Siderocalin (Lcn 2) Also Binds Carboxymycobactins, Potentially Defending against Mycobacterial Infections through Iron Sequestration

Protein turnover in mycobacterial proteomics

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

Structural basis of the catalytic mechanism operating in open-closed conformers of lipocalin type prostaglandin D synthase

Lipocalin type prostaglandin D synthase (L-PGDS) is a multifunctional protein acting as a somnogen (PGD2)-producing enzyme, an extracellular transporter of various lipophilic ligands, and an amyloid-beta chaperone in human cerebrospinal fluid. In this study, we determined the crystal structures of two different conformers of mouse L-PGDS, one with an open cavity of the beta-barrel and the other with a closed cavity due to the movement of the flexible E-F loop. The upper compartment of the central large cavity contains the catalytically essential Cys65 residue and its network of hydrogen bonds with the polar residues Ser45, Thr67, and Ser81, whereas the lower compartment is composed of hydrophobic amino acid residues that are highly conserved among other lipocalins. SH titration analysis combined with site-directed mutagenesis revealed that the Cys65 residue is activated by its interaction with Ser45 and Thr67 and that the S45A/T67A/S81A mutant showed less than 10% of the L-PGDS activity. The conformational change between the open and closed states of the cavity indicates that the mobile calyx contributes to the multiligand binding ability of L-PGDS.

The role of electrostatics in siderophore recognition by the immunoprotein Siderocalin

Iron is required for virulence of most bacterial pathogens, many of which rely on siderophores, small-molecule chelators, to scavenge iron in mammalian hosts. As an immune response, the human protein Siderocalin binds both apo and ferric siderophores in order to intercept delivery of iron to the bacterium, impeding virulence. The introduction of steric clashes into the siderophore structure is an important mechanism of evading sequestration. However, in the absence of steric incompatibilities, electrostatic interactions determine siderophore strength of binding by Siderocalin. By using a series of isosteric enterobactin analogues, the contribution of electrostatic interactions, including both charge-charge and cation-pi, to the recognition of 2,3-catecholate siderophores has been deconvoluted. The analogues used in the study incorporate a systematic combination of 2,3-catecholamide (CAM) and N-hydroxypyridinonate (1,2-HOPO) binding units on a tris(2-aminoethyl)amine (tren) backbone, [tren(CAM)(m)(1,2-HOPO)(n), where m = 0, 1, 2, or 3 and n = 3 - m]. The shape complementarity of the synthetic analogue series was determined through small-molecule crystallography, and the binding interactions were investigated through a fluorescence-based binding assay. These results were modeled and correlated through ab initio calculations of the electrostatic properties of the binding units. Although all the analogues are accommodated in the binding pocket of Siderocalin, the ferric complexes incorporating decreasing numbers of CAM units are bound with decreasing affinities (K(d) = >600, 43, 0.8, and 0.3 nM for m = 0-3). These results elucidate the role of electrostatics in the mechanism of siderophore recognition by Siderocalin.

Iron trafficking as an antimicrobial target

Iron is essential for the survival of most organisms. Microbial iron acquisition depends on multiple, sometimes complex steps, many of which are not shared by higher eukaryotes. Depriving pathogenic microbes of iron is therefore a potential antimicrobial strategy. The following minireview briefly describes general elements in microbial iron uptake pathways and summarizes some of the current work aiming at their medicinal inhibition.

Synthesis of dideoxymycobactin antigens presented by CD1a reveals T cell fine specificity for natural lipopeptide structures

Mycobacterium tuberculosis survival in cells requires mycobactin siderophores. Recently, the search for lipid antigens presented by the CD1a antigen-presenting protein led to the discovery of a mycobactin-like compound, dideoxymycobactin (DDM). Here we synthesize DDMs using solution phase and solid phase peptide synthesis chemistry. Comparison of synthetic standards to natural mycobacterial mycobactins by nuclear magnetic resonance and mass spectrometry allowed identification of an unexpected alpha-methyl serine unit in natural DDM. This finding further distinguishes these pre-siderophores as foreign compounds distinct from conventional peptides, and we provide evidence that this chemical variation influences the T cell response. One synthetic DDM recapitulated natural structures and potently stimulated T cells, making it suitable for patient studies of CD1a in infectious disease. DDM analogs differing in the stereochemistry of their butyrate or oxazoline moieties were not recognized by human T cells. Therefore, we conclude that T cells show precise specificity for both arms of the peptide, which are predicted to lie at the CD1a-T cell receptor interface.

