Dengue virus infection induces autophagy: an in vivo study
© Lee et al.; licensee BioMed Central Ltd. 2013
Received: 15 May 2013
Accepted: 4 September 2013
Published: 8 September 2013
We and others have reported that autophagy is induced by dengue viruses (DVs) in various cell lines, and that it plays a supportive role in DV replication. This study intended to clarify whether DV infection could induce autophagy in vivo. Furthermore, the effect of DV induced autophagy on viral replication and DV-related pathogenesis was investigated.
Results and conclusions
The physiopathological parameters were evaluated after DV2 was intracranially injected into 6-day-old ICR suckling mice. Autophagy-related markers were monitored by immunohistochemical/immunofluorescent staining and Western blotting. Double-membrane autophagic vesicles were investigated by transmission-electron-microscopy. DV non-structural-protein-1 (NS1) expression (indicating DV infection) was detected in the cerebrum, medulla and midbrain of the infected mice. In these infected tissues, increased LC3 puncta formation, LC3-II expression, double-membrane autophagosome-like vesicles (autophagosome), amphisome, and decreased p62 accumulation were observed, indicating that DV2 induces the autophagic progression in vivo. Amphisome formation was demonstrated by colocalization of DV2-NS1 protein or LC3 puncta and mannose-6-phosphate receptor (MPR, endosome marker) in DV2-infected brain tissues. We further manipulated DV-induced autophagy by the inducer rapamycin and the inhibitor 3-methyladenine (3MA), which accordingly promoted or suppressed the disease symptoms and virus load in the brain of the infected mice.
We demonstrated that DV2 infection of the suckling mice induces autophagy, which plays a promoting role in DV replication and pathogenesis.
DV is a positive single-strand RNA virus which belongs to the Flaviviridae family, and is composed of four serotypes (DV1 to 4). It is transmitted to vertebrate hosts via the mosquito vector Aedes aegypti and A. albopictus. DV causes over 390 million infections every year, and is one of the most important arboviruses causing human diseases. The symptoms of DV infection range from mild dengue fever (DF) to life-threatening dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS). Generally speaking, DF is self-limited, but in a small proportion of people the disease may proceed to severe DHF/DSS. Diverse hypotheses have been proposed to explain the pathologic mechanism of DHF/DSS, but the findings remain contradictory.
Autophagy participates in the degradation of long-lived and aggregated protein as well as damaged organelles in the cytoplasm to maintain homeostasis. Autophagy is characterized by a double-membrane vesicle known as an autophagosome, which recruits cytoplasmic materials and fuses with lysosome for protein degradation. Autophagy is classified as macroautophagy, microautophagy, and chaperon-mediated autophagy, depending on how it delivers the cargo to lysosome and its physiological function. Aberrant autophagic activities lead to the pathogenesis of various diseases, including diabetes, neurodegeneration, heart disease, and cancers. Tian et al., reported that autophagy was detected in brain sections of a GFP-LC3 transgenic mouse model after transient cerebral ischemia and demonstrated a relationship between autophagy and apoptosis[4, 5]. Dengue virus infection induces apoptosis in various cell lines and clinical patient specimens. However, dengue virus-induced apoptosis and its relationship with autophagy remain to be determined.
Beclin1 serves as a platform to recruit other regulatory molecules of C3-PI3K complex, including Atg14-like protein (Atg14L), UV irradiation resistance-associated gene (UVRAG), Bax-interacting factor-1 (Bif-1) and activating molecule in Beclin-1-regulated autophagy protein-1 (Ambra-1)[7–10]. During vesicle elongation, two ubiquitin-like conjugation systems are activated. First, Atg12 is covalently conjugated with Atg5 by E1-like enzyme Atg7 and E2-like enzyme Atg10. Second, Atg5 binds to Atg16L1, a coiled-coil domain-containing protein, to form a heterotrimeric complex, Atg5-Atg12-Atg16L1. This complex is responsible for the expansion of the phagophore and is dissociated from the membrane when autophagosome formation is completed. Microtubule-associated protein 1A/1B light chain 3 (LC3) (mammalian homologue of yeast Atg8) is initially cleaved by Atg4, a cysteine protease, followed by phosphatidylethanolamine (PE) modification on the carboxyl terminus of the cleaved LC3. The lipidated LC3 located on the membrane facilitates autophagosome maturation. The autophagosome may fuse with the endosome to form the amphisome or with the lysosome to form the autophagolysosome[12–14].
