Abstract
Brucella species are Gram-negative facultative intracellular bacteria responsible for brucellosis, a worldwide zoonosis affecting various hosts including humans. Along the infection in most host cells, Brucella first interacts with the endosomal pathway and eventually reaches its replicative niche inside the endoplasmic reticulum (ER), in which bacteria multiply massively. Regarding the many physical and functional interactions between the ER and mitochondria, as well as the major role of mitochondria in inflammation and host-pathogen interactions, we aim to study the effect of Brucella on the mitochondrial population of infected cells and the crosstalk existing between them which is still poorly understood. In this work, we showed that Brucella abortus induces mitophagy through an increase in LC3/mitochondria colocalizing events, as well as an increase in the number of acidified FIS1 mitochondrial fragments at 48h post-infection in host cells. Brucella abortus -induced mitophagy is accompanied by a strong mitochondrial network fragmentation which is dependent on the mitophagy receptor BNIP3L as we demonstrated that a siRNA-mediated silencing of BNIP3L prevents the mitochondrial fragmentation and mitophagy in Brucella abortus infected cells. Our results also show that the expression of BNIP3L induced by Brucella abortus relies on the activation of the hypoxia-inducible factor HIF-1α in an oxygen-independent way. Instead, HIF-1α activation appears to be iron-dependent since FeCl2 supplementation prevents HIF-1α nuclear translocation and BNIP3L expression in Brucella abortus infected cells even if the origin of a putative iron starvation response in the host cells remains to be elucidated. In an attempt to better understand the functional role of the Brucella abortus-in-duced BNIP3L- mediated mitophagy, and what would be the resulting advantage for the host cell and/or the bacteria, our results show that BNIP3L silencing drastically reduced the number of reinfection events at 72 h post-infection, suggesting that bacterial egress from the host cell could be impaired. Future research will be needed to decipher the dynamics and the role of mitophagy and mitochondrial membranes in the last steps of Brucella abortus intracellular cycle. Altogether, those results should highlight new molecular mechanisms and critical steps involved in Brucella trafficking during infection.
