Mycobacterium marinum antagonistically induces an autophagic response while repressing the autophagic flux in a TORC1- and ESX-1-dependent manner

Autophagy is a eukaryotic catabolic process also participating in cell-autonomous defence. Infected host cells generate double-membrane autophagosomes that mature in autolysosomes to engulf, kill and digest cytoplasmic pathogens. However, several bacteria subvert autophagy and benefit from its machinery and functions. Monitoring infection stages by genetics, pharmacology and microscopy, we demonstrate that the ESX-1 secretion system of Mycobacterium marinum, a close relative to M. tuberculosis, upregulates the transcription of autophagy genes, and stimulates autophagosome formation and recruitment to the mycobacteria-containing vacuole (MCV) in the host model organism Dictyostelium. Antagonistically, ESX-1 is also essential to block the autophagic flux and deplete the MCV of proteolytic activity. Activators of the TORC1 complex localize to the MCV in an ESX-1-dependent manner, suggesting an important role in the manipulation of autophagy by mycobacteria. Our findings suggest that the infection by M. marinum activates an autophagic response that is simultaneously repressed and exploited by the bacterium to support its survival inside the MCV.

细胞自噬（Autophagy）是一种真核生物分解代谢过程，同时也参与细胞自主防御。受感染的宿主细胞会生成双层膜结构的自噬体（autophagosomes），自噬体在自噬溶酶体（autolysosomes）中成熟，以此吞噬、杀伤并降解细胞质中的病原体。然而，部分细菌会破坏细胞自噬过程，并利用其相关机制与功能实现存活获益。本研究通过遗传学、药理学与显微镜成像技术监测感染进程，证实与结核分枝杆菌（M. tuberculosis）亲缘关系相近的海分枝杆菌（Mycobacterium marinum）的ESX-1分泌系统（ESX-1 secretion system）可上调自噬相关基因（autophagy genes）的转录水平，并在模式宿主生物盘基网柄菌（Dictyostelium）中刺激自噬体的形成，以及其向含分枝杆菌吞噬泡（mycobacteria-containing vacuole，MCV）的招募过程。与之拮抗的是，ESX-1分泌系统同时也是阻断自噬流（autophagic flux）、清除含分枝杆菌吞噬泡内蛋白水解活性的必需因子。TORC1复合物（TORC1 complex）的激活因子会以ESX-1依赖的方式定位至含分枝杆菌吞噬泡，这提示该复合物在分枝杆菌调控细胞自噬的过程中发挥重要作用。本研究结果表明，海分枝杆菌的感染会激活细胞自噬应答，而该细菌会同时抑制并利用这一自噬应答，以实现在含分枝杆菌吞噬泡内的存活。