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

Essential role of the response regulator Rrp2 in the infectious cycle of Borrelia burgdorferi

Alteration of surface lipoprotein profiles is a key strategy that the Lyme disease pathogen, Borrelia burgdorferi, has evolved to be maintained within its enzootic cycle between arthropods and mammals. Accumulated evidence indicates that the central regulatory pathway controlling differential gene expression by B. burgdorferi is the RpoN-RpoS pathway (the sigma(54)-sigma(S) sigma factor cascade). It was previously shown that activation of the RpoN-RpoS pathway is controlled by Rrp2, a two-component response regulator and sigma(54)-dependent transcriptional activator. The role of Rrp2 in the infectious cycle of B. burgdorferi has not been determined heretofore. In this report, we demonstrate that an rrp2 mutant defective in activating sigma(54)-dependent transcription was unable to establish infection in mice, but the rrp2 mutant was capable of surviving within ticks during and after tick feeding. Because the rrp2 mutant was defective in the production of OspC, an outer surface lipoprotein essential for mammalian host infection, we further examined whether the loss of infectivity of the rrp2 mutant was solely due to the inability to produce OspC. While transformation with a shuttle vector carrying ospC under the control of a constitutive flaB promoter restored infection to an ospC mutant in immunodeficient SCID mice, it could not rescue the avirulent phenotype of the rrp2 mutant. These data indicate that, in addition to controlling OspC, Rrp2 controls another factor(s) essential for B. burgdorferi to establish infection in mammals. Furthermore, microarray analyses revealed that 125 and 19 genes were positively and negatively regulated, respectively, by Rrp2, which provides a foundation for future identification of additional Rrp2-dependent virulence determinants in B. burgdorferi.

Real-time high resolution 3D imaging of the lyme disease spirochete adhering to and escaping from the vasculature of a living host

Pathogenic spirochetes are bacteria that cause a number of emerging and re-emerging diseases worldwide, including syphilis, leptospirosis, relapsing fever, and Lyme borreliosis. They navigate efficiently through dense extracellular matrix and cross the blood–brain barrier by unknown mechanisms. Due to their slender morphology, spirochetes are difficult to visualize by standard light microscopy, impeding studies of their behavior in situ. We engineered a fluorescent infectious strain of Borrelia burgdorferi, the Lyme disease pathogen, which expressed green fluorescent protein (GFP). Real-time 3D and 4D quantitative analysis of fluorescent spirochete dissemination from the microvasculature of living mice at high resolution revealed that dissemination was a multi-stage process that included transient tethering-type associations, short-term dragging interactions, and stationary adhesion. Stationary adhesions and extravasating spirochetes were most commonly observed at endothelial junctions, and translational motility of spirochetes appeared to play an integral role in transendothelial migration. To our knowledge, this is the first report of high resolution 3D and 4D visualization of dissemination of a bacterial pathogen in a living mammalian host, and provides the first direct insight into spirochete dissemination in vivo.

Pathogenic spirochetes are bacteria that cause a number of emerging and re-emerging diseases worldwide, including syphilis, leptospirosis, relapsing fever, and Lyme disease. They exhibit an unusual form of motility and can infect many different tissues; however, the mechanism by which they disseminate from the blood to target sites is unknown. Direct visualization of bacterial pathogens at the single cell level in living hosts is an important goal of microbiology, since this approach is likely to yield critical insight into disease processes. We engineered a fluorescent strain of Borrelia burgdorferi, a Lyme disease pathogen, and used conventional and spinning disk confocal intravital microscopy to directly visualize these bacteria in real time and 3D in living mice. We found that spirochete interaction with and dissemination out of the vasculature was a multi-stage process of unexpected complexity and that spirochete movement appeared to play an integral role in dissemination. To our knowledge, this is the first report of high resolution 3D visualization of dissemination of a bacterial pathogen in a living mammalian host, and provides the first direct insight into spirochete dissemination in vivo.

