The Role of Mammalian Lipid Transport Protein ORP1 During Coxiella Burnetii Infection

Abstract

Coxiella burnetii is an intracellular bacterium that causes the human disease Q fever. C. burnetii is transmitted from infected animals to humans through inhalation of infectious droplets. Acute Q fever is a flu-like illness lasting 10-14 days. Patients often have respiratory symptoms and present with pneumonia. Patients with suppressed immune systems or valvular heart disease can develop chronic Q fever, which causes endocarditis and vasculitis long after initial infection. Chronic Q fever is difficult to treat, and if untreated, is typically fatal. Currently, the United States lacks any vaccine for Q fever. In order to better prevent and treat this disease, it is important to understand how C. burnetii interacts with mammalian cells. Within the host cell, C. burnetii forms a large, acidic Coxiella-containing vacuole (CCV) and uses a Type 4B secretion system (T4SS) to secrete effector proteins into the host cell cytoplasm. While the CCV membrane is rich in sterols, cholesterol accumulation in the CCV is bacteriolytic, suggesting that C. burnetii regulation of lipid transport is critical for infection. The mammalian lipid transport protein ORP1L localizes to the CCV membrane and mediates CCV-ER membrane contact sites. ORP1L functions in lipid transport, including cholesterol efflux from late endosomes/lysosomes. Its sister isoform ORP1S binds cholesterol but localizes to the cytoplasm and nucleus. In ORP1- null cells, we found that CCVs were smaller than in wildtype cells, highlighting the importance of ORP1 in CCV development. CCVs in ORP1-null cells had higher cholesterol content than CCVs in wildtype cells, suggesting ORP1 functions in cholesterol efflux from the CCV. ORP1-null MH-S cells do not accumulate lipid droplets upon C. burnetii infection, supporting our hypothesis that ORP1 promotes cholesterol transfer from the CCV to the ER, as lipid droplets form from neutral lipids in the ER. While the absence of ORP1 led to a C. burnetii growth defect in MH-S cells, there was no growth defect in HeLa cells. Together, our data demonstrate that C. burnetii uses the host sterol transport protein ORP1 to promote CCV development, potentially by using ORP1 to facilitate cholesterol efflux from the CCV to diminish the bacteriolytic effects of cholesterol.