Autophagy Benefits the Replication of Egg Drop Syndrome Virus in Duck Embryo Fibroblasts

The roles and mechanisms of endoplasmic reticulum stress-mediated autophagy in animal viral infections

The endoplasmic reticulum (ER) is a unique organelle responsible for protein synthesis and processing, lipid synthesis in eukaryotic cells, and the replication of many animal viruses is closely related to ER. A considerable number of viral proteins are synthesised during viral infection, resulting in the accumulation of unfolded and misfolded proteins in ER, which in turn induces endoplasmic reticulum stress (ERS). ERS further drives three signalling pathways (PERK, IRE1, and ATF6) of the cellular unfolded protein response (UPR) to respond to the ERS. In numerous studies, ERS has been shown to mediate autophagy, a highly conserved cellular degradation mechanism to maintain cellular homeostasis in eukaryotic cells, through the UPR to restore ER homeostasis. ERS-mediated autophagy is closely linked to the occurrence and development of numerous viral diseases in animals. Host cells can inhibit viral replication by regulating ERS-mediated autophagy, restoring the ER's normal physiological process. Conversely, many viruses have evolved strategies to exploit ERS-mediated autophagy to achieve immune escape. These strategies include the regulation of PERK-eIF2α-Beclin1, PERK-eIF2α-ATF4-ATG12, IRE1α-JNK-Beclin1, and other signalling pathways, which provide favourable conditions for the replication of animal viruses in host cells. The ERS-mediated autophagy pathway has become a hot topic in animal virological research. This article reviews the most recent research regarding the regulatory functions of ERS-mediated autophagy pathways in animal viral infections, emphasising the underlying mechanisms in the context of different viral infections. Furthermore, it considers the future direction and challenges in the development of ERS-mediated autophagy targeting strategies for combating animal viral diseases, which will contribute to unveiling their pathogenic mechanism from a new perspective and provide a scientific reference for the discovery and development of new antiviral drugs and preventive strategies.

Critical Role of Viral Protein Hexon in Hypervirulent Fowl Adenovirus Serotype-4-Induced Autophagy by Interaction with BAG3 and Promotion of Viral Replication in LMH Cells

Hepatitis-pericardial syndrome (HHS) is an acute highly infectious avian disease caused by fowl adenovirus serotype 4 (FAdV-4), characterized by fulminant hepatitis and hydropericardium in broilers. Since 2015, a widespread epidemic has occurred in China due to the emergence of hypervirulent FAdV-4 (HPFAdV-4), causing huge losses to the stakeholders. However, the pathogenesis of HPFAdV-4 and the host responses to its infection remain elusive. Here, we show that infection of leghorn male hepatocellular (LMH) cells by HPFAdV-4 induced complete autophagy in cells and that the autophagy induced by recombinant HPFAdV-4-ON1 (rHPFAdV-4-ON1), a viral strain generated by replacing the hexon gene of wild-type HPFAdV-4 (HPFAdV-4-WT) with the one of nonpathogenic strain FAdV-4-ON1, was remarkably mitigated compared to that of the rHPFAdV-4-WT control, suggesting that HPFAdV-4 hexon is responsible for virus-induced autophagy. Importantly, we found that hexon interacted with a cellular protein, BAG3, a host protein that initiates autophagy, and that BAG3 expression increased in cells infected with HPFAdV-4. Furthermore, knockdown of BAG3 by RNA interference (RNAi) significantly inhibited HPFAdV-4- or hexon-induced autophagy and suppressed viral replication. On the contrary, expression of hexon markedly upregulated the expression of BAG3 via activating the P38 signaling pathway, triggering autophagy. Thus, these findings reveal that HPFAdV-4 hexon interacts with the host protein BAG3 and promotes BAG3 expression by activating P38 signaling pathway, thereby inducing autophagy and enhancing viral proliferation, which immensely furthers our understanding of the pathogenesis of HPFAdV-4 infection. IMPORTANCE HHS, mainly caused by HPFAdV-4, has caused large economic losses to the stakeholders in recent years. Infection of leghorn male hepatocellular (LMH) cells by HPFAdV-4 induced complete autophagy that is essential for HPFAdV-4 replication. By a screening strategy, the viral protein hexon was found responsible for virus-induced autophagy in cells. Importantly, hexon was identified as a factor promoting viral replication by interaction with BAG3, an initiator of host cell autophagy. These findings will help us to better understand the host response to HPFAdV-4 infection, providing a novel insight into the pathogenesis of HPFAdV-4 infection.

