Nramp1 phagocyte intracellular metal withdrawal defense

Pro-efferocytic nanotherapies reduce vascular inflammation without inducing anemia in a large animal model of atherosclerosis

Atherosclerosis is an inflammatory disorder responsible for cardiovascular disease. Reactivation of efferocytosis, the phagocytic removal of cells by macrophages, has emerged as a translational target for atherosclerosis. Systemic blockade of the key 'don't-eat-me' molecule, CD47, triggers the engulfment of apoptotic vascular tissue and potently reduces plaque burden. However, it also induces red blood cell clearance, leading to anemia. To overcome this, we previously developed a macrophage-specific nanotherapy loaded with a chemical inhibitor that promotes efferocytosis. Because it was found to be safe and effective in murine studies, we aimed to advance our nanoparticle into a porcine model of atherosclerosis. Here, we demonstrate that production can be scaled without impairing nanoparticle function. At an early stage of disease, we find our nanotherapy reduces apoptotic cell accumulation and inflammation in the atherosclerotic lesion. Notably, this therapy does not induce anemia, highlighting the translational potential of targeted macrophage checkpoint inhibitors.

The expression of Nramp1 modulates the uptake of Mycobacterium tuberculosis by macrophages through alternating inflammatory responses

Natural-resistance-associated macrophage protein-1 (NRAMP1) is a transmembrane protein of the mammalian SLC11 gene family. Previously, genome-wide association study (GWAS) have shown that the single nucleotide polymorphisms (SNPs) of NRAMP1 are associated with human susceptibility to tuberculosis (TB), and the detection of clinical samples have demonstrated that the expression levels of NRAMP1 are concomitant with the susceptibility to TB in humans and cows, but underlying mechanism is unknown. In this study, we completed a series of experiments to investigate how the expression of Nramp1 affects the infection of macrophages with Mycobacterium tuberculosis (Mtb). We found that the increase of Nramp1 expression induced the decrease of Mtb infection efficiency and the higher-level expression of pro-inflammatory cytokines and chemokines, However, the knockdown of Nramp1 promoted the efficiency of bacilli infection to macrophages and induced lower-levels of expression of pro-inflammatory cytokines and chemokines. Collectively, the results in this study demonstrated that the levels of Nramp1 expression affect Mtb infection of macrophage and regulate pro-inflammatory responses of macrophages to Mtb infection, indicating the population with the low-expression level of NRAMP1 predispose to Mtb infection and TB development, and suggesting SNPs in NRAMP1 modulate the host susceptibility to TB through its regulation of NRAMP1 expression.

Slc11 Synapomorphy: A Conserved 3D Framework Articulating Carrier Conformation Switch

Transmembrane carriers of the Slc11 family catalyze proton (H+)-dependent uptake of divalent metal ions (Me2+) such as manganese and iron-vital elements coveted during infection. The Slc11 mechanism of high-affinity Me2+ cell import is selective and conserved between prokaryotic (MntH) and eukaryotic (Nramp) homologs, though processes coupling the use of the proton motive force to Me2+ uptake evolved repeatedly. Adding bacterial piracy of Nramp genes spread in distinct environmental niches suggests selective gain of function that may benefit opportunistic pathogens. To better understand Slc11 evolution, Alphafold (AF2)/Colabfold (CF) 3D predictions for bacterial sequences from sister clades of eukaryotic descent (MCb and MCg) were compared using both native and mutant templates. AF2/CF model an array of native MCb intermediates spanning the transition from outwardly open (OO) to inwardly open (IO) carriers. In silico mutagenesis targeting (i) a set of (evolutionarily coupled) sites that may define Slc11 function (putative synapomorphy) and (ii) residues from networked communities evolving during MCb transition indicates that Slc11 synapomorphy primarily instructs a Me2+-selective conformation switch which unlocks carrier inner gate and contributes to Me2+ binding site occlusion and outer gate locking. Inner gate opening apparently proceeds from interaction between transmembrane helix (h) h5, h8 and h1a. MCg1 xenologs revealed marked differences in carrier shape and plasticity, owing partly to an altered intramolecular H+ network. Yet, targeting Slc11 synapomorphy also converted MCg1 IO models to an OO state, apparently mobilizing the same residues to control gates. But MCg1 response to mutagenesis differed, with extensive divergence within this clade correlating with MCb-like modeling properties. Notably, MCg1 divergent epistasis marks the emergence of the genus Bordetella-Achromobacter. Slc11 synapomorphy localizes to the 3D areas that deviate least among MCb and MCg1 models (either IO or OO) implying that it constitutes a 3D network of residues articulating a Me2+-selective carrier conformation switch which is maintained in fast-evolving clades at the cost of divergent epistatic interactions impacting carrier shape and dynamics.

How metals fuel fungal virulence, yet promote anti-fungal immunity

Invasive fungal infections represent a significant global health problem, and present several clinical challenges, including limited treatment options, increasing rates of antifungal drug resistance and compounding comorbidities in affected patients. Metals, such as copper, iron and zinc, are critical for various biological and cellular processes across phyla. In mammals, these metals are important determinants of immune responses, but pathogenic microbes, including fungi, also require access to these metals to fuel their own growth and drive expression of major virulence traits. Therefore, host immune cells have developed strategies to either restrict access to metals to induce starvation of invading pathogens or deploy toxic concentrations within phagosomes to cause metal poisoning. In this Review, we describe the mechanisms regulating fungal scavenging and detoxification of copper, iron and zinc and the importance of these mechanisms for virulence and infection. We also outline how these metals are involved in host immune responses and the consequences of metal deficiencies or overloads on how the host controls invasive fungal infections.

Host-directed therapy for bacterial infections -Modulation of the phagolysosome pathway

Bacterial infections still impose a significant burden on humanity, even though antimicrobial agents have long since been developed. In addition to individual severe infections, the f fatality rate of sepsis remains high, and the threat of antimicrobial-resistant bacteria grows with time, putting us at inferiority. Although tremendous resources have been devoted to the development of antimicrobial agents, we have yet to recover from the lost ground we have been driven into. Looking back at the evolution of treatment for cancer, which, like infectious diseases, has the similarity that host immunity eliminates the lesion, the development of drugs to eliminate the tumor itself has shifted from a single-minded focus on drug development to the establishment of a treatment strategy in which the de-suppression of host immunity is another pillar of treatment. In infectious diseases, on the other hand, the development of therapies that strengthen and support the immune system has only just begun. Among innate immunity, the first line of defense that bacteria encounter after invading the host, the molecular mechanisms of the phagolysosome pathway, which begins with phagocytosis to fusion with lysosome, have been elucidated in detail. Bacteria have a large number of strategies to escape and survive the pathway. Although the full picture is still unfathomable, the molecular mechanisms have been elucidated for some of them, providing sufficient clues for intervention. In this article, we review the host defense mechanisms and bacterial evasion mechanisms and discuss the possibility of host-directed therapy for bacterial infection by intervening in the phagolysosome pathway.

