Genomic structure of the mouse 2',5'-oligoadenylate synthetase gene family

Thirteen Ovary-Enriched Genes Are Individually Not Essential for Female Fertility in Mice

Infertility is considered a global health issue as it currently affects one in every six couples, with female factors reckoned to contribute to partly or solely 50% of all infertility cases. Over a thousand genes are predicted to be highly expressed in the female reproductive system and around 150 genes in the ovary. However, some of their functions in fertility remain to be elucidated. In this study, 13 ovary and/or oocyte-enriched genes (Ccdc58, D930020B18Rik, Elobl, Fbxw15, Oas1h, Nlrp2, Pramel34, Pramel47, Pkd1l2, Sting1, Tspan4, Tubal3, Zar1l) were individually knocked out by the CRISPR/Cas9 system. Mating tests showed that these 13 mutant mouse lines were capable of producing offspring. In addition, we observed the histology section of ovaries and performed in vitro fertilization in five mutant mouse lines. We found no significant anomalies in terms of ovarian development and fertilization ability. In this study, 13 different mutant mouse lines generated by CRISPR/Cas9 genome editing technology revealed that these 13 genes are individually not essential for female fertility in mice.

CD147 monoclonal antibody attenuates abdominal aortic aneurysm formation in angiotensin II-Infused apoE-/- mice

BACKGROUND

Abdominal aortic aneurysm (AAA) is a life threatening vascular disease. Our previous study reported the upregulation of CD147 expression in human aortic aneurysms.

OBJECTIVE

In this study, we injected apoE-/- mice intraperitoneally with CD147 monoclonal antibody or IgG control antibody to observe its effect on Angiotensin II (AngII) induced AAA formation.

METHODS

ApoE-/- mice were randomly divided into an AngⅡ+CD147 antibody group (n = 20) and an AngⅡ+IgG antibody group (n = 20). The Alzet osmotic minipump was implanted subcutaneously into the backs of mice to infuse AngII (1000 ng/kg/min) for 28 days and subsequently treated with CD147 monoclonal antibody or control IgG mAb (10 μg/mouse/day) beginning one day after surgery. Body weight, food intake, drinking volume and blood pressure were measured weekly throughout the study. After 4 weeks of injection, routine bloodwork measuring liver function, kidney function and lipid levels were recorded. Hematoxylin and eosin (H&E), Masson's trichrome, and Elastic van Gieson (EVG) staining were used to evaluate the pathological changes in blood vessels. In addition, Immunohistochemical assay was used to detect infiltration of inflammatory cells. Tandem mass tag (TMT)-based proteomic analysis was used to define differentially expressed proteins (DEPs) using a p-value < 0.05 and fold change > 1.2 or < 0.83 as the threshold. Subsequently, we conducted protein-protein interaction (PPI) network and GO enrichment analysis to determine the core biological function altered after CD147 antibody injection.

RESULTS

The CD147 monoclonal antibody suppresses Ang II-induced AAA formation in apoE-/- mice and reduced aortic expansion, elastic lamina degradation, and inflammatory cells accumulation. Bioinformatics analysis showed that Ptk6, Itch, Casp3, and Oas1a were the hub DEPs. These DEPs in the two group were mainly involved in collagen fibril organization, extracellular matrix organization, and muscle contraction. These data robustly demonstrated that CD147 monoclonal antibody suppresses Ang II-induced AAA formation through reduction of inflammatory response and regulation of the above defined hub proteins and biological processes. Thus, the CD147 monoclonal antibody might be a promising target in the treatment of abdominal aortic aneurysm.

Exploring the Anti-Inflammatory Effect of Inulin by Integrating Transcriptomic and Proteomic Analyses in a Murine Macrophage Cell Model

Inulin is a natural polysaccharide classified as a soluble fiber with demonstrated prebiotic activity. Prebiotics can reduce intestinal and systemic inflammation through modulation of the gut microflora and their metabolites. Additionally, extensive research is illuminating the role of macrophages in the interaction between gut microbiota and many systemic inflammatory diseases. In this study, the anti-inflammatory properties of inulin were evaluated using a murine macrophage cell model (RAW 264.7) of inflammation, and the immunomodulatory mechanism was investigated using omics technologies. The cells underwent comprehensive transcriptomic and proteomic analyses to identify the mechanisms responsible for the observed anti-inflammatory phenotype. Functional analyses of these omics results revealed two potential mechanisms that may lead to an overall reduction in cytokine and chemokine transcription: the inhibition of the NF-κB signaling pathway, leading to the downregulation of proinflammatory factors such as COX2, and the promotion of the phase II defense protein Hmox1 via the Nrf2 pathway. This study provides promising targets for research on immune modulation by dietary fibers and offers new strategies for the design of functional ingredients, foods, and nutraceutical products, which could ultimately lead to personalized nutrition and improved consumer health.

Plasma Non-transferrin-Bound Iron Could Enter into Mice Duodenum and Negatively Affect Duodenal Defense Response to Virus and Immune Responses

Plasma non-transferrin-bound iron (NTBI) exists when the plasma iron content exceeds the carrying capacity of transferrin and can be quickly cleared by the liver, pancreas, and other organs. However, whether it could enter the small intestine and its effects still remain unclear. Herein, these issues were explored. Mice were intravenously administrated of ferric citrate (treatment) or citrate acid (control) 10 min after the saturation of the transferrin. Two hours later, hepatic, duodenal, and jejunal iron content and distribution were measured and duodenal transcriptome sequencing was performed. Significant increase of duodenal and hepatic iron content was detected, indicating that plasma NTBI could be absorbed by the duodenum as well as the liver. A total of 103 differentially expressed genes were identified in the duodenum of mice in the treatment group compared to the control group. Gene Ontology (GO) functional analysis of these genes showed that they were mainly involved in defense response to virus and immune response. The results of Kyoto Encyclopedia of Genes and Genomes pathway (KEGG) analysis revealed that there were major changes in the hematopoietic cell lineage and some virus infection pathways between the two groups. Determination of 7 cytokines in the duodenum were further conducted, which demonstrated that the anti-inflammatory factors interferon (IL)-4 and IL-10 in the duodenum were significantly decreased after NTBI uptake. Our findings revealed that NTBI in plasma can enter the duodenum, which would change the duodenal hematopoietic cell lineage and have a negative impact on defense response to the virus and immune responses.

Granulosa Cell Specific Loss of Adar in Mice Delays Ovulation, Oocyte Maturation and Leads to Infertility

Adenosine deaminases acting on RNA-(ADAR) comprise one family of RNA editing enzymes that specifically catalyze adenosine to inosine (A-to-I) editing. A granulosa cell (GC) specific Adar depleted mouse model [Adar flox/flox:Cyp19a1-Cre/+ (gcAdarKO)] was used to evaluate the role of ADAR1 during the periovulatory period. Loss of Adar in GCs led to failure to ovulate at 16 h post-hCG, delayed oocyte germinal vesicle breakdown and severe infertility. RNAseq analysis of GC collected from gcAdarKO and littermate control mice at 0 and 4 h post-hCG following a super-ovulatory dose of eCG (48 h), revealed minimal differences after eCG treatment alone (0 h), consistent with normal folliculogenesis observed histologically and uterine estrogenic responses. In contrast, 300 differential expressed genes (DEGs; >1.5-fold change and FDRP < 0.1) were altered at 4 h post-hCG. Ingenuity pathway analysis identified many downstream targets of estrogen and progesterone pathways, while multiple genes involved in inflammatory responses were upregulated in the gcAdarKO GCs. Temporal expression analysis of GCs at 0, 4, 8, and 12 h post-hCG of Ifi44, Ifit1, Ifit3b, and Oas1g and Ovgp1 confirmed upregulation of these inflammatory and interferon genes and downregulation of Ovgp1 a glycoprotein involved in oocyte zona pellucida stability. Thus, loss of ADAR1 in GCs leads to increased expression of inflammatory and interferon response genes which are temporally linked to ovulation failure, alterations in oocyte developmental progression and infertility.

Values of OAS gene family in the expression signature, immune cell infiltration and prognosis of human bladder cancer

Background

Bladder cancer (BLCA) is one of the most common genitourinary malignancies in the world, but its pathogenic genes have not been fully identified and the treatment outcomes are still unsatisfactory. Although the members of 2', 5'-oligoadenylate synthetase (OAS) gene family are known involved in some tumorous biological processes, the roles of the OAS gene family in BLCA are still undetermined.

Methods

By combining vast bioinformatic datasets analyses of BLCA and the experimental verification on clinical BLCA specimen, we identified the expressions and biological functions of OAS gene family members in BLCA with comparison to normal bladder tissues.

