Abstract
Growing evidence suggests the importance of lipid metabolism in pathogenesis of tuberculosis. Neutral lipids form the majority of lipids in a caseous granuloma, a pathology characteristic of tuberculosis. Cytosolic lipid droplets (LDs) of macrophages form the store house of these lipids and have been demonstrated to contribute to the inflammatory response to infection. The proteome of lipid droplets reflects the mechanisms of lipid metabolism active under a condition. However, infection induced changes in the proteome of these dynamic organelles remains elusive. Here, we employed quantitative proteomics to identify alterations induced upon infection with live Mycobacterium tuberculosis (Mtb) in comparison with heat killed bacilli or uninfected macrophages. We found increased abundance of proteins coupled with lipid metabolism, protein synthesis, and vesicular transport function in LDs upon infection with live Mtb. Using biochemical methods and microscopy, we validated ADP-ribosyltransferase (Arf)-like 8 (ARL8B) to be increased on the lipid droplet surface of live Mtb infected macrophages and that ARL8B is a bonafide LD protein. This study provides the first proteomic evidence that the dynamic responses to infection also encompass changes at the level of LDs. This information will be important in understanding how Mtb manipulates lipid metabolism and defense mechanisms of the host macrophage.
Highlights
Mycobacterium tuberculosis (Mtb) is a successful human pathogen capable of growth within macrophages despite eliciting a robust innate and adaptive immune response
THP1 monocyte derived macrophages contain lipid droplets in the basal state and exhibit Mtb infection induced alterations in lipid metabolism that guide the inflammatory response to infection.[3,6]
lipid droplets (LDs) of THP1 macrophages are mainly coated with Perilipin 2 (PLIN2) (Figure 1a) and to some extent with Perilipin 3 (PLIN3) but devoid of Perilipin 1 (PLIN1) (Figure S1)
Summary
Mycobacterium tuberculosis (Mtb) is a successful human pathogen capable of growth within macrophages despite eliciting a robust innate and adaptive immune response. In the stimulated state, incoming fatty acids are metabolized via mitochondrial beta oxidation while in the basal state they are stored in the form of cytosolic LDs; the association of mitochondria with LDs is key to drive this process.[12] The absence of Perilipin 1A in skeletal muscle cells indicates a different mechanism for regulation of lipid storage. Emerging evidence indicates cross talk between LDs and the ER membrane from which the droplets emerge, the mitochondria to which they provide fatty acids for oxidation, and the plasma membrane which regulates vectorial transfer of exogenous fatty acids to LDs. More recently, the physical interaction between various intracellular organelles has been quantitated using live cell microscopy, revealing interaction between LDs and ER, mitochondria, and lysosomes.[19] Given the central role of LDs in nutrient balance within the cell and across organelles, the role of macrophage LDs during infection, wherein host and bacteria compete for nutrients, is an important area of investigation. Our data identifies the first proteome level evidence that Mtb hijacks the macrophage LD during infection
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