NGAL: a biomarker of acute kidney injury and other systemic conditions

Neutrophil gelatinase-associated lipocalin (NGAL) is a 25 kDa protein belonging to the lipocalin superfamily. It was initially found in activated neutrophils, however, many other cells, like kidney tubular cells, may produce NGAL in response to various insults. Recently, it has been found to have a role in iron metabolism by virtue of its binding with siderophores. It has also been found to have a role in kidney development and tubular regeneration after injury. In experimental studies, it was found to be highly expressed in response to tubular injury. In subsequent clinical studies, urine NGAL has been found to be an early predictor for acute kidney injury (AKI). Newer devices for early bedside detection of NGAL are now available. Since serum creatinine is known to be an inadequate and late marker of AKI, NGAL might soon emerge as a troponin-like early marker for AKI. Recent evidence also suggests its role as a biomarker in a variety of other renal and non-renal conditions.

High-affinity recognition of lanthanide(III) chelate complexes by a reprogrammed human lipocalin 2

Human lipocalin 2 (Lcn2), also known as neutrophil gelatinase-associated lipocalin (NGAL), which naturally scavenges bacterial ferric siderophores, has been engineered to specifically bind rare-earth and related metal ions as chelate complexes with [(R)-2-amino-3-(4-aminophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diaminepentaacetic acid (p-NH(2)-Bn-CHX-A''-DTPA). To this end, 12 amino acid residues in the ligand pocket of Lcn2, which is formed by four loops at the open end of an eight-stranded beta-barrel, were subjected to targeted random mutagenesis, and from the resulting library, variants with binding activity for the Me x DTPA group were selected using the method of bacterial phage display. One promising candidate was further developed in several cycles of in vitro affinity maturation using partial random mutagenesis and selection (via phage display and/or Escherichia coli colony screening) under conditions of increasing stringency. As result, an Lcn2 variant was obtained that binds Y x DTPA with a dissociation constant as low as 400 pM. The Lcn2 variant specifically recognizes the artificial ligand, as exemplified in (competitive) ELISA and real-time surface plasmon resonance analyses. DTPA-complexed Y(3+), Tb(3+), Gd(3+), and Lu(3+) are most tightly bound, comprising metal ions whose isotopes are in common use for radiotherapy and imaging. All of the Lcn2 variants are stably folded and can be functionally produced in high yield in E. coli. X-ray crystallographic analyses show that the new ligand is well-accommodated in the central cavity of the engineered lipocalin, whose fold is largely preserved, but that the mode of binding differs from the one seen with the natural ligand Fe x enterobactin. This structural study reveals analogies but also differences with respect to previously described antibody-metal chelate complexes. Notably, the functionalized side chain of DTPA protrudes from the ligand pocket of the lipocalin in such a way that its conjugates (with other haptens, for example) are recognized too. With their small sizes and robust fold based on a single polypeptide chain, the engineered Lcn2 variants provide novel modules and/or fusion partners for radionuclide-chelate capturing strategies that bear promise for medical diagnostics and therapy.

Ironing out the wrinkles in host defense: interactions between iron homeostasis and innate immunity

Iron is an essential micronutrient for both microbial pathogens and their mammalian hosts. Changes in iron availability and distribution have significant effects on pathogen virulence and on the immune response to infection. Recent advances in our understanding of the molecular regulation of iron metabolism have shed new light on how alterations in iron homeostasis both contribute to and influence innate immunity. In this article, we review what is currently known about the role of iron in the response to infection.

Lipocalin 2-dependent inhibition of mycobacterial growth in alveolar epithelium

Mycobacterium tuberculosis invades alveolar epithelial cells as well as macrophages. However, the role of alveolar epithelial cells in the host defense against M. tuberculosis remains unknown. In this study, we report that lipocalin 2 (Lcn2)-dependent inhibition of mycobacterial growth within epithelial cells is required for anti-mycobacterial innate immune responses. Lcn2 is secreted into the alveolar space by alveolar macrophages and epithelial cells during the early phase of respiratory mycobacterial infection. Lcn2 inhibits the in vitro growth of mycobacteria through sequestration of iron uptake. Lcn2-deficient mice are highly susceptible to intratracheal infection with M. tuberculosis. Histological analyses at the early phase of mycobacterial infection in Lcn2-deficient mice reveal increased numbers of mycobacteria in epithelial cell layers, but not in macrophages, in the lungs. Increased intracellular mycobacterial growth is observed in alveolar epithelial cells, but not in alveolar macrophages, from Lcn2-deficient mice. The inhibitory action of Lcn2 is blocked by the addition of endocytosis inhibitors, suggesting that internalization of Lcn2 into the epithelial cells is a prerequisite for the inhibition of intracellular mycobacterial growth. Taken together, these findings highlight a pivotal role for alveolar epithelial cells during mycobacterial infection, in which Lcn2 mediates anti-mycobacterial innate immune responses within the epithelial cells.

Iron behaving badly: inappropriate iron chelation as a major contributor to the aetiology of vascular and other progressive inflammatory and degenerative diseases

BACKGROUND

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular 'reactive oxygen species' (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation.