Results

The expression levels of OAS gene family members were higher in BLCA than in normal bladder tissues. The expression levels of most OAS genes had correlations with genomic mutation and methylation, and with the infiltration levels of CD4 + T cells, CD8 + T cells, neutrophils, and dendritic cells in the microenvironment of BLCA. In addition, high expressions of OAS1, OAS2, OAS3, and OASL predicted better overall survival in BLCA patients.

Conclusions

The highly expressed OAS genes in BLCA can reflect immune cells infiltration in the tumor microenvironment and predict the better overall survival of BLCA, and thus may be considered as a signature of BLCA. The study provides new insights into the diagnosis, treatment, and prognosis of BLCA.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10102-8.

Characterization, expression pattern and antiviral activities of oligoadenylate synthetase in Chinese Giant Salamander, Andrias davidianus

The enzyme 2'-5'-oligoadenylate synthetase (OAS) is an antiviral protein induced by interferons (IFNs), which plays an important role in IFN-mediated antiviral signaling pathway. In this study, the OAS of Chinese Giant Salamander, Andrias davidianus (AdOAS) was identified for the first time, and the expression profiles in vivo and the antiviral activities in vitro were investigated. The open reading frame (ORF) of AdOAS gene is 1185 bp in length, encoding a putative protein of 394 amino acids, in which a Nucleotidyltransferase (NTase) domain (40-143 aa) and a conserved OAS1 C superfamily domain (165-341 aa) are included. qRT-PCR analysis revealed a broad expression of AdOAS in vivo, with the highest expression level in intestine and heart. After infection with Chinese giant salamander iridovirus (GSIV), the mRNA level of AdOAS in liver increased significantly at 24 h and 48 h post infection and reached the peak at 72 h compared with the control group. The AdOAS mRNA level in kidney increased slightly at 6 h and 12 h post infection, declined to the initial level at 24 h and peaked at 48 h post infection, while in spleen it was slightly up-regulated at 6 h, inhibited at 12 h, 24 h and 48 h, and then significantly increased to the peak at 72 h post infection. In vitro, AdOAS mRNA level in Chinese giant salamander muscle (GSM) cells was not noticeably up-regulated until 24 h and then peaked at 48 h post GSIV infection. In antiviral activity test, the mRNA transcription and protein level of virus major capsid protein (MCP) in AdOAS over-expressed cells was significantly reduced compared with that in control cells by qRT-PCR and western blot analysis. In addition, ddPCR results showed that lower MCP gene copy was found in AdOAS over-expressed cells compared with the control group. These results collectively suggest that AdOAS plays a crucial role against GSIV infection in Chinese giant salamander, and provide a solid base for the further studies on the mechanism of immune defense and the control of the disease in this animal.

OASL1-Mediated Inhibition of Type I IFN Reduces Influenza A Infection-Induced Airway Inflammation by Regulating ILC2s

Purpose

Three observations drove this study. First, 2′-5′-oligoadenylate synthetase-like protein (OASL) is a negative regulator of type I interferon (IFN). Second, type I IFN plays a central role during virus infections and the pathogenesis of various diseases, including asthma. Third, influenza A virus (IAV) causes non-eosinophilic asthma. To evaluate the potential relationships between OASL, type I IFN, and pulmonary innate immune cells in IAV-induced acute airway inflammation by using Oasl1^-/- mice.

Methods

Asthma was induced in wild-type (WT) and Oasl1^-/- mice with IAV or ovalbumin (OVA). Airway hyperreactivity (AHR) and immune cell infiltration in the bronchoalveolar lavage (BAL) fluids were measured. The immune cells in the lungs were analyzed by flow cytometry. To investigate the ability of type I IFN to shape the response of lung type 2 innate lymphoid cells (ILC2s), IFN-α was treated intratracheally. Plasmacytoid dendritic cells (pDCs) sorted from bone marrow and ILC2s sorted from lungs of naive mice were co-cultured with/without interferon-alpha receptor subunit 1 (IFNAR-1)-blocking antibodies.

Results

In the IAV-induced asthma model, Oasl1^-/- mice developed greater AHR and immune cell infiltration in the BAL fluids than WT mice. This was not observed in OVA-induced asthma, a standard model of allergen-induced asthma. The lungs of infected Oasl1^-/- mice also had elevated DC numbers and Ifna expression and depressed IAV-induced ILC2 responses, namely, proliferation and type 2 cytokine and amphiregulin production. Intratracheal administration of type I IFN in naïve mice suppressed lung ILC2 production of type 2 cytokines and amphiregulin. Co-culture of ILC2s with pDCs showed that pDCs inhibit the function of ILC2s by secreting type I IFN.

Conclusions

OASL1 may impede the IAV-induced acute airway inflammation that drives AHR by inhibiting IAV-induced type I IFN production from lung DCs, thereby preserving the functions of lung ILC2s, including their amphiregulin production.

Specificity and Mechanism of Coronavirus, Rotavirus, and Mammalian Two-Histidine Phosphoesterases That Antagonize Antiviral Innate Immunity

The 2′,5′-oligoadenylate (2-5A)-dependent endoribonuclease, RNase L, is a principal mediator of the interferon (IFN) antiviral response. Therefore, the regulation of cellular levels of 2-5A is a key point of control in antiviral innate immunity. Cellular 2-5A levels are determined by IFN-inducible 2′,5′-oligoadenylate synthetases (OASs) and by enzymes that degrade 2-5A. Importantly, many coronaviruses (CoVs) and rotaviruses encode 2-5A-degrading enzymes, thereby antagonizing RNase L and its antiviral effects. A-kinase-anchoring protein 7 (AKAP7), a mammalian counterpart, could possibly limit tissue damage from excessive or prolonged RNase L activation during viral infections or from self-double-stranded RNAs that activate OAS. We show that these enzymes, members of the two-histidine phosphoesterase (2H-PE) superfamily, constitute a subfamily referred here as 2′,5′-PEs. 2′,5′-PEs from the mouse CoV mouse hepatitis virus (MHV) (NS2), Middle East respiratory syndrome coronavirus (MERS-CoV) (NS4b), group A rotavirus (VP3), and mouse (AKAP7) were investigated for their evolutionary relationships and activities. While there was no activity against 3′,5′-oligoribonucleotides, they all cleaved 2′,5′-oligoadenylates efficiently but with variable activity against other 2′,5′-oligonucleotides. The 2′,5′-PEs are shown to be metal ion-independent enzymes that cleave trimer 2-5A (2′,5′-p₃A₃) producing mono- or diadenylates with 2′,3′-cyclic phosphate termini. Our results suggest that the elimination of 2-5A might be the sole function of viral 2′,5′-PEs, thereby promoting viral escape from innate immunity by preventing or limiting the activation of RNase L.

Inborn errors of immunity-recent advances in research on the pathogenesis

Primary immunodeficiency (PID) is a genetic disorder with a defect of one of the important components of our immune system. Classical PID has been recognized as a disorder with loss of function of the immune system. Recent studies have unveiled disorders with immune dysfunction with autoimmunity, autoinflammation, allergy, or predisposition to malignancy. Some of them were caused by an augmented immune function or a defect in immune regulation. With this background, the term inborn errors of immunity (IEI) is now used to refer to PID in the International Union of Immunological Societies (IUIS) classification. More than 400 responsible genes have been identified in patients with IEI so far, and importantly, many of them identified lately were caused by a heterologous mutation. Moreover, the onset is not necessarily in childhood, and we started seeing more and more IEI patients diagnosed in adulthood in the clinical settings. Recent advances in genetic analysis, including whole-exome analysis, whole-genome analysis, and RNA-seq have contributed to the identification of the disease-causing gene mutation. We also started to find heterogeneity of phenotype even in the patients with the same mutation in the same family, leading us to wonder if modifier gene or epigenetic modification is involved in the pathogenesis. In contrast, we accumulated many cases suggesting genetic heterogeneity is associated with phenotypic homogeneity. It has thus become difficult to deduce a responsible gene only from the phenotype in a certain type of IEI. Current curative therapy for IEI includes hematopoietic cell transplantation and gene therapy. Other curative therapeutic modalities have been long waited and are to be introduced in the future. These include a small molecule that inhibits the gain-of-function of the molecule- and genome-editing technology. Research on IEI will surely lead to a better understanding of other immune-related disorders including rheumatic diseases and atopic disorders.

Lipid Storage and Interferon Response Determine the Phenotype of Ground Glass Hepatocytes in Mice and Humans

BACKGROUND AND AIMS

A histopathological hallmark of chronic hepatitis B virus (HBV) infection is the presence of ground glass hepatocytes (GGHs). GGHs are liver cells that exhibit eosinophilic, granular, glassy cytoplasm in light microscopy and are characterized by accumulation of HBV surface (HBs) proteins in the endoplasmic reticulum (ER). More important, GGHs have been accepted as a precursor of HCC and may represent preneoplastic lesions of the liver.

METHODS

Here we show that the reason for ground glass phenotype of hepatocytes in patients with chronic hepatitis B (CHB) and in HBs transgenic mice is a complex formation between HBs proteins and lipid droplets (LDs) within the ER.