Assessment of decorin-binding protein A to the infectivity of Borrelia burgdorferi in the murine models of needle and tick infection

Background

Decorin-binding proteins (Dbps) A and B of Borrelia burgdorferi, the agent of Lyme disease, are surface-exposed lipoproteins that presumably bind to the extracellular matrix proteoglycan, decorin. B. burgdorferi infects various tissues including the bladder, heart, joints, skin and the central nervous system, and the ability of B. burgdorferi to bind decorin has been hypothesized to be important for this disseminatory pathogenic strategy.

Results

To determine the role of DbpBA in the infectious lifecycle of B. burgdorferi, we created a DbpBA-deficient mutant of B. burgdorferi strain 297 and compared the infectious phenotype of the mutant to the wild-type strain in the experimental murine model of Lyme borreliosis. The mutant strain exhibited a 4-log decrease in infectivity, relative to the wild-type strain, when needle inoculated into mice. Upon complementation of the DbpBA-mutant strain with DbpA, the wild-type level of infectivity was restored. In addition, we demonstrated that the DbpBA-deficient mutant was able to colonize Ixodes scapularis larval ticks after feeding on infected mice and persist within the ticks during the molt to the nymphal state. Moreover, surprisingly, the DbpBA-mutant strain was capable of being transmitted to naïve mice via tick bite, giving rise to infected mice.

Conclusion

These results suggest that DbpBA is not required for the natural tick-transmission process to mammals, despite inferences from needle-inoculation experiments implying a requirement for DbpBA during mammalian infection. The combined findings also send a cautionary note regarding how results from needle-inoculation experiments with mice should be interpreted.

The pathogenesis of lyme neuroborreliosis: from infection to inflammation

This review describes the current knowledge of the pathogenesis of acute Lyme neuroborreliosis (LNB), from invasion to inflammation of the central nervous system. Borrelia burgdorferi (B.b.) enters the host through a tick bite on the skin and may disseminate from there to secondary organs, including the central nervous system. To achieve this, B.b. first has to evade the hostile immune system. In a second step, the borrelia have to reach the central nervous system and cross the blood-brain barrier. Once in the cerebrospinal fluid (CSF), the spirochetes elicit an inflammatory response. We describe current knowledge about the infiltration of leukocytes into the CSF in LNB. In the final section, we discuss the mechanisms by which the spirochetal infection leads to the observed neural dysfunction. To conclude, we construct a stringent concept of the pathogenesis of LNB.

Flagellin and outer surface proteins from Borrelia burgdorferi are not glycosylated

We investigated the presence of glycoproteins in Borrelia burgdorferi. We did not find any evidence for glycosylation of the major outer membrane proteins OspA and OspB or the structural flagellar proteins FlaB and FlaA. We suggest that glycoproteins present on the surface of B. burgdorferi may be tightly bound culture medium glycoproteins.

Both decorin-binding proteins A and B are critical for the overall virulence of Borrelia burgdorferi

Both decorin-binding proteins (DbpA and DbpB) of the Lyme disease spirochete Borrelia burgdorferi bind decorin and glycosaminoglycans, two important building blocks of proteoglycans that are abundantly found in the extracellular matrix (ECM) and connective tissues as well as on cell surfaces of mammals. As an extracellular pathogen, B. burgdorferi resides primarily in the ECM and connective tissues and between host cells during mammalian infection. The interactions of B. burgdorferi with these host ligands mediated by DbpA and DbpB potentially influence various aspects of infection. Here, we show that both DbpA and DbpB are critical for the overall virulence of B. burgdorferi in the murine host. Disruption of the dbpBA locus led to nearly a 10(4)-fold increase in the 50% infectious dose (ID50). Complementation of the mutant with either dbpA or dbpB reduced the ID50 from over 10(4) to roughly 10(3) organisms. Deletion of the dbpBA locus affected colonization in all tissues of infected mice. The lack of dbpA alone precluded the pathogen from colonizing the heart tissue, and B. burgdorferi deficient for DbpB was recovered only from 42% of the heart specimens of infected mice. Although B. burgdorferi lacking either dbpA or dbpB was consistently grown from joint specimens of almost all infected mice, it generated bacterial loads significantly lower than the control. The deficiency in either DbpA or DbpB did not reduce the bacterial load in skin, but lack of both significantly did. Taken together, the study results indicate that neither DbpA nor DbpB is essential for mammalian infection but that both are critical for the overall virulence of B. burgdorferi.

