Two independent transcriptional units control the complex and simultaneous expression of the bmp paralogous chromosomal gene family in Borrelia burgdorferi

Predicting the ligand-binding properties of Borrelia burgdorferi s.s. Bmp proteins in light of the conserved features of related Borrelia proteins

Bacteria of the genus Borrelia cause vector-borne infections like the most important hard tick-borne disease in the northern hemisphere, Lyme borreliosis (LB), and soft tick or louse transmitted relapsing fevers (RF), prevalent in temperate and tropical areas. Borrelia burgdorferi sensu lato (s.l.) includes several genospecies and causes LB in humans. In infected patients, Borrelia burgdorferi sensu stricto (s.s.) expresses the BmpA, BmpB, BmpC and BmpD proteins. The role of these proteins in the pathogenesis of LB remains incompletely characterized, but they are, however, closely related to Treponema pallidum PnrA (Purine nucleoside receptor A), a substrate-binding lipoprotein of the ATP-binding cassette (ABC) transporter family preferentially binding purine nucleosides. Based on 3D homology modeling, the Bmp proteins share the typical fold of the substrate-binding protein family and the ligand-binding properties of BmpA, BmpB and BmpD are highly similar, whereas those of BmpC differ markedly. Nevertheless, these residues are highly conserved within the genus Borrelia and the inferred phylogenetic tree also reveals that the RF Borrelia lack BmpB proteins but has an additional Bmp protein (BmpA2) missing in LB-causing Borrelia burgdorferi s.l. Our results indicate that the Bmp proteins could bind nucleosides, although BmpC might have a different ligand-binding specificity and, therefore, a distinct function. Furthermore, the work provides a means for classifying the Bmp proteins and supports further elucidation of the roles of these proteins.

Borrelia burgdorferi surface-located Lmp1 protein processed into region-specific polypeptides that are critical for microbial persistence

One of the Borrelia burgdorferi virulence determinants, annotated as Lmp1, is a surface-exposed, conserved, and potential multi-domain protein involved in various functions in spirochete infectivity. Lmp1 contributes to host-pathogen interactions and evasion of host adaptive immunity by spirochetes. Here, we show that in diverse B. burgdorferi species, Lmp1 exists as distinct, region-specific, and lower molecular mass polypeptides encompassing 1 or more domains, including independent N-terminal and middle regions and a combined middle and C-terminal region. These polypeptides originate from complex posttranslational maturation events, partly supported by a periplasmic serine protease termed as BbHtrA. Although spirochete persistence in mice is independently supported by domain-specific Lmp1 polypeptides, transmission of B. burgdorferi from ticks to mammals requires essential contributions from both N-terminal and middle regions. Interference with the functions of Lmp1 domains or their complex posttranslational maturation events may aid in development of novel therapeutic strategies to combat infection and transmission of pathogens.

RNA-Seq of Borrelia burgdorferi in Multiple Phases of Growth Reveals Insights into the Dynamics of Gene Expression, Transcriptome Architecture, and Noncoding RNAs

Borrelia burgdorferi, the agent of Lyme disease, differentially expresses numerous genes and proteins as it cycles between mammalian hosts and tick vectors. Insights on regulatory mechanisms have been provided by earlier studies that examined B. burgdorferi gene expression patterns during cultivation. However, prior studies examined bacteria at only a single time point of cultivation, providing only a snapshot of what is likely a dynamic transcriptional program driving B. burgdorferi adaptations to changes during culture growth phases. To address that concern, we performed RNA sequencing (RNA-Seq) analysis of B. burgdorferi cultures at early-exponential, mid-exponential, and early-stationary phases of growth. We found that expression of nearly 18% of annotated B. burgdorferi genes changed significantly during culture maturation. Moreover, genome-wide mapping of the B. burgdorferi transcriptome in different growth phases enabled insight on transcript boundaries, operon structures, and identified numerous putative non-coding RNAs. These RNA-Seq data are discussed and presented as a resource for the community of researchers seeking to better understand B. burgdorferi biology and pathogenesis.

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.

