The nature of extracellular iron influences iron acquisition by Mycobacterium tuberculosis residing within human macrophages

Mycobacterium tuberculosis exploits SIRT2 to trap iron for its intracellular survival

Iron is a critical nutrient for all organisms ranging from bacteria to humans. Ensuring control of this strategic vital resource significantly influences the dynamics of the struggle between host and invading pathogen. Mycobacterium tuberculosis (Mtb), the causative agent of the pulmonary disease tuberculosis (TB), has been plaguing humans for millennia and has evolved to successfully persist and multiply within host cells evading the mammalian immune defences. Invading Mtb appropriates host iron for its survival while the host innate immune response attempts to prevent its stores of this strategic mineral from being appropriated. SIRT2 is a member of the Sirtuin family. These are evolutionary conserved NAD+-dependent deacetylases involved in various cellular processes including regulation of cellular iron homeostasis. Upon Mtb infection of macrophages, SIRT2 expression is enhanced and it translocates from cytosol to nucleus. This is accompanied with a breakdown of the host's iron restriction strategy that compromises host defence mechanisms. However, the underlying mechanism as to how invading Mtb exploits SIRT2 for commandeering host iron remains unknown. In the current study, we report that the decreased bacillary load in cells wherein SIRT2 had been chemically inhibited or knocked down is due to diminished availability of iron. Inhibition or knockdown of SIRT2 in infected cells displays differential modulation of iron import and export proteins suggesting an ongoing struggle by host to limit the bioavailability of iron to pathogen. Flow cytometry analysis of infected macrophages revealed that these cells utilize a non-canonical pathway for evacuation of intracellular iron. This involves the recruitment of a specific pleioform of the moonlighting protein glyceraldehyde-3 phosphate dehydrogenase (GAPDH) to cell surface for capture of iron transporter protein apo-transferrin. Collectively, our findings reveal the process of SIRT2-mediated iron regulation in Mtb pathogenesis and could provide leads for design of novel host-targeted therapeutics.

Strongyloidiasis stercoralis coinfection is associated with altered iron status biomarkers in tuberculous lymphadenitis

Soil-transmitted helminth [mainly Strongyloidiasis stercoralis (Ss)] and tuberculous lymphadenitis (TBL) coinfection in humans is a significant public health problem. We have previously shown that TBL+Ss+ coinfection significantly alters diverse cytokine, matrix metalloproteinase, and tissue inhibitors of metalloproteinase profiles. However, no data is available to understand the influence of Ss coinfection in TBL disease with respect to iron status biomarkers. Hence, we have studied the effect of Ss coinfection on the circulating levels of iron status (ferritin, transferrin [TF], apotransferrin [ApoT], hepcidin, hemopexin) biomarkers in TBL disease. Our results show that TBL+Ss+ and/or TBL+Ss- individuals are associated with significantly altered biochemical and hematological (red blood cell (RBC) counts, hemoglobin (Hb), hematocrit (HCT), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) were decreased, and platelets were increased) parameters compared to TBL-Ss+ individuals. Our results also show that TBL+Ss+ coinfection is associated with diminished circulating levels of ferritin, ApoT, hepcidin, and hemopexin compared to TBL+Ss- individuals. TBL+Ss+ and TBL+Ss- groups are associated with altered iron status biomarkers (decreased ferritin [TBL+Ss+ alone] and increased TF, ApoT, hepcidin and hemopexin [TBL+Ss- alone]) compared to TBL-Ss+ group. The heat map expression profile and principal component analysis (PCA) analysis of iron status biomarkers were significantly altered in TBL+Ss+ compared to TBL+Ss- and/or TBL-Ss+ individuals. A significant correlation (positive/negative) was obtained among the biochemical and hematological parameters (white blood cells (WBC)/ferritin, TF, and hepcidin, mean corpuscular hemoglobin concentration (MCHC)/ferritin and hemopexin) with iron status biomarkers. Finally, receiver operating characteristic (ROC) analysis revealed that hemopexin was significantly associated with greater specificity and sensitivity in discriminating TBL+Ss+ and TBL+Ss- coinfected individuals. Thus, our data conclude that Ss coinfection is associated with altered iron status biomarkers indicating that coinfection might alter the host-Mtb interface and could influence the disease pathogenesis.

Recent Developments in Drug Delivery for Treatment of Tuberculosis by Targeting Macrophages

Tuberculosis (TB) is among the greatest public health and safety concerns in the 21st century, Mycobacterium tuberculosis, which causes TB, infects alveolar macrophages and uses these cells as one of its primary sites of replication. The current TB treatment regimen, which consist of chemotherapy involving a combination of 3-4 antimicrobials for a duration of 6-12 months, is marked with significant side effects, toxicity, and poor compliance. Targeted drug delivery offers a strategy that could overcome many of the problems of current TB treatment by specifically targeting infected macrophages. Recent advances in nanotechnology and material science have opened an avenue to explore drug carriers that actively and passively target macrophages. This approach can increase the drug penetration into macrophages by using ligands on the nanocarrier that interact with specific receptors for macrophages. This review encompasses the recent development of drug carriers specifically targeting macrophages actively and passively. Future directions and challenges associated with development of effective TB treatment is also discussed.

Host glyceraldehyde-3-phosphate dehydrogenase-mediated iron acquisition is hijacked by intraphagosomal Mycobacterium tuberculosis

Availability of iron is a key factor in the survival and multiplication of Mycobacterium tuberculosis (M.tb) within host macrophage phagosomes. Despite host cell iron regulatory machineries attempts to deny supply of this essential micronutrient, intraphagosomal M.tb continues to access extracellular iron. In the current study, we report that intracellular M.tb exploits mammalian secreted Glyceraldehyde 3-phosphate dehydrogenase (sGAPDH) for the delivery of host iron carrier proteins lactoferrin (Lf) and transferrin (Tf). Studying the trafficking of iron carriers in infected cells we observed that sGAPDH along with the iron carrier proteins are preferentially internalized into infected cells and trafficked to M.tb containing phagosomes where they are internalized by resident mycobacteria resulting in iron delivery. Collectively our findings provide a new mechanism of iron acquisition by M.tb involving the hijack of host sGAPDH. This may contribute to its successful pathogenesis and provide an option for targeted therapeutic intervention.

