Analysis of the cellular localization of Bdr paralogs in Borrelia burgdorferi, a causative agent of lyme disease: evidence for functional diversity

Erp and Rev Adhesins of the Lyme Disease Spirochete's Ubiquitous cp32 Prophages Assist the Bacterium during Vertebrate Infection

Almost all spirochetes in the genus Borrelia (sensu lato) naturally contain multiple variants of closely related prophages. In the Lyme disease borreliae, these prophages are maintained as circular episomes that are called circular plasmid 32 kb (cp32s). The cp32s of Lyme agents are particularly unique in that they encode two distinct families of lipoproteins, namely, Erp and Rev, that are expressed on the bacterial outer surface during infection of vertebrate hosts. All identified functions of those outer surface proteins involve interactions between the spirochetes and host molecules, as follows: Erp proteins bind plasmin(ogen), laminin, glycosaminoglycans, and/or components of complement and Rev proteins bind fibronectin. Thus, cp32 prophages provide their bacterial hosts with surface proteins that can enhance infection processes, thereby facilitating their own survival. Horizontal transfer via bacteriophage particles increases the spread of beneficial alleles and creates diversity among Erp and Rev proteins.

Protein Secretion in Spirochetes

Spirochetes form a separate phylum of bacteria with two membranes but otherwise unusual morphologies and envelope structures. Distinctive common features of Borrelia, Leptospira, and Treponema include the sequestration of flagella to the periplasm and thin peptidoglycan cell walls that are more closely associated with the inner membrane. Outer membrane compositions differ significantly between the genera. Leptospira most closely track Gram-negative bacteria due to the incorporation of lipopolysaccharides. Treponema and Borrelia outer membranes lack lipopolysaccharide, with treponemes expressing only a few outer membrane proteins and Borrelia displaying a dizzying diversity of abundant surface lipoproteins instead. Phylogenetic and experimental evidence indicates that spirochetes have adapted various modules of bacterial export and secretion pathways to build and maintain their envelopes. Export and insertion pathways in the inner membrane appear conserved, while spirochetal experimentation with various envelope architectures over time has led to variations in secretion pathways in the periplasm and outer membrane. Classical type I to III secretion systems have been identified, with demonstrated roles in drug efflux and export of flagellar proteins only. Unique activities of periplasmic proteases, including a C-terminal protease, are involved in maturation of some periplasmic proteins. Proper lipoprotein sorting within the periplasm appears to be dependent on functional Lol pathways that lack the outer membrane lipoprotein insertase LolB. The abundance of surface lipoproteins in Borrelia and detailed protein sorting studies suggest a lipoprotein secretion pathway that either extends Lol through the outer membrane or bypasses it altogether. Proteins can be released from cells in outer membrane vesicles or, rarely, as soluble proteins.

Cyclic di-GMP modulates gene expression in Lyme disease spirochetes at the tick-mammal interface to promote spirochete survival during the blood meal and tick-to-mammal transmission

Borrelia burgdorferi, the Lyme disease spirochete, couples environmental sensing and gene regulation primarily via the Hk1/Rrp1 two-component system (TCS) and Rrp2/RpoN/RpoS pathways. Beginning with acquisition, we reevaluated the contribution of these pathways to spirochete survival and gene regulation throughout the enzootic cycle. Live imaging of B. burgdorferi caught in the act of being acquired revealed that the absence of RpoS and the consequent derepression of tick-phase genes impart a Stay signal required for midgut colonization. In addition to the behavioral changes brought on by the RpoS-off state, acquisition requires activation of cyclic di-GMP (c-di-GMP) synthesis by the Hk1/Rrp1 TCS; B. burgdorferi lacking either component is destroyed during the blood meal. Prior studies attributed this dramatic phenotype to a metabolic lesion stemming from reduced glycerol uptake and utilization. In a head-to-head comparison, however, the B. burgdorferi Δglp mutant had a markedly greater capacity to survive tick feeding than B. burgdorferi Δhk1 or Δrrp1 mutants, establishing unequivocally that glycerol metabolism is only one component of the protection afforded by c-di-GMP. Data presented herein suggest that the protective response mediated by c-di-GMP is multifactorial, involving chemotactic responses, utilization of alternate substrates for energy generation and intermediary metabolism, and remodeling of the cell envelope as a means of defending spirochetes against threats engendered during the blood meal. Expression profiling of c-di-GMP-regulated genes through the enzootic cycle supports our contention that the Hk1/Rrp1 TCS functions primarily, if not exclusively, in ticks. These data also raise the possibility that c-di-GMP enhances the expression of a subset of RpoS-dependent genes during nymphal transmission.

