Calcium binds to leptospiral immunoglobulin-like protein, LigB, and modulates fibronectin binding

Unveiling the Secrets of Calcium-Dependent Proteins in Plant Growth-Promoting Rhizobacteria: An Abundance of Discoveries Awaits

The role of Calcium ions (Ca2+) is extensively documented and comprehensively understood in eukaryotic organisms. Nevertheless, emerging insights, primarily derived from studies on human pathogenic bacteria, suggest that this ion also plays a pivotal role in prokaryotes. In this review, our primary focus will be on unraveling the intricate Ca2+ toolkit within prokaryotic organisms, with particular emphasis on its implications for plant growth-promoting rhizobacteria (PGPR). We undertook an in silico exploration to pinpoint and identify some of the proteins described in the existing literature, including prokaryotic Ca2+ channels, pumps, and exchangers that are responsible for regulating intracellular Calcium concentration ([Ca2+]i), along with the Calcium-binding proteins (CaBPs) that play a pivotal role in sensing and transducing this essential cation. These investigations were conducted in four distinct PGPR strains: Pseudomonas chlororaphis subsp. aurantiaca SMMP3, P. donghuensis SVBP6, Pseudomonas sp. BP01, and Methylobacterium sp. 2A, which have been isolated and characterized within our research laboratories. We also present preliminary experimental data to evaluate the influence of exogenous Ca2+ concentrations ([Ca2+]ex) on the growth dynamics of these strains.

Crystal structure of a variable region segment of Leptospira host-interacting outer surface protein, LigA, reveals the orientation of Ig-like domains

Leptospiral immunoglobulin-like (Lig) protein family is a surface-exposed protein from the pathogenic Leptospira. The Lig protein family has been identified as an essential virulence factor of L. interrogan. One of the family members, LigA, contains 13 homologous tandem repeats of bacterial Ig-like (Big) domains in its extracellular portion. It is crucial in binding with the host's Extracellular matrices (ECM) and complement factors. However, its vital role in the invasion and evasion of pathogenic Leptospira, structural details, and domain organization of the extracellular portion of this protein are not explored thoroughly. Here, we described the first high-resolution crystal structure of a variable region segment (LigA8-9) of LigA at 1.87 Å resolution. The structure showed some remarkably distinctive aspects compared with the most closely related Immunoglobulin superfamily (IgSF) members. The structure illustrated the relative orientation of two domains and highlighted the role of the linker region in the domain orientation. We also observed an apparent electron density of Ca²⁺ ions coordinated with a proper interacting geometry within the protein. Molecular dynamic simulations demonstrated the involvement of a linker salt bridge in providing rigidity between the two domains. Our study proposes an overall arrangement of Ig-like domains in the LigA protein. The structural understanding of the extracellular portion of LigA and its interaction with the ECM provides insight into developing new therapeutics directed toward leptospirosis.

Characterization of novel nuclease and protease activities among Leptospiral immunoglobulin-like proteins

Bacterial immunoglobulin-like (BIg) domain containing proteins play a variety of biological functions. Leptospiral Immunoglobulin-like (Lig) proteins are well-known virulence factors located on the surface of the pathogenic Leptospira that act during adhesion, invasion, and immune evasion. The Lig proteins have many roles and have been designated as multifaceted proteins. However, the hydrolyzing function of Lig proteins is not yet investigated in detail. Here, we report novel in-vitro nuclease and protease activities in the Ig-like domain of LigA protein. All Ig-like domains were able to cleave DNA in the presence of a divalent ion, but not RNA. Site-directed mutagenesis revealed Mg⁺² binding residues in the Ig-like domain of LigA7. The basis of novel nuclease activity may be associated with protein adopting different conformation in the presence of divalent ions and substrate as investigated by change of intrinsic fluorescence. The docking of a stretch of double-strand DNA shows the binding on the positive surface of the protein. In addition, the protein is also observed to cleave a general protease substrate, β-casein, in our experimental condition. Our results proposed that the novel functions may be associated with neutrophil extracellular Trap (NET) evasion. Overall this study enhances the basic knowledge of non-nuclease proteins involved in the DNA cleavage activity and makes the foundation to explore its in-vivo activity in pathogenic Leptospira and other pathogens as well. Moreover, this information may be utilized to develop preventive strategies to interfere with Leptospira immune evasion.

Leptospiral Infection, Pathogenesis and Its Diagnosis-A Review

Leptospirosis is a perplexing conundrum for many. In the existing literature, the pathophysiological mechanisms pertaining to leptospirosis is still not understood in full. Considered as a neglected tropical zoonotic disease, leptospirosis is culminating as a serious problem worldwide, seemingly existing as co-infections with various other unrelated diseases, including dengue and malaria. Misdiagnosis is also common as non-specific symptoms are documented extensively in the literature. This can easily lead to death, as the severe form of leptospirosis (Weil's disease) manifests as a complex of systemic complications, especially renal failure. The virulence of Leptospira sp. is usually attributed to the outer membrane proteins, including LipL32. With an armament of virulence factors at their disposal, their ability to easily adhere, invade and replicate within cells calls for a swift refinement in research progress to establish their exact pathophysiological framework. As an effort to reconstitute the current knowledge on leptospirosis, the basis of leptospiral infection, including its risk factors, classification, morphology, transmission, pathogenesis, co-infections and clinical manifestations are highlighted in this review. The various diagnostic techniques are also outlined with emphasis on their respective pros and cons.

Leptospiral Immunoglobulin-Like Domain Proteins: Roles in Virulence and Immunity

The virulence mechanisms required for infection and evasion of immunity by pathogenic Leptospira species remain poorly understood. A number of L. interrogans surface proteins have been discovered, lying at the interface between the pathogen and host. Among these proteins, the functional properties of the Lig (leptospiral immunoglobulin-like domain) proteins have been examined most thoroughly. LigA, LigB, and LigC contain a series of, 13, 12, and 12 closely related domains, respectively, each containing a bacterial immunoglobulin (Big) -like fold. The multidomain region forms a mostly elongated structure that exposes a large surface area. Leptospires wield the Lig proteins to promote interactions with a range of specific host proteins, including those that aid evasion of innate immune mechanisms. These diverse binding events mediate adhesion of L. interrogans to the extracellular matrix, inhibit hemostasis, and inactivate key complement proteins. These interactions may help L. interrogans overcome the physical, hematological, and immunological barriers that would otherwise prevent the spirochete from establishing a systemic infection. Despite significant differences in the affinities of the LigA and LigB proteins for host targets, their functions overlap during lethal infection of hamsters; virulence is lost only when both ligA and ligB transcription is knocked down simultaneously. Lig proteins have been shown to be promising vaccine antigens through evaluation of a variety of different adjuvant strategies. This review serves to summarize current knowledge of Lig protein roles in virulence and immunity and to identify directions needed to better understand the precise functions of the Lig proteins during infection.

