Abrogation of ospAB constitutively activates the Rrp2-RpoN-RpoS pathway (sigmaN-sigmaS cascade) in Borrelia burgdorferi

A systemic approach to identify non-abundant immunogenic proteins in Lyme disease pathogens

Borrelia burgdorferi, the pathogen of Lyme disease, differentially produces many outer surface proteins (Osp), some of which represent the most abundant membrane proteins, such as OspA, OspB, and OspC. In cultured bacteria, these proteins can account for a substantial fraction of the total cellular or membrane proteins, posing challenges to the identification and analysis of non-abundant proteins, which could serve as novel pathogen detection markers or as vaccine candidates. Herein, we introduced serial mutations to remove these abundant Osps and generated a B. burgdorferi mutant deficient in OspA, OspB, and OspC in an infectious 297-isolate background, designated as OspABC- mutant. Compared to parental isolate, the mutant did not reflect growth defects in the cultured medium but showed differential mRNA expression of representative tested genes, in addition to gross changes in cellular and membrane protein profiles. The analysis of differentially detectable protein contents of the OspABC- mutant, as compared to the wild type, by two-dimensional gel electrophoresis followed by liquid chromatography-mass spectrometry, identified several spirochete proteins that are dominated by proteins of unknown functions, as well as membrane transporters, chaperons, and metabolic enzymes. We produced recombinant forms of two of these represented proteins, BBA34 and BB0238, and showed that these proteins are detectable during spirochete infection in the tick-borne murine model of Lyme borreliosis and thus serve as potential antigenic markers of the infection.IMPORTANCEThe present manuscript employed a systemic approach to identify non-abundant proteins in cultured Borrelia burgdorferi that are otherwise masked or hidden due to the overwhelming presence of abundant Osps like OspA, OspB, and OspC. As these Osps are either absent or transiently expressed in mammals, we performed a proof-of-concept study in which their removal allowed the analysis of otherwise less abundant antigens in OspABC-deficient mutants and identified several immunogenic proteins, including BBA34 and BB0238. These antigens could serve as novel vaccine candidates and/or genetic markers of Lyme borreliosis, promoting new research in the clinical diagnosis and prevention of Lyme disease.

The Rrp2-RpoN-RpoS pathway plays an important role in the blood-brain barrier transmigration of the Lyme disease pathogen

Lyme disease, caused by Borrelia (or Borreliella) burgdorferi, is a complex multisystemic disorder that includes Lyme neuroborreliosis resulting from the invasion of both the central and peripheral nervous systems. However, factors that enable the pathogen to cross the blood-brain barrier (BBB) and invade the central nervous system (CNS) are still not well understood. The objective of this study was to identify the B. burgdorferi factors required for BBB transmigration. We utilized a transwell BBB model based on human brain-microvascular endothelial cells and focused on investigating the Rrp2-RpoN-RpoS pathway, a central regulatory pathway that is essential for mammalian infection by B. burgdorferi. Our results demonstrated that the Rrp2-RpoN-RpoS pathway is crucial for BBB transmigration. Furthermore, we identified OspC, a major surface lipoprotein controlled by the Rrp2-RpoN-RpoS pathway, as a significant contributor to BBB transmigration. Constitutive production of OspC in a mutant defective in the Rrp2-RpoN-RpoS pathway did not rescue the impairment in BBB transmigration, indicating that this pathway controls additional factors for this process. Two other major surface lipoproteins controlled by this pathway, DbpA/B and BBK32, appeared to be dispensable for BBB transmigration. In addition, both the surface lipoprotein OspA and the Rrp1 pathway, which are required B. burgdorferi colonization in the tick vector, were found not required for BBB transmigration. Collectively, our findings using in vitro transwell assays uncover another potential role of the Rrp2-RpoN-RpoS pathway in BBB transmigration of B. burgdorferi and invasion into the CNS.

Gene Regulation and Transcriptomics

Borrelia (Borreliella) burgdorferi, along with closely related species, is the etiologic agent of Lyme disease. The spirochete subsists in an enzootic cycle that encompasses acquisition from a vertebrate host to a tick vector and transmission from a tick vector to a vertebrate host. To adapt to its environment and persist in each phase of its enzootic cycle, B. burgdorferi wields three systems to regulate the expression of genes: the RpoN-RpoS alternative sigma factor cascade, the Hk1/Rrp1 two-component system and its product c-di-GMP, and the stringent response mediated by Rel_Bbu and DksA. These regulatory systems respond to enzootic phase-specific signals and are controlled or fine- tuned by transcription factors, including BosR and BadR, as well as small RNAs, including DsrABb and Bb6S RNA. In addition, several other DNA-binding and RNA-binding proteins have been identified, although their functions have not all been defined. Global changes in gene expression revealed by high-throughput transcriptomic studies have elucidated various regulons, albeit technical obstacles have mostly limited this experimental approach to cultivated spirochetes. Regardless, we know that the spirochete, which carries a relatively small genome, regulates the expression of a considerable number of genes required for the transitions between the tick vector and the vertebrate host as well as the adaptation to each.

