Cross-species surface display of functional spirochetal lipoproteins by recombinant Borrelia burgdorferi

A Borrelia burgdorferi LptD homolog is required for flipping of surface lipoproteins through the spirochetal outer membrane

Borrelia spirochetes are unique among diderm bacteria in their lack of lipopolysaccharide (LPS) in the outer membrane (OM) and their abundance of surface-exposed lipoproteins with major roles in transmission, virulence, and pathogenesis. Despite their importance, little is known about how surface lipoproteins are translocated through the periplasm and the OM. Here, we characterized Borrelia burgdorferi BB0838, a distant homolog of the OM LPS assembly protein LptD. Using a CRISPR interference approach, we showed that BB0838 is required for cell growth and envelope stability. Upon BB0838 knockdown, surface lipoprotein OspA was retained in the inner leaflet of the OM, as determined by its inaccessibility to in situ proteolysis but its presence in OM vesicles. The topology of the OM porin/adhesin P66 remained unaffected. Quantitative mass spectrometry of the B. burgdorferi membrane-associated proteome confirmed the selective periplasmic retention of surface lipoproteins under BB0838 knockdown conditions. Additional analysis identified a single in situ protease-accessible BB0838 peptide that mapped to a predicted β-barrel surface loop. Alphafold Multimer modeled a B. burgdorferi LptB2 FGCAD complex spanning the periplasm. Together, this suggests that BB0838/LptDBb facilitates the essential terminal step in spirochetal surface lipoprotein secretion, using an orthologous OM component of a pathway that secretes LPS in proteobacteria.

Identification of a novel transport system in Borrelia burgdorferi that links the inner and outer membranes

Borrelia burgdorferi, the spirochete that causes Lyme disease, is a diderm organism that is similar to Gram-negative organisms in that it contains both an inner and outer membrane. Unlike typical Gram-negative organisms, however, B. burgdorferi lacks lipopolysaccharide (LPS). Using computational genome analyses and structural modeling, we identified a transport system containing six proteins in B. burgdorferi that are all orthologs to proteins found in the lipopolysaccharide transport (LPT) system that links the inner and outer membranes of Gram-negative organisms and is responsible for placing LPS on the surface of these organisms. While B. burgdorferi does not contain LPS, it does encode over 100 different surface-exposed lipoproteins and several major glycolipids, which like LPS are also highly amphiphilic molecules, though no system to transport these molecules to the borrelial surface is known. Accordingly, experiments supplemented by molecular modeling were undertaken to determine whether the orthologous LPT system identified in B. burgdorferi could transport lipoproteins and/or glycolipids to the borrelial outer membrane. Our combined observations strongly suggest that the LPT transport system does not transport lipoproteins to the surface. Molecular dynamic modeling, however, suggests that the borrelial LPT system could transport borrelial glycolipids to the outer membrane.

Pathogenesis of Relapsing Fever

Relapsing fever (RF) is caused by several species of Borrelia; all, except two species, are transmitted to humans by soft (argasid) ticks. The species B. recurrentis is transmitted from one human to another by the body louse, while B. miyamotoi is vectored by hard-bodied ixodid tick species. RF Borrelia have several pathogenic features that facilitate invasion and dissemination in the infected host. In this article we discuss the dynamics of vector acquisition and subsequent transmission of RF Borrelia to their vertebrate hosts. We also review taxonomic challenges for RF Borrelia as new species have been isolated throughout the globe. Moreover, aspects of pathogenesis including symptomology, neurotropism, erythrocyte and platelet adhesion are discussed. We expound on RF Borrelia evasion strategies for innate and adaptive immunity, focusing on the most fundamental pathogenetic attributes, multiphasic antigenic variation. Lastly, we review new and emerging species of RF Borrelia and discuss future directions for this global disease.

Lyme Disease Pathogenesis

Lyme disease Borrelia are obligately parasitic, tick- transmitted, invasive, persistent bacterial pathogens that cause disease in humans and non-reservoir vertebrates primarily through the induction of inflammation. During transmission from the infected tick, the bacteria undergo significant changes in gene expression, resulting in adaptation to the mammalian environment. The organisms multiply and spread locally and induce inflammatory responses that, in humans, result in clinical signs and symptoms. Borrelia virulence involves a multiplicity of mechanisms for dissemination and colonization of multiple tissues and evasion of host immune responses. Most of the tissue damage, which is seen in non-reservoir hosts, appears to result from host inflammatory reactions, despite the low numbers of bacteria in affected sites. This host response to the Lyme disease Borrelia can cause neurologic, cardiovascular, arthritic, and dermatologic manifestations during the disseminated and persistent stages of infection. The mechanisms by which a paucity of organisms (in comparison to many other infectious diseases) can cause varied and in some cases profound inflammation and symptoms remains mysterious but are the subjects of diverse ongoing investigations. In this review, we provide an overview of virulence mechanisms and determinants for which roles have been demonstrated in vivo, primarily in mouse models of infection.

The BB0345 Hypothetical Protein of Borrelia burgdorferi Is Essential for Mammalian Infection

During the natural enzootic life cycle of Borrelia burgdorferi (also known as Borreliella burgdorferi), the bacteria must sense conditions within the vertebrate and arthropod and appropriately regulate expression of genes necessary to persist within these distinct environments. bb0345 of B. burgdorferi encodes a hypothetical protein of unknown function that is predicted to contain an N-terminal helix-turn-helix (HTH) domain. Because HTH domains can mediate protein-DNA interactions, we hypothesized that BB0345 might represent a previously unidentified borrelial transcriptional regulator with the ability to regulate events critical for the B. burgdorferi enzootic cycle. To study the role of BB0345 within mammals, we generated a bb0345 mutant and assessed its virulence potential in immunocompetent mice. The bb0345 mutant was able to initiate localized infection and disseminate to distal tissues but was cleared from all sites by 14 days postinfection. In vitro growth curve analyses revealed that the bb0345 mutant grew similar to wild-type bacteria in standard Barbour-Stoenner-Kelley II (BSK-II) medium; however, the mutant was not able to grow in dilute BSK-II medium or dialysis membrane chambers (DMCs) implanted in rats. Proteinase K accessibility assays and whole-cell partitioning indicated that BB0345 was intracellular and partially membrane associated. Comparison of protein production profiles between the wild-type parent and the bb0345 mutant revealed no major differences, suggesting BB0345 may not be a global transcriptional regulator. Taken together, these data show that BB0345 is essential for B. burgdorferi survival in the mammalian host, potentially by aiding the spirochete with a physiological function that is required by the bacterium during infection.

