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. 2011 May;7(5):e1002053.
doi: 10.1371/journal.ppat.1002053. Epub 2011 May 19.

AAV exploits subcellular stress associated with inflammation, endoplasmic reticulum expansion, and misfolded proteins in models of cystic fibrosis

Affiliations

AAV exploits subcellular stress associated with inflammation, endoplasmic reticulum expansion, and misfolded proteins in models of cystic fibrosis

Jarrod S Johnson et al. PLoS Pathog. 2011 May.

Abstract

Barriers to infection act at multiple levels to prevent viruses, bacteria, and parasites from commandeering host cells for their own purposes. An intriguing hypothesis is that if a cell experiences stress, such as that elicited by inflammation, endoplasmic reticulum (ER) expansion, or misfolded proteins, then subcellular barriers will be less effective at preventing viral infection. Here we have used models of cystic fibrosis (CF) to test whether subcellular stress increases susceptibility to adeno-associated virus (AAV) infection. In human airway epithelium cultured at an air/liquid interface, physiological conditions of subcellular stress and ER expansion were mimicked using supernatant from mucopurulent material derived from CF lungs. Using this inflammatory stimulus to recapitulate stress found in diseased airways, we demonstrated that AAV infection was significantly enhanced. Since over 90% of CF cases are associated with a misfolded variant of Cystic Fibrosis Transmembrane Conductance Regulator (ΔF508-CFTR), we then explored whether the presence of misfolded proteins could independently increase susceptibility to AAV infection. In these models, AAV was an order of magnitude more efficient at transducing cells expressing ΔF508-CFTR than in cells expressing wild-type CFTR. Rescue of misfolded ΔF508-CFTR under low temperature conditions restored viral transduction efficiency to that demonstrated in controls, suggesting effects related to protein misfolding were responsible for increasing susceptibility to infection. By testing other CFTR mutants, G551D, D572N, and 1410X, we have shown this phenomenon is common to other misfolded proteins and not related to loss of CFTR activity. The presence of misfolded proteins did not affect cell surface attachment of virus or influence expression levels from promoter transgene cassettes in plasmid transfection studies, indicating exploitation occurs at the level of virion trafficking or processing. Thus, we surmised that factors enlisted to process misfolded proteins such as ΔF508-CFTR in the secretory pathway also act to restrict viral infection. In line with this hypothesis, we found that AAV trafficked to the microtubule organizing center and localized near Golgi/ER transport proteins. Moreover, AAV infection efficiency could be modulated with siRNA-mediated knockdown of proteins involved in processing ΔF508-CFTR or sorting retrograde cargo from the Golgi and ER (calnexin, KDEL-R, β-COP, and PSMB3). In summary, our data support a model where AAV exploits a compromised secretory system and, importantly, underscore the gravity with which a stressed subcellular environment, under internal or external insults, can impact infection efficiency.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Human airway epithelium under stress is more susceptible to AAV infection.
HAE cultures were exposed to SMM an inflammatory stimulant derived from CF airways by application to the apical surface to induce cellular stress and model diseased airway. 24 hr after application, SMM was removed and HAE infected with an AAV variant generated by molecular directed evolution. A. Representative en face images of three HAE cultures one week after inoculation with AAV packaging an eGFP transgene, showing GFP positive cells (green) in HAE cultures exposed to PBS (panels i, ii, and iii) or SMM (iv, v, and vi). B. Parallel experiments performed on HAE cultures with an AAV packaging a Luc transgene. Cells were harvested one week after inoculation and luciferase activity was assessed. Error bars represent standard deviations from three separate inoculations. Data shown are representative from three independent experiments (* p-value<0.05).
Figure 2
Figure 2. Cells expressing misfolded ΔF508-CFTR are more permissive to infection with human respiratory viruses.
Images of GFP-positive cells after inoculation with recombinant self-complementary AAV (scAAV2, 100 vg/cell: eGFP transgene), adenovirus (Ad; 20 vg/cell), or respiratory syncytial virus (RSV; 200 vg/cell) into control BHK-21 cells (panel i), BHK-21 cells overexpressing CFTR (ii), or BHK-21 cells overexpressing ΔF508-CFTR (iii). Representative phase contrast images are shown to demonstrate equivalent levels of confluence.
Figure 3
Figure 3. Cells expressing misfolded ΔF508-CFTR are more permissive to AAV infection.