RESULTS

As fat is a main component of LDs their presence reduces the protein density of HBs aggregates. Therefore, they adsorb less amount of eosin during hematoxylin-eosin staining and appear dull in light microscopy. However, after induction of interferon response in the liver LDs were not only co-localized with HBs but also distributed throughout the cytoplasm of hepatocytes. The uniform distribution of LDs weakens the contrast between HBs aggregates and the rest of the cytoplasm and complicates the identification of GGHs. Suppression of interferon response restored the ground glass phenotype of hepatocytes.

CONCLUSIONS

Complex formation between HBs and LDs represents a very important feature of CHB that could affect LDs functions in hepatocytes. The strain specific activation of the interferon response in the liver of HBs/c mice prevented the development of GGHs. Thus, manipulation of LDs could provide a new treatment strategy in the prevention of liver cancer.

Alternative splicing coupled with transcript degradation modulates OAS1g antiviral activity

At the heart of an innate immune response lies a tightly regulated gene expression program. This precise regulation is crucial because small changes can shift the balance from protective to destructive immunity. Here we identify a frequently used alternative splice site in the gene oligoadenylate synthetase 1g (Oas1g), a key component of the 2-5A antiviral system. Usage of this splice site leads to the generation of a transcript subject to decay, and removal of the site leads to increased expression of Oas1g and an improved antiviral response. However, removal of the splice site also leads to an increase in apoptotic cell death, suggesting this splicing event exists as a compromise between the pathogen protective benefits and collateral damage associated with OAS1g activity. Across the innate immune response, we show that a multitude of alternative splicing events predicted to lead to decay exist, and thus have the potential to play a significant role in the regulation of gene expression in innate immunity.

In vitro and bioinformatics mechanistic-based approach for cadmium carcinogenicity understanding

Cadmium is a toxic metal able to enter the cells through channels and transport pathways dedicated to essential ions, leading, among others, to the dysregulation of divalent ions homeostasis. Despite its recognized human carcinogenicity, the mechanisms are still under investigation. A powerful tool for mechanistic studies of carcinogenesis is the Cell Transformation Assay (CTA). We have isolated and characterized by whole genome microarray and bioinformatics analysis of differentially expressed genes (DEGs) cadmium-transformed cells from different foci (F1, F2, and F3) at the end of CTA (6 weeks). The systematic analysis of up- and down-regulated transcripts and the comparison of DEGs in transformed cells evidence different functional targets and the complex picture of cadmium-induced transformation. Only 34 in common DEGs are found in cells from all foci, and among these, only 4 genes are jointly up-regulated (Ccl2, Ccl5, IL6 and Spp1), all responsible for cytokines/chemokines coding. Most in common DEGs are down-regulated, suggesting that the switching-off of specific functions plays a major role in this process. In addition, the comparison of dysregulated pathways immediately after cadmium treatment with those in transformed cells provides a valuable means to the comprehension of the overall process.

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Cell transformation Assay and toxicogenomics are integrated to study cadmium carcinogenesis mechanisms

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Inflammatory response is the only common feature in Cd-transformed cells from all different foci

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Switching-off of specific functions plays a major role in Cd-induced carcinogenesis

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Comparison of triggering signals and deregulated pathways in transformed cells provides hints on cadmium mechanisms

Bioinformatics Analysis of Potential Key Genes in Trastuzumab-Resistant Gastric Cancer

Background

This study was performed to identify genes related to acquired trastuzumab resistance in gastric cancer (GC) and to analyze their prognostic value.

Methods

The gene expression profile GSE77346 was downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were obtained by using GEO2R. Functional and pathway enrichment was analyzed by using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). Search Tool for the Retrieval of Interacting Genes (STRING), Cytoscape, and MCODE were then used to construct the protein-protein interaction (PPI) network and identify hub genes. Finally, the relationship between hub genes and overall survival (OS) was analyzed by using the online Kaplan-Meier plotter tool.

Results

A total of 327 DEGs were screened and were mainly enriched in terms related to pathways in cancer, signaling pathways regulating stem cell pluripotency, HTLV-I infection, and ECM-receptor interactions. A PPI network was constructed, and 18 hub genes (including one upregulated gene and seventeen downregulated genes) were identified based on the degrees and MCODE scores of the PPI network. Finally, the expression of four hub genes (ERBB2, VIM, EGR1, and PSMB8) was found to be related to the prognosis of HER2-positive (HER2+) gastric cancer. However, the prognostic value of the other hub genes was controversial; interestingly, most of these genes were interferon- (IFN-) stimulated genes (ISGs).

Conclusions

Overall, we propose that the four hub genes may be potential targets in trastuzumab-resistant gastric cancer and that ISGs may play a key role in promoting trastuzumab resistance in GC.

Bioinformatics analysis of Ewing's sarcoma: Seeking key candidate genes and pathways

Ewing's sarcoma (ES) is the second most common bone tumor among children and adolescents worldwide. However, the genes and signaling pathways involved in ES tumorigenesis and progression remain unclear. The present study used two gene-expression profile datasets (GSE17674 and GSE31215) to elucidate key potential candidate genes and pathways in ES. Differentially expressed genes (DEGs) were identified and a functional enrichment analysis was performed. A protein-protein interaction (PPI) network was constructed, and the most significant module in the PPI network was selected from the Search Tool for the Retrieval of Interacting Genes/Proteins database. A total of 278 genes were identified by comparing the tumor samples with non-cancerous samples; these included 272 upregulated and 6 downregulated genes. The pathway analysis demonstrated significant enrichment in the positive regulation of transcription in the DEGs coding for RNA polymerase II promoter, plasma membrane and chromatin binding pathways in cancer in general. There were 269 nodes and 292 edges in the PPI network. Finally, MYC, IGF1, OAS1, EZH2 and ISG15 were identified as the hub genes according to the degree levels. The survival analysis revealed that EZH2 is associated with a poor prognosis in patients with ES. In conclusion, the DEGs, associated pathways and hub genes identified in the present study help elucidate the underlying molecular mechanisms of ES carcinogenesis and progression, and provide potential molecular targets and biomarkers for ES.

Activation of RNase L in Egyptian Rousette Bat-Derived RoNi/7 Cells Is Dependent Primarily on OAS3 and Independent of MAVS Signaling

Many RNA viruses that are highly pathogenic in humans are relatively apathogenic in their bat reservoirs, making it important to compare innate immune responses in bats to those well characterized in humans. One such antiviral response is the OAS-RNase L pathway. OASs, upon sensing dsRNA, produce 2-5A, leading to activation of RNase L which degrades viral and host RNA, limiting viral replication. Analysis of Egyptian Rousette bat sequences revealed three OAS genes expressing OAS1, OAS2, and OAS3 proteins. Interferon treatment or viral infection induces all three bat OAS mRNAs. In these bat cells as in human cells, RNase L activation and its antiviral activity are dependent primarily on OAS3 while MAVS signaling is not required. Importantly, our findings indicate the OAS-RNase L system is a primary response to virus rather than a secondary effect of interferon signaling and therefore can be activated early in infection or while interferon signaling is antagonized.

Bats are reservoirs for many RNA viruses that are highly pathogenic in humans yet relatively apathogenic in the natural host. It has been suggested that differences in innate immunity are responsible. The antiviral OAS-RNase L pathway is well characterized in humans, but there is little known about its activation and antiviral activity in bats. During infection, OASs, upon sensing double-stranded RNA (dsRNA), produce 2′-5′ oligoadenylates (2-5A), leading to activation of RNase L which degrades viral and host RNA, limiting viral replication. Humans encode three active OASs (OAS1 to -3). Analysis of the Egyptian Rousette bat genome combined with mRNA sequencing from bat RoNi/7 cells revealed three homologous OAS proteins. Interferon alpha treatment or viral infection induced all three OAS mRNAs, but RNase L mRNA is constitutively expressed. Sindbis virus (SINV) or vaccinia virus (VACVΔE3L) infection of wild-type (WT) or OAS1-KO (knockout), OAS2-KO, or MAVS-KO RoNi/7 cells, but not RNase L-KO or OAS3-KO cells, induces robust RNase L activation. SINV replication is 100- to 200-fold higher in the absence of RNase L or OAS3 than in WT cells. However, MAVS-KO had no detectable effect on RNA degradation or replication. Thus, in RoNi/7 bat cells, as in human cells, activation of RNase L during infection and its antiviral activity are dependent primarily on OAS3 while MAVS signaling is not required for the activation of RNase L and restriction of infection. Our findings indicate that OAS proteins serve as pattern recognition receptors (PRRs) to recognize viral dsRNA and that this pathway is a primary response to virus rather than a secondary effect of interferon signaling.

