Coxiella burnetii modulates Beclin 1 and Bcl-2, preventing host cell apoptosis to generate a persistent bacterial infection

Vázquez, C L; Colombo, M I

doi:10.1038/cdd.2009.129

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Original Paper
Published: 02 October 2009

Coxiella burnetii modulates Beclin 1 and Bcl-2, preventing host cell apoptosis to generate a persistent bacterial infection

C L Vázquez¹ &
M I Colombo¹

Cell Death & Differentiation volume 17, pages 421–438 (2010)Cite this article

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Abstract

Coxiella burnetii is the etiological agent of the human disease, Q fever, and is an obligate intracellular bacterium that invades and multiplies in a vacuole with lysosomal characteristics. We have previously shown that Coxiella interacts with the autophagic pathway as a strategy for its survival and replication. In addition, recent studies have shown that Coxiella exerts anti-apoptotic activity to maintain the host cell viability, thus generating a persistent infection. In the present report, we have explored the role of Beclin 1 and Bcl-2 in C. burnetii infection to elucidate how this bacterium modulates autophagy and apoptosis to its own benefit. Beclin 1, a Bcl-2 interacting protein, is required for autophagy. In this study, we show that Beclin 1 is recruited to the Coxiella-membrane vacuole, favoring its development and bacterial replication. In contrast, the anti-apoptotic protein Bcl-2 alters the normal development of the Coxiella-replicative compartment, in spite of also being recruited to the vacuole membrane. Furthermore, both vacuole development and the anti-apoptotic effect of C. burnetii are affected by Beclin 1 depletion and by the expression of a Beclin 1 mutant defective in Bcl-2 binding. Overall, these findings indicate that C. burnetii infection modulates autophagy and apoptotic pathways through Beclin 1/Bcl-2 interplay to establish a successful infection in the host cell.

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Main

The macroautophagy pathway (hereafter, autophagy) is a physiological process, ubiquitous in eukaryotic cells. It is involved in the sequestration of cytoplasm or damaged organelles in double-membrane vesicles called autophagosomes. Subsequently, these vesicles fuse with late endosomes and lysosomes, which supply the hydrolytic enzymes, to generate the autolysosomes, in which the sequestered material is degraded.^{1, 2} Autophagy can be stimulated in response to cell stress, such as nutrient deprivation and pathogen invasion, or it may occur at a basal level operating as a cell quality control mechanism to eliminate aged organelles and unnecessary structures.³ Many Atg proteins (i.e. autophagy related) function during the different stages of autophagosome biogenesis.⁴ The early events in the autophagy pathway involve the complex formed by Atg6 (orthologous to mammalian Beclin 1), Atg14, Vps15 and the class III PI-3 kinase (hVps34), which are essential for the recruitment of other Atg proteins to the isolation membrane or phagophore (autophagosome precursors).^{5, 6, 7} Two ubiquitin-like modification systems, the Atg12–Atg5 complex and the Atg8 (also known as LC3), act in autophagosome formation.⁸ Initially, Atg12 is activated and conjugated to Atg5 to localize at the isolation membrane. Then, the Atg12–Atg5 complex is dissociated with the completion of the autophagic vesicle formation,^{9, 10, 11} for which the LC3 protein is also required. LC3 exists in two forms: the cytosolic form (LC3-I) and the lipidated form (LC3-II) targeted to the autophagosomal membrane. LC3 remains associated, at least in part, even after fusion with lysosomes, when LC3-II is delipidated and recycled back to the cytoplasm.^{12, 13}

The gene Beclin 1 is a haploinsufficient tumor suppressor in mice^{14, 15} and the human gene is monoallelically deleted in several cancers.¹⁶ Initially, Beclin 1 was identified as a Bcl-2 interacting protein¹⁷ and has been implicated as an important regulator of autophagy. The suppression of Beclin 1 expression impairs autophagy and sensitizes the cells to starvation-induced apoptosis.¹⁸ Beclin 1 has a BH3 domain by which it interacts with proteins of the Bcl-2 family, including Bcl-2 and Bcl-X_L, both of which are inhibitors of apoptosis.^{19, 20} It has been shown that the interaction between Bcl-2 and Beclin 1 leads to inhibition of autophagy by interfering with the formation and activity of the autophagy promoter complex, Beclin 1/hVps34.²¹

Multiple intracellular pathogens, such as Porphyromonas gingivalis,²² Brucella abortus²³ and Legionella pneumophila,²⁴ use the autophagy pathway to their own benefit. Coxiella burnetii (hereafter Cb) is a gram-negative bacterium that causes the Q fever in humans. It has a biphasic life cycle, involving the infectious particles (small cell variant (SCV)) and the metabolic and replicative active form (large cell variant (LCV)).^{25, 26} In addition, Cb presents a type IV secretion system (T4SS) that injects bacterial effectors into host cells. These bacterial molecules likely orchestrate host cell processes, allowing the establishment of a productive infection and intracellular survival of this microorganism.²⁷ Our group has shown that Cb survives and replicates in a large replicative vacuole (C. burnetii-replicative vacuoles (CRVs)) with clear autophagic features.²⁸ Besides, autophagy induction before infection, as well as the overexpression of proteins involved in this pathway (i.e. LC3 and Rab24), favors the development of the CRV at early post-infection (p.i.) times.²⁹ Moreover, we have shown that Cb invades host cells through a classical phagocytosis involving normal components of the endocytic/phagocytic pathway. In addition, this pathogen actively interacts with autophagosomes to delay the arrival of hydrolytic enzymes facilitating vacuole development.³⁰ The generation and maturation of the large CRV require a sustained bacterial protein synthesis.³¹

Cb has the ability to induce persistent infections in host cells;^{32, 33} therefore, recent studies have been designed to understand the strategies that Cb uses to control the fate of the host cell. It has been shown that Cb infection inhibits apoptosis in host cells, allowing bacterial survival and multiplication.^{34, 35} One of the proposed mechanisms by which Cb exerts its anti-apoptotic effect is by preventing the release of cytocrome c, thus avoiding the activation of executioner caspases.³⁵ Besides, a recent study has shown that Cb infection modulates the expression of apoptosis-related genes.³⁴

In view of these results, an interesting point to analyze is how Cb modulates both autophagy and apoptosis, two pathways that are intimately associated, to develop a successful infection. In this study, we show that Cb recruits Beclin 1 to the CRV and that the development of this vacuole is favored by the overexpressed Beclin 1. In contrast, overexpressed Bcl-2 alters the development of the normal CRV, in spite of also being recruited to the vacuole membrane. We propose that Cb modulates the interplay between Beclin 1 and Bcl-2 to exert its anti-apoptotic activity.

