Human calprotectin is an iron-sequestering host-defense protein

On the Antibacterial Activity of Azacarboxylate Ligands: Lowered Metal Ion Affinities for Bis-amide Derivatives of EDTA do not mean Reduced Activity

EDTA is widely used as an inhibitor of bacterial growth, affecting the uptake and control of metal ions by microorganisms. We describe the synthesis and characterisation of two symmetrical bis-amide derivatives of EDTA, featuring glycyl or pyridyl substituents: AmGly₂ and AmPy₂ . Metal ion affinities (logK) have been evaluated for a range of metals (Mg²⁺ , Ca²⁺ , Fe³⁺ , Mn²⁺ , Zn²⁺ ), revealing less avid binding compared to EDTA. The solid-state structures of AmGly₂ and of its Mg²⁺ complex have been determined crystallographically. The latter shows an unusual 7-coordinate, capped octahedral Mg²⁺ centre. The antibacterial activities of the two ligands and of EDTA have been evaluated against a range of health-relevant bacterial species, three Gram negative (Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae) and a Gram positive (Staphylococcus aureus). The AmPy₂ ligand is the only one that displays a significant inhibitory effect against K. pneumoniae, but is less effective against the other organisms. AmGly₂ exhibits a more powerful inhibitory effect against E. coli at lower concentrations than EDTA (<3 mm) or AmPy₂ , but loses its efficacy at higher concentrations. The growth inhibition of EDTA and AmGly₂ on mutant E. coli strains with defects in outer-membrane lipopolysaccharide (LPS) structures has been assessed to provide insight into the unexpected behaviour. Taken together, the results contradict the assumption of a simple link between metal ion affinity and antimicrobial efficacy.

Initial Biochemical and Functional Evaluation of Murine Calprotectin Reveals Ca(II)-Dependence and Its Ability to Chelate Multiple Nutrient Transition Metal Ions

Calprotectin (CP) is an abundant host-defense protein that contributes to the metal-withholding innate immune response by sequestering nutrient metal ions from microbial pathogens in the extracellular space. Over the past decade, murine models of infectious disease have advanced understanding of the physiological functions of CP and its ability to compete with microbes for essential metal nutrients. Despite this extensive work, murine CP (mCP) has not been biochemically evaluated, and structural and biophysical understanding of CP is currently limited to the human orthologue. We present the reconstitution, purification, and characterization of mCP as well as the cysteine-null variant mCP-Ser. Apo mCP is a mS100A8/mS100A9 heterodimer, and Ca(II) binding causes two heterodimers to self-associate and form a heterotetramer. Initial metal-depletion studies demonstrate that mCP depletes multiple first-row transition metal ions, including Mn, Fe, Ni, Cu, and Zn, from complex microbial growth medium, indicating that mCP binds multiple nutrient metals with high affinity. Moreover, antibacterial activity assays show that mCP inhibits the growth of a variety of bacterial species. The metal-depletion and antibacterial activity studies also provide evidence that Ca(II) ions enhance these functional properties of mCP. This contribution provides the groundwork for understanding the similarities and differences between the human and murine orthologues of CP and for further elucidation of its biological coordination chemistry.

Copper Acquisition and Utilization in Fungi

Fungal cells colonize and proliferate in distinct niches, from soil and plants to diverse tissues in human hosts. Consequently, fungi are challenged with the goal of obtaining nutrients while simultaneously elaborating robust regulatory mechanisms to cope with a range of availability of nutrients, from scarcity to excess. Copper is essential for life but also potentially toxic. In this review we describe the sophisticated homeostatic mechanisms by which fungi acquire, utilize, and control this biochemically versatile trace element. Fungal pathogens, which can occupy distinct host tissues that have their own intrinsic requirements for copper homeostasis, have evolved mechanisms to acquire copper to successfully colonize the host, disseminate to other tissues, and combat host copper bombardment mechanisms that would otherwise mitigate virulence.

Iron Redox Chemistry Promotes Antiparallel Oligomerization of α-Synuclein

Brain metal dyshomeostasis and altered structural dynamics of the presynaptic protein α-synuclein (αS) are both implicated in the pathology of Parkinson's disease (PD), yet a mechanistic understanding of disease progression in the context of αS structure and metal interactions remains elusive. In this Communication, we detail the influence of iron, a prevalent redox-active brain biometal, on the aggregation propensity and secondary structure of N-terminally acetylated αS (^NAcαS), the physiologically relevant form in humans. We demonstrate that under aerobic conditions, Fe(II) commits ^NAcαS to a PD-relevant oligomeric assembly, verified by the oligomer-selective A11 antibody, that does not have any parallel β-sheet character but contains a substantial right-twisted antiparallel β-sheet component based on CD analyses and descriptive deconvolution of the secondary structure. This ^NAcαS-Fe^II oligomer does not develop into the β-sheet fibrils that have become hallmarks of PD, even after extended incubation, as verified by TEM imaging and the fibril-specific OC antibody. Thioflavin T (ThT), a fluorescent probe for β-sheet fibril formation, also lacks coordination to this antiparallel conformer. We further show that this oligomeric state is not observed when O₂ is excluded, indicating a role for iron(II)-mediated O₂ chemistry in locking this dynamic protein into a conformation that may have physiological or pathological implications.

Calprotectin in Daily Practice: Where Do We Stand in 2017?

BACKGROUND

To make a distinction between organic and functional disease is essential for gastroenterologists in their daily practice, but it may be challenging, given the variety and aspecificity of gastrointestinal symptoms among the general population. The clinician aim is to avoid diagnostic delay and to restrict unnecessary invasive and expensive exams.

SUMMARY

Faecal markers, in particular faecal calprotectin (FC), have given proof of being reliable markers of intestinal inflammation with good clinical sensitivity. Calprotectin is useful in the differential diagnosis between inflammatory bowel disease and irritable bowel syndrome, as well as in the follow-up of inflammatory bowel disease patients and in predicting treatment response, with an excellent correlation with endoscopic activity. Its role in collagenous colitis and infectious colitis is less clear and still under investigation. Key Message: Despite the growing evidence supporting its use, many clinicians are uncomfortable in dosing FC, due to its low specificity and the variability of cut-off values. Indeed there are no clear guidelines about how to manage patients with intermediate levels of FC. The aim of this article is to review recent literature on calprotectin and its use. The strong points and the limits of FC measurement will be analysed, and a practical approach in the daily clinical routine will be proposed.

Synovial Calprotectin: An Inexpensive Biomarker to Exclude a Chronic Prosthetic Joint Infection

BACKGROUND

To diagnose or exclude a chronic prosthetic joint infection (PJI) can be a clinical challenge. Therefore, sensitive and specific biomarkers are needed in the diagnostic work-up. Calprotectin is a protein with antimicrobial properties and is released by activated neutrophils, making it a specific marker for infection. Because of its low costs and ability to obtain a quantitative value as a point of care test, it is an attractive marker to use in clinical practice. In addition, the test is already used in routine care in most hospitals for other indications and therefore easy to implement.

METHODS

Between June 2015 and June 2017 we collected synovial fluid of all consecutive patients who underwent revision surgery of a prosthetic joint because of chronic pain with or without prosthetic loosening. Synovial calprotectin was measured using a lateral flow immunoassay. A PJI was defined by the diagnostic criteria described by the Musculoskeletal Infection Society.

RESULTS

Fifty-two patients with chronic pain were included. A PJI was diagnosed in 15 of 52 (29%) patients. The median calprotectin in the PJI group was 859 mg/L (interquartile range 86-1707) vs 7 mg/L (interquartile range 3-25) in the control group (P < .001). With a cut-off value of 50 mg/L, synovial calprotectin showed a sensitivity, specificity, positive predictive value, and negative predictive value of 86.7%, 91.7%, 81.3%, and 94.4%, respectively.

CONCLUSION

Synovial calprotectin is a useful and cheap biomarker to use in the diagnostic work-up of patients with chronic pain, especially to exclude a PJI prior to revision surgery.