RNase L Antiviral Activity Is Not a Critical Component of the Oas1b-Mediated Flavivirus Resistance Phenotype

In mice, resistance to central nervous system (CNS) disease induced by members of the genus Flavivirus is conferred by an allele of the 2'-5' oligoadenylate synthetase 1b gene that encodes the inactive full-length protein (Oas1b-FL). The susceptibility allele encodes a C-terminally truncated protein (Oas1b-tr). We show that the efficiency of neuron infection in the brains of resistant and susceptible mice is similar after an intracranial inoculation of two flaviviruses, but amplification of viral proteins and double-stranded RNA (dsRNA) is inhibited in infected neurons in resistant mouse brains at later times. Active OAS proteins detect cytoplasmic dsRNA and synthesize short 2'-5'-linked oligoadenylates (2'-5'A) that interact with the latent endonuclease RNase L, causing it to dimerize and cleave single-stranded RNAs. To evaluate the contribution of RNase L to the resistance phenotype in vivo, we created a line of resistant RNase L^-/- mice. Evidence of RNase L activation in infected RNase L^+/+ mice was indicated by higher levels of viral RNA in the brains of infected RNase L^-/- mice. Activation of type I interferon (IFN) signaling was detected in both resistant and susceptible brains, but Oas1a and Oas1b mRNA levels were lower in RNase L^+/+ mice of both types, suggesting that activated RNase L also has a proflaviviral effect. Inhibition of virus replication was robust in resistant RNase L^-/- mice, indicating that activated RNase L is not a critical factor in mediating this phenotype.IMPORTANCE The mouse genome encodes a family of Oas proteins that synthesize 2'-5'A in response to dsRNA. 2'-5'A activates the endonuclease RNase L to cleave single-stranded viral and cellular RNAs. The inactive, full-length Oas1b protein confers flavivirus-specific disease resistance. Although similar numbers of neurons were infected in resistant and susceptible brains after an intracranial virus infection, viral components amplified only in susceptible brains at later times. A line of resistant RNase L^-/- mice was used to evaluate the contribution of RNase L to the resistance phenotype in vivo Activation of RNase L antiviral activity by flavivirus infection was indicated by increased viral RNA levels in the brains of RNase L^-/- mice. Oas1a and Oas1b mRNA levels were higher in infected RNase L^-/- mice, indicating that activated RNase L also have a proflaviviral affect. However, the resistance phenotype was equally robust in RNase L^-/- and RNase L^+/+ mice.

Biochemical characterization of the mouse ABCF3 protein, a partner of the flavivirus-resistance protein OAS1B

Mammalian ATP-binding cassette (ABC) subfamily F member 3 (ABCF3) is a class 2 ABC protein that has previously been identified as a partner of the mouse flavivirus resistance protein 2',5'-oligoadenylate synthetase 1B (OAS1B). The functions and natural substrates of ABCF3 are not known. In this study, analysis of purified ABCF3 showed that it is an active ATPase, and binding analyses with a fluorescent ATP analog suggested unequal contributions by the two nucleotide-binding domains. We further showed that ABCF3 activity is increased by lipids, including sphingosine, sphingomyelin, platelet-activating factor, and lysophosphatidylcholine. However, cholesterol inhibited ABCF3 activity, whereas alkyl ether lipids either inhibited or resulted in a biphasic response, suggesting small changes in lipid structure differentially affect ABCF3 activity. Point mutations in the two nucleotide-binding domains of ABCF3 affected sphingosine-stimulated ATPase activity differently, further supporting different roles for the two catalytic pockets. We propose a model in which pocket 1 is the site of basal catalysis, whereas pocket 2 engages in ligand-stimulated ATP hydrolysis. Co-localization of the ABCF3-OAS1B complex to the virus-remodeled endoplasmic reticulum membrane has been shown before. We also noted that co-expression of ABCF3 and OAS1B in bacteria alleviated growth inhibition caused by expression of OAS1B alone, and ABCF3 significantly enhanced OAS1B levels, indirectly showing interaction between these two proteins in bacterial cells. As viral RNA synthesis requires large amounts of ATP, we conclude that lipid-stimulated ATP hydrolysis may contribute to the reduction in viral RNA production characteristic of the flavivirus resistance phenotype.

Activation of innate immunity by mitochondrial dsRNA in mouse cells lacking p53 protein

Viral and cellular double-stranded RNA (dsRNA) is recognized by cytosolic innate immune sensors, including RIG-I-like receptors. Some cytoplasmic dsRNA is commonly present in cells, and one source is mitochondrial dsRNA, which results from bidirectional transcription of mitochondrial DNA (mtDNA). Here we demonstrate that Trp53 mutant mouse embryonic fibroblasts contain immune-stimulating endogenous dsRNA of mitochondrial origin. We show that the immune response induced by this dsRNA is mediated via RIG-I-like receptors and leads to the expression of type I interferon and proinflammatory cytokine genes. The mitochondrial dsRNA is cleaved by RNase L, which cleaves all cellular RNA including mitochondrial mRNAs, increasing activation of RIG-I-like receptors. When mitochondrial transcription is interrupted there is a subsequent decrease in this immune-stimulatory dsRNA. Our results reveal that the role of p53 in innate immunity is even more versatile and complex than previously anticipated. Our study, therefore, sheds new light on the role of endogenous RNA in diseases featuring aberrant immune responses.

RNA regulation of the antiviral protein 2'-5'-oligoadenylate synthetase

The innate immune system is a broad collection of critical intra- and extra-cellular processes that limit the infectivity of diverse pathogens. The 2'-5'-oligoadenylate synthetase (OAS) family of enzymes are important sensors of cytosolic double-stranded RNA (dsRNA) that play a critical role in limiting viral infection by activating the latent ribonuclease (RNase L) to halt viral replication and establish an antiviral state. Attesting to the importance of the OAS/RNase L pathway, diverse viruses have developed numerous distinct strategies to evade the effects of OAS activation. How OAS proteins are regulated by viral or cellular RNAs is not fully understood but several recent studies have provided important new insights into the molecular mechanisms of OAS activation by dsRNA. Other studies have revealed unanticipated features of RNA sequence and structure that strongly enhance activation of at least one OAS family member. While these discoveries represent important advances, they also underscore the fact that much remains to be learned about RNA-mediated regulation of the OAS/RNase L pathway. In particular, defining the full complement of RNA molecular signatures that activate OAS is essential to our understanding of how these proteins maximize their protective role against pathogens while still accurately discriminating host molecules to avoid inadvertent activation by cellular RNAs. A more complete knowledge of OAS regulation may also serve as a foundation for the development of novel antiviral therapeutic strategies and lead the way to a deeper understanding of currently unappreciated cellular functions of the OAS/RNase L pathway in the absence of infection. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Translation Regulation.

Molecular cloning and functional characterization of porcine 2',5'-oligoadenylate synthetase 1b and its effect on infection with porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has previously been shown to increase porcine 2'-5'-oligoadenylate synthase (OAS) 1a expression, but the specific role of porcine OAS1b (pOAS1b) in PRRSV replication remains unknown. In this study, we conducted sequence analysis of the porcine OAS1b gene and studied the effects of its overexpression or silencing on PRRSV replication. OAS1b, localized mainly in the cytoplasm, was found to contain conserved protein domains, such as the P-Loop and D-Box, indicating that its nucleotidyl transferase activity was complete and the antiviral effect depended on ribonuclease L (RNase L). OAS1b overexpression inhibited PRRSV replication, whereas small-interfering-RNA silencing of OAS1b resulted in increased virus titers. Additionally, OAS1b promoted expression of interferons as well as interferon-β promoter activity. These results lay the theoretical foundation for the development of new anti-PRRSV strategies.

Recurrent Loss-of-Function Mutations Reveal Costs to OAS1 Antiviral Activity in Primates

Immune responses counteract infections but also cause collateral damage to hosts. Oligoadenylate synthetase 1 (OAS1) binds double-stranded RNA from invading viruses and produces 2'-5' linked oligoadenylate (2-5A) to activate ribonuclease L (RNase L), which cleaves RNA to inhibit virus replication. OAS1 can also undergo autoactivation by host RNAs, a potential trade-off to antiviral activity. We investigated functional variation in primate OAS1 as a model for how immune pathways evolve to mitigate costs and observed a surprising frequency of loss-of-function variation. In gorillas, we identified a polymorphism that severely decreases catalytic function, mirroring a common variant in humans that impairs 2-5A synthesis through alternative splicing. OAS1 loss-of-function variation is also common in monkeys, including complete loss of 2-5A synthesis in tamarins. The frequency of loss-of-function alleles suggests that costs associated with OAS1 activation can be so detrimental to host fitness that pathogen-protective effects are repeatedly forfeited.