Sequence divergence in the Treponema denticola FhbB protein and its impact on factor H binding

Treponema denticola is an anaerobic spirochete whose abundance in the subgingival crevice correlates with the development and severity of periodontal disease. The ability of T. denticola to survive and thrive in the hostile environment of the periodontal pocket is due, at least in part, to its ability to bind factor H (FH), a negative regulator of the alternative complement pathway. The FH binding protein of T. denticola has been identified as FhbB and its atomic structure has been determined. The interaction of FH with T. denticola is unique in that FH bound to the cell surface is cleaved by the T. denticola protease, dentilisin. It has been postulated that FH cleavage by T. denticola leads to immune dysregulation in periodontal pockets. In this study, we conduct a comparative assessment of the sequence, properties, structure and ligand binding kinetics of the FhbB proteins of strains 33521 and 35405. The biological outcome of the interaction of these strains with FH could differ significantly as 33521 lacks dentilisin activity. The data presented here offer insight into our understanding of the interactions of T. denticola with the host and its potential to influence disease progression.

Borrelia burgdorferi BBA52 is a potential target for transmission blocking Lyme disease vaccine

The surface-exposed antigens of Borrelia burgdorferi represent important targets for induction of protective host immune responses. BBA52 is preferentially expressed by B. burgdorferi in the feeding tick, and a targeted deletion of bba52 interferes with vector-host transitions in vivo. In this study, we demonstrate that BBA52 is an outer membrane surface-exposed protein and that disulfide bridges take part in the homo-oligomeric assembly of native protein. BBA52 antibodies lack detectable borreliacidal activities in vitro. However, active immunization studies demonstrated that BBA52 vaccinated mice were significantly less susceptible to subsequent tick-borne challenge infection. Similarly, passive transfer of BBA52 antibodies in ticks completely blocked B. burgdorferi transmission from feeding ticks to naïve mice. Taken together, these studies highlight the role of BBA52 in spirochete dissemination from ticks to mice and demonstrate the potential of BBA52 antibody-mediated strategy to complement the ongoing efforts to develop vaccines for blocking the transmission of B. burgdorferi.

Comparative cryo-electron tomography of pathogenic Lyme disease spirochetes

Spirochetes of the Borrelia burgdorferi sensu lato group, the causative agents of Lyme borreliosis, exhibit a complex biology evolved in its zoonotic cycle. Cryo-electron tomography was used to investigate structural features of three species, B. burgdorferi, B. garinii and B. afzelii, known to cause different clinical manifestations in humans. All three organisms revealed an overall similar architecture and showed different numbers of periplasmic flagellar filaments, polar periplasmic void regions, vesicles budding from the outer membrane sheath, which was covered by an amorphous slime layer. The latter was shown to be distinct in its density when comparing the three human-pathogenic Lyme disease spirochetes and Borrelia hermsii, a species causing relapsing fever. Tomograms of dividing bacteria revealed vesicles near the site of division and new basal bodies that were attached at each end of newly establishing cytoplasmic cylinder poles, while periplasmic flagellar filaments still passed the impending site of division. Two different kinds of cytoplasmic filaments showed similarities to MreB or FtsZ filaments of other bacteria. The similar and distinct structural features of Borrelia and the previously investigated pathogenic and non-pathogenic Treponema species emphasize the importance of further studying phylogenetically distant spirochetes.

The BBA01 protein, a member of paralog family 48 from Borrelia burgdorferi, is potentially interchangeable with the channel-forming protein P13

The Borrelia burgdorferi genome exhibits redundancy, with many plasmid-carried genes belonging to paralogous gene families. It has been suggested that certain paralogs may be necessary in various environments and that they are differentially expressed in response to different conditions. The chromosomally located p13 gene which codes for a channel-forming protein belongs to paralog family 48, which consists of eight additional genes. Of the paralogous genes from family 48, the BBA01 gene has the highest homology to p13. Herein, we have inactivated the BBA01 gene in B. burgdorferi strain B31-A. This mutant shows no apparent phenotypic difference compared to the wild type. However, analysis of BBA01 in a C-terminal protease A (CtpA)-deficient background revealed that like P13, BBA01 is posttranslationally processed at its C terminus. Elevated BBA01 expression was obtained in strains with the BBA01 gene introduced on the shuttle vector compared to the wild-type strain. We could further demonstrate that BBA01 is a channel-forming protein with properties surprisingly similar to those of P13. The single-channel conductance, of about 3.5 nS, formed by BBA01 is comparable to that of P13, which together with the high degree of sequence similarity suggests that the two proteins may have similar and interchangeable functions. This is further strengthened by the up-regulation of the BBA01 protein and its possible localization in the outer membrane in a p13 knockout strain, thus suggesting that P13 can be replaced by BBA01.

