Identification and characterization of duck plague virus glycoprotein C gene and gene product

DPV gC gene from the genomic DNA was amplified and cloned into a T/A cloning vector pMD18-T, generating a recombinant cloning plasmid pMD18-T/gC (Figure 1). The recombinant plasmid was confirmed by DNA sequencing, PCR and restriction digestion (Figure 2a). The gC gene fragment, which was obtained by digestion of pMD18-T/gC with EcoRI and XhoI, was ligated into the fusion expression vector pET32a(+) (Figure 3) and identified by PCR and restriction digestion (Figure 2b). After confirmation, a positive clone was submitted to DNA sequencing and the result confirmed that the gC gene was in frame with the N-terminal His6 tag within the pET32a(+) multiple cloning sites (data not shown). Then this recombinant plasmid, pET32a-gC, was transformed into Escherichia coli BL21 (DE3) which, following induction with IPTG, expressed large quantities of the pET32a-gC recombinant protein (Figure 4a), and this recombinant protein was purified by gel and electric elution (Figure 4b). In order to examine the reactivity and specificity of the recombinant fusion protein, Western blot analysis was carried out. As shown in Figure 4b, the anti-DPV serum specifically recognized a 65 kDa band, which corresponded to the theoretical molecular mass of pET32a-gC.

FQ-PCRs were used to detect the expression of the DPV gC gene during viral infection. Total RNA was isolated from mock-or DPV-infected cells at indicated times, and then cDNA was synthesized using reverse transcriptase. Aliquots of cDNA at each time point were used as template for real-time PCR reactions containing primers either for gC gene or for β-actin. Table 1 presents data from experiments where the target (gC) and reference (β-actin) were amplified in separate wells. The 2^-ΔΔCt method was used to calculate relative changes in the gene expression determined from quantitative real-time PCR experiments. As shown in Figure 5 the level of gC mRNA had been increasing since 4 hpi, peaked between 28 and 36 hpi, and then declined.

In this experiment, DEF cells were mock infected or infected with DPV, and at 4, 16, 32, 48, 60, 72 hpi, cell suspensions were harvested and lysed in RIPA buffer. Equal amounts of cell lysates were resolved by SDS-PAGE, and proteins on the gel were electrophoretically transferred to PVDF membrane and subjected to Western blot analysis with rabbit anti-DPV gC serum. The result, shown in Figure 6 revealed that a 45 kDa protein was readily detected as early as 4 hpi and seemed to be present at increased levels at 48 hpi.

The subcellular localization of gC protein was examined by indirect immunofluorescence staining. At various times after infection, DEF cells infected with DPV were fixed with 4% paraformaldehyde, treated with 5% bovine serum albumin (BSA) to block nonspecific binding and reacted with the DPV gC antiserum. Specific fluorescence became detectable only in the cytoplasm of infected cells as early as 4 hpi. At later times of infection, the protein converged at the perinuclear region of the cytoplasm. And after 60 hr, fluorescence was gently dispersed (Figure 7).

Duck embryo fibroblasts (DEF) were cultured at 37°C with 5% CO₂ in minimal essential medium (MEM) containing 10% fetal bovine serum (FBS) (Hyclone, Logan, Utah, USA), 100 U/ml penicillin, and 100 μg/ml streptomycin.

DPV CH virulent strain was obtained from the Avian Disease Research Center of Sichuan Agricultural University. For infection, DPV of 2.2 × 10⁷ TCID50/ml was employed.

After DPV inoculation, the DEF were incubated in MEM containing 3% FBS. Usually, the maximum virus titers could be obtained 72 h postinfection (hpi) when the cytopathic effect (CPE) was over 75%.

A pair of primers (5'-CGGAATTCCAAAACGCCGCACAGATGAC-3' and 5'-CCCTCGAGGTATTCAAATAATATTGTCTGC-3') was designed and used to amplify DPV gC gene from the genomic DNA. The amplified PCR product was cloned into a T/A cloning vector pMD18-T (TaKaRa), generating a recombinant cloning plasmid pMD18-T/gC (Figure 1). After verified by PCR, restriction analysis and DNA sequencing (TaKaRa), the gC gene fragment, which was obtained by digestion of pMD18-T/gC with EcoRI and XhoI, was ligated into prokaryotic vector pET32a(+) (Novagen) (Figure 3), which was digested previously with the same restriction enzymes. The recombinant plasmid, named pET32a-gC, was confirmed by PCR, restriction enzyme digestion and DNA sequencing (TaKaRa).

