Figure 1.
M45 inhibits TLR- and IL-1R-mediated NF-κB but not p38 activation.
(A) NIH-3T3 cells were transduced with retroviral vectors expressing M45 or GFP. Two days after transduction cells were stimulated with the TLR4 agonist LPS (10 µg/ml), the TLR2 agonist LTA-SA (10 µg/ml), or IL-1β (20 ng/ml). IκBα levels were determined by immunoblotting. (B) NIH-3T3 cells were transduced with an M45-expressing or an empty retroviral vector. 30 min after stimulation with IL-1β (10 ng/ml) cells were fixed, and subcellular localization of the NF-κB p65 subunit was analyzed by immunofluorescence. (C) NIH-3T3 cells expressing an NF-κB-dependent secreted alkaline phosphatase (SEAP) reporter were transduced with retroviral vectors expressing M45 or GFP and stimulated 78 h later with the TLR2 agonist Pam3CSK4 (Pam.,1 µg/ml), the TLR4 agonist LPS (0.1 µg/ml), or IL-1β (5 ng/ml). SEAP activity in the supernatant was quantified 15 h after stimulation and is shown as fold induction of SEAP activity of stimulated cells compared to non-stimulated cells (mean ± SD) (D) NIH-3T3 cells transduced with retroviral vectors expressing M45 or GFP were stimulated with IL-1β (20 ng/ml, 15 min) or TNFα (10 ng/ml, 5 min). Phosphorylated and total p38 in cell lysates was detected by immunoblotting.
Figure 2.
M45 inhibits IκBα degradation, TNFα and IL-6 production during MCMV infection.
(A) NIH-3T3 cells were infected with wt MCMV-GFP, an M45 deletion mutant (ΔM45) or a revertant virus (RM45) at an MOI of 10 and treated 5 h postinfection with the TLR2 agonist Pam3CSK4 (Pam., 0.1 µg/ml), the TLR4 agonist LPS (10 µg/ml), or IL-1β (20 ng/ml) for the indicated times. Levels of the indicated proteins were analyzed by immunoblotting. (B) BMDMs were infected with wt MCMV-GFP, ΔM45, or RM45 at an MOI of 3 and stimulated 8 h postinfection for 16 h with the TLR7 agonist R848 (0.1 µM). TNFα and IL-6 levels in the supernatant were determined by ELISA (mean ± SD). (C) RAW264.7 macrophages were infected with wt MCMV-GFP or ΔM45 at an MOI of 0.1, stimulated 24 h postinfection for 4 hours with TLR agonists Pam3CSK4 (Pam.) or Malp-2 (TLR2), LPS (TLR4), R837 (TLR7), or CpG (TLR9), in the presence of brefeldin A. Cells were fixed, permeabilized, and stained with a TNFα-specific antibody. The percentages of TNFα-positive cells within infected (GFP-positive) cell populations were determined by FACS analysis (mean ± SD).
Figure 3.
M45 interacts with the IKK complex.
(A) 293A cells were transfected with plasmids expressing HA-tagged M45 or an unrelated MCMV control protein (m143) and Flag-tagged IKK subunits. Lysates were subjected to immunoprecipitation (IP) using an anti-HA antibody. Immunoprecipitates were analyzed by immunoblotting (IB) using anti-Flag antibody. Whole cell lysates (WCL) were immunoblotted using Flag-, HA-, and actin-specific antibodies, respectively. (B) Nemo−/− MEFs were transfected with Flag-tagged IKK subunits and HA-tagged M45 or an unrelated MCMV control protein (m142), respectively. IP and IB were performed using anti-HA and anti-Flag antibodies, respectively. WCL were immunoblotted with anti-NEMO antibody and as described for panel A. (C) rip1−/− MEFs were transfected with HA-tagged M45 or an unrelated MCMV control protein (m142) and Flag-tagged NEMO. IP was done with an anti-HA antibody. Immunoprecipitates and WCL were analyzed by immunoblotting using the indicated antibodies. (D) 293A cells were transfected with plasmids expressing HA-tagged M45 or m143 (as a control). Lysates were subjected to anti-HA immunoprecipitation. Immunoblotting of precipitates and the WCL was done with antibodies as indicated. (E) NIH-3T3 cells were infected for 4 hours with RM45 (MCMV expressing HA-tagged M45), ΔM45, or Rm142 (MCMV expressing HA-tagged m142) at an MOI of 5. Cell lysates were subjected to IP and IB with antibodies as indicated.
Figure 4.
The C-terminal RNR R1 homology domain of M45 is required for the inhibition of NF-κB activation and interaction with NEMO.
(A) Schematic representation of M45 truncation mutants used in this and a previous [36] study. The C-terminal RNR R1 homology domain is shown in white, the unique N terminus in grey. The RHIM is marked black. (B) NIH-3T3 cells were transduced with retroviruses expressing full length M45, truncated M45 proteins, or GFP. After stimulation with IL-1β (20 ng/ml, 15 min), IκBα levels were determined by immunoblotting. (C) 293A cells were transfected with plasmids encoding Flag-tagged NEMO and HA-tagged full-length M45, truncated M45, or an unrelated MCMV control protein (m142), respectively. Lysates were subjected to immunoprecipitation (IP) with an anti-HA antibody. Immunoprecipitates and the whole cell lysates (WCL) were analyzed by immunoblotting (IB) with the indicated antibodies. (D) Primary BMDMs were mock infected or infected with GFP-expressing wt MCMV (wt), ΔM45 mutant (ΔM45), M45 revertant (RM45), or MCMVs expressing Ct or Nt3 (RCt and RNt3) at an MOI of 1. 17 h postinfection cells were stimulated for 4 hours with TLR9 agonist CpG (0.5 µM) or TLR7 agonist R838 (0.1 µM) in the presence of brefeldin A. Cells were fixed, permeabilized, and stained with a TNFα-specific antibody. The percentages of TNFα-positive cells within infected (GFP-positive) cell populations were determined by FACS analysis (mean ± SEM).