Autophagy is also involved in the host immunity against pathogen infection. Autophagy acts as an anti-viral component of the innate immune system and is induced by the ligands of the toll-like receptors. Furthermore, autophagy enhances the presentation of viral antigens by dendritic cells during the infection of Sendai and vesicular stomatitis viruses. Autophagy can also function in the adaptive immune response by enhancing the presentation of antigen onto MHC class II molecules[18–20]. Autophagy not only plays an antiviral role, but also shows pro-viral functions[21, 22]. Poliovirus, coxsackievirus B3, hepatitis C virus (HCV), coronavirus, enterovirus 71 and DV activate autophagy to elevate viral replication[23–28]. HCV uses autophagy for the early protein translation and suppresses the innate antiviral immunity[23, 29]. The double membrane of the autophagosome may support poliovirus replication, and the autophagic machinery is utilized for the replication of coronaviruses[25, 31]. DV infection increases autophagic activity to enhance viral replication, indicating the use of autophagosome as the docking site for viral replication complex or as the organelle for lipid metabolism to provide ATP energy for DV replication[25, 32–35]. Autophagy induction by NS4A protein of DV prevents the infected cell from death and enhances viral replication. While it is known that autophagy plays an important role in DV replication in vitro, the role of autophagy in vivo has not been reported. This study focused on autophagic activity, virus titer and pathogenesis in DV2 infection of the suckling mice.
Dengue virus and mice
The DV2 (strain PL046) was routinely maintained in A. albopictus-derived cell line C6/36 (purchased from ATCC). Breeder mice of the ICR strain were purchased from the National Laboratory Animal Center, Taiwan. The mice were maintained at the Animal Facility of National Cheng Kung University, Taiwan, and were manipulated according to the animal experiment guidelines of the National Science Council, Taiwan. Six-day-old suckling mice were inoculated intracerebrally with 2.5×105 pfu of active or heat-inactivated DV2 or control Dulbecco’s Modified Eagle Medium (DMEM) (GIBCO BRL, Gaithersburg, MD, USA) containing 2% fetal bovine serum (FBS). The mice were sacrificed and perfused with isotonic saline containing EDTA. For plaque assay, the brain tissues were collected, weighed and homogenized in 1 ml of DMEM containing 2% FBS. The supernatant was collected by centrifugation at 8000 rpm for 15 min at 4°C and frozen at −70°C. For Western blot analysis, the brain tissues were homogenized with 1 ml of Radio-immunoprecipitation assay (RIPA) lysis buffer (50mM Tris, 150mM NaCl, 0.1% SDS, 0.5% sodium deoxycholate, 1% Triton X-100, PMSF, pH7-8) (Sigma-Aldrich, St. Louis, MO, USA). The supernatant was collected by centrifugation at 14000 rpm for 20 min at 4°C and frozen at −70°C. For IFA and IHC assays, the brain tissues were embedded in Tissue-Tek O. C. T. compound (Sankura Finetek, Torrance, CA, USA), frozen in liquid nitrogen and stored at −70°C. Serial coronal sections (5 μm) were cut on a cryostat (Leica CM1800, Heidelberg, Germany) and mounted on a silanized slide (Dako, Carpinteria, CA, USA).