The Leptospira interrogans LIC10774 is a multifunctional surface protein that binds calcium and interacts with host components

Leptospirosis is a global re-emerging zoonosis, caused by pathogenic bacteria of the genus Leptospira. Humans are infected mainly through contact with contaminated water or soil. The understanding of the molecular mechanisms of leptospirosis through the characterization of unknown outer membrane proteins may contribute to the development of new treatments, diagnostic methods and vaccines. We have identified using bioinformatics analysis a protein that is encoded by the gene LIC10774, predicted to be localized at the leptospiral outer membrane and exhibit beta-roll folding. Surface exposure was confirmed by flow cytometry, ELISA and immunofluorescence-based confocal microscopy. Through circular dichroism spectroscopy and hydrophobic dye binding we have shown that rLIC10774 binds calcium ions, which imposes changes to secondary and tertiary structures. The recombinant protein was capable of binding to several host extracellular matrix and serum components. Therefore, we describe LIC10774 as a calcium-binding protein exposed in the outer surface of pathogenic leptospires with possible multifunctional roles in adhesion to host tissues, evasion of the immune system and participation in dissemination processes during leptospirosis. In addition, we hypothesize that the calcium binding is important for temperature-dependent functional roles during leptospirosis.

Functional and structural investigations of fibronectin-binding protein Apa from Mycobacterium tuberculosis

BACKGROUND

Alanine and proline-rich protein (Apa) is a secreted antigen of Mycobacterium spp. which involves in stimulating immune responses and adhering to host cells by binding to fibronectin (Fn). Here, we report the crystal structure of Apa from Mycobacterium tuberculosis (Mtb) and its Fn-binding characteristics.

METHODS

The crystal structure of Mtb Apa was determined at resolutions of 1.54 Å. The dissociation constants (K_D) of Apa and individual modules of Fn were determined by surface plasmon resonance and enzyme-linked immunosorbent assay. Site-directed mutagenesis was performed to investigate the putative Fn-binding motif of Apa.

RESULTS

Mtb Apa folds into a large seven-stranded anti-parallel β-sheet which is flanked by three α-helices. The binding affinity of Mtb Apa to individual Fn modules was assessed and the results indicated that the Mtb Apa binds to FnIII-4 and FnIII-5 of Fn CBD segment. Notably, structure analysis suggested that the previously proposed Fn-binding motif ²⁵⁸RWFV²⁶¹ is buried within the protein and may not be accessible to the binding counterpart.

CONCLUSIONS

The structural and Fn-binding characteristics we reported here provide molecular insights into the multifunctional protein Mtb Apa. FnIII-4 and FnIII-5 of CBD are the only two modules contributing to Apa-Fn interaction.

GENERAL SIGNIFICANCE

This is the first study to report the structure and Fn-binding characteristics of mycobacterial Apa. Since Apa plays a central role in stimulating immune responses and host cells adhesion, these results are of great importance in understanding the pathogenesis of mycobacterium. This information shall provide a guidance for the development of anti-mycobacteria regimen.

Calcium sequestration by fungal melanin inhibits calcium-calmodulin signalling to prevent LC3-associated phagocytosis

LC3-associated phagocytosis (LAP) is a non-canonical autophagy pathway regulated by Rubicon, with an emerging role in immune homeostasis and antifungal host defence. Aspergillus cell wall melanin protects conidia (spores) from killing by phagocytes and promotes pathogenicity through blocking nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent activation of LAP. However, the signalling regulating LAP upstream of Rubicon and the mechanism of melanin-induced inhibition of this pathway remain incompletely understood. Herein, we identify a Ca²⁺ signalling pathway that depends on intracellular Ca²⁺ sources from endoplasmic reticulum, endoplasmic reticulum-phagosome communication, Ca²⁺ release from phagosome lumen and calmodulin (CaM) recruitment, as a master regulator of Rubicon, the phagocyte NADPH oxidase NOX2 and other molecular components of LAP. Furthermore, we provide genetic evidence for the physiological importance of Ca²⁺-CaM signalling in aspergillosis. Finally, we demonstrate that Ca²⁺ sequestration by Aspergillus melanin inside the phagosome abrogates activation of Ca²⁺-CaM signalling to inhibit LAP. These findings reveal the important role of Ca²⁺-CaM signalling in antifungal immunity and identify an immunological function of Ca²⁺ binding by melanin pigments with broad physiological implications beyond fungal disease pathogenesis.

Is NS-EDTA Effective in Clearing Bacteria From Infected Wounds in a Rat Model?

BACKGROUND

Irrigation is one of the key procedures in open fracture management to eliminate pathogens and prevent infection. Metal ion deprivation could inhibit bacterial adhesins and weaken adhesion to the host tissue. EDTA in solution can competitively bind to a metal ion and thus might be able to inhibit bacterial adhesins.

QUESTIONS/PURPOSES

(1) Is normal saline-EDTA toxic to fibroblasts and endothelial cells? (2) In a contaminated wound rat model, does irrigation with normal saline-EDTA solution decrease the risk of positive bacterial cultures and infection when compared with normal saline and soap solutions? (3) In an infected wound rat model, are fewer surgical débridements and irrigations with normal saline-EDTA solution required to obtain culture-free wounds when compared with normal saline and soap controls?

METHODS

Normal saline-EDTA solution refers to 1 mmol/L EDTA dissolved in normal saline (pH adjusted to 7.4). Normal saline and soap solutions acted as controls. The toxicity of these solutions to fibroblasts and endothelial cells was assessed in vitro by Annexin V/propidium iodide staining and flow cytometer counting (a well-established method to quantitatively measure the number of dead cells). We established contaminated and infected wound models (bone-exposed or not) with either Staphylococcus aureus or Escherichia coli in rats to investigate the efficacy of normal saline-EDTA solution (n = 30 for the contaminated model and n = 50 for the infected model). For contaminated wounds, the proportion of positive bacterial cultures and infections was compared after irrigation and débridement among the three groups. For infected wounds, we performed irrigation and débridement every 48 hours until the cultures were negative and compared the number of débridements required to achieve a negative culture with survival analysis.

RESULTS

Normal saline-EDTA showed no additional toxicity to fibroblasts and endothelial cells when compared with normal saline (normal saline [97%] versus EDTA [98%] on fibroblasts, p = 0.654; normal saline [97%] versus EDTA [98%] on endothelial cells, p = 0.711). When bone was exposed in the contaminated models, EDTA irrigation resulted in fewer positive bacterial cultures with S aureus (EDTA: 23%, normal saline: 67%, soap: 40%, p = 0.003) and with E coli (EDTA: 27%, normal saline: 57%, soap: 30%, p = 0.032); however, infection risk was only lower with EDTA irrigation (S aureus with EDTA: 10%, normal saline: 33%, soap: 37%, p = 0.039; E coli with EDTA: 3%, normal saline: 27%, soap: 23%, p = 0.038). In the infected wound model, EDTA irrigation resulted in earlier culture-negative wounds (fewer surgical sessions) compared with normal saline and soap solutions (nonbone-exposed wounds infected by S aureus: p = 0.003, infected by E coli: p = 0.001; bone-exposed wounds infected by S aureus: p = 0.012, infected by E coli: p = 0.022).

CONCLUSIONS

After in vitro assessment of toxicity and in vivo evaluation of efficacy, we concluded that normal saline-EDTA is superior to normal saline and soap solution in our laboratory models.

CLINICAL RELEVANCE

The use of normal-saline EDTA as an irrigation solution may reduce the infection rate of wounds. Future studies in large animals and humans might prove our observation in rat models that normal saline-EDTA has an advantage over normal saline as an irrigation solution.

Strategizing for the purification of a multiple Big domain-containing protein in native conformation is worth it!