Bioinorganic Explorations of Zn(II) Sequestration by Human S100 Host-Defense Proteins

The human innate immune system launches a metal-withholding response to starve invading microbial pathogens of essential metal nutrients. Zn(II)-sequestering proteins of the human S100 family contribute to this process and include calprotectin (CP, S100A8/S100A9 oligomer, calgranulin A/B oligomer), S100A12 (calgranulin C), and S100A7 (psoriasin). This Perspective highlights recent advances in the Zn(II) coordination chemistry of these three proteins, as well as select studies that evaluate Zn(II) sequestration as an antimicrobial mechanism.

Salivary metals, age, and gender correlate with cultivable oral Candida carriage levels

Background: Little is known about the normal range of metal levels in unstimulated saliva, nor whether these might impact Candida carriage in healthy individuals. Both are important in determining which populations are at risk for candidiasis, as the availability of metal ions can influence the growth and pathogenesis of Candida albicans.

Objective: We quantified salivary metals of healthy individuals to determine the correlation with C. albicans oral colonization.

Design: Unstimulated whole saliva was collected from healthy adults and plated to determine fungal carriage, and metal content was measured using ICP-mass spectrometry (ICP-MS).

Results: Zinc was most abundant, followed by iron, copper, manganese, and nickel. Cultivable oral Candida carriage was found in 48% of people. Total protein levels did not differ in salivas from donors with or without carriage. However, innate fungicidal activity was increased in donors with carriage; correlations between levels of several metals were stronger in salivas with fungal carriage, indicating a shift in the oral environment. Concentrations of copper and manganese, as well as age and gender, were significantly predictive of carriage levels in a multiple regression model.

Conclusions: Salivary copper and manganese content along with age and gender could be used as a predictive metric for individuals that are more susceptible to Candida overgrowth.

Intestinal permeability and Ménière's disease

PURPOSE

Ménière disease (MD) is a multifactorial chronic disabling condition characterized by episodic vertigo, ear fullness, and hearing loss. MD patients often complain of aspecific gastrointestinal symptoms associated with autonomic dysregulation, frequently outweighed by the otological manifestations. Dietary modifications have been reported to improve the typical MD symptoms in some cases. Our purpose was to test the urinary levels of lactulose and mannitol (double sugar test) and the fecal calprotectin, both markers of altered intestinal permeability, in subjects with definite MD in an active and inactive stage.

MATERIALS AND METHODS

Twenty-six with definite unilateral MD were studied: 14 patients were symptomatic for at least 3months with moderate to severe vertigo spells and a functional level ≥4; 12 patients had been asymptomatic (no vertigo spells) for at least 3months and had a functional level=1 at the time of testing. Twenty healthy volunteers were recruited as "control group".

RESULTS

Lactulose and mannitol absorption was significantly increased in the symptomatic M patients compared to the asymptomatic group (p<0.02 and p<0.004, respectively) and to the controls. FC were also higher than normal only in the symptomatic group. (p<0.01).

CONCLUSIONS

An altered intestinal permeability, according to the two assays, was found only in symptomatic MD patients. The rationale for a possible relationship between MD and intestinal permeability is forwarded. The double-sugar test and FC quantification might be implemented in the MD diagnostic workup.

Role of Calprotectin in Withholding Zinc and Copper from Candida albicans

The opportunistic fungal pathogen Candida albicans acquires essential metals from the host, yet the host can sequester these micronutrients through a process known as nutritional immunity. How the host withholds metals from C. albicans has been poorly understood; here we examine the role of calprotectin (CP), a transition metal binding protein. When CP depletes bioavailable Zn from the extracellular environment, C. albicans strongly upregulates ZRT1 and PRA1 for Zn import and maintains constant intracellular Zn through numerous cell divisions. We show for the first time that CP can also sequester Cu by binding Cu(II) with subpicomolar affinity. CP blocks fungal acquisition of Cu from serum and induces a Cu starvation stress response involving SOD1 and SOD3 superoxide dismutases. These transcriptional changes are mirrored when C. albicans invades kidneys in a mouse model of disseminated candidiasis, although the responses to Cu and Zn limitations are temporally distinct. The Cu response progresses throughout 72 h, while the Zn response is short-lived. Notably, these stress responses were attenuated in CP null mice, but only at initial stages of infection. Thus, Zn and Cu pools are dynamic at the host-pathogen interface and CP acts early in infection to restrict metal nutrients from C. albicans.

Biochemical and Spectroscopic Observation of Mn(II) Sequestration from Bacterial Mn(II) Transport Machinery by Calprotectin

Human calprotectin (CP, S100A8/S100A9 oligomer) is a metal-sequestering host-defense protein that prevents bacterial acquisition of Mn(II). In this work, we investigate Mn(II) competition between CP and two solute-binding proteins that Staphylococcus aureus and Streptococcus pneumoniae, Gram-positive bacterial pathogens of significant clinical concern, use to obtain Mn(II) when infecting a host. Biochemical and electron paramagnetic resonance (EPR) spectroscopic analyses demonstrate that CP outcompetes staphylococcal MntC and streptococcal PsaA for Mn(II). This behavior requires the presence of excess Ca(II) ions, which enhance the Mn(II) affinity of CP. This report presents new spectroscopic evaluation of two Mn(II) proteins important for bacterial pathogenesis, direct observation of Mn(II) sequestration from bacterial Mn(II) acquisition proteins by CP, and molecular insight into the extracellular battle for metal nutrients that occurs during infection.

Metals in fungal virulence

Metals are essential for life, and they play a central role in the struggle between infecting microbes and their hosts. In fact, an important aspect of microbial pathogenesis is the 'nutritional immunity', in which metals are actively restricted (or, in an extended definition of the term, locally enriched) by the host to hinder microbial growth and virulence. Consequently, fungi have evolved often complex regulatory networks, uptake and detoxification systems for essential metals such as iron, zinc, copper, nickel and manganese. These systems often differ fundamentally from their bacterial counterparts, but even within the fungal pathogens we can find common and unique solutions to maintain metal homeostasis. Thus, we here compare the common and species-specific mechanisms used for different metals among different fungal species-focusing on important human pathogens such as Candida albicans, Aspergillus fumigatus or Cryptococcus neoformans, but also looking at model fungi such as Saccharomyces cerevisiae or A. nidulans as well-studied examples for the underlying principles. These direct comparisons of our current knowledge reveal that we have a good understanding how model fungal pathogens take up iron or zinc, but that much is still to learn about other metals and specific adaptations of individual species-not the least to exploit this knowledge for new antifungal strategies.

Human calprotectin affects the redox speciation of iron

We report that the metal-sequestering human host-defense protein calprotectin (CP, S100A8/S100A9 oligomer) affects the redox speciation of iron (Fe) in bacterial growth media and buffered aqueous solution. Under aerobic conditions and in the absence of an exogenous reducing agent, CP-Ser (S100A8(C42S)/S100A9(C3S) oligomer) depletes Fe from three different bacterial growth media preparations over a 48 h timeframe (T = 30 °C). The presence of the reducing agent β-mercaptoethanol accelerates this process and allows CP-Ser to deplete Fe over a ≈1 h timeframe. Fe-depletion assays performed with metal-binding-site variants of CP-Ser show that the hexahistidine (His₆) site, which coordinates Fe(ii) with high affinity, is required for Fe depletion. An analysis of Fe redox speciation in buffer containing Fe(iii) citrate performed under aerobic conditions demonstrates that CP-Ser causes a time-dependent increase in the [Fe(ii)]/[Fe(iii)] ratio. Taken together, these results indicate that the hexahistidine site of CP stabilizes Fe(ii) and thereby shifts the redox equilibrium of Fe to the reduced ferrous state under aerobic conditions. We also report that the presence of bacterial metabolites affects the Fe-depleting activity of CP-Ser. Supplementation of bacterial growth media with an Fe(iii)-scavenging siderophore (enterobactin, staphyloferrin B, or desferrioxamine B) attenuates the Fe-depleting activity of CP-Ser. This result indicates that formation of Fe(iii)-siderophore complexes blocks CP-mediated reduction of Fe(iii) and hence the ability of CP to coordinate Fe(ii). In contrast, the presence of pyocyanin (PYO), a redox-cycling phenazine produced by Pseudomonas aeruginosa that reduces Fe(iii) to Fe(ii), accelerates Fe depletion by CP-Ser under aerobic conditions. These findings indicate that the presence of microbial metabolites that contribute to metal homeostasis at the host/pathogen interface can affect the metal-sequestering function of CP.