Tracking of Borrelia afzelii Transmission from Infected Ixodes ricinus Nymphs to Mice

Quantitative and microscopic tracking of Borrelia afzelii transmission from infected Ixodes ricinus nymphs has shown a transmission cycle different from that of Borrelia burgdorferi and Ixodes scapularis. Borrelia afzelii organisms are abundant in the guts of unfed I. ricinus nymphs, and their numbers continuously decrease during feeding.

Quantitative and microscopic tracking of Borrelia afzelii transmission from infected Ixodes ricinus nymphs has shown a transmission cycle different from that of Borrelia burgdorferi and Ixodes scapularis. Borrelia afzelii organisms are abundant in the guts of unfed I. ricinus nymphs, and their numbers continuously decrease during feeding. Borrelia afzelii spirochetes are present in murine skin within 1 day of tick attachment. In contrast, spirochetes were not detectable in salivary glands at any stage of tick feeding. Further experiments demonstrated that tick saliva is not essential for B. afzelii infectivity, the most important requirement for successful host colonization being a change in expression of outer surface proteins that occurs in the tick gut during feeding. Spirochetes in vertebrate mode are then able to survive within the host even in the absence of tick saliva. Taken together, our data suggest that the tick gut is the decisive organ that determines the competence of I. ricinus to vector B. afzelii. We discuss possible transmission mechanisms of B. afzelii spirochetes that should be further tested in order to design effective preventive and therapeutic strategies against Lyme disease.

Borrelia Host Adaptation Protein (BadP) Is Required for the Colonization of a Mammalian Host by the Agent of Lyme Disease

Borrelia burgdorferi, the agent of Lyme disease (LD), uses host-derived signals to modulate gene expression during the vector and mammalian phases of infection. Microarray analysis of mutants lacking the B orrelia host adaptation regulator (BadR) revealed the downregulation of genes encoding enzymes whose role in the pathophysiology of B. burgdorferi is unknown. Immunoblot analysis of the badR mutants confirmed reduced levels of these enzymes, and one of these enzymes, encoded by bb0086, shares homology to prokaryotic magnesium chelatase and Lon-type proteases. The BB0086 levels in B. burgdorferi were higher under conditions mimicking those in fed ticks. Mutants lacking bb0086 had no apparent in vitro growth defect but were incapable of colonizing immunocompetent C3H/HeN or immunodeficient SCID mice. Immunoblot analysis revealed reduced levels of proteins critical for the adaptation of B. burgdorferi to the mammalian host, such as OspC, DbpA, and BBK32. Both RpoS and BosR, key regulators of gene expression in B. burgdorferi, were downregulated in the bb0086 mutants. Therefore, we designated BB0086 the B orrelia host adaptation protein (BadP). Unlike badP mutants, the control strains established infection in C3H/HeN mice at 4 days postinfection, indicating an early colonization defect in mutants due to reduced levels of the lipoproteins/regulators critical for initial stages of infection. However, badP mutants survived within dialysis membrane chambers (DMCs) implanted within the rat peritoneal cavity but, unlike the control strains, did not display complete switching of OspA to OspC, suggesting incomplete adaptation to the mammalian phase of infection. These findings have opened a novel regulatory mechanism which impacts the virulence potential of B burgdorferi.

Investigation of ospC Expression Variation among Borrelia burgdorferi Strains

Outer surface protein C (OspC) is the most studied major virulence factor of Borrelia burgdorferi, the causative agent of Lyme disease. The level of OspC varies dramatically among B. burgdorferi strains when cultured in vitro, but little is known about what causes such variation. It has been proposed that the difference in endogenous plasmid contents among strains contribute to variation in OspC phenotype, as B. burgdorferi contains more than 21 endogenous linear (lp) and circular plasmids (cp), and some of which are prone to be lost. In this study, we analyzed several clones isolated from B. burgdorferi strain 297, one of the most commonly used strains for studying ospC expression. By taking advantage of recently published plasmid sequence of strain 297, we developed a multiplex PCR method specifically for rapid plasmid profiling of B. burgdorferi strain 297. We found that some commonly used 297 clones that were thought having a complete plasmid profile, actually lacked some endogenous plasmids. Importantly, the result showed that the difference in plasmid profiles did not contribute to the ospC expression variation among the clones. Furthermore, we found that B. burgdorferi clones expressed different levels of BosR, which in turn led to different levels of RpoS and subsequently, resulted in OspC level variation among B. burgdorferi strains.