Iron homeostasis during anemia of inflammation: a prospective study in patients with tuberculosis

Anemia of inflammation is a hallmark of tuberculosis. Factors controlling iron metabolism during anemia of inflammation and its resolution are uncertain. Whether iron supplements should be given during anti-tuberculosis treatment to support Hb recovery is unclear. Before and during treatment of tuberculosis, we assessed iron kinetics, and changes in inflammation and iron metabolism indices. In a 26-wk prospective study, Tanzanian adults with tuberculosis (n=18) were studied before treatment and then every two weeks during treatment; oral and intravenous iron tracers were administered before treatment, after intensive phase (8/12 wk) and complete treatment (24 wk); no iron supplements were given. Before treatment, hepcidin and erythroferrone (ERFE) were greatly elevated, erythrocyte iron utilization was high (~80%) and iron absorption was negligible (<1%). During treatment, hepcidin and IL-6 decreased ~70% after only 2 wk (p<0.001); in contrast, ERFE did not significantly decrease until 8 wk (p<0.01). ERFE and IL-6 were the main opposing determinants of hepcidin (p<0.05) and greater ERFE was associated with reticulocytosis and hemoglobin (Hb) repletion (p<0.01). Dilution of baseline tracer concentration was 2.6-fold higher during intensive phase treatment (p<0.01) indicating enhanced erythropoiesis. After treatment completion, iron absorption increased ~20-fold (p<0.001); Hb increased ~25% (p<0.001). In tuberculosis-associated anemia of inflammation, our findings suggest elevated ERFE is unable to suppress hepcidin and iron absorption is negligible. During treatment, as inflammation resolves, ERFE may remain elevated, contributing to hepcidin suppression and Hb repletion. Iron is well-absorbed only after tuberculosis treatment and supplementation should be reserved for patients remaining anemic after treatment. (ClinicalTrials.gov Identifier:NCT02176772).

The Role of Metallic Nanoparticles in Inhibition of Mycobacterium Tuberculosis and Enhances Phagosome Maturation into the Infected Macrophage

This review focuses on the role of gallium (Ga) nanoparticles (NPs) to enhance phagosome maturation into the Mycobacterium tuberculosis-infected macrophage and the role of magnetic iron NPs as nanocarriers of antituberculosis drugs. The literature shows that silver (Ag) and zinc oxide (ZnO) NPs with dimensions less than 10 nm can penetrate directly through the macrophage bilayer membrane. Ag NPs increase the permeability membrane by motiving the aggregation of proteins in the periplasmic space and forming nano-sized pores. ZnO NPs can interact with the membrane of M. tuberculosis, which leads to the formation of surface pores and the release of intracellular nucleotides. The colloidal Ag:ZnO mixture NPs with 1:1 ratio can eliminate M. tuberculosis and shows the lowest cytotoxicity effects on MCF-7 and THP-1 cell lines. Ag/ZnO nanocrystals are not able to kill M. tuberculosis alone ex-vivo. Hence, bimetallic gold (Au)/Ag NPs possessed high efficiency to inhibit M. tuberculosis in an ex-vivo THP-1 infection model. Co-delivery of mixed MeNPs into a polymeric carrier collaborated to selective uptake by macrophages through passive targeting, initial burst release of ions from the encapsulated metallic (Me) NPs, and eventually, reduction of MeNPs toxicity, and plays a pivotal role in increasing the antitubercular activity compared to use alone. In addition, Ga NPs can import drugs to the macrophage, inhibit M. tuberculosis growth, and reduce the inhibition of phagosome maturation. Magnetic encapsulated NPs exhibited good drug release properties and might be suitable as carriers of antituberculosis drugs.

Biomarkers of iron metabolism facilitate clinical diagnosis in M ycobacterium tuberculosis infection

BACKGROUND

Perturbed iron homeostasis is a risk factor for tuberculosis (TB) progression and an indicator of TB treatment failure and mortality. Few studies have evaluated iron homeostasis as a TB diagnostic biomarker.

METHODS

We recruited participants with TB, latent TB infection (LTBI), cured TB (RxTB), pneumonia (PN) and healthy controls (HCs). We measured serum levels of three iron biomarkers including serum iron, ferritin and transferrin, then established and validated our prediction model.

RESULTS

We observed and verified that the three iron biomarker levels correlated with patient status (TB, HC, LTBI, RxTB or PN) and with the degree of lung damage and bacillary load in patients with TB. We then built a TB prediction model, neural network (NNET), incorporating the data of the three iron biomarkers. The model showed good performance for diagnosis of TB, with 83% (95% CI 77 to 87) sensitivity and 86% (95% CI 83 to 89) specificity in the training data set (n=663) and 70% (95% CI 58 to 79) sensitivity and 92% (95% CI 86 to 96) specificity in the test data set (n=220). The area under the curves (AUCs) of the NNET model to discriminate TB from HC, LTBI, RxTB and PN were all >0.83. Independent validation of the NNET model in a separate cohort (n=967) produced an AUC of 0.88 (95% CI 0.85 to 0.91) with 74% (95% CI 71 to 77) sensitivity and 92% (95% CI 87 to 96) specificity.

CONCLUSIONS

The established NNET TB prediction model discriminated TB from HC, LTBI, RxTB and PN in a large cohort of patients. This diagnostic assay may augment current TB diagnostics.

Dual Inhibition of Klebsiella pneumoniae and Pseudomonas aeruginosa Iron Metabolism Using Gallium Porphyrin and Gallium Nitrate