The reliability and accuracy of conformational or functional studies of any novel multidomain protein rely on the quality of protein. The bottleneck in structural studies with the complete Big_2 domain containing proteins like LigA, LigB or MpIBP is usually their large molecular size owing to their multidomain (>10-12 domains) architectures. Interestingly, a soil bacterium Paenarthrobacter aurescens TC1, harbours a gene that encodes a protein comprising of four predicted Big_2 domains. We report here the expression and purification of this novel, multiple Big_2 domains containing protein, Arig of P. aurescens TC1. During overexpression, recombinant Arig formed inclusion bodies and hence was purified by on-column refolding. The refolded Arig revealed a β-sheet conformation and a well-resolved near-UV CD spectra but did not exhibit a well-dispersed 2D [¹H-¹⁵N]-HSQC NMR spectrum, as expected for a well-folded β-sheet native conformation. We, therefore, further optimized Arig overexpression in the soluble fraction by including osmolytes. CD spectroscopic and 2D [¹H-¹⁵N]-HSQC analyses consolidate that Arig purified alternatively has a well-folded native conformation. While we describe different strategies for purification of Arig, we also present the spectral properties of this novel all-β-sheet protein.

Active Components of Leptospira Outer Membrane Protein LipL32 to Toll-Like Receptor 2

Proteins belonging to the toll-like receptor (TLR) family, particularly TLR2, are the major components of innate immunity against Leptospira infection. The ligands for TLR2 harbor several conserved patterns such as lipidation molecules, leucine-rich repeat (LRR) domains, TLR2 binding motifs, and TLR2 binding structure. In Leptospira, LipL32 interacts with TLR2 on human kidney cells concomitantly stimulating inflammatory responses. However, the binding mechanism of LipL32 to TLR2 is unknown. The computational prediction suggests that β1β2, loop-α3-loop, and α4 domains of LipL32 play vital roles in LipL32-TLR2 complex formation. To test these predictions, protein truncation experiments revealed that LipL32ΔNβ1β2 significantly decreased the affinity to TLR2 while LipL32ΔCα4 slightly reduced it. Interestingly, LipL32ΔCenα3 retained affinity to TLR2 in the absence of Ca2+ ions, indicating that Cenα3 play a role preventing the interaction between LipL32 and TLR2. Furthermore, the critical residues of LipL32 involved in interacting with TLR2 suggested that V35S, L36S and L263S variants significantly decreased the affinity to TLR2. The results further confirm that LipL32 interacts with TLR2 through Nβ1β2 and Cα4 domains of LipL32 as well as LipL32-TLR2 complex formation results from hydrophobic interactions. This study provides a detailed mechanism of the interaction between LipL32 and TLR2 and the residues involved in complex formation.

Acquisition of negative complement regulators by the saprophyte Leptospira biflexa expressing LigA or LigB confers enhanced survival in human serum

Leptospiral immunoglobulin-like (Lig) proteins are surface exposed molecules present in pathogenic but not in saprophytic Leptospira species. We have previously shown that Lig proteins interact with the soluble complement regulators Factor H (FH), FH like-1 (FHL-1), FH related-1 (FHR-1) and C4b Binding Protein (C4BP). In this study, we used the saprophyte L. biflexa serovar Patoc as a surrogate host to address the specific role of LigA and LigB proteins in leptospiral complement evasion. L. biflexa expressing LigA or LigB was able to acquire FH and C4BP. Bound complement regulators retained their cofactor activities of FI in the proteolytic cleavage of C3b and C4b. Moreover, heterologous expression of ligA and ligB genes in the saprophyte L. biflexa enhanced bacterial survival in human serum. Complement deposition on lig-transformed L. biflexa was assessed by flow cytometry analysis. With regard to MAC deposition, L. biflexa expressing LigA or LigB presented an intermediate profile: MAC deposition levels were greater than those found in the pathogenic L. interrogans, but lower than those observed for L. biflexa wildtype. In conclusion, Lig proteins contribute to in vitro control of complement activation on the leptospiral surface, promoting an increased bacterial survival in human serum.

Whole genome sequencing revealed host adaptation-focused genomic plasticity of pathogenic Leptospira

Leptospirosis, caused by pathogenic Leptospira spp., has recently been recognized as an emerging infectious disease worldwide. Despite its severity and global importance, knowledge about the molecular pathogenesis and virulence evolution of Leptospira spp. remains limited. Here we sequenced and analyzed 102 isolates representing global sources. A high genomic variability were observed among different Leptospira species, which was attributed to massive gene gain and loss events allowing for adaptation to specific niche conditions and changing host environments. Horizontal gene transfer and gene duplication allowed the stepwise acquisition of virulence factors in pathogenic Leptospira evolved from a recent common ancestor. More importantly, the abundant expansion of specific virulence-related protein families, such as metalloproteases-associated paralogs, were exclusively identified in pathogenic species, reflecting the importance of these protein families in the pathogenesis of leptospirosis. Our observations also indicated that positive selection played a crucial role on this bacteria adaptation to hosts. These novel findings may lead to greater understanding of the global diversity and virulence evolution of Leptospira spp.

Calcium-dependent antigen binding as a novel modality for antibody recycling by endosomal antigen dissociation

The pH-dependent antigen binding antibody, termed a recycling antibody, has recently been reported as an attractive type of second-generation engineered therapeutic antibody. A recycling antibody can dissociate antigen in the acidic endosome, and thus bind to its antigen multiple times. As a consequence, a recycling antibody can neutralize large amounts of antigen in plasma. Because this approach relies on histidine residues to achieve pH-dependent antigen binding, which could limit the epitopes that can be targeted and affect the rate of antigen dissociation in the endosome, we explored an alternative approach for generating recycling antibodies. Since calcium ion concentration is known to be lower in endosome than in plasma, we hypothesized that an antibody with antigen-binding properties that are calcium-dependent could be used as recycling antibody. Here, we report a novel anti-interleukin-6 receptor (IL-6R) antibody, identified from a phage library that binds to IL-6R only in the presence of a calcium ion. Thermal dynamics and a crystal structure study revealed that the calcium ion binds to the heavy chain CDR3 region (HCDR3), which changes and possibly stabilizes the structure of HCDR3 to make it bind to antigen calcium dependently (PDB 5AZE). In vitro and in vivo studies confirmed that this calcium-dependent antigen-binding antibody can dissociate its antigen in the endosome and accelerate antigen clearance from plasma, making it a novel approach for generating recycling antibody.

Calcium binding proteins and calcium signaling in prokaryotes

With the continued increase of genomic information and computational analyses during the recent years, the number of newly discovered calcium binding proteins (CaBPs) in prokaryotic organisms has increased dramatically. These proteins contain sequences that closely resemble a variety of eukaryotic calcium (Ca(2+)) binding motifs including the canonical and pseudo EF-hand motifs, Ca(2+)-binding β-roll, Greek key motif and a novel putative Ca(2+)-binding domain, called the Big domain. Prokaryotic CaBPs have been implicated in diverse cellular activities such as division, development, motility, homeostasis, stress response, secretion, transport, signaling and host-pathogen interactions. However, the majority of these proteins are hypothetical, and only few of them have been studied functionally. The finding of many diverse CaBPs in prokaryotic genomes opens an exciting area of research to explore and define the role of Ca(2+) in organisms other than eukaryotes. This review presents the most recent developments in the field of CaBPs and novel advancements in the role of Ca(2+) in prokaryotes.