Results

Beclin 1 decorates CRVs and favors its development

At 48 h p.i., Cb develops a large CRV that presents the features of a phagolysosomal vacuole.^{25, 32} To examine the role of Beclin 1 in Cb infection, cells were infected with Cb for 48 h and both the bacterium and Beclin 1 were detected. Interestingly, we observed the accumulation of endogenous Beclin 1 in the surrounding membrane of the CRV (Figure 1A). We have previously shown that LC3 also associates with the large CRV;²⁹ therefore, we next examined whether both Beclin 1 and LC3 were recruited to the same vacuole. Figure 1B clearly shows both endogenous Beclin 1 and GFP-LC3 decorating the large CRV. To examine in detail the distribution and function of Beclin 1 in Cb infection, we overexpressed this protein in HeLa cells. After a transient transfection with FLAG-Beclin 1, cells were infected with Cb for 24 or 48 h (Figure 1C). At 24 h p.i., panels a–c show several CRVs decorated with Beclin 1, and by 48 h (panels d–f) two typical large CRVs, also decorated by Beclin 1, are depicted, which probably were formed by the fusion of smaller ones. Figure 1D shows the quantification of the fluorescence intensity along the white arrow across the CRV depicted in Figure 1C, panel c. The graph shows the accumulation of Beclin 1 (red) at the surrounding CRV membrane.

We next assessed whether CRV development was affected by the overexpression of Beclin 1, by determining the parameters related to Cb infection, such as the number of infected cells, vacuole size and number of CRVs per cell. No differences were observed in the percentage of infected cells (cells that contain at least one intracellular bacterium) at 24 and 48 h in cells overexpressing Beclin 1, GFP-LC3 or GFP alone as a control (data not shown). However, at 6 h a significant increase was observed in the number of infected cells overexpressing Beclin 1 as well as GFP-LC3 compared with GFP (Supplementary Figure 1a). In contrast, by 12 h p.i. these differences were lost. Therefore, the overexpression of these proteins benefits the infection at earlier times, suggesting that the entry of the bacterium to transfected cells could be enhanced. In addition, we found a marked increase in the CRV number in cells overexpressing either LC3 or Beclin 1, both at 24 and 48 h, compared with cells overexpressing the vector GFP alone (Figures 1E and G). At 24 h, the diameter of CRVs in cells overexpressing Beclin 1 was also increased (Figure 1F). However, by 48 h the CRV diameter was similar in all cases (Figure 1H), indicating again that at later infection times these differences were lost. When the vacuole diameter data at 24 and 48 h were arranged in groups according to vacuoles size (Supplementary Figures 1b and c, respectively), we observed that in cells overexpressing Beclin 1 the amount of larger vacuoles was increased. These results indicate that, similar to LC3, overexpressed Beclin 1 favors CRV development at early times after infection, and confirm that the compartment in which Cb resides is targeted by autophagic proteins.

The complex Beclin 1/hVps34 is recruited to the CRVs

We next analyzed the distribution and activity of the Beclin 1/hVps34 complex, using GFP-2xFYVE, a probe that labels PI3P, the PI-3 kinase product (Figure 2A). Our findings indicated that from early p.i. times, both Beclin 1 and GFP-FYVE were decorating the limiting membrane of small vacuoles (insets in panels a–d) and even the large CRVs generated at 24 and 48 h (panels e and f). GFP-FYVE was also found in small vesicles dispersed throughout the cytoplasm, which likely represent early endosomes/autophagosomes. In addition, the percentage of CRVs associated with both Beclin 1 and FYVE increased with the infection time, being maximal at 6 h (Figure 2B), and were clearly present even at 48 h. These results show that Beclin 1 colocalizes with the active product of the PI-3 kinase, suggesting an active engagement of this kinase during Cb infection even after prolonged times of infection. Furthermore, when the triple colocalization among Beclin 1, the FYVE domain and RFP-LC3 was analyzed, the association of the markers was maintained at all p.i. times analyzed, corroborating that the relationship between Beclin 1 and the FYVE domain was indicative of a functional autophagic activity at the CRV.

To further corroborate that autophagy was indeed induced by Cb infection, we analyzed the processing of LC3 using western blot. As shown in Figure 2C, upon infection with this bacterium, a marked increase in the LC3-II form was observed, confirming the autophagy induction.

Suppression of Beclin 1 affects the normal CRV development

To determine whether Beclin 1 is actually required for the normal development of the CRVs, we analyzed whether a decrease in Beclin 1 expression had an effect on Cb infection. We used a plasmid encoding a small interference RNA against Beclin 1 (pSuper Beclin 1-KD, referred to as Beclin 1-KD), which causes the knockdown of transfected cells after 48 h. Cells were first infected with Cb, then they were transfected with the indicated plasmids (see the diagram in Figure 3A) and incubated for 48 h to diminish Beclin 1 levels. The expression levels of Beclin 1 in individual cells were detected with a specific antibody. As expected, in control cells (Figure 3B, panels a–c), endogenous Beclin 1 was recruited to the CRVs. However, in cells transfected with Beclin 1-KD, a very low detection of endogenous Beclin 1 was observed (panels d–f), corroborating the suppression of the protein expression. The number of CRVs in the co-transfected cells with Beclin 1-KD and GFP decreased when compared with cells overexpressing Beclin 1, but not with the control GFP (Figure 3C). However, we observed a significant decrease in vacuole size in cells transfected with Beclin 1-KD versus the control GFP (Figure 3D). These results suggest that Beclin 1 suppression disturbs the establishment of a normal CRV.

The no alterations in vesicle size observed in Figure 3C (comparing Beclin 1-KD with the control GFP) may be a consequence of a partial depletion in the endogenous Beclin 1 protein. Therefore, to achieve a more efficient knockdown of the protein, we used a different experimental strategy. Cells were first transfected twice (double-hit protocol) with the Beclin 1-KD plasmid before Cb infection (see Materials and Methods and the diagram in Figure 3E). Using western blot analysis, we confirmed that double transfection with the Beclin 1-KD plasmid resulted in the knockdown of Beclin 1 (Figure 3F), and that this double-hit protocol was more efficient in depleting endogenous Beclin 1 than the simple one (not shown). For the last 24 h, cells were infected with Cb. Figure 3G shows infected cells overexpressing GFP (control) or Beclin 1-KD and GFP, wherein Beclin 1 was detected with a specific antibody. In the control cells, Beclin 1 was recruited to the small CRVs (panels a–c, white arrows). As expected, cells overexpressing Beclin 1-KD (cell marked with an asterisk, panels d–f), showed a low detection of the endogenous protein. With this experimental approach, we observed a significant decrease in both the number of CRVs and vacuole diameter in cells co-transfected with Beclin 1-KD and GFP (Figures 3H and I).

Previously published work indicates that the knockdown of Beclin 1 leads to autophagy inhibition.²¹ Therefore, in order to address whether Beclin 1 depletion also inhibited autophagy in our system, cells were subjected to the double-hit protocol, but were co-transfected with GFP-LC3 to analyze the number of autophagosomes. Cells were subsequently incubated in full nutrient media (Ctrl) or under starvation conditions (Stv) to induce autophagy. As shown in Figure 3J, the number of GFP-LC3-positive dots was significantly reduced in Beclin 1-KD-overexpressing cells.

