Ferritinophagy drives uropathogenic Escherichia coli persistence in bladder epithelial cells

Autophagy is a cellular recycling pathway, which in many cases, protects host cells from infections by degrading pathogens. However, uropathogenic Escherichia coli (UPEC), the predominant cause of urinary tract infections (UTIs), persist within the urinary tract epithelium (urothelium) by forming reservoirs within autophagosomes. Iron is a critical nutrient for both host and pathogen, and regulation of iron availability is a key host defense against pathogens. Iron homeostasis depends on the shuttling of iron-bound ferritin to the lysosome for recycling, a process termed ferritinophagy (a form of selective autophagy). Here, we demonstrate for the first time that UPEC shuttles with ferritin-bound iron into the autophagosomal and lysosomal compartments within the urothelium. Iron overload in urothelial cells induces ferritinophagy in an NCOA4-dependent manner causing increased iron availability for UPEC, triggering bacterial overproliferation and host cell death. Addition of even moderate levels of iron is sufficient to increase and prolong bacterial burden. Furthermore, we show that lysosomal damage due to iron overload is the specific mechanism causing host cell death. Significantly, we demonstrate that host cell death and bacterial burden can be reversed by inhibition of autophagy or inhibition of iron-regulatory proteins, or chelation of iron. Together, our findings suggest that UPEC persist in host cells by taking advantage of ferritinophagy. Thus, modulation of iron levels in the bladder may provide a therapeutic avenue to controlling UPEC persistence, epithelial cell death, and recurrent UTIs.

细胞自噬（Autophagy）是一种细胞内循环降解途径，在多数情况下可通过降解病原体以保护宿主细胞免受感染。然而，作为尿路感染（urinary tract infections, UTIs）的主要致病菌，尿路致病性大肠埃希菌（uropathogenic Escherichia coli, UPEC）可通过在自噬体（autophagosomes）内形成定植储库，在尿路上皮（urothelium）内存留。铁是宿主与病原体均必需的关键营养物质，宿主对铁生物利用度的调控是抵御病原体感染的核心防御策略之一。铁稳态（iron homeostasis）依赖于结合铁的铁蛋白（ferritin）被转运至溶酶体（lysosome）进行循环降解，这一过程被称为铁蛋白噬（ferritinophagy，一种选择性自噬亚型）。本研究首次证实，UPEC可与结合铁的铁蛋白一同被转运至尿路上皮细胞内的自噬体与溶酶体区室中。尿路上皮细胞内的铁过载可通过NCOA4依赖的方式诱导铁蛋白噬，提升UPEC可利用的铁含量，进而触发细菌过度增殖与宿主细胞死亡。即使仅添加中等剂量的铁，也足以加重并持续维持细菌载量。此外，本研究证实，铁过载引发的溶酶体损伤是导致宿主细胞死亡的特异性机制。值得注意的是，本研究表明，通过抑制细胞自噬、铁调控蛋白或实施铁螯合治疗，可逆转宿主细胞死亡与细菌载量升高的表型。综上，本研究结果显示UPEC通过利用铁蛋白噬在宿主细胞内存留定植。因此，调控膀胱内铁水平或可为控制UPEC定植、上皮细胞死亡及复发性尿路感染提供全新的治疗思路。