Solution structure of a bacterial immunoglobulin-like domain of the outer membrane protein (LigB) from Leptospira

Leptospiral immunoglobulin-like (Lig) proteins are surface proteins expressed in pathogenic strains of Leptospira. LigB, an outer membrane protein containing tandem repeats of bacterial Ig-like (Big) domains and a no-repeat tail, has been identified as a virulence factor involved in adhesion of pathogenic Leptospira interrogans to host cells. A Big domain of LigB, LigBCen2R, was reported previously to bind the GBD domain of fibronectin, suggesting its important role in leptospiral infections. In this study, we determined the solution structure of LigBCen2R by nuclear magnetic resonance (NMR) spectroscopy. LigBCen2R adopts a canonical immunoglobulin-like fold which is comprised of a beta-sandwich of ten strands in three sheets. We indicated that LigBCen2R is able to bind to Ca(2+) with a high affinity by isothermal titration calorimetry assay. NMR perturbation experiment identified a number of residues responsible for Ca(2+) binding. Structural comparison of it with other Big domains demonstrates that they share a similar fold pattern, but vary in some structural characters. Since Lig proteins play a vital role in the infection to host cells, our study will contribute a structural basis to understand the interactions between Leptospira and host cells.

Staphylococcus aureus manganese transport protein C (MntC) is an extracellular matrix- and plasminogen-binding protein

Infections caused by Staphylococcus aureus – particularly nosocomial infections - represent a great concern. Usually, the early stage of pathogenesis consists on asymptomatic nasopharynx colonization, which could result in dissemination to other mucosal niches or invasion of sterile sites, such as blood. This pathogenic route depends on scavenging of nutrients as well as binding to and disrupting extracellular matrix (ECM). Manganese transport protein C (MntC), a conserved manganese-binding protein, takes part in this infectious scenario as an ion-scavenging factor and surprisingly as an ECM and coagulation cascade binding protein, as revealed in this work. This study showed a marked ability of MntC to bind to several ECM and coagulation cascade components, including laminin, collagen type IV, cellular and plasma fibronectin, plasminogen and fibrinogen by ELISA. The MntC binding to plasminogen appears to be related to the presence of surface-exposed lysines, since previous incubation with an analogue of lysine residue, ε-aminocaproic acid, or increasing ionic strength affected the interaction between MntC and plasminogen. MntC-bound plasminogen was converted to active plasmin in the presence of urokinase plasminogen activator (uPA). The newly released plasmin, in turn, acted in the cleavage of the α and β chains of fibrinogen. In conclusion, we describe a novel function for MntC that may help staphylococcal mucosal colonization and establishment of invasive disease, through the interaction with ECM and coagulation cascade host proteins. These data suggest that this potential virulence factor could be an adequate candidate to compose an anti-staphylococcal human vaccine formulation.

A novel calcium supplement prepared by phytoferritin nanocages protects against absorption inhibitors through a unique pathway

The consumption of milk is declining in industrialized countries, leading to inadequate calcium intake. Therefore, it is important to explore a new class of Ca-enriched nutrient for the fortification of food. In this work, we prepared a novel class of soluble and edible Ca-protein complexes where approximately 140 calcium ions were encapsulated within a phytoferritin nanocage. As an alternative to other organic and/or inorganic carriers, protein nanocages were found to provide a unique vehicle of biological origin for the intracellular delivery of calcium ions for supplementation. Such encapsulation can protect calcium ions within protein cages against dietary factors such as tannic acid (TA), oxalic acid (OA), and other divalent metal ions in foodstuffs. We demonstrated that the calcium-containing ferritin composites can be absorbed by Caco-2 cells through a process where a TfR1 receptor is involved, whereas the uptake of free calcium ions has been known to be associated with another receptor, DMT1, indicating that the calcium ions encapsulated in supramolecular protein cages can be internalized by the Caco-2 cells through a different pathway from its free analogs for calcium supplementation.

Ca2+-binding motif of βγ-crystallins

βγ-Crystallin-type double clamp (N/D)(N/D)XX(S/T)S motif is an established but sparsely investigated motif for Ca(2+) binding. A βγ-crystallin domain is formed of two Greek key motifs, accommodating two Ca(2+)-binding sites. βγ-Crystallins make a separate class of Ca(2+)-binding proteins (CaBP), apparently a major group of CaBP in bacteria. Paralleling the diversity in βγ-crystallin domains, these motifs also show great diversity, both in structure and in function. Although the expression of some of them has been associated with stress, virulence, and adhesion, the functional implications of Ca(2+) binding to βγ-crystallins in mediating biological processes are yet to be elucidated.

Interaction of Leptospira elongation factor Tu with plasminogen and complement factor H: a metabolic leptospiral protein with moonlighting activities

The elongation factor Tu (EF-Tu), an abundant bacterial protein involved in protein synthesis, has been shown to display moonlighting activities. Known to perform more than one function at different times or in different places, it is found in several subcellular locations in a single organism, and may serve as a virulence factor in a range of important human pathogens. Here we demonstrate that Leptospira EF-Tu is surface-exposed and performs additional roles as a cell-surface receptor for host plasma proteins. It binds plasminogen in a dose-dependent manner, and lysine residues are critical for this interaction. Bound plasminogen is converted to active plasmin, which, in turn, is able to cleave the natural substrates C3b and fibrinogen. Leptospira EF-Tu also acquires the complement regulator Factor H (FH). FH bound to immobilized EF-Tu displays cofactor activity, mediating C3b degradation by Factor I (FI). In this manner, EF-Tu may contribute to leptospiral tissue invasion and complement inactivation. To our knowledge, this is the first description of a leptospiral protein exhibiting moonlighting activities.

The perturbation of tryptophan fluorescence by phenylalanine to alanine mutations identifies the hydrophobic core in a subset of bacterial Ig-like domains

Many host-parasite interactions are mediated via surface-exposed proteins containing bacterial immunoglobulin-like (Big) domains. Here, we utilize the spectral properties of a conserved Trp to provide evidence that, along with a Phe, these residues are positioned within the hydrophobic core of a subset of Big_2 domains. The mutation of the Phe to Ala decreases Big_2 domain stability and impairs the ability of LigBCen2 to bind to the host protein, fibronectin.

Development of an enzyme-linked immunosorbent assay using a recombinant LigA fragment comprising repeat domains 4 to 7.5 as an antigen for diagnosis of equine leptospirosis

Leptospira immunoglobulin (Ig)-like (Lig) proteins are a novel family of surface-associated proteins in which the N-terminal 630 amino acids are conserved. In this study, we truncated the LigA conserved region into 7 fragments comprising the 1st to 3rd (LigACon1-3), 4th to 7.5th (LigACon4-7.5), 4th (LigACon4), 4.5th to 5.5th (LigACon4.5-5.5), 5.5th to 6.5th (LigACon5.5-6.5), 4th to 5th (LigACon4-5), and 6th to 7.5th (LigACon6-7.5) repeat domains. All 7 recombinant Lig proteins were screened using a slot-shaped dot blot assay for the diagnosis of equine leptospirosis. Our results showed that LigACon4-7.5 is the best candidate diagnostic antigen in a slot-shaped dot blot assay. LigACon4-7.5 was further evaluated as an indirect enzyme-linked immunosorbent assay (ELISA) antigen for the detection of Leptospira antibodies in equine sera. This assay was evaluated with equine sera (n = 60) that were microscopic agglutination test (MAT) negative and sera (n = 220) that were MAT positive to the 5 serovars that most commonly cause equine leptospirosis. The indirect ELISA results showed that at a single serum dilution of 1:250, the sensitivity and specificity of ELISA were 80.0% and 87.2%, respectively, compared to those of MAT. In conclusion, an indirect ELISA was developed utilizing a recombinant LigA fragment comprising the 4th to 7.5th repeat domain (LigACon4-7.5) as a diagnostic antigen for equine leptospirosis. This ELISA was found to be sensitive and specific, and it yielded results that concurred with those of the standard MAT.

