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Protein Aggregation in Ehrlichia chaffeensis during Infection of Mammalian Cells

M. Żółkiewski,D. Kuczyńska-Wiśnik,Chuanmin Cheng,R. Ganta

Published 2017 in FEMS Microbiology Letters

ABSTRACT

Ehrlichia chaffeensis is an obligatory intracellular pathogen transmitted through infected ticks to humans and several other mammals. Human monocytic ehrlichiosis (HME) caused by E. chaffeensis occurs mainly in the U.S.A, but also has been reported in other parts of the world. The severity of HME varies from asymptomatic to fatal. To establish infections in the host cells, E. chaffeensis needs the capability to evade the host defenses, but it is still not clear how the pathogen responds to and overcomes the host‐induced stress. We have previously shown that the expression of the molecular chaperone clpB in E. chaffeensis is strongly upregulated immediately after infection of mammalian cells. ClpB is an ATP‐dependent heat‐shock protein with a unique disaggregase activity that reactivates aggregated proteins under cellular stress conditions. Induction of clpB in E. chaffeensis during its replication in mammalian cells suggests that the ClpB activity supports the pathogen survival inside the host, but no link between protein aggregation and a pathogen life cycle has been established so far. We investigated the extent of protein aggregation in E. chaffeensis during infection of canine macrophage cell line, DH82. We discovered that the mammalian cell environment induced misfolding and aggregation of a significant fraction of E. chaffeensis proteins. The size of the aggregated fraction of E. chaffeensis proteins increased during the first 48 h post infection. The 48‐h post infection window corresponds to the upregulated production of the clpB mRNA. To disrupt the activity of ClpB in E. chaffeensis, we incubated the infected cells with up to 0.5 mM guanidinium chloride (GuHCl), which inhibits the ClpB ATPase activity in vitro. 0.5 mM GuHCl had no impact on the host cells, whereas the viability of the pathogen was reduced by ~80% in the presence of the inhibitor. Importantly, we found that the size of the aggregated protein fraction in E. chaffeensis increased significantly in cultures supplemented with 0.5 mM GuHCl. We also found that ClpB accumulated with protein aggregates formed in E. chaffeensis in the presence of GuHCl. Thus, we discovered that an exposure of E. chaffeensis to the stressful host environment during infection results in aggregation of the pathogen's proteins. Moreover, we showed that the molecular chaperone ClpB supports the survival of E. chaffeensis during the infectious stage by reducing the amount of protein aggregates. Altogether, our studies establish a novel link between the protein aggregation control and the pathogen survival.

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