mRNAs. A recombinant virus, rBUNdelNSs, that is unable to express the NSs protein does not inhibit cellular transcription and is a strong IFN inducer. We report here that cells stimulated into the antiviral state by IFN-β treatment were protected against wild-type BUNV and rBUNdelNSs infection but addition of IFN-β after infection had little effect on the replication cycle of either virus. By screening a panel of cell lines that overexpressed individual IFN-stimulated genes, we found that protein kinase R (PKR), MTAP44, and particularly viperin appreciably restricted BUNV replication. The enzymatic activities of PKR and viperin were required for their inhibitory activities. Taken together, our data show that the restriction of BUNV replication mediated by IFN is an accumulated effect of at least three IFN-stimulated genes that probably act on different stages of the viral replication cycle.

HIV-1 infection of human macrophages directly induces viperin which inhibits viral production

Macrophages are key target cells for HIV-1. HIV-1(BaL) induced a subset of interferon-stimulated genes in monocyte-derived macrophages (MDMs), which differed from that in monocyte-derived dendritic cells and CD4 T cells, without inducing any interferons. Inhibition of type I interferon induction was mediated by HIV-1 inhibition of interferon-regulated factor (IRF3) nuclear translocation. In MDMs, viperin was the most up-regulated interferon-stimulated genes, and it significantly inhibited HIV-1 production. HIV-1 infection disrupted lipid rafts via viperin induction and redistributed viperin to CD81 compartments, the site of HIV-1 egress by budding in MDMs. Exogenous farnesol, which enhances membrane protein prenylation, reversed viperin-mediated inhibition of HIV-1 production. Mutagenesis analysis in transfected cell lines showed that the internal S-adenosyl methionine domains of viperin were essential for its antiviral activity. Thus viperin may contribute to persistent noncytopathic HIV-1 infection of macrophages and possibly to biologic differences with HIV-1-infected T cells.

Viperin: a multifunctional, interferon-inducible protein that regulates virus replication

Viperin is an interferon-inducible protein that inhibits the replication of a variety of viruses by apparently diverse mechanisms. In some circumstances, it also plays a role in intracellular signaling pathways. Its expression in mitochondria, revealed by infection with human cytomegalovirus, also affects cellular metabolic pathways. We review here the current status of our understanding of this unusual molecule.

Emerging roles for lipid droplets in immunity and host-pathogen interactions

Lipid droplets (LDs) are neutral lipid storage organelles ubiquitous to eukaryotic cells. It is increasingly recognized that LDs interact extensively with other organelles and that they perform functions beyond passive lipid storage and lipid homeostasis. One emerging function for LDs is the coordination of immune responses, as these organelles participate in the generation of prostaglandins and leukotrienes, which are important inflammation mediators. Similarly, LDs are also beginning to be recognized as playing a role in interferon responses and in antigen cross presentation. Not surprisingly, there is emerging evidence that many pathogens, including hepatitis C and Dengue viruses, Chlamydia, and Mycobacterium, target LDs during infection either for nutritional purposes or as part of an anti-immunity strategy. We here review recent findings that link LDs to the regulation and execution of immune responses in the context of host-pathogen interactions.

The presence of the iron-sulfur motif is important for the conformational stability of the antiviral protein, Viperin

Viperin, an antiviral protein, has been shown to contain a CX(3)CX(2)C motif, which is conserved in the radical S-adenosyl-methionine (SAM) enzyme family. A triple mutant which replaces these three cysteines with alanines has been shown to have severe deficiency in antiviral activity. Since the crystal structure of Viperin is not available, we have used a combination of computational methods including multi-template homology modeling and molecular dynamics simulation to develop a low-resolution predicted structure. The results show that Viperin is an α-β protein containing iron-sulfur cluster at the center pocket. The calculations suggest that the removal of iron-sulfur cluster would lead to collapse of the protein tertiary structure. To verify these predictions, we have prepared, expressed and purified four mutant proteins. In three mutants individual cysteine residues were replaced by alanine residues while in the fourth all the cysteines were replaced by alanines. Conformational analyses using circular dichroism and steady state fluorescence spectroscopy indicate that the mutant proteins are partially unfolded, conformationally unstable and aggregation prone. The lack of conformational stability of the mutant proteins may have direct relevance to the absence of their antiviral activity.

Viperin, a key player in the antiviral response

Viperin is an antiviral protein that is induced by different viruses, type I interferon, poly(I:C) and lipopolysaccharide, which is localized to the endoplasmic reticulum and lipid droplets. Recently, our knowledge on the mechanism by which viperin inhibits viral replication has strongly increased. Interestingly, it also became clear that viperin can be used by viruses to increase their infectivity. Here, our current knowledge on the induction of viperin and its effect on virus replication will be reviewed.

Early antiviral response and virus-induced genes in fish

In fish as in mammals, virus infections induce changes in the expression of many host genes. Studies conducted during the last fifteen years revealed a major contribution of the interferon system in fish antiviral response. This review describes the screening methods applied to compare the impact of virus infections on the transcriptome in different fish species. These approaches identified a "core" set of genes that are strongly induced in most viral infections. The "core" interferon-induced genes (ISGs) are generally conserved in vertebrates, some of them inhibiting a wide range of viruses in mammals. A selection of ISGs -PKR, vig-1/viperin, Mx, ISG15 and finTRIMs - is further analyzed here to illustrate the diversity and complexity of the mechanisms involved in establishing an antiviral state. Most of the ISG-based pathways remain to be directly determined in fish. Fish ISGs are often duplicated and the functional specialization of multigenic families will be of particular interest for future studies.

The antiviral protein viperin inhibits hepatitis C virus replication via interaction with nonstructural protein 5A

The interferon-stimulated gene, viperin, has been shown to have antiviral activity against hepatitis C virus (HCV) in the context of the HCV replicon, although the molecular mechanisms responsible are not well understood. Here, we demonstrate that viperin plays an integral part in the ability of interferon to limit the replication of cell-culture-derived HCV (JFH-1) that accurately reflects the complete viral life cycle. Using confocal microscopy and fluorescence resonance energy transfer (FRET) analysis, we demonstrate that viperin localizes and interacts with HCV nonstructural protein 5A (NS5A) at the lipid-droplet (LD) interface. In addition, viperin also associates with NS5A and the proviral cellular factor, human vesicle-associated membrane protein-associated protein subtype A (VAP-A), at the HCV replication complex. The ability of viperin to limit HCV replication was dependent on residues within the C-terminus, as well as an N-terminal amphipathic helix. Removal of the amphipathic helix-redirected viperin from the cytosolic face of the endoplasmic reticulum and the LD to a homogenous cytoplasmic distribution, coinciding with a loss of antiviral effect. C-terminal viperin mutants still localized to the LD interface and replication complexes, but did not interact with NS5A proteins, as determined by FRET analysis.

CONCLUSION

In conclusion, we propose that viperin interacts with NS5A and the host factor, VAP-A, to limit HCV replication at the replication complex. This highlights the complexity of the host control of viral replication by interferon-stimulated gene expression.