Figure 5.
M45 induces lysosomal degradation of NEMO.
(A) NIH-3T3 cells were infected with MCMV ΔM45 or the revertant virus, RM45, at an MOI of 3. Cells were harvested at indicated time points postinfection, and the levels of IKK subunits, M45, IE1, and actin in lysates were analyzed by immunoblotting. (B) Murine fibroblasts (NIH-3T3, 10.1) or human embryonic kidney 293A cells were transduced with retroviral vectors expressing M45 or GFP, harvested 72 hours later, and analyzed by immunoblotting. (C) ts20 cells harboring a temperature-sensitive E1 enzyme were incubated for 14 h at a permissive (35°C) or a restrictive (40.5°C) temperature. Cells were then mock-infected or infected with RM45, and analyzed 10 h postinfection by immunoblotting (D) NIH-3T3 cells were mock infected or infected with RM45 (MOI of 5) and treated 2 hpi with 10 mM ammonium chloride (NH4Cl) or a mix of lysosomal protease inhibitors (PI-mix). 9 hpi, levels of IKK subunits, M45, IE1, and actin were determined by immunoblotting. (E) NIH-3T3 cells were stably transduced with retroviral vectors expressing M45 or GFP, and treated for the indicated times with PI-mix, 10 mM NH4Cl, or 5 µM lactacystin, respectively. Levels of the indicated proteins were analyzed by immunoblotting.
Figure 6.
M45 targets NEMO to autophagosomes.
(A) NIH-3T3 cells stably expressing Flag-tagged NEMO were mock infected or infected with wt MCMV-GFP or MCMV-GFP-ΔM45 at an MOI of 6. Cells were fixed 6 hpi, and NEMO distribution was analyzed by immunofluorescence. The arrow indicates the region which is shown in higher magnification in the right corner of the picture. (B) NIH-3T3 cells were transfected with Flag-NEMO and M45-HA expressing plasmids as indicated. 24 h later, cells were fixed and double immunofluorescence staining was performed using antibodies against the HA and Flag tags. Regions shown in higher magnification in the left corner of the pictures are indicated by arrows. (C) NIH-3T3 cells stably expressing Flag-NEMO were mock infected or infected with MCMV-M45-HA or MCMV-ΔM45 at an MOI of 7. Seven hpi, cells were fixed and analyzed as described for panel B. (D) NIH-3T3 cells were transfected with expression plasmids for Flag-NEMO and M45-HA or empty vector as indicated. Plasmids encoding GFP-tagged LC3, Rab5, or Rab7 were cotransfected. 24 hours posttransfection cells were fixed and used for anti-Flag immunofluorescence staining.
Figure 7.
Loss of NEMO degradation in autophagy-deficient MEFs, and MCMV-induced accumulation of LC3-II.
(A) atg5−/− and atg5+/+ MEFs were mock infected or infected with MCMVΔM45 or the revertant virus, RM45, at an MOI of 10. Eight hpi cells were harvested and levels of the indicated proteins were analyzed by immunoblotting. (B) 10.1 fibroblasts were mock infected or infected with wt MCMV or ΔM45 at an MOI of 5. Cells were left untreated or treated 30 min after infection with 10 mM NH4Cl to block lysosomal degradation of LC3-II. Cells were harvested at the indicated time points, and LC3-II levels in cell lysates were analyzed by immunoblotting using an LC3-II-specific antibody. LC3-II levels were quantified by densitometric analysis and normalized using the corresponding actin levels. Fold increases are shown relative to mock-infected cells.
Figure 8.
M45 induces the formation of autophagosomes and aggregates.
(A) NIH-3T3 cells stably expressing GFP-LC3 were transduced with retroviral vectors expressing M45 or Ct3. Three days later, cells were analyzed by confocal laser scanning microscopy. GFP-LC3 dots were counted in 50 cells per sample. Results are shown as box and whisker plots. The bottom and top of the box represent the first and third quartile, respectively. The horizontal line within the box represents the median. The minima and maxima within 1.5-fold interquartile range are shown as whiskers. Dots represent outliers. Significance was determined using ANOVA (***, p<0.001). M45 and Ct3 were detected by immunoblot. (B) Fluorescence images of cells described above. Large GFP-LC3 aggregates are indicated by arrow heads. (C) Electron micrograph of a double-membrane autophagosome in an M45-expressing cell. (D) Autophagosomes were counted in 10 randomly selected M45- and Ct3-expressing cells (2–3 TEM sections per cell). Results are shown in box and whisker plots as described for panel A. Significance was determined using the student's t-test (***, p<0.001). (E) Epifluorescence image of an M45-expressing cell with large GFP-LC3 structures. (F) TEM image of the same cell reveals large aggregates marked with ‘A’. (G, H) Successive enlargement of the region marked in panel F shows an example of an aggregate-associated autophagosome.
Figure 9.
M45 targets RIP1 and NEMO, two crucial components of the inflammatory cascade.