Iron- and heme-uptake pathways and metabolism are promising targets for the development of new antimicrobial agents, as their disruption would lead to nutritional iron starvation and inhibition of bacterial growth. Salts of gallium(III) (Ga), an iron mimetic metal, disrupt iron-dependent biological processes by binding iron-utilizing proteins and competing with iron for uptake by bacterial siderophore-mediated iron uptake systems. Ga porphyrins, heme mimetic complexes, disrupt heme-utilizing hemoproteins. Because Ga(NO₃)₃ and Ga porphyrin disrupt different pathways of bacterial ion acquisition and utilization, we hypothesized that if used in combination, they would result in enhanced antimicrobial activity. Antimicrobial activity of Ga porphyrins (Ga protoporphyrin, GaPP, or Ga mesoporphyrin, GaMP) alone and in combination with Ga(NO₃)₃ were evaluated against Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter baumannii, and methicillin-resistant Staphylococcus aureus (MRSA) under iron-limited conditions. The Ga porphyrin/Ga(NO₃)₃ combination demonstrated substantial synergism against K. pneumoniae, P. aeruginosa, and MRSA. Time-kill assays revealed that the synergistic combination of GaPP/Ga(NO₃)₃ was bacteriostatic against K. pneumoniae and MRSA and bactericidal against P. aeruginosa. The GaPP/Ga(NO₃)₃ combination significantly disrupted K. pneumoniae and P. aeruginosa biofilms on plasma-coated surfaces and increased the survival of Caenorhabditis elegans infected with K. pneumoniae or P. aeruginosa. When assessing the antibacterial activity of the Ga(III)/antibiotic combinations, GaPP/colistin and Ga(NO₃)₃/colistin combinations also showed synergistic activity against K. pneumoniae and P. aeruginosa. Our results demonstrate that GaPP and Ga(NO₃)₃ have significant synergistic effects against several important human bacterial pathogens through dual inhibition of iron and heme metabolism.

Ferritin H Deficiency in Myeloid Compartments Dysregulates Host Energy Metabolism and Increases Susceptibility to Mycobacterium tuberculosis Infection

Iron is an essential factor for the growth and virulence of Mycobacterium tuberculosis (Mtb). However, little is known about the mechanisms by which the host controls iron availability during infection. Since ferritin heavy chain (FtH) is a major intracellular source of reserve iron in the host, we hypothesized that the lack of FtH would cause dysregulated iron homeostasis to exacerbate TB disease. Therefore, we used knockout mice lacking FtH in myeloid-derived cell populations to study Mtb disease progression. We found that FtH plays a critical role in protecting mice against Mtb, as evidenced by increased organ burden, extrapulmonary dissemination, and decreased survival in Fth-/- mice. Flow cytometry analysis showed that reduced levels of FtH contribute to an excessive inflammatory response to exacerbate disease. Extracellular flux analysis showed that FtH is essential for maintaining bioenergetic homeostasis through oxidative phosphorylation. In support of these findings, RNAseq and mass spectrometry analyses demonstrated an essential role for FtH in mitochondrial function and maintenance of central intermediary metabolism in vivo. Further, we show that FtH deficiency leads to iron dysregulation through the hepcidin-ferroportin axis during infection. To assess the clinical significance of our animal studies, we performed a clinicopathological analysis of iron distribution within human TB lung tissue and showed that Mtb severely disrupts iron homeostasis in distinct microanatomic locations of the human lung. We identified hemorrhage as a major source of metabolically inert iron deposition. Importantly, we observed increased iron levels in human TB lung tissue compared to healthy tissue. Overall, these findings advance our understanding of the link between iron-dependent energy metabolism and immunity and provide new insight into iron distribution within the spectrum of human pulmonary TB. These metabolic mechanisms could serve as the foundation for novel host-directed strategies.

Regulation of Three Virulence Strategies of Mycobacterium tuberculosis: A Success Story

Tuberculosis remains one of the deadliest diseases. Emergence of drug-resistant and multidrug-resistant M. tuberculosis strains makes treating tuberculosis increasingly challenging. In order to develop novel intervention strategies, detailed understanding of the molecular mechanisms behind the success of this pathogen is required. Here, we review recent literature to provide a systems level overview of the molecular and cellular components involved in divalent metal homeostasis and their role in regulating the three main virulence strategies of M. tuberculosis: immune modulation, dormancy and phagosomal rupture. We provide a visual and modular overview of these components and their regulation. Our analysis identified a single regulatory cascade for these three virulence strategies that respond to limited availability of divalent metals in the phagosome.

Elemental Ingredients in the Macrophage Cocktail: Role of ZIP8 in Host Response to Mycobacterium tuberculosis

Tuberculosis (TB) is a global epidemic caused by the infection of human macrophages with the world's most deadly single bacterial pathogen, Mycobacterium tuberculosis (M.tb). M.tb resides in a phagosomal niche within macrophages, where trace element concentrations impact the immune response, bacterial metal metabolism, and bacterial survival. The manipulation of micronutrients is a critical mechanism of host defense against infection. In particular, the human zinc transporter Zrt-/Irt-like protein 8 (ZIP8), one of 14 ZIP family members, is important in the flux of divalent cations, including zinc, into the cytoplasm of macrophages. It also has been observed to exist on the membrane of cellular organelles, where it can serve as an efflux pump that transports zinc into the cytosol. ZIP8 is highly inducible in response to M.tb infection of macrophages, and we have observed its localization to the M.tb phagosome. The expression, localization, and function of ZIP8 and other divalent cation transporters within macrophages have important implications for TB prevention and dissemination and warrant further study. In particular, given the importance of zinc as an essential nutrient required for humans and M.tb, it is not yet clear whether ZIP-guided zinc transport serves as a host protective factor or, rather, is targeted by M.tb to enable its phagosomal survival.

Mycobacterium tuberculosis Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH) Functions as a Receptor for Human Lactoferrin

Iron is crucial for the survival of living cells, particularly the human pathogen Mycobacterium tuberculosis (M.tb) which uses multiple strategies to acquire and store iron. M.tb synthesizes high affinity iron chelators (siderophores), these extract iron from host iron carrier proteins such as transferrin (Tf) and lactoferrin (Lf). Recent studies have revealed that M.tb may also relocate several housekeeping proteins to the cell surface for capture and internalization of host iron carrier protein transferrin. One of the identified receptors is the glycolytic enzyme Glyceraldehyde-3-phosphate dehydrogenase (GAPDH). This conserved multifunctional protein has been identified as a virulence factor in several other bacterial species. Considering the close structural and functional homology between the two major human iron carrier proteins (Tf and Lf) and the fact that Lf is abundantly present in lung fluid (unlike Tf which is present in plasma), we evaluated whether GAPDH also functions as a dual receptor for Lf. The current study demonstrates that human Lf is sequestered at the bacterial surface by GAPDH. The affinity of Lf-GAPDH (31.7 ± 1.68 nM) is higher as compared to Tf-GAPDH (160 ± 24 nM). Two GAPDH mutants were analyzed for their enzymatic activity and interaction with Lf. Lastly, the present computational studies offer the first significant insights for the 3D structure of monomers and assembled tetramer with the associated co-factor NAD⁺. Sequence analysis and structural modeling identified the surface exposed, evolutionarily conserved and functional residues and predicted the effect of mutagenesis on GAPDH.