BHK-21 cells were plated in a 12-well plate (9×104 cells/well) and cultured in DMEM (GIBCO). After adsorption for 2 h with serially diluted virus solutions, the solution was replaced with fresh DMEM containing 2% FBS and 0.8% methyl cellulose (Sigma-Aldrich). At four days post-infection, the medium was removed and the cells were fixed and stained with the crystal violet solution consisting of 1% crystal violet, 0.64% NaCl, and 2% formalin for 1 h at room temperature (RT). Finally, the crystal violet was removed and the plate was washed with the tap water. The viral titer was determined by the plaque assay.
Immunohistochemostry staining (IHC) and immunofluorescence assay (IFA)
Mice brain sections were fixed with acetone and blocked with 3% H2O2 (Merck, Darmstadt, Germany) in methanol. The sections were blocked with SuperBlock (SuperBlock blocking buffer in PBS; Thermo Scientific, Rockford, IL, USA) or Vector M.O.M mouse immunoglobulin G blocking reagent (Vector Laboratories, Burlingame, CA, USA) for 1 h at RT. The sections were further incubated with the anti-DV2-NS1 antibody (ab41632, Abcam, Cambridge, MA, USA), anti-autophagy LC3 antibody (AP1802a, Abgent, San Diego, CA, USA) or anti-Beclin 1 antibody (ab62472, Abcam), which was diluted in blocking buffer overnight at 4°C. The sections were further incubated with biotinylated secondary antibody (Dako, Glostrup, Denmark) for 1 h at RT and stained with AEC Substrate Chromogen (Dako). DAPI (4′-6-Diamidino-2-phenylindole; Sigma-Aldrich, St. Louis, MO, USA) was used to stain the nuclei of the brain cells. Subsequently, the slides were immersed in hematoxylin (Merck, Darmstadt, Germany) for counterstaining and then rinsed in tap water for 10 min. Finally, the slides were mounted with Dako Faramount Aqueous Mounting Medium (Dako).
For the immunofluorescence assay, the section after the primary antibody treatment was incubated with the secondary antibody conjugated with red (A11004, Invitrogen, Eugene, Oregon, USA) or green (A11008, Invitrogen) fluorescence. Subsequently, the slide was stained with Hoechst 33258 (Sigma-Aldrich). Finally, the sections were mounted and examined under a laser confocal scanning microscope (Olympus FluoView FV1000, CenterValley, PA, USA).
Transmission electron microscopy
DV2 (2.5 x 105 pfu/mouse) was intracranially injected into the brain of six-day-old ICR suckling mice. The clinical score and body weight were measured every day. Five mice were treated with live DV2, three were treated with heat inactive DV2 (iDV2), and three mice were treated with the culture medium. At five days post-infection, mice were sacrificed after anesthesia with 7% chloral hydrate followed by perfusion with 4% paraformaldehyde in 0.1M phosphate buffer (PB). After fixation, the cervical spinal cord was removed and kept in PB solution overnight at 4°C. The cervical cord was cut into 100 μm sections and fixed with 2.5% glutaraldehyde in 0.1 M cacodylate buffer (4% sucrose, 1 mM MgCl2 and 1 mM CaCl2) and post-fixed in 1% osmium tetroxide (Electron Microscopy Sciences, Hatfield, PA, USA). The sections were then dehydrated in a series dilutions of ethanol and embedded with LR White (Agar Scientific, Stansted, UK). Ultrathin sections were obtained using an ultramicrotome (Reichert-Jung, Heidelberg, Germany) and stained with saturated aqueous uranyl acetate (Electron Microscopy Sciences) and lead citrate (Electron Microscopy Sciences) at RT, and then investigated under a Hitachi H-7650 transmission electron microscope (Hitachi, Tokyo, Japan).