High Amounts of S100-Alarmins Confer Antimicrobial Activity on Human Breast Milk Targeting Pathogens Relevant in Neonatal Sepsis

Sepsis is a leading cause of perinatal mortality worldwide. Breast milk (BM) feeding is protective against neonatal sepsis, but the molecular mechanisms remain unexplained. Despite various supplementations with potential bioactive components from BM formula feeding cannot protect from sepsis. S100-alarmins are important immunoregulators in newborns preventing excessive inflammation. At high concentrations, the S100A8/A9 protein complex also has antimicrobial properties due to its metal ion chelation capacity. To assess whether BM contains S100-alarmins that might mediate the sepsis-protective effect of BM 97 human BM samples stratified for gestational age, mode of delivery and sampling after birth were collected and analyzed. S100A8/A9 levels were massively elevated after birth (p < 0.0005). They slowly decreased during the first month of life, then reaching levels comparable to normal values in adult serum. The concentration of S100A8/A9 in BM was significantly higher after term compared with preterm birth (extremely preterm, p < 0.005; moderate preterm, p < 0.05) and after vaginal delivery compared with cesarean section (p < 0.0005). In newborn s100a9^−/− mice, enterally supplied S100-alarmins could be retrieved systemically in the plasma. To explore the antimicrobial activity against common causal pathogens of neonatal sepsis, purified S100-alarmins and unmodified as well as S100A8/A9-depleted BM were used in growth inhibition tests. The high amount of S100A8/A9 proved to be an important mediator of the antimicrobial activity of BM, especially inhibiting the growth of manganese (Mn) sensitive bacteria such as Staphylococcus aureus (p < 0.00005) and group B streptococci (p < 0.005). Depletion of S100A8/A9 significantly reduced this effect (p < 0.05, respectively). The growth of Escherichia coli was also inhibited by BM (p < 0.00005) as well as by S100A8/A9 in culture assays (p < 0.05). But its growth in BM remained unaffected by the removal of S100A8/A9 and was neither dependent on Mn suggesting that the antimicrobial effects of S100A8/A9 in BM are primarily mediated by its Mn chelating capacity. In summary, the enteral supply of bioavailable, antimicrobially active amounts of S100-alarmins might be a promising option to protect newborns at high risk from infections and sepsis.

The Metallophore Staphylopine Enables Staphylococcus aureus To Compete with the Host for Zinc and Overcome Nutritional Immunity

During infection, the host sequesters essential nutrients, such as zinc, to combat invading microbes. Despite the ability of the immune effector protein calprotectin to bind zinc with subpicomolar affinity, Staphylococcus aureus is able to successfully compete with the host for zinc. However, the zinc importers expressed by S. aureus remain unknown. Our investigations have revealed that S. aureus possesses two importers, AdcABC and CntABCDF, which are induced in response to zinc limitation. While AdcABC is similar to known zinc importers in other bacteria, CntABCDF has not previously been associated with zinc acquisition. Concurrent loss of the two systems severely impairs the ability of S. aureus to obtain zinc and grow in zinc-limited environments. Further investigations revealed that the Cnt system is responsible for the ability of S. aureus to compete with calprotectin for zinc in culture and contributes to acquisition of zinc during infection. The cnt locus also enables S. aureus to produce the broad-spectrum metallophore staphylopine. Similarly to the Cnt transporter, loss of staphylopine severely impairs the ability of S. aureus to resist host-imposed zinc starvation, both in culture and during infection. Further investigations revealed that together staphylopine and the Cnt importer function analogously to siderophore-based iron acquisition systems in order to facilitate zinc acquisition by S. aureus Analogous systems are found in a broad range of Gram-positive and Gram-negative bacterial pathogens, suggesting that this new type of zinc importer broadly contributes to the ability of bacteria to cause infection.IMPORTANCE A critical host defense against infection is the restriction of zinc availability. Despite the subpicomolar affinity of the immune effector calprotectin for zinc, Staphylococcus aureus can successfully compete for this essential metal. Here, we describe two zinc importers, AdcABC and CntABCDF, possessed by S. aureus, the latter of which has not previously been associated with zinc acquisition. The ability of S. aureus to compete with the host for zinc is dependent on CntABCDF and the metallophore staphylopine, both in culture and during infection. These results expand the mechanisms utilized by bacteria to obtain zinc, beyond Adc-like systems, and demonstrate that pathogens utilize strategies similar to siderophore-based iron acquisition to obtain other essential metals during infection. The staphylopine synthesis machinery is present in a diverse collection of bacteria, suggesting that this new family of zinc importers broadly contributes to the ability of numerous pathogens to cause infection.

Biochemical and Functional Evaluation of the Intramolecular Disulfide Bonds in the Zinc-Chelating Antimicrobial Protein Human S100A7 (Psoriasin)

Human S100A7 (psoriasin) is a metal-chelating protein expressed by epithelial cells. It is a 22-kDa homodimer with two EF-hand domains per subunit and two transition-metal-binding His3Asp sites at the dimer interface. Each subunit contains two cysteine residues that can exist as free thiols (S100A7red) or as an intramolecular disulfide bond (S100A7ox). Herein, we examine the disulfide bond redox behavior, the Zn(II) binding properties, and the antibacterial activity of S100A7, as well as the effect of Ca(II) ions on these properties. In agreement with prior work [Hein, K. Z., et al. (2013) Proc. Natl. Acad. Sci. U. S. A. 112, 13039-13044], we show that apo S100A7ox is a substrate for the mammalian thioredoxin system; however, negligible reduction of the disulfide bond is observed for Ca(II)- and Zn(II)-bound S100A7ox. Furthermore, metal binding depresses the midpoint potential of the disulfide bond. S100A7ox and S100A7red each coordinate 2 equiv of Zn(II) with subnanomolar affinity in the absence and presence of Ca(II) ions, and the cysteine thiolates in S100A7red do not form a third high-affinity Zn(II) site. These results refute a prior model implicating the Cys thiolates of S100A7red in high-affinity Zn(II) binding [Hein, K. Z., et al. (2013) Proc. Natl. Acad. Sci. U. S. A. 112, 13039-13044]. S100A7ox and the disulfide-null variants show comparable Zn(II)-depletion profiles; however, only S100A7ox exhibits antibacterial activity against select bacterial species. Metal substitution experiments suggest that the disulfide bonds in S100A7 may enhance metal sequestration by the His3Asp sites and thereby confer growth inhibitory properties to S100A7ox.

The Capacity of Mycobacterium tuberculosis To Survive Iron Starvation Might Enable It To Persist in Iron-Deprived Microenvironments of Human Granulomas

This study was conducted to investigate the role of iron deprivation in the persistence of Mycobacterium tuberculosis. We present evidence of iron restriction in human necrotic granulomas and demonstrate that under iron starvation M. tuberculosis persists, refractive to antibiotics and capable of restarting replication when iron is made available. Transcriptomics and metabolomic analyses indicated that the persistence of M. tuberculosis under iron starvation is dependent on strict control of endogenous Fe utilization and is associated with upregulation of pathogenicity and intrinsic antibiotic resistance determinants. M. tuberculosis mutants compromised in their ability to survive Fe starvation were identified. The findings of this study advance the understanding of the physiological settings that may underpin the chronicity of human tuberculosis (TB) and are relevant to the design of effective antitubercular therapies.