Molecular basis of Klebsiella pneumoniae colonization in host

Klebsiella pneumoniae (K. pneumoniae) is a common cause of nosocomial infection, which causing disseminated infections such as cystitis, pneumonia and sepsis. K. pneumoniae is intrinsic resistant to penicillin, and members of the population usually have acquired resistance to a variety of antibiotics, which makes it a major threat to clinical and public health. Bacteria can colonize on or within the hosts, accompanied by growth and reproduction of the organisms, but no clinical symptoms are presented. As the "first step" of bacterial infection, colonization in the hosts is of great importance. Colonization of bacteria can last from days to years, with resolution influenced by immune response to the organism, competition at the site from other organisms and, sometimes, use of antimicrobials. Colonized pathogenic bacteria cause healthcare-associated infections at times of reduced host immunity, which is an important cause of clinical occurrence of postoperative complications and increased mortality in ICU patients. Though, K. pneumoniae is one of the most common conditional pathogens of hospital-acquired infections, the mechanisms of K. pneumoniae colonization in humans are not completely clear. In this review, we made a brief summary of the molecular basis of K. pneumoniae colonization in the upper respiratory tract and intestinal niche, and provided new insights for understanding the pathogenesis of K. pneumoniae.

Why is manganese so valuable to bacterial pathogens?

Apart from oxygenic photosynthesis, the extent of manganese utilization in bacteria varies from species to species and also appears to depend on external conditions. This observation is in striking contrast to iron, which is similar to manganese but essential for the vast majority of bacteria. To adequately explain the role of manganese in pathogens, we first present in this review that the accumulation of molecular oxygen in the Earth's atmosphere was a key event that linked manganese utilization to iron utilization and put pressure on the use of manganese in general. We devote a large part of our contribution to explanation of how molecular oxygen interferes with iron so that it enhances oxidative stress in cells, and how bacteria have learned to control the concentration of free iron in the cytosol. The functioning of iron in the presence of molecular oxygen serves as a springboard for a fundamental understanding of why manganese is so valued by bacterial pathogens. The bulk of this review addresses how manganese can replace iron in enzymes. Redox-active enzymes must cope with the higher redox potential of manganese compared to iron. Therefore, specific manganese-dependent isoenzymes have evolved that either lower the redox potential of the bound metal or use a stronger oxidant. In contrast, redox-inactive enzymes can exchange the metal directly within the individual active site, so no isoenzymes are required. It appears that in the physiological context, only redox-inactive mononuclear or dinuclear enzymes are capable of replacing iron with manganese within the same active site. In both cases, cytosolic conditions play an important role in the selection of the metal used. In conclusion, we summarize both well-characterized and less-studied mechanisms of the tug-of-war for manganese between host and pathogen.

The cGAS-STING-autophagy pathway: Novel perspectives in neurotoxicity induced by manganese exposure

Chronic high-level heavy metal exposure increases the risk of developing different neurodegenerative diseases. Chronic excessive manganese (Mn) exposure is known to lead to neurodegenerative diseases. In addition, some evidence suggests that autophagy dysfunction plays an important role in the pathogenesis of various neurodegenerative diseases. Over the past decade, the DNA-sensing receptor cyclic GMP-AMP synthase (cGAS) and its downstream signal-efficient interferon gene stimulator (STING), as well as the molecular composition and regulatory mechanisms of this pathway have been well understood. The cGAS-STING pathway has emerged as a crucial mechanism to induce effective innate immune responses by inducing type I interferons in mammalian cells. Moreover, recent studies have found that Mn²⁺ is the second activator of the cGAS-STING pathway besides dsDNA, and inducing autophagy is a primitive function for the activation of the cGAS-STING pathway. However, overactivation of the immune response can lead to tissue damage. This review discusses the mechanism of neurotoxicity induced by Mn exposure from the cGAS-STING-autophagy pathway. Future work exploiting the cGAS-STING-autophagy pathway may provide a novel perspective for manganese neurotoxicity.

Nutritional immunity: the battle for nutrient metals at the host-pathogen interface

Trace metals are essential micronutrients required for survival across all kingdoms of life. From bacteria to animals, metals have critical roles as both structural and catalytic cofactors for an estimated third of the proteome, representing a major contributor to the maintenance of cellular homeostasis. The reactivity of metal ions engenders them with the ability to promote enzyme catalysis and stabilize reaction intermediates. However, these properties render metals toxic at high concentrations and, therefore, metal levels must be tightly regulated. Having evolved in close association with bacteria, vertebrate hosts have developed numerous strategies of metal limitation and intoxication that prevent bacterial proliferation, a process termed nutritional immunity. In turn, bacterial pathogens have evolved adaptive mechanisms to survive in conditions of metal depletion or excess. In this Review, we discuss mechanisms by which nutrient metals shape the interactions between bacterial pathogens and animal hosts. We explore the cell-specific and tissue-specific roles of distinct trace metals in shaping bacterial infections, as well as implications for future research and new therapeutic development.

Manganese Utilization in Salmonella Pathogenesis: Beyond the Canonical Antioxidant Response

The metal ion manganese (Mn²⁺) is equally coveted by hosts and bacterial pathogens. The host restricts Mn²⁺ in the gastrointestinal tract and Salmonella-containing vacuoles, as part of a process generally known as nutritional immunity. Salmonella enterica serovar Typhimurium counteract Mn²⁺ limitation using a plethora of metal importers, whose expression is under elaborate transcriptional and posttranscriptional control. Mn²⁺ serves as cofactor for a variety of enzymes involved in antioxidant defense or central metabolism. Because of its thermodynamic stability and low reactivity, bacterial pathogens may favor Mn²⁺-cofactored metalloenzymes during periods of oxidative stress. This divalent metal catalyzes metabolic flow through lower glycolysis, reductive tricarboxylic acid and the pentose phosphate pathway, thereby providing energetic, redox and biosynthetic outputs associated with the resistance of Salmonella to reactive oxygen species generated in the respiratory burst of professional phagocytic cells. Combined, the oxyradical-detoxifying properties of Mn²⁺ together with the ability of this divalent metal cation to support central metabolism help Salmonella colonize the mammalian gut and establish systemic infections.

Managing Manganese: The Role of Manganese Homeostasis in Streptococcal Pathogenesis

Pathogenic streptococci require manganese for survival in the host. In response to invading pathogens, the host recruits nutritional immune effectors at infection sites to withhold manganese from the pathogens and control bacterial growth. The manganese scarcity impairs several streptococcal processes including oxidative stress defenses, de novo DNA synthesis, bacterial survival, and virulence. Emerging evidence suggests that pathogens also encounter manganese toxicity during infection and manganese excess impacts streptococcal virulence by manganese mismetallation of non-cognate molecular targets involved in bacterial antioxidant defenses and cell division. To counter host-imposed manganese stress, the streptococcal species employ a sophisticated sensory system that tightly coordinates manganese stress-specific molecular strategies to negate host induced manganese stress and proliferate in the host. Here we review the molecular details of host-streptococcal interactions in the battle for manganese during infection and the significance of streptococcal effectors involved to bacterial pathophysiology.

A review on bacterial redox dependent iron transporters and their evolutionary relationship

Iron is an essential yet toxic micronutrient and its transport across biological membranes is tightly regulated in all living organisms. One such iron transporter, the Ftr-type permeases, is found in both eukaryotic and prokaryotic cells. These Ftr-type transporters are required for iron transport, predicted to form α-helical transmembrane structures, and conserve two ArgGluxxGlu (x = any amino acid) motifs. In the yeast Ftr transporter (Ftr1p), a ferroxidase (Fet3p) is required for iron transport in an oxidation coupled transport step. None of the bacterial Ftr-type transporters (EfeU and FetM from E. coli; cFtr from Campylobacter jejuni; FtrC from Brucella, Bordetella, and Burkholderia spp.) contain a ferroxidase protein. Bioinformatics report predicted periplasmic EfeO and FtrB (from the EfeUOB and FtrABCD systems) as novel cupredoxins. The Cu²⁺ binding and the ferrous oxidation properties of these proteins are uncharacterized and the other two bacterial Ftr-systems are expressed without any ferroxidase/cupredoxin, leading to controversy about the mode of function of these transporters. Here, we review published data on Ftr-type transporters to gain insight into their functional diversity. Based on original bioinformatics data presented here evolutionary relations between these systems are presented.