Western blot analysis
Cells were cultured and infected with DV2, and the total cell extracts harvested at various time points were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS PAGE). The separated proteins in the gel were electrically transferred to a PVDF membrane (Millipore, Bedford, MA, USA), followed by hybridization with their corresponding specific primary antibodies (anti-LC3: PM036, MBL, Woburn, MA, USA; anti-Beclin 1: sc-11427, Santa Cruz, CA, USA; anti-p62: Santa Cruz; anti-β-actin: A5441, Sigma-Aldrich) and the secondary antibodies (Goat anti-mouse IgG peroxidase conjugated Ab: AP124P and Goat anti-rabbit IgG HRP conjugated: AP132P, Chemicon, Billerica, MA, USA). After incubation with enhanced chemiluminescence (ECL) solution (Millipore) for 1 min, the membrane was exposed to an X-ray film (Eastman Kodak, NY, USA).
Data are presented as the mean ± standard deviation. Differences between the test and control groups were analyzed by the Student’s t test using the Prism software. Significance was set at p < 0.05 (*), p < 0.01 (**) and p < 0.05 (***).
Dengue virus type 2 infection of the ICR suckling mice causes physiopathological changes
Dengue virus induces amphisome and autophagosome formation as well as autophagic flux in the brain of infected mice
Regulation of autophagy affects DV-related pathogenesis of suckling mice during dengue virus infection
This study showed that in ICR suckling mice, DV2 can indeed infect the brain tissue at various regions detected by anti-DV-NS1 antibody at five days p.i. (Figure 1D), which is consistent with the results of our previous report that showed DV2 antigen was detected in the brain and liver of the infected mice. However, DV-NS1 antigen was not detected in the cerebellum and pons of the infected mice (Figure 1D). DV2 infection of the brain of 2-day-old Swiss mice induces apoptosis, and dengue virus antigen was detected in the cortical and hippocampal regions. Amaral et al. reported that NS3-positive cells could be visualized throughout the parenchyma including the cerebrum, brainstem, and cerebellum in the 6-week-old C57BL/6 mice. The discrepancy between our results and those reported in other studies may be attributed to differences in the strain of mouse or virus, age disparity of the mice, and inoculation titer of the viruses. Taken together, the findings described above indicate that dengue virus could infect various regions of the brain and cause disease symptoms. Our findings are consistent with those of previous reports[39, 40].
The results showed that DV2 infection of mouse brain induces autophagy as demonstrated by increased LC3-II protein expression in the brain tissues of DV2-infected mice (Figure 2B), LC3 protein aggregation, and colocalization of DV2-NS1 and LC3 in DV2-infected brain section (Figure 2A), as well as the double-membrane (autophagosome) vesicle formation under TEM (Figure 3C, 3D, and 3E), which was similar to the findings of our previous in vitro report. Fusion of autophagosome with endosome to form amphisome was also detected in dengue virus-infected mouse brain (Figure 4), which was consistent with the result of an in vitro investigation by Panyasrivanit et al. that showed that the endosome harboring dengue viruses fuses with the autophagosome to form amphisome, which serves as the docking site of the viral replication complex. Welsch et al. revealed that dengue virus modifies the endoplasmic reticulum (ER) membrane structure to promote its replication and efficient encapsidation of the genome into progeny virus under electron tomography. Miller and Krijnse-Locker reported that viral replication complexes form clusters around the double membrane vesicles, which are formed by contiguous invagination of the ER. The aforementioned reports indicate that DV may replicate at diverse locations, including the autophagosome membrane. Therefore, autophagy may play a role in enhancing viral replication. In this study, we demonstrated the formation of autophagosome and amphisome in vivo during DV infection (Figures 2,3 and4). Whether these vesicles are also involved in DV replication requires further confirmation.
