Dual Regulation of Host TRAIP Post-translation and Nuclear/Plasma Distribution by Porcine Reproductive and Respiratory Syndrome Virus Non-structural Protein 1α Promotes Viral Proliferation

PRRSV degrades MDA5 via dual autophagy receptors P62 and CCT2 to evade antiviral innate immunity

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major economically devastating pathogen that has evolved various strategies to evade innate immunity. Downregulation of antiviral interferon largely promotes PRRSV immunoevasion by utilizing cytoplasmic melanoma differentiation-associated gene 5 (MDA5), a receptor that senses viral RNA. In this study, the downregulated transcription and expression levels of porcine MDA5 in PRRSV infection were observed, and the detailed mechanisms were explored. We found that the interaction between P62 and MDA5 is enhanced due to two factors: the phosphorylation modification of the autophagic receptor P62 by the upregulated kinase CK2α and the K63 ubiquitination of porcine MDA5 catalyzed by the E3 ubiquitinase TRIM21 in PRRSV-infected cells. As a result of these modifications, the classic P62-mediated autophagy is triggered. Additionally, porcine MDA5 interacts with the chaperonin containing TCP1 subunit 2 (CCT2), which is enhanced by PRRSV nsp3. This interaction promotes the aggregate formation and autophagic clearance of MDA5-CCT2-nsp3 independently of ubiquitination. In summary, enhanced MDA5 degradation occurs in PRRSV infection via two autophagic pathways: the binding of MDA5 with the autophagy receptor P62 and the aggrephagy receptor CCT2, leading to intense innate immune suppression. The research reveals a novel mechanism of immune evasion in PRRSV infection and provides fundamental insights for the development of new vaccines or therapeutic strategies.

N-Acetyltransferase 9 Inhibits Porcine Reproductive and Respiratory Syndrome Virus Proliferation by N-Terminal Acetylation of the Structural Protein GP5

Porcine reproductive and respiratory syndrome virus (PRRSV) is a serious threat to the global swine industry. As a typical immunosuppressive virus, PRRSV has developed a variety of complex mechanisms to escape the host innate immunity. In this study, we uncovered a novel immune escape mechanism of PRRSV infection. Here, we demonstrate for the first time that the endoplasmic reticulum (ER)-resident N-acetyltransferase Nat9 is an important host restriction factor for PRRSV infection. Nat9 inhibited PRRSV proliferation in an acetyltransferase activity-dependent manner. Mechanistically, glycoprotein 5 (GP5) of PRRSV was identified as interacting with Nat9 and being N-terminally acetylated by it, which generates a GP5 degradation signal, promoting the K27-linked-ubiquitination degradation of GP5 to decrease virion assembly. Meanwhile, the expression of Nat9 was inhibited during PRRSV infection. In detail, two transcription factors, ETV5 and SP1, were screened out as the key transcription factors binding to the core promoter region of Nat9, and the PRRSV nonstructural protein 1β (Nsp1β), Nsp4, Nsp9, and nucleocapsid (N) proteins were found to interfere significantly with the expression of ETV5 and SP1, thereby regulating the transcription activity of Nat9 and inhibiting the expression of Nat9. The findings suggest that PRRSV decreases the N-terminal acetylation of GP5 to support virion assembly by inhibiting the expression of Nat9. Taken together, our findings showed that PRRSV has developed complex mechanisms to inhibit Nat9 expression and trigger virion assembly. IMPORTANCE To ensure efficient replication, a virus must hijack or regulate multiple host factors for its own benefit. Understanding virus-host interactions and the molecular mechanisms of host resistance to PRRSV infection is necessary to develop effective strategies to control PRRSV. The N-acetyltransferase Nat9 plays important roles during virus infection. Here, we demonstrate that Nat9 exhibits an antiviral effect on PRRSV proliferation. The GP5 protein of PRRSV is targeted specifically by Nat9, which mediates GP5 N-terminal acetylation and degradation via a ubiquitination-dependent proteasomal pathway. However, PRRSV manipulates the transcription factors ETV5 and SP1 to inhibit the expression of Nat9 and promote virion assembly. Thus, we report a novel function of Nat9 in PRRSV infection and elucidate a new mechanism by which PRRSV can escape the host innate immunity, which may provide novel insights for the development of antiviral drugs.

