Cryo-Electron Tomography Elucidates the Molecular Architecture of Treponema pallidum, the Syphilis Spirochete

Cryo-electron tomography (CET) was used to examine the native cellular organization of Treponema pallidum, the syphilis spirochete. T. pallidum cells appeared to form flat waves, did not contain an outer coat and, except for bulges over the basal bodies and widening in the vicinity of flagellar filaments, displayed a uniform periplasmic space. Although the outer membrane (OM) generally was smooth in contour, OM extrusions and blebs frequently were observed, highlighting the structure’s fluidity and lack of attachment to underlying periplasmic constituents. Cytoplasmic filaments converged from their attachment points opposite the basal bodies to form arrays that ran roughly parallel to the flagellar filaments along the inner surface of the cytoplasmic membrane (CM). Motile treponemes stably attached to rabbit epithelial cells predominantly via their tips. CET revealed that T. pallidum cell ends have a complex morphology and assume at least four distinct morphotypes. Images of dividing treponemes and organisms shedding cell envelope-derived blebs provided evidence for the spirochete’s complex membrane biology. In the regions without flagellar filaments, peptidoglycan (PG) was visualized as a thin layer that divided the periplasmic space into zones of higher and lower electron densities adjacent to the CM and OM, respectively. Flagellar filaments were observed overlying the PG layer, while image modeling placed the PG-basal body contact site in the vicinity of the stator–P-collar junction. Bioinformatics and homology modeling indicated that the MotB proteins of T. pallidum, Treponema denticola, and Borrelia burgdorferi have membrane topologies and PG binding sites highly similar to those of their well-characterized Escherichia coli and Helicobacter pylori orthologs. Collectively, our results help to clarify fundamental differences in cell envelope ultrastructure between spirochetes and gram-negative bacteria. They also confirm that PG stabilizes the flagellar motor and enable us to propose that in most spirochetes motility results from rotation of the flagellar filaments against the PG

CD14 Modulates PI3K/AKT/p38-MAPK Licensing of Negative Regulators of TLR Signaling to Restrain Chronic Inflammation

Current thinking emphasizes the primacy of CD14 in facilitating TLR recognition of microbes to initiate proinflammatory signaling events and the importance of p38-MAPK in augmenting such responses. Herein, this paradigm is challenged by demonstrating that recognition of _Borrelia burgdorferi_ not only triggers an inflammatory response in the absence of CD14, but one that is uncontrolled as a consequence of impaired PI3K/AKT/p38-MAPK signaling and negative regulation of TLR2. CD14 deficiency results in hyperphosphorylation of AKT and reduced activation of p38. Such aberrant signaling leads to decreased negative regulation by SOCS1, SOCS3, and CIS thereby engendering a more severe and persistent inflammatory response to _B. burgdorferi_. Perturbation of this CD14/p38-MAPK-dependent mechanism of immune regulation may underlie development of infectious chronic inflammatory syndromes

Use of Epivolve phage display to generate a monoclonal antibody with opsonic activity directed against a subdominant epitope on extracellular loop 4 of Treponema pallidum BamA (TP0326)

IntroductionSyphilis, a sexually transmitted infection caused by the spirochete Treponema pallidum (Tp), is resurging globally. Tp’s repertoire of outer membrane proteins (OMPs) includes BamA (β-barrel assembly machinery subunit A/TP0326), a bipartite protein consisting of a 16-stranded β-barrel with nine extracellular loops (ECLs) and five periplasmic POTRA (polypeptide transport-associated) domains. BamA ECL4 antisera promotes internalization of Tp by rabbit peritoneal macrophages.MethodsThree overlapping BamA ECL4 peptides and a two-stage, phage display strategy, termed “Epivolve” (for epitope evolution) were employed to generate single-chain variable fragments (scFvs). Additionally, antisera generated by immunizing mice and rabbits with BamA ECL4 displayed by a Pyrococcus furiosus thioredoxin scaffold (PfTrxBamA/ECL4). MAbs and antisera reactivities were evaluated by immunoblotting and ELISA. A comparison of murine and rabbit opsonophagocytosis assays was conducted to evaluate the functional ability of the Abs (e.g., opsonization) and validate the mouse assay. Sera from Tp-infected mice (MSS) and rabbits (IRS) were evaluated for ECL4-specific Abs using PfTrxBamA/ECL4 and overlapping ECL4 peptides in immunoblotting and ELISA assays.ResultsEach of the five mAbs demonstrated reactivity by immunoblotting and ELISA to nanogram amounts of PfTrxBamA/ECL4. One mAb, containing a unique amino acid sequence in both the light and heavy chains, showed activity in the murine opsonophagocytosis assay. Mice and rabbits hyperimmunized with PfTrxBamA/ECL4 produced opsonic antisera that strongly recognized the ECL presented in a heterologous scaffold and overlapping ECL4 peptides, including S2. In contrast, Abs generated during Tp infection of mice and rabbits poorly recognized the peptides, indicating that S2 contains a subdominant epitope.DiscussionEpivolve produced mAbs target subdominant opsonic epitopes in BamA ECL4, a top syphilis vaccine candidate. The murine opsonophagocytosis assay can serve as an alternative model to investigate the opsonic potential of vaccinogens. Detailed characterization of BamA ECL4-specific Abs provided a means to dissect Ab responses elicited by Tp infection