Role of the BBA64 locus of Borrelia burgdorferi in early stages of infectivity in a murine model of Lyme disease

Borrelia burgdorferi, the causative agent of Lyme disease, undergoes rapid adaptive gene expression in response to environmental signals encountered during different stages of its life cycle in the arthropod vector or the mammalian host. Among all the plasmid-encoded genes of B. burgdorferi, several linear plasmid 54 (lp54)-encoded open reading frames (ORFs) exhibit the greatest differential expression in response to mammalian host-specific temperature, pH, and other uncharacterized signals. These ORFs include members of the paralogous gene family 54 (pgf 54), such as BBA64, BBA65, and BBA66, present on lp54. In an attempt to correlate transcriptional up-regulation of these pgf 54 members to their role in infectivity, we inactivated BBA64 and characterized the phenotype of this mutant both in vitro and in vivo. There were no major differences in the protein profiles between the BBA64 mutant and the control strains, while immunoblot analysis indicated that inactivation of BBA64 resulted in increased levels of BBA65. Moreover, there was no significant difference in the ability of the BBA64 mutant to infect C3H/HeN mice compared to that of its parental or complemented control strains as determined by culturing of viable spirochetes from infected tissues. However, enumeration of spirochetes using quantitative real-time PCR revealed tissue-specific differences, suggesting a minimal role for BBA64 in the survival of B. burgdorferi in select tissues. Infectivity analysis of the BBA64 mutant suggests that B. burgdorferi may utilize multiple determinants to establish infection in mammalian hosts.

Regulation of expression of the fibronectin-binding protein BBK32 in Borrelia burgdorferi

The BBK32 protein binds to host extracellular ligand fibronectin and contributes to the pathogenesis of Borrelia burgdorferi. Here we showed that expression of the BBK32 gene is influenced by multiple environmental factors and that its regulation is governed by the response regulator Rrp2 and RpoN-RpoS (sigma(54)-sigma(S)) sigma cascade in B. burgdorferi.

Hidden in plain sight: Borrelia burgdorferi and the extracellular matrix

Borrelia burgdorferi, the tick-transmitted etiologic agent of Lyme borreliosis, can colonize and persist in multiple tissue sites despite vigorous host immune responses. The extracellular matrix appears to provide a protective niche for the spirochete. Recent studies in mice suggest that B. burgdorferi interacts in various ways with collagen and its associated molecules, exploiting molecular and structural features to establish microcolonial refugia. Better knowledge of the genetic and structural bases for these interactions of B. burgdorferi with the extracellular matrix will be required before an understanding of the persistence of B. burgdorferi in the tissues and development of chronic infections can be achieved.

Le diagnostic clinique des manifestations articulaires et musculaires de la borréliose de Lyme

The diagnosis of Lyme borreliosis in case of joint and muscular presentations is generally suggested by epidemiological factors. However, as a rule, laboratory testing is required to confirm the diagnosis. When considering the epidemiology of Lyme borreliosis in France, the only areas free of ticks infected by Borrelia burgdorgeri sl, are those close to the Mediterranean sea or at high altitude. The risk is greatest in the Alsace region. Exposure is particularly high among forest workers and people who use the countryside for their leisure activities. The likelihood of infection following a tick bite is difficult to assess; indeed, the bite site may remain unnoticed. A medical history of erythema migrans, if untreated, is a major diagnostic clue, although the association appears to be less consistent in France than in the US. Lyme arthritis generally arises apparently spontaneously. It is characteristically mono- or oligo-articular, asymmetrical, predominantly affects the knee, and has an intermittent course. Synovial cysts and enthesitis are common. Myositis is rare, polymorphic, and has been linked to other symptoms in the same localizations. Minor arthralgia and myalgia frequently occur, principally early in the course of the infection. It was suggested that sequels of the disease include so-called fibromyalgic syndromes. The principal differential diagnosis as far as arthritis is concerned, is made on spondylarthropathy and chronic juvenile arthritis. Rheumatoid arthritis is another pathology, although Lyme arthritis does sometimes evolve to chronicity.