The beneficial effect of Beclin 1 in CRV development is not exerted by a Beclin 1 mutant unable to interact with Bcl-2

Previous studies have shown that Beclin 1 interacts with the anti-apoptotic protein Bcl-2 and this interaction is important for autophagy regulation.²¹ Therefore, we next explored whether the infection with Cb was affected by the overexpression of a mutant of Beclin 1 (Beclin 1-BD), which is defective in Bcl-2 binding.²¹ Figure 4A (panels a–c) shows Beclin 1-WT decorating the CRV membrane. In cells overexpressing Beclin 1-BD (panels d–f), the protein was also recruited to the CRVs. A quantitative analysis of the number of CRVs per cell and vacuole diameter showed a significant reduction in both parameters, in cells overexpressing Beclin 1-BD compared with Beclin 1-WT (Figures 4B and C). However, we did not detect differences among cells transfected with Beclin 1 mutant and cells overexpressing GFP or GFP-Rab11 as controls.

We next quantified the effect of the expression of Beclin 1-BD in Cb infection at earlier times after infection compared with vector-transfected cells. We observed that, as shown before, whereas in Beclin 1-WT-overexpressing cells the percentage of infection significantly increased compared with GFP-overexpressing cells (mainly at 6 h p.i.), overexpression of the Beclin 1 mutant markedly decreased the percentage of infected cells at both 6 and 12 h p.i. (Figures 4D). These results indicate that at early p.i. times, the CRV development is affected by the expression of this mutated form of Beclin 1, which is unable to interact with Bcl-2. In addition, the beneficial effect of Beclin 1 in Cb infection is apparently affected by the lack of communication between Beclin 1 and Bcl-2.

Bcl-2 colocalizes with the CRVs but alters the normal development of these vacuoles, without affecting the recruitment of Beclin 1 and LC3

Beclin 1 interacts with Bcl-2 through a BH3 domain that is present in Beclin 1.^{19, 20} This interaction is reduced when autophagy is stimulated by nutrient deprivation, suggesting that the dissociation of the Beclin 1–Bcl-2 complex is important for autophagy activation.²¹ In this study, we have shown that an adequate expression of Beclin 1 is required for the establishment of the large CRV; therefore, we analyzed whether Bcl-2 also has a role in Cb infection. Figure 5A shows a cell overexpressing GFP-Bcl-2, which decorates the membrane of several CRVs. Panel d shows the graph of the fluorescence intensity quantification along the white arrow across the CRV depicted in the inset, confirming the accumulation of Bcl-2 at the surrounding membrane. No differences were observed in the percentage of infected cells overexpressing GFP-Bcl-2 at 24 h p.i. (Figure 5B). However, a significant increase in the number of CRVs was detected in cells overexpressing GFP-Bcl-2 (Figure 5C), whereas the vacuole size was markedly reduced (Figure 5D, panel a). A distribution histogram of the vacuole size shows an increase in the small vacuole population in cells overexpressing Bcl-2 (Figure 5D, panel b).

To exclude the possibility that only overexpressed Bcl-2 was recruited to the membrane vacuole, the endogenous protein was detected. As shown in Figure 5E, endogenous Bcl-2 was also found decorating the CRV limiting membrane. These results indicate that Bcl-2 is targeted to CRVs and that overexpression of this protein leads to the formation of numerous CRVs, but of smaller size than in control cells.

We next evaluated whether either LC3 or Beclin 1 was also associated with the CRVs in the cells overexpressing GFP-Bcl-2. Figure 5F shows that both GFP-Bcl-2 and endogenous LC3 were accumulated at the CRV membrane. In addition, the images depicted in Figure 5G, panels a–d, show the recruitment of GFP-Bcl-2 and endogenous Beclin 1 to the same CRV. Figure 5H shows the graph of the quantification of the fluorescence intensity along the white arrow across the CRV depicted in panel e, demonstrating the recruitment of both proteins at the surrounding membrane. Confocal microscopy analysis (Figure 5I) of midplane Z-sections through the center of the vacuole (see white line 2, in panel e) revealed that both proteins were indeed decorating the vacuole. The distribution pattern of endogenous Beclin 1 and LC3 suggests that despite the overexpression of Bcl-2 altered the normal establishment of the CRV, the autophagic proteins were still recruited.

As, in vivo, Cb has a tropism for the mononuclear phagocyte system, we also explored the distribution of either overexpressed Beclin 1-WT or GFP-Bcl-2 in a macrophage cell line (RAW cells) infected with Cb. Supplementary Figure 2, panel a, shows infected RAW cells with a marked recruitment of Beclin 1-WT (arrows in panel f) or GFP-Bcl-2 (arrows in panel i) to the CRVs generated after 24 h p.i. In addition, the number of CRVs after 24 and 48 h of infection was highly increased in cells overexpressing either Beclin 1-WT or GFP-Bcl-2 compared with control GFP (Supplementary Figure 2, panels b and c). Therefore, our results indicated that both Beclin 1-WT and GFP-Bcl-2 have similar effects in macrophages to the ones observed in HeLa cells.

As indicated before, the CRVs are typically described as phagolysosomal structures characterized by the presence of lysosomal enzymes.²⁵ We have previously shown that early after infection, Cb phagosomes delay the arrival of the lysosomal enzymes. In addition, we have also presented evidence that the interaction with the autophagic pathway contributes to this delay, likely favoring bacterial replication.³⁰ It is believed that this delay benefits the transition from the SCV to the LCV form, which is the metabolically active and replicative competent bacterium. Therefore, to examine whether the compartment in which Cb survives was affected by the overexpression of the studied proteins in the present report, we analyzed the presence of the lysosomal enzyme, cathepsin D. For this purpose, cells were transfected with Beclin 1-WT, Beclin 1-BD, Bcl-2 or with the GFP vector as a control, and cathepsin D was detected by indirect immunofluorescence at 6 or 12 h p.i. (Supplementary Figure 3a). The images in panel S3A clearly show that the CRVs generated by Cb infection contain the lysosomal enzyme and the bacteria. When the percentage of CRVs containing cathepsin D was determined, we observed that by 6 h p.i. 80% of the vacuoles were labeled by the enzyme, whereas in the cells overexpressing Beclin 1-WT this percentage was decreased (panel S3b). Interestingly, these differences were lost at 12 h p.i. (panel S3c). No variations were observed in cells overexpressing either Beclin 1-BD or Bcl-2 at any p.i. time studied. The results suggest that overexpression of the fully functional Beclin 1 temporarily delays the fusion with lysosomes, likely to promote bacterial replication. Therefore, we next analyzed whether Cb survival was affected by the overexpression of Beclin 1-WT and its mutant form, or by overexpressed Bcl-2. As shown in panel S3d, the overexpressed Beclin 1-WT notably enhanced Cb replication and viability as determined by an infectious focus-forming unit (FFU) assay. In contrast, overexpressed Bcl-2 or Beclin 1-BD failed to modify bacterial replication.