Immune Evasion Strategies of Relapsing Fever Spirochetes

Relapsing fever (RF) is claimed a neglected arthropod-borne disease caused by a number of diverse human pathogenic Borrelia (B.) species. These RF borreliae are separated into the groups of tick-transmitted species including B. duttonii, B. hermsii, B. parkeri, B. turicatae, B. hispanica, B. persica, B. caucasica, and B. myiamotoi, and the louse-borne Borrelia species B. recurrentis. As typical blood-borne pathogens achieving high cell concentrations in human blood, RF borreliae (RFB) must outwit innate immunity, in particular complement as the first line of defense. One prominent strategy developed by RFB to evade innate immunity involves inactivation of complement by recruiting distinct complement regulatory proteins, e.g., C1 esterase inhibitor (C1-INH), C4b-binding protein (C4BP), factor H (FH), FH-like protein-1 (FHL-1), and factor H-related proteins FHR-1 and FHR-2, or binding of individual complement components and plasminogen, respectively. A number of multi-functional, complement and plasminogen-binding molecules from distinct Borrelia species have previously been identified and characterized, exhibiting considerable heterogeneity in their sequences, structures, gene localization, and their capacity to bind host-derived proteins. In addition, RFB possess a unique system of antigenic variation, allowing them to change the composition of surface-exposed variable major proteins, thus evading the acquired immune response of the human host. This review focuses on the current knowledge of the immune evasion strategies by RFB and highlights the role of complement-interfering and infection-associated molecules for the pathogenesis of RFB.

The RpoS Gatekeeper in Borrelia burgdorferi: An Invariant Regulatory Scheme That Promotes Spirochete Persistence in Reservoir Hosts and Niche Diversity

Maintenance of Borrelia burgdorferi within its enzootic cycle requires a complex regulatory pathway involving the alternative σ factors RpoN and RpoS and two ancillary trans-acting factors, BosR and Rrp2. Activation of this pathway occurs within ticks during the nymphal blood meal when RpoS, the effector σ factor, transcribes genes required for tick transmission and mammalian infection. RpoS also exerts a 'gatekeeper' function by repressing σ⁷⁰-dependent tick phase genes (e.g., ospA, lp6.6). Herein, we undertook a broad examination of RpoS functionality throughout the enzootic cycle, beginning with modeling to confirm that this alternative σ factor is a 'genuine' RpoS homolog. Using a novel dual color reporter system, we established at the single spirochete level that ospA is expressed in nymphal midguts throughout transmission and is not downregulated until spirochetes have been transmitted to a naïve host. Although it is well established that rpoS/RpoS is expressed throughout infection, its requirement for persistent infection has not been demonstrated. Plasmid retention studies using a trans-complemented ΔrpoS mutant demonstrated that (i) RpoS is required for maximal fitness throughout the mammalian phase and (ii) RpoS represses tick phase genes until spirochetes are acquired by a naïve vector. By transposon mutant screening, we established that bba34/oppA5, the only OppA oligopeptide-binding protein controlled by RpoS, is a bona fide persistence gene. Lastly, comparison of the strain 297 and B31 RpoS DMC regulons identified two cohorts of RpoS-regulated genes. The first consists of highly conserved syntenic genes that are similarly regulated by RpoS in both strains and likely required for maintenance of B. burgdorferi sensu stricto strains in the wild. The second includes RpoS-regulated plasmid-encoded variable surface lipoproteins ospC, dbpA and members of the ospE/ospF/elp, mlp, revA, and Pfam54 paralogous gene families, all of which have evolved via inter- and intra-strain recombination. Thus, while the RpoN/RpoS pathway regulates a 'core' group of orthologous genes, diversity within RpoS regulons of different strains could be an important determinant of reservoir host range as well as spirochete virulence.

NMR metabolome of Borrelia burgdorferi in vitro and in vivo in mice

Lyme borreliosis (LB), caused by bacteria of the Borrelia burgdorferi sensu lato (Borrelia) species, is the most common tick-borne infection in the northern hemisphere. LB diagnostics is based on clinical evaluation of the patient and on laboratory testing, where the main method is the detection of Borrelia specific antibodies in patient samples. There are, however, shortcomings in the current serology based LB diagnostics, especially its inability to differentiate ongoing infection from a previously treated one. Identification of specific biomarkers of diseases is a growing application of metabolomics. One of the main methods of metabolomics is nuclear magnetic resonance (NMR) spectroscopy. In the present study, our aim was to analyze whether Borrelia growth in vitro and infection in vivo in mice causes specific metabolite differences, and whether NMR can be used to detect them. For this purpose, we performed NMR analyses of in vitro culture medium samples, and of serum and urine samples of Borrelia infected and control mice. The results show, that there were significant differences in the concentrations of several amino acids, energy metabolites and aromatic compounds between Borrelia culture and control media, and between infected and control mouse serum and urine samples. For example, the concentration of L-phenylalanine increases in the Borrelia growth medium and in serum of infected mice, whereas the concentrations of allantoin and trigonelline decrease in the urine of infected mice. Therefore, we conclude that Borrelia infection causes measurable metabolome differences in vitro and in Borrelia infected mouse serum and urine samples, and that these can be detected with NMR.

