Ehrlichia secretes Etf-1 to induce autophagy and capture nutrients for its growth through RAB5 and class III phosphatidylinositol 3-kinase

Lin, Mingqun; Liu, Hongyan; Xiong, Qingming; Niu, Hua; Cheng, Zhihui; Yamamoto, Akitsugu; Rikihisa, Yasuko

Ehrlichia chaffeensis, a Gram-negative obligatory intracellular bacterium in the family Anaplasmataceae, primarily infects monocytes and macrophages in mammals and causes the emerging tick-borne zoonosis called human monocytic ehrlichiosis. [1] [2] [3] This serious and sometimes fatal disease is characterized by fever, headache, myalgia, thrombocytopenia, leukopenia, and elevated liver enzyme levels. [3] [4] [5] One fundamental virulence factor of microbial pathogens is "nutritional virulence," 6 i.e., the ability to acquire nutrients for pathogen proliferation in competition with hosts and possible other microbes. Many pathogens thrive on extracellular nutrients available in the blood and mucosal surface and on the host cellular lysate. However, E. chaffeensis, as an obligatory intracellular bacterium, acquires nutrients inside host cells which are kept alive until bacteria fully proliferate and mature (reviewed in ref. 7) . The host cell-dependency of E. chaffeensis is so extreme that unlike facultative intracellular bacteria such as Legionella pneumophila or Mycobacterium tuberculosis, which can be cultured axenically, E. chaffeensis cannot replicate or even survive outside of host cells for more than a few hours. The intracellular membrane compartment (inclusion) that contains E. chaffeensis has early endosome-like characteristics, including the small GTPase RAB5, a RAB5 effector EEA1 (early endosome antigen 1), TF (transferrin), TFRC (transferrin receptor), and vacuolar-type H C -ATPase, but it lacks late endosomal or lysosomal markers or NADPH oxidase components. [8] [9] [10] Within this compartment, E. chaffeensis acquires all nutrients for its reproduction to form numerous mature infectious forms. E. chaffeensis has a small genome of 1.176 Mb with a limited capacity for biosynthesis and metabolism. 11 Consequently it must depend mostly on host-synthesized nutrients for replication. Although the host cell cytoplasm is rich with these nutrients, it is unlikely that the inclusion membrane is leaky, as ehrlichial inclusions maintain a weakly acidic intralumenal pH. 8 It is also unlikely that varieties of active transporters are synthesized and assembled in the proper orientation on the inclusion membrane to import host nutrients during ehrlichial replication. Considering these limitations, it is possible that a novel mechanism has evolved in this group of obligatory intracellular bacteria to acquire nutrients.