Fusion-related host proteins are actively regulated by NA during influenza infection as revealed by quantitative proteomics analysis

Abstract

Three recombinant influenza A viruses with different neuraminidases (NAs) in the background of A/PR/8/34 (PR8), named rPR8-H5N1NA, rPR8-H9N2NA, and rPR8-H1N1NA, derived from H5N1, H9N2, H1N1 (swine) viruses, respectively, were constructed. We performed a quantitative proteomics analysis to investigate differential protein expression in Madin-Darby canine kidney (MDCK) cells infected with recombinant and wild-type influenza viruses to determine whether NA replacement would alter host cell gene expression. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-TOF MS) and two-dimensional gel electrophoresis (2-DE), we identified 12 up-regulated and 49 down-regulated protein spots, including cytoskeletal proteins, molecular biosynthesis proteins, ubiquitin-proteasome pathway proteins, and heat shock proteins. The most significant changes in infected cells were observed for molecular biosynthesis proteins. We found more differentially expressed protein spots in cells infected with rPR8-H5N1NA or rPR8-H9N2NA viruses than cells infected with wild-type virus. Many of those proteins are postulated to be involved in cell-cell fusion, but the full mechanism remains to be explored. Meanwhile, our data demonstrate that the wild-type virus has evolutionary advantages over recombinant viruses.

Figure 1. Protein expression profiles of the influenza- and mock-infected MDCK cells.

Cell lysates (120 µg) were separated on 13-cm (isoelectric point [pI] 4–7) linear gradient IPG strips using 12.5% SDS-PAGE. Differentially expressed protein spots are indicated with green squares. (A) Representative 2-DE gels of influenza- and mock-infected MDCK cells. T1/C: PR8-wt infected/mock infected, T3/C: rH1N1NA infected/mock infected, T4/C: rH9N2NA infected/Mock infected, T5/C: rH5N1NA infected/mock infected, T3/T1: rH1N1NA infected/PR8-wt infected, T4/T1: rH9N2NA infected/PR8-wt infected, T5/T1: rH5N1NA infected/PR8-wt infected. (B) Numbers of differentially expressed protein spots detected by 2-DE in virus-infected MDCK cells compared with mock-infected MDCK cells. The number of spots ≥0 indicated the proteins were upregulated, and the number <0 indicated the proteins were downregulated. (C) Numbers of differentially expressed protein spots detected by 2-DE in recombinant viruses compared with wild-type virus (wt-PR8)-infected MDCK cells.

Figure 2. Classification of the identified proteins based on their functional annotations using Gene Ontology (GO) categories.

The proteins were annotated into three main categories: cellular component, biological process, or molecular function. The Y-axis indicates the number and percentages of genes, the X-axis indicates the GO category.

Figure 3. Transcriptional profiles of differentially expressed proteins in influenza virus-infected MDCKs.

Total cellular RNA from MDCKs with or without influenza virus infection was subjected to real-time RT-PCR. Samples were normalized to mock-infected MDCKs using β-actin as the reference gene.

Figure 4. Western blots of representative proteins in influenza virus-infected MDCKs.

The samples were prepared from MDCK cells that were virus-infected or mock-infected cells at 6 h p.i.. The β-actin protein was used as a control. (A) Western blot confirmation of differentially expressed proteins for PSMC2 (C08) and UBE2NL (C26). (B) ImageJ software analysis of the ratios of proteins changes according to Fig. 4A.

Figure 5. UBE2NL protein in virus-infected or mock-infected MDCK cells at 6 h p.i..

MDCK cells were infected with the viruses at MOI of 0.1 in the presence of 1 µg/ml TPCK-trypsin. After adsorption for 1 h at 37°C, the inocula were removed and the cultures were incubated for 6 h at 37°C in the maintenance media. Then, the cells were processed for indirect immunofluorescence assay, and the infected cells were detected with polyclonal antisera to UBE2NL protein and NP protein. (A) The fluorescence images (10×) of the infected and mock-infected cells at 6 h p.i. The FITC-fluorescence signal was expressed as UBE2NL protein and TRITC-fluorescence signal was expressed as the infected cells. (B) The fluorescence images (60×) of the cells infected by rPR8-H9N2NA or rPR8-H5N1NA viruses.