Comprehensive Spatial Analysis of the Borrelia burgdorferi Lipoproteome Reveals a Compartmentalization Bias toward the Bacterial Surface

The Lyme disease spirochete Borrelia burgdorferi is unique among bacteria in its large number of lipoproteins that are encoded by a small, exceptionally fragmented, and predominantly linear genome. Peripherally anchored in either the inner or outer membrane and facing either the periplasm or the external environment, these lipoproteins assume varied roles. A prominent subset of lipoproteins functioning as the apparent linchpins of the enzootic tick-vertebrate infection cycle have been explored as vaccine targets. Yet, most of the B. burgdorferi lipoproteome has remained uncharacterized. Here, we comprehensively and conclusively localize the B. burgdorferi lipoproteome by applying established protein localization assays to a newly generated epitope-tagged lipoprotein expression library and by validating the obtained individual protein localization results using a sensitive global mass spectrometry approach. The derived consensus localization data indicate that 86 of the 125 analyzed lipoproteins encoded by B. burgdorferi are secreted to the bacterial surface. Thirty-one of the remaining 39 periplasmic lipoproteins are retained in the inner membrane, with only 8 lipoproteins being anchored in the periplasmic leaflet of the outer membrane. The localization of 10 lipoproteins was further defined or revised, and 52 surface and 23 periplasmic lipoproteins were newly localized. Cross-referencing prior studies revealed that the borrelial surface lipoproteome contributing to the host-pathogen interface is encoded predominantly by plasmids. Conversely, periplasmic lipoproteins are encoded mainly by chromosomal loci. These studies close a gap in our understanding of the functional lipoproteome of an important human pathogen and set the stage for more in-depth studies of thus-far-neglected spirochetal lipoproteins.IMPORTANCE The small and exceptionally fragmented genome of the Lyme disease spirochete Borrelia burgdorferi encodes over 120 lipoproteins. Studies in the field have predominantly focused on a relatively small number of surface lipoproteins that play important roles in the transmission and pathogenesis of this global human pathogen. Yet, a comprehensive spatial assessment of the entire borrelial lipoproteome has been missing. The current study newly identifies 52 surface and 23 periplasmic lipoproteins. Overall, two-thirds of the B. burgdorferi lipoproteins localize to the surface, while outer membrane lipoproteins facing the periplasm are rare. This analysis underscores the dominant contribution of lipoproteins to the spirochete's rather complex and adaptable host-pathogen interface, and it encourages further functional exploration of its lipoproteome.

Surface-Exposed Lipoproteins: An Emerging Secretion Phenomenon in Gram-Negative Bacteria

Bacterial lipoproteins are hydrophilic proteins that are anchored to a cell membrane by N-terminally linked fatty acids. It is widely believed that nearly all lipoproteins produced by Gram-negative bacteria are either retained in the inner membrane (IM) or transferred to the inner leaflet of the outer membrane (OM). Lipoproteins that are exposed on the cell surface have also been reported but are generally considered to be rare. Results from a variety of recent studies, however, now suggest that the prevalence of surface-exposed lipoproteins has been underestimated. In this review we describe the evidence that the surface exposure of lipoproteins in Gram-negative bacteria is a widespread phenomenon and discuss possible mechanisms by which these proteins might be transported across the OM.

Whole-Chain Tick Saliva Proteins Presented on Hepatitis B Virus Capsid-Like Particles Induce High-Titered Antibodies with Neutralizing Potential

Ticks are vectors for various, including pathogenic, microbes. Tick saliva contains multiple anti-host defense factors that enable ticks their bloodmeals yet also facilitate microbe transmission. Lyme disease-causing borreliae profit specifically from the broadly conserved tick histamine release factor (tHRF), and from cysteine-rich glycoproteins represented by Salp15 from Ixodes scapularis and Iric-1 from Ixodes ricinus ticks which they recruit to their outer surface protein C (OspC). Hence these tick proteins are attractive targets for anti-tick vaccines that simultaneously impair borrelia transmission. Main obstacles are the tick proteins´ immunosuppressive activities, and for Salp15 orthologs, the lack of efficient recombinant expression systems. Here, we exploited the immune-enhancing properties of hepatitis B virus core protein (HBc) derived capsid-like particles (CLPs) to generate, in E. coli, nanoparticulate vaccines presenting tHRF and, as surrogates for the barely soluble wild-type proteins, cysteine-free Salp15 and Iric-1 variants. The latter CLPs were exclusively accessible in the less sterically constrained SplitCore system. Mice immunized with tHRF CLPs mounted a strong anti-tHRF antibody response. CLPs presenting cysteine-free Salp15 and Iric-1 induced antibodies to wild-type, including glycosylated, Salp15 and Iric-1. The broadly distributed epitopes included the OspC interaction sites. In vitro, the anti-Salp15 antibodies interfered with OspC binding and enhanced human complement-mediated killing of Salp15 decorated borreliae. A mixture of all three CLPs induced high titered antibodies against all three targets, suggesting the feasibility of combination vaccines. These data warrant in vivo validation of the new candidate vaccines´ protective potential against tick infestation and Borrelia transmission.