Conserved Active-Site Residues Associated with OAS Enzyme Activity and Ubiquitin-Like Domains Are Not Required for the Antiviral Activity of goOASL Protein against Avian Tembusu Virus

Interferon (IFN)-induced 2′-5′-oligoadenylate synthetase (OAS) proteins exhibit an extensive and efficient antiviral effect against flavivirus infection in mammals and birds. Only the 2′-5′-oligoadenylate synthetase-like (OASL) gene has been identified thus far in birds, except for ostrich, which has both OAS1 and OASL genes. In this study, we first investigated the antiviral activity of goose OASL (goOASL) protein against a duck-origin Tembusu virus (DTMUV) in duck embryo fibroblast cells (DEFs). To investigate the relationship of conserved amino acids that are related to OAS enzyme activity and ubiquitin-like (UBL) domains with the antiviral activity of goOASL, a series of mutant goOASL plasmids was constructed, including goOASL-S64C/D76E/D78E/D144T, goOASL∆UBLs and goOASL∆UBLs-S64C/D76E/D78E/D144T. Interestingly, all these mutant proteins significantly inhibited the replication of DTMUV in DEFs in a dose-dependent manner. Immunofluorescence analysis showed that the goOASL, goOASL-S64C/D76E/D78E/D144T, goOASL∆UBLs and goOASL∆UBLs-S64C/D76E/D78E/D144T proteins were located not only in the cytoplasm where DTMUV replicates but also in the nucleus of DEFs. However, the goOASL and goOASL mutant proteins were mainly colocalized with DTMUV in the cytoplasm of infected cells. Our data indicated that goOASL could significantly inhibit DTMUV replication in vitro, while the active-site residues S64, D76, D78 and D144, which were associated with OAS enzyme activity, the UBL domains were not required for the antiviral activity of goOASL protein.

Intracerebroventricular Administration of a 2'-O-Methyl Phosphorothioate Antisense Oligonucleotide Results in Activation of the Innate Immune System in Mouse Brain

Antisense oligonucleotides (AONs) are versatile molecules that can be used to modulate gene expression by binding to RNA. The therapeutic potential of AONs appears particularly high in the central nervous system, due to excellent distribution and uptake in brain cells, as well as good tolerability in clinical trials thus far. Nonetheless, immune stimulation in response to AON treatment in the brain remains a concern. For this reason we performed RNA sequencing analysis of brain tissue from mice treated intracerebroventricularly with phosphorothioate, 2′-O-methyl modified AONs. A significant upregulation of immune system associated genes was observed in brains of AON treated mice, with the striatum showing largest transcriptional changes. Strongest upregulation was seen for the antiviral enzyme 2′-5′-oligoadenylate synthase-like protein 2 (Oasl2) and Bone marrow stromal antigen 2 (Bst2). Histological analysis confirmed activation of microglia and astrocytes in striatum. The upregulation of immune system associated genes was detectable for at least 2 months after the last AON administration, consistent with a continuous immune response to the AON.

Identification of 2'-5'-Oligoadenylate Synthetase-Like Gene in Goose: Gene Structure, Expression Patterns, and Antiviral Activity Against Newcastle Disease Virus

2'-5'-oligoadenylate synthetase-like (OASL) is a kind of antiviral protein induced by interferons (IFNs), which plays an important role in the IFNs-mediated antiviral signaling pathway. In this study, we cloned and identified OASL in the Chinese goose for the first time. Goose 2'-5'-oligoadenylate synthetase-like (goOASL), including an ORF of 1527bp, encoding a protein of 508 amino acids. GoOASL protein contains 3 conserved motifs: nucleotidyltransferase (NTase) domain, 2'-5'-oligoadenylate synthetase (OAS) domain, and 2 ubiquitin-like (UBL) repeats. The tissue distribution profile of goOASL in 2-week-old gosling and adult goose were identified by Real-Time quantitative PCR, which revealed that the highest level of goOASL mRNA transcription was detected in the blood of adult goose and gosling. The mRNA transcription level of goOASL was upregulated in all tested tissues of duck Tembusu virus (DTMUV)-infected 3-day-old goslings, compared with control groups. Furthermore, using the stimulus Poly(I: C), ODN2006, R848, and lipopolysaccharide (LPS) as well as the viral pathogens DTMUV, H9N2 avian influenza virus (AIV), and gosling plague virus (GPV) to treat goose peripheral blood mononuclear cells (PBMCs) for 6 h, goOASL transcripts level was significantly upregulated in all treated groups. To further investigate the antiviral activity of goOASL, pcDNA3.1(+)-goOASL-His plasmid was constructed, and goOASL was expressed by the goose embryo fibroblast cells (GEFs) transfected with pcDNA3.1(+)-goOASL-His. Our research data suggested that Newcastle disease virus (NDV) replication (viral copies and viral titer) in GEFs was significantly reduced by the overexpression of goOASL protein. These data were meaningful for the antiviral immunity research of goose and shed light on the future prevention of NDV in fowl.

Genomic variations in the 2'-5' oligoadenylate synthetase 1 (OAS1) gene in zebu cattle and its crossbreds of Indian origin

In the antiviral host defense mechanisms, the role of mammalian OAS/RNASEL pathway is very significant. These enzymes are interferon-inducible and activated by binding to double-stranded RNA (dsRNA) which are present in virus infected cells. The OAS proteins functions through its receptor, the 2-5Adependent ribonuclease (RNaseL) and activated OAS-RNaseL system degrades viral and cellular RNA and subsequently inhibits protein synthesis. Polymorphisms in the human and equine OAS gene cluster have been previously utilized for casecontrol analysis of virus-induced disease. But no polymorphisms have yet been identified in the bovine OAS1 genes for use in similar case-control studies. The promoter and coding regions of the OAS1 gene was amplified and screened for polymorphisms by PCR-SSCP and sequencing in Sahiwal and Frieswal animals. Two SNPs have been identified in the promoter region of OAS1 gene, which have predicted to create/delete sites for transcription factors. Specific amplification of the exonic regions of the OAS1 gene have identified 26 SNPs and one dinucleotide repeats, among them 14 are mis-sense variants. These polymorphisms are the first to be reported in OAS1 gene and will facilitate future case-control studies of cattle susceptibility to infectious diseases.

Rhinovirus Infection of ORMDL3 Transgenic Mice Is Associated with Reduced Rhinovirus Viral Load and Airway Inflammation

Orosomucoid like 3 (ORMDL3), a gene localized to chromosome 17q21, has been linked in epidemiologic studies to childhood asthma and rhinovirus (RV) infections. As the single nucleotide polymorphisms linking ORMDL3 to asthma are associated with increased expression of ORMDL3, we have used hORMDL3^zp3-Cre mice (which have universal increased expression of human ORMDL3) to determine whether infection of these transgenic mice with RV influences levels of airway inflammation or RV viral load. RV infection of hORMDL3^zp3-Cre mice resulted in reduced RV viral load assessed by quantitative real-time PCR (lung and airway epithelium), as well as reduced airway inflammation (total bronchoalveolar lavage cells, neutrophils, macrophages, and lymphocytes) compared with RV-infected wild-type mice. Levels of the antiviral pathways including IFNs (IFN-α, IFN-β, IFN-λ) and RNAse L were significantly increased in the lungs of RV-infected hORMDL3^zp3-Cre mice. Levels of the antiviral mouse oligoadenylate synthetase (mOas)1g pathway and RNAse L were upregulated in the lungs of unchallenged hORMDL3^zp3-Cre mice. In addition, levels of mOas2, but not mOas1 (mOas1a, mOas1b, mOas1g), or mOas3 pathways were significantly more upregulated by IFNs (IFN-α, IFN-β, IFN-λ) in epithelial cells from hORMDL3^zp3-Cre mice compared with RV-infected wild-type mouse epithelial cells. RNAse L-deficient mice infected with RV had increased RV viral load. Overall, these studies suggest that increased levels of ORMDL3 contribute to antiviral defense to RV infection in mice through pathways that may include IFNs (IFN-α, IFN-β, IFN-λ), OAS, and RNAse L.

Molecular cloning, characterization, and expression of duck 2'-5'-oligoadenylate synthetase-like gene

2'-5'-Oligoadenylate synthetase-like protein (OASL) is an interferon-inducible antiviral protein that exerts antiviral effects through the RNase L- or retinoic acid-inducible gene I (RIG-I)-dependent signalling pathway. In this study, we identified and cloned the OASL gene (named duOASL) from healthy adult Cherry Valley ducks. Full-length duOASL cDNA (1630bp) encoded a 504-amino acid polypeptide containing three conserved domains, namely, nucleotidyltransferase domain, 2'-5'-oligoadenylate synthetase domain, and two ubiquitin-like repeats. DuOASL mRNA expression was quantified by performing quantitative reverse transcription-PCR (qRT-PCR). Results of qRT-PCR showed that duOASL was broadly expressed in all examined tissues, with the highest mRNA expression in the large intestine. Antiviral activity of duOASL was measured by determining its effect on Duck Tembusu virus (DTMUV) replication in vitro. We found that duOASL overexpression slightly inhibited DTMUV replication, whereas duOASL knockdown by using a specific small interfering RNA increased DTMUV replication in DF-1 cells. Thus, we successfully cloned and characterized the antiviral protein duOASL from Cherry Valley ducks and found that it exerted antiviral effects against DTMUV. These results provide a solid foundation for performing further studies to determine the mechanism underlying the antiviral effect of duOASL at the cellular level.