One-third of the world population may harbor persistent M. tuberculosis, causing an asymptomatic infection that is refractory to treatment and can reactivate to become potentially lethal tuberculosis disease. However, little is known about the factors that trigger and maintain M. tuberculosis persistence in infected individuals. Iron is an essential nutrient for M. tuberculosis growth. In this study, we show, first, that in human granulomas the immune defense creates microenvironments in which M. tuberculosis likely experiences drastic Fe deprivation and, second, that Fe-starved M. tuberculosis is capable of long-term persistence without growth. Together, these observations suggest that Fe deprivation in the lung might trigger a state of persistence in M. tuberculosis and promote chronic TB. We also identified vulnerabilities of iron-restricted persistent M. tuberculosis, which can be exploited for the design of new antitubercular therapies.

Fecal total iron concentration is inversely associated with fecal Lactobacillus in preschool children

BACKGROUND AND AIM

Iron deficiency is associated with stunting and poor performance in children. Oral iron supplementation is widely promoted to correct iron deficiency. However, excess iron may be toxic to beneficial luminal gut bacteria and could support growth of pathobionts. The aim of this study is to analyze the fecal total iron concentration and fecal Lactobacillus levels in a cohort of stunted and normal children.

METHODS

The study was undertaken in two different locations. One of them is a rural area, and the other is a semi-urban-slum area; both areas are located in the Vellore district of Tamilnadu state. Twenty children (10 stunted and 10 normal growth) aged 2 to 5 years from each area were recruited. Both groups were nearly identical demographically. Fecal samples were collected. Fecal total iron was estimated, and fecal DNA was extracted and subjected to 16S rDNA-targeted real-time PCR to determine the relative predominance of Lactobacillus and Escherichia coli.

RESULTS

The fecal total iron concentration in rural children (3656 μg/g wet wt. of feces) was significantly higher when compared with semi-urban-slum children (114.9 μg/g wet wt. of feces, P < 0.005). Inversely, fecal Lactobacillus in rural children (median 3.18 × 10^-3 relative difference compared with total bacteria) was significantly lower when compared with semi-urban-slum children (median 59.33 × 10^-3 , p < 0.005). There was no significant change observed between normal and stunted children. E. coli levels remained unaffected.

CONCLUSION

The present study documents an inverse relationship between fecal iron concentration and fecal Lactobacillus concentration in children belonging to two different localities independent of their nutritional status.

Nickel Sequestration by the Host-Defense Protein Human Calprotectin

The human innate immune protein calprotectin (CP, S100A8/S100A9 oligomer, calgranulin A/calgranulin B oligomer, MRP-8/MRP-14 oligomer) chelates a number of first-row transition metals, including Mn(II), Fe(II), and Zn(II), and can withhold these essential nutrients from microbes. Here we elucidate the Ni(II) coordination chemistry of human CP. We present a 2.6-Å crystal structure of Ni(II)- and Ca(II)-bound CP, which reveals that CP binds Ni(II) ions at both its transition-metal-binding sites: the His3Asp motif (site 1) and the His6 motif (site 2). Further biochemical studies establish that coordination of Ni(II) at the hexahistidine site is thermodynamically preferred over Zn(II). We also demonstrate that CP can sequester Ni(II) from two human pathogens, Staphylococcus aureus and Klebsiella pneumoniae, that utilize this metal nutrient during infection, and inhibit the activity of the Ni(II)-dependent enzyme urease in bacterial cultures. In total, our findings expand the biological coordination chemistry of Ni(II)-chelating proteins in nature and provide a foundation for evaluating putative roles of CP in Ni(II) homeostasis at the host-microbe interface and beyond.

A Critical Role of Zinc Importer AdcABC in Group A Streptococcus-Host Interactions During Infection and Its Implications for Vaccine Development

Bacterial pathogens must overcome host immune mechanisms to acquire micronutrients for successful replication and infection. Streptococcus pyogenes, also known as group A streptococcus (GAS), is a human pathogen that causes a variety of clinical manifestations, and disease prevention is hampered by lack of a human GAS vaccine. Herein, we report that the mammalian host recruits calprotectin (CP) to GAS infection sites and retards bacterial growth by zinc limitation. However, a GAS-encoded zinc importer and a nuanced zinc sensor aid bacterial defense against CP-mediated growth inhibition and contribute to GAS virulence. Immunization of mice with the extracellular component of the zinc importer confers protection against systemic GAS challenge. Together, we identified a key early stage host-GAS interaction and translated that knowledge into a novel vaccine strategy against GAS infection. Furthermore, we provided evidence that a similar struggle for zinc may occur during other streptococcal infections, which raises the possibility of a broad-spectrum prophylactic strategy against multiple streptococcal pathogens.

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Host employs calprotectin to impose zinc (Zn) limitation on the human pathogen group A streptococcus (GAS) during infection.

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As a defense, GAS uses a sensor, AdcR, to monitor Zn availability, and a high-affinity transporter, AdcABC, to acquire Zn.

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Finally, we characterized the extracellular subunit of AdcA as a vaccine candidate to protect mice from GAS infections.

There is an urgent need for a human vaccine to protect against diseases caused by human pathogen, group A streptococcus (GAS). Herein, we identified the key molecular players involved in the battle between the host and invading bacteria for the critical nutrient zinc. The host recruits calprotectin at GAS infection sites to limit zinc availability to the pathogen. The pathogen senses the alterations in zinc availability using a sensor, AdcR, and outcompetes calprotectin by employing a high-affinity zinc uptake system, AdcABC. Using this knowledge, we developed a successful vaccination strategy by immunization with AdcA and demonstrated protection against GAS infections.

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.

Expression, Purification, and Antimicrobial Activity of S100A12

Calgranulin proteins are important mediators of innate immunity and are members of the S100 class of the EF-hand family of calcium binding proteins. Some S100 proteins have the capacity to bind transition metals with high affinity and effectively sequester them away from invading microbial pathogens in a process that is termed "nutritional immunity". S100A12 (EN-RAGE) binds both zinc and copper and is highly abundant in innate immune cells such as macrophages and neutrophils. We report a refined method for the expression, enrichment and purification of S100A12 in its active, metal-binding configuration. Utilization of this protein in bacterial growth and viability analyses reveals that S100A12 has antimicrobial activity against the bacterial pathogen, Helicobacter pylori. The antimicrobial activity is predicated on the zinc-binding activity of S100A12, which chelates nutrient zinc, thereby starving H. pylori which requires zinc for growth and proliferation.

Transition metals at the host-pathogen interface: how Neisseria exploit human metalloproteins for acquiring iron and zinc

Transition metals are essential nutrients for all organisms and important players in the host-microbe interaction. During bacterial infection, a tug-of-war between the host and microbe for nutrient metals occurs: the host innate immune system responds to the pathogen by reducing metal availability and the pathogen tries to outmaneuver this response. The outcome of this competition, which involves metal-sequestering host-defense proteins and microbial metal acquisition machinery, is an important determinant for whether infection occurs. One strategy bacterial pathogens employ to overcome metal restriction involves hijacking abundant host metalloproteins. The obligate human pathogens Neisseria meningitidis and N. gonorrhoeae express TonB-dependent transport systems that capture human metalloproteins, extract the bound metal ions, and deliver these nutrients into the bacterial cell. This review highlights structural and mechanistic investigations that provide insights into how Neisseria acquire iron from the Fe(III)-transport protein transferrin (TF), the Fe(III)-chelating host-defense protein lactoferrin (LF), and the oxygen-transport protein hemoglobin (Hb), and obtain zinc from the metal-sequestering antimicrobial protein calprotectin (CP).

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.

Synovial calprotectin

Aims

Recently, several synovial biomarkers have been introduced into the algorithm for the diagnosis of a prosthetic joint infection (PJI). Alpha defensin is a promising biomarker, with a high sensitivity and specificity, but it is expensive. Calprotectin is a protein that is present in the cytoplasm of neutrophils, is released upon neutrophil activation and exhibits anti-microbial activity. Our aim, in this study, was to determine the diagnostic potential of synovial calprotectin in the diagnosis of a PJI.