Secretion of bacterial lipoproteins: through the cytoplasmic membrane, the periplasm and beyond

Bacterial lipoproteins are peripherally anchored membrane proteins that play a variety of roles in bacterial physiology and virulence in monoderm (single membrane-enveloped, e.g., gram-positive) and diderm (double membrane-enveloped, e.g., gram-negative) bacteria. After export of prolipoproteins through the cytoplasmic membrane, which occurs predominantly but not exclusively via the general secretory or Sec pathway, the proteins are lipid-modified at the cytoplasmic membrane in a multistep process that involves sequential modification of a cysteine residue and cleavage of the signal peptide by the signal II peptidase Lsp. In both monoderms and diderms, signal peptide processing is preceded by acylation with a diacylglycerol through preprolipoprotein diacylglycerol transferase (Lgt). In diderms but also some monoderms, lipoproteins are further modified with a third acyl chain through lipoprotein N-acyl transferase (Lnt). Fully modified lipoproteins that are destined to be anchored in the inner leaflet of the outer membrane (OM) are selected, transported and inserted by the Lol (lipoprotein outer membrane localization) pathway machinery, which consists of the inner-membrane (IM) ABC transporter-like LolCDE complex, the periplasmic LolA chaperone and the OM LolB lipoprotein receptor. Retention of lipoproteins in the cytoplasmic membrane results from Lol avoidance signals that were originally described as the "+2 rule". Surface localization of lipoproteins in diderms is rare in most bacteria, with the exception of several spirochetal species. Type 2 (T2SS) and type 5 (T5SS) secretion systems are involved in secretion of specific surface lipoproteins of γ-proteobacteria. In the model spirochete Borrelia burgdorferi, surface lipoprotein secretion does not follow established sorting rules, but remains dependent on N-terminal peptide sequences. Secretion through the outer membrane requires maintenance of lipoproteins in a translocation-competent unfolded conformation, likely through interaction with a periplasmic holding chaperone, which delivers the proteins to an outer membrane lipoprotein flippase. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Fibronectin-binding protein of Borrelia hermsii expressed in the blood of mice with relapsing fever

To identify and characterize surface proteins expressed by the relapsing fever (RF) agent Borrelia hermsii in the blood of infected mice, we used a cell-free filtrate of their blood to immunize congenic naive mice. The resultant antiserum was used for Western blotting of cell lysates, and gel slices corresponding to reactive bands were subjected to liquid chromatography-tandem mass spectrometry, followed by a search of the proteome database with the peptides. One of the immunogens was identified as the BHA007 protein, which is encoded by a 174-kb linear plasmid. BHA007 had sequence features of lipoproteins, was surface exposed by the criteria of in situ protease susceptibility and agglutination of Vtp(-) cells by anti-BHA007 antibodies, and was not essential for in vitro growth. BHA007 elicited antibodies during experimental infection of mice, but immunization with recombinant protein did not confer protection against needle-delivered infection. Open reading frames (ORFs) orthologous to BHA007 were found on large plasmids of other RF species, including the coding sequences for the CihC proteins of Borrelia duttonii and B. recurrentis, but not in Lyme disease Borrelia species. Recombinant BHA007 bound both human and bovine fibronectin with Kd (dissociation constant) values of 22 and 33 nM, respectively, and bound to C4-binding protein with less affinity. The distant homology of BHA007 and its orthologs to BBK32 proteins of Lyme disease species, as well as to previously described BBK32-like proteins in relapsing fever species, indicates that BHA007 is a member of a large family of multifunctional proteins in Borrelia species that bind to fibronectin as well as other host proteins.

Mapping the ligand-binding region of Borrelia hermsii fibronectin-binding protein

Many pathogenic microorganisms express fibronectin-binding molecules that facilitate their adherence to the extracellular matrix and/or entry into mammalian cells. We have previously described a Borrelia recurrentis gene, cihC that encodes a 40-kDa surface receptor for both, fibronectin and the complement inhibitors C4bp and C1-Inh. We now provide evidence for the expression of a group of highly homologues surface proteins, termed FbpA, in three B. hermsii isolates and two tick-borne relapsing fever spirochetes, B. parkeri and B. turicatae. When expressed in Escherichia coli or B. burgdorferi, four out of five proteins were shown to selectively bind fibronectin, whereas none of five proteins were able to bind the human complement regulators, C4bp and C1-Inh. By applying deletion mutants of the B. hermsii fibronectin-binding proteins a putative high-affinity binding site for fibronectin was mapped to its central region. In addition, the fibronectin-binding proteins of B. hermsii were found to share sequence homology with BBK32 of the Lyme disease spirochete B. burgdorferi with similar function suggesting its involvement in persistence and/or virulence of relapsing fever spirochetes.

Contribution of the infection-associated complement regulator-acquiring surface protein 4 (ErpC) to complement resistance of Borrelia burgdorferi

Borrelia burgdorferi evades complement-mediated killing by interacting with complement regulators through distinct complement regulator-acquiring surface proteins (CRASPs). Here, we extend our analyses to the contribution of CRASP-4 in mediating complement resistance of B. burgdorferi and its interaction with human complement regulators. CRASP-4 (also known as ErpC) was immobilized onto magnetic beads and used to capture proteins from human serum. Following Western blotting, factor H (CFH), CFH-related protein 1 (CFHR1), CFHR2, and CFHR5 were identified as ligands of CRASP-4. To analyze the impact of native CRASP-4 on mediating survival of serum-sensitive cells in human serum, a B. garinii strain was generated that ectopically expresses CRASP-4. CRASP-4-producing bacteria bound CFHR1, CFHR2, and CFHR5 but not CFH. In addition, transformed spirochetes deposited significant amounts of lethal complement components on their surface and were susceptible to human serum, thus indicating that CRASP-4 plays a subordinate role in complement resistance of B. burgdorferi.