Antibody profiling of canine IgG responses to the OspC protein of the Lyme disease spirochetes supports a multivalent approach in vaccine and diagnostic assay development

OspC performs essential functions during the enzootic cycle of the Lyme disease (LD) spirochetes. In this study, the specificity of antibody (Ab) responses to OspC was profiled to define the antigenic determinants during infection and after vaccination. Several OspC variants or 'types' were screened with serum from SNAP4Dx C6 positive dogs and with serum from rabbits hyperimmunized with OspC proteins. The OspC type-specific nature of the Ab response revealed that variable domains of OspC are immunodominant during infection and upon vaccination. To assess the potential of OspC to elicit Ab in the context of a bacterin vaccine, OspC production in strains cultivated in vitro was assessed. Immunoblot and indirect immunofluorescent antibody analyses demonstrated that production is low and that only a subset of cells actively produces OspC in vitro, raising questions about the potential of bacterin vaccines to stimulate significant anti-OspC Ab responses. The specificity of the OspC Ab response in experimentally infected mice over time was assessed to determine if domains shielded in the OspC homodimer become accessible and stimulate Ab production as infection progresses. The results demonstrate that the OspC Ab response remains focused on surface exposed variable regions of the protein throughout infection. In contrast to some earlier studies, it is concluded that conserved domains of OspC, including the C7 or C10 domain, do not elicit significant Ab responses during infection or upon vaccination. Collectively, the results indicate that OspC diversity must be considered in vaccine design and in the interpretation of diagnostic assays that employ OspC as a diagnostic antigen.

Outer surface protein OspC is an antiphagocytic factor that protects Borrelia burgdorferi from phagocytosis by macrophages

Outer surface protein C (OspC) is one of the major lipoproteins expressed on the surface of Borrelia burgdorferi during tick feeding and the early phase of mammalian infection. OspC is required for B. burgdorferi to establish infection in both immunocompetent and SCID mice and has been proposed to facilitate evasion of innate immune defenses. However, the exact biological function of OspC remains elusive. In this study, we showed that the ospC-deficient spirochete could not establish infection in NOD-scid IL2rγ(null) mice that lack B cells, T cells, NK cells, and lytic complement. The ospC mutant also could not establish infection in anti-Ly6G-treated SCID and C3H/HeN mice (depletion of neutrophils). However, depletion of mononuclear phagocytes at the skin site of inoculation in SCID and C3H/HeN mice allowed the ospC mutant to establish infection in vivo. In phagocyte-depleted mice, the ospC mutant was able to colonize the joints and triggered neutrophilia during dissemination. Furthermore, we found that phagocytosis of green fluorescent protein (GFP)-expressing ospC mutant spirochetes by murine peritoneal macrophages and human THP-1 macrophage-like cells, but not in PMN-HL60, was significantly higher than parental wild-type B. burgdorferi strains, suggesting that OspC has an antiphagocytic property. In addition, overproduction of OspC in spirochetes also decreased the uptake of spirochetes by murine peritoneal macrophages. Together, our findings provide evidence that mononuclear phagocytes play a key role in clearance of the ospC mutant and that OspC promotes spirochetes' evasion of macrophages during early Lyme borreliosis.

σ(N) -dependent control of acid resistance and the locus of enterocyte effacement in enterohemorrhagic Escherichia coli is activated by acetyl phosphate in a manner requiring flagellar regulator FlhDC and the σ(S) antagonist FliZ

In enterohemorrhagic Escherichia coli (EHEC), sigma factor N (σ^N) regulates glutamate-dependent acid resistance (GDAR) and the locus of enterocyte effacement (LEE); discrete genetic systems that are required for transmission and virulence of this intestinal pathogen. Regulation of these systems requires nitrogen regulatory protein C, NtrC, and is a consequence of NtrC-σ^N-dependent reduction in the activity of sigma factor S (σ^S). This study elucidates pathway components and stimuli for σ^N-directed regulation of GDAR and the LEE in EHEC. Deletion of fliZ, the product of which reduces σ^S activity, phenocopied rpoN (σ^N) and ntrC null strains for GDAR and LEE control, acid resistance, and adherence. Upregulation of fliZ by NtrC-σ^N was shown to be indirect and required an intact flagellar regulator flhDC. Activation of flhDC by NtrC-σ^N and FlhDC-dependent regulation of GDAR and the LEE was dependent on σ^N-promoter flhD_P2, and a newly described NtrC upstream activator sequence. Addition of ammonium chloride significantly altered expression of GDAR and LEE, acid resistance, and adherence, independently of rpoN, ntrC, and the NtrC sensor kinase, ntrB. Altering the availability of NtrC phosphodonor acetyl phosphate by growth without glucose, with acetate addition, or by deletion of acetate kinase ackA, abrogated NtrC-σ^N-dependent control of flhDC, fliZ, GDAR, and the LEE.