The HtrA protease of Borrelia burgdorferi degrades outer membrane protein BmpD and chemotaxis phosphatase CheX

Borrelia burgdorferi, the spirochaetal agent of Lyme disease, codes for a single HtrA protein, HtrABb (BB0104) that is homologous to DegP of Escherichia coli (41% amino acid identity). HtrABb shows physical and biochemical similarities to DegP in that it has the trimer as its fundamental unit and can degrade casein via its catalytic serine. Recombinant HtrABb exhibits proteolytic activity in vitro, while a mutant (HtrABbS198A) does not. However, HtrABb and DegP have some important differences as well. Native HtrABb occurs in both membrane-bound and soluble forms. Despite its homology to DegP, HtrABb could not complement an E. coli DegP deletion mutant. Late stage Lyme disease patients, as well as infected mice and rabbits developed a robust antibody response to HtrABb, indicating that it is a B-cell antigen. In co-immunoprecipitation studies, a number of potential binding partners for HtrABb were identified, as well as two specific proteolytic substrates, basic membrane protein D (BmpD/BB0385) and chemotaxis signal transduction phosphatase CheX (BB0671). HtrABb may function in regulating outer membrane lipoproteins and in modulating the chemotactic response of B. burgdorferi.

Adhesion mechanisms of Borrelia burgdorferi

The Borrelia are widely distributed agents of Lyme disease and Relapsing Fever. All are vector-borne zoonotic pathogens, have segmented genomes, and enigmatic mechanisms of pathogenesis. Adhesion to mammalian and tick substrates is one pathogenic mechanism that has been widely studied. At this point, the primary focus of research in this area has been on Borrelia burgdorferi, one agent of Lyme disease, but many of the adhesins of B. burgdorferi are conserved in other Lyme disease agents, and some are conserved in the Relapsing Fever Borrelia. B. burgdorferi adhesins that mediate attachment to cell-surface molecules may influence the host response to the bacteria, while adhesins that mediate attachment to soluble proteins or extracellular matrix components may cloak the bacterial surface from recognition by the host immune system as well as facilitate colonization of tissues. While targeted mutations in the genes encoding some adhesins have been shown to affect the infectivity and pathogenicity of B. burgdorferi, much work remains to be done to understand the roles of the adhesins in promoting the persistent infection required to maintain the bacteria in reservoir hosts.

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.

Borrelia burgdorferi BmpA is a laminin-binding protein

The Borrelia burgdorferi BmpA outer surface protein plays a significant role in mammalian infection by the Lyme disease spirochete and is an important antigen for the serodiagnosis of human infection. B. burgdorferi adheres to host extracellular matrix components, including laminin. The results of our studies indicate that BmpA and its three paralogous proteins, BmpB, BmpC, and BmpD, all bind to mammalian laminin. BmpA did not bind mammalian type I or type IV collagens or fibronectin. BmpA-directed antibodies significantly inhibited the adherence of live B. burgdorferi to laminin. The laminin-binding domain of BmpA was mapped to the carboxy-terminal 80 amino acids. Solubilized collagen inhibited BmpA-laminin binding, suggesting interactions through the collagen-binding domains of laminin. These results, together with previous data, indicate that BmpA and its paralogs are targets for the development of preventative and curative therapies for Lyme disease.

Borrelia burgdorferi small lipoprotein Lp6.6 is a member of multiple protein complexes in the outer membrane and facilitates pathogen transmission from ticks to mice

