Characterization of the genes encoding complete US10, SORF3, and US2 proteins from duck enteritis virus

The alphaherpesvirus conserved pUS10 is important for natural infection and its expression is regulated by the conserved Herpesviridae protein kinase (CHPK)

Conserved Herpesviridae protein kinases (CHPK) are conserved among all members of the Herpesviridae. Herpesviruses lacking CHPK propagate in cell culture at varying degrees, depending on the virus and cell culture system. CHPK is dispensable for Marek's disease herpesvirus (MDV) replication in cell culture and experimental infection in chickens; however, CHPK-particularly its kinase activity-is essential for horizontal transmission in chickens, also known as natural infection. To address the importance of CHPK during natural infection in chickens, we used liquid chromatography-tandem mass spectrometry (LC-MS/MS) based proteomics of samples collected from live chickens. Comparing modification of viral proteins in feather follicle epithelial (FFE) cells infected with wildtype or a CHPK-null virus, we identified the US10 protein (pUS10) as a potential target for CHPK in vivo. When expression of pUS10 was evaluated in cell culture and in FFE skin cells during in vivo infection, pUS10 was severely reduced or abrogated in cells infected with CHPK mutant or CHPK-null viruses, respectively, indicating a potential role for pUS10 in transmission. To test this hypothesis, US10 was deleted from the MDV genome, and the reconstituted virus was tested for replication, horizontal transmission, and disease induction. Our results showed that removal of US10 had no effect on the ability of MDV to transmit in experimentally infected chickens, but disease induction in naturally infected chickens was significantly reduced. These results show CHPK is necessary for pUS10 expression both in cell culture and in the host, and pUS10 is important for disease induction during natural infection.

US10 Protein Is Crucial but not Indispensable for Duck Enteritis Virus Infection in Vitro

To investigate the function of the duck enteritis virus (DEV) tegument protein US10, we generated US10 deletion and revertant mutants (ΔUS10 and US10FRT) via two-step RED recombination based on an infectious BAC clone of DEV CHv-BAC-G (BAC-G). In multistep growth kinetic analyses, ΔUS10 showed an approximately 100-fold reduction in viral titer, while the genome copies decreased only 4-fold compared to those of BAC-G. In one-step growth kinetic analyses, there were no significant differences in genome copies among BAC-G, ΔUS10 and US10FRT, but ΔUS10 still showed a 5- to 20-fold reduction in viral titer, and the replication defect of ΔUS10 was partially reversed by infection of US10-expressing cells. The transcription levels of Mx, OASL, IL-4, IL-6 and IL-10 in ΔUS10-infected duck embryo fibroblasts (DEFs) were significantly upregulated, while TLR3 was downregulated compared with those in BAC-G-infected DEFs. Taken together, these data indicated that US10 is vital for DEV replication and is associated with transcription of some immunity genes.

Duck enteritis virus activates CaMKKβ-AMPK to trigger autophagy in duck embryo fibroblast cells via increased cytosolic calcium

Background

The results of our previous study showed that impaired cellular energy metabolism contributes to duck enteritis virus-induced autophagy via the 5`-adenosine monophosphate-activated protein kinase (AMPK)/tuberous sclerosis complex 2/mammalian target of rapamycin pathway in duck embryo fibroblast (DEF) cells. However, it remains unknown whether any other underlying mechanisms of AMPK activation are involved in autophagy induction.

Methods

The activity of CaMKKβ and AMPK in DEF cells infected with DEV were evaluated.The Effect of inhibitory activity of CaMKKβ on DEV-induced autophagy was investigated. In addtion to, the cytosolic calcium level in DEF cells infected with DEV were evaluated.The Effect of inhibitory cytosolic calcium level on DEV-induced autophagy was investigated.

Results

In this study, duck enteritis virus (DEV) infection activated CaMKKβ and its substrate molecule AMPK at 36, 48, and 60 h post-infection (hpi). STO-609, a CaMKKβ inhibitor, or CaMKKβ siRNA significantly inhibited the activation of DEV to AMPK, LC3I to LC3II transformation, and GFP-LC3 puncta distribution. In addition, inhibition of CaMKKβ activity also significantly reduced progeny DEV titer and gB protein expression. Besides, cytosolic calcium (Ca²⁺) was higher in DEV-infected cells than mock controls at 36, 48, and 60 hpi, respectively. Treatment of DEV-infected cells with 1,2-Bis (2-aminophenoxy) ethane-N, N, N′, N-tetraacetic acid (BAPTA-AM) significantly reduced intracellular Ca²⁺ ion concentrations, as well as CaMKKβ and AMPK activities, and subsequent autophagy, in addition to viral protein synthesis and viral titer.

Conclusions

These results showed that elevated [Ca²⁺]cyto-mediated activation of CaMKKβ managed the activation of AMPK, which then positively regulated autophagy, thereby providing further insight into DEV–host interactions.