Insights into the biology of Borrelia burgdorferi gained through the application of molecular genetics

Borrelia burgdorferi, the vector-borne bacterium that causes Lyme disease, was first identified in 1982. It is known that much of the pathology associated with Lyme borreliosis is due to the spirochete's ability to infect, colonize, disseminate, and survive within the vertebrate host. Early studies aimed at defining the biological contributions of individual genes during infection and transmission were hindered by the lack of adequate tools and techniques for molecular genetic analysis of the spirochete. The development of genetic manipulation techniques, paired with elucidation and annotation of the B. burgdorferi genome sequence, has led to major advancements in our understanding of the virulence factors and the molecular events associated with Lyme disease. Since the dawn of this genetic era of Lyme research, genes required for vector or host adaptation have garnered significant attention and highlighted the central role that these components play in the enzootic cycle of this pathogen. This chapter covers the progress made in the Borrelia field since the application of mutagenesis techniques and how they have allowed researchers to begin ascribing roles to individual genes. Understanding the complex process of adaptation and survival as the spirochete cycles between the tick vector and vertebrate host will lead to the development of more effective diagnostic tools as well as identification of novel therapeutic and vaccine targets. In this chapter, the Borrelia genes are presented in the context of their general biological roles in global gene regulation, motility, cell processes, immune evasion, and colonization/dissemination.

Synthesis of RpoS is dependent on a putative enhancer binding protein Rrp2 in Borrelia burgdorferi

The RpoN-RpoS regulatory pathway plays a central role in governing adaptive changes by B. burgdorferi when the pathogen shuttles between its tick vector and mammalian hosts. In general, transcriptional activation of bacterial RpoN (σ54)-dependent genes requires an enhancer binding protein. B. burgdorferi encodes the putative enhancer binding protein Rrp2. Previous studies have revealed that the expression of σ54-dependent rpoS was abolished in an rrp2 point mutant. However, direct evidence linking the production of Rrp2 in B. burgdorferi and the expression of rpoS has been lacking, primarily due to the inability to inactivate rrp2 via deletion or insertion mutagenesis. Herein we introduced a regulatable (IPTG-inducible) rrp2 expression shuttle plasmid into B. burgdorferi, and found that the controlled up-regulation of Rrp2 resulted in the induction of σ54-dependent rpoS expression. Moreover, we created an rrp2 conditional lethal mutant in virulent B. burgdorferi. By exploiting this conditional mutant, we were able to experimentally manipulate the temporal level of Rrp2 expression in B. burgdorferi, and examine its direct impact on activation of the RpoN-RpoS regulatory pathway. Our data revealed that the synthesis of RpoS was coincident with the IPTG-induced Rrp2 levels in B. burgdorferi. In addition, the synthesis of OspC, a lipoprotein required by B. burgdorferi to establish mammalian infection, was rescued in the rrp2 point mutant when RpoS production was restored, suggesting that Rrp2 influences ospC expression indirectly via its control over RpoS. These data demonstrate that Rrp2 is required for the synthesis of RpoS, presumably via its action as an enhancer binding protein for the activation of RpoN and subsequent transcription of rpoS in B. burgdorferi.

Global repression of host-associated genes of the Lyme disease spirochete through post-transcriptional modulation of the alternative sigma factor RpoS

Borrelia burgdorferi, the agent of Lyme disease, is a vector-borne pathogen that transits between Ixodes ticks and vertebrate hosts. During the natural infectious cycle, spirochetes must globally adjust their transcriptome to survive in these dissimilar environments. One way B. burgdorferi accomplishes this is through the use of alternative sigma factors to direct transcription of specific genes. RpoS, one of only three sigma factors in B. burgdorferi, controls expression of genes required during tick-transmission and infection of the mammalian host. How spirochetes switch between different sigma factors during the infectious cycle has remained elusive. Here we establish a role for a novel protein, BBD18, in the regulation of the virulence-associated sigma factor RpoS. Constitutive expression of BBD18 repressed transcription of RpoS-dependent genes to levels equivalent to those observed in an rpoS mutant. Consistent with the global loss of RpoS-dependent transcripts, we were unable to detect RpoS protein. However, constitutive expression of BBD18 did not diminish the amount of rpoS transcript, indicating post-transcriptional regulation of RpoS by BBD18. Interestingly, BBD18-mediated repression of RpoS is independent of both the rpoS promoter and the 5’ untranslated region, suggesting a mechanism of protein destabilization rather than translational control. We propose that BBD18 is a novel regulator of RpoS and its activity likely represents a first step in the transition from an RpoS-ON to an RpoS-OFF state, when spirochetes transition from the host to the tick vector.

DhhP, a cyclic di-AMP phosphodiesterase of Borrelia burgdorferi, is essential for cell growth and virulence