pET32a-gC was transformed into Escherichia coli BL21 (DE3) and the bacteria were induced for 4 h with 0.6 mM IPTG at 37°C to express the fusion protein. The fusion protein was purified from inclusion bodies by gel and electric elution. To test the antigenicity of the recombinant fusion protein, proteins separated by 12% SDS-PAGE were subsequently subjected to Western blot analysis with rabbit anti-DPV serum. The purified recombinant protein was then mixed with an equal volume of Freund's complete adjuvant (Sigma) and used to immunize rabbits by intradermal injection, followed by two additional intradermal inoculations with Freund's incomplete adjuvant once every 7 days and the last inoculation with the purified recombinant protein. After the fourth immunization, anti-DPV gC serum was collected. Then, the purified IgG polyclonal antibodies were obtained by purification using caprylic acid and ammonium sulfate precipitation and High-Q anion exchange chromatography.

Total RNA was isolated from mock-or DPV-infected cells at different times (1, 4, 7, 10, 14, 20, 28, 36, 54, 72 hpi) and was reverse transcribed at 37°C for 120 min using random primer according to the manufacturer's instructions. The real-time PCR assays were performed using an iCycler iQ™ real-time PCR detection system (Bio-Rad Lab., Hercules, CA, USA). The primers used for PCR amplification were as follows: forward primer 5'-GAAGGACGGAATGGTGGAAG-3' and reverse primer 5'-AGCGGGTAACGAGATCTAATATTGA-3', which amplify a 78-base pair (bp) fragment of DPV gC gene, and for the endogenous control gene β-actin, forward primer 5'-CCGGGCATCGCTGACA-3' and reverse primer 5'-GGATTCATCATACTCCTGCTTGCT-3'. The amplification was performed in a 20 μl reaction mixture containing 9 μl of POWER High-Capacity cDNA Reverse Transcription Kits SYBR Green PCR master mix (Applied Biosystems), 0.5 μl of each primer, 1 μl of cDNA template and 9 μl of sterile ultra pure water. Three replicates of each reaction were performed. The PCR condition consisted of one cycle of 1 min at 95°C followed by 40 two-step cycles of 30 sec at 94°C and 30 sec at 60°C. Homogeneity of products from each reaction was confirmed by melt curve analysis. Analysis of the real-time PCR data was carried out using the comparative ΔΔCt method[50]. The fold change in expression of gC gene relative to the endogenous control gene (β-actin) at various time points was calculated as Fold change = Log(2^-ΔΔCt), where ΔΔCt = (Ct, _Target-Ct, _Reference)_{Time x}- (Ct, _Target-Ct, _Reference)_{Time 0}.

DEF were either mock infected or infected with DPV of 2.2 × 10⁷ TCID50/ml, harvested at various indicated times (4, 16, 32, 48, 60, 72 hpi), lysed on ice for 30 min with an equal volume of radioimmunoprecipitation assay (RIPA) buffer (50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1% Triton X-100, 1% sodium deoxycholate, 0.1% SDS, and 1 mmol/l phenylmethylsulfonyl fluoride) and centrifugated at 13,000×rpm for 15 min at 4°C[13]. Then equivalent amounts of the cell lysates were electrophoresed on 12% SDS-PAGE followed by staining with Coomassie Brilliant Blue R-250 and simultaneously electrophoretically transferred to a polyvinylidene fluoride (PVDF) membrane (Bio-Rad Lab., Hercus, CA, USA) in a transfer buffer at 120 V for 90 min. For Western blot analysis, purified rabbit polyclonal antibodies IgG was used as the primary antibody at a dilution of 1:50, followed by horse peroxidase (HRP) conjugated goat anti-rabbit IgG at a dilution of 1:5000 (KPL Inc., Gaithersburg, Maryland, USA) as the secondary antibody.

DEF cells, grown on coverslips in six-well plates, were mock infected or infected with DPV of 2.2 × 10⁷ TCID50/ml and then fixed with 4% paraformaldehyde for 15 min at room temperature at different times (4, 12, 24, 36, 48, 60 hpi). After blocking in PBS containing 5% bovine serum albumin (BSA) at 37°C for 1 h, the cells were incubated with purified rabbit polyclonal antibodies IgG (1:100 dilution) specific for recombinant DPV gC at 4°C overnight, rinsed three times for 10 min each with PBS and incubated with fluorescein isothiocyanate (FITC)-conjugated goat anti-rabbit IgG (Sino-American Biotechnology Co., Shanghai, China) for 1 h at 37°C. 4,6-Diamidino-2-phenylindole (DAPI; Sigma) staining was used to visualize the cell nuclei. Fluorescent images were viewed and recorded with the Bio-Rad MRC 1024 imaging system.