High-affinity manganese coordination by human calprotectin is calcium-dependent and requires the histidine-rich site formed at the dimer interface

Recent developments in copper and zinc homeostasis in bacterial pathogens

Copper and zinc homeostasis systems in pathogenic bacteria are required to resist host efforts to manipulate the availability and toxicity of these metal ions. Central to this microbial adaptive response is the involvement of metal-trafficking and metal-sensing proteins that ultimately exercise control of metal speciation in the cell. Cu-specific and Zn-specific metalloregulatory proteins regulate the transcription of metal-responsive genes while metallochaperones and related proteins ensure that these metals are appropriately buffered by the intracellular milieu and delivered to correct intracellular targets. In this review, we summarize recent findings on how bacterial pathogens mount a metal-specific response to derail host efforts to win the 'fight over metals.'

Manganese acquisition and homeostasis at the host-pathogen interface

Pathogenic bacteria acquire transition metals for cell viability and persistence of infection in competition with host nutritional defenses. The human host employs a variety of mechanisms to stress the invading pathogen with both cytotoxic metal ions and oxidative and nitrosative insults while withholding essential transition metals from the bacterium. For example, the S100 family protein calprotectin (CP) found in neutrophils is a calcium-activated chelator of extracellular Mn and Zn and is found in tissue abscesses at sites of infection by Staphylococcus aureus. In an adaptive response, bacteria have evolved systems to acquire the metals in the face of this competition while effluxing excess or toxic metals to maintain a bioavailability of transition metals that is consistent with a particular inorganic "fingerprint" under the prevailing conditions. This review highlights recent biological, chemical and structural studies focused on manganese (Mn) acquisition and homeostasis and connects this process to oxidative stress resistance and iron (Fe) availability that operates at the human host-pathogen interface.

The ZrfC alkaline zinc transporter is required for Aspergillus fumigatus virulence and its growth in the presence of the Zn/Mn-chelating protein calprotectin

Aspergillus fumigatus can invade the lungs of immunocompromised individuals causing a life-threatening disease called invasive pulmonary aspergillosis (IPA). To grow in the lungs, A. fumigatus obtains from the host all nutrients, including zinc. In living tissues, however, most zinc is tightly bound to zinc-binding proteins. Moreover, during infection the bioavailability of zinc can be further decreased by calprotectin, an antimicrobial Zn/Mn-chelating protein that is released by neutrophils in abscesses. Nevertheless, A. fumigatus manages to uptake zinc from and grow within the lungs of susceptible individuals. Thus, in this study we investigated the role of the zrfA, zrfB and zrfC genes, encoding plasma membrane zinc transporters, in A. fumigatus virulence. We showed that zrfC is essential for virulence in the absence of zrfA and zrfB, which contribute to fungal pathogenesis to a lesser extent than zrfC and are dispensable for virulence in the presence of zrfC. The special ability of ZrfC to scavenge and uptake zinc efficiently from lungtissue depended on its N-terminus, which is absent in the ZrfA and ZrfB transporters. In addition, under Zn- and/or Mn-limiting conditions zrfC enables A. fumigatus to grow in the presence of calprotectin, which is detected in fungal abscesses of non-leucopenic animals. This study extends our knowledge about the pathobiology of A. fumigatus and suggests that fungal zinc uptake could be a promising target for new antifungals.

Contributions of the S100A9 C-terminal tail to high-affinity Mn(II) chelation by the host-defense protein human calprotectin