A. Images of GFP-positive cells after administration of recombinant AAV (AAV2, 1,000 vg/cell: eGFP transgene) to BHK-21 cells (panel i), BHK-21 cells overexpressing CFTR (ii), or BHK-21 cells overexpressing ΔF508-CFTR (iii). B. Luciferase activity of cells after transduction by AAV (AAV2, 1,000 vg/cell: Luc transgene) in control BHK-21 cells, or cells expressing CFTR, or expressing ΔF508-CFTR. C. Flow cytometry analysis of AAV transduction at increasing particle numbers showing mean fluorescence intensity (MFI) of GFP expression from positive cells after inoculation with AAV2-GFP at 100, 1000, or 10000 vg/cell. D. Plots show percent of GFP-positive cells after infection with AAV2-GFP (1,000 vg/cell) in control BHK-21 cells, CFTR cells, or ΔF508-CFTR cells. Samples that were statistically different from controls are marked (*p<0.05; **p<0.01).
Figure 4
Figure 4. Effect of low temperature conditioning or expression of other misfolded proteins on AAV2 infection.
A. Western blot showing immature form of CFTR and ΔF508 (black arrow) and mature glycosylated form (arrowhead). Following low temperature conditioning (2 d, 27°C), a significant amount of ΔF508-CFTR becomes more complexly glycosylated. Asterisks (*) represent non-specific banding and demonstrate equal loading. B. Luciferase assay of transduction after low temp conditioning, and a comparative control at 27°C. Cells were infected with AAV2 (10,000 vg/cell) as indicated and luciferase activity was measured 24 hr later. C. Comparative analysis of mutant CFTR processing with AAV2 transduction. Western blot of proteins expressed in BHK-21 cell lines depicting level of glycosylation of CFTR, and mutants ΔF508 (misfolded), G551D (properly folded), D572N (misfolded), 1410× (intermediate folding defect). Related ABC transporter proteins MRP1 and an analogous mutant ΔF728-MRP1 are also shown. D. Luciferase assay for transduction in these cell lines following administration of AAV2 (10,000 vg/cell). Samples that were statistically different from controls are indicated with p-values marked (*p<0.05; **p<0.01).
Figure 5
Figure 5. Perinuclear accumulation of AAV capsids in control, CFTR, or ΔF508-CFTR cells.
A. Confocal immunofluorescence staining of capsids (green) 16 hr post-inoculation (50,000 vg/cell) do not demonstrate obvious differences in subcellular trafficking at this level of resolution. B. Single cell 3D composites were rendered using volume imaging software on z-stacked images taken after co-staining virus capsids with CFTR (panel ii, red) or ΔF508-CFTR (iii, red). Absence of yellow color suggests no direct interaction between AAV and wt or mutant CFTR, yet does not rule out that they could be degraded or processed through the same pathway. Nuclei (blue) were stained with DAPI.
Figure 6
Figure 6. AAV capsids traffic to locations near Golgi/ER transport components.
AAV2 virions (10,000 vg/cell) were administered to HeLa cells for 16 hr and prepped for confocal immunofluorescence with antibodies directed toward cellular markers (red) for ER-resident chaperones (BiP, calnexin, calreticulin), Golgi sorting components (KDEL-R,β-COP), or AAV2 capsids (green). Nuclei (blue) were stained with DAPI. White arrows indicate partial colocalization with KDEL-R and β-COP, yet it is difficult to assess whether colocalization occurs with ER markers.
Figure 7
Figure 7. Knockdown of secretory pathway components or proteins involved in CFTR processing impacts AAV infection.
Transduction efficiency of AAV following siRNA-mediated knockdown of proteins in HEK-293 cells (A & C) or HeLa cells (B & D). A margin of error for nonsignificant perturbations of infection (roughly ±30% relative to controls) is indicated by dashed lines. Cells were mock transfected or transfected with non-targeting control siRNA, or siRNA targeting BiP, calnexin, calreticulin, KDEL-R, β-COP, HDAC6, dynactin, or PSMB3. Between 36 and 48 hr after transfection, cells were infected with AAV2-GFP (200 vg/cell, HEK-293; 500 vg/cell HeLa) and fixed for flow cytometry 24 hr post-infection. Data is reported as the percentage of cells infected (% GFP positive, A & B) or mean fluorescence intensity (MFI, C & D) from cells that are positive. Differences in cell-type specificity are apparent with β-COP, but ultimately these results suggest AAV infection can be influenced by components of the secretory pathway and the proteasome. Cells infected with lenti-GFP as a control, do not display significant differences in MFI (C & D, black columns), ruling out aberrant effects on reporter gene expression. Error bars represent standard deviation and graphs are representative data sets of at least three independent experiments. Samples that were statistically different from controls are marked (*p<0.05; **p<0.01).

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