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.

Cytokine-Mediated Regulation of ARG1 in Macrophages and Its Impact on the Control of Salmonella enterica Serovar Typhimurium Infection

Arginase 1 (ARG1) is a cytosolic enzyme that cleaves L-arginine, the substrate of inducible nitric oxide synthase (iNOS), and thereby impairs the control of various intracellular pathogens. Herein, we investigated the role of ARG1 during infection with Salmonella enterica serovar Typhimurium (S.tm). To study the impact of ARG1 on Salmonella infections in vitro, bone marrow-derived macrophages (BMDM) from C57BL/6N wild-type, ARG1-deficient Tie2Cre+/-ARG1fl/fl and NRAMPG169 C57BL/6N mice were infected with S.tm. In wild-type BMDM, ARG1 was induced by S.tm and further upregulated by the addition of interleukin (IL)-4, whereas interferon-γ had an inhibitory effect. Deletion of ARG1 did not result in a reduction in bacterial numbers. In vivo, Arg1 mRNA was upregulated in the spleen, but not in the liver of C57BL/6N mice following intraperitoneal S.tm infection. The genetic deletion of ARG1 (Tie2Cre+/-ARG1fl/fl) or its pharmacological inhibition with CB-1158 neither affected the numbers of S.tm in spleen, liver and blood nor the expression of host response genes such as iNOS, IL-6 or tumour necrosis factor (TNF). Furthermore, ARG1 was dispensable for pathogen control irrespective of the presence or absence of the phagolysosomal natural resistance-associated macrophage protein 1 (NRAMP1). Thus, unlike the detrimental function of ARG1 seen during infections with other intraphagosomal microorganisms, ARG1 did not support bacterial survival in systemic salmonellosis, indicating differential roles of arginine metabolism for host immune response and microbe persistence depending on the type of pathogen.

Mycobacterium tuberculosis Binds Human Serum Amyloid A, and the Interaction Modulates the Colonization of Human Macrophages and the Transcriptional Response of the Pathogen

As a very successful pathogen with outstanding adaptive properties, Mycobacterium tuberculosis (Mtb) has developed a plethora of sophisticated mechanisms to subvert host defenses and effectively enter and replicate in the harmful environment inside professional phagocytes, namely, macrophages. Here, we demonstrated the binding interaction of Mtb with a major human acute phase protein, namely, serum amyloid A (SAA1), and identified AtpA (Rv1308), ABC (Rv2477c), EspB (Rv3881c), TB 18.6 (Rv2140c), and ThiC (Rv0423c) membrane proteins as mycobacterial effectors responsible for the pathogen-host protein interplay. SAA1-opsonization of Mtb prior to the infection of human macrophages favored bacterial entry into target phagocytes accompanied by a substantial increase in the load of intracellularly multiplying and surviving bacteria. Furthermore, binding of human SAA1 by Mtb resulted in the up- or downregulation of the transcriptional response of tubercle bacilli. The most substantial changes were related to the increased expression level of the genes of two operons encoding mycobacterial transporter systems, namely, mmpL5/mmpS5 (rv0676c), and rv1217c, rv1218c. Therefore, we postulate that during infection, Mtb-SAA1 binding promotes the infection of host macrophages by tubercle bacilli and modulates the functional response of the pathogen.

Molecular Mechanism of Nramp-Family Transition Metal Transport

The Natural resistance-associated macrophage protein (Nramp) family of transition metal transporters enables uptake and trafficking of essential micronutrients that all organisms must acquire to survive. Two decades after Nramps were identified as proton-driven, voltage-dependent secondary transporters, multiple Nramp crystal structures have begun to illustrate the fine details of the transport process and provide a new framework for understanding a wealth of preexisting biochemical data. Here we review the relevant literature pertaining to Nramps' biological roles and especially their conserved molecular mechanism, including our updated understanding of conformational change, metal binding and transport, substrate selectivity, proton transport, proton-metal coupling, and voltage dependence. We ultimately describe how the Nramp family has adapted the LeuT fold common to many secondary transporters to provide selective transition-metal transport with a mechanism that deviates from the canonical model of symport.

Uncovering the Hidden Credentials of Brucella Virulence

Bacteria in the genus Brucella are important human and veterinary pathogens. The abortion and infertility they cause in food animals produce economic hardships in areas where the disease has not been controlled, and human brucellosis is one of the world's most common zoonoses. Brucella strains have also been isolated from wildlife, but we know much less about the pathobiology and epidemiology of these infections than we do about brucellosis in domestic animals. The brucellae maintain predominantly an intracellular lifestyle in their mammalian hosts, and their ability to subvert the host immune response and survive and replicate in macrophages and placental trophoblasts underlies their success as pathogens. We are just beginning to understand how these bacteria evolved from a progenitor alphaproteobacterium with an environmental niche and diverged to become highly host-adapted and host-specific pathogens. Two important virulence determinants played critical roles in this evolution: (i) a type IV secretion system that secretes effector molecules into the host cell cytoplasm that direct the intracellular trafficking of the brucellae and modulate host immune responses and (ii) a lipopolysaccharide moiety which poorly stimulates host inflammatory responses. This review highlights what we presently know about how these and other virulence determinants contribute to Brucella pathogenesis. Gaining a better understanding of how the brucellae produce disease will provide us with information that can be used to design better strategies for preventing brucellosis in animals and for preventing and treating this disease in humans.

Manganese homeostasis at the host-pathogen interface and in the host immune system

Manganese serves as an indispensable catalytic center and the structural core of various enzymes that participate in a plethora of biological processes, including oxidative phosphorylation, glycosylation, and signal transduction. In pathogenic microorganisms, manganese is required for survival by maintaining basic biochemical activity and virulence; in contrast, the host utilizes a process known as nutritional immunity to sequester manganese from invading pathogens. Recent epidemiological and animal studies have shown that manganese increases the immune response in a wide range of vertebrates, including humans, rodents, birds, and fish. On the other hand, excess manganese can cause neurotoxicity and other detrimental effects. Here, we review recent data illustrating the essential role of manganese homeostasis at the host-pathogen interface and in the host immune system. We also discuss the accumulating body of evidence that manganese modulates various signaling pathways in immune processes. Finally, we discuss the key molecular players involved in manganese's immune regulatory function, as well as the clinical implications with respect to cancer immunotherapy.