DEV induce autophagy via the endoplasmic reticulum stress related unfolded protein response

Duck enteritis virus (DEV) can infect ducks, geese, and many other poultry species and leads to acute, septic and highly fatal infectious disease. Autophagy is an evolutionarily ancient pathway that plays an important role in many viral infections. We previously reported that DEV infection induces autophagy for its own benefit, but how this occurs remains unclear. In this study, endoplasmic reticulum (ER) stress was triggered by DEV infection, as demonstrated by the increased expression of the ER stress marker glucose-regulated protein 78 (GRP78) and the dilated morphology of the ER. Pathways associated with the unfolded protein response (UPR), including the PKR-like ER protein kinase (PERK) and inositol-requiring enzyme 1 (IRE1) pathways, but not the activating transcription factor 6 (ATF6) pathway, were activated in DEV-infected duck embryo fibroblast (DEF) cells. In addition, the knockdown of both PERK and IRE1 by small interfering RNAs (siRNAs) reduced the level of LC3-II and viral yields, which suggested that the PERK-eukaryotic initiation factor 2α (eIF2α) and IRE1-x-box protein1 (XBP1) pathways may contribute to DEV-induced autophagy. Collectively, these data offer new insight into the mechanisms of DEV -induced autophagy through activation of the ER stress-related UPR pathway.

Molecular characterization of the duck enteritis virus US10 protein

BACKGROUND

There is little information regarding the duck enteritis virus (DEV) US10 gene and its molecular characterization.

METHODS

Duck enteritis virus US10 was amplified and cloned into the recombinant vector pET32a(+). The recombinant US10 protein was expressed in Escherichia coli BL21 cells and used to immunize rabbits for the preparation of polyclonal antibodies. The harvested rabbit antiserum against DEV US10 was detected and analyzed by agar immunodiffusion. Using this antibody, western blotting and indirect immunofluorescence analysis were used to analyze the expression level and subcellular localization of US10 in infected cells at different time points. Quantitative reverse-transcription PCR (qRT-PCR) and pharmacological inhibition tests were used to ascertain the kinetic class of the US10 gene. A mass spectrometry-based strategy was used to identify US10 in purified DEV virions and quantify its abundance.

RESULTS

The recombinant pET32a(+)/US10 protein was expressed as inclusion bodies, purified by gradient urea washing, and used to prepare specific antibodies. The results of qRT-PCR, western blotting, and pharmacological inhibition tests revealed that US10 is mainly transcribed in the late stage of viral replication. However, the presence of the DNA polymerase inhibitor ganciclovir and the protein synthesis inhibitor cycloheximide blocked transcription. Therefore, US10 is a γ2 (true late) gene. Indirect immunofluorescence analysis showed that US10 proteins were initially diffusely distributed throughout the cytoplasm, but with the passage of time, they gradually relocated to a perinuclear region. The US10 protein was detected in purified DEV virions by mass spectrometry, but was not detected by western blotting, indicating that DEV US10 is a minor virion protein.

CONCLUSIONS

The DEV US10 gene is a γ2 gene and the US10 protein is localized in the perinuclear region. DEV US10 is a virion component.

Comparative genomic analysis of duck enteritis virus strains

The icosahedral virion of duck enteritis virus (DEV) is roughly spherical and approximately 150 nm in diameter. Here, we describe the genomic features of DEV CHv together with a draft genome sequence and its annotation, highlighting the homogeneity and heterogeneity of this genome in comparison with its reference genomes.

Different linkages in the long and short regions of the genomes of duck enteritis virus Clone-03 and VAC strains

BACKGROUND

Duck enteritis virus (DEV) is an unassigned member in the family Herpesviridae. To demonstrate further the evolutionary position of DEV in the family Herpesviridae, we have described a 42,897-bp fragment. We demonstrated novel genomic organization at one end of the long (L) region and in the entire short (S) region in the Clone-03 strain of DEV.

RESULTS

A 42,897-bp fragment located downstream of the LOFR11 gene was amplified from the Clone-03 strain of DEV by using 'targeted gene walking PCR'. Twenty-two open reading frames (ORFs) were predicted and determined in the following order: 5'-LORF11-RLORF1-ORF1-ICP4-S1-S2-US1-US10-SORF3-US2-MDV091.5-like-US3-US4-US5-US6-US7-US8-ORFx-US1-S2-S1-ICP4 -3'. This was different from that of the published VAC strain, both in the linkage of the L region and S region, and in the length of the US10 and US7 proteins. The MDV091.5-like gene, ORFx gene, S1 gene and S2 gene were first observed in the DEV genome. The lengths of DEV US10 and US7 were determined to be 311 and 371 amino acids, respectively, in the Clone-03 strain of DEV, and these were different from those of other strains. The comparison of genomic organization in the fragment studied herein with those of other herpesviruses showed that DEV possesses some unique characteristics, such as the duplicated US1 at each end of the US region, and the US5, which showed no homology with those of other herpesviruses. In addition, the results of phylogenetic analysis of ORFs in the represented fragment indicated that DEV is closest to its counterparts VZV (Varicellovirus) and other avian herpesviruses.

CONCLUSION

The molecular characteristics of the 42,897-bp fragment of Clone-03 have been found to be different from those of the VAC strain. The phylogenetic analysis of genes in this region showed that DEV should be a separate member of the subfamily Alphaherpesvirinae.