Cyclic di-AMP (c-di-AMP) is a recently discovered second messenger in bacteria. Most of work on c-di-AMP signaling has been done in Gram-positive bacteria, firmicutes, and actinobacteria, where c-di-AMP signaling pathways affect potassium transport, cell wall structure, and antibiotic resistance. Little is known about c-di-AMP signaling in other bacteria. Borrelia burgdorferi, the causative agent of Lyme disease, is a spirochete that has a Gram-negative dual membrane. In this study, we demonstrated that B. burgdorferi BB0619, a DHH-DHHA1 domain protein (herein designated DhhP), functions as c-di-AMP phosphodiesterase. Recombinant DhhP hydrolyzed c-di-AMP to pApA in a Mn(2+)- or Mg(2+)-dependent manner. In contrast to c-di-AMP phosphodiesterases reported thus far, DhhP appears to be essential for B. burgdorferi growth both in vitro and in the mammalian host. Inactivation of the chromosomal dhhP gene could be achieved only in the presence of a plasmid-encoded inducible dhhP gene. The conditional dhhP mutant had a dramatic increase in intracellular c-di-AMP level in comparison to the isogenic wild-type strain. Unlike what has been observed in Gram-positive bacteria, elevated cellular c-di-AMP in B. burgdorferi did not result in an increased resistance to β-lactamase antibiotics, suggesting that c-di-AMP's functions in spirochetes differ from those in Gram-positive bacteria. In addition, the dhhP mutant was defective in induction of the σ(S) factor, RpoS, and the RpoS-dependent outer membrane virulence factor OspC, which uncovers an important role of c-di-AMP in B. burgdorferi virulence.

Borrelia burgdorferi oxidative stress regulator BosR directly represses lipoproteins primarily expressed in the tick during mammalian infection

Differential gene expression is a key strategy adopted by the Lyme disease spirochaete, Borrelia burgdorferi, for adaptation and survival in the mammalian host and the tick vector. Many B. burgdorferi surface lipoproteins fall into two distinct groups according to their expression patterns: one group primarily expressed in the tick and the other group primarily expressed in the mammal. Here, we show that the Fur homologue in this bacterium, also known as Borrelia oxidative stress regulator (BosR), is required for repression of outer surface protein A (OspA) and OspD in the mammal. Furthermore, BosR binds directly to sequences upstream of the ospAB operon and the ospD gene through recognition of palindromic motifs similar to those recognized by other Fur homologues but with a 1 bp variation in the spacer length. Putative BosR binding sites have been identified upstream of 156 B. burgdorferi genes. Some of these genes share the same expression pattern as ospA and ospD. Most notably, 12 (67%) of the 18 genes previously identified in a genome-wide microarray study to be most significantly repressed in the mammal are among the putative BosR regulon. These data indicate that BosR may directly repress transcription of many genes that are downregulated in the mammal.

Contributions of environmental signals and conserved residues to the functions of carbon storage regulator A of Borrelia burgdorferi

Carbon storage regulator A of Borrelia burgdorferi (CsrABb) contributes to vertebrate host-specific adaptation by modulating activation of the Rrp2-RpoN-RpoS pathway and is critical for infectivity. We hypothesized that the functions of CsrABb are dependent on environmental signals and on select residues. We analyzed the phenotype of csrABb deletion and site-specific mutants to determine the conserved and pathogen-specific attributes of CsrABb. Levels of phosphate acetyltransferase (Pta) involved in conversion of acetyl phosphate to acetyl-coenzyme A (acetyl-CoA) and posttranscriptionally regulated by CsrABb in the csrABb mutant were reduced from or similar to those in the control strains under unfed- or fed-tick conditions, respectively. Increased levels of supplemental acetate restored vertebrate host-responsive determinants in the csrABb mutant to parental levels, indicating that both the levels of CsrABb and the acetyl phosphate and acetyl-CoA balance contribute to the activation of the Rrp2-RpoN-RpoS pathway. Site-specific replacement of 8 key residues of CsrABb (8S) with alanines resulted in increased levels of CsrABb and reduced levels of Pta and acetyl-CoA, while levels of RpoS, BosR, and other members of rpoS regulon were elevated. Truncation of 7 amino acids at the C terminus of CsrABb (7D) resulted in reduced csrABb transcripts and posttranscriptionally reduced levels of FliW located upstream of CsrABb. Electrophoretic mobility shift assays revealed increased binding of 8S mutant protein to the CsrA binding box upstream of pta compared to the parental and 7D truncated protein. Two CsrABb binding sites were also identified upstream of fliW within the flgK coding sequence. These observations reveal conserved and unique functions of CsrABb that regulate adaptive gene expression in B. burgdorferi.

Manganese and zinc regulate virulence determinants in Borrelia burgdorferi

Borrelia burgdorferi, the causative agent of Lyme disease, must adapt to two diverse niches, an arthropod vector and a mammalian host. RpoS, an alternative sigma factor, plays a central role in spirochetal adaptation to the mammalian host by governing expression of many genes important for mammalian infection. B. burgdorferi is known to be unique in metal utilization, and little is known of the role of biologically available metals in B. burgdorferi. Here, we identified two transition metal ions, manganese (Mn(2+)) and zinc (Zn(2+)), that influenced regulation of RpoS. The intracellular Mn(2+) level fluctuated approximately 20-fold under different conditions and inversely correlated with levels of RpoS and the major virulence factor OspC. Furthermore, an increase in intracellular Mn(2+) repressed temperature-dependent induction of RpoS and OspC; this repression was overcome by an excess of Zn(2+). Conversely, a decrease of intracellular Mn(2+) by deletion of the Mn(2+) transporter gene, bmtA, resulted in elevated levels of RpoS and OspC. Mn(2+) affected RpoS through BosR, a Fur family homolog that is required for rpoS expression: elevated intracellular Mn(2+) levels greatly reduced the level of BosR protein but not the level of bosR mRNA. Thus, Mn(2+) and Zn(2+) appeared to be important in modulation of the RpoS pathway that is essential to the life cycle of the Lyme disease spirochete. This finding supports the emerging notion that transition metals such as Mn(2+) and Zn(2+) play a critical role in regulation of virulence in bacteria.