Identification and functional characterization of natural resistance-associated macrophage protein 2 from sea cucumber Apostichopus japonicus

As a member of natural resistance-associated macrophage protein (Nramp) family, Nramp2 conservatively exists in the cell membrane across species and is essential for normal iron homeostasis in an H⁺-dependent manner. Withholding available iron represents an important host defense strategy. However, the function of Nramp2 in response to invading pathogens is largely unknown in invertebrates. In this study, a unique echinoderm Nramp2 was identified from sea cucumber Apostichopus japonicus (designated as AjNramp2). The cDNA sequence of AjNramp2 was 2360 bp, with a putative open reading frame of 1713 bp, encoding a typical Nramp domain containing protein with 570 amino acid residues. Structural analysis revealed that AjNramp2 consisted of highly conserved helix regions similar with the human Nramp2. Spatial expression analysis revealed that AjNramp2 was ubiquitously expressed in all examined tissues, with the highest level found in the intestine. Immunohistochemistry assay showed that AjNramp2 was mainly located in the cellular membrane in coelomocytes. Vibrio splendidus challenge and lipopolysaccharide (LPS) stimulation could significantly promote the expression of AjNramp2, which was consistent with the cellular iron level in coelomocytes. Moreover, when the expression of AjNramp2 was knocked down by siRNA-AjNramp2, the cellular iron level was coordinately decreased in coelomocytes under LPS stimulation. Taken together, results indicated that AjNramp2 serves as an iron transport receptor to withhold available iron and may contribute to the nutritional immunity defense system of sea cucumber.

Investigating the roles of the conserved Cu2+-binding residues on Brucella FtrA in producing conformational stability and functionality

Brucella is a zoonotic pathogen requiring iron for its survival and acquires this metal through the expression of several high-affinity uptake systems. Of these, the newly discovered ferrous iron transporter, FtrABCD, is proposed to take part in ferrous iron uptake. Sequence homology shows that, FtrA, the proposed periplasmic ferrous-binding component, is a P19-type protein (a periplasmic protein from C. jejuni which shows Cu2+ dependent iron affinity). Previous structural and biochemical studies on other P19 systems have established a Cu2+ dependent Mn2+ affinity as well as formation of homodimers for these systems. The Cu2+ coordinating amino acids from these proteins are conserved in Brucella FtrA, hinting towards similar properties. However, there has been no experimental evidence, till date, establishing metal affinities and the possibility of dimer formation by Brucella FtrA. Using wild-type FtrA and Cu2+-binding mutants (H65A, E67A, H118A, and H151A) we investigated the metal affinities, folding stabilities, dimer forming abilities, and the molecular basis of the Cu2+ dependence for this P19-type protein employing homology modeling, analytical gel filtration, calorimetric, and spectroscopic methods. The data reported here confirm a Cu2+-dependent, low-μM Mn2+ (Fe2+ mimic) affinity for the wild-type FtrA. In addition, our data clearly show the loss of Mn2+ affinity, and the formation of less stable protein conformations as a result of mutating these conserved Cu2+-binding residues, indicating the important roles these residues play in producing a native and functional fold of Brucella FtrA.

Analysis of Genetic Variation in the Bovine SLC11A1 Gene, Its Influence on the Expression of NRAMP1 and Potential Association With Resistance to Bovine Tuberculosis

Bovine tuberculosis (bTB), caused by Mycobacterium bovis, is a chronic zoonotic disease where host genetics is thought to contribute to susceptibility or resistance. One of the genes implicated is the SLC11A1 gene, that encodes for the natural resistance-associated macrophage protein 1 (NRAMP1). The aim of this study was to identify SLC11A1 polymorphisms and to investigate any resulting functional differences in NRAMP1 expression that might be correlated with resistance/susceptibility to M. bovis infection. Sequencing of the SLC11A1 gene in cDNA isolated from Brown Swiss, Holstein Friesian, and Sahiwal cattle identified five single nucleotide polymorphisms (SNPs) in the coding region, but only one of these (SNP4, c.1066C>G, rs109453173) was present in all three cattle breeds and therefore warranted further investigation. Additionally, variations of 10, 11, and 12 GT repeats were identified in a microsatellite (MS1) in the SLC11A1 3′UTR. Measurement of NRAMP1 expression in bovine macrophages by ELISA showed no differences between cells generated from the different breeds. Furthermore, variations in the length of the MS1 microsatellite did not impact on NRAMP1 protein expression as analyzed by luciferase reporter assay. However, further analysis of the ELISA data identified that the presence of the alternative G allele at SNP4 was associated with increased expression of NRAMP1 in bovine macrophages. Since NRAMP1 has been shown to influence the survival of intracellular pathogens such as M. bovis through the sequestering of iron, it is possible that cattle expressing the alternative G allele might have an increased resistance to bTB through increased NRAMP1 expression in their macrophages.

Biochemical Characterization of Oyster and Clam Galectins: Selective Recognition of Carbohydrate Ligands on Host Hemocytes and Perkinsus Parasites

Both vertebrates and invertebrates display active innate immune mechanisms for defense against microbial infection, including diversified repertoires of soluble and cell-associated lectins that can effect recognition and binding to potential pathogens, and trigger downstream effector pathways that clear them from the host internal milieu. Galectins are widely distributed and highly conserved lectins that have key regulatory effects on both innate and adaptive immune responses. In addition, galectins can bind to exogenous (“non-self”) carbohydrates on the surface of bacteria, enveloped viruses, parasites, and fungi, and function as recognition receptors and effector factors in innate immunity. Like most invertebrates, eastern oysters (Crassostrea virginica) and softshell clams (Mya arenaria) can effectively respond to most immune challenges through soluble and hemocyte-associated lectins. The protozoan parasite Perkinsus marinus, however, can infect eastern oysters and cause “Dermo” disease, which is highly detrimental to both natural and farmed oyster populations. The sympatric Perkinsus chesapeaki, initially isolated from infected M. arenaria clams, can also be present in oysters, and there is little evidence of pathogenicity in either clams or oysters. In this review, we discuss selected observations from our studies on the mechanisms of Perkinsus recognition that are mediated by galectin-carbohydrate interactions. We identified in the oyster two galectins that we designated CvGal1 and CvGal2, which strongly recognize P. marinus trophozoites. In the clam we also identified galectin sequences, and focused on one (that we named MaGal1) that also recognizes Perkinsus species. Here we describe the biochemical characterization of CvGal1, CvGal2, and MaGal1 with focus on the detailed study of the carbohydrate specificity, and the glycosylated moieties on the surfaces of the oyster hemocytes and the two Perkinsus species (P. marinus and P. chesapeaki). Our goal is to gain further understanding of the biochemical basis for the interactions that lead to recognition and opsonization of the Perkinsus trophozoites by the bivalve hemocytes. These basic studies on the biology of host-parasite interactions may contribute to the development of novel intervention strategies for parasitic diseases of biomedical interest.

Bacillus subtilis Regulators MntR and Zur Participate in Redox Cycling, Antibiotic Sensitivity, and Cell Wall Plasticity

The Bacillus subtilis MntR and Zur transcriptional regulators control homeostasis of manganese and zinc, two essential elements required in various cellular processes. In this work, we describe the global impact of mntR and zur deletions at the protein level. Using a comprehensive proteomic approach, we showed that 33 and 55 proteins are differentially abundant in ΔmntR and Δzur cells, respectively, including proteins involved in metal acquisition, translation, central metabolism, and cell wall homeostasis. In addition, both mutants showed modifications in intracellular metal ion pools, with significant Mg²⁺ accumulation in the ΔmntR mutant. Phenotypic and morphological analyses of ΔmntR and Δzur mutants revealed their high sensitivity to lysozyme, beta-lactam antibiotics, and external oxidative stress. Mutant strains had a modified cell wall thickness and accumulated lower levels of intracellular reactive oxygen species (ROS) than the wild-type strain. Remarkably, our results highlight an intimate connection between MntR, Zur, antibiotic sensitivity, and cell wall structure.IMPORTANCE Manganese and zinc are essential transition metals involved in many fundamental cellular processes, including protection against external oxidative stress. In Bacillus subtilis, Zur and MntR are key transcriptional regulators of zinc and manganese homeostasis, respectively. In this work, proteome analysis of B. subtilis wild-type, ΔmntR, and Δzur strains provided new insights into bacterial adaptation to deregulation of essential metal ions. Deletions of mntR and zur genes increased bacterial sensitivity to lysozyme, beta-lactam antibiotics, and external oxidative stress and impacted the cell wall thickness. Overall, these findings highlight that Zur and MntR regulatory networks are connected to antibiotic sensitivity and cell wall plasticity.