Essential calcium-binding cluster of Leptospira LipL32 protein for inflammatory responses through the Toll-like receptor 2 pathway

Leptospirosis is the most widespread zoonosis caused by the pathogenic Leptospira worldwide. LipL32, a 32-kDa lipoprotein, is the most abundant protein on the outer membrane of Leptospira and has an atypical poly(Asp) motif ((161)DDDDDGDD(168)). The x-ray crystallographic structure of LipL32 revealed that the calcium-binding cluster of LipL32 includes several essential residues Asp(132), Thr(133), Asp(164), Asp(165), and Tyr(178). The goals of this study were to determine possible roles of the Ca(2+)-binding cluster for the interaction of LipL32 and Toll-like receptor 2 (TLR2) in induced inflammatory responses of human kidney cells. Site-directed mutagenesis was employed to individually mutate Ca(2+)-binding residues of LipL32 to Ala, and their effects subsequently were observed. These mutations abolished primarily the structural integrity of the calcium-binding cluster in LipL32. The binding assay and atomic force microscopy analysis further demonstrated the decreased binding capability of LipL32 mutants to TLR2. Inflammatory responses induced by LipL32 variants, as determined by TLR2 pathway intermediates hCXCL8/IL-8, hCCL2/MCP-1, hMMP7, and hTNF-α, were also lessened. In conclusion, the calcium-binding cluster of LipL32 plays essential roles in presumably sustaining LipL32 conformation for its proper association with TLR2 to elicit inflammatory responses in human renal cells.

Elastin, a novel extracellular matrix protein adhering to mycobacterial antigen 85 complex

The antigen 85 complex (Ag85) consists of three predominantly secreted proteins (Ag85A, Ag85B, and Ag85C), which play a key role in the mycobacterial pathogenesis and also possess enzymatic mycolyltransferase activity involved in cell wall synthesis. Ag85 is not only considered to be a virulence factor because its expression is essential for intracellular survival within macrophages, but also because it contributes to adherence, invasion, and dissemination of mycobacteria in host cells. In this study, we report that the extracellular matrix components, elastin and its precursor (tropoelastin) derived from human aorta, lung, and skin, serve as binding partners of Ag85 from Mycobacterium tuberculosis. The binding affinity of M. tuberculosis Ag85 to human tropoelastin was characterized (K(D) = 0.13 ± 0.006 μm), and a novel Ag85-binding motif, AAAKAA(K/Q)(Y/F), on multiple tropoelastin modules was identified. In addition, the negatively charged Glu-258 of Ag85 was demonstrated to participate in an electrostatic interaction with human tropoelastin. Moreover, binding of Ag85 on elastin siRNA-transfected Caco-2 cells was significantly reduced (34.3%), implying that elastin acts as an important ligand contributing to mycobacterial invasion.

Leptospira interrogans shotgun phage display identified LigB as a heparin-binding protein

LigB is an adhesin from pathogenic Leptospira that is able to bind to extracellular matrix and is considered a virulence factor. A shotgun phage display genomic library was constructed and used for panning against Heparan Sulfate Proteoglycan (HSPG). A phage clone encoding part of LigB protein was selected in panning experiments and showed specific binding to heparin. To validate the selected clone, fragments of LigB were produced as recombinant proteins and showed affinity to heparin and to mammalian cells. Heparin was also able to reduce the binding of rLB-Ct to mammalian cells. Our data suggests that the glycosaminoglycan moiety of the HSPG is responsible for its binding and could mediate the attachment of the recombinant protein rLB-Ct. Thus, heparin may act as a receptor for Leptospira to colonize and to invade the host tissue.

CNG-modulin: a novel Ca-dependent modulator of ligand sensitivity in cone photoreceptor cGMP-gated ion channels

The transduction current in several different types of sensory neurons arises from the activity of cyclic nucleotide-gated (CNG) ion channels. The channels in these sensory neurons vary in structure and function, yet each one demonstrates calcium-dependent modulation of ligand sensitivity mediated by the interaction of the channel with a soluble modulator protein. In cone photoreceptors, the molecular identity of the modulator protein was previously unknown. We report the discovery and characterization of CNG-modulin, a novel 301 aa protein that interacts with the N terminus of the β subunit of the cGMP-gated channel and modulates the cGMP sensitivity of the channels in cone photoreceptors of striped bass (Morone saxatilis). Immunohistochemistry and single-cell PCR demonstrate that CNG-modulin is expressed in cone but not rod photoreceptors. Adding purified recombinant CNG-modulin to cone membrane patches containing the native CNG channels shifts the midpoint of cGMP dependence from ∼91 μM in the absence of Ca(2+) to ∼332 μM in the presence of 20 μM Ca(2+). At a fixed cGMP concentration, the midpoint of the Ca(2+) dependence is ∼857 nM Ca(2+). These restored physiological features are statistically indistinguishable from the effects of the endogenous modulator. CNG-modulin binds Ca(2+) with a concentration dependence that matches the calcium dependence of channel modulation. We conclude that CNG-modulin is the authentic Ca(2+)-dependent modulator of cone CNG channel ligand sensitivity. CNG-modulin is expressed in other tissues, such as brain, olfactory epithelium, and the inner ear, and may modulate the function of ion channels in those tissues as well.

Novel mycobacteria antigen 85 complex binding motif on fibronectin

The members of the antigen 85 protein family (Ag85), consisting of members Ag85A, Ag85B, and Ag85C, are the predominantly secreted proteins of mycobacteria and possess the ability to specifically interact with fibronectin (Fn). Because Fn-binding proteins are likely to be important virulence factors of Mycobacterium spp., Ag85 may contribute to the adherence, invasion, and dissemination of organisms in host tissue. In this study, we reported the Fn binding affinity of Ag85A, Ag85B, and Ag85C from Mycobacterium avium subsp. paratuberculosis (MAP) (K(D) values were determined from 33.6 to 68.4 nm) and mapped the Ag85-binding motifs of Fn. Fn14, a type III module located on the heparin-binding domain II (Hep-2) of Fn, was discovered to interact with Ag85 from MAP. The peptide inhibition assay subsequently demonstrated that a peptide consisting of residues 17-26 from Fn14 ((17)SLLVSWQPPR(26), termed P17-26) could interfere with Ag85B binding to Fn (73.3% reduction). In addition, single alanine substitutions along the sequence of P17-26 revealed that the key residues involved in Ag85-Fn binding likely contribute through hydrophobic and charge interactions. Moreover, binding of Ag85 on Fn siRNA-transfected Caco2 cells was dramatically reduced (44.6%), implying the physiological significance of the Ag85-Fn interaction between mycobacteria and host cells during infection. Our results indicate that Ag85 binds to Fn at a novel motif and plays a critical role in mycobacteria adherence to host cells by initiating infection. Ag85 might serve as an important colonization factor potentially contributing to mycobacterial virulence.

