Inhibition of TFEB activation promotes Coxiella burnetii growth

Abstract

Coxiella burnetii is the etiologic agent of Q fever, a zoonotic disease characterized by flu-like sickness in acute cases; endocarditis may occur and turn deadly if not treated correctly in chronic patients. Coxiella, an obligate intracellular bacterium, requires establishment of a replicative niche in the host cell. After being phagocytosed by the eukaryotic cell, the bacterium resides in a tight-fitting nascent phagosome which matures through the host canonical endocytic pathway, acquiring endosomal/lysosomal markers as well as acidic pH. Initial acidification of the Coxiella containing vacuole (CCV) is central to the bacterium’s pathogenesis because translocation of bacterial effector proteins into the host cell by the type 4B secretion system (T4BSS) initiates only after it senses the acidic environment. The effector proteins are required for subverting different host cell functions in favor of Coxiella growth, CCV maturation and are crucial for bacterial virulence. Contrary to the belief that since CCV matures through the host endocytic pathway, CCV is as acidic as lysosome, we found that CCV is significantly less acidic (pH~5.2) than lysosomes (pH~4.8) and inducing further CCV acidification causes Coxiella lysis. Furthermore, increasing lysosomal biogenesis in the host cell is detrimental for Coxiella growth. So, we hypothesized that Coxiella blocks lysosomal biogenesis in host cells to maintain the CCV pH just optimal for its growth. Lysosomal biogenesis is regulated by the master transcription factor EB (TFEB). Its ability to act as a transcription factor depends on its subcellular localization, which relies on its phosphorylation state. TFEB, when phosphorylated is cytosolic and inactive, whereas dephosphorylated TFEB translocates to the nucleus and is active, binding to promoter regions of lysosomal genes of the CLEAR network, thus controlling lysosome biogenesis. Therefore, we hypothesized that Coxiella blocks TFEB translocation to the nucleus, thus inhibiting lysosome biogenesis. We determined that Coxiella grows significantly better in TFEB-KO cells than they do in parentals. Also, using a torin-induced TFEB translocation model, we observed remarkably decreased TFEB activation in the Coxiella infected cells as was evident by less TFEB translocation to nucleus. Overall, data obtained from this work suggest that Coxiella inhibits lysosome biogenesis by blocking TFEB nuclear translocation.