REVIEW

We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation).The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible.This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, since in some circumstances (especially the presence of poorly liganded iron) molecules that are nominally antioxidants can actually act as pro-oxidants. The reduction of redox stress thus requires suitable levels of both antioxidants and effective iron chelators. Some polyphenolic antioxidants may serve both roles.Understanding the exact speciation and liganding of iron in all its states is thus crucial to separating its various pro- and anti-inflammatory activities. Redox stress, innate immunity and pro- (and some anti-)inflammatory cytokines are linked in particular via signalling pathways involving NF-kappaB and p38, with the oxidative roles of iron here seemingly involved upstream of the IkappaB kinase (IKK) reaction. In a number of cases it is possible to identify mechanisms by which ROSs and poorly liganded iron act synergistically and autocatalytically, leading to 'runaway' reactions that are hard to control unless one tackles multiple sites of action simultaneously. Some molecules such as statins and erythropoietin, not traditionally associated with anti-inflammatory activity, do indeed have 'pleiotropic' anti-inflammatory effects that may be of benefit here.

CONCLUSION

Overall we argue, by synthesising a widely dispersed literature, that the role of poorly liganded iron has been rather underappreciated in the past, and that in combination with peroxide and superoxide its activity underpins the behaviour of a great many physiological processes that degrade over time. Understanding these requires an integrative, systems-level approach that may lead to novel therapeutic targets.

The siderocalin/enterobactin interaction: a link between mammalian immunity and bacterial iron transport

The siderophore enterobactin (Ent) is produced by enteric bacteria to mediate iron uptake. Ent scavenges iron and is taken up by the bacteria as the highly stable ferric complex [Fe (III)(Ent)] (3-). This complex is also a specific target of the mammalian innate immune system protein, Siderocalin (Scn), which acts as an antibacterial agent by specifically sequestering siderophores and their ferric complexes during infection. Recent literature suggesting that Scn may also be involved in cellular iron transport has increased the importance of understanding the mechanism of siderophore interception and clearance by Scn; Scn is observed to release iron in acidic endosomes and [Fe (III)(Ent)] (3-) is known to undergo a change from catecholate to salicylate coordination in acidic conditions, which is predicted to be sterically incompatible with the Scn binding pocket (also referred to as the calyx). To investigate the interactions between the ferric Ent complex and Scn at different pH values, two recombinant forms of Scn with mutations in three residues lining the calyx were prepared: Scn-W79A/R81A and Scn-Y106F. Binding studies and crystal structures of the Scn-W79A/R81A:[Fe (III)(Ent)] (3-) and Scn-Y106F:[Fe (III)(Ent)] (3-) complexes confirm that such mutations do not affect the overall conformation of the protein but do weaken significantly its affinity for [Fe (III)(Ent)] (3-). Fluorescence, UV-vis, and EXAFS spectroscopies were used to determine Scn/siderophore dissociation constants and to characterize the coordination mode of iron over a wide pH range, in the presence of both mutant proteins and synthetic salicylate analogues of Ent. While Scn binding hinders salicylate coordination transformation, strong acidification results in the release of iron and degraded siderophore. Iron release may therefore result from a combination of Ent degradation and coordination change.

Synthesis of carbapyochelins via diastereoselective azidation of 5-(ethoxycarbonyl)methylproline derivatives

Two configurationally stable carbon-based analogues of pyochelin have been prepared from Boc-pyroglutamic acid-tert-butyl ester in 11 and 13 steps. Introduction of the amino group was achieved by a highly diastereoselective electrophilic azidation reaction to afford novel bis-alpha-amino acid proline derivatives.

Biosynthetic tailoring of microcin E492m: post-translational modification affords an antibacterial siderophore-peptide conjugate

The present work reveals that four proteins, MceCDIJ, encoded by the MccE492 gene cluster are responsible for the remarkable post-translational tailoring of microcin E492 (MccE492), an 84-residue protein toxin secreted by Klebsiella pneumonaie RYC492 that targets neighboring Gram-negative species. This modification results in attachment of a linearized and monoglycosylated derivative of enterobactin, a nonribosomal peptide and iron scavenger (siderophore), to the MccE492m C-terminus. MceC and MceD derivatize enterobactin by C-glycosylation at the C5 position of a N-(2,3-dihydroxybenzoyl)serine (DHB-Ser) moiety and regiospecific hydrolysis of an ester linkage in the trilactone scaffold, respectively. MceI and MceJ form a protein complex that attaches C-glycosylated enterobactins to the C-terminal serine residue of both a C10 model peptide and full-length MccE492. In the enzymatic product, the C-terminal serine residue is covalently attached to the C4' oxygen of the glucose moiety. Nonenzymatic and base-catalyzed migration of the peptide to the C6' position affords the C6' glycosyl ester linkage observed in the mature toxin, MccE492m, isolated from bacterial cultures.