Ca-binding and spectral properties of the common region of surface-exposed Lig proteins of leptospira

Pathogenic Leptospira protein LigA and LigB are conserved at the N-terminal sequence. In our earlier report, we have presented the spectral properties of individual Big domain of Lig proteins, and showed that an individual domain binds Ca(2+). Here we demonstrate that apart from Ca(2+)-binding properties, the spectral properties (such as doublet Trp fluorescence) shown by an individual domain are almost retained in the protein with many such domains (which could easily be called a multimer of an individual tandem repeat). Presence of Asp and Asn in a stretch of sequence in all tandem repeats points towards the possibility of their involvement in Ca(2+)-binding.

Monoclonal antibodies against the leptospiral immunoglobulin-like proteins A and B conserved regions

Leptospirosis is an infectious disease caused by pathogenic spirochetes of the genus Leptospira that affects humans and a wide variety of animals. Recently the genomes of Leptospira interrogans, Leptospira borgpetersenii and Leptospira biflexa species were sequenced allowing the identification of new virulence factors involved in survival and pathogenesis of bacteria. LigA and LigB are surface-exposed bacterial adhesins whose expression is correlated with the virulence of Leptospira strains. In this study, we produced and characterized five monoclonal antibodies (MAbs) against a recombinant fragment of LigB (rLigBrep) with approximately 54kDa that comprise the portions of LigA and LigB (domains 2-7). The 5 MAbs obtained were of the IgG1 (2) and IgG2b (3) isotypes and their affinity constants for rLigBrep ranged from 7×10(7) M(-1) to 4×10(8) M(-1). The MAbs were able to react with the native antigen on the L. interrogans, L. borgpetersenii and Leptospira noguchii surfaces by indirect immunofluorescence, immunoblotting and immunoelectron microscopy. These results demonstrate that the MAbs anti-rLigBrep can be useful to complement genetic studies and to aid studies aiming understanding the role of Lig proteins in Leptospira pathogenesis and the development of Lig-based vaccines and improved diagnostic tests for leptospirosis.

Evaluation of novel fusion proteins derived from extracellular matrix binding domains of LigB as vaccine candidates against leptospirosis in a hamster model

Leptospira binds to host extracellular matrix (ECM) through surface exposed outer membrane proteins called adhesin in order to initiate infection. Of various adhesins present on the surface of the spirochete, Leptospira-immunoglobulin like proteins (Lig proteins) and LipL32 are most abundant, widely distributed among pathogenic serovars and well characterized. Various fragments of Lig proteins (Ligcon4, Ligcon4-7.5, LigBcen2) and C-terminus fragment of LipL32 all of that bind to host ECM were fused, expressed and purified in soluble form as fusion proteins. Four week hamsters were immunized subcutaneously with various fusion proteins emulsified in EMULSIGEN-D adjuvant and subsequently boosted at 3 weeks. The protective efficacy of these novel fusion proteins was evaluated against subsequent challenge with highly virulent L. interrogans serovar Pomona (MLD50-100). Our results indicate that fusion protein based vaccine induced significant protection against acute infection with respect to PBS-adjuvant vaccinated controls as revealed by enhanced survival and reduced pulmonary hemorrhage. Moreover, the protection mediated by these novel proteins was higher than that of conserved region of Lig protein (Ligcon, established protective antigen) and correlated to the level of antibodies. LipL32 failed to impart significant protection, however fusing its immunogenic C-terminus domain to Lig fragments slightly delayed the morbidity of the infected animals. Our results demonstrate that this novel strategy could be promising in developing effective subunit vaccine to combat this zoonotic infection.

Heterologous expression of pathogen-specific genes ligA and ligB in the saprophyte Leptospira biflexa confers enhanced adhesion to cultured cells and fibronectin

Background

In comparison to other bacterial pathogens, our knowledge of the molecular basis of the pathogenesis of leptospirosis is extremely limited. An improved understanding of leptospiral pathogenetic mechanisms requires reliable tools for functional genetic analysis. Leptospiral immunoglobulin-like (Lig) proteins are surface proteins found in pathogenic Leptospira, but not in saprophytes. Here, we describe a system for heterologous expression of the Leptospira interrogans genes ligA and ligB in the saprophyte Leptospira biflexa serovar Patoc.

Results

The genes encoding LigA and LigB under the control of a constitutive spirochaetal promoter were inserted into the L. biflexa replicative plasmid. We were able to demonstrate expression and surface localization of LigA and LigB in L. biflexa. We found that the expression of the lig genes significantly enhanced the ability of transformed L. biflexa to adhere in vitro to extracellular matrix components and cultured cells, suggesting the involvement of Lig proteins in cell adhesion.

Conclusions

This work reports a complete description of the system we have developed for heterologous expression of pathogen-specific proteins in the saprophytic L. biflexa. We show that expression of LigA and LigB proteins from the pathogen confers a virulence-associated phenotype on L. biflexa, namely adhesion to eukaryotic cells and fibronectin in vitro. This study indicates that L. biflexa can serve as a surrogate host to characterize the role of key virulence factors of the causative agent of leptospirosis.

Comparative proteogenomic analysis of the Leptospira interrogans virulence-attenuated strain IPAV against the pathogenic strain 56601

The virulence-attenuated Leptospira interrogans serovar Lai strain IPAV was derived by prolonged laboratory passage from a highly virulent ancestral strain isolated in China. We studied the genetic variations of IPAV that render it avirulent via comparative analysis against the pathogenic L. interrogans serovar Lai strain 56601. The complete genome sequence of the IPAV strain was determined and used to compare with, and then rectify and reannotate the genome sequence of strain 56601. Aside from their highly similar genomic structure and gene order, a total of 33 insertions, 53 deletions and 301 single-nucleotide variations (SNVs) were detected throughout the genome of IPAV directly affecting 101 genes, either in their 5' upstream region or within their coding region. Among them, the majority of the 44 functional genes are involved in signal transduction, stress response, transmembrane transport and nitrogen metabolism. Comparative proteomic analysis based on quantitative liquid chromatography (LC)-MS/MS data revealed that among 1 627 selected pairs of orthologs, 174 genes in the IPAV strain were upregulated, with enrichment mainly in classes of energy production and lipid metabolism. In contrast, 228 genes in strain 56601 were upregulated, with the majority enriched in the categories of protein translation and DNA replication/repair. The combination of genomic and proteomic approaches illustrated that altered expression or mutations in critical genes, such as those encoding a Ser/Thr kinase, carbon-starvation protein CstA, glutamine synthetase, GTP-binding protein BipA, ribonucleotide-diphosphate reductase and phosphate transporter, and alterations in the translational profile of lipoproteins or outer membrane proteins are likely to account for the virulence attenuation in strain IPAV.

Leptospira as an emerging pathogen: a review of its biology, pathogenesis and host immune responses

Leptospirosis, the most widespread zoonosis in the world, is an emerging public health problem, particularly in large urban centers of developing countries. Several pathogenic species of the genus Leptospira can cause a wide range of clinical manifestations, from a mild, flu-like illness to a severe disease form characterized by multiorgan system complications leading to death. However, the mechanisms of pathogenesis of Leptospira are largely unknown. This article will address the animal models of acute and chronic leptospire infections, and the recent developments in the genetic manipulation of the bacteria, which facilitate the identification of virulence factors involved in pathogenesis and the assessment of their potential values in the control and prevention of leptospirosis.

