Mycobacterial Metabolic Syndrome: Triglyceride Accumulation Decreases Growth Rate and Virulence of Mycobacterium Tuberculosis

Abstract

Mycobacterium tuberculosis (Mtb) mutants lacking the operon Rv1411c-1410c encoding a lipoprotein, Rv1411c (LprG) and a putative transporter, Rv1410c (Rv1410) are dramatically attenuated for growth in mice. Previous work in our lab, using the model organism Mycobacterium smegmatis, suggested that this operon regulated the lipid content of the cell wall. Work in other laboratories characterizing LprG as a lipid-binding lipoprotein lead us to hypothesize that these bacteria grew poorly due to loss of a key lipid important in the host-pathogen interaction. Based on structural and biochemical studies we hypothesized that this attenuation was due to a lipid transport defect. Using whole cell lipidomic analysis, we found changes in LprG-1410 mutants including accumulation of triacylglyceride (TAG) species in the absence of the transport system. We have identified TAG in outer membrane fractions and supernatants of Mtb, have demonstrated the ability of LprG to transport TAG in an in vitro vesicle transfer assay, and have co-crystallized LprG with TAG. Moreover, accumulation of intracellular TAG substantially decreases growth under carbon stress in vitro and in vivo in the mouse model. Our results suggest a far different model – that TAG is ordinarily transported out of the cell and, in the absence of a transporter, limits cell proliferation independent of the host immune response. This suggests that TAG is a key metabolic regulator of cellular growth within the host.