Association of Single-Nucleotide Polymorphism in the Hepcidin Promoter Gene with Susceptibility to Extrapulmonary Tuberculosis

BACKGROUND

Hepcidin is a 25-amino acid peptide produced by the liver in response to inflammation and iron overload. It is encoded by the hepcidin antimicrobial peptide (HAMP) gene and plays a key role in innate immunity. Previous studies have reported that a -582 A>G polymorphism in the HAMP promoter (HAMP-P) affects hepcidin expression, causing susceptibility to various bacterial and viral pathogens. However, it is not known whether the HAMP-P -582 A>G polymorphism is associated with tuberculosis (TB) susceptibility.

AIMS

The objective of the current study was to examine the relationship between the HAMP-P -582 A>G polymorphism and TB susceptibility in a Chinese Han population.

METHODS

Han Chinese subjects examined included 500 pulmonary TB, 386 extrapulmonary TB, and 600 healthy control subjects. We analyzed correlations between the hepcidin promoter -582 A>G polymorphism and disease susceptibility and then examined the regulatory effects of the -582 A>G variant on hepcidin production in CD14+ monocyte cultures stimulated with lipoarabinomannan derived from Mycobacterium tuberculosis.

RESULTS

Our findings indicate that the HAMP-P -582 A>G polymorphism (rs10421768) is associated with susceptibility to extrapulmonary TB, but not pulmonary TB. CD14+ monocytes from individuals with the rs10421768 GG genotype secreted significantly less hepcidin in response to M. tuberculosis lipoarabinomannan compared with cells from individuals with either the AA or AG genotypes.

CONCLUSIONS

The G allele of the HAMP-P -582 A>G gene may play a critical role in TB susceptibility.

Gallium nanoparticles facilitate phagosome maturation and inhibit growth of virulent Mycobacterium tuberculosis in macrophages

New treatments and novel drugs are required to counter the growing problem of drug-resistant strains of Mycobacterium tuberculosis (M.tb). Our approach against drug resistant M.tb, as well as other intracellular pathogens, is by targeted drug delivery using nanoformulations of drugs already in use, as well as drugs in development. Among the latter are gallium (III) (Ga)-based compounds. In the current work, six different types of Ga and rifampin nanoparticles were prepared in such a way as to enhance targeting of M.tb infected-macrophages. They were then tested for their ability to inhibit growth of a fully pathogenic strain (H37Rv) or a non-pathogenic strain (H37Ra) of M.tb. Encapsulating Ga in folate- or mannose-conjugated block copolymers provided sustained Ga release for 15 days and significantly inhibited M.tb growth in human monocyte-derived macrophages. Nanoformulations with dendrimers encapsulating Ga or rifampin also showed promising anti-tuberculous activity. The nanoparticles co-localized with M.tb containing phagosomes, as measured by detection of mature cathepsin D (34 kDa, lysosomal hydrogenase). They also promoted maturation of the phagosome, which would be expected to increase macrophage-mediated killing of the organism. Delivery of Ga or rifampin in the form of nanoparticles to macrophages offers a promising approach for the development of new therapeutic anti-tuberculous drugs.

Mycobacterium tuberculosis Rv1474c is a TetR-like transcriptional repressor that regulates aconitase, an essential enzyme and RNA-binding protein, in an iron-responsive manner

Mycobacterium tuberculosis (M.tb), tuberculosis (TB) causing bacteria, employs several mechanisms to maintain iron homeostasis which is critical for its survival and pathogenesis. M.tb aconitase (Acn), a [4Fe-4S] cluster-containing essential protein, apart from participating in energy cycle, also binds to predicted iron-responsive RNA elements. In this study, we identified Rv1474c as a regulator of its operonic partner acn and carried out its biochemical and functional characterization. The binding motif for Rv1474c in the upstream region of acn (Rv1475c)-Rv1474c operon was verified by gel-shift assays. Reporter assays in E. coli followed by over-expression studies in mycobacteria, using both wild type and a DNA-binding defective mutant, demonstrated Rv1474c as a Tet-R like repressor of acn. Rv1474c, besides binding tetracycline, could also bind iron which negatively influenced its DNA binding activity. Further, a consistent decrease in the relative transcript levels of acn when M.tb was grown in iron-deficient conditions as compared to either normal or other stress conditions, indicated regulation of acn by Rv1474c in an iron-responsive manner in vivo. The absence of homologs in the human host and its association with indispensable iron homeostasis makes Rv1474c an attractive target for designing novel anti-mycobacterials.

Low Dose BCG Infection as a Model for Macrophage Activation Maintaining Cell Viability

Mycobacterium bovis BCG, the current vaccine against tuberculosis, is ingested by macrophages promoting the development of effector functions including cell death and microbicidal mechanisms. Despite accumulating reports on M. tuberculosis, mechanisms of BCG/macrophage interaction remain relatively undefined. In vivo, few bacilli are sufficient to establish a mycobacterial infection; however, in vitro studies systematically use high mycobacterium doses. In this study, we analyze macrophage/BCG interactions and microenvironment upon infection with low BCG doses and propose an in vitro model to study cell activation without affecting viability. We show that RAW macrophages infected with BCG at MOI 1 activated higher and sustained levels of proinflammatory cytokines and transcription factors while MOI 0.1 was more efficient for early stimulation of IL-1β, MCP-1, and KC. Both BCG infection doses induced iNOS and NO in a dose-dependent manner and maintained nuclear and mitochondrial structures. Microenvironment generated by MOI 1 induced macrophage proliferation but not MOI 0.1 infection. In conclusion, BCG infection at low dose is an efficient in vitro model to study macrophage/BCG interactions that maintains macrophage viability and mitochondrial structures. This represents a novel model that can be applied to BCG research fields including mycobacterial infections, cancer immunotherapy, and prevention of autoimmunity and allergies.