Leptospira immunoglobulin-like protein B (LigB) binding to the C-terminal fibrinogen αC domain inhibits fibrin clot formation, platelet adhesion and aggregation

Leptospira immunoglobulin-like (Lig) proteins including LigA and LigB are adhesins that bind to fibronectin, collagen, laminin and elastin. In addition, Lig proteins are fibrinogen (Fg)-binding proteins, although the physiological role of the Lig-Fg interaction is unclear. In this study, a previously identified Fg-binding region, LigBCen2 (amino acids 1014-1165 of LigB), has been further localized to LigBCen2R, which consists of the partial 11th and entire 12th Ig-like domain (amino acids 1014-1119). LigBCen2R was found to bind to the C-terminal αC domain of Fg (FgαCC; amino acids 392-644 in Fg α chain; isothermal titration calorimetry, K(D) = 0.375 µM; fluorescence spectrometry, K(D) = 0.364 µM). The quenching and blue shift observed for the maximum wavelength intensities of the tryptophan fluorescence spectra for FgαCCY570W upon LigBCen2RW1073C binding suggested an RGD motif close to the sole tryptophan on FgαCCY570W was buried in LigBCen2R upon saturation with FgαCC. A conformational change in LigBCen2R when bound to the FgαCC RGD motif blocked further binding to integrin α(IIb) β3 on platelets, thus preventing their aggregation. LigBCen2R binding to FgαCC reduced clot formation but did not affect plasminogen and tissue-type plasminogen activator interactions with FgαCC. This study is the first to report that a spirochaetal protein binds to the C-terminal αC domain of Fg, which regulates thrombosis and fibrinolysis, and may help explain the pulmonary haemorrhage and thrombocytopenia seen in clinical cases of leptospirosis.

Big domains are novel Ca²+-binding modules: evidences from big domains of Leptospira immunoglobulin-like (Lig) proteins

BACKGROUND

Many bacterial surface exposed proteins mediate the host-pathogen interaction more effectively in the presence of Ca²+. Leptospiral immunoglobulin-like (Lig) proteins, LigA and LigB, are surface exposed proteins containing Bacterial immunoglobulin like (Big) domains. The function of proteins which contain Big fold is not known. Based on the possible similarities of immunoglobulin and βγ-crystallin folds, we here explore the important question whether Ca²+ binds to a Big domains, which would provide a novel functional role of the proteins containing Big fold.

PRINCIPAL FINDINGS

We selected six individual Big domains for this study (three from the conserved part of LigA and LigB, denoted as Lig A3, Lig A4, and LigBCon5; two from the variable region of LigA, i.e., 9(th) (Lig A9) and 10(th) repeats (Lig A10); and one from the variable region of LigB, i.e., LigBCen2. We have also studied the conserved region covering the three and six repeats (LigBCon1-3 and LigCon). All these proteins bind the calcium-mimic dye Stains-all. All the selected four domains bind Ca²+ with dissociation constants of 2-4 µM. Lig A9 and Lig A10 domains fold well with moderate thermal stability, have β-sheet conformation and form homodimers. Fluorescence spectra of Big domains show a specific doublet (at 317 and 330 nm), probably due to Trp interaction with a Phe residue. Equilibrium unfolding of selected Big domains is similar and follows a two-state model, suggesting the similarity in their fold.

CONCLUSIONS

We demonstrate that the Lig are Ca²+-binding proteins, with Big domains harbouring the binding motif. We conclude that despite differences in sequence, a Big motif binds Ca²+. This work thus sets up a strong possibility for classifying the proteins containing Big domains as a novel family of Ca²+-binding proteins. Since Big domain is a part of many proteins in bacterial kingdom, we suggest a possible function these proteins via Ca²+ binding.

Manganese binds to Clostridium difficile Fbp68 and is essential for fibronectin binding

Clostridium difficile is an etiological agent of pseudomembranous colitis and antibiotic-associated diarrhea. Adhesion is the crucial first step in bacterial infection. Thus, in addition to toxins, the importance of colonization factors in C. difficile-associated disease is recognized. In this study, we identified Fbp68, one of the colonization factors that bind to fibronectin (Fn), as a manganese-binding protein (K(D) = 52.70 ± 1.97 nM). Furthermore, the conformation of Fbp68 changed dramatically upon manganese binding. Manganese binding can also stabilize the structure of Fbp68 as evidenced by the increased T(m) measured by thermodenatured circular dichroism and differential scanning calorimetry (CD, T(m) = 58-65 °C; differential scanning calorimetry, T(m) = 59-66 °C). In addition, enhanced tolerance to protease K also suggests greatly improved stability of Fbp68 through manganese binding. Fn binding activity was found to be dependent on manganese due to the lack of binding by manganese-free Fbp68 to Fn. The C-terminal 194 amino acid residues of Fbp68 (Fbp68C) were discovered to bind to the N-terminal domain of Fn (Fbp68C-NTD, K(D) = 233 ± 10 nM, obtained from isothermal titration calorimetry). Moreover, adhesion of C. difficile to Caco-2 cells can be partially blocked if cells are pretreated with Fbp68C, and the binding of Fbp68C on Fn siRNA-transfected cells was significantly reduced. These results raise the possibility that Fbp68 plays a key role in C. difficile adherence on host cells to initiate infection.

Transcriptional responses of Leptospira interrogans to host innate immunity: significant changes in metabolism, oxygen tolerance, and outer membrane

Background

Leptospira interrogans is the major causative agent of leptospirosis. Phagocytosis plays important roles in the innate immune responses to L. interrogans infection, and L. interrogans can evade the killing of phagocytes. However, little is known about the adaptation of L. interrogans during this process.

Methodology/Principal Findings

To better understand the interaction of pathogenic Leptospira and innate immunity, we employed microarray and comparative genomics analyzing the responses of L. interrogans to macrophage-derived cells. During this process, L. interrogans altered expressions of many genes involved in carbohydrate and lipid metabolism, energy production, signal transduction, transcription and translation, oxygen tolerance, and outer membrane proteins. Among them, the catalase gene expression was significantly up-regulated, suggesting it may contribute to resisting the oxidative pressure of the macrophages. The expressions of several major outer membrane protein (OMP) genes (e.g., ompL1, lipL32, lipL41, lipL48 and ompL47) were dramatically down-regulated (10–50 folds), consistent with previous observations that the major OMPs are differentially regulated in vivo. The persistent down-regulations of these major OMPs were validated by immunoblotting. Furthermore, to gain initial insight into the gene regulation mechanisms in L. interrogans, we re-defined the transcription factors (TFs) in the genome and identified the major OmpR TF gene (LB333) that is concurrently regulated with the major OMP genes, suggesting a potential role of LB333 in OMPs regulation.

Conclusions/Significance

This is the first report on global responses of pathogenic Leptospira to innate immunity, which revealed that the down-regulation of the major OMPs may be an immune evasion strategy of L. interrogans, and a putative TF may be involved in governing these down-regulations. Alterations of the leptospiral OMPs up interaction with host antigen-presenting cells (APCs) provide critical information for selection of vaccine candidates. In addition, genome-wide annotation and comparative analysis of TFs set a foundation for further studying regulatory networks in Leptospira spp.