Research Progress in Porcine Reproductive and Respiratory Syndrome Virus–Host Protein Interactions

Porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease caused by porcine reproductive and respiratory syndrome virus (PRRSV), which has been regarded as a persistent challenge for the pig industry in many countries. PRRSV is internalized into host cells by the interaction between PRRSV proteins and cellular receptors. When the virus invades the cells, the host antiviral immune system is quickly activated to suppress the replication of the viruses. To retain fitness and host adaptation, various viruses have evolved multiple elegant strategies to manipulate the host machine and circumvent against the host antiviral responses. Therefore, identification of virus–host interactions is critical for understanding the host defense against viral infections and the pathogenesis of the viral infectious diseases. Most viruses, including PRRSV, interact with host proteins during infection. On the one hand, such interaction promotes the virus from escaping the host immune system to complete its replication. On the other hand, the interactions regulate the host cell immune response to inhibit viral infections. As common antiviral drugs become increasingly inefficient under the pressure of viral selectivity, therapeutic agents targeting the intrinsic immune factors of the host protein are more promising because the host protein has a lower probability of mutation under drug-mediated selective pressure. This review elaborates on the virus–host interactions during PRRSV infection to summarize the pathogenic mechanisms of PRRSV, and we hope this can provide insights for designing effective vaccines or drugs to prevent and control the spread of PRRS.

Mapping the Key Residues within the Porcine Reproductive and Respiratory Syndrome Virus nsp1α Replicase Protein Required for Degradation of Swine Leukocyte Antigen Class I Molecules

The nonstructural protein 1α (nsp1α) of the porcine reproductive and respiratory syndrome virus (PRRSV) has been shown to target swine leukocyte antigen class I (SLA-I) for degradation, but the molecular details remain unclear. In this report, we further mapped the critical residues within nsp1α by site-directed mutagenesis. We identified a cluster of residues (i.e., Phe17, Ile81, Phe82, Arg86, Thr88, Gly90, Asn91, Phe94, Arg97, Thr160, and Asn161) necessary for this function. Interestingly, they are all located in a structurally relatively concentrated region. Further analysis by reverse genetics led to the generation of two viable viral mutants, namely, nsp1α-G90A and nsp1α-T160A. Compared to WT, nsp1α-G90A failed to co-localize with either chain of SLA-I within infected cells, whereas nsp1α-T160A exhibited a partial co-localization relationship. Consequently, the mutant nsp1α-G90A exhibited an impaired ability to downregulate SLA-I in infected macrophages as demonstrated by Western blot, indirect immunofluorescence, and flow cytometry analysis. Consistently, the ubiquitination level of SLA-I was significantly reduced in the conditions of both infection and transfection. Together, our results provide further insights into the mechanism underlying PRRSV subversion of host immunity and have important implications in vaccine development.

The dUTPase of caprine arthritis-encephalitis virus negatively regulates interferon signaling pathway

Background

Deoxyuracil triphosphate nucleotide (dUTP) pyrophosphatase (dUTPase, DU) is an enzyme of caprine arthritis-encephalitis virus (CAEV) that minimizes incorporation of dUTP into the DNA. Caprine arthritis-encephalitis virus relies partly on its ability to escape from innate immunity to cause persistent infections. Interferon β (IFN-β) is an important marker for evaluating the innate immune system, and it has a broad spectrum of antiviral activity.

Aims

This study was conducted to investigate the details of the IFN-β response to CAEV infection.

Methods

The expression of IFN-β and the proliferation of Sendai virus (SeV) and vesicular stomatitis virus (VSV) were determined by real-time quantitative polymerase chain reaction (qPCR). The effect of DU on the IFN signaling pathway was evaluated using luciferase reporter assays.

Results

In our study, the expression of IFN-β was significantly inhibited and the proliferation of SeV and VSV was promoted in cells overexpressing CAEV-DU. DU affected interferon stimulated response element (ISRE) and IFN-β promoter activities induced by RIG-I/MDA5/MAVS/TBK1 pathway, while did not affect them induced by interferon regulatory factor 3 (IRF3-5D).

Conclusion

DU protein downregulated the production of IFN-β by inhibiting the activity of the signal transduction molecules upstream of IRF3, thereby, helping CAEV escape innate immunity. Findings of this work provide an evidence to understand the persistent infection and multiple system inflammation of CAEV.

The Vif protein of caprine arthritis encephalitis virus inhibits interferon production

Caprine arthritis-encephalitis (CAE) is a chronic progressive infectious disease caused by caprine arthritis-encephalitis virus (CAEV) that seriously threatens the goat industry. Chronic infection and life-long multi-tissue inflammation are the typical features of the disease. Innate antiviral immunity is essential for the host defense system that rapidly recognizes and eliminates invading viruses. Interferon β (IFN-β) is important for innate immunity and regulates immunity against a broad spectrum of viruses. To investigate the details of the IFN-β response to CAEV infection, the effects of six viral proteins and the molecular mechanisms by which they affect IFN-β production were analyzed. Overexpression of DU and Vif promote virus proliferation and inhibit the production of IFN-β. qRT-PCR and luciferase reporter assays showed that overexpression of Vif inhibits the expression of luciferase under the control of the ISRE, NF-κB or IFN-β promoter but does not affect the expression of IFN-β activated by IRF3, indicating that Vif negatively regulates IFN-β production by affecting upstream signal transduction of IRF3. Amino acids 149-164 of Vif were found to be necessary for the inhibitory effect of IFN-β production. Our results indicate that CAEV evades surveillance and clearance by intracellular innate immunity by downregulating IFN-β production.