Siderophore-based iron acquisition and pathogen control

High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as "Trojan horse" toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

Structural Studies of the Cpx Pathway Activator NlpE on the Outer Membrane of Escherichia coli

NlpE, an outer membrane lipoprotein, functions during envelope stress responses in Gram-negative bacteria. In Escherichia coli, adhesion to abiotic surfaces has been reported to activate the Cpx pathway in an NlpE-dependent manner. External copper ions are also thought to activate the Cpx pathway mediated by NlpE. We determined the crystal structure of NlpE from E. coli at 2.6 A resolution. The structure showed that NlpE consists of two beta barrel domains. The N-terminal domain resembles the bacterial lipocalin Blc, and the C-terminal domain has an oligonucleotide/oligosaccharide-binding (OB) fold. From both biochemical analyses and the crystal structure, it can be deduced that the cysteine residues in the CXXC motif may be chemically active. Furthermore, two monomers in the asymmetric unit form an unusual 3D domain-swapped dimer. These findings indicate that tertiary and/or quaternary structural instability may be responsible for Cpx pathway activation.

Iron metabolism at the host pathogen interface: lipocalin 2 and the pathogen-associated iroA gene cluster

The host innate immune defense protein lipocalin 2 binds bacterial enterobactin siderophores to limit bacterial iron acquisition. To counteract this host defense mechanism bacteria have acquired the iroA gene cluster, which encodes enzymatic machinery and transporters that revitalize enterobactin in the form of salmochelin. The iroB enzyme introduces glucosyl residues at the C5 site on 2,3-dihydroxybenzoylserine moieties of enterobactin and thereby prevents lipocalin 2 binding. Additional strategies to evade lipocalin 2 have evolved in other bacteria, such as Mycobacteria tuberculosis and Bacillus anthracis. Targeting these specialized bacterial evasion strategy may provide a mechanism to reinvigorate lipocalin 2 in defense against specific pathogens.

Neutrophil granules: a library of innate immunity proteins

Gene expression profiling has revealed that circulating neutrophils rest between two major bursts of transcriptional and protein synthetic activities. The first occurs in the bone marrow. This equips the neutrophil with stocks of innate defense armory that are packaged into different granule subsets. The second burst occurs when the neutrophil exits circulation and migrates into tissues to find, capture and phagocytose microorganisms. This burst results in the synthesis and secretion of cytokines and chemokines that support resolution of inflammation and healing of damaged tissue. Gene expression profiling has revealed that neutrophils express a variety of innate immunity proteins, known previously only to be expressed in other cells. Likewise, it has become clear that some proteins previously thought to be specific to the neutrophil are expressed in epithelial cells during inflammation.

Neutrophil-mediated innate immune resistance to mycobacteria

Neutrophils contain antimicrobial peptides with antituberculous activity, but their contribution to immune resistance to tuberculosis (TB) infection has not been previously investigated to our knowledge. We determined differential white cell counts in peripheral blood of 189 adults who had come into contact with patients diagnosed with active TB in London, United Kingdom, and evaluated them for evidence of TB infection and capacity to restrict mycobacterial growth in whole-blood assays. Risk of TB infection was inversely and independently associated with peripheral blood neutrophil count in contacts of patients diagnosed with pulmonary TB. The ability of whole blood to restrict growth of Mycobacterium bovis bacille Calmette Guérin and Mycobacterium tuberculosis was impaired 7.3- and 3.1-fold, respectively, by neutrophil depletion. In microbiological media, human neutrophil peptides (HNPs) 1-3 killed M. tuberculosis. The neutrophil peptides cathelicidin LL-37 and lipocalin 2 restricted growth of the organism, the latter in an iron-dependent manner. Black African participants had lower neutrophil counts and lower circulating concentrations of HNP1-3 and lipocalin 2 than south Asian and white participants. Neutrophils contribute substantially to innate resistance to TB infection, an activity associated with their antimicrobial peptides. Elucidation of the regulation of neutrophil antimicrobial peptides could facilitate prevention and treatment of TB.

Neutrophil gelatinase-associated lipocalin as the real-time indicator of active kidney damage

Neutrophil gelatinase-associated lipocalin (Ngal, 24p3, SIP24, lipocalin 2, or siderocalin) was originally purified from neutrophils, but with unknown function. Recently, it was identified that Ngal activates nephron formation in the embryonic kidney, is rapidly and massively induced in renal failure and possesses kidney-protective activity. We would like to propose that blood, urine, and kidney Ngal levels are the real-time indicators of active kidney damage, rather than one of many markers of functional nephron number (as Forest Fire Theory). Ngal is a novel iron-carrier protein exerting pleiotropic actions including the upregulation of epithelial marker E-cadherin expression, opening an exciting field in cell biology.

Expression of the genes dual oxidase 2, lipocalin 2 and regenerating islet-derived 1 alpha in Crohn's disease

OBJECTIVE

A global gene expression profile of non-inflamed colonic mucosal cells from patients with Crohn's disease (CD) and of colonic mucosal cells from controls was performed.