Borrelia burgdorferi lipoprotein Lp6.6 is a differentially produced spirochete antigen. An assessment of lp6.6 expression covering representative stages of the infectious cycle of spirochetes demonstrates that the gene is solely expressed during pathogen persistence in ticks. Deletion of lp6.6 in infectious B. burgdorferi did not influence in vitro growth, or its ability to persist and induce inflammation in mice, migrate to larval or nymphal ticks or survive through the larval-nymphal molt. However, Lp6.6-deficient spirochetes displayed significant impairment in their ability to transmit from infected ticks to naïve mice, which was restored upon genetic complementation of the mutant with a wild-type copy of lp6.6, establishing that Lp6.6 plays a role in pathogen transmission from ticks to mammals. Lp6.6 is a subsurface, yet highly abundant, outer membrane antigen. Two-dimensional blue native/SDS-PAGE coupled with liquid chromatography-mass spectrometry (LC-MS/MS) analysis and protein cross-linking studies independently shows that Lp6.6 exists in multiple protein complexes in the outer membrane. We speculate that the function of Lp6.6 is connected to the physiological processes of these membrane complexes. Further characterization of differentially produced membrane antigens and associated protein complexes will likely aid in our understanding of the molecular details of B. burgdorferi persistence and transmission through a complex enzootic cycle.

The Borrelia burgdorferi flaB promoter has an extended -10 element and includes a T-rich -35/-10 spacer sequence that is essential for optimal activity

In this study, we investigated the functional elements of the flaB promoter of Borrelia burgdorferi. Promoter function was examined in a high-passage variant of strain JD1 using a set of 5' deletions and mutations within the flaB promoter. Expression from the modified flaB promoters was assayed using the gene for green fluorescent protein (gfp) as a reporter. Although the -35 element of the promoter stimulated promoter activity, its disruption did not negate expression. Sequences upstream of the -35 had no effect on expression. The -35/-10 spacer region composed of a T-rich sequence was critical for optimal promoter function. Surprisingly, a cytosine at the -13 site was found to be more favorable for transcription compared with a guanosine at the same site. Based on these results and other characteristics, we propose that the B. burgdorferi flaB promoter is an example of an extended -10 promoter. Further, the T-rich spacer is a key element of the flaB promoter that contributes to the abundance of the flagellar core protein in Borrelia species.

Borrelia burgdorferi lipoprotein BmpA activates pro-inflammatory responses in human synovial cells through a protein moiety

Borrelia burgdorferi invasion of mammalian joints results in genesis of Lyme arthritis. Other than spirochete lipids, existence of protein antigens, which are abundant in joints and participate in B. burgdorferi-induced host inflammatory response, is unknown. Here, we report that major products of the B. burgdorferi basic membrane protein (bmp) A/B operon that are induced in murine and human joints, possess inflammatory properties. Compared to the wild type B. burgdorferi, an isogenic bmpA/B mutant induced significantly lower levels of pro-inflammatory cytokines TNF-alpha and IL-1beta in cultured human synovial cells, which could be restored using bmpA/B-complemented mutants, and more directly, upon addition of recombinant BmpA, but not BmpB or control spirochete proteins. Non-lipidated and lipidated versions of BmpA induced similar levels of cytokines, and remained unaffected by treatment with lipopolysaccharide inhibitor, polymyxin B. The bmpA/B mutant was also impaired in the induction of NF-kappaB and p38 MAP kinase signaling pathways in synovial cells, which were activated by non-lipidated BmpA. These results show that a protein moiety of BmpA can induce cytokine responses in synovial cells via activation of the NF-kappaB and p38 MAP kinase pathways and thus, could potentially contribute to the genesis of Lyme arthritis.

Borrelia burgdorferi basic membrane proteins A and B participate in the genesis of Lyme arthritis

Lyme arthritis results from colonization of joints by Borrelia burgdorferi and the ensuing host response. Using gene array–based differential analysis of B. burgdorferi gene expression and quantitative reverse trancription-polymerase chain reaction, we identified two paralogous spirochete genes, bmpA and bmpB, that are preferentially up-regulated in mouse joints compared with other organs. Transfer of affinity-purified antibodies against either BmpA or BmpB into B. burgdorferi–infected mice selectively reduced spirochete numbers and inflammation in the joints. B. burgdorferi lacking bmpA/B were therefore generated to further explore the role of these proteins in the pathogenesis of Lyme disease. B. burgdorferi lacking bmpA/B were infectious in mice, but unable to persist in the joints, and they failed to induce severe arthritis. Complementation of the mutant spirochetes with a wild-type copy of the bmpA and bmpB genes partially restored the original phenotype. These data delineate a role for differentially produced B. burgdorferi antigens in spirochete colonization of mouse joints, and suggest new strategies for the treatment of Lyme arthritis.