Demonstration of cotranscription and 1-methyl-3-nitroso-nitroguanidine induction of a 30-gene operon of Borrelia burgdorferi: evidence that the 32-kilobase circular plasmids are prophages

The Borrelia genome is comprised of linear and circular elements, including a group of 32-kb circular plasmids (cp32s). Earlier analyses identified a bacteriophage, varphiBB-1, that may package cp32s, suggesting that these plasmids are prophages. cp32-8, cp32-9, and cp32-1 (plasmids L, N, and P, respectively) encode virulence factors such as the factor H binding, OspE proteins (BBL39, BBN38, and BBP38). Here the expression patterns of cp32-8 open reading frames (ORFs) in in vitro-cultivated 1-methyl-3-nitroso-nitroguanidine (MNNG)-treated and untreated spirochetes and during infection were assessed. ORFs BBL42 through BBL28, which encode several bacteriophage protein homologs, were found to be cotranscribed and expression was upregulated by MNNG. Immunoblotting revealed that MNNG-induced transcription led to increased protein production. The expression of several genes that reside outside of the BBL42-BBL28 operon was not affected by MNNG. Some of these genes, including OspE (BBL39), appear to represent morons. Real-time reverse transcription-PCR of spirochetes in mouse tissue revealed that although the phage operon was not induced during infection, transcription of BBL23 (previously designated BlyA), a putative holin, was upregulated. This observation indicates that some genes within the operon can be independently transcribed from internal promoters. Additional transcriptional analyses of the operon identified multiple transcriptional start sites and provided evidence for the expression of a homologous operon from other cp32s. The data support the hypothesis put forth by C. Eggers and D. S. Samuels (J. Bacteriol. 181:7308-7313, 1999) that the cp32s are prophages, a finding with broad implications for our understanding of Borrelia pathogenesis and Borrelia genome evolution.

bdrF2 of Lyme disease spirochetes is coexpressed with a series of cytoplasmic proteins and is produced specifically during early infection

The Bdr proteins are polymorphic inner membrane proteins produced by most Borrelia species. In Borrelia burgdorferi B31MI, the18 bdr genes form three subfamilies, bdrD, bdrE, and bdrF. The production of at least one of the Bdr paralogs, BdrF2, is up-regulated in host-adapted spirochetes, suggesting a role for the protein in the mammalian environment. Here, we demonstrate using reverse transcriptase (RT) PCR that BBG29, BBG30, BBG31, and BBG32, which reside upstream of bdrF2, are cotranscribed with bdrF2 as a five-gene operon. While the functions of most of these proteins are unknown, BBG32 encodes a putative DNA helicase. Real-time RT-PCR analyses demonstrated higher levels of bdrF2 transcript relative to other genes of the operon, suggesting that bdrF2 may also be transcribed independently from an internal promoter. Internal promoters were detected using the 5' rapid amplification of cDNA ends system. The putative promoter associated with bdrF2 was found to be highly similar in sequence to the multiple promoters associated with the ospC gene. Real-time RT-PCR analyses, performed to assess the expression of these genes in infected mice, revealed that genes of the bdrF2 locus are expressed only during early infection, suggesting a role in the establishment of infection. To further characterize the proteins encoded by the bdrF2 locus, which have unknown functions, the cellular localizations of these proteins were determined by Triton X-114 extraction and phase partitioning. BBG29 and BBG31 were found to be cytoplasmic. To determine if these proteins elicit an antibody (Ab) response during infection, immunoblot analyses were performed. Abs to these proteins were not detected. Based on the analyses presented here, we offer the hypothesis that BdrF2 and other proteins encoded by the operon form an inner-membrane-associated protein complex that may interact with DNA and which carries out its functional role during transmission or the early stages of infection.

Molecualar survival strategies of the Lyme disease spirochete Borrelia burgdorferi

Lyme disease is a tick-transmitted disease caused by the spirochete Borrelia burgdorferi. The bacterium adopts different strategies for its survival inside the immunocompetent host from the time of infection until dissemination in different parts of body tissues. The success of this spirochete depends on its ability to colonise the host tissues and counteract the host's defence mechanisms. During this process borrelia seems to maintain its vitality to ensure long-term survival in the host. Borrelia's proteins are encoded by plasmid and chromosomal genes. These genes are differentially regulated and expressed by different environmental factors in ticks as well as in the mammalian host during infection. In addition, antigenic diversity enables the spirochete to escape host defence mechanisms and maintain infection. In this review we focus on the differential expression of proteins and genes, and further molecular mechanisms used by borrelia to maintain its survival in the host. In light of these pathogenetic mechanisms, further studies on spirochete host interaction are needed to understand the complex interplay that finally lead to host autoimmunity.