Iron Homeostasis in Mycobacterium tuberculosis: Mechanistic Insights into Siderophore-Mediated Iron Uptake

Mycobacterium tuberculosis requires iron for normal growth but faces a limitation of the metal ion due to its low solubility at biological pH and the withholding of iron by the mammalian host. The pathogen expresses the Fe(3+)-specific siderophores mycobactin and carboxymycobactin to chelate the metal ion from insoluble iron and the host proteins transferrin, lactoferrin, and ferritin. Siderophore-mediated iron uptake is essential for the survival of M. tuberculosis, as knockout mutants, which were defective in siderophore synthesis or uptake, failed to survive in low-iron medium and inside macrophages. But as excess iron is toxic due to its catalytic role in the generation of free radicals, regulation of iron uptake is necessary to maintain optimal levels of intracellular iron. The focus of this review is to present a comprehensive overview of iron homeostasis in M. tuberculosis that is discussed in the context of mycobactin biosynthesis, transport of iron across the mycobacterial cell envelope, and storage of excess iron. The clinical significance of the serum iron status and the expression of the iron-regulated protein HupB in tuberculosis (TB) patients is presented here, highlighting the potential of HupB as a marker, notably in extrapulmonary TB cases.

Prolonged-acting, multi-targeting gallium nanoparticles potently inhibit growth of both HIV and mycobacteria in co-infected human macrophages

Human immunodeficiency virus (HIV) infection and Mycobacterium tuberculosis (TB) are responsible for two of the major global human infectious diseases that result in significant morbidity, mortality and socioeconomic impact. Furthermore, severity and disease prevention of both infections is enhanced by co-infection. Parallel limitations also exist in access to effective drug therapy and the emergence of resistance. Furthermore, drug-drug interactions have proven problematic during treatment of co-incident HIV and TB infections. Thus, improvements in drug access and simplified treatment regimens are needed immediately. One of the key host cells infected by both HIV and TB is the mononuclear phagocyte (MP; monocyte, macrophage and dendritic cell). Therefore, we hypothesized that one way this can be achieved is through drug-targeting by a nanoformulated drug that ideally would be active against both HIV and TB. Accordingly, we validated macrophage targeted long acting (sustained drug release) gallium (Ga) nanoformulation against HIV-mycobacterium co-infection. The multi-targeted Ga nanoparticle agent inhibited growth of both HIV and TB in the macrophage. The Ga nanoparticles reduced the growth of mycobacterium and HIV for up to 15 days following single drug loading. These results provide a potential new approach to treat HIV-TB co-infection that could eventually lead to improved clinical outcomes.

The exochelins of pathogenic mycobacteria: unique, highly potent, lipid- and water-soluble hexadentate iron chelators with multiple potential therapeutic uses

SIGNIFICANCE

Exochelins are lipid- and water-soluble siderophores of Mycobacterium tuberculosis with unique properties that endow them with exceptional pharmacologic utility. Exochelins can be utilized as probes to decipher the role of iron in normal and pathological states, and, since they rapidly cross cell membranes and chelate intracellular iron with little or no toxicity, exochelins are potentially useful for the treatment of a number of iron-dependent pathological phenomena.

RECENT ADVANCES

In animal models, exochelins have been demonstrated to have promise for the treatment of transfusion-related iron overload, restenosis after coronary artery angioplasty, cancer, and oxidative injury associated with acute myocardial infarction and transplantation.

CRITICAL ISSUES

To be clinically effective, iron chelators should be able to rapidly enter cells and chelate iron at key intracellular sites. Desferri-exochelins, and other lipid-soluble chelators, can readily cross cell membranes and remove intracellular free iron; whereas deferoxamine, which is lipid insoluble, cannot do so. Clinical utility also requires that the chelators be nontoxic, which, we hypothesize, includes the capability to prevent iron from catalyzing free radical reactions which produce •OH or other reactive oxygen species. Lipid-soluble iron chelators currently available for clinical application are bidentate (deferiprone) or tridentate (desferasirox) molecules that do not block all six sites on the iron molecule capable of catalyzing free radical reactions. In contrast, desferri-exochelins are hexadentate molecules, and by forming a one-to-one binding relationship with iron, they prevent free radical reactions.

FUTURE DIRECTIONS

Clinical studies are needed to assess the utility of desferri-exochelins in the treatment of iron-dependent pathological disorders.

Iron in intracellular infection: to provide or to deprive?

Due to their chemical versatility, transition metals were incorporated as cofactors for several basic metabolic pathways in living organisms. This same characteristic makes them potentially harmful, since they can be engaged in deleterious reactions like Fenton chemistry. As such, organisms have evolved highly specialized mechanisms to supply their own metal needs while keeping their toxic potential in check. This dual character comes into play in host-pathogen interactions, given that the host can either deprive the pathogen of these key nutrients or exploit them to induce toxicity toward the invading agent. Iron stands as the prototypic example of how a metal can be used to limit the growth of pathogens by nutrient deprivation, a mechanism widely studied in Mycobacterium infections. However, the host can also take advantage of iron-induced toxicity to control pathogen proliferation, as observed in infections caused by Leishmania. Whether we may harness either of the two pathways for therapeutical purposes is still ill-defined. In this review, we discuss how modulation of the host iron availability impacts the course of infections, focusing on those caused by two relevant intracellular pathogens, Mycobacterium and Leishmania.

Gallium nitrate is efficacious in murine models of tuberculosis and inhibits key bacterial Fe-dependent enzymes

Acquiring iron (Fe) is critical to the metabolism and growth of Mycobacterium tuberculosis. Disruption of Fe metabolism is a potential approach for novel antituberculous therapy. Gallium (Ga) has many similarities to Fe. Biological systems are often unable to distinguish Ga(3+) from Fe(3+). Unlike Fe(3+), Ga(3+) cannot be physiologically reduced to Ga(2+). Thus, substituting Ga for Fe in the active site of enzymes may render them nonfunctional. We previously showed that Ga inhibits growth of M. tuberculosis in broth and within cultured human macrophages. We now report that Ga(NO3)3 shows efficacy in murine tuberculosis models. BALB/c SCID mice were infected intratracheally with M. tuberculosis, following which they received daily intraperitoneal saline, Ga(NO3)3, or NaNO3. All mice receiving saline or NaNO3 died. All Ga(NO3)3-treated mice survived. M. tuberculosis CFU in the lungs, liver, and spleen of the NaNO3-treated or saline-treated mice were significantly higher than those in Ga-treated mice. When BALB/c mice were substituted for BALB/c SCID mice as a chronic (nonlethal) infection model, Ga(NO3)3 treatment significantly decreased lung CFU. To assess the mechanism(s) whereby Ga inhibits bacterial growth, the effect of Ga on M. tuberculosis ribonucleotide reductase (RR) (a key enzyme in DNA replication) and aconitase activities was assessed. Ga decreased M. tuberculosis RR activity by 50 to 60%, but no additional decrease in RR activity was seen at Ga concentrations that completely inhibited mycobacterial growth. Ga decreased aconitase activity by 90%. Ga(NO3)3 shows efficacy in murine M. tuberculosis infection and leads to a decrease in activity of Fe-dependent enzymes. Additional work is warranted to further define Ga's mechanism of action and to optimize delivery forms for possible therapeutic uses in humans.