Probing the Borrelia burgdorferi surface lipoprotein secretion pathway using a conditionally folding protein domain

Surface lipoproteins of Borrelia spirochetes are important virulence determinants in the transmission and pathogenesis of Lyme disease and relapsing fever. To further define the conformational secretion requirements and to identify potential lipoprotein translocation intermediates associated with the bacterial outer membrane (OM), we generated constructs in which Borrelia burgdorferi outer surface lipoprotein A (OspA) was fused to calmodulin (CaM), a conserved eukaryotic protein undergoing calcium-dependent folding. Protein localization assays showed that constructs in which CaM was fused to full-length wild-type (wt) OspA or to an intact OspA N-terminal "tether" peptide retained their competence for OM translocation even in the presence of calcium. In contrast, constructs in which CaM was fused to truncated or mutant OspA N-terminal tether peptides were targeted to the periplasmic leaflet of the OM in the presence of calcium but could be flipped to the bacterial surface upon calcium chelation. This indicated that in the absence of an intact tether peptide, unfolding of the CaM moiety was required in order to facilitate OM traversal. Together, these data further support a periplasmic tether peptide-mediated mechanism to prevent premature folding of B. burgdorferi surface lipoproteins. The specific shift in the OM topology of sequence-identical lipopeptides due to a single-variable change in environmental conditions also indicates that surface-bound Borrelia lipoproteins can localize transiently to the periplasmic leaflet of the OM.

Determination of Borrelia surface lipoprotein anchor topology by surface proteolysis

We used a surface trypsinolysis assay to probe accessibility of the membrane-proximal N-terminal tether peptides of Borrelia surface lipoproteins OspA and Vsp1. Our findings with both wild-type and mutant proteins are only compatible with the anchoring of these surface lipoproteins in the outer leaflet of the outer spirochetal membrane.

Surface localization determinants of Borrelia OspC/Vsp family lipoproteins

The dimeric OspC/Vsp family surface lipoproteins of Borrelia spirochetes are crucial to the transmission and persistence of Lyme borreliosis and tick-borne relapsing fever. However, the requirements for their proper surface display remained undefined. In previous studies, we showed that localization of Borrelia burgdorferi monomeric surface lipoprotein OspA was dependent on residues in the N-terminal "tether" peptide. Here, site-directed mutagenesis of the B. burgdorferi OspC tether revealed two distinct regions affecting either release from the inner membrane or translocation through the outer membrane. Determinants of both of these steps appear consolidated within a single region of the Borrelia turicatae Vsp1 tether. Periplasmic OspC mutants still were able to form dimers. Their localization defect could be rescued by the addition of an apparently structure-destabilizing C-terminal epitope tag but not by coexpression with wild-type OspC. Furthermore, disruption of intermolecular Vsp1 salt bridges blocked dimerization but not surface localization of the resulting Vsp1 monomers. Together, these results suggest that Borrelia OspC/Vsp1 surface lipoproteins traverse the periplasm and the outer membrane as unfolded monomeric intermediates and assemble into their functional multimeric folds only upon reaching the spirochetal surface.

Interaction of variable bacterial outer membrane lipoproteins with brain endothelium

Background

Previously we reported that the variable outer membrane lipoprotein Vsp1 from the relapsing fever spirochete Borrelia turicatae disseminates from blood to brain better than the closely related Vsp2 [1]. Here we studied the interaction between Vsp1 and Vsp2 with brain endothelium in more detail.

Methodology/Principal Findings

We compared Vsp1 to Vsp2 using human brain microvascular endothelial cell (HBMEC) association assays with aminoacid radiolabeled Vsp-expressing clones of recombinant Borrelia burgdorferi and lanthanide-labeled purified lipidated Vsp1 (LVsp1) and Vsp2 (LVsp2) and inoculations of the lanthanide-labeled proteins into mice. The results showed that heterologous expression of LVsp1 or LVsp2 in B. burgdorferi increased its association with HBMEC to a similar degree. Purified lanthanide-labeled lipidated Vsp1 (LVsp1) and LVsp2 by themselves were capable of associating with HBMEC. The association of LVsp1 with brain endothelium was time-dependent, saturable, and required the lipidation. The association of Vsp1 with HBMEC was inhibited by incubation at lower temperature or with excess unlabeled LVsp1 or LVsp2 but not with excess rVsp1 or mouse albumin or an anti Vsp1 monoclonal antibody. The association of LVsp2 with HBMEC and its movement from blood to brain parenchyma significantly increased in the presence of LVsp1.

Conclusions/Significance

Variable bacterial outer membrane lipoproteins interact with brain endothelium differently; the lipidation and variable features at the protein dome region are key modulators of this interaction.

Human complement regulators C4b-binding protein and C1 esterase inhibitor interact with a novel outer surface protein of Borrelia recurrentis

The spirochete Borrelia recurrentis is the causal agent of louse-borne relapsing fever and is transmitted to humans by the infected body louse Pediculus humanus. We have recently demonstrated that the B. recurrentis surface receptor, HcpA, specifically binds factor H, the regulator of the alternative pathway of complement activation, thereby inhibiting complement mediated bacteriolysis. Here, we show that B. recurrentis spirochetes express another potential outer membrane lipoprotein, termed CihC, and acquire C4b-binding protein (C4bp) and human C1 esterase inhibitor (C1-Inh), the major inhibitors of the classical and lectin pathway of complement activation. A highly homologous receptor for C4bp was also found in the African tick-borne relapsing fever spirochete B. duttonii. Upon its binding to B. recurrentis or recombinant CihC, C4bp retains its functional potential, i.e. facilitating the factor I-mediated degradation of C4b. The additional finding that ectopic expression of CihC in serum sensitive B. burgdorferi significantly increased spirochetal resistance against human complement suggests this receptor to substantially contribute, together with other known strategies, to immune evasion of B. recurrentis.

Borrelia recurrentis, the causal agent of louse-borne relapsing fever is transmitted to humans via infected body lice. Infection with B. recurrentis has been achieved only in humans and is accompanied by a systemic inflammatory disease, multiple relapses of fever and massive spirochetemia. A key virulence factor of B. recurrentis is their potential to undergo antigenic variation. However, for survival in the blood during the early phase of infection and for persistence in human tissues, spirochetes must be endowed with robust tools to escape innate immunity. We have recently shown that B. recurrentis acquires the serum-derived regulator factor H, thereby blocking the alternative complement pathway. Here, we show that B. recurrentis expresses in addition a novel outer surface lipoprotein that selectively binds serum-derived C4b-binding protein and C1 esterase inhibitor, two endogenous regulators of the classical and lectin pathway of complement activation. The combined data underscore the versatility of B. recurrentis to effectively evade innate and adaptive immunity, including serum resistance. Thus, the present study elucidates a new mechanism of B. recurrentis important for its evasion from complement attack and will be helpful for the development of new drugs against this fatal infection.