Evolving models of Lyme disease spirochete gene regulation

The spirochete Borrelia burgdorferi, the causative agent of Lyme disease (Lyme borreliosis), is well-adapted to maintain a natural cycle of alternately infecting vertebrates and blood-sucking ticks. During this cycle, B. burgdorferi interacts with a broad spectrum of vertebrate and arthropod tissues, acquires nutrients in diverse environments and evades killing by vertebrate and tick immune systems. The bacterium also senses when situations occur that necessitate transmission between hosts, such as when an infected tick is taking a blood meal from a potential host. To accurately accomplish the requirements necessary for survival in nature, B. burgdorferi must be keenly aware of its surroundings and respond accordingly. In this review, we trace studies performed to elucidate regulatory mechanisms employed by B. burgdorferi to control gene expression, and the development of models or "paradigms" to explain experimental results. Through comparisons of five borrelial gene families, it is readily apparent that each is controlled through a distinct mechanism. Furthermore, those results indicate that current models of interpreting in vitro data cannot accurately predict all aspects of B. burgdorferi environmental sensing and gene regulation in vivo.

Borrelia burgdorferi BmpA, BmpB, and BmpD proteins are expressed in human infection and contribute to P39 immunoblot reactivity in patients with Lyme disease

The Bmp proteins are a paralogous family of chromosomally encoded Borrelia burgdorferi lipoproteins. They have similar predicted immunogenicities and similar electrophoretic mobilities by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. P39 reactivity against Borrelia burgdorferi lysate in immunoblots of Lyme disease patients has long been identified with reactivity to BmpA, but responses to other Bmp proteins have not been examined. To determine if patients with Lyme disease developed such responses, immunoglobulin G (IgG) anti-Bmp reactivity in patient and control sera was studied by using soluble recombinant Bmp (rBmp) proteins expressed in Escherichia coli. Although some patient sera contained IgG immunoblot and immunodot reactivities against all four Bmp proteins, analysis of IgG anti-Bmp fine specificity by a competitive enzyme-linked immunosorbent assay with graded doses of soluble homologous and heterologous rBmp proteins showed that only the responses to BmpA, BmpB, and BmpD were specific. This suggests that at least three of the four Bmp proteins are expressed by B. burgdorferi in infected patients and that specific antibodies to them are likely to be present in the P39 band in some patients.

Gene organization and transcriptional analysis of the tprJ, tprI, tprG, and tprF loci in Treponema pallidum strains Nichols and Sea 81-4

The tpr gene family of Treponema pallidum subsp. pallidum, the causative agent of syphilis, has recently become the focus of intensive investigation. TprF and TprI sequences are highly conserved among different isolates and are the targets of strong humoral and cellular immune responses of the host, and immunization with a recombinant peptide from the amino terminus of these antigens has been shown to alter significantly lesion development following homologous challenge. This indicates that these antigens are expressed during infection and strongly suggests a key functionality. tprF and tprI are located immediately downstream of the tprG and tprJ genes, respectively, separated by very short intergenic spacers (55 nucleotides for G-F and 56 nucleotides for J-I). Preliminary analysis using gene-specific primers failed to amplify tprJ in the Sea 81-4 isolate. In this study, sequence and transcriptional analysis of these loci showed a similar gene organization in the Nichols and Sea 81-4 strains, a complex pattern of transcription, and the presence of G homopolymeric repeats of variable lengths upstream of the tprF, tprI, tprG, and tprJ transcriptional start sites. However, distinctive features were also identified in the Sea 81-4 isolate, including a tprG-like open reading frame in the tprJ locus, a frameshift and a premature termination in the tprG coding sequence, a longer tprG-tprF intergenic spacer, and absence of cotranscription of the tprG-tprF genes.

