Intracellular <i>Salmonella</i> induces aggrephagy of host endomembranes in persistent infections

Xenophagy has been studied in epithelial cells infected with <i>Salmonella enterica</i> serovar Typhimurium (<i>S</i>. Typhimurium). Distinct autophagy receptors target this pathogen to degradation after interacting with ubiquitin on the surface of cytosolic bacteria, and the phagophore- and autophagosome-associated protein MAP1LC3/LC3. Glycans exposed in damaged phagosomal membranes and diacylglycerol accumulation in the phagosomal membrane also trigger <i>S</i>. Typhimurium xenophagy. How these responses control intraphagosomal and cytosolic bacteria remains poorly understood. Here, we examined <i>S</i>. Typhimurium interaction with autophagy in fibroblasts, in which the pathogen displays limited growth and does not escape into the cytosol. Live-cell imaging microscopy revealed that <i>S</i>. Typhimurium recruits late endosomal or lysosomal compartments that evolve into a membranous aggregate connected to the phagosome. Active dynamics and integrity of the phagosomal membrane are requisite to induce such aggregates. This membranous structure increases over time to become an aggresome that engages autophagy machinery at late infection times (&gt; 6 h postentry). The newly formed autophagosome harbors LC3 and the autophagy receptor SQSTM1/p62 but is devoid of ubiquitin and the receptor CALCOCO2/NDP52. Live-cell imaging showed that this autophagosome captures and digests within the same vacuole the aggresome and some apposed intraphagosomal bacteria. Other phagosomes move away from the aggresome and avoid destruction. Thus, host endomembrane accumulation resulting from activity of intracellular <i>S</i>. Typhimurium stimulates a novel type of aggrephagy that acts independently of ubiquitin and CALCOCO2, and destroys only a few bacteria. Such selective degradation might allow the pathogen to reduce its progeny and, as a consequence, to establish persistent infections.

异体吞噬（xenophagy）已在被肠炎沙门氏菌鼠伤寒血清型（Salmonella enterica serovar Typhimurium，简称S. Typhimurium）感染的上皮细胞中得到研究。多种特异性自噬受体可识别胞质细菌表面的泛素，并与吞噬泡及自噬体相关蛋白微管相关蛋白1轻链3（MAP1LC3/LC3）结合，将该病原体靶向降解。吞噬体膜受损后暴露的聚糖，以及吞噬体膜中二酰甘油的积累，同样可触发S. Typhimurium的异体吞噬。目前人们对这些应答如何调控吞噬体内及胞质内的细菌仍知之甚少。
本研究针对成纤维细胞中的S. Typhimurium与自噬的相互作用展开分析，该病原体在成纤维细胞中生长受限且不会逃逸至胞质中。活细胞成像显微镜观察显示，S. Typhimurium可招募晚期内体或溶酶体区室，这些区室会演化形成与吞噬体相连的膜聚集体。吞噬体膜的动态活性与完整性是诱导此类聚集体形成的必要条件。该膜结构随时间推移不断增多，最终形成聚集体（aggresome），并在感染后期（入侵后＞6小时）招募自噬机制。
新形成的自噬体含有LC3与自噬受体SQSTM1/p62，但不含泛素及受体CALCOCO2/NDP52。活细胞成像结果表明，该自噬体可在同一液泡内捕获并降解上述膜聚集体，以及部分邻近的吞噬体内细菌。另有部分吞噬体远离该聚集体，从而避免被降解。综上，胞内S. Typhimurium活动引发的宿主内膜系统积累，可诱导一种新型的聚集体自噬（aggrephagy），该过程不依赖泛素与CALCOCO2，且仅能清除少量细菌。这种选择性降解或许可帮助病原体减少子代数量，进而建立持续性感染。