Solute carrier transporters: the metabolic gatekeepers of immune cells

Solute carrier (SLC) transporters meditate many essential physiological functions, including nutrient uptake, ion influx/efflux, and waste disposal. In its protective role against tumors and infections, the mammalian immune system coordinates complex signals to support the proliferation, differentiation, and effector function of individual cell subsets. Recent research in this area has yielded surprising findings on the roles of solute carrier transporters, which were discovered to regulate lymphocyte signaling and control their differentiation, function, and fate by modulating diverse metabolic pathways and balanced levels of different metabolites. In this review, we present current information mainly on glucose transporters, amino-acid transporters, and metal ion transporters, which are critically important for mediating immune cell homeostasis in many different pathological conditions.

A Calcium-Rich Multimineral Intervention to Modulate Colonic Microbial Communities and Metabolomic Profiles in Humans: Results from a 90-Day Trial

Aquamin is a calcium-, magnesium-, and multiple trace element-rich natural product with colon polyp prevention efficacy based on preclinical studies. The goal of this study was to determine the effects of Aquamin on colonic microbial community and attendant metabolomic profile. Thirty healthy human participants were enrolled in a 90-day trial in which Aquamin (delivering 800 mg of calcium per day) was compared with calcium alone or placebo. Before and after the intervention, colonic biopsies and stool specimens were obtained. All 30 participants completed the study without serious adverse event or change in liver and renal function markers. Compared with pretreatment values, intervention with Aquamin led to a reduction in total bacterial DNA (P = 0.0001) and a shift in the microbial community measured by thetaYC (θYC; P = 0.0087). Treatment with calcium also produced a decline in total bacteria, but smaller than seen with Aquamin, whereas no reduction was observed with placebo in the colon. In parallel with microbial changes, a reduction in total bile acid levels (P = 0.0375) and a slight increase in the level of the short-chain fatty acid (SCFA) acetate in stool specimens (P < 0.0001) from Aquamin-treated participants were noted. No change in bile acids or SCFAs was observed with calcium or placebo. We conclude that Aquamin is safe and tolerable in healthy human participants and may produce beneficial alterations in the colonic microbial community and the attendant metabolomic profile. Because the number of participants was small, the findings should be considered preliminary.

Whole genome profiling refines a panel of correlates to predict vaccine efficacy against Mycobacterium tuberculosis

New vaccines are needed to combat the public health threat posed by M. tuberculosis (M. tb), but no correlates have been defined to aid vaccine development. Using mouse models, we previously developed an in vitro system that measures the ability of M. tb-immune lymphocytes to control bacterial replication during co-culture with M. tb-infected macrophages. We demonstrated that the degree of in vitro growth control by lymphocytes from mice given vaccines of varying efficacy reflected the relative degree of in vivo protection against lethal challenge. Further, using targeted analyses of gene expression in lymphocytes recovered from co-cultures, we found mediators whose relative expression also correlated with in vitro and in vivo outcomes. Here we advanced those findings by employing genome-wide expression analyses. We first screened splenocytes recovered from co-cultures by microarray, revealing additional genes whose expression correlated with protection. After applying pathway analyses to down-select gene candidates, we used both splenocytes and peripheral blood lymphocytes to validate microarray findings by qRT-PCR. We then subjected data from top candidates to rigorous statistical analyses. Resulting correlate candidates, including CXCL9, IFN-γ, and CCL5, significantly predicted protection with high specificity. These findings therefore refine and extend a panel of relevant immune correlates to advance vaccine development.

The Cation Diffusion Facilitator Family Protein EmfA Confers Resistance to Manganese Toxicity in Brucella abortus 2308 and Is an Essential Virulence Determinant in Mice

The gene designated bab_rs23470 in the Brucella abortus 2308 genome encodes an ortholog of the cation diffusion facilitator family protein EmfA which has been linked to resistance to Mn toxicity in Rhizobium etli A B. abortus emfA null mutant derived from strain 2308 displays increased sensitivity to elevated levels of Mn in the growth medium compared to that of the parent strain but wild-type resistance to Fe, Mg, Zn, Cu, Co, and Ni. Inductively coupled plasma mass spectroscopy also indicates that the B. abortus emfA mutant retains significantly higher levels of cellular Mn after exposure to this metal than the parent strain, which is consistent with the proposed role of EmfA as a Mn exporter. Phenotypic analysis of mutants indicates that EmfA plays a much more important role in maintaining Mn homeostasis and preventing the toxicity of this metal in Brucella than does the Mn-responsive transcriptional regulator Mur. EmfA is also an essential virulence determinant for B. abortus 2308 in C57BL/6 and C57BL/6^Nramp1+/+ mice, which suggests that avoiding Mn toxicity plays a critical role in Brucella pathogenesis.IMPORTANCE Mn nutrition is essential for the basic physiology and virulence of Brucella strains. The results of the study presented here demonstrate that the cation diffusion facilitator (CDF)-type metal exporter EmfA plays critical roles in maintaining Mn homeostasis and preventing Mn toxicity in Brucella and is an essential virulence determinant for these bacteria. EmfA and other cellular components involved in Mn homeostasis represent attractive targets for the development of improved vaccines and chemotherapeutic strategies for preventing and treating brucellosis in humans and animals.

The regulation of iron homeostasis in the fungal human pathogen Candida glabrata

Iron is an essential element to most microorganisms, yet an excess of iron is toxic. Hence, living cells have to maintain a tight balance between iron uptake and iron consumption and storage. The control of intracellular iron concentrations is particularly challenging for pathogens because mammalian organisms have evolved sophisticated high-affinity systems to sequester iron from microbes and because iron availability fluctuates among the different host niches. In this review, we present the current understanding of iron homeostasis and its regulation in the fungal pathogen Candida glabrata. This yeast is an emerging pathogen which has become the second leading cause of candidemia, a life-threatening invasive mycosis. C. glabrata is relatively poorly studied compared to the closely related model yeast Saccharomyces cerevisiae or to the pathogenic yeast Candida albicans. Still, several research groups have started to identify the actors of C. glabrata iron homeostasis and its transcriptional and post-transcriptional regulation. These studies have revealed interesting particularities of C. glabrata and have shed new light on the evolution of fungal iron homeostasis.

Iron in Lung Pathology

The lung presents a unique challenge for iron homeostasis. The entire airway is in direct contact with the environment and its iron particulate matter and iron-utilizing microbes. However, the homeostatic and adaptive mechanisms of pulmonary iron regulation are poorly understood. This review provides an overview of systemic and local lung iron regulation, as well as the roles of iron in the development of lung infections, airway disease, and lung injury. These mechanisms provide an important foundation for the ongoing development of therapeutic applications.