Increasing the interaction of Borrelia burgdorferi with decorin significantly reduces the 50 percent infectious dose and severely impairs dissemination

Tight regulation of surface antigenic expression is crucial for the pathogenic strategy of the Lyme disease spirochete, Borrelia burgdorferi. Here, we report the influence of increasing expression of decorin-binding protein A (DbpA), one of the most investigated spirochetal surface adhesins, on the 50% infectious dose (ID(50)), dissemination, tissue colonization, pathogenicity, and persistence of B. burgdorferi in the murine host. Our in vitro assays showed that increasing DbpA expression dramatically increased the interaction of B. burgdorferi with decorin and sensitivity to growth inhibition/killing by anti-DbpA antibodies; however, this increased interaction did not affect spirochetal growth and replication in the presence of decorin. Increasing DbpA expression significantly reduced ID(50) values and severely impaired dissemination in severe combined immunodeficiency (SCID) and immunocompetent mice. During infection of SCID mice, B. burgdorferi with increased DbpA expression was able to effectively colonize heart and skin tissues, but not joint tissues, completely abrogating arthritis virulence. Although increasing DbpA expression did not affect spirochetal persistence in the skin, it diminished the ability of B. burgdorferi to persist in the heart and joint tissues during chronic infection of immunocompetent mice. Taken together, the study highlights the importance of controlling surface antigen expression in the infectivity, dissemination, tissue colonization, pathogenicity, and persistence of B. burgdorferi during mammalian infection.

Borrelia burgdorferi BBA74, a periplasmic protein associated with the outer membrane, lacks porin-like properties

The outer membrane of Borrelia burgdorferi, the causative agent of Lyme disease, contains very few integral membrane proteins, in contrast to other gram-negative bacteria. BBA74, a Borrelia burgdorferi plasmid-encoded protein, was proposed to be an integral outer membrane protein with putative porin function and designated as a 28-kDa outer membrane-spanning porin (Oms28). In this study, the biophysical properties of BBA74 and its subcellular localization were investigated. BBA74 is posttranslationally modified by signal peptidase I cleavage to a mature 25-kDa protein. The secondary structure of BBA74 as determined by circular dichroism spectroscopy consists of at least 78% alpha-helix with little beta-sheet structure. BBA74 in intact B. burgdorferi cells was insensitive to proteinase K digestion, and indirect immunofluorescence microscopy showed that BBA74 was not exposed on the cell surface. Triton X-114 extraction of outer membrane vesicle preparations indicated that BBA74 is not an integral membrane protein. Taken together, the data indicate that BBA74 is a periplasmic, outer membrane-associated protein that lacks properties typically associated with porins.

Invasion of human neuronal and glial cells by an infectious strain of Borrelia burgdorferi

Human infection by Borrelia burgdorferi, the etiological agent for Lyme disease, can result in serious acute and late-term disorders including neuroborreliosis, a degenerative condition of the peripheral and central nervous systems. To examine the mechanisms involved in the cellular pathogenesis of neuroborreliosis, we investigated the ability of B. burgdorferi to attach to and/or invade a panel of human neuroglial and cortical neuronal cells. In all neural cells tested, we observed B. burgdorferi in association with the cell by confocal microscopy. Further analysis by differential immunofluorescent staining of external and internal organisms, and a gentamicin protection assay demonstrated an intracellular localization of B. burgdorferi. A non-infectious strain of B. burgdorferi was attenuated in its ability to associate with these neural cells, suggesting that a specific borrelial factor related to cellular infectivity was responsible for the association. Cytopathic effects were not observed following infection of these cell lines with B. burgdorferi, and internalized spirochetes were found to be viable. Invasion of neural cells by B. burgdorferi provides a putative mechanism for the organism to avoid the host's immune response while potentially causing functional damage to neural cells during infection of the CNS.