Iron acquisition by Mycobacterium tuberculosis residing within myeloid dendritic cells

The pathophysiology of Mycobacterium tuberculosis (M.tb) infection is linked to the ability of the organism to grow within macrophages. Lung myeloid dendritic cells are a newly recognized reservoir of M.tb during infection. Iron (Fe) acquisition is critical for M.tb growth. In vivo, extracellular Fe is chelated to transferrin (TF) and lactoferrin (LF). We previously reported that M.tb replicating in human monocyte-dervied macrophages (MDM) can acquire Fe bound to TF, LF, and citrate, as well as from the MDM cytoplasm. Access of M.tb to Fe may influence its growth in macrophages and dendritic cells. In the present work we confirmed the ability of different strains of M.tb to grow in human myeloid dendritic cells in vitro. Fe acquired by M.tb replicating within dendritic cells from externally added Fe chelates varied with the Fe chelate present in the external media: Fe-citrate > Fe-LF > Fe-TF. Fe acquisition rates from each chelate did not vary over 7 days. M.tb within dendritic cells also acquired Fe from the dendritic cell cytoplasm, with the efficiency of Fe acquisition greater from cytoplasmic Fe sources, regardless of the initial Fe chelate from which that cytoplasmic Fe was derived. Growth and Fe acquisition results with human MDM were similar to those with dendritic cells. M.tb grow and replicate within myeloid dendritic cells in vitro. Fe metabolism of M.tb growing in either MDM or dendritic cells in vitro is influenced by the nature of Fe available and the organism appears to preferentially access cytoplasmic rather than extracellular Fe sources. Whether these in vitro data extend to in vivo conditions should be examined in future studies.

Profiling early lung immune responses in the mouse model of tuberculosis

Tuberculosis (TB) is caused by the intracellular bacteria Mycobacterium tuberculosis, and kills more than 1.5 million people every year worldwide. Immunity to TB is associated with the accumulation of IFNγ-producing T helper cell type 1 (Th1) in the lungs, activation of M.tuberculosis-infected macrophages and control of bacterial growth. However, very little is known regarding the early immune responses that mediate accumulation of activated Th1 cells in the M.tuberculosis-infected lungs. To define the induction of early immune mediators in the M.tuberculosis-infected lung, we performed mRNA profiling studies and characterized immune cells in M.tuberculosis-infected lungs at early stages of infection in the mouse model. Our data show that induction of mRNAs involved in the recognition of pathogens, expression of inflammatory cytokines, activation of APCs and generation of Th1 responses occurs between day 15 and day 21 post infection. The induction of these mRNAs coincides with cellular accumulation of Th1 cells and activation of myeloid cells in M.tuberculosis-infected lungs. Strikingly, we show the induction of mRNAs associated with Gr1+ cells, namely neutrophils and inflammatory monocytes, takes place on day 12 and coincides with cellular accumulation of Gr1+ cells in M.tuberculosis-infected lungs. Interestingly, in vivo depletion of Gr1+ neutrophils between days 10-15 results in decreased accumulation of Th1 cells on day 21 in M.tuberculosis-infected lungs without impacting overall protective outcomes. These data suggest that the recruitment of Gr1+ neutrophils is an early event that leads to production of chemokines that regulate the accumulation of Th1 cells in the M.tuberculosis-infected lungs.

Gallium disrupts iron uptake by intracellular and extracellular Francisella strains and exhibits therapeutic efficacy in a murine pulmonary infection model

Francisella tularensis requires iron (Fe) for growth, but the biologic sources of Fe for this organism are largely unknown. We found that Francisella sp. growing in broth culture or within human macrophages can acquire Fe from the two major host Fe-binding proteins, lactoferrin (Lf) and transferrin (Tf). Fe acquisition is a potential target for novel therapies. Gallium (Ga) is a transition metal that interferes with cellular Fe metabolism by competing with Fe for uptake/utilization. Growth of either F. tularensis live vaccine strain (LVS) or Francisella novicida was inhibited by >or=2 microM Ga chelated to Tf or Lf, with GaLf being somewhat more potent. Francisella spp. express two Fe-containing antioxidant enzymes, catalase (KatG) and Fe cofactored superoxide dismutase (FeSOD). Growth of LVS with 10 muM GaTf or GaLf led to a dramatic decrease in bacterial catalase activity and in FeSOD activity that was associated with an increased susceptibility to H(2)O(2). Ga also protected mice from intranasal challenge with F. novicida. Whereas 100% of the F. novicida-infected mice died by day 9, 75% of the mice receiving Ga continued to survive to at least day 15. Thus, a single intranasal dose of Ga followed by daily intraperitoneal Ga at a dose tolerated by the animals resulted in prolonged survival. These data support the potential utility of Ga as a therapy for F. tularensis infection of the lung.

Protein turnover in mycobacterial proteomics

Understanding the biology of Mycobacterium tuberculosis is one of the primary challenges in current tuberculosis research. Investigation of mycobacterial biology using the systems biology approach has deciphered much information with regard to the bacilli and tuberculosis pathogenesis. The modulation of its environment and the ability to enter a dormant phase are the hallmarks of M. tuberculosis. Until now, proteome studies have been able to understand much about the role of various proteins, mostly in growing M. tuberculosis cells. It has been difficult to study dormant M. tuberculosis by conventional proteomic techniques with very few proteins being found to be differentially expressed. Discrepancy between proteome and transcriptome studies lead to the conclusion that a certain aspect of the mycobacterial proteome is not being explored. Analysis of protein turnover may be the answer to this dilemma. This review, while giving a gist of the proteome response of mycobacteria to various stresses, analyzes the data obtained from abundance studies versus data from protein turnover studies in M. tuberculosis. This review brings forth the point that protein turnover analysis is capable of discerning more subtle changes in protein synthesis, degradation, and secretion activities. Thus, turnover studies could be incorporated to provide a more in-depth view into the proteome, especially in dormant or persistent cells. Turnover analysis might prove helpful in drug discovery and a better understanding of the dynamic nature of the proteome of mycobacteria.