Genome-Wide Response to Drugs and Stress in the Pathogenic Yeast Candida glabrata

Candida glabrata is the second most common cause of candidemia worldwide and its prevalence has continuously increased over the last decades. C. glabrata infections are especially worrisome in immunocompromised patients, resulting in serious systemic infections, associated to high mortality rates. Intrinsic resistance to azole antifungals, widely used drugs in the clinical setting, and the ability to efficiently colonize the human host and medical devices, withstanding stress imposed by the immune system, are thought to underlie the emergence of C. glabrata. There is a clear clinical need to understand drug and stress resistance in C. glabrata. The increasing prevalence of multidrug resistant isolates needs to be addressed in order to overcome the decrease of viable therapeutic strategies and find new therapeutic targets. Likewise, the understanding of the mechanisms underlying its impressive ability thrive under oxidative, nitrosative, acidic and metabolic stresses, is crucial to design drugs that target these pathogenesis features. The study of the underlying mechanisms that translate C. glabrata plasticity and its competence to evade the immune system, as well as survive host stresses to establish infection, will benefit from extensive scrutiny. This chapter provides a review on the contribution of genome-wide studies to uncover clinically relevant drug resistance and stress response mechanisms in the human pathogenic yeast C. glabrata.

SLC11A1 is expressed in the human placenta across multiple gestational ages

The human placenta functions as an innate immune barrier to prevent fetal infection. However, the molecular mechanisms accounting for placental resistance to pathogens are currently poorly understood. The solute carrier family 11 member 1 (SLC11A1) is a divalent cation transporter expressed primarily by macrophages and neutrophils that is essential for controlling infections by intracellular pathogens such as Salmonella, Leishmania and Mycobacteria. This report demonstrates that SLC11A1 is expressed in the syncytiotrophoblast of the human placenta at multiple gestational ages. These results suggest that SLC11A1 may play a role in blocking productive placental infections by certain intracellular pathogens.

The 'Checkmate' for Iron Between Human Host and Invading Bacteria: Chess Game Analogy

Iron is an essential nutrient for all living organisms with critical roles in many biological processes. The mammalian host maintains the iron requirements by dietary intake, while the invading pathogenic bacteria compete with the host to obtain those absorbed irons. In order to limit the iron uptake by the bacteria, the human host employs numerous iron binding proteins and withholding defense mechanisms that capture iron from the microbial invaders. To counteract, the bacteria cope with the iron limitation imposed by the host by expressing various iron acquisition systems, allowing them to achieve effective iron homeostasis. The armamentarium used by the human host and invading bacteria, leads to the dilemma of who wins the ultimate war for iron.

A Functional Study Identifying Critical Residues Involving Metal Transport Activity and Selectivity in Natural Resistance-Associated Macrophage Protein 3 in Arabidopsis thaliana

Arabidopsis thaliana natural resistance-associated macrophage protein 3 (AtNRAMP3) is involved in the transport of cadmium (Cd), iron (Fe), and manganese (Mn). Here, we present a structure-function analysis of AtNRAMP3 based on site-directed mutagenesis and metal toxicity growth assays involving yeast mutants, combined with three-dimensional (3D) structure modeling based on the crystal structure of the Eremococcus coleocola NRAMP family transporter, EcoDMT. We demonstrated that two conservative sites, D72 and N75, are essential for the transport activity. The M248A mutation resulted in a decrease in Cd sensitivity, while maintaining Mn transport. The mutation involving G61 caused a significant impairment of Fe and Mn transport, thereby indicating the importance of the conserved residue for proper protein function. The mutation involving G171 disrupted Fe transport activity but not that of Mn and Cd, suggesting that G171 is essential to metal binding and selectivity. Two residues, E194 and R262, may play an important role in stabilizing outward-facing conformation, which is essential for transport activity. Deletion assays indicated that the N-terminus is necessary for the function of AtNRAMP3. The findings of the present study revealed the structure-function relationship of AtNRAMP3 and metal transport activity and selectivity, which may possibly be applied to other plant NRAMP proteins.

[Iron and susceptibility to infections]

Iron is essential for proper functioning of the host immune system as well as an essential nutrient for growth of various pathogens. Iron deficiency increases infection susceptibility, specially those due to intracellular pathogens. At the opposite, excess iron stores could increase the virulence of some pathogens. Hepcidin synthesis is increased during the acute inflammation phase; leading to decreased iron intestinal absorption and retention of the metal within macrophages. This is considered to result from a defense mechanism of the child to limit the availability of iron for extracellular pathogens. On the other hand, iron affect innate immune responses by influencing IFN-γ or NF-kB pathways in macrophages. Consequently, iron enhances host resistance to intracellular pathogens but excess iron may alter immune system.

Investigation of the Role of Genes Encoding Zinc Exporters zntA, zitB, and fieF during Salmonella Typhimurium Infection

The transition metal zinc is involved in crucial biological processes in all living organisms and is essential for survival of Salmonella in the host. However, little is known about the role of genes encoding zinc efflux transporters during Salmonella infection. In this study, we constructed deletion mutants for genes encoding zinc exporters (zntA, zitB, and fieF) in the wild-type (WT) strain Salmonella enterica serovar Typhimurium (S. Typhimurium) 4/74. The mutants 4/74ΔzntA and 4/74ΔzntA/zitB exhibited a dramatic growth delay and abrogated growth ability, respectively, in Luria Bertani medium supplemented with 0.25 mM ZnCl₂ or 1.5 mM CuSO₄ compared to the WT strain. In order to investigate the role of genes encoding zinc exporters on survival of S. Typhimurium inside cells, amoeba and macrophage infection models were used. No significant differences in uptake or survival were detected for any of the mutants compared to the WT during infection of amoebae. In natural resistance-associated macrophage protein 1 (Nramp1)-negative J774.1 murine macrophages, significantly higher bacterial counts were observed for the mutant strains 4/74ΔzntA and 4/74ΔzntA/zitB compared to the WT at 4 h post-infection although the fold net replication was similar between all the strains. All four tested mutants (4/74ΔzntA, 4/74ΔzitB, 4/74ΔfieF, and 4/74ΔzntA/zitB) showed enhanced intracellular survival capacity within the modified Nramp1-positive murine RAW264.7 macrophages at 20 h post-infection. The fold net replication was also significantly higher for 4/74ΔzntA, 4/74ΔzitB, and 4/74ΔzntA/zitB mutants compared to the WT. Intriguingly, the ability to survive and cause infection was significantly impaired in all the three mutants tested (4/74ΔzntA, 4/74ΔzitB, and 4/74ΔzntA/zitB) in C3H/HeN mice, particularly the double mutant 4/74ΔzntA/zitB was severely attenuated compared to the WT in all the three organs analyzed. These findings suggest that these genes encoding zinc exporters, especially zntA, contribute to the resistance of S. Typhimurium to zinc and copper stresses during infection.

Control of Phagocytosis by Microbial Pathogens

Phagocytosis is a fundamental process of cells to capture and ingest foreign particles. Small unicellular organisms such as free-living amoeba use this process to acquire food. In pluricellular organisms, phagocytosis is a universal phenomenon that all cells are able to perform (including epithelial, endothelial, fibroblasts, etc.), but some specialized cells (such as neutrophils and macrophages) perform this very efficiently and were therefore named professional phagocytes by Rabinovitch. Cells use phagocytosis to capture and clear all particles larger than 0.5 µm, including pathogenic microorganisms and cellular debris. Phagocytosis involves a series of steps from recognition of the target particle, ingestion of it in a phagosome (phagocytic vacuole), maturation of this phagosome into a phagolysosome, to the final destruction of the ingested particle in the robust antimicrobial environment of the phagolysosome. For the most part, phagocytosis is an efficient process that eliminates invading pathogens and helps maintaining homeostasis. However, several pathogens have also evolved different strategies to prevent phagocytosis from proceeding in a normal way. These pathogens have a clear advantage to perpetuate the infection and continue their replication. Here, we present an overview of the phagocytic process with emphasis on the antimicrobial elements professional phagocytes use. We also summarize the current knowledge on the microbial strategies different pathogens use to prevent phagocytosis either at the level of ingestion, phagosome formation, and maturation, and even complete escape from phagosomes.