Patients and Methods

In this pilot study, we prospectively collected synovial fluid from the hip, knee, shoulder and elbow of 19 patients with a proven PJI and from a control group of 42 patients who underwent revision surgery without a PJI. PJI was diagnosed according to the current diagnostic criteria of the Musculoskeletal Infection Society. Synovial fluid was centrifuged and the supernatant was used to measure the level of calprotectin after applying a lateral flow immunoassay.

Results

The median synovial calprotectin level was 991 mg/L (interquartile range (IQR) 154 to 1787) in those with a PJI and 11 mg/L (IQR 3 to 29) in the control group (p < 0.0001). Using a cut-off value of 50 mg/L, this level showed an excellent diagnostic accuracy, with an area under the curve of 0.94. The overall sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) was 89%, 90%, 81% and 95% respectively. The NPV was 97% in the nine patients with a chronic PJI.

Conclusion

Synovial calprotectin may be a valuable biomarker in the diagnosis of a PJI, especially in the exclusion of an infection. With a lateral flow immunoassay, a relatively rapid quantitative diagnosis can be made. The measurement is cheap and is easy to use. Cite this article: Bone Joint J 2017;99-B:660–5.

A Pseudomonas T6SS effector recruits PQS-containing outer membrane vesicles for iron acquisition

Iron sequestration by host proteins contributes to the defence against bacterial pathogens, which need iron for their metabolism and virulence. A Pseudomonas aeruginosa mutant lacking all three known iron acquisition systems retains the ability to grow in media containing iron chelators, suggesting the presence of additional pathways involved in iron uptake. Here we screen P. aeruginosa mutants defective in growth in iron-depleted media and find that gene PA2374, proximal to the type VI secretion system H3 (H3-T6SS), functions synergistically with known iron acquisition systems. PA2374 (which we have renamed TseF) appears to be secreted by H3-T6SS and is incorporated into outer membrane vesicles (OMVs) by directly interacting with the iron-binding Pseudomonas quinolone signal (PQS), a cell–cell signalling compound. TseF facilitates the delivery of OMV-associated iron to bacterial cells by engaging the Fe(III)-pyochelin receptor FptA and the porin OprF. Our results reveal links between type VI secretion, cell–cell signalling and classic siderophore receptors for iron acquisition in P. aeruginosa.

Pathogens require iron for their metabolism and virulence. Here the authors identify an iron acquisition system in Pseudomonas aeruginosa involving a protein secreted by a type VI secretion system, the PQS signalling compound and siderophore receptors.

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.

Magnetic circular dichroism studies of iron(ii) binding to human calprotectin

Calprotectin (CP) is an abundant metal-chelating protein involved in host defense, and the ability of human CP to bind Fe(ii) in a calcium-dependent manner was recently discovered. In the present study, near-infrared magnetic circular dichroism spectroscopy is employed to investigate the nature of Fe(ii) coordination at the two transition-metal-binding sites of CP that are a His3Asp motif (site 1) and a His6 motif (site 2). Upon the addition of sub-stoichiometric Fe(ii), a six-coordinate (6C) Fe(ii) center associated with site 2 is preferentially formed in the presence of excess Ca(ii). This site exhibits an exceptionally large ligand field (10Dq = 11 045 cm-1) for a non-heme Fe(ii) protein. Analysis of CP variants lacking residues of the His6 motif supports that CP coordinates Fe(ii) at site 2 by employing six His ligands. In the presence of greater than one equiv. of Fe(ii) or upon mutation of the His6 motif, the metal ion also binds at site 1 of CP to form a five-coordinate (5C) Fe(ii)-His3Asp motif that was previously unidentified in this system. Notably, the introduction of His-to-Ala mutations at the His6 motif results in a mixture of 6C (site 2) and 5C (site 1) signals in the presence of sub-stoichiometric Fe(ii). These results are consistent with a reduced Fe(ii)-binding affinity of site 2 as more weakly coordinating water-derived ligands complete the 6C site. In the absence of Ca(ii), both sites 1 and 2 are occupied upon addition of sub-stoichiometric Fe(ii), and a stronger ligand field is observed for the 5C site. These spectroscopic studies provide further evaluation of a unique non-heme Fe(ii)-His6 site for metalloproteins and support the notion that Ca(ii) ions influence the Fe(ii)-binding properties of CP.

Role of divalent metals in infectious disease susceptibility and outcome

BACKGROUND

Divalent metals play important roles in maintaining metabolism and cellular growth of both eukaryotic hosts and invading microbes. Both metal deficiency and overload can result in abnormal cellular function or damage. Given its central role in host-pathogen interactions, subtle alterations of divalent metal homeostasis can occur in the course of infectious diseases which aim, from the host perspective, either to reduce the availability of respective metals to microbes or to use toxic metal accumulation to eliminate pathogens.

AIMS

To provide the reader with background information and clinical data on divalent metal homeostasis in host-pathogen interactions, how this affects the course of infectious disease and whether correction of metal disturbances has shown benefit in infections.

SOURCES

An in-depth analysis of PubMed articles related to the topic of this review published in English between 1970 and 2016 was performed.

CONTENT

From the microbial perspective, divalent metals are essential for growth and pathogenicity and to mount effective protection against antimicrobial host responses, including toxic radical formation. Microbes have evolved multiple strategies to control their access to divalent metals. From the clinical perspective, alterations of divalent metal levels may result in increased or decreased susceptibility to infection and often occur in response to infections. However, keeping in mind the strategies underlying such alterations, for which the term 'nutritional immunity' was coined, the uncritical correction of such divalent metal imbalances may cause harm to patients. This review addresses the role of the divalent metals iron, selenium, zinc, manganese and copper in infectious diseases from a mechanistic and clinical perspective.

IMPLICATIONS

We point out areas of research needed to expand our limited knowledge, hoping to improve the clinical management of patients with infections and to identify promising new targets for treatment by modulation of host or microbe divalent metal metabolism.

Fecal calprotectin level in healthy children aged less than 4 years in South Korea

BACKGROUND

Fecal calprotectin (FC) is non-invasive inflammatory marker indicating various bowel diseases. However, the median-specific cut-off values and the standard deviations (SD) of the FC levels in each age group <4 years were not elucidated.

METHODS

Healthy volunteers were enrolled from four kindergartens. A questionnaire was used to confirm that the children met the inclusion criteria, and several demographics and history of bowel symptoms were collected. The FC level was measured.

RESULTS

A total of 234 healthy children aged between 6 months and 4 years were recruited. The median FC concentration of all participants was 245 μg/g (range 12-1033 μg/g, mean 68.5 μg/g, SD 123.12 μg/g). The children were divided into six age groups. The upper limit of 95% CI of median FC values was 135 μg/g in 7-12 months group, 65 μg/g in 13-18 months group, 55 μg/g in 19-24 months group, 40 μg/g in 25-30 months group, 21 μg/g in 31-36 months group, and 12 μg/g in 37-48 months group. A negative correlation trend was found between the age and the FC concentration.

CONCLUSION

This is the first study to present the FC median levels in the specific age groups <4 years in Korea. We found a FC level reduction with age, indicating a bowel maturation process and decreased intestinal permeability of the intestinal mucosa. In our study, FC levels reached the values of 50 μg/g around the age of 2 years.

The Role of Intermetal Competition and Mis-Metalation in Metal Toxicity

The metals manganese, iron, cobalt, nickel, copper and zinc are essential for almost all bacteria, but their precise metal requirements vary by species, by ecological niche and by growth condition. Bacteria thus must acquire each of these essential elements in sufficient quantity to satisfy their cellular demand, but in excess these same elements are toxic. Metal toxicity has been exploited by humanity for centuries, and by the mammalian immune system for far longer, yet the mechanisms by which these elements cause toxicity to bacteria are not fully understood. There has been a resurgence of interest in metal toxicity in recent decades due to the problematic spread of antibiotic resistance amongst bacterial pathogens, which has led to an increased research effort to understand these toxicity mechanisms at the molecular level. A recurring theme from these studies is the role of intermetal competition in bacterial metal toxicity. In this review, we first survey biological metal usage and introduce some fundamental chemical concepts that are important for understanding bacterial metal usage and toxicity. Then we introduce a simple model by which to understand bacterial metal homeostasis in terms of the distribution of each essential metal ion within cellular 'pools', and dissect how these pools interact with each other and with key proteins of bacterial metal homeostasis. Finally, using a number of key examples from the recent literature, we look at specific metal toxicity mechanisms in model bacteria, demonstrating the role of metal-metal competition in the toxicity mechanisms of diverse essential metals.