Genome stability of Lyme disease spirochetes: comparative genomics of Borrelia burgdorferi plasmids

Lyme disease is the most common tick-borne human illness in North America. In order to understand the molecular pathogenesis, natural diversity, population structure and epizootic spread of the North American Lyme agent, Borrelia burgdorferi sensu stricto, a much better understanding of the natural diversity of its genome will be required. Towards this end we present a comparative analysis of the nucleotide sequences of the numerous plasmids of B. burgdorferi isolates B31, N40, JD1 and 297. These strains were chosen because they include the three most commonly studied laboratory strains, and because they represent different major genetic lineages and so are informative regarding the genetic diversity and evolution of this organism. A unique feature of Borrelia genomes is that they carry a large number of linear and circular plasmids, and this work shows that strains N40, JD1, 297 and B31 carry related but non-identical sets of 16, 20, 19 and 21 plasmids, respectively, that comprise 33–40% of their genomes. We deduce that there are at least 28 plasmid compatibility types among the four strains. The B. burgdorferi ∼900 Kbp linear chromosomes are evolutionarily exceptionally stable, except for a short ≤20 Kbp plasmid-like section at the right end. A few of the plasmids, including the linear lp54 and circular cp26, are also very stable. We show here that the other plasmids, especially the linear ones, are considerably more variable. Nearly all of the linear plasmids have undergone one or more substantial inter-plasmid rearrangements since their last common ancestor. In spite of these rearrangements and differences in plasmid contents, the overall gene complement of the different isolates has remained relatively constant.

Gene regulation in Borrelia burgdorferi

Borrelia burgdorferi, the spirochete that causes Lyme disease, is maintained in nature via an enzootic cycle that comprises a tick vector and a vertebrate host. Transmission from the tick to the mammal, acquisition from the mammal back to the tick, and adaptation to the two disparate environments require sensing signals and responding by regulating programs of gene expression. The molecular mechanisms utilized to effect these lifestyle changes have begun to be elucidated and feature an alternative sigma factor cascade in which RpoN (σ(54)) and RpoS (σ(S)) globally control the genes required for the different phases of the enzootic cycle. The RpoN-RpoS pathway is surprisingly complex, entailing Rrp2, an unusual enhancer-binding protein and two-component regulatory system response regulator activated by acetyl phosphate; BosR, an unorthodox DNA-binding protein; DsrA(Bb), a small noncoding RNA; and Hfq and CsrA, two RNA-binding proteins. B. burgdorferi also has a c-di-GMP signaling system that regulates the tick side of the enzootic cycle and whose function is only beginning to be appreciated.

Regulation of the virulence determinant OspC by bbd18 on linear plasmid lp17 of Borrelia burgdorferi

Persistent infection of a mammalian host by Borrelia burgdorferi, the spirochete that causes Lyme disease, requires specific downregulation of an immunogenic outer surface protein, OspC. Although OspC is an essential virulence factor needed by the spirochete to establish infection in the mammal, it represents a potent target for the host acquired immune response, and constitutive expression of OspC results in spirochete clearance. In this study, we demonstrate that a factor encoded on a linear plasmid of B. burgdorferi, lp17, can negatively regulate ospC transcription from the endogenous gene on the circular plasmid cp26 and from an ospC promoter-lacZ fusion on a shuttle vector. Furthermore, we have identified bbd18 as the gene on lp17 that is responsible for this effect. These data identify a novel component of ospC regulation and provide the basis for determining the molecular mechanisms of ospC repression in vivo.

Oligopeptide permease A5 modulates vertebrate host-specific adaptation of Borrelia burgdorferi