MATERIAL AND METHODS

Tissue specimens from macroscopically non-inflamed descending colon were obtained colonoscopically from 33 CD patients and from 17 control subjects. All controls and 10 CD patients were medication-free at the time of colonoscopy. The Human Genome U133 Plus 2.0 GeneChip Array was used for gene profiling. Hybridization data were analysed with dChip software. Results were confirmed by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Protein product expression of selected genes was assessed by immunohistochemistry using the Envision+ visualization technique.

RESULTS

The expression profile was not homogeneous with the statistical cut-point settings applied. In comparison with controls, it was found that 19 CD patients had three differentially expressed genes, two of them related to the innate immune system: dual oxidase 2 on chromosome 15 (DUOX2, fold change 4.1) and lipocalin 2 on chromosome 9 (LCN2, fold change 3.1). The third gene, regenerating islet-derived 1 alpha (REG1A, fold change 3.9), codes for a mitogenic protein; this could not be confirmed by RT-PCR. Medication-free patients had no differentially expressed genes as compared with controls. Immunohistochemistry indicated that these proteins were produced by epithelial cells (REG1A, LCN2) and leucocytes (DUOX2 and LCN2).

CONCLUSIONS

As compared with controls, non-inflamed colonic mucosal cells contain two up-regulated genes related to the innate immune system. Up-regulation of these genes, known to be induced by microorganisms, suggests either increased microflora antigenicity or an altered function in mucosal barrier defence.

Structural features of the ligand binding site on human complement protein C8gamma: a member of the lipocalin family

Human C8 is one of five components of the cytolytic membrane attack complex of complement. It contains three subunits (C8alpha, C8beta, C8gamma) arranged as a disulfide-linked C8alpha-gamma heterodimer that is noncovalently associated with C8beta. C8gamma has the distinction of being the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small hydrophobic ligand. A natural ligand for C8gamma has not been identified; however previous structural studies indicate C8gamma has a typical lipocalin fold that is suggestive of a ligand-binding capability. A distinctive feature of C8gamma is the division of its putative ligand binding pocket into a hydrophilic upper portion and a large hydrophobic lower cavity. Access to the latter is restricted by the close proximity of two tyrosine side chains (Y83 and Y131). In the present study, binding experiments were performed using lauric acid as a pseudoligand to investigate the potential accessibility of the lower cavity. The crystal structure of a C8gamma.laurate complex revealed that Y83 and Y131 can move to allow penetration of the hydrocarbon chain of laurate into the lower cavity. Introducing a Y83W mutation blocked access but had no effect on the ability of C8gamma to enhance C8 cytolytic activity. Together, these results indicate that the lower cavity in C8gamma could accommodate a ligand if such a ligand has a narrow hydrophobic moiety at one end. Entry of that moiety into the lower cavity would require movement of Y83 and Y131, which act as a gate at the cavity entrance.

Metal sensor proteins: nature's metalloregulated allosteric switches

Metalloregulatory proteins control the expression of genes that allow organisms to quickly adapt to chronic toxicity or deprivation of both biologically essential metal ions and heavy metal pollutants found in their microenvironment. Emerging evidence suggests that metal ion homeostasis and resistance defines an important tug-of-war in human host-bacterial pathogen interactions. This adaptive response originates with the formation of "metal receptor" complexes of exquisite selectivity. In this perspective, we summarize consensus structural features of metal sensing coordination complexes and the evolution of distinct metal selectivities within seven characterized metal sensor protein families. In addition, we place recent efforts to understand the structural basis of metal-induced allosteric switching of these metalloregulatory proteins in a thermodynamic framework, and review the degree to which coordination chemistry drives changes in protein structure and dynamics in selected metal sensor systems. New insights into how metal sensor proteins function in the complex intracellular milieu of the cytoplasm of cells will require a more sophisticated understanding of the "metallome" and will benefit greatly from ongoing collaborative efforts in bioinorganic, biophysical and analytical chemistry, structural biology and microbiology.

Neutrophil gelatinase–associated lipocalin is expressed in osteoarthritis and forms a complex with matrix metalloproteinase 9

OBJECTIVE

Expression of matrix metalloproteinase 9 (MMP-9) is up-regulated in osteoarthritis (OA) and usually presents as multiple bands when synovial fluid (SF) from OA patients is analyzed by zymography. Among these bands is an approximately 125-130-kd band for high molecular weight (HMW) gelatinase, which has not been characterized. This study was undertaken to characterize the HMW MMP activity in OA SF.

METHODS

MMP activity in OA SF was determined by gelatin zymography. Recombinant MMPs were used to identify MMP activity on the zymogram. Western immunoblotting, immunoprecipitation, and immunodepletion analyses were performed using antibodies specific for human MMP-9 and human neutrophil gelatinase-associated lipocalin (NGAL). Human cartilage matrix degradation was determined by dimethylmethylene blue assay.