Competition for Iron Between Host and Pathogen: A Structural Case Study on Helicobacter pylori

Helicobacter pylori (H. pylori) is a highly successful bacterial pathogen, which colonizes the stomach of more than half of the world's population. To colonize and survive in such an acidic and inhospitable niche, H. pylori cells have evolved complex mechanisms to acquire nutrients from human hosts, including iron, an essential nutrient for both the pathogens and host cells. However, human cells also utilize diverse strategies in withholding of irons to prevent the bacterial outgrowth. The competition for iron is the central battlefield between pathogen and host. This mini-review summarizes the updated scenarios of the battle for iron between H. pylori and human host from a structural biology perspective.

A Superoxide Dismutase Capable of Functioning with Iron or Manganese Promotes the Resistance of Staphylococcus aureus to Calprotectin and Nutritional Immunity

Staphylococcus aureus is a devastating mammalian pathogen for which the development of new therapeutic approaches is urgently needed due to the prevalence of antibiotic resistance. During infection pathogens must overcome the dual threats of host-imposed manganese starvation, termed nutritional immunity, and the oxidative burst of immune cells. These defenses function synergistically, as host-imposed manganese starvation reduces activity of the manganese-dependent enzyme superoxide dismutase (SOD). S. aureus expresses two SODs, denoted SodA and SodM. While all staphylococci possess SodA, SodM is unique to S. aureus, but the advantage that S. aureus gains by expressing two apparently manganese-dependent SODs is unknown. Surprisingly, loss of both SODs renders S. aureus more sensitive to host-imposed manganese starvation, suggesting a role for these proteins in overcoming nutritional immunity. In this study, we have elucidated the respective contributions of SodA and SodM to resisting oxidative stress and nutritional immunity. These analyses revealed that SodA is important for resisting oxidative stress and for disease development when manganese is abundant, while SodM is important under manganese-deplete conditions. In vitro analysis demonstrated that SodA is strictly manganese-dependent whereas SodM is in fact cambialistic, possessing equal enzymatic activity when loaded with manganese or iron. Cumulatively, these studies provide a mechanistic rationale for the acquisition of a second superoxide dismutase by S. aureus and demonstrate an important contribution of cambialistic SODs to bacterial pathogenesis. Furthermore, they also suggest a new mechanism for resisting manganese starvation, namely populating manganese-utilizing enzymes with iron.

The Two-Component System ArlRS and Alterations in Metabolism Enable Staphylococcus aureus to Resist Calprotectin-Induced Manganese Starvation

During infection the host imposes manganese and zinc starvation on invading pathogens. Despite this, Staphylococcus aureus and other successful pathogens remain capable of causing devastating disease. However, how these invaders adapt to host-imposed metal starvation and overcome nutritional immunity remains unknown. We report that ArlRS, a global staphylococcal virulence regulator, enhances the ability of S. aureus to grow in the presence of the manganese-and zinc-binding innate immune effector calprotectin. Utilization of calprotectin variants with altered metal binding properties revealed that strains lacking ArlRS are specifically more sensitive to manganese starvation. Loss of ArlRS did not alter the expression of manganese importers or prevent S. aureus from acquiring metals. It did, however, alter staphylococcal metabolism and impair the ability of S. aureus to grow on amino acids. Further studies suggested that relative to consuming glucose, the preferred carbon source of S. aureus, utilizing amino acids reduced the cellular demand for manganese. When forced to use glucose as the sole carbon source S. aureus became more sensitive to calprotectin compared to when amino acids are provided. Infection experiments utilizing wild type and calprotectin-deficient mice, which have defects in manganese sequestration, revealed that ArlRS is important for disease when manganese availability is restricted but not when this essential nutrient is freely available. In total, these results indicate that altering cellular metabolism contributes to the ability of pathogens to resist manganese starvation and that ArlRS enables S. aureus to overcome nutritional immunity by facilitating this adaptation.

The ubiquitous pathogen Staphylococcus aureus is a serious threat to human health due to the continued spread of antibiotic resistance. This spread has made it challenging to treat staphylococcal infections and led to the call for new approaches to treat this devastating pathogen. One approach is to disrupt the ability of S. aureus to adapt to nutrient availability during infection. During infection, the host imposes manganese and zinc starvation on invading pathogens. However, the mechanisms utilized by Staphylococcus aureus to overcome this host defense are unknown. We report that ArlRS, a global staphylococcal virulence regulator, is important for resisting manganese starvation during infection. Loss of ArlRS does not prevent S. aureus from acquiring metals but instead renders the bacterium incapable of adapting to limited manganese availability. ArlRS mutants also have metabolic defects and a reduced ability to grow on amino acids. When using glucose as a carbon source S. aureus is more sensitive to manganese starvation and increases the expression of manganese transporters relative to when amino acids are provided suggesting a higher demand for manganese. These observations indicate that ArlRS contributes to resisting nutritional immunity by altering metabolism to reduce the staphylococcal demand for manganese.

Transition Metals and Virulence in Bacteria

Transition metals are required trace elements for all forms of life. Due to their unique inorganic and redox properties, transition metals serve as cofactors for enzymes and other proteins. In bacterial pathogenesis, the vertebrate host represents a rich source of nutrient metals, and bacteria have evolved diverse metal acquisition strategies. Host metal homeostasis changes dramatically in response to bacterial infections, including production of metal sequestering proteins and the bombardment of bacteria with toxic levels of metals. In response, bacteria have evolved systems to subvert metal sequestration and toxicity. The coevolution of hosts and their bacterial pathogens in the battle for metals has uncovered emerging paradigms in social microbiology, rapid evolution, host specificity, and metal homeostasis across domains. This review focuses on recent advances and open questions in our understanding of the complex role of transition metals at the host-pathogen interface.

Acinetobacter baumannii Coordinates Urea Metabolism with Metal Import To Resist Host-Mediated Metal Limitation

During infection, bacterial pathogens must adapt to a nutrient metal-limited environment that is imposed by the host. The innate immune protein calprotectin inhibits bacterial growth in vitro by chelating the divalent metal ions zinc (Zn²⁺, Zn) and manganese (Mn²⁺, Mn), but pathogenic bacteria are able to cause disease in the presence of this antimicrobial protein in vivo. One such pathogen is Acinetobacter baumannii, a Gram-negative bacterium that causes pneumonia and bloodstream infections that can be complicated by resistance to multiple antibiotics. A. baumannii inhibition by calprotectin is dependent on calprotectin Mn binding, but the mechanisms employed by A. baumannii to overcome Mn limitation have not been identified. This work demonstrates that A. baumannii coordinates transcription of an NRAMP family Mn transporter and a urea carboxylase to resist the antimicrobial activities of calprotectin. This NRAMP family transporter facilitates Mn accumulation and growth of A. baumannii in the presence of calprotectin. A. baumannii is found to utilize urea as a sole nitrogen source, and urea utilization requires the urea carboxylase encoded in an operon with the NRAMP family transporter. Moreover, urea carboxylase activity is essential for calprotectin resistance in A. baumannii. Finally, evidence is provided that this system combats calprotectin in vivo, as deletion of the transporter impairs A. baumannii fitness in a mouse model of pneumonia, and this fitness defect is modulated by the presence of calprotectin. These findings reveal that A. baumannii has evolved mechanisms to subvert host-mediated metal sequestration and they uncover a connection between metal starvation and metabolic stress.