Borrelia burgdorferi rel is responsible for generation of guanosine-3'-diphosphate-5'-triphosphate and growth control

The global transcriptional regulator (p)ppGpp (guanosine-3'-diphosphate-5'-triphosphate and guanosine-3',5'-bisphosphate, collectively) produced by the relA and spoT genes in Escherichia coli allows bacteria to adapt to different environmental stresses. The genome of Borrelia burgdorferi encodes a single chromosomal rel gene (BB0198) (B. burgdorferi rel [rel(Bbu)]) homologous to relA and spoT of E. coli. Its role in (p)ppGpp synthesis, bacterial growth, and modulation of gene expression has not been studied in detail. We constructed a rel(Bbu) deletion mutant in an infectious B. burgdorferi 297 strain and isolated an extrachromosomally complemented derivative of this mutant. The mutant did not synthesize rel(Bbu) mRNA, Rel(Bbu) protein, or (p)ppGpp. This synthesis was restored in the complemented derivative, confirming that rel(Bbu) is necessary and sufficient for (p)ppGpp synthesis and degradation in B. burgdorferi. The rel(Bbu) mutant grew well during log phase in complete BSK-H but reached lower cell concentrations in the stationary phase than the wild-type parent, suggesting that (p)ppGpp may be an important factor in the ability of B. burgdorferi to adapt to stationary phase. Deletion of rel(Bbu) did not eliminate the temperature-elicited OspC shift, nor did it alter bmp gene expression or B. burgdorferi antibiotic susceptibility. Although deletion of rel(Bbu) eliminated B. burgdorferi virulence for mice, which was not restored by complementation, we suggest that rel(Bbu)-dependent accumulation of (p)ppGpp may be important for in vivo survival of this pathogen.

Expression of the bmpB gene of Borrelia burgdorferi is modulated by two distinct transcription termination events

bmp gene family 36 of Borrelia burgdorferi, the agent of Lyme disease, comprises four paralogs: bmpA, bmpB, bmpC, and bmpD. The bmpA and bmpB genes constitute an operon. All four genes have been found to be transcribed in cultured spirochetes. Expression from the bmpAB operon results in three distinct transcripts of 1.1, 1.6, and 2.4 kb, and the relative expression of bmpA mRNA is three- to fourfold greater than that of bmpB mRNA. However, thus far only expression of the BmpA protein has been demonstrated. Therefore, in this study we characterized the origins of the three transcripts and compared the relative expression of the BmpA and BmpB proteins. Northern blotting revealed that the three distinct transcripts originated from a single promoter located upstream of bmpA but terminated either 3' to the bmpA (1.1-kb RNA) or bmpB (2.4-kb RNA) gene or, most unusually, within the bmpB gene (1.6-kb RNA). Termination within the bmpB gene was associated with a functional Rho-independent transcription terminator. At the protein level, we also observed a 4.3-fold greater abundance of BmpA compared to that of BmpB. These studies identify a transcription termination mechanism in B. burgdorferi resulting in the disparate expression of the two genes of the bmpAB operon.

Analysis of promoter elements involved in the transcriptional initiation of RpoS-dependent Borrelia burgdorferi genes

Borrelia burgdorferi, the causative agent of Lyme disease, encodes an RpoS ortholog (RpoS(Bb)) that controls the temperature-inducible differential expression of at least some of the spirochete's lipoprotein genes, including ospC and dbpBA. To begin to dissect the determinants of RpoS(Bb) recognition of, and selectivity for, its dependent promoters, we linked a green fluorescent protein reporter to the promoter regions of several B. burgdorferi genes with well-characterized expression patterns. Consistent with the expression patterns of the native genes/proteins in B. burgdorferi strain 297, we found that expression of the ospC, dbpBA, and ospF reporters in the spirochete was RpoS(Bb) dependent, while the ospE and flaB reporters were RpoS(Bb) independent. To compare promoter recognition by RpoS(Bb) with that of the prototype RpoS (RpoS(Ec)), we also introduced our panel of constructs into Escherichia coli. In this surrogate, maximal expression from the ospC, dbpBA, and ospF promoters clearly required RpoS, although in the absence of RpoS(Ec) the ospF promoter was weakly recognized by another E. coli sigma factor. Furthermore, RpoS(Bb) under the control of an inducible promoter was able to complement an E. coli rpoS mutant, although RpoS(Ec) and RpoS(Bb) each initiated greater activity from their own dependent promoters than they did from those of the heterologous sigma factor. Genetic analysis of the ospC promoter demonstrated that (i) the T(-14) in the presumptive -10 region plays an important role in sigma factor recognition in both organisms but is not as critical for transcriptional initiation by RpoS(Bb) as it is for RpoS(Ec); (ii) the nucleotide at the -15 position determines RpoS or sigma(70) selectivity in E. coli but does not serve the same function in B. burgdorferi; and (iii) the 110-bp region upstream of the core promoter is not required for RpoS(Ec)- or RpoS(Bb)-dependent activity in E. coli but is required for maximal expression from this promoter in B. burgdorferi. Taken together, the results of our studies suggest that the B. burgdorferi and E. coli RpoS proteins are able to catalyze transcription from RpoS-dependent promoters of either organism, but at least some of the nucleotide elements involved in transcriptional initiation and sigma factor selection in B. burgdorferi play a different role than has been described for E. coli.

