Identification of a novel gene product that promotes survival of Mycobacterium smegmatis in macrophages

Tweaking host immune responses for novel therapeutic approaches against Mycobacterium tuberculosis

In TB, combat between the human host and Mycobacterium tuberculosis involves intricate interactions with immune cells. M. tuberculosis has evolved a complex evasion system to circumvent immune cells, leading to persistence and limiting its clearance by the host. Host-directed therapies are emerging approaches to modulate host responses, including inflammatory responses, cytokine responses, and autophagy, by using small molecules to curb mycobacterial infections. Targeting host immune pathways reduces the chances of antibiotic resistance to M. tuberculosis because, unlike antibiotics, this approach acts directly on the cells of the host. In this review, we discuss the role of immune cells during M. tuberculosis proliferation, provide a updated understanding of immunopathogenesis, and explore the range of host-modulating options for the clearance of this pathogen.

Transcript analysis and expression of the glbO gene, encoding truncated hemoglobin,O, of M. smegmatis implicate its role under hypoxia and oxidative stress

Although truncated hemoglobin O, (trHbO), is ubiquitous among mycobacteria, its physiological function is not very obvious and may be diverse. In an attempt to understand role of trHbO in cellular metabolism of a non-pathogenic mycobacterium, we analysed expression profile of the glbO gene, encoding trHbO, in M. smegmatis and studied implications of its overexpression on physiology of its host under different environmental conditions. Quantitative RT-PCR indicated that transcript level of the glbO gene remains low at a basal level under aerobic growth cycle of M. smegmatis but its level gets induced significantly during low oxygen, oxidative stress and macrophage infection. Overexpression of the glbO gene enhanced growth of M. smegmatis under hypoxia, promoted pellicle biofilm formation and provided resistance towards oxidative stress. Additionally, glbO gene overexpressing M. smegmatis exhibited enhanced cell survival over isogenic control cells and altered the level of pro- and anti- inflammatory cytokines during intracellular infection. These results suggested important role of trHbO, in supporting the cellular metabolism and survival of M, smegmatis both under low oxygen and oxidative stress.

Mycobacterium tuberculosis Rv3717 enhances the survival of Mycolicibacterium smegmatis by inhibiting host innate immune and caspase-dependent apoptosis

Tuberculosis caused by Mycobacterium tuberculosis (M. tuberculosis) infection remains a serious public threat despite decades of creative endeavors. There are few reports on the roles of M. tuberculosis enzymes involved in cell envelope biosynthesis in pathogen survival and persistence. M. tuberculosis Rv3717 encodes N-acetylmuramoyl-l-alanine amidase, a cell-wall hydrolase that hydrolyzes the bond between N-acetylmuramic acid and l-alanine in cell-wall peptidoglycan. In this paper, we demonstrated the Rv3717 promoted the survival of Mycolicibacterium smegmatis(M. smegmatis) within macrophages. More importantly, we demonstrated that this effect is because MS_Rv3717 reduces the release of host pro-inflammatory cytokines such as IL-1β, IL-6, IL-12 p40, TNF-α, and increased transcription of anti-inflammatory cytokine IL-10. At the same time, MS_Rv3717 inhibits apoptosis by inhibiting the activation of Caspase-3/9, reducing the host's elimination of M. smegmatis. Finally, from a bacterial perspective, we found Rv3717 decreased the survival of M. smegmatis under stresses such as SDS and low pH. This is the first report of the involvement of Mycobacterium cell envelope biosynthetic enzyme in host-pathogen interaction.

Rv0807, a putative phospholipase A2 of Mycobacterium tuberculosis; Elucidation through sequence analysis, homology modeling, molecular docking and molecular dynamics studies of potential substrates and inhibitors

Mycobacterium tuberculosis (Mtb) has the ability to scrounge off the host macrophages and create a cordial environment for its survival. Identification of mechanisms favoring this purpose leads to novel treatment strategies for tuberculosis. In this study through in silico approaches, we intend to identify the putative role for Rv0807 from Mtb and its essentiality for mycobacterium survival within the macrophages. Through sequence analysis, we hypothesize that Rv0807 could be a Phospholipase A2 of Mtb. Moreover, through in silico mutation studies we have predicted certain residues to be a part of the catalytic process of the Rv0807 homodimer. Rv0807 could be a potential drug target as it binds phosphatidylinositol-3-phosphate (PI3P) and could be involved in processing the host cell PI3Ps, thereby blocking the phagosomal maturation. A pharmacophore hypothesis was generated for the Rv0807 homodimer based on the ligand binding site and a set of Pretomanid related compounds were screened against the Rv0807 homodimer. The top five compounds which had better docking scores and good ADME properties were selected as best inhibitory compounds and analyzed further. Molecular dynamics (MD) studies of Rv0807 homodimer with PI3P and with the top scored compound in docking studies, demonstrated a lot of conformational changes in the protein structure as it gets occluded through the course of simulation. The movement of a loop atop the ligand binding site, suggests of a lid-like region as seen in many other phospholipases. MD simulation of the mutant structures was also performed and its effect on the protein conformational changes was discussed.Communicated by Ramaswamy H. Sarma.