The biogenesis of the CRVs requires a proper relationship between Beclin 1 and Bcl-2

As our results indicate that overexpressed Bcl-2 increases the number of small CRVs and that Beclin 1-BD mutant alters the normal establishment of the CRVs, we analyzed whether the co-expression of Beclin 1-BD and GFP-Bcl-2 had an effect on CRV development. Figure 6A (panels a–c) shows an infected cell (left) co-transfected with Beclin 1-BD and GFP-Bcl-2 with an evident recruitment of both proteins to the vacuole membrane, and a cell (right) just transfected with GFP-Bcl-2. As previously shown in Figure 5C, overexpression of Bcl-2 highly increased the number of CRVs (Figure 6B, light blue bar) and this effect was not reverted by co-expression of the Beclin 1 mutant, unable to bind Bcl-2 (Figure 6B, violet bar). In contrast, the size of the vacuoles was not affected by overexpression of the different plasmids after 48 h of infection, indicating that the Bcl-2 effect on vacuole size is transient (Figure 6C).

In order to analyze these observations further, we used the plasmid Beclin 1-KD, which causes the knockdown of Beclin 1. We used the same experimental approach indicated in Figure 3A; the infected cells were transfected with the indicated plasmids and incubated for additional 48 h to suppress Beclin 1 expression. Figure 6D (panels d–f) shows cells co-transfected with GFP-Bcl-2 and Beclin 1-KD. In cells marked with asterisks (panel e), in which Beclin 1 depletion is clearly observed, overexpressed GFP-Bcl-2 colocalized with the CRV membrane (inset panels e and f), such as in cells in which endogenous Beclin 1 was not depleted (panels a–c). The number of CRVs per cell was reduced in cells co-expressing Bcl-2 and Beclin 1-KD compared with cells overexpressing either Bcl-2 or GFP (Figure 6E). However, overexpression of Bcl-2 reverted the vacuole diameter alteration caused by Beclin 1 depletion (Figure 6F, compare blue and black bars). Taken together, these results indicate that the recruitment of Bcl-2 to the vacuole membrane is independent on the Beclin 1 presence. In addition, a proper ratio between Bcl-2 and Beclin 1 is required for the normal development of the CRVs.

The interplay between Beclin 1 and Bcl-2 is required for the anti-apoptotic effect of Cb

Cumulative evidence indicates that the apoptotic and autophagic pathways are interconnected.³⁶ Besides, the interaction of Beclin 1 and Bcl-2 has been implicated in the regulation of this crosstalk.³⁷ Recent results have shown that Cb infection is able to inhibit the apoptosis induced by UV light and staurosporine in host cells.^{34, 35} As Cb infection is affected by Beclin 1 and Bcl-2, we wondered whether overexpression of these proteins would have an effect on the inhibition of apoptosis by Cb. Supplementary Figure 4a shows the basal percentage of the apoptotic nuclei of infected cells, transfected with the different plasmids treated or not with chloramphenicol (Ch), but unexposed to UV. Supplementary Figure 4b shows the anti-apoptotic effect of Cb, indicated by the significant difference observed between uninfected and infected cells overexpressing GFP and UV irradiated. Consistent with published observations,^{34, 35} the apoptotic nuclei significantly increased in infected cells treated with Ch, showing that the anti-apoptotic function of Cb requires bacterial protein synthesis. The percentage of apoptotic nuclei in cells overexpressing GFP-Bcl-2 (Figure 7a, light blue bars), infected with Cb untreated or treated with Ch (bars 5 and 6) was similar to the infected cells overexpressing GFP (green bar 2), indicating that, as expected, overexpressed Bcl-2 has a protective role in the survival of the host cell. Interestingly, the anti-apoptotic effect of Bcl-2 was not affected by bacterial infection (compare light blue bars 4 and 5), suggesting that the Cb pro-survival activity is likely through Bcl-2. In contrast, no major differences were observed in cells overexpressing Beclin 1-WT (blue bars 7–9 compared with green bars 1–3), whereas in cells overexpressing Beclin 1-BD a marked increase of apoptosis in infected cells, untreated with Ch (red bar 11), was observed (compare bars 2, 8 and 11). This latter result indicates that in cells overexpressing the Beclin 1 mutant, the protective effect of Cb is lost. To confirm that the type of death was indeed apoptotic, we incubated the cells with Z-VAD-fmk (apoptosis inhibitor) for 2 h before UV irradiation. Supplementary Figure 4c shows that Z-VAD-fmk reduced apoptotic cell mortality in all cases. In addition, we corroborated these results using a TUNEL assay to detect apoptotic nuclei in cells overexpressing RFP (as control), Beclin 1-WT or Beclin 1-BD (Supplementary Figure 5). With this detection method the results obtained were very similar to the ones observed when the apoptotic nuclei were determined using Hoechst staining (compare Supplementary Figure 5b with Figure 7a).

We next assessed whether Cb was able to prevent cell death when Beclin 1 expression was reduced (Figure 7b). Infected and uninfected cells were transfected with GFP (green bars 1–3), GFP-Bcl-2 (light blue bars 4–6) or co-transfected with GFP-Bcl-2 and Beclin 1-WT (blue bars 7–9) or BD mutant (red bars 16–18). To suppress Beclin 1, cells were also co-transfected with Beclin 1-KD and GFP (yellow bars 10–12), or with Beclin 1-KD and GFP-Bcl-2 (grey bars 13–15), and then exposed to UV light. When Beclin 1-WT and GFP-Bcl-2 were co-expressed, the number of apoptotic cells was similar to the cells overexpressing GFP-Bcl-2 alone (compare light blue and blue bars). Interestingly, the apoptotic nuclei highly increased in infected cells depleted of Beclin 1 overexpressing either GFP or GFP-Bcl-2, compared with cells overexpressing only GFP-Bcl-2 (Figure 7b, compare hatch bar 5 with 11 and 14). In addition, the mutant protein of Beclin 1, which was unable to interact with Bcl-2, not only prevented the anti-apoptotic effect of Bcl-2, but also the one induced by Cb infection (compare bars 4 with 16 and 5 with 17, respectively). Similarly, the anti-apoptotic effect of overexpressed Bcl-2 was lost in Beclin 1-depleted cells (compare uninfected bars 4 and 13). These results support the idea that a proper relationship between Beclin 1 and Bcl-2 is necessary for the inhibition of apoptosis by Cb. Furthermore, these findings suggest that the anti-apoptotic effect of Bcl-2 requires the presence of a fully functional Beclin 1.

Discussion

To date, very little is known about the role of Beclin 1 in pathogen infections. In this study, we present evidence that Cb infection induces the recruitment of endogenous and overexpressed Beclin 1 to the large CRV, even at extended p.i. times (Figure 1). These observations are consistent with our previous results that two other proteins involved in autophagy, LC3 and Rab24, decorate the CRV.²⁹ In addition, we observed that both Beclin 1 and LC3 were clearly decorating the same vacuole membrane. Interestingly, our data show that the development of CRVs was favored by Beclin 1 expression, indicated by the increase in the number and size of CRVs in cells overexpressing Beclin 1 (Figure 1). These findings support the concept that Cb exploits autophagy to its own benefit and that overexpression of proteins involved in this process are enriched at the CRV membrane. It is noteworthy the presence of Beclin 1 in a compartment with the phagolysosomal characteristics of CRVs, given that Beclin 1 is required at early events, as the nucleation of pre-autophagosomal structures.⁵ We have observed colocalization of endogenous Beclin 1 with Rab7 at the membrane of the CRVs at 48 h after infection, consistent with the late/lysosomal nature of this compartment (data not shown).