Temporal analysis of the antigenic composition of Borrelia burgdorferi during infection in rabbit skin

The numbers of host-adapted Borrelia burgdorferi (HAB) organisms in rabbit skin were assessed by real-time PCR over the first 3 weeks of infection. Maximal numbers were found at day 11, while spirochete numbers decreased by more than 30-fold by day 21. The antigenic composition of HAB in skin biopsy samples was determined by use of a procedure termed hydrophobic antigen tissue Triton extraction. Immune sera from rabbits, sera from chronically infected mice, and monospecific antiserum to the antigenic variation protein, VlsE, were used to probe parallel two-dimensional immunoblots representing each time point. Individual proteins were identified using either specific antisera or by matching protein spots to mass spectrometry-identified protein spots from in vitro-cultivated Borrelia. There were significant changes in the relative expression of a variety of known and previously unrecognized HAB antigens during the 21-day period. OspC and the outer membrane proteins OspA and OspB were prominent at the earliest time point, day 5, when the antigenic variation protein VlsE was barely detected. OspA and OspB were not detected after day 5. OspC was not detected after day 9. VlsE was the most prominent antigen from day 7 through day 21. BmpA, ErpN, ErpP, LA7, OppA-2, DbpA, and an unidentified 15-kDa protein were also detected from day 7 through day 21. Immunoblot analysis using monospecific anti-VlsE revealed the presence of prominent distinct VlsE lower forms in HAB at days 9, 11, and 14; however, these lower forms were no longer detected at day 21. This marked diminution in VlsE lower forms paralleled the clearance of the spirochete from skin.

Localization of BmpA on the exposed outer membrane of Borrelia burgdorferi by monospecific anti-recombinant BmpA rabbit antibodies

BmpA (P39) is an immunodominant chromosomally encoded Borrelia burgdorferi protein. The potential strong cross-reactivity of anti-BmpA antibodies with the other members of this paralogous protein family and the previous use of antibodies whose reactivity to the other Bmp proteins was uncharacterized have resulted in continued controversy over its localization in B. burgdorferi. In an effort to provide a definitive demonstration of the localization of BmpA, rabbit antibodies raised to recombinant BmpA (rBmpA) were rendered monospecific by absorption with rBmpB. This reagent did not react with rBmpB, rBmpC, or rBmpD in dot immunobinding, detected only a single 39-kDa band and a single 39-kDa, pI 5.0 spot on one- and two-dimensional immunoblots of B. burgdorferi lysates, respectively, and immunoprecipitated a single 39-kDa protein from these lysates. It detected BmpA in the Triton X-114-soluble and -insoluble fractions of B. burgdorferi, suggesting association with both inner and outer bacterial cell membranes. Treatment of intact B. burgdorferi with proteinase K partially digested BmpA, consistent with a limited surface exposure on the outer bacterial membrane, a suggestion confirmed by immunofluorescence of unfixed B. burgdorferi cultured in vitro and in vivo. Anti-rBmpA antibody was bacteriostatic for B. burgdorferi B31 in culture, again suggesting localization of BmpA on the exposed spirochetal outer surface. Surface localization of BmpA, growth inhibition by anti-rBmpA antibodies, and the previously reported conservation of bmpA in different B. burgdorferi sensu lato strains may indicate that BmpA plays an essential role in B. burgdorferi biology.