β-catenin facilitates fowl adenovirus serotype 4 replication through enhancing virus-induced autophagy

β-catenin is a key component of the Wnt/β-catenin signal transduction cascade which is a highly conserved signaling pathway in eukaryotes. Increasing evidence suggests that the Wnt/β-catenin signaling pathway is involved in the infection of many viruses. However, its role in fowl adenovirus serotype 4 (FAdV-4) replication remains unclear. In the present study, we showed that FAdV-4 infection increased the expression of β-catenin and promoted the nuclear translocation of β-catenin. Overexpression of β-catenin and LiCl treatment stimulated the accumulation of β-catenin in the nucleus, and then facilitated FAdV-4 replication. Conversely, repression of β-catenin by inhibitors and siRNA significantly inhibited FAdV-4 replication. Furthermore, inhibition of autophagy by 3-Methyladenine (3-MA) suppressed the FAdV-4 replication, and repression of β-catenin inhibited the FAdV-4-triggered autophagy. In conclusion, the nuclear translocation of β-catenin benefits FAdV-4 replication, and suppression of β-catenin limits FAdV-4 production by inhibiting FAdV-4-induced autophagy. These findings indicated that β-catenin is an important regulator of FAdV-4 replication which can serve as a potential target for anti-FAdV-4 agents.

The Multi-Faceted Role of Autophagy During Animal Virus Infection

Autophagy is a process of degradation to maintain cellular homeostatic by lysosomes, which ensures cellular survival under various stress conditions, including nutrient deficiency, hypoxia, high temperature, and pathogenic infection. Xenophagy, a form of selective autophagy, serves as a defense mechanism against multiple intracellular pathogen types, such as viruses, bacteria, and parasites. Recent years have seen a growing list of animal viruses with autophagy machinery. Although the relationship between autophagy and human viruses has been widely summarized, little attention has been paid to the role of this cellular function in the veterinary field, especially today, with the growth of serious zoonotic diseases. The mechanisms of the same virus inducing autophagy in different species, or different viruses inducing autophagy in the same species have not been clarified. In this review, we examine the role of autophagy in important animal viral infectious diseases and discuss the regulation mechanisms of different animal viruses to provide a potential theoretical basis for therapeutic strategies, such as targets of new vaccine development or drugs, to improve industrial production in farming.

Clinical, Immunological, and Genetic Features in Patients with Activated PI3Kδ Syndrome (APDS): a Systematic Review

Activated phosphoinositide 3-kinase delta syndrome (APDS) is a novel primary immunodeficiency (PID) caused by heterozygous gain of function mutations in PI3Kδ catalytic p110δ (PIK3CD) or regulatory p85α (PIK3R1) subunits leading to APDS1 and APDS2, respectively. Patients with APDS present a spectrum of clinical manifestations, particularly recurrent respiratory infections and lymphoproliferation. We searched PubMed, Web of Science, and Scopus databases for APDS patients and screened for eligibility criteria. A total of 243 APDS patients were identified from 55 articles. For all patients, demographic, clinical, immunologic, and molecular data were collected. Overall, 179 APDS1 and 64 APDS2 patients were identified. The most common clinical manifestations were respiratory tract infections (pneumonia (43.6%), otitis media (28.8%), and sinusitis (25.9%)), lymphoproliferation (70.4%), autoimmunity (28%), enteropathy (26.7%), failure to thrive (20.6%), and malignancy (12.8%). The predominant immunologic phenotype was hyper-IgM syndrome (48.1%). Immunologic profiling showed decreased B cells in 74.8% and CD4⁺ T cells in 64.8% of APDS patients. The c.3061 G>A (p. E1021K) mutation in APDS1 with 85% frequency and c.1425+1 G> (A, C, T) (p.434-475del) mutation in APDS2 with 79% frequency were hotspot mutations. The majority of APDS patients were placed on long-term immunoglobulin replacement therapy. Immunosuppressive agents such as rituximab, tacrolimus, rapamycin, and leniolisib were also administered for autoimmunity and inflammatory complications. In addition, hematopoietic stem cell transplantation (HSCT) was used in 12.8% of patients. APDS has heterogynous clinical manifestations. It should be suspected in patients with history of recurrent respiratory infections, lymphoproliferation, and raised IgM levels. Moreover, HSCT should be considered in patients with severe and complicated clinical manifestations with no or insufficient response to the conventional therapies.

