Chalkophore mediated respiratory oxidase flexibility controls M. tuberculosis virulence

Reviewer #1 (Public review):

Summary:

It is essential for Mycobacterium tuberculosis (Mtb) to scavenge trace metals from its host to survive. In this study, the authors explore the effects of copper limitation on Mtb. Mtb synthesizes small molecular diisonitrile lipopeptides termed chalkophores, that chelate host copper for import, whereby the copper is incorporated into Mtb metalloproteins. However, the role of chalkophores in Mtb biology and their targeted metalloproteins are unknown. This study investigates Mtb proteins that require chalkophores for copper incorporation and their effect on Mtb virulence. It is known that the nrp operon is induced by copper deprivation and encodes the synthesis of chalkophores. A genetic analysis revealed transcriptional differences for WT and Mtb∆nrp when exposed to the copper chelator tetrathiomolybdate (TTM). The authors found that copper chelation results in upregulation of genes in the chalkophore cluster as well as genes involved in the respiratory chain: specifically, components of the heme-dependent oxidase CytBD and subunits of the bcc:aa3 heme-copper oxidase. Interestingly, treatment of Mtb∆nrp with an inhibitor of the QcrB subunit of the bcc:aa3 oxidase (Q203) resulted in similar transcriptional changes. The bcc:aa3 oxidase and CytBD are functionally redundant, and while both utilize heme as a cofactor, only the first utilizes heme and copper. Utilizing Mtb∆nrp, Mtb∆cydAB and MtbΔnrpΔcydAB along with single gene complementation, the authors showed that copper starvation survival requires diisonitrile chalkophore synthesis and that copper starvation results in dysfunctional bcc:aa3 oxidase. Further genetic analysis combined with inhibitor studies indicate that bcc:aa3 oxidase is the only target impacted by copper starvation. By monitoring oxygen consumption for mutants in combination with inhibitors, the authors show that copper deprivation inhibits respiration through the bcc:aa3 oxidase. Similarly, they show that TTM or Q203 treatment inhibits ATP production in MtbΔnrpΔcydAB, but not in WT, showing that chalkophores maintain oxidative phosphorylation. Lastly, the authors compare the virulence of WT Mtb, Mtb∆nrp and MtbΔnrpΔcydAB strains in mice spleen and lung. The Mtb∆nrp strain showed mild attenuation, but virulence in MtbΔnrpΔcydAB was severely attenuated, and complementation with the chalkophore biosynthetic pathway restored Mtb virulence. These results suggest that chalkophore mediated protection of the respiratory chain is critical to Mtb virulence, and the that redundant respiratory oxidases within Mtb provides respiratory chain flexibility that may promote host adaptation.

Strengths:

Overall, the paper is very clear and well-written, with thorough and well-thought-out experimentation.

The methods are all quite standard, so there are no weaknesses identified with regard to methodology.

Reviewer #2 (Public review):

Summary:

This is a well-written manuscript that clearly demonstrates that the nrp encoded diisonitrile chalkophore is necessary for the function of the bcc-aa3 oxidase supercomplex under low copper conditions. In addition, the study demonstrates that the chlakophore is important early during infection when copper sequestration is employed by the host as a method of nutritional immunity.

Strengths:

The authors use genetic approaches including single and double mutants of chalkophore biosynthesis, and both the Mtb oxidases. They use copper chelators to restrict copper in vitro. A strength of the work was the use of a synthesized a Mtb chalkophore analogue to show chemical complementation of the mutant nrp locus. Oxphos metabolic activity was measuered by oxygen consumption and ATP levels. Importantly, the study demonstrated that chalkophore, especially in a strain lacking the secondary oxidase, was necessary for early infection and ruled out a role for adaptive immunity in the chalkophore lacking Mtb by use of SCID mice. It is interesting that after two weeks of infection and onset of adaptive immunity, the chalkophore is not required, which is consistent with the host environment switching from a copper-restricted to copper overload in phagosomes.

Weaknesses:

Most claims in the manuscript are soundly justified. The one exception is the claim that "maintenance of respiration is the only cellular target of chalkophore mediated copper acquisition." While under the in vitro conditions tested this does appear to be the case; however, it can't be ruled out that the chalkophore is important in other situations. In particular, for maintenance of the periplasmic superoxide dismustase, SodC, which is the other M. tuberculosis enzyme known to require copper.

Reviewer #3 (Public review):

Summary:

In this manuscript, the group of Glickman expands on their previous studies on the function of chalkophores during the growth of and infection by Mycobacterium tuberculosis. Previously, the group had shown that chalkophores, which are metallophores specific for the scavenging of copper, are induced by M. tuberculosis under copper deprivation conditions. Here, they show that chalkophores, under copper limiting conditions, are essential for the uptake of copper and maturation of a terminal oxidase, the heme-copper oxidase, cytochrome bcc:aa3. As M. tuberculosis has two redundant terminal oxidases, growth of and infection by M. tuberculosis is only moderated if both the chalkophores and the second terminal oxidase, cytochrome bd, are inhibited.

Strengths:

A strength of this work is that the lab-culture experiments are expanded upon with mice infection models, providing strong indications that host-inflicted copper deprivation is a condition that M. tuberculosis has adapted to for virulence.

Weaknesses:

Because the phenotype of M. tuberculosis lacking chalkophores is similar, if not identical, to using Q203, an inhibitor of cytochrome bcc:aa3, the authors propose that the copper-containing cytochrome bcc:aa3 is the only recipient of copper-uptake by chalkophores. A minor weakness of the work is that this latter conclusion is not verified under infection conditions and other copper-enzymes might still be functionally required during one or more stages of infection.

Author response:

We thank the reviewers for their careful evaluation of our manuscript and appreciate the suggestions for improvement. We will outline our planned revisions in response to these reviews.

Reviewer 2:

“The one exception is the claim that "maintenance of respiration is the only cellular target of chalkophore mediated copper acquisition." While under the in vitro conditions tested this does appear to be the case; however, it can't be ruled out that the chalkophore is important in other situations. In particular, for maintenance of the periplasmic superoxide dismutase, SodC, which is the other M. tuberculosis enzyme known to require copper.”

And

Reviewer 3:

“Because the phenotype of M. tuberculosis lacking chalkophores is similar, if not identical, to using Q203, an inhibitor of cytochrome bcc:aa3, the authors propose that the copper-containing cytochrome bcc:aa3 is the only recipient of copper-uptake by chalkophores. A minor weakness of the work is that this latter conclusion is not verified under infection conditions and other copper-enzymes might still be functionally required during one or more stages of infection.

Both comments concern the question of whether the bcc:aa3 respiratory oxidase supercomplex is the only target of chalkophore delivered copper. In culture, our experiments suggest that bcc:aa3 is the only target. The evidence for this claim is in Figure 2E and F. In 2E, we show that M. tuberculosis DctaD (a subunit of bcc:aa3) is growth impaired, copper chelation with TTM does not exacerbate that growth defect, and that a DctaDDnrp double mutant is no more sensitive to TTM than DctaD. These data indicate that role of the chalkophore in protecting against copper deprivation is absent when the bcc:aa3 oxidase is missing. Similar results were obtained with Q203 (Figure 2F). Q203 or TTM arrest growth of M. tuberculosis Dnrp, but the combination has no additional effect, indicating that when Q203 is inhibiting the bcc:aa3 oxidase, the chalkophore has no additional role. However, we agree with the reviewers that we cannot exclude the possibility that during infection, there is an additional target of chalkophore mediated Cu acquisition. We will add this caveat to the revised version of this manuscript.