Human calprotectin (CP) is an antimicrobial protein that coordinates Mn(II) with high affinity in a Ca(II)-dependent manner at an unusual histidine-rich site (site 2) formed at the S100A8/S100A9 dimer interface. We present a 16-member CP mutant family where mutations in the S100A9 C-terminal tail (residues 96-114) are employed to evaluate the contributions of this region, which houses three histidines and four acidic residues, to Mn(II) coordination at site 2. The results from analytical size-exclusion chromatography, Mn(II) competition titrations, and electron paramagnetic resonance spectroscopy establish that the C-terminal tail is essential for high-affinity Mn(II) coordination by CP in solution. The studies indicate that His103 and His105 (HXH motif) of the tail complete the Mn(II) coordination sphere in solution, affording an unprecedented biological His6 site. These solution studies are in agreement with a Mn(II)-CP crystal structure reported recently (Damo, S. M.; et al. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 3841). Remarkably high-affinity Mn(II) binding is retained when either H103 or H105 are mutated to Ala, when the HXH motif is shifted from positions 103-105 to 104-106, and when the human tail is substituted by the C-terminal tail of murine S100A9. Nevertheless, antibacterial activity assays employing human CP mutants reveal that the native disposition of His residues is important for conferring growth inhibition against Escherichia coli and Staphylococcus aureus. Within the S100 family, the S100A8/S100A9 heterooligomer is essential for providing high-affinity Mn(II) binding; the S100A7, S100A9(C3S), S100A12, and S100B homodimers do not exhibit such Mn(II)-binding capacity.

Physical characterization of the manganese-sensing pneumococcal surface antigen repressor from Streptococcus pneumoniae

Transition metals, including manganese, are required for the proper virulence and persistence of many pathogenic bacteria. In Streptococcus pneumoniae (Spn), manganese homeostasis is controlled by a high-affinity Mn(II) uptake complex, PsaBCA, and a constitutively expressed efflux transporter, MntE. psaBCA expression is transcriptionally regulated by the DtxR/MntR family metalloregulatory protein pneumococcal surface antigen repressor (PsaR) in Spn. Here, we present a comprehensive analysis of the metal and DNA binding properties of PsaR. PsaR is a homodimer in the absence and presence of metals and binds two manganese or zinc atoms per protomer (four per dimer) in two pairs of structurally distinct sites, termed site 1 and site 2. Site 1 is likely filled with Zn(II) in vivo (K(Zn1) ≥ 10¹³ M⁻¹; K(Mn1) ≈ 10⁸ M⁻¹). The Zn(II)-site 1 complex adopts a pentacoordinate geometry as determined by X-ray absorption spectroscopy containing a single cysteine and appears to be analogous to the Cd(II) site observed in Streptococcus gordonii ScaR. Site 1 is necessary but not sufficient for full positive allosteric activation of DNA operator binding by metals as measured by ΔGc, the allosteric coupling free energy, because site 1 mutants show an intermediate ΔGc. Site 2 is the primary regulatory site and governs specificity for Mn(II) over Zn(II) in PsaR, where ΔGc(Zn,Mn) >> ΔGc(Zn,Zn) despite the fact that Zn(II) binds site 2 with an affinity 40-fold higher than that of Mn(II); i.e., K(Zn2) > K(Mn2). Mutational studies reveal that Asp7 in site 2 is a critical ligand for Mn(II)-dependent allosteric activation of DNA binding. These findings are discussed in the context of other well-studied DtxR/MntR Mn(II)/Fe(II) metallorepressors.

The copper(II) and zinc(II) coordination mode of HExxH and HxxEH motif in small peptides: the role of carboxylate location and hydrogen bonding network

Copper(II) and zinc(II) complexes with two hexapeptides encompassing HExxH and HxxEH motif were characterized by means of a combined experimental and theoretical approach. Parallel tempering and density functional theory (DFT) investigations show the presence of different hydrogen bonding networks between the copper(II) and zinc(II) complexes with the two peptides, suggesting a significant contribution of these non-covalent interactions to the stability constant values. The glutamate carboxylate group has a direct role in metal ion binding. The location of this amino acid along the sequence of the investigated peptides is critical to determine thermodynamic and spectroscopic features of the copper(II) complex species, whereas is less relevant in the zinc(II) complexes formation. Electrospray ionization mass spectrometry (ESI-MS) characterization of the zinc(II) complex species show that in the [ZnH-2L] two deprotonated amide nitrogen atoms are involved in the metal coordination environment, an uncommon behavior in zinc(II) complexes for multi-histidine ligands.