Antigenic composition of Borrelia burgdorferi during infection of SCID mice

The general concept that during infection of mice the Borrelia burgdorferi surface protein composition differs profoundly from that of tick-borne or in vitro-cultivated spirochetes is well established. Specific knowledge concerning the differences is limited because the small numbers of spirochetes present in tissue have not been amenable to direct compositional analysis. In this report we describe novel means for studying the antigenic composition of host-adapted Borrelia (HAB). The detergent Triton X-114 was used to extract the detergent-phase HAB proteins from mouse ears, ankles, knees, and hearts. Immunoblot analysis revealed a profile distinct from that of in vitro-cultivated Borrelia (IVCB). OspA and OspB were not found in the tissues of SCID mice 17 days after infection. The amounts of antigenic variation protein VlsE and the relative amounts of its transcripts were markedly increased in ear, ankle, and knee tissues but not in heart tissue. VlsE existed as isoforms having both different unit sizes and discrete lower molecular masses. The hydrophobic smaller forms of VlsE were also found in IVCB. The amounts of the surface protein (OspC) and the decorin binding protein (DbpA) were increased in ear, ankle, knee, and heart tissues, as were the relative amounts of their transcripts. Along with these findings regarding VlsE, OspC, and DbpA, two-dimensional immunoblot analysis with immune sera also revealed additional details of the antigenic composition of HAB extracted from ear, heart, and joint tissues. A variety of novel antigens, including antigens with molecular masses of 65 and 30 kDa, were found to be upregulated in mouse tissues. Extraction of hydrophobic B. burgdorferi antigens from tissue provides a powerful tool for determining the antigenic composition of HAB.

Characterization of the stringent response and rel(Bbu) expression in Borrelia burgdorferi

The stringent response is a global bacterial response to nutritional stress mediated by (p)ppGpp. We previously found that both noninfectious Borrelia burgdorferi strain B31 and infectious B. burgdorferi strain N40 produced large amounts of (p)ppGpp during growth in BSK-H medium and suggested that the stringent response was triggered in B. burgdorferi under these conditions. Here we report that (p)ppGpp levels in B. burgdorferi growing in BSK-II or BSK-H medium are not further increased by nutrient limitation or by serine hydroxamate-induced inhibition of protein synthesis and that the presence of (p)ppGpp during growth of N40 in BSK-H medium is not associated with decreased 16S rRNA synthesis. Decreased 16S rRNA synthesis was associated with the decreased growth rate of N40 seen during coculture with tick cells, which are growth conditions that were previously shown to decrease (p)ppGpp levels. One-half as much of the mRNA of the gene encoding the Rel protein of B. burgdorferi (rel(Bbu)) was produced by B31 as by N40 during in vitro growth (2 +/- 0.5 and 4 +/- 0.8 fg of rel(Bbu) mRNA/ng of total Borrelia RNA, respectively). Although the amounts of N40 rel(Bbu) mRNA were identical during growth in vitro and in rat peritoneal chambers, they were markedly decreased during growth in nymphal ticks. In contrast to the lack of change in rel(Bbu) mRNA levels, larger amounts of a 78-kDa protein that was cross-reactive with antibodies to Bacillus subtilis Rel(Bsu) were detected in immunoblots of N40 lysates after growth in rat peritoneal chambers than after growth in vitro. Differences in the level of production of (p)ppGpp between B31 and N40 could not be explained by differences in rel(Bbu) promoters since identical transcriptional start sites 309 nucleotides upstream from the B31 and N40 rel(Bbu) ATG start codon and identical sigma(70)-like promoters were identified by primer extension and sequencing analysis. rel(Bbu) complemented an Escherichia coli CF1693 relA spoT double mutant for growth on M9 minimal medium, and the transformed cells produced rel(Bbu) mRNA. These results indicate that rel(Bbu) is functional and that its transcription and translation and production of (p)ppGpp are affected by environmental conditions in strains N40 and B31. They also suggest that in B. burgdorferi, an organism with few rRNA operons that grows slowly, the role of (p)ppGpp may differ from the classic role played by this molecule in E. coli and that (p)ppGpp may not be responsible for growth rate control.

