Iron robbery by intracellular pathogen via bacterial effector–induced ferritinophagy

Abstract

Iron is essential for survival and proliferation of Ehrlichia chaffeensis, an obligatory intracellular bacterium that causes an emerging zoonosis, human monocytic ehrlichiosis. However, how Ehrlichia acquires iron in the host cells is poorly understood. Here, we found that native and recombinant (cloned into the Ehrlichia genome) Ehrlichia translocated factor-3 (Etf-3), a previously predicted effector of the Ehrlichia type IV secretion system (T4SS), is secreted into the host cell cytoplasm. Secreted Etf-3 directly bound ferritin light chain with high affinity and induced ferritinophagy by recruiting NCOA4, a cargo receptor that mediates ferritinophagy, a selective form of autophagy, and LC3, an autophagosome biogenesis protein. Etf-3−induced ferritinophagy caused ferritin degradation and significantly increased the labile cellular iron pool, which feeds Ehrlichia. Indeed, an increase in cellular ferritin by ferric ammonium citrate or overexpression of Etf-3 or NCOA4 enhanced Ehrlichia proliferation, whereas knockdown of Etf-3 in Ehrlichia via transfection with a plasmid encoding an Etf-3 antisense peptide nucleic acid inhibited Ehrlichia proliferation. Excessive ferritinophagy induces the generation of toxic reactive oxygen species (ROS), which could presumably kill both Ehrlichia and host cells. However, during Ehrlichia proliferation, we observed concomitant up-regulation of Ehrlichia Fe-superoxide dismutase, which is an integral component of Ehrlichia T4SS operon, and increased mitochondrial Mn-superoxide dismutase by cosecreted T4SS effector Etf-1. Consequently, despite enhanced ferritinophagy, cellular ROS levels were reduced in Ehrlichia-infected cells compared with uninfected cells. Thus, Ehrlichia safely robs host cell iron sequestered in ferritin. Etf-3 is a unique example of a bacterial protein that induces ferritinophagy to facilitate pathogen iron capture.