RESULTS

Zymographic analysis showed that the HMW gelatinase in OA SF comigrated with a purified NGAL-MMP-9 complex. Results of Western immunoblotting showed that the HMW gelatinase was also recognized by antibodies specific for human NGAL or human MMP-9. These same antibodies also immunoprecipitated the HMW gelatinase activity from OA SF. The NGAL-MMP-9 complex was reconstituted in vitro in gelatinase buffer. In the presence of NGAL, MMP-9 activity was stabilized; in the absence of NGAL, rapid loss of MMP-9 activity occurred. MMP-9-mediated release of cartilage matrix proteoglycans was significantly higher in the presence of NGAL (P < 0.05).

CONCLUSION

Our findings demonstrate that the HMW gelatinase activity in OA SF represents a complex of NGAL and MMP-9. The ability of NGAL to protect MMP-9 activity is relevant to cartilage matrix degradation in OA and may represent an important mechanism by which NGAL may contribute to the loss of cartilage matrix proteins in OA.

Neutrophil gelatinase-associated lipocalin-mediated iron traffic in kidney epithelia

PURPOSE OF REVIEW

Neutrophil gelatinase-associated lipocalin (NGAL) is a member of the lipocalin superfamily of carrier proteins. NGAL is the first known mammalian protein which specifically binds organic molecules called siderophores, which are high-affinity iron chelators. Here, we review the expression, siderophore-dependent biological activities and clinical significance of NGAL in epithelial development and in kidney disease.

RECENT FINDINGS

NGAL expression is rapidly induced in the nephron in response to renal epithelial injury. This has led to the establishment of NGAL assays that detect renal damage in the human. Additionally, only when complexed with siderophore and iron as a trimer, NGAL induces mesenchymal-epithelial transition (or nephron formation) in embryonic kidney in vitro and protects adult kidney from ischemia-reperfusion injury in vivo. While the structure of the NGAL: siderophore: iron complex has thus far only been solved for bacterially synthesized siderophores, new evidence suggests the presence of mammalian siderophore-like molecules.

SUMMARY

NGAL is rapidly and massively induced in renal epithelial injury and NGAL: siderophore: iron complexes may comprise a physiological renoprotective mechanism. The data have implications for the diagnosis and treatment of acute renal injury.

Anthrax pathogen evades the mammalian immune system through stealth siderophore production

Systemic anthrax, caused by inhalation or ingestion of Bacillus anthracis spores, is characterized by rapid microbial growth stages that require iron. Tightly bound and highly regulated in a mammalian host, iron is scarce during an infection. To scavenge iron from its environment, B. anthracis synthesizes by independent pathways two small molecules, the siderophores bacillibactin (BB) and petrobactin (PB). Despite the great efficiency of BB at chelating iron, PB may be the only siderophore necessary to ensure full virulence of the pathogen. In the present work, we show that BB is specifically bound by siderocalin, a recently discovered innate immune protein that is part of an antibacterial iron-depletion defense. In contrast, neither PB nor its ferric complex is bound by siderocalin. Although BB incorporates the common 2,3-dihydroxybenzoyl iron-chelating subunit, PB is novel in that it incorporates the very unusual 3,4-dihydroxybenzoyl chelating subunit. This structural variation results in a large change in the shape of both the iron complex and the free siderophore that precludes siderocalin binding, a stealthy evasion of the immune system. Our results indicate that the blockade of bacterial siderophore-mediated iron acquisition by siderocalin is not restricted to enteric pathogenic organisms and may be a general defense mechanism against several different bacterial species. Significantly, to evade this innate immune response, B. anthracis produces PB, which plays a key role in virulence of the organism. This analysis argues for antianthrax strategies targeting siderophore synthesis and uptake.

The pathogen-associated iroA gene cluster mediates bacterial evasion of lipocalin 2

Numerous bacteria cope with the scarcity of iron in their microenvironment by synthesizing small iron-scavenging molecules known as siderophores. Mammals have evolved countermeasures to block siderophore-mediated iron acquisition as part of their innate immune response. Secreted lipocalin 2 (Lcn2) sequesters the Escherichia coli siderophore enterobactin (Ent), preventing E. coli from acquiring iron and protecting mammals from infection by E. coli. Here, we show that the iroA gene cluster, found in many pathogenic strains of Gram-negative enteric bacteria, including E. coli, Salmonella spp., and Klebsiella pneumoniae, allows bacteria to evade sequestration of Ent by Lcn2. We demonstrate that C-glucosylated derivatives of Ent produced by iroA-encoded enzymes do not bind purified Lcn2, and an iroA-harboring strain of E. coli is insensitive to the growth inhibitory effects of Lcn2 in vitro. Furthermore, we show that mice rapidly succumb to infection by an iroA-harboring strain of E. coli but not its wild-type counterpart, and that this increased virulence depends on evasion of host Lcn2. Our findings indicate that the iroA gene cluster allows bacteria to evade this component of the innate immune system, rejuvenating their Ent-mediated iron-acquisition pathway and playing an important role in their virulence.