Interferon-Inducible Oligoadenylate Synthetase-Like Protein Acts as an Antiviral Effector against Classical Swine Fever Virus via the MDA5-Mediated Type I Interferon-Signaling Pathway

Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), which poses a serious threat to the global pig industry. Interferons (IFNs) and IFN-stimulated genes (ISGs) play a key role in host antiviral defense. We have previously screened the porcine 2′-5′-oligoadenylate synthetase-like protein (pOASL) as a potential anti-CSFV ISG using a reporter CSFV. This study aimed to clarify the underlying antiviral mechanism of pOASL against CSFV. We confirmed that CSFV replication was significantly suppressed in lentivirus-delivered, pOASL-overexpressing PK-15 cells, whereas silencing the expression of endogenous pOASL by small interfering RNAs markedly enhanced CSFV growth. In addition, the transcriptional level of pOASL was upregulated both in vitro and in vivo upon CSFV infection. Interestingly, the anti-CSFV effects of pOASL are independent of the canonical RNase L pathway but depend on the activation of the type I IFN response. Glutathione S-transferase pulldown and coimmunoprecipitation assays revealed that pOASL interacts with MDA5, a double-stranded RNA sensor, and further enhances MDA5-mediated type I IFN signaling. Moreover, we showed that pOASL exerts anti-CSFV effects in an MDA5-dependent manner. In conclusion, pOASL suppresses CSFV replication via the MDA5-mediated type I IFN-signaling pathway.

IMPORTANCE The host innate immune response plays an important role in mounting the initial resistance to viral infection. Here, we identify the porcine 2′-5′-oligoadenylate synthetase-like protein (pOASL) as an interferon (IFN)-stimulated gene (ISG) against classical swine fever virus (CSFV). We demonstrate that the anti-CSFV effects of pOASL depend on the activation of type I IFN response. In addition, we show that pOASL, as an MDA5-interacting protein, is a coactivator of MDA5-mediated IFN induction to exert anti-CSFV actions. This work will be beneficial to the development of novel anti-CSFV strategies by targeting pOASL.

Antagonism of RNase L Is Required for Murine Coronavirus Replication in Kupffer Cells and Liver Sinusoidal Endothelial Cells but Not in Hepatocytes

Mouse hepatitis virus strain A59 infection of mice is a useful tool for studying virus-host interaction during hepatitis development. The NS2^H126R mutant is attenuated in liver replication due to loss of phosphodiesterase activity, which the wild-type (WT) virus uses to block the 2',5'-oligoadenylate synthetase (OAS)-RNase L (RNase L) antiviral pathway. The activation of RNase L by NS2^H126R is cell type dependent and correlates with high basal expression levels of OAS, as found in myeloid cells. We tested the hypothesis that the resident liver macrophages, Kupffer cells (KC), represent the cell type most likely to restrict NS2^H126R and prevent hepatitis. As found previously, A59 and NS2^H126R replicate similarly in hepatocytes and neither activates RNase L, as assessed by an rRNA degradation assay. In contrast, in KC, A59 exhibited a 100-fold-higher titer than NS2^H126R and NS2^H126R induced rRNA degradation. Interestingly, in liver sinusoidal endothelial cells (LSEC), the cells that form a barrier between blood and liver parenchymal cells, NS2^H126R activates RNase L, which limits viral replication. Similar growth kinetics were observed for the two viruses in KC and LSEC from RNase L^-/- mice, demonstrating that both use RNase L to limit NS2^H126R replication. Depletion of KC by gadolinium(III) chloride or of LSEC by cyclophosphamide partially restores liver replication of NS2^H126R, leading to hepatitis. Thus, during mouse hepatitis virus (MHV) infection, hepatitis, which damages the parenchyma, is prevented by RNase L activity in both KC and LSEC but not in hepatocytes. This may be explained by the undetectable levels of RNase L as well as by the OASs expressed in hepatocytes.

IMPORTANCE

Mouse hepatitis virus infection of mice provides a useful tool for studying virus-host interactions during hepatitis development. The NS2^H126R mutant is attenuated in liver replication due to loss of phosphodiesterase activity, by which the wild-type virus blocks the potent OAS-RNase L antiviral pathway. RNase L activation by NS2^H126R is cell type dependent and correlates with high basal expression levels of OAS, as found in myeloid cells. We showed that the hepatocytes that comprise the liver parenchyma do not activate RNase L when infected with NS2^H126R or restrict replication. However, both Kupffer cells (KC) (i.e., the liver-resident macrophages) and the liver sinusoidal endothelial cells (LSEC) which line the sinusoids activate RNase L in response to NS2^H126R These data suggest that KC and LSEC prevent viral spread into the parenchyma, preventing hepatitis. Furthermore, hepatocytes express undetectable levels of OASs and RNase L, which likely explains the lack of RNase L activation during NS2^H126R infection.

The role of mouse 2',5'-oligoadenylate synthetase 1 paralogs

The interferon-induced oligoadenylate synthetase (OAS) family is one of the most important immune response proteins to the viral infection. The OAS protein binds dsRNA and is activated to produce 2',5'-oligoadenylates, which lead to the activation of latent form of RNase L, resulting in degradation of cellular and viral RNA and inhibition of viral replication. In mice, the Oas gene family locates on chromosome 5. The mouse Oas gene locus undergoes a recent series of duplication event, leading to the presence of eight paralogs of Oas1 genes (Oas1a through Oas1h) that forms Oas gene cluster with the Oas2, Oas3 and two OasL (OasL1 and OasL2) genes. Previous studies demonstrated that the mouse Oas1b gene conferred resistance to the flavivirus infection in mice; however, the antiviral activity of other mouse Oas1 gene family is still unknown. Therefore, in the present study, we have evaluated the mouse Oas1 paralogs regarding the enzymatic activity and antiviral activity against the two neurotropic flaviviruses, West Nile virus and tick-borne encephalitis virus. The mouse Oas1 genes were cloned from C57BL/6J (B6) as well as the Oas1b derived from feral mouse strain, MSM. The obtained results demonstrated that only Oas1a and Oas1g showed enzymatic activity. Although MSM-derived Oas1b showed antiviral activity to both viruses, all B6-derived OAS paralogs did not show antiviral activity. These results suggest that Oas1a and Oas1g play a role in potentiating viral RNA-induced interferon response in the cell, whereas the Oas1b works as a specific anti-flavivirus factor unless it is mutated. However, the role of other paralogs is unknown and should wait for further investigation.

RNase L and the NLRP3-inflammasome: An old merchant in a new trade

The type I/III interferon (IFN)-inducible 2'-5'- oligoadenylate synthetase (OAS)/endoribonuclease L (RNase L) is a classical innate immune pathway that has been implicated in antiviral and antibacterial defense and also in hereditary prostate cancer. The OAS/RNase L pathway is activated when OAS senses double-stranded RNA and catalyzes the synthesis of 2'-5' linked oligodenylates (2-5A) from ATP. 2-5A then binds and activates RNase L, resulting cleavage of single-stranded RNAs. RNase L cleavage products are capable of activating RIG-like receptors such as RIG-I and MDA5 that leads to IFN-β expression during viral infection. Our recent findings suggest that beside the RLR pathway, RNase L cleavage products can also activate the NLRP3-inflammasome pathway, which requires DHX33 (DExD/H-box helicase) and the mitochondrial adaptor protein MAVS. Here we discuss this newly identified role of OAS-RNase L pathway in regulation of inflammasome signaling as an alternative antimicrobial mechanism that has potential as a target for development of new broad-spectrum antimicrobial and anti-inflammatory therapies.

