Platensimycin activity against mycobacterial beta-ketoacyl-ACP synthases

Abstract

Background: There is an urgent need for the discovery and development of new drugs against Mycobacterium tuberculosis, the causative agent of tuberculosis, especially due to the recent emergence of multi-drug and extensively-drug resistant strains. Herein, we have examined the susceptibility of mycobacteria to the natural product platensimycin.

Methods and findings: We have demonstrated that platensimycin has bacteriostatic activity against the fast growing Mycobacterium smegmatis (MIC = 14 microg/ml) and against Mycobacterium tuberculosis (MIC = 12 microg/ml). Growth in the presence of paltensimycin specifically inhibited the biosynthesis of mycolic acids suggesting that the antibiotic targeted the components of the mycolate biosynthesis complex. Given the inhibitory activity of platensimycin against beta-ketoacyl-ACP synthases from Staphylococcus aureus, M. tuberculosis KasA, KasB or FabH were overexpressed in M. smegmatis to establish whether these mycobacterial KAS enzymes were targets of platensimycin. In M. smegmatis overexpression of kasA or kasB increased the MIC of the strains from 14 microg/ml, to 30 and 124 microg/ml respectively. However, overexpression of fabH on did not affect the MIC. Additionally, consistent with the overexpression data, in vitro assays using purified proteins demonstrated that platensimycin inhibited Mt-KasA and Mt-KasB, but not Mt-FabH.

Significance: Our results have shown that platensimycin is active against mycobacterial KasA and KasB and is thus an exciting lead compound against M. tuberculosis and the development of new synthetic analogues.

Figure 1. Structure of platensimycin (A) and platencin (B).

Figure 2. Structures of the major mycolic acids of M. tuberculosis and M. smegmatis.

Figure 3. In vivo effect of platensimycin against M. smegmatis.

(A) Clarification of cultures due to clumping and cellular lysis at time point 72 h. (B) Cultures were grown to an OD_{600 nm} of 0.4 upon which 14 µg/ml of platensimycin was added, samples were take over a 72 h period. Viable counts were calculated as per the methods where the mean CFU per millilitre from three independent experiments was calculated. •, M. smegmatis; ○, M. smegmatis + platensimycin.

Figure 4. Generation of a M. bovis BCG kasB null mutant.

Maps of the kasB region in the M. bovis BCG genome and its corresponding region in the conditional mutant ΔkasB are shown on the left, with the corresponding Southern blot on the right. The regions used as porbes are indicated by solid lines with square ends while the expected bands are indicated by broken lines (with sizes indicated). hyg, hygromycin resistance gene from Streptomyces hygroscopicus; res, γδ resolvase recognition sites.

Figure 5. TLC-autoradiography of FAMEs and MAMEs from M. smegmatis strains overexpressing Mt-KasA, Mt-KasB and Mt-FabH following platensimycin treatment.

Platensimycin (0–60 µg/ml) was titred into M. smegmatis cultures at an OD_{600 nm} of 0.4 prior to labelling with 1 µCi/ml [1,2-¹⁴C]acetate for 12 h. [¹⁴C]-FAMEs and MAMEs were extracted and resolved by TLC. An equivalent aliquot of the resulting solution of FAMEs and MAMEs was subjected to TLC using silica gel plates developed twice in petroleum ether-acetone (95∶5). Autoradiograms were produced by overnight exposure to Kodak X-Omat film to reveal [¹⁴C]labeled FAMEs and MAMEs. (A) M. smegmatis pVV16, (B) M. smegmatis pVV16-Mt-KasA, (C) M. smegmatis pVV16-Mt-KasB, and (D) M. smegmatis pVV16-Mt-KasAB.

Figure 6. TLC-autoradiography of M. smegmatis lipid extracts and cell wall bound mycolates following platensimycin treatment.

Platensimycin (0, 20, 30, 40 µg/ml) were added to M. smegmatis cultures at an OD_{600 nm} of 0.4 for 8 h prior to labelling with 1 µCi/ml [1,2-¹⁴C]acetate for 12 h. (A) 2D-Ag²⁺ TLC using silica gel plates developed twice in hexane-ethyl acetate (95∶5) (direction I) then thrice in petroleum ether-diethyl ether (85∶15) (direction Ag II). (B) Cell wall bound mycolate profiles were revealed following two developments in petroleum ether-acetone (95∶5). (C) [¹⁴C]-Apolar lipids were extracted and resolved by TLC; direction 1, chloroform-methanol-water (100∶14∶0.8); direction 2, chloroform-acetone-methanol-water (50∶60∶2.5∶3). DAT; diacyltrehalose, GMM; glucose monomycolate, TDM; trehalose dimycolate. Autoradiograms were produced by overnight exposure to film to reveal [¹⁴C]-labelled lipids.