Borrelia burgdorferi, the agent of Lyme disease, undergoes rapid adaptive gene expression in response to signals unique to its arthropod vector or vertebrate hosts. Among the upregulated genes under vertebrate host conditions is one of the five annotated homologs of oligopeptide permease A (OppA5, BBA34). A mutant lacking oppA5 was constructed in an lp25-deficient isolate of B. burgdorferi strain B31, and the minimal regions of infectivity were restored via a shuttle vector pBBE22 with or without an intact copy of bba34. Immunoblot analysis of the bba34 mutant revealed a reduction in the levels of RpoS, BosR, and CsrA(Bb) with a concomitant reduction in the levels of OspC, DbpA, BBK32, and BBA64. There were no changes in the levels of OspA, NapA, P66, and three other OppA orthologs. Quantitative transcriptional analysis correlated with the changes in the protein levels. However, the bba34 mutant displayed comparable infectivities in the C3H/HeN mice and the wild-type strain, despite the reduction in several pathogenesis-related proteins. Supplementation of the growth medium with increased levels of select components, notably sodium acetate and sodium bicarbonate, restored the levels of several proteins in the bba34 mutant to wild-type levels. We speculate that the transport of acetate appears to contribute to the accumulation of key metabolites, like acetyl phosphate, that facilitate the adaptation of B. burgdorferi to the vertebrate host by the activation of the Rrp2-RpoN-RpoS pathway. These studies underscore the importance of solute transport to host-specific adaptation of B. burgdorferi.

CsrA modulates levels of lipoproteins and key regulators of gene expression critical for pathogenic mechanisms of Borrelia burgdorferi

Carbon storage regulator A (CsrA) is an RNA binding protein that has been characterized in many bacterial species to play a central regulatory role by modulating several metabolic processes. We recently showed that a homolog of CsrA in Borrelia burgdorferi (CsrA(Bb), BB0184) was upregulated in response to propagation of B. burgdorferi under mammalian host-specific conditions. In order to further delineate the role of CsrA(Bb), we generated a deletion mutant designated ES10 in a linear plasmid 25-negative isolate of B. burgdorferi strain B31 (ML23). The deletion mutant was screened by PCR and Southern blot hybridization, and a lack of synthesis of CsrA(Bb) in ES10 was confirmed by immunoblot analysis. Analysis of ES10 propagated at pH 6.8/37°C revealed a significant reduction in the levels of OspC, DbpA, BBK32, and BBA64 compared to those for the parental wild-type strain propagated under these conditions, while there were no significant changes in the levels of either OspA or P66. Moreover, the levels of two regulatory proteins, RpoS and BosR, were also found to be lower in ES10 than in the control strain. Quantitative real-time reverse transcription-PCR analysis of total RNA extracted from the parental strain and csrA(Bb) mutant revealed significant differences in gene expression consistent with the changes at the protein level. Neither the csrA(Bb) mutant nor the trans-complemented strain was capable of infection following intradermal needle inoculation in C3H/HeN mice at either 10³ or 10⁵ spirochetes per mouse. The further characterization of molecular basis of regulation mediated by CsrA(Bb) will provide significant insights into the pathophysiology of B. burgdorferi.

Infectious arthritis and immune dysregulation: lessons from Lyme disease

PURPOSE OF REVIEW

Borrelia burgdorferi colonization of the joints induces an inflammatory response, which in some individuals progresses to chronic arthritis. In this review, we discuss novel pathways that are implicated in disease development by modulating host defenses to B. burgdorferi infection.

RECENT FINDINGS

The use of transgenic mice and gene expression analyses has revealed novel pathways involved in pathogenesis of Lyme disease. It is now clear that B. burgdorferi exploits an array of salivary gland proteins of the tick to evade immune responses in the mammalian host. The spirochete also modulates its surface protein profile upon infection and induces anti-inflammatory cytokines, favoring survival of the pathogen. The host defense involves toll-like receptors (TLRs), such as TLR2 and others, in B. burgdorferi recognition. To further dissect the genetic predisposition to treatment-refractory Lyme arthritis, HLA-DR transgenic mice have been used.

SUMMARY

The cause and pathogenesis of Lyme arthritis are complex. Elucidating the mechanisms that govern this chronic inflammatory response will provide direct insights into other infectious arthritides and the development of novel therapeutic approaches against B. burgdorferi infection.

BmpA is a surface-exposed outer-membrane protein of Borrelia burgdorferi

BmpA is an immunodominant protein of Borrelia burgdorferi as well as an arthritogenic factor. Rabbit antirecombinant BmpA (rBmpA) antibodies were raised, characterized by assaying their cross reactivity with rBmpB, rBmpC and rBmpD, and then rendered monospecific by absorption with rBmpB. This monospecific reagent reacted only with rBmpA in dot immunobinding and detected a single 39 kDa, pI 5.0, spot on two-dimensional immunoblots. It was used to assess the BmpA cellular location. BmpA was present in both detergent-soluble and -insoluble fractions of Triton X-114 phase-partitioned borrelial cells, suggesting that it was a membrane lipoprotein. Immunoblots of proteinase K-treated intact and Triton X-100 permeabilized cells showed digestion of BmpA in intact cells, consistent with surface exposure. This exposure was confirmed by dual-label immunofluorescence microscopy of intact and permeabilized borrelial cells. Conservation and surface localization of BmpA in all B. burgdorferi sensu lato genospecies could point to its playing a key role in this organism's biology and pathobiology.