Neutrophil gelatinase-associated lipocalin, a siderophore-binding eukaryotic protein

NGAL (neutrophil gelatinase-associated lipocalin) also known as lcn2 or siderochalin is constitutively expressed in myelocytes and stored in specific granules of neutrophils. It is highly induced in a variety of epithelial cells during inflammation. Analysis of the crystal structure of NGAL expressed in E.coli showed that NGAL has the ability to bind catecholate type siderophores and in this way prevent bacteria from acquisition of siderophore-bound iron. NGAL (or 24p3 as the highly homologous murine orthologue is named) knock out mice have a profound defect in defense against E.coli after intraperitoneal injection. This defect can be mimicked in wild-type mice by providing siderophore iron, which cannot be sequestered by NGAL, testifying to the specific role of NGAL as a siderophore binding protein in innate immunity. Megalin, a scavenger receptor functions as a receptor for NGAL and mediates uptake into endosomes, but other NGAL receptors are likely to exist.

Binding of the lipocalin C8γ to human complement protein C8α is mediated by loops located at the entrance to the C8γ ligand binding site

Human C8 is one of five complement components (C5b, C6, C7, C8 and C9) that interact to form the membrane attack complex (MAC). C8 is composed of a disulfide-linked C8alpha-gamma heterodimer and a noncovalently associated C8beta chain. C8alpha and C8beta are homologous to C6, C7 and C9, whereas C8gamma is the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small molecule. In C8gamma, the calyx opening is surrounded by four loops that connect beta-strands. Loop 1 is the largest and contains Cys40 that links to Cys164 in C8alpha. To determine if these loops mediate binding of C8alpha prior to interchain disulfide bond formation in C8alpha-gamma, the loops were substituted separately and in combination for the corresponding loops in siderocalin (NGAL, Lcn2), a lipocalin that is structurally similar to C8gamma. The siderocalin-C8gamma chimeric constructs were expressed in E. coli, purified, and assayed for their ability to bind C8alpha. Results indicate at least three of the four loops surrounding the entrance to the C8gamma calyx are involved in binding C8alpha. Binding near the calyx entrance suggests C8alpha may restrict and possibly regulate access to the C8gamma ligand binding site.

Microbial Evasion of the Immune System: Structural Modifications of Enterobactin Impair Siderocalin Recognition1

The mammalian protein siderocalin binds and inactivates the ferric complex of the bacterial siderophore enterobactin with a Kd value similar to that of the bacterial receptor FepA. However, microorganisms can evade this immune response by structural modifications of the siderophore. The binding of siderophores by siderocalin relies in part on electrostatic interactions and does not depend greatly on what metal is in the complex. It is also sterically limited by the rigid conformation of the protein calyx; methylation of the three catecholate rings of enterobactin hinders siderocalin recognition. The siderocalin binding has been probed for a series of enterobactin analogues in order to investigate in detail the specificity of siderocalin recognition.

Perspectives on interactions between lactoferrin and bacteriaThis paper is one of a selection of papers published in this Special Issue, entitled 7th International Conference on Lactoferrin: Structure, Function, and Applications, and has undergone the Journal's usual peer review process

Lactoferrin has long been recognized for its antimicrobial properties, initially attributed primarily to iron sequestration. It has since become apparent that interaction between the host and bacteria is modulated by a complex series of interactions between lactoferrin and bacteria, lactoferrin and bacterial products, and lactoferrin and host cells. The primary focus of this review is the interaction between lactoferrin and bacteria, but interactions with the lactoferrin-derived cationic peptide lactoferricin will also be discussed. We will summarize what is currently known about the interaction between lactoferrin (or lactoferricin) and surface or secreted bacterial components, comment on the potential physiological relevance of the findings, and identify key questions that remain unanswered.

How pathogenic bacteria evade mammalian sabotage in the battle for iron

Many bacteria, including numerous human pathogens, synthesize small molecules known as siderophores to scavenge iron. Enterobactin, a siderophore produced by enteric bacteria, is surprisingly ineffective as an iron-scavenging agent for bacteria growing in animals because of its hydrophobicity and its sequestration by the mammalian protein siderocalin, a component of the innate immune system. However, pathogenic strains of Escherichia coli and Salmonella use enzymes encoded by the iroA gene cluster to tailor enterobactin by glycosylation and linearization. The resulting modified forms of enterobactin, known as salmochelins, can evade siderocalin and are less hydrophobic than enterobactin, restoring this siderophore's iron-scavenging ability in mammals.

24p3 and its receptor: dawn of a new iron age?

24p3 is a secreted protein that induces apoptosis in leukocytes. Recently, 24p3 has been shown to bind to iron-containing bacterial siderophores. In this issue of Cell, a receptor that internalizes 24p3 is identified. Internalization of iron bound to 24p3 prevents apoptosis. In contrast, internalization of the apo form of 24p3 that does not contain iron leads to cellular iron efflux and apoptosis via the proapoptotic protein Bim.