Interestingly, using the probe GFP-2xFYVE that binds to PI3P, the PI3-kinase product, our results indicate that the Beclin 1/hVps34 complex recruited to the CRV membrane was functionally active at all times analyzed (Figure 2). These data suggest an active engagement of the autophagic and endocytic/phagocytic pathways during Cb infection, in order to thrive on the harsh environment in which this bacterium survives. A recent publication³⁸ shows that Toll-like-receptor (TLR) signaling induces the association of autophagic proteins to the phagosome, apparently without autophagosome formation, enhancing the fusion with lysosomes and, subsequently, leading to the degradation of the engulfed organism. As Cb actually replicates in a lysosomal-like compartment, our earlier^{29, 30} and present data suggest that the recruited autophagic proteins are effectively and efficiently used to promote Cb survival. Indeed, we have observed that in cells overexpressing Beclin 1, there was a delay in the arrival of the lysosomal enzyme, cathepsin D (Supplementary Figure 3b), which likely favors bacterial growth. In addition, the results presented in this report indicate that at early p.i. times, overexpression of Beclin 1 enhances the percentage of Cb-infected cells (Supplementary Figure 1a) and promotes bacterial replication (Supplementary Figure 3d).

In addition, we have observed that the knockdown of Beclin 1 leads to a reduction in the number and size of the generated CRVs (Figure 3). These results indicate that Beclin 1 is required for the normal establishment of CRVs, confirming the positive effect of autophagy in Cb infection, as a reduced number of LC3-positive dots was observed in Beclin 1-suppressed cells (Figure 3J), corroborating that reduced levels of Beclin 1 by siRNA lead to an inhibition of autophagy.¹⁸

As indicated above, Beclin 1 promotes autophagy by acting as a platform for the formation of the complex with hVps34 and several interacting proteins that positively regulate autophagy. Moreover, Beclin 1 possess a BH3 domain that allows the interaction with the anti-apoptotic proteins, Bcl-2 and Bcl-X_L.^{19, 20} In this report, we have observed that both endogenous and overexpressed Bcl-2 were clearly decorating the membrane of the CRV (Figures 5A–E). In addition, the overexpressed Bcl-2 increased the number of CRVs but the vacuole size was reduced (Figures 5C and D). However, at 48 h of infection the vacuole size was comparable with the control GFP (Figure 6C), suggesting that excess Bcl-2 somehow alters CRV development early at infection. This would be consistent with the observation that overexpressed Bcl-2 interferes with the formation of the Beclin 1/hVps34 complex.²¹ Indeed, the colocalization of Bcl-2 with the CRV membrane was unexpected, considering that no previous reports have indicated the recruitment of Bcl-2 to a pathogen-containing compartment. It is intriguing that the CRV was decorated not only by GFP-Bcl-2, but also by endogenous Beclin 1 and LC3 (Figures 5F–I), strongly suggesting an active engagement with autophagy. These results may seem paradoxical because, as indicated above, Bcl-2 is capable of inhibiting starvation-induced autophagy.²¹

We have analyzed the development of CRVs in cells transfected with a mutant of Beclin 1 (Beclin 1-BD), which is defective in Bcl-2 binding. Our data showed that Beclin 1-BD was indeed recruited to the membrane of the CRV, but it decreased the percentage of infection (Figures 4A and D, respectively). Pattingre et al.²¹ showed that Beclin 1-BD induces excessive levels of autophagy in nutrient-rich conditions, leading to autophagic cell death, suggesting that the Bcl-2 interaction with Beclin 1 is crucial to avoid inappropriate autophagy activation. Indeed, Gottlieb and collaborators³⁹ showed that under rich- and low-nutrient conditions, the Beclin 1-BD expression impairs the normal autophagic flux, preventing the autolysosomal clearance rather than increasing autophagy. This suggests that Beclin 1-Bcl-2 interaction is required to promote autophagy. Given that autophagy benefits Cb infection and bacterial replication, the altered infection pattern observed in Beclin 1-BD-expressing cells can be explained by a defective autophagic pathway. Interestingly, in infected cells co-transfected with GFP-Bcl-2 and Beclin 1-KD (to deplete the endogenous protein), we have observed that GFP-Bcl-2 was present on the CRV membrane even in cells in which Beclin 1 was suppressed. However, the number of CRVs in these cells decreased (Figures 6D–F). Therefore, despite that either Beclin 1 or Bcl-2 are independently recruited to the vacuole membrane, we propose that both proteins are necessary for the proper establishment of the CRV, likely by the regulation of adequate autophagy levels and normal flux.

Several pathogens have developed different strategies to govern the fate of the host cells. For example, Chlamydia trachomatis and Toxoplasma gondii are known to possess anti-apoptotic activity.^{40, 41} Recently, it has been shown that a substrate (SidF) of the Dot/Icm T4SS is involved in preventing apoptosis in infected cells with Legionella pneumophila. Apparently, SidF would abolish the activities of two pro-apoptotic proteins members of the Bcl-2 family: BNIP3 and Bcl-rambo.⁴² Similarly, recent studies show that Cb inhibits apoptosis induction in host cells and this effect is mediated by the bacterial protein synthesis.^{34, 35} Cumulative evidence indicates that apoptosis and autophagy are interconnected pathways, implying that a common stimulus triggers the response of both pathways or that the activation of one of them excludes the other.³⁶ Consequently, we explored a possible role for Beclin 1 and Bcl-2 in the anti-apoptotic effect of Cb. Our results reveled that Cb infection protects host cells overexpressing GFP, as well as Beclin 1-WT, of apoptotic cell death and this effect was dependent on an active bacterial protein synthesis (Figure 7a). It is noteworthy that in infected cells overexpressing GFP-Bcl-2, the number of apoptotic nuclei was similar to the control (GFP cells); however, treatment of the cells with Ch did not yield differences compared with untreated cells (Figure 7a), indicating that overexpressed Bcl-2 compensates the anti-apoptotic activity of Cb having a protective role in the survival of the host cell. On the other hand, a marked increase in apoptotic nuclei was observed in infected cells overexpressing Beclin 1-BD, indicating that the anti-apoptotic effect of Cb is abolished by the presence of the Beclin 1-BD mutant (Figure 7a). Altogether, these results suggest that the anti-apoptotic activity of Cb requires an appropriate interplay between Beclin 1 and Bcl-2. Therefore, it is tempting to speculate that a protein(s) synthesized by Cb or effectors of the Cb-T4SS are necessary to maintain the link between Beclin 1 and Bcl-2, to induce autophagy and prevent apoptosis, preserving both the integrity and the metabolic activities of the infected cell and allowing the normal development of the CRV. Thus, the results of this study in the Cb-infected cells seem somehow contradictory to previous results showing that overexpressed Bcl-2 inhibits starvation-induced autophagy.²¹ However, another report has indicated that overexpressed Bcl-2 or Bcl-X_L increases autophagy in MEF cells Bax/Bak^−/− treated with etoposide.⁴³ Consequently, it is probable that the regulatory effect of the Beclin 1-Bcl-2 complex depends on the nature of the stimulus; in our case, it is tempting to speculate that Cb would be modulating the activity and the relationship between both proteins for its own benefit and survival.