Downregulation of microRNA-30a-5p contributes to the replication of duck enteritis virus by regulating Beclin-1-mediated autophagy

BACKGROUND

MicroRNAs (miRNAs) is increasingly recognized as an important element in regulating virus-host interactions. Our previous results showed that cellular miR-30a-5p was significantly downregulated after duck enteritis virus (DEV) infection cell. However, whehter or not the miR-30a-5p is involved in DEV infection has not been known.

METHODS

Quantitative reverse-transcription PCR (qRT-PCR) was used to measure the expression levels of miRNAs(miR-30a-5p) and Beclin-1 mRNA. The miR-30a-5p - Beclin-1 target interactions were determined by Dual luciferase reporter assay (DLRA). Western blotting was utilized to analyze Beclin-1-mediated duck embryo fibroblast (DEF) cells autophagy activity. DEV titers were estimated by the median tissue culture infective dose (TCID50).

RESULTS

The miR-30a-5p was significantly downregulated and the Beclin-1 mRNA was significantly upregulated in DEV-infected DEF cells. DLRA confirmed that miR-30a-5p directly targeted the 3'- UTR of the Beclin-1 gene. Overexpression of miR-30a-5p significantly reduced the expression level of Beclin-1protein (p < 0.05), leading to the decrease of Beclin-1-mediated autophagy activity, which ultimately suppressed DEV replication (P < 0.05). Whereas transfection of miR-30a-5p inhibitor increased Beclin-1-mediated autophagy and triggered DEV replication during the whole process of DEV infection (P < 0.01).

CONCLUSIONS

This study shows that miR-30a-5p can inhibit DEV replication through reducing autophagy by targeting Beclin-1. These findings suggest a new insight into virus-host interaction during DEV infection and provide a potential new antiviral therapeutic strategy against DEV infection.

Unravelling the receptor binding property of egg drop syndrome virus (EDSV) from the crystal structure of EDSV fiber head

Egg drop syndrome virus (EDSV) is an avian adenovirus that causes markedly decrease in egg production, and in the quality of the eggs when it infects chickens. Until now, EDSV virus-cell interactions are poorly understood, and the cellular receptor is still unknown. In the present study, we determined the atomic structure of the fiber head of EDSV (residues 377-644) at 2.74 Å resolution. Structure comparison with the (chick embryo lethal orphan) CELO long fiber head and human adenovirus fiber heads reveals that the avian adenovirus may interact with the same attachment factor in a unique fashion. Based on the previous studies of CELO virus, we assumed that the chicken coxsackievirus and adenovirus receptor (CAR) may be the attachment factor. We then demonstrate that the chicken CAR serves as a cellular attachment factor for EDSV based on three lines of evidences. Taken together, the results presented here are helpful for further exploring the pathogenesis related to the interaction between EDSV and host cells, and may be used for vaccine development and intervention strategies against EDSV infection.

Glutamine Deficiency Promotes PCV2 Infection through Induction of Autophagy via Activation of ROS-Mediated JAK2/STAT3 Signaling Pathway

Porcine circovirus type 2 (PCV2) is an important pathogen in swine herds. We previously reported that glutamine (Gln) deficiency promoted PCV2 infection in vitro. Here, we established a Gln deficiency model in vivo and further investigated the detailed molecular mechanisms. In vivo and in vitro, Gln deficiency promoted PCV2 infection, which was evident through increased viral yields and PCV2 Cap protein synthesis. It also induced autophagy, as demonstrated by the increases in LC3-II conversion, SQSTM1 degradation, and GFP-LC3 dot accumulation. Autophagy inhibition abolished the effects of Gln deficiency on PCV2 infection. Inhibition of ROS generation alleviated the Gln deficiency-activated JAK2/STAT3 signaling pathway, thereby inhibiting autophagy induction. In vitro, the inhibition of STAT3 by an inhibitor or RNA interference blocked autophagy, thus reversing the effects of Gln deficiency on PCV2 infection. These results indicate that Gln deficiency activates autophagy by upregulating ROS-medicated JAK2/STAT3 signaling and thereby promoting PCV2 infection.