Enzymatic tailoring of enterobactin alters membrane partitioning and iron acquisition

Enterobactin (Ent), a prototypic bacterial siderophore, is modified by both the C-glucosyltransferase IroB and the macrolactone hydrolase IroE in pathogenic bacteria that contain the iroA cluster. To investigate the possible effects of glucosylation and macrolactone hydrolysis on the physical properties of Ent, the membrane affinities and iron acquisition rates of Ent and Ent-derived siderophores were measured. The data obtained indicate that Ent has a high membrane affinity (K(x) = 1.5 x 10(4)) similar to that of ferric acinetoferrin, an amphiphile containing two eight-carbon hydrophobic chains. Glucosylation and macrolactone hydrolysis decrease the membrane affinity of Ent by 5-25-fold. Furthermore, in the presence of phospholipid vesicles, the iron acquisition rate is significantly increased by glucosylation and macrolactone hydrolysis, due to the resultant decrease in membrane sequestration of the siderophore. These results suggest that IroB and IroE enhance the ability of Ent-producing pathogens to acquire iron in membrane-rich microenvironments.

Modulation of Iron Availability at the Host-Pathogen Interface in Phagocytic Cells

This review summarizes recent data on iron metabolism in macrophages, with a special emphasis on possible bacteriostatic and bactericidal consequences for intracellular pathogens. It includes the role of biological chelators and transporters in normal macrophage physiology and antimicrobial defense. Iron is an essential metal cofactor for many biochemical pathways in mammals. However, excess iron promotes the formation of cytotoxic oxygen derivatives so that systemic iron levels must be tightly regulated. The mechanism of iron recycling by macrophages including iron efflux from erythrocyte-containing phagosomes, iron release from macrophages, and entry into the transferrin (Tf) cycle remain poorly understood. Ferroportin expression in the liver, spleen, and bone marrow cells appears to be restricted to macrophages. Mutant mice bearing a conditional deletion of the ferroportin gene in macrophages show retention of iron by hepatic Kupffer cells and splenic macrophages. Hepcidin is induced by lipopolysaccharide (LPS) in mouse spleens and splenic macrophage in vitro and appears to mediate the LPS-induced down-regulation of ferroportin in the intestine and in splenic macrophages, suggesting that inflammatory agents may regulate iron metabolism through modulation of ferroportin expression. The host transporter Nramp1 may compete directly with bacterial divalent-metal transport systems for the acquisition of divalent metals within the phagosomal space. The ultimate outcome of these competing interactions influences the ability of pathogens to survive and replicate intracellularly. This seems particularly relevant to the Salmonella, Leishmania, and Mycobacterium spp., in which inactivating mutations in Nramp1 abrogate the natural resistance of macrophages to these pathogens.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.

Bacterial colonization of nasal mucosa induces expression of siderocalin, an iron-sequestering component of innate immunity

Host-microbe interactions often begin with colonization of mucosal surfaces. These relationships are highly specific, as certain microbial species are found only in particular microenvironments. Transcriptional microarrays were used to screen host genes whose expression in the murine nasal mucosa was affected by colonization with the Gram-positive bacterium Streptococcus pneumoniae. Siderocalin (Scn, or lipocalin 2 or neutrophil gelatinase-associated lipocalin) expression was increased up to 65-fold during colonization by real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR). Western analysis showed that Scn was secreted into airway surface fluid in colonized animals. Immunohistochemical analysis localized Scn expression primarily to secretory Bowman's glands. Similar results were observed during colonization with the Gram-negative bacterium Haemophilus influenzae, suggesting that Scn secretion is a general response. Western analysis of human nasal secretions also demonstrated secretion of Scn at potentially bacteriostatic levels. This is a previously unrecognized response that may have a role in determining the establishment or maintenance of mucosal colonization. Scn contributes to antimicrobial defence by sequestration of a subset of microbial siderophores. As neither S. pneumoniae nor H. influenzae are known to produce or utilize siderophores, successful colonizers of the nasal passages may have evolved siderophore-independent mechanisms to acquire essential iron and to evade the inhibitory effects of Scn.

Anticalins in Drug Development

Anticalins are a novel class of engineered ligand-binding proteins that are prepared from lipocalins--conventional plasma proteins in humans--via targeted random mutagenesis and selection against prescribed haptens or antigens. The first anticalins were selected to bind to small ligands, such as the cardioactive drug digoxin. Recently, libraries that also permit the generation of anticalins with high affinities and specificities for protein targets, especially disease-related cell-surface receptors, have been constructed. Anticalins are much smaller than antibodies or their antigen-binding fragments, lack glycosylation as well as immunological effector functions, and consist of a single, stably folded polypeptide chain. Thus, they offer benefits as biopharmaceuticals in several areas of medical therapy, for example as receptor antagonists or as effective antidotes against toxic compounds.