Regulation of OspE-Related, OspF-Related, and Elp Lipoproteins of Borrelia burgdorferi Strain 297 by Mammalian Host-Specific Signals

This is the published version. Copyright 2001 by the American Society for Microbiology.In previous studies we have characterized the cp32/18 loci inBorrelia burgdorferi 297 which encode OspE and OspF orthologs and a third group of lipoproteins which possess OspE/F-like leader peptides (Elps). To further these studies, we have comprehensively analyzed their patterns of expression throughout the borrelial enzootic cycle. Serial dilution reverse transcription-PCR analysis indicated that although a shift in temperature from 23 to 37°C induced transcription for all nine genes analyzed, this effect was often markedly enhanced in mammalian host-adapted organisms cultivated within dialysis membrane chambers (DMCs) implanted within the peritoneal cavities of rats. Indirect immunofluorescence assays performed on temperature-shifted, in vitro-cultivated spirochetes and organisms in the midguts of unfed and fed ticks revealed distinct expression profiles for many of the OspE-related, OspF-related, and Elp proteins. Other than BbK2.10 and ElpA1, all were expressed by temperature-shifted organisms, while only OspE, ElpB1, OspF, and BbK2.11 were expressed in the midguts of fed ticks. Additionally, although mRNA was detected for all nine lipoprotein-encoding genes, two of these proteins (BbK2.10 and ElpA1) were not expressed by spirochetes cultivated in vitro, within DMCs, or by spirochetes within tick midguts. However, the observation that B. burgdorferi-infected mice generated specific antibodies against BbK2.10 and ElpA1 indicated that these antigens are expressed only in the mammalian host and that a form of posttranscriptional regulation is involved. Analysis of the upstream regions of these genes revealed several differences between their promoter regions, the majority of which were found in the −10 and −35 hexamers and the spacer regions between them. Also, rather than undergoing simultaneous upregulation during tick feeding, these genes and the corresponding lipoproteins appear to be subject to progressive recruitment or enhancement of expression as B. burgdorferi is transmitted from its tick vector to the mammalian host. These findings underscore the potential relevance of these molecules to the pathogenic events of early Lyme disease

Tromp1, a putative rare outer membrane protein, is anchored by an uncleaved signal sequence to the Treponema pallidum cytoplasmic membrane.

Treponema pallidum rare outer membrane protein 1 (Tromp1) has extensive sequence homology with substrate-binding proteins of ATP-binding cassette transporters. Because such proteins typically are periplasmic or cytoplasmic membrane associated, experiments were conducted to clarify Tromp1's physicochemical properties and cellular location in T. pallidum. Comparison of the sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobilities of (i) native Tromp1 and Tromp1 synthesized by coupled in vitro transcription-translation and (ii) native Tromp1 and recombinant Tromp1 lacking the N-terminal signal sequence revealed that the native protein is not processed. Other studies demonstrated that recombinant Tromp1 lacks three basic porin-like properties: (i) the ability to form aqueous channels in liposomes which permit the influx of small hydrophilic solutes, (ii) an extensive beta-sheet secondary structure, and (iii) amphiphilicity. Subsurface localization of native Tromp1 was demonstrated by immunofluorescence analysis of treponemes encapsulated in gel microdroplets, while opsonization assays failed to detect surface-exposed Tromp1. Incubation of motile treponemes with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)-diazarine, a photoactivatable, lipophilic probe, also did not result in the detection of Tromp1 within the outer membranes of intact treponemes but, instead, resulted in the labeling of a basic 30.5-kDa presumptive outer membrane protein. Finally, analysis of fractionated treponemes revealed that native Tromp1 is associated predominantly with cell cylinders. These findings comprise a body of evidence that Tromp1 actually is anchored by an uncleaved signal sequence to the periplasmic face of the T. pallidum cytoplasmic membrane, where it likely subserves a transport-related function

Macrophage mediated recognition and clearance of Borrelia burgdorferi elicits MyD88-dependent and -independent phagosomal signals that contribute to phagocytosis and inflammation.