Modulation of Borrelia burgdorferi stringent response and gene expression during extracellular growth with tick cells

Borrelia burgdorferi N40 multiplied extracellularly when it was cocultured with tick cells in L15BS medium, a medium which by itself did not support B. burgdorferi N40 growth. Growth of B. burgdorferi N40 in the presence of tick cells was associated with decreased production of (p)ppGpp, the stringent response global regulator, a fourfold decrease in relA/spoT mRNA, an eightfold net decrease in bmpD mRNA, and a fourfold increase in rpsL-bmpD mRNA compared to growth of B. burgdorferi in BSK-H medium. As a result, the polycistronic rpsL-bmpD mRNA level increased from 3 to 100% of the total bmpD message. These observations demonstrate that there are reciprocal interactions between B. burgdorferi and tick cells in vitro and indicate that the starvation-associated stringent response mediated by (p)ppGpp present in B. burgdorferi growing in BSK-H medium is ameliorated in B. burgdorferi growing in coculture with tick cell lines. These results suggest that this system can provide a useful model for identifying genes controlling interactions of B. burgdorferi with tick cells in vitro when it is coupled with genetic methods to isolate and complement B. burgdorferi mutants.

Complementation of a nonmotile flaB mutant of Borrelia burgdorferi by chromosomal integration of a plasmid containing a wild-type flaB allele

With the recent identification of antibiotic resistance phenotypes, the use of reporter genes, the isolation of null mutants by insertional inactivation, and the development of extrachromosomal cloning vectors, genetic analysis of Borrelia burgdorferi is becoming a reality. A previously described nonmotile, rod-shaped, kanamycin-resistant B. burgdorferi flaB::Km null mutant was complemented by electroporation with the erythromycin resistance plasmid pED3 (a pGK12 derivative) containing the wild-type flaB sequence and 366 bp upstream from its initiation codon. The resulting MS17 clone possessed erythromycin and kanamycin resistance, flat-wave morphology, and microscopic and macroscopic motility. Several other electroporations with plasmids containing wild-type flaB and various lengths (198, 366, or 762 bp) of sequence upstream from the flaB gene starting codon did not lead to functional restoration of the nonmotile flaB null mutant. DNA hybridization, PCR analysis, and sequencing indicated that the wild-type flaB gene in nonmotile clones was present in the introduced extrachromosomal plasmids, while the motile MS17 clone was a merodiploid containing single tandem chromosomal copies of mutated flaB::Km and wild-type flaB with a 366-bp sequence upstream from its starting codon. Complementation was thus achieved only when wild-type flaB was inserted into the borrelial chromosome. Several possible mechanisms for the failure of complementation for extrachromosomally located flaB are discussed.

Successful vaccination for Lyme disease: a novel mechanism?

Borrelia burgdorferi sensu lato is the aetiologic agent of Lyme disease, which is a multi-system disorder resulting from the transmission of organisms from an infected tick. According to the US Centers for Disease Control, the incidence of Lyme disease in the US has increased 25-fold since national surveillance began and the geographical spread of Lyme disease causing spirochetes would indicate that the annual number of cases will continue to rise. Humoral immunity has been shown to play a role in protection and this has spurred efforts towards developing a Lyme disease vaccine. A number of protective immunogens have been characterised to date, but due to the heterogeneity of Lyme disease Borreliae, no single molecule has proven to be completely effective as a vaccinogen. This review will describe the immunogens that have been used to protect against B. burgdorferi infection, with a focus on the inherent challenges involved with providing successful immunity to B. burgdorferi. In addition, the promising aspects and the limitations of each protective immunogen will be discussed.