Shigella hijacks Ago2 function to promote rupture of its vacuole

MicroRNAs (miRNAs) are critical fine-tuners of host immune responses in both plants and animals. Each small RNA associates with an Argonaute (Ago) protein which is the central component of a multiprotein RNA-induced silencing complex (RISC). The small RNA further directs AGO-RISC onto sequence complementary mRNA targets to trigger their post-transcriptional silencing. This is manifested by endonucleolytic cleavage (slicing), and/or translational inhibition of the mRNA targets. The functional relevance of individual miRNAs in controlling bacterial infection just begins to be elucidated. However, there is no evidence implicating the human miRNA machinery in this process, as previously reported in plants. Here, we have used Shigella and HeLa cells as an in vitro infection model system to study the role and regulation of Ago2 in host-bacterial interactions. During the course of Shigella infection, the bacterium induces its host cell internalization in a macropinocytic-like process by means of its type III secretion system. It further rapidly escapes from its phagocytic vacuole to reach the cytosol, a step which is critical for Shigella intracellular lifestyle and that requires the bacterial type III effector IpgD. Using live microscopy, we found that Ago2 is transiently recruited at Shigella entry foci and that Ago2 knocked-down or knocked-out cells exhibit a delay in Shigella-induced vacuole rupture. A similar delay in vacuole rupture was observed in cells depleted of other miRNA factors, supporting a role for the miRNA pathway in this process. Using a complementation assay in ago2-/- cells, we further show that the miRNA-mediated translational inhibition activity of Ago2 is essential for rapid Shigella-induced vacuole rupture, while its slicing activity is dispensable for this process. Additionally, we show that the phosphorylation at Ser387 of Ago2 (Ago2 S387P) is required for vacuole rupture. This phosphorylation is dependent on the Akt survival pathway, which is known to be targeted by IpgD. Finally, we provide genetic evidence suggesting that IpgD has evolved to trigger Ago2 S387P to promote vacuole rupture. Altogether, this study demonstrates for the first time a critical role of Ago2, and of the human miRNA pathway, in host-bacterial interactions. This work also provides novel insights into the process of Shigella-induced vacuole rupture, a phenomenon that remains poorly characterized at the mechanistic level.