To support our model, we suppressed the endogenous Beclin 1 (Beclin 1-KD) and overexpressed GFP or GFP-Bcl-2 in the same infected cells to subsequently evaluate apoptosis. As expected, a marked increase in apoptotic cell death was observed in infected cells with Beclin 1 depleted (Figure 7b, yellow bars). However, the anti-apoptotic effect of Cb was also lost in cells doubly transfected with GFP-Bcl-2 and Beclin 1-KD (Figure 7b, grey bars). In addition, cells co-transfected with GFP-Bcl-2 and Beclin 1-BD also presented a marked increased in the apoptotic nuclei (red bars). Thus, these results confirm our hypothesis that the intracellular survival of Cb and the persistence of this bacterium in the host cell require a proper interplay between Beclin 1 and Bcl-2. Interestingly, when cell death was determined in uninfected cells, overexpression of Bcl-2 was unable to protect cells from apoptosis in Beclin 1-supressed cells or in cells co-expressing the Beclin 1 mutant unable to interact with Bcl-2 (compare bars 4, 13 and 16 in Figure 7b), suggesting that Beclin 1 would be modulating the anti-apoptotic action of Bcl-2. This notion is supported by results obtained in myeloid leukemia cells, in which the knockdown of Beclin 1 activates apoptosis.⁴⁴ These researchers have shown that Beclin 1 acts as a negative regulator of apoptosis because it competes with the pro-apoptotic protein Bad, for binding to the anti-apoptotic protein Bcl-X_L, implying that the complex Beclin 1-Bcl-X_L has indeed an anti-apoptotic role.⁴⁴ In addition, it has recently been reported that dexamethasone induces autophagy in murine T-cell lymphoma line. Furthermore, this autophagy induction is enhanced by Bcl-2 overexpression, apparently because of the inhibition of dexamethasone-induced apoptosis.⁴⁵ Also, a recent publication shows that overexpressed Beclin 1 does not prevent the anti-apoptotic effect of Bcl-2.⁴⁶ This is consistent with the results that we have obtained in cells co-expressing Bcl-2 and Beclin 1-WT (see Figure 7b, blue bars). Thus, it is likely that the interplay between apoptosis and autophagy would be dependent on the different stimulus, in order to lead to survival or cell death, according to the desired beneficial effect.

According to the data discussed above, we propose a working model to explain the behavior of Cb during infection (Figure 8). Cb is internalized by phagocytosis but induces autophagy in the host cell. At very early p.i. times, the phagosome fuses with autophagosomes, recruiting LC3 and the Beclin 1/hVps34 complex. This interaction with the autophagic pathway delays the arrival of the lysosomal enzymes, allowing the residence of Cb in a less harmful compartment and the transition from SCV to LCV.^{29, 30} Then, the phagosomes fuse with late endosome/lysosomes, generating the large CRV that occupies almost the whole cell, in which Cb survives. However, the establishment of this infection does not hamper host cell viability. Thus, Cb is able to produce a persistent infection by means of inhibiting apoptosis.^{34, 35} Interestingly, we have observed that in Cb-infected cells numerous mitochondria are in close apposition to the CRV (Colombo MI et al. unpublished observation). This intimate association suggests that the bacteria may actively manipulate critical mitochondria functions, such as the intrinsic apoptotic pathway. Therefore, we propose that the anti-apoptotic effect of Cb is due, at least in part, to the recruitment of Bcl-2 to the vacuole membrane and to the modulation of Bcl-2 by Beclin 1. Once the Cb infection has been settled, a fraction of Beclin 1 and Bcl-2 would be interacting at the vacuole membrane and probably at the outer mitochondria membrane, preventing the binding of sensitizer pro-apoptotic proteins (e.g. BH3-only proteins) to Bcl-2 and thus inhibiting the activation of Bax/Bak and the release of cytochrome c. Recent evidence has shown that Cb leads to a sustained activation of Akt and Erk1/2 pathways.⁴⁷ It is known that Akt is a pro-survival kinase that inhibits activation of pro-apoptotic mitochondrial proteins and transcription factors.⁴⁸ Consistent with this finding, it is likely that the protein(s) secreted by the Cb-T4SS might be inhibiting a BH3-only protein activity and therefore the binding of this protein to Bcl-2. Thus, Beclin 1 is a crucial regulator of autophagy and apoptosis in infected cells, because of its role as a member of the Beclin 1/Vps34 complex and its participation in the anti-apoptotic complex Beclin 1-Bcl-2, competing with Bad for the binding to Bcl-2.⁴⁴

To summarize, the data presented here show that Cb infection is favored by overexpressed Beclin 1 and that depletion of this protein, as well as of the overexpressed Bcl-2, alters the normal development of CRVs. Furthermore, our results suggest the existence of a functional interaction between the autophagic and apoptotic pathways in infected cells, to avoid apoptosis induction and to trigger the autophagy pathway, allowing the survival and development of Cb. In addition, the present report emphasizes the importance of exploiting the infection with a microorganism to study a complex process, which allowed us to show a key role of Beclin 1 and Bcl-2 in modulating the fate of the cells.

Materials and Methods

Materials

DMEM medium was obtained from Gibco (Invitrogen, Buenos Aires, Argentina) and fetal bovine serum (FBS) from Natocor (Córdoba, Argentina). Plasmids pSG5-FLAG- epitope-tagged human Beclin 1 and pCR3.1 Beclin 1-BD were kindly provided by Dr. Beth Levine (Columbia University, New York, USA). pEGFP-LC3 was generously provided by Dr. Noboru Mizushima (Department of Physiology and Cell Biology, Tokyo Medical and Dental University, Tokyo, Japan) and Dr. Tamotsu Yoshimori (Department of Cellular Regulation, Research Institute for Microbial Diseases, Osaka University, Suita, Osaka, Japan). pGFP-Bcl-2 was provided by Dr. Michel Rabinovitch (Escola Paulista de Medicina, Sao Paulo, Brazil) and pGFP-FYVE by Dr. Sergio Grinstein (Hospital of Sick's Kids, Toronto, Canada). pBeclin 1-KD (pSuper.retro.puro vector in which the oligonucleotide sequence used for siRNA interference with Beclin1 expression has been inserted) was provided by Dr. William S Maltese (Medical University of Ohio, Toledo, Ohio, USA). The antibodies used in this work were rabbit polyclonal anti-Coxiella, generously provided by Dr. Robert Heinzen (Rocky Mountain Laboratories, NIH, MT, USA), and monoclonal mouse anti-LC3 developed in our lab. Beclin 1 was detected with goat anti-Beclin 1 (Santa Cruz Biotechnology, Inc., CA, USA) and an antibody against FLAG (Sigma, Buenos Aires, Argentina). Endogenous Bcl-2 was detected with a rabbit anti-Bcl-2 (Chemicon International, CA, USA). Rabbit anti-cathepsin D was kindly provided by Dr. Maximiliano Gutierrez (Helmholtz Centre for Infection Research, Germany). Mouse anti-Actin was from BD (Franklin Lakes, NJ, USA). The secondary antibodies were donkey anti-rabbit conjugated to Cy5, donkey anti-goat conjugated to Cy3, donkey anti-rabbit conjugated to HRP, donkey anti-goat conjugated to HRP and donkey anti-mouse conjugated to HRP (Jackson ImmunoResearch Laboratories, PA, USA). Other secondary antibodies used were goat anti-rabbit conjugated to Texas red and goat anti-mouse conjugated to Cy3 from Amersham Biosciences Argentina SA (Buenos Aires, Argentina). The secondary antibody goat anti-rabbit conjugated to Alexa Fluor 488 and the DNA markers, Hoechst 33258 and TOPRO, were obtained from Molecular Probes (Invitrogen). Z-VAD-fmk, the Earle's balanced salts solution, EBSS (starvation) media and vinblastine were from Sigma. TUNEL assay was from Promega (Madison, WI, USA).