Structural and Functional Analysis of E. coli Cyclopropane Fatty Acid Synthase

Cell membranes must adapt to different environments. In Gram-negative bacteria, the inner membrane can be remodeled directly by modification of lipids embedded in the bilayer. For example, when Escherichia coli enters stationary phase, cyclopropane fatty acid (CFA) synthase converts most double bonds in unsaturated inner-membrane lipids into cyclopropyl groups. Here we report the crystal structure of E. coli CFA synthase. The enzyme is a dimer in the crystal and in solution, with each subunit containing a smaller N-domain that associates tightly with a larger catalytic C-domain, even following cleavage of the inter-domain linker or co-expression of each individual domain. Efficient catalysis requires dimerization and proper linkage of the two domains. These findings support an avidity-based model in which one subunit of the dimer stabilizes membrane binding, while the other subunit carries out catalysis.

A structural and functional investigation of a novel protein from Mycobacterium smegmatis implicated in mycobacterial macrophage survivability

The success of pathogenic mycobacterial species is owing in part to their ability to parasitize the generally inhospitable phagosomal environment of host macrophages, utilizing a variety of strategies to avoid their antimycobacterial capabilities and thereby enabling their survival. A recently identified gene target in Mycobacterium smegmatis, highly conserved within Mycobacterium spp. and denoted MSMEG_5817, has been found to be important for bacterial survival within host macrophages. To gain insight into its function, the crystal structure of MSMEG_5817 has been solved to 2.40 Å resolution. The structure reveals a high level of structural homology to the sterol carrier protein (SCP) family, suggesting a potential role of MSMEG_5817 in the binding and transportation of biologically relevant lipids required for bacterial survival. The lipid-binding capacity of MSMEG_5817 was confirmed by ELISA, revealing binding to a number of phospholipids with varying binding specificities compared with Homo sapiens SCP. A potential lipid-binding site was probed by alanine-scanning mutagenesis, revealing structurally relevant residues and a binding mechanism potentially differing from that of the SCPs.

Mycobacterium tuberculosis Rv3402c enhances mycobacterial survival within macrophages and modulates the host pro-inflammatory cytokines production via NF-kappa B/ERK/p38 signaling

Intracellular survival plays a central role in the pathogenesis of Mycobacterium tuberculosis, a process which depends on an array of virulence factors to colonize and replicate within the host. The M. tuberculosis iron regulated open reading frame (ORF) rv3402c, encoding a conserved hypothetical protein, was shown to be up-regulated upon infection in both human and mice macrophages. To explore the function of this ORF, we heterologously expressed the rv3402c gene in the non-pathogenic fast-growing Mycobacterium smegmatis strain, and demonstrated that Rv3402c, a cell envelope-associated protein, was able to enhance the intracellular survival of recombinant M. smegmatis. Enhanced growth was not found to be the result of an increased resistance to intracellular stresses, as growth of the Rv3402c expressing strain was unaffected by iron depletion, H₂O₂ exposure, or acidic conditions. Colonization of macrophages by M. smegmatis expressing Rv3402c was associated with substantial cell death and significantly greater amount of TNF-α and IL-1β compared with controls. Rv3402c-induced TNF-α and IL-1β production was found to be mediated by NF-κB, ERK and p38 pathway in macrophages. In summary, our study suggests that Rv3402c delivered in a live M. smegmatis vehicle can modify the cytokines profile of macrophage, promote host cell death and enhance the persistence of mycobacterium within host cells.