Structural basis for assembly of the Mn(IV)/Fe(III) cofactor in the class Ic ribonucleotide reductase from Chlamydia trachomatis

The class Ic ribonucleotide reductase (RNR) from Chlamydia trachomatis (Ct) employs a Mn(IV)/Fe(III) cofactor in each monomer of its β2 subunit to initiate nucleotide reduction. The cofactor forms by reaction of Mn(II)/Fe(II)-β2 with O2. Previously, in vitro cofactor assembly from apo β2 and divalent metal ions produced a mixture of two forms, with Mn at site 1 (Mn(IV)/Fe(III)) or site 2 (Fe(III)/Mn(IV)), of which the more active Mn(IV)/Fe(III) product predominates. Here we have addressed the basis for metal site selectivity by determining X-ray crystal structures of apo, Mn(II), and Mn(II)/Fe(II) complexes of Ct β2. A structure obtained anaerobically with equimolar Mn(II), Fe(II), and apoprotein reveals exclusive incorporation of Mn(II) at site 1 and Fe(II) at site 2, in contrast to the more modest site selectivity achieved previously. Site specificity is controlled thermodynamically by the apoprotein structure, as only minor adjustments of ligands occur upon metal binding. Additional structures imply that, by itself, Mn(II) binds in either site. Together, the structures are consistent with a model for in vitro cofactor assembly in which Fe(II) specificity for site 2 drives assembly of the appropriately configured heterobimetallic center, provided that Fe(II) is substoichiometric. This model suggests that use of a Mn(IV)/Fe(III) cofactor in vivo could be an adaptation to Fe(II) limitation. A 1.8 Å resolution model of the Mn(II)/Fe(II)-β2 complex reveals additional structural determinants for activation of the cofactor, including a proposed site for side-on (η(2)) addition of O2 to Fe(II) and a short (3.2 Å) Mn(II)-Fe(II) interionic distance, promoting formation of the Mn(IV)/Fe(IV) activation intermediate.

Production and release of antimicrobial and immune defense proteins by mammary epithelial cells following Streptococcus uberis infection of sheep

Investigating the innate immune response mediators released in milk has manifold implications, spanning from elucidation of the role played by mammary epithelial cells (MECs) in fighting microbial infections to the discovery of novel diagnostic markers for monitoring udder health in dairy animals. Here, we investigated the mammary gland response following a two-step experimental infection of lactating sheep with the mastitis-associated bacterium Streptococcus uberis. The establishment of infection was confirmed both clinically and by molecular methods, including PCR and fluorescent in situ hybridization of mammary tissues. Proteomic investigation of the milk fat globule (MFG), a complex vesicle released by lactating MECs, enabled detection of enrichment of several proteins involved in inflammation, chemotaxis of immune cells, and antimicrobial defense, including cathelicidins and calprotectin (S100A8/S100A9), in infected animals, suggesting the consistent involvement of MECs in the innate immune response to pathogens. The ability of MECs to produce and release antimicrobial and immune defense proteins was then demonstrated by immunohistochemistry and confocal immunomicroscopy of cathelicidin and the calprotectin subunit S100A9 on mammary tissues. The time course of their release in milk was also assessed by Western immunoblotting along the course of the experimental infection, revealing the rapid increase of these proteins in the MFG fraction in response to the presence of bacteria. Our results support an active role of MECs in the innate immune response of the mammary gland and provide new potential for the development of novel and more sensitive tools for monitoring mastitis in dairy animals.

Molecular basis for manganese sequestration by calprotectin and roles in the innate immune response to invading bacterial pathogens

The S100A8/S100A9 heterodimer calprotectin (CP) functions in the host response to pathogens through a mechanism termed "nutritional immunity." CP binds Mn(2+) and Zn(2+) with high affinity and starves bacteria of these essential nutrients. Combining biophysical, structural, and microbiological analysis, we identified the molecular basis of Mn(2+) sequestration. The asymmetry of the CP heterodimer creates a single Mn(2+)-binding site from six histidine residues, which distinguishes CP from all other Mn(2+)-binding proteins. Analysis of CP mutants with altered metal-binding properties revealed that, despite both Mn(2+) and Zn(2+) being essential metals, maximal growth inhibition of multiple bacterial pathogens requires Mn(2+) sequestration. These data establish the importance of Mn(2+) sequestration in defense against infection, explain the broad-spectrum antimicrobial activity of CP relative to other S100 proteins, and clarify the impact of metal depletion on the innate immune response to infection.