Leptospirosis is an important tropical disease around the world, particularly in humid tropical and subtropical countries. As a major pathogen of this disease, Leptospira interrogans can be shed from the urine of reservoir hosts, survive in soil and water, and infect humans through broken skin or mucous membranes. Recently, host adaptability and immune evasion of L. interrogans to host innate immunity was partially elucidated in infection or animal models. A better understanding of the molecular mechanisms of L. interrogans in response to host innate immunity is required to learn the nature of early leptospirosis. This study focused on the transcriptome of L. interrogans during host immune cells interaction. Significant changes in energy metabolism, oxygen tolerance and outer membrane protein profile were identified as potential immune evasion strategies by pathogenic Leptospira during the early stage of infection. The major outer membrane proteins (OMPs) of L. interrogans may be regulated by the major OmpR specific transcription factor (LB333). These results provide a foundation for further studying the pathogenesis of leptospirosis, as well as identifying gene regulatory networks in Leptospira spp.

Leptospira: a spirochaete with a hybrid outer membrane

Leptospira is a genus of spirochaetes that includes organisms with a variety of lifestyles ranging from aquatic saprophytes to invasive pathogens. Adaptation to a wide variety of environmental conditions has required leptospires to acquire a large genome and a complex outer membrane with features that are unique among bacteria. The most abundant surface-exposed outer membrane proteins are lipoproteins that are integrated into the lipid bilayer by amino-terminal fatty acids. In contrast to many spirochaetes, the leptospiral outer membrane also includes lipopolysaccharide and many homologues of well-known beta-barrel transmembrane outer membrane proteins. Research on leptospiral transmembrane outer membrane proteins has lagged behind studies of lipoproteins because of their aberrant behaviour by Triton X-114 detergent fractionation. For this reason, transmembrane outer membrane proteins are best characterized by assessing membrane integration and surface exposure. Not surprisingly, some outer membrane proteins that mediate host-pathogen interactions are strongly regulated by conditions found in mammalian host tissues. For example, the leptospiral immunoglobulin-like (Lig) repeat proteins are dramatically induced by osmolarity and mediate interactions with host extracellular matrix proteins. Development of molecular genetic tools are making it possible to finally understand the roles of these and other outer membrane proteins in mechanisms of leptospiral pathogenesis.

The terminal immunoglobulin-like repeats of LigA and LigB of Leptospira enhance their binding to gelatin binding domain of fibronectin and host cells

Leptospira spp. are pathogenic spirochetes that cause the zoonotic disease leptospirosis. Leptospiral immunoglobulin (Ig)-like protein B (LigB) contributes to the binding of Leptospira to extracellular matrix proteins such as fibronectin, fibrinogen, laminin, elastin, tropoelastin and collagen. A high-affinity Fn-binding region of LigB has been localized to LigBCen2, which contains the partial 11th and full 12th Ig-like repeats (LigBCen2R) and 47 amino acids of the non-repeat region (LigBCen2NR) of LigB. In this study, the gelatin binding domain of fibronectin was shown to interact with LigBCen2R (K(D) = 1.91+/-0.40 microM). Not only LigBCen2R but also other Ig-like domains of Lig proteins including LigAVar7'-8, LigAVar10, LigAVar11, LigAVar12, LigAVar13, LigBCen7'-8, and LigBCen9 bind to GBD. Interestingly, a large gain in affinity was achieved through an avidity effect, with the terminal domains, 13th (LigA) or 12th (LigB) Ig-like repeat of Lig protein (LigAVar7'-13 and LigBCen7'-12) enhancing binding affinity approximately 51 and 28 fold, respectively, compared to recombinant proteins without this terminal repeat. In addition, the inhibited effect on MDCKs cells can also be promoted by Lig proteins with terminal domains, but these two domains are not required for gelatin binding domain binding and cell adhesion. Interestingly, Lig proteins with the terminal domains could form compact structures with a round shape mediated by multidomain interaction. This is the first report about the interaction of gelatin binding domain of Fn and Lig proteins and provides an example of Lig-gelatin binding domain binding mediating bacterial-host interaction.

Global transcriptomic response of Leptospira interrogans serovar Copenhageni upon exposure to serum

Background

Leptospirosis is a zoonosis of worldwide distribution caused by infection with pathogenic serovars of Leptospira spp. The most common species, L. interrogans, can survive in the environment for lengthy periods of time in between infection of mammalian hosts. Transmission of pathogenic Leptospira to humans mostly occurs through abraded skin or mucosal surfaces after direct or indirect contact with infected animals or contaminated soil or water. The spirochete then spreads hematogenously, resulting in multi-organ failure and death in severe cases. Previous DNA microarray studies have identified differentially expressed genes required for adaptation to temperature and osmolarity conditions inside the host compared to those of the environment.

Results

In order to identify genes involved in survival in the early spirochetemic phase of infection, we performed a transcriptional analysis of L. interrogans serovar Copenhageni upon exposure to serum in comparison with EMJH medium. One hundred and sixty-eight genes were found to be differentially expressed, of which 55 were up-regulated and 113 were down-regulated. Genes of known or predicted function accounted for 54.5 and 45.1% of up- and down-regulated genes, respectively. Most of the differentially expressed genes were predicted to be involved in transcriptional regulation, translational process, two-component signal transduction systems, cell or membrane biogenesis, and metabolic pathways.

Conclusions

Our study showed global transcriptional changes of pathogenic Leptospira upon exposure to serum, representing a specific host environmental cue present in the bloodstream. The presence of serum led to a distinct pattern of gene expression in comparison to those of previous single-stimulus microarray studies on the effect of temperature and osmolarity upshift. The results provide insights into the pathogenesis of leptospirosis during the early bacteremic phase of infection.

Calcium binds to LipL32, a lipoprotein from pathogenic Leptospira, and modulates fibronectin binding

Tubulointerstitial nephritis is a cardinal renal manifestation of leptospirosis. LipL32, a major lipoprotein and a virulence factor, locates on the outer membrane of the pathogen Leptospira. It evades immune response by recognizing and adhering to extracellular matrix components of the host cell. The crystal structure of Ca(2+)-bound LipL32 was determined at 2.3 A resolution. LipL32 has a novel polyD sequence of seven aspartates that forms a continuous acidic surface patch for Ca(2+) binding. A significant conformational change was observed for the Ca(2+)-bound form of LipL32. Calcium binding to LipL32 was determined by isothermal titration calorimetry. The binding of fibronectin to LipL32 was observed by Stains-all CD and enzyme-linked immunosorbent assay experiments. The interaction between LipL32 and fibronectin might be associated with Ca(2+) binding. Based on the crystal structure of Ca(2+)-bound LipL32 and the Stains-all results, fibronectin probably binds near the polyD region on LipL32. Ca(2+) binding to LipL32 might be important for Leptospira to interact with the extracellular matrix of the host cell.

Leptospira: the dawn of the molecular genetics era for an emerging zoonotic pathogen

Leptospirosis is a zoonotic disease that has emerged as an important cause of morbidity and mortality among impoverished populations. One hundred years after the discovery of the causative spirochaetal agent, little is understood about Leptospira spp. pathogenesis, which in turn has hampered the development of new intervention strategies to address this neglected disease. However, the recent availability of complete genome sequences for Leptospira spp. and the discovery of genetic tools for their transformation have led to important insights into the biology of these pathogens and their pathogenesis. We discuss the life cycle of the bacterium, the recent advances in our understanding and the implications for the future prevention of leptospirosis.