Tuberculosis (Tb) is a wide-spread infectious disease caused by Mycobacterium tuberculosis. One of the main characteristics of Tb is the formation of lipid droplet (LD)-loaded, foamy macrophages during chronic infection. Using Dictyostelium as surrogate foamy macrophage and M. marinum as pathogen, we found that pathogenic mycobacteria access host LDs to build up their own lipid storage organelles and exploit ER-derived phospholipids when LDs are lacking. We hypothesize that mycobacteria not only hijack LDs, but also lipid metabolic enzymes and various components of the lipid trafficking machinery.
A recurrent strategy is the induction of membrane contact sites (MCS) between biogenic host organelles and bacteria-containing vacuoles to create focal points for lipid exchange. Using the Dictyostelium/M. marinum system we observed that pathogenic mycobacteria selectively recruit host lipid transfer proteins to their vacuole: Oxysterol binding protein (OSBP) 8, one of the 12 OSBPs in Dictyostelium, is found in the cytosol, at the Golgi apparatus and the perinuclear ER in non-infected cells, and is enriched at ER-MCV-MCS during infection. Strikingly, OSBP8 is not mobilized by bacteria that lack ESAT-6, a secreted peptide that forms pores at the membrane of the MCV. This indicates that mycobacteria effector proteins that get access to the cytosol via ESAT-6 pores might be involved in the induction of MCS between the ER and the MCV and/or the recruitment of OSBP8. Many LTPs are recruited to MCSs via interaction with phosphoinositides (PIPs). Using fluorescent probes, we and others have found that the MCV membrane accumulates various PIPs including PI4P. We postulate that OSBP8 mediates sterol/PI4P counter transport at ER-Golgi-MCS analogous to yeast Osh4p in line with the observation that the PI4P and sterol distribution is altered in OSBP8 mutants. Consequently, during infection OSBP8 may deliver sterols from the ER to the MCV in exchange for PI4P providing the bacteria with sterols to fuel their central metabolism. We are now investigating how the MCV membrane composition is altered in various Dictyostelium and M. marinum mutants to understand the impact of sterol trafficking by OSBP8 during infection.