Translocation of Borrelia burgdorferi surface lipoprotein OspA through the outer membrane requires an unfolded conformation and can initiate at the C-terminus

Borrelia burgdorferi surface lipoproteins are essential to the pathogenesis of Lyme borreliosis, but the mechanisms responsible for their localization are only beginning to emerge. We have previously demonstrated the critical nature of the amino-terminal 'tether' domain of the mature lipoprotein for sorting a fluorescent reporter to the Borrelia cell surface. Here, we show that individual deletion of four contiguous residues within the tether of major surface lipoprotein OspA results in its inefficient translocation across the Borrelia outer membrane. Intriguingly, C-terminal epitope tags of these N-terminal deletion mutants were selectively surface-exposed. Fold-destabilizing C-terminal point mutations and deletions did not block OspA secretion, but rather restored one of the otherwise periplasmic tether mutants to the bacterial surface. Together, these data indicate that disturbance of a confined tether feature leads to premature folding of OspA in the periplasm and thereby prevents secretion through the outer membrane. Furthermore, they suggest that OspA emerges tail-first on the bacterial surface, yet independent of a specific C-terminal targeting peptide sequence.

Molecular characterization of the interaction of Borrelia parkeri and Borrelia turicatae with human complement regulators

In North America, tick-borne relapsing fever is caused by the species Borrelia hermsii, B. parkeri, and B. turicatae, which are transmitted to humans through the bite of the respective infected tick vectors. Here we describe the identification and functional characterization of a surface lipoprotein of B. parkeri, designated BpcA, that binds the human complement regulators factor H and factor H-related protein 1 and, simultaneously, the host protease plasminogen. In contrast, the homologous B. turicatae protein failed to bind human factor H and factor H-related protein 1 but retained its plasminogen binding capacity. Factor H bound to BpcA maintains its regulatory capacity to control C3b deposition and C3 convertase activity. Ectopic expression of BpcA in a serum-sensitive B. burgdorferi strain protects transformed cells from complement-mediated killing. Furthermore, bound plasminogen/plasmin endows B. parkeri and B. turicatae with the potential to degrade extracellular matrix components. These findings expand our understanding of the putative recent evolutionary separation of Borrelia parkeri and Borrelia turicatae, provide evidence that B. parkeri differs from B. turicatae in its ability to resist complement attack, and may help in understanding the pathological processes underlying tick-borne relapsing fever.

Structure-function investigation of vsp serotypes of the spirochete Borrelia hermsii

Background

Relapsing fever (RF) spirochetes are notable for multiphasic antigenic variation of polymorphic outer membrane lipoproteins, a phenomenon responsible for immune evasion. An additional role in tissue localization is suggested by the finding that isogenic serotypes 1 (Bt1) and 2 (Bt2) of the RF spirochete Borrelia turicatae, which differ only in the Vsp they express, exhibit marked differences in clinical disease severity and tissue localization during infection.

Methodology/Principal Findings

Here we used known vsp DNA sequences encoding for B. turicatae and Borrelia hermsii Vsp proteins with variable regions and then studied whether there are differences in disease expression and tissue localization of their corresponding serotypes during mouse infection. For sequence and structural comparisons we focused exclusively on amino acid residues predicted to project away from the spirochetes surface, referred to as the Vsp dome. Disease severity and tissue localization were studied during persistent infection with individual or mixed serotypes in SCID mice. The results showed that all Vsp domes clustered into 3 main trunks, with the domes for B. turicatae Vsp1 (BtVsp1) and BtVsp2 clustering into separate ones. B. hermsii serotypes whose Vsp domes clustered with the BtVsp1 dome were less virulent but localized to the brain more. The BtVsp2 dome was the oddball among all and Bt2 was the only serotype that caused severe arthritis.

Conclusion/Significance

These findings indicate that there is significant variability in Vsp dome structure, disease severity, and tissue localization among serotypes of B. hermsii.

Development of a single-plasmid-based regulatable gene expression system for Borrelia burgdorferi

We developed a single-plasmid-based regulatable protein expression system for Borrelia burgdorferi. Expression of a target gene is driven by P(ost), a hybrid B. burgdorferi ospA-tetO promoter, from a recombinant B. burgdorferi plasmid constitutively expressing TetR. The system was tested using the green fluorescent protein (GFP) as a reporter. Under noninducing conditions, recombinant B. burgdorferi cells were nonfluorescent, no GFP protein was detected, and residual, small amounts of transcript were detectable only by reverse transcription-PCR but not by Northern blot hybridization. Upon induction with anhydrotetracycline, increasing levels of GFP transcript, protein, and fluorescence were observed. This tight and titratable promoter system will be invaluable for the study of essential borrelial proteins. Since target protein, operator, and repressor are carried by a single plasmid, the system's application is independent of a particular strain background.