BACKGROUND: Macrophages play prominent roles in bacteria recognition and clearance, including Borrelia burgdorferi (Bb), the Lyme disease spirochete. To elucidate mechanisms by which MyD88/TLR signaling enhances clearance of Bb by macrophages, we studied wildtype (WT) and MyD88 RESULTS: MyD88 CONCLUSION: Our findings show that MyD88 signaling enhances, but is not required, for bacterial uptake or phagosomal maturation and provide mechanistic insights into how MyD88-mediated phagosomal signaling enhances Bb uptake and clearance

Two Distinct Mechanisms Govern RpoS-mediated Repression of Tick-phase Genes During Mammalian Host Adaptation by Borrelia burgdorferi, the Lyme Disease Spirochete

The alternative sigma factor RpoS plays a key role modulating gene expression in Borrelia burgdorferi, the Lyme disease spirochete, by transcribing mammalian host-phase genes and repressing sigma(70)-dependent genes required within the arthropod vector. To identify cis regulatory elements involved in RpoS-dependent repression, we analyzed green fluorescent protein (GFP) transcriptional reporters containing portions of the upstream regions of the prototypical tick-phase genes ospAB, the glp operon, and bba74 As RpoS-mediated repression occurs only following mammalian host adaptation, strains containing the reporters were grown in dialysis membrane chambers (DMCs) implanted into the peritoneal cavities of rats. Wild-type spirochetes harboring ospAB- and glp-gfp constructs containing only the minimal (-35/-10) sigma(70) promoter elements had significantly lower expression in DMCs relative to growth in vitro at 37 degrees C; no reduction in expression occurred in a DMC-cultivated RpoS mutant harboring these constructs. In contrast, RpoS-mediated repression of bba74 required a stretch of DNA located between -165 and -82 relative to its transcriptional start site. Electrophoretic mobility shift assays employing extracts of DMC-cultivated B. burgdorferi produced a gel shift, whereas extracts from RpoS mutant spirochetes did not. Collectively, these data demonstrate that RpoS-mediated repression of tick-phase borrelial genes occurs by at least two distinct mechanisms. One (e.g., ospAB and the glp operon) involves primarily sequence elements near the core promoter, while the other (e.g., bba74) involves an RpoS-induced transacting repressor. Our results provide a genetic framework for further dissection of the essential gatekeeper role of RpoS throughout the B. burgdorferi enzootic cycle.IMPORTANCEBorrelia burgdorferi, the Lyme disease spirochete, modulates gene expression to adapt to the distinctive environments of its mammalian host and arthropod vector during its enzootic cycle. The alternative sigma factor RpoS has been referred to as a gatekeeper due to its central role in regulating the reciprocal expression of mammalian host- and tick-phase genes. While RpoS-dependent transcription has been studied extensively, little is known regarding the mechanism(s) of RpoS-mediated repression. We employed a combination of green fluorescent protein transcriptional reporters along with an in vivo model to define cis regulatory sequences responsible for RpoS-mediated repression of prototypical tick-phase genes. Repression of ospAB and the glp operon requires only sequences near their core promoters, whereas modulation of bba74 expression involves a putative RpoS-dependent repressor that binds upstream of the core promoter. Thus, Lyme disease spirochetes employ at least two different RpoS-dependent mechanisms to repress tick-phase genes within the mammal

Outer Surface Protein C Is a Dissemination-Facilitating Factor of Borrelia burgdorferi during Mammalian Infection

The Lyme disease spirochete Borrelia burgdorferi dramatically upregulates outer surface protein C (OspC) in response to fresh bloodmeal during transmission from the tick vector to a mammal, and abundantly produces the antigen during early infection. As OspC is an effective immune target, to evade the immune system B. burgdorferi downregulates the antigen once the anti-OspC humoral response has developed, suggesting an important role for OspC during early infection.In this study, a borrelial mutant producing an OspC antigen with a 5-amino-acid deletion was generated. The deletion didn't significantly increase the 50% infectious dose or reduce the tissue bacterial burden during infection of the murine host, indicating that the truncated OspC can effectively protect B. burgdorferi against innate elimination. However, the deletion greatly impaired the ability of B. burgdorferi to disseminate to remote tissues after inoculation into mice.The study indicates that OspC plays an important role in dissemination of B. burgdorferi during mammalian infection