BBA52 facilitates Borrelia burgdorferi transmission from feeding ticks to murine hosts

Borrelia burgdorferi, the pathogen of Lyme borreliosis, persists in nature through a tick-rodent transmission cycle. A selective assessment of the microbial transcriptome, limited to gene-encoding putative membrane proteins, reveals that bba52 transcription in vivo is strictly confined to the vector-specific portion of the microbial life cycle, with the highest levels of expression noted in feeding ticks and with swift down-regulation noted in mice. bba52 deletion did not affect murine disease as assessed by the genesis of arthritis and carditis or long-term persistence of pathogens in mice or ticks. However, bba52 deficiency did impair microbial transitions between hosts and vector, defects that could be fully rescued when bba52 expression was genetically restored to the original genomic locus. These studies establish that BBA52 facilitates vector-host transitions by the pathogen and therefore is a potential antigenic target for interference with transmission of B. burgdorferi from ticks to mammalian hosts.

Two regulatory elements required for enhancing ospA expression in Borrelia burgdorferi grown in vitro but repressing its expression during mammalian infection

During cycling between the tick vector and a mammal, the Lyme disease spirochaete Borrelia burgdorferi must coordinate expression of outer-surface proteins (Osps) A and B to quickly respond to environmental changes. The pathogen abundantly produces OspA/B in the tick, but represses their expression during mammalian infection. This paper reports a regulatory structure, consisting of two sequences flanking the ospAB promoter, that is required for enhancing ospA expression in B. burgdorferi grown in vitro, but repressing its expression during murine infection. Deletion or replacement of either the upstream or downstream sequence of the ospAB promoter caused a significant decrease in ospA expression in vitro, but a dramatic increase during murine infection. Fusion of either sequence with the flaB reporter promoter led to increased expression of an ospA reporter gene in vitro, but a decrease in the murine host. Furthermore, simultaneous fusion of both sequences with the reporter promoter showed a synergistic effect in enhancing expression of the ospA reporter in vitro, but repressing its expression during murine infection. Taken together, the results demonstrate that the regulatory structure functions oppositely in the two different environments and potentially provides B. burgdorferi with a molecular mechanism to quickly adapt to the distinct environments during its enzootic life cycle.

Overexpression of CsrA (BB0184) alters the morphology and antigen profiles of Borrelia burgdorferi

Borrelia burgdorferi, the agent of Lyme disease, alters its gene expression in response to highly disparate environmental signals encountered in its hosts. Among the relatively few regulators of adaptive gene expression present in the borrelial genome is an open reading frame (ORF), BB0184, annotated as CsrA (carbon storage regulator A). CsrA, in several bacterial species, has been characterized as a small RNA binding protein that functions as a global regulator affecting mRNA stability or levels of translation of multiple ORFs. Consistent with known functions of CsrA, overexpression of CsrA from B. burgdorferi (CsrABb) in Escherichia coli resulted in reduced accumulation of glycogen. We determined that csrABb is part of the flgK motility operon and that the synthesis of CsrABb was increased when B. burgdorferi was propagated under fed-tick conditions. Overexpression of CsrABb in B. burgdorferi strain B31 (ML23, lp25-negative clonal isolate) resulted in a clone, designated ES25, which exhibited alterations in colony morphology and a significant reduction in the levels of FlaB. Several lipoproteins previously characterized as playing a role in infectivity were also altered in ES25. Real-time reverse transcription-PCR analysis of RNA revealed significant differences in the transcriptional levels of ospC in ES25, while there were no such differences in the levels of other transcripts, suggesting posttranscriptional regulation of expression of these latter genes. These observations indicate that CsrABb plays a role in the regulation of expression of pathophysiological determinants of B. burgdorferi, and further characterization of CsrABb will help in better understanding of the regulators of gene expression in B. burgdorferi.

OspC-independent infection and dissemination by host-adapted Borrelia burgdorferi

Borrelia burgdorferi OspC is required for the spirochete to establish infection in a mammal by tick transmission or needle inoculation. After a brief essential period, the protein no longer is required and the gene can be shut off. Using a system in which spirochetes contain only an unstable wild-type copy of the ospC gene, we can obtain mice persistently infected with bacteria lacking OspC. We implanted pieces of infected mouse skin subcutaneously in naïve mice, using donors carrying wild-type or ospC mutant spirochetes, and found that both could infect mice by this method, with similar numbers of wild-type or ospC mutant spirochetes disseminated throughout the tissues of recipient mice. Recipient mouse immune responses to tissue transfer-mediated infection with wild-type or ospC mutant spirochetes were similar. These experiments demonstrate that mammalian host-adapted spirochetes can infect and disseminate in mice in the absence of OspC, thereby circumventing this hallmark of tick-derived or in vitro-grown spirochetes. We propose a model in which OspC is one of a succession of functionally equivalent, essential proteins that are synthesized at different stages of mammalian infection. In this model, another protein uniquely present on host-adapted spirochetes performs the same essential function initially fulfilled by OspC. The strict temporal control of B. burgdorferi outer surface protein gene expression may reflect immunological constraints rather than distinct functions.