Borrelia recurrentis employs a novel multifunctional surface protein with anti-complement, anti-opsonic and invasive potential to escape innate immunity

Borrelia recurrentis, the etiologic agent of louse-borne relapsing fever in humans, has evolved strategies, including antigenic variation, to evade immune defence, thereby causing severe diseases with high mortality rates. Here we identify for the first time a multifunctional surface lipoprotein of B. recurrentis, termed HcpA, and demonstrate that it binds human complement regulators, Factor H, CFHR-1, and simultaneously, the host protease plasminogen. Cell surface bound factor H was found to retain its activity and to confer resistance to complement attack. Moreover, ectopic expression of HcpA in a B. burgdorferi B313 strain, deficient in Factor H binding proteins, protected the transformed spirochetes from complement-mediated killing. Furthermore, HcpA-bound plasminogen/plasmin endows B. recurrentis with the potential to resist opsonization and to degrade extracellular matrix components. Together, the present study underscores the high virulence potential of B. recurrentis. The elucidation of the molecular basis underlying the versatile strategies of B. recurrentis to escape innate immunity and to persist in human tissues, including the brain, may help to understand the pathological processes underlying louse-borne relapsing fever.

Laboratory maintenance of Borrelia burgdorferi

This unit describes the propagation of Borrelia burgdorferi as well as other "cultivable" Borrelia species in Barbour-Stoenner-Kelly-II (BSK-II) medium. It supplies a detailed recipe for BSK-II, as well as protocols for standard liquid culture, the generation of frozen Borrelia stocks, and the plating of B. burgdorferi cells in solid BSK-II medium.

Dual binding specificity of a Borrelia hermsii-associated complement regulator-acquiring surface protein for factor H and plasminogen discloses a putative virulence factor of relapsing fever spirochetes

Tick-borne relapsing fever in North America is primarily caused by the spirochete Borrelia hermsii. The pathogen employs multiple strategies, including the acquisition of complement regulators and antigenic variation, to escape innate and humoral immunity. In this study we identified in B. hermsii a novel member of the complement regulator-acquiring surface protein (CRASP) family, designated BhCRASP-1, that binds the complement regulators factor H (FH) and FH-related protein 1 (FHR-1) but not FH-like protein 1 (FHL-1). BhCRASP-1 specifically interacts with the short consensus repeat 20 of FH, thereby maintaining FH-associated cofactor activity for factor I-mediated C3b inactivation. Furthermore, ectopic expression of BhCRASP- 1 converted the serum-sensitive Borrelia burgdorferi B313 strain into an intermediate complement-resistant strain. Finally, we report for the first time that BhCRASP-1 binds plasminogen/plasmin in addition to FH via, however, distinct nonoverlapping domains. The fact that surface-bound plasmin retains its proteolytic activity suggest that the dual binding specificity of BhCRASP-1 for FH and plasminogen/plasmin contributes to both the dissemination/invasion of B. hermsii and its resistance to innate immunity.

Molecular characterization and subcellular localization of macrophage infectivity potentiator, a Chlamydia trachomatis lipoprotein

Macrophage infectivity potentiator (MIP) was originally reported to be a chlamydial lipoprotein from experiments showing incorporation of radiolabeled palmitic acid into native and recombinant MIP; inhibition of posttranslational processing of recombinant MIP by globomycin, known to inhibit signal peptidase II; and solubility of native MIP in Triton X-114. However, the detailed structural characterization of the lipid moiety on MIP has never been fully elucidated. In this study, bioinformatics and mass spectrometry analysis, as well as radiolabeling and immunochemical experiments, were conducted to further characterize MIP structure and subcellular localization. In silico analysis showed that the amino acid sequence of MIP is conserved across chlamydial species. A potential signal sequence with a contained lipobox was identified, and a recombinant C20A variant was prepared by replacing the probable lipobox cysteine with an alanine. Both incorporation of U-(14)C-esterified glycerol and [U-(14)C]palmitic acid and posttranslational processing that was inhibitable by globomycin were observed for recombinant wild-type MIP but not for the recombinant C20A MIP variant. The fatty acid contents of native and recombinant MIP were analyzed by gas chromatography-mass spectrometry, and the presence of amide-linked fatty acids in recombinant MIP was investigated by alkaline methanolysis. These results demonstrated a lipid modification in MIP similar to that of other prokaryotic lipoproteins. In addition, MIP was detected in an outer membrane preparation of Chlamydia trachomatis elementary bodies and was shown to be present at the surfaces of elementary bodies by surface biotinylation and surface immunoprecipitation experiments.

Functional characterization of BbCRASP-2, a distinct outer membrane protein of Borrelia burgdorferi that binds host complement regulators factor H and FHL-1

Borrelia burgdorferi, the aetiological agent of Lyme disease, employs sophisticated means to survive in diverse mammalian hosts. Recent studies demonstrated that acquisition of complement regulators factor H and factor H-like protein-1 (FHL-1) allows spirochetes to resist complement-mediated killing. Serum-resistant B. burgdorferi express up to five distinct complement regulator-acquiring surface proteins (CRASPs) that bind factor H and/or FHL-1. In this study we have identified and characterized one of those B. burgdorferi proteins, named BbCRASP-2. BbCRASP-2 is distinct from the four previously identified factor H/FHL-1-binding CRASPs of B. burgdorferi strains. The single copy of the gene encoding BbCRASP-2, cspZ, is located on the linear plasmid lp28-3. BbCRASP-2 is highly divergent from the factor H/FHL-1-binding protein BbCRASP-1 and from members of the factor H-binding Erp (OspE/F-related) protein family. Peptide mapping analysis revealed that the factor H/FHL-1 binding site is discontinuous and it was found that C-terminal truncations abrogate factor H and FHL-1 binding. The predominant BbCRASP-2 binding site of both host complement regulators was mapped to the short consensus repeat 7 (SCR 7). Factor H and FHL-1 bound to BbCRASP-2 maintain cofactor activity for factor I-mediated C3b inactivation and accelerate the decay of the C3 convertase. Expression of BbCRASP-2 in serum-sensitive B. burgdorferi mutant B313 increased resistance to complement-mediated lysis. The characterization of BbCRASP-2 now provides a complete picture of the three diverse complement regulator-binding protein families of B. burgdorferi yielding new insights into the pathogenesis of Lyme disease.