Activation of RNase L by Murine Coronavirus in Myeloid Cells Is Dependent on Basal Oas Gene Expression and Independent of Virus-Induced Interferon

The oligoadenylate synthetase (OAS)-RNase L pathway is a potent interferon (IFN)-induced antiviral activity. Upon sensing double-stranded RNA, OAS produces 2',5'-oligoadenylates (2-5A), which activate RNase L. Murine coronavirus (mouse hepatitis virus [MHV]) nonstructural protein 2 (ns2) is a 2',5'-phosphodiesterase (PDE) that cleaves 2-5A, thereby antagonizing RNase L activation. PDE activity is required for robust replication in myeloid cells, as a mutant of MHV (ns2(H126R)) encoding an inactive PDE fails to antagonize RNase L activation and replicates poorly in bone marrow-derived macrophages (BMM), while ns2(H126R) replicates to high titer in several types of nonmyeloid cells, as well as in IFN receptor-deficient (Ifnar1(-/-)) BMM. We reported previously that myeloid cells express significantly higher basal levels of OAS transcripts than nonmyeloid cells. Here, we investigated the contributions of Oas gene expression, basal IFN signaling, and virus-induced IFN to RNase L activation. Infection with ns2(H126R) activated RNase L in Ifih1(-/-) BMM to a similar extent as in wild-type (WT) BMM, despite the lack of IFN induction in the absence of MDA5 expression. However, ns2(H126R) failed to induce RNase L activation in BMM treated with IFNAR1-blocking antibody, as well as in Ifnar1(-/-) BMM, both expressing low basal levels of Oas genes. Thus, activation of RNase L does not require virus-induced IFN but rather correlates with adequate levels of basal Oas gene expression, maintained by basal IFN signaling. Finally, overexpression of RNase L is not sufficient to compensate for inadequate basal OAS levels.

IMPORTANCE

The oligoadenylate synthetase (OAS)-RNase L pathway is a potent antiviral activity. Activation of RNase L during murine coronavirus (mouse hepatitis virus [MHV]) infection of myeloid cells correlates with high basal Oas gene expression and is independent of virus-induced interferon secretion. Thus, our data suggest that cells with high basal Oas gene expression levels can activate RNase L and thereby inhibit virus replication early in infection upon exposure to viral double-stranded RNA (dsRNA) before the induction of interferon and prior to transcription of interferon-stimulated antiviral genes. These findings challenge the notion that activation of the OAS-RNase L pathway requires virus to induce type I IFN, which in turn upregulates OAS gene expression, as well as to provide dsRNA to activate OAS. Our data further suggest that myeloid cells may serve as sentinels to restrict viral replication, thus protecting other cell types from infection.

Viral phosphodiesterases that antagonize double-stranded RNA signaling to RNase L by degrading 2-5A

The host interferon (IFN) antiviral response involves a myriad of diverse biochemical pathways that disrupt virus replication cycles at many different levels. As a result, viruses have acquired and evolved genes that antagonize the host antiviral proteins. IFNs inhibit viral infections in part through the 2',5'-oligoadenylate (2-5A) synthetase (OAS)/RNase L pathway. OAS proteins are pathogen recognition receptors that exist at different basal levels in different cell types and that are IFN inducible. Upon activation by the pathogen-associated molecular pattern viral double-stranded RNA, certain OAS proteins synthesize 2-5A from ATP. 2-5A binds to the antiviral enzyme RNase L causing its dimerization and activation. Recently, disparate RNA viruses, group 2a betacoronaviruses, and group A rotaviruses, have been shown to produce proteins with 2',5'-phosphodiesterase (PDE) activities that eliminate 2-5A thereby evading the antiviral activity of the OAS/RNase L pathway. These viral proteins are members of the eukaryotic-viral LigT-like group of 2H phosphoesterases, so named for the presence of 2 conserved catalytic histidine residues. Here, we will review the biochemistry, biology, and implications of viral and cellular 2',5'-PDEs that degrade 2-5A. In addition, we discuss alternative viral and cellular strategies for limiting the activity of OAS/RNase L.

Oral administration of Lactococcus lactis subsp. lactis JCM5805 enhances lung immune response resulting in protection from murine parainfluenza virus infection

When activated by viral infection, plasmacytoid dendritic cells (pDCs) play a primary role in the immune response through secretion of IFN-α. Lactococcus lactis subsp. lactis JCM5805 (JCM5805) is a strain of lactic acid bacteria (LAB) that activates murine and human pDCs to express type I and type III interferons (IFNs). JCM5805 has also been shown to activate pDCs via a Toll-like receptor 9 (TLR9) dependent pathway. In this study, we investigated the anti-viral effects of oral administration of JCM5805 using a mouse model of murine parainfluenza virus (mPIV1) infection. JCM5805-fed mice showed a drastic improvement in survival rate, prevention of weight loss, and reduction in lung histopathology scores compared to control mice. We further examined the mechanism of anti-viral effects elicited by JCM5805 administration using naive mice. Microscopic observations showed that JCM5805 was incorporated into CD11c⁺ immune cells in Peyer’s patches (PP) and PP pDCs were significantly activated and the expression levels of IFNs were significantly increased. Interestingly, nevertheless resident pDCs at lung were not activated and expressions levels of IFNs at whole lung tissue were not influenced, the expressions of anti-viral factors induced by IFNs, such as Isg15, Oasl2, and Viperin, at lung were up-regulated in JCM5805-fed mice compared to control mice. Therefore expressed IFNs from intestine might be delivered to lung and IFN stimulated genes might be induced. Furthermore, elevated expressions of type I IFNs from lung lymphocytes were observed in response to mPIV1 ex vivo stimulation in JCM5805-fed mice compared to control. This might be due to increased ratio of pDCs located in lung were significantly increased in JCM5805 group. Taken together, a specific LAB strain might be able to affect anti-viral immunological profile in lung via activation of intestinal pDC leading to enhanced anti-viral phenotype in vivo.

Enzymatically active 2',5'-oligoadenylate synthetases are widely distributed among Metazoa, including protostome lineage

2',5'-Oligoadenylate synthetases (OASs) belong to the nucleotidyl transferase family together with poly(A) polymerases, CCA-adding enzymes and the recently discovered cyclic-GMP-AMP synthase (cGAS). Mammalian OASs have been thoroughly characterized as components of the interferon-induced antiviral system. The OAS activity and the respective genes were also discovered in marine sponges where the interferon system is absent. In this study the recombinant OASs from several multicellular animals and their closest unicellular relative, a choanoflagellate, were expressed in a bacterial expression system and their enzymatic activities were examined. We demonstrated 2-5A synthesizing activities of OASs from the marine sponge Tedania ignis, a representative of the phylogenetically oldest metazoan phylum (Porifera), from an invertebrate of the protostome lineage, the mollusk Mytilus californianus (Mollusca), and from a vertebrate species, a cartilaginous fish Leucoraja erinacea (Chordata). However, the expressed proteins from an amphibian, the salamander Ambystoma mexicanum (Chordata), and from a protozoan, the marine choanoflagellate Monosiga brevicollis (Choanozoa), did not show 2-5A synthesizing activity. Differently from other studied OASs, OAS from the marine sponge T. ignis was able to catalyze the formation of oligomers having both 2',5'- and 3',5'-phosphodiester linkages. Our data suggest that OASs from sponges and evolutionarily higher animals have similar activation mechanisms which still include different affinities and possibly different structural requirements for the activating RNAs. Considering their 2'- and 3'-specificities, sponge OASs could represent a link between evolutionarily earlier nucleotidyl transferases and 2'-specific OASs from higher animals.

Role of ribonuclease L in viral pathogen-associated molecular pattern/influenza virus and cigarette smoke-induced inflammation and remodeling

Interactions between cigarette smoke (CS) exposure and viral infection play an important role(s) in the pathogenesis of chronic obstructive pulmonary disease and a variety of other disorders. A variety of lines of evidence suggest that this interaction induces exaggerated inflammatory, cytokine, and tissue remodeling responses. We hypothesized that the 2'-5' oligoadenylate synthetase (OAS)/RNase L system, an innate immune antiviral pathway, plays an important role in the pathogenesis of these exaggerated responses. To test this hypothesis, we characterize the activation of 2'-5' OAS in lungs from mice exposed to CS and viral pathogen-associated molecular patterns (PAMPs)/live virus, alone and in combination. We also evaluated the inflammatory and remodeling responses induced by CS and virus/viral PAMPs in lungs from RNase L null and wild-type mice. These studies demonstrate that CS and viral PAMPs/live virus interact in a synergistic manner to stimulate the production of select OAS moieties. They also demonstrate that RNase L plays a critical role in the pathogenesis of the exaggerated inflammatory, fibrotic, emphysematous, apoptotic, TGF-β1, and type I IFN responses induced by CS plus virus/viral PAMP in combination. These studies demonstrate that CS is an important regulator of antiviral innate immunity, highlight novel roles of RNase L in CS plus virus induced inflammation, tissue remodeling, apoptosis, and cytokine elaboration and highlight pathways that may be operative in chronic obstructive pulmonary disease and mechanistically related disorders.

Ribonuclease L is not critical for innate restriction and adaptive immunity against Friend retrovirus infection

Ribonuclease L (RNase L) is a type I interferon regulated factor that can significantly inhibit retroviruses in vitro and may activate cytoplasmic sensing pathways to augment adaptive immunity. However, the antiretroviral activity of RNase L remains to be validated in vivo. We investigated the role of RNaseL in counteracting Friend retrovirus (FV) infection relative to a well-described restriction factor, Apobec3. C57BL/6 wild-type (WT) and RNaseL knock-out (KO) mice exhibited similar acute FV infection levels despite significant transcriptional induction of oligoadenylate synthetase 1, which produces activators of RNase L. Apobec3 KO mice showed higher FV infection levels relative to WT mice, but deletion of RNaseL in Apobec3 KO mice did not augment FV infection. Moreover, RNaseL did not influence FV-specific IgG responses and recovery from viremia by 28 days post-infection. The results suggest that RNase L is not an evolutionarily-conserved host defense mechanism to counteract retroviruses in vivo.