Cell culture and propagation of Cb phase II

HeLa cells were maintained in DMEM 10% FBS (full nutrient medium) at 37 °C in an atmosphere of 95% air and 5% CO₂.

Clone 4 phase II Nine Mile strain of Cb bacteria, which are infective for cells but not for mammals, were provided by Ted Hackstadt (Rocky Mountain Laboratories). Non-confluent Vero cells were cultured in T75 flasks at 37 °C in DMEM medium supplemented with 10% FBS, 0.22 g/l sodium bicarbonate and 20 mM Hepes, pH 7 (infection media). Cultures were infected with phase II Cb suspensions for 7 days at 37 °C. Afterwards, cells were lysed, scrapped and centrifuged at 800 g for 10 min at 4 °C to eliminate cellular debris. The supernatants were centrifuged at 24 000 g for 30 min at 4 °C, and pellets containing Cb were resuspended in DMEM, passed 20 times through a 27-gauge needle connected to a syringe, aliquoted and frozen at −70 °C.

Cell transfection

HeLa cells were transfected with the indicated plasmid (1 mg/ml) using LipofectAMINE 2000 (Invitrogen) as indicated by the manufacturer. The transfected cells were incubated for 24 h in DMEM supplemented with 10% FBS in 24-well plates at 80% confluence and then infected.

Infection of HeLa cells with Cb

HeLa cells grown in a T25 flask were washed three times with PBS and detached with trypsin-EDTA. After resuspending with infection media, cells were plated on coverslips distributed in 24-well plates. Subsequently, cells were transfected with the corresponding plasmids. For long infection times, 120 μl aliquot of Cb suspension was diluted with 12 ml of infection media, and 500 μl of the dilution was added per well. With this dilution an MOI of 10 was obtained.²⁹ Cells were centrifuged at 500 g for 10 min and incubated for different periods of time at 37 °C in an atmosphere of 95% air and 5% CO₂. Cells were fixed and analyzed using indirect immunofluorescence as described below.

Infection with Cb and Beclin 1 knockdown with the plasmid pBeclin 1-KD

HeLa cells were plated in 24-well plates at 80% confluence and then infected with Cb. A 200 μl aliquot of Cb suspension was diluted in 12 ml of infection media, and 0.5 ml of this dilution was added per well (an MOI of 50). Cells were centrifuged at 500 g for 10 min and incubated for 3 h at 37 °C. Afterwards, cells were washed three times with PBS to eliminate extracellular bacteria and transfected with pGFP (as a control) or co-transfected with pBeclin 1-KD and pGFP or with pBeclin 1-KD and pGFP-Bcl-2, in another set of experiments. Cells were incubated for 48 h at 37 °C to knockdown Beclin 1 expression and to allow the development of the CRVs. The cells were fixed and analyzed using indirect immunofluorescence as described below.

Double-hit protocol

HeLa cells were plated in six-well plates at 40% confluence and then transfected with GFP (control) or GFP and Beclin 1-KD. After 48 h, cells were transfected for a second time with the same plasmids. Cells were incubated for additional 48 h and infected with Cb during the last 24-h period. Next, cells were fixed and subjected to indirect immunofluorescence as described below. To check knockdown efficiency, transfected cells were processed to detect endogenous Beclin 1 using western blot assays (described below).

Autophagy assays

To check autophagy activity in Beclin 1-depleted cells, cells were transfected with either Beclin 1-KD and GFP-LC3 or GFP-LC3 by the double-hit protocol. Then, cells were subjected to starvation for 2 h to induce autophagy. Afterward, cells were fixed and analyzed using indirect immunofluorescence detecting endogenous Beclin 1 as described below. The number of GFP-LC3-positive dots per cell was determined in starved cells compared with the same cells but incubated with full nutrient media. The GFP-LC3-positive dots were scored using ImageJ software (NIH, Bethesda, MD, USA).

Indirect immunofluorescence

HeLa cells were fixed with 0.5 ml of 3% paraformaldehyde solution in PBS for 10 min at room temperature, washed with PBS and quenched with NH4Cl 50 mM in PBS for 20 min. Then, cells were permeabilized with 1% saponin in PBS containing 1% BSA (blocking buffer). After blocking, cells were incubated with the indicated primary antibodies for 1 h. The cells were washed three times in blocking buffer and subsequently incubated with the indicated fluorophore-conjugated secondary antibody for 1 h.

For double immunolabeling, primary antibodies against Cb phase II (1 : 800) and against FLAG (1 : 200) were used. Bound antibodies were detected by incubation with secondary antibodies: goat anti-rabbit conjugated with Alexa 488 (1 : 600) or goat anti-mouse Cy3 (1 : 800), respectively. Endogenous LC3 was detected with a monoclonal antibody developed in our lab (1 : 10). Endogenous Beclin 1 was detected with a goat anti-Beclin 1 (1 : 50). Endogenous Bcl-2 was detected with a rabbit anti-Bcl-2 (1 : 100). Endogenous cathepsin D was detected with a rabbit anti-cathepsin D (1 : 50). Coverslips were mounted on glass slides using Mowiol (Calbiochem-Merck, Darmstadt, Germany), Mowiol-Hoechst 33258 or Mowiol-TOPRO (Molecular Probes, Invitrogen) and examined using fluorescence confocal microscopy.

Fluorescence microscopy

HeLa cells transfected with the different plasmids were analyzed by confocal microscopy using a Nikon C1 Confocal Microscope System or a FV100 Olympus Confocal Microscope (Olympus, Tokyo, Japan). Images were processed using Adobe CS (Adobe Systems Inc., San Jose, CA, USA), ImageJ software, Metamorph Program, series 4.5 (Universal Images Corporation, Tokyo, Japan), EZ-C1 program (Nikon, Tokyo, Japan) and FV10-ASW program (Olympus, Japan). For apoptosis assays an inverted microscope (Nikon Eclipse 2000, Japan) and a FV100 Olympus Confocal Microscope (Olympus, Japan) were used. Images were obtained using a CCD camera (Orca I, Hamamatsu, Hamamatsu City, Japan) and processed using the Metamorph Program, series 4.5 (Universal Images Corporation).