Crystal structure of neurotropism-associated variable surface protein 1 (Vsp1) of Borrelia turicatae

Vsp surface lipoproteins are serotype-defining antigens of relapsing fever spirochetes that undergo multiphasic antigenic variation to allow bacterial persistence in spite of an immune response. Two isogenic serotypes of Borrelia turicatae strain Oz1 differ in their Vsp sequences and in disease manifestations in infected mice: Vsp1 is associated with the selection of a neurological niche, while Vsp2 is associated with blood and skin infection. We report here crystal structures of the Vsp1 dimer at 2.7 and 2.2 A. The structures confirm that relapsing fever Vsp proteins share a common helical fold with OspCs of Lyme disease-causing Borrelia. The fold features an inner stem formed by highly conserved N and C termini and an outer "dome" formed by the variable central residues. Both Vsp1 and OspC structures possess small water-filled cavities, or pockets, that are lined largely by variable residues and are thus highly variable in shape. These features appear to signify tolerance of the Vsp-OspC fold for imperfect packing of residues at its antigenic surface. Structural comparison of Vsp1 with a homology model for Vsp2 suggests that observed differences in disease manifestation may arise in part from distinct differences in electrostatic surface properties; additional predicted positively charged surface patches on Vsp2 compared to Vsp1 may be sufficient to explain the relative propensity of Vsp2 to bind to acidic glycosaminoglycans.

Borrelia burgdorferi lipoproteins are secreted to the outer surface by default

Borrelia spirochaetes are unique among diderm bacteria in their abundance of surface-displayed lipoproteins, some of which play important roles in the pathogenesis of Lyme disease and relapsing fever. To identify the lipoprotein-sorting signals in Borrelia burgdorferi, we generated chimeras between the outer surface lipoprotein OspA, the periplasmic oligopeptide-binding lipoprotein OppAIV and mRFP1, a monomeric red fluorescent reporter protein. Localization of OspA and OppAIV point mutants showed that Borrelia lipoproteins do not follow the '+2' sorting rule which targets lipoproteins to the cytoplasmic or outer membrane of Gram-negative bacteria via the Lol pathway. Fusions of mRFP1 to short N-terminal lipopeptides of OspA, and surprisingly OppAIV, were targeted to the spirochaetal surface. Mutagenesis of the OspA N-terminus defined less than five N-terminal amino acids as the minimal secretion-facilitating signal. With the exception of negative charges, which can act as partial subsurface retention signals in certain peptide contexts, lipoprotein secretion occurs independent of N-terminal sequence. Together, these data indicate that Borrelia lipoproteins are targeted to the bacterial surface by default, but can be retained in the periplasm by sequence-specific signals.

Fibronectin binding protein BBK32 of the Lyme disease spirochete promotes bacterial attachment to glycosaminoglycans

Borrelia burgdorferi, the agent of Lyme disease, causes a multisystemic illness that can affect the skin, heart, joints, and nervous system and is capable of attachment to diverse cell types. Among the host components recognized by this spirochete are fibronectin and glycosaminoglycans (GAGs). Three surface-localized GAG-binding bacterial ligands, Bgp, DbpA, and DbpB, have been previously identified, but recent studies suggested that at least one additional GAG-binding ligand is expressed on the spirochetal surface when the spirochete is adapted to the mammalian host environment. BBK32 is a surface lipoprotein that is produced during infection and that has been shown to bind to fibronectin. In this study, we show that, when BBK32 was produced from a shuttle vector in an otherwise nonadherent high-passage B. burgdorferi strain, the protein localized on the bacterial surface and conferred attachment to fibronectin and to mammalian cell monolayers. In addition, the high-passage strain producing BBK32 bound to purified preparations of the GAGs dermatan sulfate and heparin, as well as to these GAGs on the surfaces of cultured mammalian cells. Recombinant BBK32 recognized purified heparin, indicating that the bacterial attachment to GAGs was due to direct binding by BBK32. This GAG-binding activity of BBK32 is apparently independent of fibronectin recognition, because exogenous heparin had no effect on BBK32-mediated bacterial binding to fibronectin.

Solving a sticky problem: new genetic approaches to host cell adhesion by the Lyme disease spirochete

The Lyme disease spirochetes, comprised of at least three closely related species, Borrelia burgdorferi, Borrelia garinii and Borrelia afzelii, are fascinating and enigmatic bacterial pathogens. They are maintained by tick-mediated transmission between mammalian hosts, usually small rodents. The ability of these bacteria, which have relatively small genomes, to survive and disseminate in both an immunocompetent mammal and in an arthropod vector suggests that they have evolved elegant and indispensable strategies for interacting with their hosts. Recognition of specific mammalian and tick tissues is likely to be essential for successful completion of the enzootic life cycle but, given the historical difficulties in genetic manipulation of these organisms, characterization of factors promoting cell adhesion has until recently largely been confined to either the manipulation of host cells or the analysis of potential bacterial ligands in the form of recombinant proteins. These studies have led to the identification of several mammalian receptors for Lyme disease spirochetes, including glycosaminoglycans, decorin, fibronectin and integrins, as well as a tick receptor for the bacterium, and also candidate cognate bacterial ligands. Recent advances in our ability to genetically manipulate Lyme disease spirochetes, particularly B. burgdorferi, are now providing us with firm evidence that these ligands indeed do promote bacterial adherence to host cells, and with new insights into the roles of these multifacted Borrelia-host cell interactions during mammalian and arthropod infection.

Research opportunities on human neuroborreliosis

A workshop, sponsored by the National Institutes of Health, was convened in September 2001 to evaluate the current knowledge in neurological Lyme disease. The meeting was centered into discussion of both clinical and basic aspects of the disease. Participants included researchers from the fields of infectious diseases, neurology, rheumatology, autoimmunity and basic immunology, largely but not exclusively focused on Lyme disease. This report summarizes the presentations made at the meeting.