OASL1 inhibits translation of the type I interferon-regulating transcription factor IRF7

The production of type I interferon is essential for viral clearance but is kept under tight control to avoid unnecessary tissue damage from hyperinflammatory responses. Here we found that OASL1 inhibited translation of IRF7, the master transcription factor for type I interferon, and thus negatively regulated the robust production of type I interferon during viral infection. OASL1 inhibited the translation of IRF7 mRNA by binding to the 5' untranslated region (UTR) of IRF7 and possibly by inhibiting scanning of the 43S preinitiation complex along the message. Oasl1-/- mice were resistant to viral infection because of the greater abundance of type I interferon, which suggests that OASL1 could be a potential therapeutic target for boosting the production of type I interferon during viral infection.

Susceptibility to flavivirus-specific antiviral response of Oas1b affects the neurovirulence of the Far-Eastern subtype of tick-borne encephalitis virus

Tick-borne encephalitis virus (TBEV) is a zoonotic agent that causes fatal encephalitis in humans. 2'-5'-oligoadenylate synthetase 1b (Oas1b) has been identified as a flavivirus resistance gene, but most inbred laboratory mice do not possess a functional Oas1b gene. In this study, a congenic strain carrying a functional Oas1b gene, B6.MSM-Oas, was used to evaluate the pathogenicity of Far-Eastern TBEV. Although intracerebral infection of B6.MSM-Oas mice by Oshima 5-10 resulted in limited signs of illness, infection by Sofjin-HO resulted in death with severe neurologic signs. While Oshima 5-10 was cleared from the brain, Sofjin-HO was not cleared despite a similar level of expression of the intact Oas1b gene. Necrotic neurons with viral antigens and inflammatory reactions were observed in the brain infected with Sofjin-HO. These data indicate that the different susceptibility to the antiviral activity of Oas1b resulted in a difference in neurovirulence in the two TBEV strains.

Antagonism of the interferon-induced OAS-RNase L pathway by murine coronavirus ns2 protein is required for virus replication and liver pathology

Many viruses induce hepatitis in humans, highlighting the need to understand the underlying mechanisms of virus-induced liver pathology. The murine coronavirus, mouse hepatitis virus (MHV), causes acute hepatitis in its natural host and provides a useful model for understanding virus interaction with liver cells. The MHV accessory protein, ns2, antagonizes the type I interferon response and promotes hepatitis. We show that ns2 has 2′,5′-phosphodiesterase activity, which blocks the interferon inducible 2′,5′-oligoadenylate synthetase (OAS)-RNase L pathway to facilitate hepatitis development. Ns2 cleaves 2′,5′-oligoadenylate, the product of OAS, to prevent activation of the cellular endoribonuclease RNase L and consequently block viral RNA degradation. An ns2 mutant virus was unable to replicate in the liver or induce hepatitis in wild-type mice, but was highly pathogenic in RNase L deficient mice. Thus, RNase L is a critical cellular factor for protection against viral infection of the liver and the resulting hepatitis.

Regulation of the interferon-inducible 2'-5'-oligoadenylate synthetases by adenovirus VA(I) RNA

Foreign double-stranded RNA (dsRNA) generated during the normal course of the viral life cycle serves as a key infection recognition element by proteins of the innate immune response. To circumvent this response, all adenoviruses synthesize at least one highly structured RNA (VA(I)), which, after processing by the RNA silencing machinery, inhibits the innate immune response via a series of interactions with specific protein partners. Surprisingly, VA(I) positively regulates the activity of the interferon-induced 2'-5'-oligoadenylate synthetase (OAS) enzymes, which typically represent a key mechanism whereby host-cell protein translation is attenuated in response to foreign dsRNA. We present data investigating the regulation of the OAS1 isoform by VA(I) derivatives and demonstrate that a processed version of VA(I) lacking the terminal stem behaves as a pseudo-inhibitor of OAS1. A combination of electrophoretic mobility shift assays, dynamic light scattering, and non-denaturing mass spectrometry was used to quantitate binding affinity and characterize OAS1:VA(I) complex stoichiometry. Enzyme assays characterized the ability of VA(I) derivatives to activate OAS1. Finally, the importance of RNA 5'-end phosphorylation state is investigated, and it emphasizes its potential importance in the activation or inhibition of OAS enzymes. Taken together, these data suggest a plausible strategy whereby the virus produces a single RNA transcript capable of inhibiting a variety of members of the innate immune response.

Identification of novel host cell binding partners of Oas1b, the protein conferring resistance to flavivirus-induced disease in mice

Oas1b was previously identified as the product of the Flv(r) allele that confers flavivirus-specific resistance to virus-induced disease in mice by an uncharacterized, RNase L-independent mechanism. To gain insights about the mechanism by which Oas1b specifically reduces the efficiency of flavivirus replication, cellular protein interaction partners were identified and their involvement in the Oas1b-mediated flavivirus resistance mechanism was analyzed. Initial difficulties in getting the two-hybrid assay to work with full-length Oas1b led to the discovery that this Oas protein uniquely has a C-terminal transmembrane domain that targets it to the endoplasmic reticulum (ER). Two peptides matching to oxysterol binding protein-related protein 1L (ORP1L) and ATP binding cassette protein 3, subfamily F (ABCF3), were identified as Oas1b interaction partners in yeast two-hybrid assays, and both in vitro-transcribed/translated peptides and full-length proteins in mammalian cell lysates coimmunoprecipitated with Oas1b. Knockdown of a partner involved in Oas1b-mediated antiflavivirus activity would be expected to increase flavivirus replication but not that of other types of viruses. However, RNA interference (RNAi) knockdown of ORP1L decreased the replication of the flavivirus West Nile virus (WNV) as well as that of other types of RNA viruses. This virus-nonspecific effect may be due to the recently reported dysregulation of late endosome movement by ORP1L knockdown. Knockdown of ABCF3 protein levels increased the replication of WNV but not that of other types of RNA viruses, and this effect on WNV replication was observed only in Oas1b-expressing cells. The results suggest that Oas1b is part of a complex located in the ER and that ABCF3 is a component of the Flv(r)-mediated resistance mechanism.

Activation of Oas1a gene expression by type I IFN requires both STAT1 and STAT2 while only STAT2 is required for Oas1b activation

The murine 2'-5' oligoadenylate synthetase 1a (Oas1a) and Oas1b genes are type 1 IFN responsive genes. Oas1a is an active synthetase with broad antiviral activity mediated through RNase L. Oas1b is inactive but can inhibit Oas1a synthetase activity and mediate a flavivirus-specific antiviral activity through an unknown RNase L-independent mechanism. Analysis of promoter elements regulating gene transcription confirmed that an IFN-stimulated response element (ISRE) is required for IFN beta-activation but neither the overlapping IRF binding site present in both promoters nor the adjacent Oas1b NF-kappa B site is required. Mutation of the overlapping STAT site negatively affected IFN beta-induction of Oas1a but not of Oas1b. Also, IFN beta induction of Oas1a was STAT1- and STAT2-dependent, while induction of Oas1b was STAT1-independent but STAT2-dependent. The two promoters differ at a single nucleotide in the STAT site. The data indicate that these two duplicated genes can be differentially regulated by IFN beta.

Expressed 2-5A synthetase genes and pseudogenes in the marine sponge Geodia barretti

The 2',5'-oligoadenylate synthetases (2-5A synthetases, OAS) form a family of proteins presented in many branches of Metazoa. The phylum Porifera (sponges) contains OAS proteins which are different from those in vertebrates and form a distinct OAS subfamily. In turn, OAS proteins from different genera of Demospongia show rather low similarities in their primary structures. To ascertain divergence of the OAS genes within a particular sponge genus, we identified the OAS genes from the marine sponge Geodia barretti and compared them with those from another member of the genus Geodia, Geodia cydonium. The identity and similarity of the OAS sequences found in G. barretti with those from G. cydonium were considerably higher than identities and similarities compared with those from other sponges, 75% and 85% versus 27-30% and 42-47%, respectively. We also established the presence of a transcriptionally active polymorphic OAS pseudogene in the genome of G. barretti. The transcripts of the OAS pseudogene(s) lack several internal exons encoding necessary motifs for OAS enzymatic activity. The maintenance and further diversification of OAS gene(s) and pseudogene(s) suggest the prevalence of gene duplication events over the loss of gene duplicates in Geodia genomes during the evolution.