Acinetobacter baumannii is a bacterium that causes bloodstream, wound, urinary tract, and pneumonia infections, with a high disease burden in intensive care units. Treatment of A. baumannii infection is complicated by resistance to most antibiotics in use today, and resistance to last-resort therapies has become commonplace. New treatments for A. baumannii infection are desperately needed, but our current understanding of the bacterial factors required to cause infection is limited. We previously found that the abundant innate immune protein calprotectin inhibits the growth of A. baumannii by withholding essential metals. Despite this, A. baumannii is still able to infect wild-type mice, which produce calprotectin during infection. Here, we identify factors employed by A. baumannii during infection to overcome calprotectin-mediated metal sequestration. Moreover, we expose a connection between metal starvation and metabolism that may be a “chink in the armor” of A. baumannii and lead to new treatment options.

The Hexahistidine Motif of Host-Defense Protein Human Calprotectin Contributes to Zinc Withholding and Its Functional Versatility

Human calprotectin (CP, S100A8/S100A9 oligomer, MRP-8/MRP-14 oligomer) is an abundant host-defense protein that is involved in the metal-withholding innate immune response. CP coordinates a variety of divalent first-row transition metal ions, which is implicated in its antimicrobial function, and its ability to sequester nutrient Zn(II) ions from microbial pathogens has been recognized for over two decades. CP has two distinct transition-metal-binding sites formed at the S100A8/S100A9 dimer interface, including a histidine-rich site composed of S100A8 residues His17 and His27 and S100A9 residues His91 and His95. In this study, we report that CP binds Zn(II) at this site using a hexahistidine motif, completed by His103 and His105 of the S100A9 C-terminal tail and previously identified as the high-affinity Mn(II) and Fe(II) coordination site. Zn(II) binding at this unique site shields the S100A9 C-terminal tail from proteolytic degradation by proteinase K. X-ray absorption spectroscopy and Zn(II) competition titrations support the formation of a Zn(II)-His6 motif. Microbial growth studies indicate that the hexahistidine motif is important for preventing microbial Zn(II) acquisition from CP by the probiotic Lactobacillus plantarum and the opportunistic human pathogen Candida albicans. The Zn(II)-His6 site of CP expands the known biological coordination chemistry of Zn(II) and provides new insight into how the human innate immune system starves microbes of essential metal nutrients.

Binding of transition metals to S100 proteins

The S100 proteins are a unique class of EF-hand Ca(2+) binding proteins distributed in a cell-specific, tissue-specific, and cell cycle-specific manner in humans and other vertebrates. These proteins are distinguished by their distinctive homodimeric structure, both intracellular and extracellular functions, and the ability to bind transition metals at the dimer interface. Here we summarize current knowledge of S100 protein binding of Zn(2+), Cu(2+) and Mn(2+) ions, focusing on binding affinities, conformational changes that arise from metal binding, and the roles of transition metal binding in S100 protein function.

Bacterial Strategies to Maintain Zinc Metallostasis at the Host-Pathogen Interface

Among the biologically required first row, late d-block metals from Mn^II to Zn^II, the catalytic and structural reach of Zn^II ensures that this essential micronutrient touches nearly every major metabolic process or pathway in the cell. Zn is also toxic in excess, primarily because it is a highly competitive divalent metal and will displace more weakly bound transition metals in the active sites of metalloenzymes if left unregulated. The vertebrate innate immune system uses several strategies to exploit this "Achilles heel" of microbial physiology, but bacterial evolution has responded in kind. This review highlights recent insights into transcriptional, transport, and trafficking mechanisms that pathogens use to "win the fight" over zinc and thrive in an otherwise hostile environment.

Competition for Manganese at the Host-Pathogen Interface

Transition metals such as manganese are essential nutrients for both pathogen and host. Vertebrates exploit this necessity to combat invading microbes by restricting access to these critical nutrients, a defense known as nutritional immunity. During infection, the host uses several mechanisms to impose manganese limitation. These include removal of manganese from the phagolysosome, sequestration of extracellular manganese, and utilization of other metals to prevent bacterial acquisition of manganese. In order to cause disease, pathogens employ a variety of mechanisms that enable them to adapt to and counter nutritional immunity. These adaptations include, but are likely not limited to, manganese-sensing regulators and high-affinity manganese transporters. Even though successful pathogens can overcome host-imposed manganese starvation, this defense inhibits manganese-dependent processes, reducing the ability of these microbes to cause disease. While the full impact of host-imposed manganese starvation on bacteria is unknown, critical bacterial virulence factors such as superoxide dismutases are inhibited. This chapter will review the factors involved in the competition for manganese at the host-pathogen interface and discuss the impact that limiting the availability of this metal has on invading bacteria.

The innate immune protein calprotectin promotes Pseudomonas aeruginosa and Staphylococcus aureus interaction

Microorganisms form biofilms containing differentiated cell populations. To determine factors driving differentiation, we herein visualize protein and metal distributions within Pseudomonas aeruginosa biofilms using imaging mass spectrometry. These in vitro experiments reveal correlations between differential protein distribution and metal abundance. Notably, zinc- and manganese-depleted portions of the biofilm repress the production of anti-staphylococcal molecules. Exposure to calprotectin (a host protein known to sequester metal ions at infectious foci) recapitulates responses occurring within metal-deplete portions of the biofilm and promotes interaction between P. aeruginosa and Staphylococcus aureus. Consistent with these results, the presence of calprotectin promotes co-colonization of the murine lung, and polymicrobial communities are found to co-exist in calprotectin-enriched airspaces of a cystic fibrosis lung explant. These findings, which demonstrate that metal fluctuations are a driving force of microbial community structure, have clinical implications because of the frequent occurrence of P. aeruginosa and S. aureus co-infections.

Co-infections with Pseudomonas aeruginosa and Staphylococcus aureus are common in cystic fibrosis patients. Here, the authors show that metal depletion induced by a host protein, calprotectin, promotes co-existence of both pathogens by inhibiting production of anti-staphylococcal molecules by P. aeruginosa.

Salmonella Mitigates Oxidative Stress and Thrives in the Inflamed Gut by Evading Calprotectin-Mediated Manganese Sequestration

Neutrophils hinder bacterial growth by a variety of antimicrobial mechanisms, including the production of reactive oxygen species and the secretion of proteins that sequester nutrients essential to microbes. A major player in this process is calprotectin, a host protein that exerts antimicrobial activity by chelating zinc and manganese. Here we show that the intestinal pathogen Salmonella enterica serovar Typhimurium uses specialized metal transporters to evade calprotectin sequestration of manganese, allowing the bacteria to outcompete commensals and thrive in the inflamed gut. The pathogen's ability to acquire manganese in turn promotes function of SodA and KatN, enzymes that use the metal as a cofactor to detoxify reactive oxygen species. This manganese-dependent SodA activity allows the bacteria to evade neutrophil killing mediated by calprotectin and reactive oxygen species. Thus, manganese acquisition enables S. Typhimurium to overcome host antimicrobial defenses and support its competitive growth in the intestine.

Crystal structure of human S100A8 in complex with zinc and calcium

Background

S100 proteins are a large family of calcium binding proteins present only in vertebrates. They function intra- and extracellularly both as regulators of homeostatic processes and as potent effectors during inflammation. Among these, S100A8 and S100A9 are two major constituents of neutrophils that can assemble into homodimers, heterodimers and higher oligomeric species, including fibrillary structures found in the ageing prostate. Each of these forms assumes specific functions and their formation is dependent on divalent cations, notably calcium and zinc. In particular, zinc appears as a major regulator of S100 protein function in a disease context. Despite this central role, no structural information on how zinc bind to S100A8/S100A9 and regulates their quaternary structure is yet available.

Results

Here we report two crystallographic structures of calcium and zinc-loaded human S100A8. S100A8 binds two zinc ions per homodimer, through two symmetrical, all-His tetracoordination sites, revealing a classical His-Zn binding mode for the protein. Furthermore, the presence of a (Zn)₂-cacodylate complex in our second crystal form induces ligand swapping within the canonical His₄ zinc binding motif, thereby creating two new Zn-sites, one of which involves residues from symmetry-related molecules. Finally, we describe the calcium-induced S100A8 tetramer and reveal how zinc stabilizes this tetramer by tightening the dimer-dimer interface.