Zinc'ing it out: zinc homeostasis mechanisms and their impact on the pathogenesis of human pathogen group A streptococcus

Group A Streptococccus (GAS) is a major human pathogen that causes significant morbidity and mortality. Zinc is an essential trace element required for GAS growth, however, zinc can be toxic at excess concentrations. The bacterial strategies to maintain zinc sufficiency without incurring zinc toxicity play a crucial role in host-GAS interactions and have a significant impact on GAS pathogenesis. The host deploys nutritional immune mechanisms to retard GAS growth by causing either zinc deprivation or zinc poisoning. However, GAS overcomes the zinc-dependent host defenses and survives in the hostile environment by employing complex adaptive strategies. In this review, we describe the different host immune strategies that employ either zinc limitation or zinc toxicity in different host environments to control GAS infection. We also discuss the molecular mechanisms and machineries used by GAS to evade host nutritional defenses and establish successful infection. Emerging evidence suggests that the metal transporters are major GAS virulence factors as they compete against host nutritional immune mechanisms to acquire or expel metals and promote bacterial survival in the host. Thus, identification of GAS molecules and elucidation of the mechanisms by which GAS combats host-mediated alterations in zinc availability may lead to novel interference strategies targeting GAS metal acquisition systems.

Duplication and Sub/Neofunctionalization of Malvolio, an Insect Homolog of Nramp, in the Subsocial Beetle Nicrophorus vespilloides

With growing numbers of sequenced genomes, increasing numbers of duplicate genes are being uncovered. Here we examine Malvolio, a gene in the natural resistance-associated macrophage protein (Nramp) family, that has been duplicated in the subsocial beetle, Nicrophorus vespilloides, which exhibits advanced parental behavior. There is only one copy of Mvl in honey bees and Drosophila, whereas in vertebrates there are two copies that are subfunctionalized. We first compared amino acid sequences for Drosophila, beetles, mice, and humans. We found a high level of conservation between the different species, although there was greater variation in the C-terminal regions. A phylogenetic analysis across multiple insect orders suggested that Mvl has undergone several independent duplications. To examine the potential for different functions where it has been duplicated, we quantified expression levels of Mvl1 and Mvl2 in eight tissues in N. vespilloides We found that while Mvl1 was expressed ubiquitously, albeit at varying levels, expression of Mvl2 was limited to brain and midgut. Because Mvl has been implicated in behavior, we examined expression during different behavioral states that reflected differences in opportunity for social interactions and expression of parental care behaviors. We found differing expression patterns for the two copies, with Mvl1 increasing in expression during resource preparation and feeding offspring, and Mvl2 decreasing in these same states. Given these patterns of expression, along with the protein analysis, we suggest that Mvl in N. vespilloides has experienced sub/neofunctionalization following its duplication, and may be evolving differing and tissue-specific roles in behavior and physiology.

Bacterial Metabolism Shapes the Host-Pathogen Interface

Bacterial pathogens have evolved to exploit humans as a rich source of nutrients to support survival and replication. The pathways of bacterial metabolism that permit successful colonization are surprisingly varied and highlight remarkable metabolic flexibility. The constraints and immune pressures of distinct niches within the human body set the stage for understanding the mechanisms by which bacteria acquire critical nutrients. In this article we discuss how different bacterial pathogens carry out carbon and energy metabolism in the host and how they obtain or use key nutrients for replication and immune evasion.

Nramp1 and NrampB Contribute to Resistance against Francisella in Dictyostelium

The Francisella genus comprises highly pathogenic bacteria that can cause fatal disease in their vertebrate and invertebrate hosts including humans. In general, Francisella growth depends on iron availability, hence, iron homeostasis must be tightly regulated during Francisella infection. We used the system of the professional phagocyte Dictyostelium and the fish pathogen F. noatunensis subsp. noatunensis (F.n.n.) to investigate the role of the host cell iron transporters Nramp (natural resistance associated macrophage proteins) during Francisella infection. Like its mammalian ortholog, Dictyostelium Nramp1 transports iron from the phagosome into the cytosol, whereas the paralog NrampB is located on the contractile vacuole and controls, together with Nramp1, the cellular iron homeostasis. In Dictyostelium, Nramp1 localized to the F.n.n.-phagosome but disappeared from the compartment dependent on the presence of IglC, an established Francisella virulence factor. In the absence of Nramp transporters the bacteria translocated more efficiently from the phagosome into the host cell cytosol, its replicative niche. Increased escape rates coincided with increased proteolytic activity in bead-containing phagosomes indicating a role of the Nramp transporters for phagosomal maturation. In the nramp mutants, a higher bacterial load was observed in the replicative phase compared to wild-type host cells. Upon bacterial access to the cytosol of wt cells, mRNA levels of bacterial iron uptake factors were transiently upregulated. Decreased iron levels in the nramp mutants were compensated by a prolonged upregulation of the iron scavenging system. These results show that Nramps contribute to host cell immunity against Francisella infection by influencing the translocation efficiency from the phagosome to the cytosol but not by restricting access to nutritional iron in the cytosol.

The Rcs-Regulated Colanic Acid Capsule Maintains Membrane Potential in Salmonella enterica serovar Typhimurium

The Rcs phosphorelay and Psp (phage shock protein) systems are envelope stress responses that are highly conserved in gammaproteobacteria. The Rcs regulon was found to be strongly induced during metal deprivation of Salmonella enterica serovar Typhimurium lacking the Psp response. Nineteen genes activated by the RcsA-RcsB response regulator make up an operon responsible for the production of colanic acid capsular polysaccharide, which promotes biofilm development. Despite more than half a century of research, the physiological function of colanic acid has remained elusive. Here we show that Rcs-dependent colanic acid production maintains the transmembrane electrical potential and proton motive force in cooperation with the Psp response. Production of negatively charged exopolysaccharide covalently bound to the outer membrane may enhance the surface potential by increasing the local proton concentration. This provides a unifying mechanism to account for diverse Rcs/colanic acid-related phenotypes, including susceptibility to membrane-damaging agents and biofilm formation.

Colanic acid is a negatively charged polysaccharide capsule produced by Escherichia coli, Salmonella, and other gammaproteobacteria. Research conducted over the 50 years since the discovery of colanic acid suggests that this exopolysaccharide plays an important role for bacteria living in biofilms. However, a precise physiological role for colanic acid has not been defined. In this study, we provide evidence that colanic acid maintains the transmembrane potential and proton motive force during envelope stress. This work provides a new and fundamental insight into bacterial physiology.