Increased expression of host iron-binding proteins precedes iron accumulation and calcification of primary lung lesions in experimental tuberculosis in the guinea pig

The growth and virulence of Mycobacterium tuberculosis depends on its ability to scavenge host iron, an essential and limited micronutrient in vivo. In this study, we show that ferric iron accumulates both intra- and extra-cellularly in the primary lung lesions of guinea pigs aerosol-infected with the H37Rv strain of M. tuberculosis. Iron accumulated within macrophages at the periphery of the primary granulomatous lesions while extra-cellular ferric iron was concentrated in areas of lesion necrosis. Accumulation of iron within primary lesions was preceded by an increase in expression of heavy chain (H) ferritin, lactoferrin and receptors for transferrin, primarily by macrophages and granulocytes. The increased expression of intra-cellular H ferritin and extra-cellular lactoferrin, more so than transferrin receptor, paralleled the development of necrosis within primary lesions. The deposition of extra-cellular ferric iron within necrotic foci coincided with the accumulation of calcium and phosphorus and other cations in the form of dystrophic calcification. Primary lung lesions from guinea pigs vaccinated with Mycobactrium bovis BCG prior to experimental infection, had reduced iron accumulation as well as H ferritin, lactoferrin and transferrin receptor expression. The amelioration of primary lesion necrosis and dystrophic calcification by BCG vaccination was coincident with the lack of extra-cellular ferric iron and lactoferrin accumulation. These data demonstrate that BCG vaccination ameliorates primary lesion necrosis, dystrophic mineralization and iron accumulation, in part by down-regulating the expression of macrophage H ferritin, lactoferrin and transferrin receptors, in vivo.

Expression and localization of hepcidin in macrophages: a role in host defense against tuberculosis

Hepcidin is an antimicrobial peptide produced by the liver in response to inflammatory stimuli and iron overload. Hepcidin regulates iron homeostasis by mediating the degradation of the iron export protein ferroportin 1, thereby inhibiting iron absorption from the small intestine and release of iron from macrophages. Here, we examined the expression of hepcidin in macrophages infected with the intracellular pathogens Mycobacterium avium and Mycobacterium tuberculosis. Stimulation of the mouse RAW264.7 macrophage cell line and mouse bone marrow-derived macrophages with mycobacteria and IFN-gamma synergistically induced high levels of hepcidin mRNA and protein. Similar results were obtained using the human THP-1 monocytic cell line. Stimulation of macrophages with the inflammatory cytokines IL-6 and IL-beta did not induce hepcidin mRNA expression. Iron loading inhibited hepcidin mRNA expression induced by IFN-gamma and M. avium, and iron chelation increased hepcidin mRNA expression. Intracellular protein levels and secretion of hepcidin were determined by a competitive chemiluminescence ELISA. Stimulation of RAW264.7 cells with IFN-gamma and M. tuberculosis induced intracellular expression and secretion of hepcidin. Furthermore, confocal microscopy analyses showed that hepcidin localized to the mycobacteria-containing phagosomes. As hepcidin has been shown to possess direct antimicrobial activity, we investigated its activity against M. tuberculosis. We found that hepcidin inhibited M. tuberculosis growth in vitro and caused structural damage to the mycobacteria. In summary, our data show for the first time that hepcidin localizes to the phagosome of infected, IFN-gamma-activated cells and has antimycobacterial activity.

Neisseria gonorrhoeae requires expression of TonB and the putative transporter TdfF to replicate within cervical epithelial cells

Neisseria gonorrhoeae has evolved a repertoire of iron acquisition systems that facilitate essential iron uptake in the human host. Acquisition of iron requires both the energy-harnessing cytoplasmic membrane protein, TonB, as well as specific outer membrane TonB-dependent transporters (TdTs.) Survival within host epithelial cells is important to the pathogenesis of gonococcal disease and may contribute to the persistence of infection. However, the mechanisms by which gonococci acquire iron within this intracellular niche are not currently understood. In this study, we investigated the survival of gonococcal strain FA1090 within ME180 human cervical epithelial cells with respect to high affinity iron acquisition. Intracellular survival was dependent upon iron supplied by the host cell. TonB was expressed in the host cell environment and this protein was critical to gonococcal intracellular survival. Furthermore, expression of the characterized outer membrane transporters TbpA, FetA and LbpA and putative transporters TdfG, TdfH and TdfJ were not necessary for intracellular survival. Conversely, intracellular survival was dependent on expression of the putative transporter, TdfF. Expression of TdfF was detected in the presence of epithelial cell culture media containing fetal bovine serum. Expression was further modulated by iron availability. To our knowledge, this study is the first to demonstrate the specific requirement for a single iron transporter in the survival of a bacterial pathogen within host epithelial cells.

Hereditary hemochromatosis results in decreased iron acquisition and growth by Mycobacterium tuberculosis within human macrophages

Iron (Fe) acquisition is essential for the growth of intracellular Mycobacterium tuberculosis (M.tb). How this occurs is poorly understood. Hereditary hemochromatosis is an inherited disease in which most cells become overloaded with Fe. However, hereditary hemochromatosis macrophages have lower than normal levels of intracellular Fe. This suggests M.tb growth should be slower in those cells if macrophage intracellular Fe is used by M.tb. Therefore, we compared trafficking and acquisition of transferrin (Tf)- and lactoferrin (Lf)-chelated Fe by M.tb within the phagosome of monocyte-derived macrophages (MDM) from healthy controls and subjects with hereditary hemochromatosis. M.tb in both sets of macrophages acquired more Fe from Lf than Tf. Fe acquisition by M.tb within hereditary hemochromatosis macrophages was decreased by 84% from Tf and 92% from Lf relative to that in healthy control macrophages. There was no difference in Fe acquired from Tf and Lf by the two macrophage phenotypes. Both acquired 3 times more Fe from Lf than Tf. M.tb infection and incubation with interferon gamma (IFN-gamma) reduced macrophage Fe acquisition by 20% and 50%, respectively. Both Tf and Lf colocalized with M.tb phagosomes to a similar extent, independent of macrophage phenotype. M.tb growth was 50% less in hereditary hemochromatosis macrophages. M.tb growing within macrophages from subjects with hereditary hemochromatosis acquire less Fe compared with healthy controls. This is associated with reduced growth of M.tb. These data support a role for macrophage intracellular Fe as a source for M.tb growth.

