Assays, method and compositions that exploit a mycobacterial iron acquistion pathway in cells
Assays for the identification of antibacterial agents that inhibit mycobacterial growth by interfering with the iron acquisition pathway in host cells are disclosed. Also disclosed are methods of inhibiting mycobacterial growth by interfering with iron uptake or by delivering antibacterial agents to mycobacteria in host cells through the iron uptake pathway, as well as certain compounds that are useful in the assay methods, and also as antibacterial agents.
1. A method of identifying compounds that interfere with mycobactin-mediated iron acquisition by mycobacteria within phagosomes of host cells, wherein interference with the mycobactin-mediated iron acquisition results from interference with one or more of: transport of apo-mycobactin across phagosome membranes into the host cytoplasm, formation of ferric mycobactin complexes in the host cytoplasm, accumulation of ferric mycobactin complexes in lipid droplets, or delivery of ferric mycobactin complexes to phagosomes from lipid droplets, the method comprising the steps of:
a) exposing a first set of host cells, in the absence of a test compound, to an isolated mycobactin component selected from apo-mycobactin, ferric mycobactin complex, or a ferric mycobactin complex analog or derivative that binds to lipid droplets, under conditions that permit entry of the mycobactin component into the host cell cytoplasm, formation of ferric mycobactin complexes within the host cells, accumulation of ferric mycobactin complexes in lipid droplets and delivery of ferric mycobactin complexes to phagosomes from lipid droplets;
b) exposing a second set of host cells, in the presence of the test compound, to the isolated mycobactin component of step a) under equivalent conditions as applied in step a);
c) measuring in each of the first and second sets of host cells the amount of one or more of:
i) mycobactin components present in the host cytoplasm;
ii) ferric mycobactin complexes formed from apo-mycobactin;
iii) ferric mycobactin complexes or ferric mycobactin complex analogs or derivatives associated with lipid droplets; and
iv) ferric mycobactin complexes or ferric mycobactin analogs or derivatives present within phagosomes;
d) comparing the measured amounts from step c) for the first and second sets of host cells; and
e) identifying whether the test compound interfered with mycobactin-mediated iron acquisition, wherein a decrease in any of (i)-(iv) in the presence of the test compound, as compared with in the absence of the test compound, is indicative that the test compound interferes with mycobactin-mediated iron acquisition by mycobacteria within phagosomes of host cells.
2. The method of claim 1 , wherein the host cells are macrophage or macrophage-like cells.
3. The method of claim 2 , wherein the host cells are cultured cells.
4. The method of claim 1 , wherein the mycobactin component is mycobactin J or mycobactic acid J.
5. The method of claim 1 , wherein the mycobactin component is apo-mycobactin and the amount of apo-mycobactin together with ferric mycobactin complex within host cells is measured to provide a total amount of mycobactin components in the host cells, wherein a decrease in the total amount of mycobactin components in the presence of the test compound, as compared with in the absence of the test compound, is indicative that the test compound can interfere with transport of apo-mycobactin across phagosome membranes into the host cytoplasm.
6. The method of claim 1 , wherein the mycobactin component is apo-mycobactin and the amount of ferric mycobactin complex within host cells is measured, wherein a decrease in amount of ferric mycobactin complex in the presence of the test compound, as compared with in the absence of the test compound, is indicative that the test compound interferes with formation of ferric mycobactin complexes in host cells.
7. The method of claim 1 , wherein the mycobactin component is a ferric mycobactin complex or a ferric mycobactin complex analog or derivative that binds to lipid droplets, and the amount of ferric mycobactin complex or ferric mycobactin complex analog or derivatives accumulated in the lipid droplets is measured, wherein a decrease in the amount of ferric mycobactin complex or ferric mycobactin complex analog or derivatives accumulated in the lipid droplets in the presence of the test compound, as compared with in the absence of the test compound, is indicative that the test compound interferes with accumulation of ferric mycobactin complexes in host cell lipid droplets.
8. The method of claim 1 , wherein the mycobactin component is a ferric mycobactin complex or a ferric mycobactin complex analog or derivative that binds to lipid droplets, and the amount of ferric mycobactin complex or ferric mycobactin complex analog or derivatives delivered to phagosomes is measured, wherein a decrease in the amount of ferric mycobactin complex or ferric mycobactin complex analog or derivatives delivered to phagosomes in the presence of the test compound, as compared with in the absence of the test compound, is indicative that the test compound interferes with delivery of ferric mycobactin complexes from the lipid droplets to the phagosomes.
9. The method of claim 1 , wherein the mycobactin component is a ferric mycobactin complex analog or derivative selected from gallium-mycobactin or gallium-mycobactic acid.