The dbpBA locus of Borrelia burgdorferi is not essential for infection of mice

The Lyme disease spirochete Borrelia burgdorferi expresses a broad array of adhesive molecules, including the decorin-binding proteins A and B (DbpA and DbpB), which are believed to play important roles in mammalian infection. The dbpBA locus was deleted; resulting mutants were able to infect both immunodeficient and immunocompetent mice, indicating that neither DbpA nor DbpB is essential for the infection of mammals, although the DbpAB deficiency may significantly attenuate infectivity potential.

Borrelia burgdorferi lacking BBK32, a fibronectin-binding protein, retains full pathogenicity

BBK32, a fibronectin-binding protein of Borrelia burgdorferi, is one of many surface lipoproteins that are differentially expressed by the Lyme disease spirochete at various stages of its life cycle. The level of BBK32 expression in B. burgdorferi is highest during infection of the mammalian host and lowest in flat ticks. This temporal expression profile, along with its fibronectin-binding activity, strongly suggests that BBK32 may play an important role in Lyme pathogenesis in the host. To test this hypothesis, we constructed an isogenic BBK32 deletion mutant from wild-type B. burgdorferi B31 by replacing the BBK32 gene with a kanamycin resistance cassette through homologous recombination. We examined both the wild-type strain and the BBK32 deletion mutant extensively in the experimental mouse-tick model of the Borrelia life cycle. Our data indicated that B. burgdorferi lacking BBK32 retained full pathogenicity in mice, regardless of whether mice were infected artificially by syringe inoculation or naturally by tick bite. The loss of BBK32 expression in the mutant had no adverse effect on spirochete acquisition (mouse-to-tick) and transmission (tick-to-mouse) processes. These results suggest that additional B. burgdorferi proteins can complement the function of BBK32, fibronectin binding or otherwise, during the natural spirochete life cycle.

Inactivation of the fibronectin-binding adhesin gene bbk32 significantly attenuates the infectivity potential of Borrelia burgdorferi

Borrelia burgdorferi, the aetiological agent of Lyme disease, utilizes multiple adhesins to interact with both the arthropod vector and mammalian hosts it colonizes. One such adhesive molecule is a surface-exposed fibronectin-binding lipoprotein, designated BBK32. Previous characterization of BBK32-mediated fibronectin binding has been limited to biochemical analyses due to the difficulty in mutagenizing infectious isolates of B. burgdorferi. Here we report an alternative method to inactivate bbk32 via allelic exchange through use of a low-passage variant of B. burgdorferi strain B31 that is more readily transformed. The resulting mutant does not synthesize BBK32, exhibits reduced fibronectin binding in solid phase assays and manifests decreased interactions with mouse fibroblast cells relative to both the infectious parent and genetic complement. Furthermore, the bbk32 knockout was significantly attenuated in the murine model of Lyme disease, whereas a genetically complemented control was not, indicating that BBK32 is necessary for maximal B. burgdorferi infection in the mouse. To our knowledge this is the first mutational analysis of a surface exposed, functional borrelial lipoprotein adhesin whose activity is associated with the mammalian host environment. By analogy with other pathogens that utilize fibronectin binding as an important virulence determinant, the borrelial fibronectin-BBK32 interaction is likely to be important in B. burgdorferi-specific pathogenic mechanisms, particularly in the context of dissemination, secondary colonization and/or persistence.

Borrelia burgdorferi–Traveling incognito?

We outline in this review how Borrelia burgdorferi, the causative agent of Lyme disease, moves from the tick to the vertebrate host, and what molecules are potentially involved in this challenging commute. The survival strategies utilized by the spirochete during transmission and the initial stages of infection are discussed.