Modulation of phagosomal pH by Candida albicans promotes hyphal morphogenesis and requires Stp2p, a regulator of amino acid transport

Candida albicans, the most important fungal pathogen of humans, has a unique interaction with macrophages in which phagocytosis induces a switch from the yeast to hyphal form, allowing it to escape by rupturing the immune cell. While a variety of factors induce this switch in vitro, including neutral pH, it is not clear what triggers morphogenesis within the macrophage where the acidic environment should inhibit this transition. In vitro, C. albicans grown in similar conditions in which amino acids are the primary carbon source generate large quantities of ammonia to raise the extracellular pH and induce the hyphal switch. We show here that C. albicans cells neutralize the macrophage phagosome and that neutral pH is a key inducer of germination in phagocytosed cells by using a mutant lacking STP2, a transcription factor that regulates the expression of multiple amino acid permeases, that is completely deficient in alkalinization in vitro. Phagocytosed stp2Δ mutant cells showed significant reduction in hypha formation and escaped from macrophages less readily compared to wild type cells; as a result stp2Δ mutant cells were killed at a higher rate and caused less damage to RAW264.7 macrophages. Stp2p-regulated import leads to alkalinization of the phagosome, since the majority of the wild type cells fail to co-localize with acidophilic dyes, whereas the stp2Δ mutant cells were located in acidic phagosomes. Furthermore, stp2Δ mutant cells were able to form hyphae and escape from neutral phagosomes, indicating that the survival defect in these cells was pH dependent. Finally, these defects are reflected in an attenuation of virulence in a mouse model of disseminated candidiasis. Altogether our results suggest that C. albicans utilizes amino acids to promote neutralization of the phagosomal pH, hyphal morphogenesis, and escape from macrophages.

The innate immune system represents a key barrier that fungal pathogens such as Candida albicans must overcome in order to disseminate through the host. C. albicans cells phagocytosed by macrophages initiate a complex program that involves a large-scale reprogramming of metabolism and transcription and results in the switch to a hyphal form that can penetrate and kill the macrophage. Though a number of signals are known to induce this morphological transition in vitro, what does so following phagocytosis has been unclear. We previously showed that C. albicans rapidly neutralizes acidic, nutrient-poor media that resembles the phagolysosome and that this is deficient in mutants impaired in amino acid import due to a mutation in STP2. In this paper, we investigate whether this happens within the macrophage as well. We show here that, in contrast to wild-type cells, stp2Δ mutants occupy an acidic phagosome and are unable to initiate hyphal differentiation. Because of this, they are more sensitive to killing and do less damage to the macrophages than cells that can neutralize the phagolysosome. We conclude that alteration of phagosomal pH is an important virulence adaptation in this species.

Mycobacterium tuberculosis Rv3402c enhances mycobacterial survival within macrophages and modulates the host pro-inflammatory cytokines production via NF-kappa B/ERK/p38 signaling

Intracellular survival plays a central role in the pathogenesis of Mycobacterium tuberculosis, a process which depends on an array of virulence factors to colonize and replicate within the host. The M. tuberculosis iron regulated open reading frame (ORF) rv3402c, encoding a conserved hypothetical protein, was shown to be up-regulated upon infection in both human and mice macrophages. To explore the function of this ORF, we heterologously expressed the rv3402c gene in the non-pathogenic fast-growing Mycobacterium smegmatis strain, and demonstrated that Rv3402c, a cell envelope-associated protein, was able to enhance the intracellular survival of recombinant M. smegmatis. Enhanced growth was not found to be the result of an increased resistance to intracellular stresses, as growth of the Rv3402c expressing strain was unaffected by iron depletion, H2O2 exposure, or acidic conditions. Colonization of macrophages by M. smegmatis expressing Rv3402c was associated with substantial cell death and significantly greater amount of TNF-α and IL-1β compared with controls. Rv3402c-induced TNF-α and IL-1β production was found to be mediated by NF-κB, ERK and p38 pathway in macrophages. In summary, our study suggests that Rv3402c delivered in a live M. smegmatis vehicle can modify the cytokines profile of macrophage, promote host cell death and enhance the persistence of mycobacterium within host cells.

Cloning, expression, purification and preliminary X-ray diffraction studies of a mycobacterial protein implicated in bacterial survival in macrophages

Mycobacterium species have developed numerous strategies to avoid the antimycobacterial actions of macrophages, enabling them to survive within the generally inhospitable environment of the cell. The recently identified MSMEG_5817 protein from M. smegmatis is highly conserved in Mycobacterium spp. and is required for bacterial survival in macrophages. Here, the cloning, expression, purification and crystallization of MSMEG_5817 is reported. Crystals of MSMEG_5817 were grown in 1.42 M Li2SO4, 0.1 M Tris-HCl pH 7.7, 0.1 M sodium citrate tribasic dihydrate. Native and multiple-wavelength anomalous dispersion (MAD) data sets have been collected and structure determination is in progress.