Iron Efflux by PmtA Is Critical for Oxidative Stress Resistance and Contributes Significantly to Group A Streptococcus Virulence

Group A Streptococcus (GAS) is a human-only pathogen that causes a spectrum of disease conditions. Given its survival in inflamed lesions, the ability to sense and overcome oxidative stress is critical for GAS pathogenesis. PerR senses oxidative stress and coordinates the regulation of genes involved in GAS antioxidant defenses. In this study, we investigated the role of PerR-controlled metal transporter A (PmtA) in GAS pathogenesis. Previously, PmtA was implicated in GAS antioxidant defenses and suggested to protect against zinc toxicity. Here, we report that PmtA is a P_1B4-type ATPase that functions as an Fe(II) exporter and aids GAS defenses against iron intoxication and oxidative stress. The expression of pmtA is specifically induced by excess iron, and this induction requires PerR. Furthermore, a pmtA mutant exhibited increased sensitivity to iron toxicity and oxidative stress due to an elevated intracellular accumulation of iron. RNA-sequencing analysis revealed that GAS undergoes significant alterations in gene expression to adapt to iron toxicity. Finally, using two mouse models of invasive infection, we demonstrated that iron efflux by PmtA is critical for bacterial survival during infection and GAS virulence. Together, these data demonstrate that PmtA is a key component of GAS antioxidant defenses and contributes significantly to GAS virulence.

Developmental Control of NRAMP1 (SLC11A1) Expression in Professional Phagocytes

NRAMP1 (SLC11A1) is a professional phagocyte membrane importer of divalent metals that contributes to iron recycling at homeostasis and to nutritional immunity against infection. Analyses of data generated by several consortia and additional studies were integrated to hypothesize mechanisms restricting NRAMP1 expression to mature phagocytes. Results from various epigenetic and transcriptomic approaches were collected for mesodermal and hematopoietic cell types and compiled for combined analysis with results of genetic studies associating single nucleotide polymorphisms (SNPs) with variations in NRAMP1 expression (eQTLs). Analyses establish that NRAMP1 is part of an autonomous topologically associated domain delimited by ubiquitous CCCTC-binding factor (CTCF) sites. NRAMP1 locus contains five regulatory regions: a predicted super-enhancer (S-E) key to phagocyte-specific expression; the proximal promoter; two intronic areas, including 3' inhibitory elements that restrict expression during development; and a block of upstream sites possibly extending the S-E domain. Also the downstream region adjacent to the 3' CTCF locus boundary may regulate expression during hematopoiesis. Mobilization of the locus 14 predicted transcriptional regulatory elements occurs in three steps, beginning with hematopoiesis; at the onset of myelopoiesis and through myelo-monocytic differentiation. Basal expression level in mature phagocytes is further influenced by genetic variation, tissue environment, and in response to infections that induce various epigenetic memories depending on microorganism nature. Constitutively associated transcription factors (TFs) include CCAAT enhancer binding protein beta (C/EBPb), purine rich DNA binding protein (PU.1), early growth response 2 (EGR2) and signal transducer and activator of transcription 1 (STAT1) while hypoxia-inducible factors (HIFs) and interferon regulatory factor 1 (IRF1) may stimulate iron acquisition in pro-inflammatory conditions. Mouse orthologous locus is generally conserved; chromatin patterns typify a de novo myelo-monocytic gene whose expression is tightly controlled by TFs Pu.1, C/ebps and Irf8; Irf3 and nuclear factor NF-kappa-B p 65 subunit (RelA) regulate expression in inflammatory conditions. Functional differences in the determinants identified at these orthologous loci imply that species-specific mechanisms control gene expression.

"Pumping iron"-how macrophages handle iron at the systemic, microenvironmental, and cellular levels

Macrophages reside in virtually every organ. First arising during embryogenesis, macrophages replenish themselves in the adult through a combination of self-renewal and influx of bone marrow-derived monocytes. As large phagocytic cells, macrophages participate in innate immunity while contributing to tissue-specific homeostatic functions. Among the key metabolic tasks are senescent red blood cell recycling, free heme detoxification, and provision of iron for de novo hemoglobin synthesis. While this systemic mechanism involves the shuttling of iron between spleen, liver, and bone marrow through the concerted function of defined macrophage populations, similar circuits appear to exist within the microenvironment of other organs. The high turnover of iron is the prerequisite for continuous erythropoiesis and tissue integrity but challenges macrophages’ ability to maintain cellular iron homeostasis and immune function.

This review provides a brief overview of systemic, microenvironmental, and cellular aspects of macrophage iron handling with a focus on exciting and unresolved questions in the field.

Transition Metal Homeostasis in Streptococcus pyogenes and Streptococcus pneumoniae

Trace metals such as Fe, Mn, Zn and Cu are essential for various biological functions including proper innate immune function. The host immune system has complicated and coordinated mechanisms in place to either starve and/or overload invading pathogens with various metals to combat the infection. Here, we discuss the roles of Fe, Mn and Zn in terms of nutritional immunity, and also the roles of Cu and Zn in metal overload in relation to the physiology and pathogenesis of two human streptococcal species, Streptococcus pneumoniae and Streptococcus pyogenes. S. pneumoniae is a major human pathogen that is carried asymptomatically in the nasopharynx by up to 70% of the population; however, transition to internal sites can cause a range of diseases such as pneumonia, otitis media, meningitis and bacteraemia. S. pyogenes is a human pathogen responsible for diseases ranging from pharyngitis and impetigo, to severe invasive infections. Both species have overlapping capacity with respect to metal acquisition, export and regulation and how metal homeostasis relates to their virulence and ability to invade and survive within the host. It is becoming more apparent that metals have an important role to play in the control of infection, and with further investigations, it could lead to the potential use of metals in novel antimicrobial therapies.

Self-harm to preferentially harm the pathogens within: non-specific stressors in innate immunity

Therapies with increasing specificity against pathogens follow the immune system's evolutionary course in maximizing host defence while minimizing self-harm. Nevertheless, even completely non-specific stressors, such as reactive molecular species, heat, nutrient and oxygen deprivation, and acidity can be used to preferentially harm pathogens. Strategic use of non-specific stressors requires exploiting differences in stress vulnerability between pathogens and hosts. Two basic vulnerabilities of pathogens are: (i) the inherent vulnerability to stress of growth and replication (more immediately crucial for pathogens than for host cells) and (ii) the degree of pathogen localization, permitting the host's use of locally and regionally intense stress. Each of the various types of non-specific stressors is present during severe infections at all levels of localization: (i) ultra-locally within phagolysosomes, (ii) locally at the infected site, (iii) regionally around the infected site and (iv) systemically as part of the acute-phase response. We propose that hosts strategically use a coordinated system of non-specific stressors at local, regional and systemic levels to preferentially harm the pathogens within. With the rising concern over emergence of resistance to specific therapies, we suggest more scrutiny of strategies using less specific therapies in pathogen control. Hosts' active use of multiple non-specific stressors is likely an evolutionarily basic defence whose retention underlies and supplements the well-recognized immune defences that directly target pathogens.

Leishmania and its quest for iron: An update and overview

Parasites of genus Leishmania are the causative agents of complex neglected diseases called leishmaniasis and continue to be a significant health concern globally. Iron is a vital nutritional requirement for virtually all organisms, including pathogenic trypanosomatid parasites, and plays a crucial role in many facets of cellular metabolism as a cofactor of several enzymes. Iron acquisition is essential for the survival of parasites. Yet parasites are also vulnerable to the toxicity of iron and reactive oxygen species. The aim of this review is to provide an update on the current knowledge about iron acquisition and usage by Leishmania species. We have also discussed about host strategy to modulate iron availability and the strategies deployed by Leishmania parasites to overcome iron withholding defences and thus favour parasite growth within host macrophages. Since iron plays central roles in the host's response and parasite metabolism, a comprehensive understanding of the iron metabolism is beneficial to identify potential viable therapeutic opportunities against leishmaniasis.