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.

Environmental regulation and differential production of members of the Bdr protein family of Borrelia burgdorferi

Borrelia burgdorferi B31MI carries 18 plasmid-carried genes that form the bdr gene family. The bdr genes of B. burgdorferi encode proteins that form three distinct subfamilies, the BdrD, BdrE, and BdrF subfamilies. bdr orthologs have been demonstrated to be carried by all Borrelia species analyzed, and their widespread distribution suggests that they play an important genus-wide functional role. The biological rationale for maintaining 18 bdr alleles has not been defined. It is our hypothesis that specific paralogs function in different environments and are differentially expressed in response to environmental conditions. As a first step in testing this hypothesis, the production patterns of the Bdr proteins in spirochetes grown under a variety of conditions were assessed through immunoblot analyses. The influence of temperature, serum deprivation, tick feeding, and the mammalian environment on Bdr production was evaluated. These analyses revealed that the synthesis of some Bdr paralogs is environmentally regulated. The production of BdrF(2,) BdrF(1), BdrE(4), and BdrE(5) were upregulated in host-adapted bacteria, while the production levels of other Bdr paralogs were influenced by temperature and serum starvation. These observations suggest that different Bdr paralogs function in different biological environments and provide insight into the biological basis for maintaining multiple members of this gene family.

Analysis of mechanisms associated with loss of infectivity of clonal populations of Borrelia burgdorferi B31MI

Numerous studies have provided suggestive evidence that the loss of plasmids correlates with the loss of infectivity of the Lyme disease spirochetes. In this study we have further investigated this correlation. Clonal populations were obtained from the skin of a mouse infected for 3 months with a clonal population of Borrelia burgdorferi B31MI. The complete plasmid compositions of these populations were determined using a combination of PCR and Southern hybridization. The infectivities of clones differing in plasmid composition were tested using the C3H-HeJ murine model for Lyme disease. While several clones were found to be noninfectious, a correlation between the loss of a specific plasmid and loss of infectivity in the clones analyzed in this report was not observed. While it is clear from recent studies that the loss of some specific plasmids results in attenuated virulence, this study demonstrates that additional mechanisms also contribute to the loss of infectivity.

Decreased infectivity in Borrelia burgdorferi strain B31 is associated with loss of linear plasmid 25 or 28-1

Previous reports indicated a correlation between loss of plasmids and decreased infectivity of Borrelia burgdorferi strain B31, suggesting that plasmids may encode proteins that are required for pathogenesis. In this study, we expand on this correlation. Using the B. burgdorferi genomic sequence, we designed primers specific for each plasmid, and by using PCR we catalogued 11 linear and 2 circular plasmids from 49 clonal isolates of a mid-passage B. burgdorferi strain B31, initially derived from infected mouse skin, and 20 clones obtained from mouse skin infected with a low-passage isolate of B. burgdorferi strain B31. Among the 69 clones analyzed, nine distinct genotypes were identified relative to wild-type B. burgdorferi strain B31. Among the nine clonal genotypes obtained, only the 9-kb circular plasmid (cp9), the 25-kb linear plasmid (lp25), and either the 28-kb linear plasmid 1 or 4 (lp28-1 and lp28-4, respectively) were missing, in different combinations. We compared the infectivity of the wild-type strain, containing all known B. burgdorferi plasmids, with those of single mutants lacking either lp28-1, lp28-4, or lp25 and a double mutant missing both cp9 and lp28-1. The infectivity data indicated that B. burgdorferi strain B31 cells lacking lp28-4 were modestly attenuated in all tissues analyzed, whereas samples missing lp25 were completely attenuated in all tissues, even at the highest inoculum tested. Isolates without lp28-1 infected the joint tissue yet were not able to infect other tissues as effectively. In addition, we have observed a selection in vivo in the skin, bladder, and joint for cells containing lp25 and in the skin and bladder for cells containing lp28-1, indicating that lp25 and lp28-1 encode proteins required for colonization and short-term maintenance in these mammalian tissues. In contrast, there was no selection in the joint for cells containing lp28-1, suggesting that genes on lp28-1 are not required for colonization of B. burgdorferi within the joint. These observations imply that the dynamic nature of the B. burgdorferi genome may provide the genetic heterogeneity necessary for survival in the diverse milieus that this pathogen occupies in nature and may contribute to tropism in certain mammalian host tissues.