Iron and infection: effects of host iron status and the iron-regulatory genes haptoglobin and NRAMP1 (SLC11A1) on host-pathogen interactions in tuberculosis and HIV

There are many lines of evidence illustrating that iron plays a pivotal role in modulating the battle for survival between mammalian hosts and their pathogens. Each displays considerable genetic investment in a wide range of mechanisms for acquiring and maintaining iron. These competitive mechanisms are highly complex, existing within an interacting matrix of absorption, transport, storage and detoxification systems, each of which are iron-responsive and thus able to adapt to the different phases of infection. Considerable genetic polymorphism in some of these systems, with signals of geographic selection in the hosts, and niche selection in the pathogens, indicates that they are critical for species survival. In this review we briefly summarize the role of iron in host immune function before reviewing the available evidence that iron modulates susceptibility and disease outcomes in HIV and TB (tuberculosis). We then examine the putative role of iron-related host genes by focussing on two candidate genes, haptoglobin and NRAMP1, for which there are common polymorphic variants in humans with strong evidence of functionally distinct biochemical phenotypes that would be predicted to influence the course of HIV and TB infections. Finally, we examine the limited evidence so far available that nutrient-gene interactions are likely to influence the way in which gene variants can protect against infection. We conclude that there is a wealth of evidence associating alterations in iron balance and in iron-regulatory systems with disease progression, but that many issues related to the direction of causality, mechanisms of action and sensitivity to pharmacological intervention remain to be elucidated. Since iron is probably the most widely prescribed compound throughout the world, used in both preventative and treatment regimens, a deeper understanding of the host-pathogen interactions relating to iron constitutes an important area for both basic and clinical research.

Iron-binding drugs targeted to lysosomes: a potential strategy to treat inflammatory lung disorders

In many inflammatory lung disorders, an abnormal assimilation of redox-active iron will exacerbate oxidative tissue damage. It may be that the most important cellular pool of redox-active iron exists within lysosomes, making these organelles vulnerable to oxidative stress. In experiments employing respiratory epithelial cells and macrophages, the chelation of intra-lysosomal iron efficiently prevented lysosomal rupture and the ensuing cell death induced by hydrogen peroxide, ionising radiation or silica particles. Furthermore, cell-permeable iron-binding agents (weak bases) that accumulate within lysosomes due to proton trapping were much more efficient for cytoprotection than the chelator, desferrioxamine. On a molar basis, the weak base alpha-lipoic acid plus was 5000 times more effective than desferrioxamine at preventing lysosomal rupture and apoptotic cell death in cell cultures exposed to hydrogen peroxide. Thus, iron-chelating therapy that targets the lysosome might be a future treatment strategy for inflammatory pulmonary diseases.

Iron acquisition from transferrin by Candida albicans depends on the reductive pathway

Host-pathogen interactions that alter virulence are influenced by critical nutrients such as iron. In humans, free iron is unavailable, being present only in high-affinity iron binding proteins such as transferrin. The fungal pathogen Candida albicans grows as a saprophyte on mucosal surfaces. Occasionally it invades systemically, and in this circumstance it will encounter transferrin iron. Here we report that C. albicans is able to acquire iron from transferrin. Iron-loaded transferrin restored growth to cultures arrested by iron deprivation, whereas apotransferrin was unable to promote growth. By using congenic strains, we have been able to show that iron uptake by C. albicans from transferrin was mediated by the reductive pathway (via FTR1). The genetically separate siderophore and heme uptake systems were not involved. FRE10 was required for a surface reductase activity and for efficient transferrin iron uptake activity in unbuffered medium. Other reductase genes were apparently up-regulated in medium buffered at pH 6.3 to 6.4, and the fre10(-/-) mutant had no effect under these conditions. Experiments in which transferrin was sequestered in a dialysis bag demonstrated that cell contact with the substrate was required for iron reduction and release. The requirement of FTR1 for virulence in a systemic infection model and its role in transferrin iron uptake raise the possibility that transferrin is a source of iron during systemic C. albicans infections.

Iron modulates the replication of virulent Mycobacterium bovis in resting and activated bovine and possum macrophages

Bovine and possum macrophages were infected in vitro with a virulent strain of Mycobacterium bovis, and mycobacterial replication was measured in the infected macrophages cultured under a variety of conditions. Virulent M. bovis replicated substantially in alveolar possum macrophages as well as in bovine blood monocyte-derived macrophages. Addition of recombinant bovine interferon-gamma (IFN-gamma) with low concentrations of lipopolysaccharide (LPS) rendered bovine macrophages significantly more resistant to M. bovis replication. Disruption of iron levels in infected macrophages by addition of apotransferrin or bovine lactoferrin blocked replication of M. bovis in both bovine and possum macrophages. On the other hand, addition of exogenous iron, either in the form of iron citrate or iron-saturated transferrin, rendered macrophages of both species much more permissive for the replication of M. bovis. The impact of iron deprivation/loading on the mycobacteriostatic activity of cells was independent of nitric-oxide release, as well as independent of the generation of oxygen radical species in both possum and bovine macrophages. Exogenous iron was shown to reverse the ability of IFN-gamma/LPS pulsed bovine macrophages to restrict M. bovis replication. When autologous possum lymphocytes from animals vaccinated with M. bovis strain BCG were added to infected macrophages, they rendered the macrophages less permissive for virulent M. bovis replication. Loading the cells with iron prior to this macrophage-lymphocyte interaction, reversed this immune effect induced by sensitized cells. We conclude that, in two important animal species, intracellular iron level plays an important role in M. bovis replication in macrophages, irrespective of their activation status.