Conclusions

Our structures of Zn²⁺/Ca²⁺-bound hS100A8 demonstrate that S100A8 is a genuine His-Zn S100 protein. Furthermore, they show how zinc stabilizes S100A8 tetramerization and potentially mediates the formation of novel interdimer interactions. We propose that these zinc-mediated interactions may serve as a basis for the generation of larger oligomers in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s12900-016-0058-4) contains supplementary material, which is available to authorized users.

Calprotectin is a potential prognostic marker for polycystic ovary syndrome

Background Calprotectin is an antimicrobial, calcium and zinc-binding heterocomplex protein and has been proposed as a marker to rule out inflammatory conditions. The aim of this study was to evaluate the role of calprotectin in the diagnosis of polycystic ovary syndrome and to investigate the association between calprotectin and insulin resistance. Methods A total of 41 females with polycystic ovary syndrome and 54 age-matched without polycystic ovary syndrome were eligible for the study. Serum concentration of calprotectin was determined using enzyme-linked immunosorbent assay. Clinical characteristics, hormone and metabolic parameters were evaluated in each subject. The predictive value of serum calprotectin was assessed using receiver operating characteristic curves. Correlations between the serum calprotectin concentrations and insulin resistance were examined using Spearman's correlation. Results We found that the serum calprotectin concentrations were significantly higher in polycystic ovary syndrome compared with the non-polycystic ovary syndrome group ( P < 0.001). The area under the receiver operating characteristic curve assay yielded a satisfactory result of 0.88 (95% confidence interval 0.81-0.95; P < 0.001). The optimum cut-off was 2.4  µg/mL with a 85.2% specificity and 75.6% sensitivity for polycystic ovary syndrome diagnosis. A significant positive correlation was found between the serum calprotectin and insulin resistance. Conclusions These results suggest that calprotectin might be a useful adjunct in the diagnosis of polycystic ovary syndrome, especially those with insulin resistance.

The Yin and Yang of copper during infection

Copper is an essential micronutrient for both pathogens and the animal hosts they infect. However, copper can also be toxic in cells due to its redox properties and ability to disrupt active sites of metalloproteins, such as Fe-S enzymes. Through these toxic properties, copper is an effective antimicrobial agent and an emerging concept in innate immunity is that the animal host intentionally exploits copper toxicity in antimicrobial weaponry. In particular, macrophages can attack invading microbes with high copper and this metal is also elevated at sites of lung infection. In addition, copper levels in serum rise during infection with a wide array of pathogens. To defend against this toxic copper, the microbial intruder is equipped with a battery of copper detoxification defenses that promote survival in the host, including copper exporting ATPases and copper binding metallothioneins. However, it is important to remember that copper is also an essential nutrient for microbial pathogens and serves as important cofactor for enzymes such as cytochrome c oxidase for respiration, superoxide dismutase for anti-oxidant defense and multi-copper oxidases that act on metals and organic substrates. We therefore posit that the animal host can also thwart pathogen growth by limiting their copper nutrients, similar to the well-documented nutritional immunity effects for starving microbes of essential zinc, manganese and iron micronutrients. This review provides both sides of the copper story and evaluates how the host can exploit either copper-the-toxin or copper-the-nutrient in antimicrobial tactics at the host-pathogen battleground.

Antimicrobial peptides and wound healing: biological and therapeutic considerations

Repair of tissue wounds is a fundamental process to re-establish tissue integrity and regular function. Importantly, infection is a major factor that hinders wound healing. Multicellular organisms have evolved an arsenal of host-defense molecules, including antimicrobial peptides (AMPs), aimed at controlling microbial proliferation and at modulating the host's immune response to a variety of biological or physical insults. In this brief review, we provide the evidence for a role of AMPs as endogenous mediators of wound healing and their promising therapeutic potential for the treatment of non-life-threatening skin and other epithelial injuries.

Calcium-induced Tetramerization and Zinc Chelation Shield Human Calprotectin from Degradation by Host and Bacterial Extracellular Proteases

Calprotectin (CP, S100A8/S100A9 oligomer, MRP-8/14 oligomer, calgranulins A and B) is a protein component of the innate immune system that contributes to the metal-withholding response by sequestering bioavailable transition metal ions at sites of infection. Human CP employs Ca(II) ions to modulate its quaternary structure, transition metal binding properties, and antimicrobial activity. In this work, we report the discovery that Ca(II)-induced self-association of human CP to afford heterotetramers protects the protein scaffold from degradation by host serine proteases. We present the design and characterization of two new human CP-Ser variants, S100A8(C42S)(I60E)/S100A9(C3S) and S100A8(C42S)(I60K)/S100A9(C3S), that exhibit defective tetramerization properties. Analytical size exclusion chromatography and analytical ultracentrifugation reveal that both variants, hereafter I60E and I60K, persist as heterodimers in the presence of Ca(II) only, and form heterotetramers in the presence of Mn(II) only and both Ca(II) and Mn(II). Coordination to Fe(II) also causes I60E and I60K to form heterotetramers in both the absence and presence of Ca(II). The Ca(II)-bound I60E and I60K heterodimers are readily degraded by trypsin, chymotrypsin and human neutrophil elastase, whereas the Ca(II)-bound CP-Ser heterotetramers and the Ca(II)- and Mn(II)-bound I60E and I60K heterotetramers are resistant to degradation by these host proteases. The staphylococcal extracellular protease GluC cuts the S100A8 subunit of CP-Ser at the C-terminal end of residue 89 to afford a ΔSKHE variant. The GluC cleavage site is in close proximity to the His3Asp metal-binding site, which coordinates Zn(II) with high affinity, and Zn(II) chelation protects the S100A8 subunit from GluC cleavage. Taken together, these results provide new insight into how Ca(II) ions and transition metals modulate the chemistry and biology of CP, and indicate that coordination to divalent cations transforms human CP into a protease-resistant form and enables innate immune function in the hostile conditions of an infection site.

Calcium Ions Tune the Zinc-Sequestering Properties and Antimicrobial Activity of Human S100A12

Human S100A12 is a host-defense protein expressed and released by neutrophils that contributes to innate immunity. Apo S100A12 is a 21-kDa antiparallel homodimer that harbors two Ca(II)-binding EF-hand domains per subunit and exhibits two His3Asp motifs for chelating transition metal ions at the homodimer interface. In this work, we present results from metal-binding studies and microbiology assays designed to ascertain whether Ca(II) ions modulate the Zn(II)-binding properties of S100A12 and further evaluate the antimicrobial properties of this protein. Our metal depletion studies reveal that Ca(II) ions enhance the ability of S100A12 to sequester Zn(II) from microbial growth media. We report that human S100A12 has antifungal activity against Candida albicans, C. krusei, C. glabrata and C. tropicalis, all of which cause human disease. This antifungal activity is Ca(II)-dependent and requires the His3Asp metal-binding sites. We expand upon prior studies of the antibacterial activity of S100A12 and report Ca(II)-dependent and strain-selective behavior. S100A12 exhibited in vitro growth inhibitory activity against Listeria monocytogenes. In contrast, S100A12 had negligible effect on the growth of Escherichia coli K-12 and Pseudomonas aeruginosa PAO1. Loss of functional ZnuABC, a high-affinity Zn(II) import system, increased the susceptibility of E. coli and P. aeruginosa to S100A12, indicating that S100A12 deprives these mutant strains of Zn(II). To evaluate the Zn(II)-binding sites of S100A12 in solution, we present studies using Co(II) as a spectroscopic probe and chromophoric small-molecule chelators in Zn(II) competition titrations. We confirm that S100A12 binds Zn(II) with a 2:1 stoichiometry, and our data indicate sub-nanomolar affinity binding. Taken together, these data support a model whereby S100A12 uses Ca(II) ions to tune its Zn(II)-chelating properties and antimicrobial activity.