Autophagy is a dynamic, multi-step process that can be modulated at several steps, both positively and negatively. An accumulation of autophagosomes (measured by TEM, as fluorescent GFP-LC3 dots, or as LC3II lipidation on a Western blot), could reflect either increased autophagosome formation due to increases in autophagic activity, or to reduced turnover of autophagosomes. The latter can occur with the inhibition of their maturation to amphisomes or autolysosomes, which happens if there are defects in the fusion with endosomes or lysosomes, respectively, or following inefficient degradation of the cargo once fusion has occurred. In our study, we demonstrated that the fusion of autophagosome and endosome occurred to form amphisome in vivo during DV infection together with p62 degradation, indicating that DV2 infection induces the autophagy flux in vivo (Figures 4 and5). This funding is consistent with our previous in vitro report.
It has been widely reported that the infection of many viruses affects autophagic flux. Coxsackievirus B3 (CVB3) induces the formation of autophagosomes without promoting lysosome-mediated protein degradation[27, 45]. In contrast, HIV-1, Influenza A virus, and HCV infection impair autophagic flux. HCV does not eliminate long-lived protein through autophagic degradation[45, 46]. We have demonstrated that DV2 infection triggers autophagic flux and DV2-induced autophagosome is favorable for viral replication in hepatoma cells. Panyasrivanit et al. further showed that DV2 titer is increased by blocking autophagic flux using fusion blocker L-asparagine, suggesting that the autophagic flux process decreases DV2 titer.
Some viral proteins regulating autophagic activity have been reported. Overexpression of the hepatitis B virus X gene (HBx) enhances starvation-induced autophagy through the upregulation of Beclin 1 expression. Poliovirus 2BC and 3A proteins regulate LC3 modification and membrane induction[30, 48]. Furthermore, DV2 NS4A protein induces LC3 cleavage and translocation in epithelial cells. However, whether DV2 NS4A alone in vivo is capable of inducing autophagy requires further investigation.
We also demonstrated that expression of Beclin 1 was not changed either in DV-infected or in mock-infected mice (Additional file1). This result is consistent with the finding of our previous in vitro study. Nevertheless, whether Beclin 1 participates in DV-mediated autophagy needs further investigation. We further demonstrated that manipulation of autophagy affects DV2 infection-related pathogenesis, including disease symptoms, survival rate, and virus titer by the autophagy inhibitor, 3-MA, which suppresses the PI3K class III signaling pathway and autophagic activity, and the autophagy inducer rapamycin, which blocks the mTOR signaling pathway. Although encephalitis and neuronal involvement in dengue virus infection are rare, this suckling mice model provides a unique system to clarify dengue-mediated autophagy in vivo. While the autophagy inhibitor 3-MA suppressed LC3II expression, it could not significantly suppress virus titer at days 3 and day 5 p.i. in vivo (Figures 5 and6A). It is known that 3-MA plays dual roles in autophagy. Under starvation conditions, 3-MA suppresses PI3K class III and inhibits autophagy; however, under normal conditions, 3-MA promotes autophagic flux. Therefore, the treatment conditions of 3-MA needed to be further optimized to increase its inhibitory effect on autophagy. In summary, manipulation of autophagy by 3-MA and rapamycin affects clinical scores, survival rate, as well as DV2 titer in vivo. A recent report demonstrated that DV-NS4A expression induces autophagosome formation during dengue virus infection, and promotes the infected cells to avoid apoptotic cell death, which thus contributes to viral replication. Therefore, whether suppression of DV-mediated autophagy by 3-MA induces infection-mediated apoptosis and further reduces the replication of dengue virus requires further confirmation.
We demonstrated herein that DV2 infection can induce autophagy both in vitro and in vivo. Furthermore, manipulation of autophagy by 3-MA or rapamycin affects the replication of DV2 and symptoms of dengue infection in vivo. Our results suggest that the DV2-related pathogenesis and survival rate of the suckling mice were enhanced by autophagy, possibly by promoting viral replication.
We thank P. Wilder for his critical reading of the manuscript. This work was supported by grants from National Science Council, Taiwan (NSC-99-2745-B-006-002 and NSC- 101-2321-B-006-029-) and Center of Infectious Disease and Signaling Research, NCKU, Tainan, Taiwan.
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