Measurements of the percentage of infected cells, number and size of CRVs

At different times after infection, cells were fixed for 10 min in 3% paraformaldehyde. At least 100 cells were scored in, at least, three independent experiments. The percentage of infection was calculated as the ratio between the number of infected cells and the number of total cells counted. Infected cells were defined as those with at least one clear identifiable bacterium inside. The number and size of vacuoles was determined by a morphometric analysis using Adobe Photoshop CS program (Adobe Systems Inc.) and EZ-C1 program (Nikon, Japan).

Bacterial viability and replication

The effect of overexpressed Beclin 1-WT, Beclin 1-BD and GFP-Bcl-2 on Cb replication and viability were quantified using a fluorescent infectious FFU assay on Vero cell monolayers as described.⁴⁹ In brief, transfected HeLa cells were infected for 24 h and then lysed by several passages through a 27-gauge needle connected to a syringe, a step that does not reduce the bacterial infectivity. Lysates were sonicated and serially diluted, and used to infect monolayers of Vero cells grown on coverslips in 24-well tissue culture plates in infection media at 37 °C in a 5% CO₂ atmosphere. At 48 h after infection, Vero cells were washed with PBS, fixed in 3% paraformaldehyde and processed for immunofluorescence with a rabbit anti-Coxiella antibody as described above. The average number of FFU in 50 fields was determined for each sample using a FV100 Olympus Confocal Microscope (Olympus, Japan). Approximately 500 cells were scored per coverslip. The cells were scored using ImageJ software.

Western blot analysis

To detect the knockdown efficiency of Beclin 1-KD, HeLa cells were transfected after the double-hit protocol with GFP alone or with GFP and Beclin 1-KD. After 72 h of the second transfection, cells were washed, scraped, resuspended in sample buffer containing 1% 2-mercaptoethanol and sonicated for 10 min at 4 °C. The samples were frozen until use. For western blots analysis, the protein extracts were subjected to electrophoresis in 12.5% SDS-PAGE gels, transferred to a nitrocellulose membrane and blocked with PBS supplemented with 0.1% v/v of Tween-20 and 5% of milk. The nitrocellulose membrane was incubated for 2 h with a goat anti-Beclin 1 (1 : 200), washed and incubated with a secondary HRP-conjugated anti-goat antibody (Jackson ImmunoResearch Laboratories) at a 1 : 10 000 dilution. Equal proteins loading were confirmed using mouse anti-Actin antibody (1 : 10 000). The bands were visualized using the ECL reagent (GE Healthcare, Buckinghamshire, UK), and analyzed using ImageJ software.

To detect LC3 conversion in Cb-infected cells, HeLa cells were infected or left uninfected for 48 h. Uninfected cells were subjected to control conditions, starvation with EBSS media for 2 h or starvation and 100 μM Vinblastine (Sigma) for 2 h. After incubation times, cells were washed, scraped and resuspended in sample buffer containing 1% 2-mercaptoethanol and sonicated for 10 min at 4 °C. The samples were frozen until use. For western blots analysis, the protein extracts were subjected to electrophoresis in 12.5% SDS-PAGE gels, transferred to a nitrocellulose membrane and blocked. The nitrocellulose membrane was incubated overnight at 4 °C with rabbit anti-LC3 (1 : 300), washed and incubated with a secondary HRP-conjugated anti-rabbit antibody (Jackson ImmunoResearch Laboratories) at a 1 : 10 000 dilution. Equal proteins loading were confirmed using mouse anti-Actin antibody (1 : 10 000). The bands were visualized with the ECL reagent (GE Healthcare), and analyzed using ImageJ software.

Apoptosis determinations

Nuclear fragmentation detected with Hoechst 33258

HeLa cells were plated on coverslips distributed in 24-well plates and transfected with different plasmids. Transfected cells uninfected or infected with Cb for 24 h were treated with or without chloramphenicol (80 μg/ml) for 18 h. Cells were exposed to UV light for 15 min to induce apoptosis. Afterward, cells were washed with PBS, fresh infection media was added and the incubation was continued for additional 6 h at 37 °C. Next, cells were subjected to indirect immunofluorescence. The bacteria and apoptotic nuclei were stained with Hoechst 33258. Cells were analyzed by fluorescence microscopy using an inverted microscope (Nikon Eclipse 2000). The percentages of apoptotic cells were evaluated as the ratio between the number of apoptotic cells and the number of total cells.

Tunel assay

HeLa cells were plated on coverslips distributed in 24-well plates and transfected with different plasmids. Transfected cells were uninfected or infected with Cb for 24 h and treated with or without chloramphenicol (80 μg/ml) for 18 h. Cells were exposed to UV light for 15 min to induce apoptosis. Then, cells were washed with PBS, fresh infection media was added, and the incubation was continued for additional 6 h at 37 °C. Next, cells were fixed with 3% paraformaldehyde in PBS and subjected to indirect immunofluorescence to detect Beclin 1. Then, cells were re-permeabilized with 0.1% Triton X-100 in PBS and incubated with TUNEL reaction mixture according to the manufacturer's protocol (Promega). The bacteria and apoptotic nuclei were stained with Hoechst 33258. Cells were analyzed using fluorescence microscopy with a FV100 Olympus Confocal Microscope. The percentages of apoptotic cells were evaluated as the ratio between the number of apoptotic cells and the number of total cells.

Statistical analysis

The results are represented as the mean±S.E.M. from at least three independent experiments. The comparisons were performed using ANOVA in conjunction with Tuckey's and Dunnett's tests. The significant differences were *P<0.05; **P<0.01; and ***P<0.001.

Abbreviations

Cb :: Coxiella burnetii
CRV:: Coxiella-replicative vacuoles
T4SS:: type IV secretion system

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Acknowledgements

We are grateful to Dr. Luis Mayorga and Cecilia Lerena for critical reading of this paper. We also thank Alejandra Medero for excellent technical assistance with tissue culture, and Marcelo Furlán and Graciela Gutiérrez for technical assistance. This work was partly supported by grants from Agencia Nacional de Promoción Científica y Tecnológica (PICT2004 no. 20711 and PICT2005 no. 38420), CONICET (PIP no. 5943) and SECTyP (Universidad Nacional de Cuyo).

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Laboratorio de Biología Celular y Molecular- Instituto de Histología y Embriología (IHEM), Facultad de Ciencias Médicas, Universidad Nacional de Cuyo-CONICET, Mendoza, Argentina
C L Vázquez & M I Colombo

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C L Vázquez
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M I Colombo
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Correspondence to M I Colombo.

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Vázquez, C., Colombo, M. Coxiella burnetii modulates Beclin 1 and Bcl-2, preventing host cell apoptosis to generate a persistent bacterial infection. Cell Death Differ 17, 421–438 (2010). https://doi.org/10.1038/cdd.2009.129

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Received: 12 December 2008
Revised: 16 July 2009
Accepted: 09 August 2009
Published: 02 October 2009
Issue Date: March 2010
DOI: https://doi.org/10.1038/cdd.2009.129

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