The Essential Role of Cholesterol Metabolism in the Intracellular Survival of Mycobacterium leprae Is Not Coupled to Central Carbon Metabolism and Energy Production

Abstract

Mycobacterium leprae induces the formation of lipid droplets, which are recruited to pathogen-containing phagosomes in infected macrophages and Schwann cells. Cholesterol is among the lipids with increased abundance in M. leprae-infected cells, and intracellular survival relies on cholesterol accumulation. The present study investigated the capacity of M. leprae to acquire and metabolize cholesterol. In silico analyses showed that oxidation of cholesterol to cholest-4-en-3-one (cholestenone), the first step of cholesterol degradation catalyzed by the enzyme 3β-hydroxysteroid dehydrogenase (3β-HSD), is apparently the only portion of the cholesterol catabolic pathway seen in Mycobacterium tuberculosis preserved by M. leprae. Incubation of bacteria with radiolabeled cholesterol confirmed the in silico predictions. Radiorespirometry and lipid analyses performed after incubating M. leprae with [4-14C]cholesterol or [26-14C]cholesterol showed the inability of this pathogen to metabolize the sterol rings or the side chain of cholesterol as a source of energy and carbon. However, the bacteria avidly incorporated cholesterol and, as expected, converted it to cholestenone both in vitro and in vivo. Our data indicate that M. leprae has lost the capacity to degrade and utilize cholesterol as a nutritional source but retains the enzyme responsible for its oxidation to cholestenone. Thus, the essential role of cholesterol metabolism in the intracellular survival of M. leprae is uncoupled from central carbon metabolism and energy production. Further elucidation of cholesterol metabolism in the host cell during M. leprae infection will establish the mechanism by which this lipid supports M. leprae intracellular survival and will open new avenues for novel leprosy therapies.IMPORTANCE Our study focused on the obligate intracellular pathogen Mycobacterium leprae and its capacity to metabolize cholesterol. The data make an important contribution for those interested in understanding the mechanisms of mycobacterial pathogenesis, since they indicate that the essential role of cholesterol for M. leprae intracellular survival does not rely on its utilization as a nutritional source. Our findings reinforce the complexity of cholesterol's role in sustaining M. leprae infection. Further elucidation of cholesterol metabolism in the host cell during M. leprae infection will establish the mechanism by which this lipid supports M. leprae intracellular survival and will open new avenues for novel leprosy therapies.

GSMN-ML- a genome scale metabolic network reconstruction of the obligate human pathogenMycobacterium leprae

“…Similarly, and in contrast to M. tuberculosis , GSMN-ML was incapable of in silico growth with cholesterol as a carbon source (Figure 2A). This is in agreement with previous studies that indicate that, despite retaining an ability to oxidize cholesterol, M. leprae is unable to use cholesterol as an energy or carbon source [37]. This is consistent with the genome studies, which indicate that evolutionary gene-decay in M. leprae has resulted in the loss of the mce4 operon and that encodes for active transport system for cholesterol [10], [37], as well as many genes required for cholesterol catabolism.…”

“…The results, (Figure 2A) shows that, perhaps surprisingly, M. leprae has retained the ability to utilize many carbon sources; yet, in comparison with M. tuberculosis , has lost the ability to utilize acetate and glycolate due to pseudogenization of acetate kinase, phosphate acetate transferase and acetyl-coA synthase. M. leprae has also lost almost the entire pathway involved in cholesterol utilization and cannot thereby utilize host-derived cholesterol [37], which appears to be a growth substrate for M. tuberculosis in vivo . Catabolism of amino acids, such as valine, threonine and isoleucine as carbon sources was also impaired in M. leprae (Figure 2B).…”

Fat, fight, and beyond: The multiple roles of lipid droplets in infections and inflammation

“…Usurpation of fatty acids released from the host triglycerides stored in LDs is a vital source of energy for mycobacteria, and the storage of fatty acids in the form of triglycerides in bacterial LDs could be linked to the dormancy and reactivation of M. tuberculosis . Cholesterol is another lipid essential to mycobacterial survival . M. tuberculosis has the capacity to use cholesterol as an energy source, which is important during latent‐phase infection .…”

“…40,113 Cholesterol is another lipid essential to mycobacterial survival. 118,119 M. tuberculosis has the capacity to use cholesterol as an energy source, which is important during latent-phase infection. 119,120 Despite being essential for building blocks for M. lepra growth and survival, cholesterol metabolism is not coupled with energy production.…”

“…119,120 Despite being essential for building blocks for M. lepra growth and survival, cholesterol metabolism is not coupled with energy production. 118 During the mycobacterium infections, the LDs are redistributed or recruited to vicinity of bacterium-containing phagosome. 115,121 Interfering with cytoskeletal function affects not only the impairment of recruitment of host LDs to the phagosome but also the decline of M. leprae survival in infected cells.…”